Reprogramming retinal pigment epithelium to differentiate toward retinal neurons with Sox2

Guiding non-neural, retinal pigment epithelium (RPE) to produce retinal neurons may offer a source of developing neurons for cell-replacement. Sox2 plays important roles in maintaining neural progenitor/stem cell properties and in converting fibroblasts into pluripotent stem cells. This study tests the possibility of using Sox2 to reprogram RPE to differentiate toward retinal neurons in vivo and in vitro. Expression of Sox2 in the chick retina was detected in progenitor cells, in cells at a discrete location in the layers of amacrine and ganglion cells, and in Muller glia. Overexpression of Sox2 in the developing eye resulted in hypopigmentation of the RPE. In the affected regions, expression of retinal ganglion cell markers became apparent in the RPE layer. In RPE cell culture, Sox2 promoted the expression of retinal ganglion and amacrine markers, and suppressed the expression of genes associated with RPE properties. Mechanistic investigation using the developing retina revealed a coexpression of Sox2 and basic fibroblast growth factor (bFGF), a growth factor commonly used in stem cell culture and capable of inducing RPE-to-retina transdifferentiation (or reprogramming) during early development. Similar patterns of changes in Sox2 expression and in bFGF expression were observed in atrophic retina and in injured retina. In RPE cell culture, Sox2 and bFGF mutually enhanced one another's expression. Upregulation of bFGF expression by Sox2 also occurred in the retina. These results suggest that Sox2 can initiate a reprogramming of RPE cells to differentiate toward retinal neurons and may engage bFGF during the process.