Munc18b Is a Major Mediator of Insulin Exocytosis in Rat Pancreatic β-Cells

  1. Herbert Y. Gaisano1,2
  1. 1Department of Medicine, University of Toronto, Toronto, Ontario, Canada
  2. 2Department of Physiology, University of Toronto, Toronto, Ontario, Canada
  3. 3Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, University of Tokyo, Tokyo, Japan
  4. 4National Institute for Health and Welfare, Biomedicum, Helsinki, Finland
  5. 5Division of Fundamental Neurobiology, University Health Network, Toronto, Ontario, Canada
  6. 6Minerva Foundation Institute for Medical Research, Biomedicum, Helsinki, Finland
  1. Corresponding authors: Haruo Kasai, hkasai{at}m.u-tokyo.ac.jp, and Herbert Y. Gaisano, herbert.gaisano{at}utoronto.ca.
  1. P.P.L.L., M.O., and S.D. contributed equally to this study. H.K. and H.Y.G. are equally contributing corresponding authors.

Abstract

Sec1/Munc18 proteins facilitate the formation of trans-SNARE (soluble N-ethylmaleimide–sensitive factor attachment protein receptor) complexes that mediate fusion of secretory granule (SG) with plasma membrane (PM). The capacity of pancreatic β-cells to exocytose insulin becomes compromised in diabetes. β-Cells express three Munc18 isoforms of which the role of Munc18b is unknown. We found that Munc18b depletion in rat islets disabled SNARE complex formation formed by syntaxin (Syn)-2 and Syn-3. Two-photon imaging analysis revealed in Munc18b-depleted β-cells a 40% reduction in primary exocytosis (SG-PM fusion) and abrogation of almost all sequential SG-SG fusion, together accounting for a 50% reduction in glucose-stimulated insulin secretion (GSIS). In contrast, gain-of-function expression of Munc18b wild-type and, more so, dominant-positive K314L/R315L mutant promoted the assembly of cognate SNARE complexes, which caused potentiation of biphasic GSIS. We found that this was attributed to a more than threefold enhancement of both primary exocytosis and sequential SG-SG fusion, including long-chain fusion (6–8 SGs) not normally (2–3 SG fusion) observed. Thus, Munc18b-mediated exocytosis may be deployed to increase secretory efficiency of SGs in deeper cytosolic layers of β-cells as well as additional primary exocytosis, which may open new avenues of therapy development for diabetes.

Footnotes

  • Received October 5, 2012.
  • Accepted February 14, 2013.

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  1. Diabetes vol. 62 no. 7 2416-2428
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