Biochemistry of the Retina and Pigment Epithelium

Saari, John

Abstract

Light activates the visual pigments in photoreceptor cells by converting the chromophore, 11-cis-retinal, to all-trans-retinal (bleaching). The visual pigment is regenerated in adjacent retinal pigment epithelial (RPE) cells where all-trans-retinal is converted to 11-cis-retinal in a series of chemical reactions. The reactions in the two cells are coupled into the visual cycle by intercellular diffusion of the retinoids. At any given physiologic level of illumination, a steady state is established in which the bleach rate is equal to the regeneration rate. The reactions, enzymes and processes of the visual cycle are as important to vision as those of phototransduction; however, they remain poorly characterized and present models of the cycle do not even include several important components. The long-range goal of this research is to use techniques of biochemistry, molecular biology and cell biology to provide new molecular information about the visual cycle. During the proposed project period, we shall: (1) Examine visual cycle function in mice with a targeted disruption of the CRALBP gene. Preliminary results suggest that these animals regenerate 11-cis-retinal very slowly, relative to a control. Thus, the phenotype of the CRALBP-/-mouse appears to resemble that of humans with a form of congenital stationary night blindness. (2) Explore mechanisms that could account for the apparent modulation of the rate of all-trans-retinal reduction, the rate-limiting step of the mouse visual cycle. One likely possibility, which will be tested, is that production of NADPH determined the rate of reduction of all-trans-retinal. (3) Employ HPLC retinoid analysis to determine whether the rate-determining step of the mouse visual cycle changes as the fractional bleach increases. In addition, the phenotype of mice bearing a targeted disruption of the gene encoding cellular retinal-binding protein (CRBP) will be examined. (4) Determine the subcellular localization in RPE of enzymes and proteins associated with visual cycle activity and examine the mechanism of retinoid secretion from cultured RPE cells. The proposed studies will provide information necessary for an understanding of visual cycle function and will provide a new animal model for human retinal disease (the CRALBLP-/- mouse).