Genetic analysis of Runx2 function during intramembranous ossification

Runt-related transcription factor 2 (Runx2) is an essential transcriptional regulator of osteoblast differentiation and its haploinsufficiency leads to cleidocranial dysplasia because of a defect in osteoblast differentiation during bone formation through intramembranous ossification. The cellular origin and essential period for Runx2 function during osteoblast differentiation in intramembranous ossification remain poorly understood. Paired related homeobox 1 (Prx1) is expressed in craniofacial mesenchyme, and Runx2 deficiency in Prx1⁺-derived cells (Runx2_prx1^−/− mice) resulted in defective intramembranous ossification. Runx2 was heterogeneously expressed in Prx1-green fluorescent protein (GFP)⁺ cells located at the intrasutural mesenchyme in the calvaria of transgenic mice expressing GFP under the control of the Prx1 promoter. Double-positive cells for Prx1-GFP and stem cell antigen-1 (Sca1) (Prx1⁺Sca1⁺ cells) in the calvaria expressed Runx2 at lower levels and were more homogeneous and primitive as compared with Prx1⁺Sca1⁻ cells. Osterix (Osx) is another transcriptional determinant of osteoblast lineages expressed by osteoblast precursors; therefore, Osx is highly expressed by Prx1⁻Runx2⁺ cells at the osteogenic front and on the surface of mineralized bone in the calvaria. Runx2 deficiency in Osx⁺-derived cells (Runx2_osx^−/− mice) resulted in severe defects in intramembranous ossification. These findings indicate that the essential period of Runx2 function on intramembranous ossification would begin at the Prx1⁺Sca1⁺ mesenchymal stem cell stage and end at the Osx⁺Prx1⁻Sca1⁻ osteoblast precursor stage.