Osteoporosis is a disease characterized by a low BMD leading to higher bone fragility, where fractures occur after minimal trauma. Our previous studies in girls and boys, and recent studies in mice, have shown that 40-50% of bone accretion occurs during the period of puberty. Therefore, of considerable importance in the prevention and treatment of osteoporosis are those studies that focus on understanding the mechanisms regulating bone accretion that occurs during puberty. Our recent finding that puberty-induced increases in BMD failed to occur in mice lacking functional IGF-I gene provide direct evidence for a role of IGF-I in the acquisition of peak BMD. Uniquely, IGF-I functions as a local growth factor and as a systemic hormone. The systemic action of IGF-I could be different from that of the local action of IGF-I since (1) the mechanisms involved in regulating the actions of systemic (liver-derived) and local (bone-derived) IGF-I are different, and (2) a deficiency in endocrine IGF-I action failed to reduce peak BMD in mice. Studies of gene function are being advanced by the development of new molecular techniques. One such technique is the Cre/IoxP system, which allows the conditional disruption of gene function (i.e., allows cell specific disruption of IGF-I production). We propose to apply this method to evaluate the effect of IGF-I gene-knock out exclusively in skeletal cells (i.e., osteoblasts (OBs) and also chondroblasts) on skeletal phenotype. The specific hypotheses that we propose to test in this grant proposal are: (1) disruption of the IGF-I gene in OBs will lead to a significant reduction in peak BMD and bone size; and (2) disruption of the IGF-I gene in chondroblasts will lead to reduction in longitudinal growth. In order to evaluate the above hypotheses, transgenic mice expressing Cre recombinase under the direction of osteocalcin promoter, type I collagen promoter (OB-specific), or type II collagen promoter (chondroblast-specific) will be crossed with IoxP IGF-I mice to generate Cre/IoxP IGF-I mice with disruption of IGF-I gene in OBs or chondroblasts. The efficiency of Cre recombinase to cause excision of IGF-I gene in cell types of interest will be evaluated by measurement of IGF-I mRNA by real-time PCR, and by in situ hybridization. The phenotypic manifestations in the skeletal tissues caused by disruption of IGF-I gene specifically in bone during various growth phases will be examined by skeletal phenotypic measurements, including BMD, bone strength, biochemical assays of bone turnover and histomorphometric analyses. If our hypothesis, that locally produced IGFs are more important than systemically produced IGFs, is proven to be correct, this could lead to exploration of mechanisms regulating local production of IGFs as a means to increase bone formation and peak BMD. Since small differences in peak bone mass and BMD at maturity could contribute to substantial difference in osteoporotic fractures, future therapies to increase peak BMD could reduce health care costs by decreasing the incidence of osteoporotic fractures.
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