Store-operated Ca2+ entry in malignant hyperthermia-susceptible human skeletal muscle

Ryanodine receptor activity and store-operated Ca2+ entry: Critical regulators of Ca2+ content and function in skeletal muscle

Store-operated Ca2+ entry (SOCE) is critical to cell function. In skeletal muscle, SOCE has evolved alongside excitation-contraction coupling (EC coupling); as a result, it displays unique properties compared to SOCE in other cells. The plasma membrane of skeletal muscle is mostly internalized as the tubular system, with the tubules meeting the sarcoplasmic reticulum (SR) terminal cisternae, forming junctions where the proteins that regulate EC coupling and SOCE are positioned. In this review, we describe the properties and roles of SOCE based on direct measurements of Ca2+ influx during SR Ca2+ release and leak. SOCE is activated immediately and locally as the [Ca2+ ] of the junctional SR terminal cisternae ([Ca2+ ]jSR ) depletes. [Ca2+ ]jSR changes rapidly and steeply with increasing activity of the SR ryanodine receptor isoform 1 (RyR1). The high fidelity of [Ca2+ ]jSR with RyR1 activity probably depends on the SR Ca2+ -buffer calsequestrin that is located immediately behind RyR1 inside the SR. This arrangement provides in-phase activation and deactivation of SOCE with a large dynamic range, allowing precise grading of SOCE flux. The in-phase activation of SOCE as the SR partially depletes traps Ca2+ in the cytoplasm, preventing net Ca2+ loss. Mild presentation of RyR1 leak can occur under physiological conditions, providing fibre Ca2+ redistribution without changing fibre Ca2+ content. This condition preserves normal contractile function at the same time as increasing basal metabolic rate. However, higher RyR1 leak drives excess cytoplasmic and mitochondrial Ca2+ load, setting a deleterious intracellular environment that compromises the function of the skeletal muscle.

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.

Malignant Hyperthermia in PICU—From Diagnosis to Treatment in the Light of Up-to-Date Knowledge

Malignant Hyperthermia (MH) is a rare, hereditary, life-threatening disease triggered by volatile anesthetics and succinylcholine. Rarely, MH can occur after non-pharmacological triggers too. MH was detected more often in children and young adults, which makes this topic very important for every pediatric specialist, both anesthesiologists and intensivists. MH crisis is a life-threatening severe hypermetabolic whole-body reaction. Triggers of MH are used in pediatric intensive care unit (PICU) as well, volatile anesthetics in difficult sedation, status asthmaticus or epilepticus, and succinylcholine still sometimes in airway management. Recrudescence or delayed onset of MH crisis hours after anesthesia was previously described. MH can also be a cause of rhabdomyolysis and hyperpyrexia in the PICU. In addition, patients with neuromuscular diseases are often admitted to PICU and they might be at risk for MH. The most typical symptoms of MH are hypercapnia, tachycardia, hyperthermia, and muscle rigidity. Thinking of the MH as the possible cause of deterioration of a patient’s clinical condition is the key to early diagnosis and treatment. The sooner the correct treatment is commenced, the better patient´s outcome. This narrative review article aims to summarize current knowledge and guidelines about recognition, treatment, and further management of MH in PICU.

Gene Panel Sequencing Identifies a Novel RYR1 p.Ser2300Pro Variant as Candidate for Malignant Hyperthermia with Multi-Minicore Myopathy

Malignant hyperthermia (MH), a rare autosomal dominant pharmacogenetic disorder of skeletal muscle calcium regulation, is triggered by sevoflurane in susceptible individuals. We report a Korean having MH with multi-minicore myopathy functionally supported by RYR1-mediated intracellular Ca2+ release testing in B lymphocytes. A 14-year-old boy was admitted for the evaluation of progressive torticollis accompanied by cervicothoracic scoliosis. During the preoperative drape of the patient for the release of the sternocleidomastoid muscle under general anesthesia, his wrist and ankle were observed to have severe flexion contracture. The body temperature was 37.1 °C. To treat MH, the patient was administered a bolus of dantrolene intravenously (1.5 mg/kg) and sodium bicarbonate. After a few minutes, muscle rigidity, tachycardia, and EtCO2 all resolved. Next-generation panel sequencing for hereditary myopathy identified a novel RYR1 heterozygous missense variant (NM_000540.2: c.6898T > C; p.Ser2300Pro), which mapped to the MH2 domain of the protein, a hot spot for MH mutations. Ex vivo RYR1-mediated intracellular Ca2+ release testing in B lymphocytes showed hypersensitive Ca2+ responses to isoflurane and caffeine, resulting in an abnormal Ca2+ release only in the proband, not in his family members. Our findings expand the clinical and pathological spectra of information associated with MH with multi-minicore myopathy.

Store-Operated Ca2+ Entry in Skeletal Muscle Contributes to the Increase in Body Temperature during Exertional Stress

Exertional heat stroke (HS) is a hyperthermic crisis triggered by an excessive accumulation of Ca²⁺ in skeletal muscle fibers. We demonstrated that exercise leads to the formation of calcium entry units (CEUs), which are intracellular junctions that reduce muscle fatigue by promoting the recovery of extracellular Ca²⁺ via store-operated Ca²⁺ entry (SOCE). Here, we tested the hypothesis that exercise-induced assembly of CEUs may increase the risk of HS when physical activity is performed in adverse environmental conditions (high temperature and humidity). Adult mice were: (a) first, divided into three experimental groups: control, trained-1 month (voluntary running in wheel cages), and acutely exercised-1 h (incremental treadmill run); and (b) then subjected to an exertional stress (ES) protocol, a treadmill run in an environmental chamber at 34 °C and 40% humidity. The internal temperature of the mice at the end of the ES was higher in both pre-exercised groups. During an ES ex-vivo protocol, extensor digitorum longus(EDL) muscles from the trained-1 month and exercised-1 h mice generated greater basal tension than in the control and were those that contained a greater number of CEUs, assessed by electron microscopy. The data collected suggest that the entry of Ca²⁺ from extracellular space via CEUs could contribute to exertional HS when exercise is performed in adverse environmental conditions.

Enhanced Store-Operated Ca2+ Signal of Small Intestinal Smooth Muscle Cells Accelerates Small Bowel Transit Speed in Type 1 Diabetic Mouse

Aims: The underlying mechanism of diabetic enteropathy, a common complication of type 1 diabetes, remains unclear. Store-operated Ca²⁺ entry (SOCE) is a ubiquitous type of Ca²⁺ influx involved in various cellular functions. Here, we show that SOCE-related stromal interaction molecule 1 (STIM1) and Orai1 participate in inappropriate cellular Ca²⁺ homeostasis, augmenting agonist-induced small intestinal smooth muscle contraction and small bowel transit speed in a mouse model of type 1 diabetes.

Methods and Results: We used small interfering (si)RNA to suppress STIM1 and Orai1 proteins, and employed intracellular Ca²⁺, small intestinal contraction and intestinal transit speed measurement to investigate the functional change. We found that SOCE activity and Orai1 and STIM1 expression levels of small intestinal smooth muscle were significantly increased in cells cultured in high glucose medium or in diabetic mice. Gastrointestinal transit speed and SOCE-mediated contractions were markedly increased in diabetic mice; Knocking down Orai1 or STIM1 with siRNA rescued both alterations in diabetic mice. However, the Orai1-large conductance Ca²⁺-activated K⁺ (BK_Ca) channel interaction was decreased in diabetic mice, and suppressing Orai1 expression or inhibiting the BK_Ca channel increased agonist-induced small intestinal contractions in normal mice.

Conclusion: We concluded that the increased SOCE caused by excessive STIM1 and Orai1 expression and decreased Orai1-BK_Ca interaction augmented small intestinal smooth muscle contraction and accelerated small bowel transit speed in diabetic mice. This finding demonstrates a pathological role for SOCE in diabetic enteropathy and provides a potential therapeutic target for diabetic enteropathy.

Altered Ca2+ Handling and Oxidative Stress Underlie Mitochondrial Damage and Skeletal Muscle Dysfunction in Aging and Disease

Skeletal muscle contraction relies on both high-fidelity calcium (Ca²⁺) signals and robust capacity for adenosine triphosphate (ATP) generation. Ca²⁺ release units (CRUs) are highly organized junctions between the terminal cisternae of the sarcoplasmic reticulum (SR) and the transverse tubule (T-tubule). CRUs provide the structural framework for rapid elevations in myoplasmic Ca²⁺ during excitation-contraction (EC) coupling, the process whereby depolarization of the T-tubule membrane triggers SR Ca²⁺ release through ryanodine receptor-1 (RyR1) channels. Under conditions of local or global depletion of SR Ca²⁺ stores, store-operated Ca²⁺ entry (SOCE) provides an additional source of Ca²⁺ that originates from the extracellular space. In addition to Ca²⁺, skeletal muscle also requires ATP to both produce force and to replenish SR Ca²⁺ stores. Mitochondria are the principal intracellular organelles responsible for ATP production via aerobic respiration. This review provides a broad overview of the literature supporting a role for impaired Ca²⁺ handling, dysfunctional Ca²⁺-dependent production of reactive oxygen/nitrogen species (ROS/RNS), and structural/functional alterations in CRUs and mitochondria in the loss of muscle mass, reduction in muscle contractility, and increase in muscle damage in sarcopenia and a wide range of muscle disorders including muscular dystrophy, rhabdomyolysis, central core disease, and disuse atrophy. Understanding the impact of these processes on normal muscle function will provide important insights into potential therapeutic targets designed to prevent or reverse muscle dysfunction during aging and disease.

Molecular Modification of Transient Receptor Potential Canonical 6 Channels Modulates Calcium Dyshomeostasis in a Mouse Model Relevant to Malignant Hyperthermia

BACKGROUND

Pharmacologic modulation has previously shown that transient receptor potential canonical (TRPC) channels play an important role in the pathogenesis of malignant hyperthermia. This study tested the hypothesis that genetically suppressing the function of TRPC6 can partially ameliorate muscle cation dyshomeostasis and the response to halothane in a mouse model relevant to malignant hyperthermia.

METHODS

This study examined the effect of overexpressing a muscle-specific nonconducting dominant-negative TRPC6 channel in 20 RYR1-p.R163C and 20 wild-type mice and an equal number of nonexpressing controls, using calcium- and sodium-selective microelectrodes and Western blots.

RESULTS

RYR1-p.R163C mouse muscles have chronically elevated intracellular calcium and sodium levels compared to wild-type muscles. Transgenic expression of the nonconducting TRPC6 channel reduced intracellular calcium from 331 ± 34 nM (mean ± SD) to 190 ± 27 nM (P < 0.0001) and sodium from 15 ± 1 mM to 11 ± 1 mM (P < 0.0001). Its expression lowered the increase in intracellular Ca2+ of the TRPC6-specific activator hyperforin in RYR1-p.R163C muscle fibers from 52% (348 ± 37 nM to 537 ± 70 nM) to 14% (185 ± 11 nM to 210 ± 44 nM). Western blot analysis of TRPC3 and TRPC6 expression showed the expected increase in TRPC6 caused by overexpression of its dominant-negative transgene and a compensatory increase in expression of TRPC3. Although expression of the muscle-specific dominant-negative TRPC6 was able to modulate the increase in intracellular calcium during halothane exposure and prolonged life (35 ± 5 min vs. 15 ± 3 min; P < 0.0001), a slow, steady increase in calcium began after 20 min of halothane exposure, which eventually led to death.

CONCLUSIONS

These data support previous findings that TRPC channels play an important role in causing the intracellular calcium and sodium dyshomeostasis associated with RYR1 variants that are pathogenic for malignant hyperthermia. However, they also show that modulating TRPC channels alone is not sufficient to prevent the lethal effect of exposure to volatile anesthetic malignant hyperthermia-triggering agents.

EDITOR’S PERSPECTIVE

Malignant hyperthermia 2020: Guideline from the Association of Anaesthetists

Malignant hyperthermia is defined in the International Classification of Diseases as a progressive life-threatening hyperthermic reaction occurring during general anaesthesia. Malignant hyperthermia has an underlying genetic basis, and genetically susceptible individuals are at risk of developing malignant hyperthermia if they are exposed to any of the potent inhalational anaesthetics or suxamethonium. It can also be described as a malignant hypermetabolic syndrome. There are no specific clinical features of malignant hyperthermia and the condition may prove fatal unless it is recognised in its early stages and treatment is promptly and aggressively implemented. The Association of Anaesthetists has previously produced crisis management guidelines intended to be displayed in all anaesthetic rooms as an aide memoire should a malignant hyperthermia reaction occur. The last iteration was produced in 2011 and since then there have been some developments requiring an update. In these guidelines we will provide background information that has been used in updating the crisis management recommendations but will also provide more detailed guidance on the clinical diagnosis of malignant hyperthermia. The scope of these guidelines is extended to include practical guidance for anaesthetists dealing with a case of suspected malignant hyperthermia once the acute reaction has been reversed. This includes information on care and monitoring during and after the event; appropriate equipment and resuscitative measures within the operating theatre and ICU; the importance of communication and teamwork; guidance on counselling of the patient and their family; and how to make a referral of the patient for confirmation of the diagnosis. We also review which patients presenting for surgery may be at increased risk of developing malignant hyperthermia under anaesthesia and what precautions should be taken during the peri-operative management of the patients.

Physiological and Pathological Relevance of Selective and Nonselective Ca2+ Channels in Skeletal and Cardiac Muscle

Contraction of the striated muscle is fundamental for human existence. The action of voluntary skeletal muscle enables activities such as breathing, establishing body posture, and diverse body movements. Additionally, highly precise motion empowers communication, artistic expression, and other activities that define everyday human life. The involuntary contraction of striated muscle is the core function of the heart and is essential for blood flow. Several ion channels are important in the transduction of action potentials to cytosolic Ca2+ signals that enable muscle contraction; however, other ion channels are involved in the progression of muscle pathologies that can impair normal life or threaten it. This chapter describes types of selective and nonselective Ca2+ permeable ion channels expressed in the striated muscle, their participation in different aspects of muscle excitation and contraction, and their relevance to the progression of some pathological states.

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.

Calsequestrin: a well-known but curious protein in skeletal muscle

Calsequestrin (CASQ) was discovered in rabbit skeletal muscle tissues in 1971 and has been considered simply a passive Ca²⁺-buffering protein in the sarcoplasmic reticulum (SR) that provides Ca²⁺ ions for various Ca²⁺ signals. For the past three decades, physiologists, biochemists, and structural biologists have examined the roles of the skeletal muscle type of CASQ (CASQ1) in skeletal muscle and revealed that CASQ1 has various important functions as (1) a major Ca²⁺-buffering protein to maintain the SR with a suitable amount of Ca²⁺ at each moment, (2) a dynamic Ca²⁺ sensor in the SR that regulates Ca²⁺ release from the SR to the cytosol, (3) a structural regulator for the proper formation of terminal cisternae, (4) a reverse-directional regulator of extracellular Ca²⁺ entries, and (5) a cause of human skeletal muscle diseases. This review is focused on understanding these functions of CASQ1 in the physiological or pathophysiological status of skeletal muscle.

Excessive Accumulation of Ca2 + in Mitochondria of Y522S-RYR1 Knock-in Mice: A Link Between Leak From the Sarcoplasmic Reticulum and Altered Redox State

Mice (Y522S or YS), carrying a mutation of the sarcoplasmic reticulum (SR) Ca2+ release channel of skeletal muscle fibers (ryanodine receptor type-1, RyR1) which causes Ca2+ leak, are a widely accepted and intensively studied model for human malignant hyperthermia (MH) susceptibility. Since the involvement of reactive oxygen species (ROS) and of mitochondria in MH crisis has been previously debated, here we sought to determine Ca2+ uptake in mitochondria and its possible link with ROS production in single fibers isolated from flexor digitorum brevis (FDB) of YS mice. We found that Ca2+ concentration in the mitochondrial matrix, as detected with the ratiometric FRET-based 4mtD3cpv probe, was higher in YS than in wild-type (WT) fibers at rest and after Ca2+ release from SR during repetitive electrical stimulation or caffeine administration. Also mitochondrial ROS production associated with contractile activity (detected with Mitosox probe) was much higher in YS fibers than in WT. Importantly, the inhibition of mitochondrial Ca2+ uptake achieved by silencing MCU reduced ROS accumulation in the matrix and Ca2+ release from SR. Finally, inhibition of mitochondrial ROS accumulation using Mitotempo reduced SR Ca2+ release in YS fibers exposed to caffeine. The present results support the view that mitochondria take up larger amounts of Ca2+ in YS than in WT fibers and that mitochondrial ROS production substantially contributes to the increased caffeine-sensitivity and to the enhanced Ca2+ release from SR in YS fibers.

Myopathology of Congenital Myopathies: Bridging the Old and the New

Congenital myopathies (CM) are a genetically heterogeneous group of neuromuscular disorders most commonly presenting with neonatal/childhood-onset hypotonia and muscle weakness, a relatively static or slowly progressive disease course, and originally classified into subcategories based on characteristic histopathologic findings in muscle biopsies. This enduring concept of disease definition and classification based on the clinicopathologic phenotype was pioneered in the premolecular era. Advances in molecular genetics have brought into focus the increased blurring of the original seemingly "watertight" categories through broadening of the clinical phenotypes in existing genes, and continuous identification of novel genetic backgrounds. This review summarizes the histopathologic landscape of the 4 "classical" subtypes of CM-nemaline myopathies, core myopathies, centronuclear myopathies, and congenital fiber type disproportion and some of the emerging and novel genetic diseases with a CM presentation.

Masseter muscle rigidity and the role of DNA analysis to confirm malignant hyperthermia susceptibility

Malignant hyperthermia (MH) is an uncommon, autosomal dominant disorder of skeletal muscle, triggered by inhalational anaesthetics or depolarizing muscle relaxants. Masseter muscle rigidity (MMR) can be regarded as potentially a preceding sign for an MH reaction. Susceptibility to MH can be determined by the in vitro contracture test (IVCT) or DNA analysis where a familial variant is known. Our aims were to review patients with MMR, where IVCT and DNA analysis had been undertaken, to determine if DNA analysis could be used as an initial screening tool for MH susceptibility, and, by reviewing standard monitored variables (SMVs), to determine if any clinical characteristics could be used to differentiate between MMR patients who are MH susceptible (MHS) and those who are not. Patients with MMR were identified from the Palmerston North Hospital MH Reactions Database. IVCT and DNA analysis results were documented. DNA testing was performed retrospectively in the majority of patients as many patients had presented before DNA analysis was available. Forty-one patients were analysed. Fourteen were DNA positive/IVCT positive and six DNA positive only (48% in total), seven were IVCT positive/DNA negative and 14 were IVCT normal. Increased creatine kinase (>18,000 units/L) was consistent with MH susceptibility. Severity of MMR was not linked to MH susceptibility. This study confirmed that DNA analysis can be used as a first-line test for MH susceptibility in patients presenting with MMR (consistent with European MH Group recommendations). Creatine kinase was the only SMV that was significantly different between MHS and MH normal individuals.

Role of STIM1/ORAI1-mediated store-operated Ca2+ entry in skeletal muscle physiology and disease

Store-operated Ca²⁺ entry (SOCE) is a Ca²⁺ entry mechanism activated by depletion of intracellular Ca²⁺ stores. In skeletal muscle, SOCE is mediated by an interaction between stromal-interacting molecule-1 (STIM1), the Ca²⁺ sensor of the sarcoplasmic reticulum, and ORAI1, the Ca²⁺-release-activated-Ca²⁺ (CRAC) channel located in the transverse tubule membrane. This review focuses on the molecular mechanisms and physiological role of SOCE in skeletal muscle, as well as how alterations in STIM1/ORAI1-mediated SOCE contribute to muscle disease. Recent evidence indicates that SOCE plays an important role in both muscle development/growth and fatigue. The importance of SOCE in muscle is further underscored by the discovery that loss- and gain-of-function mutations in STIM1 and ORAI1 result in an eclectic array of disorders with clinical myopathy as central defining component. Despite differences in clinical phenotype, all STIM1/ORAI1 gain-of-function mutations-linked myopathies are characterized by the abnormal accumulation of intracellular membranes, known as tubular aggregates. Finally, dysfunctional STIM1/ORAI1-mediated SOCE also contributes to the pathogenesis of muscular dystrophy, malignant hyperthermia, and sarcopenia. The picture to emerge is that tight regulation of STIM1/ORAI1-dependent Ca²⁺ signaling is critical for optimal skeletal muscle development/function such that either aberrant increases or decreases in SOCE activity result in muscle dysfunction.

With the greatest care, stromal interaction molecule (STIM) proteins verify what skeletal muscle is doing

Skeletal muscle contracts or relaxes to maintain the body position and locomotion. For the contraction and relaxation of skeletal muscle, Ca²⁺ in the cytosol of skeletal muscle fibers acts as a switch to turn on and off a series of contractile proteins. The cytosolic Ca²⁺ level in skeletal muscle fibers is governed mainly by movements of Ca²⁺ between the cytosol and the sarcoplasmic reticulum (SR). Store-operated Ca²⁺ entry (SOCE), a Ca²⁺ entryway from the extracellular space to the cytosol, has gained a significant amount of attention from muscle physiologists. Orai1 and stromal interaction molecule 1 (STIM1) are the main protein identities of SOCE. This mini-review focuses on the roles of STIM proteins and SOCE in the physiological and pathophysiological functions of skeletal muscle and in their correlations with recently identified proteins, as well as historical proteins that are known to mediate skeletal muscle function.

SOCE Is Important for Maintaining Sarcoplasmic Calcium Content and Release in Skeletal Muscle Fibers

Store-operated Ca²⁺ entry (SOCE) is a Ca²⁺-entry process activated by the depletion of intracellular stores and has an important role in many cell types. In skeletal muscle, however, its role during physiological muscle activation has been controversial. To address this question, sarcoplasmic reticulum (SR) calcium release in a mouse strain with a naturally occurring mutation in the myostatin gene (Compact (Cmpt)) leading to a hypermuscular yet reduced muscle-force phenotype was compared to that in wild-type mice. To elicit Ca²⁺ release from the SR of flexor digitorum brevis (FDB) fibers, either a ryanodine receptor agonist (4-chloro-meta-cresol) or depolarizing pulses were used. In muscles from Cmpt mice, endogenous protein levels of STIM1 and Orai1 were reduced, and consequently, SOCE after 4-chloro-meta-cresol-induced store depletion was suppressed. Although the voltage dependence of SR calcium release was not statistically different between wild-type and Cmpt fibers, the amount of releasable calcium was significantly reduced in the latter, indicating a smaller SR content. To assess the immediate role of SOCE in replenishing the SR calcium store, the evolution of intracellular calcium concentration during a train of long-lasting depolarizations to a maximally activating voltage was monitored. Cmpt mice exhibited a faster decline in calcium release, suggesting a compromised ability to refill the SR. A simple model that incorporates a reduced SOCE as an important partner in regulating immediate calcium influx through the surface membrane readily accounts for the steady-state reduction in SR calcium content and its more pronounced decline after calcium release.

A focus on extracellular Ca2+ entry into skeletal muscle

The main task of skeletal muscle is contraction and relaxation for body movement and posture maintenance. During contraction and relaxation, Ca²⁺ in the cytosol has a critical role in activating and deactivating a series of contractile proteins. In skeletal muscle, the cytosolic Ca²⁺ level is mainly determined by Ca²⁺ movements between the cytosol and the sarcoplasmic reticulum. The importance of Ca²⁺ entry from extracellular spaces to the cytosol has gained significant attention over the past decade. Store-operated Ca²⁺ entry with a low amplitude and relatively slow kinetics is a main extracellular Ca²⁺ entryway into skeletal muscle. Herein, recent studies on extracellular Ca²⁺ entry into skeletal muscle are reviewed along with descriptions of the proteins that are related to extracellular Ca²⁺ entry and their influences on skeletal muscle function and disease.

Strenuous exercise triggers a life-threatening response in mice susceptible to malignant hyperthermia

In humans, hyperthermic episodes can be triggered by halogenated anesthetics [malignant hyperthermia (MH) susceptibility] and by high temperature [environmental heat stroke (HS)]. Correlation between MH susceptibility and HS is supported by extensive work in mouse models that carry a mutation in ryanodine receptor type-1 (RYR1^Y522S/WT) and calsequestrin-1 knockout (CASQ1-null), 2 proteins that control Ca²⁺ release in skeletal muscle. As overheating episodes in humans have also been described during exertion, here we subjected RYR1^Y522S/WT and CASQ1-null mice to an exertional-stress protocol (incremental running on a treadmill at 34°C and 40% humidity). The mortality rate was 80 and 78.6% in RYR1^Y522S/WT and CASQ1-null mice, respectively, vs. 0% in wild-type mice. Lethal crises were characterized by hyperthermia and rhabdomyolysis, classic features of MH episodes. Of importance, pretreatment with azumolene, an analog of the drug used in humans to treat MH crises, reduced mortality to 0 and 12.5% in RYR1^Y522S/WT and CASQ1-null mice, respectively, thanks to a striking reduction of hyperthermia and rhabdomyolysis. At the molecular level, azumolene strongly prevented Ca²⁺-dependent activation of calpains and NF-κB by lowering myoplasmic Ca²⁺ concentration and nitro-oxidative stress, parameters that were elevated in RYR1^Y522S/WT and CASQ1-null mice. These results suggest that common molecular mechanisms underlie MH crises and exertional HS in mice.-Michelucci, A., Paolini, C., Boncompagni, S., Canato, M., Reggiani, C., Protasi, F. Strenuous exercise triggers a life-threatening response in mice susceptible to malignant hyperthermia.

Dantrolene requires Mg2+ to arrest malignant hyperthermia

Malignant hyperthermia (MH) is a clinical syndrome of skeletal muscle that presents as a hypermetabolic response to volatile anesthetic gases, where susceptible persons may develop lethally high body temperatures. Genetic predisposition mainly arises from mutations on the skeletal muscle ryanodine receptor (RyR). Dantrolene is administered to alleviate MH symptoms, but its mechanism of action and its influence on the Ca²⁺ transients elicited by MH triggers are unknown. Here, we show that Ca²⁺ release in the absence of Mg²⁺ is unaffected by the presence of dantrolene but that dantrolene becomes increasingly effective as cytoplasmic-free [Mg²⁺] (free [Mg²⁺]_cyto) passes mM levels. Furthermore, we found in human muscle susceptible to MH that dantrolene was ineffective at reducing halothane-induced repetitive Ca²⁺ waves in the presence of resting levels of free [Mg²⁺]_cyto (1 mM). However, an increase of free [Mg²⁺]_cyto to 1.5 mM could increase the period between Ca²⁺ waves. These results reconcile previous contradictory reports in muscle fibers and isolated RyRs, where Mg²⁺ is present or absent, respectively, and define the mechanism of action of dantrolene is to increase the Mg²⁺ affinity of the RyR (or "stabilize" the resting state of the channel) and suggest that the accumulation of the metabolite Mg²⁺ from MgATP hydrolysis is required to make dantrolene administration effective in arresting an MH episode.

Pharmacogenetics and pathophysiology of CACNA1S mutations in malignant hyperthermia

A review of the pharmacogenetics (PGt) and pathophysiology of calcium voltage-gated channel subunit alpha1 S (CACNA1S) mutations in malignant hyperthermia susceptibility type 5 (MHS5; MIM #60188) is presented. Malignant hyperthermia (MH) is a life-threatening hypermetabolic state of skeletal muscle usually induced by volatile, halogenated anesthetics and/or the depolarizing neuromuscular blocker succinylcholine. In addition to ryanodine receptor 1 (RYR1) mutations, several CACNA1S mutations are known to be risk factors for increased susceptibility to MH (MHS). However, the presence of these pathogenic CACNA1S gene variations cannot be used to positively predict MH since the condition is genetically heterogeneous with variable expression and incomplete penetrance. At present, one or at most six CACNA1S mutations display significant linkage or association either to clinically diagnosed MH or to MHS as determined by contracture testing. Additional pathogenic variants in CACNA1S, either alone or in combination with genes affecting Ca²⁺ homeostasis, are likely to be discovered in association to MH as whole exome sequencing becomes more commonplace.

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

A novel category of diseases of striated muscle is proposed, the couplonopathies, as those that affect components of the couplon and thereby alter its operation. Couplons are the functional units of intracellular calcium release in excitation–contraction coupling. They comprise dihydropyridine receptors, ryanodine receptors (Ca²⁺ release channels), and a growing list of ancillary proteins whose alteration may lead to disease. Within a generally similar plan, the couplons of skeletal and cardiac muscle show, in a few places, marked structural divergence associated with critical differences in the mechanisms whereby they fulfill their signaling role. Most important among these are the presence of a mechanical or allosteric communication between voltage sensors and Ca²⁺ release channels, exclusive to the skeletal couplon, and the smaller capacity of the Ca stores in cardiac muscle, which results in greater swings of store concentration during physiological function. Consideration of these structural and functional differences affords insights into the pathogenesis of several couplonopathies. The exclusive mechanical connection of the skeletal couplon explains differences in pathogenesis between malignant hyperthermia (MH) and catecholaminergic polymorphic ventricular tachycardia (CPVT), conditions most commonly caused by mutations in homologous regions of the skeletal and cardiac Ca²⁺ release channels. Based on mechanistic considerations applicable to both couplons, we identify the plasmalemma as a site of secondary modifications, typically an increase in store-operated calcium entry, that are relevant in MH pathogenesis. Similar considerations help explain the different consequences that mutations in triadin and calsequestrin have in these two tissues. As more information is gathered on the composition of cardiac and skeletal couplons, this comparative and mechanistic approach to couplonopathies should be useful to understand pathogenesis, clarify diagnosis, and propose tissue-specific drug development.

Store-operated calcium entry and diabetic complications

Store-operated Ca(2+) entry (SOCE) is mediated by the store-operated Ca(2+) channel (SOC) that opens upon depletion of internal Ca(2+) stores following activation of G protein-coupled receptors or receptor tyrosine kinases. Over the past two decades, the physiological and pathological relevance of SOCE has been extensively studied. Recently, accumulating evidence suggests associations of altered SOCE with diabetic complications. This review focuses on the implication of SOCE as it pertains to various complications resulting from diabetes. We summarize recent findings by us and others on the involvement of abnormal SOCE in the development of diabetic complications, such as diabetic nephropathy and diabetic vasculopathy. The underlying mechanisms that mediate the diabetes-associated alterations of SOCE are also discussed. The SOCE pathway may be considered as a potential therapeutic target for diabetes-associated diseases.

Bilateral congenital lumbar hernias in a patient with central core disease--A case report

Congenital lumbar hernias are rare malformations caused by defects in the development of the posterior abdominal wall. A known association exists with lumbocostovertebral syndrome; however other associated anomalies, including one case with arthrogryposis, have been previously reported. We present an infant girl with bilateral congenital lumbar hernias, multiple joint contractures, decreased muscle bulk and symptoms of malignant hyperthermia. Molecular testing revealed an R4861C mutation in the ryanodine receptor 1 (RYR1) gene, known to be associated with central core disease. This is the first reported case of the co-occurrence of congenital lumbar hernias and central core disease. We hypothesize that ryanodine receptor 1 mutations may interrupt muscle differentiation and development. Further, this case suggests an expansion of the ryanodine receptor 1-related myopathy phenotype to include congenital lumbar hernias.

Antioxidants protect calsequestrin-1 knockout mice from halothane- and heat-induced sudden death

BACKGROUND

Mice lacking calsequestrin-1 (CASQ1-null), a Ca-binding protein that modulates the activity of Ca release in the skeletal muscle, exhibit lethal hypermetabolic episodes that resemble malignant hyperthermia in humans when exposed to halothane or heat stress.

METHODS

Because oxidative species may play a critical role in malignant hyperthermia crises, we treated CASQ1-null mice with two antioxidants, N-acetylcysteine (NAC, Sigma-Aldrich, Italy; provided ad libitum in drinking water) and (±)-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox, Sigma-Aldrich; administered by intraperitoneal injection), before exposure to halothane (2%, 1 h) or heat (41°C, 1 h).

RESULTS

NAC and Trolox significantly protected CASQ1-null mice from lethal episodes, with mortality being 79% (n = 14), 25% (n = 16), and 20% (n = 5) during halothane exposure and 86% (n = 21), 29% (n = 21), and 33% (n = 6) during heat stress in untreated, NAC-treated, and Trolox-treated mice, respectively. During heat challenge, an increase in core temperature in CASQ1-null mice (42.3° ± 0.1°C, n=10) was significantly reduced by both NAC and Trolox (40.6° ± 0.3°C, n = 6 and 40.5° ± 0.2°C, n = 6). NAC treatment of CASQ1-null muscles/mice normalized caffeine sensitivity during in vitro contracture tests, Ca transients in single fibers, and significantly reduced the percentage of fibers undergoing rhabdomyolysis (37.6 ± 2.5%, 38/101 fibers in 3 mice; 11.6 ± 1.1%, 21/186 fibers in 5 mice). The protective effect of antioxidant treatment likely resulted from mitigation of oxidative stress, because NAC reduced mitochondrial superoxide production, superoxide dismutase type-1 expression, and 3-nitrotyrosine expression, and increased both reduced glutathione and reduced glutathione/oxidized glutathione ratio.

CONCLUSION

These studies provide a deeper understanding of the mechanisms that underlie hyperthermic crises in CASQ1-deficient muscle and demonstrate that antioxidant pretreatment may prevent them.

Malignant hyperthermia: a review

Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle that presents as a hypermetabolic response to potent volatile anesthetic gases such as halothane, sevoflurane, desflurane, isoflurane and the depolarizing muscle relaxant succinylcholine, and rarely, in humans, to stressors such as vigorous exercise and heat. The incidence of MH reactions ranges from 1:10,000 to 1: 250,000 anesthetics. However, the prevalence of the genetic abnormalities may be as great as one in 400 individuals. MH affects humans, certain pig breeds, dogs and horses. The classic signs of MH include hyperthermia, tachycardia, tachypnea, increased carbon dioxide production, increased oxygen consumption, acidosis, hyperkalaemia, muscle rigidity, and rhabdomyolysis, all related to a hypermetabolic response. The syndrome is likely to be fatal if untreated. An increase in end-tidal carbon dioxide despite increased minute ventilation provides an early diagnostic clue. In humans the syndrome is inherited in an autosomal dominant pattern, while in pigs it is autosomal recessive. Uncontrolled rise of myoplasmic calcium, which activates biochemical processes related to muscle activation leads to the pathophysiologic changes. In most cases, the syndrome is caused by a defect in the ryanodine receptor. Over 400 variants have been identified in the RYR1 gene located on chromosome 19q13.1, and at least 34 are causal for MH. Less than 1 % of variants have been found in CACNA1S but not all of these are causal. Diagnostic testing involves the in vitro contracture response of biopsied muscle to halothane, caffeine, and in some centres ryanodine and 4-chloro-m-cresol. Elucidation of the genetic changes has led to the introduction of DNA testing for susceptibility to MH. Dantrolene sodium is a specific antagonist and should be available wherever general anesthesia is administered. Increased understanding of the clinical manifestation and pathophysiology of the syndrome, has lead to the mortality decreasing from 80 % thirty years ago to <5 % in 2006.

Store-operated Ca2+ entry in muscle physiology and diseases

Ca(2+) release from intracellular stores and influx from extracellular reservoir regulate a wide range of physiological functions including muscle contraction and rhythmic heartbeat. One of the most ubiquitous pathways involved in controlled Ca(2+) influx into cells is store-operated Ca(2+) entry (SOCE), which is activated by the reduction of Ca(2+) concentration in the lumen of endoplasmic or sarcoplasmic reticulum (ER/SR). Although SOCE is pronounced in non-excitable cells, accumulating evidences highlight its presence and important roles in skeletal muscle and heart. Recent discovery of STIM proteins as ER/SR Ca(2+) sensors and Orai proteins as Ca(2+) channel pore forming unit expedited the mechanistic understanding of this pathway. This review focuses on current advances of SOCE components, regulation and physiologic and pathophysiologic roles in muscles. The specific property and the dysfunction of this pathway in muscle diseases, and new directions for future research in this rapidly growing field are discussed.

Triadopathies: an emerging class of skeletal muscle diseases

The triad is a skeletal muscle substructure responsible for the regulation of excitation-contraction coupling. It is formed by the close apposition of the T-tubule and the terminal sarcoplasmic reticulum. A rapidly growing list of skeletal myopathies, here referred to as triadopathies, are caused by gene mutations in components of the triad. These disorders, at their root, are caused by defects in excitation contraction coupling and intracellular calcium homeostasis. Secondary abnormalities in triad structure and/or function are also reported in several muscle diseases, most notably certain muscular dystrophies. This review highlights the current understanding of both primary and secondary triadopathies, and identifies important concepts yet to be fully addressed in the field. The emphasis of the review is both on the pathogenesis of triadopathies and their potential treatment.

Orai1-dependent calcium entry promotes skeletal muscle growth and limits fatigue

Store-operated Ca²⁺ entry (SOCE) in skeletal muscle involves signaling between stromal interaction molecule 1 (STIM1) in the sarcoplasmic reticulum and Ca²⁺ selective Orai1 channels in the sarcolemma. Here we generate transgenic mice with muscle-specific expression of dominant-negative Orai1 (dnOrai1) and demonstrate that Orai1-dependent SOCE promotes growth and limits fatigue in adult skeletal muscle. dnOrai1 mice lack SOCE specifically in muscle but are fertile and thrive well into adulthood. Although muscle ultrastructure, excitation-contraction coupling fiber type, and expression of other Ca²⁺ regulatory proteins are unaltered, dnOrai1 mice exhibit reduced body weight, muscle mass, and fiber cross-sectional area. Importantly, during intense repetitive activity, dnOrai1 mice display increased susceptibility to fatigue at the single fibre, excised muscle, and whole animal levels. We further show that STIM1 and Orai1 proteins colocalise within the triad junction but do not exist in a preassembled context. These results show that Orai1-dependent SOCE has an important physiological role in muscles of adult mice.

Activation and propagation of Ca2+ release from inside the sarcoplasmic reticulum network of mammalian skeletal muscle

Skeletal muscle fibres are large and highly elongated cells specialized for producing the force required for posture and movement. The process of controlling the production of force within the muscle, known as excitation-contraction coupling, requires virtually simultaneous release of large amounts of Ca(2+) from the sarcoplasmic reticulum (SR) at the level of every sarcomere within the muscle fibre. Here we imaged Ca(2+) movements within the SR, tubular (t-) system and in the cytoplasm to observe that the SR of skeletal muscle is a connected network capable of allowing diffusion of Ca(2+) within its lumen to promote the propagation of Ca(2+) release throughout the fibre under conditions where inhibition of SR ryanodine receptors (RyRs) was reduced. Reduction of cytoplasmic [Mg(2+)] ([Mg(2+)]cyto) induced a leak of Ca(2+) through RyRs, causing a reduction in SR Ca(2+) buffering power argued to be due to a breakdown of SR calsequestrin polymers, leading to a local elevation of [Ca(2+)]SR. The local rise in [Ca(2+)]SR, an intra-SR Ca(2+) transient, induced a local diffusely rising [Ca(2+)]cyto. A prolonged Ca(2+) wave lasting tens of seconds or more was generated from these events. Ca(2+) waves were dependent on the diffusion of Ca(2+) within the lumen of the SR and ended as [Ca(2+)]SR dropped to low levels to inactivate RyRs. Inactivation of RyRs allowed re-accumulation of [Ca(2+)]SR and the activation of secondary Ca(2+) waves in the persistent presence of low [Mg(2+)]cyto if the threshold [Ca(2+)]SR for RyR opening could be reached. Secondary Ca(2+) waves occurred without an abrupt reduction in SR Ca(2+) buffering power. Ca(2+) release and wave propagation occurred in the absence of Ca(2+)-induced Ca(2+) release. These observations are consistent with the activation of Ca(2+) release through RyRs of lowered cytoplasmic inhibition by [Ca(2+)]SR or store overload-induced Ca(2+) release. Restitution of SR Ca(2+) buffering power to its initially high value required imposing normal resting ionic conditions in the cytoplasm, which re-imposed the normal resting inhibition on the RyRs, allowing [Ca(2+)]SR to return to endogenous levels without activation of store overload-induced Ca(2+) release. These results are discussed in the context of how pathophysiological Ca(2+) release such as that occurring in malignant hyperthermia can be generated.

DNA testing for malignant hyperthermia: the reality and the dream

The advent of the polymerase chain reaction and the availability of data from various global human genome projects should make it possible, using a DNA sample isolated from white blood cells, to diagnose rapidly and accurately almost any monogenic condition resulting from single nucleotide changes. DNA-based diagnosis for malignant hyperthermia (MH) is an attractive proposition, because it could replace the invasive and morbid caffeine-halothane/in vitro contracture tests of skeletal muscle biopsy tissue. Moreover, MH is preventable if an accurate diagnosis of susceptibility can be made before general anesthesia, the most common trigger of an MH episode. Diagnosis of MH using DNA was suggested as early as 1990 when the skeletal muscle ryanodine receptor gene (RYR1), and a single point mutation therein, was linked to MH susceptibility. In 1994, a single point mutation in the α 1 subunit of the dihydropyridine receptor gene (CACNA1S) was identified and also subsequently shown to be causative of MH. In the succeeding years, the number of identified mutations in RYR1 has grown, as has the number of potential susceptibility loci, although no other gene has yet been definitively associated with MH. In addition, it has become clear that MH is associated with either of these 2 genes (RYR1 and CACNA1S) in only 50% to 70% of affected families. While DNA testing for MH susceptibility has now become widespread, it still does not replace the in vitro contracture tests. Whole exome sequence analysis makes it potentially possible to identify all variants within human coding regions, but the complexity of the genome, the heterogeneity of MH, the limitations of bioinformatic tools, and the lack of precise genotype/phenotype correlations are all confounding factors. In addition, the requirement for demonstration of causality, by in vitro functional analysis, of any familial mutation currently precludes DNA-based diagnosis as the sole test for MH susceptibility. Nevertheless, familial DNA testing for MH susceptibility is now widespread although limited to a positive diagnosis and to those few mutations that have been functionally characterized. Identification of new susceptibility genes remains elusive. When new genes are identified, it will be the role of the biochemists, physiologists, and biophysicists to devise functional assays in appropriate systems. This will remain the bottleneck unless high throughput platforms can be designed for functional work. Analysis of entire genomes from several individuals simultaneously is a reality. DNA testing for MH, based on current criteria, remains the dream.

The ryanodine receptor agonist 4-chloro-3-ethylphenol blocks ORAI store-operated channels

BACKGROUND

Depletion of the Ca(2+) store by ryanodine receptor (RyR) agonists induces store-operated Ca(2+) entry (SOCE). 4-Chloro-3-ethylphenol (4-CEP) and 4-chloro-m-cresol (4-CmC) are RyR agonists commonly used as research tools and diagnostic reagents for malignant hyperthermia. Here, we investigated the effects of 4-CEP and its analogues on SOCE.

EXPERIMENTAL APPROACH

SOCE and ORAI1-3 currents were recorded by Ca(2+) imaging and whole-cell patch recordings in rat L6 myoblasts and in HEK293 cells overexpressing STIM1/ORAI1-3.

KEY RESULTS

4-CEP induced a significant release of Ca(2+) in rat L6 myoblasts, but inhibited SOCE. The inhibitory effect was concentration-dependent and more potent than its analogues 4-CmC and 4-chlorophenol (4-ClP). In the HEK293 T-REx cells overexpressing STIM1/ORAI1-3, 4-CEP inhibited the ORAI1, ORAI2 and ORAI3 currents evoked by thapsigargin. The 2-APB-induced ORAI3 current was also blocked by 4-CEP. This inhibitory effect was reversible and independent of the Ca(2+) release. The two analogues, 4-CmC and 4-ClP, also inhibited the ORAI1-3 channels. Excised patch and intracellular application of 4-CEP demonstrated that the action site was located extracellularly. Moreover, 4-CEP evoked STIM1 translocation and subplasmalemmal clustering through its Ca(2+) store-depleting effect via the activation of RyR, but no effect on STIM1 redistribution was observed in cells co-expressing STIM1/ORAI1-3.

CONCLUSION AND IMPLICATIONS

4-CEP not only acts as a RyR agonist to deplete the Ca(2+) store and trigger STIM1 subplasmalemmal translocation and clustering, but also directly inhibits ORAI1-3 channels. These findings demonstrate a novel pharmacological property for the chlorophenol derivatives that act as RyR agonists.

Ryanodine receptors as leak channels

Ryanodine receptors are Ca(2+) release channels of internal stores. This review focuses on those situations and conditions that transform RyRs from a finely regulated ion channel to an unregulated Ca(2+) leak channel and the pathological consequences of this alteration. In skeletal muscle, mutations in either CaV1.1 channel or RyR1 results in a leaky behavior of the latter. In heart cells, RyR2 functions normally as a Ca(2+) leak channel during diastole within certain limits, the enhancement of this activity leads to arrhythmogenic situations that are tackled with different pharmacological strategies. In smooth muscle, RyRs are involved more in reducing excitability than in stimulating contraction so the leak activity of RyRs in the form of Ca(2+) sparks, locally activates Ca(2+)-dependent potassium channels to reduce excitability. In neurons the enhanced activity of RyRs is associated with the development of different neurodegenerative disorders such as Alzheimer and Huntington diseases. It appears then that the activity of RyRs as leak channels can have both physiological and pathological consequences depending on the cell type and the metabolic condition.

Store-operated Ca²⁺ entry is not required for store refilling in skeletal muscle

The present review describes store-operated Ca²⁺ entry (SOCE) in skeletal muscle. Fundamental discoveries in the field of skeletal muscle SOCE are described and the techniques that were used to make these. The advantages and limitations in these techniques are discussed to provide a means of questioning and determining the physiological role(s) of SOCE in skeletal muscle. It is concluded that SOCE has little or no role in the filling of the sarcoplasmic reticulum with Ca²⁺ at rest or during a single contracture. It is likely that SOCE is activated during fatigue, although direct measurements of SOCE are lacking and the physiological significance remains uncertain.

Accelerated activation of SOCE current in myotubes from two mouse models of anesthetic- and heat-induced sudden death

Store-operated calcium entry (SOCE) channels play an important role in Ca²⁺ signaling. Recently, excessive SOCE was proposed to play a central role in the pathogenesis of malignant hyperthermia (MH), a pharmacogenic disorder of skeletal muscle. We tested this hypothesis by characterizing SOCE current (I_SkCRAC) magnitude, voltage dependence, and rate of activation in myotubes derived from two mouse models of anesthetic- and heat-induced sudden death: 1) type 1 ryanodine receptor (RyR1) knock-in mice (Y524S/+) and 2) calsequestrin 1 and 2 double knock-out (dCasq-null) mice. I_SkCRAC voltage dependence and magnitude at -80 mV were not significantly different in myotubes derived from wild type (WT), Y524S/+ and dCasq-null mice. However, the rate of I_SkCRAC activation upon repetitive depolarization was significantly faster at room temperature in myotubes from Y524S/+ and dCasq-null mice. In addition, the maximum rate of I_SkCRAC activation in dCasq-null myotubes was also faster than WT at more physiological temperatures (35-37°C). Azumolene (50 µM), a more water-soluble analog of dantrolene that is used to reverse MH crises, failed to alter I_SkCRAC density or rate of activation. Together, these results indicate that while an increased rate of I_SkCRAC activation is a common characteristic of myotubes derived from Y524S/+ and dCasq-null mice and that the protective effects of azumolene are not due to a direct inhibition of SOCE channels.

STIM1 negatively regulates Ca²⁺ release from the sarcoplasmic reticulum in skeletal myotubes

STIM1 (stromal interaction molecule 1) mediates SOCE (store-operated Ca²⁺ entry) in skeletal muscle. However, the direct role(s) of STIM1 in skeletal muscle, such as Ca²⁺ release from the SR (sarcoplasmic reticulum) for muscle contraction, have not been identified. The times required for the maximal expression of endogenous STIM1 or Orai1, or for the appearance of puncta during the differentiation of mouse primary skeletal myoblasts to myotubes, were all different, and the formation of puncta was detected with no stimulus during differentiation, suggesting that, in skeletal muscle, the formation of puncta is a part of the differentiation. Wild-type STIM1 and two STIM1 mutants (Triple mutant, missing Ca²⁺-sensing residues but possessing the intact C-terminus; and E136X, missing the C-terminus) were overexpressed in the myotubes. The wild-type STIM1 increased SOCE, whereas neither mutant had an effect on SOCE. It was interesting that increases in the formation of puncta were observed in the Triple mutant as well as in wild-type STIM1, suggesting that SOCE-irrelevant puncta could exist in skeletal muscle. On the other hand, overexpression of wild-type or Triple mutant, but not E136X, attenuated Ca²⁺ releases from the SR in response to KCl [evoking ECC (excitation-contraction coupling) via activating DHPR (dihydropyridine receptor)] in a dominant-negative manner. The attenuation was removed by STIM1 knockdown, and STIM1 was co-immunoprecipitated with DHRP in a Ca²⁺-independent manner. These results suggest that STIM1 negatively regulates Ca²⁺ release from the SR through the direct interaction of the STIM1 C-terminus with DHPR, and that STIM1 is involved in both ECC and SOCE in skeletal muscle.

Involvement of TRPV2 and SOCE in calcium influx disorder in DMD primary human myotubes with a specific contribution of α1-syntrophin and PLC/PKC in SOCE regulation

Calcium homeostasis is critical for several vital functions in excitable and nonexcitable cells and has been shown to be impaired in many pathologies including Duchenne muscular dystrophy (DMD). Various studies using murine models showed the implication of calcium entry in the dystrophic phenotype. However, alteration of store-operated calcium entry (SOCE) and transient receptor potential vanilloid 2 (TRPV2)-dependant cation entry has not been investigated yet in human skeletal muscle cells. We pharmacologically characterized basal and store-operated cation entries in primary cultures of myotubes prepared from muscle of normal and DMD patients and found, for the first time, an increased SOCE in DMD myotubes. Moreover, this increase cannot be explained by an over expression of the well-known SOCE actors: TRPC1/4, Orai1, and stromal interaction molecule 1 (STIM1) mRNA and proteins. Thus we investigated the modes of regulation of this cation entry. We firstly demonstrated the important role of the scaffolding protein α1-syntrophin, which regulates SOCE in primary human myotubes through its PDZ domain. We also studied the implication of phospholipase C (PLC) and protein kinase C (PKC) in SOCE and showed that their inhibition restores normal levels of SOCE in DMD human myotubes. In addition, the involvement of TRPV2 in calcium deregulation in DMD human myotubes was explored. We showed an abnormal elevation of TRPV2-dependant cation entry in dystrophic primary human myotubes compared with normal ones. These findings show that calcium homeostasis mishandling in DMD myotubes depends on SOCE under the influence of Ca2+/PLC/PKC pathway and α1-syntrophin regulation as well as on TRPV2-dependant cation influx.

Temperature and RyR1 regulate the activation rate of store-operated Ca²+ entry current in myotubes

Store-operated calcium entry (SOCE) is an important Ca(2+) entry pathway in skeletal muscle. However, direct electrophysiological recording and full characterization of the underlying SOCE current in skeletal muscle cells (I(SkCRAC)) has not been reported. Here, we characterized the biophysical properties, pharmacological profile, and molecular identity of I(SkCRAC) in skeletal myotubes, as well as the regulation of its rate of activation by temperature and the type I ryanodine receptor (RyR1). I(SkCRAC) exhibited many hallmarks of Ca(2+) release activated Ca(2+) currents (I(CRAC)): store dependence, strong inward rectification, positive reversal potential, limited cesium permeability, and sensitivity to SOCE channel blockers. I(SkCRAC) was reduced by siRNA knockdown of stromal interaction molecule 1 and expression of dominant negative Orai1. Average I(SkCRAC) current density at -80mV was 1.00 ± 0.05 pA/pF. In the presence of 20 mM intracellular EGTA, I(SkCRAC) activation occurred over tens of seconds during repetitive depolarization at 0.5Hz and was inhibited by treatment with 100 μM ryanodine. The rate of SOCE activation was reduced threefold in myotubes from RyR1-null mice and increased 4.6-fold at physiological temperatures (35-37°C). These results show that I(SkCRAC) exhibits similar biophysical, pharmacological, and molecular properties as I(CRAC) in nonexcitable cells and its rate of activation during repetitive depolarization is strongly regulated by temperature and RyR1 activity.

Changes in plasma membrane Ca-ATPase and stromal interacting molecule 1 expression levels for Ca(2+) signaling in dystrophic mdx mouse muscle

The majority of the skeletal muscle plasma membrane is internalized as part of the tubular (t-) system, forming a standing junction with the sarcoplasmic reticulum (SR) membrane throughout the muscle fiber. This arrangement facilitates not only a rapid and large release of Ca(2+) from the SR for contraction upon excitation of the fiber, but has also direct implications for other interdependent cellular regulators of Ca(2+). The t-system plasma membrane Ca-ATPase (PMCA) and store-operated Ca(2+) entry (SOCE) can also be activated upon release of SR Ca(2+). In muscle, the SR Ca(2+) sensor responsible for rapidly activated SOCE appears to be the stromal interacting molecule 1L (STIM1L) isoform of STIM1 protein, which directly interacts with the Orai1 Ca(2+) channel in the t-system. The common isoform of STIM1 is STIM1S, and it has been shown that STIM1 together with Orai1 in a complex with the partner protein of STIM (POST) reduces the activity of the PMCA. We have previously shown that Orai1 and STIM1 are upregulated in dystrophic mdx mouse muscle, and here we show that STIM1L and PMCA are also upregulated in mdx muscle. Moreover, we show that the ratios of STIM1L to STIM1S in wild-type (WT) and mdx muscle are not different. We also show a greater store-dependent Ca(2+) influx in mdx compared with WT muscle for similar levels of SR Ca(2+) release while normal activation and deactivation properties were maintained. Interestingly, the fiber-averaged ability of WT and mdx muscle to extrude Ca(2+) via PMCA was found to be the same despite differences in PMCA densities. This suggests that there is a close relationship among PMCA, STIM1L, STIM1S, Orai1, and also POST expression in mdx muscle to maintain the same Ca(2+) extrusion properties as in the WT muscle.

STIM1-Ca(2+) signaling is required for the hypertrophic growth of skeletal muscle in mice

Immediately after birth, skeletal muscle must undergo an enormous period of growth and differentiation that is coordinated by several intertwined growth signaling pathways. How these pathways are integrated remains unclear but is likely to involve skeletal muscle contractile activity and calcium (Ca(2+)) signaling. Here, we show that Ca(2+) signaling governed by stromal interaction molecule 1 (STIM1) plays a central role in the integration of signaling and, therefore, muscle growth and differentiation. Conditional deletion of STIM1 from the skeletal muscle of mice (mSTIM1(-/-) mice) leads to profound growth delay, reduced myonuclear proliferation, and perinatal lethality. We show that muscle fibers of neonatal mSTIM1(-/-) mice cannot support the activity-dependent Ca(2+) transients evoked by tonic neurostimulation, even though excitation contraction coupling (ECC) remains unperturbed. In addition, disruption of tonic Ca(2+) signaling in muscle fibers attenuates downstream muscle growth signaling, such as that of calcineurin, mitogen-activated protein (MAP) kinases, extracellular signal-regulated kinase 1 and 2 (ERK1/2), and AKT. Based on our findings, we propose a model wherein STIM1-mediated store-operated calcium entry (SOCE) governs the Ca(2+) signaling required for cellular processes that are necessary for neonatal muscle growth and differentiation.

AICAR prevents heat-induced sudden death in RyR1 mutant mice independent of AMPK activation

Mice with a knock-in mutation (Y524S) in the type I ryanodine receptor (RyR1) die when exposed to short periods of temperature elevation (≥ 37 °C). We demonstrate that treatment with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) prevents heat-induced sudden death in Y524S mice. The AICAR protection is independent of AMPK activation and results from a newly identified action on the mutant RyR1 to reduce Ca²⁺ leak, preventing Ca²⁺ dependent increases in both reactive oxygen and reactive nitrogen species that act to further increase resting Ca²⁺ concentrations. If unchecked, the temperature driven increases in resting Ca²⁺ and ROS/RNS create an amplifying cycle that ultimately triggers sustained muscle contractions, rhabdomyolysis and death. Although antioxidants are effective in reducing this cycle in vitro, only AICAR prevents the heat induced death in vivo. Our findings suggest that AICAR is likely to be effective in prophylactic treatment of humans with enhanced susceptibility to exercise/heat-induced sudden death associated with RyR1 mutations.

Role of Junctin protein interactions in cellular dynamics of calsequestrin polymer upon calcium perturbation

Calsequestrin (CSQ), the major intrasarcoplasmic reticulum calcium storage protein, undergoes dynamic polymerization and depolymerization in a Ca(2+)-dependent manner. However, no direct evidence of CSQ depolymerization in vivo with physiological relevance has been obtained. In the present study, live cell imaging analysis facilitated characterization of the in vivo dynamics of the macromolecular CSQ structure. CSQ2 appeared as speckles in the presence of normal sarcoplasmic reticulum (SR) Ca(2+) that were decondensed upon Ca(2+) depletion. Moreover, CSQ2 decondensation occurred only in the stoichiometric presence of junctin (JNT). When expressed alone, CSQ2 speckles remained unchanged, even after Ca(2+) depletion. FRET analysis revealed constant interactions between CSQ2 and JNT, regardless of the SR Ca(2+) concentration, implying that JNT is an essential component of the CSQ scaffold. In vitro solubility assay, electron microscopy, and atomic force microscopy studies using purified recombinant proteins confirmed Ca(2+) and JNT-dependent disassembly of the CSQ2 polymer. Accordingly, we conclude that reversible polymerization and depolymerization of CSQ are critical in SR Ca(2+) homeostasis.

Store-operated calcium entry remains fully functional in aged mouse skeletal muscle despite a decline in STIM1 protein expression

Store-operated Ca(2+) entry (SOCE) is a robust mechanism in skeletal muscle, supported by abundant STIM1 and Orai1 in the junctional membranes. The precise role of SOCE in skeletal muscle Ca(2+) homeostasis and excitation-contraction coupling remains to be defined. Regardless, it remains important to determine whether the function and capacity of SOCE changes in aged skeletal muscle. We identified an approximate 40% decline in the expression of the integral SOCE protein, stromal interacting molecule 1 (STIM1), but no such decline in its coupling partner, Orai1, in muscle fibers from aged mice. To determine whether this changed aspects of SOCE functionality in skeletal muscle in aged mice, Ca(2+) in the cytoplasm and t-system were continuously and simultaneously imaged on a confocal microscope during sarcoplasmic reticulum Ca(2+) release and compared to experiments under identical conditions using muscle fibers from young mice. Normal activation, deactivation, Ca(2+) influx, and spatiotemporal characteristics of SOCE were found to persist in skeletal muscle from aged mice. Thus, SOCE remains a robust mechanism in aged skeletal muscle despite the decline in STIM1 protein expression, suggesting STIM1 is in excess in young skeletal muscle.

Special article: Future directions in malignant hyperthermia research and patient care

Malignant hyperthermia (MH) is a complex pharmacogenetic disorder of muscle metabolism. To more closely examine the complexities of MH and other related muscle disorders, the Malignant Hyperthermia Association of the United States (MHAUS) recently sponsored a scientific conference at which an interdisciplinary group of experts gathered to share new information and ideas. In this Special Article, we highlight key concepts and theories presented at the conference along with exciting new trends and challenges in MH research and patient care.