Molecular basis for Kir6.2 channel inhibition by adenine nucleotides

Dynamic duo: Kir6 and SUR in KATP channel structure and function

KATP channels are ligand-gated potassium channels that couple cellular energetics with membrane potential to regulate cell activity. Each channel is an eight subunit complex comprising four central pore-forming Kir6 inward rectifier potassium channel subunits surrounded by four regulatory subunits known as the sulfonylurea receptor, SUR, which confer homeostatic metabolic control of KATP gating. SUR is an ATP binding cassette (ABC) protein family homolog that lacks membrane transport activity but is essential for KATP expression and function. For more than four decades, understanding the structure-function relationship of Kir6 and SUR has remained a central objective of clinical significance. Here, we review progress in correlating the wealth of functional data in the literature with recent KATP cryoEM structures.

The Importance of Molecular Genetic Testing for Precision Diagnostics, Management, and Genetic Counseling in MODY Patients

Maturity-onset diabetes of the young (MODY) is part of the heterogeneous group of monogenic diabetes (MD) characterized by the non-immune dysfunction of pancreatic β-cells. The diagnosis of MODY still remains a challenge for clinicians, with many cases being misdiagnosed as type 1 or type 2 diabetes mellitus (T1DM/T2DM), and over 80% of cases remaining undiagnosed. With the introduction of modern technologies, important progress has been made in deciphering the molecular mechanisms and heterogeneous etiology of MD, including MODY. The aim of our study was to identify genetic variants associated with MODY in a group of patients with early-onset diabetes/prediabetes in whom a form of MD was clinically suspected. Genetic testing, based on next-generation sequencing (NGS) technology, was carried out either in a targeted manner, using gene panels for monogenic diabetes, or by analyzing the entire exome (whole-exome sequencing). GKC-MODY 2 was the most frequently detected variant, but rare forms of KCNJ11-MODY 13, specifically, HNF4A-MODY 1, were also identified. We have emphasized the importance of genetic testing for early diagnosis, MODY subtype differentiation, and genetic counseling. We presented the genotype-phenotype correlations, especially related to the clinical evolution and personalized therapy, also emphasizing the particularities of each patient in the family context.

Cryo-electron microscopy structures and progress toward a dynamic understanding of KATP channels

Puljung reviews recent cryo-EM K_ATP channel structures and proposes a mechanism by which ligand binding results in channel opening.

Adenosine triphosphate (ATP)–sensitive K⁺ (K_ATP) channels are molecular sensors of cell metabolism. These hetero-octameric channels, comprising four inward rectifier K⁺ channel subunits (Kir6.1 or Kir6.2) and four sulfonylurea receptor (SUR1 or SUR2A/B) subunits, detect metabolic changes via three classes of intracellular adenine nucleotide (ATP/ADP) binding site. One site, located on the Kir subunit, causes inhibition of the channel when ATP or ADP is bound. The other two sites, located on the SUR subunit, excite the channel when bound to Mg nucleotides. In pancreatic β cells, an increase in extracellular glucose causes a change in oxidative metabolism and thus turnover of adenine nucleotides in the cytoplasm. This leads to the closure of K_ATP channels, which depolarizes the plasma membrane and permits Ca²⁺ influx and insulin secretion. Many of the molecular details regarding the assembly of the K_ATP complex, and how changes in nucleotide concentrations affect gating, have recently been uncovered by several single-particle cryo-electron microscopy structures of the pancreatic K_ATP channel (Kir6.2/SUR1) at near-atomic resolution. Here, the author discusses the detailed picture of excitatory and inhibitory ligand binding to K_ATP that these structures present and suggests a possible mechanism by which channel activation may proceed from the ligand-binding domains of SUR to the channel pore.

Functional mapping of the N-terminal arginine cluster and C-terminal acidic residues of Kir6.2 channel fused to a G protein-coupled receptor

Ion channel-coupled receptors (ICCRs) are original man-made ligand-gated ion channels created by fusion of G protein-coupled receptors (GPCRs) to the inward-rectifier potassium channel Kir6.2. GPCR conformational changes induced by ligand binding are transduced into electrical current by the ion channel. This functional coupling is closely related to the length of the linker region formed by the GPCR C-terminus (C-ter) and Kir6.2N-terminus (N-ter). Manipulating the GPCR C-ter length allows to finely tune the channel regulation, both in amplitude and sign (opening or closing Kir6.2). In this work, we demonstrate that the primary sequence of the channel N-terminal domain is an additional parameter for the functional coupling with GPCRs. As for all Kir channels, a cluster of basic residues is present in the N-terminal domain of Kir6.2 and is composed of 5 arginines which are proximal to the GPCR C-ter in the fusion proteins. Using a functional mapping approach, we demonstrate the role of specific arginines (R27 and R32) for the function of ICCRs, indicating that the position and not the cluster of positively-charged arginines is critical for the channel regulation by the GPCR. Following observations provided by molecular dynamics simulation, we explore the hypothesis of interaction of these arginines with acidic residues, and using site-directed mutagenesis, we identified aspartate D307 and glutamate E308 residues as critical for the function of ICCRs. These results demonstrate the critical role of the N-terminal and C-terminal charged residues of Kir6.2 for its allosteric regulation by the fused GPCR.

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.

Adenosine triphosphate-sensitive potassium channels and cardiomyopathies (Review)

Cardiomyopathies have been indicated to be one of the leading causes of heart failure. Though it was indicated that genetic defects, viral infection and trace element deficiency were among the causes of cardiomyopathy, the etiology has remained to be fully elucidated. Cardiomyocytes require large amounts of energy to maintain their normal biological functions. Adenosine triphosphate-sensitive potassium channels (KATP), composed of inward-rectifier potassium ion channel and sulfonylurea receptor subunits, are present on the cell surface and mitochondrial membrane of cardiac muscle cells. As metabolic sensors sensitive to changes in intracellular energy levels, KATP adapt electrical activities to metabolic challenges, maintaining normal biological functions of myocytes. It is implied that malfunctions, mutations and altered expression of KATP are associated with the pathogenesis of conditions including c hypertrophy, diabetes as well as dilated, ischemic and endemic cardiomyopathy. However, the current knowledge is only the tip of the iceberg and the roles of KATP in cardiomyopathies largely remain to be elucidated in future studies.

Parametrisation of the free energy of ATP binding to wild-type and mutant Kir6.2 potassium channels

ATP-sensitive K(+) (K(ATP)) channels, comprised of pore-forming Kir6.x and regulatory SURx subunits, play important roles in many cellular functions; because of their sensitivity to inhibition by intracellular ATP, K(ATP) channels provide a link between cell metabolism and membrane electrical activity. We constructed structural homology models of Kir6.2 and a series of Kir6.2 channels carrying mutations within the putative ATP-binding site. Computational docking was carried out to determine the conformation of ATP in its binding site. The Linear Interaction Energy (LIE) method was used to estimate the free-energy of ATP binding to wild-type and mutant Kir6.2 channels. Comparisons of the theoretical binding free energies for ATP with those determined from mutational experiments enabled the identification of the most probable conformation of ATP bound to the Kir6.2 channel. A set of LIE parameters was defined that may enable prediction of the effects of additional Kir6.2 mutations within the ATP binding site on the affinity for ATP.

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 first clinical case of a mutation at residue K185 of Kir6.2 (KCNJ11): a major ATP-binding residue

BACKGROUND

Closure of the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel plays a key role in insulin secretion from the pancreatic beta-cells. Many mutations in KCNJ11 and ABCC8, which respectively encode the pore-forming (Kir6.2) and regulatory (SUR1) subunits of the K(ATP) channel, cause neonatal diabetes. All such mutations impair the ability of metabolically generated ATP to close the channel. Although lysine 185 is predicted to be a major contributor to the ATP-binding site of Kir6.2, no mutations at this residue have been found to cause neonatal diabetes to date.

METHODS

We report a 3-year-old girl with permanent neonatal diabetes (PNDM) caused by a novel heterozygous mutation (K185Q) at residue K185 of KCNJ11. The patient presented with marked hyperglycaemia and ketoacidosis at 70 days after birth, and insulin therapy was commenced.

RESULTS

Wild-type and mutant K(ATP) channels were expressed in Xenopus oocytes and the effects of intracellular ATP on macroscopic K(ATP) currents in inside-out membrane patches were measured. In the simulated heterozygous state, the K185Q mutation caused a substantial reduction in the ability of MgATP to inhibit the channel. Heterozygous K185Q channels were still blocked effectively by the sulphonylurea tolbutamide.

CONCLUSIONS

We report the first clinical case of a PNDM caused by a mutation at K185. Functional studies indicate that the K185Q mutation causes PNDM by reducing the ATP sensitivity of the K(ATP) channel, probably via a reduction in ATP binding to Kir6.2. Based on the experimental data, the patient was successfully transferred to sulphonylurea therapy.

Impact of disease-causing SUR1 mutations on the KATP channel subunit interface probed with a rhodamine protection assay

The function of the ATP-sensitive potassium (K(ATP)) channel relies on the proper coupling between its two subunits: the pore-forming Kir6.2 and the regulator SUR. The conformation of the interface between these two subunits can be monitored using a rhodamine 123 (Rho) protection assay because Rho blocks Kir6.2 with an efficiency that depends on the relative position of transmembrane domain (TMD) 0 of the associated SUR (Hosy, E., Dérand, R., Revilloud, J., and Vivaudou, M. (2007) J. Physiol. 582, 27-39). Here we find that the natural and synthetic K(ATP) channel activators MgADP, zinc, and SR47063 induced a Rho-insensitive conformation. The activating mutation F132L in SUR1, which causes neonatal diabetes, also rendered the channel resistant to Rho block, suggesting that it stabilized an activated conformation by uncoupling TMD0 from the rest of SUR1. At a nearby residue, the SUR1 mutation E128K impairs trafficking, thereby reducing surface expression and causing hyperinsulinism. To augment channel density at the plasma membrane to investigate the effect of mutating this residue on channel function, we introduced the milder mutation E126A at the matching residue of SUR2A. Mutation E126A imposed a hypersensitive Rho phenotype indicative of a functional uncoupling between TMD0 and Kir6.2. These results suggest that the TMD0-Kir6.2 interface is mobile and that the gating modes of Kir6.2 correlate with distinct positions of TMD0. They further demonstrate that the second intracellular loop of SUR, which contains the two residues studied here, is a key structural element of the TMD0-Kir6.2 interface.

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.

How ATP inhibits the open K(ATP) channel

ATP-sensitive potassium (K_ATP) channels are composed of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits. Binding of ATP to Kir6.2 leads to inhibition of channel activity. Because there are four subunits and thus four ATP-binding sites, four binding events are possible. ATP binds to both the open and closed states of the channel and produces a decrease in the mean open time, a reduction in the mean burst duration, and an increase in the frequency and duration of the interburst closed states. Here, we investigate the mechanism of interaction of ATP with the open state of the channel by analyzing the single-channel kinetics of concatenated Kir6.2 tetramers containing from zero to four mutated Kir6.2 subunits that possess an impaired ATP-binding site. We show that the ATP-dependent decrease in the mean burst duration is well described by a Monod-Wyman-Changeux model in which channel closing is produced by all four subunits acting in a single concerted step. The data are inconsistent with a Hodgkin-Huxley model (four independent steps) or a dimer model (two independent dimers). When the channel is open, ATP binds to a single ATP-binding site with a dissociation constant of 300 μM.

Neonatal hyperglycaemia and abnormal development of the pancreas

Transient and permanent neonatal diabetes mellitus (TNDM and PNDM) are rare conditions occurring in around 1 per 300,000 live births. In TNDM, growth-retarded infants develop diabetes in the first few weeks of life, only to go into remission after a few months with possible relapse to permanent diabetes usually around adolescence or in adulthood. In PNDM, insulin secretory failure occurs in the late fetal or early postnatal period. The very recently elucidated mutations in KCNJ11 and ABCC8 genes, encoding the Kir6.2 and SUR1 subunits of the pancreatic K(ATP) channel involved in regulation of insulin secretion, account for a third to a half of the PNDM cases. Molecular analysis of chromosome 6 anomalies and the KCNJ11 and ABCC8 genes encoding Kir6.2 and SUR1 provides a tool for distinguishing transient from permanent neonatal diabetes mellitus in the neonatal period. Some patients (those with mutations in KCNJ11 and ABCC8) may be transferred from insulin therapy to sulphonylureas.

Molecular basis of neonatal diabetes in Japanese patients

CONTEXT

Neonatal diabetes mellitus (NDM) is classified clinically into a transient form (TNDM), in which insulin secretion recovers within several months, and a permanent form (PNDM), requiring lifelong medication. However, these conditions are genetically heterogeneous.

OBJECTIVE

Our objective was to evaluate the contribution of the responsible gene and delineate their clinical characteristics.

PATIENTS AND METHODS

The chromosome 6q24 abnormality and KCNJ11 and ABCC8 mutations were analyzed in 31 Japanese patients (16 with TNDM and 15 with PNDM). Moreover, FOXP3 and IPF1 mutations were analyzed in a patient with immune dysregulation, polyendocrinopathy, enteropathy X-linked syndrome and with pancreatic agenesis, respectively.

RESULTS

A molecular basis for NDM was found in 23 patients: 6q24 in eleven, KCNJ11 in nine, ABCC8 in two, and FOXP3 in one. All the patients with the 6q24 abnormality and two patients with the KCNJ11 mutation proved to be TNDM. Five mutations were novel: two (p.A174G and p.R50G) [corrected] in KCNJ11, two (p.A90V and p.N1122D) in ABCC8, and one (p.P367L) in FOXP3. Comparing the 6q24 abnormality and KCNJ11 mutation, there were some significant clinical differences: the earlier onset of diabetes, the lower frequency of diabetic ketoacidosis at onset, and the higher proportion of the patients with macroglossia at initial presentation in the patients with 6q24 abnormality. In contrast, two patients with the KCNJ11 mutations manifested epilepsy and developmental delay.

CONCLUSIONS

Both the 6q24 abnormality and KCNJ11 mutation are major causes of NDM in Japanese patients. Clinical differences between them could provide important insight into the decision of which gene to analyze in affected patients first.

Activation of inwardly rectifying potassium (Kir) channels by phosphatidylinosital-4,5-bisphosphate (PIP2): Interaction with other regulatory ligands

All members of the inwardly rectifying potassium channels (Kir1-7) are regulated by the membrane phospholipid, phosphatidylinosital-4,5-bisphosphate (PIP(2)). Some are also modulated by other regulatory factors or ligands such as ATP and G-proteins, which give them their common names, such as the ATP sensitive potassium (K(ATP)) channel and the G-protein gated potassium channel. Other more non-specific regulators include polyamines, kinases, pH and Na(+) ions. Recent studies have demonstrated that PIP(2) acts cooperatively with other regulatory factors to modulate Kir channels. Here we review how PIP(2) and co-factors modulate channel activities in each subfamily of the Kir channels.

Remodelling of the SUR-Kir6.2 interface of the KATP channel upon ATP binding revealed by the conformational blocker rhodamine 123

ATP-sensitive K+ channels (K(ATP) channels) are metabolic sensors formed by association of a K+ channel, Kir6, and an ATP-binding cassette (ABC) protein, SUR, which allosterically regulates channel gating in response to nucleotides and pharmaceutical openers and blockers. How nucleotide binding to SUR translates into modulation of Kir6 gating remains largely unknown. To address this issue, we have used a novel conformational KATP channel inhibitor, rhodamine 123 (Rho123) which targets the Kir6 subunit in a SUR-dependent manner. Rho123 blocked SUR-less Kir6.2 channels with an affinity of approximately 1 microM, regardless of the presence of nucleotides, but it had no effect on channels formed by the association of Kir6.2 and the N-terminal transmembrane domain TMD0 of SUR. Rho123 blocked SUR + Kir6.2 channels with the same affinity as Kir6.2 but this effect was antagonized by ATP. Protection from Rho123 block by ATP was due to direct binding of ATP to SUR and did not entail hydrolysis because it was not mimicked by AMP, did not require Mg2+ and was reduced by mutations in the nucleotide-binding domains of SUR. These results suggest that Rho123 binds at the TMD0-Kir6.2 interface and that binding of ATP to SUR triggers a change in the structure of the contact zone between Kir6.2 and domain TMD0 of SUR that causes masking of the Rho123 site on Kir6.2.

Diabetes and hypoglycaemia in young children and mutations in the Kir6.2 subunit of the potassium channel: therapeutic consequences

ATP-sensitive potassium channels (K(ATP)) couple cell metabolism to electrical activity by regulating potassium movement across the membrane. These channels are octameric complex with two kind of subunits: four regulatory sulfonylurea receptor (SUR) embracing four poreforming inwardly rectifying potassium channel (Kir). Several isoforms exist for each type of subunits: SUR1 is found in the pancreatic beta-cell and neurons, whereas SUR2A is in heart cells and SUR2B in smooth muscle; Kir6.2 is in the majority of tissues as pancreatic beta-cells, brain, heart and skeletal muscle, and Kir6.1 can be found in smooth vascular muscle and astrocytes. The K(ATP) channels play multiple physiological roles in the glucose metabolism regulation, especially in beta-cells where it regulates insulin secretion, in response to an increase in ATP concentration. They also seem to be critical metabolic sensors in protection against metabolic stress as hypo or hyperglycemia, hypoxia, ischemia. Persistent hyperinsulinemic hypoglycaemia (HI) of infancy is a heterogeneous disorder which may be divided into two histopathological forms (diffuse and focal lesions). Different inactivating mutations have been implicated in both forms: the permanent inactivation of the K(ATP) channels provokes inappropriate insulin secretion, despite low ATP. Diazoxide, used efficiently in certain cases of HI, opens the K(ATP) channels and therefore overpass the mutation effect on the insulin secretion. Conversely, several studies reported sequencing of KCNJ11, coding for Kir6.2, in patients with permanent neonatal diabetes mellitus and found different mutations in 30 to 50% of the cases. More than 28 heterozygous activating mutations have now been identified, the most frequent mutation being in the aminoacid R201. These mutations result in reduced ATP-sensitivity of the K(ATP) channels compared with the wild-types and the level of channel block is responsible for different clinical features: the "mild" form confers isolated permanent neonatal diabetes whereas the severe form combines diabetes and neurological symptoms such as epilepsy, deve-lopmental delay, muscle weakness and mild dimorphic features. Sulfonylureas close K(ATP) channels by binding with high affinity to SUR suggesting they could replace insulin in these patients. Subsequently, more than 50 patients have been reported as successfully and safely switched from subcutaneous insulin injections to oral sulfonylurea therapy, with an improvement in their glycated hemoglobin. We therefore designed a protocol to transfer and evaluate children who have insulin treated neonatal diabetes due to KCNJ11 mutation, from insulin to sulfonylurea. The transfer from insulin injections to oral glibenclamide therapy seems highly effective for most patients and safe. This shows how the molecular understan-ding of some monogenic form of diabetes may lead to an unexpected change of the treatment in children. This is a spectacular example by which a pharmacogenomic approach improves the quality of life of our young diabetic patients in a tremendous way.

Subunit-stoichiometric evidence for kir6.2 channel gating, ATP binding, and binding-gating coupling

ATP-sensitive K(+) channels are gated by intracellular ATP, allowing them to couple intermediary metabolism to cellular excitability, whereas the gating mechanism remains unclear. To understand subunit stoichiometry for the ATP-dependent channel gating, we constructed tandem-multimeric Kir6.2 channels by selective disruption of the binding or gating mechanism in certain subunits. Stepwise disruptions of channel gating caused graded losses in ATP sensitivity and increases in basal P(open), with no effect on maximum ATP inhibition. Prevention of ATP binding lowered the ATP sensitivity and maximum inhibition without affecting basal P(open). The ATP-dependent gating required a minimum of two functional subunits. Two adjacent subunits are more favorable for ATP binding than two diagonal ones. Subunits showed negative cooperativity in ATP binding and positive cooperativity in channel gating. Joint disruptions of the binding and gating mechanisms in the same or alternate subunits of a concatemer revealed that both intra- and intersubunit couplings contributed to channel gating, although the binding-gating coupling preferred the intrasubunit to intersubunit configuration within the C terminus. No such preference was found between the C and N termini. These phenomena are well-described with the operational model used widely for ligand-receptor interactions.

Inhibition of ATP binding to the carboxyl terminus of Kir6.2 by epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs), the cytochrome P450 metabolites of arachidonic acid (AA), are potent and stereospecific activators of cardiac ATP-sensitive K(+)(K(ATP)) channels. EETs activate K(ATP) channels by reducing channel sensitivity to ATP. In this study, we determined the direct effects of EETs on the binding of ATP to K(ATP) channel protein. A fluorescent ATP analog, 2,4,6-trinitrophenyl (TNP)-ATP, which increases its fluorescence emission significantly upon binding with proteins, was used for binding studies with glutathione-S-transferase (GST) Kir6.2 fusion proteins. TNP-ATP bound to GST fusion protein containing the C-terminus of Kir6.2 (GST-Kir6.2C), but not to the N-terminus of Kir6.2, or to GST alone. 11,12-EET (5 muM) did not change TNP-ATP binding K(D) to GST-Kir6.2C, but B(max) was reduced by half. The effect of 11,12-EET was dose-dependent, and 8,9- and 14,15-EETs were as effective as 11,12-EET in inhibiting TNP-ATP binding to GST-Kir6.2C. AA and 11,12-dihydroxyeicosatrienoic acid (11,12-DHET), the parent compound and metabolite of 11,12-EET, respectively, were not effective inhibitors of TNP-ATP binding to GST-Kir6.2C, whereas the methyl ester of 11,12-EET was. These findings suggest that the epoxide group in EETs is important for modulation of ATP binding to Kir6.2. We conclude that EETs bind to the C-terminus of K(ATP) channels, inhibiting binding of ATP to the channel.

ATP-sensitive K+ channels: regulation of bursting by the sulphonylurea receptor, PIP2 and regions of Kir6.2

ATP-sensitive K+ channels composed of the pore-forming protein Kir6.2 and the sulphonylurea receptor SUR1 are inhibited by ATP and activated by Phosphatidylinositol Bisphosphate (PIP2). Residues involved in binding of these ligands to the Kir6.2 cytoplasmic domain have been identified, and it has been hypothesized that gating mechanisms involve conformational changes in the regions of the bundle crossing and/or the selectivity filter of Kir6.2. Regulation of Kir6.2 by SUR1, however, is not well-understood, even though this process is ATP and PIP2 dependent. In this study, we investigated the relationship between channel regulation by SUR1 and PIP2 by comparing a number of single and double mutants known to affect open probability (P(o)), PIP2 affinity, and sulphonylurea and MgADP sensitivity. When coexpressed with SUR1, the Kir6.2 mutant C166A, which is characterized by a P(o) value close to 0.8, exhibits no sulphonylurea or MgADP sensitivity. However, when P(o) was reduced by combining mutations at the PIP2-sensitive residues R176 and R177 with C166A, sulphonylurea and MgADP sensitivities were restored. These effects correlated with a dramatic decrease in PIP2 affinity, as assessed by PIP2-induced channel reactivation and inhibition by neomycin, an antagonist of PIP2 binding. Based on macroscopic and single-channel data, we propose a model in which entry into the high-P(o) bursting state by the C166A mutation or by SUR1 depends on the interaction of PIP2 with R176 and R177 and, to a lesser extent, R54. In conjunction with this PIP2-dependent process, SUR1 also regulates channel activity via a PIP2-independent, but MgADP-dependent process.

ATP sensitivity of ATP-sensitive K+ channels: role of the gamma phosphate group of ATP and the R50 residue of mouse Kir6.2

ATP-sensitive K (K(ATP)) channels are composed of Kir6, the pore-forming protein, and the sulphonylurea receptor SUR, a regulatory protein. We and others have previously shown that positively charged residues in the C terminus of Kir6.2, including R201 and K185, interact with the alpha and beta phosphate groups of ATP, respectively, to induce channel closure. A positively charged residue in the N terminus, R50, is also important, and has been proposed to interact with either the gamma or beta phosphate group of ATP. To examine this issue, we systematically mutated R50 to residues of different size, charge and hydropathy, and examined the effects on adenine nucleotide sensitivity in the absence and presence of SUR1. In the absence of SUR1, only the size of residue 50 significantly altered ATP sensitivity, with smaller side chains decreasing ATP sensitivity. In the presence of SUR1, however, hydrophathy and charge also played a role. Hydrophilic residues decreased ATP sensitivity more than hydrophobic residues for small size residues, and, surprisingly, negatively charged residues E and D preserved ATP sensitivity and increased ADP sensitivity relative to the wild-type residue R. These observations suggest that a negative charge near position 50, due to either mutation of R50 or the interaction of the gamma phosphate group of ATP with R50, facilitates closure of the ATP-dependent gate. Mutation of the nearby positively charged residue R54, known to be involved in stabilizing channel opening via electrostatic interactions with phosphatidylinositol 4,5-bisphosphate (PIP2), also caused increased ADP sensitivity as compared with ATP, suggesting a loss of function of ATP's gamma phosphate. Based on these results, we propose that a phosphate group or a negative charge at position 50 initiates channel closure by destabilizing the electrostatic interactions between negative phosphate groups of PIP2 and residues such as R54.

ATP and sulfonylurea sensitivity of mutant ATP-sensitive K+ channels in neonatal diabetes: implications for pharmacogenomic therapy

The prediction that overactivity of the pancreatic ATP-sensitive K(+) channel (K(ATP) channel) underlies reduced insulin secretion and causes a diabetic phenotype in humans has recently been borne out by genetic studies implicating "activating" mutations in the Kir6.2 subunit of K(ATP) as causal in both permanent and transient neonatal diabetes. Here we characterize the channel properties of Kir6.2 mutations that underlie transient neonatal diabetes (I182V) or more severe forms of permanent neonatal diabetes (V59M, Q52R, and I296L). In all cases, the mutations result in a significant decrease in sensitivity to inhibitory ATP, which correlates with channel "overactivity" in intact cells. Mutations can be separated into those that directly affect ATP affinity (I182V) and those that stabilize the open conformation of the channel and indirectly reduce ATP sensitivity (V59M, Q52R, and I296L). With respect to the latter group, alterations in channel gating are also reflected in a functional "uncoupling" of sulfonylurea (SU) block: SU sensitivity of I182V is similar to that of wild-type mutants, but the SU sensitivity of all gating mutants is reduced, with the I296L mutant being resistant to block by tolbutamide (</=10 mmol/l). These results have important implications for the use of insulinotropic SU drugs as an alternative therapy to insulin injections.

Modeling regulation of cardiac KATP and L-type Ca2+ currents by ATP, ADP, and Mg2+

Changes in cytosolic free Mg(2+) and adenosine nucleotide phosphates affect cardiac excitability and contractility. To investigate how modulation by Mg(2+), ATP, and ADP of K(ATP) and L-type Ca(2+) channels influences excitation-contraction coupling, we incorporated equations for intracellular ATP and MgADP regulation of the K(ATP) current and MgATP regulation of the L-type Ca(2+) current in an ionic-metabolic model of the canine ventricular myocyte. The new model: 1), quantitatively reproduces a dose-response relationship for the effects of changes in ATP on K(ATP) current, 2), simulates effects of ADP in modulating ATP sensitivity of K(ATP) channel, 3), predicts activation of Ca(2+) current during rapid increase in MgATP, and 4), demonstrates that decreased ATP/ADP ratio with normal total Mg(2+) or increased free Mg(2+) with normal ATP and ADP activate K(ATP) current, shorten action potential, and alter ionic currents and intracellular Ca(2+) signals. The model predictions are in agreement with experimental data measured under normal and a variety of pathological conditions.

Novel nucleotide-binding sites in ATP-sensitive potassium channels formed at gating interfaces

The coupling of cell metabolism to membrane electrical activity is a vital process that regulates insulin secretion, cardiac and neuronal excitability and the responses of cells to ischemia. ATP-sensitive potassium channels (K(ATP); Kir6.x) are a major part of this metabolic-electrical coupling system and translate metabolic signals such as the ATP:ADP ratio to changes in the open or closed state (gate) of the channel. The localization of the nucleotide-binding site (NBS) on Kir6.x channels and how nucleotide binding gates these K(ATP) channels remain unclear. Here, we use fluorescent nucleotide binding to purified Kir6.x proteins to define the peptide segments forming the NBS on Kir6.x channels and show that unique N- and C-terminal interactions from adjacent subunits are required for high-affinity nucleotide binding. The short N- and C-terminal segments comprising the novel intermolecular NBS are next to helices that likely move with channel opening/closing, suggesting a lock-and-key model for ligand gating.

Molecular determinants of cardiac K(ATP) channel activation by epoxyeicosatrienoic acids

We have previously reported that epoxyeicosatrienoic acids (EETs), the cytochrome P450 epoxygenase metabolites of arachidonic acid, are potent stereospecific activators of the cardiac K(ATP) channel. The epoxide group in EET is critical for reducing channel sensitivity to ATP, thereby activating the channel. This study is to identify the molecular sites on the K(ATP) channels for EET-mediated activation. We investigated the effects of EETs on Kir6.2delta C26 with or without the coexpression of SUR2A and on Kir6.2 mutants of positively charged residues known to affect channel activity coexpressed with SUR2A in HEK293 cells. The ATP IC50 values were significantly increased in Kir6.2 R27A, R50A, K185A, and R201A but not in R16A, K47A, R54A, K67A, R192A, R195A, K207A, K222A, and R314A mutants. Similar to native cardiac K(ATP) channel, 5 microM 11,12-EET increased the ATP IC50 by 9.6-fold in Kir6.2/SUR2A wild type and 8.4-fold in Kir6.2delta C26. 8,9- and 14,15-EET regioisomers activated the Kir6.2 channel as potently as 11,12-EET. 8,9- and 11,12-EET failed to change the ATP sensitivity of Kir6.2 K185A, R195A, and R201A, whereas their effects were intact in the other mutants. 14,15-EET had a similar effect with K185A and R201A mutants, but instead of R195A, it failed to activate Kir6.2R192A. These results indicate that activation of Kir6.2 by EETs does not require the SUR2A subunit, and the region in the Kir6.2 C terminus from Lys-185 to Arg-201 plays a critical role in EET-mediated Kir6.2 channel activation. Based on computer modeling of the Kir6.2 structure, we infer that the EET-Kir6.2 interaction may allosterically change the ATP binding site on Kir6.2, reducing the channel sensitivity to ATP.

Functional analysis of a structural model of the ATP-binding site of the KATP channel Kir6.2 subunit

ATP-sensitive potassium (KATP) channels couple cell metabolism to electrical activity by regulating K+ flux across the plasma membrane. Channel closure is mediated by ATP, which binds to the pore-forming subunit (Kir6.2). Here we use homology modelling and ligand docking to construct a model of the Kir6.2 tetramer and identify the ATP-binding site. The model is consistent with a large amount of functional data and was further tested by mutagenesis. Ligand binding occurs at the interface between two subunits. The phosphate tail of ATP interacts with R201 and K185 in the C-terminus of one subunit, and with R50 in the N-terminus of another; the N6 atom of the adenine ring interacts with E179 and R301 in the same subunit. Mutation of residues lining the binding pocket reduced ATP-dependent channel inhibition. The model also suggests that interactions between the C-terminus of one subunit and the 'slide helix' of the adjacent subunit may be involved in ATP-dependent gating. Consistent with a role in gating, mutations in the slide helix bias the intrinsic channel conformation towards the open state.

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

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

Concerted gating mechanism underlying KATP channel inhibition by ATP

KATP channels assemble from four regulatory SUR1 and four pore-forming Kir6.2 subunits. At the single-channel current level, ATP-dependent gating transitions between the active burst and the inactive interburst conformations underlie inhibition of the KATP channel by intracellular ATP. Previously, we identified a slow gating mutation, T171A in the Kir6.2 subunit, which dramatically reduces rates of burst to interburst transitions in Kir6.2DeltaC26 channels without SUR1 in the absence of ATP. Here, we constructed all possible mutations at position 171 in Kir6.2DeltaC26 channels without SUR1. Only four substitutions, 171A, 171F, 171H, and 171S, gave rise to functional channels, each increasing Ki,ATP for ATP inhibition by >55-fold and slowing gating to the interburst by >35-fold. Moreover, we investigated the role of individual Kir6.2 subunits in the gating by comparing burst to interburst transition rates of channels constructed from different combinations of slow 171A and fast T171 "wild-type" subunits. The relationship between gating transition rate and number of slow subunits is exponential, which excludes independent gating models where any one subunit is sufficient for inhibition gating. Rather, our results support mechanisms where four ATP sites independently can control a single gate formed by the concerted action of all four Kir6.2 subunit inner helices of the KATP channel.

Activating mutations in the KCNJ11 gene encoding the ATP-sensitive K+ channel subunit Kir6.2 are rare in clinically defined type 1 diabetes diagnosed before 2 years

We have recently shown that permanent neonatal diabetes can be caused by activating mutations in KCNJ11 that encode the Kir6.2 subunit of the beta-cell ATP-sensitive K(+) channel. Some of these patients were diagnosed after 3 months of age and presented with ketoacidosis and marked hyperglycemia, which could have been diagnosed as type 1 diabetes. We hypothesized that KCNJ11 mutations could present clinically as type 1 diabetes. We screened the KCNJ11 gene for mutations in 77 U.K. type 1 diabetic subjects diagnosed under the age of 2 years. One patient was found to be heterozygous for the missense mutation R201C. She had low birth weight, was diagnosed at 5 weeks, and did not have a high risk predisposing HLA genotype. A novel variant, R176C, was identified in one diabetic subject but did not cosegregate with diabetes within the family. In conclusion, we have shown that heterozygous activating mutations in the KCNJ11 gene are a rare cause of clinically defined type 1 diabetes diagnosed before 2 years. Although activating KCNJ11 mutations are rare in patients diagnosed with type 1 diabetes, the identification of a KCNJ11 mutation may have important treatment implications.

Permanent neonatal diabetes due to paternal germline mosaicism for an activating mutation of the KCNJ11 Gene encoding the Kir6.2 subunit of the beta-cell potassium adenosine triphosphate channel

Activating mutations in the KCNJ11 gene encoding for the Kir6.2 subunit of the beta-cell ATP-sensitive potassium channel have recently been shown to be a common cause of permanent neonatal diabetes. In 80% of probands, these are isolated cases resulting from de novo mutations. We describe a family in which two affected paternal half-siblings were found to be heterozygous for the previously reported R201C mutation. Direct sequencing of leukocyte DNA showed that their clinically unaffected mothers and father were genotypically normal. Quantitative real-time PCR analysis of the father's leukocyte DNA detected no trace of mutant DNA. These results are consistent with the father being a mosaic for the mutation, which is restricted to his germline. This is the first report of germline mosaicism in any form of monogenic diabetes. The high percentage of permanent neonatal diabetes cases due to de novo KCNJ11 mutations suggests that germline mosaicism may be common. The possibility of germline mosaicism should be considered when counseling recurrence risks for the parents of a child with an apparently de novo KCNJ11 activating mutation.

Long-chain acyl-CoA esters and phosphatidylinositol phosphates modulate ATP inhibition of KATP channels by the same mechanism

Phosphatidylinositol phosphates (PIPs, e.g. PIP2) and long-chain acyl-CoA esters (e.g. oleoyl-CoA) are potent activators of KATP channels that are thought to link KATP channel activity to the cellular metabolism of PIPs and fatty acids. Here we show that the two types of lipid act by the same mechanism: oleoyl-CoA potently reduced the ATP sensitivity of cardiac (Kir6.2/SUR2A) and pancreatic (Kir6.2/SUR1) KATP channels in a way very similar to PIP2. Mutations (R54Q, R176A) in the C- and N-terminus of Kir6.2 that greatly reduced the PIP2 modulation of ATP sensitivity likewise reduced the modulation by oleoyl-CoA, indicating that the two lipids interact with the same site. Polyvalent cations reduced the effect of oleoyl-CoA and PIP2 on the ATP sensitivity with similar potency suggesting that electrostatic interactions are of similar importance. However, experiments with differently charged inhibitory adenosine phosphates (ATP4-, ADP3- and 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-monophosphate (TNP-AMP2-)) and diadenosine tetraphosphate (Ap4A5-) ruled out a mechanism where oleoyl-CoA or PIP2 attenuate ATP inhibition by reducing ATP binding through electrostatic repulsion. Surprisingly, CoA (the head group of oleoyl-CoA) did not activate but inhibited KATP channels (IC50 = 265 +/- 33 muM). We provide evidence that CoA and diadenosine polyphosphates (e.g. Ap4A) are ligands of the inhibitory ATP-binding site on Kir6.2.

Activating mutations in the gene encoding the ATP-sensitive potassium-channel subunit Kir6.2 and permanent neonatal diabetes

BACKGROUND

Patients with permanent neonatal diabetes usually present within the first three months of life and require insulin treatment. In most, the cause is unknown. Because ATP-sensitive potassium (K(ATP)) channels mediate glucose-stimulated insulin secretion from the pancreatic beta cells, we hypothesized that activating mutations in the gene encoding the Kir6.2 subunit of this channel (KCNJ11) cause neonatal diabetes.

METHODS

We sequenced the KCNJ11 gene in 29 patients with permanent neonatal diabetes. The insulin secretory response to intravenous glucagon, glucose, and the sulfonylurea tolbutamide was assessed in patients who had mutations in the gene.

RESULTS

Six novel, heterozygous missense mutations were identified in 10 of the 29 patients. In two patients the diabetes was familial, and in eight it arose from a spontaneous mutation. Their neonatal diabetes was characterized by ketoacidosis or marked hyperglycemia and was treated with insulin. Patients did not secrete insulin in response to glucose or glucagon but did secrete insulin in response to tolbutamide. Four of the patients also had severe developmental delay and muscle weakness; three of them also had epilepsy and mild dysmorphic features. When the most common mutation in Kir6.2 was coexpressed with sulfonylurea receptor 1 in Xenopus laevis oocytes, the ability of ATP to block mutant K(ATP) channels was greatly reduced.

CONCLUSIONS

Heterozygous activating mutations in the gene encoding Kir6.2 cause permanent neonatal diabetes and may also be associated with developmental delay, muscle weakness, and epilepsy. Identification of the genetic cause of permanent neonatal diabetes may facilitate the treatment of this disease with sulfonylureas.

Molecular mechanism for ATP-dependent closure of the K+ channel Kir6.2

In the ATP-dependent K+ (KATP) channel pore-forming protein Kir6.2, mutation of three positively charged residues, R50, K185 and R201, impairs the ability of ATP to close the channel. The mutations do not change the channel open probability (Po) in the absence of ATP, supporting the involvement of these residues in ATP binding. We recently proposed that at least two of these positively charged residues, K185 and R201, interact with ATP phosphate groups to cause channel closure: the beta phosphate group of ATP interacts with K185 to initiate closure, while the alpha phosphate interacts with R201 to stabilize the channel's closed state. In the present study we replaced these three positive residues with residues of different charge, size and hydropathy. For K185 and R201, we found that charge, more than any other property, controls the interaction of ATP with Kir6.2. At these positions, replacement with another positive residue had minor effects on ATP sensitivity. In contrast, replacement of K185 with a negative residue (K185D/E) decreased ATP sensitivity much more than neutral substitutions, suggesting that an electrostatic interaction between the beta phosphate group of ATP and K185 destabilizes the open state of the channel. At R201, replacement with a negative charge (R201E) had multiple effects, decreasing ATP sensitivity and preventing full channel closure at high concentrations. In contrast, the R50E mutation had a modest effect on ATP sensitivity, and only residues such as proline and glycine that affect protein structure caused major decreases in ATP sensitivity at the R50 position. Based on these results and the recently published structure of Kir3.1 cytoplasmic domain, we propose a scheme where binding of the beta phosphate group of ATP to K185 induces a motion of the surrounding region, which destabilizes the open state, favouring closure of the M2 gate. Binding of the alpha phosphate group of ATP to R201 then stabilizes the closed state. R50 on the N-terminus controls ATP binding by facilitating the interaction of the beta phosphate group of ATP with K185 to destabilize the open state.

Identification of residues contributing to the ATP binding site of Kir6.2

The ATP-sensitive potassium (K(ATP)) channel links cell metabolism to membrane excitability. Intracellular ATP inhibits channel activity by binding to the Kir6.2 subunit of the channel, but the ATP binding site is unknown. Using cysteine-scanning mutagenesis and charged thiol-modifying reagents, we identified two amino acids in Kir6.2 that appear to interact directly with ATP: R50 in the N-terminus, and K185 in the C-terminus. The ATP sensitivity of the R50C and K185C mutant channels was increased by a positively charged thiol reagent (MTSEA), and was reduced by the negatively charged reagent MTSES. Comparison of the inhibitory effects of ATP, ADP and AMP after thiol modification suggests that K185 interacts primarily with the beta-phosphate, and R50 with the gamma-phosphate, of ATP. A molecular model of the C-terminus of Kir6.2 (based on the crystal structure of Kir3.1) was constructed and automated docking was used to identify residues interacting with ATP. These results support the idea that K185 interacts with the beta-phosphate of ATP. Thus both N- and C-termini may contribute to the ATP binding site.