Recently, leucine-rich repeat transmembrane proteins (LRRTMs) were found to be synaptic cell-adhesion molecules that, when expressed in non-neuronal cells, induce presynaptic differentiation in contacting axons. We now demonstrate that LRRTM2 induces only excitatory synapses, and that it also acts in transfected neurons similar to neuroligin-1. Using affinity chromatography, we identified α- and β-neurexins as LRRTM2 ligands, again rendering LRRTM2 similar to neuroligin-1. However, whereas neuroligins bind neurexins containing or lacking an insert in splice site #4, LRRTM2 only binds neurexins lacking an insert in splice site #4. Binding of neurexins to LRRTM2 can produce cell-adhesion junctions, consistent with a trans-interaction regulated by neurexin alternative splicing, and recombinant neurexin-1β blocks LRRTM2's ability to promote presynaptic differentiation. Thus, our data suggest that two unrelated postsynaptic cell-adhesion molecules, LRRTMs and neuroligins, unexpectedly bind to neurexins as the same presynaptic receptor, but that their binding is subject to distinct regulatory mechanisms.

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.leucine-rich repeat | neuropsychiatic disorder | synaptic cell-adhesion

Synaptic cell adhesion molecules (CAMs) are known to play key roles in various aspects of synaptic structures and functions, including early differentiation, maintenance, and plasticity. We herein report the identification of a family of cell adhesion-like molecules termed SALM that interacts with the abundant postsynaptic density (PSD) protein PSD-95. SALM2, a SALM isoform, distributes to excitatory, but not inhibitory, synaptic sites. Overexpression of SALM2 increases the number of excitatory synapses and dendritic spines. Mislocalized expression of SALM2 disrupts excitatory synapses and dendritic spines. Bead-induced direct aggregation of SALM2 results in coclustering of PSD-95 and other postsynaptic proteins, including GKAP and AMPA receptors. Knockdown of SALM2 by RNA interference reduces the number of excitatory synapses and dendritic spines and the frequency, but not amplitude, of miniature excitatory postsynaptic currents. These results suggest that SALM2 is an important regulator of the differentiation of excitatory synapses.

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Neuroligins (NLs) are postsynaptic cell-adhesion molecules essential for normal synapse function. Mutations in neuroligin-4 (NL4; gene symbol: NLGN4) have been reported in some patients with autism spectrum disorder (ASD) and other neurodevelopmental impairments. However, the low frequency of NL4 mutations, and the limited information about the affected patients and the functional consequences of their mutations, cast doubt on the causal role of NL4 mutations in these disorders. Here, we describe two brothers with classical ASD who carry a single amino acid substitution in NL4 (R87W). This substitution was absent from the brothers’ asymptomatic parents, suggesting that it arose in the maternal germline. R87 is conserved in all NL isoforms, and the R87W substitution is not observed in control individuals. At the protein level, the R87W-substitution impaired glycosylation processing of NL4 expressed in HEK293 and COS cells, destabilized NL4, caused NL4 retention in the endoplasmic reticulum in non-neuronal cells and neurons, and blocked NL4 transport to the cell-surface. As a result, the R87W-substitution inactivated the synapse-formation activity of NL4, and abolished the functional effect of NL4 on synapse strength. Viewed together, these observations suggest that a point mutation in NL4 can cause ASD by a loss-of-function mechanism.

Liprin-␣/SYD-2 is a family of multidomain proteins with four known isoforms. One of the reported functions of liprin-␣ is to regulate the development of presynaptic active zones, but the underlying mechanism is poorly understood. Here we report that liprin-␣ directly interacts with the ERC (ELKS-Rab6-interacting protein-CAST) family of proteins, members of which are known to bind RIMs, the active zone proteins that regulate neurotransmitter release. In vitro results indicate that ERC2/CAST, an active zone-specific isoform, interacts with all of the known isoforms of liprin-␣ and that liprin-␣1 associates with both ERC2 and ERC1b, a splice variant of ERC1 that distributes to both cytosolic and active zone regions. ERC2 colocalizes with liprin-␣1 in cultured neurons and forms a complex with liprin-␣1 in brain. Liprin-␣1, when expressed alone in cultured neurons, shows a partial synaptic localization. When coexpressed with ERC2, however, liprin-␣1 is redistributed to synaptic sites. Moreover, roughly the first half of ERC2, which contains the liprin-␣-binding region, is sufficient for the synaptic localization of liprin-␣1 while the second half is not. These results suggest that the interaction between ERC2 and liprin-␣ may be involved in the presynaptic localization of liprin-␣ and the molecular organization of presynaptic active zones.

Postsynaptic neuroligins are thought to perform essential functions in synapse validation and synaptic transmission by binding to, and dimerizing, presynaptic a-and b-neurexins. To test this hypothesis, we examined the functional effects of neuroligin-1 mutations that impair only a-neurexin binding, block both a-and b-neurexin binding, or abolish neuroligin-1 dimerization. Abolishing a-neurexin binding abrogated neuroligin-induced generation of neuronal synapses onto transfected non-neuronal cells in the so-called artificial synapse-formation assay, even though b-neurexin binding was retained. Thus, in this assay, neuroligin-1 induces apparent synapse formation by binding to presynaptic a-neurexins. In transfected neurons, however, neither a-nor b-neurexin binding was essential for the ability of postsynaptic neuroligin-1 to dramatically increase synapse density, suggesting a neurexin-independent mechanism of synapse formation. Moreover, neuroligin-1 dimerization was not required for either the nonneuronal or the neuronal synapse-formation assay. Nevertheless, both a-neurexin binding and neuroligin-1 dimerization were essential for the increase in apparent synapse size that is induced by neuroligin-1 in transfected neurons. Thus, neuroligin-1 performs diverse synaptic functions by mechanisms that include as essential components of a-neurexin binding and neuroligin dimerization, but extend beyond these activities.

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