Munc18-1 regulates first-phase insulin release by promoting granule docking to multiple syntaxin isoforms.
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.
Department of Pediatrics, Herman B. Wells Center, Indiana University School of Medicine, Indianapolis, Indiana 46202, USA.
Eunjin
E
Oh
Michael A
MA
Kalwat
Min-Jung
MJ
Kim
Matthijs
M
Verhage
Debbie C
DC
Thurmond
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D002478:Cells, Cultured; D005947:Glucose / Q000378:metabolism / Q000502:physiology; D057895:Haploinsufficiency; D006706:Homeostasis; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism / Q000648:ultrastructure; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D051938:Munc18 Proteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D020033:Protein Isoforms / Q000378:metabolism; D050765:Qa-SNARE Proteins / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D050600:SNARE Proteins / Q000235:genetics / Q000378:metabolism; D055571:Secretory Pathway; D022142:Secretory Vesicles / Q000378:metabolism* / Q000648:ultrastructure; D050988:Vesicle-Associated Membrane Protein 2 / Q000378:metabolism
D007328:Insulin; D051938:Munc18 Proteins; D020033:Protein Isoforms; D050765:Qa-SNARE Proteins; D050600:SNARE Proteins; D050988:Vesicle-Associated Membrane Protein 2; D005947:Glucose