Atrial cardiomyocyte calcium signalling

Contractile and Genetic Characterization of Cardiac Constructs Engineered from Human Induced Pluripotent Stem Cells: Modeling of Tuberous Sclerosis Complex and the Effects of Rapamycin

The implementation of three-dimensional tissue engineering concurrently with stem cell technology holds great promise for in vitro research in pharmacology and toxicology and modeling cardiac diseases, particularly for rare genetic and pediatric diseases for which animal models, immortal cell lines, and biopsy samples are unavailable. It also allows for a rapid assessment of phenotype-genotype relationships and tissue response to pharmacological manipulation. Mutations in the TSC1 and TSC2 genes lead to dysfunctional mTOR signaling and cause tuberous sclerosis complex (TSC), a genetic disorder that affects multiple organ systems, principally the brain, heart, skin, and kidneys. Here we differentiated healthy (CC3) and tuberous sclerosis (TSP8-15) human induced pluripotent stem cells (hiPSCs) into cardiomyocytes to create engineered cardiac tissue constructs (ECTCs). We investigated and compared their mechano-elastic properties and gene expression and assessed the effects of rapamycin, a potent inhibitor of the mechanistic target of rapamycin (mTOR). The TSP8-15 ECTCs had increased chronotropy compared to healthy ECTCs. Rapamycin induced positive inotropic and chronotropic effects (i.e., increased contractility and beating frequency, respectively) in the CC3 ECTCs but did not cause significant changes in the TSP8-15 ECTCs. A differential gene expression analysis revealed 926 up- and 439 down-regulated genes in the TSP8-15 ECTCs compared to their healthy counterparts. The application of rapamycin initiated the differential expression of 101 and 31 genes in the CC3 and TSP8-15 ECTCs, respectively. A gene ontology analysis showed that in the CC3 ECTCs, the positive inotropic and chronotropic effects of rapamycin correlated with positively regulated biological processes, which were primarily related to the metabolism of lipids and fatty and amino acids, and with negatively regulated processes, which were predominantly associated with cell proliferation and muscle and tissue development. In conclusion, this study describes for the first time an in vitro TSC cardiac tissue model, illustrates the response of normal and TSC ECTCs to rapamycin, and provides new insights into the mechanisms of TSC.

NADPH Oxidases and Oxidative Stress in the Pathogenesis of Atrial Fibrillation

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia and its prevalence increases with age. The irregular and rapid contraction of the atria can lead to ineffective blood pumping, local blood stasis, blood clots, ischemic stroke, and heart failure. NADPH oxidases (NOX) and mitochondria are the main sources of reactive oxygen species in the heart, and dysregulated activation of NOX and mitochondrial dysfunction are associated with AF pathogenesis. NOX- and mitochondria-derived oxidative stress contribute to the onset of paroxysmal AF by inducing electrophysiological changes in atrial myocytes and structural remodeling in the atria. Because high atrial activity causes cardiac myocytes to expend extremely high energy to maintain excitation-contraction coupling during persistent AF, mitochondria, the primary energy source, undergo metabolic stress, affecting their morphology, Ca2+ handling, and ATP generation. In this review, we discuss the role of oxidative stress in activating AF-triggered activities, regulating intracellular Ca2+ handling, and functional and anatomical reentry mechanisms, all of which are associated with AF initiation, perpetuation, and progression. Changes in the extracellular matrix, inflammation, ion channel expression and function, myofibril structure, and mitochondrial function occur during the early transitional stages of AF, opening a window of opportunity to target NOX and mitochondria-derived oxidative stress using isoform-specific NOX inhibitors and mitochondrial ROS scavengers, as well as drugs that improve mitochondrial dynamics and metabolism to treat persistent AF and its transition to permanent AF.

A detailed mathematical model of the human atrial cardiomyocyte: integration of electrophysiology and cardiomechanics

Mechano-electric regulations (MER) play an important role in the maintenance of cardiac performance. Mechano-calcium and mechano-electric feedback (MCF and MEF) pathways adjust the cardiomyocyte contractile force according to mechanical perturbations and affects electro-mechanical coupling. MER integrates all these regulations in one unit resulting in a complex phenomenon. Computational modelling is a useful tool to accelerate the mechanistic understanding of complex experimental phenomena. We have developed a novel model that integrates the MER loop for human atrial cardiomyocytes with proper consideration of feedforward and feedback pathways. The model couples a modified version of the action potential (AP) Koivumäki model with the contraction model by Quarteroni group. The model simulates iso-sarcometric and isometric twitches and the feedback effects on AP and Ca2+ -handling. The model showed a biphasic response of Ca2+ transient (CaT) peak to increasing pacing rates and highlights the possible mechanisms involved. The model has shown a shift of the threshold for AP and CaT alternans from 4.6 to 4 Hz under post-operative atrial fibrillation, induced by depressed SERCA activity. The alternans incidence was dependent on a chain of mechanisms including RyRs availability time, MCF coupling, CaMKII phosphorylation, and the stretch levels. As a result, the model predicted a 10% slowdown of conduction velocity for a 20% stretch, suggesting a role of stretch in creation of substrate formation for atrial fibrillation. Overall, we conclude that the developed model provides a physiological CaT followed by a physiological twitch. This model can open pathways for the future studies of human atrial electromechanics. KEY POINTS: With the availability of human atrial cellular data, interest in atrial-specific model integration has been enhanced. We have developed a detailed mathematical model of human atrial cardiomyocytes including the mechano-electric regulatory loop. The model has gone through calibration and evaluation phases against a wide collection of available human in-vitro data. The usefulness of the model for analysing clinical problems has been preliminaryly tested by simulating the increased incidence of Ca2+ transient and action potential alternans at high rates in post-operative atrial fibrillation condition. The model determines the possible role of mechano-electric feedback in alternans incidence, which can increase vulnerability to atrial arrhythmias by varying stretch levels. We found that our physiologically accurate description of Ca2+ handling can reproduce many experimental phenomena and can help to gain insights into the underlying pathophysiological mechanisms.

The 'Reverse FDUF' Mechanism of Atrial Excitation-Contraction Coupling Sustains Calcium Alternans-A Hypothesis

Cardiac calcium alternans is defined as beat-to-beat alternations of Ca transient (CaT) amplitude and has been linked to cardiac arrhythmia, including atrial fibrillation. We investigated the mechanism of atrial alternans in isolated rabbit atrial myocytes using high-resolution line scan confocal Ca imaging. Alternans was induced by increasing the pacing frequency until stable alternans was observed (1.6-2.5 Hz at room temperature). In atrial myocytes, action potential-induced Ca release is initiated in the cell periphery and subsequently propagates towards the cell center by Ca-induced Ca release (CICR) in a Ca wave-like fashion, driven by the newly identified 'fire-diffuse-uptake-fire' (FDUF) mechanism. The development of CaT alternans was accompanied by characteristic changes of the spatio-temporal organization of the CaT. During the later phase of the CaT, central [Ca]_i exceeded peripheral [Ca]_i that was indicative of a reversal of the subcellular [Ca]_i gradient from centripetal to centrifugal. This gradient reversal resulted in a reversal of CICR propagation, causing a secondary Ca release during the large-amplitude alternans CaT, thereby prolonging the CaT, enhancing Ca-release refractoriness and reducing Ca release on the subsequent beat, thus enhancing the degree of CaT alternans. Here, we propose the 'reverse FDUF' mechanism as a novel cellular mechanism of atrial CaT alternans, which explains how the uncoupling of central from peripheral Ca release leads to the reversal of propagating CICR and to alternans.

Avian cardiomyocyte architecture and what it reveals about the evolution of the vertebrate heart

Bird cardiomyocytes are long, thin and lack transverse (t)-tubules, which is akin to the cardiomyocyte morphology of ectothermic non-avian reptiles, who are typified by low maximum heart rates and low pressure development. However, birds can achieve greater contractile rates and developed pressures than mammals, whose wide cardiomyocytes contain a dense t-tubular network allowing for uniform excitation-contraction coupling and strong contractile force. To address this apparent paradox, this paper functionally links recent electrophysiological studies on bird cardiomyocytes with decades of ultrastructure measurements. It shows that it is the strong transsarcolemmal Ca2+ influx via the L-type Ca2+ current (ICaL) and the high gain of Ca2+-induced Ca2+ release from the sarcoplasmic reticulum (SR), coupled with an internal SR Ca2+ release relay system, that facilitates the strong fast contractions in the long thin bird cardiomyocytes, without the need for t-tubules. The maintenance of an elongated myocyte morphology following the post-hatch transition from ectothermy to endothermy in birds is discussed in relation to cardiac load, myocyte ploidy, and cardiac regeneration potential in adult cardiomyocytes. Overall, the paper shows how little we know about cellular Ca2+ dynamics in the bird heart and suggests how increased research efforts in this area would provide vital information in our quest to understand the role of myocyte architecture in the evolution of the vertebrate heart. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'. Please see glossary at the end of the paper for definitions of specialized terms.

CMYA5 establishes cardiac dyad architecture and positioning

Cardiac excitation-contraction coupling requires dyads, the nanoscopic microdomains formed adjacent to Z-lines by apposition of transverse tubules and junctional sarcoplasmic reticulum. Disruption of dyad architecture and function are common features of diseased cardiomyocytes. However, little is known about the mechanisms that modulate dyad organization during cardiac development, homeostasis, and disease. Here, we use proximity proteomics in intact, living hearts to identify proteins enriched near dyads. Among these proteins is CMYA5, an under-studied striated muscle protein that co-localizes with Z-lines, junctional sarcoplasmic reticulum proteins, and transverse tubules in mature cardiomyocytes. During cardiac development, CMYA5 positioning adjacent to Z-lines precedes junctional sarcoplasmic reticulum positioning or transverse tubule formation. CMYA5 ablation disrupts dyad architecture, dyad positioning at Z-lines, and junctional sarcoplasmic reticulum Ca2+ release, leading to cardiac dysfunction and inability to tolerate pressure overload. These data provide mechanistic insights into cardiomyopathy pathogenesis by demonstrating that CMYA5 anchors junctional sarcoplasmic reticulum to Z-lines, establishes dyad architecture, and regulates dyad Ca2+ release.

The Oxidative Balance Orchestrates the Main Keystones of the Functional Activity of Cardiomyocytes

This review is aimed at providing an overview of the key hallmarks of cardiomyocytes in physiological and pathological conditions. The main feature of cardiac tissue is the force generation through contraction. This process requires a conspicuous energy demand and therefore an active metabolism. The cardiac tissue is rich of mitochondria, the powerhouses in cells. These organelles, producing ATP, are also the main sources of ROS whose altered handling can cause their accumulation and therefore triggers detrimental effects on mitochondria themselves and other cell components thus leading to apoptosis and cardiac diseases. This review highlights the metabolic aspects of cardiomyocytes and wanders through the main systems of these cells: (a) the unique structural organization (such as different protein complexes represented by contractile, regulatory, and structural proteins); (b) the homeostasis of intracellular Ca²⁺ that represents a crucial ion for cardiac functions and E-C coupling; and (c) the balance of Zn²⁺, an ion with a crucial impact on the cardiovascular system. Although each system seems to be independent and finely controlled, the contractile proteins, intracellular Ca²⁺ homeostasis, and intracellular Zn²⁺ signals are strongly linked to each other by the intracellular ROS management in a fascinating way to form a "functional tetrad" which ensures the proper functioning of the myocardium. Nevertheless, if ROS balance is not properly handled, one or more of these components could be altered resulting in deleterious effects leading to an unbalance of this "tetrad" and promoting cardiovascular diseases. In conclusion, this "functional tetrad" is proposed as a complex network that communicates continuously in the cardiomyocytes and can drive the switch from physiological to pathological conditions in the heart.

Automatic Activity Arising in Cardiac Muscle Sleeves of the Pulmonary Vein

Ectopic activity in the pulmonary vein cardiac muscle sleeves can both induce and maintain human atrial fibrillation. A central issue in any study of the pulmonary veins is their difference from the left atrial cardiac muscle. Here, we attempt to summarize the physiological phenomena underlying the occurrence of ectopic electrical activity in animal pulmonary veins. We emphasize that the activation of multiple signaling pathways influencing not only myocyte electrophysiology but also the means of excitation–contraction coupling may be required for the initiation of triggered or automatic activity. We also gather information regarding not only the large-scale structure of cardiac muscle sleeves but also recent studies suggesting that cellular heterogeneity may contribute to the generation of arrythmogenic phenomena and to the distinction between pulmonary vein and left atrial heart muscle.

Calcium Signaling Silencing in Atrial Fibrillation: Implications for Atrial Sodium Homeostasis

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia, affecting more than 33 million people worldwide. Despite important advances in therapy, AF's incidence remains high, and treatment often results in recurrence of the arrhythmia. A better understanding of the cellular and molecular changes that (1) trigger AF and (2) occur after the onset of AF will help to identify novel therapeutic targets. Over the past 20 years, a large body of research has shown that intracellular Ca2+ handling is dramatically altered in AF. While some of these changes are arrhythmogenic, other changes counteract cellular arrhythmogenic mechanisms (Calcium Signaling Silencing). The intracellular Na+ concentration ([Na+])i is a key regulator of intracellular Ca2+ handling in cardiac myocytes. Despite its importance in the regulation of intracellular Ca2+ handling, little is known about [Na+]i, its regulation, and how it might be changed in AF. Previous work suggests that there might be increases in the late component of the atrial Na+ current (INa,L) in AF, suggesting that [Na+]i levels might be high in AF. Indeed, a pharmacological blockade of INa,L has been suggested as a treatment for AF. Here, we review calcium signaling silencing and changes in intracellular Na+ homeostasis during AF. We summarize the proposed arrhythmogenic mechanisms associated with increases in INa,L during AF and discuss the evidence from clinical trials that have tested the pharmacological INa,L blocker ranolazine in the treatment of AF.

Modulated Calcium Homeostasis and Release Events Under Atrial Fibrillation and Its Risk Factors: A Meta-Analysis

Background: Atrial fibrillation (AF) is associated with calcium (Ca²⁺) handling remodeling and increased spontaneous calcium release events (SCaEs). Nevertheless, its exact mechanism remains unclear, resulting in suboptimal primary and secondary preventative strategies.

Methods: We searched the PubMed database for studies that investigated the relationship between SCaEs and AF and/or its risk factors. Meta-analysis was used to examine the Ca²⁺ mechanisms involved in the primary and secondary AF preventative groups.

Results: We included a total of 74 studies, out of the identified 446 publications from inception (1982) until March 31, 2020. Forty-five were primary and 29 were secondary prevention studies for AF. The main Ca²⁺ release events, calcium transient (standardized mean difference (SMD) = 0.49; I² = 35%; confidence interval (CI) = 0.33–0.66; p < 0.0001), and spark amplitude (SMD = 0.48; I² = 0%; CI = −0.98–1.93; p = 0.054) were enhanced in the primary diseased group, while calcium transient frequency was increased in the secondary group. Calcium spark frequency was elevated in both the primary diseased and secondary AF groups. One of the key cardiac currents, the L-type calcium current (I_CaL) was significantly downregulated in primary diseased (SMD = −1.07; I² = 88%; CI = −1.94 to −0.20; p < 0.0001) and secondary AF groups (SMD = −1.28; I² = 91%; CI = −2.04 to −0.52; p < 0.0001). Furthermore, the sodium–calcium exchanger (I_NCX) and NCX1 protein expression were significantly enhanced in the primary diseased group, while only NCX1 protein expression was shown to increase in the secondary AF studies. The phosphorylation of the ryanodine receptor at S2808 (pRyR-S2808) was significantly elevated in both the primary and secondary groups. It was increased in the primary diseased and proarrhythmic subgroups (SMD = 0.95; I² = 64%; CI = 0.12–1.79; p = 0.074) and secondary AF group (SMD = 0.66; I² = 63%; CI = 0.01–1.31; p < 0.0001). Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) expression was elevated in the primary diseased and proarrhythmic drug subgroups but substantially reduced in the secondary paroxysmal AF subgroup.

Conclusions: Our study identified that I_CaL is reduced in both the primary and secondary diseased groups. Furthermore, pRyR-S2808 and NCX1 protein expression are enhanced. The remodeling leads to elevated Ca²⁺ functional activities, such as increased frequencies or amplitude of Ca²⁺ spark and Ca²⁺ transient. The main difference identified between the primary and secondary diseased groups is SERCA expression, which is elevated in the primary diseased group and substantially reduced in the secondary paroxysmal AF subgroup. We believe our study will add new evidence to AF mechanisms and treatment targets.

A Computational Study of the Effects of Tachycardia-Induced Remodeling on Calcium Wave Propagation in Rabbit Atrial Myocytes

In atrial cardiomyocytes without a well-developed T-tubule system, calcium diffuses from the periphery toward the center creating a centripetal wave pattern. During atrial fibrillation, rapid activation of atrial myocytes induces complex remodeling in diffusion properties that result in failure of calcium to propagate in a fully regenerative manner toward the center; a phenomenon termed “calcium silencing.” This has been observed in rabbit atrial myocytes after exposure to prolonged rapid pacing. Although experimental studies have pointed to possible mechanisms underlying calcium silencing, their individual effects and relative importance remain largely unknown. In this study we used computational modeling of the rabbit atrial cardiomyocyte to query the individual and combined effects of the proposed mechanisms leading to calcium silencing and abnormal calcium wave propagation. We employed a population of models obtained from a newly developed model of the rabbit atrial myocyte with spatial representation of intracellular calcium handling. We selected parameters in the model that represent experimentally observed cellular remodeling which have been implicated in calcium silencing, and scaled their values in the population to match experimental observations. In particular, we changed the maximum conductances of ICaL, INCX, and INaK, RyR open probability, RyR density, Serca2a density, and calcium buffering strength. We incorporated remodeling in a population of 16 models by independently varying parameters that reproduce experimentally observed cellular remodeling, and quantified the resulting alterations in calcium dynamics and wave propagation patterns. The results show a strong effect of ICaL in driving calcium silencing, with INCX, INaK, and RyR density also resulting in calcium silencing in some models. Calcium alternans was observed in some models where INCX and Serca2a density had been changed. Simultaneously incorporating changes in all remodeled parameters resulted in calcium silencing in all models, indicating the predominant role of decreasing ICaL in the population phenotype.

Excitation-contraction coupling and calcium release in atrial muscle

In cardiac muscle, the process of excitation-contraction coupling (ECC) describes the chain of events that links action potential induced myocyte membrane depolarization, surface membrane ion channel activation, triggering of Ca²⁺ induced Ca²⁺ release from the sarcoplasmic reticulum (SR) Ca²⁺ store to activation of the contractile machinery that is ultimately responsible for the pump function of the heart. Here we review similarities and differences of structural and functional attributes of ECC between atrial and ventricular tissue. We explore a novel "fire-diffuse-uptake-fire" paradigm of atrial ECC and Ca²⁺ release that assigns a novel role to the SR SERCA pump and involves a concerted "tandem" activation of the ryanodine receptor Ca²⁺ release channel by cytosolic and luminal Ca²⁺. We discuss the contribution of the inositol 1,4,5-trisphosphate (IP₃) receptor Ca²⁺ release channel as an auxiliary pathway to Ca²⁺ signaling, and we review IP₃ receptor-induced Ca²⁺ release involvement in beat-to-beat ECC, nuclear Ca²⁺ signaling, and arrhythmogenesis. Finally, we explore the topic of electromechanical and Ca²⁺ alternans and its ramifications for atrial arrhythmia.

A combined Langendorff-injection technique for simultaneous isolation of single cardiomyocytes from atria and ventricles of the rat heart

Single cardiomyocytes are widely used for investigations of the cellular and molecular mechanisms of regulation and modulation of cardiac performance. Intact cardiomyocytes allow one to study in detail cell function avoiding the effects of extracellular matrix and neighboring cells. The most established protocols of cardiomyocyte isolation are based on the isolated heart perfusion using a Langendorff-apparatus or on intraventricular perfusion using a syringe. However, the yield of single cardiomyocytes obtained by these methods may be low due to the cell injury following non-uniform enzyme digestion of connective tissue in different heart chambers. Moreover, isolation of atrial cardiomyocytes is challenging because of their small size and complex geometric shape. Here we present a new protocol for simultaneous isolation of high quality cardiomyocytes from the atria, ventricular free walls and interventricular septum. The protocol is based on the combination of the Langendorff perfusion method with the intraventricular and intra-atrial injection technique taking into account the collagen content variation between the different heart chambers. Obtained cells demonstrate rod-shaped morphology, a clear and regular sarcomere striation pattern and rat-specific frequency-dependence of contraction and calcium transient parameters. Our protocol provides gentle cell isolation that increases the yield of single cardiomyocytes suitable for biophysical researches .

Nanoscale imaging of rat atrial myocytes by scanning ion conductance microscopy reveals heterogeneity of T-tubule openings and ultrastructure of the cell membrane

In contrast to ventricular myocytes, the structural and functional importance of atrial transverse tubules (T-tubules) is not fully understood. Therefore, we investigated the ultrastructure of T-tubules of living rat atrial myocytes in comparison with ventricular myocytes. Nanoscale cell surface imaging by scanning ion conductance microscopy (SICM) was accompanied by confocal imaging of intracellular T-tubule network, and the effect of removal of T-tubules on atrial excitation-contraction coupling (EC-coupling) was observed. By SICM imaging, we classified atrial cell surface into 4 subtypes. About 38% of atrial myocytes had smooth cell surface with no clear T-tubule openings and intracellular T-tubules (smooth-type). In 33% of cells, we found a novel membrane nanostructure running in the direction of cell length and named it 'longitudinal fissures' (LFs-type). Interestingly, T-tubule openings were often found inside the LFs. About 17% of atrial cells resembled ventricular myocytes, but they had smaller T-tubule openings and a lower Z-groove ratio than the ventricle (ventricular-type). The remaining 12% of cells showed a mixed structure of each subtype (mixed-type). The LFs-, ventricular-, and mixed-type had an appreciable amount of reticular form of intracellular T-tubules. Formamide-induced detubulation effectively removed atrial T-tubules, which was confirmed by both confocal images and decreased cell capacitance. However, the LFs remained intact after detubulation. Detubulation reduced action potential duration and L-type Ca²⁺channel (LTCC) density, and prolonged relaxation time of the myocytes. Taken together, we observed heterogeneity of rat atrial T-tubules and membranous ultrastructure, and the alteration of atrial EC-coupling by disruption of T-tubules.

A Novel Computational Model of the Rabbit Atrial Cardiomyocyte With Spatial Calcium Dynamics

Models of cardiac electrophysiology are widely used to supplement experimental results and to provide insight into mechanisms of cardiac function and pathology. The rabbit has been a particularly important animal model for studying mechanisms of atrial pathophysiology and atrial fibrillation, which has motivated the development of models for the rabbit atrial cardiomyocyte electrophysiology. Previously developed models include detailed representations of membrane currents and intracellular ionic concentrations, but these so-called “common-pool” models lack a spatially distributed description of the calcium handling system, which reflects the detailed ultrastructure likely found in cells in vivo. Because of the less well-developed T-tubular system in atrial compared to ventricular cardiomyocytes, spatial gradients in intracellular calcium concentrations may play a more significant role in atrial cardiomyocyte pathophysiology, rendering common-pool models less suitable for investigating underlying electrophysiological mechanisms. In this study, we developed a novel computational model of the rabbit atrial cardiomyocyte incorporating detailed compartmentalization of intracellular calcium dynamics, in addition to a description of membrane currents and intracellular processes. The spatial representation of calcium was based on dividing the intracellular space into eighteen different compartments in the transversal direction, each with separate systems for internal calcium storage and release, and tracking ionic fluxes between compartments in addition to the dynamics driven by membrane currents and calcium release. The model was parameterized employing a population-of-models approach using experimental data from different sources. The parameterization of this novel model resulted in a reduced population of models with inherent variability in calcium dynamics and electrophysiological properties, all of which fall within the range of observed experimental values. As such, the population of models may represent natural variability in cardiomyocyte electrophysiology or inherent uncertainty in the underlying experimental data. The ionic model population was also able to reproduce the U-shaped waveform observed in line-scans of triggered calcium waves in atrial cardiomyocytes, characteristic of the absence of T-tubules, resulting in a centripetal calcium wave due to subcellular calcium diffusion. This novel spatial model of the rabbit atrial cardiomyocyte can be used to integrate experimental findings, offering the potential to enhance our understanding of the pathophysiological role of calcium-handling abnormalities under diseased conditions, such as atrial fibrillation.

Long noncoding RNA in cardiac aging and disease

Cardiovascular diseases (CVDs) are the main cause of morbidity and mortality in Western society and present an important age-related risk. With the constant rise in life expectancy, prevalence of CVD in the population will likely increase further. New therapies, especially in the elderly, are needed to combat CVD. This review is focused on the role of long noncoding RNA (lncRNA) in CVD. RNA sequencing experiments in the past decade showed that most RNA does not code for protein, but many RNAs function as ncRNA. Here, we summarize the recent findings of lncRNA regulation in the diseased heart. The potential use of these RNAs as biomarkers of cardiac disease prediction is also discussed.

ER membranes associated with mitochondria: Possible therapeutic targets in heart-associated diseases

Cardiovascular system cell biology is tightly regulated and mitochondria play a relevant role in maintaining heart function. In recent decades, associations between such organelles and the sarco/endoplasmic reticulum (SR) have been raised great interest. Formally identified as mitochondria-associated SR membranes (MAMs), these structures regulate different cellular functions, including calcium management, lipid metabolism, autophagy, oxidative stress, and management of unfolded proteins. In this review, we highlight MAMs' alterations mainly in cardiomyocytes, linked with cardiovascular diseases, such as cardiac ischemia-reperfusion, heart failure, and dilated cardiomyopathy. We also describe proteins that are part of the MAMs' machinery, as the FUN14 domain containing 1 (FUNDC1), the sigma 1 receptor (Sig-1R) and others, which might be new molecular targets to preserve the function and structure of the heart in such diseases. Understanding the machinery of MAMs and its function demands our attention, as such knowledge might contribute to strengthen the role of these relative novel structures in heart diseases.

Influence of the tubular network on the characteristics of calcium transients in cardiac myocytes

Transverse and axial tubules (TATS) are an essential ingredient of the excitation-contraction machinery that allow the effective coupling of L-type Calcium Channels (LCC) and ryanodine receptors (RyR2). They form a regular network in ventricular cells, while their presence in atrial myocytes is variable regionally and among animal species We have studied the effect of variations in the TAT network using a bidomain computational model of an atrial myocyte with variable density of tubules. At each z-line the t-tubule length is obtained from an exponential distribution, with a given mean penetration length. This gives rise to a distribution of t-tubules in the cell that is characterized by the fractional area (F.A.) occupied by the t-tubules. To obtain consistent results, we average over different realizations of the same mean penetration length. To this, in some simulations we add the effect of a network of axial tubules. Then we study global properties of calcium signaling, as well as regional heterogeneities and local properties of sparks and RyR2 openings. In agreement with recent experiments in detubulated ventricular and atrial cells, we find that detubulation reduces the calcium transient and synchronization in release. However, it does not affect sarcoplasmic reticulum (SR) load, so the decrease in SR calcium release is due to regional differences in Ca²⁺ release, that is restricted to the cell periphery in detubulated cells. Despite the decrease in release, the release gain is larger in detubulated cells, due to recruitment of orphaned RyR2s, i.e, those that are not confronting a cluster of LCCs. This probably provides a safeguard mechanism, allowing physiological values to be maintained upon small changes in the t-tubule density. Finally, we do not find any relevant change in spark properties between tubulated and detubulated cells, suggesting that the differences found in experiments could be due to differential properties of the RyR2s in the membrane and in the t-tubules, not incorporated in the present model. This work will help understand the effect of detubulation, that has been shown to occur in disease conditions such as heart failure (HF) in ventricular cells, or atrial fibrillation (AF) in atrial cells.

Four Dimensions of the Cardiac Myocyte Epigenome: from Fetal to Adult Heart

PURPOSE OF REVIEW

Development, physiological growth and the response of the heart to injury are accompanied by changes of the transcriptome and epigenome of cardiac myocytes. Recently, cell sorting and next generation sequencing techniques have been applied to determine cardiac myocyte-specific transcriptional and epigenetic mechanisms. This review provides a comprehensive overview of studies analysing the transcriptome and epigenome of cardiac myocytes in mouse and human hearts during development, physiological growth and disease.

RECENT FINDINGS

Adult cardiac myocytes express > 12,600 genes, and their expression levels correlate positively with active histone marks and inversely with gene body DNA methylation. DNA methylation accompanied the perinatal switch in sarcomere or metabolic isoform gene expression in cardiac myocytes, but remained rather stable in heart disease. DNA methylation and histone marks identified > 100,000 cis-regulatory regions in the cardiac myocyte epigenome with a dynamic spectrum of transcription factor binding sites. The ETS-related transcription factor ETV1 was identified as an atrial-specific element involved in the pathogenesis of atrial fibrillation. Thus, dynamic development of the atrial vs. ventricular cardiac myocyte epigenome provides a basis to identify location and time-dependent mechanisms of epigenetic control to shape pathological gene expression during heart disease. Identifying the four dimensions of the cardiac myocyte epigenome, atrial vs. ventricular location, time during development and growth, and disease-specific signals, may ultimately lead to new treatment strategies for heart disease.

Cardiomyocyte calcium handling in health and disease: Insights from in vitro and in silico studies

Calcium (Ca²⁺) plays a central role in cardiomyocyte excitation-contraction coupling. To ensure an optimal electrical impulse propagation and cardiac contraction, Ca²⁺ levels are regulated by a variety of Ca²⁺-handling proteins. In turn, Ca²⁺ modulates numerous electrophysiological processes. Accordingly, Ca²⁺-handling abnormalities can promote cardiac arrhythmias via various mechanisms, including the promotion of afterdepolarizations, ion-channel modulation and structural remodeling. In the last 30 years, significant improvements have been made in the computational modeling of cardiomyocyte Ca²⁺ handling under physiological and pathological conditions. However, numerous questions involving the Ca²⁺-dependent regulation of different macromolecular complexes, cross-talk between Ca²⁺-dependent regulatory pathways operating over a wide range of time scales, and bidirectional interactions between electrophysiology and mechanics remain to be addressed by in vitro and in silico studies. A better understanding of disease-specific Ca²⁺-dependent proarrhythmic mechanisms may facilitate the development of improved therapeutic strategies. In this review, we describe the fundamental mechanisms of cardiomyocyte Ca²⁺ handling in health and disease, and provide an overview of currently available computational models for cardiomyocyte Ca²⁺ handling. Finally, we discuss important uncertainties and open questions about cardiomyocyte Ca²⁺ handling and highlight how synergy between in vitro and in silico studies may help to answer several of these issues.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Examining Cardiomyocyte Dysfunction Using Acute Chemical Induction of an Ageing Phenotype

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

Mouse HSA+ immature cardiomyocytes persist in the adult heart and expand after ischemic injury

The assessment of the regenerative capacity of the heart has been compromised by the lack of surface signatures to characterize cardiomyocytes (CMs). Here, combined multiparametric surface marker analysis with single-cell transcriptional profiling and in vivo transplantation identify the main mouse fetal cardiac populations and their progenitors (PRGs). We found that CMs at different stages of differentiation coexist during development. We identified a population of immature heat stable antigen (HSA)/ cluster of differentiation 24 (CD24)+ CMs that persists throughout life and that, unlike other CM subsets, actively proliferates up to 1 week of age and engrafts cardiac tissue upon transplantation. In the adult heart, a discrete population of HSA/CD24+ CMs appears as mononucleated cells that increase in frequency after infarction. Our work identified cell surface signatures that allow the prospective isolation of CMs at all developmental stages and the detection of a subset of immature CMs throughout life that, although at reduced frequencies, are poised for activation in response to ischemic stimuli. This work opens new perspectives in the understanding and treatment of heart pathologies.

Cardiac fibrosis - A short review of causes and therapeutic strategies

Fibrotic diseases cause annually more than 800,000 deaths worldwide, whereof the majority accounts for lung and cardiac fibrosis. A pathological remodeling of the extracellular matrix either due to ageing or as a result of an injury or disease leads to fibrotic scars. In the heart, these scars cause several cardiac dysfunctions either by reducing the ejection fraction due to a stiffened myocardial matrix, or by impairing electric conductance, or they can even lead to death. Today it is known that there are several different types of cardiac scars depending on the underlying cause of fibrosis. In this review, we present an overview of what is known about cardiac fibrosis including the role of cardiac cells and extracellular matrix in this disease. We will further summarize current diagnostic tools and highlight pre-clinical or clinical therapeutic strategies to address cardiac fibrosis.

Modeling cardiac complexity: Advancements in myocardial models and analytical techniques for physiological investigation and therapeutic development in vitro

Cardiomyopathies, heart failure, and arrhythmias or conduction blockages impact millions of patients worldwide and are associated with marked increases in sudden cardiac death, decline in the quality of life, and the induction of secondary pathologies. These pathologies stem from dysfunction in the contractile or conductive properties of the cardiomyocyte, which as a result is a focus of fundamental investigation, drug discovery and therapeutic development, and tissue engineering. All of these foci require in vitro myocardial models and experimental techniques to probe the physiological functions of the cardiomyocyte. In this review, we provide a detailed exploration of different cell models, disease modeling strategies, and tissue constructs used from basic to translational research. Furthermore, we highlight recent advancements in imaging, electrophysiology, metabolic measurements, and mechanical and contractile characterization modalities that are advancing our understanding of cardiomyocyte physiology. With this review, we aim to both provide a biological framework for engineers contributing to the field and demonstrate the technical basis and limitations underlying physiological measurement modalities for biologists attempting to take advantage of these state-of-the-art techniques.

Long-Term Regulation of Excitation-Contraction Coupling and Oxidative Stress in Cardiac Myocytes by Pirfenidone

Pirfenidone (PFD) is used to treat human pulmonary fibrosis. Its administration to animals with distinct forms of cardiovascular disease results in striking improvement in cardiac performance. Here, its functional impact on cardiac myocytes was investigated. Cells were kept 1–2 days under either control culture conditions or the presence of PFD (1 mM). Subsequently, they were subjected to electrical stimulation to assess the levels of contractility and intracellular Ca²⁺. The PFD treatment promoted an increase in both peak contraction and kinetics of shortening and relaxation. Moreover, the amplitude and kinetics of Ca²⁺ transients were enhanced as well. Excitation–contraction coupling (ECC) was also investigated, under whole-cell patch-clamp conditions. In keeping with a previous report, PFD increased twofold the density of Ca²⁺ current (I_Ca). Notably, a similar increase in the magnitude of Ca²⁺ transients was also observed. Thus, the gain of ECC was unaltered. Likewise, PFD did not alter the peak amplitude of caffeine-induced Ca²⁺ release, indicating stimulation of Ca²⁺-induced–Ca²⁺-release (CICR) at constant sarcoplasmic reticulum Ca²⁺ load. A phase-plane analysis indicated that PFD promotes myofilament Ca²⁺ desensitization, which is being compensated by higher levels of Ca²⁺ to promote contraction. Interestingly, although the expression of the Na⁺/Ca²⁺ exchanger (NCX) was unaffected, the decay of Ca²⁺ signal in the presence of caffeine was 50% slower in PFD-treated cells (compared with controls), suggesting that PFD downregulates the activity of the exchanger. PFD also inhibited the production of reactive oxygen species, under both, basal conditions and the presence of oxidative insults (acetaldehyde and peroxide hydrogen). Conversely, the production of nitric oxide was either increased (in atrial myocytes) or remained unchanged (in ventricular myocytes). Protein levels of endothelial and neuronal nitric oxide synthases (eNOS and nNOS) were also investigated. eNOS values did not exhibit significant changes. By contrast, a dual regulation was observed for nNOS, which consisted of inhibition and stimulation, in ventricular and atrial myocytes, respectively. In the latter cells, therefore, an up-regulation of nNOS was sufficient to stimulate the synthesis of NO. These findings improve our knowledge of molecular mechanisms of PFD action and may also help in explaining the corresponding cardioprotective effects.

Stress Coping Strategies in the Heart: An Integrated View

The heart is made up of an ordered amalgam of cardiac cell types that work together to coordinate four major processes, namely energy production, electrical conductance, mechanical work, and tissue remodeling. Over the last decade, a large body of information has been amassed regarding how different cardiac cell types respond to cellular stress that affect the functionality of their elaborate intracellular membrane networks, the cellular reticular network. In the context of the heart, the manifestations of stress coping strategies likely differ depending on the coping strategy outcomes of the different cardiac cell types, and thus may underlie the development of distinct cardiac disorders. It is not clear whether all cardiac cell types have similar sensitivity to cellular stress, how specific coping response strategies modify their unique roles, and how their metabolic status is communicated to other cells within the heart. Here we discuss our understanding of the roles of specialized cardiac cells that together make the heart function as an organ with the ability to pump blood continuously and follow a regular rhythm.

Axial Tubule Junctions Activate Atrial Ca2+ Release Across Species

Rationale: Recently, abundant axial tubule (AT) membrane structures were identified deep inside atrial myocytes (AMs). Upon excitation, ATs rapidly activate intracellular Ca²⁺ release and sarcomeric contraction through extensive AT junctions, a cell-specific atrial mechanism. While AT junctions with the sarcoplasmic reticulum contain unusually large clusters of ryanodine receptor 2 (RyR2) Ca²⁺ release channels in mouse AMs, it remains unclear if similar protein networks and membrane structures exist across species, particularly those relevant for atrial disease modeling.

Objective: To examine and quantitatively analyze the architecture of AT membrane structures and associated Ca²⁺ signaling proteins across species from mouse to human.

Methods and Results: We developed superresolution microscopy (nanoscopy) strategies for intact live AMs based on a new custom-made photostable cholesterol dye and immunofluorescence imaging of membraneous structures and membrane proteins in fixed tissue sections from human, porcine, and rodent atria. Consistently, in mouse, rat, and rabbit AMs, intact cell-wide tubule networks continuous with the surface membrane were observed, mainly composed of ATs. Moreover, co-immunofluorescence nanoscopy showed L-type Ca²⁺ channel clusters adjacent to extensive junctional RyR2 clusters at ATs. However, only junctional RyR2 clusters were highly phosphorylated and may thus prime Ca²⁺ release at ATs, locally for rapid signal amplification. While the density of the integrated L-type Ca²⁺ current was similar in human and mouse AMs, the intracellular Ca²⁺ transient showed quantitative differences. Importantly, local intracellular Ca²⁺ release from AT junctions occurred through instantaneous action potential propagation via transverse tubules (TTs) from the surface membrane. Hence, sparse TTs were sufficient as electrical conduits for rapid activation of Ca²⁺ release through ATs. Nanoscopy of atrial tissue sections confirmed abundant ATs as the major network component of AMs, particularly in human atrial tissue sections.

Conclusion: AT junctions represent a conserved, cell-specific membrane structure for rapid excitation-contraction coupling throughout a representative spectrum of species including human. Since ATs provide the major excitable membrane network component in AMs, a new model of atrial “super-hub” Ca²⁺ signaling may apply across biomedically relevant species, opening avenues for future investigations about atrial disease mechanisms and therapeutic targeting.

Distinct Occurrence of Proarrhythmic Afterdepolarizations in Atrial Versus Ventricular Cardiomyocytes: Implications for Translational Research on Atrial Arrhythmia

Background: Principal mechanisms of arrhythmia have been derived from ventricular but not atrial cardiomyocytes of animal models despite higher prevalence of atrial arrhythmia (e.g., atrial fibrillation). Due to significant ultrastructural and functional differences, a simple transfer of ventricular proneness toward arrhythmia to atrial arrhythmia is critical. The use of murine models in arrhythmia research is widespread, despite known translational limitations. We here directly compare atrial and ventricular mechanisms of arrhythmia to identify critical differences that should be considered in murine models for development of antiarrhythmic strategies for atrial arrhythmia.

Methods and Results: Isolated murine atrial and ventricular myocytes were analyzed by wide field microscopy and subjected to a proarrhythmic protocol during patch-clamp experiments. As expected, the spindle shaped atrial myocytes showed decreased cell area and membrane capacitance compared to the rectangular shaped ventricular myocytes. Though delayed afterdepolarizations (DADs) could be evoked in a similar fraction of both cell types (80% of cells each), these led significantly more often to the occurrence of spontaneous action potentials (sAPs) in ventricular myocytes. Interestingly, numerous early afterdepolarizations (EADs) were observed in the majority of ventricular myocytes, but there was no EAD in any atrial myocyte (EADs per cell; atrial myocytes: 0 ± 0; n = 25/12 animals; ventricular myocytes: 1.5 [0–43]; n = 20/12 animals; p < 0.05). At the same time, the action potential duration to 90% decay (APD₉₀) was unaltered and the APD₅₀ even increased in atrial versus ventricular myocytes. However, the depolarizing L-type Ca²⁺ current (I_Ca) and Na⁺/Ca²⁺-exchanger inward current (I_NCX) were significantly smaller in atrial versus ventricular myocytes.

Conclusion: In mice, atrial myocytes exhibit a substantially distinct occurrence of proarrhythmic afterdepolarizations compared to ventricular myocytes, since they are in a similar manner susceptible to DADs but interestingly seem to be protected against EADs and show less sAPs. Key factors in the generation of EADs like I_Ca and I_NCX were significantly reduced in atrial versus ventricular myocytes, which may offer a mechanistic explanation for the observed protection against EADs. These findings may be of relevance for current studies on atrial level in murine models to develop targeted strategies for the treatment of atrial arrhythmia.

Simultaneous study of mechanobiology and calcium dynamics on hESC-derived cardiomyocytes clusters

Calcium ions act like ubiquitous second messengers in a wide amount of cellular processes. In cardiac myocytes, Ca²⁺ handling regulates the mechanical contraction necessary to the heart pump function. The field of intracellular and intercellular Ca²⁺ handling, employing in vitro models of cardiomyocytes, has become a cornerstone to understand the role and adaptation of calcium signalling in healthy and diseased hearts. Comprehensive in vitro systems and cell-based biosensors are powerful tools to enrich and speed up cardiac phenotypic and drug response evaluation. We have implemented a combined setup to measure contractility and calcium waves in human embryonic stem cells-derived cardiomyocyte 3D clusters, obtained from embryoid body differentiation. A combination of atomic force microscopy to monitor cardiac contractility, and sensitive fast scientific complementary metal-oxide-semiconductor camera for epifluorescence video recording, provided correlated signals in real time. To speed up the integrated data processing, we tested several post-processing algorithms, to improve the automatic detection of relevant functional parameters. The validation of our proposed method was assessed by caffeine stimulation (10mM) and detection/characterization of the induced cardiac response. We successfully report the first simultaneous recording of cardiac contractility and calcium waves on the described cardiac 3D models. The drug stimulation confirmed the automatic detection capabilities of the used algorithms, measuring expected physiological response, such as elongation of contraction time and Ca²⁺ cytosolic persistence, increased calcium basal fluorescence, and transient peaks. These results contribute to the implementation of novel, integrated, high-information, and reliable experimental systems for cardiac models and drug evaluation.

Structural heterogeneity of the rat pulmonary vein myocardium: consequences on intracellular calcium dynamics and arrhythmogenic potential

Mechanisms underlying ectopic activity in the pulmonary vein (PV) which triggers paroxysmal atrial fibrillation are unknown. Although several studies have suggested that calcium signalling might be involved in these arrhythmias, little is known about calcium cycling in PV cardiomyocytes (CM). We found that individual PV CM showed a wide range of transverse tubular incidence and organization, going from their virtual absence, as described in atrial CM, to well transversally organised tubular systems, like in ventricular CM. These different types of CM were found in groups scattered throughout the tissue. The variability of the tubular system was associated with cell to cell heterogeneity of calcium channel (Ca_v1.2) localisation and, thereby, of Ca_v1.2-Ryanodine receptor coupling. This was responsible for multiple forms of PV CM calcium transient. Spontaneous calcium sparks and waves were not only more abundant in PV CM than in LA CM but also associated with a higher depolarising current. In conclusion, compared with either the atrium or the ventricle, PV myocardium presents marked structural and functional heterogeneity.

Functional local crosstalk of inositol 1,4,5-trisphosphate receptor- and ryanodine receptor-dependent Ca2+ release in atrial cardiomyocytes

Aims

Enhanced inositol 1,4,5-trisphosphate receptor (InsP3R2) expression has been associated with a variety of proarrhythmogenic cardiac disorders. The functional interaction between the two major Ca2+ release mechanisms in cardiomyocytes, Ca2+ release mediated by ryanodine receptors (RyR2s) and InsP3-induced intracellular Ca2+ release (IP3ICR) remains enigmatic. We aimed at identifying characterizing local IP3ICR events, and elucidating functional local crosstalk mechanisms between cardiac InsP3R2s and RyR2s under conditions of enhanced cardiac specific InsP3R2 activity.

Methods and results

Using confocal imaging and two-dimensional spark analysis, we demonstrate in atrial myocytes (mouse model cardiac specific overexpressing InsP3R2s) that local Ca2+ release through InsP3Rs (Ca2+ puff) directly activates RyRs and triggers elementary Ca2+ release events (Ca2+ sparks). In the presence of increased intracellular InsP3 concentrations IP3ICR can modulate RyRs openings and Ca2+ spark probability. We show as well that IP3ICR remains under local control of Ca2+ release through RyRs.

Conclusions

Our results support the concept of bidirectional interaction between RyRs and InsP3Rs (i.e. Ca2+ sparks and Ca2+ puffs) in atrial myocytes. We conclude that highly efficient InsP3 dependent SR-Ca2+ flux constitute the main mechanism of functional crosstalk between InsP3Rs and RyRs resulting in more Ca2+ sensitized RyRs to trigger subsequent Ca2+-induced Ca2+ release activation. In this way, bidirectional local interaction of both SR-Ca2+ release channels may contribute to the shaping of global Ca2+ transients and thereby to contractility in cardiac myocytes.

The intricacies of atrial calcium cycling during excitation-contraction coupling

Blatter discusses the initiation and spread of Ca release, Ca store depletion, and release termination in atrial myocytes.

Alternans in atria: Mechanisms and clinical relevance

Atrial fibrillation is the most common sustained arrhythmia and its prevalence is rapidly rising with the aging of the population. Cardiac alternans, defined as cyclic beat-to-beat alternations in contraction force, action potential (AP) duration and intracellular Ca²⁺ release at constant stimulation rate, has been associated with the development of ventricular arrhythmias. Recent clinical data also provide strong evidence that alternans plays a central role in arrhythmogenesis in atria. The aim of this article is to review the mechanisms that are responsible for repolarization alternans and contribute to the transition from spatially concordant alternans to the more arrhythmogenic spatially discordant alternans in atria.

Assessing Cardiomyocyte Subtypes Following Transcription Factor-mediated Reprogramming of Mouse Embryonic Fibroblasts

Direct reprogramming of one cell type into another has recently emerged as a powerful paradigm for regenerative medicine, disease modeling, and lineage specification. In particular, the conversion of fibroblasts into induced cardiomyocyte-like myocytes (iCLMs) by Gata4, Hand2, Mef2c, and Tbx5 (GHMT) represents an important avenue for generating de novo cardiac myocytes in vitro and in vivo. Recent evidence suggests that GHMT generates a greater diversity of cardiac subtypes than previously appreciated, thus underscoring the need for a systematic approach to conducting additional studies. Before direct reprogramming can be used as a therapeutic strategy, however, the mechanistic underpinnings of lineage conversion must be understood in detail to generate specific cardiac subtypes. Here we present a detailed protocol for generating iCLMs by GHMT-mediated reprogramming of mouse embryonic fibroblasts (MEFs). We outline methods for MEF isolation, retroviral production, and MEF infection to accomplish efficient reprogramming. To determine the subtype identity of reprogrammed cells, we detail a step-by-step approach for performing immunocytochemistry on iCLMs using a defined set of compatible antibodies. Methods for confocal microscopy, identification, and quantification of iCLMs and individual atrial (iAM), ventricular (iVM), and pacemaker (iPM) subtypes are also presented. Finally, we discuss representative results of prototypical direct reprogramming experiments and highlight important technical aspects of our protocol to ensure efficient lineage conversion. Taken together, our optimized protocol should provide a stepwise approach for investigators to conduct meaningful cardiac reprogramming experiments that require identification of individual CM subtypes.

A novel mechanism of tandem activation of ryanodine receptors by cytosolic and SR luminal Ca2+ during excitation-contraction coupling in atrial myocytes

KEY POINTS

In atrial myocytes excitation-contraction coupling is strikingly different from ventricle because atrial myocytes lack a transverse tubule membrane system: Ca²⁺ release starts in the cell periphery and propagates towards the cell centre by Ca²⁺ -induced Ca²⁺ release from the sarcoplasmic reticulum (SR) Ca²⁺ store. The cytosolic Ca²⁺ sensitivity of the ryanodine receptor (RyRs) Ca²⁺ release channel is low and it is unclear how Ca²⁺ release can be activated in the interior of atrial cells. Simultaneous confocal imaging of cytosolic and intra-SR calcium revealed a transient elevation of store Ca²⁺ that we termed 'Ca²⁺ sensitization signal'. We propose a novel paradigm of atrial ECC that is based on tandem activation of the RyRs by cytosolic and luminal Ca²⁺ through a 'fire-diffuse-uptake-fire' (or FDUF) mechanism: Ca²⁺ uptake by SR Ca²⁺ pumps at the propagation front elevates Ca²⁺ inside the SR locally, leading to luminal RyR sensitization and lowering of the cytosolic Ca²⁺ activation threshold.

ABSTRACT

In atrial myocytes Ca²⁺ release during excitation-contraction coupling (ECC) is strikingly different from ventricular myocytes. In many species atrial myocytes lack a transverse tubule system, dividing the sarcoplasmic reticulum (SR) Ca²⁺ store into the peripheral subsarcolemmnal junctional (j-SR) and the much more abundant central non-junctional (nj-SR) SR. Action potential (AP)-induced Ca²⁺ entry activates Ca²⁺ -induced Ca²⁺ release (CICR) from j-SR ryanodine receptor (RyR) Ca²⁺ release channels. Peripheral elevation of [Ca²⁺ ]_i initiates CICR from nj-SR and sustains propagation of CICR to the cell centre. Simultaneous confocal measurements of cytosolic ([Ca²⁺ ]_i ; with the fluorescent Ca²⁺ indicator rhod-2) and intra-SR ([Ca²⁺ ]_SR ; fluo-5N) Ca²⁺ in rabbit atrial myocytes revealed that Ca²⁺ release from j-SR resulted in a cytosolic Ca²⁺ transient of higher amplitude compared to release from nj-SR; however, the degree of depletion of j-SR [Ca²⁺ ]_SR was smaller than nj-SR [Ca²⁺ ]_SR . Similarly, Ca²⁺ signals from individual release sites of the j-SR showed a larger cytosolic amplitude (Ca²⁺ sparks) but smaller depletion (Ca²⁺ blinks) than release from nj-SR. During AP-induced Ca²⁺ release the rise of [Ca²⁺ ]_i detected at individual release sites of the nj-SR preceded the depletion of [Ca²⁺ ]_SR , and during this latency period a transient elevation of [Ca²⁺ ]_SR occurred. We propose that Ca²⁺ release from nj-SR is activated by cytosolic and luminal Ca²⁺ (tandem RyR activation) via a novel 'fire-diffuse-uptake-fire' (FDUF) mechanism. This novel paradigm of atrial ECC predicts that Ca²⁺ uptake by sarco-endoplasmic reticulum Ca²⁺ -ATPase (SERCA) at the propagation front elevates local [Ca²⁺ ]_SR , leading to luminal RyR sensitization and lowering of the activation threshold for cytosolic CICR.

Tissue Specificity: Store-Operated Ca2+ Entry in Cardiac Myocytes

Calcium (Ca²⁺) is a key regulator of cardiomyocyte contraction. The Ca²⁺ channels, pumps, and exchangers responsible for the cyclical cytosolic Ca²⁺ signals that underlie contraction are well known. In addition to those Ca²⁺ signaling components responsible for contraction, it has been proposed that cardiomyocytes express channels that promote the influx of Ca²⁺ from the extracellular milieu to the cytosol in response to depletion of intracellular Ca²⁺ stores. With non-excitable cells, this store-operated Ca²⁺ entry (SOCE) is usually easily demonstrated and is essential for prolonging cellular Ca²⁺ signaling and for refilling depleted Ca²⁺ stores. The role of SOCE in cardiomyocytes, however, is rather more elusive. While there is published evidence for increased Ca²⁺ influx into cardiomyocytes following Ca²⁺ store depletion, it has not been universally observed. Moreover, SOCE appears to be prominent in embryonic cardiomyocytes but declines with postnatal development. In contrast, there is overwhelming evidence that the molecular components of SOCE (e.g., STIM, Orai, and TRPC proteins) are expressed in cardiomyocytes from embryo to adult. Moreover, these proteins have been shown to contribute to disease conditions such as pathological hypertrophy, and reducing their expression can attenuate hypertrophic growth. It is plausible that SOCE might underlie Ca²⁺ influx into cardiomyocytes and may have important signaling functions perhaps by activating local Ca²⁺-sensitive processes. However, the STIM, Orai, and TRPC proteins appear to cooperate with multiple protein partners in signaling complexes. It is therefore possible that some of their signaling activities are not mediated by Ca²⁺ influx signals, but by protein-protein interactions.

Modeling Inherited Arrhythmia Disorders Using Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Inherited arrhythmia disorders (IADs) are a group of potentially lethal diseases that remain diagnostic and management challenges. Although the genetic basis for many of these disorders is well known, the pathogenicity of individual mutations and the resulting clinical outcomes are difficult to predict. Treatment options remain imperfect, and optimizing therapy for individual patients can be difficult. Recent advances in the derivation of induced pluripotent stem cells (iPSCs) from patients and creation of genetically engineered human models using CRISPR/Cas9 has the potential to dramatically advance translational arrhythmia research. In this review, we discuss the current state of modeling IADs using human iPSC-derived cardiomyocytes. We also discuss current limitations and areas for further study.

Doxorubicin Regulates Autophagy Signals via Accumulation of Cytosolic Ca2+ in Human Cardiac Progenitor Cells

Doxorubicin (DOXO) is widely used to treat solid tumors. However, its clinical use is limited by side effects including serious cardiotoxicity due to cardiomyocyte damage. Resident cardiac progenitor cells (hCPCs) act as key regulators of homeostasis in myocardial cells. However, little is known about the function of hCPCs in DOXO-induced cardiotoxicity. In this study, we found that DOXO-mediated hCPC toxicity is closely related to calcium-related autophagy signaling and was significantly attenuated by blocking mTOR signaling in human hCPCs. DOXO induced hCPC apoptosis with reduction of SMP30 (regucalcin) and autophagosome marker LC3, as well as remarkable induction of the autophagy-related markers, Beclin-1, APG7, and P62/SQSTM1 and induction of calcium-related molecules, CaM (Calmodulin) and CaMKII (Calmodulin kinase II). The results of an LC3 puncta assay further indicated that DOXO reduced autophagosome formation via accumulation of cytosolic Ca²⁺. Additionally, DOXO significantly induced mTOR expression in hCPCs, and inhibition of mTOR signaling by rapamycin, a specific inhibitor, rescued DOXO-mediated autophagosome depletion in hCPCs with significant reduction of DOXO-mediated cytosolic Ca²⁺ accumulation in hCPCs, and restored SMP30 and mTOR expression. Thus, DOXO-mediated hCPC toxicity is linked to Ca²⁺-related autophagy signaling, and inhibition of mTOR signaling may provide a cardio-protective effect against DOXO-mediated hCPC toxicity.

Dyssynchronous calcium removal in heart failure-induced atrial remodeling

We tested the hypothesis that in atrial myocytes from a rabbit left ventricular heart failure (HF) model, spatial inhomogeneity and temporal dyssynchrony of Ca removal during excitation-contraction coupling together with increased Na/Ca exchange (NCX) activity generate a substrate for proarrhythmic Ca release. Ca removal occurs via Ca reuptake into the sarcoplasmic reticulum and extrusion via NCX exclusively in the cell periphery since rabbit atrial myocytes lack transverse tubules. Ca removal kinetics were assessed by the time constant τ of decay of local peripheral subsarcolemmal (SS) and central (CT) action potential (AP)-induced Ca transients (CaTs) recorded in confocal line scan mode (using Fluo-4). Spatial and temporal dyssynchrony of Ca removal was quantified by CV TAU, defined as the standard deviation of local τ along the transverse cell axis divided by mean τ. In normal cells CT CaT decline was slower compared with the SS domain, while in HF cells decline was accelerated, became equal in SS and CT regions, and a significant increase of CV TAU indicated an increased Ca removal dyssynchrony. In HF atrial cells NCX upregulation was accompanied by an overall higher incidence of spontaneous Ca waves and a higher propensity of arrhythmogenic Ca waves, defined as waves that triggered APs due to NCX-mediated membrane depolarization. NCX inhibition normalized CV TAU in HF atrial cells and decreased the propensity of Ca waves. In summary, HF atrial myocytes show accelerated but dyssynchronous diastolic Ca removal and altered sarcoplasmic reticulum Ca-ATPase (SERCA) and NCX activity that result in increased susceptibility to arrhythmia.

Regional distribution of T-tubule density in left and right atria in dogs

BACKGROUND

The peculiarities of transverse tubule (T-tubule) morphology and distribution in the atrium-and how they contribute to excitation-contraction coupling-are just beginning to be understood.

OBJECTIVES

The objectives of this study were to determine T-tubule density in the intact, live right and left atria in a large animal and to determine intraregional differences in T-tubule organization within each atrium.

METHODS

Using confocal microscopy, T-tubules were imaged in both atria in intact, Langendorf-perfused normal dog hearts loaded with di-4-ANEPPS. T-tubules were imaged in large populations of myocytes from the endocardial surface of each atrium. Computerized data analysis was performed using a new MatLab (Mathworks, Natick, MA) routine, AutoTT.

RESULTS

There was a large percentage of myocytes that had no T-tubules in both atria with a higher percentage in the right atrium (25.1%) than in the left atrium (12.5%) (P < .02). The density of transverse and longitudinal T-tubule elements was low in cells that did contain T-tubules, but there were no significant differences in density between the left atrial appendage, the pulmonary vein-posterior left atrium, the right atrial appendage, and the right atrial free wall. In contrast, there were significant differences in sarcomere spacing and cell width between different regions of the atria.

CONCLUSION

There is a sparse T-tubule network in atrial myocytes throughout both dog atria, with significant numbers of myocytes in both atria-the right atrium more so than the left atrium-having no T-tubules at all. These regional differences in T-tubule distribution, along with differences in cell width and sarcomere spacing, may have implications for the emergence of substrate for atrial fibrillation.

Myocardial plasticity: cardiac development, regeneration and disease

The adult mammalian heart is unable to recover from myocardial cell loss due to cardiac ischemia and infarction because terminally differentiated cardiomyocytes proliferate at a low rate. However, cardiomyocytes in other vertebrate animal models such as zebrafish, axolotls, newts and mammalian mouse neonates are capable of de-differentiating in order to promote cardiomyocyte proliferation and subsequent cardiac regeneration after injury. Although de-differentiation may occur in adult mammalian cardiomyocytes, it is typically associated with diseased hearts and pathologic remodeling rather than repair and regeneration. Here, we review recent studies of cardiac development, regeneration and disease that highlight how changes in myocardial identity (plasticity) is regulated and impacts adaptive and maladaptive cardiac responses.

ErbB Receptor Tyrosine Kinase: A Molecular Switch Between Cardiac and Neuroectoderm Specification in Human Pluripotent Stem Cells

Mechanisms determining intrinsic differentiation bias inherent to human pluripotent stem cells (hPSCs) toward cardiogenic fate remain elusive. We evaluated the interplay between ErbB4 and Epidemal growth factor receptor (EGFR or ErbB1) in determining cardiac differentiation in vitro as these receptor tyrosine kinases are key to heart and brain development in vivo. Our results demonstrate that during cardiac differentiation, cell fate biases exist in hPSCs due to cardiac/neuroectoderm divergence post cardiac mesoderm stage. Stage-specific up-regulation of EGFR in concert with persistent Wnt3a signaling post cardiac mesoderm favors commitment toward neural progenitor cells (NPCs). Inhibition of EGFR abrogates these effects with enhanced (>twofold) cardiac differentiation efficiencies by increasing proliferation of Nkx2-5 expressing cardiac progenitors while reducing proliferation of Sox2 expressing NPCs. Forced overexpression of ErbB4 rescued cardiac commitment by augmenting Wnt11 signaling. Convergence between EGFR/ErbB4 and canonical/noncanonical Wnt signaling determines cardiogenic fate in hPSCs. Stem Cells 2016;34:2461-2470.

Cytosolic and nuclear calcium signaling in atrial myocytes: IP3-mediated calcium release and the role of mitochondria

In rabbit atrial myocytes Ca signaling has unique features due to the lack of transverse (t) tubules, the spatial arrangement of mitochondria and the contribution of inositol-1,4,5-trisphosphate (IP3) receptor-induced Ca release (IICR). During excitation-contraction coupling action potential-induced elevation of cytosolic [Ca] originates in the cell periphery from Ca released from the junctional sarcoplasmic reticulum (j-SR) and then propagates by Ca-induced Ca release from non-junctional (nj-) SR toward the cell center. The subsarcolemmal region between j-SR and the first array of nj-SR Ca release sites is devoid of mitochondria which results in a rapid propagation of activation through this domain, whereas the subsequent propagation through the nj-SR network occurs at a velocity typical for a propagating Ca wave. Inhibition of mitochondrial Ca uptake with the Ca uniporter blocker Ru360 accelerates propagation and increases the amplitude of Ca transients (CaTs) originating from nj-SR. Elevation of cytosolic IP3 levels by rapid photolysis of caged IP3 has profound effects on the magnitude of subcellular CaTs with increased Ca release from nj-SR and enhanced CaTs in the nuclear compartment. IP3 uncaging restricted to the nucleus elicites 'mini'-Ca waves that remain confined to this compartment. Elementary IICR events (Ca puffs) preferentially originate in the nucleus in close physical association with membrane structures of the nuclear envelope and the nucleoplasmic reticulum. The data suggest that in atrial myocytes the nucleus is an autonomous Ca signaling domain where Ca dynamics are primarily governed by IICR.

Pulmonary vein sleeve cell excitation-contraction-coupling becomes dysynchronized by spontaneous calcium transients

Atrial fibrillation (AF) is the most common form of sustained cardiac arrhythmia. Substantial evidence indicates that cardiomyocytes located in the pulmonary veins [pulmonary vein sleeve cells (PVCs)] cause AF by generating ectopic electrical activity. Electrical ablation, isolating PVCs from their left atrial junctions, is a major treatment for AF. In small rodents, the sleeve of PVCs extends deep inside the lungs and is present in lung slices. Here we present data, using the lung slice preparation, characterizing how spontaneous Ca2+ transients in PVCs affect their capability to respond to electrical pacing. Immediately after a spontaneous Ca2+ transient the cell is in a refractory period and it cannot respond to electrical stimulation. Consequently, we observe that the higher the level of spontaneous activity in an individual PVC, the less likely it is that this PVC responds to electrical field stimulation. The spontaneous activity of neighbouring PVCs can be different from each other. Heterogeneity in the Ca2+ signalling of cells and in their responsiveness to electrical stimuli are known pro-arrhythmic events. The tendency of PVCs to show spontaneous Ca2+ transients and spontaneous action potentials (APs) underlies their potential to cause AF.

ErbB4 Activated p38γ MAPK Isoform Mediates Early Cardiogenesis Through NKx2.5 in Human Pluripotent Stem Cells

Activation of ErbB4 receptor signaling is instrumental in heart development, lack of which results in embryonic lethality. However, mechanism governing its intracellular signaling remains elusive. Using human pluripotent stem cells, we show that ErbB4 is critical for cardiogenesis whereby its genetic knockdown results in loss of cardiomyocytes. Phospho-proteome profiling and Western blot studies attribute this loss to inactivation of p38γ MAPK isoform which physically interacts with NKx2.5 and GATA4 transcription factors. Post-cardiomyocyte formation p38γ/NKx2.5 downregulation is followed by p38α/MEF2c upregulation suggesting stage-specific developmental roles of p38 MAPK isoforms. Knockdown of p38γ MAPK similarly disrupts cardiomyocyte formation in spite of the presence of NKx2.5. Cell fractionation and NKx2.5 phosphorylation studies suggest inhibition of ErbB4-p38γ signaling hinders NKx2.5 nuclear translocation during early cardiogenesis. This study reveals a novel pathway that directly links ErbB4 and p38γ to the transcriptional machinery of NKx2.5-GATA4 complex which is critical for cardiomyocyte formation during mammalian heart development.