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Proteoglycan aggregation in human intervertebral disc and bovine nasal cartilage Emes, John Hayward

Abstract

Herniation of the intervertebral disc is a pathological condition characterized by protrusion of the tissue posterio-laterally, often impinging on the spinal chord or nerve roots. The disease is accompanied by a reduction in the average molecular weight and viscosity of the disc proteoglycans, in excess of that which normally occurs with increasing age. The proteoglycans of disc however have not been examined in terms of the modern concepts of cartilage matrix structure. Bovine nasal cartilage, has been shown to contain proteoglycan aggregates, trapped in the intersticess of a collagen network, which can be dissociated with 4M guanidine hydrochloride into diffusible proteoglycan subunits and a multicomponent "linking" fraction. A similar system was thought to occur in the intervertebral disc. It seemed possible that, if such a system was present in the disc, the reduction in the molecular weight and viscosity of the proteoglycans with increasing age and herniation could be due to a decrease in proteoglycan aggregation. The present study showed that proteoglycan aggregates similar to those of bovine nasal cartilage are found in the human intervertebral disc, but that they only represent 5% of the total proteoglycans in the tissue. In contrast, bovine nasal cartilage contained 70% of the proteoglycans in the aggregated form. A novel modification of the extraction procedure was devised by which it was possible to assess the degree of proteoglycan aggregation. Sequential extraction of the tissue with a weak and strong electrolyte (0.4 M and 4M guanidine hydrochloride) selectively removed non-aggregated and aggregated proteoglycans respectively. This procedure provides a new and rapid method for assessing the degree of proteoglycan aggregation in a variety of connective tissues. The small proportion of aggregate in the disc was almost exclusively located in the annulus fibrosus. Re-aggregation studies suggested that both disc and cartilage contain two proteoglycans, only one of which is capable of forming aggregates. Examination of the proteoglycans in a limited number of discs suggested that the degree of aggregation did not change with increasing age. Since, in addition, aggregates represent only a small proportion of the disc proteoglycans, it appeared unlikely that a decrease in the degree of aggregation could account for the decrease in molecular weight and viscosity of the disc proteoglycans observed with increasing age and/or degenerative disc disease.

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