Synchrony of cardiomyocyte Ca(2+) release is controlled by T-tubule organization, SR Ca(2+) content, and ryanodine receptor Ca(2+) sensitivity

Distinguishing pathophysiological features of heart failure with reduced and preserved ejection fraction: A comparative analysis of two mouse models

Heart failure (HF) is a heterogeneous condition that can be categorized according to the left ventricular ejection fraction (EF) into HF with reduced (HFrEF) or preserved (HFpEF) EF. Although HFrEF and HFpEF share some common clinical manifestations, the mechanisms underlying each phenotype are often found to be distinct. Identifying shared and divergent pathophysiological features might expand our insights on HF pathophysiology and assist the search for therapies for each HF subtype. In this study, we evaluated and contrasted two new murine models of non-ischaemic HFrEF and cardiometabolic HFpEF in terms of myocardial structure, left ventricular function, gene expression, cardiomyocyte calcium handling, mitochondrial polarization and protein acetylation in a head-to-head fashion. We found that in conditions of similar haemodynamic stress, the HFrEF myocardium underwent a more pronounced hypertrophic and fibrotic remodelling, whereas inflammation was greater in the HFpEF myocardium. We observed opposing features on calcium release, which was diminished in the HFrEF cardiomyocyte but enhanced in the HFpEF cardiomyocyte. Mitochondria were less polarized in both HFrEF and HFpEF cardiomyocytes, reflecting similarly impaired metabolic capacity. Hyperacetylation of cardiac proteins was observed in both models, but it was more accentuated in the HFpEF heart. Despite shared features, unique triggering mechanisms (neurohormonal overactivation in HFrEF vs. inflammation in HFpEF) appear to determine the distinct phenotypes of HF. The findings of the present research stress the need for further exploration of the differential mechanisms underlying each HF subtype, because they might require specific therapeutic interventions. KEY POINTS: The mechanisms underlying heart failure with either reduced (HFrEF) or preserved (HFpEF) ejection fraction are often found to be different. Previous studies comparing pathophysiological traits between HFrEF and HFpEF have been conducted on animals of different ages and strains. The present research contrasted two age-matched mouse models of non-ischaemic HFrEF and cardiometabolic HFpEF to uncover divergent and shared features. We found that upon similar haemodynamic stress, the HFrEF heart experienced a more pronounced hypertrophic and fibrotic remodelling, whereas inflammation appeared to be greater in the HFpEF myocardium. Calcium release was diminished in the HFrEF cardiomyocyte and enhanced in the HFpEF cardiomyocyte. Mitochondria were comparably less polarized in both HFrEF and HFpEF myocytes. Hyperacetylation of proteins was common to both models, but stronger in the HFpEF heart. Casting light on common and distinguishing features might ease the quest for phenotype-specific therapies for heart failure patients.

Restoration of calcium release synchrony: A novel target for heart failure and ventricular arrhythmia

Myocardial calcium (Ca2+) signaling plays a crucial role in contractile function and membrane electrophysiology. An abnormal myocardial Ca2+ transient is linked to heart failure and ventricular arrhythmias. At the subcellular level, the synchronous release of Ca2+ sparks from sarcoplasmic Ca2+ release units determines the configuration and amplitude of the global Ca2+ transient. This narrative review evaluates the role of aberrant Ca2+ release synchrony in the pathophysiology of cardiomyopathies and ventricular arrhythmias. The potential therapeutic benefits of restoration of Ca2+ release synchrony in heart failure and ventricular arrhythmias are also discussed.

Ryanodine Receptor Staining Identifies Viable Cardiomyocytes in Human and Rabbit Cardiac Tissue Slices

In terms of preserving multicellularity and myocardial function in vitro, the cultivation of beating myocardial slices is an emerging technique in basic and translational cardiac research. It can be used, for example, for drug screening or to study pathomechanisms. Here, we describe staining for viable cardiomyocytes based on the immunofluorescence of ryanodine receptors (RyRs) in human and rabbit myocardial slices. Biomimetic chambers were used for culture and measurements of contractile force. Fixable fluorophore-conjugated dextran, entering cells with a permeable membrane, was used for death staining. RyRs, nuclei and the extracellular matrix, including the t-system, were additionally stained and analyzed by confocal microscopy and image processing. We found the mutual exclusion of the RyR and dextran signals in cultivated slices. T-System density and nucleus size were reduced in RyR-negative/dextran-positive myocytes. The fraction of RyR-positive myocytes and pixels correlated with the contractile force. In RyR-positive/dextran-positive myocytes, we found irregular RyR clusters and SERCA distribution patterns, confirmed by an altered power spectrum. We conclude that RyR immunofluorescence indicates viable cardiomyocytes in vibratome-cut myocardial slices, facilitating the detection and differential structural analysis of living vs. dead or dying myocytes. We suggest the loss of sarcoplasmic reticulum integrity as an early event during cardiomyocyte death.

Changes in cellular Ca2+ and Na+ regulation during the progression towards heart failure

In adapting to disease and loss of tissue, the heart shows great phenotypic plasticity that involves changes to its structure, composition and electrophysiology. Together with parallel whole body cardiovascular adaptations, the initial decline in cardiac function resulting from the insult is compensated. However, in the long term, the heart muscle begins to fail and patients with this condition have a very poor prognosis, with many dying from disturbances of rhythm. The surviving myocytes of these hearts gain Na+ , which is positively inotropic because of alterations to Ca2+ fluxes mediated by the Na+ /Ca2+ exchange, but compromises Ca2+ -dependent energy metabolism in mitochondria. Uptake of Ca2+ into the sarcoplasmic reticulum (SR) is reduced because of diminished function of SR Ca2+ ATPases. The result of increased Ca2+ influx and reduced SR Ca2+ uptake is an increase in the diastolic cytosolic Ca2+ concentration, which promotes spontaneous SR Ca2+ release and induces delayed afterdepolarisations. Action potential duration prolongs because of increased late Na+ current and changes in expression and function of other ion channels and transporters increasing the probability of the formation of early afterdepolarisations. There is a reduction in T-tubule density and so the normal spatial arrangements required for efficient excitation-contraction coupling are compromised and lead to temporal delays in Ca2+ release from the SR. Therefore, the structural and electrophysiological responses that occur to provide compensation do so at the expense of (1) increasing the likelihood of arrhythmogenesis; (2) activating hypertrophic, apoptotic and Ca2+ signalling pathways; and (3) decreasing the efficiency of SR Ca2+ release.

Prolonged β-adrenergic stimulation disperses ryanodine receptor clusters in cardiomyocytes

Ryanodine receptors (RyRs) exhibit dynamic arrangements in cardiomyocytes, and we previously showed that ‘dispersion’ of RyR clusters disrupts Ca²⁺ homeostasis during heart failure (HF) (Kolstad et al., eLife, 2018). Here, we investigated whether prolonged β-adrenergic stimulation, a hallmark of HF, promotes RyR cluster dispersion and examined the underlying mechanisms. We observed that treatment of healthy rat cardiomyocytes with isoproterenol for 1 hr triggered progressive fragmentation of RyR clusters. Pharmacological inhibition of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) reversed these effects, while cluster dispersion was reproduced by specific activation of CaMKII, and in mice with constitutively active Ser2814-RyR. A similar role of protein kinase A (PKA) in promoting RyR cluster fragmentation was established by employing PKA activation or inhibition. Progressive cluster dispersion was linked to declining Ca²⁺ spark fidelity and magnitude, and slowed release kinetics from Ca²⁺ propagation between more numerous RyR clusters. In healthy cells, this served to dampen the stimulatory actions of β-adrenergic stimulation over the longer term and protect against pro-arrhythmic Ca²⁺ waves. However, during HF, RyR dispersion was linked to impaired Ca²⁺ release. Thus, RyR localization and function are intimately linked via channel phosphorylation by both CaMKII and PKA, which, while finely tuned in healthy cardiomyocytes, underlies impaired cardiac function during pathology.

DNA Tension Probes Show that Cardiomyocyte Maturation Is Sensitive to the Piconewton Traction Forces Transmitted by Integrins

Cardiac muscle cells (CMCs) are the unit cells that comprise the heart. CMCs go through different stages of differentiation and maturation pathways to fully mature into beating cells. These cells can sense and respond to mechanical cues through receptors such as integrins which influence maturation pathways. For example, cell traction forces are important for the differentiation and development of functional CMCs, as CMCs cultured on varying substrate stiffness function differently. Most work in this area has focused on understanding the role of bulk extracellular matrix stiffness in mediating the functional fate of CMCs. Given that stiffness sensing mechanisms are mediated by individual integrin receptors, an important question in this area pertains to the specific magnitude of integrin piconewton (pN) forces that can trigger CMC functional maturation. To address this knowledge gap, we used DNA adhesion tethers that rupture at specific thresholds of force (∼12, ∼56, and ∼160 pN) to test whether capping peak integrin tension to specific magnitudes affects CMC function. We show that adhesion tethers with greater force tolerance lead to functionally mature CMCs as determined by morphology, twitching frequency, transient calcium flux measurements, and protein expression (F-actin, vinculin, α-actinin, YAP, and SERCA2a). Additionally, sarcomeric actinin alignment and multinucleation were significantly enhanced as the mechanical tolerance of integrin tethers was increased. Taken together, the results show that CMCs harness defined pN integrin forces to influence early stage development. This study represents an important step toward biophysical characterization of the contribution of pN forces in early stage cardiac differentiation.

Decrease in Ca2+ Concentration in Quail Cardiomyocytes Is Faster than That in Rat Cardiomyocytes

Mammals and birds have quicker heart rates compared to other species. Mammalian cardiomyocytes have T-tubule membranes that facilitate rapid changes in Ca2+ concentrations. In contrast, bird cardiomyocytes do not possess T-tubule membranes, which raises the question of how birds achieve fast heartbeats. In this study, we compared the changes in Ca2+ concentration in cardiomyocytes isolated from adult quails and rats to elucidate the mechanism resulting in rapid heart rates in birds. Cardiomyocytes isolated from quails were significantly narrower than those isolated from rats. When Ca2+ concentration changes in the entire cardiomyocytes were measured using Fura-2 acetoxymethyl ester (AM), the time to peak was statistically longer in quails than in rats. In contrast, the decay time was markedly shorter in quails than in rats. As a result, the total time of Ca2+ concentration change was shorter in quails than in rats. A spatiotemporal analysis of Ca2+ concentration changes in quail cardiomyocytes showed that the decrease in Ca2+ concentration was faster in the center of the cell than near the cell membrane. These results suggest that avian cardiomyocytes achieve rapid changes in Ca2+ concentration by increasing the Ca2+ removal capacity in the central part of the cell compared to mammalian cardiomyocytes.

The Physiology and Pathophysiology of T-Tubules in the Heart

In cardiomyocytes, invaginations of the sarcolemmal membrane called t-tubules are critically important for triggering contraction by excitation-contraction (EC) coupling. These structures form functional junctions with the sarcoplasmic reticulum (SR), and thereby enable close contact between L-type Ca²⁺ channels (LTCCs) and Ryanodine Receptors (RyRs). This arrangement in turn ensures efficient triggering of Ca²⁺ release, and contraction. While new data indicate that t-tubules are capable of exhibiting compensatory remodeling, they are also widely reported to be structurally and functionally compromised during disease, resulting in disrupted Ca²⁺ homeostasis, impaired systolic and/or diastolic function, and arrhythmogenesis. This review summarizes these findings, while highlighting an emerging appreciation of the distinct roles of t-tubules in the pathophysiology of heart failure with reduced and preserved ejection fraction (HFrEF and HFpEF). In this context, we review current understanding of the processes underlying t-tubule growth, maintenance, and degradation, underscoring the involvement of a variety of regulatory proteins, including junctophilin-2 (JPH2), amphiphysin-2 (BIN1), caveolin-3 (Cav3), and newer candidate proteins. Upstream regulation of t-tubule structure/function by cardiac workload and specifically ventricular wall stress is also discussed, alongside perspectives for novel strategies which may therapeutically target these mechanisms.

Role of t-tubule remodeling on mechanisms of abnormal calcium release during heart failure development in canine ventricle

The goal of this work was to investigate the role of t-tubule (TT) remodeling in abnormal Ca2+ cycling in ventricular myocytes of failing dog hearts. Heart failure (HF) was induced using rapid right ventricular pacing. Extensive changes in echocardiographic parameters, including left and right ventricular dilation and systolic dysfunction, diastolic dysfunction, elevated left ventricular filling pressures, and abnormal cardiac mechanics, indicated that severe HF developed. TT loss was extensive when measured as the density of total cell volume, derived from three-dimensional confocal image analysis, and significantly increased the distances in the cell interior to closest cell membrane. Changes in Ca2+ transients indicated increases in heterogeneity of Ca2+ release along the cell length. When critical properties of Ca2+ release variability were plotted as a function of TT organization, there was a complex, nonlinear relationship between impaired calcium release and decreasing TT organization below a certain threshold of TT organization leading to increased sensitivity in Ca2+ release below a TT density threshold of 1.5%. The loss of TTs was also associated with a greater incidence of triggered Ca2+ waves during rapid pacing. Finally, virtually all of these observations were replicated by acute detubulation by formamide treatment, indicating an important role of TT remodeling in impaired Ca2+ cycling. We conclude that TT remodeling itself is a major contributor to abnormal Ca2+ cycling in HF, reducing myocardial performance. The loss of TTs is also responsible for a greater incidence of triggered Ca2+ waves that may play a role in ventricular arrhythmias arising in HF.NEW & NOTEWORTHY Three-dimensional analysis of t-tubule density showed t-tubule disruption throughout the whole myocyte in failing dog ventricle. A double-linear relationship between Ca2+ release and t-tubule density displays a steeper slope at t-tubule densities below a threshold value (∼1.5%) above which there is little effect on Ca2+ release (T-tubule reserve). T-tubule loss increases incidence of triggered Ca2+ waves. Chemically induced t-tubule disruption suggests that t-tubule loss alone is a critical component of abnormal Ca2+ cycling in heart failure.

Relative role of T-tubules disruption and decreased SERCA2 on contractile dynamics of isolated rat ventricular myocytes

AIMS

Ventricular myocytes (VM) depolarization activates L-type Ca2+ channels (LCC) allowing Ca2+ influx (ICa) to synchronize sarcoplasmic reticulum (SR) Ca2+ release, via Ca2+-release channels (RyR2). The resulting whole-cell Ca2+ transient triggers contraction, while cytosolic Ca2+ removal by SR Ca2+ pump (SERCA2) and sarcolemmal Na+/Ca2+ exchanger (NCX) allows relaxation. In diseased hearts, extensive VM remodeling causes heterogeneous, blunted and slow Ca2+ transients. Among remodeling changes are: A) T-tubules disorganization. B) Diminished SERCA2 and low SR Ca2+. However, those often overlap, hindering their relative contribution to contractile dysfunction (CD). Furthermore, few studies have assessed their specific impact on the spatiotemporal Ca2+ transient properties and contractile dynamics simultaneously. Therefore, we sought to perform a quantitative comparison of how heterogeneous and slow Ca2+ transients, with different underlying determinants, affect contractile performance.

METHODS

We used two experimental models: A) formamide-induced acute "detubulation", where VM retain functional RyR2 and SERCA2, but lack T-tubules-associated LCC and NCX. B) Intact VM from hypothyroid rats, presenting decreased SERCA2 and SR Ca2+, but maintained T-tubules. By confocal imaging of Fluo-4-loaded VM, under field-stimulation, simultaneously acquired Ca2+ transients and shortening, allowing direct correlations.

KEY FINDINGS

We found near-linear correlations among key parameters of altered Ca2+ transients, caused independently by T-tubules disruption or decreased SR Ca2+, and shortening and relaxation, SIGNIFICANCE: Unrelated structural and molecular alterations converge in similarly abnormal Ca2+ transients and CD, highlighting the importance of independently reproduce disease-specific alterations, to quantitatively assess their impact on Ca2+ signaling and contractility, which would be valuable to determine potential disease-specific therapeutic targets.

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.

Imipramine as an alternative to formamide to detubulate rat ventricular cardiomyocytes

NEW FINDINGS

What is the central question of this study? Can imipramine, an antidepressant agent that is a cationic amphiphilic drug that interferes with the phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂ ) interactions with proteins maintaining the tubular system, be validated as a new detubulating tool? What is the main finding and its importance? Imipramine was validated as a more efficient and less toxic detubulating agent of cardiomyocytes than formamide. New insights are provided on how PI(4,5)P₂ is crucial to maintaining T-tubule attachment to the cell surface and on the cardiotoxic effects of imipramine overdoses.

ABSTRACT

Cardiac T-tubules are membrane invaginations essential for excitation-contraction coupling (ECC). Imipramine, like other cationic amphiphilic drugs, interferes with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂ ) interactions with proteins maintaining the tubular system connected to the cell surface. Our main purpose was to validate imipramine as a new detubulating agent in cardiomyocytes. Staining adult rat ventricular myocytes (ARVMs) with di-4-ANEPPS, we showed that unlike formamide, imipramine induces a complete detubulation with no impact on cell viability. Using the patch-clamp technique, we observed a ∼40% decrease in cell capacitance after imipramine pretreatment and a reduction of I_Ca,L amplitude by ∼72%. These parameters were not affected in atrial cells, excluding direct side effects of imipramine. β-Adrenergic receptor (β-AR) stimulation of the remaining I_Ca,L with isoproterenol (Iso) was still effective. ECC was investigated in ARVMs loaded with Fura-2 and paced at 1 Hz, allowing simultaneous measurement of the Ca²⁺ transient (CaT) and sarcomere shortening (SS). Amplitude of both CaT and SS was decreased by imipramine and partially restored by Iso. Furthermore, detubulated cells exhibited Ca²⁺ homeostasis perturbations. Real-time cAMP variations induced by Iso using a Förster resonance energy transfer biosensor revealed ∼27% decreased cAMP elevation upon β-AR stimulation. To conclude, we validated a new cardiomyocyte detubulation method using imipramine, which is more efficient and less toxic than formamide. This antidepressant agent induces the hallmark effects of detubulation on ECC and its β-AR stimulation. Besides, we provide new insights on how an imipramine overdose may affect cardiac function and suggest that PI(4,5)P₂ is crucial for maintaining T-tubule structure.

A Matched-Filter-Based Algorithm for Subcellular Classification of T-System in Cardiac Tissues

In mammalian ventricular cardiomyocytes, invaginations of the surface membrane form the transverse tubular system (T-system), which consists of transverse tubules (TTs) that align with sarcomeres and Z-lines as well as longitudinal tubules (LTs) that are present between Z-lines in some species. In many cardiac disease etiologies, the T-system is perturbed, which is believed to promote spatially heterogeneous, dyssynchronous Ca2+ release and inefficient contraction. In general, T-system characterization approaches have been directed primarily at isolated cells and do not detect subcellular T-system heterogeneity. Here, we present MatchedMyo, a matched-filter-based algorithm for subcellular T-system characterization in isolated cardiomyocytes and millimeter-scale myocardial sections. The algorithm utilizes "filters" representative of TTs, LTs, and T-system absence. Application of the algorithm to cardiomyocytes isolated from rat disease models of myocardial infarction (MI), dilated cardiomyopathy induced via aortic banding, and sham surgery confirmed and quantified heterogeneous T-system structure and remodeling. Cardiomyocytes from post-MI hearts exhibited increasing T-system disarray as proximity to the infarct increased. We found significant (p < 0.05, Welch's t-test) increases in LT density within cardiomyocytes proximal to the infarct (12 ± 3%, data reported as mean ± SD, n = 3) versus sham (4 ± 2%, n = 5), but not distal to the infarct (7 ± 1%, n = 3). The algorithm also detected decreases in TTs within 5° of the myocyte minor axis for isolated aortic banding (36 ± 9%, n = 3) and MI cardiomyocytes located intermediate (37 ± 4%, n = 3) and proximal (34 ± 4%, n = 3) to the infarct versus sham (57 ± 12%, n = 5). Application of bootstrapping to rabbit MI tissue revealed distal sections comprised 18.9 ± 1.0% TTs, whereas proximal sections comprised 10.1 ± 0.8% TTs (p < 0.05), a 46.6% decrease. The matched-filter approach therefore provides a robust and scalable technique for T-system characterization from isolated cells through millimeter-scale myocardial sections.

Diffusional and Electrical Properties of T-Tubules Are Governed by Their Constrictions and Dilations

Cardiac t-tubules (TTs) form a network of complex surface membrane invaginations that is essential for proper excitation-contraction coupling. Although electron and optical microscopy studies provided a wealth of important information about the structure of TTs, assessing their functional properties remains a challenge. In this study, we investigated the diffusional accessibility of TTs in intact isolated adult mouse ventricular myocytes using, to our knowledge, a novel fluorescence-based assay. In this approach, a small part of TTs is first locally filled with fluorescent dextran and then its diffusion out of TTs is monitored after rapid removal of extracellular dextran. In normal cells, diffusion of 3 kDa dextran is characterized by an average time constant of 3.9 ± 1.2 s with the data ranging from 1.8 to 10.5 s. The data are consistent with essentially free diffusion of dextran in TTs although measurable contribution of binding is also evident. TT fluorescence is abolished in cells treated with high concentration of formamide or after hyposmotic stress. Importantly, the assay we use allows for quantitative, repetitive measurements of subtle dynamic changes in TT structure of the same cell that are not possible to observe with other approaches. In particular, dextran diffusion rate decreases two-to-threefold during cell swelling, suggesting significant structural remodeling of TTs. Computer modeling shows that diffusional accessibility and electrical properties of TTs are primarily determined by the constrictions and dilations of individual TTs and that, from a functional perspective, TTs cannot be considered as a network of cylinders of the same average diameter. Constriction/dilation model of cardiac TTs is in a quantitative agreement with previous high-resolution microscopy studies of TT structure and alternative measurements of diffusional and electrical time constants of TTs. The data also show that the apparent electrical length constant of cardiac TTs is likely several-fold smaller than that estimated in earlier studies.

Dyadic Plasticity in Cardiomyocytes

Contraction of cardiomyocytes is dependent on sub-cellular structures called dyads, where invaginations of the surface membrane (t-tubules) form functional junctions with the sarcoplasmic reticulum (SR). Within each dyad, Ca2+ entry through t-tubular L-type Ca2+ channels (LTCCs) elicits Ca2+ release from closely apposed Ryanodine Receptors (RyRs) in the SR membrane. The efficiency of this process is dependent on the density and macroscale arrangement of dyads, but also on the nanoscale organization of LTCCs and RyRs within them. We presently review accumulating data demonstrating the remarkable plasticity of these structures. Dyads are known to form gradually during development, with progressive assembly of both t-tubules and junctional SR terminals, and precise trafficking of LTCCs and RyRs. While dyads can exhibit compensatory remodeling when required, dyadic degradation is believed to promote impaired contractility and arrythmogenesis in cardiac disease. Recent data indicate that this plasticity of dyadic structure/function is dependent on the regulatory proteins junctophilin-2, amphiphysin-2 (BIN1), and caveolin-3, which critically arrange dyadic membranes while stabilizing the position and activity of LTCCs and RyRs. Indeed, emerging evidence indicates that clustering of both channels enables "coupled gating", implying that nanoscale localization and function are intimately linked, and may allow fine-tuning of LTCC-RyR crosstalk. We anticipate that improved understanding of dyadic plasticity will provide greater insight into the processes of cardiac compensation and decompensation, and new opportunities to target the basic mechanisms underlying heart disease.

Computational Model of Calcium Signaling in Cardiac Atrial Cells at the Submicron Scale

In cardiac cells, calcium is the mediator of excitation-contraction coupling. Dysfunctions in calcium handling have been identified as the origin of some cardiac arrhythmias. In the particular case of atrial myocytes, recent available experimental data has found links between these dysfunctions and structural changes in the calcium handling machinery (ryanodine cluster size and distribution, t-tubular network, etc). To address this issue, we have developed a computational model of an atrial myocyte that takes into account the detailed intracellular structure. The homogenized macroscopic behavior is described with a two-concentration field model, using effective diffusion coefficients of calcium in the sarcoplasmic reticulum (SR) and in the cytoplasm. The model reproduces the right calcium transients and dependence with pacing frequency. Under basal conditions, the calcium rise is mostly restricted to the periphery of the cell, with a large concentration ratio between the periphery and the interior. We have then studied the dependence of the speed of the calcium wave on cytosolic and SR diffusion coefficients, finding an almost linear relation with the former, in agreement with a diffusive and fire mechanism of propagation, and little dependence on the latter. Finally, we have studied the effect of a change in RyR cluster microstructure. We find that, under resting conditions, the spark frequency decreases slightly with RyR cluster spatial dispersion, but markedly increases when the RyRs are distributed in clusters of larger size, stressing the importance of RyR cluster organization to understand atrial arrhythmias, as recent experimental results suggest (Macquaide et al., 2015).

Ryanodine receptor dispersion disrupts Ca2+ release in failing cardiac myocytes

Reduced cardiac contractility during heart failure (HF) is linked to impaired Ca²⁺ release from Ryanodine Receptors (RyRs). We investigated whether this deficit can be traced to nanoscale RyR reorganization. Using super-resolution imaging, we observed dispersion of RyR clusters in cardiomyocytes from post-infarction HF rats, resulting in more numerous, smaller clusters. Functional groupings of RyR clusters which produce Ca²⁺ sparks (Ca²⁺ release units, CRUs) also became less solid. An increased fraction of small CRUs in HF was linked to augmented ‘silent’ Ca²⁺ leak, not visible as sparks. Larger multi-cluster CRUs common in HF also exhibited low fidelity spark generation. When successfully triggered, sparks in failing cells displayed slow kinetics as Ca²⁺ spread across dispersed CRUs. During the action potential, these slow sparks protracted and desynchronized the overall Ca²⁺ transient. Thus, nanoscale RyR reorganization during HF augments Ca²⁺ leak and slows Ca²⁺ release kinetics, leading to weakened contraction in this disease.

The muscle cells of the heart coordinate how they contract and relax in order to produce the heartbeat. During heart failure, these cells become less able to contract. As a result the heart becomes inefficient, pumping less blood around the body.

For the cardiac muscle cells to contract, the levels of calcium ions in the cells needs to rapidly increase. In failing hearts, these increases in calcium ion levels are smaller, slower and less well coordinated. It was not known what causes these changes, making it difficult to treat heart failure.

Calcium ions are released in cardiac muscle cells through protein channels called ryanodine receptors. These receptors form clusters that allow them to synchronize when they open and close. Could the reorganization of ryanodine receptors account for the problems seen in failing hearts? To investigate, Kolstad et al. examined rat hearts using a technique called super-resolution microscopy. This showed that the clusters of ryanodine receptors break apart during heart failure to form smaller clusters. Further experiments showed that calcium ions ‘leak’ from these smaller clusters, reducing the amount of calcium that can be released into cardiac muscle cells during each heartbeat. Released calcium also spreads between the dispersed clusters, resulting in a slower rise of the calcium levels in the cells. Both changes contribute to weakened contractions of cells in failing hearts.

Therefore, heart failure can be traced back to very small rearrangements of the ryanodine receptors. This understanding will help researchers as they investigate new ways to treat heart failure.

Quantitative systems models illuminate arrhythmia mechanisms in heart failure: Role of the Na+ -Ca2+ -Ca2+ /calmodulin-dependent protein kinase II-reactive oxygen species feedback

Quantitative systems modeling aims to integrate knowledge in different research areas with models describing biological mechanisms and dynamics to gain a better understanding of complex clinical syndromes. Heart failure (HF) is a chronic complex cardiac disease that results from structural or functional disorders impairing the ability of the ventricle to fill with or eject blood. Highly interactive and dynamic changes in mechanical, structural, neurohumoral, metabolic, and electrophysiological properties collectively predispose the failing heart to cardiac arrhythmias, which are responsible for about a half of HF deaths. Multiscale cardiac modeling and simulation integrate structural and functional data from HF experimental models and patients to improve our mechanistic understanding of this complex arrhythmia syndrome. In particular, they allow investigating how disease-induced remodeling alters the coupling of electrophysiology, Ca²⁺ and Na⁺ handling, contraction, and energetics that lead to rhythm derangements. The Ca²⁺ /calmodulin-dependent protein kinase II, which expression and activity are enhanced in HF, emerges as a critical hub that modulates the feedbacks between these various subsystems and promotes arrhythmogenesis. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Models of Systems Properties and Processes > Mechanistic Models Models of Systems Properties and Processes > Cellular Models Models of Systems Properties and Processes > Organ, Tissue, and Physiological Models.

Regulation of Cardiomyocyte T-Tubular Structure: Opportunities for Therapy

PURPOSE OF REVIEW

Membrane invaginations called t-tubules play an integral role in triggering cardiomyocyte contraction, and their disruption during diseases such as heart failure critically impairs cardiac performance. In this review, we outline the growing understanding of the malleability of t-tubule structure and function, and highlight emerging t-tubule regulators which may be exploited for novel therapies.

RECENT FINDINGS

New technologies are revealing the nanometer scale organization of t-tubules, and their functional junctions with the sarcoplasmic reticulum called dyads, which generate Ca²⁺ sparks. Recent data have indicated that the dyadic anchoring protein junctophilin-2, and the membrane-bending protein BIN1 are key regulators of dyadic formation and maintenance. While the underlying signals which control expression and localization of these proteins remain unclear, accumulating data support an important role of myocardial workload. Although t-tubule alterations are believed to be a key cause of heart failure, the plasticity of these structures also creates an opportunity for therapy. Promising recent data suggest that such therapies may specifically target junctophilin-2, BIN1, and/or mechanotransduction.

T-tubule remodeling and increased heterogeneity of calcium release during the progression to heart failure in intact rat ventricle

A highly organized transverse‐tubule (TT) system is essential to normal Ca²⁺ cycling and cardiac function. We explored the relationship between the progressive disruption of TTs and resulting Ca²⁺ cycling during the development of heart failure (HF). Confocal imaging was used to measure Ca²⁺ transients and 2‐D z‐stack images in left ventricular epicardial myocytes of intact hearts from spontaneously hypertensive rats (SHR) and Wistar‐Kyoto control rats. TT organization was measured as the organizational index (OI) derived from a fast Fourier transform of TT organization. We found little decrease in the synchrony of Ca²⁺ release with TT loss until TT remodeling was severe, suggesting a TT “reserve” characterized by a wide range of TT remodeling with little effect on synchrony of release but beyond which variability in release shows an accelerating sensitivity to TT loss. To explain this observation, we applied a computational model of spatially distributed Ca²⁺ signaling units to investigate the relationship between OI and excitation‐contraction coupling. Our model showed that release heterogeneity exhibits a nonlinear relationship on both the spatial distribution of release units and the separation between L‐type Ca²⁺ channels and ryanodine receptors. Our results demonstrate a unique relationship between the synchrony of Ca²⁺ release and TT organization in myocytes of intact rat ventricle that may contribute to both the compensated and decompensated phases of heart failure.

Studying dyadic structure-function relationships: a review of current modeling approaches and new insights into Ca2+ (mis)handling

Excitation–contraction coupling in cardiac myocytes requires calcium influx through L-type calcium channels in the sarcolemma, which gates calcium release through sarcoplasmic reticulum ryanodine receptors in a process known as calcium-induced calcium release, producing a myoplasmic calcium transient and enabling cardiomyocyte contraction. The spatio-temporal dynamics of calcium release, buffering, and reuptake into the sarcoplasmic reticulum play a central role in excitation–contraction coupling in both normal and diseased cardiac myocytes. However, further quantitative understanding of these cells’ calcium machinery and the study of mechanisms that underlie both normal cardiac function and calcium-dependent etiologies in heart disease requires accurate knowledge of cardiac ultrastructure, protein distribution and subcellular function. As current imaging techniques are limited in spatial resolution, limiting insight into changes in calcium handling, computational models of excitation–contraction coupling have been increasingly employed to probe these structure–function relationships. This review will focus on the development of structural models of cardiac calcium dynamics at the subcellular level, orienting the reader broadly towards the development of models of subcellular calcium handling in cardiomyocytes. Specific focus will be given to progress in recent years in terms of multi-scale modeling employing resolved spatial models of subcellular calcium machinery. A review of the state-of-the-art will be followed by a review of emergent insights into calcium-dependent etiologies in heart disease and, finally, we will offer a perspective on future directions for related computational modeling and simulation efforts.

Electrical defects of the transverse-axial tubular system in cardiac diseases

Electrical excitability is an essential feature of cardiomyocytes and the homogenous propagation of the action potential is guaranteed by a complex network of membrane invaginations called the transverse-axial tubular system (TATS). TATS structural remodelling is a hallmark of cardiac diseases and we demonstrated that this can be accompanied by electrical defects at single T-tubular level. Using a random-access multi-photon (RAMP) microscope, we found that pathological T-tubules can fail to conduct action potentials, which delays local Ca²⁺ release. Although the underlying causes for T-tubular electrical failure are still unknown, our findings suggest that they are likely to be related to local ultrastructural alterations. Here, we first review the experimental approach that allowed us to observe and dissect the consequences of TATS electrical dysfunction and then propose two different strategies to unveil the reasons for T-tubular electrical failures. The first strategy consists in a correlative approach, in which the failing T-tubule identified with the RAMP microscope is then imaged with electron microscopy. The second approach exploits the diffusion of molecules within TATS to gain insights into the local TATS structure, even without a thorough reconstruction of the tubular network. Although challenging, the local electrical failure occurring at single T-tubules is a fundamental question that needs to be addressed and could provide novel insights in cardiac pathophysiology.

New Insights in Cardiac Calcium Handling and Excitation-Contraction Coupling

Excitation-contraction (EC) coupling denotes the conversion of electric stimulus in mechanic output in contractile cells. Several studies have demonstrated that calcium (Ca²⁺) plays a pivotal role in this process. Here we present a comprehensive and updated description of the main systems involved in cardiac Ca²⁺ handling that ensure a functional EC coupling and their pathological alterations, mainly related to heart failure.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Elevated ventricular wall stress disrupts cardiomyocyte t-tubule structure and calcium homeostasis

AIMS

Invaginations of the cellular membrane called t-tubules are essential for maintaining efficient excitation-contraction coupling in ventricular cardiomyocytes. Disruption of t-tubule structure during heart failure has been linked to dyssynchronous, slowed Ca(2+) release and reduced power of the heartbeat. The underlying mechanism is, however, unknown. We presently investigated whether elevated ventricular wall stress triggers remodelling of t-tubule structure and function.

METHODS AND RESULTS

MRI and blood pressure measurements were employed to examine regional wall stress across the left ventricle of sham-operated and failing, post-infarction rat hearts. In failing hearts, elevated left ventricular diastolic pressure and ventricular dilation resulted in markedly increased wall stress, particularly in the thin-walled region proximal to the infarct. High wall stress in this proximal zone was associated with reduced expression of the dyadic anchor junctophilin-2 and disrupted cardiomyocyte t-tubular structure. Indeed, local wall stress measurements predicted t-tubule density across sham and failing hearts. Elevated wall stress and disrupted cardiomyocyte structure in the proximal zone were also associated with desynchronized Ca(2+) release in cardiomyocytes and markedly reduced local contractility in vivo. A causative role of wall stress in promoting t-tubule remodelling was established by applying stretch to papillary muscles ex vivo under culture conditions. Loads comparable to wall stress levels observed in vivo in the proximal zone reduced expression of junctophilin-2 and promoted t-tubule loss.

CONCLUSION

Elevated wall stress reduces junctophilin-2 expression and disrupts t-tubule integrity, Ca(2+) release, and contractile function. These findings provide new insight into the role of wall stress in promoting heart failure progression.

Cardiomyocyte Ca2+ dynamics: clinical perspectives

In the heart, Ca(2+) signals regulate a variety of biological functions ranging from contractility to gene expression, cellular hypertrophy and death. In this review, we summarize the role of local Ca(2+) homeostasis in these processes in healthy cardiac muscle cells, and highlight how mismanaged Ca(2+) handling contributes to the pathophysiology of conditions such as cardiac arrhythmia, ischemic heart disease, cardiac hypertrophy and heart failure. Aiming to provide an introduction to the field with a clinical perspective, we also indicate how current and future therapies may modulate cardiomyocytes Ca(2+) handling for the treatment of patients.

An integrated finite element simulation of cardiomyocyte function based on triphasic theory

In numerical simulations of cardiac excitation-contraction coupling, the intracellular potential distribution and mobility of cytosol and ions have been mostly ignored. Although the intracellular potential gradient is small, during depolarization it can be a significant driving force for ion movement, and is comparable to diffusion in terms of net flux. Furthermore, fluid in the t-tubules is thought to advect ions to facilitate their exchange with the extracellular space. We extend our previous finite element model that was based on triphasic theory to examine the significance of these factors in cardiac physiology. Triphasic theory allows us to study the behavior of solids (proteins), fluids (cytosol) and ions governed by mechanics and electrochemistry in detailed subcellular structures, including myofibrils, mitochondria, the sarcoplasmic reticulum, membranes, and t-tubules. Our simulation results predicted an electrical potential gradient inside the t-tubules at the onset of depolarization, which corresponded to the Na(+) channel distribution therein. Ejection and suction of fluid between the t-tubules and the extracellular compartment during isometric contraction were observed. We also examined the influence of t-tubule morphology and mitochondrial location on the electrophysiology and mechanics of the cardiomyocyte. Our results confirm that the t-tubule structure is important for synchrony of Ca(2+) release, and suggest that mitochondria in the sub-sarcolemmal region might serve to cancel Ca(2+) inflow through surface sarcolemma, thereby maintaining the intracellular Ca(2+) environment in equilibrium.

Targeting cardiomyocyte Ca2+ homeostasis in heart failure

Improved treatments for heart failure patients will require the development of novel therapeutic strategies that target basal disease mechanisms. Disrupted cardiomyocyte Ca²⁺ homeostasis is recognized as a major contributor to the heart failure phenotype, as it plays a key role in systolic and diastolic dysfunction, arrhythmogenesis, and hypertrophy and apoptosis signaling. In this review, we outline existing knowledge of the involvement of Ca²⁺ homeostasis in these deficits, and identify four promising targets for therapeutic intervention: the sarcoplasmic reticulum Ca²⁺ ATPase, the Na⁺-Ca²⁺ exchanger, the ryanodine receptor, and t-tubule structure. We discuss experimental data indicating the applicability of these targets that has led to recent and ongoing clinical trials, and suggest future therapeutic approaches.

Preservation of cardiac function by prolonged action potentials in mice deficient of KChIP2

Inherited ion channelopathies and electrical remodeling in heart disease alter the cardiac action potential with important consequences for excitation-contraction coupling. Potassium channel-interacting protein 2 (KChIP2) is reduced in heart failure and interacts under physiological conditions with both Kv4 to conduct the fast-recovering transient outward K(+) current (Ito,f) and with CaV1.2 to mediate the inward L-type Ca(2+) current (ICa,L). Anesthetized KChIP2(-/-) mice have normal cardiac contraction despite the lower ICa,L, and we hypothesized that the delayed repolarization could contribute to the preservation of contractile function. Detailed analysis of current kinetics shows that only ICa,L density is reduced, and immunoblots demonstrate unaltered CaV1.2 and CaVβ₂ protein levels. Computer modeling suggests that delayed repolarization would prolong the period of Ca(2+) entry into the cell, thereby augmenting Ca(2+)-induced Ca(2+) release. Ca(2+) transients in disaggregated KChIP2(-/-) cardiomyocytes are indeed comparable to wild-type transients, corroborating the preserved contractile function and suggesting that the compensatory mechanism lies in the Ca(2+)-induced Ca(2+) release event. We next functionally probed dyad structure, ryanodine receptor Ca(2+) sensitivity, and sarcoplasmic reticulum Ca(2+) load and found that increased temporal synchronicity of the Ca(2+) release in KChIP2(-/-) cardiomyocytes may reflect improved dyad structure aiding the compensatory mechanisms in preserving cardiac contractile force. Thus the bimodal effect of KChIP2 on Ito,f and ICa,L constitutes an important regulatory effect of KChIP2 on cardiac contractility, and we conclude that delayed repolarization and improved dyad structure function together to preserve cardiac contraction in KChIP2(-/-) mice.

Calcium signalling in developing cardiomyocytes: implications for model systems and disease

Adult cardiomyocytes exhibit complex Ca(2+) homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca(2+) sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca(2+) homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as cultured neonatal cells and stem cells.

Ca(2+) cycling properties are conserved despite bradycardic effects of heart failure in sinoatrial node cells

BACKGROUND

In animal models of heart failure (HF), heart rate decreases due to an increase in intrinsic cycle length (CL) of the sinoatrial node (SAN). Pacemaker activity of SAN cells is complex and modulated by the membrane clock, i.e., the ensemble of voltage gated ion channels and electrogenic pumps and exchangers, and the Ca(2+) clock, i.e., the ensemble of intracellular Ca(2+) ([Ca(2+)]i) dependent processes. HF in SAN cells results in remodeling of the membrane clock, but few studies have examined its effects on [Ca(2+)]i homeostasis.

METHODS

SAN cells were isolated from control rabbits and rabbits with volume and pressure overload-induced HF. [Ca(2+)]i concentrations, and action potentials (APs) and Na(+)-Ca(2+) exchange current (INCX) were measured using indo-1 and patch-clamp methodology, respectively.

RESULTS

The frequency of spontaneous [Ca(2+)]i transients was significantly lower in HF SAN cells (3.0 ± 0.1 (n = 40) vs. 3.4 ± 0.1 Hz (n = 45); mean ± SEM), indicating that intrinsic CL was prolonged. HF slowed the [Ca(2+)]i transient decay, which could be explained by the slower frequency and reduced sarcoplasmic reticulum (SR) dependent rate of Ca(2+) uptake. Other [Ca(2+)]i transient parameters, SR Ca(2+) content, INCX density, and INCX-[Ca(2+)]i relationship were all unaffected by HF. Combined AP and [Ca(2+)]i recordings demonstrated that the slower [Ca(2+)]i transient decay in HF SAN cells may result in increased INCX during the diastolic depolarization, but that this effect is likely counteracted by the HF-induced increase in intracellular Na(+). β-adrenergic and muscarinic stimulation were not changed in HF SAN cells, except that late diastolic [Ca(2+)]i rise, a prominent feature of the Ca(2+) clock, is lower during β-adrenergic stimulation.

CONCLUSIONS

HF SAN cells have a slower [Ca(2+)]i transient decay with limited effects on pacemaker activity. Reduced late diastolic [Ca(2+)]i rise during β-adrenergic stimulation may contribute to an impaired increase in intrinsic frequency in HF SAN cells.

Impact of detubulation on force and kinetics of cardiac muscle contraction

T-tubule uncoupling from the plasma membrane leads to myocardial contractile abnormalities.

Action potential–driven Ca²⁺ currents from the transverse tubules (t-tubules) trigger synchronous Ca²⁺ release from the sarcoplasmic reticulum of cardiomyocytes. Loss of t-tubules has been reported in cardiac diseases, including heart failure, but the effect of uncoupling t-tubules from the sarcolemma on cardiac muscle mechanics remains largely unknown. We dissected intact rat right ventricular trabeculae and compared force, sarcomere length, and intracellular Ca²⁺ in control trabeculae with trabeculae in which the t-tubules were uncoupled from the plasma membrane by formamide-induced osmotic shock (detubulation). We verified disconnection of a consistent fraction of t-tubules from the sarcolemma by two-photon fluorescence imaging of FM4-64–labeled membranes and by the absence of tubular action potential, which was recorded by random access multiphoton microscopy in combination with a voltage-sensitive dye (Di-4-AN(F)EPPTEA). Detubulation reduced the amplitude and prolonged the duration of Ca²⁺ transients, leading to slower kinetics of force generation and relaxation and reduced twitch tension (1 Hz, 30°C, 1.5 mM [Ca²⁺]_o). No mechanical changes were observed in rat left atrial trabeculae after formamide shock, consistent with the lack of t-tubules in rodent atrial myocytes. Detubulation diminished the rate-dependent increase of Ca²⁺-transient amplitude and twitch force. However, maximal twitch tension at high [Ca²⁺]_o or in post-rest potentiated beats was unaffected, although contraction kinetics were slower. The ryanodine receptor (RyR)2 Ca-sensitizing agent caffeine (200 µM), which increases the velocity of transverse Ca²⁺ release propagation in detubulated cardiomyocytes, rescued the depressed contractile force and the slower twitch kinetics of detubulated trabeculae, with negligible effects in controls. We conclude that partial loss of t-tubules leads to myocardial contractile abnormalities that can be rescued by enhancing and accelerating the propagation of Ca²⁺-induced Ca²⁺ release to orphan RyR2 clusters.