Islet Complexins are Complex

Abstract

While Ca2+ activity is required for islet hormone secretion, proper glucose stimulated glucagon, insulin and somatostatin secretion requires a synergistic combination of electrical, paracrine, juxtacrine and cell intrinsic signaling pathways. This complexity can be unraveled by reconciling islet cell electrical activity, second messenger response and secretory vesicle mobilization in the presence and absence of paracrine regulatory factors. One molecule of interest is Complexin 2, a Protein Kinase A (PKA) dependent SNARE binding protein known to be expressed in alpha, beta and delta cells. Previous alpha cell studies showed that paracrine signaling by insulin and somatostatin combine to reduce glucagon secretion by lowering cAMP and PKA activity independent of Ca2+ activity. In contrast, beta cell insulin secretion is increased by cAMP/PKA and Ca2+ activity. Here, we test the hypothesis that Complexin 2 regulates insulin and glucagon secretion at the secretory vesicle level via phosphorylation by Protein Kinase A (PKA) which is in turn regulated by somatostatin paracrine signaling. Using islets from mice that are null for Complexin 2 gene (Cplxn2 KO), we measured insulin, glucagon and somatostatin secretion in response to glucose. We found increases in both glucagon and insulin output despite a concomitant increase in somatostatin secretion. In order to image Ca2+ signaling in Cplxn2 KO mice we used 2D confocal and 3D dual inverted selective plane illumination microscopy (diSPIM), which performs at high speed using light sheet illumination and camera detection. Live islet imaging experiments with GCaMP6 expressed in Cplxn 2 KO alpha cells showed that somatostatin, which normally inhibits Ca2+ oscillations, was unable to attenuate Ca2+ oscillations in Cplxn2 KO alpha cells during direct alpha cell cAMP/PKA activation.