Aged muscle stem cells epigenetically drive the expansion of mesenchymal progenitors [single cell RNA-Seq]
Sarcopenia, the age-related loss of muscle mass and strength, is characterized by impaired muscle repair and increased deposition of fibrotic tissue. Yet, the specific role of individual cell types in the development of fibrosis remains poorly defined. Here, we report that aged muscle stem cells (MuSCs) directly instruct fibro-adipogenic progenitors (FAPs) to proliferate and adopt a fibrogenic phenotype. Polycomb Ezh2-/- or aged mice exhibited regenerative defects, FAP expansion, fibrosis, and elevated levels of MuSC-secreted interleukin 6 (IL-6) and Spp1/Osteopontin.  In aged MuSCs, H3K27me3 erosion at the NF-kB gene correlated with increased expression and enhanced chromatin recruitment at the IL-6 and Spp1 genes, leading to their activation. Blocking IL-6 and Spp1 signaling in co-cultures of aged MuSC and FAPs, or aged mice, reduced FAP proliferation and muscle fibrosis. In summary, our results indicate that aged MuSCs instruct FAPs to proliferate and acquire a fibrogenic phenotype through a mechanism involving epigenetically-mediated derepression of NF-kB and increased activation of IL-6 and Spp1- both of which are potential pharmacological targets. Overall design: Two single cell experiments are included. For the first study, FAPs were isolate from mouse muscle tissue in both Control and iEZh2-/- mice, 7 days post injury, and single cell RNAseq analysis was performed. In the second study (coculture experiments), Control or iEzh2-/- MuSCs FACS-isolated from 7 dpi mice were co-cultured with control PDGFR?EGFP FAPs, also isolated from 7 dpi mice. The cells were cocultured for 72 hrs and performed scRNAseq.