The Organization of Aggrecan in Human Articular Cartilage

Abstract

The effect of age on the incorporation of newly synthesized aggrecan into the extracellular matrix of human articular cartilage was investigated. This property was measured in a pulse-chase explant culture system by determining the distribution of radiolabeled molecules ([ 35 S]sulfate-labeled) between a nondissociating extract (phosphate-buffered saline), which extracts mainly nonaggregated macromolecules, and a dissociating extract (4 M GnHCl) containing mainly aggrecan that was complexed in situ with hyaluronan. The rate of incorporation of aggrecan into aggregates was much slower in mature cartilage than in tissue obtained from younger individuals. Furthermore, autoradiography showed that in mature cartilage, newly synthesized aggrecan is not transported from the pericellular environment within the first 18 h of chase culture, whereas in immature cartilage, it moves into the intercellular space during the same period, i.e. aggrecan is processed in the extracellular space very differently in young and adult articular cartilage. Experiments were also performed to show that the interaction of link protein with newly synthesized aggrecan depends on the maturity of the G 1 domain of aggrecan. This investigation has shown that the extracellular aggregation of aggrecan in adult human articular cartilage involves a number of intermediate structures. These have not been identified in the very young cartilage obtained from laboratory animals or in porcine and bovine articular cartilage obtained from the abattoir.Aggrecan is one of the major macromolecules in the extracellular matrix of articular cartilage and endows the tissue with its characteristic water imbibing properties and its ability to undergo reversible compression. During normal aging of cartilage, aggrecan undergoes many posttranslational modifications to its protein core and to the number, size, and proportion of the chondroitin sulfate and keratan sulfate chains that are covalently associated with it (1). These events are mostly catabolic in nature and are a consequence of the relatively long resident time of aggrecan in the matrix (2). However, it is clear that a number of these age-related changes could only have come about by an altered anabolic response of the chondrocyte (3).A recent publication (3) has emphasized that the rate of aggrecan synthesis (sulfate incorporation) is dependent on the age of the specimen from which the tissue was obtained. However, the ability of the chondrocyte to make this molecule is only one part of the maintenance process; it also has to be assembled into aggregates in the extracellular matrix. This process involves the interaction of aggrecan with link protein(s) and hyaluronan, which are themselves under their own biosynthetic control, and the complex must then be deposited within the correct compartment of the tissue (4). Thus, extracellular assembly must be a rate-limiting step in the formation of a stable matrix. The speed at which aggregation happens is important for normal tissue homeostasis, but it would be part...

Hyaluronan and CD44: Strategic players for cell–matrix interactions during chondrogenesis and matrix assembly

“…The PTR loop contains the functional site of the binding of aggrecan to hyaluronan (Bajorath et al,1998). Studies by Bayliss et al (2000) suggest that newly synthesized aggrecan is processed in pools with different capacities for aggregation with hyaluronan and stabilization by link protein, and that tissue compartments (perhaps defined by extracellular pH) show differences with age and disease state. The interglobular domains between G2 and G3 contain the amino acids substituted with glycosaminoglycan chains of both chondroitin sulfate and keratan sulfate.…”

Dwarfism in homozygous Agc1CreERT mice is associated with decreased expression of aggrecan

“…Healthy and proper functioning of articular cartilage require a high amount of sulfonated aggrecan to form aggregates. The size of Acan aggregates is decreased in both aging and osteoarthritis (Bayliss, Howat, Davidson, & Dudhia, 2000;Stern, Kogan, Jedrzejas, & Solt es, 2007).…”

Cartilage proteoglycans

“…The nanomelic aggrecan core protein is modified by N-linked oligosaccharides and accumulates in the ER of embryonic chondrocytes. 18 The cartilage matrix deficiency (cmd) locus in the mouse maps to the aggrecan gene, in which a truncated aggrecan core protein in the result. These mouse embryos also have shortened skeletal elements and no aggrecan in the highly cellular cartilage extracellular matrix.…”

“…17 Recent studies by Bayliss et al suggest that newly synthesized aggrecan is processed in pools with different capacities for aggregation with hyaluronan and stabilization by link protein and that tissue compartments, perhaps defined by extracellular pH, show differences with age and with disease state. 18 The C-terminus comprises the G3 domain that includes an EGF module, a C-type lectin module and a complement regulatory protein (CRP) module. With aging, there is an increase in the population of aggrecan monomers that lack the G3 domain, most likely due to extracellular proteolytic degradation.…”