Senescent Cells Block Aged Muscle Repair After Disuse — and Removing Them Helps
A new study maps senescent cell types in aged mouse muscle and shows senolytics can restore mass and function after disuse atrophy.
Summary
Researchers used flow cytometry and single-cell RNA sequencing to identify which cells become senescent in aged skeletal muscle during recovery from disuse atrophy. Young and old mice underwent 14 days of hindlimb unloading followed by 7 or 14 days of reloading. Aged mice showed smaller muscle fibers, dysregulated macrophage responses, excess collagen, and elevated senescence markers. Single-cell profiling revealed that senescent cells were concentrated in the muscle interstitial niche — spanning stromal, vascular, and immune populations — and expressed a senescence-associated secretory phenotype (SASP). Treating aged mice with senolytics cleared these cells, normalized macrophage dynamics, and restored muscle mass and function. The study establishes multicellular senescence as a key driver of impaired muscle recovery in aging.
Detailed Summary
Age-related muscle wasting is worsened by episodes of disuse, and older adults often fail to fully regain lost muscle mass during recovery. While macrophages and satellite cells are known players in muscle remodeling, the upstream mechanisms that derail this process in aging have remained unclear. This study, published in Science Advances, investigates whether accumulation of senescent cells in the muscle interstitial niche underlies impaired recovery in aged mice.
Young (5–6 month) and aged (20–22 month) male mice underwent 14 days of hindlimb unloading — a well-validated model of disuse atrophy — followed by 7 or 14 days of reloading. Muscle senescence was assessed using multiple markers: p21 (cyclin-dependent kinase inhibitor), γH2AX (DNA damage marker), and SPiDER-β-Gal, a fluorescent probe for senescence-associated β-galactosidase activity suitable for live-cell flow cytometry. Single-cell RNA sequencing (scRNAseq) of sorted SPiDER-β-Gal-positive cells was then used to resolve their identities and transcriptional states at high resolution.
Aged mice displayed significantly smaller myofibers and greater collagen deposition during recovery compared to young mice, indicating poor structural remodeling. Macrophage dynamics were also abnormal: aged muscle showed a failure to transition from pro-inflammatory to anti-inflammatory macrophage phenotypes at the expected timepoints during reloading. The number of SPiDER-β-Gal-positive cells was elevated in aged muscle and inversely correlated with muscle mass recovery — meaning more senescent cells corresponded to worse outcomes.
ScRNAseq of SPiDER-positive cells uncovered a diverse multicellular senescence landscape in the muscle interstitial niche. Senescent populations spanned stromal cells (including fibro-adipogenic progenitors), vascular cells (endothelial cells and pericytes), and immune cells (including macrophages). These populations were enriched in aged muscle and showed transcriptional signatures consistent with SASP — secreting pro-inflammatory cytokines, matrix-remodeling factors, and other paracrine signals that could disrupt the local regenerative environment.
To test causality, aged mice were treated with a senolytic drug combination (dasatinib and quercetin) during the reloading period. Senolytic treatment reduced overall senescent cell burden, attenuated macrophage accumulation, and significantly restored both muscle mass and physical function in aged mice. These results causally link interstitial senescent cell accumulation to impaired muscle recovery and demonstrate that targeting senescent cells is a viable strategy to improve outcomes in aged muscle after disuse.
Key Findings
- Aged mice showed smaller myofibers, excess collagen, and abnormal macrophage polarization during muscle reloading.
- SPiDER-β-Gal+ senescent cells were elevated in aged muscle and inversely correlated with muscle mass recovery.
- scRNAseq identified senescent stromal, vascular, and immune cell populations enriched in aged muscle interstitium.
- All senescent interstitial populations displayed SASP transcriptional signatures suggesting paracrine disruption of repair.
- Senolytic treatment (dasatinib + quercetin) restored muscle mass and function in aged mice after disuse atrophy.
Methodology
Young (5–6 mo) and aged (20–22 mo) male mice underwent 14-day hindlimb unloading followed by 7- or 14-day reloading. Senescent cells were identified via flow cytometry using SPiDER-β-Gal and sorted for scRNAseq to resolve cell-type-specific transcriptional states. Senolytic intervention (dasatinib + quercetin) was administered during the reloading phase to test causal contribution of senescent cells.
Study Limitations
The study used only male mice, limiting generalizability to females. The mouse hindlimb unloading model, while well-validated, may not fully recapitulate human disuse scenarios such as bed rest or limb casting. The specific SASP factors driving macrophage dysregulation and fibrosis were not individually validated in vivo.
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