LRRTM2 Functions as a Neurexin Ligand in Promoting Excitatory Synapse Formation

Abstract

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 Leucine-Rich Repeat Superfamily of Synaptic Adhesion Molecules: LRRTMs and Slitrks

“…All four LRRTM family members display synaptogenic activities in these assays, although the induction strength of LRRTM2 is the strongest (de Wit et al, 2009;Ko et al, 2009;Linhoff et al, 2009). LRRTM2 specifically localizes in excitatory synapses, and not in inhibitory synapses (Linhoff et al, 2009).…”

“…A systematic bioinformatics study has shown that extracellular LRR domain-containing proteins have greatly expanded in mammals (135 in mouse, 139 in human) compared with worms (29 extracellular LRR proteins) and flies (66 extracellular LRR proteins) (Dolan et al, 2007). The ligands of these extracellular LRR proteins remain largely unknown, although the process of ligand identification has recently accelerated (de Wit et al, 2009;Ko et al, 2009;Siddiqui et al, 2010;Takahashi et al, 2011;Woo et al, 2009). Mounting evidence from invertebrate systems shows that extracellular LRR proteins control key phases of neuron development and neural circuit formation including axon guidance, target-cell recognition, and synapse formation.…”

“…The localizations of the other LRRTMs have not yet been determined due to a lack of reliable antibodies suitable for immunohistochemical analyses. Overexpression of LRRTM1 or -2 specifically increases the number of excitatory synapses (de Wit et al, 2009;Ko et al, 2009;Linhoff et al, 2009). In addition, LRRTMs interact with neurexins, a well-known family of presynaptic adhesion molecules whose members bind to neuroligins (de Wit et al, 2009;Ko et al, 2009;Siddiqui et al, 2010).…”

“…Overexpression of LRRTM1 or -2 specifically increases the number of excitatory synapses (de Wit et al, 2009;Ko et al, 2009;Linhoff et al, 2009). In addition, LRRTMs interact with neurexins, a well-known family of presynaptic adhesion molecules whose members bind to neuroligins (de Wit et al, 2009;Ko et al, 2009;Siddiqui et al, 2010). This interaction is modulated by the insertion status of alternative splice site #4 found in both α-and β-neurexins (Ko et al, 2009;Siddiqui et al, 2010).…”

“…In addition, LRRTMs interact with neurexins, a well-known family of presynaptic adhesion molecules whose members bind to neuroligins (de Wit et al, 2009;Ko et al, 2009;Siddiqui et al, 2010). This interaction is modulated by the insertion status of alternative splice site #4 found in both α-and β-neurexins (Ko et al, 2009;Siddiqui et al, 2010). LRRTM2 specifically binds to neurexins lacking this insert, and mediates cell adhesion in the trans-configuration (Ko et al, 2009).…”