Expression of bone morphogenetic protein-6 in dental follicle stem cells and its effect on osteogenic differentiation

The dental follicle (DF) plays an essential role in tooth eruption via regulation of bone resorption and bone formation. Bone morphogenetic protein-6 (BMP6) expression in the DF is coincident with bone growth in the tooth crypt. DF stem cells (DFSCs) have been shown to possess strong osteogenic capability. This study aims to determine the expression of BMP6 in DFSCs and to elucidate the role of BMP6 in the osteogenesis of DFSCs. DFSCs and their non-stem cell counterpart, DF cells (DFCs), were obtained from the DFs of rat pups. We showed that expression of BMP6 was significantly higher in the DFSCs than in the DFCs. DFSCs lost osteogenic capability during in vitro expansion, and DFSCs in late passages had reduced BMP6 expression as compared to early passages of DFSCs when they were subjected to osteogenic induction. Addition of exogenous human recombinant BMP6 (hrBMP6) to the osteogenic medium dramatically enhanced the osteogenesis of the late-passage DFSCs. Knockdown of BMP6 by short interfering RNA in the DFSCs in early passages resulted in a decrease in osteogenesis, which could be restored by addition of hrBMP6. We concluded that DFSCs need to express high levels of BMP6 to maintain their osteogenesis capability. Increased BMP6 expression seen in vivo in the DF may reflect the activation of DFSCs for osteogenic differentiation for bone growth during tooth eruption.