Alekseev, Alexey E.

Metabolic-sensing ATP-sensitive K+ channels (KATP channels) adjust membrane excitability to match cellular energetic demand. In the heart, KATP channel activity has been linked to homeostatic shortening of the action potential under stress, yet the requirement of channel function in securing cardiac electrical stability is only partially understood. Here, upon catecholamine challenge, disruption of KATP channels, by genetic deletion of the pore-forming Kir6.2 subunit, produced defective cardiac action potential shortening, predisposing the myocardium to early afterdepolarizations. This deficit in repolarization reserve, demonstrated in Kir6.2-knockout hearts, translated into a high risk for induction of triggered activity and ventricular dysrhythmia. Thus, intact KATP channel function is mandatory for adequate repolarization under sympathetic stress providing electrical tolerance against triggered arrhythmia.

Embryonic stem cells have the distinct potential for tissue regeneration, including cardiac repair. Their propensity for multilineage differentiation carries, however, the liability of neoplastic growth, impeding therapeutic application. Here, the tumorigenic threat associated with embryonic stem cell transplantation was suppressed by cardiac-restricted transgenic expression of the reprogramming cytokine TNF-α, enhancing the cardiogenic competence of recipient heart. The in vivo aptitude of TNF-α to promote cardiac differentiation was recapitulated in embryoid bodies in vitro. The procardiogenic action required an intact endoderm and was mediated by secreted cardio-inductive signals. Resolved TNF-α–induced endoderm-derived factors, combined in a cocktail, secured guided differentiation of embryonic stem cells in monolayers produce cardiac progenitors termed cardiopoietic cells. Characterized by a down-regulation of oncogenic markers, up-regulation, and nuclear translocation of cardiac transcription factors, this predetermined population yielded functional cardiomyocyte progeny. Recruited cardiopoietic cells delivered in infarcted hearts generated cardiomyocytes that proliferated into scar tissue, integrating with host myocardium for tumor-free repair. Thus, cardiopoietic programming establishes a strategy to hone stem cell pluripotency, offering a tumor-resistant approach for regeneration.

Reaction to stress requires feedback adaptation of cellular functions to secure a response without distress, but the molecular order of this process is only partially understood. Here, we report a previously unrecognized regulatory element in the general adaptation syndrome. Kir6.2, the ion-conducting subunit of the metabolically responsive ATP-sensitive potassium (K ATP ) channel, was mandatory for optimal adaptation capacity under stress. Genetic deletion of Kir6.2 disrupted K ATP channel-dependent adjustment of membrane excitability and calcium handling, compromising the enhancement of cardiac performance driven by sympathetic stimulation, a key mediator of the adaptation response. In the absence of Kir6.2, vigorous sympathetic challenge caused arrhythmia and sudden death, preventable by calcium-channel blockade. Thus, this vital function identifies a physiological role for K ATP channels in the heart.

ABSTRACT ATP‐sensitive K + (K ATP ) channels are unique metabolic sensors formed by association of Kir6.2, an inwardly rectifying K + channel, and the sulfonylurea receptor SUR, an ATP binding cassette protein. We identified an ATPase activity in immunoprecipitates of cardiac K ATP channels and in purified fusion proteins containing nucleotide binding domains NBD1 and NBD2 of the cardiac SUR2A isoform. NBD2 hydrolyzed ATP with a twofold higher rate compared to NBD1. The ATPase required Mg 2+ and was insensitive to ouabain, oligomycin, thapsigargin, or levamisole. K1348A and D1469N mutations in NBD2 reduced ATPase activity and produced channels with increased sensitivity to ATP. K ATP channel openers, which bind to SUR, promoted ATPase activity in purified sarcolemma. At higher concentrations, openers reduced ATPase activity, possibly through stabilization of MgADP at the channel site. K1348A and D1469N mutations attenuated the effect of openers on K ATP channel activity. Opener‐induced channel activation was also inhibited by the creatine kinase/creatine phosphate system that removes ADP from the channel complex. Thus, the K ATP channel complex functions not only as a K + conductance, but also as an enzyme regulating nucleotide‐dependent channel gating through an intrinsic ATPase activity of the SUR subunit. Modulation of the channel ATPase activity and/or scavenging the product of the ATPase reaction provide novel means to regulate cellular functions associated with K ATP channel opening.—Bienengraeber, M., Alekseev, A. E., Abraham, M. R., Carrasco, A. J., Moreau, C., Vivaudou, M., Dzeja, P. P., Terzic, A. ATPase activity of the sulfonylurea receptor: a catalytic function for the K ATP channel complex. FASEB J. 14, 1943–1952 (2000)

The mechanism by which ATP-sensitive K+ (KATP) channels open in the presence of inhibitory concentrations of ATP remains unknown. Herein, using a four-state kinetic model, we found that the nucleotide diphosphate UDP directed cardiac KATP channels to operate within intraburst transitions. These transitions are not targeted by ATP, nor the structurally unrelated sulfonylurea glyburide, which inhibit channel opening by acting on interburst transitions. Therefore, the channel remained insensitive to ATP and glyburide in the presence of UDP. “Rundown” of channel activity decreased the efficacy with which UDP could direct and maintain the channel to operate within intraburst transitions. Under this condition, the channel was sensitive to inhibition by ATP and glyburide despite the presence of UDP. This behavior of the KATP channel could be accounted for by an allosteric model of ligand-channel interaction. Thus, the response of cardiac KATP channels towards inhibitory ligands is determined by the relative lifetime the channel spends in a ligand-sensitive versus -insensitive state. Interconversion between these two conformational states represents a novel basis for KATP channel opening in the presence of inhibitory concentrations of ATP in a cardiac cell.

Transduction of energetic signals into membrane electrical events governs vital cellular functions, ranging from hormone secretion and cytoprotection to appetite control and hair growth. Central to the regulation of such diverse cellular processes are the metabolism sensing ATP-sensitive K + (K ATP ) channels. However, the mechanism that communicates metabolic signals and integrates cellular energetics with K ATP channel-dependent membrane excitability remains elusive. Here, we identify that the response of K ATP channels to metabolic challenge is regulated by adenylate kinase phosphotransfer. Adenylate kinase associates with the K ATP channel complex, anchoring cellular phosphotransfer networks and facilitating delivery of mitochondrial signals to the membrane environment. Deletion of the adenylate kinase gene compromised nucleotide exchange at the channel site and impeded communication between mitochondria and K ATP channels, rendering cellular metabolic sensing defective. Assigning a signal processing role to adenylate kinase identifies a phosphorelay mechanism essential for efficient coupling of cellular energetics with K ATP channels and associated functions.

The pharmacological phenotype of ATP-sensitive potassium (K ATP ) channels is defined by their tissue-specific regulatory subunit, the sulfonylurea receptor (SUR), which associates with the pore-forming channel core, Kir6.2. The potassium channel opener diazoxide has hyperglycemic and hypotensive properties that stem from its ability to open K ATP channels in pancreas and smooth muscle. Diazoxide is believed not to have any significant action on cardiac sarcolemmal K ATP channels. Yet, diazoxide can be cardioprotective in ischemia and has been found to bind to the presumed cardiac sarcolemmal K ATP channel-regulatory subunit, SUR2A. Here, in excised patches, diazoxide (300 μM) activated pancreatic SUR1/Kir6.2 currents and had little effect on native or recombinant cardiac SUR2A/Kir6.2 currents. However, in the presence of cytoplasmic ADP (100 μM), SUR2A/Kir6.2 channels became as sensitive to diazoxide as SUR1/Kir6.2 channels. This effect involved specific interactions between MgADP and SUR, as it required Mg 2+ , but not ATP, and was abolished by point mutations in the second nucleotide-binding domain of SUR, which impaired channel activation by MgADP. At the whole-cell level, in cardiomyocytes treated with oligomycin to block mitochondrial function, diazoxide could also activate K ATP currents only after cytosolic ADP had been raised by a creatine kinase inhibitor. Thus, ADP serves as a cofactor to define the responsiveness of cardiac K ATP channels toward diazoxide. The present demonstration of a pharmacological plasticity of K ATP channels identifies a mechanism for the control of channel activity in cardiac cells depending on the cellular ADP levels, which are elevated under ischemia.