Similar arrhythmicity in hypertrophic and fibrotic cardiac cultures caused by distinct substrate-specific mechanisms

Cardiomyocyte-specific loss of plasma membrane calcium ATPase 1 impacts cardiac rhythm and is associated with ventricular repolarisation dysfunction

Plasma membrane calcium ATPase 1 (PMCA1, Atp2b1) is emerging as a key contributor to cardiac physiology, involved in calcium handling and myocardial signalling. In addition, genome wide association studies have associated PMCA1 in several areas of cardiovascular disease including hypertension and myocardial infarction. Here, we investigated the role of PMCA1 in basal cardiac function and heart rhythm stability. Cardiac structure, heart rhythm and arrhythmia susceptibility were assessed in a cardiomyocyte-specific PMCA1 deletion (PMCA1^CKO) mouse model. PMCA1^CKO mice developed abnormal heart rhythms related to ventricular repolarisation dysfunction and displayed an increased susceptibility to ventricular arrhythmias. We further assessed the levels of cardiac ion channels using qPCR and found a downregulation of the voltage-dependent potassium channels, K_v4.2, with a corresponding reduction in the transient outward potassium current which underlies ventricular repolarisation in the murine heart. The changes in heart rhythm were found to occur in the absence of any structural cardiomyopathy. To further assess the molecular changes occurring in PMCA1^CKO hearts, we performed proteomic analysis. Functional characterisation of differentially expressed proteins suggested changes in pathways related to metabolism, protein-binding, and pathways associated cardiac function including β-adrenergic signalling. Together, these data suggest an important role for PMCA1 in basal cardiac function in relation to heart rhythm control, with reduced cardiac PMCA1 expression resulting in an increased risk of arrhythmia development.

Engineered bacterial voltage-gated sodium channel platform for cardiac gene therapy

Therapies for cardiac arrhythmias could greatly benefit from approaches to enhance electrical excitability and action potential conduction in the heart by stably overexpressing mammalian voltage-gated sodium channels. However, the large size of these channels precludes their incorporation into therapeutic viral vectors. Here, we report a platform utilizing small-size, codon-optimized engineered prokaryotic sodium channels (BacNav) driven by muscle-specific promoters that significantly enhance excitability and conduction in rat and human cardiomyocytes in vitro and adult cardiac tissues from multiple species in silico. We also show that the expression of BacNav significantly reduces occurrence of conduction block and reentrant arrhythmias in fibrotic cardiac cultures. Moreover, functional BacNav channels are stably expressed in healthy mouse hearts six weeks following intravenous injection of self-complementary adeno-associated virus (scAAV) without causing any adverse effects on cardiac electrophysiology. The large diversity of prokaryotic sodium channels and experimental-computational platform reported in this study should facilitate the development and evaluation of BacNav-based gene therapies for cardiac conduction disorders.

The Effects of Repetitive Use and Pathological Remodeling on Channelrhodopsin Function in Cardiomyocytes

Aim: Channelrhodopsins (ChRs) are a large family of light-gated ion channels with distinct properties, which is of great importance in the selection of a ChR variant for a given application. However, data to guide such selection for cardiac optogenetic applications are lacking. Therefore, we investigated the functioning of different ChR variants in normal and pathological hypertrophic cardiomyocytes subjected to various illumination protocols.

Methods and Results: Isolated neonatal rat ventricular cardiomyocytes (NRVMs) were transduced with lentiviral vectors to express one of the following ChR variants: H134R, CatCh, ReaChR, or GtACR1. NRVMs were treated with phenylephrine (PE) to induce pathological hypertrophy (PE group) or left untreated [control (CTL) group]. In these groups, ChR currents displayed unique and significantly different properties for each ChR variant on activation by a single 1-s light pulse (1 mW/mm²: 470, 565, or 617 nm). The concomitant membrane potential (V_m) responses also showed a ChR variant-specific profile, with GtACR1 causing a slight increase in average V_m during illumination (V_plateau: −38 mV) as compared with a V_plateau > −20 mV for the other ChR variants. On repetitive activation at increasing frequencies (10-ms pulses at 1–10 Hz for 30 s), peak currents, which are important for cardiac pacing, decreased with increasing activation frequencies by 17–78% (p < 0.05), while plateau currents, which are critical for arrhythmia termination, decreased by 10–75% (p < 0.05), both in a variant-specific manner. In contrast, the corresponding V_plateau remained largely stable. Importantly, current properties and V_m responses were not statistically different between the PE and CTL groups, irrespective of the variant used (p > 0.05).

Conclusion: Our data show that ChR variants function equally well in cell culture models of healthy and pathologically hypertrophic myocardium but show strong, variant-specific use-dependence. This use-dependent nature of ChR function should be taken into account during the design of cardiac optogenetic studies and the interpretation of the experimental findings thereof.

Multicellular In vitro Models of Cardiac Arrhythmias: Focus on Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia in clinical practice with a large socioeconomic impact due to its associated morbidity, mortality, reduction in quality of life and health care costs. Currently, antiarrhythmic drug therapy is the first line of treatment for most symptomatic AF patients, despite its limited efficacy, the risk of inducing potentially life-threating ventricular tachyarrhythmias as well as other side effects. Alternative, in-hospital treatment modalities consisting of electrical cardioversion and invasive catheter ablation improve patients' symptoms, but often have to be repeated and are still associated with serious complications and only suitable for specific subgroups of AF patients. The development and progression of AF generally results from the interplay of multiple disease pathways and is accompanied by structural and functional (e.g., electrical) tissue remodeling. Rational development of novel treatment modalities for AF, with its many different etiologies, requires a comprehensive insight into the complex pathophysiological mechanisms. Monolayers of atrial cells represent a simplified surrogate of atrial tissue well-suited to investigate atrial arrhythmia mechanisms, since they can easily be used in a standardized, systematic and controllable manner to study the role of specific pathways and processes in the genesis, perpetuation and termination of atrial arrhythmias. In this review, we provide an overview of the currently available two- and three-dimensional multicellular in vitro systems for investigating the initiation, maintenance and termination of atrial arrhythmias and AF. This encompasses cultures of primary (animal-derived) atrial cardiomyocytes (CMs), pluripotent stem cell-derived atrial-like CMs and (conditionally) immortalized atrial CMs. The strengths and weaknesses of each of these model systems for studying atrial arrhythmias will be discussed as well as their implications for future studies.

Atrial fibrillation and cardiac fibrosis: A review on the potential of extracellular matrix proteins as biomarkers

The involvement of fibrosis as an underlying pathology in heart diseases is becoming increasingly clear. In recent years, fibrosis has been granted a causative role in heart diseases and is now emerging as a major contributor to Atrial Fibrillation (AF) pathogenesis. AF is the most common arrhythmia encountered in the clinic, but the substrate for AF is still being debated. Consensus in the field is a combination of cardiac tissue remodeling, inflammation and genetic predisposition. The extracellular matrix (ECM) is subject of growing investigation, since measuring circulatory biomarkers of ECM formation and degradation provides both diagnostic and prognostic information. However, fibrosis is not just fibrosis. Each specific collagen biomarker holds information on regulatory mechanisms, as well as information about which section of the ECM is being remodeled, providing a detailed description of cardiac tissue homeostasis. This review entails an overview of the implication of fibrosis in AF, the different collagens and their significance, and the potential of using biomarkers of ECM remodeling as tools for understanding AF pathogenesis and identifying patients at risk for further disease progression.

Myofibroblast modulation of cardiac myocyte structure and function

After myocardial infarction, resident fibroblasts (Fb) differentiate towards myofibroblasts (MyoFb), generating the scar tissue and the interstitial fibrosis seen in the adjacent myocardium. Fb and MyoFb have the potential to interact with cardiac myocytes (CMs) but insight into the phenotype-specific role and mode of interaction is still incomplete. Our objectives are to further define the modulation of CMs by MyoFbs compared to Fbs, as well as the role of direct contact through gap junctions vs. soluble mediators, using Fbs and CMs from pig left ventricle. Fbs were treated to maintain an undifferentiated state (SD-208) or to attain full differentiation to MyoFb (TGF-β1). Fbs and MyoFbs were co-cultured with CMs, with the possibility of direct contact or separated by a Thincert membrane. Only in direct co-culture, both Fbs and MyoFbs were able to decrease CM viability after 2 days. Only MyoFbs induced significant distal spreading of CMs in both direct and indirect co-culture. MyoFbs, but not Fbs, readily made connections with CMs in direct co-culture and connexin 43 expression in MyoFb was higher than in Fb. When coupled to CMs, MyoFbs reduced the CM action potential duration and hyperpolarized the CM resting membrane potential. Uncoupling reversed these effects. In conclusion, MyoFbs, but not Fbs, alter the CM structural phenotype. MyoFbs, but not Fbs, are likely to electrically connect to CMs and thereby modulate the CM membrane potential. These data provide further support for an active role of MyoFbs in the arrhythmogenic substrate after cardiac remodelling.

Biomarkers Associated with Atrial Fibrosis and Remodeling

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

Virtual cardiac monolayers for electrical wave propagation

The complex structure of cardiac tissue is considered to be one of the main determinants of an arrhythmogenic substrate. This study is aimed at developing the first mathematical model to describe the formation of cardiac tissue, using a joint in silico-in vitro approach. First, we performed experiments under various conditions to carefully characterise the morphology of cardiac tissue in a culture of neonatal rat ventricular cells. We considered two cell types, namely, cardiomyocytes and fibroblasts. Next, we proposed a mathematical model, based on the Glazier-Graner-Hogeweg model, which is widely used in tissue growth studies. The resultant tissue morphology was coupled to the detailed electrophysiological Korhonen-Majumder model for neonatal rat ventricular cardiomyocytes, in order to study wave propagation. The simulated waves had the same anisotropy ratio and wavefront complexity as those in the experiment. Thus, we conclude that our approach allows us to reproduce the morphological and physiological properties of cardiac tissue.

CYP2J2 metabolites, epoxyeicosatrienoic acids, attenuate Ang II-induced cardiac fibrotic response by targeting Gα12/13

The arachidonic acid-cytochrome P450 2J2-epoxyeicosatrienoic acid (AA-CYP2J2-EET) metabolic pathway has been identified to be protective in the cardiovascular system. This study explored the effects of the AA-CYP2J2-EET metabolic pathway on cardiac fibrosis from the perspective of cardiac fibroblasts and underlying mechanisms. In in vivo studies, 8-week-old male CYP2J2 transgenic mice (aMHC-CYP2J2-Tr) and littermates were infused with angiotensin II (Ang II) or saline for 2 weeks. Results showed that CYP2J2 overexpression increased EET production. Meanwhile, impairment of cardiac function and fibrotic response were attenuated by CYP2J2 overexpression. The effects of CYP2J2 were associated with reduced activation of the α subunits of G12 family G proteins (Gα_12/13)/RhoA/Rho kinase (ROCK) cascade and elevation of the NO/cyclic guanosine monophosphate (cGMP) level in cardiac tissue. In in vitro studies, cardiac fibroblast activation, proliferation, migration, and collagen production induced by Ang II were associated with activation of the Gα_12/13/RhoA/ROCK pathway, which was inhibited by exogenous 11,12-EET. Moreover, silencing of Gα_12/13 or RhoA exerted similar effects as 11,12-EET. Furthermore, inhibitory effects of 11,12-EET on Gα_12/13 were blocked by NO/cGMP pathway inhibitors. Our findings indicate that enhancement of the AA-CYP2J2-EET metabolic pathway by CYP2J2 overexpression attenuates Ang II-induced cardiac dysfunction and fibrosis by reducing the fibrotic response of cardiac fibroblasts by targeting the Gα_12/13/RhoA/ROCK pathway via NO/cGMP signaling.

A Mathematical Model of Neonatal Rat Atrial Monolayers with Constitutively Active Acetylcholine-Mediated K+ Current

Atrial fibrillation (AF) is the most frequent form of arrhythmia occurring in the industrialized world. Because of its complex nature, each identified form of AF requires specialized treatment. Thus, an in-depth understanding of the bases of these arrhythmias is essential for therapeutic development. A variety of experimental studies aimed at understanding the mechanisms of AF are performed using primary cultures of neonatal rat atrial cardiomyocytes (NRAMs). Previously, we have shown that the distinct advantage of NRAM cultures is that they allow standardized, systematic, robust re-entry induction in the presence of a constitutively-active acetylcholine-mediated K⁺ current (I_KACh-c). Experimental studies dedicated to mechanistic explorations of AF, using these cultures, often use computer models for detailed electrophysiological investigations. However, currently, no mathematical model for NRAMs is available. Therefore, in the present study we propose the first model for the action potential (AP) of a NRAM with constitutively-active acetylcholine-mediated K⁺ current (I_KACh-c). The descriptions of the ionic currents were based on patch-clamp data obtained from neonatal rats. Our monolayer model closely mimics the action potential duration (APD) restitution and conduction velocity (CV) restitution curves presented in our previous in vitro studies. In addition, the model reproduces the experimentally observed dynamics of spiral wave rotation, in the absence and in the presence of drug interventions, and in the presence of localized myofibroblast heterogeneities.

A fundamentally important element in cardiac in silico research is a model for the cardiac cell. It provides a link between measurable characteristics at the subcellular level and biological processes at the whole cell level, thereby allowing the researcher to study mechanisms of cardiac arrhythmias from a molecular cell biological perspective. Such studies are of vast importance for the advancement of understanding of living systems from cells to patient populations. This paper is a joint in silico-experimental study in which we propose the first model for the action potential of an NRAM. To develop this model, we fitted patch-clamp data from recent literature, while additionally performing specific measurements of I_KACh-c in NRAMs. I_KACh-c is an important factor in atrial arrhythmogenesis and a promising target for pharmacological AF-management. The model reproduces in vitro results such as standard characteristics of AP morphology, restitution, and spiral wave dynamics in monolayers, with effects of a subsequent drug-intervention and in the presence of localized myofibroblast heterogeneities. Thus it can be used as a tool to provide computational support to a variety of systematic experimental studies that investigate the mechanisms underlying atrial fibrillation (AF) in NRAM cultures.

Integrins and integrin-related proteins in cardiac fibrosis

Cardiac fibrosis is one of the major components of the healing mechanism following any injury of the heart and as such may contribute to both systolic and diastolic dysfunction in a range of pathophysiologic conditions. Canonically, it can occur as part of the remodeling process that occurs following myocardial infarction or that follows as a response to pressure overload. Integrins are cell surface receptors which act in both cellular adhesion and signaling. Most importantly, in the context of the continuously contracting myocardium, they are recognized as mechanotransducers. They have been implicated in the development of fibrosis in several organs, including the heart. This review will focus on the involvement of integrins and integrin-related proteins, in cardiac fibrosis, outlining the roles of these proteins in the fibrotic responses in specific cardiac pathologies, discuss some of the common end effectors (angiotensin II, transforming growth factor beta 1 and mechanical stress) through which integrins function and finally discuss how manipulation of this set of proteins may lead to new treatments which could prove useful to alter the deleterious effects of cardiac fibrosis.

Cardiomyocyte-specific expression of CYP2J2 prevents development of cardiac remodelling induced by angiotensin II

AIMS

Cardiac remodelling is one of the key pathological changes that occur with cardiovascular disease. Previous studies have demonstrated the beneficial effects of CYP2J2 expression on cardiac injury. In the present study, we investigated the effects of cardiomyocyte-specific CYP2J2 expression and EET treatment on angiotensin II-induced cardiac remodelling and sought to determine the underlying molecular mechanisms involved in this process.

METHODS AND RESULTS

Eight-week-old mice with cardiomyocyte-specific CYP2J2 expression (αMHC-CYP2J2-Tr) and wild-type (WT) control mice were treated with Ang-II. Ang-II treatment of WT mice induced changes in heart morphology, cardiac hypertrophy and dysfunction, as well as collagen accumulation; however, cardiomyocyte-specific expression of CYP2J2 attenuated these effects. The cardioprotective effects observed in α-MHC-CYP2J2-Tr mice were associated with peroxisome proliferator-activated receptor (PPAR)-γ activation, reduced oxidative stress, reduced NF-κB p65 nuclear translocation, and inhibition of TGF-β1/smad pathway. The effects seen with cardiomyocyte-specific expression of CYP2J2 were partially blocked by treatment with PPAR-γ antagonist GW9662. In in vitro studies, 11,12-EET(1 μmol/L) treatment attenuated cardiomyocyte hypertrophy and remodelling-related protein (collagen I, TGF-β1, TIMP1) expression by inhibiting the oxidative stress-mediated NF-κB pathway via PPAR-γ activation. Furthermore, conditioned media from neonatal cardiomyocytes treated with 11,12-EET inhibited activation of cardiac fibroblasts and TGF-β1/smad pathway.

CONCLUSION

Cardiomyocyte-specific expression of CYP2J2 or treatment with EETs protects against cardiac remodelling by attenuating oxidative stress-mediated NF-κBp65 nuclear translocation via PPAR-γ activation.

A study of early afterdepolarizations in a model for human ventricular tissue

Sudden cardiac death is often caused by cardiac arrhythmias. Recently, special attention has been given to a certain arrhythmogenic condition, the long-QT syndrome, which occurs as a result of genetic mutations or drug toxicity. The underlying mechanisms of arrhythmias, caused by the long-QT syndrome, are not fully understood. However, arrhythmias are often connected to special excitations of cardiac cells, called early afterdepolarizations (EADs), which are depolarizations during the repolarizing phase of the action potential. So far, EADs have been studied mainly in isolated cardiac cells. However, the question on how EADs at the single-cell level can result in fibrillation at the tissue level, especially in human cell models, has not been widely studied yet. In this paper, we study wave patterns that result from single-cell EAD dynamics in a mathematical model for human ventricular cardiac tissue. We induce EADs by modeling experimental conditions which have been shown to evoke EADs at a single-cell level: by an increase of L-type Ca currents and a decrease of the delayed rectifier potassium currents. We show that, at the tissue level and depending on these parameters, three types of abnormal wave patterns emerge. We classify them into two types of spiral fibrillation and one type of oscillatory dynamics. Moreover, we find that the emergent wave patterns can be driven by calcium or sodium currents and we find phase waves in the oscillatory excitation regime. From our simulations we predict that arrhythmias caused by EADs can occur during normal wave propagation and do not require tissue heterogeneities. Experimental verification of our results is possible for experiments at the cell-culture level, where EADs can be induced by an increase of the L-type calcium conductance and by the application of I blockers, and the properties of the emergent patterns can be studied by optical mapping of the voltage and calcium.