VPS33B-dependent exosomes modulate cellular senescence of mesenchymal stem cells via an autocrine signaling pathway.

Mesenchymal stem cell (MSC)-derived exosomes have been intensively studied for their therapeutic effects on tissue repair and regeneration. However, the specific contributions of exosomes derived from endogenous bone marrow MSCs to the maintenance of bone tissue homeostasis remain unclear. In this study, we impaired MSC-derived exosome secretion by specifically deleting vascular protein sorting 33B (VPS33B). Mice deficient in VPS33B (VPS33B-cKO mice) exhibited premature bone loss and imbalanced bone remodeling processes, which were associated with a reduction in MSC number and an increase in bone marrow inflammation. MSCs derived from VPS33B-cKO mice exhibited impaired self-renewal, proliferation, osteoblastic differentiation, and increased cellular senescence. Incubation with exosomes (Y-Exo) derived from MSCs of wildtype young mice greatly ameliorated senescent phenotypes observed in VPS33B-deficient MSCs. We further demonstrated exosome autocrine pathway through a fluorescent-labeled uptake assay and observed a significant association between autocrinal exosomes and the senescence of MSCs. Mechanistically, miR-136-3p and miR-146a-5p were highly enriched in Y-Exo but not in exosomes from senescent MSCs, which promoted cell proliferation while inhibiting inflammation by targeting the PI3K-Akt and NF-κB pathway, respectively. Furthermore, intramedullary transplantation of Y-Exo successfully mitigated age-related MSC exhaustion and bone loss. Our findings indicate that endogenous MSC-derived exosomes play a crucial regulatory role in the maintenance of bone homeostasis, and propose the potential therapeutic application of young MSC-derived exosomes for the treatment of senile osteoporosis.