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Effect on Murine Growth of BDNF Receptor and p38 Knockouts at the Growth Plate

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Short stature in children is defined as those below the 3rd percentile for age;of the 3% of children with short stature, 5-10% have an identifiable cause such as growth hormone (GH) deficiency, or a deficiency of the hormone regulated by GH, insulin-like growth factor (IGF)-1. Despite the fact that many millions of dollars are spent each year in the U.S. alone on recombinant human GH and IGF-I, we still know relatively little about how long bone growth is regulated at the site of the growth plate. The growth plate is a specialized and highly organized cartilaginous tissue wherein chondrocytes progress from proliferation to maturation, hypertrophy, matrix synthesis and mineralization and finally cell death. Using large numbers of growth plate chondrocytes isolated from a bovine source, we have identified brain-derived neurotrophic factor (BDNF) as a novel regulator of growth plate maturation, and have begun to examine how BDNF, acting through its receptor TrkB, modulates IGF-I action in growth plate chondrocytes by differentially regulating two MAPK pathways. Growth plate chondrocyte proliferation and differentiation are both dependent on the activation of the classic MAPK, ERK1/2. IGF-I stimulates chondrocyte proliferation via ERK activation, but BDNF inhibits IGF-I-stimulated chondrocyte proliferation. Whereas neurotrophins like BDNF stimulate neuronal cell development by causing sustained ERK activation, BDNF instead inhibits ERK activity in our isolated chondrocytes. Moreover, BDNF activates the p38 MAPK pathway, which has been shown to inhibit proliferation in other cell types. Other groups have implicated p38 in the regulation of growth by showing that p38 inhibition delays chondrocyte maturation. We proposed a model wherein IGF-I-stimulated ERK activation is required for both proliferation and differentiation of chondrocytes, but p38 activation by BDNF is required for the cells to make the transition from proliferation to differentiation. The overall goal of the proposed project is to demonstrate the importance of p38 and TrkB signaling to growth plate development by disrupting their expression in developing chondrocytes, and to further examine the relevant signaling mechanisms in chondrocytes with in vitro studies.

PUBLIC HEALTH RELEVANCE: Despite the fact that many millions of dollars are spent each year in the U.S. alone on growth promoting therapies in children with growth failure, relatively little is known about how these therapies affect the part of the bone where growth occurs, known as the growth plate. We have isolated these specialized cells from a bovine source, and used them to identify a novel signaling pathway in them which consists of a hormone normally found in neural tissue called BDNF, and an intracellular kinase called p38. Our goal is to specifically delete the genes for these two proteins in the growth plates of mice to demonstrate the importance of BDNF and p38 signaling in mammalian growth.

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