Physiological remodelling of potassium channels in the heart

Histone deacetylase 2-dependent ventricular electrical remodeling in a porcine model of early heart failure

AIMS

Heart failure (HF) is linked to electrical remodeling that promotes ventricular arrhythmias. Underlying molecular signaling is insufficiently understood, in particular concerning patients with early disease stages. Previous observations suggest a key role for epigenetic mechanisms in cardiac remodeling processes. We hypothesized that histone deacetylases (HDACs) 1 and 2 contribute to cellular electrophysiological dysregulation in ventricular cardiomyocytes during HF development.

MATERIALS AND METHODS

HDAC and ion channel expression was quantified in a porcine model of early HF induced by short-term atrial tachypacing, resulting in atrial fibrillation with rapid ventricular rate response. Anti-Hdac1 and anti-Hdac2 siRNA treatment was employed in neonatal murine cardiomyocytes (NMCM) to study effects of HDACs on ion channel mRNA expression and action potential duration (APD).

KEY FINDINGS

Early HF was characterized by mild reduction of left ventricular ejection fraction, prolonged QTc intervals, and increased ventricular effective refractory periods. Delayed repolarization was linked to significant downregulation of HDAC2 in left ventricular (LV) tissue. In addition, there was a tendency towards reduced transcript expression of KCNJ2/K_ir2.1 K⁺ channels. In NMCM, knock-down of Hdac2 recapitulated AP prolongation. Finally, siRNA-mediated suppression of Hdac2 reduced Kcnh2/K_v11.1 K⁺ channel expression.

SIGNIFICANCE

Suppression of HDAC2 is linked to ventricular electrical remodeling of APD and ion channel expression in early stages of heart failure. This previously unrecognized mechanism may serve as basis for future approaches to prevention and treatment of ventricular arrhythmias.

The Beneficial Effects of Electroacupuncture at PC6 Acupoints (Neiguan) on Myocardial Ischemia in ASIC3 -/- mice

This study aims to investigate the possible mechanisms of electroacupuncture (EA) at PC6 to improve myocardial ischemia (MI) by regulating the cardiac transient outward potassium current channel (Ito). According to the random number table, the mice were divided into six groups of six mice each: control group, MI group, PC6, LU7 (Lieque-point), ST36 (Zusanli-point), and nonacupoint group. Mice in the control group were injected with saline (20 mg/kg, 24 hours interval), and the other ASIC3 -/- mice were injected subcutaneously twice with isoproterenol (ISO) (20 mg/kg, 24 hours interval). In the preexperiment, 5 mg/kg, 10 mg/kg, 20 mg/kg, and 30 mg/kg of ISO were used, and the results showed that 5 mg/kg and 10 mg/kg of ISO both could induce acute MI, but shorter duration of sustained MI. On the other hand, an injection of 30 mg/kg can make the mice experience arrhythmia or die immediately, and EA was operated at PC6, LU7, ST36 acupoints, and nonacupoint in the mice of PC6, LU7, ST36, and nonacupoint groups, respectively, after injecting twice. Then Western blotting techniques (Western Blot) were used to analyze the protein expressions of Kv1.4, Kv4.2, Kv4.3, and KchIP2. The results of this experiment showed that the protein expressions of Kv1.4, Kv4.2, Kv4.3, and KChIP2 in MI group were significantly lower than those in the control group (p < 0.01). Compared with MI group, the results of PC6, LU7, and ST36 groups obviously increased (p < 0.05). Furthermore, the expressions of PC6 group were higher than LU7 group and ST36 group (p < 0.05). And electrocardiogram's T-waves showed obvious pathological changes in the MI group compared to the control group (p < 0.01). After EA, the abnormal T-waves voltage of ECG in PC6, LU7, and ST36 groups was improved (p < 0.05). In addition, the rate change of PC6 group was larger than that of both LU7 and ST36 groups (p < 0.05). But the T-waves voltage of the nonacupoint group was not significantly different than that of the MI group (p > 0.05).

Orphan nuclear receptor Nur77 affects cardiomyocyte calcium homeostasis and adverse cardiac remodelling

Distinct stressors may induce heart failure. As compensation, β-adrenergic stimulation enhances myocardial contractility by elevating cardiomyocyte intracellular Ca²⁺ ([Ca²⁺]_i). However, chronic β-adrenergic stimulation promotes adverse cardiac remodelling. Cardiac expression of nuclear receptor Nur77 is enhanced by β-adrenergic stimulation, but its role in cardiac remodelling is still unclear. We show high and rapid Nur77 upregulation in cardiomyocytes stimulated with β-adrenergic agonist isoproterenol. Nur77 knockdown in culture resulted in hypertrophic cardiomyocytes. Ventricular cardiomyocytes from Nur77-deficient (Nur77-KO) mice exhibited elevated diastolic and systolic [Ca²⁺]_i and prolonged action potentials compared to wild type (WT). In vivo, these differences resulted in larger cardiomyocytes, increased expression of hypertrophic genes, and more cardiac fibrosis in Nur77-KO mice upon chronic isoproterenol stimulation. In line with the observed elevated [Ca²⁺]_i, Ca²⁺-activated phosphatase calcineurin was more active in Nur77-KO mice compared to WT. In contrast, after cardiac pressure overload by aortic constriction, Nur77-KO mice exhibited attenuated remodelling compared to WT. Concluding, Nur77-deficiency results in significantly altered cardiac Ca²⁺ homeostasis and distinct remodelling outcome depending on the type of insult. Detailed knowledge on the role of Nur77 in maintaining cardiomyocyte Ca²⁺ homeostasis and the dual role Nur77 plays in cardiac remodelling will aid in developing personalized therapies against heart failure.

Mechanisms of altered Ca²⁺ handling in heart failure

Ca²⁺ plays a crucial role in connecting membrane excitability with contraction in myocardium. The hallmark features of heart failure are mechanical dysfunction and arrhythmias; defective intracellular Ca²⁺ homeostasis is a central cause of contractile dysfunction and arrhythmias in failing myocardium. Defective Ca²⁺ homeostasis in heart failure can result from pathological alteration in the expression and activity of an increasingly understood collection of Ca²⁺ homeostatic and structural proteins, ion channels, and enzymes. This review focuses on the molecular mechanisms of defective Ca²⁺ cycling in heart failure and considers how fundamental understanding of these pathways may translate into novel and innovative therapies.