title: Synaptotagmin-7 Functions to Replenish Insulin Granules for Exocytosis in Human Islet β-Cells.
abstract: Synaptotagmin (Syt)-7, a major component of the exocytotic machinery in neurons, is also the major Syt in rodent pancreatic β-cells shown to mediate glucose-stimulated insulin secretion (GSIS). However, Syt-7's precise exocytotic actions in β-cells remain unknown. We show that Syt-7 is abundant in human β-cells. Adenovirus-short hairpin RNA knockdown (KD) of Syt-7 in human islets reduced first- and second-phase GSIS attributed to the reduction of exocytosis of predocked and newcomer insulin secretory granules (SGs). Glucose stimulation expectedly induced Syt-7 association in a Ca(2+)-dependent manner with syntaxin-3 and syntaxin-1A soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes known to mediate exocytosis of newcomer and predocked SGs, respectively. However, Syt-7-KD did not disrupt SNARE complex assembly. Instead, electron microscopy analysis showed that Syt-7-KD reduced the recruitment of SGs to the plasma membrane after glucose-stimulated depletion, which could not be rescued by glucagon-like peptide 1 pretreatment. To assess the possibility that this new action of Syt-7 on SG recruitment may involve calmodulin (CaM), pretreatment of islets with CaM blocker calmidazolium showed effects very similar to those of Syt-7-KD. Syt-7 therefore plays a novel more dominant function in the replenishment of releasable SG pools in human β-cells than its previously purported role in exocytotic fusion per se.
authors.affiliation: Department of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Subhankar
authors.initials: S
authors.lastname: Dolai
authors.affiliation: Department of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Li
authors.initials: L
authors.lastname: Xie
authors.affiliation: Department of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Dan
authors.initials: D
authors.lastname: Zhu
authors.affiliation: Department of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Tao
authors.initials: T
authors.lastname: Liang
authors.affiliation: Department of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Tairan
authors.initials: T
authors.lastname: Qin
authors.affiliation: Department of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Huanli
authors.initials: H
authors.lastname: Xie
authors.affiliation: Department of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Youhou
authors.initials: Y
authors.lastname: Kang
authors.affiliation: Department of Neuroscience, Howard Hughes Medical Institute, University of Wisconsin-Madison, Madison, WI.
authors.forename: Edwin R
authors.initials: ER
authors.lastname: Chapman
authors.affiliation: Department of Medicine, University of Toronto, Toronto, ON, Canada Department of Physiology, University of Toronto, Toronto, ON, Canada herbert.gaisano@utoronto.ca.
authors.forename: Herbert Y
authors.initials: HY
authors.lastname: Gaisano
mesh_terms: D000328:Adult; D000368:Aged; D000818:Animals; D002118:Calcium / Q000378:metabolism; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D005260:Female; D052216:Glucagon-Like Peptide 1 / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D006801:Humans; D007093:Imidazoles / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D050417:Insulin-Secreting Cells / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D050765:Qa-SNARE Proteins / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D050600:SNARE Proteins / Q000378:metabolism; D022142:Secretory Vesicles / Q000378:metabolism*; D050857:Synaptotagmins / Q000378:metabolism*
chemical_list: D007093:Imidazoles; D007328:Insulin; D050765:Qa-SNARE Proteins; D050600:SNARE Proteins; D050857:Synaptotagmins; C031938:calmidazolium; D052216:Glucagon-Like Peptide 1; D005947:Glucose; D002118:Calcium