Potential Links of Age-Related Changes of Cystatin C Expression and Polarized Secretion by RPE to AMD
Cystatin C, a potent cysteine proteinase inhibitor, is abundantly secreted by the retinal pigment epithelium (RPE) and may contribute to regulating protein turnover in the Bruch's membrane. A variant associated with increased risk of developing age-related macular degeneration (AMD) and Alzheimer's disease presents reduced secretion levels from RPE. The purpose of this study was to analyze the effects of age and the accumulation of advanced glycation end-products (AGEs) on the expression and secretion of cystatin C by the RPE.
Confluent monolayers of human fetal RPE (hfRPE) cells were cultured using an in vitro
model mimicking extracellular AGE accumulation. Cystatin C expression, secretion and its polarity were analyzed following culture on AGE-containing Bruch's membrane mimics (AGEd vs non-AGEd). Additionally, monolayer barrier properties were assessed by transepithelial resistance measurements and the relative level of cystatin C protein expression in human RPE in situ
was assessed immunohistochemically in relation to age.
It was reported that AGE-exposed RPE monolayers presented significantly decreased cystatin C expression and secretion. Basolateral secretion was fully established by week eight in non-AGEd conditions, and in AGEd cultures, polarity of secretion was impaired despite maintenance of physiological barrier properties of the monolayer. Furthermore, in the macula region of RPE/choroid segments from human eyes, the level of cystatin C protein was reduced with increasing donor age.
It was determined that exposure to AGEs reduces expression of cystatin C and affects its normal secretion in cultured RPE. Age-related changes of cystatin C in the RPE from the posterior pole may compromise its extracellular functions, potentially contributing to AMD pathogenesis.