Biology of cardiac sodium channel Nav1.5 expression

Upstream alternative polyadenylation in SCN5A produces a short transcript isoform encoding a mitochondria-localized NaV1.5 N-terminal fragment that influences cardiomyocyte respiration

SCN5A encodes the cardiac voltage-gated Na+ channel, NaV1.5, that initiates action potentials. SCN5A gene variants cause arrhythmias and increased heart failure risk. Mechanisms controlling NaV1.5 expression and activity are not fully understood. We recently found a well-conserved alternative polyadenylation (APA) signal downstream of the first SCN5A coding exon. This yields a SCN5A-short transcript isoform expressed in several species (e.g. human, pig, and cat), though rodents lack this upstream APA. Reanalysis of transcriptome-wide cardiac APA-seq and mRNA-seq data shows reductions in both upstream APA usage and short/full-length SCN5A mRNA ratios in failing hearts. Knock-in of the human SCN5A APA sequence into mice is sufficient to enable expression of SCN5A -short transcript, while significantly decreasing expression of full-length SCN5A mRNA. Notably, SCN5A -short transcript encodes a novel protein (NaV1.5-NT), composed of an N-terminus identical to NaV1.5 and a unique C-terminus derived from intronic sequence. AAV9 constructs were able to achieve stable NaV1.5-NT expression in mouse hearts, and western blot of human heart tissues showed bands co-migrating with NaV1.5-NT transgene-derived bands. NaV1.5-NT is predicted to contain a mitochondrial targeting sequence and localizes to mitochondria in cultured cardiomyocytes and in mouse hearts. NaV1.5-NT expression in cardiomyocytes led to elevations in basal oxygen consumption rate, ATP production, and mitochondrial ROS, while depleting NADH supply. Native PAGE analyses of mitochondria lysates revealed that NaV1.5-NT expression resulted in increased levels of disassembled complex V subunits and accumulation of complex I-containing supercomplexes. Overall, we discovered that APA-mediated regulation of SCN5A produces a short transcript encoding NaV1.5-NT. Our data support that NaV1.5-NT plays a multifaceted role in influencing mitochondrial physiology: 1) by increasing basal respiration likely through promoting complex V conformations that enhance proton leak, and 2) by increasing overall respiratory efficiency and NADH consumption by enhancing formation and/or stability of complex I-containing respiratory supercomplexes, though the specific molecular mechanisms underlying each of these remain unresolved.

Discovery of Novel Pyrimidine-Based Derivatives as Nav1.2 Inhibitors with Efficacy in Mouse Models of Epilepsy

Dysfunction of voltage-gated sodium channel Nav1.2 causes various epileptic disorders, and inhibition of the channel has emerged as an attractive therapeutic strategy. However, currently available Nav1.2 inhibitors exhibit low potency and limited structural diversity. In this study, a novel series of pyrimidine-based derivatives with Nav1.2 inhibitory activity were designed, synthesized, and evaluated. Compounds 14 and 35 exhibited potent activity against Nav1.2, boasting IC50 values of 120 and 65 nM, respectively. Compound 14 displayed favorable pharmacokinetics (F = 43%) following intraperitoneal injection and excellent brain penetration potency (B/P = 3.6). Compounds 14 and 35 exhibited robust antiepileptic activities in the maximal electroshock test, with ED50 values of 3.2 and 11.1 mg/kg, respectively. Compound 35 also demonstrated potent antiepileptic activity in a 6 Hz (32 mA) model, with an ED50 value of 18.5 mg/kg. Overall, compounds 14 and 35 are promising leads for the development of new small-molecule therapeutics for epilepsy.

Requirement of β subunit for the reduced voltage-gated Na+ current of a Brugada syndrome patient having novel double missense mutation (p.A385T/R504T) of SCN5A

Mutations within the SCN5A gene, which encodes the α-subunit 5 (NaV1.5) of the voltage-gated Na+ channel, have been linked to three distinct cardiac arrhythmia disorders: long QT syndrome type 3, Brugada syndrome (BrS), and cardiac conduction disorder. In this study, we have identified novel missense mutations (p.A385T/R504T) within SCN5A in a patient exhibiting overlap arrhythmia phenotypes. This study aims to elucidate the functional consequences of SCN5A mutants (p.A385T/R504T) to understand the clinical phenotypes. Whole-cell patch-clamp technique was used to analyze the NaV1.5 current (INa) in HEK293 cells transfected with the wild-type and mutant SCN5A with or without SCN1B co-expression. The amplitude of INa was not altered in mutant SCN5A (p.A385T/R504T) alone. Furthermore, a rightward shift of the voltage-dependent inactivation and faster recovery from inactivation was observed, suggesting a gain-of-function state. Intriguingly, the coexpression of SCN1B with p.A385T/R504T revealed significant reduction of INa and slower recovery from inactivation, consistent with the loss-of-function in Na+ channels. The SCN1B dependent reduction of INa was also observed in a single mutation p.R504T, but p.A385T co-expressed with SCN1B showed no reduction. In contrast, the slower recovery from inactivation with SCN1B was observed in A385T while not in R504T. The expression of SCN1B is indispensable for the electrophysiological phenotype of BrS with the novel double mutations; p.A385T and p.R504T contributed to the slower recovery from inactivation and reduced current density of NaV1.5, respectively.

Opioids-Induced Long QT Syndrome: A Challenge to Cardiac Health

The challenge posed by opioid overdose has become a significant concern for health systems due to the complexities associated with drug prohibition, widespread clinical use, and potential abuse. In response, healthcare professionals have primarily concentrated on mitigating the hallucinogenic and respiratory depressant consequences of opioid overdose to minimize associated risks. However, it is crucial to acknowledge that most opioids possess the capacity to prolong the QT interval, particularly in cases of overdose, thereby potentially resulting in severe ventricular arrhythmias and even sudden death if timely intervention is not implemented. Consequently, alongside addressing the typical adverse effects of opioids, it is imperative to consider their cardiotoxicity. To enhance comprehension of the correlation between opioids and arrhythmias, identify potential targets for prompt intervention, and mitigate the hazards associated with clinical utilization, an exploration of the interaction between drugs and ion channels, as well as their underlying mechanisms, becomes indispensable. This review primarily concentrates on elucidating the impact of opioid drugs on diverse ion channels, investigating recent advancements in this domain, and attaining a deeper understanding of the mechanisms underlying the prolongation of the QT interval by opioid drugs, along with potential interventions.

Persistent PKA activation redistributes NaV1.5 to the cell surface of adult rat ventricular myocytes

During chronic stress, persistent activation of cAMP-dependent protein kinase (PKA) occurs, which can contribute to protective or maladaptive changes in the heart. We sought to understand the effect of persistent PKA activation on NaV1.5 channel distribution and function in cardiomyocytes using adult rat ventricular myocytes as the main model. PKA activation with 8CPT-cAMP and okadaic acid (phosphatase inhibitor) caused an increase in Na+ current amplitude without altering the total NaV1.5 protein level, suggesting a redistribution of NaV1.5 to the myocytes' surface. Biotinylation experiments in HEK293 cells showed that inhibiting protein trafficking from intracellular compartments to the plasma membrane prevented the PKA-induced increase in cell surface NaV1.5. Additionally, PKA activation induced a time-dependent increase in microtubule plus-end binding protein 1 (EB1) and clustering of EB1 at myocytes' peripheral surface and intercalated discs (ICDs). This was accompanied by a decrease in stable interfibrillar microtubules but an increase in dynamic microtubules along the myocyte surface. Imaging and coimmunoprecipitation experiments revealed that NaV1.5 interacted with EB1 and β-tubulin, and both interactions were enhanced by PKA activation. We propose that persistent PKA activation promotes NaV1.5 trafficking to the peripheral surface of myocytes and ICDs by providing dynamic microtubule tracks and enhanced guidance by EB1. Our proposal is consistent with an increase in the correlative distribution of NaV1.5, EB1, and β-tubulin at these subcellular domains in PKA-activated myocytes. Our study suggests that persistent PKA activation, at least during the initial phase, can protect impulse propagation in a chronically stressed heart by increasing NaV1.5 at ICDs.

In silico validation revealed the role of SCN5A mutations and their genotype-phenotype correlations in Brugada syndrome

BACKGROUND

Brugada syndrome (BrS) is a rare genetic disease that causes sudden cardiac death (SCD) and arrhythmia. SCN5A pathogenic variants (about 30% of diagnosed patients) are responsible for BrS.

AIMS

Lack of knowledge regarding molecular characteristics and the correlation between genotype and phenotype interfere with the risk stratification and finding the optimal treatment in Vietnam. Therefore, we identified SCN5A variants and evaluated the genotype-phenotype correlation of BrS on 117 Vietnamese probands.

MATERIALS AND METHODS

The clinical characteristics and blood samples of BrS patients were collected. To determine SCN5A variants, Sanger sequencing was conducted, and subsequently, these variants were analyzed by bioinformatic tools.

RESULTS

In this cohort, the overall rate of detected variants in SCN5A was 25.6%, which could include both pathogenic and benign variants. In genetic testing, 21 SCN5A variants were identified, including eight novels and 15 published variants. Multiple bioinformatic tools were used to predict variant effect with c.551A>G, c.1890+14G>A, c.3338C>T, c.3578G>A, and c.5484C>T as benign, while other variants were predicted as disease-causing. The family history of SCD (risk ratio [RR] = 4.324, 95% CI: 2.290-8.269, p < 0.001), syncope (RR = 3.147, 95% CI: 1.668-5.982, p = 0.0004), and ventricular tachycardia/ventricular fibrillation (RR = 3.406, 95% CI: 1.722-5.400, p = 0.0035) presented a significantly higher risk in the SCN5A (+) group, consisting of individuals carrying any variant in the SCN5A gene, compared to SCN5A (-) individuals.

CONCLUSION

The results contribute to clarifying the impact of SCN5A variants on these phenotypes. Further follow-up studies need to be carried out to understand the functional effects of these SCN5A variants on the severity of BrS.

Biophysical properties of NaV1.5 channels from atrial-like and ventricular-like cardiomyocytes derived from human induced pluripotent stem cells

Generating atrial-like cardiomyocytes derived from human induced pluripotent stem cells (hiPSCs) is crucial for modeling and treating atrial-related diseases, such as atrial arrythmias including atrial fibrillations. However, it is essential to obtain a comprehensive understanding of the electrophysiological properties of these cells. The objective of the present study was to investigate the molecular, electrical, and biophysical properties of several ion channels, especially NaV1.5 channels, in atrial hiPSC cardiomyocytes. Atrial cardiomyocytes were obtained by the differentiation of hiPSCs treated with retinoic acid (RA). The quality of the atrial specification was assessed by qPCR, immunocytofluorescence, and western blotting. The electrophysiological properties of action potentials (APs), Ca2+ dynamics, K+ and Na+ currents were investigated using patch-clamp and optical mapping approaches. We evaluated mRNA transcript and protein expressions to show that atrial cardiomyocytes expressed higher atrial- and sinoatrial-specific markers (MYL7, CACNA1D) and lower ventricular-specific markers (MYL2, CACNA1C, GJA1) than ventricular cardiomyocytes. The amplitude, duration, and steady-state phase of APs in atrial cardiomyocytes decreased, and had a shape similar to that of mature atrial cardiomyocytes. Interestingly, NaV1.5 channels in atrial cardiomyocytes exhibited lower mRNA transcripts and protein expression, which could explain the lower current densities recorded by patch-clamp. Moreover, Na+ currents exhibited differences in activation and inactivation parameters. These differences could be explained by an increase in SCN2B regulatory subunit expression and a decrease in SCN1B and SCN4B regulatory subunit expressions. Our results show that a RA treatment made it possible to obtain atrial cardiomyocytes and investigate differences in NaV1.5 channel properties between ventricular- and atrial-like cells.

Pathogenic SCN5A Mutation and Thyrotoxicosis-Related Neurological Syndrome: Casual or Causal Relationship?

BACKGROUND

Various neurologic complications of hyperthyroidism are reported, and most of these complications are reversible with the amelioration of thyrotoxicosis. We report a previously undescribed concurrence of hyperthyroid-associated exercise-induced myalgia and stiffness, pyramidal tract dysfunction, and myoclonic movements that make an initial clinical diagnosis difficult.

CASE PRESENTATION

A 17-year-old male was hospitalized in the department of neurology, presenting with a 4-year history of severe exercise-induced myalgia and stiffness, weakness of lower limbs, and myoclonic movements. Laboratory investigations unexpectedly revealed hyperthyroidism. MRI of the brain and spine, electrophysiology, and whole exome sequencing were also performed. Antithyroid therapy led to marked improvement of neurologic symptoms, accompanied by a significant improvement of the time-dependent decline in compound muscle action potentials (CMAP) amplitudes after exercise and normalization of the prolonged QTc interval. Genetic analysis identified a rare variant in SCN5A.

CONCLUSION

This case report provides important insights into the relationship between hyperthyroidism and neurologic/cardiac complications, particularly in those with a genetic predisposition. SCN5A mutation possibly plays a role in the complex neurological syndrome associated with hyperthyroidism. Further studies are warranted to better understand the underlying mechanisms and potential therapeutic options for these complex conditions.

SCN5A channelopathy: arrhythmia, cardiomyopathy, epilepsy and beyond

Influx of sodium ions through voltage-gated sodium channels in cardiomyocytes is essential for proper electrical conduction within the heart. Both acquired conditions associated with sodium channel dysfunction (myocardial ischaemia, heart failure) as well as inherited disorders secondary to mutations in the gene SCN5A encoding for the cardiac sodium channel Nav1.5 are associated with life-threatening arrhythmias. Research in the last decade has uncovered the complex nature of Nav1.5 distribution, function, in particular within distinct subcellular subdomains of cardiomyocytes. Nav1.5-based channels furthermore display previously unrecognized non-electrogenic actions and may impact on cardiac structural integrity, leading to cardiomyopathy. Moreover, SCN5A and Nav1.5 are expressed in cell types other than cardiomyocytes as well as various extracardiac tissues, where their functional role in, e.g. epilepsy, gastrointestinal motility, cancer and the innate immune response is increasingly investigated and recognized. This review provides an overview of these novel insights and how they deepen our mechanistic knowledge on SCN5A channelopathies and Nav1.5 (dys)function. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Inhibition of Wnt/β-catenin signaling upregulates Nav 1.5 channels in Brugada syndrome iPSC-derived cardiomyocytes

The voltage-gated Nav 1.5 channels mediate the fast Na+ current (INa ) in cardiomyocytes initiating action potentials and cardiac contraction. Downregulation of INa , as occurs in Brugada syndrome (BrS), causes ventricular arrhythmias. The present study investigated whether the Wnt/β-catenin signaling regulates Nav 1.5 in human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). In healthy male and female iPSC-CMs, activation of Wnt/β-catenin signaling by CHIR-99021 reduced (p < 0.01) both Nav 1.5 protein and SCN5A mRNA. In iPSC-CMs from a BrS patient, both Nav 1.5 protein and peak INa were reduced compared to those in healthy iPSC-CMs. Treatment of BrS iPSC-CMs with Wnt-C59, a small-molecule Wnt inhibitor, led to a 2.1-fold increase in Nav 1.5 protein (p = 0.0005) but surprisingly did not affect SCN5A mRNA (p = 0.146). Similarly, inhibition of Wnt signaling using shRNA-mediated β-catenin knockdown in BrS iPSC-CMs led to a 4.0-fold increase in Nav 1.5, which was associated with a 4.9-fold increase in peak INa but only a 2.1-fold increase in SCN5A mRNA. The upregulation of Nav 1.5 by β-catenin knockdown was verified in iPSC-CMs from a second BrS patient. This study demonstrated that Wnt/β-catenin signaling inhibits Nav 1.5 expression in both male and female human iPSC-CMs, and inhibition of Wnt/β-catenin signaling upregulates Nav 1.5 in BrS iPSC-CMs through both transcriptional and posttranscriptional mechanisms.

Myocardial Inflammation as a Manifestation of Genetic Cardiomyopathies: From Bedside to the Bench

Over recent years, preclinical and clinical evidence has implicated myocardial inflammation (M-Infl) in the pathophysiology and phenotypes of traditionally genetic cardiomyopathies. M-Infl resembling myocarditis on imaging and histology occurs frequently as a clinical manifestation of classically genetic cardiac diseases, including dilated and arrhythmogenic cardiomyopathy. The emerging role of M-Infl in disease pathophysiology is leading to the identification of druggable targets for molecular treatment of the inflammatory process and a new paradigm in the field of cardiomyopathies. Cardiomyopathies constitute a leading cause of heart failure and arrhythmic sudden death in the young population. The aim of this review is to present, from bedside to bench, the current state of the art about the genetic basis of M-Infl in nonischemic cardiomyopathies of the dilated and arrhythmogenic spectrum in order to prompt future research towards the identification of novel mechanisms and treatment targets, with the ultimate goal of lowering disease morbidity and mortality.

The Effects of Moxifloxacin and Gemifloxacin on the ECG Morphology in Healthy Volunteers: A Phase 1 Randomized Clinical Trial

Moxifloxacin and gemifloxacin are the two newer broad-spectrum 8-methoxy-quinolone derivatives that are used to treat various bacterial infections in cardiac patients. In this research study, we assessed the impact of moxifloxacin and gemifloxacin on the QT intervals of electrocardiograms in normal adult doses and draw a comparison, in a controlled environment, on healthy volunteers. Additionally, the effect of both test drugs on the QRS complex was checked. Sixty healthy volunteers were randomly assigned to two groups via R-software, and each respectively received moxifloxacin and gemifloxacin for five days. The research ethics committee approved the research, and it was registered for clinical trial under NCT04692623. The participants' electrocardiograms were obtained before the start of the dose (baseline) and on the fifth day. Significant prolongation of QT interval was noted in moxifloxacin (p < 0.0001) as compared to gemifloxacin treated groups. There were no cases of QTc prolongation over the usual limits (450-470 ms) in the gemifloxacin-treated group, however, QTc prolongations at the rate of 30 and 60 ms from the baseline were noted, interpreted as per the EMEA guidelines. These findings indicate that moxifloxacin caused significant (p < 0.0001) QT interval prolongation (QTIP) as compared to gemifloxacin. In contrast to the previously reported literature, the prominent effect of moxifloxacin on the widening of the QRS-complex was noted with no such effect on QRS-widening in the gemifloxacin-treated group. It is concluded that both drugs have the potential for considerable QT interval prolongation (QTIP) effects, which is one of the risk factors for developing torsade de pointes (TdPs) in cardiac patients. Thus, clinicians should exercise caution when prescribing moxifloxacin and gemifloxacin to cardiac patients and should consider alternate treatment options.

A Proposed New Model to Explain the Role of Low Dose Non-DNA Targeted Radiation Exposure in Chronic Fatigue and Immune Dysfunction Syndrome

Chronic Fatigue and Immune Dysfunction Syndrome (CFIDS) is considered to be a multidimensional illness whose etiology is unknown. However, reports from Chernobyl, as well as those from the United States, have revealed an association between radiation exposure and the development of CFIDS. As such, we present an expanded model using a systems biology approach to explain the etiology of CFIDS as it relates to this cohort of patients. This paper proposes an integrated model with ionizing radiation as a suggested trigger for CFIDS mediated through UVA induction and biophoton generation inside the body resulting from radiation-induced bystander effects (RIBE). Evidence in support of this approach has been organized into a systems view linking CFIDS illness markers with the initiating events, in this case, low-dose radiation exposure. This results in the formation of reactive oxygen species (ROS) as well as important immunologic and other downstream effects. Furthermore, the model implicates melanoma and subsequent hematopoietic dysregulation in this underlying process. Through the identification of this association with melanoma, clinical medicine, including dermatology, hematology, and oncology, can now begin to apply its expansive knowledge base to provide new treatment options for an illness that has had few effective treatments.

Improved Generation of Human Induced Pluripotent Stem Cell-Derived Cardiac Pacemaker Cells Using Novel Differentiation Protocols

Current protocols for the differentiation of human-induced pluripotent stem cells (hiPSC) into cardiomyocytes only generate a small amount of cardiac pacemaker cells. In previous work, we reported the generation of high amounts of cardiac pacemaker cells by co-culturing hiPSC with mouse visceral endoderm-like (END2) cells. However, potential medical applications of cardiac pacemaker cells generated according to this protocol, comprise an incalculable xenogeneic risk. We thus aimed to establish novel protocols maintaining the differentiation efficiency of the END2 cell-based protocol, yet eliminating the use of END2 cells. Three protocols were based on the activation and inhibition of the Wingless/Integrated (Wnt) signaling pathway, supplemented either with retinoic acid and the Wnt activator CHIR99021 (protocol B) or with the NODAL inhibitor SB431542 (protocol C) or with a combination of all three components (protocol D). An additional fourth protocol (protocol E) was used, which was originally developed by the manufacturer STEMCELL Technologies for the differentiation of hiPSC or hESC into atrial cardiomyocytes. All protocols (B, C, D, E) were compared to the END2 cell-based protocol A, serving as reference, in terms of their ability to differentiate hiPSC into cardiac pacemaker cells. Our analysis revealed that protocol E induced upregulation of 12 out of 15 cardiac pacemaker-specific genes. For comparison, reference protocol A upregulated 11, while protocols B, C and D upregulated 9, 10 and 8 cardiac pacemaker-specific genes, respectively. Cells differentiated according to protocol E displayed intense fluorescence signals of cardiac pacemaker-specific markers and showed excellent rate responsiveness to adrenergic and cholinergic stimulation. In conclusion, we characterized four novel and END2 cell-independent protocols for the differentiation of hiPSC into cardiac pacemaker cells, of which protocol E was the most efficient.

Research progress of Nedd4L in cardiovascular diseases

Post-translational modifications (PTMs) are a covalent processing process of proteins after translation. Proteins are capable of playing their roles only after being modified, so as to maintain the normal physiological function of cells. As a key modification of protein post-translational modification, ubiquitination is an essential element, which forms an enzyme-linked reaction through ubiquitin-activating enzyme, ubiquitin binding enzyme, and ubiquitin ligase, aiming to regulate the expression level and function of cellular proteins. Nedd4 family is the largest group of ubiquitin ligases, including 9 members, such as Nedd4-1, Nedd4L (Nedd4-2), WWP1, WWP2, ITCH, etc. They could bind to substrate proteins through their WW domain and play a dominant role in the ubiquitination process, and then participate in various pathophysiological processes of cardiovascular diseases (such as hypertension, myocardial hypertrophy, heart failure, etc.). At present, the role of Nedd4L in the cardiovascular field is not fully understood. This review aims to summarize the progress and mechanism of Nedd4L in cardiovascular diseases, and provide potential perspective for the clinical treatment or prevention of related cardiovascular diseases by targeting Nedd4L.

Bifurcations and Proarrhythmic Behaviors in Cardiac Electrical Excitations

The heart is a hierarchical dynamic system consisting of molecules, cells, and tissues, and acts as a pump for blood circulation. The pumping function depends critically on the preceding electrical activity, and disturbances in the pattern of excitation propagation lead to cardiac arrhythmia and pump failure. Excitation phenomena in cardiomyocytes have been modeled as a nonlinear dynamical system. Because of the nonlinearity of excitation phenomena, the system dynamics could be complex, and various analyses have been performed to understand the complex dynamics. Understanding the mechanisms underlying proarrhythmic responses in the heart is crucial for developing new ways to prevent and control cardiac arrhythmias and resulting contractile dysfunction. When the heart changes to a pathological state over time, the action potential (AP) in cardiomyocytes may also change to a different state in shape and duration, often undergoing a qualitative change in behavior. Such a dynamic change is called bifurcation. In this review, we first summarize the contribution of ion channels and transporters to AP formation and our knowledge of ion-transport molecules, then briefly describe bifurcation theory for nonlinear dynamical systems, and finally detail its recent progress, focusing on the research that attempts to understand the developing mechanisms of abnormal excitations in cardiomyocytes from the perspective of bifurcation phenomena.

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.

Discovering the Triad between Nav1.5, Breast Cancer, and the Immune System: A Fundamental Review and Future Perspectives

Nav1.5 is one of the nine voltage-gated sodium channel-alpha subunit (VGSC-α) family members. The Nav1.5 channel typically carries an inward sodium ion current that depolarises the membrane potential during the upstroke of the cardiac action potential. The neonatal isoform of Nav1.5, nNav1.5, is produced via VGSC-α alternative splicing. nNav1.5 is known to potentiate breast cancer metastasis. Despite their well-known biological functions, the immunological perspectives of these channels are poorly explored. The current review has attempted to summarise the triad between Nav1.5 (nNav1.5), breast cancer, and the immune system. To date, there is no such review available that encompasses these three components as most reviews focus on the molecular and pharmacological prospects of Nav1.5. This review is divided into three major subsections: (1) the review highlights the roles of Nav1.5 and nNav1.5 in potentiating the progression of breast cancer, (2) focuses on the general connection between breast cancer and the immune system, and finally (3) the review emphasises the involvements of Nav1.5 and nNav1.5 in the functionality of the immune system and the immunogenicity. Compared to the other subsections, section three is pretty unexploited; it would be interesting to study this subsection as it completes the triad.

Engineered bacterial voltage-gated sodium channel platform for cardiac gene therapy

Therapies for cardiac arrhythmias could greatly benefit from approaches to enhance electrical excitability and action potential conduction in the heart by stably overexpressing mammalian voltage-gated sodium channels. However, the large size of these channels precludes their incorporation into therapeutic viral vectors. Here, we report a platform utilizing small-size, codon-optimized engineered prokaryotic sodium channels (BacNav) driven by muscle-specific promoters that significantly enhance excitability and conduction in rat and human cardiomyocytes in vitro and adult cardiac tissues from multiple species in silico. We also show that the expression of BacNav significantly reduces occurrence of conduction block and reentrant arrhythmias in fibrotic cardiac cultures. Moreover, functional BacNav channels are stably expressed in healthy mouse hearts six weeks following intravenous injection of self-complementary adeno-associated virus (scAAV) without causing any adverse effects on cardiac electrophysiology. The large diversity of prokaryotic sodium channels and experimental-computational platform reported in this study should facilitate the development and evaluation of BacNav-based gene therapies for cardiac conduction disorders.

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

Protein Kinase C-Mediated Hyperphosphorylation and Lateralization of Connexin 43 Are Involved in Autoimmune Myocarditis-Induced Prolongation of QRS Complex

Myocarditis is a serious and potentially life-threatening disease, which leads to cardiac dysfunction and sudden cardiac death. An increasing number of evidence suggests that myocarditis is also a malignant complication of coronavirus pneumonia, associated with heart failure and sudden cardiac death. Prolonged QRS complexes that are related to malignant arrhythmias caused by myocarditis significantly increase the risk of sudden cardiac death in patients. However, the molecular mechanisms are not fully known at present. In this study, we identify protein kinase C (PKC) as a new regulator of the QRS complex. In isolated hearts of normal rats, the PKC agonist, phorbol-12-myristate-13-acetate (PMA), induced prolongation of the QRS complex. Mechanistically, hyperphosphorylation and lateralization of connexin 43 (Cx43) by PKC induced depolymerization and internalization of Cx43 gap junction channels and prolongation of the QRS duration. Conversely, administration of the PKC inhibitor, Ro-32-0432, in experimental autoimmune myocarditis (EAM) rats after the most severe inflammation period still significantly rescued the stability of the Cx43 gap junction and alleviated prolongation of the QRS complex. Ro-32-0432 reduced phosphorylation and blocked translocation of Cx43 in EAM rat heart but did not regulate the mRNA expression level of ventricular ion channels and the other regulatory proteins, which indicates that the inhibition of PKC might have no protective effect on ion channels that generate ventricular action potential in EAM rats. These results suggest that the pharmacological inhibition of PKC ameliorates the prolongation of the QRS complex via suppression of Cx43 hyperphosphorylation, lateralization, and depolymerization of Cx43 gap junction channels in EAM rats, which provides a potential therapeutic strategy for myocarditis-induced arrhythmias.

Inhibition of minor intron splicing reduces Na+ and Ca2+ channel expression and function in cardiomyocytes

Eukaryotic genomes contain a tiny subset of ‘minor class’ introns with unique sequence elements that require their own splicing machinery. These minor introns are present in certain gene families with specific functions, such as voltage-gated Na⁺ and voltage-gated Ca²⁺ channels. Removal of minor introns by the minor spliceosome has been proposed as a post-transcriptional regulatory layer, which remains unexplored in the heart. Here, we investigate whether the minor spliceosome regulates electrophysiological properties of cardiomyocytes by knocking down the essential minor spliceosome small nuclear snRNA component U6atac in neonatal rat ventricular myocytes. Loss of U6atac led to robust minor intron retention within Scn5a and Cacna1c, resulting in reduced protein levels of Na_v1.5 and Ca_v1.2 channels. Functional consequences were studied through patch-clamp analysis, and revealed reduced Na⁺ and L-type Ca²⁺ currents after loss of U6atac. In conclusion, minor intron splicing modulates voltage-dependent ion channel expression and function in cardiomyocytes. This may be of particular relevance in situations in which minor splicing activity changes, such as in genetic diseases affecting minor spliceosome components, or in acquired diseases in which minor spliceosome components are dysregulated, such as heart failure.

Summary: Knockdown of minor spliceosome component U6atac in cardiomyocytes reveals that expression of the Na⁺ channel Scn5a and the L-type Ca²⁺ channel Cacna1c critically depend on minor intron splicing.

Functional Analysis of SCN5A Genetic Variants Associated with Brugada Syndrome

BACKGROUND

Brugada syndrome (BrS) is a rare inherited cardiac arrhythmia with increased risk of sudden cardiac death. Mutations in gene SCN5A, which encodes the α-subunit of cardiac voltage-gated sodium channel NaV1.5, have been identified in over 20% of patients with BrS. However, only a small fraction of NaV1.5 variants, which are associated with BrS, are characterized in electrophysiological experiments.

RESULTS

Here we explored variants V281A and L1582P, which were found in our patients with BrS, and variants F543L and K1419E, which are reportedly associated with BrS. Heterologous expression of the variants in CHO-K1 cells and the Western blot analysis demonstrated that each variant appeared at the cell surface. We further measured sodium current in the whole-cell voltage clamp configuration. Variant F543L produced robust sodium current with a hyperpolarizing shift in the voltage dependence of steady-state fast inactivation. Other variants did not produce detectable sodium currents, indicating a complete loss of function. In a recent cryoEM structure of the hNaV1.5 channel, residues V281, K1419, and L1582 are in close contacts with residues whose mutations are reportedly associated with BrS, indicating functional importance of respective contacts.

CONCLUSIONS

Our results support the notion that loss of function of NaV1.5 or decrease of the channel activity is involved in the pathogenesis of BrS.

Ventricular voltage-gated ion channels: Detection, characteristics, mechanisms, and drug safety evaluation

Cardiac voltage-gated ion channels (VGICs) play critical roles in mediating cardiac electrophysiological signals, such as action potentials, to maintain normal heart excitability and contraction. Inherited or acquired alterations in the structure, expression, or function of VGICs, as well as VGIC-related side effects of pharmaceutical drug delivery can result in abnormal cellular electrophysiological processes that induce life-threatening cardiac arrhythmias or even sudden cardiac death. Hence, to reduce possible heart-related risks, VGICs must be acknowledged as important targets in drug discovery and safety studies related to cardiac disease. In this review, we first summarize the development and application of electrophysiological techniques that are employed in cardiac VGIC studies alone or in combination with other techniques such as cryoelectron microscopy, optical imaging and optogenetics. Subsequently, we describe the characteristics, structure, mechanisms, and functions of various well-studied VGICs in ventricular myocytes and analyze their roles in and contributions to both physiological cardiac excitability and inherited cardiac diseases. Finally, we address the implications of the structure and function of ventricular VGICs for drug safety evaluation. In summary, multidisciplinary studies on VGICs help researchers discover potential targets of VGICs and novel VGICs in heart, enrich their knowledge of the properties and functions, determine the operation mechanisms of pathological VGICs, and introduce groundbreaking trends in drug therapy strategies, and drug safety evaluation.

Heritable arrhythmia syndromes associated with abnormal cardiac sodium channel function: ionic and non-ionic mechanisms

The cardiac sodium channel NaV1.5, encoded by the SCN5A gene, is responsible for the fast upstroke of the action potential. Mutations in SCN5A may cause sodium channel dysfunction by decreasing peak sodium current, which slows conduction and facilitates reentry-based arrhythmias, and by enhancing late sodium current, which prolongs the action potential and sets the stage for early afterdepolarization and arrhythmias. Yet, some NaV1.5-related disorders, in particular structural abnormalities, cannot be directly or solely explained on the basis of defective NaV1.5 expression or biophysics. An emerging concept that may explain the large disease spectrum associated with SCN5A mutations centres around the multifunctionality of the NaV1.5 complex. In this alternative view, alterations in NaV1.5 affect processes that are independent of its canonical ion-conducting role. We here propose a novel classification of NaV1.5 (dys)function, categorized into (i) direct ionic effects of sodium influx through NaV1.5 on membrane potential and consequent action potential generation, (ii) indirect ionic effects of sodium influx on intracellular homeostasis and signalling, and (iii) non-ionic effects of NaV1.5, independent of sodium influx, through interactions with macromolecular complexes within the different microdomains of the cardiomyocyte. These indirect ionic and non-ionic processes may, acting alone or in concert, contribute significantly to arrhythmogenesis. Hence, further exploration of these multifunctional effects of NaV1.5 is essential for the development of novel preventive and therapeutic strategies.

Functional Characterization of Two Novel Mutations in SCN5A Associated with Brugada Syndrome Identified in Italian Patients

Background. Brugada syndrome (BrS) is an autosomal dominantly inherited cardiac disease characterized by “coved type” ST-segment elevation in the right precordial leads, high susceptibility to ventricular arrhythmia and a family history of sudden cardiac death. The SCN5A gene, encoding for the cardiac voltage-gated sodium channel Nav1.5, accounts for ~20–30% of BrS cases and is considered clinically relevant. Methods. Here, we describe the clinical findings of two Italian families affected by BrS and provide the functional characterization of two novel SCN5A mutations, the missense variant Pro1310Leu and the in-frame insertion Gly1687_Ile1688insGlyArg. Results. Despite being clinically different, both patients have a family history of sudden cardiac death and had history of arrhythmic events. The Pro1310Leu mutation significantly reduced peak sodium current density without affecting channel membrane localization. Changes in the gating properties of expressed Pro1310Leu channel likely account for the loss-of-function phenotype. On the other hand, Gly1687_Ile1688insGlyArg channel, identified in a female patient, yielded a nearly undetectable sodium current. Following mexiletine incubation, the Gly1687_Ile1688insGlyArg channel showed detectable, albeit very small, currents and biophysical properties similar to those of the Nav1.5 wild-type channel. Conclusions. Overall, our results suggest that the degree of loss-of-function shown by the two Nav1.5 mutant channels correlates with the aggressive clinical phenotype of the two probands. This genotype-phenotype correlation is fundamental to set out appropriate therapeutical intervention.

The Role of HECT-Type E3 Ligase in the Development of Cardiac Disease

Despite advances in medicine, cardiac disease remains an increasing health problem associated with a high mortality rate. Maladaptive cardiac remodeling, such as cardiac hypertrophy and fibrosis, is a risk factor for heart failure; therefore, it is critical to identify new therapeutic targets. Failing heart is reported to be associated with hyper-ubiquitylation and impairment of the ubiquitin–proteasome system, indicating an importance of ubiquitylation in the development of cardiac disease. Ubiquitylation is a post-translational modification that plays a pivotal role in protein function and degradation. In 1995, homologous to E6AP C-terminus (HECT) type E3 ligases were discovered. E3 ligases are key enzymes in ubiquitylation and are classified into three families: really interesting new genes (RING), HECT, and RING-between-RINGs (RBRs). Moreover, 28 HECT-type E3 ligases have been identified in human beings. It is well conserved in evolution and is characterized by the direct attachment of ubiquitin to substrates. HECT-type E3 ligase is reported to be involved in a wide range of human diseases and health. The role of HECT-type E3 ligases in the development of cardiac diseases has been uncovered in the last decade. There are only a few review articles summarizing recent advancements regarding HECT-type E3 ligase in the field of cardiac disease. This study focused on cardiac remodeling and described the role of HECT-type E3 ligases in the development of cardiac disease. Moreover, this study revealed that the current knowledge could be exploited for the development of new clinical therapies.

In vivo Dominant-Negative Effect of an SCN5A Brugada Syndrome Variant

Loss-of-function mutations in the cardiac Na⁺ channel α-subunit Na_v1.5, encoded by SCN5A, cause Brugada syndrome (BrS), a hereditary disease characterized by sudden cardiac death due to ventricular fibrillation. We previously evidenced in vitro the dominant-negative effect of the BrS Na_v1.5-R104W variant, inducing retention of wild-type (WT) channels and leading to a drastic reduction of the resulting Na⁺ current (I_Na). To explore this dominant-negative effect in vivo, we created a murine model using adeno-associated viruses (AAVs).

T cell self-reactivity during thymic development dictates the timing of positive selection

Functional tuning of T cells based on their degree of self-reactivity is established during positive selection in the thymus, although how positive selection differs for thymocytes with relatively low versus high self-reactivity is unclear. In addition, preselection thymocytes are highly sensitive to low-affinity ligands, but the mechanism underlying their enhanced T cell receptor (TCR) sensitivity is not fully understood. Here we show that murine thymocytes with low self-reactivity experience briefer TCR signals and complete positive selection more slowly than those with high self-reactivity. Additionally, we provide evidence that cells with low self-reactivity retain a preselection gene expression signature as they mature, including genes previously implicated in modulating TCR sensitivity and a novel group of ion channel genes. Our results imply that thymocytes with low self-reactivity downregulate TCR sensitivity more slowly during positive selection, and associate membrane ion channel expression with thymocyte self-reactivity and progress through positive selection.

Cardioprotective Effect of Glycyrrhizin on Myocardial Remodeling in Diabetic Rats

Myocardial fibrosis is one of the major complications of long-term diabetes. Hyperglycemia induced cardiomyocyte atrophy is a frequent pathophysiological indicator of diabetic heart. The objective of this study was to investigate the cardioprotective effect of glycyrrhizin (GLC) on myocardial damage in diabetic rats and assess the anti-inflammatory and anti-fibrotic effect of GLC. Our study demonstrates that hyperglycemia can elevate cardiac atrophy in diabetic animals. Type 2 diabetic fatty and the lean control rats were evaluated for cardiac damage and inflammation at 8-12 weeks after the development of diabetes. Western blot and immunohistochemical studies revealed that gap junction protein connexin-43 (CX43), cardiac injury marker troponin I, cardiac muscle specific voltage gated sodium channel NaV1.5 were significantly altered in the diabetic heart. Furthermore, oxidative stress mediator receptor for advanced glycation end-products (RAGE), as well as inflammatory mediator phospho-p38 MAPK and chemokine receptor CXCR4 were increased in the diabetic heart whereas the expression of nuclear factor erythroid-2-related factor 2 (Nrf2), the antioxidant proteins that protect against oxidative damage was reduced. We also observed an increase in the expression of the pleiotropic cytokine, transforming growth factor beta (TGF-β) in the diabetic heart. GLC treatment exhibited a decrease in the expression of phospho-p38 MAPK, RAGE, NaV1.5 and TGF-β and it also altered the expression of CX43, CXCR4, Nrf2 and troponin I. These observations suggest that GLC possesses cardioprotective effects in diabetic cardiac atrophy and that these effects could be mediated through activation of Nrf2 and inhibition of CXCR4/SDF1 as well as TGF-β/p38MAPK signaling pathway.

The voltage-gated sodium channel β2 subunit associates with lipid rafts by S-palmitoylation

The voltage-gated sodium channel is critical for cardiomyocyte function. It consists of a protein complex comprising a pore-forming α subunit and associated β subunits. In polarized Madin-Darby canine kidney cells, we show evidence by acyl-biotin exchange that β2 is S-acylated at Cys-182. Interestingly, we found that palmitoylation increases β2 association with detergent-resistant membranes. β2 localizes exclusively to the apical surface. However, depletion of plasma membrane cholesterol, or blocking intracellular cholesterol transport, caused mislocalization of β2, as well as of the non-palmitoylable C182S mutant, to the basolateral domain. Apical β2 did not undergo endocytosis and displayed limited diffusion within the plane of the membrane; such behavior suggests that, at least in part, it is cytoskeleton anchored. Upon acute cholesterol depletion, its mobility was greatly reduced, and a slight reduction was also measured as a result of lack of palmitoylation, supporting β2 association with cholesterol-rich lipid rafts. Indeed, lipid raft labeling confirmed a partial overlap with apical β2. Although β2 palmitoylation was not required to promote surface localization of the α subunit, our data suggest that it is likely implicated in lipid raft association and the polarized localization of β2.

Improved sequence-based prediction of interaction sites in α-helical transmembrane proteins by deep learning

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Fast and accurate prediction of transmembrane protein interaction sites.

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First ever computational survey of interaction sites in membrane proteins.

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10-30% of amino acid positions predicted to be involved in interactions.

Interactions between transmembrane (TM) proteins are fundamental for a wide spectrum of cellular functions, but precise molecular details of these interactions remain largely unknown due to the scarcity of experimentally determined three-dimensional complex structures. Computational techniques are therefore required for a large-scale annotation of interaction sites in TM proteins. Here, we present a novel deep-learning approach, DeepTMInter, for sequence-based prediction of interaction sites in α-helical TM proteins based on their topological, physiochemical, and evolutionary properties. Using a combination of ultra-deep residual neural networks with a stacked generalization ensemble technique DeepTMInter significantly outperforms existing methods, achieving the AUC/AUCPR values of 0.689/0.598. Across the main functional families of human transmembrane proteins, the percentage of amino acid sites predicted to be involved in interactions typically ranges between 10% and 25%, and up to 30% in ion channels. DeepTMInter is available as a standalone package at https://github.com/2003100127/deeptminter. The training and benchmarking datasets are available at https://data.mendeley.com/datasets/2t8kgwzp35.

Epigenetic Changes Governing Scn5a Expression in Denervated Skeletal Muscle

The SCN5A gene encodes the α-subunit of the voltage-gated cardiac sodium channel (Na_V1.5), a key player in cardiac action potential depolarization. Genetic variants in protein-coding regions of the human SCN5A have been largely associated with inherited cardiac arrhythmias. Increasing evidence also suggests that aberrant expression of the SCN5A gene could increase susceptibility to arrhythmogenic diseases, but the mechanisms governing SCN5A expression are not yet well understood. To gain insights into the molecular basis of SCN5A gene regulation, we used rat gastrocnemius muscle four days following denervation, a process well known to stimulate Scn5a expression. Our results show that denervation of rat skeletal muscle induces the expression of the adult cardiac Scn5a isoform. RNA-seq experiments reveal that denervation leads to significant changes in the transcriptome, with Scn5a amongst the fifty top upregulated genes. Consistent with this increase in expression, ChIP-qPCR assays show enrichment of H3K27ac and H3K4me3 and binding of the transcription factor Gata4 near the Scn5a promoter region. Also, Gata4 mRNA levels are significantly induced upon denervation. Genome-wide analysis of H3K27ac by ChIP-seq suggest that a super enhancer recently described to regulate Scn5a in cardiac tissue is activated in response to denervation. Altogether, our experiments reveal that similar mechanisms regulate the expression of Scn5a in denervated muscle and cardiac tissue, suggesting a conserved pathway for SCN5A expression among striated muscles.

Multiple arrhythmic and cardiomyopathic phenotypes associated with an SCN5A A735E mutation

BACKGROUND

SCN5A mutations are associated with multiple arrhythmic and cardiomyopathic phenotypes including Brugada syndrome (BrS), sinus node dysfunction (SND), atrioventricular block, supraventricular tachyarrhythmias (SVTs), long QT syndrome (LQTS), dilated cardiomyopathy and left ventricular noncompaction. Several single SCN5A mutations have been associated with overlap of some of these phenotypes, but never with overlap of all the phenotypes.

OBJECTIVE

We encountered two pedigrees with multiple arrhythmic phenotypes with or without cardiomyopathic phenotypes, and sought to identify a responsible mutation and reveal its functional abnormalities.

METHODS

Target panel sequencing of 72 genes, including inherited arrhythmia syndromes- and cardiomyopathies-related genes, was employed in two probands. Cascade screening was performed by Saner sequencing. Wild-type or identified mutant SCN5A were expressed in tsA201 cells, and whole-cell sodium currents (I_Na) were recorded using patch-clamp techniques.

RESULTS

We identified an SCN5A A735E mutation in these probands, but did not identify any other mutations. All eight mutation carriers exhibited at least one of the arrhythmic phenotypes. Two patients exhibited multiple arrhythmic phenotypes: one (15-year-old girl) exhibited BrS, SND, and exercise and epinephrine-induced QT prolongation, the other (4-year-old boy) exhibited BrS, SND, and SVTs. Another one (30-year-old male) exhibited all arrhythmic and cardiomyopathic phenotypes, except for LQTS. One male suddenly died at age 22. Functional analysis revealed that the mutant did not produce functional I_Na.

CONCLUSIONS

A non-functional SCN5A A735E mutation could be associated with multiple arrhythmic and cardiomyopathic phenotypes, although there remains a possibility that other unidentified factors may be involved in the phenotypic variability of the mutation carriers.

Diabetes-induced changes in cardiac voltage-gated ion channels

Diabetes mellitus affects the heart through various mechanisms such as microvascular defects, metabolic abnormalities, autonomic dysfunction and incompatible immune response. Furthermore, it can also cause functional and structural changes in the myocardium by a disease known as diabetic cardiomyopathy (DCM) in the absence of coronary artery disease. As DCM progresses it causes electrical remodeling of the heart, left ventricular dysfunction and heart failure. Electrophysiological changes in the diabetic heart contribute significantly to the incidence of arrhythmias and sudden cardiac death in diabetes mellitus patients. In recent studies, significant changes in repolarizing K⁺ currents, Na⁺ currents and L-type Ca²⁺ currents along with impaired Ca²⁺ homeostasis and defective contractile function have been identified in the diabetic heart. In addition, insulin levels and other trophic factors change significantly to maintain the ionic channel expression in diabetic patients. There are many diagnostic tools and management options for DCM, but it is difficult to detect its development and to effectively prevent its progress. In this review, diabetes-associated alterations in voltage-sensitive cardiac ion channels are comprehensively assessed to understand their potential role in the pathophysiology and pathogenesis of DCM.

The endosomal trafficking regulator LITAF controls the cardiac Nav1.5 channel via the ubiquitin ligase NEDD4-2

The QT interval is a recording of cardiac electrical activity. Previous genome-wide association studies identified genetic variants that modify the QT interval upstream of LITAF (lipopolysaccharide-induced tumor necrosis factor-α factor), a protein encoding a regulator of endosomal trafficking. However, it was not clear how LITAF might impact cardiac excitation. We investigated the effect of LITAF on the voltage-gated sodium channel Nav1.5, which is critical for cardiac depolarization. We show that overexpressed LITAF resulted in a significant increase in the density of Nav1.5-generated voltage-gated sodium current INa and Nav1.5 surface protein levels in rabbit cardiomyocytes and in HEK cells stably expressing Nav1.5. Proximity ligation assays showed co-localization of endogenous LITAF and Nav1.5 in cardiomyocytes, whereas co-immunoprecipitations confirmed they are in the same complex when overexpressed in HEK cells. In vitro data suggest that LITAF interacts with the ubiquitin ligase NEDD4-2, a regulator of Nav1.5. LITAF overexpression down-regulated NEDD4-2 in cardiomyocytes and HEK cells. In HEK cells, LITAF increased ubiquitination and proteasomal degradation of co-expressed NEDD4-2 and significantly blunted the negative effect of NEDD4-2 on INa We conclude that LITAF controls cardiac excitability by promoting degradation of NEDD4-2, which is essential for removal of surface Nav1.5. LITAF-knockout zebrafish showed increased variation in and a nonsignificant 15% prolongation of action potential duration. Computer simulations using a rabbit-cardiomyocyte model demonstrated that changes in Ca2+ and Na+ homeostasis are responsible for the surprisingly modest action potential duration shortening. These computational data thus corroborate findings from several genome-wide association studies that associated LITAF with QT interval variation.

Life Cycle of the Cardiac Voltage-Gated Sodium Channel NaV1.5

Cardiac voltage-gated sodium channel Na_V1.5, encoded by SCN5A, is crucial for the upstroke of action potential and excitation of cardiomyocytes. Na_V1.5 undergoes complex processes before it reaches the target membrane microdomains and performs normal functions. A variety of protein partners are needed to achieve the balance between SCN5A transcription and mRNA decay, endoplasmic reticulum retention and export, Golgi apparatus retention and export, selective anchoring and degradation, activation, and inactivation of sodium currents. Subtle alterations can impair Na_V1.5 in terms of expression or function, eventually leading to Na_V1.5-associated diseases such as lethal arrhythmias and cardiomyopathy.

Characterization of an N-terminal Nav1.5 channel variant - a potential risk factor for arrhythmias and sudden death?

Background

Alterations in the SCN5A gene encoding the cardiac sodium channel Na_v1.5 have been linked to a number of arrhythmia syndromes and diseases including long-QT syndrome (LQTS), Brugada syndrome (BrS) and dilative cardiomyopathy (DCM), which may predispose to fatal arrhythmias and sudden death. We identified the heterozygous variant c.316A > G, p.(Ser106Gly) in a 35-year-old patient with survived cardiac arrest. In the present study, we aimed to investigate the functional impact of the variant to clarify the medical relevance.

Methods

Mutant as well as wild type GFP tagged Na_v1.5 channels were expressed in HEK293 cells. We performed functional characterization experiments using patch-clamp technique.

Results

Electrophysiological measurements indicated, that the detected missense variant alters Nav1.5 channel functionality leading to a gain-of-function effect. Cells expressing S106G channels show an increase in Na_v1.5 current over the entire voltage window.

Conclusion

The results support the assumption that the detected sequence aberration alters Na_v1.5 channel function and may predispose to cardiac arrhythmias and sudden cardiac death.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12881-020-01170-3.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Machine learning-based QSAR models to predict sodium ion channel (Nav 1.5) blockers

Aim: Conventional experimental approaches used for the evaluation of the proarrhythmic potential of compounds in the drug discovery process are expensive and time consuming but an integral element in the safety profile required for a new drug to be approved. The voltage-gated sodium ion channel 1.5 (Na_v 1.5), a target known for arrhythmic drugs, causes adverse cardiac complications when the channel is blocked. Results: Machine learning classification and regression models were built to predict the possibility of blocking these channels by small molecules. The finalized models tested with balanced accuracies of 0.88, 0.93 and 0.94 at three thresholds (1, 10 and 30 µmol, respectively). The regression model built to predict the pIC₅₀ of compounds had q² of 0.84 (root-mean-square error = 0.46). Conclusion: The machine learning models that have been built can act as effective filters to screen out the potentially toxic compounds in the early stages of drug discovery.

Biophysical Investigation of Sodium Channel Interaction with β-Subunit Variants Associated with Arrhythmias

Background: Voltage-gated sodium (Na_V) channels help regulate electrical activity of the plasma membrane. Mutations in associated subunits can result in pathological outcomes. Here we examined the interaction of Na_V channels with cardiac arrhythmia-linked mutations in SCN2B and SCN4B, two genes that encode auxiliary β-subunits. Materials and Methods: To investigate changes in SCN2B ^R137H and SCN4B ^I80T function, we combined three-dimensional X-ray crystallography with electrophysiological measurements on Na_V1.5, the dominant subtype in the heart. Results: SCN4B ^I80T alters channel activity, whereas SCN2B ^R137H does not have an apparent effect. Structurally, the SCN4B ^I80T perturbation alters hydrophobic packing of the subunit with major structural changes and causes a thermal destabilization of the folding. In contrast, SCN2B ^R137H leads to structural changes but overall protein stability is unaffected. Conclusion: SCN4B ^I80T data suggest a functionally important region in the interaction between Na_V1.5 and β4 that, when disrupted, could lead to channel dysfunction. A lack of apparent functional effects of SCN2B ^R137H on Na_V1.5 suggests an alternative working mechanism, possibly through other Na_V channel subtypes present in heart tissue. Indeed, mapping the structural variations of SCN2B ^R137H onto neuronal Na_V channel structures suggests altered interaction patterns.

Overexpression of TBX3 in human induced pluripotent stem cells (hiPSCs) increases their differentiation into cardiac pacemaker-like cells

BACKGROUD

The TBX3(T-box 3)transcription factor is considered as an essential factor in sinoatrial node formation. While the effect of TBX3 in the differentiation of sinoatrial node cells from embryonic stem cells(ESCs) has been recognized, its role in human induced pluripotent stem cell derived cardiomyocytes(hiPSCMs) has not been addressed. Therefore, the purpose of the present study was to investigate whether overexpression of TBX3 in hiPSCs could increase their differentiation into pacemaker-like cells.

METHODS

The hiPSCs were transfected with TBX3 gene during differentiation into cardiomyocytes(CMs). The hiPSCMs were analyzed using immunofluorescence, RT-qPCR, flow cytometry, whole-cell patch clamp recording to identify the differentiation effect exerted by TBX3. We discovered that hiPSCs transfected with TBX3 showed more proportions of NKX2.5-cTNT + sinoatrial node cells and faster contracting rates.

RESULTS

The results showed increment in transcription factor TBX18, SHOX2; hyperpolarization-activated cyclic nucleotide (HCN) channel: HCN1, HCN2, HCN4, connexin 45(CX45), Na + Ca2+ exchanger(NCX) in TBX3 transfected hiPSCMs. Sinoatrial node cell specific If current and action potential were also confirmed by patch clamp in TBX3 transfected hiPSCMs and the pacemaker-like cells were able to pace hiPSCMs ex vivo.

CONCLUSION

In conclusion, the present study demonstrated that overexpression of TBX3 could increase the differentiation of hiPSCs into pacemaker-like cells. Our study provide new strategy to construct a biological pacemaker, however, further study is still needed to identify the efficacy and safety of using the pacemaker-like cells to produce biological pacemaker in vivo.

Safety, Tolerability, Pharmacokinetics, and Concentration-QTc Analysis of Tetrodotoxin: A Randomized, Dose Escalation Study in Healthy Adults

Tetrodotoxin (TTX) is a highly specific voltage-gated sodium channel (VGSC) blocker in clinical evaluation as a peripheral-acting analgesic for chronic pain. This study presents the first published results of the safety including cardiac liability of TTX at therapeutic-relevant concentrations in twenty-five healthy adults. Randomized, double-blind, placebo-, and positive- (moxifloxacin) controlled study evaluated single ascending doses of 15 µg, 30 µg, and 45 µg TTX over 3 periods with a 7-day washout between each period. Subcutaneous injections of TTX were readily absorbed, reaching maximum plasma concentration (C_max) within 1.5 h. Both extent of exposure (AUC) and C_max increased in proportion to dose. No QT prolongation was identified by concentration-QTc analysis and the upper bounds of the two-sided 90% confidence interval of predicted maximum baseline and placebo corrected QTcF (ΔΔQTcF) value did not exceed 10 ms for all tetrodotoxin doses, thereby meeting the criteria of a negative QT study. Safety assessments showed no clinically relevant changes with values similar between all groups and no subject withdrawing due to adverse events. Paresthesia, oral-paresthesia, headache, dizziness, nausea, and myalgia were the most common TEAEs (overall occurrence ≥5%) in the TTX treatment groups. TTX doses investigated in this study are safe, well-tolerated, and lack proarrhythmic proclivity.

microRNA overexpression in slow transit constipation leads to reduced NaV1.5 current and altered smooth muscle contractility

OBJECTIVE

This study was designed to evaluate the roles of microRNAs (miRNAs) in slow transit constipation (STC).

DESIGN

All human tissue samples were from the muscularis externa of the colon. Expression of 372 miRNAs was examined in a discovery cohort of four patients with STC versus three age/sex-matched controls by a quantitative PCR array. Upregulated miRNAs were examined by quantitative reverse transcription qPCR (RT-qPCR) in a validation cohort of seven patients with STC and age/sex-matched controls. The effect of a highly differentially expressed miRNA on a custom human smooth muscle cell line was examined in vitro by RT-qPCR, electrophysiology, traction force microscopy, and ex vivo by lentiviral transduction in rat muscularis externa organotypic cultures.

RESULTS

The expression of 13 miRNAs was increased in STC samples. Of those miRNAs, four were predicted to target SCN5A, the gene that encodes the Na⁺ channel Na_V1.5. The expression of SCN5A mRNA was decreased in STC samples. Let-7f significantly decreased Na⁺ current density in vitro in human smooth muscle cells. In rat muscularis externa organotypic cultures, overexpression of let-7f resulted in reduced frequency and amplitude of contraction.

CONCLUSIONS

A small group of miRNAs is upregulated in STC, and many of these miRNAs target the SCN5A-encoded Na⁺ channel Na_V1.5. Within this set, a novel Na_V1.5 regulator, let-7f, resulted in decreased Na_V1.5 expression, current density and reduced motility of GI smooth muscle. These results suggest Na_V1.5 and miRNAs as novel diagnostic and potential therapeutic targets in STC.

Molecular mechanisms of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy is a genetic disorder characterized by the risk of life-threatening arrhythmias, myocardial dysfunction and fibrofatty replacement of myocardial tissue. Mutations in genes that encode components of desmosomes, the adhesive junctions that connect cardiomyocytes, are the predominant cause of arrhythmogenic cardiomyopathy and can be identified in about half of patients with the condition. However, the molecular mechanisms leading to myocardial destruction, remodelling and arrhythmic predisposition remain poorly understood. Through the development of animal, induced pluripotent stem cell and other models of disease, advances in our understanding of the pathogenic mechanisms of arrhythmogenic cardiomyopathy over the past decade have brought several signalling pathways into focus. These pathways include canonical and non-canonical WNT signalling, the Hippo-Yes-associated protein (YAP) pathway and transforming growth factor-β signalling. These studies have begun to identify potential therapeutic targets whose modulation has shown promise in preclinical models. In this Review, we summarize and discuss the reported molecular mechanisms underlying the pathogenesis of arrhythmogenic cardiomyopathy.

Spontaneous Recurrent Seizures Mediated Cardiac Dysfunction via mTOR Pathway Upregulation: A Putative Target for SUDEP Management

Background:

Alteration in electrophysiology, leading to cardiac dysfunction and subsequently a nontraumatic death is a complication of epilepsy known as “SUDEP” (Sudden Unexpected Death in Epilepsy).

Aims:

The present study was designed to understand the molecular changes and cardiac parameters during different phases of epileptogenesis in lithium-pilocarpine (Li-pilo) rat model of epilepsy.

Methods:

The animals were exposed to Li-pilo to induce Spontaneous Recurrent Seizures (SRS). Noninvasive blood pressure and electrocardiography was recorded at 7th, 28th and 75th day following pilocarpine administration, considered as latent, initial and late SRS phases, respectively. The serum biochemistry, cardiac histopathology, protein and mRNA expressions were studied, following electrocardiography on day 75.

Results:

The mean arterial pressure decreased during the latent phase, thereafter it progressively increased during the initial and the late SRS phases, as compared to the basal and the latent phase. Histopathological analysis of the heart sections indicated hypertrophy, degenerative changes and fibrous tissue deposition in epileptic animals, along with increased levels of lactate dehydrogenase and creatine kinase-MB in the serum. The expression of HIF-1α, phospho-S6, phospho-mTOR, TGF-β, collagen I and Na+/K+-ATPase α1 proteins, and mRNA levels of HIF-1α, mTOR, Rps6, Scn1b, Scn3b, Nav1.5 and TGF-β were increased in the cardiac tissue of epileptic animals, as compared to control.

Conclusion:

Our results conclusively showed that Li-pilo-induced SRS leads to cardiac dysfunction via mTOR pathway upregulation, thus suggested the regulatory control of mTOR pathway as a potential target for SUDEP management.

Combination Formulation of Tetrodotoxin and Lidocaine as a Potential Therapy for Severe Arrhythmias

Severe arrhythmias-such as ventricular arrhythmias-can be fatal, but treatment options are limited. The effects of a combined formulation of tetrodotoxin (TTX) and lidocaine (LID) on severe arrhythmias were studied. Patch clamp recording data showed that the combination of LID and TTX had a stronger inhibitory effect on voltage-gated sodium channel 1.5 (Nav1.5) than that of either TTX or LID alone. LID + TTX formulations were prepared with optimal stability containing 1 μg of TTX, 5 mg of LID, 6 mg of mannitol, and 4 mg of dextran-40 and then freeze dried. This formulation significantly delayed the onset and shortened the duration of arrhythmia induced by aconitine in rats. Arrhythmia-originated death was avoided by the combined formulation, with a decrease in the mortality rate from 64% to 0%. The data also suggests that the anti-arrhythmic effect of the combination was greater than that of either TTX or LID alone. This paper offers new approaches to develop effective medications against arrhythmias.

Trafficking and Function of the Voltage-Gated Sodium Channel β2 Subunit

The voltage-gated sodium channel is vital for cardiomyocyte function, and consists of a protein complex containing a pore-forming α subunit and two associated β subunits. A fundamental, yet unsolved, question is to define the precise function of β subunits. While their location in vivo remains unclear, large evidence shows that they regulate localization of α and the biophysical properties of the channel. The current data support that one of these subunits, β2, promotes cell surface expression of α. The main α isoform in an adult heart is Na_V1.5, and mutations in SCN5A, the gene encoding Na_V1.5, often lead to hereditary arrhythmias and sudden death. The association of β2 with cardiac arrhythmias has also been described, which could be due to alterations in trafficking, anchoring, and localization of Na_V1.5 at the cardiomyocyte surface. Here, we will discuss research dealing with mechanisms that regulate β2 trafficking, and how β2 could be pivotal for the correct localization of Na_V1.5, which influences cellular excitability and electrical coupling of the heart. Moreover, β2 may have yet to be discovered roles on cell adhesion and signaling, implying that diverse defects leading to human disease may arise due to β2 mutations.