Increased late sodium current (INaL) is a central mechanism underlying both inherited and acquired cardiac arrhythmias. Although Nav1.5 is the dominant cardiac sodium channel, multiple tetrodotoxin-sensitive (TTX-S) Nav isoforms are also expressed in cardiomyocytes, particularly within transverse tubules, where they influence excitation-contraction coupling. Whether these channels contribute directly to pathological INaL and arrhythmogenesis remains unresolved, in part because available pharmacological tools lack isoform selectivity. Herein, our objective was to determine the contribution of TTX-S Nav channels to pathological cardiac INaL and arrhythmia, and to establish selective pharmacological tools to dissect their role. Using automated patch-clamp and human Nav isoform profiling, we show that the reference INaL inducer ATX-II predominantly activates TTX-resistant Nav1.5 Nav channels. However, ATX-II lacks Nav isoform selectivity when inappropriately used, questioning the conclusions reached by numerous cardiac INaL studies. In contrast, AaH-II, a peptide from Androctonus australis hector scorpion venom, more selectively and potently enhances INaL through TTX-S Nav isoforms. In human iPS-derived cardiomyocytes TTX-S INaL leads to action potential prolongation. In adult ventricular cardiomyocytes, AaH-II induces abnormal Ca2+ handling and spontaneous Ca2+ release events. In isolated hearts and in vivo, selective TTX-S INaL activation produces conduction abnormalities, QT prolongation, and ventricular proarrhythmic events, which are prevented by nanomolar tetrodotoxin concentrations that spare Nav1.5. These findings demonstrate that TTX-S sodium channels are sufficient to generate arrhythmogenic late Na+ current in the heart, independently of Nav1.5. AaH-II provides a powerful new tool to selectively probe TTX-S INaL and reveals these channels as previously underappreciated contributors to cardiac electrical instability. Targeting TTX-S Nav channels may therefore represent a novel and potentially safer strategy for antiarrhythmic therapy.

Abstract Background Clinical interpretation of missense variants in the hERG potassium channel encoded by the KCNH2 gene remains a major challenge in inherited arrhythmia syndromes. Functional studies often rely on a minimal set of channel properties, mainly current amplitude measurements, which do not capture the multidimensional nature of channel gating and its impact on ventricular repolarization. We developed a multiscale computational framework to quantitatively link multiparametric channel dysfunction to ECG phenotypes. Methods We generated multiparametric electrophysiological profiles for KCNH2 variants using high-throughput patch-clamp, quantifying nine biophysical properties including conductance, voltage dependence, and gating kinetics. These parameters were incorporated into a modified formulation of I Kr embedded in a human ventricular electrophysiology model. The resulting framework, termed MeTAL (Multiscale-enriched Transformation and Analysis for Long-QT), produces physiologically calibrated pseudo-ECGs enabling quantitative evaluation of QT dynamics. We systematically analyzed the contribution of individual and combined gating parameters and applied the model to 41 KCNH2 variants across ACMG classes, comparing simulated QTc values with clinical data. Results Multiparametric profiling revealed complex functional signatures in most variants, with concurrent gain- and loss-of-function effects affecting distinct gating processes. Simulations demonstrated that ventricular repolarization, though strongly determined by current amplitude, is substantially influenced by inactivation-related parameters, particularly the slope and voltage dependence of inactivation. Interaction analyses showed nonlinear relationships between gating parameters, explaining why variants with similar current density can produce divergent QT phenotypes. In heterozygous simulations, MeTAL reproduced clinically observed QTc distributions across variant classes and accurately predicted the repolarization regime (normal, long-QT, or short-QT) in most cases. Conclusions Multiparametric integration of ion-channel function within a multiscale electrophysiological model enables mechanistic prediction of QT behavior beyond conductance-based metrics. This approach provides a scalable framework for interpretation of KCNH2 variants and solves the issue of risk stratification in inherited arrhythmia syndromes while offering new opportunities for variant-specific and pharmacological modeling of repolarization.

Introduction Cardiac arrhythmias are linked to abnormal function of voltage-gated sodium channels, particularly Nav1.5, encoded by SCN5A gene, a key determinant of action potential in cardiomyocytes. Mutations or dysregulations of SCN5A gene can lead to severe arrhythmias, including Brugada syndrome (BrS), while current therapies remain largely symptomatic, often relying on cardiac defibrillator implantation. Novel strategies enabling precise modulation of Nav1.5 expression hold considerable translational potential. Peptide Nucleic Acid (PNA) have emerged as promising tools for transcriptional and post-transcriptional gene regulation, but are limited by poor cellular uptake. Conjugation to Cell-Penetrating Peptides (CPPs) represents a promising non-viral and non-toxic delivery strategy. Objective This project evaluates the uptake, intracellular localization, and functional potential of CPP-PNA conjugates targeting SCN5A in human induced Pluripotent Stem Cells-derived cardiomyocytes (hiPSC-CMs). Method Human iPSCs were differentiated into cardiomyocytes using established cardiac differentiation protocols. CPP-mediated delivery was first evaluated using eleven different fluorescently labelled CPPs in different cardiac cell models. Cellular uptake and distribution were assessed by fluorescence microscopy and quantitative analyses, to optimize delivery conditions. Functional effects of CPP-PNA treatment on Nav1.5 activity are currently being investigated using high-throughput patch clamp electrophysiology and molecular biology approaches. Results Several CPPs showed efficient uptake in cardiomyocytes, with peptide- and cell type-dependent internalization patterns. Quantitative analyses identified CPPs with favorable and reproducible delivery profile in hiPSC-CMs. These CPPs were selected for conjugation to PNAs targeting SCN5A sequence. Preliminary CPP-PNA treatments were well tolerated, supporting feasibility in human cardiomyocytes. Ongoing analyses aim to determine early molecular responses and functional effects on Nav1.5 sodium currents. Conclusion This work establishes a robust delivery strategy for CPP-PNA conjugates as a promising therapeutic approach for cardiac arrhythmias.

The long QT syndrome type 3 (LQT3) is a cardiac channelopathy caused by gain-of-function mutations in the SCN5A gene, encoding the sodium channel Nav1.5. As Nav1.5 is expressed in cardiomyocytes but also in cardiac fibroblasts, we investigated whether the LQT3-causing p.ΔQKP1507-1509 (ΔQKP) SCN5A mutation alters cardiac fibroblast phenotype. Primary cultured ventricular fibroblasts from Scn5a+/ΔQKP knock-in mice showed increased proliferation, survival, expression of transforming growth factor-β (TGF-β) and activation of its canonical pathway, and reduced α-smooth muscle actin expression. Ventricular tissue from Scn5a+/ΔQKP mice exhibited augmented fibroblast populations and fibrosis. Inhibiting TGF-β receptor, sodium current or Scn5a expression decreased Scn5a+/ΔQKP fibroblast proliferation, while veratridine increased proliferation of control fibroblasts, mimicking Nav1.5 gain-of-function. Lastly, abnormal calcium signaling underlied the increased proliferation of Scn5a+/ΔQKP fibroblasts. Our study shows that cardiac fibroblasts carrying the ΔQKP-SCN5A mutation exhibit an abnormal, proliferative phenotype, paving the way for better understanding the role of cardiac fibroblasts in LQT3.

Introduction Brugada syndrome (BrS) is a rare inherited cardiac arrhythmia characterized by an increased risk of sudden death due to ventricular arrhythmias originating from the right ventricle (RV) and its outflow tract. Rare genetic variants in SCN5A gene, which generates the main cardiac sodium current (INa), are involved in only 20% of BrS cases. Genome-wide association studies identified strong polygenic influence of specific common risk haplotypes on BrS occurrence. The present study focuses on the risk allele located in an intronic region of ZFPM2 transcription factor gene, known to be involved in cardiac development but which has not been linked so far to the regulation of cardiac electrical activity. Objective To study the role of ZFPM2 risk haplotype in cardiac electrical activity using RV cells differentiated from human induced pluripotent stem cells (hiPSCs). Method A locus depicting epigenetic characteristics of distal enhancer, and that includes the ZFPM2 risk haplotype, has been identified using both CHIP-seq and ATAC-seq data. This locus has been precisely CRISPR-deleted (i-ZFPM2) in a control (WT) hiPSC line. Both lines were differentiated into RV cells using a cardioid protocol adapted from Schmidt C. et al (Cell, 2023). Gene expression, voltage clamp and current clamp experiments were conducted on differentiated cells. Results Differentiated cells displayed a gene expression profile characteristic of RV tissue. In accordance with the hypothesis that the locus is an enhancer, the expression of ZFPM2 was reduced. Interestingly, the expression of SCN5A was also reduced. Accordingly, the i-ZFPM2 RV cells showed a significant reduction of INa as compared to WT (−61%, P<0.01) with no alteration of biophysical properties of the channel. The amplitude and the dV/dtmax of the action potential were also decreased. Conclusion Our results suggest that the investigated region regulates ZFPM2 expression in human RV cells and that ZFPM2 participates in the control of cardiac electrical activity through SCN5A regulation. These data may unveil new mechanisms involved in BrS pathophysiology. Further studies will decipher the precise mechanisms by which this regulation occurs.

Introduction Cardiac arrhythmias affect approximately 5% of the population and are associated with high morbidity and mortality. With the aging of the population, their prevalence is increasing, posing a major health challenge. Treatments for preventing or terminating arrhythmias exist (Antiarrhythmic drugs, Implantable cardioverter-defibrillators, ablation techniques…) but they come with important side effects (pro-arrhythmic effects, systemic toxicity…). Photopharmacology, with its high spatio-temporal resolution, emerged as a promising approach for modulating cardiac electrical activity. Objective We successfully demonstrated its ability to regulate ion channel activity ex vivo and in vivo using an external illumination source. The objective of this study, to consider further clinical application, is to demonstrate the ability of implantable cardiac device to activate photoactivatable peptides and interfere with cardiac activity. Method A wireless and battery free cardiac device has been previously developed and designed for rats allowing electrical and optical stimulations. We implanted the device on the right ventricular free wall. A photoactivatable analogue of AaHII, an arrhythmogenic peptide, has been intravenously injected and activated locally thanks to 380nm illumination in anesthetized rats. Results We first successfully refined the surgical procedure and validated their tolerance over one month by analyzing electrical and mechanical parameters on ECG and echocardiography, circulating markers of heart injury and histology. The device is well tolerated over one month without any signs of inflammation or arrhythmias. We next demonstrated that 70% of the device-mediated photoactivations (5/7 rats), leads to a significant change in the electrical activity of the heart characterized by an increase of the T wave area on the ECG. Interestingly, all animals recovered quickly validating the local activation of the peptide. Conclusion This study is very promising for future applications of photopharmacology, and pave the road for local photoactivation of antiarrhythmic peptides to prevent or terminate arrhythmias in preclinical models.

Aims Late sodium current (INaL) is a key contributor to cardiac arrhythmias, but its precise origin and arrhythmogenic potential from tetrodotoxin-sensitive (TTX-S) sodium (Nav) channels remain unclear. While the FDA-endorsed toxin ATX-II has been widely used to model INaL-associated arrhythmogenesis, it lacks selectivity, limiting its utility in dissecting the roles of individual Nav channel subtypes. This study investigates the proarrhythmic impact of TTX-S Nav channel activation using AaH-II*, a scorpion venom-derived peptide with selective efficacy for TTX-S channels. Methods and Results Using automated patch-clamp recordings, we characterized AaH-II* selectivity across human Nav isoforms and demonstrated potent, preferential activation of INaL in hNav1.1, 1.2, 1.3, and 1.6 over the TTX-resistant cardiac isoform hNav1.5. Calcium imaging in isolated adult rat cardiomyocytes showed that low nanomolar concentrations of AaH-II* induced spontaneous calcium release events and arrhythmogenic calcium transients, even in the absence of Nav1.5 activation. Ex vivo multielectrode array recordings in Langendorff-perfused rat hearts confirmed dose-dependent ventricular conduction slowing, prolonged repolarization, and increased arrhythmia burden, all mitigated by TTX. In vivo , intravenous AaH-II* administration in rats elicited QTc prolongation, atrioventricular block, and ventricular tachyarrhythmias, which were significantly suppressed by TTX pretreatment. Conclusion We identify AaH-II* as a powerful and selective tool to study INaL from TTX-S Nav channels in cardiac tissue. Our findings reveal that TTX-S channel-mediated INaL alone is sufficient to induce arrhythmias and that pharmacological inhibition of these channels offers a promising antiarrhythmic strategy. These results advocate for broader consideration of TTX-S Nav channels as targets in arrhythmia research and drug safety screening. ### Competing Interest Statement The authors have declared no competing interest.

Background PDE2 (phosphodiesterase 2) is upregulated in human heart failure. Cardiac PDE2‐transgenic mice are protected against contractile dysfunction and arrhythmias in heart failure but whether an acute elevation of PDE2 could be of therapeutic value remains elusive. This hypothesis was tested using cardiac PDE2 gene transfer in preclinical models of heart failure. Methods and Results C57BL/6 male mice were injected with serotype 9 adeno‐associated viruses encoding for PDE2A. This led to a ≈10‐fold rise of PDE2A protein levels that affected neither cardiac structure nor function in healthy mice. Two weeks after inoculation with serotype 9 adeno‐associated viruses, mice were implanted with minipumps delivering either NaCl, isoproterenol (60 mg/kg per day), or isoproterenol and phenylephrine (30 mg/kg per day each) for 2 weeks. In mice injected with serotype 9 adeno‐associated viruses encoding for LUC (luciferase), isoproterenol or isoproterenol+phenylephrine infusion induced left ventricular hypertrophy, decreased ejection fraction unveiled by echocardiography, and promoted fibrosis and apoptosis assessed by Masson's trichrome and Tunel, respectively. Furthermore, inotropic responses to isoproterenol of ventricular cardiomyocytes isolated from isoproterenol+phenylephrine‐LUC mice loaded with 1 μmol/L Fura‐2AM and stimulated at 1 Hz to record calcium transients and sarcomere shortening were dampened. Spontaneous calcium waves at the cellular level were promoted as well as ventricular arrhythmias evoked in vivo by catheter‐mediated ventricular pacing after isoproterenol (1.5 mg/kg) and atropine (1 mg/kg) injection. However, increased PDE2A blunted these adverse outcomes evoked by sympathomimetic amines. Conclusions Cardiac gene therapy with PDE2A limits left ventricle remodeling, dysfunction, and arrhythmias evoked by catecholamines, providing evidence that increasing PDE2A activity acutely could prevent progression toward heart failure.

Brugada syndrome (BrS) is a rare cardiac arrhythmic disorder with high risk of sudden cardiac death. Recent advances have identified more than 20 risk loci with complex inheritance suggesting a polygenic model for BrS inheritance. These loci are in non-coding regions located in the vicinity of cardiac-expressed genes. This complex genetic architecture and the limited understanding of BrS genetic and molecular mechanisms hinder the development of efficient prevention strategies in the context of this syndrome and are unfavorable to the implementation of therapeutic interventions. In this context, understanding the functional impact of the identified putative risk alleles is a prerequisite. Here, we used the fly model to systematically test whether orthologues of genes located near risk alleles for BrS participate to cardiac function. The fly is the simplest model with a heart muscle and is a powerful genetic model suitable for efficient screening of candidate genes, providing a whole organism-based assessment of cardiac development, structure and function. Using high-speed heart imaging platform on intact flies, we invalided the cardiac expression of the fly orthologues of human genes associated to BrS and characterized whether they are cell autonomously implicated in heart functioning. Our results provide an overview of cardiac phenotypes associated with genes potentially involved in BrS, enabling their prioritization for further investigations in mammalian models. ### Competing Interest Statement The authors have declared no competing interest. ANR, ANR-22-CE17-0051-03

Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Sorbonne Université doctoral scolarship INSERM Introduction Brugada syndrome (BrS) is an inherited cardiac arrhythmia that greatly increases the risk of sudden death as a result of ventricular fibrillation. Approximately 20% of BrS cases can be attributed to a mutation in SCN5A, the gene encoding the alpha-subunit Nav1.5 of the cardiac sodium-channel, which carries the sodium current INa. These mutations typically lead to loss-of-function of Nav1.5, resulting in a decrease in INa and most of them (»70 %) are responsible for haploinsufficiency. In the Scn5a+/- mouse model, a sodium-channel dysfunction phenotype is observed, characterized by conduction abnormalities (PR and QRS interval prolongation), a 50% reduction in INa, and an increased susceptibility to arrhythmias. Aim The aim of this study was to assess the impact of the overexpression of a Nav1.5-peptide, which has been found to enhance INa, in Scn5a+/- mice, as a novel approach of therapy. Methods We used AAV9-systemic injection to overexpress the Nav1.5-peptide into the heart of Scn5a+/- mice to gauge their susceptibility to arrhythmias induced by intracardiac pacing. We also examined the Nav1.5-peptide effects on endogenous Nav1.5 expression through molecular biology and biochemistry analyses. Results The duration of PR and QRS intervals in the hearts of Scn5a+/- mice was restored to normal by overexpression of the Nav1.5-peptide (PR interval = 46.3 ± 0.9 ms, n=22 in Scn5a+/- mice vs 40.4 ± 0.8 ms, n= 23 in WT mice, P<0.0001 and 42.6 ± 0.9 ms, n=11 in Scn5a+/- +Nav1.5-peptide, P<0.05; QRS interval = 15.2 ± 0.4, n=22 ms in Scn5a+/- mice vs 13.2 ± 0.4 ms, n=23 in WT mice, P<0.01 and 13.6 ± 0.4 ms, n=14 in Scn5a+/- +Nav1.5-peptide, P<0.05). Additionally, overexpression of the Nav1.5-peptide seemed to protect Scn5a+/- mice from induced arrhythmic events caused by programmed-ventricular stimulation (total premature beats = 1.3 ± 0.4 in Scn5a+/- mice, n = 13 vs 0.2 ± 0.2, n = 6 in injected Scn5a+/- mice, NS and 0.2 ± 0.2, n = 12 in WT mice, P< 0.05). There were no significant differences in total Nav1.5 expression between injected and non-injected mice as shown by RT-qPCR and Western-blots, but the expression of Nav1.5 at the plasma membrane appeared to be restored by overexpression of the Nav1.5-peptide (Nav1.5/N-cadherin ratio = 0.50 ± 0.08, n = 11 in Scn5a+/- mice vs 1 ± 0.15, n= 11 in WT mice, P< 0.05 and 0.94 ± 0.13, n = 11 in injected Scn5a+/- mice, P< 0.05). Conclusion Gene therapy with Nav1.5-peptide can attenuate both conduction defects and increased arrhythmia inducibility observed in Scn5a+/- mice. Although additional studies will be needed to confirm and explore further our results, this could be promising to treat patients with « sodium-channel dysfunction » such as BrS.

Desmin, the most abundant intermediate filament in cardiomyocytes, plays a key role in maintaining cardiomyocyte structure by interconnecting intracellular organelles, and facilitating cardiomyocyte interactions with the extracellular matrix and neighboring cardiomyocytes. As a consequence, mutations in the desmin gene (DES) can lead to desminopathies, a group of diseases characterized by variable and often severe cardiomyopathies along with skeletal muscle disorders. The basic desmin intermediate filament structure is composed of four segments separated by linkers that further assemble into dimers, tetramers and eventually unit-length filaments that compact radially to give the final form of the filament. Each step in this process is critical for proper filament formation and allow specific interactions within the cell. Mutations within the desmin gene can disrupt filament formation, as seen by aggregate formation, and thus have severe cardiac and skeletal outcomes, depending on the locus of the mutation. The focus of this review is to outline the cardiac molecular consequences of mutations located in the C-terminal part of segment 2B. This region is crucial for ensuring proper desmin filament formation and is a known hotspot for mutations that significantly impact cardiac function.

BACKGROUND: Cardiac channelopathies, caused by mutations in ion-channel genes, can lead to sudden cardiac death (SCD) via ventricular arrhythmias. Brugada syndrome (BrS) is a rare inherited channelopathy characterized by a unique ECG pattern and a high incidence of ventricular fibrillation leading to SCD in the absence of structural heart defects. The main gene responsible for 20-25% of BrS cases is SCN5A, encoding the cardiac sodium channel alpha-subunit Nav1.5, which carries the sodium current (INa) responsible for the rapid depolarization phase of the action potential (AP). While current treatments do not target the genetic cause of channelopathies, this study explores the therapeutic potential of overexpressing the N-terminal region of Nav1.5 (Nter) to restore electrical activity by rescuing INa, in the context of SCN5A deficiency. METHODS: We overexpressed the Nter peptide using viral vectors in Scn5a+/- mice, in CRISPR/Cas9 edited-SCN5A+/- cardiomyocytes derived from induced-pluripotent stem cells (iPSC-CMs) and in BrS patient iPSC-CMs. We assessed molecular and functional effects of Nter overexpression in vitro and in vivo by measuring Nav1.5 subcellular expression and electrophysiological activity and by recording ECGs and arrhythmias. RESULTS: Whereas Scn5a+/- mice showed an impaired INa associated with a slowed-cardiac conduction characteristic of the BrS phenotype, cardiac-specific overexpression of Nter corrected AP parameters by restoring INa density in Scn5a+/- mouse cardiomyocytes. This increase in INa density was caused by a translocation of Nav1.5 to the cell membrane in Nter-overexpressing mice. Most importantly, Nter overexpression normalized atrioventricular and ventricular conduction and protected Scn5a+/- mice from arrhythmias triggered by programmed electrical stimulation. Similarly, Nter overexpression in SCN5A+/- human iPSC-CMs led to a 2-fold increase in Nav1.5 cell-surface expression, resulting in normalization of INa and AP parameters and abolition of early after depolarizations observed during spontaneous AP recordings. Similar results were obtained in iPSC-CMs derived from a BrS patient, confirming the potential of this therapy in human models. CONCLUSIONS: This study identified a novel therapeutic peptide effective in restoring cardiac excitability in animal and cellular models of BrS, paving the way for future development of therapies for life-threatening arrhythmias in patients with SCN5A deficiency. ### Competing Interest Statement The authors have declared no competing interest.

Introduction: Sudden cardiac death (SCD) and ventricular fibrillation are rare but severe complications of many cardiovascular diseases and represent a major health issue worldwide. Although the primary causes are often acute or chronic coronary diseases, genetic conditions, such as inherited channelopathies or non-ischemic cardiomyopathies are leading causes of SCD among the young. However, relevant experimental models to study the underlying mechanisms of arrhythmias and develop new therapies are still needed. The number of genetically engineered mouse models with cardiac phenotype is growing, making electrophysiological studies in mice essential tools to study arrhythmogenicity and arrhythmia mechanisms and to test novel treatments. Recently, intracardiac catheterization via the jugular vein was described to induce and record ventricular arrhythmias in living anesthetized mice. Several strategies have been reported, developed in healthy wild-type animals and based on aggressive right ventricular stimulation.Methods: Here, we report a protocol based on programmed electrical stimulation (PES) performed in clinical practice in patients with cardiac rhythm disorders, adapted to two transgenic mice models of arrhythmia - Brugada syndrome and cardiolaminopathy.Results: We show that this progressive protocol, based on a limited number of right ventricular extrastimuli, enables to reveal different rhythmic phenotypes between control and diseased mice. In this study, we provide detailed information on PES in mice, including catheter positioning, stimulation protocols, intracardiac and surface ECG interpretation and we reveal a higher susceptibility of two mouse lines to experience triggered ventricular arrhythmias, when compared to control mice.Discussion: Overall, this technique allows to characterize arrhythmias and provides results in phenotyping 2 arrhythmogenic-disease murine models.

Mutations in the DES gene, which encodes the intermediate filament desmin, lead to desminopathy, a rare disease characterized by skeletal muscle weakness and different forms of cardiomyopathies associated with cardiac conduction defects and arrhythmias. We generated human induced pluripotent stem cells (hiPSC) from a patient carrying the DES p.R406W mutation, and employed CRISPR/Cas9 to rectify the mutation in the patient's hiPSC line and introduced the mutation in an hiPSC line from a control individual unrelated to the patient. These hiPSC lines represent useful models for delving into the mechanisms of desminopathy and developing new therapeutic approaches.

Introduction The Brugada syndrome (BrS) is a rare cardiac arrhythmic disorder associated with an increased risk of sudden cardiac death. The predominant genetic cause involves mutations in the SCN5A gene, contributing to sodium channel dysfunction. Recently, a missense mutation (p.R211H) in the RRAD gene was identified in a BrS-affected family. Objective In this study, we investigated the consequences of this mutation using a knock-in mouse model and human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). Method Mouse phenotyping was performed using ECG recordings, and patch-clamp technique on ventricular cardiomyocytes. Western blot and immunostaining were performed on mouse ventricular tissue and hiPSC-CMs. Results The knock-in mice carrying the equivalent p.R210H mutation exhibited significant alterations in electrocardiographic parameters, including QRS complex duration prolongation and ventricular fibrosis, recapitulating features observed in BrS. Notably, these mice show increased CTGF and TGF-β expression, contributing to fibrosis development. In iPSC-CMs harboring the p.R211H mutation, a ∼50% reduction in sodium current amplitude was observed, consistent with findings in the mouse model. Additionally, correction of the mutation in iPSCs using CRISPR/Cas9 restored sodium current density, highlighting the causal link between the RRAD mutation and sodium channel dysfunction. Further investigation revealed a down-regulation of protein expression for Rad, Nav1.5, and C × 43 in both iPSC-CMs and mouse ventricles carrying the mutation, without significant changes in mRNA levels. Immunostaining indicated altered localization of C × 43 in iPSC-CMs and in knock-in homozygous mice, suggesting potential disruptions in intercellular junctions. Conclusion These findings underscore the pathogenic impact of the RRAD p.R211H mutation on sodium channel function, cardiac fibroblast proliferation, and intercellular junctions, providing valuable insights into the molecular mechanisms underlying Brugada syndrome.

EDITORIAL article Front. Physiol., 06 September 2023Sec. Cardiac Electrophysiology Volume 14 - 2023 | https://doi.org/10.3389/fphys.2023.1254596

AbstractRATIONALECardiac rhythm, conduction and synchronization of electrical activity require the coordinated action of different types of ion channels that differ according to transmural and regional specificities. Classical pharmacology affects these ion channels in a non-regionalized way which explains why treating arrhythmias, that often occur in specific foci, has often limited efficacy in addition to negative side-effects on non-targeted organs. Photopharmacology is an emergent technology that has the potential to counteract all the negative aspects of classical pharmacology by restricting drug activity in a spatio-temporal manner.OBJECTIVEWe tested the potential of photopharmacology in specifically regulating heart activity by using a caged derivative of a natural peptide inhibitor of the ERG channel, BeKm1. The peptide was uncaged and activity monitoredin vitroon a cell line expressing the hERG channel, on human cardiomyocytes derived from iPS cells, andex vivoandin vivoon zebrafish larvae and rat hearts.METHODS AND RESULTSCaged BeKm-1 is inactive and fully active upon uncaging. Uncaging of the peptide on human iPS-derived cardiomyocytes enlarges the action potential duration and triggers arrhythmias. Uncaging also triggers bradycardia and disturbs cardiac conduction within the atria in perfused rat hearts upon illumination. The potency of photopharmacology for cardiac electrical modulation was further validated in zebrafish larvae where illumination of the caged compound induces bradycardia and atrio-ventricular desynchrony. Finally, in anesthetized rats, illumination of the caged peptide in the right atria, containing the sino-atrial node, leads to bradycardia without arrhythmia.CONCLUSIONSThis report demonstrates that photopharmacology, using the caged peptide strategy, can be used for dynamically regulating cardiac electrical activityin vivoand that spatial illumination restriction can dissociate the bradycardic effect from the arrhythmic one. The technology is applicable to all kinds of cardiac ion channels and regions of interest to create arrhythmogenic models or investigate new clinical applications.