Junctional Sarcoplasmic Reticulum Research Articles

Blink nadir measurements of sarcoplasmic reticulum are consistent with strong local Ca2+ depletion

Ca2+ blinks measure the exit of Ca2+ from the junctional sarcoplasmic reticulum (JSR) in a cardiac myocyte during a Ca2+ spark. Here, the relationship between experimental blink fluorescence measurements and the [Ca2+] in the JSR is explored using long 3D simulations of diastolic Ca2+ release. For a fast intra-SR Ca2+-activated fluorophore like Fluo-5N, we show that a simple mathematical formula relates the two for an ideal blink (i.e., when fluorescence signals come only from the JSR). The formula shows that normalized JSR [Ca2+] is much lower than the normalized fluorescence and that JSR Ca2+ depletes ∼40–50% more than previously inferred from blink fluorescence measurements. In addition, we show that stray fluorescence signals (e.g., from other parts of the sarcoplasmic reticulum network) can mask even deeper Ca2+ depletion. Overall, the simulations show that strong JSR Ca2+ depletion like that seen in many simulations is consistent with the relatively moderate fluorescence changes seen in experiments.

A Feature Selection-Incorporated Simulation Study to Reveal the Effect of Calcium Ions on Cardiac Repolarization Alternans during Myocardial Ischemia

(1) Background: The main factors and their interrelationships contributing to cardiac repolarization alternans (CRA) remain unclear. This study aimed to elucidate the calcium (Ca2+)-related mechanisms underlying myocardial ischemia (MI)-induced CRA. (2) Materials and Methods: CRA was induced using S1 stimuli for pacing in an in silico ventricular model with MI. The standard deviations of nine Ca2+-related subcellular parameters among heartbeats from 100 respective nodes with and without alternans were chosen as features, including the maximum systole and end-diastole and corresponding differences in the Ca2+ concentration in the intracellular region([Ca2+]i) and junctional sarcoplasmic reticulum ([Ca2+]jsr), as well as the maximum opening of the L-type Ca2+ current (ICaL) voltage-dependent activation gate (d-gate), maximum closing of the inactivation gate (ff-gate), and the gated channel opening time (GCOT). Feature selection was applied to determine the importance of these features. (3) Results: The major parameters affecting CRA were the differences in [Ca2+]i at end-diastole, followed by the extent of d-gate activation and GCOT among beats. (4) Conclusions: MI-induced CRA is primarily characterized by functional changes in Ca2+ re-uptake, leading to alternans of [Ca2+]i and subsequent alternans of ICaL-dependent properties. The combination of computational simulation and machine learning shows promise in researching the underlying mechanisms of cardiac electrophysiology.

Modeling sarcoplasmic reticulum Ca2+ in rat cardiomyocytes.

The sarcoplasmic reticulum (SR) primarily serves as the intracellular Ca2+ store in cardiac myocytes, mediating cellular function under cardiac physiology and diseases. However, the properties of cardiac SR Ca2+ have not yet been fully determined, particularly in rats and mice, which are the most commonly used experimental species in studies on cardiac physiology and diseases. Here, we developed a spatially detailed numerical model to deduce Ca2+ movements inside the junctional SR (jSR) cisternae of rat cardiomyocytes. Our model accurately reproduced the jSR Ca2+ kinetics of local and global SR Ca2+ releases reported in a recent experimental study. With this model, we revealed that jSR Ca2+ kinetics was mostly determined by the total release flux via type 2 ryanodine receptor (RyR2) channels but not by RyR2 positioning. Although Ca2+ diffusion in global SR was previously reported to be slow, our simulation demonstrated that Ca2+ diffused very quickly inside local jSR cisternae and the decrease in the diffusion coefficient resulted in a significant reduction of jSR Ca2+ depletion amplitude. Intracellular Ca2+ was typically experimentally detected with fluorescence dye. Our simulation revealed that when the dynamical characteristics of fluorescence dye exerted a minimal effect on actual Ca2+ mobility inside jSR, the reaction rate of the dye with Ca2+ could significantly affect apparent jSR Ca2+ kinetics. Therefore, loading a chemical fluorescence dye with fast kinetics, such as Fluo-5N, into SR is important for Ca2+ measurement inside SR. Overall, our model provides new insights into deciphering Ca2+ handling inside nanoscopic jSR cisternae in rat cardiomyocytes.

Abstract P2067: Loss Of Cardiomyocyte β2 Spectrin Promotes Cardiac Dyad Dysfunction And Accelerated Heart Failure

Background: The cardiac dyad is composed of the transverse-tubules, junctional sarcoplasmic reticulum (jSR), and its associated calcium (Ca 2+ ) release units, which are integral for cardiac excitation-contraction (E-C). Proper E-C coupling requires Ca 2+ influx via L-type Ca 2+ channels (LTCC) which triggers Ca 2+ release from adjacent ryanodine receptor 2 (RyR2) receptors located in jSR. Loss of the βII-spectrin scaffolding protein alters RyR2 localization; however, its contribution to cardiac dyad organization and E-C coupling remains unclear. Hypothesis: We hypothesized that βII-spectrin is required for cardiac dyad organization and proper cardiomyocyte calcium signaling. Methods: Cardiomyocyte-specific βII-spectrin knockout (βII-cKO) and control (βII-flox) mice were generated and hearts were harvested for immunoblotting, qRT-PCR, and transmission electron microscopy (TEM). Cardiomyocytes were isolated and stained using proximity ligation assay (PLA) and visualized by confocal microscopy. To assess heart response to pressure overload, control and βII-cKO mice were subjected to transaortic constriction surgery followed by treatment with or without, diltiazem and monitored by echocardiography, and electrocardiography. Results: Cardiac dyad structure and components are significantly altered in βII-cKO hearts. Ultrastructural analysis of βII-cKO hearts showed fragmented cardiac dyads, dilated jSR and reduced jSR contact with T-tubules. Quantitative RT-PCR and immunoblotting analysis revealed altered expression of dyad mRNAs and proteins. Preliminary PLA demonstrated reduced density of LTCC-RyR2 complexes. Furthermore, pressure-induced overload of βII-cKO hearts promoted accelerated heart failure (HF), which can be rescued by treatment with diltiazem. Conclusions: We demonstrate that βII-spectrin is required for cardiac dyad integrity, and that βII-spectrin-mediated HF can be attenuated by diltiazem.

Structural Adaptation of the Excitation-Contraction Coupling Apparatus in Calsequestrin1-Null Mice during Postnatal Development.

The precise arrangement and peculiar interaction of transverse tubule (T-tubule) and sarcoplasmic reticulum (SR) membranes efficiently guarantee adequate contractile properties of skeletal muscle fibers. Fast muscle fibers from mice lacking calsequestrin 1 (CASQ1) are characterized by the profound ultrastructural remodeling of T-tubule/SR junctions. This study investigates the role of CASQ1, an essential component of calcium release units (CRUs), in the postnatal development of muscle fibers. By using CASQ1-knockout mice, we examined the maturation of CRUs and the involvement of different junctional proteins in the juxtaposition of the membrane system. Our morphological investigation of both wild-type (WT) and CASQ1-null extensor digitorum longus (EDL) fibers, from 1 week to 4 months of age, yielded noteworthy findings. Firstly, we observed that the absence of CASQ1 hindered the full maturation of CRUs, despite the correct localization of key junctional components (ryanodine receptor, dihydropyridine receptor, and triadin) to the junctional SR in adult animals. Furthermore, analysis of protein expression profiles related to T-tubule biogenesis and organization (junctophilin 1, amphiphysin 2, caveolin 3, and mitsugumin 29) demonstrated delayed progression in their expression during postnatal development in the absence of CASQ1, suggesting the impaired maturation of CRUs. The absence of CASQ1 directly impacts the proper assembly of CRUs during development and influences the expression and coordination of other proteins involved in T-tubule biogenesis and organization.

Enhanced Mitochondria-SR Tethering Triggers Adaptive Cardiac Muscle Remodeling.

Cardiac contractile function requires high energy from mitochondria, and Ca2+ from the sarcoplasmic reticulum (SR). Via local Ca2+ transfer at close mitochondria-SR contacts, cardiac excitation feedforward regulates mitochondrial ATP production to match surges in demand (excitation-bioenergetics coupling). However, pathological stresses may cause mitochondrial Ca2+ overload, excessive reactive oxygen species production and permeability transition, risking homeostatic collapse and myocyte loss. Excitation-bioenergetics coupling involves mitochondria-SR tethers but the role of tethering in cardiac physiology/pathology is debated. Endogenous tether proteins are multifunctional; therefore, nonselective targets to scrutinize interorganelle linkage. Here, we assessed the physiological/pathological relevance of selective chronic enhancement of cardiac mitochondria-SR tethering. We introduced to mice a cardiac muscle-specific engineered tether (linker) transgene with a fluorescent protein core and deployed 2D/3D electron microscopy, biochemical approaches, fluorescence imaging, in vivo and ex vivo cardiac performance monitoring and stress challenges to characterize the linker phenotype. Expressed in the mature cardiomyocytes, the linker expanded and tightened individual mitochondria-junctional SR contacts; but also evoked a marked remodeling with large dense mitochondrial clusters that excluded dyads. Yet, excitation-bioenergetics coupling remained well-preserved, likely due to more longitudinal mitochondria-dyad contacts and nanotunnelling between mitochondria exposed to junctional SR and those sealed away from junctional SR. Remarkably, the linker decreased female vulnerability to acute massive β-adrenergic stress. It also reduced myocyte death and mitochondrial calcium-overload-associated myocardial impairment in ex vivo ischemia/reperfusion injury. We propose that mitochondria-SR/endoplasmic reticulum contacts operate at a structural optimum. Although acute changes in tethering may cause dysfunction, upon chronic enhancement of contacts from early life, adaptive remodeling of the organelles shifts the system to a new, stable structural optimum. This remodeling balances the individually enhanced mitochondrion-junctional SR crosstalk and excitation-bioenergetics coupling, by increasing the connected mitochondrial pool and, presumably, Ca2+/reactive oxygen species capacity, which then improves the resilience to stresses associated with dysregulated hyperactive Ca2+ signaling.

Activation of Ca2+ transport in cardiac microsomes enriches functional sets of ER and SR proteins

The importance of sarcoplasmic reticulum (SR) Ca2+-handling in heart has led to detailed understanding of Ca2+-release and re-uptake protein complexes, while less is known about other endoplasmic reticulum (ER) functions in the heart. To more fully understand cardiac SR and ER functions, we analyzed cardiac microsomes based on their increased density through the actions of the SR Ca2+-ATPase (SERCA) and the ryanodine receptor that are highly active in cardiomyocytes. Crude cardiac microsomal vesicles loaded with Ca oxalate produced two higher density subfractions, MedSR and HighSR. Proteins from 20.0 μg of MV, MedSR, and HighSR protein were fractionated using SDS-PAGE, then trypsinized from 20 separate gel pieces, and analyzed by LC–MS/MS to determine protein content. From 62,000 individual peptide spectra obtained, we identified 1105 different proteins, of which 354 were enriched ≥ 2.0-fold in SR fractions compared to the crude membrane preparation. Previously studied SR proteins were all enriched, as were proteins associated with canonical ER functions. Contractile, mitochondrial, and sarcolemmal proteins were not enriched. Comparing the levels of SERCA-positive SR proteins in MedSR versus HighSR vesicles produced a range of SR subfraction enrichments signifying differing levels of Ca2+ leak co-localized in the same membrane patch. All known junctional SR proteins were more enriched in MedSR, while canonical ER proteins were more enriched in HighSR membrane. Proteins constituting other putative ER/SR subdomains also exhibited average Esub enrichment values (mean ± S.D.) that spanned the range of possible Esub values, suggesting that functional sets of proteins are localized to the same areas of the ER/SR membrane. We conclude that active Ca2+ loading of cardiac microsomes, reflecting the combined activities of Ca2+ uptake by SERCA, and Ca2+ leak by RyR, permits evaluation of multiple functional ER/SR subdomains. Sets of proteins from these subdomains exhibited similar enrichment patterns across membrane subfractions, reflecting the relative levels of SERCA and RyR present within individual patches of cardiac ER and SR.

Live-cell photoactivated 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 photoactivated label on RyR2. This allowed correlative imaging of RyR localization, by super-resolution photoactivated localization microscopy, and Ca2+ sparks, by high-speed imaging. Two populations of Ca2+ sparks were observed: stationary events and ‘traveling’ events that spread between neighboring RyR clusters. Traveling sparks exhibited up to eight 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 intracluster RyR recruitment to generate larger events. In contrast, RyR ‘dispersion’ during heart failure facilitates the generation of traveling sparks. Thus, RyRs cooperatively generate Ca2+ sparks in a complex, malleable fashion, and channel organization regulates the propensity for local propagation of Ca2+ release.

Sarcoplasmic Reticulum Ca2+ Buffer Proteins: A Focus on the Yet-To-Be-Explored Role of Sarcalumenin in Skeletal Muscle Health and Disease.

Sarcalumenin (SAR) is a luminal Ca2+ buffer protein with high capacity but low affinity for calcium binding found predominantly in the longitudinal sarcoplasmic reticulum (SR) of fast- and slow-twitch skeletal muscles and the heart. Together with other luminal Ca2+ buffer proteins, SAR plays a critical role in modulation of Ca2+ uptake and Ca2+ release during excitation-contraction coupling in muscle fibers. SAR appears to be important in a wide range of other physiological functions, such as Sarco-Endoplasmic Reticulum Calcium ATPase (SERCA) stabilization, Store-Operated-Calcium-Entry (SOCE) mechanisms, muscle fatigue resistance and muscle development. The function and structural features of SAR are very similar to those of calsequestrin (CSQ), the most abundant and well-characterized Ca2+ buffer protein of junctional SR. Despite the structural and functional similarity, very few targeted studies are available in the literature. The present review provides an overview of the role of SAR in skeletal muscle physiology, as well as of its possible involvement and dysfunction in muscle wasting disorders, in order to summarize the current knowledge on SAR and drive attention to this important but still underinvestigated/neglected protein.

IP3R activity increases propensity of RyR-mediated sparks by elevating dyadic [Ca2＋

Calcium (Ca2＋) plays a critical role in the excitation contraction coupling (ECC) process that mediates the contraction of cardiomyocytes during each heartbeat. While ryanodine receptors (RyRs) are the primary Ca2＋ channels responsible for generating the cell-wide Ca2＋ transients during ECC, Ca2＋ release, via inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) are also reported in cardiomyocytes to elicit ECC-modulating effects. Recent studies suggest that the localization of IP3Rs at dyads grant their ability to modify the occurrence of Ca2＋ sparks (elementary Ca2＋ release events that constitute cell wide Ca2＋ releases associated with ECC) which may underlie their modulatory influence on ECC. Here, we aim to uncover the mechanism by which dyad-localized IP3Rs influence Ca2＋ spark dynamics. To this end, we developed a mathematical model of the dyad that incorporates the behaviour of IP3Rs, in addition to RyRs, to reveal the impact of their activity on local Ca2＋ handling and consequent Ca2＋ spark occurrence and its properties. Consistent with published experimental data, our model predicts that the propensity for Ca2＋ spark formation increases in the presence of IP3R activity. Our simulations support the hypothesis that IP3Rs elevate Ca2＋ in the dyad, sensitizing proximal RyRs towards activation and hence Ca2＋ spark formation. The stochasticity of IP3R gating is an important aspect of this mechanism. However, dyadic IP3R activity lowers the Ca2＋ available in the junctional sarcoplasmic reticulum (JSR) for release, thus resulting in Ca2＋ sparks with similar durations but lower amplitudes.

Abstract 13200: Loss of Cardiomyocyte β2 Spectrin Promotes Cardiac Dyad Dysfunction and Accelerated Heart Failure

Background: The cardiac dyad and associated calcium (Ca 2+ ) release units (CRUs) are integral for cardiac excitation-contraction (E-C) coupling. The cardiac dyad is a multicomponent complex composed of the transverse-tubules, junctional sarcoplasmic reticulum (jSR), and CRU-associated proteins. Proper E-C coupling requires Ca 2+ influx via long-lasting-L-type Ca 2+ channels (Cav1.2), which triggers Ca 2+ release from adjacent ryanodine receptor 2 (RyR2) located in jSR. βII-spectrin loss alters RyR2 localization, but its contribution to cardiac dyad organization and E-C coupling remains unclear. Hypothesis: We hypothesized that βII-spectrin is required for cardiac dyad organization and proper cardiomyocyte calcium signaling. Methods: Cardiomyocyte-specific βII-spectrin knockout (βII-cKO) and control mice (βII-flox) were generated and harvested for immunoblotting, qRT-PCR, and imaging by transmission electron microscopy (TEM). Mice hearts were also interrogated by echocardiography following transaortic constriction. Results: Cardiac dyad structure and CRU components are significantly altered in βII-cKO hearts. Ultrastructural analysis of βII-cKOhearts by TEM showed fragmented cardiac dyads, dilated jSR and reduced jSR contact with T-tubules. Quantitative RT-PCR showed increased expression of calsequestrin 1 and 2, decreased phospholambam, Ncx and Cacna1c (major Cav1.2 subunit), while Ryr2 , Jph2 , Serca2a are unchanged. Protein expression of NCX was decreased, while expression of CAV1.2, RYR2, CASQ2, and SERCA2A remained unchanged. Furthermore, pressure-induced overload of βII-cKO hearts promoted accelerated heart failure (HF). Conclusions: This is the first demonstration that the βII-spectrin cytoskeletal protein is critically important for maintaining the structural integrity of the cardiac dyad. βII-spectrin loss and the resulting alterations in CRU structure, genes, and protein composition promoting accelerated HF.

RYR2 phosphomutants show that the tetramers and the junctional sarcoplasmic reticulum form a structural syncytium.

We previously showed that RYR2 tetramers are distributed nonuniformly within ventricular dyads, and that physiological and pathological factors can alter their relative positions. Agents that decreased Ca2+ spark frequency, high Mg2+, and saturating concentrations of the immunophilins FKBP12 and FKBP12.6 drew the receptors together, minimizing their nearest-neighbor distance and reducing the size of the clusters. Activating kinases with a phosphorylation cocktail did the opposite. The purpose of this study is to test the hypothesis that phosphorylation of RYR2 is required for the structural changes we have observed. We measured junctional sarcoplasmic reticulum (jSR) lengths using 2-D transmission electron microscopy (TEM) and directly visualized RYR2 distribution using dual-tilt electron tomography in phosphomutant mice S2808A, S2814A, S2814D, and S2030A. Mouse hearts were hung on a Langendorff and treated with either saline or 300 nmol/liter isoproterenol (ISO) for 2 min before being fixed and sectioned for analysis. We found that (1) RYR2 distribution in mouse ventricles is comparable to that reported for rats and humans, (2) the response to ISO applied to an intact, beating heart is identical to a phosphorylation cocktail applied to isolated permeabilized myocytes, and (3) all of the mutations produced significant changes in the tetramer arrangements and/or NND relative to wild-type (WT) mice. Our 2-D TEM measurements showed that (1) in WT mice, ISO significantly increased the length of the jSR, (2) ISO significantly increased the jSR lengths of WT, S2814A, and S2808A mice, but not the S2030A mouse, and (3) the jSR length of the S2814D mouse was significantly greater than WT, but not WT + ISO or S2814D + ISO, indicating that a mutation of the RYR2 alone caused a significant change in the jSR length. These results indicate that the tetramers and the jSR form a structural syncytium.

Biological noise is a key determinant of the reproducibility and adaptability of cardiac pacemaking and EC coupling.

Each heartbeat begins with the generation of an action potential in pacemaking cells in the sinoatrial node. This signal triggers contraction of cardiac muscle through a process termed excitation-contraction (EC) coupling. EC coupling is initiated in dyadic structures of cardiac myocytes, where ryanodine receptors in the junctional sarcoplasmic reticulum come into close apposition with clusters of CaV1.2 channels in invaginations of the sarcolemma. Cooperative activation of CaV1.2 channels within these clusters causes a local increase in intracellular Ca2+ that activates the juxtaposed ryanodine receptors. A salient feature of healthy cardiac function is the reliable and precise beat-to-beat pacemaking and amplitude of Ca2+ transients during EC coupling. In this review, we discuss recent discoveries suggesting that the exquisite reproducibility of this system emerges, paradoxically, from high variability at subcellular, cellular, and network levels. This variability is attributable to stochastic fluctuations in ion channel trafficking, clustering, and gating, as well as dyadic structure, which increase intracellular Ca2+ variance during EC coupling. Although the effects of these large, local fluctuations in function and organization are sometimes negligible at the macroscopic level owing to spatial-temporal summation within and across cells in the tissue, recent work suggests that the "noisiness" of these intracellular Ca2+ events may either enhance or counterintuitively reduce variability in a context-dependent manner. Indeed, these noisy events may represent distinct regulatory features in the tuning of cardiac contractility. Collectively, these observations support the importance of incorporating experimentally determined values of Ca2+ variance in all EC coupling models. The high reproducibility of cardiac contraction is a paradoxical outcome of high Ca2+ signaling variability at subcellular, cellular, and network levels caused by stochastic fluctuations in multiple processes in time and space. This underlying stochasticity, which counterintuitively manifests as reliable, consistent Ca2+ transients during EC coupling, also allows for rapid changes in cardiac rhythmicity and contractility in health and disease.

CMYA5 establishes cardiac dyad architecture and positioning

Cardiac excitation-contraction coupling requires dyads, the nanoscopic microdomains formed adjacent to Z-lines by apposition of transverse tubules and junctional sarcoplasmic reticulum. Disruption of dyad architecture and function are common features of diseased cardiomyocytes. However, little is known about the mechanisms that modulate dyad organization during cardiac development, homeostasis, and disease. Here, we use proximity proteomics in intact, living hearts to identify proteins enriched near dyads. Among these proteins is CMYA5, an under-studied striated muscle protein that co-localizes with Z-lines, junctional sarcoplasmic reticulum proteins, and transverse tubules in mature cardiomyocytes. During cardiac development, CMYA5 positioning adjacent to Z-lines precedes junctional sarcoplasmic reticulum positioning or transverse tubule formation. CMYA5 ablation disrupts dyad architecture, dyad positioning at Z-lines, and junctional sarcoplasmic reticulum Ca2+ release, leading to cardiac dysfunction and inability to tolerate pressure overload. These data provide mechanistic insights into cardiomyopathy pathogenesis by demonstrating that CMYA5 anchors junctional sarcoplasmic reticulum to Z-lines, establishes dyad architecture, and regulates dyad Ca2+ release.

Occurrence of early afterdepolarization under healthy or hypertrophic cardiomyopathy conditions in the human ventricular endocardial myocyte: In silico study using 109 torsadogenic or non-torsadogenic compounds

The goal of the CiPA initiative (Comprehensive in vitro Proarrhythmia Assay) was to assess a more accurate prediction of new drug candidate proarrhythmic severe liabilities such as torsades de pointes, for example. This new CiPA paradigm was partly based on in silico reconstruction of human ventricular cardiomyocyte action potential useful to identify repolarization abnormalities such early afterdepolarization (EAD), for example. Using the ToR-ORd algorithm (Tomek-Rodriguez-O'Hara-Rudy dynamic model), the aim of the present work was (i) to identify intracellular parameters leading to EAD occurrence under healthy and hypertrophic cardiomyopathy (HCM) conditions and (ii) to evaluate the prediction accuracy of compound torsadogenic risk based on EAD occurrence using a large set of 109 torsadogenic and non-torsadogenic compounds under both experimental conditions. In silico results highlighted the crucial involvement of Ca++ handling in the ventricular cardiomyocyte intracellular subspace compartment for the initiation of EAD, demonstrated by a higher amplitude of Ca++ release from junctional sarcoplasmic reticulum to subspace compartments (Jrel) measured at EAD take-off voltage in the presence vs. the absence of EAD initiated either by high IKr inhibition or by high enough concentration of a torsadogenic compound under both experimental conditions. Under healthy or HCM conditions, the prediction accuracy of the torsadogenic risk of compound based on EAD occurrence was observed to be 61 or 92%, respectively. This high accuracy under HCM conditions was discussed regarding its usefulness for cardiac safety pharmacology at least at early drug screening/preclinical stage of the drug development process.

Multiple regions within junctin drive its interaction with calsequestrin-1 and its localization to triads in skeletal muscle.

Junctin is a transmembrane protein of striated muscles, located at the junctional sarcoplasmic reticulum (SR). It is characterized by a luminal C-terminal tail, through which it functionally interacts with calsequestrin and the ryanodine receptor (RyR). Interaction with calsequestrin was ascribed to the presence of stretches of charged amino acids (aa). However, the regions able to bind calsequestrin have not been defined in detail. We report here that, in non-muscle cells, junctin and calsequestrin assemble in long linear regions within the endoplasmic reticulum, mirroring the formation of calsequestrin polymers. In differentiating myotubes, the two proteins colocalize at triads, where they assemble with other proteins of the junctional SR. By performing GST pull-down assays with distinct regions of the junctin tail, we identified two KEKE motifs that can bind calsequestrin. In addition, stretches of charged aa downstream these motifs were found to also bind calsequestrin and the RyR. Deletion of even one of these regions impaired the ability of junctin to localize at the junctional SR, suggesting that interaction with other proteins at this site represents a key element in junctin targeting.

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.

Cardiac Alternans Occurs through the Synergy of Voltage- and Calcium-Dependent Mechanisms.

Cardiac alternans is characterized by alternating weak and strong beats of the heart. This signaling at the cellular level may appear as alternating long and short action potentials (APs) that occur in synchrony with alternating large and small calcium transients, respectively. Previous studies have suggested that alternans manifests itself through either a voltage dependent mechanism based upon action potential restitution or as a calcium dependent mechanism based on refractoriness of calcium release. We use a novel model of cardiac excitation-contraction (EC) coupling in the rat ventricular myocyte that includes 20,000 calcium release units (CRU) each with 49 ryanodine receptors (RyR2s) and 7 L-type calcium channels that are all stochastically gated. The model suggests that at the cellular level in the case of alternans produced by rapid pacing, the mechanism requires a synergy of voltage- and calcium-dependent mechanisms. The rapid pacing reduces AP duration and magnitude reducing the number of L-type calcium channels activating individual CRUs during each AP and thus increases the population of CRUs that can be recruited stochastically. Elevated myoplasmic and sarcoplasmic reticulum (SR) calcium, [Ca2+]myo and [Ca2+]SR respectively, increases ryanodine receptor open probability (Po) according to our model used in this simulation and this increased the probability of activating additional CRUs. A CRU that opens in one beat is less likely to open the subsequent beat due to refractoriness caused by incomplete refilling of the junctional sarcoplasmic reticulum (jSR). Furthermore, the model includes estimates of changes in Na+ fluxes and [Na+]i and thus provides insight into how changes in electrical activity, [Na+]i and sodium-calcium exchanger activity can modulate alternans. The model thus tracks critical elements that can account for rate-dependent changes in [Na+]i and [Ca2+]myo and how they contribute to the generation of Ca2+ signaling alternans in the heart.

Desmin interacts with STIM1 and coordinates Ca2+ signaling in skeletal muscle.

Stromal interaction molecule 1 (STIM1), the sarcoplasmic reticulum (SR) transmembrane protein, activates store-operated Ca2+ entry (SOCE) in skeletal muscle and, thereby, coordinates Ca2+ homeostasis, Ca2+-dependent gene expression, and contractility. STIM1 occupies space in the junctional SR membrane of the triads and the longitudinal SR at the Z-line. How STIM1 is organized and is retained in these specific subdomains of the SR is unclear. Here, we identified desmin, the major type III intermediate filament protein in muscle, as a binding partner for STIM1 based on a yeast 2-hybrid screen. Validation of the desmin-STIM1 interaction by immunoprecipitation and immunolocalization confirmed that the CC1-SOAR domains of STIM1 interact with desmin to enhance STIM1 oligomerization yet limit SOCE. Based on our studies of desmin-KO mice, we developed a model wherein desmin connected STIM1 at the Z-line in order to regulate the efficiency of Ca2+ refilling of the SR. Taken together, these studies showed that desmin-STIM1 assembles a cytoskeletal-SR connection that is important for Ca2+ signaling in skeletal muscle.

RyR2 Gain-of-Function and Not So Sudden Cardiac Death

RyR2 Gain-of-Function and Not So Sudden Cardiac Death