Candidate Gene Association Study in Type 2 Diabetes Indicates a Role for Genes Involved in β-Cell Function as Well as Insulin Action

Type 2 diabetes is an increasingly common, serious metabolic disorder with a substantial inherited component. It is characterised by defects in both insulin secretion and action. Progress in identification of specific genetic variants predisposing to the disease has been limited. To complement ongoing positional cloning efforts, we have undertaken a large-scale candidate gene association study. We examined 152 SNPs in 71 candidate genes for association with diabetes status and related phenotypes in 2,134 Caucasians in a case-control study and an independent quantitative trait (QT) cohort in the United Kingdom. Polymorphisms in five of 15 genes (33%) encoding molecules known to primarily influence pancreatic β-cell function—ABCC8 (sulphonylurea receptor), KCNJ11 (KIR6.2), SLC2A2 (GLUT2), HNF4A (HNF4α), and INS (insulin)—significantly altered disease risk, and in three genes, the risk allele, haplotype, or both had a biologically consistent effect on a relevant physiological trait in the QT study. We examined 35 genes predicted to have their major influence on insulin action, and three (9%)—INSR, PIK3R1, and SOS1—showed significant associations with diabetes. These results confirm the genetic complexity of Type 2 diabetes and provide evidence that common variants in genes influencing pancreatic β-cell function may make a significant contribution to the inherited component of this disease. This study additionally demonstrates that the systematic examination of panels of biological candidate genes in large, well-characterised populations can be an effective complement to positional cloning approaches. The absence of large single-gene effects and the detection of multiple small effects accentuate the need for the study of larger populations in order to reliably identify the size of effect we now expect for complex diseases.     

β-cell Smad2 null mice have improved β-cell function and are protected from diet-induced hyperglycemia

Understanding signaling pathways that regulate pancreatic β-cell function to produce, store, and release insulin, as well as pathways that control β-cell proliferation, is vital to find new treatments for diabetes mellitus. Transforming growth factor-beta (TGF-β) signaling is involved in a broad range of β-cell functions. The canonical TGF-β signaling pathway functions through intracellular smads, including smad2 and smad3, to regulate cell development, proliferation, differentiation, and function in many organs. Here, we demonstrate the role of TGF-β/smad2 signaling in regulating mature β-cell proliferation and function using β-cell-specific smad2 null mutant mice. β-cell-specific smad2-deficient mice exhibited improved glucose clearance as demonstrated by glucose tolerance testing, enhanced in vivo and ex vivo glucose-stimulated insulin secretion, and increased β-cell mass and proliferation. Furthermore, when these mice were fed a high-fat diet to induce hyperglycemia, they again showed improved glucose tolerance, insulin secretion, and insulin sensitivity. In addition, ex vivo analysis of smad2-deficient islets showed that they displayed increased glucose-stimulated insulin secretion and upregulation of genes involved in insulin synthesis and insulin secretion. Thus, we conclude that smad2 could represent an attractive therapeutic target for type 2 diabetes mellitus.     

β-Cell Expansion for Therapeutic Compensation of Insulin Resistance in Type 2 Diabetes

Insulin resistance is the primary cause of type 2 diabetes. However, if compensated by increased insulin production, insulin resistance by itself does not lead to overt disease. Type 2 diabetes develops when this compensation is insufficient, due to defects in β-cell function and in regulation of the β-cell mass. β-Cell transplantation, as well as approaches that replenish or preserve the endogenous β-cell mass, may facilitate the treatment of type 2 diabetes in patients requiring exogenous insulin.     

Genetic Deficiency of Glycogen Synthase Kinase-3β Corrects Diabetes in Mouse Models of Insulin Resistance

Despite treatment with agents that enhance β-cell function and insulin action, reduction in β-cell mass is relentless in patients with insulin resistance and type 2 diabetes mellitus. Insulin resistance is characterized by impaired signaling through the insulin/insulin receptor/insulin receptor substrate/PI-3K/Akt pathway, leading to elevation of negatively regulated substrates such as glycogen synthase kinase-3β (Gsk-3β). When elevated, this enzyme has antiproliferative and proapoptotic properties. In these studies, we designed experiments to determine the contribution of Gsk-3β to regulation of β-cell mass in two mouse models of insulin resistance. Mice lacking one allele of the insulin receptor (Ir^+/−) exhibit insulin resistance and a doubling of β-cell mass. Crossing these mice with those having haploinsufficiency for Gsk-3β (Gsk-3β^+/−) reduced insulin resistance by augmenting whole-body glucose disposal, and significantly reduced β-cell mass. In the second model, mice missing two alleles of the insulin receptor substrate 2 (Irs2^−/−), like the Ir^+/− mice, are insulin resistant, but develop profound β-cell loss, resulting in early diabetes. We found that islets from these mice had a 4-fold elevation of Gsk-3β activity associated with a marked reduction of β-cell proliferation and increased apoptosis. Irs2^−/− mice crossed with Gsk-3β^+/− mice preserved β-cell mass by reversing the negative effects on proliferation and apoptosis, preventing onset of diabetes. Previous studies had shown that islets of Irs2^−/− mice had increased cyclin-dependent kinase inhibitor p27^kip1 that was limiting for β-cell replication, and reduced Pdx1 levels associated with increased cell death. Preservation of β-cell mass in Gsk-3β^+/−Irs2^−/− mice was accompanied by suppressed p27^kip1 levels and increased Pdx1 levels. To separate peripheral versus β-cell–specific effects of reduction of Gsk3β activity on preservation of β-cell mass, mice homozygous for a floxed Gsk-3β allele (Gsk-3^F/F) were then crossed with rat insulin promoter-Cre (RIP-Cre) mice to produce β-cell–specific knockout of Gsk-3β (βGsk-3β^−/−). Like Gsk-3β^+/− mice, βGsk-3β^−/− mice also prevented the diabetes of the Irs2^−/− mice. The results of these studies now define a new, negatively regulated substrate of the insulin signaling pathway specifically within β-cells that when elevated, can impair replication and increase apoptosis, resulting in loss of β-cells and diabetes. These results thus form the rationale for developing agents to inhibit this enzyme in obese insulin-resistant individuals to preserve β-cells and prevent diabetes onset.     

Insulin-like Growth Factor 2 Overexpression Induces β-Cell Dysfunction and Increases Beta-cell Susceptibility to Damage

The human insulin-like growth factor 2 (IGF2) and insulin genes are located within the same genomic region. Although human genomic studies have demonstrated associations between diabetes and the insulin/IGF2 locus or the IGF2 mRNA-binding protein 2 (IGF2BP2), the role of IGF2 in diabetes pathogenesis is not fully understood. We previously described that transgenic mice overexpressing IGF2 specifically in β-cells (Tg-IGF2) develop a pre-diabetic state. Here, we characterized the effects of IGF2 on β-cell functionality. Overexpression of IGF2 led to β-cell dedifferentiation and endoplasmic reticulum stress causing islet dysfunction in vivo. Both adenovirus-mediated overexpression of IGF2 and treatment of adult wild-type islets with recombinant IGF2 in vitro further confirmed the direct implication of IGF2 on β-cell dysfunction. Treatment of Tg-IGF2 mice with subdiabetogenic doses of streptozotocin or crossing these mice with a transgenic model of islet lymphocytic infiltration promoted the development of overt diabetes, suggesting that IGF2 makes islets more susceptible to β-cell damage and immune attack. These results indicate that increased local levels of IGF2 in pancreatic islets may predispose to the onset of diabetes. This study unravels an unprecedented role of IGF2 on β-cells function.     

Black Ginseng Extract Counteracts Streptozotocin-Induced Diabetes in Mice

Black ginseng, a new type of processed ginseng that has a unique ginsenoside profile, has been shown to display potent pharmacological activities in in vitro and in vivo models. Although red ginseng is considered beneficial for the prevention of diabetes, the relationship between black ginseng and diabetes is unknown. Therefore, this study was designed to evaluate the anti-diabetic potential of black ginseng extract (BGE) in streptozotocin (STZ)-induced insulin-deficient diabetic mice, in comparison with red ginseng extract (RGE). HPLC analyses showed that BGE has a different ginsenoside composition to RGE; BGE contains Rg5 and compound k as the major ginsenosides. BGE at 200 mg/kg reduced hyperglycemia, increased the insulin/glucose ratio and improved islet architecture and β-cell function in STZ-treated mice. The inhibition of β-cell apoptosis by BGE was associated with suppression of the cytokine—induced nuclear factor–κB—mediated signaling pathway in the pancreas. Moreover, these anti-diabetic effects of BGE were more potent than those of RGE. Collectively, our data indicate that BGE, in part by suppressing cytokine—induced apoptotic signaling, protects β-cells from oxidative injury and counteracts diabetes in mice.     

Candidate gene association study in type 2 diabetes indicates a role for genes involved in beta-cell function as well as insulin action

Type 2 diabetes is an increasingly common, serious metabolic disorder with a substantial inherited component. It is characterised by defects in both insulin secretion and action. Progress in identification of specific genetic variants predisposing to the disease has been limited. To complement ongoing positional cloning efforts, we have undertaken a large-scale candidate gene association study. We examined 152 SNPs in 71 candidate genes for association with diabetes status and related phenotypes in 2,134 Caucasians in a case-control study and an independent quantitative trait (QT) cohort in the United Kingdom. Polymorphisms in five of 15 genes (33%) encoding molecules known to primarily influence pancreatic beta-cell function-ABCC8 (sulphonylurea receptor), KCNJ11 (KIR6.2), SLC2A2 (GLUT2), HNF4A (HNF4alpha), and INS (insulin)-significantly altered disease risk, and in three genes, the risk allele, haplotype, or both had a biologically consistent effect on a relevant physiological trait in the QT study. We examined 35 genes predicted to have their major influence on insulin action, and three (9%)-INSR, PIK3R1, and SOS1-showed significant associations with diabetes. These results confirm the genetic complexity of Type 2 diabetes and provide evidence that common variants in genes influencing pancreatic beta-cell function may make a significant contribution to the inherited component of this disease. This study additionally demonstrates that the systematic examination of panels of biological candidate genes in large, well-characterised populations can be an effective complement to positional cloning approaches. The absence of large single-gene effects and the detection of multiple small effects accentuate the need for the study of larger populations in order to reliably identify the size of effect we now expect for complex diseases.     

A Syntenic Cross Species Aneuploidy Genetic Screen Links RCAN1 Expression to β-Cell Mitochondrial Dysfunction in Type 2 Diabetes

Type 2 diabetes (T2D) is a complex metabolic disease associated with obesity, insulin resistance and hypoinsulinemia due to pancreatic β-cell dysfunction. Reduced mitochondrial function is thought to be central to β-cell dysfunction. Mitochondrial dysfunction and reduced insulin secretion are also observed in β-cells of humans with the most common human genetic disorder, Down syndrome (DS, Trisomy 21). To identify regions of chromosome 21 that may be associated with perturbed glucose homeostasis we profiled the glycaemic status of different DS mouse models. The Ts65Dn and Dp16 DS mouse lines were hyperglycemic, while Tc1 and Ts1Rhr mice were not, providing us with a region of chromosome 21 containing genes that cause hyperglycemia. We then examined whether any of these genes were upregulated in a set of ~5,000 gene expression changes we had identified in a large gene expression analysis of human T2D β-cells. This approach produced a single gene, RCAN1, as a candidate gene linking hyperglycemia and functional changes in T2D β-cells. Further investigations demonstrated that RCAN1 methylation is reduced in human T2D islets at multiple sites, correlating with increased expression. RCAN1 protein expression was also increased in db/db mouse islets and in human and mouse islets exposed to high glucose. Mice overexpressing RCAN1 had reduced in vivo glucose-stimulated insulin secretion and their β-cells displayed mitochondrial dysfunction including hyperpolarised membrane potential, reduced oxidative phosphorylation and low ATP production. This lack of β-cell ATP had functional consequences by negatively affecting both glucose-stimulated membrane depolarisation and ATP-dependent insulin granule exocytosis. Thus, from amongst the myriad of gene expression changes occurring in T2D β-cells where we had little knowledge of which changes cause β-cell dysfunction, we applied a trisomy 21 screening approach which linked RCAN1 to β-cell mitochondrial dysfunction in T2D.     

Deletion of the Mitochondrial Flavoprotein Apoptosis Inducing Factor (AIF) Induces β-Cell Apoptosis and Impairs β-Cell Mass

Background

Apoptosis is a hallmark of β-cell death in both type 1 and type 2 diabetes mellitus. Understanding how apoptosis contributes to β-cell turnover may lead to strategies to prevent progression of diabetes. A key mediator of apoptosis, mitochondrial function, and cell survival is apoptosis inducing factor (AIF). In the present study, we investigated the role of AIF on β-cell mass and survival using the Harlequin (Hq) mutant mice, which are hypomorphic for AIF.

Methodology/Principal Findings

Immunohistochemical evaluation of pancreata from Hq mutant mice displayed much smaller islets compared to wild-type mice (WT). Analysis of β-cell mass in these mice revealed a greater than 4-fold reduction in β-cell mass together with an 8-fold increase in β-cell apoptosis. Analysis of cell cycle dynamics, using BrdU pulse as a marker for cells in S-phase, did not detect significant differences in the frequency of β-cells in S-phase. In contrast, double staining for phosphorylated Histone H3 and insulin showed a 3-fold increase in β-cells in the G2 phase in Hq mutant mice, but no differences in M-phase compared to WT mice. This suggests that the β-cells from Hq mutant mice are arrested in the G2 phase and are unlikely to complete the cell cycle. β-cells from Hq mutant mice display increased sensitivity to hydrogen peroxide-induced apoptosis, which was confirmed in human islets in which AIF was depleted by siRNA. AIF deficiency had no effect on glucose stimulated insulin secretion, but the impaired effect of hydrogen peroxide on β-cell function was potentiated.

Conclusions/Significance

Our results indicate that AIF is essential for maintaining β-cell mass and for oxidative stress response. A decrease in the oxidative phosphorylation capacity may counteract the development of diabetes, despite its deleterious effects on β-cell survival.     

Pancreatic β-Cell Death due to Pdx-1 Deficiency Requires Multi-BH Domain Protein Bax but Not Bak

Diabetes develops in Pdx1-haploinsufficient mice due to an increase in β-cell death leading to reduced β-cell mass and decreased insulin secretion. Knockdown of Pdx1 gene expression in mouse MIN6 insulinoma cells induced apoptotic cell death with an increase in Bax activation and knockdown of Bax reduced apoptotic β-cell death. In Pdx1 haploinsufficient mice, Bax ablation in β-cells increased β-cell mass, decreased the number of TUNEL positive cells and improved glucose tolerance after glucose challenge. These changes were not observed with Bak ablation in Pdx1-haploinsufficient mice. These results suggest that Bax mediates β-cell apoptosis in Pdx1-deficient diabetes.     

Hexarelin Protects Rodent Pancreatic Β-Cells Function from Cytotoxic Effects of Streptozotocin Involving Mitochondrial Signalling Pathways In Vivo and In Vitro

Mitochondrial functions are crucial for pancreatic β-cell survival and glucose-induced insulin secretion. Hexarelin (Hex) is a synthetic small peptide ghrelin analogue, which has been shown to protect cardiomyocytes from the ischemia-reperfusion process. In this study, we used in vitro and in vivo models of streptozotocin (STZ)-induced β-cell damage to study the protective effect of Hex and the associated mechanisms. We found that STZ produced a cytotoxic effect in a dose- and time-dependent manner in MIN6 cells (a mouse β-cell line). Hex (1.0 μM) decreased the STZ-induced damage in β-cells. Rhodamine 123 assay and superoxide DHE production assay revealed that Hex ameliorated STZ-induced mitochondrial damage and excessive superoxide activity in β-cells. In addition, Hex significantly reduced STZ-induced expression of cleaved Caspases-3, Caspases-9 and the ratio of pro-apoptotic protein Bax to anti-apoptotic protein Bcl-2 in MIN6 cells. We further examined the in vivo effect of Hex in a rat model of type 1 diabetes induced by STZ injection. Hex ameliorated STZ-induced decrease in plasma insulin and protected the structure of islets from STZ-induced disruption. Hex also ameliorated STZ-induced expression of cleaved Caspase-9 and the Bax in β-cells. In conclusion, our data indicate that Hex is able to protects β-cell mass from STZ-caused cytotoxic effects involving mitochondrial pathways in vitro and in vivo. Hex may serve as a potential protective agent for the management of diabetes.     

Moderate Hypoxia Induces β-Cell Dysfunction with HIF-1–Independent Gene Expression Changes

Pancreatic β-cell failure is central to the development and progression of type 2 diabetes. We recently demonstrated that β-cells become hypoxic under high glucose conditions due to increased oxygen consumption and that the pancreatic islets of diabetic mice but not those of control mice are moderately hypoxic. However, the impact of moderate hypoxia on β-cell number and function is unknown. In the present study, moderate hypoxia induced a hypoxic response in MIN6 cells, as evidenced by increased levels of HIF-1α protein and target genes. Under these conditions, a selective downregulation of Mafa, Pdx1, Slc2a2, Ndufa5, Kcnj11, Ins1, Wfs1, Foxa2, and Neurod1, which play important roles in β-cells, was also observed in both MIN6 cells and isolated pancreatic islets. Consistent with the altered expression of these genes, abnormal insulin secretion was detected in hypoxic MIN6 cells. Most of the hypoxia-induced gene downregulation in MIN6 cells was not affected by the suppression of HIF-1α, suggesting a HIF-1–independent mechanism. Moderate hypoxia also induced apoptosis in MIN6 cells. These results suggest that hypoxia is a novel stressor of β-cells and that hypoxic stress may play a role in the deterioration of β-cell function.     

Novel Zn2+ Modulated GPR39 Receptor Agonists Do Not Drive Acute Insulin Secretion in Rodents

Type 2 diabetes (T2D) occurs when there is insufficient insulin release to control blood glucose, due to insulin resistance and impaired β-cell function. The GPR39 receptor is expressed in metabolic tissues including pancreatic β-cells and has been proposed as a T2D target. Specifically, GPR39 agonists might improve β-cell function leading to more adequate and sustained insulin release and glucose control. The present study aimed to test the hypothesis that GPR39 agonism would improve glucose stimulated insulin secretion in vivo. A high throughput screen, followed by a medicinal chemistry program, identified three novel potent Zn²⁺ modulated GPR39 agonists. These agonists were evaluated in acute rodent glucose tolerance tests. The results showed a lack of glucose lowering and insulinotropic effects not only in lean mice, but also in diet-induced obese (DIO) mice and Zucker fatty rats. It is concluded that Zn²⁺ modulated GPR39 agonists do not acutely stimulate insulin release in rodents.     

Chronic Ethanol Consumption-induced Pancreatic β-Cell Dysfunction and Apoptosis through Glucokinase Nitration and Its Down-regulation

Chronic ethanol consumption is known as an independent risk factor for type 2 diabetes, which is characterized by impaired glucose homeostasis and insulin resistance; however, there is a great deal of controversy concerning the relationships between alcohol consumption and the development of type 2 diabetes. We investigated the effects of chronic ethanol consumption on pancreatic β-cell dysfunction and whether generated peroxynitrite participates in the impaired glucose homeostasis. Here we show that chronic ethanol feeding decreases the ability of pancreatic β-cells to mediate insulin secretion and ATP production in coordination with the decrease of glucokinase, Glut2, and insulin expression. Specific blockade of ATF3 using siRNA or C-terminally deleted ATF3(ΔC) attenuated ethanol-induced pancreatic β-cell apoptosis or dysfunction and restored the down-regulation of glucokinase (GCK), insulin, and pancreatic duodenal homeobox-1 induced by ethanol. GCK inactivation and down-regulation were predominantly mediated by ethanol metabolism-generated peroxynitrite, which were suppressed by the peroxynitrite scavengers N^γ-monomethyl-l-arginine, uric acid, and deferoxamine but not by the S-nitrosylation inhibitor DTT, indicating that tyrosine nitration is the predominant modification associated with GCK down-regulation and inactivation rather than S-nitrosylation of cysteine. Tyrosine nitration of GCK prevented its association with pBad, and GCK translocation into the mitochondria results in subsequent proteasomal degradation of GCK following ubiquitination. This study identified a novel and efficient pathway by which chronic ethanol consumption may induce GCK down-regulation and inactivation by inducing tyrosine nitration of GCK, resulting in pancreatic β-cell apoptosis and dysfunction. Peroxynitrite-induced ATF3 may also serve as a potent upstream regulator of GCK down-regulation and β-cell apoptosis.     

GAD2 on chromosome 10p12 is a candidate gene for human obesity

The gene GAD2 encoding the glutamic acid decarboxylase enzyme (GAD65) is a positional candidate gene for obesity on Chromosome 10p11–12, a susceptibility locus for morbid obesity in four independent ethnic populations. GAD65 catalyzes the formation of γ-aminobutyric acid (GABA), which interacts with neuropeptide Y in the paraventricular nucleus to contribute to stimulate food intake. A case-control study (575 morbidly obese and 646 control subjects) analyzing GAD2 variants identified both a protective haplotype, including the most frequent alleles of single nucleotide polymorphisms (SNPs) +61450 C>A and +83897 T>A (OR = 0.81, 95% CI [0.681–0.972], p = 0.0049) and an at-risk SNP (−243 A>G) for morbid obesity (OR = 1.3, 95% CI [1.053–1.585], p = 0.014). Furthermore, familial-based analyses confirmed the association with the obesity of SNP +61450 C>A and +83897 T>A haplotype (χ² = 7.637, p = 0.02). In the murine insulinoma cell line βTC3, the G at-risk allele of SNP −243 A>G increased six times GAD2 promoter activity (p < 0.0001) and induced a 6-fold higher affinity for nuclear extracts. The −243 A>G SNP was associated with higher hunger scores (p = 0.007) and disinhibition scores (p = 0.028), as assessed by the Stunkard Three-Factor Eating Questionnaire. As GAD2 is highly expressed in pancreatic β cells, we analyzed GAD65 antibody level as a marker of β-cell activity and of insulin secretion. In the control group, −243 A>G, +61450 C>A, and +83897 T>A SNPs were associated with lower GAD65 autoantibody levels (p values of 0.003, 0.047, and 0.006, respectively). SNP +83897 T>A was associated with lower fasting insulin and insulin secretion, as assessed by the HOMA-B% homeostasis model of β-cell function (p = 0.009 and 0.01, respectively). These data support the hypothesis of the orexigenic effect of GABA in humans and of a contribution of genes involved in GABA metabolism in the modulation of food intake and in the development of morbid obesity.     

Non-Invasive Bioluminescence Imaging of β-Cell Function in Obese-Hyperglycemic [ob/ob] Mice

Background

Type 2 diabetes results from failure of the β-cells to compensate for increased insulin demand due to abnormal levels of metabolic factors. The ob/ob(lep-/-) mouse has been extensively studied as an animal model of type 2 diabetes. Previous studies have shown a correlation between β-cell function and bioluminescent imaging in lean genetically engineered mice. The ability to noninvasively monitor β-cell function in ob/ob mice could provide new information on β-cell regulation in type 2 diabetes.

Methods

To create the B6 Albino ob/ob MIP-luc mice (ob/ob-luc), the ob/ob mouse was crossed with the CD1 MIP-luc mouse. All mice were backcrossed over multiple generations to ensure the genetic background of the transgenic mice was over 96% similar to the background of the original ob/ob mouse. Animal weight, blood glucose levels, insulin in plasma, and in vivo bioluminescence (BLI) were monitored weekly or biweekly for up to 70 weeks of age. BL imaging was performed using IVIS Spectrum (Perkin Elmer) and calculated by integrating the bioluminescence signal between 5 and 10 min after i.v. injection of D-luciferin. Insulin immunohistochemistry determined islet beta cell count and insulin secretion assay determined islet insulin function.

Results

There were significant increases in BLI and insulin levels as the ob/ob-luc mice aged while glucose levels gradually decreased. Ob/ob-luc were sacrificed at different time points to determine ex vivo BLI, islet function and total β-cell numbers using a cell counting training algorithm developed for the Vectra image analysis system (Perkin Elmer). The number of β-cells increased as the mice aged and all three ex vivo measurements correlated with BLI.

Conclusions

The ob/ob-luc mice can serve as a model of metabolic stress, similar to human type 2 diabetes using BLI as a surrogate marker for β-cell function.     

Transgenic Overexpression of Active Calcineurin in β-Cells Results in Decreased β-Cell Mass and Hyperglycemia

Background

Glucose modulates β-cell mass and function through an initial depolarization and Ca²⁺ influx, which then triggers a number of growth regulating signaling pathways. One of the most important downstream effectors in Ca²⁺ signaling is the calcium/Calmodulin activated serine threonine phosphatase, calcineurin. Recent evidence suggests that calcineurin/NFAT is essential for β-cell proliferation, and that in its absence loss of β-cells results in diabetes. We hypothesized that in contrast, activation of calcineurin might result in expansion of β-cell mass and resistance to diabetes.

Methodology/Principal Findings

To determine the role of activation of calcineurin signaling in the regulation of pancreatic β-cell mass and proliferation, we created mice that expressed a constitutively active form of calcineurin under the insulin gene promoter (caCn^RIP). To our surprise, these mice exhibited glucose intolerance. In vitro studies demonstrated that while the second phase of Insulin secretion is enhanced, the overall insulin secretory response was conserved. Islet morphometric studies demonstrated decreased β-cell mass suggesting that this was a major component responsible for altered Insulin secretion and glucose intolerance in caCn^RIP mice. The reduced β-cell mass was accompanied by decreased proliferation and enhanced apoptosis.

Conclusions

Our studies identify calcineurin as an important factor in controlling glucose homeostasis and indicate that chronic depolarization leading to increased calcineurin activity may contribute, along with other genetic and environmental factors, to β-cell dysfunction and diabetes.