Mitochondria transfer enhances bone marrow mesenchymal stem cell functions and promotes bone defect healing

Abstract Background: Bone marrow-derived mesenchymal stem cells (BMSCs) transplantation is considered a promising therapeutic approach for bone defect repair. However, during the transplantation procedure, the functions and viability of BMSCs may be impaired due to extended durations of in vitro culture, aging and disease conditions of patients. Inspired by spontaneous intercellular mitochondria transfer that naturally occurs within injured tissues to rescue cellular or tissue function, we investigated whether artificial mitochondria transfer into pre-transplant BMSCs in vitro could improve cellular function and enhance their therapeutic effects on bone defect repair in situ. Methods: First, mitochondria were isolated from donor BMSCs and transferred into recipient BMSCs of the same passage. Afterwards, changes in proliferative capability was evaluated by Cell Counting Kit-8, Ki67 staining, etc., while Transwell, wound scratch healing and cell motility tests were conducted to determine migration ability. Then, alkaline phosphatase (ALP) staining, Alizarin Red staining, combined with qPCR and Western Blot experiments of Runx2 and BMP2 were performed to elucidate the effect of mitochondria transfer on the osteogenic potential of BMSCs in vitro. After that, the in vivo experiments were completed by transplanting mitochondria-recipient BMSCs into a rat cranial critical-size bone defect model. Micro CT scanning and histological analysis were conducted 4 weeks and 8 weeks after transplantation to evaluate the osteogenesis effect in situ. Finally, in order to discover the potential connection between cellular behavioral changes and aerobic metabolism, OXPHOS (oxidative phosphorylation) and ATP production were assessed and inhibition of aerobic respiration by oligomycin was proceeded. Results: Mitochondria-recipient BMSCs exhibited significantly enhanced proliferation and migration, and increased osteogenic differentiation upon osteogenic induction. The in vivo results showed more new bone formation after transplantation of mitochondria-recipient BMSCs in situ. Increased OXPHOS activity and ATP production were further observed, whereas the inhibition of which impaired the enhancement of proliferation, migration and osteogenic differentiation induced by mitochondria transfer. Conclusions: Mitochondria transfer is a feasible technique to enhance BMSCs function in vitro and promote bone defect repair in situ through the up-regulation of aerobic metabolism. The results indicated that mitochondria transfer may be a novel promising technique for optimizing stem cell function.