Expression of mannose receptors for pinocytosis and phagocytosis on rat retinal pigment epithelium.

We report here the presence of a mannose-specific receptor on apical membranes of rat retinal pigment epithelial (RPE) cells. For pinocytic studies, 125I-Mannose-BSA (125I-Man-BSA) was incubated with RPE explants from normal (Long Evans) and dystrophic (pigmented RCS) rat retinas. Normal RPE bound 36.1 ng of ligand and, in the presence of mannan competitor, the amount bound was 18.3 ng. In a similar assay, total ligand uptake by dystrophic RPE was 25.9 ng with 9.8 ng specific for mannose recognition. Comparing the amounts of ligand bound, dystrophic RPE recognized 55% of the amount recognized by normal RPE. The presence of the mannose receptor was localized on both normal and dystrophic RPE apical plasma membranes by autoradiographic techniques using 125I-Man-BSA. Normal RPE showed a greater number of silver grains present at the apical cell membrane as compared to dystrophic RPE. Silver grains were significantly reduced when incubation with the ligand was carried out in the presence of a mannan competitor. Further, in phagocytic studies, latex beads coated with mannan were used as phagocytic particles. Normal RPE phagocytized 4.52 mannan-beads per cell profile by a mannose-specific mechanism, whereas dystrophic RPE did not recognize mannan-beads. Our data suggest that RPE cells express surface receptors which recognize both soluble and particulate mannose ligands. The pinocytic and autoradiographic studies suggest that normal RPE binds more soluble ligand than does dystrophic RPE. If the mannose receptors mediate both pinocytosis and phagocytosis, a possible reduction in number of soluble mannose binding sites on the dystrophic RPE may be related to the diminished phagocytic recognition of particulate ligand by the dystrophic RPE.(ABSTRACT TRUNCATED AT 250 WORDS)