Induction of chondrogenesis and superficial zone protein accumulation in synovial side population cells by BMP-7 and TGF-beta1.

Synovial cells are known to contain a sub-population of cells with multipotent differentiating capacity including chondrocytes. However, the stem/progenitor cells in synovial cells have not been well characterized. Stem/progenitor cells can exclude Hoechst 33342 dye, and the cell fraction with this property is called "side population (SP)." SP cells are present in many adult tissues. The aim of this investigation was to identify, isolate, and characterize SP cells from bovine synovium. Hoechst dye efflux and fluorescence activated cell sorting showed that synovial cells contained 0.60% SP cells. In the presence of verapamil, an inhibitor of ABC transporters critical for the dye efflux property, the SP cell fraction was not observed, indicating the critical role of ABC transporters. Isoforms of ABC transporters (ABCG2 and ABCB1 mRNA) were highly expressed in SP cells derived from the synovial cells by real-time RT-PCR analysis. Bone morphogenetic protein-7 (BMP-7) induced type II collagen mRNA expression characteristic of chondrogenesis in articular cartilage with both SP and non-SP cells. In addition, expression of SZP mRNA, a marker of the surface layer of articular cartilage, was significantly up-regulated by BMP-7, and the protein accumulation of SZP was stimulated by both BMP-7 and TGF-beta1. Thus, synovial cells contain ABC transporter-dependent SP cells. These findings demonstrate that side population cells of synovium differentiate toward an articular chondrocyte phenotype of the surface layer and have direct implications for tissue engineering and regeneration of articular cartilage.