Stem Cell Exosomes Reverse Muscle Atrophy by Reactivating Estrogen Signaling

Sarcopenia — the age-related loss of skeletal muscle mass and strength — is driven not only by aging but also by chronic inflammation, immobility, and endocrine decline, particularly falling estrogen levels after menopause. Current management (exercise, nutritional supplementation, hormone replacement) offers only modest benefit, creating demand for novel biologics. This preclinical study systematically evaluated whether hUC-MSCs and their secreted exosomes could reverse glucocorticoid-induced muscle atrophy and clarified the molecular pathways involved.

Male C57BL/6J mice received intraperitoneal dexamethasone (20 mg/kg/day, 7 days) to induce muscle atrophy, then were treated for 14 days with bilateral intramuscular injections of hUC-MSCs (1×10⁶ cells/kg), MSC-Exos (100 µg), or the estrogen analog SNG162 (40 mg/kg i.p.) as a mechanistic comparator. In parallel, dexamethasone-treated C2C12 myotubes were co-cultured with hUC-MSCs or exosomes for 24 hours. Outcomes included grip strength, treadmill endurance, muscle weight ratios, histological fiber cross-sectional area (CSA), TUNEL apoptosis staining, EdU proliferation assays, Western blotting, ELISA for cytokines and estradiol, and RNA sequencing.

All three treatments — hUC-MSCs, MSC-Exos, and SNG162 — meaningfully restored grip strength and endurance and increased gastrocnemius CSA and muscle-to-body-weight ratio without affecting total body weight. At the molecular level, they upregulated the muscle differentiation marker MyHC, autophagy initiator Beclin-1, and the pro-survival ratio Bcl-2/Bax, while significantly suppressing the atrophy E3 ubiquitin ligases MAFbX and MURF1 and the transcription factor FOXO3. Inflammatory cytokines TNF-α, IL-6, and IL-1β were also reduced. Estrogen receptors ERα46, ERα36, and ERβ, as well as circulating estradiol levels, were elevated in treated groups, providing the first direct evidence that hUC-MSC paracrine activity modulates the estrogen signaling axis in skeletal muscle.

RNA sequencing of gastrocnemius tissue identified the PI3K/AKT/mTOR and ERK1/2 pathways as central mediators of the observed protective effects, consistent with Western blot data. Importantly, MSC-Exos outperformed whole hUC-MSCs in stimulating C2C12 myotube proliferation (EdU assay) and differentiation (MyHC immunofluorescence), suggesting that the therapeutic cargo of exosomes — likely microRNAs, growth factors, and signaling proteins — is sufficient to replicate and even exceed the benefits of cell transplantation.

The findings support hUC-MSCs and MSC-Exos as promising interventions for sarcopenia, acting through a previously underappreciated estrogen-dependent mechanism. Because exosomes avoid tumorigenesis risk, survive freeze-thaw cycles, and can be mass-produced, they represent a particularly attractive off-the-shelf therapeutic format. However, this study used only male mice and a pharmacological (dexamethasone) rather than aging model, limiting direct translation to clinical sarcopenia, especially in postmenopausal women.