Neuroligin-1 performs neurexin-dependent and neurexin-independent functions in synapse validation

Synaptogenic Assays Using Neurons Cultured on Micropatterned Substrates

One of the difficulties for studying the mechanisms of synaptogenesis stems from the spatial unpredictability of contact formation between neurons, and the involvement of many parallel adhesive pathways mediating axon/dendrite recognition. To circumvent these limitations, we describe here a method allowing the investigation of synaptic contacts at controlled locations with high precision and statistics. Specifically, primary neurons are cultured on micropatterned substrates comprising arrays of micron-scale dots coated with purified synaptogenic adhesion molecules. Coating the substrates with the homophilic adhesion molecule SynCAM triggers the formation of functional presynaptic structures in axons, while neurexin elicits postsynapses in dendrites from neurons expressing the counter receptor neuroligin. This assay can be combined with various imaging techniques including immunocytochemistry to screen the accumulation of synaptic components, long-term live cell recordings to probe the kinetics of neurite growth and synapse differentiation, as well as high resolution single molecule tracking.

Conformational Plasticity in the Transsynaptic Neurexin-Cerebellin-Glutamate Receptor Adhesion Complex

Synaptic specificity is a defining property of neural networks. In the cerebellum, synapses between parallel fiber neurons and Purkinje cells are specified by the simultaneous interactions of secreted protein cerebellin with pre-synaptic neurexin and post-synaptic delta-type glutamate receptors (GluD). Here, we determined the crystal structures of the trimeric C1q-like domain of rat cerebellin-1, and the first complete ectodomain of a GluD, rat GluD2. Cerebellin binds to the LNS6 domain of α- and β-neurexin-1 through a high-affinity interaction that involves its highly flexible N-terminal domain. In contrast, we show that the interaction of cerebellin with isolated GluD2 ectodomain is low affinity, which is not simply an outcome of lost avidity when compared with binding with a tetrameric full-length receptor. Rather, high-affinity capture of cerebellin by post-synaptic terminals is likely controlled by long-distance regulation within this transsynaptic complex. Altogether, our results suggest unusual conformational flexibility within all components of the complex.

Distinct roles for extracellular and intracellular domains in neuroligin function at inhibitory synapses

Neuroligins (NLGNs) are postsynaptic cell adhesion molecules that interact trans-synaptically with neurexins to mediate synapse development and function. NLGN2 is only at inhibitory synapses while NLGN3 is at both excitatory and inhibitory synapses. We found that NLGN3 function at inhibitory synapses in rat CA1 depends on the presence of NLGN2 and identified a domain in the extracellular region that accounted for this functional difference between NLGN2 and 3 specifically at inhibitory synapses. We further show that the presence of a cytoplasmic tail (c-tail) is indispensible, and identified two domains in the c-tail that are necessary for NLGN function at inhibitory synapses. These domains point to a gephyrin-dependent mechanism that is disrupted by an autism-associated mutation at R705 and a gephyrin-independent mechanism reliant on a putative phosphorylation site at S714. Our work highlights unique and separate roles for the extracellular and intracellular regions in specifying and carrying out NLGN function respectively.

DOI:http://dx.doi.org/10.7554/eLife.19236.001

Neuroligin 1 regulates spines and synaptic plasticity via LIMK1/cofilin-mediated actin reorganization

The C-terminal domain of NLG1 is sufficient to enhance spine and synapse number and to modulate synaptic plasticity, and it exerts these effects via its interaction with SPAR and the subsequent activation of LIMK1/cofilin-mediated actin reorganization.

Neuroligin (NLG) 1 is important for synapse development and function, but the underlying mechanisms remain unclear. It is known that at least some aspects of NLG1 function are independent of the presynaptic neurexin, suggesting that the C-terminal domain (CTD) of NLG1 may be sufficient for synaptic regulation. In addition, NLG1 is subjected to activity-dependent proteolytic cleavage, generating a cytosolic CTD fragment, but the significance of this process remains unknown. In this study, we show that the CTD of NLG1 is sufficient to (a) enhance spine and synapse number, (b) modulate synaptic plasticity, and (c) exert these effects via its interaction with spine-associated Rap guanosine triphosphatase–activating protein and subsequent activation of LIM-domain protein kinase 1/cofilin–mediated actin reorganization. Our results provide a novel postsynaptic mechanism by which NLG1 regulates synapse development and function.

The balancing act of GABAergic synapse organizers

GABA (γ-aminobutyric acid) is the main neurotransmitter at inhibitory synapses in the mammalian brain. It is essential for maintaining the excitation and inhibition (E/I) ratio, whose imbalance underlies various brain diseases. Emerging information about inhibitory synapse organizers provides a novel molecular framework for understanding E/I balance at the synapse, circuit, and systems levels. This review highlights recent advances in deciphering these components of the inhibitory synapse and their roles in the development, transmission, and circuit properties of inhibitory synapses. We also discuss how their dysfunction may lead to a variety of brain disorders, suggesting new therapeutic strategies based on balancing the E/I ratio.

Neuroligins Sculpt Cerebellar Purkinje-Cell Circuits by Differential Control of Distinct Classes of Synapses

Neuroligins are postsynaptic cell-adhesion molecules that bind presynaptic neurexins and are genetically linked to autism. Neuroligins are proposed to organize synaptogenesis and/or synaptic transmission, but no systematic analysis of neuroligins in a defined circuit is available. Here, we show that conditional deletion of all neuroligins in cerebellar Purkinje cells caused loss of distal climbing-fiber synapses and weakened climbing-fiber but not parallel-fiber synapses, consistent with alternative use of neuroligins and cerebellins as neurexin ligands for the excitatory climbing-fiber versus parallel-fiber synapses. Moreover, deletion of neuroligins increased the size of inhibitory basket/stellate-cell synapses but simultaneously severely impaired their function. Multiple neuroligin isoforms differentially contributed to climbing-fiber and basket/stellate-cell synapse functions, such that inhibitory synapse-specific neuroligin-2 was unexpectedly essential for maintaining normal climbing-fiber synapse numbers. Using systematic analyses of all neuroligins in a defined neural circuit, our data thus show that neuroligins differentially contribute to various Purkinje-cell synapses in the cerebellum in vivo.

Developmental expression of the neuroligins and neurexins in fragile X mice

Neuroligins and neurexins are transsynaptic proteins involved in the maturation of glutamatergic and GABAergic synapses. Research has identified synaptic proteins and function as primary contributors to the development of fragile X syndrome. Fragile X mental retardation protein (FMRP), the protein that is lacking in fragile X syndrome, binds neuroligin-1 and -3 mRNA. Using in situ hybridization, we examined temporal and spatial expression patterns of neuroligin (NLGN) and neurexin (NRXN) mRNAs in the somatosensory (S1) cortex and hippocampus in wild-type (WT) and fragile X knockout (FMR1-KO) mice during the first 5 weeks of postnatal life. Genotype-based differences in expression included increased NLGN1 mRNA in CA1 and S1 cortex, decreased NLGN2 mRNA in CA1 and dentate gyrus (DG) regions of the hippocampus, and increased NRXN3 mRNA in CA1, DG, and S1 cortex between female WT and FMR1-KO mice. In male mice, decreased expression of NRXN3 mRNA was observed in CA1 and DG regions of FMR1-KO mice. Sex differences in hippocampal expression of NLGN2, NRXN1, NRXN2, and NRXN3 mRNAs and in S1 cortex expression of NRXN3 mRNAs were observed WT mice, whereas sex differences in NLGN3, NRXN1, NRXN2, and NRXN3 mRNA expression in the hippocampus and in NLGN1, NRXN2 and NRXN3 mRNA expression in S1 cortex were detected in FMR1-KO mice. These results provide a neuroanatomical map of NLGN and NRXN expression patterns over postnatal development in WT and FMR1-KO mice. The differences in developmental trajectory of these synaptic proteins could contribute to long-term differences in CNS wiring and synaptic function.

Neuroligin 1 modulates striatal glutamatergic neurotransmission in a pathway and NMDAR subunit-specific manner

Together with its presynaptic partner Neurexin 1 (Nxn1), Neuroligin 1 (NL1) participates in synapse specification and synapse maintenance. We and others have shown that NL1 can also modulate glutamatergic synaptic function in the central nervous system of rodent models. These molecular/cellular changes can translate into altered animal behaviors that are thought to be analogous to symptomatology of neuropsychiatric disorders. For example, in dorsal striatum of NL1 deletion mice, we previously reported that the ratio N-methyl-D-aspartate receptor (NMDAR) mediated synaptic currents to α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor (AMPAR) mediated synaptic currents (NMDA/AMPA) is reduced in medium spiny neuron (MSNs). Importantly, this reduction in NMDA/AMPA ratio correlated with increased repetitive grooming. The striatum is the input nucleus of the basal ganglia (BG). Classical models of this circuitry imply that there are two principal pathways that render distinct and somewhat opposite striatal outputs critical to the function of these nuclei in modulating motor behavior. Thus, we set out to better characterize the effects of NL1 deletion on direct and indirect pathways of the dorsal striatum by genetically labeling MSNs participating in the direct and indirect pathways. We demonstrate that a decrease in NMDAR-mediated currents is limited to MSNs of the direct pathway. Furthermore, the decrease in NMDAR-mediated currents is largely due to a reduction in function of NMDARs containing the GluN2A subunit. In contrast, indirect pathway MSNs in NL1 knockout (KO) mice showed a reduction in the frequency of miniature excitatory neurotransmission not observed in the direct pathway. Thus, NL1 deletion differentially affects direct and indirect pathway MSNs in dorsal striatum. These findings have potential implications for striatal function in NL1 KO mice.

The cellular and molecular landscape of neuroligins

A fundamental physical interaction exists across the synapse. It is mediated by synaptic adhesion molecules, and is among the earliest and most indispensable of molecular events occurring during synaptogenesis. The regulation of adhesion molecules and their interactions with other synaptic proteins likely affect not only on synapse formation but also on ongoing synaptic function. We review research on one major family of postsynaptic adhesion molecules, neuroligins, which bind to their presynaptic partner neurexin across the synaptic cleft. We move from a structural overview to the broad cellular and synaptic context of neuroligins, intermolecular interactions, and molecular modifications that occur within a synapse. Finally, we examine evidence concerning the physiological functions of neuroligin in a cell and highlight areas requiring further investigation.

Using Gene Ontology to describe the role of the neurexin-neuroligin-SHANK complex in human, mouse and rat and its relevance to autism

Background

People with an autistic spectrum disorder (ASD) display a variety of characteristic behavioral traits, including impaired social interaction, communication difficulties and repetitive behavior. This complex neurodevelopment disorder is known to be associated with a combination of genetic and environmental factors. Neurexins and neuroligins play a key role in synaptogenesis and neurexin-neuroligin adhesion is one of several processes that have been implicated in autism spectrum disorders.

Results

In this report we describe the manual annotation of a selection of gene products known to be associated with autism and/or the neurexin-neuroligin-SHANK complex and demonstrate how a focused annotation approach leads to the creation of more descriptive Gene Ontology (GO) terms, as well as an increase in both the number of gene product annotations and their granularity, thus improving the data available in the GO database.

Conclusions

The manual annotations we describe will impact on the functional analysis of a variety of future autism-relevant datasets. Comprehensive gene annotation is an essential aspect of genomic and proteomic studies, as the quality of gene annotations incorporated into statistical analysis tools affects the effective interpretation of data obtained through genome wide association studies, next generation sequencing, proteomic and transcriptomic datasets.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-015-0622-0) contains supplementary material, which is available to authorized users.

Neurexin-Neuroligin Synaptic Complex Regulates Schizophrenia-Related DISC1/Kal-7/Rac1 "Signalosome"

Neurexins (NXs) and neuroligins (NLs) are cell adhesion molecules that are localized at opposite sites of synaptic membranes. They interact with each other to promote the assembly, maintenance, and function of synapses in the central nervous system. Both NX and NL are cleaved from a membrane-attached intracellular domain in an activity-dependent manner, generating the soluble ectodomain of NX or NL. Expression of the NX1 and NX3 genes in the brain appears to be regulated by a schizophrenia-related protein, DISC1. Here, we show that soluble ecto-NX1β can regulate the expression of DISC1 and induce signaling downstream of DISC1. We also show that NL1 binds to a well-characterized DISC1 interaction partner, Kal-7, and this interaction can be compromised by DISC1. Our results indicate that the NX/NL synaptic complex is intrinsically involved in the regulation of DISC1 function, thus contributing to a better understanding of the pathology of schizophrenia.

PTPσ functions as a presynaptic receptor for the glypican-4/LRRTM4 complex and is essential for excitatory synaptic transmission

Leukocyte common antigen-related receptor protein tyrosine phosphatases--comprising LAR, PTPδ, and PTPσ--are synaptic adhesion molecules that organize synapse development. Here, we identify glypican 4 (GPC-4) as a ligand for PTPσ. GPC-4 showed strong (nanomolar) affinity and heparan sulfate (HS)-dependent interaction with the Ig domains of PTPσ. PTPσ bound only to proteolytically cleaved GPC-4 and formed additional complex with leucine-rich repeat transmembrane protein 4 (LRRTM4) in rat brains. Moreover, single knockdown (KD) of PTPσ, but not LAR, in cultured neurons significantly reduced the synaptogenic activity of LRRTM4, a postsynaptic ligand of GPC-4, in heterologous synapse-formation assays. Finally, PTPσ KD dramatically decreased both the frequency and amplitude of excitatory synaptic transmission. This effect was reversed by wild-type PTPσ, but not by a HS-binding-defective PTPσ mutant. Our results collectively suggest that presynaptic PTPσ, together with GPC-4, acts in a HS-dependent manner to maintain excitatory synapse development and function.

Structural basis for LAR-RPTP/Slitrk complex-mediated synaptic adhesion

Synaptic adhesion molecules orchestrate synaptogenesis. The presynaptic leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs) regulate synapse development by interacting with postsynaptic Slit- and Trk-like family proteins (Slitrks), which harbour two extracellular leucine-rich repeats (LRR1 and LRR2). Here we identify the minimal regions of the LAR-RPTPs and Slitrks, LAR-RPTPs Ig1-3 and Slitrks LRR1, for their interaction and synaptogenic function. Subsequent crystallographic and structure-guided functional analyses reveal that the splicing inserts in LAR-RPTPs are key molecular determinants for Slitrk binding and synapse formation. Moreover, structural comparison of the two Slitrk1 LRRs reveal that unique properties on the concave surface of Slitrk1 LRR1 render its specific binding to LAR-RPTPs. Finally, we demonstrate that lateral interactions between adjacent trans-synaptic LAR-RPTPs/Slitrks complexes observed in crystal lattices are critical for Slitrk1-induced lateral assembly and synaptogenic activity. Thus, we propose a model in which Slitrks mediate synaptogenic functions through direct binding to LAR-RPTPs and the subsequent lateral assembly of LAR-RPTPs/Slitrks complexes.

Direct visualization of trans-synaptic neurexin-neuroligin interactions during synapse formation

Neurexins and neuroligins are synaptic cell-adhesion molecules that are essential for normal synapse specification and function and are thought to bind to each other trans-synaptically, but such interactions have not been demonstrated directly. Here, we generated neurexin-1β and neuroligin-1 and neuroligin-2 fusion proteins containing complementary "split" GFP fragments positioned such that binding of neurexin-1β to neuroligin-1 or neuroligin-2 allowed GFP reconstitution without dramatically changing their binding affinities. GFP fluorescence was only reconstituted from split-GFP-modified neurexin-1β and neuroligin-1 if and after neurexin-1β bound to its neuroligin partner; reassociation of the split-GFP components with each other did not mediate binding. Using trans-cellular reconstitution of GFP fluorescence from split-GFP-modified neurexin-1β and neuroligins as an assay, we demonstrate that trans-synaptic neurexin/neuroligin binding indeed occurred when mouse hippocampal neurons formed synapses onto non-neuronal COS-7 cells expressing neuroligins or when mouse hippocampal neurons formed synapses with each other. This visualization of synapses by neurexin/neuroligin binding prompted us to refer to this approach as "SynView." Our data demonstrate that neurexin-1β forms a trans-synaptic complex with neuroligin-1 and neuroligin-2 and that this interaction can be used to label synapses in a specific fashion in vivo.

Calsyntenin-3 molecular architecture and interaction with neurexin 1α

Calsyntenin 3 (Cstn3 or Clstn3), a recently identified synaptic organizer, promotes the development of synapses. Cstn3 localizes to the postsynaptic membrane and triggers presynaptic differentiation. Calsyntenin members play an evolutionarily conserved role in memory and learning. Cstn3 was recently shown in cell-based assays to interact with neurexin 1α (n1α), a synaptic organizer that is implicated in neuropsychiatric disease. Interaction would permit Cstn3 and n1α to form a trans-synaptic complex and promote synaptic differentiation. However, it is contentious whether Cstn3 binds n1α directly. To understand the structure and function of Cstn3, we determined its architecture by electron microscopy and delineated the interaction between Cstn3 and n1α biochemically and biophysically. We show that Cstn3 ectodomains form monomers as well as tetramers that are stabilized by disulfide bonds and Ca(2+), and both are probably flexible in solution. We show further that the extracellular domains of Cstn3 and n1α interact directly and that both Cstn3 monomers and tetramers bind n1α with nanomolar affinity. The interaction is promoted by Ca(2+) and requires minimally the LNS domain of Cstn3. Furthermore, Cstn3 uses a fundamentally different mechanism to bind n1α compared with other neurexin partners, such as the synaptic organizer neuroligin 2, because Cstn3 does not strictly require the sixth LNS domain of n1α. Our structural data suggest how Cstn3 as a synaptic organizer on the postsynaptic membrane, particularly in tetrameric form, may assemble radially symmetric trans-synaptic bridges with the presynaptic synaptic organizer n1α to recruit and spatially organize proteins into networks essential for synaptic function.

Elfn1 recruits presynaptic mGluR7 in trans and its loss results in seizures

GABAergic interneurons are highly heterogeneous, and much is unknown about the specification and functional roles of their neural circuits. Here we show that a transinteraction of Elfn1 and mGluR7 controls targeted interneuron synapse development and that loss of Elfn1 results in hyperactivity and sensory-triggered epileptic seizures in mice. Elfn1 protein increases during postnatal development and localizes to postsynaptic sites of somatostatin-containing interneurons (SOM-INs) in the hippocampal CA1 stratum oriens and dentate gyrus (DG) hilus. Elfn1 knockout (KO) mice have deficits in mGluR7 recruitment to synaptic sites on SOM-INs, and presynaptic plasticity is impaired at these synapses. In patients with epilepsy and attention deficit hyperactivity disorder (ADHD), we find damaging missense mutations of ELFN1 that are clustered in the carboxy-terminal region required for mGluR7 recruitment. These results reveal a novel mechanism for interneuron subtype-specific neural circuit establishment and define a common basis bridging neurological disorders.

Reduced inhibitory gate in the barrel cortex of Neuroligin3R451C knock-in mice, an animal model of autism spectrum disorders

Neuroligins are postsynaptic adhesion molecules that interacting with presynaptic neurexins ensure the cross‐talk between pre‐ and postsynaptic specializations. Rare mutations in neurexin–neuroligin genes have been linked to autism spectrum disorders (ASDs). One of these, the R451C mutation of the gene encoding for Neuroligin3 (Nlgn3), has been found in patients with familial forms of ASDs. Animals carrying this mutation (NL3^R451C knock‐in mice) exhibit impaired social behaviors, reminiscent of those observed in ASD patients, associated with major alterations in both GABAergic and glutamatergic transmission, which vary among different brain regions and at different developmental stages. Here, pair recordings from parvalbumin‐ (PV) expressing basket cells and spiny neurons were used to study GABAergic synaptic signaling in layer IV barrel cortex of NL3^R451C mutant mice. We found that the R451C mutation severely affects the probability of GABA release from PV‐expressing basket cells, responsible for controlling via thalamo‐cortical inputs the feed‐forward inhibition. No changes in excitatory inputs to parvalbumin‐positive basket cells or spiny neurons were detected. These data clearly show that primary targets of the NL3 mutation are PV‐expressing basket cells, independently of the brain region where they are localized. Changes in the inhibitory gate of layer IV somatosensory cortex may alter sensory processing in ASD patients leading to misleading sensory representations with difficulties to combine pieces of information into a unified perceptual whole.

Autism spectrum disorders (ASDs) comprise a heterogeneous group of neurodevelopmental abnormalities, characterized by impaired social interactions. Here, using mice carrying the R451C mutation of the Nlgn3 gene (found in few familial cases of ASDs), and exhibiting behavioral deficits reminiscent of those present in ASD patients, we found that, in layer IV barrel cortex microcircuit, parvalbumin‐expressing basket cells display a reduced probability of GABA release into spiny neurons. Alterations of GABAergic signaling in layer IV somatosensory cortex of NL3R451C knock‐in mice may alter sensory processing leading to misleading sensory representations with difficulties to combine pieces of information into a unified perceptual whole.

Micropatterned substrates coated with neuronal adhesion molecules for high-content study of synapse formation

Studying the roles of different proteins and the mechanisms involved in synaptogenesis is hindered by the complexity and heterogeneity of synapse types, and by the spatial and temporal unpredictability of spontaneous synapse formation. Here we demonstrate a robust and high-content method to induce selectively presynaptic or postsynaptic structures at controlled locations. Neurons are cultured on micropatterned substrates comprising arrays of micron-scale dots coated with various synaptogenic adhesion molecules. When plated on neurexin-1β-coated micropatterns, neurons expressing neuroligin-1 exhibit specific dendritic organization and selective recruitment of the postsynaptic scaffolding molecule PSD-95. Furthermore, functional AMPA receptors are trapped at neurexin-1β dots, as revealed by live-imaging experiments. In contrast, neurons plated on SynCAM1-coated substrates exhibit strongly patterned axons and selectively assemble functional presynapses. N-cadherin coating, however, is not able to elicit synapses, indicating the specificity of our system. This method opens the way to both fundamental and therapeutic studies of various synaptic systems.

Calsyntenins function as synaptogenic adhesion molecules in concert with neurexins

Multiple synaptic adhesion molecules govern synapse formation. Here, we propose calsyntenin-3/alcadein-β as a synapse organizer that specifically induces presynaptic differentiation in heterologous synapse-formation assays. Calsyntenin-3 (CST-3) is highly expressed during various postnatal periods of mouse brain development. The simultaneous knockdown of all three CSTs, but not CST-3 alone, decreases inhibitory, but not excitatory, synapse densities in cultured hippocampal neurons. Moreover, the knockdown of CSTs specifically reduces inhibitory synaptic transmission in vitro and in vivo. Remarkably, the loss of CSTs induces a concomitant decrease in neuron soma size in a non-cell-autonomous manner. Furthermore, α-neurexins (α-Nrxs) are components of a CST-3 complex involved in CST-3-mediated presynaptic differentiation. However, CST-3 does not directly bind to Nrxs. Viewed together, these data suggest that the three CSTs redundantly regulate inhibitory synapse formation, inhibitory synapse function, and neuron development in concert with Nrxs.

Assembly of synapses: biomimetic assays to control neurexin/neuroligin interactions at the neuronal surface

The role of adhesion molecules in the assembly of synapses in the nervous system is an important issue. To characterize the role of neurexin/neuroligin adhesion complexes in synapse differentiation, various imaging assays can be performed in primary hippocampal cultures. First, to temporally control contact formation, biomimetic assays can be performed using microspheres coated with purified neurexin or with antibody clusters that aggregate neurexin. These models are combined with live fluorescence imaging to study the dynamics of accumulation of post-synaptic components, including scaffolding molecules and glutamate receptors. To demonstrate that AMPA receptors can be recruited to nascent neurexin/neuroligin contacts through lateral diffusion, the mobility of AMPA receptors in the neuronal membrane is monitored by tracking individual quantum dots (QDs) conjugated to antibodies against AMPA receptors. Experiments monitoring the attachment and detachment of Nrx-coated QDs to measure the rates of neurexin/neuroligin interaction can also be performed. Each of these assays is detailed in this unit.

CaMKII phosphorylation of neuroligin-1 regulates excitatory synapses

Neuroligins are postsynaptic cell adhesion molecules that are important for synaptic function through their trans-synaptic interaction with neurexins (NRXNs). The localization and synaptic effects of neuroligin-1 (NL-1, also called NLGN1) are specific to excitatory synapses with the capacity to enhance excitatory synapses dependent on synaptic activity or Ca(2+)/calmodulin kinase II (CaMKII). Here we report that CaMKII robustly phosphorylates the intracellular domain of NL-1. We show that T739 is the dominant CaMKII site on NL-1 and is phosphorylated in response to synaptic activity in cultured rodent neurons and sensory experience in vivo. Furthermore, a phosphodeficient mutant (NL-1 T739A) reduces the basal and activity-driven surface expression of NL-1, leading to a reduction in neuroligin-mediated excitatory synaptic potentiation. To the best of our knowledge, our results are the first to demonstrate a direct functional interaction between CaMKII and NL-1, two primary components of excitatory synapses.

Membrane-tethered monomeric neurexin LNS-domain triggers synapse formation

Neurexins are presynaptic cell-adhesion molecules that bind to postsynaptic cell-adhesion molecules such as neuroligins and leucine-rich repeat transmembrane proteins (LRRTMs). When neuroligins or LRRTMs are expressed in a nonneuronal cell, cocultured neurons avidly form heterologous synapses onto that cell. Here we show that knockdown of all neurexins in cultured hippocampal mouse neurons did not impair synapse formation between neurons, but blocked heterologous synapse formation induced by neuroligin-1 or LRRTM2. Rescue experiments demonstrated that all neurexins tested restored heterologous synapse formation in neurexin-deficient neurons. Neurexin-deficient neurons exhibited a decrease in the levels of the PDZ-domain protein CASK (a calcium/calmodulin-activated serine/threonine kinase), which binds to neurexins, and mutation of the PDZ-domain binding sequence of neurexin-3β blocked its transport to the neuronal surface and impaired heterologous synapse formation. However, replacement of the C-terminal neurexin sequence with an unrelated PDZ-domain binding sequence that does not bind to CASK fully restored surface transport and heterologous synapse formation in neurexin-deficient neurons, suggesting that no particular PDZ-domain protein is essential for neurexin surface transport or heterologous synapse formation. Further mutagenesis revealed, moreover, that the entire neurexin cytoplasmic tail was dispensable for heterologous synapse formation in neurexin-deficient neurons, as long as the neurexin protein was transported to the neuronal cell surface. Furthermore, the single LNS-domain (for laminin/neurexin/sex hormone-binding globulin-domain) of neurexin-1β or neurexin-3β, when tethered to the presynaptic plasma membrane by a glycosylinositolphosphate anchor, was sufficient for rescuing heterologous synapse formation in neurexin-deficient neurons. Our data suggest that neurexins mediate heterologous synapse formation via an extracellular interaction with presynaptic and postsynaptic ligands without the need for signal transduction by the neurexin cytoplasmic tail.

Unbiased discovery of glypican as a receptor for LRRTM4 in regulating excitatory synapse development

Leucine-rich repeat (LRR) proteins have recently been identified as important regulators of synapse development and function, but for many LRR proteins the ligand-receptor interactions are not known. Here we identify the heparan sulfate (HS) proteoglycan glypican as a receptor for LRRTM4 using an unbiased proteomics-based approach. Glypican binds LRRTM4, but not LRRTM2, in an HS-dependent manner. Glypican 4 (GPC4) and LRRTM4 localize to the pre- and postsynaptic membranes of excitatory synapses, respectively. Consistent with a trans-synaptic interaction, LRRTM4 triggers GPC4 clustering in contacting axons and GPC4 induces clustering of LRRTM4 in contacting dendrites in an HS-dependent manner. LRRTM4 positively regulates excitatory synapse development in cultured neurons and in vivo, and the synaptogenic activity of LRRTM4 requires the presence of HS on the neuronal surface. Our results identify glypican as an LRRTM4 receptor and indicate that a trans-synaptic glypican-LRRTM4 interaction regulates excitatory synapse development.

Neurexin-1β binding to neuroligin-1 triggers the preferential recruitment of PSD-95 versus gephyrin through tyrosine phosphorylation of neuroligin-1

Adhesion between neurexin-1β (Nrx1β) and neuroligin-1 (Nlg1) induces early recruitment of the postsynaptic density protein 95 (PSD-95) scaffold; however, the associated signaling mechanisms are unknown. To dissociate the effects of ligand binding and receptor multimerization, we compared conditions in which Nlg1 in neurons was bound to Nrx1β or nonactivating HA antibodies. Time-lapse imaging, fluorescence recovery after photobleaching, and single-particle tracking demonstrated that in addition to aggregating Nlg1, Nrx1β binding stimulates the interaction between Nlg1 and PSD-95. Phosphotyrosine immunoblots and pull-down of gephyrin by Nlg1 peptides in vitro showed that Nlg1 can be phosphorylated at a unique tyrosine (Y782), preventing gephyrin binding. Expression of Nlg1 point mutants in neurons indicated that Y782 phosphorylation controls the preferential binding of Nlg1 to PSD-95 versus gephyrin, and accordingly the formation of inhibitory and excitatory synapses. We propose that ligand-induced changes in the Nlg1 phosphotyrosine level control the balance between excitatory and inhibitory scaffold assembly during synapse formation and stabilization.

Developmental regulation of GABAergic signalling in the hippocampus of neuroligin 3 R451C knock-in mice: an animal model of Autism

Autism Spectrum Disorders (ASDs) comprise an heterogeneous group of neuro-developmental abnormalities, mainly of genetic origin, characterized by impaired social interactions, communications deficits, and stereotyped behaviors. In a small percentage of cases, ASDs have been found to be associated with single mutations in genes involved in synaptic function. One of these involves the postsynaptic cell adhesion molecule neuroligin (NL) 3. NLs interact with presynaptic neurexins (Nrxs) to ensure a correct cross talk between post and presynaptic specializations. Here, transgenic mice carrying the human R451C mutation of Nlgn3, were used to study GABAergic signaling in the hippocampus early in postnatal life. Whole cell recordings from CA3 pyramidal neurons in slices from NL3^R451C knock-in mice revealed an enhanced frequency of Giant Depolarizing Potentials (GDPs), as compared to controls. This effect was probably dependent on an increased GABAergic drive to principal cells as demonstrated by the enhanced frequency of miniature GABA_A-mediated (GPSCs), but not AMPA-mediated postsynaptic currents (EPSCs). Changes in frequency of mGPSCs were associated with an acceleration of their decay kinetics, in the absence of any change in unitary synaptic conductance or in the number of GABA_A receptor channels, as assessed by peak scaled non-stationary fluctuation analysis. The enhanced GABAergic but not glutamatergic transmission early in postnatal life may change the excitatory/inhibitory balance known to play a key role in the construction and refinement of neuronal circuits during postnatal development. This may lead to behavioral deficits reminiscent of those observed in ASDs patients.

Slitrks control excitatory and inhibitory synapse formation with LAR receptor protein tyrosine phosphatases

The balance between excitatory and inhibitory synaptic inputs, which is governed by multiple synapse organizers, controls neural circuit functions and behaviors. Slit- and Trk-like proteins (Slitrks) are a family of synapse organizers, whose emerging synaptic roles are incompletely understood. Here, we report that Slitrks are enriched in postsynaptic densities in rat brains. Overexpression of Slitrks promoted synapse formation, whereas RNAi-mediated knockdown of Slitrks decreased synapse density. Intriguingly, Slitrks were required for both excitatory and inhibitory synapse formation in an isoform-dependent manner. Moreover, Slitrks required distinct members of the leukocyte antigen-related receptor protein tyrosine phosphatase (LAR-RPTP) family to trigger synapse formation. Protein tyrosine phosphatase σ (PTPσ), in particular, was specifically required for excitatory synaptic differentiation by Slitrks, whereas PTPδ was necessary for inhibitory synapse differentiation. Taken together, these data suggest that combinatorial interactions of Slitrks with LAR-RPTP family members maintain synapse formation to coordinate excitatory-inhibitory balance.

MDGAs interact selectively with neuroligin-2 but not other neuroligins to regulate inhibitory synapse development

The MAM domain-containing GPI anchor proteins MDGA1 and MDGA2 are Ig superfamily adhesion molecules composed of six IG domains, a fibronectin III domain, a MAM domain, and a GPI anchor. MDGAs contribute to the radial migration and positioning of a subset of cortical neurons during early neural development. However, MDGAs continue to be expressed in postnatal brain, and their functions during postnatal neural development remain unknown. Here, we demonstrate that MDGAs specifically and with a nanomolar affinity bind to neuroligin-2, a cell-adhesion molecule of inhibitory synapses, but do not bind detectably to neuroligin-1 or neuroligin-3. We observed no cell adhesion between cells expressing neuroligin-2 and MDGA1, suggesting a cis interaction. Importantly, RNAi-mediated knockdown of MDGAs increased the abundance of inhibitory but not excitatory synapses in a neuroligin-2-dependent manner. Conversely, overexpression of MDGA1 decreased the numbers of functional inhibitory synapses. Likewise, coexpression of both MDGA1 and neuroligin-2 reduced the synaptogenic capacity of neuroligin-2 in an artificial synapse-formation assay by abolishing the ability of neuroligin-2 to form an adhesion complex with neurexins. Taken together, our data suggest that MDGAs inhibit the activity of neuroligin-2 in controlling the function of inhibitory synapses and that MDGAs do so by binding to neuroligin-2.

Dimerization of postsynaptic neuroligin drives synaptic assembly via transsynaptic clustering of neurexin

The transsynaptic complex of neuroligin (NLGN) and neurexin forms a physical connection between pre- and postsynaptic neurons that occurs early in the course of new synapse assembly. Both neuroligin and neurexin have, indeed, been proposed to exhibit active, instructive roles in the formation of synapses. However, the process by which these instructive roles play out during synaptogenesis is not well understood. Here, we examine one aspect of postsynaptic neuroligin with regard to its synaptogenic properties: its basal state as a constitutive dimer. We show that dimerization is required for the synaptogenic properties of neuroligin and likely serves to induce presynaptic differentiation via a transsynaptic clustering of neurexin. Further, we introduce chemically inducible, exogenous dimerization domains to the neuroligin molecule, effectively bestowing chemical control of neuroligin dimerization. This allows us to identify the acute requirements of neuroligin dimerization by chemically manipulating the monomeric-to-dimeric conversion of neuroligin. Based on the results of the inducible dimerization experiments, we propose a model in which dimerized neuroligin induces the mechanical clustering of presynaptic molecules as part of a requisite step in the coordinated assembly of a chemical synapse.

Transsynaptic signaling by activity-dependent cleavage of neuroligin-1

Adhesive contact between pre- and postsynaptic neurons initiates synapse formation during brain development and provides a natural means of transsynaptic signaling. Numerous adhesion molecules and their role during synapse development have been described in detail. However, once established, the mechanisms of adhesive disassembly and its function in regulating synaptic transmission have been unclear. Here, we report that synaptic activity induces acute proteolytic cleavage of neuroligin-1 (NLG1), a postsynaptic adhesion molecule at glutamatergic synapses. NLG1 cleavage is triggered by NMDA receptor activation, requires Ca2+ /calmodulin-dependent protein kinase, and is mediated by proteolytic activity of matrix metalloprotease 9 (MMP9). Cleavage of NLG1 occurs at single activated spines, is regulated by neural activity in vivo, and causes rapid destabilization of its presynaptic partner neurexin-1β (NRX1β). In turn, NLG1 cleavage depresses synaptic transmission by abruptly reducing presynaptic release probability. Thus, local proteolytic control of synaptic adhesion tunes synaptic transmission during brain development and plasticity.

Activity-dependent proteolytic cleavage of neuroligin-1

Neuroligin (NLG), a postsynaptic adhesion molecule, is involved in the formation of synapses by binding to a cognate presynaptic ligand, neurexin. Here we report that neuroligin-1 (NLG1) undergoes ectodomain shedding at the juxtamembrane stalk region to generate a secreted form of NLG1 and a membrane-tethered C-terminal fragment (CTF) in adult rat brains in vivo as well as in neuronal cultures. Pharmacological and genetic studies identified ADAM10 as the major protease responsible for NLG1 shedding, the latter being augmented by synaptic NMDA receptor activation or interaction with soluble neurexin ligands. NLG1-CTF was subsequently cleaved by presenilin/γ-secretase. Secretion of soluble NLG1 was significantly upregulated under a prolonged epileptic seizure condition, and inhibition of NLG1 shedding led to an increase in numbers of dendritic spines in neuronal cultures. Collectively, neuronal activity-dependent proteolytic processing of NLG1 may negatively regulate the remodeling of spines at excitatory synapses.

Transcellular neuroligin-2 interactions enhance insulin secretion and are integral to pancreatic β cell function

Normal glucose-stimulated insulin secretion is dependent on interactions between neighboring β cells. Elucidation of the reasons why this cell-to-cell contact is essential will probably yield critical insights into β cell maturation and function. In the central nervous system, transcellular protein interactions (i.e. interactions between proteins on the surfaces of different cells) involving neuroligins are key mediators of synaptic functional development. We previously demonstrated that β cells express neuroligin-2 and that insulin secretion is affected by changes in neuroligin-2 expression. Here we show that the effect of neuroligin-2 on insulin secretion is mediated by transcellular interactions. Neuroligin-2 binds with nanomolar affinity to a partner on the β cell surface and contributes to the increased insulin secretion brought about by β cell-to-β cell contact. It does so in a manner seemingly independent of interactions with neurexin, a known binding partner. As in the synapse, transcellular neuroligin-2 interactions enhance the functioning of the submembrane exocytic machinery. Also, as in the synapse, neuroligin-2 clustering is important. Neuroligin-2 in soluble form, rather than presented on a cell surface, decreases insulin secretion by rat islets and MIN-6 cells, most likely by interfering with endogenous neuroligin interactions. Prolonged contact with neuroligin-2-expressing cells increases INS-1 β cell proliferation and insulin content. These results extend the known parallels between the synaptic and β cell secretory machineries to extracellular interactions. Neuroligin-2 interactions are one of the few transcellular protein interactions thus far identified that directly enhance insulin secretion. Together, these results indicate a significant role for transcellular neuroligin-2 interactions in the establishment of β cell function.

Synaptic cell adhesion

Chemical synapses are asymmetric intercellular junctions that mediate synaptic transmission. Synaptic junctions are organized by trans-synaptic cell adhesion molecules bridging the synaptic cleft. Synaptic cell adhesion molecules not only connect pre- and postsynaptic compartments, but also mediate trans-synaptic recognition and signaling processes that are essential for the establishment, specification, and plasticity of synapses. A growing number of synaptic cell adhesion molecules that include neurexins and neuroligins, Ig-domain proteins such as SynCAMs, receptor phosphotyrosine kinases and phosphatases, and several leucine-rich repeat proteins have been identified. These synaptic cell adhesion molecules use characteristic extracellular domains to perform complementary roles in organizing synaptic junctions that are only now being revealed. The importance of synaptic cell adhesion molecules for brain function is highlighted by recent findings implicating several such molecules, notably neurexins and neuroligins, in schizophrenia and autism.

The role of neurexins and neuroligins in the formation, maturation, and function of vertebrate synapses

Neurexins (NXs) and neuroligins (NLs) are transsynaptically interacting cell adhesion proteins that play a key role in the formation, maturation, activity-dependent validation, and maintenance of synapses. As complex alternative splicing processes in nerve cells generate a large number of NX and NLs variants, it has been proposed that a combinatorial interaction code generated by these variants may determine synapse identity and network connectivity during brain development. The functional importance of NXs and NLs is exemplified by the fact that mutations in NX and NL genes are associated with several neuropsychiatric disorders, most notably with autism. Accordingly, major research efforts have focused on the molecular mechanisms by which NXs and NLs operate at synapses. In this review, we summarize recent progress in this field and discuss emerging topics, such as the role of alternative interaction partners of NXs and NLs in synapse formation and function, and their relevance for synaptic plasticity in the mature brain. The novel findings highlight the fundamental importance of NX-NL interactions in a wide range of synaptic functions.

Sex-dependent novelty response in neurexin-1α mutant mice

Neurexin-1 alpha (NRXN1α) belongs to the family of cell adhesion molecules (CAMs), which are involved in the formation of neuronal networks and synapses. NRXN1α gene mutations have been identified in neuropsychiatric diseases including Schizophrenia (SCZ) and Autism Spectrum Disorder (ASD). In order to get a better understanding of the pleiotropic behavioral manifestations caused by NRXN1α gene mutations, we performed a behavioral study of Nrxn1α heterozygous knock-out (+/−) mice and observed increased responsiveness to novelty and accelerated habituation to novel environments compared to wild type (+/+) litter-mates. However, this effect was mainly observed in male mice, strongly suggesting that gender-specific mechanisms play an important role in Nrxn1α-induced phenotypes.

High affinity neurexin binding to cell adhesion G-protein-coupled receptor CIRL1/latrophilin-1 produces an intercellular adhesion complex

The G-protein-coupled receptor CIRL1/latrophilin-1 (CL1) and the type-1 membrane proteins neurexins represent distinct neuronal cell adhesion molecules that exhibit no similarities except for one common function: both proteins are receptors for α-latrotoxin, a component of black widow spider venom that induces massive neurotransmitter release at synapses. Unexpectedly, we have now identified a direct binding interaction between the extracellular domains of CL1 and neurexins that is regulated by alternative splicing of neurexins at splice site 4 (SS4). Using saturation binding assays, we showed that neurexins lacking an insert at SS4 bind to CL1 with nanomolar affinity, whereas neurexins containing an insert at SS4 are unable to bind. CL1 competed for neurexin binding with neuroligin-1, a well characterized neurexin ligand. The extracellular sequences of CL1 contain five domains (lectin, olfactomedin-like, serine/threonine-rich, hormone-binding, and G-protein-coupled receptor autoproteolysis-inducing (GAIN) domains). Of these domains, the olfactomedin-like domain mediates neurexin binding as shown by deletion mapping. Cell adhesion assays using cells expressing neurexins and CL1 revealed that their interaction produces a stable intercellular adhesion complex, indicating that their interaction can be trans-cellular. Thus, our data suggest that CL1 constitutes a novel ligand for neurexins that may be localized postsynaptically based on its well characterized interaction with intracellular SH3 and multiple ankyrin repeats adaptor proteins (SHANK) and could form a trans-synaptic complex with presynaptic neurexins.

Neurexin-neuroligin adhesions capture surface-diffusing AMPA receptors through PSD-95 scaffolds

The mechanisms governing the recruitment of functional glutamate receptors at nascent excitatory postsynapses following initial axon-dendrite contact remain unclear. We examined here the ability of neurexin/neuroligin adhesions to mobilize AMPA-type glutamate receptors (AMPARs) at postsynapses through a diffusion/trap process involving the scaffold molecule PSD-95. Using single nanoparticle tracking in primary rat and mouse hippocampal neurons overexpressing or lacking neuroligin-1 (Nlg1), a striking inverse correlation was found between AMPAR diffusion and Nlg1 expression level. The use of Nlg1 mutants and inhibitory RNAs against PSD-95 demonstrated that this effect depended on intact Nlg1/PSD-95 interactions. Furthermore, functional AMPARs were recruited within 1 h at nascent Nlg1/PSD-95 clusters assembled by neurexin-1β multimers, a process requiring AMPAR membrane diffusion. Triggering novel neurexin/neuroligin adhesions also caused a depletion of PSD-95 from native synapses and a drop in AMPAR miniature EPSCs, indicating a competitive mechanism. Finally, both AMPAR level at synapses and AMPAR-dependent synaptic transmission were diminished in hippocampal slices from newborn Nlg1 knock-out mice, confirming an important role of Nlg1 in driving AMPARs to nascent synapses. Together, these data reveal a mechanism by which membrane-diffusing AMPARs can be rapidly trapped at PSD-95 scaffolds assembled at nascent neurexin/neuroligin adhesions, in competition with existing synapses.

Neurexins and neuroligins: recent insights from invertebrates

During brain development, each neuron must find and synapse with the correct pre- and postsynaptic partners. The complexity of these connections and the relatively large distances some neurons must send their axons to find the correct partners makes studying brain development one of the most challenging, and yet fascinating disciplines in biology. Furthermore, once the initial connections have been made, the neurons constantly remodel their dendritic and axonal arbours in response to changing demands. Neurexin and neuroligin are two cell adhesion molecules identified as important regulators of this process. The importance of these genes in the development and modulation of synaptic connectivity is emphasised by the observation that mutations in these genes in humans have been associated with cognitive disorders such as Autism spectrum disorders, Tourette syndrome and Schizophrenia. The present review will discuss recent advances in our understanding of the role of these genes in synaptic development and modulation, and in particular, we will focus on recent work in invertebrate models, and how these results relate to studies in mammals.

Neuroligins/LRRTMs prevent activity- and Ca2+/calmodulin-dependent synapse elimination in cultured neurons

Neuroligins and leucine-rich repeat transmembrane proteins are necessary to prevent activity-dependent elimination of excitatory synapses in cultured neurons, with synapse elimination operating by a Ca²⁺/calmodulin-dependent pathway.

Neuroligins (NLs) and leucine-rich repeat transmembrane proteins (LRRTMs) are postsynaptic cell adhesion molecules that bind to presynaptic neurexins. In this paper, we show that short hairpin ribonucleic acid–mediated knockdowns (KDs) of LRRTM1, LRRTM2, and/or NL-3, alone or together as double or triple KDs (TKDs) in cultured hippocampal neurons, did not decrease synapse numbers. In neurons cultured from NL-1 knockout mice, however, TKD of LRRTMs and NL-3 induced an ∼40% loss of excitatory but not inhibitory synapses. Strikingly, synapse loss triggered by the LRRTM/NL deficiency was abrogated by chronic blockade of synaptic activity as well as by chronic inhibition of Ca²⁺ influx or Ca²⁺/calmodulin (CaM) kinases. Furthermore, postsynaptic KD of CaM prevented synapse loss in a cell-autonomous manner, an effect that was reversed by CaM rescue. Our results suggest that two neurexin ligands, LRRTMs and NLs, act redundantly to maintain excitatory synapses and that synapse elimination caused by the absence of NLs and LRRTMs is promoted by synaptic activity and mediated by a postsynaptic Ca²⁺/CaM-dependent signaling pathway.

The synaptic protein neuroligin-1 interacts with the amyloid β-peptide. Is there a role in Alzheimer's disease?

Amyloid β-peptide (Aβ) is the main component of the amyloid plaques associated with Alzheimer's disease (AD). In the early steps of the disease soluble Aβ oligomers are produced. According to the current "amyloid hypothesis" these oligomers can accumulate over time, leading progressively to the loss of synaptic function and the cognitive failure characteristic of AD. To understand the role of oligomeric Aβ species in AD pathology, it is important to understand the mechanism by which Aβ oligomers are targeted to synaptic junction. We report here the interaction between Aβ with neuroligin-1 (NL-1), a postsynaptic cell-adhesion protein specific for excitatory synapses, which shares a high degree of similarity with acetylcholinesterase, the first synaptic protein described to interact with Aβ. Using intrinsic fluorescence and surface plasmon resonance, we found that Aβ binds to the extracellular domain of NL-1 with a K(d) in the nanomolar range. In the case of NL-2, a postsynaptic cell-adhesion protein specific for inhibitory synapses, just a very weak interaction with Aβ was observed. Aβ polymerization analysis-studied by thioflavin-T assay and electron microscopy-indicated that NL-1 stabilized Aβ aggregates in vitro. Moreover, NL-1 acts as a nucleating factor during the Aβ aggregation process, stimulating the formation of Aβ oligomers. Besides, immunoprecipitation assays confirm that Aβ oligomers interact with NL-1 but not with NL-2. In conclusion, our results show that NL-1 interacts with Aβ increasing the formation of Aβ oligomers, suggesting that this interaction could triggers the targeting of Aβ oligomer to the postsynaptic regions of excitatory synapses.

Autism-linked neuroligin-3 R451C mutation differentially alters hippocampal and cortical synaptic function

Multiple independent mutations in neuroligin genes were identified in patients with familial autism, including the R451C substitution in neuroligin-3 (NL3). Previous studies showed that NL3(R451C) knock-in mice exhibited modestly impaired social behaviors, enhanced water maze learning abilities, and increased synaptic inhibition in the somatosensory cortex, and they suggested that the behavioral changes in these mice may be caused by a general shift of synaptic transmission to inhibition. Here, we confirm that NL3(R451C) mutant mice behaviorally exhibit social interaction deficits and electrophysiologically display increased synaptic inhibition in the somatosensory cortex. Unexpectedly, however, we find that the NL3(R451C) mutation produced a strikingly different phenotype in the hippocampus. Specifically, in the hippocampal CA1 region, the NL3(R451C) mutation caused an ∼1.5-fold increase in AMPA receptor-mediated excitatory synaptic transmission, dramatically altered the kinetics of NMDA receptor-mediated synaptic responses, induced an approximately twofold up-regulation of NMDA receptors containing NR2B subunits, and enhanced long-term potentiation almost twofold. NL3 KO mice did not exhibit any of these changes. Quantitative light microscopy and EM revealed that the NL3(R451C) mutation increased dendritic branching and altered the structure of synapses in the stratum radiatum of the hippocampus. Thus, in NL3(R451C) mutant mice, a single point mutation in a synaptic cell adhesion molecule causes context-dependent changes in synaptic transmission; these changes are consistent with the broad impact of this mutation on murine and human behaviors, suggesting that NL3 controls excitatory and inhibitory synapse properties in a region- and circuit-specific manner.

Neurexins and neuroligins: synapses look out of the nervous system

The scientific interest in the family of the so-called nervous vascular parallels has been growing steadily for the past 15 years, either by addition of new members to the group or, lately, by deepening the analysis of established concepts and mediators. Proteins governing both neurons and vascular cells are known to be involved in events such as cell fate determination and migration/guidance but not in the last and apparently most complex step of nervous system development, the formation and maturation of synapses. Hence, the recent addition to this family of the specific synaptic proteins, Neurexin and Neuroligin, is a double innovation. The two proteins, which were thought to be "simple" adhesive links between the pre- and post-synaptic sides of chemical synapses, are in fact extremely complex and modulate the most subtle synaptic activities. We will discuss the relevant data and the intriguing challenge of transferring synaptic activities to vascular functions.

Understanding the molecular diversity of GABAergic synapses

GABAergic synapses exhibit a high degree of subcellular and molecular specialization, which contrasts with their apparent simplicity in ultrastructural appearance. Indeed, when observed in the electron microscope, GABAergic synapses fit in the symmetric, or Gray’s type II category, being characterized by a relatively simple postsynaptic specialization. The inhibitory postsynaptic density cannot be readily isolated, and progress in understanding its molecular composition has lagged behind that of excitatory synapses. However, recent studies have brought significant progress in the identification of new synaptic proteins, revealing an unexpected complexity in the molecular machinery that regulates GABAergic synaptogenesis. In this article, we provide an overview of the molecular diversity of GABAergic synapses, and we consider how synapse specificity may be encoded by selective trans-synaptic interactions between pre- and postsynaptic adhesion molecules and secreted factors that reside in the synaptic cleft. We also discuss the importance of developing cataloguing tools that could be used to decipher the molecular diversity of synapses and to predict alterations of inhibitory transmission in the course of neurological diseases.

An autism-associated point mutation in the neuroligin cytoplasmic tail selectively impairs AMPA receptor-mediated synaptic transmission in hippocampus

Neuroligins are evolutionarily conserved postsynaptic cell-adhesion molecules that function, at least in part, by forming trans-synaptic complexes with presynaptic neurexins. Different neuroligin isoforms perform diverse functions and exhibit distinct intracellular localizations, but contain similar cytoplasmic sequences whose role remains largely unknown. Here, we analysed the effect of a single amino-acid substitution (R704C) that targets a conserved arginine residue in the cytoplasmic sequence of all neuroligins, and that was associated with autism in neuroligin-4. We introduced the R704C mutation into mouse neuroligin-3 by homologous recombination, and examined its effect on synapses in vitro and in vivo. Electrophysiological and morphological studies revealed that the neuroligin-3 R704C mutation did not significantly alter synapse formation, but dramatically impaired synapse function. Specifically, the R704C mutation caused a major and selective decrease in AMPA receptor-mediated synaptic transmission in pyramidal neurons of the hippocampus, without similarly changing NMDA or GABA receptor-mediated synaptic transmission, and without detectably altering presynaptic neurotransmitter release. Our results suggest that the cytoplasmic tail of neuroligin-3 has a central role in synaptic transmission by modulating the recruitment of AMPA receptors to postsynaptic sites at excitatory synapses.

Neurexins, neuroligins and LRRTMs: synaptic adhesion getting fishy

Recent studies have identified the leucine rich repeat protein LRRTM2 as a post-synaptic ligand of Neurexins. Neurexins also bind the post-synaptic adhesion molecules, Neuroligins. All three families of genes have been implicated in the etiologies of neurodevelopmental disorders, specifically autism spectrum disorders and schizophrenia. Does the binding promiscuity of Neurexins now suggest complex cooperativity or redundancy at the synapse? While recent studies in primary neuronal cultures and also systematic extracellular protein interaction screens suggest summative effects of these systems, we propose that studying these interactions in the developing zebrafish embryo or larvae may shed more light on their functions during synaptogenesis in vivo. These gene families have recently been extensively characterized in zebrafish, demonstrating high sequence conservation with the human genes. The simpler circuitry of the zebrafish, together with the characterization of the expression patterns down to single, identifiable neurons and the ability to knock-down or over-express multiple genes in a rapid way lend themselves to dissecting complex interaction pathways. Furthermore, the capability of performing high-throughput drug screens suggests that these small vertebrates may prove extremely useful in identifying pharmacological approaches to treating autism spectrum disorders.

The structure of neurexin 1α reveals features promoting a role as synaptic organizer

α-neurexins are essential synaptic adhesion molecules implicated in autism spectrum disorder and schizophrenia. The α-neurexin extracellular domain consists of six LNS domains interspersed by three EGF-like repeats and interacts with many different proteins in the synaptic cleft. To understand how α-neurexins might function as synaptic organizers, we solved the structure of the neurexin 1α extracellular domain (n1α) to 2.65 Å. The L-shaped molecule can be divided into a flexible repeat I (LNS1-EGF-A-LNS2), a rigid horseshoe-shaped repeat II (LNS3-EGF-B-LNS4) with structural similarity to so-called reelin repeats, and an extended repeat III (LNS5-EGF-B-LNS6) with controlled flexibility. A 2.95 Å structure of n1α carrying splice insert SS#3 in LNS4 reveals that SS#3 protrudes as a loop and does not alter the rigid arrangement of repeat II. The global architecture imposed by conserved structural features enables α-neurexins to recruit and organize proteins in distinct and variable ways, influenced by splicing, thereby promoting synaptic function.

Differential regulation of the postsynaptic clustering of γ-aminobutyric acid type A (GABAA) receptors by collybistin isoforms

Collybistin promotes submembrane clustering of gephyrin and is essential for the postsynaptic localization of gephyrin and γ-aminobutyric acid type A (GABA(A)) receptors at GABAergic synapses in hippocampus and amygdala. Four collybistin isoforms are expressed in brain neurons; CB2 and CB3 differ in the C terminus and occur with and without the Src homology 3 (SH3) domain. We have found that in transfected hippocampal neurons, all collybistin isoforms (CB2(SH3+), CB2(SH3-), CB3(SH3+), and CB3(SH3-)) target to and concentrate at GABAergic postsynapses. Moreover, in non-transfected neurons, collybistin concentrates at GABAergic synapses. Hippocampal neurons co-transfected with CB2(SH3-) and gephyrin developed very large postsynaptic gephyrin and GABA(A) receptor clusters (superclusters). This effect was accompanied by a significant increase in the amplitude of miniature inhibitory postsynaptic currents. Co-transfection with CB2(SH3+) and gephyrin induced the formation of many (supernumerary) non-synaptic clusters. Transfection with gephyrin alone did not affect cluster number or size, but gephyrin potentiated the clustering effect of CB2(SH3-) or CB2(SH3+). Co-transfection with CB2(SH3-) or CB2(SH3+) and gephyrin did not affect the density of presynaptic GABAergic terminals contacting the transfected cells, indicating that collybistin is not synaptogenic. Nevertheless, the synaptic superclusters induced by CB2(SH3-) and gephyrin were accompanied by enlarged presynaptic GABAergic terminals. The enhanced clustering of gephyrin and GABA(A) receptors induced by collybistin isoforms was not accompanied by enhanced clustering of neuroligin 2. Moreover, during the development of GABAergic synapses, the clustering of gephyrin and GABA(A) receptors preceded the clustering of neuroligin 2. We propose a model in which the SH3- isoforms play a major role in the postsynaptic accumulation of GABA(A) receptors and in GABAergic synaptic strength.

Functional dependence of neuroligin on a new non-PDZ intracellular domain

Neuroligins, a family of postsynaptic adhesion molecules, are important in synaptogenesis through a well-characterized trans-synaptic interaction with neurexin. In addition, neuroligins are thought to drive postsynaptic assembly through binding of their intracellular domain to PSD-95. However, there is little direct evidence to support the functional necessity of the neuroligin intracellular domain in postsynaptic development. We found that presence of endogenous neuroligin obscured the study of exogenous mutated neuroligin. We therefore used chained microRNAs in rat organotypic hippocampal slices to generate a reduced background of endogenous neuroligin. On this reduced background, we found that neuroligin function was critically dependent on the cytoplasmic tail. However, this function required neither the PDZ ligand nor any other previously described cytoplasmic binding domain, but rather required a previously unknown conserved region. Mutation of a single critical residue in this region inhibited neuroligin-mediated excitatory synaptic potentiation. Finally, we found a functional distinction between neuroligins 1 and 3.

Structural basis for variant-specific neuroligin-binding by α-neurexin

Neurexins (Nrxs) are presynaptic membrane proteins with a single membrane-spanning domain that mediate asymmetric trans-synaptic cell adhesion by binding to their postsynaptic receptor neuroligins. α-Nrx has a large extracellular region comprised of multiple copies of laminin, neurexin, sex-hormone-binding globulin (LNS) domains and epidermal growth factor (EGF) modules, while that of β-Nrx has but a single LNS domain. It has long been known that the larger α-Nrx and the shorter β-Nrx show distinct binding behaviors toward different isoforms/variants of neuroligins, although the underlying mechanism has yet to be elucidated. Here, we describe the crystal structure of a fragment corresponding to the C-terminal one-third of the Nrx1α ectodomain, consisting of LNS5-EGF3-LNS6. The 2.3 Å-resolution structure revealed the presence of a domain configuration that was rigidified by inter-domain contacts, as opposed to the more common flexible "beads-on-a-string" arrangement. Although the neuroligin-binding site on the LNS6 domain was completely exposed, the location of the α-Nrx specific LNS5-EGF3 segment proved incompatible with the loop segment inserted in the B+ neuroligin variant, which explains the variant-specific neuroligin recognition capability observed in α-Nrx. This, combined with a low-resolution molecular envelope obtained by a single particle reconstruction performed on negatively stained full-length Nrx1α sample, allowed us to derive a structural model of the α-Nrx ectodomain. This model will help us understand not only how the large α-Nrx ectodomain is accommodated in the synaptic cleft, but also how the trans-synaptic adhesion mediated by α- and β-Nrxs could differentially affect synaptic structure and function.