CSF-1, released by osteoblasts and stromal cells, stimulates the proliferation and differentiation of osteoclast progenitors and enhances osteoclast survival. In the op/op mouse model, a thymidine insertion in the coding osteopetrosis. The long-term goal of this proposal is to determine the effect of soluble(s) and membrane-bound (m) CSF-1 isoforms on osteoclastogenesis and define cell-specific cis-acting elements in the 5' flanking region of the CSF-1 promoter that regulate their expression. Our first hypothesis is that sCSF-1 and mCSF-1 are differentially synthesized and stimulate osteoclast formation due, in part, to their interaction with the c-fms receptor. To address this issue in vitro, op/op stromal cells will be transfected with sCSF-1 or mCSF-1 cDNA and stable transfectants examined for CSF-1 synthesis, bioactivity, osteoclast support and c-fms tyrosine kinase activation in target cells. The effect of each isoform on osteoclast formation in vivo will be explored by targeting isoform to ameliorate osteopetrosis in op/op mice, genetic crosses between sCSF-1 and mCSF-1 transgenic mice and heterozygous op/wt mice, genetic crosses between sCSF-1 and mCSF-1 transgenic mice and heterozygous op/wt mice will be carried out to establish op/op mutants expressing each transgene. Mice will be examined for serum CSF-1 bone growth, incisor eruption and resolution of osteopetrosis. Bone sections will be assessed for osteoclast activity; histomorphometric analysis will evaluate both static and dynamic indices of bone remodeling. The in vivo therapeutic effect of targeting each isoform to the bone in op/op mice will be assessed using adenoviral vectors designed to limit CSF-1 expression in osteoblasts. Our second hypothesis is that specific regulatory elements in the 5' flanking region of the CSF-1 promoter direct cell-specific gene expression in vitro and in vivo during murine development. The temporal and spacial expression of CSF-1 during murine skeletal development will be assessed by in situ hybridization and immunohistochemistry. To determine potential cis-acting elements that control cell-specific expression of CSF-1, deletion constructs generated from the 5' flanking region of CSF- 1 will be tested for their ability to direct transcription in osteoblast, stromal, liver, muscle, epithelial and B cell lines. Relevant CSF-1 cell specific promoter sequences will be assessed in vivo by generating transgenic mice harboring cell-specific CSF-1 promoter segment(s) linked to the bacterial lacZ reporter gene. These studies should increase our understanding of the molecular mechanisms that activate CSF-1 during development and may suggest novel therapeutic strategies designed to regulate osteoclast formation in a variety of bone disorders such as osteoporosis and bone fracture.
