Peropsin Effects Light-dependent Modulation of Retinyl Esters in the RPE

Abstract

Purpose : Peropsin (Rrh) is a non-visual opsin of unknown function expressed solely in the apical microvilli of the retinal pigment epithelium (RPE). The aim of the current study is to understand the function of peropsin. Its unique expression pattern suggests that peropsin conveys the illumination status to RPE cells, possibly to affect light-dependent regulation of visual-retinoid metabolism.

Methods : In vivo studies were performed on dark-adapted mice homozygous for a null mutation in the Rrh gene (Rrh-/-) compared to age-matched 129/Sv wild-type (WT) mice. Retinoid dynamics experiments were performed on overnight dark-adapted mice. Subsets were subjected to photobleach followed by incremental amounts of dark recovery. Retinoids were analyzed from RPE and retina separately. Retinoid uptake was measured in whole eyecups incubated with IRBP-bound [³H]-retinol in dark or in light conditions. LRAT and retinyl-ester hydrolase (REH) activity were measured using RPE homogenates and all-trans-retinol or all-trans-retinyl acetate as substrates, respectively. Retina sections were studied by light and fluorescence microscopy.

Results : Retinas from Rrh-/- mice had normal morphology, with no evidence of degeneration. All-trans-retinyl ester levels were several-fold lower in RPE from Rrh-/- versus WT mice under dark-adapted and immediate post-bleach conditions. These retinoids returned to WT levels after five minutes of recovery in the dark. Possible explanations for this biochemical phenotype include (i) impaired uptake of retinol into RPE cells in the dark; (ii) impaired synthesis of retinyl esters in the RPE; or (iii) accelerated hydrolysis of retinyl esters in the dark. To test these possibilities, we measured uptake of [³H] retinol into dark- and light-exposed eyecups. We observed no difference between WT and Rrh-/- in either [³H] retinol uptake or [³H] retinyl-ester synthesis. In vitro ester synthesis was decreased in homogenate prepared from Rrh-/- mice. Finally, we assayed for differences in REH activity in eye cup homogenates, and we observed a moderate increase in REH activity in Rrh-/- samples.

Conclusions : Loss of peropsin in dark-adapted Rrh-/- mice causes accelerated hydrolysis of retinyl esters by the RPE. Therefore, peropsin appears to inhibit the hydrolysis of retinyl esters in the dark. Peropsin may act to sequester substrate for Rpe65-isomerase and hence the RPE visual cycle under dim light conditions.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.