The epidermal growth factor-like domain of the large proteoglycans from articular cartilage (aggrecans). Estimate of content at different ages and in osteoarthritis

Abstract

Sequencing of cDNA clones has shown that the carboxy terminal domain of the core protein of large proteoglycans (aggrecans) from human cartilage contains an epidermal growth factor-like (EGF-like) domain which is alternatively spliced. In a previous study it was found that the domain of the translated protein can be recognized by polyclonal antibodies to mouse EGF. A competitive enzyme-linked immunoabsorbent (ELISA) assay has been developed to evaluate the EGF-like domain content of aggrecans at various ages and in osteoarthritis. Fetal aggrecans digested with protease free chondroitinase ABC were adsorbed on polyvinyl chloride microtiter plates followed by blocking with bovine serum albumin and goat serum. Mixtures of known amounts of protein of digested aggrecans and constant amounts of anti-mouse EGF antibodies were incubated and added to plates. The second antibody was peroxidase-conjugate F(ab')2. Fetal, newborn and child aggrecan proteins have a higher content of EGF-like domain than aggrecan proteins from cartilage of older humans. Three areas of cartilages from osteoarthritic joints were separated: cartilages with normal macroscopic appearance, erosion border cartilage and osteophytic cartilage. Values derived from these samples were compared with values derived from nonosteoarthritic aged humans. The content of aggrecans from osteoarthritic cartilage with normal macroscopic appearance was similar to or slightly lower than the latter. The aggrecans from osteophytes had a higher EGF-like domain content. The aggrecans from the erosion border had a variable content, close to noneroded cartilages, to osteophytes or in between the values obtained for noneroded cartilages and for osteophytes. Variations in the amount of newly synthesized aggrecans, in the proteolysis of the carboxy terminal domain of aggrecans and in the alternating splicing of the EGF-like domain might explain the results shown here.