The Interactive Fly

Zygotically transcribed genes

Genes coding for Neuromuscular Junctional proteins

Presynaptic Active Zone

Five evolutionarily conserved proteins - RIM (Drosophila Rab3 interacting molecule) , Munc13 (Drosophila unc-13), RIM-BP (Drosophila Rim-binding protein), &alpha-liprin (Drosophila Liprin-α), and ELKS (Drosophila Bruchpilot) proteins - form the core of active zones. SYD-1 is a Rho GAP that is essential for synapse assembly in invertebrates but its functional homolog is unknown in vertebrates. RIM, Munc13, and RIM-BP are multidomain proteins composed of a string of identifiable modules, whereas α-liprin and ELKS exhibit a simpler structure. The five core active zone proteins form a single large protein complex that docks and primes synaptic vesicles, recruits Ca2+ channels to the docked and primed vesicles, tethers the vesicles and Ca2+ channels to synaptic cell-adhesion molecules, and mediates synaptic plasticity (Südhof, 2013). Imaging of developing Drosophila glutamatergic synapses revealed that the Unc13B isoform was recruited to nascent active zones by the scaffolding proteins Syd-1 and Liprin-α, and Unc13A is positioned by Bruchpilot and Rim-binding protein complexes at maturing active zones (Bohme, 2016).

Postsynapse of the larval neuromuscular junction

(A) At the postsynaptic membrane, shown at the bottom, Discs large (Dlg) localizes to spectrin-actin complexes. Homophilic adhesion between Fasciclin 2 (Fas2) transmembrane proteins links the presynaptic and postsynaptic sides, with the intracellular C-terminal domains anchored to the first and second PDZ domains of Dlg. The adducin Hu-li tai shao (Hts) is in a complex with Dlg at the postsynaptic membrane, though the interaction may not be direct. Hts also binds to the lipid Phosphatidylinositol 4,5-bisphosphate (PIP2) via the MARCKS-homology domain. (B) Hts promotes the accumulation of par-1 and camkII transcripts in the muscle cytoplasm through an as of yet identified mechanism. PAR-1 and CaMKII phosphorylate Dlg. Phosphorylation disrupts Dlg postsynaptic targeting. (C) Phosphorylation translocates Hts away from the postsynaptic membrane and hinders Hts' ability to regulate Dlg localization, presumably through the control of PAR-1 and CaMKII at the transcriptional level. Phosphorylation of the MARCKS-homology domain also inhibits Hts' ability to bind to PIP2 (Wang, T., 2014).

Neuropilin and tolloid-like (Neto) engages the iGluR complexes extrajunctionally and together they traffic and cluster at the synapses, opposite from the active zones marked by T-bars. Neto and the essential iGluR subunits are limiting for formation of functional iGluR complexes at the NMJ and for growth of synaptic structures (Kim, 2014).

  • Transmission, Development, and Plasticity of Synapses - a review by Kathryn Harris & Troy Littleton

    Biology of the Presynapse
  • The schizophrenia susceptibility gene dysbindin controls synaptic homeostasis
  • Genetic analysis in Drosophila reveals a role for the mitochondrial protein p32 in synaptic transmission
  • The Bruchpilot cytomatrix determines the size of the readily releasable pool of synaptic vesicles
  • Pathogenic Huntington alters BMP signaling and synaptic growth through local disruptions of endosomal compartments
  • Myosin VI contributes to synaptic transmission and development at the Drosophila neuromuscular junction
  • Anterograde Transport of Rab4-Associated Vesicles Regulates Synapse Organization in Drosophila
  • The KIF1A homolog Unc-104 is important for spontaneous release, postsynaptic density maturation and perisynaptic scaffold organization
  • Inhibitory control of synaptic and behavioral plasticity by octopaminergic signaling
  • A Presynaptic ENaC Channel Drives Homeostatic Plasticity
  • Extended synaptotagmin localizes to presynaptic ER and promotes neurotransmission and synaptic growth in Drosophila
  • RIM-binding protein links synaptic homeostasis to the stabilization and replenishment of high release probability vesicles
  • A presynaptic glutamate receptor subunit confers robustness to neurotransmission and homeostatic potentiation
  • Presynaptic spinophilin tunes Neurexin signalling to control active zone architecture and function
  • Neurexin, neuroligin and wishful thinking coordinate synaptic cytoarchitecture and growth at neuromuscular junctions
  • A neuroprotective role for microRNA miR-1000 mediated by limiting glutamate excitotoxicity
  • Loss of skywalker reveals synaptic endosomes as sorting stations for synaptic vesicle proteins
  • Reduced synaptic vesicle protein degradation at lysosomes curbs TBC1D24/sky-induced neurodegeneration
  • Heterotrimeric Go protein links Wnt-Frizzled signaling with ankyrins to regulate the neuronal microtubule cytoskeleton
  • Drosophila Cbp53E regulates axon growth at the meuromuscular junction
  • Distinct regulation of transmitter release at the Drosophila NMJ by different isoforms of nemy
  • The guanine exchange factor Gartenzwerg and the small GTPase Arl1 function in the same pathway with Arfaptin during synapse growth
  • Drosophila neuronal injury follows a temporal sequence of cellular events leading to degeneration at the neuromuscular junction
  • Diminished MTORC1-dependent JNK activation underlies the neurodevelopmental defects associated with lysosomal dysfunction
  • σ2-adaptin facilitates basal synaptic transmission and is required for regenerating endo-exo cycling pool under high frequency nerve stimulation in Drosophila
  • Hsc70-4 deforms membranes to promote synaptic protein turnover by endosomal microautophagy
  • Rab3-GEF controls active zone development at the Drosophila neuromuscular junction
  • A pre-synaptic regulatory system acts trans-synaptically via Mon1 to regulate Glutamate receptor levels in Drosophila
  • Drosophila homolog of human KIF22 at the autism-linked 16p11.2 loci influences synaptic connectivity at larval neuromuscular junctions
  • Na+ /H+ -exchange via the Drosophila vesicular glutamate transporter (DVGLUT) mediates activity-induced acid efflux from presynaptic terminals
  • The Strip-Hippo pathway regulates synaptic terminal formation by modulating actin organization at the Drosophila neuromuscular synapses
  • The innate immune receptor PGRP-LC controls presynaptic homeostatic plasticity
  • An auxiliary subunit of the presynaptic calcium channel, α2δ-3, is required for rapid transsynaptic homeostatic signaling
  • Presynaptic CamKII regulates activity-dependent axon terminal growth
  • The equilibrium between antagonistic signaling pathways determines the number of synapses in Drosophila
  • Skywalker-TBC1D24 has a lipid-binding pocket mutated in epilepsy and required for synaptic function
  • High-probability neurotransmitter release sites represent an energy-efficient design
  • Activity induces Fmr1-sensitive synaptic capture of anterograde circulating neuropeptide vesicles
  • Phosphorylation of Complexin by PKA regulates activity-dependent spontaneous neurotransmitter release and structural synaptic plasticity
  • Fife organizes synaptic vesicles and calcium channels for high-probability neurotransmitter release
  • Input-specific plasticity and homeostasis at the Drosophila larval neuromuscular junction
  • The long 3'UTR mRNA of CaMKII is essential for translation-dependent plasticity of spontaneous release in Drosophila melanogaster
  • Presynaptic DLG regulates synaptic function through the localization of voltage-activated Ca(2+) channels
  • The Ih channel gene promotes synaptic transmission and coordinated movement in Drosophila melanogaster
  • A Ca2+ channel differentially regulates Clathrin-mediated and activity-dependent bulk endocytosis
  • Neuroligin 4 regulates synaptic growth via the Bone morphogenetic protein (BMP) signaling pathway at the Drosophila neuromuscular junction

    Biology of the Postsynapse
  • Drosophila Syncrip modulates the expression of mRNAs encoding key synaptic proteins required for morphology at the neuromuscular junction
  • The EHD protein Past1 controls postsynaptic membrane elaboration and synaptic function
  • Mucin-type core 1 glycans regulate the localization of neuromuscular junctions and establishment of muscle cell architecture in Drosophila
  • Shank modulates postsynaptic wnt signaling to regulate synaptic development
  • The influence of postsynaptic structure on missing quanta at the Drosophila neuromuscular junction
  • Dbo/Henji modulates synaptic dPAK to gate glutamate receptor abundance and postsynaptic response
  • Filamin, a synaptic organizer in Drosophila, determines glutamate receptor composition and membrane growth
  • The Drosophila postsynaptic DEG/ENaC channel ppk29 contributes to excitatory neurotransmission
  • Regulation of neuromuscular junction organization by Rab2 and its effector ICA69 in Drosophila

    Signaling between Pre- and Post-synapse
  • Retrograde signaling by Syt 4 induces presynaptic release and synapse-specific growth
  • Postsynaptic glutamate receptors regulate local BMP signaling at the Drosophila neuromuscular junction
  • MAN1 restricts BMP signaling during synaptic growth in Drosophila
  • Jelly belly trans-synaptic signaling to anaplastic lymphoma kinase regulates neurotransmission strength and synapse architecture
  • Modeling spinal muscular atrophy in Drosophila links Smn to FGF signaling
  • Drosophila Neuroligin3 regulates neuromuscular junction development and synaptic differentiation
  • Anterograde Activin signaling regulates postsynaptic membrane potential and GluRIIA/B abundance at the Drosophila neuromuscular junction
  • Regulation of postsynaptic retrograde signaling by presynaptic exosome release
  • A distinct perisynaptic glial cell type forms tripartite neuromuscular synapses in the Drosophila adult
  • The extracellular-regulated kinase effector Lk6 is required for Glutamate receptor localization at the Drosophila neuromuscular junction
  • Drosophila ortholog of intellectual disability-related ACSL4, inhibits synaptic growth by altered lipids
  • Activity-induced synaptic structural modifications by an activator of integrin signaling at the Drosophila neuromuscular junction
  • Two algorithms for high-throughput and multi-parametric quantification of Drosophila neuromuscular junction morphology
  • Myostatin-like proteins regulate synaptic function and neuronal morphology
  • Secreted tissue inhibitor of matrix metalloproteinase restricts trans-synaptic signaling to coordinate synaptogenesis
  • Notum coordinates synapse development via extracellular regulation of Wnt Wingless trans-synaptic signaling
  • Retrograde semaphorin-plexin signalling drives homeostatic synaptic plasticity

    The Extracellular Matrix
  • A targeted glycan-related gene screen reveals heparan sulfate proteoglycan sulfation regulates WNT and BMP trans-synaptic signaling
  • Endostatin is a trans-synaptic signal for homeostatic synaptic plasticity
  • N-glycosylation requirements in neuromuscular synaptogenesis
  • Three-dimensional imaging of Drosophila motor synapses reveals ultrastructural organizational patterns

  • RNAi-mediated reverse genetic screen identified Drosophila chaperones regulating eye and neuromuscular junction morphology
  • Ultrastructural comparison of the Drosophila larval and adult ventral abdominal neuromuscular junction

    Genes coding for neuromuscular junction proteins

  • The schizophrenia susceptibility gene dysbindin controls synaptic homeostasis

    The molecular mechanisms that achieve homeostatic stabilization of neural function remain largely unknown. To better understand how neural function is stabilized during development and throughout life, an electrophysiology-based forward genetic screen was used, and the function of more than 250 neuronally expressed genes was assessed for a role in the homeostatic modulation of synaptic transmission in Drosophila. This screen ruled out the involvement of numerous synaptic proteins and identified a critical function for dysbindin, a gene linked to schizophrenia in humans. dysbindin was found to be required presynaptically for the retrograde, homeostatic modulation of neurotransmission, and functions in a dose-dependent manner downstream or independently of calcium influx. Thus, dysbindin is essential for adaptive neural plasticity and may link altered homeostatic signaling with a complex neurological disease (Dickman, 2009).

    At glutamatergic synapses of species ranging from Drosophila to human, disruption of postsynaptic neurotransmitter receptor function can be precisely offset by an increase in presynaptic neurotransmitter release to homeostatically maintain normal postsynaptic excitation. The Drosophila neuromuscular junction (NMJ) is a glutamatergic synapse that is used as a model for this form of homeostatic signaling in the nervous system. Efficient homeostatic modulation of presynaptic release at the Drosophila NMJ can occur in ten min following bath application of philanthotoxin-433 (PhTx; a polyamine toxin present in the venom sac of the solitary digger wasp Philanthus triangulum), which persistently and specifically inhibits postsynaptic glutamate receptors (Dickman, 2009).

    This study has systematically screened for mutations that block the rapid, PhTx-dependent induction of synaptic homeostasis. Mutations in 276 genes were screened electrophysiologically. For each mutant, an average value was calculated for the amplitude of both the spontaneous miniature excitatory junctional potential (mEJP) and evoked excitatory junctional potential (EJP) following treatment of the dissected neuromuscular preparation with PhTx for 10 min. 14 mutants were isolated with average EJP amplitudes more than two standard deviations smaller than the distribution mean. From these candidates, 7 mutants were identified that block synaptic homeostasis without an obvious effect on NMJ morphology or baseline synaptic transmission. It is concluded that the molecular mechanisms of synaptic homeostasis can be genetically separated from the mechanisms responsible for normal neuromuscular development and baseline synaptic transmission (Dickman, 2009).

    A fraction of the mutants assayed (19.5%) are previously published genetic lesions. This allows ruling out of the involvement of numerous genes and associated biochemical processes. Mutations that disrupt RNA-interference/micro-RNA processing, retrograde trans-synaptic signaling, synaptic transmission, active zone assembly, synaptic vesicle endocytosis and mitochondria all showed reliable homeostatic compensation. Therefore, synaptic homeostasis is a robust phenomenon, unperturbed by a broad spectrum of synaptic mutations. In addition, significant homeostatic compensation in synaptojanin and endophilin mutants argues against the involvement of synaptic vesicle endocytosis and indicates that the size of the recycling synaptic vesicle pool is not a limiting factor for synaptic homeostasis. These data also emphasize the importance and specificity of those identified mutations that do block synaptic homeostasis. These include four ion channels, two of which are of unknown function, and two calcium-binding proteins of unknown function. Thus, homeostatic signaling at the NMJ may include previously unexplored mechanisms of synaptic modulation (Dickman, 2009).

    One mutation that was identified with a specific defect in homeostatic compensation is a transposon insertion that resides in the Drosophila homologue of dysbindin (CG6856). The DTNBP1 (dysbindin) locus is linked with schizophrenia in humans. A transposon insertion was identified within the dysbindin locus (pBace01028, referred to as dysb1, that showed a complete absence of homeostatic compensation following application of PhTx. A similar effect was observed when dysb1 was placed in trans to a deficiency that uncovers the dysb locus, indicating that the dysb1 mutant was a strong loss of function or null mutation. No significant change in baseline synaptic transmission was observed in dysb1 mutant animals (0.5 mM extracellular calcium). Thus, under these recording conditions, this mutation disrupted synaptic homeostasis without altering baseline neurotransmission. As a control, synaptic homeostasis was normal in animals in which the pBace01028 transposon was precisely excised (Dickman, 2009).

    The dysb gene is ubiquitously expressed in Drosophila embryos. Therefore, a dysbindin transgene was generated and expressed in the dysb1 mutant. Presynaptic expression of dysb fully restored homeostatic compensation in the dysb1 mutant background, whereas muscle-specific expression of dysb did not. Thus, Dysbindin is necessary presynaptically for the rapid induction of synaptic homeostasis (Dickman, 2009).

    It was next asked whether Dysbindin is also required for the sustained expression of synaptic homeostasis. Double mutant animals were generated harboring both the dysb1 mutation and a mutation in a gene encoding a postsynaptic glutamate receptor (GluRIIA). GluRIIA mutant animals normally show robust homeostatic compensation. However, homeostatic compensation was blocked in GluRIIA; dysb1 double mutant animals. Thus, dysbindin was also necessary for the sustained expression of synaptic homeostasis over several days of larval development (Dickman, 2009).

    Synapse morphology was qualitatively normal in dysb mutants including both the shape of the presynaptic nerve terminal and the levels, localization and organization of synaptic markers including futsch-positive microtubules, synapsin and synaptotagmin. Bouton number and active zone density are also normal in dysb mutants. Thus, the disruption of synaptic homeostasis in dysb1 mutants is not a secondary consequence of altered or impaired NMJ development (Dickman, 2009).

    In the vertebrate nervous system, Dysbindin is associated with synaptic vesicles. The localization was examined of a Venus-tagged dysb transgene (ven-dysb) that rescues the dysb1 mutant. Ven-Dysb showed extensive overlap with synaptic vesicle associated proteins when expressed in neurons. Thus, Dysbindin functions presynaptically, potentially at or near the synaptic vesicle pool (Dickman, 2009).

    To further define the function of Dysbindin, baseline synaptic transmission in the dysb mutant was investigated in greater detail. At 0.5 mM extracellular calcium, synaptic transmission in dysb1 mutant animals was indistinguishable from wild type. However, when extracellular calcium was reduced, baseline synaptic transmission was significantly impaired in dysb compared to wild type and this defect was rescued by presynaptic expression of dysb. Thus, there is an alteration of the calcium dependence of synaptic transmission in the dysb mutant. Indeed, at reduced extracellular calcium, both paired-pulse facilitation and facilitation that occurs during a prolonged stimulus train were increased in dysb mutants (Dickman, 2009).

    In vertebrates, the levels of dysb expression correlate with parallel changes in extracellular glutamate concentration. Therefore, whether dysb overexpression might increase presynaptic release was tested. In wild-type animals overexpressing dysb in neurons, synaptic transmission is normal at low extracellular calcium (0.2 and 0.3 mM Ca2+) but was enhanced at relatively higher extracellular calcium (0.5 mM Ca2+). The complementary effects of dysb loss-of-function and overexpression confirm that Dysbindin has an important influence on calcium-dependent vesicle release (Dickman, 2009).

    The presynaptic CaV2.1 calcium channel, encoded by cacophony (cac), is required for synaptic vesicle release at the Drosophila NMJ. cac mutations decrease presynaptic calcium influx and also block synaptic homeostasis. Genetic interaction between dysb and cac was tested during synaptic homeostasis. Because homozygous cac and dysb mutations individually block synaptic homeostasis, analysis of double mutant combinations would not be informative. An analysis of heterozygous mutant combinations and gene overexpression were examined. Synaptic homeostasis was suppressed by a heterozygous mutation in cac. However, this suppression was not enhanced by the presence of a heterozygous mutation in dysb. In addition, neuronal overexpression of cac did not restore homeostatic compensation in dysb mutant animals and the enhancement of presynaptic release caused by neuronal dysb overexpression still occurs in a heterozygous cac mutant background. Thus, Dysbindin may function downstream or independently of Cac during synaptic homeostasis (Dickman, 2009).

    To further explore the relationship between Dysbindin and Cac, it was asked whether dysb mutations might directly influence presynaptic calcium influx. The spatially averaged calcium signal in dysb1 was indistinguishable from wild type, indicating no difference in presynaptic calcium influx. Thus, Dysbindin appears to function downstream or independently of calcium influx to control synaptic homeostasis (Dickman, 2009).

    Through a systematic electrophysiological analysis of more than 250 mutants this study could rule out the involvement of numerous synaptic proteins and biochemical processes in the mechanisms of synaptic homeostasis and demonstrate that this phenomenon is separable from the molecular mechanisms that specify structural and functional synapse development. Dysbindin is therefore identified as an essential presynaptic component within a homeostatic signaling system that regulates and stabilizes synaptic efficacy. Dysbindin functions downstream or independently of the presynaptic CaV2.1 calcium channel in the mechanisms of synaptic homeostasis (Dickman, 2009).

    Emerging lines of evidence suggest that glutamate hypofunction could be related to the etiology of schizophrenia. Likewise, reduced levels of dysbindin expression were associated with schizophrenia. The sandy mouse, which lacks Dysbindin, has a decreased rate of vesicle release (~30% decrease), a correlated decrease in vesicle pool size and an increased thickness of the postsynaptic density. This study confirms a modest, facilitatory function for Dysbindin during baseline transmission. However, numerous mutations with similar or more severe defects in baseline transmission show normal synaptic homeostasis. By contrast, loss of Dysbindin completely blocks the adaptive, homeostatic modulation of vesicle release, suggesting that the potential contribution of dysbindin mutations to schizophrenia may be derived from altered homeostatic plasticity as opposed to decreased baseline glutamatergic transmission (Dickman, 2009).

    Genetic analysis in Drosophila reveals a role for the mitochondrial protein p32 in synaptic transmission

    Mitochondria located within neuronal presynaptic terminals have been shown to play important roles in the release of chemical neurotransmitters. In the present study, a genetic screen for synaptic transmission mutants of Drosophila has identified the first mutation in a Drosophila homolog of the mitochondrial protein P32. Although P32 is highly conserved and has been studied extensively, its physiological role in mitochondria remains unknown and it has not previously been implicated in neural function. The Drosophila P32 mutant, referred to as dp32EC1, exhibited a temperature-sensitive (TS) paralytic behavioral phenotype. Moreover, electrophysiological analysis at adult neuromuscular synapses revealed a TS reduction in the amplitude of excitatory postsynaptic currents (EPSC) and indicated that dP32 functions in neurotransmitter release. These studies are the first to address P32 function in Drosophila and expand the knowledge of mitochondrial proteins contributing to synaptic transmission (Lutas, 2012).

    A genetic screen for synaptic transmission mutants in Drosophila isolated a new mutation in a Drosophila homolog of the mitochondrial protein P32, which represents the first P32 mutation in a multicellular organism. Although P32 is highly conserved and has been studied extensively, its physiological function in mitochondria remains unknown. This new mutant, referred to as dP32EC1, exhibited a temperature-sensitive (TS) paralytic behavioral phenotype. Moreover, electrophysiological analysis at adult neuromuscular synapses revealed a TS reduction in neurotransmitter release, indicating that dP32 serves an important function in synaptic transmission. Immunocytochemical analysis has shown that dP32 is located within presynaptic mitochondria, which are known to be important in ATP production and calcium signaling at synapses. Furthermore, the basic molecular and structural organization of synapses appears to be normal in the dP32 mutant, suggesting a direct role for this protein in synaptic function. At the molecular level, biochemical studies indicated conserved homomultimeric interactions of dP32 subunits. Finally, assessment of presynaptic mitochondrial function was examined in the dP32 mutant through measurement of ATP levels and imaging studies of mitochondrial membrane potential and presynaptic calcium. This work indicated that mitochondrial ATP production and membrane potential in the dP32 mutant resembled wild-type, whereas the mutant exhibited a TS increase in both resting and evoked presynaptic calcium concentration. Taken together, the preceding findings reveal a role for dP32 in synaptic transmission and mitochondrial regulation of presynaptic calcium (Lutas, 2012).

    Mitochondrial localization of P32 proteins involves an N-terminal targeting domain that is cleaved from the mature targeted protein. Comparison of Drosophila and vertebrate P32 sequences indicates conservation of the proteolytic cleavage site in dP32. In the present study, an equivalent targeting function for the N-terminal domain of dP32 was demonstrated through its ability to mediate mitochondrial targeting. When the first 71 amino acids of dP32, including the proteolytic cleavage site, was fused to GCaMP3, this fusion protein (mito-GCaMP3) was efficiently targeted to mitochondria. Although only modest sequence conservation was observed between the N-terminal domains of dP32 and vertebrate P32 proteins, previous studies suggest that mitochondrial targeting domains vary in amino acid sequence but often share an amphipathic helical structure (Lutas, 2012).

    Structural studies have established that P32 is a homotrimer in which monomers are arranged around a central pore in a donut-like structure. In the present study, homomultimerization of dP32 subunits was demonstrated in co-immunoprecipitation experiments. The trimeric structure of P32 exhibits a highly asymmetric charge distribution that creates a concentration of negatively charged residues along one side of the donut, raising the possibility that P32 may participate in calcium binding within the mitochondrial matrix. Notably, five residues that are spatially clustered to form a pocket on the negatively charged side of human P32, Glu-89, Leu-231, Asp-232, Glu-264, and Tyr-268, are identical in the Drosophila protein. Further genetic analysis may address the importance of these clustered residues in dP32 function at synapses (Lutas, 2012).

    Several possible mechanisms of dP32 function in mitochondria and synaptic transmission were considered and investigated in this paper, most notably possible roles in supporting mitochondrial membrane potential, ATP production, and presynaptic calcium signaling. Among these, the observations favor a function for dP32 in mitochondrial mechanisms regulating presynaptic calcium. Although neurotransmitter release was reduced at restrictive temperatures in dP32EC1, the presynaptic calcium concentration was increased both at rest and in response to synaptic stimulation. It is of interest to consider why the increase in presynaptic calcium in dP32EC1 is TS in what appears to be a complete loss-of-function mutant. Previous studies at Drosophila larval neuromuscular synapses at elevated temperatures have observed a TS increase in resting cytosolic calcium and associated inhibition of neurotransmitter release. This calcium increase was enhanced by pharmacological inhibition of presynaptic calcium clearance mechanisms or genetic removal of presynaptic mitochondria, but it remained dependent on temperature. The present findings may reflect a similar TS process involving calcium-dependent inhibition of neurotransmitter release and dP32-dependent mitochondrial mechanisms. Efforts to further address these mechanisms were pursued by employing a calcium indicator targeted to the mitochondrial matrix, mito-GCaMP3. Although this approach was successful for imaging mitochondrial calcium transients elicited by motor axon stimulation in both WT and dP32EC1 at 20°C, robust calcium transients could not be observed in either genotype when the temperature was increased to the restrictive temperatures of 33° or 36° (Lutas, 2012).

    The preceding observations suggest that sustained elevation of presynaptic calcium in the dP32 mutant may lead to reduced neurotransmitter release. Such a calcium-dependent mechanism has been reported previously in the squid giant synapse and attributed to calcium-dependent adaptation of the neurotransmitter release apparatus. Understanding the precise mechanism by which loss of dP32 impairs neurotransmitter release will require further investigation. One interesting question is how the absence of dP32 in the mitochondrial matrix leads to increased presynaptic calcium and whether this reflects the putative calcium binding capacity of this protein. Finally, while the present study is focused on the newly discovered role for P32 in neurotransmitter release, the resulting research materials are expected to facilitate in vivo analysis of P32 function in a broad range of biological processes (Lutas, 2012).

    The Bruchpilot cytomatrix determines the size of the readily releasable pool of synaptic vesicles

    Synaptic vesicles (SVs) fuse at a specialized membrane domain called the active zone (AZ), covered by a conserved cytomatrix. How exactly cytomatrix components intersect with SV release remains insufficiently understood. Previous studies have shown that loss of the Drosophila ELKS family protein Bruchpilot (BRP) eliminates the cytomatrix (T bar) and declusters Ca2+ channels. This paper explores additional functions of the cytomatrix, starting with the biochemical identification of two BRP isoforms. Both isoforms alternated in a circular array and are important for proper T-bar formation. Basal transmission is decreased in isoform-specific mutants, attributable to a reduction in the size of the readily releasable pool (RRP) of SVs. A corresponding reduction was found in the number of SVs docked close to the remaining cytomatrix. It is proposed that the macromolecular architecture created by the alternating pattern of the BRP isoforms determines the number of Ca2+ channel-coupled SV release slots available per AZ and thereby sets the size of the RRP (Matkovic, 2013).

    An elaborate protein cytomatrix covering the AZ membrane is meant to facilitate and control the SV release process. Quantitative analysis of neurotransmitter release has provided evidence that the number of SV release sites per AZ might be fixed. Although these sites are thought to be located in close proximity to presynaptic Ca2+ channels, ultrastructural and molecular information is largely missing here. Potentially, specific interactions between SVs and certain cytomatrix components might be involved. This study provides evidence that the BRP-based cytomatrix plays a role in defining the number of readily releasable SVs, possibly by offering morphological and molecular-determined 'release slots' (Matkovic, 2013).

    Previous studies have characterized the role of BRP based on null alleles, which result in a complete absence of AZ cytomatrix (T bar), partially declustered Ca2+ channels, and likely as a direct consequence, reduced vesicular release probability. In contrast, in the analysis of BRP isoform-specific mutants, the current study neither observed any Ca2+ channel clustering deficits nor changes in vesicular release probability (Matkovic, 2013).

    Previous studies have found a binding site between the intracellular C terminus of the Cac Ca2+ channel and an N-terminal stretch of BRP, which is unique to BRP-190 (Fouquet, 2009). That solely losing BRP-190 is not sufficient to affect Ca2+ channel clustering could possibly be explained by the presence of redundant binding sites within BRP-170. Ca2+ channel clustering might well be a collective feature of the cytomatrix, and Ca2+ channels likely use multiple simultaneous interactions with several cytomatrix proteins to anchor within the AZ membrane (Matkovic, 2013).

    In fact, RIM-binding protein family proteins at rodent and Drosophila AZs bind Ca2+ channels, and loss of the only RIM-binding protein in Drosophila results in partial loss of Ca2+ channels from AZs. RIM-binding protein levels at AZ were slightly but significantly reduced in the BRP isoform mutants. Clearly, it remains a possibility that RIM-binding protein is a major scaffold determinant of the release slots and that e.g., subtle mislocalizations of RIM-binding protein might in part contribute to the BRP isoform mutant phenotype. The brp-null phenotype can now be interpreted as a 'catastrophic event' in which a complete loss of this large scaffold protein leads to a severe decrease of cytomatrix avidity (potentially mediated via a loss of RIM-binding protein) below a critical level, resulting in a 'collapse' of the normal cytomatrix architecture. Thus, functionalities associated with discrete regions of BRP and RIM-binding protein can apparently be masked when the BRP-based AZ scaffold is completely eliminated (Matkovic, 2013).

    The distal cytomatrix in brpnude is bare of SVs in EM, and SV replenishment is defective, resulting in short-term depression (and not facilitation as in brp nulls). However, no change of short-term plasticity could be detected in the brp isoform alleles with the same analyses, consistent with neither a change in Ca2+ channel clustering nor in SV clustering at the distal cytomatrix. Nevertheless, a basal release deficit was observed, which can be explained by a reduction in the size of the readily releasable vesicle pool, assigning an additional function to the BRP cytomatrix (Matkovic, 2013).

    Release-ready SVs are meant to be molecularly and positionally primed for release. Important factors are the equipment with or the attachment to the proteins of the core release machinery and the localization of the SV in proximity to the Ca2+ source. At the Drosophila NMJ, SV release is insensitive to slow Ca2+ buffers such as EGT; therefore, SVs are thought to be spatially tightly coupled to Ca2+ channels (nanodomain coupling; Eggermann, 2012). Since Ca2+ channels are found localized directly underneath the T-bar pedestal composed of the N-terminal region of BRP (Fouquet, 2009), release-ready SVs might well correspond to the SVs that were found docked at the pedestal of the T bar and thus in very close proximity to the Ca2+ channels. This in turn is in agreement with BRP itself being important for defining the number of release-ready SVs determined by electrophysiology and EM (Matkovic, 2013).

    Light microscopic inspection of an AB directed against the C terminus of BRP, common to both isoforms, with 50-nm STED resolution, typically revealed approximately five dots arranged as a circle or regular pentagon. Both isoforms were labelled individually, and it was found that (1) both isoforms seem to localize with their C termini similarly toward the distal edge of the cytomatrix and (2) both isoforms typically form an identical number of dots per AZ similar to the number of dots observed with the BRPC-Term AB recognizing both isoforms. Thus, the BRP isoforms seem to be arranged in neighboring but not overlapping clusters, forming a circular array. Consistent with both BRP isoforms not overlapping in space, there was neither efficient co-IP between them nor did elimination of one isoform substantially interfere with the AZ localization of the respective other isoform. Thus, BRP-190 and -170 seem to form discrete oligomers. The alternating pattern of BRP-190 and -170 appears to set a typical cytomatrix size, as both isoform mutants had a reduced T-bar width in EM and a reduced mean number of BRP dots per AZ. As this corresponded with a similar reduction in the number of SVs in the RRP, this AZ architecture could set a typical number of Ca2+ nanodomain-coupled RRP slots possibly located between BRP clusters. However, beyond providing a discrete morphological architecture, the two BRP isoforms described in this study might harbor additional functionalities. The brpΔ190 phenotype was more pronounced than the brpΔ170, leaving the possibility that the highly conserved N terminus of BRP-190 promotes release by further mechanisms going beyond the points analyzed in this study. Future analysis will also have to address whether localization and regulation of additional cytomatrix and release components, such as RIM-binding protein, Unc-13 family proteins, or RIM, contribute to the formation of release slots as well (Matkovic, 2013).

    Ultimately, functional differences between individual synaptic sites must be defined by variances in their molecular organization. Functional features of a synapse can be extracted electrophysiologically. Thereby, the number of Ca2+ channels was recently identified as a major determinant of the release probability of single vesicles, Pvr, in rat calyces (Sheng, 2012). Furthermore, AZ size seems to scale with the overall likelihood of release from a given AZ (Holderith, 2012). The current results suggest that the BRP-based cytomatrix should be a general determinant of the release likelihood per AZ by establishing Pvr, through Ca2+ channel clustering, as shown previously, and, as shown in this study, by determining the size of the RRP. The genetic results show that the cytomatrix can, in principle, control the RRP size independent of Ca2+ channel clustering. A coupled increase in the size of the T-bar cytomatrix together with increasing SV release was previously observed at NMJs compensating for loss of the glutamate receptor subunit glurIIA. Moreover, an increase in the number of release-ready SVs together with an increase in the amount of BRP was recently described as part of a homeostatic presynaptic response after pharmacological block of postsynaptic GluRIIA (Weyhersmuller, 2011). In line with this scenario, it was recently shown that lack of acetylation of BRP in elp3 mutants led to an increase in the complexity of the AZ cytomatrix along with an increase in RRP size (Miskiewicz, 2011). Furthermore, in vivo imaging of synaptic transmission with single synapse resolution revealed that the likelihood of release correlates with the amount of BRP present at an individual AZ (Peled, 2011). This cytomatrix size-SV release scaling might be a general principle, as a correlation between the amount of SV exocytosis, measured by an optical assay, and the amount of the AZ protein Bassoon at individual synapses of cultured rat hippocampal neurons has also been observed (Matz, 2010). The current results suggest that not only the mere size, but also the distinct architecture of the cytomatrix influence release at individual synapses through determining RRP size (Matkovic, 2013).

    Pathogenic Huntington alters BMP signaling and synaptic growth through local disruptions of endosomal compartments

    Huntington's disease (HD) is a neurodegenerative disorder caused by expansion of a polyglutamine (polyQ) stretch within the Huntingtin (Htt) protein. Pathogenic Htt disrupts multiple neuronal processes, including gene expression, axonal trafficking, proteasome and mitochondrial activity, and intracellular vesicle trafficking. However, the primary pathogenic mechanism and subcellular site of action for mutant Htt are still unclear. Using a Drosophila HD model, this study found that pathogenic Htt expression leads to a profound overgrowth of synaptic connections that directly correlates with the levels of Htt at nerve terminals. Branches of the same nerve containing different levels of Htt show distinct phenotypes, indicating Htt acts locally to disrupt synaptic growth. The effects of pathogenic Htt on synaptic growth arise from defective synaptic endosomal trafficking, leading to expansion of a recycling endosomal signaling compartment containing Sorting Nexin 16, and a reduction in late endosomes containing Rab11. The disruption of endosomal compartments leads to elevated BMP signaling within nerve terminals, driving excessive synaptic growth. Blocking aberrant signaling from endosomes or reducing BMP activity (see Wishfull thinking) ameliorates the severity of HD pathology and improves viability. Pathogenic Htt is present largely in a non-aggregated form at synapses, indicating cytosolic forms of the protein are likely to be the toxic species that disrupt endosomal signaling. These data indicate that pathogenic Htt acts locally at nerve terminals to alter trafficking between endosomal compartments, leading to defects in synaptic structure that correlate with pathogenesis and lethality in the Drosophila HD model (Akbergenova, 2017).

    Myosin VI contributes to synaptic transmission and development at the Drosophila neuromuscular junction

    Myosin VI, encoded by jaguar (jar) in Drosophila, is a unique member of the myosin superfamily of actin-based motor proteins. Myosin VI is the only myosin known to move towards the minus or pointed ends of actin filaments. Although Myosin VI has been implicated in numerous cellular processes as both an anchor and a transporter, little is known about the role of Myosin VI in the nervous system. Previous studies recovered jar in a screen for genes that modify neuromuscular junction (NMJ) development and this study reports on the genetic analysis of Myosin VI in synaptic development and function using loss of function jar alleles. The experiments on Drosophila third instar larvae revealed decreased locomotor activity, a decrease in NMJ length, a reduction in synaptic bouton number, and altered synaptic vesicle localization in jar mutants. Furthermore, studies of synaptic transmission revealed alterations in both basal synaptic transmission and short-term plasticity at the jar mutant neuromuscular synapse. Altogether these findings indicate that Myosin VI is important for proper synaptic function and morphology. Myosin VI may be functioning as an anchor to tether vesicles to the bouton periphery and, thereby, participating in the regulation of synaptic vesicle mobilization during synaptic transmission (Kisiel, 2011).

    Although Myosin VI function in the vesicle cycle has been implicated in mammalian cells, this report provides the first evidence that Myosin VI is important for maintaining normal peripheral vesicle localization at the bouton. In Drosophila, there are four types of boutons, which are the sites of neurotransmitter release at the NMJ, and they differ in their morphological and chemical properties. Of interest for this study were the largest synaptic boutons found at type I axon terminals, which are present at all NMJs of mature larvae. Visualization of synaptotagmin staining using confocal imaging revealed a mislocalization of synaptic vesicles in jar mutant boutons. An increasing number of jar mutant boutons, corresponding to the severity of Myosin VI loss of function, were found to exhibit diffuse staining over the entire bouton area as opposed to the doughnut-shaped staining pattern present in control boutons. Bouton centre occupancy has previously been observed at Drosophila NMJs of larvae lacking synapsin, a phosphoprotein that reversibly associates with vesicles, using FM1-43 loading under low frequencies. EM analysis confirmed that in synapsin knockouts there was a spread of vesicles into the bouton centre, accompanied by a reduction in the size of the reserve pool. Thus, synapsin is thought to function in maintaining the peripheral distribution of vesicles in Ib boutons. Likewise, the unexpected diffuse synaptotagmin staining of jar mutant boutons suggests Myosin VI participates in restricting vesicles to the bouton periphery. It is possible that Myosin VI is functioning as a regulator of the actin cytoskeleton at the synapse. Mutant studies have revealed that the presynaptic actin cytoskeleton is required for proper synaptic morphogenesis. Myosin VI has already been shown to function in regulating the actin cytoskeleton during the process of spermatid individualization, by acting either as a structural cross-linker or as an anchor at the front edge of the actin cone, and during nuclear divisions in the syncytial blastoderm. However, live imaging of actin dynamics at the synaptic boutons revealed no major defects in the actin cytoskeleton at jar loss of function mutant nerve terminals (Kisiel, 2011).

    To assess whether the morphological defects in vesicle localization observed at jar mutant synapses impact synaptic transmission, electrophysiological assays with different stimulation paradigms were used to recruit vesicles from different functional pools. The data add to the knowledge of this protein's physiological role at synapses. Myosin VI mutant mouse hippocampal neurons exhibit defects in the internalization of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-type glutamate receptor, responsible for fast glutamatergic transmission, suggesting Myosin VI normally plays a role in AMPAR endocytosis. In addition, basal synaptic transmission is reduced in Myosin VI deficient mouse hippocampal slices compared to wild-type controls. Electrophysiological experiments also indicate that Myosin VI mediates glutamate release induced by brain-derived neurotrophic factor, which is known to modulate synaptic transmission and plasticity in the mammalian central and peripheral nervous system (Kisiel, 2011).

    This study is the first to show that Myosin VI's role in synaptic transmission involves mobilization of vesicles from different functional pools, indicating that Myosin VI is important for synaptic plasticity. At the Drosophila NMJ, three pools of vesicles with differential release properties have been identified using FM1-43 staining loaded by various stimulation protocols. The immediately releasable pool (IRP), representing approximately 1% of all vesicles at the NMJ, consists of vesicles docked and primed at active zones for immediate release and experiences rapid depletion within a few stimuli. The readily releasable pool (RRP), making up 14% to 19% of all vesicles at the NMJ, is mobilized by moderate stimulation of ≤3 Hz and maintains exo/endocytosis at these stimulation frequencies. The reserve pool (RP) represents the vast majority of vesicles, 80% to 90%, and is mobilized upon depletion of the RRP. Recruitment from the RP occurs with high frequency stimulation of ≥10 Hz. Spontaneous release was reduced in the most severe jar loss of function mutants. Evoked response at 1 Hz stimulation was also reduced in the jar maternal null mutant. Although less severe jar mutants exhibited a significant decrease in bouton number, they did not experience an accompanying reduction in evoked potential amplitude at low frequency stimulation, suggesting that other homeostatic mechanisms are important for maintaining synaptic strength. The impaired synaptic response in the jar maternal null mutant may be due to a reduction in the probability of RRP vesicle release or in RRP size. If Myosin VI functions to anchor synaptic vesicles, it may act on the RRP to ensure vesicles are localized in manner that makes them readily available for release. Thus, in jar maternal null mutants the reduction in EJP amplitude may occur because a significant number of vesicles were displaced from areas of higher probability release. Alternately, RRP pool size may be reduced at jar mutant synapses (Kisiel, 2011).

    Different synaptic vesicle pool properties, such as rate of recruitment of the RP in response to high frequency stimuli, may translate to changes in short-term synaptic plasticity. The increase in EJP amplitude observed at 10 Hz stimulation in 1 mM Ca2+ saline may be attributable to enhanced mobilization of the RP for jar322/Df(3R)crb87-5 NMJs. Filamentous actin has been implicated in RP mobilization as cytochalasin D, an inhibitor of actin polymerization, has been shown to reduce RP dynamics. This suggests translocation from the RP to the RRP may be mediated by an actin-based myosin motor protein. If Myosin VI functions as a synaptic vesicle tether to regulate recruitment from the RP pool, RP vesicles would be more readily mobilized and transitioned into the RRP upon high frequency stimulation in jar loss of function mutants. Consistent with the idea that RP vesicles were more rapidly incorporated into the RRP, a greater initial depression is observed at jar322/Df(3R)crb87-5 mutant synapses during high frequency stimulation in 10 mM Ca2+ saline corresponding to the depletion of vesicles at high calcium concentrations. Taken together, the data suggest that Myosin VI mediates synaptic transmission and short-term plasticity by regulating the mobilization of synaptic vesicles from different functional pools. In mammalian cells, Myosin VI has been implicated as a mediator of vesicle endocyctosis and has been shown to transport uncoated vesicles through the actin-rich periphery to the early endosome. The current experiments, however, indicate that endocytosis is not likely affected at jar mutant synapses. Typically, endocytotic mutants are unable to maintain synaptic transmission in response to high frequency stimulation, whereas the Myosin VI loss of function mutants exhibited enhanced EJP amplitude observed at 10 Hz stimulation in 1 mM Ca2+ saline. Additional experiments are required to confirm that Myosin VI is functioning as a vesicle tether. Fluorescence recovery after photobleaching analysis can be used to examine the effect of Myosin VI on synaptic vesicle mobility. If Myosin VI is functioning as a vesicle tether, synaptic vesicle mobility is expected to be increased in jar mutants compared to controls (Kisiel, 2011).

    In summary, the present work shows that Myosin VI is important for proper synaptic morphology and physiology at the Drosophila NMJ. Myosin VI function in peripheral vesicle localization at the bouton may underlie its contribution to basal synaptic transmission and expression of synaptic plasticity. Future work will address the mechanism by which Myosin VI performs its roles at the synapse, whether as a vesicle tether or by some other involvement in vesicle trafficking (Kisiel, 2011).

    Anterograde Transport of Rab4-Associated Vesicles Regulates Synapse Organization in Drosophila

    Local endosomal recycling at synapses is essential to maintain neurotransmission. Rab4 GTPase, found on sorting endosomes, is proposed to balance the flow of vesicles among endocytic, recycling, and degradative pathways in the presynaptic compartment. This study reports that Rab4-associated vesicles move bidirectionally in Drosophila axons but with an anterograde bias, resulting in their moderate enrichment at the synaptic region of the larval ventral ganglion. Results from FK506 binding protein (FKBP) and FKBP-Rapamycin binding domain (FRB) conjugation assays in rat embryonic fibroblasts together with genetic analyses in Drosophila indicate that an association with Kinesin-2 (mediated by the tail domain of Kinesin-2α/KIF3A/KLP64D subunit) moves Rab4-associated vesicles toward the synapse. Reduction in the anterograde traffic of Rab4 causes an expansion of the volume of the synapse-bearing region in the ventral ganglion and increases the motility of Drosophila larvae. These results suggest that Rab4-dependent vesicular traffic toward the synapse plays a vital role in maintaining synaptic balance in this neuronal network (Dey, 2017).

    The KIF1A homolog Unc-104 is important for spontaneous release, postsynaptic density maturation and perisynaptic scaffold organization

    The kinesin-3 family member KIF1A has been shown to be important for experience dependent neuroplasticity. In Drosophila, amorphic mutations in the KIF1A homolog unc-104 disrupt the formation of mature boutons. Disease associated KIF1A mutations have been associated with motor and sensory dysfunctions as well as non-syndromic intellectual disability in humans. A hypomorphic mutation in the forkhead-associated domain of Unc-104, unc-104bris, impairs active zone maturation resulting in an increased fraction of post-synaptic glutamate receptor fields that lack the active zone scaffolding protein Bruchpilot. This study shows that the unc-104bris mutation causes defects in synaptic transmission as manifested by reduced amplitude of both evoked and miniature excitatory junctional potentials. Structural defects observed in the postsynaptic compartment of mutant NMJs include reduced glutamate receptor field size, and altered glutamate receptor composition. In addition, there is a marked loss of postsynaptic scaffolding proteins and reduced complexity of the sub-synaptic reticulum, which can be rescued by pre- but not postsynaptic expression of unc-104. These results highlight the importance of kinesin-3 based axonal transport in synaptic transmission and provide novel insights into the role of Unc-104 in synapse maturation (Y. V. Zhang, 2017).

    Inhibitory control of synaptic and behavioral plasticity by octopaminergic signaling

    Adrenergic receptors and their ligands are important regulators of synaptic plasticity and metaplasticity, but the exact mechanisms underlying their action are still poorly understood. Octopamine, the invertebrate homolog of mammalian adrenaline or noradrenaline, plays important roles in modulating behavior and synaptic functions. Previous work (Koon, 2011) uncovered an octopaminergic positive-feedback mechanism to regulate structural synaptic plasticity during development and in response to starvation. Under this mechanism, activation of Octβ2R autoreceptors by octopamine at octopaminergic neurons initiated a cAMP-dependent cascade that stimulated the development of new synaptic boutons at the Drosophila larval neuromuscular junction (NMJ). However, the regulatory mechanisms that served to brake such positive feedback were not known. This study reports the presence of an alternative octopamine autoreceptor, Octβ1R, with antagonistic functions on synaptic growth. Mutations in octβ1r result in the overgrowth of both glutamatergic and octopaminergic NMJs, suggesting that Octβ1R is a negative regulator of synaptic expansion. As does Octβ2R, Octβ1R functions in a cell-autonomous manner at presynaptic motorneurons. However, unlike Octβ2R, which activates a cAMP pathway, Octβ1R likely inhibits cAMP production through inhibitory Goα. Despite its inhibitory role, Octβ1R is required for acute changes in synaptic structure in response to octopamine and for starvation-induced increase in locomotor speed. These results demonstrate the dual action of octopamine on synaptic growth and behavioral plasticity, and highlight the important role of inhibitory influences for normal responses to physiological stimuli (Koon, 2012).

    Adrenaline/noradrenaline and their receptors have emerged as important modulators of synaptic plasticity, metaplasticity, and behavior in the mammalian brain. However, the mechanisms underlying this regulation of synaptic structure are not known (Koon, 2012 and references therein).

    In insects, adrenergic signaling is accomplished through octopamine and octopamine receptors and is a powerful modulator of behaviors such as appetitive behavior and aggression. It also regulates synaptic function and synaptic structure (Koon, 2011; Koon, 2012).

    A previous study has demonstrated that at the Drosophila larval neuromuscular junction (NMJ) octopamine regulates the expansion of both modulatory and excitatory nerve terminals (Koon, 2011). Larval NMJs are innervated by glutamatergic, octopaminergic, and peptidergic motorneurons. Of these, glutamatergic nerve terminals provide classical excitatory transmission, while octopaminergic nerve endings support global modulation of excitability and synaptic growth. Larval NMJs are continuously expanding to compensate for muscle cell growth and respond to acute changes in activity by extending new synaptic boutons. By binding to the octopamine autoreceptor Octβ2R, octopamine activates a cAMP second messenger pathway that leads to CREB activation and transcription, which in turn promotes the extension of new octopaminergic nerve endings (Koon, 2011). This positive-feedback mechanism was required for an increase in locomotor activity in response to starvation. In addition, this mechanism positively regulated the growth of glutamatergic nerve endings through Octβ2R receptors present in glutamatergic neurons. An important question regards the mechanisms that serve to brake such positive feedback (Koon, 2012).

    This study demonstrates the presence of a second octopamine receptor, Octβ1R, which serves as such a brake. Octβ1R is also an autoreceptor in octopaminergic neurons that serves to inhibit synaptic growth. This inhibitory influence is excerpted through the activation of the inhibitory G-protein subunit, Goα, and thus by limiting cAMP production. Like Octβ2R receptors, Octβ1R receptors are also present at excitatory glutamatergic endings. Thus, octopamine release induces a dual excitatory (through Octβ2R) and inhibitory (through Octβ1R) function on the growth of both octopaminergic and glutamatergic endings. The presence of both the excitatory and inhibitory receptors is required for normal structural plasticity at octopaminergic terminals and for normal responses to starvation, as obliterating Octβ1R (this study) or Octβ2R (Koon, 2011) prevents the acute growth of octopaminergic ending in response to octopamine and the increase in locomotor speed in response to starvation. Thus, this study highlights the requirement of both excitatory and inhibitory influences for normal synaptic and behavioral plasticity (Koon, 2012).

    The previous study demonstrated that octopamine regulates synaptic and behavioral plasticity through an autoregulatory positive-feedback mechanism involving Octβ2R, which promotes both type I and type II outgrowth (Koon, 2011). This study has now identified an octopamine receptor, Octβ1R, which antagonizes the function of Octβ2R. It is proposed that Octβ1R may serve as a brake for the positive feedback induced by Octβ2R. Octβ1R receptors inhibit the cAMP pathway via the inhibitory G-protein Goα, as loss of Octβ1R or Goα function results in synaptic overgrowth of type I and type II endings in an octopamine cell-autonomous manner, and as octβ1r and goα interact genetically. Notably, defective Octβ1R signaling appears to saturate cAMP levels, occluding the function of Octβ2R. Thus, the loss of Octβ1R function results in insensitivity to octopamine stimulation. In turn, this abolishes starvation-induced behavioral changes that require Octβ2R signaling. While this study centered primarily on Octβ1R function at octopaminergic NMJ terminals, it is important to emphasize that octopamine neurons are also present in the larval brain. Thus, with current tools it is not possible to discern whether the defects are exclusively due to the function of octopaminergic motorneurons, or whether other central octopaminergic neurons contribute to these effects. While the phenotypes on NMJ development are most parsimoniously explained by a local function at NMJ terminals, it is likely that the behavioral effects are more complex, also involving important contribution from brain octopaminergic neurons (Koon, 2012).

    At the Drosophila larval NMJ, three type II motorneurons innervate most of the body wall muscles in each segment (Koon, 2011). This layout suggests that octopamine is likely to globally regulate plasticity, by tuning the excitability levels of multiple excitatory synapses on the body wall muscles. Together, the observations in the previous study (Koon, 2011) and this investigation identify the presence of excitatory and inhibitory octopamine receptors that are coexpressed in the same cells. This suggests that global regulation of synapses and behavior by octopamine can be tipped toward excitation or inhibition depending on receptor expression levels, affinity of the receptors for octopamine, and availability of these receptors for binding octopamine on the target cells. This dual mode of controlling excitability likely provides enhanced flexibility, allowing a broader level of control over synaptic functions (Koon, 2012).

    An important question is how can Octβ1R and Octβ2R regulate development of innervation and behavior given that they are activated by the same ligand, are localized in the same cells, and their functions are antagonistic. Several alternatives can be proposed. Octβ1R and Octβ2R might have different affinities for octopamine binding. Thus, different levels of octopamine release could differentially activate the receptors. For instance, if Octβ1R receptors have higher affinity for octopamine, and octopamine is normally released at low levels, a stable degree of innervation could be maintained by continuous inhibition of synaptic growth-promoting signals. High levels of octopamine release, as would occur during starvation, would then activate the lower affinity Octβ2R, eliciting synaptic growth. Precedence for this type of regulation has been obtained in honeybees and olive fruit flies, where low concentrations of octopamine are inhibitory while high concentrations are excitatory to cardiac contraction (Koon, 2012).

    An alternative possibility is based on the well known internalization of GPCRs upon ligand binding. It is possible that such a mechanism would maintain an appropriate ratio of Octβ1R and Octβ2R at the cell surface, actively keeping or removing octopamine receptor-mediated excitation or inhibition, depending on physiological states. A third alternative is that receptors could be posttranslationally modified upon ligand binding, which might also affect their downstream functions. For example, dimerization of β2-adrenergic receptors can inhibit its adenylate cyclase-activating activity and phosphorylation of β1-adrenergic receptor by PKA reduces its affinity for Gsα and increases its affinity for Giα/oα. Last, Octβ1R and Octβ2R receptors could be spatially separated in neurons, with one receptor being closer and the other distant to sites of octopamine release. In this scenario, the receptors would likely be exposed to different octopamine concentration (Koon, 2012).

    Simultaneous expression of excitatory and inhibitory GPCRs in the same neuron has been reported previously. For instance, mammalian dopamine receptors can couple to both stimulatory and inhibitory G-proteins, with the D1 receptor-like family being coupled to Gsα and the D2-like family being coupled to Giα/oα (Koon, 2012).

    Previous studies have investigated the effect of octopamine on synaptic transmission at the Drosophila first-instar and third-instar larval NMJ. While the studies at the third-instar larval NMJ demonstrated an excitatory effect of octopamine in neurotransmission, the study on the first-instar larval stage substantiated an inhibitory effect. A recent study now provides a potential explanation for such discrepancy between the responses to octopamine at the two larval stages). In particular, it was found that that Octβ1R is expressed at high levels in first instar and at low levels in third instar. In contrast, Octβ2R is expressed at low levels in first instar and at high levels in third instar. The current studies demonstrating an inhibitory role for Octβ1R (this study) and an excitatory role for Octβ2R (Koon, 2011) are in agreement with the idea that octopamine may play an inhibitory role during first instar, but an excitatory role during third instar (Koon, 2012).

    Octopamine receptors have been shown to elicit intracellular Ca2+ and/or cAMP increase. OAMB, the only α-adrenergic-like receptor in Drosophila, has been implicated to function via Ca2+ signaling in the Drosophila oviduct. However, OAMB is expressed in the oviduct epithelium, and not in the oviduct muscle cells. Given that octopamine induces relaxation of oviduct muscles, the presence of an alternative, inhibitory octopamine receptor in oviduct muscles was proposed. The identification of Octβ1R receptor as an inhibitory receptor raises the possibility that this is the inhibitory receptor in the oviduct (Koon, 2012 and references therein).

    In apparent contradiction to these findings, a previous study has shown that Octβ1R (also known as OA2) is capable of increasing cAMP. In that study, HEK293 cells transfected with Octβ1R were exposed to different octopamine concentrations, which resulted in an increase in cAMP levels. A potential explanation for the disparate results is that GPCR overexpression might alter its coupling to downstream pathways. For instance, mammalian β2-adrenergic receptors are known to couple to both Gsα and Giα/oα proteins. However, overexpression of β2-adrenergic receptors constitutively couples the receptor to Gsα and not to Giα or Goα. Furthermore, analysis of its binding specificity through immunoprecipitation shows that, when the receptor was overexpressed in transgenic mice, it coprecipitated with Gsα but not with Giα/oα in the absence of agonist. An additional explanation is that human embryonic kidney HEK293 cells are unlikely to express the same transduction pathways as endogenous Drosophila cells. Indeed, a recent study showed that HEK293 cells express virtually no Goα, which could also explain the lack of inhibitory response of overexpressed Octβ1R in this cell line (Koon, 2012).

    Goα is expressed in the nervous system of Drosophila and shows a marked increase in levels during the development of axonal tracts. Goα levels are altered in memory mutants including dunce and rutabaga, and Goα is necessary for associative learning. PTX overexpression in mushroom bodies of adult Drosophila severely disrupts memory, suggesting a role of Goα in synaptic plasticity. However, homozygous goα mutants are lethal due to defective development of the heart preventing the use of null mutants in studies of the NMJ or the adult brain. Moreover, overexpression of inhibitory G-proteins is known to sequester available Gβ and Gγ subunits, resulting in unspecific downregulation of other G-protein signaling. Thus, there are significant problems associated with the use of an overexpression approach to study Goα function. Fortunately, the availability of PTX and multiple Goα-RNAi strains allowed downregulation of Goα function in a cell-specific manner to examine synaptic development at the NMJ, which was found to phenocopy defects observed at the NMJ of octβ1r mutants. The presence of genetic interactions between the octβ1r and goα genes further support the notion that the two proteins act in the same signaling pathway to inhibit synaptic growth. These results provide strong evidence for the involvement of Goα in synaptic plasticity at the NMJ (Koon, 2012).

    In summary, these studies reveal that octopamine acts both as an inhibitory and excitatory transmitter to regulate synaptic growth and behavior. Thus, the inhibitory function of octopamine in global synaptic growth is as crucial as its excitatory function in maintaining plasticity in a dynamic range (Koon, 2012).

    A Presynaptic ENaC Channel Drives Homeostatic Plasticity

    An electrophysiology-based forward genetic screen has identified two genes, pickpocket11 (ppk11) and pickpocket16 (ppk16), as being necessary for the homeostatic modulation of presynaptic neurotransmitter release at the Drosophila neuromuscular junction (NMJ). Pickpocket genes encode Degenerin/Epithelial Sodium channel subunits (DEG/ENaC). This study demonstrates that ppk11 and ppk16 are necessary in presynaptic motoneurons for both the acute induction and long-term maintenance of synaptic homeostasis. ppk11 and ppk16 are cotranscribed as a single mRNA that is upregulated during homeostatic plasticity. Acute pharmacological inhibition of a PPK11- and PPK16-containing channel abolishes the expression of short- and long-term homeostatic plasticity without altering baseline presynaptic neurotransmitter release, indicating remarkable specificity for homeostatic plasticity rather than NMJ development. Finally, presynaptic calcium imaging experiments support a model in which a PPK11- and PPK16-containing DEG/ENaC channel modulates presynaptic membrane voltage and, thereby, controls calcium channel activity to homeostatically regulate neurotransmitter release (Younger, 2013).

    Homeostatic signaling systems are believed to interface with the mechanisms of learning-related plasticity to achieve stable, yet flexible, neural function and animal behavior. Experimental evidence from organisms as diverse as Drosophila, mouse, and humans demonstrates that homeostatic signaling systems stabilize neural function through the modulation of synaptic transmission, ion channel abundance, and neurotransmitter receptor trafficking. In each experiment, the cells respond to an experimental perturbation by modulating ion channel abundance or synaptic transmission to counteract the perturbation and re-establish baseline function. Altered homeostatic signaling is hypothesized to contribute to the cause or progression of neurological disease. For example, impaired or maladaptive homeostatic signaling may participate in the progression of autism-spectrum disorders, posttraumatic epilepsy, and epilepsy (Younger, 2013).

    The homeostatic modulation of presynaptic neurotransmitter release has been observed at mammalian central synapses and at neuromuscular synapses in species ranging from Drosophila to mouse and human. The Drosophila neuromuscular junction (NMJ) is a prominent model system for the study of this form of homeostatic plasticity. At the Drosophila NMJ, decreased postsynaptic neurotransmitter receptor sensitivity is precisely counteracted by a homeostatic potentiation of neurotransmitter release, thereby maintaining appropriate muscle excitation. The homeostatic enhancement of presynaptic release is due to increased vesicle release without a change in active zone number (Younger, 2013 and references therein).

    This process is referred to as 'synaptic homeostasis,' recognizing that it reflects a subset of homeostatic regulatory mechanisms that have been shown to stabilize neural function through modulation of ion channel gene expression and neurotransmitter receptor abundance (quantal scaling) (Younger, 2013 and references therein).

    An electrophysiology-based, forward genetic screen has been pioneered to identify the mechanisms of synaptic homeostasis. To date, a role has been ascribed for several genes in the mechanism of synaptic homeostasis including the Eph receptor, the schizophrenia-associated gene dysbindin, the presynaptic CaV2.1 calcium channel, presynaptic Rab3 GTPase-activating protein (Rab3-GAP), and Rab3-interacting molecule (RIM). An emerging model suggests that, in response to inhibition of postsynaptic glutamate receptor function, a retrograde signal acts upon the presynaptic nerve terminal to enhance the number of synaptic vesicles released per action potential to precisely offset the severity of glutamate receptor inhibition. Two components of the presynaptic release mechanism are necessary for the execution of synaptic homeostasis, increased calcium influx through presynaptic CaV2.1 calcium channels and a RIM-dependent increase in the readily releasable pool of synaptic vesicles. Many questions remain unanswered. In particular, how is a change in presynaptic calcium influx induced and sustained during synaptic homeostasis (Younger, 2013)?

    This study reports the identification of two genes, pickpocket16 and pickpocket11, that, when mutated, block homeostatic plasticity. Drosophila pickpocket genes encode Degenerin/Epithelial Sodium channel (DEG/ENaC) subunits. Channels in this superfamily are voltage insensitive and are assembled as either homomeric or heteromeric trimers. Each channel subunit has two transmembrane domains with short cytoplasmic N and C termini and a large extracellular loop implicated in responding to diverse extracellular stimuli (Younger, 2013).

    Little is known regarding the function of pH-insensitive DEG/ ENaC channels in the nervous system. DEG/ENaC channels have been implicated as part of the mechanotransduction machinery and in taste perception in both invertebrate and vertebrate systems. In Drosophila, PPK11 has been shown to function as an ENaC channel subunit that is required for the perception of salt taste and fluid clearance in the tracheal system, a function that may be considered analogous to ENaC channel activity in the mammalian lung (Younger, 2013 and references therein).

    This study demonstrates that ppk11 and ppk16 are coregulated during homeostatic synaptic plasticity and that homeostatic plasticity is blocked when gene is genetically deleted, when gene expression is disrupted in motoneurons, or when pickpocket channel function is pharmacologically inhibited. Advantage was taken of the fact that presynaptic homeostasis can be blocked pharmacologically to demonstrate that the persistent induction of homeostatic plasticity does not interfere with synapse growth and development. Homeostatic plasticity can be acutely and rapidly erased, leaving behind otherwise normal synaptic transmission. Finally, pharmacological inhibition of this pickpocket channel was demonstrated to abolish the homeostatic modulation of presynaptic calcium influx that was previously shown to be necessary for the homeostatic increase in neurotransmitter release (Younger, 2013).

    A model for DEG/ENaC channel function can be based on the well-established regulation of ENaC channel trafficking in the kidney during the homeostatic control of salt balance. Enhanced sodium reabsorption in the principle cells of the cortical collecting duct of the kidney is achieved by increased ENaC channel transcription and trafficking to the apical cell surface, which enhances sodium influx. Sodium is then pumped out of the basolateral. By analogy, a model is proposed for synaptic homeostasis in which the trafficking of DEG/ENaC channels to the neuronal membrane, at or near the NMJ, modulates presynaptic membrane potential to potentiate presynaptic calcium channel activity and thereby achieve precise homeostatic modulation of neurotransmitter release (Younger, 2013).

    This study provides evidence that a presynaptic DEG/ENaC channel composed of PPK11 and PPK16 is required for the rapid induction, expression, and continued maintenance of homeostatic synaptic plasticity at the Drosophila NMJ. Remarkably, ppk11 and ppk16 genes are not only required for homeostatic plasticity but are among the first homeostatic plasticity genes shown to be differentially regulated during homeostatic plasticity. Specifically, it was shown that expression of both ppk11 and ppk16 is increased 4-fold in the GluRIIA mutant background. It was also demonstrated that ppk11 and ppk16 are transcribed together in a single transcript and behave genetically as an operon-like, single genetic unit. This molecular organization suggests a model in which ppk11 and ppk16 are cotranscribed to generate DEG/ ENaC channels with an equal stoichiometric ratio of PPK11 and PPK16 subunits. This is consistent with previous models for gene regulation in Drosophila. However, the possibility cannot be ruled out that two independent DEG/ ENaC channels are upregulated, one containing PPK11 and one containing PPK16. The upregulation of ppk11 and ppk16 together with the necessity of DEG/ENaC channel function during the time when synaptic homeostasis is assayed, indicates that these genes are probably part of the homeostat and not merely necessary for the expression of synaptic homeostasis (Younger, 2013).

    DEG/ENaC channels are voltage-insensitive cation channels that are primarily permeable to sodium and can carry a sodium leak current. A model for DEG/ENaC channel function during synaptic homeostasis can be based on the well-established regulation of ENaC channel trafficking in the kidney during the homeostatic control of salt balance. Enhanced sodium reabsorption in the principle cells of the cortical collecting duct of the kidney is triggered by aldosterone binding to the mineralocorticoid receptor. This increases ENaC channel transcription and trafficking to the apical cell surface, which enhances sodium influx. Sodium is then pumped out of the basolateral side of the cell, accomplishing sodium reabsorption (Younger, 2013 and references therein).

    It is speculated that a retrograde, homeostatic signal from muscle triggers increased trafficking of a PPK11/16-containing DEG/ENaC channel to the neuronal plasma membrane, at or near the NMJ. Since the rapid induction of synaptic homeostasis is protein synthesis independent, the existence is hypothesized of a resting pool of PPK11/16 channels that are inserted in the membrane in response to postsynaptic glutamate receptor inhibition. If postsynaptic glutamate receptor inhibition is sustained, as in the GluRIIA mutant, then increased transcription of ppk11/16 supports a persistent requirement for this channel at the developing NMJ. Once on the plasma membrane, the PPK11/16 channel would induce a sodium leak and cause a moderate depolarization of the nerve terminal. This subthreshold depolarization would lead, indirectly, to an increase in action potential-induced presynaptic calcium influx through the CaV2.1 calcium channel and subsequent neurotransmitter release (Younger, 2013).

    There are two major possibilities for how ENaC-dependent depolarization of the nerve terminal could potentiate calcium influx and evoked neurotransmitter release. One possibility, based on work in the ferret prefrontal cortex and Aplysia central synapses, is that presynaptic membrane depolarization causes action potential broadening through potassium channel inactivation, thereby enhancing both calcium influx and release. A second possibility is that subthreshold depolarization of the nerve terminal causes an increase in resting calcium that leads to calcium-dependent calcium channel facilitation. Consistent with this model, it has been shown at several mammalian synapses that subthreshold depolarization of the presynaptic nerve terminal increases resting calcium and neurotransmitter release through low-voltage modulation of presynaptic P/Q-type calcium channels. However, at these mammalian synapses, the change in resting calcium does not lead to an increase in action potential-evoked calcium influx, highlighting a difference between the mechanisms of homeostatic potentiation and the type of presynaptic modulation observed at these other synapses. The mechanism by which elevated basal calcium potentiates release at these mammalian synapses remains under debate. It should be noted that small, subthreshold depolarization of the presynaptic resting potential, as small as 5 mV, are sufficient to cause a 2-fold increase in release at both neuromuscular and mammalian central synapses. This is within a reasonable range for modulation of presynaptic membrane potential by pickpocket channel insertion. Unfortunately, it is not technically feasible to record directly from the presynaptic terminal at the Drosophila NMJ. Finally, it is noted that it remains formally possible that a PPK11/16-containing DEG/ENaC channel passes calcium, based upon the ability of mammalian ASIC channels to flux calcium (Younger, 2013).

    This model might provide insight regarding how accurate tuning of presynaptic neurotransmitter release can be achieved. There is a supralinear relationship between calcium influx and release. Therefore, if changing calcium channel number is the mechanism by which synaptic homeostasis is achieved, then there must be very tight and tunable control of calcium channel number within each presynaptic active zone. By contrast, if homeostatic plasticity is achieved by ENaC-dependent modulation of membrane voltage, then variable insertion of ENaC channels could uniformly modulate calcium channel activity, simultaneously across all of the active zones of the presynaptic nerve terminal. Furthermore, if the ENaC channel sodium leak is small, and if presynaptic calcium channels are moderately influenced by small changes in resting membrane potential, then relatively coarse modulation of ENaC channel trafficking could be used to achieve precise, homeostatic control of calcium influx and neurotransmitter release. Again, these are testable hypotheses that will be addressed in the future (Younger, 2013).

    Extended synaptotagmin localizes to presynaptic ER and promotes neurotransmission and synaptic growth in Drosophila

    The endoplasmic reticulum (ER) is an extensive organelle in neurons with important roles at synapses including the regulation of cytosolic Ca2+, neurotransmission, lipid metabolism, and membrane trafficking. Despite intriguing evidence for these crucial functions, how presynaptic ER influences synaptic physiology remains enigmatic. To gain insight into this question, mutations were generated and characterized in the single Extended Synaptotagmin (Esyt) ortholog in Drosophila melanogaster. Esyts are evolutionarily conserved ER proteins with Ca2+-sensing domains that have recently been shown to orchestrate membrane tethering and lipid exchange between the ER and plasma membrane. Esyt was shown to localize to presynaptic ER structures at the neuromuscular junction. It was shown that synaptic growth, structure, and homeostatic plasticity are surprisingly unperturbed at synapses lacking Esyt expression. However, neurotransmission is reduced in Esyt mutants, consistent with a presynaptic role in promoting neurotransmitter release. Finally, neuronal overexpression of Esyt enhances synaptic growth and the sustainment of the vesicle pool during intense activity, suggesting that increased Esyt levels may modulate the membrane trafficking and/or resting calcium pathways that control synapse extension. Thus, this study has identified Esyt as a presynaptic ER protein that can promote neurotransmission and synaptic growth, revealing the first in vivo neuronal functions of this conserved gene family (Kikuma, 2017).

    RIM-binding protein links synaptic homeostasis to the stabilization and replenishment of high release probability vesicles

    This study defines activities of RIM-binding protein (RBP) that are essential for baseline neurotransmission and presynaptic homeostatic plasticity. At baseline, rbp mutants have a approximately 10-fold decrease in the apparent Ca(2+) sensitivity of release that this study attributes to (1) impaired presynaptic Ca(2+) influx, (2) looser coupling of vesicles to Ca(2+) influx, and (3) limited access to the readily releasable vesicle pool (RRP). During homeostatic plasticity, RBP is necessary for the potentiation of Ca(2+) influx and the expansion of the RRP. Remarkably, rbp mutants also reveal a rate-limiting stage required for the replenishment of high release probability (p) vesicles following vesicle depletion. This rate slows approximately 4-fold at baseline and nearly 7-fold during homeostatic signaling in rbp. These effects are independent of altered Ca(2+) influx and RRP size. It is proposed that RBP stabilizes synaptic efficacy and homeostatic plasticity through coordinated control of presynaptic Ca(2+) influx and the dynamics of a high-p vesicle pool (Muller, 2015).

    A presynaptic glutamate receptor subunit confers robustness to neurotransmission and homeostatic potentiation

    Homeostatic signaling systems are thought to interface with other forms of plasticity to ensure flexible yet stable levels of neurotransmission. The role of neurotransmitter receptors in this process, beyond mediating neurotransmission itself, is not known. Through a forward genetic screen, the Drosophila kainate-type ionotropic glutamate receptor subunit DKaiR1D was identified as being required for the retrograde, homeostatic potentiation of synaptic strength. DKaiR1D is necessary in presynaptic motor neurons, localized near active zones, and confers robustness to the calcium sensitivity of baseline synaptic transmission. Acute pharmacological blockade of DKaiR1D disrupts homeostatic plasticity, indicating that this receptor is required for the expression of this process, distinct from developmental roles. Finally, this study demonstrates that calcium permeability through DKaiR1D is necessary for baseline synaptic transmission, but not for homeostatic signaling. It is proposed that DKaiR1D is a glutamate autoreceptor that promotes robustness to synaptic strength and plasticity with active zone specificity (Kiragasi, 2017).

    The nervous system is endowed with potent and adaptive homeostatic signaling systems that maintain stable functionality despite the myriad changes that occur during neural development and maturation. The importance of homeostatic regulation in the nervous system is underscored by associations with a variety of neurological diseases, yet the genes and mechanisms involved remain enigmatic. A powerful model of presynaptic homeostatic plasticity has been established at the Drosophila neuromuscular junction (NMJ), a model glutamatergic synapse with molecular machinery that parallels central synapses in mammals. Here, genetic and pharmacological manipulations that reduce postsynaptic (muscle) glutamate receptor function trigger a trans-synaptic, retrograde feedback signal to the neuron that increases presynaptic release to precisely compensate for this perturbation. This process is referred to as 'presynaptic homeostatic potentiation' (PHP), because the expression mechanism requires a presynaptic increase in neurotransmitter release (Kiragasi, 2017).

    In recent years, forward and candidate genetic approaches have revealed several new and unanticipated genes necessary for PHP expression. While perturbations to the glutamate receptors in muscle are crucial events in the induction of PHP, whether other ionotropic glutamate receptors (iGluRs) function in PHP or are even expressed at the Drosophila NMJ is unknown. Finally, although evidence has emerged that homeostatic modulation is synapse specific, no roles for neurotransmitter receptors or other factors have been found to enable the presynaptic tuning of release efficacy at individual synapses (Kiragasi, 2017).

    This study has identified the kainate-type iGluR subunit DKaiR1D to be necessary for PHP expression at the Drosophila NMJ. DKaiR1D is necessary for the calcium sensitivity of baseline synaptic transmission, as well as for the acute and chronic expression of homeostatic potentiation. Recently, the functional reconstitution of DKaiR1D was achieved in heterologous cells, revealing that these receptors form homomeric calcium-permeable channels with atypical pharmacological properties compared to their vertebrate homologs (Li, 2016). This study has found that DKaiR1D is expressed in the nervous system and not the muscle, is present near presynaptic active zones, and is required specifically in motor neurons to enable the robustness of baseline neurotransmission and homeostatic plasticity. It is proposd that glutamate activates DKaiR1D at presynaptic release sites to translate autocrine activity into the robust stabilization of synaptic strength with active zone specificity (Kiragasi, 2017).

    This unexpected role for iGluRs in sensing glutamate at presynaptic terminals indicates an autocrine mechanism that responds to glutamate release to adaptively modulate presynaptic activity at individual active zones (Kiragasi, 2017).

    Glutamate receptors have diverse functions in modulating presynaptic excitability and short-term plasticity in addition to their established roles in postsynaptic excitation. Similar to what was observed with DKaiR1D at the Drosophila NMJ, rodent iGluRs also localize to presynaptic active zone, are activated by high concentrations of glutamate, and can modulate release during single action potentials. This suggests conserved autocrine modulatory mechanisms shared between these systems (Kiragasi, 2017).

    Rodent autoreceptors are known to modulate presynaptic activity on rapid timescales. In these cases, most of the impact on release is likely to derive from a calcium store-dependent mechanism, or from modulation of the action potential during the repolarization phase, when most of the calcium influx that drives vesicle release occurs. In a similar fashion, activation of presynaptic DKaiR1D during a single action potential could lead to a rapid additional source of presynaptic calcium influx from DKaiR1D itself and/or through modulation of presynaptic membrane potential to drive increased vesicle release. Voltage imaging at the Drosophila NMJ has found the half width of the action potential waveform to be ~5 ms (Ford, 2014), which is sufficient time to be modulated through such a mechanism. Therefore, dynamic changes in voltage or calcium influx at or near active zones could, in principle, drive additional vesicle release during a single action potential. This modulation may be restricted to nearby active zones and compartments relative to the site of glutamate release. Indeed, presynaptic kainate autoreceptors have the capacity to confer short-range, synapse-specific modulation to synaptic transmission (Scott, 2008), while presynaptic ligand-gated ion channels in C. elegans can also rapidly modulate synaptic transmission (Takayanagi-Kiya, 2016). Local activation of DKaiR1D could, therefore, subserve a powerful and flexible means of tuning presynaptic efficacy at or near individual release sites (Kiragasi, 2017).

    How does DKaiR1D promote the expression of presynaptic homeostatic plasticity? In contrast to the role of DKaiR1D in baseline release, the data indicate that the DKaiR1D-dependent mechanism that drives PHP is calcium independent. This implies two changes to DKaiR1D functionality that are unique to homeostatic adaptation compared to baseline transmission. First, because presynaptic release is acutely potentiated following application of PhTx, the activity, levels, and/or localization of DKaiR1D receptors must change to acquire a novel influence on neurotransmitter release following PHP induction. The activity of synaptic glutamate receptors can change through associations with additional subunits and auxiliary factors such as Neto. Furthermore, various forms of plasticity are expressed through dynamic changes in the levels and localization of glutamate receptors trafficking between active zones and endosome pools or extra-synaptic membrane. Indeed, when DKaiR1D is overexpressed in motor neurons, it rescues baseline transmission and PHP expression while localizing to heterogeneous puncta of varying distances relative to active zones. Notably, there is evidence that DKaiR1D interacts with other glutamate receptor subunits in vivo (Karuppudurai, 2014), which may contribute to the pharmacological differences observed in this study compared with the in vitro characterization (Li, 2016) and may also be targets of modulation during PHP (Kiragasi, 2017).

    Second, calcium signaling through DKaiR1D differentially drives baseline release and homeostatic plasticity. Therefore, mechanisms distinct from calcium permeability of the channel must contribute to PHP expression. One possibility is that DKaiR1D signals through an undefined metabotropic mechanism during PHP, which might contribute to the ability of the calcium impermeable DKaiR1DR transgene, with reduced conductance (Li, 2016), to rescue PHP expression. Alternatively, an attractive possibility is that following PHP induction, glutamate released from nearby active zones may dynamically modulate the presynaptic membrane potential and/or action potential waveform to promote additional synaptic vesicle release. Indeed, small, sub-threshold depolarizations of the presynaptic resting potential, as small as 5 mV, are sufficient to induce a 2-fold increase in presynaptic release. The timescale of this activity could occur within a few milliseconds as discussed above, and studies at the Drosophila NMJ have revealed that glutamate is released from single synaptic vesicles over timescales of milliseconds. Interestingly, epithelial sodium channels (ENaCs) have been proposed to enable PHP expression through changes in the presynaptic membrane potential, and such a mechanism could be shared by DKaiR1D but gated by glutamate release at individual active zones. Thus, DKaiR1D may serve to homeostatically modulate presynaptic release through modulation of presynaptic voltage and, intriguingly, with active zone specificity (Kiragasi, 2017).

    This characterization of DKaiR1D has revealed a role for presynaptic glutamate signaling in homeostatic plasticity. In the mammalian CNS, glutamate signaling drives the adaptive regulation of postsynaptic AMPA receptor insertion and removal, known as homeostatic scaling. Further, kainate receptors were recently demonstrated to regulate postsynaptic homeostatic scaling. Together with the present study, these results demonstrate that glutamatergic signaling through kainate receptors orchestrate the potent and adaptive homeostatic control of synaptic strength on both sides of the synapse. Future studies will reveal the integration between synaptic glutamate signaling and other forces that modulate synaptic strength to enable robust, flexible, and stable neurotransmission (Kiragasi, 2017).

    Presynaptic spinophilin tunes neurexin signalling to control active zone architecture and function

    Assembly and maturation of synapses at the Drosophila neuromuscular junction (NMJ) depend on trans-synaptic neurexin/neuroligin signalling, which is promoted by the scaffolding protein Syd-1 binding to neurexin. This study reports that the scaffold protein spinophilin binds to the C-terminal portion of neurexin and is needed to limit neurexin/neuroligin signalling by acting antagonistic to Syd-1 (RhoGAP100F). Loss of presynaptic spinophilin results in the formation of excess, but atypically small active zones. Neuroligin-1/neurexin-1/Syd-1 levels are increased at spinophilin mutant NMJs, and removal of single copies of the neurexin-1, Syd-1 or neuroligin-1 genes suppresses the spinophilin-active zone phenotype. Evoked transmission is strongly reduced at spinophilin terminals, owing to a severely reduced release probability at individual active zones. It is concluded that presynaptic spinophilin fine-tunes neurexin/neuroligin signalling to control active zone number and functionality, thereby optimizing them for action potential-induced exocytosis (Muhammad, 2015).

    Chemical synapses release synaptic vesicles (SVs) at specialized presynaptic membranes, so-called active zones (AZs), which are characterized by electron-dense structures, reflecting the presence of extended molecular protein scaffolds. These AZ scaffolds confer stability and facilitate SV release. Importantly, at individual AZs, scaffold size is found to scale with the propensity to engage in action potential-evoked release. An evolutionarily conserved set of large multi-domain proteins operating as major building blocks for these scaffolds has been identified over the last years: Syd-2/Liprin-α, RIM, RIM-binding-protein (RBP) and ELKS family proteins (of which the the Drosophila homologue is called Bruchpilot (BRP)). However, how presynaptic scaffold assembly and maturation are controlled and coupled spatiotemporally to the postsynaptic assembly of neurotransmitter receptors remains largely unknown, although trans-synaptic signalling via Neurexin-1 (Nrx-1)-Neuroligin-1 (Nlg1) adhesion molecules is a strong candidate for a conserved 'master module' in this context, based on Nrx-Nlg signalling promoting synaptogenesis in vitro, synapses of rodents, Caenorhabditis elegans10 and Drosophila (Muhammad, 2015).

    With respect to scaffolding proteins, Syd-1 was found to promote synapse assembly in C. elegans, Drosophila and rodents. In Drosophila, the Syd-1-PDZ domain binds the Nrx-1 C terminus and couples pre- with postsynaptic maturation at nascent synapses of glutamatergic neuromuscular junctions (NMJs) in Drosophila larvae. Syd-1 cooperates with Nrx-1/Nlg1 to stabilize newly formed AZ scaffolds, allowing them to overcome a 'threshold' for synapse formation. Additional factors tuning scaffold assembly, however, remain to be identified. This study shows that the conserved scaffold protein spinophilin (Spn) is able to fine-tune Nrx-1 function by binding the Nrx-1 C terminus with micromolar affinity via its PDZ domain. In the absence of presynaptic Spn, 'excessive seeding' of new AZs occurred over the entire NMJ due to elevated Nrx-1/Nlg1 signalling. Apart from structural changes, this study shows that Spn plays an important role in neurotransmission since it is essential to establish proper SV release probability, resulting in a changed ratio of spontaneous versus evoked release at Spn NMJ terminals. The trans-synaptic dialogue between Nrx-1 and Nlg1 aids in the initial assembly, specification and maturation of synapses, and is a key component in the modification of neuronal networks. Regulatory factors and processes that fine-tune and coordinate Nrx-1/Nlg1 signalling during synapse assembly process are currently under investigation. These data indicate that Drosophila Spn-like protein acts presynaptically to attenuate Nrx-1/Nlg1 signalling and protects from excessive seeding of new AZ scaffolds at the NMJ. In Spn mutants, excessive AZs suffered from insufficient evoked release, which may be partly explained by their reduced size, and partly by a genuine functional role of Spn (potentially mediated via Nrx-1 binding). In mice, loss of Spn (Neurabin II), one of the two Neurabin protein families present in mammals, was reported to provoke a developmental increase in synapse numbers (Feng, 2000). While Spinophilin was found to be expressed both pre- and post-synaptically (Muly, 2004; Muly, 2004a; Muly, 2004b), its function, so far, has only been analysed in the context of postsynaptic spines (Feng, 2000; Terry-Lorenzo 2005; Allen, 2006; Sarrouilhe, 2005). Given the conserved Spn/Nrx-1 interaction reported in this study, Spn family proteins might execute a generic function in controlling Nrx-1/Nlg1-dependent signalling during synapse assembly (Muhammad, 2015).

    This study consistently found that Spn counteracts another multi-domain synaptic regulator, Syd-1, in the control of Nrx-1/Nlg1 signalling. Previous genetic work in C. elegans identified roles of Syd-1 epistatic to Syd-2/Liprin-α in synaptogenesis. Syd-1 also operates epistatic to Syd-2/Liprin-α at Drosophila NMJs. Syd-1 immobilizes Nrx-1, positioning Nlg1 at juxtaposed postsynaptic sites, where it is needed for efficient incorporation of GluR complexes. Intravital imaging suggested an early checkpoint for synapse assembly, involving Syd-1, Nrx-1/Nlg1 signalling and oligomerization of Liprin-α in the formation of an early nucleation lattice, which is followed later by ELKS/BRP-dependent scaffolding events. As Spn promotes the diffusional motility of Nrx-1 over the terminal surface and limits Nrx-1/Nlg1 signalling, and as its phenotype is reversed by loss of a single gene copy of nrx-1, nlg1 or syd-1, Spn displays all the features of a 'negative' element mounting, which effectively sets the threshold for AZ assembly. As suggested by FRAP experiments, Spn might withdraw a population of Nrx-1 from the early assembly process, establishing an assembly threshold that ensures a 'typical' AZ design and associated postsynaptic compartments. As a negative regulatory element, Spn might allow tuning of presynaptic AZ scaffold size and function (Muhammad, 2015).

    The C. elegans Spn homologue NAB-1 (NeurABin1) was previously shown to bind Syd-1 in cell culture recruitment assays (Chia, 2012). This study found consistent evidence for Syd-1/Nrx-1/Spn tripartite complexes in salivary gland experiments. Moreover, the PDZ domain containing regions of Spn and Syd-1 interacted in Y2H experiments. It would be interesting to dissect whether the interaction of Spn/Syd-1 plays a role in controlling the access of Nrx-1 to one or both factors. For C. elegans HSN synapses, a previous study showed that loss of NAB-1 results in a deficit of synaptic markers, such as Syd-1 and Syd-2/Liprin-α, while NAB-1 binding to F-actin was also found to be important for synapse assembly. Though at first glance rather contradictory to the results described in this study, differences might result from Chia (2012) studying synapse assembly executed over a short time window, when partner cells meet for the first time. In contrast, this study used a model (Drosophila larval NMJs) where an already functional neuronal terminal adds novel AZs. Despite the efforts of this study, no role of F-actin in the assembly of AZs of late larval Drosophila NMJs was demonstrated. F-actin patches might be particularly important to establish the first synaptic contacts between partner cells. Both the study by Chia et al. and this study, however, point clearly towards important regulatory roles of Spn family members in the presynaptic control of synapse assembly. Further, this study described a novel interaction between the Spn-PDZ domain and the intracellular C-term of Nrx-1 at the atomic level. Interestingly, it was found that all functions of Spn reported in this study, structural as well as functional, were strictly dependent on the ligand-binding integrity of this PDZ domain. It is noteworthy that the Spn-PDZ domain binds other ligands as well, for example, Kalirin-7 and p70S6K , and further elucidation of its role as a signal 'integrator' in synapse plasticity should be interesting. The fact that Nrx-1 levels were increased at Spn NMJs and, most importantly, that genetic removal of a single nrx-1 gene copy effectively suppressed the Spn AZ phenotype, indicates an important role of the Spn/Nrx-1 interaction in this context. Affinity of Spn-PDZ for the Nrx-1 C-term was somewhat lower than that of the Syd-1-PDZ, both in ITC and Y2H experiments. Nonetheless, overexpression of Spn was successful in reducing the targeting effect of Syd-1 on overexpressed Nrx-1GFP. It will be interesting to see whether this interaction can be differentially regulated, for example, by (de)phosphorylation. It is worth noting that apart from Syd-1 and Spn, several other proteins containing PDZ domains, including CASK, Mint1/X11, CIPP and Syntenin, were found to bind to the Nrxs C-termini. CASK was previously shown to interact genetically with Nrx-1, controlling endocytic function at Drosophila NMJs. However, when this study tested for an influence of CASK on Nrx-1GFP motility using FRAP, genetic ablation of CASK had no effect (Muhammad, 2015).

    Thus, CASK function seemingly resembles neither Syd-1 nor Spn. Clearly, future work will have to address and integrate the role of other synaptic regulators converging on the Nrx-1 C-term. In particular, CASK (which displays a kinase function that phosphorylates certain motifs within the Nrx-1 C-term) might alternately control Spn- and Syd-1-dependent functions. Presynaptic Nrx-1, through binding to postsynaptic Nlg1 at developing Drosophila NMJ terminals, is important for the proper assembly of new synaptic sites. It is of note, however, that while mammalian Nrxs display robust synaptogenetic activity in cellular in vitro systems, direct genetic evidence for synaptogenetic activity of Nrxs in the mammalian CNS remained rather scarce. Triple knockout mice lacking all α-Nrxs display no gross synaptic defects at the ultrastructural level. Future analysis will have to investigate whether differences here might be explained by specific compensation mechanisms in mammals; for example, by β-Nrxs, or other parallel trans-synaptic communication modules. Genuine functional deficits in neurotransmitter release were also observed after the elimination of presynaptic Spn. Elimination of ligand binding to the PDZ domain rendered the protein completely nonfunctional, without affecting its synaptic targeting. Thus, the Spn functional defects are likely to be mediated via a lack of Nrx-1 binding. Notably, ample evidence connects Nrx-1 function with both the functional and structural maturation of Drosophila presynaptic AZs. This work now promotes the possibility that binding of Spn to Nrx-1 is important for establishing correct release probability, independent of absolute AZ scaffold size. It is noteworthy that Nrx-1 function was previously shown to be important for proper Ca2+ channel function and, as a result, properly evoked SV release. Thus, it will be interesting to investigate whether the specific functional contributions of Spn are mediated via deficits in the AZ organization of voltage-gated Ca2+ channels or Ca2+ sensors, such as synaptotagmin. Taken together, this study found an unexpected function for Spn in addition of AZs at Drosophila glutamatergic terminals, through the integration of signals from both the pre- and postsynaptic compartment. Given that the Spn/Nrx-1 interaction is found to be conserved from Drosophila to rodents, addressing similar roles of presynaptic Spn in mammalian brain physiology and pathophysiology might be informative (Muhammad, 2015).

    Neurexin, neuroligin and wishful thinking coordinate synaptic cytoarchitecture and growth at neuromuscular junctions

    Using full length and truncated forms of Neurexin (Dnrx) and Neuroligins (Dnlg) together with cell biological analyses and genetic interactions, this study reports novel functions of dnrx and dnlg1 in clustering of pre- and postsynaptic proteins, coordination of synaptic growth and ultrastructural organization. dnrx and dnlg1 extracellular and intracellular regions are required for proper synaptic growth and localization of dnlg1 and Dnrx, respectively. dnrx and dnlg1 single and double mutants display altered subcellular distribution of Discs large (Dlg), which is the homolog of mammalian post-synaptic density protein, PSD95. dnrx and dnlg1 mutants also display ultrastructural defects ranging from abnormal active zones, misformed pre- and post-synaptic areas with underdeveloped subsynaptic reticulum. Interestingly, dnrx and dnlg1 mutants have reduced levels of the BMP receptor Wishful thinking (Wit), and dnrx and dnlg1 are required for proper localization and stability of Wit. In addition, the synaptic overgrowth phenotype resulting from the overexpression of dnrx fails to manifest in wit mutants. Phenotypic analyses of dnrx/wit and dnlg1/wit mutants indicate that Dnrx/Dnlg1/Wit coordinate synaptic growth and architecture at the NMJ. These findings also demonstrate that loss of dnrx and dnlg1 leads to decreased levels of the BMP co-receptor, Thickveins and the downstream effector phosphorylated Mad at the NMJ synapses indicating that Dnrx/Dnlg1 regulate components of the BMP signaling pathway. Together these findings reveal that Dnrx/Dnlg are at the core of a highly orchestrated process that combines adhesive and signaling mechanisms to ensure proper synaptic organization and growth during NMJ development (Banerjee, 2016).

    Neurexins and Neuroligins have emerged as major players in the organization of excitatory and inhibitory synapses across species. Mutational analyses in Drosophila uncovered specific functions of dnrx and dnlg1 in NMJ synapse organization and growth, where dnrx is expressed pre-synaptically and dnlg1 post-synaptically. Deletion of the extracellular and intracellular regions of dnrx or dnlg1 revealed that both regions are necessary for dnrx and dnlg1 clustering and function at the synapse. Most importantly, the data presented here suggest that dnrx and dnlg1 genetically interact with wit as well as the downstream effector of BMP signaling, Mad to allow both the organization and growth of NMJ synapses. The surprising finding that loss of dnrx and dnlg1 leads to decreased wit stability and that dnrx and dnlg1 are required for proper wit localization raises the possibility that these proteins function to coordinate trans-synaptic adhesion and synaptic growth. This is further strengthened from the observations that, in the absence of Wit, the synaptogenic activity from overexpression of dnrx did not manifest into an increased synaptic growth as is seen in the presence of wit. Loss of dnrx and dnlg1 also led to a reduction in the levels of other components of the BMP pathway, namely the Tkv and pMad. Together these findings are the first to demonstrate a functional coordination between trans-synaptic adhesion proteins dnrx and dnlg1 with wit receptor and BMP pathway members to allow precise synaptic organization and growth at NMJ. It would be of immense interest to investigate whether similar mechanisms might be operating in vertebrate systems (Banerjee, 2016).

    The trans-synaptic cell adhesion complex formed by heterophilic binding of pre-synaptic Neurexins (Nrxs) and post-synaptic Neuroligins (Nlgs), displayed synaptogenic function in cell culture experiments. In vertebrates and invertebrates alike, Nrxs and Nlgs belong to one of the most extensively studied synaptic adhesion molecules with a specific role in synapse organization and function. In Drosophila, dnrx and dnlg1 mutations cause reductions in bouton number, perturbation in active zone organization and severe reduction in synaptic transmission. These phenotypes are essentially phenocopied in both dnrx and dnlg1 mutants, and double mutants do not cause any significant enhancement in the single mutant phenotypes illustrating that they likely function in the same pathway. From immunohistochemical and biochemical analyses, it is evident that lack of dnrx or dnlg1 causes their diffuse localization and protein instability in each other's mutant backgrounds. These data suggest that trans-synaptic interaction between dnrx and dnlg1 is required for their proper localization and stability, and that these proteins have a broader function in the context of general synaptic machinery involving Nrx-Nlg across phyla. It also raises the possibility that the trans-synaptic molecular complex involving Nrx-Nlg may alter stability of other synaptic proteins and lead to impairments in synaptic function without completely abolishing synaptic structure and neurotransmission. It is therefore not surprising that Nrx and Nlg have been recently reported to be associated with many non-lethal cognitive and neurological disorders, such as schizophrenia, ASD, and learning disability (Banerjee, 2016).

    The rescue studies of dnrx and dnlg1 localization using their respective N- and C-terminal domain truncations emphasize a requirement of the full-length protein for proper synaptic clustering. Given that dnrx and dnlg1 likely interact via their extracellular domains, it is somewhat expected that an N-terminal deletion as seen in genotypic combinations of elav-Gal4/UAS-DnrxΔ N;dnrx-/- and mef2-Gal4/UAS-dnlg1Δ N;dnlg1-/- would fail to rescue the synaptic clustering of Dnlg1 and Dnrx, respectively. However, the inability of the C-terminal deletions of these proteins as seen in elav-Gal4/UAS-DnrxΔ c;dnrx-/- and mef2-Gal4/UAS-dnlg1Δ Cdnlg1-/- to cluster Dnlg1 and Dnrx, respectively, to wild type localization and/or levels suggest that the lack of the cytoplasmic domains of these proteins may render the remainder portion of the protein unstable, thereby leading to its inability to be either recruited or held at the synaptic apparatus (Banerjee, 2016).

    Finally, the subcellular localization of Dlg at the SSR and its levels in the dnrx and dnlg1 single and double mutants as well as in the rescue genotypes provides key insights into the stoichiometry of Dnrx-Dnlg1 interactions, and how Dnrx-Dnlg1 might be functioning with other synaptic proteins in their vicinity to organize the synaptic machinery. A significant reduction in Dlg levels in dnrx mutants raises the question of whether Dlg localization/levels are controlled presynaptically? Alternatively, it is possible that Dnrx-Dnlg1 is not mutually exclusive for all their synaptic functions and there might be other Neuroligins that might function with Dnrx. One attractive candidate could be Dnlg2, which is both pre- and postsynaptic. dnrx could also function through the recently identified Neuroligins 3 and 4. Although dnlg1 mutants did not show any significant difference in Dlg levels, a diffuse subcellular localization nevertheless raised the possibility of a structural disorganization of the postsynaptic terminal and defects in SSR morphology, which were confirmed by ultrastructural studies. The rescue analysis of the subcellular localization of Dlg demonstrates that Dlg localization and levels could be restored fully in a presynaptic rescue of dnrx mutants by expression of full length Dnrx, however the localization of Dlg could not be restored by expression of DnrxΔNT. These observations suggest that the extracellular domain of dnrx is essential for the localization and also the levels of Dlg. Whether the extracellular domain influences Dlg via dnlg1 or any of the other three Dnlgs (2, 3 and 4) at the NMJ remains to be elucidated (Banerjee, 2016).

    Most studies on Nrx/Nlg across species offer clues as to how these proteins assemble synapses and how they might function in the brain to establish and modify neuronal network properties and cognition, however, very little is known on the signaling pathways that these proteins may potentially function in. It has been previously reported that Neurexin 1 is induced by BMP growth factors in vitro and in vivo and that could possibly allow regulation of synaptic growth and development. In Drosophila, both dnrx and dnlg1 mutants showed reduced synaptic growth similar to wit pathway mutants. Reduced wit levels both from immunolocalization studies in dnrx and dnlg1 mutant backgrounds and biochemical studies from immunoblot and sucrose density gradient sedimentation analyses present compelling evidence towards the requirement of dnrx and dnlg1 for wit stability. Reduction of synaptic dnrx levels in wit mutants argue for interdependence in the localization of these presynaptic proteins at the NMJ synaptic boutons. The findings that synaptic boutons of dnrx and dnlg1 mutants also show reduction in the levels of the co-receptor Tkv suggest that this effect may not be exclusively Wit-specific, and possibly due to the overall integrity of trans-synaptic adhesion complex that ensures Wit and Tkv stability at the NMJ (Banerjee, 2016).

    Genetic interaction studies displayed a significant reduction in bouton numbers resulting from trans-heterozygous combinations of wit/+,dnrx/+ and wit/+,dnlg1/+ compared to the single heterozygotes strongly favoring the likelihood of these genes functioning together in the same pathway. Although double mutants of wit,dnrx and wit,dnlg1 are somewhat severe than dnrx and dnlg1 single mutants, they did not reveal any significant differences in their bouton counts compared to wit single mutants. Moreover, genetic interactions between Mad;dnrx and Mad;dnlg1 together with decreased levels of pMad in dnrx and dnlg1 mutants provided further evidence that dnrx and dnlg1 regulate components of the BMP pathway. Interestingly, reduced levels of pMad were observed in dnrx and dnlg1 mutants, in contrast to a recent study that reported higher level of synaptic pMad in dnrx and dnlg1 mutants. The differences in pMad levels encountered in these two studies could be attributed to differences in rearing conditions, nature of the food and genetic backgrounds, as these factors have been invoked to affect synaptic pMad levels. These findings strongly support that dnrx, dnlg1 and wit function cooperatively to coordinate synaptic growth and signaling at the NMJ (Banerjee, 2016).

    Loss of either dnrx or dnlg1 does not completely abolish the apposition of pre- and post-synaptic membrane at the NMJ synapses but detachments that occur at multiple sites along the synaptic zone. These observations point to either unique clustering of the Dnlg1/Dnrx molecular complexes or preservation of trans-synaptic adhesion by other adhesion molecules at the NMJ synapses. Indeed, recent studies show nanoscale organization of synaptic adhesion molecules Neurexin 1Β, NLG1 and LRRTM2 to form trans-synaptic adhesive structures (Chamma, 2016). In addition to dnrx and dnlg1 mutants, synaptic ultrastructural analysis showed similarity in presynaptic membrane detachments with a characteristic ruffling morphology in wit mutants as well suggesting that these proteins are required for maintaining trans-synaptic adhesion. Interestingly, double mutants from any combinations of these three genes such as dnlg1,dnrx or wit,dnrx and wit,dnlg1 did not show any severity in the detachment/ruffling of the presynaptic membrane suggesting that there might be a phenotypic threshold that cannot be surpassed as part of an intrinsic synaptic machinery to preserve its very structure and function. It would be interesting to test this possibility if more than two genes are lost simultaneously as in a triple mutant combination. Alternatively, there could be presence of other distinct adhesive complexes that remain intact and function outside the realm of Dnrx, dnlg1 and wit proteins thus preventing a complete disintegration of the synaptic apparatus (Banerjee, 2016).

    The same holds true for most of the ultrastructural pre- and postsynaptic differentiation defects observed in the single and double mutants of dnrx, dnlg1 and wit. Barring dnrx,dnlg1 double mutants in which the SSR width showed a significant reduction compared to the individual single mutants, all other phenotypes documented from the ultrastructural analysis showed no difference in severity between single and double mutants. Another common theme that emerged from the ultrastructural analysis was that loss of either pre- or postsynaptic proteins or any combinations thereof, showed a mixture of defects that spanned both sides of the synaptic terminal. For example, presynaptic phenotypes such as increased number of active zones or abnormally long active zones as well as postsynaptic phenotypes such as reduction in width and density of the SSR were observed when presynaptic proteins such as dnrx and wit and postsynaptic dnlg1 were lost individually or in combination. These studies suggest that pre- and postsynaptic differentiation is tightly regulated and not mutually exclusive where loss of presynaptic proteins would result only in presynaptic deficits and vice-versa (Banerjee, 2016).

    The postsynaptic SSR phenotypes seen in the single and double mutants of dnrx, dnlg1 and wit might be due to their interaction/association with postsynaptic or perisynaptic protein complexes such as Dlg and Fasciclin II. Alternatively, the postsynaptic differentiation or maturation deficits in these mutants could also be due to a failure of postsynaptic GluRs to be localized properly or their levels maintained sufficiently. It has been shown previously that lack of GluR complexes interferes with the formation of SSR. Deficits in postsynaptic density assembly have been previously reported for dnlg1 mutants, including a misalignment of the postsynaptic GluR fields with the presynaptic transmitter release sites. GluR distribution also showed profound abnormalities in dnrx mutants. These observations suggest that trans-synaptic adhesion and synapse organization and growth is highly coordinated during development, and that multiple molecular complexes may engage in ensuring proper synaptic development. Some of these questions need to be addressed in future investigations (Banerjee, 2016).

    In Drosophila, the postsynaptic muscle-derived BMP ligand, Glass bottom boat (Gbb), binds to type II receptor Wit, and type I receptors Tkv, and Saxophone (Sax) at the NMJ. Receptor activation by Gbb leads to the recruitment and phosphorylation of Mad at the NMJ terminals followed by nuclear translocation of pMad with the co-Smad, Medea, and transcriptional initiation of other downstream targets. It is interesting to note that previously published studies revealed that a postsynaptic signaling event occurs during larval development mediated by Type I receptor Tkv and Mad. Based on findings from this study, it is speculated that Dnrx and Dnlg1-mediated trans-synaptic adhesive complex allows recruitment and stabilization of wit and associated components to assemble a larger BMP signaling complex to ensure proper downstream signaling. Loss of dnrx and/or dnlg1 results in loss of adhesion and a decrease in the levels of Wit/Tkv receptors as well as decreased phosphorylation of Mad. Thus a combination of trans-synaptic adhesion and signaling mediated by Dnrx, Dnlg1 and components of the BMP pathway orchestrate the assembly of the NMJ and coordinate proper synaptic growth and architecture (Banerjee, 2016).

    The data presented in this study address fundamental questions of how the interplay of pre- and postsynaptic proteins contributes towards the trans-synaptic adhesion, synapse differentiation and growth during organismal development. dnrx and dnlg1 establish trans-synaptic adhesion and functionally associate with the presynaptic signaling receptor wit to engage as a molecular machinery to coordinate synaptic growth, cytoarchitecture and signaling. dnrx and dnlg1 also function in regulating BMP receptor levels (Wit and Tkv) as well as the downstream effector, Mad, at the NMJ. It is thus conceivable that at the molecular level setting up of a Dnrx-Dnlg1 mediated trans-synaptic adhesion is a critical component for molecules such as Wit and Tkv to perform signaling function. It would be of immense interest to investigate how mammalian Neurexins and Neuroligins are engaged with signaling pathways that not only are involved in synapse formation but also their functional modulation, as the respective genetic loci show strong associations with cognitive and neurodevelopmental disorders (Banerjee, 2016).

    A neuroprotective role for microRNA miR-1000 mediated by limiting glutamate excitotoxicity

    Evidence has begun to emerge for microRNAs as regulators of synaptic signaling, specifically acting to control postsynaptic responsiveness during synaptic transmission. This report provides evidence that Drosophila melanogaster miR-1000 acts presynaptically to regulate glutamate release at the synapse by controlling expression of the vesicular glutamate transporter (VGlut). Genetic deletion of miR-1000 led to elevated apoptosis in the brain as a result of glutamatergic excitotoxicity. The seed-similar miR-137 regulates VGluT2 expression in mouse neurons. These conserved miRNAs share a neuroprotective function in the brains of flies and mice. Drosophila miR-1000 showed activity-dependent expression, which might serve as a mechanism to allow neuronal activity to fine-tune the strength of excitatory synaptic transmission (Verma, 2015).

    miRNAs have emerged in recent years as important regulators of homeostatic mechanisms. Changes in miRNA expression and activity have been linked to neurodegenerative disorders. A growing body of evidence suggests that miRNAs are neuroprotective in the aging brain, as well as in the control of synaptic function and plasticity. Mouse miR-134 acts postsynaptically to regulate synapse strength, and miR-181 and miR-223 regulate glutamate receptors, thereby affecting postsynaptic responsiveness to glutamate. miR-1, a muscle-specific miRNA, acts in a retrograde fashion at the neuromuscular junction to regulate the kinetics of synaptic vesicle exocytosis. However, there are few examples of miRNAs acting directly in the presynaptic terminal to control synaptic strength. miR-485, which is found presynaptically, has been shown to control the expression of synaptic vesicle protein SV2A, thereby affecting synapse density and GluR2 receptor levels (Verma, 2015).

    This paper reports that Drosophila miR-1000 regulates neurotransmitter release from presynaptic terminals. miR-1000 regulates expression of the VGlut, which loads glutamate into synaptic vesicles. Genetic ablation of miR-1000 leads to glutamate excitotoxicity, resulting in early-onset neuronal death. Presynaptic regulation of miR-1000 is activity dependent and may serve as a mechanism for tuning synaptic transmission. Evidence is presented that this regulatory relationship is conserved in the mammalian CNS, with a seed-similar miRNA, miR-137, conferring neuroprotection through regulation of VGluT2. The consequences of misregulation of glutamatergic signaling can be severe: excitotoxicity due to excessive glutamate release has been implicated in ischemia and traumatic brain injury, as well as in neurodegenerative conditions such as Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis (Verma, 2015).

    Although postsynaptic regulation of glutamate receptor activity has been well studied, much less is known about presynaptic regulation of glutamatergic signaling. These findings suggest that miR-1000 acts presynaptically to regulate VGlut expression and thereby control synaptic glutamate release. It is tempting to speculate that this could provide a mechanism for tuning synaptic output and locally modulating synaptic strength. Such a mechanism would be most useful if the miRNA itself could be regulated in an activity-dependent manner. Evidence is provided that miR-1000 expression is regulated by light in vivo, presumably reflecting photoreceptor activity in the eye. This in turn leads to light-regulated regulation of VGlut reporter levels. These findings lend support to the notion of activity-dependent regulation of miR-1000 activity. An in depth exploration of these issues awaits the development of methods to monitor changes in presynaptic miRNA levels in real time (Verma, 2015).

    Failure of miR-1000-mediated regulation of VGlut led to excess glutamate release and resulted in excitotoxicity. Consistent with these findings, Gal4-directed overexpression of VGlut has been reported to cause neurodegeneration. Notably, elevated levels of vertebrate VGluTs have been associated with excitotoxicity in animal models of epilepsy and traumatic brain injury. The GAERS rat epilepsy model shows elevated levels of VGluT2 but not of VGluT1. Similarly, in a model of stroke, ischemic injury was found to result in elevated expression of VGluT1 but not of VGluT2. VGluT1 levels are regulated by methamphetamine treatment, likely contributing to excitotoxic consequences of methamphetamine abuse. VGluT1 levels have also been reported to increase in rat brains following antidepressant treatment (Verma, 2015).

    In the mouse, miR-223 acts on postsynaptic glutamate receptors and has a neuroprotective role in vivo. These findings provide evidence that miR-1000 has a neuroprotective role mediated through regulation of presynaptic glutamate release and that this regulatory mechanism is conserved for miR-137 and VGluT2 in the mouse. Together, these studies show that miRNA-mediated regulation of glutamatergic activity acts pre- and post-synaptically to modulate synaptic transmission and to protect against excitotoxicity. In this context, it is noteworthy that miR-137 is reported to be enriched at synapses. miR-137 levels were found to be low in a subset of Alzheimer's patients with elevated serine palmitoyltransferase 1 expression leading to increased ceramide production. Single nucleotide polymorphisms affecting miR-137 target sites could lead to low-level constitutive overexpression of its targets, even when the SNP is present in a single copy. A single nucleotide polymorphism affecting miR-137 has also been identified as a risk factor for schizophrenia. It will be of interest to learn whether misregulation of VGluT2 expression contributes to these neurological conditions. The current findings raise the possibility that miRNA mediated regulation makes VGluT and other miRNA targets possible risk factors in neurodegenerative disease (Verma, 2015).

    Loss of skywalker reveals synaptic endosomes as sorting stations for synaptic vesicle proteins

    Exchange of proteins at sorting endosomes is not only critical to numerous signaling pathways but also to receptor-mediated signaling and to pathogen entry into cells; however, how this process is regulated in synaptic vesicle cycling remains unexplored. This work presents evidence that loss of function of a single neuronally expressed GTPase activating protein (GAP), Skywalker (Sky) facilitates endosomal trafficking of synaptic vesicles at Drosophila neuromuscular junction boutons, chiefly by controlling Rab35 GTPase activity. Analyses of genetic interactions with the ESCRT machinery as well as chimeric ubiquitinated synaptic vesicle proteins indicate that endosomal trafficking facilitates the replacement of dysfunctional synaptic vesicle components. Consequently, sky mutants harbor a larger readily releasable pool of synaptic vesicles and show a dramatic increase in basal neurotransmitter release. Thus, the trafficking of vesicles via endosomes uncovered using sky mutants provides an elegant mechanism by which neurons may regulate synaptic vesicle rejuvenation and neurotransmitter release (Uytterhoeven, 2011).

    Synaptic vesicles recycle locally at the synapse, and this study now provides genetic evidence that the synapse holds the capacity to regulate the sorting of synaptic vesicle proteins at 2xFYVE and Rab5 positive endosomes. 2xFYVE-GFP positive endosomes involved in signaling pathways exist at nerve endings. Likewise, membrane invaginations and endosomal-like cisternae form as a result of bulk membrane uptake during intense nerve stimulation, and in various endocytic mutants. However, the current data indicate that endosomes that accumulate in sky mutants are fundamentally different from these 'endocytic cisternae.' First, endocytic cisternae are not enriched for the endosomal markers 2xFYVE and Rab5. Second, endosomal structures in sky mutants do not appear to directly form at the plasma membrane. Third, endocytic cisternae may directly fuse with the membrane, however, based on mEJC and TEM analyses, sky induced endosomes appear to function as an intermediate station. Note that FM 1-43 dye can enter and leave endosomes in sky mutants, suggesting small synaptic vesicles can form at these structures. Together with time-course experiments in sky mutants, endosomes specifically accumulate upon stimulation and dissipate during rest. Therefore, the data suggest that endosomes in sky mutants constitute an intermediate step in the synaptic vesicle cycle. Such a compartment may also be operational in wild-type synapses because a 2xFYVE-GFP endosomal compartment can be largely depleted from synaptic vesicles upon endocytic blockade and because synaptic vesicles harbor proteins that are also commonly found on endosomes (Uytterhoeven, 2011).

    Rab GTPases are involved in numerous intracellular trafficking events. In particular in synaptic vesicle traffic, Rab3, and its close isoform Rab27 have been implicated in vesicle tethering and/or priming, while Rab5 has been also implicated in endocytosis. However, an involvement of Rab proteins in other aspects of the synaptic vesicle cycle, including recycling, remains enigmatic. This systematic analysis of CA Rabs now implicates several Rabs in synaptic recycling. First, consistent with previous results, the data suggest that Rab5 mediates transport to synaptic endosomes. Second, Rab7CA and Rab11CA appear to inhibit new vesicle formation, likely by controlling post endosome trafficking. Finally, this study found that Rab23CA and Rab35CA mediate transport to or retention of synaptic vesicles at sub-boutonic structures. Neither of these Rabs had yet been implicated in synaptic vesicle traffic and hence, these in vivo studies identified different Rab proteins involved in aspects of the synaptic vesicle cycle (Uytterhoeven, 2011).

    The expression of Rab5CA and Rab35CA results in increased trafficking of vesicles to -or retention of vesicles at- sub-boutonic structures, and unlike expression of Rab23CA, Rab5CA and Rab35CA also result in a facilitation of neurotransmitter release similar to sky mutants. Rab35 has been found to localize to the plasma membrane (Sato, 2008) and this study shows enrichment at NMJ boutons close to the membrane as well. Rab35 has been implicated in endosomal, clathrin-dependent traffic, phagocytic membrane uptake in non-neuronal cells and actin filament assembly during Drosophila bristle development (Allaire, 2010, Sato, 2008, Shim, 2010, Zhang, 2009). Given these roles, Rab35 is ideally posed to also play a role in the endosomal traffic of synaptic vesicles, and the current in vitro and in vivo genetic interaction studies indicate Sky to be a Rab35 GAP in synaptic vesicle trafficking. Although Rab5 mediates endosomal traffic in different cell types, the data indicate that Sky is not a GAP for Rab5 in vivo at the synapse. While this study does not exclude Sky activating the GTPase activity of alternative Rabs in different contexts, the results suggest a Sky-Rab35 partnership that restricts endosomal trafficking of synaptic vesicles (Uytterhoeven, 2011).

    In most cell types the ESCRT complex mediates sorting of ubiquitinated proteins into multivesicular bodies; however, such a function has not been characterized for synaptic vesicle components. This study provide evidence that the ESCRT complex mediates synaptic vesicle protein sorting in sky mutants where synaptic vesicles cycle excessively via endosomes. This study found genetic interactions with ESCRT genes. In addition, more efficient clearance of an artificially ubiquitinated synaptic vesicle protein, Ub-nSybHA, was also found in sky mutants, and this effect is ESCRT dependent. Combined, the data indicate endosomal sorting to control the quality of proteins in the synaptic vesicle cycle in sky mutants (Uytterhoeven, 2011).

    Increased transmitter release as a result of endosomal sorting e.g. in sky mutants, may appear beneficial in some instances, however in inhibitory neurons for example, such an effect could result in reduced neuronal signaling while in excitatory neurons this feature could over activate postsynaptic cells. Clearly, fine-tuned neurotransmission in neuronal populations may ensure optimal information flow, and misregulation of Sky activity in neuronal populations may cause systemic defects including larval paralysis and death. Further underscoring this notion, in humans, mutations in the Sky orthologue TBC1D24 are causative of focal epilepsy (Uytterhoeven, 2011).

    The sorting mechanism uncovered by loss of Sky function or upon expression of Rab35CA may address the use-dependent decline in protein- or lipid-function at synapses, and the continuous need to rejuvenate the synaptic vesicle protein pool. In addition, it may also constitute a mechanism to remove inappropriately endocytosed membrane components from the vesicle pool. The trafficking pathway inhibited by Sky may also yield the ability of synapses to adapt the functional properties of their synaptic vesicles, thus controlling the nature or abundance of proteins involved in vesicle fusion and neuronal signaling (Uytterhoeven, 2011).

    Reduced synaptic vesicle protein degradation at lysosomes curbs TBC1D24/sky-induced neurodegeneration

    Synaptic demise and accumulation of dysfunctional proteins are thought of as common features in neurodegeneration. However, the mechanisms by which synaptic proteins turn over remain elusive. This paper studied Drosophila melanogaster lacking active TBC1D24/Skywalker (Sky), a protein that in humans causes severe neurodegeneration, epilepsy, and DOOR (deafness, onychdystrophy, osteodystrophy, and mental retardation) syndrome; endosome-to-lysosome trafficking was identified as a mechanism for degradation of synaptic vesicle-associated proteins. In fly sky mutants, synaptic vesicles traveled excessively to endosomes. Using chimeric fluorescent timers, synaptic vesicle-associated proteins were shown to be younger on average, suggesting that older proteins are more efficiently degraded. Using a genetic screen, it was found that reducing endosomal-to-lysosomal trafficking, controlled by the homotypic fusion and vacuole protein sorting (HOPS) complex, rescued the neurotransmission and neurodegeneration defects in sky mutants. Consistently, synaptic vesicle proteins were older in HOPS complex mutants, and these mutants also showed reduced neurotransmission. These findings define a mechanism in which synaptic transmission is facilitated by efficient protein turnover at lysosomes and identify a potential strategy to suppress defects arising from TBC1D24 mutations in humans (Fernandes, 2014).

    High-probability neurotransmitter release sites represent an energy-efficient design

    Nerve terminals contain multiple sites specialized for the release of neurotransmitters. This study tested the hypothesis that high-probability release sites represent an energy-efficient design. Release site probabilities and energy efficiency were examined at the terminals of two glutamatergic motor neurons synapsing on the same muscle fiber in Drosophila larvae. Through electrophysiological and ultrastructural measurements, calculated release site probabilities were found to differ considerably between terminals (0.33 versus 0.11). The energy required to release and recycle glutamate were calculated from the same measurements. The energy required to remove calcium and sodium ions subsequent to nerve excitation was estimated through microfluorimetric and morphological measurements. Energy efficiency was calculated as the number of glutamate molecules released per ATP molecule hydrolyzed, and high-probability release site terminals were found to be more efficient (0.13 versus 0.06). This analytical model indicates that energy efficiency is optimal (~ 0.15) at high release site probabilities (~0.76). As limitations in energy supply constrain neural function, high-probability release sites might ameliorate such constraints by demanding less energy. Energy efficiency can be viewed as one aspect of nerve terminal function, in balance with others, because high-efficiency terminals depress significantly during episodic bursts of activity (Lu, 2016).

    Activity induces Fmr1-sensitive synaptic capture of anterograde circulating neuropeptide vesicles

    Synaptic neuropeptide and neurotrophin stores are maintained by constitutive bidirectional capture of dense-core vesicles (DCVs) as they circulate in and out of the nerve terminal. Activity increases DCV capture to rapidly replenish synaptic neuropeptide stores following release. However, it is not known whether this is due to enhanced bidirectional capture. Experiments at the Drosophila neuromuscular junction, where DCVs contain neuropeptides and a bone morphogenic protein, show that activity-dependent replenishment of synaptic neuropeptides following release is evident after inhibiting the retrograde transport with the dynactin disruptor mycalolide B or photobleaching DCVs entering a synaptic bouton by retrograde transport. In contrast, photobleaching anterograde transport vesicles entering a bouton inhibits neuropeptide replenishment after activity. Furthermore, tracking of individual DCVs moving through boutons shows that activity selectively increases capture of DCVs undergoing anterograde transport. Finally, upregulating fragile X mental retardation 1 protein (Fmr1, also called FMRP) acts independently of futsch/MAP-1B to abolish activity-dependent, but not constitutive, capture. Fmr1 also reduces presynaptic neuropeptide stores without affecting activity-independent delivery and evoked release. Therefore, presynaptic motoneuron neuropeptide storage is increased by a vesicle capture mechanism that is distinguished from constitutive bidirectional capture by activity dependence, anterograde selectivity, and Fmr1 sensitivity. These results show that activity recruits a separate mechanism than used at rest to stimulate additional synaptic capture of DCVs for future release of neuropeptides and neurotrophins (Cavolo, 2016).

    Synapses are supplied by anterograde axonal transport from the soma, the site of synthesis of synaptic vesicle proteins and dense-core vesicles (DCVs) that contain neuropeptides and neurotrophins. Delivery to synapses was thought to be based on a one-way anterograde trip until it was discovered that DCVs are subject to sporadic capture while traveling bidirectionally through en passant boutons as part of long-distance vesicle circulation (Wong, 2012). Interestingly, constitutive DCV capture occurs both during fast anterograde and retrograde transport, which are mediated by different motors (i.e., the kinesin 3 family member unc-104/Kif1A and the dynein/dynactin complex, respectively). Balanced capture in both directions is advantageous because DCVs are distributed equally among en passant boutons (Wong, 2012). In principle, bidirectional capture could occur by parallel regulation of anterograde and retrograde motors or by modification of the microtubules that both anterograde and retrograde DCV motors travel on (Cavolo, 2016).

    Before the discovery of bidirectional capture of circulating vesicles, activity was shown to replenish the presynaptic neuropeptide pool following release by inducing Ca2+-dependent capture of DCVs being transported through boutons. This result and subsequent experiments (Bulgari, 2014) established that capture, rather than delivery or DCV turnover, limits synaptic neuropeptide stores. Activity-dependent capture was first described with a GFP-tagged neuropeptide in the Drosophila neuromuscular junction (NMJ), but also occurs with neurotrypsin, wnt/wingless, and brain-derived neurotrophic factor. Mechanistically, activity-dependent capture was characterized in terms of the rebound in presynaptic GFP-tagged peptide content following release and correlated with decreased retrograde transport. However, it is now evident that the reduction in retrograde flux could be caused by enhanced bidirectional capture as DCVs travel back and forth through the terminal as part of vesicle circulation (Wong, 2012). Therefore, prior studies support the hypothesis that there is only one synaptic capture mechanism, which is bidirectional and facilitated by activity (Cavolo, 2016).

    This study tested the above hypothesis by investigating the directionality of activity-dependent capture. Experiments were performed with multiple approaches, including inhibiting retrograde transport, particle tracking, and simultaneous photobleaching and imaging (SPAIM; Wong, 2012). Furthermore, the effect of fragile X retardation protein (Fmr1, also called FMRP) was examined because it is known to affect bouton size and neuropeptide release. Together, these studies establish that different mechanisms mediate synaptic capture at rest and in response to activity (Cavolo, 2016).

    Until recently, it was thought that presynaptic neuropeptide stores were set by controlling synthesis and delivery by fast one-way axonal transport of DCVs. However, studies of the Drosophila NMJ have shown that there is an excess of DCVs delivered to type-I boutons by long-distance vesicle circulation. Therefore, because DCV delivery is not limiting, the presynaptic neuropeptide pool is determined by capture, which was found to be bidirectional (Wong, 2012). However, in addition to constitutive capture, activity induces Ca2+-dependent capture. This is advantageous because tapping into the circulating vesicle pool removes delays associated with synthesis and transport, which can take days in humans, to rapidly replace released peptides. Surprisingly, experiments presented in this study demonstrate that activity-dependent capture is unidirectional and selectively sensitive to a genetic perturbation (i.e., Fmr1 overexpression). Therefore, activity does not simply enhance constitutive bidirectional capture that operates at rest, but instead stimulates an independent synaptic capture mechanism (Cavolo, 2016).

    Previously, it was not possible to genetically block activity-dependent capture to determine its contribution to steady-state presynaptic stores. However, this study documented inhibition of activity-dependent capture by Fmr1 overexpression. As this was accompanied by a dramatic decrease in presynaptic DCV number, it is concluded that activity-dependent capture makes a large contribution to steady-state presynaptic peptide stores and hence the capacity for future release. At the Drosophila NMJ, DCVs contain a bone morphogenic protein and neuropeptides. Thus, it is possible that activity-dependent capture affects development and acute synaptic function (Cavolo, 2016).

    Capture efficiency measurements revealed that the previously detected decrease in retrograde traffic following activity was an indirect effect of vesicle circulation; activity-induced capture of only anterograde DCVs at each en passant bouton simply leaves fewer DCVs for the retrograde trip back into the axon without changing retrograde capture. Of interest, anterograde selectivity for activity-induced capture rules out mechanisms that would perturb transport in both directions (e.g., microtubule breaks). DCV anterograde transport is mediated by the unc-104/Kif1A motor, which also transports SSV proteins and is required for formation of boutons. Therefore, activity-dependent capture may regulate unc-104/Kif1A to affect synaptic release of both small-molecule transmitters and peptides. However, alternative targets could be involved, including proteins that mediate DCV interaction with this anterograde motor or alter the DCV itself (e.g., its phosphoinositides, which may bind to the unc-104/Kif1A pleckstrin homology domain) (Cavolo, 2016).

    Phosphorylation of Complexin by PKA regulates activity-dependent spontaneous neurotransmitter release and structural synaptic plasticity

    Synaptic plasticity is a fundamental feature of the nervous system that allows adaptation to changing behavioral environments. Most studies of synaptic plasticity have examined the regulated trafficking of postsynaptic glutamate receptors that generates alterations in synaptic transmission. Whether and how changes in the presynaptic release machinery contribute to neuronal plasticity is less clear. The SNARE complex mediates neurotransmitter release in response to presynaptic Ca(2+) entry. This study shows that the SNARE fusion clamp Complexin undergoes activity-dependent phosphorylation that alters the basic properties of neurotransmission in Drosophila. Retrograde signaling following stimulation activates PKA-dependent phosphorylation of the Complexin C terminus that selectively and transiently enhances spontaneous release. Enhanced spontaneous release is required for activity-dependent synaptic growth. These data indicate that SNARE-dependent fusion mechanisms can be regulated in an activity-dependent manner and highlight the key role of spontaneous neurotransmitter release as a mediator of functional and structural plasticity (Cho, 2015).

    These findings indicate that the SNARE-binding protein Cpx is a key PKA target that regulates spontaneous fusion rates and presynaptic plasticity at Drosophila NMJs. Cpx's function can be modified to regulate activity-dependent functional and structural plasticity. In vivo experiments using Cpx phosphomimetic rescues demonstrate that Cpx phosphorylation at residue S126 selectively alters its ability to act as a synaptic vesicle fusion clamp. In addition, S126 is critical for the expression of HFMR, an activity-dependent form of acute functional plasticity that modulates mini frequency at Drosophila synapses. These data indicate a Syt 4-dependent retrograde signaling pathway converges on Cpx to regulate its synaptic function. Additionally, it was found that elevated spontaneous fusion rates correlate with enhanced synaptic growth. This pathway requires Syt 4 retrograde signaling to enhance spontaneous release and to trigger synaptic growth. Moreover, the Cpx S126 PKA phosphorylation site is required for activity-dependent synaptic growth, suggesting acute regulation of minis via Cpx phosphorylation is likely to contribute to structural synaptic plasticity. Together, these data identify a novel mechanism of acute synaptic plasticity that impinges directly on the presynaptic release machinery to regulate spontaneous release rates and synaptic maturation (Cho, 2015).

    How does acute phosphorylation of S126 alter Cpx's function? The Cpx C-terminus associates with lipid membranes through a prenylation domain (CAAX motif) and/or the presence of an amphipathic helix. The Drosophila Cpx isoform used in this study (Cpx 7B) lacks a CAAX-motif, but contains a C-terminal amphipathic helix flanked by the S126 phosphorylation site. S126 phosphorylation does not alter synaptic targeting of Cpx or its ability to associate with SNARE complexes in vitro. As such, phosphorylation may instead alter interactions of the amphipathic helix region with lipid membranes and/or alter Cpx interactions with other proteins that modulate synaptic release. Given the well-established role of the Cpx C-terminus in regulating membrane binding and synaptic vesicle tethering of Cpx, phosphorylation at this site would be predicted to alter the subcellular localization of the protein and its accessibility to the SNARE complex. However, no large differences between WT Cpx and CpxS126D were observed in liposome binding. This assay is unlikely to reveal subtle changes in lipid interactions by Cpx, as this study found that C-terminal deletions (CTD) maintained its ability to bind membranes. The ability of the CTD versions of Drosophila Cpx to associate with liposomes is unlike that observed with C. elegans Cpx, and indicate domains outside of Drosophila Cpx's C-terminus contribute to lipid membrane association as well, potentially masking effects from S126 phosphorylation that might occur in vivo. Alternatively, phosphorylation of the Cpx C-terminus could alter its association with other SNARE complex modulators such as Syt 1 (Cho, 2015).

    The data indicate that enhanced minis regulate synaptic growth through several previously identified NMJ maturation pathways. The Wit signaling pathway is required for synaptic growth in the background of enhanced minis. The wit gene encodes a presynaptic type II BMP receptor that receives retrograde, transsynaptic BMP signals from postsynaptic muscles. Consistent with these data, other studies have demonstrated that downstream signaling components of the BMP pathway are necessary and sufficient for mini-dependent synaptic growth at the Drosophila NMJ. Additionally, it was found that the postsynaptic Ca++ sensor, Syt 4, is required for enhancing spontaneous release and increasing synaptic growth. The data does not currently distinguish the interdependence of the BMP and Syt 4 retrograde signaling pathways, and other retrograde signaling pathways might contribute to mini-dependent synaptic growth as well. Recently, several retrograde pathways have been identified that regulate functional homeostatic plasticity at the Drosophila NMJ. Future work will be required to fully define the retrograde signaling pathways necessary to mediate mini-dependent synaptic growth (Cho, 2015).

    How might elevated spontaneous release through Cpx phosphorylation regulate synaptic growth? It is hypothesized that the switch in synaptic vesicle release mode to a constitutive fusion pathway that occurs over several minutes following stimulation serves as a synaptic tagging mechanism. By continuing to activate postsynaptic glutamate receptors in the absence of incoming action potentials, the elevation in mini frequency would serve to enhance postsynaptic calcium levels by prolonging glutamate receptor stimulation. This would prolong retrograde signaling that initiates downstream cascades to directly alter cytoskeletal architecture required for synaptic bouton budding. Given that elevated rates of spontaneous fusion still occur in cpx and syx3-69 in the absence of Syt 4 and BMP signaling, yet synaptic overgrowth is suppressed in these conditions, it is unlikely that spontaneous fusion itself directly drives synaptic growth. Rather, the transient enhancement in spontaneous release may serve to prolong postsynaptic calcium signals that engage distinct effectors for structural remodeling that fail to be activated in the absence of elevated spontaneous release. Results from mammalian studies indicate spontaneous release can uniquely regulate postsynaptic protein translation and activate distinct populations of NMDA receptors compared to evoked release, so it is possible that spontaneous fusion may engage unique postsynaptic effectors at Drosophila NMJs as well (Cho, 2015).

    In the last few decades, intense research efforts have elucidated several molecular mechanisms of classic Hebbian forms of synaptic plasticity that include long-term potentiation (LTP) and long-term depression (LTD), alterations in synaptic function that lasts last minutes to hours. The most widely studied expression mechanism for these forms of synaptic plasticity involve modulation of postsynaptic AMPA-type glutamate receptor (AMPAR) function and membrane trafficking. In contrast, molecular mechanisms of short-term synaptic plasticity remain poorly understood. Several forms of short-term plasticity have been described, such as post-tetanic potentiation (PTP), which involves stimulation-dependent increases in synaptic strength, including changes in mini frequency. Short-term plasticity expression is likely to impinge on the alterations to the presynaptic release machinery downstream of activated effector molecules. For example, Munc 18, a presynaptic protein involved in the priming step of vesicle exocytosis via its ability to associate with members of the SNARE machinery, is dynamically regulated by Ca++-dependent protein kinase C (PKC), and its regulation is required to express PTP at the Calyx of Held. This study demonstrates that the presynaptic vesicle fusion machinery can also be directly modified to alter spontaneous neurotransmission via activity-dependent modification of Cpx function by PKA. Protein kinase CK2 and PKC phosphorylation sites within the C-terminus of mammalian and C. elegans Cpx have been identified. Therefore, activity-dependent regulation of Cpx function via C-terminal phosphorylation may be an evolutionarily conserved mechanism to regulate synaptic plasticity. Moreover, Cpx may lie downstream of multiple effector pathways to modulate various forms of short-term plasticity, including PTP, in a synapse-specific manner. Interestingly, Cpx is expressed both pre- and postsynaptically in mammalian hippocampal neurons and is required to express LTP via regulation of AMPAR delivery to the synapse, suggesting Cpx-mediated synaptic plasticity expression mechanisms may also occur postsynaptically (ACho, 2015 and references therein).

    In summary, these results indicate minis serve as an independent and regulated neuronal signaling pathway that contributes to activity-dependent synaptic growth. Previous studies found Cpx’s function as a facilitator and clamp for synaptic vesicle fusion is genetically separable, demonstrating distinct molecular mechanisms regulate evoked and spontaneous release. Evoked and spontaneous release are also separable beyond Cpx regulation, as other studies have demonstrated that minis can utilize distinct components of the SNARE machinery, distinct vesicle pools, and distinct individual synaptic release sites . These findings suggest diverse regulatory mechanisms for spontaneous release that might be selectively modulated at specific synapses (Cho, 2015).

    Fife organizes synaptic vesicles and calcium channels for high-probability neurotransmitter release

    The strength of synaptic connections varies significantly and is a key determinant of communication within neural circuits. Mechanistic insight into presynaptic factors that establish and modulate neurotransmitter release properties is crucial to understanding synapse strength, circuit function, and neural plasticity. Drosophila Fife , a Piccolo-RIM homolog. has been shown to regulate neurotransmission and motor behavior through an unknown mechanism. This study demonstrates that Fife localizes and interacts with RIM (Rab3 interacting molecule) at the active zone cytomatrix to promote neurotransmitter release. Loss of Fife results in the severe disruption of active zone cytomatrix architecture and molecular organization. Through electron tomographic and electrophysiological studies, a decrease was found in the accumulation of release-ready synaptic vesicles and their release probability caused by impaired coupling to Ca2+ channels. Finally, Fife was found to be essential for the homeostatic modulation of neurotransmission. It is proposed that Fife organizes active zones to create synaptic vesicle release sites within nanometer distance of Ca2+ channel clusters for reliable and modifiable neurotransmitter release (Bruckner 2016).

    The strength of synaptic transmission is a critical determinant of information processing in neural circuits. Evoked neurotransmission depends on localized Ca2+ influx triggering neurotransmitter release from synaptic vesicles at specialized domains of presynaptic terminals called active zones. At the active zone membrane, synaptic vesicles are docked and molecularly primed to respond to a rise in Ca2+ concentration by fusing with the membrane to release neurotransmitter. A conserved complex of active zone-associated proteins makes up the active zone cytomatrix. In Drosophila melanogaster, these proteins include the ELKS family protein Bruchpilot, Rab3-interacting molecule (RIM), RIM-binding protein, Unc13, and Fife. Active zone cytomatrix proteins contain many lipid- and protein-binding domains that mediate diverse interactions with key players in synaptic transmission, leading to the model that the active zone cytomatrix spatially organizes presynaptic terminals for the millisecond coupling of neurotransmitter release to action potentials (Bruckner, 2016).

    The specific neurotransmitter release properties of an active zone are determined by two key parameters acting in concert: (1) the number of synaptic vesicles docked at the membrane and molecularly primed for Ca2+-triggered release, termed the readily releasable pool, and (2) the release probability of these vesicles. Vesicle release probability is established by multiple parameters, including Ca2+ channel levels, localization and function at active zones, the spatial coupling of Ca2+ channels and release-ready vesicles, and the intrinsic Ca2+ sensitivity of individual vesicles. The observation that the presynaptic parameters of synaptic strength vary significantly even between the synapses of an individual neuron indicates that neurotransmitter release properties are determined locally at active zones and raises the question of how this complex regulation is achieved. Genetic studies in Drosophila, Caenorhabditis elegans, and mice are revealing a key role for the active zone cytomatrix in determining the functional parameters underlying synaptic strength. A mechanistic understanding of how the active zone cytomatrix achieves local control of synaptic release properties will yield fundamental insights into neural circuit function (Bruckner, 2016).

    Fife, a Piccolo-RIM-related protein is required for proper neurotransmitter release and motor behavior. This study demonstrates that Fife localizes to the active zone cytomatrix, where it interacts with RIM to promote neurotransmitter release. The active zone cytomatrix is diminished and molecularly disorganized at Fife mutant synapses, and Fife is critical for vesicle docking at the active zone membrane. Not only are the number of release-ready vesicles reduced in the absence of Fife, but their probability of release is also significantly impaired because of disrupted coupling to calcium channels. These results suggest that Fife promotes high-probability neurotransmitter release by organizing the active zone cytomatrix to create vesicle release sites in nanometer proximity to clustered Ca2+ channels. Finally, it was found that in addition to its role in determining baseline synaptic strength, Fife plays an essential role in presynaptic homeostatic plasticity. Together, these findings provide mechanistic insight into how synaptic strength is established and modified to tune communication in neural circuits (Bruckner, 2016).

    This study demonstrates that Fife plays a key role in organizing presynaptic terminals to determine synaptic release properties. Fife functions with RIM at the active zone cytomatrix to promote neurotransmitter release. Functional and ultrastructural imaging studies demonstrate that Fife regulates the docking of release-ready synaptic vesicles and, through nanodomain coupling to Ca2+ channels, their high probability of release. It was further found that Fife is required for the homeostatic increase in neurotransmitter release that maintains circuit function when postsynaptic receptors are disrupted. These findings uncover Fife's role as a local determinant of synaptic strength and add to understanding of how precise communication in neural circuits is established and modulated (Bruckner, 2016).

    The finding that Fife interacts with RIM provides insight into how Fife functions within the network of cytomatrix proteins. RIM is a central active zone protein that was recently shown to facilitate vesicle priming at mammalian synapses by relieving autoinhibition of the priming factor Munc13. Like Fife, Drosophila RIM promotes Ca2+ channel accumulation at active zones and exhibits EGTA-sensitive neurotransmitter release at the Drosophila NMJ. This suggests that Fife and RIM may promote high-probability neurotransmitter release by acting together to dock and prime synaptic vesicles in close proximity to Ca2+ channels clustered at the cytomatrix. The findings are consistent with previous work in pancreatic β cells, where it was found that Piccolo and RIM2α form a complex that promotes insulin secretion through an unknown mechanism. To date, functional studies at mammalian synapses have focused on investigating interactions between Piccolo and Bassoon, which bind through their common coiled-coil regions. Thus, it will be of interest to investigate the functional relationship between Piccolo and RIM in promoting neurotransmitter release in mouse models. Piccolo also binds CAST1 through coiled-coil domain interactions. Although the Piccolo coiled-coil region is not present in Fife, future studies to determine whether this interaction is preserved through distinct interacting domains will be important, as Fife and Drosophila CAST-related Bruchpilot carry out overlapping functions. Similarly, although neither Fife nor Piccolo contains the conserved SH3-binding domain that mediates the interaction between RIM and RIM-binding protein, the overlap between Fife and RIM-binding protein phenotypes raises the possibility of functional interactions that will also be important to investigate in future experiments (Bruckner, 2016).

    Significant alterations to active zone cytomatrix size and structure were observed in Fife mutants, whereas none have been detected in RIM mutants, indicating that Fife carries out this function independently of RIM. Previous ultrastructural analysis in aldehyde-fixed samples revealed occasional free-floating electron-dense structures that resemble active zone cytomatrix material and cluster synaptic vesicles. These unanchored electron-dense structures have also been observed at low frequency in Drosophila RIM-binding protein mutants, which, like Fife mutants, exhibit smaller active zone cytomatrices, and at higher frequency in rodent ribbon synapses lacking Bassoon. These structures were not visible in HPF/FS-prepared electron microscopy samples, likely because protein components of the synapse are not cross-linked upon fixation. That these structures are not observed in control synapses but have been found in multiple active zone cytomatrix mutants argues that the extensive cross-linking of proteins in chemically fixed preparations may enable the visualization of biologically relevant complexes missed with cryofixation. This supports the idea that the two fixation methods may offer different advantages for ultrastructural studies of synapses. In any case, the active zone cytomatrix is significantly reduced in size at Fife active zones in both HPF/FS- and aldehyde-fixed electron micrographs (Bruckner, 2016).

    Although diminished or absent cytomatrices have been observed in electron micrographs of RIM-binding protein and Bruchpilot mutants, this phenotype has not been observed in electron micrographs of other active zone cytomatrix mutants, suggesting it represents more than the loss of a single component protein. Rather, the reduced complexity visible in electron micrographs likely reflects a broader underlying molecular disorganization. Further support for this model comes from superresolution imaging, which reveals molecular disorganization at Fife active zones as indicated by the loss of the characteristic ring-shaped localization pattern of Bruchpilot's C terminus. Similar disorganization of Bruchpilot was observed at active zones lacking RIM-binding protein. Superresolution imaging was used to investigate the localization of active zone proteins Cacophony and RIM-binding protein and, although the levels of Cacophony are reduced at Fife active zones, no apparent differences were observed in the patterns of these proteins. Cacophony and RIM-binding protein both localize in smaller puncta than Bruchpilot, so future studies with higher resolution imaging modalities such as stimulated emission depletion microscopy may reveal more subtle abnormalities. Correlations between active zone molecular composition and release probability have been observed at diverse synapses. At the Drosophila NMJ, functional imaging with genetically encoded Ca2+ indicators has demonstrated that active zones display a wide range of release probabilities. Active zones with high release probability contain higher levels of Bruchpilot, which may in turn correlate with higher Ca2+ channel levels. At mouse hippocampal synapses, Bassoon and RIM levels directly correlate with neurotransmitter release probability. Consistently, synaptic probability of release is significantly decreased in Fife mutants (Bruckner, 2016)

    Through a combination of morphological and functional studies, this study found that Fife acts to promote the active zone docking of synaptic vesicles and regulates their probability of release. Because the number of readily releasable vesicles appears to scale with active zone cytomatrix size and molecular composition at diverse synapses, a conserved function of the active zone cytomatrix may be to establish release sites for synaptic vesicles. Consistent with this view, the number of release-ready vesicles is also reduced in Drosophila RIM-binding protein-null mutants and isoform-specific bruchpilotΔ170 and bruchpilotΔ190 mutants, which share similar active zone structural abnormalities with Fife. By combining rapid preservation of intact Drosophila larvae by HPF/FS fixation, electron tomography, and extensive segmentation of active zone structures, this study obtained a detailed view of the 3D organization of active zones in near-native state that allowed further dissection of Fife's role in determining the size of the readily releasable vesicle pool. Membrane-docked vesicles are significantly decreased in Fife mutants, whereas more distant vesicles attached to the membrane by long tethers appear unaffected. Correlating physiological and morphological parameters of neurotransmission is an ongoing challenge in the field. It has been proposed that docking and priming are not separable events in the establishment of the readily releasable vesicle pool, but rather the morphological and physiological manifestations of a single process. Although approximately one third of docked vesicles in these preparations lack obvious short connections to the membrane, which are thought to represent priming factors, the possibility cannot be excluded that these filaments are present but obscured, perhaps because the vesicles are more tightly linked to the membrane. As this proportion is unchanged in Fife mutants, it is concluded that Fife acts to promote vesicle docking and may simultaneously facilitate molecular priming-possibly through its interactions with RIM (Bruckner, 2016).

    The data indicate that neurotransmitter release at Fife synapses is highly sensitive to EGTA, a slow Ca2+ chelator that has been used to investigate the coupling of Ca2+ influx at voltage-gated Ca2+ channels and Ca2+ sensors on synaptic vesicles. At synapses with high release probability, including inhibitory synapses in the mammalian hippocampus and cerebellum, excitatory synapses of the mature Calyx of held, and the Drosophila NMJ, molecularly primed synaptic vesicles and Ca2+ channel clusters are thought to be positioned within ~100 nm of one another to ensure the tight coupling of Ca2+ influx and Ca2+ sensors that explains observed release properties. The EGTA sensitivity of release at Fife, but not wild-type, NMJs indicates that Fife likely regulates the probability that a docked vesicle is released by positionally coupling release-ready vesicles to Ca2+ channels clustered beneath the active zone cytomatrix. The trend toward fewer docked vesicles associated with the active zone cytomatrix in tomograms of Fife synapses provides morphological support for this model. Building on detailed tomographic studies of the Drosophila NMJ to visualize how Ca2+ channels and vesicles are spatially organized at active zones in different genetic backgrounds will be an important step in advancing understanding of the geometry of release probability and how it is established (Bruckner, 2016).

    Finally, this study found that Fife is required for presynaptic homeostasis. In response to decreases in glutamate receptor levels or function, Drosophila motoneurons rapidly increase synaptic vesicle release to maintain postsynaptic excitation. This homeostatic increase in presynaptic neurotransmission is accompanied by an increase in the number of dense projections per active zone and Bruchpilot levels. Cytomatrix proteins RIM, RIM-binding protein, and now Fife have all been shown to function in presynaptic homeostasis, indicating a critical role for the active zone cytomatrix as a substrate for synaptic plasticity. These studies provide insight into the molecular mechanisms through which the active zone cytomatrix determines neurotransmitter release parameters to modulate how information flows in neural circuits (Bruckner, 2016).

    Input-specific plasticity and homeostasis at the Drosophila larval neuromuscular junction

    Synaptic connections undergo activity-dependent plasticity during development and learning, as well as homeostatic re-adjustment to ensure stability. Little is known about the relationship between these processes, particularly in vivo. This was addressed with novel quantal resolution imaging of transmission during locomotive behavior at glutamatergic synapses of the Drosophila larval neuromuscular junction. Two motor input types, Ib and Is, were found to provide distinct forms of excitatory drive during crawling and differ in key transmission properties. Although both inputs vary in transmission probability, active Is synapses are more reliable. High-frequency firing 'wakes up' silent Ib synapses and depresses Is synapses. Strikingly, homeostatic compensation in presynaptic strength only occurs at Ib synapses. This specialization is associated with distinct regulation of postsynaptic CaMKII. Thus, basal synaptic strength, short-term plasticity, and homeostasis are determined input-specifically, generating a functional diversity that sculpts excitatory transmission and behavioral function (Newman, 2017).

    The transfer of information between neurons throughout the nervous system relies on communication across inherently unreliable chemical synapses. Synaptic communication is further complicated by the fact that individual neurons can receive inputs from many functionally diverse neurons. Even synapses formed by one presynaptic neuron and one or more postsynaptic target neurons can vary greatly in neurotransmitter release, postsynaptic sensitivity, and plasticity. Additionally, postsynaptic cells are not passive receivers of information but can produce retrograde signals, including homeostatic signals that modulate synaptic release. The mechanisms that regulate diversity in transmission at individual synapses are not well understood, nor are their relationships to the plasticity and homeostatic mechanisms that adjust synaptic strength. In particular, it is unclear whether retrograde signaling is input or synapse specific and able to maintain input context among diverse convergent synapses (Newman, 2017).

    The Drosophila larval neuromuscular junction (NMJ) is a model system for studying glutamatergic transmission, with pre- and postsynaptic molecular machinery similar to that of central excitatory synapses in vertebrates, while also possessing activity-dependent adjustments in synaptic strength, including short- and long-term plasticity, as well as homeostatic plasticity. Two morphologically distinct glutamatergic motor neurons, larger type Ib and smaller type Is, converge onto most of the larval body wall muscles used for locomotion. There is evidence that the transmission properties of these inputs differ (Kurdyak, 1994; Lnenicka, 2000; Lu, 2016), thus providing a powerful system for investigating the role of input and synapse specificity in the regulation of basal synaptic strength, plasticity, and homeostasis. In addition, there is great heterogeneity, in that both the basal release probability (Pr) of evoked release and the frequency of spontaneous release differ greatly between synapses of the same Ib axon (Peled, 2011; Peled, 2014). This study set out to understand how plasticity and homeostasis function in a diverse pool of synapses and to determine whether synaptic homeostasis operates globally or has input specificity (Newman, 2017).

    To address these questions requires high-resolution, high- speed analysis of function at many synapses simultaneously. Quantal resolution measurements of excitatory transmission through Ca2+-permeant glutamate receptors (GluRs) has been achieved by imaging chemical or genetically encoded Ca2+ indicators (GECIs) in the postsynaptic cell (Cho, 2015; Guerrero, 2005; Lin, 2016; Melom, 2013; Muhammad, 2015; Peled, 2011; Peled, 2014; Reese, 2015, Reese, 2016; Siegel, 2013). This study has generated a vastly improved postsynaptically targeted GECI based on GCaMP6f. When expressed in Drosophila larval muscle, 'SynapGCaMP6f' enables quantal imaging without voltage clamping. SynapGCaMP6f was combined with an optical platform that immobilizes larvae without anesthetics to measure synaptic transmission simultaneously at hundreds of Ib and Is synapses in the intact, behaving animal (Newman, 2017).

    The quantal imaging makes it possible to connect the elementary properties of transmission at single synapses, to the synaptic drive that is generated by convergent synaptic inputs, to the operation of muscle groups in large parts of the animal, and finally to behavior. These observations show that basal synaptic strength, short-term plasticity, and, most strikingly, synaptic homeostasis are input specific, diversifying excitatory transmission and behavioral output (Newman, 2017).

    The Ib and Is inputs to the Drosophila larval muscle respectively resemble tonic (low release probability, facilitating) and phasic (high release probability, depressing) inputs seen at neuro-muscular and neuro-neuronal connections in other organisms. Given these differences, the Is input has been proposed to initiate strong, phasic muscle contractions (Schaefer, 2010). However, this study found that the primary excitatory drive and main cause of muscle contraction in larval Drosophila is actually the Ib input. This is despite the fact that Is synapses have a higher basal release probability (Pr) and larger quantal sizes. These functional differences likely reflect the relatively short duration of the Is bursts of activity and the relatively small number of active zones of the Is terminal. Whereas elimination of transmission from Is inputs had little or no effect on muscle contraction during restrained locomotion, just as it had earlier been shown to have negligible effect on general behavior, this study found that the Is input is needed for normal intersegmental coordination of contraction waves. This is consistent with the broad, multi-muscle Is innervation pattern (Newman, 2017).

    Ib synaptic activity begins before and ends after Is activity, consistent with their common presynaptic inputs and different intrinsic excitabilities. Recent work has shown that excitatory and inhibitory interneurons regulate the differential recruitment of MNs between hemi- segments and within hemisegments, respectively (Heckscher, 2015; Zwart, 2016). While Ib inputs ramp up their synaptic drive during locomotor bursts, Is inputs abruptly reach a maximum, which they often sustain for the duration of the shorter burst. Given their substantial depression during high-frequency stimulation, the ability of the Is input to sustain a fixed output level during locomotion suggests that it increases the firing frequency during the bursts. The common interneuron drive within the VNC suggests that the progressive increase in synaptic drive during the Ib burst may also be partly due to an increase in frequency during the burst, in addition to the recruitment of silent synapses and increase in Pr of active synapses, which is observed during facilitating high-frequency trains. Thus, a combination of circuit-level and cell-autonomous differences combine to generate the complex behavioral output of the larva from a limited number MNs and muscles (Newman, 2017).

    Spontaneous glutamate release has been shown to play a role in synapse development (Choi, 2014). However, this study found that spontaneous release in vivo represents only ~1% of total release, suggesting that evoked release would drown out the influence of spontaneous release. One possible explanation is that in late third-instar larvae, spontaneous release may represent a larger fraction of total synaptic transmission than earlier in development when the developmental influences are exerted. An intriguing alternative explanation derives from the finding that, at both inputs, synapses preferentially participate in either evoked or spontaneous release. This suggests that developmental signals could occur at a subset synapses that are dominated by spontaneous release (Newman, 2017).

    Synapses of both inputs were found to differ in key properties, with Is synapses having larger quantal sizes, less total spontaneous release, higher Pr, and short-term depression (as opposed to facilitation in Ib synapses) during high-frequency trains. The difference in short-term plasticity between the inputs is consistent with electrophysiological analysis of responses to separate stimulation of Is or Ib axons (Lnenicka, 2000; Lu, 2016), as well as modeling. These resemble input-specific differences seen in the mammalian brain, such as between parallel and climbing fibers converging on Purkinje cells (Dittman, 2000; Mapelli, 2015) or interneurons in the cortex. The tendency to facilitate or depress is shaped by basal Pr, where high Pr results in greater depletion of immediately releasable vesicles leading to depression. Although overall this agrees with the observation that Is synapses have higher Pr and depress, it was found that despite overlapping distributions of single-synapse Pr, overall release tended to depress in Is and facilitate in Ib, suggesting additional differences in regulation. This greater complexity comes into stark relief when the behavior of hundreds of individual synapses is examined during high-frequency trains of presynaptic firing. This study found that the heterogeneity of short-term plasticity within an input is much greater than was previously appreciated. Local plasticity dynamics were catagorized by how transmission changed during a stimulus train, and it was found that the inputs differ only in two categories, Is inputs having a larger number of active sites that depress and Ib having a larger number of silent sites that are recruited to a releasing state. These observations reveal a previously unknown level of specialization in basal release and plasticity between neighboring synapses of a common input (Newman, 2017).

    The Drosophila larval NMJ has proven to be a powerful system for studying synaptic homeostasis. The mechanism of this homeostasis is that reduction in quantal size (because of reduced GluR conductance that results from either mutation of a GluR subunit or partial pharmacological block) triggers a retrograde signal that leads to increased transmitter release, restoring the normal the normal level of excitatory drive (Davis, 2015). The current results reveal a new aspect to homeostatic plasticity by showing that it is specific to input and acts primarily at synapses with certain properties of basal transmission, short-term plasticity and physiological function. Only glutamate release from the Ib input is boosted during homeostatic compensation. This is attributed to the fact that despite the smaller quantal size and unitary Ca2+ influx at Ib synapses, Ib inputs have longer bouts of activity, resulting in much larger aggregate Ca2+ elevation in the Ib postsynapse during locomotion. The larger Ca2+ influx drives a higher activation of CaMKII in the Ib postsynapse. Critically, mutation of the GluR subunit to reduce quantal size at both Ib and Is synapses exclusively reduces activated CaMKII at the Ib postsynapse. Why there is no reduction in activated CaMKII at the Is postsynapse is not clear, though nonlinearity in the relationship between the Ca2+ concentration profile and CaMKII activation (Stratton, 2013) and differences in GluR subunit composition that influence Ca2+ influx may contribute (Newman, 2017).

    A second mechanistic difference ensures exclusivity for homeostatic signaling to the Ib input: even when CaMKII activity is inhibited throughout the muscle, only Ib presynapses are boosted in Pr. This could mean that Is presynapses are not responsive to the homeostatic signal. Alternatively, it could mean that Is postsynapses are incapable of generating the homeostatic signal, an idea that would be consistent with the finding that elimination of release from Is axons does not induce homeostatic compensation at Ib synapses. However, this would also require that the retrograde homeostatic signal act very locally so that it could not travel even a few microns from signaling-capable Ib postsynapses to Is boutons that are often located very close by. Another possibility is that basal Pr is high at Is synapses partly due to lower levels of activity in vivo resulting in greater homeostatic enhancement in presynaptic release, which in turn could occlude further increases in synaptic reliability following changes in GluR composition or global postsynaptic CaMKII inhibition (Newman, 2017).

    Evidence for signaling compartmentalization can be seen in the structural differences between Ib and Is NMJs, particularly on the postsynaptic side. Ib axons are surrounded by a significantly thicker and more elaborate SSR. There are also substantial differences in the localization of key organizing postsynaptic proteins such as Dlg which is present at higher levels in Ib postsynapses. CaMKII activity has also been shown to affect SSR structure and Dlg localization. Thus, the different activity patterns during native behaviors may also directly regulate postsynaptic properties other than presynaptic release to maintain functional diversity (Newman, 2017).

    The presynaptic changes that mediate homeostatic compensation in Pr are only partly known. Homeostasis has been shown to be accompanied by an increase in presynaptic AP-evoked Ca2+ elevation in Ib boutons. Although a number of molecules have been demonstrated to play a role in mediating presynaptic homeostatic compensation (Davis 2015), it remains to be determined how these mechanisms interact with input-specific differences in strength and plasticity (Newman, 2017).

    Although homeostasis has been generally thought to be a global mechanism to regulate synaptic strength, evidence has emerged that homeostasis can be regulated with variable degrees of specificity. This study has now demonstrated that homeostatic changes can also propagate retrogradely to the presynaptic neuron in an input-specific and synapse-autonomous manner. Given the divergent innervation of multiple muscles by Is inputs and unique innervation by Ib inputs, a Ib-specific homeostatic mechanism can provide greater functional flexibility (Newman, 2017).

    By combining in vivo, physiological measurements of glutamate release with high-resolution quantal analysis at single synapses in the semi-dissected preparation, this study has clarified the relative properties of convergent glutamatergic inputs in the larva during native behaviors. Importantly, this study has demonstrated that the regulation of basal synaptic strength, short-term plasticity, and homeostasis are shaped in a precise input-specific manner. The Ib input has higher levels of in vivo activity, lower Pr synapses, and a propensity for facilitation by recruiting silent synapses during spike trains. Furthermore, the Ib input is the primary determinant for contraction dynamics in the behaving animal. Consistent with its dominant role, when postsynaptic sensitivity to glutamate is altered at both inputs, there is a selective homeostatic adjustment in the amount of neurotransmitter released from Ib input. CaMKII, localized to the postsynaptic density, is ideally placed to detect local changes in postsynaptic activity at both inputs, and postsynaptic inhibition of CaMKII activity is sufficient to enhance release at the Ib input, demonstrating a high degree of signaling compartmentalization within a single muscle cell. Together, these results demonstrate how synaptic activity at the Drosophila larval NMJ is precisely regulated to ensure both functional diversity and stability (Newman, 2017).

    The long 3'UTR mRNA of CaMKII is essential for translation-dependent plasticity of spontaneous release in Drosophila melanogaster

    A null mutation of the Drosophila calcium/calmodulin-dependent protein kinase II gene (CaMKII) was generated using homologous recombination. Null animals survive to larval and pupal stages due to a large maternal contribution of CaMKII mRNA, which consists of a short 3'-UTR form lacking regulatory elements that guide local translation. The selective loss of the long 3'UTR mRNA in CaMKII null larvae allows testing its role in plasticity. Development and evoked function of the larval neuromuscular junction are surprisingly normal, but the resting rate of miniature excitatory junctional potentials (mEJPs) is significantly lower in CaMKII mutants. Mutants also lack the ability to increase mEJP rate in response to spaced depolarization, a type of activity-dependent plasticity shown to require both transcription and translation. Consistent with this, overexpression of miR-289 in wild-type animals blocks plasticity of spontaneous release. In addition to the defects in regulation of mEJP rate, CaMKII protein is largely lost from synapses in the mutant. All phenotypes are non-sex-specific and rescued by a fosmid containing the entire wild-type CaMKII locus, but only viability and CaMKII localization are rescued by genomic fosmids lacking the long 3'UTR. This suggests that synaptic CaMKII accumulates by two distinct mechanisms: local synthesis requiring the long 3'UTR form of CaMKII mRNA and a process which requires zygotic transcription of CaMKII mRNA. The origin of synaptic CaMKII also dictates its functionality. Locally translated CaMKII has a privileged role in regulation of spontaneous release which cannot be fulfilled by synaptic CaMKII from the other pool (Kuklin, 2017).

    CaMKII is both ubiquitous and abundant. In mammals, CaMKII constitutes around 1% of 96 total brain protein and it is also highly expressed in fly heads. Unsurprisingly, CaMKII has been shown to have a plethora of important functions in the nervous system including roles in multiple stages and forms of learning and memory. At the Drosophila neuromuscular junction (NMJ) the roles of CaMKII encompass both development of the synapse and its activity-dependent plasticity. To date, these functions have been revealed using transgenes encoding CaMK II inhibitors or RNAi to decrease kinase activity or activated forms of the kinase to increase activity (Kuklin, 2017).

    Early studies at the larval NMJ showed that global inhibition of CaMKII with a heat shock-inducible inhibitor peptide transgene (hs-ala lines) increased branching and bouton number. This same manipulation also increased the amplitude of evoked currents and blocked paired pulse facilitation. Global inhibition of CaMKII was also associatedwith increased presynaptic excitability while expression of constitutively active CaMKII in motor neurons suppressed excitability. Subsequent studies looking at postsynaptic inhibition of CaMKII with both ala peptide and another inhibitor (CaMKIINtide) showed that muscle CaMKII activity could stimulate a retrograde signaling pathway which increased quantal content without changes in mEJP amplitude. The larger quantal content correlated with an increase in morphologically identified release sites and, for high levels of CaMKIINtide expression, an increase in mini frequency (Kuklin, 2017).

    More recently, presynaptic expression of either ala peptide or CaMKII RNAi was shown to block activity-dependent bouton sprouting. The abundance of roles is consistent with the presence of the kinase at high levels on both sides of the NMJ, and in multiple cellular compartments (Kuklin, 2017).

    The use of genetics to investigate the role of signal transduction molecules in neuronal function has been standard practice for many years in Drosophila. Although Drosophila CaMKII was cloned over 20 years ago, the location of the gene on heterochromatin-rich chromosome complicated standard mutational approaches. To begin genetic analysis of CaMKII, a null mutation was generated in the CaMKII gene by homologous recombination, inserting two stop codons into the N-terminal coding sequence. This study shows that this mutation is comp letely lethal before adulthood in the homozygous state (Kuklin, 2017).

    Homozygous mutant animals survive into late larval and pupal stages, due to a large amount of maternally contributed CaMKII mRNA which has a short 3'UTR lacking regulatory information, including binding site for miR-289 which are present in the long form. The fact that null animals survive to pupate, and show essentially normal morphological development of the neuromuscular junction (NMJ), implies that maternally-derived short 3'UTR CaMKII is able to support the majority of basic processes. Indeed transgenic expression of the short 3'UTR form can partially rescue viability indicating that lack of CaMKII protein is the cause of lethality rather than some critical role of the long 3'UTR form of the mRNA. Third instar null animals, however, lack the synaptic enrichment of CaMKII seen in wild-type animals. They also show very specific defects in miniature excitatory junctional potentials (mEJPs) and do not exhibit transcription/translation-dependent plasticity of mEJP frequency. CaMKII derived from newly transcribed mRNA can rescue synaptic localization and viability independent of the 3'UTR, but plasticity of mEJPs requires the long 3'UTR mRNA. Consistent with this, suppression of CaMKII translation in wild-type animals by overexpression of miR-289 also blocks mEJP plasticity. These results argue that synaptic localization of CaMKII can occur via multiple mechanisms, and that locally translated kinase has a special role in plasticity (Kuklin, 2017).

    Maternal CaMKII mRNA allows initiation of normal development in null larvae but cannot support metamorphosis. Given the numerous and important functions of CaMKII, it is not unexpected that loss of CaMKII is lethal by adulthood. Surprisingly, however, these mutants appear to be fairly normal with respect to the structure and function of the nervous system in larval stages. This is likely because Drosophila embryos receive large amounts of mRNA from maternally-derived support cells in the ovary. Because the maternal genotype is CaMKII w+/+, this means that even genetically null oocytes will contain mRNA encoding CaMKII. This mRNA is able to provide normal initial levels of the protein, and accordingly there is no lethality during embryonic development. By late larval stages the amount of CaMKII falls, reflecting either degradation or dilution of maternally encoded kinase. By the time animals reach third instar, functional problems can beseen at the NMJ and there is significant lethality. The severely reduced CaMKII level in pupal stages blocks the ability to complete metamorphosis. Production of new CaMKII mRNA is clearly necessary for continued viability as the animal enters this stage (Kuklin, 2017).

    One major difference between the maternal and zygotic mRNAs is that the maternal message has a truncated 3'UTR and lacks sequences known to confer post-transcriptional regulation. Interestingly, viability does not seem to require the long 3'UTR . Animals containing either a rescue fosmid lacking long UTR sequences or expressing a neuronal transgene with a truncated UTR are able to reach adulthood and reproduce. This suggests that producing new protein is sufficient for survival through metamorphosis and that the long UTR form has a specialized role. This underscores the need for future studies to utilize cell-specific and temporally-controlled genetic manipulations of kinase protein and mRNA structure (Kuklin, 2017).

    The CaMKII null NMJ phenotype differs from that of animals expressing CaMKII inhibitors. The ability to obtain CaMKII null third instar larvae allowed characterization both the structure of the NMJ and its function in the absence of zygotic transcription. What was immediately obvious was that the phenotypes of the null animals did not resemble the phenotypes reported for animals expressing CaMK II inhibitors or RNAi, manipulations that should affect CaMKII activity levels regardless of the mRNA template. Null animals had no obvious morphological defects or changes in excitability or evoked release, only a decrease in the rate of spontaneous release. In contrast, in animals with strong postsynaptic inhibition of CaMKII an increase in mini rate was reported, likely due to an increase in the number of presynaptic release sites. In the CaMKII null, the number of release sites, as assessed by staining for Brp, is unchanged indicating that the decrease in minis is due to an alteration in release probability (Kuklin, 2017).

    These qualitatively distinct phenotypes suggest that inhibition of CaMKII enzymatic activity is not the same as loss of zygotic transcription on a background of maternally-provided kinase. On the face of it, these differences are surprising since both types of manipulation, transgenic inhibition of CaMKII and loss of new transcription of the gene, should produce animals with reduced CaMKII enzyme activity. What might account for these differences? One possibility is that the absolute levels of CaMKII activity might be different. This would imply that different magnitudes of activity loss have qualitatively distinct effects. This possibility seems unlikely, however, given previous findings with the heat shock driven ala lines where different levels of peptide inhibitor were tested: high and low levels of ala expression did not differ qualitatively, only in severity. A second possibility is that the time window in which CaMKII activity is lost is the key difference between the two manipulations. Expression of inhibitors using GAL4 lines that turn on early in development could reduce activity earlier than slow depletion of maternal mRNA does. This would imply that early loss of CaMKII activity has qualitatively different effects than later loss. A third possibility is that RNAi and inhibitor peptides have off-target effects, perhaps on CaMKI, a poorly studied enzyme in the fly. A fourth possibility, and one that is favored, is that inhibition and mutation might be different because they disrupt distinct pools of CaMKII which are specified by both transcriptional and translational mechanisms (Kuklin, 2017).

    How is CaMKII from newly transcribed mRNA distinct from that encoded by maternal mRNA? Maternal CaMKII mRNA differs in two ways from zygotic mRNA. First, it differs in structure. Drosophila CaMKII has multiple polyadenylation sites and can have either a short or long 3'UTR. Based on publicly available RNA seq data sets and 3' RACE PCR, mRNA from 0-2 h embryos (which reflects maternal contribution) contains exclusively the short 3'UTR. The RNA seq data also suggest it originates from a distinct transcription start site and differs in its 3'UTR. The second difference is that zygotic mRNA has a different history. Maternal mRNA is synthesized in the nuclei of ovarian nurse cells and never sees the inside of a neuronal nucleus. For CaMK II and other mRNAs the association with mRNA transport machinery occurs in the nucleus. Newly transcribed nuclear mRNAs therefore have preferential access to the machinery that mediates RNA localization. This machinery can be cell type-specific and change over development, meaning that maternal mRNA, even if it has the correct regulatory sequences, may not be competent to localize correctly. Thus while maternal and zygotic CaMKII mRNAs encode the same protein, they do not contain the same regulatory information and may not have the same access to localization or processing factors (Kuklin, 2017).

    How do these two differences influence neuronal structure and function? CaMKII null mutants have two obvious defects: a decrease in basal and stimulated mEJP rate and a lack of synaptically localized CaMKII. These two deficits appear to be mechanistically distinct. Rescue of synaptic localization was seen with both the WT gene and a fosmid lacking long UTR sequences. Localization is therefore 3'UTR -independent but appears to require newly transcribed mRNA. Whether this is due to an mRNA-based mechanism (transport of mRNA to synaptic sites and local translation), or whether it is due to preferential transport or diffusion of protein synthesized in the soma from new mRNA templates, will require further investigation (Kuklin, 2017).

    In contrast to synaptic CaMKII localization, the presence of the long 3'UTR is absolutely required for establishing a normal basal level of spontaneous release, and for activity-dependent increases in mEJP rate. This plasticity is translation-dependent and is suppressed by miR-289 which has been previously shown to regulate activity-dependent presynaptic synthesis of CaMKII at the NMJr. The partial suppression seen with pre synaptic miR-289 could be due to relative expression levels of the mRNA and miR or to a requirement for other regulators. The postsynaptic suppression of plasticity points to involvement of CaMKII- dependent retrograde signaling in spontaneous release. Taken together, however, these data imply that there is a population of synaptic long 3'UTR CaMKII mRNA that is locally translated and acts to increase the probability of release. Why newly translated CaMKII is required is unknown, but in rodent neurons, synaptically synthesized CaMKII has preferential access to certain binding partners. These results also revealed differences be tween the NMJ and adult olfactory system synapses. In adult projection neurons, the 3'UTR was required for both localization and activity- dependent regulation of CaMKII translation in dendrites. Further investigation of the mechanisms of RNA and protein localization will be required to resolve these differences, but it is likely that there will be multiple mechanisms for regulation of synaptic CaMKII levels (Kuklin, 2017).

    Activity-dependent synthesis of CaMKII is clearly a critical feature of the enzyme and is conserved across species and developmental stages. Previous work in the adult fly brain has shown that CaMKII mRNA contains sequences that regulate activity-dependent translation. Importantly, this conserved in mammals where 3'UTR sequences in the CAMK2A gene have been shown to drive localization and activity-dependent translation, though it has been suggested that there may also be a role for 3'UTR sequences. The fly will provide a powerful model system for understanding how and why CaMKII is targeted to multiple subcellular compartments. The discovery that local translation of CaMKII is a key driver of plasticity of mini rate also provides a foothold for obtaining an understanding of this process. Spontaneous release is increasingly being recognized as mechanistically and functionally distinct from evoked release (Kuklin, 2017).

    The regulation of spontaneous release, and even the sites at which it occurs, are separate from action potential evoked activity. These miniature events can regulate nuclear gene expression, local translation and participate in developmental processes such as circuit wiring. The dependence of activity-dependent plasticity of spontaneous release on local translation of CaMKII on both sides of the synapse suggests that there are complex mechanisms for fine tuning this important type of synaptic activity (Kuklin, 2017).

    Presynaptic DLG regulates synaptic function through the localization of voltage-activated Ca(2+) channels

    The DLG-MAGUK subfamily of proteins plays a role on the recycling and clustering of glutamate receptors (GLUR) at the postsynaptic density. discs-large1 (dlg) is the only DLG-MAGUK gene in Drosophila and originates two main products, DLGA and DLGS97 which differ by the presence of an L27 domain. Combining electrophysiology, immunostaining and genetic manipulation at the pre and postsynaptic compartments, this study examined the DLG contribution to the basal synaptic-function at the Drosophila larval neuromuscular junction. The results reveal a specific function of DLGS97 in the regulation of the size of GLUR fields and their subunit composition. Strikingly the absence of any of DLG proteins at the presynaptic terminal disrupts the clustering and localization of the calcium channel DmCa1A subunit (Cacophony), decreases the action potential-evoked release probability and alters short-term plasticity. These results show for the first time a crucial role of DLG proteins in the presynaptic function in vivo (Astorga, 2016).

    dlg1 is the only gene of the DLG-MAGUK subfamily in Drosophila. Similar to vertebrate genes, two forms of the gene are expressed as the result of two transcription start sites. DLGA (α form) and DLGS97 (β form) are distinguished by the inclusion of an L27 domain in beta forms located in the amino terminus of the protein. In vertebrates DLG4/PSD95 is predominantly expressed as α form while DLG1/SAP97 is mainly expressed as β form. DLGA is expressed in epithelial tissues, somatic muscle and neurons; in turn, DLGS97 is not expressed in the epithelial tissue. In the larval neuromuscular junction (NMJ), a glutamatergic synapse, both dlg products are expressed pre and postsynaptically. Hypomorphic dlg alleles display underdeveloped subsynaptic reticulum, bigger glutamate receptors fields and an increased size of synaptic boutons, active zones and vesicles. Additionally to these morphological defects, altered synaptic responses such as increased excitatory junction currents (EJC) and increased amplitude of miniature excitatory junction potentials have been observed. The strong morphological defects make difficult to distinguish developmental defects from the role of DLGs in the basal function of the mature synapse. Previously studies have reported form-specific null mutant strains for DLGA (dlgA40.2) and DLGS97, (dlgS975). These mutants do not show the gross morphological defects observed in hypomorphic mutants, although still show functional synaptic defects, supporting a role of DLG proteins in the mature synaptic function (Astorga, 2016).

    Combining genetic, electrophysiology and immunostaining techniques this study dissected the role of DLG proteins at the pre and postsynaptic compartments. The results show the specific requirement of postsynaptic DLGS97 for normal glutamate receptor (GLUR) distribution. In turn, both DLG proteins increase the release probability by promoting voltage-dependent Ca2+ channel localization. The results demonstrate for the first time a relevant role to DLG proteins in the presynaptic function contributing to Ca2+ mediated short-term plasticity and probability of release (Astorga, 2016).

    Flies carrying the severe hypomorph dlg1 mutant allele, dlgXI-2 and the isoform specific dlgS97 null mutant displayed increased amplitude of the spontaneous excitatory postsynaptic (junctional) potential (mEJP) without changes in frequency. In addition all mutants displayed a decreased quantal content as measured by evoked post-synaptic potentials. The specific defects behind these results were explored. To characterize the synaptic transmission in WT and dlg mutants, post synaptic currents were recorded in HL3.1 solution on muscles 6 or 7 of third instar male larvae of the various genotypes. Recordings of spontaneous excitatory postsynaptic currents (mEJC) were obtained after blocking the voltage activated sodium channels. Thereafter, histogram distributions were constructed with the amplitudes of the miniature events and the quantal size was estimated by the peak value obtained adjusting a Log-Normal distribution in each genotype. It is worth to emphasize that finding a phenotype on dlgA or dlgS97 mutants means that DLGA or DLGS97 proteins by themselves cannot replace DLG function (Astorga, 2016).

    The average amplitude of spontaneous postsynaptic potentials were compared and, supporting previous results, it was found that the average amplitude of the mEJC of the mutants dlgXI-2 (0.99 ± 0.05 nA) and dlgS97 (0.98 ± 0.03 nA) were significantly larger compared to the average amplitudes of the mEJC of Canton-S strain used as WT control (0.81 ± 0.04 nA) and of dlgA (0.78 ± 0.02 nA) specific mutant. The same result was obtained comparing the quantal size. None of the mutants showed a significant change compared to the WT in the frequency of the mEJC. As an additional control, all mutants were recorded over a deficiency covering the dlg gene, finding similar results. These findings are in accordance with the idea that DLGS97 protein and not DLGA is necessary for the quantal size determination (Astorga, 2016).

    Bigger quantal size could be of pre or postsynaptic origin as the result of increased neurotransmitter (NT) content in vesicles or increased glutamate receptor field's size respectively. First, to determine the pre or post-synaptic origin of this phenotype, a UAS-dsRNA construct that targets all dlg products, was expressed under the control of the motoneuron promoter OK6-GAL4 or the muscle promoter C57-GAL4. As expected for a post-synaptic defect, the increased quantal size observed in dlgS97 mutants was mimicked only by the decrease of DLG in the muscle. The specific role of DLGS97 in the muscle is supported by the rescue of the dlgS97 mutant phenotype only by the expression of DLGS97 in the muscle and not in the motor neuron. The effect of GAL4 expression was examined in the mutant background in all experiments; neither of the GAL4 lines without the specific UAS constructs changed the phenotype of the mutants. Again, none of the genotypes studied displayed differences with the WT in the frequency of the minis (Astorga, 2016).

    Changes in quantal size of postsynaptic origin could be due to higher number of post-synaptic receptors and/or a different composition of the postsynaptic receptors. An increase in the size of glutamate receptors fields has been described in dlg hypomorphic alleles including dlgXI-2 mutants. Therefore, the size of the glutamate receptor fields was compared among the mutants and with WT, and also the active zones were measured using antibodies for the active zone protein Bruchpilot. Consistently with previous results bigger glutamate receptors fields were found compared to WT only in dlgXI-2 and dlgS97 mutants but not in dlgA mutants. Surprisingly, an increased number of active zones per bouton was also found in all mutants, a phenotype usually associated with an increase in the frequency of minis that were not observe. In addition, an increased active zone area was found in dlgA and dlgXI-2 mutants (Astorga, 2016).

    As expected for a postsynaptic defect, the bigger size of the glutamate fields in dlgS97 mutants was rescued by the expression of DLGS97 in the muscle but not by its expression in the motor neuron. These results confirm that DLGS97, but not DLGA is responsible for the regulation of the size of the receptors fields in the muscle (Astorga, 2016).

    The strict requirement of DLGS97 in the regulation of the size of GLUR fields supports results that have involved other DLGS97 interacting proteins in the regulation of the size of the glutamate receptors fields. METRO, an MPP-like MAGUK protein, has been shown to form a complex with DLGS97 and LIN-7 through the L27 domains present in each of the three proteins. metro mutants display decreased DLGS97 at the synapse and larger GLUR receptors fields than WT, even bigger than dlgS97 mutants. METRO and DLGS97 depend on each other for their stability on the synapse, thus, in dlgS97 mutants, METRO and dLIN-7 are highly reduced at the synapse. The similar post-synaptic phenotype of metro and dlgS97 and the reported interaction between these two proteins suggests the proposal that the increase size of GLUR fields is consequence of the loss of METRO due to the loss of DLGS97 protein (Astorga, 2016).

    As stated before, changes in quantal size of postsynaptic origin can also reflect a different composition of the receptors. Drosophila NMJ GLUR receptors are tetramers composed by obligatory subunits and two alternative subunits, GLURIIA and GLURIIB. Receptors composed by one of these two subunits differ in their kinetics; GLURIIB receptors desensitizes faster than GLURIIA receptors. Thus, the kinetic of the spontaneous currents (mEJCs), is associated to the relative abundance of these two types of receptors in the GLUR fields. It has been shown that the abundance of GLURIIB but not of GLURIIA in the synapse is associated with the expression of dlg. To analyze if dlg mutants display a change in the composition of the subunits abundance relative to the control, the kinetics of the mEJCs were studied. Kinetics analyses of the mEJCs revealed that only dlgS97 and the double mutant display a slower kinetic in the off response, which is compatible with a different composition of the glutamate receptors fields regarding the proportion between GLURIIA and B receptors. The value of tau also increased in larvae expressing dsRNA-dlg in the muscle, but not by its expression in the motor neuron. Finally tau-off values recovered the WT value only with the expression of DLGS97 in the muscle. As slower mEJCs were observed, the results suggest an increase in the ratio of GLURIIA/GLURIIB. It is known that receptors containing the GLURIIA subunit display bigger conductance and slower inactivation kinetics than receptors containing the GLURIIB subunit. Thus, synapses with post-synaptic receptors fields containing proportionally less GLURIIB subunits would display bigger and slower mEJCs similar to the phenotype observed in dlgS97 mutants. To confirm this hypothesis, the abundance of GLURIIA and GLURIIB receptors was evaluated by immunofluorescence in the NMJ of WT and dlg mutant larvae. The immunofluorescence that allowed the detection and quantification of GLURIII and GLURIIB fields was performed with paraformaldehyde (PFA) fixative. However, the immunofluorescence to detect GLURIIA receptors only works fixating the tissue with Bouin reagent. Thus, in order to be able to compare between these two fixations, the size of the GLUR fields was normalized by the HRP staining that labels the whole presynaptic bouton. First, as a control, GLURIIA and GLURIII were double stained in the same larvae. The results show that using PFA fixative, GLURIII fields display bigger size only in dlgS97 mutants and not in dlgA mutants. Even more, as predicted from the kinetic data, only dlgS97 and not dlgA mutants display bigger GLURIIA fields while there are not difference in the size of GLURIIB fields between WT and the mutants. Additionally the results show no difference in the number of GLURIIA or GLURIIB clusters between WT and dlg mutants. Immunohistochemical results confirm the prediction from the electrophysiological data revealing that in dlgS97 mutants, GLURIIA subunits are proportionally more abundant in GLUR fields than in control larvae. In conclusion, the results show that dlgS97 mutants display larger quanta and mEJCs with slower kinetic establishing its participation in the regulation of the size of GLUR fields where the increased size is obtained mainly through the recruitment of receptors containing GLURIIA subunits. As a similar result was obtained in another study that observed that the loss of GLURIIB receptors in the NMJ of dlgXI-2 mutant embryos, these observation suggest that either of the two DLG proteins are necessary for the localization of GLURIIB in the synapse but only DLGS97 is actively limiting the size of the clusters by regulating the number of GLURIIA receptors (Astorga, 2016).

    Taking into account previous reports that show the regulation of the synaptic localization of DLG by CAMKII, the regulation of the subunit composition by CAMKII and these results, a mechanism is proposed by which, after a strong activation of CAMKII, the phosphorylation of DLGS97 would detach it from the synapse allowing the increase of the size of the GLUR fields by the recruitment of GLURIIA over GLURIIB. These changes should increase the synaptic response by two different mechanisms (Astorga, 2016).

    To determine if DLG proteins modulate the presynaptic release probability, excitatory junction currents (EJC) were recorded in the muscle by stimulating the nerve at 0.5 Hz in low extracellular Ca2+ (0.2 mM), both conditions to avoid synaptic depression. For all mutant genotypes the average peak amplitude and quantal content (EJC amplitude/quantal size) of the evoked responses were significantly smaller than WT. In congruence with previous results, the lower amplitude of the current response is accompanied by a decrease in the quantal content. Taking into account the results on the size of the GLUR fields in the mutant's muscles, these results are compatible with a reduction of the neurotransmitter release in dlg mutants. A decreased neurotransmitter release could be associated with a decreased number of release sites in the boutons. However, the number of active zones per bouton is increased in all dlg mutants with bigger active zones in dlgXI-2 and dlgA mutants (Astorga, 2016).

    The decrease in the evoked response could be a consequence of the absence of the specific form of DLG in the postsynaptic side, transmitted by unknown mechanisms or, alternatively, it could be the result of an effect of DLG on the probability of release. In order to explore where this phenotype originates (pre or post-synaptically) DLG levels were downregulated by expressing dsRNA against all forms of dlg. Compatible with a presynaptic defect, the expression of UAS-dsRNA-dlg presynaptically decreases the amplitude of the evoked response while the same construct expressed postsynaptically using C57 promoter did not changed the amplitude of the EJCs. The presynaptic expression of the dsRNA-dlg also mimics the reduction in quantal content of the mutants, displaying a severe reduction in this parameter. On the other hand, the postsynaptic expression of the dsRNA-dlg associates to a moderate but significant decrease on the quantal content, as expected from the effect already reported of the postsynaptic dsRNA-dlg on the quantal size and the lack of effect on the amplitude of the EJCs. The presynaptic effect of DLG is supported further by the rescue experiments. Thus, the amplitude of the evoked response and the quantal content in dlgA40.2 mutant is completely rescued by the selective expression of DLGA in the presynaptic compartment but not by its expression in the postsynaptic compartment. The pre-synaptic expression of DLGS97 improves the synaptic function increasing the average size of the EJCs such that the difference between the WT and the presynaptic-rescue is not significant, suggesting a complete rescue. However, the average EJC in the presynaptic rescue is not different either from the control mutant animal, which is interpreted as the rescue not being complete and thus the term partial rescue is used. DLGS97 does not, however rescued at al the quantal content. This is explained because although the amplitude of the current increased, the quantal size remains unchanged by the presynaptic expression of DLGS97. In consequence the quantal content does not increase as much as the current. On the other hand, the postsynaptic expression did not increase the amplitude of the evoked current. However, since it does rescue the quantal size the quantal content augmented enough to be different from the mutant control. Notably, DLGA expressed presynaptically in dlgA40.2mutants not only rescued the EJC amplitude but also the number of active zones per bouton and the size of the active zones. On the other hand DLGS97 expression only partially rescued the increased number of active zones in dlgS975 mutants. These results support a role of DLG proteins in the presynaptic function where DLGA seems to regulate more aspects than DLGS97. Despite the fact that both forms of DLG share most of their protein domains, neither of the two-forms is able to fully rescue the absence of the other, suggesting that both of them participate in a complex. The binding between the SH3 and GUK domains of MAGUK proteins has been described; this interaction (at least in vitro) is able to form intra or intermolecular associations and offers a mechanism by which DLGA and DLGS97 proteins could be associated to recruit proteins to a complex (Astorga, 2016).

    Changes in the overall quantal content at these synapses may reveal presynaptic defects. However, genetic background and other independent modification could alter apparent release. To independently scrutinize alteration in the presynaptic release probability two presynaptic properties were examined, the short-term plasticity and the calcium dependency of quantal release (Astorga, 2016).

    To explore the EJC phenotype observed in dlg mutants, stimulation paradigms were carried out that allow characterization of aspects of the short term plasticity that are known to depend on presynaptic functionality and give clues about the mechanisms involved in the observed defects. First, the response were studied of the mutants to high frequency stimulation, 150 stimuli at 20 Hz. WT responses at high frequency stimulation show a fast increase in the amplitude of the response that then slows down. The fast initial increase is called facilitation and the second phase with smaller slope is called augmentation. The time constant of the facilitation is believed to reflex the calcium dynamics in the terminal and its slope to be the product of the accumulation of calcium and the consequent calcium dependent increase in the probability of release. The fractional increment in the mutants' responses showed an increased facilitation in all mutants, while an increased augmentation was only significant in dlgA mutants compared to WT. Additionally all mutants showed a trend toward steeper slopes than WT, but only the augmentation slope in dlgA mutants reached statistical significance. These results support that the mutants display a lower probability of release than WT, which could reflect defects in the calcium dynamics or in the response to calcium (Astorga, 2016).

    Previous work in dlg mutants did not report defects in short-term plasticity. These works differ from the current one in methodological aspects, mainly that they were carried out in a media with high concentration of magnesium (20 mM) and calcium (1.5 mM). This work was carried out in a media containing low magnesium (4 mM) and calcium (0.2 mM) concentration. It is known that magnesium reduces neurotransmitter release, probably due to partial blockade of VGCC. Additionally, magnesium permeates more than sodium and potassium through GLURs (Astorga, 2016).

    To better evaluate the calcium dynamics in the terminal pair pulse (PP) experiments, a well-known paradigm to evaluate presynaptic calcium dynamics, were carried out. In PP, a second depolarization shortly after the first one carried out in low extracellular calcium concentration elicits an increased release of neurotransmitter thought to reflect the increased calcium concentration in the terminal reached after the first stimulus. According to this, and posing as the working hypothesis that DLG affects presynaptic calcium dynamics, a second pulse would be expected to increase the release in a bigger proportion, since the first stimulus did not release much of the ready releasable pool. Conversely, a second stimulus given at high calcium concentration produces a decrease in the release of neurotransmitter, which is considered to originate in the partial depletion of the ready releasable pool at the release sites. Thus, a second pulse at high calcium concentration should elicit a smaller decrease of the release since an inferior entrance of calcium should produce less depletion of the ready releasable pool of vesicles (Astorga, 2016).

    Consistently with a decreased calcium entrance, all mutants displayed increased pair pulse facilitation at low calcium concentration and decreased pair pulse depression at high calcium concentration. These results support a defect in the calcium entrance to the terminal as the underlying defect in dlg mutants causing the evoked stimuli defects. To characterize the calcium dependency of the release in the mutants the evoked responses were measured at different calcium concentrations. It can be observed that for all the mutants and at most calcium concentrations, the quantal content of the evoked response is lower than the control. The only exception is seen at 2 mM calcium where the quantal content of the dlgA mutants and the control are not different to each other. However, even at this calcium concentration the quantal content of dlgS97 and the double mutant dlgXI-2 are significantly lower than the control. To get insight about the release process the responses were fit to a Hill equation. This type of fitting better estimate the maximum responses and the EC50, which is masked in the overall release of different backgrounds. This is observed in the graph with the normalized responses by the maximal quantal predicted. The adjusted curves show that mutants reach the theoretical maximal quantal content at higher calcium concentration than the WT and that the EC50 for the mutants is diminished respect to the WT. To confirm the presynaptic origin of the defect in the calcium dependency, the calcium dependency was carried out in the mutant genotypes expressing DLGA or DLGS97 pre or postsynaptically. The quantal content analysis shows that only the presynaptic expression of DLGA in dlgA mutants completely rescued the calcium dependency, in line with previous results that show the importance of DLGA in the presynaptic compartment. On the other hand the presynaptic expression of DLGS97, although it rescued the calcium dependency, failed to rescue the maximal quantal content. Observing the graph with the normalized responses, DLGA as well as DLGS97 both are able to restore the WT calcium dependency. The inability to rescue the maximal quantal content could be explained by the existence of synaptic compensatory mechanisms that allow to counterweigh the bigger quantal size in dlgS97 mutants, which were shown before not to be rescued by the presynaptic expression of DLGS97 (Astorga, 2016).

    Facilitation is thought to depend on the resultant of the calcium entrance, calcium release from intracellular stores and the clearance of cytosolic calcium. So, the defects in facilitation observed in the mutants could be due to a decreased calcium entrance but also they could be due to a defect on the clearance of calcium. In a preliminary experiment, the relative changes were measured of the total intracellular calcium concentration in the bouton using the genetically encoded calcium indicator GCamp6f. GCamp6f expressed in control flies (OK6-GAL4/UAS-GCamp6f) respond with a fast and transitory change in the cytoplasmic calcium of the boutons when they are exposed to a local pulse of potassium. The same experimental approach in dlgS97 mutant larvae reveals that the rise of the calcium response is significantly slower than the control; additionally the recovery of the response is also significantly slower. These preliminary experiments suggest a defect in calcium entrance in the mutants but they also support a defect in the extrusion that hint to additional defects. Further experiments are needed to clarify the calcium kinetics involved since these experiments were measuring the bulk of calcium change and in doing this approximation, the nanodomain changes that are known to be the ones that regulate the neurotransmitter release are being lost (Astorga, 2016).

    Since the results described above including the calcium dependency of the release as well as the parameters of the short-term plasticity suggest that the calcium entrance to the terminal is impaired, a view that is supported by the preliminary data measuring the cytosolic calcium, the next experiments focused on the calcium entrance. The main calcium entry to the terminal is the voltage gated calcium channel (VGCC) encoded by the Drosophila gene cacophony. Advantage was taken of a UAS-cacophony1-EGFP transgenic fly (CAC-GFP) to study the distribution of the channel in WT and mutant genotypes. CAC-GFP overexpressed in WT background localizes in the synapse in a strictly plasma membrane-associated manner in big clusters closely associated with release sites. However, CAC-GFP overexpressed in dlgS97 or dlgA mutant background displays a significant decrease in the expression accompanied by a more disperse localization with significantly smaller clusters, suggesting that the Cacophony protein might not be properly delivered or anchored to the plasma membrane in dlg mutants (Astorga, 2016).

    It was reasoned that if dlg mutants had a defect on calcium entrance, the over expression of calcium channels should rescue at least partially the phenotype. Advantage was taken of the fact that CAC-GFP construct encodes a functional channel, and recordings were taken from control and dlg mutants overexpressing CAC. As expected and supporting a decreased calcium entrance in the mutants, dlgS97 and dlgA mutants that overexpress CAC-GFP display significantly bigger evoked EJCs compared to dlg mutants, without a change of phenotype in the spontaneous currents. Additionally, the over expression of CAC-GFP partially rescued the pair pulse facilitation and the pair pulse depression as well as the calcium curve (Astorga, 2016).

    The disrupted localization of CAC could result from the disturbance of a normal direct association to DLG or it could be affected indirectly. To test an immunoprecipitation assay was carried out using flies that express CAC-EGFP in all neurons. Antibodies against GFP were able to precipitate DLG together with Cacophony-GFP, supporting that Cacophony channel is part of the DLG complex in the boutons (Astorga, 2016).

    A possible interaction between DLG and voltage-gated calcium channels (VGCCs is) the VGCC auxiliary subunits. The α2δ auxiliary subunit (Straightjacket in Drosophila) increases calcium channel activity and plasma membrane expression of CaV2 α1 subunits and Cacophony. The β auxiliary subunit increases plasma membrane expression of several mammalian VGCC classes. Intriguing β subunits are also MAGUK proteins and they are able to release the VGCC α subunit from the endoplasmic reticulum retention. It may be speculated that DLG through their SH3-GUK domain might be playing the role of the β subunit (Astorga, 2016).

    On the other hand, in mammalian cultured neurons it has been proposed that a complex formed by the scaffold proteins LIN-2/CASK, LIN-10/MINT and LIN-7 is involved in the localization of VGCCs at the synapse and that SAP97 forms a complex with CASK. The association of DLGS97 with LIN7 has been reported in the postsynaptic compartment in the Drosophila NMJ. Furthermore, an association between the L27 domain of DLGS97 and the L27 domain of Drosophila CASK has been shown in vitro, however there are no reports of this type of association with DLGA. Another protein involved in the localization of calcium channels in the active zone is RIM. Drosophila rim has been involved in synaptic homeostasis and the modulation of vesicle pools. Surprisingly rim mutants, display low probability of release and altered responses to different calcium concentrations. Recently it was shown that spinophilin mutants display a phenotype with bigger quantal size and GLUR fields size with a higher proportion of GLURIIA subtype of receptors as well as decreased EJCs and decreased pair pulse facilitation. This is a phenotype very similar to the one described here for dlg mutants. The authors in this report did not explore the calcium channels abundance or distribution and the current study did not explore the link of DLG to Neuroligins, Neurexins and Syd. It would be interesting to determine if there is a link between Spinophilin and DLG (Astorga, 2016).

    Taken together these results show that dlgS97 is the main isoform responsible for the postsynaptic defects in the dlgXI-2 mutants; which comprise the increase in the size of the receptors fields and the change in the ratio of GLURIIA/GLURIIB. The results as well support a model in which DLG forms a presynaptic complex that includes Cacophony where the absence of either form of DLG leads to defects in the localization of the voltage dependent calcium channel and to a decrease in the entrance of calcium to the bouton; which in turn affect the probability of release and the short-term plasticity in the mutants. The results described in this study highlight the specificity of the function of DLGS97 and DLGA proteins and describe for the first time an in vivo presynaptic role of DLG proteins (Astorga, 2016).

    The Ih channel gene promotes synaptic transmission and coordinated movement in Drosophila melanogaster

    Hyperpolarization-activated cyclic nucleotide-gated "HCN" channels, which underlie the hyperpolarization-activated current (Ih), have been proposed to play diverse roles in neurons. The presynaptic HCN channel is thought to both promote and inhibit neurotransmitter release from synapses, depending upon its interactions with other presynaptic ion channels. In larvae of Drosophila melanogaster, inhibition of the presynaptic HCN channel by the drug ZD7288 reduces the enhancement of neurotransmitter release at motor terminals by serotonin but this drug has no effect on basal neurotransmitter release, implying that the channel does not contribute to firing under basal conditions. This study shows that genetic disruption of the sole HCN gene (Ih) reduces the amplitude of the evoked response at the neuromuscular junction (NMJ) of third instar larvae by decreasing the number of released vesicles. The anatomy of the (NMJ) is not notably affected by disruption of the Ih gene. It is proposed that the presynaptic HCN channel is active under basal conditions and promotes neurotransmission at larval motor terminals. Finally, it was demonstrated that Ih partial loss-of-function mutant adult flies have impaired locomotion, and, thus, it is hypothesized that the presynaptic HCN channel at the (NMJ) may contribute to coordinated movement (Hegle, 2017).

    A Ca2+ channel differentially regulates Clathrin-mediated and activity-dependent bulk endocytosis

    Clathrin-mediated endocytosis (CME) and activity-dependent bulk endocytosis (ADBE) are two predominant forms of synaptic vesicle (SV) endocytosis, elicited by moderate and strong stimuli, respectively. They are tightly coupled with exocytosis for sustained neurotransmission. However, the underlying mechanisms are ill defined. Previous work has shown that the Flower (Fwe) Ca2+ channel present in SVs is incorporated into the periactive zone upon SV fusion, where it triggers CME, thus coupling exocytosis to CME. This study shows that Fwe also promotes ADBE. Intriguingly, the effects of Fwe on CME and ADBE depend on the strength of the stimulus. Upon mild stimulation, Fwe controls CME independently of Ca2+ channeling. However, upon strong stimulation, Fwe triggers a Ca2+ influx that initiates ADBE. Moreover, knockout of rodent fwe in cultured rat hippocampal neurons impairs but does not completely abolish CME, similar to the loss of Drosophila fwe at the neuromuscular junction, suggesting that Fwe plays a regulatory role in regulating CME across species. In addition, the function of Fwe in ADBE is conserved at mammalian central synapses. Hence, Fwe exerts different effects in response to different stimulus strengths to control two major modes of endocytosis (Yao, 2017).

    A tight coupling of exocytosis and endocytosis is critical for supporting continuous exocytosis of neurotransmitters. CME and ADBE are well-characterized forms of SV endocytosis triggered by moderate and strong nerve stimuli, respectively. However, how they are coupled with exocytosis under distinct stimulation paradigms remains less explored. A model is proposed based on the present data. When presynaptic terminals are mildly stimulated, SV release leads to neurotransmitter release and the transfer of Fwe channel from SVs to the periactive zone where CME and ADBE occur actively. The data suggest that this channel does not supply Ca2+ for CME to proceed. However, intense activity promotes Fwe to elevate presynaptic Ca2+ levels near endocytic zones where ADBE is subsequently triggered. Thus, Fwe exerts different activities and properties in response to different stimuli to couple exocytosis to different modes of endocytosis (Yao, 2017).

    It has been previously concluded that Fwe-dependent Ca2+ influx triggers CME (Yao, 2009). However, the current results suggest alternative explanations. First, the presynaptic Ca2+ concentrations elicited by moderate activity conditions, i.e., 1-min 90 mM K+/0.5 mM Ca2+ or 20-s 10-20 Hz electric stimulation, are not dependent on Fwe. Second, expression of 4% FweE79Q, a condition that abolishes Ca2+ influx via Fwe, rescues the CME defects associated with fwe mutants, including decreased FM1-43 dye uptake, a reduced number of SVs, and enlarged SVs. Third, raising the presynaptic Ca2+ level has no beneficial impact on the reduced number of SVs observed in fwe mutants. These data are consistent with the observations that a Ca2+ influx dependent on VGCCs triggers CME at a mammalian synapse. Hence, Fwe acts in parallel with or downstream to VGCC-mediated Ca2+ influx during CME (Yao, 2017).

    ADBE is triggered by intracellular Ca2+ elevation, which has been assumed to be driven by VGCCs that are located at the active zones. However, the data strongly support a role for Fwe as an important Ca2+ channel for ADBE. First, following exocytosis, Fwe is enriched at the periactive zone where ADBE predominates. Second, Fwe selectively supplies Ca2+ to the presynaptic compartment during intense activity stimulation, which is highly correlated with the rapid formation of ADBE upon stimulation. Third, 4% FweE79Q expression, which induces very subtle or no Ca2+ upon strong stimulation, fails to rescue the ADBE defect associated with loss of fwe. Fourth, treatment with a low concentration of La3+ solution that specifically blocks the Ca2+ conductance of Fwe significantly abolishes ADBE. Lastly, the role of Fwe-derived Ca2+ influx in the initiation of ADBE mimics the effect of Ca2+ on ADBE at the rat Calyx of Held. As loss of fwe does not completely eliminate ADBE, the results do not exclude the possibility that VGCC may function in parallel with Fwe to promote ADBE following intense stimulation (Yao, 2017).

    Interestingly, Ca2+ influx via Fwe does not control SV exocytosis during mild and intense stimulations. How do VGCC and Fwe selectively regulate SV exocytosis and ADBE, respectively? One potential mechanism is that VGCC triggers a high, transient Ca2+ influx around the active zone that elicits SV exocytosis. In contrast, Fwe is activated at the periactive zone to create a spatially and temporally distinct Ca2+ microdomain. A selective failure to increase the presynaptic Ca2+ level during strong stimulation is evident upon loss of fwe. This pinpoints to an activity-dependent gating of the Fwe channel. Consistent with this finding, an increase in the level of Fwe in the plasma membrane does not lead to presynaptic Ca2+ elevation at the Calyx of Held when the presynaptic terminals are at rest or subject to mild stimulation. However, previous studies showed that, in shits terminals, blocking CME results in the accumulation of the Fwe channel in the plasma membrane, elevating Ca2+ levels. It is possible that Dynamin is also involved in regulating the channel activity of Fwe or that the effects other than Fwe accumulation associated with shits mutants may affect intracellular Ca2+ handling. Further investigation of how neuronal activity gates the channel function of Fwe should advance knowledge on the activity-dependent exo-endo coupling (Yao, 2017).

    Although a proteomic analysis did not identify ratFwe2 in SVs purified from rat brain, biochemical analyses show that ratFwe2 is indeed associated with the membrane of SVs. The data show that 4% of the total endogenous Fwe channels efficiently promotes CME and ADBE at the Drosophila NMJ. If a single SV needs at least one functional Fwe channel complex during exo-endo coupling, and one functional Fwe complex comprises at least four monomers, similar to VGCCs, transient receptor potential cation channel subfamily V members (TRPV) 5 and 6, and calcium release-activated channel (CRAC)/Orai1, then it is anticipated that each SV contains ~100 Fwe proteins (4 monomers x 25). This suggests that Fwe is highly abundant on the SVs. It is unlikely that many SVs do not have the Fwe, as a 25-fold reduction of the protein is enough to ensure functional integrity during repetitive neurotransmission. Finally, the results for the SypHy and dextran uptake assays at mammalian central synapses indicate the functional conservation of the Fwe channel in promoting different modes of SV retrieval. In summary, the Fwe-mediated exo-endo coupling seems to be of broad importance for sustained synaptic transmission across species. (Yao, 2017).

    Neuroligin 4 regulates synaptic growth via the Bone morphogenetic protein (BMP) signaling pathway at the Drosophila neuromuscular junction

    The neuroligin (Nlg) family of neural cell adhesion molecules is thought to be required for synapse formation and development, and has been linked to the development of autism spectrum disorders in humans. In Drosophila melanogaster, mutations in the neuroligin 1-3 genes have been reported to induce synapse developmental defects at neuromuscular junctions (NMJs), but the role of neuroligin 4 (dnlg4) in synapse development has not been determined. This study reports that the Drosophila Neuroligin 4 (DNlg4) is different from DNlg1-3 in that it presynaptically regulates NMJ synapse development. Loss of dnlg4 results in reduced growth of NMJs with fewer synaptic boutons. The morphological defects caused by dnlg4 mutant are associated with a corresponding decrease in synaptic transmission efficacy. All of these defects could only be rescued when DNlg4 was expressed in the presynapse of NMJs. To understand the basis of DNlg4 function, genetic interactions were sought, and connections were found with the components of the bone morphogenetic protein (BMP) signaling pathway. Immunostaining and western blot analyses demonstrated that the regulation of NMJ growth by DNlg4 was due to the positive modulation of BMP signaling by DNlg4. Specifically, BMP type I receptor Tkv abundance was reduced in dnlg4 mutants, and immunoprecipitation assays showed that DNlg4 and Tkv physically interacted in vivo. This study demonstrates that DNlg4 presynaptically regulates neuromuscular synaptic growth via the BMP signaling pathway by modulating Tkv (X. Zhang, 2017).

    The formation, development, and plasticity of synapses are critical for the construction of neural circuits, and the Drosophila larval neuromuscular junction (NMJ) is an ideal model system to dissect these processes. In the past few decades, several subcellular events and signaling pathways have been reported to be involved in regulating synaptic growth at Drosophila NMJs, such as local actin assembly, endocytosis, ubiquitin-mediated protein degradation, the Wingless pathway, and the bone morphogenetic protein (BMP) pathway. Among these, BMP signaling is thought to be a major retrograde pathway that promotes the synaptic growth of NMJs (X. Zhang, 2017).

    At the Drosophila NMJ, the BMP homolog Glass bottom boat (Gbb) is released by muscle cells and binds to the presynaptic type II BMP receptor Wishful thinking (Wit). Wit is a constitutively active serine/threonine kinase and, upon binding to Gbb, forms a complex with the type I BMP receptor Thickvein (Tkv) or saxophone (Sax), which results in their activation by phosphorylation. The activated type I receptor subsequently phosphorylates the downstream R-Smad protein Mothers against decapentaplegic (Mad). Phosphorylated Mad (pMad) then binds to the co-Smad Medea (Med). This complex translocates to the nucleus of motoneurons to activate or repress the transcription of target genes required for NMJ growth. Mutation of any component in the BMP signaling pathway results in a striking deficiency of NMJ growth. In addition, many molecules are reported to affect NMJ growth by negatively regulating BMP signaling at different points in the pathway. This study report that Drosophila Neuroligin 4 (DNlg4), a trans-synaptic adhesion protein, acts as a positive regulator of BMP signaling to regulate NMJ growth (X. Zhang, 2017).

    Neuroligins (Nlgs) were initially reported to be the postsynaptic ligands of the presynaptic adhesion proteins neurexins (Nrxs), and loss of function of Nlgs in humans is thought to be associated with several mental disorders, including autism and schizophrenia. Nlgs are an evolutionarily conserved family of proteins encoded by four independent genes in rodents and five independent genes in humans. Nlgs have been reported to induce synapse assembly by co-cultured neurons when expressed in nonneuronal cells, and overexpression of Nlgs in neurons increases synapse density. These in vitro cell culture studies suggest a role of Nlgs in inducing the formation of synaptic contacts. However, an in vivo study showed, despite severe defects in synaptic transmission, that there was no alteration of synapse number in neurons from nlg1-3 triple knock-out mice. Similarly, loss of Nlg1 specifically in the hippocampus or amygdala did not alter the synapse number, suggesting that the role of Nlgs is not to trigger the initial synapse formation. Rather, it is more likely that upon binding to Nrx, Nlg functions in maturation of nascent synapses, including differentiation and stabilization by recruiting scaffolding proteins, postsynaptic receptors, and signaling proteins (X. Zhang, 2017).

    In Drosophila, four Nlgs have been identified. Mutations in dnlg1-3 result in defective synapse differentiation that is primarily characterized by abnormal protein levels or the ectopic postsynaptic localization of glutamate receptors in larval NMJs. In addition, loss of dnlg1-3 separately leads to impairment in NMJ synapse development, as indicated by abnormal synaptic bouton number, but the precise underlying mechanism is poorly understood. A recent study showed that flies with a dnlg4 mutation exhibit an autism-related phenotype of behavioral inflexibility, as indicated by impaired reversal learning. DNlg4 also regulates sleep by recruiting the GABA receptor to clock neurons and thus modulating GABA transmission, which suggests a role of DNlg4 in synapse differentiation. However, potential molecular mechanisms underlying the behavioral defects caused by dnlg4 mutation and the role of DNlg4 in synapse development have not been reported (X. Zhang, 2017).

    This paper reports the generation of an independent null allele of dnlg4 and characterized the role of DNlg4 in neuromuscular synaptic growth. Loss of DNlg4 led to impaired NMJ synapse growth, as indicated by decreased synaptic bouton numbers and increased bouton size. Presynaptic knockdown of DNlg4 mimicked these phenotypes. These morphological abnormalities in dnlg4 mutants led to corresponding impairment in synaptic transmission efficacy. Unexpectedly, all of these defects were only rescued when DNlg4 was expressed in presynaptic, instead of postsynaptic, areas of NMJs.DNlg4 genetically interacted with components of the BMP pathway and that the presynaptic BMP signaling at NMJs was decreased in dnlg4 mutants. A reduction of the BMP type I receptor Tkv was observed in dnlg4 mutants and DNlg4 physically interacted with Tkv in vivo. Altogether, this study revealed that DNlg4 regulated neuromuscular synaptic growth by positively modulating BMP signaling though Tkv (X. Zhang, 2017).

    Sequence analyses showed that there are four nlg genes in the Drosophila genome, and all four DNlgs share significant amino acid sequence homology and protein structures with vertebrate Nlgs. DNlg1 and DNlg2 have a positive effect on synaptic growth of NMJs, as indicated by an obvious reduction in synaptic boutons in the dnlg1 and dnlg2 mutants. Conversely, loss of DNlg3 led to increased numbers of synaptic boutons at NMJs. In the present study, a dnlg4 null mutant was generated by gene targeting, and this mutant exhibited significant defects in NMJ morphology, including fewer synaptic boutons and increased bouton size. However, neuronal overexpression of two copies of the dnlg4 transgene induced a pronounced increase in bouton number. These results demonstrated a positive role of DNlg4 in regulating synapse development. Interestingly, neuronal overexpression of one copy of dnlg4 in the WT background did not induce an increase in bouton number, but it did when expressed in a dnlg4 mutant background. These results suggested a homeostatic adjustment during synapse development in Drosophila, which could somewhat counteract the effect caused by increased DNlg4. As a result, a moderate increase of DNlg4 in WT flies did not lead to increased synaptic growth of NMJs (X. Zhang, 2017).

    In Drosophila, loss of DNlgs in the dnlg1-3 mutants also induced synaptic differentiation defects that were characterized by decreased protein levels or impaired distribution of glutamate receptors and other postsynaptic proteins at NMJs. In contrast to other dnlg mutants, statistical alteration in the distribution or protein level of glutamate receptors at NMJs in was not observed dnlg4 mutants. In addition, the distribution and protein level of some presynaptic proteins, such as BRP, CSP, and SYT, were normal in the dnlg4 mutants. However, the ultrastructural analyses of the NMJs showed that there were still some defects in synaptic ultrastructure in the dnlg4 mutants, including the increased bouton area per active zone, longer single PSD, and reduced postsynaptic SSR regions. One striking ultrastructural defect in the dnlg4 mutants was the partial detachment of presynaptic membranes from postsynaptic membranes within the active zone, which was rarely observed in WT flies, suggesting the adhesion function of DNlg4 during synaptogenesis. It was interesting that this defect also appeared in dnlg1 mutants and dnrx mutants, suggesting that the DNlg4 might affect the synaptic architecture by a mechanism similar to that of DNlg1 and DNrx (X. Zhang, 2017).

    Functionally, dnlg4 mutants showed a mild impairment in transmitter release at NMJs, as characterized by slightly reduced evoked EJP amplitude and quantal contents. This phenotype was consistent with the morphological impairments and the ultrastructural defects of NMJs, suggesting a probable reduction in the number of total synapses or functional synapses at NMJs. The amplitude of mEJP was not changed in the dnlg4 mutants, which was consistent with the normal protein levels of postsynaptic glutamate receptors. Interestingly, the frequency of mEJPs in the dnlg4 mutants was dramatically increased, indicating that DNlg4 affects spontaneous transmitter release. The detailed mechanism underlying this should be addressed in future work (X. Zhang, 2017).

    In mammals, Nlgs are generally considered to function as postsynaptic adhesion molecules and to help form trans-synaptic complexes with presynaptic Nrxs. In Drosophila, DNlg1 and DNlg3 are reported to be located in postsynaptic membranes. However, there are some exceptions to the postsynaptic localization of Nlgs. For example, an Nlg in Caenorhabditis elegans is reported to be present in both presynaptic and postsynaptic regions. DNlg2 is also required both presynaptically and postsynaptically for regulating neuromuscular synaptic growth. These studies support a more complex mechanism of Nlgs in synapse modulation and function. These data add to this complexity by suggesting a presynaptic role of DNlg4 in larval neuromuscular synaptic growth and synaptic functions (X. Zhang, 2017).

    First, in the VNC of third-instar larvae, DNlg4 was concentrated in the neuropil region where the synapses aggregated. In NMJs, although endogenous DNlg4 was not detected by anti-DNlg4 antibodies, the exogenous DNlg4 that was expressed in motoneurons was located in type I synaptic boutons of NMJs, suggesting a reasonable presynaptic location of DNlg4 at NMJs. In another assay, DNlg4 was expressed using a DNlg4-Gal4 driver to mimic the endogenous expression pattern of the dnlg4 gene. The DNlg4 promoted by DNlg4-Gal4 was distributed in type I boutons of NMJs and was located in presynaptic areas of boutons, which also supported the presynaptic location of endogenous DNlg4 at NMJs, although a simultaneous postsynaptic localization of DNlg4 at NMJs could not be excluded. Second, knockdown of DNlg4 presynaptically led to morphological defects in NMJs similar to those observed in the dnlg4 mutants, indicated by decreased bouton numbers and increased bouton size. However, knockdown of DNlg4 postsynaptically did not cause the same phenotypes. These morphological defects of NMJs in the dnlg4 mutants could be completely rescued when DNlg4 was expressed in presynaptic neurons, but not when it was expressed in postsynaptic muscles. Third, dnlg4 mutants had a significant increase in spontaneous transmitter release frequency, which was usually interpreted as a presynaptic defect. In addition, all of the functional defects in the dnlg4 mutants, including decreased amplitude of EJPs, reduced quantal contents, and increased mEJP frequency, could be rescued by presynaptic expression of DNlg4. These results indicated that presynaptic DNlg4 was essential for proper proliferation and function of synapses at NMJs. Finally, the number of synaptic boutons at NMJs was significantly increased when two copies of UAS-dnlg4 were overexpressed in presynaptic neurons, whereas this phenomenon was not observed when two copies of UAS-dnlg4 were overexpressed in muscles, suggesting that presynaptic DNlg4 alone was sufficient to promote synaptic growth. Altogether, these data provided convincing evidence that DNlg4 functions as a presynaptic molecule in regulating synaptic growth and transmitter release at NMJs (X. Zhang, 2017).

    The BMPs are major retrograde trans-synaptic signals that affect presynaptic growth and neurotransmission both in the CNS and at NMJs. This study presents immunohistochemical and genetic data showing that DNlg4 regulates NMJ growth via the BMP signaling pathway (X. Zhang, 2017).

    First, the dnlg4 mutants shared similar phenotypes with the components of the BMP signaling pathway in synapse development, synapse architecture, and synapse functions of NMJs, including reduced synaptic bouton number, increased presynaptic membrane ruffles, and decreased synaptic transmission efficacy. Second, several genetic crosses followed by neuromuscular bouton number analyses showed a definite dosage-sensitive genetic interaction between dnlg4 and the components of the BMP signaling pathway, such as tkv, wit, mad, and dad, suggesting that DNlg4 is required for BMP signaling in regulating NMJ growth. Third, both immunohistochemical and Western blot analyses showed that the pMad, which serves as an indicator of BMP signaling, is decreased in both the synaptic boutons of NMJs and motoneuronal nuclei in the dnlg4 mutants, whereas it is increased in dnlg4-overexpressing flies. Finally, the expression of the BMP signaling target gene trio, is significantly reduced in the dnlg4 mutants, but it was increased in dnlg4-overexpressing flies. Together, these results supported the hypothesis that DNlg4 promotes synaptic growth by positively regulating BMP signaling (X. Zhang, 2017).

    The critical question to be addressed, therefore, is the mechanism underlying this regulation. Through Western blot analyses of larval brain homogenates, it is found that the protein level of Tkv (as assessed by ectopically expressed Tkv-GFP) in the dnlg4 mutants is significantly decreased, whereas Wit and Mad, the other two components of the BMP pathway, were not altered. The immunohistochemical assay also showed that the Tkv protein in the dnlg4 mutants is reduced in both the VNC and the synaptic boutons of NMJs. These data indicated the specific positive regulation of Tkv by DNlg4. Previous studies reported that tkv mutants had decreased synaptic bouton number, increased presynaptic membrane ruffles, reduced amplitude of EJP, and unchanged mEJP amplitude. In addition, neuronal overexpression of one copy of transgenic tkv did not induce NMJ overgrowth, but it induced such overgrowth when two copies of transgenic tkv were expressed in neurons. These phenotypes were similar to what was observed in the dnlg4 mutants and dnlg4-overexpressing flies. All of these results strongly demonstrated that DNlg4 regulates BMP signaling by modulating Tkv protein levels (X. Zhang, 2017).

    Because the dnlg4 mutants had a level of tkv mRNA comparable with that of the WT controls, DNlg4 did not affect the transcription of tkv. The possibility of Tkv trafficking defects from the cell body to the axon terminal could also be excluded because no retention of Tkv in the soma of motoneurons was observed. Thus, it seems reasonable to speculate that DNlg4 affects the recruitment or stability of Tkvs at the presynapse. In support of this hypothesis, co-localization of Tkv and DNlg4 at presynaptic regions of NMJs was observed by immunostaining. As further confirmation, it was shown that Tkv could be co-immunoprecipitated with an antibody against DNlg4, and a reverse co-immunoprecipitation assay using anti-GFP antibodies also resulted in the precipitation of DNlg4 by Tkv-GFP, suggesting a physical interaction between DNlg4 and Tkv in vivo. Furthermore, using pull-down assay, the acetylcholinesterase-like domain of the N terminus was shown to be essential for DNlg4 interacting with Tkv (X. Zhang, 2017).

    If the recruitment of Tkv to the presynaptic membrane entirely depends on DNlg4, the existence of Tkv in the presynaptic membrane would not be detected, and accumulation of Tkv proteins in the presynaptic areas of NMJs would probably be observed in the dnlg4 mutants. However, minor Tkv protein level was detected at the presynaptic membrane, and no accumulation of Tkv was observed at NMJs in the dnlg4 mutants. Thus, a more reasonable hypothesis is that DNlg4 affected the stability of Tkv at the presynaptic membranes. The protein level of Tkv in the presynapses of NMJs was reported to be regulated by several pathways, including direct proteasomal degradation, ubiquitin-mediated degradation, and endocytosis. The simplest model is that the DNlg4 might stabilize Tkv via inhibiting its degradation. Several protein kinases, such as the ribosomal protein S6 kinase-like protein (S6KL) and the serine/threonine kinase Fused, have been reported to interact physically with Tkv in vitro and to facilitate its proteasomal degradation. In particular, S6KL has been demonstrated to degrade Tkv at NMJs. DNlg4 might stabilize Tkv by inhibiting these protein kinases, but the detailed mechanism of such activity still needs to be addressed (X. Zhang, 2017).

    In summary, this study demonstrated that DNlg4 positively regulated neuromuscular synaptic growth by modulating BMP signaling through the maintenance of Tkv protein levels in the presynapse of NMJs. To accomplish this function, DNlg4 acted as a presynaptic molecule instead of a postsynaptic molecule. This study further suggested a relationship between Nlgs and BMP signaling and provided a new understanding of the exact role of Nlgs during synapse formation and development (X. Zhang, 2017).

    Heterotrimeric Go protein links Wnt-Frizzled signaling with ankyrins to regulate the neuronal microtubule cytoskeleton

    Drosophila neuromuscular junctions (NMJs) represent a powerful model system with which to study glutamatergic synapse formation and remodeling. Several proteins have been implicated in these processes, including components of canonical Wingless (Drosophila Wnt1) signaling and the giant isoforms of the membrane-cytoskeleton linker Ankyrin 2, but possible interconnections and cooperation between these proteins were unknown. This study demonstrates that the heterotrimeric G protein Go functions as a transducer of Wingless-Frizzled 2 signaling in the synapse. Ankyrin 2 was identified as a target of Go signaling required for NMJ formation. Moreover, the Go-ankyrin interaction is conserved in the mammalian neurite outgrowth pathway. Without ankyrins, a major switch in the Go-induced neuronal cytoskeleton program is observed, from microtubule-dependent neurite outgrowth to actin-dependent lamellopodial induction. These findings describe a novel mechanism regulating the microtubule cytoskeleton in the nervous system. This work in Drosophila and mammalian cells suggests that this mechanism might be generally applicable in nervous system development and function (Luchtenborg, 2014).

    Ankyrins (Ank) are highly abundant modular proteins that mediate protein-protein interactions, mainly serving as adaptors for linking the cytoskeleton to the plasma membrane. Mammalian genomes encode three Ank genes [AnkR (Ank1), AnkB (Ank2) and AnkG (Ank3)], whereas Drosophila has two [Ank1 (also known as Ank - FlyBase) and Ank2]. Ank2 is expressed exclusively in neurons and exists in several splicing variants. The larger isoforms (Ank2M, Ank2L and Ank2XL) are localized to axons and play important roles in NMJ formation and function. The C-terminal part of Ank2L can bind to microtubules. Despite the well-established role of Ank2 in NMJ formation, its function has been considered somewhat passive and its mode of regulation has not been clarified. This study shows that Gαo binds to Ank2 and that these proteins and the Wg pathway components Wg, Fz2, and Sgg jointly coordinate the formation of the NMJ. The functional Gαo-Ank interaction is conserved from insects to mammals (Luchtenborg, 2014).

    Synaptic plasticity underlies learning and memory. Both in invertebrates and vertebrates, activation of Wnt signaling is involved in several aspects of synapse formation and remodeling, and defects in this pathway may be causative of synaptic loss and neurodegeneration. Thus, understanding the molecular mechanisms of synaptic Wnt signaling is of fundamental as well as medical importance. The Drosophila NMJ is a powerful model system with which to study glutamatergic synapses, and the Wnt pathway has been widely identified as one of the key regulators of NMJ formation.

    This study provides important mechanistic insights into Wnt signal transduction in the NMJ, identifying the heterotrimeric Go protein as a crucial downstream transducer of the Wg-Fz2 pathway in the presynapse. It was further demonstrated that Ank2, a known player in the NMJ, is a target of Gαo in this signaling (Luchtenborg, 2014).

    This study found that the α subunit of Go is strongly expressed in the presynaptic cell, and that under- or overactivation of this G protein leads to neurotransmission and behavioral defects. At the level of NMJ morphology, presynaptic downregulation or Ptx-mediated inactivation of Gαo was found to recapitulate the phenotypes obtained by similar silencing of wg and fz2. These data confirm that presynaptic Wg signaling, in addition to the Wg pathway active in the muscle, is crucial for proper NMJ formation, and that Go is required for this process. Furthermore, neuronal Gαo overexpression can rescue the wg and fz2 loss-of-function phenotypes, demonstrating that, as in other contexts of Wnt/Fz signaling, Go acts as a transducer of Wg/Fz2 in NMJ formation. In contrast to its evident function and clear localization in the presynapse, Gαo localization on the muscle side of the synapse is much less pronounced or absent. Unlike Gαo, the main Drosophila Gβ subunit is strongly expressed in both the pre- and postsynapse. Thus, a heterotrimeric G protein other than Go might be involved in the postsynaptic Fz2 transduction, as has been implicated in Fz signaling in some other contexts (Luchtenborg, 2014).

    A recent study proposed a role for Gαo downstream of the octopamine receptor Octβ1R. This signaling was proposed to regulate the acute behavioral response to starvation both on type II NMJs (octapaminergic) and on the type I NMJs (glutamatergic) analyzed in this study. In contrast to the current observations, downregulation of Gαo in these NMJs was proposed to increase, rather than decrease, type I bouton numbers. It is suspected that the main reason for the discrepancy lies in the Gal4 lines used. The BG439-Gal4 and C380-Gal4 lines of Koon are poorly characterized and, unlike the well-analyzed pan-neuronal elav-Gal4 and motoneuron-specific OK371-Gal4 and D42-Gal4 driver lines used in the current study, might mediate a more acute expression. In this case, this study reflects the positive role of Gαo in the developmental formation of glutamatergic boutons, as opposed to a role in acute fine-tuning in response to environmental factors as studied by Koon (Luchtenborg, 2014).

    Postsynaptic expression of fz2 was found to fully rescue fz2 null NMJs. This study found that presynaptic knockdown of Fz2 (and other components of Wg-Fz2-Gαo signaling) recapitulates fz2 null phenotypes, whereas presynaptic overactivation of this pathway increases bouton numbers; furthermore, presynaptic overexpression of fz2 or Gαo rescues the fz2 nulls, just as postsynaptic overexpression of fz2 does. The current data thus support a crucial role for presynaptic Wg-Fz2-Gαo signaling in NMJ formation. Interestingly, both pre- and postsynaptic re-introduction of Arrow, an Fz2 co-receptor that is normally present both pre- and postsynaptically, as is Fz2 itself, can rescue arrow mutant NMJs. Thus, it appears that the pre- and postsynaptic branches of Fz2 signaling are both involved in NMJ development. A certain degree of redundancy between these branches must exist. Indeed, wild-type levels of Fz2 in the muscle are not sufficient to rescue the bouton defects induced by presynaptic expression of RNAi-fz2, yet overexpression of fz2 in the muscle can restore the bouton integrity of fz2 nulls. One might hypothesize that postsynaptic Fz2 overexpression activates a compensatory pathway - such as that mediated by reduction in laminin A signaling - that leads to restoration in bouton numbers in fz2 mutants. The current data showing that the targeted downregulation of Fz2 in the presynapse is sufficient to recapitulate the fz2 null phenotype underpin the crucial function of presynaptic Fz2 signaling in NMJ formation (Luchtenborg, 2014).

    This study found that downregulation of Ank2 produces NMJ defects similar to those of wg, fz2 or Gαo silencing. However, Ank2 mutant phenotypes appear more pronounced, indicating that Wg-Fz2-Gαo signaling might control a subset of Ank2-mediated activities in the NMJ. Ank2 was proposed to play a structural role in NMJ formation, binding to microtubules through its C-terminal region. However, since the C-terminal region was insufficient to rescue Ank2L mutant phenotypes, additional domains are likely to mediate Ank2 function through binding to other proteins. This study demonstrates in the yeast two-hybrid system and in pull-down experiments that the ankyrin repeat region of Ank2 physically binds Gαo, suggesting that the function of Ank2 in NMJ formation might be regulated by Wg-Fz2-Gαo signaling. Indeed, epistasis experiments place Ank2 downstream of Gαo in NMJ formation (Luchtenborg, 2014).

    Upon dissociation of the heterotrimeric Go protein by activated GPCRs such as Fz2, the liberated Gαo subunit can signal to its downstream targets both in the GTP- and GDP-bound state (the latter after hydrolysis of GTP and before re-association with Gβγ). The free signaling Gαo-GDP form is predicted to be relatively long lived, and a number of Gαo target proteins have been identified that interact equally well with both of the nucleotide forms of this G protein. In the context of NMJ formation, this study found that Gαo-GTP and -GDP are efficient in the activation of downstream signaling, and identifies Ank2 as a binding partner of Gαo that interacts with both nucleotide forms. The importance of signaling by Gα-GDP released from a heterotrimeric complex by the action of GPCRs has also been demonstrated in recent studies of mammalian chemotaxis, planar cell polarity and cancer (Luchtenborg, 2014).

    Gαo[G203T], which largely resides in the GDP-binding state owing to its reduced affinity for GTP, might be expected to act as a dominant-negative. However, in canonical Wnt signaling, regulation of asymmetric cell division as well as in planar cell polarity (PCP) signaling in the wing, Gαo[G203T] displays no dominant-negative activity but is simply silent, whereas in eye PCP signaling this form acts positively but is weaker than other Gαo forms. Biochemical characterization of the mammalian Gαi2[G203T] mutant revealed that it can still bind Gβγ and GTP, but upon nucleotide exchange Gαi2[G203T] fails to adopt the activated confirmation and can further lose GTP. The current biochemical characterization confirms that Gαo[G203T] still binds GTP. Interestingly, Gαi2[G203T] inhibited only a fraction of Gαi2-mediated signaling, suggesting that the dominant-negative effects of the mutant are effector specific. Thus, it is inferred that a portion of Gαo[G203T] can form a competent Fz2-transducing complex, and a portion of overexpressed Gαo[G203T] resides in a free GDP-loaded form that is also competent to activate downstream targets - Ank2 in the context of NMJ formation (Luchtenborg, 2014).

    These experiments place Ank2 downstream of Gαo and also of Sgg (GSK3β). It remains to be investigated whether Ank2 can directly interact with and/or be phosphorylated by Sgg. Meanwhile, it is proposed that the microtubule-binding protein Futsch might be a linker between Sgg and Ank2. Futsch is involved in NMJ formation and is placed downstream of Wg-Sgg signaling, being the target of phosphorylation and negative regulation by Sgg as the alternative target to β-catenin, which is dispensable in Wg NMJ signaling. Abnormal Futsch localization has been observed in Ank2 mutants. In Drosophila wing and mammalian cells in culture, Gαo acts upstream of Sgg/GSK3β. Cumulatively, these data might suggest that the Wg-Fz2-Gαo cascade sends a signal to Futsch through Sgg, parallel to that mediated by Ank2 (Luchtenborg, 2014).

    The importance of the Gαo-Ank2 interaction for Drosophila NMJ development is corroborated by findings in mammalian neuronal cells, where it was demonstrated that the ability of Gαo to induce neurite outgrowth is critically dependent on AnkB and AnkG. Knockdown of either or both ankyrin reduces neurite production. Remarkably, upon AnkB/G downregulation, Gαo switches its activity from the induction of microtubule-dependent processes (neurites) to actin-dependent protrusions (lamellopodia). Furthermore, Gαo recruits AnkB to the growing neurite tips. These data demonstrate that the Gαo-ankyrin mechanistic interactions are conserved from insects to mammals and are important for control over the neuronal tubulin cytoskeleton in the context of neurite growth and synapse formation. The novel signaling mechanism that were uncovered might thus be of general applicability in animal nervous system development and function (Luchtenborg, 2014).

    Drosophila Cbp53E regulates axon growth at the meuromuscular junction

    Calcium is a primary second messenger in all cells that functions in processes ranging from cellular proliferation to synaptic transmission. Proper regulation of calcium is achieved through numerous mechanisms involving channels, sensors, and buffers notably containing one or more EF-hand calcium binding domains. The Drosophila genome encodes only a single 6 EF-hand domain containing protein, Cbp53E, which is likely the prototypic member of a small family of related mammalian proteins that act as calcium buffers and calcium sensors. Like the mammalian homologs, Cbp53E is broadly though discretely expressed throughout the nervous system. Despite the importance of calcium in neuronal function and growth, nothing is known about Cbp53E's function in neuronal development. To address this deficiency, novel null alleles of Drosophila Cbp53E were generated, and neuronal development was examined at the well-characterized larval neuromuscular junction. Loss of Cbp53E resulted in increases in axonal branching at both peptidergic and glutamatergic neuronal terminals. This overgrowth could be completely rescued by expression of exogenous Cbp53E. Overexpression of Cbp53E, however, only affected the growth of peptidergic neuronal processes. These findings indicate that Cbp53E plays a significant role in neuronal growth and suggest that it may function in both local synaptic and global cellular mechanisms (Hagel, 2015).

    Distinct regulation of transmitter release at the Drosophila NMJ by different isoforms of nemy

    Synaptic transmission is highly plastic and subject to regulation by a wide variety of neuromodulators and neuropeptides. The present study examined the role of isoforms of the cytochrome b561 homologue called no extended memory (nemy) in regulation of synaptic strength and plasticity at the neuromuscular junction (NMJ) of third instar larvae in Drosophila. Specifically, two independent excisions of nemy were generated that differentially affect the expression of nemy isoforms. The nemy45 excision, which specifically reduces the expression of the longest splice form of nemy, leads to an increase in stimulus evoked transmitter release and altered synaptic plasticity at the NMJ. Conversely, the nemy26.2 excision, which appears to reduce the expression of all splice forms except the longest splice isoform, shows a reduction in stimulus evoked transmitter release, and enhanced synaptic plasticity. It was further shown that nemy45 mutants have reduced levels of amidated peptides similar to that observed in peptidyl-glycine hydryoxylating mono-oxygenase (PHM) mutants. In contrast, nemy26.2 mutants show no defects in peptide amidation but rather display a decrease in Tyramine beta hydroxylase activity (TbetaH). Taken together, these results show non-redundant roles for the different nemy isoforms and shed light on the complex regulation of neuromodulators (Knight, 2015).

    The guanine exchange factor Gartenzwerg and the small GTPase Arl1 function in the same pathway with Arfaptin during synapse growth

    The generation of neuronal morphology requires transport vesicles originating from the Golgi apparatus (GA) to deliver specialized components to the axon and dendrites. Drosophila Arfaptin is a membrane-binding protein localized to the GA that is required for the growth of the presynaptic nerve terminal. This study provides biochemical, cellular and genetic evidence that the small GTPase Arl1 and the guanine-nucleotide exchange factor (GEF) Gartenzwerg are required for Arfaptin function at the Golgi during synapse growth. These data define a new signaling pathway composed of Arfaptin, Arl1, and Garz, required for the generation of normal synapse morphology (Chang, 2015).

    Drosophila neuronal injury follows a temporal sequence of cellular events leading to degeneration at the neuromuscular junction

    There is a critical need to improve understanding of the molecular and cellular mechanisms that drive neurodegeneration. At the molecular level, neurodegeneration involves the activation of complex signaling pathways that drive the active destruction of neurons and their intracellular components. This study used an in vivo motor neuron injury assay to acutely induce neurodegeneration in order to follow the temporal order of events that occur following injury in Drosophila. Sites of injury can be rapidly identified based on structural defects to the neuronal cytoskeleton that result in disrupted axonal transport. Additionally, the neuromuscular junction accumulates ubiquitinated proteins prior to the neurodegenerative events, occurring at 24 hours post injury. These data provide insights into the early molecular events that occur during axonal and neuromuscular degeneration in a genetically tractable model organism. Importantly, the mechanisms that mediate neurodegeneration in flies are conserved in humans. Thus, these studies will facilitate the identification of biomedically relevant targets for future treatments (Lincoln, 2015).

    Diminished MTORC1-dependent JNK activation underlies the neurodevelopmental defects associated with lysosomal dysfunction

    This study evaluated the mechanisms underlying the neurodevelopmental deficits in Drosophila and mouse models of lysosomal storage diseases (LSDs). Lysosomes promote the growth of neuromuscular junctions (NMJs) via Rag GTPases and mechanistic target of rapamycin complex 1 (MTORC1). However, rather than employing S6K/4E-BP1, MTORC1 stimulates NMJ growth via JNK, a determinant of axonal growth in Drosophila and mammals. This role of lysosomal function in regulating JNK phosphorylation is conserved in mammals. Despite requiring the amino-acid-responsive kinase MTORC1, NMJ development is insensitive to dietary protein. This paradox is attributed to anaplastic lymphoma kinase (ALK), which restricts neuronal amino acid uptake, and the administration of an ALK inhibitor couples NMJ development to dietary protein. These findings provide an explanation for the neurodevelopmental deficits in LSDs and suggest an actionable target for treatment (Wong, 2015).

    Mucolipidosis type IV (MLIV) and Batten disease are untreatable lysosomal storage diseases (LSDs) that cause childhood neurodegeneration. MLIV arises from loss-of-function mutations in the gene encoding TRPML1, an endolysosomal cation channel belonging to the TRP superfamily. The absence of TRPML1 leads to defective lysosomal storage and autophagy, mitochondrial damage, and macromolecular aggregation, which together initiate the protracted neurodegeneration observed in MLIV). Batten disease arises from the absence of a lysosomal protein, CLN3), and results in psychomotor retardation. Both diseases cause early alterations in neuronal function. For instance, brain imaging studies revealed that MLIV and Batten patients display diminished axonal development in the cortex and corpus callosum, the causes of which remain unknown (Wong, 2015).

    To better understand the etiology of MLIV in a genetically tractable model, flies were generated lacking the TRPML1 ortholog. The trpml-deficient (trpml1) flies have led to insight into the mechanisms of neurodegeneration and lysosomal storage (Wong, 2015).

    This study reports that trpml1 larvae exhibit diminished synaptic growth at the NMJ, a well-studied model synapse. Lysosomal function supports Rag GTPases and MTORC1 activation, and this is essential for JNK phosphorylation and synapse development (Wong, 2015).

    Drosophila larvae and mice lacking CLN3 also exhibit diminished Rag/ MTORC1 and JNK activation, suggesting that alterations in neuronal signaling are similar in different LSDs and are evolution- arily conserved. Interestingly, the NMJ defects in the two fly LSD models were suppressed by the administration of a high-protein diet and a drug that is currently in clinical trials to treat certain forms of cancer. These findings inform a pharmacotherapeutic strategy that may suppress the neurodevelopmental defects observed in LSD patients (Wong, 2015).

    This study shows that lysosomal dysfunction in Drosophila MNs results in diminished bouton numbers at the larval NMJ. Evidence is presented that lysosomal dysfunction results in decreased activation of the amino-acid-responsive cascade involving Rag/MTORC1, which are critical for normal NMJ development (Wong, 2015).

    Despite the requirement for MTORC1 in NMJ synapse development, previous studies and the current findings show that bouton numbers are independent of S6K and 4E-BP1. Rather, MTORC1 promotes NMJ growth via a MAP kinase cascade culminating in JNK activation. Therefore, decreasing lysosomal function or Rag/MTORC1 activation in hiwND8 suppressed the associated synaptic overgrowth. However, the 'small-bouton' phenotype of hiwND8 was independent of MTORC1. Thus, MTORC1 is required for JNK-dependent regulation of bouton numbers, whereas bouton morphology is independent of MTORC1. Furthermore, although both rheb expression and hiw loss result in Wnd-dependent elevation in bouton numbers, the supernumerary boutons in each case show distinct morphological features. Additional studies are needed for deciphering the complex interplay between MTORC1-JNK in regulating the NMJ morphology (Wong, 2015).

    Biochemical analyses revealed that both JNK phosphorylation and its transcriptional output correlated with the activity of MTORC1, which are consistent with prior observations that cln3 overexpression promotes JNK activation and that tsc1/tsc2 deletion in flies result in increased JNK-dependent transcription. These findings point to the remarkable versatility of MTORC1 in controlling both protein trans lation and gene transcription (Wong, 2015).

    Using an in vitro kinase assay, this study demonstrates that Wnd is a target of MTORC1. Because axonal injury activates both MTORC1 and DLK/JNK, these findings imply a functional connection between these two pathways. Interestingly, the data also suggest that MTORC1 contains additional kinases besides MTOR that can phosphorylate Wnd. One possibility is that ULK1/Atg1, which associates with MTORC1, could be the kinase that phosphorylates Wnd. Consistent with this notion, overexpression of Atg1 in the Drosophila neurons has been shown to promote JNK signaling and NMJ synapse overgrowth via Wnd) (Wong, 2015).

    This study also found that developmental JNK activation in axonal tracts of the CC and pJNK levels in cortical neurons were compromised in a mouse model of Batten disease. Thus, the signaling deficits identified in Drosophila are also conserved in mammals. The activity of DLK (the mouse homolog of Wnd) and JNK signaling are critical for axonal development in the mouse CNS. Therefore, decreased neuronal JNK activation during development might underlie the thinning of the axonal tracts observed in many LSDs (Wong, 2015).

    Although the findings of this study demonstrate a role for an amino-acid- responsive cascade in the synaptic defects associated with lysosomal dysfunction, simply elevating the dietary protein content was not sufficient to rescue these defects. These findings were reminiscent of an elegant study that showed that the growth of Drosophila neuroblasts is uncoupled from dietary amino acids owing to the function of ALK, which suppresses the uptake of amino acids into the neuroblasts (Cheng, 2011). Indeed, simultaneous administration of an ALK inhibitor and a high-protein diet partially rescued the synaptic growth defects associated with the lysosomal dysfunction, and improved the rescue of pupal lethality associated with trpml1. Although these studies do not causally link the defects in synapse development with pupal lethality, they do raise the intriguing possibility that multiple phenotypes associated with LSDs could be targeted using ALK inhibitors along with a protein-rich diet (Wong, 2015).

    Although LSDs result in lysosomal dysfunction throughout the body, neurons are exceptionally sensitive to these alterations. The cause for this sensitivity remains incompletely understood. Given the findings of this study that mature neurons do not efficiently take up amino acids from the extracellular medium, lysosomal degradation of proteins serves as a major source of free amino acids in these cells. Therefore, disruption of lysosomal degradation leads to severe shortage of free amino acids in neurons, regardless of the quantity of dietary proteins, thus explaining the exquisite sensitivity of neurons to lysosomal dysfunction (Wong, 2015).

    σ2-adaptin facilitates basal synaptic transmission and is required for regenerating endo-exo cycling pool under high frequency nerve stimulation in Drosophila

    The functional requirement of AP2 complex (see AP-2α) in synaptic membrane retrieval by clathrin mediated endocytosis (CME) is not fully understood. This study isolated and functionally characterized a mutation that dramatically altered synaptic development. Based on the aberrant neuromuscular junction synapse, this mutation was named angur (a Hindi dialect meaning grapes). Loss-of-function alleles of angur show more than two-fold overgrowth in bouton numbers and dramatic decrease in bouton size. angur mutation was mapped to σ2-adaptin, the smallest subunit of the adapter complex 2. Reducing neuronal level of any of the subunits of AP2 complex or disrupting AP2 complex assembly in neurons phenocopied σ2-adaptin mutation. Genetic perturbation of σ2-adaptin in neurons leads to a reversible temperature sensitive paralysis at 38 degrees °C. Electrophysiological analysis of the mutants revealed reduced evoked junction potentials and quantal content. Interestingly, high frequency nerve stimulation caused prolonged synaptic fatigue at the NMJs. The synaptic level of subunits of AP2 complex and clathrin but not other endocytic proteins were reduced in the mutants. Moreover, the BMP/TGFβ signalling was altered in these mutants and was restored by normalizing σ2-adaptin in neurons. Thus, these data suggest that - 1) while σ2-adaptin facilitates SV recycling for basal synaptic transmission, its activity is also required for regenerating SV during high frequency nerve stimulation; and 2) σ2-adaptin regulates NMJ morphology by attenuating TGFβ signalling (Choudhury, 2016).

    Synaptic transmission requires fusion of synaptic vesicles (SVs) at the active zones followed by their efficient retrieval and recycling through endocytic mechanisms. Retrieval and sorting of membrane lipids and vesicular proteins at the synapse are mediated by a well-orchestrated and coordinated action of several adapter and endocytic proteins. Clathrin-mediated endocytosis (CME) is the primary pathway operative at the synapses for membrane retrieval. Genetic analysis of the components of the CME pathway in Caenorhabditis elegans and Drosophila has revealed that this pathway is required for SV re-formation, and in many cases, blocking CME at synapses results in temperature-sensitive paralysis. Additionally, CME plays a crucial role in regulating synaptic morphology. At Drosophila NMJs, blocking CME results in enhanced bone morphogenetic protein (BMP) signaling and affects synaptic growth (Choudhury, 2016).

    The heterotetrameric adapter protein 2 (AP2) complex is a major effector of the CME pathway. AP2 serves as a major hub for a large number of molecular interactions and links plasma membrane, cargo/signaling molecules, clathrin, and accessory proteins in the CME pathway and hence can directly influence synaptic signaling. The AP2 complex is pseudo-asymmetric and contains four subunits-one each of large α and β2 subunits, one medium μ2 subunit, and a small σ2 subunit. Depletion of clathrin or its major adapter, AP2, in either Drosophila or mammalian central synapses results in accumulation of endosome-like vacuoles and reduction of SVs, suggesting that CME may not be essential for membrane retrieval. Similarly, genetic perturbation of μ2-adaptin or α-adaptin shows only mild defects in vesicle biogenesis at C. elegans synapses, but simultaneous loss of both adaptins leads to severely compromised SV biogenesis and accumulation of large vacuoles at nerve terminals. While Drosophila loss-of-function mutations in α-adaptin are embryonic lethal, hypomorphic mutants exhibit reduced FM1-43 uptake, suggesting a compromised endocytosis in these mutants. Whether reduced endocytosis reflects a defect in membrane retrieval or a defect in SV biogenesis remains unclear. Moreover, the consequences of AP2 reduction on synaptic morphology and physiology remain unknown (Choudhury, 2016).

    This study presents an analysis of Drosophila σ2-adaptin in the context of regulating NMJ morphological plasticity and physiology. A mutation was identified that dramatically altered NMJ morphology. Next, this mutation was mapped to σ2-adaptin by deficiency mapping. This study shows that AP2-dependent vesicle endocytosis regulates both synaptic growth and transmitter release. The AP2 complex is a heterotetramer, and these studies in Drosophila show that the four subunits are obligate partners of each other and are required for a functional AP2 complex. This finding is in contrast to the hemicomplex model in C. elegans, in which α/σ2 and β22 can sustain the function, if any one of the subunits is mutated. Loss of AP2 disrupts stable microtubule loops of the presynaptic cytoskeleton and exacerbates growth signaling through the phosphorylated Mothers Against Decapentaplegic (pMAD) pathway, suggesting that normal AP2 constrains the TGFβ signaling module. Reducing σ2-adaptin level results in synaptic fatigue at the larval NMJ synapses during high-frequency stimulation and causes temperature-sensitive paralysis in adults. Based on these results, it is suggestd that AP2 is essential for attenuating synaptic growth signaling mediated by the TGFβ pathway in addition to its requirement in regenerating SVs under high-frequency nerve firing (Choudhury, 2016).

    AP2/CME has been proposed to play an essential role in SV endocytosis. Moreover, mutation in the proteins affecting CME also results in altered NMJ development in Drosophila, suggesting regulation of synaptic signaling by CME. This study analyzed the role of AP2, one of the major adapters for clathrin at synapses, in the context of its physiological relevance and synaptic signaling. The results demonstrate that AP2 facilitates basal synaptic transmission and SV endocytosis but also is essential during high-frequency nerve firing to regenerate SVs. Moreover, evidence is provided that AP2 regulates morphological plasticity at the Drosophila NMJ by stabilizing the microtubule loops and attenuating the BMP signaling pathway (Choudhury, 2016).

    The biogenesis of rhabdomeres is regulated by endocytosis and intracellular trafficking. Components of the endocytic machinery, when perturbed, lead to defects in rhabdomere formation. One such example is the disruption of the rhabdomere base when shits flies are grown at restrictive temperature for 4 hr. Rh1-Gal4-driven shits1 flies raised at 19° led to an enlargement of photoreceptor cell bodies with the inter-rhabdomeric space being reduced. Similarly, when α-adaptin was knocked down using GMR-Gal4, rhabdomere biogenesis was disrupted, with smaller rhabdomeres. In accordance with these observations, the TEM analysis of angur mutant eye clones showed no detectable rhabdomeres. Consistent with the defect in rhabdomere formation, no membrane depolarization was observed in angur mutant eye clones. The visual cascade in Drosophila begins with rhodopsin being converted to meta-rhodopsin by light, followed by a subsequent phosphorylation by GPRK1. The phosphorylated meta-rhodopsin binds to arrestins, where its activity is quenched. The rhodopsin-arrestin complex is endocytosed, and rhodopsin is degraded in the lysosomes. Thus, endocytosis regulates rhodopsin turnover in the photoreceptor cells, and blocking of the endocytic pathway components by mutations in Arr1, Arr2, or AP2 leads to retinal degeneration and photoreceptor cell death. This further establishes a strong link between endocytosis and retinal degeneration. Thus, it is likely that defective endocytosis in angur mutant eye clones affects meta-rhodopsin turnover, resulting into defective rhabdomeres and disorganized ommatidia. Moreover, endocytosis of several signaling receptors also has been shown to regulate morphogenesis during Drosophila eye development. The Notch signaling pathway has been studied extensively, and it has been shown that the Notch extracellular domain is transendocytosed during this process into the Delta-expressing cells. The localization of Notch and Delta is disrupted when endocytosis is acutely blocked, and this prevents internalization of Notch in the Delta-expressing cells. It is thus speculated that the endocytic defect in angur mutant clones affects turnover and internalization of signaling molecules that may lead to defective rhabdomere development (Choudhury, 2016).

    The functional analysis of the σ2-adaptin mutants as well as of other subunits of AP2 reveals its requirement in maintaining basal synaptic transmission and regeneration of synaptic vesicles under high-frequency nerve firing at Drosophila NMJ synapses. The data are consistent with previous reports showing the requirement for Drosophila α-adaptin in SV recycling at NMJ synapses. However, in contrast to its role at mammalian central synapses, this study found that loss of AP2 at Drosophila NMJ synapses affects basal synaptic transmission and SV endocytosis, albeit not very strongly. Moreover, the kinetics of SV re-formation in σ2-adaptin mutants was only mildly affected. Consistent with the recent observation that loss of the AP2 complex at mammalian central synapses or its subunits in C. elegans affects synaptic vesicle biogenesis, compromised synaptic transmission and SV re-formation was observed after high-frequency stimulation, suggesting that at Drosophila NMJ synapses, AP2 function is required not only for SV trafficking but also for SV regeneration under high-frequency nerve stimulation. A direct estimation of vesicle pool size in σ2-adaptin mutants further supports its requirement in the regulation of SV pool size and is consistent with a recent report. Because loss of AP2 in Drosophila does not completely abrogate SV re-formation, it remains possible that other known synaptic clathrin adapters such as AP180, Eps15, and Epsin might partially compensate for loss of the AP2 complex. This is corroborated by the observation that even a 96% reduction in synaptic α-adaptin level caused only an ~50% reduction in clathrin, suggesting that other clathrin adapters might contribute to retention and/or stability of synaptic clathrin (Choudhury, 2016).

    The striking physiological defect in σ2-adaptin mutants or neuronally reduced α-adaptin was their inability to recover from synaptic depression even after 4 min of cessation of high-frequency stimulation. Consistent with these observations, the endo-exo cycling pool did not recover to its initial value even after 4 min of rest following high-frequency nerve stimulation. This suggests that in addition to its requirement in synaptic vesicle endocytosis, AP2 complex functions at a relatively slower step downstream of membrane retrieval, possibly during membrane sorting from endosomes to regenerate fusion-competent synaptic vesicles (Gu, 2013; Kononenko, 2014; Choudhury, 2016 and references therein).

    Several studies in cell culture support a model that indicates that the AP2 complex is an obligate heterotetramer for subunit stability and function. However, studies of AP2 function in C. elegans suggest that α/σ2 subunits and β22 subunits may constitute two hemicomplexes that can carry out the minimal function of the AP2 complex independent of one another. Does AP2 in Drosophila form hemicomplexes and contribute to vesicle trafficking? The results do not support this model for Drosophila. First, unlike C. elegans, in which individual mutants for subunits of the AP2 complex are viable, the Drosophila mutant for α-adaptin is early larval lethal. Consistent with these findings, neuronal knockdown of α-adaptin or β2-adaptin resulted in third instar lethality. Second, if the hemicomplexes were functional in Drosophila, one would observe significant levels of synaptic β2-adaptin in α-adaptin knockdown or σ2-adaptin mutants. However, it was observed that removing any of the subunits of the Drosophila AP2 complex in neurons resulted into an unstable AP2 complex and degradation of α- and β2-adaptins. Third, inhibiting AP2 complex assembly by reducing synaptic PI(4,5)P2 levels also resulted into an unstable AP2 complex. These observations strongly suggest that in contrast to C. elegans, all the subunits of AP2 in Drosophila are obligate partners for a stable AP2 complex for clathrin-dependent SV trafficking (Choudhury, 2016).

    Mutations in genes implicated in regulating CME, BMP signaling, or actin cytoskeleton dynamics all show abnormal NMJ development in Drosophila, characterized by either supernumerary boutons or an increased number but smaller boutons. The NMJ phenotype due to loss of σ2-adaptin is consistent with that of other known endocytic mutants implicated in CME. The contrasting synaptic overgrowth observed in σ2-adaptin mutants suggests a role for AP2 in mediating presynaptic growth signaling. Such synaptic overgrowth also was observed when any of the subunits of AP2 were downregulated or its assembly was interfered with by downregulating neuronal PI(4,5)P2. This suggests a pathway in which PI(4,5)P2 and the AP2 complex interact obligatorily to regulate synapse growth. The NMJ phenotype in angur mutants is strikingly more severe from that of the other synaptic mutants and points toward multiple growth signaling pathways that are possibly affected by an AP2-dependent endocytic deficit (Choudhury, 2016).

    The morphology of the synapse is a consequence of the neuronal cytoskeleton network that shapes the growing synapse. Futsch, a protein with MAP1B homology, regulates the synaptic microtubule cytoskeleton, thereby controlling synaptic growth at the Drosophila NMJ. The hypothesis that the microtubule organization could be altered was strengthened by the dramatic decrease in the number of Futsch-positive loops in the σ2-adaptin mutants. Futsch-positive loops have long been known to be associated with stable synaptic boutons, while the absence or disruption of these loops is indicative of boutons undergoing division or sprouting. The dramatic increase in the number of boutons in these mutants with a corresponding decrease in the number of loops correlates well with the fact that these boutons might be undergoing division. The phenotype associated with futsch mutants is fewer and larger boutons with impaired microtubule organization, which is an expected phenotype when bouton division is impaired. The σ2-adaptin mutants, however, show a larger number but smaller-sized boutons, indicating that bouton division is enhanced. One of the signaling events that has been shown to dictate this process is BMP signaling. Further, BMP signaling is also required during developmental synaptic growth. Consistent with this, elevated pMAD levels were found in σ2-adaptin mutants, indicating that BMP signaling is upregulated in these mutants. It has also been demonstrated that BMP signaling plays a role in maintenance of the presynaptic microtubule network. Drosophila spichthyin and spartin mutants have upregulated BMP signaling with significantly increased microtubule loops. It has been proposed that Futsch acts downstream of BMP signaling to regulate synaptic growth. The σ2-adaptin mutants also show upregulated BMP signaling, but in contrast to spichthyin and spartin mutants, σ2-adaptin mutants have fewer Futsch loops, which also appeared fragmented. This suggests that deregulated BMP signaling, whether upregulation or downregulation, impairs microtubule stability. dap160 mutants also show supernumerary boutons, with fragmented Futsch staining suggesting that the microtubule dynamics are misregulated in these mutants. Interestingly, Nwk levels are drastically altered in dap160 mutants, suggesting that Nwk levels are important for maintaining microtubule stability. Because σ2-adaptin mutants have normal Nwk levels, the observed synaptic overgrowth is independent of Nwk and suggests regulation of Futsch through Nwk-independent pathways (Choudhury, 2016).

    Hsc70-4 deforms membranes to promote synaptic protein turnover by endosomal microautophagy

    Synapses are often far from their cell bodies and must largely independently cope with dysfunctional proteins resulting from synaptic activity and stress. To identify membrane-associated machines that can engulf synaptic targets destined for degradation, a large-scale in vitro liposome-based screen was performed followed by functional studies. A presynaptically enriched chaperone Hsc70-4 was identified that bends membranes based on its ability to oligomerize. This activity promotes endosomal microautophagy and the turnover of specific synaptic proteins. Loss of microautophagy slows down neurotransmission while gain of microautophagy increases neurotransmission. Interestingly, Sgt, a cochaperone of Hsc70-4, is able to switch the activity of Hsc70-4 from synaptic endosomal microautophagy toward chaperone activity. Hence, Hsc70-4 controls rejuvenation of the synaptic protein pool in a dual way: either by refolding proteins together with Sgt, or by targeting them for degradation by facilitating endosomal microautophagy based on its membrane deforming activity (Uytterhoeven, 2016).

    Rab3-GEF controls active zone development at the Drosophila neuromuscular junction

    Synaptic signaling involves the release of neurotransmitter from presynaptic active zones (AZs). Proteins that regulate vesicle exocytosis cluster at AZs, composing the cytomatrix at the active zone (CAZ). At the Drosophila neuromuscular junction (NMJ), the small GTPase Rab3 controls the distribution of CAZ proteins across release sites, thereby regulating the efficacy of individual AZs. This study identifies Rab3-GEF as a second protein that acts in conjunction with Rab3 to control AZ protein composition. At rab3-GEF mutant NMJs, Bruchpilot (Brp) and Ca(2+) channels are enriched at a subset of AZs, leaving the remaining sites devoid of key CAZ components in a manner that is indistinguishable from rab3 mutant NMJs. As the Drosophila homologue of mammalian DENN/MADD and Caenorhabditis elegans AEX-3, Rab3-GEF is a guanine nucleotide exchange factor (GEF) for Rab3 that stimulates GDP to GTP exchange. Mechanistic studies reveal that although Rab3 and Rab3-GEF act within the same mechanism to control AZ development, Rab3-GEF is involved in multiple roles. It was shown that Rab3-GEF is required for transport of Rab3. However, the synaptic phenotype in the rab3-GEF mutant cannot be fully explained by defective transport and loss of GEF activity. A transgenically expressed GTP-locked variant of Rab3 accumulates at the NMJ at wild-type levels and fully rescues the rab3 mutant but is unable to rescue the rab3-GEF mutant. These results suggest that although Rab3-GEF acts upstream of Rab3 to control Rab3 localization and likely GTP-binding, it also acts downstream to regulate CAZ development, potentially as a Rab3 effector at the synapse (Bae, 2016).

    A pre-synaptic regulatory system acts trans-synaptically via Mon1 to regulate Glutamate receptor levels in Drosophila

    Mon1 is an evolutionarily conserved protein involved in the conversion of Rab5 positive early endosomes to late endosomes through the recruitment of Rab7. This study has identified a role for Drosophila Mon1 in regulating glutamate receptor levels at the larval neuromuscular junction. Mutants were generated in Dmon1 through P-element excision. These mutants are short-lived with strong motor defects. At the synapse, the mutants show altered bouton morphology with several small supernumerary or satellite boutons surrounding a mature bouton; a significant increase in expression of GluRIIA and reduced expression of Bruchpilot. Neuronal knockdown of Dmon1 is sufficient to increase GluRIIA levels suggesting its involvement in a pre-synaptic mechanism that regulates post-synaptic receptor levels. Ultrastructural analysis of mutant synapses reveals significantly smaller synaptic vesicles. Overexpression of vglut suppresses the defects in synaptic morphology and also downregulates GluRIIA levels in Dmon1 mutants suggesting that homeostatic mechanisms are not affected in these mutants. It is proposed that DMon1 is part of a pre-synaptically regulated trans-synaptic mechanism that regulates GluRIIA levels at the larval neuromuscular junction (Deivasigamani, 2015).

    Neurotransmitter release at the synapse is modulated by factors that control synaptic growth, synaptic vesicle recycling, and receptor turnover at postsynaptic sites. Endolysosomal trafficking modulates the function of these factors and therefore plays an important role in regulating synaptic development and function. Intracellular trafficking is regulated by Rabs, which are small GTPases. These proteins control specific steps in the trafficking process. A clear understanding of the role of Rabs at the synapse is still nascent. Drosophila has 31 Rabs, and most of these are expressed in the nervous system. Rab5 and Rab7, present on early and late endosomes, respectively, are critical regulators of endolysosomal trafficking and loss of this regulation affects neuronal viability underscored by the fact that mutations in Rab7 are associated with neurodegeneration. Rab5 along with Rab3 is present on synaptic vesicles, and both play a role in regulating neurotransmitter release. In Drosophila, Rab3 is involved in the assembly of active zones by controlling the level of both Bruchpilot-a core active zone protein-and the calcium channels surrounding the active zone. In hippocampal and cortex neurons, Rab5 facilitates LTD through removal of AMPA receptors from the synapse. In Drosophila, Rab5 regulates neurotransmission; it also functions to maintain synaptic vesicle size by preventing homotypic fusion. Compared to Rab5 or Rab3, less is known about the roles of Rab7 at the synapse. In spinal cord motor neurons, Rab7 mediates sorting and retrograde transport of neurotrophin-carrying vesicles. In Drosophila, tbc1D17-a known GAP for Rab7-affects GluRIIA levels; the effect of this on neurotransmission has not been evaluated. Excessive trafficking via the endolysosomal pathway also affects neurotransmission. This has been observed in mutants for tbc1D24-a GAP for Rab35. A high rate of turnover of synaptic vesicle proteins in these mutants is seen to increase neurotransmitter release (Deivasigamani, 2015 and references therein).

    This study has examined the synaptic role of DMon1-a key regulator of endosomal maturation. Multiple synaptic phenotypes are found associated with Dmon1 loss of function, and one of these is altered synaptic morphology. Boutons in Dmon1 mutants are larger with more satellite or supernumerary boutons-a phenotype strongly associated with endocytic mutants. Formation of satellite boutons is thought to occur due to loss of bouton maturation, with the initial step of bouton budding being controlled postsynaptically and the maturation step being regulated presynaptically. Supporting this, a recent study shows that miniature neurotransmission is required for bouton maturation. The presence of excess satellite boutons in Dmon1 mutants suggests that the number of 'miniature' events is likely to be affected in these mutants. The fact that this phenotype can be rescued upon expression of vGlut supports this possibility. However, this does not fit with the observed decrease in size and intensity of Brp positive puncta in these mutants. Active zones with low or nonfunctional Brp are known to be more strongly associated with increased spontaneous neurotransmission. Considering the involvement of postsynaptic signaling in initiating satellite bouton formation, it is thought that altered neurotransmission possibly together with impaired postsynaptic or retrograde signaling, contributes to the altered synaptic morphology in Dmon1 mutants. This may also explain why no satellite boutons are observed in neuronal RNAi animals (Deivasigamani, 2015).

    A striking phenotype associated with loss of Dmon1 is the increase in GluRIIA levels. This phenotype seems presynaptic in origin since neuronal loss of Dmon1 is sufficient to increase GluRIIA levels. Is the increase in GluRIIA due to trafficking defects in the neuron? This seems unlikely for the following reasons: First, it has been shown that although neuronal overexpression of wild-type and dominant negative Rab5 alters evoked response in a reciprocal manner, there is no change in synaptic morphology, glutamate receptor localization and density, or change in synaptic vesicle size. The role of Rab7 at the synapse is less clear. In a recent study, loss of tbc1D15-17, which functions as a GAP for Rab7, was shown to increase GluRIIA levels at the synapse. Selective knockdown of the gene in muscles, and not neurons, was seen to increase GluRIIA levels, indicating that the function of the gene is primarily postsynaptic. These data are not consistent with the current results from neuronal knockdown of Dmon1, suggesting that the presynaptic role of Dmon1 in regulating GluRIIA levels is likely to be independent of Rab5 and Rab7 and therefore novel (Deivasigamani, 2015).

    The current experiments to evaluate the postsynaptic role of Dmon1 have been less clear. Although a modest increase in GluRIIA levels are seen upon knockdown in muscles, the increase is not always significant when compared to controls. However, the fact that muscle expression of Dmon1 can rescue the GluRIIA phenotype in the mutant suggests that it is likely to be one of the players in regulating GluRIIA postsynaptically. Further, it is to be noted, that while overexpression of vGlut leads to down-regulation of the receptor at the synapse, the receptors do not seem to get trapped in the muscle, suggesting that multiple pathways are likely to be involved in regulating receptor turnover in the muscle, and the DMon1-Rab7-mediated pathway may be just one of them (Deivasigamani, 2015).

    How might neuronal Dmon1 regulate receptor expression? One possibility is that the increase in receptor levels is a postsynaptic homeostatic response to defects in neurotransmission, given that Dmon1Δ181 mutants have smaller synaptic vesicles. However, in dvglut mutants, presence of smaller synaptic vesicles does not lead to any change in GluRIIA levels, given that receptors at the synapse are generally expressed at saturating levels. Therefore, it seems unlikely that the increase in GluRIIA is part of a homeostatic response, although one cannot rule this out completely. The other possibility is that DMon1 is part of a transsynaptic signaling mechanism that regulates GluRIIA levels in a post-transcriptional manner. The observation that presynaptically expressed DMon1 localizes to postsynaptic regions and the results from neuronal RNAi and rescue experiments support this possibility. The involvement of transsynaptic signaling in regulating synaptic growth and function has been demonstrated in the case of signaling molecules such as Ephrins, Wingless, and Syt4. In Drosophila, both Wingless and Syt4 are released by the presynaptic terminal via exosomes to mediate their effects in the postsynaptic compartment. It was hypothesized that DMon1 released from the boutons either directly regulates GluRIIA levels or facilitates the release of an unknown factor required to maintain receptor levels. The function of DMon1 in the muscle is likely to be more consistent with its role in cellular trafficking and may mediate one of the pathways regulating GluRIIA turnover. These possibilities will need to be tested to gain a mechanistic understanding of receptor regulation by Dmon1 (Deivasigamani, 2015).

    Drosophila homolog of human KIF22 at the autism-linked 16p11.2 loci influences synaptic connectivity at larval neuromuscular junctions

    Copy number variations at multiple chromosomal loci, including 16p11.2, have recently been implicated in the pathogenesis of autism spectrum disorder (ASD), a neurodevelopmental disease that affects ~1%-3% of children worldwide. The aim of this study was to investigate the roles of human genes at the 16p11.2 loci in synaptic development using Drosophila larval neuromuscular junctions (NMJ), a well-established model synapse with stereotypic innervation patterns. A preliminary genetic screen based on RNA interference was conducted in combination with the GAL4-UAS system, followed by mutational analyses. The result indicated that disruption of klp68D, a gene closely related to human KIF22, caused ectopic innervations of axon branches forming type III boutons in muscle 13, along with less frequent re-routing of other axon branches. In addition, mutations in klp68D, of which gene product forms Kinesin-2 complex with KLP68D, led to similar targeting errors of type III axons. Mutant phenotypes were at least partially reproduced by knockdown of each gene via RNA interference. Taken together, these data suggest the roles of Kinesin-2 proteins, including KLP68D and KLP64D, in ensuring proper synaptic wiring (Park, 2016).

    Recent clinical studies on ASD have revealed gross alterations in the structure of nervous system. For instance, total brain size and the rate of neuronal proliferation in the prefrontal cortex are significantly increased in ASD patients. Such structural changes may reflect altered neuronal connectivity between specific brain regions. Moreover, the proposed candidate genes responsible for ASD include various synaptic proteins that play important roles in neurite outgrowth, axonal guidance, axonal targeting and synaptogenesis, suggesting structural abnormalities at a synaptic level responsible for expression of ASD phenotypes. Based on these findings, it was hypothesized that genetic perturbation at the 16p11.2 loci would lead to aberrant synaptic connectivity, thus underlying functional disturbances that lead to ASD. Results demonstrate significant axon targeting errors caused by defects in KLP68D, a Drosophila Kinesin-2 protein closely related to human KIF22 at the autism-linked 16p11.2 loci (Park, 2016).

    Hetero-trimeric Kinesin-2 complex in Drosophila, consisting of KLP68D, KLP64D and DmKAP, has been implicated in microtubule organization and axonal transport of synaptic proteins such as choline acetyltransferase. However, experimental evidence is missing to support the idea that Kinesin-2 complex may participate in delivering molecules important for axon targeting. Potential cargos of KLP68D and KLP64D motors have been estimated to include Unc-51/ATG1, Fasciclin II, EB1, Armadillo, Bazooka, and DE-cadherin, most of whom have been well characterized for their roles in synaptogenesis. It will be important to investigate whether disruptions of any of these potential cargos lead to aberrant axon targeting phenotypes observed in Klp68D and Klp64D mutants (Park, 2016).

    It should be noted that Drosophila Nod ("no distributive disjunction"), mostly involved in chromosomal segregation has been recognized as a homolog for human KIF22. However, similar levels of sequence homology to KIF22 were found in both Nod and KLP68D. In fact, a blast analysis results in higher sequence identity between KIF22 and KLP68D than Nod (41% vs. 33%). The specificity of motor protein cargos is often predicted to depend on the amino acid composition of motor proteins outside their core motor domain. Therefore, relatively lower level of homology between human KIF22 and Drosophila KLP68D may correspond to their distinct molecular functions. In contrast to KLP68D, the role of KIF22 in the mammalian nervous system has not been extensively investigated, but only limited to chromosomal segregation and genomic stability. Whether Drosophila KLP68D can be functionally replaced by human KIF22 in transgenic animals awaits further investigations (Park, 2016).

    Na+ /H+ -exchange via the Drosophila vesicular glutamate transporter (DVGLUT) mediates activity-induced acid efflux from presynaptic terminals

    Neuronal activity can result in transient acidification of presynaptic terminals and such shifts in cytosolic pH (pHcyto) likely influence mechanisms underlying forms of synaptic plasticity with a presynaptic locus. As neuronal activity drives acid loading in presynaptic terminals it was hypothesized that the same activity might drive acid efflux mechanisms to maintain pHcyto homeostasis. To better understand the integration of neuronal activity and pHcyto regulation this study investigated the acid extrusion mechanisms at Drosophila glutamatergic motorneuron terminals. Expression of a fluorescent genetically-encoded pH-indicator (GEpHI), named "pHerry', in the presynaptic cytosol revealed acid efflux following nerve activity to be greater than that predicted from measurements of the intrinsic rate of acid efflux. Analysis of activity-induced acid transients in terminals deficient in either endocytosis or exocytosis revealed an acid efflux mechanism reliant upon synaptic vesicle exocytosis. Pharmacological and genetic dissection in situ and in a heterologous expression system indicate that this acid efflux is mediated by conventional plasmamembrane acid transporters, and also by previously unrecognized intrinsic H+ /Na+ exchange via the Drosophila vesicular glutamate transporter (DVGLUT). DVGLUT functions not only as a vesicular glutamate transporter but also serves as an acid extruding protein when deposited on the plasma membrane (Rossano, 2016).

    The Strip-Hippo pathway regulates synaptic terminal formation by modulating actin organization at the Drosophila neuromuscular synapses

    Synapse formation requires the precise coordination of axon elongation, cytoskeletal stability, and diverse modes of cell signaling. The underlying mechanisms of this interplay, however, remain unclear. This study demonstrates that Strip, a component of the striatin-interacting phosphatase and kinase (STRIPAK) complex that regulates these processes, is required to ensure the proper development of synaptic boutons at the Drosophila neuromuscular junction. In doing so, Strip negatively regulates the activity of the Hippo (Hpo) pathway, an evolutionarily conserved regulator of organ size whose role in synapse formation is currently unappreciated. Strip functions genetically with Enabled, an actin assembly/elongation factor and the presumptive downstream target of Hpo signaling, to modulate local actin organization at synaptic termini. This regulation occurs independently of the transcriptional co-activator Yorkie, the canonical downstream target of the Hpo pathway. This study identifies a previously unanticipated role of the Strip-Hippo pathway in synaptic development, linking cell signaling to actin organization (Sakuma, 2016).

    Since the Hippo (Hpo) pathway was discovered as the key regulator to ensure the appropriate final tissue size by coordinating cell proliferation and cell death, large-scale genetics studies have identified numerous regulators of the Hpo pathway. While most pathway components identified thus far are positive regulators of Hpo, some negative regulators were recently reported. One such negative regulator is the STRIPAK (STRiatin-Interacting Phosphatase And Kinase) complex, which is evolutionarily conserved and regulates various cellular processes including cell cycle control and cell polarity. The core component of the STRIPAK complex is the striatin family of proteins: striatins serve as B‴ subunits (one of the subfamily of regulatory B subunits) of the protein phosphatase 2A (PP2A) complex. Beyond this, the A and C subunits of PP2A, Mob3, Mst3, Mst4, Ysk1, Ccm3, Strip1, and Strip2 form the core mammalian STRIPAK complex. It has been previously reported that Strip, the Drosophila homolog of mammalian Strip1 and 2, is involved in early endosome formation, which is essential for axon elongation. Building on these findings, it was hypothesized that the Strip-Hpo pathway may also be involved in neuronal synaptic development (Sakuma, 2016).

    The Drosophila larval neuromuscular junction (NMJ) is an ideal model for studying synaptic development because of its identifiable, stereotyped morphology, accessibility, broad complement of available reagents, and suitability for a wide range of experimental approaches. Furthermore, the Drosophila NMJ, like vertebrate central synapses, is glutamatergic, suggesting that the molecular mechanisms that regulate synaptic development in Drosophila NMJ might be applicable to vertebrates. Motor neuron axons are genetically hardwired to target specific muscles by the end of the embryonic stage. There, axonal growth cones subsequently differentiate into presynaptic termini, called boutons, each of which contains multiple active zones). During the larval stage, muscle size increases nearly 100-fold and boutons are continuously and proportionately added to maintain constant innervation strength. Various molecules can negatively or positively regulate the growth of synaptic termini. Amongst the many factors, elements of the actin cytoskeleton are key effectors of morphological change, functioning downstream of several cell surface receptors and signaling pathways. Of the two types of actin filaments (branched and linear), the activity of Arp2/3 complex, responsible for nucleation of branched F-actin, the first step of actin polymerization, should be strictly regulated. Arp2/3 hyperactivation results in synaptic terminal overgrowth characterized by excess small boutons emanating from the main branch that are termed satellite boutons (Sakuma, 2016).

    This study shows that Strip negatively regulates the synapse terminal development through tuning the activity of the core Hpo kinase cassette. Loss or reduction of strip function in motor neurons increased the number of satellite boutons, which could be suppressed by reducing the genetic dosage of hpo. Similarly, activation of the core Hpo kinase cassette also increased satellite boutons. In this context, the presumptive downstream target of the core Hpo kinase cassette is Enabled (Ena), a regulator of F-actin assembly and elongation that was reported to antagonize the activity of Arp2/3. The canonical downstream transcriptional co-activator, Yorkie (Yki), appears dispensable for Hpo-mediated synaptic terminal development. It is proposed that the evolutionarily conserved Strip-Hpo pathway regulates local actin organization by modulating Ena activity during synaptic development (Sakuma, 2016).

    This study has identified Strip and components of the Hpo pathway as regulators of synaptic morphology. In addition to the intensely investigated function of Hpo in growth control in mitotic cells, a few postmitotic roles of the Hpo pathway have recently been uncovered, such as dendrite tiling in Drosophila sensory neurons and cell fate specification of photoreceptor cells in Drosophila retina. This study now finds an additional postmitotic role for Hpo in synaptic terminal development. The results indicate that Strip and the core Hpo kinase cassette regulate satellite bouton formation by regulating the activity of Ena, an actin regulator that is involved in the initiation, extension, and maintenance of linear actin filaments at the barbed end. Although it cannot be excluded that there might be other targets of Yki in motor neurons than diap1 or bantam whose transcriptional activations were not observed in this study, Yki, a well-known downstream target of the core Hpo kinase cassette, was dispensable for proper synaptic morphology. Ena phosphorylation causes its inactivation; therefore, it was reasoned that Strip can act as a positive regulator of Ena by inactivating the Hpo pathway. A model is proposed for the regulation of satellite bouton formation by Strip and Hpo pathway components (see The Strip-Hpo pathway regulates satellite bouton formation with Ena, a regulator of F-actin organization). As the presynaptic localization of endogenous Strip was punctate and non-uniform, it is expected that Strip localization could be critical for regulating the phosphorylation status of Hpo, Wts, and Ena, which locally alters actin organization and eventually specifies the position of satellite bouton formation that could be a marker for new bouton outgrowth. When Strip is present, the core Hpo kinase cassette is inactivated, which in turn locally increases the expression of the active (unphosphorylated) form of Ena. However, the core Hpo kinase cassette can be activated in the absence of Strip, which subsequently phosphorylates and inactivates Ena. Ena prevents Arp2/3-induced branching, suggesting that Ena inactivation activates Arp2/3 and results in satellite bouton formation, similar to Rac activation. It is reported that Arp2/3 is involved in bouton formation and axon terminal branching downstream of WAVE/SCAR complex in NMJ. Indeed, the cureent findings support this hypothesis. F-actin organization was altered by strip knockdown in motor neurons. When the GFP-moe reporter was expressed in motor neuron, the GFP fused to the C-terminal actin-binding domain of Moesin, which is widely used as an F-actin reporter. The intensity of actin puncta became higher and puncta were unevenly distributed when strip was knocked down. This data suggests that Strip function is important for the proper organization of F-actin (Sakuma, 2016).

    There are many indications that Strip and other STRIPAK components (Mst3, Mst4, and Ccm3) regulate the actin network. For example, Strip1, Strip2, Mst3, and Mst4 regulate the actomyosin contractions which regulate cell migration in cancer cells. In addition to regulating the actin network, STRIPAK has been suggested to function in microtubule organization. Mutants of Drosophila Mob4, a member of the core STRIPAK complex and homolog of mammalian Mob3, show abnormal microtubule morphology at NMJs and muscles. Furthermore, it has been reported that Strip forms a complex with Glued, the homolog of mammalian p150Glued, a component of the dynactin complex required for dynein motor-mediated retrograde transport along microtubules. Strip also affects microtubule stability. As previously mentioned, microtubules are also key effectors of synaptic development downstream of several receptors and signaling pathways. Taken together, the STRIPAK complex can act as a regulatory hub for multiple cellular signals including Hpo pathway-mediated actin organization, endocytic pathway-dependent BMP signals, and microtubule stability for proper synaptic development (Sakuma, 2016).

    The Hpo pathway has been reported to act as a sensor of the local cellular microenvironment, such as mechanical cues, apico-basal polarity and actin architecture to balance cell proliferation and cell death. Although synaptic morphogenesis is a postmitotic process, it is very plastic and depends on a dynamically changing extracellular environment, as exemplified by the nearly 100-fold expansion of muscle size during larval development. Thus, this study demonstrates an intriguing function for the Strip-Hippo pathway in the homeostatic control of neuronal synaptic morphology and function (Sakuma, 2016).

    The innate immune receptor PGRP-LC controls presynaptic homeostatic plasticity

    It is now appreciated that the brain is immunologically active. Highly conserved innate immune signaling responds to pathogen invasion and injury and promotes structural refinement of neural circuitry. However, it remains generally unknown whether innate immune signaling has a function during the day-to-day regulation of neural function in the absence of pathogens and irrespective of cellular damage or developmental change. This study shows that an innate immune receptor, a member of the peptidoglycan pattern recognition receptor family (PGRP-LC), is required for the induction and sustained expression of homeostatic synaptic plasticity. This receptor functions presynaptically, controlling the homeostatic modulation of the readily releasable pool of synaptic vesicles following inhibition of postsynaptic glutamate receptor function. Thus, PGRP-LC is a candidate receptor for retrograde, trans-synaptic signaling, a novel activity for innate immune signaling and the first known function of a PGRP-type receptor in the nervous system of any organism (Harris, 2015).

    The homeostatic modulation of presynaptic neurotransmitter release has been observed at mammalian central synapses and at neuromuscular synapses in species ranging from Drosophila to mouse and human. The homeostatic enhancement of presynaptic release following inhibition of postsynaptic glutamate receptors is achieved by an increase in presynaptic calcium influx through presynaptic CaV2.1 calcium channels and the simultaneous expansion of the readily releasable pool (RRP) of synaptic vesicles. To date, the retrograde, trans-synaptic signaling system that initiates these presynaptic changes following disruption of postsynaptic glutamate receptors remains largely unknown (Harris, 2015).

    A large-scale forward genetic screen for homeostatic plasticity genes, using synaptic electrophysiology at the neuromuscular junction (NMJ) as the primary assay, identified mutations in the PGRP-LC locus that block presynaptic homeostasis. In Drosophila, PGRP-LC is the primary receptor that initiates an innate immune response through the immune deficiency (IMD) pathway. In mammals, there are four PGRPs including a 'long' isoform (PGRP-L or PGlyRP2) that appears to have both secreted and membrane-associated activity. In mice, the PGlyRP2 protein has two functions in the innate immune response: a well-documented extracellular enzymatic (amidase) activity and a pro-inflammatory signaling function that is independent of its extracellular enzymatic activity. As yet, there are no known functions for PGRPs in the nervous system of any organism (Harris, 2015).

    Innate immune signaling has been found to participate in neural development and disease including a role for the C1q component of the complement cascade, Toll-like receptor signaling, and tumor necrosis factor signaling. In these examples, however, the innate immune response is induced within microglia or astrocytes. Far less clear is the role of innate immune signaling within neurons, either centrally or peripherally, although there are clear examples of downstream signaling components such as Rel and NFκB having important functions during learning related neural plasticity. This study places the innate immune receptor, PGRP-LC at the presynaptic terminal of Drosophila motoneurons and demonstrate that this receptor is essential for robust presynaptic homeostatic plasticity. It is speculated, based on the current data, that PGRP-LC could function as a receptor for the long-sought retrograde signal that mediates homeostatic signaling from muscle to nerve (Harris, 2015).

    A distinguishing feature of presynaptic homeostatic plasticity is that it can be both rapidly induced and sustained for prolonged periods. It is possible that innate immune signaling activity is adjusted to fit the requirements of presynaptic homeostasis in motoneurons. This would be consistent with the established diversity of innate immune signaling functions during development such as dorso-ventral patterning. Alternatively, the induction of innate immune signaling might serve a unique function during presynaptic homeostasis. Many homeostatic signaling systems incorporate feed-forward signaling elements. By analogy, PGRP-LCx could function as a feed-forward signaling receptor that acts more like a 'switch' to enable presynaptic homeostasis in the nerve terminal. The accuracy of the homeostatic response would be determined by other signaling elements. In favor of this idea, the induction of innate immune signaling is rapid, occurring in seconds to minutes, and can be maintained for the duration of an inducing stimulus, consistent with recent evidence that presynaptic homeostasis is rapidly and continually induced at synapses in the presence of a persistent postsynaptic perturbation. Finally, it remains formally possible that PGRP-dependent signaling is a permissive signal that allows expression presynaptic homeostasis (Harris, 2015).

    An auxiliary subunit of the presynaptic calcium channel, α2δ-3, is required for rapid transsynaptic homeostatic signaling

    The homeostatic modulation of neurotransmitter release, termed presynaptic homeostatic potentiation (PHP), is a fundamental type of neuromodulation, conserved from Drosophila to humans, that stabilizes information transfer at synaptic connections throughout the nervous system. This study demonstrates that α2δ-3 (straitjacket), an auxiliary subunit of the presynaptic calcium channel, Cacophony, is required for PHP. The α2δ gene family has been linked to chronic pain, epilepsy, autism, and the action of two psychiatric drugs: gabapentin and pregabalin. Loss of α2δ-3 blocks both the rapid induction and sustained expression of PHP due to a failure to potentiate presynaptic calcium influx and the RIM-dependent readily releasable vesicle pool. These deficits are independent of α2δ-3-mediated regulation of baseline calcium influx and presynaptic action potential waveform. α2δ proteins reside at the extracellular face of presynaptic release sites throughout the nervous system, a site ideal for mediating rapid, transsynaptic homeostatic signaling in health and disease (Wang, 2016).

    Presynaptic homeostatic potentiation (PHP) can be initiated by disruption of postsynaptic neurotransmitter receptors and is expressed as a change in presynaptic vesicle release. As such, PHP requires retrograde, transsynaptic signaling. The homeostatic potentiation of presynaptic release is mediated by increased presynaptic calcium influx without a change in the presynaptic action potential waveform. A remarkable property of presynaptic homeostatic plasticity is that it can be induced in a time frame of seconds to minutes and can be stably maintained throughout the life of an organism -- months in Drosophila and decades in humans. Equally remarkable, presynaptic homeostasis can precisely offset the magnitude of postsynaptic perturbations that vary widely in severity. This implies the existence of profoundly stable and remarkably precise homeostatic modifications to the presynaptic release apparatus. Transsynaptic signaling systems that are capable of achieving the rapid, accurate, and persistent control of presynaptic vesicle release are generally unknown (Wang, 2016).

    In a large-scale forward genetic screen for homeostatic plasticity genes, mutations were identified in the α2δ-3 auxiliary subunit of the CaV2.1 calcium channel. α2δ genes encode a family of proteins that are post-translationally processed into a large glycosylated extracellular α2 domain that is linked through disulfide bonding to a short, membrane-associated δ domain. Existing loss-of-function data are consistent with the primary function of α2δ being the trafficking and synaptic stabilization of pore-forming α1 calcium channel subunits, with which they associate in the ER. There is also evidence that α2δ subunits control calcium channel kinetics in a channel-type- and cell-type-specific manner. However, the function of the α2δ gene family extends beyond calcium channel trafficking and membrane stabilization, including activities related to synapse formation and stability. As such, the large, glycosylated extracellular domain in α2δ may have additional, potent signaling activities at the active zone (Wang, 2016).

    Importantly, the α2δ gene family is associated with a wide range of neurological diseases, including autism spectrum disorders (ASDs), neuropathic pain, and epilepsy. The α2δ-1 and α2δ-2 proteins are the primary targets of gabapentin and pregabalin, two major drugs used to treat neuropathic pain and epilepsy. This study demonstrates that α2δ-3 is essential for PHP. Thus, while α2δ-3 is an extracellular component of the extended presynaptic calcium channel complex (Davies, 2010), it nonetheless has a profound ability to modulate the intracellular neurotransmitter release mechanism. It is proposed that α2δ-3 relays signaling information from the synaptic cleft to the cytoplasmic face of the presynaptic active zone during PHP, an activity that could reasonably be related to the function of α2δ-3 during neurological disease (Wang, 2016).

    This study demonstrates that α2δ-3 is essential for the rapid induction and sustained expression of presynaptic homeostatic potentiation (PHP). α2δ-3 encodes a glycosylated extracellular protein known to interact with matrix proteins that reside within the synaptic cleft. As such, it is proposed that α2δ-3 mediates homeostatic, retrograde signaling by connecting signaling within synaptic cleft to effector proteins within the presynaptic terminal, such as RIM. Since α2δ-3 associates with the pore-forming α1 subunit of calcium channels, it is ideally positioned to relay signaling to the site of high-release probability vesicle fusion adjacent to the presynaptic calcium channels (Wang, 2016).

    It was previously demonstrated that PHP requires not only potentiation of presynaptic calcium influx but also a parallel homeostatic expansion of the readily releasable pool (RRP). Several lines of evidence argue against the possibility that the homeostatic potentiation of presynaptic calcium influx fully accounts for the observed potentiation of the RRP. First, it is well established in mammalian systems and the Drosophila NMJ (Müller, 2015) that the calcium-dependence of the RRP is sub-linear. Therefore, the relatively small change in presynaptic calcium influx that occurs during PHP (12%-25%) would not be sufficient to account for the observed doubling of the RRP, an effect that has been quantified across a wide range of extracellular calcium (0.3-15 mM [Ca2+]e) (Müller, 2015). Second, the homeostatic increase of presynaptic calcium influx and the homeostatic expansion of RRP are genetically separable processes (Harris, 2015). Since loss of α2δ-3 completely blocks the homeostatic expansion of the RRP, it appears that α2δ-3 has an additional activity that is directed at the homeostatic modulation of the RRP (Wang, 2016).

    Collectively, these data argue that α2δ-3 functions with Rab3 interacting molecule (rim), either directly or indirectly, to achieve a homeostatic potentiation of the RRP. First, the loss of function phenotype of α2δ-3 is strikingly similar to that observed in rim mutants. Both mutations cause a deficit in presynaptic release that is associated with diminished baseline presynaptic calcium influx, diminished size of the baseline RRP, and enhanced sensitivity to application of EGTA-AM. Second, this study demonstrates a strong trans-heterozygous interaction between rim/+ and α2δ-3/+, suggesting that both genes function to control the same presynaptic processes during PHP. Since the rim mutation selectively disrupts the homeostatic modulation of the RRP, this genetic interaction could reflect a failure to homeostatically modulate the RRP (Wang, 2016).

    Both RIM and α2δ-3 bind the pore-forming α1 subunit of the CaV2.1 calcium channel. As such, signaling could be relayed from α2δ-3 to RIM through molecular interactions within the extended CaV2.1 calcium channel complex. However, not all evidence is consistent with this possibility. For example, RNAi-mediated depletion of CaV2.1 channels, sufficient to decrease release by ∼80%, does not prevent presynaptic homeostasis. Thus, loss of α2δ-3 blocks PHP, whereas loss of the CaV2.1 α1 subunit does not. In addition, the double-heterozygous mutant of rim/+ and α2δ-3/+ blocks PHP but does not disrupt baseline vesicle release, arguing that this genetic interaction is not due to a decrease in the number or organization of presynaptic α1 calcium channel subunits. Thus, it is speculated that α2δ-3 conveys signaling through a co-receptor on the plasma membrane to participate in the homeostatic modulation of the RRP. There are very few extracellular proteins known to establish baseline levels of primed, fusion-competent synaptic vesicles. Since α2δ proteins should reside at chemical synapses throughout the nervous system, this signaling could reasonably be related to the neurological and psychiatric diseases associated with α2δ genes (Wang, 2016).

    Presynaptic CamKII regulates activity-dependent axon terminal growth

    Spaced synaptic depolarization induces rapid axon terminal growth and the formation of new synaptic boutons at the Drosophila larval neuromuscular junction (NMJ). This study identified a novel presynaptic function for the Calcium/Calmodulin-dependent Kinase II (CamKII) protein in the control of activity-dependent synaptic growth. Consistent with this function, both total and phosphorylated CamKII (p-CamKII) are were found to be enriched in axon terminals. Interestingly, p-CamKII appears to be enriched at the presynaptic axon terminal membrane. Moreover, levels of total CamKII protein within presynaptic boutons globally increase within one hour following stimulation. These effects correlate with the activity-dependent formation of new presynaptic boutons. The increase in presynaptic CamKII levels is inhibited by treatment with cyclohexamide suggesting a protein-synthesis dependent mechanism. Previous work has found that acute spaced stimulation rapidly downregulates levels of neuronal microRNAs (miRNAs) that are required for the control of activity-dependent axon terminal growth at this synapse. The rapid activity-dependent accumulation of CamKII protein within axon terminals is inhibited by overexpression of activity-regulated miR-289 in motor neurons. Experiments in vitro using a CamKII translational reporter show that miR-289 can directly repress the translation of CamKII via a sequence motif found within the CamKII 3' untranslated region (UTR). Collectively, these studies support the idea that presynaptic CamKII acts downstream of synaptic stimulation and the miRNA pathway to control rapid activity-dependent changes in synapse structure (Nesler, 2016).

    Acute spaced synaptic depolarization rapidly induces the formation of new synaptic boutons at the larval NMJ. These immature presynaptic outgrowths, also known as "ghost boutons", are characterized by the presence of synaptic vesicles but by a lack of active zones and postsynaptic specializations. IA wild-type third instar larval NMJ will typically have about 2 ghost boutons. Using an established synaptic growth protocol, a robust increase in the number of ghost boutons was observed following 5 x K+ spaced stimulation. It has been shown that activity-dependent ghost bouton formation involves both new gene transcription and protein synthesis. Furthermore, new presynaptic expansions can form within 30 min of stimulation even after the axon innervating the NMJ has been severed. These findings suggest that a local mechanism (i.e. local signaling and/or translation) is required for the budding and outgrowth of new axon terminals. As expected, application of the translational inhibitor cyclohexamide during the recovery phase prevented the formation of new ghost boutons (Nesler, 2016).

    It has been shown previously that the outgrowth of new synaptic boutons in response to spaced depolarization requires the function of activity-regulated neuronal miRNAs including miR-8, miR-289, and miR-958 (Nesler, 2013). This implies that mRNAs encoding for synaptic proteins might be targets for regulation by these miRNAs. Focused was placed on CamKII for three reasons. (1) CamKII has been shown to have a conserved role in the control of long-term synaptic plasticity and its expression at synapses requires components of the miRNA pathway. Furthermore, the fly CamKII mRNA contains two predicted binding sites for activity-regulated miR-289. (2) CamKII and PKA both phosphorylate and actives synapsin. At the fly NMJ, a synapsin-dependent mechanism is required for a transient increase in neurotransmitter release in response to tetanic stimulation. Synapsin also redistributes to sites of activity-dependent axon terminal growth and regulates outgrowth via a PKA-dependent pathway. (3) Presynaptic CamKII has been shown to function in axon pathfinding in cultured Xenopus neurons. It seemed likely that activity-dependent ghost bouton formation and axon guidance might share similar molecular machinery (Nesler, 2016).

    It was postulated that presynaptic CamKII was required to control activity-dependent axon terminal growth at the larval NMJ. To address this question, CamKII expression was disrupted in motor neurons using two transgenic RNAi constructs. Depletion of presynaptic CamKII with both transgenes prevented the formation of new ghost boutons in response to spaced stimulation. Thus, presynaptic CamKII is necessary to control the formation of new synaptic boutons (Nesler, 2016).

    To further confirm that presynaptic CamKII function was required for activity-dependent growth, a transgenic line was used that inducibly expressed an inhibitory peptide (UAS-CamKIIAla). As in mammals, the activation of Drosophila CamKII by exposure to calcium leads to the autophosphorylation of a conserved threonine residue within the autoinhibitory domain (T287 in Drosophila). Activation of CamKII then confers an independence to calcium levels that persists until threonine-287 is dephosphorylated. The synthetic Ala peptide mimics the autoinhibitory domain and its transgenic expression is sufficient to substantially inhibit endogenous CamKII activity. Expression of the Ala inhibitory peptide in larval motor neurons disrupted the formation of new ghost boutons following spaced synaptic depolarization. These observations are consistent with results from CamKII RNAi (Nesler, 2016).

    Together, these data suggest that presynaptic CamKII function is required to control new ghost bouton formation in response to acute synaptic activity. Similarly, presynaptic CamKII has been implicated in controlling both bouton number and morphology during development of the larval NMJ. Reducing neuronal CamKII levels by RNAi has recently been shown to significantly reduce the number of type 1b boutons at the larval NMJ suggesting that presynaptic CamKII is required to control normal synapse development (Gillespie, 2013). In contrast, presynaptic expression of the Ala inhibitory peptide has no effect on the total number of type 1 synaptic boutons. Given that the Ala peptide does not completely inhibit CamKII autophosphorylation, it is suggested that the activation of CamKII in response to acute spaced synaptic depolarization is likely to be more sensitive to disruption then during NMJ development (Nesler, 2016).

    It was next asked if presynaptic CamKII could induce activity-dependent axon terminal growth at the NMJ. The overexpression of genes that are necessary for the control of ghost bouton formation generally does not cause an increase in the overall number of new synaptic boutons following 5 x high K+ spaced. Instead, overexpression often leads to an increased sensitization of the synapse to subsequent stimuli (for example, significant growth is observed after 3 x instead of 5 x high K+). The overexpression of a wild-type CamKII transgene in motor neurons caused an increase of 71% in ghost bouton numbers in 3 x K+ spaced stimulation larvae compared to 0 x K+ controls. While this is trending towards an increase, it did not reach statistical significance, even though expression levels were substantially higher than endogenous CamKII. Thus, increased CamKII is not sufficient to stimulate activity-dependent axon terminal growth (Nesler, 2016).

    To further investigate the role of presynaptic CamKII in activity-dependent axon terminal growth, the effect of transgenic neuronal overexpression of either an overactive form of CamKII (CamKIIT287D) or a form that is incapable of remaining active in the absence of elevated calcium (CamKIIT287A). Much like C380-Gal4/+ controls, presynaptic expression of either transgene had no significant effect on the number of ghost boutons in 3 x high K+ stimulation. Again, levels of CamKII protein in axon terminals in both transgenic lines were elevated relative to controls. Collectively, these data suggest that constitutive activation of CamKII is not sufficient to sensitize the NMJ to stimulation (Nesler, 2016).

    The results suggest that the temporal and/or spatial regulation of CamKII expression or activation is likely required to control activity-dependent growth. In support, the Drosophila CamKII protein has been shown to phosphorylate and regulate the activity of the Ether-a-go-go (Eag) potassium channel in motor neuron axon terminals. In turn, CamKII is bound and locally activated by phosphorylated Eag. This local activation can persist even after calcium levels have been reduced. CamKII autophosphorylation and Eag localization to synapses requires the activity of the membrane-associated Calcium/Calmodulin-associated Serine Kinase, CASK. The presynaptic coexpression of CASK with CamKIIT287D reverses (to wild-type levels) the increase in type 1b boutons observed when CamKIIT287D is overexpressed alone. Thus, a mechanism exists at the larval NMJ that allows for the persistence of local CamKII activation in the absence of additional stimul (Nesler, 2016).

    After establishing that CamKII has a novel presynaptic function in activity-dependent ghost bouton formation, the distribution of CamKII protein at the larval NMJ was examined closely. It has previously been reported that CamKII strongly colocalizes with postsynaptic Discs large (DLG), the Drosophila ortholog of mammalian PSD-95, around the borders of type 1 synaptic boutons. In support, an anti-CamKII antibody coimmunoprecipitates DLG from larval body wall extracts. Interestingly, while DLG is pre-dominantly postsynaptic at the developing NMJ it is also initially expressed in the presynaptic cell and at least partially overlaps with presynaptic membrane markers in axon terminals. It has been demonstrated that while fly CamKII colocalizes with DLG within dendrites of adult olfactory projection neurons (PNs), it also localizes to presynaptic boutons within those same neurons. Consistent with the latter observations (using a different CamKII antibody), it has been shown that CamKII is substantially enriched in presynaptic terminals of type 1b boutons. To resolve these inconsistent results, both antibodies against CamKII were used to more closely analyze the localization of CamKII at the third instar larval NMJ. First, double labeling of wild-type NMJs with a monoclonal CamKII antibody and anti-horseradish peroxidase (HRP), a marker for Drosophila neurons, confirmed that CamKII was enriched in presynaptic boutons in a pattern very similar to that of HRP. A closer examination of confocal optical sections revealed that almost all CamKII localized to the presynaptic terminal and was not significantly enriched either (1) at sites surrounding presynaptic boutons, or (2) in the axons innervating synaptic arbors (Nesler, 2016).

    Within boutons, CamKII appeared to be predominantly cytoplasmic but was sometimes localized to discrete puncta that were reminiscent of antibody staining for active zones. Prolonged depolarization of hippocampal neurons with K+ leads to mobilization of CamKII from the cytoplasm to sites near active zones. Moreover, using a fluorescent reporter for CamKII activity, high frequency stimulation causes the very rapid (on the order of minutes) activation of presynaptic CamKII and promotes its translocation from the cytoplasm to sites near active zones. To address this possibility, larval NMJs were double labeled with antibodies targeting both CamKII and DVGLUT, the Drosophila vesicular glutamate transporter, in order to visualize active zones. As predicted, it was found that some presynaptic CamKII colocalized with DVGLUT in type 1b and 1s boutons. Thus, in some type 1 synaptic boutons, CamKII protein is enriched in or near active zones (Nesler, 2016).

    To confirm that CamKII was enriched in presynaptic boutons, wild-type NMJs were double labelled with a polyclonal CamKII antibody and anti-DLG. CamKII did partially colocalize with DLG at the border of type 1 synaptic boutons. However, in this study, CamKII was primarily localized to the presynaptic side of the synapse. Collectively, this study provides strong evidence that CamKII is expressed on both the pre- and postsynaptic side of the synapse but that it is clearly enriched within presynaptic boutons at the larval NMJ. This localization is analogous to CamKII distribution in mammalian axons (Nesler, 2016).

    After demonstrating that total CamKII was enriched in presynaptic axon terminals, it was next asked if any of this protein was active by assessing phosphorylation of threonine-287 using a phospho-specific polyclonal antibody. It was found that pT287 CamKII staining intensity was strong and fairly uniform in presynaptic boutons and weakly stained axons innervating synaptic arbor. Closer examination of confocal optical sections revealed that almost all p-CamKII colocalized with HRP in the presynaptic terminal and only sparsely stained the body wall muscle (Fig. 3A′). Presynaptic CamKII RNAi almost completely disrupted p-CamKII in axon terminals leaving some residual staining in the presynaptic bouton and surrounding muscle suggesting that the antibody is specific. To further demonstrate this presynaptic localization, it was found that p-CamKII staining clearly does not overlap with postsynaptic DLG but does colocalize strongly with immunostaining using the monoclonal total CamKII antibody (Nesler, 2016).

    Collectively, three different antibodies were used to show that CamKII enriched in presynaptic axon terminals. Next, it was asked as to how this enrichment was occurring. In Drosophila and mammalian neurons, the CamKII mRNA is transported to dendritic compartments and locally translated in response to synaptic stimulation. This spatial and temporal regulation requires sequence motifs found within the 5' and 3' UTRs of the CamKII transcript. In contrast, the localization of CamKII to axon terminals of Drosophila PNs does not strictly require the CamKII 3'UTR suggesting that enrichment in presynaptic boutons occurs through a mechanism that does not strictly require local translation. In mammalian neurons, CamKII is enriched in axon terminals where it can associate with synaptic vesicles and synapsin I. Recently, it has been shown that mammalian CamKII and the synapsin proteins are both conveyed to distal axons at rates consistent with slow axonal transport, with a small fraction of synapsin cotransported with vesicles via fast transport (Nesler, 2016).

    Because activity-dependent growth at the larval NMJ requires the miRNA pathway and new protein synthesis, it was asked if the localization of CamKII protein to axon terminals might require the CamKII 3'UTR. As expected, when expression was specifically driven in larval motor neurons, a transgenic CamKII:EYFP fusion protein regulated by the CamKII 3'UTR localized strongly to presynaptic boutons at the larval NMJ. However, very similar results were observed using the same CamKII:EYFP fusion protein regulated by a heterologous 3'UTR. Taken together, these data suggest that localization of CamKII protein to presynaptic boutons at the NMJ does not require mRNA transport and local translation. Thus, it is concluded that most of the Drosophila CamKII protein found in motoneuron axon terminals is likely there due to the transport of cytosolic CamKII from the cell body to synapses via a mechanism involving axonal transport. (Nesler, 2016).

    It was of interest to determining how CamKII might be regulating activity-dependent axon terminal growth, and it was speculated that either the levels or distribution of CamKII protein might be altered in response to spaced depolarization. It first asked if high K+ stimulation resulted in an increase in CamKII protein within motoneuron axon terminals. Larval preparations were stimulated, and changes in the levels of CamKII protein in presynaptic boutons was examined by immunohistochemistry and quantitative confocal microscopy. Following spaced stimulation, CamKII staining within boutons rapidly increased (in ~ 1 h) by an average of 26%. This increase in immunofluorescence was global and did not appear to be localized to particular regions of the NMJ (i.e., near obvious presynaptic outgrowths). CamKII has been reported to very rapidly translocate to regions near active zones in response to high frequency stimulation. However, when compared to DVGLUT levels in unstimulated and stimulated larvae, no significant increase in CamKII immunofluorescence was observed indicating that translocation does not occur or does not persist in the current assay. To determine if this increase in CamKII enrichment required new protein synthesis, larval preparations were incubated with the translational inhibitor cyclohexamide during the recovery phase. Surprisingly, this treatment completely blocked the activity-dependent affects on presynaptic CamKII enrichment within axon terminals. Thus, spaced high K+ stimulation results in a rapid increase in CamKII levels in presynaptic boutons via some mechanism that requires activity-dependent protein synthesis (Nesler, 2016).

    Next, it was asked if the levels or distribution of p-CamKII changed in response to spaced stimulation. Larval preparations were stimulated exactly as described above and analyzed by confocal microscopy. Interestingly, p-CamKII staining was enriched at the presynaptic membrane of many axon terminals following spaced depolarization (Nesler, 2016).

    Given the requirement for new protein synthesis, it was speculated that the additional CamKII protein in axon terminals could be derived from a pool of CamKII mRNA that is rapidly transcribed and translated in the soma in response to spaced depolarization. This newly translated CamKII would then be actively transported out to axon terminals via standard mechanisms. If this were true, it would be expected that elevated CamKII levels could be detected in the larval ventral ganglion. To examine this process more closely, global total CamKII expression levels within the larval ventral ganglion were assayed by Western blot analysis. It was found that two distinct isoforms of CamKII are expressed in explanted larval ventral ganglia, Surprisingly, no increase was observed in CamKII protein levels in the ventral ganglion (Nesler, 2016).

    What is the source of this new presynaptic CamKII protein? Three possible explanations are proposed. First, new CamKII protein might be transcribed and translated in the motor neuron cell body. However, this new protein would be rapidly transported away to axon terminals in response to spaced depolarization. Second, some CamKII protein is found in the axons innervating the NMJ (seen using the p-CamKII antibody). It is possible that activity stimulates the rapid transport of an existing pool of CamKII protein from distal axons into axon terminals. This process would be sensitive to translational inhibitors. Finally, a pool of CamKII mRNA might be actively transported into axon terminals and then locally translated in response to spaced depolarization. This would account for the both the dependence on translation and for increased CamKII enrichment in presynaptic boutons (Nesler, 2016).

    Thus far, this study has shown that activity-dependent ghost bouton formation correlates with a protein synthesis-dependent increase in CamKII levels within presynaptic boutons at the larval NMJ. The activity-dependent translation of CamKII in olfactory neuron dendrites in the adult Drosophila brain requires components of the miRNA pathway. Within the CamKII 3'UTR, there are two putative binding sites for activity-regulated miR-289. These two binding sites were of particular interest. It was previously shown that levels of mature miR-289 are rapidly downregulated in the larval brain in response to 5 x high K+ spaced training (Nesler, 2013). Moreover, presynaptic overexpression of miR-289 significantly inhibits activity-dependent ghost bouton formation at the larval NMJ (Nesler, 2013). Based on these data, it was speculated that CamKII might be a target for regulation by miR-289 (Nesler, 2016).

    To determine if CamKII is a target for repression by miR-289 in vivo, a transgenic construct containing the primary miR-289 transcript was overexpressed in motor neurons and CamKII enrichment was examined by anti-CamKII immunostaining and quantitative confocal microscopy. Relative to controls, the presynaptic overexpression of miR-289 completely abolished the observed activity-dependent increase in CamKII immunofluorescence. When analyzing global CamKII levels within axon terminals during NMJ development, presynaptic miR-289 expression led to a slight decrease in CamKII immunofluorescence. This trend is similar to results observed following treatment with cyclohexamide during the recover period. The lack of full repression by miR-289 is not surprising given that one miRNA alone is often not sufficient to completely repress target gene expression (Nesler, 2016).

    To directly test the ability of miR-289 to repress translation of CamKII, a reporter was developed where the coding sequence for firefly luciferase (FLuc) was fused to the regulatory CamKII 3'UTR (FLuc-CamKII 3'UTR). When this wild-type reporter was coexpressed with miR-289 in Drosophila S2 cells, expression of FLuc was significantly reduced. In contrast, when this reporter was coexpressed with miR-279a, a miRNA not predicted to bind to the CamKII 3'UTR, no repression was observed. To confirm that repression of the FLuc-CamKII reporter by miR-289 was via a specific interaction, the second of two predicted miR-289 binding sites was mutagenized. Binding site 2 (BS2) was a stronger candidate for regulation because it is flanked by AU-rich elements (AREs) and miR-289 has been shown to promote ARE-mediated mRNA instability through these sequences. Moreover, it is well established that the stabilization and destabilization of neuronal mRNAs via interactions between AREs and ARE-binding factors plays a significant role in the establishment and maintenance of long-term synaptic plasticity in both vertebrates and invertebrates. Altering three nucleotides within BS2 in the required seed region binding site was sufficient to significantly disrupt repression of the reporter by miR-289. The minimal predicted BS2 sequence was cloned into an unrelated 3'UTR and it was asked if miR-289 could repress translation. Coexpression of the FLuc-SV-mBS2 reporter with miR-289 led to significant repression. Taken together, these results indicate that the BS2 sequence is both necessary and sufficient for miR-289 regulation via the CamKII 3′UTR (Nesler, 2016).

    The most important conclusion of this study is that presynaptic CamKII is required to control activity-dependent axon terminal growth at the Drosophila larval NMJ. First, it was shown that CamKII is necessary to control ghost bouton formation in response to spaced synaptic depolarization. Next, it was demonstrated that spaced stimulation correlates with a rapid protein synthesis dependent increase in CamKII immunofluorescence in presynaptic boutons. This increase is suppressed by presynaptic overexpression of activity-regulated miR-289. Previous work has shown that overexpression of miR-289 in larval motor neurons can suppress activity-dependent axon terminal growth (Nesler, 2013). This study demonstrated that miR-289 can repress the translation of a FLuc-CamKII 3'UTR reporter via a specific interaction with a binding site within the CamKII 3'UTR ( Fig. 6C-E). Collectively, this experimental evidence suggests that CamKII functions downstream of the miRNA pathway to control activity-dependent changes in synapse structure. (Nesler, 2016).

    Thus, CamKII protein is expressed in the right place to regulate rapid events that are occurring within presynaptic boutons. Several questions remain regarding CamKII function in the control of activity-dependent axon terminal growth. First, it is unclear what the significance might be of a rapid increase of total CamKII in presynaptic terminals. Why is the pool of CamKII protein that is already present not sufficient to control these processes? Similar questions have been asked regarding activity-dependent processes occurring within dendrites. It is postulated that the CamKII mRNA might be locally translated in axon terminals. It has been proposed that local mRNA translation might be (1) required for efficient targeting of some synaptic proteins to specific sites, or (2) local translation may in and of itself be required to control activity-dependent processes at the synapse. Second, the impact of spaced depolarization on CamKII function needs to be assessed and downstream targets of CamKII phosphorylation involved in these processes need to be identified. One very strong candidate is synapsin which, at the Drosophila NMJ, has been shown to rapidly redistribute to sites of new ghost bouton outgrowth in response to spaced stimulation. Finally, the idea that CamKII might work through a Eag/CASK-dependent mechanism to control activity-dependent axon terminal growth needs to be examined (Nesler, 2016).

    The equilibrium between antagonistic signaling pathways determines the number of synapses in Drosophila

    Using the Drosophila larval neuromuscular junction, this study shows a PI3K-dependent pathway for synaptogenesis which is functionally connected with other previously known elements including the Wit receptor, its ligand Gbb, and the MAPkinases cascade. Based on epistasis assays, the functional hierarchy within the pathway was determined. Wit seems to trigger signaling through PI3K, and Ras85D also contributes to the initiation of synaptogenesis. However, contrary to other signaling pathways, PI3K does not require Ras85D binding in the context of synaptogenesis. In addition to the MAPK cascade, Bsk/JNK undergoes regulation by Puc and Ras85D which results in a narrow range of activity of this kinase to determine normalcy of synapse number. The transcriptional readout of the synaptogenesis pathway involves the Fos/Jun complex and the repressor Cic. In addition, an antagonistic pathway was identified that uses the transcription factors Mad and Medea and the microRNA bantam to down-regulate key elements of the pro-synaptogenesis pathway. Like its counterpart, the anti-synaptogenesis signaling uses small GTPases and MAPKs including Ras64B, Ras-like-a, p38a and Licorne. Bantam downregulates the pro-synaptogenesis factors PI3K, Hiw, Ras85D and Bsk, but not AKT. AKT, however, can suppress Mad which, in conjunction with the reported suppression of Mad by Hiw, closes the mutual regulation between both pathways. Thus, the number of synapses seems to result from the balanced output from these two pathways (Jordan-Alvarez, 2017).

    The epistasis assays have determined the in vivo functional links between PI3K and other previously known pro-synaptogenesis factors. Epistasis assays are based on the combined expression of two or more UAS constructs. Several double combinations in this study have produced a phenotype in spite of the apparent ineffectiveness of the single constructs. This type of results underscores the necessity to use epistasis assays in order to reveal functional interactions in vivo, hence, biologically relevant. In addition to the pro-synaptogenesis signaling, the study has revealed an anti-synaptogenesis pathway that composes a signaling equilibrium to determine the actual number of synapses. The magnitude of the synapse number changes elicited by the factors tested here are mostly within the range of 20%-50%. Are these values significant to cause behavioral changes? Reductions in the order of 30% of excitatory or inhibitory synapses in adult Drosophila local olfactory interneurons transform perception of certain odorants from attraction to repulsion and vice versa. In schizophrenia patients, a 16% loss of inhibitory synapses in the brain cortex has been reported. In Rhesus monkeys, the pyramidal neurons in layer III of area 46 in dorsolateral prefrontal cortex show a 33% spine loss, and a significant reduction in learning task performance during normal aging. Thus, it seems that behavior is rather sensitive to small changes in synapse number irrespective of the total brain mass (Jordan-Alvarez, 2017).

    The signaling interactions analyzed here were chosen because they were reported in other cellular systems and species previously. Some of these interactions have been confirmed (e.g., Gbb/Wit), while others have proven ineffective in the context of synaptogenesis (e.g., Ras85D/PI3K binding). Likely, the two signaling pathways, pro- and anti-synaptogenesis, are not the only ones relevant for synapse formation. For example, in spite of the null condition of the gbb and wit mutant alleles used here, the resulting synaptic phenotypes are far less extreme than expected if these two factors would be the only source of signaling for synaptogenesis. Although it could be argued that the incomplete absence of synapses in the mutant phenotypes could result from maternal perdurance, Wit is not part of the oocyte endowment while Gbb is. Three alternative possibilities may be considered, additional ligands for Wit, additional receptors for Gbb, and a combination of the previous two. Beyond the identity of these putative additional ligands and receptors, the stoichiometry between ligands and receptors may certainly be relevant. Actually, Gbb levels are titrated by Crimpy. An equivalent quantitative regulation could operate on Wit. The reported data on Wit illustrate already the diversity of the functional repertoire of this receptor. Wit can form heteromeric complexes with Thick veins (Tkv) or Saxophone (Sax) receptors to receive Dpp/BMP4 or Gbb/BMP7 as ligands. However, the same study also showed that Wit could dimerize with another receptor, Baboon, upon binding of Myoglianin to activate a different and antagonistic signaling pathway, TGFβ/activin-like (Jordan-Alvarez, 2017).

    The Gbb/Wit/PI3K signaling analyzed in this study is likely not the only pro-synaptogenesis pathway in flies and vertebrates. The ligand Wingless (Wg), member of the Wnt family, and the receptors Frizzled have been widely documented as relevant in neuromuscular junction development, albeit data on synapse number are scant. Interestingly, however, the downstream intermediaries can be as diverse as those mentioned above for Wit. Although generally depicted as linear pathways, a more realistic image would be a network of cross-interacting signaling events whose in situ regulation and cellular compartmentalization remains fully unexplored (Jordan-Alvarez, 2017).

    The quantitative regulation of receptors is most relevant to understand their biological effects. In that context, is worth noting that Tkv levels are distinctly regulated from those of Wit and Sax through ubiquitination in the context of neurite growth. On the other hand, although the receptor Wit is considered a RSTK type, the functional link with PI3K is a feature usually associated to the RTK type instead. The link of Wit with a kinase has a precedent with LIMK1 that binds to, and is functionally downstream from, Wit in the context of synapse stabilization. Thus, Wit should be considered a wide spectrum receptor in terms of its ligands, co-receptor partners and, consequently, signaling pathways elicited. Actually, the Wit amino acid sequence shows both, Tyr and Ser/Thr motifs justifying its initial classification as a 'dual' type of receptor. In this report this study did not determine if Wit heterodimerizes with other receptors, as canonical RSTKs do, or if it forms homodimers, as canonical RTKs do. However, the lack of synaptogenesis effects by the putative co-receptors, Tkv and Sax, and the phenotypic similarity with the manipulation of the standard RTK signaling effector Cic, leaves open the possibility that Wit could play RTK-like functions, at least in the context of synaptogenesis (Jordan-Alvarez, 2017).

    Consistent with the proposal of a dual mechanism for Wit, its activation seems to be a requirement to elicit two independent signaling steps, PI3K and Ras85D, that could reflect RTK and RSTK mechanisms, respectively. Both steps are independent because the mutated form of PI3K unable to bind Ras85D, PI3KΔRBD, is as effective as the normal PI3K to elicit synaptogenesis. PI3K and Ras85D signaling, however, seem to converge on Bsk revealing a novel feature of this crossroad point. The activity level of Bsk is known to be critical in many signaling processes. The peculiarities of Bsk/JNK activity include its coordinated regulation by p38a and Slpr in the context of stress heat response without interference on the developmental context. Another modulator, Puc, was described as a negative feed-back loop in the context of oxidative stress. The Puc mediated loop is operative also for synaptogenesis, while that of p38a/Slpr is relevant for p38a only, as shown here. Further, Ras85D represents an additional regulator in the neural scenario. The triple regulation of Bsk/JNK by Ras85D, Puc and the MAPKs seems to stablish a narrow range of activity thresholds within which normal number of synapses is determined (Jordan-Alvarez, 2017).

    The concept of signaling thresholds is also unveiled in this study by the identification of another signaling pathway that opposes synapse formation. The pro- and anti-synaptogenesis pathways have similar constituents, including small GTPases, MAPKs and transcriptional effectors, Mad/Smad, which are canonical for RSTK receptors. The RSTK type II receptor Put, which can mediate diverse signaling pathways according to the co-receptor bound can be discarded in either the pro- or the anti-synaptogenesis pathways. Thus, the main receptor for the anti-synaptogenesis pathway remains to be identified (Jordan-Alvarez, 2017).

    Concerning small GTPases, the pro-synaptogenesis pathway uses Ras85D while its counterpart uses the poorly studied Ras64B. The anti-synaptogenesis pathway includes an additional member of this family of enzymes, Rala. This small GTPase plays a role in the exocyst-mediated growth of the muscle membrane specialization that surrounds the synaptic bouton as a consequence of synapse activity. That is, Rala can influence synapse physiology acting from the postsynaptic side. The experimental expression of a constitutively active form of Rala in the neuron does not seem to affect the overall synaptic terminal branching. However, the null ral mutant shows reduced synapse branching and its vertebrate homolog is expressed in the central nervous system. This study found that Rala under-expression in neurons yields an elevated number of synapses. Thus, it is likely that this small GTPase acts as a break to synaptogenesis, hence its inclusion in the antagonistic pathway (Jordan-Alvarez, 2017).

    Synaptogenesis and neuritogenesis are distinct processes since each one can be differentially affected by the same mutant (e.g.: Hiw). Both features, however, share some signals (e.g., Wnd, Hep). This signaling overlap is akin to the case of axon specification versus spine formation for constituents of the apico/basal polarity complex Par3-6/aPKC [127]. These and other examples illustrated in this study underscore the need to discriminate between synapses and boutons. This study is focused on the cell autonomous signaling that takes place in the neuron. Non-cell autonomous signals (e.g., originated in the glia or hemolymph circulating) have not been considered. The active role of glia in axon pruning and bouton number has been the subject of other studies. Considering the reported role of Hiw through the midline glia in the remodeling of the giant fiber interneuron it is not unlikely that the glia-to-neuron signaling may share components with the neuron autonomous signaling addressed here (Jordan-Alvarez, 2017).

    The summary scheme (see Summary diagram of antagonistic signaling pathways for synaptogenesis and their interactions) describes the scenario where two signaling pathways mutually regulate each other. Epistasis assays are the only experimental approach for in vivo studies of more than one signaling component, albeit this type of assay is only feasible in Drosophila Thus, it is plausible that vertebrate synaptogenesis will be regulated by a similar antagonistic signaling (Jordan-Alvarez, 2017).

    The regulatory equilibrium as a mechanism to determine a biological parameter is the most relevant feature in this scenario for several reasons. First, because this type of mechanism can respond very fast to changes in the physiological status of the cell, and, second because it provides remarkable precision to the trait to be regulated, synapse number in this case. Although bi-stable regulatory mechanisms are known in other contexts, the case of synapse number may seem unexpected because the highly dynamic nature of synapse number has been recognized only recently. Consequently, a molecular signaling mechanism endowed with proper precision and time resolution must sustain this dynamic process. The balanced equilibrium uncovered in this study, although most likely still incomplete in terms of its components, offers such a mechanism (Jordan-Alvarez, 2017).

    Skywalker-TBC1D24 has a lipid-binding pocket mutated in epilepsy and required for synaptic function

    Mutations in TBC1D24 cause severe epilepsy and DOORS syndrome, but the molecular mechanisms underlying these pathologies are unresolved. This study solved the crystal structure of the TBC domain of the Drosophila ortholog Skywalker, revealing an unanticipated cationic pocket conserved among TBC1D24 homologs. Cocrystallization and biochemistry showed that this pocket binds phosphoinositides phosphorylated at the 4 and 5 positions. The most prevalent patient mutations affect the phosphoinositide-binding pocket and inhibit lipid binding. Using in vivo photobleaching of Skywalker-GFP mutants, including pathogenic mutants, it was shown that membrane binding via this pocket restricts Skywalker diffusion in presynaptic terminals. Additionally, the pathogenic mutations cause severe neurological defects in flies, including impaired synaptic-vesicle trafficking and seizures, and these defects are reversed by genetically increasing synaptic PI(4,5)P2 concentrations through synaptojanin mutations. Hence, this study has discovered that a TBC domain affected by clinical mutations directly binds phosphoinositides through a cationic pocket and that phosphoinositide binding is critical for presynaptic function (Fischer, 2016).

    Drosophila Syncrip modulates the expression of mRNAs encoding key synaptic proteins required for morphology at the neuromuscular junction

    Localized mRNA translation is thought to play a key role in synaptic plasticity, but the identity of the transcripts and the molecular mechanism underlying their function are still poorly understood. This study shows that Syncrip, a regulator of localized translation in the Drosophila oocyte and a component of mammalian neuronal mRNA granules, is also expressed in the Drosophila larval neuromuscular junction, where it regulates synaptic growth. RNA-immunoprecipitation followed by high-throughput sequencing and qRT-PCR were used to show that Syncrip associates with a number of mRNAs encoding proteins with key synaptic functions, including msp-300, syd-1 (RhoGAP100F), neurexin-1, futsch, highwire, discs large, and alpha-spectrin. The protein levels of MSP-300, Discs large, and a number of others are significantly affected in syncrip null mutants. Furthermore, syncrip mutants show a reduction in MSP-300 protein levels and defects in muscle nuclear distribution characteristic of msp-300 mutants. These results highlight a number of potential new players in localized translation during synaptic plasticity in the neuromuscular junction. It is proposed that Syncrip acts as a modulator of synaptic plasticity by regulating the translation of these key mRNAs encoding synaptic scaffolding proteins and other important components involved in synaptic growth and function (McDermott, 2014).

    Localized translation is a widespread and evolutionarily ancient strategy used to temporally and spatially restrict specific proteins to their site of function and has been extensively studied during early development and in polarized cells in a variety of model systems. It is thought to be of particular importance in the regulation of neuronal development and in the plastic changes at neuronal synapses that underlie memory and learning, allowing rapid local changes in gene expression to occur independently of new transcriptional programs. The Drosophila neuromuscular junction (NMJ) is an excellent model system for studying the general molecular principles of the regulation of synaptic development and plasticity. Genetic or activity-based manipulations of synaptic translation at the NMJ has previously been shown to affect the morphological and electrophysiological plasticity of NMJ synapses. However, neither the mRNA targets nor the molecular mechanism by which such translational regulation occurs are fully understood (McDermott, 2014).

    Previous work identified CG17838, the fly homolog of the mammalian RNA binding protein SYNCRIP/hnRNPQ, which was named Syncrip (Syp). Mammalian SYNCRIP/hnRNPQ is a component of neuronal RNA transport granules that contain CamKIIα, Arc, and IP3R1 mRNAs and is thought to regulate translation via an interaction with the noncoding RNA BC200/BC1, itself a translational repressor. Moreover, SYNCRIP/hnRNPQ competes with poly(A) binding proteins to inhibit translation in vitro and regulates dendritic morphology via association with, and localization of, mRNAs encoding components of the Cdc-42/N-WASP/Arp2/3 actin nucleation-promoting complex. Drosophila Syp has a domain structure similar to its mammalian homolog, containing RRM RNA binding domains and nuclear localization signal(s), as well as encoding a number of protein isoforms. It was previously shown that Syp binds specifically to the gurken (grk) mRNA localization signal together with a number of factors previously shown to be required for grk mRNA localization and translational regulation. Furthermore, syp loss-of-function alleles lead to patterning defects indicating that syp is required for grk and oskar (osk) mRNA localization and translational regulation in the Drosophila oocyte (McDermott, 2014).

    This study shows that Syp is detected in the Drosophila third instar larval muscle nuclei and also postsynaptically at the NMJ. Syp is required for proper synaptic morphology at the NMJ, as syp loss-of-function mutants show a synaptic overgrowth phenotype, while overexpression of Syp in the muscle can suppress NMJ growth. Syp protein associates with a number of mRNAs encoding proteins with key roles in synaptic growth and function including, msp-300, syd-1, neurexin-1 (nrx-1), futsch, highwire (hiw), discs large 1 (dlg1), and α-spectrin (α-spec). The protein levels of a number of these mRNA targets, including msp-300 and dlg1, are significantly affected in syp null mutants. Furthermore, in addition to regulating MSP-300 protein levels, Syp is required for correct MSP-300 protein localization, and syp null mutants have defects in myonuclear distribution and morphology that resemble those observed in msp-300 mutants. It is proposed that Syp coordinates the protein levels from a number of transcripts with key roles in synaptic growth and is a mediator of synaptic morphology and growth at the Drosophila NMJ (McDermott, 2014).

    The results demonstrate that Syp is required for the appropriate branching of the motoneurons and the number of synapses they make at the muscle. These observations are potentially explained by the finding that Syp is also required for the correct level of expression of msp-300, dlg1 and other mRNA targets. Given that it was previously shown that Syp regulates mRNA localization and localized translation in the Drosophila oocyte, and studies by others have shown that mammalian SYNCRIP/hnRNPQ inhibits translation initiation by competitively binding poly(A) sequences, these functions of Syp as occurring at the level of translational regulation of the mRNAs to which Syp binds. The data are also consistent with other work in mammals showing that SYNCRIP/hnRNPQ is a component of neuronal RNA transport granules that can regulate dendritic morphology via the localized expression of mRNAs encoding components of the Cdc-42/N-WASP/Arp2/3 actin nucleation-promoting complex (McDermott, 2014 and references therein).

    Translation at the Drosophila NMJ is thought to provide a mechanism for the rapid assembly of synaptic components and synaptic growth during larval development, in response to rapid increases in the surface area of body wall muscles or in response to changes in larval locomotion. The phenotypes observed in this study resemble, and are comparable to, those seen when subsynaptic translation is altered genetically or by increased locomotor activity. In syp null mutants, NMJ synaptic terminals are overgrown, containing more branches and synaptic boutons. Similarly, bouton numbers are increased by knocking down Syp in the muscle using RNAi. In contrast, overexpression of Syp in the muscle has the opposite phenotype, resulting in an inhibition of synaptic growth and branching. Furthermore, expressing RNAi against syp in motoneurons alone does not result in a change in NMJ morphology, indicating that Syp acts postsynaptically in muscle, but not presynaptically at the NMJ to regulate morphology. Interestingly, pan-neuronal syp knockdown or overexpression using Elav-GAL4 also results in NMJ growth defects, revealing that some of the defects observed in the syp null mutant may be attributed to Syp function in neuronal cell types other than the motoneurons, such as glial cells, which are known to influence NMJ morphology. Finally, while Syp is not required in the motoneuron to regulate synapse growth and is not detected in the motoneuron, the possibility cannot be excluded that Syp is present at low levels in the presynapse and regulates processes independent of synapse morphology. A further detailed characterization of the cell types and developmental stages in which Syp is expressed and functions is required to better understand the complex phenotypes that were observe (McDermott, 2014).

    RNA binding proteins have emerged as critical regulators of both neuronal morphology and synaptic transmision, suggesting that protein production modulates synapse efficacy. Consistent with this, it has been shown in a parallel study that Syp is required for proper synaptic transmission and vesicle release and regulates the presynapse through expression of retrograde Bone Morphogenesis Protein (BMP) signals in the postsynapse. In this role, Syp may coordinate postsynaptic translation with presynaptic neurotransmitter release. These observations provide a good explanation for how Syp influences the presynapse despite being only detectable in the postsynapse. This study has shown that Syp associates with a large number of mRNAs within third instar larvae, many of which encode proteins with key roles in synaptic growth and function. Syp mRNA targets include msp-300, syd-1, nrx-1, futsch, hiw, dlg1, and α-spec. Syp negatively regulates Syd-1, Hiw, and DLG protein levels in the larval body wall but positively regulates MSP-300 and Nrx-1 protein levels. Dysregulation of these multiple mRNA targets likely accounts for the phenotypes that were observed. Postsynaptically expressed targets with key synaptic roles that could explain the synaptic phenotypes that were observed in syp alleles include MSP-300, α-Spec, and DLG. For example, mutants in dlg1 and mutants where postsynaptic DLG is destabilized or delocalized have NMJ morphology phenotypes similar to those observed upon overexpression of Syp in the muscle. Presynaptically expressed targets include syd-1, nrx-1, and hiw. However, this study has shown that syp knockdown in presynaptic motoneurons does not result in any defects in NMJ morphology. The RIP-Seq experiments were carried out using whole larvae and will, therefore, identify Syp targets in a range of different tissues and cells, the regulation of which may or may not contribute to the phenotype that were observed in syp mutants. It is, therefore, possible that Syp associates with these presynaptic targets in other neuronal cell types such as the DA neurons of the larval peripheral nervous system. It is also possible that Nrx-1 or Hiw are expressed and required postsynaptically in the muscle, but this has not been definitively determined. syp alleles may provide useful tools to examine where key synaptic genes are expressed and how they are regulated (McDermott, 2014).

    The identity of localized mRNAs and the mechanism of localized translation at the NMJ are major outstanding questions in the field. To date, studies have shown that GluRIIA mRNA aggregates are distributed throughout the muscle. The Syp targets identified in this study, such as msp-300, hiw, nrx-1, α-spec, and dlg1, are now excellent candidates for localized expression at the NMJ. Ultimately, conclusive demonstration of localized translation will involve the visualization of new protein synthesis of targets during activity-dependent synaptic plasticity. Biochemical experiments will also be required to establish the precise mode of binding of Syp to its downstream mRNA targets, the basis for interaction specificity, and the molecular mechanism by which Syp differentially regulates the protein levels of its mRNA targets at the Drosophila NMJ. Despite the fact that mammalian SYNCRIP is known to associate with poly(A) tails, this study and other published work have revealed that Syp can associate with specific transcripts. How Syp associates with specific mRNAs is unknown, and future studies are needed to uncover whether the interaction of Syp with specific transcripts is dictated by direct binding of the three Syp RRM RNA binding domains or by binding to other specific mRNA binding proteins. It is also possible that specific mRNA stem–loops, similar to the gurken localization signal, are required for Syp to bind to its mRNA targets (McDermott, 2014).

    This study shows that msp-300 is the most significant mRNA target of Syp. MSP-300 is the Drosophila ortholog of human Nesprin proteins. These proteins have been genetically implicated in various human myopathies. For example, Nesprin/Syne-1 or Nesprin/Syne-2 is associated with Emery-Dreifuss muscular dystrophy (EDMD) as well as severe cardiomyopathies. Moreover, Syp itself is increasingly linked with factors and targets that can cause human neurodegenerative disorders. Recent work has revealed that SYNCRIP/hnRNPQ and Fragile X mental retardation protein (FMRP) are present in the same mRNP granule, and loss of expression of FMRP or the ability of FMRP to interact with mRNA and polysomes can cause cases of Fragile X syndrome. Separate studies have also shown that SYNCRIP interacts with wild-type survival of motor neuron (SMN) protein but not the truncated or mutant forms found to cause spinal muscular atrophy, and Syp genetically interacts with Smn mutations in vivo. Understanding Syp function in the regulation of such diverse and complex targets may, therefore, provide new avenues for understanding the molecular basis of complex disease phenotypes and potentially lead to future therapeutic approaches (McDermott, 2014).

    The EHD protein Past1 controls postsynaptic membrane elaboration and synaptic function

    Membranes form elaborate structures that are highly tailored to their specialized cellular functions, yet the mechanisms by which these structures are shaped remain poorly understood. This study shows that the conserved membrane-remodeling C-terminal Eps15 Homology Domain (EHD) protein Past1 is required for the normal assembly of the subsynaptic muscle membrane reticulum (SSR) at the Drosophila melanogaster larval neuromuscular junction (NMJ). past1 mutants exhibit altered NMJ morphology, decreased synaptic transmission, reduced glutamate receptor levels, and a deficit in synaptic homeostasis. The membrane-remodeling proteins Amphiphysin and Syndapin colocalize with Past1 in distinct SSR subdomains, and collapse into Amphiphysin-dependent membrane nodules in the SSR of past1 mutants. These results suggest a mechanism by which the coordinated actions of multiple lipid-binding proteins lead to the elaboration of increasing layers of the SSR, and uncover new roles for an EHD protein at synapses (Koles, 2015).

    Mucin-type core 1 glycans regulate the localization of neuromuscular junctions and establishment of muscle cell architecture in Drosophila

    T antigen (Galβ1-3GalNAcα1-Ser/Thr), a core 1 mucin-type O-glycan structure, is synthesized by Drosophila core 1 β1,3-galactosyltrasferase 1 (dC1GalT1) and is expressed in various tissues. dC1GalT1 synthesizes T antigen expressed in hemocytes, lymph glands, and the central nervous system (CNS) and dC1GalT1 mutant larvae display decreased numbers of circulating hemocytes and excessive differentiation of hematopoietic stem cells in lymph glands. dC1GalT1 mutant larvae have also been shown to have morphological defects in the CNS. However, the functions of T antigen in other tissues remain largely unknown. This study found that glycans contributed to the localization of neuromuscular junction (NMJ) boutons. In dC1GalT1 mutant larvae, NMJs were ectopically formed in the cleft between muscles 6 and 7 and connected with these two muscles. dC1GalT1 synthesized T antigen, which was expressed at NMJs. In addition, the function of mucin-type O-glycans in muscle cells was determined. In dC1GalT1 mutant muscles, myofibers and basement membranes were disorganized. Moreover, ultrastructural defects in NMJs and accumulation of large endosome-like structures within both NMJ boutons and muscle cells were observed in dC1GalT1 mutants. Taken together, these results demonstrated that mucin-type O-glycans synthesized by dC1GalT1 were involved in the localization of NMJ boutons, synaptogenesis of NMJs, establishment of muscle cell architecture, and endocytosis (Itoh, 2016).

    Shank modulates postsynaptic wnt signaling to regulate synaptic development

    Prosap/Shank scaffolding proteins regulate the formation, organization, and plasticity of excitatory synapses. Mutations in SHANK family genes are implicated in autism spectrum disorder and other neuropsychiatric conditions. However, the molecular mechanisms underlying Shank function are not fully understood, and no study to date has examined the consequences of complete loss of all Shank proteins in vivo. This study characterized the single Drosophila Prosap/Shank family homolog. Shank is enriched at the postsynaptic membrane of glutamatergic neuromuscular junctions and controls multiple parameters of synapse biology in a dose-dependent manner. Both loss and overexpression of Shank result in defects in synaptic bouton number and maturation. It was found that Shank regulates a noncanonical Wnt signaling pathway in the postsynaptic cell by modulating the internalization of the Wnt receptor Fz2. This study identifies Shank as a key component of synaptic Wnt signaling, defining a novel mechanism for how Shank contributes to synapse maturation during neuronal development (Harris, 2016).

    The postsynaptic density (PSD) of excitatory synapses contains a complex and dynamic arrangement of proteins, allowing the cell to respond to neurotransmitter and participate in bidirectional signaling to regulate synaptic function. Prosap/Shank family proteins are multidomain proteins that form an organizational scaffold at the PSD. Human genetic studies have implicated SHANK family genes as causative for autism spectrum disorder (ASD) (Uchino and Waga, 2013; Guilmatre, 2014), with haploinsufficiency of SHANK3 considered one of the most prevalent causes (Betancur and Buxbaum, 2013). Investigations of Shank in animal models have identified several functions for the protein at synapses, including regulation of glutamate receptor trafficking, the actin cytoskeleton, and synapse formation, transmission, and plasticity (Grabrucker, 2011; Jiang and Ehlers, 2013). However, phenotypes associated with loss of Shank are variable, and it has been challenging to fully remove Shank protein function in vivo as a result of redundancy between three Shank family genes and the existence of multiple isoforms of each Shank. There is a single homolog of Shank in Drosophila (Liebl and Featherstone, 2008), presenting the opportunity to characterize the function of Shank at synapses in vivo in null mutant animals (Harris, 2016).

    Wnt pathways play important roles in synaptic development, function, and plasticity. Like Shank and several other synaptic genes, deletions and duplications of canonical Wnt signaling components have been identified in individuals with ASD. A postsynaptic noncanonical Wnt pathway has been characterized at the Drosophila glutamatergic neuromuscular junction (NMJ), linking release of Wnt by the presynaptic neuron to plastic responses in the postsynaptic cell. In this Frizzled-2 (Fz2) nuclear import (FNI) pathway, Wnt1/Wg is secreted by the neuron and binds its receptor Fz2 in the postsynaptic membrane. Surface Fz2 is then internalized and cleaved, and a C-terminal fragment of Fz2 (Fz2-C) is imported into the nucleus in which it interacts with ribonucleoprotein particles containing synaptic transcripts. Mutations in this pathway result in defects of synaptic development at the NMJ (Harris, 2016 and references therein).

    In this study, a null allele of Drosophila Shank was created, allowing investigation of the consequences of removing all Shank protein in vivo. Loss of Shank is shown to impair synaptic bouton number and maturity and results in defects in the organization of the subsynaptic reticulum (SSR), a complex system of infoldings of the postsynaptic membrane at the NMJ. It was also demonstrated that overexpression of Shank has morphological consequences similar to loss of Shank and that Shank dosage is critical to synaptic development. Finally, the results indicate that Shank regulates the internalization of Fz2 to affect the FNI signaling pathway, revealing a novel connection between the scaffolding protein Shank and synaptic Wnt signaling (Harris, 2016).

    By generating Drosophila mutants completely lacking any Shank protein, this study identified a novel function of this synaptic scaffolding protein in synapse development. Aberrant expression of Shank results in defects affecting synapse number, maturity, and ultrastructure, and a subset of these defects is attributable to a downregulation of a noncanonical Wnt signaling pathway in the postsynaptic cell (Harris, 2016).

    The defects observed in Shank mutants are mostly consistent with defects described from in vivo and in vitro rodent models of Shank. Synaptic phenotypes reported from Shank mutants vary, likely reflecting incomplete knockdown of Shank splice variants, and heterogeneity in the requirement for Shank between the different brain regions and developmental stages analyzed (for review, see Jiang and Ehlers, 2013). Nevertheless, taken collectively, analyses of Shank1-Shank3 mutant mice indicate that Shank genes regulate multiple parameters of the structure and function of glutamatergic synapses, including the morphology of dendritic spines and the organization of proteins in the PSD (Harris, 2016 and references therein).

    By removing all Shank protein in Drosophila, this study identified essential functions for Shank at a model glutamatergic synapse. Shank mutants exhibit prominent abnormalities in synaptic structure, including a decrease in the total number of synaptic boutons, which results in an overall decrease in the number of AZs. In addition, a subset of synaptic boutons fails to assemble a postsynaptic apparatus. Finally, even in mature boutons, the SSR has fewer membranous folds and makes less frequent contact with the presynaptic membrane, indicating a defect in postsynaptic development. The SSR houses and concentrates important synaptic components near the synaptic cleft, including scaffolding proteins, adhesion molecules, and glutamate receptors. Thus, defects in SSR development can affect the assembly and regulation of synaptic signaling platforms. These findings indicate that Shank is a key regulator of synaptic growth and maturation (Harris, 2016).

    The findings also indicate that gene dosage of Shank is critical for normal synapse development at Drosophila glutamatergic NMJs. The morphological phenotypes that were observed scale with the level of Shank expression, with mild phenotypes seen with both 50% loss and moderate overexpression of Shank, and severe phenotypes seen with both full loss and strong overexpression of Shank. The observation of synapse loss in heterozygotes of the Shank null allele is significant, because haploinsufficiency of SHANK3 is well established as a monogenic cause of ASD (Harris, 2016).

    Consistent with the observation that excess Shank is detrimental, duplications of the SHANK3 genomic region (22q13) are known to cause a spectrum of neuropsychiatric disorders. Large duplications spanning SHANK3 and multiple neighboring genes have been reported in individuals with attention deficit-hyperactivity disorder (ADHD), schizophrenia, and ASD. Smaller duplications, spanning SHANK3 and only one or two adjacent genes, have been reported in individuals with ADHD, epilepsy, and bipolar disorder. Furthermore, duplication of the Shank3 locus in mice results in manic-like behavior, seizures, and defects in neuronal excitatory/inhibitory balance. Thus, the requirement for proper Shank dosage for normal synaptic function may be a conserved feature (Harris, 2016).

    One unexpected finding from this study was the identification of a previously unappreciated aspect of Shank as a regulator of Wnt signaling. Shank regulates the internalization of the transmembrane Fz2 receptor, thus affecting transduction of Wnt signaling from the plasma membrane to the nucleus. Downregulation of this pathway is implicated in impaired postsynaptic organization, including supernumerary GBs and SSR defects. The physical proximity of Shank and Fz2 at the postsynaptic membrane suggests that Shank directly or indirectly modulates the internalization of Fz2. Shank is a scaffolding protein with many binding partners that could contribute to such an interaction. One intriguing possibility is the PDZ-containing protein Grip. Shank2 and Shank3 have been reported to bind Grip1. Furthermore, Drosophila Grip transports Fz2 to the nucleus on microtubules to facilitate the FNI pathway (Ataman, 2006). Thus, an interaction between Shank, Fz2, and Grip to regulate synaptic signaling is an attractive model (Harris, 2016).

    Although loss of Shank is associated with impaired internalization of the Fz2 receptor, how excess Shank leads to FNI impairment remains an open question. One possibility is that an increase in the concentration of the Shank scaffold at the synapse physically impedes the transport of Fz2 or other components of the pathway or saturates binding partners that are essential for Fz2 trafficking. Both overexpression and loss of function of Shank ultimately lead to a failure to accumulate the cleaved Fz2 C terminus within the nucleus, in which it is required to interact with RNA binding proteins that facilitate transport of synaptic transcripts to postsynaptic compartments. Although Shank and Wnt both play important synaptic roles, this study is the first demonstration of a functional interaction between Shank and Wnt signaling at the synapse (Harris, 2016).

    Intriguingly, no obvious defects were found in glutamate receptor levels or distribution in the absence of Shank. This was surprising given the role for Shank in regulating the FNI pathway, and downregulation of FNI was shown previously to lead to increased GluR field size. Several studies have reported changes in the levels of AMPA or NMDA receptor subunits in Shank mutant mice, although others have also observed no changes. Levels of metabotropic glutamate receptors are also affected in some Shank mutant models. Moreover, transfected Shank3 can recruit functional glutamate receptors in cultured cerebellar neurons. It is possible that Drosophila Shank mutants have defects in GluRs that are too subtle to detect with current methodology. Another possibility is that Shank is involved in signaling mechanisms that are secondary to FNI and that lead to compensatory changes in GluRs at individual synapses. Indeed, the results are consistent with Shank having additional functions at the synapse in addition to its role in FNI, particularly affecting synaptic bouton number. In conclusion, this study has fpind that the sole Drosophila Shank homolog functions to regulate synaptic development in a dose-dependent manner, providing a new model system to further investigate how loss of this scaffolding protein may underlie neurodevelopmental disease (Harris, 2016).

    The influence of postsynaptic structure on missing quanta at the Drosophila neuromuscular junction

    Synaptic transmission requires both pre- and post-synaptic elements for neural communication. The postsynaptic structure contributes to the ability of synaptic currents to induce voltage changes in postsynaptic cells. At the Drosophila neuromuscular junction (NMJ), the postsynaptic structure, known as the subsynaptic reticulum (SSR), consists of elaborate membrane folds that link the synaptic contacts to the muscle, but its role in synaptic physiology is poorly understood. This study investigated the role of the SSR with simultaneous intra- and extra-cellular recordings that allow identification of the origin of spontaneously occurring synaptic events. Data from Type 1b and 1s synaptic boutons, which have naturally occurring variations of the SSR, were compared with genetic mutants that up or down-regulate SSR complexity. Some synaptic currents do not result in postsynaptic voltage changes, events that were called 'missing quanta'. The frequency of missing quanta is positively correlated with SSR complexity in both natural and genetically-induced variants. Rise-time and amplitude data suggest that passive membrane properties contribute to the observed differences in synaptic effectiveness. It is concluded that electrotonic decay within the postsynaptic structure contributes to the phenomenon of missing quanta. Further studies directed at understanding the role of the SSR in synaptic transmission and the potential for regulating 'missing quanta' will yield important information about synaptic transmission at the Drosophila NMJ (Nguyen, 2016).

    Dbo/Henji modulates synaptic dPAK to gate glutamate receptor abundance and postsynaptic response

    In response to environmental and physiological changes, the synapse manifests plasticity while simultaneously maintains homeostasis. This study analyzed mutant synapses of henji, also known as diablo (dbo), at the Drosophila neuromuscular junction (NMJ). In henji mutants, NMJ growth is defective with appearance of satellite boutons. Transmission electron microscopy analysis indicates that the synaptic membrane region is expanded. The postsynaptic density (PSD) houses glutamate receptors GluRIIA and GluRIIB, which have distinct transmission properties. In henji mutants, GluRIIA abundance is upregulated but of GluRIIB is not. Electrophysiological results also support a GluR compositional shift towards a higher IIA/IIB ratio at henji NMJs. Strikingly, dPAK, a positive regulator for GluRIIA synaptic localization, accumulates at the henji PSD. Reducing the dpak gene dosage suppresses satellite boutons and GluRIIA accumulation at henji NMJs. In addition, dPAK associated with Henji through the Kelch repeats which is the domain essential for Henji localization and function at postsynapses. It is proposed that Henji acts at postsynapses to restrict both presynaptic bouton growth and postsynaptic GluRIIA abundance by modulating dPAK (Wang, 2016).-

    Coordinated action and communication between pre- and postsynapses are essential in maintaining synaptic strength and plasticity. Presynaptic strength or release probability of synaptic vesicles involves layers of regulation including vesicle docking, fusion, and recycling, as well as endocytosis and exocytosis. Also, how postsynapses interpret the signal strength from presynapses depends largely on the abundance of neurotransmitter receptors at the synaptic membrane. During long-term potentiation, lateral diffusion of extrasynaptic AMPA receptor to synaptic sites is accelerated and the exocytosis of AMPAR is enhanced near the postsynaptic density (PSD), causing an accumulation of synaptic receptors. In contrast, under the long-term depression condition, synaptic AMPAR is reduced by hastened endocytosis. While molecular mechanisms are proposed to play roles in regulating and fine-tuning postsynaptic glutamate receptor (GluR) abundance in plasticity models, the developmental regulation of GluR abundance at the synaptic surface still needs to be elucidated. Synapses at the Drosophila neuromuscular junction (NMJ) use glutamate as the neurotransmitter, and have properties reminiscent of mammalian central excitatory synapses. Homologous to vertebrate AMPAR and kainate receptors, Drosophila GluR subunits assemble as tetramers to gate ion influx. Each functional receptor contains essential subunits (GluRIIC, GluRIID and GluRIIE) and either GluRIIA or GluRIIB; therefore, synaptic GluRs can be classified according to their subunit compositions as either A- or B-type receptors. These two types of receptors exhibit distinct developmental and functional properties. Newly-formed PSDs tend to accumulate more GluRIIA channels, while the IIA/IIB ratio becomes more balanced when PSDs mature. In addition, GluRIIB channels have much faster desensitization kinetics, which results in smaller quantal size than GluRIIA channels. Therefore, the synaptic composition of these two types of GluRs greatly influences the postsynaptic interpretation of neuronal activities. The Drosophila homolog of p21-activated kinase (dPAK) regulates GluRIIA abundance at the PSD; GluRIIA receptor clusters at the postsynaptic membrane are strongly reduced in dpak mutants. However, overexpression of dPAK in postsynapses is not sufficient to increase GluRIIA cluster size, suggesting that dPAK activity in regulating GluRIIA abundance is tightly controlled (Wang, 2016).

    Ubiquitination and deubiquitination play critical roles in regulating synaptic functions. In loss-of-function mutants for highwire, a gene encoding a conserved E3 ubiquitin ligase, NMJs overgrow, producing supernumerary synaptic boutons. This phenotype is duplicated by overexpression of the deubiquitinating enzyme Fat facets (Faf) in presynapses. These studies underline the importance of balanced ubiquitination in synapse formation and function. Cullin-RING ubiquitin ligases (CRLs) are large protein complexes that confer substrate ubiquitination. Importantly, CRLs promote ubiquitination through substrate receptors that provide specific recognition of substrates for ubiquitination. The BTB-Kelch proteins are suggested to be the substrate receptors for Cul3-scaffolded CRLs. This study identified a BTB-Kelch-containing protein, Henji, also known as Dbo, which regulates NMJ growth and synaptic activity by restricting the clustering of GluRIIA. Synaptic size of henji mutants was significantly expanded, as viewed under transmission electron microscopy (TEM). Immunostaining for dPAK and GluRIIA also suggests larger areas of PSDs in the absence of Henji, and the intensity of each fluorescent punctum becomes stronger, indicating abnormal accumulation of these PSD proteins. By genetically reducing one gene dosage of dpak in henji mutants, GluRIIA accumulation and abnormal bouton morphology was suppressed. In contrast, reducing the gluriia gene dosage in henji mutants restored bouton morphology but failed to suppress dPAK accumulation. Thus, Henji regulates bouton morphology and GluRIIA clustering levels likely through a control of dPAK. Interestingly, while overexpression of dPAK, either constitutively active or dominantly negative, had no effects on GluRIIA clustering, overexpression of these dPAK forms in henji mutants modulated GluRIIA levels, indicating that Henji limits the action of dPAK to regulate GluRIIA synaptic abundance. Henji localized to the subsynaptic reticulum (SSR) surrounding synaptic sites, consistent with the idea that Henji functions as a gatekeeper for synaptic GluRIIA abundance (Wang, 2016).

    This study shows that Henji functions at the postsynapse to regulate synaptic development and function at the NMJ. The PSD area is expanded and GluRIIA clusters abnormally accumulate at the PSD. Genetic evidences are provided to support that the elevation of GluRIIA synaptic abundance is at least partially caused by a corresponding accumulation of dPAK in henji mutants. Henji is sufficient to downregulate dPAK and GluRIIA levels and the Kelch repeats of Henji play the most critical role in this process. Henji tightly gates dPAK in regulating GluRIIA abundance, as dPAK enhances GluRIIA cluster abundance only when Henji is absent. Therefore, this study has identified a specific negative regulation of dPAK at the postsynaptic sites that contributes to the PSD formation and GluR cluster formation at the NMJ (Wang, 2016).

    PAK proteins transduce various signaling activities to impinge on cytoskeleton dynamics. Through kinase activity-dependent and -independent mechanisms, PAK regulates not only actin- and microtubule-based cytoskeletal rearrangement but also the activity of motors acting on these cytoskeletal tracks. In mammalian systems, PAKs participate in many synaptic events including dendrite morphogenesis, neurotransmitter receptor trafficking, synaptic strength modulation, and activity-dependent plasticity. Pathologically, PAK dysregulation also contributes to serious neurodegenerative diseases, Huntington's disease and X-linked mental retardation (Wang, 2016).

    At Drosophila NMJs, dPAK has divergent functions; loss of dpak causes a dramatic reduction in both Dlg and GluRIIA synaptic abundance, but the underlying molecular mechanisms have not been revealed. The current data show that Henji functions to restrict GluRIIA clustering but has no effect on Dlg levels, suggesting that Henji regulates one aspect of dPAK activities, probably via the SH2/SH3 adaptor protein Dock. Alternatively, Henji may function to limit dPAK protein levels locally near the postsynaptic region, rendering its influence on GluRIIA clustering, while dPAK that regulates Dlg may localize outside of the Henji-enriched region. Supporting this idea, Henji is specifically enriched around the SSR region instead of dispersed throughout the muscle cytosol. Moreover, ectopic Myc-dPAK localized at the postsynapse only when henji was mutated, indicating that Henji regulates dPAK postsynaptic localization (Wang, 2016).

    The interaction with Rac, Cdc42, or both triggers autophosphorylation and subsequent conformational changes of PAK, resulting in kinase activation. The myristoylated dPAK that has been shown to be active in growth cones failed to enhance GluRIIA abundance at the NMJ. This result shows that dPAK is necessary to regulate GluRIIA synaptic abundance, but is itself tightly regulated at the synaptic protein level or the kinase activity. Indeed, evidence is provided to show specific negative regulation of dPAK by Henji; overexpression of dPAK CA that could not enhance GluRIIA abundance in WT larvae further increased the already enhanced GluRIIA levels in the henji mutant. Similar to the CA form, the DN form also showed no effect on GluRIIA when simply overexpressed in the WT background, but exhibited strong suppression of GluRIIA in the henji mutant background. Thus, regardless of the possible conformational differences between the CA and DN forms, Henji appears to confer a constitutive negative regulation of dPAK at postsynapses, suggesting a tight control that could be at subcellular localization. In contrast to CA and DN forms, activation of dPAK requires binding to Rac1 and Cdc42, and subsequent protein phosphorylation. This additional layer of regulation may serve as a limiting factor rendering dPAK WT from recruiting GluRIIA to PSDs regardless in WT or henji mutant background (Wang, 2016).

    The structural feature suggests that Henji could function as a conventional substrate receptor of the Cul3-based E3 ligase complex. At Drosophila wing discs, Dbo functions as a Cul3-based E3 ligase to promote Dishevelled (Dsh) downregulation. Similar to the henji alleles, it was confirmed that the dbo [Δ25.1] allele and dbo RNAi were competent to induce dPAK and GluRIIA accumulation at the postsynapse. An immunoprecipitation experiment detected Henji and dPAK in the same complex, and dPAK also forms a complex with the C-terminal substrate-binding Kelch-repeats region. However, no notable or consistent increase was detected in Henji-dependent dPAK poly-ubiquitination in both S2 cells and larval extracts. Also, the Cul3-binding BTB domain of Henji seems dispensable in the suppression of dPAK levels in henji mutants. Importantly, Cul3 knockdown in muscle cells failed to cause any accumulation of GluRIIA and dPAK at the NMJ. Sensitive genetic interaction between henji and Cul3 failed to induced dPAK and GluRIIA accumulation. Dbo functions together with another BTB-Kelch protein Kelch (Kel) to downregulate Dsh. However, Kel negatively regulates GluRIIA levels without affecting dPAK localization at the postsynaptic site. This data argues that Kel functions in a distinct pathway to Henji in postsynaptic regulation of GluRIIA. Taken together, no direct evidence was found to support that dPAK is downregulated by Henji through ubiquitination-dependent degradation. Alternately, Henji could bind dPAK near the postsynaptic region and this interaction may block the recruitment or localization of dPAK onto postsynaptic sites. Under this model, dPAK is less restricted and has a higher propensity to localize at postsynaptic sites in the absence of Henji, resulting in synaptic accumulation of dPAK and GluRIIA expansions (Wang, 2016).

    As many synaptic events require rapid responses, local regulation of protein levels becomes crucial in synapses. To achieve accurate modulation, certain synaptic proteins should be selectively controlled under different developmental or environmental contexts. Indeed, emerging evidence shows that various aspects of synapse formation and function are under the control of the ubiquitin proteasome system (UPS), including synapse formation, morphogenesis, synaptic pruning and elimination, neurotransmission, and activity-dependent plasticity. In particular, the membrane abundance of postsynaptic GluR that modulates synaptic function can be regulated by components of the UPS. When Apc2, the gene encoding Drosophila APC/C E3 ligase, is mutated, GluRIIA shows excess accumulation but the molecular mechanism was not elucidated. Similarly, loss of the substrate adaptor BTB-Kelch protein KEL-8 in C. elegans also results in the stabilization of GLR-1-ubiquitin conjugates. However, no evidence shows direct ubiquitination and degradation of GLR-1 by KEL-8. Also, absence of the LIN-23-APC/C complex in C. elegans affects GLR-1 abundance at postsynaptic sites without altering the level of ubiquitinated GLR-1. Therefore, GLR-1 receptor endocytosis and recycling or ubiquitination and degradation of GLR-1-associated scaffold proteins are proposed to be the underlying mechanism for E3 ligase regulation. In mammals, endocytosis of AMPAR can be influenced by poly-ubiquitination and degradation of the prominent postsynaptic scaffold protein PSD-95 (Wang, 2016).

    This study describes a novel regulation by the BTB-Kelch protein Henji on synaptic GluRIIA levels. By limiting GluRIIA synaptic levels, Henji modulates the postsynaptic output in response to presynaptic glutamate release. In the absence of Henji, quantal size is elevated, coinciding with an increase in the postsynaptic GluRIIA/GluRIIB ratio. In a previous study, increases in the GluRIIA/GluRIIB ratio by overexpressing a GluRIIA transgene in the muscle or by reducing the gene copy of gluriib promote NMJ growth, but co-expression of both GluRIIA and GluRIIB did not alter the bouton number. Combined with the current findings, those data provide a link between an increased GluRIIA-mediated postsynaptic response and bouton addition at NMJs. However, satellite boutons were not detected following GluRIIA overexpression. One possibility is that satellite boutons are considered as immature boutons and their appearance may indicate the tendency for NMJ expansion, as in the case of excess BMP signaling. Failure to become mature boutons may be caused by the lack of cooperation with other factors such as components of the presynaptic endocytic pathway, actin cytoskeleton rearrangement or neuronal activity. No significant alterations in endocytosis and the BMP pathway in the henji mutant. Nevertheless, it cannot be ruled out that Henji may modulate other presynaptic events that are defective in henji mutants to interfere with bouton maturation (Wang, 2016).

    Filamin, a synaptic organizer in Drosophila, determines glutamate receptor composition and membrane growth

    Filamin is a scaffolding protein that functions in many cells as an actin-crosslinker. FLN90, an isoform of the Drosophila ortholog Filamin/cheerio that lacks the actin-binding domain, is shown in this study to govern the growth of postsynaptic membrane folds and the composition of glutamate receptor clusters at the larval neuromuscular junction. Genetic and biochemical analyses reveal that FLN90 is present surrounding synaptic boutons. FLN90 is required in the muscle for localization of the kinase dPak and, downstream of dPak, for localization of the GTPase Ral and the exocyst complex to this region. Consequently, Filamin is needed for growth of the subsynaptic reticulum. In addition, in the absence of filamin, type-A glutamate receptor subunits are lacking at the postsynapse, while type-B subunits cluster correctly. Receptor composition is dependent on dPak, but independent of the Ral pathway. Thus two major aspects of synapse formation, morphological plasticity and subtype-specific receptor clustering, require postsynaptic Filamin (Lee, 2016).

    Proper postsynaptic function depends on appropriate localization of receptors and signaling molecules. Scaffolds such as PSD-95/SAP90 and members of the Shank family are critical to achieving this organization. While usually without intrinsic enzymatic activity, scaffolds recruit, assemble, and stabilize receptors and protein networks through multiple protein-protein interactions: they can bind to receptors, postsynaptic signaling complexes, and the cytoskeleton at the postsynaptic density. Mutations in these proteins are associated with neuropsychiatric disorders. While understanding synapse assembly has begun, much remains to be investigated (Lee, 2016).

    The Drosophila larval neuromuscular junction (NMJ) is a well-studied and genetically accessible glutamatergic synapse. Transmission is mediated by AMPA-type receptors, and several postsynaptic proteins important for its development and function have related proteins at mammalian synapses, including the PDZ-containing protein Discs-Large (DLG) and the kinase Pak. In addition, the postsynaptic membrane forms deep invaginations and folds called the subsynaptic reticulum (SSR), which are hypothesized to create subsynaptic compartments comparable to dendritic spines. Recently, we found that the SSR is a plastic structure whose growth is regulated by synaptic activity. This phenomenon may be akin to the use-dependent morphological changes, such as growth of dendritic spines, that occur postsynaptically in mammalian brain. The addition of membrane and growth of the SSR requires the exocyst complex to be recruited to the synapse by the small GTPase Ral; the SSR fails to form in ral mutant larvae. Moreover, the localization of Ral to a region surrounding synaptic boutons is likely to direct the selective addition of membrane to this domain. Ral thus provided a tractable entry point for better understanding postsynaptic assembly. The mechanism for localizing the Ral pathway, however, was unknown. The present study determined that Ral localization is dependent on cheerio, a gene encoding filamin, which is critical for proper development of the postsynapse (Lee, 2016).

    Filamin is a family of highly conserved protein scaffolds with a long rod-like structure of Ig-like repeats. With over 90 identified binding partners, some of which are present also at the synapse, mammalian filamin A (FLNA) is the most abundant and commonly studied filamin. Filamin can bind actin and has received the most attention in the context of actin cytoskeletal organization. Drosophila filamin, encoded by the gene cheerio (cher), shares its domain organization and 46% identity in amino acid sequence with human FLNA. Drosophila filamin has a well-studied role in ring canal formation during oocyte development, where it recruits and organizes actin filaments. This study now shows that filamin has an essential postsynaptic role at the fly NMJ. An isoform of this scaffold protein that lacks the actin-binding domain acts via dPak to localize GluRIIA receptors and Ral; filamin thereby orchestrates both receptor composition and membrane growth at the synapse (Lee, 2016).

    Filamin is a highly conserved protein whose loss of function is associated with neurodevelopmental disorders. In humans, mutations in the X-linked FLNA cause periventricular heterotopia, a disorder of cortical malformation with a wide range of clinical manifestations such as epilepsy and neuropsychiatric disturbances. Studies in rodent models have shown that abnormal filamin expression causes dendritic arborization defects in a TSC mouse and that filamin influences neuronal proliferation. Filamin is present in acetylcholine receptor clusters at the mammalian NMJ, but its function there is unknown. In lysates of the mammalian brain, filamin associates with known synaptic proteins such as Shank3, Neuroligin 3, and Kv4.2. A recent report indicated that filamin degradation promotes a transition from immature filopodia to mature dendritic spines, a phenomenon that is likely to be related to the actin-bundling properties of the long isoform of filamin. Data in the present study have uncovered a novel pathway that does not require the actin-binding domain of filamin. In this pathway, postsynaptically localized filamin, via Pak, directs two distinct effector modules governing synapse development and plasticity: (1) the Ral-exocyst pathway for activity-dependent membrane addition and (2) the composition of glutamate receptor clusters. These pathways determine key structural and physiological properties of the postsynapse (Lee, 2016).

    Although loss of filamin had diverse effects on synapse assembly, they were selective. Muscle-specific knockdown or the cherQ1415sd allele disrupted type-A but not type-B GluR localization at the postsynaptic density. Likewise, the phenotypes for muscle filamin were confined to the postsynaptic side: the presynaptic active zone protein Brp and overall architecture of the nerve endings were not altered by muscle-specific knockdown. The specificity of its effect on particular synaptic proteins, and the absence of the actin-binding region in FLN90, suggests that filamin's major mode of action here is not overall cytoskeletal organization, but rather to serve as a scaffold for particular protein-protein interactions (Lee, 2016).

    Analysis of the distribution of the SSR marker Syndapin and direct examination of the subsynaptic membrane by electron microscopy revealed that formation of the SSR required filamin. Genetic analysis uncovered a sequential pathway for SSR formation from filamin to the Pak/Pix/Rac signaling complex, to Ral, to the exocyst complex and consequent membrane addition. The SSR is formed during the second half of larval life and may be an adaptation for the low input resistance of third-instar muscles. Like dendritic spines, the infoldings of the SSR create biochemically isolated compartments in the vicinity of postsynaptic receptors and may shape physiological responses, although first-order properties of the synapse, such as mini- or EPSP amplitude are little altered in mutants that lack an SSR. The formation of the SSR requires transcriptional changes driven by Wnt signaling and nuclear import, proteins that induce membrane curvature (such as Syndapin, Amphiphysin, and Past1), and Ral-driven, exocyst-dependent membrane addition. The activation of Ral by Ca2+ influx during synaptic transmission allows the SSR to grow in an activity-dependent fashion. The localization of Ral to the region surrounding the bouton appears crucial to determining the site of membrane addition because Ral localization precedes SSR formation and exocyst recruitment and because exocyst recruitment occurred selectively surrounding boutons even when Ca2+-influx occurred globally in response to calcimycin. This study has now shown that Ral localization, and consequently exocyst recruitment, membrane growth, and the presence of the SSR marker Syndapin, are all dependent on a local action of filamin at the synapse. FLN90, the filamin short isoform, localized to sites of synaptic contact and indeed surrounded the boutons just as does Ral. When this postsynaptic filamin was removed by muscle-specific filamin knockdown or the cherQ1415sd allele, the downstream elements of the pathway, Pak, Rac, Ral, the exocyst, and Syndapin, were no longer synaptically targeted. The mislocalization is not a secondary effect of loss of the SSR but likely a direct consequence of filamin loss, as Pak and Ral can localize subsynaptically even in the absence of the SSR. Unlike the likely mode of action of nuclear signaling by Wnt, the delocalization of Ral was not a consequence of altered protein production; its expression levels did not change. Thus filamin may be viewed as orchestrating the formation of the SSR and directing it to the region surrounding synaptic boutons (Lee, 2016).

    The second major feature of the filamin phenotype was the large reduction in the levels of the GluRIIA receptor subunit from the postsynaptic membranes. GluRIIA and GluRIIB differ in their electrophysiological properties and subsynaptic distribution. Because type B GluRs, which contain the IIB subunit, desensitize more rapidly than type A, the relative abundance of type A and type B GluRs is a key determinant of postsynaptic responses and changes with synapse maturation. The selective decrease in GluRIIA at filamin-null NMJs is likely a consequence of dPak mislocalization: filamin-null NMJs lack synaptic dPak, and dPak null NMJs lack synaptic GluRIIA. Moreover, the first-order electrophysiological properties at NMJs lacking filamin resembled those reported at NMJs missing dPak. In the current study, though, only the change in mEPSP frequency was statistically significant. At filamin-null NMJs, the decrease in GluRIIA is accompanied by an increase in GluRIIB, suggestive of a partial compensation that could account for the relatively normal synaptic transmission. Because the IIA and IIB subunits differ in desensitization kinetics and regulation by second messengers, functional consequences of filamin loss may become more apparent with more extensive physiological characterizations at longer time scales (Lee, 2016).

    While both SSR growth and receptor composition required the kinase activity of dPak, receptor composition was independent of Ral and thus represents a distinct branch of the pathway downstream of dPak. As with Ral, the loss of GluRIIA from the synapse was due to delocalization and not a change in expression of the protein, consistent with unaltered GluRIIA transcripts in dPak null animals. Thus filamin, via dPak, alters proteins with functional significance for the synapse as well as its structural maturation (Lee, 2016).

    Mammalian filamin, via its many Ig-like repeats, has known scaffold functions in submembrane cellular compartments and filamin is therefore likely also to serve as a scaffold at the fly NMJ. These epistasis data indicate that filamin recruits Ral through recruitment of a signaling complex already known to function at the fly NMJ: dPak and its partners dPix and Rac. Mammalian filamin is reported to directly interact with Ral during filopodia formation, however the details of their interaction at the fly NMJ are less clear. Because Ral localization requires filamin to recruit dPak and dPix and specifically requires the kinase activity of dPak, it is possible that either Ral or filamin need to be phosphorylated by dPak to bind one another. Mammalian filamin interacts with some components of the Pak signaling complex and is a substrate of Pak. This study has now shown that Drosophila filamin and PAK interact when coexpressed in HEK cells, and thus a direct scaffolding role for FLN90 in the recruitment of Pak and the organization of the postsynapse is likely (Lee, 2016).

    The overlapping but different distributions of filamin and its downstream targets indicate that its scaffolding functions must undergo regulation by additional factors. The proteins discussed in this study take on either of two patterns at the synapse. Some, like Ral, Syndapin, and filamin itself, are what can be termed subsynaptic and, like the SSR, envelope the entire synaptic bouton. Others, like dPak and its partners and the GluRIIA proteins, are concentrated in much smaller regions, immediately opposite presynaptic active zones, where the postsynaptic density (PSD) is located. It is hypothesized that filamin interacts with additional proteins, including potentially transsynaptic adhesion proteins, that localize filamin to the subsynaptic region and also govern to which of the downstream effectors it will bind. Indeed, it appears paradoxical that dPak, though predominantly at the postsynaptic density is nonetheless required for Ral localization throughout the subsynaptic region. If dPak is needed to phosphorylate either filamin or Ral to permit Ral localization, the phosphorylations outside the PSD may be due to low levels of the dPak complex in that region; synaptic dPak was previously shown to be a relatively mobile component of the PSD (Lee, 2016).

    Filamin was the first nonmuscle actin-crosslinking protein to be discovered. With an actin-binding domain at the N terminus, the long isoform of filamin and its capacity to integrate cellular signals with cytoskeletal dynamics have subsequently been the focus of the majority of the filamin literature. At the NMJ, however, this was not the case. Several lines of evidence indicated that the short FLN90 isoform of filamin, which lacks the actin-binding domain, plays an essential role in postsynaptic assembly. First, the short FLN90 isoform was the predominant and perhaps the only isoform of filamin found expressed in the muscles. Second, both endogenous and overexpressed FLN90 localized subsynaptically. Third, loss of the short isoform disrupted localization of postsynaptic components while lack of just the long isoform had little or no effect. Lastly, exogenous expression of just the short isoform in filamin null background sufficiently rescued the defect in SSR growth. The modest postsynaptic phenotypes of the cher1allele, which predominantly disrupts the long isoform, may be due to small effects of the allele on expression of the short isoform or may be an indirect consequence of the presence of the long isoform in the nerve terminals (Lee, 2016).

    The existence of the short isoform has been reported in both flies and mammals and may be produced either by transcriptional regulation or calpain-mediated cleavage. The short isoform can be a transcriptional co-activator, but its functional significance and mechanisms of action have been largely elusive. The short isoform has little or no affinity for actin, but most of the known sites for other protein-protein interactions are shared by both isoforms. Thus the structure of FLN90, with nine predicted Ig repeats and likely protein-protein interactions, is consistent with a scaffolding function to localize key synaptic molecules independent of interactions with the actin cytoskeleton (Lee, 2016).

    This study has introduced filamin as a major contributor to synapse development and organization. The severity of the phenotypes indicates filamin has a crucial role that is not redundant with other scaffolding proteins. The effects of filamin encompass several much-studied aspects of the Drosophila NMJ: the clustering and subunit subtype of glutamate receptors and the plastic assembly of specialized postsynaptic membrane structures. The pathways that govern these two phenomena diverge downstream of Pak kinase activity and are dependent on filamin for the proper localization of key signaling modules in the pathways. By likely acting as a scaffold protein, the short isoform of filamin may function as a link between cell surface proteins, as yet unidentified, and postsynaptic proteins with essential localizations to and functions at the synapse. Because many of the components of these pathways at the fly NMJ are also present at mammalian synapses and can interact with mammalian filamin, a parallel set of functions in CNS dendrites merits investigation (Lee, 2016).

    The Drosophila postsynaptic DEG/ENaC channel ppk29 contributes to excitatory neurotransmission

    The protein family of Degenerin/Epithelial Sodium Channels (DEG/ENaC) is comprised of diverse animal-specific, non-voltage-gated ion channels that play important roles in regulating cationic gradients across epithelial barriers. However, the specific neurophysiological functions of most DEG/ENaC-encoding genes remain poorly understood. This study demonstrates that ppk29 contributes specifically to the postsynaptic modulation of excitatory synaptic transmission at the larval neuromuscular junction (NMJ). Electrophysiological data indicate that the function of ppk29 in muscle is necessary for normal postsynaptic responsivity to neurotransmitter release, and for normal coordinated larval movement. The ppk29 mutation does not affect gross synaptic morphology and ultrastructure, which indicates the observed phenotypes are likely due to defects in glutamate receptor function. Together, these data indicate that DEG/ENaC ion channels play a fundamental role in the postsynaptic regulation of excitatory neurotransmission (Hill, 2007).

    Regulation of neuromuscular junction organization by Rab2 and its effector ICA69 in Drosophila

    Mechanisms underlying synaptic differentiation, which involves neuronal membrane and cytoskeletal remodeling, are not completely understood. This study performed a targeted RNAi-mediated screen of Drosophila BAR-domain proteins and identified islet cell autoantigen 69 kDa (dICA69) as one of the key regulators of morphological differentiation of larval neuromuscular junction (NMJ). Drosophila ICA69 colocalizes with α-Spectrin at the NMJ. The conserved N-BAR domain of dICA69 deforms liposomes in vitro. Full length and ICAC but not the N-BAR domain of dICA69 which induces filopodia in cultured cells. Consistent with its cytoskeleton regulatory role, dICA69 mutants show reduced α-Spectrin immunoreactivity at the larval NMJ. Manipulating levels of dICA69 or its interactor dPICK1 alters synaptic level of ionotropic glutamate receptors (iGluRs). Moreover, reducing dPICK1 or dRab2 levels phenocopies dICA69 mutation. Interestingly, dRab2 regulates not only synaptic iGluR but also dICA69 levels. Thus, these data suggest that: a) dICA69 regulates NMJ organization through a pathway that involves dPICK1 and dRab2, and b) dRab2 genetically functions upstream of dICA69 and regulates NMJ organization and targeting/retention of iGluRs by regulating dICA69 levels (Mallik 2017).

    This study demonstrates that ICA69 regulates NMJ structural organization and synaptic levels of glutamate receptor clusters. The findings suggest a model in which Rab2 functions genetically upstream of ICA69 to regulate its synaptic level, which in turn regulates the Spectrin cytoskeleton and iGluRs at the NMJ (Mallik, 2017).

    The requirement of ICA69 for Drosophila NMJ organization is strongly supported by its enrichment in the postsynaptic Spectrin-rich scaffold. Consistent with this idea, ICA69 mutants or animals with downregulated ICA69 levels show reduced arborization and bouton numbers at the NMJ. Several studies have shown that cytoskeletal regulation is a key process for NMJ development. Multiple lines of evidence suggest that ICA69 promotes NMJ growth by regulating the cytoskeletal network surrounding the SSR. First, ICA69 is highly enriched at the NMJ in the same microdomain as Spectrin. Second, ICA69 induces filopodia in cultured cells and relocalizes positive regulators of actin polymerization at the filopodia. Third, mutation in ICA69 significantly reduces α-Spectrin levels. The Actin-Spectrin scaffold at the postsynapse has been implicated in regulation of NMJ organization in postembryonic development in Drosophila. This study reveals a crucial requirement of ICA69 in regulating synaptic α-Spectrin levels and indicates that ICA69 is required for the assembly of Actin-Spectrin scaffolds surrounding the SSR. Whether localization and/or stability of postsynaptic Spectrin-Actin scaffold depends on direct interaction between scaffold components and ICA69 or on some unknown signaling mechanism needs to be further investigated (Mallik, 2017).

    For the proper establishment of NMJ connections, neurons as well as muscles require trafficking of various synaptic proteins. Rab GTPases and their regulators are considered to be some of the most important signaling molecules for intracellular trafficking. Interestingly, nearly half of all the Drosophila Rab proteins function specifically in neurons and a few of them localize to the NMJs. ICA69 physically associates with Rab2 and has been suggested as one of its effectors in regulating dense core vesicle maturation in Caenorhabditis elegans. This study found that Rab2 endogenous regulatory sequence-driven Rab2EYFP is detectable in the larval muscles as punctate structures, suggesting its involvement in NMJ organization. This idea is supported by four compelling pieces of evidence. First, ubiquitous or muscle-specific knockdown of Rab2 phenocopies ICA69 mutants. Second, knockdown of Rab2 significantly reduces synaptic α-Spectrin levels. Third, Rab2 directly regulates synaptic ICA69 levels. Fourth, co-expressing an ICA69 transgene and Rab2 RNAi rescues the morphological defects of Rab2 RNAi. Based on these observations, it is suggested that Drosophila Rab2 functions genetically upstream of ICA69. Like Rab2, PICK1 depletion reduced synaptic ICA69 levels and phenocopied the NMJ morphological defects observed in ICA69 mutants or after Rab2 depletion. Moreover, simultaneous knockdown of ICA69 and PICK1 or of ICA69 and Rab2 did not show an additive effect on the NMJ structural defects. These observations support the notion that ICA69, PICK1 and Rab2 might function in the same genetic pathway to regulate NMJ structural organization (Mallik, 2017).

    In mammalian neurons, ICA69 is, surprisingly, not enriched at the synapses and negatively regulates AMPA receptor trafficking. Hence, it is expected that ICA69 mutants would have normal, if not more, iGluR clusters at the NMJ. Contrary to this expectation, reducing the ICA69 level resulted in reduced GluRIIA as well as GluRIIB glutamate receptor clusters. A recent study has shown that ICA69 and PICK1 stability is interdependent in Drosophila brain. Thus, it is likely that iGluR clusters at the NMJ are regulated by levels of ICA69 and PICK1 in muscles (Mallik, 2017).

    How does ICA69 reduce iGluR levels both in knockdown and overexpression scenarios? It is suggested that the endogenous stoichiometry of ICA69 and PICK1 is crucial for normal synaptic targeting of iGluRs at the Drosophila NMJ. Reducing ICA69 destabilizes the ICA69-PICK1 heteromeric complex thereby reducing PICK1 availability for synaptic targeting of iGluRs. Overexpression of ICA69 forms more of the ICA69-PICK1 inhibitory complexes, which reduces synaptic targeting of iGluRs. Hence, the idea that the endogenous level of ICA69 is crucial for maintaining normal glutamate receptor clusters at the synapses is supported (Mallik, 2017).

    The data suggest that ~40% simultaneous reduction of GluRIIA/IIB/III at Drosophila NMJ synapses has no major consequence on larval synaptic physiology. Three possibilities are suggested to explain this. First, the relative levels of GluRIIA and GluRIIB subunits are crucial for determining the efficacy of synaptic transmission at the Drosophila NMJ synapse . The decrease for each of the GluRIIA, -IIB and -III subunits in the ICA69 mutant is almost identical; ~40% compared with controls. This hints towards a homeostatic compensatory mechanism whereby ~60% of the receptor subunits are sufficient to form enough functional receptor complexes, which can maintain normal synaptic strength. Second, the amount of GluRIII is reflective of the sum of GluRIIA and -IIB complexes together, and GluRIII is essential for the localization of GluRIIA and IIB subunits. A 40% decrease in GluRIII staining correlates well with an identical decrease in GluRIIA and -IIB staining. It is plausible that there is essentially negligible change in functional glutamate receptor assembly at ICA69 mutant synapses. Third, ICA69 possibly plays a role in trafficking glutamate receptors to the postsynaptic density and is not rate limiting in the formation of functional glutamate receptor complexes. Thus, ICA69 mutants exhibit normal synaptic physiology without embracing other compensatory mechanisms such as reduced quantal size or increased quantal content (Mallik, 2017).

    How might the iGluR levels relate to the NMJ growth? A tight correlation exists between the amount of synaptic glutamate receptors and the NMJ morphology. Downregulation of iGluRs in muscles has been shown to reduce the number of boutons. Similarly, hypomorphic mutants of GluRIII or GluRIIA have reduced bouton numbers. Consistent with this, overexpression of GluRIIA induces arborization and bouton number. Moreover, mutants with altered synaptic iGluR levels also show altered bouton numbers. For instance, neto and filamin (cheerio) mutants show reduced iGluR levels and bouton numbers. One of the possible mechanisms by which glutamate receptors can alter the NMJ morphology is through regulation of synaptic phospho-MAD levels. As the iGluRs (for instance, GluRIID) have also been shown to localize in central neuropil, it remains a possibility that the endogenous pattern of central electrical activity could also play crucial roles in sculpting the NMJ during development (Mallik, 2017).

    Retrograde signaling by Syt 4 induces presynaptic release and synapse-specific growth

    The molecular pathways involved in retrograde signal transduction at synapses and the function of retrograde communication are poorly understood. This study demonstrates that postsynaptic calcium 2+ ion (Ca2+) influx through glutamate receptors and subsequent postsynaptic vesicle fusion trigger a robust induction of presynaptic miniature release after high-frequency stimulation at Drosophila neuromuscular junctions. An isoform of the synaptotagmin family, synaptotagmin 4 (Syt 4), serves as a postsynaptic Ca2+ sensor to release retrograde signals that stimulate enhanced presynaptic function through activation of the cyclic adenosine monophosphate (cAMP)-cAMP-dependent protein kinase pathway. Postsynaptic Ca2+ influx also stimulates local synaptic differentiation and growth through Syt 4-mediated retrograde signals in a synapse-specific manner (Yoshihara, 2005).

    Neuronal development requires coordinated signaling to orchestrate pre- and postsynaptic maturation of synaptic connections. Synapse-specific enhancement of synaptic strength as occurs during long-term potentiation, as well as compensatory homeostatic synaptic changes, have been suggested to require retrograde signals for their induction. Although retrograde signaling has been implicated widely in synaptic plasticity, the molecular mechanisms that transduce postsynaptic Ca2+ signals during enhanced synaptic activity to alterations in presynaptic function are poorly characterized. Because postsynaptic Ca2+ is essential for synapse-specific potentiation, it is important to characterize how Ca2+ can regulate retrograde communication at synapses (Yoshihara, 2005).

    To dissect the mechanisms underlying activity-dependent synaptic plasticity, test were performed to see whether newly formed Drosophila glutamatergic neuromuscular junctions (NMJs), which have ∼30 active zones, show physiological changes after 100-Hz stimulation (5-1552+ chelator EGTA from the patch pipette caused a modest suppression of HFMR, whereas the fast Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) induced strong suppression by 2.5 min of perfusion. Longer perfusion with BAPTA for 5 min before stimulation abolished HFMR, indicating HFMR is induced after postsynaptic Ca2+ influx (Yoshihara, 2005).

    Ca2+-induced vesicle fusion in presynaptic terminals provides a temporally controlled and spatially restricted signal essential for synaptic communication. Postsynaptic vesicles within dendrites have been visualized by transmission electron microscopy, and dendritic release of several neuromodulators has been reported. To test whether postsynaptic vesicle fusion might underlie the Ca2+-dependent release of retrograde signals, postsynaptic vesicle recycling was blocked by using the dominant negative shibirets1 mutation, which disrupts endocytosis at elevated temperatures. shibirets1 was expressed specifically in postsynaptic muscles by driving a UAS-shibirets1 transgene with muscle-specific myosin heavy chain (Mhc)-Gal4, keeping presynaptic activity intact. At the permissive temperature (23°C), high-frequency stimulation induced normal HFMR. However, raising the temperature to 31°C suppressed HFMR in the presence of postsynaptic shibirets1, whereas wild-type animals displayed normal HFMR at 31°C. Basic synaptic properties in Mhc-Gal4, UAS-shibirets1 animals were not affected at either the permissive or the restrictive temperature. The suppression of HFMR is not due to irreversible damage induced by postsynaptic UAS-shibirets1 expression, because a second high-frequency stimulation after recovery to the permissive temperature induced normal HFMR (Yoshihara, 2005).

    The synaptic vesicle protein synaptotagmin 1 (Syt 1) is the major Ca2+ sensor for vesicle fusion at presynaptic terminals but is not localized postsynaptically. It has recently been shown that another isoform of the synaptotagmin family, synaptotagmin 4 (Syt 4), is present in the postsynaptic compartment (Adolfsen, 2004), suggesting Syt 4 might function as a postsynaptic Ca2+ sensor. Syt 4 immunoreactivity is observed in a punctate pattern surrounding presynaptic terminals, suggesting Syt 4 is present on postsynaptic vesicles. Postsynaptic vesicle recycling was blocked by using the UAS-shibirets1 transgene driven with Mhc-Gal4. Without a temperature shift, Syt 4-containing vesicles showed their normal postsynaptic distribution surrounding presynaptic terminals. When the temperature was shifted to 37oC for 10 min in the presence of high-K+ saline containing 1.5 mM Ca2+ to drive synaptic activity, Syt 4-containing vesicles translocated to the plasma membrane. After recovery at 18oC for 20 min, postsynaptic vesicles returned to their normal position. Removing extracellular Ca2+ during the high-K+ stimulation resulted in vesicles that did not translocate to the postsynaptic membrane (Yoshihara, 2005).

    To further test whether the Syt 4 vesicle population undergoes fusion with the postsynaptic membrane as opposed to mediating fusion between intracellular compartments, transgenic animals were constructed expressing a pH-sensitive green fluorescent protein (GFP) variant (ecliptic pHluorin) fused at the intravesicular N terminus of Syt 4. Ecliptic pHluorin increases its fluorescence 20-fold when exposed to the extracellular space from the acidic lumen of intracellular vesicles during fusion. Expression of Syt 4-pHluorin in postsynaptic muscles resulted in intense fluorescence at specific subdomains in the postsynaptic membrane, defining regions where Syt 4 vesicles undergo exocytosis. The fluorescence was not diffusely present over the postsynaptic membrane but directed to restricted compartments. Mhc-Gal4, UAS-Syt 4-pHluorin larvae were costained with antibodies against the postsynaptic density protein, DPAK, and nc82, a monoclonal antibody against a presynaptic active zone protein. Syt 4-pHluorin colocalized with DPAK and localized adjacent to nc82, demonstrating that Syt 4-pHluorin translocates from postsynaptic vesicles to the plasma membrane at postsynaptic densities opposite presynaptic active zones (Yoshihara, 2005).

    To examine the function of Syt 4-dependent postsynaptic vesicle fusion, the phenotypes of a syt 4 null mutant (syt 4BA1) and a syt 4 deficiency (rn16) were tested. Mutants lacking Syt 4 hatch from the egg case 21 hours after egg laying at 25oC, similar to wild type, and grow to fully mature larvae that pupate and eclose with a normal time course. To determine whether postsynaptic vesicle fusion triggered by Ca2+ influx is required for HFMR, the effects of high-frequency stimulation in syt 4 mutants were analyzed. In contrast to controls, the increase of miniature release was eliminated in syt 4 mutants. Postsynaptic expression of a UAS-syt 4 transgene completely restored HFMR in the null mutant, demonstrating that postsynaptic Syt 4 is required for triggering enhanced presynaptic function. Presynaptic expression of a UAS-syt 4 transgene did not restore HFMR. In addition, postsynaptic expression of a mutant Syt 4 with neutralized Ca2+-binding sites in both C2A and C2B domains did not rescue HFMR, indicating that retrograde signaling by Syt 4 requires Ca2+ binding (Yoshihara, 2005).

    The large increase in miniature frequency observed during HFMR is similar to the enhancement of presynaptic release after activation of cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) described in Aplysia and Drosophila. Bath application of forskolin, an activator of adenylyl cyclase, results in a robust enhancement of miniature frequency at Drosophila NMJs similar to that observed during HFMR, suggesting retrograde signals may function to increase presynaptic cAMP. To test the role of the cAMP-PKA pathway in HFMR, DC0 mutants were assayed for the presence of HFMR. DC0 encodes the major catalytic subunit of PKA in Drosophila and has been implicated in olfactory learning. Similar to the lack of forskolin-induced miniature induction, DC0 null mutants lacked HFMR. Bath application of forskolin in syt 4 mutants resulted in enhanced miniature frequency, suggesting activation of the cAMP pathway can bypass the requirement for Syt 4 in synaptic potentiation (Yoshihara, 2005).

    To further explore the role of retrograde signaling at Drosophila synapses, the role of activity was tested in synapse differentiation and growth. During Drosophila embryonic development, presynaptic terminals undergo a stereotypical structural change from a flat path-finding growth cone into varicose synaptic terminals through dynamic reconstruction. Such developmental changes in synaptic structure may share common molecular mechanisms with morphological changes induced during activity-dependent plasticity. Synaptic transmission was eliminated by using a deletion mutation that removes the postsynaptic glutamate receptors, DGluRIIA and DGluRIIB (referred to as GluRs). Postsynaptic currents normally induced by nerve stimulation were completely absent in the mutants (gluR). Miniatures were also eliminated, even at elevated extracellular Ca2+ concentrations of 4 mM. In the absence of GluRs, the presynaptic morphology of motor terminals is abnormal, even though GluRs are only expressed in postsynaptic muscles. GluR-deficient terminals maintain a flattened growth cone-like structure and fail to constrict into normal synaptic varicosities. Synaptic development was also assayed in a null mutant of the presynaptic plasma membrane t-SNARE [SNAP (soluble N-ethylmaleimide-sensitive factor attachment protein) receptor], syntaxin (syx), which eliminates neurotransmitter release, providing an inactive synapse similar to that in the gluR mutant. syx null mutants also have abnormal growth cone-like presynaptic terminals with less varicose structure (Yoshihara, 2005).

    Because activity is required for synapse development, whether Syt 4-dependent vesicle fusion may be required, similar to its role in acute retrograde signaling during HFMR, was tested. Physiological analysis revealed that the amplitude of evoked currents in mutants lacking Syt 4 was moderately reduced compared with wild type, suggesting weaker synaptic function or development. Similar to the morphological phenotype of the gluR mutant, syt 4 null mutant embryos showed defective presynaptic differentiation. Nerve terminals lacking Syt 4 displayed reduced varicose structure, whereas wild-type terminals have already formed individual varicosities at this stage of development. Postsynaptic expression with a UAS-syt 4 transgene rescued the physiological and morphological phenotypes. Syt 4 Ca2+-binding deficient mutant transgenes did not rescue either the morphological immaturity or the reduced amplitude of evoked currents, even though Syt 4 immunoreactivity at the postsynaptic compartment was restored by muscle-specific expression of the mutant syt 4 transgene, similar to the wild-type syt 4 transgene and endogenous Syt 4 immunoreactivity (Yoshihara, 2005).

    Mammalian syt 4 was originally identified as an immediate-early gene that is transcriptionally up-regulated by nerve activity in certain brain regions. Therefore, this study analyzed gain-of-function phenotypes caused by postsynaptic Syt 4 overexpression specifically in muscle cells to increase the probability of postsynaptic vesicle fusion. Syt 4 overexpression induced overgrowth of presynaptic terminals in mature third instar larvae, in contrast to overexpression of Syt 1, which does not traffic to Syt 4-containing postsynaptic vesicles. In addition to synaptic overgrowth, Syt 4 overexpression occasionally induced the formation of abnormally large varicosities. Postsynaptic overexpression of the Syt 4 Ca2+-binding mutant did not induce synaptic overgrowth, indicating that retrograde signaling by Syt 4 also requires Ca2+ binding to promote synaptic growth (Yoshihara, 2005).

    To determine whether the cAMP-PKA pathway is important in activity-dependent synaptic growth, the effects of PKA on synaptic morphology were assayed. Expression of constitutively active PKA presynaptically using a motor neuron-specific Gal4 driver induced not only synaptic overgrowth but also larger individual varicosities in mature third instar larvae, similar to those induced by postsynaptic overexpression of Syt 4. These observations are consistent with the presynaptic overgrowth observed in the learning mutant, dunce, which disrupts the enzyme that degrades cAMP, and with studies in Aplysia implicating PKA in synaptic varicosity formation. The loss-of-function phenotype of PKA mutants (DC0B3) were characterized at the embryonic NMJ to compare with gluR and syt 4 mutants. Presynaptic terminals in the DC0 mutant were morphologically aberrant, with abnormal growth cone-like features and less varicose structure. Postsynaptic expression of a constitutively active PKA transgene in the DC0 or syt 4 mutant backgrounds rescued the immature morphology, suggesting activation of PKA is downstream of Syt 4-dependent release of retrograde signals (Yoshihara, 2005).

    Similar to the role of Syt 1-dependent synaptic vesicle fusion in triggering synaptic transmission at individual synapses, Syt 4-dependent vesicle fusion might trigger synapse-specific plasticity and growth. To test synapse specificity, advantage was taken of the specific properties of the Drosophila NMJ at muscle fibers 6 and 7, where two motorneurons innervate both muscle fibers 6 and 7 during development. Syt 4 was expressed specifically in embryonic muscle fiber 6 but not muscle fiber 7 by using the H94-Gal4 driver. If Syt 4-dependent retrograde signals induce general growth of the motorneuron, one would expect to see a proliferation of synapses on both muscle fibers. Alternatively, if Syt 4 promoted local synaptic growth, one would expect specific activation of synapse proliferation only on target muscle 6, releasing the Syt 4-dependent signal. UAS-syt 4 driven by H94-Gal4 increased innervation on muscle fiber 6 compared with that on muscle fiber 7 in third instar larvae. Control experiments with Syt 4 Ca2+-binding deficient mutant transgenes, or a transgene encoding Syt 1, did not result in proliferation. Thus, synaptic growth can be preferentially directed to specific postsynaptic targets where Syt 4-dependent retrograde signals predominate, allowing differential strengthening of active synapses via local rewiring (Yoshihara, 2005).

    On the basis of the results described in this study, a local feedback model is proposed for activity-dependent synaptic plasticity and growth at Drosophila NMJs. Synapse-specific Ca2+ influx triggers postsynaptic vesicle fusion through Syt 4. Fusion of Syt 4-containing vesicles with the postsynaptic membrane releases locally acting retrograde signals that activate the presynaptic terminal, likely through the cAMP pathway. Active PKA then triggers cytoskeletal changes by unknown effectors to induce presynaptic growth and differentiation. Moreover, PKA is well known to facilitate neurotransmitter release directly, triggering a local synaptic enhancement of presynaptic release as shown in HFMR. Therefore, postsynaptic vesicular fusion might initiate a positive feedback loop, providing a localized activated synaptic state that can be maintained beyond the initial trigger (Yoshihara, 2005).

    As a general mechanism for memory storage, Hebb postulated that potentiated synapses maintain an activated state until structural changes occur to consolidate alterations in synaptic strength. The current results demonstrate that acute plasticity and synapse-specific growth require Syt 4-dependent retrograde signaling at Drosophila NMJs. The feedback mechanism described in this study could be a molecular basis for both input-specific postsynaptic tagging and an output-specific presynaptic mark or tag for long-lasting potentiation. The regenerative nature of a positive feedback signal allows individual synapses to be tagged in a discrete all-or-none manner until synaptic rewiring is completed. The synaptic tag is maintained as a large increase in miniature frequency at Drosophila NMJs, suggesting a previously unknown role for miniature release in neuronal function. The spatial resolution for input and output specificity would result from the accuracy insured by Ca2+-dependent vesicle fusion and subsequent diffusion, similar to the precision of presynaptic neurotransmitter release (Yoshihara, 2005).

    Postsynaptic glutamate receptors regulate local BMP signaling at the Drosophila neuromuscular junction

    Effective communication between pre- and post-synaptic compartments is required for proper synapse development and function. At the Drosophila neuromuscular junction (NMJ), a retrograde BMP signal functions to promote synapse growth, stability and homeostasis and coordinates the growth of synaptic structures. Retrograde BMP signaling triggers accumulation of the pathway effector pMad in motoneuron nuclei and at synaptic termini. Nuclear pMad, in conjunction with transcription factors, modulates the expression of target genes and instructs synaptic growth; a role for synaptic pMad remains to be determined. This study reports that pMad signals are selectively lost at NMJ synapses with reduced postsynaptic sensitivities. Despite this loss of synaptic pMad, nuclear pMad persisted in motoneuron nuclei, and expression of BMP target genes was unaffected, indicating a specific impairment in pMad production/maintenance at synaptic termini. During development, synaptic pMad accumulation followed the arrival and clustering of ionotropic glutamate receptors (iGluRs) at NMJ synapses. Synaptic pMad was lost at NMJ synapses developing at suboptimal levels of iGluRs and Neto, an auxiliary subunit required for functional iGluRs. Genetic manipulations of non-essential iGluR subunits revealed that synaptic pMad signals specifically correlate with the postsynaptic type-A glutamate receptors. Altering type-A receptor activities via protein kinase A (PKA) revealed that synaptic pMad depends on the activity and not the net levels of postsynaptic type-A receptors. Thus, synaptic pMad functions as a local sensor for NMJ synapse activity and has the potential to coordinate synaptic activity with a BMP retrograde signal required for synapse growth and homeostasis (Sulkowski, 2013).

    Previous work has described Neto as the first nonchannel subunit required for the clustering of iGluRs and formation of functional synapses at the Drosophila NMJ. Neto and iGluR complexes associate in the striated muscle and depend on each other for targeting and clustering at postsynaptic specializations. This study shows that Neto/iGluR synaptic complexes induce accumulation of pMad at synaptic termini in an activity-dependent manner. The effect of Neto/iGluR clusters on BMP signaling is selective, and limited to synaptic pMad; nuclear accumulation of pMad appears largely independent of postsynaptic glutamate receptors. This study demonstrates that synaptic pMad mirrors the activity of postsynaptic type-A receptors. As such, synaptic pMad may function as an acute sensor for postsynaptic sensitivity. Local fluctuations in synaptic pMad may provide a versatile means to relay changes in synapse activity to presynaptic neurons and coordinate synapse activity status with synapse growth and homeostasis (Sulkowski, 2013).

    Drosophila NMJs maintain their evoked potentials remarkably constant during development, from late embryo to the third instar larval stages. This coordination between motoneuron and muscle properties requires active trans-synaptic signaling, including a retrograde BMP signal, which promotes synaptic growth and confers synaptic homeostasis. Nuclear pMad accumulates in motoneurons during late embryogenesis. However, embryos mutant for BMP pathway components hatch into the larval stages, indicating that BMP signaling is not required for the initial assembly of NMJ synapses and instead modulates NMJ growth and development. This study demonstrates that synaptic accumulation of pMad follows GluRIIA arrival at nascent NMJs and depends on optimal levels of synaptic Neto and iGluRs. As type-A receptors have been associated with nascent synapses, and type-B receptors mark mature NMJs, accumulation of synaptic pMad appears to correlate with a growing phase at NMJ synapses. Furthermore, synaptic pMad correlates with the activity and not the net levels of postsynaptic type-A receptors. In fact, expression of a GluRIIA variant with a mutation in the putative ion conduction pore triggered reduction of synaptic pMad levels. Thus, synaptic pMad functions as a molecular sensor for synapse activity and may constitute an important element in synapse plasticity (Sulkowski, 2013).

    The synaptic pMad pool has been localized primarily to the presynaptic compartment. However, a contribution for postsynaptic pMad to the pool of synaptic pMad is also possible. Postsynaptic pMad accumulates in response to glia-secreted Mav, which regulates gbb expression and indirectly modulates the Gbb-mediated retrograde signaling (Fuentes-Medel, 2012). RNAi experiments revealed that knockdown of mad in muscle induces a decrease in synaptic pMad, albeit much reduced in amplitude compared with knockdown of mad in motoneurons (Fuentes-Medel, 2012). Also, knockdown of wit in motoneurons, but not in muscle, and knockdown of put in muscle, but not in motoneurons, triggers reduction of synaptic pMad (Fuentes-Medel, 2012). Intriguingly, the synaptic pMad is practically abolished in GluRIIA and neto109 mutants and cannot be further reduced by additional decrease in Mad levels. Whereas loss of postsynaptic pMad could be due to a Mav-dependent feedback mechanism that controls Gbb secretion from the muscle, the absence of presynaptic pMad demonstrates a role for GluRIIA and Neto in modulation of BMP retrograde signaling (Sulkowski, 2013).

    As BMP signals are generally short lived, synaptic pMad probably reflects accumulation of active BMP/receptor complexes at synaptic termini. Recent evidence suggests that BMP receptors traffic along the motoneuron axons, with Gbb/receptors complexes moving preferentially in a retrograde direction. By contrast, Mad does not appear to traffic. Thus, Mad is likely to be phosphorylated and maintained locally by a pool of active Gbb/BMP receptor complexes that remain at synaptic termini for the time postsynaptic type-A receptors are active (Sulkowski, 2013).

    The activity of type-A glutamate receptors may control synaptic pMad accumulation (1) indirectly via activity-dependent changes that are relayed to both pre- and postsynaptic cells, or (2) directly by influencing the production and signaling of varied Gbb ligand forms or by localizing Gbb activities. For example, inhibition of postsynaptic receptor activity induces trans-synaptic modulation of presynaptic Ca2+ influx. Such Ca2+ influx changes may trigger events that induce a local change in synaptic pMad accumulation. One possibility is that changes in Ca2+ influx may recruit Importin-β11 at presynaptic termini, which in turn mediate synaptic pMad accumulation (Sulkowski, 2013).

    At the Drosophila NMJ, Gbb is secreted in the synaptic cleft from both pre- and postsynaptic compartments. The secretion of Gbb is regulated at multiple levels, transcriptionally and post-translationally. Furthermore, the Gbb prodomain could be processed at several cleavage sites to generate Gbb ligands with varying activities. The longer, more active Gbb ligand retains a portion of the prodomain that could influence the formation of Gbb/BMP receptor complexes. Synaptic pMad may result from signaling by selective forms of Gbb. Or type-A receptors could modulate secretion and processing of Gbb in an activity-dependent manner. Understanding the function of different pools and active forms of Gbb within the synaptic cleft will help explain the multiple roles for Gbb at Drosophila NMJs (Sulkowski, 2013).

    Alternatively, active postsynaptic type-A receptor complexes may directly engage and stabilize presynaptic Gbb/BMP receptor signaling complexes via trans-synaptic interactions. CUB domains can directly bind BMPs; thus Neto may utilize its extracellular CUB domains to engage Gbb and/or presynaptic BMP receptors. As synaptic pMad mirrors active type-A receptors, such trans-synaptic complexes will depend on Neto in complexes with active type-A receptors. No capture has yet been shown of a direct interaction between Gbb and Neto CUB domains in co-immunoprecipitation experiments. Nonetheless, a trans-synaptic complex that depends on the activity of type-A receptors could offer a versatile means for relaying synapse activity status to the presynaptic neuron via fast assembly and disassembly (Sulkowski, 2013).

    Irrespective of the strategy that correlates synaptic pMad pool with the active type-A receptor/Neto complexes, further mechanisms must act to maintain the Gbb/BMP receptor complexes at synapses and protect them from endocytosis and retrograde transport. Such mechanisms must be specific, as general modulators of BMP receptors endocytosis impact both synaptic and nuclear pMad. A candidate for differential control of BMP/receptor complexes is Importin-β11. Loss of synaptic pMad in importin-β11 is rescued by neuronal expression of activated BMP receptors, by blocking retrograde transport, but not by neuronal expression of Mad. As Mad does not appear to traffic, presynaptic Importin-β11 must act upstream of the BMP receptors, perhaps to stabilize active Gbb/BMP receptor complexes at the neuron membrane. By contrast, local pMad cannot be restored at Neto-deprived NMJs by overactivation of presynaptic BMP receptors or by blocking retrograde transport. As neto and gbb interact genetically, it is tempting to speculate that postsynaptic Neto/type-A complexes localize Gbb activities and stabilize Gbb/BMP receptor complexes from the extracellular side. Additional extracellular factors, for example heparan proteoglycans, or intracellular modulators, such as Nemo kinase, may control the distribution of sticky Gbb molecules within the synaptic cleft and their binding to BMP receptors, or may stabilize Gbb/BMP receptor complexes at synaptic termini (Sulkowski, 2013).

    Synaptic pMad may act locally and/or in coordination with the transcriptional control of BMP target genes to ensure proper growth and development of the synaptic structures. A presynaptic pool of pMad maintained by Importin-β11 neuronal activities ensures normal NMJ structure and function. Like importin-β11, GluRIIA and Neto-deprived synapses show a significantly reduced number of boutons. Intriguingly, the absence of GluRIIA induces up to 20% reduction in bouton numbers, whereas knockdown of GluRIIB does not appear to affect NMJ growth. Although the amplitude of the growth phenotypes observed in normal culturing conditions (25°C) was modest, this phenomenon may explain the requirement for GluRIIA reported for activity-dependent NMJ development (at 29°C). Furthermore, knockdown of Neto or any iGluR essential subunit affect synaptic pMad and NMJ growth in a dose-dependent manner. Not significant changes were found in nuclear pMad or expression of BMP target genes in GluRIIA or Neto-deprived animals, but the restoration of synaptic pMad by presynaptic constitutively active BMP receptors rescues the morphology and physiology of importin-β11 mutant NMJs. The smaller NMJs observed in the absence of local pMad may reflect a direct contribution of synaptic pMad to retrograde BMP signaling, a pathway that provides an instructive signal for NMJ growth. Thus, BMP signaling may integrate synapse activity status with the control of synapse growth (Sulkowski, 2013).

    Synaptic pMad may also contribute to synapse stability. Mutants in BMP signaling pathway have an increased number of 'synaptic footprints': regions of the NMJ where the terminal nerve once resided and has retracted. It has been proposed that Gbb binding to its receptors activates the Williams Syndrome-associated Kinase LIMK1 to stabilize the NMJ. Synaptic pMad may further contribute to the stabilization of synapse contacts by engaging in interactions that anchor the Gbb/BMP receptor complexes at synaptic termini. During neural tube closure, local pSmad1/5/8 mediates stabilization of BMP signaling complexes at tight junction via binding to apical polarity complexes. Flies may utilize a similar anchor mechanism that relies on pMad-mediated interactions for stabilizing BMP signaling complexes and other components at synaptic junctions. Local active BMP signaling complexes are thought to function in this manner in the maintenance of stemness and in epithelial-to-mesenchymal transition (Sulkowski, 2013).

    Separate from its role in synapse growth and stability, BMP signaling is required presynaptically to maintain the competence of motoneurons to express homeostatic plasticity. The requirements for BMP signaling components for the rapid induction of presynaptic response may include a role for synaptic pMad in relaying acute perturbations of postsynaptic receptor function to the presynaptic compartment. At the very least, attenuation of local pMad signals, when postsynaptic type-A receptors are lost or inactive, may release local Gbb/BMP receptor complexes and allow them to traffic to neuron soma and increase the BMP transcriptional response, promoting expression of presynaptic components and neurotransmitter release. In addition, synaptic pMad-dependent complexes may influence the composition and/or activity of postsynaptic glutamate receptors. Although future experiments will be needed to address the nature and function of local pMad-containing complexes, the current findings clearly demonstrate that synaptic pMad constitutes an exquisite monitor of synapse activity status, which has the potential to relay information about synapse activity to both pre- and postsynaptic compartments and contribute to synaptic plasticity. As BMP signaling plays a crucial role in synaptic growth and homeostasis at the Drosophila NMJ, the use of synaptic pMad as a sensor for synapse activity may enable the BMP signaling pathway to monitor synapse activity then function to adjust synaptic growth and stability during development and homeostasis (Sulkowski, 2013).

    MAN1 restricts BMP signaling during synaptic growth in Drosophila

    Bone morphogenic protein (BMP) signaling is crucial for coordinated synaptic growth and plasticity. This study shows that the nuclear LEM-domain protein MAN1 is a negative regulator of synaptic growth at Drosophila larval and adult neuromuscular junctions (NMJs). Loss of MAN1 is associated with synaptic structural defects, including floating T-bars, membrane attachment defects, and accumulation of vesicles between perisynaptic membranes and membranes of the subsynaptic reticulum. In addition, MAN1 mutants accumulate more heterogeneously sized vesicles and multivesicular bodies in larval and adult synapses, the latter indicating that MAN1 may function in synaptic vesicle recycling and endosome-to-lysosome trafficking. Synaptic overgrowth in MAN1 is sensitive to BMP signaling levels, and loss of key BMP components attenuate BMP-induced synaptic overgrowth. Based on these observations, it is proposed that MAN1 negatively regulates accumulation and distribution of BMP signaling components to ensure proper synaptic growth and integrity at larval and adult NMJs (Laugks, 2016).

    Jelly belly trans-synaptic signaling to anaplastic lymphoma kinase regulates neurotransmission strength and synapse architecture

    In Drosophila, the secreted signaling molecule Jelly Belly (Jeb) activates anaplastic lymphoma kinase (Alk), a receptor tyrosine kinase, in multiple developmental and adult contexts. Jeb and Alk are highly enriched at Drosophila synapses within the CNS neuropil and neuromuscular junction (NMJ), and a conserved intercellular signaling function was been postulated. At the embryonic and larval NMJ, Jeb is localized in the motor neuron presynaptic terminal whereas Alk is concentrated in the muscle postsynaptic domain surrounding boutons, consistent with anterograde trans-synaptic signaling. This study shows that neurotransmission is regulated by Jeb secretion by functional inhibition of Jeb-Alk signaling. Jeb is a novel negative regulator of neuromuscular transmission. Reduction or inhibition of Alk function results in enhanced synaptic transmission. Activation of Alk conversely inhibits synaptic transmission. Restoration of wild-type postsynaptic Alk expression in Alk partial loss-of-function mutants rescues NMJ transmission phenotypes and confirms that postsynaptic Alk regulates NMJ transmission. The effects of impaired Alk signaling on neurotransmission are observed in the absence of associated changes in NMJ structure. Complete removal of Jeb in motor neurons, however, disrupts both presynaptic bouton architecture and postsynaptic differentiation. Nonphysiologic activation of Alk signaling also negatively regulates NMJ growth. Activation of Jeb-Alk signaling triggers the Ras-MAP kinase cascade in both pre- and postsynaptic compartments. These novel roles for Jeb-Alk signaling in the modulation of synaptic function and structure have potential implications for recently reported Alk functions in human addiction, retention of spatial memory, cognitive dysfunction in neurofibromatosis, and pathogenesis of amyotrophic lateral sclerosis (Rohrbough, 2013).

    The results support an anterograde signaling model in which presynaptically secreted Jeb activates postsynaptic Alk. The data to support this hypothesis derives from multiple tests. First, immunolabeling shows Jeb is concentrated within presynaptic boutons, while Alk is present in the surrounding postsynaptic subsynaptic reticulum (SSR) (Rohrbough, 2011). Second, targeted postsynaptic Alk expression in Alk LOF mutants is sufficient to rescue synaptic transmission defects, a strong demonstration that Alk is required in the postsynaptic muscle to regulate neurotransmission. Third, post-synaptic inhibtion of Alk by tissue specific RNAi results in 2- fold increased accumulation of perisynaptic Jeb. Fourth, the MARCM clonal approach demonstrates Jeb may be required within presynaptic motor neurons to regulate postsynaptic molecular assembly. Fifth, elevated presynaptic Jeb expression activates postsynaptic Ras/MAPK/ERK activation, while inhibition of postsynaptic Alk reduces Ras/MAPK/ERK activitation (Rohrbough, 2013).

    In structurally normal NMJs, strong effects on neurotransmission were found as a consequence of perturbations in Jeb-Alk signaling. The clearest, most consistent results derive from techniques that activate or inhibit Jeb-Alk signaling postsynaptically. Postsynaptic hyperactivation of Alk weakens NMJ synaptic transmission. This functional phenotype parallels the negative regulation of synaptic growth by postsynaptic Alk activation. Consistent with the inhibitory effect of Alk activation on neurotransmission, enhanced neurotransmission was observed as a consequence of muscle specific reductions in Alk levels by transgenic RNAi. Additional confirmation for Alk-dependent inhibition of neurotransmission is provided by analysis of a hypomorphic temperature sensitive allele of Alk. Partial loss of Alk function results in strongly increased NMJ neurotransmission. The implication is that Alk activity limits or negatively regulates synaptic strength. It was also shown that muscle-specific Alk expression in the strongest alkts/alkf01491 partial loss of function genotype rescues reduced neurotransmission to near wild-type levels, a conclusive demonstration that postsynaptic Alk function negatively regulates the strength of NMJ neurotransmission. This function is novel: Jeb-Alk transynaptic signaling is the only known negative regulator of synaptic transmission (Rohrbough, 2013).

    Presynaptic manipulation of Jeb yields less strong though still consistent results. Transmission is uneffected by increased pan-neuronal Jeb expression, though this activates Ras/MAPK/ERK both centrally and presynaptically at the NMJ and, to a lesser degree, within the postsynaptic muscle. Motor neuron electrical activity activates neuronal Ras/MAPK/ERK signaling, and this presynaptic Ras/MAPK/ERK activation is positively linked to both structural and functional NMJ synaptic remodeling. Motor neuron specific over expression of Jeb does produce a modest but statistically significant reduction in neuromuscular transmission. Ectopic expression of Jeb in muscle results in substantial inhibiton of neuromuscular transmission. One hypothesis that may account for the diffence between panneuronal and motor neuron or muscle specific manipulation of Jeb-Alk signalling is that the effects of manipulating pan-neuronal Jeb represent a composite of central and peripheral effects on the motor neuron. In first instar larvae it was found that both jeb and alk mutants display impaired central output to motor neurons most consistent with a central synaptic defect (Rohrbough, 2011). The integrated physiologic function subserved by Jeb-Alk signaling in the NMJ, which has yet to be determined, will provide the essential context for interpretting these results (Rohrbough, 2013).

    The novel inhibitory role of Jeb-Alk signaling in NMJ transmission implies that it is part of a transynaptic regulatory network that integrates neuronal activity and responses with other homeostatic mechanisms. This study provides indirect evidence that Jeb secretion is regulated. The physiologic regulation of Jeb secretion is a critical missing component of understanding how Jeb-Alk signaling fits into the regulation of synaptic plasticity. Jeb-Alk signaling regulates postembryonic NMJ synaptic growth and patterning Jeb-Alk signaling is not required for embryonic NMJ synaptogenesis or differentiation, although jeb and alk null mutants display impaired locomotion and reduced NMJ transmission (Rohrbough and Broadie 2011). At later developmental stages, removing Jeb in motor neurons strongly disrupts late larval NMJ synaptic terminal architecture and bouton morphology. Postsynaptic Dlg scaffolding and GluR clustering are strongly perturbed in association with jeb mutant terminals. The mosaic analysis supports a cell-autonomous, anterograde signaling function for Jeb. One mechanistic hypothesis is that Jeb-Alk nerve-to-muscle signaling regulates NMJ morphogenesis by recruiting or regulating cell adhesion molecules (CAMs). In the developing adult visual system, anterograde Jeb-Alk signaling induces the expression of postsynaptic adhesion molecules Dumbfounded/Kirre, Roughest/IrreC and Flamingo to shape the optic neuropil target environment. At the larval NMJ, adhesion molecules such as fasciclins and integrins regulate activity-dependent synaptic growth and structural remodeling. The current results imply that Jeb-Alk signaling either directly regulates Dlg localization or indirectly drives Dlg-dependent postsynaptic differentiation. Dlg has demonstrated roles in NMJ morphogenesis and GluR expression and field regulation, and directly binds and regulates fasciclin II and βPS integrin. Future work will test the hypothesis that Jeb-Alk signaling organizes or regulates adhesion receptors and postsynaptic scaffolding to control bouton differentiation and shape functional synaptic architecture (Rohrbough, 2013).

    In other systems, Jeb-Alk signaling has been studied primarily at the level of behavior. In C. elegans, the Jeb homolog Hen-1 was identified in a forward genetic behavioral screen for impaired ability to integrate conflicting sensory input (Ishihara, 2002). The Hen-1 phenotype is non-developmental and can be rescued only by adult Hen-1 expression. There is no uniquely identified mammalian Jeb/Hen-1 homolog, but ALK is expressed in the mammalian nervous system during development and at maturity. Alk is expressed in the mouse hippocampus and Alk loss of function enhances behavioral performance in tests dependent on hippocampal function. Similarly, Drosophila learning has shown a dependence on the Ras/MAPK/ERK cascade, which is activated by Jeb-Alk signaling and is probably inhibited by Drosophila neurofibromin (dNf1). Genetic or pharmacologic inhibtion of Jeb-Alk signaling enhances associative learning while increased Jeb-Alk signaling or loss of dNf1 impairs learning. Inhibition of Alk rescues dNf1 mutant learning deficits. These studies suggest that the Jeb-Alk trans-synaptic pathway acts in concert with other, negative regulators of Ras/MAPK/ERK signaling to balance developmental and learning-related synaptic structural and functional changes. Strikingly, a whole-genome association study recently identified human ALK as one of a small number of genes associated with sporadic amyotrophic lateral sclerosis (ALS), a devistating neurodegerative disease of central motor units. If Alk has a conserved inhibitory role in synaptic physiological regulation, hypofunctional human Alk variants may result in augmented motor unit activity and contribute to excitotoxicity and progressive motor unit degeneration in ALS. Pharmacologic activation of Alk has already been hypothesized to have therapeutic benefit in treating ALS. Further insight from future studies should be gained into the mechanism by which the Jeb-Alk signaling pathway regulates synaptic adaptivity in both normal and pathological states (Rohrbough, 2013).

    Modeling spinal muscular atrophy in Drosophila links Smn to FGF signaling

    Spinal muscular atrophy (SMA), a devastating neurodegenerative disorder characterized by motor neuron loss and muscle atrophy, has been linked to mutations in the Survival Motor Neuron (SMN) gene. Based on an SMA model developed in Drosophila, which displays features that are analogous to the human pathology and vertebrate SMA models, the fibroblast growth factor (FGF) signaling pathway was functionally linked to the Drosophila homologue of SMN, Smn through interaction of the FGF receptor breathless with Smn. This study functionally characterize this relationship and demonstrates that Smn activity regulates the expression of FGF signaling components and thus FGF signaling. Furthermore, it was shown that alterations in FGF signaling activity are able to modify the neuromuscular junction defects caused by loss of Smn function and that muscle-specific activation of FGF is sufficient to rescue Smn-associated abnormalities (Sen, 2011).

    Given the variability of the SMA phenotype and the proven relationship between the severity of the disease and small changes in wild-type SMN activity, there is a significant possibility that any modifiers of SMN activity, either direct or indirect, will have therapeutic value. To systematically explore the genome for genes that are capable of modulating SMN function in vivo, advantage was taken of the existence of an SMA model offered by Drosophila to search for Smn genetic interactors. The model that was developed is based on the lethality and an associated neuromuscular junction phenotype linked to loss of Smn function, a phenotype remarkably similar to the NMJ phenotype reported for human patients. Though the role of SMN in biogenesis of snRNPs has been well documented, its regulators and downstream effectors have not been systematically delineated, nor has the link between mutations in SMN and the specific loss of motor neurons seen in SMA patients been uncovered. It may be the case that the specificity of this phenotype is reflective of either specialized SMN functions at the NMJ or a particular sensitivity of motor neurons to the loss of SMN activity. Among the genes the genetic strategy revealed as Smn loss of function modifiers was breathless, encoding an FGF receptor, thus establishing a link between Smn and the FGF pathway (Sen, 2011).

    Importantly, in addition to this link, it was also found that FGF signaling is independently involved in NMJ morphogenesis, a function demonstrated in vertebrates but not previously attributed to this pathway in Drosophila despite extensive characterization of its essential role in branching morphogenesis of the tracheal system, migration of multiple cell types, as well as the proper patterning of the mesoderm. The morphological effects that were observed, caused by the modulation of several pathway elements, plainly reveal an involvement of FGF signaling at the NMJ, a role confirmed by the electrophysiological analyses. The down-regulation of FGF signals in muscle results in a reduction of bouton numbers and is associated with increased mEJP amplitudes. The opposite effect is observed when FGF signaling is increased in muscles, suggesting that FGF signaling inversely regulates quantal size. Thus, FGF perturbation in muscle alters both presynaptic growth and specific aspects of synaptic transmission. These observations imply the existence of functional trans-synaptic homeostatic mechanisms, which have been previously shown to compensate for similar changes by increasing presynaptic bouton numbers and transmitter release. However, in this specific instance, only synaptic growth (bouton number) but not transmitter release (quantal content) is affected, the precise mechanisms for which remain unclear. Moreover, the fact that mEJP amplitudes are affected suggests that postsynaptic receptivity to glutamate release from the presynapse is altered. Similar quantal size phenotypes have been observed in several instances previously. For instance, postsynaptic PKA and NF-kappaB are known to regulate quantal size through changes in DGluRs. Directly altering the expression of various GluR subunits also predictably influences quantal size. The genetic interaction this study has demonstrated between FGF and Smn can be described as an epistatic relationship in which the FGF pathway functions downstream of Smn and is consistent with the observation that neuromuscular defects associated with loss of Smn function in muscle can be rescued by muscle-specific activation of FGF signaling. Intriguingly, the relationship described in this study between Smn and FGF is valid beyond the NMJ, as loss of Smn function genetic mosaics in the wing disc clearly result in the down-regulation of FGF signaling. Although the precise molecular mechanism underlying this relationship is still elusive, Smn activity affects transcript and protein levels of the FGF receptor, as well as the expression of additional elements of the FGF pathway. Whether this defines a cascade of interrelated events or whether each of these changes reflects an independent Smn-related regulatory event remains to be determined. Given the fact that Smn mutants in Drosophila display altered postsynaptic currents and severely compromised postsynaptic receptor clustering in muscles, it is conceivable that FGF signaling represents a link between Smn activity and postsynaptic glutamate receptor levels (Sen, 2011).

    It should be noted that a link between SMN and the FGF pathway has been suggested by a series of studies in vertebrates where a molecular interaction between an FGF-2 isoform and the SMN protein has been described.These studies raise the possibility that FGF-2 may negatively interfere with SMN complex function through SMN itself. Such observations would, on first appearance, suggest that the epistatic relationship between SMN and FGF signaling in vertebrate cells may be the reverse of what was observed in Drosophila. In point of fact however, the differences in the experimental parameters and approaches between these studies do not allow meaningful comparisons (Sen, 2011).

    An important question raised by the above phenotypic analyses is whether the abnormalities associated with FGF and/or Smn perturbations reflect developmental or maintenance issues. It may be the case that the larval system in Drosophila is not ideally suited to differentiate between these alternatives as larval tissue is destined to undergo programmed cell death (histolysis) during metamorphosis. One advantage that flies do offer, however, is the ability to dissociate the development of the adult neuromuscular system from its maintenance as the entirety of its development occurs during the pupal stage, before emergence of the adult. Thus, the Drosophila pupa/adult may provide a platform to address these issues, as Drosophila displays Smn-dependent adult phenotypes. In light of the relationship that was established between Smn and FGF signaling and the known involvement of FGF signaling in the development of both the larval and adult musculature, it will be particularly interesting to examine the effects of modulating FGF activity on the aforementioned processes. Such studies may be of particular relevance to SMA where it is quite difficult to discern the developmental consequences of SMN loss in humans, as neurodegenerative symptoms displayed by patients may obscure basic problems resulting from altered developmental programs such as neuronal pathfinding, initial NMJ formation, etc (Sen, 2011).

    In vertebrates, synaptic development and maintenance use at least three distinct signaling mechanisms: the TGF-β, wingless, and FGF pathways. In Drosophila, it is noteworthy that the first two have been demonstrated to function in a similar fashion at the NMJ. Remarkably, the genetic screens involving Smn have identified elements of all three of these pathways as modifiers of Smn-related phenotypes. These connections are considered particularly significant as they raise the possibility that Smn may serve as a node, integrating signaling events crucial for NMJ function, potentially leaving this structure particularly vulnerable to the loss of Smn. Though further correspondence between the Drosophila model and the human condition remains to be determined, the Smn-FGF relationship observed in Drosophila raises the possibility that pharmacological manipulation of FGF signals might mitigate SMN motor neuron-related abnormalities (Sen, 2011).

    Drosophila Neuroligin3 regulates neuromuscular junction development and synaptic differentiation

    Neuroligins (Nlgs) are a family of cell adhesion molecules thought to be important for synapse maturation and function. Studies in mammals have shown that different Nlgs have different roles in synaptic maturation and function. The functions of Drosophila Neuroligin1 (DNlg1), DNlg2, and DNlg4 have also been examined. This study reports the role of DNlg3 in synaptic development and function by using Drosophila neuromuscular junctions (NMJs) as a model system. DNlg3 was found to be expressed in both CNS and NMJs where it was largely restricted to the postsynaptic site. By generating and examining dnlg3 mutants, the mutants mutants were found to exhibit an increased bouton number and reduced bouton size compared to the wild-type. Consistent with alterations in bouton properties, pre- and postsynaptic differentiations were also affected including abnormal synaptic vesicle endocytosis, increased PSD length and reduced GluRIIA recruitment. Additionally, synaptic transmission was reduced. Altogether, this study shows that DNlg3 is required for NMJ development, synaptic differentiation and function (Xing, 2014).

    Anterograde Activin signaling regulates postsynaptic membrane potential and GluRIIA/B abundance at the Drosophila neuromuscular junction

    Members of the TGF-beta superfamily play numerous roles in nervous system development and function. In Drosophila, retrograde BMP signaling at the neuromuscular junction (NMJ) is required presynaptically for proper synapse growth and neurotransmitter release. This study analyzed whether the Activin branch of the TGF-beta superfamily also contributes to NMJ development and function. Elimination of the Activin/TGF-beta type I receptor babo, or its downstream signal transducer smox, does not affect presynaptic NMJ growth or evoked excitatory junctional potentials (EJPs), but instead results in a number of postsynaptic defects including depolarized membrane potential, small size and frequency of miniature excitatory junction potentials (mEJPs), and decreased synaptic densities of the glutamate receptors GluRIIA and B. The majority of the defective smox synaptic phenotypes were rescued by muscle-specific expression of a smox transgene. Furthermore, a mutation in actβ, an Activin-like ligand that is strongly expressed in motor neurons, phenocopies babo and smox loss-of-function alleles. These results demonstrate that anterograde Activin/TGF-beta signaling at the Drosophila NMJ is crucial for achieving normal abundance and localization of several important postsynaptic signaling molecules and for regulating postsynaptic membrane physiology. Together with the well-established presynaptic role of the retrograde BMP signaling via Glass bottom boat and Wishful thinking, these findings indicate that the two branches of the TGF-beta superfamily are differentially deployed on each side of the Drosophila NMJ synapse to regulate distinct aspects of its development and function (Kim, 2014).

    Numerous reports have now implicated the Activin/TGF-β and BMP branches of the TGF-β superfamily in regulating neuronal development, synaptic plasticity and cognitive behavior. Accordingly, members from both subfamilies are widely expressed in the nervous system and are co-expressed in multiple regions of vertebrate and invertebrate brains. It is therefore quite likely that ligands of both subfamilies co-exist within the extracellular space and in some cases, act on the same neurons. Lending support to this idea, pyramidal neurons in the CA3 region of the rat hippocampus are known to accumulate both phosphorylated Smad2 and Smad1/5/8, transcriptional transducers of the canonical Activin/TGF-β and BMP-type signaling, respectively. The activation of these two closely-related signaling pathways in common sets of neurons, or different cells of a common neuronal circuit raises the intriguing question of whether the two pathways play different or redundant roles during neuronal development and function (Kim, 2014).

    This study utilized the Drosophila neuromuscular junction to address this issue since ligands of both Activin/TGF-β and BMP families are expressed in both muscle and motor neurons. The data, together with previous studies on the role of BMP signaling at the NMJ, clearly demonstrate that the two pathways influence NMJ synaptogenesis in different ways. The Activin/TGF-β pathway is necessary for achieving the proper densities of GluRIIA, GluRIIB and Dlg in postsynaptic muscle membrane, while the BMP pathway has a smaller effect on the distribution of these postsynaptic proteins. In addition, the Activin/TGF-β pathway was dispensable for maintaining overall synaptic growth and homeostasis, both of which are strongly affected by mutations in the BMP pathway. In addition, tissue-specific rescue experiments indicate that the postsynaptic reception of Activin/TGF-β signaling is important in regulating synaptic GluR abundance, whereas BMP signal reception is known to act in the presynaptic motor neurons to promote synaptic growth. These observations suggest that each pathway influences NMJ synapse development and function by acting mainly in either the pre- or postsynaptic cell (Kim, 2014).

    Interestingly, the BMP and Activin/TGF-β pathways have also been recently found to control different aspects of the Drosophila innate immune response (Clark, 2011). In this case BMP signaling suppresses the expression of multiple antimicrobial peptide genes following wounding, whereas the Activin/TGF-β pathway limits melanization after bacterial infection in adult flies. Therefore, it appears that the division of labor between these subpathways is not limited to just the nervous system, rather it may be the norm when these related signaling pathways act in concert to regulate a common biological process (Kim, 2014).

    The fact that the pathways actually differ in how they affect a complex biological process is not surprising given that the different R-Smads are likely to have different selectivity in gene activation. Within motor neurons, BMP signaling promotes microtubule formation in axons and directly regulates expression of trio, a Rac GEF, that acts as a major regulator of actin cytoskeleton in many types of cells. Thus, it is likely that BMP signaling modulates synaptic growth, in part, by changing the structure and dynamics of the actin and microtubule cytoskeleton within motor neurons. BMP signaling also regulates the transcription of twit, a gene encoding a L-6 neurotoxin-like molecule that controls the frequency of presynaptic spontaneous vesicle release (Kim, 2012; Kim, 2014 and references therein).

    Targets of Drosophila Activin/TGF-β signaling in any tissue are less well characterized. Within the central brain, glial-derived Myo signals through Smox to control expression of the Ecdysone B1 receptors in remodeling mushroom body neurons. However, it is not clear if EcR-B1 is a direct or indirect target of smox transcriptional regulation. It is also unclear if Ecdysone signaling plays a role in regulating synaptogenesis at the NMJ, although it may play a role during metamorphic remodeling of the NMJ as it does for the mushroom body neurons. The only other known targets of Smox are InR, Pi3K and Akt, all of which are Insulin signaling components and are reduced in the Drosophila prothoracic gland in the absence of Activin/TGF-β signaling. Once again the effect may be indirect, but this finding is interesting since Insulin signaling components have been shown to control synaptic clustering of GluRs (Kim, 2014).

    The clustering of GluRs and Dlg at the NMJ have been shown to be regulated by both transcriptional and post-transcriptional mechanisms. For example, a recent genetic screen identified longitudinals lacking (lola), a BTN-Zn finger transcription factor, as an essential regulator of GluR and dPak expression in muscles. In contrast, the current studies on Activin/TGF-β signaling suggest, at least for GluRIIA, that this pathway functions at the post-transcriptional level since this study found that overexpression of glurIIA-gfp using an exogenous promotor and transcriptional activator does not lead to an enrichment of GluRIIAGFP at synaptic sites of Activin/TGF-β pathway mutants. This phenotype is reminiscent of that found for certain mutants in the NF-κB signaling system. Loss of Dorsal (an NF-κB homolog), Cactus (an IκB related factor), or Pelle (an IRAK kinase) leads to a substantial reduction of GluRIIA and a slight reduction of Dlg postsynaptic localization at the NMJ and a concomitant reduction in mEJP size. In addition, as was found for loss of Activin/TGF-β signaling, exogenously-expressed GluRIIA-myc did not reach the synaptic surface in NF-κB signaling mutants consistent with a possible role of Activin/TGF-β signaling in regulating NF-κB signaling. However, even if future studies show that the relationship is true, the Activin/TGF-β pathway likely regulates additional factors since its loss also affects GluRIIB levels and muscle resting potential, neither of which is altered in NF-κB pathway mutants. Interestingly, the regulation of GluRIIB levels by Activin/TGF-β signaling does appear to be at the level of transcription, indicating that this signaling pathway likely affects GluR clustering at the NMJ via both transcriptional and post-transcriptional mechanisms (Kim, 2014).

    Analysis of Activin/TGF-β signaling at the NMJ, coupled with previous studies on BMP signaling and the novel ligand Maverick, indicates that TGF-β ligands are produced in, and act upon, all three cell types that contribute to NMJ function, specifically the motor neuron, wrapping glia, and muscle (see Model of controlling NMJ development and function by Activin/TGF-β and BMP pathways). This leads to the important issue of how directionality of TGF-β signaling at the NMJ is regulated. One possibility is that ligands are sequestered, either inside the secreting cells or on their surfaces, so that they have limited access to receptors on the opposing pre or postsynaptic membrane. For example, Gbb is produced both in muscle and motor neurons, leading to the issue of how directional signaling from muscle to motor neurons is achieved. On the postsynaptic muscle, Gbb release is potentiated by dRich (Rho GTPase activating protein at 92B), a Cdc42 selective Gap while in the presynaptic neuron Crimpy, a Drosophila homolog of the vertebrate Crim1 gene, has been shown to bind to a precursor form of Gbb. The Gbb/Crimpy complex is thought to either interfere with secretion or activation of motor neuron-derived Gbb thus ensuring that only muscle-derived Gbb activates the retrograde BMP signal at the NMJ. Since there are a large number of characterized TGF-β superfamily binding proteins, Drosophila homologs of some of these factors such as the BMP binding proteins Cv-2, Sog, Tsg and Dally, or the Activin-binding protein Follistatin, may sequester and regulate levels of active ligands within the NMJ. Sequestering mechanisms may also provide direction control by facilitating autocrine as opposed to juxtacrine signaling. If ligand-binding proteins are associated with the membrane surface of the ligand-producing cell, they may facilitate delivery of the ligand to receptors on the producing cell, thus enhancing autocrine signaling. It is interesting in this regard that in the developing Drosophila retina, Actβ appears to signal in an autocrine fashion to control photoreceptor connectivity in the brain (Kim, 2014).

    Activin-type ligands are secreted from glia, motor neuron and muscle. The Activin-type ligands induce Babo-mediated phosphorylation of Smox that facilitates association with Med. In the muscle, the phospho-Smox/Med complexes activate the transcription of glurIIB and an unknown factor controlling post-transcriptional process or stability of glurIIA mRNA. In the motor neuron, the phospo-Smox/Med complex controls spontaneous release of synaptic vesicles via unknown mechanism(s). On the other hand, glia-secreted Mav stimulates Mad phosphorylation in the muscle resulting in increased gbb transcription. Gbb protein is released from the muscle and binds Tkv/Sax and Wit complex on the motor neuron leading to an accumulation of phospho-Mad in the nuclei by an unknown mechanism. The resultant phospho-Mad/Med complex activates the transcription of trio whose product promotes synaptic bouton formation (Kim, 2014).

    Another important mechanism to control signal direction is likely to be tissue-specific receptor expression. For example, Wit is highly enriched in motor neurons compared to muscle, and this may help ensure that Gbb released from the postsynaptic muscle signals to the presynaptic motor neuron. Type I receptor diversity may be even more important in controlling directionality since at least 2 isoforms of Tkv and three isoforms of Babo have been identified. In the case of Babo, Activin-like ligands have a clear preference for signaling through different receptor isoforms, and these isoforms show differential tissue expression (Kim, 2014).

    An additional factor to be considered in understanding TGF-β superfamily signal integration within different NMJ cell types is the possibility of canonical versus non-canonical and/or cross-pathway signaling. For example, in mushroom body neurons Babo can signal in a non-Smad dependent manner through Rho1, Rac and LIM kinase1 (LIMK1) to regulate axon growth and target recognition. Whether this mechanism, or another non-canonical pathway is operative at the NMJ is unclear. Cross-pathway signaling has also recently been identified in Drosophila. In this example, loss of Smox protein in the wing disc has been shown to up-regulate Mad activity in a Babo-dependent manner. Double mutants of babo and smox suppress the cross-pathway signal. As is described in this study, smox protein null mutations lead to significantly more severe GluR and mEJP defects than strong babo mutations alone, and this phenotype is suppressed in double mutants. Thus, as in wing discs, loss of Smox protein likely leads to ectopic Mad activity in muscles that further decrease GluR expression and/or localization at the NMJ. Consistent with this view, this study found that loss of Mad actually increases GluRIIB localization, suggesting that Mad acts negatively to regulate GluRIIB in muscle. One possible model to explain the Smox/Mad data is that normally the Babo/Smox signal inhibits Mad signaling which is itself a repressive signal for GluR accumulation. Thus, in babo mutants, total GluR levels decrease due to the loss of smox and therefore an increase in the repressive Mad signal. In the smox protein null mutant even more repressive Mad signal is generated by Babo further hyperactivating Mad activity leading to even lower levels of GluR accumulation. In medea mutants the activity of both pathways is reduced thereby returning the level of GluR levels close to normal. Additional experiments employing various single and double mutants, together with tissue-specific expression of various ligands, receptor isoforms and ligand-binding proteins will be needed to fully elucidate how vectorial TGF-β signaling is accomplished at the NMJ. Likewise, the identifcation of directly responding target genes and how they are influenced by both Smox and Mad signals is needed to fully appreciate how these two TGF-β signaling branches regulates NMJ functional activity (Kim, 2014).

    Regulation of postsynaptic retrograde signaling by presynaptic exosome release

    Retrograde signals from postsynaptic targets are critical during development and plasticity of synaptic connections. These signals serve to adjust the activity of presynaptic cells according to postsynaptic cell outputs and to maintain synaptic function within a dynamic range. Despite their importance, the mechanisms that trigger the release of retrograde signals and the role of presynaptic cells in this signaling event are unknown. This study shows that a retrograde signal mediated by Synaptotagmin 4 (Syt4) is transmitted to the postsynaptic cell through anterograde delivery of Syt4 via exosomes. Thus, by transferring an essential component of retrograde signaling through exosomes, presynaptic cells enable retrograde signaling (Korkut, 2013).

    This study shows that Syt4 protein functions in postsynaptic muscles to mediate activity-dependent presynaptic growth and potentiation of quantal release. However, to mediate this function Syt4 needs to be transferred from presynaptic terminals to postsynaptic muscle sites. Evidence is presented that, most likely, the entire pool of postsynaptic Syt4 is derived from presynaptic cells. Like the Wnt binding protein, Evi, Syt4 is packaged in exosomes, which provides a mechanism for the unusual transfer of transmembrane proteins across cells. Taken together, these studies support a novel mechanism for the presynaptic control of a retrograde signal, through the presynaptic release of exosomes containing Syt4 (Korkut, 2013).

    Larval NMJs continuously generate new synaptic boutons and their corresponding postsynaptic specializations, ensuring constant synaptic efficacy despite the continuous growth of muscle cells. This precise matching of pre- and postsynaptic compartments is regulated by electrical activity, which induces a retrograde signal in muscle to stimulate new presynaptic growth. This process is likely to fine-tune the magnitude of the retrograde signal in specific nerve terminal-muscle cell pairs, each with a characteristic size. Given that most larval muscle cells are innervated by multiple motorneurons, this mechanism may also enable spatial coincidence to ensure the synaptic specificity of plasticity, making certain that only those activated synapses within a cell become structurally regulated (Korkut, 2013).

    A distinct perisynaptic glial cell type forms tripartite neuromuscular synapses in the Drosophila adult

    Studies of Drosophila flight muscle neuromuscular synapses have revealed their tripartite architecture and established an attractive experimental model for genetic analysis of glial function in synaptic transmission. This study defined a new Drosophila glial cell type, designated peripheral perisynaptic glia (PPG), which resides in the periphery and interacts specifically with fine motor axon branches forming neuromuscular synapses. Identification and specific labeling of PPG was achieved through cell type-specific RNAi-mediated knockdown (KD) of a glial marker, Glutamine Synthetase 2 (GS2). In addition, comparison among different Drosophila neuromuscular synapse models from adult and larval developmental stages indicated the presence of tripartite synapses on several different muscle types in the adult. In contrast, PPG appear to be absent from larval body wall neuromuscular synapses, which do not exhibit a tripartite architecture but rather are imbedded in the muscle plasma membrane. Evolutionary conservation of tripartite synapse architecture and peripheral perisynaptic glia in vertebrates and Drosophila suggests ancient and conserved roles for glia-synapse interactions in synaptic transmission (Strauss, 2015).

    The extracellular-regulated kinase effector Lk6 is required for Glutamate receptor localization at the Drosophila neuromuscular junction

    The proper localization and synthesis of postsynaptic glutamate receptors are essential for synaptic plasticity. Synaptic translation initiation is thought to occur via the target of rapamycin (TOR) and mitogen-activated protein kinase signal-integrating kinase (Mnk) signaling pathways, which is downstream of extracellular-regulated kinase (ERK). This study used the model glutamatergic synapse, the Drosophila neuromuscular junction, to better understand the roles of the Mnk and TOR signaling pathways in synapse development. These synapses contain non-NMDA receptors that are most similar to AMPA receptors. The data show that Lk6, the Drosophila homolog of Mnk1 and Mnk2, is required in either presynaptic neurons or postsynaptic muscle for the proper localization of the GluRIIA glutamate receptor subunit. Lk6 may signal through eukaryotic initiation factor (eIF) 4E to regulate the synaptic levels of GluRIIA as either interfering with eIF4E binding to eIF4G or expression of a nonphosphorylatable isoform of eIF4E resulted in a significant reduction in GluRIIA at the synapse. It was also found that Lk6 and TOR may independently regulate synaptic levels of GluRIIA. (Hussein, 2016).

    This study is the first to provide information on the properties and regulation of the Drosophila protein kinase LK6. Its catalytic domain is strikingly similar to those of mammalian Mnks; similar to them, in mammalian cells LK6 can bind to ERK, can be activated by ERK signalling and can phosphorylate eIF4E. This occurs at the physiological site, Ser209. The MAPK-binding motif of LK6 is of the type previously shown to bind ERK but not p38 MAPK. Consistent with this, when expressed in mammalian cells, LK6 is not activated by stimuli that turn on p38 MAPK (Hussein, 2016).

    It is more challenging to perform similar experiments in Drosophila cells owing to the difficulty in transfecting, e.g. S2 cells with high efficiency. However, importantly, this study shows that LK6 also interacts with the ERK homologue Rolled, but not with the Drosophila p38 homologue. The results, furthermore, show that LK6 is activated by Phorbol myristate acetate (PMA), but not by arsenite, which activates p38 MAPK. The regulatory properties of LK6 thus appear to be similar in mammalian and Drosophila cells, indicating that the specificity of the MAPK-interaction motifs is probably similar in both mammals and Diptera. Similar to Mnk1 and Mnk2a, LK6 is primarily, if not exclusively, cytoplasmic. It does contain a basic region of the type that, in Mnk1 and Mnk2, can bind to the nuclear shuttling protein importin-α. It therefore seems probable that either (1) it contains an NES, which ensures its efficient re-export from the nucleus, or (2) the basic region is not accessible to importin-α. The lack of effect of LMB on the localization of LK6 rules out the operation of a CRM1-type NES of the kind found in Mnk1, although the very long C-terminal extension of LK6 might contain an LMB-insensitive NES (Hussein, 2016).

    By analogy with the Mnks, it is probable that the N-terminal polybasic region of LK6 mediates its binding to eIF4G and could also interact with importin-α. Given that full-length LK6 shows less efficient binding to eIF4G when compared with Mnk1, it also seems possible that it binds importin-α less efficiently, which may contribute to the finding that LK6 is cytoplasmic. It has been shown previously that even the much shorter C-terminus of Mnk2a impedes access to the N-terminal basic region in that protein, so it is entirely possible that the much larger C-terminal part of LK6 has a similar effect. This could explain why the fragment of LK6 that lacks the C-terminus bound better to eIF4G than did the full-length protein. It may also be that the low degree of binding reflects the fact that the association of LK6 with the heterologous human protein was being studied, rather than with Drosophila eIF4G. Repeated attempts have been made to use the available antisera to examine the association of LK6 with eIF4G in S2 cells, but without success. Comparison of the polybasic region of LK6 with those of Mnk1 and Mnk2a (which do bind eIF4G and importin-α), and recent results for mutants with alterations in these features, do not reveal any difference that might obviously explain the decreased ability of LK6 to bind mammalian eIF4G. As argued above, the C-terminus of LK6 may also impair its activation by ERK, based on the observation that the catalytic domain is more effectively activated than a mutant of the full-length protein that also lacks the ERK-binding motif (Hussein, 2016).

    The results support the idea that LK6 is a Drosophila eIF4E kinase. LK6 can phosphorylate eIF4E in vitro and its overexpression in cells leads to increased phosphorylation of endogenous eIF4E. Furthermore, the activation of LK6 by ERK signalling but not by p38 MAPK signalling correlates well with the observed behaviour of the phosphorylation of eIF4E in PMA- or arsenite-treated Drosophila cells, and the fact that LK6 is activated by stimuli that stimulate ERK but is not activated by stimuli that activate p38 MAPK, in HEK-293 cells. The ability of LK6 to bind eIF4G also supports the contention that it can act as an eIF4E kinase in vivo (Hussein, 2016).

    The observation that phosphorylation of the endogenous eIF4E in S2 cells is increased by PMA but not by arsenite is consistent with the regulatory properties of LK6 and with the notion that LK6 may phosphorylate eIF4E in these cells. The fact that it is the only close homologue of the Mnks in the fruitfly genome is also consistent with this notion. Phosphorylation of eIF4E has previously been shown to play an important role in growth in this organism and in its normal development. The current data show that LK6 can phosphorylate Drosophila eIF4E in vitro, consistent with the idea that LK6 acts as an eIF4E kinase in this organism. The dsRNAi data that was obtained, which show that two different interfering dsRNAs directed against LK6 each markedly decrease eIF4E phosphorylation in S2 cells, offer strong support to the conclusion that LK6 acts as an eIF4E kinase in Drosophila. Unfortunately, the poor quality of the available anti-LK6 antisera prevented assessing whether the incomplete nature of the loss of phosphorylation of eIF4E reflects incomplete elimination of LK6 expression (Hussein, 2016).

    Previous genetic studies have linked LK6 to Ras signalling in Drosophila. This agrees very well with the finding that LK6 is activated by ERK signalling, since ERK lies downstream of Ras. LK6 was first identified as interacting with microtubules and centrosomes. Overexpression of LK6 led to defects in microtubule organization, indicative of their increased stability. The connections between the phosphorylations of eIF4E and microtubules are not immediately obvious. However, it is entirely possible that LK6 has additional substrates that interact with microtubules or are components of centrosomes and their phosphorylation may be important in the regulation of, for example, mitosis. Numerous microtubule-associated proteins are indeed phosphorylated. Microtubules undergo massive reorganization during mitosis and this involves an array of phosphorylation events and protein kinases. It may therefore be relevant that LK6 is activated by mitogenic signalling (i.e. through ERK and thus Ras) (Hussein, 2016).

    Drosophila ortholog of intellectual disability-related ACSL4, inhibits synaptic growth by altered lipids

    Nervous system development and function are tightly regulated by metabolic processes, including the metabolism of lipids such as fatty acids (FAs). Mutations in long-chain acyl-CoA synthetase 4 (ACSL4) are associated with non-syndromic intellectual disabilities. A previous study reported that Acsl, the Drosophila ortholog of mammalian ACSL3 and ACSL4, inhibits neuromuscular synapse growth by suppressing transforming growth factor-beta/bone morphogenetic protein (BMP) signaling. This study reports that Acsl regulates the composition of FAs and membrane lipid, which in turn affect neuromuscular junction (NMJ) synapse development. Acsl mutant brains had decreased abundance of C16:1 fatty acyls; restoration of Acsl expression abrogated NMJ overgrowth and the increase in BMP signaling. A lipidomic analysis revealed that Acsl suppressed the levels of three lipid raft components in the brain, including mannosyl glucosylceramide (MacCer), phosphoethanolamine ceramide, and ergosterol. MacCer level was elevated in Acsl mutant NMJs and along with sterol promoted NMJ overgrowth but was not associated with the increase in BMP signaling in the mutants. These findings suggest that Acsl inhibits NMJ growth by stimulating C16:1 and concomitantly suppressing raft-associated lipid levels (Huang, 2016).

    Lipids are essential membrane components that have crucial roles in neural development and function. Dysregulation of lipid metabolism underlies a wide range of human neurological diseases including neurodegeneration and intellectual disability. Acyl-CoA synthetase long-chain family member 4 (ACSL4) is the first gene in fatty acid metabolism associated with non-syndromic intellectual disability. ACSL4 protein has two variants: a ubiquitously expressed short form, and a brain-specific long form that is highly expressed in the hippocampus, a crucial region for memory. Indeed, ACSL4 has been shown to play an important role in synaptic spine formation. However, it is unclear how mutations in ACSL4 lead to intellectual disability (Huang, 2016).

    There are 26 genes encoding acyl-CoA synthetases (ACSs) in humans. Each of the enzymes has distinct substrate preferences for fatty acid with various lengths of aliphatic carbon chains. In contrast, there are 13 ACS genes in the Drosophila genome. ACSs convert free fatty acids into acyl-CoAs for lipid synthesis, fatty acid degradation or membrane lipid remodeling. For example, ACSL4 converts long-chain fatty acids (LCFAs; aliphatic tails longer than 12 carbons), preferentially arachidonic acid (C20:4), into LCFA-CoAs that are incorporated into glycerol-phospholipids (GPLs) and neutral lipids in non-neuronal cells. The mechanism of how fatty acids and fatty-acid-modifying enzymes affect lipid composition, and thereby modulate development processes, is beginning to be understood in lower model organisms (Huang, 2016 and references therein).

    Synaptic growth is required for normal brain function such as learning and memory. Many neurological disorders including intellectual disability are associated with synaptic defects. The Drosophila neuromuscular junction (NMJ) is a powerful system for studying the mechanisms that regulate synaptic growth. Drosophila Acsl, also known as dAcsl, is the ortholog of mammalian ACSL3 and ACSL4 (Zhang, 2009). It has been reported that Acsl affects axonal transport of synaptic vesicles and inhibits NMJ growth by inhibiting bone morphogenetic protein (BMP) signaling However, how Acsl affects lipid metabolism and the role of Acsl-regulated lipid metabolism in synapse development are largely unknown (Huang, 2016).

    This study demonstrates that Acsl positively regulates the abundance of the LCFA palmitoleic acid (C16:1) in the brain. Reduced levels of C16:1 in Acsl mutants led to NMJ overgrowth and enhanced BMP signaling. A lipidomic analysis revealed that mannosyl glucosylceramide (MacCer), phosphoethanolamine ceramide (CerPE, the Drosophila analog of sphingomyelin) and ergosterol levels were increased in Acsl mutant brains. Genetic and pharmacological analyses further showed that the increased level of MacCer and sterol underlie the NMJ overgrowth in Acsl mutants in a pathway parallel to BMP signaling. These results indicate that Acsl regulates fatty acid and sphingolipid levels to modulate growth signals and NMJ growth, providing insight into the pathogenesis of ACSL4-related intellectual disability (Huang, 2016).

    Like ACSL4, Acsl primarily associates with the ER and facilitates fatty acid incorporation into lipids ; impairment of Acsl activity reduces the abundance of its substrate LCFAs in lipids. Although this study did not directly determine the substrate preference of Acsl, fatty acid analysis suggests that both Acsl and ACSL4 positively regulate C16:1 abundance in Drosophila brain. The two proteins also have conserved functions in other processes, such as lipid storage, axonal transport and synaptic development (Huang, 2016 and references therein).

    The similar NMJ overgrowth in desat1 and Acsl mutants suggests that the normal fatty acid composition is essential for proper development of synapses. The rescue effect of C16:1, together with the genetic interaction between Acsl and desat1, indicates that reduced C16:1 contributes to NMJ overgrowth in Acsl mutants. It has been previously reported that the synaptic overgrowth in Acsl mutants is due in part to an elevation of BMP signaling resulting from defects in endocytic recycling and BMP receptor inactivation. Endosomes are membrane compartments that are regulated by various membrane lipids, particularly the conversion between different PtdIns species, such as PI(3)P, PI(4,5)P2, PI(3,4,5)P3 and so on. The current findings suggest that increased BMP signaling in Acsl mutants is associated with an imbalance in fatty acid composition, specifically, a decrease in C16:1. It is thus possible that proper fatty acid composition is necessary for the normal conversion and localization of endosomal lipids (e.g., PtdIns species), affecting endosomal recycling and BMP receptor inactivation. Future studies will examine the regulation of specific fatty acids such as C16:1 in endosomal recycling and BMP signaling (Huang, 2016).

    Acsl primarily associates with the ER in multiple cell types including motor neurons and participates in lipid synthesis. In Drosophila, most of acyl chains in GPLs are C16 and C18 LCFAs without VLCFAs. In contrast, sphingolipids contain higher levels of VLCFAs than LCFAs as acyl chains. Thus, most LCFA-CoAs are channeled into GPLs whereas VLCFA-CoAs are mainly incorporated into sphingolipids in Drosophila. This study found that Acsl positively regulates the production of C16:1-containing GPLs, as well as the level of PtdEth, the most abundant GPL in the brain. Presumably, fatty acids that are less preferred by Acsl could be channeled into lipids by other ACSs, and might show increased abundance because of a compensatory mechanism in Acsl mutants, which could contribute to the elevation in VLCFA-containing sphingolipids (Huang, 2016).

    In addition to ER localization, ACSL4 and Acsl also localize to peroxisomes in a few non-neuronal cells, suggesting a role for these proteins in the activation of VLCFAs for peroxisomal degradation. Indeed, in animal models and patients with impaired peroxisomal function, accumulation of VLCFAs or increased levels of lipid species with longer fatty acid chains is observed. Thus, a defect in peroxisomal VLCFA degradation might underlie the elevation in sphingolipid species with VLCFA chains in Acsl mutant brains (Huang, 2016).

    Alternatively, Acsl might affect lipid composition through a pathway that is independent of fatty acid incorporation. For example, as degradation of sphingolipids occurs primarily within lysosomes, a defect in lysosomal degradation might lead to an accumulation of sphingolipids and sterols in Acsl mutants. In addition, fatty acids and acyl-CoAs are ligands of many transcription factors, and ACSL3 activates the transcription of lipogenic genes in rat hepatocytes. Thus, Acsl might transcriptionally regulate genes encoding enzymes involved in the metabolism of lipids such as MacCer, CerPE and PtdEth. However, the detailed mechanism of how Acsl affects lipid class composition, especially the downregulation of raft-related MacCer and CerPE by Acsl in the nervous system, remains to be elucidated (Huang, 2016).

    The data showed that elevation of the raft-related lipids MacCer and sterol facilitate NMJ overgrowth in Acsl mutants. Moreover, MacCer promotes bouton formation in a pathway parallel to BMP signaling, at least in part. It is unclear how these raft-related lipids regulate synaptic growth. It is likely that MacCer and sterol might interact with raft-associated growth signaling pathways. Larval NMJ development is mediated by multiple growth factors and downstream signaling cascades. For instance, Wingless (Wg) (Wnt1 in mammals) is a raft-associated protein that activates signaling pathways essential for NMJ growth and synaptic differentiation. It is therefore possible that the level or activity of some raft-associated growth factors is increased in Acsl mutants, thereby promoting NMJ overgrowth. Further investigation is needed to dissect the regulatory mechanisms of raft-related lipids, particularly MacCer, in promoting synaptic growth and bouton formation. It also would be of interest to address how Acsl-regulated lipids regulate neurotransmission in conjunction with synapse development (Huang, 2016).

    Activity-induced synaptic structural modifications by an activator of integrin signaling at the Drosophila neuromuscular junction

    Activity-induced synaptic structural modification is crucial for neural development and synaptic plasticity, but the molecular players involved in this process are not well defined. This study reports that a protein named Shriveled, Shv, regulates synaptic growth and activity-dependent synaptic remodeling at the Drosophila neuromuscular junction. Depletion of Shv causes synaptic overgrowth and an accumulation of immature boutons. Shv physically and genetically interacts with βPS integrin. Furthermore, Shv is secreted during intense, but not mild, neuronal activity to acutely activate integrin signaling, induce synaptic bouton enlargement, and increase postsynaptic glutamate receptor abundance. Consequently, loss of Shv prevents activity-induced synapse maturation and abolishes post-tetanic potentiation, a form of synaptic plasticity. These data identify Shv as a novel trans-synaptic signal secreted upon intense neuronal activity to promote synapse remodeling through integrin receptor signaling (Yeun Lee, 2017)

    Two algorithms for high-throughput and multi-parametric quantification of Drosophila neuromuscular junction morphology

    The Drosophila larval neuromuscular junction (NMJ), a well-established model for glutamatergic synapses, has been extensively studied for decades. Identification of mutations causing NMJ morphological defects revealed a repertoire of genes that regulate synapse development and function. Many of these were identified in large-scale studies that focused on qualitative approaches to detect morphological abnormalities of the Drosophila NMJ. This protocol describes in detail two image analysis algorithms "Drosophila NMJ Morphometrics" and "Drosophila NMJ Bouton Morphometrics", available as Fiji-compatible macros, for quantitative, accurate and objective morphometric analysis of the Drosophila NMJ. This methodology is developed to analyze NMJ terminals immunolabeled with the commonly used markers Dlg-1 and Brp. Additionally, its wider application to other markers such as Hrp, Csp and Syt is presented in this protocol. The macros are able to assess nine morphological NMJ features: NMJ area, NMJ perimeter, number of boutons, NMJ length, NMJ longest branch length, number of islands, number of branches, number of branching points and number of active zones in the NMJ terminal (Castells-Nobau, 2017).

    Myostatin-like proteins regulate synaptic function and neuronal morphology

    Growth factors of the TGF-beta superfamily play key roles in regulating neuronal and muscle function. Myostatin (or GDF8) and GDF11 are potent negative regulators of skeletal muscle mass. However, expression of both Myostatin and its cognate receptors in other tissues, including brain and peripheral nerves, suggests a potential wider biological role. This study shows that Myoglianin (MYO), the Drosophila homolog of Myostatin and GDF11, regulates not only body weight and muscle size, but also inhibits neuromuscular synapse strength and composition in a Smad2-dependent manner. Both Myostatin and GDF11 affected synapse formation in isolated rat cortical neuron cultures, suggesting an effect on synaptogenesis beyond neuromuscular junctions. This study also shows that Myoglianin acts in vivo to inhibit synaptic transmission between neurons in the escape response neural circuit of adult flies. Thus, these anti-myogenic proteins act as important inhibitors of synapse function and neuronal growth (Augustin, 2017).

    Growth factors regulate many aspects of tissue development, growth and metabolism. Myostatin and GDF11 are highly homologous members of the TGF-β superfamily of growth factors. While GDF11 plays a role in a variety of systems, the role of Myostatin appears to be confined to skeletal and cardiac muscles (Augustin, 2017).

    Despite the previously described roles of MYO in neural remodelling and synapse refinement (Awasaki, 2011; Yu, 2013) very little is known about the impact of Myoglianin on synaptic physiology. This study first established muscle-derived MYO as a negative regulator of both spontaneous and evoked response at the NMJ, demonstrating its role as a broad regulator of synaptic transmission. The highly coordinated apposition of active zones and glutamate receptors underlies their ability to regulate synaptic strength and plasticity of the larval NMJ. This study has shown that muscle expression of myo inversely affects the NMJ quantity of Brp and GluRIIA, critical pre-and post-synaptic proteins, and determinants of evoked neurotransmitter release and quantal size (i.e., postsynaptic sensitivity to presynaptically released transmitter), respectively. While it is possible that MYO exerts its influence on synaptic strength through other mediators, GluRIIA and Brp are their likely downstream effectors. The electrophysiological results, obtained using the GAL4-UAS system for targeted manipulation of myo, differ from the ones obtained recently using a genetic null myo mutant showing slightly reduced miniature amplitudes (Kim, 2014). The likely explanation is that compensatory effects happen in other tissues in the tissue-specific knockdown animals that cannot occur in genetic nulls, especially for systemic type factors. The other possible explanation is differential cross regulation between different (MYO-like) ligands in genetic null vs tissue knockdown animals. These results thus indicate the relevance of tissue specificity of MYO action, and of myo expression levels, in regulating synaptic function, and emphasize the need for caution when interpreting results from different types of gene manipulations (Augustin, 2017).

    myo expression was detected in the glial cells of the larval neuromuscular junction. While Drosophila NMJ contains at least 2 subtypes of glia, myo expression appears confined to the 'repo-positive' subtype both in the central (Awasaki, 2011) and peripheral nervous system. The dual muscle and glial presence makes MYO ideally positioned for regulating NMJ function. Due to the small size of the compartment, however, glia-derived MYO likely has a modulatory role at the neuromuscular junction (Augustin, 2017).

    This study also found that muscle suppression of Myoglianin, a Drosophila homolog of Myostatin and GDF11, promotes increased larval weight and body-wall muscle size in developing larvae, resembling the effect of Myostatin knockdown in mammals. Interestingly, pan-glial expression of myo negatively affected larval wet weight, but not the size of somatic myofibers, suggesting previously unsuspected systemic roles for glial cells (Augustin, 2017).

    Smad2 is a mediator of MYO action on both evoked response and postsynaptic sensitivity, with MAD having a minor effect on the latter. While MAD primarily functions as a cytoplasmic transducer of BMP signalling, it has been demonstrated that, under certain conditions, MAD can be phosphorylated in response to Activin pathway activation (Peterson, 2012; Augustin, 2017 and references therein).

    This study detected elevated levels of phosphorylated Akt and GSK-3/Shaggy in larval somatic muscles of animals with reduced myo expression in this tissue. In flies and mammals, the Akt- mTOR axis promotes skeletal muscle growth, and phosphorylation-induced inhibition of GSK-3/Shaggy induces hypertrophy in skeletal myotube. The effects of attenuated myo expression on larval tissue size, however, do not appear to be mediated by Smad2 (or MAD) activation as their overexpression does not reverse the weight phenotype in 'low myo' background. Indeed, 'non-Smad' signalling pathways have been demonstrated for various TGF-β ligands in vertebrates and Drosophila. In addition to its role as an inhibitor of the NMJ growth and active zone formation in developing Drosophila larvae, GSK-3β is also a critical promoter of synaptic plasticity, possibly through regulation of glutamate receptor function or trafficking. This work has revealed Shaggy as a mediator of reduced MYO action, and as a negative regulator of synaptic strength at the larval NMJ. While MYO likely affects both sides of the synapse directly, an unlikely but possible scenario is that presynaptic motoneuron responds to a retrograde signal released from muscle/glial cells at the NMJ in response to an induction by MYO. An attractive hypothesis is that MYO negatively regulates presynaptic release directly, in conjunction with muscle-secreted Gbb, a positive regulator of neuromuscular synapse development and growth. The effects of MYO could also be mediated through the transmembrane protein Plum, previously proposed to regulate connectivity at the larval NMJ by sequestrating Myoglianin (Yu, 2013; Augustin, 2017 and references therein).

    Myostatin negatively regulates synaptic function and neuronal morphology This study found that injections of Myostatin into rapidly growing larvae abolish the positive effect of myo down-regulation on NMJ strength and composition, and reverse the elevated muscle p-Akt levels. Furthermore, both Myostatin and GDF11 surpressed the growth of neuronal processes and perturbed the formation of synapses in cultured brain neurons, suggesting a direct action on neurons and regulation of synaptogenesis beyond neuromuscular junctions. Recently, Myostatin transcript and protein were detected in the mouse hippocampus and olfactory system neurons, respectively, and Myostatin type I (Alk4/5) and type II (ActIIB) receptors were found to be expressed in the mammalian nervous system. The current results therefore expand on these findings, suggesting functional relevance for Myostatin in both peripheral and central nervous system, and beyond its action as a canonical regulator of skeletal muscle growth. These novel roles remain to be further explored (Augustin, 2017).

    This study expanded analysis of the functional relevance of MYO in the nervous system by demonstrating its importance in a non-NMJ synapse. Specifically, Myoglianin plays a role in the development of a mixed electro-chemical synapse in the Drosophila escape response pathway, likely by regulating the density of shakB-encoded gap junctions at the GF-TTMn synapse (Blagburn, 1999). These findings implicate MYO as a broad negative regulator of neuronal function across the nervous system and developmental stages (Augustin, 2017).

    This work thus reveals broad and novel roles for anti-myogenic TGF-β superfamily of proteins in the nervous system and suggests new targets for interventions into synaptic function across species (Augustin, 2017).

    Secreted tissue inhibitor of matrix metalloproteinase restricts trans-synaptic signaling to coordinate synaptogenesis

    Synaptogenesis is coordinated by trans-synaptic signals that traverse the specialized synaptomatrix between pre- and postsynaptic cells. Matrix metalloproteinase (Mmp) activity sculpts this environment, balanced by secreted Tissue inhibitors of Mmp (Timp). This study used the reductionist Drosophila matrix metalloproteome to test consequences of eliminating all Timp regulatory control of Mmp activity at the neuromuscular junction (NMJ). Using in situ zymography, Timp was found to limit Mmp activity at the NMJ terminal and shape extracellular proteolytic dynamics surrounding individual synaptic boutons. In newly-generated timp null mutants, NMJs exhibit architectural overelaboration with supernumerary synaptic boutons. With cell-targeted RNAi and rescue studies, postsynaptic Timp was found to limit presynaptic architecture. Functionally, timp nulls exhibit compromised synaptic vesicle cycling, with reduced, lower fidelity activity. NMJ defects manifest in impaired locomotor function. Mechanistically, Timp was found to limit BMP trans-synaptic signaling and the downstream synapse-to-nucleus signal transduction. Pharmacologically restoring Mmp inhibition in timp nulls corrects BMP signaling and synaptic properties. Genetically restoring BMP signaling in timp nulls corrects NMJ structure and motor function. Thus, Timp inhibition of Mmp proteolytic activity restricts BMP trans-synaptic signaling to coordinate synaptogenesis (Shilts, 2017).

    The synaptic cleft is populated with a complex extracellular network of secreted and transmembrane proteins, yet little is known about the extracellular mechanisms that act to shape this critical cellular interface. This synaptomatrix contains integrin, heparan sulfate proteoglycan and cognate receptors for a host of known secreted and transmembrane ligands. Extracellular proteins in the cleft are extensively remodeled in parallel with intercellular changes that accompany synaptic maturation and activity-dependent plasticity. Extracellular matrix metalloproteinase (Mmp) enzymes catalyze synaptic remodeling by proteolytically cleaving the secreted and transmembrane substrates regulating synapse structural integrity, and modulating intercellular signaling between presynaptic and postsynaptic partners. Given the powerful organizing effects of these proteases, their activity must be tightly regulated. One key mechanism is secretion of tissue inhibitors of Mmps (Timps), which restrict Mmp activity to proper spatial and temporal windows. Whenever Mmp regulation is disrupted, developmental abnormalities and disease often result. Improper Mmp expression and activity is implicated in a range of neurological disorders, including schizophrenia, addiction, epilepsy and autism spectrum disorder (ASD). As the most common heritable ASD and intellectual disability, Fragile X syndrome (FXS) underscores the importance of preserving Mmp balance to control proper synaptic structure and function. Importantly, the Mmp inhibitor minocycline alleviates synaptic and behavioral phenotypes in FXS disease models, and has shown promise in clinical trials for human patients, showing that elevated Mmp activity is causally linked to FXS neuropathology (Shilts, 2017).

    Since Mmp dysregulation produces pronounced synaptic defects in disease states, it was hypothesized that loss of the endogenous Mmp control mediated by Timp would disrupt synapse architecture and function. This study took advantage of the simplified Drosophila melanogaster matrix metalloproteome to test consequences of genetically ablating all Timp regulatory control over Mmp activity. In contrast to mammals, which have at least 24 Mmps and four partially redundant Timps, Drosophila has a single secreted Mmp (Mmp1), a single membrane-anchored Mmp (Mmp2) and a single Timp, all of which are highly conserved and can interact with their respective human homologs. In the Drosophila nervous system, Mmps direct both axonal targeting and dendritic remodeling. Recently, Mmp1 and Mmp2 were found to regulate trans-synaptic signaling at the neuromuscular junction (NMJ) to modulate synaptic structure and function. Moreover, trans-synaptic signaling dysregulation has been causally linked to synaptic defects in the Drosophila FXS model. One trans-synaptic pathway important for both synaptic structure and function involves bone morphogenetic protein (BMP) signaling via the Glass Bottom Boat (Gbb) ligand. Gbb secreted from the muscle regulates NMJ structure, whereas Gbb secreted from the motor neuron regulates neurotransmission. Gbb ligand in the extracellular space surrounding synaptic boutons activates downstream phosphorylated Mothers Against Decapentaplegic (pMAD) signal transduction locally at the synapse and distantly within motor neuron nuclei of the central nervous system. Synaptic pMAD is associated with assembly of functional neurotransmission machinery at the NMJ, whereas the accumulation of nuclear pMAD promotes NMJ growth. It is hypothesized that the balance of Mmp proteolytic activity controlled by Timp guides trans-synaptic signaling pathways to modulate both NMJ synaptic structure and function (Shilts, 2017).

    To test this hypothesis, the first ever timp-specific loss-of-function null alleles were generated using CRISPR/Cas9 genome editing. Generation of specific mutations was previously intractable due to a conserved nested relationship that places the timp gene within an intron of the important synaptic synapsin gene. Previously work has shown that Timp localizes in the NMJ perisynaptic space (Dear, 2016), where it shows a co-dependent relationship with both secreted Mmp1 and membrane-anchored Mmp2. The current study employed in situ zymography in living NMJs to show that Timp inhibits synaptic Mmp function and regulates the dynamics of Mmp proteolytic activity in the extracellular space surrounding synaptic boutons. Loss of Timp regulation removes a restraint on synaptic architecture, resulting in the overelaboration of boutons. Using transgenic RNA interference (RNAi) and rescue, Timp secretion from the postsynaptic muscle was shown to be required to regulate the presynaptic motor neuron architecture. In parallel, Timp was also found to control synaptic function. By employing FM1-43 dye imaging, this study found Timp modulates the speed and fidelity of the synaptic vesicle (SV) cycle driving synaptic neurotransmission, and impairs the coordinated muscle peristalsis output of neuromuscular activity. In testing trans-synaptic signaling pathways, Timp function was found to act to restrict BMP signaling, with timp null mutants showing elevated Gbb ligand levels in the extracellular space surrounding synaptic boutons and increased downstream pMAD signal transduction at both the synapse and within motor neuron nuclei. Inhibiting proteolytic activity with minocycline treatment in timp null mutants restores normal BMP signaling and significantly corrects NMJ properties and output motor function. Genetically restoring normal BMP signaling in timp null mutants corrects NMJ architecture and functional motor output, indicating that aberrant trans-synaptic signaling is the causal mechanism. Taken together, these results show that neuromuscular synapses require a responsive balance of Mmp activity controlled by Timp inhibition to restrict the BMP trans-synaptic signaling that modulates NMJ structure and function (Shilts, 2017).

    Remodeling of the synaptic extracellular environment is a highly dynamic process, demanding precise spatiotemporal control in response to specific developmental and activity-dependent signals. Mmp proteolytic activity is an ideal node of regulation for the necessary responsive kinetics and specificity, with Timps controlling the timing, duration and spatial specificity of enzyme function. Taking advantage of the simplified matrix metalloproteome of Drosophila, with only a single functionally conserved Timp, it was possible to eliminate all Timp function with one mutation. Using site-directed CRISPR/Cas9, the first timp null allele was generated, with targeted mutation of the timp gene, without disrupting the synapsin gene in which it is nested. Although nested genetic placement does not imply a functional relationship, the highly conserved nesting of timp in synapsin occurs across vertebrates and invertebrates, which are separated by hundreds of millions of years of evolution. The evolutionary conservation of timp nesting in synapsin is interesting, since synapsin encodes a key synaptic regulator and there is evidence of co-regulation of genes nested with Timps. In addition to timp loss-of-function mutants, no viable Mmp gain-of-function has yet been reported in Drosophila. Thus, the new CRISPR-induced timp null is a tool to characterize total Timp function as well as generally elevated Mmp activity as seen in the nervous system. Currently, there are relatively few reports concerning Timp loss in the nervous system. In mice, TIMP-2 knockout causes motor deficits and expanded NMJ branching, and TIMP-1 overexpression reduces outgrowth in cortical cells, supporting findings of this tudy. In Drosophila, Timp overexpression inhibits NMJ growth, which again complements the current findings (Shilts, 2017).

    This study uncovered key roles for Timp in controlling synaptic Mmp activity, thereby regulating NMJ structure, function and output. Muscle-secreted Timp limits synaptic Mmp proteolytic activity and shapes the distribution of Mmp activity within the synaptomatrix. This local regulation of Mmp functional dynamics has not been reported in neuronal synapses, but is consistent with known roles of Timp in non-neuronal contexts. This study found that postsynaptic Timp limits presynaptic NMJ architecture and bouton formation. This is surprising given that individual Mmp knockdown similarly limits synaptic structure in flies and mice, but may suggest that both loss and gain of Mmp function converge phenotypically or that, collectively, Timp repression of multiple Mmp activities acts as a brake on synaptic growth. Timp also was found to regulate synaptic function, by facilitating SV endocytosis and maintaining SV cycle fidelity. In comparison, mmp mutants elevate transmission strength, also by altered SV cycling dynamics, consistent with Timp repression of Mmp function. Timp enables faster and higher fidelity muscle contraction peristalsis, driving coordinated locomotion. Motor defects have consistently been found across a range of Mmp manipulations, although molecular mechanisms had not been identified. Taken together, these results complete a characterization of the entire Drosophila matrix metalloproteome in controlling neuromuscular synapse structure and function. The timp null synaptic phenotypes prompt a re-assessment: Mmps are not simply negative regulators of synaptic differentiation, but can promote structural development within a context-dependent mechanism. This work shows Timp and Mmps interact to sculpt synapse form and function (Shilts, 2017).

    Timp limits BMP trans-synaptic signals mediating communication between the muscle and motor neuron, with Timp loss elevating Gbb ligand levels. BMP ligands are well known to be sequestered by extracellular molecules, and proteolytic cleavage of these extracellular antagonists controls the distributions of signaling activity in multiple cellular contexts. In Drosophila neurons, Mmp2 regulates motor axon pathfinding and fasciculation via Mmp2-mediated proteolytic cleavage of the ECM Fibrillin/Fibulin-related Faulty Attraction (Frac) protein to enable BMP signaling. Similarly, this study found elevated BMP trans-synaptic signaling in timp mutants with Mmp proteolytic hyperactivity. Gbb secretion from the postsynaptic muscle regulates NMJ architecture, whereas Gbb released from the presynaptic motor neuron regulates neurotransmission function. These roles are consistent with the misregulation of synaptic structure and SV cycle function, respectively, seen in timp mutants with elevated Gbb signaling. The accumulation of Gbb in the perisynaptic synaptomatrix of timp null mutants drives downstream activation of pMAD signal transduction in both motor neuron synaptic terminals and motor neuron nuclei. This is consistent with pMAD activation of transcriptional programs for coordinating synapse structural and functional differentiation. Gbb secreted from the postsynaptic muscle is regulated by Timp that is also secreted from the muscle, which provides control for motor neuron terminals to expand in response to muscle growth and activity-dependent plasticity. In contrast, Mmps come from both presynaptic and postsynaptic cells. Thus, directional Timp control acts as a specific muscle-derived mechanism to regulate Gbb trans-synaptic signaling (Shilts, 2017).

    Elevated BMP Gbb signaling in a Drosophila model of Troyer syndrome, a hereditary spastic paraplegia (HSP) disease, causes strikingly similar NMJ synaptic structural and functional defects to those seen upon loss of Timp. Like the timp null mutants, spartin mutants that are causatively associated with Troyer syndrome exhibit expanded synaptic arbors and decreased FM1-43 dye SV endocytic loading with impaired motor function. Importantly, Fragile X Mental Retardation Protein (FMRP) is a downstream effector of Spartin function, limiting BMP Gbb signaling. Loss of FMRP causes Fragile X syndrome (FXS), and reducing non-canonical BMP signaling alleviates the synaptic defects in Drosophila and mouse FXS disease models. Likewise, targeted mutation of the FXS-related S6 kinase (S6K) similarly results in both expanded synaptic architecture and decreased SV endocytosis at the Drosophila NMJ, once again resembling timp null phenotypes. As in timp mutants, there are also clear precedents for mutations of other key regulatory proteins increasing NMJ functional variability to compromise motor output function. These findings with timp demonstrate the utility of variability as a metric to uncover regulatory nodes that preserve the functional resiliency of the nervous system (Shilts, 2017).

    By pharmacologically correcting timp null phenotypes with the characterized Mmp inhibitor minocycline, this study has shown that mutant defects are causally linked to Mmp hyperactivity. Alleviation of timp null phenotypes is robust, albeit partial, which may reflect experimental limitations of the drug administration, or possibly reveal other Mmp-independent Timp functions. In particular, behavioral assays of motor function show conspicuous, albeit partial, rescue, which may be evidence of an Mmp-independent contribution to motor function or, more likely, that the precise spatiotemporal dynamics of Timp at the NMJ are necessary for proper motor function. In rats, transient proteolytic activity in the synaptomatrix accompanies long-term potentiation and dendrite maturation , which corroborates the current model that Timp dynamically restricts synaptic modulation through localized ECM proteolysis. Crucially, pharmacologically balancing Mmp activity in timp null mutants with minocycline treatment restores BMP trans-synaptic signaling, and genetically correcting BMP signaling prevent synaptic and movement defects. These findings support the model that Mmp activity in the synaptomatrix, under regulation by Timp, limits BMP trans-synaptic signals, thereby controlling NMJ synaptogenesis and functional motor output (Shilts, 2017).

    These studies provide an avenue for possible therapeutic treatments in a range of neurological disease states with elevated Mmp activity. In particular, Mmp hyperactivity has been causally implicated in FXS and related ASD conditions. The synaptic cytoarchitectural phenotypes of timp mutants phenocopy the Drosophila FXS model, and trans-synaptic signaling defects are causative in synaptic structural and functional defects in this disease model, including BMP signaling. By re-creating the elevated Mmp activity characterizing neurological disease conditions such as FXS, the timp genetic tools developed in this study provide insights into fundamental synaptic mechanisms with direct clinical relevance. In future studies, timp manipulations will be combined with established Drosophila disease models in order to more fully dissect contributions of Mmp-dependent trans-synaptic signaling impairments in different neurological disease states (Shilts, 2017).

    Notum coordinates synapse development via extracellular regulation of Wnt Wingless trans-synaptic signaling

    Synaptogenesis requires orchestrated communication between pre- and postsynaptic cells via coordinated trans-synaptic signaling across the extracellular synaptomatrix. The first discovered Wnt signaling ligand Drosophila Wingless (Wg; Wnt-1 in mammals) plays critical roles in synaptic development, regulating synapse architecture as well as functional differentiation. This study investigated synaptogenic functions of the secreted extracellular deacylase Notum, which restricts Wg signaling by cleaving an essential palmitoleate moiety. At the glutamatergic neuromuscular junction (NMJ) synapse, Notum secreted from the postsynaptic muscle was found to act to strongly modulate synapse growth, structural architecture, ultrastructural development and functional differentiation. In notum nulls, upregulated extracellular Wg ligand and nuclear trans-synaptic signal transduction was found, as well as downstream misregulation of both pre- and postsynaptic molecular assembly. Structural, functional and molecular synaptogenic defects are all phenocopied by Wg over-expression, suggesting Notum acts solely through inhibiting Wg trans-synaptic signaling. Moreover, these synaptic development phenotypes are suppressed by genetically correcting Wg levels in notum null mutants, indicating that Notum normally functions to coordinate synaptic structural and functional differentiation via negative regulation of Wg trans-synaptic signaling in the extracellular synaptomatrix (Kopke, 2017).

    In the developing nervous system, Wnt signaling ligands act as potent regulators of multiple stages of neuronal connectivity maturation, stabilization and synaptogenesis, including sculpting structural architecture and determining neurotransmission strength. Drosophila Wingless is secreted from presynaptic neurons and glia at the developing glutamatergic neuromuscular junction (NMJ), to bind Frizzled-2 (Fz2) receptors in both anterograde and autocrine signaling. In the postsynaptic muscle, Wg binding to Fz2 activates the Frizzled Nuclear Import (FNI) signaling pathway, which involves Fz2 endocytosis followed by Fz2 cleavage and Fz2 C-terminus nuclear import (Mathew, 2005). Fz2-C trafficked in nuclear ribonucleoprotein (RNP) granules regulates translation of synaptic mRNAs, thereby driving expression changes that modulate synapse structural and functional differentiation (Speese, 2012). In the presynaptic neuron, Wg binding to Fz2 activates a divergent canonical pathway inhibiting the Glycogen Synthase Kinase 3β (GSK3β) homolog Shaggy (Sgg) to regulate microtubule cytoskeleton dynamics via Microtubule-Associated Protein 1B (MAP1B) homolog Futsch. Futsch binding to microtubules regulates architectural changes in synaptic branching and bouton formation. Such multifaceted Wg functions require tight management throughout synaptic development (Kopke, 2017).

    A highly conserved extracellular Wg regulator is the secreted deacylase Notum. The notum gene was discovered in a Drosophila gain-of-function (GOF) mutant screen targeting wing development. Under scalloped-Gal4 control, notum GOF causes loss of the wing and duplication of the dorsal thorax. In the developing wing disc, Notum acts as a secreted, extracellular feedback inhibitor of Wg signaling. Notum function was recently re-defined as a carboxylesterase that cleaves an essential Wg lipid moiety (palmitoleic acid attached to conserved serine), leaving it unable to bind to Fz2 and activate downstream signaling (Kakugawa, 2015). This Wnt palmitoleate moiety is similarly cleaved by human Notum acting as a highly conserved secreted feedback antagonist in the extracellular space to inactivate Wnt signaling (Langton, 2016; Kakugawa, 2015). At the Drosophila NMJ, extracellular regulation of Wg trans-synaptic signaling has been found to play key roles in synaptogenesis (Dani, 2012b; Parkinson et al., 2013). For example, extracellular matrix metalloproteinase (MMP) enzymes cleave heparan sulfate proteoglycan (HSPG) co-receptors to regulate Wg trans-synaptic signaling that controls structural and functional synaptic development. Impairment of this mechanism is causative for Fragile X syndrome (FXS) synaptogenic defects. Similarly, misregulated extracellular mechanisms impair Wg trans-synaptic signaling in both Congenital Disorder of Glycosylation (CDG) and Galactosemia disease states, causing NMJ synaptogenic defects underlying coordinated movement disorders. Given these insights, this study investigated the putative roles for Notum as a new secreted Wg antagonist regulating synaptogenesis (Kopke, 2017).

    This study utilized the well-characterized Drosophila NMJ glutamate synapse model to study Notum requirements in synaptic development. Notum, secreted from muscle and glia, is resident in the extracellular space surrounding developing synaptic boutons, where it negatively regulates Wg trans-synaptic signaling. In notum mutants, extracellular Wg ligand levels and downstream Wg signaling are elevated. Null mutants display both increased synapse number and strength, altered synaptic vesicle cycling, and synaptic ultrastructural defects including a decrease in SSR/bouton ratio, decreased synaptic vesicle density and an increase in the size of vesicular organelles. Cell-targeted RNAi studies reveal both postsynaptic and perisynaptic requirements, with muscle and glial notum knockdown resulting in overelaborated NMJ architecture, but neuronal-driven notum knockdown causing no detectable effects on synaptogenesis. Null notum defects are all phenocopied by neuronal Wg overexpression, suggesting that synaptogenic phenotypes arise from lack of Wg inhibition. Consistently, genetically correcting Wg levels at the synapse in notum nulls alleviates synaptogenic phenotypes, demonstrating that Notum functions solely as a negative regulator of Wg signaling. Taken together, these results identify Notum as a secreted Wnt inhibitor resident in the extracellular synaptomatrix with critical functions regulating trans-synaptic Wnt signaling to coordinate structural and functional synaptogenesis (Kopke, 2017).

    Tightly coordinated trans-synaptic signals are required for proper development of the pre- and postsynaptic apparatus to ensure efficient communication at the synapse. This signaling is both coordinated and controlled in the extracellular space through the actions of secreted and transmembrane glycans, heparan sulfate proteoglycan (HSPG) co-receptors and secreted enzymes, such as matrix metalloproteinase (Mmp) classes. Wg (Wnt-1) mediates a critical trans-synaptic signaling pathway regulated by these extracellular synaptic mechanisms, with key roles in both structural and functional synaptogenesis. This study proposes that Notum is a novel extracellular regulator limiting Wg trans-synaptic signaling to control NMJ synaptogenesis. Wg is post-translationally modified by addition of palmitoleate on a conserved serine (S239) by membrane-bound O-acyltransferase (MBOAT) Porcupine. This lipidation event is required for Fz2 receptor binding and essential for signaling. At the synaptic interface, the GPI-anchored glypican Dally-like Protein (Dlp) regulates Wg trans-synaptic signaling, and Notum was initially described as cleaving such GPI-anchored glypicans from the cell surface, affecting their ability to interact with the Wg ligand. However, Notum was recently redefined as a secreted carboxylesterase, not a phospholipase (Kakugawa, 2015), with structural studies showing a hydrophobic pocket that binds and then cleaves palmitoleate (Kopke, 2017).

    Notum is consistently reported to act primarily as an extracellular Wg feedback inhibitor. The current studies support this function within the synaptomatrix during synaptogenesis. At the Drosophila NMJ, Wg is secreted from both presynaptic neurons and associated peripheral glia (Kerr, 2014), with the glial function specifically regulating synaptic transmission strength and postsynaptic glutamate receptor clustering. This analyses suggest that Notum is secreted from both postsynaptic muscle and peripheral glia, establishing a dynamic, Wg-like expression pattern surrounding synaptic boutons. In notum null mutants, Wg signaling is increased at the developing NMJ, revealed by both decreased Fz2 receptor in the synaptic membrane (Wg-driven endocytosis) and an increase in nuclear Fz2-C punctae (FNI pathway). These findings are consistent with Notum function limiting Wg signaling, as established in other developmental contexts. Notum appears to provide a fascinating directional regulation of Wg trans- synaptic signaling, affecting the anterograde FNI signaling pathway in muscles, but not the autocrine divergent canonical pathway in neurons. Despite the strong elevation in synaptic Wg ligand levels in notum null mutants, no evidence is seen of altered presynaptic MAP1B homolog Futsch or changes in the microtubule cytoskeleton. However, Notum strongly limits Fz2 C-terminus nuclear import into the postsynaptic nuclei, which is known to drive ribonucleoprotein (RNP) translational regulation of synaptic mRNAs to control synapse structural and functional differentiation (Kopke, 2017).

    Synaptic morphogenesis and architectural development is strongly perturbed in notum null mutants, including increased NMJ area, branching and bouton formation, consistent with Notum function inhibiting Wg trans-synaptic signaling. Elevating presynaptic Wg closely phenocopies notum synaptic defects, including expanded innervation area, more branching and supernumerary synaptic boutons. The results show that Notum secreted from muscle and peripheral glia controls Wg in the extracellular space, with targeted notum RNAi resulting in a similar NMJ expansion to notum nulls, whereas neuronal notum knockdown produces no effects. Interestingly, the glial-targeted RNAi increases boutons with no change in branching, whereas muscle knockdown has a stronger impact also affecting branching. Presynaptic Futsch/Map1B microtubule loops have been proposed to mediate Wg-dependent branching and bouton formation. However, neuronal Wg overexpression has no discernable effect on Futsch-positive microtubule loops. Consistently, Notum LOF also does not impact this pathway, with notum mutants displaying no change in Futsch-labeled looped, bundled, punctate or splayed microtubules. Wg binding to the presynaptic Fz2 receptor may activate another divergent Wnt pathway that does not involve Futsch. Alternatively, Wg signaling via muscle Fz2 may produce a retrograde signal back to the neuron to alter presynaptic development. To test these two possibilities, future studies will employ cell-targeted Fz2 knockdown in notum nulls to assay for suppression of the synaptic overgrowth phenotypes (Kopke, 2017).

    Measures of functional synaptic differentiation reveal elevated neurotransmission and faster motor output function with both notum knockout and Wg over-expression. These results are consistent with Notum function inhibiting Wg trans-synaptic signaling, and consistent with previously characterized roles of Wg in NMJ functional development. Notum LOF increases presynaptic function selectively with an elevated mEJC frequency, greater EJC quantal content and heightened synaptic vesicle release during maintained high- frequency stimulation. Some of these effects may map to the increased synaptic bouton numbers. Both Notum LOF and Wg GOF also cause NMJ boutons to spatially clump together, with ultrastructural studies showing multiple boutons sharing one SSR profile. These are not satellite boutons, but rather aberrantly developing boutons that may result in functional defects. Notum knockdown in glia does not cause detectable mEJC/EJC changes, although Wg from glia regulates NMJ functional properties. Interestingly, loss of Notum appears to improve motor performance, and repo-targeted notum RNAi shows that glial Notum contributes to this function. This is an unusual outcome in a mutant condition, and it is assumed that there must be a counter-balancing cost for increasing neuromuscular function. Live FM dye imaging reveals that notum mutants load less dye into synaptic boutons upon nerve stimulation, indicating a role in synaptic vesicle endocytosis and/or the developmental regulation of synaptic vesicle pool size. These results show Notum function limits Wg trans-synaptic signaling to control presynaptic differentiation critical for synapse function and motor output. As with Wg, the source of Notum (muscle vs. glia) appears to be important for distinct synaptogenic functions. Notum from peripheral glia regulates only bouton formation, whereas Notum from muscle regulates both NMJ growth and function (Kopke, 2017).

    Electron microscopy reveals a very strong decrease in synaptic vesicle density in notum null boutons, providing an explanation for the live FM1-43 dye imaging defects. One of the most striking ultrastructural phenotypes is numerous, enlarged synaptic vesicular bodies. These organelles are highly reminiscent of bulk endosomes, in which a large area of presynaptic membrane is internalized, and will subsequently bud off synaptic vesicles. This pathway is usually driven by intense stimulation during activity-dependent bulk endocytosis (ADBE), as first observed at the frog neuromuscular junction. This pathway is induced by high frequency trains of stimulation, and several proteins have been identified that affect the formation of bulk endosomes, including Syndapin and Rolling Blackout (RBO). At the Drosophila NMJ, conditional rbots mutants block ADBE, reducing the number and size of bulk endosomes (Vijayakrishnan, 2009). It will be interesting to test Wg GOF for enlarged endosomal structures, and study their involvement in Wg-dependent synaptic maturation. On the postsynaptic side, Notum also drives proper differentiation. Notum LOF reduces the postsynaptic DLG scaffold and postsynaptic SSR layering. The reduced SSR area in notum mutants is surprising, given that a reduction in postsynaptic Wg signaling also results in fewer SSR layers. However, SSR architecture has not been studied following Wg over-expression. Postsynaptic SSR formation may be sensitive to bidirectional Wg changes, and may be reduced if Wg is tipped in either direction (Kopke, 2017).

    Mechanistically, Notum controls both pre- and postsynaptic molecular assembly, with LOF defects phenocopied by Wg over-expression. The results are consistent with Notum function inhibiting Wg trans-synaptic signaling, and consistent with previously characterized roles for Wg in synaptic molecular development. This study analyzed both the presynaptic active zone protein Bruchpilot and the two postsynaptic GluR classes. Both presynaptic Brp and postsynaptic GluRs are misregulated in notum nulls, with an increase in synapse number but not density. Importantly, both Notum LOF and Wg GOF elevates synapse number. Consistently, Wnt7a over-expression in mouse cerebellar cells also increases the number of synaptic sites and causes accumulation of presynaptic proteins required for synaptic vesicle function. The increased synapse density per NMJ may compensate for reduced neurotransmission per bouton, leading to a net stronger overall NMJ function. In notum mutants, this could reconcile the elevated synaptic strength measured by electrophysiology compared to compromised single bouton function measured by FM dye imaging and impaired TEM ultrastructure. In any case, synaptic assembly during development is regulated by Notum function limiting Wg trans-synaptic signaling (Kopke, 2017).

    Genetically reducing Wg by combining a heterozygous wg null mutation into the homozygous notum null background reduces extracellular synaptic Wg back to control levels. Wg reduction suppresses synaptogenic defects, restoring increased NMJ area, branching and bouton numbers completely back to normal. Both notumKO and Wg GOF causes hyperactive movement, with roll-over speeds supporting synaptogenic defects of larger, stronger NMJs in both mutant conditions. However, notumKO motor function is only partially restored by correcting Wg levels. One explanation for incomplete rescue is that multiple Wnts may contribute to motor behavior. Serine lipidation is conserved for all Wnts, and at least two other Wnts have been suggested to act at the Drosophila NMJ (Wnt2, Wnt5). Wnts are the only secreted ligands suggested to be O-palmitoleated on a serine to function as Notum substrates (Kopke, 2017).

    Retrograde semaphorin-plexin signalling drives homeostatic synaptic plasticity

    Homeostatic signalling systems ensure stable but flexible neural activity and animal behaviour. Presynaptic homeostatic plasticity is a conserved form of neuronal homeostatic signalling that is observed in organisms ranging from Drosophila to human. Defining the underlying molecular mechanisms of neuronal homeostatic signalling will be essential in order to establish clear connections to the causes and progression of neurological disease. During neural development, semaphorin-plexin signalling instructs axon guidance and neuronal morphogenesis. However, semaphorins and plexins are also expressed in the adult brain. This study shows that semaphorin 2b (Sema2b) is a target-derived signal that acts upon presynaptic plexin B (PlexB) receptors to mediate the retrograde, homeostatic control of presynaptic neurotransmitter release at the neuromuscular junction in Drosophila. Further, Sema2b-PlexB signalling regulates presynaptic homeostatic plasticity through the cytoplasmic protein Mical and the oxoreductase-dependent control of presynaptic actin. It is proposed that semaphorin-plexin signalling is an essential platform for the stabilization of synaptic transmission throughout the developing and mature nervous system. These findings may be relevant to the aetiology and treatment of diverse neurological and psychiatric diseases that are characterized by altered or inappropriate neural function and behaviour (Orr, 2017).

    Semaphorins are a large family of secreted or membrane-associated signalling proteins and plexins serve as signal-transducing semaphorin receptors. Semaphorin-plexin signalling was initially described as mediating growth cone collapse. But, semaphorin-plexin signalling is far more diverse. Notably, semaphorins and plexins continue to be expressed in the mature brain, where their function remains mostly unknown. Semaphorins have been shown to be synaptic signalling proteins, but the activity of semaphorins has been limited to the control of neuroanatomical synapse formation and elimination. This study demonstrates that semaphorin-plexin signalling achieves retrograde, trans-synaptic control of presynaptic neurotransmitter release and homeostatic plasticity (Orr, 2017).

    A well-documented assay was used to induce presynaptic homeostatic plasticity (PHP), applying a sub-blocking concentration of the glutamate-receptor antagonist philanthotoxin-433 (PhTx; 15 μM) to significantly decrease the amplitude of average miniature excitatory postsynaptic potentials (mEPSPs; 0.3 μM [Ca2+]e) or miniature excitatory postsynaptic currents (mEPSCs; 1.5 μM [Ca2+]e). This postsynaptic perturbation induces a significant increase in presynaptic neurotransmitter release (the quantal content) that offsets the postsynaptic perturbation and restores normal muscle excitation. This offsetting increase in presynaptic neurotransmitter release is characteristic of PHP1. When this assay was used in larvae containing a null mutation in either the sema2b gene (sema2bC4) or the PlexB gene (PlexBKG0088), PHP was blocked. Consistent with this being a loss-of-function phenotype, heterozygous mutations (either sema2b/+ or PlexB/+) have normal PHP. Remarkably, a double-heterozygous mutant combination of sema2b/+ and PlexB/+ blocks PHP, consistent with both genes acting in concert to drive the expression of PHP (Orr, 2017).

    The long-term maintenance of PHP was investigated and the involvement of other semaphorin or Plexin gene family members. Deletion of a non-essential glutamate-receptor subunit (GluRIIA) induces a long-lasting form of PHP1. Long-term PHP is blocked in a sema2b;GluRIIA double mutant as well as in GluRIIA larvae expressing transgenic RNA interference (RNAi) to knockdown PlexB selectively in motor neurons. Next, the effect of mutations was separately tested in all of the remaining semaphorin and Plexin genes encoded in the Drosophila genomet. The sema2b and PlexB mutants are the only mutants that show disruption of PHP (Orr, 2017).

    Tissue-specific RNAi and transgenic rescue experiments were performed. Expression of UAS-Sema2b-RNAi in motor neurons (OK371-Gal4) had no effect on PHP, whereas expression in muscle (BG57-Gal4) blocked PHP. In addition, expression of UAS-sema2b in muscle rescues PHP in the sema2b-mutant background. Consistent with these data, sema2b was found to be expressed in muscle and Sema2b protein, expressed under endogenous promoter sequences, concentrates at postsynaptic membranes. Next, it was shown that motor neuron-specific expression of UAS-PlexB-RNAi blocks PHP, whereas muscle-specific expression does not. Motor neuron-specific expression of a previously characterized UAS-PlexBDN dominant-negative transgene, lacking the intracellular signalling domain, blocks PHP. RNA-sequencing analysis of purified motor neurons demonstrates PlexB expression in motor neurons. Finally, motor neuron-specific expression of a PlexB-myc transgene shows that PlexB traffics to the presynaptic nerve terminal. Taken together, these data indicate that Sema2b is a ligand originating in the muscle that acts via presynaptic PlexB to drive expression of PHP (Orr, 2017).

    If Sema2b is a retrograde signal that acts upon the presynaptic PlexB receptor, then it should be possible to reconstitute this retrograde signalling by acute application of Sema2b protein. Purified Sema2b protein was acutely applied to the neuromuscular junction (NMJ) of sema2b mutants following PhTx treatment to induce PHP. Sema2b protein (100 nM) was found to completely restores PHP in the sema2b mutant, but fails to restore PHP in the PlexB mutant. In addition, application of Sema2b protein is sufficient to potentiate baseline release, and this effect is also dependent upon PlexB. Finally, a membrane-tethered UAS-sema2b transgene, expressed in muscle, fails to rescue PHP, even though it is concentrated on the postsynaptic membranes. Together, these results indicate that Sema2b is a secreted, postsynaptic ligand that acts upon presynaptic PlexB to enable the expression of PHP. The possibility is acknowledged that PlexB could require a presynaptic co-receptor of, as yet, unknown identity (Orr, 2017).

    Given that acute application of Sema2b protein rescues PHP in the sema2b mutant, the failure of PHP in sema2b-mutant larvae cannot be a secondary consequence of altered NMJ development. Nonetheless, Sema2b-PlexB signalling is required for normal NMJ growth. Axon-targeting errors are rare at muscles 6/7, analysed at the third instar larval stage. This study demonstrated that the NMJs in sema2b and PlexB mutants are composed of fewer, larger synaptic boutons with no change in total NMJ area. The abundance of the active-zone-associated protein Bruchpilot (Brp) is unaltered in the sema2b mutant and the sema2b/+;;PlexB/+ double-heterozygous larvae, both of which block PHP. There is a significant decrease in total Brp staining in the PlexB mutant, an effect of unknown consequence. Qualitatively, the ring-like organization of Brp staining was similar across all genotypes, indicative of normal active-zone organization. Finally, there is no consistent difference in synapse ultrastructure across genotypes. Therefore, the Sema2b-PlexB-dependent control of bouton size may be a separate function of Sema2b-PlexB signalling, analogous to anatomical regulation by semaphorins in mammalian systems (Orr, 2017).

    PHP occurs through the potentiation of the readily releasable pool (RRP) of synaptic vesicles. Application of PhTx induces a doubling of the apparent RRP in wild-type larvae, an effect that is disrupted in both sema2b and PlexB mutants. Failure to potentiate the RRP is also shown as a failure to maintain the cumulative EPSC amplitude after PhTx application. A strong genetic interaction was subsequently shown with a mutation in the presynaptic scaffolding gene rab3-interacting molecule (rim), a PHP gene. Heterozygous mutations in rim, or in sema2b or PlexB have no effect on PHP. However, double-heterozygous combinations of rim/+ with either sema2b/+ or PlexB/+ strongly impaired the expression of PHP (sema2b/+,rim/+) or abolished PHP (rim/+;;PlexB/+). These data do not, however, reflect direct signalling between PlexB and Rim (Orr, 2017).

    To define how PlexB could modulate the RRP, known downstream signalling elements were tested. Mical is necessary for PHP. In Drosophila a single mical gene encodes a highly conserved multi-domain cytoplasmic protein that mediates actin depolymerization, achieved through redox modification of a specific methionine residue (Met44) in actin. Notably, prior genetic evidence has placed Mical downstream of both PlexA and PlexB signalling during axon guidance (Orr, 2017).

    An analysis of multiple mical mutations in larvae as well as transgenic rescue animals demonstrates that mical is necessary presynaptically for PHP. Mical protein is present presynaptically and presynaptic expression of a Mical-resistant UAS-Actin5C transgene, which interferes with Mical-mediated actin depolymerization, blocks PHP. This transgenic protein also concentrates within presynaptic boutons. Additional experiments reveal that the homeostatic expansion of RRP is blocked in mical mutants and when Mical-resistant UAS-Act5 is expressed presynaptically. Strong genetic interactions were found between mical and both the PlexB and rim mutants. Finally, anatomical experiments demonstrate that active zones are normal in the mical mutant, including in both light and electron microscopy experiments. It is proposed that Mical enables PlexB-mediated control of the RRP through the regulation of presynaptic actin (Orr, 2017).

    For half a century, evidence has underscored the importance of target-derived, retrograde signalling that controls presynaptic neurotransmitter release1. Gene discovery, based on forward genetics, indicates that PHP is controlled by the coordinated action of at least three parallel signalling systems. These data regarding Sema2b, PlexB and Mical can be generalized, then semaphorin-plexin signalling could represent a platform for retrograde, trans-synaptic, homeostatic control of presynaptic release, thereby stabilizing synaptic transmission and information transfer throughout the nervous systems of organisms ranging from Drosophila to humans (Orr, 2017).

    A targeted glycan-related gene screen reveals heparan sulfate proteoglycan sulfation regulates WNT and BMP trans-synaptic signaling

    A Drosophila transgenic RNAi screen targeting the glycan genome, including all N/O/GAG-glycan biosynthesis/modification enzymes and glycan-binding lectins, was conducted to discover novel glycan functions in synaptogenesis. As proof-of-product,functionally paired heparan sulfate (HS) 6-O-sulfotransferase (hs6st) and sulfatase (sulf1), which bidirectionally control HS proteoglycan (HSPG) sulfation, were characterized. RNAi knockdown of hs6st and sulf1 causes opposite effects on functional synapse development, with decreased (hs6st) and increased (sulf1) neurotransmission strength confirmed in null mutants. HSPG co-receptors for WNT and BMP intercellular signaling, Dally-like Protein and Syndecan, are differentially misregulated in the synaptomatrix of these mutants. Consistently, hs6st and sulf1 nulls differentially elevate both WNT (Wingless; Wg) and BMP (Glass Bottom Boat; Gbb) ligand abundance in the synaptomatrix. Anterograde Wg signaling via Wg receptor dFrizzled2 C-terminus nuclear import and retrograde Gbb signaling via synaptic MAD phosphorylation and nuclear import are differentially activated in hs6st and sulf1 mutants. Consequently, transcriptional control of presynaptic glutamate release machinery and postsynaptic glutamate receptors is bidirectionally altered in hs6st and sulf1 mutants, explaining the bidirectional change in synaptic functional strength. Genetic correction of the altered WNT/BMP signaling restores normal synaptic development in both mutant conditions, proving that altered trans-synaptic signaling causes functional differentiation defects (Dani, 2012b).

    It is well known that synaptic interfaces harbor heavily-glycosylated membrane proteins, glycolipids and ECM molecules, but understanding of glycan-mediated mechanisms within this synaptomatrix is limited. A genomic screen aimed to systematically interrogate glycan roles in both structural and functional development in the genetically-tractable Drosophila NMJ synapse. 130 candidate genes were screened, classified into 8 functional families: N-glycan biosynthesis, O-glycan biosynthesis, GAG biosynthesis, glycoprotein/proteoglycan core proteins, glycan modifying/degrading enzymes, glycosyltransferases, sugar transporters and glycan-binding lectins. From this screen, 103 RNAi knockdown conditions were larval viable, whereas 27 others produced early developmental lethality. 35 genes had statistically significant effects on different measures of morphological development: 27 RNAi-mediated knockdowns increased synaptic bouton number, 9 affected synapse area (2 increased, 7 decreased) and 2 genes increased synaptic branch number. These data suggest that overall glycan mechanisms predominantly serve to limit synaptic morphogenesis. 13 genes had significant effects on the functional differentiation of the synapse, with 12 increasing transmission strength and only 1 decreasing function upon RNAi knockdown. Thus, glycan-mediated mechanisms also predominantly limit synaptic functional development. A very small fraction of tested genes (CG1597; pgant35A, CG7480; veg, CG6657; hs6st, CG4451; sulf1, CG6725 and CG11874) had effects on both morphology and function. A large percentage of genes (~30%) showed morphological defects with no corresponding effect on function, while only 7% of genes showed functional alterations without morphological defects, and <5% of all genes affect both. These results suggest that glycans have clearly separable roles in modulating morphological and functional development of the NMJ synapse (Dani, 2012b).

    A growing list of neurological disorders linked to the synapse are attributed to dysfunctional glycan mechanisms, including muscular dystrophies, cognitive impairment and autism spectrum disorders. Drosophila homologs of glycosylation genes implicated in neural disease states include ALG3 (CG4084), ALG6 (CG5091), DPM1 (CG10166), FUCT1 (CG9620), GCS1 (CG1597), MGAT2 (CG7921), MPDU1 (CG3792), PMI (CG33718) and PPM2 (CG12151). Two of these genes, Gfr (CG9620) and CG1597, showed synaptic morphology phenotypes in the RNAi screen. Given that connectivity defects are clearly implicated in cognitive impairment and autism spectrum disorders, it would be of interest to explore the glycan mechanism affecting synapse morphology in Drosophila models of these disease states. Glycans are well known to modulate extracellular signaling, including ligands of integrin receptors, to regulate intercellular communication. In the genetic screen, several O-glycosyltransferases mediating this mechanism were identified to show morphological (GalNAc-T2, CG6394; pgant35A, CG7480, O-fut2, CG14789; rumi, CG31152) and functional (pgant5, CG31651; pgant35A, CG7480) synaptic defects upon RNAi knockdown. These findings suggest that known integrin-mediated signaling pathways controlling NMJ synaptic structural and functional development are modulated by glycan mechanisms. The screen showed CG6657 RNAi knockdown affects functional differentiation, consistent with reports that this gene regulates peripheral nervous system development. The corroboration of the screen results with published reports underscores the utility of RNAi-mediated screening to identify glycan mechanisms, and supports use of the screen results for bioinformatic/meta-analysis to link observed phenotypes to neurophysiological/pathological disease states and to direct future glycan mechanism studies at the synapse (Dani, 2012b).

    From this screen, the two functionally-paired genes sulf1 and hs6st were selected for further characterization. As in the RNAi screen, null alleles of these two genes had opposite effects on synaptic functional differentiation but similar effects on synapse morphogenesis, validating the corresponding screen results. The two gene products have functionally-paired roles; Hs6st is a heparan sulfate (HS) 6-O-sulfotransferase, and Sulf1 is a HS 6-O-endosulfatase. These activities control sulfation of the same C6 on the repeated glucosamine moiety in HS GAG chains found on heparan sulfate proteoglycans (HSPGs). At the Drosophila NMJ, two HSPGs are known to regulate synapse assembly; the GPI-anchored glypican Dally-like protein (Dlp), and the transmembrane Syndecan (Sdc). In contrast, the secreted HSPG Perlecan (Trol) is not detectably enriched at the NMJ, and indeed appears to be selectively excluded from the perisynaptic domain. In other developmental contexts, the membrane HSPGs Dlp and Sdc are known to act as co-receptors for WNT and BMP ligands, regulating ligand abundance, presentation to cognate receptors and therefore signaling. Importantly, the regulation of HSPG co-receptor abundance has been shown to be dependent on sulfation state mediated by extracellular sulfatases. Consistently, upregulation of Dlp and Sdc was observed in sulf1 null synapses, whereas Dlp was reduced in hs6st null synapses. In the developing Drosophila wing disc, HSPG co-receptors increase levels of the Wg ligand due to extracellular stabilization, and the primary function of Dlp in this developmental context is to retain Wg at the cell surface. Likewise, in developing Drosophila embryos, a significant fraction of Wg ligand is retained on the cell surfaces in a HSPG-dependent manner, with the HSPG acting as an extracellular co-receptor. Syndecan also modulates ligand-dependent activation of cell-surface receptors by acting as a co-receptor. At the NMJ, regulation of both these HSPG co-receptors occurs in the closely juxtaposed region between presynaptic bouton and muscle subsynaptic reticulum, in the exact same extracellular space traversed by the secreted trans-synaptic Wg and Gbb signals. It is therefore proposed that altered Dlp and Sdc HSPG co-receptors in sulf1 and hs6st mutants differentially trap/stabilize Wg and Gbb trans-synaptic signals at the interface between motor neuron and muscle, to modulate the extent and efficacy of intercellular signaling driving synaptic development (Dani, 2012b).

    HS sulfation modification is linked to modulating the intercellular signaling driving neuronal differentiation . In particular, WNT and BMP ligands are both regulated via HS sulfation of their extracellular co-receptors, and both signals have multiple functions directing neuronal differentiation, including synaptogenesis. In the Drosophila wing disc, extracellular WNT (Wg) ligand abundance and distribution was recently shown to be strongly elevated in sulf1 null mutants. Moreover, sulf1 has also recently been shown to modulate BMP signaling in other cellular contexts. Consistently, this study has shown increased WNT Wg and the BMP Gbb abundance and distribution in sulf1 null NMJ synapses. The hs6st null also exhibits elevated Wg and Gbb at the synaptic interface, albeit the increase is lower and results in differential signaling consequences. In support of this contrasting effect, extracellular signaling ligands are known to bind HSPG HS chains differentially dependent on specific sulfation patterns. It is important to note that the sulf1 and hs6st modulation of trans-synaptic signals is not universal, as Jelly Belly (Jeb) ligand abundance and distribution was not altered in the sulf1 and hs6st null conditions. This indicates that discrete classes of secreted trans-synaptic molecules are modulated by distinct glycan mechanisms to control NMJ structure and function (Dani, 2012b).

    At the Drosophila NMJ, Wg is very well characterized as an anterograde trans-synaptic signal and Gbb is very well characterized as a retrograde trans-synaptic signal. In Wg signaling, the dFz2 receptor is internalized upon Wg binding and then cleaved so that the dFz2-C fragment is imported into muscle nuclei. In hs6st nulls, increased Wg ligand abundance at the synaptic terminal corresponds to an increase in dFz2C punctae in muscle nuclei as expected. In contrast, the increase in Wg at the sulf1 null synapse did not correspond to an increase in the dFz2C-terminus nuclear internalization, but rather a significant decrease. One explanation for this apparent discrepancy is the 'exchange factor' model based on the biphasic ability of the HSPG co-receptor Dlp to modulate Wg signaling. In the Drosophila wing disc, this model suggests that the transition of Dlp co-receptor from an activator to repressor of signaling depends on Wg cognate receptor dFz2 levels, such that a low ratio of Dlp:dFz2 potentiates Wg-dFz2 interaction, whereas a high ratio of Dlp:dFz2 prevents dFz2 from capturing Wg. In sulf1 null synapses, a very great increase was observed in Dlp abundance (~40% elevated) with no significant change in the dFz2 receptor. In contrast, at hs6st null synapses there is a decrease in Dlp abundance (15% decreased) together with a significant increase in dFz2 receptor abundance (~25% elevated). Thus, the higher Dlp:dFz2 ratio in sulf1 nulls could explain the decrease in Wg signal activation, evidenced by decreased dFz2-C terminus import into the muscle nucleus. In contrast, the Dlp:Fz2 ratio in hs6st is much lower, supporting activation of the dFz2-C terminus nuclear internalization pathway. This previously proposed competitive binding mechanism dependent on Dlp co-receptor and dFz2 receptor ratios predicts the observed synaptic Wg signaling pathway modulation in sulf1 and hs6st dependent manner (Dani, 2012b).

    At the Drosophila NMJ, Gbb is very well characterized as a retrograde trans-synaptic signal, with muscle-derived Gbb causing the receptor complex Wishful thinking (Wit), Thickveins (Tkv) and Saxaphone (Sax) to induce phosphorylation of the transcription factor mothers against Mothers against decapentaplegic (P-Mad). Mutation of Gbb ligand, receptors or regulators of this pathway have shown that Gbb-mediated retrograde signaling is required for proper synaptic differentiation and functional development. Further, loss of Gbb signaling results in significantly decreased levels of P-Mad in the motor neurons. This study shows that accumulation of Gbb in sulf1 and hs6st null synapses causes elevated P-Mad signaling at the synapse and P-Mad accumulation in motor neuron nuclei. Importantly, sulf1 null synapses show a significantly higher level of P-Mad signaling compared to hs6st null synapses, and this same change is proportionally found in P-Mad accumulation within the motor neuron nuclei. These findings indicate differential activation of Gbb trans-synaptic signaling dependent on the HS sulfation state is controlled by the sulf1 and hs6st mechanism, similar to the differential effect observed on Wg trans-synaptic signaling. Genetic interaction studies show that these differential effects on trans-synaptic signaling have functional consequences, and exert a causative action on the observed bi-directional functional differentiation phenotypes in sulf1 and hs6st nulls. Genetic correction of Wg and Gbb defects in the sulf1 null background restores elevated transmission back to control levels. Similarly, genetic correction of Wg and Gbb in hs6st nulls restores the decreased transmission strength back to control levels. These results demonstrate that the Wg and Gbb trans-synaptic signaling pathways are differentially regulated and, in combination, induce opposite effects on synaptic differentiation (Dani, 2012b).

    Both wg and gbb pathway mutants display disorganized and mislocalized presynaptic components at the active zone (e.g. Bruchpilot; Brp) and postsynaptic components including glutamate receptors (e.g. Bad reception; Brec/GluRIID). Consistently, the bi-directional effects on neurotransmission strength in sulf1 and hs6st mutants are paralleled by dysregulation of these same synaptic components. Changes in presynaptic Brp and postsynaptic GluR abundance/distribution causally explain the bi-directional effects on synaptic functional strength between sulf1 and hs6st null mutant states. Alterations in active zone Brp and postsynaptic GluRs also agree with assessment of spontaneous synaptic activity. Null sulf1 and hs6st synapses showed opposite effects on miniature evoked junctional current (mEJC) frequency (presynaptic component) and amplitude (postsynaptic component). Further, quantal content measurements also support the observation of bidirectional synaptic function in the two functionally paired nulls. Genetic correction of Wg and Gbb defects in both sulf1 and hs6st nulls restores the molecular composition of the pre- and postsynaptic compartments back to wildtype levels. When both trans-synaptic signaling pathways are considered together, these data suggest that HSPG sulfate modification under the control of functionally-paired sulf1 and hs6st jointly regulates both WNT and BMP trans-synaptic signaling pathways in a differential manner to modulate synaptic functional development on both sides of the cleft (Dani, 2012b).

    This paper has presented the first systematic investigation of glycan roles in the modulation of synaptic structural and functional development. A host of glycan-related genes were identified that are important for modulating neuromuscular synaptogenesis, and these genes are now available for future investigations, to determine mechanistic requirements at the synapse, and to explore links to neurological disorders. As proof for the utilization of these screen results, this study has identified extracellular heparan sulfate modification as a critical platform of the intersection for two secreted trans-synaptic signals, and differential control of their downstream signaling pathways that drive synaptic development. Other trans-synaptic signaling pathways are independent and unaffected by this mechanism, although it is of course possible that a larger assortment of signals could be modulated by this or similar mechanisms. This study supports the core hypothesis that the extracellular space of the synaptic interface, the heavily-glycosylated synaptomatrix, forms a domain where glycans coordinately mediate regulation of trans-synaptic pathways to modulate synaptogenesis and subsequent functional maturation (Dani, 2012b).

    Endostatin is a trans-synaptic signal for homeostatic synaptic plasticity

    At synapses in organisms ranging from fly to human, a decrease in postsynaptic neurotransmitter receptor function elicits a homeostatic increase in presynaptic release that restores baseline synaptic efficacy. This process, termed presynaptic homeostasis, requires a retrograde, trans-synaptic signal of unknown identity. Multiplexin was identified in a forward genetic screen for homeostatic plasticity genes. Multiplexin is the Drosophila homolog of Collagen XV/XVIII, a matrix protein that can be proteolytically cleaved to release Endostatin, an antiangiogenesis signaling factor. This study demonstrates that Multiplexin is required for normal calcium channel abundance, presynaptic calcium influx, and neurotransmitter release. Remarkably, Endostatin has a specific activity, independent of baseline synapse development, that is required for the homeostatic modulation of presynaptic calcium influx and neurotransmitter release. These data support a model in which proteolytic release of Endostatin signals trans-synaptically, acting in concert with the presynaptic CaV2.1 calcium channel, to promote presynaptic homeostasis (Wang, S. J.,2014).

    The nervous system is continually modified by experience. Given the tremendous complexity of the nervous system, it is astounding that robust and reproducible neural function can be sustained throughout life. It is now apparent that homeostatic signaling systems stabilize the excitable properties of nerve and muscle and, thereby, constrain how the nervous system can be altered by experience or crippled by disease. The Drosophila neuromuscular junction (NMJ) has emerged as a powerful model system to dissect the underlying mechanisms that achieve the homeostatic modulation of presynaptic neurotransmitter release. At the Drosophila NMJ, inhibition of postsynaptic glutamate receptor function causes a homeostatic increase in presynaptic neurotransmitter release that precisely restores muscle excitation to baseline levels. This phenomenon is conserved from fly to human. Importantly, presynaptic homeostasis has also been observed at mammalian central synapses in vitro in response to differences in target innervation and altered postsynaptic excitability and following chronic inhibition of neural activity (Wang, S. J.,2014).

    Despite progress in identifying presynaptic effector proteins that are required for the expression of presynaptic homeostasis, the identity of the retrograde signaling system remains unknown. Numerous neurotrophic factors, such as nerve growth factor; brain-derived neurotrophic factor (BDNF); and glia-derived neurotrophic factor, as well as nitric oxide, endocannabinoids, and adhesion molecules, are identified as retrograde signals that regulate presynaptic cell survival, differentiation, and biophysical properties in an activity-dependent manner. Among these molecules, BDNF has been implicated in the trans-synaptic control of presynaptic release in cultured hippocampal neurons. Previous work demonstrated that a bone morphogenetic protein (BMP) ligand (Glass bottom boat) is released from muscle, activates a type II BMP receptor at the presynaptic terminal, and is required for the growth of the presynaptic nerve terminal. This BMP signaling system is also necessary for presynaptic homeostasis. However, the BMP signaling system is a permissive signal that acts at the motoneuron cell body (Wang, S. J.,2014).

    A large-scale, electrophysiology-based forward genetic screen for mutations that block presynaptic homeostasis identified multiplexin as a candidate homeostatic plasticity gene. Drosophila Multiplexin is the homolog of human Collagen XV and XVIII, matrix molecules that are expressed ubiquitously in various vascular and epithelial basement membranes throughout the body. Mutations in the human COL18A1 gene cause Knobloch syndrome, characterized by retinal detachment, macular abnormalities, and occipital encephalocele. Patients with Knobloch syndrome are also predisposed to epilepsy, highlighting the critical function of Collagen XVIII in the central nervous system. Moreover, the C-terminal of Collagen XVIII, encoding an Endostatin domain, can be cleaved proteolytically and functions as an antiangiogenesis factor to inhibit tumor progression. Endostatin inhibits angiogenesis by interacting with various downstream signaling factors, including vascular endothelial growth factor receptors, integrins, and Wnt signaling molecules. Little is known regarding the function of multiplexin in the nervous system. This study provides evidence that Endostatin, a proteolytic cleavage product of Drosophila Multiplexin, functions as a trans-synaptic signaling molecule that is essential for the homeostatic modulation of presynaptic neurotransmitter release at the Drosophila NMJ (Wang, S. J.,2014).

    Loss of Endostatin blocks the homeostatic modulation of presynaptic calcium influx and presynaptic neurotransmitter release. This activity is remarkably specific to presynaptic homeostasis, since loss of Endostatin has no effect on baseline neurotransmission or synapse morphology. Endostatin also interacts genetically with the pore-forming subunit of the CaV2.1 calcium channel and is required for the homeostatic increase of presynaptic calcium influx during synaptic homeostatic plasticity. Finally, transgenic overexpression of Endostatin is sufficient to rescue synaptic homeostasis and baseline neurotransmitter release when it is supplied to either the presynaptic or postsynaptic side of the synapse. Although deletion of Endostatin does not impair baseline transmission, overexpression of Endostatin in the dmpf07253 mutant is sufficient to restore baseline transmission release even in the absence of the Thrombospondin-like domain. As a working model, it is proposed that inhibition of postsynaptic glutamate receptors initiates the proteolytic cleavage of Multiplexin, which resides in the synaptic cleft. It is further proposed that release of Endostatin acts upon presynaptic calcium channels, directly or indirectly, to potentiate calcium influx and presynaptic neurotransmitter release. This model is consistent with data from other systems demonstrating that activation of Endostatin requires proteolytic cleavage of Collagen XVIII by matrix metalloproteases (MMPs) and cysteine cathepsins. Moreover, only free Endostatin released by cleavage functions as an antiangiogenesis factor (Wang, S. J.,2014).

    The means by which presynaptic calcium channel function is modulated by Endostatin remains to be elucidated. Recently, it has been shown that a presynaptic Deg/ENaC channel is also necessary for the homeostatic modulation of presynaptic release. In this previous study, a model is presented in which ENaC channel insertion causes a sodium leak and modest depolarization of the presynaptic resting membrane potential that, in turn, potentiates presynaptic calcium influx. One possibility is that the interaction of Endostatin with the presynaptic CaV2.1 channels enables the channels to respond to low-voltage modulation. This would be consistent with both Endostatin and the ENaC channel being strictly necessary for presynaptic homeostasis. It remains formally possible that Endostatin stabilizes presynaptic ENaC channels and, thereby, influences presynaptic calcium influx. For example, it was demonstrated that the interaction between ENaC channels and extracellular collagens mediates the mechanosensory transduction in the touch reception systems (Wang, S. J.,2014).

    Activation of Endostatinin in other systems requires proteolytical cleavage of Collagen XVIII by MMPs and cysteine cathepsins. This raises an intriguing possibility that, during synaptic homeostasis, Multiplexin could be cleaved by synaptic MMPs, releasing Endostatin to trigger a homeostatic change in presynaptic release. In this model, inhibition of postsynaptic glutamate receptors would lead to the activation of MMPs within the synaptic cleft. Thus, the retrograde signal would be a multistage system, providing opportunity for both amplification and multilevel control of the signaling event. At glutamatergic synapses in hippocampal neurons, proteolytic cleavage of neuroligin-1, a synaptic adhesion molecule residing in postsynaptic terminals, is triggered by postsynaptic NMDA receptor activation. Cleavage of neuroligin-1 depresses presynaptic transmission by reducing presynaptic release probability in a trans-synaptic manner. Thus, the activity-dependent cleavage of cell adhesion and extracellular matrix proteins could provide a robust and evolutionarily conserved feedback paradigm for trans-synaptic signaling to regulate synaptic efficacy in diverse neuronal circuits (Wang, S. J.,2014).

    N-glycosylation requirements in neuromuscular synaptogenesis

    Neural development requires N-glycosylation regulation of intercellular signaling, but the requirements in synaptogenesis have not been well tested. All complex and hybrid N-glycosylation requires MGAT1 (UDP-GlcNAc:alpha-3-D-mannoside-beta1,2-N-acetylglucosaminyl-transferase I) function, and Mgat1 nulls are the most compromised N-glycosylation condition that survive long enough to permit synaptogenesis studies. At the Drosophila neuromuscular junction (NMJ), Mgat1 mutants display selective loss of lectin-defined carbohydrates in the extracellular synaptomatrix, and an accompanying accumulation of the secreted endogenous Mind the gap (MTG) lectin, a key synaptogenesis regulator. Null Mgat1 mutants exhibit strongly overelaborated synaptic structural development, consistent with inhibitory roles for complex/hybrid N-glycans in morphological synaptogenesis, and strengthened functional synapse differentiation, consistent with synaptogenic MTG functions. Synapse molecular composition is surprisingly selectively altered, with decreases in presynaptic active zone Bruchpilot (BRP) and postsynaptic Glutamate receptor subtype B (GLURIIB), but no detectable change in a wide range of other synaptic components. Synaptogenesis is driven by bidirectional trans-synaptic signals that traverse the glycan-rich synaptomatrix, and Mgat1 mutation disrupts both anterograde and retrograde signals, consistent with MTG regulation of trans-synaptic signaling. Downstream of intercellular signaling, pre- and postsynaptic scaffolds are recruited to drive synaptogenesis, and Mgat1 mutants exhibit loss of both classic Discs large 1 (DLG1) and newly defined Lethal (2) giant larvae [L(2)gl] scaffolds. It is concluded that MGAT1-dependent N-glycosylation shapes the synaptomatrix carbohydrate environment and endogenous lectin localization within this domain, to modulate retention of trans-synaptic signaling ligands driving synaptic scaffold recruitment during synaptogenesis (Parkinson, 2013).

    This study began with the hypothesis that disruption of synaptomatrix N-glycosylation would alter trans-synaptic signaling underlying NMJ synaptogenesis (Dani, 2012a). MGAT1 loss transforms the synaptomatrix glycan environment. Complete absence of the HRP epitope, α1-3-fucosylated N-glycans, is expected to require MGAT1 activity: key HRP epitope synaptic proteins include fasciclins, Neurotactin and Neuroglian, among others. This study shows that HRP epitope modification of the key synaptogenic regulator Fasciclin 2 is not required for stabilization or localization, suggesting a role in protein function. However, complete loss of Vicia villosa (VVA) lectin reactivity synaptomatrix labeling is surprising because the epitope is a terminal β-GalNAc. This result suggests that the N-glycan LacdiNAc is enriched at the NMJ, and that the terminal GalNAc expected on O-glycans/glycosphingolipids may be present on N-glycans in this synaptic context. Importantly, VVA labels Dystroglycan and loss of Dystroglycan glycosylation blocks extracellular ligand binding and complex formation in Drosophila, and causes muscular dystrophies in humans. This study shows that VVA-recognized Dystroglycan glycosylation is not required for protein stabilization or synaptic localization, but did not test functionality or complex formation, which probably requires MGAT1-dependent modification. Conversely, the secreted endogenous lectin MTG is highly elevated in Mgat1 null synaptomatrix, probably owing to attempted compensation for complex and hybrid N-glycan losses that serve as MTG binding sites. MTG binds GlcNAc in a calcium-dependent manner and pulls down a number of HRP-epitope proteins by immunoprecipitation (Rushton, 2012), although the specific proteins have not been identified. It will be of interest to perform immunoprecipitation on Mgat1 samples to identify changes in HRP bands. Importantly, MTG is crucial for synaptomatrix glycan patterning and functional synaptic development. MTG regulates VVA synaptomatrix labeling, suggesting a mechanistic link between the VVA and MTG changes in Mgat1 mutants. The MTG elevation observed in Mgat1 nulls provides a plausible causative mechanism for strengthened functional differentiation (Parkinson, 2013).

    Consistent with recent glycosylation gene screen findings (Dani, 2012a), Mgat1 nulls exhibit increased synaptic growth and structural overelaboration. Therefore, complex and hybrid N-glycans overall provide a brake on synaptic morphogenesis, although individual N-glycans may provide positive regulation. Likely players include MGAT1-dependent HRP-epitope proteins (e.g., fasciclins, Neurotactin, Neuroglian), and position-specific (PS) integrin receptors and their ligands, all of which are heavily glycosylated and have well-characterized roles regulating synaptic architecture. An alternative hypothesis is that Mgat1 phenotypes may result from the presence of high-mannose glycans on sites normally carrying complex/hybrid structures, suggesting possible gain of function rather than loss of function of specific N-glycan classes. NMJ branch and bouton number play roles in determining functional strength, although active zones and GluRs are also regulated independently. Thus, the increased functional strength could be caused by increased structure at Mgat1 null NMJs. However, muscle-targeted UAS-Mgat1 rescues otherwise Mgat1 null function, but has no effect on structural defects, demonstrating that these two roles are separable. Presynaptic Mgat1 RNAi also causes strong functional defects, showing there is additionally a presynaptic requirement in functional differentiation. Neuron-targeted Mgat1 causes lethality, indicating that MGAT1 levels must be tightly regulated, but preventing independent assessment of Mgat1 presynaptic rescue of synaptogenesis defects (Parkinson, 2013).

    Presynaptic glutamate release and postsynaptic glutamate receptor responses drive synapse function. Using lipophilic dye to visualize SV cycling, this study found Mgat1 null mutants endogenously cycle less than controls, but have greater cycling capacity upon depolarizing stimulation. The endogenous cycling defect is consistent with the sluggish locomotion of Mgat1 mutants, whereas the elevated stimulation-evoked cycling is consistent with electrophysiological measures of neurotransmission. Similarly, mutation of dPOMT1, which glycosylates VVA-labeled Dystroglycan, decreases SV release probability (Wairkar, 2008), although dPOMT1 adds mannose not GalNAc. Null Mgat1 mutants display no change in SV cycle components (e.g. Synaptobrevin, Synaptotagmin, Synaptogyrin, etc.), but exhibit reduced expression of the key active zone component Bruchpilot. Other examples of presynaptic glycosylation requirements include the Drosophila Fuseless (FUSL) glycan transporter, which is critical for Cacophony (CAC) voltage-gated calcium channel recruitment to active zones, and the mammalian GalNAc transferase (GALGT2), whose overexpression causes decreased active zone assembly. Postsynaptically, Mgat1 nulls show specific loss of GLURIIB-containing receptors. Similarly, dPOMT1 mutants exhibit specific GLURIIB loss (Wairkar, 2008), although dystroglycan nulls display GLURIIA loss. Selective GLURIIB loss in Mgat1 nulls may drive increased neurotransmission owing to channel kinetics differences in GLURIIA versus GLURIIB receptors (Parkinson, 2013).

    Bidirectional trans-synaptic signaling regulates NMJ structure, function and pre/postsynaptic composition. This intercellular signaling requires ligand passage through, and containment within, the heavily glycosylated synaptomatrix, which is strongly compromised in Mgat1 mutants. In testing three well-characterized signaling pathways, this study found that Wingless (Wg) accumulates, whereas both GBB and JEB are reduced in the Mgat1 null synaptomatrix. WG has two N-glycosylation sites, but these do not regulate ligand expression, suggesting WG build-up occurs owing to lost synaptomatrix N-glycosylation. Importantly, WG overexpression increases NMJ bouton formation similarly to the phenotype of Mgat1 nulls, suggesting a possible causal mechanism. GBB is predicted to be N-glycosylated at four sites, but putative glycosylation roles have not yet been tested. Importantly, GBB loss impairs presynaptic active zone development similarly to Mgat1 nulls, suggesting a separable causal mechanism. JEB is not predicted to be N-glycosylated, indicating that JEB loss is caused by lost synaptomatrix N-glycosylation. Importantly, it has been shown that loss of JEB signaling increases functional synaptic differentiation similarly to Mgat1 nulls (Rohrbough, 2013). In addition, jeb mutants exhibit strongly suppressed NMJ endogenous activity, similarly to the reduced endogenous SV cycling in Mgat1 nulls. Moreover, the MTG lectin negatively regulates JEB accumulation in NMJ synaptomatrix, consistent with elevated MTG causing JEB downregulation in Mgat1 nulls (Parkinson, 2013).

    Trans-synaptic signaling drives recruitment of scaffolds that, in turn, recruit pre- and postsynaptic molecular components. Specifically, DLG1 and L(2)GL scaffolds regulate the distribution and density of both active zone components (e.g. BRP) and postsynaptic GluRs, and both of these scaffolds are reduced at Mgat1 null NMJs. Importantly, dlg1 mutants display selective loss of GLURIIB, with GLURIIA unchanged, similar to Mgat1 nulls, suggesting a causal mechanism. Moreover, l(2)gl mutants display both a selective GLURIIB impairment as well as reduction of BRP aggregation in active zones, similarly to Mgat1 nulls, suggesting a separable involvement for this synaptic scaffold. DLG1 and L(2)GL are known to interact in other developmental contexts, indicating a likely interaction at the developing synapse. Although synaptic ultrastructure has not been examined in l(2)gl mutants, dlg1 mutants exhibit impaired NMJ development, including a deformed SSR. These synaptogenesis requirements predict similar ultrastructural defects in Mgat1 mutants, albeit presumably due to the combined loss of both DLG1 and L(2)GL scaffolds. Future work will focus on electron microscopy analyses to probe N-glycosylation mechanisms of synaptic development (Parkinson, 2013).

    Three-dimensional imaging of Drosophila motor synapses reveals ultrastructural organizational patterns

    This study combined cryo-preservation of intact Drosophila larvae and electron tomography with comprehensive segmentation of key features to reconstruct the complete ultrastructure of a model glutamatergic synapse in a near-to-native state. Presynaptically, a complex network of filaments was detailed that connects and organizes synaptic vesicles. The complexity of this synaptic vesicle network was linked to proximity to the active zone cytomatrix, consistent with the model that these protein structures function together to regulate synaptic vesicle pools. A net-shaped network of electron-dense filaments spanning the synaptic cleft was identified that suggests conserved organization of trans-synaptic adhesion complexes at excitatory synapses. Postsynaptically, a regular pattern of macromolecules was characterized that yields structural insights into the scaffolding of neurotransmitter receptors. Together, these analyses reveal an unexpected level of conservation in the nanoscale organization of diverse glutamatergic synapses and provide a structural foundation for understanding the molecular machines that regulate synaptic communication at a powerful model synapse (Zhan, 2016).

    RNAi-mediated reverse genetic screen identified Drosophila chaperones regulating eye and neuromuscular junction morphology

    Accumulation of toxic proteins in neurons have been linked with the onset of neurodegenerative diseases, which in many cases, are characterized by altered neuronal function and synapse loss. Molecular chaperones help protein folding and resolubilization of unfolded proteins thereby reducing the protein aggregation stress. While most of the chaperones are expressed in neurons, their functional relevance largely remains unknown. Using bioinformatics analysis, this study identified 95 Drosophila chaperones and classified them into seven different classes. Ubiquitous actin5C-Gal4 mediated RNAi knockdown revealed that about 50% of the chaperones are essential in Drosophila. Knocking down these genes in eyes revealed that about 30% of the essential chaperones are crucial for eye development. Using neuron-specific knockdown, immunocytochemistry and robust behavioural assays, a new set of chaperones were identified that play critical roles in the regulation of Drosophila NMJ structural organization. Together, these data presents the first classification and comprehensive analysis of Drosophila chaperones. The screen identified new set of chaperones that regulate eye and NMJ morphogenesis. Outcome of the screen reported here provides a useful resource for further elucidating the role of individual chaperones in Drosophila eye morphogenesis and synaptic development (Raut, 2017).

    Ultrastructural comparison of the Drosophila larval and adult ventral abdominal neuromuscular junction

    Drosophila melanogaster has recently emerged as model system for studying synaptic transmission and plasticity during adulthood, aging and neurodegeneration. However, still little is known about the basic neuronal mechanisms of synaptic function in the adult fly. Per se, adult Drosophila neuromuscular junctions should be highly suited for studying these aspects as they allow for genetic manipulations in combination with ultrastructural and electrophysiological analyses. Although different neuromuscular junctions of the adult fly have been described during the last years, a direct ultrastructural comparison with their larval counterpart is lacking. The present study was designed to close this gap by providing a detailed ultrastructural comparison of the larval and the adult neuromuscular junction of the ventrolongitudinal muscle. Assessment of several parameters revealed similarities but also major differences in the ultrastructural organisation of the two model neuromuscular junctions. While basic morphological parameters are retained from the larval into the adult stage, the analysis discovered major differences of potential functional relevance in the adult: The electron-dense membrane apposition of the presynaptic and postsynaptic membrane is shorter, the subsynaptic reticulum is less elaborated and the number of synaptic vesicles at a certain distance of the presynaptic membrane is higher (Wagner, 2017).


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    Zygotically transcribed genes

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