Distribution of anionic sites on the surface of retinal pigment epithelial and rod photoreceptor cells.

The distribution of anionic (negatively charged) groups on the surface of retinal pigment epithelial cells (RPE), rod photoreceptor cells (RP) and discarded rod outer segments (R0S) was studied by labeling these groups with cationic ferritin (CF) at different pH values. In normal neonatal rats, characteristic CF binding patterns were observed on the RPE cell surface at pH values ranging from 1.8-7.2. Similar distributions were seen on the surface of RPE cells from RCS-p+ rats with inherited retinal degeneration. Treatment of RPE cells from normal neonates with several different degradative enzyme preparations prior to CF labeling, did not alter the density or distribution of CF molecules at any of the pH values tested. In normal adult rats, CF binding to RPE cells was essentially similar to that observed in neonates. In RP cells, heavy CF labeling at pH 1.8 was seen on the surface of the inner and outer segments and connecting cilium. CF labeling patterns on discarded R0S undergoing phagocytosis were similar to those on RPE cells at all pH values except pH 5.5. The findings indicate that anionic sites are present on the surface of RPE and RP cells and on discarded R0S undergoing phagocytosis. There is no evidence that such sites are involved in the phagocytic defect characteristic of RCS-p+ rats. Adult-like patterns of anionic sites on RPE cells appear early in postnatal development; their resistance to a wide range of degradative enzymes suggests that the carbohydrates or proteins containing these anionic moieties are organized in a complex fashion in the RPE cell surface.