Identifying the mechanism of decorin action in the intervertebral disc through an in vitro decorin over-expression system: Understanding the role of decorin in disc degeneration

The intervertebral disc consists of two tissues: the annulus fibrosus and the nucleus pulposus. During disc degeneration there is a breakdown of the major structural molecules of the disc, collagen and proteoglycan. Conversely, the small proteoglycan, decorin, which may or may not possesses a large glycosaminoglycan chain, exhibits an increase in its protein and mRNA levels, particularly in the annulus fibrosus. Whether the elevated decorin has any effect on the annulus fibrosus cells, however, has yet to be delineated. To better understand the role decorin may play in intervertebral disc homeostasis, decorin (both glycated and non-glycated forms) was over-expressed in bovine annulus fibrosus cells and studied in monolayer cultures. The data presented in this study supports our hypothesis that as the disc degenerates, the increased presence of decorin functions to restore the integrity of the intervertebral disc. We identified that decorin interacts primarily with the epidermal growth factor receptor in the annulus fibrosus cells, activating the MAPK and PI3K signaling pathways (Figure 35), which resulted in an increased expression of the receptor (Figures 37 and 38) and its mRNA (Figure 39). Additionally, decorin over-expression increased mRNA expression of matrix molecules (Figures 40 and 42), decreased mRNA expression of matrix metalloproteinases, MMP-1 and MMP-13 (Figure 44), and enhanced proteoglycan production as measured by a sulfated-glycosaminoglycan dimethylmethylene blue assay (Figures 45 and 46). Furthermore, decorin over-expression resulted in an enhanced migration of cells into an in vitro wound, which was dependent on the glycosaminoglycan chain (Figures 48 and 49). Addition of tumor necrosis factor-alpha, to decorin over-expressing cells resulted in decreased mRNA level for metalloproteinases (MMP-1 and MMP-13), relative to transfected control cells (Figure 50). Furthermore, the mRNA levels for the tumor necrosis factor receptor were lower in both types of decorin over-expressing cells (Figure 56). The conclusion of this study is that increased levels of decorin in the annulus fibrosus cell environment may influence remodeling of the extracellular matrix of the tissue. Ultimately, a better understanding of decorin's effect on the resident cells of the intervertebral disc may lead to novel therapeutic solutions to ailments associated with disc degeneration.