Aims/hypothesis: C57BL/6J mice exhibit impaired glucose tolerance. The aims of this study were to map the genetic loci underlying this phenotype, to further characterise the physiological defects and to identify candidate genes. Methods: Glucose tolerance was measured in an intraperitoneal glucose tolerance test and genetic determinants mapped in an F2 intercross. Insulin sensitivity was measured by injecting insulin and following glucose disposal from the plasma. To measure beta cell function, insulin secretion and electrophysiological studies were carried out on isolated islets. Candidate genes were investigated by sequencing and quantitative RNA analysis. Results: C57BL/6J mice showed normal insulin sensitivity and impaired insulin secretion. In beta cells, glucose did not stimulate a rise in intracellular calcium and its ability to close K ATP channels was impaired. We identified three genetic loci responsible for the impaired glucose tolerance. Nicotinamide nucleotide transhydrogenase (Nnt) lies within one locus and is a nuclear-encoded mitochondrial proton pump. Expression of Nnt is more than sevenfold and fivefold lower respectively in C57BL/6J liver and islets. There is a missense mutation in exon 1 and a multi-exon deletion in the C57BL/6J gene. Glucokinase lies within the Gluchos2 locus and shows reduced enzyme activity in liver. Conclusions/interpretation: The C57BL/6J mouse strain exhibits plasma glucose intolerance reminiscent of human type 2 diabetes. Our data suggest a defect in beta cell glucose metabolism that results in reduced electrical activity and insulin secretion. We have identified three loci that are responsible for the inherited impaired plasma glucose tolerance and identified a novel candidate gene for contribution to glucose intolerance through reduced beta cell activity.

Peroxisome proliferator activated receptor gamma 2 (PPARg2) is the nutritionally regulated isoform of PPARg. Ablation of PPARg2 in the ob/ob background, PPARg2−/− Lepob/Lepob (POKO mouse), resulted in decreased fat mass, severe insulin resistance, β-cell failure, and dyslipidaemia. Our results indicate that the PPARg2 isoform plays an important role, mediating adipose tissue expansion in response to positive energy balance. Lipidomic analyses suggest that PPARg2 plays an important antilipotoxic role when induced ectopically in liver and muscle by facilitating deposition of fat as relatively harmless triacylglycerol species and thus preventing accumulation of reactive lipid species. Our data also indicate that PPARg2 may be required for the β-cell hypertrophic adaptive response to insulin resistance. In summary, the PPARg2 isoform prevents lipotoxicity by (a) promoting adipose tissue expansion, (b) increasing the lipid-buffering capacity of peripheral organs, and (c) facilitating the adaptive proliferative response of β-cells to insulin resistance.

Diabetes is characterized by hyperglycaemia due to impaired insulin secretion and aberrant glucagon secretion resulting from changes in pancreatic islet cell function and/or mass. The extent to which hyperglycaemia per se underlies these alterations remains poorly understood. Here we show that β-cell-specific expression of a human activating KATP channel mutation in adult mice leads to rapid diabetes and marked alterations in islet morphology, ultrastructure and gene expression. Chronic hyperglycaemia is associated with a dramatic reduction in insulin-positive cells and an increase in glucagon-positive cells in islets, without alterations in cell turnover. Furthermore, some β-cells begin expressing glucagon, whilst retaining many β-cell characteristics. Hyperglycaemia, rather than KATP channel activation, underlies these changes, as they are prevented by insulin therapy and fully reversed by sulphonylureas. Our data suggest that many changes in islet structure and function associated with diabetes are attributable to hyperglycaemia alone and are reversed when blood glucose is normalized.

ATP-sensitive potassium (K ATP ) channels conduct potassium ions across cell membranes and thereby couple cellular energy metabolism to membrane electrical activity. Here, we report the heterologous expression and purification of a functionally active K ATP channel complex composed of pore-forming Kir6.2 and regulatory SUR1 subunits, and determination of its structure at 18 Å resolution by single-particle electron microscopy. The purified channel shows ATP-ase activity similar to that of ATPbinding cassette proteins related to SUR1, and supports Rb þ fluxes when reconstituted into liposomes. It has a compact structure, with four SUR1 subunits embracing a central Kir6.2 tetramer in both transmembrane and cytosolic domains. A cleft between adjacent SUR1s provides a route by which ATP may access its binding site on Kir6.2. The nucleotide-binding domains of adjacent SUR1 appear to interact, and form a large docking platform for cytosolic proteins. The structure, in combination with molecular modelling, suggests how SUR1 interacts with Kir6.2.

Heterozygous activating mutations in the KCNJ11 gene encoding the pore-forming Kir6.2 subunit of the pancreatic beta cell K(ATP) channel are the most common cause of permanent neonatal diabetes (PNDM). Patients with PNDM due to a heterozygous activating mutation in the ABCC8 gene encoding the SUR1 regulatory subunit of the K(ATP) channel have recently been reported. We studied a cohort of 59 patients with permanent diabetes who received a diagnosis before 6 mo of age and who did not have a KCNJ11 mutation. ABCC8 gene mutations were identified in 16 of 59 patients and included 8 patients with heterozygous de novo mutations. A recessive mode of inheritance was observed in eight patients with homozygous, mosaic, or compound heterozygous mutations. Functional studies of selected mutations showed a reduced response to ATP consistent with an activating mutation that results in reduced insulin secretion. A novel mutational mechanism was observed in which a heterozygous activating mutation resulted in PNDM only when a second, loss-of-function mutation was also present.

Neonatal diabetes is a rare monogenic form of diabetes that usually presents within the first six months of life. It is commonly caused by gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of the plasmalemmal ATP-sensitive K + (K ATP ) channel. To better understand this disease, we generated a mouse expressing a Kir6.2 mutation (V59M) that causes neonatal diabetes in humans and we used Cre-lox technology to express the mutation specifically in pancreatic β cells. These β-V59M mice developed severe diabetes soon after birth, and by 5 weeks of age, blood glucose levels were markedly increased and insulin was undetectable. Islets isolated from β-V59M mice secreted substantially less insulin and showed a smaller increase in intracellular calcium in response to glucose. This was due to a reduced sensitivity of K ATP channels in pancreatic β cells to inhibition by ATP or glucose. In contrast, the sulfonylurea tolbutamide, a specific blocker of K ATP channels, closed K ATP channels, elevated intracellular calcium levels, and stimulated insulin release in β-V59M β cells, indicating that events downstream of K ATP channel closure remained intact. Expression of the V59M Kir6.2 mutation in pancreatic β cells alone is thus sufficient to recapitulate the neonatal diabetes observed in humans. β-V59M islets also displayed a reduced percentage of β cells, abnormal morphology, lower insulin content, and decreased expression of Kir6.2, SUR1, and insulin mRNA. All these changes are expected to contribute to the diabetes of β-V59M mice. Their cause requires further investigation.

The C57BL/6J mouse displays glucose intolerance and reduced insulin secretion. QTL mapping identified Nicotinamide Nucleotide Transhydrogenase (Nnt), a nuclear-encoded mitochondrial protein thought to be involved in free radical detoxification, as a candidate gene. To investigate its functional role, we used siRNA to knock down Nnt in insulin-secreting MIN6 cells. This produced a dramatic reduction in insulin secretion and the rise in [Ca2+]i evoked by glucose, but not tolbutamide. We identified two ENU-induced point mutations in Nnt (N68K, G745D). Nnt mutant mice were glucose intolerant and secreted less insulin during a glucose tolerance test. Isolated islets showed impaired insulin secretion in response to glucose, but not to tolbutamide, and glucose failed to enhance ATP levels. Glucose utilization and production of reactive oxygen species were increased in Nnt beta cells. We hypothesize that Nnt mutations/deletion uncouple beta cell mitochondrial metabolism leading to less ATP production, enhanced KATP channel activity, and consequently impaired insulin secretion.

Insulin secretion from pancreatic ␤ cells is stimulated by glucose metabolism. However, the relative importance of metabolizing glucose via mitochondrial oxidative phosphorylation versus glycolysis for insulin secretion remains unclear. von Hippel-Lindau (VHL) tumor suppressor protein, pVHL, negatively regulates hypoxia-inducible factor HIF1␣, a transcription factor implicated in promoting a glycolytic form of metabolism. Here we report a central role for the pVHL-HIF1␣ pathway in the control of ␤-cell glucose utilization, insulin secretion, and glucose homeostasis. Conditional inactivation of Vhlh in ␤ cells promoted a diversion of glucose away from mitochondria into lactate production, causing cells to produce high levels of glycolytically derived ATP and to secrete elevated levels of insulin at low glucose concentrations. Vhlh-deficient mice exhibited diminished glucose-stimulated changes in cytoplasmic Ca 2+ concentration, electrical activity, and insulin secretion, which culminate in impaired systemic glucose tolerance. Importantly, combined deletion of Vhlh and Hif1␣ rescued these phenotypes, implying that they are the result of HIF1␣ activation. Together, these results identify pVHL and HIF1␣ as key regulators of insulin secretion from pancreatic ␤ cells. They further suggest that changes in the metabolic strategy of glucose metabolism in ␤ cells have profound effects on whole-body glucose homeostasis.[Keywords: HIF; VHL; glucose intolerance; islet; pancreas] Supplemental material is available at http://www.genesdev.org. Received July 14, 2008; revised version accepted September 5, 2008. During adulthood, cell type-specific growth that exceeds the normal physiological constraints is a common feature of adaptive processes of tissues to changes in metabolic homeostasis and underlies the development of many human diseases, including cancer, heart disease, and diabetes (De Boer et al. 2003;Bouwens and Rooman 2005). Adaptive cell mass expansion, whether neoplastic or nonneoplastic, creates a requirement for compensatory neovascularization to supply oxygen, metabolic substances, and growth/survival factors to the growing tissue (Marti 2005). Therefore, adaptive cell growth responses are generally accompanied, at least initially, by relative states of hypoxia as a result of a mismatch between oxygen demand caused by tissue expansion and oxygen supply provided by the vasculature. An immediate consequence of decreased tissue oxygen availability is that cells shift cellular fuel metabolism from mitochondrial respiration to glycolysis and activate an angiogenic program to increase oxygen delivery in order to overcome the imbalance between tissue mass and vascularization (Semenza 2001;Brahimi-Horn et al. 2007). In this way, tissue function is supported and further mass expansion can occur.At the molecular level, the central regulators of the cellular response to low-oxygen availability are the hypoxia-inducible transcription factors (HIF). HIF are heterodimeric transcription factors composed of HIF1␣, HIF2␣, or HI...