High Molecular-Weight Gelatinase Species of Human Bruch's Membrane: Compositional Analyses and Age-Related Changes

Abstract

The structural and functional demise of aging Bruch's membrane is associated with a reduction in the activity of the matrix metalloproteinase (MMP) degradation system. The gelatinase component of the MMP system consists of MMP2 and MMP9 and two high molecular-weight (HMW1, HMW2) species that are yet to be characterized and whose roles in the aging process are yet to be elucidated. The purpose of this study was to determine the age-related changes in levels of expression and subunit characterization of the HMW gelatinase species of Bruch's membrane. Gelatin zymography followed by densitometric scanning was used to quantify the level of the HMW species present. Gel-filtration chromatography allowed the fractionation of the gelatinases according to their molecular weight, and subsequent degradation of the HMW species with a mino-phenyl acetate activation, reduction, and alkylation produced subunit fragments for analysis. Most of the HMW1 and HMW2 pool (80% and 87%, respectively) were tightly bound to the matrix. Aging was associated with significant increases in the levels of HMW1 and HMW2 (P < 0.005 and P < 0.05 respectively). On gel filtration, a single large macromolecular complex (LMMC) was observed containing HMW1, HMW2, MMP9, and some MMP2. Activation-mediated fragmentation of HMW1 and HMW2 showed them to be composed of heteropolymers of MMP2 and MMP9. The age-related increase of HMW1 and HMW2, together with the formation of LMMC, resulted in the sequestration of MMP2 and MMP9, thereby reducing the free pool for activation. This is likely to contribute to reduced matrix degradation and turnover of Bruch's membrane in both normal aging and age-related macular degeneration.