Cartilage matrix remodelling differs by disease state and joint type.

Dramatic alterations in mechanical properties have been documented for osteoarthritic (OA) cartilage. However, the matrix composition underlying these changes has not been mapped and their aetiology is not entirely understood. We hypothesised that an understanding of the cartilage matrix heterogeneity could provide insights into the origin of these OA-related alterations. We generated serial transverse cryo sections for 7 different cartilage conditions: 2 joint sites (knee and hip), 2 disease states (healthy and OA) and 3 tissue depths (superficial, middle and deep). By laser capture microscopy, we acquired ~200 cartilage matrix specimens from territorial (T) and interterritorial (IT) regions for all 7 conditions. A standardised matrix area was collected for each condition for a total of 0.02 ± 0.001 mm3 (corresponding to 20 µg of tissue) from a total of 4800 specimens. Extracted proteins were analysed for abundance by targeted proteomics. For most proteins, a lower IT/T ratio was observed for the OA disease state and knee joint type. A major cause of the altered IT/T ratios was the decreased protein abundance in IT regions. The collagenase-derived type III collagen neo-epitope, indicative of collagen proteolysis, was significantly more abundant in OA cartilage. In addition, it was enriched on average of 1.45-fold in IT relative to T matrix. These results were consistent with an elevated proteolysis in IT regions of OA cartilage, due to degenerative influences originating from synovial tissue and/or produced locally by chondrocytes. In addition, they offered direct evidence for dynamic remodelling of cartilage and provided a cogent biochemical template for understanding the alterations of matrix mechanical properties.