Cartilage Extracellular Matrix Fragments and Trauma-Induced Chondrocytes

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Severe damage to cartilage from blunt trauma is thought to trigger a degenerative cascade that leads to post-traumatic osteoarthritis (PTOA). Cartilage degeneration in PTOA initiates at injury sites and, over a period of years, spreads to engulf previously healthy cartilage. It seems likely that this slowly advancing process of local chondrolysis starts with catabolic factors produced at injury sites, but it is not clear how chondrolytic activity is sustained and how lesions spread over such long time frames. Although this is obviously a multifactorial process influenced substantially by post- trauma mechanical conditions (Project 2), the hypothesis that proteolytic fragments of the cartilage extracellular matrix (ECM) play a role is well-supported by in vitro and in vivo studies. These show that ECM fragments with strong catabolic effects are enriched in arthritic cartilage and joint fluids and that their removal from joints inhibits the progression of arthritis. ECM fragments are capable of limited diffusion through normal cartilage and can induce matrix protease expression by chondrocytes deep in the tissue. The potential to create new zones of degeneration and ECM fragmentation suggests a vicious cycle that could lead to sustained chondrolytic activity around injury sites. Here we propose to evaluate the role of ECM fragments in this process, focusing on fibronectin fragments as a likely source of catabolic stimulation in this context. An in vitro cartilage injury model was developed to study these local catabolic processes. We expect that the mechanisms of post-traumatic chondrolysis in this in vitro system will reflect processes that occur in vivo, however, this assumption will be tested in an animal cartilage trauma model. These systems will be used to test the following hypotheses: (1) Fibronectin fragments (Fn-fs) generated at impact injury sites, stimulate chondrolysis in healthy cartilage; (2) Post-traumatic chondrolysis is self- sustaining and propagates over time; (3) Inhibitors of Fn-f-induced signaling will impede post- traumatic chondrolysis; (4) Post-traumatic chondrolysis is driven by factors intrinsic to cartilage. At the conclusion of this project we expect to have identified one or more inhibitors that show strong chondroprotective activity, providing a sound basis for further tests to determine their therapeutic value.