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Objective: Juvenile idiopathic inflammatory/immune myopathies (IIMs) constitute a highly heterogeneous group of disorders with diagnostic difficulties and prognostic uncertainties. Circulating myositis‐specific autoantibodies (MSAs) have been recognized as reliable tools for patient substratification. Considering the key role of type I interferon (IFN) up‐regulation in juvenile IIM, we undertook the present study to investigate whether IFN‐induced 15‐kd protein (ISG‐15) could be a reliable biomarker for stratification and diagnosis and to better elucidate its role in juvenile IIM pathophysiology. Methods: The study included 56 patients: 24 with juvenile dermatomyositis (DM), 12 with juvenile overlap myositis (OM), 10 with Duchenne muscular dystrophy, and 10 with congenital myopathies. Muscle biopsy samples were assessed by immunohistochemistry, immunoblotting, and real‐time quantitative polymerase chain reaction. Negative regulators of type I IFN (ISG15 and USP18) and positive regulators of type I IFN (DDX58 and IFIH1) were analyzed. Results: ISG15 expression discriminated patients with juvenile IIM from those with nonimmune myopathies and, among patients with juvenile IIM, discriminated those with DM from those with OM. Among patients with juvenile DM, up‐regulation of the type I IFN positive regulators DDX58 and IFIH1 was similar regardless of MSA status. In contrast, the highest levels of the type I IFN negative regulator ISG15 were observed in patients who were positive for melanoma differentiation–associated gene 5 (MDA‐5). Finally, ISG15 levels were inversely correlated with the severity of muscle histologic abnormalities and positively correlated with motor performance as evaluated by the Childhood Myositis Assessment Scale and by manual muscle strength testing. Conclusion: Muscle ISG15 expression is strongly associated with juvenile DM, with patients exhibiting a different ISG‐15 muscle signature according to their MSA class. Patients with juvenile DM who are positive for MDA‐5 have higher expression of ISG15 in both gene form and protein form compared to the other subgroups. Moreover, our data show that negative regulation of type I IFN correlates with milder muscle involvement. [ABSTRACT FROM AUTHOR]

Obesity triggers skeletal muscle physio-pathological alterations. However, the crosstalk between adipose tissue and myogenic cells remains poorly understood during obesity. We identified NID-1 among the adipose tissue secreted factors impairing myogenic potential of human myoblasts and murine muscle stem cells in vitro. Mice under High Fat Diet (HFD) displayed increased NID-1 expression in the skeletal muscle endomysium associated with intramuscular fat adipose tissue expansion and compromised muscle stem cell function. We show that NID-1 is highly secreted by skeletal muscle fibro-adipogenic/mesenchymal progenitors (FAPs) during obesity. We demonstrate that increased muscle NID-1 impairs muscle stem cells proliferation and primes the fibrogenic differentiation of FAPs, giving rise to an excessive deposition of extracellular matrix. Finally, we propose a model in which obesity leads to skeletal muscle extracellular matrix remodeling by FAPs, mediating the alteration of myogenic function by adipose tissue and highlighting the key role of NID-1 in the crosstalk between adipose tissue and skeletal muscle. A. Adult skeletal muscle mesenchymal progenitor fibro-adipogenic potential in regular diet conditions. Undifferentiated multipotent PDGFRα-expressing progenitors (FAPs) are able to generate pre-adipocytes (NID-1 low) and fibrocytes (NID-1 high) committed to become adipocytes and fibroblast-like cells, respectively, under homeostatic and regeneration condition. B. Adult MuSCs myogenic potential under regular diet. During homeostasis, MuSCs are quiescent Pax7-expressing cells. Their role is maintaining the number of skeletal muscle progenitors throughout life. However, certain stimuli, such as muscle injury, launch the muscle regeneration program. Pax7 expression is downregulated at the time MyoD is expressed. Activated myoblasts undergo several cell cycles (Ki67+) to generate abundant number of myoblasts. Following differentiation, myoblasts decrease MyoD expression, and upregulate the commitment marker MyoG. Finally, myocytes fuse to repair or generate new myotubes. C. Adult skeletal muscle mesenchymal progenitor fibro-adipogenic potential after High-Fat Diet (HFD). HFD promotes FAP infiltration into obese mouse skeletal muscle. The surplus of energy stimulates transformation of the NID-1 Low FAP subpopulation into adipocytes and therefore, intramuscular adipose tissue deposition. The overstimulation of low NID-1 expressing FAPs forces their differentiation or trans-differentiation to NID-1 high expression FAPs. In obese mice under homeostatic conditions, most FAPs are expressing high levels of NID-1 which give rise to fibroblast-like cells upon regeneration. This in turn leads to excessive extracellular matrix deposition once the muscle damage is repaired. D. Obese mice adipose tissue secretome enhances FAP's fibrogenic fate during regeneration. E. Adult MuSCs myogenic potential after HFD. The increase in NID-1 secreting-FAPs alters the MuSCs niche. In the steady state, NID-1 may maintain the quiescence of Pax7 -expressing MuSCs, giving rise to a higher number of skeletal muscle progenitors. However, upon regeneration the persistence of NID-1 may blunt MuSCs clonal expansion and favor myoblast differentiation. [Display omitted] [ABSTRACT FROM AUTHOR]

Hemifacial myohyperplasia (HFMH) is a rare cause of facial asymmetry exclusively involving facial muscles. The underlying cause and the mechanism of disease progression are unknown. Here, we identified a somatic gain-of-function mutation of PIK3CA in five pediatric patients with HFMH. To understand the physiopathology of muscle hypertrophy in this context, we created a mouse model carrying specifically a PIK3CA mutation in skeletal muscles. PIK3CA gain-of-function mutation led to striated muscle cell hypertrophy, mitochondria dysfunction, and hypoglycemia with low circulating insulin levels. Alpelisib treatment, an approved PIK3CA inhibitor, was able to prevent and reduce muscle hypertrophy in the mouse model with correction of endocrine anomalies. Based on these findings, we treated the five HFMH patients. All patients demonstrated clinical, esthetical, and radiological improvement with proof of target engagement. In conclusion, we show that HFMH is due to somatic alteration of PIK3CA and is accessible to pharmacological intervention., (© 2023 Bayard et al.)

The necrosis of muscle fibres (myonecrosis) plays a central role in the pathogenesis of several muscle conditions, including muscular dystrophies. Therapeutic options addressing the causes of muscular dystrophy pathogenesis are expected to alleviate muscle degeneration. Therefore, a method to assay and quantify the extent of cell death in muscle biopsies is needed. Conventional methods to observe myofiber degeneration in situ are either poorly quantitative or rely on the injection of vital dyes. In this article, an immunofluorescence protocol is described that stains necrotic myofibers by targeting immunoglobulin G (IgG) uptake by myofibers. The IgG uptake method is based on cell features characterizing the necrotic demise, including 1) the loss of plasma membrane integrity with the release of damage-associated molecular patterns and 2) the uptake of plasmatic proteins. In murine cross-sections, the co-immunolabelling of myofibers, extracellular matrix proteins, and mouse IgG allows clean and straightforward identification of myofibers with necrotic fate. This simple method is suitable for quantitative analysis and applicable to all species, including human samples, and does not require the injection of vital dye. The staining of necrotic myofibers by IgG uptake can also be paired with other co-immunolabelling.