Munc18b Is a Major Mediator of Insulin Exocytosis in Rat Pancreatic β-Cells
- Patrick P.L. Lam1,2,
- Mitsuyo Ohno3,
- Subhankar Dolai1,
- Yu He1,
- Tairan Qin1,
- Tao Liang1,
- Dan Zhu1,
- Youhou Kang1,
- Yunfeng Liu1,
- Maria Kauppi4,
- Li Xie1,
- Wilson C.Y. Wan1,
- Na-Rhum Bin2,5,
- Shuzo Sugita2,5,
- Vesa M. Olkkonen6,
- Noriko Takahashi3,
- Haruo Kasai3⇑ and
- Herbert Y. Gaisano1,2⇑
- 1Department of Medicine, University of Toronto, Toronto, Ontario, Canada
- 2Department of Physiology, University of Toronto, Toronto, Ontario, Canada
- 3Laboratory of Structural Physiology, Center for Disease Biology and Integrative Medicine, University of Tokyo, Tokyo, Japan
- 4National Institute for Health and Welfare, Biomedicum, Helsinki, Finland
- 5Division of Fundamental Neurobiology, University Health Network, Toronto, Ontario, Canada
- 6Minerva Foundation Institute for Medical Research, Biomedicum, Helsinki, Finland
- Corresponding authors: Haruo Kasai, , and Herbert Y. Gaisano, .
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.
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.
This article contains Supplementary Data online at http://diabetes.diabetesjournals.org/lookup/suppl/doi:10.2337/db12-1380/-/DC1.
- Received October 5, 2012.
- Accepted February 14, 2013.
- © 2013 by the American Diabetes Association.
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