Enhanced sarcoplasmic reticulum Ca2+ leak and increased Na+-Ca2+ exchanger function underlie delayed afterdepolarizations in patients with chronic atrial fibrillation

Disease mechanism and novel drug therapies for atrial fibrillation

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia, affects over 3 % of adults globally, increasing risks for stroke, heart failure, and cognitive decline. Early rhythm control shows promise in improving AF prognosis, and catheter ablation remains an effective, safe option, especially for paroxysmal AF. However, high recurrence rates with antiarrhythmic drugs and ablation persist, particularly in cases of persistent AF. Emerging research on molecular mechanisms has led to innovative therapeutic strategies targeting these pathways, offering hope for more effective AF management. This review explores recent insights into the complex pathophysiology of AF, with a particular focus on ion channel dysfunction, calcium mishandling, oxidative stress, and fibrosis. It further considers how these factors will inspire new therapeutic options.

Beneficial effects of doxycycline on atrial electrical remodelling in a rat model of atrial fibrillation

BACKGROUND

Previous studies showed that doxycycline (Dox) can attenuate chronic intermittent hypoxia (CIH)-induced atrial fibrosis in rats. On this basis, we further investigated the effects of Dox on CIH-induced atrial electrical remodelling.

METHODS

Rats were randomised into 3 groups: Control group, CIH group, and CIH with Dox treatment (CIH-D) group (n = 30). CIH and CIH-D rats were subjected to CIH 8 h/d for 6 weeks. After collecting the basic parameters of the rats, atrial fibrillation (AF) inducibility, conduction inhomogeneity, and epicardial conduction velocity were examined by in vitro cardiac electrophysiology experiments. The expression levels of ion channel subunits in the atrium were detected by Western blotting. Whole-cell patch clamp experiments were used to recorded action potential (AP), ICa-L, Ito, and the kinetic parameters.

RESULTS

Compared to the Control rats, CIH rats showed increased AF inducibility, conduction inhomogeneity, and expression levels of p-RyR2, p-CaMKII, Kv11.1, Kir2.3, KCa3.1, whereas the epicardial conduction velocity, ICa-L, Ito, and expression levels of Cav1.2, Kv1.5, Kv4.3 were decreased. Dox-treatment significantly improved the expression levels of Kv1.5, Kv4.3 and Kir2.3 in CIH-D rats.

CONCLUSION

CIH caused atrial electrical remodelling in our rats, which was improved by Dox treatment. These changes indicated the potential effects of Dox in AF.

Propionic Acidemia-Induced Proarrhythmic Electrophysiological Alterations in Human iPSC-Derived Cardiomyocytes

Propionic acidemia (PA) is a metabolic disorder caused by a deficiency of the mitochondrial enzyme propionyl-CoA carboxylase (PCC) due to mutations in the PCCA or PCCB genes, which encode the two PCC subunits. PA may lead to several types of cardiomyopathy and has been linked to cardiac electrical abnormalities such as QT interval prolongation, life-threatening arrhythmias, and sudden cardiac death. To gain insights into the mechanisms underlying PA-induced proarrhythmia, we recorded action potentials (APs) and ion currents using whole-cell patch-clamp in ventricular-like induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CMs) from a PA patient carrying two pathogenic mutations in the PCCA gene (p.Cys616_Val633del and p.Gly477Glufs*9) (PCCA cells) and from a healthy subject (healthy cells). In cells driven at 1 Hz, PCC deficiency increased the latency and prolonged the AP duration (APD) measured at 20% of repolarization, without modifying resting membrane potential or AP amplitude. Moreover, delayed afterdepolarizations appeared at the end of the repolarization phase in unstimulated and paced PCCA cells. PCC deficiency significantly reduced peak sodium current (INa) but increased the late INa (INaL) component. In addition, L-type Ca2+ current (ICaL) density was reduced, while the inward and outward density of the Na+/Ca2+ exchanger current (INCX) was increased in PCCA cells compared to healthy ones. In conclusion, our results demonstrate that at the cellular level, PCC deficiency can modify the ion currents controlling cardiac excitability, APD, and intracellular Ca2+ handling, increasing the risk of arrhythmias independently of the progressive late-onset cardiomyopathy induced by PA disease.

Macrophage-mediated IL-6 signaling drives ryanodine receptor-2 calcium leak in postoperative atrial fibrillation

Postoperative atrial fibrillation (poAF) is AF occurring days after surgery, with a prevalence of 33% among patients undergoing open-heart surgery. The degree of postoperative inflammation correlates with poAF risk, but less is known about the cellular and molecular mechanisms driving postoperative atrial arrhythmogenesis. We performed single-cell RNA-seq comparing atrial nonmyocytes from mice with and without poAF, which revealed infiltrating CCR2+ macrophages to be the most altered cell type. Pseudotime trajectory analyses identified Il-6 as a gene of interest driving in macrophages, which we confirmed in pericardial fluid collected from human patients after cardiac surgery. Indeed, macrophage depletion and macrophage-specific Il6ra conditional knockout (cKO) prevented poAF in mice. Downstream STAT3 inhibition with TTI-101 and cardiomyocyte-specific Stat3 cKO rescued poAF, indicating a proarrhythmogenic role of STAT3 in poAF development. Confocal imaging in isolated atrial cardiomyocytes (ACMs) uncovered what we believe to be a novel link between STAT3 and CaMKII-mediated ryanodine receptor-2 (RyR2)-Ser(S)2814 phosphorylation. Indeed, nonphosphorylatable RyR2S2814A mice were protected from poAF, and CaMKII inhibition prevented arrhythmogenic Ca2+ mishandling in ACMs from mice with poAF. Altogether, we provide multiomic, biochemical, and functional evidence from mice and humans that IL-6-STAT3-CaMKII signaling driven by infiltrating atrial macrophages is a pivotal driver of poAF, which portends therapeutic utility for poAF prevention.

Long Non-coding RNA Involved in the Pathophysiology of Atrial Fibrillation

BACKGROUND

Atrial fibrillation (AF) is a prevalent and chronic cardiovascular disorder associated with various pathophysiological alterations, including atrial electrical and structural remodeling, disrupted calcium handling, autonomic nervous system dysfunction, aberrant energy metabolism, and immune dysregulation. Emerging evidence suggests that long non-coding RNAs (lncRNAs) play a significant role in the pathogenesis of AF.

OBJECTIVE

This discussion aims to elucidate the involvement of AF-related lncRNAs, with a specific focus on their role as miRNA sponges that modulate crucial signaling pathways, contributing to the progression of AF. We also address current limitations in AF-related lncRNA research and explore potential future directions in this field. Additionally, we summarize feasible strategies and promising delivery systems for targeting lncRNAs in AF therapy.

CONCLUSION

In conclusion, targeting AF-related lncRNAs holds substantial promise for future investigations and represents a potential therapeutic avenue for managing AF.

T-World: A highly general computational model of a human ventricular myocyte

Cardiovascular disease is the leading cause of death, demanding new tools to improve mechanistic understanding and overcome limitations of stem cell and animal-based research. We introduce T-World, a highly general virtual model of human ventricular cardiomyocyte suitable for multiscale studies. T-World shows comprehensive agreement with human physiology, from electrical activation to contraction, and is the first to replicate all key cellular mechanisms driving life-threatening arrhythmias. Extensively validated on unseen data, it demonstrates strong predictivity across applications and scales. Using T-World we revealed a likely sex-specific arrhythmia risk in females related to restitution properties, identified arrhythmia drivers in type 2 diabetes, and describe unexpected pro-arrhythmic role of NaV1.8 in heart failure. T-World demonstrates strong performance in predicting drug-induced arrhythmia risk and opens new opportunities for predicting and explaining drug efficacy, demonstrated by unpicking effects of mexiletine in Long QT syndrome 2. T-World is available as open-source code and an online app.

TAX1BP3 Causes TRPV4-Mediated Autosomal Recessive Arrhythmogenic Cardiomyopathy

BACKGROUND

Arrhythmogenic cardiomyopathy (ACM) is one of the leading causes of sudden cardiac death in children, young adults, and athletes and is characterized by the fibro-fatty replacement of the myocardium, predominantly of the right ventricle. Sixty percent of patients with ACM have a known genetic cause, but for the remainder, the pathogenesis is unknown. This lack of mechanistic understanding has slowed the development of disease-modifying therapies, and children with ACM have a high degree of morbidity and mortality.

METHODS

Induced pluripotent stem cells (iPSCs) from 3 family members were differentiated into cardiac myocytes (CMs). Calcium imaging was conducted by labeling calcium with CAL-520 and confocal imaging to capture calcium sparks after iPSC-CMs were electrically paced. A cardiac-specific, inducible knockout mouse (Tax1bp3-/-) was made and intracardiac electrophysiology studies conducted to observe arrhythmia inducibility following pacing.

RESULTS

We identified a kindred with multiple members affected by ACM cosegregating with biallelic variants in the gene TAX1BP3, which encodes the protein TAX1BP3 (Tax1-binding protein 3). iPSC-CMs derived from this kindred demonstrated increased intracellular lipid droplets, induction of TRPV4 (transient receptor potential vanilloid type 4) expression, and inducible TRPV4 current. This was associated with depletion of the intracellular sarcoplasmic reticulum Ca2+ store and increased RyR2 (ryanodine receptor 2)-mediated store Ca2+ leak and delayed afterdepolarizations, a known mechanism of Ca2+-mediated arrhythmogenesis. Similarly, Tax1bp3 cardiac-specific knockout mice had increased Ca2+ leak and were predisposed to ventricular arrhythmias compared with wild-type mice. Ca2+ leak in both the iPSC-CMs and mouse ventricular myocytes was rescued by small molecule TRPV4 inhibition. This strategy also effectively reduced Ca2+ leak in a PKP2 (plakophilin 2) p.His773AlafsX8 iPSC-CM model of ACM.

CONCLUSIONS

We conclude that TAX1BP3 is associated with rare autosomal recessive ACM through TRPV4-mediated Ca2+ leak from RyR2. Further, TRPV4 current inhibition has the potential to be a new therapeutic target for ACM.

Cancer Is a Major Determinant of Postoperative Atrial Fibrillation After Cardiac Surgery

Background/Objectives: Postoperative atrial fibrillation (POAF) occurs frequently after cardiac surgery and is associated with increased morbidity and mortality. The pathogenesis of POAF in this setting is complex and not completely understood. Since cancer is a well-known risk factor for AF, the aim of this study was to identify potential predictors, including cancer, of POAF after cardiac surgery. Methods: This prospective study included 400 consecutive patients in sinus rhythm who underwent elective cardiac surgery in Aretaeio Hospital (Nicosia, Cyprus) from January 2020 till January 2023. The primary outcome was the development of POAF during hospitalization, defined as any documented AF episode lasting >30 s. Predictors of the primary outcome were studied using univariable and multivariable logistic regression analysis. Results: Of the 400 patients (68 [61-73] years, 64 [16%] females) studied, 66 (16.5%) developed POAF. Among the variables examined, the only predictors of POAF were cardiopulmonary bypass time (odds ratio [OR] = 1.001, 95% confidence interval = [95% CI, 1.000-1.001], p = -0.031) and cancer (OR = 3.852, 95% CI = [1.535-9.664], p = 0.004). Cancer was present in 13 (4%) and in 10 (15%) of patients without and with POAF, respectively (p < 0.001). Conclusions: Cancer was associated with a dramatic increase in POAF risk early after elective cardiac surgery in this study. Whether patients developing POAF after cardiac surgery should be searched for cancer deserves further investigation.

Cardioprotective effects of semaglutide on isolated human ventricular myocardium

AIMS

Semaglutide, a glucagon-like peptide-1 (GLP-1) receptor agonist, has shown promising effects in reducing cardiovascular events in patients with obesity and heart failure (HF) with preserved ejection fraction (HFpEF) irrespective of concomitant diabetes. However, the exact mechanisms underlying its cardioprotective actions remain unclear. Our study investigates the direct effects of semaglutide on human cardiomyocytes, focusing on calcium (Ca) and sodium (Na) handling and its potential to improve myocardial contractility.

METHODS AND RESULTS

Human left ventricular cardiomyocytes were isolated from non-failing (NF) hearts, patients with aortic stenosis and a HFpEF-like phenotype (AS), and those with end-stage HF with reduced ejection fraction (HFrEF). Late Na current (INa), sarcoplasmic reticulum (SR) Ca leak, and contractility were assessed in isolated cardiomyocytes treated with semaglutide. CaMKII inhibitor autocamtide-2-related inhibitory peptide and GLP-1 receptor antagonist exendin 9-39 (Ex-9-39) were used to elucidate signalling pathways. Semaglutide reduced late INa in AS and HFrEF cardiomyocytes to levels comparable to NF. Additionally, semaglutide decreased diastolic SR Ca leak and improved systolic Ca transients and contractility in AS and HFrEF tissue. These effects were mediated through GLP-1 receptor agonism and were comparable to CaMKII inhibition. In multicellular preparations, semaglutide differentially improved myocardial contractility in AS and HFrEF in a dose-dependent manner.

CONCLUSION

Semaglutide directly modulates ion homeostasis in human cardiomyocytes, reducing proarrhythmic diastolic SR Ca leak and enhancing systolic function, which may explain its observed clinical benefits. These findings provide mechanistic insights into the cardioprotective effects of semaglutide and suggest its potential therapeutic use in HF.

Cardiac FGF23 Increases Intracellular Calcium in Atrial Myocytes and the Susceptibility to Atrial Fibrillation Decreased in FGF23f / fMyHCCre /+ Mice

Exogenous fibroblast growth factor (FGF) 23 is closely associated with atrial fibrillation (AF) and is able to alter the cardiac electrophysiological activity by increasing intracellular calcium. While its arrhythmogenic mechanism remains unclear, this study aims to investigate the electrophysiological effects of cardiac FGF23 on intracellular calcium in atrial cells and its underlying mechanism. The incidence of AF was significantly decreased in FGF23f/fMyHCCre/+ mice compared to Cre mice. A significant increase in the incidence of triggering activity (TA), L-type calcium currents (ICa,L) and systolic calcium transient was induced in neonatal mice atrial myocytes (NMAMs) from the overexpression of FGF23. Conversely, the opposite effects were exhibited as a reduced diastolic spontaneous calcium leak and weakened Na+/Ca2+ exchange (NCX) function in cardiac myocytes from FGF23f/fMyHCCre/+ mice, which can reduce incidences of AF induced by delayed after depolarization (DAD). In addition, ryanodine-receptor 2 (RyR2) of calcium regulatory proteins was significantly downregulated in FGF23f/fMyHCCre/+ mice and upregulated in FGF23 overexpression of NMAMs. In conclusion, overexpression of cardiac FGF23 may increase the susceptibility to AF due to DAD or TA induced by increasing intracellular calcium in atrial myocytes.

Atrial Fibrillation in End-Stage Heart Failure

End-stage heart failure (HF) accounts for 1% to 10% of all HF cases. It is frequently associated with coexistent atrial fibrillation due in part to structural changes related to altered hemodynamics, increased wall stress, and neurohormonal activation. The treatment remains challenging because of frequent atrial remodeling, comorbidities, and high recurrence rates. Studies have found no benefit of medical rhythm control compared with rate control. However, recent data suggest that catheter ablation might be associated with a reduction in HF symptomatology and all-cause mortality. Alternative treatment options including AV nodal ablation are available for refractory cases, but should not delay urgent heart transplantation.

Mitochondrial calcium homeostasis and atrial fibrillation: Mechanisms and therapeutic strategies review

Atrial fibrillation (AF) is tightly linked to mitochondrial dysfunction, calcium (Ca²⁺) imbalance, and oxidative stress. Mitochondrial Ca²⁺ is essential for regulating metabolic enzymes, maintaining the tricarboxylic acid (TCA) cycle, supporting the electron transport chain (ETC), and producing ATP. Additionally, Ca²⁺ modulates oxidative balance by regulating antioxidant enzymes and reactive oxygen species (ROS) clearance. However, Ca²⁺ homeostasis disruptions, particularly overload, result in excessive ROS production, mitochondrial permeability transition pore (mPTP) opening, and oxidative stress-induced damage. These changes lead to mitochondrial dysfunction, Ca²⁺ leakage, and cardiomyocyte apoptosis, driving AF progression and atrial remodeling. Therapeutically, targeting mitochondrial Ca²⁺ homeostasis shows promise in mitigating AF. Moderate Ca²⁺ regulation enhances energy metabolism, stabilizes mitochondrial membrane potential, and bolsters antioxidant defenses by upregulating enzymes like superoxide dismutase and glutathione peroxidase. This reduces ROS generation and facilitates clearance. Proper Ca²⁺ levels also prevent electron leakage and promote mitophagy, aiding in damaged mitochondria removal and reducing ROS accumulation. Future strategies include modulating Ryanodine receptor 2 (RyR2), mitochondrial calcium uniporter (MCU), and sodium-calcium exchanger (NCLX) to control Ca²⁺ overload and oxidative damage. Addressing mitochondrial Ca²⁺ dynamics offers a compelling approach to breaking the cycle of Ca²⁺ overload, oxidative stress, and AF progression. Further research is needed to clarify the mechanisms of mitochondrial Ca²⁺ regulation and its role in AF pathogenesis. This knowledge will guide the development of innovative treatments to improve outcomes and quality of life for AF patients.

The role of SGLT1 in atrial fibrillation and its relationship with endothelial-mesenchymal transition

Atrial fibrillation (AF) is a prevalent arrhythmia closely associated with atrial fibrosis, posing significant challenges to cardiovascular health. Recent studies have identified the sodium-glucose co-transporter 1 (SGLT1) as a potential key player in the pathophysiology of heart diseases, particularly in the context of AF and atrial fibrosis. This review aims to synthesize current knowledge regarding the mechanisms by which SGLT1 influences the development of AF and atrial fibrosis, with a specific focus on its relationship with endothelial-mesenchymal transition (EMT). By analyzing the latest research findings, this paper discusses how SGLT1 may modulate structural and functional changes in the atria, thereby enhancing our understanding of the underlying mechanisms driving AF.

The Role of HIF-1α in Atrial Fibrillation: Recent Advances and Therapeutic Potentials

The steady increase in life expectancy throughout the world is contributing to an increased incidence of atrial fibrillation (AF), which imposes a significant socioeconomic toll on affected patients and societies. The mechanisms underlying atrial fibrillation are multifaceted and vary among individuals. Hypoxia is a process that is closely linked to AF onset and progression. Hypoxia-inducible factor 1-alpha (HIF-1α) is a transcription factor that serves as a key regulator of oxygen homeostasis within cells through its activation under hypoxic conditions and subsequently coordinates various pathophysiological responses. High levels of HIF-1α expression are evident in AF patients, and facilitate the progression from persistent AF to permanent AF. Thus, HIF-1α may serve as a promising target for novel therapeutic strategies aimed at the prevention and treatment of AF. This review provides an overview and synthesis of recent studies probing the relationship between HIF-1α and AF, providing a foundation for future studies and the development targeted drug therapies.

Constitutive deletion of the obscurin-Ig58/59 domains induces atrial remodeling and Ca2+-based arrhythmogenesis

Obscurin is a giant protein that coordinates diverse aspects of striated muscle physiology. Obscurin immunoglobulin domains 58/59 (Ig58/59) associate with essential sarcomeric and Ca2+ cycling proteins. To explore the pathophysiological significance of Ig58/59, we generated the Obscn-ΔIg58/59 mouse model, expressing obscurin constitutively lacking Ig58/59. Males in this line develop atrial fibrillation by 6 months, with atrial and ventricular dilation by 12 months. As Obscn-ΔIg58/59 left ventricles at 6 months exhibit no deficits in sarcomeric ultrastructure or Ca2+ signaling, we hypothesized that susceptibility to arrhythmia may emanate from the atria. Ultrastructural evaluation of male Obscn-ΔIg58/59 atria uncovered prominent Z-disk streaming by 6 months and further misalignment by 12 months. Relatedly, isolated Obscn-ΔIg58/59 atrial cardiomyocytes exhibited increased Ca2+ spark frequency and age-specific alterations in Ca2+ cycling dynamics, coinciding with arrhythmia onset and progression. Quantitative analysis of the transverse-axial tubule (TAT) network using super-resolution microscopy demonstrated significant TAT depletion in Obscn-ΔIg58/59 atria. These structural and Ca2+ signaling deficits were accompanied by age-specific alterations in the expression or phosphorylation of T-cap protein, which links transverse tubules to Z-disks, and junctophilin 2, which connects transverse tubules to the sarcoplasmic reticulum. Collectively, our work establishes the Obscn-ΔIg58/59 model as a reputable genetic model for atrial cardiomyopathy and provides mechanistic insights into atrial fibrillation and remodeling.

Inhibitors of Intracellular RyR2 Calcium Release Channels as Therapeutic Agents in Arrhythmogenic Heart Diseases

Ryanodine receptor type 2 (RyR2) is the principal intracellular calcium release channel in the cardiac sarcoplasmic reticulum (SR). Pathological RyR2 hyperactivity generates arrhythmia risk in genetic and structural heart diseases. RYR2 gain-of-function mutations cause catecholaminergic polymorphic ventricular tachycardia. In structural heart diseases (i.e., heart failure), posttranslation modifications render RyR2 channels leaky, resulting in pathologic calcium release during diastole, contributing to arrhythmogenesis and contractile dysfunction. Hence, RyR2 represents a therapeutic target in arrhythmogenic heart diseases. We provide an overview of the structure and function of RyR2, and then review US Food and Drug Administration-approved and investigational RyR2 inhibitors. A therapeutic classification of RyR2 inhibitors is proposed based on their mechanism of action. Class I RyR2 inhibitors (e.g., flecainide) do not change SR calcium content and are primarily antiarrhythmic. Class II RyR2 inhibitors (e.g., dantrolene) increase SR calcium content, making them less effective as antiarrhythmics but preferable in conditions with reduced SR calcium content such as heart failure.

From Atrial Small-conductance Calcium-activated Potassium Channels to New Antiarrhythmics

Despite significant advances in its management, AF remains a major healthcare burden affecting millions of individuals. Rhythm control with antiarrhythmic drugs or catheter ablation has been shown to improve symptoms and outcomes in AF patients, but current treatment options have limited efficacy and/or significant side-effects. Novel mechanism-based approaches could potentially be more effective, enabling improved therapeutic strategies for managing AF. Small-conductance calcium-activated potassium (SK or KCa2.x) channels encoded by KCNN1-3 have recently gathered interest as novel antiarrhythmic targets with potential atrial-predominant effects. Here, the molecular composition of smallconductance calcium-activated potassium channels and their complex regulation in AF as the basis for understanding the distinct mechanism of action of pore-blockers (apamin, UCL1684, ICAGEN) and modulators of calcium-dependent activation (NS8593, AP14145, AP30663) are summarised. Furthermore, the preclinical and early clinical evidence for the role of small-conductance calcium-activated potassium channel inhibitors in the treatment of AF are reviewed.

Genetic and Molecular Underpinnings of Atrial Fibrillation

Atrial fibrillation (AF), the most common sustained arrhythmia, increases stroke and heart failure risks. Here we review genes linked to AF and mechanisms by which they alter AF risk. We highlight gene expression differences between atrial and ventricular cardiomyocytes, regulatory mechanisms responsible for these differences, and their potential contribution to AF. Understanding AF mechanisms through the lens of atrial gene regulation is crucial to improving AF treatment.

QTc interval prolongation and risk of atrial fibrillation recurrence: a meta-analysis and observational cohort study

Introduction

Corrected QT interval (QTc) is a ventricular repolarization marker on electrocardiography. Previous studies evaluated its value in predicting atrial fibrillation (AF) occurrence. However, its predictive efficacy for AF recurrence remains controversial.

Methods

We searched PubMed and Google databases for studies before January 2024 evaluating the association between QTc interval and AF incidence. A meta-analysis of the eligible datasets was conducted using Bazett's formula, with subgroup analysis to explore the heterogeneity. Additionally, thirty-eight patients with AF who underwent radiofrequency catheter ablation were enrolled and followed-up for 3-36 months. Univariate and multivariate Cox models were used to calculate the hazard ratios (HRs) and determine the relationship between clinical factors and AF recurrence. Kaplan-Meier survival analysis and ROC curve were conducted to assess the impact and predictive efficacy of individual factors.

Results

Eleven datasets from nine eligible studies were enrolled and meta-analysed. We found that patients with prolonged QTc interval was associated with a significantly higher AF incidence risk, and the risk increased with every 10-ms prolongation. However, this association was not significant in the AF recurrence subgroup. In our prospective cohort, the preoperative body mass index, QTc, left atrial diameter (LAD), and uric acid levels influenced AF recurrence. Multivariate Cox regression analysis identified LAD as an independent factor affecting AF recurrence in patients with a high predictive efficiency. Kaplan-Meier survival analysis showed that increased LAD (>4.5 cm) was associated with postoperative AF recurrence.

Discussion

Therefore, LAD has better predictive power and can be an indicator for predicting postoperative AF recurrence.

Impact of Overnight Storage of Human Atrial Myocytes on Intracellular Calcium Homeostasis and Electrophysiological Utility

Human atrial myocytes afford an attractive experimental model to investigate mechanisms underlying electrophysiological alterations in cardiovascular disease. However, this model presents limitations, such as the availability of human atrial tissue and a variable yield of myocytes isolation. Therefore, we aimed to determine whether overnight storage can increase the time window where the electrophysiological properties of human atrial myocytes can be determined. To address this issue, human atrial myocytes isolated from patients undergoing cardiac surgery were used for patch-clamp experiments on the day of cell isolation (Day 1) and the following day (Day 2). The shape of the current-voltage (I-V) relationship for the calcium current (ICa) depended on the access resistance and the cell capacitance, with large cells (>75 pF) requiring a lower access resistance (<15 MΩ) than small cells (<40 pF) to avoid distortion of the I-V curve. Importantly, overnight storage did not significantly affect (1) the ICa amplitude or properties, (2) sarcoplasmic reticulum calcium homeostasis or (3) the frequency-dependency of the beat-to-beat response. In conclusion, overnight storage of isolated human atrial myocytes at 4 °C does not affect essential features of intracellular calcium homeostasis and, therefore, affords a simple protocol to extend the experimental lifetime of human atrial myocytes.

Enhanced Ca2+-Driven Arrhythmogenic Events in Female Patients With Atrial Fibrillation: Insights From Computational Modeling

BACKGROUND

Substantial sex-based differences have been reported in atrial fibrillation (AF), but the underlying mechanisms are poorly understood.

OBJECTIVES

This study sought to gain a mechanistic understanding of Ca2+-handling disturbances and Ca2+-driven arrhythmogenic events in male vs female atrial cardiomyocytes and establish their responses to Ca2+-targeted interventions.

METHODS

We integrated reported sex differences and AF-associated changes (ie, expression and phosphorylation of Ca2+-handling proteins, cardiomyocyte ultrastructural characteristics, and dimensions) into our human atrial cardiomyocyte model that couples electrophysiology with spatially detailed Ca2+-handling processes. Sex-specific responses of atrial cardiomyocytes to arrhythmia-provoking protocols and Ca2+-targeted interventions were evaluated.

RESULTS

Simulated quiescent cardiomyocytes showed increased incidence of Ca2+ sparks in female vs male myocytes in AF, in agreement with previous experimental reports. Additionally, our female model exhibited elevated propensity to develop pacing-induced spontaneous Ca2+ releases (SCRs) and augmented beat-to-beat variability in action potential (AP)-elicited Ca2+ transients compared with the male model. Sensitivity analysis uncovered distinct arrhythmogenic contributions of each component involved in sex and/or AF alterations. Specifically, increased ryanodine receptor phosphorylation emerged as the major SCR contributor in female AF cardiomyocytes, whereas reduced L-type Ca2+ current was protective against SCRs for male AF cardiomyocytes. Furthermore, simulated Ca2+-targeted interventions identified potential strategies (eg, t-tubule restoration, and inhibition of ryanodine receptor and sarcoplasmic/endoplasmic reticulum Ca2⁺-ATPase) to attenuate Ca2+-driven arrhythmogenic events in women, and revealed enhanced efficacy when applied in combination.

CONCLUSIONS

Sex-specific modeling uncovers increased Ca2+-driven arrhythmogenic events in female vs male atria in AF, and suggests combined Ca2+-targeted interventions are promising therapeutic approaches in women.

Ubiquitin-specific protease 38 promotes atrial fibrillation in diabetic mice by stabilizing iron regulatory protein 2

BACKGROUND

Atrial fibrillation (AF) is a common cardiovascular disease often observed in diabetes mellitus, and there is currently no satisfactory therapeutic option. Ubiquitin-specific protease 38 (USP38) has been implicated in the degradation of numerous substrate proteins in the myocardium. Herein, we aim to investigate the role of USP38 in AF induced by diabetes.

METHODS

Cardiac-specific transgenic USP38 mice and cardiac-specific knockout USP38 mice were constructed, and streptozotocin was used to establish diabetic mouse model. Functional, electrophysiological, histologic, biochemical studies were performed.

RESULTS

The expression of USP38 was upregulated in atrial tissues of diabetic mice and HL-1 cells exposed to high glucose. USP38 overexpression increased susceptibility to AF, accompanied by aberrant expression of calcium-handling protein, heightened iron load and oxidation stress in diabetic mice. Conversely, USP38 deficiency reduced vulnerability to AF by hampering ferroptosis. Mechanistically, USP38 bound to iron regulatory protein 2 (IRP2), stabilizing it and remove K48-linked polyubiquitination chains, thereby increasing intracellular iron overload, lipid peroxidation, and ultimately contributing to ferroptosis. In addition, reduced iron overload by deferoxamine treatment alleviated oxidation stress and decreased vulnerability to AF in diabetic mice.

CONCLUSION

Overall, our findings reveal the detrimental role of USP38 in diabetes-related AF, manifested by increased level of iron overload and oxidation stress.

Oxidative stress and atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia in clinical practice. Though the pathogenesis of AF is complex and is not completely understood, many studies suggest that oxidative stress is a major mechanism in pathophysiology of AF. Through multiple mechanisms, reactive oxygen species (ROS) lead to the formation of an AF substrate that facilitates the development and maintenance of AF. In this review article, we provide an update on the different mechanisms by which oxidative stress promotes atrial remodeling. We then discuss several therapeutic strategies targeting oxidative stress for the prevention or treatment of AF. Considering the complex biology of ROS induced remodeling, and the evolution of ROS sources and compartmentalization during AF progression, there is a definite need for improvement in timing, targeting and reduction of off-target effects of therapeutic strategies targeting oxidative injury in AF.

Mitochondrial dysfunction is a key link involved in the pathogenesis of sick sinus syndrome: a review

Sick sinus syndrome (SSS) is a grave medical condition that can precipitate sudden death. The pathogenesis of SSS remains incompletely understood. Existing research postulates that the fundamental mechanism involves increased fibrosis of the sinoatrial node and its surrounding tissues, as well as disturbances in the coupled-clock system, comprising the membrane clock and the Ca2+ clock. Mitochondrial dysfunction exacerbates regional tissue fibrosis and disrupts the functioning of both the membrane and calcium clocks. This plays a crucial role in the underlying pathophysiology of SSS, including mitochondrial energy metabolism disorders, mitochondrial oxidative stress damage, calcium overload, and mitochondrial quality control disorders. Elucidating the mitochondrial mechanisms involved in the pathophysiology of SSS and further investigating the disease's mechanisms is of great significance.

Atrial cardiomyopathy resulting from loss of plakophilin-2 expression: Response to adrenergic stimulation and implications for the exercise response

Atrial arrhythmias occur in 20-40% of patients with arrhythmogenic right ventricular cardiomyopathy (ARVC) and are associated with an increased risk of sustained ventricular arrhythmias and inappropriate implantable cardioverter-defibrillator shocks. The pathophysiology of atrial arrhythmias in ARVC remains unclear. Most cases of gene-positive ARVC are linked to pathogenic variants in the desmosomal gene plakophilin-2 (PKP2). Here, we test the hypothesis that loss of PKP2 expression leads to pro-arrhythmic changes in atrial cardiomyocytes. Atrial cells/tissue were obtained from a cardiac-specific, tamoxifen-activated model of PKP2 deficiency (PKP2cKO). By contrast to PKP2cKO ventricular myocytes, PKP2cKO atrial cardiomyocytes presented no significant differences in intracellular calcium (Ca2+ i) transient dynamics, sarcoplasmic reticulum load or action potential morphology. PKP2cKO atrial cardiomyocytes showed elevated reactive oxygen species levels, increased frequency and amplitude of Ca2+ sparks, and increased diastolic [Ca2+]i compared to control; the latter two parameters were further increased by isoproterenol exposure and reversed by exposure to ryanodine receptor blocker dantrolene. We speculate that these isoproterenol-dependent effects may impact on the exercise-related atrial arrhythmia risk in ARVC patients. Despite absence of changes in Ca2+ i transient dynamics, PKP2cKO atrial cardiomyocytes showed enhanced sarcomere shortening and impaired sarcomere relaxation. Orthogonal transcriptomic analysis of human(GTEx) and PKP2cKO atrial tissue led to identification of 41 transcripts depending on PKP2 expression. Biochemical follow-up confirmed reduced abundance of sarcomeric protein myosin binding protein C, potentially playing a role in cellular shortening and relaxation changes observed. Our findings provide novel insights into the role of PKP2 in atrial myocardium with potential implications to therapeutic management of atrial fibrillation in patients with PKP2-related ARVC. KEY POINTS: Atrial arrhythmias occur in a large group of patients with arrhythmogenic right ventricular cardiomyopathy (ARVC), a cardiac disease mostly caused by pathogenic variants in the desmosomal gene plakophilin-2 (PKP2). Exercise is considered to be an independent risk factor for arrhythmias consequent to PKP2 deficiency. We show that loss of PKP2 expression affects cellular calcium handling and electrophysiology differently in left atrial vs. ventricular myocardium and causes extensive atrial fibrosis. PKP2-deficient atrial cardiomyocytes present increased spontaneous sarcoplasmic reticulum calcium release events, further enhanced by isoproterenol exposure and reversible by a ryanodine receptor blocker (dantrolene). In addition, PKP2-deficient atrial myocytes exhibit impaired relaxation and enhanced sarcomere shortening, most probably related to reduced abundance of myosin binding protein C. We speculate that cellular effects reported upon isoproterenol impact on the exercise-related atrial arrhythmia risk in ARVC patients. We further propose that therapeutic approaches aimed at mitigating ventricular damage may be effective to treat the atrial disease in ARVC.

Structure-activity optimization of ryanodine receptor modulators for the treatment of catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmia disorder associated with lethal arrhythmias. Most CPVT cases are caused by inherited variants in the gene encoding ryanodine receptor type 2 (RYR2).

OBJECTIVE

The goal of this study was to investigate the structure-activity relationship of tetracaine derivatives and to test a lead compound in a mouse model of CPVT.

METHODS

We synthesized >200 tetracaine derivatives and characterized 11 of those. The effects of these compounds on Ca2+ handling in cardiomyocytes from R176Q/+ mice was tested with confocal microscopy. The effects of lead compound MSV1302 on arrhythmia inducibility and cardiac contractility were tested by programmed electrical stimulation and echocardiography, respectively. Plasma and microsomal stability and cytotoxicity assays were also performed.

RESULTS

Ca2+ imaging revealed that 3 of 11 compounds suppressed sarcoplasmic reticulum Ca2+ leak through mutant RyR2. Two compounds selected for further testing exhibited a half-maximal effective concentration of 146 nM (MSV1302) and 49 nM (MSV1406). Whereas neither compound altered baseline electrocardiogram intervals, only MSV1302 suppressed stress- and pacing-induced ventricular tachycardia in vivo in R176Q/+ mice. Echocardiography revealed that the lead compound MSV1302 did not negatively affect cardiac inotropy and chronotropy. Finally, compound MSV1302 did not block INa, ICa,L, or IKr; it exhibited excellent stability in plasma and microsomes, and it was not cytotoxic.

CONCLUSION

Structure-activity relationship studies of second-generation tetracaine derivatives identified lead compound MSV1302 with a favorable pharmacokinetic profile. MSV1302 normalized aberrant RyR2 activity in vitro and in vivo, without altering cardiac inotropy, chronotropy, or off-target effects on other ion channels. This compound may be a strong candidate for future clinical studies to determine its efficacy in CPVT patients.

Metabolic remodelling in atrial fibrillation: manifestations, mechanisms and clinical implications

Atrial fibrillation (AF) is a continually growing health-care burden that often presents together with metabolic disorders, including diabetes mellitus and obesity. Current treatments often fall short of preventing AF and its adverse outcomes. Accumulating evidence suggests that metabolic disturbances can promote the development of AF through structural and electrophysiological remodelling, but the underlying mechanisms that predispose an individual to AF are aetiology-dependent, thus emphasizing the need for tailored therapeutic strategies to treat AF that target an individual's metabolic profile. AF itself can induce changes in glucose, lipid and ketone metabolism, mitochondrial function and myofibrillar energetics (as part of a process referred to as 'metabolic remodelling'), which can all contribute to atrial dysfunction. In this Review, we discuss our current understanding of AF in the setting of metabolic disorders, as well as changes in atrial metabolism that are relevant to the development of AF. We also describe the potential of available and emerging treatment strategies to target metabolic remodelling in the setting of AF and highlight key questions and challenges that need to be addressed to improve outcomes in these patients.

Simulation of cardiac arrhythmias in human induced pluripotent stem cell-derived cardiomyocytes

The effects and mechanisms of cardiac arrhythmias are still incompletely understood and an important subject of cardiovascular research. A major difficulty for investigating arrhythmias is the lack of appropriate human models. Here, we present a protocol for a translational simulation of different types of arrhythmias using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) and electric cell culture pacing. The protocol comprises the handling of ventricular and atrial hiPSC-CM before and during in vitro arrhythmia simulation and possible arrhythmia simulation protocols mimicking clinical arrhythmias like atrial fibrillation. Isolated or confluent hiPSC-CM can be used for the simulation. In vitro arrhythmia simulation did not impair cell viability of hiPSC-CM and could reproduce arrhythmia associated phenotypes of patients. The use of hiPSC-CM enables patient-specific studies of arrhythmias, genetic interventions, or drug-screening. Thus, the in vitro arrhythmia simulation protocol may offer a versatile tool for translational studies on the mechanisms and treatment options of cardiac arrhythmias.

A detailed mathematical model of the human atrial cardiomyocyte: integration of electrophysiology and cardiomechanics

Mechano-electric regulations (MER) play an important role in the maintenance of cardiac performance. Mechano-calcium and mechano-electric feedback (MCF and MEF) pathways adjust the cardiomyocyte contractile force according to mechanical perturbations and affects electro-mechanical coupling. MER integrates all these regulations in one unit resulting in a complex phenomenon. Computational modelling is a useful tool to accelerate the mechanistic understanding of complex experimental phenomena. We have developed a novel model that integrates the MER loop for human atrial cardiomyocytes with proper consideration of feedforward and feedback pathways. The model couples a modified version of the action potential (AP) Koivumäki model with the contraction model by Quarteroni group. The model simulates iso-sarcometric and isometric twitches and the feedback effects on AP and Ca2+-handling. The model showed a biphasic response of Ca2+ transient (CaT) peak to increasing pacing rates and highlights the possible mechanisms involved. The model has shown a shift of the threshold for AP and CaT alternans from 4.6 to 4 Hz under post-operative atrial fibrillation, induced by depressed SERCA activity. The alternans incidence was dependent on a chain of mechanisms including RyRs availability time, MCF coupling, CaMKII phosphorylation, and the stretch levels. As a result, the model predicted a 10% slowdown of conduction velocity for a 20% stretch, suggesting a role of stretch in creation of substrate formation for atrial fibrillation. Overall, we conclude that the developed model provides a physiological CaT followed by a physiological twitch. This model can open pathways for the future studies of human atrial electromechanics. KEY POINTS: With the availability of human atrial cellular data, interest in atrial-specific model integration has been enhanced. We have developed a detailed mathematical model of human atrial cardiomyocytes including the mechano-electric regulatory loop. The model has gone through calibration and evaluation phases against a wide collection of available human in-vitro data. The usefulness of the model for analysing clinical problems has been preliminaryly tested by simulating the increased incidence of Ca2+ transient and action potential alternans at high rates in post-operative atrial fibrillation condition. The model determines the possible role of mechano-electric feedback in alternans incidence, which can increase vulnerability to atrial arrhythmias by varying stretch levels. We found that our physiologically accurate description of Ca2+ handling can reproduce many experimental phenomena and can help to gain insights into the underlying pathophysiological mechanisms.

Atrial fibrillation in cancer, anticancer therapies, and underlying mechanisms

Atrial fibrillation (AF) is a common arrhythmic complication in cancer patients and can be exacerbated by traditional cytotoxic and targeted anticancer therapies. Increased incidence of AF in cancer patients is independent of confounding factors, including preexisting myocardial arrhythmogenic substrates, type of cancer, or cancer stage. Mechanistically, AF is characterized by fast unsynchronized atrial contractions with rapid ventricular response, which impairs ventricular filling and results in various symptoms such as fatigue, chest pain, and shortness of breath. Due to increased blood stasis, a consequence of both cancer and AF, concern for stroke increases in this patient population. To compound matters, cardiotoxic anticancer therapies themselves promote AF; thereby exacerbating AF morbidity and mortality in cancer patients. In this review, we examine the relationship between AF, cancer, and cardiotoxic anticancer therapies with a focus on the shared molecular and electrophysiological mechanisms linking these disease processes. We also explore the potential role of sodium-glucose co-transporter 2 inhibitors (SGLT2i) in the management of anticancer-therapy-induced AF.

Impaired Intracellular Calcium Buffering Contributes to the Arrhythmogenic Substrate in Atrial Myocytes From Patients With Atrial Fibrillation

BACKGROUND

Alterations in the buffering of intracellular Ca2+, for which myofilament proteins play a key role, have been shown to promote cardiac arrhythmia. It is interesting that although studies report atrial myofibrillar degradation in patients with persistent atrial fibrillation (persAF), the intracellular Ca2+ buffering profile in persAF remains obscure. Therefore, we aimed to investigate the intracellular buffering of Ca2+ and its potential arrhythmogenic role in persAF.

METHODS

Transmembrane Ca2+ fluxes (patch-clamp) and intracellular Ca2+ signaling (fluo-3-acetoxymethyl ester) were recorded simultaneously in myocytes from right atrial biopsies of sinus rhythm (Ctrl) and patients with persAF, alongside human atrial subtype induced pluripotent stem cell-derived cardiac myocytes (iPSC-CMs). Protein levels were quantified by immunoblotting of human atrial tissue and induced pluripotent stem cell-derived cardiac myocytes. Mouse whole heart and atrial electrophysiology were measured on a Langendorff system.

RESULTS

Cytosolic Ca2+ buffering was decreased in atrial myocytes of patients with persAF because of a depleted amount of Ca2+ buffers. In agreement, protein levels of selected Ca2+ binding myofilament proteins, including cTnC (cardiac troponin C), a major cytosolic Ca2+ buffer, were significantly lower in patients with persAF. Small interfering RNA (siRNA)-mediated knockdown of cTnC (si-cTNC) in atrial iPSC-CM phenocopied the reduced cytosolic Ca2+ buffering observed in persAF. Si-cTnC treated atrial iPSC-CM exhibited a higher predisposition to spontaneous Ca2+ release events and developed action potential alternans at low stimulation frequencies. Last, indirect reduction of cytosolic Ca2+ buffering using blebbistatin in an ex vivo mouse whole heart model increased vulnerability to tachypacing-induced atrial arrhythmia, validating the direct mechanistic link between impaired cytosolic Ca2+ buffering and atrial arrhythmogenesis.

CONCLUSIONS

Our findings suggest that loss of myofilament proteins, particularly reduced cTnC protein levels, causes diminished cytosolic Ca2+ buffering in persAF, thereby potentiating the occurrence of spontaneous Ca2+ release events and atrial fibrillation susceptibility. Strategies targeting intracellular buffering may represent a promising therapeutic lead in persAF management.

Cell-cell communication: new insights and clinical implications

Multicellular organisms are composed of diverse cell types that must coordinate their behaviors through communication. Cell-cell communication (CCC) is essential for growth, development, differentiation, tissue and organ formation, maintenance, and physiological regulation. Cells communicate through direct contact or at a distance using ligand-receptor interactions. So cellular communication encompasses two essential processes: cell signal conduction for generation and intercellular transmission of signals, and cell signal transduction for reception and procession of signals. Deciphering intercellular communication networks is critical for understanding cell differentiation, development, and metabolism. First, we comprehensively review the historical milestones in CCC studies, followed by a detailed description of the mechanisms of signal molecule transmission and the importance of the main signaling pathways they mediate in maintaining biological functions. Then we systematically introduce a series of human diseases caused by abnormalities in cell communication and their progress in clinical applications. Finally, we summarize various methods for monitoring cell interactions, including cell imaging, proximity-based chemical labeling, mechanical force analysis, downstream analysis strategies, and single-cell technologies. These methods aim to illustrate how biological functions depend on these interactions and the complexity of their regulatory signaling pathways to regulate crucial physiological processes, including tissue homeostasis, cell development, and immune responses in diseases. In addition, this review enhances our understanding of the biological processes that occur after cell-cell binding, highlighting its application in discovering new therapeutic targets and biomarkers related to precision medicine. This collective understanding provides a foundation for developing new targeted drugs and personalized treatments.

Calcium-mediated DAD in membrane potentials and triggered activity in atrial myocytes of ETV1f / fMyHCCre /+ mice

The E-twenty-six variant 1 (ETV1)-dependent transcriptome plays an important role in atrial electrical and structural remodelling and the occurrence of atrial fibrillation (AF), but the underlying mechanism of ETV1 in AF is unclear. In this study, cardiomyocyte-specific ETV1 knockout (ETV1f/fMyHCCre/+, ETV1-CKO) mice were constructed to observe the susceptibility to AF and the underlying mechanism in AF associated with ETV1-CKO mice. AF susceptibility was examined by intraesophageal burst pacing, induction of AF was increased obviously in ETV1-CKO mice than WT mice. Electrophysiology experiments indicated shortened APD50 and APD90, increased incidence of DADs, decreased density of ICa,L in ETV1-CKO mice. There was no difference in VINACT,1/2 and VACT,1/2, but a significantly longer duration of the recovery time after inactivation in the ETV1-CKO mice. The recording of intracellular Ca2+ showed that there was significantly increased in the frequency of calcium spark, Ca2+ transient amplitude, and proportion of SCaEs in ETV1-CKO mice. Reduction of Cav1.2 rather than NCX1 and SERCA2a, increase RyR2, p-RyR2 and CaMKII was reflected in ETV1-CKO group. This study demonstrates that the increase in calcium spark and SCaEs corresponding to Ca2+ transient amplitude may trigger DAD in membrane potential in ETV1-CKO mice, thereby increasing the risk of AF.

Mechanisms of stretch-induced electro-anatomical remodeling and atrial arrhythmogenesis

Atrial fibrillation (AF) is the most common cardiac rhythm disorder, often occurring in the setting of atrial distension and elevated myocardialstretch. While various mechano-electrochemical signal transduction pathways have been linked to AF development and progression, the underlying molecular mechanisms remain poorly understood, hampering AF therapies. In this review, we describe different aspects of stretch-induced electro-anatomical remodeling as seen in animal models and in patients with AF. Specifically, we focus on cellular and molecular mechanisms that are responsible for mechano-electrochemical signal transduction and the development of ectopic beats triggering AF from pulmonary veins, the most common source of paroxysmal AF. Furthermore, we describe structural changes caused by stretch occurring before and shortly after the onset of AF as well as during AF progression, contributing to longstanding forms of AF. We also propose mechanical stretch as a new dimension to the concept "AF begets AF", in addition to underlying diseases. Finally, we discuss the mechanisms of these electro-anatomical alterations in a search for potential therapeutic strategies and the development of novel antiarrhythmic drugs targeted at the components of mechano-electrochemical signal transduction not only in cardiac myocytes, but also in cardiac non-myocyte cells.

Spexin Diminishes Atrial Fibrillation Vulnerability by Acting on Galanin Receptor 2

BACKGROUND

G protein-coupled receptors play a critical role in atrial fibrillation (AF). Spexin is a novel ligand of galanin receptors (GALRs). In this study, we investigated the regulation of spexin and GALRs on AF and the underlying mechanisms.

METHODS

Global spexin knockout (SPX-KO) and cardiomyocyte-specific GALRs knockout (GALR-cKO) mice underwent burst pacing electrical stimulation. Optical mapping was used to determine atrial conduction velocity and action potential duration. Atrial myocyte action potential duration and inward rectifying K+ current (IK1) were recorded using whole-cell patch clamps. Isolated cardiomyocytes were stained with Fluo-3/AM dye, and intracellular Ca2+ handling was examined by CCD camera. A mouse model of AF was established by Ang-II (angiotensin II) infusion.

RESULTS

Spexin plasma levels in patients with AF were lower than those in subjects without AF, and knockout of spexin increased AF susceptibility in mice. In the atrium of SPX-KO mice, potassium inwardly rectifying channel subfamily J member 2 (KCNJ2) and sarcolipin (SLN) were upregulated; meanwhile, IK1 current was increased and Ca2+ handling was impaired in isolated atrial myocytes of SPX-KO mice. GALR2-cKO mice, but not GALR1-cKO and GALR3-cKO mice, had a higher incidence of AF, which was associated with higher IK1 current and intracellular Ca2+ overload. The phosphorylation level of CREB (cyclic AMP responsive element binding protein 1) was upregulated in atrial tissues of SPX-KO and GALR2-cKO mice. Chromatin immunoprecipitation confirmed the recruitment of p-CREB to the proximal promoter regions of KCNJ2 and SLN. Finally, spexin treatment suppressed CREB signaling, decreased IK1 current and decreased intracellular Ca2+ overload, which thus reduced the inducibility of AF in Ang-II-infused mice.

CONCLUSIONS

Spexin reduces atrial fibrillation susceptibility by inhibiting CREB phosphorylation and thus downregulating KCNJ2 and SLN transcription by GALR2 receptor. The spexin/GALR2/CREB signaling pathway represents a novel therapeutic avenue in the development of agents against atrial fibrillation.

Micronano Synergetic Three-Dimensional Bioelectronics: A Revolutionary Breakthrough Platform for Cardiac Electrophysiology

Cardiovascular diseases (CVDs) are the leading cause of mortality and therefore pose a significant threat to human health. Cardiac electrophysiology plays a crucial role in the investigation and treatment of CVDs, including arrhythmia. The long-term and accurate detection of electrophysiological activity in cardiomyocytes is essential for advancing cardiology and pharmacology. Regarding the electrophysiological study of cardiac cells, many micronano bioelectric devices and systems have been developed. Such bioelectronic devices possess unique geometric structures of electrodes that enhance quality of electrophysiological signal recording. Though planar multielectrode/multitransistors are widely used for simultaneous multichannel measurement of cell electrophysiological signals, their use for extracellular electrophysiological recording exhibits low signal strength and quality. However, the integration of three-dimensional (3D) multielectrode/multitransistor arrays that use advanced penetration strategies can achieve high-quality intracellular signal recording. This review provides an overview of the manufacturing, geometric structure, and penetration paradigms of 3D micronano devices, as well as their applications for precise drug screening and biomimetic disease modeling. Furthermore, this review also summarizes the current challenges and outlines future directions for the preparation and application of micronano bioelectronic devices, with an aim to promote the development of intracellular electrophysiological platforms and thereby meet the demands of emerging clinical applications.

NaV1.8 as Proarrhythmic Target in a Ventricular Cardiac Stem Cell Model

The sodium channel NaV1.8, encoded by the SCN10A gene, has recently emerged as a potential regulator of cardiac electrophysiology. We have previously shown that NaV1.8 contributes to arrhythmogenesis by inducing a persistent Na+ current (late Na+ current, INaL) in human atrial and ventricular cardiomyocytes (CM). We now aim to further investigate the contribution of NaV1.8 to human ventricular arrhythmogenesis at the CM-specific level using pharmacological inhibition as well as a genetic knockout (KO) of SCN10A in induced pluripotent stem cell CM (iPSC-CM). In functional voltage-clamp experiments, we demonstrate that INaL was significantly reduced in ventricular SCN10A-KO iPSC-CM and in control CM after a specific pharmacological inhibition of NaV1.8. In contrast, we did not find any effects on ventricular APD90. The frequency of spontaneous sarcoplasmic reticulum Ca2+ sparks and waves were reduced in SCN10A-KO iPSC-CM and control cells following the pharmacological inhibition of NaV1.8. We further analyzed potential triggers of arrhythmias and found reduced delayed afterdepolarizations (DAD) in SCN10A-KO iPSC-CM and after the specific inhibition of NaV1.8 in control cells. In conclusion, we show that NaV1.8-induced INaL primarily impacts arrhythmogenesis at a subcellular level, with minimal effects on systolic cellular Ca2+ release. The inhibition or knockout of NaV1.8 diminishes proarrhythmic triggers in ventricular CM. In conjunction with our previously published results, this work confirms NaV1.8 as a proarrhythmic target that may be useful in an anti-arrhythmic therapeutic strategy.

Stress-Induced Proteasome Sub-Cellular Translocation in Cardiomyocytes Causes Altered Intracellular Calcium Handling and Arrhythmias

The ubiquitin-proteasome system (UPS) is an essential mechanism responsible for the selective degradation of substrate proteins via their conjugation with ubiquitin. Since cardiomyocytes have very limited self-renewal capacity, as they are prone to protein damage due to constant mechanical and metabolic stress, the UPS has a key role in cardiac physiology and pathophysiology. While altered proteasomal activity contributes to a variety of cardiac pathologies, such as heart failure and ischemia/reperfusion injury (IRI), the environmental cues affecting its activity are still unknown, and they are the focus of this work. Following a recent study by Ciechanover's group showing that amino acid (AA) starvation in cultured cancer cell lines modulates proteasome intracellular localization and activity, we tested two hypotheses in human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs, CMs): (i) AA starvation causes proteasome translocation in CMs, similarly to the observation in cultured cancer cell lines; (ii) manipulation of subcellular proteasomal compartmentalization is associated with electrophysiological abnormalities in the form of arrhythmias, mediated via altered intracellular Ca2+ handling. The major findings are: (i) starving CMs to AAs results in proteasome translocation from the nucleus to the cytoplasm, while supplementation with the aromatic amino acids tyrosine (Y), tryptophan (W) and phenylalanine (F) (YWF) inhibits the proteasome recruitment; (ii) AA-deficient treatments cause arrhythmias; (iii) the arrhythmias observed upon nuclear proteasome sequestration(-AA+YWF) are blocked by KB-R7943, an inhibitor of the reverse mode of the sodium-calcium exchanger NCX; (iv) the retrograde perfusion of isolated rat hearts with AA starvation media is associated with arrhythmias. Collectively, our novel findings describe a newly identified mechanism linking the UPS to arrhythmia generation in CMs and whole hearts.

Exploring the role of TRPM4 in calcium-dependent triggered activity and cardiac arrhythmias

Cardiac arrhythmias pose a major threat to a patient's health, yet prove to be often difficult to predict, prevent and treat. A key mechanism in the occurrence of arrhythmias is disturbed Ca2+ homeostasis in cardiac muscle cells. As a Ca2+-activated non-selective cation channel, TRPM4 has been linked to Ca2+-induced arrhythmias, potentially contributing to translating an increase in intracellular Ca2+ concentration into membrane depolarisation and an increase in cellular excitability. Indeed, evidence from genetically modified mice, analysis of mutations in human patients and the identification of a TRPM4 blocking compound that can be applied in vivo further underscore this hypothesis. Here, we provide an overview of these data in the context of our current understanding of Ca2+-dependent arrhythmias.

High-throughput methods for cardiac cellular electrophysiology studies: the road to personalized medicine

Personalized medicine refers to the tailored application of medical treatment at an individual level, considering the specific genotype or phenotype of each patient for targeted therapy. In the context of cardiovascular diseases, implementing personalized medicine is challenging due to the high costs involved and the slow pace of identifying the pathogenicity of genetic variants, deciphering molecular mechanisms of disease, and testing treatment approaches. Scalable cellular models such as human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) serve as useful in vitro tools that reflect individual patient genetics and retain clinical phenotypes. High-throughput functional assessment of these constructs is necessary to rapidly assess cardiac pathogenicity and test new therapeutics if personalized medicine is to become a reality. High-throughput photometry recordings of single cells coupled with potentiometric probes offer cost-effective alternatives to traditional patch-clamp assessments of cardiomyocyte action potential characteristics. Importantly, automated patch-clamp (APC) is rapidly emerging in the pharmaceutical industry and academia as a powerful method to assess individual membrane-bound ionic currents and ion channel biophysics over multiple cells in parallel. Now amenable to primary cell and hiPSC-CM measurement, APC represents an exciting leap forward in the characterization of a multitude of molecular mechanisms that underlie clinical cardiac phenotypes. This review provides a summary of state-of-the-art high-throughput electrophysiological techniques to assess cardiac electrophysiology and an overview of recent works that successfully integrate these methods into basic science research that could potentially facilitate future implementation of personalized medicine at a clinical level.

Left Atrial Glucose Metabolism Evaluation by 18F-FDG-PET in Persistent Atrial Fibrillation and in Sinus Rhythm

The role of atrial metabolism alterations for initiation and atrial fibrillation (AF) persistence remains poorly understood. Therefore, we evaluated left atrial glucose metabolism by nicotinic acid derivative stimulated 18-fluorodeoxyglucose positron emission tomography in 36 patients with persistent AF undergoing catheter ablation before and 3 months after return to sinus rhythm and compared values against healthy controls. Under identical hemodynamics and metabolic conditions, and although left ventricular FDG uptake remained unchanged, patients in persistent AF presented significantly higher total left atrial and left atrial appendage uptake, which decreased significantly after return to sinus rhythm, despite improvement of passive and active atrial contractile function. These findings support a role of altered glucose metabolism and metabolic wasting underlying the pathophysiology of persistent AF.

Pathophysiology and clinical relevance of atrial myopathy

Atrial myopathy is a condition that consists of electrical, structural, contractile, and autonomic remodeling of the atria and is the substrate for development of atrial fibrillation, the most common arrhythmia. Pathophysiologic mechanisms driving atrial myopathy are inflammation, oxidative stress, atrial stretch, and neurohormonal signals, e.g., angiotensin-II and aldosterone. These mechanisms initiate the structural and functional remodeling of the atrial myocardium. Novel therapeutic strategies are being developed that target the pathophysiologic mechanisms of atrial myopathy. In this review, we will discuss the pathophysiology of atrial myopathy, as well as diagnostic and therapeutic strategies.

Enhanced Ca2+-Driven Arrhythmias in Female Patients with Atrial Fibrillation: Insights from Computational Modeling

BACKGROUND AND AIMS

Substantial sex-based differences have been reported in atrial fibrillation (AF), with female patients experiencing worse symptoms, increased complications from drug side effects or ablation, and elevated risk of AF-related stroke and mortality. Recent studies revealed sex-specific alterations in AF-associated Ca2+ dysregulation, whereby female cardiomyocytes more frequently exhibit potentially proarrhythmic Ca2+-driven instabilities compared to male cardiomyocytes. In this study, we aim to gain a mechanistic understanding of the Ca2+-handling disturbances and Ca2+-driven arrhythmogenic events in males vs females and establish their responses to Ca2+-targeted interventions.

METHODS AND RESULTS

We incorporated known sex differences and AF-associated changes in the expression and phosphorylation of key Ca2+-handling proteins and in ultrastructural properties and dimensions of atrial cardiomyocytes into our recently developed 3D atrial cardiomyocyte model that couples electrophysiology with spatially detailed Ca2+-handling processes. Our simulations of quiescent cardiomyocytes show increased incidence of Ca2+ sparks in female vs male myocytes in AF, in agreement with previous experimental reports. Additionally, our female model exhibited elevated propensity to develop pacing-induced spontaneous Ca2+ releases (SCRs) and augmented beat-to-beat variability in action potential (AP)-elicited Ca2+ transients compared with the male model. Parameter sensitivity analysis uncovered precise arrhythmogenic contributions of each component that was implicated in sex and/or AF alterations. Specifically, increased ryanodine receptor phosphorylation in female AF cardiomyocytes emerged as the major SCR contributor, while reduced L-type Ca2+ current was protective against SCRs for male AF cardiomyocytes. Furthermore, simulations of tentative Ca2+-targeted interventions identified potential strategies to attenuate Ca2+-driven arrhythmogenic events in female atria (e.g., t-tubule restoration, and inhibition of ryanodine receptor and sarcoplasmic/endoplasmic reticulum Ca2+-ATPase), and revealed enhanced efficacy when applied in combination.

CONCLUSIONS

Our sex-specific computational models of human atrial cardiomyocytes uncover increased propensity to Ca2+-driven arrhythmogenic events in female compared to male atrial cardiomyocytes in AF, and point to combined Ca2+-targeted interventions as promising approaches to treat AF in female patients. Our study establishes that AF treatment may benefit from sex-dependent strategies informed by sex-specific mechanisms.

Implications for neutrophils in cardiac arrhythmias

Cardiac arrhythmias commonly occur as a result of aberrant electrical impulse formation or conduction in the myocardium. Frequently discussed triggers include underlying heart diseases such as myocardial ischemia, electrolyte imbalances, or genetic anomalies of ion channels involved in the tightly regulated cardiac action potential. Recently, the role of innate immune cells in the onset of arrhythmic events has been highlighted in numerous studies, correlating leukocyte expansion in the myocardium to increased arrhythmic burden. Here, we aim to call attention to the role of neutrophils in the pathogenesis of cardiac arrhythmias and their expansion during myocardial ischemia and infectious disease manifestation. In addition, we will elucidate molecular mechanisms associated with neutrophil activation and discuss their involvement as direct mediators of arrhythmogenicity.

Zbtb16 increases susceptibility of atrial fibrillation in type 2 diabetic mice via Txnip-Trx2 signaling

Atrial fibrillation (AF) is the most prevalent sustained cardiac arrhythmia, and recent epidemiological studies suggested type 2 diabetes mellitus (T2DM) is an independent risk factor for the development of AF. Zinc finger and BTB (broad-complex, tram-track and bric-a-brac) domain containing 16 (Zbtb16) serve as transcriptional factors to regulate many biological processes. However, the potential effects of Zbtb16 in AF under T2DM condition remain unclear. Here, we reported that db/db mice displayed higher AF vulnerability and Zbtb16 was identified as the most significantly enriched gene by RNA sequencing (RNA-seq) analysis in atrium. In addition, thioredoxin interacting protein (Txnip) was distinguished as the key downstream gene of Zbtb16 by Cleavage Under Targets and Tagmentation (CUT&Tag) assay. Mechanistically, increased Txnip combined with thioredoxin 2 (Trx2) in mitochondrion induced excess reactive oxygen species (ROS) release, calcium/calmodulin-dependent protein kinase II (CaMKII) overactivation, and spontaneous Ca2+ waves (SCWs) occurrence, which could be inhibited through atrial-specific knockdown (KD) of Zbtb16 or Txnip by adeno-associated virus 9 (AAV9) or Mito-TEMPO treatment. High glucose (HG)-treated HL-1 cells were used to mimic the setting of diabetic in vitro. Zbtb16-Txnip-Trx2 signaling-induced excess ROS release and CaMKII activation were also verified in HL-1 cells under HG condition. Furthermore, atrial-specific Zbtb16 or Txnip-KD reduced incidence and duration of AF in db/db mice. Altogether, we demonstrated that interrupting Zbtb16-Txnip-Trx2 signaling in atrium could decrease AF susceptibility via reducing ROS release and CaMKII activation in the setting of T2DM.

Atrial fibrillation: pathophysiology, genetic and epigenetic mechanisms

Atrial fibrillation (AF) is the most common supraventricular arrhythmia affecting up to 1% of the general population. Its prevalence dramatically increases with age and could reach up to ∼10% in the elderly. The management of AF is a complex issue that is object of extensive ongoing basic and clinical research, it depends on its genetic and epigenetic causes, and it varies considerably geographically and also according to the ethnicity. Mechanistically, over the last decade, Genome Wide Association Studies have uncovered over 100 genetic loci associated with AF, and have shown that European ancestry is associated with elevated risk of AF. These AF-associated loci revolve around different types of disturbances, including inflammation, electrical abnormalities, and structural remodeling. Moreover, the discovery of epigenetic regulatory mechanisms, involving non-coding RNAs, DNA methylation and histone modification, has allowed unravelling what modifications reshape the processes leading to arrhythmias. Our review provides a current state of the field regarding the identification and functional characterization of AF-related genetic and epigenetic regulatory networks, including ethnic differences. We discuss clear and emerging connections between genetic regulation and pathophysiological mechanisms of AF.

hiPSC-derived cardiomyocytes as a model to study the role of small-conductance Ca2+-activated K+ (SK) ion channel variants associated with atrial fibrillation

Atrial fibrillation (AF), the most common arrhythmia, has been associated with different electrophysiological, molecular, and structural alterations in atrial cardiomyocytes. Therefore, more studies are required to elucidate the genetic and molecular basis of AF. Various genome-wide association studies (GWAS) have strongly associated different single nucleotide polymorphisms (SNPs) with AF. One of these GWAS identified the rs13376333 risk SNP as the most significant one from the 1q21 chromosomal region. The rs13376333 risk SNP is intronic to the KCNN3 gene that encodes for small conductance calcium-activated potassium channels type 3 (SK3). However, the functional electrophysiological effects of this variant are not known. SK channels represent a unique family of K+ channels, primarily regulated by cytosolic Ca2+ concentration, and different studies support their critical role in the regulation of atrial excitability and consequently in the development of arrhythmias like AF. Since different studies have shown that both upregulation and downregulation of SK3 channels can lead to arrhythmias by different mechanisms, an important goal is to elucidate whether the rs13376333 risk SNP is a gain-of-function (GoF) or a loss-of-function (LoF) variant. A better understanding of the functional consequences associated with these SNPs could influence clinical practice guidelines by improving genotype-based risk stratification and personalized treatment. Although research using native human atrial cardiomyocytes and animal models has provided useful insights, each model has its limitations. Therefore, there is a critical need to develop a human-derived model that represents human physiology more accurately than existing animal models. In this context, research with human induced pluripotent stem cells (hiPSC) and subsequent generation of cardiomyocytes derived from hiPSC (hiPSC-CMs) has revealed the underlying causes of various cardiovascular diseases and identified treatment opportunities that were not possible using in vitro or in vivo studies with animal models. Thus, the ability to generate atrial cardiomyocytes and atrial tissue derived from hiPSCs from human/patients with specific genetic diseases, incorporating novel genetic editing tools to generate isogenic controls and organelle-specific reporters, and 3D bioprinting of atrial tissue could be essential to study AF pathophysiological mechanisms. In this review, we will first give an overview of SK-channel function, its role in atrial fibrillation and outline pathophysiological mechanisms of KCNN3 risk SNPs. We will then highlight the advantages of using the hiPSC-CM model to investigate SNPs associated with AF, while addressing limitations and best practices for rigorous hiPSC studies.

The autonomic nervous system in atrial fibrillation-pathophysiology and non-invasive assessment

The autonomic nervous system plays a crucial role in atrial fibrillation pathophysiology. Parasympathetic hyperactivity result in a shortening of the action potential duration, a reduction of the conduction wavelength, and as such facilitates reentry in the presence of triggers. Further, autonomic remodeling of atrial myocytes in AF includes progressive sympathetic hyperinnervation by increased atrial sympathetic nerve density and sympathetic atrial nerve sprouting. Knowledge on the pathophysiological process in AF, including the contribution of the autonomic nervous system, may in the near future guide personalized AF management. This review focuses on the role of the autonomic nervous system in atrial fibrillation pathophysiology and non-invasive assessment of the autonomic nervous system.

2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation: A Report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines

AIM

The "2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation" provides recommendations to guide clinicians in the treatment of patients with atrial fibrillation.

METHODS

A comprehensive literature search was conducted from May 12, 2022, to November 3, 2022, encompassing studies, reviews, and other evidence conducted on human subjects that were published in English from PubMed, EMBASE, the Cochrane Library, the Agency for Healthcare Research and Quality, and other selected databases relevant to this guideline. Additional relevant studies, published through November 2022, during the guideline writing process, were also considered by the writing committee and added to the evidence tables, where appropriate.

STRUCTURE

Atrial fibrillation is the most sustained common arrhythmia, and its incidence and prevalence are increasing in the United States and globally. Recommendations from the "2014 AHA/ACC/HRS Guideline for the Management of Patients With Atrial Fibrillation" and the "2019 AHA/ACC/HRS Focused Update of the 2014 AHA/ACC/HRS Guideline for the Management of Patients With Atrial Fibrillation" have been updated with new evidence to guide clinicians. In addition, new recommendations addressing atrial fibrillation and thromboembolic risk assessment, anticoagulation, left atrial appendage occlusion, atrial fibrillation catheter or surgical ablation, and risk factor modification and atrial fibrillation prevention have been developed.