Fibulin–5 and Fibulin–6 (Hemicentin–1) Are Expressed by RPE Cells and Found in Drusen

Abstract

Abstract: : Purpose: Mutations in genes encoding fibulin–5 (Fib–5) and fibulin–6 (Fib–6) have been associated with Age–Related Macular Degeneration, yet little is known about their distribution or function in the eye. The purpose of this study was to examine the expression of these novel extracellular matrix proteins and to begin to investigate their functions. Methods: Expression of Fib–5 and Fib–6 mRNA and protein were assessed in human donor eyes with and without AMD, and in cultured ARPE–19 cells. mRNA levels were measured using real time RT–PCR, and proteins were localized in sections using antibodies to unique peptides and imaging with a confocal microscope. Adhesion of ARPE–19 cells to recombinant Fib–5 was measured, and secretion of mutant Fib–5 proteins was analyzed in 293–T cells. Results: In cultured ARPE–19 cells, and in RPE–choroid tissue samples, Fib–5 and Fib–6 mRNA could be readily detected by quantitative RT–PCR. In some individuals with AMD, significantly higher levels of Fib–5 were detected in samples of RPE–choroid. In tissue sections, Fib–5 protein immunoreactivity was detected in Bruch’s membrane, RPE, choroid, and some drusen. Fib–6 was detected in Bruch’s membrane, choroid, and some drusen. Recombinant wildtype Fib–5 supported adhesion of cultured ARPE–19 cells. Mutations in Fib–5 associated with AMD altered their secretion properties when expressed in 293–T cells. Conclusions: These results show for the first time that Fib–5 and Fib–6 synthesized by RPE cells and are components of Bruch’s membrane and some drusen. Fib–5 may be a novel adhesion molecule for RPE cells, and mutations may alter secretion of Fib–5, which could contribute to AMD.

Keywords: age-related macular degeneration • extracellular matrix • retinal pigment epithelium