Animal models for atrial fibrillation: clinical insights and scientific opportunities

Calcium-mediated DAD in membrane potentials and triggered activity in atrial myocytes of ETV1f / fMyHCCre /+ mice

The E-twenty-six variant 1 (ETV1)-dependent transcriptome plays an important role in atrial electrical and structural remodelling and the occurrence of atrial fibrillation (AF), but the underlying mechanism of ETV1 in AF is unclear. In this study, cardiomyocyte-specific ETV1 knockout (ETV1f/fMyHCCre/+, ETV1-CKO) mice were constructed to observe the susceptibility to AF and the underlying mechanism in AF associated with ETV1-CKO mice. AF susceptibility was examined by intraesophageal burst pacing, induction of AF was increased obviously in ETV1-CKO mice than WT mice. Electrophysiology experiments indicated shortened APD50 and APD90, increased incidence of DADs, decreased density of ICa,L in ETV1-CKO mice. There was no difference in VINACT,1/2 and VACT,1/2, but a significantly longer duration of the recovery time after inactivation in the ETV1-CKO mice. The recording of intracellular Ca2+ showed that there was significantly increased in the frequency of calcium spark, Ca2+ transient amplitude, and proportion of SCaEs in ETV1-CKO mice. Reduction of Cav1.2 rather than NCX1 and SERCA2a, increase RyR2, p-RyR2 and CaMKII was reflected in ETV1-CKO group. This study demonstrates that the increase in calcium spark and SCaEs corresponding to Ca2+ transient amplitude may trigger DAD in membrane potential in ETV1-CKO mice, thereby increasing the risk of AF.

Clonal Hematopoiesis of Indeterminate Potential With Loss of Tet2 Enhances Risk for Atrial Fibrillation Through Nlrp3 Inflammasome Activation

BACKGROUND

Clonal hematopoiesis of indeterminate potential (CHIP), a common age-associated phenomenon, associates with increased risk of both hematological malignancy and cardiovascular disease. Although CHIP is known to increase the risk of myocardial infarction and heart failure, the influence of CHIP in cardiac arrhythmias, such as atrial fibrillation (AF), is less explored.

METHODS

CHIP prevalence was determined in the UK Biobank, and incident AF analysis was stratified by CHIP status and clone size using Cox proportional hazard models. Lethally irradiated mice were transplanted with hematopoietic-specific loss of Tet2, hematopoietic-specific loss of Tet2 and Nlrp3, or wild-type control and fed a Western diet, compounded with or without NLRP3 (NLR [NACHT, LRR {leucine rich repeat}] family pyrin domain containing protein 3) inhibitor, NP3-361, for 6 to 9 weeks. Mice underwent in vivo invasive electrophysiology studies and ex vivo optical mapping. Cardiomyocytes from Ldlr-/- mice with hematopoietic-specific loss of Tet2 or wild-type control and fed a Western diet were isolated to evaluate calcium signaling dynamics and analysis. Cocultures of pluripotent stem cell-derived atrial cardiomyocytes were incubated with Tet2-deficient bone marrow-derived macrophages, wild-type control, or cytokines IL-1β (interleukin 1β) or IL-6 (interleukin 6).

RESULTS

Analysis of the UK Biobank showed individuals with CHIP, in particular TET2 CHIP, have increased incident AF. Hematopoietic-specific inactivation of Tet2 increases AF propensity in atherogenic and nonatherogenic mouse models and is associated with increased Nlrp3 expression and CaMKII (Ca2+/calmodulin-dependent protein kinase II) activation, with AF susceptibility prevented by inactivation of Nlrp3. Cardiomyocytes isolated from Ldlr-/- mice with hematopoietic inactivation of Tet2 and fed a Western diet have impaired calcium release from the sarcoplasmic reticulum into the cytosol, contributing to atrial arrhythmogenesis. Abnormal sarcoplasmic reticulum calcium release was recapitulated in cocultures of cardiomyocytes with the addition of Tet2-deficient macrophages or cytokines IL-1β or IL-6.

CONCLUSIONS

We identified a modest association between CHIP, particularly TET2 CHIP, and incident AF in the UK Biobank population. In a mouse model of AF resulting from hematopoietic-specific inactivation of Tet2, we propose altered calcium handling as an arrhythmogenic mechanism, dependent on Nlrp3 inflammasome activation. Our data are in keeping with previous studies of CHIP in cardiovascular disease, and further studies into the therapeutic potential of NLRP3 inhibition for individuals with TET2 CHIP may be warranted.

Atrial fibrillation-associated electrical remodelling in human induced pluripotent stem cell-derived atrial cardiomyocytes: a novel pathway for antiarrhythmic therapy development

AIMS

Atrial fibrillation (AF) is associated with tachycardia-induced cellular electrophysiology alterations which promote AF chronification and treatment resistance. Development of novel antiarrhythmic therapies is hampered by the absence of scalable experimental human models that reflect AF-associated electrical remodelling. Therefore, we aimed to assess if AF-associated remodelling of cellular electrophysiology can be simulated in human atrial-like cardiomyocytes derived from induced pluripotent stem cells in the presence of retinoic acid (iPSC-aCM), and atrial-engineered human myocardium (aEHM) under short term (24 h) and chronic (7 days) tachypacing (TP).

METHODS AND RESULTS

First, 24-h electrical pacing at 3 Hz was used to investigate whether AF-associated remodelling in iPSC-aCM and aEHM would ensue. Compared to controls (24 h, 1 Hz pacing) TP-stimulated iPSC-aCM presented classical hallmarks of AF-associated remodelling: (i) decreased L-type Ca2+ current (ICa,L) and (ii) impaired activation of acetylcholine-activated inward-rectifier K+ current (IK,ACh). This resulted in action potential shortening and an absent response to the M-receptor agonist carbachol in both iPSC-aCM and aEHM subjected to TP. Accordingly, mRNA expression of the channel-subunit Kir3.4 was reduced. Selective IK,ACh blockade with tertiapin reduced basal inward-rectifier K+ current only in iPSC-aCM subjected to TP, thereby unmasking an agonist-independent constitutively active IK,ACh. To allow for long-term TP, we developed iPSC-aCM and aEHM expressing the light-gated ion-channel f-Chrimson. The same hallmarks of AF-associated remodelling were observed after optical-TP. In addition, continuous TP (7 days) led to (i) increased amplitude of inward-rectifier K+ current (IK1), (ii) hyperpolarization of the resting membrane potential, (iii) increased action potential-amplitude and upstroke velocity as well as (iv) reversibly impaired contractile function in aEHM.

CONCLUSIONS

Classical hallmarks of AF-associated remodelling were mimicked through TP of iPSC-aCM and aEHM. The use of the ultrafast f-Chrimson depolarizing ion channel allowed us to model the time-dependence of AF-associated remodelling in vitro for the first time. The observation of electrical remodelling with associated reversible contractile dysfunction offers a novel platform for human-centric discovery of antiarrhythmic therapies.

25 years of basic and translational science in EP Europace: novel insights into arrhythmia mechanisms and therapeutic strategies

In the last 25 years, EP Europace has published more than 300 basic and translational science articles covering different arrhythmia types (ranging from atrial fibrillation to ventricular tachyarrhythmias), different diseases predisposing to arrhythmia formation (such as genetic arrhythmia disorders and heart failure), and different interventional and pharmacological anti-arrhythmic treatment strategies (ranging from pacing and defibrillation to different ablation approaches and novel drug-therapies). These studies have been conducted in cellular models, small and large animal models, and in the last couple of years increasingly in silico using computational approaches. In sum, these articles have contributed substantially to our pathophysiological understanding of arrhythmia mechanisms and treatment options; many of which have made their way into clinical applications. This review discusses a representative selection of EP Europace manuscripts covering the topics of pacing and ablation, atrial fibrillation, heart failure and pro-arrhythmic ventricular remodelling, ion channel (dys)function and pharmacology, inherited arrhythmia syndromes, and arrhythmogenic cardiomyopathies, highlighting some of the advances of the past 25 years. Given the increasingly recognized complexity and multidisciplinary nature of arrhythmogenesis and continued technological developments, basic and translational electrophysiological research is key advancing the field. EP Europace aims to further increase its contribution to the discovery of arrhythmia mechanisms and the implementation of mechanism-based precision therapy approaches in arrhythmia management.

Mouse models of spontaneous atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in adults, with a prevalence increasing with age. Current clinical management of AF is focused on tertiary prevention (i.e., treating the symptoms and sequelae) rather than addressing the underlying molecular pathophysiology. Robust animal models of AF, particularly those that do not require supraphysiologic stimuli to induce AF (i.e., showing spontaneous AF), enable studies that can uncover the underlying mechanisms of AF. Several mouse models of AF have been described to exhibit spontaneous AF, but pathophysiologic drivers of AF differ among models. Here, we describe relevant AF mechanisms and provide an overview of large and small animal models of AF. We then provide an in-depth review of the spontaneous mouse models of AF, highlighting the relevant AF mechanisms for each model.

Passive and Flexible Wireless Electronics Fabricated on Parylene/PDMS Substrate for Stimulation of Human Stem Cell-Derived Cardiomyocytes

In this paper, we report the development of a wireless, passive, biocompatible, and flexible system for stimulation of human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMS). Fabricated on a transparent parylene/PDMS substrate, the proposed stimulator enables real-time excitation and characterization of hiPSC-CMs cultured on-board. The device comprises a rectenna operating at 2.35 GHz which receives radio frequency (RF) energy from an external transmitter and converts it into DC voltage to deliver monophasic stimulation. The operation of the stimulator was primarily verified by delivering monophasic voltage pulses through gold electrodes to hiPSC-CMs cultured on the Matrigel-coated substrates. Stimulated hiPSC-CMs beat in accordance with the monophasic pulses when delivered at 0.5, 1, and 2 Hz pulsing frequency, while no significant cell death was observed. The wireless stimulator could generate monophasic pulses with an amplitude of 8 V at a distance of 15 mm. These results demonstrated the proposed wireless stimulator's efficacy for providing electrical stimulation to engineered cardiac tissues. The proposed stimulator will have a wide application in tissue engineering where a fully wireless stimulation of electroconductive cells is needed. The device also has potential to be employed as a cardiac stimulator by delivering external stimulation and regulating the contractions of cardiac tissue.

A novel murine model of atrial fibrillation by diphtheria toxin-induced injury

The treatment of atrial fibrillation (AF) continues to be a significant clinical challenge. While genome-wide association studies (GWAS) are beginning to identify AF susceptibility genes (Gudbjartsson et al., Nature, 2007, 448, 353-357; Choi et al., Circ. Res., 2020, 126, 200-209; van Ouwerkerk et al., Circ. Res., 2022, 127, 229-243), non-genetic risk factors including physical, chemical, and biological environments remain the major contributors to the development of AF. However, little is known regarding how non-genetic risk factors promote the pathogenesis of AF (Weiss et al., Heart Rhythm, 2016, 13, 1868-1877; Chakraborty et al., Heart Rhythm, 2020, 17, 1,398-1,404; Nattel et al., Circ. Res., 2020, 127, 51-72). This is, in part, due to the lack of a robust and reliable animal model induced by non-genetic factors. The currently available models using rapid pacing protocols fail to generate a stable AF phenotype in rodent models, often requiring additional genetic modifications that introduce potential sources of bias (Schüttler et al., Circ. Res., 2020, 127, 91-110). Here, we report a novel murine model of AF using an inducible and tissue-specific activation of diphtheria toxin (DT)-mediated cellular injury system. By the tissue-specific and inducible expression of human HB-EGF in atrial myocytes, we developed a reliable, robust and scalable murine model of AF that is triggered by a non-genetic inducer without the need for AF susceptibility gene mutations.

A Mechanistic Clinical Trial Using (R)- Versus (S)-Propafenone to Test RyR2 (Ryanodine Receptor) Inhibition for the Prevention of Atrial Fibrillation Induction

BACKGROUND

Experimental data suggest ryanodine receptor-mediated intracellular calcium leak is a mechanism for atrial fibrillation (AF), but evidence in humans is still needed. Propafenone is composed of two enantiomers that are equally potent sodium-channel blockers; however, (R)-propafenone is an ryanodine receptor inhibitor whereas (S)-propafenone is not. This study tested the hypothesis that ryanodine receptor inhibition with (R)-propafenone prevents induction of AF compared to (S)-propafenone or placebo in patients referred for AF ablation.

METHODS

Participants were randomized 4:4:1 to a one-time intravenous dose of (R)-propafenone, (S)-propafenone, or placebo. The study drug was given at the start of the procedure and an AF induction protocol using rapid atrial pacing was performed before ablation. The primary endpoint was 30 s of AF or atrial flutter.

RESULTS

A total of 193 participants were enrolled and 165 (85%) completed the study protocol (median age: 63 years, 58% male, 95% paroxysmal AF). Sustained AF and/or atrial flutter was induced in 60 participants (84.5%) receiving (R)-propafenone, 60 (80.0%) receiving (S)-propafenone group, and 12 (63.2%) receiving placebo. Atrial flutter occurred significantly more often in the (R)-propafenone (N=23, 32.4%) and (S)-propafenone (N=26, 34.7%) groups compared to placebo (N=1, 5.3%, P=0.029). There was no significant difference between (R)-propafenone and (S)-propafenone for the primary outcome of AF and/or atrial flutter induction in univariable (P=0.522) or multivariable analysis (P=0.199, adjusted for age and serum drug level).

CONCLUSIONS

There is no difference in AF inducibility between (R)-propafenone and (S)-propafenone at clinically relevant concentrations. These results are confounded by a high rate of inducible atrial flutter due to sodium-channel blockade.

REGISTRATION

https://clinicaltrials.gov; Unique Identifier: NCT02710669.

Shunt and pace: a novel experimental model of atrial fibrillation with a volume-loaded left atrium

BACKGROUND

Atrial fibrillation (AF) is frequently seen in patients with a volume-loaded left atrium (LA) such as mitral valve regurgitation (MR). Previous animal models have incomplete relevance to human AF associated with MR.

METHODS

A novel experimental model with a combination of volume loading of LA by creating a shunt from the subclavian artery to pulmonary artery and electrical remodeling induced by continuous rapid LA pacing was designed and the electrophysiological effects were examined in 10 canines. Five weeks after the shunt surgery, the entire atrial epicardium was mapped during sustained AF with form-fitted electrode patches with 246 bipolar electrodes and a three-dimensional dynamic mapping system to characterize the induced AF.

RESULTS

Three animals died of severe heart failure and pacing failure occurred in one. Remaining six animals were subjected to the analysis. The LA diameter increased progressively after the shunt surgery. Sustained AF was induced after 3 weeks of continuous rapid LA pacing in all animals. The activation maps revealed repetitive focal activations arising from the pulmonary veins, right or left atrial regions, and reentrant activations in the RA, which patterns of atrial activations are the same as those seen in human AF.

CONCLUSION

The animal model with a combination of LA volume load and electrical remodeling was relevant to human AF associated with LA volume load. Studies using the present model may provide further knowledges of AF and may be useful in examining the effects of pharmacological and non-pharmacological therapies.

Gut microbiota, dysbiosis and atrial fibrillation. Arrhythmogenic mechanisms and potential clinical implications

Recent preclinical and observational cohort studies have implicated imbalances in gut microbiota composition as a contributor to atrial fibrillation (AF). The gut microbiota is a complex and dynamic ecosystem containing trillions of microorganisms, which produces bioactive metabolites influencing host health and disease development. In addition to host-specific determinants, lifestyle-related factors such as diet and drugs are important determinants of the gut microbiota composition. In this review, we discuss the evidence suggesting a potential bidirectional association between AF and gut microbiota, identifying gut microbiota-derived metabolites as possible regulators of the AF substrate. We summarize the effect of gut microbiota on the development and progression of AF risk factors, including heart failure, hypertension, obesity, and coronary artery disease. We also discuss the potential anti-arrhythmic effects of pharmacological and diet-induced modifications of gut microbiota composition, which may modulate and prevent the progression to AF. Finally, we highlight important gaps in knowledge and areas requiring future investigation. Although data supporting a direct relationship between gut microbiota and AF are very limited at the present time, emerging preclinical and clinical research dealing with mechanistic interactions between gut microbiota and AF is important as it may lead to new insights into AF pathophysiology and the discovery of novel therapeutic targets for AF.

Stereotactic Radiotherapy Ablation and Atrial Fibrillation: Technical Issues and Clinical Expectations Derived From a Systematic Review

Aim

The purpose of this study is to collect available evidence on the feasibility and efficacy of stereotactic arrhythmia radio ablation (STAR), including both photon radiotherapy (XRT) and particle beam therapy (PBT), in the treatment of atrial fibrillation (AF), and to provide cardiologists and radiation oncologists with a practical overview on this topic.

Methods

Three hundred and thirty-five articles were identified up to November 2021 according to preferred reporting items for systematic reviews and meta-analyses criteria; preclinical and clinical studies were included without data restrictions or language limitations. Selected works were analyzed for comparing target selection, treatment plan details, and the accelerator employed, addressing workup modalities, acute and long-term side-effects, and efficacy, defined either by the presence of scar or by the absence of AF recurrence.

Results

Twenty-one works published between 2010 and 2021 were included. Seventeen studies concerned XRT, three PBT, and one involved both. Nine studies (1 in silico and 8 in vivo; doses ranging from 15 to 40 Gy) comprised a total of 59 animals, 12 (8 in silico, 4 in vivo; doses ranging from 16 to 50 Gy) focused on humans, with 9 patients undergoing STAR: average follow-up duration was 5 and 6 months, respectively. Data analysis supported efficacy of the treatment in the preclinical setting, whereas in the context of clinical studies the main favorable finding consisted in the detection of electrical scar in 4/4 patients undergoing specific evaluation; the minimum dose for efficacy was 25 Gy in both humans and animals. No acute complication was recorded; severe side-effects related to the long-term were observed only for very high STAR doses in 2 animals. Significant variability was evidenced among studies in the definition of target volume and doses, and in the management of respiratory and cardiac target motion.

Conclusion

STAR is an innovative non-invasive procedure already applied for experimental treatment of ventricular arrhythmias. Particular attention must be paid to safety, rather than efficacy of STAR, given the benign nature of AF. Uncertainties persist, mainly regarding the definition of the treatment plan and the role of the target motion. In this setting, more information about the toxicity profile of this new approach is compulsory before applying STAR to AF in clinical practice.

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

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

Peptide Inhibitors of Kv1.5: An Option for the Treatment of Atrial Fibrillation

The human voltage gated potassium channel Kv1.5 that conducts the IKur current is a key determinant of the atrial action potential. Its mutations have been linked to hereditary forms of atrial fibrillation (AF), and the channel is an attractive target for the management of AF. The development of IKur blockers to treat AF resulted in small molecule Kv1.5 inhibitors. The selectivity of the blocker for the target channel plays an important role in the potential therapeutic application of the drug candidate: the higher the selectivity, the lower the risk of side effects. In this respect, small molecule inhibitors of Kv1.5 are compromised due to their limited selectivity. A wide range of peptide toxins from venomous animals are targeting ion channels, including mammalian channels. These peptides usually have a much larger interacting surface with the ion channel compared to small molecule inhibitors and thus, generally confer higher selectivity to the peptide blockers. We found two peptides in the literature, which inhibited IKur: Ts6 and Osu1. Their affinity and selectivity for Kv1.5 can be improved by rational drug design in which their amino acid sequences could be modified in a targeted way guided by in silico docking experiments.

Size matters in atrial fibrillation: the underestimated importance of reduction of contiguous electrical mass underlying the effectiveness of catheter ablation

Evidence has accumulated over the last century of the importance of a critical electrical mass in sustaining atrial fibrillation (AF). AF ablation certainly reduces electrically contiguous atrial mass, but this is not widely accepted to be an important part of its mechanism of action. In this article, we review data showing that atrial size is correlated in many settings with AF propensity. Larger mammals are more likely to exhibit AF. This is seen both in the natural world and in animal models, where it is much easier to create a goat model than a mouse model of AF, for example. This also extends to humans-athletes, taller people, and obese individuals all have large atria and are more likely to exhibit AF. Within an individual, risk factors such as hypertension, valvular disease and ischaemia can enlarge the atrium and increase the risk of AF. With respect to AF ablation, we explore how variations in ablation strategy and the relative effectiveness of these strategies may suggest that a reduction in electrical atrial mass is an important mechanism of action. We counter this with examples in which there is no doubt that mass reduction is less important than competing theories such as ganglionated plexus ablation. We conclude that, when considering future strategies for the ablative therapy of AF, it is important not to discount the possibility that contiguous electrical mass reduction is the most important mechanism despite the disappointing consequence being that enhancing success rates in AF ablation may involve greater tissue destruction.

Are Interactions between Epicardial Adipose Tissue, Cardiac Fibroblasts and Cardiac Myocytes Instrumental in Atrial Fibrosis and Atrial Fibrillation?

Atrial fibrillation is very common among the elderly and/or obese. While myocardial fibrosis is associated with atrial fibrillation, the exact mechanisms within atrial myocytes and surrounding non-myocytes are not fully understood. This review considers the potential roles of myocardial fibroblasts and myofibroblasts in fibrosis and modulating myocyte electrophysiology through electrotonic interactions. Coupling with (myo)fibroblasts in vitro and in silico prolonged myocyte action potential duration and caused resting depolarization; an optogenetic study has verified in vivo that fibroblasts depolarized when coupled myocytes produced action potentials. This review also introduces another non-myocyte which may modulate both myocardial (myo)fibroblasts and myocytes: epicardial adipose tissue. Epicardial adipocytes are in intimate contact with myocytes and (myo)fibroblasts and may infiltrate the myocardium. Adipocytes secrete numerous adipokines which modulate (myo)fibroblast and myocyte physiology. These adipokines are protective in healthy hearts, preventing inflammation and fibrosis. However, adipokines secreted from adipocytes may switch to pro-inflammatory and pro-fibrotic, associated with reactive oxygen species generation. Pro-fibrotic adipokines stimulate myofibroblast differentiation, causing pronounced fibrosis in the epicardial adipose tissue and the myocardium. Adipose tissue also influences myocyte electrophysiology, via the adipokines and/or through electrotonic interactions. Deeper understanding of the interactions between myocytes and non-myocytes is important to understand and manage atrial fibrillation.

Heart failure with preserved ejection fraction in humans and mice: embracing clinical complexity in mouse models

Heart failure (HF) with preserved ejection fraction (HFpEF) is a multifactorial disease accounting for a large and increasing proportion of all clinical HF presentations. As a clinical syndrome, HFpEF is characterized by typical signs and symptoms of HF, a distinct cardiac phenotype and raised natriuretic peptides. Non-cardiac comorbidities frequently co-exist and contribute to the pathophysiology of HFpEF. To date, no therapy has proven to improve outcomes in HFpEF, with drug development hampered, at least partly, by lack of consensus on appropriate standards for pre-clinical HFpEF models. Recently, two clinical algorithms (HFA-PEFF and H₂FPEF scores) have been developed to improve and standardize the diagnosis of HFpEF. In this review, we evaluate the translational utility of HFpEF mouse models in the context of these HFpEF scores. We systematically recorded evidence of symptoms and signs of HF or clinical HFpEF features and included several cardiac and extra-cardiac parameters as well as age and sex for each HFpEF mouse model. We found that most of the pre-clinical HFpEF models do not meet the HFpEF clinical criteria, although some multifactorial models resemble human HFpEF to a reasonable extent. We therefore conclude that to optimize the translational value of mouse models to human HFpEF, a novel approach for the development of pre-clinical HFpEF models is needed, taking into account the complex HFpEF pathophysiology in humans.

Integrated Quantitative Phosphoproteomics and Cell-based Functional Screening Reveals Specific Pathological Cardiac Hypertrophy-related Phosphorylation Sites

Cardiac hypertrophic signaling cascades resulting in heart failure diseases are mediated by protein phosphorylation. Recent developments in mass spectrometry-based phosphoproteomics have led to the identification of thousands of differentially phosphorylated proteins and their phosphorylation sites. However, functional studies of these differentially phosphorylated proteins have not been conducted in a large-scale or high-throughput manner due to a lack of methods capable of revealing the functional relevance of each phosphorylation site. In this study, an integrated approach combining quantitative phosphoproteomics and cell-based functional screening using phosphorylation competition peptides was developed. A pathological cardiac hypertrophy model, junctate-1 transgenic mice and control mice, were analyzed using label-free quantitative phosphoproteomics to identify differentially phosphorylated proteins and sites. A cell-based functional assay system measuring hypertrophic cell growth of neonatal rat ventricle cardiomyocytes (NRVMs) following phenylephrine treatment was applied, and changes in phosphorylation of individual differentially phosphorylated sites were induced by incorporation of phosphorylation competition peptides conjugated with cell-penetrating peptides. Cell-based functional screening against 18 selected phosphorylation sites identified three phosphorylation sites (Ser-98, Ser-179 of Ldb3, and Ser-1146 of palladin) displaying near-complete inhibition of cardiac hypertrophic growth of NRVMs. Changes in phosphorylation levels of Ser-98 and Ser-179 in Ldb3 were further confirmed in NRVMs and other pathological/physiological hypertrophy models, including transverse aortic constriction and swimming models, using site-specific phospho-antibodies. Our integrated approach can be used to identify functionally important phosphorylation sites among differentially phosphorylated sites, and unlike conventional approaches, it is easily applicable for large-scale and/or high-throughput analyses.

ESC working group on cardiac cellular electrophysiology position paper: relevance, opportunities, and limitations of experimental models for cardiac electrophysiology research

Cardiac arrhythmias are a major cause of death and disability. A large number of experimental cell and animal models have been developed to study arrhythmogenic diseases. These models have provided important insights into the underlying arrhythmia mechanisms and translational options for their therapeutic management. This position paper from the ESC Working Group on Cardiac Cellular Electrophysiology provides an overview of (i) currently available in vitro, ex vivo, and in vivo electrophysiological research methodologies, (ii) the most commonly used experimental (cellular and animal) models for cardiac arrhythmias including relevant species differences, (iii) the use of human cardiac tissue, induced pluripotent stem cell (hiPSC)-derived and in silico models to study cardiac arrhythmias, and (iv) the availability, relevance, limitations, and opportunities of these cellular and animal models to recapitulate specific acquired and inherited arrhythmogenic diseases, including atrial fibrillation, heart failure, cardiomyopathy, myocarditis, sinus node, and conduction disorders and channelopathies. By promoting a better understanding of these models and their limitations, this position paper aims to improve the quality of basic research in cardiac electrophysiology, with the ultimate goal to facilitate the clinical translation and application of basic electrophysiological research findings on arrhythmia mechanisms and therapies.

Differential Effect of Three Macrolide Antibiotics on Cardiac Pathology and Electrophysiology in a Myocardial Infarction Rat Model: Influence on Sodium Nav1.5 Channel Expression

Macrolides were reported to have cardiotoxic effects presented mainly by electrocardiogram (ECG) changes with increased risk in cardiac patients. We aimed to determine the impact of three macrolides, azithromycin, clarithromycin and erythromycin, on cardiac electrophysiology, cardiac enzyme activities, histopathological changes, and sodium voltage-gated alpha subunit 5 (Nav1.5) channel expression. We used eight experimental groups of male albino rats: vehicle, azithromycin (100 mg/kg), clarithromycin (100 mg/kg), erythromycin (100 mg/kg), MI + vehicle, MI + azithromycin (100 mg/kg), MI + clarithromycin (100 mg/kg) and MI + erythromycin (100 mg/kg); each group received chronic oral doses of the vehicle/drugs for seven weeks. ECG abnormalities and elevated serum cardiac enzymes were observed particularly in rats with AMI compared to healthy rats. Microscopic examination revealed elevated pathology scores for rats treated with clarithromycin in both experiments following treatment with erythromycin in healthy rats. Although rats with MI did not show further elevations in fibrosis score on treatment with macrolides, they produced significant fibrosis in healthy rats. Downregulation of cardiac Nav1.5 transcript was observed following macrolides treatment in both groups (healthy rats and rats with MI). In conclusion, the current findings suggested the potential cardiotoxic effects of chronic doses of macrolide antibiotics in rats with MI as manifested by abnormal ECG changes and pathological findings in addition to downregulation of Nav1.5 channels. Furthermore, in the current dose ranges, azithromycin produced the least toxicity compared to clarithromycin and erythromycin.

The inflammation-resolution promoting molecule resolvin-D1 prevents atrial proarrhythmic remodelling in experimental right heart disease

AIMS

Inflammation plays a role in atrial fibrillation (AF), but classical anti-inflammatory molecules are ineffective. Recent evidence suggests that failure of inflammation-resolution causes persistent inflammatory signalling and that a novel drug-family called resolvins promotes inflammation-resolution. Right heart disease (RHD) is associated with AF; experimental RHD shows signs of atrial inflammatory-pathway activation. Here, we evaluated resolvin-therapy effects on atrial arrhythmogenic remodelling in experimental RHD.

METHODS AND RESULTS

Pulmonary hypertension and RHD were induced in rats with an intraperitoneal injection of 60 mg/kg monocrotaline (MCT). An intervention group received daily resolvin-D1 (RvD1), starting 1 day before MCT administration. Right atrial (RA) conduction and gene-expression were analysed respectively by optical mapping and qPCR/gene-microarray. RvD1 had no or minimal effects on MCT-induced pulmonary artery or right ventricular remodelling. Nevertheless, in vivo transoesophageal pacing induced atrial tachyarrhythmias in no CTRL rats vs. 100% MCT-only rats, and only 33% RvD1-treated MCT rats (P < 0.001 vs. MCT-only). Conduction velocity was significantly decreased by MCT, an effect prevented by RvD1. RHD caused RA dilation and fibrosis. RvD1 strongly attenuated RA fibrosis but had no effect on RA dilation. MCT increased RA expression of inflammation- and fibrosis-related gene-expression pathways on gene-microarray transcriptomic analysis, effects significantly attenuated by RvD1 (334 pathways enriched in MCT-rats vs. control; only 177 dysregulated by MCT with RvD1 treatment). MCT significantly increased RA content of type 1 (proinflammatory) CD68-positive M1 macrophages without affecting type 2 (anti-inflammatory) M2 macrophages. RvD1-treated MCT-rat RA showed significant reductions in proinflammatory M1 macrophages and increases in anti-inflammatory M2 macrophages vs. MCT-only. MCT caused statistically significant increases in protein-expression (western blot) of COL3A1, ASC, CASP1, CASP8, IL1β, TGFβ3, CXCL1, and CXCL2, and decreases in MMP2, vs. control. RvD1-treatment suppressed all these MCT-induced protein-expression changes.

CONCLUSION

The inflammation-resolution enhancing molecule RvD1 prevents AF-promoting RA remodelling, while suppressing inflammatory changes and fibrotic/electrical remodelling, in RHD. Resolvins show potential promise in combating atrial arrhythmogenic remodelling by suppressing ongoing inflammatory signalling.

Transcriptome and proteome mapping in the sheep atria reveal molecular featurets of atrial fibrillation progression

AIMS

Atrial fibrillation (AF) is a progressive cardiac arrhythmia that increases the risk of hospitalization and adverse cardiovascular events. There is a clear demand for more inclusive and large-scale approaches to understand the molecular drivers responsible for AF, as well as the fundamental mechanisms governing the transition from paroxysmal to persistent and permanent forms. In this study, we aimed to create a molecular map of AF and find the distinct molecular programmes underlying cell type-specific atrial remodelling and AF progression.

METHODS AND RESULTS

We used a sheep model of long-standing, tachypacing-induced AF, sampled right and left atrial tissue, and isolated cardiomyocytes (CMs) from control, intermediate (transition), and late time points during AF progression, and performed transcriptomic and proteome profiling. We have merged all these layers of information into a meaningful three-component space in which we explored the genes and proteins detected and their common patterns of expression. Our data-driven analysis points at extracellular matrix remodelling, inflammation, ion channel, myofibril structure, mitochondrial complexes, chromatin remodelling, and genes related to neural function, as well as critical regulators of cell proliferation as hallmarks of AF progression. Most important, we prove that these changes occur at early transitional stages of the disease, but not at later stages, and that the left atrium undergoes significantly more profound changes than the right atrium in its expression programme. The pattern of dynamic changes in gene and protein expression replicate the electrical and structural remodelling demonstrated previously in the sheep and in humans, and uncover novel mechanisms potentially relevant for disease treatment.

CONCLUSIONS

Transcriptomic and proteomic analysis of AF progression in a large animal model shows that significant changes occur at early stages, and that among others involve previously undescribed increase in mitochondria, changes to the chromatin of atrial CMs, and genes related to neural function and cell proliferation.

Characterization of the HCN Interaction Partner TRIP8b/PEX5R in the Intracardiac Nervous System of TRIP8b-Deficient and Wild-Type Mice

The tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b/PEX5R) is an interaction partner and auxiliary subunit of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which are key for rhythm generation in the brain and in the heart. Since TRIP8b is expressed in central neurons but not in cardiomyocytes, the TRIP8b-HCN interaction has been studied intensely in the brain, but is deemed irrelevant in the cardiac conduction system. Still, to date, TRIP8b has not been studied in the intrinsic cardiac nervous system (ICNS), a neuronal network located within epicardial fat pads. In vitro electrophysiological studies revealed that TRIP8b-deficient mouse hearts exhibit increased atrial refractory and atrioventricular nodal refractory periods, compared to hearts of wild-type littermates. Meanwhile, heart rate, sino-nodal recovery time, and ventricular refractory period did not differ between genotypes. Trip8b mRNA was detected in the ICNS by quantitative polymerase chain reaction. RNAscope in situ hybridization confirmed Trip8b localization in neuronal somata and nerve fibers. Additionally, we found a very low amount of mRNAs in the sinus node and atrioventricular node, most likely attributable to the delicate fibers innervating the conduction system. In contrast, TRIP8b protein was not detectable. Our data suggest that TRIP8b in the ICNS may play a role in the modulation of atrial electrophysiology beyond HCN-mediated sino-nodal control of the heart.

Cannabinoid Receptor Agonist Inhibits Atrial Electrical Remodeling in a Tachypaced Ex Vivo Rat Model

Introduction: Atrial fibrillation (AF) leads to rate-dependent atrial changes collectively defined as atrial remodelling (AR). Shortening of the atrial effective refractory period (AERP) and decreased conduction velocity are among the hallmarks of AR. Pharmacological strategies to inhibit AR, thereby reducing the self-perpetual nature of AF, are of great clinical value. Cannabinoid receptor (CBR) ligands may exert cardioprotective effects; CB13, a dual CBR agonist with limited brain penetration, protects cardiomyocytes from mitochondrial dysfunction induced by endothelin-1. Here, we examined the effects of CB13 on normal physiology of the rat heart and development of tachypacing-induced AR.

Methods: Rat hearts were perfused in a Langendorff set-up with CB13 (1 µM) or vehicle. Hemodynamic properties of non-paced hearts were examined conventionally. In a different set of hearts, programmed stimulation protocol was performed before and after atrial tachypacing for 90 min using a mini-hook platinum quadrupole electrode inserted on the right atrium. Atrial samples were further assessed by western blot analysis.

Results: CB13 had no effects on basal hemodynamic properties. However, the compound inhibited tachypacing-induced shortening of the AERP. Protein expression of PGC1α was significantly increased by CB13 compared to vehicle in paced and non-paced hearts. Phosphorylation of AMPKα at residue threonine 172 was increased suggesting upregulation of mitochondrial biogenesis. Connexin43 was downregulated by tachypacing. This effect was diminished in the presence of CB13.

Conclusion: Our findings support the notion that peripheral activation of CBR may be a new treatment strategy to prevent AR in patients suffering from AF, and therefore warrants further study.

Patch-Clamp Recordings of Action Potentials From Human Atrial Myocytes: Optimization Through Dynamic Clamp

Introduction: Atrial fibrillation (AF) is the most common cardiac arrhythmia. Consequently, novel therapies are being developed. Ultimately, the impact of compounds on the action potential (AP) needs to be tested in freshly isolated human atrial myocytes. However, the frequent depolarized state of these cells upon isolation seriously hampers reliable AP recordings.

Purpose: We assessed whether AP recordings from single human atrial myocytes could be improved by providing these cells with a proper inward rectifier K⁺ current (I_K1), and consequently with a regular, non-depolarized resting membrane potential (RMP), through “dynamic clamp”.

Methods: Single myocytes were enzymatically isolated from left atrial appendage tissue obtained from patients with paroxysmal AF undergoing minimally invasive surgical ablation. APs were elicited at 1 Hz and measured using perforated patch-clamp methodology, injecting a synthetic I_K1 to generate a regular RMP. The injected I_K1 had strong or moderate rectification. For comparison, a regular RMP was forced through injection of a constant outward current. A wide variety of ion channel blockers was tested to assess their modulatory effects on AP characteristics.

Results: Without any current injection, RMPs ranged from −9.6 to −86.2 mV in 58 cells. In depolarized cells (RMP positive to −60 mV), RMP could be set at −80 mV using I_K1 or constant current injection and APs could be evoked upon stimulation. AP duration differed significantly between current injection methods (p < 0.05) and was shortest with constant current injection and longest with injection of I_K1 with strong rectification. With moderate rectification, AP duration at 90% repolarization (APD₉₀) was similar to myocytes with regular non-depolarized RMP, suggesting that a synthetic I_K1 with moderate rectification is the most appropriate for human atrial myocytes. Importantly, APs evoked using each injection method were still sensitive to all drugs tested (lidocaine, nifedipine, E-4031, low dose 4-aminopyridine, barium, and apamin), suggesting that the major ionic currents of the atrial cells remained functional. However, certain drug effects were quantitatively dependent on the current injection approach used.

Conclusion: Injection of a synthetic I_K1 with moderate rectification facilitates detailed AP measurements in human atrial myocytes. Therefore, dynamic clamp represents a promising tool for testing novel antiarrhythmic drugs.

Anti-arrhythmic investigations in large animal models of atrial fibrillation

Atrial fibrillation (AF) constitutes an increasing health problem in the aging population. Animal models reflecting human phenotypes are needed to understand the mechanisms of AF, as well as to test new pharmacological interventions. In recent years, a number of large animal models, primarily pigs, goats, dog and horses have been used in AF research. These animals can to a certain extent recapitulate the human pathophysiological characteristics and serve as valuable tools in investigating new pharmacological interventions for treating AF. This review focuses on anti-arrhythmic investigations in large animals. Initially, spontaneous AF in small and large mammals is discussed. This is followed by a short presentation of frequently used methods for inducing short- and long-term AF. The major focus of the review is on anti-arrhythmic compounds either frequently used in the human clinic (ranolazine, flecainide, vernakalant and amiodarone) or being promising new AF medicine candidates (I_K,Ach , I_SK,Ca and I_K2P blockers).

Identification of two preclinical canine models of atrial fibrillation to facilitate drug discovery

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmia occurring in humans, and new treatment strategies are critically needed. The lack of reliable preclinical animal models of AF is a major limitation to drug development of novel antiarrhythmic compounds.

OBJECTIVE

The purpose of this study was to provide a comprehensive head-to-head assessment of 5 canine AF models.

METHODS

Five canine models were evaluated for the efficacy of AF induction and AF duration. We tested 2 acute models: short-term atrial tachypacing (AT) for 6 hours with analysis of AF at hourly increments, and carbachol injection into a cardiac fat pad followed by short-term AT. We also tested 3 chronic models: pacemaker implantation followed by either 4 weeks of AT and subsequent atrial burst pacing or intermittent long-term AT for up to 4-5 months to generate AF ≥4.5 hours, and finally ventricular tachypacing to induce heart failure followed by atrial burst pacing to induce AF.

RESULTS

Careful evaluation showed that acute AT, AT for 4 weeks, and the heart failure model all were unsuccessful in generating reproducible AF episodes of sufficient duration to study antiarrhythmic drugs. In contrast, intermittent long-term AT generated AF lasting ≥4.5 hours in ∼30% of animals. The acute model using carbachol and short-term AT resulted in AF induction of ≥15 minutes in ≥75% of animals, thus enabling testing of antiarrhythmic drugs.

CONCLUSION

Intermittent long-term AT and the combination of local carbachol injection with successive short-term AT may contribute to future drug development efforts for AF treatment.

ECG Restitution Analysis and Machine Learning to Detect Paroxysmal Atrial Fibrillation: Insight from the Equine Athlete as a Model for Human Athletes

Atrial fibrillation is the most frequent arrhythmia in both equine and human athletes. Currently, this condition is diagnosed via electrocardiogram (ECG) monitoring which lacks sensitivity in about half of cases when it presents in paroxysmal form. We investigated whether the arrhythmogenic substrate present between the episodes of paroxysmal atrial fibrillation (PAF) can be detected using restitution analysis of normal sinus-rhythm ECGs. In this work, ECG recordings were obtained during routine clinical work from control and horses with PAF. The extracted QT, TQ, and RR intervals were used for ECG restitution analysis. The restitution data were trained and tested using k-nearest neighbor (k-NN) algorithm with various values of neighbors k to derive a discrimination tool. A combination of QT, RR, and TQ intervals was used to analyze the relationship between these intervals and their effects on PAF. A simple majority vote on individual record (one beat) classifications was used to determine the final classification. The k-NN classifiers using two-interval measures were able to predict the diagnosis of PAF with area under the receiving operating characteristic curve close to 0.8 (RR, TQ with k ≥ 9) and 0.9 (RR, QT with k ≥ 21 or TQ, QT with k ≥ 25). By simultaneously using all three intervals for each beat and a majority vote, mean area under the curves of 0.9 were obtained for all tested k-values (3-41). We concluded that 3D ECG restitution analysis can potentially be used as a metric of an automated method for screening of PAF.

Comprehensive evaluation of electrophysiological and 3D structural features of human atrial myocardium with insights on atrial fibrillation maintenance mechanisms

Atrial fibrillation (AF) occurrence and maintenance is associated with progressive remodeling of electrophysiological (repolarization and conduction) and 3D structural (fibrosis, fiber orientations, and wall thickness) features of the human atria. Significant diversity in AF etiology leads to heterogeneous arrhythmogenic electrophysiological and structural substrates within the 3D structure of the human atria. Since current clinical methods have yet to fully resolve the patient-specific arrhythmogenic substrates, mechanism-based AF treatments remain underdeveloped. Here, we review current knowledge from in-vivo, ex-vivo, and in-vitro human heart studies, and discuss how these studies may provide new insights on the synergy of atrial electrophysiological and 3D structural features in AF maintenance. In-vitro studies on surgically acquired human atrial samples provide a great opportunity to study a wide spectrum of AF pathology, including functional changes in single-cell action potentials, ion channels, and gene/protein expression. However, limited size of the samples prevents evaluation of heterogeneous AF substrates and reentrant mechanisms. In contrast, coronary-perfused ex-vivo human hearts can be studied with state-of-the-art functional and structural technologies, such as high-resolution near-infrared optical mapping and contrast-enhanced MRI. These imaging modalities can resolve atrial arrhythmogenic substrates and their role in reentrant mechanisms maintaining AF and validate clinical approaches. Nonetheless, longitudinal studies are not feasible in explanted human hearts. As no approach is perfect, we suggest that combining the strengths of direct human atrial studies with high fidelity approaches available in the laboratory and in realistic patient-specific computer models would elucidate deeper knowledge of AF mechanisms. We propose that a comprehensive translational pipeline from ex-vivo human heart studies to longitudinal clinically relevant AF animal studies and finally to clinical trials is necessary to identify patient-specific arrhythmogenic substrates and develop novel AF treatments.

Fragmented sinoatrial dynamics in the prediction of atrial fibrillation: the Multi-Ethnic Study of Atherosclerosis

Heart rate fragmentation (HRF), a marker of abnormal sinoatrial dynamics, was shown to be associated with incident cardiovascular events in the Multi-Ethnic Study of Atherosclerosis (MESA). Here, we test the hypothesis that HRF is also associated with incident atrial fibrillation (AF) in the MESA cohort of participants who underwent in-home polysomnography (PSG) and in two high-risk subgroups: those ≥70 yr taking antihypertensive medication and those with serum concentrations of NH₂-terminal prohormone B-type natriuretic peptide (NT-proBNP) >125 pg/ml (top quartile). Heart rate time series (n = 1,858) derived from the ECG channel of the PSG were analyzed using newly developed HRF metrics, traditional heart rate variability (HRV) indices and two widely used nonlinear measures. Eighty-three participants developed AF over a mean follow-up period of 3.83 ± 0.87 yr. A one-standard deviation increase in HRF was associated with a 31% (95% CI: 3-66%) increase in risk of incident AF, in Cox models adjusted for age, height, NT-proBNP, and frequent premature supraventricular complexes. Furthermore, HRF added value to the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE)-AF models. Traditional HRV and nonlinear indices were not significantly associated with incident AF. In the two high-risk subgroups defined above, HRF was also significantly associated with incident AF in unadjusted and adjusted models. These findings support the translational utility of HRF metrics for short-term (∼4-yr) prediction of AF. In addition, they support broadening the concept of atrial remodeling to include electrodynamical remodeling, a term used to refer to pathophysiological alterations in sinus interbeat interval dynamics.NEW & NOTEWORTHY This study is the first demonstration that heart rate fragmentation (HRF), a marker of anomalous sinoatrial dynamics, is an independent predictor of atrial fibrillation (AF). Traditional measures of heart rate variability and two widely used nonlinear measures were not associated with incident AF in the Multi-Ethnic Study of Atherosclerosis. Fragmentation measures added value to the strongest contemporary predictors of AF, including ECG-derived parameters, coronary calcification score, serum concentrations of NH₂-terminal prohormone B-type natriuretic peptide, and supraventricular ectopy. The computational algorithms for quantification of HRF could be readily incorporated into wearable ECG monitoring devices.

Ion Channel and Structural Remodeling in Obesity-Mediated Atrial Fibrillation

BACKGROUND

Epidemiological studies have established obesity as an independent risk factor for atrial fibrillation (AF), but the underlying pathophysiological mechanisms remain unclear. Reduced cardiac sodium channel expression is a known causal mechanism in AF. We hypothesized that obesity decreases Nav1.5 expression via enhanced oxidative stress, thus reducing INa, and enhancing susceptibility to AF.

METHODS

To elucidate the underlying electrophysiological mechanisms a diet-induced obese mouse model was used. Weight, blood pressure, glucose, F2-isoprostanes, NOX2 (NADPH oxidase 2), and PKC (protein kinase C) were measured in obese mice and compared with lean controls. Invasive electrophysiological, immunohistochemistry, Western blotting, and patch clamping of membrane potentials was performed to evaluate the molecular and electrophysiological phenotype of atrial myocytes.

RESULTS

Pacing-induced AF in 100% of diet-induced obese mice versus 25% in controls (P<0.01) with increased AF burden. Cardiac sodium channel expression, INa and atrial action potential duration were reduced and potassium channel expression (Kv1.5) and current (IKur) and F2-isoprostanes, NOX2, and PKC-α/δ expression and atrial fibrosis were significantly increased in diet-induced obese mice as compared with controls. A mitochondrial antioxidant reduced AF burden, restored INa, ICa,L, IKur, action potential duration, and reversed atrial fibrosis in diet-induced obese mice as compared with controls.

CONCLUSIONS

Inducible AF in obese mice is mediated, in part, by a combined effect of sodium, potassium, and calcium channel remodeling and atrial fibrosis. Mitochondrial antioxidant therapy abrogated the ion channel and structural remodeling and reversed the obesity-induced AF burden. Our findings have important implications for the management of obesity-mediated AF in patients. Graphic Abstract: A graphic abstract is available for this article.

The molecular genetic basis of atrial fibrillation

As the most common cardiac arrhythmia, atrial fibrillation (AF) is a major risk factor for stroke, heart failure, and premature death with considerable associated costs. However, no available treatment options have optimal benefit-harm profiles currently, reflecting an incomplete understanding of the biological mechanisms underlying this complex arrhythmia. Recently, molecular epidemiological studies, especially genome-wide association studies, have emphasized the substantial genetic component of AF etiology. A comprehensive mapping of the genetic underpinnings for AF can expand our knowledge of AF mechanism and further facilitate the process of locating novel therapeutics for AF. Here we provide a state-of-the-art review of the molecular genetics of AF incorporating evidence from linkage analysis and candidate gene, as well as genome-wide association studies of common variations and rare copy number variations; potential epigenetic modifications (e.g., DNA methylation, histone modification, and non-coding RNAs) are also involved. We also outline the challenges in mechanism investigation and potential future directions in this article.

Colchicine prevents atrial fibrillation promotion by inhibiting IL-1β-induced IL-6 release and atrial fibrosis in the rat sterile pericarditis model

A few clinical trials have recently reported the potential effect of colchicine in preventing post-operative atrial fibrillation (POAF) and early atrial fibrillation (AF) recurrence after catheter pulmonary vein isolation. However, the molecular mechanisms through which colchicine inhibits AF remain unclear. We aim to assess the anti-AF effect of colchicine in the rat sterile pericarditis (SP) model and to investigate its molecular mechanisms. SP was induced in Sprague-Dawley rats by the epicardial application of sterile talc. Treatment with colchicine or vehicle began 1 d before pericardiotomy. AF was induced by transesophageal burst pacing on day 3 after surgery. Treatment with colchicine reduced the duration of AF and the probability of induction of AF in SP rats. The dose of 0.5 mg kg^-1·day^-1 had the best effect. Such treatment also reduced neutrophil infiltration, the mRNA expression of IL-6, TGF-β, and TNF-α, atrial fibrosis, fibrosis related genes, and signal molecules (STAT3, P38, and AKT). Meanwhile, the release of IL-1β (4-24 h) and IL-6 (4-72 h) in atria after surgery was significantly inhibited by colchicine. In cultured rat cardiac fibroblasts, colchicine treatment inhibited IL-1β-induced expression of IL-6, which was accompanied by significantly decreased phosphorylation of P38, AKT, JNK, and NFκB. Interestingly, the supplementation of IL-6 abolished the anti-AF effect of colchicine in SP rats. Colchicine prevents AF in SP rats through the inhibition of IL-1β-induced IL-6 release and subsequent atrial fibrosis. However, further studies are required to investigate whether colchicine inhibits POAF through other mechanisms.

Inhibition of Adenosine Pathway Alters Atrial Electrophysiology and Prevents Atrial Fibrillation

Background

Adenosine leads to atrial action potential (AP) shortening through activation of adenosine 1 receptors (A₁-R) and subsequent opening of G-protein-coupled inwardly rectifying K⁺ channels. Extracellular production of adenosine is drastically increased during stress and ischemia.

Objective

The aim of this study was to address whether the pharmacological blockade of endogenous production of adenosine and of its signaling prevents atrial fibrillation (AF).

Methods

The role of A₁-R activation on atrial action potential duration, refractoriness, and AF vulnerability was investigated in rat isolated beating heart preparations (Langendorff) with an A₁-R agonist [2-chloro-N⁶-cyclopentyladenosine (CCPA), 50 nM] and antagonist [1-butyl-3-(3-hydroxypropyl)-8-(3-noradamantyl)xanthine (PSB36), 40 nM]. Furthermore, to interfere with the endogenous adenosine release, the ecto-5′-nucleotidase (CD73) inhibitor was applied [5′-(α,β-methylene) diphosphate sodium salt (AMPCP), 500 μM]. Isolated trabeculae from human right atrial appendages (hRAAs) were used for comparison.

Results

As expected, CCPA shortened AP duration at 90% of repolarization (APD₉₀) and effective refractory period (ERP) in rat atria. PSB36 prolonged APD₉₀ and ERP in rat atria, and CD73 inhibition with AMPCP prolonged ERP in rats, confirming that endogenously produced amount of adenosine is sufficiently high to alter atrial electrophysiology. In human atrial appendages, CCPA shortened APD₉₀, while PSB36 prolonged it. Rat hearts treated with CCPA are prone to AF. In contrast, PSB36 and AMPCP prevented AF events and reduced AF duration (vehicle, 11.5 ± 2.6 s; CCPA, 40.6 ± 16.1 s; PSB36, 6.5 ± 3.7 s; AMPCP, 3.0 ± 1.4 s; P < 0.0001).

Conclusion

A₁-R activation by intrinsic adenosine release alters atrial electrophysiology and promotes AF. Inhibition of adenosine pathway protects atria from arrhythmic events.

Improving translational research in sex-specific effects of comorbidities and risk factors in ischaemic heart disease and cardioprotection: position paper and recommendations of the ESC Working Group on Cellular Biology of the Heart

Ischaemic heart disease (IHD) is a complex disorder and a leading cause of death and morbidity in both men and women. Sex, however, affects several aspects of IHD, including pathophysiology, incidence, clinical presentation, diagnosis as well as treatment and outcome. Several diseases or risk factors frequently associated with IHD can modify cellular signalling cascades, thus affecting ischaemia/reperfusion injury as well as responses to cardioprotective interventions. Importantly, the prevalence and impact of risk factors and several comorbidities differ between males and females, and their effects on IHD development and prognosis might differ according to sex. The cellular and molecular mechanisms underlying these differences are still poorly understood, and their identification might have important translational implications in the prediction or prevention of risk of IHD in men and women. Despite this, most experimental studies on IHD are still undertaken in animal models in the absence of risk factors and comorbidities, and assessment of potential sex-specific differences are largely missing. This ESC WG Position Paper will discuss: (i) the importance of sex as a biological variable in cardiovascular research, (ii) major biological mechanisms underlying sex-related differences relevant to IHD risk factors and comorbidities, (iii) prospects and pitfalls of preclinical models to investigate these associations, and finally (iv) will provide recommendations to guide future research. Although gender differences also affect IHD risk in the clinical setting, they will not be discussed in detail here.

Analysis of electropharmacological effects of AVE0118 on the atria of chronic atrioventricular block dogs: characterization of anti-atrial fibrillatory action by atrial repolarization-delaying agent

AVE0118, an inhibitor of I_Kur, I_to and I_K,ACh, was in the drug pipeline for atrial fibrillation. To investigate the limitation of AVE0118 as an anti-atrial fibrillatory drug, we studied its electropharmacological effects particularly focusing on the anti-atrial fibrillatory action as reverse translational research. We adopted the chronic atrioventricular block beagle dogs (n = 4), having a pathophysiology of bradycardia-associated, volume overload-induced chronic heart failure, in which the atrial fibrillation was induced by 10 s of burst pacing on atrial septum. AVE0118 in doses of 0.24 and 1.2 mg/kg, i.v. over 10 min hardly altered electrophysiological variables. Meanwhile, AVE0118 in a dose of 6 mg/kg, i.v. over 10 min delayed the inter-atrial conduction in a frequency-dependent manner and prolonged the atrial effective refractory period in a reverse frequency-dependent manner, whereas it did not significantly alter the duration of atrial fibrillation or its cycle length. The increment of atrial effective refractory period was 3.3 times greater compared with that of ventricular one at a basic cycle length of 400 ms. Torsade de pointes was not induced during the experimental period. Thus, AVE0118 may possess a favorable cardiac safety pharmacological profile, but its weak anti-atrial fibrillatory effect would indicate the limitation of atrial repolarization-delaying agents for suppressing atrial fibrillation.

Altered calcium handling produces reentry-promoting action potential alternans in atrial fibrillation-remodeled hearts

Atrial fibrillation (AF) alters atrial cardiomyocyte (ACM) Ca2+ handling, promoting ectopic beat formation. We examined the effects of AF-associated remodeling on Ca2+-related action potential dynamics and consequences for AF susceptibility. AF was maintained electrically in dogs by right atrial (RA) tachypacing. ACMs isolated from AF dogs showed increased Ca2+ release refractoriness, spontaneous Ca2+ spark frequency, and cycle length (CL) threshold for Ca2+ and action potential duration (APD) alternans versus controls. AF increased the in situ CL threshold for Ca2+/APD alternans and spatial dispersion in Ca2+ release recovery kinetics, leading to spatially discordant alternans associated with reentrant rotor formation and susceptibility to AF induction/maintenance. The clinically available agent dantrolene reduced Ca2+ leak and CL threshold for Ca2+/APD alternans in ACMs and AF dog right atrium, while suppressing AF susceptibility; caffeine increased Ca2+ leak and CL threshold for Ca2+/APD alternans in control dog ACMs and RA tissues. In vivo, the atrial repolarization alternans CL threshold was increased in AF versus control, as was AF vulnerability. Intravenous dantrolene restored repolarization alternans threshold and reduced AF vulnerability. Immunoblots showed reduced expression of total and phosphorylated ryanodine receptors and calsequestrin in AF and unchanged phospholamban/SERCA expression. Thus, along with promoting spontaneous ectopy, AF-induced Ca2+ handling abnormalities favor AF by enhancing vulnerability to repolarization alternans, promoting initiation and maintenance of reentrant activity; dantrolene provides a lead molecule to target this mechanism.

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

Background

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

Material/Methods

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

Results

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

Conclusions

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

Animal models of arrhythmia: classic electrophysiology to genetically modified large animals

Arrhythmias are common and contribute substantially to cardiovascular morbidity and mortality. The underlying pathophysiology of arrhythmias is complex and remains incompletely understood, which explains why mostly only symptomatic therapy is available. The evaluation of the complex interplay between various cell types in the heart, including cardiomyocytes from the conduction system and the working myocardium, fibroblasts and cardiac immune cells, remains a major challenge in arrhythmia research because it can be investigated only in vivo. Various animal species have been used, and several disease models have been developed to study arrhythmias. Although every species is useful and might be ideal to study a specific hypothesis, we suggest a practical trio of animal models for future use: mice for genetic investigations, mechanistic evaluations or early studies to identify potential drug targets; rabbits for studies on ion channel function, repolarization or re-entrant arrhythmias; and pigs for preclinical translational studies to validate previous findings. In this Review, we provide a comprehensive overview of different models and currently used species for arrhythmia research, discuss their advantages and disadvantages and provide guidance for researchers who are considering performing in vivo studies.

Wireless, battery-free, fully implantable multimodal and multisite pacemakers for applications in small animal models

Small animals support a wide range of pathological phenotypes and genotypes as versatile, affordable models for pathogenesis of cardiovascular diseases and for exploration of strategies in electrotherapy, gene therapy, and optogenetics. Pacing tools in such contexts are currently limited to tethered embodiments that constrain animal behaviors and experimental designs. Here, we introduce a highly miniaturized wireless energy-harvesting and digital communication electronics for thin, miniaturized pacing platforms weighing 110 mg with capabilities for subdermal implantation and tolerance to over 200,000 multiaxial cycles of strain without degradation in electrical or optical performance. Multimodal and multisite pacing in ex vivo and in vivo studies over many days demonstrate chronic stability and excellent biocompatibility. Optogenetic stimulation of cardiac cycles with in-animal control and induction of heart failure through chronic pacing serve as examples of modes of operation relevant to fundamental and applied cardiovascular research and biomedical technology.

Pacing tools that support small animals and can serve as models for pathogenesis of cardiovascular diseases are currently not available. Here, the authors report a miniaturized wireless battery-free implantable multimodal and multisite pacemaker that provides unlimited stimulation to test subjects.

Optocardiography and Electrophysiology Studies of Ex Vivo Langendorff-perfused Hearts

Small animal models are most commonly used in cardiovascular research due to the availability of genetically modified species and lower cost compared to larger animals. Yet, larger mammals are better suited for translational research questions related to normal cardiac physiology, pathophysiology, and preclinical testing of therapeutic agents. To overcome the technical barriers associated with employing a larger animal model in cardiac research, we describe an approach to measure physiological parameters in an isolated, Langendorff-perfused piglet heart. This approach combines two powerful experimental tools to evaluate the state of the heart: electrophysiology (EP) study and simultaneous optical mapping of transmembrane voltage and intracellular calcium using parameter sensitive dyes (RH237, Rhod2-AM). The described methodologies are well suited for translational studies investigating the cardiac conduction system, alterations in action potential morphology, calcium handling, excitation-contraction coupling and the incidence of cardiac alternans or arrhythmias.

Analysis of a coupled fluid-structure interaction model of the left atrium and mitral valve

We present a coupled left atrium-mitral valve model based on computed tomography scans with fibre-reinforced hyperelastic materials. Fluid-structure interaction is realised by using an immersed boundary-finite element framework. Effects of pathological conditions, eg, mitral valve regurgitation and atrial fibrillation, and geometric and structural variations, namely, uniform vs non-uniform atrial wall thickness and rule-based vs atlas-based fibre architectures, on the system are investigated. We show that in the case of atrial fibrillation, pulmonary venous flow reversal at late diastole disappears, and the filling waves at the left atrial appendage orifice during systole have reduced magnitude. In the case of mitral regurgitation, a higher atrial pressure and disturbed flows are seen, especially during systole, when a large regurgitant jet can be found with the suppressed pulmonary venous flow. We also show that both the rule-based and atlas-based fibre defining methods lead to similar flow fields and atrial wall deformations. However, the changes in wall thickness from non-uniform to uniform tend to underestimate the atrial deformation. Using a uniform but thickened wall also lowers the overall strain level. The flow velocity within the left atrial appendage, which is important in terms of appendage thrombosis, increases with the thickness of the left atrial wall. Energy analysis shows that the kinetic and dissipation energies of the flow within the left atrium are altered differently by atrial fibrillation and mitral valve regurgitation, providing a useful indication of the atrial performance in pathological situations.

Longitudinal study of electrical, functional and structural remodelling in an equine model of atrial fibrillation

Background

Large animal models are important in atrial fibrillation (AF) research, as they can be used to study the pathophysiology of AF and new therapeutic approaches. Unlike other animal models, horses spontaneously develop AF and could therefore serve as a bona fide model in AF research. We therefore aimed to study the electrical, functional and structural remodelling caused by chronic AF in a horse model.

Method

Nine female horses were included in the study, with six horses tachypaced into self-sustained AF and three that served as a time-matched sham-operated control group. Acceleration in atrial fibrillatory rate (AFR), changes in electrocardiographic and echocardiographic variables and response to medical treatment (flecainide 2 mg/kg) were recorded over a period of 2 months. At the end of the study, changes in ion channel expression and fibrosis were measured and compared between the two groups.

Results

AFR increased from 299 ± 33 fibrillations per minute (fpm) to 376 ± 12 fpm (p < 0.05) and atrial function (active left atrial fractional area change) decreased significantly during the study (p < 0.05). No changes were observed in heart rate or ventricular function. The AF group had more atrial fibrosis compared to the control group (p < 0.05). No differences in ion channel expression were observed.

Conclusion

Horses with induced AF show signs of atrial remodelling that are similar to humans and other animal models.

A Miniaturized, Programmable Pacemaker for Long-Term Studies in the Mouse

RATIONALE

Cardiac pacing is a critical technology for the treatment of arrhythmia and heart failure. The impact of specific pacing strategies on myocardial function is an area of intense research and high clinical significance. Mouse models have proven extremely useful for probing mechanisms of heart disease, but there is currently no reliable technology for long-term pacing in the mouse.

OBJECTIVE

We sought to develop a device for long-term pacing studies in mice. We evaluated the device for (1) treating third-degree atrioventricular block after macrophage depletion, (2) ventricular pacing-induced cardiomyopathy, and (3) high-rate atrial pacing.

METHODS AND RESULTS

We developed a mouse pacemaker by refashioning a 26 mm×6.7 mm clinical device powered by a miniaturized, highly efficient battery. The electrode was fitted with a single flexible lead, and custom software extended the pacing rate to up to 1200 bpm. The wirelessly programmable device was implanted in the dorsal subcutaneous space of 39 mice. The tunneled lead was passed through a left thoracotomy incision and attached to the epicardial surface of the apex (for ventricular pacing) or the left atrium (for atrial pacing). Mice tolerated the implantation and both long-term atrial and ventricular pacing over weeks. We then validated the pacemaker's suitability for the treatment of atrioventricular block after macrophage depletion in Cd11b ^DTR mice. Ventricular pacing increased the heart rate from 313±59 to 550 bpm ( P<0.05). In addition, we characterized tachypacing-induced cardiomyopathy in mice. Four weeks of ventricular pacing resulted in reduced left ventricular function, fibrosis, and an increased number of cardiac leukocytes and endothelial activation. Finally, we demonstrated the feasibility of chronic atrial pacing at 1200 bpm.

CONCLUSIONS

Long-term pacing with a fully implantable, programmable, and battery-powered device enables previously impossible investigations of arrhythmia and heart failure in the mouse.

Translational Models of Arrhythmia Mechanisms and Susceptibility: Success and Challenges of Modeling Human Disease

We discuss large animal translational models of arrhythmia susceptibility and sudden cardiac death, focusing on important considerations when interpreting the data derived before applying them to human trials. The utility of large animal models of arrhythmia and the pros and cons of specific translational large animals used will be discussed, including the necessary tradeoffs between models designed to derive mechanisms vs. those to test therapies. Recent technical advancements which can be applied to large animal models of arrhythmias to better elucidate mechanistic insights will be introduced. Finally, some specific examples of past successes and challenges in translating the results of large animal models of arrhythmias to clinical trials and practice will be examined, and common themes regarding the success and failure of translating studies to therapy in man will be discussed.

Addressing challenges of quantitative methodologies and event interpretation in the study of atrial fibrillation

Atrial fibrillation (AF) is the commonest arrhythmia, yet the mechanisms of its onset and persistence are incompletely known. Although techniques for quantitative assessment have been investigated, there have been few attempts to integrate this information to advance disease treatment protocols. In this review, key quantitative methods for AF analysis are described, and suggestions are provided for the coordination of the available information, and to develop foci and directions for future research efforts. Quantitative biologists may have an interest in this topic in order to develop machine learning and tools for arrhythmia characterization, but they may perhaps have a minimal background in the clinical methodology and in the types of observed events and mechanistic hypotheses that have thus far been developed. We attempt to address these issues via exploration of the published literature. Although no new data is presented in this review, examples are shown of current lines of investigation, and in particular, how electrogram analysis and whole-chamber quantitative modeling of the left atrium may be useful to characterize fibrillatory patterns of activity, so as to propose avenues for more efficacious acquisition and interpretation of AF data.

Minimally Invasive Delivery of Hydrogel-Encapsulated Amiodarone to the Epicardium Reduces Atrial Fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common cardiac arrhythmia. Although treatment options for AF exist, many patients cannot be maintained in normal sinus rhythm. Amiodarone is an effective medication for AF but has limited clinical utility because of off-target tissue toxicity.

METHODS

Here, we use a pig model of AF to test the efficacy of an amiodarone-containing polyethylene glycol-based hydrogel. The gel is placed directly on the atrial epicardium through the pericardial space in a minimally invasive procedure using a specially designed catheter.

RESULTS

Implantation of amiodarone-containing gel significantly reduced the duration of sustained AF at 21 and 28 days; inducibility of AF was reduced 14 and 21 days post-delivery. Off-target organ drug levels in the liver, lungs, thyroid, and fat were significantly reduced in animals treated with epicardial amiodarone gel compared with systemic controls in small-animal distribution studies.

CONCLUSIONS

The pericardium is an underutilized therapeutic site and may be a new treatment strategy for AF and other cardiovascular diseases.