Aged Muscle Stem Cells Drive Fibrosis Through Epigenetic Rewiring

Sarcopenia—the progressive loss of muscle mass and strength with aging—involves both diminished regenerative capacity of muscle stem cells (MuSCs) and pathological accumulation of fibrotic tissue. While it was known that aged MuSCs are functionally compromised, how they actively contribute to fibrosis in surrounding tissue was poorly understood. This study, published in Nature Aging, provides a mechanistic explanation rooted in epigenetic dysregulation.

The researchers generated an inducible mouse model (iEzh2−/−) in which the polycomb methyltransferase Ezh2—responsible for depositing the repressive H3K27me3 histone mark—was deleted specifically in adult MuSCs using a tamoxifen-Cre system. After muscle injury, iEzh2−/− mice showed ~65% fewer MuSCs, reduced muscle fiber cross-sectional area, expanded FAP populations at both 7 and 28 days post-injury, and significantly elevated fibrosis compared to controls. Critically, the number of FAPs was similar between groups in uninjured muscle, confirming injury-induced expansion rather than baseline differences.

RNA-sequencing and proteomic analyses of FACS-isolated MuSCs revealed that both iEzh2−/− and aged MuSCs upregulate and secrete IL-6 and Spp1 (osteopontin). Co-culture experiments demonstrated that aged MuSCs alone—independent of macrophages, immune cells, or other muscle-resident cell types—are sufficient to drive FAP proliferation and a fibrogenic phenotype. This established a direct, cell-autonomous paracrine mechanism. ChIP-seq and ATAC-seq analyses showed that loss of H3K27me3 at the Nfkb1 locus in aged MuSCs correlates with increased NF-κB expression and enhanced NF-κB chromatin recruitment to the IL6 and Spp1 gene promoters. Reducing NF-κB expression in aged MuSCs by siRNA decreased both IL-6 and Spp1 levels, confirming the regulatory axis.

Pharmacological intervention validated therapeutic relevance: antibody-mediated blockade of IL-6 receptor (tocilizumab) and Spp1 neutralization in co-culture systems reduced FAP proliferation. In aged mice, dual inhibition of IL-6 and Spp1 signaling following muscle injury reduced FAP numbers and fibrotic deposition, and partially restored muscle regeneration metrics. These results demonstrate that the epigenetic state of MuSCs is not merely a passive reflection of aging but is an active driver of the inflammatory and fibrotic microenvironment.

The study positions epigenetic restoration of MuSCs—or downstream blockade of IL-6 and Spp1—as plausible therapeutic strategies for sarcopenia-associated fibrosis. Importantly, the iEzh2−/− model validated as a useful surrogate for studying epigenetic aging in muscle without requiring geriatric animals, though direct comparison with naturally aged mice remains essential for translational conclusions.