Molecular and functional remodeling of Ito by angiotensin II in the mouse left ventricle

Nucleoporin 50 mediates Kcna4 transcription to regulate cardiac electrical activity

Emerging evidence has demonstrated that nucleoporins (Nups) play a pivotal role in cell-type-specific gene regulation, but how they control the expression and activity of ion channel genes in the heart remains unclear. Here, we show that Nup50, which is localized in the nucleus of cardiomyocytes, selectively induces an increase in the transcription and translation of Kcna4. The Kcna4 gene encodes a K+ voltage-gated channel of shaker-related subfamily member 4 and is essential for regulating the action potential in cardiac membranes. Using immunofluorescence imaging, luciferase assays and chromatin immunoprecipitation assays, we identified that the direct binding of the FG-repeat domain within Nup50 to the proximity of the Kcna4 promoter was required to activate the transcription and subsequent translation of Kcna4. Functionally, Nup50 overexpression increased the currents of KCNA4-encoded Ito,s channels, and reverse knockdown of Nup50 resulted in a remarkable decrease in the amplitude of Ito,s currents in cardiomyocytes. Moreover, a positive correlation between Nup50 and Kcna4 mRNA and protein expression was observed in heart tissues subjected to ischemic insults. These findings provide insights into the homeostatic control of cardiac electrophysiology through Nup-mediated regulation.

Acquired cardiac channelopathies in epilepsy: Evidence, mechanisms, and clinical significance

There is growing evidence that cardiac dysfunction in patients with chronic epilepsy could play a pathogenic role in sudden unexpected death in epilepsy (SUDEP). Recent animal studies have revealed that epilepsy secondarily alters the expression of cardiac ion channels alongside abnormal cardiac electrophysiology and remodeling. These molecular findings represent novel evidence for an acquired cardiac channelopathy in epilepsy, distinct from inherited ion channels mutations associated with cardiocerebral phenotypes. Specifically, seizure activity has been shown to alter the messenger RNA (mRNA) and protein expression of voltage-gated sodium channels (Na_v 1.1, Na_v 1.5), voltage-gated potassium channels (K_v 4.2, K_v 4.3), sodium-calcium exchangers (NCX1), and nonspecific cation-conducting channels (HCN2, HCN4). The pathophysiology may involve autonomic dysfunction and structural cardiac disease, as both are independently associated with epilepsy and ion channel dysregulation. Indeed, in vivo and in vitro studies of cardiac pathology reveal a complex network of signaling pathways and transcription factors regulating ion channel expression in the setting of sympathetic overactivity, cardiac failure, and hypertrophy. Other mechanisms such as circulating inflammatory mediators or exogenous effects of antiepileptic medications lack evidence. Moreover, an acquired cardiac channelopathy may underlie the electrophysiologic cardiac abnormalities seen in chronic epilepsy, potentially contributing to the increased risk of malignant arrhythmias and sudden death. Therefore, further investigation is necessary to establish whether cardiac ion channel dysregulation similarly occurs in patients with epilepsy, and to characterize any pathogenic relationship with SUDEP.

Distinct patterns of atrial electrical and structural remodeling in angiotensin II mediated atrial fibrillation

Atrial fibrillation (AF) is prevalent in hypertension and elevated angiotensin II (Ang II); however, the mechanisms by which Ang II leads to AF are poorly understood. Here, we investigated the basis for this in mice treated with Ang II or saline for 3 weeks. Ang II treatment increased susceptibility to AF compared to saline controls in association with increases in P wave duration and atrial effective refractory period, as well as reductions in right and left atrial conduction velocity. Patch-clamp studies demonstrate that action potential (AP) duration was prolonged in right atrial myocytes from Ang II treated mice in association with a reduction in repolarizing K⁺ currents. In contrast, APs in left atrial myocytes from Ang II treated mice showed reductions in upstroke velocity and overshoot, as well as greater prolongations in AP duration. Ang II reduced Na⁺ current (I_Na) in the left, but not the right atrium. This reduction in I_Na was reversible following inhibition of protein kinase C (PKC) and PKCα expression was increased selectively in the left atrium in Ang II treated mice. The transient outward K⁺ current (I_to) showed larger reductions in the left atrium in association with a shift in the voltage dependence of activation. Finally, Ang II caused fibrosis throughout the atria in association with changes in collagen expression and regulators of the extracellular matrix. This study demonstrates that hypertension and elevated Ang II cause distinct patterns of electrical and structural remodeling in the right and left atria that collectively create a substrate for AF.

KChIP2 genotype dependence of transient outward current (Ito) properties in cardiomyocytes isolated from male and female mice

The transient outward current (I_to) in cardiomyocytes is largely mediated by Kv4 channels associated with Kv Channel Interacting Protein 2 (KChIP2). A knockout model has documented the critical role of KChIP2 in I_to expression. The present study was conducted to characterize in both sexes the dependence of I_to properties, including current magnitude, inactivation kinetics, recovery from inactivation and voltage dependence of inactivation, on the number of functional KChIP2 alleles. For this purpose we performed whole-cell patch-clamp experiments on isolated left ventricular cardiomyocytes from male and female mice which had different KChIP2 genotypes; i.e., wild-type (KChIP2^+/+), heterozygous knockout (KChIP2^+/-) or complete knockout of KChIP2 (KChIP2^-/-). We found in both sexes a KChIP2 gene dosage effect (i.e., a proportionality between number of alleles and phenotype) on I_to magnitude, however, concerning other I_to properties, KChIP2^+/- resembled KChIP2^+/+. Only in the total absence of KChIP2 (KChIP2^-/-) we observed a slowing of I_to kinetics, a slowing of recovery from inactivation and a negative shift of a portion of the voltage dependence of inactivation. In a minor fraction of KChIP2^-/- myocytes I_to was completely lost. The distinct KChIP2 genotype dependences of I_to magnitude and inactivation kinetics, respectively, seen in cardiomyocytes were reproduced with two-electrode voltage-clamp experiments on Xenopus oocytes expressing Kv4.2 and different amounts of KChIP2. Our results corroborate the critical role of KChIP2 in controlling I_to properties. They demonstrate that the Kv4.2/KChIP2 interaction in cardiomyocytes is highly dynamic, with a clear KChIP2 gene dosage effect on Kv4 channel surface expression but not on inactivation gating.

Chronic enalapril treatment increases transient outward potassium current in cardiomyocytes isolated from right ventricle of spontaneously hypertensive rats

It has been well established that chronic pressure overload resulting from hypertension leads to ventricular hypertrophy and electrophysiological remodeling. The transient outward potassium current (I _to) reduction described in hypertensive animals delays ventricular repolarization, leading to complex ventricular arrhythmias and sudden death. Antihypertensive drugs, as angiotensin-converting enzyme inhibitors (ACEi), can restore I _to and reduce the incidence of arrhythmic events. The purpose of this study was to evaluate the differential effects of long-term treatment with ACEi or direct-acting smooth muscle relaxant on the I _to of left and right ventricle myocytes of spontaneously hypertensive rats (SHR). Animals were divided into four groups: normotensive Wistar-Kyoto rats (WKY), hypertensive (SHR), SHR treated for 6 weeks with enalapril 10 mg/kg/day (SHRE), or hydralazine 20 mg/kg/day (SHRH). Systolic blood pressure (SBP) and hypertrophy index (heart weight/body weight (HW/BW)) were determined at the end of treatment period. Cell membrane capacitance (C _m) and I _to were assessed in cardiomyocytes isolated from left and right ventricles. The SHR exhibited significantly increased SBP and HW/BW when compared to the WKY. The treated groups, SHRE and SHRH, restored normal SBP but not HW/BW. The SHR group exhibited a diminished I _to in the left but not the right ventricle. Both the treated groups restored I _to in the left ventricle. However, in the right ventricle, only enalapril treatment modified I _to. The SHRE group exhibited a significant increase in I _to compared to all the other groups. These findings suggest that enalapril may increase I _to by a pressure overload independent mechanism.

Down-regulation of ether-a-go-go-related gene potassium channel protein through sustained stimulation of AT1 receptor by angiotensin II

We investigated the effects of AT1 receptor stimulation by angiotensin II (Ang II) on human ether-a-go-go-related gene (hERG) potassium channel protein in a heterogeneous expression system with the human embryonic kidney (HEK) 293 cells which stably expressed hERG channel protein and were transiently transfected with the human AT1 receptors (HEK293/hERG). Western-blot analysis showed that Ang II significantly decreased the expression of mature hERG channel protein (155-kDa band) in a time- and dose-dependent manner without affecting the level of immature hERG channel protein (135-kDa band). The relative intensity of 155-kDa band was 64.7±6.8% of control (P<0.01) after treatment of Ang II at 100nM for 24h. To investigate the effect of Ang II on the degradation of mature hERG channel protein, we blocked forward trafficking from ER to Golgi with a Golgi transit inhibitor brefeldin A (10μM). Ang II significantly enhanced the time-dependent reduction of mature hERG channel protein. In addition, the proteasomal inhibitor lactacystin (5μM) inhibited Ang II-mediated the reduction of mature hERG channel protein, but the lysosomal inhibitor bafilomycin A1 (1μM) had no effect on the protein. The protein kinase C (PKC) inhibitor bisindolylmaleimide 1 (1μM) antagonized the reduction of mature hERG channel protein induced by Ang II. The results indicate that sustained stimulation of AT1 receptors by Ang II reduces the mature hERG channel protein via accelerating channel proteasomal degradation involving the PKC pathway.

Effect of fosinopril on the transient outward potassium current of hypertrophied left ventricular myocardium in the spontaneously hypertensive rat

To investigate fosinopril's effect on the transient outward potassium current (Ito) of differing degrees of hypertrophied myocytes in the spontaneously hypertensive rat (SHR). Ten- and 24-week-old SHRs were used as models for cardiac hypertrophy. Hypertrophied ventricular myocytes were exposed to 1, 10, and 100 μmol/L fosinopril; the whole-cell patch-clamp technique was used to study the effects on the transient outward potassium current. Ito current density was decreased in SHR myocytes relative to controls (14.17 ± 0.31 and 11.62 ± 0.08 pA/pF in 10- and 24-week-old SHR versus 16.73 ± 0.15 pA/pF, p < 0.01). Higher concentrations of fosinopril (10 and 100 μmol/L) increased Ito peak current density in 10-week-old SHR myocytes compared with controls (14.92 ± 0.14 and 15.27 ± 0.13 pA/pF versus 14.17 ± 0.31 pA/pF, p < 0.01). Fosinopril increased Ito peak current density in 24-week-old SHR myocytes at all doses (12.70 ± 0.07, 13.74 ± 0.10, and 14.53 ± 0.13 versus 11.62 ± 0.08 pA/pF for controls, p < 0.01). Fosinopril had a greater Ito elevation potential on hypertrophied myocytes in 24-week-old compared with 10-week-old SHR for each dose (1.08 ± 0.09 versus 0.37 ± 0.26 pA/pF, p < 0.01; 2.13 ± 0.05 versus 0.75 ± 0.35 pA/pF, p < 0.01; 2.92 ± 0.07 versus 1.10 ± 0.40 pA/pF, p < 0.01). Fosinopril increased Ito current density in hypertrophied myocytes. This effect was more pronounced in myocytes with a greater degree of hypertrophy.

Bone morphogenetic protein-4 contributes to the down-regulation of Kv4.3 K+ channels in pathological cardiac hypertrophy

Kv4.3 K(+) channels contributing to Ito are involved in the repolarization of cardiac action potential. Kv4.3 K(+) channels decrease in pathological cardiac hypertrophy, but the mechanism remains unclear. Our previous study found that the expression of bone morphogenetic protein 4 (BMP4) increased in pressure-overload and Ang II constant infusion induced cardiac hypertrophy. Since the downregulation of Kv4.3 K(+) channels and the upregulation of BMP4 simultaneously occur in pathological cardiac hypertrophy, we hypothesize that the up-regulated BMP4 would contribute to the downregulation of Kv4.3 K(+) channels in cardiac hypertrophy. We found that BMP4 treatment reduced Kv4.3 but not Kv4.2 and Kv1.4 K(+) channel protein expression, and BMP4-induced decrease of Kv4.3 K(+) channel protein expression was reversed by BMP4 inhibitor noggin and DMH1 in cultured cardiomyocytes in vitro. BMP4-induced decrease of Kv4.3 K(+) channel protein expression was also reversed by the NADPH oxidase inhibitor apocynin and the radical scavenger tempol. In in vivo transverse aortic constriction (TAC)-induced cardiac hypertrophy, constant infusion of DMH1 completely rescued TAC-induced down-regulation of Kv4.3 K(+) channel protein expression. We conclude that BMP4 contributes to the downregulation of Kv4.3 K(+) channels in pathological cardiac hypertrophy and the underlying mechanism might be through increasing ROS production.

Growth factor-induced mobilization of cardiac progenitor cells reduces the risk of arrhythmias, in a rat model of chronic myocardial infarction

Heart repair by stem cell treatment may involve life-threatening arrhythmias. Cardiac progenitor cells (CPCs) appear best suited for reconstituting lost myocardium without posing arrhythmic risks, being commissioned towards cardiac phenotype. In this study we tested the hypothesis that mobilization of CPCs through locally delivered Hepatocyte Growth Factor and Insulin-Like Growth Factor-1 to heal chronic myocardial infarction (MI), lowers the proneness to arrhythmias. We used 133 adult male Wistar rats either with one-month old MI and treated with growth factors (GFs, n = 60) or vehicle (V, n = 55), or sham operated (n = 18). In selected groups of animals, prior to and two weeks after GF/V delivery, we evaluated stress-induced ventricular arrhythmias by telemetry-ECG, cardiac mechanics by echocardiography, and ventricular excitability, conduction velocity and refractoriness by epicardial multiple-lead recording. Invasive hemodynamic measurements were performed before sacrifice and eventually the hearts were subjected to anatomical, morphometric, immunohistochemical, and molecular biology analyses. When compared with untreated MI, GFs decreased stress-induced arrhythmias and concurrently prolonged the effective refractory period (ERP) without affecting neither the duration of ventricular repolarization, as suggested by measurements of QTc interval and mRNA levels for K-channel α-subunits Kv4.2 and Kv4.3, nor the dispersion of refractoriness. Further, markers of cardiomyocyte reactive hypertrophy, including mRNA levels for K-channel α-subunit Kv1.4 and β-subunit KChIP2, interstitial fibrosis and negative structural remodeling were significantly reduced in peri-infarcted/remote ventricular myocardium. Finally, analyses of BrdU incorporation and distribution of connexin43 and N-cadherin indicated that cytokines generated new vessels and electromechanically-connected myocytes and abolished the correlation of infarct size with deterioration of mechanical function. In conclusion, local injection of GFs ameliorates electromechanical competence in chronic MI. Reduced arrhythmogenesis is attributable to prolongation of ERP resulting from improved intercellular coupling via increased expression of connexin43, and attenuation of unfavorable remodeling.