Notch signaling pathway promotes osteogenic differentiation of mesenchymal stem cells by enhancing BMP9/Smad signaling

Notch is an important pathway in that it regulates cell-to-cell signal transduction, which plays an essential role in skeletal remodeling. Bone morphogenetic protein (BMP) 9 has been regarded as one of the most efficient BMPs by which to induce osteogenic differentiation in mesenchymal stem cells (MSCs). Understanding the interaction between Notch and BMP9 signaling is a critical issue for optimizing the application of MSCs and BMPs in bone tissue engineering. In the present study, we investigated the role of Notch signaling in the BMP9-induced osteogenic differentiation of MSCs. Our data demonstrated that Notch signaling obviously enhanced BMP9-induced osteogenic differentiation in MSCs in vitro and in vivo. Notch signaling augmented the activity of BMP9-induced BMP/Smad signaling and increased the gene expression of essential osteogenic factors induced by BMP9 in MSCs, such as runt-related transcription factor 2 (Runx2), type I collagen (Colla1) and inhibitor of differentiation (Id) 1. We also found that Notch signaling promoted the expression of activin-like kinase 2 (ALK2) induced by BMP9, and the inhibitory effect of dnALK2 on BMP9-induced osteogenic differentiation was rescued by constitutive overexpression of Delta-like 1 (DLL1). Notch signaling also exhibited an apparent effect on the proliferation of mouse embryo fibroblasts (MEFs) during BMP9-induced osteogenic differentiation. These results indicate that Notch plays a significant role in mediating BMP9-induced osteogenic differentiation in MSCs, which may be partly regulated by upregulation of the expression of ALK2.