Removal of aging microenvironment of bone defects can effectively promote bone defect repair

BACKGROUND: Bone defects remain a significant challenge in orthopedics, and despite the widespread use of mesenchymal stem cells (MSCs) in regenerative medicine, their therapeutic performance in vivo often falls short of expectations. Emerging evidence suggests that the local microenvironment particularly age-related or injury-induced cellular senescence may compromise MSC function. In this study, we investigated how a senescent vascular niche arising after bone injury influences MSC proliferation, differentiation, and its possible mechanisms involved in regeneration and clearing of the aging microenvironment. METHODS: A femoral trochlear defect was created in 3-month-old SD rats (n = 6/group) to characterize temporal senescence changes using SA-β-Gal staining, p16/CD31 immunofluorescence, and expression of Cdkn1a/Cdkn2a. Primary endothelial cells (ECs) were isolated and senescence-induced with 400 µM H₂O₂ for 45 min. MSCs were co-cultured with senescent ECs (SnECs) in 3D collagen I hydrogels (2 mg/mL, ~ 5 kPa). Quercetin, selected from DrugAge screening (20 µM), was incorporated into a 4 wt% thermosensitive hydrogel (TSH-Q) to enable 7-day sustained release. Bone regeneration was assessed by µCT, histology, and immunofluorescence at 1 and 4 weeks post-injury. RESULTS: Bone defects triggered a biphasic senescence response: early senescence occurred predominantly in peri-defect osteocytes at 1 week, while robust senescence was later observed in neovascular endothelial cells by week 4. SnECs significantly impaired MSC biological functions, reducing migration, chondrogenic differentiation (Safranin O intensity), and mineralization (Alizarin Red) (all p < 0.01). Local delivery of quercetin via TSH-Q cleared approximately 81% of p16⁺ endothelial cells in vivo and enhanced bone repair, increasing BV/TV compared with unloaded TSH (p < 0.001). In the early stage of bone defects, aging cells mainly represent bone cells in the tissues surrounding the bone defect. At later staged, with no changes in the microenvironment, the aging of vascular endothelial cells in the new tissues and blood vessels was most significant. We successfully induced endothelial cell senescence and further explored the functional impact of SnECs on MSCs and found that aging ECs led to a decline in MSCs effects in aging, including reduced proliferation, chondrogenic differentiation, osteogenic differentiation, tissue repair, and mineralization. The therapeutic effects of MSCs and the repair of bone defects were effectively promoted by constructing a quercetin thermosensitive hydrogel sustained-release system to improve the aging microenvironment of bone defects. CONCLUSIONS: Bone injury generates a senescent vascular niche that markedly disrupts MSC-mediated regeneration. Targeted rejuvenation of this niche using sustained-release quercetin effectively restores MSC function and significantly accelerates bone reconstruction. These findings highlight the aging microenvironment as a key therapeutic target for improving MSC-based treatments for bone defects.