My laboratory's research is focused on the molecules that regulate the differentiation of osteoblasts, which are the cells responsible for bone formation. We are using the a1 Type I collagen (Col1a1) promoter, which is very highly expressed in osteoblasts, as a model system for these studies. By studying promoter-reporter constructs in transgenic mice, we identified a homeodomain protein binding site which is necessary for high level expression of the gene. We are currently studying four proteins which bind to this site, Msx2, Dlx5 , Prx1 and Prx2. Although knockout and overexpression studies in mice have suggested that these proteins play a role in skeletal development, the precise mechanistic description of this role is difficult because these proteins are expressed in many cell types during development. My laboratory has focused on development and exploitation of retroviral vectors which allow studies of protein and promoter function in primary osteoblasts derived directly from chick embryos or mice, which more closely mimic cells in intact animals than immortalized osteoblastic cell lines. Our results led us to propose a model for the role of Msx2 in skull development, in which Msx2 inhibits osteoblast differentiation yet causes more rapid bone growth. This model explains the results obtained from overexpression and knockout studies. Our ongoing studies on Dlx5, Prx1 and Prx2 are leading to similar insights. Dlx5 stongly stimulates osteoblast differentiation, while Prx1 and 2 may be inhibitors. A second area of research is focused on developing retrovirus vectors and techniques for gene therapy of bone. We have developed a vector in which the Col1a1 promoter drives expression of a marker gene specifically in osteoblasts of transgenic mice, which has previously proved difficult because of the inactivation of retroviruses in embryonic stem cells. We are also developing methods for re-introducing engineered osteoblast precursers into the bone. Thus we hope to be able to treat mouse models of human diseases, and eventually to contribute to treatment of human diseases such as osteoporosis.