An assay has been establised to quantify synaptic retraction at the Drosophila NMJ and it has been used to identify molecules involved in synaptic stability. This assay is based on the demonstration that the formation of organized postsynaptic muscle membrane folds, termed the subsynaptic reticulum (SSR), requires the presence of the presynaptic nerve terminal. Therefore, the SSR and proteins that localize to this structure will only be present at sites where the nerve terminal resides, or where it has recently resided. Thus, observed sites of organized postsynaptic SSR that lack opposing presynaptic neuronal markers identify regions of the neuromuscular junction (NMJ) where the nerve terminal once resided and has since retracted. This interpretation has been confirmed in studies using light-level, ultrastructural, and electrophysiological analyses. Sites of synapse retraction are referred to as 'retraction events' or 'synaptic footprints' and represent a quantitative assay for synaptic stability. A wide array of pre- and postsynaptic markers have been used to clearly define synaptic retraction events. Synaptic retractions can be identified with equal efficiency using antibodies that recognize diverse presynaptic antigens including cytoplasmic, membrane-associated, cytoskeleton, or vesicle-associated proteins. Several postsynaptic markers have also been used to quantify synapse retractions, including Discs-large, Shaker-GFP, and the clustered postsynaptic glutamate receptors (Eaton, 2005).
Presynaptic BMP signaling is necessary for synapse stabilization: The retraction assay was used to test whether BMP signaling in Drosophila motoneurons is required for synapse stabilization. It was first confirmed that mutations in the BMP type II receptor Wit and the BMP ligand Gbb cause a significant decrease in bouton number. These mutations also cause a significant increase in synaptic footprints, demonstrating that synaptic stability is significantly compromised in the absence of BMP signaling. Synaptic footprints were identified in equal numbers using multiple presynaptic markers, including anti-Synapsin and anti-nc82, which recognizes an antigen at the presynaptic active zone. Synaptic footprints can be rescued in the wit mutant background by neuronal expression of the full-length wit transgene, demonstrating that synapse destabilization is caused by the absence of the presynaptic Wit receptor. The number of synapse retractions is slightly less in gbb, but still statistically significantly when compared with the number in wit. This could be due to the fact that wit is a null mutation, whereas the gbb genotype that was used is not. Null mutations in gbb do not survive through larval development and, therefore, the genetic combination was used that had been previously used in studies of its effects on synaptic growth (Eaton, 2005).
The analysis was extended to mutations that disrupt additional downstream components of the canonical BMP signaling cascade. Mutations in the BMP type I receptor thick-veins (tkv), the Smad homolog mad, and the co-Smad medea were tested. All three mutations decrease bouton numbers to levels that are statistically identical to those observed in the wit and gbb mutations. All three mutations also cause a statistically significant increase in synaptic footprints compared to wild-type, demonstrating that canonical BMP signaling is necessary for synaptic stability as well as for growth (Eaton, 2005).
The fact that synaptic footprints in wit can be rescued by neuronal expression of UAS-wit suggests that BMP signaling is necessary in the motoneuron for synaptic stability. To confirm that Smad-mediated signaling is also required within the motoneuron for synaptic stabilization, the inhibitory Smad dad (dad gain of function, DAD GOF) was neuronally overexpressed. Genetic evidence suggests that this manipulation can block both Mad and DSmad2 signaling. Neuronal DAD GOF decreases bouton numbers and increases synaptic footprints to levels that are near those observed in tkv, mad, and medea. Finally, in order to examine whether Smad signaling is required in the cell soma for synaptic stabilization, a genetic interaction was examined between the wit mutation and the overexpression of a dominant-negative Glued transgene (DN-Glued) that disrupts retrograde axonal transport. Impairment of dynactin function in motoneurons disrupts retrograde axonal transport and causes synapse retraction at the NMJ. The number of retractions in the DN-Glued; wit double mutant is not additive with respect to the wit mutation or DN-Glued overexpression. Although the overexpression of DN-Glued cannot be considered a null mutant condition, it is sufficient to block the accumulation of nuclear phospho-Mad and therefore prevents BMP signaling to the nucleus. Thus, it is concluded that Smad-mediated signaling to the motoneuron cell soma is necessary for both synaptic growth and synaptic stability. Together, these data establish an in vivo link between retrograde axonal transport, an essential trophic signaling system, and the mechanisms of synaptic stabilization (Eaton, 2005).
Analysis of synaptic stability in synaptic growth mutations: Additional experiments were performed to test whether mutations that decrease the number of boutons always increase the number of footprints. In these experiments, mutations were analyzed that decrease bouton number but which have not been implicated in the BMP signaling system. Bouton numbers are significantly decreased in these three independent mutations: the cell adhesion molecule fasciclin II (fasII), the tyrosine phosphatase Dlar, and the microtubule binding protein futsch. In each of these three mutations, bouton numbers are significantly decreased whereas the number of synaptic footprints remains unchanged. It has also been shown that impaired synaptic growth in a presynaptic calcium channel mutant does not alter synaptic stability. From these data it is concluded that impaired synaptic growth does not necessarily impair synaptic stabilization. By extension, it is concluded that BMP signaling is required for two separable processes, synaptic growth and synaptic stability (Eaton, 2005).
Identification of a Smad-independent synapse-stabilizing activity for the BMP receptor: While all of the mutations in canonical BMP signaling molecules decrease bouton numbers to the same extent, there are significantly fewer synaptic footprints in the tkv, mad, and medea mutations (as well as in DAD GOF) when compared to the wit mutation. Thus, while canonical BMP signaling is required for both synaptic growth and stability, the Wit receptor has an additional stabilizing influence on the NMJ that cannot be accounted for by the downstream Smad signaling system. Experiments were pursued to investigate the mechanism of Wit-mediated synaptic stability that appears to be independent of Smad-mediated signaling (Eaton, 2005).
It was first determined whether the signaling associated with synaptic stabilization and synaptic growth might map to different regions of the cytoplasmic tail of the Wit receptor. To do so, a transgenic rescue approach was taken that involves neuronal expression of either the full-length wit transgene or a truncated wit transgene in the wit mutant background. The truncated Wit receptor lacks a C-terminal portion that is not required for Smad signaling in mammalian systems and has been shown to restore viability to the wit mutation. First, it was demonstrated that presynaptic expression of the full-length wit transgene in the wit mutant background rescues synaptic growth and synaptic stability. However, while presynaptic expression of the truncated wit transgene (wit-dCT) completely rescues synaptic growth, it is unable to fully restore synaptic stability to wild-type levels. It has been confirmed that the Wit-dCT receptor is able to activate downstream Smad signaling, by showing that neuronal expression of the wit-dCT construct rescues the presence of nuclear phospho-Mad in the wit mutant background. Possible differences in expression levels of the wit-dCT transgene that could account for incomplete rescue of synaptic stability were controlled for. Together, these data identify a region of the Wit receptor that is necessary for synapse stabilization, but is not required for nuclear Smad signaling, synaptic growth, or synaptic function. Experiments were pursued to investigate how this region of the Wit receptor influences synaptic stability (Eaton, 2005).
LIMK1 functions presynaptically to control synaptic stability: To support the conclusion that LIMK1 function is necessary for synapse stabilization and to determine whether LIMK1 functions in the nerve or the muscle to control synaptic stability, transgenic rescue experiments and overexpression experiments were pursued using a dominant-negative LIMK1 transgene. Neuronal expression of a dominant-negative, kinase-inactive LIMK1 (DN-LIMK1) transgene was found to significantly increase synaptic retractions while muscle-specific expression of DN-LIMK1 has no effect. There was a slight decrease in bouton number observed when DN-LIMK1 was expressed neuronally that was not observed in the LIMK1 mutations. This may represent a dominant effect of this transgene. However, this decrease in bouton number is significantly less than that observed following disruption of BMP signaling. Finally, in agreement with experiments using a dominant-negative transgene, it was found that neuronal expression of a wild-type LIMK1 transgene (UAS-LIMK1) restores synapse stability to the hemizygotic DLIMKP1/Y loss-of-function mutation without altering other aspects of synapse development. Together, these data support the conclusion that LIMK1 is specifically required in the presynaptic motoneuron for synaptic stabilization (Eaton, 2005).
LIMK1 functions downstream of the Wit receptor to control synaptic stability: The observation that LIMK1 binds the Wit receptor and is required presynaptically for synapse stabilization suggests that LIMK1 functions downstream of the Wit receptor and may confer the added synapse-stabilizing activity of the Wit receptor. To address this possibility, genetic interactions were examined between the wit and LIMK1 mutations. Animals harboring one mutant copy of wit and one mutant copy of LIMK1 (DLIMKP1/+; witA12/+) show a significant increase in synaptic footprints without a change in bouton number and without a change in synaptic function compared to wild-type. In comparison, heterozygous mutations in either gene alone do not show a significant increase in synaptic footprints compared to wild-type and have normal bouton numbers. Also the DN-LIMK1 was neuronally overexpressed in the wit mutant background and no additive effect was observed on the number of NMJs with retractions with respect to the wit mutation or the overexpression of DN-LIMK1 alone. The strong transheterozygous interaction between the witA12 mutation and LIMK1P1, as well as the lack of an additive effect in the number of NMJs with footprints when DN-LIMK1 is overexpressed in the wit mutant background, supports the conclusion that LIMK1 functions in the same genetic pathway as wit to control synaptic stability (Eaton, 2005).
Whether overexpression of UAS-LIMK1 could rescue synaptic stability was investigated in the wit mutant background. To do so, the wild-type LIMK1 transgene (UAS-LIMK1) was neuronally expressed in the wit mutant background. When UAS-LIMK1 was expressed in the wit mutant background using either a pan-neuronal (elav-GAL4) or motoneuron-specific (OK6-GAL4) GAL4 driver, it was possible to rescue defects in both synaptic growth and synaptic stability. Importantly, strong presynaptic expression of UAS-LIMK1 by elav-GAL4 raised at 29°C resulted in complete rescue of synaptic footprints to wild-type levels. In all rescue experiments, including overexpression of LIMK1 in the wit background, the presence of the wit mutation was confirmed. These data, in combination with the demonstration that LIMK1 binds the Wit receptor and the genetic interactions observed between wit and LIMK1, support the conclusion that LIMK1 functions downstream of wit to control synaptic stability (Eaton, 2005).
Remarkably, rescue of the wit mutation by UAS-LIMK1 not only restores synaptic growth and stability, but also rescues animal viability. wit mutant animals normally die during midlarval life and are never observed to mature into adult flies. Neuronal expression of LIMK1 (with elav-GAL4) in the wit mutant background restores adult viability. Adult flies emerge in large numbers and are able to climb normally the vertical sides of a plastic vial (Eaton, 2005).
A further analysis demonstrated that LIMK1 expression restores synaptic function to the wit receptor mutation. Electrophysiological analysis of the wit mutant synapse demonstrated a severe impairment of synaptic transmission including a dramatic decrease in the excitatory postsynaptic potential (EPSP) amplitude, a significant decrease in the average amplitude of the spontaneous miniature release events (mEPSP), and a dramatic decrease in presynaptic release as estimated by the average EPSP/average mEPSP. Expression of UAS-LIMK1 restores all aspects of synaptic function to near wild-type levels, including EPSP amplitude, mEPSP amplitude, and quantal content. The degree of rescue with UAS-LIMK1 is not statistically different from that observed when wit is rescued by the full-length wit transgene. A slight but significant decrease was observed in quantal content in the DLIMKP1/DLIMKP1 homozygotes compared to wild-type, although this decrease is much less than that observed in wit or mad mutants. It is therefore conclude that LIMK1 is able to rescue the wit electrophysiological defects even though it is not normally involved in BMP-dependent regulation of synaptic function. Thus, LIMK1 expression can restore all of the essential functions of the Wit receptor in the wit mutant background (Eaton, 2005).
Evidence that LIMK1 functions in parallel to Mad-mediated signaling: An investigation was carried out to see whether the rescue of synaptic growth and stability by LIMK1 expression in the wit mutant background is caused by activation of downstream Mad signaling that is independent of the Wit receptor. mad mutations have a significant increase in synaptic footprints and a significant reduction in bouton number. Neuronal expression of LIMK1 in the mad mutant background rescues both synaptic growth and stability to wild-type levels. However, unlike expression of LIMK1 in the wit mutant background, expression of LIMK1 does not restore viability to the mad mutant background. This difference is likely due to the expression of mad outside of the nervous system, whereas wit expression is largely restricted to the nervous system. Thus, LIMK1 is sufficient to promote synaptic growth and stability in the absence of Mad-mediated signaling. These data support the conclusion that LIMK1 functions in parallel to Mad-mediated signaling in the motoneuron to control synaptic stability and growth (Eaton, 2005).
Presynaptic localization of LIMK1: The localization of LIMK1 in the presence and absence of the Wit receptor was investigated. The UAS-LIMK1 transgene used in rescue experiments harbors an HA epitope tag that was used to visualize LIMK1 at the NMJ. Staining for HA in animals with neuronal expression of UAS-LIMK1 revealed a filamentous LIMK1 localization throughout the presynaptic nerve terminal. This staining pattern is identical in wild-type or wit mutant animals that neuronally express UAS-LIMK1. Surprisingly, LIMK1-HA is fond in very close association with the synaptic microtubules identified by anti-Futsch (Map1B-like protein). However, although LIMK1-HA is closely associated with anti-Futsch, the two markers do not colocalize. Rather, the LIMK1-HA staining appears as an independent continuous filament that extends throughout the NMJ. Two additional experiments were pursued to test whether the LIMK1-HA staining pattern requires the integrity of the synaptic microtubule cytoskeleton. (1) LIMK1-HA was overexpressed in the futsch mutant background, which severely disrupts the synaptic microtubule cytoskeleton. Filamentous LIMK1-HA staining remains in the futsch mutation, indicating that LIMK1 filaments are independent of stable synaptic microtubules. (2) Whether LIMK1-HA is perturbed following treatment of the synapse with nocadozole at a concentration that eliminates dynamic microtubule plus ends was investigated. It was found that nocadozole treatment slightly decreases the intensity of LIMK1-HA staining, but clear filamentous staining remains. These data suggest that LIMK1-HA filaments are closely associated with synaptic microtubules, but can be stable in the absence of a continuous synaptic microtubule cytoskeleton. Finally, this staining pattern does not resemble the synaptic localization of actin-GFP, which concentrates into randomly distributed patches throughout the NMJ. Thus, LIMK1 concentrates into a unique compartment within the presynaptic nerve terminal that has not been previously described. In addition, instances were found in which this compartment approached the synaptic plasma membrane, where it could associate with membrane receptors such as the Wit receptor. Finally, the possibility cannnot be ruled out that low levels of LIMK1 are present throughout the cytoplasm and in association with synaptic actin, since steady-state protein distribution was studiied in fixed tissue (Eaton, 2005).
Evidence that LIMK1 functions independently of ADF/Cofilin during synapse stabilization: The role of LIMK1 during axon outgrowth and growth cone motility is largely due to LIMK1 phosphorylation and inactivation of ADF/cofilin, which alters actin turnover. In these studies, the phosphorylation of ADF/cofilin by LIMK1 is antagonized by the Slingshot (Ssh) family of phosphatases (Niwa, 2002). Therefore, if ADF/cofilin is the target of LIMK1, then the overexpression of the Ssh phosphatase should mimic LIMK1 loss of function in this signaling cascade. It was found that overexpression of the ssh cDNA in the motoneuron (elav UAS-SSH wt) has no effect on synaptic growth, synaptic stability, or synaptic function. It was confirmed that Ssh protein traffics to the synapse in these experiments and it was found that Ssh has a cytoplasmic localization that does not resemble what was observed for LIMK1-HA. Finally, overexpression of a constitutively activated cofilin transgene (elav UAS-tsr-S3A) also has no effect on synaptic growth or stabilization. In combination with the observation that the distribution of synaptic LIMK1 does not resemble the distribution of synaptic actin, these data suggest that LIMK1 is not acting through ADF/cofilin to control synaptic stability. These data highlight differences in LIMK1 function during the rapid, dynamic process of axon outgrowth versus the slower, more prolonged mechanisms that govern synapse stabilization at the NMJ (Eaton, 2005).
The Drosophila neuromuscular junction (NMJ) is capable of rapidly budding new presynaptic varicosities over the course of minutes in response to elevated neuronal activity. Using live imaging of synaptic growth, this dynamic process was characterized, and it was demonstrated that rapid bouton budding requires retrograde bone morphogenic protein (BMP) signaling and local alteration in the presynaptic actin cytoskeleton. BMP acts during development to provide competence for rapid synaptic growth by regulating the levels of the Rho-type guanine nucleotide exchange factor Trio, a transcriptional output of BMP-Smad signaling. In a parallel pathway, it was found that the BMP type II receptor Wit signals through the effector protein LIM domain kinase 1 (Limk) to regulate bouton budding. Limk interfaces with structural plasticity by controlling the activity of the actin depolymerizing protein Cofilin. Expression of constitutively active or inactive Cofilin (Twinstar) in motor neurons demonstrates that increased Cofilin activity promotes rapid bouton formation in response to elevated synaptic activity. Correspondingly, the overexpression of Limk, which inhibits Cofilin, inhibits bouton budding. Live imaging of the presynaptic F-actin cytoskeleton reveals that activity-dependent bouton addition is accompanied by the formation of new F-actin puncta at sites of synaptic growth. Pharmacological disruption of actin turnover inhibits bouton budding, indicating that local changes in the actin cytoskeleton at pre-existing boutons precede new budding events. It is proposed that developmental BMP signaling potentiates NMJs for rapid activity-dependent structural plasticity that is achieved by muscle release of retrograde signals that regulate local presynaptic actin cytoskeletal dynamics (Piccioli, 2014).
Activity-dependent changes in synaptic structure play an important role in developmental wiring of the nervous system. The Drosophila larval neuromuscular junction (NMJ) has emerged as a model glutamatergic synapse that is well suited to study activity-dependent structural plasticity. The NMJ can be imaged in vivo during developmental periods of rapid synaptic growth when the axonal terminal expands ~5- to 10-fold in size over 5 d. Forward genetic screens to identify mutations that alter synaptic growth have revealed essential roles for retrograde bone morphogenic protein (BMP) signaling mediated by the secreted ligand Glass bottom boat (Gbb). Mutations that disrupt BMP signaling lead to synaptic undergrowth and neurotransmitter release defects. Multiple pathways downstream of retrograde BMP signaling through the type II receptor Wishful thinking (Wit) have been linked to synaptic growth, synapse stability, and homeostatic plasticity in Drosophila. BMP signaling via the Smad transcription factor Mothers against Dpp (Mad) regulates the expression of the Rho-type guanine nucleotide exchange factor (GEF) trio to control normal synaptic growth. Wit also interacts with LIM domain kinase 1 (Limk) to enhance synaptic stabilization in a pathway parallel to canonical Smad-dependent signaling. BMP signaling through Wit also potentiates synapses for homeostatic plasticity in a pathway that is independent of limk and synaptic growth regulation (Piccioli, 2014).
The NMJ displays acute structural plasticity in the form of rapid presynaptic bouton budding in response to elevated levels of neuronal activity. These rapidly generated presynaptic varicosities, referred to as ghost boutons, lack presynaptic and postsynaptic transmission machinery when initially formed. The budding of ghost boutons requires retrograde signaling mediated by the postsynaptic Ca2+-sensitive vesicle trafficking regulator synaptotagmin (Syt) 4 (Korkut, 2013). Syt4 also participates in developmental synaptic growth and controls retrograde signaling that mediates enhanced spontaneous release at the NMJ (Yoshihara, 2005; Barber, 2009). Beyond the role of Syt4 in ghost bouton budding, little is known about the signaling pathways that underlie this rapid form of structural synaptic plasticity. In particular, it is unclear whether pathways that regulate synaptic growth over the longer time scales of larval development also trigger acute structural plasticity. To address these issues, this study identified synaptic pathways that are required for rapid structural plasticity at Drosophila NMJs. Ghost bouton budding was found to be locally regulated at the synapse level, occurring in axons that have been severed from the neuronal cell body. In addition, activity-induced ghost bouton formation requires Syt1-mediated neurotransmitter release and postsynaptic glutamate receptor function. Like developmental growth, retrograde BMP signaling is required for ghost bouton budding. BMP signaling functions through a permissive role mediated by developmental Smad and Trio signaling, as well as through a local Wit-dependent modulation of Limk and Cofilin (Twinstar) activity that alters presynaptic actin dynamics (Piccioli, 2014).
Experimental analysis of ghost bouton budding at the Drosophila NMJ indicates that rapid activity-dependent synaptic growth requires retrograde BMP signaling at this synapse. The current data support a model in which BMP signaling through the type II receptor Wit is required developmentally to potentiate synapses for budding in response to elevated synaptic activity. This pathway requires Smad-dependent expression of the Rho-type GEF trio, and parallels a requirement for BMP signaling and Trio in developmental synaptic growth that occurs during the larval stages. In a parallel pathway, Wit interaction with Limk inhibits bouton budding through regulation of Cofilin activity. Both pathways regulate the synaptic actin cytoskeleton and may converge on similar actin regulatory molecules such as Limk and Cofilin via Rac1 or RhoA. Manipulating Cofilin activity levels by the overexpression of Limk or the expression of constitutively active/inactive Cofilin demonstrates that high Cofilin activity favors bouton budding, while low Cofilin activity inhibits budding. Local changes in the actin cytoskeleton that accompany activity-dependent bouton budding were also observed at sites of new synaptic growth. In addition, pharmacological disruption of normal actin turnover inhibits budding, suggesting that increased actin turnover mediated by Cofilin potentiates rapid activity-dependent synaptic plasticity (Piccioli, 2014).
Multiple genetic perturbations of BMP signaling were identified that altered the frequency of activity-dependent bouton budding at the NMJ. Although several of these mechanisms are shared with those previously characterized to control BMP-mediated developmental synaptic growth, several manipulations separated rapid activity-dependent BMP-mediated bouton budding from the slower forms of developmental growth. In the case of wit mutants or motor neuron overexpression of dad, a reduction in baseline bouton number was observed that showed varying degrees of severity. Wit mutants displayed strongly undergrown synapses, while dad overexpression animals had only modest synaptic undergrowth. In contrast, both these manipulations strongly suppressed ghost bouton budding. Additionally, synaptic undergrowth with partial knockdown of Gbb using postsynaptic RNAi was not observed, while this manipulation caused a strong reduction in ghost bouton budding. These observations indicate that rapid ghost bouton budding is more sensitive to modest perturbations in BMP signaling compared with developmental synaptic growth. One explanation for this differential sensitivity is that BMP signaling potentiates NMJs for activity-dependent bouton budding via transcriptional regulation of molecular components that are not required for normal synaptic growth. Alternatively, similar molecular pathways are required, but at different levels of output. In particular, trio mutants display a less severe synaptic undergrowth phenotype than wit mutants, but show similarly severe defects in ghost bouton budding. Because trio expression is strongly dependent on BMP signaling (Ball, 2010), a modest reduction in BMP output could reduce Trio levels such that ghost bouton budding is significantly reduced, while normal synaptic growth is less affected. It will be interesting to determine in future studies whether the developmental role for BMP signaling for acute structural plasticity shares a critical period as has recently been found for BMP function during developmental synaptic growth (Piccioli, 2014).
Given the requirement of the postsynaptic Ca2+ sensor Syt4 for normal levels of ghost bouton budding, an attractive model is that BMP is released acutely in response to elevated activity through the fusion of Syt4-positive postsynaptic vesicles. However, the current analysis indicates that retrograde BMP signaling through trio transcriptional upregulation is unlikely to be an instructive cue for bouton budding, as the severing of axons and the inhibition of retrograde trafficking of P-Mad before stimulation does not reduce budding in response to elevated activity. It is possible that synaptic P-Mad may play an instructional role in ghost bouton budding, as a local decrease in budding frequency was observed when Gbb expression was specifically reduced in muscle 6. Neuronal overexpression of dad also reduced synaptic P-Mad. Therefore, dad overexpression could inhibit ghost bouton budding by decreasing synaptic P-Mad signaling, in addition to decreasing nuclear Smad signaling. However, no dosage-dependent genetic interactions were observed between syt4 and wit, suggesting that Syt4 may participate in a separate pathway to regulate ghost bouton budding. Activity-dependent fusion of Syt4 postsynaptic vesicles (Yoshihara, 2005) could release a separate unidentified retrograde signal that provides an instructive cue for budding that would function in parallel to a developmental requirement for retrograde BMP signaling (Piccioli, 2014).
In addition to instructive cues from the postsynaptic compartment that trigger ghost bouton budding, the presynaptic nerve terminal must have molecular machines in place to read out these signals and execute the budding event. The regulation of Rho GTPases via Rho GEFs and GAPs downstream of extracellular cues is an attractive mechanism, as these proteins play critical roles in the regulation of neuronal morphology and axonal guidance. Several studies have shown that retrograde synaptic signaling regulates Rho GTPase activity to alter synaptic function and growth in Drosophila (Tolias, 2011). Ghost bouton budding mediated by developmental BMP signaling also shares some similarities with mechanisms underlying homeostatic plasticity at Drosophila NMJs. The Eph receptor is required for synaptic homeostasis at the NMJ, and it interfaces with developmental BMP signaling via Wit. While Eph receptor-mediated homeostatic plasticity predominantly requires the downstream RhoA-type GEF Ephexin, the Eph receptor may also signal through Rac1. Drosophila VAP-33A may also act as a ligand for synaptic Eph receptors, as it has been shown to regulate NMJ morphology and growth, while preferentially localizing to sites of bouton budding. The current analysis indicates that the levels of Trio, which functions as a Rho-type GEF, are bidirectionally correlated with ghost bouton budding activity and that overexpressed Trio is localized to ghost boutons after budding. As such, acute Trio regulation represents another attractive pathway for rapidly modifying bouton budding activity (Piccioli, 2014).
Rho GTPase signaling can produce distinct effects in differing systems and cell types depending on the presence or absence of downstream effectors, although most of these pathways ultimately impinge on regulation of the actin cytoskeletal. Indeed, this study has found a key role for Limk regulation of Cofilin activity in the control of ghost bouton budding. The current findings indicate that Limk activity normally functions to inhibit the formation of ghost boutons, as neuronal overexpression of Limk strongly suppressed activity-dependent bouton budding. Consistent with an inhibitory role for Limk, Cofilin activity promotes budding, while the overexpression of an inactive Cofilin inhibited budding. The expression of mutant Cofilin transgenes resulted in visible changes to the presynaptic actin cytoskeleton at NMJs, indicating that these manipulations likely alter rapid budding events by changing local actin dynamics at sites of potential growth. Using live imaging of F-actin dynamics before and after bouton budding, the formation of new F-actin puncta was observed at sites of bouton budding. Elevated Cofilin activity is sufficient to increase ghost bouton budding frequency, and is predicted to increase actin turnover and the formation of F-actin structures. Pharmacological disruption of actin polymerization dynamics also disrupts rapid bouton addition in response to elevated activity (Piccioli, 2014).
These findings support a model whereby Wit has opposing signaling roles with respect to bouton budding. Providing a permissive role via Smad signaling and an inhibitory role via Limk activation may provide for a system in which increased potential for rapid synaptic expansion is directly coupled to enhanced synaptic stability. This coupling could set a threshold for ghost bouton budding downstream of synaptic activity. In the background of moderate or low synaptic activity, Limk prevents ghost bouton budding. When synaptic activity is elevated, additional signaling events promote new synaptic growth by either reducing or outcompeting Limk activity, with a concurrent activation of Cofilin. Decreased Limk activity downstream of extracellular cues has been shown to regulate cell morphology in other systems as well, providing an attractive mechanism for rapid activity-dependent regulation of synaptic structure at Drosophila NMJs (Piccioli, 2014).
Mammalian LIM-kinases (LIMKs) phosphorylate cofilin and induce actin cytoskeletal reorganization. To elucidate the functional roles of LIMKs in vivo during developmental processes, the cDNA encoding a Drosophila homolog of LIMK (DLIMK) was isolated and two isoforms of DLIMK transcripts were identified coding for proteins with 1235 and 1257 amino acids, possessing the structure composed of two LIM domains, a PDZ domain, a protein kinase domain, and an unusual long C-terminal extension. In situ hybridization analysis in Drosophila embryos detected the uniformly distributed DLIMK mRNA in stages 2 to 5. In vitro kinase reaction revealed that DLIMK efficiently phosphorylates Drosophila cofilin (Twinstar) specifically at Ser-3, the site responsible for inactivation of its actin-depolymerizing activity. When expressed in cultured cells, wild-type DLIMK, but not its kinase-inactive form, induces changes in actin cytoskeletal organization. These observations suggest that the LIMK-cofilin signaling pathway for regulating actin filament dynamics is evolutionarily conserved between Drosophila and mammals (Ohashi, 2000).
Since mammalian LIMKs phosphorylate cofilin specifically at Ser-3, it was asked if DLIMK could phosphorylate cofilin. HA-tagged DLIMK(short) and its D500A mutant, in which the presumptive catalytic residue Asp-500 is replaced by Ala, were expressed in COS-7 cells, immunoprecipitated with anti-HA antibody, and subjected to in vitro kinase reaction, using mouse and Drosophila cofilin as substrates. When the immunoprecipitates were separated on SDS-PAGE and immunoblotted with anti-HA antibody, two major bands with estimated molecular masses of about 140 and 120 kDa were detected. The 140-kDa protein, with a molecular mass similar to that calculated from HAtagged DLIMK (140,426 Da), seems to be the product with a full-length amino acid sequence of HA-DLIMK. A rapidly migrating 120-kDa protein may be a proteolytically processed, C-terminally truncated product, since it retains the N-terminal HA-epitope. In vitro kinase reaction revealed that DLIMK, but not DLIMK(D500A), phosphorylated mouse and Drosophila cofilin, but the S3A mutants with replacement of Ser-3 by Ala were not phosphorylated by DLIMK, which means that DLIMK phosphorylates cofilin specifically at Ser-3, the site for inactivating its activity (Ohashi, 2000).
The effect of DLIMK on actin organization was examined in cultured cells. HA-tagged DLIMK was expressed in HeLa cells and actin filaments were visualized by rhodamine-conjugated phalloidin staining. Accumulation of actin filaments was noted at the periphery of the DLIMK-expressing cells, compared with findings in surrounding cells that did not express DLIMK. In contrast to wild-type DLIMK, expression of a kinase-negative form of DLIMK, DLIMK(D500A), had no apparent effect on actin organization. Thus, similar to mammalian LIMKs, DLIMK has the potential to induce actin reorganization in a manner dependent on its kinase activity. The findings in this study that DLIMK has the potential to phosphorylate cofilin and induce actin cytoskeletal rearrangements suggest that the LIMK-mediated signaling pathway used to regulate actin filament dynamics and reorganization is evolutionarily conserved in both Drosophila and vertebrates. In addition, it is also likely that mechanisms of activation of LIMK by Rho-ROCK and Rac-PAK signaling pathways are conserved in Drosophila and vertebrates. In Drosophila, homologs of Rho family GTPases and their effectors, ROCK and PAK, play a role in morphogenetic events, such as gastrulation, dorsal closure, neurite outgrowth, axon guidance, myoblast fusion, oogenesis and tissue polarity. Mutations in the PAK gene result in defects in photoreceptor axon guidance and targeting in Drosophila visual system. The Drosophila genetic approaches will provide insights into physiological functions of LIMK and LIMK-mediated signaling pathways in morphological changes and migrations of cells during developmental processes. Considering that the LIMK1 gene hemizygosity is linked to impairment of visuospatial cognition in a human genetic disorder, it is particularly important to explore the functions of LIMKs in neuronal development and neural network formation (Ohashi, 2000).
The ADF (actin-depolymerizing factor)/cofilin family, represented by Twinstar in Drosophila is a stimulus-responsive mediator of actin dynamics. In contrast to the mechanisms of inactivation of ADF/cofilin by kinases such as LIM-kinase 1 (LIMK1: see Drosophila LIM-kinase1), much less is known about its reactivation through dephosphorylation. Slingshot (Ssh), a family of phosphatases that have the property of F actin binding, is described. In Drosophila, loss of ssh function dramatically increases levels of both F actin and phospho-cofilin (P cofilin) and disorganizes epidermal cell morphogenesis. In mammalian cells, human Ssh homologs (hSSHs) suppress LIMK1-induced actin reorganization. Furthermore, Ssh and the hSSHs dephosphorylate P cofilin in cultured cells and in cell-free assays. These results strongly suggest that the SSH family plays a pivotal role in actin dynamics by reactivating ADF/cofilin in vivo (Niwa, 2002).
Regulation of the dynamics of the actin cytoskeleton is fundamental in the construction and remodeling of a variety of polarized subcellular structures. The reorganization of the actin cytoskeleton is controlled at multiple levels. When focused on elongation of actin filaments, profilin (chickadee in Drosophila) promotes the elongation at barbed ends, and capping protein (CP: capping protein beta in Drosophila) stabilizes the barbed ends; a member of the ADF (actin-depolymerizing factor)/cofilin family accelerates depolymerization at pointed ends and severs long actin filaments (Niwa, 2002 and references therein).
Among actin binding proteins, ADF/cofilin is the most-characterized stimulus-responsive mediator of actin dynamics. In response to insulin and lysophosphatidic acid, LIM-kinases (LIMKs), activated by effectors of Rho family GTPases, phosphorylate ADF/cofilin specifically at Ser-3, and thereby inhibit filament-severing and monomer binding activities of ADF/cofilin proteins. Testicular protein kinases (TESKs) also phosphorylate Ser-3 of ADF/cofilin and inhibit its activity, although upstream pathways of TESKs activation appear to be separate from that of LIMKs. ADF/cofilin undergoes rapid dephosphorylation as well, in response to several extracellular stimuli, which could result from downregulation of the kinases, upregulation of a phosphatase(s), or both. In contrast to characterization of the kinases, however, much less is known about the phosphatases that reactivate ADF/cofilin (Niwa, 2002 and references therein).
Actin dynamics in vivo have been studied genetically in several systems, including Drosophila. Bristles and hairs on appendages are actin-based protrusions exposed on body surfaces; therefore, they are easily scored landmarks for isolating mutations that cause malformations. In fact, loci reported to be involved in such malformations include chickadee (chic) encoding Profilin, capping protein beta (cpb) encoding the subunit of CP, and twinstar (tsr) encoding ADF/cofilin. This study reports another locus, which has been named slingshot (ssh) after the bifurcation phenotypes of the bristles and hairs in these mutants. The ssh gene encodes a phosphatase that is conserved among several animal species. Loss of ssh function dramatically increases the level of actin filaments, which is similar to a phenotype of tsr mutant cells. In cultured mammalian cells, such a strong enhancement of actin polymerization is induced by overproduction of LIMKs or TESKs (Niwa, 2002 and references therein).
One hypothesis would be that Ssh and these kinases share the same substrate, ADF/cofilin, regulate its phosphorylation level, and consequently control its actin-depolymerizing activity. To pursue this possibility, multiple approaches were undertaken. Evidence is presented that loss of ssh function in Drosophila increases the level of phospho-cofilin (P cofilin) and that ssh genetically interacts with the Drosophila LIMK gene in actin-based cell morphogenesis. In cultured cells, expression of either of two human Ssh homologs (hSSHs) with LIMK1 or TESK1 suppresses actin reorganization induced by these kinases; and Ssh or hSSHs expression reduces the level of P cofilin. Finally, Ssh and the hSSHs dephosphorylate P cofilin as judged from the results of cell-free assays. These results suggest that the SSH family plays a critical role in controlling actin dynamics, presumably through dephosphorylating and thus reactivating cofilin in cellular and developmental contexts (Niwa, 2002).
In the course of genetic screening for loci that affect bristle number and/or morphology, a focus was placed on l(3)01207, which has one copy of PZ inserted into CG6238. Lethality of l(3)01207 was due to loss of CG6238 function, as shown by the facts that the lethality was recovered by remobilization of the P element and by CG6238 cDNA expression using a widely expressed GAL4 driver, daughterless (da)-GAL4. Through subsequent analysis, it was found that adult escapers of hypomorphic alleles had bifurcated and twisted bristles. This locus was renamed slingshot (ssh) (Niwa, 2002).
To study exactly how actin reorganization is affected by ssh mutations, ssh mutant clones were stained with dye-conjugated phalliodin. Loss of ssh function causes a dramatic increase in the level of F actin in a cell-autonomous manner. This phenotype is found in mutant clones in larval imaginal discs, pupal wings, and follicle epithelia of the egg chamber. Along the apico-basal cell axis, overaccumulation of actin filaments was not necessarily restricted to the apical subcellular region; basolateral positions were also brightly stained. At 30-36 hr after puparium formation (h APF) at 25°C, each epidermal cell in the wing localizes an assembly of actin bundles to its distal-most vertex, producing a single prehair in the wild-type. Prehairs of the mutant cells were much more intensely stained with dye-conjugated phalliodin than those of normal cells, indicating that individual mutant hairs contained more actin filaments. ssh null cells seem to show no obvious defect in proliferation or cell-cell adhesion when compared with cells in wild-type twin spots (Niwa, 2002).
A dramatic increase in the F actin level has also been reported in twinstar (tsr)/cofilin clones (Baum, 2000; Baum, 2001); in cultured mammalian cells, excessive actin polymerization is caused by overproduction of LIMKs or TESKs. Therefore, the possibility was examined that those kinases and the SSH family share the same substrate, ADF/cofilin, and thus control the level of phospho-ADF/cofilin in vivo (Niwa, 2002).
Whether loss of ssh function increases the level of phosphorylated Drosophila cofilin (P Dcofilin) in fly tissues was examined. For this purpose, binding and specificity of anti-P cofilin antibody to P Dcofilin was first characterized. Anti-P cofilin antibody was originally made against a phosphopeptide of mammalian cofilin. When ssh1-11 clones were analyzed by staining or Western blotting with this anti-P cofilin antibody, the endogenous P Dcofilin level in ssh/ssh mutant cells was significantly elevated compared with that in the surrounding ssh/+ and +/+ wild-type cells, whereas the total amount of cofilin was not altered. This result is consistent with the hypothesis that a physiological expression level of Ssh is required for dephosphorylating P Dcofilin in vivo (Niwa, 2002).
Loss of ssh function and expression of Drosophila LIMK (DLIMK) exerts synergistic effects on wing hair morphogenesis and on the levels of F actin and P Dcofilin. Double-stranded RNA (dsRNA) of ssh sequences was expressed in dorsal cells of the wing. This phenocopied a hypomorphic ssh condition through a phenomenon called RNA interference (RNAi). DLIMK production on its own, whether the wild-type form (wt) or a kinase-inactive form (KI), does not give detectable phenotypes under the experimental conditions employed. The weak RNAi phenotype of ssh is strongly enhanced by expression of DLIMK-wt, but DLIMK-KI does not show this enhancement. Similarly, drastic increases in F actin and P Dcofilin levels result from coexpression of the dsRNA and DLIMK-wt. In those cells, unusually thick actin bundles are observed when images are collected with a low gain. Cells on ventral surfaces are not affected, where the transgene was silent (Niwa, 2002).
In addition to experiments using Drosophila, the approach of transfecting mammalian cell lines with hSSH plasmids was taken to test the following hypothesis: if hSSH dephosphorylates P cofilin, coexpression of hSSH together with LIMKs or TESKs is expected to counterbalance the promotion effect of those kinases on actin polymerization. To evaluate the degree of actin polymerization, F actin patterns were classified into three categories. In control cells expressing GFP alone, strong F actin signals were seen infrequently and most of the cells were scored as class 3, whereas LIMK1 or TESK1 expression made class 1 (strongest F actin signals) and class 2 (intermediate signal) predominant. Coexpression of hSSH-1L(wt) or hSSH-2(wt) with either kinase restored the cells to class 3 category; the CS phosphatase mutant forms, however, failed to counteract the kinases. These results support that the hSSH phosphatase activity inhibits LIMK1- or TESK1-dependent actin reorganization (Niwa, 2002).
The above coexpression results could be explained by the hypothesis that P cofilin is a direct target of the hSSHs; however, another interpretation is also possible. The kinase activity of LIMK1 is enhanced by phosphorylation at Thr-508 (Ohashi, 2000); therefore, the hSSHs might dephosphorylate LIMK1 and make it less active. This second hypothesis is less likely; a phosphomimetic form of LIMK1, LIMK1(T508EE), which is no longer phosphorylated at the residue of 508, induces actin polymerization. Nevertheless, hSSH-1L is still able to counteract LIMK1(T508EE) since it is LIMK1(wt) (Niwa, 2002).
Coexpression of hSSH-1L(CS) with LIMK1 evokes robust assembly of actin filaments in a way that is hardly induced by LIMK1 expression alone. When compared to F actin patterns in LIMK1-expressing cells, it looked as if almost all actin filaments in the coexpressing cell are assembled to build a limited number of thick bundles. These F-actin patterns were counted as class 2. These unusual structures were also induced by expressing hSSH-1L(CS) alone, but not at all by expressing the wild-type form. This abnormal assembly of the filaments is reminiscent of the consequence of coexpression of ssh-dsRNA and DLIMK in Drosophila epidermal cells, suggesting the possibility that hSSH-1L(CS) overexpression exerts a dominant-negative effect. hSSH-2(CS) also tends to induce actin reorganization, as shown by a slight expansion of the class 2 population when compared with the effect of its wild-type expression; however, unlike hSSH-1L(CS), hSSH-2(CS) can not generate the thick actin bundles (Niwa, 2002).
A straightforward prediction of the above results would be that expression of hSSH-1L(wt), hSSH-2(wt), or Ssh(wt) reduces the P ADF/cofilin level in cells. This was indeed found to be the case. Expression of hSSH-1L, hSSH-2, or Ssh causes a prominent decrease in the level of P cofilin or P Dcofilin; in contrast, hSSH-3(wt) expression is ineffective. Neither of the two CS forms nor MKP-5, which shows a limited sequence similarity to the catalytic domain of the SSH family, is able to decrease the P cofilin level. Complementary experiments have indicated that hSSH-1L is unable to dephosphorylate p38alpha and JNK2, which are efficiently inactivated by MKP-5. These results suggest that the SSH family displays distinct substrate specificity from the MKP family (Niwa, 2002).
Whether hSSH-1L, hSSH-2, and/or Ssh dephosphorylate P cofilin was directly assayed in cell-free systems. The hSSHs and cofilin were expressed in COS-7 cells; Ssh and Dcofilin were expressed in S2 cells. The immunoprecipitated enzymes and the purified substrates, which contain phosphorylated forms, were subject to in vitro reactions. Each wild-type form of the hSSHs or Ssh dephosphorylated P cofilin, but the CS forms do not, supporting the notion that cofilin is a substrate of hSSH-1L, hSSH-2, and Ssh. In the cell-free assay, 90% of P cofilin was dephosphorylated by Ssh after 15 min reaction (Niwa, 2002).
Under the experimental conditions used, hSSH-2 was reproducibly less active than hSSH-1L. When comparable amounts of hSSH-1L(wt) and hSSH-2(wt) were reacted with the same amount of P cofilin, over 99% of the substrate was dephosphorylated by hSSH-1L(wt), whereas only 60% was by hSSH-2(wt). ADF (also called destrin) is a close relative of cofilin in the vertebrate ADF/cofilin family and is also phosphorylated at Ser-3 by LIMKs and TESKs; hSSH-1L dephosphorylates phospho-ADF as well (Niwa, 2002).
In a transfection experiment, it was found that hSSH-1L(CS) predominantly colocalizes with F actin. This result prompted an investigation to see whether hSSHs and Ssh have the property of binding F actin. Tagged hSSHs and Ssh were incubated with actin filaments and centrifuged to examine whether each of them cosediment with F actin or not. Both wild-type and CS forms of hSSH-1L and Ssh preferentially cosedimented with actin filaments, whereas hSSH-2(wt), hSSH-2(CS), and hSSH-3(wt) exhibit weaker binding capabilities. A fraction of hSSH-3(CS) precipitates in the absence of actin filaments; thus, whether hSSH-3(CS) binds F actin or not was difficult to address with this method. Consistent with the result of the cosedimentation assay, Ssh(wt) and Ssh(CS) appear to colocalize with cortical actin in wing epithelia at 30 h APF (Niwa, 2002).
These studies using the whole animal and cell lines have demonstrated that Ssh and two human homologs (hSSH-1L and -2) prevent excessive actin polymerization and these SSH family members reduce the P cofilin level in cells. These studies have also shown in cell-free assays that Ssh and the hSSHs dephosphorylate cofilin efficiently and bind to actin filaments. All of these results are consistent with the hypothesis that Ssh and the hSSHs control reorganization of actin cytoskeleton by reactivating cofilin. Both the cofilin-phosphatase activity and the F actin binding ability of hSSH-2 are weaker than those of hSSH-1L under the experimental conditions used. Extensive domain-swapping between hSSH-1L and -2 may reveal whether the two activities are related to one another (Niwa, 2002).
In genomes of S. cerevisiae, C. elegans, and Arabidopsis, appear to have no ortholog of genes encoding LIMK, TESK, and Ssh, although cofilin in all of these species has a Ser-3 or equivalent serine residue. This is suggestive of a model in which the actin-depolymerizing activity of cofilin in those species may be regulated in vivo by a set of enzymes structurally distinct from LIMK, TESK, and Ssh. Alternatively, the activity may not be regulated by serine phosphorylation, but by other means such as PIP2 binding or intracellular pH changes. The latter possibility is supported in S. cerevisiae and D. discoideum by the report that P cofilin is not detected in vegetatively growing cells (Niwa, 2002).
The analysis of ssh mutants shows that a critical role of Ssh is in the building of wing hair, bristle, and arista, presumably through reactivating P cofilin. These cellular extensions in insects share a number of structural features with those found throughout the animal kingdom, such as the brush border of intestinal epithelial cell. Formation of all those protrusions on apical cell surfaces requires packing of parallel filaments through actin-bundling proteins. Therefore, thickening and/or splitting phenotypes in ssh mutants are most likely due to overaccumulated filaments that were not arranged in a normal array. Ssh does not appear to be required for cell proliferation or viability, which provides a contrast to an effect of loss of a generic enzyme PP1 or PP2A. All these results are consistent with the hypothesis that cofilin is the major substrate of Ssh and that Ssh does not display broad substrate specificity (Niwa, 2002).
It is worth noting that mutations of some other loci cause phenotypes similar to or reminiscent of those of ssh and twinstar (tsr)/cofilin. In act up (acu)/caplet (capt) clones in imaginal discs and follicle cells, the level of actin filaments is elevated as in ssh clones, and bristle malformation occurs. acu/capt encodes cyclase-associated protein (CAP), and CAP limits filament formation catalyzed by Enabled at apical cell junctions. The splitting or branching of wing hair, bristle, and arista is caused by loss of tricornered (trc) or furry (fry). The Trc protein is an evolutionally conserved kinase; Fry is also conserved among species, but its biochemical function is unknown. It would be intriguing to explore whether trc or fry phenotypes are associated with altered levels of actin filaments and P cofilin (Niwa, 2002).
The current studies did not focus on pursuing whether or not Ssh regulates cofilin functions in cell behaviors other than epithelial cell morphogenesis. For example, one of the important functions of ADF/cofilin is the dynamic regulation of the cortical ring at the cleavage furrow to complete cytokinesis. During ana/telophase in tsr mutants, aberrantly large actin-based structures appear at the site of contractile ring formation and fail to disassemble at the end of telophase. The apparent dispensability of Ssh in mitosis in the wing could be explained by the possibility that only a small fraction of active cofilin molecules are required for dynamics of the contractile ring and/or that a cofilin-phosphatase(s) other than Ssh is responsible for the progression of cytokinesis (Niwa, 2002).
In response to extracellular stimuli, some cells are known to undergo rapid dephosphorylation of ADF/cofilin (Moon, 1995); all of these stimuli result in cytoskeletal reorganization, possibly by way of reactivation of ADF/cofilin. During semaphorin-3A (Sema-3A)-induced growth cone collapse, a rapid increase and subsequent decrease in P cofilin occurs in growth cones within 5 min after exposure to Sema-3A (Aizawa, 2001). This supports a mechanism of local and transient regulation of phosphorylation-dephosphorylation cycling. In some systems, stimulation of cortical actin dynamics occurs without a net change in the ratio of P to non-P ADF/cofilin. Instead, what may be crucial is the turnover rate of the P ADF/cofilin pools or differential distributions of phosphorylation and dephosphorylation spots within cells (Meberg, 1998; Yang, 1998). The studies reported here raise an obvious question to be addressed, that is, whether the Ssh family is involved in the above stimuli-driven actin reorganizations. It is thus necessary to explore whether the activity of the SSH family is regulated in response to the stimuli, and if so, to determine the components of such regulatory mechanisms (Niwa, 2002).
Rho GTPases are essential regulators of cytoskeletal reorganization, but how they do so during neuronal morphogenesis in vivo is poorly understood. The actin-depolymerizing and actin-severing protein factor cofilin, encoded by twinstar, is essential for axon growth in Drosophila neurons. Cofilin function in axon growth is inhibited by LIM kinase and activated by Slingshot phosphatase. Dephosphorylating cofilin appears to be the major function of Slingshot in regulating axon growth in vivo. Genetic data provide evidence that Rho or Rac/Cdc42, via effector kinases Rho-associated kinase (Rok, also named Rho kinase or ROCK), or p21-activated kinase (Pak), respectively, activate LIM kinase to inhibit axon growth. Importantly, Rac also activates a Pak-independent pathway that promotes axon growth, and different RacGEFs regulate these distinct pathways. These genetic analyses reveal convergent and divergent pathways from Rho GTPases to the cytoskeleton during axon growth in vivo and suggest that different developmental outcomes could be achieved by biases in pathway selection (Ng, 2004).
Previous biochemical studies have established cofilin as a direct target of LIM kinase. To determine if Drosophila LIMK1 is the cofilin kinase in MB neurons, LIMK1 was overexpressed in all MB neurons, which resulted in axon growth defects. In 'severe' cases, all axon lobes were truncated. In addition, there were axon guidance defects at the peduncle, where axons formed a ball-like structure. In cases of 'strong' phenotypes, no guidance defects were detected, but both dorsal and medial axons were truncated. In cases of 'weak' phenotypes, axon growth defects were detected only in the dorsal lobes, while the medial lobes appeared normal. The expression level of individual transgenic lines caused the variability in the extent of axon defects. As determined from the epitope tag (HA) staining, stronger expression lines were associated with highly penetrant, severe axon growth and guidance defects, while weaker expression lines were associated with a mixture of severe, strong, weak, and, in some cases, no axon defects. Overexpression of kinase-inactive forms of LIMK1 (UAS-LIMK1 KI) at comparable levels (as determined by anti-HA staining where LIMK1 KI is similarly localized to axons as wild-type LIMK1) resulted in normal axon projections. This indicates that the axon growth defects caused by LIMK1 overexpression depend on its kinase activity (Ng, 2004).
Developmental studies indicate that the LIMK1 overexpression phenotype is similar to the ssh-/- phenotype. For at least γ and alpha/ß neurons, these phenotypes are also a result of axon extension failure. To test whether endogenous LIMK1 plays a role in MB morphogenesis, LIMK1 RNAi was expressed in MB neurons. This results in axon guidance and some axon growth defects. Therefore, LIMK1 is likely to play an endogenous role in MB morphogenesis (Ng, 2004).
The similarities between the ssh-/- and LIMK1 overexpression phenotypes suggest that LIMK1 and Ssh exert their effects by regulating a common substrate, cofilin. To genetically test whether LIMK1 overexpression leads to the inactivation of cofilin through phosphorylation, either Ssh or cofilin were coexpressed in a LIMK1 overexpression background. These experiments focussed on the transgenic line UAS-LIMK1 WT F4, which has an intermediate level of expression, allowing sensitive genetic interactions to be detected. Consistent with the idea that LIMK1 and Ssh regulate the phosphorylation of a common substrate, coexpression of wild-type Ssh fully suppresses the LIMK1 phenotype. In addition, coexpression of either wild-type or S3A cofilin also suppressed the LIMK1 overexpression phenotype. These experiments support the model that axon growth defects caused by LIMK1 overexpression are in large part a result of cofilin inactivation (Ng, 2004).
So far, this work has demonstrated that axon growth requires cofilin and that cofilin activity is regulated positively by Ssh phosphatase and negatively by LIM kinase. How is this cytoskeletal pathway directed by Rho GTPases in vivo? The role of the Rho-Rok pathway in regulating LIMK was examined first. Since MB axon growth is sensitive to the level of LIMK1 activity, genetic interaction studies were performed, either reducing the gene dose of endogenous Rho signaling components by half or overexpressing components of the Rho pathway in a LIMK1 overexpression background. Removing one copy of Rho1 (also called RhoA) strongly suppresses the axon growth phenotype caused by LIMK1 overexpression. Halving the gene dose of the Drosophila Rho effector kinase Rok (also called Drok) also suppresses the LIMK1 overexpression phenotype. However, this effect is much weaker, possibly because the Rok level is not as dose sensitive. In contrast, coexpressing Rok or Rho1 with LIMK1 strongly enhances the axon growth defects, even though overexpressing either Rho1 or Rok alone does not disrupt axon growth. These results suggest that Rho1 and Rok positively regulate LIMK1 activity (Ng, 2004).
The possible involvement of upstream regulators of Rho1 was also tested. Introducing one mutant copy of RhoGEF2 or pebble, two guanine nucleotide exchange factors known to positively regulate Rho1 or overexpression of the Rho1 inhibitor p190 RhoGAP, all resulted in suppressing the LIMK1 overexpression phenotype. Taken together with known biochemical activities of these signaling components, these genetic interaction data suggest that Rho1 activates Rok, which then directly activates LIMK1. In turn, this pathway is likely to be regulated positively by at least two RhoGEFs (RhoGEF2 and Pbl) and negatively by p190 RhoGAP in MB neurons in vivo (Ng, 2004).
While LIMK1 overexpression in MB neurons provides a sensitive assay for genetic interactions, the model was also tested using endogenous components. According to the model, overexpression of LIMK1 results in cofilin hyperphosphorylation, which is phenocopied in ssh mutants. Therefore, modulating the level of Rho1 signaling should also modify the ssh-/- phenotype in a similar manner. Indeed, loss of one copy of either Rho1 or RhoGEF2 also markedly suppresses the ssh-/- defects (Ng, 2004).
Biochemical studies have shown that LIM kinase is activated by Pak; a downstream effector kinase for Rac and Cdc42. Whether Pak activation could affect the LIMK pathway in axon growth was tested. In similar genetic interaction experiments, it was found that introducing three independently generated Drosophila Pak mutant alleles suppresses the LIMK1 overexpression phenotype. In addition, Pak overexpression also results in axon growth and guidance defects similar to those seen with LIMK1 overexpression, consistent with the model in which Pak activates LIMK1, leading to cofilin hyperphosphorylation. As predicted from this model, coexpression of active versions of cofilin with Pak results in a partial suppression of the growth and guidance defects (Ng, 2004).
Next it was determined which Rho GTPase pathway (Cdc42, Rac1/Rac2, or Rho1) regulates Drosophila Pak. It was found that loss of one copy of Cdc42 results in a strong suppression of the Pak overexpression phenotypes. Similar reduction of Rac1 and Rac2 (Rac1J10, Rac2Δ) results in weaker suppression. In contrast, reducing Rho1 results in a slight enhancement of the Pak overexpression phenotype. This suggests that the Pak overexpression phenotype specifically reflects the positive signaling input from Cdc42 and Rac in vivo (Ng, 2004).
Next it was tested whether mutations in the Cdc42 or Rac genes (Rac1, Rac2, Mtl, and the different combinations) would modify the LIMK1 overexpression phenotype. Reducing the Cdc42 activity leads to the suppression of the LIMK1 phenotype, suggesting that Cdc42 acts through Pak to activate LIMK1. Loss of one copy of both Rac2 and Mtl does not alter the LIMK1 phenotype. However, the LIMK1 phenotype is very sensitive to the levels of endogenous Rac1, since introducing one copy of a strong hypomorphic allele of Rac1 (Rac1J10) significantly suppresses the LIMK1 phenotype. This suppression is further enhanced when one copy of a null allele of Rac2 is introduced. These results suggest that Rac signaling (in particular, Rac1 and Rac2) also acts to activate LIMK1. This suggestion was also supported by the overexpression experiment. Overexpression of wild-type Rac1 (UAS-Rac1 WT) results in a weak LIMK1-like phenotype. However, when Rac1 and LIMK1 are coexpressed, axon growth defects were enhanced. Together with previous biochemical studies, these results suggest that Cdc42 and Rac act via Pak to activate LIMK1 (Ng, 2004).
To verify these genetic interactions with endogenous components, whether reducing either Cdc42 or Rac activity would modify the ssh-/- phenotype was examined. As with Rho1, reducing Cdc42 or Rac activity also partially suppresses the ssh-/- defects. Thus, these data indicate that Rho1, Cdc42, and Rac can all act through distinct downstream kinases to activate LIMK1 (Ng, 2004).
Surprisingly, reducing Rac GTPase activity can also result in enhancing the LIMK1 overexpression phenotype. For instance, the suppression effect of Rac1J10 Rac2Δ/+ was reverted by heterozygosity of Mtl (Rac1J10 Rac2Δ MtlΔ/+). This enhancement is more evident when one copy of the strongest allele of Rac1 was introduced into the LIMK1 overexpression background (Rac1J11/+). This effect is not due to dominant effects of this Rac1 allele, since in the same genetic background Rac1J11/+ animals did not display LIMK1-like growth defects without the LIMK1 expression transgene. Introduction of another hypomorphic allele of Rac1 (Rac1J6) together with Rac2 (Rac1J6 Rac2Δ/+) also strongly enhances the LIMK1 overexpression phenotype (Ng, 2004).
One interpretation of these results is that, while Rac can activate LIMK1 (via Pak), there is an alternative pathway downstream of Rac, acting antagonistically to LIMK1, that promotes axon growth. This is consistent with the finding that loss of Rac GTPase activity leads to axon growth defects in MB neurons (Ng, 2002). This pathway is likely to be Pak independent, since the Rac axon growth-promoting activity does not require direct binding to Pak (Ng, 2002), and Pak activation leads to axon growth inhibition. To further verify the existence of a Pak-independent pathway in regulating axon growth, use was made of a well-described Rac1 effector domain mutant. In mammalian fibroblasts, Rac1 activates Pak1 and, through an independent downstream pathway, promotes lamellipodia formation. A Y40C mutation in the effector binding domain of Rac1 results in the loss of Pak1 activation, but the lamellipodia promoting activity is maintained. Transgenic overexpression of Rac1 Y40C alone at levels comparable to those of wild-type Rac did not result in gross axon phenotypes. However, in contrast to wild-type Rac1 that strongly enhances LIMK1, coexpression of Rac1 Y40C strongly suppresses the LIMK1 overexpression phenotypes. These results suggest that Rac1 activates a Pak-independent pathway to counteract the effects of LIMK1 activity on axon growth (Ng, 2004).
Whether upstream activators of Rac (RacGEFs), could regulate these distinct axon growth pathways was tested. Trio encodes a RacGEF essential for axon guidance in MB neurons. Introducing one mutant copy of trio significantly suppressed the LIMK1 overexpression phenotype, suggesting that Trio acts to activate LIMK1. This was further verified by the overexpression experiments in which overexpression of wild-type Trio (UAS-trio) alone results in a mild LIMK1-like phenotype, while coexpression with LIMK1 results in a strong enhancement of the axon growth defects. Trio has two GEF domains—GEF1 is specific for Rac1, Rac2, and Mtl in vitro and in vivo, and GEF2 can activate Rho1/RhoA in vitro. Overexpression of the isolated Trio GEF1 domain (UAS-trio GEF1) in MB neurons results in severe axon growth defects, whereas overexpression of the isolated Trio GEF2 domain (UAS-trio GEF2) does not result in any gross defects. These results support the model that, in MB neurons, Trio, via its GEF1 domain, acts through Rac and Pak to activate LIMK1. These findings are also consistent with those of previous studies in which Trio acted via Rac/Pak to regulate Drosophila photoreceptor axon guidance (Ng, 2004).
In contrast to Trio, loss of one copy of still life (sif), encoding a different RacGEF, markedly enhances the LIMK1 phenotype. In overexpression experiments, UAS-sif alone does not result in gross axon defects. However, coexpression of Sif resulted in a strong suppression of the LIMK1 phenotype. These experiments suggest that Sif activates the pathway that acts antagonistically to LIMK1 (Ng, 2004).
The data suggest that Rac promotes axon growth via a pathway antagonistic to Pak and LIMK1. How does this pathway act to promote axon growth? One strong possibility is that Rac stimulates actin polymerization to promote axon growth. Therefore a number of candidate genes known to promote actin polymerization were tested. The following genetic criteria were establised by which these candidate pathways should work: (1) like Rac, loss of the candidate gene should result in axon growth defects; (2) genetic interactions with LIMK1, either through loss- or gain-of-function analyses, should show that they act antagonistically to LIMK1 (Ng, 2004).
The role of the actin nucleation factor SCAR-Arp2/3 complex was tested. Rac has been shown to promote de novo actin polymerization through interactions via the SCAR-Arp2/3 complex. Activation of the SCAR-Arp2/3 complex is required to establish cell protrusions during lamellipodia and filopodia formation, making it a good candidate pathway for promoting axon growth. This hypothesis was tested by making MARCM clones in MB neurons using null alleles of SCAR, WASp (a protein related to SCAR, also called Wsp), double mutants for SCAR and WASp, or Arpc1 (Flybase-Sop2), an essential component of the Arp2/3 complex. No axon growth defects were detected in single-cell or neuroblast clones. In addition, reduction of SCAR or Sop2 levels did not modify the LIMK1 overexpression phenotype. Furthermore, overexpression of SCAR did not suppress, but mildly enhanced, the LIMK1 overexpression phenotype. Taken together, these results suggest that the SCAR/WASp-Arp2/3 pathway does not play an essential role in axon growth of MB neurons, and it is unlikely that this pathway contributes to the Rac pathway that promotes axon growth (Ng, 2004).
Several other known regulators of actin polymerization were tested for their contribution to MB axon growth. The actin polymerization stimulators profilin (encoded by chickadee or chic) and Enabled (Ena) have been shown to be essential for axon growth and guidance in Drosophila. In addition, genetic interaction studies suggest that these proteins may be involved in Rac GTPase signaling. When assayed for MB axon growth, both ena-/- and chic-/- single-cell and neuroblast clones exhibited drastic axon growth defects. Interestingly, when examined at higher resolutions, neither ena nor chic axons displayed filopodia-like and lamellipodia-like protrusions at the axon termini characteristic of tsr-/- neurons. These genetic interaction experiments found no evidence that ena or chic act antagonistically to LIMK1. In fact, Ena overexpression strongly enhances the LIMK1 overexpression phenotype. Thus, although both Ena and profilin are essential for MB axon growth, they do not appear to constitute the Rac-mediated axon growth-promoting pathway (Ng, 2004).
Finally, the Formin-class protein Diaphanous (Dia) was tested, since it has been implicated in regulating actin polymerization downstream of Rho GTPases. dia-/- MB neurons do not exhibit axon growth defects in single-cell clones or in neuroblast clones, which exhibit strong cell proliferation defects. Interestingly, reducing Dia activity appears to suppress the LIMK1 overexpression phenotype, suggesting that Dia can act in a pathway that enhances, but does not antagonize, LIMK1 (Ng, 2004).
The C-terminal tail of mammalian BMP type II receptors has been shown to interact with and regulate the activity of LIMK1 in vitro (Foletta, 2003; Lee-Hoeflich, 2004). This study demonstrates that Drosophila LIM Kinase1 (LIMK1) binds the C-terminal tail of the Wit receptor that is deleted in the wit-dCT transgene. Peptides corresponding to the N-terminal region of LIMK1 containing either the tandem LIM and PDZ domains together (LIM-PDZ) or containing the tandem LIM domains alone (LIM only) interact with the C-terminal region of the Wit receptor (C-Term), but not with the kinase domain (Kinase) in a yeast two-hybrid binding assay. No interactions were detected between the PDZ domain of LIMK1 and the Wit receptor in this analysis. In addition, no interaction was detected between the C-terminal region of the Wit receptor (Wit-CT-FLAG) and full-length LIMK1 (LIMK1-HA) when these proteins are coexpressed in Drosophila S2 cells. These observations suggest that the LIM domains of LIMK1 mediate specific binding to the C-terminal tail of the Wit receptor, consistent with findings reported for LIMK1 binding to mammalian BMP receptors (Foletta, 2003; Lee-Hoeflich, 2004).
Whether LIMK1 is required for synaptic growth and/or stabilization was tested. A P element insertion (DLIMKP1) that resides in a 5′ untranslated exon of the LIMK1 gene as well as two deficiency chromosomes that uncover the LIMK1 locus were used. Northern analysis demonstrated that DLIMKP1/DLIMKP1 results in the near absence of detectable message, indicating that this is a strong-hypomorphic or null mutation. The LIMK1 mutations, including DLIMKP1/DLIMKP1, DLIMKP1/Y, DLIMKP1/Df(1)JA26, and DLIMKP1/Df(1)HF368 are all viable. This is consistent with the observation that LIMK1 knockout mice are homozygous viable (Meng, 2002). DLIMKP1/DLIMKP1, DLIMKP1/Y, DLIMKP1/Df(1)JA26, and DLIMKP1/Df(1)HF368 all cause a significant increase in synaptic footprints without decreasing synaptic bouton number. These genetic data indicate that LIMK1 is necessary for synaptic stability, but is not required for normal synaptic growth. Since the number of synaptic footprints is comparable when DLIMKP1 is analyzed in trans to deficiency chromosomes that uncover the LIMK1 locus, it is concluded that this P element insertion represents a strong loss-of-function mutation (Eaton, 2005).
The Rho-kinases (ROCKs) are major effector targets of the activated Rho GTPase that have been implicated in many of the Rho-mediated effects on cell shape and movement via their ability to affect acto-myosin contractility. The role of ROCKs in cell shape change and motility suggests a potentially important role for Rho-ROCK signaling in tissue morphogenesis during development. Indeed, in Drosophila, a single ROCK ortholog, DRok, has been identified and has been found to be required for establishing planar cell polarity. A potential role for DRok in additional aspects of tissue morphogenesis was examined using an activated form of the protein in transgenic flies. The findings demonstrate that DRok activity can influence multiple morphogenetic processes, including eye and wing development. Furthermore, genetic studies reveal that Drok interacts with multiple downstream effectors of the Rho GTPase signaling pathway, including non-muscle myosin heavy chain, adducin, and Diaphanous in those developmental processes. Finally, in overexpression studies, it was determined that Drok and Drosophila Lim-kinase interact in the developing nervous system. These findings indicate widespread diverse roles for DRok in tissue morphogenesis during Drosophila development, in which multiple DRok substrates appear to be required (Verdier, 2006; full text of article).
Among the Rho-kinase substrates that have been strongly implicated in neural development are the Lim-kinases. The single Drosophila Lim-kinase (DLimk) is required for proper synapse formation and proper regulation of its activity is necessary for normal axon growth. To determine whether DRok-mediated activation of DLimk plays a role in proper neural development, transgenic flies expressing activated DRok were crossed with flies over-expressing DLimk to examine phenotypes in the developing nervous system. First, using GMR-driven transgenes to identify a potential interaction in the developing eye, it was observed that while overexpression of DLimk causes no detectable effects on eye development, co-expression of DLimk with activated DRok results in a dramatic disruption of eye development associated with a severe morphology defect of the external eye and a reduced overall eye size. Since the effects of a single-copy DRok-cat transgene on exterior eye structures in this setting are relatively mild, this finding is consistent with a synergistic interaction between these two proteins, and suggests that a DRok-DLimk signal may be influencing normal development. Second, a similar synergistic interaction between DRok and DLimk was observed in the developing central nervous system. Using an elav-GAL4 driver to express UAS-linked Drok-cat and Dlimk in developing neurons, it was observed that neither protein alone causes any detectable effect on the appearance of the embryonic nervous system, whereas co-expression of the proteins results in the appearance of breaks along the ventral nerve cord. These findings suggest that DLimk is likely to mediate at least some of the DRok-dependent functions in the developing nervous system (Verdier, 2006).
In conclusion, genetic analysis of DRok in development, using ovexpression studies in the eye, in the wing and in the CNS indicates that stringent regulation of DRok activity is required for various developmental processes, such as photoreceptor maintenance and wing vein formation. In addition, the overexpression system has revealed zipper, the Drosophila nonmuscle myosin heavy chain, as a strong genetic interactor of DRok, as seen in other reported developmental events such as dorsal closure and wing planar cell polarity, confirming that myosin II is a key downstream mediator of Rho-kinase biological effects in several morphogenetic processes. Moreover, DRok interacts with another target protein, DLimk, to influence some other aspects of tissue morphogenesis, including CNS development (Verdier, 2006).
How learned experiences persist as memory for a long time is an important question. In Drosophila the persistence of memory is dependent upon amyloid-like oligomers of the Orb2 protein. However, it is not clear how the conversion of Orb2 to the amyloid-like oligomeric state is regulated. The Orb2 has two protein isoforms, and the rare Orb2A isoform is critical for oligomerization of the ubiquitous Orb2B isoform. This study reports the discovery of a protein network comprised of protein phosphatase 2A (PP2A), Transducer of Erb-B2 (Tob), and Lim Kinase (LimK) that controls the abundance of Orb2A. PP2A maintains Orb2A in an unphosphorylated and unstable state, whereas Tob-LimK phosphorylates and stabilizes Orb2A. Mutation of LimK abolishes activity-dependent Orb2 oligomerization in the adult brain. Moreover, Tob-Orb2 association is modulated by neuronal activity and Tob activity in the mushroom body is required for stable memory formation. These observations suggest that the interplay between PP2A and Tob-LimK activity may dynamically regulate Orb2 amyloid-like oligomer formation and the stabilization of memories (White-Grindley, 2014).
Previous work suggested that conversion of neuronal CPEB to an amyloid-like oligomeric state provides a molecular mechanism for the persistence of memory. However, it is not known how Orb2 oligomerization is regulated so that it will occur in a neuron/synapse-specific and activity-dependent manner. This study reports that factors that influence Orb2A stability and thereby abundance regulate Orb2 oligomerization (White-Grindley, 2014)
Tob, a previously known regulator of SMAD-dependent transcription and CPEB-mediated translation, associates with both forms of Orb2, but increases the half-life of only Orb2A. Stimulation with tyramine or activation of mushroom body neurons enhances the association of Tob with Orb2, and overexpression of Tob enhances Orb2 oligomerization. Both Orb2 and Tob are phosphoproteins. Phosphorylation destabilizes Orb2-associated Tob, whereas it stabilizes Orb2A. Tob promotes Orb2 phosphorylation by recruiting LimK, and PP2A controls the phosphorylation status of Orb2A and Orb2B (White-Grindley, 2014).
PP2A, an autocatalytic phosphatase, is known to act as a bidirectional switch in activity-dependent changes in synaptic activity. PP2A activity is down-regulated upon induction of long-term potentiation of hippocampal CA1 synapses (LTP) and up-regulated during long-term depression (LTD). Similarly, Lim Kinase, which is synthesized locally at the synapse in response to synaptic activation, is also critical for long-term changes in synaptic activity and synaptic growth (White-Grindley, 2014).
Based on these observations a model is proposed for activity-dependent and synapse-specific regulation of amyloid-like oligomerization of Orb2. It is postulated that in the basal state synaptic PP2A keeps the available Orb2A in an unphosphorylated and thereby unstable state. Neuronal stimulation results in synthesis of Orb2A by a yet unknown mechanism. The Tob protein that is constitutively present at the synapse binds to and stabilizes the unphosphorylated Orb2A and recruits the activated LimK to the Tob-Orb2 complex, allowing Orb2 phosphorylation. Concomitant decreases in PP2A activity and phosphorylation by other kinases enhances and increases Orb2A half-life. The increase in Orb2A level as well as phosphorylation may induce conformational change in Orb2A, which allows Orb2A to act as a seed. Alternatively, accumulation and oligomerization of Orb2A may create an environment that is conducive to overall Orb2 oligomerization. In the absence of an in vitro Orb2A-Orb2B oligomerization assay, it is not possible to distinguish between these two possibilities (White-Grindley, 2014).
For Tob, initial Orb2 association stabilizes Tob. However, association with Orb2 as well as suppression of PP2A activity leads to additional phosphorylation, which results in dissociation of Tob from the Orb2-Tob complex and destabilization. The destabilization of Orb2-associated Tob provides a temporal restriction to the Orb2 oligomerization process. The coincident inactivation of PP2A and activation of LimK may also provide a mechanism for stimulus specificity and synaptic restriction (White-Grindley, 2014).
Orb2A and Orb2B are phosphorylated at multiple sites, including serine/threonine and presumably tyrosine residues. These phosphorylation events are likely mediated by multiple kinases because overexpression of LimK did not affect Orb2 phosphorylation to the extent observed with the inhibition or activation of PP2A, raising several interesting questions. In what order do these phosphorylations occur? What function do they serve individually and in combination? What kinases are involved? Moreover, similar to mammalian CPEB family members, in addition to changing stability, phosphorylation may also influence the function of Orb2A and Orb2B (White-Grindley, 2014).
Does Tob regulate Orb2 function? In mammals Tob has been shown to recruit Caf1 to CPEB3 target mRNA, resulting in deadenylation, and CPEB3 is known to act as a translation repressor when ectopically expressed. This study found Drosophila Tob also interacts with Pop2/Caf1 and Orb2A and Orb2B can repress translation of some mRNA. Orb2 has also been identified as a modifier of Fragile-X Mental Retardation Protein (FMRP)-dependent translation, and Fragile-X is believed to act in translation repression (Cziko, 2009). Therefore, the Tob-Orb2 association may contribute to Orb2-dependent translation repression, and the degradation of Orb2-associated Tob may relieve translation repression. Additionally, if the oligomeric Orb2 has an altered affinity for either mRNA or other translation regulators, Tob can affect Orb2 function by inducing oligomerization. However, the relationship between Tob phosphorylation and its function is unclear at this point (White-Grindley, 2014).
Does involvement of Tob both in transcription and translation serve a specific purpose in the nervous system? Tob inhibits BMP-mediated activation of the Smad-family transcription activators (Smad 1/5/8) by promoting association of inhibitory Smads (Smad 6/7) with the activated receptor. In Drosophila BMP induces synaptic growth via activation of the Smad-family of transcriptional activators, and subsequent stabilization of these newly formed synapses via activation of LimK. These studies suggest Tob and LimK also regulate Orb2-dependent translation, raising the possibility Tob may coordinate transcriptional activation in the cell body to translational regulation in the synapse (White-Grindley, 2014).
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