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• We confirm the presence of an increase in internalized nuclei, fibre type 1 hypertrophy and NAHD central clearances/cores. • One third of our FSHD1 samples showed membrane attack complex (MAC) deposits. • Abnormal MHC i expression was observed in 72 % of samples at the membrane but also at cytoplasmic level. • Increased muscle regeneration in FSHD1 correlates with the inflammatory score. The aim of this study was to identify key routinely used myopathologic biomarkers of FSHD1. Needle muscle biopsies were taken in 34 affected muscles (m. quadriceps femoris (QF), n = 20, m. tibialis anterior (TA), n = 13, m. biceps brachii, n = 1) from 22 patients (age, 53.5 (10) years; M = 12, F = 10). Eleven patients had more than one biopsy (2xQF, n = 1; QF+TA, n = 9; 2xQF+TA, n = 1). Histochemistry, immunoperoxidase, and immunofluorescence stainings were performed and compared to age and muscle type matched muscle specimens of 11 healthy controls. Myopathologic features observed in our FSHD1 cohort were internalized nuclei, type 1 fibre hypertrophy and NADH central clearances/cores. We observed a prominent inflammatory response with MAC deposits, MHC I expression, and muscle regeneration that correlated with the inflammatory score. Our up-to-date characterization of FSHD1 points towards MHC I, MAC, and embryonic Myosin Heavy Chain/muscle regeneration as useful myopathologic readouts of FSHD1. [ABSTRACT FROM AUTHOR]

Background: The quantitative analysis of muscle histomorphometry has been growing in importance in both research and clinical settings. Accurate and stringent assessment of myofibers' changes in size and number, and alterations in the proportion of oxidative (type I) and glycolytic (type II) fibers is essential for the appropriate study of aging and pathological muscle, as well as for diagnosis and follow-up of muscle diseases. Manual and semi-automated methods to assess muscle morphometry in sections are time-consuming, limited to a small field of analysis, and susceptible to bias, while most automated methods have been only tested in rodent muscle. Methods: We developed a new macro script for Fiji-ImageJ to automatically assess human fiber morphometry in digital images of the entire muscle. We tested the functionality of our method in deltoid muscle biopsies from a heterogeneous population of subjects with histologically normal muscle (male, female, old, young, lean, obese) and patients with dermatomyositis, necrotizing autoimmune myopathy, and anti-synthetase syndrome myopathy. Results: Our macro is fully automated, requires no user intervention, and demonstrated improved fiber segmentation by running a series of image pre-processing steps before the analysis. Likewise, our tool showed high accuracy, as compared with manual methods, for identifying the total number of fibers (r = 0.97, p < 0.001), fiber I and fiber II proportion (r = 0.92, p < 0.001), and minor diameter (r = 0.86, p < 0.001) while conducting analysis in ~ 5 min/sample. The performance of the macro analysis was maintained in pectoral and deltoid samples from subjects of different age, gender, body weight, and muscle status. The output of the analyses includes excel files with the quantification of fibers' morphometry and color-coded maps based on the fiber's size, which proved to be an advantageous feature for the fast and easy visual identification of location-specific atrophy and a potential tool for medical diagnosis. Conclusion: Our macro is reliable and suitable for the study of human skeletal muscle for research and for diagnosis in clinical settings providing reproducible and consistent analysis when the time is of the utmost importance. [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]