Teaching cardiac excitation-contraction coupling using a mathematical computer simulation model of human ventricular myocytes

To understand the excitation-contraction (E-C) coupling of cardiomyocytes, including the electrophysiological mechanism of their characteristically long action potential duration, is one of the major learning goals in medical physiology. However, the integrative interpretation of the responses occurring during the contraction-relaxation cycle is challenging due to the dynamic interaction of underlying factors. Starting in 2017, we adopted the mathematical computer simulation model of human ventricular myocyte (Cardiac E-C_Sim), hypothesizing that this educational technology may facilitate students' learning of cardiac physiology. Here, we describe the overall process for the educational application of Cardiac E-C_Sim in the human physiology practicum of Seoul National University College of Medicine. We also report the results from questionnaires covering detailed assessment of the practicum class. The analysis of results and feedback opinions enabled us to understand how the students had approached the problem-solving process. As a whole, the students could better accomplish the learning goals using Cardiac E-C_Sim, followed by constructive discussions on the complex and dynamic mechanisms of cardiac E-C coupling. We suggest that the combined approach of lecture-based teaching and computer simulations guided by a manual containing clinical context would be broadly applicable in physiology education.

DMD Pluripotent Stem Cell Derived Cardiac Cells Recapitulate in vitro Human Cardiac Pathophysiology

Duchenne muscular dystrophy (DMD) is a severe genetic disorder characterized by the lack of functional dystrophin. DMD is associated with progressive dilated cardiomyopathy, eventually leading to heart failure as the main cause of death in DMD patients. Although several molecular mechanisms leading to the DMD cardiomyocyte (DMD-CM) death were described, mostly in mouse model, no suitable human CM model was until recently available together with proper clarification of the DMD-CM phenotype and delay in cardiac symptoms manifestation. We obtained several independent dystrophin-deficient human pluripotent stem cell (hPSC) lines from DMD patients and CRISPR/Cas9-generated DMD gene mutation. We differentiated DMD-hPSC into cardiac cells (CC) creating a human DMD-CC disease model. We observed that mutation-carrying cells were less prone to differentiate into CCs. DMD-CCs demonstrated an enhanced cell death rate in time. Furthermore, ion channel expression was altered in terms of potassium (Kir2.1 overexpression) and calcium handling (dihydropyridine receptor overexpression). DMD-CCs exhibited increased time of calcium transient rising compared to aged-matched control, suggesting mishandling of calcium release. We observed mechanical impairment (hypocontractility), bradycardia, increased heart rate variability, and blunted β-adrenergic response connected with remodeling of β-adrenergic receptors expression in DMD-CCs. Overall, these results indicated that our DMD-CC models are functionally affected by dystrophin-deficiency associated and recapitulate functional defects and cardiac wasting observed in the disease. It offers an accurate tool to study human cardiomyopathy progression and test therapies in vitro.

Modeling and simulation of excitation- contraction coupling of fast-twitch skeletal muscle fibers

BACKGROUND:

The current excitation-contraction coupling model of fast-twitch skeletal muscle fibers cannot completely simulate the excitation-contraction process.

OBJECTIVE:

To solve this problem, this study proposes an excitation-contraction model of fast-twitch skeletal muscle fibers based on the physiological structure and contractile properties of half-sarcomeres.

METHODS:

The model includes the action potential model of fast-twitch fiber membranes and transverse tubule membranes, the cycle model of 𝐶𝑎2+ in myofibril, the cross-bridge cycle model, and the fatigue model of metabolism.

RESULTS:

Finally, detailed analyses of the results from the simulation are conducted using the Simulink toolbox in MATLAB. Two conditions, non-coincidence and coincidence, are analyzed for both the thick and thin myofilaments.

CONCLUSIONS:

The simulation results of two groups of models are the same as the previous research results, which validates the accuracy of models.

Effect of carvedilol on atrial excitation-contraction coupling, Ca2+ release, and arrhythmogenicity

Carvedilol is an FDA-approved β-blocker commonly used for treatment of high blood pressure, congestive heart failure, and cardiac tachyarrhythmias, including atrial fibrillation. We investigated at the cellular level the mechanisms through which carvedilol interferes with sarcoplasmic reticulum (SR) Ca²⁺ release during excitation-contraction coupling (ECC) in single rabbit atrial myocytes. Carvedilol caused concentration-dependent (1-10 µM) failure of SR Ca²⁺ release. Failure of ECC and Ca²⁺ release was the result of dose-dependent inhibition of voltage-gated Na⁺ (I_Na) and L-type Ca²⁺ (I_Ca) currents that are responsible for the rapid depolarization phase of the cardiac action potential (AP) and the initiation of Ca²⁺-induced Ca²⁺ release from the SR, respectively. Carvedilol (1 µM) led to AP duration shortening, AP failures, and peak I_Na inhibition by ~80%, whereas I_Ca was not markedly affected. Carvedilol (10 µM) blocked I_Na almost completely and reduced I_Ca by ~40%. No effect on Ca²⁺-transient amplitude, I_Ca, and I_Na was observed in control experiments with the β-blocker metoprolol, suggesting that the carvedilol effect on ECC is unlikely the result of its β-blocking property. The effects of carvedilol (1 µM) on subcellular SR Ca²⁺ release was spatially inhomogeneous, where a selective inhibition of peripheral subsarcolemmal Ca²⁺ release from the junctional SR accounted for the cell-averaged reduction in Ca²⁺-transient amplitude. Furthermore, carvedilol significantly reduced the probability of spontaneous arrhythmogenic Ca²⁺ waves without changes of SR Ca²⁺ load. The data suggest a profound antiarrhythmic action of carvedilol in atrial myocytes resulting from an inhibitory effect on the SR Ca²⁺ release channel.NEW & NOTEWORTHY Here we show that the clinically widely used β-blocker carvedilol has profound effects on Ca²⁺ signaling and ion currents, but also antiarrhythmic effects in adult atrial myocytes. Carvedilol inhibits sodium and calcium currents and leads to failure of ECC but also prevents spontaneous Ca²⁺ release from cellular sarcoplasmic reticulum (SR) Ca²⁺ stores in form of arrhythmogenic Ca²⁺ waves. The antiarrhythmic effect occurs by carvedilol acting directly on the SR ryanodine receptor Ca²⁺ release channel.

Mechano-electric and mechano-chemo-transduction in cardiomyocytes

Cardiac excitation-contraction (E-C) coupling is influenced by (at least) three dynamic systems that couple and feedback to one another (see Abstract Figure). Here we review the mechanical effects on cardiomyocytes that include mechano-electro-transduction (commonly referred to as mechano-electric coupling, MEC) and mechano-chemo-transduction (MCT) mechanisms at cell and molecular levels which couple to Ca2+ -electro and E-C coupling reviewed elsewhere. These feedback loops from muscle contraction and mechano-transduction to the Ca2+ homeodynamics and to the electrical excitation are essential for understanding the E-C coupling dynamic system and arrhythmogenesis in mechanically loaded hearts. This white paper comprises two parts, each reflecting key aspects from the 2018 UC Davis symposium: MEC (how mechanical load influences electrical dynamics) and MCT (how mechanical load alters cell signalling and Ca2+ dynamics). Of course, such separation is artificial since Ca2+ dynamics profoundly affect ion channels and electrogenic transporters and vice versa. In time, these dynamic systems and their interactions must become fully integrated, and that should be a goal for a comprehensive understanding of how mechanical load influences cell signalling, Ca2+ homeodynamics and electrical dynamics. In this white paper we emphasize current understanding, consensus, controversies and the pressing issues for future investigations. Space constraints make it impossible to cover all relevant articles in the field, so we will focus on the topics discussed at the symposium.

Abnormal Excitation-Contraction Coupling and Calcium Homeostasis in Myopathies and Cardiomyopathies

Muscle contraction requires specialized membrane structures with precise geometry and relies on the concerted interplay of electrical stimulation and Ca2+ release, known as excitation-contraction coupling (ECC). The membrane structure hosting ECC is called triad in skeletal muscle and dyad in cardiac muscle, and structural or functional defects of triads and dyads have been observed in a variety of myopathies and cardiomyopathies. Based on their function, the proteins localized at the triad/dyad can be classified into three molecular pathways: the Ca2+ release complex (CRC), store-operated Ca2+ entry (SOCE), and membrane remodeling. All three are mechanistically linked, and consequently, aberrations in any of these pathways cause similar disease entities. This review provides an overview of the clinical and genetic spectrum of triad and dyad defects with a main focus of attention on the underlying pathomechanisms.

Activation of endogenous protein phosphatase 1 enhances the calcium sensitivity of the ryanodine receptor type 2 in murine ventricular cardiomyocytes

KEY POINTS

Increased protein phosphatase 1 (PP-1) activity has been found in end stage human heart failure. Although PP-1 has been extensively studied, a detailed understanding of its role in the excitation-contraction coupling mechanism, in normal and diseased hearts, remains elusive. The present study investigates the functional effect of the PP-1 activity on local Ca²⁺ release events in ventricular cardiomyocytes, by using an activating peptide (PDP3) for the stimulation of the endogenous PP-1 protein. We report that acute de-phosphorylation may increase the sensitivity of RyR2 channels to Ca²⁺ in situ, and that the RyR2-serine2808 phosphorylation site may mediate such a process. Our approach unmasks the functional importance of PP-1 in the regulation of RyR2 activity, suggesting a potential role in the generation of a pathophysiological sarcoplasmic reticulum Ca²⁺ leak in the diseased heart.

ABSTRACT

Changes in cardiac ryanodine receptor (RyR2) phosphorylation are considered to be important regulatory and disease related post-translational protein modifications. The extent of RyR2 phosphorylation is mainly determined by the balance of the activities of protein kinases and phosphatases, respectively. Increased protein phosphatase-1 (PP-1) activity has been observed in heart failure, although the regulatory role of this enzyme on intracellular Ca²⁺ handling remains poorly understood. To determine the physiological and pathophysiological significance of increased PP-1 activity, we investigated how the PP-1 catalytic subunit (PP-1c) alters Ca²⁺ sparks in permeabilized cardiomyocytes and we also applied a PP-1-disrupting peptide (PDP3) to specifically activate endogenous PP-1, including the one anchored on the RyR2 macromolecular complex. We compared wild-type and transgenic mice in which the usually highly phosphorylated site RyR2-S2808 has been ablated to investigate its involvement in RyR2 modulation (S2808A^+/+ ). In wild-type myocytes, PP-1 increased Ca²⁺ spark frequency by two-fold, followed by depletion of the sarcoplasmic reticulum Ca²⁺ store. Similarly, PDP3 transiently increased spark frequency and decreased sarcoplasmic reticulum Ca²⁺ load. RyR2 Ca²⁺ sensitivity, which was assessed by Ca²⁺ spark recovery analysis, was increased in the presence of PDP3 compared to a negative control peptide. S2808A^+/+ cardiomyocytes did not respond to both PP-1c and PDP3 treatment. Our results suggest an increased Ca²⁺ sensitivity of RyR2 upon de-phosphorylation by PP-1. Furthermore, we have confirmed the S2808 site as a target for PP-1 and as a potential link between RyR2s modulation and the cellular response.

Cardiac ryanodine receptor distribution is dynamic and changed by auxiliary proteins and post-translational modification

The effects of the immunophilins, FKBP12 and FKBP12.6, and phosphorylation on type II ryanodine receptor (RyR2) arrangement and function were examined using correlation microscopy (line scan confocal imaging of Ca²⁺ sparks and dual-tilt electron tomography) and dSTORM imaging of permeabilized Wistar rat ventricular myocytes. Saturating concentrations (10 µmol/L) of either FKBP12 or 12.6 significantly reduced the frequency, spread, amplitude and Ca²⁺ spark mass relative to control, while the tomograms revealed both proteins shifted the tetramers into a largely side-by-side configuration. Phosphorylation of immunophilin-saturated RyR2 resulted in structural and functional changes largely comparable to phosphorylation alone. dSTORM images of myocyte surfaces demonstrated that both FKBP12 and 12.6 significantly reduced RyR2 cluster sizes, while phosphorylation, even of immunophilin-saturated RyR2, increased them. We conclude that both RyR2 cluster size and the arrangement of tetramers within clusters is dynamic and respond to changes in the cellular environment. Further, these changes affect Ca²⁺ spark formation.

Ca2+ Channels Mediate Bidirectional Signaling between Sarcolemma and Sarcoplasmic Reticulum in Muscle Cells

The skeletal muscle and myocardial cells present highly specialized structures; for example, the close interaction between the sarcoplasmic reticulum (SR) and mitochondria—responsible for excitation-metabolism coupling—and the junction that connects the SR with T-tubules, critical for excitation-contraction (EC) coupling. The mechanisms that underlie EC coupling in these two cell types, however, are fundamentally distinct. They involve the differential expression of Ca²⁺ channel subtypes: Ca_V1.1 and RyR1 (skeletal), vs. Ca_V1.2 and RyR2 (cardiac). The Ca_V channels transform action potentials into elevations of cytosolic Ca²⁺, by activating RyRs and thus promoting SR Ca²⁺ release. The high levels of Ca²⁺, in turn, stimulate not only the contractile machinery but also the generation of mitochondrial reactive oxygen species (ROS). This forward signaling is reciprocally regulated by the following feedback mechanisms: Ca²⁺-dependent inactivation (of Ca²⁺ channels), the recruitment of Na⁺/Ca²⁺ exchanger activity, and oxidative changes in ion channels and transporters. Here, we summarize both well-established concepts and recent advances that have contributed to a better understanding of the molecular mechanisms involved in this bidirectional signaling.

Myotonic dystrophy type 1 alters muscle twitch properties, spinal reflexes, and perturbation-induced trans-cortical reflexes

BACKGROUND

Neurophysiologic biomarkers are needed for clinical trials of therapies for myotonic dystrophy (DM1). We characterized muscle properties, spinal reflexes (H-reflexes), and trans-cortical long-latency reflexes (LLRs) in a cohort with mild/moderate DM1.

METHODS

Twenty-four people with DM1 and 25 matched controls underwent assessment of tibial nerve H-reflexes and soleus muscle twitch properties. Quadriceps LLRs were elicited by delivering an unexpected perturbation during a single-limb squat (SLS) visuomotor tracking task.

RESULTS

DM1 was associated with decreased H-reflex depression. The efficacy of doublet stimulation was enhanced, yielding an elevated double-single twitch ratio. DM1 participants demonstrated greater error during the SLS task. DM1 individuals with the least-robust LLR responses showed the greatest loss of spinal H-reflex depression.

CONCLUSIONS

DM1 is associated with abnormalities of muscle twitch properties. Co-occurring alterations of spinal and trans-cortical reflex properties underscore the central nervous system manifestations of this disorder and may assist in gauging efficacy during clinical trials.

Assessing Cardiomyocyte Excitation-Contraction Coupling Site Detection From Live Cell Imaging Using a Structurally-Realistic Computational Model of Calcium Release

Calcium signaling plays a pivotal role in cardiomyocytes, coupling electrical excitation to mechanical contraction of the heart. Determining locations of active calcium release sites, and how their recruitment changes in response to stimuli and in disease states is therefore of central interest in cardiac physiology. Current algorithms for detecting release sites from live cell imaging data are however not easily validated against a known “ground truth,” which makes interpretation of the output of such algorithms, in particular the degree of confidence in site detection, a challenging task. Computational models are capable of integrating findings from multiple sources into a consistent, predictive framework. In cellular physiology, such models have the potential to reveal structure and function beyond the temporal and spatial resolution limitations of individual experimental measurements. Here, we create a spatially detailed computational model of calcium release in an eight sarcomere section of a ventricular cardiomyocyte, using electron tomography reconstruction of cardiac ultrastructure and confocal imaging of protein localization. This provides a high-resolution model of calcium diffusion from intracellular stores, which can be used as a platform to simulate confocal fluorescence imaging in the context of known ground truth structures from the higher resolution model. We use this capability to evaluate the performance of a recently proposed method for detecting the functional response of calcium release sites in live cells. Model permutations reveal how calcium release site density and mitochondria acting as diffusion barriers impact the detection performance of the algorithm. We demonstrate that site density has the greatest impact on detection precision and recall, in particular affecting the effective detectable depth of sites in confocal data. Our findings provide guidance on how such detection algorithms may best be applied to experimental data and give insights into limitations when using two-dimensional microscopy images to analyse three-dimensional cellular structures.

The contribution of low-frequency fatigue to the loss of quadriceps contractile function following repeated drop jumps

NEW FINDINGS

What is the central question of this study? Why do some subjects recover slowly following a bout of eccentric exercise and why is recovery faster following a repeated bout? What is the main finding and its importance? The results are consistent with two major causes of the reduction of quadriceps torque, the onset of low-frequency fatigue which recovered relatively fast and a second, delayed form of damage. Differences in the delayed damage process largely accounted for the differences in the rate of torque recovery between subjects after a first bout and it was suppression of the delayed damage which accounted for the faster recovery following a repeated bout of eccentric exercise.

ABSTRACT

The purpose of this study was to determine the extent to which low-frequency fatigue (LFF) accounts for the loss of quadriceps strength and time course of recovery following a series of drop jumps (DJs). Seventeen female subjects (20.8 ± 1.4 years) undertook 100 DJs, which were repeated 4 weeks later. Maximum isometric torque (MIT) and the ratio of torque generated by 20 and 100 Hz electrical stimulation (20/100), as a measure of LFF, were measured over 7 days following each series of DJs. After the first series the 20/100 ratio fell to a greater extent than MIT (to 35 ± 8.7% and 69 ± 11%, respectively) but recovered over 2-3 days, while MIT showed little recovery over this time. Changes of the 20/100 ratio were similar between subjects with fast or slow MIT recovery. Following the second series of DJs, changes in the 20/100 ratio were similar to those of the first bout and there were no differences between fast and slow recovering subjects. MIT, however, recovered more rapidly than after the first bout; the faster recovery was confined to the subjects who recovered slowly following the first bout. The results are consistent with two major causes of the reduction of quadriceps torque, the onset of low-frequency fatigue which recovered relatively fast and a second, delayed, form of damage. The latter largely accounted for the differences in MIT recovery between subjects after the first bout, while suppression of the delayed damage accounted for the faster recovery following the repeated bout.

Functional Electrical Stimulation: A Possible Strategy to Improve Muscle Function in Central Core Disease?

Central Core Disease (CCD) is a congenital myopathy characterized by presence of amorphous central areas (or cores) lacking glycolytic/oxidative enzymes and mitochondria in skeletal muscle fibers. Most CCD families are linked to mutations in ryanodine receptor type-1 (RYR1), the gene encoding for the sarcoplasmic reticulum (SR) Ca²⁺ release channel of skeletal muscle. As no treatments are available for CCD, currently management of patients is essentially based on a physiotherapic approaches. Functional electrical stimulation (FES) is a technique used to deliver low energy electrical impulses to artificially stimulate selected skeletal muscle groups. Here we tested the efficacy of FES in counteracting muscle loss and improve function in the lower extremities of a 55-year-old female patient which was diagnosed with CCD at the age of 44. Genetic screening of the RyR1 gene identified a missense mutation (c.7354C>T) in exon 46 resulting in an amino acid substitution (p.R2452W) and a duplication (c.12853_12864dup12) in exon 91. The patient was treated with FES for 26 months and subjected before, during, and after training to a series of functional and structural assessments: measurement of maximum isometric force of leg extensor muscles, magnetic resonance imaging, a complete set of functional tests to assess mobility in activities of daily living, and analysis of muscle biopsies by histology and electron microscopy. All results point to an improvement in muscle structure and function induced by FES suggesting that this approach could be considered as an additional supportive measure to maintain/improve muscle function (and possibly reduce muscle loss) in CCD patients.

Cardiac-specific overexpression of caveolin-3 preserves t-tubular ICa during heart failure in mice

NEW FINDINGS

What is the central question of this study? What is the cellular basis of the protection conferred on the heart by overexpression of caveolin-3 (Cav-3 OE) against many of the features of heart failure normally observed in vivo? What is the main finding and its importance? Cav-3 overexpression has little effect in normal ventricular myocytes but reduces cellular hypertrophy and preserves t-tubular ICa , but not local t-tubular Ca2+ release, in heart failure induced by pressure overload in mice. Thus Cav-3 overexpression provides specific but limited protection following induction of heart failure, although other factors disrupt Ca2+ release.

ABSTRACT

Caveolin-3 (Cav-3) is an 18 kDa protein that has been implicated in t-tubule formation and function in cardiac ventricular myocytes. During cardiac hypertrophy and failure, Cav-3 expression decreases, t-tubule structure is disrupted and excitation-contraction coupling (ECC) is impaired. Previous work has suggested that Cav-3 overexpression (OE) is cardio-protective, but the effect of Cav-3 OE on these cellular changes is unknown. We therefore investigated whether Cav-3 OE in mice is protective against the cellular effects of pressure overload induced by 8 weeks' transverse aortic constriction (TAC). Cav-3 OE mice developed cardiac dilatation, decreased stroke volume and ejection fraction, and hypertrophy and pulmonary congestion in response to TAC. These changes were accompanied by cellular hypertrophy, a decrease in t-tubule regularity and density, and impaired local Ca2+ release at the t-tubules. However, the extent of cardiac and cellular hypertrophy was reduced in Cav-3 OE compared to WT mice, and t-tubular Ca2+ current (ICa ) density was maintained. These data suggest that Cav-3 OE helps prevent hypertrophy and loss of t-tubular ICa following TAC, but that other factors disrupt local Ca2+ release.

Aberrant regulation of epigenetic modifiers contributes to the pathogenesis in patients with selenoprotein N-related myopathies

Congenital myopathies are early onset, slowly progressive neuromuscular disorders of variable severity. They are genetically and phenotypically heterogeneous and caused by pathogenic variants in several genes. Multi-minicore Disease, one of the more common congenital myopathies, is frequently caused by recessive variants in either SELENON, encoding the endoplasmic reticulum glycoprotein selenoprotein N or RYR1, encoding a protein involved in calcium homeostasis and excitation-contraction coupling. The mechanism by which recessive SELENON variants cause Multiminicore disease (MmD) is unclear. Here, we extensively investigated muscle physiological, biochemical and epigenetic modifications, including DNA methylation, histone modification, and noncoding RNA expression, to understand the pathomechanism of MmD. We identified biochemical changes that are common in patients harboring recessive RYR1 and SELENON variants, including depletion of transcripts encoding proteins involved in skeletal muscle calcium homeostasis, increased levels of Class II histone deacetylases (HDACs) and DNA methyltransferases. CpG methylation analysis of genomic DNA of patients with RYR1 and SELENON variants identified >3,500 common aberrantly methylated genes, many of which are involved in calcium signaling. These results provide the proof of concept for the potential use of drugs targeting HDACs and DNA methyltransferases to treat patients with specific forms of congenital myopathies.

Long-term testosterone deficiency modifies myofilament and calcium-handling proteins and promotes diastolic dysfunction in the aging mouse heart

The impact of long-term gonadectomy (GDX) on cardiac contractile function was explored in the setting of aging. Male mice were subjected to bilateral GDX or sham operation (4 wk) and investigated at 16-18 mo of age. Ventricular myocytes were field stimulated (2 Hz, 37°C). Peak Ca²⁺ transients (fura 2) and contractions were similar in GDX and sham-operated mice, although Ca²⁺ transients (50% decay time: 45.2 ± 2.3 vs. 55.6 ± 3.1 ms, P < 0.05) and contractions (time constant of relaxation: 39.1 ± 3.2 vs. 69.5 ± 9.3 ms, P < 0.05) were prolonged in GDX mice. Action potential duration was increased in myocytes from GDX mice, but this did not account for prolonged responses, as Ca²⁺ transient decay was slow even when cells from GDX mice were voltage clamped with simulated "sham" action potentials. Western blots of proteins involved in Ca²⁺ sequestration and efflux showed that Na⁺/Ca²⁺ exchanger and sarco(endo)plasmic reticulum Ca²⁺-ATPase type 2 protein levels were unaffected, whereas phospholamban was dramatically higher in ventricles from aging GDX mice (0.24 ± 0.02 vs. 0.86 ± 0.13, P < 0.05). Myofilament Ca²⁺ sensitivity at physiological Ca²⁺ was similar, but phosphorylation of essential myosin light chain 1 was reduced by ≈50% in ventricles from aging GDX mice. M-mode echocardiography showed no change in systolic function (e.g., ejection fraction). Critically, pulse-wave Doppler echocardiography showed that GDX slowed isovolumic relaxation time (12.9 ± 0.9 vs. 16.9 ± 1.0 ms, P < 0.05), indicative of diastolic dysfunction. Thus, dysregulation of intracellular Ca²⁺ and myofilament dysfunction contribute to deficits in contraction in hearts from testosterone-deficient aging mice. This suggests that low testosterone helps promote diastolic dysfunction in the aging heart. NEW & NOTEWORTHY The influence of long-term gonadectomy on contractile function was examined in aging male hearts. Gonadectomy slowed the decay of Ca²⁺ transients and contractions in ventricular myocytes and slowed isovolumic relaxation time, demonstrating diastolic dysfunction. Underlying mechanisms included Ca²⁺ dysregulation, elevated phospholamban protein levels, and hypophosphorylation of a myofilament protein, essential myosin light chain. Testosterone deficiency led to intracellular Ca²⁺ dysregulation and myofilament dysfunction, which may facilitate diastolic dysfunction in the setting of aging.

3D dSTORM imaging reveals novel detail of ryanodine receptor localization in rat cardiac myocytes

KEY POINTS

Using 3D direct stochastic optical reconstruction microscopy (dSTORM), we developed novel approaches to quantitatively describe the nanoscale, 3D organization of ryanodine receptors (RyRs) in cardiomyocytes. Complex arrangements of RyR clusters were observed in 3D space, both at the cell surface and within the cell interior, with allocation to dyadic and non-dyadic pools. 3D imaging importantly allowed discernment of clusters overlapping in the z-axis, for which detection was obscured by conventional 2D imaging techniques. Thus, RyR clusters were found to be significantly smaller than previous 2D estimates. Ca2+ release units (CRUs), i.e. functional groupings of neighbouring RyR clusters, were similarly observed to be smaller than earlier reports. Internal CRUs contained more RyRs in more clusters than CRUs on the cell surface, and yielded longer duration Ca2+ sparks.

ABSTRACT

Cardiomyocyte contraction is dependent on Ca2+ release from ryanodine receptors (RyRs). However, the precise localization of RyRs remains unknown, due to shortcomings of imaging techniques which are diffraction limited or restricted to 2D. We aimed to determine the 3D nanoscale organization of RyRs in rat cardiomyocytes by employing direct stochastic optical reconstruction microscopy (dSTORM) with phase ramp technology. Initial observations at the cell surface showed an undulating organization of RyR clusters, resulting in their frequent overlap in the z-axis and obscured detection by 2D techniques. Non-overlapping clusters were imaged to create a calibration curve for estimating RyR number based on recorded fluorescence blinks. Employing this method at the cell surface and interior revealed smaller RyR clusters than 2D estimates, as erroneous merging of axially aligned RyRs was circumvented. Functional groupings of RyR clusters (Ca2+ release units, CRUs), contained an average of 18 and 23 RyRs at the surface and interior, respectively, although half of all CRUs contained only a single 'rogue' RyR. Internal CRUs were more tightly packed along z-lines than surface CRUs, contained larger and more numerous RyR clusters, and constituted ∼75% of the roughly 1 million RyRs present in an average cardiomyocyte. This complex internal 3D geometry was underscored by correlative imaging of RyRs and t-tubules, which enabled quantification of dyadic and non-dyadic RyR populations. Mirroring differences in CRU size and complexity, Ca2+ sparks originating from internal CRUs were of longer duration than those at the surface. These data provide novel, nanoscale insight into RyR organization and function across cardiomyocytes.

Action potential shortening rescues atrial calcium alternans

KEY POINTS

Cardiac alternans refers to a beat-to-beat alternation in contraction, action potential (AP) morphology and Ca²⁺ transient (CaT) amplitude, and represents a risk factor for cardiac arrhythmia, including atrial fibrillation. We developed strategies to pharmacologically manipulate the AP waveform with the goal to reduce or eliminate the occurrence of CaT and contraction alternans in atrial tissue. With combined patch-clamp and intracellular Ca²⁺ measurements we investigated the effect of specific ion channel inhibitors and activators on alternans. In single rabbit atrial myocytes, suppression of Ca²⁺ -activated Cl^- channels eliminated AP duration alternans, but prolonged the AP and failed to eliminate CaT alternans. In contrast, activation of K⁺ currents (I_Ks and I_Kr ) shortened the AP and eliminated both AP duration and CaT alternans. As demonstrated also at the whole heart level, activation of K⁺ conductances represents a promising strategy to suppress alternans, and thus reducing a risk factor for atrial fibrillation.

ABSTRACT

At the cellular level alternans is observed as beat-to-beat alternations in contraction, action potential (AP) morphology and magnitude of the Ca²⁺ transient (CaT). Alternans is a well-established risk factor for cardiac arrhythmia, including atrial fibrillation. This study investigates whether pharmacological manipulation of AP morphology is a viable strategy to reduce the risk of arrhythmogenic CaT alternans. Pacing-induced AP and CaT alternans were studied in rabbit atrial myocytes using combined Ca²⁺ imaging and electrophysiological measurements. Increased AP duration (APD) and beat-to-beat alternations in AP morphology lowered the pacing frequency threshold and increased the degree of CaT alternans. Inhibition of Ca²⁺ -activated Cl^- channels reduced beat-to-beat AP alternations, but prolonged APD and failed to suppress CaT alternans. In contrast, AP shortening induced by activators of two K⁺ channels (ML277 for Kv7.1 and NS1643 for Kv11.1) abolished both APD and CaT alternans in field-stimulated and current-clamped myocytes. K⁺ channel activators had no effect on the degree of Ca²⁺ alternans in AP voltage-clamped cells, confirming that suppression of Ca²⁺ alternans was caused by the changes in AP morphology. Finally, activation of Kv11.1 channel significantly attenuated or even abolished atrial T-wave alternans in isolated Langendorff perfused hearts. In summary, AP shortening suppressed or completely eliminated both CaT and APD alternans in single atrial myocytes and atrial T-wave alternans at the whole heart level. Therefore, we suggest that AP shortening is a potential intervention to avert development of alternans with important ramifications for arrhythmia prevention and therapy.

Interplay Between Sub-Cellular Alterations of Calcium Release and T-Tubular Defects in Cardiac Diseases

Asynchronous Ca²⁺ release promotes non-homogeneous myofilament activation, leading to mechanical dysfunction, as well as initiation of propagated calcium waves and arrhythmias. Recent advances in microscopy techniques have allowed for optical recordings of local Ca²⁺ fluxes and action potentials from multiple sub-cellular domains within cardiac cells with unprecedented spatial and temporal resolution. Since then, sub-cellular local information of the spatio-temporal relationship between Ca²⁺ release and action potential propagation have been unlocked, providing novel mechanistic insights in cardiac excitation-contraction coupling (ECC). Here, we review the promising perspectives arouse from repeatedly probing Ca²⁺ release at the same sub-cellular location while simultaneously probing multiple locations at the same time within a single cardiac cell. We also compare the results obtained in three different rodent models of cardiac diseases, highlighting disease-specific mechanisms. Slower local Ca²⁺ release has been observed in regions with defective action potential conduction in diseased cardiac cells. Moreover, significant increment of Ca²⁺ variability (both in time and in space) has been found in diseased cardiac cells but does not directly correlate with local electrical defects nor with the degree of structural aberrations of the cellular membrane system, suggesting a role for other players of the ECC machinery. We finally explore exciting opportunities provided by the technology for studying different cardiomyocyte populations, as well as for dissecting the mechanisms responsible for subcellular spatio-temporal variability of Ca²⁺ release.

STAC3 variants cause a congenital myopathy with distinctive dysmorphic features and malignant hyperthermia susceptibility

SH3 and cysteine-rich domain-containing protein 3 (STAC3) is an essential component of the skeletal muscle excitation-contraction coupling (ECC) machinery, though its role and function are not yet completely understood. Here, we report 18 patients carrying a homozygous p.(Trp284Ser) STAC3 variant in addition to a patient compound heterozygous for the p.(Trp284Ser) and a novel splice site change (c.997-1G > T). Clinical severity ranged from prenatal onset with severe features at birth, to a milder and slowly progressive congenital myopathy phenotype. A malignant hyperthermia (MH)-like reaction had occurred in several patients. The functional analysis demonstrated impaired ECC. In particular, KCl-induced membrane depolarization resulted in significantly reduced sarcoplasmic reticulum Ca²⁺ release. Co-immunoprecipitation of STAC3 with Ca_V 1.1 in patients and control muscle samples showed that the protein interaction between STAC3 and Ca_V 1.1 was not significantly affected by the STAC3 variants. This study demonstrates that STAC3 gene analysis should be included in the diagnostic work up of patients of any ethnicity presenting with congenital myopathy, in particular if a history of MH-like episodes is reported. While the precise pathomechanism remains to be elucidated, our functional characterization of STAC3 variants revealed that defective ECC is not a result of Ca_V 1.1 sarcolemma mislocalization or impaired STAC3-Ca_V 1.1 interaction.

Obstruction of ventricular Ca2+ -dependent arrhythmogenicity by inositol 1,4,5-trisphosphate-triggered sarcoplasmic reticulum Ca2+ release

KEY POINTS

Augmented inositol 1,4,5-trisphosphate (IP3 ) receptor (IP3 R2) expression has been linked to a variety of cardiac pathologies. Although cardiac IP3 R2 function has been in the focus of research for some time, a detailed understanding of its potential role in ventricular myocyte excitation-contraction coupling under pathophysiological conditions remains elusive. The present study focuses on mechanisms of IP3 R2-mediated sarcoplasmic reticulum (SR)-Ca2+ release in ventricular excitation-contraction coupling under IP3 R2-overexpressing conditions by studying intracellular Ca2+ events. We report that, upon IP3 R2 overexpression in ventricular myocytes, IP3 -induced Ca2+ release (IP3 ICR) modulates the SR-Ca2+ content via "eventless" SR-Ca2+ release, affecting the global SR-Ca2+ leak. Thus, IP3 R2 activation could act as a SR-Ca2+ gateway mechanism to escape ominous SR-Ca2+ overload. Our approach unmasks a so far unrecognized mechanism by which "eventless" IP3 ICR plays a protective role against ventricular Ca2+ -dependent arrhythmogenicity.

ABSTRACT

Augmented inositol 1,4,5-trisphosphate (IP3 ) receptor (IP3 R2) function has been linked to a variety of cardiac pathologies including cardiac arrhythmias. The functional role of IP3 -induced Ca2+ release (IP3 ICR) within ventricular excitation-contraction coupling (ECC) remains elusive. As part of pathophysiological cellular remodelling, IP3 R2s are overexpressed and have been repeatedly linked to enhanced Ca2+ -dependent arrhythmogenicity. In this study we test the hypothesis that an opposite scenario might be plausible in which IP3 ICR is part of an ECC protecting mechanism, resulting in a Ca2+ -dependent anti-arrhythmogenic response on the cellular scale. IP3 R2 activation was triggered via endothelin-1 or IP3 -salt application in single ventricular myocytes from a cardiac-specific IP3 R type 2 overexpressing mouse model. Upon IP3 R2 overexpression, IP3 R activation reduced Ca2+ -wave occurrence (46 vs. 21.72%; P < 0.001) while its block increased SR-Ca2+ content (∼29.4% 2-aminoethoxydiphenyl borate, ∼16.4% xestospongin C; P < 0.001), suggesting an active role of IP3 ICR in SR-Ca2+ content regulation and anti-arrhythmogenic function. Pharmacological separation of ryanodine receptor RyR2 and IP3 R2 functions and two-dimensional Ca2+ event analysis failed to identify local IP3 ICR events (Ca2+ puffs). SR-Ca2+ leak measurements revealed that under pathophysiological conditions, "eventless" SR-Ca2+ efflux via enhanced IP3 ICR maintains the SR-Ca2+ content below Ca2+ spark threshold, preventing aberrant SR-Ca2+ release and resulting in a protective mechanism against SR-Ca2+ overload and arrhythmias. Our results support a so far unrecognized modulatory mechanism in ventricular myocytes working in an anti-arrhythmogenic fashion.

Targeting Calpain for Heart Failure Therapy: Implications From Multiple Murine Models

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Calpain is hyperactivated in human failing hearts and rodent heart failure models of different etiologies.

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Inhibition of calpain activity with MDL-28170 protects against cardiac dysfunction by preserving JP2 expression and T-tubule ultrastructural integrity in murine models of heart failure.

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Overexpression of JP2 delays the onset of early cardiac sudden death and heart failure, induced by calpain overactivation.

Heart failure remains a major cause of morbidity and mortality in developed countries. There is still a strong need to devise new mechanism-based treatments for heart failure. Numerous studies have suggested the importance of the Ca²⁺-dependent protease calpain in cardiac physiology and pathology. However, no drugs are currently under development or testing in human patients to target calpain for heart failure treatment. Herein the data demonstrate that inhibition of calpain activity protects against deleterious ultrastructural remodeling and cardiac dysfunction in multiple rodent models of heart failure, providing compelling evidence that calpain inhibition is a promising therapeutic strategy for heart failure treatment.

Model discrimination for Ca2+ -dependent regulation of myosin light chain kinase in smooth muscle contraction

Excitation-contraction coupling in smooth muscle is mediated by the Ca²⁺ - and calmodulin-dependent regulation of myosin light chain kinase. The precise mechanism of this regulation remains controversial, and several mathematical models have been proposed for the interaction of the three species. These models have previously been analyzed at steady state primarily by numerical simulation of differential equations, for which parameter values must be estimated from data. Here, we use the linear framework for timescale separation to demonstrate that models of this general kind can be solved analytically for an equilibrium steady state, without having to determine parameter values. This analysis leads to parameter-independent methods for discriminating between the models, for which we propose experiments that could be performed with existing methods.