PROTEOGLYCANS ISOLATED FROM DISSOCIATIVE EXTRACTS OF DIFFERENTLY AGEDHUMAN ARTICULAR-CARTILAGE - CHARACTERIZATION OF NATURALLY-OCCURRING HYALURONAN-BINDING FRAGMENTS OF AGGRECAN

Proteoglycans extracted with 4 M guanidinium chloride from young (mean 20 years) or old (mean 79 years) macroscopically normal human articul ar cartilage were separated by density gradient centrifugation and Q-S epharose chromatography and characterized by gradient gel SDS/PAGE and immunodetection before and after removal of glycosaminoglycan chains. The extracts contained two large populations of aggrecan, a populatio n of small N-terminal aggrecan fragments, as well as decorin, biglycan and fibromodulin. The distribution of all these species in density gr adient fractions has been determined. The large aggrecan populations c omprised four different chondroitin sulphate-bearing core proteins whi le the population of smaller fragments comprised eight different compo nents. The two smallest fragments (35 and 42 kDa), identified as the f irst globular domain of aggrecan (N-terminal) (G1) and containing no g lycosaminoglycan, were detected only in extracts of old cartilage. A 5 5 and a 70 kDa fragment of G1 were present in both keratan sulphate-co ntaining and non-keratan sulphate-containing forms. Four other fragmen ts, each containing keratan sulphate epitopes, were identified and the se contained either G1 epitopes (one 95 kDa species), or G1 and G2 epi topes (three species). These results have suggested that proteolytic p rocessing at the N-terminus is more extensive than has previously been recognized and raises the possibility that more than one proteinase m ay be involved in aggrecan degradation in vivo. With the exception of the two smallest G1 fragments, the repertoire of proteoglycan fragment s found in young and old human articular cartilage is essentially the same, although the relative abundance of various species differed. The older tissue contains a larger proportion of C-terminally truncated a ggrecan fragments and a significantly decreased content of decorin and biglycan.