Age-dependent variation in metalloproteinase activity of isolated human Bruch's membrane and choroid

To characterize and determine the effect of aging on the matrix metalloproteinase (MMP) component of the extracellular matrix-remodeling mechanism of isolated human Bruch's-choroid. Immunohistochemical techniques and western blot analysis were used to detect and localize various members of the MMP family of proteolytic enzymes in the Bruch's-choroid complex. Gelatin substrate zymography was used to detect and quantify the levels of MMP-2 and -9 in homogenates of Bruch's-choroid from both macular and peripheral regions of the human fundus. Aging alterations in these enzymes were quantified by densitometric analysis of photographic negatives of the zymography gels. Intact preparations of Bruch's-choroid showed the presence of inactive forms of two gelatinases (MMP-2, 65 kDa, and MMP-9, 92 kDa), interstitial collagenase (MMP-1, 52 kDa) and stromelysin (MMP-3, 57 kDa). MMP-1 and -3 were localized primarily to Bruch's membrane. MMP-9 was distributed evenly in Bruch's membrane with some patchy presence in the choroidal mass. Distribution of MMP-2 was similar to that of MMP-9, but the staining in Bruch's was much fainter. On gelatin zymography, an active form of MMP-2 (58-kDa species) was frequently observed in peripheral samples but only occasionally in macular regions. The levels of MMP-2 and -9 increased with aging in both the macular and the peripheral regions of the fundus (P < 0.05). MMP-2 levels were lower in macular regions than in the periphery but no such variation was observed with MMP-9. Both these inactive gelatinases could be activated in vitro. A matrix-degrading mechanism essential for extracellular remodeling was shown to be present in Bruch's membrane. In macular regions, increasing levels of inactive forms of metalloproteinase and scarcity of active forms of MMP-2 suggests possible involvement of impaired extracellular degradation in both aging and macular degeneration.