Modulation of cardiac ERG1 K(+) channels by cGMP signaling

Vericiguat suppresses ventricular tachyarrhythmias inducibility in a rabbit myocardial infarction model

BACKGROUND

The VICTORIA trial demonstrated a significant decrease in cardiovascular events through vericiguat therapy. This study aimed to assess the potential mechanisms responsible for the reduction of cardiovascular events with vericiguat therapy in a rabbit model of myocardial infarction (MI).

METHODS

A chronic MI rabbit model was created through coronary artery ligation. Following 4 weeks, the hearts were harvested and Langendorff perfused. Subsequently, electrophysiological examinations and dual voltage-calcium optical mapping studies were conducted at baseline and after administration of vericiguat at a dose of 5 μmol/L.

RESULTS

Acute vericiguat therapy demonstrated a significant reduction in premature ventricular beat burden and effectively suppressed ventricular arrhythmic inducibility. The electrophysiological influences of vericiguat therapy included an increased ventricular effective refractory period, prolonged action potential duration, and accelerated intracellular calcium (Cai) homeostasis, leading to the suppression of action potential and Cai alternans. The pacing-induced ventricular arrhythmias exhibited a reentrant pattern, attributed to fixed or functional conduction block in the peri-infarct zone. Vericiguat therapy effectively mitigated the formation of cardiac alternans as well as the development of reentrant impulses, providing additional anti-arrhythmic benefits.

CONCLUSIONS

In the MI rabbit model, vericiguat therapy demonstrates anti-ventricular arrhythmia effects. The vericiguat therapy reduces ventricular ectopic beats, inhibiting the initiation of ventricular arrhythmias. Furthermore, the therapy successfully suppresses cardiac alternans, preventing conduction block and, consequently, the formation of reentry circuits.

hERG1 channel subunit composition mediates proton inhibition of rapid delayed rectifier potassium current (IKr) in cardiomyocytes derived from hiPSCs

The voltage-gated channel, hERG1, conducts the rapid delayed rectifier potassium current (I_Kr) and is critical for human cardiac repolarization. Reduced I_Kr causes long QT syndrome and increases the risk for cardiac arrhythmia and sudden death. At least two subunits form functional hERG1 channels, hERG1a and hERG1b. Changes in hERG1a/1b abundance modulate I_Kr kinetics, magnitude, and drug sensitivity. Studies from native cardiac tissue suggest that hERG1 subunit abundance is dynamically regulated, but the impact of altered subunit abundance on I_Kr and its response to external stressors is not well understood. Here, we used a substrate-driven human-induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) maturation model to investigate how changes in relative hERG1a/1b subunit abundance impact the response of native I_Kr to extracellular acidosis, a known component of ischemic heart disease and sudden infant death syndrome. I_Kr recorded from immatured hiPSC-CMs displays a 2-fold greater inhibition by extracellular acidosis (pH 6.3) compared with matured hiPSC-CMs. Quantitative RT-PCR and immunocytochemistry demonstrated that hERG1a subunit mRNA and protein were upregulated and hERG1b subunit mRNA and protein were downregulated in matured hiPSC-CMs compared with immatured hiPSC-CMs. The shift in subunit abundance in matured hiPSC-CMs was accompanied by increased I_Kr. Silencing hERG1b's impact on native I_Kr kinetics by overexpressing a polypeptide identical to the hERG1a N-terminal Per-Arnt-Sim domain reduced the magnitude of I_Kr proton inhibition in immatured hiPSC-CMs to levels comparable to those observed in matured hiPSC-CMs. These data demonstrate that hERG1 subunit abundance is dynamically regulated and determines I_Kr proton sensitivity in hiPSC-CMs.

A Mathematical Model of the Mouse Atrial Myocyte With Inter-Atrial Electrophysiological Heterogeneity

Biophysically detailed mathematical models of cardiac electrophysiology provide an alternative to experimental approaches for investigating possible ionic mechanisms underlying the genesis of electrical action potentials and their propagation through the heart. The aim of this study was to develop a biophysically detailed mathematical model of the action potentials of mouse atrial myocytes, a popular experimental model for elucidating molecular and cellular mechanisms of arrhythmogenesis. Based on experimental data from isolated mouse atrial cardiomyocytes, a set of mathematical equations for describing the biophysical properties of membrane ion channel currents, intracellular Ca²⁺ handling, and Ca²⁺-calmodulin activated protein kinase II and β-adrenergic signaling pathways were developed. Wherever possible, membrane ion channel currents were modeled using Markov chain formalisms, allowing detailed representation of channel kinetics. The model also considered heterogeneous electrophysiological properties between the left and the right atrial cardiomyocytes. The developed model was validated by its ability to reproduce the characteristics of action potentials and Ca²⁺ transients, matching quantitatively to experimental data. Using the model, the functional roles of four K⁺ channel currents in atrial action potential were evaluated by channel block simulations, results of which were quantitatively in agreement with existent experimental data. To conclude, this newly developed model of mouse atrial cardiomyocytes provides a powerful tool for investigating possible ion channel mechanisms of atrial electrical activity at the cellular level and can be further used to investigate mechanisms underlying atrial arrhythmogenesis.

SUMO co-expression modifies KV 11.1 channel activity

AIM

The voltage-gated potassium channel K_V 11.1 is the molecular basis for the I_Kr current, which plays an important role in cardiac physiology. Its malfunction is associated with both inherited and acquired cardiac arrhythmias. Native currents differ from those in experimental models, suggesting additional regulatory mechanisms. We hypothesized that the post-translational modification sumoylation fine-tunes channel activity.

METHODS

The functional effects of sumoylation on K_V 11.1 were addressed by employing two-electrode voltage-clamp (TEVC) experiments in Xenopus laevis oocytes. Site-directed mutagenesis enabled a further analysis of the SUMO-target amino acids. We assessed protein expression levels and used confocal imaging for localization studies.

RESULTS

Co-expression with Ubc9 and SUMO alters the electrophysiological properties of K_V 11.1 leading to a decrease in steady-state current amplitude largely due to faster inactivation and alteration of deactivation kinetics. We identified three lysines (K21, K93 and K116) in the PAS domain as the putative SUMO-targets.

CONCLUSION

This study indicates K_V 11.1 as a sumoylation target and offers three main targets: K21, K93, and K116. Furthermore, it proposes an underlying mechanism for the observed kinetic impact of the PAS domain.

Effects of Temperature on Heteromeric Kv11.1a/1b and Kv11.3 Channels

Kv11.1 channels are crucial in cardiac physiology, and there is increasing evidence of physiological roles of different Kv11 channels outside the heart. The HERG (human Kv11.1a) channel has previously been shown to carry substantially more current at elevated temperatures, and we have now comparably investigated the temperature dependence of neuronal Kv11.3 channels and the more ubiquitous heteromeric Kv11.1a/1b channels. Transiently expressed rat Kv11 channels were studied at 21°C, 30°C, and 35°C. At near-physiological temperature, the maximal sustained outward current density was almost three times the mean value obtained at room temperature for Kv11.1a/1b, and increased by ∼150% for Kv11.3. For both channels, reduced inactivation contributed to the current increase at higher temperature. Elevated temperature moved Kv11.1a/1b isochronal activation curves to more negative potentials, but shifted the potential of half-maximal Kv11.3 channel activation to more depolarized values and reduced its voltage sensitivity. Thus, increased temperature stabilized the open state over the closed state of Kv11.1a/1b channels and exerted the opposite effect on Kv11.3 channel activation. Both Kv11 channels exhibited an overall high temperature sensitivity of most gating parameters, with remarkably high Q10 factors of ∼5 for the rate of Kv11.1a/1b activation. The Q10 factors for Kv11.3 gating were more uniform, but still higher for activation than for inactivation kinetics. The results demonstrate that characteristic differences between Kv11.1a/1b and Kv11.3 determined at room temperature do not necessarily apply to physiological conditions. The data provided here can aid in the design of models that will enhance our understanding of the role of Kv11 currents in excitable cells.

High-fat diet-dependent modulation of the delayed rectifier K(+) current in adult guinea pig atrial myocytes

Obesity is associated with hyperlipidemia, electrical remodeling of the heart, and increased risk of supraventricular arrhythmias in both male and female patients. The delayed rectifier K(+) current (IK), is an important regulator of atrial repolarization. There is a paucity of studies on the functional role of IK in response to obesity. Here, we assessed the obesity-mediated functional modulation of IK in low-fat diet (LFD), and high-fat diet (HFD) fed adult guinea pigs. Guinea pigs were randomly divided into control and obese groups fed, ad libitum, with a LFD (10 kcal% fat) or a HFD (45 kcal% fat) respectively. Action potential duration (APD), and IK were studied in atrial myocytes and IKr and IKs in HEK293 cells using whole-cell patch clamp electrophysiology. HFD guinea pigs displayed a significant increase in body weight, total cholesterol and total triglycerides within 50 days. Atrial APD at 30% (APD30) and 90% (APD90) repolarization were shorter, while atrial IK density was significantly increased in HFD guinea pigs. Exposure to palmitic acid (PA) increased heterologously expressed IKr and IKs densities, while oleic acid (OA), severely reduced IKr and had no effect on IKs. The data are first to show that in obese guinea pigs abbreviated APD is due to increased IK density likely through elevations of PA. Our findings may have crucial implications for targeted treatment options for obesity-related arrhythmias.

The Therapeutic Potential of hERG1 K+ Channels for Treating Cancer and Cardiac Arrhythmias

hERG potassium channels present pharmacologists and medicinal chemists with a dilemma. On the one hand hERG is a major reason for drugs being withdrawn from the market because of drug induced long QT syndrome and the associated risk of inducing sudden cardiac death, and yet hERG blockers are still widely used in the clinic to treat cardiac arrhythmias. Moreover, in the last decade overwhelming evidence has been provided that hERG channels are aberrantly expressed in cancer cells and that they contribute to tumour cell proliferation, resistance to apoptosis, and neoangiogenesis. Here we provide an overview of the properties of hERG channels and their role in excitable cells of the heart and nervous system as well as in cancer. We consider the therapeutic potential of hERG, not only with regard to the negative impact due to drug induced long QT syndrome, but also its future potential as a treatment in the fight against cancer.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Deciphering hERG channels: molecular basis of the rapid component of the delayed rectifier potassium current

The rapid component of the delayed rectifier potassium current (I(Kr)), encoded by the ether-a-go-go-related gene (ERG1, officially denominated as KCNH2), is a major contributor to repolarization in the mammalian heart. Acute (e.g. drug-induced) and chronic (e.g. inherited genetic disorder) disruptions of this current can lead to prolongation of the action potential and potentiate occurrence of lethal arrhythmias. Many cardiac and non-cardiac drugs show high affinity for the I(Kr) channel and it is therefore extensively studied during safety pharmacology. The unique biophysical and pharmacological properties of the I(Kr) channel are largely recapitulated by expressing the human variant (hERG1a) in overexpressing systems. hERG1a channels are tetramers consisting of four 1159 amino acid long proteins and have electrophysiological properties similar, but not identical, to native I(Kr). In the search for an explanation to the discrepancies between I(Kr) and hERG1a channels, two alternative hERG1 proteins have been found. Alternative transcription of hERG1 leads to a protein with a 56 amino acid shorter N-terminus, known as hERG1b. hERG1b can form channels alone or coassemble with hERG1a. Alternative splicing leads to an alternate C-terminus and a protein known as hERGuso. hERGuso and hERG1b regulate hERG1a channel trafficking, functional expression and channel kinetics. Expression of hERGuso leads to a reduced number of channels at the plasma membrane and thereby reduces current density. On the contrary, co-assembly with hERG1b alters channel kinetics resulting in more available channels and a larger current. These findings have implication for understanding mechanisms of disease, acute and chronic drug effects, and potential gender differences.

Expression pattern of Kv11 (Ether à-go-go-related gene; erg) K+ channels in the mouse retina

In response to light, most retinal neurons exhibit gradual changes in membrane potential. Therefore K+ channels that mediate threshold currents are well-suited for the fine-tuning of signal transduction. In the present study we demonstrate the expression of the different Kv11 (ether-à-go-go related gene; erg) channel subunits in the human and mouse retina by RT PCR and quantitative PCR, respectively. Immunofluorescence analysis with cryosections of mouse retinae revealed the following local distribution of the three Kv11 subunits: Kv11.1 (m-erg1) displayed the most abundant expression with the strongest immunoreactivity in rod bipolar cells. In addition, immunoreactivity was found in the inner part of the outer plexiform layer (OPL), in the inner plexiform layer (IPL) and in the inner segments of photoreceptors. Immunoreactivity for Kv11.2 (m-erg2) was observed in the outer part of the OPL and throughout the IPL. Double-labeling for vGluT1 or synaptophysin indicated a mainly presynaptic localization of Kv11.2. While no significant staining for Kv11.3 (m-erg3) was detected in the neuronal retina, strong Kv11.3 immunoreactivity was present in the apical membrane of the retinal pigment epithelium. The different expression levels were confirmed by real-time PCR showing almost equal levels of Kv11.1 and Kv11.2, while Kv11.3 mRNA expression was significantly lower. The two main splice variants of Kv11.1, isoforms a and b were detected in comparable levels suggesting a possible formation of cGMP/cGK-sensitive Kv11.1 channels in photoreceptors and rod bipolar cells. Taken together, the immunohistological results revealed different expression patterns of the three Kv11 channels in the mouse retina supposing distinct physiological roles.

The role of atrial natriuretic peptide in modulating cardiac electrophysiology

Since the discovery of atrial natriuretic peptide (ANP) in 1981, significant progress has been made in understanding the mechanism of its release and its role in salt and water balance in the body. It has also become clear that ANP plays a key role in cardiac electrophysiology, modulating the autonomic nervous system and regulating the function of cardiac ion channels. The clinical importance of this role was established when mutations in NPPA, the gene encoding ANP, were identified as a cause of familial atrial fibrillation. This review examines our current understanding of the electrophysiological effects of ANP, and their physiological relationship to clinical studies linking ANP and atrial fibrillation.