Collagen Turnover in Normal and Degenerate Human Intervertebral Discs as Determined by the Racemization of Aspartic Acid

Abstract

Knowledge of rates of protein turnover is important for a quantitative understanding of tissue synthesis and catabolism. In this work, we have used the racemization of aspartic acid as a marker for the turnover of collagen obtained from healthy and pathological human intervertebral disc matrices. We measured the ratio of the D-and L-isomers in collagen extracted from these tissues as a function of age between 16 and 77 years. For collagen taken from healthy discs, the fractional increase of D-Asp was found to be 6.74 ؋ 10 ؊4 /year; for degenerate discs, the corresponding rate was 5.18 ؋ 10 ؊4 /year. Using the racemization rate found previously for the stable population of collagen molecules in dentin, we found that the rate of collagen turnover (k T ) in discs is not constant but rather a decreasing function of age. The average turnover rate in normal disc between the ages of 20 and 40 is 0.00728 ؎ 0.00275/year, and that between the ages of 50 and 80 is 0.00323 ؎ 0.000947/year, which correspond to average half-lives of 95 and 215 years, respectively. Turnover of collagen from degenerate discs may be more rapid than that found for normal discs; however, statistical analysis leaves this point uncertain. The finding of a similar correlation between the accumulation of D-Asp and that of pentosidine for three normal collagenous tissues further supports the idea that the accumulation of pentosidine in a particular tissue can, along with the racemization of aspartic acid, be used as a reliable measure of protein turnover.The intervertebral disc (IVD), 2 the largest avascular cartilaginous structure (1, 2), plays a primary mechanical role in transmitting loads through the spine and providing flexibility to the spinal column. The disc has a complex structure and contains very few cells embedded in an extracellular matrix. These cells have the essential function of maintaining and repairing the matrix by synthesizing matrix macromolecules and by producing degradative enzymes, including metalloproteinases and their inhibitors (TIMPs (tissue inhibitors of metalloproteinase)), which are all involved in tissue metabolism and turnover. Thus, normal disc function is dependent on a balance between synthesis and matrix breakdown.The fine balance between synthesis and degradation determines the concentrations of tissue components and hence the composition of the tissue. In vivo (3, 4) and in vitro (5) environmental factors such as mechanical stress (6 -8) and nutrient levels have been found to affect matrix composition, presumably by modulating rates of macromolecular biosynthesis and degradation. Normally, when a balance is maintained, damaged tissue can be restored by cellular repair responses; otherwise, the matrix composition and organization are altered, and the cellular repair responses become inadequate. Hence, the degraded matrix can no longer carry loads effectively, which leads to the degeneration of the disc.During aging, many changes involving the proportions and biochemical properties of the matrix occur. These proces...

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“…In contrast, the half-lives of many ECM proteins are measured in years. Aspartic acid racemization (AAR) studies estimate that the half-lives of types I and II collagen in human skin, articular cartilage and intervertebral disc are 15, 95 and 117 years, respectively (Sivan et al 2008;Verzijl et al 2000). The lower turnover rates and hence longer half-lives of cartilage collagens compared with dermal collagens may be a consequence of the lower cell densities and hence lower catabolic and anabolic rates which prevail in mature cartilage (Antoniou et al 1996).…”

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