OBJECTIVEZinc ions are essential for the formation of hexameric insulin and hormone crystallization. A nonsynonymous single nucleotide polymorphism rs13266634 in the SLC30A8 gene, encoding the secretory granule zinc transporter ZnT8, is associated with type 2 diabetes. We describe the effects of deleting the ZnT8 gene in mice and explore the action of the at-risk allele.RESEARCH DESIGN AND METHODSSlc30a8 null mice were generated and backcrossed at least twice onto a C57BL/6J background. Glucose and insulin tolerance were measured by intraperitoneal injection or euglycemic clamp, respectively. Insulin secretion, electrophysiology, imaging, and the generation of adenoviruses encoding the low- (W325) or elevated- (R325) risk ZnT8 alleles were undertaken using standard protocols.RESULTSZnT8−/− mice displayed age-, sex-, and diet-dependent abnormalities in glucose tolerance, insulin secretion, and body weight. Islets isolated from null mice had reduced granule zinc content and showed age-dependent changes in granule morphology, with markedly fewer dense cores but more rod-like crystals. Glucose-stimulated insulin secretion, granule fusion, and insulin crystal dissolution, assessed by total internal reflection fluorescence microscopy, were unchanged or enhanced in ZnT8−/− islets. Insulin processing was normal. Molecular modeling revealed that residue-325 was located at the interface between ZnT8 monomers. Correspondingly, the R325 variant displayed lower apparent Zn2+ transport activity than W325 ZnT8 by fluorescence-based assay.CONCLUSIONSZnT8 is required for normal insulin crystallization and insulin release in vivo but not, remarkably, in vitro. Defects in the former processes in carriers of the R allele may increase type 2 diabetes risks.

OBJECTIVEThe inability of pancreatic β-cells to appropriately respond to glucose and secrete insulin are primary defects associated with β-cell failure in type 2 diabetes. Mitochondrial dysfunction has been implicated as a key factor in the development of type 2 diabetes; however, a link between mitochondrial dysfunction and defective insulin secretion is unclear.RESEARCH DESIGN AND METHODSWe investigated the changes in islet mitochondrial function and morphology during progression from insulin resistance (3 weeks old), immediately before hyperglycemia (5 weeks old), and after diabetes onset (10 weeks old) in transgenic MKR mice compared with controls. The molecular and protein changes at 10 weeks were determined using microarray and iTRAQ proteomic screens.RESULTSAt 3 weeks, MKR mice were hyperinsulinemic but normoglycemic and β-cells showed negligible mitochondrial or morphological changes. At 5 weeks, MKR islets displayed abrogated hyperpolarization of mitochondrial membrane potential (ΔΨm), reduced mitochondrial Ca2+ uptake, slightly enlarged mitochondria, and reduced glucose-stimulated insulin secretion. By 10 weeks, MKR mice were hyperglycemic and hyperinsulinemic and β-cells contained swollen mitochondria with disordered cristae. β-Cells displayed impaired stimulus-secretion coupling including reduced hyperpolarization of ΔΨm, impaired Ca2+-signaling, and reduced glucose-stimulated ATP/ADP and insulin release. Furthermore, decreased cytochrome c oxidase–dependent oxygen consumption and signs of oxidative stress were observed in diabetic islets. Protein profiling of diabetic islets revealed that 36 mitochondrial proteins were differentially expressed, including inner membrane proteins of the electron transport chain.CONCLUSIONSWe provide novel evidence for a critical role of defective mitochondrial oxidative phosphorylation and morphology in the pathology of insulin resistance–induced β-cell failure.

The role of the mitochondrial permeability transition pore (MPTP) in apoptosis of nucleated cells is well documented. In contrast, the role of MPTP in apoptosis of anucleated platelets is largely unknown. The aim of this study was to elucidate the contribution of MPTP in the control of different manifestations of platelet apoptosis by analyzing the effect of cyclosporin A (CsA), a potent inhibitor of MPTP formation. Using flow cytometry, we studied the effect of pretreatment of platelets with CsA on apoptotic responses in human platelets stimulated with calcium ionophore A23187. We found that CsA inhibited A23187-stimulated platelet apoptosis, completely preventing (i) depolarization of mitochondrial inner membrane potential (DCm), (ii) activation of cytosolic apoptosis executioner caspase-3, (iii) platelet shrinkage, and (iv) fragmentation of platelets to microparticles, but (v) only partially (E25%), inhibiting phosphatidylserine (PS) exposure on the platelet surface. This study shows that MPTP formation is upstream of DCm depolarization, caspase-3 activation, platelet shrinkage and microparticle formation, and stringently controls these apoptotic events in A23187-stimulated platelets but is less involved in PS externalization. These data also indicate that CsA may rescue platelets from apoptosis, preventing caspase-3 activation and inhibiting the terminal cellular manifestations of platelet apoptosis, such as platelet shrinkage and degradation to microparticles. Furthermore, the results suggest a novel potentially useful application of CsA as an inhibitor of platelet demise through apoptosis in thrombocytopenias associated with enhanced platelet apoptosis.

Aims/hypothesis: The role of gamma-aminobutyric acid (GABA) and A-type GABA receptors (GABA A Rs) in modulating islet endocrine function has been actively investigated since the identification of GABA and GABA A Rs in the pancreatic islets. However, the reported effects of GABA A R activation on insulin secretion from islet beta cells have been controversial. Methods: This study examined the hypothesis that the effect of GABA on beta cell insulin secretion is dependent on glucose concentration. Results: Perforated patchclamp recordings in INS-1 cells demonstrated that GABA, at concentrations ranging from 1 to 1,000 μmol/l, induced a transmembrane current (I GABA ) which was sensitive to the GABA A R antagonist bicuculline. The current-voltage relationship revealed that I GABA reversed at −42±2.2 mV, independently of glucose concentration. Nevertheless, the glucose concentration critically controlled the membrane potential (V M ), i.e., at low glucose (0 or 2.8 mmol/l) the endogenous V M of INS-1 cells was below the I GABA reversal potential and at high glucose (16.7 or 28 mmol/l), the endogenous V M of INS-1 cells was above the I GABA reversal potential. Therefore, GABA dose-dependently induced membrane depolarisation at a low glucose concentration, but hyperpolarisation at a high glucose concentration. Consistent with electrophysiological findings, insulin secretion assays demonstrated that at 2.8 mmol/l glucose, GABA increased insulin secretion in a dose-dependent fashion (p<0.05, n=7). This enhancement was blocked by bicuculline (p<0.05, n=4). In contrast, in the presence of 28 mmol/l glucose, GABA suppressed the secretion of insulin (p<0.05, n=5). Conclusions/interpretation: These findings indicate that activation of GABA A Rs in beta cells regulates insulin secretion in concert with changes in glucose levels.

OBJECTIVE-Prolonged elevation of glucose can adversely affect ␤-cell function. In vitro studies have linked glucoseinduced ␤-cell dysfunction to oxidative stress; however, whether oxidative stress plays a role in vivo is unclear. Therefore, our objective was to investigate the role of oxidative stress in an in vivo model of glucose-induced ␤-cell dysfunction.RESEARCH DESIGN AND METHODS-Wistar rats were infused intravenously with glucose for 48 h to achieve 20 mmol/l hyperglycemia with/without co-infusion of one of the following antioxidants: taurine (2-amino ethanesulfonic acid) (TAU), an aldehyde scavenger; N-acetylcysteine (NAC), a precursor of glutathione; or tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (TPO), a superoxide dismutase mimetic. This was followed by islet isolation or hyperglycemic clamp.RESULTS-A 48-h glucose infusion decreased glucose-stimulated insulin secretion (GSIS) and elevated reactive oxygen species (ROS), total superoxide, and mitochondrial superoxide in freshly isolated islets. TPO prevented the increase in total and mitochondrial superoxide and the ␤-cell dysfunction induced by high glucose. However, TAU and NAC, despite completely normalizing H 2 DCF-DA (dihydro-dichlorofluorescein diacetate)-measured ROS, did not prevent the increase in superoxide and the decrease in ␤-cell function induced by high glucose. TPO but not TAU also prevented ␤-cell dysfunction induced by less extreme hyperglycemia (15 mmol/l) for a longer period of time (96 h). To further investigate whether TPO is effective in vivo, a hyperglycemic clamp was performed. Similar to the findings in isolated islets, prolonged glucose elevation (20 mmol/l for 48 h) decreased ␤-cell function as assessed by the disposition index (insulin secretion adjusted for insulin sensitivity), and coinfusion of TPO with glucose completely restored ␤-cell function.CONCLUSIONS-These findings implicate superoxide generation in ␤-cell dysfunction induced by prolonged hyperglycemia.