title: Munc18b is a major mediator of insulin exocytosis in rat pancreatic β-cells.
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.
authors: Department of Medicine, University of Toronto, Toronto, Ontario, Canada.
authors: Patrick P L
authors: PP
authors: Lam
authors: Mitsuyo
authors: M
authors: Ohno
authors: Subhankar
authors: S
authors: Dolai
authors: Yu
authors: Y
authors: He
authors: Tairan
authors: T
authors: Qin
authors: Tao
authors: T
authors: Liang
authors: Dan
authors: D
authors: Zhu
authors: Youhou
authors: Y
authors: Kang
authors: Yunfeng
authors: Y
authors: Liu
authors: Maria
authors: M
authors: Kauppi
authors: Li
authors: L
authors: Xie
authors: Wilson C Y
authors: WC
authors: Wan
authors: Na-Rhum
authors: NR
authors: Bin
authors: Shuzo
authors: S
authors: Sugita
authors: Vesa M
authors: VM
authors: Olkkonen
authors: Noriko
authors: N
authors: Takahashi
authors: Haruo
authors: H
authors: Kasai
authors: Herbert Y
authors: HY
authors: Gaisano
mesh_terms: D000818:Animals; D005089:Exocytosis / Q000502:physiology*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000378:metabolism*; D008297:Male; D051938:Munc18 Proteins / Q000235:genetics / Q000378:metabolism*; D050765:Qa-SNARE Proteins / Q000235:genetics / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D022142:Secretory Vesicles / Q000378:metabolism*; D050827:Syntaxin 1 / Q000235:genetics / Q000378:metabolism
chemical_list: D007328:Insulin; D051938:Munc18 Proteins; D050765:Qa-SNARE Proteins; D050827:Syntaxin 1