Expression of the cell surface antigens RET-PE2 and N-CAM by rat retinal pigment epithelial cells during development and in tissue culture

Invagination of the optic vesicle to form the optic cup results in the formation of two apposed layers of neuroepithelium which follow divergent developmental pathways. Changes in the expression of cell surface molecules may be either the cause or result of important inductive signals during this process. We have used immunological reagents to study the expression of two molecules in the rat: the neural cell adhesion molecule N-CAM, and a cell membrane-associated protein which is specific for pigment epithelium in the adult, RET-PE2. Both N-CAM and RET-PE2 are present in both layers of the optic cup at embryonic age E13, but they become restricted to inner retina and pigment epithelium, respectively, by E17 and maintain that pattern of expression in the normal adult. Culture of pigment epithelial cells results in the reexpression of N-CAM and the continued expression of RET-PE2. Western blotting reveals that the size and relative proportions of the 180- and 140-kDa N-CAM molecules synthesized by rat pigment epithelial cells in vitro differ from that made by rat brain, retina and liver. Embryonic RPE N-CAM contains the sulfated carbohydrate recognized by the HNK-1 antibody, but this epitope was not present on N-CAM synthesized by cultured RPE cells. The reexpression of an embryonic antigen when placed in culture suggests that pigment epithelial cells retain some degree of plasticity in the adult state.