LRRTM3 Regulates Excitatory Synapse Development through Alternative Splicing and Neurexin Binding

The four members of the LRRTM family (LRRTM1-4) are postsynaptic adhesion molecules essential for excitatory synapse development. They have also been implicated in neuropsychiatric diseases. Here, we focus on LRRTM3, showing that two distinct LRRTM3 variants generated by alternative splicing regulate LRRTM3 interaction with PSD-95, but not its excitatory synapse-promoting activity. Overexpression of either LRRTM3 variant increased excitatory synapse density in dentate gyrus (DG) granule neurons, whereas LRRTM3 knockdown decreased it. LRRTM3 also controlled activity-regulated AMPA receptor surface expression in an alternative splicing-dependent manner. Furthermore, Lrrtm3-knockout mice displayed specific alterations in excitatory synapse density, excitatory synaptic transmission and excitability in DG granule neurons but not in CA1 pyramidal neurons. Lastly, LRRTM3 required only specific splice variants of presynaptic neurexins for their synaptogenic activity. Collectively, our data highlight alternative splicing and differential presynaptic ligand utilization in the regulation of LRRTMs, revealing key regulatory mechanisms for excitatory synapse development. [Display omitted] •Alternative splicing of Lrrtm3 and Lrrtm4 mRNAs produces distinct protein variants•LRRTM3 is required for DG excitatory synapse development in vitro and in vivo•LRRTM3 regulates activity-dependent AMPAR surface expression•Neurexins are universal ligands for all LRRTMs In this study, Um et al. identify leucine-rich repeat transmembrane protein 3 (LRRTM3) as a crucial regulator of excitatory synapse development in dentate gyrus granule neurons. Two different splice variants of LRRTM3 promote excitatory synapse development, but a shorter variant of LRRTM3 that can bind to PSD-95 controls activity-dependent surface expression of the AMPA-type glutamate receptor. Further characterization of LRRTM3-knockout animals revealed that LRRTM3 is required for excitatory synapse structure and function in vivo. Lastly, presynaptic neurexins are required for the LRRTM3 synaptogenic activity that induces presynaptic differentiation, suggesting that LRRTM3 performs synapse-organizing functions that are both redundant and non-redundant relative to those of other LRRTM family members.