Retinal pigment epithelial cells play a central role in the conservation of docosahexaenoic acid by photoreceptor cells after shedding and phagocytosis

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Abstract
The involvement of retinal pigment epithelial (RPE) cells in the recycling of docosahexaenoic acid (DHA), from phagocytized disc membranes back to the retina, was studied in frogs subsequent to injection of [5H]DHA via the dorsal lymph sac. Rod outer segments (ROS) gradually accumulated [3H]DHA as a dense, heavily labeled region that arrived at the distal tips by 28 days post-injection. Autoradiographic analysis at the time of maximal shedding and phagocytosis (1–2 hr after the onset of light) showed diffusely (before 28 days) and heavily (after 28 days) labeled phagosomes in RPE cells. Biochemical analysis of the [3H]DHA-containing lipids of discs that contribute to the labeling of RPE cells after phagocytosis was also performed. Between 27 and 34 days, when 12% of retinal [3H]DHA-lipids present in disc membranes are phagocytized by RPE cells, total retinal labeling remained unchanged. Taken together, these data suggest that the [3H]DHA of the densely labeled region of the ROS was recycled back to the photoreceptor cells only after it had reached the RPE cells following 28 days post-injection. We conclude that, following daily phagocytosis of ROS tips, RPE cells play a central role in the conservation and redelivery of ROS-derived DHA back to photoreceptor cells through the interphotoreceptor matrix.