Topographical variation in the catabolism of aggrecan in an ovine annular lesion model of experimental disc degeneration.

An established model of experimental disc degeneration (Osti et al., Spine 15:762, 1990; Melrose et al., J Orthop Res 10:655, 1992) was used in this study. Four 2-year-old sheep received anterolateral incisions (4 x 10 mm) in the outer one-third of the annulus fibrosus of their L2-L3 and L4-L5 discs (lesion group). The annulus was not incised in another four sham-operated animals. After 6 months the sheep were killed, lumbar discs were dissected into lateral halves of the annulus fibrosus and the nucleus pulposus. Cells were isolated from disc tissues enzymatically and were grown in alginate bead culture to examine the proteoglycan metabolism of cells from lesion and control zones. The media of lesion zone cultures contained relatively high levels (compared with sham cultures) of catabolic fragments of the large, high-buoyant-density proteoglycans as demonstrated by Western blotting using monoclonal antibodies (5-D-4, 3-B-3, 1-C-6) and biotinylated hyaluronan and also by gel chromatography. Furthermore, cells from the vicinity of the lesion site also synthesized significantly lower levels (compared with sham cultures) of aggrecan that was retained within the alginate beads. Collectively, these data indicated that focal depletion of large, high-buoyant-density proteoglycans was evident within lesion sites in this model of experimental disc degeneration. The introduction of an annular lesion therefore significantly affected the proteoglycan metabolism of endogenous disc cell populations. The unique hydrodynamic and viscoelastic properties of the intervertebral disc are dependent to a large degree on the tissue levels of aggrecan. The focal depletion of aggrecan by annular lesions therefore may represent an important predisposing factor to the subsequent degeneration of these intervertebral discs.