The prevalent Glu23Lys polymorphism in the potassium inward rectifier 6.2 (KIR6.2) gene is associated with impaired glucagon suppression in response to hyperglycemia

Association between Genotype and the Glycemic Response to an Oral Glucose Tolerance Test: A Systematic Review

The inter-individual variability of metabolic response to foods may be partly due to genetic variation. This systematic review aims to assess the associations between genetic variants and glucose response to an oral glucose tolerance test (OGTT). Three databases (PubMed, Web of Science, Embase) were searched for keywords in the field of genetics, OGTT, and metabolic response (PROSPERO: CRD42021231203). Inclusion criteria were available data on single nucleotide polymorphisms (SNPs) and glucose area under the curve (gAUC) in a healthy study cohort. In total, 33,219 records were identified, of which 139 reports met the inclusion criteria. This narrative synthesis focused on 49 reports describing gene loci for which several reports were available. An association between SNPs and the gAUC was described for 13 gene loci with 53 different SNPs. Three gene loci were mostly investigated: transcription factor 7 like 2 (TCF7L2), peroxisome proliferator-activated receptor gamma (PPARγ), and potassium inwardly rectifying channel subfamily J member 11 (KCNJ11). In most reports, the associations were not significant or single findings were not replicated. No robust evidence for an association between SNPs and gAUC after an OGTT in healthy persons was found across the identified studies. Future studies should investigate the effect of polygenic risk scores on postprandial glucose levels.

The KCNJ11-E23K Gene Variant Hastens Diabetes Progression by Impairing Glucose-Induced Insulin Secretion

The ATP-sensitive K⁺ (K_ATP) channel controls blood glucose levels by coupling glucose metabolism to insulin secretion in pancreatic β-cells. E23K, a common polymorphism in the pore-forming K_ATP channel subunit (KCNJ11) gene, has been linked to increased risk of type 2 diabetes. Understanding the risk-allele-specific pathogenesis has the potential to improve personalized diabetes treatment, but the underlying mechanism has remained elusive. Using a genetically engineered mouse model, we now show that the K23 variant impairs glucose-induced insulin secretion and increases diabetes risk when combined with a high-fat diet (HFD) and obesity. K_ATP-channels in β-cells with two K23 risk alleles (KK) showed decreased ATP inhibition, and the threshold for glucose-stimulated insulin secretion from KK islets was increased. Consequently, the insulin response to glucose and glycemic control was impaired in KK mice fed a standard diet. On an HFD, the effects of the KK genotype were exacerbated, accelerating diet-induced diabetes progression and causing β-cell failure. We conclude that the K23 variant increases diabetes risk by impairing insulin secretion at threshold glucose levels, thus accelerating loss of β-cell function in the early stages of diabetes progression.

Diabetes Mellitus and Ischemic Heart Disease: The Role of Ion Channels

Diabetes mellitus is one the strongest risk factors for cardiovascular disease and, in particular, for ischemic heart disease (IHD). The pathophysiology of myocardial ischemia in diabetic patients is complex and not fully understood: some diabetic patients have mainly coronary stenosis obstructing blood flow to the myocardium; others present with coronary microvascular disease with an absence of plaques in the epicardial vessels. Ion channels acting in the cross-talk between the myocardial energy state and coronary blood flow may play a role in the pathophysiology of IHD in diabetic patients. In particular, some genetic variants for ATP-dependent potassium channels seem to be involved in the determinism of IHD.

Alpha cell dysfunction in type 1 diabetes

Type 1 diabetes is characterized by selective loss of beta cells and insulin secretion, which significantly impact glucose homeostasis. However, this progressive disease is also associated with dysfunction of the alpha cell component of the islet, which can exacerbate hyperglycemia due to paradoxical hyperglucagonemia or lead to severe hypoglycemia as a result of failed counterregulation. In this review, the physiology of alpha cell secretion and the potential mechanisms underlying alpha cell dysfunction in type 1 diabetes will be explored. Because type 1 diabetes is a progressive disease, a synthesized timeline of aberrant alpha cell function will be presented as an attempt to delineate the natural history of type 1 diabetes with respect to the alpha cell.

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.

Obstacles to Translating Genotype-Phenotype Correlates in Metabolic Disease

Type 2 diabetes mellitus is a polygenic disease with a variable phenotype. Many genetic associations have been described; however, understanding their underlying pathophysiological role in Type 2 diabetes mellitus is important for development of future therapeutic targets. Here, we review the physiological mechanisms of diabetes-associated variants that affect glycemia.

α-Cell Dysfunctions and Molecular Alterations in Male Insulinopenic Diabetic Mice Are Not Completely Corrected by Insulin

Glucagon and α-cell dysfunction are critical in the development of hyperglycemia during diabetes both in humans and rodents. We hypothesized that α-cell dysfunction leading to dysregulated glucagon secretion in diabetes is due to both a lack of insulin and intrinsic defects. To characterize α-cell dysfunction in diabetes, we used glucagon-Venus transgenic male mice and induced insulinopenic hyperglycemia by streptozotocin administration leading to alterations of glucagon secretion. We investigated the in vivo impact of insulinopenic hyperglycemia on glucagon-producing cells using FACS-sorted α-cells from control and diabetic mice. We demonstrate that increased glucagonemia in diabetic mice is mainly due to increases of glucagon release and biosynthesis per cell compared with controls without changes in α-cell mass. We identified genes coding for proteins involved in glucagon biosynthesis and secretion, α-cell differentiation, and potential stress markers such as the glucagon, Arx, MafB, cMaf, Brain4, Foxa1, Foxa3, HNF4α, TCF7L2, Glut1, Sglt2, Cav2.1, Cav2.2, Nav1.7, Kir6.2/Sur1, Pten, IR, NeuroD1, GPR40, and Sumo1 genes, which were abnormally regulated in diabetic mice. Importantly, insulin treatment partially corrected α-cell function and expression of genes coding for proglucagon, or involved in glucagon secretion, glucose transport and insulin signaling but not those coding for cMAF, FOXA1, and α-cell differentiation markers as well as GPR40, NEUROD1, CAV2.1, and SUMO1. Our results indicate that insulinopenic diabetes induce marked α-cell dysfunction and molecular alteration, which are only partially corrected by in vivo insulin treatment.

Physiology of proglucagon peptides: role of glucagon and GLP-1 in health and disease

The preproglucagon gene (Gcg) is expressed by specific enteroendocrine cells (L-cells) of the intestinal mucosa, pancreatic islet α-cells, and a discrete set of neurons within the nucleus of the solitary tract. Gcg encodes multiple peptides including glucagon, glucagon-like peptide-1, glucagon-like peptide-2, oxyntomodulin, and glicentin. Of these, glucagon and GLP-1 have received the most attention because of important roles in glucose metabolism, involvement in diabetes and other disorders, and application to therapeutics. The generally accepted model is that GLP-1 improves glucose homeostasis indirectly via stimulation of nutrient-induced insulin release and by reducing glucagon secretion. Yet the body of literature surrounding GLP-1 physiology reveals an incompletely understood and complex system that includes peripheral and central GLP-1 actions to regulate energy and glucose homeostasis. On the other hand, glucagon is established principally as a counterregulatory hormone, increasing in response to physiological challenges that threaten adequate blood glucose levels and driving glucose production to restore euglycemia. However, there also exists a potential role for glucagon in regulating energy expenditure that has recently been suggested in pharmacological studies. It is also becoming apparent that there is cross-talk between the proglucagon derived-peptides, e.g., GLP-1 inhibits glucagon secretion, and some additive or synergistic pharmacological interaction between GLP-1 and glucagon, e.g., dual glucagon/GLP-1 agonists cause more weight loss than single agonists. In this review, we discuss the physiological functions of both glucagon and GLP-1 by comparing and contrasting how these peptides function, variably in concert and opposition, to regulate glucose and energy homeostasis.

Pancreatic α-Cell Dysfunction in Type 2 Diabetes: Old Kids on the Block

Type 2 diabetes (T2D) has been known as 'bi-hormonal disorder' since decades ago, the role of glucagon from α-cell has languished whereas β-cell taking center stage. Recently, numerous findings indicate that the defects of glucagon secretion get involve with development and exacerbation of hyperglycemia in T2D. Aberrant α-cell responses exhibit both fasting and postprandial states: hyperglucagonemia contributes to fasting hyperglycemia caused by inappropriate hepatic glucose production, and to postprandial hyperglycemia owing to blunted α-cell suppression. During hypoglycemia, insufficient counter-regulation response is also observed in advanced T2D. Though many debates still remained for exact mechanisms behind the dysregulation of α-cell in T2D, it is clear that the blockade of glucagon receptor or suppression of glucagon secretion from α-cell would be novel therapeutic targets for control of hyperglycemia. Whereas there have not been remarkable advances in developing new class of drugs, currently available glucagon-like peptide-1 and dipeptidyl peptidase-IV inhibitors could be options for treatment of hyperglucagonemia. In this review, we focus on α-cell dysfunction and therapeutic potentials of targeting α-cell in T2D.

Foxa1 and Foxa2 regulate α-cell differentiation, glucagon biosynthesis, and secretion

The Forkhead box A transcription factors are major regulators of glucose homeostasis. They show both distinct and redundant roles during pancreas development and in adult mouse β-cells. In vivo ablation studies have revealed critical implications of Foxa1 on glucagon biosynthesis and requirement of Foxa2 in α-cell terminal differentiation. In order to examine the respective role of these factors in mature α-cells, we used small interfering RNA (siRNA) directed against Foxa1 and Foxa2 in rat primary pancreatic α-cells and rodent α-cell lines leading to marked decreases in Foxa1 and Foxa2 mRNA levels and proteins. Both Foxa1 and Foxa2 control glucagon gene expression specifically through the G2 element. Although we found that Foxa2 controls the expression of the glucagon, MafB, Pou3f4, Pcsk2, Nkx2.2, Kir6.2, and Sur1 genes, Foxa1 only regulates glucagon gene expression. Interestingly, the Isl1 and Gipr genes were not controlled by either Foxa1 or Foxa2 alone but by their combination. Foxa1 and Foxa2 directly activate and bind the promoter region the Nkx2.2, Kir6.2 and Sur1, Gipr, Isl1, and Pou3f4 genes. We also demonstrated that glucagon secretion is affected by the combined effects of Foxa1 and Foxa2 but not by either one alone. Our results indicate that Foxa1 and Foxa2 control glucagon biosynthesis and secretion as well as α-cell differentiation with both common and unique target genes.

ATP-regulated potassium channels and voltage-gated calcium channels in pancreatic alpha and beta cells: similar functions but reciprocal effects on secretion

Closure of ATP-regulated K(+) channels (K(ATP) channels) plays a central role in glucose-stimulated insulin secretion in beta cells. K(ATP) channels are also highly expressed in glucagon-producing alpha cells, where their function remains unresolved. Under hypoglycaemic conditions, K(ATP) channels are open in alpha cells but their activity is low and only ~1% of that in beta cells. Like beta cells, alpha cells respond to hyperglycaemia with K(ATP) channel closure, membrane depolarisation and stimulation of action potential firing. Yet, hyperglycaemia reciprocally regulates glucagon (inhibition) and insulin secretion (stimulation). Here we discuss how this conundrum can be resolved and how reduced K(ATP) channel activity, via membrane depolarisation, paradoxically reduces alpha cell Ca(2+) entry and glucagon exocytosis. Finally, we consider whether the glucagon secretory defects associated with diabetes can be attributed to impaired K(ATP) channel regulation and discuss the potential for remedial pharmacological intervention using sulfonylureas.

Role of KATP channels in glucose-regulated glucagon secretion and impaired counterregulation in type 2 diabetes

Glucagon, secreted by pancreatic islet α cells, is the principal hyperglycemic hormone. In diabetes, glucagon secretion is not suppressed at high glucose, exacerbating the consequences of insufficient insulin secretion, and is inadequate at low glucose, potentially leading to fatal hypoglycemia. The causal mechanisms remain unknown. Here we show that α cell KATP-channel activity is very low under hypoglycemic conditions and that hyperglycemia, via elevated intracellular ATP/ADP, leads to complete inhibition. This produces membrane depolarization and voltage-dependent inactivation of the Na(+) channels involved in action potential firing that, via reduced action potential height and Ca(2+) entry, suppresses glucagon secretion. Maneuvers that increase KATP channel activity, such as metabolic inhibition, mimic the glucagon secretory defects associated with diabetes. Low concentrations of the KATP channel blocker tolbutamide partially restore glucose-regulated glucagon secretion in islets from type 2 diabetic organ donors. These data suggest that impaired metabolic control of the KATP channels underlies the defective glucose regulation of glucagon secretion in type 2 diabetes.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

K(ATP) channels and islet hormone secretion: new insights and controversies

ATP-sensitive potassium channels (K(ATP) channels) link cell metabolism to electrical activity by controlling the cell membrane potential. They participate in many physiological processes but have a particularly important role in systemic glucose homeostasis by regulating hormone secretion from pancreatic islet cells. Glucose-induced closure of K(ATP) channels is crucial for insulin secretion. Emerging data suggest that K(ATP) channels also play a key part in glucagon secretion, although precisely how they do so remains controversial. This Review highlights the role of K(ATP) channels in insulin and glucagon secretion. We discuss how K(ATP) channels might contribute not only to the initiation of insulin release but also to the graded stimulation of insulin secretion that occurs with increasing glucose concentrations. The various hypotheses concerning the role of K(ATP) channels in glucagon release are also reviewed. Furthermore, we illustrate how mutations in K(ATP) channel genes can cause hyposecretion or hypersecretion of insulin, as in neonatal diabetes mellitus and congenital hyperinsulinism, and how defective metabolic regulation of the channel may underlie the hypoinsulinaemia and the hyperglucagonaemia that characterize type 2 diabetes mellitus. Finally, we outline how sulphonylureas, which inhibit K(ATP) channels, stimulate insulin secretion in patients with neonatal diabetes mellitus or type 2 diabetes mellitus, and suggest their potential use to target the glucagon secretory defects found in diabetes mellitus.

Impairment of GLP1-induced insulin secretion: role of genetic background, insulin resistance and hyperglycaemia

One major risk factor of type 2 diabetes is the impairment of glucose-induced insulin secretion which is mediated by the individual genetic background and environmental factors. In addition to impairment of glucose-induced insulin secretion, impaired glucagon-like peptide (GLP)1-induced insulin secretion has been identified to be present in subjects with diabetes and impaired glucose tolerance, but little is known about its fundamental mechanisms. The state of GLP1 resistance is probably an important mechanism explaining the reduced incretin effect observed in type 2 diabetes. In this review, we address methods that can be used for the measurement of insulin secretion in response to GLP1 in humans, and studies showing that specific diabetes risk genes are associated with resistance of the secretory function of the β-cell in response to GLP1 administration. Furthermore, we discuss other factors that are associated with impaired GLP1-induced insulin secretion, for example, insulin resistance. Finally, we provide evidence that hyperglycaemia per se, the genetic background and their interaction result in the development of GLP1 resistance of the β-cell. We speculate that the response or the non-response to therapy with GLP1 analogues and/or dipeptidyl peptidase-4 (DPP-IV) inhibitors is critically dependent on GLP1 resistance.

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 effect of KCNJ11 polymorphism on the risk of type 2 diabetes: a global meta-analysis based on 49 case-control studies

Potassium inwardly rectifying channel, subfamily-J, member 11 (KCNJ11) gene encodes Kir6.2 subunits of the adenosine triphosphate (ATP)-sensitive potassium channel involved in glucose-mediated metabolic signaling pathway and has attracted considerable attention as a candidate gene for type 2 diabetes (T2D) based on its function in glucose-stimulated insulin secretion. In the past decade, a number of case-control studies have been conducted to investigate the relationship between the KCNJ11 polymorphisms and T2D. However, these studies have yielded contradictory results. To investigate this inconsistency and derive a more precise estimation of the relationship, we conducted a comprehensive meta-analysis of 64,403 cases and 122,945 controls from 49 published studies. Using the random-effects model, we found a significant association between E23K (rs5219) polymorphism and T2D risk with per-allele odds ratio of 1.13 (95% confidence interval: 1.10-1.15; p<10(-5)). Significant results were found in East Asians and Caucasians when stratified by ethnicity; whereas no significant associations were found among South Asians and other ethnic populations. In subgroup analysis by sample size, mean age and body mass index (BMI) of cases, mean BMI of controls and diagnostic criterion, significantly increased risks were found in all genetic models. This meta-analysis suggests that the E23K polymorphism in KCNJ11 is associated with elevated T2D risk, but these associations vary in different ethnic populations.

The role of dysregulated glucagon secretion in type 2 diabetes

Excessive production of glucose by the liver contributes to fasting and postprandial hyperglycaemia, hallmarks of type 2 diabetes. A central feature of this pathologic response is insufficient hepatic insulin action, due to a combination of insulin resistance and impaired β-cell function. However, a case can be made that glucagon also plays a role in dysregulated hepatic glucose production and abnormal glucose homeostasis. Plasma glucagon concentrations are inappropriately elevated in diabetic individuals, and α-cell suppression by hyperglycaemia is blunted. Experimental evidence suggests that this contributes to greater rates of hepatic glucose production in the fasting state and attenuated reduction after meals. Recent studies in animal models indicate that reduction of glucagon action can have profound effects to mitigate hyperglycaemia even in the face of severe hypoinsulinaemia. While there are no specific treatments for diabetic patients yet available that act specifically on the glucagon signalling pathway, newer agents including glucagon-like peptide-1 (GLP-1) receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors reduce plasma glucagon and this is thought to contribute to their action to lower blood glucose. The α-cell and glucagon receptor remain tempting targets for novel diabetes treatments, but it is important to understand the magnitude of benefit new strategies would provide as preclinical models suggest that chronic interference with glucagon action could entail adverse effects as well.

Effect of genetic polymorphisms on the development of secondary failure to sulfonylurea in egyptian patients with type 2 diabetes

OBJECTIVE

This study investigated the possibility that genetic factors, such as polymorphism of K inward rectifier subunit (Kir6.2), E23K, and Arg(972) polymorphism of insulin receptor sub-strate-1 (IRS-1), may predispose patients to sulfonylurea failure.

METHODS

A total of 100 unrelated Egyptian patients with type 2 diabetes were recruited. They were divided into two equal groups: group I consisted of patients with secondary failure to sulfonylurea (hemoglobin A(1c) ≥ 8% despite sulfonylurea therapy) while group II consisted of patients whose condition was controlled with oral therapy.

RESULTS

Of all the patients, 45% and 14% were carriers of the K allele and Arg(972) variants respectively. The frequency of the K allele was 34% among patients with diabetes that was controlled with oral therapy and 56% among patients with secondary failure to sulfonylurea. The frequency of the Arg(972) IRS-1 variant was 6% among patients with diabetes controlled with oral therapy and 22% among patients with secondary failure.

CONCLUSION

The E23K variant of the Kir6.2 gene and Arg(972) IRS-1 variants are associated with increased risk for secondary failure to sulfonylurea.

New type 2 diabetes risk genes provide new insights in insulin secretion mechanisms

Type 2 diabetes results from the inability of beta cells to increase insulin secretion sufficiently to compensate for insulin resistance. Insulin resistance is thought to result mainly from environmental factors, such as obesity. However, there is compelling evidence that the decline of both insulin sensitivity and insulin secretion have also a genetic component. Recent genome-wide association studies identified several novel risk genes for type 2 diabetes. The vast majority of these genes affect beta cell function by molecular mechanisms that remain unknown in detail. Nevertheless, we and others could show that a group of genes affect glucose-stimulated insulin secretion, a group incretin-stimulated insulin secretion (incretin sensitivity or secretion) and a group proinsulin-to-insulin conversion. The most important so far type 2 diabetes risk gene, TCF7L2, interferes with all three mechanisms. In addition to advancing knowledge in the pathophysiology of type 2 diabetes, the discovery of novel genetic determinants of diabetes susceptibility may help understanding of gene-environment, gene-therapy and gene-gene interactions. It was also hoped that it could make determination of the individual risk for type 2 diabetes feasible. However, the allelic relative risks of most genetic variants discovered so far are relatively low. Thus, at present, clinical criteria assess the risk for type 2 diabetes with greater sensitivity and specificity than the combination of all known genetic variants.

Glycemia determines the effect of type 2 diabetes risk genes on insulin secretion

OBJECTIVE

Several single nucleotide polymorphisms (SNPs) in diabetes risk genes reduce glucose- and/or incretin-induced insulin secretion. Here, we investigated interactions between glycemia and such diabetes risk polymorphisms.

RESEARCH DESIGN AND METHODS

Insulin secretion was assessed by insulinogenic index and areas under the curve of C-peptide/glucose in 1,576 subjects using an oral glucose tolerance test (OGTT). Participants were genotyped for 10 diabetes risk SNPs associated with β-cell dysfunction: rs5215 (KCNJ11), rs13266634 (SLC30A8), rs7754840 (CDKAL1), rs10811661 (CDKN2A/2B), rs10830963 (MTNR1B), rs7903146 (TCF7L2), rs10010131 (WFS1), rs7923837 (HHEX), rs151290 (KCNQ1), and rs4402960 (IGF2BP2). Furthermore, the impact of the interaction between genetic variation in TCF7L2 and glycemia on changes in insulin secretion was tested in 315 individuals taking part in a lifestyle intervention study.

RESULTS

For the SNPs in TCF7L2 and WFS1, we found a significant interaction between glucose control and insulin secretion (all P ≤ 0.0018 for glucose × genotype). When plotting insulin secretion against glucose at 120 min OGTT, the compromising SNP effects on insulin secretion are most apparent under high glucose. In the longitudinal study, rs7903146 in TCF7L2 showed a significant interaction with baseline glucose tolerance upon change in insulin secretion (P = 0.0027). Increased glucose levels at baseline predicted an increase in insulin secretion upon improvement of glycemia by lifestyle intervention only in carriers of the risk alleles.

CONCLUSIONS

For the diabetes risk genes TCF7L2 and WFS1, which are associated with impaired incretin signaling, the level of glycemia determines SNP effects on insulin secretion. This indicates the increasing relevance of these SNPs during the progression of prediabetes stages toward clinically overt type 2 diabetes.

Physiologic characterization of type 2 diabetes-related loci

For the past two decades, genetics has been widely explored as a tool for unraveling the pathogenesis of diabetes. Many risk alleles for type 2 diabetes and hyperglycemia have been detected in recent years through massive genome-wide association studies and evidence exists that most of these variants influence pancreatic β-cell function. However, risk alleles in five loci seem to have a primary impact on insulin sensitivity. Investigations of more detailed physiologic phenotypes, such as the insulin response to intravenous glucose or the incretion hormones, are now emerging and give indications of more specific pathologic mechanisms for diabetes-related risk variants. Such studies have shed light on the function of some loci but also underlined the complex nature of disease mechanism. In the future, sequencing-based discovery of low-frequency variants with higher impact on intermediate diabetes-related traits is a likely scenario and identification of new pathways involved in type 2 diabetes predisposition will offer opportunities for the development of novel therapeutic and preventative approaches.

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.

Combined risk allele score of eight type 2 diabetes genes is associated with reduced first-phase glucose-stimulated insulin secretion during hyperglycemic clamps

OBJECTIVE

At least 20 type 2 diabetes loci have now been identified, and several of these are associated with altered beta-cell function. In this study, we have investigated the combined effects of eight known beta-cell loci on insulin secretion stimulated by three different secretagogues during hyperglycemic clamps.

RESEARCH DESIGN AND METHODS

A total of 447 subjects originating from four independent studies in the Netherlands and Germany (256 with normal glucose tolerance [NGT]/191 with impaired glucose tolerance [IGT]) underwent a hyperglycemic clamp. A subset had an extended clamp with additional glucagon-like peptide (GLP)-1 and arginine (n = 224). We next genotyped single nucleotide polymorphisms in TCF7L2, KCNJ11, CDKAL1, IGF2BP2, HHEX/IDE, CDKN2A/B, SLC30A8, and MTNR1B and calculated a risk allele score by risk allele counting.

RESULTS

The risk allele score was associated with lower first-phase glucose-stimulated insulin secretion (GSIS) (P = 7.1 x 10(-6)). The effect size was equal in subjects with NGT and IGT. We also noted an inverse correlation with the disposition index (P = 1.6 x 10(-3)). When we stratified the study population according to the number of risk alleles into three groups, those with a medium- or high-risk allele score had 9 and 23% lower first-phase GSIS. Second-phase GSIS, insulin sensitivity index and GLP-1, or arginine-stimulated insulin release were not significantly different.

CONCLUSIONS

A combined risk allele score for eight known beta-cell genes is associated with the rapid first-phase GSIS and the disposition index. The slower second-phase GSIS, GLP-1, and arginine-stimulated insulin secretion are not associated, suggesting that especially processes involved in rapid granule recruitment and exocytosis are affected in the majority of risk loci.

Pathomechanisms of type 2 diabetes genes

Type 2 diabetes mellitus is a complex metabolic disease that is caused by insulin resistance and beta-cell dysfunction. Furthermore, type 2 diabetes has an evident genetic component and represents a polygenic disease. During the last decade, considerable progress was made in the identification of type 2 diabetes risk genes. This was crucially influenced by the development of affordable high-density single nucleotide polymorphism (SNP) arrays that prompted several successful genome-wide association scans in large case-control cohorts. Subsequent to the identification of type 2 diabetes risk SNPs, cohorts thoroughly phenotyped for prediabetic traits with elaborate in vivo methods allowed an initial characterization of the pathomechanisms of these SNPs. Although the underlying molecular mechanisms are still incompletely understood, a surprising result of these pathomechanistic investigations was that most of the risk SNPs affect beta-cell function. This favors a beta-cell-centric view on the genetics of type 2 diabetes. The aim of this review is to summarize the current knowledge about the type 2 diabetes risk genes and their variants' pathomechanisms.

Genetic variations in the pancreatic ATP-sensitive potassium channel, beta-cell dysfunction, and susceptibility to type 2 diabetes

The KCNJ11 and ABCC8 genes encode the components of the pancreatic ATP-sensitive potassium (KATP) channel, which regulates insulin secretion by beta-cells and hence could be involved in the pathogenesis of type 2 diabetes (T2D). The KCNJ11 E23K and ABCC8 exon 31 variants have been studied in 127 Russian T2D patients and 117 controls using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) approach. The KCNJ11 E23 variant and the ABCC8 exon 31 allele A were associated with higher risk of T2D [Odds ratio (OR) of 1.53 (P=0.023) and 2.41 (P=1.95 x 10(-5))], respectively. Diabetic carriers of the ABCC8 G/G variant had reduced 2 h glucose compared to A/A+A/G (P=0.031). The G/G genotype of ABCC8 was also significantly associated with increased both fasting and 2 h serum insulin in diabetic and non-diabetic patients. A HOMA-beta value characterizing the beta-cell homeostasis was higher in the non-diabetic carriers homozygous for G/G (98.0+/-46.9) then for other genotypes (HOMA-beta = 85.6+/-45.5 for A/A+A/G, P=0.0015). The KCNJ11 E23K and ABCC8 exon 31 variants contribute to susceptibility to T2D diabetes, glucose intolerance and altered insulin secretion in a Russian population.

K(ATP)-channels and glucose-regulated glucagon secretion

Glucagon, secreted by the alpha-cells of the pancreatic islets, is the most important glucose-increasing hormone of the body. The precise regulation of glucagon release remains incompletely defined but has been proposed to involve release of inhibitory factors from neighbouring beta-cells (paracrine control). However, the observation that glucose can regulate glucagon secretion under conditions when insulin secretion does not occur argues that the alpha-cell is also equipped with its own intrinsic (exerted within the alpha-cell itself) glucose sensing. Here we consider the possible mechanisms involved with a focus on ATP-regulated K(+)-channels and changes in alpha-cell membrane potential.

Variants of CDKAL1 and IGF2BP2 affect first-phase insulin secretion during hyperglycaemic clamps

AIMS/HYPOTHESIS

Genome-wide association studies have recently identified novel type 2 diabetes susceptibility gene regions. We assessed the effects of six of these regions on insulin secretion as determined by a hyperglycaemic clamp.

METHODS

Variants of the HHEX/IDE, CDKAL1, SLC30A8, IGF2BP2 and CDKN2A/CDKN2B genes were genotyped in a cohort of 146 participants with NGT and 126 with IGT from the Netherlands and Germany, who all underwent a hyperglycaemic clamp at 10 mmol/l glucose.

RESULTS

Variants of CDKAL1 and IGF2BP2 were associated with reductions in first-phase insulin secretion (34% and 28%, respectively). The disposition index was also significantly reduced. For gene regions near HHEX/IDE, SLC30A8 and CDKN2A/CDKN2B we did not find significant associations with first-phase insulin secretion (7-18% difference between genotypes; all p > 0.3). None of the variants showed a significant effect on second-phase insulin secretion in our cohorts (2-8% difference between genotypes, all p > 0.3). Furthermore, the gene variants were not associated with the insulin sensitivity index.

CONCLUSIONS

Variants of CDKAL1 and IGF2BP2 attenuate the first phase of glucose-stimulated insulin secretion but show no effect on the second phase of insulin secretion. Our results, based on hyperglycaemic clamps, provide further insight into the pathogenic mechanism behind the association of these gene variants with type 2 diabetes.

Neonatal hyperglycaemia and abnormal development of the pancreas

Transient and permanent neonatal diabetes mellitus (TNDM and PNDM) are rare conditions occurring in around 1 per 300,000 live births. In TNDM, growth-retarded infants develop diabetes in the first few weeks of life, only to go into remission after a few months with possible relapse to permanent diabetes usually around adolescence or in adulthood. In PNDM, insulin secretory failure occurs in the late fetal or early postnatal period. The very recently elucidated mutations in KCNJ11 and ABCC8 genes, encoding the Kir6.2 and SUR1 subunits of the pancreatic K(ATP) channel involved in regulation of insulin secretion, account for a third to a half of the PNDM cases. Molecular analysis of chromosome 6 anomalies and the KCNJ11 and ABCC8 genes encoding Kir6.2 and SUR1 provides a tool for distinguishing transient from permanent neonatal diabetes mellitus in the neonatal period. Some patients (those with mutations in KCNJ11 and ABCC8) may be transferred from insulin therapy to sulphonylureas.

Impaired glucagon-like peptide-1-induced insulin secretion in carriers of transcription factor 7-like 2 (TCF7L2) gene polymorphisms

AIMS/HYPOTHESIS

Polymorphisms in the transcription factor 7-like 2 (TCF7L2) gene are associated with type 2 diabetes and reduced insulin secretion. The transcription factor TCF7L2 is an essential factor for glucagon-like peptide-1 (GLP-1) secretion from intestinal L cells. We studied whether a defect in the enteroinsular axis contributes to impaired insulin secretion in carriers of TCF7L2 polymorphisms.

METHODS

We genotyped 1,110 non-diabetic German participants for five single nucleotide polymorphisms in TCF7L2. All participants underwent an OGTT; GLP-1 secretion was measured in 155 participants. In 210 participants, an IVGTT combined with a hyperinsulinaemic-euglycaemic clamp was performed. In another 160 participants from the Netherlands and 73 from Germany, a hyperglycaemic clamp (10 mmol/l) was performed. In 73 German participants this clamp was combined with a GLP-1 infusion and an arginine bolus.

RESULTS

The OGTT data confirmed that variants in TCF7L2 are associated with reduced insulin secretion. In contrast, insulin secretion induced by an i.v. glucose challenge in the IVGTT and hyperglycaemic clamp was not different between the genotypes. GLP-1 concentrations during the OGTT were not influenced by the TCF7L2 variants. However, GLP-1-infusion combined with a hyperglycaemic clamp showed a significant reduction in GLP-1-induced insulin secretion in carriers of the risk allele in two variants (rs7903146, rs12255372, p < 0.02).

CONCLUSIONS/INTERPRETATION

Variants of TCF7L2 specifically impair GLP-1-induced insulin secretion. This seems to be rather the result of a functional defect in the GLP-1 signalling in beta cells than a reduction in GLP-1 secretion. This defect might explain the impaired insulin secretion in carriers of the risk alleles and confers the increased risk of type 2 diabetes.

A K ATP channel-dependent pathway within alpha cells regulates glucagon release from both rodent and human islets of Langerhans

Glucagon, secreted from pancreatic islet α cells, stimulates gluconeogenesis and liver glycogen breakdown. The mechanism regulating glucagon release is debated, and variously attributed to neuronal control, paracrine control by neighbouring β cells, or to an intrinsic glucose sensing by the α cells themselves. We examined hormone secretion and Ca²⁺ responses of α and β cells within intact rodent and human islets. Glucose-dependent suppression of glucagon release persisted when paracrine GABA or Zn²⁺ signalling was blocked, but was reversed by low concentrations (1–20 μM) of the ATP-sensitive K⁺ (K_ATP) channel opener diazoxide, which had no effect on insulin release or β cell responses. This effect was prevented by the K_ATP channel blocker tolbutamide (100 μM). Higher diazoxide concentrations (≥30 μM) decreased glucagon and insulin secretion, and α- and β-cell Ca²⁺ responses, in parallel. In the absence of glucose, tolbutamide at low concentrations (<1 μM) stimulated glucagon secretion, whereas high concentrations (>10 μM) were inhibitory. In the presence of a maximally inhibitory concentration of tolbutamide (0.5 mM), glucose had no additional suppressive effect. Downstream of the K_ATP channel, inhibition of voltage-gated Na⁺ (TTX) and N-type Ca²⁺ channels (ω-conotoxin), but not L-type Ca²⁺ channels (nifedipine), prevented glucagon secretion. Both the N-type Ca²⁺ channels and α-cell exocytosis were inactivated at depolarised membrane potentials. Rodent and human glucagon secretion is regulated by an α-cell K_ATP channel-dependent mechanism. We propose that elevated glucose reduces electrical activity and exocytosis via depolarisation-induced inactivation of ion channels involved in action potential firing and secretion.

Glucagon is a critical regulator of glucose homeostasis. Its major action is to mobilize glucose from the liver. Glucagon secretion from α cells of the pancreatic islets of Langerhans is suppressed by elevated blood sugar, a response that is often perturbed in diabetes. Much work has focused on the regulation of α-cell glucagon secretion by neuronal factors and by paracrine factors from neighbouring cells, including the important islet hormone insulin. In contrast, we provide evidence in support of a direct effect of glucose on α cells within intact rodent and human islets. Notably, our work implicates an α-cell glucose-sensing pathway similar to that found in insulin-secreting β cells, involving closure of ATP-dependent K⁺ channels in the presence of glucose. Furthermore, we find that membrane depolarisation results in inhibition of Na⁺ and Ca²⁺ channel activity and α-cell exocytosis. Thus, we propose that elevated blood glucose reduces α-cell electrical activity and glucagon secretion by inactivating the ion channels involved in action potential firing and secretion.

Elevated glucose levels reduce electrical activity and the release of glucagon via inactivation of ion channels in pancreatic islet cells.

Diabetes and hypoglycaemia in young children and mutations in the Kir6.2 subunit of the potassium channel: therapeutic consequences

ATP-sensitive potassium channels (K(ATP)) couple cell metabolism to electrical activity by regulating potassium movement across the membrane. These channels are octameric complex with two kind of subunits: four regulatory sulfonylurea receptor (SUR) embracing four poreforming inwardly rectifying potassium channel (Kir). Several isoforms exist for each type of subunits: SUR1 is found in the pancreatic beta-cell and neurons, whereas SUR2A is in heart cells and SUR2B in smooth muscle; Kir6.2 is in the majority of tissues as pancreatic beta-cells, brain, heart and skeletal muscle, and Kir6.1 can be found in smooth vascular muscle and astrocytes. The K(ATP) channels play multiple physiological roles in the glucose metabolism regulation, especially in beta-cells where it regulates insulin secretion, in response to an increase in ATP concentration. They also seem to be critical metabolic sensors in protection against metabolic stress as hypo or hyperglycemia, hypoxia, ischemia. Persistent hyperinsulinemic hypoglycaemia (HI) of infancy is a heterogeneous disorder which may be divided into two histopathological forms (diffuse and focal lesions). Different inactivating mutations have been implicated in both forms: the permanent inactivation of the K(ATP) channels provokes inappropriate insulin secretion, despite low ATP. Diazoxide, used efficiently in certain cases of HI, opens the K(ATP) channels and therefore overpass the mutation effect on the insulin secretion. Conversely, several studies reported sequencing of KCNJ11, coding for Kir6.2, in patients with permanent neonatal diabetes mellitus and found different mutations in 30 to 50% of the cases. More than 28 heterozygous activating mutations have now been identified, the most frequent mutation being in the aminoacid R201. These mutations result in reduced ATP-sensitivity of the K(ATP) channels compared with the wild-types and the level of channel block is responsible for different clinical features: the "mild" form confers isolated permanent neonatal diabetes whereas the severe form combines diabetes and neurological symptoms such as epilepsy, deve-lopmental delay, muscle weakness and mild dimorphic features. Sulfonylureas close K(ATP) channels by binding with high affinity to SUR suggesting they could replace insulin in these patients. Subsequently, more than 50 patients have been reported as successfully and safely switched from subcutaneous insulin injections to oral sulfonylurea therapy, with an improvement in their glycated hemoglobin. We therefore designed a protocol to transfer and evaluate children who have insulin treated neonatal diabetes due to KCNJ11 mutation, from insulin to sulfonylurea. The transfer from insulin injections to oral glibenclamide therapy seems highly effective for most patients and safe. This shows how the molecular understan-ding of some monogenic form of diabetes may lead to an unexpected change of the treatment in children. This is a spectacular example by which a pharmacogenomic approach improves the quality of life of our young diabetic patients in a tremendous way.

Alpha-cells of the endocrine pancreas: 35 years of research but the enigma remains

Glucagon, a hormone secreted from the alpha-cells of the endocrine pancreas, is critical for blood glucose homeostasis. It is the major counterpart to insulin and is released during hypoglycemia to induce hepatic glucose output. The control of glucagon secretion is multifactorial and involves direct effects of nutrients on alpha-cell stimulus-secretion coupling as well as paracrine regulation by insulin and zinc and other factors secreted from neighboring beta- and delta-cells within the islet of Langerhans. Glucagon secretion is also regulated by circulating hormones and the autonomic nervous system. In this review, we describe the components of the alpha-cell stimulus secretion coupling and how nutrient metabolism in the alpha-cell leads to changes in glucagon secretion. The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions. We also highlight the pathophysiology of the alpha-cell and how hyperglucagonemia represents an important component of the metabolic abnormalities associated with diabetes mellitus. Therapeutic inhibition of glucagon action in patients with type 2 diabetes remains an exciting prospect.

The E23K variant of KCNJ11 encoding the pancreatic beta-cell adenosine 5'-triphosphate-sensitive potassium channel subunit Kir6.2 is associated with an increased risk of secondary failure to sulfonylurea in patients with type 2 diabetes

CONTEXT

Several studies suggest that genetic factors may play a role in the different responses to antidiabetic therapy; however, conclusive evidence is still lacking.

OBJECTIVE

The objective of the study was to investigate whether diabetic patients carrying the E23K variant in KCNJ11 are at increased risk for secondary sulfonylurea failure.

DESIGN

Secondary sulfonylurea failure was defined as fasting plasma glucose greater than 300 mg/dl despite sulfonylurea-metformin combined therapy and appropriate diet, in the absence of other conditions causing hyperglycemia.

SETTING

The study was conducted in an ambulatory care facility.

PATIENTS

A total of 525 Caucasian type 2 diabetic patients were enrolled in the study.

INTERVENTION

Sulfonylurea treatment was followed by sulfonylurea-metformin combined therapy and then insulin treatment.

MAIN OUTCOME MEASURE

Secondary failure was the main outcome measure.

RESULTS

Of the diabetic patients enrolled in the study, 38.5% were E23E homozygous, 51.4% were E23K heterozygous, and 10.1% were K23K homozygous. The frequency of carriers of the K allele was 58 and 66.8% among patients treated with oral therapy or secondary sulfonylurea failure, respectively (odds ratio, 1.45; 95% confidence interval, 1.01-2.09; P = 0.04). Adjustment for age, gender, fasting glycemia, glycosylated hemoglobin, age at diagnosis, and duration of diabetes in a logistic regression analysis did not change this association (odds ratio, 1.69; 95% confidence interval, 1.02-2.78; P = 0.04). Islets isolated from carriers of the K allele showed no differences in glucose-stimulated insulin secretion and a tendency toward reduced response upon glibenclamide stimulation (P = 0.09). After 24-h exposure to high (16.7 mmol/liter) glucose concentration, impairment of glibenclamide-induced insulin release was significantly (P = 0.01) worse with the E23K variant.

CONCLUSIONS

These data suggest that the E23K variant in KCNJ11 may influence the variability in the response of patients to sulfonylureas, thus representing an example of pharmacogenetics in type 2 diabetes.

Adiponectin is functionally active in human islets but does not affect insulin secretory function or beta-cell lipoapoptosis

CONTEXT

The adipokine adiponectin has insulin-sensitizing, antiatherogenic, and antiinflammatory properties. Mouse and human adiponectin receptor-1 and -2 have been cloned, both of which are expressed in various tissues and mediate effects of globular and full-length adiponectin. Whether adiponectin affects insulin secretion and beta-cell apoptosis and whether plasma adiponectin is associated with beta-cell function in humans is under investigation.

DESIGN AND METHODS

In human islets from multiorgan donors, we investigated expression of adiponectin receptor-1 and -2. Furthermore, glucose-stimulated insulin secretion was determined by RIA. In addition, we investigated fatty acid-induced beta-cell apoptosis by terminal dUTP nick end labeling and flow-cytometric cell cycle analysis (sub-G1 formation). In humans in vivo, insulin secretory function was measured during hyperglycemic clamps in 65 normal glucose-tolerant subjects. We determined first and second phase of glucose-stimulated, glucagon-like peptide-1-stimulated, and arginine-stimulated insulin secretion.

RESULTS

Adiponectin receptor-1 and -2 are expressed in human islets at the mRNA and protein level. Moreover, full-length adiponectin induces phosphorylation of acetyl coenzyme A carboxylase. However, adiponectin did not affect basal or glucose-stimulated insulin secretion or basal or fatty acid-induced beta-cell apoptosis. In vivo, fasting plasma adiponectin concentrations were not associated with glucose-stimulated first- and second-phase insulin secretion or with glucagon-like peptide-1- or arginine-stimulated insulin secretion (all P > 0.42).

CONCLUSIONS

These data support a regulatory role of adiponectin in human islets; however, adiponectin does not seem to affect insulin secretion or basal/fatty acid-induced beta-cell apoptosis in humans.

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 potassium channelopathies: focus on insulin secretion

ATP-sensitive potassium (K(ATP)) channels, so named because they are inhibited by intracellular (ATP), play key physiological roles in many tissues. In pancreatic beta cells, these channels regulate glucose-dependent insulin secretion and serve as the target for sulfonylurea drugs used to treat type 2 diabetes. This review focuses on insulin secretory disorders, such as congenital hyperinsulinemia and neonatal diabetes, that result from mutations in K(ATP) channel genes. It also considers the extent to which defective regulation of K(ATP) channel activity contributes to the etiology of type 2 diabetes.

Activating mutations in Kir6.2 and neonatal diabetes: new clinical syndromes, new scientific insights, and new therapy

Closure of ATP-sensitive K(+) channels (K(ATP) channels) in response to metabolically generated ATP or binding of sulfonylurea drugs stimulates insulin release from pancreatic beta-cells. Heterozygous gain-of-function mutations in the KCJN11 gene encoding the Kir6.2 subunit of this channel are found in approximately 47% of patients diagnosed with permanent diabetes at <6 months of age. There is a striking genotype-phenotype relationship with specific Kir6.2 mutations being associated with transient neonatal diabetes, permanent neonatal diabetes alone, and a novel syndrome characterized by developmental delay, epilepsy, and neonatal diabetes (DEND) syndrome. All mutations appear to cause neonatal diabetes by reducing K(ATP) channel ATP sensitivity and increasing the K(ATP) current, which inhibits beta-cell electrical activity and insulin secretion. The severity of the clinical symptoms is reflected in the ATP sensitivity of heterozygous channels in vitro with wild type > transient neonatal diabetes > permanent neonatal diabetes > DEND syndrome channels. Sulfonylureas still close mutated K(ATP) channels, and many patients can discontinue insulin injections and show improved glycemic control when treated with high-dose sulfonylurea tablets. In conclusion, the finding that Kir6.2 mutations can cause neonatal diabetes has enabled a new therapeutic approach and shed new light on the structure and function of the Kir6.2 subunit of the K(ATP) channel.

Free fatty acid-induced insulin resistance in the obese is not prevented by rosiglitazone treatment

OBJECTIVE

Elevation of free fatty acids (FFAs) by the infusion of triglyceride-heparin emulsion infusion (TG-Hep) causes insulin resistance (IR). We examined the effect of insulin sensitizer (rosiglitazone) on FFA-induced IR.

DESIGN

Nine obese subjects underwent a 6-h infusion of TG-Hep before and after 6 wk of rosiglitazone (8 mg/d) treatment. Hyperinsulinemic euglycemic clamps were performed during 0-2 and 4-6 h of TG-Hep.

RESULTS

After rosiglitazone for 6 wk, fasting FFA concentration fell, but not significantly (489 +/- 63 at 0 wk; 397 +/- 58 micromol/liter at 6 wk; P = 0.16), whereas C-reactive protein (4.26 +/- 0.95 at 0 wk; 2.03 +/- 0.45 microg/ml at 6 wk) and serum amyloid A (17.36 +/- 4.63 at 0 wk; 8.77 +/- 1.63 microg/ml at 6 wk) decreased significantly. At 0 wk, TG-Hep infusion caused a decrease in glucose infusion rate (GIR) from 4.49 +/- 0.95 mg/kg.min to 3.02 +/- 0.59 mg/kg.min (P = 0.018). Rosiglitazone treatment resulted in an increase in baseline GIR to 6.29 +/- 0.81 mg/kg.min (P = 0.03 vs. 0 wk), which decreased to 4.52 +/- 0.53 mg/kg.min (P = 0.001) after 6 h of TG-Hep infusion. The decrease in GIR induced by TG-Hep infusion was similar before and after rosiglitazone therapy [1.47 +/- 0.50 vs. 1.77 0.3 mg/kg.min (28.9 +/- 6.5 vs. 26.4 +/- 3.7%); P = 0.51]. The rise in FFAs and triglycerides after TG-Hep infusion was significantly lower at 6 wk (P = 0.006 for FFAs; P = 0.024 for triglycerides).

CONCLUSIONS

We conclude that rosiglitazone: 1) causes a significant increase in GIR; 2) induces a decrease in inflammatory mediators, C-reactive protein, and serum amyloid A; 3) decreases the rise in FFAs and triglycerides after TG-Hep infusion; and 4) does not prevent FFA-induced IR.

Analysis of separate and combined effects of common variation in KCNJ11 and PPARG on risk of type 2 diabetes

The separate and combined effects of the PPARG Pro(12)Ala polymorphism and the KCNJ11 Glu(23)Lys polymorphisms on risk of type 2 diabetes were investigated in relatively large-scale, case-control studies. Separate effects of the variants were examined among 1187/1461 type 2 diabetic patients and 4791/4986 middle-aged, glucose-tolerant subjects. The combined analysis involved 1164 type 2 diabetic patients and 4733 middle-aged, glucose-tolerant subjects. In the separate analyses, the K allele of the KCNJ11 Glu(23)Lys associated with type 2 diabetes (odds ratio, 1.19; P = 0.0002), whereas the PPARG Pro(12)Ala showed no significant association with type 2 diabetes. The combined analysis indicated that the two polymorphisms acted in an additive manner to increase the risk of type 2 diabetes, and we found no evidence for a synergistic interaction between them. Analysis of a model with equal additive effects of the two variants showed that the odds ratio for type 2 diabetes increased with 1.14/risk allele (P = 0.003). Together, the two polymorphisms conferred a population-attributable risk for type 2 diabetes of 28%. In conclusion, our results showed no evidence of a synergistic interaction between the KCNJ11 Glu(23)Lys and PPARG Pro(12)Ala polymorphisms, but indicated that they may act in an additive manner to increase the risk of type 2 diabetes.

Roles of KATP channels as metabolic sensors in acute metabolic changes

Physiological and pathophysiological roles of K(ATP) channels have been clarified recently in genetically engineered mice. The Kir6.2-containing K(ATP) channels in pancreatic ss-cells and the hypothalamus are essential in the regulation of glucose-induced insulin secretion and hypoglycemia-induced glucagon secretion, respectively, and are involved in glucose uptake in skeletal muscles, thus playing a key role in the maintenance of glucose homeostasis. Disruption of Kir6.1-containing K(ATP) channels in mice leads to spontaneous vascular spasm mimicking vasospastic (Prinzmetal) angina in humans, indicating that the Kir6.1-containing K(ATP) channels in vascular smooth muscles participate in the regulation of vascular tonus, especially in coronary arteries. Together with protective roles of K(ATP) channels against cardiac ischemia and hypoxia-induced seizure propagation, it is now clear that K(ATP) channels, as metabolic sensors, are critical in the maintenance of homeostasis against acute metabolic changes.

Common variants in the ATP-sensitive K+ channel genes KCNJ11 (Kir6.2) and ABCC8 (SUR1) in relation to glucose intolerance: population-based studies and meta-analyses

AIMS

To evaluate the relation between common variants in the ATP-sensitive K+ channel genes and glucose intolerance.

METHODS

We conducted a meta-analysis of reported association studies in Caucasian populations for common variants in the ABCC8 (exons 16 and 18) and the KCNJ11 (E23K) gene and examined sources of heterogeneity in the results. The meta-analysis was based on 7768-10216 subjects (depending on the gene variant), and included two new population-based studies in the Netherlands with 725 cases and 742 controls.

RESULTS

For the KCNJ11 variant, the summary odds ratio (OR) for glucose intolerance was 1.12 (1.01-1.23, P=0.03) for the EK genotype and 1.44 (1.17-1.78, P=0.0007) for the KK genotype, as compared with the EE genotype. For the ABCC8 exon 16 variant, the OR was 1.06 (0.94-1.19, P=0.34) for ct and 0.93 (0.71-1.20, P=0.56) for tt, as compared with the cc genotype. For ABCC8 exon 18, the OR was 1.20 (0.97-1.49, P=0.10) for CT/TT, as compared with the CC genotype. Studies of the ABCC8 variants that were published first or had smaller sample sizes (for the exon 18 variant) showed stronger associations, which may indicate publication bias. For the ABCC8 exon 18 and the KCNJ11 variant, associations were stronger for studies of clinical diabetes than newly detected glucose intolerance. The population attributable risk for clinical Type 2 diabetes was 6.2% for the KCNJ11 KK genotype and 10.1% for the KCNJ11 EK and KK genotype combined.

CONCLUSIONS

The common KCNJ11 E23K gene variant, but not the ABCC8 exon 16 or exon 18 variant, was consistently associated with Type 2 diabetes.

Genetics of type 2 diabetes mellitus: status and perspectives

Throughout the last decade, molecular genetic studies of non-autoimmune diabetes mellitus have contributed significantly to our present understanding of this disease's complex aetiopathogenesis. Monogenic forms of diabetes (maturity-onset diabetes of the young, MODY) have been identified and classified into MODY1-6 according to the mutated genes that by being expressed in the pancreatic beta-cells confirm at the molecular level the clinical presentation of MODY as a predominantly insulin secretory deficient form of diabetes mellitus. Genomewide linkage studies of presumed polygenic type 2 diabetic populations indicate that loci on chromosomes 1q, 5q, 8p, 10q, 12q and 20q contain susceptibility genes. Yet, so far, the only susceptibility gene, calpain-10 (CAPN10), which has been identified using genomewide linkage studies, is located on chromosome 2q37. Mutation analyses of selected 'candidate' susceptibility genes in various populations have also identified the widespread Pro12Ala variant of the peroxisome proliferator-activated receptor-gamma and the common Glu23Lys variant of the ATP-sensitive potassium channel, Kir6.2 (KCNJ11). These variants may contribute significantly to the risk type 2 diabetes conferring insulin resistance of liver, muscle and fat (Pro12Ala) and a relative insulin secretory deficiency (Glu23Lys). It is likely that, in the near future, the recent more detailed knowledge of the human genome and insights into its haploblocks together with the developments of high-throughput and cheap genotyping will facilitate the discovery of many more type 2 diabetes gene variants in study materials, which are statistically powered and phenotypically well characterized. The results of these efforts are likely to be the platform for major progress in the development of personalized antidiabetic drugs with higher efficacy and few side effects.

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

The ATP-sensitive potassium (KATP) channel couples membrane excitability to cellular metabolism and is a critical mediator in the process of glucose-stimulated insulin secretion. Increasing numbers of KATP channel polymorphisms are being described and linked to altered insulin secretion indicating that genes encoding this ion channel could be susceptibility markers for type-2 diabetes. Genetic variation of KATP channels may result in altered beta-cell electrical activity, glucose homeostasis, and increased susceptibility to type-2 diabetes. Of particular interest is the Kir6.2 E23K polymorphism, which is linked to increased susceptibility to type-2 diabetes in Caucasian populations and may also be associated with weight gain and obesity, both of which are major diabetes risk factors. This association highlights the potential contribution of both genetic and environmental factors to the development and progression of type-2 diabetes. In addition, the common occurrence of the E23K polymorphism in Caucasian populations may have conferred an evolutionary advantage to our ancestors. This review will summarize the current status of the association of KATP channel polymorphisms with type-2 diabetes, focusing on the possible mechanisms by which these polymorphisms alter glucose homeostasis and offering insights into possible evolutionary pressures that may have contributed to the high prevalence of KATP channel polymorphisms in the Caucasian population.

Kir6.2 polymorphisms sensitize beta-cell ATP-sensitive potassium channels to activation by acyl CoAs: a possible cellular mechanism for increased susceptibility to type 2 diabetes?

The commonly occurring E23K and I337V Kir6.2 polymorphisms in the ATP-sensitive potassium (KATP) channel are more frequent in Caucasian type 2 diabetic populations. However, the underlying cellular mechanisms contributing to the pathogenesis of type 2 diabetes remain uncharacterized. Chronic elevation of plasma free fatty acids observed in obese and type 2 diabetic subjects leads to cytosolic accumulation of long-chain acyl CoAs (LC-CoAs) in pancreatic beta-cells. We postulated that the documented stimulatory effects of LC-CoAs on KATP channels might be enhanced in polymorphic KATP channels. Patch-clamp experiments were performed on inside-out patches containing recombinant KATP channels (Kir6.2/SUR1) to record macroscopic currents. KATP channels containing Kir6.2 (E23K/I337V) showed significantly increased activity in response to physiological palmitoyl-CoA concentrations (100-1,000 nmol/l) compared with wild-type KATP channels. At physiological intracellular ATP concentrations (mmol/l), E23K/I337V polymorphic KATP channels demonstrated significantly enhanced activity in response to palmitoyl-CoA. The observed increase in KATP channel activity may result in multiple defects in glucose homeostasis, including impaired insulin and glucagon-like peptide-1 secretion and increased glucagon release. In summary, these results suggest that the E23K/I337V polymorphism may have a diabetogenic effect via increased KATP channel activity in response to endogenous levels of LC-CoAs in tissues involved in the maintenance of glucose homeostasis.

NN414, a SUR1/Kir6.2-selective potassium channel opener, reduces blood glucose and improves glucose tolerance in the VDF Zucker rat

A novel potassium channel opener compound, NN414, selective for the SUR1/Kir6.2 subtype of the ATP-sensitive potassium channel, was used to examine the effect of reducing beta-cell workload in the male Vancouver diabetic fatty (VDF) Zucker rat model of mild type 2 diabetes. Two chronic dosing protocols of NN414 of 3 weeks' duration were compared with appropriate vehicle-treated controls. In the first group, rats received NN414 (continued group; 1.5 mg/kg p.o. twice daily), during which an oral glucose tolerance test (OGTT) (on day 19 of dosing) was performed and insulin secretion from an in situ perfused pancreas preparation (on day 21) was measured. The second group received NN414 (discontinued group; same dose), but active treatment was replaced by vehicle treatment 2 days before the OGTT and for a further 2 days before the perfused pancreas study. Basal glucose was significantly reduced by NN414, with the fall averaging 0.64 mmol/l after 3 weeks of treatment (P < 0.0001). The glucose excursion and hyperinsulinemia during the OGTT were significantly different between the continued, discontinued, and vehicle groups (glucose area under the curve [AUC]: 640 +/- 29, 740 +/- 27, and 954 +/- 82 mmol. l(-1). min(-1), respectively, P < 0.0001; insulin AUC: 38.9 +/- 4.2, 44.2 +/- 4.2, and 55.1 +/- 2.6 nmol.l(-1).min(-1), respectively, P < 0.0001). Hyperinsulinemia during the pancreas perfusion with 4.4 mmol/l glucose was significantly reduced in both treatment groups versus vehicle (P < 0.0005). Insulin secretory responsiveness to a step increase in glucose from 4.4 to 16.6 mmol/l, calculated relative to basal, was significantly improved in the continued group versus vehicle (P < 0.01). In conclusion, administration of NN414 for 3 weeks in VDF rats reduces basal hyperglycemia, improves glucose tolerance, and reduces hyperinsulinemia during an OGTT and improves insulin secretory responsiveness ex vivo. NN414 may therefore represent a novel approach to the prevention and treatment of impaired glucose tolerance and type 2 diabetes.