Carriers of an inactivating beta-cell ATP-sensitive K(+) channel mutation have normal glucose tolerance and insulin sensitivity and appropriate insulin secretion

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

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

Novel dominant KATP channel mutations in infants with congenital hyperinsulinism: Validation by in vitro expression studies and in vivo carrier phenotyping

Inactivating mutations in the genes encoding the two subunits of the pancreatic beta-cell KATP channel, ABCC8 and KCNJ11, are the most common finding in children with congenital hyperinsulinism (HI). Interpreting novel missense variants in these genes is problematic, because they can be either dominant or recessive mutations, benign polymorphisms, or diabetes mutations. This report describes six novel missense variants in ABCC8 and KCNJ11 that were identified in 11 probands with congenital HI. One of the three ABCC8 mutations (p.Ala1458Thr) and all three KCNJ11 mutations were associated with responsiveness to diazoxide. Sixteen family members carried the ABCC8 or KCNJ11 mutations; only two had hypoglycemia detected at birth and four others reported symptoms of hypoglycemia. Phenotype testing of seven adult mutation carriers revealed abnormal protein-induced hypoglycemia in all; fasting hypoketotic hypoglycemia was demonstrated in four of the seven. All of six mutations were confirmed to cause dominant pathogenic defects based on in vitro expression studies in COSm6 cells demonstrating normal trafficking, but reduced responses to MgADP and diazoxide. These results indicate a combination of in vitro and in vivo phenotype tests can be used to differentiate dominant from recessive KATP channel HI mutations and personalize management of children with congenital HI.

The unusual stoichiometry of ADP activation of the KATP channel

K_ATP channels, oligomers of 4 pore-forming Kir6.2 proteins and 4 sulfonylurea receptors (SUR), sense metabolism by monitoring both cytosolic ATP, which closes the channel by interacting with Kir6.2, and ADP, which opens it via SUR. SUR mutations that alter activation by ADP are a major cause of K_ATP channelopathies. We examined the mechanism of ADP activation by analysis of single-channel and macropatch recordings from Xenopus oocytes expressing various mixtures of wild-type SUR2A and an ADP-activation-defective mutant. Evaluation of the data by a binomial distribution model suggests that wild-type and mutant SURs freely co-assemble and that channel activation results from interaction of ADP with only 2 of 4 SURs. This finding explains the heterozygous nature of most K_ATP channelopathies linked to mutations altering ADP activation. It also suggests that the channel deviates from circular symmetry and could function as a dimer-of-dimers.

The diabetic β-cell: hyperstimulated vs. hyperexcited

Hyperglycaemia has multiple effects on β-cells, some clearly prosecretory, including hyperplasia and elevated insulin content, but eventually, a 'glucotoxic' effect which leads to pancreatic β-cell dysfunction, reduced β-cell mass and insulin deficiency, is an important part of diabetes pathophysiology. Myriad underlying cellular and molecular processes could lead to such dysfunction. High glucose will stimulate glycolysis and oxidative phosphorylation, which will in turn increase β-cell membrane excitability through K(ATP) channel closure. Chronic hyperexcitability will then lead to persistently elevated [Ca(2+)](i), a key trigger to insulin secretion. Thus, at least a part of the consequence of 'hyperstimulation' by glucose has been suggested to be a result of 'hyperexcitability' and chronically elevated [Ca(2+)](i). This link is lost when the [glucose], K(ATP) -channel activity link is broken, either pharmacologically or genetically. In isolated islets, such studies reveal that hyperexcitability causes a largely reversible chronic loss of insulin content, but in vivo chronic hyperexcitability per se does not lead to β-cell death or loss of insulin content. On the other hand, chronic inexcitability in vivo leads to systemic diabetes and consequential β-cell death, even while [Ca(2+)](i) remains low.

In vitro insulin secretion by pancreatic tissue from infants with diazoxide-resistant congenital hyperinsulinism deviates from model predictions

Congenital hyperinsulinism (CHI) is the major cause of persistent neonatal hypoglycemia. CHI most often occurs due to mutations in the ABCC8 (which encodes sulfonylurea receptor 1) or KCNJ11 (which encodes the potassium channel Kir6.2) gene, which result in a lack of functional KATP channels in pancreatic β cells. Diffuse forms of CHI (DiCHI), in which all β cells are abnormal, often require subtotal pancreatectomy, whereas focal forms (FoCHI), which are characterized by localized hyperplasia of abnormal β cells, can be cured by resection of the lesion. Here, we characterized the in vitro kinetics of insulin secretion by pancreatic fragments from 6 DiCHI patients and by focal lesion and normal adjacent pancreas from 18 FoCHI patients. Responses of normal pancreas were similar to those reported for islets from adult organ donors. Compared with normal pancreas, basal insulin secretion was elevated in both FoCHI and DiCHI tissue. Affected tissues were heterogeneous in their secretory responses, with increased glucose levels often producing a rapid increase in insulin secretion that could be followed by a paradoxical decrease below prestimulatory levels. The KATP channel blocker tolbutamide was consistently ineffective in stimulating insulin secretion; conversely, the KATP channel activator diazoxide often caused an unanticipated increase in insulin secretion. These observed alterations in secretory behavior were similar in focal lesion and DiCHI tissue, and independent of the specific mutation in ABCC8 or KCNJ11. They cannot be explained by classic models of β cell function. Our results provide insight into the excessive and sometimes paradoxical changes in insulin secretion observed in CHI patients with inactivating mutations of KATP channels.

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

OBJECTIVE

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

RESEARCH DESIGN AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Hyperinsulinism and diabetes: genetic dissection of beta cell metabolism-excitation coupling in mice

The role of metabolism-excitation coupling in insulin secretion has long been apparent, but in recent years, in parallel with studies of human hyperinsulinism and diabetes, genetic manipulation of proteins involved in glucose transport, metabolism, and excitability in mice has brought the central importance of this pathway into sharp relief. We focus on these animal studies and how they provide important insights into not only metabolic and electrical regulation of insulin secretion, but also downstream consequences of alterations in this pathway and the etiology and treatment of insulin-secretion diseases in humans.

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

BACKGROUND

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

METHODS AND FINDINGS

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

CONCLUSIONS

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

Electrocardiographic alterations during hyperinsulinemic hypoglycemia in healthy subjects

BACKGROUND

We evaluated the arrhythmogenic potential of hypoglycemia by studying electrocardiographic (ECG) changes in response to hyperinsulinemic hypoglycemia and associated sympathoadrenal counterregulatory responses in healthy subjects.

METHODS

The study population consisted of 18 subjects, aged 30-40 years. Five-minute ECG recordings and blood samplings were performed at baseline and during the euglycemic and hypoglycemic hyperinsulinemic clamp studies. PR, QT, and QTc intervals of electrocardiogram and ECG morphology were assessed from signal-averaged ECG.

RESULTS

Although cardiac beat interval remained unchanged, PR interval decreased (P < 0.01) and QTc interval (P < 0.001) increased in response to hyperinsulinemic hypoglycemia. Concomitant morphological alterations consisted of slight increases in R-wave amplitude and area (P < 0.01 for both), significant decreases in T-wave amplitude and area (P < 0.001 for both), and moderate ST depression (P < 0.001). Counterregulatory norepinephrine response correlated with amplification of the R wave (r =-0.620, P < 0.05) and epinephrine response correlated with flattening of the T wave (r =-0.508, P < 0.05).

CONCLUSIONS

Hyperinsulinemic hypoglycemia with consequent sympathetic humoral activation is associated with several ECG alterations in atrioventricular conduction, ventricular depolarization, and ventricular repolarization. Such alterations in cardiac electrical function may be of importance in provoking severe arrhythmias and "dead-in-bed" syndrome in diabetic patients with unrecognized hypoglycemic episodes.

beta-cell hyperexcitability: from hyperinsulinism to diabetes

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

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

AIMS/HYPOTHESIS

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS/INTERPRETATION

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

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

ATP-sensitive K+ channels (K(ATP) channels) control electrical activity in beta-cells and therefore are key players in excitation-secretion coupling. Partial suppression of beta-cell K(ATP) channels in transgenic (AAA) mice causes hypersecretion of insulin and enhanced glucose tolerance, whereas complete suppression of these channels in Kir6.2 knockout (KO) mice leads to hyperexcitability, but mild glucose intolerance. To test the interplay of hyperexcitability and dietary stress, we subjected AAA and KO mice to a high-fat diet. After 3 months on the diet, both AAA and KO mice converted to an undersecreting and markedly glucose-intolerant phenotype. Although Kir6.2 is expressed in multiple tissues, its primary functional consequence in both AAA and KO mice is enhanced beta-cell electrical activity. The results of our study provide evidence that, when combined with dietary stress, this hyperexcitability is a causal diabetic factor. We propose an "inverse U" model for the response to enhanced beta-cell excitability: the expected initial hypersecretion can progress to undersecretion and glucose-intolerance, either spontaneously or in response to dietary stress.

The role of sulphonylureas in the management of type 2 diabetes mellitus

The sulphonylureas act by triggering insulin release from the pancreatic beta cell. A specific site on the adenosine triphosphate (ATP)-sensitive potassium channels is occupied by sulphonylureas leading to closure of the potassium channels and subsequent opening of calcium channels. This results in exocytosis of insulin. The meglitinides are not sulphonylureas but also occupy the sulphonylurea receptor unit coupled to the ATP-sensitive potassium channel. Glibenclamide (glyburide), gliclazide, glipizide and glimepiride are the primary sulphonylureas in current clinical use for type 2 diabetes mellitus. Glibenclamide has a higher frequency of hypoglycaemia than the other agents. With long-term use, there is a progressive decrease in the effectiveness of sulphonylureas. This loss of effect is the result of a reduction in insulin-producing capacity by the pancreatic beta cell and is also seen with other antihyperglycaemic agents. The major adverse effect of sulphonylureas is hypoglycaemia. There is a theoretical concern that sulphonylureas may affect cardiac potassium channels resulting in a diminished response to ischaemia. There are now many choices for initial therapy of type 2 diabetes in addition to sulphonylureas. Metformin and thiazolidinediones affect insulin sensitivity by independent mechanisms. Disaccharidase inhibitors reduce rapid carbohydrate absorption. No single agent appears capable of achieving target glucose levels in the majority of patients with type 2 diabetes. Combinations of agents are successful in lowering glycosylated haemoglobin levels more than with a single agent. Sulphonylureas are particularly beneficial when combined with agents such as metformin that decrease insulin resistance. Sulphonylureas can also be given with a basal insulin injection to provide enhanced endogenous insulin secretion after meals. Sulphonylureas will continue to be used both primarily and as part of combined therapy for most patients with type 2 diabetes.

Hyperinsulinism in Infancy: From Basic Science to Clinical Disease

Dunne, Mark J., Karen E. Cosgrove, Ruth M. Shepherd, Albert Aynsley-Green, and Keith J. Lindley. Hyperinsulinism in Infancy: From Basic Science to Clinical Disease. Physiol Rev 84: 239–275, 2004; 10.1152/physrev.00022.2003.—Ion channelopathies have now been described in many well-characterized cell types including neurons, myocytes, epithelial cells, and endocrine cells. However, in only a few cases has the relationship between altered ion channel function, cell biology, and clinical disease been defined. Hyperinsulinism in infancy (HI) is a rare, potentially lethal condition of the newborn and early childhood. The causes of HI are varied and numerous, but in almost all cases they share a common target protein, the ATP-sensitive K+channel. From gene defects in ion channel subunits to defects in β-cell metabolism and anaplerosis, this review describes the relationship between pathogenesis and clinical medicine. Until recently, HI was generally considered an orphan disease, but as parallel defects in ion channels, enzymes, and metabolic pathways also give rise to diabetes and impaired insulin release, the HI paradigm has wider implications for more common disorders of the endocrine pancreas and the molecular physiology of ion transport.

Variations in insulin secretion in carriers of the E23K variant in the KIR6.2 subunit of the ATP-sensitive K(+) channel in the beta-cell

An association between type 2 diabetes and genetic variation in the KIR6.2 gene has been reported in several populations. Based on in vitro studies with cell lines expressing the Glu(23)Lys (E23K) mutation, it was recently suggested that this mutation might result in altered insulin secretion. We have examined glucose-stimulated insulin secretion in relation to this KIR6.2 gene variant in two independent Dutch cohorts. Subjects with normal (n = 65) or impaired (n = 94) glucose tolerance underwent 3-h hyperglycemic clamps at 10 mmol/l glucose. We did not observe significant differences in first- or second-phase insulin secretion between carriers and noncarriers of the gene variant in either of the study populations (all P > 0.45). Furthermore, we found no evidence for a significant interaction with disease-associated gene variants in the sulfonylurea receptor (SUR1) gene. We conclude that the E23K mutation in the KIR6.2 gene is not associated with detectable alterations in glucose-stimulated insulin secretion in two independent populations from the Netherlands.

K(ATP) channels and insulin secretion disorders

ATP-sensitive potassium (K(ATP)) channels are inhibited by intracellular ATP and activated by ADP. Nutrient oxidation in beta-cells leads to a rise in [ATP]-to-[ADP] ratios, which in turn leads to reduced K(ATP) channel activity, depolarization, voltage-dependent Ca(2+) channel activation, Ca(2+) entry, and exocytosis. Persistent hyperinsulinemic hypoglycemia of infancy (HI) is a genetic disorder characterized by dysregulated insulin secretion and, although rare, causes severe mental retardation and epilepsy if left untreated. The last five or six years have seen rapid advance in understanding the molecular basis of K(ATP) channel activity and the molecular genetics of HI. In the majority of cases for which a genotype has been uncovered, causal HI mutations are found in one or the other of the two genes, SUR1 and Kir6.2, that encode the K(ATP) channel. This article will review studies that have defined the link between channel activity and defective insulin release and will consider implications for future understanding of the mechanisms of control of insulin secretion in normal and diseased states.