title: Syntaxin-3 regulates newcomer insulin granule exocytosis and compound fusion in pancreatic beta cells.
abstract: OBJECTIVE
  The molecular basis of the exocytosis of secretory insulin-containing granules (SGs) during biphasic glucose-stimulated insulin secretion (GSIS) from pancreatic beta cells remains unclear. Syntaxin (SYN)-1A and SYN-4 have been shown to mediate insulin exocytosis. The insulin-secretory function of SYN-3, which is particularly abundant in SGs, is unclear.
  
  
  METHODS
  Mouse pancreatic islets and INS-1 cells were treated with adenovirus carrying Syn-3 (also known as Stx3) or small interfering RNA targeting Syn-3 in order to examine insulin secretion by radioimmunoassay. The localisation and distribution of insulin granules were examined by confocal and electron microscopy. Dynamic single-granule fusion events were assessed using total internal reflection fluorescence microscopy (TIRFM).
  
  
  RESULTS
  Depletion of endogenous SYN-3 inhibited insulin release. TIRFM showed no change in the number or fusion competence of previously docked SGs but, instead, a marked reduction in the recruitment of newcomer SGs and their subsequent exocytotic fusion during biphasic GSIS. Conversely, overexpression of Syn-3 enhanced both phases of GSIS, owing to the increase in newcomer SGs and, remarkably, to increased SG-SG fusion, which was confirmed by electron microscopy.
  
  
  CONCLUSIONS
  In insulin secretion, SYN-3 plays a role in the mediation of newcomer SG exocytosis and SG-SG fusion that contributes to biphasic GSIS.
authors.affiliation: Department of Medicine, University of Toronto, Toronto, Ontario, Canada.
authors.forename: D
authors.initials: D
authors.lastname: Zhu
authors.forename: E
authors.initials: E
authors.lastname: Koo
authors.forename: E
authors.initials: E
authors.lastname: Kwan
authors.forename: Y
authors.initials: Y
authors.lastname: Kang
authors.forename: S
authors.initials: S
authors.lastname: Park
authors.forename: H
authors.initials: H
authors.lastname: Xie
authors.forename: S
authors.initials: S
authors.lastname: Sugita
authors.forename: H Y
authors.initials: HY
authors.lastname: Gaisano
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002460:Cell Line; D005089:Exocytosis / Q000235:genetics / Q000502:physiology*; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000378:metabolism*; D007515:Islets of Langerhans / Q000378:metabolism*; D051379:Mice; D018613:Microscopy, Confocal; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence; D050765:Qa-SNARE Proteins / Q000235:genetics / Q000378:metabolism*; D034741:RNA, Small Interfering; D011863:Radioimmunoassay
chemical_list: D007328:Insulin; D050765:Qa-SNARE Proteins; D034741:RNA, Small Interfering