Crosslinking of cartilage matrix protein to aggrecan increases with maturation

Cartilage oligomeric matrix protein enhances matrix assembly during chondrogenesis of human mesenchymal stem cells

Cartilage oligomeric matrix protein/thrombospondin-5 (COMP/TSP5) is an abundant cartilage extracellular matrix (ECM) protein that interacts with major cartilage ECM components, including aggrecan and collagens. To test our hypothesis that COMP/TSP5 functions in the assembly of the ECM during cartilage morphogenesis, we have employed mesenchymal stem cell (MSC) chondrogenesis in vitro as a model to examine the effects of COMP over-expression on neo-cartilage formation. Human bone marrow-derived MSCs were transfected with either full-length COMP cDNA or control plasmid, followed by chondrogenic induction in three-dimensional pellet or alginate hydrogel culture. MSC chondrogenesis and ECM production was estimated based on quantitation of sulfated glycosaminoglycan (sGAG) accumulation, immunohistochemistry of the presence and distribution of cartilage ECM proteins, and real-time RT-PCR analyis of mRNA expression of cartilage markers. Our results showed that COMP over-expression resulted in increased total sGAG content during the early phase of MSC chondrogenesis, and increased immuno-detectable levels of aggrecan and collagen type II in the ECM of COMP-transfected pellet and alginate cultures, indicating more abundant cartilaginous matrix. COMP transfection did not significantly increase the transcript levels of the early chondrogenic marker, Sox9, or aggrecan, suggesting that enhancement of MSC cartilage ECM was effected at post-transcriptional levels. These findings strongly suggest that COMP functions in mesenchymal chondrogenesis by enhancing cartilage ECM organization and assembly. The action of COMP is most likely mediated not via direct changes in cartilage matrix gene expression but via interactions of COMP with other cartilage ECM proteins, such as aggrecan and collagens, that result in enhanced assembly and retention.

Interaction of cartilage oligomeric matrix protein/thrombospondin 5 with aggrecan

Cartilage oligomeric matrix protein/thrombospondin 5 (COMP/TSP5) is a major component of the extracellular matrix (ECM) of the musculoskeletal system. Its importance is underscored by its association with several growth disorders. In this report, we investigated its interaction with aggrecan, a major component of cartilage ECM. We also tested a COMP/TSP5 mutant, designated MUT3 that accounts for 30% of human pseudoachondroplasia cases, to determine if the mutation affects function. Using a solid-phase binding assay, we have shown that COMP/TSP5 can bind aggrecan. This binding was decreased with MUT3, or when COMP/TSP5 was treated with EDTA, indicating the presence of a conformation-dependent aggrecan binding site. Soluble glycosaminoglycans (GAGs) partially inhibited binding, suggesting that the interaction was mediated in part through aggrecan GAG side chains. Using affinity co-electrophoresis, we showed that COMP/TSP5, in its calcium-replete conformation, bound to heparin, chondroitin sulfates, and heparan sulfate; this binding was reduced with EDTA treatment of COMP/TSP5. MUT3 showed weaker binding than calcium-repleted COMP/TSP5. Using recombinant COMP/TSP5 fragments, we found that the "signature domain" could bind to aggrecan, suggesting that this domain can mediate the interaction of COMP/TSP5 and aggrecan. In summary, our data indicate that COMP/TSP5 is an aggrecan-binding protein, and this interaction is regulated by the calcium-sensitive conformation of COMP/TSP5; interaction of COMP with aggrecan can be mediated through the GAG side chains on aggrecan and the "signature domain" of COMP/TSP5. Our results suggest that COMP/TSP5 may function to support matrix interactions in cartilage ECM.

Cartilage oligomeric matrix protein/thrombospondin 5 supports chondrocyte attachment through interaction with integrins

Cartilage oligomeric matrix protein/thrombospondin 5 (COMP/TSP5) is a major component of the extracellular matrix of the musculoskeletal system. Although COMP/TSP5 abnormalities are associated with several pathological conditions, its normal function remains unclear. This study was undertaken to delineate the function(s) of COMP/TSP5 in cartilage, especially regarding its interaction with chondrocytes. We show that COMP/TSP5 can support chondrocyte attachment and that the RGD sequence in COMP/TSP5 and the integrin receptors alpha5beta1 and alphaVbeta3 on the chondrocytes are involved in mediating this attachment. The interactions of COMP/TSP5 with the integrins are dependent on COMP/TSP5 conformation. Chondrocyte attachment to COMP/TSP5 in the calcium-replete conformation was inhibited by function-blocking integrin alpha5 and beta1 antibodies, suggesting the involvement of the alpha5beta1 integrin. Under this condition, a function-blocking antibody against alphaVbeta3 did not have any effect on cell attachment. On the other hand, chondrocyte attachment to reduced COMP/TSP5 was instead sensitive to alphaVbeta3 function-blocking antibodies, suggesting that COMP/TSP5 mediates attachment through chondrocyte alphaVbeta3 integrin under this condition. Cell attachment to reduced COMP/TSP5 was not inhibited by beta1 antibodies. These data indicate that COMP/TSP5 in different conformations can utilize different integrin receptors. These results are the first to demonstrate that COMP/TSP5 can mediate chondrocyte attachment through interactions with integrins. Through these interactions, COMP/TSP5 may be able to regulate cellular activities and respond to environment in the surrounding cartilage matrix.

Synovial fluid markers in osteoarthritis

Exploration of the mechanisms responsible for breakdown and repair of articular cartilage in osteoarthritis has promoted interest in the potential use of cartilage-derived molecules as markers of cartilage metabolism in this disease. Many of the macromolecules detectable in synovial fluid have been evaluated as possible surrogate markers of the severity or activity of disease processes. Measurements of the synovial fluid concentration of any marker without due consideration of the fate of the molecule in the joint cavity are likely to be misleading and insufficient to cement a relationship between marker concentration and the severity of articular pathology.