The function of Runx2 after commitment into the osteoblast lineage

Objectives: Runx2 is an essential transcription factor for osteoblast differentiation, and Cbfb is required for the Runx2-dependent osteblast differentiation. However, the functions of Runx2 and Cbfb after the commitment into the osteoblast lineage remain to be clarified. Previous in vitro and in vivo studies showed that Runx2 plays an essential role in the expression of major bone matrix protein genes after the commitment into the osteoblast lineage. As Runx2 transgenic mice under the control of Col1a1 promoter showed severe osteopenia, however, it needs to be reevaluated. Methods: Runx2 has two isoforms (type I Runx2 and type II Runx2) with different N-termini. We generated type I Runx2, type II Runx2, type I Runx2/Cbfb double, type II Runx2/Cbfb double, and dominant negative (dn)-Runx2 transgenic mice under the control of Col1a1 promoter, which directs the transgene expression to osteoblasts. Results: Although the expression level of the transgene in type I Runx2 transgenic mice was much higher than that in type II Runx2 transgenic mice, the severity of osteopenia in type I Runx2 transgenic mice was milder than that in type II Runx2 transgenic mice. However, type I Runx2/Cbfb double transgenic mice showed severe osteopenia with multiple fractures, which was similar to type II Runx2/Cbfb double transgenic mice. In these nice, osteoblast maturation was severely inhibited. In dn-Runx2 transgenic mice, the number of osteoblasts was reduced, but the osteoblasts expressed normal levels of major bone matrix protein genes and the trabecular bone gradually increased during aging. Conclusion: These findings indicate that Runx2 isoforms exert different activities on osteobalsts, which is partly due to the different dependencies of Runx2 isoforms on Cbfb. Further, Runx2 inhibits osteoblast maturation but is not essential for the regulation of expression of major bone matrix protein genes.