Structural basis for synaptic adhesion mediated by neuroligin-neurexin interactions

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Abstract
The heterophilic synaptic adhesion molecules neuroligins and neurexins are essential for establishing and maintaining neuronal circuits by modulating the formation and maturation of synapses. The neuroligin-neurexin adhesion is Ca2+-dependent and regulated by alternative splicing. We report a structure of the complex at a resolution of 2.4 Å between the mouse neuroligin-1 (NL1) cholinesterase-like domain and the mouse neurexin-1β (NX1β) LNS (laminin, neurexin and sex hormone–binding globulin–like) domain. The structure revealed a delicate neuroligin-neurexin assembly mediated by a hydrophilic, Ca2+-mediated and solvent-supplemented interface, rendering it capable of being modulated by alternative splicing and other regulatory factors. Thermodynamic data supported a mechanism wherein splicing site B of NL1 acts by modulating a salt bridge at the edge of the NL1-NX1β interface. Mapping neuroligin mutations implicated in autism indicated that most such mutations are structurally destabilizing, supporting deficient neuroligin biosynthesis and processing as a common cause for this brain disorder.