The origin of tubular aggregates in human myopathies

An Orai1 gain-of-function tubular aggregate myopathy mouse model phenocopies key features of the human disease

Tubular aggregate myopathy (TAM) is a heritable myopathy primarily characterized by progressive muscle weakness, elevated levels of creatine kinase (CK), hypocalcemia, exercise intolerance, and the presence of tubular aggregates (TAs). Here, we generated a knock-in mouse model based on a human gain-of-function mutation which results in a severe, early-onset form of TAM, by inducing a glycine-to-serine point mutation in the ORAI1 pore (Orai1G100S/+ or GS mice). By 8 months of age, GS mice exhibited significant muscle weakness, exercise intolerance, elevated CK levels, hypocalcemia, and robust TA presence. Unexpectedly, constitutive Ca2+ entry in mutant mice was observed in muscle only during early development and was abolished in adult skeletal muscle, partly due to reduced ORAI1 expression. Consistent with proteomic results, significant mitochondrial damage and dysfunction was observed in skeletal muscle of GS mice. Thus, GS mice represent a powerful model for investigation of the pathophysiological mechanisms that underlie key TAM symptoms, as well as those compensatory responses that limit the damaging effects of uncontrolled ORAI1-mediated Ca2+ influx.

A Gain-of-Function Mutation in the Ca2+ Channel ORAI1 Causes Stormorken Syndrome with Tubular Aggregates in Mice

Store-operated Ca2+ entry (SOCE) controls Ca2+ homeostasis and mediates multiple Ca2+-dependent signaling pathways and cellular processes. It relies on the concerted activity of the reticular Ca2+ sensor STIM1 and the plasma membrane Ca2+ channel ORAI1. STIM1 and ORAI1 gain-of-function (GoF) mutations induce SOCE overactivity and excessive Ca2+ influx, leading to tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK), two overlapping disorders characterized by muscle weakness and a variable occurrence of multi-systemic anomalies affecting spleen, skin, and platelets. To date, different STIM1 mouse models exist, but only a single ORAI1 mouse model with muscle-specific TAM/STRMK phenotype has been described, precluding a comparative analysis of the physiopathology in all affected tissues. Here, we generated and characterized mice harboring a prevalent ORAI1 TAM/STRMK mutation and we provide phenotypic, physiological, biochemical, and functional data. Examination of Orai1V109M/+ mice revealed smaller size, spleen enlargement, reduced muscle force, and decreased platelet numbers. Morphological analyses of muscle sections evidenced the presence of tubular aggregates, the histopathological hallmark on biopsies from TAM/STRMK patients absent in all reported STIM1 models. Overall, Orai1V109M/+ mice reliably recapitulate the human disorder and highlight the primary physiological defects caused by ORAI1 gain-of-function mutations. They also provide the possibility to investigate the formation of tubular aggregates and to develop a common therapy for different TAM/STRMK forms.

Tubular Aggregates as a Marker of Aging in Skeletal Muscle

Tubular aggregates (TA) are skeletal muscle structures that arise from the progressive accumulation of sarcoplasmic reticulum proteins, mainly with aging. Muscle regeneration plays a role in TA formation. TA quantification may aid in the evaluation of muscle aging and genetic muscle degeneration. TA form over time, appears in aging in normal murine muscles. TA reduction in injured conditions may be due to the degeneration-regeneration process in muscles, with loss of damaged muscle fibers and formation of new fibers that do not present protein aggregation. These new regenerated fibers do not improve the function capacity of the aged muscle. Here, we present a methodology for labeling and identifying tubular aggregates in muscle fibers and also the standardization of its quantification.

Constitutive, Muscle-Specific Orai1 Knockout Results in the Incomplete Assembly of Ca2+ Entry Units and a Reduction in the Age-Dependent Formation of Tubular Aggregates

Store-operated Ca2+ entry (SOCE) is a ubiquitous cellular mechanism that cells use to activate extracellular Ca2+ entry when intracellular Ca2+ stores are depleted. In skeletal muscle, SOCE occurs within Ca2+ entry units (CEUs), intracellular junctions between stacks of SR membranes containing STIM1 and transverse tubules (TTs) containing ORAI1. Gain-of-function mutations in STIM1 and ORAI1 are linked to tubular aggregate (TA) myopathy, a disease characterized by the atypical accumulation of tubes of SR origin. Moreover, SOCE and TAs are increased in the muscles of aged male mice. Here, we assessed the longitudinal effects (from 4-6 months to 10-14 months of age) of constitutive, muscle-specific Orai1 knockout (cOrai1 KO) on skeletal muscle structure, function, and the assembly of TAs and CEUs. The results from these studies indicate that cOrai1 KO mice exhibit a shorter lifespan, reduced body weight, exercise intolerance, decreased muscle-specific force and rate of force production, and an increased number of structurally damaged mitochondria. In addition, electron microscopy analyses revealed (i) the absence of TAs with increasing age and (ii) an increased number of SR stacks without adjacent TTs (i.e., incomplete CEUs) in cOrai1 KO mice. The absence of TAs is consistent with TAs being formed as a result of excessive ORAI1-dependent Ca2+ entry.

Branched-chain amino acids and L-alanine supplementation ameliorate calcium dyshomeostasis in sarcopenia: New insights for nutritional interventions

Introduction: During aging, sarcopenia and decline in physiological processes lead to partial loss of muscle strength, atrophy, and increased fatigability. Muscle changes may be related to a reduced intake of essential amino acids playing a role in proteostasis. We have recently shown that branched-chain amino acid (BCAA) supplements improve atrophy and weakness in models of muscle disuse and aging. Considering the key roles that the alteration of Ca2+-related homeostasis and store-operated calcium entry (SOCE) play in several muscle dysfunctions, this study has been aimed at gaining insight into the potential ability of BCAA-based dietary formulations in aged mice on various players of Ca2+ dyshomeostasis. Methods: Seventeen-month-old male C57BL/6J mice received a 12-week supplementation with BCAAs alone or boosted with two equivalents of L-alanine (2-Ala) or with dipeptide L-alanyl-L-alanine (Di-Ala) in drinking water. Outcomes were evaluated on ex vivo skeletal muscles indices vs. adult 3-month-old male C57BL/6J mice. Results: Ca2+ imaging confirmed a decrease in SOCE and an increase of resting Ca2+ concentration in aged vs. adult mice without alteration in the canonical components of SOCE. Aged muscles vs. adult muscles were characterized by a decrease in the expression of ryanodine receptor 1 (RyR1), the Sarco-Endoplasmic Reticulum Calcium ATPase (SERCA) pump, and sarcalumenin together with an alteration of the expression of mitsugumin 29 and mitsugumin 53, two recently recognized players in the SOCE mechanism. BCAAs, particularly the formulation BCAAs+2-Ala, were able to ameliorate all these alterations. Discussion: These results provide evidence that Ca2+ homeostasis dysfunction plays a role in the functional deficit observed in aged muscle and supports the interest of dietary BCAA supplementation in counteracting sarcopenia-related SOCE dysregulation.

ORAI1 inhibition as an efficient preclinical therapy for tubular aggregate myopathy and Stormorken syndrome

Tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK) are clinically overlapping disorders characterized by childhood-onset muscle weakness and a variable occurrence of multisystemic signs, including short stature, thrombocytopenia, and hyposplenism. TAM/STRMK is caused by gain-of-function mutations in the Ca2+ sensor STIM1 or the Ca2+ channel ORAI1, both of which regulate Ca2+ homeostasis through the ubiquitous store-operated Ca2+ entry (SOCE) mechanism. Functional experiments in cells have demonstrated that the TAM/STRMK mutations induce SOCE overactivation, resulting in excessive influx of extracellular Ca2+. There is currently no treatment for TAM/STRMK, but SOCE is amenable to manipulation. Here, we crossed Stim1R304W/+ mice harboring the most common TAM/STRMK mutation with Orai1R93W/+ mice carrying an ORAI1 mutation partially obstructing Ca2+ influx. Compared with Stim1R304W/+ littermates, Stim1R304W/+Orai1R93W/+ offspring showed a normalization of bone architecture, spleen histology, and muscle morphology; an increase of thrombocytes; and improved muscle contraction and relaxation kinetics. Accordingly, comparative RNA-Seq detected more than 1,200 dysregulated genes in Stim1R304W/+ muscle and revealed a major restoration of gene expression in Stim1R304W/+Orai1R93W/+ mice. Altogether, we provide physiological, morphological, functional, and molecular data highlighting the therapeutic potential of ORAI1 inhibition to rescue the multisystemic TAM/STRMK signs, and we identified myostatin as a promising biomarker for TAM/STRMK in humans and mice.

The Effects of Aging on Sarcoplasmic Reticulum-Related Factors in the Skeletal Muscle of Mice

The pathogenesis of sarcopenia includes the dysfunction of calcium homeostasis associated with the sarcoplasmic reticulum; however, the localization in sarcoplasmic reticulum-related factors and differences by myofiber type remain unclear. Here, we investigated the effects of aging on sarcoplasmic reticulum-related factors in the soleus (slow-twitch) and gastrocnemius (fast-twitch) muscles of 3- and 24-month-old male C57BL/6J mice. There were no notable differences in the skeletal muscle weight of these 3- and 24-month-old mice. The expression of Atp2a1, Atp2a2, Sln, and Pln increased with age in the gastrocnemius muscles, but not in the soleus muscles. Subsequently, immunohistochemical analysis revealed ectopic sarcoplasmic reticulum calcium ion ATPase (SERCA) 1 and SERCA2a immunoreactivity only in the gastrocnemius muscles of old mice. Histochemical and transmission electron microscope analysis identified tubular aggregate (TA), an aggregation of the sarcoplasmic reticulum, in the gastrocnemius muscles of old mice. Dihydropyridine receptor α1, ryanodine receptor 1, junctophilin (JPH) 1, and JPH2, which contribute to sarcoplasmic reticulum function, were also localized in or around the TA. Furthermore, JPH1 and JPH2 co-localized with matrix metalloproteinase (MMP) 2 around the TA. These results suggest that sarcoplasmic reticulum-related factors are localized in or around TAs that occur in fast-twitch muscle with aging, but some of them might be degraded by MMP2.

Voluntary wheel running mitigates disease in an Orai1 gain-of-function mouse model of tubular aggregate myopathy

Tubular aggregate myopathy (TAM) is an inherited skeletal muscle disease associated with progressive muscle weakness, cramps, and myalgia. Tubular aggregates (TAs) are regular arrays of highly ordered and densely packed SR straight-tubes in muscle biopsies; the extensive presence of TAs represent a key histopathological hallmark of this disease in TAM patients. TAM is caused by gain-of-function mutations in proteins that coordinate store-operated Ca2+ entry (SOCE): STIM1 Ca2+ sensor proteins in the sarcoplasmic reticulum (SR) and Ca2+-permeable ORAI1 channels in the surface membrane. We have previously shown that voluntary wheel running (VWR) prevents formation of TAs in aging mice. Here, we assessed the therapeutic potential of endurance exercise (in the form of VWR) in mitigating the functional and structural alterations in a knock-in mouse model of TAM (Orai1G100S/+ or GS mice) based on a gain-of-function mutation in the ORAI1 pore. WT and GS mice were singly-housed for six months (from two to eight months of age) with either free-spinning or locked low profile wheels. Six months of VWR exercise significantly increased soleus peak tetanic specific force production, normalized FDB fiber Ca2+ store content, and markedly reduced TAs in EDL muscle from GS mice. Six months of VWR exercise normalized the expression of mitochondrial proteins found to be altered in soleus muscle of sedentary GS mice in conjunction with a signature of increased protein translation and biosynthetic processes. Parallel proteomic analyses of EDL muscles from sedentary WT and GS mice revealed changes in a tight network of pathways involved in formation of supramolecular complexes, which were also normalized following six months of VWR. In summary, sustained voluntary endurance exercise improved slow twitch muscle function, reduced the presence of TAs in fast twitch muscle, and normalized the muscle proteome of GS mice consistent with protective adaptions in proteostasis, mitochondrial structure/function, and formation of supramolecular complexes.

MYTHO is a novel regulator of skeletal muscle autophagy and integrity

Autophagy is a critical process in the regulation of muscle mass, function and integrity. The molecular mechanisms regulating autophagy are complex and still partly understood. Here, we identify and characterize a novel FoxO-dependent gene, d230025d16rik which we named Mytho (Macroautophagy and YouTH Optimizer), as a regulator of autophagy and skeletal muscle integrity in vivo. Mytho is significantly up-regulated in various mouse models of skeletal muscle atrophy. Short term depletion of MYTHO in mice attenuates muscle atrophy caused by fasting, denervation, cancer cachexia and sepsis. While MYTHO overexpression is sufficient to trigger muscle atrophy, MYTHO knockdown results in a progressive increase in muscle mass associated with a sustained activation of the mTORC1 signaling pathway. Prolonged MYTHO knockdown is associated with severe myopathic features, including impaired autophagy, muscle weakness, myofiber degeneration, and extensive ultrastructural defects, such as accumulation of autophagic vacuoles and tubular aggregates. Inhibition of the mTORC1 signaling pathway in mice using rapamycin treatment attenuates the myopathic phenotype triggered by MYTHO knockdown. Skeletal muscles from human patients diagnosed with myotonic dystrophy type 1 (DM1) display reduced Mytho expression, activation of the mTORC1 signaling pathway and impaired autophagy, raising the possibility that low Mytho expression might contribute to the progression of the disease. We conclude that MYTHO is a key regulator of muscle autophagy and integrity.

Histological Methods to Assess Skeletal Muscle Degeneration and Regeneration in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease caused by the loss of function of the protein dystrophin. This protein contributes to the stabilisation of striated cells during contraction, as it anchors the cytoskeleton with components of the extracellular matrix through the dystrophin-associated protein complex (DAPC). Moreover, absence of the functional protein affects the expression and function of proteins within the DAPC, leading to molecular events responsible for myofibre damage, muscle weakening, disability and, eventually, premature death. Presently, there is no cure for DMD, but different treatments help manage some of the symptoms. Advances in genetic and exon-skipping therapies are the most promising intervention, the safety and efficiency of which are tested in animal models. In addition to in vivo functional tests, ex vivo molecular evaluation aids assess to what extent the therapy has contributed to the regenerative process. In this regard, the later advances in microscopy and image acquisition systems and the current expansion of antibodies for immunohistological evaluation together with the development of different spectrum fluorescent dyes have made histology a crucial tool. Nevertheless, the complexity of the molecular events that take place in dystrophic muscles, together with the rise of a multitude of markers for each of the phases of the process, makes the histological assessment a challenging task. Therefore, here, we summarise and explain the rationale behind different histological techniques used in the literature to assess degeneration and regeneration in the field of dystrophinopathies, focusing especially on those related to DMD.

Mutations in proteins involved in E-C coupling and SOCE and congenital myopathies

In skeletal muscle, Ca2+ necessary for muscle contraction is stored and released from the sarcoplasmic reticulum (SR), a specialized form of endoplasmic reticulum through the mechanism known as excitation-contraction (E-C) coupling. Following activation of skeletal muscle contraction by the E-C coupling mechanism, replenishment of intracellular stores requires reuptake of cytosolic Ca2+ into the SR by the activity of SR Ca2+-ATPases, but also Ca2+ entry from the extracellular space, through a mechanism called store-operated calcium entry (SOCE). The fine orchestration of these processes requires several proteins, including Ca2+ channels, Ca2+ sensors, and Ca2+ buffers, as well as the active involvement of mitochondria. Mutations in genes coding for proteins participating in E-C coupling and SOCE are causative of several myopathies characterized by a wide spectrum of clinical phenotypes, a variety of histological features, and alterations in intracellular Ca2+ balance. This review summarizes current knowledge on these myopathies and discusses available knowledge on the pathogenic mechanisms of disease.

Chronic inhibition of the mitochondrial ATP synthase in skeletal muscle triggers sarcoplasmic reticulum distress and tubular aggregates

Tubular aggregates (TA) are honeycomb-like arrays of sarcoplasmic-reticulum (SR) tubules affecting aged glycolytic fibers of male individuals and inducing severe sarcomere disorganization and muscular pain. TA develop in skeletal muscle from Tubular Aggregate Myopathy (TAM) patients as well as in other disorders including endocrine syndromes, diabetes, and ageing, being their primary cause unknown. Nowadays, there is no cure for TA. Intriguingly, both hypoxia and calcium dyshomeostasis prompt TA formation, pointing to a possible role for mitochondria in their setting. However, a functional link between mitochondrial dysfunctions and TA remains unknown. Herein, we investigate the alteration in muscle-proteome of TAM patients, the molecular mechanism of TA onset and a potential therapy in a preclinical mouse model of the disease. We show that in vivo chronic inhibition of the mitochondrial ATP synthase in muscle causes TA. Upon long-term restrained oxidative phosphorylation (OXPHOS), oxidative soleus experiments a metabolic and structural switch towards glycolytic fibers, increases mitochondrial fission, and activates mitophagy to recycle damaged mitochondria. TA result from the overresponse of the fission controller DRP1, that upregulates the Store-Operate-Calcium-Entry and increases the mitochondria-SR interaction in a futile attempt to buffer calcium overloads upon prolonged OXPHOS inhibition. Accordingly, hypoxic muscles cultured ex vivo show an increase in mitochondria/SR contact sites and autophagic/mitophagic zones, where TA clusters grow around defective mitochondria. Moreover, hypoxia triggered a stronger TA formation upon ATP synthase inhibition, and this effect was reduced by the DRP1 inhibitor mDIVI. Remarkably, the muscle proteome of TAM patients displays similar alterations in mitochondrial dynamics and in ATP synthase contents. In vivo edaravone treatment in mice with restrained OXPHOS restored a healthy phenotype by prompting mitogenesis and mitochondrial fusion. Altogether, our data provide a functional link between the ATP synthase/DRP1 axis and the setting of TA, and repurpose edaravone as a possible treatment for TA-associated disorders.

Ryanodine receptor 1 (RYR1) mutations in two patients with tubular aggregate myopathy

Two likely causative mutations in the RYR1 gene were identified in two patients with myopathy with tubular aggregates, but no evidence of cores or core‐like pathology on muscle biopsy. These patients were clinically evaluated and underwent routine laboratory investigations, electrophysiologic tests, muscle biopsy and muscle magnetic resonance imaging (MRI). They reported stiffness of the muscles following sustained activity or cold exposure and had serum creatine kinase elevation. The identified RYR1 mutations (p.Thr2206Met or p.Gly2434Arg, in patient 1 and patient 2, respectively) were previously identified in individuals with malignant hyperthermia susceptibility and are reported as causative according to the European Malignant Hyperthermia Group rules. To our knowledge, these data represent the first identification of causative mutations in the RYR1 gene in patients with tubular aggregate myopathy and extend the spectrum of histological alterations caused by mutation in the RYR1 gene.

The Sarcoplasmic Reticulum of Skeletal Muscle Cells: A Labyrinth of Membrane Contact Sites

The sarcoplasmic reticulum of skeletal muscle cells is a highly ordered structure consisting of an intricate network of tubules and cisternae specialized for regulating Ca2+ homeostasis in the context of muscle contraction. The sarcoplasmic reticulum contains several proteins, some of which support Ca2+ storage and release, while others regulate the formation and maintenance of this highly convoluted organelle and mediate the interaction with other components of the muscle fiber. In this review, some of the main issues concerning the biology of the sarcoplasmic reticulum will be described and discussed; particular attention will be addressed to the structure and function of the two domains of the sarcoplasmic reticulum supporting the excitation-contraction coupling and Ca2+-uptake mechanisms.

Pathophysiological Effects of Overactive STIM1 on Murine Muscle Function and Structure

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous mechanism regulating extracellular Ca²⁺ entry to control a multitude of Ca²⁺-dependent signaling pathways and cellular processes. SOCE relies on the concerted activity of the reticular Ca²⁺ sensor STIM1 and the plasma membrane Ca²⁺ channel ORAI1, and dysfunctions of these key factors result in human pathologies. STIM1 and ORAI1 gain-of-function (GoF) mutations induce excessive Ca²⁺ influx through SOCE over-activation, and cause tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK), two overlapping disorders characterized by muscle weakness and additional multi-systemic signs affecting growth, platelets, spleen, skin, and intellectual abilities. In order to investigate the pathophysiological effect of overactive SOCE on muscle function and structure, we combined transcriptomics with morphological and functional studies on a TAM/STRMK mouse model. Muscles from Stim1^R304W/+ mice displayed aberrant expression profiles of genes implicated in Ca²⁺ handling and excitation-contraction coupling (ECC), and in vivo investigations evidenced delayed muscle contraction and relaxation kinetics. We also identified signs of reticular stress and abnormal mitochondrial activity, and histological and respirometric analyses on muscle samples revealed enhanced myofiber degeneration associated with reduced mitochondrial respiration. Taken together, we uncovered a molecular disease signature and deciphered the pathomechanism underlying the functional and structural muscle anomalies characterizing TAM/STRMK.

Congenital myasthenic syndrome in China: genetic and myopathological characterization

OBJECTIVE

We aimed to summarize the clinical, genetic, and myopathological features of a cohort of Chinese patients with congenital myasthenic syndrome, and follow up on therapeutic outcomes.

METHODS

The clinical spectrum, mutational frequency of genes, and pathological diagnostic clues of various subtypes of patients with congenital myasthenic syndrome were summarized. Therapeutic effects were followed up.

RESULTS

Thirty-five patients from 29 families were recruited. Ten genes were identified: GFPT1 (27.6%), AGRN (17.2%), CHRNE (17.2%), COLQ (13.8%), GMPPB (6.9%), CHAT, CHRNA1, DOK7, COG7, and SLC25A1 (3.4% each, respectively). Sole limb-girdle weakness was found in patients with AGRN (1/8) and GFPT1 (7/8) mutations, whereas distal weakness was all observed in patients with AGRN (6/8) mutations. Tubular aggregates were only found in patients with GFPT1 mutations (5/6). The patients with GMPPB mutations (2/2) had decreased alpha-dystroglycan. Acetylcholinesterase inhibitor therapy resulted in no response or worsened symptoms in patients with COLQ mutations, a diverse response in patients with AGRN mutations, and a good response in patients with other subtypes. Albuterol therapy was effective or harmless in most subtypes. Therapy effects became attenuated with long-term use in patients with COLQ or AGRN mutations.

INTERPRETATION

The genetic distribution of congenital myasthenic syndrome in China is distinct from that of other ethnic origins. The appearance of distal weakness, selective limb-girdle myasthenic syndrome, tubular aggregates, and decreased alpha-dystroglycan were indicative of the specific subtypes. Based on the follow-up findings, we suggest cautious evaluation of the long-term efficacy of therapeutic agents in congenital myasthenic syndrome.

Gain-of-Function STIM1 L96V Mutation Causes Myogenesis Alteration in Muscle Cells From a Patient Affected by Tubular Aggregate Myopathy

Tubular Aggregate Myopathy (TAM) is a hereditary ultra-rare muscle disorder characterized by muscle weakness and cramps or myasthenic features. Biopsies from TAM patients show the presence of tubular aggregates originated from sarcoplasmic reticulum due to altered Ca²⁺ homeostasis. TAM is caused by gain-of-function mutations in STIM1 or ORAI1, proteins responsible for Store-Operated-Calcium-Entry (SOCE), a pivotal mechanism in Ca²⁺ signaling. So far there is no cure for TAM and the mechanisms through which STIM1 or ORAI1 gene mutation lead to muscle dysfunction remain to be clarified. It has been established that post-natal myogenesis critically relies on Ca²⁺ influx through SOCE. To explore how Ca²⁺ homeostasis dysregulation associated with TAM impacts on muscle differentiation cascade, we here performed a functional characterization of myoblasts and myotubes deriving from patients carrying STIM1 L96V mutation by using fura-2 cytofluorimetry, high content imaging and real-time PCR. We demonstrated a higher resting Ca²⁺ concentration and an increased SOCE in STIM1 mutant compared with control, together with a compensatory down-regulation of genes involved in Ca²⁺ handling (RyR1, Atp2a1, Trpc1). Differentiating STIM1 L96V myoblasts persisted in a mononuclear state and the fewer multinucleated myotubes had distinct morphology and geometry of mitochondrial network compared to controls, indicating a defect in the late differentiation phase. The alteration in myogenic pathway was confirmed by gene expression analysis regarding early (Myf5, Mef2D) and late (DMD, Tnnt3) differentiation markers together with mitochondrial markers (IDH3A, OGDH). We provided evidences of mechanisms responsible for a defective myogenesis associated to TAM mutant and validated a reliable cellular model usefull for TAM preclinical studies.

Long-Term Exercise Reduces Formation of Tubular Aggregates and Promotes Maintenance of Ca2+ Entry Units in Aged Muscle

Tubular aggregates (TAs) in skeletal muscle fibers are unusual accumulation of sarcoplasmic reticulum (SR) tubes that are found in different disorders including TA myopathy (TAM). TAM is a muscular disease characterized by muscle pain, cramping, and weakness that has been recently linked to mutations in STIM1 and ORAI1. STIM1 and ORAI1 are the two main proteins mediating store-operated Ca2+ entry (SOCE), a mechanism activated by depletion of intracellular Ca2+ stores (e.g., SR) that allows recovery of Ca2+ from the extracellular space during repetitive muscle activity. We have recently shown that exercise triggers the formation of unique intracellular junctions between SR and transverse tubules named Ca 2+ entry units (CEUs). CEUs promote colocalization of STIM1 with ORAI1 and improve muscle function in presence of external Ca2+. TAs virtually identical to those of TAM patients are also found in fast-twitch fibers of aging male mice. Here, we used a combination of electron and confocal microscopy, Western blotting, and ex vivo stimulation protocols (in presence or absence of external Ca2+) to evaluate the presence of TAs, STIM1-ORAI1 localization and expression and fatigue resistance of intact extensor digitorum longus (EDL) muscles in wild-type male adult (4-month-old) and aged (24-month-old) mice and in mice trained in wheel cages for 15 months (from 9 to 24 months of age). The results collected indicate that (i) aging causes STIM1 and ORAI1 to accumulate in TAs and (ii) long-term exercise significantly reduced formation of TAs. In addition, (iii) EDL muscles from aged mice exhibited a faster decay of contractile force than adult muscles, likely caused by their inability to refill intracellular Ca2+ stores, and (iv) exercise in wheel cages restored the capability of aged EDL muscles to use external Ca2+ by promoting maintenance of CEUs. In conclusion, exercise prevented improper accumulation of STIM1 and ORAI1 in TAs during aging, maintaining the capability of aged muscle to refill intracellular Ca2+ stores via SOCE.

STIM1/ORAI1 Loss-of-Function and Gain-of-Function Mutations Inversely Impact on SOCE and Calcium Homeostasis and Cause Multi-Systemic Mirror Diseases

Store-operated Ca²⁺ entry (SOCE) is a ubiquitous and essential mechanism regulating Ca²⁺ homeostasis in all tissues, and controls a wide range of cellular functions including keratinocyte differentiation, osteoblastogenesis and osteoclastogenesis, T cell proliferation, platelet activation, and muscle contraction. The main SOCE actors are STIM1 and ORAI1. Depletion of the reticular Ca²⁺ stores induces oligomerization of the luminal Ca²⁺ sensor STIM1, and the oligomers activate the plasma membrane Ca²⁺ channel ORAI1 to trigger extracellular Ca²⁺ entry. Mutations in STIM1 and ORAI1 result in abnormal SOCE and lead to multi-systemic disorders. Recessive loss-of-function mutations are associated with CRAC (Ca²⁺ release-activated Ca²⁺) channelopathy, involving immunodeficiency and autoimmunity, muscular hypotonia, ectodermal dysplasia, and mydriasis. In contrast, dominant STIM1 and ORAI1 gain-of-function mutations give rise to tubular aggregate myopathy and Stormorken syndrome (TAM/STRMK), forming a clinical spectrum encompassing muscle weakness, thrombocytopenia, ichthyosis, hyposplenism, short stature, and miosis. Functional studies on patient-derived cells revealed that CRAC channelopathy mutations impair SOCE and extracellular Ca²⁺ influx, while TAM/STRMK mutations induce excessive Ca²⁺ entry through SOCE over-activation. In accordance with the opposite pathomechanisms underlying both disorders, CRAC channelopathy and TAM/STRMK patients show mirror phenotypes at the clinical and molecular levels, and the respective animal models recapitulate the skin, bones, immune system, platelet, and muscle anomalies. Here we review and compare the clinical presentations of CRAC channelopathy and TAM/STRMK patients and the histological and molecular findings obtained on human samples and murine models to highlight the mirror phenotypes in different tissues, and to point out potentially undiagnosed anomalies in patients, which may be relevant for disease management and prospective therapeutic approaches.

Functional analyses of STIM1 mutations reveal a common pathomechanism for tubular aggregate myopathy and Stormorken syndrome

Tubular aggregate myopathy (TAM) is a progressive disorder characterized by muscle weakness, cramps, and myalgia. TAM clinically overlaps with Stormorken syndrome (STRMK), combining TAM with miosis, thrombocytopenia, hyposplenism, ichthyosis, short stature, and dyslexia. TAM and STRMK arise from gain-of-function mutations in STIM1 (stromal interaction molecule 1) or ORAI1, both encoding key regulators of Ca²⁺ homeostasis, and mutations in either gene result in excessive extracellular Ca²⁺ entry. The pathomechanistic similarities and differences between TAM and STRMK are only partially understood. Here we provide functional in vitro experiments demonstrating that STIM1 harboring the TAM D84G or the STRMK R304W mutation similarly cluster and exert a dominant effect on the wild-type protein. Both mutants recruit ORAI1 to the clusters, increase cytosolic Ca²⁺ levels, promote major nuclear import of the Ca²⁺ -dependent transcription factor NFAT (nuclear factor of activated T cells), and trigger the formation of circular membrane stacks. In conclusion, the analyzed TAM and STRMK mutations have a comparable impact on STIM1 protein function and downstream effects of excessive Ca²⁺ entry, highlighting that TAM and STRMK involve a common pathomechanism.

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.

Biallelic loss of function variants in SYT2 cause a treatable congenital onset presynaptic myasthenic syndrome

Synaptotagmins are integral synaptic vesicle membrane proteins that function as calcium sensors and regulate neurotransmitter release at the presynaptic nerve terminal. Synaptotagmin-2 (SYT2), is the major isoform expressed at the neuromuscular junction. Recently, dominant missense variants in SYT2 have been reported as a rare cause of distal motor neuropathy and myasthenic syndrome, manifesting with stable or slowly progressive distal weakness of variable severity along with presynaptic NMJ impairment. These variants are thought to have a dominant-negative effect on synaptic vesicle exocytosis, although the precise pathomechanism remains to be elucidated. Here we report seven patients of five families, with biallelic loss of function variants in SYT2, clinically manifesting with a remarkably consistent phenotype of severe congenital onset hypotonia and weakness, with variable degrees of respiratory involvement. Electrodiagnostic findings were consistent with a presynaptic congenital myasthenic syndrome (CMS) in some. Treatment with an acetylcholinesterase inhibitor pursued in three patients showed clinical improvement with increased strength and function. This series further establishes SYT2 as a CMS-disease gene and expands its clinical and genetic spectrum to include recessive loss-of-function variants, manifesting as a severe congenital onset presynaptic CMS with potential treatment implications.

Clinical and muscle MRI features in a family with tubular aggregate myopathy and novel STIM1 mutation

Heterozygous mutations in the stromal interaction molecule-1-gene (STIM1) cause a clinical phenotype varying from tubular aggregate myopathy with single or multiple signs of Stormorken syndrome to the full Stormorken phenotype. We identified a novel heterozygous mutation c.325C > T (p.H109Y) in the EF-hand domain of STIM1 in six patients of a large Belgian family, and performed a detailed clinical (N = 6), histopathological (N = 2) and whole-body muscle MRI (N = 3) study. The clinical phenotype was characterized by a slowly progressive, predominant proximal muscle weakness in all patients (100%), and additional exercise-induced myalgia in three (60%). Patients experienced symptom onset between 10 and 20 years, remained ambulatory into late adulthood, showed elevated serum creatine kinase levels and tubular aggregates in type 1 and type 2 fibers on muscle biopsy. Interestingly, jaw contractures and hyperlaxity, as well as non-muscular multisystemic features such as menorrhagia, easy bruising and ichthyosis occurred in one patient, and miosis in another. Whole-body muscle MRI revealed predominant involvement of superficial neck extensors, subscapularis, obliquus abdominis externus, lumbar extensors, rectus femoris, biceps femoris longus, medial head of gastrocnemius and flexor hallucis longus. Our findings in patients with myopathy with tubular aggregates and a STIM1 mutation further support the concept of a continuous spectrum with Stormorken syndrome.

Abnormal Excitation-Contraction Coupling and Calcium Homeostasis in Myopathies and Cardiomyopathies

Muscle contraction requires specialized membrane structures with precise geometry and relies on the concerted interplay of electrical stimulation and Ca2+ release, known as excitation-contraction coupling (ECC). The membrane structure hosting ECC is called triad in skeletal muscle and dyad in cardiac muscle, and structural or functional defects of triads and dyads have been observed in a variety of myopathies and cardiomyopathies. Based on their function, the proteins localized at the triad/dyad can be classified into three molecular pathways: the Ca2+ release complex (CRC), store-operated Ca2+ entry (SOCE), and membrane remodeling. All three are mechanistically linked, and consequently, aberrations in any of these pathways cause similar disease entities. This review provides an overview of the clinical and genetic spectrum of triad and dyad defects with a main focus of attention on the underlying pathomechanisms.

Cylindrical spirals in two families: Clinical and genetic investigations

Cylindrical spirals are a rare ultrastructural finding on muscle biopsy, with fewer than 20 reported cases since its first description in 1979. These structures are sometimes observed with tubular aggregates and are thought to comprise longitudinal sarcoplasmic reticulum. While mutations in genes encoding key components of Ca²⁺ handling (ORAI1 and STIM1) underlie tubular aggregate myopathy, no causative genes have been associated with cylindrical spirals. Here we describe two families with cylindrical spirals on muscle biopsy with a suspected genetic cause. In one family we identified a known truncating variant in EBF3, previously associated with a neurodevelopmental disorder. The affected individuals in this family present with clinical features overlapping with those described for EBF3 disease. An isolated proband in the second family harbours bi-allelic truncating variants in TTN and her clinical course and other features on biopsy are highly concordant for titinopathy. From experimental studies, EBF3 is known to be involved in Ca²⁺ regulation in muscle, thus EBF3 dysregulation may represent a novel mechanism of impaired Ca²⁺ handling leading to cylindrical spirals. Additional cases of EBF3 disease or titinopathy with cylindrical spirals need to be identified to support the involvement of these genes in the pathogenesis of cylindrical spirals.

Peroxisome proliferator-activated receptor γ coactivator 1α regulates mitochondrial calcium homeostasis, sarcoplasmic reticulum stress, and cell death to mitigate skeletal muscle aging

Age-related impairment of muscle function severely affects the health of an increasing elderly population. While causality and the underlying mechanisms remain poorly understood, exercise is an efficient intervention to blunt these aging effects. We thus investigated the role of the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), a potent regulator of mitochondrial function and exercise adaptation, in skeletal muscle during aging. We demonstrate that PGC-1α overexpression improves mitochondrial dynamics and calcium buffering in an estrogen-related receptor α-dependent manner. Moreover, we show that sarcoplasmic reticulum stress is attenuated by PGC-1α. As a result, PGC-1α prevents tubular aggregate formation and cell death pathway activation in old muscle. Similarly, the pro-apoptotic effects of ceramide and thapsigargin were blunted by PGC-1α in muscle cells. Accordingly, mice with muscle-specific gain-of-function and loss-of-function of PGC-1α exhibit a delayed and premature aging phenotype, respectively. Together, our data reveal a key protective effect of PGC-1α on muscle function and overall health span in aging.

Tubular aggregate myopathy and Stormorken syndrome: Mutation spectrum and genotype/phenotype correlation

Calcium (Ca²⁺ ) acts as a ubiquitous second messenger, and normal cell and tissue physiology strictly depends on the precise regulation of Ca²⁺ entry, storage, and release. Store-operated Ca²⁺ entry (SOCE) is a major mechanism controlling extracellular Ca²⁺ entry, and mainly relies on the accurate interplay between the Ca²⁺ sensor STIM1 and the Ca²⁺ channel ORAI1. Mutations in STIM1 or ORAI1 result in abnormal Ca²⁺ homeostasis and are associated with severe human disorders. Recessive loss-of-function mutations impair SOCE and cause combined immunodeficiency, while dominant gain-of-function mutations induce excessive extracellular Ca²⁺ entry and cause tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK). TAM and STRMK are spectra of the same multisystemic disease characterized by muscle weakness, miosis, thrombocytopenia, hyposplenism, ichthyosis, dyslexia, and short stature. To date, 42 TAM/STRMK families have been described, and here we report five additional families for which we provide clinical, histological, ultrastructural, and genetic data. In this study, we list and review all new and previously reported STIM1 and ORAI1 cases, discuss the pathomechanisms of the mutations based on the known functions and the protein structure of STIM1 and ORAI1, draw a genotype/phenotype correlation, and delineate an efficient screening strategy for the molecular diagnosis of TAM/STRMK.

A pattern-based approach to the interpretation of skeletal muscle biopsies

The interpretation of muscle biopsies is complex and provides the most useful information when integrated with the clinical presentation of the patient. These biopsies are performed for workup of a wide range of diseases including dystrophies, metabolic diseases, and inflammatory processes. Recent insights have led to changes in the classification of inflammatory myopathies and have changed the role that muscle biopsies have in the workup of inherited diseases. These changes will be reviewed. This review follows a morphology-driven approach by discussing diseases of skeletal muscle based on a few basic patterns that include cases with (1) active myopathic damage and inflammation, (2) active myopathic damage without associated inflammation, (3) chronic myopathic changes, (4) myopathies with distinctive inclusions or vacuoles, (5) biopsies mainly showing atrophic changes, and (6) biopsies that appear normal on routine preparations. Each of these categories goes along with certain diagnostic considerations and pitfalls. Individual biopsy features are only rarely pathognomonic. Establishing a firm diagnosis therefore typically requires integration of all of the biopsy findings and relevant clinical information. With this approach, a muscle biopsy can often provide helpful information in the diagnostic workup of patients presenting with neuromuscular problems.

GFPT1 deficiency in muscle leads to myasthenia and myopathy in mice

Glutamine-fructose-6-phosphate transaminase 1 (GFPT1) is the rate-limiting enzyme in the hexosamine biosynthetic pathway which yields precursors required for protein and lipid glycosylation. Mutations in GFPT1 and other genes downstream of this pathway cause congenital myasthenic syndrome (CMS) characterized by fatigable muscle weakness owing to impaired neurotransmission. The precise pathomechanisms at the neuromuscular junction (NMJ) owing to a deficiency in GFPT1 is yet to be discovered. One of the challenges we face is the viability of Gfpt1^−/− knockout mice. In this study, we use Cre/LoxP technology to generate a muscle-specific GFPT1 knockout mouse model, Gfpt1^tm1d/tm1d, characteristic of the human CMS phenotype. Our data suggest a critical role for muscle derived GFPT1 in the development of the NMJ, neurotransmission, skeletal muscle integrity and highlight that a deficiency in skeletal muscle alone is sufficient to cause morphological postsynaptic NMJ changes that are accompanied by presynaptic alterations despite the conservation of neuronal GFPT1 expression. In addition to the conventional morphological NMJ changes and fatigable muscle weakness, Gfpt1^tm1d/tm1d mice display a progressive myopathic phenotype with the presence of tubular aggregates in muscle, characteristic of the GFPT1-CMS phenotype. We further identify an upregulation of skeletal muscle proteins glypican-1, farnesyltransferase/geranylgeranyltransferase type-1 subunit α and muscle-specific kinase, which are known to be involved in the differentiation and maintenance of the NMJ. The Gfpt1^tm1d/tm1d model allows for further investigation of pathophysiological consequences on genes and pathways downstream of GFPT1 likely to involve misglycosylation or hypoglycosylation of NMJs and muscle targets.

[Tubular aggregate myopathy and Stormorken syndrome]

Calcium (Ca²⁺) is an essential regulator for a large number of cellular functions in various tissues and organs, and small disturbances of Ca²⁺ homeostasis can severely compromise normal physiology. Intracellular Ca²⁺ balance is mainly controlled by the reticular Ca²⁺ sensor STIM1 and the plasma membrane Ca²⁺ channel ORAI1 through a mechanism known as store-operated Ca²⁺ entry (SOCE). Gain-of-function mutations in STIM1 or ORAI1 cause excessive extracellular Ca²⁺ influx, resulting in tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK). Both disorders are spectra of the same disease and involve muscle weakness, miosis, thrombocytopenia, hyposplenism, ichthyosis, dyslexia, and short stature. Here we summarize the clinical and histological characteristics of both disorders, provide an overview on the genetic causes, and recapitulate the current knowledge on the pathomechanisms leading to the multi-systemic phenotype of tubular aggregate myopathy and Stormorken syndrome.

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.

Stormorken Syndrome Caused by a p.R304W STIM1 Mutation: The First Italian Patient and a Review of the Literature

Stormorken syndrome is a rare autosomal dominant disease that is characterized by a complex phenotype that includes tubular aggregate myopathy (TAM), bleeding diathesis, hyposplenism, mild hypocalcemia and additional features, such as miosis and a mild intellectual disability (dyslexia). Stormorken syndrome is caused by autosomal dominant mutations in the STIM1 gene, which encodes an endoplasmic reticulum Ca²⁺ sensor. Here, we describe the clinical and molecular aspects of a 21-year-old Italian female with Stormorken syndrome. The STIM1 gene sequence identified a c.910C > T transition in a STIM1 allele (p.R304W). The p.R304W mutation is a common mutation that is responsible for Stormorken syndrome and is hypothesized to cause a gain of function action associated with a rise in Ca²⁺ levels. A review of published STIM1 mutations (n = 50) and reported Stormorken patients (n = 11) indicated a genotype-phenotype correlation with mutations in a coiled coil cytoplasmic domain associated with complete Stormorken syndrome, and other pathological variants outside this region were more often linked to an incomplete phenotype. Our study describes the first Italian patient with Stormorken syndrome, contributes to the genotype/phenotype correlation and highlights the possibility of directly investigating the p.R304W mutation in the presence of a typical phenotype.

Highlights

- Stormorken syndrome is a rare autosomal dominant disease.

- Stormoken syndrome is caused by autosomal dominant mutations in the STIM1 gene.

- We present the features of a 21-year-old Italian female with Stormorken syndrome.

- Our review of published STIM1 mutations suggests a genotype-phenotype correlation.

- The p.R304W mutation should be investigated in the presence of a typical phenotype.

Gain-of-function mutations in STIM1 and ORAI1 causing tubular aggregate myopathy and Stormorken syndrome

Calcium (Ca²⁺) is a key regulator for a large number of cellular functions in all kinds of cells, and small disturbances of Ca²⁺ homeostasis can severely compromise normal physiology in various tissues and organs. A major mechanism controlling Ca²⁺ homeostasis is store-operated Ca²⁺ entry (SOCE), which relies on the concerted action of the reticular Ca²⁺ sensor STIM1 and the plasma membrane Ca²⁺ channel ORAI1. Gain-of-function mutations in the respective genes induce excessive Ca²⁺ entry, and cause tubular aggregate myopathy (TAM) and Stormorken syndrome. Both disorders are part of a clinical continuum and involve muscle weakness and additional variably pronounced features including miosis, thrombocytopenia, hyposplenism, ichthyosis, dyslexia, and short stature. Mutations in the reticular Ca²⁺ buffer calsequestrin (CASQ1) have moreover been associated with the mild end of the TAM/Stormorken syndrome spectrum. Here we review the clinical and histological characteristics of both disorders, provide an overview on the genetic causes, and thereby focus on the pathomechanisms leading to muscle dysfunction and the multi-systemic phenotype of tubular aggregate myopathy and Stormorken syndrome.

Comparative proteomic profiling reveals a role for Cisd2 in skeletal muscle aging

Skeletal muscle has emerged as one of the most important tissues involved in regulating systemic metabolism. The gastrocnemius is a powerful skeletal muscle composed of predominantly glycolytic fast‐twitch fibers that are preferentially lost among old age. This decrease in gastrocnemius muscle mass is remarkable during aging; however, the underlying molecular mechanism is not fully understood. Strikingly, there is a ~70% decrease in Cisd2 protein, a key regulator of lifespan in mice and the disease gene for Wolfram syndrome 2 in humans, within the gastrocnemius after middle age among mice. A proteomics approach was used to investigate the gastrocnemius of naturally aged mice, and this was compared to the autonomous effect of Cisd2 on gastrocnemius aging using muscle‐specific Cisd2 knockout (mKO) mice as a premature aging model. Intriguingly, dysregulation of calcium signaling and activation of UPR/ER stress stand out as the top two pathways. Additionally, the activity of Serca1 was significantly impaired and this impairment is mainly attributable to irreversibly oxidative modifications of Serca. Our results reveal that the overall characteristics of the gastrocnemius are very similar when naturally aged mice and the Cisd2 mKO mice are compared in terms of pathological alterations, ultrastructural abnormalities, and proteomics profiling. This suggests that Cisd2 mKO mouse is a unique model for understanding the aging mechanism of skeletal muscle. Furthermore, this work substantiates the hypothesis that Cisd2 is crucial to the gastrocnemius muscle and suggests that Cisd2 is a potential therapeutic target for muscle aging.

Identification and characterization of three novel mutations in the CASQ1 gene in four patients with tubular aggregate myopathy

Here, we report the identification of three novel missense mutations in the calsequestrin-1 (CASQ1) gene in four patients with tubular aggregate myopathy. These CASQ1 mutations affect conserved amino acids in position 44 (p.(Asp44Asn)), 103 (p.(Gly103Asp)), and 385 (p.(Ile385Thr)). Functional studies, based on turbidity and dynamic light scattering measurements at increasing Ca²⁺ concentrations, showed a reduced Ca²⁺ -dependent aggregation for the CASQ1 protein containing p.Asp44Asn and p.Gly103Asp mutations and a slight increase in Ca²⁺ -dependent aggregation for the p.Ile385Thr. Accordingly, limited trypsin proteolysis assay showed that p.Asp44Asn and p.Gly103Asp were more susceptible to trypsin cleavage in the presence of Ca²⁺ in comparison with WT and p.Ile385Thr. Analysis of single muscle fibers of a patient carrying the p.Gly103Asp mutation showed a significant reduction in response to caffeine stimulation, compared with normal control fibers. Expression of CASQ1 mutations in eukaryotic cells revealed a reduced ability of all these CASQ1 mutants to store Ca²⁺ and a reduced inhibitory effect of p.Ile385Thr and p.Asp44Asn on store operated Ca²⁺ entry. These results widen the spectrum of skeletal muscle diseases associated with CASQ1 and indicate that these mutations affect properties critical for correct Ca²⁺ handling in skeletal muscle fibers.

ORAI1 Mutations with Distinct Channel Gating Defects in Tubular Aggregate Myopathy

Calcium (Ca²⁺ ) is a physiological key factor, and the precise modulation of free cytosolic Ca²⁺ levels regulates multiple cellular functions. Store-operated Ca²⁺ entry (SOCE) is a major mechanism controlling Ca²⁺ homeostasis, and is mediated by the concerted activity of the Ca²⁺ sensor STIM1 and the Ca²⁺ channel ORAI1. Dominant gain-of-function mutations in STIM1 or ORAI1 cause tubular aggregate myopathy (TAM) or Stormorken syndrome, whereas recessive loss-of-function mutations are associated with immunodeficiency. Here, we report the identification and functional characterization of novel ORAI1 mutations in TAM patients. We assess basal activity and SOCE of the mutant ORAI1 channels, and we demonstrate that the G98S and V107M mutations generate constitutively permeable ORAI1 channels, whereas T184M alters the channel permeability only in the presence of STIM1. These data indicate a mutation-dependent pathomechanism and a genotype/phenotype correlation, as the ORAI1 mutations associated with the most severe symptoms induce the strongest functional cellular effect. Examination of the non-muscle features of our patients strongly suggests that TAM and Stormorken syndrome are spectra of the same disease. Overall, our results emphasize the importance of SOCE in skeletal muscle physiology, and provide new insights in the pathomechanisms involving aberrant Ca²⁺ homeostasis and leading to muscle dysfunction.

Calcium Dyshomeostasis in Tubular Aggregate Myopathy

Calcium is a crucial mediator of cell signaling in skeletal muscles for basic cellular functions and specific functions, including contraction, fiber-type differentiation and energy production. The sarcoplasmic reticulum (SR) is an organelle that provides a large supply of intracellular Ca²⁺ in myofibers. Upon excitation, it releases Ca²⁺ into the cytosol, inducing contraction of myofibrils. During relaxation, it takes up cytosolic Ca²⁺ to terminate the contraction. During exercise, Ca²⁺ is cycled between the cytosol and the SR through a system by which the Ca²⁺ pool in the SR is restored by uptake of extracellular Ca²⁺ via a specific channel on the plasma membrane. This channel is called the store-operated Ca²⁺ channel or the Ca²⁺ release-activated Ca²⁺ channel. It is activated by depletion of the Ca²⁺ store in the SR by coordination of two main molecules: stromal interaction molecule 1 (STIM1) and calcium release-activated calcium channel protein 1 (ORAI1). Recently, myopathies with a dominant mutation in these genes have been reported and the pathogenic mechanism of such diseases have been proposed. This review overviews the calcium signaling in skeletal muscles and role of store-operated Ca²⁺ entry in calcium homeostasis. Finally, we discuss the phenotypes and the pathomechanism of myopathies caused by mutations in the STIM1 and ORAI1 genes.

Diseases caused by mutations in ORAI1 and STIM1

Ca(2+) release-activated Ca(2+) (CRAC) channels mediate a specific form of Ca(2+) influx called store-operated Ca(2+) entry (SOCE) that contributes to the function of many cell types. CRAC channels are composed of ORAI1 proteins located in the plasma membrane, which form its ion-conducting pore. ORAI1 channels are activated by stromal interaction molecule (STIM) 1 and STIM2 located in the endoplasmic reticulum. Loss- and gain-of-function gene mutations in ORAI1 and STIM1 in human patients cause distinct disease syndromes. CRAC channelopathy is caused by loss-of-function mutations in ORAI1 and STIM1 that abolish CRAC channel function and SOCE; it is characterized by severe combined immunodeficiency (SCID)-like disease, autoimmunity, muscular hypotonia, and ectodermal dysplasia, with defects in sweat gland function and dental enamel formation. The latter defect emphasizes an important role of CRAC channels in tooth development. By contrast, autosomal dominant gain-of-function mutations in ORAI1 and STIM1 result in constitutive CRAC channel activation, SOCE, and increased intracellular Ca(2+) levels that are associated with an overlapping spectrum of diseases, including nonsyndromic tubular aggregate myopathy (TAM) and York platelet and Stormorken syndromes. The latter two syndromes are defined, besides myopathy, by thrombocytopenia, thrombopathy, and bleeding diathesis. The fact that myopathy results from both loss- and gain-of-function mutations in ORAI1 and STIM1 highlights the importance of CRAC channels for Ca(2+) homeostasis in skeletal muscle function. The cellular dysfunction and clinical disease spectrum observed in mutant patients provide important information about the molecular regulation of ORAI1 and STIM1 proteins and the role of CRAC channels in human physiology.

Muscle imaging in patients with tubular aggregate myopathy caused by mutations in STIM1

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We characterized muscle imaging pattern in STIM1-related myopathy.

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The subscapularis muscle was preferentially affected in the upper girdle.

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Flexor hallucis longus was consistently affected in the lower limbs.

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Muscle involvement is homogeneous and different from non-STIM1 patients.

Tubular aggregate myopathy is a genetically heterogeneous disease characterized by tubular aggregates as the hallmark on muscle biopsy. Mutations in STIM1 have recently been identified as one genetic cause in a number of tubular aggregate myopathy cases. To characterize the pattern of muscle involvement in this disease, upper and lower girdles and lower limbs were imaged in five patients with mutations in STIM1, and the scans were compared with two patients with tubular aggregate myopathy not caused by mutations in STIM1. A common pattern of involvement was found in STIM1-mutated patients, although with variable extent and severity of lesions. In the upper girdle, the subscapularis muscle was invariably affected. In the lower limbs, all the patients showed a consistent involvement of the flexor hallucis longus, which is very rarely affected in other muscle diseases, and a diffuse involvement of thigh and posterior leg with sparing of gracilis, tibialis anterior and, to a lesser extent, short head of biceps femoris. Mutations in STIM1 are associated with a homogeneous involvement on imaging despite variable clinical features. Muscle imaging can be useful in identifying STIM1-mutated patients especially among other forms of tubular aggregate myopathy.

50 years to diagnosis: Autosomal dominant tubular aggregate myopathy caused by a novel STIM1 mutation

Tubular aggregates in human muscle biopsies have been reported to occur in a variety of acquired and hereditary neuromuscular conditions since 1964. Recently mutations in the gene encoding the main calcium sensor in the sarcoplasmic reticulum, stromal interaction molecule 1 (STIM1), have been identified as a cause of autosomal dominant tubular aggregate myopathy. We studied a German family with tubular aggregate myopathy and defined cellular consequences of altered STIM1 function. Both patients in our family had early progressive myopathy with proximal paresis of arm and leg muscles, scapular winging, ventilatory failure, joint contractures and external ophthalmoplegia. One patient had a well-documented disease course over 50 years. Sequencing of the STIM1 gene revealed a previously unreported missense mutation (c.242G>A; p.Gly81Asp) located in the first calcium binding EF domain. Functional characterization of the new STIM1 mutation by calcium imaging revealed that calcium influx was significantly increased in primary myoblasts of the index patient compared to controls pointing at a severe alteration of intracellular calcium homeostasis. This new family widens the spectrum of STIM1-associated myopathies to a more severe phenotype.

The RNA-binding protein Rbfox1 regulates splicing required for skeletal muscle structure and function

The Rbfox family of RNA-binding proteins is highly conserved with established roles in alternative splicing (AS) regulation. High-throughput studies aimed at understanding transcriptome remodeling have revealed skeletal muscle as displaying one of the largest number of AS events. This finding is consistent with requirements for tissue-specific protein isoforms needed to sustain muscle-specific functions. Rbfox1 is abundant in vertebrate brain, heart and skeletal muscle. Genome-wide genetic approaches have linked the Rbfox1 gene to autism, and a brain-specific knockout mouse revealed a critical role for this splicing regulator in neuronal function. Moreover, a Caenorhabditis elegans Rbfox1 homolog regulates muscle-specific splicing. To determine the role of Rbfox1 in muscle function, we developed a conditional knockout mouse model to specifically delete Rbfox1 in adult tissue. We show that Rbfox1 is required for muscle function but a >70% loss of Rbfox1 in satellite cells does not disrupt muscle regeneration. Deep sequencing identified aberrant splicing of multiple genes including those encoding myofibrillar and cytoskeletal proteins, and proteins that regulate calcium handling. Ultrastructure analysis of Rbfox1(-/-) muscle by electron microscopy revealed abundant tubular aggregates. Immunostaining showed mislocalization of the sarcoplasmic reticulum proteins Serca1 and Ryr1 in a pattern indicative of colocalization with the tubular aggregates. Consistent with mislocalization of Serca1 and Ryr1, calcium handling was drastically altered in Rbfox1(-/-) muscle. Moreover, muscle function was significantly impaired in Rbfox1(-/-) muscle as indicated by decreased force generation. These results demonstrate that Rbfox1 regulates a network of AS events required to maintain multiple aspects of muscle physiology.

Clinical, histological and genetic characterisation of patients with tubular aggregate myopathy caused by mutations in STIM1

BACKGROUND

Tubular aggregate myopathies (TAMs) are muscle disorders characterised by abnormal accumulations of densely packed single-walled or double-walled membrane tubules in muscle fibres. Recently, STIM1, encoding a major calcium sensor of the endoplasmic reticulum, was identified as a TAM gene.

METHODS

The present study aims to define the clinical, histological and ultrastructural phenotype of tubular aggregate myopathy and to assess the STIM1 mutation spectrum.

RESULTS

We describe six new TAM families harbouring one known and four novel STIM1 mutations. All identified mutations are heterozygous missense mutations affecting highly conserved amino acids in the calcium-binding EF-hand domains, demonstrating the presence of a mutation hot spot for TAM. We show that the mutations induce constitutive STIM1 clustering, strongly suggesting that calcium sensing and consequently calcium homoeostasis is impaired. Histological and ultrastructural analyses define a common picture with tubular aggregates labelled with Gomori trichrome and Nicotinamide adenine dinucleotide (NADH) tetrazolium reductase, substantiating their endoplasmic reticulum origin. The aggregates were observed in both fibre types and were often accompanied by nuclear internalisation and fibre size variability. The phenotypical spectrum ranged from childhood onset progressive muscle weakness and elevated creatine kinase levels to adult-onset myalgia without muscle weakness and normal CK levels.

CONCLUSIONS

The present study expands the phenotypical spectrum of STIM1-related tubular aggregate myopathy. STIM1 should therefore be considered for patients with tubular aggregate myopathies involving either muscle weakness or myalgia as the first and predominant clinical sign.