Munc18-1 Regulates First-Phase Insulin Release by Promoting Granule Docking to Multiple Syntaxin Isoforms

J Biol Chem. 2012 Jul 27;287(31):25821-33. doi: 10.1074/jbc.M112.361501. Epub 2012 Jun 8.

Abstract

Attenuated levels of the Sec1/Munc18 (SM) protein Munc18-1 in human islet β-cells is coincident with type 2 diabetes, although how Munc18-1 facilitates insulin secretion remains enigmatic. Herein, using conventional Munc18-1(+/-) and β-cell specific Munc18-1(-/-) knock-out mice, we establish that Munc18-1 is required for the first phase of insulin secretion. Conversely, human islets expressing elevated levels of Munc18-1 elicited significant potentiation of only first-phase insulin release. Insulin secretory changes positively correlated with insulin granule number at the plasma membrane: Munc18-1-deficient cells lacked 35% of the normal component of pre-docked insulin secretory granules, whereas cells with elevated levels of Munc18-1 exhibited a ∼20% increase in pre-docked granule number. Pre-docked syntaxin 1-based SNARE complexes bound by Munc18-1 were detected in β-cell lysates but, surprisingly, were reduced by elevation of Munc18-1 levels. Paradoxically, elevated Munc18-1 levels coincided with increased binding of syntaxin 4 to VAMP2 at the plasma membrane. Accordingly, syntaxin 4 was a requisite for Munc18-1 potentiation of insulin release. Munc18c, the cognate SM isoform for syntaxin 4, failed to bind SNARE complexes. Given that Munc18-1 does not pair with syntaxin 4, these data suggest a novel indirect role for Munc18-1 in facilitating syntaxin 4-mediated granule pre-docking to support first-phase insulin exocytosis.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Membrane / metabolism
  • Cell Membrane / ultrastructure
  • Cells, Cultured
  • Glucose / metabolism
  • Glucose / physiology
  • Haploinsufficiency
  • Homeostasis
  • Humans
  • Insulin / metabolism*
  • Insulin Secretion
  • Islets of Langerhans / metabolism
  • Islets of Langerhans / ultrastructure
  • Male
  • Mice
  • Mice, Inbred C57BL
  • Mice, Transgenic
  • Munc18 Proteins / genetics
  • Munc18 Proteins / metabolism*
  • Munc18 Proteins / physiology
  • Protein Isoforms / metabolism
  • Qa-SNARE Proteins / genetics
  • Qa-SNARE Proteins / metabolism*
  • Rats
  • SNARE Proteins / genetics
  • SNARE Proteins / metabolism
  • Secretory Pathway
  • Secretory Vesicles / metabolism*
  • Secretory Vesicles / ultrastructure
  • Vesicle-Associated Membrane Protein 2 / metabolism

Substances

  • Insulin
  • Munc18 Proteins
  • Protein Isoforms
  • Qa-SNARE Proteins
  • SNARE Proteins
  • Vesicle-Associated Membrane Protein 2
  • Glucose