Neuroligins (NLGNs) are brain-specific cell adhesion molecules. They are expressed on the postsynaptic membrane and bind to presynaptic neurexins (NRXNs) spanning the synaptic cleft. Interestingly, mutations in both NLGNs and NRXNs have been identified in Autism Spectrum Disorder (ASD) patients. This has led researchers to develop genetically engineered NLGN mouse models to study the etiology of ASDs. These studies have shown that NLGN dysfunction can shift the balance of inhibition and excitation in the brain. NLGN isoforms are highly conserved, yet display distinct synaptic localizations. However, the molecular mechanisms that regulate isoform-specific targeting and localization are not well understood. We focus on the role of protein-protein interactions and post-translational modifications in dictating NLGN trafficking and functional regulation. The localization and synaptic effects of NLGN1 are specific to excitatory synapses at which NLGN1 enhances the recruitment of synaptic proteins and potentiates excitatory synaptic transmission. These NLGN1 effects are dependent on synaptic activity and CaMKII. We recently found that CaMKII robustly and specifically phosphorylates the intracellular domain of NLGN1. We showed that NLGN1 has a single dominant CaMKII site, T739, which is phosphorylated in response to synaptic activity in cultured neurons and in vivo in response to visual stimulation. Furthermore, a phospho-deficient mutant (T739A) reduces the basal and activity-driven surface expression of NLGN1, leading to a reduction in NLGN-mediated excitatory synaptic potentiation. Our findings are the first to show a direct functional interplay between CaMKII and NLGN1, two primary components of excitatory synapses. ASDs are a group of neurodevelopmental disorders that have a high genetic predisposition and higher occurrence rates in males than females. A variety of point mutations have been identified in X-linked NLGN3 and 4X in patients with intellectual disability and symptoms characteristic of ASDs. Interestingly, all of the autism-associated point mutations in NLGN3 and NLGN4X reported thus far reside in their extracellular domains except for a single point mutation in the intracellular domain of NLGN4X at arginine (R) 704, which is modified to a cysteine (C). We discovered that endogenous NLGN4X is robustly phosphorylated by protein kinase C (PKC) at T707 in human embryonic neurons. This autism mutation (R704C) eliminates T707 phosphorylation, which is critical for NLGN4X-mediated excitatory enhancement. Interestingly, unlike other NLGN ASD-associated mutations, R704C, did not disrupt the stability or surface expression of NLGN4X, yet still led to synaptic dysfunction. Mouse models of autism have uncovered a role for an imbalance of excitatory/inhibitory transmission, often resulting in direct increases in inhibitory transmission. Our results establish a potential causality between a genetic mutation, a key posttranslational modification, and robust synaptic changes and will provide insights in elucidating the pathophysiology of ASDs.
| Jeong, Jaehoon; Paskus, Jeremiah D; Roche, Katherine W (2017) Posttranslational modifications of neuroligins regulate neuronal and glial signaling. Curr Opin Neurobiol 45:130-138 |
| Bemben, Michael A; Shipman, Seth L; Nicoll, Roger A et al. (2015) The cellular and molecular landscape of neuroligins. Trends Neurosci 38:496-505 |
| Bemben, Michael A; Nguyen, Quynh-Anh; Wang, Tongguang et al. (2015) Autism-associated mutation inhibits protein kinase C-mediated neuroligin-4X enhancement of excitatory synapses. Proc Natl Acad Sci U S A 112:2551-6 |
| Bemben, Michael A; Shipman, Seth L; Hirai, Takaaki et al. (2014) CaMKII phosphorylation of neuroligin-1 regulates excitatory synapses. Nat Neurosci 17:56-64 |
