Cartilage degradation, the hallmark of rheumatoid arthritis and osteoarthritis, is closely linked to the tissue's state of mechanical usage. Cartilage breakdown can be induced by mechanical overuse and exacerbated by disuse;however, mechanical loading at physiologically relevant levels can protect joint tissues from degradation. Yet the cellular mechanisms that mediate both the protective effects of physiological loading and the damaging effects of inappropriate mechanical usage remain unclear. We recently identified a novel transcriptional regulator CITED2 (CBP/p300-lnteracting Transactivator with ED-rich tail 2) that is induced in chondrocytes and other cells by physiologically relevant levels of mechanical loading, and whose expression is inversely correlated with the production of matrix metalloproteinases (MMPs) implicated in cartilage degradation. We therefore hypothesize that CITED2 is a key element of the regulatory pathway that mediates the effects of mechanical loading on cartilage degradation. The proposed studies have three specific aims directed at characterizing this signaling pathway: (1) to determine the relationship between changes in CITED2 expression and the regulation of MMPs in chondrocytes responding to mechanical loading (intermittent hydrostatic pressure);(2) to identify molecular mechanisms by which CITED2 regulates the expression of MMP genes;and (3) to identify specific mechanisms by which mechanical loading regulates the expression of CITED2. In these studies we will experimentally modify the expression or activity of molecular components participating in this signaling pathway and monitor resulting changes in the expression of the appropriate target genes (CITED2, MMPs). We will also characterize interactions between molecular members of these regulatory pathways by in vitro and in vivo binding assays. Finally we will test whether the effects of mechanical loading on MMP expression can be mimicked using a biological regulator that has been found to target a specific molecular component of this novel molecular switching mechanism. The results of these studies will thus identify novel targets for therapeutic approaches to combat joint degenerative diseases, and will improve our understanding of the fundamental biological processes underlying both normal connective tissue turnover and pathologic degradation.