Aims/hypothesis: We have previously reported that the haplodeficient Munc13-1+/- mouse exhibits impaired biphasic glucose-stimulated insulin secretion (GSIS), causing glucose intolerance mimicking type 2 diabetes. Glucagon-like peptide-1 (GLP-1) can bypass these insulin-secretory defects in type 2 diabetes, but the mechanism of exocytotic events mediated by GLP-1 in rescuing insulin secretion is unclear.
Methods: The total internal reflection fluorescence microscopy (TIRFM) technique was used to examine single insulin granule fusion events in mouse islet beta cells.
Results: There was no difference in the density of docked granules in the resting state between Munc13-1+/+ and Munc13-1+/- mouse islet beta cells. While exocytosis of previously docked granules in Munc13-1+/- beta cells is reduced during high-K+ stimulation as expected, we now find a reduction in additional exocytosis events that account for the major portion of GSIS, namely two types of newcomer granules, one which has a short docking time (short-dock) and another undergoing no docking before exocytosis (no-dock). As mammalian homologue of Caenorhabditis elegans unc-13-1 (Munc13-1) is a phorbol ester substrate, phorbol ester could partially rescue biphasic GSIS in Munc13-1-deficient beta cells by enhancing recruitment of short-dock newcomer granules for exocytosis. The more effective rescue of biphasic GSIS by GLP-1 than by phorbol was due to increased recruitment of both short-dock and no-dock newcomer granules.
Conclusions/interpretation: Phorbol ester and GLP-1 potentiation of biphasic GSIS are brought about by recruitment of distinct populations of newcomer granules for exocytosis, which may be mediated by Munc13-1 interaction with syntaxin–SNARE complexes other than that formed by syntaxin-1A.