The composition of the beta-cell exocytic machinery is very similar to that of neuronal synapses, and the developmental pathway of beta-cells and neurons substantially overlap. beta-Cells secrete gamma-aminobutyric acid and express proteins that, in the brain, are specific markers of inhibitory synapses. Recently, neuronal coculture experiments have identified three families of synaptic cell-surface molecules (neurexins, neuroligins, and SynCAM) that drive synapse formation in vitro and that control the differentiation of nascent synapses into either excitatory or inhibitory fully mature nerve terminals. The inhibitory synapse-like character of the beta-cells led us to hypothesize that members of these families of synapse-inducing adhesion molecules would be expressed in beta-cells and that the pattern of expression would resemble that associated with neuronal inhibitory synaptogenesis. Here, we describe beta-cell expression of the neuroligins, neurexins, and SynCAM, and show that neuroligin expression affects insulin secretion in INS-1 beta-cells and rat islet cells. Our findings demonstrate that neuroligins and neurexins are expressed outside the central nervous system and help confer an inhibitory synaptic-like phenotype onto the beta-cell surface. Analogous to their role in synaptic neurotransmission, neurexin-neuroligin interactions may play a role in the formation of the submembrane insulin secretory apparatus.

Background:The cell surface, transmembrane protein neurexin may play a role in insulin secretion. Results: Knockdown and knock-out of neurexin-1␣, the predominant ␤-cell isoform, increased insulin secretion and impaired secretory granule docking. Conclusion: Neurexin-1␣ helps mediate insulin granule docking and thereby constrains secretion. Significance: Neurexin-1␣ could couple localization and functioning of the insulin docking and secretory machinery to extracellular protein interactions.

Background:The synaptic protein neuroligin-2 is present on the pancreatic ␤ cell surface and affects insulin secretion. Results: Insulin secretion increases when ␤ cells contact adjacent cells expressing neuroligin; soluble neuroligin inhibits secretion. Conclusion: Clustered neuroligin-2 enhances insulin secretion in a transcellular manner, likely by promoting secretory microdomain formation. Significance: Transcellular protein-protein interactions paralleling the transsynaptic interactions that drive synapse formation may guide ␤ cell functional maturation.