Syntaxin-1A actions on sulfonylurea receptor 2A can block acidic pH-induced cardiac K(ATP) channel activation

KATP Channels in the Cardiovascular System

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

Adenosine triphosphate-sensitive potassium channels and cardiomyopathies (Review)

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

Syntaxin-1A inhibits KATP channels by interacting with specific conserved motifs within sulfonylurea receptor 2A

We previously demonstrated that syntaxin (Syn)-1A is present in the sarcolemma of rat cardiomyocytes and binds sulfonylurea receptor (SUR) 2A nucleotide binding folds (NBFs) to inhibit ATP-sensitive potassium (K(ATP)) channel. Here, we examined for the precise domains within the NBFs of SUR2A that may interact with Syn-1A. Specifically, we tested truncated NBF protein segments encompassing the conserved motifs Walker A (W(A)), signature/Linker (L), and Walker B (W(B)). In vitro binding results indicate that the domains encompassing W(A) and L of NBF-1 and all three conserved motifs of NBF-2 bound Syn-1A. Electrophysiological studies, employing inside-out patch-clamp recordings from SUR2A/Kir6.2 expressing HEK cells and mouse cardiomyocytes, show that W(B) and L of NBF-1 and all three NBF-2 truncated protein segments reduced Syn-1A inhibition of SUR2A/K(ATP) channels. Remarkably, these same NBF-1 and -2 truncated proteins could independently disrupt the intimate FRET interactions of full length SUR2A (-mCherry) and Syn-1A (-EGFP). These results taken together indicate that Syn-1A possibly maintains inhibition of cardiac ventricular K(ATP) channels by binding to large regions of NBF-1 and NBF-2 to stabilize the NBF-1-NBF-2 heterodimer formation and prevent ATP-binding and ATP hydrolysis. Since K(ATP) channels are closely coupled to metabolic states, we postulate that these very intimate Syn-1A-SUR2A interactions are critically important for myocardial protection during stress, in which profound changes in metabolic factors (pH, ATP) could modulate these Syn-1A-SUR2A interactions.

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.

Sulfonylurea receptor 1 subunits of ATP-sensitive potassium channels and myocardial ischemia/reperfusion injury

K(ATP) channels are generally cardioprotective under conditions of metabolic impairment, consisting of pore-forming (Kir6.1 and/or Kir6.2) and sulphonylurea-binding, modulatory subunits [sulfonylurea receptor (SUR) 1, 2A, or 2B]. Cardiovascular K(ATP) channels are generally thought to consist of Kir6.2/SUR2A subunits (in the case of heart muscle) or Kir6.1/SUR2B subunits (smooth muscle), whereas SUR1-containing channels have well-documented roles in pancreatic insulin release. Recent data, however, demonstrated the presence of SUR1 subunits in mouse cardiac tissue (particularly in atria) and a surprising protection from myocardial ischemia/reperfusion in SUR1-null mice. Here, we review some of the extra-pancreatic roles assigned to SUR1 subunits and consider whether these might be involved in the sequelae of ischemia/reperfusion.

Functional interaction of the SNARE protein NtSyp121 in Ca2+ channel gating, Ca2+ transients and ABA signalling of stomatal guard cells

There is now growing evidence that membrane vesicle trafficking proteins, especially of the superfamily of SNAREs, are critical for cellular signalling in plants. Work from this laboratory first demonstrated that a soluble, inhibitory (dominant-negative) fragment of the SNARE NtSyp121 blocked K+ and Cl- channel responses to the stress-related hormone abscisic acid (ABA), but left open a question about functional impacts on signal intermediates, especially on Ca2+-mediated signalling events. Here, we report one mode of action for the SNARE mediated directly through alterations in Ca2+ channel gating and its consequent effects on cytosolic-free [Ca2+] ([Ca2+]i) elevation. We find that expressing the same inhibitory fragment of NtSyp121 blocks ABA-evoked stomatal closure, but only partially suppresses stomatal closure in the presence of the NO donor, SNAP, which promotes [Ca2+]i elevation independently of the plasma membrane Ca2+ channels. Consistent with these observations, Ca2+ channel gating at the plasma membrane is altered by the SNARE fragment in a manner effective in reducing the potential for triggering a rise in [Ca2+]i, and we show directly that its expression in vivo leads to a pronounced suppression of evoked [Ca2+]i transients. These observations offer primary evidence for the functional coupling of the SNARE with Ca2+ channels at the plant cell plasma membrane and, because [Ca2+]i plays a key role in the control of K+ and Cl- channel currents in guard cells, they underscore an important mechanism for SNARE integration with ion channel regulation during stomatal closure.