New concepts for reconstruction of retinal and pigment epithelial tissues

The rise of stem cell-based regenerative medicine has created great hopes for novel therapies for major blinding diseases. Intensive relevant research is grounded on a deep cellular and molecular knowledge of the complex embryonic development of the neural retina and retinal pigmented epithelium (RPE) from the eye vesicle. This research similarly relies on a long history of transdifferentiation studies, having revealed an innate capacity to regenerate a more or less complete retinal tissue from RPE. To analyze principles of self-organization that govern retinal tissue (re-)construction under normal or regenerative conditions on a ‘cell-by-cell’ basis, the reaggregate approach of dispersed embryonic progenitor cells into retinotypic cellular spheres has been instrumental. Based on this knowledge, a multitude of fascinating studies using embryonic, induced pluripotent, adult stem cells, or permanent cell lines from various species have been carried out over the past two decades, and directed production of human retinal and RPE cell types has become possible. Moreover, reconstruction of complete retinal tissue, of functioning RPE monolayers, or even eye-like structures has become feasible. After their implantation into appropriate animal models for blinding diseases, some functional recovery has been observed. Here, we review some historical, cellular and molecular perspectives of this vast research program

Epithelia–Mesenchyme Interaction Plays an Essential Role in Transdifferentiation of Retinal Pigment Epithelium of silver Mutant Quail: Localization of FGF and Related Molecules and Aberrant Migration Pattern of Neural Crest Cells during Eye Rudiment Formation

AbstractHomozygotes of the quail silver mutation, which have plumage color changes, also display a unique phenotype in the eye: during early embryonic development, the retinal pigment epithelium (RPE) spontaneously transdifferentiates into neural retinal tissue. Mitf is considered to be the responsible gene and to function similarly to the mouse microphthalmia mutation, and tissue interaction between RPE and surrounding mesenchymal tissue in organ culture has been shown to be essential for the initiation of the transdifferentiation process in which fibroblast growth factor (FGF) signaling is involved. The immunohistochemical results of the present study show that laminin and heparan sulfate proteoglycan, both acting as cofactors for FGF binding, are localized in the area of transdifferentiation of silver embryos much more abundantly than in wild-type embryos. More intense immunohistochemical staining with FGF-1 antibody, but not with FGF-2 antibody, is also found in the neural retina, RPE, and choroidal tissue of silver embryos than in wild-type embryos. HNK-1 immunohistochemistry revealed that clusters of HNK-1-positive cells (presumptive migrating neural crest cells) are frequently located around the developing eyes and in the posterior region of the silver embryonic eye. Finally, chick–quail chimerical eyes were made by grafting silver quail optic vesicles to chicken host embryos: in most cases, no transdifferentiation occurs in the silver RPE, but in a few cases, transdifferentiation occurs where silver quail cells predominate in the choroid tissue. These observations together with our previous in vitro study indicate that the silver mutation affects not only RPE cells but also cephalic neural crest cells, which migrate to the eye rudiment, and that these crest cells play an essential role in the transdifferentiation of RPE, possibly by modifying the FGF signaling pathway. The precise molecular mechanism involved in RPE–neural crest cell interaction is still unknown, and the quail silver mutation is considered to be a good experimental model for studying the role of neural crest cells in vertebrate eye development

RPE specification in the chick is mediated by surface ectoderm-derived BMP and Wnt signalling.

The retinal pigment epithelium (RPE) is indispensable for vertebrate eye development and vision. In the classical model of optic vesicle patterning, the surface ectoderm produces fibroblast growth factors (FGFs) that specify the neural retina (NR) distally, whereas TGFβ family members released from the proximal mesenchyme are involved in RPE specification. However, we previously proposed that bone morphogenetic proteins (BMPs) released from the surface ectoderm are essential for RPE specification in chick. We now show that the BMP- and Wnt-expressing surface ectoderm is required for RPE specification. We reveal that Wnt signalling from the overlying surface ectoderm is involved in restricting BMP-mediated RPE specification to the dorsal optic vesicle. Wnt2b is expressed in the dorsal surface ectoderm and subsequently in dorsal optic vesicle cells. Activation of Wnt signalling by implanting Wnt3a-soaked beads or inhibiting GSK3β at optic vesicle stages inhibits NR development and converts the entire optic vesicle into RPE. Surface ectoderm removal at early optic vesicle stages or inhibition of Wnt, but not Wnt/β-catenin, signalling prevents pigmentation and downregulates the RPE regulatory gene Mitf. Activation of BMP or Wnt signalling can replace the surface ectoderm to rescue MITF expression and optic cup formation. We provide evidence that BMPs and Wnts cooperate via a GSK3β-dependent but β-catenin-independent pathway at the level of pSmad to ensure RPE specification in dorsal optic vesicle cells. We propose a new dorsoventral model of optic vesicle patterning, whereby initially surface ectoderm-derived Wnt signalling directs dorsal optic vesicle cells to develop into RPE through a stabilising effect of BMP signalling