Aggrecan degradation in human intervertebral disc and articular cartilage

Aggrecan degradation in human intervertebral disc and articular cartilage has been studied by using anti-neoepitope antibodies specific for the N-terminal degradation products generated by cleavage within the interglobular domain at the metalloproteinase and aggrecanase sites. Immunoblot analysis of extracts of annulus fibrosus, nucleus pulposus and articular cartilage demonstrated age-related patterns in the abundance of both degradation products. In all three tissues the metalloproteinase-generated fragment was present at very low levels in young individuals but increased in abundance with age. In the disc tissues, the abundance of this degradation product levelled off in the juvenile; for cartilage this occurred in early adulthood. Despite these temporal differences, the levels attained in adults were comparable for the three tissues. In contrast, the aggrecanase-generated degradation product exhibited tissue-specific differences in the variation of its abundance with age. Whereas this degradation product increased with age in annulus fibrosus and articular cartilage and had levelled off by adulthood, in nucleus pulposus it was present in greatest abundance in young individuals and decreased to very low levels with age. Examination of discs exhibiting various degrees of degeneration did not reveal any differences in the levels of the metalloproteinase and aggrecanase-generated cleavage products that could not be accounted for by differences in age. In adults the product of aggrecanase action was much more abundant in articular cartilage than in either of the disc tissues, despite the age-related increase also observed for annulus fibrosus. Analysis of tissue extracts with an antibody recognizing the G1 domain of aggrecan identified two major degradation products whose abundance and size were correlated with the fragments detected by the anti-neoepitope antibodies. Taken together, these results indicate that cleavage at the metalloproteinase and aggrecanase sites are quantitatively important events in aggrecan catabolism in both articular cartilage and intervertebral disc in vivo. Moreover the two enzyme systems act independently and exhibit differences in the degree to which they contribute to aggrecan degradation in these tissues.