BMSC-derived exosomes facilitate osteogenesis and ameliorate ageing-related bone loss through restoring Th17/Treg homeostasis via the miR-21/Skp2/FoxO1 axis

Osteoporosis (OP) is among the most prevalent systemic skeletal disorders worldwide and is characterized by decreased bone mass and microarchitectural deterioration, leading to increased fracture risk and significant impairment of quality of life, particularly among elderly individuals. Recently, exosomes derived from bone marrow mesenchymal stem cells (BMSCs), termed BMSC-exosomes, have emerged as promising therapeutic agents for OP because of their regenerative and immunomodulatory potential. In this study, we used senescence-accelerated mouse prone 6 (SAMP6) mice, MC3T3-E1 osteoblastic cells, and CD4(+) T cells to investigate the effects of BMSC-exosomes on osteogenesis and to elucidate the underlying molecular mechanisms. Our results demonstrate that BMSC-derived exosomes enhance osteogenic differentiation in vitro and ameliorate age-related bone loss in vivo. We identified miR-21-5p as a highly enriched microRNA within BMSC-exosomes, which plays a central role in mediating their pro-osteogenic effects and protecting against OP progression. Flow cytometry analysis revealed that BMSC-exosome treatment effectively restored the imbalance between T helper 17 cells (Th17) and regulatory T cells (Treg cells)-a key immune dysregulation observed in OP-in both SAMP6 mice and cultured CD4(+) T cells. Through integrated bioinformatics analysis and experimental validation, we showed that BMSC-derived miR-21-5p directly targeted S-phase kinase-associated protein 2 (SKP2), leading to its downregulation. SKP2 then promotes the ubiquitination and subsequent degradation of Forkhead Box O1 (FoxO1), a transcription factor essential for maintaining Th17/Treg homeostasis. By suppressing SKP2, miR-21-5p stabilizes FoxO1, thereby promoting immune balance and enhancing osteogenic activity. Collectively, these findings indicate that miR-21-5p-enriched BMSC-exosomes alleviate OP by modulating the SKP2/ubiquitination/FoxO1 signalling axis and restoring the Th17/Treg balance. This dual action-promoting bone formation and correcting immune dysfunction-highlights the therapeutic potential of BMSC-exosomes. Thus, the use of miR-21-5p-loaded BMSC-exosomes represents a novel and promising strategy for the prevention and treatment of OP.