FNDC5/irisin-enriched sEVs conjugated with bone-targeting aptamer alleviate osteoporosis: a potential alternative to exercise

Exercise maintains bone health and produces protective effects on bone loss. In this study, we investigated the potential protective effects of circulating small extracellular vesicles (sEVs) generated under endurance exercise training (Exe-sEVs) on ovariectomized (OVX)-induced bone loss. Inhibition of sEVs secretion by GW4869 partially reversed exercised-induced protection against OVX-induced bone loss. Importantly, Exe-sEVs was internalized by bone tissue and alleviated bone loss in OVX-mice. The increased levels of fibronectin type-III domain-containing protein 5 (FNDC5/irisin) in Exe-sEVs contributed to the promotion of osteogenesis in bone marrow mesenchymal stem cells (BM-MSCs). However, systemic knockdown of FNDC5, the precursor of irisin, abolished the exercise-induced protective effects against bone loss in OVX-mice. Moreover, incubation of irisin enhanced osteogenesis and attenuated adipogenesis in BM-MSCs. Intriguingly, implantation of BM-MSCs overexpressing FNDC5 accelerated osteogenesis and chondrogenesis in BALB/c immunodeficiency mice. Mechanistically, irisin promoted phosphorylation of p38MAPK and JNK, but not ERK. Blocking the JNK and p38MAPK signaling pathway with specific inhibitors abolished the pro-osteogenesis and anti-adipogenesis effects of irisin on BM-MSCs. However, inhibition of β-arrestin-2 rescued the irisin-induced activation of p38MAPK and JNK. Finally, aptamer-modified FNDC5-sEVs (Apt-FNDC5-sEVs) exhibited higher enrichment in bone tissues and enhanced bone formation. In conclusion, exercise-induced circulating FNDC5/irisin-enriched sEVs promote osteogenesis of mouse BM-MSCs both in vitro and in vivo, partially through a β-arrestin-2-dependent p38MAPK and JNK signaling pathway. Apt-FNDC5-sEVs represent a promising strategy for the treatment of osteoporosis.