Calcium sparks

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.

NaV1.8 as Proarrhythmic Target in a Ventricular Cardiac Stem Cell Model

The sodium channel NaV1.8, encoded by the SCN10A gene, has recently emerged as a potential regulator of cardiac electrophysiology. We have previously shown that NaV1.8 contributes to arrhythmogenesis by inducing a persistent Na+ current (late Na+ current, INaL) in human atrial and ventricular cardiomyocytes (CM). We now aim to further investigate the contribution of NaV1.8 to human ventricular arrhythmogenesis at the CM-specific level using pharmacological inhibition as well as a genetic knockout (KO) of SCN10A in induced pluripotent stem cell CM (iPSC-CM). In functional voltage-clamp experiments, we demonstrate that INaL was significantly reduced in ventricular SCN10A-KO iPSC-CM and in control CM after a specific pharmacological inhibition of NaV1.8. In contrast, we did not find any effects on ventricular APD90. The frequency of spontaneous sarcoplasmic reticulum Ca2+ sparks and waves were reduced in SCN10A-KO iPSC-CM and control cells following the pharmacological inhibition of NaV1.8. We further analyzed potential triggers of arrhythmias and found reduced delayed afterdepolarizations (DAD) in SCN10A-KO iPSC-CM and after the specific inhibition of NaV1.8 in control cells. In conclusion, we show that NaV1.8-induced INaL primarily impacts arrhythmogenesis at a subcellular level, with minimal effects on systolic cellular Ca2+ release. The inhibition or knockout of NaV1.8 diminishes proarrhythmic triggers in ventricular CM. In conjunction with our previously published results, this work confirms NaV1.8 as a proarrhythmic target that may be useful in an anti-arrhythmic therapeutic strategy.

Mitochondrial Calcium Waves by Electrical Stimulation in Cultured Hippocampal Neurons

Mitochondria are critical to cellular Ca2+ homeostasis via the sequestering of cytosolic Ca2+ in the mitochondrial matrix. Mitochondrial Ca2+ buffering regulates neuronal activity and neuronal death by shaping cytosolic and presynaptic Ca2+ or controlling energy metabolism. Dysfunction in mitochondrial Ca2+ buffering has been implicated in psychological and neurological disorders. Ca2+ wave propagation refers to the spreading of Ca2+ for buffering and maintaining the associated rise in Ca2+ concentration. We investigated mitochondrial Ca2+ waves in hippocampal neurons using genetically encoded Ca2+ indicators. Neurons transfected with mito-GCaMP5G, mito-RCaMP1h, and CEPIA3mt exhibited evidence of mitochondrial Ca2+ waves with electrical stimulation. These waves were observed with 200 action potentials at 40 Hz or 20 Hz but not with lower frequencies or fewer action potentials. The application of inhibitors of mitochondrial calcium uniporter and oxidative phosphorylation suppressed mitochondrial Ca2+ waves. However, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and N-methyl-d-aspartate receptor blockade had no effect on mitochondrial Ca2+ wave were propagation. The Ca2+ waves were not observed in endoplasmic reticula, presynaptic terminals, or cytosol in association with electrical stimulation of 200 action potentials at 40 Hz. These results offer novel insights into the mechanisms underlying mitochondrial Ca2+ buffering and the molecular basis of mitochondrial Ca2+ waves in neurons in response to electrical stimulation.

Tissue-specific differences in Ca2+ sensitivity of the mitochondrial permeability transition pore (PTP). Experiments in male rat liver and heart

Permeability transition pore (PTP) opening dissipates ion and electron gradients across the internal mitochondrial membrane (IMM), including excess Ca2+ in the mitochondrial matrix. After opening, immediate PTP closure must follow to prevent outer membrane disruption, loss of cytochrome c, and eventual apoptosis. Flickering, defined as the rapid alternative opening/closing of PTP, has been reported in heart, which undergoes frequent, large variations in Ca2+. In contrast, in tissues that undergo depolarization events less often, such as the liver, PTP would not need to be as dynamic and thus these tissues would not be as resistant to stress. To evaluate this idea, it was decided to follow the reversibility of the permeability transition (PT) in isolated murine mitochondria from two different tissues: the very dynamic heart, and the liver, which suffers depolarizations less frequently. It was observed that in heart mitochondria PT remained reversible for longer periods and at higher Ca2+ loads than in liver mitochondria. In all cases, Ca2+ uptake was inhibited by ruthenium red and PT was delayed by Cyclosporine A. Characterization of this phenomenon included measuring the rate of oxygen consumption, organelle swelling and Ca2+ uptake and retention. Results strongly suggest that there are tissue-specific differences in PTP physiology, as it resists many more Ca2+ additions before opening in a highly active organ such as the heart than in an organ that seldom suffers Ca2+ loading, such as the liver.

Hyperglycemia triggers RyR2-dependent alterations of mitochondrial calcium homeostasis in response to cardiac ischemia-reperfusion: Key role of DRP1 activation

Hyperglycemia increases the heart sensitivity to ischemia-reperfusion (IR), but the underlying cellular mechanisms remain unclear. Mitochondrial dynamics (the processes that govern mitochondrial morphology and their interactions with other organelles, such as the reticulum), has emerged as a key factor in the heart vulnerability to IR. However, it is unknown whether mitochondrial dynamics contributes to hyperglycemia deleterious effect during IR. We hypothesized that (i) the higher heart vulnerability to IR in hyperglycemic conditions could be explained by hyperglycemia effect on the complex interplay between mitochondrial dynamics, Ca2+ homeostasis, and reactive oxygen species (ROS) production; and (ii) the activation of DRP1, a key regulator of mitochondrial dynamics, could play a central role. Using transmission electron microscopy and proteomic analysis, we showed that the interactions between sarcoplasmic reticulum and mitochondria and mitochondrial fission were increased during IR in isolated rat hearts perfused with a hyperglycemic buffer compared with hearts perfused with a normoglycemic buffer. In isolated mitochondria and cardiomyocytes, hyperglycemia increased mitochondrial ROS production and Ca2+ uptake. This was associated with higher RyR2 instability. These results could contribute to explain the early mPTP activation in mitochondria from isolated hearts perfused with a hyperglycemic buffer and in hearts from streptozotocin-treated rats (to increase the blood glucose). DRP1 inhibition by Mdivi-1 during the hyperglycemic phase and before IR induction, normalized Ca2+ homeostasis, ROS production, mPTP activation, and reduced the heart sensitivity to IR in streptozotocin-treated rats. In conclusion, hyperglycemia-dependent DRP1 activation results in higher reticulum-mitochondria calcium exchange that contribute to the higher heart vulnerability to IR.

Alpha herpesvirus exocytosis from neuron cell bodies uses constitutive secretory mechanisms, and egress and spread from axons is independent of neuronal firing activity

Alpha herpesviruses naturally infect the peripheral nervous system, and can spread to the central nervous system, causing severe debilitating or deadly disease. Because alpha herpesviruses spread along synaptic circuits, and infected neurons exhibit altered electrophysiology and increased spontaneous activity, we hypothesized that alpha herpesviruses use activity-dependent synaptic vesicle-like regulated secretory mechanisms for egress and spread from neurons. Using live-cell fluorescence microscopy, we show that Pseudorabies Virus (PRV) particles use the constitutive Rab6 post-Golgi secretory pathway to exit from the cell body of primary neurons, independent of local calcium signaling. Some PRV particles colocalize with Rab6 in the proximal axon, but we did not detect colocalization/co-transport in the distal axon. Thus, the specific secretory mechanisms used for viral egress from axons remains unclear. To address the role of neuronal activity more generally, we used a compartmentalized neuron culture system to measure the egress and spread of PRV from axons, and pharmacological and optogenetics approaches to modulate neuronal activity. Using tetrodotoxin to silence neuronal activity, we observed no inhibition, and using potassium chloride or optogenetics to elevate neuronal activity, we also show no increase in virus spread from axons. We conclude that PRV egress from neurons uses constitutive secretory mechanisms: generally, activity-independent mechanisms in axons, and specifically, the constitutive Rab6 post-Golgi secretory pathway in cell bodies.

Strain rate of stretch affects crossbridge detachment during relaxation of intact cardiac trabeculae

Mechanical Control of Relaxation refers to the dependence of myocardial relaxation on the strain rate just prior to relaxation, but the mechanisms of enhanced relaxation are not well characterized. This study aimed to characterize how crossbridge kinetics varied with strain rate and time-to-stretch as the myocardium relaxed in early diastole. Ramp-stretches of varying rates (amplitude = 1% muscle length) were applied to intact rat cardiac trabeculae following a load-clamp at 50% of the maximal developed twitch force, which provides a first-order estimate of ejection and coupling to an afterload. The resultant stress-response was calculated as the difference between the time-dependent stress profile between load-clamped twitches with and without a ramp-stretch. The stress-response exhibited features of the step-stretch response of activated, permeabilized myocardium, such as distortion-dependent peak stress, rapid force decay related to crossbridge detachment, and stress recovery related to crossbridge recruitment. The peak stress was strain rate dependent, but the minimum stress and the time-to-minimum stress values were not. The initial rapid change in the stress-response indicates enhanced crossbridge detachment at higher strain rates during relaxation in intact cardiac trabeculae. Physiologic considerations, such as time-varying calcium, are discussed as potential limitations to fitting these data with traditional distortion-recruitment models of crossbridge activity.

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.

Improved Cardiac Performance and Decreased Arrhythmia in Hypertrophic Cardiomyopathy With Non-β-Blocking R-Enantiomer Carvedilol

BACKGROUND

Hypercontractility and arrhythmia are key pathophysiologic features of hypertrophic cardiomyopathy (HCM), the most common inherited heart disease. β-Adrenergic receptor antagonists (β-blockers) are the first-line therapy for HCM. However, β-blockers commonly selected for this disease are often poorly tolerated in patients, where heart-rate reduction and noncardiac effects can lead to reduced cardiac output and fatigue. Mavacamten, myosin ATPase inhibitor recently approved by the US Food and Drug Administration, has demonstrated the ability to ameliorate hypercontractility without lowering heart rate, but its benefits are so far limited to patients with left ventricular (LV) outflow tract obstruction, and its effect on arrhythmia is unknown.

METHODS

We screened 21 β-blockers for their impact on myocyte contractility and evaluated the antiarrhythmic properties of the most promising drug in a ventricular myocyte arrhythmia model. We then examined its in vivo effect on LV function by hemodynamic pressure-volume loop analysis. The efficacy of the drug was tested in vitro and in vivo compared with current therapeutic options (metoprolol, verapamil, and mavacamten) for HCM in an established mouse model of HCM (Myh6R403Q/+ and induced pluripotent stem cell (iPSC)-derived cardiomyocytes from patients with HCM (MYH7R403Q/+).

RESULTS

We identified that carvedilol, a β-blocker not commonly used in HCM, suppresses contractile function and arrhythmia by inhibiting RyR2 (ryanodine receptor type 2). Unlike metoprolol (a β1-blocker), carvedilol markedly reduced LV contractility through RyR2 inhibition, while maintaining stroke volume through α1-adrenergic receptor inhibition in vivo. Clinically available carvedilol is a racemic mixture, and the R-enantiomer, devoid of β-blocking effect, retains the ability to inhibit both α1-receptor and RyR2, thereby suppressing contractile function and arrhythmias without lowering heart rate and cardiac output. In Myh6R403Q/+ mice, R-carvedilol normalized hyperdynamic contraction, suppressed arrhythmia, and increased cardiac output better than metoprolol, verapamil, and mavacamten. The ability of R-carvedilol to suppress contractile function was well retained in MYH7R403Q/+ iPSC-derived cardiomyocytes.

CONCLUSIONS

R-enantiomer carvedilol attenuates hyperdynamic contraction, suppresses arrhythmia, and at the same time, improves cardiac output without lowering heart rate by dual blockade of α1-adrenergic receptor and RyR2 in mouse and human models of HCM. This combination of therapeutic effects is unique among current therapeutic options for HCM and may particularly benefit patients without LV outflow tract obstruction.

Metabolic profiling of Mytilus coruscus mantle in response of shell repairing under acute acidification

Mytilus coruscus is an economically important marine bivalve mollusk found in the Yangtze River estuary, which experiences dramatic pH fluctuations due to seasonal freshwater input and suffer from shell fracture or injury in the natural environment. In this study, we used intact-shell and damaged-shell M. coruscus and performed metabolomic analysis, free amino acids analysis, calcium-positive staining, and intracellular calcium level tests in the mantle to investigate whether the mantle-specific metabolites can be induced by acute sea-water acidification and understand how the mantle responds to acute acidification during the shell repair process. We observed that both shell damage and acute acidification induced alterations in phospholipids, amino acids, nucleotides, organic acids, benzenoids, and their analogs and derivatives. Glycylproline, spicamycin, and 2-aminoheptanoic acid (2-AHA) are explicitly induced by shell damage. Betaine, aspartate, and oxidized glutathione are specifically induced by acute acidification. Our results show different metabolic patterns in the mussel mantle in response to different stressors, which can help elucidate the shell repair process under ocean acidification. furthermore, metabolic processes related to energy supply, cell function, signal transduction, and amino acid synthesis are disturbed by shell damage and/or acute acidification, indicating that both shell damage and acute acidification increased energy consumption, and disturb phospholipid synthesis, osmotic regulation, and redox balance. Free amino acid analysis and enzymatic activity assays partially confirmed our findings, highlighting the adaptation of M. coruscus to dramatic pH fluctuations in the Yangtze River estuary.

NADPH Oxidases and Oxidative Stress in the Pathogenesis of Atrial Fibrillation

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

Oxidative stress in early metabolic syndrome impairs cardiac RyR2 and SERCA2a activity and modifies the interplay of these proteins during Ca2+ waves

We investigated how oxidative stress (OS) alters Ca2+ handling in ventricular myocytes in early metabolic syndrome (MetS) in sucrose-fed rats. The effects of N-acetyl cysteine (NAC) or dl-Dithiothreitol (DTT) on systolic Ca2+ transients (SCaTs), diastolic Ca2+ sparks (CaS) and Ca2+ waves (CaW), recorded by confocal techniques, and L-type Ca2+ current (ICa), assessed by whole-cell patch clamp, were evaluated in MetS and Control cells. MetS myocytes exhibited decreased SCaTs and CaS frequency but unaffected CaW propagation. In Control cells, NAC/DTT reduced RyR2/SERCA2a activity blunting SCaTs, CaS frequency and CaW propagation, suggesting that basal ROS optimised Ca2+ signalling by maintaining RyR2/SERCA2a function and that these proteins facilitate CaW propagation. Conversely, NAC/DTT in MetS recovered RyR2/SERCA2a function, improving SCaTs and CaS frequency, but unexpectedly decreasing CaW propagation. We hypothesised that OS decreases RyR2/SERCA2a activity at early MetS, and while decreased SERCA2a favours CaW propagation, diminished RyR2 restrains it.

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.

L-Carnitine improves mechanical responses of cardiomyocytes and restores Ca2+ homeostasis during aging

L-Carnitine (β-hydroxy-γ-trimethylaminobutyric acid, LC) is a crucial molecule for the mitochondrial oxidation of fatty acids. It facilitates the transport of long-chain fatty acids into the mitochondrial matrix. The reduction in LC levels during the aging process has been linked to numerous cardiovascular disorders, including contractility dysfunction, and disrupted intracellular Ca2+ homeostasis. The aim of this study was to examine the effects of long-term (7 months) LC administration on cardiomyocyte contraction and intracellular Ca2+ transients ([Ca2+]i) in aging rats. Male albino Wistar rats were randomly assigned to either the control or LC-treated groups. LC (50 mg/kg body weight/day) was dissolved in distilled water and orally administered for a period of 7 months. The control group received distilled water alone. Subsequently, ventricular single cardiomyocytes were isolated, and the contractility and Ca2+ transients were recorded in aging (18 months) rats. This study demonstrates, for the first time, a novel inotropic effect of long-term LC treatment on rat ventricular cardiomyocyte contraction. LC increased cardiomyocyte cell shortening and resting sarcomere length. Furthermore, LC supplementation led to a reduction in resting [Ca2+]i level and an increase in the amplitude of [Ca2+]i transients, indicative of enhanced contraction. Consistent with these results, decay time of Ca2+ transients also decreased significantly in the LC-treated group. The long-term administration of LC may help restore the Ca2+ homeostasis altered during aging and could be used as a cardioprotective medication in cases where myocyte contractility is diminished.

Elementary intracellular Ca signals approximated as a transition of release channel system from a metastable state

Cardiac muscle contraction is initiated by an elementary Ca signal (called Ca spark) which is achieved by collective action of Ca release channels in a cluster. The mechanism of this synchronization remains uncertain. We approached Ca spark activation as an emergent phenomenon of an interactive system of release channels. We constructed a weakly lumped Markov chain that applies an Ising model formalism to such release channel clusters and probable open channel configurations and demonstrated that spark activation is described as a system transition from a metastable to an absorbing state, analogous to the pressure required to overcome surface tension in bubble formation. This yielded quantitative estimates of the spark generation probability as a function of various system parameters. We performed numerical simulations to find spark probabilities as a function of sarcoplasmic reticulum Ca concentration, obtaining similar values for spark activation threshold as our analytic model, as well as those reported in experimental studies. Our parametric sensitivity analyses also showed that the spark activation threshold decreased as Ca sensitivity of RyR activation and RyR cluster size increased.

Cardiomyocytes from female compared to male mice have larger ryanodine receptor clusters and higher calcium spark frequency

Sex differences in cardiac physiology are receiving increased attention as it has become clear that men and women have different aetiologies of cardiac disease and require different treatments. There are experimental data suggesting that male cardiomyocytes exhibit larger Ca2+ transients due to larger Ca2+ sparks and a higher excitation-contraction coupling gain; in addition, they exhibit a larger response to adrenergic stimulation with isoprenaline (ISO). Here, we studied whether there are sex differences relating to structural organization of the transverse tubular network and ryanodine receptors (RyRs). Surprisingly, we found that female cardiomyocytes exhibited a higher spark frequency in a range of spark magnitudes. While overall RyR expression and phosphorylation were the same, female cardiomyocytes had larger but fewer RyR clusters. The density of transverse t-tubules was the same, but male cardiomyocytes had more longitudinal t-tubules. The Ca2+ transients were similar in male and female cardiomyocytes under control conditions and in the presence of ISO. The synchrony of the Ca2+ transients was similar between sexes as well. Overall, our data suggest subtle sex differences in the Ca2+ influx and efflux pathways and their response to ISO, but these differences are balanced, resulting in similar Ca2+ transients in field-stimulated male and female cardiomyocytes. The higher spark frequency in female cardiomyocytes is related to the organization of RyRs into larger, but fewer clusters. KEY POINTS: During a heartbeat, the force of contraction depends on the amplitude of the calcium transient, which in turn depends on the amount of calcium released as calcium sparks through ryanodine receptors in the sarcoplasmic reticulum. Previous studies suggest that cardiomyocytes from male compared to female mice exhibit larger calcium sparks, larger sarcoplasmic reticulum calcium release and greater response to adrenergic stimulation triggering a fight-or-flight response. In contrast, we show that cardiomyocytes from female mice have a higher spark frequency during adrenergic stimulation and similar spark morphology. The higher spark frequency is related to the organization of ryanodine receptors into fewer, but larger clusters in female compared to male mouse cardiomyocytes. Despite subtle sex differences in cardiomyocyte structure and calcium fluxes, the differences are balanced, leading to similar calcium transients in cardiomyocytes from male and female mice.

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.

Combining nitric oxide and calcium sensing for the detection of endothelial dysfunction

Cardiovascular diseases are the leading cause of death worldwide and are not typically diagnosed until the disease has manifested. Endothelial dysfunction is an early, reversible precursor in the irreversible development of cardiovascular diseases and is characterized by a decrease in nitric oxide production. We believe that more reliable and reproducible methods are necessary for the detection of endothelial dysfunction. Both nitric oxide and calcium play important roles in the endothelial function. Here we review different types of molecular sensors used in biological settings. Next, we review the current nitric oxide and calcium sensors available. Finally, we review methods for using both sensors for the detection of endothelial dysfunction.

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.

Diversity of cells and signals in the cardiovascular system

This white paper is the outcome of the seventh UC Davis Cardiovascular Research Symposium on Systems Approach to Understanding Cardiovascular Disease and Arrhythmia. This biannual meeting aims to bring together leading experts in subfields of cardiovascular biomedicine to focus on topics of importance to the field. The theme of the 2022 Symposium was 'Cell Diversity in the Cardiovascular System, cell-autonomous and cell-cell signalling'. Experts in the field contributed their experimental and mathematical modelling perspectives and discussed emerging questions, controversies, and challenges in examining cell and signal diversity, co-ordination and interrelationships involved in cardiovascular function. This paper originates from the topics of formal presentations and informal discussions from the Symposium, which aimed to develop a holistic view of how the multiple cell types in the cardiovascular system integrate to influence cardiovascular function, disease progression and therapeutic strategies. The first section describes the major cell types (e.g. cardiomyocytes, vascular smooth muscle and endothelial cells, fibroblasts, neurons, immune cells, etc.) and the signals involved in cardiovascular function. The second section emphasizes the complexity at the subcellular, cellular and system levels in the context of cardiovascular development, ageing and disease. Finally, the third section surveys the technological innovations that allow the interrogation of this diversity and advancing our understanding of the integrated cardiovascular function and dysfunction.

IP3 receptors: An "elementary" journey from structure to signals

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are large tetrameric channels which sit mostly in the membrane of the endoplasmic reticulum (ER) and mediate Ca2+ release from intracellular stores in response to extracellular stimuli in almost all cells. Dual regulation of IP3Rs by IP3 and Ca2+ itself, upstream "licensing", and the arrangement of IP3Rs into small clusters in the ER membrane, allow IP3Rs to generate spatially and temporally diverse Ca2+ signals. The characteristic biphasic regulation of IP3Rs by cytosolic Ca2+ concentration underpins regenerative Ca2+ signals by Ca2+-induced Ca2+-release, while also preventing uncontrolled explosive Ca2+ release. In this way, cells can harness a simple ion such as Ca2+ as a near-universal intracellular messenger to regulate diverse cellular functions, including those with conflicting outcomes such as cell survival and cell death. High-resolution structures of the IP3R bound to IP3 and Ca2+ in different combinations have together started to unravel the workings of this giant channel. Here we discuss, in the context of recently published structures, how the tight regulation of IP3Rs and their cellular geography lead to generation of "elementary" local Ca2+ signals known as Ca2+ "puffs", which form the fundamental bottleneck through which all IP3-mediated cytosolic Ca2+ signals must first pass.

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.

Endoplasmic Reticulum-Plasma Membrane Junctions as Sites of Depolarization-Induced Ca2+ Signaling in Excitable Cells

Membrane contact sites between endoplasmic reticulum (ER) and plasma membrane (PM), or ER-PM junctions, are found in all eukaryotic cells. In excitable cells they play unique roles in organizing diverse forms of Ca2+ signaling as triggered by membrane depolarization. ER-PM junctions underlie crucial physiological processes such as excitation-contraction coupling, smooth muscle contraction and relaxation, and various forms of activity-dependent signaling and plasticity in neurons. In many cases the structure and molecular composition of ER-PM junctions in excitable cells comprise important regulatory feedback loops linking depolarization-induced Ca2+ signaling at these sites to the regulation of membrane potential. Here, we describe recent findings on physiological roles and molecular composition of native ER-PM junctions in excitable cells. We focus on recent studies that provide new insights into canonical forms of depolarization-induced Ca2+ signaling occurring at junctional triads and dyads of striated muscle, as well as the diversity of ER-PM junctions in these cells and in smooth muscle and neurons.

Cardiac-Specific Expression of Cre Recombinase Leads to Age-Related Cardiac Dysfunction Associated with Tumor-like Growth of Atrial Cardiomyocyte and Ventricular Fibrosis and Ferroptosis

Transgenic expression of Cre recombinase driven by a specific promoter is normally used to conditionally knockout a gene in a tissue- or cell-type-specific manner. In αMHC-Cre transgenic mouse model, expression of Cre recombinase is controlled by the myocardial-specific α-myosin heavy chain (αMHC) promoter, which is commonly used to edit myocardial-specific genes. Toxic effects of Cre expression have been reported, including intro-chromosome rearrangements, micronuclei formation and other forms of DNA damage, and cardiomyopathy was observed in cardiac-specific Cre transgenic mice. However, mechanisms associated with Cardiotoxicity of Cre remain poorly understood. In our study, our data unveiled that αMHC-Cre mice developed arrhythmias and died after six months progressively, and none of them survived more than one year. Histopathological examination showed that αMHC-Cre mice had aberrant proliferation of tumor-like tissue in the atrial chamber extended from and vacuolation of ventricular myocytes. Furthermore, the αMHC-Cre mice developed severe cardiac interstitial and perivascular fibrosis, accompanied by significant increase of expression levels of MMP-2 and MMP-9 in the cardiac atrium and ventricular. Moreover, cardiac-specific expression of Cre led to disintegration of the intercalated disc, along with altered proteins expression of the disc and calcium-handling abnormality. Comprehensively, we identified that the ferroptosis signaling pathway is involved in heart failure caused by cardiac-specific expression of Cre, on which oxidative stress results in cytoplasmic vacuole accumulation of lipid peroxidation on the myocardial cell membrane. Taken together, these results revealed that cardiac-specific expression of Cre recombinase can lead to atrial mesenchymal tumor-like growth in the mice, which causes cardiac dysfunction, including cardiac fibrosis, reduction of the intercalated disc and cardiomyocytes ferroptosis at the age older than six months in mice. Our study suggests that αMHC-Cre mouse models are effective in young mice, but not in old mice. Researchers need to be particularly careful when using αMHC-Cre mouse model to interpret those phenotypic impacts of gene responses. As the Cre-associated cardiac pathology matched mostly to that of the patients, the model could also be employed for investigating age-related cardiac dysfunction.

Cyclical Stretching Induces Excess Intracellular Ca2+ Influx in Human Keloid-Derived Fibroblasts In Vitro

BACKGROUND

The incidence of keloids is higher in the case of darker skin. It is more common in the parts exposed to stretching (thorax, abdomen, and joints). Cyclical stretching reportedly induced each Ca2+ spike through differential mechanosensitive channels in human synovial and dermal fibroblasts. Therefore, the authors hypothesized that cyclical stretching also induces a specific Ca2+ spike in keloid-derived fibroblasts.

METHODS

This in vitro study compared the intracellular calcium dynamics induced by cyclical stretching between control (human dermal fibroblasts) and keloid (human keloid-derived fibroblasts) groups. Each group was exposed to two-dimensional stretch using an originally developed stretch microdevice. Intracellular Ca2+ was observed for 5 minutes, including 30 seconds of baseline, under a fluorescent confocal laser microscope. The intracellular Ca2+ concentration was evaluated every 0.5 second using the fluorescence intensity ratio. A positive cellular response was defined as a rise of the ratio by greater than or equal to 20%. The normal response cutoff value was determined by receiver operating characteristic analysis.

RESULTS

The keloid groups were significantly more responsive than the control groups (15.7% versus 8.2%; P = 0.029). In the cellular response-positive cells, the keloid groups reached significantly higher intracellular Ca2+ concentration peaks than the control groups (2.20 versus 1.26; P = 0.0022). The cutoff value was 1.77, and 10.4% of the keloid-derived fibroblasts exhibited a hyper-Ca2+ spike above the normal range.

CONCLUSIONS

Keloid-derived fibroblasts with a hyper-Ca2+ spike might constitute a keloid-specific subpopulation. Hereafter, the authors will study whether the normalization of excessive intracellular Ca2+ concentration leads to keloid treatment in vivo.

CLINICAL RELEVANCE STATEMENT

This study result provided a clue to the onset mechanism of keloids, which the authors hope will lead to the development of new therapy in the future.

Calcium oscillation on homogeneous and heterogeneous networks of ryanodine receptor

Calcium oscillation is an important calcium homeostasis, imbalance of which is the key mechanism of initiation and progression of many major diseases. The formation and maintenance of calcium homeostasis are closely related to the spatial distribution of calcium channels on endoplasmic reticulum, whose complex structure was unveiled by recent observations with superresolution imaging techniques. In the current paper, a theoretical framework is established by abstracting the spatial distribution of the calcium channels as a nonlinear biological complex network with calcium channels as nodes and Ca^{2+} as edges. A dynamical model for a ryanodine receptor (RyR) is adopted to investigate the effect of spatial distribution on calcium oscillation. The mean-field model can be well reproduced from the complete graph and dense Erdös-Rényi network. The synchronization of RyRs is found important to generate a global calcium oscillation. Below a critical density of the Erdös-Rényi or BaraBási-Albert network, the amplitude and interspike interval decrease rapidly with the end of disappearance of oscillation due to the desynchronization. The clique graph with a cluster structure cannot produce a global oscillation due to the failure of synchronization between clusters. A more realistic geometric network is constructed in a two-dimensional plane based on the experimental information about the RyR arrangement of clusters and the frequency distribution of cluster sizes. Different from the clique graph, the global oscillation can be generated with reasonable parameters on the geometric network. The simulation also suggests that existence of small clusters and rogue RyRs plays an important role in the maintenance of global calcium oscillation through keeping synchronization between large clusters. Such results support the heterogeneous distribution of RyRs with different-size clusters, which is helpful to understand recent observations with superresolution nanoscale imaging techniques. The current theoretical framework can also be extent to investigate other phenomena in calcium signal transduction.

Trapping of single diffusing particles by a circular disk on a reflecting flat surface. Absorbing hemisphere approximation

Recent progress in biophysics (for example, in studies of chemical sensing and spatiotemporal cell-signaling) poses new challenges to statistical theory of trapping of single diffusing particles. Here we deal with one of them, namely, trapping kinetics of single particles diffusing in a half-space bounded by a reflecting flat surface containing an absorbing circular disk. This trapping problem is essentially two-dimensional and the question of the angular dependence of the kinetics on the particle starting point is highly nontrivial. We propose an approximate approach to the problem that replaces the absorbing disk by an absorbing hemisphere of a properly chosen radius. This replacement makes the problem angular-independent and essentially one-dimensional. After the replacement one can find an exact solution for the particle propagator (Green's function) that allows one to completely characterize the kinetics. Extensive testing of the theoretical predictions based on the absorbing hemisphere approximation against three-dimensional Brownian dynamics simulations shows excellent agreement between the analytical and simulation results when the particle starts sufficiently far away from the disk. Our approach fails and the angular dependence of the kinetics is important when the distance of the particle starting point from the disk center is comparable with the disk radius. However, even when the initial distance is only two disk radii, the maximum relative error of the theoretical predictions is about 10%. The relative error rapidly decreases as the initial distance increases.

Ca2+-Activated K+ Channels and the Regulation of the Uteroplacental Circulation

Adequate uteroplacental blood supply is essential for the development and growth of the placenta and fetus during pregnancy. Aberrant uteroplacental perfusion is associated with pregnancy complications such as preeclampsia, fetal growth restriction (FGR), and gestational diabetes. The regulation of uteroplacental blood flow is thus vital to the well-being of the mother and fetus. Ca²⁺-activated K⁺ (K_Ca) channels of small, intermediate, and large conductance participate in setting and regulating the resting membrane potential of vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) and play a critical role in controlling vascular tone and blood pressure. K_Ca channels are important mediators of estrogen/pregnancy-induced adaptive changes in the uteroplacental circulation. Activation of the channels hyperpolarizes uteroplacental VSMCs/ECs, leading to attenuated vascular tone, blunted vasopressor responses, and increased uteroplacental blood flow. However, the regulation of uteroplacental vascular function by K_Ca channels is compromised in pregnancy complications. This review intends to provide a comprehensive overview of roles of K_Ca channels in the regulation of the uteroplacental circulation under physiological and pathophysiological conditions.

The role of subunit cooperativity on ryanodine receptor 2 calcium signaling

The ryanodine receptor type 2 (RyR2) is composed of four subunits that control calcium (Ca) release in cardiac cells. RyR2 serves primarily as a Ca sensor and can respond to rapid sub-millisecond pulses of Ca while remaining shut at resting concentrations. However, it is not known how the four subunits interact for the RyR2 to function as an effective Ca sensor. To address this question, and to understand the role of subunit cooperativity in Ca-mediated signal transduction, we have developed a computational model of the RyR2 composed of four interacting subunits. We first analyze the statistical properties of a single RyR2 tetramer, where each subunit can exist in a closed or open conformation. Our findings indicate that the number of subunits in the open state is a crucial parameter that dictates RyR2 kinetics. We find that three or four open subunits are required for the RyR2 to harness cooperative interactions to respond to sub-millisecond changes in Ca, while at the same time remaining shut at the resting Ca levels in the cardiac cell. If the required number of open subunits is lowered to one or two, the RyR2 cannot serve as a robust Ca sensor, as the large cooperativity required to stabilize the closed state prevents channel activation. Using this four-subunit model, we analyze the kinetics of Ca release from a RyR2 cluster. We show that the closure of a cluster of RyR2 channels is highly sensitive to the balance of cooperative interactions between closed and open subunits. Based on this result, we analyze how specific interactions between RyR2 subunits can induce persistent Ca leak from the sarcoplasmic reticulum (SR), which is believed to be arrhythmogenic. Thus, these results provide a framework to analyze how a pharmacologic or genetic modification of RyR2 subunit cooperativity can induce abnormal Ca cycling that can potentially lead to life-threatening arrhythmias.

The Importance of Integrated Regulation Mechanism of Coronary Microvascular Function for Maintaining the Stability of Coronary Microcirculation: An Easily Overlooked Perspective

Coronary microvascular dysfunction (CMD) refers to a group of disorders affecting the structure and function of coronary microcirculation and is associated with an increased risk of major adverse cardiovascular events. At present, great progress has been made in the diagnosis of CMD, but there is no specific treatment for it because of the complexity of CMD pathogenesis. Vascular dysfunction is one of the important causes of CMD, but previous reviews mostly considered microvascular dysfunction as a whole abnormality so the obtained conclusions are skewed. The coronary microvascular function is co-regulated by multiple mechanisms, and the mechanisms by which microvessels of different luminal diameters are regulated vary. The main purpose of this review is to revisit the mechanisms by which coronary microvessels at different diameters regulate coronary microcirculation through integrated sequential activation and briefly discuss the pathogenesis, diagnosis, and treatment progress of CMD from this perspective.

Binding and Functional Folding (BFF): A Physiological Framework for Studying Biomolecular Interactions and Allostery

EF-hand Ca2+-binding proteins (CBPs), such as S100 proteins (S100s) and calmodulin (CaM), are signaling proteins that undergo conformational changes upon increasing intracellular Ca2+. Upon binding Ca2+, S100 proteins and CaM interact with protein targets and induce important biological responses. The Ca2+-binding affinity of CaM and most S100s in the absence of target is weak (CaKD > 1 μM). However, upon effector protein binding, the Ca2+ affinity of these proteins increases via heterotropic allostery (CaKD < 1 μM). Because of the high number and micromolar concentrations of EF-hand CBPs in a cell, at any given time, allostery is required physiologically, allowing for (i) proper Ca2+ homeostasis and (ii) strict maintenance of Ca2+-signaling within a narrow dynamic range of free Ca2+ ion concentrations, [Ca2+]free. In this review, mechanisms of allostery are coalesced into an empirical "binding and functional folding (BFF)" physiological framework. At the molecular level, folding (F), binding and folding (BF), and BFF events include all atoms in the biomolecular complex under study. The BFF framework is introduced with two straightforward BFF types for proteins (type 1, concerted; type 2, stepwise) and considers how homologous and nonhomologous amino acid residues of CBPs and their effector protein(s) evolved to provide allosteric tightening of Ca2+ and simultaneously determine how specific and relatively promiscuous CBP-target complexes form as both are needed for proper cellular function.

Mechanisms of Ventricular Arrhythmias and Implications for Catheter Ablation

Ventricular arrhythmias present with a wide spectrum of clinical manifestations, from mildly symptomatic frequent premature ventricular contractions to life-threatening events. Pathophysiologically, idiopathic ventricular arrhythmias occur in the absence of structural heart disease or ion channelopathies. Ventricular arrhythmias in the context of structural heart disease are usually determined by scar-related reentry and are associated with increased mortality. Catheter ablation is safe and highly effective in treating ventricular arrhythmias. The proper characterization of the arrhythmogenic substrate is essential for accurate procedural planning. We provide an overview on the main mechanisms of ventricular arrhythmias and their implications for catheter ablation.

Antiarrhythmic and Inotropic Effects of Selective Na+/Ca2+ Exchanger Inhibition: What Can We Learn from the Pharmacological Studies?

Life-long stable heart function requires a critical balance of intracellular Ca²⁺. Several ion channels and pumps cooperate in a complex machinery that controls the influx, release, and efflux of Ca²⁺. Probably one of the most interesting and most complex players of this crosstalk is the Na⁺/Ca²⁺ exchanger, which represents the main Ca²⁺ efflux mechanism; however, under some circumstances, it can also bring Ca²⁺ into the cell. Therefore, the inhibition of the Na⁺/Ca²⁺ exchanger has emerged as one of the most promising possible pharmacological targets to increase Ca²⁺ levels, to decrease arrhythmogenic depolarizations, and to reduce excessive Ca²⁺ influx. In line with this, as a response to increasing demand, several more or less selective Na⁺/Ca²⁺ exchanger inhibitor compounds have been developed. In the past 20 years, several results have been published regarding the effect of Na⁺/Ca²⁺ exchanger inhibition under various circumstances, e.g., species, inhibitor compounds, and experimental conditions; however, the results are often controversial. Does selective Na⁺/Ca²⁺ exchanger inhibition have any future in clinical pharmacological practice? In this review, the experimental results of Na⁺/Ca²⁺ exchanger inhibition are summarized focusing on the data obtained by novel highly selective inhibitors.

Modeling Calcium Cycling in the Heart: Progress, Pitfalls, and Challenges

Intracellular calcium (Ca) cycling in the heart plays key roles in excitation-contraction coupling and arrhythmogenesis. In cardiac myocytes, the Ca release channels, i.e., the ryanodine receptors (RyRs), are clustered in the sarcoplasmic reticulum membrane, forming Ca release units (CRUs). The RyRs in a CRU act collectively to give rise to discrete Ca release events, called Ca sparks. A cell contains hundreds to thousands of CRUs, diffusively coupled via Ca to form a CRU network. A rich spectrum of spatiotemporal Ca dynamics is observed in cardiac myocytes, including Ca sparks, spark clusters, mini-waves, persistent whole-cell waves, and oscillations. Models of different temporal and spatial scales have been developed to investigate these dynamics. Due to the complexities of the CRU network and the spatiotemporal Ca dynamics, it is challenging to model the Ca cycling dynamics in the cardiac system, particularly at the tissue sales. In this article, we review the progress of modeling of Ca cycling in cardiac systems from single RyRs to the tissue scale, the pros and cons of the current models and different modeling approaches, and the challenges to be tackled in the future.

Sec61 complex/translocon: The role of an atypical ER Ca2+-leak channel in health and disease

The heterotrimeric Sec61 protein complex forms the functional core of the so-called translocon that forms an aqueous channel in the endoplasmic reticulum (ER). The primary role of the Sec61 complex is to allow protein import in the ER during translation. Surprisingly, a completely different function in intracellular Ca2+ homeostasis has emerged for the Sec61 complex, and the latter is now accepted as one of the major Ca2+-leak pathways of the ER. In this review, we first discuss the structure of the Sec61 complex and focus on the pharmacology and regulation of the Sec61 complex as a Ca2+-leak channel. Subsequently, we will pay particular attention to pathologies that are linked to Sec61 mutations, such as plasma cell deficiency and congenital neutropenia. Finally, we will explore the relevance of the Sec61 complex as a Ca2+-leak channel in various pathophysiological (ER stress, apoptosis, ischemia-reperfusion) and pathological (type 2 diabetes, cancer) settings.

Shaped by leaky ER: Homeostatic Ca2+ fluxes

At any moment in time, cells coordinate and balance their calcium ion (Ca²⁺) fluxes. The term ‘Ca²⁺ homeostasis’ suggests that balancing resting Ca²⁺ levels is a rather static process. However, direct ER Ca²⁺ imaging shows that resting Ca²⁺ levels are maintained by surprisingly dynamic Ca²⁺ fluxes between the ER Ca²⁺ store, the cytosol, and the extracellular space. The data show that the ER Ca²⁺ leak, continuously fed by the high-energy consuming SERCA, is a fundamental driver of resting Ca²⁺ dynamics. Based on simplistic Ca²⁺ toolkit models, we discuss how the ER Ca²⁺ leak could contribute to evolutionarily conserved Ca²⁺ phenomena such as Ca²⁺ entry, ER Ca²⁺ release, and Ca²⁺ oscillations.

How does flecainide impact RyR2 channel function?

Flecainide, a cardiac class 1C blocker of the surface membrane sodium channel (NaV1.5), has also been reported to reduce cardiac ryanodine receptor (RyR2)-mediated sarcoplasmic reticulum (SR) Ca2+ release. It has been introduced as a clinical antiarrhythmic agent for catecholaminergic polymorphic ventricular tachycardia (CPVT), a condition most commonly associated with gain-of-function RyR2 mutations. Current debate concerns both cellular mechanisms of its antiarrhythmic action and molecular mechanisms of its RyR2 actions. At the cellular level, it targets NaV1.5, RyR2, Na+/Ca2+ exchange (NCX), and additional proteins involved in excitation-contraction (EC) coupling and potentially contribute to the CPVT phenotype. This Viewpoint primarily addresses the various direct molecular actions of flecainide on isolated RyR2 channels in artificial lipid bilayers. Such studies demonstrate different, multifarious, flecainide binding sites on RyR2, with voltage-dependent binding in the channel pore or voltage-independent binding at distant peripheral sites. In contrast to its single NaV1.5 pore binding site, flecainide may bind to at least four separate inhibitory sites on RyR2 and one activation site. None of these binding sites have been specifically located in the linear RyR2 sequence or high-resolution structure. Furthermore, it is not clear which of the inhibitory sites contribute to flecainide's reduction of spontaneous Ca2+ release in cellular studies. A confounding observation is that flecainide binding to voltage-dependent inhibition sites reduces cation fluxes in a direction opposite to physiological Ca2+ flow from SR lumen to cytosol. This may suggest that, rather than directly blocking Ca2+ efflux, flecainide can reduce Ca2+ efflux by blocking counter currents through the pore which otherwise limit SR membrane potential change during systolic Ca2+ efflux. In summary, the antiarrhythmic effects of flecainide in CPVT seem to involve multiple components of EC coupling and multiple actions on RyR2. Their clarification may identify novel specific drug targets and facilitate flecainide's clinical utilization in CPVT.

β-Adrenergic Receptor Desensitization/Down-Regulation in Heart Failure: A Friend or Foe?

Cardiac sympathetic activation, mediated by β-adrenergic receptors (β-ARs), normally increases cardiac contraction and relaxation. Accomplishing this task requires a physiological, concerted Ca²⁺ signaling, being able to increase Ca²⁺ release from sarcoplasmic reticulum (SR) in systole and speed up Ca²⁺ re-uptake in diastole. In heart failure (HF) myocardial β-ARs undergo desensitization/down-regulation due to sustained sympathetic adrenergic activation. β-AR desensitization/down-regulation diminishes adrenergic signaling and cardiac contractile reserve, and is conventionally considered to be detrimental in HF progression. Abnormal Ca²⁺ handling, manifested as cardiac ryanodine receptor (RyR2) dysfunction and diastolic Ca²⁺ leak (due to sustained adrenergic activation) also occur in HF. RyR2 dysfunction and Ca²⁺ leak deplete SR Ca²⁺ store, diminish Ca²⁺ release in systole and elevate Ca²⁺ levels in diastole, impairing both systolic and diastolic ventricular function. Moreover, elevated Ca²⁺ levels in diastole promote triggered activity and arrhythmogenesis. In the presence of RyR2 dysfunction and Ca²⁺ leak, further activation of the β-AR signaling in HF would worsen the existing abnormal Ca²⁺ handling, exacerbating not only cardiac dysfunction, but also ventricular arrhythmogenesis and sudden cardiac death. Thus, we conclude that β-AR desensitization/down-regulation may be a self-preserving, adaptive process (acting like an intrinsic β-AR blocker) protecting the failing heart from developing lethal ventricular arrhythmias under conditions of elevated sympathetic drive and catecholamine levels in HF, rather than a conventionally considered detrimental process. This also implies that medications simply enhancing β-AR signaling (like β-AR agonists) may not be so beneficial unless they can also correct dysfunctional Ca²⁺ handling in HF.

Liquiritigenin protects against myocardial ischemic by inhibiting oxidative stress, apoptosis, and L-type Ca2+ channels

Liquiritigenin (Lq) offers cytoprotective effects against various cardiac injuries, but its beneficial effects on myocardial ischemic (MI) injury and the related mechanisms remain unclear. In the in vivo study, an animal model of MI was induced by intraperitoneal injection of isoproterenol (Iso, 85 mg/kg). ECG, heart rate, serum levels of CK and CK-MB, histopathological changes, and reactive oxygen species (ROS) levels were all measured. In vitro, H9c2 cells were divided into four groups and treated for 24 hr with liquiritigenin (30 μmol/L and 100 μmol/L) followed with CoCl₂ (800 μmol/L) for another 24 hr. Cell viability, apoptosis, mitochondrial membrane potential, and intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) were then assessed. The L-type Ca²⁺ current (I_Ca-L ) was detected using a patch clamp technique on isolated rat ventricular myocytes. The myocyte contraction and Ca²⁺ transients were measured using an IonOptix detection system. The remarkable cardiac injury and generation of intracellular ROS induced by Iso were alleviated via treatment with Lq. CoCl₂ administration induced cell apoptosis, mitochondrial dysfunction, and Ca²⁺ overload in H9c2; Lq reduces these deleterious effects of CoCl₂ . Meanwhile, Lq blocked I_Ca-L in a dose-dependent manner. The half-maximal inhibitory concentration of Lq was 110.87 μmol/L. Lq reversibly reduced the amplitude of cell contraction as well as the Ca²⁺ transients. The results show that Lq protects against MI injury by antioxidation, antiapoptosis, counteraction mitochondrial dysfunction, and inhibition of I_Ca-L , thus damping intracellular Ca²⁺ .

Mitofusin 2 positively regulates Ca2+ signaling by tethering the sarcoplasmic reticulum and mitochondria in rat aortic smooth muscle cells

Mitochondria buffer cytosolic Ca²⁺increases following Ca²⁺ influx from extracellular spaces and Ca²⁺ release from intracellular Ca²⁺ store sites under physiological circumstances. Therefore, close contact of mitochondria with the sarcoplasmic reticulum (SR) is required for maintaining Ca²⁺ homeostasis. Mitofusin 2 (Mfn2) localizes in both mitochondrial and SR membranes, and is hypothesized to optimize the distance and Ca²⁺ transfer between these organelles. However, the physiological significance of Mfn2 in vascular smooth muscle cells (VSMCs) is poorly understood. In the present study, the role of Mfn2 in the physical and functional couplings between SR and mitochondria was examined in rat aortic smooth muscle cells (rASMCs) by confocal and electron microscope imaging. When Mfn2 was knocked-down using siRNA in rASMCs, the mean distance between these organelles was extended from 16.2 to 21.6 nm. The increase in the cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) induced by 100 nM arginine vasopressin (AVP) was not affected by Mfn2 siRNA knockdown, whereas cytosolic Ca²⁺ removal was slower after Mfn2 knockdown. Following the AVP-induced [Ca²⁺]_cyt increase, mitochondrial Ca²⁺ uptake and Ca²⁺ refill into the SR were attenuated by Mfn2 knockdown. In addition, Mfn2-knockdown cells exhibited a loss of mitochondrial membrane potential (ΔΨ_mito) and lower ATP levels in mitochondria. Moreover, Mfn2 knockdown inhibited cell proliferation. In contrast, Mfn2 overexpression increased ΔΨ_mito and cell growth. This study strongly suggests that Mfn2 is responsible for SR-mitochondria Ca²⁺ signaling by tethering mitochondria to SR, thereby regulating ATP production and proliferation of VSMCs.

Critical Requirements for the Initiation of a Cardiac Arrhythmia in Rat Ventricle: How Many Myocytes?

Cardiovascular disease is the leading cause of death worldwide due in a large part to arrhythmia. In order to understand how calcium dynamics play a role in arrhythmogenesis, normal and dysfunctional Ca²⁺ signaling in a subcellular, cellular, and tissued level is examined using cardiac ventricular myocytes at a high temporal and spatial resolution using multiscale computational modeling. Ca²⁺ sparks underlie normal excitation-contraction coupling. However, under pathological conditions, Ca²⁺ sparks can combine to form Ca²⁺ waves. These propagating elevations of (Ca²⁺)_i can activate an inward Na⁺-Ca²⁺ exchanger current (I_NCX) that contributes to early after-depolarization (EADs) and delayed after-depolarizations (DADs). However, how cellular currents lead to full depolarization of the myocardium and how they initiate extra systoles is still not fully understood. This study explores how many myocytes must be entrained to initiate arrhythmogenic depolarizations in biophysically detailed computational models. The model presented here suggests that only a small number of myocytes must activate in order to trigger an arrhythmogenic propagating action potential. These conditions were examined in 1-D, 2-D, and 3-D considering heart geometry. The depolarization of only a few hundred ventricular myocytes is required to trigger an ectopic depolarization. The number decreases under disease conditions such as heart failure. Furthermore, in geometrically restricted parts of the heart such as the thin muscle strands found in the trabeculae and papillary muscle, the number of cells needed to trigger a propagating depolarization falls even further to less than ten myocytes.

RuBPY decreases intracellular calcium by decreasing influx and increasing storage

RuBPY is a ruthenium complex NO donor that presents a nitrite in its moiety and has been shown to induce vasodilation in various arteries, as well as arterial pressure reduction with no changes in heart rate. Since vascular tone is highly dependent on the cytosolic calcium concentration ([Ca²⁺ ]c), the current study aimed to investigate the effects of RuBPY on the intracellular mobilization of calcium stores of rat aortic vascular smooth muscle cells. Vascular reactivity experiments were performed in isolated aortic rings that were contracted with a high concentration of KCl or phenylephrine (Phe). Moreover, primary cultured vascular smooth muscle cells were used to measure [Ca²⁺ ]c by confocal microscopy. The NO donor RuBPY decreased the [Ca²⁺ ]c and reduced KCl and Phe -induced contractile responses. The selective inhibitor of sarco-endoplasmic Ca-ATPase (SERCA) with thapsigargin impaired the effect of RuBPY on Phe -induced contractile response. RuBPY also reduced caffeine-induced contraction, and the contraction dependent on the capacitive Ca²⁺ influx. Therefore, our results suggest that NO released from RuBPY decreased [Ca²⁺ ]c by calcium influx blockade, and activation of guanylyl-cyclase-cGMP-GK pathway. These results indicate that RuBPY increases Ca²⁺ storage in the sarcoplasmic reticulum by SERCA activation, and also by capacitive Ca²⁺ influx inhibition, which is dependent on the intracellular release of nitric oxide from this compound. This article is protected by copyright. All rights reserved.

Mechanisms of Lian-Gui-Ning-Xin-Tang in the treatment of arrhythmia: Integrated pharmacology and in vivo pharmacological assessment

BACKGROUND

Lian-Gui-Ning-Xin-Tang (LGNXT), a classical traditional Chinese medicine (TCM) formula, has been widely used in clinical practice and has shown satisfactory efficacy in the treatment of arrhythmias. However, its mechanism of action in the treatment of arrhythmias is still unknown. Moreover, the complex chemical composition and therapeutic targets of LGNXT pose a challenge in pharmacological research.

PURPOSE

To analyze the active compounds and action mechanisms of LGNXT for the treatment of arrhythmias.

METHODS

Here, we used an integrated pharmacology approach to identify the potential active compounds and mechanisms of action of LGNXT in treating arrhythmias. Potential active compounds in LGNXT were identified using ultra-performance liquid chromatography-quadrupole-time-of-flight mass spectrometry (UPLC-Q-TOF/MS) and the potential related targets of these compounds were predicted using an integrated in silico approach. The obtained targets were mapped onto relevant databases to identify their corresponding pathways, following the experiments that were conducted to confirm whether the presumptive results of systemic pharmacology were correct.

RESULTS

Eighty-three components were identified in herbal materials and in animal plasma using UPLC-Q-TOF/MS and were considered the potential active components of LGNXT. Thirty key targets and 57 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were identified as possible targets and pathways involved in LGNXT-mediated treatment using network pharmacology, with the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)/Ca2+ system pathway being the most significantly affected. This finding was validated using an adrenaline (Adr)-induced rat model of arrhythmias. Pretreatment with LGNXT delayed the occurrence, shortened the duration, and reduced the severity of arrhythmias. LGNXT exerted antiarrhythmic effects by inhibiting cAMP, PKA, CACNA1C, and RyR2.

CONCLUSIONS

The findings of this study revealed that preventing intracellular Ca2+ overload and maintaining intracellular Ca2+ homeostasis may be the primary mechanisms of LGNXT in alleviating arrhythmias. Thus, we suggest that the β-adrenergic receptor (AR)/cAMP/PKA/Ca2+ system signaling hub may constitute a promising molecular target for the development of novel antiarrhythmic therapeutic interventions. Additionally, we believe that the approach of investigation of the biological effects of a multi-herbal formula by the combination of metabolomics and network pharmacology, as used in this study, could serve as a systematic model for TCM research.

Regulation of Neural Differentiation of ADMSCs using Graphene-Mediated Wireless-Localized Electrical Signals Driven by Electromagnetic Induction

Although adipose-derived mesenchymal stem cells (ADMSCs) isolated from patients' fat are considered as the most important autologous stem cells for tissue repair, significant difficulties in the neural differentiation of ADMSCs still impede stem cell therapy for neurodegenerative diseases. Herein, a wireless-electrical stimulation method is proposed to direct the neural differentiation of ADMSCs based on the electromagnetic effect using a graphene film as a conductive scaffold. By placing a rotating magnet on the top of a culture system without any inducer, the ADMSCs cultured on graphene differentiate into functional neurons within 15 days. As a conductive biodegradable nanomaterial, graphene film acts as a wireless electrical signal generator driven by the electromagnetic induction, and millivolt-level voltage generated in situ provokes ADMSCs to differentiate into neurons, proved by morphological variation, extremely high levels of neuron-specific genes, and proteins. Most importantly, Ca²⁺ intracellular influx is observed in these ADMSC-derived neurons once exposure to neurotransmitters, indicating that these cells are functional neurons. This research enhances stem cell therapy for neurodegenerative diseases using autologous ADMSCs and overcomes the lack of neural stem cells. This nanostructure-mediated physical-signal simulation method is inexpensive, safe, and localized, and has a significant impact on neural regeneration.

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

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

Vidarabine, an anti-herpes agent, prevents occlusal-disharmony-induced cardiac dysfunction in mice

We recently reported a positive relationship between occlusal disharmony and cardiovascular disease via activation of β-adrenergic signaling in mice. Furthermore, inhibition of type 5 adenylyl cyclase (AC5), a major cardiac subtype in adults, protects the heart against oxidative stress. Here, we examined the role of AC5 in the development of occlusal-disharmony-induced cardiovascular disease in bite-opening (BO) mice, prepared by cementing a suitable appliance onto the mandibular incisor. We first examined the effects of BO treatment on cardiac function in mice treated or not treated for 2 weeks with vidarabine, which we previously identified as an inhibitor of cardiac AC. Cardiac function was significantly decreased in the BO group compared to the control group, but vidarabine ameliorated the dysfunction. Cardiac fibrosis, myocyte apoptosis and myocyte oxidative DNA damage were significantly increased in the BO group, but vidarabine blocked these changes. The BO-induced cardiac dysfunction was associated with increased phospholamban phosphorylation at threonine-17 and serine-16, as well as increased activation of the Ca2+-calmodulin-dependent protein kinase II/receptor-interacting protein 3 signaling pathway. These data suggest that AC5 inhibition with vidarabine might be a new therapeutic approach for the treatment of cardiovascular disease associated with occlusal disharmony.

Ion Channel Partnerships: Odd and Not-So-Odd Couples Controlling Neuronal Ion Channel Function

The concerted function of the large number of ion channels expressed in excitable cells, including brain neurons, shapes diverse signaling events by controlling the electrical properties of membranes. It has long been recognized that specific groups of ion channels are functionally coupled in mediating ionic fluxes that impact membrane potential, and that these changes in membrane potential impact ion channel gating. Recent studies have identified distinct sets of ion channels that can also physically and functionally associate to regulate the function of either ion channel partner beyond that afforded by changes in membrane potential alone. Here, we review canonical examples of such ion channel partnerships, in which a Ca²⁺ channel is partnered with a Ca²⁺-activated K⁺ channel to provide a dedicated route for efficient coupling of Ca²⁺ influx to K⁺ channel activation. We also highlight examples of non-canonical ion channel partnerships between Ca²⁺ channels and voltage-gated K⁺ channels that are not intrinsically Ca²⁺ sensitive, but whose partnership nonetheless yields enhanced regulation of one or the other ion channel partner. We also discuss how these ion channel partnerships can be shaped by the subcellular compartments in which they are found and provide perspectives on how recent advances in techniques to identify proteins in close proximity to one another in native cells may lead to an expanded knowledge of other ion channel partnerships.

Numb is required for optimal contraction of skeletal muscle

BACKGROUND

The role of Numb, a protein that is important for cell fate and development and that, in human muscle, is expressed at reduced levels with advanced age, was investigated; adult mice skeletal muscle and its localization and function within myofibres were determined.

METHODS

Numb expression was evaluated by western blot. Numb localization was determined by confocal microscopy. The effects of conditional knock out (cKO) of Numb and the closely related gene Numb-like in skeletal muscle fibres were evaluated by in situ physiology, transmission and focused ion beam scanning electron microscopy, three-dimensional reconstruction of mitochondria, lipidomics, and bulk RNA sequencing. Additional studies using primary mouse myotubes investigated the effects of Numb knockdown on cell fusion, mitochondrial function, and calcium transients.

RESULTS

Numb protein expression was reduced by ~70% (P < 0.01) at 24 as compared with 3 months of age in gastrocnemius and tibialis anterior muscle. Numb was localized within muscle fibres as bands traversing fibres at regularly spaced intervals in close proximity to dihydropyridine receptors. The cKO of Numb and Numb-like reduced specific tetanic force by 36% (P < 0.01), altered mitochondrial spatial relationships to sarcomeric structures, increased Z-line spacing by 30% (P < 0.0001), perturbed sarcoplasmic reticulum organization and reduced mitochondrial volume by over 80% (P < 0.01). Only six genes were differentially expressed in cKO mice: Itga4, Sema7a, Irgm2, Vezf1, Mib1, and Tmem132a. Several lipid mediators derived from polyunsaturated fatty acids through lipoxygenases were up-regulated in Numb cKO skeletal muscle: 12-HEPE was increased by ~250% (P < 0.05) and 17,18-EpETE by ~240% (P < 0.05). In mouse primary myotubes, Numb knockdown reduced cell fusion (~20%, P < 0.01) and delayed the caffeine-induced rise in cytosolic calcium concentrations by more than 100% (P < 0.01).

CONCLUSIONS

These findings implicate Numb as a critical factor in skeletal muscle structure and function and suggest that Numb is critical for calcium release. We therefore speculate that Numb plays critical roles in excitation-contraction coupling, one of the putative targets of aged skeletal muscles. These findings provide new insights into the molecular underpinnings of the loss of muscle function observed with sarcopenia.

Magnesium Ions Moderate Calcium-Induced Calcium Release in Cardiac Calcium Release Sites by Binding to Ryanodine Receptor Activation and Inhibition Sites

Ryanodine receptor channels at calcium release sites of cardiac myocytes operate on the principle of calcium-induced calcium release. In vitro experiments revealed competition of Ca2+ and Mg2+ in the activation of ryanodine receptors (RyRs) as well as inhibition of RyRs by Mg2+. The impact of RyR modulation by Mg2+ on calcium release is not well understood due to the technical limitations of in situ experiments. We turned instead to an in silico model of a calcium release site (CRS), based on a homotetrameric model of RyR gating with kinetic parameters determined from in vitro measurements. We inspected changes in the activity of the CRS model in response to a random opening of one of 20 realistically distributed RyRs, arising from Ca2+/Mg2+ interactions at RyR channels. Calcium release events (CREs) were simulated at a range of Mg2+-binding parameters at near-physiological Mg2+ and ATP concentrations. Facilitation of Mg2+ binding to the RyR activation site inhibited the formation of sparks and slowed down their activation. Impeding Mg-binding to the RyR activation site enhanced spark formation and speeded up their activation. Varying Mg2+ binding to the RyR inhibition site also dramatically affected calcium release events. Facilitation of Mg2+ binding to the RyR inhibition site reduced the amplitude, relative occurrence, and the time-to-end of sparks, and vice versa. The characteristics of CREs correlated dose-dependently with the effective coupling strength between RyRs, defined as a function of RyR vicinity, single-channel calcium current, and Mg-binding parameters of the RyR channels. These findings postulate the role of Mg2+ in calcium release as a negative modulator of the coupling strength among RyRs in a CRS, translating to damping of the positive feedback of the calcium-induced calcium-release mechanism.

Nanoscale Organization, Regulation, and Dynamic Reorganization of Cardiac Calcium Channels

The architectural specializations and targeted delivery pathways of cardiomyocytes ensure that L-type Ca²⁺ channels (CaV1.2) are concentrated on the t-tubule sarcolemma within nanometers of their intracellular partners the type 2 ryanodine receptors (RyR2) which cluster on the junctional sarcoplasmic reticulum (jSR). The organization and distribution of these two groups of cardiac calcium channel clusters critically underlies the uniform contraction of the myocardium. Ca²⁺ signaling between these two sets of adjacent clusters produces Ca²⁺ sparks that in health, cannot escalate into Ca²⁺ waves because there is sufficient separation of adjacent clusters so that the release of Ca²⁺ from one RyR2 cluster or supercluster, cannot activate and sustain the release of Ca²⁺ from neighboring clusters. Instead, thousands of these Ca²⁺ release units (CRUs) generate near simultaneous Ca²⁺ sparks across every cardiomyocyte during the action potential when calcium induced calcium release from RyR2 is stimulated by depolarization induced Ca²⁺ influx through voltage dependent CaV1.2 channel clusters. These sparks summate to generate a global Ca²⁺ transient that activates the myofilaments and thus the electrical signal of the action potential is transduced into a functional output, myocardial contraction. To generate more, or less contractile force to match the hemodynamic and metabolic demands of the body, the heart responds to β-adrenergic signaling by altering activity of calcium channels to tune excitation-contraction coupling accordingly. Recent accumulating evidence suggests that this tuning process also involves altered expression, and dynamic reorganization of CaV1.2 and RyR2 channels on their respective membranes to control the amplitude of Ca²⁺ entry, SR Ca²⁺ release and myocardial function. In heart failure and aging, altered distribution and reorganization of these key Ca²⁺ signaling proteins occurs alongside architectural remodeling and is thought to contribute to impaired contractile function. In the present review we discuss these latest developments, their implications, and future questions to be addressed.