Ectopic Msx2 overexpression inhibits and Msx2 antisense stimulates calvarial osteoblast differentiation

Msx2 is believed to play a role in regulating bone development, particularl y in sutures of cranial bone. In this study we investigated the effects of retroviral-mediated overexpression of Msx2 mRNA, in both sense and antisens e orientations, on primary cultured chick calvarial osteoblasts. Unregulate d overexpression of sense mRNA produced high levels of Msx2 protein through out the culture period, preventing the expected fall as the cells different iate. The continued high expression of Msx2 prevented osteoblastic differen tiation and mineralization of the extracellular matrix. In contrast, expres sion of antisense Msx2 RNA decreased proliferation and accelerated differen tiation. In other studies, we showed that the Msx2 promoter was widely expr essed during the proliferative phase of mouse calvarial osteoblast cultures but was preferentially downregulated in osteoblastic nodules. These result s support a model in which Msx2 prevents differentiation and stimulates pro liferation of cells at the extreme ends of the osteogenic fronts of the cal variae, facilitating expansion of the skull and closure of the suture. (C) 1999 Academic Press.