Effects of metoprolol therapy on cardiac gap junction remodelling and conduction in human chronic atrial fibrillation

Discovery of 1,2,4-Triazole-3-thione Derivatives as Potent and Selective DCN1 Inhibitors for Pathological Cardiac Fibrosis and Remodeling

DCN1, a critical co-E3 ligase during the neddylation process, is overactivated in many diseases, such as cancers, heart failure as well as fibrotic diseases, and has been regarded as a new target for drug development. Herein, we designed and synthesized a new class of 1,2,4-triazole-3-thione-based DCN1 inhibitors based the hit HD1 identified from high-throughput screening and optimized through numerous structure-activity-relationship (SAR) explorations. HD2 (IC50= 2.96 nM) was finally identified and represented a highly potent and selective DCN1 inhibitor with favorable PK properties and low toxicity. Amazingly, HD2 effectively relieved Ang II/TGFβ-induced cardiac fibroblast activation in vitro, and reduced ISO-induced cardiac fibrosis as well as remodeling in vivo, which was linked to the inhibition of cullin 3 neddylation and its substrate Nrf2 accumulation. Our findings unveil a novel 1,2,4-triazole-3-thione-based derivative HD2, which can be recognized as a promising lead compound targeting DCN1 for cardiac fibrosis and remodeling.

Interindividual Age-Independent Differences in Human CX43 Impact Ventricular Arrhythmic Risk

Connexin 43 (CX43) is one of the major components of gap junctions, the structures responsible for the intercellular communication and transmission of the electrical impulse in the left ventricle. There is limited information on the histological changes of CX43 with age and their effect on electrophysiology, especially in humans. Here, we analyzed left ventricular biopsies from living donors starting at midlife to characterize age-related CX43 remodeling. We assessed its quantity, degree of lateralization, and spatial heterogeneity together with fibrotic deposition. We observed no significant age-related remodeling of CX43. Only spatial heterogeneity increased slightly with age, and this increase was better explained by biological age than by chronological age. Importantly, we found that CX43 features varied considerably among individuals in our population with no relevant relationship to age or fibrosis content, in contrast to animal species. We used our experimental results to feed computational models of human ventricular electrophysiology and to assess the effects of interindividual differences in specific features of CX43 and fibrosis on conduction velocity, action potential duration, and arrhythmogenicity. We found that larger amounts of fibrosis were associated with the highest arrhythmic risk, with this risk being increased when fibrosis deposition was combined with a reduction in CX43 amount and/or with an increase in CX43 spatial heterogeneity. These mechanisms underlying high arrhythmic risk in some individuals were not associated with age in our study population. In conclusion, our data rule out CX43 remodeling as an age-related arrhythmic substrate in the population beyond midlife, but highlight its potential as a proarrhythmic factor at the individual level, especially when combined with increased fibrosis.

Atrial Fibrillation: Focus on Myocardial Connexins and Gap Junctions

Atrial fibrillation (AF) represents the most common type of clinical cardiac arrhythmia worldwide and contributes to substantial morbidity, mortality and socioeconomic burden. Aggregating evidence highlights the strong genetic basis of AF. In addition to chromosomal abnormalities, pathogenic mutations in over 50 genes have been causally linked to AF, of which the majority encode ion channels, cardiac structural proteins, transcription factors and gap junction channels. In the heart, gap junctions comprised of connexins (Cxs) form intercellular pathways responsible for electrical coupling and rapid coordinated action potential propagation between adjacent cardiomyocytes. Among the 21 isoforms of connexins already identified in the mammal genomes, 5 isoforms (Cx37, Cx40, Cx43, Cx45 and Cx46) are expressed in human heart. Abnormal electrical coupling between cardiomyocytes caused by structural remodeling of gap junction channels (alterations in connexin distribution and protein levels) has been associated with enhanced susceptibility to AF and recent studies have revealed multiple causative mutations or polymorphisms in 4 isoforms of connexins predisposing to AF. In this review, an overview of the genetics of AF is made, with a focus on the roles of mutant myocardial connexins and gap junctions in the pathogenesis of AF, to underscore the hypothesis that cardiac connexins are a major molecular target in the management of AF.

Analysis of buccal mucosa as a prognostic tool in children with arrhythmogenic cardiomyopathy

Background

The diagnosis of arrhythmogenic cardiomyopathy (ACM) is challenging especially in children at risk of adverse events. Analysis of cardiac myocyte junctional protein distribution may have diagnostic and prognostic implications, but its utility is limited by the need for a myocardial sample. We previously reported that buccal mucosa cells show junctional protein redistribution similar to that seen in cardiac myocytes of adult patients with ACM.

Objectives

We aimed to determine when junctional protein distribution abnormalities first occur in children with ACM variants and whether they correlate with progression of clinically apparent disease.

Methods

We analyzed buccal mucosa samples of children and adolescents with a family history of ACM (n = 13) and age-matched controls (n = 13). Samples were immunostained for plakoglobin, desmoplakin, plakophilin-1 and connexin43 and analyzed by confocal microscopy. All participants were swabbed at least twice with an average interval of 12–18 months between samplings.

Results

Junctional protein re-localization in buccal mucosa cells did not correlate with the presence of ACM-causing variants but instead occurred with clinical onset of disease. No changes in protein distribution were seen unless and until there was clinical evidence of disease. In addition, progressive shifts in the distribution of key proteins correlated with worsening of the disease phenotype. Finally, we observed restoration of junctional signal for Cx43 in patient with a favorable response to anti-arrhythmic therapy.

Conclusions

Due to ethical concerns about obtaining heart biopsies in children with no apparent disease, it has not been possible to analyze molecular changes in cardiac myocytes with the onset/progression of clinical disease. Using buccal smears as a surrogate for the myocardium may facilitate future studies of mechanisms and pathophysiological consequences of junctional protein redistribution in ACM. Buccal cells may also be a safe and inexpensive tool for risk stratification and potentially monitoring response to treatment in children bearing ACM variants.

New Insights on the Role of Connexins and Gap Junctions Channels in Adipose Tissue and Obesity

Due to the inability to curb the excessive increase in the prevalence of obesity and overweight, it is necessary to comprehend in more detail the factors involved in the pathophysiology and to appreciate more clearly the biochemical and molecular mechanisms of obesity. Thus, understanding the biological regulation of adipose tissue is of fundamental relevance. Connexin, a protein that forms intercellular membrane channels of gap junctions and unopposed hemichannels, plays a key role in adipogenesis and in the maintenance of adipose tissue homeostasis. The expression and function of Connexin 43 (Cx43) during the different stages of the adipogenesis are differentially regulated. Moreover, it has been shown that cell–cell communication decreases dramatically upon differentiation into adipocytes. Furthermore, inhibition of Cx43 degradation or constitutive overexpression of Cx43 blocks adipocyte differentiation. In the first events of adipogenesis, the connexin is highly phosphorylated, which is likely associated with enhanced Gap Junction (GJ) communication. In an intermediate state of adipocyte differentiation, Cx43 phosphorylation decreases, as it is displaced from the membrane and degraded through the proteasome; thus, Cx43 total protein is reduced. Cx is involved in cardiac disease as well as in obesity-related cardiovascular diseases. Different studies suggest that obesity together with a high-fat diet are related to the production of remodeling factors associated with expression and distribution of Cx43 in the atrium.

Remodeling of Cardiac Gap Junctional Cell-Cell Coupling

The heart works as a functional syncytium, which is realized via cell-cell coupling maintained by gap junction channels. These channels connect two adjacent cells, so that action potentials can be transferred. Each cell contributes a hexameric hemichannel (=connexon), formed by protein subuntis named connexins. These hemichannels dock to each other and form the gap junction channel. This channel works as a low ohmic resistor also allowing the passage of small molecules up to 1000 Dalton. Connexins are a protein family comprising of 21 isoforms in humans. In the heart, the main isoforms are Cx43 (the 43 kDa connexin; ubiquitous), Cx40 (mostly in atrium and specific conduction system), and Cx45 (in early developmental states, in the conduction system, and between fibroblasts and cardiomyocytes). These gap junction channels are mainly located at the polar region of the cardiomyocytes and thus contribute to the anisotropic pattern of cardiac electrical conductivity. While in the beginning the cell–cell coupling was considered to be static, similar to an anatomically defined structure, we have learned in the past decades that gap junctions are also subject to cardiac remodeling processes in cardiac disease such as atrial fibrillation, myocardial infarction, or cardiomyopathy. The underlying remodeling processes include the modulation of connexin expression by e.g., angiotensin, endothelin, or catecholamines, as well as the modulation of the localization of the gap junctions e.g., by the direction and strength of local mechanical forces. A reduction in connexin expression can result in a reduced conduction velocity. The alteration of gap junction localization has been shown to result in altered pathways of conduction and altered anisotropy. In particular, it can produce or contribute to non-uniformity of anisotropy, and thereby can pre-form an arrhythmogenic substrate. Interestingly, these remodeling processes seem to be susceptible to certain pharmacological treatment.

Molecular Insights in Atrial Fibrillation Pathogenesis and Therapeutics: A Narrative Review

The prevalence of atrial fibrillation (AF) is bound to increase globally in the following years, affecting the quality of life of millions of people, increasing mortality and morbidity, and beleaguering health care systems. Increasingly effective therapeutic options against AF are the constantly evolving electroanatomic substrate mapping systems of the left atrium (LA) and ablation catheter technologies. Yet, a prerequisite for better long-term success rates is the understanding of AF pathogenesis and maintenance. LA electrical and anatomical remodeling remains in the epicenter of current research for novel diagnostic and treatment modalities. On a molecular level, electrical remodeling lies on impaired calcium handling, enhanced inwardly rectifying potassium currents, and gap junction perturbations. In addition, a wide array of profibrotic stimuli activates fibroblast to an increased extracellular matrix turnover via various intermediaries. Concomitant dysregulation of the autonomic nervous system and the humoral function of increased epicardial adipose tissue (EAT) are established mediators in the pathophysiology of AF. Local atrial lymphomononuclear cells infiltrate and increased inflammasome activity accelerate and perpetuate arrhythmia substrate. Finally, impaired intracellular protein metabolism, excessive oxidative stress, and mitochondrial dysfunction deplete atrial cardiomyocyte ATP and promote arrhythmogenesis. These overlapping cellular and molecular alterations hinder us from distinguishing the cause from the effect in AF pathogenesis. Yet, a plethora of therapeutic modalities target these molecular perturbations and hold promise in combating the AF burden. Namely, atrial selective ion channel inhibitors, AF gene therapy, anti-fibrotic agents, AF drug repurposing, immunomodulators, and indirect cardiac neuromodulation are discussed here.

The novel ginsenoside AD2 prevents angiotensin II-induced connexin 40 and connexin 43 dysregulation by activating AMP kinase signaling in perfused beating rat atria

Connexin-40 (Cx40) and Cx43 are the principal components of gap junctions. Dysregulation of connexin expression is clinically related to cardiac pathologies. 25-Hydroxy protopanaxadiol [25-OH-PPD, 20 (R)-dammarane-3β, 12β, 20, 25-tetrol], known as AD2, is a novel protopanaxadiol extracted from Panax ginseng that exhibits many pharmacological activities, but its effects on cardiac gap junctions are poorly understood. The aim of this study was to evaluate the effects of AD2 on angiotensin II (Ang II)-induced Cx40 and Cx43 dysregulation. In this study, isolated beating rat atria were perfused with Ang II (5 μM) for 1 h to induce Cx40 and Cx43 dysregulation. The effects of AD2 (1.6, 16, and 160 μg/100 g body weight) on Ang II-induced hemodynamics in rats were analyzed by biological recorder, and changes in proteins levels were analyzed by western blotting. The results showed that AD2 ameliorated Ang II-induced hyper hemodynamics and abnormal P-waves, and prevented fibrotic collagen deposition (3.77% ± 1.64%-26.31% ± 1.64% with Ang II, 5.76% ± 0.94% with AD2). Ang II upregulated expression of nuclear factor kappa B, activator protein 1, and transforming growth factor β1, and downregulated of Cx40 and Cx43 expression, which were inhibited by AD2 concomitantly with increased of AMP-activated protein kinase (AMPK) expression via liver kinase B1 activation. The present findings suggest that AD2 inhibited Ang II-induced dysregulation of Cx40 and Cx43 via activation of AMPK signaling, thus highlighting the promise and utility of AD2 for treatment of connexin dysregulation-related heart disease.

Cardiac β-adrenergic receptor activation mediates distinct and cell type-dependent changes in the expression and distribution of connexin 43

Activation of the sympatho-β-adrenergic receptors (β-ARs) system is a hallmark of heart failure, leading to fibrosis and arrhythmias. Connexin 43 (Cx43) is the most abundant gap junctional protein in the myocardium. Current knowledge is limited regarding Cx43 remodelling in diverse cell types in the diseased myocardium and the underlying mechanism. We studied cell type-dependent changes in Cx43 remodelling due to β-AR overactivation and molecular mechanisms involved. Mouse models of isoproterenol stimulation or transgenic cardiomyocyte overexpression of β₂ -AR were used, which exhibited cardiac fibrosis and up-regulated total Cx43 abundance. In both models, whereas Cx43 expression in cardiomyocytes was reduced and more laterally distributed, fibroblasts exhibited elevated Cx43 expression and enhanced gap junction communication. Mechanistically, activation of β₂ -AR in fibroblasts in vitro elevated Cx43 expression, which was abolished by the β₂ -antagonist ICI-118551 or protein kinase A inhibitor H-89, but simulated by the adenylyl cyclase activator forskolin. Our in vitro and in vivo data showed that β-AR activation-induced production of IL-18 sequentially stimulated Cx43 expression in fibroblasts in a paracrine fashion. In summary, our findings demonstrate a pivotal role of β-AR in mediating distinct and cell type-dependent changes in the expression and distribution of Cx43, leading to pathological gap junction remodelling in the myocardium.

Metoprolol Inhibits Profibrotic Remodeling of Epicardial Adipose Tissue in a Canine Model of Chronic Obstructive Sleep Apnea

Background Whether chronic obstructive sleep apnea ( OSA ) could promote epicardial adipose tissue ( EAT ) secretion of profibrotic adipokines, and thereby contribute to atrial fibrosis, and the potential therapeutic effects of metoprolol remain unknown. Methods and Results A chronic OSA canine model was established by repeatedly clamping the endotracheal tube for and then reopening it for 4 hours every other day for 12 weeks. In a metoprolol treatment group, metoprolol succinate was administered daily for 12 weeks. The EAT infiltration and left atrial fibrosis were examined. The expressions of adipokines secreted by EAT and hypoxic 3T3-L1 adipocytes were detected. The changes in collagen synthesis, transforming growth factor-β1 expression, and cell differentiation and proliferation in cardiac fibroblasts induced by hypoxic 3T3-L1 adipocyte-derived conditioned medium were further analyzed. Chronic OSA induced infiltration of EAT into the left atrium. OSA enhanced the profibrotic effect of EAT on the adjacent atrial myocardium. Moreover, OSA induced profibrotic cytokine secretion from EAT . We also found that hypoxia induced adipokine secretion in cultured adipocytes, and the medium conditioned by the hypoxic adipocytes increased collagen and transforming growth factor-β1 protein expression and cell proliferation of cardiac fibroblasts. More importantly, metoprolol attenuated infiltration of EAT and alleviated the profibrotic effect of EAT by inhibiting adipokine secretion. Metoprolol also inhibited hypoxia-induced adipokine secretion in adipocytes and thereby blocked the hypoxic adipocyte-derived conditioned medium-induced fibrotic response of cardiac fibroblasts. Conclusions Chronic OSA enhanced the profibrotic effect of EAT on the neighboring atrial myocardium by stimulating the secretion of profibrotic adipokines from EAT , which was significantly attenuated by metoprolol. This study gives insights into mechanisms underlying OSA -induced atrial fibrillation and also provides experimental evidence for the protective effects of metoprolol.

Immunoproteasome Subunit β5i Promotes Ang II (Angiotensin II)–Induced Atrial Fibrillation by Targeting ATRAP (Ang II Type I Receptor–Associated Protein) Degradation in Mice

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia and increases the risk of stroke, heart failure, and death. Ang II (angiotensin II) triggers AF, mainly through stimulation of the AT1R (Ang II type I receptor). The immunoproteasome is a highly efficient proteolytic machine derived from the constitutive proteasome, but the role it plays in regulating AT1R activation and triggering AF remains unknown. Here, we show that among the catalytic subunits, β5i (PSMB8) expression, and chymotrypsin-like activity were the most significantly upregulated in atrial tissue of Ang II–infused mice or serum from patients with AF. β5i KO (β5i knockout) in mice markedly attenuated Ang II-induced AF incidence, atrial fibrosis, inflammatory response, and oxidative stress compared with WT (wild type) animals, but injection with recombinant adeno-associated virus serotype 9–β5i increased these effects. Moreover, we found that ATRAP (AT1R-associated protein) was a target of β5i. Overexpression of ATRAP significantly attenuated Ang II-induced atrial remodeling and AF in recombinant adeno-associated virus serotype 9–β5i-injected mice. Mechanistically, Ang II upregulated β5i expression to promote ATRAP degradation, which resulted in activation of AT1R-mediated NF-κB signaling, increased NADPH oxidase activity, increased TGF (transforming growth factor)-β1/Smad signaling, and altered the expression of Kir2.1 and CX43 (connexin 43) in the atria, thereby affecting atrial remodeling and AF. In summary, this study identifies β5i as a negative regulator of ATRAP stability that contributes to AT1R activation and to AF, highlighting that targeting β5i activity may represent a potential therapeutic approach for the treatment of hypertensive AF.

Cardiac Arrhythmias as Manifestations of Nanopathies: An Emerging View

A nanodomain is a collection of proteins localized within a specialized, nanoscale structural environment, which can serve as the functional unit of macroscopic physiologic processes. We are beginning to recognize the key roles of cardiomyocyte nanodomains in essential processes of cardiac physiology such as electrical impulse propagation and excitation–contraction coupling (ECC). There is growing appreciation of nanodomain dysfunction, i.e., nanopathy, as a mechanistic driver of life-threatening arrhythmias in a variety of pathologies. Here, we offer an overview of current research on the role of nanodomains in cardiac physiology with particular emphasis on: (1) sodium channel-rich nanodomains within the intercalated disk that participate in cell-to-cell electrical coupling and (2) dyadic nanodomains located along transverse tubules that participate in ECC. The beat to beat function of cardiomyocytes involves three phases: the action potential, the calcium transient, and mechanical contraction/relaxation. In all these phases, cell-wide function results from the aggregation of the stochastic function of individual proteins. While it has long been known that proteins that exist in close proximity influence each other’s function, it is increasingly appreciated that there exist nanoscale structures that act as functional units of cardiac biophysical phenomena. Termed nanodomains, these structures are collections of proteins, localized within specialized nanoscale structural environments. The nano-environments enable the generation of localized electrical and/or chemical gradients, thereby conferring unique functional properties to these units. Thus, the function of a nanodomain is determined by its protein constituents as well as their local structural environment, adding an additional layer of complexity to cardiac biology and biophysics. However, with the emergence of experimental techniques that allow direct investigation of structure and function at the nanoscale, our understanding of cardiac physiology and pathophysiology at these scales is rapidly advancing. Here, we will discuss the structure and functions of multiple cardiomyocyte nanodomains, and novel strategies that target them for the treatment of cardiac arrhythmias.

Omega-3 Index and Anti-Arrhythmic Potential of Omega-3 PUFAs

Omega-3 polyunsaturated fatty acids (PUFAs), namely eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are permanent subjects of interest in relation to the protection of cardiovascular health and the prevention of the incidence of both ventricular and atrial arrhythmias. The purpose of this updated review is to focus on the novel cellular and molecular effects of omega-3 PUFAs, in the context of the mechanisms and factors involved in the development of cardiac arrhythmias; to provide results of the most recent studies on the omega-3 PUFA anti-arrhythmic efficacy and to discuss the lack of the benefit in relation to omega-3 PUFA status. The evidence is in the favor of omega-3 PUFA acute and long-term treatment, perhaps with mitochondria-targeted antioxidants. However, for a more objective evaluation of the anti-arrhythmic potential of omega-3 PUFAs in clinical trials, it is necessary to monitor the basal pre-interventional omega-3 status of individuals, i.e., red blood cell content, omega-3 index and free plasma levels. In the view of evidence-based medicine, it seems to be crucial to aim to establish new approaches in the prevention of cardiac arrhythmias and associated morbidity and mortality that comes with these conditions.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

Computational Representations of Myocardial Infarct Scars and Implications for Arrhythmogenesis

Image-based computational modeling is becoming an increasingly used clinical tool to provide insight into the mechanisms of reentrant arrhythmias. In the context of ischemic heart disease, faithful representation of the electrophysiological properties of the infarct region within models is essential, due to the scars known for arrhythmic properties. Here, we review the different computational representations of the infarcted region, summarizing the experimental measurements upon which they are based. We then focus on the two most common representations of the scar core (complete insulator or electrically passive tissue) and perform simulations of electrical propagation around idealized infarct geometries. Our simulations highlight significant differences in action potential duration and focal effective refractory period (ERP) around the scar, driven by differences in electrotonic loading, depending on the choice of scar representation. Finally, a novel mechanism for arrhythmia induction, following a focal ectopic beat, is demonstrated, which relies on localized gradients in ERP directly caused by the electrotonic sink effects of the neighboring passive scar.

Association of atrial fibrillation with gene polymorphisms of connexin 40 and angiotensin II receptor type 1 in Chongming adults of Shanghai

OBJECTIVE

To characterized the gene polymorphisms of connexin 40 (cx40) and angiotensin II receptor type 1 (AT1R) in Chongming adults with atrial fibrillation (AF) and to explore their relationships with AF.

METHODS

82 patients with AF, and 82 subjects without AF were enrolled. Polymorphisms of cx40 G-44A and AT1 A1166C were detected. Moreover, several samples were randomly selected to validate the gene polymorphisms of cx40 and AT1.

RESULTS

Genotypes AA, AG and GG of cx40 G-44A were found in both AF patients and controls. The frequencies of genotypes AA, AG and GG were 39%, 29% and 32%, respectively, in AF patients and 31%, 35% and 34%, respectively in controls. The frequencies of alleles A and G were 54% and 46%, respectively in AF patients and 48% and 52%, respectively, in controls (P < 0.05). The risk for AF in patients with allele A increased 1.31 times (OR = 1.31, P < 0.05). The frequencies of genotypes AA, AC and CC were 88%, 8% and 4%, respectively in AF patients and 93%, 6% and 1%, respectively in controls. The frequencies of alleles A and C were 92% and 8%, respectively in AF patients and 96% and 4%, respectively in controls (P < 0.05). More AF patients had allele C as compared to controls. The risk for AF increased by 1.43 times in patients with allele C (OR = 1.43, P < 0.05).

CONCLUSION

There were relationships between gene polymorphisms of cx40 and AT1 and AF in Chongming adults. Allele A of cx40 G-44A and allele C of AT1 A1166C significantly increase the risk for AF.

Can we protect from malignant arrhythmias by modulation of cardiac cell-to-cell coupling?

Defects in intercellular coupling in the heart play a key role in the initiation and persistence of malignant arrhythmias. Such disorders result from abnormal expression and distribution of connexins, the major constituents of cardiac gap junction channels. The alterations of myocardial connexin are well established as a consistent feature of both human and animal heart disease and aging. Following these facts, the modulation of connexin mediated intercellular coupling is suggested as a new antiarrhythmic approach. This review provides recent data supporting this concept. It can be challenging for the development of new antiarrhythmic drugs. Moreover, findings point out the implication of some endogenous compounds in protection from life-threatening arrhythmias via preservation of myocardial connexin.

Remodeling of cardiac passive electrical properties and susceptibility to ventricular and atrial arrhythmias

Coordinated electrical activation of the heart is essential for the maintenance of a regular cardiac rhythm and effective contractions. Action potentials spread from one cell to the next via gap junction channels. Because of the elongated shape of cardiomyocytes, longitudinal resistivity is lower than transverse resistivity causing electrical anisotropy. Moreover, non-uniformity is created by clustering of gap junction channels at cell poles and by non-excitable structures such as collagenous strands, vessels or fibroblasts. Structural changes in cardiac disease often affect passive electrical properties by increasing non-uniformity and altering anisotropy. This disturbs normal electrical impulse propagation and is, consequently, a substrate for arrhythmia. However, to investigate how these structural changes lead to arrhythmias remains a challenge. One important mechanism, which may both cause and prevent arrhythmia, is the mismatch between current sources and sinks. Propagation of the electrical impulse requires a sufficient source of depolarizing current. In the case of a mismatch, the activated tissue (source) is not able to deliver enough depolarizing current to trigger an action potential in the non-activated tissue (sink). This eventually leads to conduction block. It has been suggested that in this situation a balanced geometrical distribution of gap junctions and reduced gap junction conductance may allow successful propagation. In contrast, source-sink mismatch can prevent spontaneous arrhythmogenic activity in a small number of cells from spreading over the ventricle, especially if gap junction conductance is enhanced. Beside gap junctions, cell geometry and non-cellular structures strongly modulate arrhythmogenic mechanisms. The present review elucidates these and other implications of passive electrical properties for cardiac rhythm and arrhythmogenesis.

Effects of human atrial ionic remodelling by β-blocker therapy on mechanisms of atrial fibrillation: a computer simulation

AIMS

Atrial anti-arrhythmic effects of β-adrenoceptor antagonists (β-blockers) may involve both a suppression of pro-arrhythmic effects of catecholamines, and an adaptational electrophysiological response to chronic β-blocker use; so-called 'pharmacological remodelling'. In human atrium, such remodelling decreases the transient outward (Ito) and inward rectifier (IK1) K(+) currents, and increases the cellular action potential duration (APD) and effective refractory period (ERP). However, the consequences of these changes on mechanisms of genesis and maintenance of atrial fibrillation (AF) are unknown. Using mathematical modelling, we tested the hypothesis that the long-term adaptational decrease in human atrial Ito and IK1 caused by chronic β-blocker therapy, i.e. independent of acute electrophysiological effects of β-blockers, in an otherwise un-remodelled atrium, could suppress AF.

METHODS AND RESULTS

Contemporarily, biophysically detailed human atrial cell and tissue models were used to investigate effects of the β-blocker-based pharmacological remodelling. Chronic β-blockade remodelling prolonged atrial cell APD and ERP. The incidence of small amplitude APD alternans in the CRN model was reduced. At the 1D tissue level, β-blocker remodelling decreased the maximum pacing rate at which APs could be conducted. At the three-dimensional organ level, β-blocker remodelling reduced the life span of re-entry scroll waves.

CONCLUSION

This study improves our understanding of the electrophysiological mechanisms of AF suppression by chronic β-blocker therapy. Atrial fibrillation suppression may involve a reduced propensity for maintenance of re-entrant excitation waves, as a consequence of increased APD and ERP.

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.

Prophylactic Effects of Metoprolol on the Prevention of Atrial Fibrillation after Cardiac Surgery Are Dose Dependent

Atrial fibrillation (AF) is one of the most common complications after cardiac surgery. Many studies have reported an incidence of 20%-40% in patients undergoing open heart surgery, and the peak incidence usually occurs between the postoperative days [Fuller 1989; Aranki 1996; Svedjeholm 2000; Maisel 2001]. AF is commonly self-limited and rarely results in postoperative death. However, postoperative AF (POAF) is often associated with complications, including stroke, heart failure, prolonged hospital stay, and increased costs [Maisel 2001; Bramer 2010]. Many pharmacological methods have been used to prevent this complication, and beta-blockers, which have been investigated in several studies, have demonstrated effectiveness [Ali 1997; Connolly 2003; Crystal 2004; Halonen 2006; Imren 2007]. There is currently a consensus in the use of beta-blockers for the prevention of POAF. However, whether the effect of beta-blockers on POAF is dose dependent has not been widely studied [Coleman 2004; Lucio 2004]. In addition, patients with different racial backgrounds have a different response to metoprolol based on body shape. In addition, the CYP2D6 genotypes are different among white and Asian patients. In this study dose-dependent prophylactic effects of beta-blockers, which were obtained in a single center.

Connexin Remodeling Contributes to Atrial Fibrillation

Atrial fibrillation significantly contributes to mortality and morbidity through increased risk of stroke, heart failure and myocardial infarction. Investigations of mechanisms responsible for the development and maintenance of atrial fibrillation have highlighted the importance of gap junctional remodeling. Connexins 40 and 43, the major atrial gap junctional proteins, undergo considerable alterations in expression and localization in atrial fibrillation, creating an environment conducive to sustained reentry. Atrial fibrillation is initiated and/or maintained in this reentrant substrate. This review will focus on connexin remodeling in the context of underlying mechanism and possible therapeutic target for atrial fibrillation.

Role of connexins in human congenital heart disease: the chicken and egg problem

Inborn cardiac diseases are among the most frequent congenital anomalies and are the main cause of death in infants within the first year of age in industrialized countries when not adequately treated. They can be divided into simple and complex cardiac malformations. The former ones, for instance atrial and ventricular septal defects, valvular or subvalvular stenosis or insufficiency account for up to 80% of cardiac abnormalities. The latter ones, for example transposition of the great vessels, Tetralogy of Fallot or Shone’s anomaly often do not involve only the heart, but also the great vessels and although occurring less frequently, these severe cardiac malformations will become symptomatic within the first months of age and have a high risk of mortality if the patients remain untreated. In the last decade, there is increasing evidence that cardiac gap junction proteins, the connexins (Cx), might have an impact on cardiac anomalies. In the heart, mainly three of them (Cx40, Cx43, and Cx45) are differentially expressed with regard to temporal organogenesis and to their spatial distribution in the heart. These proteins, forming gap junction channels, are most important for a normal electrical conduction and coordinated synchronous heart muscle contraction and also for the normal embryonic development of the heart. Animal and also some human studies revealed that at least in some cardiac malformations alterations in certain gap junction proteins are present but until today no particular gap junction mutation could be assigned to a specific cardiac anomaly. As gap junctions have often been supposed to transmit growth and differentiation signals from cell to cell it is reasonable to assume that they are somehow involved in misdirected growth present in many inborn heart diseases playing a primary or contributory role. This review addresses the potentional role of gap junctions in the development of inborn heart anomalies like the conotruncal heart defects.

Ischemic Conditioning and Atrial Fibrillation: Hope for a NewTherapy?

Atrial fibrillation (AF) is the most common sustained arrhythmia.It is accompanied by both structural and ion channel remodeling which underlie the propensity to perpetuate AF.The prevalence of AF is expected to increase as population ages and as more patients survival myocardial infarction. Despite pharmacological and nonpharmacological (such as ablation) therapies for AF, more effective therapy is needed. Ischemic or pharmacological conditioning offers a potential novel approach to patients with AF. This review will focus on the basic biology of ischemic pre- and postconditioning, patho-physiology of AF, potentially novel AF treatment approachs based on conditioning, and clinical situations that may be amenable to a conditioning strategy.

On the different roles of AT1 and AT2 receptors in stretch-induced changes of connexin43 expression and localisation

Cyclic mechanical stretch (CMS) and angiotensin II (ATII) play an important role in cardiac remodelling. Thus, we aimed to examine how ATII affects CMS-induced changes in localisation and expression of the gap junction protein connexin43 (Cx43). Neonatal rat cardiomyocytes cultured on gelatin-coated Flexcell cell culture plates were kept static or were exposed to CMS (110 % of resting length, 1 Hz) for 24 h with or without additional ATII (0.1 μmol/L). Moreover, inhibitors of ATII receptors (AT-R) were used (for AT(1)-R: losartan 0.1 μmol/L, for AT(2)-R: PD123177 0.1 μmol/L). Thereafter, the cardiomyocytes were investigated by immunohistology, PCR and Western blot. After 24 h of CMS, cardiomyocytes were significantly elongated and orientated 75 ± 1.6° nearly perpendicular to the stretch axis. Furthermore, CMS significantly accentuated Cx43 at the cell poles (ratio Cx43 polar/lateral static: 2.32 ± 0.17; CMS: 10.08 ± 3.2). Additional ATII application significantly reduced Cx43 polarisation (ratio Cx43 polar/lateral ATII: 4.61 ± 0.42). The combined administration of ATII and losartan to CMS further reduced Cx43 polarisation to control levels, whilst the AT(2)-R blocker PD123177 restored polarisation. Moreover, CMS and ATII application resulted in a significant Cx43 protein and Cx43 mRNA up-regulation which could be blocked by losartan but not by PD123177. Thus, CMS results in a self-organisation of the cardiomyocytes leading to elongated cells orientated transversely towards the stretch axis with enhanced Cx43 expression and Cx43 accentuation at the cell poles. ATII enhances total Cx43 mRNA and protein expression probably via AT(1)-R (=inhibitory effect of losartan) and reduces Cx43 polarisation presumably via AT(2)-R, since PD123177 (but not losartan) inhibited the negative effects of ATII on polarisation.

Effects of mechanical forces and stretch on intercellular gap junction coupling

Mechanical forces provide fundamental physiological stimulus in living organisms. Recent investigations demonstrated how various types of mechanical load, like strain, pressure, shear stress, or cyclic stretch can affect cell biology and gap junction intercellular communication (GJIC). Depending on the cell type, the type of mechanical load and on strength and duration of application, these forces can induce hypertrophic processes and modulate the expression and function of certain connexins such as Cx43, while others such as Cx37 or Cx40 are reported to be less mechanosensitive. In particular, not only expression but also subcellular localization of Cx43 is altered in cardiomyocytes submitted to cyclic mechanical stretch resulting in the typical elongated cell shape with an accentuation of Cx43 at the cell poles. In the heart both cardiomyocytes and fibroblasts can alter their GJIC in response to mechanical load. In the vasculature both endothelial cells and smooth muscle cells are subject to strain and cyclic stretch resulting from the pulsatile flow. In addition, vascular endothelial cells are mainly affected by shear stress resulting from the blood flow parallel to their surface. These mechanical forces lead to a regulation of GJIC in vascular tissue. In bones, osteocytes and osteoblasts are coupled via gap junctions, which also react to mechanical forces. Since gap junctions are involved in regulation of cell growth and differentiation, the mechanosensitivity of the regulation of these channels might open new perspectives to explain how cells can respond to mechanical load, and how stretch induces self-organization of a cell layer which might have implications for embryology and the development of organs. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.