Role of the type II diabetes-associated gene SLC30A8 in the pancreatic α-cell

The SLC30A8 locus encodes the Zn2+ transporter ZnT8, whose expression is largely restricted to α- and β-cells, endocrine cells of the pancreatic islet. Genome-wide association studies have revealed >90 loci associated with Type 2 diabetes. The first such study identified a specific single nucleotide polymorphism that results in an amino acid substitution and a transporter with reduced activity. This risk variant is associated with increased Type 2 diabetes risk. More recently, rare loss-of-function variants were found to be protective. Here, we aimed to investigate the role of SLC30A8/ZnT8 in the regulation of glucagon secretion. ZnT8 was selectively deleted in the α-cell by crossing mice bearing floxed alleles at exon 1 with mice carrying a Cre recombinase transgene under the control of the preproglucagon promoter. Additionally, these mice were crossed to Rosa26 RFP mice for identification of α-cells. Fluorescence-activated sorting of RFP+ cells revealed that recombination at the RFP locus occurred in ~30% of α-cells. ZnT8 was deleted in ~50% of these labelled cells achieving a total of ~15% deletion in the whole α-cell population. Glucose tolerance and insulin sensitivity were normal, though during hypoglycaemic clamps female KO mice required lower glucose infusion rates and had enhanced glucagon secretion compared to control mice. Similarly, isolated islets from KO mice released significantly more glucagon at 1mM glucose. In addition, inducible overexpression of ZnT8 led to a reciprocal decrease of glucagon secretion from isolated islets at 1mM glucose. Hypoglycaemic clamps confirmed the finding of lowered glucagon release following ZnT8 overexpression. Cytosolic Ca2+ responses to low glucose were comparable in WT and KO cells. Both cytoplasmic and granular free Zn2+ levels were significantly reduced in the KO α-cells compared to control cells yet there were no changes in the gene expression levels of other ZnT famlily members. Thus it appears that ZnT8 is important in the α-cell for appropriate responses to hypoglycaemia and Zn2+ homeostasis.Open Acces

Intracellular zinc in insulin secretion and action:A determinant of diabetes risk?

Zinc is an important micronutrient, essential in the diet to avoid a variety of conditions associated with malnutrition such as diarrhoea and alopecia. Lowered circulating levels of zinc are also found in diabetes mellitus, a condition which affects one in twelve of the adult population and whose treatments consume approximately 10 % of healthcare budgets. Zn2+ions are essential for a huge range of cellular functions and, in the specialised pancreatic β-cell, for the storage of insulin within the secretory granule. Correspondingly, genetic variants in theSLC30A8gene, which encodes the diabetes-associated granule-resident Zn2+transporter ZnT8, are associated with an altered risk of type 2 diabetes. Here, we focus on (i) recent advances in measuring free zinc concentrations dynamically in subcellular compartments, and (ii) studies dissecting the role of intracellular zinc in the control of glucose homeostasisin vitroandin vivo.We discuss the effects on insulin secretion and action of deleting or over-expressingSlc30a8highly selectively in the pancreatic β-cell, and the role of zinc in insulin signalling. While modulated by genetic variability, healthy levels of dietary zinc, and hence normal cellular zinc homeostasis, are likely to play an important role in the proper release and action of insulin to maintain glucose homeostasis and lower diabetes risk.</jats:p

Hypoxia lowers SLC30A8/ZnT8 expression and free cytosolic Zn2+ in pancreatic beta cells

AIMS/HYPOTHESIS: Hypoxic damage complicates islet isolation for transplantation and may contribute to beta cell failure in type 2 diabetes. Polymorphisms in the SLC30A8 gene, encoding the secretory granule zinc transporter 8 (ZnT8), influence type 2 diabetes risk, conceivably by modulating cytosolic Zn(2+) levels. We have therefore explored the role of ZnT8 and cytosolic Zn(2+) in the response to hypoxia of pancreatic islet cells. METHODS: Human, mouse or rat islets were isolated and exposed to varying O2 tensions. Cytosolic free zinc was measured using the adenovirally expressed recombinant targeted zinc probe eCALWY4. Gene expression was measured using quantitative (q)RT-PCR, western (immuno-) blotting or immunocytochemistry. Beta cells were identified by insulin immunoreactivity. RESULTS: Deprivation of O2 (1% vs 5% or 21%) for 24 h lowered free cytosolic Zn(2+) concentrations by ~40% (p 12 weeks) Slc30a8 null mice vs controls, but not younger animals. CONCLUSIONS/INTERPRETATION: The response of pancreatic beta cells to hypoxia is characterised by decreased SLC30A8 expression and lowered cytosolic Zn(2+) concentrations. The dependence on ZnT8 of hypoxia-induced changes in cell survival may contribute to the actions of SLC30A8 variants on diabetes risk in humans