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

Exome sequencing in undiagnosed congenital myopathy reveals new genes and refines genes-phenotypes correlations

BACKGROUND

Congenital myopathies are severe genetic diseases with a strong impact on patient autonomy and often on survival. A large number of patients do not have a genetic diagnosis, precluding genetic counseling and appropriate clinical management. Our objective was to find novel pathogenic variants and genes associated with congenital myopathies and to decrease diagnostic odysseys and dead-end.

METHODS

To identify pathogenic variants and genes implicated in congenital myopathies, we established and conducted the MYOCAPTURE project from 2009 to 2018 to perform exome sequencing in a large cohort of 310 families partially excluded for the main known genes.

RESULTS

Pathogenic variants were identified in 156 families (50%), among which 123 families (40%) had a conclusive diagnosis. Only 44 (36%) of the resolved cases were linked to a known myopathy gene with the corresponding phenotype, while 55 (44%) were linked to pathogenic variants in a known myopathy gene with atypical signs, highlighting that most genetic diagnosis could not be anticipated based on clinical-histological assessments in this cohort. An important phenotypic and genetic heterogeneity was observed for the different genes and for the different congenital myopathy subtypes, respectively. In addition, we identified 14 new myopathy genes not previously associated with muscle diseases (20% of all diagnosed cases) that we previously reported in the literature, revealing novel pathomechanisms and potential therapeutic targets.

CONCLUSIONS

Overall, this approach illustrates the importance of massive parallel gene sequencing as a comprehensive tool for establishing a molecular diagnosis for families with congenital myopathies. It also emphasizes the contribution of clinical data, histological findings on muscle biopsies, and the availability of DNA samples from additional family members to the diagnostic success rate. This study facilitated and accelerated the genetic diagnosis of congenital myopathies, improved health care for several patients, and opened novel perspectives for either repurposing of existing molecules or the development of novel treatments.

Activation mechanisms and structural dynamics of STIM proteins

The family of stromal interaction molecules (STIM) includes two widely expressed single-pass endoplasmic reticulum (ER) transmembrane proteins and additional splice variants that act as precise ER-luminal Ca2+ sensors. STIM proteins mainly function as one of the two essential components of the so-called Ca2+ release-activated Ca2+ (CRAC) channel. The second CRAC channel component is constituted by pore-forming Orai proteins in the plasma membrane. STIM and Orai physically interact with each other to enable CRAC channel opening, which is a critical prerequisite for various downstream signalling pathways such as gene transcription or proliferation. Their activation commonly requires the emptying of the intracellular ER Ca2+ store. Using their Ca2+ sensing capabilities, STIM proteins confer this Ca2+ content-dependent signal to Orai, thereby linking Ca2+ store depletion to CRAC channel opening. Here we review the conformational dynamics occurring along the entire STIM protein upon store depletion, involving the transition from the quiescent, compactly folded structure into an active, extended state, modulation by a variety of accessory components in the cell as well as the impairment of individual steps of the STIM activation cascade associated with disease.

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 D84G mutation in STIM1 causes nuclear envelope dysfunction and myopathy in mice

Stromal interaction molecule 1 (STIM1) is a Ca2+ sensor located in the sarcoplasmic reticulum (SR) of skeletal muscle, where it is best known for its role in store-operated Ca2+ entry (SOCE). Genetic syndromes resulting from STIM1 mutations are recognized as a cause of muscle weakness and atrophy. Here, we focused on a gain-of-function mutation that occurs in humans and mice (STIM1+/D84G mice), in which muscles exhibited constitutive SOCE. Unexpectedly, this constitutive SOCE did not affect global Ca2+ transients, SR Ca2+ content, or excitation-contraction coupling (ECC) and was therefore unlikely to underlie the reduced muscle mass and weakness observed in these mice. Instead, we demonstrate that the presence of D84G STIM1 in the nuclear envelope of STIM1+/D84G muscle disrupted nuclear-cytosolic coupling, causing severe derangement in nuclear architecture, DNA damage, and altered lamina A-associated gene expression. Functionally, we found that D84G STIM1 reduced the transfer of Ca2+ from the cytosol to the nucleus in myoblasts, resulting in a reduction of [Ca2+]N. Taken together, we propose a novel role for STIM1 in the nuclear envelope that links Ca2+ signaling to nuclear stability in skeletal muscle.

The ER stress sensor IRE1 interacts with STIM1 to promote store-operated calcium entry, T cell activation, and muscular differentiation

Store-operated Ca2+ entry (SOCE) mediated by stromal interacting molecule (STIM)-gated ORAI channels at endoplasmic reticulum (ER) and plasma membrane (PM) contact sites maintains adequate levels of Ca2+ within the ER lumen during Ca2+ signaling. Disruption of ER Ca2+ homeostasis activates the unfolded protein response (UPR) to restore proteostasis. Here, we report that the UPR transducer inositol-requiring enzyme 1 (IRE1) interacts with STIM1, promotes ER-PM contact sites, and enhances SOCE. IRE1 deficiency reduces T cell activation and human myoblast differentiation. In turn, STIM1 deficiency reduces IRE1 signaling after store depletion. Using a CaMPARI2-based Ca2+ genome-wide screen, we identify CAMKG2 and slc105a as SOCE enhancers during ER stress. Our findings unveil a direct crosstalk between SOCE and UPR via IRE1, acting as key regulator of ER Ca2+ and proteostasis in T cells and muscles. Under ER stress, this IRE1-STIM1 axis boosts SOCE to preserve immune cell functions, a pathway that could be targeted for cancer immunotherapy.

Modulation of biomolecular phase behavior by metal ions

Liquid-liquid phase separation (LLPS) appears to be a newly appreciated aspect of the cellular organization of biomolecules that leads to the formation of membraneless organelles (MLOs). MLOs generate distinct microenvironments where particular biomolecules are highly concentrated compared to those in the surrounding environment. Their thermodynamically driven formation is reversible, and their liquid nature allows them to fuse with each other. Dysfunctional biomolecular condensation is associated with human diseases. Pathological states of MLOs may originate from the mutation of proteins or may be induced by other factors. In most aberrant MLOs, transient interactions are replaced by stronger and more rigid interactions, preventing their dissolution, and causing their uncontrolled growth and dysfunction. For these reasons, there is great interest in identifying factors that modulate LLPS. In this review, we discuss an enigmatic and mostly unexplored aspect of this process, namely, the regulatory effects of metal ions on the phase behavior of biomolecules.

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

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

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.

The Ca2+ Sensor STIM in Human Diseases

The STIM family of proteins plays a crucial role in a plethora of cellular functions through the regulation of store-operated Ca2+ entry (SOCE) and, thus, intracellular calcium homeostasis. The two members of the mammalian STIM family, STIM1 and STIM2, are transmembrane proteins that act as Ca2+ sensors in the endoplasmic reticulum (ER) and, upon Ca2+ store discharge, interact with and activate the Orai/CRACs in the plasma membrane. Dysregulation of Ca2+ signaling leads to the pathogenesis of a variety of human diseases, including neurodegenerative disorders, cardiovascular diseases, cancer, and immune disorders. Therefore, understanding the mechanisms underlying Ca2+ signaling pathways is crucial for developing therapeutic strategies targeting these diseases. This review focuses on several rare conditions associated with STIM1 mutations that lead to either gain- or loss-of-function, characterized by myopathy, hematological and immunological disorders, among others, and due to abnormal activation of CRACs. In addition, we summarize the current evidence concerning STIM2 allele duplication and deletion associated with language, intellectual, and developmental delay, recurrent pulmonary infections, microcephaly, facial dimorphism, limb anomalies, hypogonadism, and congenital heart defects.

The D84G mutation in STIM1 causes nuclear envelope dysfunction and myopathy in mice

Stromal interaction molecule 1 (STIM1) is a Ca 2+ sensor located in the sarcoplasmic reticulum (SR) of skeletal muscle where it is best known for its role in store operated Ca 2+ entry (SOCE). Genetic syndromes resulting from STIM1 mutations are recognized as a cause of muscle weakness and atrophy. Here, we focus on a gain of function mutation that occurs in humans and mice (STIM1 +/D84G mice) where muscles exhibit constitutive SOCE. Unexpectedly, this constitutive SOCE did not affect global Ca 2+ transients, SR Ca 2+ content or excitation contraction coupling (ECC) and was therefore unlikely to underlie the reduced muscle mass and weakness observed in these mice. Instead, we demonstrate that the presence of D84G STIM1 in the nuclear envelope of STIM1 +/D84G muscle disrupts nuclear-cytosolic coupling causing severe derangement in nuclear architecture, DNA damage, and altered lamina A associated gene expression. Functionally, we found D84G STIM1 reduced the transfer of Ca 2+ from the cytosol to the nucleus in myoblasts resulting in a reduction of [Ca 2+ ] N . Taken together, we propose a novel role for STIM1 in the nuclear envelope that links Ca 2+ signaling to nuclear stability in skeletal muscle.

Store-operated calcium entry: From physiology to tubular aggregate myopathy

Store-Operated Ca²⁺ entry (SOCE) is recognized as a key mechanism in muscle physiology necessary to refill intracellular Ca²⁺ stores during sustained muscle activity. For many years the cell structures expected to mediate SOCE in skeletal muscle fibres remained unknown. Recently, the identification of Ca²⁺ Entry Units (CEUs) in exercised muscle fibres opened new insights into the role of extracellular Ca²⁺ in muscle contraction and, more generally, in intracellular Ca²⁺ homeostasis. Accordingly, intracellular Ca²⁺ unbalance due to alterations in SOCE strictly correlates with muscle disfunction and disease. Mutations in proteins involved in SOCE (STIM1, ORAI1, and CASQ1) have been linked to tubular aggregate myopathy (TAM), a disease that causes muscle weakness and myalgia and is characterized by a typical accumulation of highly ordered and packed membrane tubules originated from the sarcoplasmic reticulum (SR). Achieving a full understanding of the molecular pathways activated by alterations in Ca²⁺ entry mechanisms is a necessary step to design effective therapies for human SOCE-related disorders.

CASQ1-related myopathy: The first report from China and the literature review

Calsequestrin 1 (CASQ1) is the most crucial Ca²⁺ binding protein localized in the sarcoplasmic reticulum (SR) of skeletal muscle. With high capacity and low affinity for Ca²⁺, CASQ1 plays a significant role in maintaining a large amount of Ca²⁺ necessary for muscle contraction. However, only five mutations in CASQ1 have been identified to date. Here, we report a 42-year-old Chinese female patient who presented with a 12 years history of slowly progressive upper limb weakness, predominantly affecting distal muscles, which was uncommon comparing to other CASQ1-related patients. Next-generation sequencing (NGS) analysis revealed a novel heterozygous mutation (c.766G > A, p.Val256Met) in CASQ1. Functional studies confirmed the likely pathogenicity of this variant. Muscle histopathology revealed rare optically empty vacuoles in myofibers and atypical eosinophilic granules in the cytoplasm, which has not been observed before. We also performed a literature review on all the pathogenic mutations in CASQ1 and summarized their genetic and clinical characteristics. This is the first report on CASQ1-related myopathy from China, further expanding the mutation spectrum of CASQ1 gene and provides new insights into the function of CASQ1.

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.

Silencing of the Ca2+ Channel ORAI1 Improves the Multi-Systemic Phenotype of Tubular Aggregate Myopathy (TAM) and Stormorken Syndrome (STRMK) in Mice

Tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK) form a clinical continuum associating progressive muscle weakness with additional multi-systemic anomalies of the bones, skin, spleen, and platelets. TAM/STRMK arises from excessive extracellular Ca²⁺ entry due to gain-of-function mutations in the Ca²⁺ sensor STIM1 or the Ca²⁺ channel ORAI1. Currently, no treatment is available. Here we assessed the therapeutic potential of ORAI1 downregulation to anticipate and reverse disease development in a faithful mouse model carrying the most common TAM/STRMK mutation and recapitulating the main signs of the human disorder. To this aim, we crossed Stim1^R304W/+ mice with Orai1^+/− mice expressing 50% of ORAI1. Systematic phenotyping of the offspring revealed that the Stim1^R304W/+Orai1^+/− mice were born with a normalized ratio and showed improved postnatal growth, bone architecture, and partly ameliorated muscle function and structure compared with their Stim1^R304W/+ littermates. We also produced AAV particles containing Orai1-specific shRNAs, and intramuscular injections of Stim1^R304W/+ mice improved the skeletal muscle contraction and relaxation properties, while muscle histology remained unchanged. Altogether, we provide the proof-of-concept that Orai1 silencing partially prevents the development of the multi-systemic TAM/STRMK phenotype in mice, and we also established an approach to target Orai1 expression in postnatal tissues.

CIC-39Na reverses the thrombocytopenia that characterizes tubular aggregate myopathy

Store-Operated Ca2+-Entry is a cellular mechanism that governs the replenishment of intracellular stores of Ca2+ upon depletion caused by the opening of intracellular Ca2+-channels. Gain-of-function mutations of the two key proteins of Store-Operated Ca2+-Entry, STIM1 and ORAI1, are associated with several ultra-rare diseases clustered as tubular aggregate myopathies. Our group has previously demonstrated that a mouse model bearing the STIM1 p.I115F mutation recapitulates the main features of the STIM1 gain-of-function disorders: muscle weakness and thrombocytopenia. Similar findings have been found in other mice bearing different mutations on STIM1. At present, no valid treatment is available for these patients. In the present contribution, we report that CIC-39Na, a Store-Operated Ca2+-Entry inhibitor, restores platelet number and counteracts the abnormal bleeding that characterizes these mice. Subtle differences in thrombopoiesis were observed in STIM1 p.I115F mice, but the main difference between wild-type and STIM1 p.I115F mice was in platelet clearance and in the levels of platelet cytosolic basal Ca2+. Both were restored upon treatment of animals with CIC-39Na. This finding paves the way to a pharmacological treatment strategy for thrombocytopenia in tubular aggregate myopathy patients.

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.

Molecular Characterization of Skeletal Muscle Dysfunction in Sigma 1 Receptor (Sigmar1) Knockout Mice

Sigma 1 receptor (Sigmar1) is a widely expressed, multitasking molecular chaperone protein that plays functional roles in several cellular processes. Mutations in the Sigmar1 gene are associated with several distal neuropathies with strong manifestation in skeletal muscle dysfunction with phenotypes like muscle wasting and atrophy. However, the physiological function of Sigmar1 in skeletal muscle remains unknown. Herein, the physiological role of Sigmar1 in skeletal muscle structure and function in gastrocnemius, quadriceps, soleus, extensor digitorum longus, and tibialis anterior muscles was determined. Quantification of myofiber cross-sectional area showed altered myofiber size distribution and changes in myofiber type in the skeletal muscle of the Sigmar1-/- mice. Interestingly, ultrastructural analysis by transmission electron microscopy showed the presence of abnormal mitochondria, and immunostaining showed derangements in dystrophin localization in skeletal muscles from Sigmar1-/- mice. In addition, myopathy in Sigmar1-/- mice was associated with an increased number of central nuclei, increased collagen deposition, and fibrosis. Functional studies also showed reduced endurance and exercise capacity in the Sigmar1-/- mice without any changes in voluntary locomotion, markers for muscle denervation, and muscle atrophy. Overall, this study shows, for the first time, a potential physiological function of Sigmar1 in maintaining healthy skeletal muscle structure and function.

Genetic defects are common in myopathies with tubular aggregates

Objective

A group of genes have been reported to be associated with myopathies with tubular aggregates (TAs). Many cases with TAs still lack of genetic clarification. This study aims to explore the genetic background of cases with TAs in order to improve our knowledge of the pathogenesis of these rare pathological structures.

Methods

Thirty‐three patients including two family members with biopsy confirmed TAs were collected. Whole‐exome sequencing was performed on 31 unrelated index patients and a candidate gene search strategy was conducted. The identified variants were confirmed by Sanger sequencing. The wild‐type and the mutant p.Ala11Thr of ALG14 were transfected into human embryonic kidney 293 cells (HEK293), and western blot analysis was performed to quantify protein expression levels.

Results

Eleven index cases (33%) were found to have pathogenic variant or likely pathogenic variants in STIM1, ORAI1, PGAM2, SCN4A, CASQ1 and ALG14. Among them, the c.764A>T (p.Glu255Val) in STIM1 and the c.1333G>C (p.Val445Leu) in SCN4A were novel. Western blot analysis showed that the expression of ALG14 protein was severely reduced in the mutant ALG14 HEK293 cells (p.Ala11Thr) compared with wild type. The ALG14 variants might be associated with TAs in patients with complex multisystem disorders.

Interpretation

This study expands the phenotypic and genotypic spectrums of myopathies with TAs. Our findings further confirm previous hypothesis that genes related with calcium signalling pathway and N‐linked glycosylation pathway are the main genetic causes of myopathies with TAs.

Calsequestrin 1 Is an Active Partner of Stromal Interaction Molecule 2 in Skeletal Muscle

Calsequestrin 1 (CASQ1) in skeletal muscle buffers and senses Ca²⁺ in the sarcoplasmic reticulum (SR). CASQ1 also regulates store-operated Ca²⁺ entry (SOCE) by binding to stromal interaction molecule 1 (STIM1). Abnormal SOCE and/or abnormal expression or mutations in CASQ1, STIM1, or STIM2 are associated with human skeletal, cardiac, or smooth muscle diseases. However, the functional relevance of CASQ1 along with STIM2 has not been studied in any tissue, including skeletal muscle. First, in the present study, it was found by biochemical approaches that CASQ1 is bound to STIM2 via its 92 N-terminal amino acids (C1 region). Next, to examine the functional relevance of the CASQ1-STIM2 interaction in skeletal muscle, the full-length wild-type CASQ1 or the C1 region was expressed in mouse primary skeletal myotubes, and the myotubes were examined using single-myotube Ca²⁺ imaging experiments and transmission electron microscopy observations. The CASQ1-STIM2 interaction via the C1 region decreased SOCE, increased intracellular Ca²⁺ release for skeletal muscle contraction, and changed intracellular Ca²⁺ distributions (high Ca²⁺ in the SR and low Ca²⁺ in the cytosol were observed). Furthermore, the C1 region itself (which lacks Ca²⁺-buffering ability but has STIM2-binding ability) decreased the expression of Ca²⁺-related proteins (canonical-type transient receptor potential cation channel type 6 and calmodulin 1) and induced mitochondrial shape abnormalities. Therefore, in skeletal muscle, CASQ1 plays active roles in Ca²⁺ movement and distribution by interacting with STIM2 as well as Ca²⁺ sensing and buffering.

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

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

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.

Functional Calsequestrin-1 Is Expressed in the Heart and Its Deficiency Is Causally Related to Malignant Hyperthermia-Like Arrhythmia

Background: Calsequestrins (Casqs), comprising the Casq1 and Casq2 isoforms, buffer Ca²⁺ and regulate its release in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle, respectively. Human inherited diseases associated with mutations in CASQ1 or CASQ2 include malignant hyperthermia/environmental heat stroke (MH/EHS) and catecholaminergic polymorphic ventricular tachycardia. However, patients with an MH/EHS event often suffer from arrhythmia for which the underlying mechanism remains unknown. Methods: Working hearts from conventional (Casq1-KO) and cardiac-specific (Casq1-CKO) Casq1 knockout mice were monitored in vivo and ex vivo by electrocardiogram and electrical mapping, respectively. MH was induced by 2% isoflurane and treated intraperitoneally with dantrolene. Time-lapse imaging was used to monitor intracellular Ca²⁺ activity in isolated mouse cardiomyocytes or neonatal rat ventricular myocytes (NRVMs) with knockdown, over-expression or truncation of the Casq1 gene. Conformational change in both Casqs was determined by crosslinking Western blot analysis. Results: Like MH/EHS patients, Casq1-KO and Casq1-CKO mice had faster basal heart rate, and ventricular tachycardia upon exposure to 2% isoflurane, which could be relieved by dantrolene. Basal sinus tachycardia and ventricular ectopic electrical triggering also occurred in Casq1-KO hearts ex vivo. Accordingly, the ventricular cardiomyocytes from Casq1-CKO mice displayed dantrolene-sensitive increased Ca²⁺ waves and diastole premature Ca²⁺ transients/oscillations upon isoflurane. NRVMs with Casq1-knockdown had enhanced spontaneous Ca2+ sparks/transients upon isoflurane, while cells over-expressing Casq1 exhibited decreased Ca2+ sparks/transients that were absent in cells with truncation of 9 amino acids at the C-terminus of Casq1. Structural evaluation showed that most of the Casq1 protein was present as a polymer and physically interacted with RyR2 in the ventricular SR. The Casq1 isoform was also expressed in human myocardium. Mechanistically, exposure to 2% isoflurane or heating at 41ºC induced Casq1 oligomerization in mouse ventricular and skeletal muscle tissues, leading to a reduced Casq1/RyR2 interaction and increased RyR2 activity in the ventricle. Conclusions: Casq1 is expressed in the heart, where it regulates SR Ca²⁺ release and heart rate. Casq1 deficiency independently causes MH/EHS-like ventricular arrhythmia by trigger-induced Casq1 oligomerization and a relief of its inhibitory effect on RyR2-mediated Ca²⁺ release, thus revealing a new inherited arrhythmia and a novel mechanism for MH/EHS arrhythmogenesis.

Improper Remodeling of Organelles Deputed to Ca2+ Handling and Aerobic ATP Production Underlies Muscle Dysfunction in Ageing

Proper skeletal muscle function is controlled by intracellular Ca2+ concentration and by efficient production of energy (ATP), which, in turn, depend on: (a) the release and re-uptake of Ca2+ from sarcoplasmic-reticulum (SR) during excitation-contraction (EC) coupling, which controls the contraction and relaxation of sarcomeres; (b) the uptake of Ca2+ into the mitochondrial matrix, which stimulates aerobic ATP production; and finally (c) the entry of Ca2+ from the extracellular space via store-operated Ca2+ entry (SOCE), a mechanism that is important to limit/delay muscle fatigue. Abnormalities in Ca2+ handling underlie many physio-pathological conditions, including dysfunction in ageing. The specific focus of this review is to discuss the importance of the proper architecture of organelles and membrane systems involved in the mechanisms introduced above for the correct skeletal muscle function. We reviewed the existing literature about EC coupling, mitochondrial Ca2+ uptake, SOCE and about the structural membranes and organelles deputed to those functions and finally, we summarized the data collected in different, but complementary, projects studying changes caused by denervation and ageing to the structure and positioning of those organelles: a. denervation of muscle fibers-an event that contributes, to some degree, to muscle loss in ageing (known as sarcopenia)-causes misplacement and damage: (i) of membrane structures involved in EC coupling (calcium release units, CRUs) and (ii) of the mitochondrial network; b. sedentary ageing causes partial disarray/damage of CRUs and of calcium entry units (CEUs, structures involved in SOCE) and loss/misplacement of mitochondria; c. functional electrical stimulation (FES) and regular exercise promote the rescue/maintenance of the proper architecture of CRUs, CEUs, and of mitochondria in both denervation and ageing. All these structural changes were accompanied by related functional changes, i.e., loss/decay in function caused by denervation and ageing, and improved function following FES or exercise. These data suggest that the integrity and proper disposition of intracellular organelles deputed to Ca2+ handling and aerobic generation of ATP is challenged by inactivity (or reduced activity); modifications in the architecture of these intracellular membrane systems may contribute to muscle dysfunction in ageing and sarcopenia.

Neuromuscular junction instability and altered intracellular calcium handling as early determinants of force loss during unloading in humans

Key points

Few days of unloading are sufficient to induce a decline of skeletal muscle mass and function; notably, contractile force is lost at a faster rate than muscle mass.

The reasons behind this disproportionate loss of muscle force are still poorly understood.

We provide strong evidence of two mechanisms only hypothesized until now for the rapid muscle force loss in only 10 days of bed rest.

Our results show that an initial neuromuscular junction instability, accompanied by alterations in the innervation status and impairment of single fibre sarcoplasmic reticulum function contribute to the loss of contractile force in front of a preserved myofibrillar function and central activation capacity.

Early onset of neuromuscular junction instability and impairment in calcium dynamics involved in excitation–contraction coupling are proposed as eligible determinants to the greater decline in muscle force than in muscle size during unloading.

Abstract

Unloading induces rapid skeletal muscle atrophy and functional decline. Importantly, force is lost at a much higher rate than muscle mass. We aimed to investigate the early determinants of the disproportionate loss of force compared to that of muscle mass in response to unloading. Ten young participants underwent 10 days of bed rest (BR). At baseline (BR0) and at 10 days (BR10), quadriceps femoris (QF) volume (VOL) and isometric maximum voluntary contraction (MVC) were assessed. At BR0 and BR10 blood samples and biopsies of vastus lateralis (VL) muscle were collected. Neuromuscular junction (NMJ) stability and myofibre innervation status were assessed, together with single fibre mechanical properties and sarcoplasmic reticulum (SR) calcium handling. From BR0 to BR10, QFVOL and MVC decreased by 5.2% (P = 0.003) and 14.3% (P < 0.001), respectively. Initial and partial denervation was detected from increased neural cell adhesion molecule (NCAM)‐positive myofibres at BR10 compared with BR0 (+3.4%, P = 0.016). NMJ instability was further inferred from increased C‐terminal agrin fragment concentration in serum (+19.2% at BR10, P = 0.031). Fast fibre cross‐sectional area (CSA) showed a trend to decrease by 15% (P = 0.055) at BR10, while single fibre maximal tension (force/CSA) was unchanged. However, at BR10 SR Ca²⁺ release in response to caffeine decreased by 35.1% (P < 0.002) and 30.2% (P < 0.001) in fast and slow fibres, respectively, pointing to an impaired excitation–contraction coupling. These findings support the view that the early onset of NMJ instability and impairment in SR function are eligible mechanisms contributing to the greater decline in muscle force than in muscle size during unloading.

Few days of unloading are sufficient to induce a decline of skeletal muscle mass and function; notably, contractile force is lost at a faster rate than muscle mass.

The reasons behind this disproportionate loss of muscle force are still poorly understood.

We provide strong evidence of two mechanisms only hypothesized until now for the rapid muscle force loss in only 10 days of bed rest.

Our results show that an initial neuromuscular junction instability, accompanied by alterations in the innervation status and impairment of single fibre sarcoplasmic reticulum function contribute to the loss of contractile force in front of a preserved myofibrillar function and central activation capacity.

Early onset of neuromuscular junction instability and impairment in calcium dynamics involved in excitation–contraction coupling are proposed as eligible determinants to the greater decline in muscle force than in muscle size during unloading.

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.

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

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

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.

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.

Calsequestrin, a key protein in striated muscle health and disease

Calsequestrin (CASQ) is the most abundant Ca²⁺ binding protein localized in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. The genome of vertebrates contains two genes, CASQ1 and CASQ2. CASQ1 and CASQ2 have a high level of homology, but show specific patterns of expression. Fast-twitch skeletal muscle fibers express only CASQ1, both CASQ1 and CASQ2 are present in slow-twitch skeletal muscle fibers, while CASQ2 is the only protein present in cardiomyocytes. Depending on the intraluminal SR Ca²⁺ levels, CASQ monomers assemble to form large polymers, which increase their Ca²⁺ binding ability. CASQ interacts with triadin and junctin, two additional SR proteins which contribute to localize CASQ to the junctional region of the SR (j-SR) and also modulate CASQ ability to polymerize into large macromolecular complexes. In addition to its ability to bind Ca²⁺ in the SR, CASQ appears also to be able to contribute to regulation of Ca²⁺ homeostasis in muscle cells. Both CASQ1 and CASQ2 are able to either activate and inhibit the ryanodine receptors (RyRs) calcium release channels, likely through their interactions with junctin and triadin. Additional evidence indicates that CASQ1 contributes to regulate the mechanism of store operated calcium entry in skeletal muscle via a direct interaction with the Stromal Interaction Molecule 1 (STIM1). Mutations in CASQ2 and CASQ1 have been identified, respectively, in patients with catecholamine-induced polymorphic ventricular tachycardia and in patients with some forms of myopathy. This review will highlight recent developments in understanding CASQ1 and CASQ2 in health and diseases.

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.

Caffeine as a tool to investigate sarcoplasmic reticulum and intracellular calcium dynamics in human skeletal muscles

Caffeine is worldwide used for its power to increase cognitive and physical performance. The ergogenic effects of caffeine, however, do not depend on a direct action on muscles. Actually, the actions of caffeine on skeletal muscles, take place at millimolar concentrations which are far above the micromolar level reached after a regular consumption of coffee or similar drinks, and close to a lethal concentration. At millimolar concentrations caffeine exerts a powerful effect on sarcoplasmic reticulum (SR) activating the release of calcium via ryanodine receptors and, possibly, inhibiting calcium reuptake. For this reason caffeine has become a valuable tool for studying SR function and for diagnostics of SR related muscle disorders. This review aims to briefly describe the effects and the mechanism of action of caffeine on sarcoplasmic reticulum and to focus on its use to study intracellular calcium dynamics in human muscle fibers in physiological and pathological conditions.

An interplay of structure and intrinsic disorder in the functionality of peptidylarginine deiminases, a family of key autoimmunity-related enzymes

Citrullination is a post-translation modification of proteins, where the proteinaceous arginine residues are converted to non-coded citrulline residues. The immune tolerance to such citrullinated protein can be lost, leading to inflammatory and autoimmune diseases. Citrullination is a chemical reaction mediated by peptidylarginine deiminase enzymes (PADs), which are a family of calcium-dependent cysteine hydrolase enzymes that includes five isotypes: PAD1, PAD2, PAD3, PAD4, and PAD6. Each PAD has specific substrates and tissue distribution, where it modifies the arginine to produce a citrullinated protein with altered structure and function. All mammalian PADs have a sequence similarity of about 70-95%, whereas in humans, they are 50-55% homologous in their structure and amino acid sequences. Being calcium-dependent hydrolases, PADs are inactive under the physiological level of calcium, but could be activated due to distortions in calcium homeostasis, or when the cellular calcium levels are increased. In this article, we analyze some of the currently available data on the structural properties of human PADs, the mechanisms of their calcium-induced activation, and show that these proteins contain functionally important regions of intrinsic disorder. Citrullination represents an important trigger of multiple physiological and pathological processes, and as a result, PADs are recognized to play a number of important roles in autoimmune diseases, cancer, and neurodegeneration. Therefore, we also review the current state of the art in the development of PAD inhibitors with good potency and selectivity.

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.

Molecular determinants of homo- and heteromeric interactions of Junctophilin-1 at triads in adult skeletal muscle fibers

In adult skeletal muscles, 2 junctophilin isoforms (JPH1 and JPH2) tether the sarcoplasmic reticulum (SR) to transverse tubule (T-tubule) membranes, generating stable membrane contact sites known as triads. JPHs are anchored to the membrane of the SR by a C-terminal transmembrane domain (TMD) and bind the T-tubule membrane through their cytosolic N-terminal region, which contains 8 lipid-binding (MORN) motifs. By combining expression of GFP-JPH1 deletion mutants in skeletal muscle fibers with in vitro biochemical experiments, we investigated the molecular determinants of JPH1 recruitment at triads in adult skeletal muscle fibers. We found that MORN motifs bind PI(4,5)P₂ in the sarcolemma, but do not mediate the selective localization of JPH1 at the T-tubule compartment of triads. On the contrary, fusion proteins containing only the TMD of JPH1 were able to localize at the junctional SR compartment of the triad. Bimolecular fluorescence complementation experiments indicated that the TMD of JPH1 can form dimers, suggesting that the observed localization at triads may result from dimerization with the TMDs of resident JPH1. A second domain, capable of mediating homo- and heterodimeric interactions between JPH1 and JPH2 was identified in the cytosolic region. FRAP experiments revealed that removal of either one of these 2 domains in JPH1 decreases the association of the resulting mutant proteins with triads. Altogether, these results suggest that the ability to establish homo- and heterodimeric interactions with resident JPHs may support the recruitment and stability of newly synthesized JPHs at triads in adult skeletal muscle fibers.

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

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

Functional Electrical Stimulation: A Possible Strategy to Improve Muscle Function in Central Core Disease?

Central Core Disease (CCD) is a congenital myopathy characterized by presence of amorphous central areas (or cores) lacking glycolytic/oxidative enzymes and mitochondria in skeletal muscle fibers. Most CCD families are linked to mutations in ryanodine receptor type-1 (RYR1), the gene encoding for the sarcoplasmic reticulum (SR) Ca²⁺ release channel of skeletal muscle. As no treatments are available for CCD, currently management of patients is essentially based on a physiotherapic approaches. Functional electrical stimulation (FES) is a technique used to deliver low energy electrical impulses to artificially stimulate selected skeletal muscle groups. Here we tested the efficacy of FES in counteracting muscle loss and improve function in the lower extremities of a 55-year-old female patient which was diagnosed with CCD at the age of 44. Genetic screening of the RyR1 gene identified a missense mutation (c.7354C>T) in exon 46 resulting in an amino acid substitution (p.R2452W) and a duplication (c.12853_12864dup12) in exon 91. The patient was treated with FES for 26 months and subjected before, during, and after training to a series of functional and structural assessments: measurement of maximum isometric force of leg extensor muscles, magnetic resonance imaging, a complete set of functional tests to assess mobility in activities of daily living, and analysis of muscle biopsies by histology and electron microscopy. All results point to an improvement in muscle structure and function induced by FES suggesting that this approach could be considered as an additional supportive measure to maintain/improve muscle function (and possibly reduce muscle loss) in CCD patients.

[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.

The clinical spectrum of CASQ1-related myopathy

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

Exciting perspectives for Translational Myology in the Abstracts of the 2018Spring PaduaMuscleDays: Giovanni Salviati Memorial - Chapter III - Abstracts of March 16, 2018

Myologists working in Padua (Italy) were able to continue a half-century tradition of studies of skeletal muscles, that started with a research on fever, specifically if and how skeletal muscle contribute to it by burning bacterial toxin. Beside main publications in high-impact-factor journals by Padua myologists, I hope to convince readers (and myself) of the relevance of the editing Basic and Applied Myology (BAM), retitled from 2010 European Journal of Translational Myology (EJTM), of the institution of the Interdepartmental Research Center of Myology of the University of Padova (CIR-Myo), and of a long series of International Conferences organized in Euganei Hills and Padova, that is, the PaduaMuscleDays. The 2018Spring PaduaMuscleDays (2018SpPMD), were held in Euganei Hills and Padua (Italy), in March 14-17, and were dedicated to Giovanni Salviati. The main event of the “Giovanni Salviati Memorial”, was held in the Aula Guariento, Accademia Galileiana di Scienze, Lettere ed Arti of Padua to honor a beloved friend and excellent scientist 20 years after his premature passing. Using the words of Prof. Nicola Rizzuto, we all share his believe that Giovanni “will be remembered not only for his talent and originality as a biochemist, but also for his unassuming and humanistic personality, a rare quality in highly successful people like Giovanni. The best way to remember such a person is to gather pupils and colleagues, who shared with him the same scientific interests and ask them to discuss recent advances in their own fields, just as Giovanni have liked to do”. Since Giovanni’s friends sent many abstracts still influenced by their previous collaboration with him, all the Sessions of the 2018SpPMD reflect both to the research aims of Giovanni Salviati and the traditional topics of the PaduaMuscleDays, that is, basics and applications of physical, molecular and cellular strategies to maintain or recover functions of skeletal muscles. The translational researches summarized in the 2018SpPMD Abstracts are at the appropriate high level to attract approval of Ethical Committees, the interest of International Granting Agencies and approval for publication in top quality, international journals. The abstracts of the March 16, 2018 Padua Muscle Day are listed in this chapter III. All 2018SpPMD Abstracts are indexed at the end of the Chapter IV.

Disturbed Ca2+ Homeostasis in Muscle-Wasting Disorders

Ca²⁺ is essential for proper structure and function of skeletal muscle. It not only activates contraction and force development but also participates in multiple signaling pathways. Low levels of Ca²⁺ restrain muscle regeneration by limiting the fusion of satellite cells. Ironically, sustained elevations of Ca²⁺ also result in muscle degeneration as this ion promotes high rates of protein breakdown. Moreover, transforming growth factors (TGFs) which are well known for controlling muscle growth also regulate Ca²⁺ channels. Thus, therapies focused on changing levels of Ca²⁺ and TGFs are promising for treating muscle-wasting disorders. Three principal systems govern the homeostasis of Ca²⁺, namely, excitation-contraction (EC) coupling, excitation-coupled Ca²⁺ entry (ECCE), and store-operated Ca²⁺ entry (SOCE). Accordingly, alterations in these systems can lead to weakness and atrophy in many hereditary diseases, such as Brody disease, central core disease (CCD), tubular aggregate myopathy (TAM), myotonic dystrophy type 1 (MD1), oculopharyngeal muscular dystrophy (OPMD), and Duchenne muscular dystrophy (DMD). Here, the interrelationship between all these molecules and processes is reviewed.