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Objective. Muscle-derived stem cells (MDSCs) isolated from mouse skeletal muscle exhibit long-time proliferation, high self-renewal, and multipotent differentiation. This study was undertaken to investigate the ability of MDSCs that were retrovirally transduced to express bone morphogenetic protein 4 (BMP-4) to differentiate into chondrocytes in vitro and in vivo and enhance articular cartilage repair.Methods. Using monolayer and micromass pellet culture systems, we evaluated the in vitro chondrogenic differentiation of LacZ-and BMP-4-transduced MDSCs with or without transforming growth factor ␤1 (TGF␤1) stimulation. We used a nude rat model of a fullthickness articular cartilage defect to assess the duration of LacZ transgene expression and evaluate the ability of transplanted cells to acquire a chondrocytic phenotype. We evaluated cartilage repair macroscopically and histologically 4, 8, 12, and 24 weeks after surgery, and performed histologic grading of the repaired tissues.Results

Objective. To investigate the effect of vascular endothelial growth factor (VEGF) stimulation and the effect of blocking VEGF with its antagonist, soluble Flt-1 (sFlt-1), on chondrogenesis, using muscle-derived stem cells (MDSCs) isolated from mouse skeletal muscle.Methods. The direct effect of VEGF on the in vitro chondrogenic ability of mouse MDSCs was tested using a pellet culture system, followed by real-time quantitative polymerase chain reaction (PCR) and histologic analyses. Next, the effect of VEGF on chondrogenesis within the synovial joint was tested, using genetically engineered MDSCs implanted into rat osteochondral defects. In this model, MDSCs transduced with a retroviral vector to express bone morphogenetic protein 4 (BMP-4) were coimplanted with MDSCs transduced to express either VEGF or sFlt-1 (a VEGF antagonist) to provide a gain-and loss-of-function experimental design. Histologic scoring was used to compare cartilage formation among the treatment groups.Results. Hyaline-like cartilage matrix production was observed in both VEGF-treated and VEGF-blocked (sFlt-1-treated) pellet cultures, but quantitative PCR revealed that sFlt-1 treatment improved the expression of chondrogenic genes in MDSCs that were stimulated to undergo chondrogenic differentiation with BMP-4 and transforming growth factor ␤3 (TGF␤3). In vivo testing of articular cartilage repair showed that VEGFtransduced MDSCs caused an arthritic change in the knee joint, and sFlt-1 improved the MDSC-mediated repair of articular cartilage, compared with BMP-4 alone.Conclusion. Soluble Flt-1 gene therapy improved the BMP-4-and TGF␤3-induced chondrogenic gene expression of MDSCs in vitro and improved the persistence of articular cartilage repair by preventing vascularization and bone invasion into the repaired articular cartilage.