Constitutive Expression of a Dominant-Negative TGF-β Type II Receptor in the Posterior Left Atrium Leads to Beneficial Remodeling of Atrial Fibrillation Substrate

BRD4 as a therapeutic target for atrial fibrosis and atrial fibrillation

OBJECTIVE

This study aimed to elucidate the molecular mechanisms by which BRD4 play a role in atrial fibrillation (AF).

METHODS AND RESULTS

We used a discovery-driven approach to detect BRD4 expression in the atria of patients with AF and in various murine models of atrial fibrosis. We used a BRD4 inhibitor (JQ1) and atrial fibroblast (aFB)-specific BRD4-knockout mice to elucidate the role of BRD4 in AF. We further examined the underlying mechanisms using RNA-seq and ChIP-seq analyses in vitro, to identify key downstream targets of BRD4. We found that BRD4 expression is significantly increased in patients with AF, with accompanying atrial fibrosis and aFB differentiation. We showed that JQ1 treatment and shRNA-based molecular silencing of BRD4 blocked ANG-II-induced extracellular matrix production and cell-cycle progression in aFBs. BRD4-related RNA-seq and ChIP-seq analyses in aFBs demonstrated enrichment of a subset of promoters related to the expression of profibrotic and proliferation-related genes. The pharmacological inhibition of BRD4 in vivo or in aFB-specific BRD4-knockout in mice limited ANG-II-induced atrial fibrosis, atrial enlargement, and AF susceptibility.

CONCLUSION

Our findings suggest that BRD4 plays a key role in pathological AF, at least partially by activating aFB proliferation and ECM synthesis. This study provides mechanistic insights into the development of BRD4 inhibitors as targeted antiarrhythmic therapies.

Advancements in gene therapy approaches for atrial fibrillation: Targeted delivery, mechanistic insights and future prospects

Atrial fibrillation (AF) remains a complex and challenging arrhythmia to treat, necessitating innovative therapeutic strategies. This review explores the evolving landscape of gene therapy for AF, focusing on targeted delivery methods, mechanistic insights, and future prospects. Direct myocardial injection, reversible electroporation, and gene painting techniques are discussed as effective means of delivering therapeutic genes, emphasizing their potential to modulate both structural and electrical aspects of the AF substrate. The importance of identifying precise targets for gene therapy, particularly in the context of AF-associated genetic, structural, and electrical abnormalities, is highlighted. Current studies employing animal models, such as mice and large animals, provide valuable insights into the efficacy and limitations of gene therapy approaches. The significance of imaging methods for detecting atrial fibrosis and guiding targeted gene delivery is underscored. Activation mapping techniques offer a nuanced understanding of AF-specific mechanisms, enabling tailored gene therapy interventions. Future prospects include the integration of advanced imaging, activation mapping, and percutaneous catheter-based techniques to refine transendocardial gene delivery, with potential applications in both ventricular and atrial contexts. As gene therapy for AF progresses, bridging the translational gap between preclinical models and clinical applications is imperative for the successful implementation of these promising approaches.

Use of Atrial Fibrillation Electrograms and T1/T2 Magnetic Resonance Imaging to Define the Progressive Nature of Molecular and Structural Remodeling: A New Paradigm Underlying the Emergence of Persistent Atrial Fibrillation

BACKGROUND

The temporal progression states of the molecular and structural substrate in atrial fibrillation (AF) are not well understood. We hypothesized that these can be detected by AF electrograms and magnetic resonance imaging parametric mapping.

METHODS AND RESULTS

AF was induced in 43 dogs (25-35 kg, ≥1 year) by rapid atrial pacing (RAP) (3-33 weeks, 600 beats/min), and 4 controls were used. We performed high-resolution epicardial mapping (UnEmap, 6 atrial regions, both atria, 130 electrodes, distance 2.5 mm) and analyzed electrogram cycle length, dominant frequency, organization index, and peak-to-peak bipolar voltage. Implantable telemetry recordings were used to quantify parasympathetic nerve activity over RAP time. Magnetic resonance imaging native T1, postcontrast T1, T2 mapping, and extracellular volume fraction were assessed (1.5T, Siemens) at baseline and AF. In explanted atrial tissue, DNA oxidative damage (8-hydroxy-2'-deoxyguanosine staining) and percentage of fibrofatty tissue were quantified. Cycle length and organization index decreased (R=0.5, P<0.05; and R=0.5, P<0.05; respectively), and dominant frequency increased (R=0.3, P n.s.) until 80 days of RAP but not thereafter. In contrast, voltage continued to decrease throughout the duration of RAP (R=0.6, P<0.05). Parasympathetic nerve activity increased following RAP and plateaued at 80 days. Magnetic resonance imaging native T1 and T2 times increased with RAP days (R=0.5, P<0.05; R=0.6, P<0.05) in the posterior left atrium throughout RAP. Increased RAP days correlated with increasing 8-hydroxy-2'-deoxyguanosine levels and with fibrosis percentage (R=0.5, P<0.05 for both).

CONCLUSIONS

A combination of AF electrogram characteristics and T1/T2 magnetic resonance imaging can detect early-stage AF remodeling (autonomic remodeling, oxidative stress) and advanced AF remodeling due to oxidative stress and fibrosis.

Atrial myopathy

This paper delves into the progressive concept of atrial myopathy, shedding light on its development and its impact on atrial characteristics. It extensively explores the intricate connections between atrial myopathy, atrial fibrillation (AF), and strokes. Researchers have sought additional contributors to AF-related strokes due to the absence of a clear timing correlation between paroxysmal AF episodes and strokes in patients with cardiac implantable electronic devices. Through various animal models and human investigations, a close interrelation among aging, inflammation, oxidative stress, and stretching mechanisms has been identified. These mechanisms contribute to fibrosis, alterations in electrical properties, autonomic remodeling, and a heightened pro-thrombotic state. These interconnected factors establish a detrimental cycle, exacerbating atrial myopathy and elevating the risk of sustained AF and strokes. By emphasizing the significance of atrial myopathy and the risk of strokes that are distinct from AF, the paper also discusses methods for identifying patients with atrial myopathy. Moreover, it proposes an approach to incorporate the concept of atrial myopathy into clinical practice to guide anticoagulation decisions in individuals with AF.

Costunolide ameliorates angiotensin II-induced atrial inflammation and fibrosis by regulating mitochondrial function and oxidative stress in mice: A possible therapeutic approach for atrial fibrillation

Atrial fibrillation (AF) is a cardiac disease characterized by disordered atrial electrical activity. Atrial inflammation and fibrosis are involved in AF progression. Costunolide (COS) is a sesquiterpene lactone containing anti-inflammatory and anti-fibrotic activities. This study aims to explore the underlying mechanisms by which COS protects against AF. Male C57BL/6 mice (8- to 10-week-old) were infused with angiotensin (Ang) II for 3 weeks. Meanwhile, different doses of COS (COS-L: 10 mg/kg, COS-H: 20 mg/kg) were administered to mice by intragastric treatment. The results showed irregular and rapid heart rates in Ang II-treated mice. Moreover, the levels of inflammatory cytokines and fibrotic factors were elevated in mice. COS triggered a reduction of Ang II-induced inflammation and fibrosis, which conferred a protective effect. Mechanistically, mitochondrial dysfunction with mitochondrial respiration inhibition and aberrant ATP levels were observed after Ang II treatment. Moreover, Ang-II-induced excessive reactive oxygen species caused oxidative stress, which was further aggravated by inhibiting Nrf2 nuclear translocation. Importantly, COS diminished these Ang-II-mediated effects in mice. In conclusion, COS attenuated inflammation and fibrosis in Ang-II-treated mice by alleviating mitochondrial dysfunction and oxidative stress. Our findings represent a potential therapeutic option for AF treatment.

Tic-Tac: A Translational Approach in Mechanisms Associated with Irregular Heartbeat and Sinus Rhythm Restoration in Atrial Fibrillation Patients

Atrial fibrillation (AF) is a prevalent cardiac condition predominantly affecting older adults, characterized by irregular heartbeat rhythm. The condition often leads to significant disability and increased mortality rates. Traditionally, two therapeutic strategies have been employed for its treatment: heart rate control and rhythm control. Recent clinical studies have emphasized the critical role of early restoration of sinus rhythm in improving patient outcomes. The persistence of the irregular rhythm allows for the progression and structural remodeling of the atria, eventually leading to irreversible stages, as observed clinically when AF becomes permanent. Cardioversion to sinus rhythm alters this progression pattern through mechanisms that are still being studied. In this review, we provide an in-depth analysis of the pathophysiological mechanisms responsible for maintaining AF and how they are modified during sinus rhythm restoration using existing therapeutic strategies at different stages of clinical investigation. Moreover, we explore potential future therapeutic approaches, including the promising prospect of gene therapy.

Gene therapy to terminate tachyarrhythmias

INTRODUCTION

To date, the treatment option for tachyarrhythmia is classified into drug therapy, catheter ablation, and implantable device therapy. However, the efficacy of the antiarrhythmic drugs is limited. Although the indication of catheter ablation is expanding, several fatal tachyarrhythmias are still refractory to ablation. Implantable cardioverter-defibrillator increases survival, but it is not a curable treatment. Therefore, a novel therapy for tachyarrhythmias refractory to present treatments is desired. Gene therapy is being developed as a promising candidate for this purpose, and basic research and translational research have been accumulated in recent years.

AREAS COVERED

This paper reviews the current state of gene therapy for arrhythmias, including susceptible arrhythmias, the route of administration to the heart, and the type of vector to use. We also discuss the latest progress in the technology of gene delivery and genome editing.

EXPERT OPINION

Gene therapy is one of the most promising technologies for arrhythmia treatment. However, additional technological innovation to achieve safe, localized, homogeneous, and long-lasting gene transfer is required for its clinical application.

Left atrial disease and left atrial reverse remodeling across different stages of heart failure development and progression: a new target for prevention and treatment

The left atrium is a dynamic chamber with peculiar characteristics. Stressors and disease mechanisms may deeply modify its structure and function, leading to left atrial remodelling and disease. Left atrial disease is a predictor of poor outcomes. It may be a consequence of left ventricular systolic and diastolic dysfunction and neurohormonal and inflammatory activation and/or actively contribute to the progression and clinical course of heart failure through multiple mechanisms such as left ventricular filling and development of atrial fibrillation and subsequent embolic events. There is growing evidence that therapy may improve left atrial function and reverse left atrial remodelling. Whether this translates into changes in patient's prognosis is still unknown.

In this review we report current data about changes in left atrial size and function across different stages of development and progression of heart failure. At each stage, drug therapies, lifestyle interventions and procedures have been associated with improvement in left atrial structure and function, namely a reduction in left atrial volume and/or an improvement in left atrial strain function, a process that can be defined as left atrial reverse remodelling and, in some cases, this has been associated with improvement in clinical outcomes. Further evidence is still needed mainly with respect of the possible role of left atrial reverse remodelling as an independent mechanism affecting the patient's clinical course and as regards better standardization of clinically meaningful changes in left atrial measurements. Summarizing current evidence, this review may be the basis for further studies.

A New Hypothetical Concept in Metabolic Understanding of Cardiac Fibrosis: Glycolysis Combined with TGF-β and KLF5 Signaling

The accumulation of fibrosis in cardiac tissues is one of the leading causes of heart failure. The principal cellular effectors in cardiac fibrosis are activated fibroblasts and myofibroblasts, which serve as the primary source of matrix proteins. TGF-β signaling pathways play a prominent role in cardiac fibrosis. The control of TGF-β by KLF5 in cardiac fibrosis has been demonstrated for modulating cardiovascular remodeling. Since the expression of KLF5 is reduced, the accumulation of fibrosis diminishes. Because the molecular mechanism of fibrosis is still being explored, there are currently few options for effectively reducing or reversing it. Studying metabolic alterations is considered an essential process that supports the explanation of fibrosis in a variety of organs and especially the glycolysis alteration in the heart. However, the interplay among the main factors involved in fibrosis pathogenesis, namely TGF-β, KLF5, and the metabolic process in glycolysis, is still indistinct. In this review, we explain what we know about cardiac fibroblasts and how they could help with heart repair. Moreover, we hypothesize and summarize the knowledge trend on the molecular mechanism of TGF-β, KLF5, the role of the glycolysis pathway in fibrosis, and present the future therapy of cardiac fibrosis. These studies may target therapies that could become important strategies for fibrosis reduction in the future.

Analysis of potential genetic biomarkers using machine learning methods and immune infiltration regulatory mechanisms underlying atrial fibrillation

Objective

We aimed to screen out biomarkers for atrial fibrillation (AF) based on machine learning methods and evaluate the degree of immune infiltration in AF patients in detail.

Methods

Two datasets (GSE41177 and GSE79768) related to AF were downloaded from Gene expression omnibus (GEO) database and merged for further analysis. Differentially expressed genes (DEGs) were screened out using “limma” package in R software. Candidate biomarkers for AF were identified using machine learning methods of the LASSO regression algorithm and SVM-RFE algorithm. Receiver operating characteristic (ROC) curve was employed to assess the diagnostic effectiveness of biomarkers, which was further validated in another independent validation dataset of GSE14975. Moreover, we used CIBERSORT to study the proportion of infiltrating immune cells in each sample, and the Spearman method was used to explore the correlation between biomarkers and immune cells.

Results

129 DEGs were identified, and CYBB, CXCR2, and S100A4 were identified as key biomarkers of AF using LASSO regression and SVM-RFE algorithm. Both in the training dataset and the validation dataset, CYBB, CXCR2, and S100A4 showed favorable diagnostic effectiveness. Immune infiltration analysis indicated that, compared with sinus rhythm (SR), the atrial samples of patients with AF contained a higher T cells gamma delta, neutrophils and mast cells resting, whereas T cells follicular helper were relatively lower. Correlation analysis demonstrated that CYBB, CXCR2, and S100A4 were significantly correlated with the infiltrating immune cells.

Conclusions

In conclusion, this study suggested that CYBB, CXCR2, and S100A4 are key biomarkers of AF correlated with infiltrating immune cells, and infiltrating immune cells play pivotal roles in AF.

Transforming growth factor-β in myocardial disease

Transforming growth factor-β (TGFβ) isoforms are upregulated and activated in myocardial diseases and have an important role in cardiac repair and remodelling, regulating the phenotype and function of cardiomyocytes, fibroblasts, immune cells and vascular cells. Cardiac injury triggers the generation of bioactive TGFβ from latent stores, through mechanisms involving proteases, integrins and specialized extracellular matrix (ECM) proteins. Activated TGFβ signals through the SMAD intracellular effectors or through non-SMAD cascades. In the infarcted heart, the anti-inflammatory and fibroblast-activating actions of TGFβ have an important role in repair; however, excessive or prolonged TGFβ signalling accentuates adverse remodelling, contributing to cardiac dysfunction. Cardiac pressure overload also activates TGFβ cascades, which initially can have a protective role, promoting an ECM-preserving phenotype in fibroblasts and preventing the generation of injurious, pro-inflammatory ECM fragments. However, prolonged and overactive TGFβ signalling in pressure-overloaded cardiomyocytes and fibroblasts can promote cardiac fibrosis and dysfunction. In the atria, TGFβ-mediated fibrosis can contribute to the pathogenic substrate for atrial fibrillation. Overactive or dysregulated TGFβ responses have also been implicated in cardiac ageing and in the pathogenesis of diabetic, genetic and inflammatory cardiomyopathies. This Review summarizes the current evidence on the role of TGFβ signalling in myocardial diseases, focusing on cellular targets and molecular mechanisms, and discussing challenges and opportunities for therapeutic translation.

Recent advances in gene therapy for atrial fibrillation

Atrial fibrillation (AF) is the most common heart rhythm disorder in adults and a major cause of stroke. Unfortunately, current treatments for AF are suboptimal as they are not targeting the molecular mechanisms underlying AF. In this regard, gene therapy is emerging as a promising approach for mechanism-based treatment of AF. In this review, we summarize recent advances and challenges in gene therapy for this important cardiovascular disease.

Atrial fibrosis as a dominant factor for the development of atrial fibrillation: facts and gaps

Atrial fibrillation (AF), the most commonly diagnosed arrhythmia, affects a notable percentage of the population and constitutes a major risk factor for thromboembolic events and other heart-related conditions. Fibrosis plays an important role in the onset and perpetuation of AF through structural and electrical remodelling processes. Multiple molecular pathways are involved in atrial substrate modification and the subsequent maintenance of AF. In this review, we aim to recapitulate underlying molecular pathways leading to atrial fibrosis and to indicate existing gaps in the complex interplay of atrial fibrosis and AF.

Identification of key immune-related genes and immune infiltration in atrial fibrillation with valvular heart disease based on bioinformatics analysis

Background

Atrial fibrillation (AF) is the most common persistent arrhythmia. Valvular heart disease (VHD) and AF frequently coexist. In our study, from performing bioinformatics analysis, we sought to identify immune-related genes (IRGs) and explore the role of immune cell infiltration in AF-VHD in depth, aiming at investigating the potential molecular mechanism and developing new therapeutic targets for AF, including AF-VHD.

Methods

The gene expression of the GSE41177 and GSE79768 datasets were downloaded from the Gene Expression Omnibus database. Differentially expressed genes (DEGs) were analyzed via the limma package in Bioconductor with R software. Differentially expressed immune-related genes (DEIRGs) were selected via combination ImmPort database with DEGs, and the enrichment function and pathway analysis were explored. A protein-protein interaction (PPI) network was built with a Search Tool for the Retrieval of Interacting Genes/Proteins plugin in Cytoscape. The CIBERSORT algorithm was used to evaluate immune infiltration in the left atrial (LA) tissues between AF-VHD and sinus rhythm (SR) patients. Finally, a correlation analysis between key DEIRGs and infiltrating immune cells was performed.

Results

A total of 130 DEIRGs were detected. Enrichment function of DEIRGs demonstrated that they are significant in immune and inflammatory responses. The key DEIRGs assessed by the PPI network and involved in both the immune and inflammatory responses were the C-X-C motif chemokine ligand (CXCL) 1, pro-platelet basic protein (PPBP), CXCL12, and C-C motif chemokine ligand 4 (CCL4). The immune infiltration findings indicated that, compared with the LA tissues from SR patients, the tissues from AF-VHD patients contained a higher proportion of gamma delta T cells, but a lower proportion of CD8 and regulatory T cells. The results of correlation analysis demonstrated that CXCL1 was positively correlated with activated mast cells and significantly negatively correlated with resting mast cells. PPBP, CXCL12, and CCL4 were positively correlated with the infiltration of various immune cells, such as neutrophils, plasma cells, and resting dendritic cells.

Conclusions

The key immune-related genes and the differences in immune infiltration in LA tissues play an essential role in the occurrence and progression of AF-VHD.

Platelets Promote Ang II (Angiotensin II)-Induced Atrial Fibrillation by Releasing TGF-β1 (Transforming Growth Factor-β1) and Interacting With Fibroblasts

Hypertension is a risk factor of atrial fibrillation (AF), and a certain number of patients with hypertension were found with an enlarged left atrium. Platelet activation is found in patients with hypertension or pressure overload/Ang II (angiotensin II)-induced hypertensive animal models and contribute to ventricular fibrosis. Whether hypertension-induced atrial fibrosis is mediated by platelets remains unknown. Our previous experimental data showed that platelet-derived TGF-β1 (transforming growth factor-β1) was reduced in patients with hypertensive AF. The present study is to investigate whether platelet-derived TGF-β1 promotes Ang II-induced atrial fibrosis and AF. Platelet activation and atrial platelet accumulation were measured in sinus rhythm controls, normotensive AF, and patients with hypertensive AF. Ang II (1500 ng/kg per minute, 3 weeks) infused mice with pharmacological (clopidogrel) and genetic platelet inhibition (TGF-β1 deletion in platelets) were used. Platelet activation, atrial structural remodeling, atrial electrical transmission, AF inducibility, inflammation, and fibrosis were measured in mice. We found that circulating platelets were activated in patients with hypertensive AF. A large amount of platelet was accumulated in the atriums of patients with hypertensive AF. Both clopidogrel treatment and platelet-specific deletion of TGF-β1 attenuated Ang II-induced structural remodeling, atrial electrical transmission, AF inducibility, as well as atrial inflammation and fibrosis than mice without interventions. Furthermore, clopidogrel blocked atrial platelet accumulation and platelet-fibroblast conjugation. Platelets promoted atrial fibroblast differentiation in cell culture. Profibrotic actions of platelets are largely via activation of atrial fibroblasts by releasing TGF-β1 and inducing platelet-fibroblast conjugation, and platelet inhibition is sufficient to inhibit atrial fibrosis and AF inducibility.

New methodological approaches to atrial fibrillation drug discovery

INTRODUCTION

Atrial fibrillation (AF) is the most common arrhythmia encountered in clinical practice and rhythm control using pharmacological agents is required in selected patients. Nonetheless, current medication is only modestly efficacious and associated with significant cardiovascular and systemic side effects. More efficacious and safe drugs are required to restore and maintain sinus rhythm in patients with AF.

AREAS COVERED

In this review, several potential drug targets are discussed including trans-membrane ion channels, intracellular calcium signaling, gap junction signaling, atrial inflammation and fibrosis, and the autonomic nervous system. New tools and methodologies for AF drug development are also reviewed including gene therapy, genome-guided therapy, stem cell technologies, tissue engineering, and optogenetics.

EXPERT OPINION

In recent decades, there has been an increased understanding of the underlying pathogenesis of AF. As a result, there is a gradual paradigm shift from focusing only on trans-membrane ion channel inhibition to developing therapeutic agents that target other underlying arrhythmogenic mechanisms. Gene therapy and genome-guided therapy are emerging as novel treatments for AF with some success in proof-of-concept studies. Recent advances in stem cell technology, tissue engineering, and optogenetics may allow more effective in-vitro drug screening than conventional methodologies.

Attenuation of Oxidative Injury With Targeted Expression of NADPH Oxidase 2 Short Hairpin RNA Prevents Onset and Maintenance of Electrical Remodeling in the Canine Atrium: A Novel Gene Therapy Approach to Atrial Fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common heart rhythm disorder in adults and a major cause of stroke. Unfortunately, current treatments of AF are suboptimal because they are not targeted to the molecular mechanisms underlying AF. Using a highly novel gene therapy approach in a canine, rapid atrial pacing model of AF, we demonstrate that NADPH oxidase 2 (NOX2) generated oxidative injury causes upregulation of a constitutively active form of acetylcholine-dependent K+ current (IKACh), called IKH; this is an important mechanism underlying not only the genesis, but also the perpetuation of electric remodeling in the intact, fibrillating atrium.

METHODS

To understand the mechanism by which oxidative injury promotes the genesis and maintenance of AF, we performed targeted injection of NOX2 short hairpin RNA (followed by electroporation to facilitate gene delivery) in atria of healthy dogs followed by rapid atrial pacing. We used in vivo high-density electric mapping, isolation of atrial myocytes, whole-cell patch clamping, in vitro tachypacing of atrial myocytes, lucigenin chemiluminescence assay, immunoblotting, real-time polymerase chain reaction, immunohistochemistry, and Masson trichrome staining.

RESULTS

First, we demonstrate that generation of oxidative injury in atrial myocytes is a frequency-dependent process, with rapid pacing in canine atrial myocytes inducing oxidative injury through the induction of NOX2 and the generation of mitochondrial reactive oxygen species. We show that oxidative injury likely contributes to electric remodeling in AF by upregulating IKACh by a mechanism involving frequency-dependent activation of PKCε (protein kinase C epsilon). The time to onset of nonsustained AF increased by >5-fold in NOX2 short hairpin RNA-treated dogs. Furthermore, animals treated with NOX2 short hairpin RNA did not develop sustained AF for up to 12 weeks. The electrophysiological mechanism underlying AF prevention was prolongation of atrial effective refractory periods, at least in part attributable to the attenuation of IKACh. Attenuated membrane translocation of PKCε appeared to be a likely molecular mechanism underlying this beneficial electrophysiological remodeling.

CONCLUSIONS

NOX2 oxidative injury (1) underlies the onset, and the maintenance of electric remodeling in AF, as well, and (2) can be successfully prevented with a novel, gene-based approach. Future optimization of this approach may lead to a novel, mechanism-guided therapy for AF.

Atrial Myopathy Underlying Atrial Fibrillation

While AF most often occurs in the setting of atrial disease, current assessment and treatment of patients with AF does not focus on the extent of the atrial myopathy that serves as the substrate for this arrhythmia. Atrial myopathy, in particular atrial fibrosis, may initiate a vicious cycle in which atrial myopathy leads to AF, which in turn leads to a worsening myopathy. Various techniques, including ECG, plasma biomarkers, electroanatomical voltage mapping, echocardiography, and cardiac MRI, can help to identify and quantify aspects of the atrial myopathy. Current therapies, such as catheter ablation, do not directly address the underlying atrial myopathy. There is emerging research showing that by targeting this myopathy we can help decrease the occurrence and burden of AF.

A New Therapeutic Framework for Atrial Fibrillation Drug Development

Atrial fibrillation (AF) is a highly prevalent cardiac arrhythmia and cause of significant morbidity and mortality. Its increasing prevalence in aging societies constitutes a growing challenge to global healthcare systems. Despite substantial unmet needs in AF prevention and treatment, drug developments hitherto have been challenging, and the current pharmaceutical pipeline is nearly empty. In this review, we argue that current drugs for AF are inadequate because of an oversimplified system for patient classification and the development of drugs that do not interdict underlying disease mechanisms. We posit that an improved understanding of AF molecular pathophysiology related to the continuous identification of novel disease-modifying drug targets and an increased appreciation of patient heterogeneity provide a new framework to personalize AF drug development. Together with recent innovations in diagnostics, remote rhythm monitoring, and big data capabilities, we anticipate that adoption of a new framework for patient subsegmentation based on pathophysiological, genetic, and molecular subsets will improve success rates of clinical trials and advance drugs that reduce the individual patient and public health burden of AF.

Enhanced cardiomyocyte reactive oxygen species signaling promotes ibrutinib-induced atrial fibrillation

Atrial fibrillation (AF) occurs in up to 11% of cancer patients treated with ibrutinib. The pathophysiology of ibrutinib promoted AF is complicated, as there are multiple interactions involved; the detailed molecular mechanisms underlying this are still unclear. Here, we aimed to determine the electrophysiological and molecular mechanisms of burst-pacing-induced AF in ibrutinib-treated mice. The results indicated differentially expressed proteins in ibrutinib-treated mice, identified through proteomic analysis, were found to play a role in oxidative stress-related pathways. Finally, treatment with an inhibitor of NADPH oxidase (NOX) prevented and reversed AF development in ibrutinib-treated mice. It was showed that the related protein expression of reactive oxygen species (ROS) in the ibrutinib group was significantly increased, including NOX2, NOX4, p22-phox, XO and TGF-β protein expression. It was interesting that ibrutinib group also significantly increased the expression of ox-CaMKII, p-CaMKII (Thr-286) and p-RyR2 (Ser2814), causing enhanced abnormal sarcoplasmic reticulum (SR) Ca²⁺ release and mitochondrial structures, as well as atrial fibrosis and atrial hypertrophy in ibrutinib-treated mice, and apocynin reduced the expression of these proteins. Ibrutinib-treated mice were also more likely to develop AF, and AF occurred over longer periods. In conclusion, our study has established a pathophysiological role for ROS signaling in atrial cardiomyocytes, and it may be that ox-CaMKII and p-CaMKII (Thr-286) are activated by ROS to increase AF susceptibility following ibrutinib treatment. We have also identified the inhibition of NOX as a potential novel AF therapy approach.

Gene therapy for atrial fibrillation - How close to clinical implementation?

In this review we examine the current state of gene therapy for the treatment of cardiac arrhythmias. We describe advances and challenges in successfully creating and incorporating gene vectors into the myocardium. After summarizing the current scientific research in gene transfer technology we then focus on the most promising areas of gene therapy, the treatment of atrial fibrillation and ventricular tachyarrhythmias. We review the scientific literature to determine how gene therapy could potentially be used to treat patients with cardiac arrhythmias.

Atrial Myopathy

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The authors discuss the concept of atrial myopathy; its relationship to aging, electrophysiological remodeling, and autonomic remodeling; the interplay between atrial myopathy, AF, and stroke; and suggest how to identify patients with atrial myopathy and how to incorporate atrial myopathy into decisions about anticoagulation.

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Atrial myopathy seen in animal models of AF and in patients with AF is the result of a combination of factors that lead to electrical and structural remodeling in the atrium. Although AF may lead to the initiation and/or progression of this myopathy, the presence of AF is by no means essential to the development or the maintenance of the atrial myopathic state.

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Methods to identify atrial myopathy include atrial electrograms, tissue biopsy, cardiac imaging, and certain serum biomarkers. A promising modality is 4-dimensional flow cardiac magnetic resonance. The concept of atrial myopathy may help guide oral anticoagulant therapy in selected groups of patients with AF, particularly those with low to intermediate risk of strokes and those who have undergone successful AF ablation. This review highlights the need for prospective randomized trials to test these hypotheses.

This paper discusses the evolving concept of atrial myopathy by presenting how it develops and how it affects the properties of the atria. It also reviews the complex relationships among atrial myopathy, atrial fibrillation (AF), and stroke. Finally, it discusses how to apply the concept of atrial myopathy in the clinical setting—to identify patients with atrial myopathy and to be more selective in anticoagulation in a subset of patients with AF. An apparent lack of a temporal relationship between episodes of paroxysmal AF and stroke in patients with cardiac implantable electronic devices has led investigators to search for additional factors that are responsible for AF-related strokes. Multiple animal models and human studies have revealed a close interplay of atrial myopathy, AF, and stroke via various mechanisms (e.g., aging, inflammation, oxidative stress, and stretch), which, in turn, lead to fibrosis, electrical and autonomic remodeling, and a pro-thrombotic state. The complex interplay among these mechanisms creates a vicious cycle of ever-worsening atrial myopathy and a higher risk of more sustained AF and strokes. By highlighting the importance of atrial myopathy and the risk of strokes independent of AF, this paper reviews the methods to identify patients with atrial myopathy and proposes a way to incorporate the concept of atrial myopathy to guide anticoagulation in patients with AF.

Genetic Ablation of TASK-1 (Tandem of P Domains in a Weak Inward Rectifying K+ Channel-Related Acid-Sensitive K+ Channel-1) (K2P3.1) K+ Channels Suppresses Atrial Fibrillation and Prevents Electrical Remodeling

BACKGROUND

Despite an increasing understanding of atrial fibrillation (AF) pathophysiology, translation into mechanism-based treatment options is lacking. In atrial cardiomyocytes of patients with chronic AF, expression, and function of tandem of P domains in a weak inward rectifying TASK-1 (K⁺ channel-related acid-sensitive K⁺ channel-1) (K_2P3.1) atrial-specific 2-pore domain potassium channels is enhanced, resulting in action potential duration shortening. TASK-1 channel inhibition prevents action potential duration shortening to maintain values observed among sinus rhythm subjects. The present preclinical study used a porcine AF model to evaluate the antiarrhythmic efficacy of TASK-1 inhibition by adeno-associated viral anti-TASK-1-siRNA (small interfering RNA) gene transfer.

METHODS

AF was induced in domestic pigs by atrial burst stimulation via implanted pacemakers. Adeno-associated viral vectors carrying anti-TASK-1-siRNA were injected into both atria to suppress TASK-1 channel expression. After the 14-day follow-up period, porcine cardiomyocytes were isolated from right and left atrium, followed by electrophysiological and molecular characterization.

RESULTS

AF was associated with increased TASK-1 transcript, protein and ion current levels leading to shortened action potential duration in atrial cardiomyocytes compared to sinus rhythm controls, similar to previous findings in humans. Anti-TASK-1 adeno-associated viral application significantly reduced AF burden in comparison to untreated AF pigs. Antiarrhythmic effects of anti-TASK-1-siRNA were associated with reduction of TASK-1 currents and prolongation of action potential durations in atrial cardiomyocytes to sinus rhythm values. Conclusions Adeno-associated viral-based anti-TASK-1 gene therapy suppressed AF and corrected cellular electrophysiological remodeling in a porcine model of AF. Suppression of AF through selective reduction of TASK-1 currents represents a new option for antiarrhythmic therapy.

Long noncoding RNA Crnde attenuates cardiac fibrosis via Smad3-Crnde negative feedback in diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM)-ventricular dysfunction in the absence of underlying heart disease-is a common complication of diabetes and a leading cause of mortality associated with the disease. In DCM, cardiac fibrosis is the main cause of heart failure. Although it is well-established that the transforming growth factor-beta signaling pathway plays a part in inducing cardiac fibrosis in DCM, details of the molecular mechanism involved remain elusive. Therefore, it is crucial to study the gene reg;ulation of key signaling effectors in DCM-associated cardiac fibrosis. A recently emerged hotspot in the field of gene regulation is the role of long noncoding RNAs (lncRNAs). Recent evidence indicates that lncRNAs play a critical role in cardiac fibrosis; however, in DCM, the function of these regulatory RNAs have not been studied in depth. In this study, we identified a conserved cardiac-specific lncRNA named colorectal neoplasia differentially expressed (Crnde). By analyzing 376 human heart tissues, it was found that Crnde expression is negatively correlated with that of cardiac fibrosis marker genes. Moreover, Crnde expression was shown to be enriched in cardiac fibroblasts (CFs). Overexpression of Crnde attenuated cardiac fibrosis and enhanced cardiac function in mice with DCM. Further, in vitro experiments showed that Crnde negatively regulates the myofibroblast differentiation of CFs. The expression of Crnde was activated by SMAD family member 3 (Smad3), shedding light on the underlying molecular mechanism. Interestingly, Crnde also inhibited the transcriptional activation of Smad3 on target genes, thereby inhibiting the expression of myofibroblastic marker genes in CFs. Overall, our data provide valuable insights into the development of potential anti-cardiac fibrosis strategies centered on lncRNAs, for the treatment of DCM.

Gene Therapy for the Treatment of Cardiac Arrhythmias: Current and Emerging Applications

In this review, we examine the current state of gene therapy for the treatment of cardiac arrhythmias. We describe advances and challenges in successfully creating and incorporating gene vectors into the myocardium. After summarizing the current scientific research in gene transfer technology, we then focus on the most promising areas of gene therapy at this time, which is the treatment of atrial fibrillation and ventricular tachyarrhythmias. We also review the scientific literature to determine how gene therapy could potentially be used to treat patients with cardiac arrhythmias.

Oxidative stress creates a unique, CaMKII-mediated substrate for atrial fibrillation in heart failure

The precise mechanisms by which oxidative stress (OS) causes atrial fibrillation (AF) are not known. Since AF frequently originates in the posterior left atrium (PLA), we hypothesized that OS, via calmodulin-dependent protein kinase II (CaMKII) signaling, creates a fertile substrate in the PLA for triggered activity and reentry. In a canine heart failure (HF) model, OS generation and oxidized-CaMKII-induced (Ox-CaMKII-induced) RyR2 and Nav1.5 signaling were increased preferentially in the PLA (compared with left atrial appendage). Triggered Ca2+ waves (TCWs) in HF PLA myocytes were particularly sensitive to acute ROS inhibition. Computational modeling confirmed a direct relationship between OS/CaMKII signaling and TCW generation. CaMKII phosphorylated Nav1.5 (CaMKII-p-Nav1.5 [S571]) was located preferentially at the intercalated disc (ID), being nearly absent at the lateral membrane. Furthermore, a decrease in ankyrin-G (AnkG) in HF led to patchy dropout of CaMKII-p-Nav1.5 at the ID, causing its distribution to become spatially heterogeneous; this corresponded to preferential slowing and inhomogeneity of conduction noted in the HF PLA. Computational modeling illustrated how conduction slowing (e.g., due to increase in CaMKII-p-Nav1.5) interacts with fibrosis to cause reentry in the PLA. We conclude that OS via CaMKII leads to substrate for triggered activity and reentry in HF PLA by mechanisms independent of but complementary to fibrosis.

MicroRNA-30c suppresses the pro-fibrogenic effects of cardiac fibroblasts induced by TGF-β1 and prevents atrial fibrosis by targeting TGFβRII

Atrial fibrosis serves as an important contributor to atrial fibrillation (AF). Recent data have suggested that microRNA-30c (miR-30c) is involved in fibrotic remodelling and cancer development, but the specific role of miR-30c in atrial fibrosis remains unclear. The purpose of this study was to investigate the role of miR-30c in atrial fibrosis and its underlying mechanisms through in vivo and in vitro experiments. Our results indicate that miR-30c is significantly down-regulated in the rat abdominal aortic constriction (AAC) model and in the cellular model of fibrosis induced by transforming growth factor-β1 (TGF-β1). Overexpression of miR-30c in cardiac fibroblasts (CFs) markedly inhibits CF proliferation, differentiation, migration and collagen production, whereas decrease in miR-30c leads to the opposite results. Moreover, we identified TGFβRII as a target of miR-30c. Finally, transferring adeno-associated virus 9 (AAV9)-miR-30c into the inferior vena cava of rats attenuated fibrosis in the left atrium following AAC. These data indicate that miR-30c attenuates atrial fibrosis via inhibition of CF proliferation, differentiation, migration and collagen production by targeting TGFβRII, suggesting that miR-30c might be a novel potential therapeutic target for preventing atrial fibrosis.

Biological therapies targeting arrhythmias: are cells and genes the answer?

INTRODUCTION

Arrhythmias can cause symptoms ranging from simple dizziness to life-threatening circulatory collapse. Current management includes medical therapy and procedures such as catheter ablation or device implantation. However, these strategies still pose a risk of serious side effects, and some patients remain symptomatic. Advancement in our understanding of how arrhythmias develop on the cellular level has made more targeted approaches possible. In addition, contemporary studies have found that several genes are involved in the pathogenesis of arrhythmias.

AREAS COVERED

In the present review, the authors explore the cellular and genetic mechanisms leading to arrhythmias as well as the progress that has been made in using both gene and cell therapy to treat tachy- and bradyarrhythmias. They also consider why gene and cell therapy has resulted into a few clinical trials with promising results, however still not applicable in routine clinical practice.

EXPERT OPINION

The question currently is whether such biological therapies could replace current established approaches. The contemporary evidence suggests that despite recent advances in this field, it will need more work in experimental models before this is applied into clinical practice. Gene and cell studies targeting conduction and repolarization are promising, but still not ready for use in the clinical setting.

Oxidative stress and inflammation as central mediators of atrial fibrillation in obesity and diabetes

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in humans. Several risk factors promote AF, among which diabetes mellitus has emerged as one of the most important. The growing recognition that obesity, diabetes and AF are closely intertwined disorders has spurred major interest in uncovering their mechanistic links. In this article we provide an update on the growing evidence linking oxidative stress and inflammation to adverse atrial structural and electrical remodeling that leads to the onset and maintenance of AF in the diabetic heart. We then discuss several therapeutic strategies to improve atrial excitability by targeting pathways that control oxidative stress and inflammation.

Improving Atrial Fibrillation Therapy: Is There a Gene for That?

Atrial fibrillation (AF) is an all-too-common and often challenging reality of clinical care. AF leads to significant morbidity and mortality; however, currently available treatments for AF have modest efficacy and high recurrence rates. In recent years, genetic therapy approaches have been explored in preclinical models of AF, and offer potential as a treatment modality with targeted delivery, tissue specificity, and therapy tailored to address mechanisms underlying the arrhythmia. However, many challenges remain before gene therapy can advance to a clinically relevant AF treatment. In this review, we summarize the available published data on gene therapy and discuss the challenges, opportunities, and limitations of this approach.

The crucial role of activin A/ALK4 pathway in the pathogenesis of Ang-II-induced atrial fibrosis and vulnerability to atrial fibrillation

Atrial fibrosis, the hallmark of structural remodeling associated with atrial fibrillation (AF), is characterized by abnormal proliferation of atrial fibroblasts and excessive deposition of extracellular matrix. Transforming growth factor-β1 (TGF-β1)/activin receptor-like kinase 5 (ALK5)/Smad2/3/4 pathway has been reported to be involved in the process. Recent studies have implicated both activin A and its specific downstream component activin receptor-like kinase 4 (ALK4) in stimulating fibrosis in non-cardiac organs. We recently reported that ALK4 haplodeficiency attenuated the pressure overload- and myocardial infarction-induced ventricular fibrosis. However, the role of activin A/ALK4 in the pathogenesis of atrial fibrosis and vulnerability to AF remains unknown. Our study provided experimental and clinical evidence for the involvement of activin A and ALK4 in the pathophysiology of atrial fibrosis and AF. Patients with AF had higher activin A and ALK4 expression in atriums as compared to individuals devoid of AF. After angiotensin-II (Ang-II) stimulation which mimicked atrial fibrosis progression, ALK4-deficient mice showed lower expression of ALK4 in atriums, reduced activation of atrial fibroblasts, blunted atrial enlargement and atrial fibrosis, and further reduced AF vulnerability upon right atrial electrophysiological studies as compared to wild-type littermates. Moreover, we found that apart from the well-known TGF-β1/ALK5 pathway, the activation of activin A/ALK4/smad2/3 pathway played an important role in the pathogenesis of Ang-II-mediated atrial fibrosis and inducibility of AF, suggesting that targeting ALK4 might be a potential therapy for atrial fibrosis and AF.

Gene Therapy for Atrial Fibrillation in Heart Failure

Atrial fibrillation (AF) is the most common arrhythmia and a major cause of morbidity and mortality in an aging population. Unfortunately, current treatments for AF are suboptimal, in large part because the molecular mechanisms underlying AF are not well understood. Recent advances in our understanding of the AF disease state have led to the preclinical development of gene-based therapies that are targeted to key molecular mechanisms involved in the genesis and maintenance of AF.