Ectopic fat deposition in skeletal muscle is closely associated with several disorders, however, the origin of these adipocytes is not clear, nor is the mechanism of their formation. Satellite cells function as adult muscle stem cells but are proposed to possess multipotency. Here, we prospectively identify PDGFRα⁺ mesenchymal progenitors as being distinct from satellite cells and located in the muscle interstitium. We show that, of the muscle-derived cell populations, only PDGFRα⁺ cells show efficient adipogenic differentiation both in vitro and in vivo. Reciprocal transplantations between regenerating and degenerating muscles, and co-culture experiments revealed that adipogenesis of PDGFRα⁺ cells is strongly inhibited by the presence of satellite cell-derived myofibres. These results suggest that PDGFRα⁺ mesenchymal progenitors are the major contributor to ectopic fat cell formation in skeletal muscle, and emphasize that interaction between muscle cells and PDGFRα⁺ mesenchymal progenitors, not the fate decision of satellite cells, has a considerable impact on muscle homeostasis.

Accumulation of adipocytes and collagen type-I-producing cells (fibrosis) is observed in muscular dystrophies. The origin of these cells had been largely unknown, but recently we identified mesenchymal progenitors positive for platelet-derived growth factor receptor alpha (PDGFRα) as the origin of adipocytes in skeletal muscle. However, the origin of muscle fibrosis remains largely unknown. In this study, clonal analyses show that PDGFRα⁺ cells also differentiate into collagen type-I-producing cells. In fact, PDGFRα⁺ cells accumulated in fibrotic areas of the diaphragm in the mdx mouse, a model of Duchenne muscular dystrophy. Furthermore, mRNA of fibrosis markers was expressed exclusively in the PDGFRα⁺ cell fraction in the mdx diaphragm. Importantly, TGF-β isoforms, known as potent profibrotic cytokines, induced expression of markers of fibrosis in PDGFRα⁺ cells but not in myogenic cells. Transplantation studies revealed that fibrogenic PDGFRα⁺ cells mainly derived from pre-existing PDGFRα⁺ cells and that the contribution of PDGFRα⁻ cells and circulating cells was limited. These results indicate that mesenchymal progenitors are the main origin of not only fat accumulation but also fibrosis in skeletal muscle.

Skeletal muscle satellite cells play key roles in postnatal muscle growth and regeneration. To study molecular regulation of satellite cells, we directly prepared satellite cells from 8- to 12-week-old C57BL/6 mice and performed genome-wide gene expression analysis. Compared with activated/cycling satellite cells, 507 genes were highly upregulated in quiescent satellite cells. These included negative regulators of cell cycle and myogenic inhibitors. Gene set enrichment analysis revealed that quiescent satellite cells preferentially express the genes involved in cell-cell adhesion, regulation of cell growth, formation of extracellular matrix, copper and iron homeostasis, and lipid transportation. Furthermore, reverse transcription-polymerase chain reaction on differentially expressed genes confirmed that calcitonin receptor (CTR) was exclusively expressed in dormant satellite cells but not in activated satellite cells. In addition, CTR mRNA is hardly detected in nonmyogenic cells. Therefore, we next examined the expression of CTR in vivo. CTR was specifically expressed on quiescent satellite cells, but the expression was not found on activated/proliferating satellite cells during muscle regeneration. CTR-positive cells reappeared at the rim of regenerating myofibers in later stages of muscle regeneration. Calcitonin stimulation delayed the activation of quiescent satellite cells. Our data provide roles of CTR in quiescent satellite cells and a solid scaffold to further dissect molecular regulation of satellite cells. Disclosure of potential conflicts of interest is found at the end of this article.

Fatty and fibrous connective tissue formation is a hallmark of diseased skeletal muscle and deteriorates muscle function. We previously identified non-myogenic mesenchymal progenitors that contribute to adipogenesis and fibrogenesis in mouse skeletal muscle. In this study, we report the identification and characterization of a human counterpart to these progenitors. By using PDGFRα as a specific marker, mesenchymal progenitors can be identified in the interstitium and isolated from human skeletal muscle. PDGFRα⁺ cells represent a cell population distinct from CD56⁺ myogenic cells, and adipogenic and fibrogenic potentials were highly enriched in the PDGFRα⁺ population. Activation of PDGFRα stimulates proliferation of PDGFRα⁺ cells through PI3K-Akt and MEK2-MAPK signaling pathways, and aberrant accumulation of PDGFRα⁺ cells was conspicuous in muscles of patients with both genetic and non-genetic muscle diseases. Our results revealed the pathological relevance of PDGFRα⁺ mesenchymal progenitors to human muscle diseases and provide a basis for developing therapeutic strategy to treat muscle diseases.