Differential induction of gene expression by basic fibroblast growth factor and neuroD in cultured retinal pigment epithelial cells

Embryonic chick retinal pigment epithelial (RPE) cells can undergo transdifferentiation upon appropriate stimulation. For example, basic fibroblast growth factor (bFGF) induces intact RPE tissue younger than embryonic day 4.5 (E4.5) to transdifferentiate into a neural retina. NeuroD, a gene encoding a basic helix-loop–helix transcription factor, triggers de novo production of cells that resemble young photoreceptor cells morphologically and express general neuron markers (HNK-1/N-CAM and MAP2) and a photoreceptor-specific marker (visinin) from cell cultures of dissociated E6 RPE (Yan & Wang, 1998). The present study examined whether bFGF will lead to the same transdifferentiation phenomenon as neuroD when applied to dissociated, cultured E6 RPE cells, and whether interplay exists between the two factors under the culture conditions. Dissociated E6 RPE cells were cultured in the presence or absence of bFGF, and with or without the addition of retrovirus expressing neuroD. Gene expression was analyzed with immunocytochemistry and in situ hybridization. Unlike neuroD, bFGF did not induce the expression of visinin, or HNK-1/N-CAM and MAP2. However, bFGF elicited the expression of RA4 immunogenicity; yet, many of these RA4-positive cells lacked a neuronal morphology. Addition of bFGF to neuroD-expressing cultures did not alter the number of visinin-expressing cells; misexpression of neuroD in bFGF-treated cultures did not change the number of RA4-positive cells, suggesting the absence of interference or synergistic interaction between the two factors. Our data indicated that bFGF and neuroD induced the expression of different genes in cultured RPE cells.