SNAP-25b-deficiency increases insulin secretion and changes spatiotemporal profile of Ca2+oscillations in \u03b2 cell networks

Teresa Daraio,Marjan Slak Rupnik,Maša Skelin Klemen,Marko Gosak,Ismael Valladolid-Acebes,Kerstin Brismar,Lidija Križančić Bombek,Christina Bark,Essam Refai,Per-Olof Berggren

doi:10.1038/s41598-017-08082-y

Teresa Daraio, Marjan Slak Rupnik + Show 8 more

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https://doi.org/10.1038/s41598-017-08082-y

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Abstract

SNAP-25 is a protein of the core SNARE complex mediating stimulus-dependent release of insulin from pancreatic β cells. The protein exists as two alternatively spliced isoforms, SNAP-25a and SNAP-25b, differing in 9 out of 206 amino acids, yet their specific roles in pancreatic β cells remain unclear. We explored the effect of SNAP-25b-deficiency on glucose-stimulated insulin release in islets and found increased secretion both in vivo and in vitro. However, slow photo-release of caged Ca2+ in β cells within pancreatic slices showed no significant differences in Ca2+-sensitivity, amplitude or rate of exocytosis between SNAP-25b-deficient and wild-type littermates. Therefore, we next investigated if Ca2+ handling was affected in glucose-stimulated β cells using intracellular Ca2+-imaging and found premature activation and delayed termination of [Ca2+]i elevations. These findings were accompanied by less synchronized Ca2+-oscillations and hence more segregated functional β cell networks in SNAP-25b-deficient mice. Islet gross morphology and architecture were maintained in mutant mice, although sex specific compensatory changes were observed. Thus, our study proposes that SNAP-25b in pancreatic β cells, except for participating in the core SNARE complex, is necessary for accurate regulation of Ca2+-dynamics.

Highlights

A controlled insulin secretion from β cells in the islets of Langerhans is essential to preserve healthy levels of blood glucose during basal and stimulated conditions[1]
This study shows that SNAP-25b-deficiency results in increased insulin release, higher number of β cells in males, altered islet size, lost collective control of Ca2+-oscillations and defective inter-β-cell-connectivity within islets, all features of the early phase of developing type 2 diabetes
We hypothesized that SNAP-25b-deficiency in islets primarily affected the core exocytotic SNARE machinery and the mechanism of insulin secretion

Summary

Introduction

A controlled insulin secretion from β cells in the islets of Langerhans is essential to preserve healthy levels of blood glucose during basal and stimulated conditions[1]. Import and metabolize glucose from the blood, molecular processes increase cytosolic levels of Ca2+, which activate the SNARE machinery mediating insulin granule fusion with the plasma membrane. Alterations in islet morphology as well as in connexin36-dependent intercellular communication via gap junctions can result in loss of [Ca2+]i coordination, which leads to an impairment of the normal pulsatile pattern of insulin secretion[4, 26, 30,31,32]. We have analyzed the effect of SNAP-25b-deficiency during acute glucose-stimulated insulin secretion, gross islet morphology, Ca2+-dependent exocytosis in individual β cells and glucose-dependent β cell network activity

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