Phenotypic heterogeneity of retinal pigment epithelial cells in vitro and in situ

We have previously developed methods based upon differential cell adhesion to select for cells of two different phenotypes, epithelioid and fusiform, from cultures of human RPE. Here we considered whether the differences in cell shape correlated with differences in protein tyrosine phosphorylation since it is known that elevated phosphorylation perturbs the stability of the adherens junction, a major determinant of epithelial phenotype. In cultures of both phenotypes we found low tyrosine phosphorylation levels in postconfluent cultures, and the same complement of tyrosine phosphoproteins after treatment with a phosphatase inhibitor. However, in contrast to epithelioid cells, fusiform RPE failed to localize the phosphoproteins to junctional sites on the cell periphery. We also re-evaluated primary and passaged RPE cultures for additional cell shape variants. Several discrete phenotypes were identified within the same cultures. They required several weeks at confluency to develop in primary as well as in passaged cultures, but after developing they remained stable for months. Since explanted RPE cells manifest several shape variants in an identical culture environment we examined bovine RPE cells in situ to determine whether the cells were heterogeneous with regards to some cell surface and cytoskeletal proteins that might contribute to cell shape. Circumferential actin microfilament bundles and the occluding junction protein ZO-1 had fairly uniform distributions among cells in the monolayer, but the intermediate filament protein vimentin and the pericellular expression of phosphotyrosine varied among individual cells. Therefore, despite its grossly homogeneous appearance, the RPE monolayer in situ might be considered a mosaic of structurally heterogeneous cells which can give rise to phenotypically-distinct subpopulations when propagated in vitro.