Variations in aggrecan localization and gene expression patterns characterize increasing stages of human intervertebral disk degeneration

Abstract

During disk degeneration, annulus dehydration and matrix fraying culminate in the formation of tears through which nucleus and annulus disk material may rupture, causing radicular pain. Annular tears are present in more than half of the patients in early adulthood and are almost always present in the elderly. Aggrecan, which provides the disk with a shock absorber function under loading, is a key disk extracellular matrix (ECM) component. The objective of the present study was to assess the immunolocalization of aggrecan in the annulus, and to assess molecular gene expression patterns in the annulus ECM utilizing microarray analysis. Immunohistochemistry was performed on 45 specimens using an anti-human aggrecan antibody. Affymetrix microarray gene expression studies used the extracellular matrix ontology approach to evaluate an additional 6 grade I–II, 9 grade III, and 4 grade IV disks. Grade III/IV disks were compared to healthier grade I/II disks. Healthy and less degenerated disks showed a general uniform aggrecan immunolocalization; more degenerated disks contained regions with little or no identifiable aggrecan localization. In degenerated disks, molecular studies showed a significant downregulation of aggrecan, ADAMTS-like 3, and ADAMTS10. Collagen types III and VIII, fibronectin, decorin, connective tissue growth factor, TIMP-3, latent TGF-β binding protein 2 and TGF-β1 were significantly upregulated with fold changes ranging from 2.4 to 9.8. Findings here help us better understand changes in the immunohistochemical distribution of a key proteoglycan during disk aging. Such information may have application as we work towards biologic therapies to improve the aging/degenerating disk matrix.