A comparison of caveolae and caveolin-1 to folate receptor alpha in retinaand retinal pigment epithelium

Caveolae are flask-shaped membrane invaginations present in most mammalian cells. They are distinguished by the presence of a striated coat composed o f the protein, caveolin. Caveolae have been implicated in numerous cellular processes, including potocytosis in which caveolae are hypothesized to co- localize with folate receptor alpha and participate in folate uptake. Our l aboratory has recently localized folate receptor alpha to the basolateral s urface of the retinal pigment epithelium (RPE). It is present also in many other cells of the retina. In the present study, we asked whether caveolae were present in the RPE, and if so, whether their pattern of distribution w as similar to folate receptor alpha. We also examined the distribution patt ern of caveolin-1, which can be a marker of caveolae. Extensive electron mi croscopical analysis revealed caveolae associated with endothelial cells. H owever, none were detected in intact or cultured RPE. Laser scanning confoc al microscopical analysis of intact RPE localized caveolin-1 to the apical and basal surfaces, a distribution unlike folate receptor alpha. Western an alysis confirmed the presence of caveolin-1 in cultured RPE cells and laser scanning confocal microscopy localized the protein to the basal plasma mem brane of the RPE, a distribution like that of folate receptor alpha. This d istribution was confirmed by electron microscopic immunolocalization. The l ack of caveolae in the RPE suggests that these structures may not be essent ial for folate internalization in the RPE.