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Gabriel H. Travis, Christian Sanfilippo, C. Nathaniel Roybal; Visual Chromophore in Rhodopsin Re-enters the Visual Cycle Following Phagocytosis of Outer Segments by RPE Cells. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3357.
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Cells of the retinal pigment epithelium (RPE) take up all-trans-retinol (all-trans-ROL) released by bleached photoreceptors and convert it to 11-cis-retinaldehdye (11-cis-RAL) chromophore via a multi-step enzyme pathway called the visual cycle. 11-cis-RAL released by RPE cells is taken up by photoreceptors for regenerating rhodopsin and cone-opsin pigments. RPE cells phagocytose the distal 10% of photoreceptor outer segments (OS) diurnally. Thus, approximately 10% of the total ocular retinoid content is taken up daily by RPE cells in the form of rhodopsin. The fate of this apically ingested 11-cis-RAL is unknown, and the subject of the current study.
Bovine OS were isolated, photobleached, and regenerated with [3H] 11-cis-RAL or [3H] 9-cis-RAL. The resulting rhodopsin or iso-rhodopsin was immuno-affinity purified and reconstituted into proteoliposome ‘synthetic outer segments’ (SOS and iso-SOS). RPE-containing eyecups from 129/Sv wild-type (WT) and rpe65-/- mice were incubated with MFG-E8 and pulse-fed SOS or iso-SOS. Eyecups and incubation media were then analyzed for retinoid content by HPLC.
WT eyecups showed increasing [3H] labeled all-trans-retinyl palmitate (RP), 11-cis-RP, and 11-cis-RAL after a meal of SOS. 11-cis-RAL also increased in the medium during the incubation. In contrast, rpe65-/- mice, which lack the retinoid isomerase in RPE cells, showed increasing [3H] all-trans-ROL and all-trans-RP with no increase in 11-cis-RAL. After a meal of iso-SOS, WT eyecups accumulated all-trans-RP and secreted 11-cis-RAL into the medium. No accumulation of 9-cis-retinoids was seen in WT eyecups after a meal of iso-SOS.
These results suggest that following phagocytosis of rhodopsin-containing OS, the constituent 11-cis-RAL is thermally isomerized to all-trans-RAL, which is then reduced and esterified to form all-trans-RP, and is finally isomerized and oxidized back to 11-cis-RAL by enzymes of the visual cycle in RPE cells. This process represents a previously uncharacterized retinoid-recycling pathway in the RPE.
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