Diet-induced glucose intolerance in mice with decreased beta-cell ATP-sensitive K+ channels

A loss-of-function mutation in KCNJ11 causing sulfonylurea-sensitive diabetes in early adult life

AIMS/HYPOTHESIS

The ATP-sensitive potassium (KATP) channel couples beta cell electrical activity to glucose-stimulated insulin secretion. Loss-of-function mutations in either the pore-forming (inwardly rectifying potassium channel 6.2 [Kir6.2], encoded by KCNJ11) or regulatory (sulfonylurea receptor 1, encoded by ABCC8) subunits result in congenital hyperinsulinism, whereas gain-of-function mutations cause neonatal diabetes. Here, we report a novel loss-of-function mutation (Ser118Leu) in the pore helix of Kir6.2 paradoxically associated with sulfonylurea-sensitive diabetes that presents in early adult life.

METHODS

A 31-year-old woman was diagnosed with mild hyperglycaemia during an employee screen. After three pregnancies, during which she was diagnosed with gestational diabetes, the patient continued to show elevated blood glucose and was treated with glibenclamide (known as glyburide in the USA and Canada) and metformin. Genetic testing identified a heterozygous mutation (S118L) in the KCNJ11 gene. Neither parent was known to have diabetes. We investigated the functional properties and membrane trafficking of mutant and wild-type KATP channels in Xenopus oocytes and in HEK-293T cells, using patch-clamp, two-electrode voltage-clamp and surface expression assays.

RESULTS

Functional analysis showed no changes in the ATP sensitivity or metabolic regulation of the mutant channel. However, the Kir6.2-S118L mutation impaired surface expression of the KATP channel by 40%, categorising this as a loss-of-function mutation.

CONCLUSIONS/INTERPRETATION

Our data support the increasing evidence that individuals with mild loss-of-function KATP channel mutations may develop insulin deficiency in early adulthood and even frank diabetes in middle age. In this case, the patient may have had hyperinsulinism that escaped detection in early life. Our results support the importance of functional analysis of KATP channel mutations in cases of atypical diabetes.

Genetics and Natural History of Non-pancreatectomized Patients With Congenital Hyperinsulinism Due to Variants in ABCC8

CONTEXT

Patients with congenital hyperinsulinism due to ABCC8 variants generally present severe hypoglycemia and those who do not respond to medical treatment typically undergo pancreatectomy. Few data exist on the natural history of non-pancreatectomized patients.

OBJECTIVE

This work aims to describe the genetic characteristics and natural history in a cohort of non-pancreatectomized patients with congenital hyperinsulinism due to variants in the ABCC8 gene.

METHODS

Ambispective study of patients with congenital hyperinsulinism with pathogenic or likely pathogenic variants in ABCC8 treated in the last 48 years and who were not pancreatectomized. Continuous glucose monitoring (CGM) has been periodically performed in all patients since 2003. An oral glucose tolerance test was performed if hyperglycemia was detected in the CGM.

RESULTS

Eighteen non-pancreatectomized patients with ABCC8 variants were included. Seven (38.9%) patients were heterozygous, 8 (44.4%) compound heterozygous, 2 (11.1%) homozygous, and 1 patient carried 2 variants with incomplete familial segregation studies. Seventeen patients were followed up and 12 (70.6%) of them evolved to spontaneous resolution (median age 6.0 ± 4 years; range, 1-14). Five of these 12 patients (41.7%) subsequently progressed to diabetes with insufficient insulin secretion. Evolution to diabetes was more frequent in patients with biallelic variants in the ABCC8 gene.

CONCLUSION

The high remission rate observed in our cohort makes conservative medical treatment a reliable strategy for the management of patients with congenital hyperinsulinism due to ABCC8 variants. In addition, a periodic follow-up of glucose metabolism after remission is recommended, as a significant proportion of patients evolved to impaired glucose tolerance or diabetes (biphasic phenotype).

Personalized Therapeutics for KATP-Dependent Pathologies

Ubiquitously expressed throughout the body, ATP-sensitive potassium (K_ATP) channels couple cellular metabolism to electrical activity in multiple tissues; their unique assembly as four Kir6 pore-forming subunits and four sulfonylurea receptor (SUR) subunits has resulted in a large armory of selective channel opener and inhibitor drugs. The spectrum of monogenic pathologies that result from gain- or loss-of-function mutations in these channels, and the potential for therapeutic correction of these pathologies, is now clear. However, while available drugs can be effective treatments for specific pathologies, cross-reactivity with the other Kir6 or SUR subfamily members can result in drug-induced versions of each pathology and may limit therapeutic usefulness. This review discusses the background to K_ATP channel physiology, pathology, and pharmacology and considers the potential for more specific or effective therapeutic agents.

Genome-edited zebrafish model of ABCC8 loss-of-function disease

ATP-sensitive potassium channel (KATP)gain- (GOF) and loss-of-function (LOF) mutations underlie human neonatal diabetes mellitus (NDM) and hyperinsulinism (HI), respectively. While transgenic mice expressing incomplete KATP LOF do reiterate mild hyperinsulinism, KATP knockout animals do not exhibit persistent hyperinsulinism. We have shown that islet excitability and glucose homeostasis are regulated by identical KATP channels in zebrafish. SUR1 truncation mutation (K499X) was introduced into the abcc8 gene to explore the possibility of using zebrafish for modeling human HI. Patch-clamp analysis confirmed the complete absence of channel activity in β-cells from K499X (SUR1-/-) fish. No difference in random blood glucose was detected in heterozygous SUR1+/- fish nor in homozygous SUR1-/- fish, mimicking findings in SUR1 knockout mice. Mutant fish did, however, demonstrate impaired glucose tolerance, similar to partial LOF mouse models. In paralleling features of mammalian diabetes and hyperinsulinism resulting from equivalent LOF mutations, these gene-edited animals provide valid zebrafish models of KATP -dependent pancreatic diseases.

ATP-Sensitive Potassium Channels in Hyperinsulinism and Type 2 Diabetes: Inconvenient Paradox or New Paradigm?

Secretion of insulin from pancreatic β-cells is complex, but physiological glucose-dependent secretion is dominated by electrical activity, in turn controlled by ATP-sensitive potassium (KATP) channel activity. Accordingly, loss-of-function mutations of the KATP channel Kir6.2 (KCNJ11) or SUR1 (ABCC8) subunit increase electrical excitability and secretion, resulting in congenital hyperinsulinism (CHI), whereas gain-of-function mutations cause underexcitability and undersecretion, resulting in neonatal diabetes mellitus (NDM). Thus, diazoxide, which activates KATP channels, and sulfonylureas, which inhibit KATP channels, have dramatically improved therapies for CHI and NDM, respectively. However, key findings do not fit within this simple paradigm: mice with complete absence of β-cell KATP activity are not hyperinsulinemic; instead, they are paradoxically glucose intolerant and prone to diabetes, as are older human CHI patients. Critically, despite these advances, there has been little insight into any role of KATP channel activity changes in the development of type 2 diabetes (T2D). Intriguingly, the CHI progression from hypersecretion to undersecretion actually mirrors the classical response to insulin resistance in the progression of T2D. In seeking to explain the progression of CHI, multiple lines of evidence lead us to propose that underlying mechanisms are also similar and that development of T2D may involve loss of KATP activity.

High-fat diet prevents the development of autoimmune diabetes in NOD mice

AIMS

Type 1 diabetes (T1D) has a strong genetic predisposition and requires an environmental trigger to initiate the beta-cell autoimmune destruction. The rate of childhood obesity has risen in parallel to the proportion of T1D, suggesting high-fat diet (HFD)/obesity as potential environmental triggers for autoimmune diabetes. To explore this, non-obese diabetic (NOD) mice were subjected to HFD and monitored for the development of diabetes, insulitis and beta-cell stress.

MATERIALS AND METHODS

Four-week-old female NOD mice were placed on HFD (HFD-NOD) or standard chow-diet. Blood glucose was monitored weekly up to 40 weeks of age, and glucose- and insulin-tolerance tests performed at 4, 10 and 15 weeks. Pancreata and islets were analysed for insulin secretion, beta-cell mass, inflammation, insulitis and endoplasmic reticulum stress markers. Immune cell levels were measured in islets and spleens. Stool microbiome was analysed at age 4, 8 and 25 weeks.

RESULTS

At early ages, HFD-NOD mice showed a significant increase in body weight, glucose intolerance and insulin resistance; but paradoxically, they were protected from developing diabetes. This was accompanied by increased insulin secretion and beta-cell mass, decreased insulitis, increased splenic T-regulatory cells and altered stool microbiome.

CONCLUSIONS

This study shows that HFD protects NOD mice from autoimmune diabetes and preserves beta-cell mass and function through alterations in gut microbiome, increased T-regulatory cells and decreased insulitis. Further studies into the exact mechanism of HFD-mediated prevention of diabetes in NOD mice could potentially lead to interventions to prevent or delay T1D development in humans.

Preferential Gq signaling in diabetes: an electrical switch in incretin action and in diabetes progression?

Patients with type 2 diabetes (T2D) fail to secrete insulin in response to increased glucose levels that occur with eating. Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are two incretins secreted from gastrointestinal cells that amplify insulin secretion when glucose is high. In this issue of the JCI, Oduori et al. explore the role of ATP-sensitive K+ (KATP) channels in maintaining glucose homeostasis. In persistently depolarized β cells from KATP channel knockout (KO) mice, the researchers revealed a shift in G protein signaling from the Gs family to the Gq family. This shift explains why GLP-1, which signals via Gq, but not GIP, which signals preferentially via Gs, can effectively potentiate secretion in islets from the KATP channel-deficient mice and in other models of KATP deficiency, including diabetic KK-Ay mice. Their results provide one explanation for differential insulinotropic potential of incretins in human T2D and point to a potentially unifying model for T2D progression itself.

Mechanisms controlling pancreatic islet cell function in insulin secretion

Metabolic homeostasis in mammals is tightly regulated by the complementary actions of insulin and glucagon. The secretion of these hormones from pancreatic β-cells and α-cells, respectively, is controlled by metabolic, endocrine, and paracrine regulatory mechanisms and is essential for the control of blood levels of glucose. The deregulation of these mechanisms leads to various pathologies, most notably type 2 diabetes, which is driven by the combined lesions of impaired insulin action and a loss of the normal insulin secretion response to glucose. Glucose stimulates insulin secretion from β-cells in a bi-modal fashion, and new insights about the underlying mechanisms, particularly relating to the second or amplifying phase of this secretory response, have been recently gained. Other recent work highlights the importance of α-cell-produced proglucagon-derived peptides, incretin hormones from the gastrointestinal tract and other dietary components, including certain amino acids and fatty acids, in priming and potentiation of the β-cell glucose response. These advances provide a new perspective for the understanding of the β-cell failure that triggers type 2 diabetes.

Excitotoxicity and Overnutrition Additively Impair Metabolic Function and Identity of Pancreatic β-Cells

A sustained increase in intracellular Ca²⁺ concentration (referred to hereafter as excitotoxicity), brought on by chronic metabolic stress, may contribute to pancreatic β-cell failure. To determine the additive effects of excitotoxicity and overnutrition on β-cell function and gene expression, we analyzed the impact of a high-fat diet (HFD) on Abcc8 knockout mice. Excitotoxicity caused β-cells to be more susceptible to HFD-induced impairment of glucose homeostasis, and these effects were mitigated by verapamil, a Ca²⁺ channel blocker. Excitotoxicity, overnutrition, and the combination of both stresses caused similar but distinct alterations in the β-cell transcriptome, including additive increases in genes associated with mitochondrial energy metabolism, fatty acid β-oxidation, and mitochondrial biogenesis and their key regulator Ppargc1a Overnutrition worsened excitotoxicity-induced mitochondrial dysfunction, increasing metabolic inflexibility and mitochondrial damage. In addition, excitotoxicity and overnutrition, individually and together, impaired both β-cell function and identity by reducing expression of genes important for insulin secretion, cell polarity, cell junction, cilia, cytoskeleton, vesicular trafficking, and regulation of β-cell epigenetic and transcriptional program. Sex had an impact on all β-cell responses, with male animals exhibiting greater metabolic stress-induced impairments than females. Together, these findings indicate that a sustained increase in intracellular Ca²⁺, by altering mitochondrial function and impairing β-cell identity, augments overnutrition-induced β-cell failure.

Alterations in pancreatic islet cell function in response to small bowel resection

After 50% proximal small bowel resection (SBR) in mice, we have demonstrated hepatic steatosis, impaired glucose metabolism without insulin resistance, and increased pancreatic islet area. We sought to determine the consequences of SBR on pancreatic β-cell morphology, proliferation, and expression of a key regulatory hormone, glucagon-like peptide-1 (GLP-1). C57BL/6 mice underwent 50% SBR or sham operation. At 10 wk, pancreatic insulin content and secretion was measured by ELISA. Immunohistochemistry was performed to determine structural alterations in pancreatic α-and β-cells. Western blot analysis was used to measure GLP-1R expression, and immunoassay was used to measure plasma insulin and GLP-1. Experiments were repeated by administering a GLP-1 agonist (exendin-4) to a cohort of mice following SBR. After SBR, there was pancreatic islet hypertrophy and impaired glucose tolerance. The proportion of α and β cells was not grossly altered. Whole pancreas and pancreatic islet insulin content was not significantly different; however, SBR mice demonstrated decreased insulin secretion in both static incubation and islet perfusion experiments. The expression of pancreatic GLP-1R was decreased approximately twofold after SBR, compared with sham and serum GLP-1, was decreased. These metabolic derangements were mitigated after administration of the GLP-1 agonist. Following massive SBR, there is significant hypertrophy of pancreatic islet cells with morphologically intact α- and β-cells. Significantly reduced pancreatic insulin release in both static and dynamic conditions demonstrate a perturbed second phase of insulin secretion. GLP-1 is a key mediator of this amplification pathway. Decreased expression of serum GLP-1 and pancreatic GLP-1R in face of no change in insulin content presents a novel pathway for enteropancreatic glucose regulation following SBR.NEW & NOTEWORTHY Metabolic changes occur following intestinal resection; however, the effects on pancreatic function are unknown. Prior studies have demonstrated that glucagon-like protein-1 (GLP-1) signaling is a crucial player in the improved insulin sensitivity after bariatric surgery. In this study, we explore the effect of massive small bowel resection on gut hormone physiology and provide novel insights into the enteropancreatic axis.

High-fat-diet-induced remission of diabetes in a subset of KATP -GOF insulin-secretory-deficient mice

AIMS

To examine the effects of a high-fat-diet (HFD) on monogenic neonatal diabetes, without the confounding effects of compensatory hyperinsulinaemia.

METHODS

Mice expressing K_ATP channel gain-of-function (K_ATP -GOF) mutations, which models human neonatal diabetes, were fed an HFD.

RESULTS

Surprisingly, K_ATP -GOF mice exhibited resistance to HFD-induced obesity, accompanied by markedly divergent blood glucose control, with some K_ATP -GOF mice showing persistent diabetes (K_ATP -GOF-non-remitter [NR] mice) and others showing remission of diabetes (K_ATP -GOF-remitter [R] mice). Compared with the severely diabetic and insulin-resistant K_ATP -GOF-NR mice, HFD-fed K_ATP -GOF-R mice had lower blood glucose, improved insulin sensitivity, and increased circulating plasma insulin and glucagon-like peptide-1 concentrations. Strikingly, while HFD-fed K_ATP -GOF-NR mice showed increased food intake and decreased physical activity, reduced whole body fat mass and increased plasma lipids, K_ATP -GOF-R mice showed similar features to those of control littermates. Importantly, K_ATP -GOF-R mice had restored insulin content and β-cell mass compared with the marked loss observed in both HFD-fed K_ATP -GOF-NR and chow-fed K_ATP -GOF mice.

CONCLUSION

Together, our results suggest that restriction of dietary carbohydrates and caloric replacement by fat can induce metabolic changes that are beneficial in reducing glucotoxicity and secondary consequences of diabetes in a mouse model of insulin-secretory deficiency.

ABCC8 R1420H Loss-of-Function Variant in a Southwest American Indian Community: Association With Increased Birth Weight and Doubled Risk of Type 2 Diabetes

Missense variants in KCNJ11 and ABCC8, which encode the KIR6.2 and SUR1 subunits of the β-cell KATP channel, have previously been implicated in type 2 diabetes, neonatal diabetes, and hyperinsulinemic hypoglycemia of infancy (HHI). To determine whether variation in these genes affects risk for type 2 diabetes or increased birth weight as a consequence of fetal hyperinsulinemia in Pima Indians, missense and common noncoding variants were analyzed in individuals living in the Gila River Indian Community. A R1420H variant in SUR1 (ABCC8) was identified in 3.3% of the population (N = 7,710). R1420H carriers had higher mean birth weights and a twofold increased risk for type 2 diabetes with a 7-year earlier onset age despite being leaner than noncarriers. One individual homozygous for R1420H was identified; retrospective review of his medical records was consistent with HHI and a diagnosis of diabetes at age 3.5 years. In vitro studies showed that the R1420H substitution decreases KATP channel activity. Identification of this loss-of-function variant in ABCC8 with a carrier frequency of 3.3% affects clinical care as homozygous inheritance and potential HHI will occur in 1/3,600 births in this American Indian population.

Mitochondrial pyruvate carrier 2 hypomorphism in mice leads to defects in glucose-stimulated insulin secretion

Carrier-facilitated pyruvate transport across the inner mitochondrial membrane plays an essential role in anabolic and catabolic intermediary metabolism. Mitochondrial pyruvate carrier 2 (Mpc2) is believed to be a component of the complex that facilitates mitochondrial pyruvate import. Complete MPC2 deficiency resulted in embryonic lethality in mice. However, a second mouse line expressing an N-terminal truncated MPC2 protein (Mpc2(Δ16)) was viable but exhibited a reduced capacity for mitochondrial pyruvate oxidation. Metabolic studies demonstrated exaggerated blood lactate concentrations after pyruvate, glucose, or insulin challenge in Mpc2(Δ16) mice. Additionally, compared with wild-type controls, Mpc2(Δ16) mice exhibited normal insulin sensitivity but elevated blood glucose after bolus pyruvate or glucose injection. This was attributable to reduced glucose-stimulated insulin secretion and was corrected by sulfonylurea KATP channel inhibitor administration. Collectively, these data are consistent with a role for MPC2 in mitochondrial pyruvate import and suggest that Mpc2 deficiency results in defective pancreatic β cell glucose sensing.

A mouse model of human hyperinsulinism produced by the E1506K mutation in the sulphonylurea receptor SUR1

Loss-of-function mutations in the KATP channel genes KCNJ11 and ABCC8 cause neonatal hyperinsulinism in humans. Dominantly inherited mutations cause less severe disease, which may progress to glucose intolerance and diabetes in later life (e.g., SUR1-E1506K). We generated a mouse expressing SUR1-E1506K in place of SUR1. KATP channel inhibition by MgATP was enhanced in both homozygous (homE1506K) and heterozygous (hetE1506K) mutant mice, due to impaired channel activation by MgADP. As a consequence, mutant β-cells showed less on-cell KATP channel activity and fired action potentials in glucose-free solution. HomE1506K mice exhibited enhanced insulin secretion and lower fasting blood glucose within 8 weeks of birth, but reduced insulin secretion and impaired glucose tolerance at 6 months of age. These changes correlated with a lower insulin content; unlike wild-type or hetE1506K mice, insulin content did not increase with age in homE1506K mice. There was no difference in the number and size of islets or β-cells in the three types of mice, or evidence of β-cell proliferation. We conclude that the gradual development of glucose intolerance in patients with the SUR1-E1506K mutation might, as in the mouse model, result from impaired insulin secretion due a failure of insulin content to increase with age.

Genetic modulation of islet β-cell iPLA₂β expression provides evidence for its impact on β-cell apoptosis and autophagy

β-cell apoptosis is a significant contributor to β-cell dysfunction in diabetes and ER stress is among the factors that contributes to β-cell death. We previously identified that the Ca²⁺-independent phospholipase A₂β (iPLA₂β), which in islets is localized in β-cells, participates in ER stress-induced β-cell apoptosis. Here, direct assessment of iPLA₂β role was made using β-cell-specific iPLA₂β overexpressing (RIP-iPLA₂β-Tg) and globally iPLA₂β-deficient (iPLA₂β-KO) mice. Islets from Tg, but not KO, express higher islet iPLA₂β and neutral sphingomyelinase, decrease in sphingomyelins, and increase in ceramides, relative to WT group. ER stress induces iPLA₂β, ER stress factors, loss of mitochondrial membrane potential (∆Ψ), caspase-3 activation, and β-cell apoptosis in the WT and these are all amplified in the Tg group. Surprisingly, β-cells apoptosis while reduced in the KO is higher than in the WT group. This, however, was not accompanied by greater caspase-3 activation but with larger loss of ∆Ψ, suggesting that iPLA₂β deficiency impacts mitochondrial membrane integrity and causes apoptosis by a caspase-independent manner. Further, autophagy, as reflected by LC3-II accumulation, is increased in Tg and decreased in KO, relative to WT. Our findings suggest that (1) iPLA₂β impacts upstream (UPR) and downstream (ceramide generation and mitochondrial) pathways in β-cells and (2) both over- or under-expression of iPLA₂β is deleterious to the β-cells. Further, we present for the first time evidence for potential regulation of autophagy by iPLA₂β in islet β-cells. These findings support the hypothesis that iPLA₂β induction under stress, as in diabetes, is a key component to amplifying β-cell death processes.

Block of Kv1.7 potassium currents increases glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) relies on repetitive, electrical spiking activity of the beta cell membrane. Cyclic activation of voltage-gated potassium channels (K_v) generates an outward, ‘delayed rectifier’ potassium current, which drives the repolarizing phase of each spike and modulates insulin release. Although several K_v channels are expressed in pancreatic islets, their individual contributions to GSIS remain incompletely understood. We take advantage of a naturally occurring cone-snail peptide toxin, Conkunitzin-S1 (Conk-S1), which selectively blocks K_v1.7 channels to provide an intrinsically limited, finely graded control of total beta cell delayed rectifier current and hence of GSIS. Conk-S1 increases GSIS in isolated rat islets, likely by reducing K_v1.7-mediated delayed rectifier currents in beta cells, which yields increases in action potential firing and cytoplasmic free calcium. In rats, Conk-S1 increases glucose-dependent insulin secretion without decreasing basal glucose. Thus, we conclude that K_v1.7 contributes to the membrane-repolarizing current of beta cells during GSIS and that block of this specific component of beta cell K_v current offers a potential strategy for enhancing GSIS with minimal risk of hypoglycaemia during metabolic disorders such as Type 2 diabetes.

Congenital hyperinsulinism and glucose hypersensitivity in homozygous and heterozygous carriers of Kir6.2 (KCNJ11) mutation V290M mutation: K(ATP) channel inactivation mechanism and clinical management

OBJECTIVE

The ATP-sensitive K(+) channel (K(ATP)) controls insulin secretion from the islet. Gain- or loss-of-function mutations in channel subunits underlie human neonatal diabetes and congenital hyperinsulinism (HI), respectively. In this study, we sought to identify the mechanistic basis of K(ATP)-induced HI in two probands and to characterize the clinical course.

RESEARCH DESIGN AND METHODS

We analyzed HI in two probands and characterized the course of clinical treatment in each, as well as properties of mutant K(ATP) channels expressed in COSm6 cells using Rb efflux and patch-clamp methods.

RESULTS

We identified mutation V290M in the pore-forming Kir6.2 subunit in each proband. In vitro expression in COSm6 cells supports the mutation resulting in an inactivating phenotype, which leads to significantly reduced activity in intact cells when expressed homomerically, and to a lesser extent when expressed heteromerically with wild-type subunits. In one heterozygous proband, a fluoro-DOPA scan revealed a causal focal lesion, indicating uniparental disomy with loss of heterozygosity. In a second family, the proband, homozygous for the mutation, was diagnosed with severe diazoxide-unresponsive hypersinsulinism at 2 weeks of age. The patient continues to be treated successfully with octreotide and amlodipine. The parents and a male sibling are heterozygous carriers without overt clinical HI. Interestingly, both the mother and the sibling exhibit evidence of abnormally enhanced glucose tolerance.

CONCLUSIONS

V290M results in inactivating K(ATP) channels that underlie HI. Homozygous individuals may be managed medically, without pancreatectomy. Heterozygous carriers also show evidence of enhanced glucose sensitivity, consistent with incomplete loss of K(ATP) channel activity.

HMR 1098 is not an SUR isotype specific inhibitor of heterologous or sarcolemmal K ATP channels

Murine ventricular and atrial ATP-sensitive potassium (K(ATP)) channels contain different sulfonylurea receptors (ventricular K(ATP) channels are Kir6.2/SUR2A complexes, while atrial K(ATP) channels are Kir6.2/SUR1 complexes). HMR 1098, the sodium salt of HMR 1883 {1-[[5-[2-(5-chloro-o-anisamido)ethyl]-2-methoxyphenyl]sulfonyl]-3-methylthiourea}, has been considered as a selective sarcolemmal (i.e. SUR2A-dependent) K(ATP) channel inhibitor. However, it is not clear whether HMR 1098 would preferentially inhibit ventricular K(ATP) channels over atrial K(ATP) channels. To test this, we used whole-cell patch clamp techniques on mouse atrial and ventricular myocytes as well as (86)Rb(+) efflux assays and excised inside-out patch clamp techniques on Kir6.2/SUR1 and Kir6.2/SUR2A channels heterologously expressed in COSm6 cells. In mouse atrial myocytes, both spontaneously activated and diazoxide-activated K(ATP) currents were effectively inhibited by 10 μM HMR 1098. By contrast, in ventricular myocytes, pinacidil-activated K(ATP) currents were inhibited by HMR 1098 at a high concentration (100 μM) but not at a low concentration (10 μM). Consistent with this finding, HMR 1098 inhibits (86)Rb(+) effluxes through Kir6.2/SUR1 more effectively than Kir6.2/SUR2A channels in COSm6 cells. In excised inside-out patches, HMR 1098 inhibited Kir6.2/SUR1 channels more effectively, particularly in the presence of MgADP and MgATP (mimicking physiological stimulation). Finally, dose-dependent enhancement of insulin secretion from pancreatic islets and decrease of blood glucose level confirm that HMR 1098 is an inhibitor of Kir6.2/SUR1-composed K(ATP) channels.

A mutation in KCNJ11 causing human hyperinsulinism (Y12X) results in a glucose-intolerant phenotype in the mouse

AIMS/HYPOTHESIS

We identified a mouse with a point mutation (Y12STOP) in the Kcnj11 subunit of the K(ATP) channel. This point mutation is identical to that found in a patient with congenital hyperinsulinism of infancy (HI). We aimed to characterise the phenotype arising from this loss-of-function mutation and to compare it with that of other mouse models and patients with HI.

METHODS

We phenotyped an N-ethyl-N-nitrosourea-induced mutation on a C3H/HeH background (Kcnj11 ( Y12STOP )) using intraperitoneal glucose tolerance testing to measure glucose and insulin plasma concentrations. Insulin secretion and response to incretins were measured on isolated islets.

RESULTS

Homozygous male and female adult Kcnj11 ( Y12STOP ) mice exhibited impaired glucose tolerance and a defect in insulin secretion as measured in vivo and in vitro. Islets had an impaired incretin response and reduced insulin content.

CONCLUSIONS/INTERPRETATION

The phenotype of homozygous Kcnj11 ( Y12STOP ) mice is consistent with that of other Kcnj11-knockout mouse models. In contrast to the patient carrying this mutation homozygously, the mice studied did not have hyperinsulinaemia or hypoglycaemia. It has been reported that HI patients may develop diabetes and our mouse model may reflect this clinical feature. The Kcnj11 ( Y12STOP ) model may thus be useful in further studies of K(ATP) channel function in various cell types and in investigation of the development of hyperglycaemia in HI patients.

The CDK4-pRB-E2F1 pathway controls insulin secretion

CDK4-pRB-E2F1 cell-cycle regulators are robustly expressed in non-proliferating beta cells, suggesting that besides the control of beta-cell number the CDK4-pRB-E2F1 pathway has a role in beta-cell function. We show here that E2F1 directly regulates expression of Kir6.2, which is a key component of the K(ATP) channel involved in the regulation of glucose-induced insulin secretion. We demonstrate, through chromatin immunoprecipitation analysis from tissues, that Kir6.2 expression is regulated at the promoter level by the CDK4-pRB-E2F1 pathway. Consistently, inhibition of CDK4, or genetic inactivation of E2F1, results in decreased expression of Kir6.2, impaired insulin secretion and glucose intolerance in mice. Furthermore we show that rescue of Kir6.2 expression restores insulin secretion in E2f1(-/-) beta cells. Finally, we demonstrate that CDK4 is activated by glucose through the insulin pathway, ultimately resulting in E2F1 activation and, consequently, increased expression of Kir6.2. In summary we provide evidence that the CDK4-pRB-E2F1 regulatory pathway is involved in glucose homeostasis, defining a new link between cell proliferation and metabolism.

Secondary consequences of beta cell inexcitability: identification and prevention in a murine model of K(ATP)-induced neonatal diabetes mellitus

ATP-insensitive K(ATP) channel mutations cause neonatal diabetes mellitus (NDM). To explore the mechanistic etiology, we generated transgenic mice carrying an ATP-insensitive mutant K(ATP) channel subunit. Constitutive expression in pancreatic beta cells caused neonatal hyperglycemia and progression to severe diabetes and growth retardation, with loss of islet insulin content and beta cell architecture. Tamoxifen-induced expression in adult beta cells led to diabetes within 2 weeks, with similar secondary consequences. Diabetes was prevented by transplantation of normal islets under the kidney capsule. Moreover, the endogenous islets maintained normal insulin content and secretion in response to sulfonylureas, but not glucose, consistent with reduced ATP sensitivity of beta cell K(ATP) channels. In NDM, transfer to sulfonylurea therapy is less effective in older patients. This may stem from poor glycemic control or lack of insulin because glibenclamide treatment prior to tamoxifen induction prevented diabetes and secondary complications in mice but failed to halt disease progression after diabetes had developed.

Glucose controls cytosolic Ca2+ and insulin secretion in mouse islets lacking adenosine triphosphate-sensitive K+ channels owing to a knockout of the pore-forming subunit Kir6.2

Glucose-induced insulin secretion is classically attributed to the cooperation of an ATP-sensitive potassium (K ATP) channel-dependent Ca2+ influx with a subsequent increase of the cytosolic free Ca2+ concentration ([Ca2+]c) (triggering pathway) and a K ATP channel-independent augmentation of secretion without further increase of [Ca2+]c (amplifying pathway). Here, we characterized the effects of glucose in beta-cells lacking K ATP channels because of a knockout (KO) of the pore-forming subunit Kir6.2. Islets from 1-yr and 2-wk-old Kir6.2KO mice were used freshly after isolation and after 18 h culture to measure glucose effects on [Ca2+]c and insulin secretion. Kir6.2KO islets were insensitive to diazoxide and tolbutamide. In fresh adult Kir6.2KO islets, basal [Ca2+]c and insulin secretion were marginally elevated, and high glucose increased [Ca2+]c only transiently, so that the secretory response was minimal (10% of controls) despite a functioning amplifying pathway (evidenced in 30 mm KCl). Culture in 10 mm glucose increased basal secretion and considerably improved glucose-induced insulin secretion (200% of controls), unexpectedly because of an increase in [Ca2+]c with modulation of [Ca2+]c oscillations. Similar results were obtained in 2-wk-old Kir6.2KO islets. Under selected conditions, high glucose evoked biphasic increases in [Ca2+]c and insulin secretion, by inducing K ATP channel-independent depolarization and Ca2+ influx via voltage-dependent Ca2+ channels. In conclusion, Kir6.2KO beta-cells down-regulate insulin secretion by maintaining low [Ca2+]c, but culture reveals a glucose-responsive phenotype mainly by increasing [Ca2+]c. The results support models implicating a K ATP channel-independent amplifying pathway in glucose-induced insulin secretion, and show that K ATP channels are not the only possible transducers of metabolic effects on the triggering Ca2+ signal.

Chronic antidiabetic sulfonylureas in vivo: reversible effects on mouse pancreatic beta-cells

BACKGROUND

Pancreatic beta-cell ATP-sensitive potassium (K ATP) channels are critical links between nutrient metabolism and insulin secretion. In humans, reduced or absent beta-cell K ATP channel activity resulting from loss-of-function K ATP mutations induces insulin hypersecretion. Mice with reduced K ATP channel activity also demonstrate hyperinsulinism, but mice with complete loss of K ATP channels (K ATP knockout mice) show an unexpected insulin undersecretory phenotype. Therefore we have proposed an "inverse U" hypothesis to explain the response to enhanced excitability, in which excessive hyperexcitability drives beta-cells to insulin secretory failure without cell death. Many patients with type 2 diabetes treated with antidiabetic sulfonylureas (which inhibit K ATP activity and thereby enhance insulin secretion) show long-term insulin secretory failure, which we further suggest might reflect a similar progression.

METHODS AND FINDINGS

To test the above hypotheses, and to mechanistically investigate the consequences of prolonged hyperexcitability in vivo, we used a novel approach of implanting mice with slow-release sulfonylurea (glibenclamide) pellets, to chronically inhibit beta-cell K ATP channels. Glibenclamide-implanted wild-type mice became progressively and consistently diabetic, with significantly (p < 0.05) reduced insulin secretion in response to glucose. After 1 wk of treatment, these mice were as glucose intolerant as adult K ATP knockout mice, and reduction of secretory capacity in freshly isolated islets from implanted animals was as significant (p < 0.05) as those from K ATP knockout animals. However, secretory capacity was fully restored in islets from sulfonylurea-treated mice within hours of drug washout and in vivo within 1 mo after glibenclamide treatment was terminated. Pancreatic immunostaining showed normal islet size and alpha-/beta-cell distribution within the islet, and TUNEL staining showed no evidence of apoptosis.

CONCLUSIONS

These results demonstrate that chronic glibenclamide treatment in vivo causes loss of insulin secretory capacity due to beta-cell hyperexcitability, but also reveal rapid reversibility of this secretory failure, arguing against beta-cell apoptosis or other cell death induced by sulfonylureas. These in vivo studies may help to explain why patients with type 2 diabetes can show long-term secondary failure to secrete insulin in response to sulfonylureas, but experience restoration of insulin secretion after a drug resting period, without permanent damage to beta-cells. This finding suggests that novel treatment regimens may succeed in prolonging pharmacological therapies in susceptible individuals.

beta-cell hyperexcitability: from hyperinsulinism to diabetes

Nutrient oxidation in beta cells generates a rise in [ATP]:[ADP] ratio. This reduces K(ATP) channel activity, leading to depolarization, activation of voltage-dependent Ca(2+) channels, Ca(2+) entry and insulin secretion. Consistent with this paradigm, loss-of-function mutations in the genes (KCNJ11 and ABCC8) that encode the two subunits (Kir6.2 and SUR1, respectively) of the ATP-sensitive K(+) (K(ATP)) channel underlie hyperinsulinism in humans, a genetic disorder characterized by dysregulated insulin secretion. In mice with genetic suppression of K(ATP) channel subunit expression, partial loss of K(ATP) channel conductance also causes hypersecretion, but unexpectedly, complete loss results in an undersecreting, mildly glucose-intolerant phenotype. When challenged by a high-fat diet, normal mice and mice with reduced K(ATP) channel density respond with hypersecretion, but mice with more significant or complete loss of K(ATP) channels cross over, or progress further, to an undersecreting, diabetic phenotype. It is our contention that in mice, and perhaps in humans, there is an inverse U-shaped response to hyperexcitabilty, leading first to hypersecretion but with further exacerbation to undersecretion and diabetes. The causes of the overcompensation and diabetic susceptibility are poorly understood but may have broader implications for the progression of hyperinsulinism and type 2 diabetes in humans.

Alchemy in the soup: transforming metabolic signals to excitability

The intersection of cell metabolism with electrical signaling links the environment and cell function over time scales ranging from milliseconds to lifetimes. In responding to cellular metabolites, adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels are an important component of this intersection. Recent studies have begun to delineate the roles of K(ATP) channels in multiple tissues and the far-reaching consequences of aberrant K(ATP) channel activity and disturbed sensing of cell metabolism.

Glucose recruits K(ATP) channels via non-insulin-containing dense-core granules

beta cells rely on adenosine triphosphate-sensitive potassium (K(ATP)) channels to initiate and end glucose-stimulated insulin secretion through changes in membrane potential. These channels may also act as a constituent of the exocytotic machinery to mediate insulin release independent of their electrical function. However, the molecular mechanisms whereby the beta cell plasma membrane maintains an appropriate number of K(ATP) channels are not known. We now show that glucose increases K(ATP) current amplitude by increasing the number of K(ATP) channels in the beta cell plasma membrane. The effect was blocked by inhibition of protein kinase A (PKA) as well as by depletion of extracellular or intracellular Ca(2+). Furthermore, glucose promoted recruitment of the potassium inward rectifier 6.2 to the plasma membrane, and intracellular K(ATP) channels localized in chromogranin-positive/insulin-negative dense-core granules. Our data suggest that glucose can recruit K(ATP) channels to the beta cell plasma membrane via non-insulin-containing dense-core granules in a Ca(2+)- and PKA-dependent manner.

Molecular cell biology of KATPchannels: implications for neonatal diabetes

ATP-sensitive potassium (KATP) channels play a key role in the regulation of insulin secretion by coupling glucose metabolism to the electrical activity of pancreatic β-cells. To generate an electric signal of suitable magnitude, the plasma membrane of the β-cell must contain an appropriate number of channels. An inadequate number of channels can lead to congenital hyperinsulinism, whereas an excess of channels can result in the opposite condition, neonatal diabetes. KATPchannels are made up of four subunits each of Kir6.2 and the sulphonylurea receptor (SUR1), encoded by the genesKCNJ11andABCC8, respectively. Following synthesis, the subunits must assemble into an octameric complex to be able to exit the endoplasmic reticulum and reach the plasma membrane. While this biosynthetic pathway ensures supply of channels to the cell surface, an opposite pathway, involving clathrin-mediated endocytosis, removes channels back into the cell. The balance between these two processes, perhaps in conjunction with endocytic recycling, would dictate the channel density at the cell membrane. In this review, we discuss the molecular signals that contribute to this balance, and how an imbalance could lead to a disease state such as neonatal diabetes.

Transgenic expression of a dominant negative K(ATP) channel subunit in the mouse endothelium: effects on coronary flow and endothelin-1 secretion

K(ATP) channels are involved in regulating coronary function, but the contribution of endothelial K(ATP) channels remains largely uncharacterized. We generated a transgenic mouse model to specifically target endothelial K(ATP) channels by expressing a dominant negative Kir6.1 subunit only in the endothelium. These animals had no obvious overt phenotype and no early mortality. Histologically, the coronary endothelium in these animals was preserved. There was no evidence of increased susceptibility to ergonovine-induced coronary vasospasm. However, isolated hearts from these animals had a substantially elevated basal coronary perfusion pressure. The K(ATP) channel openers, adenosine and levcromakalim, decreased the perfusion pressure whereas the K(ATP) channel blocker glibenclamide failed to produce a vasoconstrictive response. The inducible endothelial nitric oxide pathway was intact, as evidenced by vasodilation caused by bradykinin. In contrast, basal endothelin-1 release was significantly elevated in the coronary effluent from these hearts. Treatment of mice with bosentan (endothelin-1 receptor antagonist) normalized the coronary perfusion pressure, demonstrating that the elevated endothelin-1 release was sufficient to account for the increased coronary perfusion pressure. Pharmacological blockade of K(ATP) channels led to elevated endothelin-1 levels in the coronary effluent of isolated mouse and rat hearts as well as enhanced endothelin-1 secretion from isolated human coronary endothelial cells. These data are consistent with a role for endothelial K(ATP) channels to control the coronary blood flow by modulating the release of the vasoconstrictor, endothelin-1.

Expression of ATP-insensitive KATP channels in pancreatic beta-cells underlies a spectrum of diabetic phenotypes

Glucose metabolism in pancreatic beta-cells elevates cytoplasmic [ATP]/[ADP], causing closure of ATP-sensitive K(+) channels (K(ATP) channels), Ca(2+) entry through voltage-dependent Ca(2+) channels, and insulin release. Decreased responsiveness of K(ATP) channels to the [ATP]/[ADP] ratio should lead to decreased insulin secretion and diabetes. We generated mice expressing K(ATP) channels with reduced ATP sensitivity in their beta-cells. Previously, we described a severe diabetes, with nearly complete neonatal lethality, in four lines (A-C and E) of these mice. We have now analyzed an additional three lines (D, F, and G) in which the transgene is expressed at relatively low levels. These animals survive past weaning but are glucose intolerant and can develop severe diabetes. Despite normal islet morphology and insulin content, islets from glucose-intolerant animals exhibit reduced glucose-stimulated insulin secretion. The data demonstrate that a range of phenotypes can be expected for a reduction in ATP sensitivity of beta-cell K(ATP) channels and provide models for the corollary neonatal diabetes in humans.

Hyperinsulinism in mice with heterozygous loss of K(ATP) channels

AIMS/HYPOTHESIS

ATP-sensitive K(+) (K(ATP)) channels couple glucose metabolism to insulin secretion in pancreatic beta cells. In humans, loss-of-function mutations of beta cell K(ATP) subunits (SUR1, encoded by the gene ABCC8, or Kir6.2, encoded by the gene KCNJ11) cause congenital hyperinsulinaemia. Mice with dominant-negative reduction of beta cell K(ATP) (Kir6.2[AAA]) exhibit hyperinsulinism, whereas mice with zero K(ATP) (Kir6.2(-/-)) show transient hyperinsulinaemia as neonates, but are glucose-intolerant as adults. Thus, we propose that partial loss of beta cell K(ATP) in vivo causes insulin hypersecretion, but complete absence may cause insulin secretory failure.

MATERIALS AND METHODS

Heterozygous Kir6.2(+/-) and SUR1(+/-) animals were generated by backcrossing from knockout animals. Glucose tolerance in intact animals was determined following i.p. loading. Glucose-stimulated insulin secretion (GSIS), islet K(ATP) conductance and glucose dependence of intracellular Ca(2+) were assessed in isolated islets.

RESULTS

In both of the mechanistically distinct models of reduced K(ATP) (Kir6.2(+/-) and SUR1(+/-)), K(ATP) density is reduced by approximately 60%. While both Kir6.2(-/-) and SUR1(-/-) mice are glucose-intolerant and have reduced glucose-stimulated insulin secretion, heterozygous Kir6.2(+/-) and SUR1(+/-) mice show enhanced glucose tolerance and increased GSIS, paralleled by a left-shift in glucose dependence of intracellular Ca(2+) oscillations.

CONCLUSIONS/INTERPRETATION

The results confirm that incomplete loss of beta cell K(ATP) in vivo underlies a hyperinsulinaemic phenotype, whereas complete loss of K(ATP) underlies eventual secretory failure.

Insulin secretory responses and phospholipid composition of pancreatic islets from mice that do not express Group VIA phospholipase A2 and effects of metabolic stress on glucose homeostasis

Studies involving pharmacologic or molecular biologic manipulation of Group VIA phospholipase A(2) (iPLA(2)beta) activity in pancreatic islets and insulinoma cells suggest that iPLA(2)beta participates in insulin secretion. It has also been suggested that iPLA(2)beta is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels and arachidonate incorporation into phosphatidylcholine (PC). We have generated iPLA(2)beta-null mice by homologous recombination and have reported that they exhibit reduced male fertility and defective motility of spermatozoa. Here we report that pancreatic islets from iPLA(2)beta-null mice have impaired insulin secretory responses to D-glucose and forskolin. Electrospray ionization mass spectrometric analyses indicate that the abundance of arachidonate-containing PC species of islets, brain, and other tissues from iPLA(2)beta-null mice is virtually identical to that of wild-type mice, and no iPLA(2)beta mRNA was observed in any tissue from iPLA(2)beta-null mice at any age. Despite the insulin secretory abnormalities of isolated islets, fasting and fed blood glucose concentrations of iPLA(2)beta-null and wild-type mice are essentially identical under normal circumstances, but iPLA(2)beta-null mice develop more severe hyperglycemia than wild-type mice after administration of multiple low doses of the beta-cell toxin streptozotocin, suggesting an impaired islet secretory reserve. A high fat diet also induces more severe glucose intolerance in iPLA(2)beta-null mice than in wild-type mice, but PLA(2)beta-null mice have greater responsiveness to exogenous insulin than do wild-type mice fed a high fat diet. These and previous findings thus indicate that iPLA(2)beta-null mice exhibit phenotypic abnormalities in pancreatic islets in addition to testes and macrophages.

Critical role of gap junction coupled KATP channel activity for regulated insulin secretion

Pancreatic beta-cells secrete insulin in response to closure of ATP-sensitive K+ (KATP) channels, which causes membrane depolarization and a concomitant rise in intracellular Ca2+ (Cai). In intact islets, beta-cells are coupled by gap junctions, which are proposed to synchronize electrical activity and Cai oscillations after exposure to stimulatory glucose (>7 mM). To determine the significance of this coupling in regulating insulin secretion, we examined islets and beta-cells from transgenic mice that express zero functional KATP channels in approximately 70% of their beta-cells, but normal KATP channel density in the remainder. We found that KATP channel activity from approximately 30% of the beta-cells is sufficient to maintain strong glucose dependence of metabolism, Cai, membrane potential, and insulin secretion from intact islets, but that glucose dependence is lost in isolated transgenic cells. Further, inhibition of gap junctions caused loss of glucose sensitivity specifically in transgenic islets. These data demonstrate a critical role of gap junctional coupling of KATP channel activity in control of membrane potential across the islet. Control via coupling lessens the effects of cell-cell variation and provides resistance to defects in excitability that would otherwise lead to a profound diabetic state, such as occurs in persistent neonatal diabetes mellitus.

Diabetes and insulin secretion: the ATP-sensitive K+ channel (K ATP) connection

The ATP-sensitive K+ channel (K ATP channel) senses metabolic changes in the pancreatic beta-cell, thereby coupling metabolism to electrical activity and ultimately to insulin secretion. When K ATP channels open, beta-cells hyperpolarize and insulin secretion is suppressed. The prediction that K ATP channel "overactivity" should cause a diabetic state due to undersecretion of insulin has been dramatically borne out by recent genetic studies implicating "activating" mutations in the Kir6.2 subunit of K ATP channel as causal in human diabetes. This article summarizes the emerging picture of K ATP channel as a major cause of neonatal diabetes and of a polymorphism in K ATP channel (E23K) as a type 2 diabetes risk factor. The degree of K ATP channel "overactivity" correlates with the severity of the diabetic phenotype. At one end of the spectrum, polymorphisms that result in a modest increase in K ATP channel activity represent a risk factor for development of late-onset diabetes. At the other end, severe "activating" mutations underlie syndromic neonatal diabetes, with multiple organ involvement and complete failure of glucose-dependent insulin secretion, reflecting K ATP channel "overactivity" in both pancreatic and extrapancreatic tissues.

ATP-sensitive K+ channel signaling in glucokinase-deficient diabetes

As the rate-limiting controller of glucose metabolism, glucokinase represents the primary beta-cell "glucose sensor." Inactivation of both glucokinase (GK) alleles results in permanent neonatal diabetes; inactivation of a single allele causes maturity-onset diabetes of the young type 2 (MODY-2). Similarly, mice lacking both alleles (GK(-/-)) exhibit severe neonatal diabetes and die within a week, whereas heterozygous GK(+/-) mice exhibit markedly impaired glucose tolerance and diabetes, resembling MODY-2. Glucose metabolism increases the cytosolic [ATP]-to-[ADP] ratio, which closes ATP-sensitive K(+) channels (K(ATP) channels), leading to membrane depolarization, Ca(2+) entry, and insulin exocytosis. Glucokinase insufficiency causes defective K(ATP) channel regulation, which may underlie the impaired secretion. To test this prediction, we crossed mice lacking neuroendocrine glucokinase (nGK(+/-)) with mice lacking K(ATP) channels (Kir6.2(-/-)). Kir6.2 knockout rescues perinatal lethality of nGK(-/-), although nGK(-/-)Kir6.2(-/-) animals are postnatally diabetic and still die prematurely. nGK(+/-) animals are diabetic on the Kir6.2(+/+) background but only mildly glucose intolerant on the Kir6.2(-/-) background. In the presence of glutamine, isolated nGK(+/-)Kir6.2(-/-) islets show improved insulin secretion compared with nGK(+/-)Kir6.2(+/+). The significant abrogation of nGK(-/-) and nGK(+/-) phenotypes in the absence of K(ATP) demonstrate that a major factor in glucokinase deficiency is indeed altered K(ATP) signaling. The results have implications for understanding and therapy of glucokinase-related diabetes.

The MODY1 gene HNF-4alpha regulates selected genes involved in insulin secretion

Mutations in the gene encoding hepatocyte nuclear factor-4alpha (HNF-4alpha) result in maturity-onset diabetes of the young (MODY). To determine the contribution of HNF-4alpha to the maintenance of glucose homeostasis by the beta cell in vivo, we derived a conditional knockout of HNF-4alpha using the Cre-loxP system. Surprisingly, deletion of HNF-4alpha in beta cells resulted in hyperinsulinemia in fasted and fed mice but paradoxically also in impaired glucose tolerance. Islet perifusion and calcium-imaging studies showed abnormal responses of the mutant beta cells to stimulation by glucose and sulfonylureas. These phenotypes can be explained in part by a 60% reduction in expression of the potassium channel subunit Kir6.2. We demonstrate using cotransfection assays that the Kir6.2 gene is a transcriptional target of HNF-4alpha. Our data provide genetic evidence that HNF-4alpha is required in the pancreatic beta cell for regulation of the pathway of insulin secretion dependent on the ATP-dependent potassium channel.

The MODY1 gene HNF-4alpha regulates selected genes involved in insulin secretion

Mutations in the gene encoding hepatocyte nuclear factor-4alpha (HNF-4alpha) result in maturity-onset diabetes of the young (MODY). To determine the contribution of HNF-4alpha to the maintenance of glucose homeostasis by the beta cell in vivo, we derived a conditional knockout of HNF-4alpha using the Cre-loxP system. Surprisingly, deletion of HNF-4alpha in beta cells resulted in hyperinsulinemia in fasted and fed mice but paradoxically also in impaired glucose tolerance. Islet perifusion and calcium-imaging studies showed abnormal responses of the mutant beta cells to stimulation by glucose and sulfonylureas. These phenotypes can be explained in part by a 60% reduction in expression of the potassium channel subunit Kir6.2. We demonstrate using cotransfection assays that the Kir6.2 gene is a transcriptional target of HNF-4alpha. Our data provide genetic evidence that HNF-4alpha is required in the pancreatic beta cell for regulation of the pathway of insulin secretion dependent on the ATP-dependent potassium channel.