Age-related changes in fibromodulin and lumican in human intervertebral discs

Study Design. An analysis of proteoglycans of the intervertebral disc using immunoblotting of tissue extracts. Objectives, To investigate the changes in structure and abundance of fibromodulin and lumican in human intervertebral discs during aging and degeneration. Summary of Background Data. Fibromodulin and lumican are keratan sulfate proteoglycan constituents of the disc's extracellular matrix, whose interaction with collagen fibrils may contribute to the mechanical properties of the tissue. Changes in their abundance and/or structure that occur with aging and degeneration therefore may have an impact on disc function. Methods. Lumbar intervertebral discs were obtained from individuals of different ages, and extracts of anulus fibrosus and nucleus pulposus were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting using antibodies specific for fibromodulin and lumican. Results. The major changes in abundance observed with age were a decrease in fibromodulin in the adult nucleus pulposus and an increase in lumican in anulus fibrosus during early juvenile development. In addition, fibromodulin in the anulus fibrosus exhibited a structural change with increasing age, characterized by a shift toward the predominance of its glycoprotein form lacking keratan sulfate. Fibromodulin was more abundant in the anulus fibrosus than in nucleus pulposus at all ages, whereas lumican was much more abundant in nucleus pulposus than in anulus fibrosus in the young juvenile; in the adult, however, lumican was present in comparable levels in both tissues. With increasing degrees of degeneration, fibromodulin exhibited an increase in abundance. Conclusions. Growth, aging, and degeneration of the intervertebral disc are associated with changes in the abundance and structure of fibromodulin and lumican, which presumably influence the functional properties of the tissue.