Characterization of the macromolecular components of the articular cartilage surface.

The intact surface of articular cartilage is a highly organized structure composed of a variety of macromolecules. The studies reported here deal with a partial characterization of the non-covalently bound components of the outermost layer of articular cartilage. Normal bovine and human cartilage articular surfaces were extracted for 5 min with 4-M guanidine HCl solution. Analysis and quantitation of small proteoglycans in the extract were carried out by PAGE (polyacrylamide gel electrophoresis), Western blot, and radioimmunoassays. The present studies indicate that the major proteins extracted from the articular surface of bovine and human cartilage are the collagen-binding small proteoglycans designated as fibromodulin and albumin. Fibronectin, decorin, and biglycan were also detected in smaller amounts. Immunoblotting of the surface material developed with a monoclonal antibody with keratan sulfate specificity confirmed the presence of fibromodulin coinciding with the major protein band of approximately 70-100-kDa molecular mass. Gel filtration chromatography of the surface material confirmed the previous results. Additional in vitro assays showed that the collagen-binding material extracted from the cartilage surface contained the small proteoglycans. Anti-human fibromodulin antibodies bound in significantly greater amounts to the intact articular surfaces than to cut surfaces of normal human cartilage. It is concluded that small, non-aggregating proteoglycans constitute the major proteoglycan species non-covalently bound to macromolecules at the articular surface of cartilage partially responsible for the interference of anti-collagen type II antibody binding and for the inhibition of cell adhesion to the intact surface.