Down-regulation of L-type calcium channel and sarcoplasmic reticular Ca(2+)-ATPase mRNA in human atrial fibrillation without significant change in the mRNA of ryanodine receptor, calsequestrin and phospholamban: an insight into the mechanism of atrial electrical remodeling

Environmental Exposome and Atrial Fibrillation: Emerging Evidence and Future Directions

There has been increased awareness of the linkage between environmental exposures and cardiovascular health and disease. Atrial fibrillation is the most common sustained cardiac arrhythmia, affecting millions of people worldwide and contributing to substantial morbidity and mortality. Although numerous studies have explored the role of genetic and lifestyle factors in the development and progression of atrial fibrillation, the potential impact of environmental determinants on this prevalent condition has received comparatively less attention. This review aims to provide a comprehensive overview of the current evidence on environmental determinants of atrial fibrillation, encompassing factors such as air pollution, temperature, humidity, and other meteorologic conditions, noise pollution, greenspace, and the social environment. We discuss the existing evidence from epidemiological and mechanistic studies, critically evaluating the strengths and limitations of these investigations and the potential underlying biological mechanisms through which environmental exposures may affect atrial fibrillation risk. Furthermore, we address the potential implications of these findings for public health and clinical practice and identify knowledge gaps and future research directions in this emerging field.

Echocardiographic evaluation of the right atrial size and function: Relevance for clinical practice

Right atrial (RA) structural and functional evaluations have recently emerged as powerful biomarkers for adverse events in various cardiovascular conditions. Quantitative analysis of the right atrium, usually performed with volume changes or speckle-tracking echocardiography (STE), has markedly changed our understanding of RA function and remodeling. Knowledge of reference echocardiographic values and measurement methods of RA volumes and myocardial function is a prerequisite to introduce RA quantitation in the clinical routine. This review describes the methodology, benefits and pitfalls of measuring RA size and function by echocardiography based on the current understanding of right atrial anatomy and physiological function and provides the current knowledge of right atrial function in related cardiac diseases.

Spatial Distribution of Calcium Sparks Determines Their Ability to Induce Afterdepolarizations in Human Atrial Myocytes

Analysis of the spatio-temporal distribution of calcium sparks showed a preferential increase in sparks near the sarcolemma in atrial myocytes from patients with atrial fibrillation (AF), linked to higher ryanodine receptor (RyR2) phosphorylation at s2808 and lower calsequestrin-2 levels. Mathematical modeling, incorporating modulation of RyR2 gating, showed that only the observed combinations of RyR2 phosphorylation and calsequestrin-2 levels can account for the spatio-temporal distribution of sparks in patients with and without AF. Furthermore, we demonstrate that preferential calcium release near the sarcolemma is key to a higher incidence and amplitude of afterdepolarizations in atrial myocytes from patients with AF.

Pharmacological nNOS inhibition modified small-conductance Ca2+-activated K+ channel without altering Ca2+ dynamics

Atrial fibrillation (AF) is associated with electrical remodeling processes that promote a substrate for the maintenance of AF. Although the small-conductance Ca²⁺-activated K⁺ (SK) channel is a key factor in atrial electrical remodeling, the mechanism of its activation remains unclear. Regional nitric oxide (NO) production by neuronal nitric oxide synthase (nNOS) is involved in atrial electrical remodeling. In this study, atrial tachyarrhythmia (ATA) induction and optical mapping were performed on perfused rat hearts. nNOS is pharmacologically inhibited by S-methylthiocitrulline (SMTC). The influence of the SK channel was examined using a specific channel inhibitor, apamin (APA). Parameters such as action potential duration (APD), conduction velocity, and calcium transient (CaT) were evaluated using voltage and calcium optical mapping. The dominant frequency was examined in the analysis of AF dynamics. SMTC (100 nM) increased the inducibility of ATA and apamin (100 nM) mitigated nNOS inhibition-induced arrhythmogenicity. SMTC caused abbreviations and enhanced the spatial dispersion of APD, which was reversed by apamin. By contrast, conduction velocity and other parameters associated with CaT were not affected by SMTC or apamin administration. Apamin reduced the frequency of SMTC-induced ATA. In summary, nNOS inhibition abbreviates APD by modifying the SK channels. A specific SK channel blocker, apamin, mitigated APD abbreviation without alteration of CaT, implying an underlying mechanism of posttranslational modification of SK channels.NEW & NOTEWORTHY We demonstrated that pharmacological nNOS inhibition increased the atrial arrhythmia inducibility and a specific small-conductance Ca²⁺-activated K⁺ channel blocker, apamin, reversed the enhanced atrial arrhythmia inducibility. Apamin mitigated APD abbreviation without alteration of Ca²⁺ transient, implying an underlying mechanism of posttranslational modification of SK channels.

Higher Na+-Ca2+ Exchanger Function and Triggered Activity Contribute to Male Predisposition to Atrial Fibrillation

Male sex is one of the most important risk factors of atrial fibrillation (AF), with the incidence in men being almost double that in women. However, the reasons for this sex difference are unknown. Accordingly, in this study, we sought to determine whether there are sex differences in intracellular Ca2+ homeostasis in mouse atrial myocytes that might help explain male predisposition to AF. AF susceptibility was assessed in male (M) and female (F) mice (4–5 months old) using programmed electrical stimulation (EPS) protocols. Males were 50% more likely to develop AF. The Ca2+ transient amplitude was 28% higher in male atrial myocytes. Spontaneous systolic and diastolic Ca2+ releases, which are known sources of triggered activity, were significantly more frequent in males than females. The time to 90% decay of Ca2+ transient was faster in males. Males had 54% higher Na+-Ca2+ exchanger (NCX1) current density, and its expression was also more abundant. L-type Ca2+ current (ICaL) was recorded with and without BAPTA, a Ca2+ chelator. ICaL density was lower in males only in the absence of BAPTA, suggesting stronger Ca2+-dependent inactivation in males. CaV1.2 expression was similar between sexes. This study reports major sex differences in Ca2+ homeostasis in mouse atria, with larger Ca2+ transients and enhanced NCX1 function and expression in males resulting in more spontaneous Ca2+ releases. These sex differences may contribute to male susceptibility to AF by promoting triggered activity.

The function and regulation of calsequestrin-2: implications in calcium-mediated arrhythmias

Cardiac arrhythmias are life-threatening events in which the heart develops an irregular rhythm. Mishandling of Ca²⁺ within the myocytes of the heart has been widely demonstrated to be an underlying mechanism of arrhythmogenesis. This includes altered function of the ryanodine receptor (RyR2)-the primary Ca²⁺ release channel located to the sarcoplasmic reticulum (SR). The spontaneous leak of SR Ca²⁺ via RyR2 is a well-established contributor in the development of arrhythmic contractions. This leak is associated with increased channel activity in response to changes in SR Ca²⁺ load. RyR2 activity can be regulated through several avenues, including interactions with numerous accessory proteins. One such protein is calsequestrin-2 (CSQ2), which is the primary Ca²⁺-buffering protein within the SR. The capacity of CSQ2 to buffer Ca²⁺ is tightly associated with the ability of the protein to polymerise in response to changing Ca²⁺ levels. CSQ2 can itself be regulated through phosphorylation and glycosylation modifications, which impact protein polymerisation and trafficking. Changes in CSQ2 modifications are implicated in cardiac pathologies, while mutations in CSQ2 have been identified in arrhythmic patients. Here, we review the role of CSQ2 in arrhythmogenesis including evidence for the indirect and direct regulation of RyR2 by CSQ2, and the consequences of a loss of functional CSQ2 in Ca²⁺ homeostasis and Ca²⁺-mediated arrhythmias.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-021-00914-6.

Abnormal Calcium Handling in Atrial Fibrillation Is Linked to Changes in Cyclic AMP Dependent Signaling

Both, the decreased L-type Ca²⁺ current (I_Ca,L) density and increased spontaneous Ca²⁺ release from the sarcoplasmic reticulum (SR), have been associated with atrial fibrillation (AF). In this study, we tested the hypothesis that remodeling of 3′,5′-cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) signaling is linked to these compartment-specific changes (up- or down-regulation) in Ca²⁺-handling. Perforated patch-clamp experiments were performed in atrial myocytes from 53 patients with AF and 104 patients in sinus rhythm (Ctl). A significantly higher frequency of transient inward currents (I_TI) activated by spontaneous Ca²⁺ release was confirmed in myocytes from AF patients. Next, inhibition of PKA by H-89 promoted a stronger effect on the I_TI frequency in these myocytes compared to myocytes from Ctl patients (7.6-fold vs. 2.5-fold reduction), while the β-agonist isoproterenol (ISO) caused a greater increase in Ctl patients (5.5-fold vs. 2.1-fold). I_Ca,L density was larger in myocytes from Ctl patients at baseline (p < 0.05). However, the effect of ISO on I_Ca,L density was only slightly stronger in AF than in Ctl myocytes (3.6-fold vs. 2.7-fold). Interestingly, a significant reduction of I_Ca,L and Ca²⁺ sparks was observed upon Ca²⁺/Calmodulin-dependent protein kinase II inhibition by KN-93, but this inhibition had no effect on I_TI. Fluorescence resonance energy transfer (FRET) experiments showed that although AF promoted cytosolic desensitization to β-adrenergic stimulation, ISO increased cAMP to similar levels in both groups of patients in the L-type Ca²⁺ channel and ryanodine receptor compartments. Basal cAMP signaling also showed compartment-specific regulation by phosphodiesterases in atrial myocytes from 44 Ctl and 43 AF patients. Our results suggest that AF is associated with opposite changes in compartmentalized PKA/cAMP-dependent regulation of I_Ca,L (down-regulation) and I_TI (up-regulation).

Influence of sex on intracellular calcium homeostasis in patients with atrial fibrillation

Aims

Atrial fibrillation (AF) has been associated with intracellular calcium disturbances in human atrial myocytes, but little is known about the potential influence of sex and we here aimed to address this issue.

Methods and results

Alterations in calcium regulatory mechanisms were assessed in human atrial myocytes from patients without AF or with long-standing persistent or permanent AF. Patch-clamp measurements revealed that L-type calcium current (I_Ca) density was significantly smaller in males with than without AF (−1.15 ± 0.37 vs. −2.06 ± 0.29 pA/pF) but not in females with AF (−1.88 ± 0.40 vs. −2.21 ± 0.0.30 pA/pF). In contrast, transient inward currents (I_Ti) were more frequent in females with than without AF (1.92 ± 0.36 vs. 1.10 ± 0.19 events/min) but not in males with AF. Moreover, confocal calcium imaging showed that females with AF had more calcium spark sites than those without AF (9.8 ± 1.8 vs. 2.2 ± 1.9 sites/µm²) and sparks were wider (3.0 ± 0.3 vs. 2.2 ± 0.3 µm) and lasted longer (79 ± 6 vs. 55 ± 8 ms), favouring their fusion into calcium waves that triggers I_TIs and afterdepolarizations. This was linked to higher ryanodine receptor phosphorylation at s2808 in women with AF, and inhibition of adenosine A_2A or beta-adrenergic receptors that modulate s2808 phosphorylation was able to reduce the higher incidence of I_TI in women with AF.

Conclusion

Perturbations of the calcium homoeostasis in AF is sex-dependent, concurring with increased spontaneous SR calcium release-induced electrical activity in women but not in men, and with diminished I_Ca density in men only.

WNT signaling in atrial fibrillation

The Wnt signaling pathway regulates physiological processes such as cell proliferation and differentiation, cell fate decisions, and stem cell maintenance and, thus, plays essential roles in embryonic development, but also in adult tissue homeostasis and repair. The Wnt signaling pathway has been associated with heart development and repair and has been shown to be crucially involved in proliferation and differentiation of progenitor cells into cardiomyocytes. The investigation of the role of the Wnt signaling pathway and the regulation of its expression/activity in atrial fibrillation has only just begun. The present minireview (I) provides original data regarding the expression of Wnt signaling components in atrial tissue of patients with atrial fibrillation or sinus rhythm and (II) summarizes the current state of knowledge of the regulation of Wnt signaling components' expression/activity and the contribution of the various levels of the Wnt signal transduction pathway to the processes of the development, maintenance, and progression of atrial fibrillation.

Proteomic Analysis of Atrial Appendages Revealed the Pathophysiological Changes of Atrial Fibrillation

Atrial fibrillation (AF), known as the most common arrhythmia in the developed world, affects 1.5–2.0% of the population. Numerous basic studies have been carried out to identify the roles of electric and structural remodeling in the pathophysiological changes of AF, but more explorations are required to further understand the mechanisms of AF development. Proteomics enables researchers to identify protein alterations responsible for the pathological developing progresses of diseases. Compared to the genome, the proteome is closely related to the disease phenotype and can better manifest the progression of diseases. In this study, AF patients proteomically analyzed to identify possible mechanisms. Totally 20 patients undergoing cardiac surgery (10 with paroxysmal AF and 10 with persistent AF) and 10 healthy subjects were recruited. The differentially expressed proteins identified here included AKR1A1, LYZ, H2AFY, DDAH1, FGA, FGB, LAMB1, LAMC1, MYL2, MYBPC3, MYL5, MYH10, HNRNPU, DKK3, COPS7A, YWHAQ, and PAICS. These proteins were mainly involved in the development of structural remodeling. The differently expressed proteins may provide a new perspective for the pathological process of AF, and may enable useful targets for drug interference. Nevertheless, more research in terms of multi-omics is required to investigate possible implicated molecular pathways of AF development.

Examining Cardiomyocyte Dysfunction Using Acute Chemical Induction of an Ageing Phenotype

Much effort is focussed on understanding the structural and functional changes in the heart that underlie age-dependent deterioration of cardiac performance. Longitudinal studies, using aged animals, have pinpointed changes occurring to the contractile myocytes within the heart. However, whilst longitudinal studies are important, other experimental approaches are being advanced that can recapitulate the phenotypic changes seen during ageing. This study investigated the induction of an ageing cardiomyocyte phenotypic change by incubation of cells with hydroxyurea for several days ex vivo. Hydroxyurea incubation has been demonstrated to phenocopy age- and senescence-induced changes in neurons, but its utility for ageing studies with cardiac cells has not been examined. Incubation of neonatal rat ventricular myocytes with hydroxyurea for up to 7 days replicated specific aspects of cardiac ageing including reduced systolic calcium responses, increased alternans and a lesser ability of the cells to follow electrical pacing. Additional functional and structural changes were observed within the myocytes that pointed to ageing-like remodelling, including lipofuscin granule accumulation, reduced mitochondrial membrane potential, increased production of reactive oxygen species, and altered ultrastructure, such as mitochondria with disrupted cristae and disorganised myofibres. These data highlight the utility of alternative approaches for exploring cellular ageing whilst avoiding the costs and co-morbid factors that can affect longitudinal studies.

Biomarkers Associated with Atrial Fibrosis and Remodeling

Atrial fibrillation is the most common rhythm disturbance encountered in clinical practice. Although often considered as solely arrhythmic in nature, current evidence has established that atrial myopathy constitutes both the substrate and the outcome of atrial fibrillation, thus initiating a vicious, self-perpetuating cycle. This myopathy is triggered by stress-induced (including pressure/volume overload, inflammation, oxidative stress) responses of atrial tissue, which in the long term become maladaptive, and combine elements of both structural, especially fibrosis, and electrical remodeling, with contemporary approaches yielding potentially useful biomarkers of these processes. Biomarker value becomes greater given the fact that they can both predict atrial fibrillation occurrence and treatment outcome. This mini-review will focus on the biomarkers of atrial remodeling (both electrical and structural) and fibrosis that have been validated in human studies, including biochemical, histological and imaging approaches.

Cardiac arrhythmias secondary to hormone therapy in trans women

Introduction: With greater social acceptance and the evolution of transgender medicine as a specialty, more trans women are seeking hormone therapy (HT). Several studies have identified an increase in cardiovascular disease in trans women, however no studies have investigated the incidence of arrhythmias. Using two cases from the authors' clinic as examples, we propose that hormone therapy in trans women may increase the risk of cardiac arrhythmias. Areas covered: A literature search of sex hormones and cardiac arrhythmias was conducted. Using sex hormone studies completed in cis individuals and animal models we identified several similarities to trans women on HT. In cis men, low levels of testosterone are associated with increased rates of atrial fibrillation and right ventricular outflow tract arrhythmias. The role of estradiol remains less clear but there is evidence to suggest that the administration of exogenous estrogen may increase the rates of cardiac arrhythmias in cis women. Expert opinion: Research in the field of transgender medicine is expanding. As more trans women initiate HT, we will have a larger database from which to collect information regarding the benefits and risks of treatment, including the potential side effect of arrhythmias.

Characterization of circRNA‑associated ceRNA networks in patients with nonvalvular persistent atrial fibrillation

Circular RNAs (circRNAs) are non-coding RNAs forming closed-loop structures, and their aberrant expression may lead to disease. However, the potential network of circRNA‑associated competing endogenous RNA (ceRNA) involved in nonvalvular persistent atrial fibrillation (NPAF) has not been previously reported. In the present study, four left atrial appendages (LAA) of patients with NPAF and four normal LAAs were examined via RNA sequencing, and their potential functions were investigated via bioinformatics analysis. The circRNA‑enriched genes were analyzed using Gene Ontology (GO) categories, while the enrichment of circRNAs was detected via the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. A total of 296 significantly dysregulated circRNA transcripts were obtained, with 238 upregulated and 58 downregulated. A number of circRNAs were further confirmed using reverse transcription‑quantitative polymerase chain reaction analysis. Furthermore, the more comprehensive circRNA‑associated ceRNA networks were examined in patients with NPAF. GO categories and KEGG annotation analysis of circRNAs revealed that the circRNA‑associated ceRNA networks were likely to influence AF though alterations in calcium and cardiac muscle contraction. The circRNA‑associated ceRNA networks revealed that dysregulated circRNAs in NPAF may be involved in regulating hsa‑microRNA (miR)‑208b and hsa‑miR‑21. To the best of our knowledge, this study presents the circRNA‑associated ceRNA networks in NPAF for the first time, which may have potential implications for the pathogenesis of AF. This study reveals a potential perspective from which to investigate circRNAs in circRNA‑associated ceRNA networks (hsa_circRNA002085, hsa_circRNA001321) in NPAF, and provides a potential biomarker for AF.

Calcium in the Pathophysiology of Atrial Fibrillation and Heart Failure

Atrial fibrillation (AF) is commonly associated with heart failure. A bidirectional relationship exists between the two-AF exacerbates heart failure causing a significant increase in heart failure symptoms, admissions to hospital and cardiovascular death, while pathological remodeling of the atria as a result of heart failure increases the risk of AF. A comprehensive understanding of the pathophysiology of AF is essential if we are to break this vicious circle. In this review, the latest evidence will be presented showing a fundamental role for calcium in both the induction and maintenance of AF. After outlining atrial electrophysiology and calcium handling, the role of calcium-dependent afterdepolarizations and atrial repolarization alternans in triggering AF will be considered. The atrial response to rapid stimulation will be discussed, including the short-term protection from calcium overload in the form of calcium signaling silencing and the eventual progression to diastolic calcium leak causing afterdepolarizations and the development of an electrical substrate that perpetuates AF. The role of calcium in the bidirectional relationship between heart failure and AF will then be covered. The effects of heart failure on atrial calcium handling that promote AF will be reviewed, including effects on both atrial myocytes and the pulmonary veins, before the aspects of AF which exacerbate heart failure are discussed. Finally, the limitations of human and animal studies will be explored allowing contextualization of what are sometimes discordant results.

Low testosterone levels are predictive for incident atrial fibrillation and ischaemic stroke in men, but protective in women - results from the FINRISK study

Background Atrial fibrillation is the most common serious abnormal heart rhythm, and a frequent cause of ischaemic stroke. Recent experimental studies, mainly in orchiectomised rats, report a relationship between sex hormones and atrial electrophysiology and electroanatomy. We aimed to evaluate whether low testosterone levels are predictive for atrial fibrillation and/or ischaemic stroke in men and women. Design and methods The serum total testosterone levels were measured at baseline in a population cohort of 7892 subjects (3876 male, 4016 female), aged 25-74 years, using a commercially available immunoassay. The main outcome measure was atrial fibrillation or ischaemic stroke, whichever came first. Results During a median follow-up of 13.8 years, a total of 629 subjects (8.0%) suffered from incident atrial fibrillation ( n = 426) and/or ischemic stroke ( n = 276). Cox regression analyses, adjusted for age (used as time-scale), geographical region, total cholesterol (log), high-density lipoprotein-cholesterol (log), hypertension medication, known diabetes, smoking status, waist-hip-ratio, and time of blood drawn, documented differential predictive value of low sex-specific testosterone levels for atrial fibrillation and/or ischaemic stroke, in men and in women: Increasing levels were associated with lower risk in men (hazard ratio per one nmol/l increase 0.98 (95% confidence interval 0.93-1.00); p = 0.049). On the other hand, increasing testosterone levels were associated with higher risk in women (hazard ratio per one nmol/l increase 1.17 (95% confidence interval 1.02-1.36); p = 0.031). Conclusion Our study indicates that low testosterone levels are associated with increased risk of future atrial fibrillation and/or ischaemic stroke in men, while they are protective in women.

Probucol prevents atrial ion channel remodeling in an alloxan-induced diabetes rabbit model

Diabetes mellitus (DM) increases the risk of developing atrial fibrillation (AF), but the molecular mechanisms of diabetes-induced atrial remodeling processes have not been fully characterized. The aim of this study was to examine the mechanisms underlying atrial ion channel remodeling in alloxan-induced diabetes model in rabbits. A total of 40 Japanese rabbits were randomly assigned to a control group (C), alloxan-induced diabetic group (DM), probucol-treated control group (Control-P), and probucol-treated diabetic group (DM-P). Using whole-cell voltage-clamp techniques, ICa,L, INa and action potential durations (APDs) were measured in cardiomyocytes isolated from the left atria in the four groups, respectively. In the DM group, increased Ica,L and decreased INa currents were reflected in prolonged APD90 and APD50 values. These changes were reversed in the DM-P group. In conclusion, probucol cured AF by alleviating the ion channel remodeling of atrial myocytes in the setting of diabetes and the promising therapeutic potential of anti-oxidative compounds in the treatment of AF warrants further study.

Reactive oxygen species mediated oxidative stress links diabetes and atrial fibrillation

Diabetes is an independent risk factor for atrial fibrillation (AF); however, the underlying mechanism linking diabetes and AF remains to be clarified. The present study aimed to explore the molecular mechanism of increased reactive oxygen species (ROS) production in AF and the ROS-mediated downstream events in diabetes. Firstly, the atrial fibroblasts were isolated from the left atrium of rabbits using enzyme digestion and differential adhesion. Then, the isolated cells were identified by morphology analysis under a microscope, collagen distribution using Masson trichrome staining and vimentin by immunofluorescence. Following this, the collected atrial fibroblasts were randomly divided into 7 groups and administered with high glucose (25 mM glucose), H₂O₂ stimulation (100 nmol/l), glucose + apocynin (100 µg/ml), H₂O₂ + apocynin, glucose + H₂O₂, and a combination of glucose, apocynin and H₂O₂, as well as the negative control (NC). An MTS assay was performed to investigate cell proliferation following the different treatments, and western blotting was conducted to explore the expression of several proteins including NAD(P)H oxidative (NOX) subunits, key factors involved in mitogen-activated protein kinase (MAPK) signaling pathways and matrix metalloproteinases (MMPs). The atrial fibroblasts were spindle-shaped with one or more protuberances. Vimentin was positively expressed in collected cells under confocal laser scanning microscopy. This result indicated that the atrial fibroblasts were successfully prepared. High glucose and H₂O₂ stimulation significantly increased the proliferation of atrial fibroblasts and apocynin markedly attenuated the promoting effects on cell proliferation induced by high glucose and H₂O₂ treatment (P<0.05). Additionally, high glucose and H₂O₂ stimulation increased the expression of Rac1, phospho(p)-c-Jun N-terminal kinase 1, p38, p-p38 and MMP9, which was markedly decreased by the addition of apocynin (P<0.05). The mechanism associated with diabetes and AF may be attributed to oxidative stress (ROS production) derived from NOX activity, and then induced activation of the MAPK signaling pathways and MMP9 expression.

Proteomics and transcriptomics in atrial fibrillation

Atrial fibrillation (AF) is the most common tachyarrhythmia. AF, due to substantial remodeling processes initiated in the atria, is a typically self-sustaining and progressive disease. Atrial remodeling has been intensively investigated at the molecular level in recent decades. Although the application of "omics" technologies has already significantly contributed to our current understanding of the pathophysiology of AF, the complexity of the latter and the large heterogeneity of AF patients remained a major limitation. With the advent of novel "omics" and by applying integrative approaches, it will be possible to extract more information and push boundaries. The present review will summarize the contribution of transcriptomics and proteomics to our understanding of the pathophysiology of AF.

Overexpressing microRNA-150 attenuates hypoxia-induced human cardiomyocyte cell apoptosis by targeting glucose-regulated protein-94

MicroRNA (miR)-150 has been demonstrated to protect the heart from ischemic injury. However, the protective effect of miR-150 in hypoxia-injured cardiomyocytes remains unclear. The present study aimed to investigate the target gene of miR-150 and the underlying molecular mechanisms of miR-150 in hypoxia-induced cardiomyocyte apoptosis. Using the hypoxia model of human cardiomyocytes (HCMs) in vitro, it was demonstrated that miR-150 was markedly inhibited in HCMs after hypoxia treatment. Overexpressing miR-150 significantly decreased hypoxia-induced HCM death and apoptosis. In addition, GRP94 was revealed to be a direct target of miR-150. Additionally, GRP94 was demonstrated to be involved in hypoxia-induced HCM apoptosis, and the protein expression levels of GRP94 were increased in HCMs in the presence of hypoxia. These findings demonstrated that miR-150 is involved in hypoxia-mediated gene regulation and apoptosis in HCMs. Furthermore, GRP94 knockout increased the cell viability of hypoxia-impaired HCMs with miR-150 mimic or miR-150 inhibitor transfection. In conclusion, miR-150 may serve a protective role in cardiomyocyte hypoxia injury, and the underlying mechanism was mediated, at least partially, by inhibiting GRP94 expression. These findings may provide a novel insight for the therapy of hypoxia-induced myocardial I/R injury.

Metabolic inhibition reduces cardiac L-type Ca2+ channel current due to acidification caused by ATP hydrolysis

Metabolic stress evoked by myocardial ischemia leads to impairment of cardiac excitation and contractility. We studied the mechanisms by which metabolic inhibition affects the activity of L-type Ca²⁺ channels (LTCCs) in frog ventricular myocytes. Metabolic inhibition induced by the protonophore FCCP (as well as by 2,4- dinitrophenol, sodium azide or antimycin A) resulted in a dose-dependent reduction of LTCC current (I_Ca,L) which was more pronounced during β-adrenergic stimulation with isoprenaline. I_Ca,L was still reduced by metabolic inhibition even in the presence of 3 mM intracellular ATP, or when the cell was dialysed with cAMP or ATP-γ-S to induce irreversible thiophosphorylation of LTCCs, indicating that reduction in I_Ca,L is not due to ATP depletion and/or reduced phosphorylation of the channels. However, the effect of metabolic inhibition on I_Ca,L was strongly attenuated when the mitochondrial F₁F₀-ATP-synthase was blocked by oligomycin or when the cells were dialysed with the non-hydrolysable ATP analogue AMP-PCP. Moreover, increasing the intracellular pH buffering capacity or intracellular dialysis of the myocytes with an alkaline solution strongly attenuated the inhibitory effect of FCCP on I_Ca,L. Thus, our data demonstrate that metabolic inhibition leads to excessive ATP hydrolysis by the mitochondrial F₁F₀-ATP-synthase operating in the reverse mode and this results in intracellular acidosis causing the suppression of I_Ca,L. Limiting ATP break-down by F₁F₀-ATP-synthase and the consecutive development of intracellular acidosis might thus represent a potential therapeutic approach for maintaining a normal cardiac function during ischemia.

Right atrial myocardial deformation by two-dimensional speckle tracking echocardiography predicts recurrence in paroxysmal atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is a bi-atrial disease yet little attention has been given to right heart function in AF. We propose that the assessment of right atrial (RA) and right ventricular function (RV) using two-dimensional speckle tracking echocardiography (2D-STE) could be valuable in predicting AF recurrence in patients with paroxysmal AF (PAF).

METHODS

Thirty patients with PAF were prospectively recruited from a dedicated AF clinic. Right atrial size, volume, and area and RV dimensions were analyzed along with RA and RV strain derived from 2D-STE at baseline and at 3 and 12 months.

RESULTS

Higher RA booster strain independently predicted sinus rhythm (SR) maintenance for up to 1 year (P = 0.001). RV strain was impaired in patients with recurrent AF compared to those in SR (P < 0.05) but did not predict AF recurrence. Two-dimensional STE for RA and RV function was simple to perform with excellent reproducibility (adjusted R ² 0.92-0.99).

CONCLUSIONS

Two-dimensional STE is useful and highly reproducible in assessing right heart function in AF patients. RA booster strain function was predictive of sinus rhythm maintenance for up to 1 year.

Gender Modulates the Expression of Calcium-Regulating Proteins in Pediatric Atrial Myocardium

A differential expression of sarcoplasmic reticulum calcium-ATPase (SERCA2a) and phospholamban (PLB) characterizes the remodeling process in heart failure and atrial arrhythmias in adult patients. Gender is known to modulate the course and Prognosis of different forms of heart disease. We hypothesized that gender plays a role in molecular changes of myocardial calcium regulating components already in childhood. Moreover, we studied the influence of volume overloaded (VO) on SERCA2a and PLB in pediatric patients. Quantitative reverse transcription-polymerase chain reaction was used to measure mRNA expression of SERCA2a and PLB in atrial myocardium from 30 pediatric patients (12 girls, 18 boys). Eighteen patients had VO right atria, and 12 patients had not-overloaded atria (NO). Protein expression was studied by Western blot. In the entire population, SERCA2a and PLB expression was not different between girls and boys. If hemodynamic overload was taken into account, SERCA2a mRNA expression was significantly reduced in the VO group compared with the NO group (P = 0.021). The VO versus NO difference was restricted to toys, which corresponds to a highly significant interaction of gender versus VO status (P = 0.002). The PLB to SERCA2a Protein ratio was significantly lower in girls (P = 0.028). The decrease in SERCA2a mRNA expression in VO atrial myocardium and the PLB to SERCA2a ratio of protein expression was modulated by gender in this pediatric population. To our knowledge, this study is the first to show the impact of gender on the differential expression of calcium-regulating components in Pediatric cardiac patients.

miR-208b upregulation interferes with calcium handling in HL-1 atrial myocytes: Implications in human chronic atrial fibrillation

MicroRNAs (miR) have considerable potential as therapeutic tools in cardiac diseases. Alterations in atrial miR are involved in the development of atrial fibrillation (AF), but the molecular mechanism underlying their contribution to atrial remodeling in chronic atrial fibrillation (CAF) is only partially understood. Here we used miR array to analyze the miR profile of atrial biopsies from sinus rhythm (SR) and CAF patients. qRT-PCR identified a distinctive CAF-miR signature and described conserved miR-208b upregulation in human and ovine AF atrial tissue. We used bioinformatics analysis to predict genes and signaling pathways as putative miR-208b targets, which highlighted genes from the cardiac muscle gene program and from canonical WNT, gap-junction and Ca²⁺ signaling networks. Results from analysis of miR-208b-overexpressing HL-1 atrial myocytes and from myocytes isolated from CAF patients showed that aberrant miR-208b levels reduced the expression and function of L-type Ca²⁺ channel subunits (CACNA1C and CACNB2) as well as the sarcoplasmic reticulum-Ca²⁺ pump SERCA2. These findings clearly pointed to CAF-specific upregulated miR-208b as an important mediator in Ca²⁺ handling impairment during atrial remodeling.

Atrial Fibrillation After Cardiac Surgery: Pathophysiology and Treatment

Atrial fibrillation (AF) occurs in 25% to 60% of patients after cardiac surgery. It is most consistently associated with advanced age and valvular heart operations. Despite improving knowledge of the pathophysiology of chronic AF, postoperative AF remains an obstinate clinical problem. It is associated with an increased risk of stroke, longer hospital stay, and higher hospital expenditure. Consequently, there has been great interest in strategies to prevent and treat this arrhythmia. Treatment for postoperative AF may require immediate electrical cardioversion for hemodynamically unstable patients. Heart rate control is useful in most patients, with anticoagulation considered after 48 hours. Antiarrhythmic therapy is often effective in restoring sinus rhythm but its use needs to be balanced against the patient's risk of proarrhythmic side effects such as torsade de pointes.

Post-operative atrial fibrillation is associated with a pre-existing structural and electrical substrate in human right atrial myocardium

Background

Post-operative atrial fibrillation (POAF) is a major health economic burden. However, the precise mechanisms in POAF remain unclear. In other forms of AF, sites of high dominant frequency (DF) in sinus rhythm (SR) may harbour ‘AF nests’. We studied AF inducibility in relation to substrate changes using epicardial electrograms and cardiomyocyte calcium handling in the atria of AF naïve patients.

Method

Bipolar electrograms were recorded from the lateral right atrial (RA) wall in 34 patients undergoing coronary surgery using a high-density array in sinus rhythm (NSR). RA burst pacing at 200/500/1000 ms cycle lengths (CL) was performed, recording episodes of AF > 30 s. Co-localised RA tissue was snap frozen for RNA and protein extraction.

Results

Electrograms prolonged during AF (76.64 ± 29.35 ms) vs. NSR/pacing (p < 0.001). Compared to NSR, electrogram amplitude was reduced during AF and during pacing at 200 ms CL (p < 0.001). Electrogram DF was significantly lower in AF (75.87 ± 23.63 Hz) vs. NSR (89.33 ± 25.99 Hz) (p < 0.05), and NSR DF higher in AF inducible patients at the site of AF initiation (p < 0.05). Structurally, POAF atrial myocardium demonstrated reduced sarcolipin gene (p = 0.0080) and protein (p = 0.0242) expression vs. NSR. Phospholamban gene and protein expression was unchanged. SERCA2a protein expression remained unchanged, but MYH6 (p = 0.0297) and SERCA2A (p = 0.0343) gene expression was reduced in POAF.

Conclusions

Human atrial electrograms prolong and reduce in amplitude in induced peri-operative AF vs. NSR or pacing. In those sustaining AF, high DF sites in NSR may indicate ‘AF nests’. This electrical remodelling is accompanied by structural remodelling with altered expression of cardiomyocyte calcium handling detectable before POAF. These novel upstream substrate changes offer a novel mechanism and manifestation of human POAF.

Exercise Capacity Correlates With Left Atrial Structural Remodeling as Detected by Late Gadolinium-Enhanced Cardiac Magnetic Resonance in Patients With Atrial Fibrillation

OBJECTIVES

This study hypothesized that left atrial structural remodeling (LA-TR) correlates with exercise capacity (EC) in a cohort of patients with atrial fibrillation (AF).

BACKGROUND

Late gadolinium-enhanced cardiac magnetic resonance (LGE-CMR) imaging provides a method of assessing LA-TR in patients with AF.

METHODS

A total of 145 patients (32% female, mean age 63.4 ± 11.6 years of age) with AF (66 paroxysmal, 71 persistent, 8 long-standing persistent) presenting for catheter ablation were included in the study. All patients underwent LGE-CMR imaging as well as maximal exercise test using the Bruce protocol prior to catheter ablation of AF. EC was quantified by minutes of exercise and metabolic equivalent (MET) level achieved. LA-TR was quantified from LGE-CMR imaging and classified according to the Utah classification of LA structural remodeling (Utah stage I: <10% LA wall enhancement; Utah II: 10% to <20%; Utah III: 20% to <30%; and Utah IV: >30%). AF recurrence was assessed at 1 year from the date of ablation.

RESULTS

The average duration of exercise was 8 ± 3 min, and the mean MET achieved was 9.7 ± 3.2. METs achieved were inversely correlated with LA-TR (R² = 0.061; p = 0.003). The duration of exercise was also inversely correlated with LA-TR (R² = 0.071; p = 0.001). Both EC and LA-TR were associated with AF recurrence post ablation in univariate analysis, but only LA-TR and age were independently predictive of recurrence in multivariate analysis (p = 0.001). For every additional minute on the treadmill, subjects were 13% more likely to be free of AF 1 year post ablation (p = 0.047).

CONCLUSIONS

EC is inversely associated with LA-TR in patients with AF and is predictive of freedom from AF post-ablation.

Left atrial appendage occlusion for stroke prevention

Atrial fibrillation (AF) remains the most common arrhythmia encountered in clinical practice. One of its more common deleterious effects is the development of thromboembolism leading to stroke. The left atrial appendage (LAA) has been shown to the site of the majority of thrombus formation leading to stroke. Anticoagulation with warfarin has been the treatment of choice for prevention of embolic events. Newer anticoagulants have been developed but they still have the potential side effect of causing major bleeding. Occlusion of the LAA has emerged as an alternative therapeutic approach to medical therapy. The aim of this article is to discuss in detail the role of the LAA in thromboembolism in AF, role of device and surgical therapies, and the current clinical data supporting their use. This is particularly timely in that there is now an approved LAA closure device approved in the US for stroke prevention in patients with nonvalvular AF.

Rate Control Strategy Elevated To Primary Treatment For Atrial Fibrillation: Has The Last Word Already Been Spoken?

In the last decade, we were able to see the light shed by several trials and observational studies that dealt with the appropriate manner of treating patients with atrial fibrillation (AF). Recently the AF management by cardiologists has become more aggressive, in part because of an improved comprehension of this rhythm disturbance, as well as, the availability of new treatment strategies. Increasing awareness of AF as a disease rather than as an acceptable alternative to sinus rhythm has led to search for clear arguments to support a certain strategy as a gold standard. In this respect, the decision of whether to restore sinus rhythm, or to control the ventricular rate and allow AF to persist is of critical importance. The results of randomized, controlled trials addressing this matter shed some light on the proper way of treatment for these AF patients. The AFFIRM and RACE trials and their respective sub-studies showed surprising results. The vast majority of physicians were surprised to learn that the rate control strategy was elevated to the position of primary treatment for the AF management instead of the all-time recognized rhythm control approach to restoration and maintenance of sinus rhythm. The use of anticoagulants in the trials was different in the treatment strategies. There was a greater anticoagulant use in the rate control arm because of the belief that anticoagulation can be discontinued in the rhythm control arm when sinus rhythm was restored and maintained for one month. On the other hand, only pharmacological agents were used to maintain sinus rhythm in those trials, however, there is increasing evidence that AF ablation can restore and maintain sinus rhythm in a great proportion of patients. Indeed, there are some limitations and several interesting aspects of these trials and other studies that will be discussed. The last word has not been spoken yet.

Novel therapeutic targets in the management of atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, contributing to increased morbidity and reduced survival through its associations with stroke and heart failure. AF contributes to a four- to fivefold increase in the risk of stroke in the general population and is responsible for 10-15 % of all ischemic strokes. Diagnosis and treatment of AF require considerable health care resources. Current therapies to restore sinus rhythm in AF are suboptimal and are limited either by their pro-arrhythmic effects or by their procedure-related complications. These limitations have necessitated identification of newer therapeutic targets to expand the treatment options. There has been a considerable amount of research interest in investigating the mechanisms of initiation and propagation of AF. Despite extensive research focused on the pathogenesis of AF, a thorough understanding of various pathways mediating initiation and propagation of AF still remains limited. Research efforts focused on the identification of these pathways and molecular mediators have generated a great degree of interest for developing more targeted therapies. This review discusses the potential therapeutic targets and the results from experimental and clinical research investigating these targets.

Atrial fibrillation from the pathologist's perspective

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia encountered in clinical practice, is associated with increased morbidity and mortality. Electrophysiologically, it is characterized by a high rate of asynchronous atrial cell depolarization causing a loss of atrial contractile function and irregular ventricular rates. For a long time, AF was considered as a pure functional disorder without any structural background. Only in recent years, have new mapping and imaging techniques identified atrial locations, which are very often involved in the initiation and maintenance of this supraventricular arrhythmia (i.e. the distal portion of the pulmonary veins and the surrounding atrial myocardium). Morphological analysis of these myocardial sites has demonstrated significant structural remodeling as well as paved the way for further knowledge of AF natural history, pathogenesis, and treatment. This architectural myocardial disarrangement is induced by the arrhythmia itself and the very frequently associated cardiovascular disorders. At the same time, the structural remodeling is also capable of sustaining AF, thereby creating a sort of pathogenetic vicious circle. This review focuses on current understanding about the structural and genetic bases of AF with reference to their classification, pathogenesis, and clinical implications.

Oxidative stress in atrial fibrillation: an emerging role of NADPH oxidase

Atrial fibrillation (AF) is the most common cardiac arrhythmia. Patients with AF have up to seven-fold higher risk of suffering from ischemic stroke. Better understanding of etiologies of AF and its thromboembolic complications are required for improved patient care, as current anti-arrhythmic therapies have limited efficacy and off target effects. Accumulating evidence has implicated a potential role of oxidative stress in the pathogenesis of AF. Excessive production of reactive oxygen species (ROS) is likely involved in the structural and electrical remodeling of the heart, contributing to fibrosis and thrombosis. In particular, NADPH oxidase (NOX) has emerged as a potential enzymatic source for ROS production in AF based on growing evidence from clinical and animal studies. Indeed, NOX can be activated by known upstream triggers of AF such as angiotensin II and atrial stretch. In addition, treatments such as statins, antioxidants, ACEI or AT1RB have been shown to prevent post-operative AF; among which ACEI/AT1RB and statins can attenuate NOX activity. On the other hand, detailed molecular mechanisms by which specific NOX isoform(s) are involved in the pathogenesis of AF and the extent to which activation of NOX plays a causal role in AF development remains to be determined. The current review discusses causes and consequences of oxidative stress in AF with a special focus on the emerging role of NOX pathways.

A functional variant in the promoter region regulates the C-reactive protein gene and is a potential candidate for increased risk of atrial fibrillation

OBJECTIVES

In a large population-based cohort, the level of C-reactive protein (CRP) in patients at baseline predicts an increased risk of future development of atrial fibrillation (AF). The mechanism of this increased risk is unknown. Furthermore, both the molecular effects of CRP on atrial myocytes and fibroblasts and whether genetic variants in the CRP gene predispose to AF are also unknown.

METHODS

A genetic association study between CRP gene polymorphisms and AF was performed in two independent populations (I: 100 AF patients and 101 controls; II: 348 AF patients and 356 controls), with functional studies to elucidate the mechanism of association.

RESULTS

Three polymorphisms (T-861C, A-821G and C-390A/C-390T) were found in the 1-kb promoter of CRP. A triallelic polymorphism (C-390A/C-390T) captured all haplotype information and determined the CRP gene promoter activity and the plasma CRP level, and was in nearly complete linkage disequilibrium with G1059C polymorphism in exon 2. The -390A variant was associated with a higher CRP gene promoter activity, a higher plasma CRP level and a higher risk of AF. Patients with AF also had a higher plasma CRP level than controls. CRP significantly increased the inward L-type calcium current in atrial myocytes with no changes in other ionic currents. CRP did not affect the expressions of type I alpha 1 (COL1A1), type III alpha 1 (COL3A1) and type 1 alpha 2 (COL1A2) procollagens in atrial fibroblasts.

CONCLUSION

A CRP gene promoter triallelic polymorphism was associated with CRP gene promoter activity, determined the plasma level of CRP, and predicted the risk of AF. The mechanism of this may be via augmention of calcium influx by CRP in atrial myocytes, but not because of atrial fibrosis.

Prevention of postoperative atrial fibrillation: novel and safe strategy based on the modulation of the antioxidant system

Postoperative atrial fibrillation (AF) is the most common arrhythmia following cardiac surgery with extracorporeal circulation. The pathogenesis of postoperative AF is multifactorial. Oxidative stress, caused by the unavoidable ischemia-reperfusion event occurring in this setting, is a major contributory factor. Reactive oxygen species (ROS)-derived effects could result in lipid peroxidation, protein carbonylation, or DNA oxidation of cardiac tissue, thus leading to functional and structural myocardial remodeling. The vulnerability of myocardial tissue to the oxidative challenge is also dependent on the activity of the antioxidant system. High ROS levels, overwhelming this system, should result in deleterious cellular effects, such as the induction of necrosis, apoptosis, or autophagy. Nevertheless, tissue exposure to low to moderate ROS levels could trigger a survival response with a trend to reinforce the antioxidant defense system. Administration of n-3 polyunsaturated fatty acids (PUFA), known to involve a moderate ROS production, is consistent with a diminished vulnerability to the development of postoperative AF. Accordingly, supplementation of n-3 PUFA successfully reduced the incidence of postoperative AF after coronary bypass grafting. This response is due to an up-regulation of antioxidant enzymes, as shown in experimental models. In turn, non-enzymatic antioxidant reinforcement through vitamin C administration prior to cardiac surgery has also reduced the postoperative AF incidence. Therefore, it should be expected that a mixed therapy result in an improvement of the cardioprotective effect by modulating both components of the antioxidant system. We present novel available evidence supporting the hypothesis of an effective prevention of postoperative AF including a two-step therapeutic strategy: n-3 PUFA followed by vitamin C supplementation to patients scheduled for cardiac surgery with extracorporeal circulation. The present study should encourage the design of clinical trials aimed to test the efficacy of this strategy to offer new therapeutic opportunities to patients challenged by ischemia-reperfusion events not solely in heart, but also in other organs such as kidney or liver in transplantation surgeries.

Sarcoplasmic reticulum and L-type Ca²⁺ channel activity regulate the beat-to-beat stability of calcium handling in human atrial myocytes

Irregularities in intracellular calcium on a beat-to-beat basis can precede cardiac arrhythmia, but the mechanisms inducing such irregularities remain elusive. This study tested the hypothesis that sarcoplasmic reticulum (SR) and L-type calcium channel activity determine the beat-to-beat response and its rate dependency. For this purpose, patch-clamp technique and confocal calcium imaging was used to record L-type calcium current (ICa) and visualize calcium in human atrial myocytes subjected to increasing stimulation frequencies (from 0.2 to 2 Hz). The beat-to-beat response was heterogeneous among a population of 133 myocytes, with 30 myocytes responding uniformly at all frequencies, while alternating and irregular responses were induced in 78 and 25 myocytes, respectively. Myocytes with uniform responses had the lowest frequency of calcium wave-induced transient inward currents (ITI; 0.4 ± 0.2 min⁻¹), ICa density (1.8 ± 0.3 pA pF⁻¹) and caffeine-releasable calcium load (6.2 ± 0.5 amol pF⁻¹), while those with alternating responses had the highest ITI frequency (1.8 ± 0.3 min⁻¹,P =0.003) and ICa density (2.4 ± 0.2 pA pF⁻¹, P =0.04). In contrast, the calcium load was highest in myocytes with irregular responses (8.5 ± 0.7 amol pF⁻¹, P =0.01). Accordingly, partial ICa inhibition reduced the incidence (from 78 to 44%, P <0.05) and increased the threshold frequency for beat-to-beat alternation (from 1.3 ± 0.2 to 1.9 ± 0.2 Hz, P <0.05). Partial inhibition of SR calcium release reduced the ITI frequency, increased calcium loading and favoured induction of irregular responses, while complete inhibition abolished beat-to-beat alternation at all frequencies. In conclusion, the beat-to-beat response was heterogeneous among human atrial myocytes subjected to increasing stimulation frequencies, and the nature and stability of the response were determined by the SR and L-type calcium channel activities, suggesting that these mechanisms are key to controlling cardiac beat-to-beat stability.

Ca2+-calmodulin-dependent protein kinase II represses cardiac transcription of the L-type calcium channel alpha(1C)-subunit gene (Cacna1c) by DREAM translocation

Recent studies have demonstrated that changes in the activity of calcium-calmodulin-dependent protein kinase II (CaMKII) induce a unique cardiomyocyte phenotype through the regulation of specific genes involved in excitation-contraction (E-C)-coupling. To explain the transcriptional effects of CaMKII we identified a novel CaMKII-dependent pathway for controlling the expression of the pore-forming α-subunit (Cav1.2) of the L-type calcium channel (LTCC) in cardiac myocytes. We show that overexpression of either cytosolic (δC) or nuclear (δB) CaMKII isoforms selectively downregulate the expression of the Cav1.2. Pharmacological inhibition of CaMKII activity induced measurable changes in LTCC current density and subsequent changes in cardiomyocyte calcium signalling in less than 24 h. The effect of CaMKII on the α1C-subunit gene (Cacna1c) promoter was abolished by deletion of the downstream regulatory element (DRE), which binds transcriptional repressor DREAM/calsenilin/KChIP3. Imaging DREAM-GFP (green fluorescent protein)-expressing cardiomyocytes showed that CaMKII potentiates the calcium-induced nuclear translocation of DREAM. Thereby CaMKII increases DREAM binding to the DRE consensus sequence of the endogenous Cacna1c gene. By mathematical modelling we demonstrate that the LTCC downregulation through the Ca2+-CaMKII-DREAM cascade constitutes a physiological feedback mechanism enabling cardiomyocytes to adjust the calcium intrusion through LTCCs to the amount of intracellular calcium detected by CaMKII.

Remodeling of ion channel expression in patients with chronic atrial fibrillation and mitral valvular heart disease

BACKGROUND/AIMS

Underlying cardiac pathology and atrial fibrillation (AF) affect the molecular remodeling of ion channels in the atria. Changes in the expression of these molecules have not been demonstrated in Korean patients with mitral valvular heart disease. Thus, the purpose of this study was to analyze ion channel expression in patients with chronic AF and mitral valvular heart disease.

METHODS

A total of 17 patients (eight males and nine females; mean age, 57 ± 14 years [range, 19 to 77]) undergoing open-heart surgery were included in the study. Twelve patients (seven with coronary artery disease and five with aortic valvular disease) had sinus rhythm, and five patients (all with mitral valvular disease) had chronic, permanent AF. A piece of right atrial appendage tissue (0.5 g) was obtained during surgery. RT-PCR was used to evaluate the expression of L-type Ca(2+) channels, ryanodine receptor (RyR2), sarcoplasmic reticular Ca(2+)-ATPase (SERCA2), gene encoding the rapid component of the delayed rectifier I(kr) (HERG), gene encoding calcium-independent transient outward current I(to1) (Kv4.3), gene encoding the ultrarapid component of the delayed rectifier I(ku) (Kv1.5), K(+) channel-interacting protein 2 (KChIP2), hyperpolarization-activated cation channel 2 associated with the pacemaker current I(f) (HCN2), and gene encoding Na(+) channel (SCN5A).

RESULTS

Reduced L-type Ca(2+) channel, RyR2, SERCA2, Kv1.5, and KChIP2 expression and borderline increased HCN2 expression were observed in the patients with AF and mitral valvular heart disease. Left atrial diameter was negatively correlated with RyR2 and KChIP2 expression. Fractional area shortening of the left atrium was positively correlated with RyR2 and KChIP2 expression.

CONCLUSIONS

Alterations in ion channel expression and the anatomical substrate may favor the initiation and maintenance of AF in patients with mitral valvular heart disease.

MicroRNA-328 contributes to adverse electrical remodeling in atrial fibrillation

BACKGROUND

A characteristic of both clinical and experimental atrial fibrillation (AF) is atrial electric remodeling associated with profound reduction of L-type Ca(2+) current and shortening of the action potential duration. The possibility that microRNAs (miRNAs) may be involved in this process has not been tested. Accordingly, we assessed the potential role of miRNAs in regulating experimental AF.

METHODS AND RESULTS

The miRNA transcriptome was analyzed by microarray and verified by real-time reverse-transcription polymerase chain reaction with left atrial samples from dogs with AF established by right atrial tachypacing for 8 weeks and from human atrial samples from AF patients with rheumatic heart disease. miR-223, miR-328, and miR-664 were found to be upregulated by >2 fold, whereas miR-101, miR-320, and miR-499 were downregulated by at least 50%. In particular, miR-328 level was elevated by 3.9-fold in AF dogs and 3.5-fold in AF patients relative to non-AF subjects. Computational prediction identified CACNA1C and CACNB1, which encode cardiac L-type Ca(2+) channel α1c- and β1 subunits, respectively, as potential targets for miR-328. Forced expression of miR-328 through adenovirus infection in canine atrium and transgenic approach in mice recapitulated the phenotypes of AF, exemplified by enhanced AF vulnerability, diminished L-type Ca(2+) current, and shortened atrial action potential duration. Normalization of miR-328 level with antagomiR reversed the conditions, and genetic knockdown of endogenous miR-328 dampened AF vulnerability. CACNA1C and CACNB1 as the cognate target genes for miR-328 were confirmed by Western blot and luciferase activity assay showing the reciprocal relationship between the levels of miR-328 and L-type Ca(2+) channel protein subunits.

CONCLUSIONS

miR-328 contributes to the adverse atrial electric remodeling in AF through targeting L-type Ca(2+) channel genes. The study therefore uncovered a novel molecular mechanism for AF and indicated miR-328 as a potential therapeutic target for AF.

MicroRNAs and atrial fibrillation: new fundamentals

Atrial fibrillation (AF) is the most commonly encountered clinical arrhythmia associated with pronounced morbidity, mortality, and socio-economic burden. This pathological entity is associated with an altered expression profile of genes that are important for atrial function. MicroRNAs (miRNAs), a new class of non-coding mRNAs of around 22 nucleotides in length, have rapidly emerged as one of the key players in the gene expression regulatory network. The potential roles of miRNAs in controlling AF have recently been investigated. The studies have provided some promising results for our better understanding of the molecular mechanisms of AF. In this review article, we provide a synopsis of the studies linking miRNAs to cardiac excitability and other processes pertinent to AF. To introduce the main topic, we discuss basic knowledge about miRNA biology and our current understanding of mechanisms for AF. The most up-to-date research data on the possible roles of miRNAs in AF initiation and maintenance are presented, and the available experimental results on miRNA and AF are discussed. Some speculations pertinent to the subject are made. Finally, perspectives on future directions of research on miRNAs in AF are provided.

The ryanodine receptor channel as a molecular motif in atrial fibrillation: pathophysiological and therapeutic implications

Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with substantial morbidity and mortality. It causes profound changes in sarcoplasmic reticulum (SR) Ca(2+) homeostasis, including ryanodine receptor channel dysfunction and diastolic SR Ca(2+) leak, which might contribute to both decreased contractile function and increased propensity to atrial arrhythmias. In this review, we will focus on the molecular basis of ryanodine receptor channel dysfunction and enhanced diastolic SR Ca(2+) leak in AF. The potential relevance of increased incidence of spontaneous SR Ca(2+) release for both AF induction and/or maintenance and the development of novel mechanism-based therapeutic approaches will be discussed.

Atorvastatin treatment affects atrial ion currents and their tachycardia-induced remodeling in rabbits

AIMS

Atrial fibrillation (AF) leads to electrical atrial remodeling including alterations of various ion channels early after arrhythmia onset. The beneficial effects of statins in AF treatment due to their influence on oxidative stress and inflammation are discussed. Our hypothesis was that statins might also alter atrial ion currents and their early tachycardia-induced remodeling.

MAIN METHODS

Effects of an atorvastatin treatment (7 days) on atrial ion currents and their tachycardia-induced alterations were studied in a rabbit model of tachycardia-induced electrical remodeling (rapid atrial pacing (600 min) for 24 and 120 h). Ion currents (L-type calcium channel [I(Ca,L)], transient outward current [I(to)]) were measured using whole cell patch clamp method and were compared with previous experiments in untreated but also tachypaced animals.

KEY FINDINGS

Atorvastatin treatment alone decreased I(Ca,L) similar to rapid atrial pacing alone, currents were also further reduced by additional atrial tachypacing. I(to) and its pacing-induced down-regulation after 24 h were not influenced by atorvastatin treatment. However, I(to) was still reduced after 120 h in atorvastatin-treated animals and did not return to control values as expected.

SIGNIFICANCE

The present study establishes that an atorvastatin treatment can affect atrial ion currents and their tachycardia-induced remodeling in a rabbit model. These results show that-amongst other positive effects on oxidative stress and inflammation-the impact of statins on ion currents and their tachycardia-induced alterations might also play a role in "upstream" treatment of AF with HMG-CoA reductase inhibitors.

Is structural remodeling of fibrillated atria the consequence of tissue hypoxia?

BACKGROUND

Matrix metalloproteinases (MMPs) play an important role in degradation of the extracellular matrix of injured tissue. MMP-9 expression increases in fibrillating atrial tissue; however, the mechanism for this increase has not been clarified.

METHODS AND RESULTS

Changes in the expression of vascular endothelial growth factor (VEGF), VEGF receptors, and hypoxia-induced transcription factor-1alpha (HIF-1alpha) in fibrillating atrial tissue were investigated. Atrial tissue samples were obtained from 13 patients with atrial fibrillation (AF) and 25 patients without a history of AF (regular sinus rhythm, RSR) undergoing cardiac operations. Western blot, real-time polymerase chain reaction, and immunofluorescence analyses of the expression of VEGF, VEGF receptors, and HIF-1alpha were performed. The VEGF mRNA and protein levels increased significantly in the AF group compared with the RSR group (P<0.05), and the expression of HIF-1alpha protein was also significantly higher in the AF group. VEGF receptor-1 mRNA, a high-affinity receptor for VEGF, but not VEGF receptor-2 mRNA, was upregulated in the atria of the AF group (P<0.05). Immunofluorescence staining revealed excess production and co-localization of HIF-1alpha, VEGF and MMP-9 in the endothelium of the atrial arteries in the AF group.

CONCLUSIONS

It is possible that upregulation of HIF-1/VEGF is involved in the enhancement of MMP-9 expression under hypoxic conditions.

Increased expression of mineralocorticoid receptor in human atrial fibrillation and a cellular model of atrial fibrillation

OBJECTIVES

This study was designed to evaluate the status of steroidogenesis proteins and de novo synthesis of aldosterone in the atrium, and relationships of these factors to atrial fibrillation (AF).

BACKGROUND

The role of mineralocorticoid in the pathogenesis of AF is unknown.

METHODS

We studied atrial expression of steroidogenesis proteins and aldosterone level in patients with and without AF, and in HL-1 atrial myocytes. We also investigated the electrophysiologic effects and signal transduction of aldosterone on atrial myocytes.

RESULTS

We found basal expressions of mineralocorticoid receptors (MRs), glucocorticoid receptors, and 11-beta-hydroxysteroid dehydrogenase type 2 (11bHSD2) but not 11-beta-hydroxylase (CYP11B1) or aldosterone synthase (CYP11B2) in human atria and HL-1 myocytes. There was no significant difference of mean atrial aldosterone level between patients with AF and those with normal sinus rhythm. However, patients with AF had a significantly higher atrial MR expression compared with those with normal sinus rhythm (1.73 +/- 0.24-fold, p < 0.001). Using mouse HL-1 atrial myocytes as a cellular AF model, we found that rapid depolarization increased MR expression (1.97 +/- 0.72-fold, p = 0.008) through a calcium-dependent mechanism, thus augmenting the genomic effect of aldosterone signaling as evaluated by MR reporter. Aldosterone increased intracellular oxidative stress through a nongenomic pathway, which was attenuated by nicotinamide adenine dinucleotide phosphate oxidase inhibitor diphenyleneiodonium, but not by MR-blockade spironolactone. Aldosterone increased expression of the alpha-1G and -1H subunits of the T-type calcium channel and thus increased the T-type calcium current (-13.6 +/- 2.9 pA/pF vs. -4.5 +/- 1.6 pA/pF, p < 0.01) and the intracellular calcium load through a genomic pathway, which were attenuated by spironolactone, but not by diphenyleneiodonium.

CONCLUSIONS

Expression of MR increased in AF, thus augmenting the genomic effects of aldosterone. Aldosterone induced atrial ionic remodeling and calcium overload through a genomic pathway, which was attenuated by spironolactone. These results suggest that aldosterone may play a role in AF electrical remodeling and provide insight into the treatment of AF with MR blockade.