Targeted effects of retinoic acid signaling upon photoreceptor development in zebrafish

Chromatic organization of cone photoreceptors in the retina of rainbow trout: single cones irreversibly switch from UV (SWS1) to blue (SWS2) light sensitive opsin during natural development

The retinas of salmonid fishes have single and double cones arranged in square to row formations termed mosaics. The square mosaic unit is formed by four double cones that make the sides of the square with a single (centre)cone in the middle, and a single (corner) cone at each corner of the square when present. Previous research using coho salmon-derived riboprobes on four species of anadromous Pacific salmon has shown that all single cones express a SWS1 (UV sensitive) visual pigment protein (opsin) at hatching, and that these cones switch to a SWS2 (blue light sensitive) opsin during the juvenile period. Whether this opsin switch applies to non-anadromous species, like the rainbow trout, is under debate as species-specific riboprobes have not been used to study opsin expression during development of a trout. As well, a postulated recovery of SWS1 opsin expression in the retina of adult rainbow trout, perhaps via a reverse process to that occurring in the juvenile, has not been investigated. Here, we used in situhybridization with species-specific riboprobes and microspectrophotometry on rainbow trout retina to show that: (1) single cones in the juvenile switch opsin expression from SWS1 to SWS2, (2) this switch is not reversed in the adult, i.e. all single cones in the main retina continue to express SWS2 opsin, and (3) opsin switches do not occur in double cones: each member expresses one opsin, maximally sensitive to green (RH2) or red (LWS) light. The opsin switch in the single cones of salmonid fishes may be a general process of chromatic organization that occurs during retinal development of most vertebrates.

Mechanisms for persistent microphthalmia following ethanol exposure during retinal neurogenesis in zebrafish embryos

The exposure of the developing human embryo to ethanol results in a spectrum of disorders involving multiple organ systems, including the visual system. One common phenotype seen in humans exposed to ethanol in utero is microphthalmia. The objective of this study was to describe the effects of ethanol during retinal neurogenesis in a model organism, the zebrafish, and to pursue the potential mechanisms by which ethanol causes microphthalmia. Zebrafish embryos were exposed to 1% or 1.5% ethanol from 24 to 48 h after fertilization, a period during which the retinal neuroepithelium undergoes rapid proliferation and differentiation to form a laminated structure composed of different retinal cell types. Ethanol exposure resulted in significantly reduced eye size immediately following the treatment, and this microphthalmia persisted through larval development. This reduced eye size could not entirely be accounted for by the accompanying general delay in embryonic development. Retinal cell death was only slightly higher in ethanol-exposed embryos, although cell death in the lens was extensive in some of these embryos, and lenses were significantly reduced in size as compared to those of control embryos. The initiation of retinal neurogenesis was not affected, but the subsequent waves of cell differentiation were markedly reduced. Even cells that were likely generated after ethanol exposure--rod and cone photoreceptors and Müller glia--were delayed in their expression of cell-specific markers by at least 24 h. We conclude that ethanol exposure over the time of retinal neurogenesis resulted in persistent microphthalmia due to a combination of an overall developmental delay, lens abnormalities, and reduced retinal cell differentiation.

Pbx homeodomain proteins pattern both the zebrafish retina and tectum

BACKGROUND

Pbx genes encode TALE class homeodomain transcription factors that pattern the developing neural tube, pancreas, and blood. Within the hindbrain, Pbx cooperates with Hox proteins to regulate rhombomere segment identity. Pbx cooperates with Eng to regulate midbrain-hindbrain boundary maintenance, and with MyoD to control fast muscle cell differentiation. Although previous results have demonstrated that Pbx is required for proper eye size, functions in regulating retinal cell identity and patterning have not yet been examined.

RESULTS

Analysis of retinal ganglion cell axon pathfinding and outgrowth in pbx2/4 null embryos demonstrated a key role for pbx genes in regulating neural cell behavior. To identify Pbx-dependent genes involved in regulating retino-tectal pathfinding, we conducted a microarray screen for Pbx-dependent transcripts in zebrafish, and detected genes that are specifically expressed in the eye and tectum. A subset of Pbx-dependent retinal transcripts delineate specific domains in the dorso-temporal lobe of the developing retina. Furthermore, we determined that some Pbx-dependent transcripts also require Meis1 and Gdf6a function. Since gdf6a expression is also dependent on Pbx, we propose a model in which Pbx proteins regulate expression of the growth factor gdf6a, which in turn regulates patterning of the dorso-temporal lobe of the retina. This, in concert with aberrant tectal patterning in pbx2/4 null embryos, may lead to the observed defects in RGC outgrowth.

CONCLUSION

These data define a novel role for Pbx in patterning the vertebrate retina and tectum in a manner required for proper retinal ganglion cell axon outgrowth.

Neurogenesis in the fish retina

The retinas of teleost fish have long been of interest to developmental neurobiologists for their persistent plasticity during growth, life history changes, and response to injury. Because the vertebrate retina is a highly conserved tissue, the study of persistent plasticity in teleosts has provided insights into mechanisms for postembryonic retinal neurogenesis in mammals. In addition, in the past 10 years there has been an explosion in the use of teleost fish-zebrafish (Danio rerio) in particular-to understand the mechanisms of embryonic retinal neurogenesis in a model vertebrate with genetic resources. This review summarizes the key features of teleost retinal neurogenesis that make it a productive and interesting experimental system, and focuses on the contributions to our knowledge of retinal neurogenesis that uniquely required or significantly benefited from the use of a fish model system.