The goal of our studies is to contribute to the understanding of the transcriptional regulation of osteoblastic differentiation. This is crucial to our understanding of the mechanisms controlling bone formation. Greater understanding of this process may lead to more rational design of therapies for osteoporosis and other bone diseases. Our studies and those of others have suggested that Dlx5, a homeodomain-containing transcription factor, plays an important role in the induction of osteoblastic differentiation. The Specific Aims of this grant application are: 1. To assess the role of Dlx5 in osteoblast differentiation in vivo by studying the effect of over expression of Dlx5 in osteoblasts off transgenic mice at different stages of osteoblast lineage progression. 2. To study the effect of inactivation of Dlx5 in cultured osteoblasts, and to assess the effect of Dlx5 on osteoblastic gene expression. We will study in vitro differentiation of osteoblasts derived from Dlx5 knockout mice. We will also carry out focused microarray analysis of cells plus and minus Dlx5 to identify genes that are induced or inhibited by Dlx5, and analyze bone development in an osteoblast directed knockout of the Dlx5 gene. This will allow us to separate the effects of Dlx5 expression in osteoblasts from the effect of this gene in other cell types. 3. To assess the potential for other members of the Dlx gene family to contribute to induction of osteoblast differentiation in vivo. We will continue to examine the expression of Dlx1, 2,3, 6 and 7 in osteoblasts. We will continue studies on Dlx3, which we already have detected in differentiating osteoblasts. We will continue studies on the ability of other Dlx genes to induce osteoblastic differentiation in the same in vitro system that we have used to study Dlx5. We will also study an osteoblast-directed knockout of the Dlx3 gene, to evaluate the role of Dlx3 in bone development in vivo. Our proposed studies will contribute to a more complete understand of the role of the Dlx protein family in osteoblast differentiation.