Insulin is released by Ca2+-dependent exocytosis of secretory granules, while many receptors, ion channels, and transporters reach the plasma membrane by constitutive exocytosis of endosomal vesicles. Both processes rely on specific sets of soluble N-ethylmaleimide attachment protein receptor (SNARE) proteins for membrane fusion. Here, we used live-cell total internal reflection fluorescence imaging to understand the role of vesicle-associated membrane protein 8 (VAMP8), a SNARE reported to be involved in both types of exocytosis, in pancreatic β-cells. Both endogenous and overexpressed VAMP8 localized to Rab5-, Rab7-, and Rab11-positive endosomal vesicles, but not to insulin granules. Depolarization evoked slow exocytosis of VAMP8/Rab11-positive vesicles that was insensitive to tetanus neurotoxin cleavage and accelerated by somatostatin but not exendin-4. VAMP8-positive vesicles fused within seconds of their arrival at the plasma membrane, in contrast to insulin granules that remained docked for several minutes. Fusion of VAMP8-positive vesicles released glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) and GLUT2 into the plasma membrane, and fusion of GLP-1R–positive vesicles, but not GLP-1R endocytosis, was suppressed by VAMP8 knockdown. Public data indicate a negative correlation between VAMP8 gene expression and secretory index in human donor islets, and we find that overexpression of full-length VAMP8, or soluble VAMP8 lacking its transmembrane domain, inhibited exocytosis of insulin granules. We conclude that VAMP8 associates with early, late, and recycling endosomes, supports exocytosis of recycling endosomes to control surface expression of physiologically important membrane proteins (GLP-1R and GLUT2), and can suppress insulin secretion. Finally, the data suggest that the previously described “newcomer” exocytosis reflects release from an endosomal compartment, rather than exocytosis of insulin-containing secretory granules.
Vesicle-associated membrane protein 8 (VAMP8) localizes to endosomal vesicles and mediates their exocytosis in pancreatic β-cells.
VAMP8 dependent vesicle fusion delivers glucagon-like peptide 1 receptor and GLUT2 to the plasma membrane.
VAMP8 overexpression inhibits insulin granule exocytosis.
“Newcomer” exocytosis likely involves endosomal compartments, not insulin granules.
This article contains supplementary material online at https://doi.org/10.2337/figshare.29316068.
