Calcium sparks: elementary events underlying excitation-contraction coupling in heart muscle

Novel Ca2+ wave mechanisms in cardiac myocytes revealed by multiscale Ca2+ release model

Integrating cellular sarcoplasmic reticulum (SR) Ca2+ release with the known Ca2+ activation properties of RyR2s remains challenging. The sharp increase in SR Ca2+ permeability above a threshold SR luminal [Ca2+] is not reflected in RyR2 kinetics from single-channel studies. Additionally, the current paradigm that global Ca2+ release (Ca2+ waves) arises from interacting local events (Ca2+ sparks) faces a key issue that these events rarely activate neighboring sites. We present a multiscale model that reproduces Ca2+ sparks and waves in skinned ventricular myocytes using experimentally validated RyR2 kinetics. The model spans spatial domains from 10-8 to 10-4 m and timescales from 10-6 to 10 s. Ca2+ release sites are distributed in cubic voxels (0.25-µm sides) informed by super-resolution micrographs. We use parallel computing to calculate Ca2+ transport, diffusion, and buffering. Substantial increases in SR Ca2+ release occur, and Ca2+ waves initiate when Ca2+ sparks become prolonged above a threshold SR [Ca2+]. These prolonged events (Ca2+ embers) are much more likely than Ca2+ sparks to activate release from neighboring sites and accumulate increases in cytoplasmic [Ca2+] along with an associated fall in Ca2+ buffering power. This primes the cytoplasm for Ca2+-induced Ca2+ release (CICR) that produces Ca2+ waves. Thus, Ca2+ ember formation and CICR are both essential for initiation and propagation of Ca2+ waves. Cell architecture, along with the differential effects of RyR2 opening and closing rates, collectively determines the SR [Ca2+] threshold for Ca2+ embers, waves, and the phenomenon of store overload-induced Ca2+ release.

Reduced voltage-activated Ca2+ release flux in muscle fibers from a rat model of Duchenne dystrophy

The potential pathogenic role of disturbed Ca2+ homeostasis in Duchenne muscular dystrophy (DMD) remains a complex, unsettled issue. We used muscle fibers isolated from 3-mo-old DMDmdx rats to further investigate the case. Most DMDmdx fibers exhibited no sign of trophic or morphology distinction as compared with WT fibers and mitochondria and t-tubule membrane networks also showed no stringent discrepancy. Under voltage clamp, values for holding current were similar in the two groups, whereas values for capacitance were larger in DMDmdx fibers, suggestive of enhanced amount of t-tubule membrane. The Ca2+ current density across the channel carried by the EC coupling voltage sensor (CaV1.1) was unchanged. The maximum rate of voltage-activated sarcoplasmic reticulum (SR) Ca2+ release was reduced by 25% in the DMDmdx fibers, with no change in voltage dependency. Imaging resting Ca2+ revealed rare spontaneous local SR Ca2+ release events with no sign of elevated activity in DMDmdx fibers. Under current clamp, DMDmdx fibers generated similar trains of action potentials as WT fibers. Results suggest that reduced peak amplitude of SR Ca2+ release is an inherent feature of this DMD model, likely contributing to muscle weakness. This occurs despite a preserved amount of releasable Ca2+ and with no change in excitability, CaV1.1 channel activity, and SR Ca2+ release at rest. Although we cannot exclude that fibers from the 3-mo-old animals do not yet display a fully developed disease phenotype, results provide limited support for pathomechanistic concepts frequently associated with DMD such as membrane fragility, excessive Ca2+ entry, or enhanced SR Ca2+ leak.

Formation and regulation of calcium sparks on a nonlinear spatial network of ryanodine receptors

Accurate regulation of calcium release is essential for cellular signaling, with the spatial distribution of ryanodine receptors (RyRs) playing a critical role. In this study, we present a nonlinear spatial network model that simulates RyR spatial organization to investigate calcium release dynamics by integrating RyR behavior, calcium buffering, and calsequestrin (CSQ) regulation. The model successfully reproduces calcium sparks, shedding light on their initiation, duration, and termination mechanisms under clamped calcium conditions. Our simulations demonstrate that RyR clusters act as on-off switches for calcium release, producing short-lived calcium quarks and longer-lasting calcium sparks based on distinct activation patterns. Spark termination is governed by calcium gradients and stochastic RyR dynamics, with CSQ facilitating RyR closure and spark termination. We also uncover the dual role of CSQ as both a calcium buffer and a regulator of RyRs. Elevated CSQ levels prolong calcium release due to buffering effects, while CSQ-RyR interactions induce excessive refractoriness, a phenomenon linked to pathological conditions such as ventricular arrhythmias. Dysregulated CSQ function disrupts the on-off switching behavior of RyRs, impairing calcium release dynamics. These findings provide new insights into RyR-mediated calcium signaling, highlighting CSQ's pivotal role in maintaining calcium homeostasis and its implications for pathological conditions. This work advances the understanding of calcium spark regulation and underscores its significance for cardiomyocyte function.

Constitutive deletion of the obscurin-Ig58/59 domains induces atrial remodeling and Ca2+-based arrhythmogenesis

Obscurin is a giant protein that coordinates diverse aspects of striated muscle physiology. Obscurin immunoglobulin domains 58/59 (Ig58/59) associate with essential sarcomeric and Ca2+ cycling proteins. To explore the pathophysiological significance of Ig58/59, we generated the Obscn-ΔIg58/59 mouse model, expressing obscurin constitutively lacking Ig58/59. Males in this line develop atrial fibrillation by 6 months, with atrial and ventricular dilation by 12 months. As Obscn-ΔIg58/59 left ventricles at 6 months exhibit no deficits in sarcomeric ultrastructure or Ca2+ signaling, we hypothesized that susceptibility to arrhythmia may emanate from the atria. Ultrastructural evaluation of male Obscn-ΔIg58/59 atria uncovered prominent Z-disk streaming by 6 months and further misalignment by 12 months. Relatedly, isolated Obscn-ΔIg58/59 atrial cardiomyocytes exhibited increased Ca2+ spark frequency and age-specific alterations in Ca2+ cycling dynamics, coinciding with arrhythmia onset and progression. Quantitative analysis of the transverse-axial tubule (TAT) network using super-resolution microscopy demonstrated significant TAT depletion in Obscn-ΔIg58/59 atria. These structural and Ca2+ signaling deficits were accompanied by age-specific alterations in the expression or phosphorylation of T-cap protein, which links transverse tubules to Z-disks, and junctophilin 2, which connects transverse tubules to the sarcoplasmic reticulum. Collectively, our work establishes the Obscn-ΔIg58/59 model as a reputable genetic model for atrial cardiomyopathy and provides mechanistic insights into atrial fibrillation and remodeling.

Inhibitors of Intracellular RyR2 Calcium Release Channels as Therapeutic Agents in Arrhythmogenic Heart Diseases

Ryanodine receptor type 2 (RyR2) is the principal intracellular calcium release channel in the cardiac sarcoplasmic reticulum (SR). Pathological RyR2 hyperactivity generates arrhythmia risk in genetic and structural heart diseases. RYR2 gain-of-function mutations cause catecholaminergic polymorphic ventricular tachycardia. In structural heart diseases (i.e., heart failure), posttranslation modifications render RyR2 channels leaky, resulting in pathologic calcium release during diastole, contributing to arrhythmogenesis and contractile dysfunction. Hence, RyR2 represents a therapeutic target in arrhythmogenic heart diseases. We provide an overview of the structure and function of RyR2, and then review US Food and Drug Administration-approved and investigational RyR2 inhibitors. A therapeutic classification of RyR2 inhibitors is proposed based on their mechanism of action. Class I RyR2 inhibitors (e.g., flecainide) do not change SR calcium content and are primarily antiarrhythmic. Class II RyR2 inhibitors (e.g., dantrolene) increase SR calcium content, making them less effective as antiarrhythmics but preferable in conditions with reduced SR calcium content such as heart failure.

Endoplasmic Reticulum Calcium Signaling in Hippocampal Neurons

The endoplasmic reticulum (ER) is a key organelle in cellular homeostasis, regulating calcium levels and coordinating protein synthesis and folding. In neurons, the ER forms interconnected sheets and tubules that facilitate the propagation of calcium-based signals. Calcium plays a central role in the modulation and regulation of numerous functions in excitable cells. It is a versatile signaling molecule that influences neurotransmitter release, muscle contraction, gene expression, and cell survival. This review focuses on the intricate dynamics of calcium signaling in hippocampal neurons, with particular emphasis on the activation of voltage-gated and ionotropic glutamate receptors in the plasma membrane and ryanodine and inositol 1,4,5-trisphosphate receptors in the ER. These channels and receptors are involved in the generation and transmission of electrical signals and the modulation of calcium concentrations within the neuronal network. By analyzing calcium fluctuations in neurons and the associated calcium handling mechanisms at the ER, mitochondria, endo-lysosome and cytosol, we can gain a deeper understanding of the mechanistic pathways underlying neuronal interactions and information transfer.

Expression level of cardiac ryanodine receptors dictates properties of Ca2+-induced Ca2+ release

The type 2 ryanodine receptor (RyR2) is the major Ca2+ release channel required for Ca2+-induced Ca2+ release (CICR) and cardiac excitation-contraction coupling. The cluster organization of RyR2 at the dyad is critical for efficient CICR. Despite its central role in cardiac Ca2+ signaling, the mechanisms that control CICR are not fully understood. As a single RyR2 Ca2+ flux dictates local CICR that underlies Ca2+ sparks, RyR2 density in a cluster, and therefore the distance between RyR2s, should have a profound impact on local CICR. Here, we studied the effect of the RyR2 expression level ([RyR2]) on CICR activation, termination, and amplitude. The endoplasmic reticulum (ER)-targeted Ca2+ sensor RCEPIA-1er was used to directly measure the ER [Ca2+] (Ca2+]ER) in the T-Rex-293 the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2a) stable cell line expressing human RyR2. Cells coexpressing RyR2 and SERCA2a produced periodic [Ca2+]ER depletions in the form of spontaneous Ca2+ waves due to propagating CICR. For each studied cell, the [Ca2+]ER at which Ca2+ waves are activated and terminated was analyzed as a function of [RyR2]. CICR parameters, such as [Ca2+]ER activation, termination, and amplitude, were inversely proportional to [RyR2] at low-intermediate levels. Increasing the sensitivity of RyR2 to cytosolic Ca2+ lowered the [Ca2+]ER at which CICR is activated and terminated. Decreasing the sensitivity of RyR2 to cytosolic Ca2+ had the opposite effect on CICR. These results suggest that RyR2 density in the release cluster should have a significant impact on local CICR activation and termination. Since SR Ca2+ load is evenly distributed throughout the SR network, clusters with higher RyR2 density would have a higher probability of initiating spontaneous CICR.

Role of ryanodine receptor cooperativity in Ca2+-wave-mediated triggered activity in cardiomyocytes

Ca2+ waves are known to trigger delayed after-depolarizations that can cause malignant cardiac arrhythmias. However, modelling Ca2+ waves using physiologically realistic models has remained a major challenge. Existing models with low Ca2+ sensitivity of ryanodine receptors (RyRs) necessitate large release currents, leading to an unrealistically large Ca2+ transient amplitude incompatible with the experimental observations. Consequently, current physiologically detailed models of delayed after-depolarizations resort to unrealistic cell architectures to produce Ca2+ waves with a normal Ca2+ transient amplitude. Here, we address these challenges by incorporating RyR cooperativity into a physiologically detailed model with a realistic cell architecture. We represent RyR cooperativity phenomenologically through a Hill coefficient within the sigmoid function of RyR open probability. Simulations in permeabilized myocytes with high Ca2+ sensitivity reveal that a sufficiently large Hill coefficient is required for Ca2+ wave propagation via the fire-diffuse-fire mechanism. In intact myocytes, propagating Ca2+ waves can occur only within an intermediate Hill coefficient range. Within this range, the spark rate is neither too low, enabling Ca2+ wave propagation, nor too high, allowing for the maintenance of a high sarcoplasmic reticulum load during diastole of the action potential. Moreover, this model successfully replicates other experimentally observed manifestations of Ca2+-wave-mediated triggered activity, including phase 2 and phase 3 early after-depolarizations and high-frequency voltage-Ca2+ oscillations. These oscillations feature an elevated take-off potential with depolarization mediated by the L-type Ca2+ current. The model also sheds light on the roles of luminal gating of RyRs and the mobile buffer ATP in the genesis of these arrhythmogenic phenomena. KEY POINTS: Existing mathematical models of Ca2+ waves use an excessively large Ca2+-release current or unrealistic diffusive coupling between release units. Our physiologically realistic model, using a Hill coefficient in the ryanodine receptor (RyR) gating function to represent RyR cooperativity, addresses these limitations and generates organized Ca2+ waves at Hill coefficients ranging from ∼5 to 10, as opposed to the traditional value of 2. This range of Hill coefficients gives a spark rate neither too low, thereby enabling Ca2+ wave propagation, nor too high, allowing for the maintenance of a high sarcoplasmic reticulum load during the plateau phase of the action potential. Additionally, the model generates Ca2+-wave-mediated phase 2 and phase 3 early after-depolarizations, and coupled membrane voltage with Ca2+ oscillations mediated by the L-type Ca2+ current. This study suggests that pharmacologically targeting RyR cooperativity could be a promising strategy for treating cardiac arrhythmias linked to Ca2+-wave-mediated triggered activity.

Structure-Function Relationship of the Ryanodine Receptor Cluster Network in Sinoatrial Node Cells

The rate of spontaneous action potentials (APs) generated by sinoatrial node cells (SANC) is regulated by local Ca2+ release (LCR) from the sarcoplasmic reticulum via Ca2+ release channels (ryanodine receptors, RyRs). LCR events propagate and self-organize within the network of RyR clusters (Ca release units, CRUs) via Ca-induced-Ca-release (CICR) that depends on CRU sizes and locations: While larger CRUs generate stronger release signals, the network's topology governs signal diffusion and propagation. This study used super-resolution structured illumination microscopy to image the 3D network of CRUs in rabbit SANC. The peripheral CRUs formed a spatial mesh, reflecting the cell surface geometry. Two distinct subpopulations of CRUs were identified within each cell, with size distributions conforming to a two-component Gamma mixture model. Furthermore, neighboring CRUs exhibited repulsive behavior. Functional properties of the CRU network were further examined in a novel numerical SANC model developed using our experimental data. Model simulations revealed that heterogeneities in both CRU sizes and locations facilitate CICR and increase the AP firing rate in a cooperative manner. However, these heterogeneities reduce the effect of β-adrenergic stimulation in terms of its relative change in AP firing rate. The presence of heterogeneities in both sizes and locations allows SANC to reach higher absolute AP firing rates during β-adrenergic stimulation. Thus, the CICR facilitation by heterogeneities in CRU sizes and locations regulates and optimizes cardiac pacemaker cell operation under various physiological conditions. Dysfunction of this optimization could be a key factor in heart rate reserve decline in aging and disease.

Electrophysical cardiac remodeling at the molecular level: Insights into ryanodine receptor activation and calcium-induced calcium release from a stochastic explicit-particle model

We present the first-ever, fully discrete, stochastic model of triggered cardiac Ca2+ dynamics. Using anatomically accurate subcellular cardiac myocyte geometries, we simulate the molecular players involved in Ca2+ handling using high-resolution stochastic and explicit-particle methods at the level of an individual cardiac dyadic junction. Integrating data from multiple experimental sources, the model not only replicates the findings of traditional in silico studies and complements in vitro experimental data but also reveals new insights into the molecular mechanisms driving cardiac dysfunction under stress and disease conditions. We improve upon older, nondiscrete models using the same realistic geometry by incorporating molecular mechanisms for spontaneous, as well as triggered calcium-induced calcium release (CICR). Action potentials are used to activate L-type calcium channels (LTCC), triggering CICR through ryanodine receptors (RyRs) on the surface of the sarcoplasmic reticulum. These improvements allow for the specific focus on the couplon: the structure-function relationship between LTCC and RyR. We investigate the electrophysical effects of normal and diseased action potentials on CICR and interrogate the effects of dyadic junction deformation through detubulation and orphaning of RyR. Our work demonstrates the importance of the electrophysical integrity of the calcium release unit on CICR fidelity, giving insights into the molecular basis of heart disease. Finally, we provide a unique, detailed, molecular view of the CICR process using advanced rendering techniques. This easy-to-use model comes complete with tutorials and the necessary software for use and analysis to maximize usability and reproducibility. Our work focuses on quantifying, qualifying, and visualizing the behavior of the molecular species that underlie the function and dysfunction of subcellular cardiomyocyte systems.

Loss-of-function W4645R mutation in the RyR2-caffeine binding site: implications for synchrony and arrhythmogenesis

AIMS

Previous studies have identified RyR2 W4645R mutation, located in the caffeine-binding site, to associate with CPVT1 pathology. Caffeine binding to its site is thought to displace the carboxyl-terminal domain to Ca2+-binding, allowing the tryptophan residue (W4645) to regulate Ca2+ sensitivity of RyR2. To gain insights into regulation of RyR2 Ca2+-binding and its interaction with caffeine-binding site, we introduced W4645R-RyR2 point mutation via CRISPR/Cas9 gene-editing in human induced pluripotent stem cell-derived cardiomyocytes (hiPSCCMs) and characterized their Ca2+-signaling phenotype compared to WT hiPSCCMs.

METHODS AND RESULTS

W4645R-RyR2 cardiomyocytes had: (1) no significant change in ICa magnitude or voltage-dependence; (2) slightly reduced CICR; (3) altered relaxation kinetics of Ca2+-transients with no change in isoproterenol sensitivity; (4) complete loss of caffeine-triggered Ca2+ release; (5) larger SR Ca2+ leak resulting in 40 % lower SR Ca2+ content, as determined by myocytes' response to 4-CmC; (6) lower incidence of calcium sparks and asynchronous spontaneous SR Ca2+ releases.

CONCLUSIONS

W4645R-RyR2 mutation induces loss of caffeine-triggered SR Ca2+ release and enhances SR Ca2+ leak that underlie asynchronous spontaneous Ca2+ releases, triggering arrhythmia and impairing cardiac function.

Calcium handling abnormalities increase arrhythmia susceptibility in DMSXL myotonic dystrophy type 1 mice

BACKGROUND

Myotonic dystrophy type 1 (DM1) is a multiorgan disorder with significant cardiac involvement. ECG abnormalities, including arrhythmias, occur in 80 % of DM1 patients and are the second-most common cause of death after respiratory complications; however, the mechanisms underlying the arrhythmogenesis remain unclear. The objective of this study was to investigate the basis of the electrophysiological abnormalities in DM1 using the DMSXL mouse model.

METHODS

ECG parameters were evaluated at baseline and post flecainide challenge. Calcium transient and action potential parameters were evaluated in Langendorff-perfused hearts using fluorescence optical mapping. Calcium transient/sparks were evaluated in ventricular myocytes via confocal microscopy. Protein and mRNA levels for calcium handling proteins were evaluated using western blot and RT-qPCR, respectively.

RESULTS

DMSXL mice showed arrhythmic events on ECG including premature ventricular contractions and sinus block. DMSXL mice showed increased calcium transient time to peak without any change to voltage parameters. Calcium alternans and both sustained and non-sustained ventricular tachyarrhythmias were readily inducible in DMSXL mice. The confocal experiments also showed calcium transient alternans and increased frequency of calcium sparks in DMSXL cardiomyocytes. These calcium abnormalities were correlated with increased RyR2 phosphorylation without changes to the other calcium handling proteins.

CONCLUSIONS

The DMSXL mouse model of DM1 exhibited enhanced arrhythmogenicity associated with abnormal intracellular calcium handling due to hyperphosphorylation of RyR2, pointing to RyR2 as a potential new therapeutic target in DM1 treatment.

Thirty years of Ca2+ spark research: digital principle of cell signaling unveiled

Calcium ions (Ca2+) are an archetypical and most versatile second messenger in virtually all cell types. Inspired by the discovery of Ca2+ sparks in the 1990s, vibrant research over the last three decades has unveiled a constellation of Ca2+ microdomains as elementary events of Ca2+ signaling and, more importantly, a digital-analog dualism as the system design principle of Ca2+ signaling. In this brief review, we present a sketchy summary on advances in the field of sparkology, and discuss how the digital subsystem can fulfill physiological roles otherwise impossible for any analog system. In addition, we attempt to address how the digital-analog dualism endows the simple cation messenger with signaling speediness, specificity, efficiency, stability, and unparalleled versatility.

Using a Failing Human Ventricular Cardiomyocyte Model to Re-Evaluate Ca2+ Cycling, Voltage Dependence, and Spark Characteristics

Previous studies have observed alterations in excitation-contraction (EC) coupling during end-stage heart failure that include action potential and calcium (Ca2+) transient prolongation and a reduction of the Ca2+ transient amplitude. Underlying these phenomena are the downregulation of potassium (K+) currents, downregulation of the sarcoplasmic reticulum Ca2+ ATPase (SERCA), increase Ca2+ sensitivity of the ryanodine receptor, and the upregulation of the sodium-calcium (Na=-Ca2+) exchanger. However, in human heart failure (HF), debate continues about the relative contributions of the changes in calcium handling vs. the changes in the membrane currents. To understand the consequences of the above changes, they are incorporated into a computational human ventricular myocyte HF model that can explore the contributions of the spontaneous Ca2+ release from the sarcoplasmic reticulum (SR). The reduction of transient outward K+ current (Ito) is the main membrane current contributor to the decrease in RyR2 open probability and L-type calcium channel (LCC) density which emphasizes its importance to phase 1 of the action potential (AP) shape and duration (APD). During current-clamp conditions, RyR2 hyperphosphorylation exhibits the least amount of Ca2+ release from the SR into the cytosol and SR Ca2+ fractional release during a dynamic slow-rapid-slow (0.5-2.5-0.5 Hz) pacing, but it displays the most abundant and more lasting Ca2+ sparks two-fold longer than a normal cell. On the other hand, under voltage-clamp conditions, HF by decreased SERCA and upregulated INCX show the least SR Ca2+ uptake and EC coupling gain, as compared to HF by hyperphosphorylated RyR2s. Overall, this study demonstrates that the (a) combined effect of SERCA and NCX, and the (b) RyR2 dysfunction, along with the downregulation of the cardiomyocyte's potassium currents, could substantially contribute to Ca2+ mishandling at the spark level that leads to heart failure.

Hypocalcemic cardiomyopathy with heart failure: A rare Case report

Hypocalcemia is one of the rarest causes of reversible cardiomyopathy. Patients with refractory heart failure need to be explored for hypocalcemia and need prompt correction.

Abstract

Hypocalcemia is a rare cause of reversible dilated cardiomyopathy. Correction of calcium is crucial to recover left ventricular function and structure. We presented the case of a 55-year-old female who was admitted to the hospital with refractory heart failure due to hypocalcemia induced by primary hypoparathyroidism and complicated by vitamin D deficiency. The patient's cardiac symptoms improved dramatically upon correction of hypocalcemia, and vitamin D. Therefore, the key clinical message of this case report is, that hypocalcemia, although rare, should be considered as one of the differential diagnoses when heart failure is refractory and early diagnosis and treatment is necessary as it is the cause of reversible cardiomyopathy and could reduce morbidity and mortality.

A deep learning-based approach for efficient detection and classification of local Ca²⁺ release events in Full-Frame confocal imaging

The release of Ca2+ ions from intracellular stores plays a crucial role in many cellular processes, acting as a secondary messenger in various cell types, including cardiomyocytes, smooth muscle cells, hepatocytes, and many others. Detecting and classifying associated local Ca2+ release events is particularly important, as these events provide insight into the mechanisms, interplay, and interdependencies of local Ca2+release events underlying global intracellular Ca2+signaling. However, time-consuming and labor-intensive procedures often complicate analysis, especially with low signal-to-noise ratio imaging data. Here, we present an innovative deep learning-based approach for automatically detecting and classifying local Ca2+ release events. This approach is exemplified with rapid full-frame confocal imaging data recorded in isolated cardiomyocytes. To demonstrate the robustness and accuracy of our method, we first use conventional evaluation methods by comparing the intersection between manual annotations and the segmentation of Ca2+ release events provided by the deep learning method, as well as the annotated and recognized instances of individual events. In addition to these methods, we compare the performance of the proposed model with the annotation of six experts in the field. Our model can recognize more than 75 % of the annotated Ca2+ release events and correctly classify more than 75 %. A key result was that there were no significant differences between the annotations produced by human experts and the result of the proposed deep learning model. We conclude that the proposed approach is a robust and time-saving alternative to conventional full-frame confocal imaging analysis of local intracellular Ca2+ events.

The NADPH oxidase inhibitor diphenyleneiodonium suppresses Ca2+ signaling and contraction in rat cardiac myocytes

Diphenyleneiodonium (DPI) has been widely used as an inhibitor of NADPH oxidase (Nox) to discover its function in cardiac myocytes under various stimuli. However, the effects of DPI itself on Ca2+ signaling and contraction in cardiac myocytes under control conditions have not been understood. We investigated the effects of DPI on contraction and Ca2+ signaling and their underlying mechanisms using video edge detection, confocal imaging, and whole-cell patch clamp technique in isolated rat cardiac myocytes. Application of DPI suppressed cell shortenings in a concentration-dependent manner (IC50 of ≅0.17 µM) with a maximal inhibition of ~70% at ~100 µM. DPI decreased the magnitude of Ca2+ transient and sarcoplasmic reticulum Ca2+ content by 20%-30% at 3 µM that is usually used to remove the Nox activity, with no effect on fractional release. There was no significant change in the half-decay time of Ca2+ transients by DPI. The L-type Ca2+ current (ICa) was decreased concentration-dependently by DPI (IC50 of ≅40.3 µM) with ≅13.1%-inhibition at 3 µM. The frequency of Ca2+ sparks was reduced by 3 µM DPI (by ~25%), which was resistant to a brief removal of external Ca2+ and Na+. Mitochondrial superoxide level was reduced by DPI at 3-100 µM. Our data suggest that DPI may suppress L-type Ca2+ channel and RyR, thereby attenuating Ca2+-induced Ca2+ release and contractility in cardiac myocytes, and that such DPI effects may be related to mitochondrial metabolic suppression.

G Protein-Coupled Receptors: A Century of Research and Discovery

GPCRs (G protein-coupled receptors), also known as 7 transmembrane domain receptors, are the largest receptor family in the human genome, with ≈800 members. GPCRs regulate nearly every aspect of human physiology and disease, thus serving as important drug targets in cardiovascular disease. Sharing a conserved structure comprised of 7 transmembrane α-helices, GPCRs couple to heterotrimeric G-proteins, GPCR kinases, and β-arrestins, promoting downstream signaling through second messengers and other intracellular signaling pathways. GPCR drug development has led to important cardiovascular therapies, such as antagonists of β-adrenergic and angiotensin II receptors for heart failure and hypertension, and agonists of the glucagon-like peptide-1 receptor for reducing adverse cardiovascular events and other emerging indications. There continues to be a major interest in GPCR drug development in cardiovascular and cardiometabolic disease, driven by advances in GPCR mechanistic studies and structure-based drug design. This review recounts the rich history of GPCR research, including the current state of clinically used GPCR drugs, and highlights newly discovered aspects of GPCR biology and promising directions for future investigation. As additional mechanisms for regulating GPCR signaling are uncovered, new strategies for targeting these ubiquitous receptors hold tremendous promise for the field of cardiovascular medicine.

Fueling the heartbeat: Dynamic regulation of intracellular ATP during excitation-contraction coupling in ventricular myocytes

The heart beats approximately 100,000 times per day in humans, imposing substantial energetic demands on cardiac muscle. Adenosine triphosphate (ATP) is an essential energy source for normal function of cardiac muscle during each beat, as it powers ion transport, intracellular Ca2+ handling, and actin-myosin cross-bridge cycling. Despite this, the impact of excitation-contraction coupling on the intracellular ATP concentration ([ATP]i) in myocytes is poorly understood. Here, we conducted real-time measurements of [ATP]i in ventricular myocytes using a genetically encoded ATP fluorescent reporter. Our data reveal rapid beat-to-beat variations in [ATP]i. Notably, diastolic [ATP]i was <1 mM, which is eightfold to 10-fold lower than previously estimated. Accordingly, ATP-sensitive K+ (KATP) channels were active at physiological [ATP]i. Cells exhibited two distinct types of ATP fluctuations during an action potential: net increases (Mode 1) or decreases (Mode 2) in [ATP]i. Mode 1 [ATP]i increases necessitated Ca2+ entry and release from the sarcoplasmic reticulum (SR) and were associated with increases in mitochondrial Ca2+. By contrast, decreases in mitochondrial Ca2+ accompanied Mode 2 [ATP]i decreases. Down-regulation of the protein mitofusin 2 reduced the magnitude of [ATP]i fluctuations, indicating that SR-mitochondrial coupling plays a crucial role in the dynamic control of ATP levels. Activation of β-adrenergic receptors decreased [ATP]i, underscoring the energetic impact of this signaling pathway. Finally, our work suggests that cross-bridge cycling is the largest consumer of ATP in a ventricular myocyte during an action potential. These findings provide insights into the energetic demands of EC coupling and highlight the dynamic nature of ATP concentrations in cardiac muscle.

Cardiac monoamine oxidase-A inhibition protects against catecholamine-induced ventricular arrhythmias via enhanced diastolic calcium control

AIMS

A mechanistic link between depression and risk of arrhythmias could be attributed to altered catecholamine metabolism in the heart. Monoamine oxidase-A (MAO-A), a key enzyme involved in catecholamine metabolism and longstanding antidepressant target, is highly expressed in the myocardium. The present study aimed to elucidate the functional significance and underlying mechanisms of cardiac MAO-A in arrhythmogenesis.

METHODS AND RESULTS

Analysis of the TriNetX database revealed that depressed patients treated with MAO inhibitors had a lower risk of arrhythmias compared with those treated with selective serotonin reuptake inhibitors. This effect was phenocopied in mice with cardiomyocyte-specific MAO-A deficiency (cMAO-Adef), which showed a significant reduction in both incidence and duration of catecholamine stress-induced ventricular tachycardia compared with wild-type mice. Additionally, cMAO-Adef cardiomyocytes exhibited altered Ca2+ handling under catecholamine stimulation, with increased diastolic Ca2+ reuptake, reduced diastolic Ca2+ leak, and diminished systolic Ca2+ release. Mechanistically, cMAO-Adef hearts had reduced catecholamine levels under sympathetic stress, along with reduced levels of reactive oxygen species and protein carbonylation, leading to decreased oxidation of Type II PKA and CaMKII. These changes potentiated phospholamban (PLB) phosphorylation, thereby enhancing diastolic Ca2+ reuptake, while reducing ryanodine receptor 2 (RyR2) phosphorylation to decrease diastolic Ca2+ leak. Consequently, cMAO-Adef hearts exhibited lower diastolic Ca2+ levels and fewer arrhythmogenic Ca2+ waves during sympathetic overstimulation.

CONCLUSION

Cardiac MAO-A inhibition exerts an anti-arrhythmic effect by enhancing diastolic Ca2+ handling under catecholamine stress.

A novel RyR2 mutation associated with co-morbid catecholaminergic polymorphic ventricular tachycardia (CPVT) and benign epilepsy with centrotemporal spikes (BECTS)

In this case report, we describe a 14-year-old patient with a novel RyR2 gene mutation (c.6577G > T/p.Val2193Leu), identified through a comprehensive review of medical history, examination findings, and follow-up data. The pathogenic potential of this mutation, which results in the loss of some interatomic forces and compromises the closure of the RyR2 protein pore leading to calcium leakage, was analyzed using the I-TASSER Suite to predict the structural changes in the protein. This mutation manifested clinically as co-morbid catecholaminergic polymorphic ventricular tachycardia (CPVT) and benign epilepsy with centrotemporal spikes (BECTS), a combination not previously documented in the same patient. While seizures were successfully managed with levetiracetam, the patient's exercise-induced syncope episodes could not be controlled with metoprolol, highlighting the complexity and challenge in managing CPVT associated with this novel RyR2 variation.

An integrated toolkit for human microglia functional genomics

BACKGROUND

Microglia, the brain's resident immune cells, play vital roles in brain development, and disorders like Alzheimer's disease (AD). Human iPSC-derived microglia (iMG) provide a promising model to study these processes. However, existing iMG generation protocols face challenges, such as prolonged differentiation time, lack of detailed characterization, and limited gene function investigation via CRISPR-Cas9.

METHODS

Our integrated toolkit for in-vitro microglia functional genomics optimizes iPSC differentiation into iMG through a streamlined two-step, 20-day process, producing iMG with a normal karyotype. We confirmed the iMG's authenticity and quality through single-cell RNA sequencing, chromatin accessibility profiles (ATAC-Seq), proteomics and functional tests. The toolkit also incorporates a drug-dependent CRISPR-ON/OFF system for temporally controlled gene expression. Further, we facilitate the use of multi-omic data by providing online searchable platform that compares new iMG profiles to human primary microglia: https://sherlab.shinyapps.io/IPSC-derived-Microglia/ .

RESULTS

Our method generates iMG that closely align with human primary microglia in terms of transcriptomic, proteomic, and chromatin accessibility profiles. Functionally, these iMG exhibit Ca2 + transients, cytokine driven migration, immune responses to inflammatory signals, and active phagocytosis of CNS related substrates including synaptosomes, amyloid beta and myelin. Significantly, the toolkit facilitates repeated iMG harvesting, essential for large-scale experiments like CRISPR-Cas9 screens. The standalone ATAC-Seq profiles of our iMG closely resemble primary microglia, positioning them as ideal tools to study AD-associated single nucleotide variants (SNV) especially in the genome regulatory regions.

CONCLUSIONS

Our advanced two-step protocol rapidly and efficiently produces authentic iMG. With features like the CRISPR-ON/OFF system and a comprehensive multi-omic data platform, our toolkit equips researchers for robust microglial functional genomic studies. By facilitating detailed SNV investigation and offering a sustainable cell harvest mechanism, the toolkit heralds significant progress in neurodegenerative disease drug research and therapeutic advancement.

Phosphorylation of RyR2 simultaneously expands the dyad and rearranges the tetramers

We have previously demonstrated that type II ryanodine receptors (RyR2) tetramers can be rapidly rearranged in response to a phosphorylation cocktail. The cocktail modified downstream targets indiscriminately, making it impossible to determine whether phosphorylation of RyR2 was an essential element of the response. Here, we used the β-agonist isoproterenol and mice homozygous for one of the following clinically relevant mutations: S2030A, S2808A, S2814A, or S2814D. We measured the length of the dyad using transmission electron microscopy (TEM) and directly visualized RyR2 distribution using dual-tilt electron tomography. We found that the S2814D mutation, by itself, significantly expanded the dyad and reorganized the tetramers, suggesting a direct link between the phosphorylation state of the tetramer and its microarchitecture. S2808A and S2814A mutant mice, as well as wild types, had significant expansions of their dyads in response to isoproterenol, while S2030A mutants did not. In agreement with functional data from these mutants, S2030 and S2808 were necessary for a complete β-adrenergic response, unlike S2814 mutants. Additionally, all mutants had unique effects on the organization of their tetramer arrays. Lastly, the correlation of structural with functional changes suggests that tetramer-tetramer contacts play an important functional role. We thus conclude that both the size of the dyad and the arrangement of the tetramers are linked to the state of the channel tetramer and can be dynamically altered by a β-adrenergic receptor agonist.

Luminal transport through intact endoplasmic reticulum limits the magnitude of localized Ca2+ signals

The endoplasmic reticulum (ER) forms an interconnected network of tubules stretching throughout the cell. Understanding how ER functionality relies on its structural organization is crucial for elucidating cellular vulnerability to ER perturbations, which have been implicated in several neuronal pathologies. One of the key functions of the ER is enabling Ca[Formula: see text] signaling by storing large quantities of this ion and releasing it into the cytoplasm in a spatiotemporally controlled manner. Through a combination of physical modeling and live-cell imaging, we demonstrate that alterations in ER shape significantly impact its ability to support efficient local Ca[Formula: see text] releases, due to hindered transport of luminal content within the ER. Our model reveals that rapid Ca[Formula: see text] release necessitates mobile luminal buffer proteins with moderate binding strength, moving through a well-connected network of ER tubules. These findings provide insight into the functional advantages of normal ER architecture, emphasizing its importance as a kinetically efficient intracellular Ca[Formula: see text] delivery system.

In situ structural insights into the excitation-contraction coupling mechanism of skeletal muscle

Excitation-contraction coupling (ECC) is a fundamental mechanism in control of skeletal muscle contraction and occurs at triad junctions, where dihydropyridine receptors (DHPRs) on transverse tubules sense excitation signals and then cause calcium release from the sarcoplasmic reticulum via coupling to type 1 ryanodine receptors (RyR1s), inducing the subsequent contraction of muscle filaments. However, the molecular mechanism remains unclear due to the lack of structural details. Here, we explored the architecture of triad junction by cryo-electron tomography, solved the in situ structure of RyR1 in complex with FKBP12 and calmodulin with the resolution of 16.7 Angstrom, and found the intact RyR1-DHPR supercomplex. RyR1s arrange into two rows on the terminal cisternae membrane by forming right-hand corner-to-corner contacts, and tetrads of DHPRs bind to RyR1s in an alternating manner, forming another two rows on the transverse tubule membrane. This unique arrangement is important for synergistic calcium release and provides direct evidence of physical coupling in ECC.

Hyperspectral imaging and dynamic region of interest tracking approaches to quantify localized cAMP signals

Cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger known to orchestrate a myriad of cellular functions over a wide range of timescales. In the last 20 years, a variety of single-cell sensors have been developed to measure second messenger signals including cAMP, Ca2+, and the balance of kinase and phosphatase activities. These sensors utilize changes in fluorescence emission of an individual fluorophore or Förster resonance energy transfer (FRET) to detect changes in second messenger concentration. cAMP and kinase activity reporter probes have provided powerful tools for the study of localized signals. Studies relying on these and related probes have the potential to further revolutionize our understanding of G protein-coupled receptor signaling systems. Unfortunately, investigators have not been able to take full advantage of the potential of these probes due to the limited signal-to-noise ratio of the probes and the limited ability of standard epifluorescence and confocal microscope systems to simultaneously measure the distributions of multiple signals (e.g. cAMP, Ca2+, and changes in kinase activities) in real time. In this review, we focus on recently implemented strategies to overcome these limitations: hyperspectral imaging and adaptive thresholding approaches to track dynamic regions of interest (ROI). This combination of approaches increases signal-to-noise ratio and contrast, and allows identification of localized signals throughout cells. These in turn lead to the identification and quantification of intracellular signals with higher effective resolution. Hyperspectral imaging and dynamic ROI tracking approaches offer investigators additional tools with which to visualize and quantify multiplexed intracellular signaling systems.

Spiking at the edge: Excitability at interfaces in reaction-diffusion systems

Excitable media, ranging from bioelectric tissues and chemical oscillators to forest fires and competing populations, are nonlinear, spatially extended systems capable of spiking. Most investigations of excitable media consider situations where the amplifying and suppressing forces necessary for spiking coexist at every point in space. In this case, spikes arise due to local bistabilities, which require a fine-tuned ratio between local amplification and suppression strengths. But, in nature and engineered systems, these forces can be segregated in space, forming structures like interfaces and boundaries. Here, we show how boundaries can generate and protect spiking when the reacting components can spread out: Even arbitrarily weak diffusion can cause spiking at the edge between two non-excitable media. This edge spiking arises due to a global bistability, which can occur even if amplification and suppression strengths do not allow spiking when mixed. We analytically derive a spiking phase diagram that depends on two parameters: i) the ratio between the system size and the characteristic diffusive length-scale and ii) the ratio between the amplification and suppression strengths. Our analysis explains recent experimental observations of action potentials at the interface between two non-excitable bioelectric tissues. Beyond electrophysiology, we highlight how edge spiking emerges in predator-prey dynamics and in oscillating chemical reactions. Our findings provide a theoretical blueprint for a class of interfacial excitations in reaction-diffusion systems, with potential implications for spatially controlled chemical reactions, nonlinear waveguides and neuromorphic computation, as well as spiking instabilities, such as cardiac arrhythmias, that naturally occur in heterogeneous biological media.

Cardiomyocyte Differentiation from Mouse Embryonic Stem Cells by WNT Switch Method

The differentiation of ESCs into cardiomyocytes in vitro is an excellent and reliable model system for studying normal cardiomyocyte development in mammals, modeling cardiac diseases, and for use in drug screening. Mouse ESC differentiation still provides relevant biological information about cardiac development. However, the current methods for efficiently differentiating ESCs into cardiomyocytes are limiting. Here, we describe the "WNT Switch" method to efficiently commit mouse ESCs into cardiomyocytes using the small molecule WNT signaling modulators CHIR99021 and XAV939 in vitro. This method significantly improves the yield of beating cardiomyocytes, reduces number of treatments, and is less laborious.

Myofilament dysfunction in diastolic heart failure

Diastolic heart failure (DHF), in which impaired ventricular filling leads to typical heart failure symptoms, represents over 50% of all heart failure cases and is linked with risk factors, including metabolic syndrome, hypertension, diabetes, and aging. A substantial proportion of patients with this disorder maintain normal left ventricular systolic function, as assessed by ejection fraction. Despite the high prevalence of DHF, no effective therapeutic agents are available to treat this condition, partially because the molecular mechanisms of diastolic dysfunction remain poorly understood. As such, by focusing on the underlying molecular and cellular processes contributing to DHF can yield new insights that can represent an exciting new avenue and propose a novel therapeutic approach for DHF treatment. This review discusses new developments from basic and clinical/translational research to highlight current knowledge gaps, help define molecular determinants of diastolic dysfunction, and clarify new targets for treatment.

Hypertrophic cardiomyopathy in MYBPC3 carriers in aging

Hypertrophic cardiomyopathy (HCM) is characterized by abnormal thickening of the myocardium, leading to arrhythmias, heart failure, and elevated risk of sudden cardiac death, particularly among the young. This inherited disease is predominantly caused by mutations in sarcomeric genes, among which those in the cardiac myosin binding protein-C3 (MYBPC3) gene are major contributors. HCM associated with MYBPC3 mutations usually presents in the elderly and ranges from asymptomatic to symptomatic forms, affecting numerous cardiac functions and presenting significant health risks with a spectrum of clinical manifestations. Regulation of MYBPC3 expression involves various transcriptional and translational mechanisms, yet the destiny of mutant MYBPC3 mRNA and protein in late-onset HCM remains unclear. Pathogenesis related to MYBPC3 mutations includes nonsense-mediated decay, alternative splicing, and ubiquitin-proteasome system events, leading to allelic imbalance and haploinsufficiency. Aging further exacerbates the severity of HCM in carriers of MYBPC3 mutations. Advancements in high-throughput omics techniques have identified crucial molecular events and regulatory disruptions in cardiomyocytes expressing MYBPC3 variants. This review assesses the pathogenic mechanisms that promote late-onset HCM through the lens of transcriptional, post-transcriptional, and post-translational modulation of MYBPC3, underscoring its significance in HCM across carriers. The review also evaluates the influence of aging on these processes and MYBPC3 levels during HCM pathogenesis in the elderly. While pinpointing targets for novel medical interventions to conserve cardiac function remains challenging, the emergence of personalized omics offers promising avenues for future HCM treatments, particularly for late-onset cases.

Channels, Transporters, and Receptors at Membrane Contact Sites

Membrane contact sites (MCSs) are specialized regions where two or more organelle membranes come into close apposition, typically separated by only 10-30 nm, while remaining distinct and unfused. These sites play crucial roles in cellular homeostasis, signaling, and metabolism. This review focuses on ion channels, transporters, and receptors localized to MCSs, with particular emphasis on those associated with the plasma membrane and endoplasmic reticulum (ER). We discuss the molecular composition and functional significance of these proteins in shaping both organelle and cellular functions, highlighting their importance in excitable cells and their influence on intracellular calcium signaling. Key MCSs examined include ER-plasma membrane, ER-mitochondria, and ER-lysosome contacts. This review addresses our current knowledge of the ion channels found within these contacts, the dynamic regulation of MCSs, their importance in various physiological processes, and their potential implications in pathological conditions.

Ca2+ dynamics in interstitial cells: foundational mechanisms for the motor patterns in the gastrointestinal tract

The gastrointestinal (GI) tract displays multiple motor patterns that move nutrients and wastes through the body. Smooth muscle cells (SMCs) provide the forces necessary for GI motility, but interstitial cells, electrically coupled to SMCs, tune SMC excitability, transduce inputs from enteric motor neurons, and generate pacemaker activity that underlies major motor patterns, such as peristalsis and segmentation. The interstitial cells regulating SMCs are interstitial cells of Cajal (ICC) and PDGF receptor (PDGFR)α+ cells. Together these cells form the SIP syncytium. ICC and PDGFRα+ cells express signature Ca2+-dependent conductances: ICC express Ca2+-activated Cl- channels, encoded by Ano1, that generate inward current, and PDGFRα+ cells express Ca2+-activated K+ channels, encoded by Kcnn3, that generate outward current. The open probabilities of interstitial cell conductances are controlled by Ca2+ release from the endoplasmic reticulum. The resulting Ca2+ transients occur spontaneously in a stochastic manner. Ca2+ transients in ICC induce spontaneous transient inward currents and spontaneous transient depolarizations (STDs). Neurotransmission increases or decreases Ca2+ transients, and the resulting depolarizing or hyperpolarizing responses conduct to other cells in the SIP syncytium. In pacemaker ICC, STDs activate voltage-dependent Ca2+ influx, which initiates a cluster of Ca2+ transients and sustains activation of ANO1 channels and depolarization during slow waves. Regulation of GI motility has traditionally been described as neurogenic and myogenic. Recent advances in understanding Ca2+ handling mechanisms in interstitial cells and how these mechanisms influence motor patterns of the GI tract suggest that the term "myogenic" should be replaced by the term "SIPgenic," as this review discusses.

Super-Resolution Analysis of the Origins of the Elementary Events of ER Calcium Release in Dorsal Root Ganglion Neurons

Coordinated events of calcium (Ca2+) released from the endoplasmic reticulum (ER) are key second messengers in excitable cells. In pain-sensing dorsal root ganglion (DRG) neurons, these events can be observed as Ca2+ sparks, produced by a combination of ryanodine receptors (RyR) and inositol 1,4,5-triphosphate receptors (IP3R1). These microscopic signals offer the neuronal cells with a possible means of modulating the subplasmalemmal Ca2+ handling, initiating vesicular exocytosis. With super-resolution dSTORM and expansion microscopies, we visualised the nanoscale distributions of both RyR and IP3R1 that featured loosely organised clusters in the subplasmalemmal regions of cultured rat DRG somata. We adapted a novel correlative microscopy protocol to examine the nanoscale patterns of RyR and IP3R1 in the locality of each Ca2+ spark. We found that most subplasmalemmal sparks correlated with relatively small groups of RyR whilst larger sparks were often associated with larger groups of IP3R1. These data also showed spontaneous Ca2+ sparks in <30% of the subplasmalemmal cell area but consisted of both these channel species at a 3.8-5 times higher density than in nonactive regions of the cell. Taken together, these observations reveal distinct patterns and length scales of RyR and IP3R1 co-clustering at contact sites between the ER and the surface plasmalemma that encode the positions and the quantity of Ca2+ released at each Ca2+ spark.

Gene editing and cardiac disease modelling for the interpretation of genetic variants of uncertain significance in congenital heart disease

BACKGROUND

Genomic sequencing in congenital heart disease (CHD) patients often discovers novel genetic variants, which are classified as variants of uncertain significance (VUS). Functional analysis of each VUS is required in specialised laboratories, to determine whether the VUS is disease causative or not, leading to lengthy diagnostic delays. We investigated stem cell cardiac disease modelling and transcriptomics for the purpose of genetic variant classification using a GATA4 (p.Arg283Cys) VUS in a patient with CHD.

METHODS

We performed high efficiency CRISPR gene editing with homology directed repair in induced pluripotent stem cells (iPSCs), followed by rapid clonal selection with amplicon sequencing. Genetic variant and healthy matched control cells were compared using cardiomyocyte disease modelling and transcriptomics.

RESULTS

Genetic variant and healthy cardiomyocytes similarly expressed Troponin T (cTNNT), and GATA4. Transcriptomics analysis of cardiomyocyte differentiation identified changes consistent with the patient's clinical human phenotype ontology terms. Further, transcriptomics revealed changes in calcium signalling, and cardiomyocyte adrenergic signalling in the variant cells. Functional testing demonstrated, altered action potentials in GATA4 genetic variant cardiomyocytes were consistent with patient cardiac abnormalities.

CONCLUSIONS

This work provides in vivo functional studies supportive of a damaging effect on the gene or gene product. Furthermore, we demonstrate the utility of iPSCs, CRISPR gene editing and cardiac disease modelling for genetic variant interpretation. The method can readily be applied to other genetic variants in GATA4 or other genes in cardiac disease, providing a centralised assessment pathway for patient genetic variant interpretation.

Clustering properties of the cardiac ryanodine receptor in health and heart failure

The cardiac ryanodine receptor (RyR2) is an intracellular Ca2+ release channel vital for the function of the heart. Physiologically, RyR2 is triggered to release Ca2+ from the sarcoplasmic reticulum (SR) which enables cardiac contraction; however, spontaneous Ca2+ leak from RyR2 has been implicated in the pathophysiology of heart failure (HF). RyR2 channels have been well documented to assemble into clusters within the SR membrane, with the organisation of RyR2 clusters recently gaining interest as a mechanism by which the occurrence of pathological Ca2+ leak is regulated, including in HF. In this review, we explain the terminology relating to key nanoscale RyR2 clustering properties as both single clusters and functionally grouped Ca2+ release units, with a focus on the advancements in super-resolution imaging approaches which have enabled the detailed study of cluster organisation. Further, we discuss proposed mechanisms for modulating RyR2 channel organisation and the debate regarding the potential impact of cluster organisation on Ca2+ leak activity. Finally, recent experimental evidence investigating the nanoscale remodelling and functional alterations of RyR2 clusters in HF is discussed with consideration of the clinical implications.

Calcium Signaling Consequences of RyR2-S4938F Mutation Expressed in Human iPSC-Derived Cardiomyocytes

Type-2 ryanodine receptor (RyR2) is the major Ca2+ release channel of the cardiac sarcoplasmic reticulum (SR) that controls the rhythm and strength of the heartbeat, but its malfunction may generate severe arrhythmia leading to sudden cardiac death or heart failure. S4938F-RyR2 mutation in the carboxyl-terminal was expressed in human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) using CRISPR/Cas9 gene-editing technique. Ca2+ signaling and electrophysiological properties of beating cardiomyocytes carrying the mutation were studied using total internal reflection fluorescence microscopy (TIRF) and patch clamp technique. In mutant cells, L-type Ca2+ currents (ICa), measured either by depolarizations to zero mV or repolarizations from +100 mV to -50 mV, and their activated Ca2+ transients were significantly smaller, despite their larger caffeine-triggered Ca2+ release signals compared to wild type (WT) cells, suggesting ICa-induced Ca2+ release (CICR) was compromised. The larger SR Ca2+ content of S4938F-RyR2 cells may underlie the higher frequency of spontaneously occurring Ca2+ sparks and Ca2+ transients and their arrhythmogenic phenotype.

T cell Ca2+ microdomains through the lens of computational modeling

Cellular Ca2+ signaling is highly organized in time and space. Locally restricted and short-lived regions of Ca2+ increase, called Ca2+ microdomains, constitute building blocks that are differentially arranged to create cellular Ca2+ signatures controlling physiological responses. Here, we focus on Ca2+ microdomains occurring in restricted cytosolic spaces between the plasma membrane and the endoplasmic reticulum, called endoplasmic reticulum-plasma membrane junctions. In T cells, these microdomains have been finely characterized. Enough quantitative data are thus available to develop detailed computational models of junctional Ca2+ dynamics. Simulations are able to predict the characteristics of Ca2+ increases at the level of single channels and in junctions of different spatial configurations, in response to various signaling molecules. Thanks to the synergy between experimental observations and computational modeling, a unified description of the molecular mechanisms that create Ca2+ microdomains in the first seconds of T cell stimulation is emerging.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Na + /K + ATPase-Ca v 1.2 nanodomain differentially regulates intracellular [Na + ], [Ca 2+ ] and local adrenergic signaling in cardiac myocytes

Background

The intracellular Na + concentration ([Na + ] i ) is a crucial but understudied regulator of cardiac myocyte function. The Na + /K + ATPase (NKA) controls the steady-state [Na + ] i and thereby determines the set-point for intracellular Ca 2+ . Here, we investigate the nanoscopic organization and local adrenergic regulation of the NKA macromolecular complex and how it differentially regulates the intracellular Na + and Ca 2+ homeostases in atrial and ventricular myocytes.

Methods

Multicolor STORM super-resolution microscopy, Western Blot analyses, and in vivo examination of adrenergic regulation are employed to examine the organization and function of Na + nanodomains in cardiac myocytes. Quantitative fluorescence microscopy at high spatiotemporal resolution is used in conjunction with cellular electrophysiology to investigate intracellular Na + homeostasis in atrial and ventricular myocytes.

Results

The NKAα1 (NKAα1) and the L-type Ca 2+ -channel (Ca v 1.2) form a nanodomain with a center-to center distance of ∼65 nm in both ventricular and atrial myocytes. NKAα1 protein expression levels are ∼3 fold higher in atria compared to ventricle. 100% higher atrial I NKA , produced by large NKA "superclusters", underlies the substantially lower Na + concentration in atrial myocytes compared to the benchmark values set in ventricular myocytes. The NKA's regulatory protein phospholemman (PLM) has similar expression levels across atria and ventricle resulting in a much lower PLM/NKAα1 ratio for atrial compared to ventricular tissue. In addition, a huge PLM phosphorylation reserve in atrial tissue produces a high ß-adrenergic sensitivity of I NKA in atrial myocytes. ß-adrenergic regulation of I NKA is locally mediated in the NKAα1-Ca v 1.2 nanodomain via A-kinase anchoring proteins.

Conclusions

NKAα1, Ca v 1.2 and their accessory proteins form a structural and regulatory nanodomain at the cardiac dyad. The tissue-specific composition and local adrenergic regulation of this "signaling cloud" is a main regulator of the distinct global intracellular Na + and Ca 2+ concentrations in atrial and ventricular myocytes.

SparkMaster 2: A New Software for Automatic Analysis of Calcium Spark Data

BACKGROUND

Calcium (Ca) sparks are elementary units of subcellular Ca release in cardiomyocytes and other cells. Accordingly, Ca spark imaging is an essential tool for understanding the physiology and pathophysiology of Ca handling and is used to identify new drugs targeting Ca-related cellular dysfunction (eg, cardiac arrhythmias). The large volumes of imaging data produced during such experiments require accurate and high-throughput analysis.

METHODS

We developed a new software tool SparkMaster 2 (SM2) for the analysis of Ca sparks imaged by confocal line-scan microscopy, combining high accuracy, flexibility, and user-friendliness. SM2 is distributed as a stand-alone application requiring no installation. It can be controlled using a simple-to-use graphical user interface, or using Python scripting.

RESULTS

SM2 is shown to have the following strengths: (1) high accuracy at identifying Ca release events, clearly outperforming previous highly successful software SparkMaster; (2) multiple types of Ca release events can be identified using SM2: Ca sparks, waves, miniwaves, and long sparks; (3) SM2 can accurately split and analyze individual sparks within spark clusters, a capability not handled adequately by prior tools. We demonstrate the practical utility of SM2 in two case studies, investigating how Ca levels affect spontaneous Ca release, and how large-scale release events may promote release refractoriness. SM2 is also useful in atrial and smooth muscle myocytes, across different imaging conditions.

CONCLUSIONS

SparkMaster 2 is a new, much-improved user-friendly software for accurate high-throughput analysis of line-scan Ca spark imaging data. It is free, easy to use, and provides valuable built-in features to facilitate visualization, analysis, and interpretation of Ca spark data. It should enhance the quality and throughput of Ca spark and wave analysis across cell types, particularly in the study of arrhythmogenic Ca release events in cardiomyocytes.

Enhancing the functional maturity of hiPSC-derived cardiomyocytes to assess inotropic compounds

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) present an attractive in vitro platform to model safety and toxicity assessments-notably screening pro-arrhythmic compounds. The utility of the platform is stymied by a hiPSC-CM contractile apparatus and calcium handling mechanism akin to fetal phenotypes, evidenced by a negative force-frequency relationship. As such, hiPSC-CMs are limited in their ability to assess compounds that modulate contraction mediated by ionotropic compounds (Robertson, Tran, & George, 2013). To address this limitation, we utilize Agilent's xCELLigence Real-Time Cell Analyzer ePacer (RTCA ePacer) to enhance hiPSC-CM functional maturity. A continuous, progressive increase of electrical pacing is applied to hiPSC-CMs for up to 15 days. Contraction and viability are recorded by measurement of impedance using the RTCA ePacer. Our data confirms hiPSC-CMs inherently demonstrate a negative impedance amplitude frequency that is reversed after long-term electrical pacing. The data also indicate positive inotropic compounds increase the contractility of paced cardiomyocytes and calcium handling machinery is improved. Increased expression of genes critical to cardiomyocyte maturation further underscores the maturity of paced cells. In summary, our data suggest the application of continuous electrical pacing can functionally mature hiPSC-CMs, enhancing cellular response to positive inotropic compounds and improving calcium handling. SUMMARY: Long-term electrical stimulation of hiPSC-CM leads to functional maturation enabling predictive assessment of inotropic compounds.

Ca2+ spark latency and control of intrinsic Ca2+ release dyssynchrony in rat cardiac ventricular muscle cells

Cardiac excitation-contraction coupling (ECC) depends on Ca2+ release from intracellular stores via ryanodine receptors (RyRs) triggered by L-type Ca2+ channels (LCCs). Uncertain numbers of RyRs and LCCs form 'couplons' whose activation produces Ca2+ sparks, which summate to form a cell-wide Ca2+ transient that switches on contraction. Voltage (Vm) changes during the action potential (AP) and stochasticity in channel gating should create variability in Ca2+ spark timing, but Ca2+ transient wavefronts have remarkable uniformity. To examine how this is achieved, we measured the Vm-dependence of evoked Ca2+ spark probability (Pspark) and latency over a wide voltage range in rat ventricular cells. With depolarising steps, Ca2+ spark latency showed a U-shaped Vm-dependence, while repolarising steps from 50 mV produced Ca2+ spark latencies that increased monotonically with Vm. A computer model based on reported channel gating and geometry reproduced our experimental data and revealed a likely RyR:LCC stoichiometry of ∼ 5:1 for the Ca2+ spark initiating complex (IC). Using the experimental AP waveform, the model revealed a high coupling fidelity (Pcpl ∼ 0.5) between each LCC opening and IC activation. The presence of ∼ 4 ICs per couplon reduced Ca2+ spark latency and increased Pspark to match experimental data. Variability in AP release timing is less than that seen with voltage steps because the AP overshoot and later repolarization decrease Pspark due to effects on LCC flux and LCC deactivation respectively. This work provides a framework for explaining the Vm- and time-dependence of Pspark, and indicates how ion channel dispersion in disease can contribute to dyssynchrony in Ca2+ release.

The ryanodine receptor microdomain in cardiomyocytes

The ryanodine receptor type 2 (RyR) is a key player in Ca2+ handling during excitation-contraction coupling. During each heartbeat, RyR channels are responsible for linking the action potential with the contractile machinery of the cardiomyocyte by releasing Ca2+ from the sarcoplasmic reticulum. RyR function is fine-tuned by associated signalling molecules, arrangement in clusters and subcellular localization. These parameters together define RyR function within microdomains and are subject to disease remodelling. This review describes the latest findings on RyR microdomain organization, the alterations with disease which result in increased subcellular heterogeneity and emergence of microdomains with enhanced arrhythmogenic potential, and presents novel technologies that guide future research to study and target RyR channels within specific microdomains.

Local Control Model of a Human Ventricular Myocyte: An Exploration of Frequency-Dependent Changes and Calcium Sparks

Calcium (Ca2+) sparks are the elementary events of excitation-contraction coupling, yet they are not explicitly represented in human ventricular myocyte models. A stochastic ventricular cardiomyocyte human model that adapts to intracellular Ca2+ ([Ca2+]i) dynamics, spark regulation, and frequency-dependent changes in the form of locally controlled Ca2+ release was developed. The 20,000 CRUs in this model are composed of 9 individual LCCs and 49 RyRs that function as couplons. The simulated action potential duration at 1 Hz steady-state pacing is ~0.280 s similar to human ventricular cell recordings. Rate-dependence experiments reveal that APD shortening mechanisms are largely contributed by the L-type calcium channel inactivation, RyR open fraction, and [Ca2+]myo concentrations. The dynamic slow-rapid-slow pacing protocol shows that RyR open probability during high pacing frequency (2.5 Hz) switches to an adapted "nonconducting" form of Ca2+-dependent transition state. The predicted force was also observed to be increased in high pacing, but the SR Ca2+ fractional release was lower due to the smaller difference between diastolic and systolic [Ca2+]SR. Restitution analysis through the S1S2 protocol and increased LCC Ca2+-dependent activation rate show that the duration of LCC opening helps modulate its effects on the APD restitution at different diastolic intervals. Ultimately, a longer duration of calcium sparks was observed in relation to the SR Ca2+ loading at high pacing rates. Overall, this study demonstrates the spontaneous Ca2+ release events and ion channel responses throughout various stimuli.

Mechanisms of spontaneous Ca2+ release-mediated arrhythmia in a novel 3D human atrial myocyte model: I. Transverse-axial tubule variation

Intracellular calcium (Ca2+ ) cycling is tightly regulated in the healthy heart ensuring effective contraction. This is achieved by transverse (t)-tubule membrane invaginations that facilitate close coupling of key Ca2+ -handling proteins such as the L-type Ca2+ channel and Na+ -Ca2+ exchanger (NCX) on the cell surface with ryanodine receptors (RyRs) on the intracellular Ca2+ store. Although less abundant and regular than in the ventricle, t-tubules also exist in atrial myocytes as a network of transverse invaginations with axial extensions known as the transverse-axial tubule system (TATS). In heart failure and atrial fibrillation, there is TATS remodelling that is associated with aberrant Ca2+ -handling and Ca2+ -induced arrhythmic activity; however, the mechanism underlying this is not fully understood. To address this, we developed a novel 3D human atrial myocyte model that couples electrophysiology and Ca2+ -handling with variable TATS organization and density. We extensively parameterized and validated our model against experimental data to build a robust tool examining TATS regulation of subcellular Ca2+ release. We found that varying TATS density and thus the localization of key Ca2+ -handling proteins has profound effects on Ca2+ handling. Following TATS loss, there is reduced NCX that results in increased cleft Ca2+ concentration through decreased Ca2+ extrusion. This elevated Ca2+ increases RyR open probability causing spontaneous Ca2+ releases and the promotion of arrhythmogenic waves (especially in the cell interior) leading to voltage instabilities through delayed afterdepolarizations. In summary, the present study demonstrates a mechanistic link between TATS remodelling and Ca2+ -driven proarrhythmic behaviour that probably reflects the arrhythmogenic state observed in disease. KEY POINTS: Transverse-axial tubule systems (TATS) modulate Ca2+ handling and excitation-contraction coupling in atrial myocytes, with TATS remodelling in heart failure and atrial fibrillation being associated with altered Ca2+ cycling and subsequent arrhythmogenesis. To investigate the poorly understood mechanisms linking TATS variation and spontaneous Ca2+ release, we built, parameterized and validated a 3D human atrial myocyte model coupling electrophysiology and spatially-detailed subcellular Ca2+ handling governed by the TATS. Simulated TATS loss causes diastolic Ca2+ and voltage instabilities through reduced Na+ -Ca2+ exchanger-mediated Ca2+ removal, cleft Ca2+ accumulation and increased ryanodine receptor open probability, resulting in spontaneous Ca2+ release and promotion of arrhythmogenic waves and delayed afterdepolarizations. At fast electrical rates typical of atrial tachycardia/fibrillation, spontaneous Ca2+ releases are larger and more frequent in the cell interior than at the periphery. Our work provides mechanistic insight into how atrial TATS remodelling can lead to Ca2+ -driven instabilities that may ultimately contribute to the arrhythmogenic state in disease.

PHOSPHORYLATION OF RyR2 SIMULTANEOUSLY EXPANDS THE DYAD AND REARRANGES THE TETRAMERS

We have previously demonstrated that type II ryanodine receptors (RyR2) tetramers can be rapidly rearranged in response to a phosphorylation cocktail. The cocktail modified downstream targets indiscriminately making it impossible to determine whether phosphorylation of RyR2 was an essential element of the response. We therefore used the β-agonist isoproterenol and mice with one of the homozygous mutations, S2030A +/+ , S2808A +/+ , S2814A +/+ , or S2814D +/+ , to address this question and to elucidate the role of these clinically relevant mutations. We measured the length of the dyad using transmission electron microscopy (TEM) and directly visualized RyR2 distribution using dual-tilt electron tomography. We found that: 1) The S2814D mutation, by itself, significantly expanded the dyad and reorganized the tetramers suggesting a direct link between the phosphorylation state of the tetramer and the microarchitecture. 2) All of the wild-type, as well as the S2808A and S2814A mice, had significant expansions of their dyads in response to ISO, while S2030A did not. 3) In agreement with functional data from the same mutants, S2030 and S2808 were necessary for a complete β-adrenergic response, whereas S2814 was not. 4) All the mutated residues had unique effects on the organization of their tetramer arrays. 5) The correlation of structure with function suggests that tetramer-tetramer contacts play an important functional role. We conclude that both the size of the dyad and the arrangement of the tetramers are linked to the state of the channel tetramer and can be dynamically altered by a β-adrenergic receptor agonist.

Summary

Analysis of RyR2 mutants suggests a direct link between the phosphorylation state of the channel tetramer and the microarchitecture of the dyad. All phosphorylation site mutations produced significant and unique effects on the structure of the dyad and its response to isoproterenol.

Correlative super-resolution analysis of cardiac calcium sparks and their molecular origins in health and disease

Rapid release of calcium from internal stores via ryanodine receptors (RyRs) is one of the fastest types of cytoplasmic second messenger signalling in excitable cells. In the heart, rapid summation of the elementary events of calcium release, 'calcium sparks', determine the contraction of the myocardium. We adapted a correlative super-resolution microscopy protocol to correlate sub-plasmalemmal spontaneous calcium sparks in rat right ventricular myocytes with the local nanoscale RyR2 positions. This revealed a steep relationship between the integral of a calcium spark and the sum of the local RyR2s. Segmentation of recurring spark sites showed evidence of repeated and triggered saltatory activation of multiple local RyR2 clusters. In myocytes taken from failing right ventricles, RyR2 clusters themselves showed a dissipated morphology and fragmented (smaller) clusters. They also featured greater heterogeneity in both the spark properties and the relationship between the integral of the calcium spark and the local ensemble of RyR2s. While fragmented (smaller) RyR2 clusters were rarely observed directly underlying the larger sparks or the recurring spark sites, local interrogation of the channel-to-channel distances confirmed a clear link between the positions of each calcium spark and the tight, non-random clustering of the local RyR2 in both healthy and failing ventricles.

Live-cell photo-activated localization microscopy correlates nanoscale ryanodine receptor configuration to calcium sparks in cardiomyocytes

Ca2+ sparks constitute the fundamental units of Ca2+ release in cardiomyocytes. Here we investigate how ryanodine receptors (RyRs) collectively generate these events by employing a transgenic mouse with a photo-activated label on RyR2. This allowed correlative imaging of RyR localization, by super-resolution Photo-Activated Localization Microscopy, and Ca2+ sparks, by high-speed imaging. Two populations of Ca2+ sparks were observed: stationary events and "travelling" events that spread between neighbouring RyR clusters. Travelling sparks exhibited up to 8 distinct releases, sourced from local or distal junctional sarcoplasmic reticulum. Quantitative analyses showed that sparks may be triggered by any number of RyRs within a cluster, and that acute β-adrenergic stimulation augments intra-cluster RyR recruitment to generate larger events. In contrast, RyR "dispersion" during heart failure facilitates the generation of travelling sparks. Thus, RyRs cooperatively generate Ca2+ sparks in a complex, malleable fashion, and channel organization regulates the propensity for local propagation of Ca2+ release.

High-speed 2D light-sheet fluorescence microscopy enables quantification of spatially varying calcium dynamics in ventricular cardiomyocytes

Introduction: Reduced synchrony of calcium release and t-tubule structure organization in individual cardiomyocytes has been linked to loss of contractile strength and arrhythmia. Compared to confocal scanning techniques widely used for imaging calcium dynamics in cardiac muscle cells, light-sheet fluorescence microscopy enables fast acquisition of a 2D plane in the sample with low phototoxicity. Methods: A custom light-sheet fluorescence microscope was used to achieve dual-channel 2D timelapse imaging of calcium and the sarcolemma, enabling calcium sparks and transients in left and right ventricle cardiomyocytes to be correlated with the cell microstructure. Imaging electrically stimulated dual-labelled cardiomyocytes immobilized with para-nitroblebbistatin, a non-phototoxic, low fluorescence contraction uncoupler, with sub-micron resolution at 395 fps over a 38 μm × 170 µm FOV allowed characterization of calcium spark morphology and 2D mapping of the calcium transient time-to-half-maximum across the cell. Results: Blinded analysis of the data revealed sparks with greater amplitude in left ventricle myocytes. The time for the calcium transient to reach half-maximum amplitude in the central part of the cell was found to be, on average, 2 ms shorter than at the cell ends. Sparks co-localized with t-tubules were found to have significantly longer duration, larger area and spark mass than those further away from t-tubules. Conclusion: The high spatiotemporal resolution of the microscope and automated image-analysis enabled detailed 2D mapping and quantification of calcium dynamics of n = 60 myocytes, with the findings demonstrating multi-level spatial variation of calcium dynamics across the cell, supporting the dependence of synchrony and characteristics of calcium release on the underlying t-tubule structure.