Store-operated calcium entry contributes to abnormal Ca²⁺ signalling in dystrophic mdx mouse myoblasts

Investigating the Involvement of C-X-C Motif Chemokine 5 and P2X7 Purinoceptor in Ectopic Calcification in Mouse Models of Duchenne Muscular Dystrophy

Ectopic calcification of myofibers is an early pathogenic feature in patients and animal models of Duchenne muscular dystrophy (DMD). In previous studies using the Dmdmdx-βgeo mouse model, we found that the dystrophin-null phenotype exacerbates this abnormality and that mineralised myofibers are surrounded by macrophages. Furthermore, the P2X7 purinoceptor, functioning in immune cells offers protection against dystrophic calcification. In the present study, by exploring transcriptomic data from Dmdmdx mice, we hypothesised these effects to be mediated by C-X-C motif chemokine 5 (CXCL5) downstream of P2X7 activation. We found that CXCL5 is upregulated in the quadriceps muscles of Dmdmdx-βgeo mice compared to wild-type controls. In contrast, at the cell level, dystrophic (SC5) skeletal muscle cells secreted less CXCL5 chemokine than wild-type (IMO) controls. Although release from IMO cells was increased by P2X7 activation, this could not explain the elevated CXCL5 levels observed in dystrophic muscle tissue. Instead, we found that CXCL5 is released by dystrophin-null macrophages in response to P2X7 activation, suggesting that macrophages are the source of CXCL5 in dystrophic muscles. The effects of CXCL5 upon mineralisation were investigated using the Alizarin Red assay to quantify calcium deposition in vitro. In basal (low phosphate) media, CXCL5 increased calcification in IMO but not SC5 myoblasts. However, in cultures treated in high phosphate media, to mimic dysregulated phosphate metabolism occurring in DMD, CXCL5 decreased calcification in both IMO and SC5 cells. These data indicate that CXCL5 is part of a homoeostatic mechanism regulating intracellular calcium, that CXCL5 can be released by macrophages in response to the extracellular ATP damage-associated signal, and that CXCL5 can be part of a damage response to protect against ectopic calcification. This mechanism is affected by DMD gene mutations.

Ablation of the dystrophin Dp71f alternative C-terminal variant increases sarcoma tumour cell aggressiveness

Alterations in Dp71 expression, the most ubiquitous dystrophin isoform, have been associated with patient survival across tumours. Intriguingly, in certain malignancies, Dp71 acts as a tumour suppressor, while manifesting oncogenic properties in others. This diversity could be explained by the expression of two Dp71 splice variants encoding proteins with distinct C-termini, each with specific properties. Expression of these variants has impeded the exploration of their unique roles. Using CRISPR/Cas9, we ablated the Dp71f variant with the alternative C-terminus in a sarcoma cell line not expressing the canonical C-terminal variant, and conducted molecular (RNAseq) and functional characterisation of the knockout cells. Dp71f ablation induced major transcriptomic alterations, particularly affecting the expression of genes involved in calcium signalling and ECM-receptor interaction pathways. The genome-scale metabolic analysis identified significant downregulation of glucose transport via membrane vesicle reaction (GLCter) and downregulated glycolysis/gluconeogenesis pathway. Functionally, these molecular changes corresponded with, increased calcium responses, cell adhesion, proliferation, survival under serum starvation and chemotherapeutic resistance. Knockout cells showed reduced GLUT1 protein expression, survival without attachment and their migration and invasion in vitro and in vivo were unaltered, despite increased matrix metalloproteinases release. Our findings emphasise the importance of alternative splicing of dystrophin transcripts and underscore the role of the Dp71f variant, which appears to govern distinct cellular processes frequently dysregulated in tumour cells. The loss of this regulatory mechanism promotes sarcoma cell survival and treatment resistance. Thus, Dp71f is a target for future investigations exploring the intricate functions of specific DMD transcripts in physiology and across malignancies.

Primary mouse myoblast metabotropic purinoceptor profiles and calcium signalling differ with their muscle origin and are altered in mdx dystrophinopathy

Mortality of Duchenne Muscular Dystrophy (DMD) is a consequence of progressive wasting of skeletal and cardiac muscle, where dystrophinopathy affects not only muscle fibres but also myogenic cells. Elevated activity of P2X7 receptors and increased store-operated calcium entry have been identified in myoblasts from the mdx mouse model of DMD. Moreover, in immortalized mdx myoblasts, increased metabotropic purinergic receptor response was found. Here, to exclude any potential effects of cell immortalization, we investigated the metabotropic response in primary mdx and wild-type myoblasts. Overall, analyses of receptor transcript and protein levels, antagonist sensitivity, and cellular localization in these primary myoblasts confirmed the previous data from immortalised cells. However, we identified significant differences in the pattern of expression and activity of P2Y receptors and the levels of the "calcium signalling toolkit" proteins between mdx and wild-type myoblasts isolated from different muscles. These results not only extend the earlier findings on the phenotypic effects of dystrophinopathy in undifferentiated muscle but, importantly, also reveal that these changes are muscle type-dependent and endure in isolated cells. This muscle-specific cellular impact of DMD may not be limited to the purinergic abnormality in mice and needs to be taken into consideration in human studies.

Searching for Mechanisms Underlying the Assembly of Calcium Entry Units: The Role of Temperature and pH

Store-operated Ca2+ entry (SOCE) is a mechanism that allows muscle fibers to recover external Ca2+, which first enters the cytoplasm andthen, via SERCA pump, also refills the depleted intracellular stores (i.e., the sarcoplasmic reticulum, SR). We recently discovered that SOCE is mediated by Calcium Entry Units (CEUs), intracellular junctions formed by: (i) SR stacks containing STIM1; and (ii) I-band extensions of the transverse tubule (TT) containing Orai1. The number and size of CEUs increase during prolonged muscle activity, though the mechanisms underlying exercise-dependent formation of new CEUs remain to be elucidated. Here, we first subjected isolated extensor digitorum longus (EDL) muscles from wild type mice to an exvivo exercise protocol and verified that functional CEUs can assemble alsoin the absence of blood supply and innervation. Then, we evaluated whetherparameters that are influenced by exercise, such as temperature and pH, may influence the assembly of CEUs. Results collected indicate that higher temperature (36 °C vs. 25 °C) and lower pH (7.2 vs. 7.4) increase the percentage of fibers containing SR stacks, the n. of SR stacks/area, and the elongation of TTs at the I band. Functionally, assembly of CEUs at higher temperature (36 °C) or at lower pH (7.2) correlates with increased fatigue resistance of EDL muscles in the presence of extracellular Ca2+. Taken together, these results indicate that CEUs can assemble in isolated EDL muscles and that temperature and pH are two of the possible regulators of CEU formation.

Loss of full-length dystrophin expression results in major cell-autonomous abnormalities in proliferating myoblasts

Duchenne muscular dystrophy (DMD) affects myofibers and muscle stem cells, causing progressive muscle degeneration and repair defects. It was unknown whether dystrophic myoblasts—the effector cells of muscle growth and regeneration—are affected. Using transcriptomic, genome-scale metabolic modelling and functional analyses, we demonstrate, for the first time, convergent abnormalities in primary mouse and human dystrophic myoblasts. In Dmd^mdx myoblasts lacking full-length dystrophin, the expression of 170 genes was significantly altered. Myod1 and key genes controlled by MyoD (Myog, Mymk, Mymx, epigenetic regulators, ECM interactors, calcium signalling and fibrosis genes) were significantly downregulated. Gene ontology analysis indicated enrichment in genes involved in muscle development and function. Functionally, we found increased myoblast proliferation, reduced chemotaxis and accelerated differentiation, which are all essential for myoregeneration. The defects were caused by the loss of expression of full-length dystrophin, as similar and not exacerbated alterations were observed in dystrophin-null Dmd^mdx-βgeo myoblasts. Corresponding abnormalities were identified in human DMD primary myoblasts and a dystrophic mouse muscle cell line, confirming the cross-species and cell-autonomous nature of these defects. The genome-scale metabolic analysis in human DMD myoblasts showed alterations in the rate of glycolysis/gluconeogenesis, leukotriene metabolism, and mitochondrial beta-oxidation of various fatty acids. These results reveal the disease continuum: DMD defects in satellite cells, the myoblast dysfunction affecting muscle regeneration, which is insufficient to counteract muscle loss due to myofiber instability. Contrary to the established belief, our data demonstrate that DMD abnormalities occur in myoblasts, making these cells a novel therapeutic target for the treatment of this lethal disease.

Lipopolysaccharide affects energy metabolism and elevates nicotinamide N-methyltransferase level in human aortic endothelial cells (HAEC)

This study aimed to investigate the putative role of nicotinamide N-methyltransferase in the metabolic response of human aortic endothelial cells. This enzyme catalyses S-adenosylmethionine-mediated methylation of nicotinamide to methylnicotinamide. This reaction is accompanied by the reduction of the intracellular nicotinamide and S-adenosylmethionine content. This may affect NAD⁺ synthesis and various processes of methylation, including epigenetic modifications of chromatin. Particularly high activity of nicotinamide N-methyltransferase is detected in liver, many neoplasms as well as in various cells in stressful conditions. The elevated nicotinamide N-methyltransferase content was also found in endothelial cells treated with statins. Although the exogenous methylnicotinamide has been postulated to induce a vasodilatory response, the specific metabolic role of nicotinamide N-methyltransferase in vascular endothelium is still unclear. Treatment of endothelial cells with bacterial lipopolysaccharide evokes several metabolic and functional consequences which built a multifaceted physiological response of endothelium to bacterial infection. Among the spectrum of biochemical changes substantially elevated protein level of nicotinamide N-methyltransferase was particularly intriguing. Here it has been shown that silencing of the nicotinamide N-methyltransferase gene influences several changes which are observed in cells treated with lipopolysaccharide. They include altered energy metabolism and rearrangement of the mitochondrial network. A complete explanation of the mechanisms behind the protective consequences of the nicotinamide N-methyltransferase deficiency in cells treated with lipopolysaccharide needs further investigation.

Postdevelopmental knockout of Orai1 improves muscle pathology in a mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD), an X-linked disorder caused by loss-of-function mutations in the dystrophin gene, is characterized by progressive muscle degeneration and weakness. Enhanced store-operated Ca2+ entry (SOCE), a Ca2+ influx mechanism coordinated by STIM1 sensors of luminal Ca2+ within the sarcoplasmic reticulum (SR) and Ca2+-permeable Orai1 channels in the sarcolemma, is proposed to contribute to Ca2+-mediated muscle damage in DMD. To directly determine the impact of Orai1-dependent SOCE on the dystrophic phenotype, we crossed mdx mice with tamoxifen-inducible, muscle-specific Orai1 knockout mice (mdx-Orai1 KO mice). Both constitutive and SOCE were significantly increased in flexor digitorum brevis fibers from mdx mice, while SOCE was absent in fibers from both Orai1 KO and mdx-Orai1 KO mice. Compared with WT mice, fibers from mdx mice exhibited (1) increased resting myoplasmic Ca2+ levels, (2) reduced total releasable Ca2+ store content, and (3) a prolonged rate of electrically evoked Ca2+ transient decay. These effects were partially normalized in fibers from mdx-Orai1 KO mice. Intact extensor digitorum longus muscles from mdx mice exhibited a significant reduction of maximal specific force, which was rescued in muscles from mdx-Orai1 KO mice. Finally, during exposure to consecutive eccentric contractions, muscles from mdx mice displayed a more pronounced decline in specific force compared with that of WT mice, which was also significantly attenuated by Orai1 ablation. Together, these results indicate that enhanced Orai1-dependent SOCE exacerbates the dystrophic phenotype and that Orai1 deficiency improves muscle pathology by both normalizing Ca2+ homeostasis and promoting sarcolemmal integrity/stability.

Disrupted Calcium Homeostasis in Duchenne Muscular Dystrophy: A Common Mechanism behind Diverse Consequences

Duchenne muscular dystrophy (DMD) leads to disability and death in young men. This disease is caused by mutations in the DMD gene encoding diverse isoforms of dystrophin. Loss of full-length dystrophins is both necessary and sufficient for causing degeneration and wasting of striated muscles, neuropsychological impairment, and bone deformities. Among this spectrum of defects, abnormalities of calcium homeostasis are the common dystrophic feature. Given the fundamental role of Ca2+ in all cells, this biochemical alteration might be underlying all the DMD abnormalities. However, its mechanism is not completely understood. While abnormally elevated resting cytosolic Ca2+ concentration is found in all dystrophic cells, the aberrant mechanisms leading to that outcome have cell-specific components. We probe the diverse aspects of calcium response in various affected tissues. In skeletal muscles, cardiomyocytes, and neurons, dystrophin appears to serve as a scaffold for proteins engaged in calcium homeostasis, while its interactions with actin cytoskeleton influence endoplasmic reticulum organisation and motility. However, in myoblasts, lymphocytes, endotheliocytes, and mesenchymal and myogenic cells, calcium abnormalities cannot be clearly attributed to the loss of interaction between dystrophin and the calcium toolbox proteins. Nevertheless, DMD gene mutations in these cells lead to significant defects and the calcium anomalies are a symptom of the early developmental phase of this pathology. As the impaired calcium homeostasis appears to underpin multiple DMD abnormalities, understanding this alteration may lead to the development of new therapies. In fact, it appears possible to mitigate the impact of the abnormal calcium homeostasis and the dystrophic phenotype in the total absence of dystrophin. This opens new treatment avenues for this incurable disease.

Treatment with a triazole inhibitor of the mitochondrial permeability transition pore fully corrects the pathology of sapje zebrafish lacking dystrophin

High-throughput screening identified isoxazoles as potent but metabolically unstable inhibitors of the mitochondrial permeability transition pore (PTP). Here we have studied the effects of a metabolically stable triazole analog, TR001, which maintains the PTP inhibitory properties with an in vitro potency in the nanomolar range. We show that TR001 leads to recovery of muscle structure and function of sapje zebrafish, a severe model of Duchenne muscular dystrophy (DMD). PTP inhibition fully restores the otherwise defective respiration in vivo, allowing normal development of sapje individuals in spite of lack of dystrophin. About 80 % sapje zebrafish treated with TR001 are alive and normal at 18 days post fertilization (dpf), a point in time when not a single untreated sapje individual survives. Time to 50 % death of treated zebrafish increases from 5 to 28 dpf, a sizeable number of individuals becoming young adults in spite of the persistent lack of dystrophin expression. TR001 improves respiration of myoblasts and myotubes from DMD patients, suggesting that PTP-dependent dysfunction also occurs in the human disease and that mitochondrial therapy of DMD with PTP-inhibiting triazoles is a viable treatment option.

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 (Mstn^{Cmpt–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.

Calcium entry units (CEUs): perspectives in skeletal muscle function and disease

In the last decades the term Store-operated Ca²⁺ entry (SOCE) has been used in the scientific literature to describe an ubiquitous cellular mechanism that allows recovery of calcium (Ca²⁺) from the extracellular space. SOCE is triggered by a reduction of Ca²⁺ content (i.e. depletion) in intracellular stores, i.e. endoplasmic or sarcoplasmic reticulum (ER and SR). In skeletal muscle the mechanism is primarily mediated by a physical interaction between stromal interaction molecule-1 (STIM1), a Ca²⁺ sensor located in the SR membrane, and ORAI1, a Ca²⁺-permeable channel of external membranes, located in transverse tubules (TTs), the invaginations of the plasma membrane (PM) deputed to propagation of action potentials. It is generally accepted that in skeletal muscle SOCE is important to limit muscle fatigue during repetitive stimulation. We recently discovered that exercise promotes the assembly of new intracellular junctions that contains colocalized STIM1 and ORAI1, and that the presence of these new junctions increases Ca²⁺ entry via ORAI1, while improving fatigue resistance during repetitive stimulation. Based on these findings we named these new junctions Ca²⁺ Entry Units (CEUs). CEUs are dynamic organelles that assemble during muscle activity and disassemble during recovery thanks to the plasticity of the SR (containing STIM1) and the elongation/retraction of TTs (bearing ORAI1). Interestingly, similar structures described as SR stacks were previously reported in different mouse models carrying mutations in proteins involved in Ca²⁺ handling (calsequestrin-null mice; triadin and junctin null mice, etc.) or associated to microtubules (MAP6 knockout mice). Mutations in Stim1 and Orai1 (and calsequestrin-1) genes have been associated to tubular aggregate myopathy (TAM), a muscular disease characterized by: (a) muscle pain, cramping, or weakness that begins in childhood and worsens over time, and (b) the presence of large accumulations of ordered SR tubes (tubular aggregates, TAs) that do not contain myofibrils, mitochondria, nor TTs. Interestingly, TAs are also present in fast twitch muscle fibers of ageing mice. Several important issues remain un-answered: (a) the molecular mechanisms and signals that trigger the remodeling of membranes and the functional activation of SOCE during exercise are unclear; and (b) how dysfunctional SOCE and/or mutations in Stim1, Orai1 and calsequestrin (Casq1) genes lead to the formation of tubular aggregates (TAs) in aging and disease deserve investigation.

TRPCs: Influential Mediators in Skeletal Muscle

Ca²⁺ itself or Ca²⁺-dependent signaling pathways play fundamental roles in various cellular processes from cell growth to death. The most representative example can be found in skeletal muscle cells where a well-timed and adequate supply of Ca²⁺ is required for coordinated Ca²⁺-dependent skeletal muscle functions, such as the interactions of contractile proteins during contraction. Intracellular Ca²⁺ movements between the cytosol and sarcoplasmic reticulum (SR) are strictly regulated to maintain the appropriate Ca²⁺ supply in skeletal muscle cells. Added to intracellular Ca²⁺ movements, the contribution of extracellular Ca²⁺ entry to skeletal muscle functions and its significance have been continuously studied since the early 1990s. Here, studies on the roles of channel proteins that mediate extracellular Ca²⁺ entry into skeletal muscle cells using skeletal myoblasts, myotubes, fibers, tissue, or skeletal muscle-originated cell lines are reviewed with special attention to the proposed functions of transient receptor potential canonical proteins (TRPCs) as store-operated Ca²⁺ entry (SOCE) channels under normal conditions and the potential abnormal properties of TRPCs in muscle diseases such as Duchenne muscular dystrophy (DMD).

Dystrophic mdx mouse myoblasts exhibit elevated ATP/UTP-evoked metabotropic purinergic responses and alterations in calcium signalling

Pathophysiology of Duchenne Muscular Dystrophy (DMD) is still elusive. Although progressive wasting of muscle fibres is a cause of muscle deterioration, there is a growing body of evidence that the triggering effects of DMD mutation are present at the earlier stage of muscle development and affect myogenic cells. Among these abnormalities, elevated activity of P2X7 receptors and increased store-operated calcium entry myoblasts have been identified in mdx mouse. Here, the metabotropic extracellular ATP/UTP-evoked response has been investigated. Sensitivity to antagonist, effect of gene silencing and cellular localization studies linked these elevated purinergic responses to the increased expression of P2Y2 but not P2Y4 receptors. These alterations have physiological implications as shown by reduced motility of mdx myoblasts upon treatment with P2Y2 agonist. However, the ultimate increase in intracellular calcium in dystrophic cells reflected complex alterations of calcium homeostasis identified in the RNA seq data and with significant modulation confirmed at the protein level, including a decrease of Gq11 subunit α, plasma membrane calcium ATP-ase, inositol-2,4,5-trisphosphate-receptor proteins and elevation of phospholipase Cβ, sarco-endoplamatic reticulum calcium ATP-ase and sodium‑calcium exchanger. In conclusion, whereas specificity of dystrophic myoblast excitation by extracellular nucleotides is determined by particular receptor overexpression, the intensity of such altered response depends on relative activities of downstream calcium regulators that are also affected by Dmd mutations. Furthermore, these phenotypic effects of DMD emerge as early as in undifferentiated muscle. Therefore, the pathogenesis of DMD and the relevance of current therapeutic approaches may need re-evaluation.

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.

The Complex Role of Store Operated Calcium Entry Pathways and Related Proteins in the Function of Cardiac, Skeletal and Vascular Smooth Muscle Cells

Cardiac, skeletal, and smooth muscle cells shared the common feature of contraction in response to different stimuli. Agonist-induced muscle's contraction is triggered by a cytosolic free Ca²⁺ concentration increase due to a rapid Ca²⁺ release from intracellular stores and a transmembrane Ca²⁺ influx, mainly through L-type Ca²⁺ channels. Compelling evidences have demonstrated that Ca²⁺ might also enter through other cationic channels such as Store-Operated Ca²⁺ Channels (SOCCs), involved in several physiological functions and pathological conditions. The opening of SOCCs is regulated by the filling state of the intracellular Ca²⁺ store, the sarcoplasmic reticulum, which communicates to the plasma membrane channels through the Stromal Interaction Molecule 1/2 (STIM1/2) protein. In muscle cells, SOCCs can be mainly non-selective cation channels formed by Orai1 and other members of the Transient Receptor Potential-Canonical (TRPC) channels family, as well as highly selective Ca²⁺ Release-Activated Ca²⁺ (CRAC) channels, formed exclusively by subunits of Orai proteins likely organized in macromolecular complexes. This review summarizes the current knowledge of the complex role of Store Operated Calcium Entry (SOCE) pathways and related proteins in the function of cardiac, skeletal, and vascular smooth muscle cells.

PPARδ modulation rescues mitochondrial fatty acid oxidation defects in the mdx model of muscular dystrophy

Duchenne muscular dystrophy (DMD) is a recessive, fatal X-linked disease that is characterized by progressive skeletal muscle wasting due to the absence of dystrophin, which is an a essential protein that bridges the inner cytoskeleton and extra-cellular matrix. This study set out to characterize the mitochondria in primary muscle satellite cell derived myoblasts from mdx mice and wild type control mice. Compared to wild type derived cells the mdx derived cells have reduced mitochondrial bioenergetics and have fewer mitochondria. Here, we demonstrate that a novel PPARδ modulator improves mitochondrial function in the mdx mice, which supports that modulating PPARδ may be therapeutically beneficial in DMD patients.

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.

STIM2 regulates both intracellular Ca2+ distribution and Ca2+ movement in skeletal myotubes

Stromal interaction molecule 1 (STIM1) along with Orai1 mediates extracellular Ca²⁺ entry into the cytosol through a store-operated Ca²⁺ entry (SOCE) mechanism in various tissues including skeletal muscle. However, the role(s) of STIM2, a homolog of STIM1, in skeletal muscle has not been well addressed. The present study, first, was focused on searching for STIM2-binding proteins from among proteins mediating skeletal muscle functions. This study used a binding assay, quadrupole time-of-flight mass spectrometry, and co-immunoprecipitation assay with bona-fide STIM2- and SERCA1a-expressing rabbit skeletal muscle. The region for amino acids from 453 to 729 of STIM2 binds to sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 1a (SERCA1a). Next, oxalate-supported ⁴⁵Ca²⁺-uptake experiments and various single-myotube Ca²⁺ imaging experiments using STIM2-knockdown mouse primary skeletal myotubes have suggested that STIM2 attenuates SERCA1a activity during skeletal muscle contraction, which contributes to the intracellular Ca²⁺ distribution between the cytosol and the SR at rest. In addition, STIM2 regulates Ca²⁺ movement through RyR1 during skeletal muscle contraction as well as SOCE. Therefore, via regulation of SERCA1a activity, STIM2 regulates both intracellular Ca²⁺ distribution and Ca²⁺ movement in skeletal muscle, which makes it both similar to, yet different from, STIM1.

Exercise-dependent formation of new junctions that promote STIM1-Orai1 assembly in skeletal muscle

Store-operated Ca²⁺ entry (SOCE), a ubiquitous mechanism that allows recovery of Ca²⁺ ions from the extracellular space, has been proposed to limit fatigue during repetitive skeletal muscle activity. However, the subcellular location for SOCE in muscle fibers has not been unequivocally identified. Here we show that exercise drives a significant remodeling of the sarcotubular system to form previously unidentified junctions between the sarcoplasmic reticulum (SR) and transverse-tubules (TTs). We also demonstrate that these new SR-TT junctions contain the molecular machinery that mediate SOCE: stromal interaction molecule-1 (STIM1), which functions as the SR Ca²⁺ sensor, and Orai1, the Ca²⁺-permeable channel in the TT. In addition, EDL muscles isolated from exercised mice exhibit an increased capability of maintaining contractile force during repetitive stimulation in the presence of 2.5 mM extracellular Ca²⁺, compared to muscles from control mice. This functional difference is significantly reduced by either replacement of extracellular Ca²⁺ with Mg²⁺ or the addition of SOCE inhibitors (BTP-2 and 2-APB). We propose that the new SR-TT junctions formed during exercise, and that contain STIM1 and Orai1, function as Ca²⁺Entry Units (CEUs), structures that provide a pathway to rapidly recover Ca²⁺ ions from the extracellular space during repetitive muscle activity.

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.

Stem Cell Differentiation Toward the Myogenic Lineage for Muscle Tissue Regeneration: A Focus on Muscular Dystrophy

Skeletal muscle tissue engineering is one of the important ways for regenerating functionally defective muscles. Among the myopathies, the Duchenne muscular dystrophy (DMD) is a progressive disease due to mutations of the dystrophin gene leading to progressive myofiber degeneration with severe symptoms. Although current therapies in muscular dystrophy are still very challenging, important progress has been made in materials science and in cellular technologies with the use of stem cells. It is therefore useful to review these advances and the results obtained in a clinical point of view. This article focuses on the differentiation of stem cells into myoblasts, and their application in muscular dystrophy. After an overview of the different stem cells that can be induced to differentiate into the myogenic lineage, we introduce scaffolding materials used for muscular tissue engineering. We then described some widely used methods to differentiate different types of stem cell into myoblasts. We highlight recent insights obtained in therapies for muscular dystrophy. Finally, we conclude with a discussion on stem cell technology. We discussed in parallel the benefits brought by the evolution of the materials and by the expansion of cell sources which can differentiate into myoblasts. We also discussed on future challenges for clinical applications and how to accelerate the translation from the research to the clinic in the frame of DMD.

Dystrophin: The dead calm of a dogma

Duchenne muscular dystrophy (DMD) is the most common inherited muscle disease leading to severe disability and death of young men. Current interventions are palliative as no treatment improves the long-term outcome. Therefore, new therapeutic modalities with translational potential are urgently needed and abnormalities downstream from the absence of dystrophin are realistic targets. It has been shown that DMD mutations alter extracellular ATP (eATP) signaling via P2RX7 purinoceptor upregulation, which leads to autophagic death of dystrophic muscle cells. Furthermore, the eATP-P2RX7 axis contributes to DMD pathology by stimulating harmful inflammatory responses. We demonstrated recently that genetic ablation or pharmacological inhibition of P2RX7 in the mdx mouse model of DMD produced functional attenuation of both muscle and non-muscle symptoms, establishing this receptor as an attractive therapeutic target. Central to the argument presented here, this purinergic phenotype affects dystrophic myoblasts. Muscle cells were believed not to be affected at this stage of differentiation, as they do not produce detectable dystrophin protein. Our findings contradict the central hypothesis stating that aberrant dystrophin expression is inconsequential in myoblasts and the DMD pathology results from effects such as sarcolemma fragility, due to the absence of dystrophin, in differentiated myofibres. However, we discuss here the evidence that, already in myogenic cells, DMD mutations produce a plethora of abnormalities, including in cell proliferation, differentiation, energy metabolism, Ca(2+) homeostasis and death, leading to impaired muscle regeneration. We hope that this discussion may bring to light further results that will help re-evaluating the established belief. Clearly, understanding how DMD mutations alter such a range of functions in myogenic cells is vital for developing effective therapies.