ATPase activity of the sulfonylurea receptor: a catalytic function for the KATP channel complex

Pulmonary hypertension associated with diazoxide: the SUR1 paradox

The ATP-sensitive potassium channels and their regulatory subunits, sulfonylurea receptor 1 (SUR1/Kir6.2) and SUR2/Kir6.1, contribute to the pathophysiology of pulmonary hypertension (PH). Loss-of-function pathogenic variants in the ABCC8 gene, which encodes for SUR1, have been associated with heritable pulmonary arterial hypertension. Conversely, activation of SUR1 and SUR2 leads to the relaxation of pulmonary arteries and reduces cell proliferation and migration. Diazoxide, a SUR1 activator, has been shown to alleviate experimental PH, suggesting its potential as a therapeutic option. However, there are paradoxical reports of diazoxide-induced PH in infants. This review explores the role of SUR1/2 in the pathophysiology of PH and the contradictory effects of diazoxide on the pulmonary vascular bed. Additionally, we conducted a comprehensive literature review of cases of diazoxide-associated PH and analysed data from the World Health Organization pharmacovigilance database (VigiBase). Significant disproportionality signals link diazoxide to PH, while no other SUR activators have been connected with pulmonary vascular disease. Diazoxide-associated PH seems to be dose-dependent and potentially related to acute effects on the pulmonary vascular bed. Further research is required to decipher the differing pulmonary vascular consequences of diazoxide in different age populations and experimental models.

Mechanisms and Characteristics of Sulfonylureas and Glinides

BACKGROUND

Type 2 diabetes mellitus is a complex progressive endocrine disease characterized by hyperglycemia and life-threatening complications. It is the most common disorder of pancreatic cell function that causes insulin deficiency. Sulfonylurea is a class of oral hypoglycemic drugs. Over the past half century, these drugs, together with the subsequent non-sulfonylureas (glinides), have been the main oral drugs for insulin secretion.

OBJECTIVE

Through in-depth study, the medical profession considers it as an important drug for improving blood sugar control.

METHODS

The mechanism, characteristics, efficacy and side effects of sulfonylureas and glinides were reviewed in detail.

RESULTS

Sulfonylureas and glinides not only stimulated the release of insulin from pancreatic cells, but also had many extrapanular hypoglycemic effect, such as reducing the clearance rate of insulin in liver, reducing the secretion of glucagon, and enhancing the sensitivity of peripheral tissues to insulin in type 2 diabetes mellitus.

CONCLUSION

Sulfonylureas and glinides are effective first-line drugs for the treatment of diabetes mellitus. Although they have the risk of hypoglycemia, weight gain and cardiovascular disease, their clinical practicability and safety can be guaranteed as long as they are reasonably used.

ATP binding without hydrolysis switches sulfonylurea receptor 1 (SUR1) to outward-facing conformations that activate KATP channels

Neuroendocrine-type ATP-sensitive K⁺ (K_ATP) channels are metabolite sensors coupling membrane potential with metabolism, thereby linking insulin secretion to plasma glucose levels. They are octameric complexes, (SUR1/Kir6.2)₄, comprising sulfonylurea receptor 1 (SUR1 or ABCC8) and a K⁺-selective inward rectifier (Kir6.2 or KCNJ11). Interactions between nucleotide-, agonist-, and antagonist-binding sites affect channel activity allosterically. Although it is hypothesized that opening these channels requires SUR1-mediated MgATP hydrolysis, we show here that ATP binding to SUR1, without hydrolysis, opens channels when nucleotide antagonism on Kir6.2 is minimized and SUR1 mutants with increased ATP affinities are used. We found that ATP binding is sufficient to switch SUR1 alone between inward- or outward-facing conformations with low or high dissociation constant, K_D , values for the conformation-sensitive channel antagonist [³H]glibenclamide ([³H]GBM), indicating that ATP can act as a pure agonist. Assembly with Kir6.2 reduced SUR1's K_D for [³H]GBM. This reduction required the Kir N terminus (KNtp), consistent with KNtp occupying a "transport cavity," thus positioning it to link ATP-induced SUR1 conformational changes to channel gating. Moreover, ATP/GBM site coupling was constrained in WT SUR1/WT Kir6.2 channels; ATP-bound channels had a lower K_D for [³H]GBM than ATP-bound SUR1. This constraint was largely eliminated by the Q1179R neonatal diabetes-associated mutation in helix 15, suggesting that a "swapped" helix pair, 15 and 16, is part of a structural pathway connecting the ATP/GBM sites. Our results suggest that ATP binding to SUR1 biases K_ATP channels toward open states, consistent with SUR1 variants with lower K_D values causing neonatal diabetes, whereas increased K_D values cause congenital hyperinsulinism.

The mechano-sensitivity of cardiac ATP-sensitive potassium channels is mediated by intrinsic MgATPase activity

Cardiac ATP-sensitive K⁺ (K_ATP) channel activity plays an important cardio-protective role in regulating excitability in response to metabolic stress. Evidence suggests that these channels are also mechano-sensitive and therefore may couple K_ATP channel activity to increased cardiac workloads. However, the molecular mechanism that couples membrane stretch to channel activity is not currently known. We hypothesized that membrane stretch may alter the intrinsic MgATPase activity of the cardiac K_ATP channel resulting in increased channel activation. The inside-out patch-clamp technique was used to record single-channel and macroscopic recombinant K_ATP channel activity in response to membrane stretch elicited by negative pipette pressure. We found that stretch activation requires the presence of the SUR subunit and that inhibition of MgATPase activity with either the non-hydrolysable ATP analog AMP-PNP or the ATPase inhibitor BeFx significantly reduced the stimulatory effect of stretch. We employed a point mutagenic approach to determine that a single residue (K1337) in the hairpin loop proximal to the major MgATPase catalytic site in the SUR2A subunit is responsible for the difference in mechano-sensitivity between SUR2A and SUR1 containing K_ATP channels. Moreover, using a double cysteine mutant substitution in the hairpin loop region revealed the importance of a key residue-residue interaction in this region that transduces membrane mechanical forces into K_ATP channel stimulation via increases in channel MgATPase activity. With respect to K_ATP channel pharmacology, glibenclamide, but not glicalizide or repaglinide, was able to completely inhibit K_ATP channel mechano-sensitivity. In summary, our results provide a highly plausible molecular mechanism by which mechanical membrane forces are rapidly converted in changes in K_ATP channel activity that have implications for our understanding of cardiac K_ATP channels in physiological or pathophysiological settings that involve increased workload.

The Nucleotide-Binding Sites of SUR1: A Mechanistic Model

ATP-sensitive potassium (K_ATP) channels comprise four pore-forming Kir6.2 subunits and four modulatory sulfonylurea receptor (SUR) subunits. The latter belong to the ATP-binding cassette family of transporters. K_ATP channels are inhibited by ATP (or ADP) binding to Kir6.2 and activated by Mg-nucleotide interactions with SUR. This dual regulation enables the K_ATP channel to couple the metabolic state of a cell to its electrical excitability and is crucial for the K_ATP channel’s role in regulating insulin secretion, cardiac and neuronal excitability, and vascular tone. Here, we review the regulation of the K_ATP channel by adenine nucleotides and present an equilibrium allosteric model for nucleotide activation and inhibition. The model can account for many experimental observations in the literature and provides testable predictions for future experiments.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Isosteviol Sensitizes sarcKATP Channels towards Pinacidil and Potentiates Mitochondrial Uncoupling of Diazoxide in Guinea Pig Ventricular Myocytes

K_ATP channel is an important mediator or factor in physiological and pathological metabolic pathway. Activation of K_ATP channel has been identified to be a critical step in the cardioprotective mechanism against IR injury. On the other hand, desensitization of the channel to its opener or the metabolic ligand ATP in pathological conditions, like cardiac hypertrophy, would decrease the adaption of myocardium to metabolic stress and is a disadvantage for drug therapy. Isosteviol, obtained by acid hydrolysis of stevioside, has been demonstrated to play a cardioprotective role against diseases of cardiovascular system, like anti-IR injury, antihypertension, antihyperglycemia, and so forth. The present study investigated the effect of isosteviol (STV) on sarcK_ATP channel current induced by pinacidil and mitochondrial flavoprotein oxidation induced by diazoxide. Our results showed that preincubating cells with STV not only increased the current amplitude and activating rate of sarcK_ATP channels induced by pinacidil but also potentiated diazoxide-elicited oxidation of flavoprotein in mitochondria.

Phosphorylation-dependent changes in nucleotide binding, conformation, and dynamics of the first nucleotide binding domain (NBD1) of the sulfonylurea receptor 2B (SUR2B)

The sulfonylurea receptor 2B (SUR2B) forms the regulatory subunit of ATP-sensitive potassium (KATP) channels in vascular smooth muscle. Phosphorylation of the SUR2B nucleotide binding domains (NBD1 and NBD2) by protein kinase A results in increased channel open probability. Here, we investigate the effects of phosphorylation on the structure and nucleotide binding properties of NBD1. Phosphorylation sites in SUR2B NBD1 are located in an N-terminal tail that is disordered. Nuclear magnetic resonance (NMR) data indicate that phosphorylation of the N-terminal tail affects multiple residues in NBD1, including residues in the NBD2-binding site, and results in altered conformation and dynamics of NBD1. NMR spectra of NBD1 lacking the N-terminal tail, NBD1-ΔN, suggest that phosphorylation disrupts interactions of the N-terminal tail with the core of NBD1, a model supported by dynamic light scattering. Increased nucleotide binding of phosphorylated NBD1 and NBD1-ΔN, compared with non-phosphorylated NBD1, suggests that by disrupting the interaction of the NBD core with the N-terminal tail, phosphorylation also exposes the MgATP-binding site on NBD1. These data provide insights into the molecular basis by which phosphorylation of SUR2B NBD1 activates KATP channels.

Carbamazepine inhibits ATP-sensitive potassium channel activity by disrupting channel response to MgADP

In pancreatic β-cells, K(ATP) channels consisting of Kir6.2 and SUR1 couple cell metabolism to membrane excitability and regulate insulin secretion. Sulfonylureas, insulin secretagogues used to treat type II diabetes, inhibit K(ATP) channel activity primarily by abolishing the stimulatory effect of MgADP endowed by SUR1. In addition, sulfonylureas have been shown to function as pharmacological chaperones to correct channel biogenesis and trafficking defects. Recently, we reported that carbamazepine, an anticonvulsant known to inhibit voltage-gated sodium channels, has profound effects on K(ATP) channels. Like sulfonylureas, carbamazepine corrects trafficking defects in channels bearing mutations in the first transmembrane domain of SUR1. Moreover, carbamazepine inhibits the activity of K(ATP) channels such that rescued mutant channels are unable to open when the intracellular ATP/ADP ratio is lowered by metabolic inhibition. Here, we investigated the mechanism by which carbamazepine inhibits K(ATP) channel activity. We show that carbamazepine specifically blocks channel response to MgADP. This gating effect resembles that of sulfonylureas. Our results reveal striking similarities between carbamazepine and sulfonylureas in their effects on K(ATP) channel biogenesis and gating and suggest that the 2 classes of drugs may act via a converging mechanism.

The role of ATP-sensitive potassium channels in cellular function and protection in the cardiovascular system

ATP-sensitive potassium channels (K(ATP)) are widely distributed and present in a number of tissues including muscle, pancreatic beta cells and the brain. Their activity is regulated by adenine nucleotides, characteristically being activated by falling ATP and rising ADP levels. Thus, they link cellular metabolism with membrane excitability. Recent studies using genetically modified mice and genomic studies in patients have implicated K(ATP) channels in a number of physiological and pathological processes. In this review, we focus on their role in cellular function and protection particularly in the cardiovascular system.

Involvement of the phosphatidylinositol kinase pathway in augmentation of ATP-sensitive K+ channel currents by hypo-osmotic stress in rat ventricular myocytes

The objective of this study was to investigate the mechanisms of increase in the efficacy of ATP-sensitive K+ channel (KATP) openings by hypo-osmotic stress. The whole-cell KATP currents (IK,ATP) stimulated by 100 μmol/L pinacidil, a K+ channel opening drug, were significantly augmented during hypo-osmotic stress (189 mOsmol/L) compared with normal conditions (303 mOsmol/L). The EC50 and Emax value for pinacidil-activated IK,ATP (measured at 0 mV) was 154 μmol/L and 844 pA, respectively, in normal solution and 16.6 μmol/L and 1266 pA, respectively, in hypo-osmotic solution. Augmentation of IK,ATP during hypo-osmotic stress was attenuated by wortmannin (50 μmol/L), an inhibitor of phosphatidylinositol 3- and 4-kinases, but not by (i) phalloidin (30 μmol/L), an actin filament stabilizer, (ii) the absence of Ca2+ from the internal and external solutions, and (iii) the presence of creatine phosphate (3 mmol/L), which affects creatine kinase regulation of the KATP channels. In the single-channel recordings, an inside-out patch was made after approximately 5 min exposure of the myocyte to hypo-osmotic solution. However, the IC50 value for ATP under such conditions was not different from that obtained in normal osmotic solution. In conclusion, hypo-osmotic stress could augment cardiac IK,ATP through intracellular mechanisms involving the phosphatidylinositol kinase pathway.

Multiplicity of effectors of the cardioprotective agent, diazoxide

Diazoxide has been identified over the past 50years to have a number of physiological effects, including lowering the blood pressure and rectifying hypoglycemia. Today it is used clinically to treat these conditions. More recently, another important mode of action emerged: diazoxide has powerful protective properties against cardiac ischemia. The heart has intrinsic protective mechanisms against ischemia injury; one of which is ischemic preconditioning. Diazoxide mimics ischemic preconditioning. The purpose of this treatise is to review the literature in an attempt to identify the many effectors of diazoxide and discuss how they may contribute to diazoxide's cardioprotective properties. Particular emphasis is placed on the concentration ranges in which diazoxide affects its different targets and how this compares with the concentrations commonly used to study cardioprotection. It is concluded that diazoxide may have several potential effectors that may potentially contribute to cardioprotection, including KATP channels in the pancreas, smooth muscle, endothelium, neurons and the mitochondrial inner membrane. Diazoxide may also affect other ion channels and ATPases and may directly regulate mitochondrial energetics. It is possible that the success of diazoxide lies in this promiscuity and that the compound acts to rebalance multiple physiological processes during cardiac ischemia.

Coronary spasm and acute myocardial infarction due to a mutation (V734I) in the nucleotide binding domain 1 of ABCC9

BACKGROUND

Alterations in coronary vasomotor tone may participate in the pathogenesis of acute myocardial infarction (AMI). Vascular ATP-sensitive K(+) (KATP) channels, formed by Kir6.x/SUR2B, are key regulators of coronary tone and mutations in cardiac (Kir6.2/SUR2A) KATP channels result in heart disease. Here we explore the pathophysiological mechanism of a rare mutation (V734I) found in exon 17 of the ABCC9 gene, estimated to cause a 6.4-fold higher risk of AMI before the age of 60.

METHODS AND RESULTS

Eleven patients carrying the mutation were identified; they presented AMI of vasospastic origin associated with increased plasma levels of endothelin-1 and increased leukocyte ROCK activity. The effects of the mutation on the functional properties of the two splice variants of ABCC9 (SUR2A and SUR2B) were studied using patch-clamp electrophysiology. The mutation reduced the sensitivity to MgATP inhibition of Kir6.2/SUR2B channels but not of Kir6.2/SUR2A and Kir6.1/SUR2B channels. Furthermore, the stimulatory effects of MgNDP (MgADP, MgGDP and MgUDP) were unaltered in mutant Kir6.2/SUR2A and Kir6.1/SUR2B channels. In contrast, mutant channels composed of Kir6.2 and SUR2B were less sensitive to MgNDP activation, assessed in the presence of MgATP. The antianginal drug nicorandil activated Kir6.2/SUR2B-V734I channels, thus substituting for the loss of MgNDP stimulation, suggesting that this drug could be of therapeutic use in the treatment of AMI associated with V734I.

CONCLUSIONS

The 734I allele in ABCC9 may influence susceptibility to AMI by impairing the response of vascular, but not cardiac, KATP channels to intracellular nucleotides. This is the first human mutation in an ion channel gene to be implicated in AMI.

Regulation of cardiac ATP-sensitive potassium channel surface expression by calcium/calmodulin-dependent protein kinase II

Cardiac ATP-sensitive potassium (K(ATP)) channels are key sensors and effectors of the metabolic status of cardiomyocytes. Alteration in their expression impacts their effectiveness in maintaining cellular energy homeostasis and resistance to injury. We sought to determine how activation of calcium/calmodulin-dependent protein kinase II (CaMKII), a central regulator of calcium signaling, translates into reduced membrane expression and current capacity of cardiac K(ATP) channels. We used real-time monitoring of K(ATP) channel current density, immunohistochemistry, and biotinylation studies in isolated hearts and cardiomyocytes from wild-type and transgenic mice as well as HEK cells expressing wild-type and mutant K(ATP) channel subunits to track the dynamics of K(ATP) channel surface expression. Results showed that activation of CaMKII triggered dynamin-dependent internalization of K(ATP) channels. This process required phosphorylation of threonine at 180 and 224 and an intact (330)YSKF(333) endocytosis motif of the K(ATP) channel Kir6.2 pore-forming subunit. A molecular model of the μ2 subunit of the endocytosis adaptor protein, AP2, complexed with Kir6.2 predicted that μ2 docks by interaction with (330)YSKF(333) and Thr-180 on one and Thr-224 on the adjacent Kir6.2 subunit. Phosphorylation of Thr-180 and Thr-224 would favor interactions with the corresponding arginine- and lysine-rich loops on μ2. We concluded that calcium-dependent activation of CaMKII results in phosphorylation of Kir6.2, which promotes endocytosis of cardiac K(ATP) channel subunits. This mechanism couples the surface expression of cardiac K(ATP) channels with calcium signaling and reveals new targets to improve cardiac energy efficiency and stress resistance.

The molecular genetics of sulfonylurea receptors in the pathogenesis and treatment of insulin secretory disorders and type 2 diabetes

Sulfonylurea receptors (SURs) form an integral part of the ATP-sensitive potassium (K(ATP)) channel complex that is present in most excitable cell types. K(ATP) channels couple cellular metabolism to electrical activity and provide a wide range of cellular functions including stimulus secretion coupling in pancreatic β cells. K(ATP) channels are composed of SURs and inward rectifier potassium channel (Kir6.x) subunits encoded by the ABCC8/9 and KCNJ8/11 genes, respectively. Recent advances in the genetics, molecular biology, and pharmacology of SURs have led to an increased understanding of these channels in the etiology and treatment of rare genetic insulin secretory disorders. Furthermore, common genetic variants in these genes are associated with an increased risk for type 2 diabetes. In this review we summarize the molecular biology, pharmacology, and physiology of SURs and K(ATP) channels, highlighting recent advances in their genetics and understanding of rare insulin secretory disorders and susceptibility to type 2 diabetes.

Reduction in number of sarcolemmal KATP channels slows cardiac action potential duration shortening under hypoxia

The cardiovascular system operates under demands ranging from conditions of rest to extreme stress. One mechanism of cardiac stress tolerance is action potential duration shortening driven by ATP-sensitive potassium (K(ATP)) channels. K(ATP) channel expression has a significant physiologic impact on action potential duration shortening and myocardial energy consumption in response to physiologic heart rate acceleration. However, the effect of reduced channel expression on action potential duration shortening in response to severe metabolic stress is yet to be established. Here, transgenic mice with myocardium-specific expression of a dominant negative K(ATP) channel subunit were compared with littermate controls. Evaluation of K(ATP) channel whole cell current and channel number/patch was assessed by patch clamp in isolated ventricular cardiomyocytes. Monophasic action potentials were monitored in retrogradely perfused, isolated hearts during the transition to hypoxic perfusate. An 80-85% reduction in cardiac K(ATP) channel current density results in a similar magnitude, but significantly slower rate, of shortening of the ventricular action potential duration in response to severe hypoxia, despite no significant difference in coronary flow. Therefore, the number of functional cardiac sarcolemmal K(ATP) channels is a critical determinant of the rate of adaptation of myocardial membrane excitability, with implications for optimization of cardiac energy consumption and consequent cardioprotection under conditions of severe metabolic stress.

The first nucleotide binding domain of the sulfonylurea receptor 2A contains regulatory elements and is folded and functions as an independent module

The sulfonylurea receptor 2A (SUR2A) is an ATP-binding cassette (ABC) protein that forms the regulatory subunit of ATP-sensitive potassium (K(ATP)) channels in the heart. ATP binding and hydrolysis at the SUR2A nucleotide binding domains (NBDs) control gating of K(ATP) channels, and mutations in the NBDs that affect ATP hydrolysis and cellular trafficking cause cardiovascular disorders. To date, there is limited information on the SUR2A NBDs and the effects of disease-causing mutations on their structure and interactions. Structural and biophysical studies of NBDs, especially from eukaryotic ABC proteins like SUR2A, have been hindered by low solubility of the isolated domains. We hypothesized that the solubility of heterologously expressed SUR2A NBDs depends on the precise definition of the domain boundaries. Putative boundaries of SUR2A NBD1 were identified by structure-based sequence alignments and subsequently tested by exploring the solubility of SUR2A NBD1 constructs with different N and C termini. We have determined boundaries of SUR2A NBD1 that allow for soluble heterologous expression of the protein, producing a folded domain with ATP binding activity. Surprisingly, our alignment and screening data indicate that SUR2A NBD1 contains two putative, previously unidentified, regulatory elements: a large insert within the β-sheet subdomain and a C-terminal extension. Our approach, which combines the use of structure-based sequence alignments and predictions of disordered regions combined with biochemical and biophysical studies, may be applied as a general method for developing suitable constructs of other NBDs of ABC proteins.

Exercise-induced expression of cardiac ATP-sensitive potassium channels promotes action potential shortening and energy conservation

Physical activity is one of the most important determinants of cardiac function. The ability of the heart to increase delivery of oxygen and metabolic fuels relies on an array of adaptive responses necessary to match bodily demand while avoiding exhaustion of cardiac resources. The ATP-sensitive potassium (K(ATP)) channel has the unique ability to adjust cardiac membrane excitability in accordance with ATP and ADP levels, and up-regulation of its expression that occurs in response to exercise could represent a critical element of this adaption. However, the mechanism by which K(ATP) channel expression changes result in a beneficial effect on cardiac excitability and function remains to be established. Here, we demonstrate that an exercise-induced rise in K(ATP) channel expression enhanced the rate and magnitude of action potential shortening in response to heart rate acceleration. This adaptation in membrane excitability promoted significant reduction in cardiac energy consumption under escalating workloads. Genetic disruption of normal K(ATP) channel pore function abolished the exercise-related changes in action potential duration adjustment and caused increased cardiac energy consumption. Thus, an expression-driven enhancement in the K(ATP) channel-dependent membrane response to alterations in cardiac workload represents a previously unrecognized mechanism for adaptation to physical activity and a potential target for cardioprotection.

The molecular mechanisms and pharmacotherapy of ATP-sensitive potassium channel gene mutations underlying neonatal diabetes

Neonatal diabetes mellitus (NDM) is a monogenic disorder caused by mutations in genes involved in regulation of insulin secretion from pancreatic β-cells. Mutations in the KCNJ11 and ABCC8 genes, encoding the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel Kir6.2 and SUR1 subunits, respectively, are found in ∼50% of NDM patients. In the pancreatic β-cell, K(ATP) channel activity couples glucose metabolism to insulin secretion via cellular excitability and mutations in either KCNJ11 or ABCC8 genes alter K(ATP) channel activity, leading to faulty insulin secretion. Inactivation mutations decrease K(ATP) channel activity and stimulate excessive insulin secretion, leading to hyperinsulinism of infancy. In direct contrast, activation mutations increase K(ATP) channel activity, resulting in impaired insulin secretion, NDM, and in severe cases, developmental delay and epilepsy. Many NDM patients with KCNJ11 and ABCC8 mutations can be successfully treated with sulfonylureas (SUs) that inhibit the K(ATP) channel, thus replacing the need for daily insulin injections. There is also strong evidence indicating that SU therapy ameliorates some of the neurological defects observed in patients with more severe forms of NDM. This review focuses on the molecular and cellular mechanisms of mutations in the K(ATP) channel that underlie NDM. SU pharmacogenomics is also discussed with respect to evaluating whether patients with certain K(ATP) channel activation mutations can be successfully switched to SU therapy.

Recognition of sulfonylurea receptor (ABCC8/9) ligands by the multidrug resistance transporter P-glycoprotein (ABCB1): functional similarities based on common structural features between two multispecific ABC proteins

ATP-sensitive K(+) (K(ATP)) channels are the target of a number of pharmacological agents, blockers like hypoglycemic sulfonylureas and openers like the hypotensive cromakalim and diazoxide. These agents act on the channel regulatory subunit, the sulfonylurea receptor (SUR), which is an ABC protein with homologies to P-glycoprotein (P-gp). P-gp is a multidrug transporter expressed in tumor cells and in some healthy tissues. Because these two ABC proteins both exhibit multispecific recognition properties, we have tested whether SUR ligands could be substrates of P-gp. Interaction with P-gp was assayed by monitoring ATPase activity of P-gp-enriched vesicles. The blockers glibenclamide, tolbutamide, and meglitinide increased ATPase activity, with a rank order of potencies that correlated with their capacity to block K(ATP) channels. P-gp ATPase activity was also increased by the openers SR47063 (a cromakalim analog), P1075 (a pinacidil analog), and diazoxide. Thus, these molecules bind to P-gp (although with lower affinities than for SUR) and are possibly transported by P-gp. Competition experiments among these molecules as well as with typical P-gp substrates revealed a structural similarity between drug binding domains in the two proteins. To rationalize the observed data, we addressed the molecular features of these proteins and compared structural models, computerized by homology from the recently solved structures of murine P-gp and bacterial ABC transporters MsbA and Sav1866. Considering the various residues experimentally assigned to be involved in drug binding, we uncovered several hot spots, which organized spatially in two main binding domains, selective for SR47063 and for glibenclamide, in matching regions of both P-gp and SUR.

K(ATP) channels process nucleotide signals in muscle thermogenic response

Uniquely gated by intracellular adenine nucleotides, sarcolemmal ATP-sensitive K(+) (K(ATP)) channels have been typically assigned to protective cellular responses under severe energy insults. More recently, K(ATP) channels have been instituted in the continuous control of muscle energy expenditure under non-stressed, physiological states. These advances raised the question of how K(ATP) channels can process trends in cellular energetics within a milieu where each metabolic system is set to buffer nucleotide pools. Unveiling the mechanistic basis of the K(ATP) channel-driven thermogenic response in muscles thus invites the concepts of intracellular compartmentalization of energy and proteins, along with nucleotide signaling over diffusion barriers. Furthermore, it requires gaining insight into the properties of reversibility of intrinsic ATPase activity associated with K(ATP) channel complexes. Notwithstanding the operational paradigm, the homeostatic role of sarcolemmal K(ATP) channels can be now broadened to a wider range of environmental cues affecting metabolic well-being. In this way, under conditions of energy deficit such as ischemic insult or adrenergic stress, the operation of K(ATP) channel complexes would result in protective energy saving, safeguarding muscle performance and integrity. Under energy surplus, downregulation of K(ATP) channel function may find potential implications in conditions of energy imbalance linked to obesity, cold intolerance and associated metabolic disorders.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

The ATPase activities of sulfonylurea receptor 2A and sulfonylurea receptor 2B are influenced by the C-terminal 42 amino acids

Unusually among ATP-binding cassette proteins, the sulfonylurea receptor (SUR) acts as a channel regulator. ATP-sensitive potassium channels are octameric complexes composed of four pore-forming Kir6.2 subunits and four regulatory SUR subunits. Two different genes encode SUR1 (ABCC8) and SUR2 (ABCC9), with the latter being differentially spliced to give SUR2A and SUR2B, which differ only in their C-terminal 42 amino acids. ATP-sensitive potassium channels containing these different SUR2 isoforms are differentially modulated by MgATP, with Kir6.2/SUR2B being activated more than Kir6.2/SUR2A. We show here that purified SUR2B has a lower ATPase activity and a 10-fold lower K(m) for MgATP than SUR2A. Similarly, the isolated nucleotide-binding domain (NBD) 2 of SUR2B was less active than that of SUR2A. We further found that the NBDs of SUR2B interact, and that the activity of full-length SUR cannot be predicted from that of either the isolated NBDs or NBD mixtures. Notably, deletion of the last 42 amino acids from NBD2 of SUR2 resulted in ATPase activity resembling that of NBD2 of SUR2A rather than that of NBD2 of SUR2B: this might indicate that these amino acids are responsible for the lower ATPase activity of SUR2B and the isolated NBD2 of SUR2B. We suggest that the lower ATPase activity of SUR2B may result in enhanced duration of the MgADP-bound state, leading to channel activation.

ATP-binding cassette proteins involved in glucose and lipid homeostasis

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

Asymmetric switching in a homodimeric ABC transporter: a simulation study

ABC transporters are a large family of membrane proteins involved in a variety of cellular processes, including multidrug and tumor resistance and ion channel regulation. Advances in the structural and functional understanding of ABC transporters have revealed that hydrolysis at the two canonical nucleotide-binding sites (NBSs) is co-operative and non-simultaneous. A conserved core architecture of bacterial and eukaryotic ABC exporters has been established, as exemplified by the crystal structure of the homodimeric multidrug exporter Sav1866. Currently, it is unclear how sequential ATP hydrolysis arises in a symmetric homodimeric transporter, since it implies at least transient asymmetry at the NBSs. We show by molecular dynamics simulation that the initially symmetric structure of Sav1866 readily undergoes asymmetric transitions at its NBSs in a pre-hydrolytic nucleotide configuration. MgATP-binding residues and a network of charged residues at the dimer interface are shown to form a sequence of putative molecular switches that allow ATP hydrolysis only at one NBS. We extend our findings to eukaryotic ABC exporters which often consist of two non-identical half-transporters, frequently with degeneracy substitutions at one of their two NBSs. Interestingly, many residues involved in asymmetric conformational switching in Sav1866 are substituted in degenerate eukaryotic NBS. This finding strengthens recent suggestions that the interplay of a consensus and a degenerate NBS in eukaroytic ABC proteins pre-determines the sequence of hydrolysis at the two NBSs.

ABC transporters are a large family of membrane proteins present in all organisms. Typically, they utilize ATP hydrolysis, the most prominent biological energy source, to translocate substrates into cells (e.g., bacterial nutritient uptake) or out of cells (e.g., multidrug exporters that contribute to antimicrobial resistance in bacteria and resistance to chemotherapeutic drugs in cancer). Also clinically relevant non-transport roles have been identified among ABC proteins. ABC transporters bind two molecules of ATP but do not hydrolyze them simultaneously. Therefore, an ABC transporter that consists of two symmetric halves must temporarily adopt asymmetric conformations at the two ATP-binding sites. Such transient conformational changes are difficult to address biochemically, but may be amenable to study by simulation methods, leading to future experiments. We employ molecular dynamics simulations to study how asymmetric switching might occur in the homodimeric bacterial ABC multidrug exporter Sav1866. The simulations suggest a mechanism of conformational switching that encompasses the ATP-binding sites and their interface towards the substrate-binding site. We extend our findings to show how asymmetric residue substitutions may render the switching process non-stochastic in mammalian Sav1866-like ABC exporters. This contributes to ongoing discussions about the role of two dissimilar ATP-binding sites in clinically relevant ABC proteins.

Substitution of the Walker A lysine by arginine in the nucleotide-binding domains of sulphonylurea receptor SUR2B: effects on ligand binding and channel activity

Sulphonylurea receptors (SURs) serve as regulatory subunits of ATP-sensitive K(+) channels. SURs are members of the ATP-binding cassette (ABC) protein superfamily and contain two conserved nucleotide-binding domains (NBDs) which bind and hydrolyse MgATP; in addition, they carry the binding sites for the sulphonylureas like glibenclamide (GBC) which close the channel and for the K(ATP) channel openers such as P1075. Here we have exchanged the conserved Lys in the Walker A motif by Arg in both NBDs of SUR2B, the regulatory subunit of the vascular K(ATP) channel. Then the effect of the mutation on the ATPase-dependent binding of GBC and P1075 to SUR2B and on the activity of the recombinant vascular (Kir6.1/SUR2B) channel was assessed. Surprisingly, in the absence of MgATP, the mutation weakened binding of P1075 and the extent of allosteric inhibition of GBC binding by P1075. The mutation abolished most, but not all, of the MgATP effects on the binding of GBC and P1075 and prevented nucleotide-induced activation of the channel which relies on SUR reaching the posthydrolytic (MgADP-bound) state; the mutant channel was, however, opened by P1075 at higher concentrations. The data provide evidence that mutant SUR2B binds MgATP but that the posthydrolytic state is insufficiently populated. This suggests that the mutation locks SUR2B in an MgATP-binding prehydrolytic-like state; binding of P1075 may induce a posthydrolytic-like conformation to open the channel.

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

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

Quaternary structure of KATP channel SUR2A nucleotide binding domains resolved by synchrotron radiation X-ray scattering

Heterodimeric nucleotide binding domains NBD1/NBD2 distinguish the ATP-binding cassette protein SUR2A, a recognized regulatory subunit of cardiac ATP-sensitive K(+) (K(ATP)) channels. The tandem function of these core domains ensures metabolism-dependent gating of the Kir6.2 channel pore, yet their structural arrangement has not been resolved. Here, purified monodisperse and interference-free recombinant particles were subjected to synchrotron radiation small-angle X-ray scattering (SAXS) in solution. Intensity function analysis of SAXS profiles resolved NBD1 and NBD2 as octamers. Implemented by ab initio simulated annealing, shape determination prioritized an oblong envelope wrapping NBD1 and NBD2 with respective dimensions of 168x80x37A(3) and 175x81x37A(3) based on symmetry constraints, validated by atomic force microscopy. Docking crystal structure homology models against SAXS data reconstructed the NBD ensemble surrounding an inner cleft suitable for Kir6.2 insertion. Human heart disease-associated mutations introduced in silico verified the criticality of the mapped protein-protein interface. The resolved quaternary structure delineates thereby a macromolecular arrangement of K(ATP) channel SUR2A regulatory domains.

Human K(ATP) channelopathies: diseases of metabolic homeostasis

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

K(ATP) channelopathies in the pancreas

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

Proteomic profiling of KATP channel-deficient hypertensive heart maps risk for maladaptive cardiomyopathic outcome

KCNJ11 null mutants, lacking Kir6.2 ATP-sensitive K(+) (K(ATP)) channels, exhibit a marked susceptibility towards hypertension (HTN)-induced heart failure. To gain insight into the molecular alterations induced by knockout of this metabolic sensor under hemodynamic stress, wild-type (WT) and Kir6.2 knockout (Kir6.2-KO) cardiac proteomes were profiled by comparative 2-DE and Orbitrap MS. Despite equivalent systemic HTN produced by chronic hyperaldosteronism, 114 unique proteins were altered in Kir6.2-KO compared to WT hearts. Bioinformatic analysis linked the primary biological function of the K(ATP) channel-dependent protein cohort to energetic metabolism (64% of proteins), followed by signaling infrastructure (36%) including oxidoreductases, stress-related chaperones, processes supporting protein degradation, transcription and translation, and cytostructure. Mapped protein-protein relationships authenticated the primary impact on metabolic pathways, delineating the K(ATP) channel-dependent subproteome within a nonstochastic network. Iterative systems interrogation of the proteomic web prioritized heart-specific adverse effects, i.e., "Cardiac Damage", "Cardiac Enlargement", and "Cardiac Fibrosis", exposing a predisposition for the development of cardiomyopathic traits in the hypertensive Kir6.2-KO. Validating this maladaptive forecast, phenotyping documented an aggravated myocardial contractile performance, a massive interstitial fibrosis and an exaggerated left ventricular size, all prognostic indices of poor outcome. Thus, Kir6.2 ablation engenders unfavorable proteomic remodeling in hypertensive hearts, providing a composite molecular substrate for pathologic stress-associated cardiovascular disease.

KCNJ11 knockout morula re-engineered by stem cell diploid aggregation

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

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

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

Recording the activity of ATP-sensitive K(+) channels in open-cell cell-attached configuration

The functional activity of ion channels is affected by their local environment, and for certain types of channels this local effect is critical. This chapter describes a method of recording the activity of ATP-sensitive K(+) channels, expressed on the plasma membrane of pancreatic beta-cells, without destroying the beta-cell architecture. In the open-cell cell-attached patch-clamp configuration, ligands and effectors of these metabolism-dependent channels are delivered to their cytoplasmic side via pores in the plasma membrane produced by bacterial toxins.

Modeling K(ATP) channel gating and its regulation

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

Functional clustering of mutations in the dimer interface of the nucleotide binding folds of the sulfonylurea receptor

ATP-sensitive K(+) (K(ATP)) channels modulate their activity as a function of inhibitory ATP and stimulatory Mg-nucleotides. They are constituted by two proteins: a pore-forming K(+) channel subunit (Kir6.1, Kir6.2) and a regulatory sulfonylurea receptor (SUR) subunit, an ATP-binding cassette (ABC) transporter that confers MgADP stimulation to the channel. Channel regulation by MgADP is dependent on nucleotide interaction with the cytoplasmic nucleotide binding folds (NBF1 and NBF2) of the SUR subunit. Crystal structures of bacterial ABC proteins indicate that NBFs form as dimers, suggesting that NBF1-NBF2 heterodimers may form in SUR and other eukaryotic ABC proteins. We have modeled SUR1 NBF1 and NBF2 as a heterodimer, and tested the validity of the predicted dimer interface by systematic mutagenesis. Engineered cysteine mutations in this region have significant effects, both positive and negative, on MgADP stimulation of K(ATP) channels in excised patches and on macroscopic channel activity in intact cells. Additionally, the mutations cluster in the model structure according to their functional effect, such that patterns of alteration emerge. Of note, three gain-of-function mutations, leading to MgADP hyperstimulation of the channel, are located in the D-loop region at the center of the predicted dimer interface. Overall, the data support the idea that SUR1 NBFs assemble as heterodimers and that this interaction is functionally critical.

Testing for violations of microscopic reversibility in ATP-sensitive potassium channel gating

In pancreatic beta cells, insulin secretion is tightly controlled by the cells' metabolic state via the ATP-sensitive potassium (KATP) channel. ATP is a key mediator in this signaling process, where its role as an inhibitor of KATP channels has been extensively studied. Since the channel contains an ATPase as an accessory subunit, the possibility that ATP hydrolysis mediates KATP channel opening has also been proposed. However, a rigorous test of coupling between ATP hydrolysis and channel gating has not previously been performed. In the present work, we examine whether KATP channel gating obeys detailed balance in order to determine whether ATP hydrolysis is strongly coupled to the gating of the KATP channel. Single-channel records were obtained from inside-out patches of transiently transfected HEK-293 cells. Channel activity in membrane patches with exactly one channel shows no violations of microscopic reversibility. Although KATP channel gating shows long closed times on the time scale where ATP hydrolysis takes place, the time symmetry of channel gating indicates that it is not tightly coupled to ATP hydrolysis. This lack of coupling suggests that channel gating operates close to equilibrium; although detailed balance is not expected to hold for ATP hydrolysis, it still does so in channel gating. On the basis of these results, the function of the ATPase active site in channel gating may be to sense nucleotides by differential binding of ATP and ADP, rather than to drive a thermodynamically unfavorable conformational change.

Role for SUR2A ED domain in allosteric coupling within the K(ATP) channel complex

Allosteric regulation of heteromultimeric ATP-sensitive potassium (K_ATP) channels is unique among protein systems as it implies transmission of ligand-induced structural adaptation at the regulatory SUR subunit, a member of ATP-binding cassette ABCC family, to the distinct pore-forming K⁺ (Kir6.x) channel module. Cooperative interaction between nucleotide binding domains (NBDs) of SUR is a prerequisite for K_ATP channel gating, yet pathways of allosteric intersubunit communication remain uncertain. Here, we analyzed the role of the ED domain, a stretch of 15 negatively charged aspartate/glutamate amino acid residues (948–962) of the SUR2A isoform, in the regulation of cardiac K_ATP channels. Disruption of the ED domain impeded cooperative NBDs interaction and interrupted the regulation of K_ATP channel complexes by MgADP, potassium channel openers, and sulfonylurea drugs. Thus, the ED domain is a structural component of the allosteric pathway within the K_ATP channel complex integrating transduction of diverse nucleotide-dependent states in the regulatory SUR subunit to the open/closed states of the K⁺-conducting channel pore.

A rare mutation in ABCC8/SUR1 leading to altered ATP-sensitive K+ channel activity and beta-cell glucose sensing is associated with type 2 diabetes in adults

OBJECTIVE

ATP-sensitive K(+) channels (K(ATP) channels) link glucose metabolism to the electrical activity of the pancreatic beta-cell to regulate insulin secretion. Mutations in either the Kir6.2 or sulfonylurea receptor (SUR) 1 subunit of the channel have previously been shown to cause neonatal diabetes. We describe here an activating mutation in the ABCC8 gene, encoding SUR1, that is associated with the development of type 2 diabetes only in adults.

RESEARCH DESIGN AND METHODS

Recombinant K(ATP) channel subunits were expressed using pIRES2-based vectors in human embryonic kidney (HEK) 293 or INS1(832/13) cells and the subcellular distribution of c-myc-tagged SUR1 channels analyzed by confocal microscopy. K(ATP) channel activity was measured in inside-out patches and plasma membrane potential in perforated whole-cell patches. Cytoplasmic [Ca(2+)] was imaged using Fura-Red.

RESULTS

A mutation in ABCC8/SUR1, leading to a Y356C substitution in the seventh membrane-spanning alpha-helix, was observed in a patient diagnosed with hyperglycemia at age 39 years and in two adult offspring with impaired insulin secretion. Single K(ATP) channels incorporating SUR1-Y356C displayed lower sensitivity to MgATP (IC(50) = 24 and 95 micromol/l for wild-type and mutant channels, respectively). Similar effects were observed in the absence of Mg(2+), suggesting an allosteric effect via associated Kir6.2 subunits. Overexpression of SUR1-Y356C in INS1(832/13) cells impaired glucose-induced cell depolarization and increased in intracellular free Ca(2+) concentration, albeit more weakly than neonatal diabetes-associated SUR1 mutants.

CONCLUSIONS

An ABCC8/SUR1 mutation with relatively minor effects on K(ATP) channel activity and beta-cell glucose sensing causes diabetes in adulthood. These data suggest a close correlation between altered SUR1 properties and clinical phenotype.

Molecular determinants for the activating/blocking actions of the 2H-1,4-benzoxazine derivatives, a class of potassium channel modulators targeting the skeletal muscle KATP channels

The 2H-1,4-benzoxazine derivatives are modulators of the skeletal muscle ATP-sensitive-K(+) channels (K(ATP)), activating it in the presence of ATP but inhibiting it in the absence of nucleotide. To investigate the molecular determinants for the activating/blocking actions of these compounds, novel molecules with different alkyl or aryl-alkyl substitutes at position 2 of the 1,4-benzoxazine ring were prepared. The effects of the lengthening of the alkyl chain and of branched substitutes, as well as of the introduction of aliphatic/aromatic rings on the activity of the molecules, were investigated on the skeletal muscle K(ATP) channels of the rat, in excised-patch experiments, in the presence or absence of internal ATP (10(-4) M). In the presence of ATP, the 2-n-hexyl analog was the most potent activator (DE(50) = 1.08 x 10(-10) M), whereas the 2-phenylethyl was not effective. The rank order of efficacy of the openers was 2-n-hexyl > or =2-cyclohexylmethyl >2-isopropyl = 2-n-butyl > or = 2-phenyl > or = 2-benzyl = 2-isobutyl analogs. In the absence of ATP, the 2-phenyl analog was the most potent inhibitor (IC(50) = 2.5 x 10(-11) M); the rank order of efficacy of the blockers was 2-phenyl > or = 2-n-hexyl > 2-n-butyl > 2-cyclohexylmethyl, whereas the 2-phenylethyl, 2-benzyl, and 2-isobutyl 1,4-benzoxazine analogs were not effective; the 2-isopropyl analog activated the K(ATP) channel even in the absence of nucleotide. Therefore, distinct molecular determinants for the activating or blocking actions for these compounds can be found. For example, the replacement of the linear with the branched alkyl substitutes at the position 2 of the 1,4-benzoxazine nucleus determines the molecular switch from blockers to openers. These compounds were 100-fold more potent and effective as openers than other KCO against the muscle K(ATP) channels.

cAMP-dependent protein kinase phosphorylation produces interdomain movement in SUR2B leading to activation of the vascular KATP channel

Vascular ATP-sensitive K(+) channels are activated by multiple vasodilating hormones and neurotransmitters via PKA. A critical PKA phosphorylation site (Ser-1387) is found in the second nucleotide-binding domain (NBD(2)) of the SUR2B subunit. To understand how phosphorylation at Ser-1387 leads to changes in channel activity, we modeled the SUR2B using a newly crystallized ABC protein SAV1866. The model showed that Ser-1387 was located on the interface of NBD2 with TMD1 and physically interacted with Tyr-506 in TMD1. A positively charged residue (Arg-1462) in NBD2 was revealed in the close vicinity of Ser-1387. Mutation of either of these three residues abolished PKA-dependent channel activation. Molecular dynamics simulations suggested that Ser-1387, Tyr-506, and Arg-1462 formed a compact triad upon Ser-1387 phosphorylation, leading to reshaping of the NBD2 interface and movements of NBD2 and TMD1. Restriction of the interdomain movements by engineering a disulfide bond between TMD1 and NBD2 prevented the channel activation in a redox-dependent manner. Thus, a channel-gating mechanism is suggested through enhancing the NBD-TMD coupling efficiency following Ser-1387 phosphorylation, which is shared by multiple vasodilators.

Interaction of asymmetric ABCC9-encoded nucleotide binding domains determines KATP channel SUR2A catalytic activity

Nucleotide binding domains (NBDs) secure ATP-binding cassette (ABC) transporter function. Distinct from traditional ABC transporters, ABCC9-encoded sulfonylurea receptors (SUR2A) form, with Kir6.2 potassium channels, ATP-sensitive K+ (K ATP) channel complexes. SUR2A contains ATPase activity harbored within NBD2 and, to a lesser degree, NBD1, with catalytically driven conformations exerting determinate linkage on the Kir6.2 channel pore. While homodomain interactions typify NBDs of conventional ABC transporters, heterodomain NBD interactions and their functional consequence have not been resolved for the atypical SUR2A protein. Here, nanoscale protein topography mapped assembly of monodisperse purified recombinant SUR2A NBD1/NBD2 domains, precharacterized by dynamic light scattering. Heterodomain interaction produced conformational rearrangements inferred by secondary structural change in circular dichroism, and validated by atomic force and transmission electron microscopy. Physical engagement of NBD1 with NBD2 translated into enhanced intrinsic ATPase activity. Molecular modeling delineated a complemental asymmetry of NBD1/NBD2 ATP-binding sites. Mutation in the predicted catalytic base residue, D834E of NBD1, altered NBD1 ATPase activity disrupting potentiation of catalytic behavior in the NBD1/NBD2 interactome. Thus, NBD1/NBD2 assembly, resolved by a panel of proteomic approaches, provides a molecular substrate that determines the optimal catalytic activity in SUR2A, establishing a paradigm for the structure-function relationship within the K ATP channel complex.

Cardiac sulfonylurea receptor short form-based channels confer a glibenclamide-insensitive KATP activity

The cardiac sarcolemmal ATP-sensitive potassium channel (K(ATP)) consists of a Kir6.2 pore and an SUR2 regulatory subunit, which is an ATP-binding cassette (ABC) transporter. K(ATP) channels have been proposed to play protective roles during ischemic preconditioning. An SUR2 mutant mouse was previously generated by disrupting the first nucleotide-binding domain (NBD1), where a glibenclamide action site was located. In the mutant ventricular myocytes, a non-conventional glibenclamide-insensitive (10 microM), ATP-sensitive current (I(KATPn)) was detected in 33% of single-channel recordings with an average amplitude of 12.3+/-5.4 pA per patch, an IC(50) to ATP inhibition at 10 microM and a mean burst duration at 20.6+/-1.8 ms. Newly designed SUR2 isoform- or variant-specific antibodies identified novel SUR2 short forms in the sizes of 28 and 68 kDa in addition to a 150-kDa long form in the sarcolemmal membrane of wild-type (WT) heart. We hypothesized that channels constituted by these short forms that lack NBD1 confer I(KATPn). The absence of the long form in the mutant corresponded to loss of the conventional glibenclamide-sensitive K(ATP) currents (I(KATP)) in isolated cardiomyocytes and vascular smooth muscle cells but the SUR2 short forms remained intact. Nested exonic RT-PCR in the mutant indicated that the short forms lacked NBD1 but contained NBD2. The SUR2 short forms co-immunoprecipitated with Kir6.1 or Kir6.2 suggesting that the short forms may function as hemi-transporters reported in other eukaryotic ABC transporter subgroups. Our results indicate that different K(ATP) compositions may co-exist in cardiac sarcolemmal membrane.

The actions of a novel potent islet beta-cell specific ATP-sensitive K+ channel opener can be modulated by syntaxin-1A acting on sulfonylurea receptor 1

Islet beta-cell-specific ATP-sensitive K(+) (K(ATP)) channel openers thiadiazine dioxides induce islet rest to improve insulin secretion, but their molecular basis of action remains unclear. We reported that syntaxin-1A binds nucleotide binding folds of sulfonylurea receptor 1 (SUR1) in beta-cells to inhibit K(ATP) channels. As a strategy to elucidate the molecular mechanism of action of these K(ATP) channel openers, we explored the possibility that 6-chloro-3-(1-methylcyclobutyl)amino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (NNC55-0462) might influence syntaxin-1A-SUR1 interactions or vice versa. Whole-cell and inside-out patch-clamp electrophysiology was used to examine the effects of glutathione S-transferase (GST)-syntaxin-1A dialysis or green fluorescence protein/syntaxin-1A cotransfection on NNC55-0462 actions. In vitro pull-down binding studies were used to examine NNC55-0462 influence on syntaxin-1A-SUR1 interactions. Dialysis of GST-syntaxin-1A into the cell cytoplasm reduced both potency and efficacy of extracellularly perfused NNC55-0462 in a HEK cell line stably expressing Kir6.2/SUR1 (BA8 cells) and in rat islet beta-cells. Moreover, inside-out membrane patches excised from BA8 cells showed that both GST-syntaxin-1A and its H3 domain inhibited K(ATP) channels previously activated by NNC55-0462. This action on K(ATP) channels is isoform-specific to syntaxin-1A because syntaxin-2 was without effect. Furthermore, the parent compound diazoxide showed similar sensitivity to GST-syntaxin-1A inhibition. NNC55-0462, however, did not influence syntaxin-1A-SUR1 binding interaction. Our results demonstrated that syntaxin-1A interactions with SUR1 at its cytoplasmic domains can modulate the actions of the K(ATP) channel openers NNC55-0462 and diazoxide on K(ATP) channels. The reduced levels of islet syntaxin-1A in diabetes would thus be expected to exert a positive influence on the therapeutic effects of this class of K(ATP) channel openers.

ATP-sensitive potassium channels: metabolic sensing and cardioprotection

The cardiovascular system operates under a wide scale of demands, ranging from conditions of rest to extreme stress. How the heart muscle matches rates of ATP production with utilization is an area of active investigation. ATP-sensitive potassium (K(ATP)) channels serve a critical role in the orchestration of myocardial energetic well-being. K(ATP) channel heteromultimers consist of inwardly-rectifying K(+) channel 6.2 and ATP-binding cassette sulfonylurea receptor 2A that translates local ATP/ADP levels, set by ATPases and phosphotransfer reactions, to the channel pore function. In cells in which the mobility of metabolites between intracellular microdomains is limited, coupling of phosphotransfer pathways with K(ATP) channels permits a high-fidelity transduction of nucleotide fluxes into changes in membrane excitability, matching energy demands with metabolic resources. This K(ATP) channel-dependent optimization of cardiac action potential duration preserves cellular energy balance at varying workloads. Mutations of K(ATP) channels result in disruption of the nucleotide signaling network and generate a stress-vulnerable phenotype with excessive susceptibility to injury, development of cardiomyopathy, and arrhythmia. Solving the mechanisms underlying the integration of K(ATP) channels into the cellular energy network will advance the understanding of endogenous cardioprotection and the development of strategies for the management of cardiovascular injury and disease progression.