Elevated amyloid β production in senescent retinal pigment epithelium, a possible mechanism of subretinal deposition of amyloid β in age-related macular degeneration

Age-related macular degeneration (AMD) is the most common cause of legal blindness in the elderly individuals in developed countries. Subretinally-deposited amyloid beta (A beta) is a main contributor of developing AMD. However, the mechanism causing A beta deposition in AMD eyes is unknown. Aging is the most significant risk of AMD, thus, we examined the effect of aging on subretinal A beta deposition. mRNAs and cell lysates were isolated from retinal pigment epithelial (RPE) cells derived from 24-month-old (24M RPE) and 2-month-old (2M RPE) C57BL/6 mice. A beta concentration in culture supernatants was measured by ELISA. Activity and expression of proteins that regulate A beta level were examined by activity assay and real time PCR. Effect of beta-secretase (BACE) on A beta production was examined by siRNA silencing. A beta amounts in supernatants of 24M RPE were significantly higher than 2M RPE. Activity and mRNA levels of neprilysin, an A beta degrading enzyme, were significantly decreased in 24M RPE compared to 2M RPE. PCR analysis found that BACE2 was significantly more abundantly expressed than BACE1 in RPE cells, however, inactivation of BACE2 gene did not affect A beta production. BACE1 protein amounts did not differ between 24M and 2M RPE, however, BACE1 activity was significantly higher in 24M RPE compared to 2M RPE. There were no significant changes in the activities of alpha- or gamma-secretase between 2M and 24M RPE. In conclusion, RPE cells produce more amounts of A beta when they are senescent, and this is probably caused by a decrease in A beta degradation due to a reduction in the expression and activity of neprilysin and an increase in A beta synthesis due to increased activity of BACE1. (C) 2012 Elsevier Inc. All rights reserved.