Direct conversion of human fibroblasts into functional osteoblasts by defined factors

Significance Osteoblasts produce bone matrix and contribute to bone remodeling. We have established a procedure to directly convert human fibroblasts into osteoblasts by transducing some defined factors and culturing in osteogenic medium. Osteoblast-specific transcription factors, Runx2 and Osterix, in combination with Oct4 and L-Myc, drastically induced fibroblasts to produce calcified bone matrix and express osteoblast-specific markers. The directly converted osteoblasts (dOBs) showed similar gene expression profiles as normal osteoblasts and contributed to bone repair after transplantation into mice with bone defects. Furthermore, dOBs did not require continuous expression of the exogenous genes to maintain their phenotype. These findings strongly suggest successful direct reprogramming of fibroblasts into osteoblasts, which may be applicable to bone regeneration therapy. Osteoblasts produce calcified bone matrix and contribute to bone formation and remodeling. In this study, we established a procedure to directly convert human fibroblasts into osteoblasts by transducing some defined factors and culturing in osteogenic medium. Osteoblast-specific transcription factors, Runt-related transcription factor 2 (Runx2), and Osterix, in combination with Octamer-binding transcription factor 3/4 (Oct4) and L-Myc (RXOL) transduction, converted ∼80% of the fibroblasts into osteocalcin-producing cells. The directly converted osteoblasts (dOBs) induced by RXOL displayed a similar gene expression profile as normal human osteoblasts and contributed to bone repair after transplantation into immunodeficient mice at artificial bone defect lesions. The dOBs expressed endogenous Runx2 and Osterix, and did not require continuous expression of the exogenous genes to maintain their phenotype. Another combination, Oct4 plus L-Myc (OL), also induced fibroblasts to produce bone matrix, but the OL-transduced cells did not express Osterix and exhibited a more distant gene expression profile to osteoblasts compared with RXOL-transduced cells. These findings strongly suggest successful direct reprogramming of fibroblasts into functional osteoblasts by RXOL, a technology that may provide bone regeneration therapy against bone disorders.