Correction: Heterogeneous expression of CFTR in insulin-secreting β-cells of the normal human islet

[This corrects the article DOI: 10.1371/journal.pone.0242749.].

Update of variants identified in the pancreatic β-cell KATP channel genes KCNJ11 and ABCC8 in individuals with congenital hyperinsulinism and diabetes

The most common genetic cause of neonatal diabetes and hyperinsulinism is pathogenic variants in ABCC8 and KCNJ11. These genes encode the subunits of the β-cell ATP-sensitive potassium channel, a key component of the glucose-stimulated insulin secretion pathway. Mutations in the two genes cause dysregulated insulin secretion; inactivating mutations cause an oversecretion of insulin, leading to congenital hyperinsulinism, whereas activating mutations cause the opposing phenotype, diabetes. This review focuses on variants identified in ABCC8 and KCNJ11, the phenotypic spectrum and the treatment implications for individuals with pathogenic variants.

Structural abnormalities in islets from very young children with cystic fibrosis may contribute to cystic fibrosis-related diabetes

Cystic fibrosis (CF)-related diabetes (CFRD) is thought to result from beta-cell injury due in part to pancreas exocrine damage and lipofibrosis. CFRD pancreata exhibit reduced islet density and altered cellular composition. To investigate a possible etiology, we tested the hypothesis that such changes are present in CF pancreata before the development of lipofibrosis. We evaluated pancreas and islet morphology in tissues from very young CF children (<4 years of age), and adult patients with CF and CFRD. The relative number of beta-cells in young CF tissues was reduced by 50% or more when compared to age-matched controls. Furthermore, young CF tissues displayed significantly smaller insulin-positive areas, lower proportion of beta-cells positive for the proliferation marker Ki67 or the ductal marker CK19 vs. control subjects, and islet inflammatory cell infiltrates, independently of the severity of the exocrine lesion and in the absence of amyloid deposits. CFRD pancreata exhibited greater islet injury with further reduction in islet density, decreased relative beta-cell number, and presence of amyloid deposits. Together, these results strongly suggest that an early deficiency in beta-cell number in infants with CF may contribute to the development of glucose intolerance in the CF pediatric population, and to CFRD, later in life.

The neuronal K+Cl- co-transporter 2 (Slc12a5) modulates insulin secretion

Intracellular chloride concentration ([Cl-]i) in pancreatic β-cells is kept above electrochemical equilibrium due to the predominant functional presence of Cl- loaders such as the Na+K+2Cl- co-transporter 1 (Slc12a2) over Cl-extruders of unidentified nature. Using molecular cloning, RT-PCR, Western blotting, immunolocalization and in vitro functional assays, we establish that the "neuron-specific" K+Cl- co-transporter 2 (KCC2, Slc12a5) is expressed in several endocrine cells of the pancreatic islet, including glucagon secreting α-cells, but particularly in insulin-secreting β-cells, where we provide evidence for its role in the insulin secretory response. Three KCC2 splice variants were identified: the formerly described KCC2a and KCC2b along with a novel one lacking exon 25 (KCC2a-S25). This new variant is undetectable in brain or spinal cord, the only and most abundant known sources of KCC2. Inhibition of KCC2 activity in clonal MIN6 β-cells increases basal and glucose-stimulated insulin secretion and Ca2+ uptake in the presence of glibenclamide, an inhibitor of the ATP-dependent potassium (KATP)-channels, thus suggesting a possible mechanism underlying KCC2-dependent insulin release. We propose that the long-time considered "neuron-specific" KCC2 co-transporter is expressed in pancreatic islet β-cells where it modulates Ca2+-dependent insulin secretion.

Increased Slc12a1 expression in β-cells and improved glucose disposal in Slc12a2 heterozygous mice

The products of the Slc12a1 and Slc12a2 genes, commonly known as Na(+)-dependent K(+)2Cl(-) co-transporters NKCC2 and NKCC1, respectively, are the targets for the diuretic bumetanide. NKCCs are implicated in the regulation of intracellular chloride concentration ([Cl(-)]i) in pancreatic β-cells, and as such, they may play a role in glucose-stimulated plasma membrane depolarization and insulin secretion. Unexpectedly, permanent elimination of NKCC1 does not preclude insulin secretion, an event potentially linked to the homeostatic regulation of additional Cl(-) transporters expressed in β-cells. In this report we provide evidence for such a mechanism. Mice lacking a single allele of Slc12a2 exhibit lower fasting glycemia, increased acute insulin response (AIR) and lower blood glucose levels 15-30 min after a glucose load when compared to mice harboring both alleles of the gene. Furthermore, heterozygous expression or complete absence of Slc12a2 associates with increased NKCC2 protein expression in rodent pancreatic β-cells. This has been confirmed by using chronic pharmacological down-regulation of NKCC1 with bumetanide in the mouse MIN6 β-cell line or permanent molecular silencing of NKCC1 in COS7 cells, which results in increased NKCC2 expression. Furthermore, MIN6 cells chronically pretreated with bumetanide exhibit increased initial rates of Cl(-) uptake while preserving glucose-stimulated insulin secretion. Together, our results suggest that NKCCs are involved in insulin secretion and that a single Slc12a2 allele may protect β-cells from failure due to increased homeostatic expression of Slc12a1.

Hypothalamic leucine metabolism regulates liver glucose production

Amino acids profoundly affect insulin action and glucose metabolism in mammals. Here, we investigated the role of the mediobasal hypothalamus (MBH), a key center involved in nutrient-dependent metabolic regulation. Specifically, we tested the novel hypothesis that the metabolism of leucine within the MBH couples the central sensing of leucine with the control of glucose production by the liver. We performed either central (MBH) or systemic infusions of leucine in Sprague-Dawley male rats during basal pancreatic insulin clamps in combination with various pharmacological and molecular interventions designed to modulate leucine metabolism in the MBH. We also examined the role of hypothalamic ATP-sensitive K(+) channels (K(ATP) channels) in the effects of leucine. Enhancing the metabolism of leucine acutely in the MBH lowered blood glucose through a biochemical network that was insensitive to rapamycin but strictly dependent on the hypothalamic metabolism of leucine to α-ketoisocaproic acid and, further, insensitive to acetyl- and malonyl-CoA. Functional K(ATP) channels were also required. Importantly, molecular attenuation of this central sensing mechanism in rats conferred susceptibility to developing hyperglycemia. We postulate that the metabolic sensing of leucine in the MBH is a previously unrecognized mechanism for the regulation of hepatic glucose production required to maintain glucose homeostasis.

Permanent diabetes during the first year of life: multiple gene screening in 54 patients

AIMS/HYPOTHESIS

The aim of this study was to investigate the genetic aetiology of permanent diabetes mellitus with onset in the first 12 months of age.

METHODS

We studied 46 probands with permanent, insulin-requiring diabetes with onset within the first 6 months of life (permanent neonatal diabetes mellitus [PNDM]/monogenic diabetes of infancy [MDI]) (group 1) and eight participants with diabetes diagnosed between 7 and 12 months of age (group 2). KCNJ11, INS and ABCC8 genes were sequentially sequenced in all patients. For those who were negative in the initial screening, we examined ERN1, CHGA, CHGB and NKX6-1 genes and, in selected probands, CACNA1C, GCK, FOXP3, NEUROG3 and CDK4. The incidence rate for PNDM/MDI was calculated using a database of Italian patients collected from 1995 to 2009.

RESULTS

In group 1 we found mutations in KCNJ11, INS and ABCC8 genes in 23 (50%), 9 (19.5%) and 4 (8.6%) patients respectively, and a single homozygous mutation in GCK (2.1%). In group 2, we identified one incidence of a KCNJ11 mutation. No genetic defects were detected in other loci. The incidence rate of PNDM/MDI in Italy is estimated to be 1:210,287.

CONCLUSIONS/INTERPRETATION

Genetic mutations were identified in ~75% of non-consanguineous probands with PNDM/MDI, using sequential screening of KCNJ11, INS and ABCC8 genes in infants diagnosed within the first 6 months of age. This percentage decreased to 12% in those with diabetes diagnosed between 7 and 12 months. Patients belonging to the latter group may either carry mutations in genes different from those commonly found in PNDM/MDI or have developed an early-onset form of autoimmune diabetes.

Conserved intramolecular disulfide bond is critical to trafficking and fate of ATP-binding cassette (ABC) transporters ABCB6 and sulfonylurea receptor 1 (SUR1)/ABCC8

The ATP-binding cassette (ABC) transporter ABCB6 is a mitochondrial porphyrin transporter that activates porphyrin biosynthesis. ABCB6 lacks a canonical mitochondrial targeting sequence but reportedly traffics to other cellular compartments such as the plasma membrane. How ABCB6 reaches these destinations is unknown. In this study, we show that endogenous ABCB6 is glycosylated in multiple cell types, indicating trafficking through the endoplasmic reticulum (ER), and has only one atypical site for glycosylation (NXC) in its amino terminus. ABCB6 remained glycosylated when the highly conserved cysteine (Cys-8) was substituted with serine to make a consensus site, NXS. However, this substitution blocked ER exit and produced ABCB6 degradation, which was mostly reversed by the proteasomal inhibitor MG132. The amino terminus of ABCB6 has an additional highly conserved ER luminal cysteine (Cys-26). When Cys-26 was mutated alone or in combination with Cys-8, it also resulted in instability and ER retention. Further analysis revealed that these two cysteines form a disulfide bond. We discovered that other ABC transporters with an amino terminus in the ER had similarly configured conserved cysteines. This analysis led to the discovery of a disease-causing mutation in the sulfonylurea receptor 1 (SUR1)/ABCC8 from a patient with hyperinsulinemic hypoglycemia. The mutant allele only contains a mutation in a conserved amino-terminal cysteine, producing SUR1 that fails to reach the cell surface. These results suggest that for ABC transporters the propensity to form a disulfide bond in the ER defines a unique checkpoint that determines whether a protein is ER-retained.

Suppression of KATP channel activity protects murine pancreatic beta cells against oxidative stress

The enhanced oxidative stress associated with type 2 diabetes mellitus contributes to disease pathogenesis. We previously identified plasma membrane-associated ATP-sensitive K+ (KATP) channels of pancreatic beta cells as targets for oxidants. Here, we examined the effects of genetic and pharmacologic ablation of KATP channels on loss of mouse beta cell function and viability following oxidative stress. Using mice lacking the sulfonylurea receptor type 1 (Sur1) subunit of KATP channels, we found that, compared with insulin secretion by WT islets, insulin secretion by Sur1-/- islets was less susceptible to oxidative stress induced by the oxidant H2O2. This was likely, at least in part, a result of the reduced ability of H2O2 to hyperpolarize plasma membrane potential and reduce cytosolic free Ca2+ concentration ([Ca2+]c) in the Sur1-/- beta cells. Remarkably, Sur1-/- beta cells were less prone to apoptosis induced by H2O2 or an NO donor than WT beta cells, despite an enhanced basal rate of apoptosis. This protective effect was attributed to upregulation of the antioxidant enzymes SOD, glutathione peroxidase, and catalase. Upregulation of antioxidant enzymes and reduced sensitivity of Sur1-/- cells to H2O2-induced apoptosis were mimicked by treatment with the sulfonylureas tolbutamide and gliclazide. Enzyme upregulation and protection against oxidant-induced apoptosis were abrogated by agents lowering [Ca2+]c. Sur1-/- mice were less susceptible than WT mice to streptozotocin-induced beta cell destruction and subsequent hyperglycemia and death, which suggests that loss of KATP channel activity may protect against streptozotocin-induced diabetes in vivo.

Sodium channel beta1 regulatory subunit deficiency reduces pancreatic islet glucose-stimulated insulin and glucagon secretion

Glucose-stimulated insulin and glucagon release regulates glucose homeostasis by an excitation-secretion coupling pathway beginning with ATP-sensitive K(+) channel closure, membrane depolarization, and entry of calcium ions to stimulate exocytosis. The contribution of voltage-gated sodium channels to this release pathway is still being elucidated. We demonstrate that loss of Scn1b, a major regulatory subunit expressed with Na(v)1.7 protein in mouse pancreatic islets, reduces glucose-stimulated insulin and glucagon secretion in vitro and in vivo, resulting in severe fed and fasting hypoglycemia. This genetic mouse model is the first to demonstrate that sodium channelopathy impairs the physiological excitation-release coupling pathway for pancreatic insulin and glucagon release.

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.

Neonatal diabetes mellitus

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

The KATP channel is critical for calcium sequestration into non-ER compartments in mouse pancreatic beta cells

K(ATP) channel activity influences beta cell Ca(2+) homeostasis by regulating Ca(2+) influx through L-type Ca(2+) channels. The present paper demonstrates that loss of K(ATP) channel activity due to pharmacologic or genetic ablation affects Ca(2+) storage in intracellular organelles. ATP depletion, by the mitochondrial inhibitor FCCP, led to Ca(2+) release from the endoplasmic reticulum (ER) of wildtype beta cells. Blockade of ER Ca(2+) ATPases by cyclopiazonic acid abolished the FCCP-induced Ca(2+) transient. In beta cells treated with K(ATP) channel inhibitors FCCP elicited a significantly larger Ca(2+) transient. Cyclopiazonic acid did not abolish this Ca(2+) transient suggesting that non-ER compartments are recruited as additional Ca(2+) stores in beta cells lacking K(ATP) channel activity. Genetic ablation of K(ATP) channels in SUR1KO mice produced identical results. In INS-1 cells transfected with a mitochondrial-targeted Ca(2+)-sensitive fluorescence dye (ratiometric pericam) the increase in mitochondrial Ca(2+) evoked by tolbutamide was 5-fold larger compared to 15 mM glucose. These data show that genetic or pharmacologic ablation of K(ATP) channel activity conveys Ca(2+) release from a non-ER store. Based on the sensitivity to FCCP and the property of tolbutamide to increase mitochondrial Ca(2+) it is suggested that mitochondria are the recruited store. The change in Ca(2+) sequestration in beta cells treated with insulinotropic antidiabetics may have implications for beta cell survival and the therapeutic use of these drugs.

Glucose stimulates Ca2+ influx and insulin secretion in 2-week-old beta-cells lacking ATP-sensitive K+ channels

In adult beta-cells glucose-induced insulin secretion involves two mechanisms (a) a K(ATP) channel-dependent Ca(2+) influx and rise of cytosolic [Ca(2+)](c) and (b) a K(ATP) channel-independent amplification of secretion without further increase of [Ca(2+)](c). Mice lacking the high affinity sulfonylurea receptor (Sur1KO), and thus K(ATP) channels, have been developed as a model of congenital hyperinsulinism. Here, we compared [Ca(2+)](c) and insulin secretion in overnight cultured islets from 2-week-old normal and Sur1KO mice. Control islets proved functionally mature: the magnitude and biphasic kinetics of [Ca(2+)](c) and insulin secretion changes induced by glucose, and operation of the amplifying pathway, were similar to adult islets. Sur1KO islets perifused with 1 mm glucose showed elevation of both basal [Ca(2+)](c) and insulin secretion. Stimulation with 15 mm glucose produced a transient drop of [Ca(2+)](c) followed by an overshoot and a sustained elevation, accompanied by a monophasic, 6-fold increase in insulin secretion. Glucose also increased insulin secretion when [Ca(2+)](c) was clamped by KCl. When Sur1KO islets were cultured in 5 instead of 10 mm glucose, [Ca(2+)](c) and insulin secretion were unexpectedly low in 1 mm glucose and increased following a biphasic time course upon stimulation by 15 mm glucose. This K(ATP) channel-independent first phase [Ca(2+)](c) rise was attributed to a Na(+)-, Cl(-)-, and Na(+)-pump-independent depolarization of beta-cells, leading to Ca(2+) influx through voltage-dependent calcium channels. Glucose indeed depolarized Sur1KO islets under these conditions. It is suggested that unidentified potassium channels are sensitive to glucose and subserve the acute and long-term metabolic control of [Ca(2+)](c) in beta-cells without functional K(ATP) channels.

ABCC8 and ABCC9: ABC transporters that regulate K+ channels

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

Activating mutations in the ABCC8 gene in neonatal diabetes mellitus

BACKGROUND

The ATP-sensitive potassium (K(ATP)) channel, composed of the beta-cell proteins sulfonylurea receptor (SUR1) and inward-rectifying potassium channel subunit Kir6.2, is a key regulator of insulin release. It is inhibited by the binding of adenine nucleotides to subunit Kir6.2, which closes the channel, and activated by nucleotide binding or hydrolysis on SUR1, which opens the channel. The balance of these opposing actions determines the low open-channel probability, P(O), which controls the excitability of pancreatic beta cells. We hypothesized that activating mutations in ABCC8, which encodes SUR1, cause neonatal diabetes.

METHODS

We screened the 39 exons of ABCC8 in 34 patients with permanent or transient neonatal diabetes of unknown origin. We assayed the electrophysiologic activity of mutant and wild-type K(ATP) channels.

RESULTS

We identified seven missense mutations in nine patients. Four mutations were familial and showed vertical transmission with neonatal and adult-onset diabetes; the remaining mutations were not transmitted and not found in more than 300 patients without diabetes or with early-onset diabetes of similar genetic background. Mutant channels in intact cells and in physiologic concentrations of magnesium ATP had a markedly higher P(O) than did wild-type channels. These overactive channels remained sensitive to sulfonylurea, and treatment with sulfonylureas resulted in euglycemia.

CONCLUSIONS

Dominant mutations in ABCC8 accounted for 12 percent of cases of neonatal diabetes in the study group. Diabetes results from a newly discovered mechanism whereby the basal magnesium-nucleotide-dependent stimulatory action of SUR1 on the Kir pore is elevated and blockade by sulfonylureas is preserved.

Regulation of glucagon secretion at low glucose concentrations: evidence for adenosine triphosphate-sensitive potassium channel involvement

Glucagon is a potent counterregulatory hormone that opposes the action of insulin in controlling glycemia. The cellular mechanisms by which pancreatic alpha-cell glucagon secretion occurs in response to hypoglycemia are poorly known. SUR1/K(IR)6.2-type ATP-sensitive K(+) (K(ATP)) channels have been implicated in the glucagon counterregulatory response at central and peripheral levels, but their role is not well understood. In this study, we examined hypoglycemia-induced glucagon secretion in vitro in isolated islets and in vivo using Sur1KO mice lacking neuroendocrine-type K(ATP) channels and paired wild-type (WT) controls. Sur1KO mice fed ad libitum have normal glucagon levels and mobilize hepatic glycogen in response to exogenous glucagon but exhibit a blunted glucagon response to insulin-induced hypoglycemia. Glucagon release from Sur1KO and WT islets is increased at 2.8 mmol/liter glucose and suppressed by increasing glucose concentrations. WT islets increase glucagon secretion approximately 20-fold when challenged with 0.1 mmol/liter glucose vs. approximately 2.7-fold for Sur1KO islets. Glucagon release requires Ca(2+) and is inhibited by nifedipine. Consistent with a regulatory interaction between K(ATP) channels and intra-islet zinc-insulin, WT islets exhibit an inverse correlation between beta-cell secretion and glucagon release. Glibenclamide stimulated insulin secretion and reduced glucagon release in WT islets but was without effect on secretion from Sur1KO islets. The results indicate that loss of alpha-cell K(ATP) channels uncouples glucagon release from inhibition by beta-cells and reveals a role for K(ATP) channels in the regulation of glucagon release by low glucose.

Serum glucagon counterregulatory hormonal response to hypoglycemia is blunted in congenital hyperinsulinism

The mechanisms involved in the release of glucagon in response to hypoglycemia are unclear. Proposed mechanisms include the activation of the autonomic nervous system via glucose-sensing neurons in the central nervous system, via the regulation of glucagon secretion by intra-islet insulin and zinc concentrations, or via direct ionic control, all mechanisms that involve high-affinity sulfonylurea receptor/inwardly rectifying potassium channel-type ATP-sensitive K(+) channels. Patients with congenital hyperinsulinism provide a unique physiological model to understand glucagon regulation. In this study, we compare serum glucagon responses to hyperinsulinemic hypoglycemia versus nonhyperinsulinemic hypoglycemia. In the patient group (n = 20), the mean serum glucagon value during hyperinsulinemic hypoglycemia was 17.6 +/- 5.7 ng/l compared with 59.4 +/- 7.8 ng/l in the control group (n = 15) with nonhyperinsulinemic hypoglycemia (P < 0.01). There was no difference between the serum glucagon responses in children with diffuse, focal, and diazoxide-responsive forms of hyperinsulinism. The mean serum epinephrine and norepinephrine concentrations in the hyperinsulinemic group were 2,779 +/- 431 pmol/l and 2.9 +/- 0.7 nmol/l and appropriately rose despite the blunted glucagon response. In conclusion, the loss of ATP-sensitive K(+) channels and or elevated intraislet insulin cannot explain the blunted glucagon release in all patients with congenital hyperinsulinism. Other possible mechanisms such as the suppressive effect of prolonged hyperinsulinemia on alpha-cell secretion should be considered.

Insulin secretagogues, sulfonylurea receptors and K(ATP) channels

ATP-sensitive K+ channels, termed K(ATP) channels, provide a link between cellular metabolism and membrane electrical activity in a variety of tissues. Channel isoforms have been identified and are targets for compounds that both stimulate and inhibit their activity resulting in membrane hyperpolarization and depolarization, respectively. Examples include relaxation of vascular smooth muscle and stimulation of insulin secretion. This article reviews the cloning, molecular biology, and structure of K(ATP) channels, with particular focus on the SUR1/K(IR)6.2 neuroendocrine channels that are important for the regulation of insulin secretion. We integrate the extensive pharmacologic structure-activity-relationship data on these channels, which defines a bipartite drug binding pocket in the SUR (sulfonylurea receptor), with recent structure-function studies that identify domains of SUR and K(IR)6.2, the channel pore, which are critical for channel assembly, for gating, and for the ligand-receptor interactions that modulate channel activity. The atomic structure of a sulfonylurea in a protein pocket is used to develop insight into the recognition of these compounds. A homology model of K(ATP) channels, based on VC-MsbA, another member of the ABC protein family, is described and used to position amino acids important for the action of channel openers and blockers within the core of SUR. The model has a central chamber which could serve as a multifaceted binding pocket.

Hypothalamic K(ATP) channels control hepatic glucose production

Obesity is the driving force behind the worldwide increase in the prevalence of type 2 diabetes mellitus. Hyperglycaemia is a hallmark of diabetes and is largely due to increased hepatic gluconeogenesis. The medial hypothalamus is a major integrator of nutritional and hormonal signals, which play pivotal roles not only in the regulation of energy balance but also in the modulation of liver glucose output. Bidirectional changes in hypothalamic insulin signalling therefore result in parallel changes in both energy balance and glucose metabolism. Here we show that activation of ATP-sensitive potassium (K(ATP)) channels in the mediobasal hypothalamus is sufficient to lower blood glucose levels through inhibition of hepatic gluconeogenesis. Finally, the infusion of a K(ATP) blocker within the mediobasal hypothalamus, or the surgical resection of the hepatic branch of the vagus nerve, negates the effects of central insulin and halves the effects of systemic insulin on hepatic glucose production. Consistent with these results, mice lacking the SUR1 subunit of the K(ATP) channel are resistant to the inhibitory action of insulin on gluconeogenesis. These findings suggest that activation of hypothalamic K(ATP) channels normally restrains hepatic gluconeogenesis, and that any alteration within this central nervous system/liver circuit can contribute to diabetic hyperglycaemia.

Crosstalk between membrane potential and cytosolic Ca2+ concentration in beta cells from Sur1-/- mice

AIMS/HYPOTHESIS

Islets or beta cells from Sur1(-/-) mice were used to determine whether changes in plasma membrane potential (V(m)) remain coupled to changes in cytosolic Ca(2+) ([Ca(2+)](i)) in the absence of K(ATP) channels and thus provide a triggering signal for insulin secretion. The study also sought to elucidate whether [Ca(2+)](i) influences oscillations in V(m) in sur1(-/-) beta cells.

METHODS

Plasma membrane potential and ion currents were measured with microelectrodes and the patch-clamp technique. [Ca(2+)](i) was monitored with the fluorescent dye fura-2. Insulin secretion from isolated islets was determined by static incubations.

RESULTS

Membrane depolarisation of Sur1(-/-) islets by arginine or increased extracellular K(+), elevated [Ca(2+)](i) and augmented insulin secretion. Oligomycin completely abolished glucose-stimulated insulin release from Sur1(-/-) islets. Oscillations in V(m) were influenced by [Ca(2+)](i) as follows: (1) elevation of extracellular Ca(2+) lengthened phases of membrane hyperpolarisation; (2) simulating a burst of action potentials induced a Ca(2+)-dependent outward current that was augmented by increased Ca(2+) influx through L-type Ca(2+) channels; (3) Ca(2+) depletion of intracellular stores by cyclopiazonic acid increased the burst frequency in Sur1(-/-) islets, elevating [Ca(2+)](i) and insulin secretion; (4) store depletion activated a Ca(2+) influx that was not inhibitable by the L-type Ca(2+) channel blocker D600.

CONCLUSIONS/INTERPRETATION

Although V(m) is largely uncoupled from glucose metabolism in the absence of K(ATP) channels, increased electrical activity leads to elevations of [Ca(2+)](i) that are sufficient to stimulate insulin secretion. In Sur1(-/-) beta cells, [Ca(2+)](i) exerts feedback mechanisms on V(m) by activating a hyperpolarising outward current and by depolarising V(m) via store-operated ion channels.

Docking and fusion of insulin secretory granules in SUR1 knock out mouse beta-cells observed by total internal reflection fluorescence microscopy

To explore how the sulfonylurea receptor (SUR1) is involved in docking and fusion of insulin granules, dynamic motion of single insulin secretory granules near the plasma membrane was examined in SUR1 knock-out (Sur1KO) beta-cells by total internal reflection fluorescence microscopy. Sur1KO beta-cells exhibited a marked reduction in the number of fusion events from previously docked granules. However, the number of docked granules declined during stimulation as a consequence of the release of docked granules into the cytoplasm vs. fusion with the plasma membrane. Thus, the impaired docking and fusion results in decreased insulin exocytosis from Sur1KO beta-cells.

Hypothalamic sensing of circulating fatty acids is required for glucose homeostasis

Increased glucose production is a hallmark of type 2 diabetes and alterations in lipid metabolism have a causative role in its pathophysiology. Here we postulate that physiological increments in plasma fatty acids can be sensed within the hypothalamus and that this sensing is required to balance their direct stimulatory action on hepatic gluconeogenesis. In the presence of physiologically-relevant increases in the levels of plasma fatty acids, negating their central action on hepatic glucose fluxes through (i) inhibition of the hypothalamic esterification of fatty acids, (ii) genetic deletion (Sur1-deficient mice) of hypothalamic K(ATP) channels or pharmacological blockade (K(ATP) blocker) of their activation by fatty acids, or (iii) surgical resection of the hepatic branch of the vagus nerve led to a marked increase in liver glucose production. These findings indicate that a physiological elevation in circulating lipids can be sensed within the hypothalamus and that a defect in hypothalamic lipid sensing disrupts glucose homeostasis.

Hyperinsulinism of infancy: novel ABCC8 and KCNJ11 mutations and evidence for additional locus heterogeneity

Hyperinsulinism of infancy is a genetically heterogeneous disease characterized by dysregulation of insulin secretion resulting in severe hypoglycemia. To date, mutations in five different genes, the sulfonylurea receptor (SUR1, ABCC8), the inward rectifying potassium channel (K(IR)6.2, KCNJ11), glucokinase (GCK), glutamate dehydrogenase (GLUD1), and short-chain 3-hydroxyacyl-coenzyme A dehydrogenase (SCHAD), have been implicated. Previous reports suggest that, in 40% of patients, no mutation can be identified in any of these genes, suggesting additional locus heterogeneity. However, previous studies did not screen all five genes using direct sequencing, the most sensitive technique available for mutation detection. We selected 15 hyperinsulinism of infancy patients and systematically sequenced the promoter and all coding exons and intron/exon boundaries of ABCC8 and KCNJ11. If no mutation was identified, the coding sequence and intron/exon boundaries of GCK, GLUD1, and SCHAD were sequenced. Seven novel mutations were found in the ABCC8 coding region, one mutation was found in the KCNJ11 coding region, and one novel mutation was found in each of the two promoter regions screened. Functional studies on beta-cells from six patients showed abnormal ATP-sensitive K+ channel function in five of the patients; the sixth had normal channel activity, and no mutations were found. Photolabeling studies using a reconstituted system showed that all missense mutations altered intracellular trafficking. Each of the promoter mutations decreased expression of a reporter gene by about 60% in a heterologous expression system. In four patients (27%), no mutations were identified. Thus, further genetic heterogeneity is suggested in this disorder. These patients represent a cohort that can be used for searching for mutations in other candidate genes.

Toward linking structure with function in ATP-sensitive K+ channels

Advances in understanding the overall structural features of inward rectifiers and ATP-binding cassette (ABC) transporters are providing novel insight into the architecture of ATP-sensitive K+ channels (KATP channels) (KIR6.0/SUR)4. The structure of the K(IR) pore has been modeled on bacterial K+ channels, while the lipid-A exporter, MsbA, provides a template for the MDR-like core of sulfonylurea receptor (SUR)-1. TMD0, an NH2-terminal bundle of five alpha-helices found in SURs, binds to and activates KIR6.0. The adjacent cytoplasmic L0 linker serves a dual function, acting as a tether to link the MDR-like core to the KIR6.2/TMD0 complex and exerting bidirectional control over channel gating via interactions with the NH2-terminus of the KIR. Homology modeling of the SUR1 core offers the possibility of defining the glibenclamide/sulfonylurea binding pocket. Consistent with 30-year-old studies on the pharmacology of hypoglycemic agents, the pocket is bipartite. Elements of the COOH-terminal half of the core recognize a hydrophobic group in glibenclamide, adjacent to the sulfonylurea moiety, to provide selectivity for SUR1, while the benzamido group appears to be in proximity to L0 and the KIR NH2-terminus.

Both triggering and amplifying pathways contribute to fuel-induced insulin secretion in the absence of sulfonylurea receptor-1 in pancreatic beta-cells

In normal beta-cells glucose induces insulin secretion by activating both a triggering pathway (closure of K(ATP) channels, depolarization, and rise in cytosolic [Ca(2+)](i)) and an amplifying pathway (augmentation of Ca(2+) efficacy on exocytosis). It is unclear if and how nutrients can regulate insulin secretion by beta-cells lacking K(ATP) channels (Sur1 knockout mice). We compared glucose- and amino acid-induced insulin secretion and [Ca(2+)](i) changes in control and Sur1KO islets. In 1 mm glucose (non-stimulatory for controls), the triggering signal [Ca(2+)](i) was high (loss of regulation) and insulin secretion was stimulated in Sur1KO islets. This "basal" secretion was decreased or increased by imposed changes in [Ca(2+)](i) and was dependent on ATP production, indicating that both triggering and amplifying signals are involved. High glucose stimulated insulin secretion in Sur1KO islets, by an unsuspected, transient increase in [Ca(2+)](i) and a sustained activation of the amplifying pathway. Unlike controls, Sur1KO islets were insensitive to diazoxide and tolbutamide, which rules out effects of either drug at sites other than K(ATP) channels. Amino acids potently increased insulin secretion by Sur1KO islets through both a further electrogenic rise in [Ca(2+)](i) and a metabolism-dependent activation of the amplifying pathway. After sulfonylurea blockade of their K(ATP) channels, control islets qualitatively behaved like Sur1KO islets, but their insulin secretion rate was consistently lower for a similar or even higher [Ca(2+)](i). In conclusion, fuel secretagogues can control insulin secretion in beta-cells without K(ATP) channels, partly by an unsuspected influence on the triggering [Ca(2+)](i) signal and mainly by the modulation of a very effective amplifying pathway.

Epinephrine-induced hyperpolarization of islet cells without KATP channels

This study examines the effect of epinephrine, a known physiological inhibitor of insulin secretion, on the membrane potential of pancreatic islet cells from sulfonylurea receptor-1 (ABCC8)-null mice (Sur1KO), which lack functional ATP-sensitive K+ (KATP) channels. These channels have been argued to be activated by catecholamines, but epinephrine effectively inhibits insulin secretion in both Sur1KO and wild-type islets and in mice. Isolated Sur1KO beta-cells are depolarized in both low (2.8 mmol/l) and high (16.7 mmol/l) glucose and exhibit Ca(2+)-dependent action potentials. Epinephrine hyperpolarizes Sur1KO beta-cells, inhibiting their spontaneous action potentials. This effect, observed in standard whole cell patches, is abolished by pertussis toxin and blocked by BaCl2. The epinephrine effect is mimicked by clonidine, a selective alpha2-adrenoceptor agonist and inhibited by alpha-yohimbine, an alpha2-antagonist. A selection of K+ channel inhibitors, tetraethylammonium, apamin, dendrotoxin, iberiotoxin, E-4130, chromanol 293B, and tertiapin did not block the epinephrine-induced hyperpolarization. Analysis of whole cell currents revealed an inward conductance of 0.11 +/- 0.04 nS/pF (n = 7) and a TEA-sensitive outward conductance of 0.55 +/- 0.08 nS/pF (n = 7) at -60 and 0 mV, respectively. Guanosine 5'-O-(3-thiotriphosphate) (100 microM) in the patch pipette did not significantly alter these currents or activate novel inward-rectifying K+ currents. We conclude that epinephrine can hyperpolarize beta-cells in the absence of KATP channels via activation of low-conductance BaCl2-sensitive K+ channels that are regulated by pertussis toxin-sensitive G proteins.

Assembly, Maturation, and Turnover of KATP Channel Subunits

ATP-sensitive K(+), or K(ATP), channels are comprised of K(IR)6.x and sulfonylurea receptor (SUR) subunits that assemble as octamers, (K(IR)/SUR)(4). The assembly pathway is unknown. Pulse-labeling studies show that when K(IR)6.2 is expressed individually, its turnover is biphasic; approximately 60% is lost with t((1/2)) approximately 36 min. The remainder converts to a long-lived species (t((1/2)) approximately 26 h) with an estimated half-time of 1.2 h. Expressed alone, SUR1 has a long half-life, approximately 25.5 h. When K(IR)6.2 and SUR1 are co-expressed, they associate rapidly and the fast degradation of K(IR)6.2 is eliminated. Based on changes in the glycosylation state of SUR1, the half-time for the maturation of K(ATP) channels, including completion of assembly, transit to the Golgi, and glycosylation, is approximately 2.2 h. Estimation of the turnover rates of mature, fully glycosylated SUR1 associated with K(IR)6.2 and of K(IR)6.2 associated with Myc-tagged SUR1 gave similar values for the half-life of K(ATP) channels, a mean value of approximately 7.3 h. K(ATP) channel subunits in INS-1 beta-cells displayed qualitatively similar kinetics. The results imply the octameric channels are stable. Two mutations, K(IR)6.2 W91R and SUR1 DeltaF1388, identified in patients with the severe form of familial hyperinsulinism, profoundly alter the rate of K(IR)6.2 and SUR1 turnover, respectively. Both mutant subunits associate with their respective partners but dissociate freely and degrade rapidly. The data support models of channel formation in which K(IR)6.2-SUR1 heteromers assemble functional channels and are inconsistent with models where SUR1 can only assemble with K(IR)6.2 tetramers.

Oscillations of membrane potential and cytosolic Ca(2+) concentration in SUR1(-/-) beta cells

AIMS/HYPOTHESIS

SUR1(ABCC8)(-/-) mice lacking functional K(ATP) channels are an appropriate model to test the significance of K(ATP) channels in beta-cell function. We examined how this gene deletion interferes with stimulus-secretion coupling. We tested the influence of metabolic inhibition and galanin, whose mode of action is controversial.

METHODS

Plasma membrane potential (Vm) and currents were measured with microelectrodes or the patch-clamp technique; cytosolic Ca(2+) concentrations ([Ca(2+)](c)) and mitochondrial membrane potential (DeltaPsi) were measured using fluorescent dyes.

RESULTS

In contrast to the controls, SUR1(-/-) beta cells showed electrical activity even at a low glucose concentration. Continuous spike activity was measured with the patch-clamp technique, but with microelectrodes slow oscillations in Vm consisting of bursts of Ca(2+)-dependent action potentials were detected. [Ca(2+)](c) showed various patterns of oscillations or a sustained increase. Sodium azide did not hyperpolarize SUR1(-/-) beta cells. The depolarization of DeltaPsi evoked by sodium azide was significantly lower in SUR1(-/-) than SUR1(+/+) cells. Galanin transiently decreased action potential frequency and [Ca(2+)](c) in cells from both SUR1(-/-) and SUR1(+/+) mice.

CONCLUSION/INTERPRETATION

The strong dependence of Vm and [Ca(2+)](c) on glucose concentration observed in SUR1(+/+) beta cells is disrupted in the knock-out cells. This demonstrates that both parameters oscillate in the absence of functional K(ATP) channels. The lack of effect of metabolic inhibition by sodium azide shows that in SUR1(-/-) beta cells changes in ATP/ADP no longer link glucose metabolism and Vm. The results with galanin suggest that this peptide affects beta cells independently of K(ATP) currents and thus could contribute to the regulation of beta-cell function in SUR1(-/-) animals.

Clinical and molecular characterization of a dominant form of congenital hyperinsulinism caused by a mutation in the high-affinity sulfonylurea receptor

Recessive mutations of sulfonylurea receptor 1 (SUR1) and potassium inward rectifier 6.2 (Kir6.2), the two adjacent genes on chromosome 11p that comprise the beta-cell plasma membrane ATP-sensitive K(+) (K(ATP)) channels, are responsible for the most common form of congenital hyperinsulinism in children. The present study was undertaken to identify the genetic defect in a family with dominantly inherited hyperinsulinism affecting five individuals in three generations. Clinical tests were carried out in three of the patients using acute insulin responses (AIRs) to intravenous stimuli to localize the site of defect in insulin regulation. The affected individuals showed abnormal positive calcium AIR, normal negative leucine AIR, subnormal positive glucose AIR, and impaired tolbutamide AIR. This AIR pattern suggested a K(ATP) channel defect because it resembled that seen in children with recessive hyperinsulinism due to two common SUR1 mutations, g3992-9a and delPhe1388. Genetic linkage to the K(ATP) locus was established using intragenic polymorphisms. Mutation analysis identified a novel trinucleotide deletion in SUR1 exon 34 that results in the loss of serine 1387. Studies of delSer1387 in COSm6 cells confirmed that the expressed mutant protein assembles with Kir6.2 and trafficks to the plasma membrane, but it had no (86)Rb efflux ion transport activity. These results indicate that hyperinsulinism in this family is caused by a SUR1 mutation that is expressed dominantly rather than recessively.

Ischemic preconditioning in the hippocampus of a knockout mouse lacking SUR1-based K(ATP) channels

BACKGROUND AND PURPOSE

ATP-sensitive K+ (K(ATP)) channels have been implicated in the mechanism of neuronal ischemic preconditioning. To evaluate the role of neuronal/beta-cell-type K(ATP) channels, SUR1 null (Sur1KO) mice lacking (K(IR)6.x/SUR1)(4) K(ATP) channels were subjected to a preconditioning protocol with the use of double carotid occlusion.

METHODS

Wild-type C57BL/6 and Sur1KO mice were subjected to a double carotid block for 40 minutes with or without a 20-minute preconditioning block. After a 10-day reperfusion period, damage was assessed histologically in the hippocampal CA1, CA2, and CA3 areas and in the dentate gyrus. The neuroprotective effects of intracerebroventricular injections of diazoxide, which selectively affects mitochondria versus opening SUR1-type K(ATP) channels, and 5-hydroxydecanoate, a selective blocker of mitoK(ATP) channels, were evaluated with the same protocol.

RESULTS

Neurons in the CA1 region of both Sur1KO and wild-type animals subjected to a 20-minute ischemic insult were protected equally from neuronal damage produced by a subsequent 40-minute ischemic period. Pretreatment with diazoxide protected both Sur1KO and wild-type neurons, while 5-hydroxydecanoate augmented neurodegeneration in both strains of animals when administered before a 20-minute bout of ischemia.

CONCLUSIONS

SUR1-based K(ATP) channels are not obligatory for neuronal preconditioning or augmentation of neurodegeneration by 5-hydroxydecanoate.

cAMP-activated protein kinase-independent potentiation of insulin secretion by cAMP is impaired in SUR1 null islets

Whereas the loss of ATP-sensitive K(+) channel (K(ATP) channel) activity in human pancreatic beta-cells causes severe hypoglycemia in certain forms of hyperinsulinemic hypoglycemia, similar channel loss in sulfonylurea receptor-1 (SUR1) and Kir6.2 null mice yields a milder phenotype that is characterized by normoglycemia, unless the animals are stressed. While investigating potential compensatory mechanisms, we found that incretins, specifically glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP), can increase the cAMP content of Sur1KO islets but do not potentiate glucose-stimulated insulin release. This impairment is secondary to a restriction in the ability of Sur1KO beta-cells to sense cAMP correctly. Potentiation does not appear to require cAMP-activated protein kinase (PKA) because H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide) and KT5720, inhibitors of PKA, do not affect stimulation by GLP-1, GIP, or exendin-4 in wild-type islets, although they block phosphorylation of cAMP-response element-binding protein. The impaired incretin response in Sur1KO islets is specific; the stimulation of insulin release by other modulators, including mastoparan and activators of protein kinase C, is conserved. The results suggest that the defect responsible for the loss of cAMP-induced potentiation of insulin secretion is PKA independent. We hypothesize that a reduced release of insulin in response to incretins may contribute to the unexpected normoglycemic phenotype of Sur1KO mice versus the pronounced hypoglycemia seen in neonates with loss of K(ATP) channel activity.

Transactivation of the mouse sulfonylurea receptor I gene by BETA2/NeuroD

The sulfonylurea receptor 1 (SUR1) plays a key role in regulation of insulin secretion in pancreatic beta-cells. In this study we investigated the mechanism for tissue-specific expression of the SUR1 gene. A -138/-20 fragment exhibited basal promoter activity while the -660/-20 fragment contained a regulatory element for tissue-specific expression of the mouse SUR1 gene. A pancreatic beta-cell-specific transcription factor, BETA2 (beta-cell E box transcription factor)/NeuroD, enhanced the promoter activity of the -660/-20 fragment in cooperation with E47. Coexpression of a dominant negative mutant of BETA2/NeuroD, BETA2(1-233), repressed the promoter activity of the -660/-20 fragment. BETA2/NeuroD bound specifically to the E3 element located at -141. The E3 sequence in a heterologous context conferred transactivation by BETA2/NeuroD in HeLa and HIT cells. Mutation of E3 eliminated the stimulatory effect of BETA2/NeuroD. Unlike BETA2/NeuroD, neurogenin 3 (ngn3) could not activate the E3 element in HeLa cells. Overexpression of ngn3 concomitantly increased expression of BETA2/NeuroD and SUR1 in HIT cells but not in HeLa cells. These results indicate that BETA2/NeuroD induces tissue-specific expression of the SUR1 gene through the E3 element. These results also suggest that E3 is specific for BETA2/NeuroD, and the stimulatory effect of ngn3 in HIT cells may require factors specifically expressed in HIT cells.

Of mice and men: K(ATP) channels and insulin secretion

K(ATP) channels are a unique, small family of potassium (K+)-selective ion channels assembled from four inward rectifier pore-forming subunits, K(IR)6.x, paired with four sulfonylurea receptors (SURs), members of the adenosine triphosphate (ATP)-binding cassette superfamily. The activity of these channels can be regulated by metabolically driven changes in the ratio of adenosine diphosphate (ADP) to ATP, providing a means to couple membrane electrical activity with metabolism. In pancreatic beta cells in the islets of Langerhans, K(ATP) channels are part of an ionic mechanism that couples glucose metabolism to insulin secretion. This chapter 1) briefly describes the properties of K(ATP) channels; 2) discusses data on a genetically recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI), caused by loss of beta-cell K(ATP) channel activity; and 3) compares the severe impairment of glucose homeostasis that characterizes the human phenotype with the near-normal phenotype observed in K(ATP) channel null mice.

Dysregulation of insulin secretion in children with congenital hyperinsulinism due to sulfonylurea receptor mutations

Mutations in the high-affinity sulfonylurea receptor (SUR)-1 cause one of the severe recessively inherited diffuse forms of congenital hyperinsulinism or, when associated with loss of heterozygosity, focal adenomatosis. We hypothesized that SUR1 mutations would render the beta-cell insensitive to sulfonylureas and to glucose. Stimulated insulin responses were compared among eight patients with diffuse hyperinsulinism (two mutations), six carrier parents, and ten normal adults. In the patients with diffuse hyperinsulinism, the acute insulin response to intravenous tolbutamide was absent and did not overlap with the responses seen in either adult group. There was positive, albeit significantly blunted, acute insulin response to intravenous dextrose in the patients with diffuse hyperinsulinism. Graded infusions of glucose, to raise and then lower plasma glucose concentrations over 4 h, caused similar rises in blood glucose but lower peak insulin levels in the hyperinsulinemic patients. Loss of acute insulin response to tolbutamide can identify children with diffuse SUR1 defects. The greater response to glucose than to tolbutamide indicates that ATP-sensitive potassium (KATP) channel-independent pathways are involved in glucose-mediated insulin release in patients with diffuse SUR1 defects. The diminished glucose responsiveness suggests that SUR1 mutations and lack of KATP channel activity may contribute to the late development of diabetes in patients with hyperinsulinism independently of subtotal pancreatectomy.

Calcium-stimulated insulin secretion in diffuse and focal forms of congenital hyperinsulinism

OBJECTIVES

To identify infants with hyperinsulinism caused by defects of the beta-cell adenosine triphosphate-dependent potassium channel complex and to distinguish focal and diffuse forms of hyperinsulinism caused by these mutations.

STUDY DESIGN

The acute insulin response to intravenous calcium stimulation (CaAIR) was determined in 9 patients <20 years with diffuse hyperinsulinism caused by defective beta-cell sulfonylurea receptor (SUR1(-/-)), 3 patients with focal congenital hyperinsulinism (6 weeks to 18 months), a 10-year-old with insulinoma, 5 with hyperinsulinism/hyperammonemia syndrome caused by defective glutamate dehydrogenase (6 months to 28 years), 4 SUR1(+/-) heterozygotes with no symptoms, and 9 normal adults. Three infants with congenital focal disease, 1 with diffuse hyperinsulinism, and the child with insulinoma underwent selective pancreatic intra-arterial calcium stimulation with hepatic venous sampling.

RESULTS

Children with diffuse SUR1(-/-) disease and infants with congenital focal hyperinsulinism responded to CaAIR, whereas the normal control group, patients with hyperinsulinism/hyperammonemia syndrome, and SUR1(+/-) carriers did not. Selective arterial calcium stimulation of the pancreas with hepatic venous sampling revealed selective, significant step-ups in insulin secretion that correlated anatomically with the location of solitary lesions confirmed surgically in 2 of 3 infants with congenital focal disease and in the child with insulinoma. Selective arterial calcium stimulation of the pancreas with hepatic venous sampling demonstrated markedly elevated baseline insulin levels throughout the pancreas of the infant with diffuse hyperinsulinism.

CONCLUSIONS

The intravenous CaAIR is a safe and simple test for identifying infants with diffuse SUR1(-/-) hyperinsulinism or with focal congenital hyperinsulinism. Preoperative selective arterial calcium stimulation of the pancreas with hepatic venous sampling can localize focal lesions causing hyperinsulinism in children. The combination of these calcium stimulation tests may help distinguish focal lesions suitable for cure by local surgical resection.

Sur1 knockout mice. A model for K(ATP) channel-independent regulation of insulin secretion

Sur1 knockout mouse beta-cells lack K(ATP) channels and show spontaneous Ca(2+) action potentials equivalent to those seen in patients with persistent hyperinsulinemic hypoglycemia of infancy, but the mice are normoglycemic unless stressed. Sur1(-/-) islets lack first phase insulin secretion and exhibit an attenuated glucose-stimulated second phase secretion. Loss of the first phase leads to mild glucose intolerance, whereas reduced insulin output is consistent with observed neonatal hyperglycemia. Loss of K(ATP) channels impairs the rate of return to a basal secretory level after a fall in glucose concentration. This leads to increased hypoglycemia upon fasting and contributes to a very early, transient neonatal hypoglycemia. Whereas persistent hyperinsulinemic hypoglycemia of infancy underscores the importance of the K(ATP)-dependent ionic pathway in control of insulin release, the Sur1(-/-) animals provide a novel model for study of K(ATP)-independent pathways that regulate insulin secretion.

Familial hyperinsulinism and pancreatic beta-cell ATP-sensitive potassium channels

Familial hyperinsulinism, also known as persistent hyperinsulinemic hypoglycemia of infancy (PHHI), is a genetic disease characterized by mild to severe hypoglycemia in the presence of inappropriately high levels of insulin. The recessive form is caused by mutations in the adenosine 5'-triphosphate (ATP)-sensitive K+ channel (KATP channel) present in the plasma membrane of pancreatic beta-cells. This channel is formed by two subunits, the high-affinity sulfonylurea receptor, SUR1, and KIR6.2, a member of the inwardly rectifying family of K+ channels. KATP channels regulate insulin secretion by linking membrane excitability with glucose metabolism. Approximately 50 mutations, in both channel subunits, that abolish or alter the regulation of beta-cell KATP channels have been identified in patients with the recessive form of PHHI.

Sulfonylurea receptors: ABC transporters that regulate ATP-sensitive K(+) channels

The association of sulfonylurea receptors (SURs) with K(IR)6.x subunits to form ATP-sensitive K(+) channels presents perhaps the most unusual function known for members of the transport ATPase family. The integration of these two protein subunits extends well beyond conferring sensitivity to sulfonylureas. Recent studies indicate SUR-K(IR)6.x interactions are critical for all of the properties associated with native K(ATP) channels including quality control over surface expression, channel kinetics, inhibition and stimulation by Mg-nucleotides and response both to channel blockers like sulfonylureas and to potassium channel openers. K(ATP) channels are a unique example of the physiologic and medical importance of a transport ATPase and provide a paradigm for how other members of the family may interact with other ion channels.

The C terminus of SUR1 is required for trafficking of KATP channels

In beta cells from the pancreas, ATP-sensitive potassium channels, or KATP channels, are composed of two subunits, SUR1 and KIR6.2, assembled in a (SUR1/KIR6.2)4 stoichiometry. The correct stoichiometry of channels at the cell surface is tightly regulated by the presence of novel endoplasmic reticulum (ER) retention signals in SUR1 and KIR6.2; incompletely assembled KATP channels fail to exit the ER/cis-Golgi compartments. In addition to these retrograde signals, we show that the C terminus of SUR1 has an anterograde signal, composed in part of a dileucine motif and downstream phenylalanine, which is required for KATP channels to exit the ER/cis-Golgi compartments and transit to the cell surface. Deletion of as few as seven amino acids, including the phenylalanine, from SUR1 markedly reduces surface expression of KATP channels. Mutations leading to truncation of the C terminus of SUR1 are one cause of a severe, recessive form of persistent hyperinsulinemic hypoglycemia of infancy. We propose that the complete loss of beta cell KATP channel activity seen in this form of hyperinsulinism is a failure of KATP channels to traffic to the plasma membrane.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

Sulfonylurea receptors set the maximal open probability, ATP sensitivity and plasma membrane density of KATP channels

KATP channels are heteromultimers of SUR and KIR6.2. C-terminal truncation of KIR6.2 allows surface expression of the pore. KIR6.2deltaC35 channels display approximately 7-fold lower maximal open probability, approximately 35-fold reduced ATP sensitivity, reduced mean open time, a markedly increased transition rate from a burst into a long-lived closed state, and have no counterpart in vivo. SUR1 and SUR2A restore wild-type bursting, ATP sensitivity and increase channel density in the plasma membrane. The high IC50(ATP) of approximately 4 mM for KIR6.2deltaCK185Q channels results from the additive effects of SUR removal and KIR6.2 modification. The results demonstrate allosteric interaction(s) are essential for normal intrinsic activity, ATP inhibition, and trafficking of KATP channels.