Microscopic characterization of rat retinal progenitor cells.

A progenitor cell line was developed from a postnatal day 2 (P2) rat retina to study the effects of secreted proteins of the retinal pigment epithelium (RPE) on isolated retinal progenitor cells and markers for immature and differentiated retinal cells. Progenitor cells were cloned from a P2 explant grown in secreted proteins of cultured RPE cells. A cell line was cloned from a single progenitor cell. During the period of RPE-secreted protein stimulation the cells were transformed with the psi AE1A virus. Progenitor cells formed extensive processes when grown in serum and proliferated from the explant when grown in secreted proteins of RPE cells as demonstrated by bromodeoxyuridine (BrdU). All progenitor cells at early and late passages including a cloned cell line (D4) expressed Pax6, a transcription factor essential for eye development, which was verified by Western blotting. All cells expressed nestin, an early neuroepithelial cell marker. These two traits showed the immature character of these rat retinal progenitor cells. All cells expressed the intermediate filament protein vimentin, an intermediate filament protein. Interestingly, most progenitor cells grown in serum expressed the mature cell markers opsin, but few cells expressed glial fibrillary acidic protein (GFAP). The progenitor cells responded to proteins secreted by cultured RPE cells by forming large clusters, while cells grown in retinoic acid formed long thin processes that extended from a round cell body. These progenitor cells, following treatment with secreted proteins of the RPE, will be tested for their therapeutic effect in diseased rat retinas.