gms | German Medical Science

Objectives: Osteoarthritis (OA) is a progressive joint disease leading to cartilage degeneration, remodeling of the subchondral bone and vascularization. During disease progression the extracellular matrix (ECM) is continuously degraded. In an early phase, proteoglycans are lost while collagens are only degraded at later stages. Other non-collagenous ECM components are less well studied. The highly abundant glycoprotein cartilage oligomeric matrix protein (COMP) was found to be increased in serum and synovial fluid of OA patients and is therefore already used as a diagnostic marker. In the present study, we investigated the expression, extractability and distribution of thrombospondin-4 (TSP-4), another member of the TSP family that is known to be expressed in injured tissues, associated with angiogenesis and regulation of collagen production.

Methods: Human articular cartilage obtained from patients undergoing endoprosthetic knee replacement surgery was used for morphological evaluation, immunohistological staining and protein expression analysis. Proteoglycans were stained with Safranin-O, the proteins COMP and TSP-4 were visualized using specific antibodies. Protein levels were analyzed by immunoblots after total protein extraction. We used the OARSI histopathology grading system, samples received from non-OA patients were used as healthy control.

Results and conclusion: In total, we analyzed at least 5 samples per disease grade. Analyses of protein levels show an increased expression of both TSP-4 and COMP in OA patients compared to healthy controls. However, a strong correlation between OA severity and protein expression is only detected for TSP-4 but not for COMP. Interestingly, a different localization of TSP-4 and COMP can be detected in cartilage of healthy controls and OA patients. In healthy controls, a weak staining for TSP-4 is detected predominantly in the superficial zone while it is much broader distributed and more intense in OA samples. In contrast, COMP is uniformly distributed in the superficial and middle zone in healthy controls but the staining pattern in OA cartilage is more irregular. Further, the signal intensity for COMP decreases in parallel with the proteoglycan loss. Surprisingly, the TSP-4 signal increases particularly in areas where proteoglycans and COMP are already absent. This study shows an increase of TSP-4 that correlates with OA progression, suggesting TSP-4 as a potential biomarker for late stage OA. However, the serum concentration of TSP-4 in OA patients remains to be determined. It is already known that TSP-4 interacts with different collagens but further studies are needed to investigate if TSP-4 has a function in assembly and organization of the cartilage ECM. It is attractive to speculate that TSP-4 could compensate for the loss of COMP and initiate repair in degenerating areas. Thus, TSP-4 might be a target to improve regeneration.