Age-Related Changes of Cystatin C Expression and Polarized Secretion by Retinal Pigment Epithelium: Potential Age-Related Macular Degeneration Links

Abstract

Cystatin C, a potent cysteine proteinase inhibitor, is abundantly secreted by the RPE and may contribute to regulating protein turnover in the Bruch's membrane (BrM). A cystatin C variant associated with increased risk of developing AMD and Alzheimer's disease (AD) 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 BrM mimics (AGEd versus non-AGEd). Monolayer barrier properties were assessed by transepithelial resistance measurements. The relative level of cystatin C protein expression in human RPE in situ was assessed immunohistochemically in relation to age. Advanced glycation end product-exposed RPE monolayers presented significantly decreased cystatin C expression and secretion. Basolateral secretion was fully established by week 8 in non-AGEd conditions. In AGEd cultures, polarity of secretion was impaired despite maintenance of physiological barrier properties of the monolayer. In the macula region of RPE/choroid segments from human eyes, the level of cystatin C protein was reduced with increasing donor age. 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.