Recovery of cardiac calcium release is controlled by sarcoplasmic reticulum refilling and ryanodine receptor sensitivity

Eu3+ detects two functionally distinct luminal Ca2+ binding sites in ryanodine receptors

Ryanodine receptors (RyRs) are Ca2+ release channels, gated by Ca2+ in the cytosol and the sarcoplasmic reticulum lumen. Their regulation is impaired in certain cardiac and muscle diseases. Although a lot of data is available on the luminal Ca2+ regulation of RyR, its interpretation is complicated by the possibility that the divalent ions used to probe the luminal binding sites may contaminate the cytoplasmic sites by crossing the channel pore. In this study, we used Eu3+, an impermeable agonist of Ca2+ binding sites, as a probe to avoid this complication and to gain more specific information about the function of the luminal Ca2+ sensor. Single-channel currents were measured from skeletal muscle and cardiac RyRs (RyR1 and RyR2) using the lipid bilayer technique. We show that RyR2 is activated by the luminal addition of Ca2+, whereas RyR1 is inhibited. These results were qualitatively reproducible using Eu3+. The luminal regulation of RyR1 carrying a mutation associated with malignant hyperthermia was not different from that of the wild-type. RyR1 inhibition by Eu3+ was extremely voltage dependent, whereas RyR2 activation did not depend on the membrane potential. These results suggest that the RyR1 inhibition site is in the membrane's electric field (channel pore), whereas the RyR2 activation site is outside. Using in silico analysis and previous results, we predicted putative Ca2+ binding site sequences. We propose that RyR2 bears an activation site, which is missing in RyR1, but both isoforms share the same inhibitory Ca2+ binding site near the channel gate.

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.

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.

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.

Cardiac Alternans: From Bedside to Bench and Back

Cardiac alternans arises from dynamical instabilities in the electrical and calcium cycling systems of the heart, and often precedes ventricular arrhythmias and sudden cardiac death. In this review, we integrate clinical observations with theory and experiment to paint a holistic portrait of cardiac alternans: the underlying mechanisms, arrhythmic manifestations and electrocardiographic signatures. We first summarize the cellular and tissue mechanisms of alternans that have been demonstrated both theoretically and experimentally, including 3 voltage-driven and 2 calcium-driven alternans mechanisms. Based on experimental and simulation results, we describe their relevance to mechanisms of arrhythmogenesis under different disease conditions, and their link to electrocardiographic characteristics of alternans observed in patients. Our major conclusion is that alternans is not only a predictor, but also a causal mechanism of potentially lethal ventricular and atrial arrhythmias across the full spectrum of arrhythmia mechanisms that culminate in functional reentry, although less important for anatomic reentry and focal arrhythmias.

Synchronization of spatially discordant voltage and calcium alternans in cardiac tissue

The heart is an excitable medium which is excited by membrane potential depolarization and propagation. Membrane potential depolarization brings in calcium (Ca) through the Ca channels to trigger intracellular Ca release for contraction of the heart. Ca also affects voltage via Ca-dependent ionic currents, and thus, voltage and Ca are bidirectionally coupled. It has been shown that the voltage subsystem or the Ca subsystem can generate its own dynamical instabilities which are affected by their bidirectional couplings, leading to complex dynamics of action potential and Ca cycling. Moreover, the dynamics become spatiotemporal in tissue in which cells are diffusively coupled through voltage. A widely investigated spatiotemporal dynamics is spatially discordant alternans (SDA) in which action potential duration (APD) or Ca amplitude exhibits temporally period-2 and spatially out-of-phase patterns, i.e., APD-SDA and Ca-SDA patterns, respectively. However, the mechanisms of formation, stability, and synchronization of APD-SDA and Ca-SDA patterns remain incompletely understood. In this paper, we use cardiac tissue models described by an amplitude equation, coupled iterated maps, and reaction-diffusion equations with detailed physiology (the ionic model) to perform analytical and computational investigations. We show that, when the Ca subsystem is stable, the Ca-SDA pattern always follows the APD-SDA pattern, and thus, they are always synchronized. When the Ca subsystem is unstable, synchronization of APD-SDA and Ca-SDA patterns depends on the stabilities of both subsystems, their coupling strengths, and the spatial scales of the initial Ca-SDA patterns. Spontaneous (initial condition-independent) synchronization is promoted by enhancing APD instability and reducing Ca instability as well as stronger Ca-to-APD and APD-to-Ca coupling, a pattern formation caused by dynamical instabilities. When Ca is more unstable and APD is less unstable or APD-to-Ca coupling is weak, synchronization of APD-SDA and Ca-SDA patterns is promoted by larger initially synchronized Ca-SDA clusters, i.e., initial condition-dependent synchronization. The synchronized APD-SDA and Ca-SDA patterns can be locked in-phase, antiphase, or quasiperiodic depending on the coupling relationship between APD and Ca. These theoretical and simulation results provide mechanistic insights into the APD-SDA and Ca-SDA dynamics observed in experimental studies.

Determinants of Ca2+ release restitution: Insights from genetically altered animals and mathematical modeling

Each heartbeat is followed by a refractory period. Recovery from refractoriness is known as Ca²⁺ release restitution (CRR), and its alterations are potential triggers of Ca²⁺ arrhythmias. Although the control of CRR has been associated with SR Ca²⁺ load and RYR2 Ca²⁺ sensitivity, the relative role of some of the determinants of CRR remains largely undefined. An intriguing point, difficult to dissect and previously neglected, is the possible independent effect of SR Ca²⁺ content versus the velocity of SR Ca²⁺ refilling on CRR. To assess these interrogations, we used isolated myocytes with phospholamban (PLN) ablation (PLNKO), knock-in mice with pseudoconstitutive CaMKII phosphorylation of RYR2 S2814 (S2814D), S2814D crossed with PLNKO mice (SDKO), and a previously validated human cardiac myocyte model. Restitution of cytosolic Ca²⁺ (Fura-2 AM) and L-type calcium current (ICaL; patch-clamp) was evaluated with a two-pulse (S₁/S₂) protocol. CRR and ICaL restitution increased as a function of the (S₂-S₁) coupling interval, following an exponential curve. When SR Ca²⁺ load was increased by increasing extracellular [Ca²⁺] from 2.0 to 4.0 mM, CRR and ICaL restitution were enhanced, suggesting that ICaL restitution may contribute to the faster CRR observed at 4.0 mM [Ca²⁺]. In contrast, ICaL restitution did not differ among the different mouse models. For a given SR Ca²⁺ load, CRR was accelerated in S2814D myocytes versus WT, but not in PLNKO and SDKO myocytes versus WT and S2814D, respectively. The model mimics all experimental data. Moreover, when the PLN ablation-induced decrease in RYR2 expression was corrected, the model revealed that CRR was accelerated in PLNKO and SDKO versus WT and S2814D myocytes, consistent with the enhanced velocity of refilling, SR [Ca²⁺] recovery, and CRR. We speculate that refilling rate might enhance CRR independently of SR Ca²⁺ load.

Local recovery of cardiac calcium-induced calcium release interrogated by ultra-effective, two-photon uncaging of calcium

Key points

In cardiac myocytes, subcellular local calcium release signals, calcium sparks, are recruited to form each cellular calcium transient and activate the contractile machinery.

Abnormal timing of recovery of sparks after their termination may contribute to arrhythmias.

We developed a method to interrogate recovery of calcium spark trigger probabilities and their amplitude over time using two‐photon photolysis of a new ultra‐effective caged calcium compound.

The findings confirm the utility of the technique to define an elevated sensitivity of the calcium release mechanism in situ and to follow hastened recovery of spark trigger probabilities in a mouse model of an inherited cardiac arrhythmia, which was used for validation.

Analogous methods are likely to be applicable to investigate other microscopic subcellular signalling systems in a variety of cell types.

Abstract

In cardiac myocytes Ca²⁺‐induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyRs) governs activation of contraction. Ca²⁺ release occurs via subcellular Ca²⁺ signalling events, Ca²⁺ sparks. Local recovery of Ca²⁺ release depends on both SR refilling and restoration of Ca²⁺ sensitivity of the RyRs. We used two‐photon (2P) photolysis of the ultra‐effective caged Ca²⁺ compound BIST‐2EGTA and laser‐scanning confocal Ca²⁺ imaging to probe refractoriness of local Ca²⁺ release in control conditions and in the presence of cAMP or low‐dose caffeine (to stimulate CICR) or cyclopiazonic acid (CPA; to slow SR refilling). Permeabilized cardiomyocytes were loaded with BIST‐2EGTA and rhod‐2. Pairs of short 2P photolytic pulses (1 ms, 810 nm) were applied with different intervals to test Ca²⁺ release amplitude recovery and trigger probability for the second spark in a pair. Photolytic and biological events were distinguished by classification with a self‐learning support vector machine (SVM) algorithm. In permeabilized myocytes data recorded in the presence of CPA showed a lower probability of triggering a second spark compared to control or cAMP conditions. Cardiomyocytes from a mouse model harbouring the arrhythmogenic RyR^R420Q mutation were used for further validation and revealed a higher Ca²⁺ sensitivity of CICR. This new 2P approach provides composite information of Ca²⁺ release amplitude and trigger probability recovery reflecting both SR refilling and restoration of CICR and RyR Ca²⁺ sensitivity. It can be used to measure the kinetics of local CICR recovery, alterations of which may be related to premature heart beats and arrhythmias.

In cardiac myocytes, subcellular local calcium release signals, calcium sparks, are recruited to form each cellular calcium transient and activate the contractile machinery.

Abnormal timing of recovery of sparks after their termination may contribute to arrhythmias.

We developed a method to interrogate recovery of calcium spark trigger probabilities and their amplitude over time using two‐photon photolysis of a new ultra‐effective caged calcium compound.

The findings confirm the utility of the technique to define an elevated sensitivity of the calcium release mechanism in situ and to follow hastened recovery of spark trigger probabilities in a mouse model of an inherited cardiac arrhythmia, which was used for validation.

Analogous methods are likely to be applicable to investigate other microscopic subcellular signalling systems in a variety of cell types.

CaMKII inhibition has dual effects on spontaneous Ca2+ release and Ca2+ alternans in ventricular cardiomyocytes from mice with a gain-of-function RyR2 mutation

In conditions with abnormally increased activity of the cardiac ryanodine receptor (RyR2), Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) can contribute to a further destabilization of RyR2 that results in triggered arrhythmias. Therefore, inhibition of CaMKII in such conditions has been suggested as a strategy to suppress RyR2 activity and arrhythmias. However, suppression of RyR2 activity can lead to the development of arrhythmogenic Ca²⁺ alternans. The aim of this study was to test whether the suppression of RyR2 activity caused by inhibition of CaMKII increases propensity for Ca²⁺ alternans. We studied spontaneous Ca²⁺ release events and Ca²⁺ alternans in isolated left ventricular cardiomyocytes from mice carrying the gain-of-function RyR2 mutation RyR2-R2474S and from wild-type mice. CaMKII inhibition by KN-93 effectively decreased the frequency of spontaneous Ca²⁺ release events in RyR2-R2474S cardiomyocytes exposed to the β-adrenoceptor agonist isoprenaline. However, KN-93-treated RyR2-R2474S cardiomyocytes also showed increased propensity for Ca²⁺ alternans and increased Ca²⁺ alternans ratio compared with both an inactive analog of KN-93 and with vehicle-treated controls. This increased propensity for Ca²⁺ alternans was explained by prolongation of Ca²⁺ release refractoriness. Importantly, the increased propensity for Ca²⁺ alternans in KN-93-treated RyR2-R2474S cardiomyocytes did not surpass that of wild type. In conclusion, inhibition of CaMKII efficiently reduces spontaneous Ca²⁺ release but promotes Ca²⁺ alternans in RyR2-R2474S cardiomyocytes with a gain-of-function RyR2 mutation. The dominant effect in RyR2-R2474S is to reduce spontaneous Ca²⁺ release, which supports this intervention as a therapeutic strategy in this specific condition. However, future studies on CaMKII inhibition in conditions with increased propensity for Ca²⁺ alternans should include investigation of both phenomena.NEW & NOTEWORTHY Genetically increased RyR2 activity promotes arrhythmogenic Ca²⁺ release. Inhibition of CaMKII suppresses RyR2 activity and arrhythmogenic Ca²⁺ release. Suppression of RyR2 activity prolongs refractoriness of Ca²⁺ release. Prolonged refractoriness of Ca²⁺ release leads to arrhythmogenic Ca²⁺ alternans. CaMKII inhibition promotes Ca²⁺ alternans by prolonging Ca²⁺ release refractoriness.

Blockade of sodium‑calcium exchanger via ORM-10962 attenuates cardiac alternans

Repolarization alternans, a periodic oscillation of long-short action potential duration, is an important source of arrhythmogenic substrate, although the mechanisms driving it are insufficiently understood. Despite its relevance as an arrhythmia precursor, there are no successful therapies able to target it specifically. We hypothesized that blockade of the sodium‑calcium exchanger (NCX) could inhibit alternans. The effects of the selective NCX blocker ORM-10962 were evaluated on action potentials measured with microelectrodes from canine papillary muscle preparations, and calcium transients measured using Fluo4-AM from isolated ventricular myocytes paced to evoke alternans. Computer simulations were used to obtain insight into the drug's mechanisms of action. ORM-10962 attenuated cardiac alternans, both in action potential duration and calcium transient amplitude. Three morphological types of alternans were observed, with differential response to ORM-10962 with regards to APD alternans attenuation. Analysis of APD restitution indicates that calcium oscillations underlie alternans formation. Furthermore, ORM-10962 did not markedly alter APD restitution, but increased post-repolarization refractoriness, which may be mediated by indirectly reduced L-type calcium current. Computer simulations reproduced alternans attenuation via ORM-10962, suggesting that it is acts by reducing sarcoplasmic reticulum release refractoriness. This results from the ORM-10962-induced sodium‑calcium exchanger block accompanied by an indirect reduction in L-type calcium current. Using a computer model of a heart failure cell, we furthermore demonstrate that the anti-alternans effect holds also for this disease, in which the risk of alternans is elevated. Targeting NCX may therefore be a useful anti-arrhythmic strategy to specifically prevent calcium driven alternans.

SPEG: a key regulator of cardiac calcium homeostasis

Proper cardiac Ca2+ homeostasis is essential for normal excitation-contraction coupling. Perturbations in cardiac Ca2+ handling through altered kinase activity has been implicated in altered cardiac contractility and arrhythmogenesis. Thus, a better understanding of cardiac Ca2+ handling regulation is vital for a better understanding of various human disease processes. 'Striated muscle preferentially expressed protein kinase' (SPEG) is a member of the myosin light chain kinase family that is key for normal cardiac function. Work within the last 5 years has revealed that SPEG has a crucial role in maintaining normal cardiac Ca2+ handling through maintenance of transverse tubule formation and phosphorylation of junctional membrane complex proteins. Additionally, SPEG has been causally impacted in human genetic diseases such as centronuclear myopathy and dilated cardiomyopathy as well as in common acquired cardiovascular disease such as heart failure and atrial fibrillation. Given the rapidly emerging role of SPEG as a key cardiac Ca2+ regulator, we here present this review in order to summarize recent findings regarding the mechanisms of SPEG regulation of cardiac excitation-contraction coupling in both physiology and human disease. A better understanding of the roles of SPEG will be important for a more complete comprehension of cardiac Ca2+ regulation in physiology and disease.

Loss of SPEG Inhibitory Phosphorylation of Ryanodine Receptor Type-2 Promotes Atrial Fibrillation

BACKGROUND

Enhanced diastolic calcium (Ca²⁺) release through ryanodine receptor type-2 (RyR2) has been implicated in atrial fibrillation (AF) promotion. Diastolic sarcoplasmic reticulum Ca²⁺ leak is caused by increased RyR2 phosphorylation by PKA (protein kinase A) or CaMKII (Ca²⁺/calmodulin-dependent kinase-II) phosphorylation, or less dephosphorylation by protein phosphatases. However, considerable controversy remains regarding the molecular mechanisms underlying altered RyR2 function in AF. We thus aimed to determine the role of SPEG (striated muscle preferentially expressed protein kinase), a novel regulator of RyR2 phosphorylation, in AF pathogenesis.

METHODS

Western blotting was performed with right atrial biopsies from patients with paroxysmal AF. SPEG atrial knockout mice were generated using adeno-associated virus 9. In mice, AF inducibility was determined using intracardiac programmed electric stimulation, and diastolic Ca²⁺ leak in atrial cardiomyocytes was assessed using confocal Ca²⁺ imaging. Phosphoproteomics studies and Western blotting were used to measure RyR2 phosphorylation. To test the effects of RyR2-S2367 phosphorylation, knockin mice with an inactivated S2367 phosphorylation site (S2367A) and a constitutively activated S2367 residue (S2367D) were generated by using CRISPR-Cas9.

RESULTS

Western blotting revealed decreased SPEG protein levels in atrial biopsies from patients with paroxysmal AF in comparison with patients in sinus rhythm. SPEG atrial-specific knockout mice exhibited increased susceptibility to pacing-induced AF by programmed electric stimulation and enhanced Ca²⁺ spark frequency in atrial cardiomyocytes with Ca²⁺ imaging, establishing a causal role for decreased SPEG in AF pathogenesis. Phosphoproteomics in hearts from SPEG cardiomyocyte knockout mice identified RyR2-S2367 as a novel kinase substrate of SPEG. Western blotting demonstrated that RyR2-S2367 phosphorylation was also decreased in patients with paroxysmal AF. RyR2-S2367A mice exhibited an increased susceptibility to pacing-induced AF, and aberrant atrial sarcoplasmic reticulum Ca²⁺ leak, as well. In contrast, RyR2-S2367D mice were resistant to pacing-induced AF.

CONCLUSIONS

Unlike other kinases (PKA, CaMKII) that increase RyR2 activity, SPEG phosphorylation reduces RyR2-mediated sarcoplasmic reticulum Ca²⁺ release. Reduced SPEG levels and RyR2-S2367 phosphorylation typified patients with paroxysmal AF. Studies in S2367 knockin mouse models showed a causal relationship between reduced S2367 phosphorylation and AF susceptibility. Thus, modulating SPEG activity and phosphorylation levels of the novel S2367 site on RyR2 may represent a novel target for AF treatment.

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

KEY POINTS

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

ABSTRACT

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

Recruiting RyRs to Open in a Ca2+ Release Unit: Single-RyR Gating Properties Make RyR Group Dynamics

In cardiac myocytes, clusters of type-2 ryanodine receptors (RyR2s) release Ca²⁺ from the sarcoplasmic reticulum (SR) via a positive feedback mechanism in which fluxed Ca²⁺ activates nearby RyRs. Although the general principles of this are understood, less is known about how single-RyR gating properties define the RyR group dynamics in an array of many channels. Here, we examine this using simulations with three models of RyR gating that have identical open probabilities: the commonly used two-state Markov gating model, one that utilizes multiple exponentials to fit single-channel open time (OT) and closed time (CT) distributions, and an extension of this multiexponential model that also includes experimentally measured correlations between single-channel OTs and CTs. The simulations of RyR clusters that utilize the multiexponential gating model produce infrequent Ca²⁺ release events with relatively few open RyRs. Ca²⁺ release events become even smaller when OT/CT correlations are included. This occurs because the correlations produce a small but consistent bias against recruiting more RyRs to open during the middle of a Ca²⁺ release event, between the initiation and termination phases (which are unaltered compared to the uncorrelated simulations). In comparison, the two-state model produces frequent, large, and long Ca²⁺ release events because it had a recruitment bias in favor of opening more RyRs. This difference stems from the two-state model's single-RyR OT and CT distributions being qualitatively different from the experimental ones. Thus, the details of single-RyR gating can profoundly affect SR Ca²⁺ release even if open probability and mean OTs and CTs are identical. We also show that Ca²⁺ release events can terminate spontaneously without any reduction in SR [Ca²⁺], luminal regulation, Ca²⁺-dependent inactivation, or physical coupling between RyRs when Ca²⁺ flux is below a threshold value. This supports and extends the pernicious attrition/induction decay hypothesis that SR Ca²⁺ release events terminate below a threshold Ca²⁺ flux.

IOCBIO Sparks detection and analysis software

Analysis of calcium sparks in cardiomyocytes can provide valuable information about functional changes of calcium handling in health and disease. As a part of the calcium sparks analysis, sparks detection and characterization is necessary. Here, we describe a new open-source platform for automatic calcium sparks detection from line scan confocal images. The developed software is tailored for detecting only calcium sparks, allowing us to design a graphical user interface specifically for this task. The software enables detecting sparks automatically as well as adding, removing, or adjusting regions of interest marking each spark. The results of the analysis are stored in an SQL database, allowing simple integration with statistical tools. We have analyzed the performance of the algorithm using a large set of synthetic images with varying spark sizes and noise levels and also compared the analysis results with results obtained by software established in the field. The use of our software is illustrated by an analysis of the effect of isoprenaline (ISO) on spark frequency, amplitude, and spatial and temporal characteristics. For that, cardiomyocytes from C57BL/6 mice were used. We demonstrated an increase in spark frequency, tendency of having larger spark amplitudes, sparks with a longer duration, and occurrence of multiple sparks from the same site in the presence of ISO. We also show that the duration and the width of sparks with the same amplitude were similar in the absence and presence of ISO. The software was released as an open source repository and is available for free use and collaborative development.

Quantitative tests reveal that microtubules tune the healthy heart but underlie arrhythmias in pathology

KEY POINTS

Our group previously discovered and characterized the microtubule mechanotransduction pathway linking diastolic stretch to NADPH oxidase 2-derived reactive oxygen species signals that regulate calcium sparks and calcium influx pathways. Here we used focused experimental tests to constrain and expand our existing computational models of calcium signalling in heart. Mechanistic and quantitative modelling revealed new insights in disease including: changes in microtubule network density and properties, elevated NOX2 expression, altered calcium release dynamics, how NADPH oxidase 2 is activated by and responds to stretch, and finally the degree to which normalizing mechano-activated reactive oxygen species signals can prevent calcium-dependent arrhythmias.

ABSTRACT

Microtubule (MT) mechanotransduction links diastolic stretch to generation of NADPH oxidase 2 (NOX2)-dependent reactive oxygen species (ROS), signals we term X-ROS. While stretch-elicited X-ROS primes intracellular calcium (Ca²⁺ ) channels for synchronized activation in the healthy heart, the dysregulated excess in this pathway underscores asynchronous Ca²⁺ release and arrhythmia. Here, we expanded our existing computational models of Ca²⁺ signalling in heart to include MT-dependent mechanotransduction through X-ROS. Informed by new focused experimental tests to properly constrain our model, we quantify the role of X-ROS on excitation-contraction coupling in healthy and pathological conditions. This approach allowed for a mechanistic investigation that revealed new insights into X-ROS signalling in disease including changes in MT network density and post-translational modifications (PTMs), elevated NOX2 expression, altered Ca²⁺ release dynamics (i.e. Ca²⁺ sparks and Ca²⁺ waves), how NOX2 is activated by and responds to stretch, and finally the degree to which normalizing X-ROS can prevent Ca²⁺ -dependent arrhythmias.

Phosphorylation of the ryanodine receptor 2 at serine 2030 is required for a complete β-adrenergic response

Phosphorylation is thought to play a role in modulation of the ryanodine receptor 2 channel. Using a S2030A knock-in mouse model, Potenza et al. reveal that phosphorylation of RyR2-S2030 mediates channel regulation during the β-adrenergic response.

During physical exercise or stress, the sympathetic system stimulates cardiac contractility via β-adrenergic receptor (β-AR) activation, resulting in protein kinase A (PKA)–mediated phosphorylation of the cardiac ryanodine receptor RyR2. PKA-dependent “hyperphosphorylation” of the RyR2 channel has been proposed as a major impairment that contributes to progression of heart failure. However, the sites of PKA phosphorylation and their phosphorylation status in cardiac diseases are not well defined. Among the known RyR2 phosphorylation sites, serine 2030 (S2030) remains highly controversial as a site of functional impact. We examined the contribution of RyR2-S2030 to Ca²⁺ signaling and excitation–contraction coupling (ECC) in a transgenic mouse with an ablated RyR2-S2030 phosphorylation site (RyR2-S2030A^+/+). We assessed ECC gain by using whole-cell patch–clamp recordings and confocal Ca²⁺ imaging during β-ARs stimulation with isoproterenol (Iso) and consistent SR Ca²⁺ loading and L-type Ca²⁺ current (I_Ca) triggering. Under these conditions, ECC gain is diminished in mutant compared with WT cardiomyocytes. Resting Ca²⁺ spark frequency (CaSpF) with Iso is also reduced by mutation of S2030. In permeabilized cells, when SR Ca²⁺ pump activity is kept constant (using 2D12 antibody against phospholamban), cAMP does not change CaSpF in S2030A^+/+ myocytes. Using Ca²⁺ spark recovery analysis, we found that mutant RyR Ca²⁺ sensitivity is not enhanced by Iso application, contrary to WT RyRs. Furthermore, ablation of RyR2-S2030 prevents acceleration of Ca²⁺ waves and increases latency to the first spontaneous Ca²⁺ release after a train of stimulations during Iso treatment. Together, these results suggest that phosphorylation at S2030 may represent an important step in the modulation of RyR2 activity during β-adrenergic stimulation and a potential target for the development of new antiarrhythmic drugs.

Modulation of Cardiac Alternans by Altered Sarcoplasmic Reticulum Calcium Release: A Simulation Study

Background: Cardiac alternans is an important precursor to arrhythmia, facilitating formation of conduction block, and re-entry. Diseased hearts were observed to be particularly vulnerable to alternans, mainly in heart failure or after myocardial infarction. Alternans is typically linked to oscillation of calcium cycling, particularly in the sarcoplasmic reticulum (SR). While the role of SR calcium reuptake in alternans is well established, the role of altered calcium release by ryanodine receptors has not yet been studied extensively. At the same time, there is strong evidence that calcium release is abnormal in heart failure and other heart diseases, suggesting that these changes might play a pro-alternans role.

Aims: To demonstrate how changes to intracellular calcium release dynamics and magnitude affect alternans vulnerability.

Methods: We used the state-of-the-art Heijman–Rudy and O’Hara–Rudy computer models of ventricular myocyte, given their detailed representation of calcium handling and their previous utility in alternans research. We modified the models to obtain precise control over SR release dynamics and magnitude, allowing for the evaluation of these properties in alternans formation and suppression.

Results: Shorter time to peak SR release and shorter release duration decrease alternans vulnerability by improved refilling of releasable calcium within junctional SR; conversely, slow release promotes alternans. Modulating the total amount of calcium released, we show that sufficiently increased calcium release may surprisingly prevent alternans via a mechanism linked to the functional depletion of junctional SR during release. We show that this mechanism underlies differences between “eye-type” and “fork-type” alternans, which were observed in human in vivo and in silico. We also provide a detailed explanation of alternans formation in the given computer models, termed “sarcoplasmic reticulum calcium cycling refractoriness.” The mechanism relies on the steep SR load–release relationship, combined with relatively limited rate of junctional SR refilling.

Conclusion: Both altered dynamics and magnitude of SR calcium release modulate alternans vulnerability. In particular, slow dynamics of SR release, such as those observed in heart failure, promote alternans. Therefore, acceleration of intracellular calcium release, e.g., via synchronization of calcium sparks, may inhibit alternans in failing hearts and reduce arrhythmia occurrence.

NCX-Mediated Subcellular Ca2+ Dynamics Underlying Early Afterdepolarizations in LQT2 Cardiomyocytes

Long QT syndrome type 2 (LQT2) is a congenital disease characterized by loss of function mutations in hERG potassium channels (I_Kr). LQT2 is associated with fatal ventricular arrhythmias promoted by triggered activity in the form of early afterdepolarizations (EADs). We previously demonstrated that intracellular Ca²⁺ handling is remodeled in LQT2 myocytes. Remodeling leads to aberrant late RyR-mediated Ca²⁺ releases that drive forward-mode Na⁺-Ca²⁺ exchanger (NCX) current and slow repolarization to promote reopening of L-type calcium channels and EADs. Forward-mode NCX was found to be enhanced despite the fact that these late releases do not significantly alter the whole-cell cytosolic calcium concentration during a vulnerable period of phase 2 of the action potential corresponding to the onset of EADs. Here, we use a multiscale ventricular myocyte model to explain this finding. We show that because the local NCX current is a saturating nonlinear function of the local submembrane calcium concentration, a larger number of smaller-amplitude discrete Ca²⁺ release events can produce a large increase in whole-cell forward-mode NCX current without increasing significantly the whole-cell cytosolic calcium concentration. Furthermore, we develop novel insights, to our knowledge, into how alterations of stochastic RyR activity at the single-channel level cause late aberrant Ca²⁺ release events. Experimental measurements in transgenic LTQ2 rabbits confirm the critical arrhythmogenic role of NCX and identify this current as a potential target for antiarrhythmic therapies in LQT2.

Gene Transfer of Engineered Calmodulin Alleviates Ventricular Arrhythmias in a Calsequestrin-Associated Mouse Model of Catecholaminergic Polymorphic Ventricular Tachycardia

Background

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a familial arrhythmogenic syndrome characterized by sudden death. There are several genetic forms of CPVT associated with mutations in genes encoding the cardiac ryanodine receptor (RyR2) and its auxiliary proteins including calsequestrin (CASQ2) and calmodulin (CaM). It has been suggested that impairment of the ability of RyR2 to stay closed (ie, refractory) during diastole may be a common mechanism for these diseases. Here, we explore the possibility of engineering CaM variants that normalize abbreviated RyR2 refractoriness for subsequent viral‐mediated delivery to alleviate arrhythmias in non–CaM‐related CPVT.

Methods and Results

To that end, we have designed a CaM protein (GSH‐M37Q; dubbed as therapeutic CaM or T‐CaM) that exhibited a slowed N‐terminal Ca dissociation rate and prolonged RyR2 refractoriness in permeabilized myocytes derived from CPVT mice carrying the CASQ2 mutation R33Q. This T‐CaM was introduced to the heart of R33Q mice through recombinant adeno‐associated viral vector serotype 9. Eight weeks postinfection, we performed confocal microscopy to assess Ca handling and recorded surface ECGs to assess susceptibility to arrhythmias in vivo. During catecholamine stimulation with isoproterenol, T‐CaM reduced isoproterenol‐promoted diastolic Ca waves in isolated CPVT cardiomyocytes. Importantly, T‐CaM exposure abolished ventricular tachycardia in CPVT mice challenged with catecholamines.

Conclusions

Our results suggest that gene transfer of T‐CaM by adeno‐associated viral vector serotype 9 improves myocyte Ca handling and alleviates arrhythmias in a calsequestrin‐associated CPVT model, thus supporting the potential of a CaM‐based antiarrhythmic approach as a therapeutic avenue for genetically distinct forms of CPVT.

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

Aims

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

Methods and results

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

Conclusions

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

Ambiguous interactions between diastolic and SR Ca2+ in the regulation of cardiac Ca2+ release

Sobie et al. highlight unresolved issues concerning the regulation of sarcoplasmic reticulum calcium release in cardiac myocytes.

The role of luminal Ca regulation in Ca signaling refractoriness and cardiac arrhythmogenesis

Györke et al. discuss the role of sarcoplasmic reticulum Ca2+ in cardiac refractoriness and pathological implications.

The role of spatial organization of Ca2+ release sites in the generation of arrhythmogenic diastolic Ca2+ release in myocytes from failing hearts

In heart failure (HF), dysregulated cardiac ryanodine receptors (RyR2) contribute to the generation of diastolic Ca2+ waves (DCWs), thereby predisposing adrenergically stressed failing hearts to life-threatening arrhythmias. However, the specific cellular, subcellular, and molecular defects that account for cardiac arrhythmia in HF remain to be elucidated. Patch-clamp techniques and confocal Ca2+ imaging were applied to study spatially defined Ca2+ handling in ventricular myocytes isolated from normal (control) and failing canine hearts. Based on their activation time upon electrical stimulation, Ca2+ release sites were categorized as coupled, located in close proximity to the sarcolemmal Ca2+ channels, and uncoupled, the Ca2+ channel-free non-junctional Ca2+ release units. In control myocytes, stimulation of β-adrenergic receptors with isoproterenol (Iso) resulted in a preferential increase in Ca2+ spark rate at uncoupled sites. This site-specific effect of Iso was eliminated by the phosphatase inhibitor okadaic acid, which caused similar facilitation of Ca2+ sparks at coupled and uncoupled sites. Iso-challenged HF myocytes exhibited increased predisposition to DCWs compared to control myocytes. In addition, the overall frequency of Ca2+ sparks was increased in HF cells due to preferential stimulation of coupled sites. Furthermore, coupled sites exhibited accelerated recovery from functional refractoriness in HF myocytes compared to control myocytes. Spatially resolved subcellular Ca2+ mapping revealed that DCWs predominantly originated from coupled sites. Inhibition of CaMKII suppressed DCWs and prevented preferential stimulation of coupled sites in Iso-challenged HF myocytes. These results suggest that CaMKII- (and phosphatase)-dependent dysregulation of junctional Ca2+ release sites contributes to Ca2+-dependent arrhythmogenesis in HF.

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

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

Stochastic initiation and termination of calcium-mediated triggered activity in cardiac myocytes

Cardiac myocytes normally initiate action potentials in response to a current stimulus that depolarizes the membrane above an excitation threshold. Aberrant excitation can also occur due to spontaneous calcium (Ca²⁺) release (SCR) from intracellular stores after the end of a preceding action potential. SCR drives the Na⁺/Ca²⁺ exchange current inducing a "delayed afterdepolarization" that can in turn trigger an action potential if the excitation threshold is reached. This "triggered activity" is known to cause arrhythmias, but how it is initiated and terminated is not understood. Using computer simulations of a ventricular myocyte model, we show that initiation and termination are inherently random events. We determine the probability of those events from statistical measurements of the number of beats before initiation and before termination, respectively, which follow geometric distributions. Moreover, we elucidate the origin of randomness by a statistical analysis of SCR events, which do not follow a Poisson process observed in other eukaryotic cells. Due to synchronization of Ca²⁺ releases during the action potential upstroke, waiting times of SCR events after the upstroke are narrowly distributed, whereas SCR amplitudes follow a broad normal distribution with a width determined by fluctuations in the number of independent Ca²⁺ wave foci. This distribution enables us to compute the probabilities of initiation and termination of bursts of triggered activity that are maintained by a positive feedback between the action potential upstroke and SCR. Our results establish a theoretical framework for interpreting complex and varied manifestations of triggered activity relevant to cardiac arrhythmias.

A unified theory of calcium alternans in ventricular myocytes

Intracellular calcium (Ca²⁺) alternans is a dynamical phenomenon in ventricular myocytes, which is linked to the genesis of lethal arrhythmias. Iterated map models of intracellular Ca²⁺ cycling dynamics in ventricular myocytes under periodic pacing have been developed to study the mechanisms of Ca²⁺ alternans. Two mechanisms of Ca²⁺ alternans have been demonstrated in these models: one relies mainly on fractional sarcoplasmic reticulum Ca²⁺ release and uptake, and the other on refractoriness and other properties of Ca²⁺ sparks. Each of the two mechanisms can partially explain the experimental observations, but both have their inconsistencies with the experimental results. Here we developed an iterated map model that is composed of two coupled iterated maps, which unifies the two mechanisms into a single cohesive mathematical framework. The unified theory can consistently explain the seemingly contradictory experimental observations and shows that the two mechanisms work synergistically to promote Ca²⁺ alternans. Predictions of the theory were examined in a physiologically-detailed spatial Ca²⁺ cycling model of ventricular myocytes.

There and back again: Iterating between population-based modeling and experiments reveals surprising regulation of calcium transients in rat cardiac myocytes

While many ion channels and transporters involved in cardiac cellular physiology have been identified and described, the relative importance of each in determining emergent cellular behaviors remains unclear. Here we address this issue with a novel approach that combines population-based mathematical modeling with experimental tests to systematically quantify the relative contributions of different ion channels and transporters to the amplitude of the cellular Ca(2+) transient. Sensitivity analysis of a mathematical model of the rat ventricular cardiomyocyte quantified the response of cell behaviors to changes in the level of each ion channel and transporter, and experimental tests of these predictions were performed to validate or invalidate the predictions. The model analysis found that partial inhibition of the transient outward current in rat ventricular epicardial myocytes was predicted to have a greater impact on Ca(2+) transient amplitude than either: (1) inhibition of the same current in endocardial myocytes, or (2) comparable inhibition of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA). Experimental tests confirmed the model predictions qualitatively but showed some quantitative disagreement. This guided us to recalibrate the model by adjusting the relative importance of several Ca(2+) fluxes, thereby improving the consistency with experimental data and producing a more predictive model. Analysis of human cardiomyocyte models suggests that the relative importance of outward currents to Ca(2+) transporters is generalizable to human atrial cardiomyocytes, but not ventricular cardiomyocytes. Overall, our novel approach of combining population-based mathematical modeling with experimental tests has yielded new insight into the relative importance of different determinants of cell behavior.

What is a Ca(2+) wave? Is it like an Electrical Wave?

Arrhythmia subcellular mechanisms are constantly being explored. Recent knowledge has shown that travelling Ca(2+) waves in cardiac cells are critical for delayed afterdepolarisations and in some cases, early afterdepolarisations. In this review, we comment on the properties of cardiac Ca(2+) waves and abnormal Ca(2+) releases in terms of properties used to describe electrical waves; propagation, excitability and refractoriness.

Calcium-voltage coupling in the genesis of early and delayed afterdepolarizations in cardiac myocytes

Early afterdepolarizations (EADs) and delayed afterdepolarizations (DADs) are voltage oscillations known to cause cardiac arrhythmias. EADs are mainly driven by voltage oscillations in the repolarizing phase of the action potential (AP), while DADs are driven by spontaneous calcium (Ca) release during diastole. Because voltage and Ca are bidirectionally coupled, they modulate each other's behaviors, and new AP and Ca cycling dynamics can emerge from this coupling. In this study, we performed computer simulations using an AP model with detailed spatiotemporal Ca cycling incorporating stochastic openings of Ca channels and ryanodine receptors to investigate the effects of Ca-voltage coupling on EAD and DAD dynamics. Simulations were complemented by experiments in mouse ventricular myocytes. We show that: 1) alteration of the Ca transient due to increased ryanodine receptor leakiness and/or sarco/endoplasmic reticulum Ca ATPase activity can either promote or suppress EADs due to the complex effects of Ca on ionic current properties; 2) spontaneous Ca waves also exhibit complex effects on EADs, but cannot induce EADs of significant amplitude without the participation of ICa,L; 3) lengthening AP duration and the occurrence of EADs promote DADs by increasing intracellular Ca loading, and two mechanisms of DADs are identified, i.e., Ca-wave-dependent and Ca-wave-independent; and 4) Ca-voltage coupling promotes complex EAD patterns such as EAD alternans that are not observed for solely voltage-driven EADs. In conclusion, Ca-voltage coupling combined with the nonlinear dynamical behaviors of voltage and Ca cycling play a key role in generating complex EAD and DAD dynamics observed experimentally in cardiac myocytes, whose mechanisms are complex but analyzable.

Biphasic decay of the Ca transient results from increased sarcoplasmic reticulum Ca leak

Key points

Ca leak from the sarcoplasmic reticulum through the ryanodine receptor (RyR) reduces the amplitude of the Ca transient and slows its rate of decay.

In the presence of β‐adrenergic stimulation, RyR‐mediated Ca leak produces a biphasic decay of the Ca transient with a fast early phase and a slow late phase.

Two forms of Ca leak have been studied, Ca‐sensitising (induced by caffeine) and non‐sensitising (induced by ryanodine) and both induce biphasic decay of the Ca transient.

Only Ca‐sensitising leak can be reversed by traditional RyR inhibitors such as tetracaine.

Ca leak can also induce Ca waves. At low levels of leak, waves occur. As leak is increased, first biphasic decay and then slowed monophasic decay is seen. The level of leak has major effects on the shape of the Ca transient.

Abstract

In heart failure, a reduction in Ca transient amplitude and contractile dysfunction can by caused by Ca leak through the sarcoplasmic reticulum (SR) Ca channel (ryanodine receptor, RyR) and/or decreased activity of the SR Ca ATPase (SERCA). We have characterised the effects of two forms of Ca leak (Ca‐sensitising and non‐sensitising) on calcium cycling and compared with those of SERCA inhibition. We measured [Ca²⁺]_i with fluo‐3 in voltage‐clamped rat ventricular myocytes. Increasing SR leak with either caffeine (to sensitise the RyR to Ca activation) or ryanodine (non‐sensitising) had similar effects to SERCA inhibition: decreased systolic [Ca²⁺]_i, increased diastolic [Ca²⁺]_i and slowed decay. However, in the presence of isoproterenol, leak produced a biphasic decay of the Ca transient in the majority of cells while SERCA inhibition produced monophasic decay. Tetracaine reversed the effects of caffeine but not of ryanodine. When caffeine (1 mmol l⁻¹) was added to a cell which displayed Ca waves, the wave frequency initially increased before waves disappeared and biphasic decay developed. Eventually (at higher caffeine concentrations), the biphasic decay was replaced by slow decay. We conclude that, in the presence of adrenergic stimulation, Ca leak can produce biphasic decay; the slow phase results from the leak opposing Ca uptake by SERCA. The degree of leak determines whether decay of Ca waves, biphasic or monophasic, occurs.

Ca leak from the sarcoplasmic reticulum through the ryanodine receptor (RyR) reduces the amplitude of the Ca transient and slows its rate of decay.

In the presence of β‐adrenergic stimulation, RyR‐mediated Ca leak produces a biphasic decay of the Ca transient with a fast early phase and a slow late phase.

Two forms of Ca leak have been studied, Ca‐sensitising (induced by caffeine) and non‐sensitising (induced by ryanodine) and both induce biphasic decay of the Ca transient.

Only Ca‐sensitising leak can be reversed by traditional RyR inhibitors such as tetracaine.

Ca leak can also induce Ca waves. At low levels of leak, waves occur. As leak is increased, first biphasic decay and then slowed monophasic decay is seen. The level of leak has major effects on the shape of the Ca transient.

Ryanodine receptor phosphorylation by CaMKII promotes spontaneous Ca(2+) release events in a rodent model of early stage diabetes: The arrhythmogenic substrate

BACKGROUND

Heart failure and arrhythmias occur more frequently in patients with type 2 diabetes (T2DM) than in the general population. T2DM is preceded by a prediabetic condition marked by elevated reactive oxygen species (ROS) and subclinical cardiovascular defects. Although multifunctional Ca2+ calmodulin-dependent protein kinase II (CaMKII) is ROS-activated and CaMKII hyperactivity promotes cardiac diseases, a link between prediabetes and CaMKII in the heart is unprecedented.

OBJECTIVES

To prove the hypothesis that increased ROS and CaMKII activity contribute to heart failure and arrhythmogenic mechanisms in early stage diabetes.

METHODS-RESULTS

Echocardiography, electrocardiography, biochemical and intracellular Ca2+ (Ca2+i) determinations were performed in fructose-rich diet-induced impaired glucose tolerance, a prediabetes model, in rodents. Fructose-rich diet rats showed decreased contractility and hypertrophy associated with increased CaMKII activity, ROS production, oxidized CaMKII and enhanced CaMKII-dependent ryanodine receptor (RyR2) phosphorylation compared to rats fed with control diet. Isolated cardiomyocytes from fructose-rich diet showed increased spontaneous Ca2+i release events associated with spontaneous contractions, which were prevented by KN-93, a CaMKII inhibitor, or addition of Tempol, a ROS scavenger, to the diet. Moreover, fructose-rich diet myocytes showed increased diastolic Ca2+ during the burst of spontaneous Ca2+i release events. Mice treated with Tempol or with sarcoplasmic reticulum-targeted CaMKII-inhibition by transgenic expression of the CaMKII inhibitory peptide AIP, were protected from fructose-rich diet-induced spontaneous Ca2+i release events, spontaneous contractions and arrhythmogenesis in vivo, despite ROS increases.

CONCLUSIONS

RyR2 phosphorylation by ROS-activated CaMKII, contributes to impaired glucose tolerance-induced arrhythmogenic mechanisms, suggesting that CaMKII inhibition could prevent prediabetic cardiovascular complications and/or evolution.

R4496C RyR2 mutation impairs atrial and ventricular contractility

A ryanodine receptor 2 mutation associated with catecholaminergic polymorphic ventricular tachycardia renders cardiomyocytes incapable of mediating a positive inotropic response.

Ryanodine receptor (RyR2) is the major Ca²⁺ channel of the cardiac sarcoplasmic reticulum (SR) and plays a crucial role in the generation of myocardial force. Changes in RyR2 gating properties and resulting increases in its open probability (P_o) are associated with Ca²⁺ leakage from the SR and arrhythmias; however, the effects of RyR2 dysfunction on myocardial contractility are unknown. Here, we investigated the possibility that a RyR2 mutation associated with catecholaminergic polymorphic ventricular tachycardia, R4496C, affects the contractile function of atrial and ventricular myocardium. We measured isometric twitch tension in left ventricular and atrial trabeculae from wild-type mice and heterozygous transgenic mice carrying the R4496C RyR2 mutation and found that twitch force was comparable under baseline conditions (30°C, 2 mM [Ca²⁺]_o, 1 Hz). However, the positive inotropic responses to high stimulation frequency, 0.1 µM isoproterenol, and 5 mM [Ca²⁺]_o were decreased in R4496C trabeculae, as was post-rest potentiation. We investigated the mechanisms underlying inotropic insufficiency in R4496C muscles in single ventricular myocytes. Under baseline conditions, the amplitude of the Ca²⁺ transient was normal, despite the reduced SR Ca²⁺ content. Under inotropic challenge, however, R4496C myocytes were unable to boost the amplitude of Ca²⁺ transients because they are incapable of properly increasing the amount of Ca²⁺ stored in the SR because of a larger SR Ca²⁺ leakage. Recovery of force in response to premature stimuli was faster in R4496C myocardium, despite the unchanged rates of recovery of L-type Ca²⁺ channel current (I_Ca-L) and SR Ca²⁺ content in single myocytes. A faster recovery from inactivation of the mutant R4496C channels could explain this behavior. In conclusion, changes in RyR2 channel gating associated with the R4496C mutation could be directly responsible for the alterations in both ventricular and atrial contractility. The increased RyR2 P_o and fractional Ca²⁺ release from the SR induced by the R4496C mutation preserves baseline contractility despite a slight decrease in SR Ca²⁺ content, but cannot compensate for the inability to increase SR Ca²⁺ content during inotropic challenge.

Ryanodine Receptor Activation Induces Long-Term Plasticity of Spine Calcium Dynamics

A key feature of signalling in dendritic spines is the synapse-specific transduction of short electrical signals into biochemical responses. Ca²⁺ is a major upstream effector in this transduction cascade, serving both as a depolarising electrical charge carrier at the membrane and an intracellular second messenger. Upon action potential firing, the majority of spines are subject to global back-propagating action potential (bAP) Ca²⁺ transients. These transients translate neuronal suprathreshold activation into intracellular biochemical events. Using a combination of electrophysiology, two-photon Ca²⁺ imaging, and modelling, we demonstrate that bAPs are electrochemically coupled to Ca²⁺ release from intracellular stores via ryanodine receptors (RyRs). We describe a new function mediated by spine RyRs: the activity-dependent long-term enhancement of the bAP-Ca²⁺ transient. Spines regulate bAP Ca²⁺ influx independent of each other, as bAP-Ca²⁺ transient enhancement is compartmentalized and independent of the dendritic Ca²⁺ transient. Furthermore, this functional state change depends exclusively on bAPs travelling antidromically into dendrites and spines. Induction, but not expression, of bAP-Ca²⁺ transient enhancement is a spine-specific function of the RyR. We demonstrate that RyRs can form specific Ca²⁺ signalling nanodomains within single spines. Functionally, RyR mediated Ca²⁺ release in these nanodomains induces a new form of Ca²⁺ transient plasticity that constitutes a spine specific storage mechanism of neuronal suprathreshold activity patterns.

A combination of two-photon calcium imaging, electrophysiology, and modelling shows how ryanodine receptors (a type of intracellular calcium channel) generate a signalling nanodomain within individual dendritic spines, enabling compartmentalized plasticity of calcium dynamics.

Experiences change neuronal circuits, and these circuit changes outlast the initial experiences. This means that, in neurons, the fast electrical activity encoding experiences needs to be transduced into longer-lived biochemical and structural changes. A key mediator between these two timescales of neuronal activity is the Ca²⁺ ion. Ca²⁺ serves both as an electric charge carrier mediating fast voltage changes at the membrane and as a second messenger activating intracellular signalling cascades. Even within the spatial confines of dendritic spines, the specialized domains of dendrites that receive synaptic connections, Ca²⁺ encodes a versatile array of specific functions. In this study, we first demonstrate that voltage-gated Ca²⁺ channels and ryanodine receptors, intracellular channels located on the membrane of the endoplasmic reticulum through which Ca²⁺ can be released into the cytosol, are electrochemically coupled in single dendritic spines. We identify how ryanodine receptors induce enhancement of the Ca²⁺ influx, mediated by the opening of voltage-gated Ca²⁺ channels, induced by action potentials in a compartmentalized, spine-specific manner. Within the femtoliter volume of a single spine, specificity of this route of Ca²⁺-signalling is achieved by a signalling nanodomain centred on the ryanodine receptor. Our work stresses the role of the ryanodine receptor not only as an ion channel releasing Ca²⁺ from the endoplasmic reticulum but also as a macromolecular complex generating specificity of Ca²⁺-signalling within the spatial constraints of a single spine.

Cytosolic Ca²⁺ buffering determines the intra-SR Ca²⁺ concentration at which cardiac Ca²⁺ sparks terminate

Single ryanodine receptor (RyR) Ca(2+) flux amplitude (i(Ca-RyR)) decreases as intra-sarcoplasmic reticulum (SR) Ca(2+) levels fall during a cardiac Ca(2+) spark. Since i(Ca-RyR) drives the inter-RyR Ca(2+)-induced Ca(2+) release (CICR) that underlies the spark, decreasing i(Ca-RyR) may contribute to spark termination because RyRs that spontaneously close may stay closed. To test this possibility, we simultaneously measured local cytosolic and intra-SR ([Ca(2+)]cyto and [Ca(2+)]SR) during Ca(2+) sparks in permeabilized rabbit ventricular myocytes. Local cytosolic or intra-SR Ca(2+) dynamics were manipulated using Ca(2+) buffers. Buffer manipulations applied in cells had no effect on individual RyR channels reconstituted in planar lipid bilayers. Presence of a fast cytosolic Ca(2+) buffer (BAPTA) significantly suppressed Ca(2+) spark activity and sparks terminated earlier at a higher than usual [Ca(2+)]SR level (∼80% vs. ∼62%). When cytosolic Ca(2+) buffer power was reduced (i.e. cytosolic EGTA level decreased), sparks terminated later and at a lower than usual [Ca(2+)]SR level (∼45% vs. ∼62%). When intra-SR Ca(2+) buffer power was increased, sparks also terminated later and at a lower than usual [Ca(2+)]SR (∼48% vs. ∼62%). These results suggest that cytosolic local control of inter-RyR CICR by i(Ca-RyR) plays a substantial role during the spark termination process. Thus, alterations in local cytosolic Ca(2+) handling dynamics in the dyadic cleft (Ca(2+) buffering, extrusion, etc.) likely influence Ca(2+) spark termination.

Cardiac alternans and intracellular calcium cycling

Cardiac alternans refers to a condition in which there is a periodic beat-to-beat oscillation in electrical activity and the strength of cardiac muscle contraction at a constant heart rate. Clinically, cardiac alternans occurs in settings that are typical for cardiac arrhythmias and has been causally linked to these conditions. At the cellular level, alternans is defined as beat-to-beat alternations in contraction amplitude (mechanical alternans), action potential duration (APD; electrical or APD alternans) and Ca(2+) transient amplitude (Ca(2+) alternans). The cause of alternans is multifactorial; however, alternans always originate from disturbances of the bidirectional coupling between membrane voltage (Vm ) and intracellular calcium ([Ca(2+) ]i ). Bidirectional coupling refers to the fact that, in cardiac cells, Vm depolarization and the generation of action potentials cause the elevation of [Ca(2+) ]i that is required for contraction (a process referred to as excitation-contraction coupling); conversely, changes of [Ca(2+) ]i control Vm because important membrane currents are Ca(2+) dependent. Evidence is mounting that alternans is ultimately caused by disturbances of cellular Ca(2+) signalling. Herein we review how two key factors of cardiac cellular Ca(2+) cycling, namely the release of Ca(2+) from internal stores and the capability of clearing the cytosol from Ca(2+) after each beat, determine the conditions under which alternans occurs. The contributions from key Ca(2+) -handling proteins (i.e. surface membrane channels, ion pumps and transporters and internal Ca(2+) release channels) are discussed.

Cardiac myocyte alternans in intact heart: Influence of cell-cell coupling and β-adrenergic stimulation

BACKGROUND

Cardiac alternans are proarrhythmic and mechanistically link cardiac mechanical dysfunction and sudden cardiac death. Beat-to-beat alternans occur when beats with large Ca(2+) transients and long action potential duration (APD) alternate with the converse. APD alternans are typically driven by Ca(2+) alternans and sarcoplasmic reticulum (SR) Ca(2+) release alternans. But the effect of intercellular communication via gap junctions (GJ) on alternans in the intact heart remains unknown.

OBJECTIVE

We assessed the effects of cell-to-cell coupling on local alternans in intact Langendorff-perfused mouse hearts, measuring single myocyte [Ca(2+)] alternans synchronization among neighboring cells, and effects of β-adrenergic receptor (β-AR) activation and reduced GJ coupling.

METHODS AND RESULTS

Mouse hearts (C57BL/6) were retrogradely perfused and loaded with Fluo8-AM to record cardiac myocyte [Ca(2+)] in situ with confocal microscopy. Single cell resolution allowed analysis of alternans within the intact organ during alternans induction. Carbenoxolone (25 μM), a GJ inhibitor, significantly increased the occurrence and amplitude of alternans in single cells within the intact heart. Alternans were concordant between neighboring cells throughout the field of view, except transiently during onset. β-AR stimulation only reduced Ca(2+) alternans in tissue that had reduced GJ coupling, matching effects seen in isolated myocytes.

CONCLUSIONS

Ca(2+) alternans among neighboring myocytes is predominantly concordant, likely because of electrical coupling between cells. Consistent with this, partial GJ uncoupling increased propensity and amplitude of Ca(2+) alternans, and made them more sensitive to reversal by β-AR activation, as in isolated myocytes. Electrical coupling between myocytes may thus limit the alternans initiation, but also allow alternans to be more stable once established.

Calcium homeostasis in a local/global whole cell model of permeabilized ventricular myocytes with a Langevin description of stochastic calcium release

Population density approaches to modeling local control of Ca2+-induced Ca2+ release in cardiac myocytes can be used to construct minimal whole cell models that accurately represent heterogeneous local Ca2+ signals. Unfortunately, the computational complexity of such “local/global” whole cell models scales with the number of Ca2+ release unit (CaRU) states, which is a rapidly increasing function of the number of ryanodine receptors (RyRs) per CaRU. Here we present an alternative approach based on a Langevin description of the collective gating of RyRs coupled by local Ca2+ concentration ([Ca2+]). The computational efficiency of this approach no longer depends on the number of RyRs per CaRU. When the RyR model is minimal, Langevin equations may be replaced by a single Fokker-Planck equation, yielding an extremely compact and efficient local/global whole cell model that reproduces and helps interpret recent experiments that investigate Ca2+ homeostasis in permeabilized ventricular myocytes. Our calculations show that elevated myoplasmic [Ca2+] promotes elevated network sarcoplasmic reticulum (SR) [Ca2+] via SR Ca2+-ATPase-mediated Ca2+ uptake. However, elevated myoplasmic [Ca2+] may also activate RyRs and promote stochastic SR Ca2+ release, which can in turn decrease SR [Ca2+]. Increasing myoplasmic [Ca2+] results in an exponential increase in spark-mediated release and a linear increase in nonspark-mediated release, consistent with recent experiments. The model exhibits two steady-state release fluxes for the same network SR [Ca2+] depending on whether myoplasmic [Ca2+] is low or high. In the later case, spontaneous release decreases SR [Ca2+] in a manner that maintains robust Ca2+ sparks.

Nonlinear and Stochastic Dynamics in the Heart

In a normal human life span, the heart beats about 2 to 3 billion times. Under diseased conditions, a heart may lose its normal rhythm and degenerate suddenly into much faster and irregular rhythms, called arrhythmias, which may lead to sudden death. The transition from a normal rhythm to an arrhythmia is a transition from regular electrical wave conduction to irregular or turbulent wave conduction in the heart, and thus this medical problem is also a problem of physics and mathematics. In the last century, clinical, experimental, and theoretical studies have shown that dynamical theories play fundamental roles in understanding the mechanisms of the genesis of the normal heart rhythm as well as lethal arrhythmias. In this article, we summarize in detail the nonlinear and stochastic dynamics occurring in the heart and their links to normal cardiac functions and arrhythmias, providing a holistic view through integrating dynamics from the molecular (microscopic) scale, to the organelle (mesoscopic) scale, to the cellular, tissue, and organ (macroscopic) scales. We discuss what existing problems and challenges are waiting to be solved and how multi-scale mathematical modeling and nonlinear dynamics may be helpful for solving these problems.

Maximal acceleration of Ca2+ release refractoriness by β-adrenergic stimulation requires dual activation of kinases PKA and CaMKII in mouse ventricular myocytes

KEY POINTS

Refractoriness of calcium release in heart cells is altered in several disease states, but the physiological mechanisms that regulate this process are incompletely understood. We examined refractoriness of calcium release in mouse ventricular myocytes and investigated how activation of different intracellular signalling pathways influenced this process. We found that refractoriness of calcium release is abbreviated by stimulation of the 'fight-or-flight' response, and that simultaneous activation of multiple intracellular signalling pathways contributes to this response. Data obtained under several conditions at the subcellular, microscopic level were consistent with results obtained at the cellular level. The results provide insight into regulation of cardiac calcium release and how alterations to this process may increase arrhythmia risk under different conditions.

ABSTRACT

Time-dependent refractoriness of calcium (Ca(2+)) release in cardiac myocytes is an important factor in determining whether pro-arrhythmic release patterns develop. At the subcellular level of the Ca(2+) spark, recent studies have suggested that recovery of spark amplitude is controlled by local sarcoplasmic reticulum (SR) refilling whereas refractoriness of spark triggering depends on both refilling and the sensitivity of the ryanodine receptor (RyR) release channels that produce sparks. Here we studied regulation of Ca(2+) spark refractoriness in mouse ventricular myocytes by examining how β-adrenergic stimulation influenced sequences of Ca(2+) sparks originating from individual RyR clusters. Our protocol allowed us to separately measure recovery of spark amplitude and delays between successive sparks, and data were interpreted quantitatively through simulations with a stochastic mathematical model. We found that, compared with spark sequences measured under control conditions: (1) β-adrenergic stimulation with isoproterenol (isoprenaline) accelerated spark amplitude recovery and decreased spark-to-spark delays; (2) activating protein kinase A (PKA) with forskolin accelerated amplitude recovery but did not affect spark-to-spark delays; (3) inhibiting PKA with H89 retarded amplitude recovery and increased spark-to-spark delays; (4) preventing phosphorylation of the RyR at serine 2808 with a knock-in mouse prevented the decrease in spark-to-spark delays seen with β-adrenergic stimulation; (5) inhibiting either PKA or Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) during β-adrenergic stimulation prevented the decrease in spark-to-spark delays seen without inhibition. The results suggest that activation of either PKA or CaMKII is sufficient to speed SR refilling, but activation of both kinases appears necessary to observe increased RyR sensitivity. The data provide novel insight into β-adrenergic regulation of Ca(2+) release refractoriness in mouse myocytes.

Life and death of a cardiac calcium spark

Calcium sparks in cardiac myocytes are brief, localized calcium releases from the sarcoplasmic reticulum (SR) believed to be caused by locally regenerative calcium-induced calcium release (CICR) via couplons, clusters of ryanodine receptors (RyRs). How such regeneration is terminated is uncertain. We performed numerical simulations of an idealized stochastic model of spark production, assuming a RyR gating scheme with only two states (open and closed). Local depletion of calcium in the SR was inevitable during a spark, and this could terminate sparks by interrupting CICR, with or without assumed modulation of RyR gating by SR lumenal calcium. Spark termination by local SR depletion was not robust: under some conditions, sparks could be greatly and variably prolonged, terminating by stochastic attrition-a phenomenon we dub "spark metastability." Spark fluorescence rise time was not a good surrogate for the duration of calcium release. Using a highly simplified, deterministic model of the dynamics of a couplon, we show that spark metastability depends on the kinetic relationship of RyR gating and junctional SR refilling rates. The conditions for spark metastability resemble those produced by known mutations of RyR2 and CASQ2 that cause life-threatening triggered arrhythmias, and spark metastability may be mitigated by altering the kinetics of the RyR in a manner similar to the effects of drugs known to prevent those arrhythmias. The model was unable to explain the distributions of spark amplitudes and rise times seen in chemically skinned cat atrial myocytes, suggesting that such sparks may be more complex events involving heterogeneity of couplons or local propagation among sub-clusters of RyRs.

Hierarchical clustering of ryanodine receptors enables emergence of a calcium clock in sinoatrial node cells

The sinoatrial node, whose cells (sinoatrial node cells [SANCs]) generate rhythmic action potentials, is the primary pacemaker of the heart. During diastole, calcium released from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) interacts with membrane currents to control the rate of the heartbeat. This "calcium clock" takes the form of stochastic, partially periodic, localized calcium release (LCR) events that propagate, wave-like, for limited distances. The detailed mechanisms controlling the calcium clock are not understood. We constructed a computational model of SANCs, including three-dimensional diffusion and buffering of calcium in the cytosol and SR; explicit, stochastic gating of individual RyRs and L-type calcium channels; and a full complement of voltage- and calcium-dependent membrane currents. We did not include an anatomical submembrane space or inactivation of RyRs, the two heuristic components that have been used in prior models but are not observed experimentally. When RyRs were distributed in discrete clusters separated by >1 µm, only isolated sparks were produced in this model and LCR events did not form. However, immunofluorescent staining of SANCs for RyR revealed the presence of bridging RyR groups between large clusters, forming an irregular network. Incorporation of this architecture into the model led to the generation of propagating LCR events. Partial periodicity emerged from the interaction of LCR events, as observed experimentally. This calcium clock becomes entrained with membrane currents to accelerate the beating rate, which therefore was controlled by the activity of the SERCA pump, RyR sensitivity, and L-type current amplitude, all of which are targets of β-adrenergic-mediated phosphorylation. Unexpectedly, simulations revealed the existence of a pathological mode at high RyR sensitivity to calcium, in which the calcium clock loses synchronization with the membrane, resulting in a paradoxical decrease in beating rate in response to β-adrenergic stimulation. The model indicates that the hierarchical clustering of surface RyRs in SANCs may be a crucial adaptive mechanism. Pathological desynchronization of the clocks may explain sinus node dysfunction in heart failure and RyR mutations.

Computational modeling of subcellular transport and signaling

Numerous signaling processes in the cell are controlled in microdomains that are defined by cellular structures ranging from nm to μm in size. Recent improvements in microscopy enable the resolution and reconstruction of these micro domains, while new computational methods provide the means to elucidate their functional roles. Collectively these tools allow for a biophysical understanding of the cellular environment and its pathological progression in disease. Here we review recent advancements in microscopy, and subcellular modeling on the basis of reconstructed geometries, with a special focus on signaling microdomains that are important for the excitation contraction coupling in cardiac myocytes.

Control of sarcoplasmic reticulum Ca2+ release by stochastic RyR gating within a 3D model of the cardiac dyad and importance of induction decay for CICR termination

The factors responsible for the regulation of regenerative calcium-induced calcium release (CICR) during Ca(2+) spark evolution remain unclear. Cardiac ryanodine receptor (RyR) gating in rats and sheep was recorded at physiological Ca(2+), Mg(2+), and ATP levels and incorporated into a 3D model of the cardiac dyad, which reproduced the time course of Ca(2+) sparks, Ca(2+) blinks, and Ca(2+) spark restitution. The termination of CICR by induction decay in the model principally arose from the steep Ca(2+) dependence of RyR closed time, with the measured sarcoplasmic reticulum (SR) lumen Ca(2+) dependence of RyR gating making almost no contribution. The start of CICR termination was strongly dependent on the extent of local depletion of junctional SR Ca(2+), as well as the time course of local Ca(2+) gradients within the junctional space. Reducing the dimensions of the dyad junction reduced Ca(2+) spark amplitude by reducing the strength of regenerative feedback within CICR. A refractory period for Ca(2+) spark initiation and subsequent Ca(2+) spark amplitude restitution arose from 1), the extent to which the regenerative phase of CICR can be supported by the partially depleted junctional SR, and 2), the availability of releasable Ca(2+) in the junctional SR. The physical organization of RyRs within the junctional space had minimal effects on Ca(2+) spark amplitude when more than nine RyRs were present. Spark amplitude had a nonlinear dependence on RyR single-channel Ca(2+) flux, and was approximately halved by reducing the flux from 0.6 to 0.2 pA. Although rat and sheep RyRs had quite different Ca(2+) sensitivities, Ca(2+) spark amplitude was hardly affected. This suggests that moderate changes in RyR gating by second-messenger systems will principally alter the spatiotemporal properties of SR release, with smaller effects on the amount released.

Selective modulation of coupled ryanodine receptors during microdomain activation of calcium/calmodulin-dependent kinase II in the dyadic cleft

RATIONALE

In ventricular myocytes of large mammals with low T-tubule density, a significant number of ryanodine receptors (RyRs) are not coupled to the sarcolemma; cardiac remodeling increases noncoupled RyRs.

OBJECTIVE

Our aim was to test the hypothesis that coupled and noncoupled RyRs have distinct microdomain-dependent modulation.

METHODS AND RESULTS

We studied single myocytes from pig left ventricle. The T-tubule network was analyzed in 3-dimension (3D) to measure distance to membrane of release sites. The rising phase of the Ca(2+) transient was correlated with proximity to the membrane (confocal imaging, whole-cell voltage-clamp, K5fluo-4 as Ca(2+) indicator). Ca(2+) sparks after stimulation were thus identified as resulting from coupled or noncoupled RyRs. We used high-frequency stimulation as a known activator of Ca(2+)/calmodulin-dependent kinase II. Spark frequency increased significantly more in coupled than in noncoupled RyRs. This specific modulation of coupled RyRs was abolished by the Ca(2+)/calmodulin-dependent kinase II blockers autocamtide-2-related inhibitory peptide and KN-93, but not by KN-92. Colocalization of Ca(2+)/calmodulin-dependent kinase II and RyR was not detectably different for coupled and noncoupled sites, but the F-actin disruptor cytochalasin D prevented the specific modulation of coupled RyRs. NADPH oxidase 2 inhibition by diphenyleneiodonium or apocynin, or global reactive oxygen species scavenging, also prevented coupled RyR modulation. During stimulated Ca(2+) transients, frequency-dependent increase of the rate of Ca(2+) rise was seen in coupled RyR regions only and abolished by autocamtide-2-related inhibitory peptide. After myocardial infarction, selective modulation of coupled RyR was lost.

CONCLUSIONS

Coupled RyRs have a distinct modulation by Ca(2+)/calmodulin-dependent kinase II and reactive oxygen species, dependent on an intact cytoskeleton and consistent with a local Ca(2+)/reactive oxygen species microdomain, and subject to modification with disease.

The emergence of subcellular pacemaker sites for calcium waves and oscillations

Calcium (Ca(2+)) waves generating oscillatory Ca(2+) signals are widely observed in biological cells. Experimental studies have shown that under certain conditions, initiation of Ca(2+) waves is random in space and time, while under other conditions, waves occur repetitively from preferred locations (pacemaker sites) from which they entrain the whole cell. In this study, we use computer simulations to investigate the self-organization of Ca(2+) sparks into pacemaker sites generating Ca(2+) oscillations. In both ventricular myocyte experiments and computer simulations of a heterogeneous Ca(2+) release unit (CRU) network model, we show that Ca(2+) waves occur randomly in space and time when the Ca(2+) level is low, but as the Ca(2+) level increases, waves occur repetitively from the same sites. Our analysis indicates that this transition to entrainment can be attributed to the fact that random Ca(2+) sparks self-organize into Ca(2+) oscillations differently at low and high Ca(2+) levels. At low Ca(2+), the whole cell Ca(2+) oscillation frequency of the coupled CRU system is much slower than that of an isolated single CRU. Compared to a single CRU, the distribution of interspike intervals (ISIs) of the coupled CRU network exhibits a greater variation, and its ISI distribution is asymmetric with respect to the peak, exhibiting a fat tail. At high Ca(2+), however, the coupled CRU network has a faster frequency and lesser ISI variation compared to an individual CRU. The ISI distribution of the coupled network no longer exhibits a fat tail and is well-approximated by a Gaussian distribution. This same Ca(2+) oscillation behaviour can also be achieved by varying the number of ryanodine receptors per CRU or the distance between CRUs. Using these results, we develop a theory for the entrainment of random oscillators which provides a unified explanation for the experimental observations underlying the emergence of pacemaker sites and Ca(2+) oscillations.

Isoproterenol increases the fraction of spark-dependent RyR-mediated leak in ventricular myocytes

Recent research suggests that the diastolic ryanodine-receptor-mediated release of Ca(2+) (J(leak)) from the sarcoplasmic reticulum of ventricular myocytes occurs in spark and nonspark forms. Further information about the role(s) of these release manifestations is scarce, however. This study addresses whether the fraction of spark-mediated J(leak) increases due to β-adrenergic stimulation. Confocal microscopy was used to simultaneously image Ca(2+) sparks and quantify J(leak) in intact rabbit myocytes, either in the absence or in the presence of 125 nM isoproterenol. It was found that isoproterenol treatment shifts the spark-frequency-J(leak) relationship toward an increased sensitivity to a [Ca(2+)] trigger. In agreement, a small but significant increase in spark width was found for cells with matched baseline [Ca(2+)] and total SR [Ca(2+)]. The reconstruction of release fluxes, when applied to the average sparks from those selected cells, yielded a wider release source in the isoproterenol event, indicating the recruitment of peripheral ryanodine receptors. Overall, the results presented here indicate that β-adrenergic stimulation increases the spark-dependent fraction of J(leak). Working together, the increased Ca(2+) sensitivity and the greater spark width found during isoproterenol treatment may increase the probability of Ca(2+) wave generation.