Triggered firing and atrial fibrillation in transgenic mice with selective atrial fibrosis induced by overexpression of TGF-β1

Pathophysiology, molecular mechanisms, and genetics of atrial fibrillation

The development of atrial fibrillation (AF) is a highly complex, multifactorial process involving pathophysiologic mechanisms, molecular pathway mechanisms and numerous genetic abnormalities. The pathophysiologic mechanisms including altered ion channels, abnormalities of the autonomic nervous system, inflammation, and abnormalities in Ca2 + handling. Molecular pathway mechanisms including, but not limited to, renin-angiotensin-aldosterone (RAAS), transforming growth factor-β (TGF-β), oxidative stress (OS). Although in clinical practice, the distinction between types of AF such as paroxysmal and persistent determines the choice of treatment options. However, it is the pathophysiologic alterations present in AF that truly determine the success of AF treatment and prognosis, but even more so the molecular mechanisms and genetic alterations that lie behind them. One tiny clue reveals the general trend, and small beginnings show how things will develop. This article will organize the development of these mechanisms and their interactions in recent years.

Translation of pathophysiological mechanisms of atrial fibrosis into new diagnostic and therapeutic approaches

Atrial fibrosis is one of the main manifestations of atrial cardiomyopathy, an array of electrical, mechanical and structural alterations associated with atrial fibrillation (AF), stroke and heart failure. Atrial fibrosis can be both a cause and a consequence of AF and, once present, it accelerates the progression of AF. The pathophysiological mechanisms leading to atrial fibrosis are diverse and include stretch-induced activation of fibroblasts, systemic inflammatory processes, activation of coagulation factors and fibrofatty infiltrations. Importantly, atrial fibrosis can occur in different forms, such as reactive and replacement fibrosis. The diversity of atrial fibrosis mechanisms and patterns depends on sex, age and comorbidity profile, hampering the development of therapeutic strategies. In addition, the presence and severity of comorbidities often change over time, potentially causing temporal changes in the mechanisms underlying atrial fibrosis development. This Review summarizes the latest knowledge on the molecular and cellular mechanisms of atrial fibrosis, its association with comorbidities and the sex-related differences. We describe how the various patterns of atrial fibrosis translate into electrophysiological mechanisms that promote AF, and critically appraise the clinical applicability and limitations of diagnostic tools to quantify atrial fibrosis. Finally, we provide an overview of the newest therapeutic interventions under development and discuss relevant knowledge gaps related to the association between clinical manifestations and pathological mechanisms of atrial fibrosis and to the translation of this knowledge to a clinical setting.

Speech-induced atrial tachycardia: A narrative review of putative mechanisms implicating the autonomic nervous system

Despite being uncommon, speech-induced atrial tachycardias carry significant morbidity and affect predominantly healthy individuals. Little is known about their mechanism, treatment, and prognosis. In this review, we seek to identify the underlying connections and pathophysiology between speech and arrhythmias while providing an informed approach to evaluation and management.

Pathophysiological Roles of the TRPV4 Channel in the Heart

The transient receptor potential vanilloid 4 (TRPV4) channel is a non-selective cation channel that is mostly permeable to calcium (Ca²⁺), which participates in intracellular Ca²⁺ handling in cardiac cells. It is widely expressed through the body and is activated by a large spectrum of physicochemical stimuli, conferring it a role in a variety of sensorial and physiological functions. Within the cardiovascular system, TRPV4 expression is reported in cardiomyocytes, endothelial cells (ECs) and smooth muscle cells (SMCs), where it modulates mitochondrial activity, Ca²⁺ homeostasis, cardiomyocytes electrical activity and contractility, cardiac embryonic development and fibroblast proliferation, as well as vascular permeability, dilatation and constriction. On the other hand, TRPV4 channels participate in several cardiac pathological processes such as the development of cardiac fibrosis, hypertrophy, ischemia-reperfusion injuries, heart failure, myocardial infarction and arrhythmia. In this manuscript, we provide an overview of TRPV4 channel implications in cardiac physiology and discuss the potential of the TRPV4 channel as a therapeutic target against cardiovascular diseases.

PI3K(p110α) as a determinant and gene therapy for atrial enlargement in atrial fibrillation

Atrial fibrillation (AF) is an irregular heart rhythm, characterised by chaotic atrial activation, which is promoted by remodelling. Once initiated, AF can also propagate the progression of itself in the so-called ''AF begets AF''. Several lines of investigation have shown that signalling molecules, including reactive oxygen species, angiotensin II, and phosphoinositide 3-kinases (PI3Ks), in presence or absence of cardiovascular disease risk factors, stabilise and promote AF maintenance. In particular, reduced cardiac-specific PI3K activity that is not associated with oncology is cardiotoxic and increases susceptibility to AF. Atrial-specific PI3K(p110α) transgene can cause pathological atrial enlargement. Highlighting the crucial importance of the p110α protein in a clinical problem that currently challenges the professional health care practice, in over forty (40) transgenic mouse models of AF (Table1), currently existing, of which some of the models are models of human genetic disorders, including PI3K(p110α) transgenic mouse model, over 70% of them reporting atrial size showed enlarged, greater atrial size. Individuals with minimal to severely dilated atria develop AF more likely. Left atrial diameter and volume stratification are an assessment for follow-up surveillance to detect AF. Gene therapy to reduce atrial size will be associated with a reduction in AF burden. In this overview, PI3K(p110α), a master regulator of organ size, was investigated in atrial enlargement and in physiological determinants that promote AF.

Intermedin 1-53 Ameliorates Atrial Fibrosis and Reduces Inducibility of Atrial Fibrillation via TGF-β1/pSmad3 and Nox4 Pathway in a Rat Model of Heart Failure

OBJECTIVE

New drugs to block the occurrence of atrial fibrillation (AF) based on atrial structural remodeling (ASR) are urgently needed. The purpose of this study was to study the role of intermedin 1-53 (IMD1-53) in ASR and AF formation in rats after myocardial infarction (MI).

MATERIAL AND METHODS

Heart failure was induced by MI in rats. Fourteen days after MI surgery, rats with heart failure were randomized into control (untreated MI group, n = 10) and IMD-treated (n = 10) groups. The MI group and sham group received saline injections. The rats in the IMD group received IMD1-53, 10 nmol/kg/day intraperitoneally for 4 weeks. The AF inducibility and atrial effective refractory period (AERP) were assessed with an electrophysiology test. Additionally, the left-atrial diameter was determined, and heart function and hemodynamic tests were performed. We detected the area changes of myocardial fibrosis in the left atrium using Masson staining. To detect the protein expression and mRNA expression of transforming growth factor-β1 (TGF-β1), α-SMA, collagen Ⅰ, collagen III, and NADPH oxidase (Nox4) in the myocardial fibroblasts and left atrium, we used the Western blot method and real-time quantitative polymerase chain reaction (PCR) assays.

RESULTS

Compared with the MI group, IMD1-53 treatment decreased the left-atrial diameter and improved cardiac function, while it also improved the left-ventricle end-diastolic pressure (LVEDP). IMD1-53 treatment attenuated AERP prolongation and reduced atrial fibrillation inducibility in the IMD group. In vivo, IMD1-53 reduced the left-atrial fibrosis content in the heart after MI surgery and inhibited the mRNA and protein expression of collagen type Ⅰ and III. IMD1-53 also inhibited the expression of TGF-β1, α-SMA, and Nox4 both in mRNA and protein. In vivo, we found that IMD1-53 inhibited the phosphorylation of Smad3. In vitro, we found that the downregulated expression of Nox4 was partly dependent on the TGF-β1/ALK5 pathway.

CONCLUSIONS

IMD1-53 decreased the duration and inducibility of AF and atrial fibrosis in the rats after MI operation. The possible mechanisms are related to the inhibition of TGF-β1/Smad3-related fibrosis and TGF-β1/Nox4 activity. Therefore, IMD1-53 may be a promising upstream treatment drug to prevent AF.

Critical Link between Calcium Regional Heterogeneity and Atrial Fibrillation Susceptibility in Sheep Left Atria

BACKGROUND

Atrial fibrillation is the most sustained form of arrhythmia in the human population that leads to important electrophysiological and structural cardiac remodeling as it progresses into a chronic form. Calcium is an established key player of cellular electrophysiology in the heart, yet to date, there is no information that maps calcium signaling across the left atrium.

OBJECTIVE

The aim of this study is to determine whether calcium signaling is homogenous throughout the different regions of the left atrium. This work tests the hypothesis that differences across the healthy left atrium contribute to a unique, region-dependent calcium cycling and participates in the pro-arrhythmic activity during atrial fibrillation.

METHODS

An animal model relevant to human cardiac function (the sheep) was used to characterize both the electrical activity and the calcium signaling of three distinct left atrium regions (appendage, free wall and pulmonary veins) in control conditions and after acetylcholine perfusion (5 μM) to induce acute atrial fibrillation. High-resolution dual calcium-voltage optical mapping on the left atria of sheep was performed to explore the spatiotemporal dynamics of calcium signaling in relation to electrophysiological properties.

RESULTS

Action potential duration (at 80% repolarization) was not significantly different in the three regions of interest for the three pacing sites. In contrast, the time to 50% calcium transient decay was significantly different depending on the region paced and recorded. Acetylcholine perfusion and burst pacing-induced atrial fibrillation when pulmonary veins and appendage regions were paced but not when the free wall region was. Dantrolene (a ryanodine receptor blocker) did not reduce atrial fibrillation susceptibility.

CONCLUSION

These data provide the first evidence of heterogenous calcium signaling across the healthy left atrium. Such basal regional differences may be exacerbated during the progression of atrial fibrillation and thus play a crucial role in focal arrhythmia initiation without ryanodine receptor gating modification.

Premature atrial contractions: A predictor of atrial fibrillation and a relevant marker of atrial cardiomyopathy

An increased burden of premature atrial contractions (PACs) has long been considered a benign phenomenon. However, strong evidence of their involvement in the occurrence of atrial fibrillation (AF), ischemic stroke, and excess mortality suggests the need for management. The central question to be resolved is whether increased ectopic atrial rhythm is only a predictor of AF or whether it is a marker of atrial cardiomyopathy and therefore of ischemic stroke. After reviewing the pathophysiology of PACs and its impact on patient prognosis, this mini-review proposes to 1) detail the physiological and clinical elements linking PACs and AF, 2) present the evidence in favor of supraventricular ectopic activity as a marker of cardiomyopathy, and 3) outline the current limitations of this concept and the potential future clinical implications.

Lack of authentic atrial fibrillation in commonly used murine atrial fibrillation models

The mouse is a useful preclinical species for evaluating disease etiology due to the availability of a wide variety of genetically modified strains and the ability to perform disease-modifying manipulations. In order to establish an atrial filtration (AF) model in our laboratory, we profiled several commonly used murine AF models. We initially evaluated a pharmacological model of acute carbachol (CCh) treatment plus atrial burst pacing in C57BL/6 mice. In an effort to observe micro-reentrant circuits indicative of authentic AF, we employed optical mapping imaging in isolated mouse hearts. While CCh reduced atrial refractoriness and increased atrial tachyarrhythmia vulnerability, the left atrial (LA) excitation patterns were rather regular without reentrant circuits or wavelets. Therefore, the atrial tachyarrhythmia resembled high frequency atrial flutter, not typical AF per se. We next examined both a chronic angiotensin II (Ang II) infusion model and the surgical model of transverse aortic constriction (TAC), which have both been reported to induce atrial and ventricular structural changes that serve as a substrates for micro-reentrant AF. Although we observed some extent of atrial remodeling such as fibrosis or enlarged LA diameter, burst pacing-induced atrial tachyarrhythmia vulnerability did not differ from control mice in either model. This again suggested that an AF-like pathophysiology is difficult to demonstrate in the mouse. To continue searching for a valid murine AF model, we studied mice with a cardiac-specific deficiency (KO) in liver kinase B1 (Cardiac-LKB1), which has been reported to exhibit spontaneous AF. Indeed, the electrocardiograms (ECG) of conscious Cardiac-LKB1 KO mice exhibited no P waves and had irregular RR intervals, which are characteristics of AF. Histological evaluation of Cardiac-LKB1 KO mice revealed dilated and fibrotic atria, again consistent with AF. However, atrial electrograms and optical mapping revealed that electrical activity was limited to the sino-atrial node area with no electrical conduction into the atrial myocardium beyond. Thus, Cardiac-LKB1 KO mice have severe atrial myopathy or atrial standstill, but not AF. In summary, the atrial tachyarrhythmias we observed in the four murine models were distinct from typical human AF, which often exhibits micro- or macro-reentrant atrial circuits. Our results suggest that the four murine AF models we examined may not reflect human AF well, and raise a cautionary note for use of those murine models to study AF.

Sleep apnea and atrial fibrillation: challenges in clinical and translational research

INTRODUCTION

Sleep-disordered breathing (SDB) is present in 21-74% of all patients with atrial fibrillation (AF). Treatment of SDB by positive airway pressure may help to prevent recurrence of AF after electrical cardioversion and help to improve AF ablation success rates in non-randomized studies.

AREAS COVERED

In this review, the current understanding of the atrial arrhythmogenic pathophysiology of SDB is summarized, and diagnostic and therapeutic challenges in AF patients are discussed. Current international recommendations are presented, and a comprehensive literature search is undertaken.

EXPERT OPINION

AF patients with SDB rarely report SDB-related symptoms such as daytime sleepiness. Therefore, systematic home sleep testing evaluation should be considered for all patients eligible for rhythm control strategy. A close interdisciplinary collaboration between the electrophysiologist/cardiologist, nurses and sleep-specialists are required for the management of SDB in AF patients. An arrhythmia-orientated assessment of SDB may better quantify SDB-related AF risk in an individual patient and may help to better guide targeted and personalized SDB treatment in AF patients as a component of rhythm and symptom control strategies. Finally, randomized controlled trials are needed to confirm the relationship between SDB and AF, and the benefits of routine testing and treatment of SDB in AF patients.

Regional Diversities in Fibrogenesis Weighed as a Key Determinant for Atrial Arrhythmogenesis

Atrial fibrosis plays a key role in atrial myopathy, resulting in the genesis of atrial fibrillation (AF). The abnormal distribution of fibrotic tissue, electrical coupling, paracrine interactions, and biomechanical–electrical interactions have all been suggested as causes of fibrosis-related arrhythmogenesis. Moreover, the regional difference in fibrogenesis, specifically the left atrium (LA) exhibiting a higher arrhythmogenesis and level of fibrosis than the right atrium (RA) in AF, is a key contributor to atrial arrhythmogenesis. LA fibroblasts have greater profibrotic cellular activities than RA fibroblasts, but knowledge about the regional diversity of atrial regional fibrogenesis remains limited. This article provides a comprehensive review of research findings on the association between fibrogenesis and arrhythmogenesis from laboratory to clinical evidence and updates the current understanding of the potential mechanism underlying the difference in fibrogenesis between the LA and RA.

Electrophysiological Consequences of Cardiac Fibrosis

For both the atria and ventricles, fibrosis is generally recognized as one of the key determinants of conduction disturbances. By definition, fibrosis refers to an increased amount of fibrous tissue. However, fibrosis is not a singular entity. Various forms can be distinguished, that differ in distribution: replacement fibrosis, endomysial and perimysial fibrosis, and perivascular, endocardial, and epicardial fibrosis. These different forms typically result from diverging pathophysiological mechanisms and can have different consequences for conduction. The impact of fibrosis on propagation depends on exactly how the patterns of electrical connections between myocytes are altered. We will therefore first consider the normal patterns of electrical connections and their regional diversity as determinants of propagation. Subsequently, we will summarize current knowledge on how different forms of fibrosis lead to a loss of electrical connectivity in order to explain their effects on propagation and mechanisms of arrhythmogenesis, including ectopy, reentry, and alternans. Finally, we will discuss a histological quantification of fibrosis. Because of the different forms of fibrosis and their diverging effects on electrical propagation, the total amount of fibrosis is a poor indicator for the effect on conduction. Ideally, an assessment of cardiac fibrosis should exclude fibrous tissue that does not affect conduction and differentiate between the various types that do; in this article, we highlight practical solutions for histological analysis that meet these requirements.

LncRNA TUG1 Regulates Proliferation of Cardiac Fibroblast via the miR-29b-3p/TGF-β1 Axis

Background: Atrial fibrillation (AF) is a very common clinical arrhythmia, accompanied by the overproliferation of cardiac fibroblasts (CFs). This study aimed to investigate the role of the long non-coding RNA(lncRNA) taurine upregulated gene 1 (TUG1) in the proliferation of CFs and further investigated its underlying mechanism.

Methods: One hundred four paroxysmal AF patients and 94 healthy controls were recruited. Human cardiac fibroblasts (HCFs) were applied to establish an AF cell model through treatment with angiotensin II (AngII). qRT-PCR was used for the measurement of gene levels. The cell proliferation was detected by cell counting kit-8 (CCK-8). Luciferase reporter assay was performed for target gene analysis.

Results: Elevated levels of TUG1 and low expression of miR-29b-3p were detected in the serum of AF patients compared with the healthy controls. Pearson's correlation analysis exhibited an inverse relationship between TUG1 and miR-29b-3p expression in AF patients (r = −7.106, p < 0.001). Knockdown of TUG1 inhibited AngII-induced CF proliferation. Taurine upregulated gene 1 (TUG1) functions as a competing endogenous RNA (ceRNA) for miR-29b-3p, and downregulation of miR-29b-3p reversed the role of TUG1 in CF proliferation. TGF-β1 is a direct target gene of miR-29b-3p.

Conclusions: Long non-coding RNA taurine upregulated gene 1 is a key regulator in the occurrence of AF. Slicing TUG1 inhibits CF proliferation by regulating the miR-29b-3p/TGF-β1 axis.

Transcriptional factors in calcium mishandling and atrial fibrillation development

Healthy cardiac conduction relies on the coordinated electrical activity of distinct populations of cardiomyocytes. Disruption of cell-cell conduction results in cardiac arrhythmias, a leading cause of morbidity and mortality worldwide. Recent genetic studies have highlighted a major heritable component and identified numerous loci associated with risk of atrial fibrillation, including transcription factor genes, particularly those important in cardiac development, microRNAs, and long noncoding RNAs. Identification of such genetic factors has prompted the search to understand the mechanisms that underlie the genetic component of AF. Recent studies have found several mechanisms by which genetic alterations can result in AF formation via disruption of calcium handling. Loss of developmental transcription factors in adult cardiomyocytes can result in disruption of SR calcium ATPase, sodium calcium exchanger, calcium channels, among other ion channels, which underlie action potential abnormalities and triggered activity that can contribute to AF. This review aims to summarize the complex network of transcription factors and their roles in calcium handling.

Impact of diagnosis-to-ablation time on non-pulmonary vein triggers and ablation outcomes in persistent atrial fibrillation

INTRODUCTION

Non-pulmonary vein (PV) triggers are a major cause of atrial tachyarrhythmia (ATA) recurrence after catheter ablation. However, the effect of the diagnosis-to-ablation time (DAT) on non-PV triggers in persistent atrial fibrillation is unknown.

METHODS AND RESULTS

This observational study evaluated 502 consecutive persistent AF patients who underwent initial ablation. We compared 408 patients whose DAT was <3 years with 94 patients whose DAT was ≥3 years. Following PV and posterior wall isolation, 193 non-PV triggers, including 50 AFs, 30 atrial tachycardias (ATs), and 113 repetitive atrial premature beats, were elicited and ablated in 137 (27%) patients. Specifically, 80 non-PV AF/AT triggers were provoked in 64 (13%) patients, being identified more frequently in the DAT ≥ 3 years group than in the DAT < 3 years group (20% vs. 11%, p = .025) especially with a higher prevalence of coronary sinus/inferior left atrial triggers. During a median follow-up of 770 days, the ATA recurrence-free rate was higher in the DAT < 3 years group than the DAT ≥ 3 years group (79% vs. 53% at 2 years, p < .001). In a multivariate analysis, female sex (odds ratio: 2.70, p = .002) and a longer DAT (odds ratio: 1.13/year, p = .008) were predictors of non-PV AF/AT triggers, and a longer DAT (hazard ratio: 1.12/year, p < .001) and non-PV AT/AF triggers (hazard ratio: 1.79, p = .009) were associated with ATA recurrence.

CONCLUSION

Early ablation after the first diagnosis of persistent AF may reduce emerging non-PV AF/AT triggers and ATA recurrence.

hsa-miR-4443 inhibits myocardial fibroblast proliferation by targeting THBS1 to regulate TGF-β1/α-SMA/collagen signaling in atrial fibrillation

Fibrosis caused by the increase in extracellular matrix in cardiac fibroblasts plays an important role in the occurrence and development of atrial fibrillation (AF). The aim of this study was to investigate the role of hsa-miR-4443 in AF, human cardiac fibroblast (HCFB) proliferation, and extracellular matrix remodeling. TaqMan Stem-loop miRNA assay was used to measure hsa-miR-4443 expression in patients with persistent AF (n=123) and healthy controls (n=100). Patients with AF were confirmed to have atrial fibrosis by late gadolinium enhancement. At the cellular level, after hsa-miR-4443 mimic and inhibitor were transfected with HCFBs, proliferation, apoptosis, migration, and invasion were analyzed. Lastly, hsa-miR-4443-targeted gene and transforming growth factor (TGF)-β1/α-SMA/collagen pathway were evaluated by dual-luciferase reporter assay and western blot, respectively. In patients with AF, hsa-miR-4443 decreased significantly and collagen metabolism level increased significantly. Logistic regression analysis showed that low hsa-miR-4443 level was a risk factor of AF (P<0.001). The receiver operating characteristic curve revealed that hsa-miR-4443 was useful for predicting AF (area under the curve: 0.828, sensitivity: 0.71, specificity: 0.78, P<0.001). In HCFBs, hsa-miR-4443 targeted thrombospondin-1 (THBS1) and downregulated TGF-β1/α-SMA/collagen pathway. The inhibition of hsa-miR-4443 expression promoted HCFB proliferation, migration, invasion, myofibroblast differentiation, and collagen production. The significant reduction of hsa-miR-4443 can be used as a biomarker for AF. hsa-miR-4443 protected AF by targeting THBS1 and regulated TGF-β1/α-SMA/collagen pathway to inhibit HCFB proliferation and collagen synthesis.

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

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

Doxycycline Improves Fibrosis-Induced Abnormalities in Atrial Conduction and Vulnerability to Atrial Fibrillation in Chronic Intermittent Hypoxia Rats

Background

The structural remodeling of atrial architecture, especially increased amounts of fibrosis, is a critical substrate to atrial fibrillation (AF). Doxycycline (Doxy) has recently been shown to exert protective effects against fibrogenic response. This study investigated whether doxycycline (Doxy) can sufficiently ameliorate the fibrosis-induced changes of atrial conduction and AF vulnerability in a chronic intermittent hypoxia (CIH) rat model.

Material/Methods

Sixty rats were randomized into 3 groups: Control, CIH, and CIH with Doxy treatment (DOXY) group. CIH rats were exposed to CIH (6 h/d) and Doxy-treated rats were treated with Doxy during processing CIH. After 6 weeks, echocardiographic and hemodynamic parameters were measured. Isolated atrial epicardial activation mapping and heart electrophysiology were performed. The extent of atrial interstitial fibrosis were estimated by Masson’s trichrome staining. The expression levels of TGF-β1 and downstream factors were determined by real-Time PCR, immunohistochemistry, and Western blot analysis.

Results

Compared to Control rats, the CIH rats showed significant atrial interstitial fibrosis, longer inter-atrial conduction time, and elevated conduction inhomogeneity and AF inducibility, and the expression of TGF-β1, TGF-βRI, TGF-βRII, P-Smad2/3, α-SMA, CTGF, and Collagen I were significantly increased, whereas the velocity of atrial conduction and the expression of miR-30c were dramatically decreased. All of these changes were significantly improved by Doxy treatment.

Conclusions

The findings suggested that Doxy can profoundly mitigate atrial fibrosis, conduction inhomogeneity as well as high AF inducibility secondary to fibrosis in a CIH rat model through suppressing the TGF-β1 signaling pathway.

AGE-RAGE Stress in the Pathophysiology of Atrial Fibrillation and Its Treatment

Atrial fibrillation (AF) is the most common of cardiac arrhythmias. Mechanisms such as atrial structural remodeling and electrical remodeling have been implicated in the pathogenesis of AF. The data to date suggest that advanced glycation end products (AGEs) and its cell receptor RAGE (receptor for AGE) and soluble receptor (sRAGE) are involved in the pathogenesis of AF. This review focuses on the role of AGE-RAGE axis in the pathogenesis of AF. Interaction of AGE with RAGE generates reactive oxygen species, cytokines, and vascular cell adhesion molecules. sRAGE is a cytoprotective agent. The data show that serum levels of AGE and sRAGE, and expression of RAGE, are elevated in AF patients. Elevated levels of sRAGE did not protect the development of AF. This might be due to greater elevation of AGE than sRAGE. Measurement of AGE-RAGE stress (AGE/sRAGE) would be appropriate as compared with measurement of AGE or RAGE or sRAGE alone in AF patients. AGE and its interaction with RAGE can induce AF through alteration in cellular protein and extracellular matrix. AGE and its interaction with RAGE induce atrial structural and electrical remodeling. The treatment strategy should be directed toward reduction in AGE levels, suppression of RAGE expression, blocking of binding of AGE to RAGE, and elevation of sRAGE and antioxidants. In conclusion, AGE-RAGE axis is involved in the development of AF through atrial structural and electrical remodeling. The treatment modalities for AF should include lowering of AGE, suppression of RAGE, elevation of sRAGE, and use of antioxidants.

Region-specific parasympathetic nerve remodeling in the left atrium contributes to creation of a vulnerable substrate for atrial fibrillation

Atrial fibrillation (AF) is the most common heart rhythm disorder and a major cause of stroke. Unfortunately, current therapies for AF are suboptimal, largely because the molecular mechanisms underlying AF are poorly understood. Since the autonomic nervous system is thought to increase vulnerability to AF, we used a rapid atrial pacing (RAP) canine model to investigate the anatomic and electrophysiological characteristics of autonomic remodeling in different regions of the left atrium. RAP led to marked hypertrophy of parent nerve bundles in the posterior left atrium (PLA), resulting in a global increase in parasympathetic and sympathetic innervation throughout the left atrium. Parasympathetic fibers were more heterogeneously distributed in the PLA when compared with other left atrial regions; this led to greater fractionation and disorganization of AF electrograms in the PLA. Computational modeling revealed that heterogeneously distributed parasympathetic activity exacerbates sympathetic substrate for wave break and reentry. We further discovered that levels of nerve growth factor (NGF) were greatest in the left atrial appendage (LAA), where AF was most organized. Preferential NGF release by the LAA - likely a direct function of frequency and regularity of atrial stimulation - may have important implications for creation of a vulnerable AF substrate.

LncRNA PVT1 regulates atrial fibrosis via miR-128-3p-SP1-TGF-β1-Smad axis in atrial fibrillation

BACKGROUND

Long non-coding RNAs (lncRNA) plasmacytoma variant translocation 1 (PVT1) has been shown to be associated with liver fibrosis. Nevertheless, the role of PVT1 in atrial fibrosis remains undefined. This study aims to elucidate the pathophysiological role of lncRNA PVT1 in the regulation of atrial fibrosis and to explore the underlying mechanism.

METHODS

Expression of PVT1, miR-128-sp, and Sp1 were examined in human atrial muscle tissues and angiotensin-II (Ang-II)-induced human atrial fibroblasts. Furthermore, the role of PVT1 in regulating atrial fibrosis in Ang-II-treated human atrial fibroblasts and Ang-II-induced atrial fibrosis in mice was investigated. Moreover, the interaction among PVT1, miR-128-3p, and Sp1 were examined using bioinformatics, expression correlation analysis, gain- or loss-of-function assays, RIP assays, and luciferase reporter assays. The involvement of transforming growth factor beta 1 (TGF-β1)/Smad pathway in this process was also explored.

RESULTS

PVT1 was increased in atrial muscle tissues from AF patients and positively with collagen I and collagen III. In vitro assay revealed that PVT1 overexpression facilitated the Ang-II-induced atrial fibroblasts proliferation, collagen production, and TGF-β1/Smad signaling activation, whereas PVT1 knockdown caused the opposite effect. In vivo assay further confirmed that PVT1 knockdown attenuated the Ang-II-induced mouse atrial fibrosis. Mechanically, PVT1 acted as a sponge for miR-128-3p to facilitate Sp1 expression, thereby activating the TGF-β1/Smad signaling pathway.

CONCLUSION

LncRNA PVT1 promotes atrial fibrosis via miR-128-3p-SP1-TGF-β1-Smad axis in atrial fibrillation.

Qiliqiangxin attenuates atrial structural remodeling in prolonged pacing-induced atrial fibrillation in rabbits

Qiliqiangxin (QL) can attenuate myocardial remodeling and improve cardiac function in some cardiac diseases, including heart failure and hypertension. This study was to explore the effects and mechanism of QL on atrial structural remodeling in atrial fibrillation (AF). Twenty-one rabbits were randomly divided into a sham-operation group, pacing group (pacing with 600 beats per minute for 4 weeks), and treatment group (2.5 g/kg/day). Before pacing, the rabbits received QL-administered p.o. for 1 week. We measured atrial electrophysiological parameters in all groups to evaluate AF inducibility and the atrial effective refractory period (AERP). Echocardiography evaluated cardiac function and structure. TUNEL detection, hematoxylin and eosin (HE) staining, and Masson's trichrome staining were performed. Immunohistochemistry and western blotting (WB) were used to detect alterations in calcium channel L-type dihydropyridine receptor α2 subunit (DHPR) and fibrosis-related regulatory factors. AF inducibility was markedly decreased after QL treatment. Furthermore, we found that AERP and DHPR were reduced significantly in pacing rabbits compared with sham rabbits; treatment with QL increased DHPR and AERP compared to the pacing group. The QL group showed significantly decreased mast cell density and improved atrial ejection fraction values compared with the pacing group. Moreover, QL decreased interventricular septum thickness (IVSd) and left ventricular end-diastolic diameter (LVEDD). Compared with the sham group, the levels of TGFβ1 and P-smad2/3 were significantly upregulated in the pacing group. QL reduced TGF-β1 and P-smad2/3 levels and downstream fibrosis-related factors. Our study demonstrated that QL treatment attenuates atrial structural remodeling potentially by inhibiting TGF-β1/P-smad2/3 signaling pathway.

TGF-β1 affects cell-cell adhesion in the heart in an NCAM1-dependent mechanism

The contractile property of the myocardium is maintained by cell-cell junctions enabling cardiomyocytes to work as a syncytium. Alterations in cell-cell junctions are observed in heart failure, a disease characterized by the activation of Transforming Growth Factor beta 1 (TGFβ1). While TGFβ1 has been implicated in diverse biologic responses, its molecular function in controlling cell-cell adhesion in the heart has never been investigated. Cardiac-specific transgenic mice expressing active TGFβ1 were generated to model the observed increase in activity in the failing heart. Activation of TGFβ1 in the heart was sufficient to drive ventricular dysfunction. To begin to understand the function of this important molecule we undertook an extensive structural analysis of the myocardium by electron microscopy and immunostaining. This approach revealed that TGFβ1 alters intercalated disc structures and cell-cell adhesion in ventricular myocytes. Mechanistically, we found that TGFβ1 induces the expression of neural adhesion molecule 1 (NCAM1) in cardiomyocytes in a p38-dependent pathway, and that selective targeting of NCAM1 was sufficient to rescue the cell adhesion defect observed when cardiomyocytes were treated with TGFβ1. Importantly, NCAM1 was upregulated in human heart samples from ischemic and non-ischemic cardiomyopathy patients and NCAM1 protein levels correlated with the degree of TGFβ1 activity in the human cardiac ventricle. Overall, we found that TGFβ1 is deleterious to the heart by regulating the adhesion properties of cardiomyocytes in an NCAM1-dependent mechanism. Our results suggest that inhibiting NCAM1 would be cardioprotective, counteract the pathological action of TGFβ1 and reduce heart failure severity.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Cross-talk between macrophages and atrial myocytes in atrial fibrillation

Increased macrophage accumulation occurs in the atria of patients with atrial fibrillation (AF). However, the phenotype and functions of the macrophages in AF remain unclear. We investigated the macrophage-atrial myocyte interaction in AF patients and found that the increased macrophages were mainly pro-inflammatory macrophages (iNOS⁺, Arg1⁻). Tachypacing of HL-1 atrial myocytes also led to pro-inflammatory macrophage polarization. In addition, lipopolysaccharide (LPS)-stimulated pro-inflammatory macrophages-induced atrial electrical remodeling, evidenced by increased AF incidence and decreased atrial effective refractory period and L-type calcium currents (I_Ca-L) in both canine and mouse AF models. Depletion of macrophages relieved LPS-induced atrial electrical remodeling, confirming the role of pro-inflammatory macrophages in the pathogenesis of AF. We also found that the effect of LPS-stimulated macrophages on atrial myocytes was mediated by secretion of interleukin 1 beta (IL-1β), which inhibited atrial myocyte quaking protein (QKI) expression. IL-1β knockout in macrophages restored the LPS-stimulated macrophage-induced inhibition of QKI and CACNA1C (α1C subunit of L-type calcium channel) in atrial myocytes. Meanwhile, QKI overexpression in atrial myocytes restored the LPS-stimulated macrophage-induced electrical remodeling through enhanced binding of QKI to CACNA1C mRNA, which upregulated the expression of CACNA1C as well as I_Ca-L. In contrast, QKI knockout inhibited CACNA1C expression. Finally, using transcription factor activation profiling plate array and chromatin immunoprecipitation, we revealed that special AT-rich sequence binding protein 1 activated QKI transcription. Taken together, our study uncovered the functional interaction between macrophages and atrial myocytes in AF. AF induced pro-inflammatory macrophage polarization while pro-inflammatory macrophages exacerbated atrial electrical remodeling by secreting IL-1β, further inhibiting QKI expression in atrial myocytes, which contributed to I_Ca-L downregulation. Our study demonstrates a novel molecular mechanism underlying the pathogenesis and progression of AF and suggests that QKI is a potential therapeutic target.

Electrophysiological and molecular mechanisms of paroxysmal atrial fibrillation

Atrial fibrillation (AF) is an extremely prevalent arrhythmia that presents a wide range of therapeutic challenges. AF usually begins in a self-terminating paroxysmal form (pAF). With time, the AF pattern often evolves to become persistent (nonterminating within 7 days). Important differences exist between pAF and persistent AF in terms of clinical features, in particular the responsiveness to antiarrhythmic drugs and ablation therapy. AF mechanisms have been extensively reviewed, but few or no Reviews focus specifically on the pathophysiology of pAF. Accordingly, in this Review, we examine the available data on the electrophysiological basis for pAF occurrence and maintenance, as well as the molecular mechanisms forming the underlying substrate. We first consider the mechanistic insights that have been obtained from clinical studies in the electrophysiology laboratory, noninvasive observations, and genetic studies. We then discuss the information about underlying molecular mechanisms that has been obtained from experimental studies on animal models and patient samples. Finally, we discuss the data available from animal models with spontaneous AF presentation, their relationship to clinical findings, and their relevance to understanding the mechanisms underlying pAF. Our analysis then turns to potential factors governing cases of progression from pAF to persistent AF and the clinical implications of the basic mechanisms we review. We conclude by identifying and discussing questions that we consider particularly important to address through future research in this area.

Assessment of atrial conduction times in patients with mild diastolic dysfunction and normal atrial size

Objective:

Abnormalities in atrial electromechanical delays (EMDs) are considered independent predictors of atrial fibrillation and can reflect atrial remodeling. The main purpose in this study was to compare inter-left and right intra-atrial EMDs of patients with mild left ventricular (LV) diastolic dysfunction, without left atrial (LA) structural remodeling in the absence of high filling pressure, with healthy individuals.

Methods:

In this prospective study, a total of 41 consecutive outpatients who were referred to our echocardiography laboratory with mild diastolic dysfunction (age: 60.9±9.6 years) and 45 healthy control subjects who were referred from an outpatient clinic for check-up (age: 32.2±10.3 years) with normal diastolic function were enrolled into this study. All subjects had normal LA volume and normal right atrial area and did not have high filling pressure. Diastolic dysfunction were determined per American Society of Echocardiography recommendations; so, the following indices were measured: peak early (E) and atrial (A) flow velocities (cm/s), E/A ratio, and deceleration time (DT) (ms) of mitral inflow, systolic (S) and diastolic (D) pulmonary vein wave velocities (cm/s) by pulse wave Doppler, and e’ in septal and lateral mitral annulus by pulse wave tissue Doppler. Time interval from the onset of P wave on the ECG to the beginning of the late diastolic wave (Am wave) on tissue Doppler trace, which is named PA, was obtained from the lateral and septal mitral annulus and right ventricular (RV) tricuspid annulus as atrial conduction times (ACTs) and were named lateral PA, septal PA, and RV PA, respectively. The difference between lateral PA and septal, PA septal and RV PA was defined as left and right intra-atrial EMD, respectively. The difference between lateral PA and RV PA was defined as inter-atrial EMD. Data analysis was done by independent student’s t-test, Mann-Whitney U test, χ² test, Spearman rank order, Pearson’s correlation coefficient, and multivariate regression analysis in the appropriate site.

Results:

A, DT, S/D ratio, and E/e’ (average) were significantly lower in the control group, and E, D, E/A ratio, e’ septal, and e’ lateral wall were significantly lower in the patient group. Atrial conduction times were longer in the patient group, but in the multivariate analysis, there was no correlation between ACTs and diastolic dysfunction. There was no significant difference in left intra-atrial EMD (14.2±9.7 ms vs. 16.4±11.4 ms; p=0.336), right intra-atrial EMD (12.8±12.2 ms vs. 15.4±12.1 ms; p=0.321), and inter-atrial EMD (26.9±13.7 ms vs. 31.7±13.7 ms; p=0.108) between the two groups. Multivariate analysis showed no correlation between inter- and intra-atrial EMDs and diastolic dysfunction.

Conclusion:

There was no significant difference in ACTs and inter-atrial and left and right intra-atrial EMD in patients with mild LV diastolic dysfunction and normal LA volume in the absence high filling pressure compared with normal subjects.

Effects of Wild-Type and Mutant Forms of Atrial Natriuretic Peptide on Atrial Electrophysiology and Arrhythmogenesis

Background—

Atrial natriuretic peptide (ANP) is a hormone with numerous beneficial cardiovascular effects. Recently, a mutation in the ANP gene, which results in the generation of a mutant form of ANP (mANP), was identified and shown to cause atrial fibrillation in people. The mechanism(s) through which mANP causes atrial fibrillation is unknown. Our objective was to compare the effects of wild-type ANP and mANP on atrial electrophysiology in mice and humans.

Methods and Results—

Action potentials (APs), L-type Ca 2+ currents ( I Ca,L ), and Na + current were recorded in atrial myocytes from wild-type or natriuretic peptide receptor C knockout (NPR-C −/− ) mice. In mice, ANP and mANP (10–100 nmol/L) had opposing effects on atrial myocyte AP morphology and I Ca,L . ANP increased AP upstroke velocity ( V max ), AP duration, and I Ca,L similarly in wild-type and NPR-C −/− myocytes. In contrast, mANP decreased V max , AP duration, and I Ca,L , and these effects were completely absent in NPR-C −/− myocytes. ANP and mANP also had opposing effects on I Ca,L in human atrial myocytes. In contrast, neither ANP nor mANP had any effect on Na + current in mouse atrial myocytes. Optical mapping studies in mice demonstrate that ANP sped electric conduction in the atria, whereas mANP did the opposite and slowed atrial conduction. Atrial pacing in the presence of mANP induced arrhythmias in 62.5% of hearts, whereas treatment with ANP completely prevented the occurrence of arrhythmias.

Conclusions—

These findings provide mechanistic insight into how mANP causes atrial fibrillation and demonstrate that wild-type ANP is antiarrhythmic.

Inflammation and the pathogenesis of atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia. However, the development of preventative therapies for AF has been disappointing. The infiltration of immune cells and proteins that mediate the inflammatory response in cardiac tissue and circulatory processes is associated with AF. Furthermore, the presence of inflammation in the heart or systemic circulation can predict the onset of AF and recurrence in the general population, as well as in patients after cardiac surgery, cardioversion, and catheter ablation. Mediators of the inflammatory response can alter atrial electrophysiology and structural substrates, thereby leading to increased vulnerability to AF. Inflammation also modulates calcium homeostasis and connexins, which are associated with triggers of AF and heterogeneous atrial conduction. Myolysis, cardiomyocyte apoptosis, and the activation of fibrotic pathways via fibroblasts, transforming growth factor-β and matrix metalloproteases are also mediated by inflammatory pathways, which can all contribute to structural remodelling of the atria. The development of thromboembolism, a detrimental complication of AF, is also associated with inflammatory activity. Understanding the complex pathophysiological processes and dynamic changes of AF-associated inflammation might help to identify specific anti-inflammatory strategies for the prevention of AF.

Electrophysiology of human cardiac atrial and ventricular telocytes

Telocytes (TCs) with exceptionally long cellular processes of telopodes have been described in human epicardium to act as structural supporting cells in the heart. We examined myocardial chamber-specific TCs identified in atrial and ventricular fibroblast culture using immunocytochemistry and studied their electrophysiological property by whole-cell patch clamp. Atrial and ventricular TCs with extended telopodes and alternating podoms and podomers that expressed CD34, c-Kit and PDGFR-β were identified. These cells expressed large conductance Ca²⁺-activated K⁺ current (BK_Ca) and inwardly rectifying K⁺ current (IK_ir), but not transient outward K⁺ current (I_to) and ATP-sensitive potassium current (K_ATP). The active channels were functionally competent with demonstrated modulatory response to H₂S and transforming growth factor (TGF)-β1 whereby H₂S significantly inhibited the stimulatory effect of TGF-β1 on current density of both BK_Ca and IK_ir. Furthermore, H₂S attenuated TGF-β1-stimulated KCa1.1/Kv1.1 (encode BK_Ca) and Kir2.1 (encode IK_ir) expression in TCs. Our results show that functionally competent K⁺ channels are present in human atrial and ventricular TCs and their modulation may have significant implications in myocardial physiopathology.

Cellular and molecular electrophysiology of atrial fibrillation initiation, maintenance, and progression

Atrial fibrillation (AF) is the most common clinically relevant arrhythmia and is associated with increased morbidity and mortality. The incidence of AF is expected to continue to rise with the aging of the population. AF is generally considered to be a progressive condition, occurring first in a paroxysmal form, then in persistent, and then long-standing persistent (chronic or permanent) forms. However, not all patients go through every phase, and the time spent in each can vary widely. Research over the past decades has identified a multitude of pathophysiological processes contributing to the initiation, maintenance, and progression of AF. However, many aspects of AF pathophysiology remain incompletely understood. In this review, we discuss the cellular and molecular electrophysiology of AF initiation, maintenance, and progression, predominantly based on recent data obtained in human tissue and animal models. The central role of Ca(2+)-handling abnormalities in both focal ectopic activity and AF substrate progression is discussed, along with the underlying molecular basis. We also deal with the ionic determinants that govern AF initiation and maintenance, as well as the structural remodeling that stabilizes AF-maintaining re-entrant mechanisms and finally makes the arrhythmia refractory to therapy. In addition, we highlight important gaps in our current understanding, particularly with respect to the translation of these concepts to the clinical setting. Ultimately, a comprehensive understanding of AF pathophysiology is expected to foster the development of improved pharmacological and nonpharmacological therapeutic approaches and to greatly improve clinical management.

Impact of Metabolic Syndrome on Left Atrial Electroanatomical Remodeling and Outcomes After Radiofrequency Ablation of Nonvalvular Atrial Fibrillation

Background—

Recent studies reported worse outcomes after atrial fibrillation (AF) ablation in patients with metabolic syndrome (MetS). However, mechanisms of AF recurrence in MetS remain unclear.

Method and Results—

We performed pulmonary vein isolation and voltage mapping in 236 patients with AF (age 61±9.6 years; persistent AF 64%; MetS 54%). Left atrial (LA) low voltage areas were semiquantitatively estimated and presented as low voltage index. MetS was defined according to National Cholesterol Education Program Adult Treatment Panel III. Follow-up for AF recurrence ≤12 months was performed. LA low voltage areas were observed in 46% of patients with MetS versus 8.2% patients without MetS ; P <0.0001. MetS was an independent predictor of LA low voltage areas: odds ratio, 11.64; 95% confidence interval, 4.381–30.903; P <0.0001. Observed AF recurrence at 12 months was 42.7% in MetS versus 36.1% in the non-MetS group ( P =0.303). The presence of LA low voltage areas was a predictor of 12-month AF recurrence: odds ratio, 2.99; 95% confidence interval, 1.36–6.56; P =0.006. Probability of 12-month AF recurrence increased with 84.5% for every unit of low voltage Index.

Conclusions—

MetS was not associated with worse outcomes after radiofrequency catheter ablation of AF, but LA low voltage areas were more frequently observed in patients with MetS. The presence and extent of LA low voltage areas may influence the long-term outcomes after catheter ablation.

TGF-β1 signal pathway in the regulation of inflammation in patients with atrial fibrillation

OBJECTIVE

To observe the expression changes of inflammatory markers TGF-β1, Smad3 and IL-6 in patients with atrial fibrillation (AF), and to explore the significance of TGF-β1 signaling pathway in the structural remodeling of AF.

METHODS

The expression of TGF-β1, Smad3 and IL-6 in 50 cases with AF and 30 normal cases were detected by RT-PCR and ELISA.

RESULTS

The TGF-β1, Smad3 and IL-6 mRNA and protein expression levels in patients with AF were significantly higher than that in the control group (P<0.05), but there was no significantly different between the paroxysmal AF group and the persistent AF group (P>0.05). The TGF-β1mRNA expression in the ⩾ 50 years subgroup was significantly higher than that in the <50 years subgroups, and it was higher in the NYHA III subgroup than in the I/II grade subgroup. It was also higher in the left ventricular ejection fraction (LVEF) <50% subgroup than in LVEF ⩾ 50% group, and it was significantly higher in the AF time ⩾ 36 months subgroup than that in <36 months subgroup (P<0.05). The Smad3 and IL-6 expressions in the in the LVEF <50% subgroup were both high that than that in LVEF ⩾ 50% group, and higher in the AF time ⩾ 36 months subgroup than that in <36 months subgroup (P<0.05). There were a positive correlation between TGF-β1, Smad3 and IL-6 (r=0.687, r=0.547). There were also a positive correlation between Smad3 and IL-6 mRNA (r=0.823).

CONCLUSIONS

AF is associated with inflammation, and the inflammation is also involved in the fibrillation and sustain of AF. The TGF-β1 signal pathway may be involved in the process of atrial structural remodeling in patients with AF, and iss related with the occurrence and maintenance of AF.

Role of the autonomic nervous system in atrial fibrillation: pathophysiology and therapy

Autonomic nervous system activation can induce significant and heterogeneous changes of atrial electrophysiology and induce atrial tachyarrhythmias, including atrial tachycardia and atrial fibrillation (AF). The importance of the autonomic nervous system in atrial arrhythmogenesis is also supported by circadian variation in the incidence of symptomatic AF in humans. Methods that reduce autonomic innervation or outflow have been shown to reduce the incidence of spontaneous or induced atrial arrhythmias, suggesting that neuromodulation may be helpful in controlling AF. In this review, we focus on the relationship between the autonomic nervous system and the pathophysiology of AF and the potential benefit and limitations of neuromodulation in the management of this arrhythmia. We conclude that autonomic nerve activity plays an important role in the initiation and maintenance of AF, and modulating autonomic nerve function may contribute to AF control. Potential therapeutic applications include ganglionated plexus ablation, renal sympathetic denervation, cervical vagal nerve stimulation, baroreflex stimulation, cutaneous stimulation, novel drug approaches, and biological therapies. Although the role of the autonomic nervous system has long been recognized, new science and new technologies promise exciting prospects for the future.

TheTGFB1 functional polymorphism rs1800469 and susceptibility to atrial fibrillation in two Chinese Han populations

Transforming growth factor-β1 (TGF-β1) is related to the degree of atrial fibrosis and plays critical roles in the induction and perpetuation of atrial fibrillation (AF). To investigate the association of the common promoter polymorphism rs1800469 in the TGF-β1 gene (TGFB1) with the risk of AF in Chinese Han population, we carried out a case-control study of two hospital-based independent populations: Southeast Chinese population (581 patients with AF and 723 controls), and Northeast Chinese population (308 AF patients and 292 controls). Two hundred and seventy-eight cases of AF were lone AF and 334 cases of AF were diagnosed as paroxysmal AF. In both populations, AF patients had larger left atrial diameters than the controls did. The rs1800469 genotypes in the TGFB1 gene were determined by polymerase chain reaction-restriction fragment length polymorphism. The genotype and allele frequencies of rs1800469 were not different between AF patients and controls of the Southeast Chinese population, Northeast Chinese population, and total Study Population. After adjustment for age, sex, hypertension and LAD, there was no association between the rs1800469 polymorphism and the risk of AF under the dominant, recessive and additive genetic models. Similar results were obtained from subanalysis of the lone and paroxymal AF subgroups. Our results do not support the role of the TGFB1 rs1800469 functional gene variant in the development of AF in the Chinese Han population.

Hydrogen sulphide suppresses human atrial fibroblast proliferation and transformation to myofibroblasts

Cardiac fibroblasts are crucial in pathophysiology of the myocardium whereby their aberrant proliferation has significant impact on cardiac function. Hydrogen sulphide (H₂S) is a gaseous modulator of potassium channels on cardiomyocytes and has been reported to attenuate cardiac fibrosis. Yet, the mechanism of H₂S in modulating proliferation of cardiac fibroblasts remains poorly understood. We hypothesized that H₂S inhibits proliferative response of atrial fibroblasts through modulation of potassium channels. Biophysical property of potassium channels in human atrial fibroblasts was examined by whole-cell patch clamp technique and their cellular proliferation in response to H₂S was assessed by BrdU assay. Large conductance Ca²⁺-activated K⁺ current (BK_Ca), transient outward K⁺ current (I_to) and inwardly rectifying K⁺ current (IK_ir) were found in human atrial fibroblasts. Current density of BK_Ca (IC₅₀ = 69.4 μM; n = 6), I_to (IC₅₀ = 55.1 μM; n = 6) and IK_ir (IC₅₀ = 78.9 μM; n = 6) was significantly decreased (P < 0.05) by acute exposure to NaHS (a H₂S donor) in atrial fibroblasts. Furthermore, NaHS (100–500 μM) inhibited fibroblast proliferation induced by transforming growth factor-β1 (TGF-β1; 1 ng/ml), Ang II (100 nM) or 20% FBS. Pre-conditioning of fibroblasts with NaHS decreased basal expression of Kv4.3 (encode I_to), but not KCa1.1 (encode BK_Ca) and Kir2.1 (encode IK_ir). Furthermore, H₂S significantly attenuated TGF-β1–stimulated Kv4.3 and α-smooth muscle actin expression, which coincided with its inhibition of TGF-β–induced myofibroblast transformation. Our results show that H₂S attenuates atrial fibroblast proliferation via suppression of K⁺ channel activity and moderates their differentiation towards myofibroblasts.

Atrial fibrosis: a risk stratifier for atrial fibrillation

Atrial fibrillation (AF), especially persistent and long-standing persistent AF, may result in electro-anatomical changes in the left atrium, resulting in remodeling and deposition of fibrous tissue. There are emerging data that atrial substrate modification may increase the risk of thromboembolic complications, including stroke. Several studies have reported that atrial fibrosis is due to complex interactions among several cellular and neurohumoral mediators. Late gadolinium enhancement MRI has been reported to allow quantitative assessment of myocardial fibrosis in patients at risk of developing a stroke. Current stroke risk stratification criteria for AF do not utilize atrial fibrosis as an independent risk factor despite its association with AF and stroke. Further research is required in developing adequate risk stratification tools for predicting the stroke risk and catheter ablation outcomes in AF.

Cardiac electrophysiology in mice: a matter of size

Over the last decade, mouse models have become a popular instrument for studying cardiac arrhythmias. This review assesses in which respects a mouse heart is a miniature human heart, a suitable model for studying mechanisms of cardiac arrhythmias in humans and in which respects human and murine hearts differ. Section I considers the issue of scaling of mammalian cardiac (electro) physiology to body mass. Then, we summarize differences between mice and humans in cardiac activation (section II) and the currents underlying the action potential in the murine working myocardium (section III). Changes in cardiac electrophysiology in mouse models of heart disease are briefly outlined in section IV, while section V discusses technical considerations pertaining to recording cardiac electrical activity in mice. Finally, section VI offers general considerations on the influence of cardiac size on the mechanisms of tachy-arrhythmias.

An introduction to murine models of atrial fibrillation

Understanding the mechanism of re-entrant arrhythmias in the past 30 years has allowed the development of almost curative therapies for many rhythm disturbances. The complex, polymorphic arrhythmias of atrial fibrillation (AF) and sudden death are, unfortunately, not yet well understood, and hence still in need of adequate therapy. AF contributes markedly to morbidity and mortality in aging Western populations. In the past decade, many genetically altered murine models have been described and characterized. Here, we review genetically altered murine models of AF; powerful tools that will enable a better understanding of the mechanisms of AF and the assessment of novel therapeutic interventions.