Increasing gap junction coupling suppresses ibutilide-induced torsades de pointes

Connexins in the Heart: Regulation, Function and Involvement in Cardiac Disease

Connexins are a family of transmembrane proteins that play a key role in cardiac physiology. Gap junctional channels put into contact the cytoplasms of connected cardiomyocytes, allowing the existence of electrical coupling. However, in addition to this fundamental role, connexins are also involved in cardiomyocyte death and survival. Thus, chemical coupling through gap junctions plays a key role in the spreading of injury between connected cells. Moreover, in addition to their involvement in cell-to-cell communication, mounting evidence indicates that connexins have additional gap junction-independent functions. Opening of unopposed hemichannels, located at the lateral surface of cardiomyocytes, may compromise cell homeostasis and may be involved in ischemia/reperfusion injury. In addition, connexins located at non-canonical cell structures, including mitochondria and the nucleus, have been demonstrated to be involved in cardioprotection and in regulation of cell growth and differentiation. In this review, we will provide, first, an overview on connexin biology, including their synthesis and degradation, their regulation and their interactions. Then, we will conduct an in-depth examination of the role of connexins in cardiac pathophysiology, including new findings regarding their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription or signaling regulation.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

Fluconazole-induced long QT syndrome via impaired human ether-a-go-go-related gene (hERG) protein trafficking in rabbits

Aims

hERG protein trafficking deficiency has long been known in drug-induced long QT syndrome (LQTS). However, validated evidence from in vivo data kept scanty. Our goal was to investigate the proarrhythmic action of fluconazole and its underlying mechanism in an animal model.

Methods and results

Twenty female Japanese long-eared white rabbits were randomly distributed into a control group and a fluconazole group for a chronic 2-week treatment. The control group was treated with 0.5% sodium carboxymethylcellulose (CMCNa), and the fluconazole group was treated with fluconazole. Electrocardiograms (ECGs) were recorded during the experimental period. Isolated arterially perfused left ventricular wedge preparations from the rabbits were made 2 weeks after treatment, and the arrhythmia events, the transmural ECG, and action potential from both the endocardium and epicardium were recorded. The changes in hERG protein expression were measured by western blot. The fluconazole group showed a longer QT interval 1 week after treatment (P < 0.05) and a higher arrhythmia occurrence 2 weeks after treatment (P < 0.05) than the control group. The fluconazole group also showed a longer transmural dispersion of repolarization and a higher occurrence of life-threatening torsades de pointes in arterially perfused left ventricular preparations. Furthermore, western blot analysis showed that the density of mature hERG protein was lower in the fluconazole group than that in the control group.

Conclusion

Fluconazole can prolong the QT interval and possess proarrhythmic activity due to its inhibition of hERG protein trafficking in our experimental model. These findings may impact the clinical potential of fluconazole in humans.

Electrophysiological mechanisms of long and short QT syndromes

The QT interval on the human electrocardiogram is normally in the order of 450 ms, and reflects the summated durations of action potential (AP) depolarization and repolarization of ventricular myocytes. Both prolongation and shortening in the QT interval have been associated with ventricular tachy-arrhythmias, which predispose affected individuals to sudden cardiac death. In this article, the molecular determinants of the AP duration and the causes of long and short QT syndromes (LQTS and SQTS) are explored. This is followed by a review of the recent advances on their arrhythmogenic mechanisms involving reentry and/or triggered activity based on experiments conducted in mouse models. Established and novel clinical risk markers based on the QT interval for the prediction of arrhythmic risk and cardiovascular mortality are presented here. It is concluded by a discussion on strategies for the future rational design of anti-arrhythmic agents.

Study of factors affecting the progression and termination of drug induced Torsade de pointes in two dimensional cardiac tissue

INTRODUCTION

To study the conditions leading to the initiation and termination of drug induced Torsade de pointes (TdP) along with QT prolongation.

METHODS

A 2D anisotropic transmural section of the ventricular myocardium is modeled using the TP06 equations and the cells are interconnected with gap junction conductances (GJC). The tissue is remodeled by reducing the repolarization reserve (by increasing calcium current (I_CaL)) of all cells thus making them vulnerable to development of early after depolarizations (EADs).

RESULTS

Clinical risk conditions like decreased potassium current (I_Kr), bradycardia, hypokalemia and short-long-short (SLS) triggering sequences are included in the tissue. A pseudo-electrocardiogram is created to realize the intensity of remodeling required in presence of risk factors to initiate TdP. On initiating TdP, the effect of increasing GJC and decreasing I_CaL is shown to terminate a non-self-limiting TdP.

CONCLUSION

Without the inclusion of underlying increase in I_CaL along with risk factors, TdP cannot be initiated.

Changes in cardiac Nav1.5 expression, function, and acetylation by pan-histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors are small molecule anticancer therapeutics that exhibit limiting cardiotoxicities including QT interval prolongation and life-threatening cardiac arrhythmias. Because the molecular mechanisms for HDAC inhibitor-induced cardiotoxicity are poorly understood, we performed whole cell patch voltage-clamp experiments to measure cardiac sodium currents (I_Na) from wild-type neonatal mouse ventricular or human-induced pluripotent stem cell-derived cardiomyocytes treated with trichostatin A (TSA), vorinostat (VOR), or romidepsin (FK228). All three pan-HDAC inhibitors dose dependently decreased peak I_Na density and shifted the voltage activation curve 3- to 8-mV positive. Increases in late I_Na were not observed despite a moderate slowing of the inactivation rate at low activating potentials (<-40 mV). Scn5a mRNA levels were not significantly altered but Na_V1.5 protein levels were significantly reduced. Immunoprecipitation with anti-Na_V1.5 and Western blotting with anti-acetyl-lysine antibodies indicated that Na_V1.5 acetylation is increased in vivo after HDAC inhibition. FK228 inhibited total cardiac HDAC activity with two apparent IC₅₀s of 5 nM and 1.75 μM, consistent with previous findings with TSA and VOR. FK228 also decreased ventricular gap junction conductance (g_j), again consistent with previous findings. We conclude that pan-HDAC inhibition reduces cardiac I_Na density and Na_V1.5 protein levels without affecting late I_Na amplitude and, thus, probably does not contribute to the reported QT interval prolongation and arrhythmias associated with pan-HDAC inhibitor therapies. Conversely, reductions in g_j may enhance the occurrence of triggered activity by limiting electrotonic inhibition and, combined with reduced I_Na, slow myocardial conduction and increase vulnerability to reentrant arrhythmias.