Epigenetic regulation of cardiac electrophysiology in atrial fibrillation: HDAC2 determines action potential duration and suppresses NRSF in cardiomyocytes.

Atrial fibrillation (AF) is associated with electrical remodeling, leading to cellular electrophysiological dysfunction and arrhythmia perpetuation. Emerging evidence suggests a key role for epigenetic mechanisms in the regulation of ion channel expression. Histone deacetylases (HDACs) control gene expression through deacetylation of histone proteins. We hypothesized that class I HDACs in complex with neuron-restrictive silencer factor (NRSF) determine atrial K+ channel expression. AF was characterized by reduced atrial HDAC2 mRNA levels and upregulation of NRSF in humans and in a pig model, with regional differences between right and left atrium. In vitro studies revealed inverse regulation of Hdac2 and Nrsf in HL-1 atrial myocytes. A direct association of HDAC2 with active regulatory elements of cardiac K+ channels was revealed by chromatin immunoprecipitation. Specific knock-down of Hdac2 and Nrsf induced alterations of K+ channel expression. Hdac2 knock-down resulted in prolongation of action potential duration (APD) in neonatal rat cardiomyocytes, whereas inactivation of Nrsf induced APD shortening. Potential AF-related triggers were recapitulated by experimental tachypacing and mechanical stretch, respectively, and exerted differential effects on the expression of class I HDACs and K+ channels in cardiomyocytes. In conclusion, HDAC2 and NRSF contribute to AF-associated remodeling of APD and K+ channel expression in cardiomyocytes via direct interaction with regulatory chromatin regions. Specific modulation of these factors may provide a starting point for the development of more individualized treatment options for atrial fibrillation.

Inhibition of Histone Deacetylases Induces K+ Channel Remodeling and Action Potential Prolongation in HL-1 Atrial Cardiomyocytes.

BACKGROUND/AIMS: Cardiac arrhythmias are triggered by environmental stimuli that may modulate expression of cardiac ion channels. Underlying epigenetic regulation of cardiac electrophysiology remains incompletely understood. Histone deacetylases (HDACs) control gene expression and cardiac integrity. We hypothesized that class I/II HDACs transcriptionally regulate ion channel expression and determine action potential duration (APD) in cardiac myocytes. METHODS: Global class I/II HDAC inhibition was achieved by administration of trichostatin A (TSA). HDAC-mediated effects on K+ channel expression and electrophysiological function were evaluated in murine atrial cardiomyocytes (HL-1 cells) using real-time PCR, Western blot, and patch clamp analyses. Electrical tachypacing was employed to recapitulate arrhythmia-related effects on ion channel remodeling in the absence and presence of HDAC inhibition. RESULTS: Global HDAC inhibition increased histone acetylation and prolonged APD90 in atrial cardiomyocytes compared to untreated control cells. Transcript levels of voltage-gated or inwardly rectifying K+ channels Kcnq1, Kcnj3 and Kcnj5 were significantly reduced, whereas Kcnk2, Kcnj2 and Kcnd3 mRNAs were upregulated. Ion channel remodeling was similarly observed at protein level. Short-term tachypacing did not induce significant transcriptional K+ channel remodeling. CONCLUSION: The present findings link class I/II HDAC activity to regulation of ion channel expression and action potential duration in atrial cardiomyocytes. Clinical implications for HDAC-based antiarrhythmic therapy and cardiac safety of HDAC inhibitors require further investigation.

Differential regulation of KCa 2.1 (KCNN1) K+ channel expression by histone deacetylases in atrial fibrillation with concomitant heart failure.

Atrial fibrillation (AF) with concomitant heart failure (HF) poses a significant therapeutic challenge. Mechanism-based approaches may optimize AF therapy. Small-conductance, calcium-activated K+ (KCa , KCNN) channels contribute to cardiac action potential repolarization. KCNN1 exhibits predominant atrial expression and is downregulated in chronic AF patients with preserved cardiac function. Epigenetic regulation is suggested by AF suppression following histone deacetylase (HDAC) inhibition. We hypothesized that HDAC-dependent KCNN1 remodeling contributes to arrhythmogenesis in AF complicated by HF. The aim of this study was to assess KCNN1 and HDAC1-7 and 9 transcript levels in AF/HF patients and in a pig model of atrial tachypacing-induced AF with reduced left ventricular function. In HL-1 atrial myocytes, tachypacing and anti-Hdac siRNAs were employed to investigate effects on Kcnn1 mRNA levels. KCNN1 expression displayed side-specific remodeling in AF/HF patients with upregulation in left and suppression in right atrium. In pigs, KCNN1 remodeling showed intermediate phenotypes. HDAC levels were differentially altered in humans and pigs, reflecting highly variable epigenetic regulation. Tachypacing recapitulated downregulation of Hdacs 1, 3, 4, 6, and 7 with a tendency towards reduced Kcnn1 levels in vitro, indicating that atrial high rates induce remodeling. Finally, Kcnn1 expression was decreased by knockdown of Hdacs 2, 3, 6, and 7 and enhanced by genetic Hdac9 inactivation, while anti-Hdac 1, 4, and 5 siRNAs did not affect Kcnn1 transcript levels. In conclusion, KCNN1 and HDAC expression is differentially remodeled in AF complicated by HF. Direct regulation of KCNN1 by HDACs in atrial myocytes provides a basis for mechanism-based antiarrhythmic therapy.

HDAC2-dependent remodeling of KCa2.2 (KCNN2) and KCa2.3 (KCNN3) K+ channels in atrial fibrillation with concomitant heart failure.

AIMS: Atrial fibrillation (AF) with concomitant heart failure (HF) is associated with prolonged atrial refractoriness. Small-conductance, calcium-activated K+ (KCa, KCNN) channels promote action potential (AP) repolarization. KCNN2 and KCNN3 variants are associated with AF risk. In addition, histone deacetylase (HDAC)-related epigenetic mechanisms have been implicated in AP regulation. We hypothesized that HDAC2-dependent remodeling of KCNN2 and KCNN3 expression contributes to atrial arrhythmogenesis in AF complicated by HF. The objectives were to assess HDAC2 and KCNN2/3 transcript levels in AF/HF patients and in a pig model, and to investigate cellular epigenetic effects of HDAC2 inactivation on KCNN expression. MATERIALS AND METHODS: HDAC2 and KCNN2/3 transcript levels were quantified in patients with AF and HF, and in a porcine model of atrial tachypacing-induced AF and reduced left ventricular function. Tachypacing and anti-Hdac2 siRNA treatment were employed in HL-1 atrial myocytes to study effects on KCNN2/3 mRNA and KCa protein abundance. KEY FINDINGS: Atrial KCNN2 and KCNN3 expression was reduced in AF/HF patients and in a corresponding pig model. HDAC2 displayed significant downregulation in humans and a tendency towards reduced expression in right atrial tissue of pigs. Tachypacing recapitulated downregulation of Kcnn2/KCa2.2, Kcnn3/KCa2.3 and Hdac2/HDAC2, indicating that high atrial rates trigger epigenetic remodeling mechanisms. Finally, knock-down of Hdac2 in vitro reduced Kcnn3/KCa2.3 expression. SIGNIFICANCE: KCNN2/3 and HDAC2 expression is suppressed in AF complicated by HF. Hdac2 directly regulates Kcnn3 mRNA levels in atrial cells. The mechanistic and therapeutic significance of epigenetic electrophysiological effects in AF requires further validation.

Trigger-Specific Remodeling of KCa2 Potassium Channels in Models of Atrial Fibrillation.

AIM: Effective antiarrhythmic treatment of atrial fibrillation (AF) constitutes a major challenge, in particular, when concomitant heart failure (HF) is present. HF-associated atrial arrhythmogenesis is distinctly characterized by prolonged atrial refractoriness. Small-conductance, calcium-activated K+ (KCa, SK, KCNN) channels contribute to cardiac action potential repolarization and are implicated in AF susceptibility and therapy. The mechanistic impact of AF/HF-related triggers on atrial KCa channels is not known. We hypothesized that tachycardia, stretch, ß-adrenergic stimulation, and hypoxia differentially determine KCa2.1-2.3 channel remodeling in atrial cells. METHODS: KCNN1-3 transcript levels were assessed in AF/HF patients and in a pig model of atrial tachypacing-induced AF with reduced left ventricular function. HL-1 atrial myocytes were subjected to proarrhythmic triggers to investigate the effects on Kcnn mRNA and KCa channel protein. RESULTS: Atrial KCNN1-3 expression was reduced in AF/HF patients. KCNN2 and KCNN3 suppression was recapitulated in the corresponding pig model. In contrast to human AF, KCNN1 remained unchanged in pigs. Channel- and stressor-specific remodeling was revealed in vitro. Lower expression levels of KCNN1/KCa2.1 were linked to stretch and ß-adrenergic stimulation. Furthermore, KCNN3/KCa2.3 expression was suppressed upon tachypacing and hypoxia. Finally, KCNN2/KCa2.2 abundance was specifically enhanced by hypoxia. CONCLUSION: Reduction of KCa2.1-2.3 channel expression might contribute to the action potential prolongation in AF complicated by HF. Subtype-specific KCa2 channel remodeling induced by tachypacing, stretch, ß-adrenergic stimulation, or hypoxia is expected to differentially determine atrial remodeling, depending on patient-specific activation of each triggering factor. Stressor-dependent KCa2 regulation in atrial myocytes provides a starting point for mechanism-based antiarrhythmic therapy.

Inhibition of cardiac Kv4.3 (Ito) channel isoforms by class I antiarrhythmic drugs lidocaine and mexiletine.

Transient outward K+ current, Ito, contributes to cardiac action potential generation and is primarily carried by Kv4.3 (KCND3) channels. Two Kv4.3 isoforms are expressed in human ventricle and show differential remodeling in heart failure (HF). Lidocaine and mexiletine may be applied in selected patients to suppress ventricular arrhythmias, without effects on sudden cardiac death or mortality. Isoform-dependent effects of antiarrhythmic drugs on Kv4.3 channels and potential implications for remodeling-based antiarrhythmic management have not been assessed to date. We sought to test the hypotheses that Kv4.3 channels are targeted by lidocaine and mexiletine, and that drug sensitivity is determined in isoform-specific manner. Expression of KCND3 isoforms was quantified using qRT-PCR in left ventricular samples of patients with HF due to either ischemic or dilated cardiomyopathies (ICM or DCM). Long (Kv4.3-L) and short (Kv4.3-S) isoforms were heterologously expressed in Xenopus laevis oocytes to study drug sensitivity and effects on biophysical characteristics activation, deactivation, inactivation, and recovery from inactivation. In the present HF patient cohort KCND3 isoform expression did not differ between ICM and DCM. In vitro, lidocaine (IC50-Kv4.3-L: 0.8 mM; IC50-Kv4.3-S: 1.2 mM) and mexiletine (IC50-Kv4.3-L: 146 µM; IC50-Kv4.3-S: 160 µM) inhibited Kv4.3 with different sensitivity. Biophysical analyses identified accelerated and enhanced inactivation combined with delayed recovery from inactivation as primary biophysical mechanisms underlying Kv4.3 current reduction. In conclusion, differential effects on Kv4.3 isoforms extend the electropharmacological profile of lidocaine and mexiletine. Patient-specific remodeling of Kv4.3 isoforms may determine individual drug responses and requires consideration during clinical application of compounds targeting Kv4.3.

Cardiovascular pharmacology of K2P17.1 (TASK-4, TALK-2) two-pore-domain K+ channels.

K2P17.1 (TASK-4, TALK-2) potassium channels are expressed in the heart and represent potential targets for pharmacological management of atrial and ventricular arrhythmias. Reduced K2P17.1 expression was found in atria and ventricles of heart failure (HF) patients. Modulation of K2P17.1 currents by antiarrhythmic compounds has not been comprehensively studied to date. The objective of this study was to investigate acute effects of clinically relevant antiarrhythmic drugs on human K2P17.1 channels to provide a more complete picture of K2P17.1 electropharmacology. Whole-cell patch clamp and two-electrode voltage clamp electrophysiology was employed to study human K2P17.1 channel pharmacology. K2P17.1 channels expressed in Xenopus laevis oocytes were screened for sensitivity to antiarrhythmic drugs, revealing significant activation by propafenone (+ 296%; 100 µM), quinidine (+ 58%; 100 µM), mexiletine (+ 21%; 100 µM), propranolol (+ 139%; 100 µM), and metoprolol (+ 17%; 100 µM) within 60 min. In addition, the currents were inhibited by amiodarone (- 13%; 100 µM), sotalol (- 10%; 100 µM), verapamil (- 21%; 100 µM), and ranolazine (- 8%; 100 µM). K2P17.1 channels were not significantly affected by ajmaline and carvedilol. Concentration-dependent K2P17.1 activation by propafenone was characterized in more detail. The onset of activation was fast, and current-voltage relationships were not modulated by propafenone. K2P17.1 activation was confirmed in mammalian Chinese hamster ovary cells, revealing 7.8-fold current increase by 100 µM propafenone. Human K2P17.1 channels were sensitive to multiple antiarrhythmic drugs. Differential pharmacological regulation of repolarizing K2P17.1 background K+ channels may be employed for personalized antiarrhythmic therapy.