In beta cells from the pancreas, ATP-sensitive potassium channels, or K ATP channels, are composed of two subunits, SUR1 and K IR 6.2, assembled in a (SUR1/ K IR 6.2) 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 K IR 6.2; incompletely assembled K ATP 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 K ATP 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 K ATP 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 K ATP channel activity seen in this form of hyperinsulinism is a failure of K ATP channels to traffic to the plasma membrane.

Whereas the loss of ATP-sensitive K؉ channel (K ATP channel) activity in human pancreatic ␤-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 glucosedependent 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 I nsulin secretion is a unique example of exocytosis controlled by metabolic, ionic, and hormonal pathways. An imbalance in insulin release due to disruption of these pathways can produce the profound changes in glucose homeostasis associated with either hyperglycemia (diabetes) or hypoglycemia (e.g., hyperinsulinemic hypoglycemia [HI]). In pancreatic ␤-cells, ATPsensitive K ϩ channels (K ATP channels), composed of Kir6.2 and the sulfonylurea receptor-1 (SUR1), and voltage-gated Ca 2ϩ channels are key players linking increased glucose metabolism to elevation of cytosolic Ca 2ϩ concentration ([Ca 2ϩ ] i ). Membrane depolarization induced by closure of K ATP channels, secondary to changes in ADP/ ATP resulting from glucose metabolism, leads to the generation of Ca 2ϩ -dependent action potentials and [Ca 2ϩ ] i oscillations, which are considered to be a major mediator of insulin secretion. Sulfonylureas that block ␤-cell K ATP channels are commonly used to restore insulin secretion in type 2 diabetes. The loss or constitutive closure of ␤-cell K ATP channels, as a result of mutations in either subunit, is a cause of both dominant and recessive forms of HI (HI-SUR1 or HI-Kir6.2), characterized by elevated plasma insulin values inconsistent with the observed hypoglycemia (reviewed in 1,2). Studies on islets isolated from patients diagnosed with HI are consistent with hypersecretion of insulin (3,4). By comparison, the clinical phenotype of mice lacking ␤-cell/neuronal K ATP channels is strikingly normal. Kir6.2 null (Kir6.2KO) (5) and Sur1KO (6) mice are normoglycemic when fed, displaying only mild glucose intolerance, consistent with their loss of first phase and attenuated second phase of insulin release. Sur1KO mice exhibit greater hypoglycemia upon fasting, consistent with their inability to rapidly repolarize their ␤-cells and reduce insulin release (6). No compensating ionic mechanisms have been identified, and the electrophysiological phenotype of isolated K ATP KO mouse ␤-cells, i.e., constant membrane depolarization, presence of Ca 2ϩ -dependent action potentials, and elevated oscillating [Ca 2ϩ ] i in low glucose, is quite similar to that of ␤-cells from HI neonates (compare 5-7); therefore, it is unclear why K ATP KO islets lack the elevated basal insulin release observed in HI islets (3,4).In a search for dif...

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.

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 ␤-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. Diabetes 52: [2403][2404][2405][2406][2407][2408][2409][2410] 2003 C ongenital hyperinsulinism in children is a disorder of persistent hypoglycemia that is caused by genetic mutations in the pathways regulating insulin secretion by pancreatic islets (1). The most common of these disorders is associated with recessive mutations of the two adjacent genes on chromosome 11p that comprise the ␤-cell ATP-sensitive K ϩ (K ATP ) channel: the high-affinity sulfonylurea receptor 1 (SUR1 or ABCC8) and its regulated ion pore, potassium inward rectifier 6.2 (Kir6.2 or KCNJ11) (2-5). Mutations in these channel genes can also cause sporadic focal hyperinsulinism through a two-hit mechanism involving loss of heterozygosity for the maternal 11p, leading to expression of a paternally transmitted K ATP mutation (6,7). In addition, Huopio et al. (8) recently reported a family with a dominantly expressed mutation of SUR1. Children with either the focal or diffuse forms of disease associated with K ATP channel mutations often present with severe hypoglycemia at birth. Because the channel is impaired, they often do not respond to medical therapy with the channel agonist diazoxide and thus frequently require near-total pancreatectomy.In 1998, we reported three families with congenital hyperinsulinism in whom the disease was transmitted in a dominant, rather than recessive, manner (9). We subsequently identified the genetic defects in two of these families as being dominantly expressed mutations of glutamate dehydrogenase (GDH; GLUD1 on 10q) (10) and ...

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.