Tissue regeneration requires coordination between resident stem cells and local niche cells1,2. Here we identify that senescent cells are integral components of the skeletal muscle regenerative niche that repress regeneration at all stages of life. The technical limitation of senescent-cell scarcity3 was overcome by combining single-cell transcriptomics and a senescent-cell enrichment sorting protocol. We identified and isolated different senescent cell types from damaged muscles of young and old mice. Deeper transcriptome, chromatin and pathway analyses revealed conservation of cell identity traits as well as two universal senescence hallmarks (inflammation and fibrosis) across cell type, regeneration time and ageing. Senescent cells create an aged-like inflamed niche that mirrors inflammation associated with ageing (inflammageing4) and arrests stem cell proliferation and regeneration. Reducing the burden of senescent cells, or reducing their inflammatory secretome through CD36 neutralization, accelerates regeneration in young and old mice. By contrast, transplantation of senescent cells delays regeneration. Our results provide a technique for isolating in vivo senescent cells, define a senescence blueprint for muscle, and uncover unproductive functional interactions between senescent cells and stem cells in regenerative niches that can be overcome. As senescent cells also accumulate in human muscles, our findings open potential paths for improving muscle repair throughout life.

A unique property of skeletal muscle is its ability to adapt its mass to changes in activity. Inactivity, as in disuse or aging, causes atrophy, the loss of muscle mass and strength, leading to physical incapacity and poor quality of life. Here, through a combination of transcriptomics and transgenesis, we identify sestrins, a family of stress-inducible metabolic regulators, as protective factors against muscle wasting. Sestrin expression decreases during inactivity and its genetic deficiency exacerbates muscle wasting; conversely, sestrin overexpression suffices to prevent atrophy. This protection occurs through mTORC1 inhibition, which upregulates autophagy, and AKT activation, which in turn inhibits FoxO-regulated ubiquitin-proteasome-mediated proteolysis. This study reveals sestrin as a central integrator of anabolic and degradative pathways preventing muscle wasting. Since sestrin also protected muscles against aging-induced atrophy, our findings have implications for sarcopenia.

Regeneration of skeletal muscle depends on a population of adult stem cells (satellite cells) that remain quiescent throughout life. Satellite cell regenerative functions decline with ageing. Here we report that geriatric satellite cells are incapable of maintaining their normal quiescent state in muscle homeostatic conditions, and that this irreversibly affects their intrinsic regenerative and self-renewal capacities. In geriatric mice, resting satellite cells lose reversible quiescence by switching to an irreversible pre-senescence state, caused by derepression of p16(INK4a) (also called Cdkn2a). On injury, these cells fail to activate and expand, undergoing accelerated entry into a full senescence state (geroconversion), even in a youthful environment. p16(INK4a) silencing in geriatric satellite cells restores quiescence and muscle regenerative functions. Our results demonstrate that maintenance of quiescence in adult life depends on the active repression of senescence pathways. As p16(INK4a) is dysregulated in human geriatric satellite cells, these findings provide the basis for stem-cell rejuvenation in sarcopenic muscles.