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We used mice lacking , a key component of the β-cell K-channel, to analyze the effects of a sustained elevation in the intracellular Ca concentration ([Ca]) on β-cell identity and gene expression. Lineage tracing analysis revealed the conversion of β-cells lacking into pancreatic polypeptide cells but not to α- or δ-cells. RNA-sequencing analysis of FACS-purified β-cells confirmed an increase in gene expression and revealed altered expression of more than 4,200 genes, many of which are involved in Ca signaling, the maintenance of β-cell identity, and cell adhesion. The expression of and , two highly upregulated genes, is closely correlated with membrane depolarization, suggesting their use as markers for an increase in [Ca] Moreover, a bioinformatics analysis predicts that many of the dysregulated genes are regulated by common transcription factors, one of which, , was confirmed to be directly controlled by Ca influx in β-cells. Interestingly, among the upregulated genes is , a putative marker of β-cell dedifferentiation, and other genes associated with β-cell failure. Taken together, our results suggest that chronically elevated β-cell [Ca] in islets contributes to the alteration of β-cell identity, islet cell numbers and morphology, and gene expression by disrupting a network of Ca-regulated genes.
© 2017 by the American Diabetes Association.
Stromal interaction molecule 1 (STIM1) regulates store-operated Ca entry (SOCE) and other ion channels either as an endoplasmic reticulum Ca-sensing protein or when present in the plasma membrane. However, the role of STIM1 in insulin-secreting β-cells is unresolved. We report that lowering expression of , the gene that encodes STIM1, in insulin-secreting MIN6 β-cells with RNA interference inhibits SOCE and ATP-sensitive K () channel activation. The effects of knockdown were reversed by transduction of MIN6 cells with an adenovirus gene shuttle vector that expressed human Immunoprecipitation studies revealed that STIM1 binds to nucleotide binding fold-1 (NBF1) of the sulfonylurea receptor 1 (SUR1) subunit of the channel. Binding of STIM1 to SUR1 was enhanced by poly-lysine. Our data indicate that SOCE and channel activity are regulated by STIM1. This suggests that STIM1 is a multifunctional signaling effector that participates in the control of membrane excitability and Ca signaling events in β-cells.
© 2017 by The American Society for Biochemistry and Molecular Biology, Inc.
ATP-regulated potassium (KATP) channel complexes of inward rectifier potassium channel (Kir) 6.2 and sulfonylurea receptor (SUR) 1 critically regulate pancreatic islet β-cell membrane potential, calcium influx, and insulin secretion, and consequently, represent important drug targets for metabolic disorders of glucose homeostasis. The KATP channel opener diazoxide is used clinically to treat intractable hypoglycemia caused by excessive insulin secretion, but its use is limited by off-target effects due to lack of potency and selectivity. Some progress has been made in developing improved Kir6.2/SUR1 agonists from existing chemical scaffolds and compound screening, but there are surprisingly few distinct chemotypes that are specific for SUR1-containing KATP channels. Here we report the serendipitous discovery in a high-throughput screen of a novel activator of Kir6.2/SUR1: VU0071063 [7-(4-(tert-butyl)benzyl)-1,3-dimethyl-1H-purine-2,6(3H,7H)-dione]. The xanthine derivative rapidly and dose-dependently activates Kir6.2/SUR1 with a half-effective concentration (EC50) of approximately 7 μM, is more efficacious than diazoxide at low micromolar concentrations, directly activates the channel in excised membrane patches, and is selective for SUR1- over SUR2A-containing Kir6.1 or Kir6.2 channels, as well as Kir2.1, Kir2.2, Kir2.3, Kir3.1/3.2, and voltage-gated potassium channel 2.1. Finally, we show that VU0071063 activates native Kir6.2/SUR1 channels, thereby inhibiting glucose-stimulated calcium entry in isolated mouse pancreatic β cells. VU0071063 represents a novel tool/compound for investigating β-cell physiology, KATP channel gating, and a new chemical scaffold for developing improved activators with medicinal chemistry.
Insulin from islet β-cells maintains glucose homeostasis by stimulating peripheral tissues to remove glucose from circulation. Persistent elevation of insulin demand increases β-cell number through self-replication or differentiation (neogenesis) as part of a compensatory response. However, it is not well understood how a persistent increase in insulin demand is detected. We have previously demonstrated that a persistent increase in insulin demand by overnutrition induces compensatory β-cell differentiation in zebrafish. Here, we use a series of pharmacological and genetic analyses to show that prolonged stimulation of existing β-cells is necessary and sufficient for this compensatory response. In the absence of feeding, tonic, but not intermittent, pharmacological activation of β-cell secretion was sufficient to induce β-cell differentiation. Conversely, drugs that block β-cell secretion, including an ATP-sensitive potassium (K ATP) channel agonist and an L-type Ca(2+) channel blocker, suppressed overnutrition-induced β-cell differentiation. Genetic experiments specifically targeting β-cells confirm existing β-cells as the overnutrition sensor. First, inducible expression of a constitutively active K ATP channel in β-cells suppressed the overnutrition effect. Second, inducible expression of a dominant-negative K ATP mutant induced β-cell differentiation independent of nutrients. Third, sensitizing β-cell metabolism by transgenic expression of a hyperactive glucokinase potentiated differentiation. Finally, ablation of the existing β-cells abolished the differentiation response. Taken together, these data establish that overnutrition induces β-cell differentiation in larval zebrafish through prolonged activation of β-cells. These findings demonstrate an essential role for existing β-cells in sensing overnutrition and compensating for their own insufficiency by recruiting additional β-cells.
AIM - The Kir 6.1 K(atp) channel is believed to play an important role in ventricular repolarization as determined from both functional and genetic studies of the potassium inwardly-rectifying channel, subfamily J, member 8 (KCNJ8)-S422L missense mutation in patients with J-wave syndromes. Although Kir6.1 is also present in atrial tissue, it is unknown whether this channel modulates atrial repolarization and hence whether the S422L mutation portends a greater risk of atrial arrhythmias. This study sought to examine whether there was an increased frequency of the KCNJ8-S422L mutation among patients with atrial fibrillation (AF) and early repolarization (ER) as a possible novel susceptibility gene for AF.
METHODS AND RESULTS - A total of 325 lone AF probands were identified from the Vanderbilt AF Registry, a collection of clinical data and DNA from consented, consecutively enrolled participants. The coding regions of KCNJ8 were sequenced, and the patient's presenting electrocardiogram (ECG) was reviewed by two independent physicians for ER abnormalities. The KCNJ8-S422L mutation was identified in two AF probands while no other candidate gene variants were identified in these cases. Twenty-two (7%) patients were found to have ER on the ECG, including the two probands carrying the S422L variant. In one small AF kindred, the S422L variant co-segregated with AF and ER.
CONCLUSIONS - The KCNJ8-S422L variant is associated with both increased AF susceptibility and ER indicating a role for Kir 6.1 K(atp) channel in both ventricular and atrial repolarization.
Maintaining blood glucose homeostasis is a complex process that depends on pancreatic islet hormone secretion. Hormone secretion from islets is coupled to calcium entry which results from regenerative islet cell electrical activity. Therefore, the ionic mechanisms that regulate calcium entry into islet cells are crucial for maintaining normal glucose homeostasis. Genome-wide association studies (GWAS) have identified single-nucleotide polymorphisms (SNPs), including five located in or near ion-channel or associated subunit genes, which show an association with human diseases characterized by dysglycemia. This review focuses on polymorphisms and mutations in ion-channel genes that are associated with perturbations in human glucose homeostasis and discusses their potential roles in modulating pancreatic islet hormone secretion.
Copyright © 2011 Elsevier Ltd. All rights reserved.
Recent studies revealed a surprising regenerative capacity of insulin-producing β cells in mice, suggesting that regenerative therapy for human diabetes could in principle be achieved. Physiologic β cell regeneration under stressed conditions relies on accelerated proliferation of surviving β cells, but the factors that trigger and control this response remain unclear. Using islet transplantation experiments, we show that β cell mass is controlled systemically rather than by local factors such as tissue damage. Chronic changes in β cell glucose metabolism, rather than blood glucose levels per se, are the main positive regulator of basal and compensatory β cell proliferation in vivo. Intracellularly, genetic and pharmacologic manipulations reveal that glucose induces β cell replication via metabolism by glucokinase, the first step of glycolysis, followed by closure of K(ATP) channels and membrane depolarization. Our data provide a molecular mechanism for homeostatic control of β cell mass by metabolic demand.
Copyright © 2011 Elsevier Inc. All rights reserved.
AIMS/HYPOTHESIS - Mutations that render ATP-sensitive potassium (K(ATP)) channels insensitive to ATP inhibition cause neonatal diabetes mellitus. In mice, these mutations cause insulin secretion to be lost initially and, as the disease progresses, beta cell mass and insulin content also disappear. We investigated whether defects in calcium signalling alone are sufficient to explain short-term and long-term islet dysfunction.
METHODS - We examined the metabolic, electrical and insulin secretion response in islets from mice that become diabetic after induction of ATP-insensitive Kir6.2 expression. To separate direct effects of K(ATP) overactivity on beta cell function from indirect effects of prolonged hyperglycaemia, normal glycaemia was maintained by protective exogenous islet transplantation.
RESULTS - In endogenous islets from protected animals, glucose-dependent elevations of intracellular free-calcium activity ([Ca(2+)](i)) were severely blunted. Insulin content of these islets was normal, and sulfonylureas and KCl stimulated increased [Ca(2+)](i). In the absence of transplant protection, [Ca(2+)](i) responses were similar, but glucose metabolism and redox state were dramatically altered; sulfonylurea- and KCl-stimulated insulin secretion was also lost, because of systemic effects induced by long-term hyperglycaemia and/or hypoinsulinaemia. In both cases, [Ca(2+)](i) dynamics were synchronous across the islet. After reduction of gap-junction coupling, glucose-dependent [Ca(2+)](i) and insulin secretion was partially restored, indicating that excitability of weakly expressing cells is suppressed by cells expressing mutants, via gap-junctions.
CONCLUSIONS/INTERPRETATION - The primary defect in K(ATP)-induced neonatal diabetes mellitus is failure of glucose metabolism to elevate [Ca(2+)](i), which suppresses insulin secretion and mildly alters islet glucose metabolism. Loss of insulin content and mitochondrial dysfunction are secondary to the long-term hyperglycaemia and/or hypoinsulinaemia that result from the absence of glucose-dependent insulin secretion.
The isoform-specific structure of the ATP-sensitive potassium (K(ATP)) channel endows it with differential fundamental properties, including physiological activation and pharmacology. Numerous studies have convincingly demonstrated that the pore-forming Kir6.2 (KCNJ11) and regulatory SUR2A (ABCC9) subunits are essential elements of the sarcolemmal K(ATP) channel in cardiac ventricular myocytes. Using a novel antibody directed against the COOH terminus of SUR1 (ABCC8), we show that this K(ATP) subunit is also expressed in mouse myocardium and is the dominant SUR isoform in the atrium. This suggests differential sarcolemmal K(ATP) composition in atria and ventricles, and, to test this, K(ATP) currents were measured in isolated atrial and ventricular myocytes from wild-type and SUR1(-/-) animals. K(ATP) conductance is essentially abolished in SUR1(-/-) atrial myocytes but is normal in SUR1(-/-) ventricular myocytes. Furthermore, pharmacological properties of wild-type atrial K(ATP) match closely the properties of heterologously expressed SUR1/Kir6.2 channels, whereas ventricular K(ATP) properties match those of heterologously expressed SUR2A/Kir6.2 channels. Collectively, the data demonstrate a previously unappreciated K(ATP) channel heterogeneity: SUR1 is an essential component of atrial, but not ventricular, K(ATP) channels. Differential molecular make-up of the 2 channels underlies differential pharmacology, with important implications when considering sulfonylurea therapy or dissecting the role of cardiac K(ATP) pharmacologically, as well as for understanding of the role of diazoxide in preconditioning.