Aggrecan fragments as biomarkers in osteoarthritis

Abstract: Background: In osteoarthritis (OA) the balance of cartilage matrix synthesis and degradation is disturbed, resulting in a gradual destruction of the articular cartilage. Matrix components released into body fluids by proteolytic cleavage can be used as biomarkers of ongoing processes. This thesis focuses on proteolytic degradation of the proteoglycan aggrecan with the overall aim to study the potential of aggrecan fragments as biomarkers in knee OA. Methodology/Principal findings: Using neoepitope specific antibodies in Western blots, aggrecan fragments were identified in knee cartilage and synovial fluid (SF) pooled from individuals with a wide spectrum of disease. Aggrecanases were found to dominate aggrecan proteolysis in disease, although a contribution of matrix metalloprotease (MMP) activity was noted. Western blot quantification in individual samples showed that the proportion of aggrecan released into SF generated by aggrecanases varied in disease, and was higher in diagnostic groups associated with high disease activity. Quantification by ELISA of SF ARGS showed that SF ARGS better distinguished samples from patients with knee joint disease from samples obtained from knee healthy individuals than aggrecan measures not specific for this neoepitope. In patients meniscectomized 18 years earlier, SF ARGS levels were inversely associated with progression of radiographic OA. Conclusions: Aggrecanase is the dominating protease in human knee OA and its activity toward the aggrecan interglobular domain (IGD) is elevated in disease. SF levels of aggrecan ARGS fragments generated by this IGD activity can be used as biomarkers and has diagnostic as well as prognostic capabilities.