Paradoxical buffering of calcium by calsequestrin demonstrated for the calcium store of skeletal muscle

Molecular regulation of calcium-sensing receptor (CaSR)-mediated signaling

Calcium-sensing receptor (CaSR), a family C G-protein-coupled receptor, plays a crucial role in regulating calcium homeostasis by sensing small concentration changes of extracellular Ca2+, Mg2+, amino acids (e.g., L-Trp and L-Phe), small peptides, anions (e.g., HCO3 - and PO4 3-), and pH. CaSR-mediated intracellular Ca2+ signaling regulates a diverse set of cellular processes including gene transcription, cell proliferation, differentiation, apoptosis, muscle contraction, and neuronal transmission. Dysfunction of CaSR with mutations results in diseases such as autosomal dominant hypocalcemia, familial hypocalciuric hypercalcemia, and neonatal severe hyperparathyroidism. CaSR also influences calciotropic disorders, such as osteoporosis, and noncalciotropic disorders, such as cancer, Alzheimer's disease, and pulmonary arterial hypertension. This study first reviews recent advances in biochemical and structural determination of the framework of CaSR and its interaction sites with natural ligands, as well as exogenous positive allosteric modulators and negative allosteric modulators. The establishment of the first CaSR protein-protein interactome network revealed 94 novel players involved in protein processing in endoplasmic reticulum, trafficking, cell surface expression, endocytosis, degradation, and signaling pathways. The roles of these proteins in Ca2+-dependent cellular physiological processes and in CaSR-dependent cellular signaling provide new insights into the molecular basis of diseases caused by CaSR mutations and dysregulated CaSR activity caused by its protein interactors and facilitate the design of therapeutic agents that target CaSR and other family C G-protein-coupled receptors.

The Structural-Functional Crosstalk of the Calsequestrin System: Insights and Pathological Implications

Calsequestrin (CASQ) is a key intra-sarcoplasmic reticulum Ca2+-handling protein that plays a pivotal role in the contraction of cardiac and skeletal muscles. Its Ca2+-dependent polymerization dynamics shape the translation of electric excitation signals to the Ca2+-induced contraction of the actin-myosin architecture. Mutations in CASQ are linked to life-threatening pathological conditions, including tubular aggregate myopathy, malignant hyperthermia, and Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT). The variability in the penetrance of these phenotypes and the lack of a clear understanding of the disease mechanisms associated with CASQ mutations pose a major challenge to the development of effective therapeutic strategies. In vitro studies have mainly focused on the polymerization and Ca2+-buffering properties of CASQ but have provided little insight into the complex interplay of structural and functional changes that underlie disease. In this review, the biochemical and structural natures of CASQ are explored in-depth, while emphasizing their direct and indirect consequences for muscle Ca2+ physiology. We propose a novel functional classification of CASQ pathological missense mutations based on the structural stability of the monomer, dimer, or linear polymer conformation. We also highlight emerging similarities between polymeric CASQ and polyelectrolyte systems, emphasizing the potential for the use of this paradigm to guide further research.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Cytosolic Ca2+ gradients and mitochondrial Ca2+ uptake in resting muscle fibers: A model analysis

Calcium ions (Ca2+) enter mitochondria via the mitochondrial Ca2+ uniporter, driven by electrical and concentration gradients. In this regard, transgenic mouse models, such as calsequestrin knockout (CSQ-KO) mice, with higher mitochondrial Ca2+ concentrations ([Ca2+]mito), should display higher cytosolic Ca2+ concentrations ([Ca2+]cyto). However, repeated measurements of [Ca2+]cyto in quiescent CSQ-KO fibers never showed a difference between WT and CSQ-KO. Starting from the consideration that fluorescent Ca2+ probes (Fura-2 and Indo-1) measure averaged global cytosolic concentrations, in this report we explored the role of local Ca2+ concentrations (i.e., Ca2+ microdomains) in regulating mitochondrial Ca2+ in resting cells, using a multicompartmental diffusional Ca2+ model. Progressively including the inward and outward fluxes of sarcoplasmic reticulum (SR), extracellular space, and mitochondria, we explored their contribution to the local Ca2+ distribution within the cell. The model predicts Ca2+ concentration gradients with hot spots or microdomains even at rest, minor but similar to those of evoked Ca2+ release. Due to their specific localization close to Ca2+ release units (CRU), mitochondria could take up Ca2+ directly from high-concentration microdomains, thus sensibly raising [Ca2+]mito, despite minor, possibly undetectable, modifications of the average [Ca2+]cyto.

Voltage sensor current, SR Ca2+ release, and Ca2+ channel current during trains of action potential-like depolarizations of skeletal muscle fibers

In skeletal muscle, CaV 1.1 serves as the voltage sensor for both excitation-contraction coupling (ECC) and L-type Ca2+ channel activation. We have recently adapted the technique of action potential (AP) voltage clamp (APVC) to monitor the current generated by the movement of intramembrane voltage sensors (IQ ) during single imposed transverse tubular AP-like depolarization waveforms (IQAP ). We now extend this procedure to monitoring IQAP , and Ca2+ currents during trains of tubular AP-like waveforms in adult murine skeletal muscle fibers, and compare them with the trajectories of APs and AP-induced Ca2+ release measured in other fibers using field stimulation and optical probes. The AP waveform remains relatively constant during brief trains (<1 sec) for propagating APs in non-V clamped fibers. Trains of 10 AP-like depolarizations at 10 Hz (900 ms), 50 Hz (180 ms), or 100 Hz (90 ms) did not alter IQAP amplitude or kinetics, consistent with previous findings in isolated muscle fibers where negligible charge immobilization occurred during 100 ms step depolarizations. Using field stimulation, Ca2+ release did exhibit a considerable decline from pulse to pulse during the train, also consistent with previous findings, indicating that the decline of Ca2+ release during a short train of APs is not correlated to modification of charge movement. Ca2+ currents during single or 10 Hz trains of AP-like depolarizations were hardly detectable, were minimal during 50 Hz trains, and became more evident during 100 Hz trains in some fibers. Our results verify predictions on the behavior of the ECC machinery in response to AP-like depolarizations and provide a direct demonstration that Ca2+ currents elicited by single AP-like waveforms are negligible, but can become more prominent in some fibers during short high-frequency train stimulation that elicits maximal isometric force.

Constitutive assembly of Ca2+ entry units in soleus muscle from calsequestrin knockout mice

Calcium (Ca2+) entry units (CEUs) are junctions within the I band of the sarcomere between stacks of sarcoplasmic reticulum (SR) cisternae and extensions of the transverse (T)-tubule. CEUs contain STIM1 and Orai1 proteins, the molecular machinery of store-operated Ca2+ entry (SOCE). In extensor digitorum longus (EDL) fibers of wild-type (WT) mice, CEUs transiently assemble during acute exercise and disassemble several hours thereafter. By contrast, calsequestrin-1 (CASQ1) ablation induces a compensatory constitutive assembly of CEUs in EDL fibers, resulting in enhanced constitutive and maximum SOCE that counteracts SR Ca2+ depletion during repetitive activity. However, whether CEUs form in slow-twitch fibers, which express both the skeletal CASQ1 and the cardiac CASQ2 isoforms, is unknown. Herein, we compared the structure and function of soleus muscles from WT and knockout mice that lack either CASQ1 (CASQ1-null) or both CASQs (dCASQ-null). Ultrastructural analyses showed that SR/T-tubule junctions at the I band, virtually identical to CEUs in EDL muscle, were present and more frequent in CASQ1-null than WT mice, with dCASQ-null exhibiting the highest incidence. The greater incidence of CEUs in soleus from dCASQ-null mice correlated with increased specific force production during repetitive, high-frequency stimulation, which depended on Ca2+ entry. Consistent with this, Orai1 expression was significantly increased in soleus of CASQ1-null mice, but even more in dCASQ-null mice, compared with WT. Together, these results strengthen the concept that CEU assembly strongly depends on CASQ expression and provides an alternative source of Ca2+ needed to refill SR Ca2+ stores to maintain specific force production during sustained muscle activity.

Ablation of Calsequestrin-1, Ca2+ unbalance, and susceptibility to heat stroke

Introduction: Ca2+ levels in adult skeletal muscle fibers are mainly controlled by excitation-contraction (EC) coupling, a mechanism that translates action potentials in release of Ca2+ from the sarcoplasmic reticulum (SR) release channels, i.e. the ryanodine receptors type-1 (RyR1). Calsequestrin (Casq) is a protein that binds large amounts of Ca2+ in the lumen of the SR terminal cisternae, near sites of Ca2+ release. There is general agreement that Casq is not only important for the SR ability to store Ca2+, but also for modulating the opening probability of the RyR Ca2+ release channels. The initial studies: About 20 years ago we generated a mouse model lacking Casq1 (Casq1-null mice), the isoform predominantly expressed in adult fast twitch skeletal muscle. While the knockout was not lethal as expected, lack of Casq1 caused a striking remodeling of membranes of SR and of transverse tubules (TTs), and mitochondrial damage. Functionally, CASQ1-knockout resulted in reduced SR Ca2+ content, smaller Ca2+ transients, and severe SR depletion during repetitive stimulation. The myopathic phenotype of Casq1-null mice: After the initial studies, we discovered that Casq1-null mice were prone to sudden death when exposed to halogenated anaesthetics, heat and even strenuous exercise. These syndromes are similar to human malignant hyperthermia susceptibility (MHS) and environmental-exertional heat stroke (HS). We learned that mechanisms underlying these syndromes involved excessive SR Ca2+ leak and excessive production of oxidative species: indeed, mortality and mitochondrial damage were significantly prevented by administration of antioxidants and reduction of oxidative stress. Though, how Casq1-null mice could survive without the most important SR Ca2+ binding protein was a puzzling issue that was not solved. Unravelling the mystery: The mystery was finally solved in 2020, when we discovered that in Casq1-null mice the SR undergoes adaptations that result in constitutively active store-operated Ca2+ entry (SOCE). SOCE is a mechanism that allows skeletal fibers to use external Ca2+ when SR stores are depleted. The post-natal compensatory mechanism that allows Casq1-null mice to survive involves the assembly of new SR-TT junctions (named Ca2+ entry units) containing Stim1 and Orai1, the two proteins that mediate SOCE.

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.

Pre-assembled Ca2+ entry units and constitutively active Ca2+ entry in skeletal muscle of calsequestrin-1 knockout mice

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous Ca²⁺ influx mechanism triggered by depletion of Ca²⁺ stores from the endoplasmic/sarcoplasmic reticulum (ER/SR). We recently reported that acute exercise in WT mice drives the formation of Ca²⁺ entry units (CEUs), intracellular junctions that contain STIM1 and Orai1, the two key proteins mediating SOCE. The presence of CEUs correlates with increased constitutive- and store-operated Ca²⁺ entry, as well as sustained Ca²⁺ release and force generation during repetitive stimulation. Skeletal muscle from mice lacking calsequestrin-1 (CASQ1-null), the primary Ca²⁺-binding protein in the lumen of SR terminal cisternae, exhibits significantly reduced total Ca²⁺ store content and marked SR Ca²⁺ depletion during high-frequency stimulation. Here, we report that CEUs are constitutively assembled in extensor digitorum longus (EDL) and flexor digitorum brevis (FDB) muscles of sedentary CASQ1-null mice. The higher density of CEUs in EDL (39.6 ± 2.1/100 µm² versus 2.0 ± 0.3/100 µm²) and FDB (16.7 ± 1.0/100 µm² versus 2.7 ± 0.5/100 µm²) muscles of CASQ1-null compared with WT mice correlated with enhanced constitutive- and store-operated Ca²⁺ entry and increased expression of STIM1, Orai1, and SERCA. The higher ability to recover Ca²⁺ ions via SOCE in CASQ1-null muscle served to promote enhanced maintenance of peak Ca²⁺ transient amplitude, increased dependence of luminal SR Ca²⁺ replenishment on BTP-2-sensitive SOCE, and increased maintenance of contractile force during repetitive, high-frequency stimulation. Together, these data suggest that muscles from CASQ1-null mice compensate for the lack of CASQ1 and reduction in total releasable SR Ca²⁺ content by assembling CEUs to promote constitutive and store-operated Ca²⁺ entry.

The Role of Orai1 in Regulating Sarcoplasmic Calcium Release, Mitochondrial Morphology and Function in Myostatin Deficient Skeletal Muscle

In mice a naturally occurring 12-bp deletion in the myostatin gene is considered responsible for the compact phenotype (MstnCmpt-dl1Abc, Cmpt) labeled by a tremendous increase in body weight along with signs of muscle weakness, easier fatigability, decreased Orai1 expression and store operated calcium entry (SOCE). Here, on the one hand, Cmpt fibers were reconstructed with venus-Orai1 but this failed to restore SOCE. On the other hand, the endogenous Orai1 was silenced in fibers from wild type C57Bl6 mice which resulted in ∼70% of Orai1 being silenced in whole muscle homogenates as confirmed by Western blot, accompanied by an inhibitory effect on the voltage dependence of SR calcium release that manifested in a slight shift toward more positive potential values. This maneuver completely hampered SOCE. Our observations are consistent with the idea that Orai1 channels are present in distinct pools responsible for either a rapid refilling of the SR terminal cisternae connected to each voltage-activated calcium transient, or a slow SOCE associated with an overall depletion of calcium in the SR lumen. Furthermore, when Cmpt cells were loaded with the mitochondrial membrane potential sensitive dye TMRE, fiber segments with depolarized mitochondria were identified covering on average 26.5 ± 1.5% of the fiber area. These defective areas were located around the neuromuscular junction and displayed significantly smaller calcium transients. The ultrastructural analysis of the Cmpt fibers revealed changes in the mitochondrial morphology. In addition, the mitochondrial calcium uptake during repetitive stimulation was higher in the Cmpt fibers. Our results favor the idea that reduced function and/or expression of SOCE partners (in this study Orai1) and mitochondrial defects could play an important role in muscle weakness and degeneration associated with certain pathologies, perhaps including loss of function of the neuromuscular junction and aging.

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.

Calsequestrin. Structure, function, and evolution

Calsequestrin is the major Ca²⁺ binding protein in the sarcoplasmic reticulum (SR), serves as the main Ca²⁺ storage and buffering protein and is an important regulator of Ca²⁺ release channels in both skeletal and cardiac muscle. It is anchored at the junctional SR membrane through interactions with membrane proteins and undergoes reversible polymerization with increasing Ca²⁺ concentration. Calsequestrin provides high local Ca²⁺ at the junctional SR and communicates changes in luminal Ca²⁺ concentration to Ca²⁺ release channels, thus it is an essential component of excitation-contraction coupling. Recent studies reveal new insights on calsequestrin trafficking, Ca²⁺ binding, protein evolution, protein-protein interactions, stress responses and the molecular basis of related human muscle disease, including catecholaminergic polymorphic ventricular tachycardia (CPVT). Here we provide a comprehensive overview of calsequestrin, with recent advances in structure, diverse functions, phylogenetic analysis, and its role in muscle physiology, stress responses and human pathology.

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.

The relationship between form and function throughout the history of excitation-contraction coupling

Franzini-Armstrong reviews the development of the excitation–contraction coupling field over time.

The concept of excitation–contraction coupling is almost as old as Journal of General Physiology. It was understood as early as the 1940s that a series of stereotyped events is responsible for the rapid contraction response of muscle fibers to an initial electrical event at the surface. These early developments, now lost in what seems to be the far past for most young investigators, have provided an endless source of experimental approaches. In this Milestone in Physiology, I describe in detail the experiments and concepts that introduced and established the field of excitation–contraction coupling in skeletal muscle. More recent advances are presented in an abbreviated form, as readers are likely to be familiar with recent work in the field.

The voltage sensor of excitation-contraction coupling in mammals: Inactivation and interaction with Ca2

In excitation–contraction coupling, voltage-sensing modules (VSMs) of Ca_V1.1 Ca²⁺ channels simultaneously gate the associated pore and Ca²⁺ release channels in the sarcoplasmic reticulum. Ferreira Gregorio et al. find that VSMs adopt two inactivated states, and the degree of inactivation is dependent on external Ca²⁺ and the mouse strain used.

In skeletal muscle, the four-helix voltage-sensing modules (VSMs) of Ca_V1.1 calcium channels simultaneously gate two Ca²⁺ pathways: the Ca_V1.1 pore itself and the RyR1 calcium release channel in the sarcoplasmic reticulum. Here, to gain insight into the mechanism by which VSMs gate RyR1, we quantify intramembrane charge movement associated with VSM activation (sensing current) and gated Ca²⁺ release flux in single muscle cells of mice and rats. As found for most four-helix VSMs, upon sustained depolarization, rodent VSMs lose the ability to activate Ca²⁺ release channels opening; their properties change from a functionally capable mode, in which the mobile sensor charge is called charge 1, to an inactivated mode, charge 2, with a voltage dependence shifted toward more negative voltages. We find that charge 2 is promoted and Ca²⁺ release inactivated when resting, well-polarized muscle cells are exposed to low extracellular [Ca²⁺] and that the opposite occurs in high [Ca²⁺]. It follows that murine VSMs are partly inactivated at rest, which establishes the reduced availability of voltage sensing as a pathogenic mechanism in disorders of calcemia. We additionally find that the degree of resting inactivation is significantly different in two mouse strains, which underscores the variability of voltage sensor properties and their vulnerability to environmental conditions. Our studies reveal that the resting and activated states of VSMs are equally favored by extracellular Ca²⁺. Promotion by an extracellular species of two states of the VSM that differ in the conformation of the activation gate requires the existence of a second gate, inactivation, topologically extracellular and therefore accessible from outside regardless of the activation state.

Impaired calcium signaling in muscle fibers from intercostal and foot skeletal muscle in a cigarette smoke-induced mouse model of COPD

INTRODUCTION

Respiratory and locomotor skeletal muscle dysfunction are common findings in chronic obstructive pulmonary disease (COPD); however, the mechanisms that cause muscle impairment in COPD are unclear. Because Ca2+ signaling in excitation-contraction (E-C) coupling is important for muscle activity, we hypothesized that Ca2+ dysregulation could contribute to muscle dysfunction in COPD.

METHODS

Intercostal and flexor digitorum brevis muscles from control and cigarette smoke-exposed mice were investigated. We used single cell Ca2+ imaging and Western blot assays to assess Ca2+ signals and E-C coupling proteins.

RESULTS

We found impaired Ca2+ signals in muscle fibers from both muscle types, without significant changes in releasable Ca2+ or in the expression levels of E-C coupling proteins.

CONCLUSIONS

Ca2+ dysregulation may contribute or accompany respiratory and locomotor muscle dysfunction in COPD. These findings are of significance to the understanding of the pathophysiological course of COPD in respiratory and locomotor muscles. Muscle Nerve 56: 282-291, 2017.

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.

The C-terminal calcium-sensitive disordered motifs regulate isoform-specific polymerization characteristics of calsequestrin

Calsequestrin (CASQ) exists as two distinct isoforms CASQ1 and CASQ2 in all vertebrates. Although the isoforms exhibit unique functional characteristic, the structural basis for the same is yet to be fully defined. Interestingly, the C-terminal region of the two isoforms exhibit significant differences both in length and amino acid composition; forming Dn-motif and DEXn-motif in CASQ1 and CASQ2, respectively. Here, we investigated if the unique C-terminal motifs possess Ca(2+)-sensitivity and affect protein function. Sequence analysis shows that both the Dn- and DEXn-motifs are intrinsically disordered regions (IDRs) of the protein, a feature that is conserved from fish to man. Using purified synthetic peptides, we show that these motifs undergo distinctive Ca(2+)-mediated folding suggesting that these disordered motifs are Ca(2+)-sensitivity. We generated chimeric proteins by swapping the C-terminal portions between CASQ1 and CASQ2. Our studies show that the C-terminal portions do not play significant role in protein folding. An interesting finding of the current study is that the switching of the C-terminal portion completely reverses the polymerization kinetics. Collectively, these data suggest that these Ca(2+)-sensitivity IDRs located at the back-to-back dimer interface influence isoform-specific Ca(2+)-dependent polymerization properties of CASQ.

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.

Oxidative stress, mitochondrial damage, and cores in muscle from calsequestrin-1 knockout mice

Background

Mutations in the gene encoding ryanodine receptor type-1 (RYR1), the calcium ion (Ca²⁺) release channel in the sarcoplasmic reticulum (SR) of skeletal muscle, are linked to central core disease (CCD) and malignant hyperthermia (MH) susceptibility. We recently reported that mice lacking the skeletal isoform of calsequestrin (CASQ1-null), the primary Ca²⁺ buffer in the SR of skeletal muscle and a modulator of RYR1 activity, exhibit lethal heat- and anesthetic-induced hypermetabolic episodes that resemble MH events in humans.

Methods

We compared ultrastructure, oxidative status, and contractile function in skeletal fibers of extensor digitorum longus (EDL) muscles in wild type (WT) and CASQ1-null mice at different ages (from 4 to 27 months) using structural, biochemical, and functional assays.

Results

About 25% of fibers in EDL muscles from CASQ1-null mice of 14 to 27 months of age exhibited large areas of structural disarray (named core-like regions), which were rarely observed in muscle from age-matched WT mice. To determine early events that may lead to the formation of cores, we analyzed EDL muscles from adult mice: at 4 to 6 months of age, CASQ1-null mice (compared to WT) displayed significantly reduced grip strength (40 ± 1 vs. 86 ± 1 mN/gr) and exhibited an increase in the percentage of damaged mitochondria (15.1% vs. 2.6%) and a decrease in average cross-sectional fiber area (approximately 37%) in EDL fibers. Finally, oxidative stress was also significantly increased (25% reduction in ratio between reduced and oxidized glutathione, or GSH/GSSG, and 35% increase in production of mitochondrial superoxide flashes). Providing ad libitum access to N-acetylcysteine in the drinking water for 2 months normalized GSH/GSSG ratio, reduced mitochondrial damage (down to 8.9%), and improved grip strength (from 46 ± 3 to 59 ± 2 mN/gr) in CASQ1-null mice.

Conclusions

Our findings: 1) demonstrate that ablation of CASQ1 leads to enhanced oxidative stress, mitochondrial damage, and the formation of structural cores in skeletal muscle; 2) provide new insights in the pathogenic mechanisms that lead to damage/disappearance of mitochondria in cores; and 3) suggest that antioxidants may provide some therapeutic benefit in reducing mitochondrial damage, limiting the development of cores, and improving muscle function.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0035-9) contains supplementary material, which is available to authorized users.

The disorders of the calcium release unit of skeletal muscles: what have we learned from mouse models?

Calcium storage, release, and reuptake are essential for normal physiological function of muscle. Several human skeletal muscle disorders can arise from dysfunction in the control and coordination of these three critical processes. The release from the Sarcoplasmic Reticulum stores (SR) is handled by a multiprotein complex called Calcium Release Unit and composed of DiHydroPyridine Receptor or DHPR, Ryanodine Receptor or RYR, Calsequestrin or CASQ, junctin, Triadin, Junctophilin and Mitsugumin 29. Malignant hyperthermia (MH), Central Core Disease (CCD), Exertional/environmental Heat Stroke (EHS) and Multiminicore disease (MmD) are inherited disorders of calcium homeostasis in skeletal muscles directly related to mutations of genes coding for proteins of the CRU, primarily ryanodine receptor (RYR1). To understand the pathophysiology of MH and CCD, four murine lines carrying point mutations of human RYR1 have been developed: Y524S, R163C, I4898T and T4826I. Mice carrying those mutations show a phenotype with the traits of MH and/or CCD. Interestingly, also ablation of skeletal muscle calsequestrin (CASQ1) leads to a phenotype with MH-like lethal episodes in response to halothane and heat stress and development of central cores. In this review, we aim to describe the murine lines with RYR mutations or CASQ ablation, which show a phenotype similar to human MH or CCD, to underline their specific phenotypes and their differences and to discuss their contribution to the understanding of the pathophysiology of the disorders and the development of therapeutic strategies.

The excitation-contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation-contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca2+ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca2+ from the SR and transient increase of Ca2+ concentration in the myoplasm, (6) activation of the myoplasmic Ca2+ buffering system and the contractile apparatus, followed by (7) Ca2+ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca2+ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na+/Ca2+ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca2+ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca2+ sparks.

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.

Altered Ca2+ concentration, permeability and buffering in the myofibre Ca2+ store of a mouse model of malignant hyperthermia

  Malignant hyperthermia (MH) is linked to mutations in the type 1 ryanodine receptor, RyR1, the Ca2+ channel of the sarcoplasmic reticulum (SR) of skeletal muscle. The Y522S MH mutation was studied for its complex presentation, which includes structurally and functionally altered cell 'cores'. Imaging cytosolic and intra-SR [Ca2+] in muscle cells of heterozygous YS mice we determined Ca2+ release flux activated by clamp depolarization, permeability (P) of the SR membrane (ratio of flux and [Ca2+] gradient) and SR Ca2+ buffering power (B). In YS cells resting [Ca2+]SR was 45% of the value in normal littermates (WT). P was more than doubled, so that initial flux was normal. Measuring [Ca2+]SR(t) revealed dynamic changes in B(t). The alterations were similar to those caused by cytosolic BAPTA, which promotes release by hampering Ca2+-dependent inactivation (CDI). The [Ca2+] transients showed abnormal 'breaks', decaying phases after an initial rise, traced to a collapse in flux and P. Similar breaks occurred in WT myofibres with calsequestrin reduced by siRNA; calsequestrin content, however, was normal in YS muscle. Thus, the Y522S mutation causes greater openness of the RyR1, lowers resting [Ca2+]SR and alters SR Ca2+ buffering in a way that copies the functional instability observed upon reduction of calsequestrin content. The similarities with the effects of BAPTA suggest that the mutation, occurring near the cytosolic vestibule of the channel, reduces CDI as one of its primary effects. The unstable SR buffering, mimicked by silencing of calsequestrin, may help precipitate the loss of Ca2+ control that defines a fulminant MH event.

Dynamic measurement of the calcium buffering properties of the sarcoplasmic reticulum in mouse skeletal muscle

The buffering power, B, of the sarcoplasmic reticulum (SR), ratio of the changes in total and free [Ca(2+)], was determined in fast-twitch mouse muscle cells subjected to depleting membrane depolarization. Changes in total SR [Ca(2+)] were measured integrating Ca(2+) release flux, determined with a cytosolic [Ca(2+)] monitor. Free [Ca(2+)](SR) was measured using the cameleon D4cpv-Casq1. In 34 wild-type (WT) cells average B during the depolarization (ON phase) was 157 (SEM 26), implying that of 157 ions released, 156 were bound inside the SR. B was significantly greater when BAPTA, which increases release flux, was present in the cytosol. B was greater early in the pulse - when flux was greatest - than at its end, and greater in the ON than in the OFF. In 29 Casq1-null cells, B was 40 (3.6). The difference suggests that 75% of the releasable calcium is normally bound to calsequestrin. In the nulls the difference in B between ON and OFF was less than in the WT but still significant. This difference and the associated decay in B during the ON were not artifacts of a slow SR monitor, as they were also found in the WT when [Ca(2+)](SR) was tracked with the fast dye fluo-5N. The calcium buffering power, binding capacity and non-linear binding properties of the SR measured here could be accounted for by calsequestrin at the concentration present in mammalian muscle, provided that its properties were substantially different from those found in solution. Its affinity should be higher, or K(D) lower than the conventionally accepted 1 mm; its cooperativity (n in a Hill fit) should be higher and the stoichiometry of binding should be at the higher end of the values derived in solution. The reduction in B during release might reflect changes in calsequestrin conformation upon calcium loss.

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.

Measurement of RyR permeability reveals a role of calsequestrin in termination of SR Ca(2+) release in skeletal muscle

The mechanisms that terminate Ca²⁺ release from the sarcoplasmic reticulum are not fully understood. D4cpv-Casq1 (Sztretye et al. 2011. J. Gen. Physiol. doi:10.1085/jgp.201010591) was used in mouse skeletal muscle cells under voltage clamp to measure free Ca²⁺ concentration inside the sarcoplasmic reticulum (SR), [Ca²⁺]_SR, simultaneously with that in the cytosol, [Ca²⁺]_c, during the response to long-lasting depolarization of the plasma membrane. The ratio of Ca²⁺ release flux (derived from [Ca²⁺]_c(t)) over the gradient that drives it (essentially equal to [Ca²⁺]_SR) provided directly, for the first time, a dynamic measure of the permeability to Ca²⁺ of the releasing SR membrane. During maximal depolarization, flux rapidly rises to a peak and then decays. Before 0.5 s, [Ca²⁺]_SR stabilized at ∼35% of its resting level; depletion was therefore incomplete. By 0.4 s of depolarization, the measured permeability decayed to ∼10% of maximum, indicating ryanodine receptor channel closure. Inactivation of the t tubule voltage sensor was immeasurably small by this time and thus not a significant factor in channel closure. In cells of mice null for Casq1, permeability did not decrease in the same way, indicating that calsequestrin (Casq) is essential in the mechanism of channel closure and termination of Ca²⁺ release. The absence of this mechanism explains why the total amount of calcium releasable by depolarization is not greatly reduced in Casq-null muscle (Royer et al. 2010. J. Gen. Physiol. doi:10.1085/jgp.201010454). When the fast buffer BAPTA was introduced in the cytosol, release flux became more intense, and the SR emptied earlier. The consequent reduction in permeability accelerated as well, reaching comparable decay at earlier times but comparable levels of depletion. This observation indicates that [Ca²⁺]_SR, sensed by Casq and transmitted to the channels presumably via connecting proteins, is determinant to cause the closure that terminates Ca²⁺ release.

Mice null for calsequestrin 1 exhibit deficits in functional performance and sarcoplasmic reticulum calcium handling

In skeletal muscle, the release of calcium (Ca²⁺) by ryanodine sensitive sarcoplasmic reticulum (SR) Ca²⁺ release channels (i.e., ryanodine receptors; RyR1s) is the primary determinant of contractile filament activation. Much attention has been focused on calsequestrin (CASQ1) and its role in SR Ca²⁺ buffering as well as its potential for modulating RyR1, the L-type Ca²⁺ channel (dihydropyridine receptor, DHPR) and other sarcolemmal channels through sensing luminal [Ca²⁺]. The genetic ablation of CASQ1 expression results in significant alterations in SR Ca²⁺ content and SR Ca²⁺ release especially during prolonged activation. While these findings predict a significant loss-of-function phenotype in vivo, little information on functional status of CASQ1 null mice is available. We examined fast muscle in vivo and in vitro and identified significant deficits in functional performance that indicate an inability to sustain contractile activation. In single CASQ1 null skeletal myofibers we demonstrate a decrease in voltage dependent RyR Ca²⁺ release with single action potentials and a collapse of the Ca²⁺ release with repetitive trains. Under voltage clamp, SR Ca²⁺ release flux and total SR Ca²⁺ release are significantly reduced in CASQ1 null myofibers. The decrease in peak Ca²⁺ release flux appears to be solely due to elimination of the slowly decaying component of SR Ca²⁺ release, whereas the rapidly decaying component of SR Ca²⁺ release is not altered in either amplitude or time course in CASQ1 null fibers. Finally, intra-SR [Ca²⁺] during ligand and voltage activation of RyR1 revealed a significant decrease in the SR[Ca²⁺]_free in intact CASQ1 null fibers and a increase in the release and uptake kinetics consistent with a depletion of intra-SR Ca²⁺ buffering capacity. Taken together we have revealed that the genetic ablation of CASQ1 expression results in significant functional deficits consistent with a decrease in the slowly decaying component of SR Ca²⁺ release.

Lessons from calsequestrin-1 ablation in vivo: much more than a Ca(2+) buffer after all

Calsequestrin type-1 (CASQ1), the main sarcoplasmic reticulum (SR) Ca(2+) binding protein, plays a dual role in skeletal fibers: a) it provides a large pool of rapidly-releasable Ca(2+) during excitation-contraction (EC) coupling; and b) it modulates the activity of ryanodine receptors (RYRs), the SR Ca(2+) release channels. We have generated a mouse lacking CASQ1 in order to further characterize the role of CASQ1 in skeletal muscle. Contrary to initial expectations, CASQ1 ablation is compatible with normal motor activity, in spite of moderate muscle atrophy. However, CASQ1 deficiency results in profound remodeling of the EC coupling apparatus: shrinkage of junctional SR lumen; proliferation of SR/transverse-tubule contacts; and increased density of RYRs. While force development during a twitch is preserved, it is nevertheless characterized by a prolonged time course, likely reflecting impaired Ca(2+) re-uptake by the SR. Finally, lack of CASQ1 also results in increased rate of SR Ca(2+) depletion and inability of muscle to sustain tension during a prolonged tetani. All modifications are more pronounced (or only found) in fast-twitch extensor digitorum longus muscle compared to slow-twitch soleus muscle, likely because the latter expresses higher amounts of calsequestrin type-2 (CASQ2). Surprisingly, male CASQ1-null mice also exhibit a marked increased rate of spontaneous mortality suggestive of a stress-induced phenotype. Consistent with this idea, CASQ1-null mice exhibit an increased susceptibility to undergo a hypermetabolic syndrome characterized by whole body contractures, rhabdomyolysis, hyperthermia and sudden death in response to halothane- and heat-exposure, a phenotype remarkably similar to human malignant hyperthermia and environmental heat-stroke. The latter findings validate the CASQ1 gene as a candidate for linkage analysis in human muscle disorders.

Calsequestrin (CASQ1) rescues function and structure of calcium release units in skeletal muscles of CASQ1-null mice

Amplitude of Ca(2+) transients, ultrastructure of Ca(2+) release units, and molecular composition of sarcoplasmic reticulum (SR) are altered in fast-twitch skeletal muscles of calsequestrin-1 (CASQ1)-null mice. To determine whether such changes are directly caused by CASQ1 ablation or are instead the result of adaptive mechanisms, here we assessed ability of CASQ1 in rescuing the null phenotype. In vivo reintroduction of CASQ1 was carried out by cDNA electro transfer in flexor digitorum brevis muscle of the mouse. Exogenous CASQ1 was found to be correctly targeted to the junctional SR (jSR), as judged by immunofluorescence and confocal microscopy; terminal cisternae (TC) lumen was filled with electron dense material and its width was significantly increased, as judged by electron microscopy; peak amplitude of Ca(2+) transients was significantly increased compared with null muscle fibers transfected only with green fluorescent protein (control); and finally, transfected fibers were able to sustain cytosolic Ca(2+) concentration during prolonged tetanic stimulation. Only the expression of TC proteins, such as calsequestrin 2, sarcalumenin, and triadin, was not rescued as judged by Western blot. Thus our results support the view that CASQ1 plays a key role in both Ca(2+) homeostasis and TC structure.

Defects in Ca2+ release associated with local expression of pathological ryanodine receptors in mouse muscle fibres

Mutations of the gene encoding the type 1 ryanodine receptor (RyR1) are associated with skeletal muscle disorders including malignant hyperthermia susceptibility (MHS) and central core disease (CCD). We used in vivo expression of EGFP-RyR1 constructs in fully differentiated mouse muscle fibres to characterize the function of several RyR1 mutants. Wild-type and Y523S, R615C, R2163H and I4897T mutants of RyR1 were separately expressed and found to be present within restricted regions of fibres with a pattern consistent with triadic localization. Confocal measurements of voltage-clamp-activated myoplasmic Ca(2+) transients demonstrated alterations of sarcoplasmic reticulum (SR) Ca(2+) release spatially correlated with the presence of exogenous RyR1s. The Y523S, R615C and R2163H RyR1 MHS-related mutants were associated with enhanced peak Ca(2+) release for low and moderate levels of depolarization, whereas the I4897T CCD mutant produced a chronic reduction of peak SR Ca(2+) release. For example, peak Ca(2+) release in response to a depolarization to -20 mV in regions of fibres expressing Y523S and I4897T was 2.0 ± 0.3 (n = 9) and 0.46 ± 0.1 (n = 5) times the corresponding value in adjacent, non-expressing regions of the same fibre, respectively. Interestingly no significant change in the estimated total amount of Ca(2+) released at the end of large depolarizing pulses was observed for any of the mutant RyR1 channels. Overall, results are consistent with an 'inherent' increase in RyR1 sensitivity to activation by the voltage sensor for the MHS-related RyR1 mutants and a partial failure of voltage-gated release for the CCD-related I4897T mutant, that occur with no sign of change in SR Ca(2+) content. Furthermore, the results indicate that RyR1 channel density is tightly regulated even under the present conditions of forced exogenous expression.

Differential effect of calsequestrin ablation on structure and function of fast and slow skeletal muscle fibers

We compared structure and function of EDL and Soleus muscles in adult (4–6 m) mice lacking both Calsequestrin (CASQ) isoforms, the main SR Ca²⁺-binding proteins. Lack of CASQ induced ultrastructural alterations in ~30% of Soleus fibers, but not in EDL. Twitch time parameters were prolonged in both muscles, although tension was not reduced. However, when stimulated for 2 sec at 100 hz, Soleus was able to sustain contraction, while in EDL active tension declined by 70–80%. The results presented in this paper unmask a differential effect of CASQ1&2 ablation in fast versus slow fibers. CASQ is essential in EDL to provide large amount of Ca²⁺ released from the SR during tetanic stimulation. In contrast, Soleus deals much better with lack of CASQ because slow fibers require lower Ca²⁺ amounts and slower cycling to function properly. Nevertheless, Soleus suffers more severe structural damage, possibly because SR Ca²⁺ leak is more pronounced.

D4cpv-calsequestrin: a sensitive ratiometric biosensor accurately targeted to the calcium store of skeletal muscle

Current fluorescent monitors of free [Ca(2+)] in the sarcoplasmic reticulum (SR) of skeletal muscle cells are of limited quantitative value. They provide either a nonratio signal that is difficult to calibrate and is not specific or, in the case of Forster resonant energy transfer (FRET) biosensors, a signal of small dynamic range, which may be degraded further by imperfect targeting and interference from endogenous ligands of calsequestrin. We describe a novel tool that uses the cameleon D4cpv, which has a greater dynamic range and lower susceptibility to endogenous ligands than earlier cameleons. D4cpv was targeted to the SR by fusion with the cDNA of calsequestrin 1 or a variant that binds less Ca(2+). "D4cpv-Casq1," expressed in adult mouse at concentrations up to 22 µmole/liter of muscle cell, displayed the accurate targeting of calsequestrin and stayed inside cells after permeabilization of surface and t system membranes, which confirmed its strict targeting. FRET ratio changes of D4cpv-Casq1 were calibrated inside cells, with an effective K(D) of 222 µM and a dynamic range [(R(max) - R(min))/R(min)] of 2.5, which are improvements over comparable sensors. Both the maximal ratio, R(max), and its resting value were slightly lower in areas of high expression, a variation that was inversely correlated to distance from the sites of protein synthesis. The average [Ca(2+)](SR) in 74 viable cells at rest was 416 µM. The distribution of individual ratio values was Gaussian, but that of the calculated [Ca(2+)](SR) was skewed, with a tail of very large values, up to 6 mM. Model calculations reproduce this skewness as the consequence of quantifiably small variations in biosensor performance. Local variability, a perceived weakness of biosensors, thus becomes quantifiable. It is demonstrably small in D4cpv. D4cpv-Casq1 therefore provides substantial improvements in sensitivity, specificity, and reproducibility over existing monitors of SR free Ca(2+) concentration.

Massive alterations of sarcoplasmic reticulum free calcium in skeletal muscle fibers lacking calsequestrin revealed by a genetically encoded probe

The cytosolic free Ca(2+) transients elicited by muscle fiber excitation are well characterized, but little is known about the free [Ca(2+)] dynamics within the sarcoplasmic reticulum (SR). A targetable ratiometric FRET-based calcium indicator (D1ER Cameleon) allowed us to investigate SR Ca(2+) dynamics and analyze the impact of calsequestrin (CSQ) on SR [Ca(2+)] in enzymatically dissociated flexor digitorum brevis muscle fibers from WT and CSQ-KO mice lacking isoform 1 (CSQ-KO) or both isoforms [CSQ-double KO (DKO)]. At rest, free SR [Ca(2+)] did not differ between WT, CSQ-KO, and CSQ-DKO fibers. During sustained contractions, changes were rather small in WT, reflecting powerful buffering of CSQ, whereas in CSQ-KO fibers, significant drops in SR [Ca(2+)] occurred. Their amplitude increased with stimulation frequency between 1 and 60 Hz. At 60 Hz, the SR became virtually depleted of Ca(2+), both in CSQ-KO and CSQ-DKO fibers. In CSQ-KO fibers, cytosolic free calcium detected with Fura-2 declined during repetitive stimulation, indicating that SR calcium content was insufficient for sustained contractile activity. SR Ca(2+) reuptake during and after stimulation trains appeared to be governed by three temporally distinct processes with rate constants of 50, 1-5, and 0.3 s(-1) (at 26 °C), reflecting activity of the SR Ca(2+) pump and interplay of luminal and cytosolic Ca(2+) buffers and pointing to store-operated calcium entry (SOCE). SOCE might play an essential role during muscle contractures responsible for the malignant hyperthermia-like syndrome in mice lacking CSQ.