Published in:
01-02-2013 | Article
Syntaxin-3 regulates newcomer insulin granule exocytosis and compound fusion in pancreatic beta cells
Published in: Diabetologia | Issue 2/2013
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Aims/hypothesis
The molecular basis of the exocytosis of secretory insulin-containing granules (SGs) during biphasic glucose-stimulated insulin secretion (GSIS) from pancreatic beta cells remains unclear. Syntaxin (SYN)-1A and SYN-4 have been shown to mediate insulin exocytosis. The insulin-secretory function of SYN-3, which is particularly abundant in SGs, is unclear.
Methods
Mouse pancreatic islets and INS-1 cells were treated with adenovirus carrying Syn-3 (also known as Stx3) or small interfering RNA targeting Syn-3 in order to examine insulin secretion by radioimmunoassay. The localisation and distribution of insulin granules were examined by confocal and electron microscopy. Dynamic single-granule fusion events were assessed using total internal reflection fluorescence microscopy (TIRFM).
Results
Depletion of endogenous SYN-3 inhibited insulin release. TIRFM showed no change in the number or fusion competence of previously docked SGs but, instead, a marked reduction in the recruitment of newcomer SGs and their subsequent exocytotic fusion during biphasic GSIS. Conversely, overexpression of Syn-3 enhanced both phases of GSIS, owing to the increase in newcomer SGs and, remarkably, to increased SG–SG fusion, which was confirmed by electron microscopy.
Conclusions/interpretation
In insulin secretion, SYN-3 plays a role in the mediation of newcomer SG exocytosis and SG–SG fusion that contributes to biphasic GSIS.