Reassessing the Role of Progenitor Cells in Muscle Wasting in Cancer Cachexia.

Skeletal muscles contain myogenic and non-myogenic progenitor cells that proliferate and differentiate after tissue damage to restore myofiber, connective tissue, and blood vessel homeostasis. We previously showed that cancer-induced muscle wasting involves myofiber damage and impaired differentiation of myogenic progenitors, coincident with the aberrant accumulation of mesenchymal progenitor cells expressing both myogenic (Pax7) and non-myogenic (Sca1, Pdgfrα) progenitor markers. Here, we combined lineage tracing and scRNA-seq to more deeply explore muscle resident progenitor cells during cancer cachexia. Colon-26 (C-26) carcinoma cells were injected into adult (≥12-weeks-old) Pax7-CreER; Rosa26-LSL-tdTomato mice, and tamoxifen was administered after tumors were established but prior to cachexia. At cachexia endpoint, scRNA-seq analysis was performed on muscle mononuclear cells. In both control and C-26 muscles, Pax7 and tdTomato transcripts were restricted to myogenic progenitors, whereas Sca1 and Pdgfrα were restricted to non-myogenic progenitors. These results were confirmed by flow cytometry and suggest that mesenchymal progenitor cells do not commit to a myogenic fate during cancer cachexia. However, consistent with earlier findings, our transcriptomic analyses validated that myogenic progenitors from tumor-bearing mice were impaired to differentiate. When we repeated Pax7-lineage tracing and scRNA-seq on young mice (6-week-old) still undergoing developmental muscle growth similar results were obtained, but interestingly, by flow cytometry we detected a small population of Sca1+; tdTomato+ cells, not present in adult muscles. Thus, non-myogenic progenitors might indeed be capable of adopting a myogenic fate during cancer cachexia, but this contributes to only a minor fraction of Pax7+ cells and likely to be age dependent.