Project Summary Osteoarthritis (OA) is a classic age-related disorder and the most common cause ofpain and disability in the elderly. It is primarily characterized by the progressive destruction of articular cartilage. This past decade has witnessed significant advances in deciphering the basic mechanisms by which OA develops. However, to date, no disease modifying drug therapy is available for preventing OA development and repairing the degenerative cartilage. The uppermost superficial zone of articular cartilage is the first line of defense against OA initiation. We recently found that epidermal growth factor receptor (EGFR), a tyrosine kinase receptor, is expressed abundantly throughout the articular cartilage with its active form (p-EGFR) predominantly located in the superficial zone. Interestingly, at the onset of OA, p-EGFR amount, along with two major EGFR ligands (TGFα and HBEGF), were markedly attenuated while the amount of Mig6, a negative inhibitor of EGFR, was enhanced, suggesting a potential role for EGFR signaling pathway in cartilage homeostasis and diseases. Using a series of mouse models with deficient or overactivated EGFR activity by genetic manipulation of Egfr and Mig6 genes, we and others have demonstrated that EGFR signaling is critical for maintaining the number and mechanical properties of superficial chondrocytes, suppressing their hypertrophy, promoting proteoglycan 4 (Prg4) expression, and stimulating surface lubrication function. Most strikingly, in aging- and surgery-induced OA models, mice with chondrocyte-specific (Col2-Cre) EGFR deficiency developed the most severe OA phenotypes, including a complete loss of articular cartilage, subchondral bone sclerosis, and escalated joint pain. Hence, we hypothesize that EGFR signaling is essential for maintaining the structure and function of the superficial layer in the articular cartilage and thus, can be targeted for OA treatment. Our objectives are to understand the role of this novel signaling pathway in articular cartilage homeostasis and diseases, and to seek approaches targeting this pathway for OA treatment. To achieve these, we will perform the following aims: 1) determine the temporal role of EGFR signaling in OA pathogenesis; 2) elucidate the mechanisms of the protective action of EGFR on articular cartilage; 3) investigate whether EGFR signaling is a promising target for OA treatment. Complementary genetic approaches, such as EGFR vs Mig6, loss of function vs gain of function, and aggrecan-CreER vs Prg4-CreER, will be used throughout the proposal. Moreover, we have designed and synthesized TGFα-conjugated nanoparticles with prolonged retention and penetration abilities in knee cartilage. A proof-of-principal experiment will be performed to examine its therapeutic effects on cartilage degeneration at different OA stages. This proposal will uncover critical EGFR actions in knee articular cartilage and provide crucial evidence for targeting this novel pathway in OA...