DESCRIPTION (Taken from the application): Despite a wealth of information regarding hypoxia and angiogenesis during fracture repair, the underlining molecular events responsible for these critical early processes remain unknown. With the recent technological advances in molecular biology and the identification of thousands of genes, it is now possible to more clearly examine the precise molecular events that underlie the fracture repair process. In essence, we strongly believe that key early stage processes, including hypoxia and angiogenesis, are ultimately responsible for determining the success (or failure) of the healing process. Thus, we propose the hypothesis that during the early stages of fracture healing, hypoxia resulting from the inevitable disruption of the bone's blood supply, induces hypoxia inducible factor I (HIF-1a), which in turn up-regulates transcription of a cascade of downstream genes that directly promote angiogenesis. The objective of this three year study is to test the hypothesis that the up-regulation of the transcription factor, HIF-la, is critical to the establishment of neovascularization within areas of chondrogenesis and endochondral ossification in the fracture callus. Our preliminary data show that HIF-la, as well as a number of angiogenic-related genes (i.e. vascular endothelial growth factor [VEGF], CYR61), are up-regulated during the stages of fracture healing, providing strong supporting evidence for our hypothesis. Experiments will be performed to systematically extend these findings through four specific aims that utilize the established in vivo femur fracture model to determine the temporal and spatial expression levels of: (i) HIF-1a (ii) its target genes known to play a role in angiogenesis (VEGF), vasodilation (nitric oxide synthase NOS, heme oxygenase HO1, and erythopoiesis (erythropoietin RPO, tranferrin), and to directly compare the (iii) angiogenesis (neovascularization) and (iv) structural integrity (strength and stiffness) of the fracture callus and healing femurs, respectively, in HIF-1a partially deficient (+/-) mice to that of their wild type (+/+) littermates. These studies will provide unique insight into critical early-stage events, and help define etiologic factors that may contribute to the incidence of delayed healing, particularly in the case of the molecular components (i.e. HIF-l a) involved in hypoxia and angiogenesis.