Atrial fibrillation (AF) is an extremely common cardiac rhythm disorder that causes substantial morbidity and contributes to mortality. The mechanisms underlying AF are complex, involving both increased spontaneous ectopic firing of atrial cells and impulse reentry through atrial tissue. Over the past ten years, there has been enormous progress in understanding the underlying molecular pathobiology. This article reviews the basic mechanisms and molecular processes causing AF. We discuss the ways in which cardiac disease states, extracardiac factors, and abnormal genetic control lead to the arrhythmia. We conclude with a discussion of the potential therapeutic implications that might arise from an improved mechanistic understanding.

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Recent advances in the molecular pathophysiology of atrial fibrillation

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.

Cellular and Molecular Electrophysiology of Atrial Fibrillation Initiation, Maintenance, and Progression

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/j.febslet.2014.01.049

Specific Cx43 phosphorylation events regulate gap junction turnover in vivo.

Atrial fibrillation is the most common type of cardiac arrhythmia, and is responsible for substantial morbidity and mortality in the general population. Current treatments have moderate efficacy and considerable risks, especially of pro-arrhythmia, highlighting the need for new therapeutic strategies. In recent years, substantial efforts have been invested in developing novel treatments that target the underlying molecular determinants of atrial fibrillation, and several new compounds are under development. This Review focuses on the mechanistic rationale for the development of new anti-atrial fibrillation drugs, on the molecular and structural motifs that they target and on the results obtained so far in experimental and clinical studies.

Novel molecular targets for atrial fibrillation therapy

Connexin 43 is an emerging therapeutic target in ischemia/reperfusion injury, cardioprotection and neuroprotection.

Co-ordinated electrical activation of the heart is maintained by intercellular coupling of cardiomyocytes via gap junctional channels located in the intercalated disks. These channels consist of two hexameric hemichannels, docked to each other, provided by either of the adjacent cells. Thus, a complete gap junction channel is made from 12 protein subunits, the connexins. While 21 isoforms of connexins are presently known, cardiomyocytes typically are coupled by Cx43 (most abundant), Cx40 or Cx45. Some years ago, antiarrhythmic peptides were discovered and synthesised, which were shown to increase macroscopic gap junction conductance (electrical coupling) and enhance dye transfer (metabolic coupling). The lead substance of these peptides is AAP10 (H-Gly-Ala-Gly-Hyp-Pro-Tyr-CONH2), a peptide with a horseshoe-like spatial structure as became evident from two-dimensional nuclear magnetic resonance studies. A stable d-amino-acid derivative of AAP10, rotigaptide, as well as a non-peptide analogue, gap-134, has been developed in recent years. Antiarrhythmic peptides act on Cx43 and Cx45 gap junctions but not on Cx40 channels. AAP10 has been shown to enhance intercellular communication in rat, rabbit and human cardiomyocytes. Antiarrhythmic peptides are effective against ventricular tachyarrhythmias, such as late ischaemic (type IB) ventricular fibrillation, CaCl2 or aconitine-induced arrhythmia. Interestingly, the effect of antiarrhythmic peptides is higher in partially uncoupled cells and was shown to be related to maintained Cx43 phosphorylation, while arrhythmogenic conditions like ischaemia result in Cx43 dephosphorylation and intercellular decoupling. It is still a matter of debate whether these drugs also act against atrial fibrillation. The present review outlines the development of this group of peptides and derivatives, their mode of action and molecular mechanisms, and discusses their possible therapeutic potential.

Improving cardiac gap junction communication as a new antiarrhythmic mechanism: the action of antiarrhythmic peptides

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.

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

Previously, it was shown that antiarrhythmic peptides and our lead substance AAP10 enhance electrical intercellular communication via gap junctions. Now, we wanted to elucidate whether AAP10 acts preferably in the ischemic area and the molecular mechanism of this peptide. Seventeen rabbit hearts were isolated, perfused according to Langendorff, and submitted to 30-min local ischemia by LAD occlusion with/without AAP10 (50 nM). Electrophysiology was assessed by 256 channel epicardial mapping. Finally, the ischemic zone, border zone, and non-ischemic zone were excised, and the cardiac gap junction protein connexin43 (Cx43), its phosphorylation state, and the distribution at the polar and lateral membrane of cardiomyocytes were determined by Western blot and immunofluorescence. Ischemia led to a decrease in activation recovery interval (ARI) homogeneity, which could be completely prevented by AAP10. Moreover, ischemia-induced activation wave slowing in the ischemic border zone was antagonized by AAP10. In ischemic center and border zone, but not in the non-ischemic area, (phospho-Cx43/dephospho-Cx43)-ratio decreased. This was also significantly antagonized by AAP10. Serine 368 was identified as one phosphorylation site for the activity of AAP10. In the non-ischemic area, AAP10 had no influence on Cx43 phosphorylation state. Interestingly, ischemia led to a loss of Cx43 from the cell poles and lateral sides in the ischemic area and border zone. AAP10 completely prevented the ischemia-induced decrease in polar Cx43 presence. In the ischemic area, AAP10 prevents from ischemia-induced Cx43 dephosphorylation and loss of Cx43 from the gap junction at cell poles and in parallel prevents the decrease in ARI homogeneity and attenuates ischemia-induced slowing of activation wave propagation. The AAP10 action seems confined to the ischemic area.

Local effects and mechanisms of antiarrhythmic peptide AAP10 in acute regional myocardial ischemia: electrophysiological and molecular findings.

Background Hypercholesterolaemia is common in patients after cardiac transplantation. Monoclonal antibodies that inhibit proprotein convertase subtilisin-kexin type 9 (PCSK9) reduce low-density lipoprotein (LDL) cholesterol levels and subsequently the risk of cardiovascular events in patients with dyslipidaemia. There are no published data on the effect of this medication class on cholesterol levels in patients after cardiac transplantation. Methods In this retrospective study we investigated patients who were treated with PCSK9 inhibitors either because of intolerance of statins or residual hypercholesterolaemia with evidence of cardiac allograft vasculopathy. We compared the data of patients prior to the start with these medications with their most recent dataset. Results Ten patients (nine men; mean age 58±6 years) underwent cardiac transplantation 8.3±4.5 (range 3-15) years ago. The treatment duration of Evolocumab or Alirocumab was on average 296±125 days and lead to a reduction of total Cholesterol (281±52 mg/dl to 197±36 mg/dl; p = 0.002) and LDL Cholesterol (170±22 mg/dl to 101±39 mg/dl; p = 0.001). No significant effects on HDL Cholesterol, BNP, Creatin Kinase or hepatic enzymes were noticed. There were no unplanned hospitalisations, episodes of rejections, change of ejection fraction or opportunistic infections. Both patients on Alirocumab developed liver pathologies: One patient died of hepatocellular carcinoma and the other developed hepatitis E. Conclusions Our study demonstrates that the PCSK9 inhibitors Evolocumab and Alirocumab lead to a significant reduction of LDL Cholesterol in heart transplantation recipients. No effect on cardiac function or episodes of rejections were noticed. Larger and long-term studies are needed to establish safety and efficacy of PCSK9 inhibitors after cardiac transplantation.

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Treatment of hypercholesterolaemia with PCSK9 inhibitors in patients after cardiac transplantation.

Uncoupling of cardiac gap junction channels is an important arrhythmogenic mechanism in ischemia/reperfusion. Antiarrhythmic peptide AAP10 (H-Gly-Ala-Gly-Hyp-Pro-Tyr-CONH2) has been shown to prevent acidosis-induced uncoupling and ischemia-related increase in dispersion. Previous structure–effect investigations and subsequent computer modeling studies indicated that the tricyclic antidepressant desipramine may exert similar effects as AAP10. We assessed the binding of 14C-AAP10 to membranes of rabbit cardiac ventricles and its displacement with desipramine in a classical radioligand binding and competition study. Gap junction currents were measured between isolated pairs of human atrial cardiomyocytes under normal and acidotic (pH 6.3) conditions with or without 1 μmol/l desipramine using dual whole-cell voltage clamp. The effect of 1 μmol/l desipramine was assessed in isolated rabbit hearts (Langendorff technique) undergoing local ischemia by coronary occlusion with 256-channel electrophysiological mapping and subsequent analysis of connexin43 (Cx43) expression, phosphorylation (Western blot), and subcellular localization (immunohistology). We found saturable 14C-AAP10 binding to cardiac membranes (K
 D, 0.29 ± 0.11 nmol/l; B
 max, 42.5 ± 7.2 pmol/mg) which could be displaced by desipramine with a K
 D.High = 0.14 μmol/l and a K
 D.Low = 22 μmol/l. Acidosis reduced the gap junction conductance in human cardiomyocyte pairs from 24.1 ± 4.7 to 11.5 ± 2.5 nS, which could be significantly reversed by desipramine (26.6 ± 4.8 nS). In isolated hearts, ischemia resulted in significantly increased dispersion of activation–recovery intervals, loss of membrane Cx43, and dephosphorylation of Cx43, which all could be prevented by desipramine. Desipramine seems to prevent the uncoupling of cardiac gap junctions and ischemia-related increase in dispersion.

Joanna Jozwiak

Papers

Improving cardiac gap junction communication as a new antiarrhythmic mechanism: the action of antiarrhythmic peptides

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

Local effects and mechanisms of antiarrhythmic peptide AAP10 in acute regional myocardial ischemia: electrophysiological and molecular findings.

Treatment of hypercholesterolaemia with PCSK9 inhibitors in patients after cardiac transplantation.

Desipramine prevents cardiac gap junction uncoupling