ABCC8 and ABCC9: ABC transporters that regulate K+ channels
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- Bryan, J., Muñoz, A., Zhang, X. et al. Pflugers Arch - Eur J Physiol (2007) 453: 703. doi:10.1007/s00424-006-0116-z
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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 KIR6.1/KCNJ8 or KIR6.2/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+ (KATP) 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 β cells, are the cause of monogenic forms of hyperinsulinemic hypoglycemia and neonatal diabetes. Additionally, the subtle dysregulation of KATP channel activity by a KIR6.2 polymorphism has been suggested as a predisposing factor in type 2 diabetes mellitus. Studies on KATP channel null mice are clarifying the roles of these metabolically sensitive channels in a variety of tissues.
The adenosine triphosphate (ATP)-sensitive K+ (KATP) channel area has been widely, some would say over, reviewed during the past dozen years. Our intention here is to comment on recent studies with an emphasis on ABCC8 and not reiterate material covered in recent extensive reviews on the muscle-type KATP channels [74, 92, 99, 101, 134, 144].
Adenosine-triphosphate-sensitive K+ channels are responsive to changes in ATP/adenosine diphosphate (ADP) and provide a means to couple movement of potassium ions and, thus, membrane potential to cellular energy status. Metabolic control of membrane potential is a key factor in the regulation of the Ca2+ triggering signals that underlie glucose homeostasis both in the endocrine pancreas and in the central nervous system (CNS). Genetic mutations that disrupt this control lead to a spectrum of changes, mild to severe, in blood glucose levels and energy balance, underscoring the importance of this dominant network.
Adenosine-triphosphate-sensitive K+ channels are assembled from two different subunits: A KIR6.x subunit that forms the ion-conducting pore and a sulfonylurea receptor (SUR), a member of the ABCC subfamily, with affinity for hypoglycemic sulfonylureas (e.g., the channel antagonists tolbutamide and glibenclamide) and hyperglycemic channel agonists (e.g., diazoxide, pinacidil, and cromakalim). KATP channels are obligate hetero-octamers (SUR/KIR6.x)4, whose subunit activities are highly integrated (34, 70, 138; see 2, 27 for review). Ordinarily, neither subunit will reach the cell surface in the absence of its partner. Trafficking from the endoplasmic reticulum to the cell surface is regulated by arginine-rich RKR motifs on both subunits  in combination with a C-terminal signal on SUR . These motifs insure the correct assembly of full-length subunits necessary for channel surface expression. The quality control mechanism is not fully understood, but interaction(s) with 14-3-3 proteins are reported to play a role (156; see 96, 107 for review).
The SUR and KIR6.x subunits are the products of two pairs of genes: ABCC8 [SUR1; Online Mendelian Inheritance in Man (OMIM) 600509] is paired with KCNJ11 (KIR6.2; OMIM 600937), which is approximately 5 kb downstream (3′ of ABCC8) on the short arm of human chromosome 11 (11p15.1). ABCC9 (SUR2; OMIM 601439), on the short arm of chromosome 12 (12p12.1), is separated from KCNJ8 (KIR6.1; OMIM 600935) by approximately 26.2 kb. These ABCC genes specify three major SUR isoforms (see 1 for review). SUR1, the receptor with the highest affinity for sulfonylureas, is commonly assembled with KIR6.2, and (SUR1/KIR6.2)4 channels are broadly distributed in the neuroendocrine system. Inhibition of pancreatic β-cell KATP channels by sulfonylurea and nonsulfonylurea hypoglycemic agents, which results in insulin secretion secondary to β-cell depolarization and increased intracellular Ca2+ levels, is the primary mechanism of action of these compounds. Differential splicing of the terminal exon of the ABCC9 gene produces two SUR2 isoforms: SUR2A, paired with KIR6.2 to assemble the KATP channels found in cardiac and skeletal muscle cells, and SUR2B, which assembles with KIR6.1 to make KATP channels in smooth muscle, particularly in parts of the vasculature where they participate in maintenance of vascular tone (see 69 for brief review).
KIR6.x subunits are members of the inward rectifier superfamily
Multiple studies have shown that engineered KIR6.x subunits can assemble functional pores in the absence of a SUR, and that their activity is sensitive to ATP [8, 43, 150]. Biochemical studies have demonstrated ATP binding to the large C-terminal domain [147, 148, 151]. Recent studies have used the coordinates of bacterial K+ channels [42, 82, 110] to make homology models the KIR6.x pore and provide novel information on the adenine-selective, nucleotide-diphosphate-binding pocket, which is composed of residues from both the N- and C-termini (7, 29; reviewed in 5). Based on modeling and earlier analysis of a KIR6.2 mutant channel with substitutions in both the KIR N- and C-termini , we proposed a specific model where the amino and carboxy termini that comprise the nucleotide binding site are from adjacent KIR subunits . This model implies that inter-KIR subunit coupling contributes to the concerted transitions characteristic of KATP channels.
Sulfonylurea receptors are typical ABC proteins
The SURs are multidomain proteins with a topology similar to other ABC proteins, including the classic ABC “core” consisting of two bundles of six transmembrane helices (TMD1 and 2) with nucleotide-binding domains (NBD1 and 2) C-terminal to each TMD (Fig. 1). The SURs, and several other ABCC proteins (i.e., ABCC1, 2, 3, 6, and 10), have an additional amino terminal module that consists of a bundle of five transmembrane helices (TMD0) connected to the core via an intracellular linker termed “L0.” In ABCC8 and 9, TMD0-L0 is the principal domain interacting with the KIR subunit as discussed below. The SUR NBDs contain the canonical phosphate-binding Walker A and B motifs, the Q-loop, the signature sequence, and the H-loop, hallmarks of the ABC family. The SURs were among the first ABC proteins recognized to have degenerate, nonsymmetric NBDs with a noncanonical signature sequence, FSQGQ vs LSGGQ, in NBD2 and an aspartate (D) in place of the usual glutamate (E) adjacent to the highly conserved D in the Walker B motif. Early studies on SUR1, without a KIR, indicated tight binding of 8-azido ATP and affinity labeling of NBD1, irrespective of the presence of Mg2+, with no indication of hydrolysis. Mg2+ potentiated nucleotide binding in NBD2 where hydrolysis is thought to occur (93, 94, 97; reviewed in 92). Although not yet explicitly demonstrated, by analogy with other ABC proteins reviewed in this issue of Pflügers Archives, ATP binding and hydrolysis are expected to drive dimerization of the SUR NBDs and produce concomitant rearrangements of TMD1 and TMD2 . In other ABC proteins, these rearrangements are associated with substrate transport, whereas in KATP channels, the challenge is to understand how they are coupled to the gating mechanism of the KIR6.x pore (see 104, 158 for discussion of specific models).
Drug binding sites
SURs are the targets for various compounds that bind and either stimulate or inhibit KATP channel activity. We  and others [72, 104, 118, 120] have reviewed this area recently. Therapeutic agents, like tolbutamide, glibenclamide, glipizide, glimeperide, nateglinide, repaglinide, etc., which antagonize the activity of the SUR1/KIR6.2 β-cell channels, are perhaps best known because of their wide use as hypoglycemic agents that restore first phase of insulin secretion in patients with type 2 diabetes. Extensive effort has gone into understanding their pharmacophore structure and the mechanism of action of these compounds along with the location of the binding site. Current evidence suggests the binding site is an extended pocket on the cytoplasmic side of the receptor, which involves residues from TMD2 and L0 and the amino terminus of KIR6.2. The binding of sulfonylureas to SUR1/KIR6.2 channels has a dual effect, partially inhibiting channel activity in the absence of nucleotides and abolishing the stimulatory action of SUR1 on the pore in the presence of Mg nucleotides. Potassium channel openers (KCOs), like diazoxide, have been used to hyperpolarize β cells and reduce insulin release, whereas openers specific for SUR2 have long been sought in an effort to control the excitability of cardiac and vascular smooth muscle (see 120 for review). Amino acid residues in TMD2, in what appears to be a cavity in the homology models of the SUR ABC core, markedly affect KCO binding (Fig. 1b; 38, 102, 103; see 104 for review). Although the properties of these binding sites, and the effects of nucleotide binding and hydrolysis, are not as well studied as the classical ABC drug transporters, the structural parallels are clear.
Various compounds have been shown to modulate KATP channel activity, particularly phosphoinositides (20, 52, 121, 130, 140; see 19 for review), long-chain acyl coenzyme As (CoAs) derived from fatty acids [23–25, 121], G proteins (127; but see 13), and phosphorylation (see 88 for review). The functions of these modulators are potentially important, but in most cases, their physiologic role(s) are only beginning to be firmly established. Lower pH has been reported to activate KATP channels by reducing their sensitivity to inhibitory ATP and may play a role in the regulation of vascular tone during hypercapnic acidosis . Several recent studies [18, 22, 119] have shown that Zn2+, cosecreted with insulin and present in high concentrations in areas of the CNS, can activate ATP-inhibited SUR1/KIR6.2 KATP channels via binding to two histidines on the extracellular face of SUR1 . In pancreatic islets, Zn2+ is coreleased with insulin, where it may play an autocrine role or serve to attenuate glucagon release from α cells [56, 71].
The TMD0-L0 domain links the SUR ABC core with the pore
Recent studies have emphasized the role of the TMD0-L0 module in both the assembly and regulation of KATP channels. In some ABCC proteins TMD0 is required for correct trafficking [53, 91], whereas in MRP1, L0, but not TMD0, is required for transport activity  and is part of the glutathione binding site . Coexpression of SUR1 TMD0 with poorly active (KIR6.2)4 pores demonstrated partial restoration of function, including an increase in their maximum channel open probability (POmax) and the restoration of bursting activity; immunoprecipitation experiments confirmed TMD0 forms complexes with KIR6.2 [7, 31]. TMD0 consists primarily of transmembrane helices, and the idea that one or more TMD0 helices interact with the KIR M1 helix is supported by experiments with chimeric KIR subunits  and the observation that KIR6.2 N-terminal fragments containing M1, but not M2 and the C-terminal domain, interact with SUR1 . Because of their smaller size, we termed the TMD0/KIR6.2 complexes “mini-KATP” channels and referred to the TMD0-L0 module as a “gatekeeper.” These mini channels lack many of the defining characteristics of full channels and are not sensitive to sulfonylureas, are not stimulated by Mg nucleotides, and retain the reduced sensitivity to inhibitory ATP characteristic of (KIR6.2)4 pores. Additionally, TMD0 does not support the trafficking of KIR6.2 with an intact ER retention motif to the cell surface, implying that these properties of the full channel require interactions between the TMD0-KIR pore complex and the ABC core .
L0 and the KIR amino terminus are critical elements of the transduction mechanism
Early studies with chimeric receptors showed that L0 played a critical role in specifying gating differences between SUR1-based neuroendocrine vs SUR2-based muscle-type KATP channels . Analysis of the mini-KATP channels underscored the importance of L0 in the control of gating. The progressive inclusion of L0 sequence into mini-KATP channels identified a proximal stimulatory segment that stabilized the pore in a continuous bursting mode . This proximal segment contains a conserved, predicted amphipathic helix ; a similar sequence in MRP1 is important for transport . The inclusion of more distal segments of L0 attenuated the strong stimulatory effect. The results implied that the TMD0-L0 pore complex had all of the structural elements necessary to affect bidirectional control of gating and dovetailed nicely with studies showing that the initial N-terminus of KIR6.2 plays a critical role in limiting the length of time a KATP channel remains in an open bursting configuration. For example, the deletion of the KIR6.2 amino terminus [9, 79] or the application of a synthetic N-terminal peptide  produced channels that burst continuously, suggesting that interaction(s) between the N terminus and a putative binding site on SUR limited burst length. We developed a model in which stimulatory interactions between the proximal helical segment of L0 and the KIR6.2 slide helix are balanced by inhibitory interactions between the more distal half of L0 and the initial N terminus of KIR6.2 (6, 7, 10; reviewed in 5, 27). In this quasimechanical model, the dimerization or reconfiguration of the ABC core as a consequence of ATP binding and hydrolysis is presumed to exert a force that repositions L0 to affect channel gating by moving the slide helix and, thus, M1 and M2. The movements are analogous to those proposed to affect gating in voltage-gated Kv1.2 channels, where changes in membrane potential produce a conformational change in the voltage sensor that moves the S4-S5 linker and repositions S5 and S6 to open or close the gate .
Dual regulation of KATP channel activity by adenine nucleotides
In most ABC proteins, hydrolysis of ATP is assumed to power substrate transport across the cell membrane. In KATP channels, adenine nucleotides can bind to both the KIR and SUR subunits and exert inhibitory and stimulatory actions, respectively. The binding of ATP or ADP to KIR6.x subunits reduces channel activity. This binding and inhibitory action are Mg2+-independent and do not require hydrolysis. The stimulatory action of SURs, on the other hand, is Mg-nucleotide-dependent, and nucleotide binding and hydrolysis increases the activity of the ATP-inhibited pore.
In pancreatic β cells, the glycolytic pathway is critically involved in signaling increases in glucose metabolism to KATP channels [46, 90, 95]. The nicotinamide adenine dinucleotide (reduced form; NADH) equivalents derived from glycolysis are transferred into mitochondria via the glycerol-phosphate, dihydroxyacetone-phosphate, and malate-aspartate shuttles. Blockage of either shuttle alone has little effect on glucose-stimulated insulin secretion, whereas blockage of both pathways in mitochondrial glycerol-phosphate dehydrogenase null mice [50, 51] strongly inhibits first-phase insulin release and abolishes second-phase secretion in response to elevated glucose. How the NADH equivalents from glycolysis, via generation of ATP, inhibit KATP channels has not been established, but the results suggest that β-cell KATP channels are not sensing “bulk” cytosolic [ATP]i. One idea is that specific respiratory chains couple the malate-aspartate and glycerol-phosphate NADH shuttles to a mitochondrial creatine kinase which transfers phosphate from matrix ATP to creatine to produce creatine phosphate . The creatine phosphate is then converted to ATP at or near KATP channels. This hypothesis is supported by reports that SUR2A, the regulatory subunit of KIR6.2/SUR2A KATP channels found in striated muscle, is physically associated with creatine kinase .
KATP channel pathologies
Multiple genetic disorders of glucose homeostasis and cardiovascular tone have been associated with mutations in KATP channel subunits. KCNJ8 has been suggested as a candidate gene for Prinzmetal angina based on the phenotype of the KIR6.1KO mouse , but human mutations have yet to be identified . ABCC9 has not been linked to single-gene disorders at this time, but it has been associated with dilated cardiomyopathy .
Mutations in SUR1 and KIR6.2 are an established cause of hyperinsulinemic hypoglycemia of infancy (HI), characterized by excess insulin release for the degree of hypoglycemia (see 1 for review). More than 40 mutations in KCNJ11 and more than 100 ABCC8 mutations have been identified in patients with HI that lead to loss of channel function by affecting subunit assembly and channel trafficking [30, 36, 136, 149] or, in the case of some ABCC8 missense mutations, by impairing the Mg-nucleotide-dependent stimulation of the pore by SUR1 (109, 139; reviewed in 55). The neuroendocrine SUR1/KIR6.2-type KATP channels are a key regulator of membrane potential; thus, their loss in HI individuals abolishes the ability of pancreatic β cells to hyperpolarize when glucose is reduced and, thus, suppress insulin release. This uncoupling results in excess insulin release that produces hypoglycemia. There is no known therapeutic strategy to enhance folding, assembly, or trafficking of mutant subunits, and these cases often require surgical intervention. Many individuals with missense mutations are responsive to diazoxide, a K+ channel opener, or to octreotide, a somatostatin analog, and can be treated pharmacologically.
Recent studies on the genetic basis of neonatal diabetes (ND) have confirmed the observation in transgenic mice  that expression of “overactive” KATP channels results in neonatal hyperglycemia secondary to reduced insulin secretion. Although ND is a rare genetic disorder (estimated at ∼1/400,000 births) , these findings are important because they confirm the general ionic mechanism and how that increased KATP channel activity and, thus, more hyperpolarized β cells result in a decrease in insulin release. Multiple mutations in KIR6.2 have been found to produce ND (59, 152; reviewed in 66, 143). Although various structural alterations can be anticipated that would result in more active channels, several of the reported KIR6.2 mutations reduce the apparent affinity for inhibitory ATP, thus leading to more active channels at a given nucleotide level [59, 152]. Although these mutations are dominant, simulating heterozygosity by expression of 1:1 mixtures of mutant, for example, the R201H mutation, and wild-type KIRs with SUR1 yielded a population whose inhibition by ATP was essentially indistinguishable from wild-type channels. The results suggest that the small percentage of homozygous mutant channels, expected to arise from random assortment during assembly of the pore, is sufficient to hyperpolarize β cells. Many of the ND-KIR6.2 mutant channels retain their sensitivity to sulfonylureas, allowing patients to be switched from insulin to sulfonylurea therapy .
The dual action of nucleotides on KATP channels outlined above anticipated SUR “gain-of-function” mutations having an enhanced stimulatory action on the pore. Sequencing of the ABCC8 gene in a small population of patients diagnosed with ND, from the French Network for the Study of Neonatal Diabetes Mellitus for Genetic Diagnosis, identified seven mutations that segregated with the disorder . Analysis of two SUR1 mutant channels, I1424V or H1023Y, demonstrated they were more active than wild-type channels both in on-cell recordings from intact mammalian cells and in isolated patches exposed to a quasiphysiologic concentration of MgATP (1 mM). In the absence of Mg2+, when the stimulatory action of SUR1 on the pore was abolished, there was no significant difference in the ATP inhibitory curves of mutant and wild-type channels, indicating the I1424V or H1023Y receptors exert an enhanced stimulatory action on the pore. The simulation of heterozygosity by expression of 1:1 mixtures of ND-SUR1 H1023Y and wild-type SUR1 with KIR6.2 produced average mean channel activities intermediate between the “homozygous” mutant and wild-type channels. The mutant ND-SUR1 channels are inhibited by sulfonylureas, allowing patients to substitute oral hypoglycemic agents for insulin therapy.
Type 2 diabetes
Hyperinsulinemic hypoglycemia and ND are clear examples of rare monogenic disorders of glucose homeostasis that provide insight into the regulation of insulin secretion. Genetic studies indicate KATP channels also may have a role in type 2 diabetes mellitus. Several studies (for example, 54, 60, 63; reviewed in 124) indicate a polymorphism, E32K, in the amino terminus of KIR6.2 is a risk factor for type 2 diabetes. This KIR6.2 polymorphism has a subtle activating effect on KATP channel activity and is reported to reduce sensitivity to inhibitory ATP  and increase the stimulatory action of long-chain acyl CoAs , particularly long-chain, saturated acyl CoAs .
Transgenic mouse models
All of the subunits of KATP channels, including SUR1, SUR2, KIR6.1, and KIR6.2, have been deleted in mice and their phenotypes studied. This area, particularly with respect to KATP channels in muscle, has been reviewed recently , and in the interest of space, we focus on the results with Sur1KO animals. Two independent SUR1 null mouse lines have been generated. Seghers et al.  replaced exon 2 with puromycin-N-acetyl-transferase, whereas Shiota et al.  used a cre recombinase strategy to delete exon 1, leaving a neomycin resistance cassette. Neither strain produces SUR1, and as expected, both strains lack SUR1/KIR6.2-type KATP channels, with no studies showing upregulation of SUR2 subunits and compensatory ionic currents.
Newborn Sur1KO mice were found to exhibit significant hypoglycemia secondary to hyperinsulinemia, but this resolved within several days , and the KO animals then remain normoglycemic [65, 108, 133]. Intraperitoneal glucose tolerance tests on adult mice showed that knockout animals fail to release insulin in response to a glucose challenge [133, 137], whereas fasted knockout animals are able to secrete insulin in response to feeding  consistent with stimulation via the CNS.
Secretion studies on isolated islets have produced divergent results, with some reports consistent with enhanced insulin release, whereas others were consistent with a defect in insulin secretion. Seghers et al.  showed a loss of first phase and an attenuated second phase of insulin secretion in response to a glucose challenge. Using animals from the same colony, Nenquin et al.  showed increased insulin release from isolated Sur1KO vs WT islets in low glucose (1 mM) consistent with the elevated Ca2+ triggering signal. This study confirmed the lack of first-phase response and showed further that increasing glucose metabolism stimulated insulin release, demonstrating the augmentation pathway (see 67, 68 for review) is intact in the Sur1KO animals. Similar glucose-stimulated insulin secretion was reported by Haspel et al. , and Muñoz et al.  showed increased insulin release from Sur1KO islets in low glucose plus amino acids. Studies on the Sur1KO animals generated by Shiota et al.  have usually failed to show insulin secretion under hypoglycemic conditions or a significant increase in secretion when glucose is elevated, thus leading to the conclusion that loss of SUR1 impairs insulin release [41, 87], although Eliasson et al.  reported glucose-stimulated insulin release using this strain. These divergent results with isolated islets, excessive insulin release in low glucose and increased secretion upon increasing glucose vs generally impaired insulin secretion, have suggested opposite compensatory mechanisms must operate to account for the normal insulin and blood sugar levels seen in the knockout animals. Nenquin et al.  suggested the need for a mechanism to suppress the excessive insulin release seen in low glucose, whereas Doliba et al.  argue for an enhanced stimulation via a neural mechanism.
Role of KATP channels in the generation of Ca2+ and electrical oscillations
A signature feature of islets in elevated glucose is a tight coupling between oscillations of β-cell membrane potential and [Ca2+]c that trigger pulsatile insulin release [58, 129]. Several mechanisms proposed to account for the generation of these oscillations involve the response of KATP channels to changes in ATP/ADP (see, for example, 40, 81). Using intracellular microelectrode recording techniques, Düfer et al.  showed that Sur1KO islets exhibit Vm and [Ca2+]c oscillations in 15 mM glucose, which, in contrast to wild-type islets, persist in 0.5 mM glucose, implying KATP channels are not essential for oscillation. The microelectrode experiments showed that the electrical activity of Sur1KO β cells in islets was modulated by glucose as illustrated in Fig. 3. In contrast to wild-type β cells, the application of sodium azide did not result in hyperpolarization of Sur1KO β cells but did reduce the amplitude of Ca2+-dependent action potentials by directly inhibiting Ca2+ channels [44, 45]. Neither tolbutamide nor diazoxide had any effect on Vm oscillations in KATP null islets. The activation of a low-conductance, Ca2+-dependent K+ current, termed “IKslow,” has been implicated in the oscillatory activity of wild-type β cells [61, 62]. Haspel et al.  showed that Sur1KO β cells have a similar Ca2+-dependent K+ current that is inhibited when [Ca2+]c is reduced using D600, an l-type Ca2+ channel blocker, and stimulated using BayK 8644, a Ca2+ channel opener. KATP channels control oscillations of Vm and [Ca2+]c in wild-type β cells, but a secondary oscillatory mechanism must exist in Sur1KO cells. We presume the two mechanisms are not active in parallel because blocking KATP channels acutely with 100 μM tolbutamide does not induce oscillations in [Ca2+]c .
Multiple compensatory mechanisms have been suggested to explain the large difference in glucose homeostasis between the HI neonates and the rodent models. Interestingly, several mouse models that exhibit partial loss of SUR1/KIR6.2 channel activity exhibit hypoglycemic phenotypes, which more nearly approximate the human disorder [64, 85].
The incretin response is impaired in Sur1KO mice
Catecholamine response in Sur1KO mice
Sieg et al.  explored the idea that elevated epinephrine might act to suppress insulin release in Sur1KO mice. Others have implied that the inhibitory action of epinephrine on insulin secretion might involve activation of KATP channels, but exogenous epinephrine hyperpolarized Sur1KO β cells via an α2-adrenoceptor mechanism, thus inhibiting insulin secretion from isolated islets and suppressing carbachol-induced insulin release in Sur1KO mice. The molecular nature of the low conductance, BaCl2-sensitive K+ channels regulated by pertussis-sensitive G proteins associated with β-cell hyperpolarization is not known . Preliminary measurements of catecholamine levels in Sur1KO vs wild-type mice have not uncovered any significant differences (unpublished data); therefore, we tentatively conclude that suppression of insulin release via elevated epinephrine does not tonically suppress insulin secretion in the knockout animals, but further study is warranted.
Acetylcholine and amino acids
Shiota et al.  showed that fasted Sur1KO mice increase their insulin level in response to feeding, suggesting that neural stimulation is an important factor. Doliba et al.  and Nakazaki et al.  reported that acetylcholine and carbachol stimulate insulin release from Sur1KO islets even in low glucose. Doliba et al.  argue secretion is impaired in the Sur1KO mice and suggest acetylcholine, released in response to feeding, enhances insulin secretion, thus contributing to their euglycemia. Amino acids are also known to potentiate insulin secretion, and several studies have reported amino acids stimulate insulin release from Sur1KO islets [65, 87, 105].
KATP channels are part of a brain–liver circuit that modulates hepatic glucose production
SUR1/KIR6.2 channels are known to be present throughout the CNS and are implicated in neuroprotection during periods of anoxia (reviewed in 16). Here we focus on their role(s) in glucose homeostasis, particularly in the hypothalamus. Early electrophysiological studies identified a reciprocal response of hypothalamic neurons to applied glucose [3, 111, 112]. In the ventromedial hypothalamus (VMH), a majority of responding neurons increased their firing rate (glucose-responsive or glucose-stimulated), whereas in the lateral hypothalamus (LH), a majority reduced their activity (glucose-sensitive or glucose-inhibited). These results have been elegantly confirmed and extended by more recent studies, for example [142, 145]. Several reports and experiments on KIR6.2KO mice (98; see 28, 86, 125 for review) indicate that glucose-stimulated neurons in the VMH underlie the counterregulatory response to hypoglycemia, although Yang et al.  have reported that glucose-inhibited neurons are involved. The behavior of glucose-stimulated neurons is consistent with a β-cell-type regulation of KATP channels in the sense that increased glucose metabolism reduces channel activity and, thus, increases the neuronal firing rate secondary to membrane depolarization. During hypoglycemia, the firing rate drops and is presumed to reduce an inhibitory effect on glucagon release.
Three recent articles delineate a novel brain–liver circuit involving neurons in the LH in which activation of KATP channels is implicated in the control of hepatic glucose production. The activity of these neurons is presumed to suppress hepatic glucose production and increased hypothalamic insulin , free fatty acids , and glucose  via intracerebroventricular (ICV) infusion, resulting in reduced blood glucose levels. These effects are mimicked by diazoxide alone and blocked by coinfusion of glibenclamide. Consistent with their lack of SUR1/KIR6.2 KATP channels, Sur1KO mice maintained their hepatic glucose output when infused with fatty acids  or insulin .
Adenosine-triphosphate-sensitive K+ channels present a unique use of ABC proteins as regulators of ion channels rather than transporters in their own right. Continuing analyses of the role of these channels in human genetic disease have provided better understanding of disorders of glucose homeostasis and their treatment and have validated the dual role nucleotides play in the regulation of channel activity. Understanding how HI and ND mutations inhibit and stimulate channel activity promises further insight into the molecular control of channel gating, whereas studies on KATP channel knockout mice continue to provide novel insights into their function.
This work was supported by grants from the Deutsche Forschungsgemeinschaft (Dr225/6-1 and Du425/1-1) and the US National Institutes of Health (DK52771, DK44311, and JDRF 1-2005-950). There are no conflicts of interest in this study.