InteractiveFly: GeneBrief

Calcium-independent receptor for α-latrotoxin: Biological Overview | References


Gene name - Calcium-independent receptor for α-latrotoxin

Synonyms - Latrotoxin receptor

Cytological map position - 44D4-44D5

Function - G-protein coupled receptor

Keywords - shapes the perception of tactile, proprioceptive, and auditory stimuli through chordotonal neurons - sensitizes these neurons for the detection of mechanical stimulation by amplifying their input-output function

Symbol - Cirl

FlyBase ID: FBgn0033313

Genetic map position - chr2R:8,614,722-8,625,203

Classification - G-protein coupled receptor 2 family - Latrophilin/CL-1-like GPS domain

Cellular location - surface transmembrane



NCBI links: Precomputed BLAST | EntrezGene
BIOLOGICAL OVERVIEW

G-protein-coupled receptors (GPCRs) are typically regarded as chemosensors that control cellular states in response to soluble extracellular cues. However, the modality of stimuli recognized through adhesion GPCR (aGPCR), the second largest class of the GPCR superfamily, is unresolved. This study study characterizes the Drosophila aGPCR Latrophilin/dCirl, a prototype member of this enigmatic receptor class. dCirl is shown to shapes the perception of tactile, proprioceptive, and auditory stimuli through chordotonal neurons, the principal mechanosensors of Drosophila. dCirl sensitizes these neurons for the detection of mechanical stimulation by amplifying their input-output function. These results indicate that aGPCR may generally process and modulate the perception of mechanical signals, linking these important stimuli to the sensory canon of the GPCR superfamily (Scholz, 2015).

Because of the nature of their activating agents, G-protein-coupled receptors (GPCRs) are established sensors of chemical compounds. The concept that GPCRs may also be fit to detect and transduce physical modalities, i.e., mechanical stimulation, has received minor support thus far. In vitro observations showed that, in addition to classical soluble agonists, mechanical impact such as stretch, osmolarity, and plasma membrane viscosity may alter the metabotropic activity of individual class A GPCR. However, the ratio and relationship between chemical and mechanical sensitivity and the physiological role of the latter remain unclear (Scholz, 2015).

Genetic studies have indicated that adhesion GPCRs (aGPCRs), a large GPCR class with more than 30 mammalian members, are essential components in developmental processes (Langenhan, 2009). Human mutations in aGPCR loci are notoriously linked to pathological conditions emanating from dysfunction of these underlying mechanisms, including disorders of the nervous and cardiovascular systems, and neoplasias of all major tissues (Langenhan, 2013). However, as the identity of aGPCR stimuli is unclear, it has proven difficult to comprehend how a GPCRs exert physiological control during these processes (Scholz, 2015).

Latrophilins constitute a prototype aGPCR subfamily because of their long evolutionary history. Latrophilins are present in invertebrate and vertebrate animals (Fredriksson, 2005), and their receptor architecture has remained highly conserved across this large phylogenetic distance. The mammalian Latrophilin 1 homolog was identified through its capacity to bind the black widow spider venom component α-latrotoxin, which induces a surge of vesicular release from synaptic terminals and neuroendocrine cells through formation of membrane pores. Latrophilin 1/ADGRL1 was suggested to partake in presynaptic calcium homeostasis by interacting with a teneurin ligand (Silva, 2011) and in trans-cellular adhesion through interaction with neurexins 1b and 2b (Boucard, 2012). Further, engagement of Latrophilin 3/ADGRL3 with FLRT proteins may contribute to synapse development (O‚ÄôSullivan, 2014). The role of Latrophilins in the nervous system thus appears complex (Scholz, 2015).

This study has used a genomic engineering approach to remove and modify the Latrophilin locus dCirl, the only Latrophilin homolog of Drosophila melanogaster. dCirl was shown to be required in chordotonal neurons for adequate sensitivity to gentle touch, sound, and proprioceptive feedback during larval locomotion. This indicates an unexpected role of the aGPCR Latrophilin in the recognition of mechanosensory stimuli and provides a unique in vivo demonstration of a GPCR in mechanoception (Scholz, 2015).

This analysis provides multiple lines of evidence to support that Latrophilin/dCirl, one of only two aGPCRs in the fly, is a critical regulator of mechanosensation through chordotonal neurons in Drosophila larvae. Larval chordotonal organs respond to tactile stimuli arising through gentle touch, mechanical deformation of the larval body wall and musculature during the locomotion cycle, and vibrational cues elicited through sound. First, this study determined that registration of all these mechanical qualities is reduced in the absence of dCirl, based on behavioral assays. Then it was established that behavioral defects can be rescued by re-expression of dCirl in chordotonal neurons, one of several cell types with endogenous dCirl expression. Mechanically stimulated lch5 neurons lacking dCirl were shown to responded with action currents at approximately half the control rate across a broad spectrum of stimulation frequencies, providing direct functional evidence for a role of dCirl in chordotonal dendrites, the site of mechanotransduction and receptor potential generation, or somata, where action potentials are likely initiated. Further, the ability of chordotonal neurons to generate mechanical responses relative to their background spike activity appears to be modulated by dCirl (Scholz, 2015).

Combining dCirl KO with strong hypomorphs of trp homologs, ion channels that are directly responsible for the conversion of mechanical stimulation into electrical signals within chordotonal neurons, implied that dCirl operates upstream of them. Intriguingly, removing dCirl from nompC or nan mutant backgrounds resulted in inverse outcomes, i.e., decreased and increased crawling distances, respectively. This suggests that dCirl enhances nompC activity while curtailing nan function. These experiments demonstrate that dCirl genetically interacts with essential elements of the mechanotransduction machinery in chordotonal cilia. Additional studies showed that the dCirl promoter contains a RFX/Fd3F transcription factor signature that implicates dCirl in the mechanosensitive specialization of sensory cilia (Newton, 2012). On the basis of these results, it is proposed that dCirl partakes in the process of mechanotransduction or spike initiation and transmission to promote sensory encoding (Scholz, 2015).

The classical model of GPCR activation has become the archetypical example for cellular perception of external signals. It comprises soluble ligands that bind to the extracellular portions of a cognate receptor, whereby receptor conformation is stabilized in a state that stimulates metabotropic effectors. Thus, GPCRs are primarily regarded as chemosensors due to the nature of their activating agents. The concept that GPCRs may also be fit to detect and transduce physical modalities, i.e., mechanical stimulation, has received little support thus far (Scholz, 2015).

aGPCRs display an exceptional property among the GPCR superfamily in that they recognize cellular or matricellular ligands. To date, only one ligand, Type 4 collagen, has proved adequate to induce intracellular signaling (Paavola, 2014), whereas for the vast majority of ligand-aGPCR interactions this proof either failed or is lacking (Langenhan, 2013). This implies that sole ligand recognition is generally not sufficient to induce a metabotropic response of aGPCRs. Thus, in addition to ligand engagement, the current results suggest that mechanical load is a co-requirement to trigger the activity of dCIRL, a prototypical aGPCR homolog (Scholz, 2015).

Recent findings place aGPCRs in the context of mechanically governed cellular functions (Yang, 2013), but how mechanical perception through aGPCR activity impinges on cell responses has not yet been established. In addition, the molecular structure of aGPCR is marked by the presence of a GPCR auto-proteolysis inducing (GAIN) domain (Arac, 2012), which plays a paramount role in signaling scenarios for aGPCRs (Promel, 2013). This domain type is also present in PKD-1/Polycystin-1-like proteins, which are required to sense osmotic stress and fluid flow in different cell types and are thus considered bona fide mechanosensors (Retailleau, 2014). In addition, studies on EGF-TM7-, BAI-, and GPR56-type aGPCRs further showed that proteolytic processing and loss of NTF may figure prominently in activation of the receptors' metabotropic signaling output and that mechanical forces exerted through receptor-ligand contact are required for receptor internalization (Scholz, 2015 and references therein).

dCirl is not the only aGPCR associated with mechanosensation. Celsr1 is required during planar cell polarity establishment of neurons of the inner ear sensory epithelium. Similarly, the very large G-protein-coupled receptor 1 (VLGR1) exerts an ill-defined developmental role in cochlear inner and outer hair cells, where the receptor connects the ankle regions of neighboring stereocilia. In addition, VLGR1 forms fibrous links between ciliary and apical inner segment membranes in photoreceptors. Both cell types are affected in a type of Usher syndrome, a congenital combination of deafness and progressive retinitis pigmentosa in humans, which is caused by loss of VLGR1 function. Although present evidence derived from studies of constitutively inactive alleles suggests a requirement for Celsr1 and VLGR1 aGPCR for sensory neuron development, their putative physiological roles after completion of tissue differ-entiation have remained unclear and should be of great interest (Scholz, 2015 and references therein).

In the current model on dCirl function, aGPCR activity, adjusted by mechanical challenge, modulates the molecular machinery gating mechanotransduction currents or the subsequent initiation of action potentials and ensures that mechanical signals are encoded distinctly from the background activity of the sen-sory organ. Thereby, dCirl shapes amplitude and kinetics of the sensory neuronal response. Linking adequate physiological receptor stimulation to downstream pathways and cell function is an essential next step to grasp the significance of aGPCR function and the consequences of their malfunction in human conditions. The versatility of the dCirl model now provides an unprecedented opportunity to study the mechanosensory properties of an exemplary aGPCR and to uncover features that might prove of general relevance for the function and regulation of the entire aGPCR class (Scholz, 2015).

Mechano-dependent signaling by Latrophilin/CIRL quenches cAMP in proprioceptive neurons

Adhesion-type G protein-coupled receptors (aGPCRs), a large molecule family with over 30 members in humans, operate in organ development, brain function and govern immunological responses. Correspondingly, this receptor family is linked to a multitude of diverse human diseases. aGPCRs have been suggested to possess mechanosensory properties, though their mechanism of action is fully unknown. This study shows that the Drosophila aGPCR Latrophilin/dCIRL acts in mechanosensory neurons by modulating ionotropic receptor currents, the initiating step of cellular mechanosensation. This process depends on the length of the extended ectodomain and the tethered agonist of the receptor, but not on its autoproteolysis, a characteristic biochemical feature of the aGPCR family. Intracellularly, dCIRL quenches cAMP levels upon mechanical activation thereby specifically increasing the mechanosensitivity of neurons. These results provide direct evidence that the aGPCR dCIRL acts as a molecular sensor and signal transducer that detects and converts mechanical stimuli into a metabotropic response (Scholz, 2017).

This study demonstrates how a GPCR can specifically shape mechanotransduction in a sensory neuron in vivo. This study thus serves a two-fold purpose. It delineates pivotal steps in the activation paradigm of aGPCRs and sheds light on the contribution of metabotropic signals to the physiology of neuronal mechanosensation (Scholz, 2017).

While there is ongoing discussion whether metabotropic pathways are suitable to sense physical or chemical stimuli with fast onset kinetics, due to the supposed inherent slowness of second messenger systems, the results demonstrate that the aGPCR dCIRL/Latrophilin is necessary for faithful mechanostimulus detection in the lch5 organ of Drosophila larvae. In these PNS neurons, dCIRL contributes to the correct setting of the neuron's mechanically-evoked receptor potential. This is in line with the location of the receptor, which is present in the dendritic membrane and the single cilium of chordotonal organ (ChO) neurons, one of the few documentations of the subcellular location of an aGPCR in its natural environment. The dendritic and ciliary membranes harbor mechanosensitive Transient Receptor Potential (TRP) channels that elicit a receptor potential in the mechanosensory neuron by converting mechanical strain into ion flux. Moreover, two mechanosensitive TRP channel subunits, TRPN1/NompC and TRPV/Nanchung, interact genetically with dCirl (Scholz, 2015). The present study further specifies this relationship by showing that the extent of the mechanosensory receptor current is controlled by dCirl. This suggests that the activity of the aGPCR directly modulates ion flux through TRP channels, and highlights that metabotropic and ionotropic signals may cooperate during the rapid sensory processes that underlie primary mechanosensation (Scholz, 2017).

The nature of this cooperation is yet unclear. Second messenger signals may alter force-response properties of ion channels through post-translational modifications to correct for the mechanical setting of sensory structures, e.g. stretch, shape or osmotic state of the neuron, before acute mechanical stimuli arrive. Indeed, there are precedents for such a direct interplay between GPCRs and channel proteins in olfactory and cardiovascular contexts (Scholz, 2017).

ChOs are polymodal sensors that can also detect thermal stimuli. This study shows that dCIRL does not influence this thermosensory response (between 15°C and 30°C) emphasizing the mechano-specific role of this aGPCR. Replacing sensory input by optogenetic stimulation supports this conclusion, since stimulation with ChR2-XXM, a mutant of Channelrhodopsin-2, evoked normal activity in dCirlKO larvae (Scholz, 2017).

Turning to the molecular mechanisms of dCIRL activation, this study shows that the length of the extracellular tail instructs receptor activity. This observation is compatible with an extracellular engagement of the dCIRL NTF with cellular or matricellular protein(s) through its adhesion domains. Mammalian latrophilins were shown to interact with teneurins, FLRTs and neurexins 1β and 2β suggesting that the receptors are anchored to opposed cell surfaces through their ligands. However, FLRTs do not exist in Drosophila and an engagement of dCIRL with the other two candidate partners could not be detected to date indicating that other interactors may engage and mechanically affix dCIRL. The data support a model where the distance between ligand-receptor contact site and signaling 7TM unit determines the mechanical load onto the receptor protein and its subsequent signal output. This scenario bears similarity to the role of the cytoplasmic ankyrin repeats of NompC, which provide a mechanical tether to the cytoskeleton of mechanosensory cells, and are essential for proper mechanoactivation of this ionotropic sensor (Scholz, 2017).

aGPCR activation occurs by means of a tethered agonist (Stachel) (Liebscher, 2014; Monk, 2015; Stoveken, 2015), which encompasses the last β-strand of the GPCR autoproteolysis-inducing (GAIN) domain. Structural concerns imply that after GAIN domain cleavage a substantial part of the Stachel remains enclosed within the GAIN domain and should thus be inaccessible to interactions with the 7TM domain. These considerations beg the question how the tethered agonist gets exposed to stimulate receptor activity, and how this process relates to the mechanosensitivity of aGPCRs. Two models account for the elusive link between these critical features. Mechanical challenge to the receptor causes: (1) physical disruption of the heterodimer at the GPS thereby exposing the tethered agonist. In this scenario, GPS cleavage is absolutely essential for receptor activity; (2) Allosteric changes of the GAIN domain, e.g., through isomerization of the tethered agonist-7TM region, that allow for the engagement of the Stachel with the 7TM. In this situation, GPS cleavage and disruption of the NTF-CTF receptor heterodimer are not necessary for receptor activity. This study found that autoproteolytic cleavage is not required for the perception and transduction of vibrational mechanical stimuli by dCIRL (Scholz, 2017).

This study further uncovered that the concomitant disruption of Stachel and autoproteolysis disables dCIRL's mechanosensory function in ChO neurons. Thus, the tethered agonist concept pertains to aGPCRs in Drosophila. Notably, these findings also demonstrate that classical GPS mutations have similar biochemical but different physiological effects in vivo (Scholz, 2017).

Finally, this study interrogated intracellular signaling by dCIRL. In contrast to previously described Gαs coupling of rat and nematode latrophilins (Müller, 2015), the mechanosensory response of ChO neurons was decreased by optogenetic augmentation of adenylyl cylcase activity, and the mechanosensory deficit of dCirlKO mutants was rescued by pharmacological inhibition of adenylyl cyclase. FRET measurements also directly demonstrated that mechanical stimulation reduces the cAMP concentration in the sensory neurons, and that this mechano-metabotropic coupling depends on dCIRL. Thus, dCIRL converts a mechanosensory signal into a drop of cAMP levels. This suggests that the Drosophila latrophilin entertains a cascade that inhibits adenylyl cyclases or stimulates phosphodiesterases in ChO neurons, and that G-protein coupling pathways by latrophilin homologs may depend on species and/or cell type (Scholz, 2017).

Members of the aGPCR family are associated with a vast range of physiological processes extending beyond canonical neuronal mechanosensation. For example, dysfunction of ADGRG1/GPR56 causes polymicrogyria, ADGRF5/GPR116 controls pulmonary surfactant production, genetic lesions in many aGPCR loci are associated with a roster of cancer types and ADGRE2/EMR2 regulates mast cell degranulation. Intriguingly, a point mutation in the GAIN domain of ADGRE2 sensitizes the receptor to mechanical stimuli in kindreds of patients suffering from vibratory urticaria. The results now provide a basis to test the generality of the concept that aGPCRs are metabotropic mechanosensors also outside classical mechanosensory structures, and aid in understanding the contribution of ailing aGPCR signaling in diseased tissues (Scholz, 2017).


Functions of Latrotoxin receptor orthologs in other species

Oriented cell division in the C. elegans embryo is coordinated by G-protein signaling dependent on the adhesion GPCR LAT-1

Orientation of spindles and cell division planes during development of many species ensures that correct cell-cell contacts are established, which is vital for proper tissue formation. This is a tightly regulated process involving a complex interplay of various signals. The molecular mechanisms underlying several of these pathways are still incompletely understood. This study identified the signaling cascade of the C. elegans latrophilin homolog LAT-1, an essential player in the coordination of anterior-posterior spindle orientation during the fourth round of embryonic cell division. The receptor mediates a G protein-signaling pathway revealing that G-protein signaling in oriented cell division is not solely GPCR-independent. Genetic analyses showed that through the interaction with a Gs protein LAT-1 elevates intracellular cyclic AMP (cAMP) levels in the C. elegans embryo. Stimulation of this G-protein cascade in lat-1 null mutant nematodes is sufficient to orient spindles and cell division planes in the embryo in the correct direction. Finally, LAT-1 was shown to be activated by an intramolecular agonist to trigger this cascade. These data support a model in which a novel, GPCR-dependent G protein-signaling cascade mediated by LAT-1 controls alignment of cell division planes in an anterior-posterior direction via a metabotropic Gs-protein/adenylyl cyclase pathway by regulating intracellular cAMP levels (Muller, 2015).

Latrophilins function as heterophilic cell-adhesion molecules by binding to teneurins: regulation by alternative splicing

Latrophilin-1, -2, and -3 are adhesion-type G protein-coupled receptors that are auxiliary alpha-latrotoxin receptors, suggesting that they may have a synaptic function. Using pulldowns, this study identified teneurins (see Tenascin major and Tenascin accessory), type II transmembrane proteins that are also candidate synaptic cell-adhesion molecules, as interactors for the lectin-like domain of latrophilins. Teneurin are shown to bind to latrophilins with nanomolar affinity, and this binding mediates cell adhesion, consistent with a role of teneurin binding to latrophilins in trans-synaptic interactions. All latrophilins are subject to alternative splicing at an N-terminal site; in latrophilin-1, this alternative splicing modulates teneurin binding but has no effect on binding of latrophilin-1 to another ligand, FLRT3. Addition to cultured neurons of soluble teneurin-binding fragments of latrophilin-1 decreased synapse density, suggesting that latrophilin binding to teneurin may directly or indirectly influence synapse formation and/or maintenance. These observations are potentially intriguing in view of the proposed role for Drosophila teneurins in determining synapse specificity. However, teneurins in Drosophila were suggested to act as homophilic cell-adhesion molecules, whereas the current findings suggest a heterophilic interaction mechanism. Thus, whether mammalian teneurins also are homophilic cell-adhesion molecules was tested, in addition to binding to latrophilins as heterophilic cell-adhesion molecules. Strikingly, it was found that although teneurins bind to each other in solution, homophilic teneurin-teneurin binding is unable to support stable cell adhesion, different from heterophilic teneurin-latrophilin binding. Thus, mammalian teneurins act as heterophilic cell-adhesion molecules that may be involved in trans-neuronal interaction processes such as synapse formation or maintenance (Boucard, 2014).

Latrophilin 1 and its endogenous ligand Lasso/teneurin-2 form a high-affinity transsynaptic receptor pair with signaling capabilities

Latrophilin 1 (LPH1), a neuronal receptor of alpha-latrotoxin, is implicated in neurotransmitter release and control of presynaptic Ca(2+). As an 'adhesion G-protein-coupled receptor,' LPH1 can convert cell surface interactions into intracellular signaling. To examine the physiological functions of LPH1, wLPH1's extracellular domain was used to purify its endogenous ligand. A single protein of approximately 275 kDa was isolated from rat brain and termed Lasso. Peptide sequencing and molecular cloning have shown that Lasso is a splice variant of teneurin-2, a brain-specific orphan cell surface receptor with a function in neuronal pathfinding and synaptogenesis. This study shows that LPH1 and Lasso interact strongly and specifically. They are always copurified from rat brain extracts. Coculturing cells expressing LPH1 with cells expressing Lasso leads to their mutual attraction and formation of multiple junctions to which both proteins are recruited. Cells expressing LPH1 form chimerical synapses with hippocampal neurons in cocultures; LPH1 and postsynaptic neuronal protein PSD-95 accumulate on opposite sides of these structures. Immunoblotting and immunoelectron microscopy of purified synapses and immunostaining of cultured hippocampal neurons show that LPH1 and Lasso are enriched in synapses; in both systems, LPH1 is presynaptic, whereas Lasso is postsynaptic. A C-terminal fragment of Lasso interacts with LPH1 and induces Ca(2+) signals in presynaptic boutons of hippocampal neurons and in neuroblastoma cells expressing LPH1. Thus, LPH1 and Lasso can form transsynaptic complexes capable of inducing presynaptic Ca(2+) signals, which might affect synaptic functions (Silva, 2011).

LPH1 has been implicated in multiple phenomena, including binding of α-LTX, release of neurotransmitters, intracellular signaling, neuronal morphogenesis, and mental conditions. However, further studies of LPH1 require the identification of its endogenous ligand. Using LPH1-affinity chromatography, this study has now isolated such a ligand. Lasso is a splice variant of teneurin-2. It interacts with LPH1 specifically and strongly, but binds very weakly to LPH2 and does not bind to LPH3. Reciprocally, sequencing results and LPH1 binding suggest that LPH1 interacts with Lasso/teneurin-2 only, and not with teneurin-1, -3, or -4 (Silva, 2011).

Both LPH1 and Lasso/teneurin-2 are highly abundant in the brain. Can they mediate neuronal cell interaction? Teneurin is proteolyzed between the TMR and EGF repeats (see Kenzelmann, 2008), and this could preclude its receptor activity. However, only a proportion of teneurin is cleaved, resulting in two bands on reducing SDS gels. The fragment remains anchored on the cell surface, probably as part of the homodimer. This makes Lasso a bona fide cell-surface receptor (Silva, 2011).

Accordingly, Lasso and LPH1 mediate heterophilic cell-cell contacts between expressing cells in cocultures and formation of artificial synapses between fibroblasts and neurons. Moreover, the data indicate that in cultured neurons and mature brain, LPH1 is localized in the presynaptic membranes, whereas Lasso is mostly postsynaptic. Given the size of LPH1 (14 nm) and Lasso (~30 nm), their complex can span the synaptic cleft (20-30 nm), allowing this receptor pair to connect neurons at synapses (Silva, 2011).

The LPH1-Lasso interaction is not purely structural. LPH1 mediates signaling induced by LTXN4C both in model cells expressing exogenous LPH1 and in organotypic hippocampal cultures. This signaling requires the CTF and involves the activation of phospholipase C, production of inositol-trisphosphate, and release of stored Ca2+. This study demonstrates that a soluble C-terminal fragment of Lasso also induces an increase in cytosolic Ca2+ in NB2a cells expressing LPH1 and in hippocampal neurons. Such a rise in cytosolic Ca2+ in neurons could modulate neurotransmitter release (Silva, 2011).

Interestingly, teneurin contains a sequence termed teneurin C-terminal-associated peptide (TCAP) that resembles corticotropin-releasing factor. It is hypothesized that TCAP is cleaved from teneurin and acts as a soluble ligand of unknown receptors (Wang, 2005). Synthetic TCAP regulates cAMP in immortalized neurons and, when injected cerebrally, affects behaviors related to stress and anxiety. However, there is no evidence that TCAP is released in vivo. The current work suggests an alternative possibility: TCAP, being part of Lasso, affects animal behavior by stimulating LPH1. This is supported by LPH being implicated in schizophrenia, anxiety (offspring killing by LPH1−/− mice), and attention deficit/hyperactivity disorder (Silva, 2011).

The current findings raise several interesting questions. Does Lasso send an intracellular signal in response to LPH1 binding? What are the relationships among the four teneurin homologs and the three LPH proteins found in most vertebrates? Can the LPH1–Lasso interaction affect synaptogenesis, neurotransmitter release, or synaptic plasticity? Answering these questions will provide important insights into the physiological functions of these two families of neuronal cell surface receptors (Silva, 2011).

ADHD-associated dopamine transporter, latrophilin and neurofibromin share a dopamine-related locomotor signature in Drosophila

Attention-deficit/hyperactivity disorder (ADHD) is a common, highly heritable neuropsychiatric disorder with hyperactivity as one of the hallmarks. Aberrant dopamine signaling is thought to be a major theme in ADHD, but how this relates to the vast majority of ADHD candidate genes is illusive. This study reports a Drosophila dopamine-related locomotor endophenotype that is shared by pan-neuronal knockdown of orthologs of the ADHD-associated genes Dopamine transporter (DAT1) and Latrophilin (LPHN3), and of a gene causing a monogenic disorder with frequent ADHD comorbidity: Neurofibromin (NF1). The locomotor signature was not found in control models and could be ameliorated by methylphenidate, validating its relevance to symptoms of the disorder. The Drosophila ADHD endophenotype can be further exploited in high throughput to characterize the growing number of candidate genes. It represents an equally useful outcome measure for testing chemical compounds to define novel treatment options (van der Voet, 2015).

Whereas in zebrafish it was found that latrophilin knockdown mutants show severe disorganization of the dopaminergic system (Lange, 2012), its development was left intact in the Drosophila knockdown model. This allowed gene function to be addressed independent of compromised circuits. In the Drosophila brain, loss of DAT, latrophilin, and Nf1 caused hyperactivity and reduced sleep in a light-dependent manner, phenocopying acute activation of dopaminergic neurons. That MPH, a drug that prolongs residence of secreted dopamine in the synaptic cleft and is thought to increase dopamine signaling, can improve ADHD-like phenotypes seems paradoxical but is a known phenomenon. It was found that expression of DAT carrying a mutation found in ADHD causes anomalous dopamine efflux, leading to elevated synaptic dopamine levels that could be rescued with MPH. Locomotor hyperactive mice lacking DAT (DAT-KO) show a marked reduction of locomotor activity in response to MPH administration, demonstrating the method of action is more complex than just DAT antagonism. This can include cross-talk between the D2 dopamine receptor (D2R), the rate-limiting enzyme for the biosynthesis of dopamine (TH) and the dopamine transporter (DAT). In Drosophila, MPH rescued defective optomotor response caused by activated but not by inhibited dopaminergic neurons. Further research is needed to understand the mechanisms linking LPHN3 and NF1 with dopamine signaling (van der Voet, 2015).


REFERENCES

Search PubMed for articles about Drosophila Latrotoxin receptor

Arac, D., Boucard, A. A., Bolliger, M. F., Nguyen, J., Soltis, S. M., Sudhof, T. C. and Brunger, A. T. (2012). A novel evolutionarily conserved domain of cell-adhesion GPCRs mediates autoproteolysis. EMBO J 31: 1364-1378. PubMed ID: 22333914

Boucard, A. A., Maxeiner, S. and Sudhof, T. C. (2014). Latrophilins function as heterophilic cell-adhesion molecules by binding to teneurins: regulation by alternative splicing. J Biol Chem 289: 387-402. PubMed ID: 24273166

Fredriksson, R. and Schioth, H. B. (2005). The repertoire of G-protein-coupled receptors in fully sequenced genomes. Mol Pharmacol 67: 1414-1425. PubMed ID: 15687224

Kenzelmann, D., Chiquet-Ehrismann, R., Leachman, N. T. and Tucker, R. P. (2008). Teneurin-1 is expressed in interconnected regions of the developing brain and is processed in vivo. BMC Dev Biol 8: 30. PubMed ID: 18366734

Langenhan, T., Promel, S., Mestek, L., Esmaeili, B., Waller-Evans, H., Hennig, C., Kohara, Y., Avery, L., Vakonakis, I., Schnabel, R. and Russ, A. P. (2009). Latrophilin signaling links anterior-posterior tissue polarity and oriented cell divisions in the C. elegans embryo. Dev Cell 17: 494-504. PubMed ID: 19853563

Lange, M., Norton, W., Coolen, M., Chaminade, M., Merker, S., Proft, F., Schmitt, A., Vernier, P., Lesch, K. P. and Bally-Cuif, L. (2012). The ADHD-linked gene Lphn3.1 controls locomotor activity and impulsivity in zebrafish. Mol Psychiatry 17: 855. PubMed ID: 22918194

Langenhan, T., Aust, G. and Hamann, J. (2013). Sticky signaling--adhesion class G protein-coupled receptors take the stage. Sci Signal 6: re3. PubMed ID: 23695165

Liebscher, I., Schon, J., Petersen, S. C., Fischer, L., Auerbach, N., Demberg, L. M., Mogha, A., Coster, M., Simon, K. U., Rothemund, S., Monk, K. R. and Schoneberg, T. (2014). A tethered agonist within the ectodomain activates the adhesion G protein-coupled receptors GPR126 and GPR133. Cell Rep 9(6): 2018-2026. PubMed ID: 25533341

Monk, K. R., Hamann, J., Langenhan, T., Nijmeijer, S., Schoneberg, T. and Liebscher, I. (2015). Adhesion G Protein-Coupled Receptors: From In Vitro Pharmacology to In Vivo Mechanisms. Mol Pharmacol 88(3): 617-623. PubMed ID: 25956432

Muller, A., Winkler, J., Fiedler, F., Sastradihardja, T., Binder, C., Schnabel, R., Kungel, J., Rothemund, S., Hennig, C., Schoneberg, T. and Promel, S. (2015). Oriented Cell Division in the C. elegans Embryo Is Coordinated by G-Protein Signaling Dependent on the Adhesion GPCR LAT-1. PLoS Genet 11(10): e1005624. PubMed ID: 26505631

O'Sullivan, M. L., Martini, F., von Daake, S., Comoletti, D. and Ghosh, A. (2014). LPHN3, a presynaptic adhesion-GPCR implicated in ADHD, regulates the strength of neocortical layer 2/3 synaptic input to layer 5. Neural Dev 9: 7. PubMed ID: 24739570

Newton, F. G., zur Lage, P. I., Karak, S., Moore, D. J., Gopfert, M. C. and Jarman, A. P. (2012). Forkhead transcription factor Fd3F cooperates with Rfx to regulate a gene expression program for mechanosensory cilia specialization. Dev Cell 22: 1221-1233. PubMed ID: 22698283

Paavola, K. J., Sidik, H., Zuchero, J. B., Eckart, M. and Talbot, W. S. (2014). Type IV collagen is an activating ligand for the adhesion G protein-coupled receptor GPR126. Sci Signal 7: ra76. PubMed ID: 25118328

Promel, S., Langenhan, T. and Arac, D. (2013). Matching structure with function: the GAIN domain of adhesion-GPCR and PKD1-like proteins. Trends Pharmacol Sci 34: 470-478. PubMed ID: 23850273

Retailleau, K. and Duprat, F. (2014). Polycystins and partners: proposed role in mechanosensitivity. J Physiol 592: 2453-2471. PubMed ID: 24687583

Scholz, N., Gehring, J., Guan, C., Ljaschenko, D., Fischer, R., Lakshmanan, V., Kittel, R. J. and Langenhan, T. (2015). The adhesion GPCR Latrophilin/CIRL shapes mechanosensation. Cell Rep. PubMed ID: 25937282

Scholz, N., Guan, C., Nieberler, M., Grotemeyer, A., Maiellaro, I., Gao, S., Beck, S., Pawlak, M., Sauer, M., Asan, E., Rothemund, S., Winkler, J., Promel, S., Nagel, G., Langenhan, T. and Kittel, R. J. (2017). Mechano-dependent signaling by Latrophilin/CIRL quenches cAMP in proprioceptive neurons. Elife 6. PubMed ID: 28784204

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Biological Overview

date revised: 20 May, 2015

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