Current status of the E23K Kir6.2 polymorphism: implications for type-2 diabetes

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.

Ischemic Heart Disease and Heart Failure: Role of Coronary Ion Channels

Heart failure is a complex syndrome responsible for high rates of death and hospitalization. Ischemic heart disease is one of the most frequent causes of heart failure and it is normally attributed to coronary artery disease, defined by the presence of one or more obstructive plaques, which determine a reduced coronary blood flow, causing myocardial ischemia and consequent heart failure. However, coronary obstruction is only an element of a complex pathophysiological process that leads to myocardial ischemia. In the literature, attention paid to the role of microcirculation, in the pathophysiology of ischemic heart disease and heart failure, is growing. Coronary microvascular dysfunction determines an inability of coronary circulation to satisfy myocardial metabolic demands, due to the imbalance of coronary blood flow regulatory mechanisms, including ion channels, leading to the development of hypoxia, fibrosis and tissue death, which may determine a loss of myocardial function, even beyond the presence of atherosclerotic epicardial plaques. For this reason, ion channels may represent the link among coronary microvascular dysfunction, ischemic heart disease and consequent heart failure.

Contribution of systemic inflammation to permanence of KATP-induced neonatal diabetes in mice

Gain-of-function (GOF) mutations in the ATP-sensitive potassium (K_ATP) channels cause neonatal diabetes. Despite the well-established genetic root of the disease, pathways modulating disease severity and treatment effectiveness remain poorly understood. Patient phenotypes can vary from severe diabetes to remission, even in individuals with the same mutation and within the same family, suggesting that subtle modifiers can influence disease outcome. We have tested the underlying mechanism of transient vs. permanent neonatal diabetes in K_ATP-GOF mice treated for 14 days with glibenclamide. Some K_ATP-GOF mice show remission of diabetes and enhanced insulin sensitivity long after diabetes treatment has ended, while others maintain severe insulin-resistance. However, insulin sensitivity is not different between the two groups before or during diabetes induction, suggesting that improved sensitivity is a consequence, rather than the cause of, remission, implicating other factors modulating glucose early in diabetes progression. Leptin, glucagon, insulin, and glucagon-like peptide-1 are not different between remitters and nonremitters. However, liver glucose production is significantly reduced before transgene induction in remitter, relative to nonremitter and nontreated, mice. Surprisingly, while subsequent remitter animals exhibited normal serum cytokines, nonremitter mice showed increased cytokines, which paralleled the divergence in blood glucose. Together, these results suggest that systemic inflammation may play a role in the remitting versus non-remitting outcome. Supporting this conclusion, treatment with the anti-inflammatory meloxicam significantly increased the fraction of remitting animals. Beyond neonatal diabetes, the potential for inflammation and glucose production to exacerbate other forms of diabetes from a compensated state to a glucotoxic state should be considered.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

Expression and function of ATP-dependent potassium channels in zebrafish islet β-cells

ATP-sensitive potassium channels (KATP channels) are critical nutrient sensors in many mammalian tissues. In the pancreas, KATP channels are essential for coupling glucose metabolism to insulin secretion. While orthologous genes for many components of metabolism-secretion coupling in mammals are present in lower vertebrates, their expression, functionality and ultimate impact on body glucose homeostasis are unclear. In this paper, we demonstrate that zebrafish islet β-cells express functional KATP channels of similar subunit composition, structure and metabolic sensitivity to their mammalian counterparts. We further show that pharmacological activation of native zebrafish KATP using diazoxide, a specific KATP channel opener, is sufficient to disturb glucose tolerance in adult zebrafish. That β-cell KATP channel expression and function are conserved between zebrafish and mammals illustrates the evolutionary conservation of islet metabolic sensing from fish to humans, and lends relevance to the use of zebrafish to model islet glucose sensing and diseases of membrane excitability such as neonatal diabetes.

Association of the polymorphic marker Glu23Lys in the KCNJ11 gene with hypertension in Kyrgyz patients

Aim. To study the association of the polymorphic marker Glu23Lys in the KCNJ11 with the development of hypertension in Kyrgyz patients. Subjects and methods. This case-control study enrolled 214 unrelated ethnic Kyrgyzes, in which a study group included 152 hypertensive patients (82 men and 70 women) and a control group consisted of 109 apparently healthy individuals (61 men and 48 women). The examinees’ mean age was 55.2±10.1 years. Hypertension was verified when blood pressure (BP) was above 140/90 mm Hg. Polymerase chain reaction-restriction fragment length polymorphism analysis was used to identify the polymorphic marker Glu23Lys in the KCNJ11 gene. Results. In the hypertension and control groups, the prevalence of 3 genotypes (Glu23Glu, Glu23Lys, and Lys23Lys) of the Glu23Lys polymorphism in the KCNJ11 gene differed significantly (χ2=8.04; p=0.018). The Lys23Lys and Glu23Lys genotypes were statistically more frequently recorded in the hypertension group and the homozygous Glu23Glu genotype was, on the contrary, more common in the control group than in the study one. In the hypertension group, the 23Lys allele frequency was statistically significantly higher than that in the control one (χ2=7.36; p=0.0067). The carriage of the 23Lys allele increased the risk of hypertension by 1.68 times (odds ratio (OR), 1.68; 95% confidence interval (CI), 1.17—2.41), that of the Glu23 allele had, on the contrary, a protective effect (OR, 0.60; 95% CI, 0.41—0.86). Conclusion. The polymorphic marker Glu23Lys in the KCNJ11 gene is associated with hypertension in the Kyrgyzes. The 23Lys allele is a marker for the higher risk of hypertension.

Neonatal diabetes caused by a homozygous KCNJ11 mutation demonstrates that tiny changes in ATP sensitivity markedly affect diabetes risk

AIMS/HYPOTHESIS

The pancreatic ATP-sensitive potassium (KATP) channel plays a pivotal role in linking beta cell metabolism to insulin secretion. Mutations in KATP channel genes can result in hypo- or hypersecretion of insulin, as in neonatal diabetes mellitus and congenital hyperinsulinism, respectively. To date, all patients affected by neonatal diabetes due to a mutation in the pore-forming subunit of the channel (Kir6.2, KCNJ11) are heterozygous for the mutation. Here, we report the first clinical case of neonatal diabetes caused by a homozygous KCNJ11 mutation.

METHODS

A male patient was diagnosed with diabetes shortly after birth. At 5 months of age, genetic testing revealed he carried a homozygous KCNJ11 mutation, G324R, (Kir6.2-G324R) and he was successfully transferred to sulfonylurea therapy (0.2 mg kg(-1) day(-1)). Neither heterozygous parent was affected. Functional properties of wild-type, heterozygous and homozygous mutant KATP channels were examined after heterologous expression in Xenopus oocytes.

RESULTS

Functional studies indicated that the Kir6.2-G324R mutation reduces the channel ATP sensitivity but that the difference in ATP inhibition between homozygous and heterozygous channels is remarkably small. Nevertheless, the homozygous patient developed neonatal diabetes, whereas the heterozygous parents were, and remain, unaffected. Kir6.2-G324R channels were fully shut by the sulfonylurea tolbutamide, which explains why the patient's diabetes was well controlled by sulfonylurea therapy.

CONCLUSIONS/INTERPRETATION

The data demonstrate that tiny changes in KATP channel activity can alter beta cell electrical activity and insulin secretion sufficiently to cause diabetes. They also aid our understanding of how the Kir6.2-E23K variant predisposes to type 2 diabetes.

Modeling K,ATP--dependent excitability in pancreatic islets

In pancreatic ?-cells, K,ATP channels respond to changes in glucose to regulate cell excitability and insulin release. Confirming a high sensitivity of electrical activity to K,ATP activity, mutations that cause gain of K,ATP function cause neonatal diabetes. Our aim was to quantitatively assess the contribution of K,ATP current to the regulation of glucose-dependent bursting by reproducing experimentally observed changes in excitability when K,ATP conductance is altered by genetic manipulation. A recent detailed computational model of single cell pancreatic ?-cell excitability reproduces the ?-cell response to varying glucose concentrations. However, initial simulations showed that the model underrepresents the significance of K,ATP activity and was unable to reproduce K,ATP conductance-dependent changes in excitability. By altering the ATP and glucose dependence of the L-type Ca(2+) channel and the Na-K ATPase to better fit experiment, appropriate dependence of excitability on K,ATP conductance was reproduced. Because experiments were conducted in islets, which contain cell-to-cell variability, we extended the model from a single cell to a three-dimensional model (10×10×10 cell) islet with 1000 cells. For each cell, the conductance of the major currents was allowed to vary as was the gap junction conductance between cells. This showed that single cell glucose-dependent behavior was then highly variable, but was uniform in coupled islets. The study highlights the importance of parameterization of detailed models of ?-cell excitability and suggests future experiments that will lead to improved characterization of ?-cell excitability and the control of insulin secretion.

ATP-dependent potassium channels and type 2 diabetes mellitus

Diabetes mellitus is a public health problem, which affects a millions worldwide. Most diabetes cases are classified as type 2 diabetes mellitus, which is highly associated with obesity. Type 2 diabetes is considered a multifactorial disorder, with both environmental and genetic factors contributing to its development. An important issue linked with diabetes development is the failure of the insulin releasing mechanism involving abnormal activity of the ATP-dependent potassium channel, KATP. This channel is a transmembrane protein encoded by the KCNJ11 and ABCC8 genes. Furthermore, polymorphisms in these genes have been linked to type 2 diabetes because of the role of KATP in insulin release. While several genetic variations have been reported to be associated with this disease, the E23K polymorphism is most commonly associated with this pathology, as well as to obesity. Here, we review the molecular genetics of the potassium channel and discusses its most described polymorphisms and their associations with type 2 diabetes mellitus.

Evidence for association of the E23K variant of KCNJ11 gene with type 2 diabetes in Tunisian population: population-based study and meta-analysis

Aims. Genetic association studies have reported the E23K variant of KCNJ11 gene to be associated with Type 2 diabetes. In Arab populations, only four studies have investigated the role of this variant. We aimed to replicate and validate the association between the E23K variant and Type 2 diabetes in Tunisian and Arab populations. Methods. We have performed a case-control association study including 250 Tunisian patients with Type 2 diabetes and 267 controls. Allelic association has also been evaluated by 2 meta-analyses including all population-based studies among Tunisians and Arabs (2 and 5 populations, resp.). Results. A significant association between the E23K variant and Type 2 diabetes was found (OR = 1.6, 95% CI = 1.14–2.27, and P = 0.007). Furthermore, our meta-analysis has confirmed the significant role of the E23K variant in susceptibility of Type 2 diabetes in Tunisian and Arab populations (OR = 1.29, 95% CI = 1.15–1.46, and P < 10⁻³ and OR = 1.33, 95% CI = 1.13–1.56, and P = 0.001, resp.). Conclusion. Both case-control and meta-analyses results revealed the significant association between the E23K variant of KCNJ11 and Type 2 diabetes among Tunisians and Arabs.

Pancreatic β cell dedifferentiation in diabetes and redifferentiation following insulin therapy

Diabetes is characterized by "glucotoxic" loss of pancreatic β cell function and insulin content, but underlying mechanisms remain unclear. A mouse model of insulin-secretory deficiency induced by β cell inexcitability (K(ATP) gain of function) demonstrates development of diabetes and reiterates the features of human neonatal diabetes. In the diabetic state, β cells lose their mature identity and dedifferentiate to neurogenin3-positive and insulin-negative cells. Lineage-tracing experiments show that dedifferentiated cells can subsequently redifferentiate to mature neurogenin3-negative, insulin-positive β cells after lowering of blood glucose by insulin therapy. We demonstrate here that β cell dedifferentiation, rather than apoptosis, is the main mechanism of loss of insulin-positive cells, and redifferentiation accounts for restoration of insulin content and antidiabetic drug responsivity in these animals. These results may help explain gradual decrease in β cell mass in long-standing diabetes and recovery of β cell function and drug responsivity in type 2 diabetic patients following insulin therapy, and they suggest an approach to rescuing "exhausted" β cells in diabetes.

Role of genetic polymorphisms of ion channels in the pathophysiology of coronary microvascular dysfunction and ischemic heart disease

Conventionally, ischemic heart disease (IHD) is equated with large vessel coronary disease. However, recent evidence has suggested a role of compromised microvascular regulation in the etiology of IHD. Because regulation of coronary blood flow likely involves activity of specific ion channels, and key factors involved in endothelium-dependent dilation, we proposed that genetic anomalies of ion channels or specific endothelial regulators may underlie coronary microvascular disease. We aimed to evaluate the clinical impact of single-nucleotide polymorphisms in genes encoding for ion channels expressed in the coronary vasculature and the possible correlation with IHD resulting from microvascular dysfunction. 242 consecutive patients who were candidates for coronary angiography were enrolled. A prospective, observational, single-center study was conducted, analyzing genetic polymorphisms relative to (1) NOS3 encoding for endothelial nitric oxide synthase (eNOS); (2) ATP2A2 encoding for the Ca²⁺/H⁺-ATPase pump (SERCA); (3) SCN5A encoding for the voltage-dependent Na⁺ channel (Nav1.5); (4) KCNJ8 and KCNJ11 encoding for the Kir6.1 and Kir6.2 subunits of K-ATP channels, respectively; and (5) KCN5A encoding for the voltage-gated K⁺ channel (Kv1.5). No significant associations between clinical IHD manifestations and polymorphisms for SERCA, Kir6.1, and Kv1.5 were observed (p > 0.05), whereas specific polymorphisms detected in eNOS, as well as in Kir6.2 and Nav1.5 were found to be correlated with IHD and microvascular dysfunction. Interestingly, genetic polymorphisms for ion channels seem to have an important clinical impact influencing the susceptibility for microvascular dysfunction and IHD, independent of the presence of classic cardiovascular risk factors.

The interactive effect of SIRT1 promoter region polymorphism on type 2 diabetes susceptibility in the North Indian population

Our previous studies have implicated genes mainly involved in the activity of pancreatic β cells in type 2 diabetes (T2D) susceptibility in the North Indian population. Recent literature on the role of SIRT1 as a potential master switch modulating insulin secretion and regulating gene expression in pancreatic β cells has warranted an evaluation of SIRT1 promoter region polymorphisms in the North Indian population, which is the main focus of the present study. 1542 samples (692 T2D patients and 850 controls) were sequenced for the 1.46 kb region upstream the translation start site of the SIRT1 gene. We performed a functional characterization of the SIRT1 promoter region polymorphisms using luciferase assay and observed a single-nucleotide polymorphism (SNP), rs12778366, in association with SIRT1 expression. We propose that TT, the high-expressing genotype of SNP rs12778366 in the SIRT1 promoter region and present in >80% of the North Indian population, was favored under conditions of feast-famine cycles in evolution, which has turned out to be a cause of concern in the present sedentary lifestyle under ad libitum conditions. Case-control association analysis did not implicate rs12778366 in T2DM per se in the studied population. However, our earlier reported risk genotype combinations of mt-ND3, PGC1α, and UCP2-866, when compared with the protective genotype combinations, in the background of the high-expressing TT genotype of SIRT1 SNP rs12778366, showed a very high additive risk [corrected odd ratio (OR) = 8.91; p = 6.5×10(-11)]. The risk level was considerably low in the genotype backgrounds of TX (OR = 6.68; p = 2.71×10(-12)) and CX (OR = 3.74; p = 4.0×10(-3)). In addition, we screened other reported T2D-associated polymorphisms: PIK3R1 rs3730089, IRS1 rs1801278, and PPP1R3 rs1799999, which did not show any significant association in North Indian population. The present paper emphasizes the importance of gene interactions in the biological pathways of T2D, a complex lifestyle disease.

The lessons of early-onset monogenic diabetes for the understanding of diabetes pathogenesis

Monogenic diabetes consists of different subtypes of single gene disorders comprising a large spectrum of phenotypes, namely neonatal diabetes mellitus or monogenic diabetes of infancy, dominantly inherited familial forms of early-onset diabetes (called Maturity-Onset Diabetes of the Young) and rarer diabetes-associated syndromic diseases. All these forms diagnosed at a very-young age are unrelated to auto-immunity. Their genetic dissection has revealed major genes in developmental and/or functional processes of the pancreatic β-cell physiology, and various molecular mechanisms underlying the primary pancreatic defects. Most of these discoveries have had remarkable consequences on the patients care and patient's long-term condition with outstanding examples of successful genomic medicine, which are highlighted in this chapter. Increasing evidence also shows that frequent polymorphisms in or near monogenic diabetes genes may contribute to adult polygenic type 2 diabetes. In this regard, unelucidated forms of monogenic diabetes represent invaluable models for identifying new targets of β-cell dysfunction.

Channeling dysglycemia: ion-channel variations perturbing glucose homeostasis

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.

The ATP-sensitive K(+) channel ABCC8 S1369A type 2 diabetes risk variant increases MgATPase activity

Pancreatic β-cell ATP-sensitive K(+) (K(ATP)) channels are composed of Kir6.2 and SUR1 subunits encoded by the KCNJ11 and ABCC8 genes, respectively. Although rare monogenic activating mutations in these genes cause overt neonatal diabetes, the common variants E23K (KCNJ11) and S1369A (ABCC8) form a tightly heritable haplotype that is associated with an increased susceptibility to type 2 diabetes (T2D) risk. However, the molecular mechanism(s) underlying this risk remain to be elucidated. A homology model of the SUR1 nucleotide-binding domains (NBDs) indicates that residue 1369 is in close proximity to the major MgATPase site. Therefore, we investigated the intrinsic MgATPase activity of K(ATP) channels containing these variants. Electrophysiological and biochemical techniques were used to study the MgATPase activity of recombinant human K(ATP) channels or glutathione S-transferase and NBD2 fusion proteins containing the E23/S1369 (nonrisk) or K23/A1369 (risk) variant haplotypes. K(ATP) channels containing the K23/A1369 haplotype displayed a significantly increased stimulation by guanosine triphosphate compared with the E23/S1369 haplotype (3.2- vs. 1.8-fold). This effect was dependent on the presence of the A1369 variant and was lost in the absence of Mg(2+) ions or in the presence of the MgATPase inhibitor beryllium fluoride. Direct biochemical assays also confirmed an increase in MgATPase activity in NBD2 fusion proteins containing the A1369 variant. Our findings demonstrate that the A1369 variant increases K(ATP) channel MgATPase activity, providing a plausible molecular mechanism by which the K23/A1369 haplotype increases susceptibility to T2D in humans homozygous for these variants.

The molecular genetics of sulfonylurea receptors in the pathogenesis and treatment of insulin secretory disorders and type 2 diabetes

Sulfonylurea receptors (SURs) form an integral part of the ATP-sensitive potassium (K(ATP)) channel complex that is present in most excitable cell types. K(ATP) channels couple cellular metabolism to electrical activity and provide a wide range of cellular functions including stimulus secretion coupling in pancreatic β cells. K(ATP) channels are composed of SURs and inward rectifier potassium channel (Kir6.x) subunits encoded by the ABCC8/9 and KCNJ8/11 genes, respectively. Recent advances in the genetics, molecular biology, and pharmacology of SURs have led to an increased understanding of these channels in the etiology and treatment of rare genetic insulin secretory disorders. Furthermore, common genetic variants in these genes are associated with an increased risk for type 2 diabetes. In this review we summarize the molecular biology, pharmacology, and physiology of SURs and K(ATP) channels, highlighting recent advances in their genetics and understanding of rare insulin secretory disorders and susceptibility to type 2 diabetes.

Acute sulfonylurea therapy at disease onset can cause permanent remission of KATP-induced diabetes

OBJECTIVE

Neonatal diabetes mellitus (NDM) can be caused by gain-of-function ATP-sensitive K(+) (K(ATP)) channel mutations. This realization has led to sulfonylurea therapy replacing insulin injections in many patients. In a murine model of K(ATP)-dependent NDM, hyperglycemia and consequent loss of β-cells are both avoided by chronic sulfonylurea treatment. Interestingly, K(ATP) mutations may underlie remitting-relapsing, transient, or permanent forms of the disease in different patients, but the reason for the different outcomes is unknown.

RESEARCH DESIGN AND METHODS

To gain further insight into disease progression and outcome, we examined the effects of very early intervention by injecting NDM mice with high-dose glibenclamide for only 6 days, at the beginning of disease onset, then after the subsequent progression with measurements of blood glucose, islet function, and insulin sensitivity.

RESULTS

Although ∼70% of mice developed severe diabetes after treatment cessation, ∼30% were essentially cured, maintaining near-normal blood glucose until killed. Another group of NDM mice was initiated on oral glibenclamide (in the drinking water), and the dose was titrated daily, to maintain blood glucose <200 mg/dL. In this case, ∼30% were also essentially cured; they were weaned from the drug after ∼4 weeks and again subsequently maintained near-normal blood glucose. These cured mice maintain normal insulin content and were more sensitive to insulin than control mice, a compensatory mechanism that together with basal insulin secretion may be sufficient to maintain near-normal glucose levels.

CONCLUSIONS

At least in a subset of animals, early sulfonylurea treatment leads to permanent remission of NDM. These cured animals exhibit insulin-hypersensitivity. Although untreated NDM mice rapidly lose insulin content and progress to permanently extremely elevated blood glucose levels, early tight control of blood glucose may permit this insulin-hypersensitivity, in combination with maintained basal insulin secretion, to provide long-term remission.

Current understanding of K ATP channels in neonatal diseases: focus on insulin secretion disorders

ATP-sensitive potassium (K(ATP)) channels are cell metabolic sensors that couple cell metabolic status to electric activity, thus regulating many cellular functions. In pancreatic beta cells, K(ATP) channels modulate insulin secretion in response to fluctuations in plasma glucose level, and play an important role in glucose homeostasis. Recent studies show that gain-of-function and loss-of-function mutations in K(ATP) channel subunits cause neonatal diabetes mellitus and congenital hyperinsulinism respectively. These findings lead to significant changes in the diagnosis and treatment for neonatal insulin secretion disorders. This review describes the physiological and pathophysiological functions of K(ATP) channels in glucose homeostasis, their specific roles in neonatal diabetes mellitus and congenital hyperinsulinism, as well as future perspectives of K(ATP) channels in neonatal diseases.

KATP channel Kir6.2 E23K variant overrepresented in human heart failure is associated with impaired exercise stress response

ATP-sensitive K+ (K(ATP)) channels maintain cardiac homeostasis under stress, as revealed by murine gene knockout models of the KCNJ11-encoded Kir6.2 pore. However, the translational significance of K(ATP) channels in human cardiac physiology remains largely unknown. Here, the frequency of the minor K23 allele of the common functional Kir6.2 E23K polymorphism was found overrepresented in 115 subjects with congestive heart failure compared to 2,031 community-based controls (69 vs. 56%, P < 0.001). Moreover, the KK genotype, present in 18% of heart failure patients, was associated with abnormal cardiopulmonary exercise stress testing. In spite of similar baseline heart rates at rest among genotypic subgroups (EE: 72.2 ± 2.3, EK: 75.0 ± 1.8 and KK:77.1 ± 3.0 bpm), subjects with the KK genotype had a significantly reduced heart rate increase at matched workload (EE: 32.8 ± 2.7%, EK: 28.8 ± 2.1%, KK: 21.7 ± 2.6%, P < 0.05), at 75% of maximum oxygen consumption (EE: 53.9 ± 3.9%, EK: 49.9 ± 3.1%, KK: 36.8 ± 5.3%, P < 0.05), and at peak V(O2) (EE: 82.8 ± 6.0%, EK: 80.5 ± 4.7%, KK: 59.7 ± 8.1%, P < 0.05). Molecular modeling of the tetrameric Kir6.2 pore structure revealed the E23 residue within the functionally relevant intracellular slide helix region. Substitution of the wild-type E residue with an oppositely charged, bulkier K residue would potentially result in a significant structural rearrangement and disrupted interactions with neighboring Kir6.2 subunits, providing a basis for altered high-fidelity K(ATP) channel gating, particularly in the homozygous state. Blunted heart rate response during exercise is a risk factor for mortality in patients with heart failure, establishing the clinical relevance of Kir6.2 E23K as a biomarker for impaired stress performance and underscoring the essential role of K(ATP) channels in human cardiac physiology.

The molecular mechanisms and pharmacotherapy of ATP-sensitive potassium channel gene mutations underlying neonatal diabetes

Neonatal diabetes mellitus (NDM) is a monogenic disorder caused by mutations in genes involved in regulation of insulin secretion from pancreatic β-cells. Mutations in the KCNJ11 and ABCC8 genes, encoding the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel Kir6.2 and SUR1 subunits, respectively, are found in ∼50% of NDM patients. In the pancreatic β-cell, K(ATP) channel activity couples glucose metabolism to insulin secretion via cellular excitability and mutations in either KCNJ11 or ABCC8 genes alter K(ATP) channel activity, leading to faulty insulin secretion. Inactivation mutations decrease K(ATP) channel activity and stimulate excessive insulin secretion, leading to hyperinsulinism of infancy. In direct contrast, activation mutations increase K(ATP) channel activity, resulting in impaired insulin secretion, NDM, and in severe cases, developmental delay and epilepsy. Many NDM patients with KCNJ11 and ABCC8 mutations can be successfully treated with sulfonylureas (SUs) that inhibit the K(ATP) channel, thus replacing the need for daily insulin injections. There is also strong evidence indicating that SU therapy ameliorates some of the neurological defects observed in patients with more severe forms of NDM. This review focuses on the molecular and cellular mechanisms of mutations in the K(ATP) channel that underlie NDM. SU pharmacogenomics is also discussed with respect to evaluating whether patients with certain K(ATP) channel activation mutations can be successfully switched to SU therapy.

The emerging genetics of type 2 diabetes

The elucidation of several genetic etiologies of both monogenic and polygenic type 2 diabetes (T2D) has revealed several key regulators of glucose homeostasis and insulin secretion in humans. Genome-wide association studies (GWAS) have been instrumental in most of these recent discoveries. The T2D susceptibility genes identified so far are mainly involved in pancreatic beta-cell maturation or function. However, common DNA variants in those genes only explain approximately 10% of T2D heritability. The resequencing of whole exomes and whole genomes with next-generation technologies should identify additional genetic changes that contribute to the monogenic forms of diabetes and possibly provide novel clues to the genetic architecture of common adult T2D.

Gestational diabetes mellitus screening based on the gene chip technique

AIM

The present study sought to validate the gene chip technique to screen for gestational diabetes mellitus (GDM).

METHODS

We selected 70 single-nucleotide polymorphisms (SNPs) associated with diabetes in previous studies, and SNP genotyping was performed in 130 cases (50 diabetes patients and 80 normal controls whose mean term of pregnancies are similarly 38-39 weeks). Relevant SNP patterns were established, and gene chips were designed after statistically analyzing the data. Peripheral blood samples were then collected from 24 healthy pregnant women and 24 pregnant women with GDM for gene chip analysis. The results of the gene chip method were also verified by DNA sequencing.

RESULTS

Four candidate SNPs (rs13266634, rs266729, rs3802177 and rs9300039) were obtained after genotyping. Primers were designed based on the four SNPs, and gene chips met the quality standard according to signal intensity. The genotypes of SNPs rs3802177, rs13266634 and rs266729 showed significant differences between healthy pregnant women and pregnant women with GDM, and these results were identical to those obtained with the DNA sequencing method.

CONCLUSIONS

We have demonstrated the feasibility of the gene chip technique in screening GDM and identified candidate loci with which to study this disease.

Electrophysiology of islet cells

Stimulus-Secretion Coupling (SSC) of pancreatic islet cells comprises electrical activity. Changes of the membrane potential (V(m)) are regulated by metabolism-dependent alterations in ion channel activity. This coupling is best explored in beta-cells. The effect of glucose is directly linked to mitochondrial metabolism as the ATP/ADP ratio determines the open probability of ATP-sensitive K(+) channels (K(ATP) channels). Nucleotide sensitivity and concentration in the direct vicinity of the channels are controlled by several factors including phospholipids, fatty acids, and kinases, e.g., creatine and adenylate kinase. Closure of K(ATP) channels leads to depolarization of beta-cells via a yet unknown depolarizing current. Ca(2+) influx during action potentials (APs) results in an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) that triggers exocytosis. APs are elicited by the opening of voltage-dependent Na(+) and/or Ca(2+) channels and repolarized by voltage- and/or Ca(2+)-dependent K(+) channels. At a constant stimulatory glucose concentration APs are clustered in bursts that are interrupted by hyperpolarized interburst phases. Bursting electrical activity induces parallel fluctuations in [Ca(2+)](c) and insulin secretion. Bursts are terminated by I(Kslow) consisting of currents through Ca(2+)-dependent K(+) channels and K(ATP) channels. This review focuses on structure, characteristics, physiological function, and regulation of ion channels in beta-cells. Information about pharmacological drugs acting on K(ATP) channels, K(ATP) channelopathies, and influence of oxidative stress on K(ATP) channel function is provided. One focus is the outstanding significance of L-type Ca(2+) channels for insulin secretion. The role of less well characterized beta-cell channels including voltage-dependent Na(+) channels, volume sensitive anion channels (VSACs), transient receptor potential (TRP)-related channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is discussed. A model of beta-cell oscillations provides insight in the interplay of the different channels to induce and maintain electrical activity. Regulation of beta-cell electrical activity by hormones and the autonomous nervous system is discussed. alpha- and delta-cells are also equipped with K(ATP) channels, voltage-dependent Na(+), K(+), and Ca(2+) channels. Yet the SSC of these cells is less clear and is not necessarily dependent on K(ATP) channel closure. Different ion channels of alpha- and delta-cells are introduced and SSC in alpha-cells is described in special respect of paracrine effects of insulin and GABA secreted from beta-cells.

Understanding the genetic basis for adverse drug effects: the calcineurin inhibitors

The calcineurin inhibitors-cyclosporine and tacrolimus-are the mainstay of immunosuppressive therapy in solid organ transplantation. These drugs produce severe adverse drug effects (ADEs) such as nephrotoxicity, posttransplantation diabetes mellitus, and hypertension. Accumulated evidence suggests that the development of type 2 diabetes, hypertension, and renal failure may be associated with specific DNA genotypes. In this review, the genes involved with the development of these disease processes are compared with those implicated in calcineurin inhibitor-induced ADEs. The renin-angiotensin system genes, cytokine-encoding genes, and plasminogen activator inhibitor type 1 genes have been implicated in calcineurin inhibitor-induced nephrotoxicity, as well as in development of renal failure. A number of genes are implicated in contributing to diabetes, and these include the vitamin D receptor gene, VDR; hepatocyte nuclear factor genes, HNF; transcription factor 7-like 2 gene, TCF7L2; angiotensin-converting enzyme gene, ACE; cytokines; peroxisome proliferator-activated receptor gamma gene, PPARG; and others. Studies have suggested that the VDR, PPARG, HNF1A, and adenosine 5'-triphosphate-binding cassette ABCC8 (which encodes the sulfonylurea receptor) genes are associated with calcineurin inhibitor-induced diabetes. The genes encoding for the angiotensin-converting enzyme, endothelial constitutive nitric oxide synthase, and cytochrome P450 3A isoenzyme have been involved in the development of hypertension and in calcineurin inhibitor-induced hypertension. The genetic study of disease states can be the stepping stones for thoroughly understanding the genetic basis of ADEs. Gene polymorphisms are implicated in the development of diseases and corresponding disease-like ADEs. The disease-associated genes provide candidate genes for exploring ADEs and may provide genomic biomarkers for assessing the risk for developing severe calcineurin inhibitor-related ADEs as well as for developing preventive strategies.

Human K(ATP) channelopathies: diseases of metabolic homeostasis

Assembly of an inward rectifier K+ channel pore (Kir6.1/Kir6.2) and an adenosine triphosphate (ATP)-binding regulatory subunit (SUR1/SUR2A/SUR2B) forms ATP-sensitive K+ (KATP) channel heteromultimers, widely distributed in metabolically active tissues throughout the body. KATP channels are metabolism-gated biosensors functioning as molecular rheostats that adjust membrane potential-dependent functions to match cellular energetic demands. Vital in the adaptive response to (patho)physiological stress, KATP channels serve a homeostatic role ranging from glucose regulation to cardioprotection. Accordingly, genetic variation in KATP channel subunits has been linked to the etiology of life-threatening human diseases. In particular, pathogenic mutations in KATP channels have been identified in insulin secretion disorders, namely, congenital hyperinsulinism and neonatal diabetes. Moreover, KATP channel defects underlie the triad of developmental delay, epilepsy, and neonatal diabetes (DEND syndrome). KATP channelopathies implicated in patients with mechanical and/or electrical heart disease include dilated cardiomyopathy (with ventricular arrhythmia; CMD1O) and adrenergic atrial fibrillation. A common Kir6.2 E23K polymorphism has been associated with late-onset diabetes and as a risk factor for maladaptive cardiac remodeling in the community-at-large and abnormal cardiopulmonary exercise stress performance in patients with heart failure. The overall mutation frequency within KATP channel genes and the spectrum of genotype-phenotype relationships remain to be established, while predicting consequences of a deficit in channel function is becoming increasingly feasible through systems biology approaches. Thus, advances in molecular medicine in the emerging field of human KATP channelopathies offer new opportunities for targeted individualized screening, early diagnosis, and tailored therapy.

K(ATP) channelopathies in the pancreas

Adenosine-triphosphate-sensitive potassium channels (KATP) are regulated by adenosine nucleotides, and, thereby, couple cellular metabolism with electrical activity in multiple tissues including the pancreatic beta-cell. The critical involvement of KATP in insulin secretion is confirmed by the demonstration that inactivating and activating mutations in KATP underlie persistent hyperinsulinemia and neonatal diabetes mellitus, respectively, in both animal models and humans. In addition, a common variant in KATP represents a risk factor in the etiology of type 2 diabetes. This review focuses on the mechanistic basis by which KATP mutations underlie insulin secretory disorders and the implications of these findings for successful clinical intervention.

Dual role of K ATP channel C-terminal motif in membrane targeting and metabolic regulation

The coordinated sorting of ion channels to specific plasma membrane domains is necessary for excitable cell physiology. K(ATP) channels, assembled from pore-forming (Kir6.x) and regulatory sulfonylurea receptor subunits, are critical electrical transducers of the metabolic state of excitable tissues, including skeletal and smooth muscle, heart, brain, kidney, and pancreas. Here we show that the C-terminal domain of Kir6.2 contains a motif conferring membrane targeting in primary excitable cells. Kir6.2 lacking this motif displays aberrant channel targeting due to loss of association with the membrane adapter ankyrin-B (AnkB). Moreover, we demonstrate that this Kir6.2 C-terminal AnkB-binding motif (ABM) serves a dual role in K(ATP) channel trafficking and membrane metabolic regulation and dysfunction in these pathways results in human excitable cell disease. Thus, the K(ATP) channel ABM serves as a previously unrecognized bifunctional touch-point for grading K(ATP) channel gating and membrane targeting and may play a fundamental role in controlling excitable cell metabolic regulation.

Kir6.2 variant E23K increases ATP-sensitive K+ channel activity and is associated with impaired insulin release and enhanced insulin sensitivity in adults with normal glucose tolerance

OBJECTIVE

The E23K variant in the Kir6.2 subunit of the ATP-sensitive K(+) channel (K(ATP) channel) is associated with increased risk of type 2 diabetes. The present study was undertaken to increase our understanding of the mechanisms responsible. To avoid confounding effects of hyperglycemia, insulin secretion and action were studied in subjects with the variant who had normal glucose tolerance.

RESEARCH DESIGN AND METHODS

Nine subjects with the E23K genotype K/K and nine matched subjects with the E/E genotype underwent 5-h oral glucose tolerance tests (OGTTs), graded glucose infusion, and hyperinsulinemic-euglycemic clamp with stable-isotope-labeled tracer infusions to assess insulin secretion, action, and clearance. A total of 461 volunteers consecutively genotyped for the E23K variant also underwent OGTTs. Functional studies of the wild-type and E23K variant potassium channels were conducted.

RESULTS

Insulin secretory responses to oral and intravenous glucose were reduced by approximately 40% in glucose-tolerant subjects homozygous for E23K. Normal glucose tolerance with reduced insulin secretion suggests a change in insulin sensitivity. The hyperinsulinemic-euglycemic clamp revealed that hepatic insulin sensitivity is approximately 40% greater in subjects with the E23K variant, and these subjects demonstrate increased insulin sensitivity after oral glucose. The reconstituted E23K channels confirm reduced sensitivity to inhibitory ATP and increase in open probability, a direct molecular explanation for reduced insulin secretion.

CONCLUSIONS

The E23K variant leads to overactivity of the K(ATP) channel, resulting in reduced insulin secretion. Initially, insulin sensitivity is enhanced, thereby maintaining normal glucose tolerance. Presumably, over time, as insulin secretion falls further or insulin resistance develops, glucose levels rise resulting in type 2 diabetes.

The effects of E23K polymorphism in Kir6.2 subunit on insulin sensitivity in skeletal muscle cells by long-chain fatty acyl CoA

ATP-sensitive K(+) (K(ATP)) channels couple intermediary metabolism to cellular activity. Genetic disruption of these channels impairs glucose homeostasis. Similar effects occur from a single-nucleotide polymorphism of the Kir6.2 subunit seen in greater than 50% of the human population, which causes a point mutation of Glu23 to lysine. This E23K variant shows higher susceptibility to diabetes due to mechanisms that are not fully understood. This study was designed to examine the dysregulation of E23K on insulin sensitivity in the presence of long-chain fatty acyl CoA (LC-CoA), a major active form of free fatty acids. Physiological concentrations of LC-CoA decreased insulin sensitivity in E23K-transfected L6 muscle cells by increasing the activation of negative regulators in the insulin signaling pathway. LC-CoA also reduced IRS-1 and Akt phosphorylation and glucose transport. This effect was not due to the expression of the E23K mutant on cell membrane. Our results indicate that E23K could impair insulin sensitivity, thus predisposing E23K carriers to insulin resistance.

Association of KCNJ11 and ABCC8 genetic polymorphisms with response to repaglinide in Chinese diabetic patients

AIM

The aim of this study was to investigate the association of KCNJ11 E23K and ABCC8 exon16-3T/C with the therapeutic effect of repaglinide in patients with type 2 diabetes.

METHODS

A total of 100 Chinese patients with newly diagnosed type 2 diabetes were treated with repaglinide for 24 weeks. Arginine stimulation tests were performed to evaluate beta cell function. Gene variations were detected with PCR-restriction fragment length polymorphism. Responders were defined by a greater than 25% decrease in fasting plasma glucose or a greater than 20% decrease in hemoglobin A1c (HbA1c) values (or both) after the 24 week repaglinide treatment.

RESULTS

Both baseline HbA1c and the decrease of HbA1c were significantly higher in patients with E/K and K/K genotypes of the KCNJ11 E23K variant when compared with E/E homozygotes (P=0.0103 and 0.0221, respectively). The decrease in 2 h postprandial plasma glucose (2hPG) was significantly greater in E/K heterozygotes than E/E homozygotes (P=0.0367). There was a significant difference in the response rate to repaglinide treatment between the E and K alleles (68% vs 82%, P=0.0324). The changes in fasting insulin and the homeostasis model assessment of insulin resistance were significantly greater in patients with ABCC8 exon16-3 C/C versus the T/C and T/T genotypes (P=0.0372 and 0.0274, respectively).

CONCLUSION

The KCNJ11 E23K variant was associated with the therapeutic effect of repaglinide. In addition, The C/C homozygotes of the ABCC8 exon16-3T/C variant responded better to repaglinide in insulin sensitivity than the T/C and T/T genotypes.

Elevation in intracellular long-chain acyl-coenzyme A esters lead to reduced beta-cell excitability via activation of adenosine 5'-triphosphate-sensitive potassium channels

Closure of pancreatic beta-cell ATP-sensitive potassium (K(ATP)) channels links glucose metabolism to electrical activity and insulin secretion. It is now known that saturated, but not polyunsaturated, long-chain acyl-coenyzme A esters (acyl-CoAs) can potently activate K(ATP) channels when superfused directly across excised membrane patches, suggesting a plausible mechanism to account for reduced beta-cell excitability and insulin secretion observed in obesity and type 2 diabetes. However, reduced beta-cell excitability due to elevation of endogenous saturated acyl-CoAs has not been confirmed in intact pancreatic beta-cells. To test this notion directly, endogenous acyl-CoA levels were elevated within primary mouse beta-cells using virally delivered overexpression of long-chain acyl-CoA synthetase-1 (AdACSL-1), and the effects on beta-cell K(ATP) channel activity and cell excitability was assessed using the perforated whole-cell and cell-attached patch-clamp technique. Data indicated a significant increase in K(ATP) channel activity in AdACSL-1-infected beta-cells cultured in medium supplemented with palmitate/oleate but not with the polyunsaturated fat linoleate. No changes in the ATP/ADP ratio were observed in any of the groups. Furthermore, AdACSL-1-infected beta-cells (with palmitate/oleate) showed a significant decrease in electrical responsiveness to glucose and tolbutamide and a hyperpolarized resting membrane potential at 5 mm glucose. These results suggest a direct link between intracellular fatty ester accumulation and K(ATP) channel activation, which may contribute to beta-cell dysfunction in type 2 diabetes.

K(ATP) channel polymorphism is associated with left ventricular size in hypertensive individuals: a large-scale community-based study

ATP-sensitive K+ (K(ATP)) channel mutations have been identified in individuals with dilated cardiomyopathy and overt heart failure. Here, a common E23K functional polymorphism in the Kir6.2 channel pore versus cardiac phenotype was studied in a cross-sectional community-based cohort (n = 2,031). The KK genotype was associated with greater left ventricular size among subjects with increased stress load due to hypertension. These findings implicate Kir6.2 K23 as a risk factor for adverse subclinical myocardial remodeling, and underscore the significance of cardiac K(ATP) channels within the population.

Monogenic diabetes in the young, pharmacogenetics and relevance to multifactorial forms of type 2 diabetes

Most valuable breakthroughs in the genetics of type 2 diabetes for the past two decades have arisen from candidate gene studies and familial linkage analysis of maturity-onset diabetes of the young (MODY), an autosomal dominant form of diabetes typically occurring before 25 years of age caused by primary insulin secretion defects. Despite its low prevalence, MODY is not a single entity but presents genetic, metabolic and clinical heterogeneity. MODY can result from mutations in at least six different genes encoding the glucose sensor enzyme glucokinase and transcription factors that participate in a regulatory network essential for adult beta-cell function. Additional genes have been described in other discrete phenotypes or syndromic forms of diabetes. Whereas common variants in the MODY genes contribute very modestly to type 2 diabetes susceptibility in adults, major findings emerging from the advent of genome-wide association studies will deliver an increasing number of genes and new pathways for the pathological events of the disease.

Neonatal diabetes mellitus

An explosion of work over the last decade has produced insight into the multiple hereditary causes of a nonimmunological form of diabetes diagnosed most frequently within the first 6 months of life. These studies are providing increased understanding of genes involved in the entire chain of steps that control glucose homeostasis. Neonatal diabetes is now understood to arise from mutations in genes that play critical roles in the development of the pancreas, of beta-cell apoptosis and insulin processing, as well as the regulation of insulin release. For the basic researcher, this work is providing novel tools to explore fundamental molecular and cellular processes. For the clinician, these studies underscore the need to identify the genetic cause underlying each case. It is increasingly clear that the prognosis, therapeutic approach, and genetic counseling a physician provides must be tailored to a specific gene in order to provide the best medical care.

Genetic study of Saudi diabetes (GSSD): significant association of the KCNJ11 E23K polymorphism with type 2 diabetes

BACKGROUND

The E23K variant of KCNJ11 has been associated with type 2 diabetes (T2D) in several but not all populations studied. Thus far, despite a high incidence of T2D, the role of this variant in Arabs has not been established.

METHODS

We performed a case-control association study using 550 T2D Saudi patients (WHO criteria), and 335 controls (age>or=60; fasting plasma glucose<7 mmol/L). E23K genotyping was performed by using molecular beacon-based real time PCR assays.

RESULTS

The difference in K or risk allele frequency of cases and controls was significant with an OR of 1.7 (p=0.0001). The K allele is more common among T2D patients (21%) than in the age and sex matched controls (13.6%). This was consistent with a likely eventual conversion to T2D of younger normoglycemic individuals as they grow older.

CONCLUSIONS

Our results report for the first time a positive association of the E23K variant with T2D in an Arab population. Confirmation by a larger study is indicated.

SNPs in the KCNJ11-ABCC8 gene locus are associated with type 2 diabetes and blood pressure levels in the Japanese population

Many genetic association studies support a contribution of genetic variants in the KCNJ11-ABCC8 gene locus to type 2 diabetes (T2D) susceptibility in Caucasians. In non-Caucasian populations, however, there have been only a few association studies, and discordant results were obtained. Herein, we selected a total of 31 SNPs covering a 211.3-kb region of the KCNJ11-ABCC8 locus, characterized the patterns of linkage disequilibrium (LD) and haplotype structure, and performed a case-control association study in a Japanese population consisting of 909 T2D patients and 893 control subjects. We found significant associations between eight SNPs, including the KCNJ11 E23K and ABCC8 S1369A variants, and T2D. These disease-associated SNPs were genetically indistinguishable because of the presence of strong LD, as found previously in Caucasians. For the KCNJ11 E23K variant, the most significant association was obtained under a dominant genetic model (OR 1.32, 95% CI 1.09-1.60, P = 0.004). A meta-analysis of East Asian studies, comprising a total of 3,357 T2D patients (77.4% Japanese) and 2,836 control subjects (77.8% Japanese), confirmed the significant role of the KCNJ11 E23K variant in T2D susceptibility. Furthermore, we found evidence suggesting that the KCNJ11 E23K genotype is independently associated with higher blood-pressure levels.

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.

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.

Protection conferred by myocardial ATP-sensitive K+ channels in pressure overload-induced congestive heart failure revealed in KCNJ11 Kir6.2-null mutant

Ventricular load can precipitate development of the heart failure syndrome, yet the molecular components that control the cardiac adaptive response to imposed demand remain partly understood. Compromised ATP-sensitive K(+) (K(ATP)) channel function renders the heart vulnerable to stress, implicating this metabolic sensor in the homeostatic response that would normally prevent progression of cardiac disease. Here, pressure overload was imposed on the left ventricle by transverse aortic constriction in the wild-type and in mice lacking sarcolemmal K(ATP) channels through Kir6.2 pore knockout (Kir6.2-KO). Despite equivalent haemodynamic loads, within 30 min of aortic constriction, Kir6.2-KO showed an aberrant prolongation of action potentials with intracellular calcium overload and ATP depletion, whereas wild-type maintained ionic and energetic handling. On catheterization, constricted Kir6.2-KO displayed compromised myocardial performance with elevated left ventricular end-diastolic pressure, not seen in the wild-type. Glyburide, a K(ATP) channel inhibitor, reproduced the knockout phenotype in the wild-type, whereas the calcium channel antagonist, verapamil, prevented abnormal outcome in Kir6.2-KO. Within 48 h following aortic constriction, fulminant biventricular congestive heart failure, characterized by exercise intolerance, cardiac contractile dysfunction, hepatopulmonary congestion and ascites, halved the Kir6.2-KO cohort, while no signs of organ failure or mortality were seen in wild-type. Surviving Kir6.2-KO developed premature and exaggerated fibrotic myocardial hypertrophy associated with nuclear up-regulation of calcium-dependent pro-remodelling MEF2 and NF-AT pathways, precipitating chamber dilatation within 3 weeks. Thus, K(ATP) channels appear mandatory in acute and chronic cardiac adaptation to imposed haemodynamic load, protecting against congestive heart failure and death.

ABCC8 and ABCC9: ABC transporters that regulate K+ channels

The sulfonylurea receptors (SURs) ABCC8/SUR1 and ABCC9/SUR2 are members of the C-branch of the transport adenosine triphosphatase superfamily. Unlike their brethren, the SURs have no identified transport function; instead, evolution has matched these molecules with K(+) selective pores, either K(IR)6.1/KCNJ8 or K(IR)6.2/KCNJ11, to assemble adenosine triphosphate (ATP)-sensitive K(+) channels found in endocrine cells, neurons, and both smooth and striated muscle. Adenine nucleotides, the major regulators of ATP-sensitive K(+) (K(ATP)) channel activity, exert a dual action. Nucleotide binding to the pore reduces the activity or channel open probability, whereas Mg-nucleotide binding and/or hydrolysis in the nucleotide-binding domains of SUR antagonize this inhibitory action to stimulate channel openings. Mutations in either subunit can alter this balance and, in the case of the SUR1/KIR6.2 channels found in neurons and insulin-secreting pancreatic beta cells, are the cause of monogenic forms of hyperinsulinemic hypoglycemia and neonatal diabetes. Additionally, the subtle dysregulation of K(ATP) channel activity by a K(IR)6.2 polymorphism has been suggested as a predisposing factor in type 2 diabetes mellitus. Studies on K(ATP) channel null mice are clarifying the roles of these metabolically sensitive channels in a variety of tissues.

KCNJ11 gene knockout of the Kir6.2 KATP channel causes maladaptive remodeling and heart failure in hypertension

Heart failure is a growing epidemic, with systemic hypertension a major risk factor for development of disease. However, the molecular determinants that prevent the transition from a state of hypertensive load to that of overt cardiac failure remain largely unknown. Here in experimental hypertension, knockout of the KCNJ11 gene, encoding the Kir6.2 pore-forming subunit of the sarcolemmal ATP-sensitive potassium (K(ATP)) channel, predisposed to heart failure and death. Defective decoding of hypertension-induced metabolic distress signals in the K(ATP) channel knockout set in motion pathological calcium overload and aggravated cardiac remodeling through a calcium/calcineurin-dependent cyclosporine-sensitive pathway. Rescue of the failing K(ATP) knockout phenotype was achieved by alternative control of myocardial calcium influx, bypassing uncoupled metabolic-electrical integration. The intact KCNJ11-encoded K(ATP) channel is thus a required safety element preventing hypertension-induced heart failure, with channel dysfunction a molecular substrate for stress-associated channelopathy in cardiovascular disease.

Human ether-a-go-go-related (HERG) gene and ATP-sensitive potassium channels as targets for adverse drug effects

Torsades de pointes (TdP) arrhythmia is a potentially fatal form of ventricular arrhythmia that occurs under conditions where cardiac repolarization is delayed (as indicated by prolonged QT intervals from electrocardiographic recordings). A likely mechanism for QT interval prolongation and TdP arrhythmias is blockade of the rapid component of the cardiac delayed rectifier K+ current (IKr), which is encoded by human ether-a-go-go-related gene (HERG). Over 100 non-cardiovascular drugs have the potential to induce QT interval prolongations in the electrocardiogram (ECG) or TdP arrhythmias. The binding site of most HERG channel blockers is located inside the central cavity of the channel. An evaluation of possible effects on HERG channels during the development of novel drugs is recommended by international guidelines. During cardiac ischaemia activation of ATP-sensitive K+ (KATP) channels contributes to action potential (AP) shortening which is either cardiotoxic by inducing re-entrant ventricular arrhythmias or cardioprotective by inducing energy-sparing effects or ischaemic preconditioning (IPC). KATP channels are formed by an inward-rectifier K+ channel (Kir6.0) and a sulfonylurea receptor (SUR) subunit: Kir6.2 and SUR2A in cardiac myocytes, Kir6.2 and SUR1 in pancreatic beta-cells. Sulfonylureas and glinides stimulate insulin secretion via blockade of the pancreatic beta-cell KATP channel. Clinical studies about cardiotoxic effects of sulfonylureas are contradictory. Sulfonylureas and glinides differ in their selectivity for pancreatic over cardiovascular KATP channels, being either selective (tolbutamide, glibenclamide) or non-selective (repaglinide). The possibility exists that non-selective KATP channel inhibitors might have cardiovascular side effects. Blockers of the pore-forming Kir6.2 subunit are insulin secretagogues and might have cardioprotective or cardiotoxic effects during cardiac ischaemia.