Proteomic Analysis of Tendon Extracellular Matrix Reveals Disease Stage- specific Fragmentation and Differential Cleavage of COMP ( Cartilage Oligomeric Matrix Protein)*

Background: Tendon disease is characterized by extensive remodeling of the extracellular matrix. Results: Novel COMP cleavage fragments were identified in both an in vitro inflammatory model and natural disease. Conclusion: Inflammatory mediators drive distinct COMP fragmentation at different stages of tendon disease. Significance: Novel COMP neo-terminal fragments provide opportunities for developing markers for tendon injury. During inflammatory processes the extracellular matrix (ECM) is extensively remodeled, and many of the constituent components are released as proteolytically cleaved fragments. These degradative processes are better documented for inflammatory joint diseases than tendinopathy even though the pathogenesis has many similarities. The aims of this study were to investigate the proteomic composition of injured tendons during early and late disease stages to identify disease-specific cleavage patterns of the ECM protein cartilage oligomeric matrix protein (COMP). In addition to characterizing fragments released in naturally occurring disease, we hypothesized that stimulation of tendon explants with proinflammatory mediators in vitro would induce fragments of COMP analogous to natural disease. Therefore, normal tendon explants were stimulated with IL-1 and prostaglandin E-2, and their effects on the release of COMP and its cleavage patterns were characterized. Analyses of injured tendons identified an altered proteomic composition of the ECM at all stages post injury, showing protein fragments that were specific to disease stage. IL-1 enhanced the proteolytic cleavage and release of COMP from tendon explants, whereas PGE(2) had no catabolic effect. Of the cleavage fragments identified in early stage tendon disease, two fragments were generated by an IL-1-mediated mechanism. These fragments provide a platform for the development of neo-epitope assays specific to injury stage for tendon disease.