Localization and distribution of cartilage oligomeric matrix protein in the rat intervertebral disc.

STUDY DESIGN: Whole rat intervertebral disc (IVD), as well as the anulus fibrosus (AF) and the nucleus pulposus (NP) were studied using immunoblot, immunohistochemistry, and reverse-transcription followed by polymerase chain reaction (RT-PCR) methods to investigate the expression and distribution of cartilage oligomeric matrix protein (COMP). OBJECTIVES: To investigate the expression and distribution patterns of COMP in normal IVD. SUMMARY OF BACKGROUND DATA: COMP is an extracellular matrix protein abundantly expressed in articular and growth plate cartilage, as well as bone, ligament, tendon, and synovium. The potential importance of COMP to the spine has been underscored by its mutations that lead to skeletal dysplasia with characteristic platyspondyly. However, the expression and distribution of COMP in spine and IVD has not been illustrated before. METHODS: The presence of COMP protein was investigated by immunoblotting using a COMP antibody F8 on protein extractions from whole IVD and AF or NP. To compare the expression levels of COMP between lumbar and tail IVDs, and between AF and NP of the IVD, wet weight of the tissues were used for normalization. To show that COMP can be made by IVD cells in situ, RT-PCR was used to investigate the COMP mRNA message. The distribution patterns of COMP in IVD were investigated using immunohistochemistry studies with COMP antibody F8. RESULTS: COMP is expressed at both the protein and mRNA levels in both the AF and NP of both the lumbar spine and tail IVD. Immunohistochemistry studies show that COMP is found in the extracellular matrix of the IVD, exhibiting lamellar distribution pattern in the AF region. When normalized to wet weight, COMP is found to be expressed at higher levels in the lumbar than the tail IVD, and within the IVD, greater in the AF than the NP region. CONCLUSIONS: Our results demonstrate the expression of COMP in both the AF and NP of the IVD. COMP is a component of the extracellular matrix of AF and NP, with a lamellar distribution pattern in the AF. Our data suggest that COMP may play a role in the normal structure of IVD.

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.