Cartilage destruction - Release of type IX collagen in joint disease

Abstract: Cartilage is of vital importance for the function of joints. It is characterized by a prominent extracellular matrix (ECM) that is produced and maintained by relatively few cells of one type, the chondrocyte. The cartilage ECM consists of collagen fibrillar networks, proteoglycan aggregates, and a number of other molecules united in numerous interactions. This work focused at identifying catabolic events leading to cartilage destruction and functional impairment and developing tools to detect such events in patients. In the first part, using a model for cartilage degradation in disease, N-terminal fragments of type IX collagen was found to be released at an intermediate time point. The two most abundant fragments were further studied and their novel C-terminals identified. These two fragments comprise most of the NC4 domain and the combined NC4 + COL3 domains, respectively. Furthermore, the identified cleavage sites were shown to be induced by matrix metalloproteinase 13 (MMP-13). In the second part, the potential degradation of type IX collagen in human cartilage was investigated. It was shown that two similar type IX collagen fragments were released from cartilage explants upon MMP-13 digestion. The novel C-terminal of the fragment comprising the NC4 domain was identified. The cleavage site was found to differ between human and bovine type IX collagen. An antiserum raised against the human C-terminal neoepitope confirmed the cleavage and was used to develop an immunoassay (inhibition ELISA). This assay was used to detect elevated levels of the neoepitope in sera from arthritis patients compared to controls, indicating that this cleavage occurs in joint pathology.