Truncation of Kir6.2 produces ATP-sensitive K+ channels in the absence of the sulphonylurea receptor

Successful development and use of a thermodynamic stability screen for optimizing the yield of nucleotide binding domains

ATP sensitive potassium (KATP) channels consist of four copies of a pore-forming inward rectifying potassium channel (Kir6.1 or Kir6.2) and four copies of a sulfonylurea receptor (SUR1, SUR2A, or SUR2B). SUR proteins are members of the ATP-binding cassette superfamily of proteins. Binding of ATP to the Kir6.x subunit mediates channel inhibition, whereas MgATP binding and hydrolysis at the SUR NBDs results in channel opening. Mutations in SUR1 and SUR2A NBDs cause diseases of insulin secretion and cardiac disorders, respectively, underlying the importance of studying the NBDs. Although purification of SUR2A NBD1 in a soluble form is possible, the lack of long-term sample stability of the protein in a concentrated form has precluded detailed studies of the protein aimed at gaining a molecular-level understanding of how SUR mutations cause disease. Here we use a convenient and cost-effective thermodynamic screening method to probe stabilizing conditions for SUR2A NBD1. Results from the screen are used to alter the purification protocol to allow for significantly increased yields of the purified protein. In addition, the screen provides strategies for long-term storage of NBD1 and generating NBD1 samples at high concentrations suitable for NMR studies. NMR spectra of NBD1 with MgAMP-PNP are of higher quality compared to using MgATP, indicating that MgAMP-PNP be used as the ligand in future NMR studies. The screen presented here can be expanded to using different additives and can be employed to enhance purification yields, sample life times, and storage of other low stability nucleotide binding domains, such as GTPases.

Imaging energy status in live cells with a fluorescent biosensor of the intracellular ATP-to-ADP ratio

The ATP:ADP ratio is a critical parameter of cellular energy status that regulates many metabolic activities. Here we report an optimized genetically-encoded fluorescent biosensor, PercevalHR, that senses the ATP:ADP ratio. PercevalHR is tuned to the range of intracellular ATP:ADP expected in mammalian cells, and it can be used with one- or two-photon microscopy in live samples. We use PercevalHR to visualize activity-dependent changes in ATP:ADP when neurons are exposed to multiple stimuli, demonstrating that it is a sensitive reporter of physiological changes in energy consumption and production. We also use PercevalHR to visualize intracellular ATP:ADP while simultaneously recording currents from ATP-sensitive potassium (K_ATP) channels in single cells, showing that PercevalHR enables the study of coordinated variation in ATP:ADP and K_ATP channel open probability in intact cells. With its ability to monitor changes in cellular energetics within seconds, PercevalHR should be a versatile tool for metabolic research.

Tuning the electrical properties of the heart by differential trafficking of KATP ion channel complexes

The copy number of membrane proteins at the cell surface is tightly regulated. Many ion channels and receptors present retrieval motifs to COPI vesicle coats and are retained in the early secretory pathway. In some cases, the interaction with COPI is prevented by binding to 14-3-3 proteins. However, the functional significance of this antagonism between COPI and 14-3-3 in terminally differentiated cells is unknown. Here, we show that ATP-sensitive K⁺ (K_ATP) channels, which are composed of Kir6.2 and SUR1 subunits, are stalled in the Golgi complex of ventricular, but not atrial, cardiomyocytes. Upon sustained β-adrenergic stimulation, which leads to activation of protein kinase A (PKA), SUR1-containing channels reach the plasma membrane of ventricular cells. We show that PKA-dependent phosphorylation of the C-terminus of Kir6.2 decreases binding to COPI and, thereby, silences the arginine-based retrieval signal. Thus, activation of the sympathetic nervous system releases this population of K_ATP channels from storage in the Golgi and, hence, might facilitate the adaptive response to metabolic challenges.

Acute exposure of methylglyoxal leads to activation of KATP channels expressed in HEK293 cells

AIM

Highly reactive carbonyl methylglyoxal (MGO) is one of the metabolites excessively produced in diabetes. We have showed that prolonged exposure of vascular smooth muscle cells to MGO leads to instability of the mRNA encoding ATP-sensitive potassium (KATP) channel. In the present study we investigated the effects of MGO on the activity of KATP channels.

METHODS

Kir6.1/ SUR2B, Kir6.2/SUR2B or Kir6.2Δ36 (a truncated Kir6.2 isoform) alone was expressed in HEK293 cells. Whole-cell currents were recorded in the cells with an Axopatch 200B amplifier. Macroscopic currents and single-channel currents were recorded in giant inside-out patches and normal inside-out patches, respectively. Data were analyzed using Clampfit 9 software.

RESULTS

The basal activity of Kir6.1/SUR2B channels was low. The specific KATP channel opener pinacidil (10 μmol/L) could fully activate Kir6.1/SUR2B channels, which was inhibited by the specific KATP channel blocker glibenclamide (10 μmol/L). MGO (0.1-10 mmol/L) dose-dependently activated Kir6.1/SUR2B channels with an EC50 of 1.7 mmol/L. The activation of Kir6.1/SUR2B channels by MGO was reversible upon washout, and could be inhibited completely by glibenclamide. Kir6.2Δ36 channels expressed in HEK293 cells could open automatically, and the channel activity was enhanced in the presence of MGO (3 mmol/L). Single channel recordings showed that MGO (3 mmol/L) markedly increased the open probability of Kir6.1/SUR2B channels, leaving the channel conductance unaltered.

CONCLUSION

Acute application of MGO activates KATP channels through direct, non-covalent and reversible interactions with the Kir6 subunits.

Chronic exposure to valproic acid promotes insulin release, reduces KATP channel current and does not affect Ca (2+) signaling in mouse islets

Hyperinsulinemia is one of the reported side effects of valproic acid (VPA), a medicine used to treat epilepsy. However, its underlying mechanism remains unknown. The present study was designed to investigate a direct effect of VPA on insulin secretion by using mouse pancreactic islets and β-cells. VPA had no acute effect on insulin secretion from islets, or on cytosolic Ca(2+) ([Ca(2+)]i) in single β-cells. However, following long-term exposure to VPA (48 h), both basal and glucose-stimulated insulin secretion were markedly elevated (5-fold), while the insulin gene expression level was unaltered. Following long-term exposure to VPA, β-cells showed a decrease in whole cell KATP channel current. However, the increase in [Ca(2+)]i in response to the sulfonylurea drug, tolbutamide was attenuated. The present study shows that VPA has no acute effects, but long-term treatment results in enhancement of both basal and glucose-stimulated insulin secretion. This long-term effect may mediate the KATP channel, while VPA can also attenuate the effect of the KATP channel blocker tolbutamide.

Pharmacological rescue of trafficking-impaired ATP-sensitive potassium channels

ATP-sensitive potassium (K_ATP) channels link cell metabolism to membrane excitability and are involved in a wide range of physiological processes including hormone secretion, control of vascular tone, and protection of cardiac and neuronal cells against ischemic injuries. In pancreatic β-cells, K_ATP channels play a key role in glucose-stimulated insulin secretion, and gain or loss of channel function results in neonatal diabetes or congenital hyperinsulinism, respectively. The β-cell K_ATP channel is formed by co-assembly of four Kir6.2 inwardly rectifying potassium channel subunits encoded by KCNJ11 and four sulfonylurea receptor 1 subunits encoded by ABCC8. Many mutations in ABCC8 or KCNJ11 cause loss of channel function, thus, congenital hyperinsulinism by hampering channel biogenesis and hence trafficking to the cell surface. The trafficking defects caused by a subset of these mutations can be corrected by sulfonylureas, K_ATP channel antagonists that have long been used to treat type 2 diabetes. More recently, carbamazepine, an anticonvulsant that is thought to target primarily voltage-gated sodium channels has been shown to correct K_ATP channel trafficking defects. This article reviews studies to date aimed at understanding the mechanisms by which mutations impair channel biogenesis and trafficking and the mechanisms by which pharmacological ligands overcome channel trafficking defects. Insight into channel structure-function relationships and therapeutic implications from these studies are discussed.

Functional assay for T4 lysozyme-engineered G protein-coupled receptors with an ion channel reporter

Structural studies of G protein-coupled receptors (GPCRs) extensively use the insertion of globular soluble protein domains to facilitate their crystallization. However, when inserted in the third intracellular loop (i3 loop), the soluble protein domain disrupts their coupling to G proteins and impedes the GPCRs functional characterization by standard G protein-based assays. Therefore, activity tests of crystallization-optimized GPCRs are essentially limited to their ligand binding properties using radioligand binding assays. Functional characterization of additional thermostabilizing mutations requires the insertion of similar mutations in the wild-type receptor to allow G protein-activation tests. We demonstrate that ion channel-coupled receptor technology is a complementary approach for a comprehensive functional characterization of crystallization-optimized GPCRs and potentially of any engineered GPCR. Ligand-induced conformational changes of the GPCRs are translated into electrical signal and detected by simple current recordings, even though binding of G proteins is sterically blocked by the added soluble protein domain.

A mouse model of human hyperinsulinism produced by the E1506K mutation in the sulphonylurea receptor SUR1

Loss-of-function mutations in the KATP channel genes KCNJ11 and ABCC8 cause neonatal hyperinsulinism in humans. Dominantly inherited mutations cause less severe disease, which may progress to glucose intolerance and diabetes in later life (e.g., SUR1-E1506K). We generated a mouse expressing SUR1-E1506K in place of SUR1. KATP channel inhibition by MgATP was enhanced in both homozygous (homE1506K) and heterozygous (hetE1506K) mutant mice, due to impaired channel activation by MgADP. As a consequence, mutant β-cells showed less on-cell KATP channel activity and fired action potentials in glucose-free solution. HomE1506K mice exhibited enhanced insulin secretion and lower fasting blood glucose within 8 weeks of birth, but reduced insulin secretion and impaired glucose tolerance at 6 months of age. These changes correlated with a lower insulin content; unlike wild-type or hetE1506K mice, insulin content did not increase with age in homE1506K mice. There was no difference in the number and size of islets or β-cells in the three types of mice, or evidence of β-cell proliferation. We conclude that the gradual development of glucose intolerance in patients with the SUR1-E1506K mutation might, as in the mouse model, result from impaired insulin secretion due a failure of insulin content to increase with age.

K(ATP) channels and islet hormone secretion: new insights and controversies

ATP-sensitive potassium channels (K(ATP) channels) link cell metabolism to electrical activity by controlling the cell membrane potential. They participate in many physiological processes but have a particularly important role in systemic glucose homeostasis by regulating hormone secretion from pancreatic islet cells. Glucose-induced closure of K(ATP) channels is crucial for insulin secretion. Emerging data suggest that K(ATP) channels also play a key part in glucagon secretion, although precisely how they do so remains controversial. This Review highlights the role of K(ATP) channels in insulin and glucagon secretion. We discuss how K(ATP) channels might contribute not only to the initiation of insulin release but also to the graded stimulation of insulin secretion that occurs with increasing glucose concentrations. The various hypotheses concerning the role of K(ATP) channels in glucagon release are also reviewed. Furthermore, we illustrate how mutations in K(ATP) channel genes can cause hyposecretion or hypersecretion of insulin, as in neonatal diabetes mellitus and congenital hyperinsulinism, and how defective metabolic regulation of the channel may underlie the hypoinsulinaemia and the hyperglucagonaemia that characterize type 2 diabetes mellitus. Finally, we outline how sulphonylureas, which inhibit K(ATP) channels, stimulate insulin secretion in patients with neonatal diabetes mellitus or type 2 diabetes mellitus, and suggest their potential use to target the glucagon secretory defects found in diabetes mellitus.

Molecular mechanism of sulphonylurea block of K(ATP) channels carrying mutations that impair ATP inhibition and cause neonatal diabetes

Sulphonylurea drugs are the therapy of choice for treating neonatal diabetes (ND) caused by mutations in the ATP-sensitive K(+) channel (KATP channel). We investigated the interactions between MgATP, MgADP, and the sulphonylurea gliclazide with KATP channels expressed in Xenopus oocytes. In the absence of MgATP, gliclazide block was similar for wild-type channels and those carrying the Kir6.2 ND mutations R210C, G334D, I296L, and V59M. Gliclazide abolished the stimulatory effect of MgATP on all channels. Conversely, high MgATP concentrations reduced the gliclazide concentration, producing a half-maximal block of G334D and R201C channels and suggesting a mutual antagonism between nucleotide and gliclazide binding. The maximal extent of high-affinity gliclazide block of wild-type channels was increased by MgATP, but this effect was smaller for ND channels; channels that were least sensitive to ATP inhibition showed the smallest increase in sulphonylurea block. Consequently, G334D and I296L channels were not fully blocked, even at physiological MgATP concentrations (1 mmol/L). Glibenclamide block was also reduced in β-cells expressing Kir6.2-V59M channels. These data help to explain why patients with some mutations (e.g., G334D, I296L) are insensitive to sulphonylurea therapy, why higher drug concentrations are needed to treat ND than type 2 diabetes, and why patients with severe ND mutations are less prone to drug-induced hypoglycemia.

ATP-sensitive potassium currents from channels formed by Kir6 and a modified cardiac mitochondrial SUR2 variant

Cardiac ATP-sensitive potassium channels (KATP) are found in both the sarcoplasmic reticulum (sarcKATP) and the inner membrane of mitochondria (mitoKATP). SarcKATP are composed of a pore containing subunit Kir6.2 and a regulatory sulfonylurea receptor subunit (SUR2), but the composition of mitoKATP remains unclear. An unusual intra-exonic splice variant of SUR2 (SUR2A-55) was previously identified in mitochondria of mammalian heart and brain, and by analogy with sarcKATP we proposed SUR2A-55 as a candidate regulatory subunit of mitoKATP. Although SUR2A-55 lacks the first nucleotide binding domain (NBD) and 2 transmembrane domains (TMD), it has a hybrid TMD and retains the second NBD. It resembles a hemi-ABC transporter suggesting it could multimerize to function as a regulatory subunit. A putative mitochondrial targeting signal in the N-terminal domain of SUR2A-55 was removed by truncation and when co-expressed with Kir6.1 and Kir6.2 it targeted to the plasma membrane and yielded KATP currents. Single channel conductance, mean open time, and burst open time of SUR2A-55 based KATP was similar to the full-length SUR2A based KATP. However, the SUR2A-55 KATP were 70-fold less sensitive to block by ATP, and twice as resistant to intracellular Ca (2+) inhibition compared with the SUR2A KATP, and were markedly insensitive to KATP drugs, pinacidil, diazoxide, and glybenclamide. These results suggest that the SUR2A-55 based channels would tend to be open under physiological conditions and in ischemia, and could account for cardiac and mitochondrial phenotypes protective for ischemia.

Leptin promotes K(ATP) channel trafficking by AMPK signaling in pancreatic β-cells

Leptin is a pivotal regulator of energy and glucose homeostasis, and defects in leptin signaling result in obesity and diabetes. The ATP-sensitive potassium (K(ATP)) channels couple glucose metabolism to insulin secretion in pancreatic β-cells. In this study, we provide evidence that leptin modulates pancreatic β-cell functions by promoting K(ATP) channel translocation to the plasma membrane via AMP-activated protein kinase (AMPK) signaling. K(ATP) channels were localized mostly to intracellular compartments of pancreatic β-cells in the fed state and translocated to the plasma membrane in the fasted state. This process was defective in leptin-deficient ob/ob mice, but restored by leptin treatment. We discovered that the molecular mechanism of leptin-induced AMPK activation involves canonical transient receptor potential 4 and calcium/calmodulin-dependent protein kinase kinase β. AMPK activation was dependent on both leptin and glucose concentrations, so at optimal concentrations of leptin, AMPK was activated sufficiently to induce K(ATP) channel trafficking and hyperpolarization of pancreatic β-cells in a physiological range of fasting glucose levels. There was a close correlation between phospho-AMPK levels and β-cell membrane potentials, suggesting that AMPK-dependent K(ATP) channel trafficking is a key mechanism for regulating β-cell membrane potentials. Our results present a signaling pathway whereby leptin regulates glucose homeostasis by modulating β-cell excitability.

Mouse Models of β-cell KATP Channel Dysfunction

ATP-sensitive K⁺ (K_ATP) channels in pancreatic β-cells couple glucose metabolism to insulin secretion. Reduced K_ATP channel activity produces excessive insulin release and hyperinsulinism whereas increased K_ATP channel activity leads to lower insulin secretion and diabetes. Paradoxically, mice with genetic deletion of K_ATP channels, or loss-of-function mutations, are only transiently hypoglycaemic during the neonatal period and often display reduced glucose-stimulated insulin secretion subsequently. Mice with K_ATP channel gain-of-function mutations are hyperglycaemic and have impaired glucose-stimulated insulin secretion, a phenotype that accurately mimics human diabetes. This review discusses how mice expressing altered K_ATP channels have provided valuable insight into β-cell function.

Targeting microglial K(ATP) channels to treat neurodegenerative diseases: a mitochondrial issue

Neurodegeneration is a complex process involving different cell types and neurotransmitters. A common characteristic of neurodegenerative disorders is the occurrence of a neuroinflammatory reaction in which cellular processes involving glial cells, mainly microglia and astrocytes, are activated in response to neuronal death. Microglia do not constitute a unique cell population but rather present a range of phenotypes closely related to the evolution of neurodegeneration. In a dynamic equilibrium with the lesion microenvironment, microglia phenotypes cover from a proinflammatory activation state to a neurotrophic one directly involved in cell repair and extracellular matrix remodeling. At each moment, the microglial phenotype is likely to depend on the diversity of signals from the environment and of its response capacity. As a consequence, microglia present a high energy demand, for which the mitochondria activity determines the microglia participation in the neurodegenerative process. As such, modulation of microglia activity by controlling microglia mitochondrial activity constitutes an innovative approach to interfere in the neurodegenerative process. In this review, we discuss the mitochondrial K_ATP channel as a new target to control microglia activity, avoid its toxic phenotype, and facilitate a positive disease outcome.

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.

Neonatal diabetes caused by activating mutations in the sulphonylurea receptor

Adenosine triphosphate (ATP)-sensitive potassium (KATP) channels in pancreatic β-cells play a crucial role in insulin secretion and glucose homeostasis. These channels are composed of two subunits: a pore-forming subunit (Kir6.2) and a regulatory subunit (sulphonylurea receptor-1). Recent studies identified large number of gain of function mutations in the regulatory subunit of the channel which cause neonatal diabetes. Majority of mutations cause neonatal diabetes alone, however some lead to a severe form of neonatal diabetes with associated neurological complications. This review focuses on the functional effects of these mutations as well as the implications for treatment.

Engineered Kir6.2 mutations that correct the trafficking defect of K(ATP) channels caused by specific SUR1 mutations

KATP channels consisting of Kir6.2 and SUR1 couple cell metabolism to membrane excitability and regulate insulin secretion. The molecular interactions between SUR1 and Kir6.2 that govern channel gating and biogenesis are incompletely understood. In a recent study, we showed that a SUR1 and Kir6.2 mutation pair, E203K-SUR1 and Q52E-Kir6.2, at the SUR1/Kir6.2 interface near the plasma membrane increases the ATP-sensitivity of the channel by nearly 100-fold. Here, we report the finding that the same mutation pair also suppresses channel folding/trafficking defects caused by select SUR1 mutations in the first transmembrane domain of SUR1. Analysis of the contributions from individual mutations, however, revealed that the correction effect is attributed largely to Q52E-Kir6.2 alone. Moreover, the correction is dependent on the negative charge of the substituting amino acid at the Q52 position in Kir6.2. Our study demonstrates for the first time that engineered mutations in Kir6.2 can correct the biogenesis defect caused by specific mutations in the SUR1 subunit.

A Kir6.2 pore mutation causes inactivation of ATP-sensitive potassium channels by disrupting PIP2-dependent gating

In the absence of intracellular nucleotides, ATP-sensitive potassium (KATP) channels exhibit spontaneous activity via a phosphatidylinositol-4,5-bisphosphate (PIP2)-dependent gating process. Previous studies show that stability of this activity requires subunit-subunit interactions in the cytoplasmic domain of Kir6.2; selective mutagenesis and disease mutations at the subunit interface result in time-dependent channel inactivation. Here, we report that mutation of the central glycine in the pore-lining second transmembrane segment (TM2) to proline in Kir6.2 causes KATP channel inactivation. Unlike C-type inactivation, a consequence of selectivity filter closure, in many K(+) channels, the rate of inactivation in G156P channels was insensitive to changes in extracellular ion concentrations or ion species fluxing through the pore. Instead, the rate of G156P inactivation decreased with exogenous application of PIP2 and increased when PIP2-channel interaction was inhibited with neomycin or poly-L-lysine. These findings indicate the G156P mutation reduces the ability of PIP2 to stabilize the open state of KATP channels, similar to mutations in the cytoplasmic domain that produce inactivation. Consistent with this notion, when PIP2-dependent open state stability was substantially increased by addition of a second gain-of-function mutation, G156P inactivation was abolished. Importantly, bath application and removal of Mg(2+)-free ATP or a nonhydrolyzable analog of ATP, which binds to the cytoplasmic domain of Kir6.2 and causes channel closure, recover G156P channel from inactivation, indicating crosstalk between cytoplasmic and transmembrane domains. The G156P mutation provides mechanistic insight into the structural and functional interactions between the pore and cytoplasmic domains of Kir6.2 during gating.

Tetrameric structure of SUR2B revealed by electron microscopy of oriented single particles

The ATP-sensitive potassium (K_ATP) channel is a hetero-octameric complex that links cell metabolism to membrane electrical activity in many cells, thereby controlling physiological functions such as insulin release, muscle contraction and neuronal activity. It consists of four pore-forming Kir6.2 and four regulatory sulfonylurea receptor (SUR) subunits. SUR2B serves as the regulatory subunit in smooth muscle and some neurones. An integrative approach, combining electron microscopy and homology modelling, has been used to obtain information on the structure of this large (megadalton) membrane protein complex. Single-particle electron microscopy of purified SUR2B tethered to a lipid monolayer revealed that it assembles as a tetramer of four SUR2B subunits surrounding a central hole. In the absence of an X-ray structure, a homology model for SUR2B based on the X-ray structure of the related ABC transporter Sav1866 was used to fit the experimental images. The model indicates that the central hole can readily accommodate the transmembrane domains of the Kir tetramer, suggests a location for the first transmembrane domains of SUR2B (which are absent in Sav1866) and suggests the relative orientation of the SUR and Kir6.2 subunits.

Rosiglitazone selectively inhibits K(ATP) channels by acting on the K(IR) 6 subunit

BACKGROUND AND PURPOSE

Rosiglitazone is an anti-diabetic drug acting as an insulin sensitizer. We recently found that rosiglitazone also inhibits the vascular isoform of ATP-sensitive K(+) channels and compromises vasodilatory effects of β-adrenoceptor activation and pinacidil. As its potency for the channel inhibition is in the micromolar range, rosiglitazone may be used as an effective K(ATP) channel inhibitor for research and therapeutic purposes. Therefore, we performed experiments to determine whether other isoforms of K(ATP) channels are also sensitive to rosiglitazone and what their sensitivities are.

EXPERIMENTAL APPROACH

K(IR) 6.1/SUR2B, K(IR) 6.2/SUR1, K(IR) 6.2/SUR2A, K(IR) 6.2/SUR2B and K(IR) 6.2ΔC36 channels were expressed in HEK293 cells and were studied using patch-clamp techniques.

KEY RESULTS

Rosiglitazone inhibited all isoforms of K(ATP) channels in excised patches and in the whole-cell configuration. Its IC(50) was 10 µmol·L(-1) for the K(IR) 6.1/SUR2B channel and ∼45 µmol·L(-1) for K(IR) 6.2/SURx channels. Rosiglitazone also inhibited K(IR) 6.2ΔC36 channels in the absence of the sulphonylurea receptor (SUR) subunit, with potency (IC(50) = 45 µmol·L(-1) ) almost identical to that for K(IR) 6.2/SURx channels. Single-channel kinetic analysis showed that the channel inhibition was mediated by augmentation of the long-lasting closures without affecting the channel open state and unitary conductance. In contrast, rosiglitazone had no effect on K(IR) 1.1, K(IR) 2.1 and K(IR) 4.1 channels, suggesting that the channel inhibitory effect is selective for K(IR) 6.x channels.

CONCLUSIONS AND IMPLICATIONS

These results suggest a novel K(ATP) channel inhibitor that acts on the pore-forming K(IR) 6.x subunit, affecting the channel gating.

The sulfonylurea receptor 1 (Sur1)-transient receptor potential melastatin 4 (Trpm4) channel

The sulfonylurea receptor 1 (Sur1)-NC_Ca-ATP channel plays a central role in necrotic cell death in central nervous system (CNS) injury, including ischemic stroke, and traumatic brain and spinal cord injury. Here, we show that Sur1-NC_Ca-ATP channels are formed by co-assembly of Sur1 and transient receptor potential melastatin 4 (Trpm4). Co-expression of Sur1 and Trpm4 yielded Sur1-Trpm4 heteromers, as shown in experiments with Förster resonance energy transfer (FRET) and co-immunoprecipitation. Co-expression of Sur1 and Trpm4 also yielded functional Sur1-Trpm4 channels with biophysical properties of Trpm4 and pharmacological properties of Sur1. Co-assembly with Sur1 doubled the affinity of Trpm4 for calmodulin and doubled its sensitivity to intracellular calcium. Experiments with FRET and co-immunoprecipitation showed de novo appearance of Sur1-Trpm4 heteromers after spinal cord injury in rats. Our findings depart from the long-held view of an exclusive association between Sur1 and K_ATP channels and reveal an unexpected molecular partnership with far-ranging implications for CNS injury.

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.

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.

Engineered interaction between SUR1 and Kir6.2 that enhances ATP sensitivity in KATP channels

The ATP-sensitive potassium (K_ATP) channel consisting of the inward rectifier Kir6.2 and SUR1 (sulfonylurea receptor 1) couples cell metabolism to membrane excitability and regulates insulin secretion. Inhibition by intracellular ATP is a hallmark feature of the channel. ATP sensitivity is conferred by Kir6.2 but enhanced by SUR1. The mechanism by which SUR1 increases channel ATP sensitivity is not understood. In this study, we report molecular interactions between SUR1 and Kir6.2 that markedly alter channel ATP sensitivity. Channels bearing an E203K mutation in SUR1 and a Q52E in Kir6.2 exhibit ATP sensitivity ∼100-fold higher than wild-type channels. Cross-linking of E203C in SUR1 and Q52C in Kir6.2 locks the channel in a closed state and is reversible by reducing agents, demonstrating close proximity of the two residues. Our results reveal that ATP sensitivity in K_ATP channels is a dynamic parameter dictated by interactions between SUR1 and Kir6.2.

A universally conserved residue in the SUR1 subunit of the KATP channel is essential for translating nucleotide binding at SUR1 into channel opening

The sulphonylurea receptor (SUR1) subunit of the ATP-sensitive potassium (KATP) channel is a member of the ATP-binding cassette (ABC) protein family. Binding of MgADP to nucleotide-binding domain 2 (NBD2) is critical for channel activation.We identified a residue in NBD2 (G1401) that is fully conserved among ABC proteins and whose functional importance is unknown. Homology modelling places G1401 on the outer surface of the protein, distant from the nucleotide-binding site. The ATPase activity of purified SUR1-NBD2-G1410R (bound to maltose-binding protein) was slightly inhibited when compared to the wild-type protein, but its inhibition by MgADP was unchanged, indicating that MgADP binding is not altered. However, MgADP activation of channel activity was abolished. This implies that the G1401R mutation impairs the mechanism by which MgADP binding to NBD2 is translated into opening of the KATP channel pore. The location of G1401 would be consistent with interaction of this residue with the pore-forming Kir6.2 subunit. Channel activity in the presence of MgATP reflects the balance between the stimulatory (at SUR1) and inhibitory (at Kir6.2) effects of nucleotides. Mutant channels were 2.5-fold less sensitive to MgATP inhibition and not activated by MgATP. This suggests that ATP block of the channel is reduced by the SUR1 mutation. Interestingly, this effect was dependent on the functional integrity of the NBDs. These results therefore suggest that SUR1 modulates both nucleotide inhibition and activation of the KATP channel.

Metabolic syndrome induces changes in KATP-channels and calcium currents in pancreatic β-cells

Metabolic syndrome (MS) can be defined as a group of signs that increases the risk of developing type 2 diabetes mellitus (DM2). These signs include obesity, hyperinsulinemia and insulin resistance. We are interested in the mechanisms that trigger hyperinsulinemia as a step to understand how β cells fail in DM2. Pancreatic β cells secrete insulin in response to glucose variations in the extracellular medium. When they are chronically over-stimulated, hyperinsulinemia is observed; but then, with time, they become incapable of maintaining normal glucose levels, giving rise to DM2. A chronic high sucrose diet for two months induces MS in adult male Wistar rats. In the present article, we analyzed the effect of the internal environment of rats with MS, on the activity of ATP-sensitive potassium channels (KATP) and calcium currents of pancreatic β cells. After 24 weeks of treatment with 20% sucrose in their drinking water, rats showed central obesity, hyperinsulinemia and insulin resistance, and their systolic blood pressure and triglycerides plasma levels increased. These signs indicate the onset of MS. KATP channels in isolated patches of β cells from MS rats, had an increased sensitivity to ATP with respect to controls. Moreover, the macroscopic calcium currents, show increased variability compared with cells from control individuals. These results demonstrate that regardless of genetic background, a high sucrose diet leads to the development of MS. The observed changes in ionic channels can partially explain the increase in insulin secretion in MS rats. However, some β cells showed smaller calcium currents. These cells may represent a β cell subpopulation as it becomes exhausted by the long-term high sucrose diet.

Mice expressing a human K(ATP) channel mutation have altered channel ATP sensitivity but no cardiac abnormalities

AIMS/HYPOTHESIS

Patients with severe gain-of-function mutations in the Kir6.2 subunit of the ATP-sensitive potassium (K(ATP)) channel, have neonatal diabetes, muscle hypotonia and mental and motor developmental delay-a condition known as iDEND syndrome. However, despite the fact that Kir6.2 forms the pore of the cardiac K(ATP) channel, patients show no obvious cardiac symptoms. The aim of this project was to use a mouse model of iDEND syndrome to determine whether iDEND mutations affect cardiac function and cardiac K(ATP) channel ATP sensitivity.

METHODS

We performed patch-clamp and in vivo cine-MRI studies on mice in which the most common iDEND mutation (Kir6.2-V59M) was targeted to cardiac muscle using Cre-lox technology (m-V59M mice).

RESULTS

Patch-clamp studies of isolated cardiac myocytes revealed a markedly reduced K(ATP) channel sensitivity to MgATP inhibition in m-V59M mice (IC(50) 62 μmol/l compared with 13 μmol/l for littermate controls). In vivo cine-MRI revealed there were no gross morphological differences and no differences in heart rate, end diastolic volume, end systolic volume, stroke volume, ejection fraction, cardiac output or wall thickening between m-V59M and control hearts, either under resting conditions or under dobutamine stress.

CONCLUSIONS/INTERPRETATION

The common iDEND mutation Kir6.2-V59M decreases ATP block of cardiac K(ATP) channels but was without obvious effect on heart function, suggesting that metabolic changes fail to open the mutated channel to an extent that affects function (at least in the absence of ischaemia). This may have implications for the choice of sulfonylurea used to treat neonatal diabetes.

Oligomerization at the membrane: potassium channel structure and function

Cell membranes present a naturally impervious barrier to aqueous solutes, such that the physiochemical environment on either side of the lipid bilayer can substantially differ. Integral membrane proteins are embedded in this heterogeneous lipid environment, wherein the juxtaposition of apolar and polar molecular surfaces defines factors such as transverse orientation, the surface area available for oligomerisation and the symmetry of resultant assemblies. This chapter focuses on potassium channels -representative molecular pores that play a critical role in electrical signalling by enabling selective transport of K(+) ions across cell membranes. Oligomerization is central to K(+) channel action; individual subunits are nonfunctional and conduction, selectivity and gating involve manipulation of the common subunit interface of the tetramer. Regulation of channel activity can be viewed from the perspective that the pore of K(+) channels has coopted other proteins, utilizing a process of hetero-oligomerisation to absorb new functions that both enable the pore to respond to extrinsic signals and provide an electrical signature.

Regulation of the ATP-sensitive potassium channel subunit, Kir6.2, by a Ca2+-dependent protein kinase C

The activity of ATP-sensitive potassium (K(ATP)) channels is governed by the concentration of intracellular ATP and ADP and is thus responsive to the metabolic status of the cell. Phosphorylation of K(ATP) channels by protein kinase A (PKA) or protein kinase C (PKC) results in the modulation of channel activity and is particularly important in regulating smooth muscle tone. At the molecular level the smooth muscle channel is composed of a sulfonylurea subunit (SUR2B) and a pore-forming subunit Kir6.1 and/or Kir6.2. Previously, Kir6.1/SUR2B channels have been shown to be inhibited by PKC, and Kir6.2/SUR2B channels have been shown to be activated or have no response to PKC. In this study we have examined the modulation of channel complexes formed of the inward rectifier subunit, Kir6.2, and the sulfonylurea subunit, SUR2B. Using a combination of biochemical and electrophysiological techniques we show that this complex can be inhibited by protein kinase C in a Ca(2+)-dependent manner and that this inhibition is likely to be as a result of internalization. We identify a residue in the distal C terminus of Kir6.2 (Ser-372) whose phosphorylation leads to down-regulation of the channel complex. This inhibitory effect is distinct from activation which is seen with low levels of channel activity.

Dexmedetomidine directly inhibits vascular ATP-sensitive potassium channels

AIMS

Dexmedetomidine is reported to have an effect on peripheral vasoconstriction; however, the exact mechanisms underlying this process are unclear. In this study, we hypothesized that dexmedetomidine-induced inhibition of vascular ATP-sensitive K(+) (K(ATP)) channels may be associated with this vasoconstriction. To test this hypothesis, we investigated the effects of dexmedetomidine on vascular K(ATP)-channel activity at the single-channel level.

MAIN METHODS

We used cell-attached and inside-out patch-clamp configurations to examine the effects of dexmedetomidine on the activities of native rat vascular K(ATP) channels, recombinant K(ATP) channels with different combinations of various inwardly rectifying potassium channels (Kir6.0 family: Kir6.1, 6.2) and sulfonylurea receptor subunits (SUR1, 2A, 2B), and SUR-deficient channels derived from a truncated isoform of Kir6.2 subunit, namely, Kir6.2ΔC36 channels.

KEY FINDINGS

Dexmedetomidine was observed to inhibit the native rat vascular K(ATP) channels in both cell-attached and inside-out configurations. This drug also inhibited the activity of all types of recombinant SUR/Kir6.0 K(ATP) channels as well as Kir6.2ΔC36 channels with equivalent potency.

SIGNIFICANCE

These results indicate that dexmedetomidine directly inhibits K(ATP) channels through the Kir6.0 subunit.

Mechanism of KATP hyperactivity and sulfonylurea tolerance due to a diabetogenic mutation in L0 helix of sulfonylurea receptor 1 (ABCC8)

Activating mutations in different domains of the ABCC8 gene-coded sulfonylurea receptor 1 (SUR1) cause neonatal diabetes. Here we show that a diabetogenic mutation in an unexplored helix preceding the ABC core of SUR1 dramatically increases open probability of (SUR1/Kir6.2)(4) channel (KATP) by reciprocally changing rates of its transitions to and from the long-lived, inhibitory ligand-stabilized closed state. This kinetic mechanism attenuates ATP and sulfonylurea inhibition, but not Mg-nucleotide stimulation, of SUR1/Kir6.2. The results suggest a key role for L0 helix in KATP gating and together with previous findings from mutant KATP clarify why many patients with neonatal diabetes require high doses of sulfonylureas.

C-terminal dimerization activates the nociceptive transduction channel transient receptor potential vanilloid 1

Covalent modification of the specific cysteine residue(s) by oxidative stress robustly potentiates transient receptor potential vanilloid 1 (TRPV1) and sensitizes nociception. Here we provide biochemical evidence of dimerization of TRPV1 subunits upon exposure to phenylarsine oxide and hydrogen peroxide (H(2)O(2)), two chemical surrogates of oxidative stress. A disulfide bond formed between apposing cysteines ligates two C termini, serving as the structural basis of channel sensitization by oxidative covalent C-terminal modification. Systematic cysteine scanning of the C terminus of a cysteineless TRPV1 channel revealed a critical region within which any cysteine introduced phenylarsine oxide activation to mutant TRPV1. Oxidative sensitization persisted even when this region is substituted with a random peptide linker containing a single cysteine. So did insertion of this region to TRPV3, a homolog lacking the corresponding region and resistant to oxidative challenge. These results suggest that the non-conserved linker in the TRPV1 C terminus senses environmental oxidative stress and adjusts channel activity during cumulative oxidative damage by lowering the activation threshold of gating elements shared by TRPV channels.

Forced gating motions by a substituted titratable side chain at the bundle crossing of a potassium channel

Numerous inwardly rectifying potassium (Kir) channels possess an aromatic residue in the helix bundle crossing region, forming the narrowest pore constriction in crystal structures. However, the role of the Kir channel bundle crossing as a functional gate remains uncertain. We report a unique phenotype of Kir6.2 channels mutated to encode glutamate at this position (F168E). Despite a prediction of four glutamates in close proximity, Kir6.2(F168E) channels are predominantly closed at physiological pH, whereas alkalization causes rapid and reversible channel activation. These findings suggest that F168E glutamates are uncharged at physiological pH but become deprotonated at alkaline pH, forcing channel opening due to mutual repulsion of nearby negatively charged side chains. The potassium channel pore scaffold likely brings these glutamates close together, causing a significant pK(a) shift relative to the free side chain (as seen in the KcsA selectivity filter). Alkalization also shifts the apparent ATP sensitivity of the channel, indicating that forced motion of the bundle crossing is coupled to the ATP-binding site and may resemble conformational changes involved in wild-type Kir6.2 gating. The study demonstrates a novel mechanism for engineering extrinsic control of channel gating by pH and shows that conformational changes in the bundle crossing region are involved in ligand-dependent gating of Kir channels.

Regulation of K(ATP) channel by 17β-estradiol in pancreatic β-cells

ATP-sensitive potassium channels (K(ATP)) regulate electrical activity and insulin secretion in pancreatic β-cells. When glucose concentration increases, the [ATP]/[ADP] ratio rises closing K(ATP) channels, and the membrane potential depolarizes, triggering insulin secretion. This pivotal role of K(ATP) channels is used not only by glucose but also by neurotransmitters, hormones and other physiological agents to modulate electrical and secretory β-cell response. In recent years, it has been demonstrated that estrogens and estrogen receptors are involved in glucose homeostasis, and that they can modulate the electrical activity and insulin secretion of pancreatic β-cells. The hormone 17β-estradiol (E2), at physiological levels, is implicated in maintaining normal insulin sensitivity for β-cell function. Long term exposure to E2 increases insulin content, insulin gene expression and insulin release via the estrogen receptor α (ERα), while rapid responses to E2 can regulate K(ATP) channels increasing cGMP levels through the estrogen receptor β (ERβ) and type A guanylate cyclase receptor (GC-A). This review summarizes the main actions of 17β-estradiol on K(ATP) channels and the subsequent insulin release in pancreatic β-cells.

The antimalarial drug mefloquine inhibits cardiac inward rectifier K+ channels: evidence for interference in PIP2-channel interaction

The antimalarial drug mefloquine was found to inhibit the KATP channel by an unknown mechanism. Because mefloquine is a Cationic amphiphilic drug and is known to insert into lipid bilayers, we postulate that mefloquine interferes with the interaction between PIP2 and Kir channels resulting in channel inhibition. We studied the inhibitory effects of mefloquine on Kir2.1, Kir2.3, Kir2.3(I213L), and Kir6.2/SUR2A channels expressed in HEK-293 cells, and on IK1 and IKATP from feline cardiac myocytes. The order of mefloquine inhibition was Kir6.2/SUR2A ≈ Kir2.3 (IC50 ≈ 2 μM) > Kir2.1 (IC50 > 30 μM). Similar results were obtained in cardiac myocytes. The Kir2.3(I213L) mutant, which enhances the strength of interaction with PIP2 (compared to WT), was significantly less sensitive (IC50 = 9 μM). In inside-out patches, continuous application of PIP2 strikingly prevented the mefloquine inhibition. Our results support the idea that mefloquine interferes with PIP2-Kir channels interactions.

ATP activates ATP-sensitive potassium channels composed of mutant sulfonylurea receptor 1 and Kir6.2 with diminished PIP2 sensitivity

ATP-sensitive potassium (K(ATP)) channels are inhibited by ATP and activated by phosphatidylinositol 4,5-bisphosphate (PIP(2)). Both channel subunits Kir6.2 and sulfonylurea receptor 1 (SUR1) contribute to gating: while Kir6.2 interacts with ATP and PIP(2), SUR1 enhances sensitivity to both ligands. Recently, we showed that a mutation, E128K, in the N-terminal transmembrane domain of SUR1 disrupts functional coupling between SUR1 and Kir6.2, leading to reduced ATP and PIP(2) sensitivities resembling channels formed by Kir6.2 alone. We show here that when E128K SUR1 was co-expressed with Kir6.2 mutants known to disrupt PIP(2) gating, the resulting channels were surprisingly stimulated rather than inhibited by ATP. To explain this paradoxical gating behavior, we propose a model in which the open state of doubly mutant channels is highly unstable; ATP binding induces a conformational change in ATP-unbound closed channels that is conducive to brief opening when ATP unbinds, giving rise to the appearance of ATP-induced stimulation.

A conserved tryptophan at the membrane-water interface acts as a gatekeeper for Kir6.2/SUR1 channels and causes neonatal diabetes when mutated

We identified a novel heterozygous mutation, W68R, in the Kir6.2 subunit of the ATP-sensitive potassium (KATP) channel, in a patient with transient neonatal diabetes. This tryptophan is absolutely conserved in mammalian Kir channels. The functional effects of mutations at residue 68 of Kir6.2 were studied by heterologous expression in Xenopus oocytes, and by homology modelling. We found the Kir6.2-W68R mutation causes a small reduction in ATP inhibition in the heterozygous state and an increase in the whole-cell KATP current. This can explain the clinical phenotype of the patient. The effect of the mutation was not charge or size dependent, the order of potency for ATP inhibition being W Collapse

Key Words Collapse

MESH Headings

• ATP-Binding Cassette Transporters/genetics
• ATP-Binding Cassette Transporters/metabolism
• Amino Acid Sequence
• Animals
• Cell Membrane/genetics
• Cell Membrane/metabolism
• Child
• Conserved Sequence/genetics
• Diabetes Mellitus/genetics
• Diabetes Mellitus/metabolism
• Female
• Genetic Carrier Screening
• Humans
• Infant, Newborn
• Infant, Newborn, Diseases/genetics
• Infant, Newborn, Diseases/metabolism
• Molecular Sequence Data
• Mutation/genetics
• Oocytes/metabolism
• Phenotype
• Potassium Channels, Inwardly Rectifying/genetics
• Potassium Channels, Inwardly Rectifying/metabolism
• Rats
• Receptors, Drug/genetics
• Receptors, Drug/metabolism
• Sulfonylurea Receptors
• Tryptophan/genetics
• Xenopus laevis

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Grants

• Wellcome Trust
• 084655 Wellcome Trust
• 089795 Wellcome Trust
• 076436 Wellcome Trust

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Affiliation(s)

Roope Männikkö Henry Wellcome Centre for Gene Function, Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
Phillip J Stansfeld
Alexandra S Ashcroft
Andrew T Hattersley
Mark S P Sansom
Sian Ellard
Frances M Ashcroft

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Mutations of the same conserved glutamate residue in NBD2 of the sulfonylurea receptor 1 subunit of the KATP channel can result in either hyperinsulinism or neonatal diabetes

OBJECTIVE

Two novel mutations (E1506D, E1506G) in the nucleotide-binding domain 2 (NBD2) of the ATP-sensitive K(+) channel (K(ATP) channel) sulfonylurea receptor 1 (SUR1) subunit were detected heterozygously in patients with neonatal diabetes. A mutation at the same residue (E1506K) was previously shown to cause congenital hyperinsulinemia. We sought to understand why mutations at the same residue can cause either neonatal diabetes or hyperinsulinemia.

RESEARCH DESIGN AND METHODS

Neonatal diabetic patients were sequenced for mutations in ABCC8 (SUR1) and KCNJ11 (Kir6.2). Wild-type and mutant K(ATP) channels were expressed in Xenopus laevis oocytes and studied with electrophysiological methods.

RESULTS

Oocytes expressing neonatal diabetes mutant channels had larger resting whole-cell K(ATP) currents than wild-type, consistent with the patients' diabetes. Conversely, no E1506K currents were recorded at rest or after metabolic inhibition, as expected for a mutation causing hyperinsulinemia. K(ATP) channels are activated by Mg-nucleotides (via SUR1) and blocked by ATP (via Kir6.2). All mutations decreased channel activation by MgADP but had little effect on MgATP activation, as assessed using an ATP-insensitive Kir6.2 subunit. Importantly, using wild-type Kir6.2, a 30-s preconditioning exposure to physiological MgATP concentrations (>300 µmol/L) caused a marked reduction in the ATP sensitivity of neonatal diabetic channels, a small decrease in that of wild-type channels, and no change for E1506K channels. This difference in MgATP inhibition may explain the difference in resting whole-cell currents found for the neonatal diabetes and hyperinsulinemia mutations.

CONCLUSIONS

Mutations in the same residue can cause either hyperinsulinemia or neonatal diabetes. Differentially altered nucleotide regulation by NBD2 of SUR1 can explain the respective clinical phenotypes.

The potassium channel KcsA: a model protein in studying membrane protein oligomerization and stability of oligomeric assembly?

Many membrane proteins are functional as stable oligomers. An understanding of the conditions that elicit and enhance oligomerization is important in many therapeutics. In this regard, protein-protein and protein-lipid interactions play crucial roles in the assembly and stability of oligomeric complexes. Recent years have seen a rapid increase in the mechanistic information on the importance of cytoplasmic termini in determining subunit assembly and stability of oligomeric complexes. In addition, the role of specific protein-lipid interaction between anionic phospholipids and "hot spots" on the protein surface has also become evident in stabilizing oligomeric assemblies. This review focuses on several contemporary developments of membrane proteins that stabilize oligomers by taking the potassium channel KcsA as an exemplary ion channel.

β2-Adrenergic ion-channel coupled receptors as conformational motion detectors

Ion Channel-Coupled Receptors (ICCRs) are artificial proteins comprised of a G protein-coupled receptor and a fused ion channel, engineered to couple channel gating to ligand binding. These novel biological objects have potential use in drug screening and functional characterization, in addition to providing new tools in the synthetic biology repertoire as synthetic K⁺-selective ligand-gated channels. The ICCR concept was previously validated with fusion proteins between the K⁺ channel Kir6.2 and muscarinic M₂ or dopaminergic D₂ receptors. Here, we extend the concept to the distinct, longer β₂-adrenergic receptor which, unlike M₂ and D₂ receptors, displayed barely detectable surface expression in our Xenopus oocyte expression system and did not couple to Kir6.2 when unmodified. Here, we show that a Kir6.2-binding protein, the N-terminal transmembrane domain of the sulfonylurea receptor, can greatly increase plasma membrane expression of β₂ constructs. We then demonstrate how engineering of both receptor and channel can produce β₂-Kir6.2 ICCRs. Specifically, removal of 62–72 residues from the cytoplasmic C-terminus of the receptor was required to enable coupling, suggesting that ligand-dependent conformational changes do not efficiently propagate to the distal C-terminus. Characterization of the β₂ ICCRs demonstrated that full and partial agonists had the same coupling efficacy, that an inverse agonist had no effect and that the stabilizing mutation E122 W reduced agonist-induced coupling efficacy without affecting affinity. Because the ICCRs are expected to report motions of the receptor C-terminus, these results provide novel insights into the conformational dynamics of the β₂ receptor.

N-terminal transmembrane domain of SUR1 controls gating of Kir6.2 by modulating channel sensitivity to PIP2

Functional integrity of pancreatic adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels depends on the interactions between the pore-forming potassium channel subunit Kir6.2 and the regulatory subunit sulfonylurea receptor 1 (SUR1). Previous studies have shown that the N-terminal transmembrane domain of SUR1 (TMD0) interacts with Kir6.2 and is sufficient to confer high intrinsic open probability (P(o)) and bursting patterns of activity observed in full-length K(ATP) channels. However, the nature of TMD0-Kir6.2 interactions that underlie gating modulation is not well understood. Using two previously described disease-causing mutations in TMD0 (R74W and E128K), we performed amino acid substitutions to study the structural roles of these residues in K(ATP) channel function in the context of full-length SUR1 as well as TMD0. Our results revealed that although R74W and E128K in full-length SUR1 both decrease surface channel expression and reduce channel sensitivity to ATP inhibition, they arrive there via distinct mechanisms. Mutation of R74 uniformly reduced TMD0 protein levels, suggesting that R74 is necessary for stability of TMD0. In contrast, E128 mutations retained TMD0 protein levels but reduced functional coupling between TMD0 and Kir6.2 in mini-K(ATP) channels formed by TMD0 and Kir6.2. Importantly, E128K full-length channels, despite having a greatly reduced P(o), exhibit little response to phosphatidylinositol 4,5-bisphosphate (PIP(2)) stimulation. This is reminiscent of Kir6.2 channel behavior in the absence of SUR1 and suggests that TMD0 controls Kir6.2 gating by modulating Kir6.2 interactions with PIP(2). Further supporting this notion, the E128W mutation in full-length channels resulted in channel inactivation that was prevented or reversed by exogenous PIP(2). These results identify a critical determinant in TMD0 that controls Kir6.2 gating by controlling channel sensitivity to PIP(2). Moreover, they uncover a novel mechanism of K(ATP) channel inactivation involving aberrant functional coupling between SUR1 and Kir6.2.

KATP channel mutations in congenital hyperinsulinism

Adenosine triphosphate (ATP)-sensitive potassium channels (K(ATP) channels) have a central role in the regulation of insulin secretion in pancreatic β cells. They are octameric complexes organized around the central core constituted by the Kir6.2 subunits. The regulation of the channel itself takes place on the sulfonylurea receptor-1 subunit. The channel opens and closes according to the balance between adenine nucleotide ATP and adenosine diphosphate. Hyperinsulinemic hypoglycemia (also named congenital hyperinsulinism, or CHI) is associated with loss-of-function K(ATP) channel mutations. Their frequency depends on the histopathological form and the responsiveness of CHI patients to diazoxide. ABCC8/KCNJ11 defects are identified in approximately 80% of patients with CHI refractory to diazoxide. Within this group, focal forms are related to a paternally inherited KCNJ11 or ABCC8 mutation and the loss of the corresponding maternal allele in some pancreatic β cells leading to a focal lesion. Diffuse forms are mostly associated with recessively inherited mutations. Some patients with diffuse forms also carried a single K(ATP) channel mutation. In contrast, K(ATP) mutations are involved in 15% of diazoxide-responsive CHI cases that are either sporadic or dominantly inherited.

Activation of the K(ATP) channel by Mg-nucleotide interaction with SUR1

The mechanism of adenosine triphosphate (ATP)-sensitive potassium (K_ATP) channel activation by Mg-nucleotides was studied using a mutation (G334D) in the Kir6.2 subunit of the channel that renders K_ATP channels insensitive to nucleotide inhibition and has no apparent effect on their gating. K_ATP channels carrying this mutation (Kir6.2-G334D/SUR1 channels) were activated by MgATP and MgADP with an EC₅₀ of 112 and 8 µM, respectively. This activation was largely suppressed by mutation of the Walker A lysines in the nucleotide-binding domains of SUR1: the remaining small (∼10%), slowly developing component of MgATP activation was fully inhibited by the lipid kinase inhibitor LY294002. The EC₅₀ for activation of Kir6.2-G334D/SUR1 currents by MgADP was lower than that for MgATP, and the time course of activation was faster. The poorly hydrolyzable analogue MgATPγS also activated Kir6.2-G334D/SUR1. AMPPCP both failed to activate Kir6.2-G334D/SUR1 and to prevent its activation by MgATP. Maximal stimulatory concentrations of MgATP (10 mM) and MgADP (1 mM) exerted identical effects on the single-channel kinetics: they dramatically elevated the open probability (P_O > 0.8), increased the mean open time and the mean burst duration, reduced the frequency and number of interburst closed states, and eliminated the short burst states. By comparing our results with those obtained for wild-type K_ATP channels, we conclude that the MgADP sensitivity of the wild-type K_ATP channel can be described quantitatively by a combination of inhibition at Kir6.2 (measured for wild-type channels in the absence of Mg²⁺) and activation via SUR1 (determined for Kir6.2-G334D/SUR1 channels). However, this is not the case for the effects of MgATP.

ATP modulates interaction of syntaxin-1A with sulfonylurea receptor 1 to regulate pancreatic beta-cell KATP channels

ATP-sensitive potassium (K(ATP)) channels are regulated by a variety of cytosolic factors (adenine nucleotides, Mg(2+), phospholipids, and pH). We previously reported that K(ATP) channels are also regulated by endogenous membrane-bound SNARE protein syntaxin-1A (Syn-1A), which binds both nucleotide-binding folds of sulfonylurea receptor (SUR)1 and 2A, causing inhibition of K(ATP) channel activity in pancreatic islet β-cells and cardiac myocytes, respectively. In this study, we show that ATP dose-dependently inhibits Syn-1A binding to SUR1 at physiological concentrations, with the addition of Mg(2+) causing a decrease in the ATP-induced inhibitory effect. This ATP disruption of Syn-1A binding to SUR1 was confirmed by FRET analysis in living HEK293 cells. Electrophysiological studies in pancreatic β-cells demonstrated that reduced ATP concentrations increased K(ATP) channel sensitivity to Syn-1A inhibition. Depletion of endogenous Syn-1A in insulinoma cells by botulinum neurotoxin C1 proteolysis followed by rescue with exogenous Syn-1A showed that Syn-1A modulates K(ATP) channel sensitivity to ATP. Thus, our data indicate that although both ATP and Syn-1A independently inhibit β-cell K(ATP) channel gating, they could also influence the sensitivity of K(ATP) channels to each other. These findings provide new insight into an alternate mechanism by which ATP regulates pancreatic β-cell K(ATP) channel activity, not only by its direct actions on Kir6.2 pore subunit, but also via ATP modulation of Syn-1A binding to SUR1.