bFGF-induced transdifferentiation of RPE to neuronal progenitors is regulated by the mechanical properties of the substratum.

The N-CAM-negative retinal pigment epithelium (RPE[N-CAM-]) can transdifferentiate into N-CAM-positive neural retina (NR[N-CAM+]) when stimulated by basic fibroblast growth factor (bFGF). In this report we examine whether the properties of the growth substrate affect the fate determination of the presumptive RPE. We used a system of biochemically related substrata with different mechanical properties, that is hydrated basement membrane gels (BM gels) and carpets of immobilized basement membrane proteins (BM carpets). We examined the effects of bFGF on RPE grown on either BM gels or BM carpets and compared them with the effects of bFGF on RPE cells grown on their native basement membrane. We show that bFGF provides the stimulus necessary to redirect the choice of fate of the presumptive RPE[N-CAM-] from the RPE pathway into the neural pathway. However, the mechanical properties of the substratum determine the extent to which a neural phenotype is expressed by the transdifferentiating cells. RPE[N-CAM-] transdifferentiates into a pleomorphic neuroepithelium[N-CAM+] on rigid, two-dimensional BM carpets, into a pseudostratified neuroepithelium[N-CAM+] on highly malleable BM gels, and into a stratified, NR[N-CAM+]-like neuroepithelium on its native basement membrane, which is of intermediate rigidity. The newly formed NR, except for the inverted polarity, has a morphology corresponding to, and expresses markers in a distribution appropriate for, the equivalent stage of retinal histodifferentiation in the embryo. We also show that bFGF is not a mitogen for the presumptive RPE cells, while it is a potent one for the presumptive NR.