Sonic hedgehog is involved in eye field separation along the proximodistal axis. We show that Hh signalling continues to be important in defining aspects of the proximodistal axis as the optic vesicle and optic cup mature. We show that two other Hedgehog proteins, Banded hedgehog and Cephalic hedgehog, related to the mouse Indian hedgehog and Desert hedgehog, respectively, are strongly expressed in the central retinal pigment epithelium but excluded from the peripheral pigment epithelium surrounding the ciliary marginal zone. By contrast, downstream components of the Hedgehog signalling pathway, Gli2, Gli3 and X-Smoothened, are expressed in this narrow peripheral epithelium. We show that this zone contains cells that are in the proliferative state. This equivalent region in the adult mammalian eye, the pigmented ciliary epithelium, has been identified as a zone in which retinal stem cells reside. These data, combined with double labelling and the use of other retinal pigment epithelium markers, show that the retinal pigment epithelium of tadpole embryos has a molecularly distinct peripheral to central axis. In addition, Gli2, Gli3 and X-Smoothened are also expressed in the neural retina, in the most peripheral region of the ciliary marginal zone, where retinal stem cells are found in Xenopus, suggesting that they are good markers for retinal stem cells. To test the role of the Hedgehog pathway at different stages of retinogenesis, we activated the pathway by injecting a dominant-negative form of PKA or blocking it by treating embryos with cyclopamine. Embryos injected or treated at early stages display clear proximodistal defects in the retina. Interestingly, the main phenotype of embryos treated with cyclopamine at late stages is a severe defect in RPE differentiation. This study thus provides new insights into the role of Hedgehog signalling in the formation of the proximodistal axis of the eye and the differentiation of retinal pigment epithelium.

Hedgehog signaling has been linked to cell proliferation in a variety of systems; however, its effects on the cell cycle have not been closely studied. In the vertebrate retina, Hedgehog's effects are controversial, with some reports emphasizing increased proliferation and others pointing to a role in cell cycle exit. Here we demonstrate a novel role for Hedgehog signaling in speeding up the cell cycle in the developing retina by reducing the length of G1 and G2 phases. These fast cycling cells tend to exit the cell cycle early. Conversely, retinal progenitors with blocked Hedgehog signaling cycle more slowly, with longer G1 and G2 phases, and remain in the cell cycle longer. Hedgehog may modulate cell cycle kinetics through activation of the key cell cycle activators cyclin D1, cyclin A2, cyclin B1, and cdc25C. These findings support a role for Hedgehog in regulating the conversion from slow cycling stem cells to fast cycling transient amplifying progenitors that are closer to cell cycle exit.[Keywords: Hedgehog; retinal stem cells; Xenopus; zebrafish; cell cycle kinetics; cell cycle exit; cyclin; Cdc25C] Supplemental material is available at http://www.genesdev.org.

The vertebrate retina has been widely used as a model to study the development of the central nervous system. Its accessibility and relatively simple organization allow analysis of basic mechanisms such as cell proliferation, differentiation and death. For this reason, it could represent an ideal place to solve the puzzle of Hh signaling during neural development. However, the extensive wealth of data, sometimes apparently discordant, has made the retina one of the most complicated models for studying the role of the Hh cascade. Given the complexity of the field, a deep analysis of the data arising from different animal models is essential. In this review, we will compare and discuss all reported roles of Hh signaling in eye development to shed light on its multiple functions.

During the development of the nervous system, after a given number of divisions, progenitors exit the cell cycle and differentiate as neurons or glial cells. Some cells however do not obey this general rule and persist in a progenitor state. These cells, called stem cells, have the ability to self-renew and to generate different lineages. Understanding the mechanisms that allow stem cells to "resist" differentiating stimuli is currently one of the most fascinating research areas for biologists. The amphibian and fish retinas, known to contain stem cell populations, have been pioneering models for neural stem cell research. The Xenopus retina enabled the characterization of the genetic processes that occur in the path from a pluripotent stem cell to a committed progenitor to a differentiated neuron. More recently, the discovery that avian and mammalian retinas also contain stem cell populations, has contributed to the definitive view of the adult nervous system of upper vertebrates as a more dynamic and plastic structure than previously thought. This has attracted the attention of clinicians who are attempting to employ stem cells for transplantation into damaged tissue. Research in this area is promising and will represent a key instrument in the fight against blindness and retinal dystrophies. In this review, we will focus primarily on describing the main characteristics of various retinal stem cell populations, highlighting their divergences during evolution, and their potential for retinal cell transplantation. We will also give an overview of the signaling cascades that could modulate their potential and plasticity.

No abstract