Matrix metalloproteinase-12 immunolocalization in the degenerating human intervertebral disc and sand rat spine: Biologic implications.

Matrix metalloproteinase-12 (MMP-12; macrophage metalloelastase) degrades a number of extracellular matrix components which are present in the intervertebral disc, including type IV collagen, fibronectin, laminin, chondroitin sulfates, elastin and fibrinogen. MMP-12 has recently discovered relationships with cytokines and chemokines which also relate to disc cell biology. To date, no study has assessed immunolocalization of MMP-12 in degenerating human intervertebral disc tissue. Immunocytochemical localization was performed on 18 human disc specimens and on lumbar spines of the sand rat, a small animal model with well-recognized age-related disc degeneration. In the human disc, intracellular localization was present in both the annulus and nucleus portions of the disc. The sand rat degenerating disc also showed MMP-12 disc localization, with additional presence in chondrocytes of the vertebral endplate of older animals. This is the initial characterization of the presence of MMP-12 in the human and sand rat disc, and in chondrocytes of the vertebral endplate in older sand rats with degenerating discs. Findings are important because they document the presence of an additional MMP-12 in disc tissue, thus expanding our understanding of disc extracellular matrix remodeling, and because they provide novel information on the presence of MMP-12 in the cartilage endplate as it undergoes sclerosis during disc degeneration in the aging sand rat.