Age-related changes in the synthesis of link protein and aggrecan in humanarticular cartilage: implications for aggregate stability

The rates of incorporation of radiolabelled leucine into aggrecan and link protein have been measured in human articular cartilage of different ages. Aggrecan and link protein were purified in the Al fraction of CsCl gradient s as a result of their ability to form high-buoyant-density proteoglycan ag gregates with hyaluronic acid. Separation of the aggrecan from the link pro tein was achieved by Mono Q anion-exchange chromatography. The rates of syn thesis of both aggrecan and link protein decreased with age, The age-relate d decrease in synthesis of aggrecan was paralleled by a decrease in the rat e of sulphate incorporation into glycosaminoglycan chains. The synthesis of link protein decreased with age to a greater extent than that of aggrecan such that the ratio of the rates of link protein to aggrecan synthesis decr eased from 1 in immature cartilage to 0.2 in mature cartilage. The age-rela ted decrease in link protein synthesis is controlled at least in part by tr anscriptional or post-transcriptional mechanisms, as shown by the accompany ing age-related decrease in link-protein mRNA. The absence of any age-relat ed decrease in aggrecan mRNA suggests that the decrease in synthesis of agg recan core protein is controlled by a translational mechanism. Measurement of the total tissue content of aggrecan and link protein by radioimmunoassa y revealed an age-related increase in the accumulation of these matrix prot eins, even though their de novo synthesis was decreasing. This illustrates the importance that the regulation of extracellular post-translational modi fication also has in controlling the overall turnover of the cartilage matr ix.