RI COBRE: ENDOCRINE AND NUTRITIONAL CONTROL OF LONG BONE GROWTH

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Linear bone growth and maintenance of articular cartilage both require the proliferation and differentiation of chondrocytes. Much of the work done on chondrogenesis has focused on the role of the growth hormone (GH)/insulin-like growth factor-l (IGF-I) axis. While it has long been known that nutrient deprivation impairs linear growth, this has widely been considered to be a function of indirect nutrient effects through modulation of the GH/IGF-I axis. However, our preliminary data indicate that the essential amino acid leucine can directly modulate chondrogenesis. Given that chondrocyte differentiation involves cellular hypertrophy, and that the recently characterized mammalian Target of Rapamycin (mTOR) pathway regulates cell size in response to nutrient availability, we hypothesized that nutrient signaling via mTOR might exert direct effects on chondrocytes. In preliminary studies, we found that rapamycin, an mTOR inhibitor, blocked differentiation of ATDC5 chondrogenic cells. However, leucine restriction also inhibited differentiation. However, a preliminary examination of the signaling effects of rapamycin versus leucine restriction indicated differences. Similar results in an intact in vivo system, fetal rat metatarsal explants, showed differences between the effects of rapamycin and leucine restriction. Based on these observations, we propose the following aims. Specific Aim 1: We will characterize and compare the induction of chondrocyte differentiation by IGF-I and insulin, focusing on actions mediated through the nutrient-sensing kinase mTOR. Specific Aim 2: We will test the hypothesis that restriction of the availability of essential amino acids has a direct effect on chondrocyte proliferation and differentiation, and that this effect is mediated through effects on mTOR as well as through mTOR-independent mechanisms, which we will characterize. Specific Aim 3: thoughout the course of the studies, the physiological significance of key findings will be assessed through parallel studies using a more physiological model of chondrogenesis and bone growth, fetal rat metatarsal explant cultures. This work has the potential to establish a role for nutrients in the indirect regulation of chondrogenesis via insulin and in direct regulation via the mTOR pathway.