title: Over-expression of Slc30a8/ZnT8 selectively in the mouse α cell impairs glucagon release and responses to hypoglycemia.
abstract: BACKGROUND
  The human SLC30A8 gene encodes the secretory granule-localised zinc transporter ZnT8 whose expression is chiefly restricted to the endocrine pancreas. Single nucleotide polymorphisms (SNPs) in the human SLC30A8 gene have been associated, through genome-wide studies, with altered type 2 diabetes risk. In addition to a role in the control of insulin release, recent studies involving targeted gene ablation from the pancreatic α cell (Solomou et al., J Biol Chem 290(35):21432-42) have also implicated ZnT8 in the control of glucagon release. Up to now, however, the possibility that increased levels of the transporter in these cells may impact glucagon secretion has not been explored.
  
  
  METHODS
  Here, we use a recently-developed reverse tetracyline transactivator promoter-regulated ZnT8 transgene to drive the over-expression of human ZnT8 selectively in the α cell in adult mice. Glucose homeostasis and glucagon secretion were subsequently assessed both in vivo during hypoglycemic clamps and from isolated islets in vitro.
  
  
  RESULTS
  Doxyclin-dependent human ZnT8 mRNA expression was apparent in both isolated islets and in fluorescence-activated cell sorting- (FACS) purified α cells. Examined at 12 weeks of age, intraperitoneal glucose (1 g/kg) tolerance was unchanged in transgenic mice versus wild-type littermates (n = 8-10 mice/genotype, p > 0.05) and sensitivity to intraperitoneal insulin (0.75U/kg) was similarly unaltered in transgenic animals. In contrast, under hyperinsulinemic-hypoglycemic clamp, a ~45 % (p < 0.001) reduction in glucose infusion rate was apparent, and glucagon release was significantly (~40 %, p < 0.01) impaired, in transgenic mice. Correspondingly, examined in vitro, glucagon secretion was significantly reduced (~30 %, p < 0.05) from transgenic versus control islets at low, stimulatory glucose concentrations (1 mM, p < 0.05) but not at high glucose (17 mM) glucose (p > 0.05). Over-expression of ZnT8 in glucagonoma-derived αTC1-9 cells increased granule free Zn(2+) concentrations consistent with a role for Zn(2+) in this compartment in the action of ZnT8 on glucagon secretion.
  
  
  CONCLUSIONS
  Increased ZnT8 expression, and a likely increase in intragranular free Zn(2+) concentration, is deleterious in pancreatic α cells for stimulated glucagon release. These data provide further evidence that type 2 diabetes-associated polymorphisms in the SLC30A8/ZnT8 gene may act in part via alterations in glucagon release and suggest that ZnT8 activation may restrict glucagon release in some settings.
authors.affiliation: Section of Cell Biology and Functional Genomics, Division of Diabetes Endocrinology and Metabolism, Department of Medicine, Imperial College London, Imperial Centre for Translational and Experimental Medicine, Hammersmith Hospital, du Cane Road, London, W12 0NN UK.
authors.forename: Antonia
authors.initials: A
authors.lastname: Solomou
authors.affiliation: University Paris Diderot-Paris 7-Unit of Functional and Adaptive Biology (BFA) UMR 8251 CNRS, Paris, France.
authors.forename: Erwann
authors.initials: E
authors.lastname: Philippe
authors.affiliation: Section of Cell Biology and Functional Genomics, Division of Diabetes Endocrinology and Metabolism, Department of Medicine, Imperial College London, Imperial Centre for Translational and Experimental Medicine, Hammersmith Hospital, du Cane Road, London, W12 0NN UK.
authors.forename: Pauline
authors.initials: P
authors.lastname: Chabosseau
authors.affiliation: University Paris Diderot-Paris 7-Unit of Functional and Adaptive Biology (BFA) UMR 8251 CNRS, Paris, France.
authors.forename: Stephanie
authors.initials: S
authors.lastname: Migrenne-Li
authors.affiliation: CNRS UMR 5248, Chimie et Biologie des Membranes et Nano-objets, Université de Bordeaux, F-33615 Pessac, France.
authors.forename: Julien
authors.initials: J
authors.lastname: Gaitan
authors.affiliation: CNRS UMR 5248, Chimie et Biologie des Membranes et Nano-objets, Université de Bordeaux, F-33615 Pessac, France.
authors.forename: Jochen
authors.initials: J
authors.lastname: Lang
authors.affiliation: University Paris Diderot-Paris 7-Unit of Functional and Adaptive Biology (BFA) UMR 8251 CNRS, Paris, France.
authors.forename: Christophe
authors.initials: C
authors.lastname: Magnan
authors.affiliation: Section of Cell Biology and Functional Genomics, Division of Diabetes Endocrinology and Metabolism, Department of Medicine, Imperial College London, Imperial Centre for Translational and Experimental Medicine, Hammersmith Hospital, du Cane Road, London, W12 0NN UK.
authors.forename: Guy A
authors.identifier: 0000-0001-6360-0343
authors.initials: GA
authors.lastname: Rutter