The retinal pigment epithelium (RPE) is developmentally and anatomically close to the neural retina. Unlike retinal neurons, RPE cells are nonneural and can reenter the cell cycle on stimulation. Furthermore, their progenies may differentiate into cell types other than RPE.
13 14 15 16 17 18 Classical experiments show that embryonic chick RPE at early developmental stages can be triggered to transdifferentiate into a neural retina.
19 This RPE-to-retina transdifferentiation occurs in vivo and in vitro under the induction of fibroblast growth factors.
20 21 22 23 24 Making use of these properties of RPE, we have recently begun to explore the possibility of using RPE as a potential source of retinal neurons. Chick RPE cells are dissociated, cultured, and infected by retrovirus RCAS expressing a proneural gene. Later, the culture is analyzed for de novo expression of neural properties. We found that cultured, dissociated RPE cells undergo transdifferentiation toward various types of retinal neurons under the induction of different proneural bHLH genes,
25 26 27 28 including
neuroD, which encodes a transcriptional factor in the basic helix-loop-helix (bHLH) family and is homologous to
Drosophila proneural gene
atonal. In the retina,
neuroD is mostly expressed in photoreceptor cells and rod progenitors of teleost fish, with some expression in amacrine cells.
25 29 30 Gain- and loss-of-function studies in the mouse and the chick show that
neuroD plays an instrumental role in photoreceptor development
25 31 and is required specifically for photoreceptor survival.
29 When ectopically expressed,
neuroD induces RPE cells to transdifferentiate selectively toward a photoreceptor phenotype. In culture, the transdifferentiation is substantial, judging from the presence of inner segment–like processes and the expression of an array of photoreceptor genes.
25 32 Nevertheless, it is unknown how these cells would behave and to what extent they would differentiate along the photoreceptor pathway once they encounter the three-dimensional milieu (or niche) offered by the local environment of the eye. As an initial step to address this question, we grafted transdifferentiating cells into the developing chick eye. We found that transdifferentiating cells continued their transdifferentiation along the photoreceptor pathway in the subretinal space, the choroid, and the vitreous. Some of the grafted cells integrated into the host ONL and emanated axons with elaborate arborization in the OPL. We also found that RPE cells exhibited remarkable self-organization activities; not only were grafted cells incorporated into the host tissue, multilayered RPE-like tissues were formed from host cells and grafted cells. The self-organization (or regeneration) and the transdifferentiation properties of RPE cells render them unique for studies of photoreceptor rescue and replacement.