Selenoprotein N is dynamically expressed during mouse development and detected early in muscle precursors

Zebrafish and cellular models of SELENON-Related Myopathy exhibit novel embryonic and metabolic phenotypes

SELENON-Related Myopathy (SELENON-RM) is a rare congenital myopathy caused by mutations of the SELENON gene characterized by axial muscle weakness and progressive respiratory insufficiency. Muscle histopathology commonly includes multiminicores or a dystrophic pattern but is often non-specific. The SELENON gene encodes selenoprotein N (SelN), a selenocysteine-containing redox enzyme located in the endo/sarcoplasmic reticulum membrane where it colocalizes with mitochondria-associated membranes. However, the molecular mechanism(s) by which SelN deficiency causes SELENON-RM are undetermined. A hurdle is the lack of cellular and animal models that show assayable phenotypes. Here we report deep-phenotyping of SelN-deficient zebrafish and muscle cells. SelN-deficient zebrafish exhibit changes in embryonic muscle function and swimming activity in larvae. Analysis of single cell RNAseq data in a zebrafish embryo-atlas revealed coexpression between selenon and genes involved in glutathione redox pathway. SelN-deficient zebrafish and mouse myoblasts exhibit changes in glutathione and redox homeostasis, suggesting a direct relationship with SelN function. We report changes in metabolic function abnormalities in SelN-null myotubes when compared to WT. These results suggest that SelN has functional roles during zebrafish early development and myoblast metabolism.

Congenital myopathies

The congenital myopathies are inherited muscle disorders characterized clinically by hypotonia and weakness, usually from birth, with a static or slowly progressive clinical course. Historically, the congenital myopathies have been classified according to major morphological features seen on muscle biopsy as nemaline myopathy, central core disease, centronuclear or myotubular myopathy, and congenital fiber type disproportion. However, in the past two decades, the genetic basis of these different forms of congenital myopathy has been further elucidated with the result being improved correlation with histological and genetic characteristics. However, these notions have been challenged for three reasons. First, many of the congenital myopathies can be caused by mutations in more than one gene that suggests an impact of genetic heterogeneity. Second, mutations in the same gene can cause different muscle pathologies. Third, the same genetic mutation may lead to different pathological features in members of the same family or in the same individual at different ages. This chapter provides a clinical overview of the congenital myopathies and a clinically useful guide to its genetic basis recognizing the increasing reliance of exome, subexome, and genome sequencing studies as first-line analysis in many patients.

Novel SEPN1 Mutations in Exon 1 Are Common in Rigid Spine With Muscular Dystrophy Type 1 in Chinese Patients

Congenital muscular dystrophy with early rigid spine, also known as the rigid spine with muscular dystrophy type 1 (RSMD1), is caused by SEPN1 mutation. We investigated the clinical manifestations, pathological features, and genetic characteristics of 8 Chinese RSMD1 patients in order to improve diagnosis and management of the disease. Eight patients presented with delayed motor development, muscle weakness, hypotonia, and a myopathic face with high palatine arches. All patients could walk independently, though with poor running and jumping, and most had a rigid spine, lordosis, or scoliosis. The symptoms of respiratory involvement were present early, and upper respiratory tract infections and pneumonia often occurred. Five patients had severe pneumonia, pulmonary hypertension, and respiratory failure. Lung function tests showed variable restrictive ventilation dysfunction. Polysomnography suggested hypoxia and hypoventilation. The serum creatine kinase (CK) level was normal or mildly increased. Muscle biopsy indicated chronic myopathic changes and minicores. Muscle magnetic resonance imaging (MRI) showed diffuse fatty infiltration of the gluteus maximus and thigh muscle. SEPN1 gene analysis revealed 16 compound heterozygous variants, 81.3% of which are unreported, including 7 exon 1 variants. Our study expands the spectrum of clinical and genetic findings in RSMD1 to improve diagnosis, management, and standards of care. SEPN1 mutations in exon 1 are common and easily missed, and exon 1 should be carefully analyzed when RSMD1 is suspected, which will provide valuable genetic counseling for the family and useful information for future natural history studies and clinical trials.

Defining and identifying satellite cell-opathies within muscular dystrophies and myopathies

Muscular dystrophies and congenital myopathies arise from specific genetic mutations causing skeletal muscle weakness that reduces quality of life. Muscle health relies on resident muscle stem cells called satellite cells, which enable life-course muscle growth, maintenance, repair and regeneration. Such tuned plasticity gradually diminishes in muscle diseases, suggesting compromised satellite cell function. A central issue however, is whether the pathogenic mutation perturbs satellite cell function directly and/or indirectly via an increasingly hostile microenvironment as disease progresses. Here, we explore the effects on satellite cell function of pathogenic mutations in genes (myopathogenes) that associate with muscle disorders, to evaluate clinical and muscle pathological hallmarks that define dysfunctional satellite cells. We deploy transcriptomic analysis and comparison between muscular dystrophies and myopathies to determine the contribution of satellite cell dysfunction using literature, expression dynamics of myopathogenes and their response to the satellite cell regulator PAX7. Our multimodal approach extends current pathological classifications to define Satellite Cell-opathies: muscle disorders in which satellite cell dysfunction contributes to pathology. Primary Satellite Cell-opathies are conditions where mutations in a myopathogene directly affect satellite cell function, such as in Progressive Congenital Myopathy with Scoliosis (MYOSCO) and Carey-Fineman-Ziter Syndrome (CFZS). Primary satellite cell-opathies are generally characterised as being congenital with general hypotonia, and specific involvement of respiratory, trunk and facial muscles, although serum CK levels are usually within the normal range. Secondary Satellite Cell-opathies have mutations in myopathogenes that affect both satellite cells and muscle fibres. Such classification aids diagnosis and predicting probable disease course, as well as informing on treatment and therapeutic development.

Calcium and Redox Liaison: A Key Role of Selenoprotein N in Skeletal Muscle

Selenoprotein N (SEPN1) is a type II glycoprotein of the endoplasmic reticulum (ER) that senses calcium levels to tune the activity of the sarcoplasmic reticulum calcium pump (SERCA pump) through a redox-mediated mechanism, modulating ER calcium homeostasis. In SEPN1-depleted muscles, altered ER calcium homeostasis triggers ER stress, which induces CHOP-mediated malfunction, altering excitation–contraction coupling. SEPN1 is localized in a region of the ER where the latter is in close contact with mitochondria, i.e., the mitochondria-associated membranes (MAM), which are important for calcium mobilization from the ER to mitochondria. Accordingly, SEPN1-depleted models have impairment of both ER and mitochondria calcium regulation and ATP production. SEPN1-related myopathy (SEPN1-RM) is an inherited congenital muscle disease due to SEPN1 loss of function, whose main histopathological features are minicores, i.e., areas of mitochondria depletion and sarcomere disorganization in muscle fibers. SEPN1-RM presents with weakness involving predominantly axial and diaphragmatic muscles. Since there is currently no disease-modifying drug to treat this myopathy, analysis of SEPN1 function in parallel with that of the muscle phenotype in SEPN1 loss of function models should help in understanding the pathogenic basis of the disease and possibly point to novel drugs for therapy. The present essay recapitulates the novel biological findings on SEPN1 and how these reconcile with the muscle and bioenergetics phenotype of SEPN1-related myopathy.

Cored in the act: the use of models to understand core myopathies

The core myopathies are a group of congenital myopathies with variable clinical expression - ranging from early-onset skeletal-muscle weakness to later-onset disease of variable severity - that are identified by characteristic 'core-like' lesions in myofibers and the presence of hypothonia and slowly or rather non-progressive muscle weakness. The genetic causes are diverse; central core disease is most often caused by mutations in ryanodine receptor 1 (RYR1), whereas multi-minicore disease is linked to pathogenic variants of several genes, including selenoprotein N (SELENON), RYR1 and titin (TTN). Understanding the mechanisms that drive core development and muscle weakness remains challenging due to the diversity of the excitation-contraction coupling (ECC) proteins involved and the differential effects of mutations across proteins. Because of this, the use of representative models expressing a mature ECC apparatus is crucial. Animal models have facilitated the identification of disease progression mechanisms for some mutations and have provided evidence to help explain genotype-phenotype correlations. However, many unanswered questions remain about the common and divergent pathological mechanisms that drive disease progression, and these mechanisms need to be understood in order to identify therapeutic targets. Several new transgenic animals have been described recently, expanding the spectrum of core myopathy models, including mice with patient-specific mutations. Furthermore, recent developments in 3D tissue engineering are expected to enable the study of core myopathy disease progression and the effects of potential therapeutic interventions in the context of human cells. In this Review, we summarize the current landscape of core myopathy models, and assess the hurdles and opportunities of future modeling strategies.

Therapeutic Aspects in Congenital Myopathies

The congenital myopathies are a genetically heterogeneous and diverse group of early-onset, nondystrophic neuromuscular disorders. While the originally reported "classical" entities within this group - Central Core Disease, Multiminicore Disease, Nemaline Myopathy, and Centronuclear Myopathy - were defined by the predominant finding on muscle biopsy, "novel" forms with multiple, subtle, and unusual histopathologic features have been described more recently, reflective of an expanding phenotypical spectrum. The main disease mechanisms concern excitation-contraction coupling, intracellular calcium homeostasis, and thin/thick filament interactions. Management to date has been mainly supportive. Therapeutic strategies currently at various stages of exploration include genetic interventions aimed at direct correction of the underlying genetic defect, enzyme replacement therapy, and pharmacologic approaches, either specifically targeting the principal effect of the underlying gene mutation, or addressing its downstream consequences more generally. Clinical trial development is accelerating but will require more robust natural history data and tailored outcome measures.

A novel mutation in SEPN1 causing rigid spine muscular dystrophy 1: a Case report

Background

Muscular dystrophies are a clinically and genetically heterogeneous group of disorders characterized by variable degrees of progressive muscle degeneration and weakness. There is a wide variability in the age of onset, symptoms and rate of progression in subtypes of these disorders. Herein, we present the results of our study conducted to identify the pathogenic genetic variation involved in our patient affected by rigid spine muscular dystrophy.

Case presentation

A 14-year-old boy, product of a first-cousin marriage, was enrolled in our study with failure to thrive, fatigue, muscular dystrophy, generalized muscular atrophy, kyphoscoliosis, and flexion contracture of the knees and elbows. Whole-exome sequencing (WES) was carried out on the DNA of the patient to investigate all coding regions and uncovered a novel, homozygous missense mutation in SEPN1 gene (c. 1379 C > T, p.Ser460Phe). This mutation has not been reported before in different public variant databases and also our database (BayanGene), so it is classified as a variation of unknown significance (VUS). Subsequently, it was confirmed that the novel variation was homozygous in our patient and heterozygous in his parents. Different bioinformatics tools showed the damaging effects of the variant on protein. Multiple sequence alignment using BLASTP on ExPASy and WebLogo, revealed the conservation of the mutated residue.

Conclusion

We reported a novel homozygous mutation in SEPN1 gene that expands our understanding of rigid spine muscular dystrophy. Although bioinformatics analyses of results were in favor of the pathogenicity of the mutation, functional studies are needed to establish the pathogenicity of the variant.

The genetics of congenital myopathies

Congenital myopathies are a clinically and genetically heterogeneous group of conditions that most commonly present at or around the time of birth with hypotonia, muscle weakness, and (often) respiratory distress. Historically, this group of disorders has been subclassified based on muscle histopathologic characteristics. There has been an explosion of gene discovery, and there are now at least 32 different genetic causes of disease. With this increased understanding of the genetic basis of disease has come the knowledge that the mutations in congenital myopathy genes can present with a wide variety of clinical phenotypes and can result in a broad spectrum of histopathologic findings on muscle biopsy. In addition, mutations in several genes can share the same histopathologic features. The identification of new genes and interpretation of different pathomechanisms at a molecular level have helped us to understand the clinical and histopathologic similarities that this group of disorders share. In this review, we highlight the genetic understanding for each subtype, its pathogenesis, and the future key issues in congenital myopathies.

Congenital myopathies: disorders of excitation-contraction coupling and muscle contraction

The congenital myopathies are a group of early-onset, non-dystrophic neuromuscular conditions with characteristic muscle biopsy findings, variable severity and a stable or slowly progressive course. Pronounced weakness in axial and proximal muscle groups is a common feature, and involvement of extraocular, cardiorespiratory and/or distal muscles can implicate specific genetic defects. Central core disease (CCD), multi-minicore disease (MmD), centronuclear myopathy (CNM) and nemaline myopathy were among the first congenital myopathies to be reported, and they still represent the main diagnostic categories. However, these entities seem to belong to a much wider phenotypic spectrum. To date, congenital myopathies have been attributed to mutations in over 20 genes, which encode proteins implicated in skeletal muscle Ca²⁺ homeostasis, excitation-contraction coupling, thin-thick filament assembly and interactions, and other mechanisms. RYR1 mutations are the most frequent genetic cause, and CCD and MmD are the most common subgroups. Next-generation sequencing has vastly improved mutation detection and has enabled the identification of novel genetic backgrounds. At present, management of congenital myopathies is largely supportive, although new therapeutic approaches are reaching the clinical trial stage.

SEPN1, an endoplasmic reticulum-localized selenoprotein linked to skeletal muscle pathology, counteracts hyperoxidation by means of redox-regulating SERCA2 pump activity

Selenoprotein N (SEPN1) is a broadly expressed resident protein of the endoplasmic reticulum (ER) whose loss-of-function inexplicably leads to human muscle disease. We found that SEPN1 levels parallel those of endoplamic reticulum oxidoreductin 1 (ERO1), an ER protein thiol oxidase, and that SEPN1's redox activity defends the ER from ERO1-generated peroxides. Moreover, we have defined the redox-regulated interactome of SEPN1 and identified the ER calcium import SERCA2 pump as a redox-partner of SEPN1. SEPN1 enhances SERCA2 activity by reducing luminal cysteines that are hyperoxidized by ERO1-generated peroxides. Cells lacking SEPN1 are hypersensitive to ERO1 overexpression and conspicuously defective in ER calcium re-uptake. After being muscle-transduced with an adeno-associated virus driving ERO1α, SEPN1 knockout mice unmasks a myopathy that resembles the dense core disease due to human mutations in SEPN1, whereas the combined attenuation of ERO1α and SEPN1 enhances cell fitness. These observations reveal the involvement of SEPN1 in ER redox and calcium homeostasis and that an ERO1 inhibitor, restoring redox-dependent calcium homeostasis, may ameliorate the myopathy of SEPN1 deficiency.

Triadopathies: an emerging class of skeletal muscle diseases

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

Selenoprotein N deficiency in mice is associated with abnormal lung development

Mutations in the human SEPN1 gene, encoding selenoprotein N (SepN), cause SEPN1-related myopathy (SEPN1-RM) characterized by muscle weakness, spinal rigidity, and respiratory insufficiency. As with other members of the selenoprotein family, selenoprotein N incorporates selenium in the form of selenocysteine (Sec). Most selenoproteins that have been functionally characterized are involved in oxidation-reduction (redox) reactions, with the Sec residue located at their catalytic site. To model SEPN1-RM, we generated a Sepn1-knockout (Sepn1(-/-)) mouse line. Homozygous Sepn1(-/-) mice are fertile, and their weight and lifespan are comparable to wild-type (WT) animals. Under baseline conditions, the muscle histology of Sepn1(-/-) mice remains normal, but subtle core lesions could be detected in skeletal muscle after inducing oxidative stress. Ryanodine receptor (RyR) calcium release channels showed lower sensitivity to caffeine in SepN deficient myofibers, suggesting a possible role of SepN in RyR regulation. SepN deficiency also leads to abnormal lung development characterized by enlarged alveoli, which is associated with decreased tissue elastance and increased quasi-static compliance of Sepn1(-/-) lungs. This finding raises the possibility that the respiratory syndrome observed in patients with SEPN1 mutations may have a primary pulmonary component in addition to the weakness of respiratory muscles.

Loss of abdominal muscle in Pitx2 mutants associated with altered axial specification of lateral plate mesoderm

Sequence specific transcription factors (SSTFs) combinatorially define cell types during development by forming recursively linked network kernels. Pitx2 expression begins during gastrulation, together with Hox genes, and becomes localized to the abdominal lateral plate mesoderm (LPM) before the onset of myogenesis in somites. The somatopleure of Pitx2 null embryos begins to grow abnormally outward before muscle regulatory factors (MRFs) or Pitx2 begin expression in the dermomyotome/myotome. Abdominal somites become deformed and stunted as they elongate into the mutant body wall, but maintain normal MRF expression domains. Subsequent loss of abdominal muscles is therefore not due to defects in specification, determination, or commitment of the myogenic lineage. Microarray analysis was used to identify SSTF families whose expression levels change in E10.5 interlimb body wall biopsies. All Hox9-11 paralogs had lower RNA levels in mutants, whereas genes expressed selectively in the hypaxial dermomyotome/myotome and sclerotome had higher RNA levels in mutants. In situ hybridization analyses indicate that Hox gene expression was reduced in parts of the LPM and intermediate mesoderm of mutants. Chromatin occupancy studies conducted on E10.5 interlimb body wall biopsies showed that Pitx2 protein occupied chromatin sites containing conserved bicoid core motifs in the vicinity of Hox 9-11 and MRF genes. Taken together, the data indicate that Pitx2 protein in LPM cells acts, presumably in combination with other SSTFs, to repress gene expression, that are normally expressed in physically adjoining cell types. Pitx2 thereby prevents cells in the interlimb LPM from adopting the stable network kernels that define sclerotomal, dermomyotomal, or myotomal mesenchymal cell types. This mechanism may be viewed either as lineage restriction or specification.

Ubiquitous expression of selenoprotein N transcripts in chicken tissues and early developmental expression pattern in skeletal muscles

Previous results revealed a ubiquitous expression pattern of selenoprotein N (SelN, SEPN1) in humans, zebrafish, and mouse, suggesting that it plays a potential role during the embryogenesis of these species. However, no information is known about the tissue distribution of SelN and mRNA expression analysis in the muscle tissues during development in birds. We analyzed the mRNA expression of SelN in 26 different tissues of 90-day-old chickens and the expression of SelN in the muscle tissues of 12-day-old chicken embryos and 15-month-old adult chickens by quantitative real-time PCR. The results showed that SelN transcripts were expressed widely in the chicken tissues. Moreover, the expression of SelN mRNA in skeletal muscles was present at a high level in whole embryos and at a lower level in postnatal stages. However, the expression of SelN mRNA in cardiac muscle showed a different expression pattern compared with skeletal muscles. Our data indicate that the expression of the SelN gene in chicken is ubiquitous, suggesting a role of SelN in the development of chick embryo skeletal muscles.

Selenoprotein N in skeletal muscle: from diseases to function

Selenoprotein N (SelN) deficiency causes several inherited neuromuscular disorders collectively termed SEPN1-related myopathies, characterized by early onset, generalized muscle atrophy, and muscle weakness affecting especially axial muscles and leading to spine rigidity, severe scoliosis, and respiratory insufficiency. SelN is ubiquitously expressed and is located in the membrane of the endoplasmic reticulum; however, its function remains elusive. The predominant expression of SelN in human fetal tissues and the embryonic muscle phenotype reported in mutant zebrafish suggest that it is involved in myogenesis. In mice, SelN is also mostly expressed during embryogenesis and especially in the myotome, but no defect was detected in muscle development and growth in the Sepn1 knock-out mouse model. By contrast, we recently demonstrated that SelN is essential for muscle regeneration and satellite cell maintenance in mice and humans, hence opening new avenues regarding the pathomechanism(s) leading to SEPN1-related myopathies. At the cellular level, recent data suggested that SelN participates in oxidative and calcium homeostasis, with a potential role in the regulation of the ryanodine receptor activity. Despite the recent and exciting progress regarding the physiological function(s) of SelN in muscle tissue, the pathogenesis leading to SEPN1-related myopathies remains largely unknown, with several unsolved questions, and no treatment available. In this review, we introduce SelN, its properties and expression pattern in zebrafish, mice, and humans, and we discuss its potential roles in muscle tissue and the ensuing clues for the development of therapeutic options.

Core myopathies

The core myopathies, Central Core Disease and Multiminicore Disease, are heterogeneous congenital myopathies with the common defining histopathological feature of focally reduced oxidative enzyme activity (central cores, multiminicores). Mutations in the gene encoding for the skeletal muscle ryanodine (RyR1) receptor are the most common cause. Mutations in the selenoprotein N (SEPN1) gene cause a less common variant. Pathogenic mechanisms underlying dominant RYR1 mutations have been extensively characterized, whereas those associated with recessive RYR1 and SEPN1 mutations are emerging. Identifying a specific genetic defect from the histopathological diagnosis of a core myopathy is complex and ought to be informed by a combined appraisal of histopathological, clinical, and, increasingly, muscle magnetic resonance imaging data. The present review aims at giving an overview of the main genetic and clinicopathological findings, with a major emphasis on features likely to inform the diagnostic process, as well as current treatments and perspectives for future research.

The PACAP-regulated gene selenoprotein T is highly induced in nervous, endocrine, and metabolic tissues during ontogenetic and regenerative processes

Selenoproteins contain the essential trace element selenium whose deficiency leads to major disorders including cancer, male reproductive system failure, or autoimmune thyroid disease. Up to now, 25 selenoprotein-encoding genes were identified in mammals, but the spatiotemporal distribution, regulation, and function of some of these selenium-containing proteins remain poorly documented. Here, we found that selenoprotein T (SelT), a new thioredoxin-like protein, is regulated by the trophic neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) in differentiating but not mature adrenomedullary cells. In fact, our analysis revealed that, in rat, SelT is highly expressed in most embryonic structures, and then its levels decreased progressively as these organs develop, to vanish in most adult tissues. In the brain, SelT was abundantly expressed in neural progenitors in various regions such as the cortex and cerebellum but was undetectable in adult nervous cells except rostral migratory-stream astrocytes and Bergmann cells. In contrast, SelT expression was maintained in several adult endocrine tissues such as pituitary, thyroid, or testis. In the pituitary gland, SelT was found in secretory cells of the anterior lobe, whereas in the testis, the selenoprotein was present only in spermatogenic and Leydig cells. Finally, we found that SelT expression is strongly stimulated in liver cells during the regenerative process that occurs after partial hepatectomy. Taken together, these data show that SelT induction is associated with ontogenesis, tissue maturation, and regenerative mechanisms, indicating that this PACAP-regulated selenoprotein may play a crucial role in cell growth and activity in nervous, endocrine, and metabolic tissues.

Increased muscle stress-sensitivity induced by selenoprotein N inactivation in mouse: a mammalian model for SEPN1-related myopathy

Selenium is an essential trace element and selenoprotein N (SelN) was the first selenium-containing protein shown to be directly involved in human inherited diseases. Mutations in the SEPN1 gene, encoding SelN, cause a group of muscular disorders characterized by predominant affection of axial muscles. SelN has been shown to participate in calcium and redox homeostasis, but its pathophysiological role in skeletal muscle remains largely unknown. To address SelN function in vivo, we generated a Sepn1-null mouse model by gene targeting. The Sepn1^−/− mice had normal growth and lifespan, and were macroscopically indistinguishable from wild-type littermates. Only minor defects were observed in muscle morphology and contractile properties in SelN-deficient mice in basal conditions. However, when subjected to challenging physical exercise and stress conditions (forced swimming test), Sepn1^−/− mice developed an obvious phenotype, characterized by limited motility and body rigidity during the swimming session, as well as a progressive curvature of the spine and predominant alteration of paravertebral muscles. This induced phenotype recapitulates the distribution of muscle involvement in patients with SEPN1-Related Myopathy, hence positioning this new animal model as a valuable tool to dissect the role of SelN in muscle function and to characterize the pathophysiological process.

Satellite cell loss and impaired muscle regeneration in selenoprotein N deficiency

Selenoprotein N (SelN) deficiency causes a group of inherited neuromuscular disorders termed SEPN1-related myopathies (SEPN1-RM). Although the function of SelN remains unknown, recent data demonstrated that it is dispensable for mouse embryogenesis and suggested its involvement in the regulation of ryanodine receptors and/or cellular redox homeostasis. Here, we investigate the role of SelN in satellite cell (SC) function and muscle regeneration, using the Sepn1(-/-) mouse model. Following cardiotoxin-induced injury, SelN expression was strongly up-regulated in wild-type muscles and, for the first time, we detected its endogenous expression in a subset of mononucleated cells by immunohistochemistry. We show that SelN deficiency results in a reduced basal SC pool in adult skeletal muscles and in an imperfect muscle restoration following a single injury. A dramatic depletion of the SC pool was detected after the first round of degeneration and regeneration that totally prevented subsequent regeneration of Sepn1(-/-) muscles. We demonstrate that SelN deficiency affects SC dynamics on isolated single fibres and increases the proliferation of Sepn1(-/-) muscle precursors in vivo and in vitro. Most importantly, exhaustion of the SC population was specifically identified in muscle biopsies from patients with mutations in the SEPN1 gene. In conclusion, we describe for the first time a major physiological function of SelN in skeletal muscles, as a key regulator of SC function, which likely plays a central role in the pathophysiological mechanism leading to SEPN1-RM.

Transcriptional regulation of selenoprotein W by MyoD during early skeletal muscle differentiation

Selenoprotein W (SelW) is expressed in various tissues, but it is especially high in the skeletal muscle of mammals. Such tissue-specific protein expression implies regulation by a tissue-specific factor. In this study, we investigated SelW expression during myogenic C2C12 cell differentiation using RT-PCR, quantitative PCR, and Western blot analysis. Both the protein and mRNA levels of SelW were increased during C2C12 cell differentiation, particularly during the early stage. Sequence analysis of the SelW promoter revealed four putative E-boxes, E1, E2, E3, and E4, which are known binding sites for MyoD, a myogenic transcriptional factor. Luciferase reporter assay showed that E1 and E4 were crucial for MyoD-dependent promoter activity. Using EMSA analysis, we observed that MyoD bound directly to E1 but not to E4, even though E4 mutation reduced SelW promoter activity in the luciferase reporter assay. Binding of MyoD to E1 was further investigated by ChIP assay. These results suggest that the SelW gene was activated by the binding of MyoD to a specific E-box during early skeletal muscle differentiation.

Selenoproteins and protection against oxidative stress: selenoprotein N as a novel player at the crossroads of redox signaling and calcium homeostasis

Healthy cells continually produce low levels of reactive oxygen species (ROS), which are buffered by multiple antioxidant systems. Imbalance between ROS production and elimination results in oxidative stress, which has been implicated in aging and in numerous human diseases, including cancer and diabetes. Selenoproteins are a family of proteins that contain the amino acid selenocysteine, encoded by an in-frame UGA. Those selenoproteins whose function is identified are catalytically active in redox processes, representing one of the main enzymatic antioxidant systems and important mediators of the beneficial role of selenium in human health. Nevertheless, the function of most selenoproteins remains unknown; this included Selenoprotein N (SelN), the only selenoprotein directly associated with a human genetic disease. Mutations of the SelN gene cause SEPN1-related myopathy, a particular early-onset muscle disorder. Recent studies have identified SelN as a key protein in cell protection against oxidative stress and redox-related calcium homeostasis. Furthermore, an effective ex vivo treatment of SelN deficiency has been identified, paving the way to a clinical therapy. In this review we discuss the physiological and pathophysiological role of SelN and the interest of SEPN1-related myopathy as a model paradigm to understand and target therapeutically other selenoproteins involved in human health and disease.