Regulation of KATP channel activity by diazoxide and MgADP. Distinct functions of the two nucleotide binding folds of the sulfonylurea receptor

Dual actions of the metabolic inhibitor, sodium azide on K(ATP) channel currents in the rat CRI-G1 insulinoma cell line

1. The effects of various inhibitors of the mitochondrial electron transport chain on the activity of ATP-sensitive K+ channels were examined in the Cambridge rat insulinoma G1 (CRI-G1) cell line using a combination of whole cell and single channel recording techniques. 2. Whole cell current clamp recordings, with 5 mM ATP in the pipette, demonstrate that the mitochondrial uncoupler sodium azide (3 mM) rapidly hyperpolarizes CRI-G1 cells with a concomitant increase in K+ conductance. This is due to activation of K(ATP) channels as the sulphonylurea tolbutamide (100 microM) completely reversed the actions of azide. Other inhibitors of the mitochondrial electron transport chain, rotenone (10 microM) or oligomycin (2 microM) did not hyperpolarize CRI-G1 cells or increase K+ conductance. 3. In cell-attached recordings, bath application of 3 mM sodium azide (in the absence of glucose) resulted in a rapid increase in K(ATP) channel activity, an action readily reversible by tolbutamide (100 microM). Application of sodium azide (3 mM), in the presence of Mg-ATP, to the intracellular surface of excised inside-out patches also increased K(ATP) channel activity, in a reversible manner. 4. In contrast, rotenone (10 microM) or oligomycin (2 microM) did not increase K(ATP) channel activity in either cell-attached, in the absence of glucose, or inside-out membrane patch recordings. 5. Addition of sodium azide (3 mM) to the intracellular surface of inside-out membrane patches in the presence of Mg-free ATP or the non-hydrolysable analogue 5'-adenylylimidodiphosphate (AMP-PNP) inhibited, rather than increased, K(ATP) channel activity. 6. In conclusion, sodium azide, but not rotenone or oligomycin, directly activates K(ATP) channels in CRI-G1 insulin secreting cells. This action of azide is similar to that reported previously for diazoxide.

ATP-sensitive potassium channels: structures, functions, and pathophysiology

ATP-sensitive potassium channels (KATP channels) play important roles in various tissues by coupling cell metabolic status to electrical activity. Recently, molecular biological and electrophysiological techniques have revealed the molecular basis of the KATP channels to be a complex of the Kir6.0 subunit, a member of the inwardly rectifying K+ channel subfamily Kir6.0, and the sulfonylurea receptor (SUR) subunit, a member of ATP-binding cassette (ABC) superfamily; the functional diversity of the various KATP channels is being determined by a combination of the Kir6.0 subunit (Kir6.1 or Kir6.2) and the SUR subunit (SUR1 or SUR2) comprising it. Recent studies of the KATP channels have suggested mechanisms of KATP channel regulation and pathophysiology and also a new model in which ABC proteins regulate the functional expression of ion channels.

KATP channel inhibition by ATP requires distinct functional domains of the cytoplasmic C terminus of the pore-forming subunit

ATP-sensitive potassium ("KATP") channels are rapidly inhibited by intracellular ATP. This inhibition plays a crucial role in the coupling of electrical activity to energy metabolism in a variety of cells. The KATP channel is formed from four each of a sulfonylurea receptor (SUR) regulatory subunit and an inwardly rectifying potassium (Kir6.2) pore-forming subunit. We used systematic chimeric and point mutagenesis, combined with patch-clamp recording, to investigate the molecular basis of ATP-dependent inhibition gating of mouse pancreatic beta cell KATP channels expressed in Xenopus oocytes. We identified distinct functional domains of the presumed cytoplasmic C-terminal segment of the Kir6.2 subunit that play an important role in this inhibition. Our results suggest that one domain is associated with inhibitory ATP binding and another with gate closure.

PIP2 and PIP as determinants for ATP inhibition of KATP channels

Adenosine triphosphate (ATP)-sensitive potassium (KATP) channels couple electrical activity to cellular metabolism through their inhibition by intracellular ATP. ATP inhibition of KATP channels varies among tissues and is affected by the metabolic and regulatory state of individual cells, suggesting involvement of endogenous factors. It is reported here that phosphatidylinositol-4, 5-bisphosphate (PIP2) and phosphatidylinositol-4-phosphate (PIP) controlled ATP inhibition of cloned KATP channels (Kir6.2 and SUR1). These phospholipids acted on the Kir6.2 subunit and shifted ATP sensitivity by several orders of magnitude. Receptor-mediated activation of phospholipase C resulted in inhibition of KATP-mediated currents. These results represent a mechanism for control of excitability through phospholipids.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

Cytoplasmic terminus domains of Kir6.x confer different nucleotide-dependent gating on the ATP-sensitive K+ channel

1. In order to investigate the structural basis for the nucleotide-dependent gating of ATP-sensitive K+ channels (KATP), Kir6.1 (uKATP-1), Kir6.2 (BIR1) and chimeric channels were co-expressed with a common subtype of sulphonylurea receptor, SUR1, in COS7 cells. Representing the amino terminal domain-transmembrane domain-carboxyl-terminal domain of Kir6.1 as 1-1-1 and of Kir6.2 as 2-2-2, chimeric Kir6.x channels were constructed by swapping the amino and/or carboxyl terminal domains between Kir6.1 and Kir6.2 to give the chimeric x-1-x channels 1-1-2, 2-1-1 and 2-1-2, and the chimeric x-2-x channels 2-2-1, 1-2-2 and 1-2-1. 2. Inside-out patch clamp experiments revealed that both wild-type Kir6.1 and Kir6.2 formed inwardly rectifying K+ channels. Single-channel conductances were 36.3 and 66.1 pS, respectively. Chimeric x-1-x channels, whose transmembrane domain was that of Kir6.1, showed similar ion-pore properties to wild-type Kir6.1. Likewise, chimeric x-2-x channels had similar ion-pore properties to wild-type Kir6.2. 3. Wild-type Kir6.1 and Kir6.2 possessed distinct gating properties towards intracellular nucleotides. The activity of Kir6.1 was entirely dependent on Mg2+ and nucleotide diphosphates (NDPs) such as UDP. In contrast, Kir6.2 was activated upon excision of patch membrane. When Kir6.2 underwent rundown, UDP reactivated the channel. 4. In order to eliminate UDP dependence from Kir6.1, it was necessary to replace both N- and C-termini; chimera 2-1-2 opened in UDP-free conditions. With Kir6.2, substitution of the N-terminus with that of Kir6.1 conferred UDP dependence on chimeras 1-2-2 and 1-2-1. Chimera 2-2-1 opened in UDP-free conditions, but UDP potentiated the channel activity by > 20-fold. 5. The kinetics of UDP-dependent activation were significantly different between Kir6.1 and Kir6.2. Kir6.1 maximally activated by UDP was sensitive to intracellular ATP, although its ATP sensitivity was significantly lower than that of Kir6.2 measured in identical conditions. The kinetics of UDP-dependent activation and ATP sensitivity could be transferred between Kir6.1 and Kir6.2 only when both N- and C-termini were replaced. We therefore concluded that nucleotide-dependent gating was regulated by the N- and C-terminal domains irrespective of the transmembrane domains.

Mechanism of cloned ATP-sensitive potassium channel activation by oleoyl-CoA

Insulin secretion from pancreatic beta cells is coupled to cell metabolism through closure of ATP-sensitive potassium (KATP) channels, which comprise Kir6.2 and sulfonylurea receptor (SUR1) subunits. Although metabolic regulation of KATP channel activity is believed to be mediated principally by the adenine nucleotides, other metabolic intermediates, including long chain acyl-CoA esters, may also be involved. We recorded macroscopic and single-channel currents from Xenopus oocytes expressing either Kir6.2/SUR1 or Kir6. 2DeltaC36 (which forms channels in the absence of SUR1). Oleoyl-CoA (1 microM) activated both wild-type Kir6.2/SUR1 and Kir6.2DeltaC36 macroscopic currents, approximately 2-fold, by increasing the number and open probability of Kir6.2/SUR1 and Kir6.2DeltaC36 channels. It was ineffective on the related Kir subunit Kir1.1a. Oleoyl-CoA also impaired channel inhibition by ATP, increasing the Ki values for both Kir6.2/SUR1 and Kir6.2DeltaC36 currents by approximately 3-fold. Our results indicate that activation of KATP channels by oleoyl-CoA results from an interaction with the Kir6.2 subunit, unlike the stimulatory effects of MgADP and diazoxide which are mediated through SUR1. The increased activity and reduced ATP sensitivity of KATP channels by oleoyl-CoA might contribute to the impaired insulin secretion observed in non-insulin-dependent diabetes mellitus.

Potassium channel openers require ATP to bind to and act through sulfonylurea receptors

KATP channels are composed of a small inwardly rectifying K+ channel subunit, either KIR6.1 or KIR6.2, plus a sulfonylurea receptor, SUR1 or SUR2 (A or B), which belong to the ATP-binding cassette superfamily. SUR1/KIR6.2 reconstitute the neuronal/pancreatic beta-cell channel, whereas SUR2A/KIR6.2 and SUR2B/KIR6.1 (or KIR6.2) are proposed to reconstitute the cardiac and the vascular-smooth-muscle-type KATP channels, respectively. We report that potassium channel openers (KCOs) bind to and act through SURs and that binding to SUR1, SUR2A and SUR2B requires ATP. Non-hydrolysable ATP-analogues do not support binding, and Mg2+ or Mn2+ are required. Point mutations in the Walker A motifs or linker regions of both nucleotide-binding folds (NBFs) abolish or weaken [3H]P1075 binding to SUR2B, rendering reconstituted SUR2B/KIR6.2 channels insensitive towards KCOs. The C-terminus of SUR affects KCO affinity with SUR2B approximately SUR1 > SUR2A. KCOs belonging to different structural classes inhibited specific [3H]P1075 binding to SUR2B in a monophasic manner, with the exception of minoxidil sulfate, which induced a biphasic displacement. The affinities of KCO binding to SUR2B were 3.5-8-fold higher than their potencies for activation of SUR2B/KIR6.2 channels. The results establish that SURs are the KCO receptors of KATP channels and suggest that KCO binding requires a conformational change induced by ATP hydrolysis in both NBFs.

Intracellular nucleotide-mediated gating of SUR/Kir6.0 complex potassium channels expressed in a mammalian cell line and its modification by pinacidil

1. We have examined the properties of intracellular nucleotide-mediated gating of K+ channel constructs composed of the sulphonylurea receptor 2B and the inwardly rectifying K+ channel subunits Kir6.1 and Kir6.2 (SUR2B/Kir6.1 and SUR2B/Kir6.2 complex K+ channels) heterologously expressed in human embryonic kidney (HEK) 293T cells. In the cell-attached form, both types of K+ channel were activated by pinacidil. 2. In inside-out (IO) patches, the SUR2B/Kir6.2 channels opened spontaneously and were inhibited by intracellular ATP (ATPi). Pinacidil attenuated the ATPi-mediated channel inhibition in a concentration-dependent manner. In contrast, the SUR2B/Kir6.1 channels required intracellular nucleoside di- or tri-, but not mono-, phosphates for opening. The potency of adenine, guanine or uracil nucleotides to activate SUR2B/Kir6.1 channels was enhanced by pinacidil. 3. In the presence of pinacidil, adenine and guanine, but not uracil, nucleotides exhibited bell-shaped concentration-dependent activating effects on SUR2B/Kir6.1 channels. This was due to channel inhibition caused by adenine and guanine nucleotides, which was unaffected by pinacidil. 4. From power density spectrum analysis of SUR2B/Kir6.1 currents, channel activation could be described by the product of two gates, a nucleotide-independent fast channel gate and a nucleotide-dependent slow gate, which controlled the number of functional channels. Pinacidil specifically increased the potency of nucleotide action on the slow gate. 5. We conclude that Kir6.0 subunits play a crucial role in the nucleotide-mediated gating of SUR/Kir6.0 complex K+ channels and may determine the molecular mode of pinacidil action.

Mechanism of ATP-sensitive K channel inhibition by sulfhydryl modification

ATP-sensitive potassium (KATP) channels are reversibly inhibited by intracellular ATP. Agents that interact with sulfhydryl moieties produce an irreversible inhibition of KATP channel activity when applied to the intracellular membrane surface. ATP appears to protect against this effect, suggesting that the cysteine residue with which thiol reagents interact may either lie within the ATP-binding site or be inaccessible when the channel is closed. We have examined the interaction of the membrane-impermeant thiol-reactive agent p-chloromercuriphenylsulphonate (pCMPS) with the cloned beta cell KATP channel. This channel comprises the pore-forming Kir6.2 and regulatory SUR1 subunits. We show that the cysteine residue involved in channel inhibition by pCMPS resides on the Kir6.2 subunit and is located at position 42, which lies within the NH2 terminus of the protein. Although ATP protects against the effects of pCMPS, the ATP sensitivity of the KATP channel was unchanged by mutation of C42 to either valine (V) or alanine (A), suggesting that ATP does not interact directly with this residue. These results are consistent with the idea that C42 is inaccessible to the intracellular solution, and thereby protected from interaction with pCMPS when the channel is closed by ATP. We also observed that the C42A mutation does not affect the ability of SUR1 to endow Kir6.2 with diazoxide sensitivity, and reduces, but does not prevent, the effects of MgADP and tolbutamide, which are mediated via SUR1. The Kir6.2-C42A (or V) mutant channel may provide a suitable background for cysteine-scanning mutagenesis studies.

Molecular analysis of ATP-sensitive K channel gating and implications for channel inhibition by ATP

The beta cell KATP channel is an octameric complex of four pore-forming subunits (Kir6.2) and four regulatory subunits (SUR1). A truncated isoform of Kir6.2 (Kir6.2DeltaC26), which expresses independently of SUR1, shows intrinsic ATP sensitivity, suggesting that this subunit is primarily responsible for mediating ATP inhibition. We show here that mutation of C166, which lies at the cytosolic end of the second transmembrane domain, to serine (C166S) increases the open probability of Kir6.2DeltaC26 approximately sevenfold by reducing the time the channel spends in a long closed state. Rundown of channel activity is also decreased. Kir6.2DeltaC26 containing the C166S mutation shows a markedly reduced ATP sensitivity: the Ki is reduced from 175 microM to 2.8 mM. Substitution of threonine, alanine, methionine, or phenylalanine at position C166 also reduced the channel sensitivity to ATP and simultaneously increased the open probability. Thus, ATP does not act as an open channel blocker. The inhibitory effects of tolbutamide are reduced in channels composed of SUR1 and Kir6.2 carrying the C166S mutation. Our results are consistent with the idea that C166 plays a role in the intrinsic gating of the channel, possibly by influencing a gate located at the intracellular end of the pore. Kinetic analysis suggests that the apparent decrease in ATP sensitivity, and the changes in other properties, observed when C166 is mutated is largely a consequence of the impaired transition from the open to the long closed state.

Genetic heterogeneity in familial hyperinsulinism

Familial hyperinsulinism (HI) is a disorder characterized by dysregulation of insulin secretion and profound hypoglycemia. Mutations in both the Kir6.2 and sulfonylurea receptor (SUR1) genes have been associated with the autosomal recessive form of this disorder. In this study, the spectrum and frequency of SUR1 mutations in HI and their significance to clinical manifestations of the disease were investigated by screening 45 HI probands of various ethnic origins for mutations in the SUR1 gene. Single-strand conformation polymorphism (SSCP) and nucleotide sequence analyses of genomic DNA revealed a total of 17 novel and three previously described mutations in SUR1 . The novel mutations comprised one nonsense and 10 missense mutations, two deletions, three mutations in consensus splice-site sequences and an in-frame insertion of six nucleotides. One mutation occurred in the first nucleotide binding domain (NBF-1) of the SUR1 molecule and another eight mutations were located in the second nucleotide binding domain (NBF-2), including two at highly conserved amino acid residues within the Walker A sequence motif. The majority of the remaining mutations was distributed throughout the three putative transmembrane domains of the SUR1 protein. With the exception of the 3993-9G-->A mutation, which was detected on 4.5% (4/88) disease chromosomes, allelic frequencies for the identified mutations varied between 1.1 and 2.3% for HI chromosomes, indicating that each mutation was rare within the patient cohort. The clinical manifestations of HI in those patients homozygous for mutations in the SUR1 gene are described. In contrast with the allelic homogeneity of HI previously described in Ashkenazi Jewish patients, these findings suggest that a large degree of allelic heterogeneity at the SUR1 locus exists in non-Ashkenazi HI patients. These data have important implications for genetic counseling and prenatal diagnosis of HI, and also provide a basis to further elucidate the molecular mechanisms underlying the pathophysiology of this disease.

Molecular determinants of KATP channel inhibition by ATP

ATP-sensitive K+ (KATP) channels are both inhibited and activated by intracellular nucleotides, such as ATP and ADP. The inhibitory effects of nucleotides are mediated via the pore-forming subunit, Kir6.2, whereas the potentiatory effects are conferred by the sulfonylurea receptor subunit, SUR. The stimulatory action of Mg-nucleotides complicates analysis of nucleotide inhibition of Kir6. 2/SUR1 channels. We therefore used a truncated isoform of Kir6.2, that expresses ATP-sensitive channels in the absence of SUR1, to explore the mechanism of nucleotide inhibition. We found that Kir6.2 is highly selective for ATP, and that both the adenine moiety and the beta-phosphate contribute to specificity. We also identified several mutations that significantly reduce ATP inhibition. These are located in two distinct regions of Kir6.2: the N-terminus preceding, and the C-terminus immediately following, the transmembrane domains. Some mutations in the C-terminus also markedly increased the channel open probability, which may account for the decrease in apparent ATP sensitivity. Other mutations did not affect the single-channel kinetics, and may reduce ATP inhibition by interfering with ATP binding and/or the link between ATP binding and pore closure. Our results also implicate the proximal C-terminus in KATP channel gating.

MgATP activates the beta cell KATP channel by interaction with its SUR1 subunit

ATP-sensitive potassium (KATP) channels in the pancreatic beta cell membrane mediate insulin release in response to elevation of plasma glucose levels. They are open at rest but close in response to glucose metabolism, producing a depolarization that stimulates Ca2+ influx and exocytosis. Metabolic regulation of KATP channel activity currently is believed to be mediated by changes in the intracellular concentrations of ATP and MgADP, which inhibit and activate the channel, respectively. The beta cell KATP channel is a complex of four Kir6.2 pore-forming subunits and four SUR1 regulatory subunits: Kir6.2 mediates channel inhibition by ATP, whereas the potentiatory action of MgADP involves the nucleotide-binding domains (NBDs) of SUR1. We show here that MgATP (like MgADP) is able to stimulate KATP channel activity, but that this effect normally is masked by the potent inhibitory effect of the nucleotide. Mg2+ caused an apparent reduction in the inhibitory action of ATP on wild-type KATP channels, and MgATP actually activated KATP channels containing a mutation in the Kir6.2 subunit that impairs nucleotide inhibition (R50G). Both of these effects were abolished when mutations were made in the NBDs of SUR1 that are predicted to abolish MgATP binding and/or hydrolysis (D853N, D1505N, K719A, or K1384M). These results suggest that, like MgADP, MgATP stimulates KATP channel activity by interaction with the NBDs of SUR1. Further support for this idea is that the ATP sensitivity of a truncated form of Kir6.2, which shows functional expression in the absence of SUR1, is unaffected by Mg2+.

A touching case of channel regulation: the ATP-sensitive K+ channel

The classical type of KATP channel is an octameric (4:4) complex of two structurally unrelated subunits, Kir6.2 and SUR. The former serves as an ATP-inhibitable pore, while SUR is a regulatory subunit endowing sensitivity to sulphonylurea and K+ channel opener drugs, and the potentiatory action of MgADP. Both subunits are required to form a functional channel.

Octameric stoichiometry of the KATP channel complex

ATP-sensitive potassium (KATP) channels link cellular metabolism to electrical activity in nerve, muscle, and endocrine tissues. They are formed as a functional complex of two unrelated subunits-a member of the Kir inward rectifier potassium channel family, and a sulfonylurea receptor (SUR), a member of the ATP-binding cassette transporter family, which includes cystic fibrosis transmembrane conductance regulators and multidrug resistance protein, regulators of chloride channel activity. This recent discovery has brought together proteins from two very distinct superfamilies in a novel functional complex. The pancreatic KATP channel is probably formed specifically of Kir6.2 and SUR1 isoforms. The relationship between SUR1 and Kir6.2 must be determined to understand how SUR1 and Kir6.2 interact to form this unique channel. We have used mutant Kir6.2 subunits and dimeric (SUR1-Kir6.2) constructs to examine the functional stoichiometry of the KATP channel. The data indicate that the KATP channel pore is lined by four Kir6.2 subunits, and that each Kir6.2 subunit requires one SUR1 subunit to generate a functional channel in an octameric or tetradimeric structure.