The effect of swainsonine on the phagocytosis of rod outer segments by rat RPE

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Abstract
From studies using inhibitors such as tunicamycin and castanospermine, it has been suggested that plasma membrane glycoproteins may function as receptors in the phagocytosis of rod outer segments (ROS) by the retinal pigment epithelium (RPE). The exact structure of the oligosaccharide side chain of the glycoprotein may not be critical for this process. We have employed another inhibitor, swainsonine, which inhibits mannosidase II, a terminal enzyme in the protein glycosylation pathway, which results in membrane glycoproteins having hybrid-type oligosaccharide chains and fewer complex oligosaccharide chains. We have examined the ability of cultured rat RPE explants to phagocytize fluorescein isothiocyanate (FTTC) labelled ROS or latex beads in the presence and absence of swainsonine. A significant (p<0.05) reduction in the phagocytosis of FTTC-ROS was found between the swainsonine treated (37.7 ± 4.1%) and untreated (85.4 ± 2.7%) RPE explants. The nonspecific uptake of latex beads in both swainsonine treated (85.3 ± 2.4%) and untreated (89.3 ± 2.0%) RPE explants indicate that the RPE cells retained their ability to phagocytize. Major differences in spectrophotometric analysis of WGA-stained blots were an absence of a peak at 201 kD, a doublet at 86 kD and an overall reduction in all peak absorbances in the swainsonine treatments as compared to the untreated controls. These results suggest that the alterations in RPE glycoprotein formation due to swainsonine alter the ability of RPE to phagocytize ROS.