Hyperinsulinism in Infancy: From Basic Science to Clinical Disease

Anemia and neutropenic fever with high dose diazoxide treatment in a case with hyperinsulinism due to Munchausen by proxy

BACKGROUND

The etiology of hyperinsulinemic hypoglycemia in adolescents is similar to that of adults. Patients resistant to medical treatment may undergo pancreatectomy. Diazoxide is the mainstay of medical treatment. Rarely bone marrow suppression is reported due to diazoxide.

PATIENT

An adolescent with severe hyperinsulinemic hypoglycemia was referred for pancreatectomy after she was treated with high doses of diazoxide, octreotide and glucose. She developed anemia and febrile neutropenia in the course of diazoxide treatment that resolved with cessation of medication. The cause of the hyperinsulinemia proved to be classical Munchausen by proxy.

CONCLUSION

This is the first report of bone marrow suppression involving erythroid series by diazoxide. Follow-up of blood count may be considered in patients on high dosages since anemia may be dose dependent. Munchausen by proxy poses a serious threat to children with significant morbidity and mortality. Awareness and a high index of suspicion in clinical settings with unusual causes are the mainstay for the diagnosis.

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.

ATP-binding cassette proteins involved in glucose and lipid homeostasis

Glucose and lipids are essential to the body, but excess glucose or lipids lead to metabolic syndrome. ATP-binding cassette (ABC) proteins are involved in the homeostasis of glucose and lipid in that they regulate insulin secretion and remove excess cholesterol from the body. Sulfonylurea receptor (SUR) is a subunit of the ATP-sensitive potassium channels, which regulate insulin secretion from pancreatic beta-cells by sensing cellular metabolic levels. ABCG1 removes excess cholesterol from peripheral tissues and functions in reverse cholesterol transport to the liver. ABCG5 and ABCG8 suppress the absorption of cholesterol in the intestine and exclude cholesterol from the liver to the bile duct. ABCG1 and ABCG4, expressed in the central nervous system, play roles in lipid metabolism in the brain. These ABC proteins are targets of drugs and functional foods to cure and prevent diabetes, hyperlipidemia, and neurodegenerative diseases. In this review, recent knowledge of the physiological function and regulation of ABC proteins in the homeostasis of glucose and lipids is discussed.

Congenital hyperinsulinism

Congenital hyperinsulinism (CHI or HI) is a condition leading to recurrent hypoglycemia due to an inappropriate insulin secretion by the pancreatic islet beta cells. HI has two main characteristics: a high glucose requirement to correct hypoglycemia and a responsiveness of hypoglycemia to exogenous glucagon. HI is usually isolated but may be rarely part of a genetic syndrome (e.g. Beckwith-Wiedemann syndrome, Sotos syndrome etc.). The severity of HI is evaluated by the glucose administration rate required to maintain normal glycemia and the responsiveness to medical treatment. Neonatal onset HI is usually severe while late onset and syndromic HI are generally responsive to a medical treatment. Glycemia must be maintained within normal ranges to avoid brain damages, initially with glucose administration and glucagon infusion then, once the diagnosis is set, with specific HI treatment. Oral diazoxide is a first line treatment. In case of unresponsiveness to this treatment, somatostatin analogues and calcium antagonists may be added, and further investigations are required for the putative histological diagnosis: pancreatic (18)F-fluoro-L-DOPA PET-CT and molecular analysis. Indeed, focal forms consist of a focal adenomatous hyperplasia of islet cells, and will be cured after a partial pancreatectomy. Diffuse HI involves all the pancreatic beta cells of the whole pancreas. Diffuse HI resistant to medical treatment (octreotide, diazoxide, calcium antagonists and continuous feeding) may require subtotal pancreatectomy which post-operative outcome is unpredictable. The genetics of focal islet-cells hyperplasia associates a paternally inherited mutation of the ABCC8 or the KCNJ11 genes, with a loss of the maternal allele specifically in the hyperplasic islet cells. The genetics of diffuse isolated HI is heterogeneous and may be recessively inherited (ABCC8 and KCNJ11) or dominantly inherited (ABCC8, KCNJ11, GCK, GLUD1, SLC16A1, HNF4A and HADH). Syndromic HI are always diffuse form and the genetics depend on the syndrome. Except for HI due to potassium channel defect (ABCC8 and KCNJ11), most of these HI are sensitive to diazoxide. The main points sum up the management of HI: i) prevention of brain damages by normalizing glycemia and ii) screening for focal HI as they may be definitively cured after a limited pancreatectomy.

Constitutive endocytic recycling and protein kinase C-mediated lysosomal degradation control K(ATP) channel surface density

Pancreatic ATP-sensitive potassium (K(ATP)) channels control insulin secretion by coupling the excitability of the pancreatic beta-cell to glucose metabolism. Little is currently known about how the plasma membrane density of these channels is regulated. We therefore set out to examine in detail the endocytosis and recycling of these channels and how these processes are regulated. To achieve this goal, we expressed K(ATP) channels bearing an extracellular hemagglutinin epitope in human embryonic kidney cells and followed their fate along the endocytic pathway. Our results show that K(ATP) channels undergo multiple rounds of endocytosis and recycling. Further, activation of protein kinase C (PKC) with phorbol 12-myristate 13-acetate significantly decreases K(ATP) channel surface density by reducing channel recycling and diverting the channel to lysosomal degradation. These findings were recapitulated in the model pancreatic beta-cell line INS1e, where activation of PKC leads to a decrease in the surface density of native K(ATP) channels. Because sorting of internalized channels between lysosomal and recycling pathways could have opposite effects on the excitability of pancreatic beta-cells, we propose that PKC-regulated K(ATP) channel trafficking may play a role in the regulation of insulin secretion.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Pharmacological stimulation and inhibition of insulin secretion in mouse islets lacking ATP-sensitive K+ channels

BACKGROUND AND PURPOSE

ATP-sensitive potassium channels (K(ATP) channels) in beta cells are a major target for insulinotropic drugs. Here, we studied the effects of selected stimulatory and inhibitory pharmacological agents in islets lacking K(ATP) channels.

EXPERIMENTAL APPROACH

We compared insulin secretion (IS) and cytosolic calcium ([Ca(2+)](c)) changes in islets isolated from control mice and mice lacking sulphonylurea receptor1 (SUR1), and thus K(ATP) channels in their beta cells (Sur1KO).

KEY RESULTS

While similarly increasing [Ca(2+)](c) and IS in controls, agents binding to site A (tolbutamide) or site B (meglitinide) of SUR1 were ineffective in Sur1KO islets. Of two non-selective blockers of potassium channels, quinine was inactive, whereas tetraethylammonium was more active in Sur1KO compared with control islets. Phentolamine, efaroxan and alinidine, three imidazolines binding to K(IR)6.2 (pore of K(ATP) channels), stimulated control islets, but only phentolamine retained weaker stimulatory effects on [Ca(2+)](c) and IS in Sur1KO islets. Neither K(ATP) channel opener (diazoxide, pinacidil) inhibited Sur1KO islets. Calcium channel blockers (nimodipine, verapamil) or diphenylhydantoin decreased [Ca(2+)](c) and IS in both types of islets, verapamil and diphenylhydantoin being more efficient in Sur1KO islets. Activation of alpha(2)-adrenoceptors or dopamine receptors strongly inhibited IS while partially (clonidine > dopamine) lowering [Ca(2+)](c) (control > Sur1KO islets).

CONCLUSIONS AND IMPLICATIONS

Those drugs retaining effects on IS in islets lacking K(ATP) channels, also affected [Ca(2+)](c), indicating actions on other ionic channels. The greater effects of some inhibitors in Sur1KO than in control islets might be relevant to medical treatment of congenital hyperinsulinism caused by inactivating mutations of K(ATP) channels.

ATP-sensitive potassium channels in health and disease

The ATP-sensitive potassium (K(ATP)) channel plays a crucial role in insulin secretion and thus glucose homeostasis. K(ATP) channel activity in the pancreatic beta-cell is finely balanced; increased activity prevents insulin secretion, whereas reduced activity stimulates insulin release. The beta-cell metabolism tightly regulates K(ATP) channel gating, and if this coupling is perturbed, two distinct disease states can result. Diabetes occurs when the K(ATP) channel fails to close in response to increased metabolism, whereas congenital hyperinsulinism results when K(ATP) channels remain closed even at very low blood glucose levels. In general there is a good correlation between the magnitude of K(ATP) current and disease severity. Mutations that cause a complete loss of K(ATP) channels in the beta-cell plasma membrane produce a severe form of congenital hyperinsulinism, whereas mutations that partially impair channel function produce a milder phenotype. Similarly mutations that greatly reduce the ATP sensitivity of the K(ATP) channel lead to a severe form of neonatal diabetes with associated neurological complications, whilst mutations that cause smaller shifts in ATP sensitivity cause neonatal diabetes alone. This chapter reviews our current understanding of the pancreatic beta-cell K(ATP) channel and highlights recent structural, functional and clinical advances.

Human K(ATP) channelopathies: diseases of metabolic homeostasis

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

Characterization and functional restoration of a potassium channel Kir6.2 pore mutation identified in congenital hyperinsulinism

The inwardly rectifying potassium channel Kir6.2 assembles with sulfonylurea receptor 1 to form the ATP-sensitive potassium (K(ATP)) channels that regulate insulin secretion in pancreatic beta-cells. Mutations in K(ATP) channels underlie insulin secretion disease. Here, we report the characterization of a heterozygous missense Kir6.2 mutation, G156R, identified in congenital hyperinsulinism. Homomeric mutant channels reconstituted in COS cells show similar surface expression as wild-type channels but fail to conduct potassium currents. The mutated glycine is in the pore-lining transmembrane helix of Kir6.2; an equivalent glycine in other potassium channels has been proposed to serve as a hinge to allow helix bending during gating. We found that mutation of an adjacent asparagine, Asn-160, to aspartate, which converts the channel from a weak to a strong inward rectifier, on the G156R background restored ion conduction in the mutant channel. Unlike N160D channels, however, G156R/N160D channels are not blocked by intracellular polyamines at positive membrane potential and exhibit wild-type-like nucleotide sensitivities, suggesting the aspartate introduced at position 160 interacts with arginine at 156 to restore ion conduction and gating. Using tandem Kir6.2 tetramers containing G156R and/or N160D in designated positions, we show that one mutant subunit in the tetramer is insufficient to abolish conductance and that G156R and N160D can interact in the same or adjacent subunits to restore conduction. We conclude that the glycine at 156 is not essential for K(ATP) channel gating and that the Kir6.2 gating defect caused by the G156R mutation could be rescued by manipulating chemical interactions between pore residues.

Molecular biology of K(ATP) channels and implications for health and disease

The ATP-sensitive potassium (K(ATP)) channel is expressed in most excitable tissues and plays a critical role in numerous physiological processes by coupling intracellular energetics to electrical activity. The channel is comprised of four Kir6.x subunits associated with four regulatory sulfonylurea receptors (SUR). Intracellular ATP acts on Kir6.x to inhibit channel activity, while MgADP stimulates channel activity through SUR. Changes in the cytosolic [ATP] to [ADP] ratio thus determine channel activity. Multiple mutations in Kir6.x and SUR genes have implicated K(ATP) channels in various diseases ranging from diabetes and hyperinsulinism to cardiac arrhythmias and cardiovascular disease. Continuing studies of channel physiology and pathology will bring new insights to the molecular basis of K(ATP) channel function, leading to a better understanding of the role that K(ATP) channels play in both health and disease.

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.

Direct observation of individual KCNQ1 potassium channels reveals their distinctive diffusive behavior

We have directly observed the trafficking and fusion of ion channel containing vesicles and monitored the release of individual ion channels at the plasma membrane of live mammalian cells using total internal reflection fluorescence microscopy. Proteins were fused in-frame with green or red fluorescent proteins and expressed at low level in HL-1 and HEK293 cells. Dual color imaging revealed that vesicle trafficking involved motorized movement along microtubules followed by stalling, fusion, and subsequent release of individual ion channels at the plasma membrane. We found that KCNQ1-KCNE1 complexes were released in batches of about 5 molecules per vesicle. To elucidate the properties of ion channel complexes at the cell membrane we tracked the movement of individual molecules and compared the diffusive behavior of two types of potassium channel complex (KCNQ1-KCNE1 and Kir6.2-SUR2A) to that of a G-protein coupled receptor, the A1 adenosine receptor. Plots of mean squared displacement against time intervals showed that mobility depended on channel type, cell type, and temperature. Analysis of the mobility of wild type KCNQ1-KCNE1 complexes showed the existence of a significant immobile subpopulation and also a significant number of molecules that demonstrated periodic stalling of diffusive movements. This behavior was enhanced in cells treated with jasplakinolide and was abrogated in a C-terminal truncated form (KCNQ1(R518X)-KCNE1) of the protein. This mutant has been identified in patients with the long QT syndrome. We propose that KCNQ1-KCNE1 complexes interact intermittently with the actin cytoskeleton via the C-terminal region and this interaction may have a functional role.

K(ATP) channelopathies in the pancreas

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

Shortcomings of current models of glucose-induced insulin secretion

Glucose-induced insulin secretion by pancreatic beta-cells is generally schematized by a 'consensus model' that involves the following sequence of events: acceleration of glucose metabolism, closure of ATP-sensitive potassium channels (K(ATP) channels) in the plasma membrane, depolarization, influx of Ca(2+) through voltage-dependent calcium channels and a rise in cytosolic-free Ca(2+) concentration that induces exocytosis of insulin-containing granules. This model adequately depicts the essential triggering pathway but is incomplete. In this article, we first make a case for a model of dual regulation in which a metabolic amplifying pathway is also activated by glucose and augments the secretory response to the triggering Ca(2+) signal under physiological conditions. We next discuss experimental evidence, largely but not exclusively obtained from beta-cells lacking K(ATP) channels, which indicates that these channels are not the only possible transducers of glucose effects on the triggering Ca(2+)signal. We finally address the identity of the widely neglected background inward current (Cl(-) efflux vs. Na(+) or Ca(2+) influx through voltage-independent channels) that is necessary to cause beta-cell depolarization when glucose closes K(ATP) channels. More attention should be paid to the possibility that some components of this background current are influenced by glucose metabolism and have their place in a model of glucose-induced insulin secretion.

Adjacent mutations in the gating loop of Kir6.2 produce neonatal diabetes and hyperinsulinism

K(ATP) channels regulate insulin secretion from pancreatic beta-cells. Loss- and gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of this channel cause hyperinsulinism of infancy and neonatal diabetes, respectively. We report two novel mutations in the gating loop of Kir6.2 which cause neonatal diabetes with developmental delay (T293N) and hyperinsulinism (T294M). These mutations increase (T293N) or decrease (T294M) whole-cell K(ATP) currents, accounting for the different clinical phenotypes. The T293N mutation increases the intrinsic channel open probability (Po((0))), thereby indirectly decreasing channel inhibition by ATP and increasing whole-cell currents. T294M channels exhibit a dramatically reduced Po((0)) in the homozygous but not in the pseudo-heterozygous state. Unlike wild-type channels, hetT294M channels were activated by MgADP in the absence but not in the presence of MgATP; however, they are activated by MgGDP in both the absence and presence of MgGTP. These mutations demonstrate the importance of the gating loop of Kir channels in regulating Po((0)) and further suggest that Mg-nucleotide interaction with SUR1 may reduce ATP inhibition at Kir6.2.

KCNJ11 knockout morula re-engineered by stem cell diploid aggregation

KCNJ11-encoded Kir6.2 assembles with ATP-binding cassette sulphonylurea receptors to generate ATP-sensitive K+ (KATP) channel complexes. Expressed in tissues with dynamic metabolic flux, these evolutionarily conserved yet structurally and functionally unique heteromultimers serve as high-fidelity rheostats that adjust membrane potential-dependent cell functions to match energetic demand. Genetic defects in channel subunits disrupt the cellular homeostatic response to environmental stress, compromising organ tolerance in the adult. As maladaptation characterizes malignant KATP channelopathies, establishment of platforms to examine progression of KATP channel-dependent adaptive behaviour is warranted. Chimeras provide a powerful tool to assay the contribution of genetic variance to stress intolerance during prenatal or post-natal development. Here, KCNJ11 KATP channel gene knockout<-->wild-type chimeras were engineered through diploid aggregation. Integration of wild-type embryonic stem cells into zona pellucida-denuded morula derived from knockout embryos achieved varying degrees of incorporation of stress-tolerant tissue within the KATP channel-deficient background. Despite the stress-vulnerable phenotype of the knockout, ex vivo derived mosaic blastocysts tolerated intrauterine transfer and implantation, followed by full-term embryonic development in pseudopregnant surrogates to produce live chimeric offspring. The development of adult chimerism from the knockout<-->wild-type mosaic embryo offers thereby a new paradigm to probe the ecogenetic control of the KATP channel-dependent stress response.

Review. SUR1: a unique ATP-binding cassette protein that functions as an ion channel regulator

SUR1 is an ATP-binding cassette (ABC) transporter with a novel function. In contrast to other ABC proteins, it serves as the regulatory subunit of an ion channel. The ATP-sensitive (KATP) channel is an octameric complex of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits, and it links cell metabolism to electrical activity in many cell types. ATPase activity at the nucleotide-binding domains of SUR results in an increase in KATP channel open probability. Conversely, ATP binding to Kir6.2 closes the channel. Metabolic regulation is achieved by the balance between these two opposing effects. Precisely how SUR1 talks to Kir6.2 remains unclear, but recent studies have identified some residues and domains that are involved in both physical and functional interactions between the two proteins. The importance of these interactions is exemplified by the fact that impaired regulation of Kir6.2 by SUR1 results in human disease, with loss-of-function SUR1 mutations causing congenital hyperinsulinism and gain-of-function SUR1 mutations leading to neonatal diabetes. This paper reviews recent data on the regulation of Kir6.2 by SUR1 and considers the molecular mechanisms by which SUR1 mutations produce disease.

Sulfonylurea receptor 1 mutations that cause opposite insulin secretion defects with chemical chaperone exposure

The beta-cell ATP-sensitive potassium (K(ATP)) channel composed of sulfonylurea receptor SUR1 and potassium channel Kir6.2 serves a key role in insulin secretion regulation by linking glucose metabolism to cell excitability. Mutations in SUR1 or Kir6.2 that decrease channel function are typically associated with congenital hyperinsulinism, whereas those that increase channel function are associated with neonatal diabetes. Here we report that two hyperinsulinism-associated SUR1 missense mutations, R74W and E128K, surprisingly reduce channel inhibition by intracellular ATP, a gating defect expected to yield the opposite disease phenotype neonatal diabetes. Under normal conditions, both mutant channels showed poor surface expression due to retention in the endoplasmic reticulum, accounting for the loss of channel function phenotype in the congenital hyperinsulinism patients. This trafficking defect, however, could be corrected by treating cells with the oral hypoglycemic drugs sulfonylureas, which we have shown previously to act as small molecule chemical chaperones for K(ATP) channels. The R74W and E128K mutants thus rescued to the cell surface paradoxically exhibited ATP sensitivity 6- and 12-fold lower than wild-type channels, respectively. Further analyses revealed a nucleotide-independent decrease in mutant channel intrinsic open probability, suggesting the mutations may reduce ATP sensitivity by causing functional uncoupling between SUR1 and Kir6.2. In insulin-secreting cells, rescue of both mutant channels to the cell surface led to hyperpolarized membrane potentials and reduced insulin secretion upon glucose stimulation. Our results show that sulfonylureas, as chemical chaperones, can dictate manifestation of the two opposite insulin secretion defects by altering the expression levels of the disease mutants.

The K(ATP) channel and neonatal diabetes

ATP-sensitive potassium (K(ATP)) channels play a key role in glucose-dependent insulin secretion in pancreatic beta-cells. Recently, activating mutations in beta-cell K(ATP) channels were found to be an important cause of neonatal diabetes. In some patients, these mutations may also affect K(ATP) channel function in muscles, nerves and brain which can result in a severe disease termed DEND syndrome (Developmental delay, Epilepsy and Neonatal Diabetes). This review focuses on mutations in the pore-forming K(ATP) channel subunit (Kir6.2) that cause neonatal diabetes and discusses recent advances in our understanding of clinical features of neonatal diabetes, its underlying molecular mechanisms and their impact on treatment.

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

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

Channel regulation of glucose sensing in the pancreatic beta-cell

Mammalian beta-cells are acutely and chronically regulated by sensing surrounding glucose levels that determine the rate at which insulin is secreted, to maintain euglycemia. Experimental research in vitro and in vivo has shown that, when these cells are exposed to adverse conditions like long periods of hypoglycemia or hyperglycemia, their capability to sense glucose is decreased. Understanding the normal physiology and identifying the main players along this route becomes paramount. In this review, we have taken on the task of looking at the role that ion channels play in the regulation of this process, delineating the different families, and describing the signaling that parallels the glucose sensing process that results in insulin release.

Insulin resistance and amyloidogenesis as common molecular foundation for type 2 diabetes and Alzheimer's disease

Characterized as a peripheral metabolic disorder and a degenerative disease of the central nervous system respectively, it is now widely recognized that type 2 diabetes mellitus (T2DM) and Alzheimer's disease (AD) share several common abnormalities including impaired glucose metabolism, increased oxidative stress, insulin resistance and amyloidogenesis. Several recent studies suggest that this is not an epiphenomenon, but rather these two diseases disrupt common molecular pathways and each disease compounds the progression of the other. For instance, in AD the accumulation of the amyloid-beta peptide (Abeta), which characterizes the disease and is thought to participate in the neurodegenerative process, may also induce neuronal insulin resistance. Conversely, disrupting normal glucose metabolism in transgenic animal models of AD that over-express the human amyloid precursor protein (hAPP) promotes amyloid-peptide aggregation and accelerates the disease progression. Studying these processes at a cellular level suggests that insulin resistance and Abeta aggregation may not only be the consequence of excitotoxicity, aberrant Ca(2+) signals, and proinflammatory cytokines such as TNF-alpha, but may also promote these pathological effectors. At the molecular level, insulin resistance and Abeta disrupt common signal transduction cascades including the insulin receptor family/PI3 kinase/Akt/GSK3 pathway. Thus both disease processes contribute to overlapping pathology, thereby compounding disease symptoms and progression.

Modeling K(ATP) channel gating and its regulation

ATP-sensitive potassium (K(ATP)) channels couple cell metabolism to plasmalemmal potassium fluxes in a variety of cell types. The activity of these channels is primarily determined by intracellular adenosine nucleotides, which have both inhibitory and stimulatory effects. The role of K(ATP) channels has been studied most extensively in pancreatic beta-cells, where they link glucose metabolism to insulin secretion. Many mutations in K(ATP) channel subunits (Kir6.2, SUR1) have been identified that cause either neonatal diabetes or congenital hyperinsulinism. Thus, a mechanistic understanding of K(ATP) channel behavior is necessary for modeling beta-cell electrical activity and insulin release in both health and disease. Here, we review recent advances in the K(ATP) channel structure and function. We focus on the molecular mechanisms of K(ATP) channel gating by adenosine nucleotides, phospholipids and sulphonylureas and consider the advantages and limitations of various mathematical models of macroscopic and single-channel K(ATP) currents. Finally, we outline future directions for the development of more realistic models of K(ATP) channel gating.

Activation of the Na+/K+-ATPase by insulin and glucose as a putative negative feedback mechanism in pancreatic beta-cells

Pancreatic beta-cells of sulfonylurea receptor type 1 knock-out (SUR1(-/-)) mice exhibit an oscillating membrane potential (V (m)) demonstrating that hyper-polarisation occurs despite the lack of K(ATP) channels. We hypothesize that glucose activates the Na(+)/K(+)-ATPase thus increasing a hyper-polarising current. Elevating glucose in SUR1(-/-) beta-cells resulted in a transient fall in V (m) and [Ca(2+)](c) independent of sarcoplasmic and endoplasmic reticulum Ca(2+)-activated ATPase (SERCA) activation. This was not affected by K(+) channel blockade but inhibited by ATP depletion and by ouabain. Increasing glucose also reduced [Na(+)](c), an effect reversed by ouabain. Exogenously applied insulin decreased [Na(+)](c) and hyper-polarised V (m). Inhibiting insulin signalling in SUR1(-/-) beta-cells blunted the glucose-induced decrease of [Ca(2+)](c). Tolbutamide (1 mmol/l) disclosed the SERCA-independent effect of glucose on [Ca(2+)](c) in wild-type beta-cells. The data show that in SUR1(-/-) beta-cells, glucose activates the Na(+)/K(+)-ATPase presumably by increasing [ATP](c). Insulin can also stimulate the pump and potentiate the effect of glucose. Pathways involving the pump may thus serve as potential drug targets in certain metabolic disorders.

[18F-fluoro-L-DOPA PET-CT imaging combined with genetic analysis for optimal classification and treatment in a child with severe congenital hyperinsulinism]

BACKGROUND

Congenital hyperinsulinism (CHI) is the most common cause of persistent hypoglycaemia in infancy. The differential diagnosis between focal and diffuse forms of CHI is of great importance when planning surgery. The aim of this article is to show the first case of focal CHI diagnosed in Spain using PET-CT imaging combined with genetic analysis.

METHODS

A 13 month child with CHI and normal conventional radiological investigations treated with diazoxide, diet control and feeding by gastrostomy is presented. Genetic analysis of ABCC8 and KCNJ11 genes and PET-TAC using 18F-fluoro-L-DOPA were performed.

RESULTS

A pathological mutation (G111R) in the paternal allele of ABCC8 was detected. PET-CT scanning using 18F-fluoro-L-DOPA showed a focus of high uptake in the body of the pancreas compatible with adenoma that was hystopathologically confirmed. After surgical resection the patient is asymptomatic without needing either pharmacological treatment or dietetic control.

CONCLUSIONS

The combination of genetic analysis and 18F-fluoro-L-DOPA PET-TAC shows a great potential for the identification, location and guideline for surgery in CHI.

Exendin-(9-39) corrects fasting hypoglycemia in SUR-1-/- mice by lowering cAMP in pancreatic beta-cells and inhibiting insulin secretion

Congenital hyperinsulinism is a disorder of pancreatic beta-cell function characterized by failure to suppress insulin secretion in the setting of hypoglycemia, resulting in brain damage or death if untreated. Loss-of-function mutations in the K(ATP) channel (composed of two subunits: Kir6.2 and SUR-1) are responsible for the most common and severe form of congenital hyperinsulinism. Most patients are unresponsive to available medical therapy and require palliative pancreatectomy. Similar to the human condition, the SUR-1(-/-) mouse is hypoglycemic when fasted and hyperglycemic when glucose-loaded. We have previously reported that the glucagon-like peptide-1 receptor antagonist exendin-(9-39) raises fasting blood glucose in normal mice. Here we examine the effect of exendin-(9-39) on fasting blood glucose in SUR-1(-/-) mice. Mice were randomized to receive exendin-(9-39) or vehicle. Fasting blood glucose levels in SUR-1(-/-) mice treated with exendin-(9-39) were significantly higher than in vehicle-treated mice and not different from wild-type littermates. Exendin-(9-39) did not further worsen glucose tolerance and had no effect on body weight and insulin sensitivity. Isolated islet perifusion studies demonstrated that exendin-(9-39) blocked amino acid-stimulated insulin secretion, which is abnormally increased in SUR-1(-/-) islets. Furthermore, cAMP content in SUR-1(-/-) islets was reduced by exendin-(9-39) both basally and when stimulated by amino acids, whereas cytosolic calcium levels were not affected. These findings suggest that cAMP plays a key role in K(ATP)-independent insulin secretion and that the GLP-1 receptor is constitutively active in SUR-1(-/-) beta-cells. Our findings indicate that exendin-(9-39) normalizes fasting hypoglycemia in SUR-1(-/-) mice via a direct effect on insulin secretion, thereby raising exendin-(9-39) as a potential therapeutic agent for K(ATP) hyperinsulinism.

Identification of the Alzheimer's disease amyloid precursor protein (APP) and its homologue APLP2 as essential modulators of glucose and insulin homeostasis and growth

The amyloid precursor protein (APP), the source of the neurotoxic amyloid beta (A beta) peptide involved in Alzheimer's disease (AD), belongs to a conserved family of related proteins. In mammals, the APP family contains amyloid precursor-like protein 1 (APLP1) and amyloid precursor-like protein 2 (APLP2). Whilst a number of activities have been attributed to the APP family, an overall function has not been definitively established. While ablating either the APP or APLP2 gene in mice produces minimal phenotypic change, the combined knockout of these genes in mice causes postnatal mortality. Postnatal survival therefore requires a shared but unknown function of APP and APLP2. To investigate the biochemical basis for the postnatal lethality, plasma was analysed from double knockout mice (APP-/- APLP2-/-) 2 days before birth, at gestational day E17, and from mice at 12-16 h after birth. The postnatal double knockouts had 66% lower plasma glucose levels than their wild-type controls and 50% lower than their single knockout counterparts. Interestingly, the postnatal double knockouts displayed hyperinsulinaemia, as shown by inappropriate plasma insulin levels, given their degree of hypoglycaemia. The single knockout mice also showed hyperinsulinaemia and had 31% lower plasma glucose than the wild-types. While the double knockouts did not survive more than 24 h after birth, the single knockouts reached adulthood and their hypoglycaemia continued. Therefore, APP and APLP2 expression modulates plasma insulin and glucose concentrations. Plasma calcium, magnesium and phosphate were also significantly reduced in the double knockouts compared to the wild-types, and they showed distinctive growth restriction, suggesting the involvement of a metabolic impairment. These results link the expression of the APP and APLP2 genes with glucose homeostasis and growth and therefore identify a novel function for the APP family.

What's new in metabolic and genetic hypoglycaemias: diagnosis and management

Hypoglycaemia in children can be a life-threatening situation that needs to be assessed rigorously in order to treat efficiently and avoid relapse that can be responsible for cerebral damage. The diagnosis of impairment in glucose homeostasis requires the knowledge of the mechanisms regulating blood glucose concentration. The clinical history and presentation, when available, especially the timing of hypoglycaemia with respect to the last meal and some simple clinical and biological tests may allow diagnosing the vast majority of patients presenting with hypoglycaemia. Recently, new metabolic and endocrinologic genetic causes of hypoglycaemia have been identified that may give new insight to the complex mechanisms of glucose regulation and thus contribute to the discovery of new genes regulating glucose homeostasis. New diagnostic tests such as the 18-fluoro-Dopa PET-scan have also been recently developed.

Glucokinase activators in diabetes management

Type 2 diabetes is a chronic metabolic disease that adversely affects both the quality and longevity of life. The disease is characterised by elevated blood glucose (hyperglycaemia) that is associated with microvascular complications and increased macrovascular risk. Existing oral agents, either alone or in combination, do not exhibit adequate or sustained glucose lowering efficacy in Type 2 diabetics. Consequently, there is an unmet medical need for improved antidiabetic agents which are both more effective at lowering glucose and which display sustained efficacy over a number of years. Such agents would allow present glycaemic treatment targets to be achieved with greater success. Glucokinase activators (GKAs) represent a novel class of glucose-lowering agents. Preclinical data supports the notion that these agents act to lower blood glucose through effects in both the liver and pancreas. It is predicted that this dual compartment mechanism of action of GKAs will translate to robust glucose lowering in diabetic patients. The potential benefits and risks associated with the pharmacological activation of glucokinase are evaluated. The status of GKAs in preclinical and clinical development is assessed are the future prospects of these agents.

Trafficking of ATP-sensitive potassium channels in health and disease

K(ATP) channels (ATP-sensitive potassium channels), comprising four subunits each of Kir6.2 (inwardly rectifying potassium channel 6.2) and the SUR1 (sulfonylurea receptor 1), play a central role in glucose-stimulated insulin secretion by the pancreatic beta-cell. Changes in the number of channels at the cell surface are associated with genetic diseases of aberrant insulin secretion, including CHI (congenital hyperinsulinism) and NDM (neonatal diabetes mellitus). The present review summarizes advances in our understanding of the vesicular trafficking of normal K(ATP) channels and how genetic mutations in Kir6.2 interfere with such trafficking. A mutation, E282K, causing CHI, was found to disrupt a DXE [di-acidic ER (endoplasmic reticulum)-exit signal], thereby preventing its assembly into COPII (coatamer protein II)-coated vesicles and subsequent ER exit. The resultant decrease in the cell-surface density of the channel could explain the disease phenotype. Two mutations, Y330C and F333I, reported in patients with NDM, disrupted an endocytic traffic signal, thereby impairing CCV (clathrin-coated vesicle) formation and endocytosis. The consequent increase in the density of K(ATP) channels, together with an attenuated sensitivity to ATP reported previously, may account for the severe form of NDM.

Destabilization of ATP-sensitive potassium channel activity by novel KCNJ11 mutations identified in congenital hyperinsulinism

The inwardly rectifying potassium channel Kir6.2 is the pore-forming subunit of the ATP-sensitive potassium (K(ATP)) channel, which controls insulin secretion by coupling glucose metabolism to membrane potential in beta-cells. Loss of channel function because of mutations in Kir6.2 or its associated regulatory subunit, sulfonylurea receptor 1, causes congenital hyperinsulinism (CHI), a neonatal disease characterized by persistent insulin secretion despite severe hypoglycemia. Here, we report a novel K(ATP) channel gating defect caused by CHI-associated Kir6.2 mutations at arginine 301 (to cysteine, glycine, histidine, or proline). These mutations in addition to reducing channel expression at the cell surface also cause rapid, spontaneous current decay, a gating defect we refer to as inactivation. Based on the crystal structures of Kir3.1 and KirBac1.1, Arg-301 interacts with several residues in the neighboring Kir6.2 subunit. Mutation of a subset of these residues also induces channel inactivation, suggesting that the disease mutations may cause inactivation by disrupting subunit-subunit interactions. To evaluate the effect of channel inactivation on beta-cell function, we expressed an alternative inactivation mutant R301A, which has equivalent surface expression efficiency as wild type channels, in the insulin-secreting cell line INS-1. Mutant expression resulted in more depolarized membrane potential and elevated insulin secretion at basal glucose concentration (3 mm) compared with cells expressing wild type channels, demonstrating that the inactivation gating defect itself is sufficient to cause loss of channel function and hyperinsulinism. Our studies suggest the importance of Kir6.2 subunit-subunit interactions in K(ATP) channel gating and function and reveal a novel gating defect underlying CHI.

Hyperinsulinism in Infancy and Childhood: When an Insulin Level Is Not Always Enough

Background: Hypoglycemia in infants and children can lead to seizures, developmental delay, and permanent brain damage. Hyperinsulinism (HI) is the most common cause of both transient and permanent disorders of hypoglycemia. HI is characterized by dysregulated insulin secretion, which results in persistent mild to severe hypoglycemia. The various forms of HI represent a group of clinically, genetically, and morphologically heterogeneous disorders.

Content: Congenital hyperinsulinism is associated with mutations of SUR-1 and Kir6.2, glucokinase, glutamate dehydrogenase, short-chain 3-hydroxyacyl-CoA dehydrogenase, and ectopic expression on β-cell plasma membrane of SLC16A1. Hyperinsulinism can be associated with perinatal stress such as birth asphyxia, maternal toxemia, prematurity, or intrauterine growth retardation, resulting in prolonged neonatal hypoglycemia. Mimickers of hyperinsulinism include neonatal panhypopituitarism, drug-induced hypoglycemia, insulinoma, antiinsulin and insulin-receptor stimulating antibodies, Beckwith-Wiedemann Syndrome, and congenital disorders of glycosylation. Laboratory testing for hyperinsulinism may include quantification of blood glucose, plasma insulin, plasma β-hydroxybutyrate, plasma fatty acids, plasma ammonia, plasma acylcarnitine profile, and urine organic acids. Genetic testing is available through commercial laboratories for genes known to be associated with hyperinsulinism. Acute insulin response (AIR) tests are useful in phenotypic characterization. Imaging and histologic tools are also available for diagnosing and classifying hyperinsulinism. The goal of treatment in infants with hyperinsulinism is to prevent brain damage from hypoglycemia by maintaining plasma glucose levels above 700 mg/L (70 mg/dL) through pharmacologic or surgical therapy.

Summary: The management of hyperinsulinism requires a multidisciplinary approach that includes pediatric endocrinologists, radiologists, surgeons, and pathologists who are trained in diagnosing, identifying, and treating hyperinsulinism.

Alanine in HI: a silent mutation cries out!

It is a widely held paradigm in molecular biology that a change in the third base of a codon is silent in terms of expression. In this investigation, results are presented that challenge that paradigm, at least in terms of one polymorphism in KCNJ11, which is one of five genes that have been implicated in the disorder Hyperinsulinism of Infancy. In two cohorts of Australian patients, an uneven distribution of KCNJ11 SNP's was observed. A silent polymorphism at codon 190 was over-represented in the patients who responded well to medical treatment and under-represented in those that required radical surgical intervention. In an attempt to investigate this polymorphism, it was expressed in vitro and western blot analysis showed that there were virtually no bands from the homozygous variant samples, while strong bands were seen in normal controls. The human genome is highly redundant in terms of tRNA species for each amino acids but enigmatically under-represents a number of specific codons. The polymorphism in question occurs within one such codon. We propose that the presence of a base change at the third position of codon that is not represented by a corresponding anti-codon within the human nuclear tRNA leads to a decreased rate of expression of the protein.

K(ATP) channel pharmacogenomics: from bench to bedside

Inheritance plays a significant role in defining drug response and toxicity. Advances in molecular pharmacology and modern genomics emphasize genetic variation in dictating inter-individual pharmacokinetics and pharmacodynamics. A case in point is the homeostatic ATP-sensitive potassium (K(ATP)) channel, an established drug target that adjusts membrane excitability to match cellular energetic demand. There is an increased recognition that genetic variability of the K(ATP) channel impacts therapeutic decision-making in human disease.

The Walter B. Cannon Physiology in Perspective Lecture, 2007. ATP-sensitive K+ channels and disease: from molecule to malady

This essay is based on a lecture given to the American Physiological Society in honor of Walter B. Cannon, an advocate of homeostasis. It focuses on the role of the ATP-sensitive potassium K(+) (K(ATP)) channel in glucose homeostasis and, in particular, on its role in insulin secretion from pancreatic beta-cells. The beta-cell K(ATP) channel comprises pore-forming Kir6.2 and regulatory SUR1 subunits, and mutations in either type of subunit can result in too little or too much insulin release. Here, I review the latest information on the relationship between K(ATP) channel structure and function, and consider how mutations in the K(ATP) channel genes lead to neonatal diabetes or congenital hyperinsulinism.

Congenital hyperinsulinism associated ABCC8 mutations that cause defective trafficking of ATP-sensitive K+ channels: identification and rescue

Congenital hyperinsulinism (CHI) is a disease characterized by persistent insulin secretion despite severe hypoglycemia. Mutations in the pancreatic ATP-sensitive K(+) (K(ATP)) channel proteins sulfonylurea receptor 1 (SUR1) and Kir6.2, encoded by ABCC8 and KCNJ11, respectively, is the most common cause of the disease. Many mutations in SUR1 render the channel unable to traffic to the cell surface, thereby reducing channel function. Previous studies have shown that for some SUR1 trafficking mutants, the defects could be corrected by treating cells with sulfonylureas or diazoxide. The purpose of this study is to identify additional mutations that cause channel biogenesis/trafficking defects and those that are amenable to rescue by pharmacological chaperones. Fifteen previously uncharacterized CHI-associated missense SUR1 mutations were examined for their biogenesis/trafficking defects and responses to pharmacological chaperones, using a combination of immunological and functional assays. Twelve of the 15 mutations analyzed cause reduction in cell surface expression of K(ATP) channels by >50%. Sulfonylureas rescued a subset of the trafficking mutants. By contrast, diazoxide failed to rescue any of the mutants. Strikingly, the mutations rescued by sulfonylureas are all located in the first transmembrane domain of SUR1, designated as TMD0. All TMD0 mutants rescued to the cell surface by the sulfonylurea tolbutamide could be subsequently activated by metabolic inhibition on tolbutamide removal. Our study identifies a group of CHI-causing SUR1 mutations for which the resulting K(ATP) channel trafficking and expression defects may be corrected pharmacologically to restore channel function.

The role of ATP sensitive channels in insulin secretion and the implications in persistent hyperinsulinemic hypoglycaemia of infancy (PHHI)

Persistent Hyperinsulinemic Hypoglycaemia of Infancy (PHHI) is a metabolic syndrome of unregulated insulin secretion. It is a heterogenous disease with causes linked to mutations of the ATP sensitive potassium channels of the beta cell, as well as to metabolism in the beta cell. 5 candidate genes--ABCC8, KCNJ11, GCK, GLUD1 and SCHAD have been implicated in the disease so far, however the aetiology of the disease remains unknown in up to 50% of all patients. We genotyped 43 subjects with PHHI (20 surgically treated and 23 medically treated) for disease associated mutations in the candidate genes. Mutations on ABCC8 were identified in 16 of the 20 (80%) of the surgically treated patients. One putative mutation was identified in the medically treated cohort. The polymorphism E23K on KCNJ11 that is associated with NIDDM was differentially distributed in the 2 cohorts. We discuss the mutations identified, emphasise the importance of the K-ATP channel in physiological processes and discuss the possibility that the disease is caused by mutations in other genes associated with insulin release, glucose metabolism in the beta cell or beta cell apoptosis and survival. We propose that these processes must be explored in order to further our understanding of PHHI.

Molecular cell biology of KATPchannels: implications for neonatal diabetes

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

Identification of two Novel Frameshift Mutations in the KCNJ11 gene in two Italian patients affected by Congenital Hyperinsulinism of Infancy

Congenital Hyperinsulinism of Infancy (CHI) is a genetically heterogeneous disorder characterized by profound hypoglycemia related to inappropriate insulin secretion. Two histopathologically and genetically distinct groups are recognized among patients with CHI due to ATP-sensitive potassium channel (KATP) defects: a diffuse type (Di-CHI), which involves the whole pancreas, and a focal form (Fo-CHI), which shows adenomatous islet-cell hyperplasia of a particular area within the normal pancreas. The beta-cell KATP channel consists of two essential subunits: Kir6.2 encoded by the KCNJ11 gene which is the pore-forming unit and belongs to the inwardly rectifying potassium channel family, and SUR1 (sulfonylurea receptor 1) encoded by the ABCC8 gene, which belongs to the ATP-binding cassette (ABC) transporter family. The KATP channel is an octameric complex of four Kir6.2 and four SUR1 subunits. More than one hundred mutations have been found in KATP channel genes ABCC8 and KCNJ11, but to date only twenty mutations have been identified in KCNJ11, most of them are missense mutations and only one is a single base deletion. The Fo-CHI has been demonstrated to arise in individuals who have a germline mutation in the paternal allele of ABCC8 or KCNJ11 in addition to a somatic loss of the maternally derived chromosome region 11p15 in adenomatous pancreatic beta-cells, while Di-CHI predominantly arises from the autosomal recessive inheritance of KATP channel gene mutations. Here we describe the molecular findings in nine children who presented, in the neonatal period, with signs and symptoms of hypoglycemia and diagnosed affected by CHI according to international diagnostic criteria. Direct sequencing of the complete coding exon and promoter region of KCNJ11 gene showed, in two Italian patients, two new heterozygous mutations which result in the appearance of premature translation termination codons resulting in the premature end of Kir6.2. Interestingly most of the CHI mutations detected in other population studies are situated in the ABCC8 gene.

The majority of cases of neonatal diabetes in Spain can be explained by known genetic abnormalities

BACKGROUND

Neonatal diabetes is a rare disease characterized by hyperglycaemia within the first 3 months of life and requiring insulin treatment; it can either be transient (TNDM) or permanent (PNDM). Alterations at band 6q24 and heterozygous activating mutations in KCNJ11, the gene encoding the pore-forming subunit of the KATP channel, can cause neonatal diabetes. Aims We screened the 6q24 region, KCNJ11, GCK, FOXP3 and IPF1 genes for mutations in families with PNDM or TNDM to establish a phenotype-genotype correlation.

METHODS

Twenty-two patients with neonatal diabetes were recruited. Inclusion criteria were insulin-treated diabetes diagnosed within the first 3 months and insulin treatment for at least 15 days. Clinical data were recorded in a questionnaire.

RESULTS

We identified 17 genetic alterations in our patients: six alterations at the 6q24 band associated with TNDM and nine mutations in KCNJ11, five of which were novel. The analysis for a phenotype-genotype correlation showed that patients with 6q24 alterations had a lower birth weight and were diagnosed earlier than patients with KCNJ11 mutations. At follow-up of the TNDM patients with genetic alterations, 43% developed diabetes or impaired glucose tolerance in later life (one with 6q24 duplication and two with N48D and E227K mutations at KCNJ11 gene). Furthermore, half the first-degree relatives who carried a genetic alteration but who had not suffered from neonatal diabetes were diagnosed with diabetes or impaired glucose tolerance before the age of 30 years.

CONCLUSIONS

KCNJ11 mutations are common in both TNDM and PNDM and are associated with a higher birth weight compared with patients with 6q24 abnormalities. Patients with TNDM should be screened for abnormalities in glucose metabolism in adult life.

Placental immaturity and hyperinsulinaemia in full-term newborns

BACKGROUND

Evidence from large studies suggests that low birthweight is a risk factor for cardiovascular disease and glucose metabolism disorders in adulthood, but the physiological mechanisms involved in intrauterine growth conditioning low birthweight are not completely understood. The objectives of this study were to determine whether placental immaturity (PI), defined as the lower quartile of placental maturity index (PMI), is associated to hyperinsulinaemia at birth and to identify the risk factors associated with PI.

MATERIALS AND METHODS

Cross-sectional study conducted at medical research units of two Mexican general hospitals. A total of 272 full-term newborns with gestational age >/= 38 and < 41 weeks were allocated into the corresponding group according to the quartile distribution of PMI. Data from the lower (PMI < 13.3) and higher quartile (PMI >/= 24.3) were compared. The PMI was estimated by dividing the number of epithelial plates by the average thickness of the epithelial plate. Serum measures included cord glucose and insulin levels of the newborns at birth.

RESULTS

A total of 74 (27.2%) children had hyperinsulinaemia at birth, of them 47 (63.5%) with PI. The adjusted multiple regression analysis showed a strong association between PI and hyperinsulinaemia at birth [odds ratio (OR) 2.6; CI 95% 1.3-4.3). Additional adjusted analysis showed that both mother's age </= 16 years (OR 1.75; CI95% 1.2-9.1) and maternal cigarette smoking (OR 2.7; CI95% 1.3-8.9) are associated to PI.

CONCLUSIONS

The PI is independently associated with hyperinsulinaemia at birth: smoking and mother's age lower than 16 years are risk factors for development of PI.

Complex ABCC8 DNA variations in congenital hyperinsulinism: lessons from functional studies

OBJECTIVE

Congenital hyperinsulinism (CHI) is a cause of persistent and severe hypoglycaemia in infancy. Mutations in the genes ABCC8 and KCNJ11 encoding SUR1 and Kir6.2, respectively, are the commonest cause of CHI. We investigated whether the possession of two DNA variants leading to coding changes in a single allele of ABCC8 can affect the potential mechanism of disease pathogenesis.

DESIGN AND PATIENTS

We studied two patients with complex mutations in the ABCC8 gene with CHI and used in vitro studies to explore the potential disease mechanism and the contribution of the various mutant allelles.

RESULTS

The first case had diffuse disease and was homozygous for the mutations D1193V and R1436Q in SUR1. Channel complexes containing the D1193V mutant were delivered to the plasma membrane and were functional and those containing R1436Q were also present at the plasma membrane but were nonfunctional. Combining the two mutations (SUR1D1193V/R1436Q) led to intracellular retention of the channel complex. In a second family, the patient had histologically focal disease and was heterozygous for two mutations from his father (G228D and D1471N) and one from his mother (V1572I). SUR1 G228D and D1471N singly or in combination led to intracellular retention of the channel complex and loss of function. By contrast, V1572I is trafficked appropriately and is functional, consistent with a mechanism of reduction to hemizygosity of paternal ABCC8 in focal disease. V1572I is likely to be a benign DNA variant.

CONCLUSION

In one patient the combination of two coding variants led to intracellular retention of channel complex. In a second patient, functional studies allowed us to unravel the DNA variants likely to be causing the abrogation of ATP-sensitive K(+) channel function.

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.