Microarray on osteoblasts made from bone marrow stromal cells from Bmp2 flox/flox and Bmp2; Prx1-Cre mice

Enhanced BMP or canonical Wnt (cWnt) signaling are therapeutic strategies employed to enhance bone formation and fracture repair, but the mechanisms each pathway utilizes to specify cell fate of bone-forming osteoblasts remain poorly understood. Among all BMPs expressed in bone, we find that singular deficiency of Bmp2 blocks the ability of cWnt signaling to specify osteoblasts from limb bud or bone marrow progenitors. When exposed to cWnts, Bmp2-deficient cells fail to progress through the Runx2/Osx1 checkpoint and thus do not upregulate multiple genes controlling mineral metabolism in osteoblasts. Cells lacking Bmp2 after induction of Osx1 differentiate normally in response to cWnts, supporting pre-Osx1+ osteoprogenitors as a critical source and target of BMP2. Our analysis furthermore reveals Grainyhead-like 3 (Grhl3) is to date an unidentified transcription factor in the osteoblast gene regulatory network that is induced during bone development and bone repair, and acts upstream of Osx in a BMP2-dependent manner. The Runx2/Osx1 transition therefore receives critical regulatory inputs from BMP2 that are not compensated for by cWnt signaling, and this is mediated at least in part by induction and activation of Grhl3.

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Salazar VS, Ohte S, Capelo LP, Gamer L, Rosen V