The couplonopathies: A comparative approach to a class of diseases of skeletal and cardiac muscle

Effects of High-Dose Cyclophosphamide on Ultrastructural Changes and Gene Expression Profiles in the Cardiomyocytes of C57BL/6J Mice

The pathogenesis of cyclophosphamide (CY)-induced cardiotoxicity remains unknown, and methods for its prevention have not been established. To elucidate the acute structural changes that take place in myocardial cells and the pathways leading to myocardial damage under high-dose CY treatments, we performed detailed pathological analyses of myocardial tissue obtained from C57BL/6J mice subjected to a high-dose CY treatment. Additionally, we analysed the genome-wide cardiomyocyte expression profiles of mice subjected to the high-dose CY treatment. Treatment with CY (400 mg/kg/day intraperitoneally for two days) caused marked ultrastructural aberrations, as observed using electron microscopy, although these aberrations could not be observed using optical microscopy. The expansion of the transverse tubule and sarcoplasmic reticulum, turbulence in myocardial fibre travel, and a low contractile protein density were observed in cardiomyocytes. The high-dose CY treatment altered the cardiomyocyte expression of 1210 genes (with 675 genes upregulated and 535 genes downregulated) associated with cell-cell junctions, inflammatory responses, cardiomyopathy, and cardiac muscle function, as determined using microarray analysis (|Z-score| > 2.0). The expression of functionally important genes related to myocardial contraction and the regulation of calcium ion levels was validated using real-time polymerase chain reaction analysis. The results of the gene expression profiling, functional annotation clustering, and Kyoto Encyclopedia of Genes and Genomes pathway functional-classification analysis suggest that CY-induced cardiotoxicity is associated with the disruption of the Ca2+ signalling pathway.

Muscle calcium stress cleaves junctophilin1, unleashing a gene regulatory program predicted to correct glucose dysregulation

Calcium ion movements between cellular stores and the cytosol govern muscle contraction, the most energy-consuming function in mammals, which confers skeletal myofibers a pivotal role in glycemia regulation. Chronic myoplasmic calcium elevation ("calcium stress"), found in malignant hyperthermia-susceptible (MHS) patients and multiple myopathies, has been suggested to underlie the progression from hyperglycemia to insulin resistance. What drives such progression remains elusive. We find that muscle cells derived from MHS patients have increased content of an activated fragment of GSK3β - a specialized kinase that inhibits glycogen synthase, impairing glucose utilization and delineating a path to hyperglycemia. We also find decreased content of junctophilin1, an essential structural protein that colocalizes in the couplon with the voltage-sensing CaV1.1, the calcium channel RyR1 and calpain1, accompanied by an increase in a 44 kDa junctophilin1 fragment (JPh44) that moves into nuclei. We trace these changes to activated proteolysis by calpain1, secondary to increased myoplasmic calcium. We demonstrate that a JPh44-like construct induces transcriptional changes predictive of increased glucose utilization in myoblasts, including less transcription and translation of GSK3β and decreased transcription of proteins that reduce utilization of glucose. These effects reveal a stress-adaptive response, mediated by the novel regulator of transcription JPh44.

Elevated Ca2+ at the triad junction underlies dysregulation of Ca2+ signaling in dysferlin-null skeletal muscle

Dysferlin-null A/J myofibers generate abnormal Ca2+ transients that are slightly reduced in amplitude compared to controls. These are further reduced in amplitude by hypoosmotic shock and often appear as Ca2+ waves (Lukyanenko et al., J. Physiol., 2017). Ca2+ waves are typically associated with Ca2+-induced Ca2+ release, or CICR, which can be myopathic. We tested the ability of a permeable Ca2+ chelator, BAPTA-AM, to inhibit CICR in injured dysferlin-null fibers and found that 10-50 nM BAPTA-AM suppressed all Ca2+ waves. The same concentrations of BAPTA-AM increased the amplitude of the Ca2+ transient in A/J fibers to wild type levels and protected transients against the loss of amplitude after hypoosmotic shock, as also seen in wild type fibers. Incubation with 10 nM BAPTA-AM led to intracellular BAPTA concentrations of ∼60 nM, as estimated with its fluorescent analog, Fluo-4AM. This should be sufficient to restore intracellular Ca2+ to levels seen in wild type muscle. Fluo-4AM was ∼10-fold less effective than BAPTA-AM, however, consistent with its lower affinity for Ca2+. EGTA, which has an affinity for Ca2+ similar to BAPTA, but with much slower kinetics of binding, was even less potent when introduced as the -AM derivative. By contrast, a dysferlin variant with GCaMP6fu in place of its C2A domain accumulated at triad junctions, like wild type dysferlin, and suppressed all abnormal Ca2+ signaling. GCaMP6fu introduced as a Venus chimera did not accumulate at junctions and failed to suppress abnormal Ca2+ signaling. Our results suggest that leak of Ca2+ into the triad junctional cleft underlies dysregulation of Ca2+ signaling in dysferlin-null myofibers, and that dysferlin's C2A domain suppresses abnormal Ca2+ signaling and protects muscle against injury by binding Ca2+ in the cleft.

Quantification of the calcium signaling deficit in muscles devoid of triadin

Triadin, a protein of the sarcoplasmic reticulum (SR) of striated muscles, anchors the calcium-storing protein calsequestrin to calcium release RyR channels at the junction with t-tubules, and modulates these channels by conformational effects. Triadin ablation induces structural SR changes and alters the expression of other proteins. Here we quantify alterations of calcium signaling in single skeletal myofibers of constitutive triadin-null mice. We find higher resting cytosolic and lower SR-luminal [Ca²⁺], 40% lower calsequestrin expression, and more Ca_V1.1, RyR1 and SERCA1. Despite the increased Ca_V1.1, the mobile intramembrane charge was reduced by ~20% in Triadin-null fibers. The initial peak of calcium release flux by pulse depolarization was minimally altered in the null fibers (revealing an increase in peak calcium permeability). The “hump” phase that followed, attributable to calcium detaching from calsequestrin, was 25% lower, a smaller change than expected from the reduced calsequestrin content and calcium saturation. The exponential decay rate of calcium transients was 25% higher, consistent with the higher SERCA1 content. Recovery of calcium flux after a depleting depolarization was faster in triadin-null myofibers, consistent with the increased uptake rate and lower SR calsequestrin content. In sum, the triadin knockout determines an increased RyR1 channel openness, which depletes the SR, a substantial loss of calsequestrin and gains in other couplon proteins. Powerful functional compensations ensue: activation of SOCE that increases [Ca²⁺]_cyto; increased SERCA1 activity, which limits the decrease in [Ca²⁺]_SR and a restoration of SR calcium storage of unknown substrate. Together, they effectively limit the functional loss in skeletal muscles.

Human BIN1 isoforms grow, maintain and regenerate excitation-contraction couplons in adult rat and human stem cell-derived cardiomyocytes

AIMS

In ventricular myocytes, Transverse-tubules (T-tubules) are instrumental for excitation-contraction (EC) coupling and their disarray is a hallmark of cardiac diseases. BIN1 is a key contributor to their biogenesis. Our study set out to investigate the role of human BIN1 splice variants in the maintenance and regeneration of EC-coupling in rat adult ventricular myocytes and human induced pluripotent stem cell-derived cardiac myocytes (hiPS-CMs).

METHODS AND RESULTS

In heart samples from healthy human donors expression patterns of 5 BIN1 splice variants were identified. Following viral transduction of human BIN1 splice variants in cellular models of T-tubular disarray we employed high-speed confocal calcium imaging and Ca-CLEAN analysis to identify functional EC-coupling sites and T-tubular architecture. Adult rat ventricular myocytes were used to investigate the regeneration after loss and maintenance of EC-coupling while we studied the enhancement of EC-coupling in hiPS-CMs. All five human BIN1 splice variants induced de novo generation of T-tubules in both cell types. Isoforms with the phosphoinositide binding motif (PI) were most potent in maintenance and regeneration of T-tubules and functional EC-coupling in adult rat myocytes. In hiPSC-CMs, BIN1 variants with PI motiv induced de-novo generation of T-tubules, functional EC-coupling sites and enhanced calcium handling.

CONCLUSION(S)

BIN1 is essential for the maintenance, regeneration, and de-novo generation of functional T-tubules, especially isoforms with PI motifs. These T-tubules trigger the development of functional EC couplons resulting in enhanced calcium handling.

TRANSLATIONAL PERSPECTIVE

Cardiomyopathy and heart failure are among the most frequent causes of death in modern societies. Gene therapies and hiPSC technology are becoming increasingly promising, both for treatment and therapy development. On the cellular level, one of the common denominators of cardiac diseases is the concurrent loss of T-tubules essential for efficient EC-coupling. While initial approaches in animal models employing gene therapy with BIN1 have depicted encouraging improvements the expression pattern of BIN1 isoforms in the human heart is still elusive. The present study identifies a unique set of five distinct BIN1 isoforms in healthy human hearts and demonstrates their potency in both, T-tubule maintenance and re-generation after loss resulting in efficient EC-coupling. Noteworthy, PI-motif containing isoforms were potent trigger of de-novo generation of T-tubules and establishment of efficient EC-coupling in hiPSC-CMs. Therefore, the expression of BIN1 might be novel and promising for pharmaceutical treatment and gene therapy.

The Orai1 inhibitor BTP2 has multiple effects on Ca2+ handling in skeletal muscle

BTP2 is an inhibitor of the Ca²⁺ channel Orai1, which mediates store-operated Ca²⁺ entry (SOCE). Despite having been extensively used in skeletal muscle, the effects of this inhibitor on Ca²⁺ handling in muscle cells have not been described. To address this question, we used intra- and extracellular application of BTP2 in mechanically skinned fibers and developed a localized modulator application approach, which provided in-preparation reference and test fiber sections to enhance detection of the effect of Ca²⁺ handling modulators. In addition to blocking Orai1-dependent SOCE, we found a BTP2-dependent inhibition of resting extracellular Ca²⁺ flux. Increasing concentrations of BTP2 caused a shift from inducing accumulation of Ca²⁺ in the t-system due to Orai1 blocking to reducing the resting [Ca²⁺] in the sealed t-system. This effect was not observed in the absence of functional ryanodine receptors (RYRs), suggesting that higher concentrations of BTP2 impair RYR function. Additionally, we found that BTP2 impaired action potential–induced Ca²⁺ release from the sarcoplasmic reticulum during repetitive stimulation without compromising the fiber Ca²⁺ content. BTP2 was found to have an effect on RYR-mediated Ca²⁺ release, suggesting that RYR is the point of BTP2-induced inhibition during cycles of EC coupling. The effects of BTP2 on the RYR Ca²⁺ leak and release were abolished by pre-exposure to saponin, indicating that the effects of BTP2 on the RYR are not direct and require a functional t-system. Our results demonstrate the presence of a SOCE channels–mediated basal Ca²⁺ influx in healthy muscle fibers and indicate that BTP2 has multiple effects on Ca²⁺ handling, including indirect effects on the activity of the RYR.

ECC meets CEU-New focus on the backdoor for calcium ions in skeletal muscle cells

In this issue, Michelucci et al. report the existence of specific sites acting as Ca2+ entry units (CEUs) in fast skeletal muscle of mice lacking calsequestrin (CASQ1), the major Ca2+ binding protein of the SR. The CEU provides constitutive and store-operated Ca2+ entry (SOCE) and resistance to force decline resulting from SR Ca2+ depletion during repetitive muscle activity.

Preserved Ca2+ handling and excitation-contraction coupling in muscle fibres from diet-induced obese mice

AIMS/HYPOTHESIS

Disrupted intracellular Ca²⁺ handling is known to play a role in diabetic cardiomyopathy but it has also been postulated to contribute to obesity- and type 2 diabetes-associated skeletal muscle dysfunction. Still, there is so far very limited functional insight into whether, and if so to what extent, muscular Ca²⁺ homeostasis is affected in this situation, so as to potentially determine or contribute to muscle weakness. In differentiated muscle, force production is under the control of the excitation-contraction coupling process: upon plasma membrane electrical activity, the Ca_V1.1 voltage sensor/Ca²⁺ channel in the plasma membrane triggers opening of the ryanodine receptor Ca²⁺ release channel in the sarcoplasmic reticulum (SR) membrane. Opening of the ryanodine receptor triggers the rise in cytosolic Ca²⁺, which activates contraction while Ca²⁺ uptake by the SR ATPase Ca²⁺-pump promotes relaxation. These are the core mechanisms underlying the tight control of muscle force by neuronal electrical activity. This study aimed at characterising their inherent physiological function in a diet-induced mouse model of obesity and type 2 diabetes.

METHODS

Intact muscle fibres were isolated from mice fed either with a standard chow diet or with a high-fat, high-sucrose diet generating obesity, insulin resistance and glucose intolerance. Properties of muscle fibres were investigated with a combination of whole-cell voltage-clamp electrophysiology and confocal fluorescence imaging. The integrity and density of the plasma membrane network (transverse tubules) that carries the membrane excitation throughout the muscle fibres was assessed with the dye Di-8-ANEPPS. Ca_V1.1 Ca²⁺ channel activity was studied by measuring the changes in current across the plasma membrane elicited by voltage-clamp depolarising pulses of increasing amplitude. SR Ca²⁺ release through ryanodine receptors was simultaneously detected with the Ca²⁺-sensitive dye Rhod-2 in the cytosol. Ca_V1.1 voltage-sensing activity was separately characterised from the properties of intra-plasma-membrane charge movement produced by short voltage-clamp depolarising pulses. Spontaneous Ca²⁺ release at rest was assessed with the Ca²⁺-sensitive dye Fluo-4. The rate of SR Ca²⁺ uptake was assessed from the time course of cytosolic Ca²⁺ recovery after the end of voltage excitation using the Ca²⁺-sensitive dye Fluo-4FF. The response to a fatigue-stimulation protocol was determined from the time course of decline of the peak Fluo-4FF Ca²⁺ transients elicited by 30 trains of 5-ms-long depolarising pulses delivered at 100 Hz.

RESULTS

The transverse tubule network architecture and density were well preserved in the fibres from the obese mice. The Ca_V1.1 Ca²⁺ current and voltage-sensing properties were also largely unaffected with mean values for maximum conductance and maximum amount of charge of 234 ± 12 S/F and 30.7 ± 1.6 nC/μF compared with 196 ± 13 S/F and 32.9 ± 2.0 nC/μF in fibres from mice fed with the standard diet, respectively. Voltage-activated SR Ca²⁺ release through ryanodine receptors also exhibited very similar properties in the two groups with mean values for maximum rate of Ca²⁺ release of 76.0 ± 6.5 and 78.1 ± 4.4 μmol l^-1 ms^-1, in fibres from control and obese mice, respectively. The response to a fatigue protocol was also largely unaffected in fibres from the obese mice, and so were the rate of cytosolic Ca²⁺ removal and the spontaneous Ca²⁺ release activity at rest.

CONCLUSIONS/INTERPRETATION

The functional properties of the main mechanisms involved in the control of muscle Ca²⁺ homeostasis are well preserved in muscle fibres from obese mice, at the level of both the plasma membrane and of the SR. We conclude that intracellular Ca²⁺ handling and excitation-contraction coupling in skeletal muscle fibres are not primary targets of obesity and type 2 diabetes. Graphical abstract.

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

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

Tricalbin-Mediated Contact Sites Control ER Curvature to Maintain Plasma Membrane Integrity

Membrane contact sites (MCS) between the endoplasmic reticulum (ER) and the plasma membrane (PM) play fundamental roles in all eukaryotic cells. ER-PM MCS are particularly abundant in Saccharomyces cerevisiae, where approximately half of the PM surface is covered by cortical ER (cER). Several proteins, including Ist2, Scs2/22, and Tcb1/2/3 are implicated in cER formation, but the specific roles of these molecules are poorly understood. Here, we use cryo-electron tomography to show that ER-PM tethers are key determinants of cER morphology. Notably, Tcb proteins (tricalbins) form peaks of extreme curvature on the cER membrane facing the PM. Combined modeling and functional assays suggest that Tcb-mediated cER peaks facilitate the transport of lipids between the cER and the PM, which is necessary to maintain PM integrity under heat stress. ER peaks were also present at other MCS, implying that membrane curvature enforcement may be a widespread mechanism to regulate MCS function.

Overactivation of the sodium-calcium exchanger and transient receptor potential in anesthesia-induced malignant hyperthermia

Malignant hyperthermia is a pharmacogenetic disorder, which is an uncommon but frequently fatal intricacy of inhalation anesthesia in man. It causes a quick rise in body temperature to highly irreversible levels, which causes death in around three of four cases. The trigger anesthetics cause an anomalous, continued ascent in myoplasmic calcium levels. Possible mechanisms by which continuous release of sodium, calcium from skeletal muscle plasma membrane and sarcoplasmic reticulum stores respectively can produce the profound hyperthermia are discussed.

Calcium Homeostasis Is Modified in Skeletal Muscle Fibers of Small Ankyrin1 Knockout Mice

Small Ankyrins (sAnk1) are muscle-specific isoforms generated by the Ank1 gene that participate in the organization of the sarcoplasmic reticulum (SR) of striated muscles. Accordingly, the volume of SR tubules localized around the myofibrils is strongly reduced in skeletal muscle fibers of 4- and 10-month-old sAnk1 knockout (KO) mice, while additional structural alterations only develop with aging. To verify whether the lack of sAnk1 also alters intracellular Ca²⁺ handling, cytosolic Ca²⁺ levels were analyzed in stimulated skeletal muscle fibers from 4- and 10-month-old sAnk1 KO mice. The SR Ca²⁺ content was reduced in sAnk1 KO mice regardless of age. The amplitude of the Ca²⁺ transients induced by depolarizing pulses was decreased in myofibers of sAnk1 KO with respect to wild type (WT) fibers, while their voltage dependence was not affected. Furthermore, analysis of spontaneous Ca²⁺ release events (sparks) on saponin-permeabilized muscle fibers indicated that the frequency of sparks was significantly lower in fibers from 4-month-old KO mice compared to WT. Furthermore, both the amplitude and spatial spread of sparks were significantly smaller in muscle fibers from both 4- and 10-month-old KO mice compared to WT. These data suggest that the absence of sAnk1 results in an impairment of SR Ca²⁺ release, likely as a consequence of a decreased Ca²⁺ store due to the reduction of the SR volume in sAnk1 KO muscle fibers.

Abnormal calcium signalling and the caffeine-halothane contracture test

BACKGROUND

The variable clinical presentation of malignant hyperthermia (MH), a disorder of calcium signalling, hinders its diagnosis and management. Diagnosis relies on the caffeine-halothane contracture test, measuring contraction forces upon exposure of muscle to caffeine or halothane (FC and FH, respectively). Patients with above-threshold FC or FH are diagnosed as MH susceptible. Many patients test positive to halothane only (termed 'HH'). Our objective was to determine the characteristics of these HH patients, including their clinical symptoms and features of cytosolic Ca2+ signalling related to excitation-contraction coupling in myotubes.

METHODS

After institutional ethics committee approval, recruited patients undergoing contracture testing at Toronto's MH centre were assigned to three groups: HH, doubly positive (HS), and negative patients (HN). A clinical index was assembled from musculoskeletal symptoms and signs. An analogous calcium index summarised four measures in cultured myotubes: resting [Ca2+]cytosol, frequency of spontaneous cytosolic Ca2+ events, Ca2+ waves, and cell-wide Ca2+ spikes after electrical stimulation.

RESULTS

The highest values of both indexes were found in the HH group; the differences in calcium index between HH and the other groups were statistically significant. The principal component analysis confirmed the unique cell-level features of the HH group, and identified elevated resting [Ca2+]cytosol and spontaneous event frequency as the defining HH characteristics.

CONCLUSIONS

These findings suggest that HH pathogenesis stems from excess Ca2+ leak through sarcoplasmic reticulum channels. This identifies HH as a separate diagnostic group and opens their condition to treatment based on understanding of pathophysiological mechanisms.

The clinical spectrum of CASQ1-related myopathy

OBJECTIVE

To identify and characterize patients with calsequestrin 1 (CASQ1)-related myopathy.

METHODS

Patients selected according to histopathologic features underwent CASQ1 genetic screening. CASQ1-mutated patients were clinically evaluated and underwent muscle MRI. Vacuole morphology and vacuolated fiber type were characterized.

RESULTS

Twenty-two CASQ1-mutated patients (12 families) were identified, 21 sharing the previously described founder mutation (p.Asp244Gly) and 1 with the p.Gly103Asp mutation. Patients usually presented in the sixth decade with exercise intolerance and myalgias and later developed mild to moderate, slowly progressive proximal weakness with quadriceps atrophy and scapular winging. Muscle MRI (n = 11) showed a recurrent fibrofatty substitution pattern. Three patients presented subclinical cardiac abnormalities. Muscle histopathology in patients with p.Asp244Gly showed vacuoles in type II fibers appearing empty in hematoxylin-eosin, Gomori, and nicotinamide adenine dinucleotide (NADH) tetrazolium reductase stains but strongly positive for sarcoplasmic reticulum proteins. The muscle histopathology of p.Gly103Asp mutation was different, showing also NADH-positive accumulation consistent with tubular aggregates.

CONCLUSIONS

We report the clinical and molecular details of the largest cohort of CASQ1-mutated patients. A possible heart involvement is presented, further expanding the phenotype of the disease. One mutation is common due to a founder effect, but other mutations are possible. Because of a paucity of symptoms, it is likely that CASQ1 mutations may remain undiagnosed if a muscle biopsy is not performed.

A 3D diffusional-compartmental model of the calcium dynamics in cytosol, sarcoplasmic reticulum and mitochondria of murine skeletal muscle fibers

Variations of free calcium concentration ([Ca2+]) are powerful intracellular signals, controlling contraction as well as metabolism in muscle cells. To fully understand the role of calcium redistribution upon excitation and contraction in skeletal muscle cells, the local [Ca2+] in different compartments needs to be taken into consideration. Fluorescent probes allow the determination of [Ca2+] in the cytosol where myofibrils are embedded, the lumen of the sarcoplasmic reticulum (SR) and the mitochondrial matrix. Previously, models have been developed describing intracellular calcium handling in skeletal and cardiac muscle cells. However, a comprehensive model describing the kinetics of the changes in free calcium concentration in these three compartments is lacking. We designed a new 3D compartmental model of the half sarcomere with radial symmetry, which accounts for diffusion of Ca2+ into the three compartments and simulates its dynamics at rest and at various rates of stimulation in mice skeletal muscle fibers. This model satisfactorily reproduces both the amplitude and time course of the variations of [Ca2+] in the three compartments in mouse fast fibers. As an illustration of the applicability of the model, we investigated the effects of Calsequestrin (CSQ) ablation. CSQ is the main Ca2+ buffer in the SR, localized in close proximity of its calcium release sites and near to the mitochondria. CSQ knock-out mice muscles still preserve a near-normal contractile behavior, but it is unclear whether this is caused by additional SR calcium buffering or a significant contribution of calcium entry from extracellular space, via stored-operated calcium entry (SOCE). The model enabled quantitative assessment of these two scenarios by comparison to measurements of local calcium in the cytosol, the SR and the mitochondria. In conclusion, the model represents a useful tool to investigate the impact of protein ablation and of pharmacological interventions on intracellular calcium dynamics in mice skeletal muscle.

Junctional membrane Ca2+ dynamics in human muscle fibers are altered by malignant hyperthermia causative RyR mutation

We used the nanometer-wide tubules of the transverse tubular (t)-system of human skeletal muscle fibers as sensitive sensors for the quantitative monitoring of the Ca²⁺-handling properties in the narrow junctional cytoplasmic space sandwiched between the tubular membrane and the sarcoplasmic reticulum cisternae in single muscle fibers. The t-system sealed with a Ca²⁺-sensitive dye trapped in it is sensitive to changes in ryanodine receptor (RyR) Ca²⁺ leak, the store operated calcium entry flux, plasma membrane Ca pump, and sodium-calcium exchanger activities, thus making the sealed t-system a nanodomain Ca²⁺ sensor of Ca²⁺ dynamics in the junctional space. The sensor was used to assess the basal Ca²⁺-handling properties of human muscle fibers obtained by needle biopsy from control subjects and from people with a malignant hyperthermia (MH) causative RyR variant. Using this approach we show that the muscle fibers from MH-susceptible individuals display leakier RyRs and a greater capacity to extrude Ca²⁺ across the t-system membrane compared with fibers from controls. This study provides a quantitative way to assess the effect of RyR variants on junctional membrane Ca²⁺ handling under defined ionic conditions.

A privileged role for neuronal Na+ channels in regulating ventricular [Ca2+] and arrhythmias

Jones et al. provide commentary on the intricate crosstalk between ion transporters that goes awry in long QT arrhythmia.

Calcium-induced release of calcium in muscle: 50 years of work and the emerging consensus

Ryanodine-sensitive intracellular Ca2+ channels (RyRs) open upon binding Ca2+ at cytosolic-facing sites. This results in concerted, self-reinforcing opening of RyRs clustered in specialized regions on the membranes of Ca2+ storage organelles (endoplasmic reticulum and sarcoplasmic reticulum), a process that produces Ca2+-induced Ca2+ release (CICR). The process is optimized to achieve large but brief and localized increases in cytosolic Ca2+ concentration, a feature now believed to be critical for encoding the multiplicity of signals conveyed by this ion. In this paper, I trace the path of research that led to a consensus on the physiological significance of CICR in skeletal muscle, beginning with its discovery. I focus on the approaches that were developed to quantify the contribution of CICR to the Ca2+ increase that results in contraction, as opposed to the flux activated directly by membrane depolarization (depolarization-induced Ca2+ release [DICR]). Although the emerging consensus is that CICR plays an important role alongside DICR in most taxa, its contribution in most mammalian muscles appears to be limited to embryogenesis. Finally, I survey the relevance of CICR, confirmed or plausible, to pathogenesis as well as the multiple questions about activation of release channels that remain unanswered after 50 years.

Voltage modulates halothane-triggered Ca2+ release in malignant hyperthermia-susceptible muscle

Malignant hyperthermia can result from mutations in the ryanodine receptor that favor anesthetic-induced Ca²⁺ release. Zullo et al. find that membrane potential modulates the effect of the volatile anesthetic halothane on skeletal muscle ryanodine receptors possessing the Y524S mutation.

Malignant hyperthermia (MH) is a fatal hypermetabolic state that may occur during general anesthesia in susceptible individuals. It is often caused by mutations in the ryanodine receptor RyR1 that favor drug-induced release of Ca²⁺ from the sarcoplasmic reticulum. Here, knowing that membrane depolarization triggers Ca²⁺ release in normal muscle function, we study the cross-influence of membrane potential and anesthetic drugs on Ca²⁺ release. We used short single muscle fibers of knock-in mice heterozygous for the RyR1 mutation Y524S combined with microfluorimetry to measure intracellular Ca²⁺ signals. Halothane, a volatile anesthetic used in contracture testing for MH susceptibility, was equilibrated with the solution superfusing the cells by means of a vaporizer system. In the range 0.2 to 3%, the drug causes significantly larger elevations of free myoplasmic [Ca²⁺] in mutant (YS) compared with wild-type (WT) fibers. Action potential–induced Ca²⁺ signals exhibit a slowing of their time course of relaxation that can be attributed to a component of delayed Ca²⁺ release turnoff. In further experiments, we applied halothane to single fibers that were voltage-clamped using two intracellular microelectrodes and studied the effect of small (10-mV) deviations from the holding potential (−80 mV). Untreated WT fibers show essentially no changes in [Ca²⁺], whereas the Ca²⁺ level of YS fibers increases and decreases on depolarization and hyperpolarization, respectively. The drug causes a significant enhancement of this response. Depolarizing pulses reveal a substantial negative shift in the voltage dependence of activation of Ca²⁺ release. This behavior likely results from the allosteric coupling between RyR1 and its transverse tubular voltage sensor. We conclude that the binding of halothane to RyR1 alters the voltage dependence of Ca²⁺ release in MH-susceptible muscle fibers such that the resting membrane potential becomes a decisive factor for the efficiency of the drug to trigger Ca²⁺ release.

Core skeletal muscle ryanodine receptor calcium release complex

The core skeletal muscle ryanodine receptor (RyR1) calcium release complex extends through three compartments of the muscle fibre, linking the extracellular environment through the cytoplasmic junctional gap to the lumen of the internal sarcoplasmic reticulum (SR) calcium store. The protein complex is essential for skeletal excitation-contraction (EC)-coupling and skeletal muscle function. Its importance is highlighted by perinatal death if any one of the EC-coupling components are missing and by myopathies associated with mutation of any of the proteins. The proteins essential for EC-coupling include the DHPR α_1S subunit in the transverse tubule membrane, the DHPR β_1a subunit in the cytosol and the RyR1 ion channel in the SR membrane. The other core proteins are triadin and junctin and calsequestrin, associated mainly with SR. These SR proteins are not essential for survival but exert structural and functional influences that modify the gain of EC-coupling and maintain normal muscle function. This review summarises our current knowledge of the individual protein/protein interactions within the core complex and their overall contribution to EC-coupling. We highlight significant areas that provide a continuing challenge for the field. Additional important components of the Ca²⁺ release complex, such as FKBP12, calmodulin, S100A1 and Stac3 are identified and reviewed elsewhere.

Coupling of excitation to Ca2+ release is modulated by dysferlin

KEY POINTS

Dysferlin, the protein missing in limb girdle muscular dystrophy 2B and Miyoshi myopathy, concentrates in transverse tubules of skeletal muscle, where it stabilizes voltage-induced Ca²⁺ transients against loss after osmotic shock injury (OSI). Local expression of dysferlin in dysferlin-null myofibres increases transient amplitude to control levels and protects them from loss after OSI. Inhibitors of ryanodine receptors (RyR1) and L-type Ca²⁺ channels protect voltage-induced Ca²⁺ transients from loss; thus both proteins play a role in injury in dysferlin's absence. Effects of Ca²⁺ -free medium and S107, which inhibits SR Ca²⁺ leak, suggest the SR as the primary source of Ca²⁺ responsible for the loss of the Ca²⁺ transient upon injury. Ca²⁺ waves were induced by OSI and suppressed by exogenous dysferlin. We conclude that dysferlin prevents injury-induced SR Ca²⁺ leak.

ABSTRACT

Dysferlin concentrates in the transverse tubules of skeletal muscle and stabilizes Ca²⁺ transients when muscle fibres are subjected to osmotic shock injury (OSI). We show here that voltage-induced Ca²⁺ transients elicited in dysferlin-null A/J myofibres were smaller than control A/WySnJ fibres. Regional expression of Venus-dysferlin chimeras in A/J fibres restored the full amplitude of the Ca²⁺ transients and protected against OSI. We also show that drugs that target ryanodine receptors (RyR1: dantrolene, tetracaine, S107) and L-type Ca²⁺ channels (LTCCs: nifedipine, verapamil, diltiazem) prevented the decrease in Ca²⁺ transients in A/J fibres following OSI. Diltiazem specifically increased transients by ∼20% in uninjured A/J fibres, restoring them to control values. The fact that both RyR1s and LTCCs were involved in OSI-induced damage suggests that damage is mediated by increased Ca²⁺ leak from the sarcoplasmic reticulum (SR) through the RyR1. Congruent with this, injured A/J fibres produced Ca²⁺ sparks and Ca²⁺ waves. S107 (a stabilizer of RyR1-FK506 binding protein coupling that reduces Ca²⁺ leak) or local expression of Venus-dysferlin prevented OSI-induced Ca²⁺ waves. Our data suggest that dysferlin modulates SR Ca²⁺ release in skeletal muscle, and that in its absence OSI causes increased RyR1-mediated Ca²⁺ leak from the SR into the cytoplasm.

Sequencing of genes involved in the movement of calcium across human skeletal muscle sarcoplasmic reticulum: continuing the search for genes associated with malignant hyperthermia

The genetic basis of malignant hyperthermia (MH) is not fully characterised and likely involves more than just the currently classified mutations in the gene encoding the skeletal muscle ryanodine receptor (RYR1) and the gene encoding the α1 subunit of the dihydropyridine receptor (CACNA1S). In this paper we sequence other genes involved in calcium trafficking within skeletal muscle in patients with positive in vitro contracture tests, searching for alternative genes associated with MH. We identified four rare variants in four different genes (CACNB1, CASQ1, SERCA1 and CASQ2) encoding proteins involved in calcium handling in skeletal muscle in a cohort of 30 Australian MH susceptible probands in whom prior complete sequencing of RYR1 and CACNA1S had yielded no rare variants. These four variants have very low minor allele frequencies and while it is tempting to speculate that they have a role in MH, they remain at present variants of unknown significance. Nevertheless they provide the basis for a new set of functional studies, which may indeed identify novel players in MH.

Calcium Dynamics Mediated by the Endoplasmic/Sarcoplasmic Reticulum and Related Diseases

The flow of intracellular calcium (Ca2+) is critical for the activation and regulation of important biological events that are required in living organisms. As the major Ca2+ repositories inside the cell, the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR) of muscle cells are central in maintaining and amplifying the intracellular Ca2+ signal. The morphology of these organelles, along with the distribution of key calcium-binding proteins (CaBPs), regulatory proteins, pumps, and receptors fundamentally impact the local and global differences in Ca2+ release kinetics. In this review, we will discuss the structural and morphological differences between the ER and SR and how they influence localized Ca2+ release, related diseases, and the need for targeted genetically encoded calcium indicators (GECIs) to study these events.

Functional impact of an oculopharyngeal muscular dystrophy mutation in PABPN1

KEY POINTS

Mutations in the gene encoding poly(A)-binding protein nuclear 1 (PABPN1) result in oculopharyngeal muscular dystrophy (OPMD). This disease is of late-onset, but the underlying mechanism is unclear. Ca²⁺ stimulates muscle growth and contraction and, because OPMD courses with muscle atrophy and weakness, we hypothesized that the homeostasis of Ca²⁺ is altered in this disorder. C2C12 myotubes were transfected with cDNAs encoding either PABPN1 or the PABPN1-17A OPMD mutation. Subsequently, they were investigated concerning not only excitation-contraction coupling (ECC) and intracellular levels of Ca²⁺ , but also differentiation stage and nuclear structure. PABPN1-17A gave rise to: inhibition of Ca²⁺ release during ECC, depletion of sarcoplasmic reticulum Ca²⁺ content, reduced expression of ryanodine receptors, altered nuclear morphology and incapability to stimulate myoblast fusion. PABPN1-17A failed to inhibit ECC in adult muscle fibres, suggesting that its effects are primarily related to muscle regeneration.

ABSTRACT

Oculopharyngeal muscular dystrophy (OPMD) is linked to mutations in the gene encoding poly(A)-binding protein nuclear 1 (PABPN1). OPMD mutations consist of an expansion of a tract that contains 10 alanines (to 12-17). This disease courses with muscle weakness that begins in adulthood, but the underlying mechanism is unclear. In the present study, we investigated the functional effects of PABPN1 and an OPMD mutation (PABPN1-17A) using myotubes transfected with cDNAs encoding these proteins (GFP-tagged). PABPN1 stimulated myoblast fusion (100%), whereas PABPN1-17A failed to mimic this effect. Additionally, the OPMD mutation markedly altered nuclear morphology; specifically, it led to nuclei with a more convoluted and ovoid shape. Although PABPN1 and PABPN1-17A modified the expression of sarcoplasmic/endoplasmic reticulum Ca²⁺ -ATPase and calsequestrin, the corresponding changes did not have a clear impact on [Ca²⁺ ]. Interestingly, neither L-type Ca²⁺ channels, nor voltage-gated sarcoplasmic reticulum (SR) Ca²⁺ release (VGCR) was altered by PABPN1. However, PABPN1-17A produced a selective inhibition of VGCR (50%). This effect probably arises from both lower expression of RyR1 and depletion of SR Ca²⁺ . The latter, however, was not related to inhibition of store-operated Ca²⁺ entry. Both PABPN1 constructs promoted a moderated decrease in cytosolic [Ca²⁺ ], which apparently results from down-regulation of excitation-coupled Ca²⁺ entry. On the other hand, PABPN1-17A did not alter ECC in muscle fibres, suggesting that adult muscle is less prone to developing deleterious effects. These results demonstrate that PABPN1 proteins regulate essential processes during myotube formation and support the notion that OPMD involves disruption of myogenesis, nuclear structure and homeostasis of Ca²⁺ .

Calsequestrin depolymerizes when calcium is depleted in the sarcoplasmic reticulum of working muscle

Calsequestrin, the only known protein with cyclical storage and supply of calcium as main role, is proposed to have other functions, which remain unproven. Voluntary movement and the heart beat require this calcium flow to be massive and fast. How does calsequestrin do it? To bind large amounts of calcium in vitro, calsequestrin must polymerize and then depolymerize to release it. Does this rule apply inside the sarcoplasmic reticulum (SR) of a working cell? We answered using fluorescently tagged calsequestrin expressed in muscles of mice. By FRAP and imaging we monitored mobility of calsequestrin as [Ca²⁺] in the SR--measured with a calsequestrin-fused biosensor--was lowered. We found that calsequestrin is polymerized within the SR at rest and that it depolymerized as [Ca²⁺] went down: fully when calcium depletion was maximal (a condition achieved with an SR calcium channel opening drug) and partially when depletion was limited (a condition imposed by fatiguing stimulation, long-lasting depolarization, or low drug concentrations). With fluorescence and electron microscopic imaging we demonstrated massive movements of calsequestrin accompanied by drastic morphological SR changes in fully depleted cells. When cells were partially depleted no remodeling was found. The present results support the proposed role of calsequestrin in termination of calcium release by conformationally inducing closure of SR channels. A channel closing switch operated by calsequestrin depolymerization will limit depletion, thereby preventing full disassembly of the polymeric calsequestrin network and catastrophic structural changes in the SR.

A Calsequestrin-1 Mutation Associated with a Skeletal Muscle Disease Alters Sarcoplasmic Ca2+ Release

An autosomal dominant protein aggregate myopathy, characterized by high plasma creatine kinase and calsequestrin-1 (CASQ1) accumulation in skeletal muscle, has been recently associated with a missense mutation in CASQ1 gene. The mutation replaces an evolutionarily-conserved aspartic acid with glycine at position 244 (p.D244G) of CASQ1, the main sarcoplasmic reticulum (SR) Ca²⁺ binding and storage protein localized at the terminal cisternae of skeletal muscle cells. Here, immunocytochemical analysis of myotubes, differentiated from muscle-derived primary myoblasts, shows that sarcoplasmic vacuolar aggregations positive for CASQ1 are significantly larger in CASQ1-mutated cells than control cells. A strong co-immuno staining of both RyR1 and CASQ1 was also noted in the vacuoles of myotubes and muscle biopsies derived from patients. Electrophysiological recordings and sarcoplasmic Ca²⁺ measurements provide evidence for less Ca²⁺ release from the SR of mutated myotubes when compared to that of controls. These findings further clarify the pathogenic nature of the p.D244G variant and point out defects in sarcoplasmic Ca²⁺ homeostasis as a mechanism underlying this human disease, which could be distinctly classified as “CASQ1-couplonopathy”.

Calsequestrins in skeletal and cardiac muscle from adult Danio rerio

Calsequestrin (Casq) is a high capacity, low affinity Ca(2+)-binding protein, critical for Ca(2+)-buffering in cardiac and skeletal muscle sarcoplasmic reticulum. All vertebrates have multiple genes encoding for different Casq isoforms. Increasing interest has been focused on mammalian and human Casq genes since mutations of both cardiac (Casq2) and skeletal muscle (Casq1) isoforms cause different, and sometime severe, human pathologies. Danio rerio (zebrafish) is a powerful model for studying function and mutations of human proteins. In this work, expression, biochemical properties cellular and sub-cellular localization of D. rerio native Casq isoforms are investigated. By quantitative PCR, three mRNAs were detected in skeletal muscle and heart with different abundances. Three zebrafish Casqs: Casq1a, Casq1b and Casq2 were identified by mass spectrometry (Data are available via ProteomeXchange with identifier PXD002455). Skeletal and cardiac zebrafish calsequestrins share properties with mammalian Casq1 and Casq2. Skeletal Casqs were found primarily, but not exclusively, at the sarcomere Z-line level where terminal cisternae of sarcoplasmic reticulum are located.

Characterization of Two Human Skeletal Calsequestrin Mutants Implicated in Malignant Hyperthermia and Vacuolar Aggregate Myopathy

Calsequestrin 1 is the principal Ca(2+) storage protein of the sarcoplasmic reticulum of skeletal muscle. Its inheritable D244G mutation causes a myopathy with vacuolar aggregates, whereas its M87T "variant" is weakly associated with malignant hyperthermia. We characterized the consequences of these mutations with studies of the human proteins in vitro. Equilibrium dialysis and turbidity measurements showed that D244G and, to a lesser extent, M87T partially lose Ca(2+) binding exhibited by wild type calsequestrin 1 at high Ca(2+) concentrations. D244G aggregates abruptly and abnormally, a property that fully explains the protein inclusions that characterize its phenotype. D244G crystallized in low Ca(2+) concentrations lacks two Ca(2+) ions normally present in wild type that weakens the hydrophobic core of Domain II. D244G crystallized in high Ca(2+) concentrations regains its missing ions and Domain II order but shows a novel dimeric interaction. The M87T mutation causes a major shift of the α-helix bearing the mutated residue, significantly weakening the back-to-back interface essential for tetramerization. D244G exhibited the more severe structural and biophysical property changes, which matches the different pathophysiological impacts of these mutations.