Introduction: Runx2 is one of the most studied transcription factors expressed in mesenchymal stem cells (MSCs) upon their commitment toward an osteogenic differentiation. During endochondral bone formation in vivo, Sox9 directly interacts with Runx2 and represses its activity; however, the role of Sox9 in direct osteogenesis in vitro has been largely overlooked.
Methods: Bone marrow-derived human MSCs (hMSCs) were cultured in vitro either in the control or osteogenic medium supplemented with dexamethasone (DEX). To further investigate the role of Sox9 in direct osteogenesis in vitro, hMSCs were treated with Sox9 siRNA.
Results: We show here that Sox9 is the key early indicator during in vitro osteogenic differentiation of hMSCs. Osteogenic induction leads to a significant decrease of Sox9 gene and protein expression by day 7. Treatment of hMSCs with Sox9 siRNA enhanced mineralization in vitro, suggesting that downregulation of Sox9 is involved in direct osteogenesis. siRNA knockdown of Sox9 did not in itself induce osteogenesis in the absence of DEX, indicating that other factors are still required.
Conclusion: Screening of not preselected donors of different ages and gender (n=12) has shown that the Runx2/Sox9 ratio on day 7 is correlated to the (45)Ca incorporation on day 28. The impact of Sox9 downregulation in the mineralization of human MSCs in vitro indicates a so far unprecedented role of Sox9 as a major regulator of direct osteogenesis. We propose that the Runx2/Sox9 ratio is a promising, early, in vitro screening method for osteogenicity of human MSCs.