Variations of the proteoglycans of the canine intervertebral disc with ageing

A group of young (2.0 +/- 0.6 years) (group 1) and old (9.7 +/- 1.5 years) (group 2) beagle dogs were given Na2 35SO4 (1.0 mCi/kg) intravenously 60 days prior to being killed to radiolabel their proteoglycans. Lumbar discs were removed and dissected into nucleus pulposus and annulus fibrosus. Proteoglycans were extracted at 4 degrees C from these tissues with buffered 4.0 M Gdn-HCl containing proteinase inhibitors, and purified by CsCl density gradient ultracentrifugation. The average hydrodynamic size and ability of the purified proteoglycans to aggregate in the presence of excess hyaluronic acid was determined by Sepharose CL-2B chromatography. The galactosamine/glucosamine ratios of these proteoglycans as well as their non-aggregating fractions were also ascertained. The proteoglycan content of discs of old animals was significantly less than in the young. The proportion of 35S-labelled, or non-labelled proteoglycans which could aggregate in the presence of hyaluronic acid was also much lower in the preparations isolated from the older discs. In contrast, the average hydrodynamic size of the non-aggregating proteoglycans isolated from the annuli fibrosi of group 2 animals were larger than the corresponding population of group 1 animals. Aminosugar analysis of these same proteoglycan fractions from older animals afforded galactosamine/glucosamine ratios (mean 1.81 +/- 0.14) which were less than the younger age group (mean 2.63 +/- 0.40). These data suggest that with ageing and degeneration the proteoglycans of the beagle disc undergo increased degradation with the accumulation in the annulus fibrosus of a population which is of larger average hydrodynamic size and richer in keratan sulphate than proteoglycans present in younger tissues.