Zebrafish <italic>Rx</italic>, a paired -class homeobox gene, and retinal development.

Abstract

The <italic>paired</italic>-class homeobox gene, <italic> Rx</italic>, is essential for vertebrate eye development. Targeted disruption of the <italic>Rx</italic> gene completely blocks formation of the optic primordia and produces forebrain abnormalities in mouse embryos while overexpression of <italic>Rx</italic> leads to formation of ectopic retinal tissue and duplication of retina and forebrain in <italic>Xenopus</italic>. In my thesis research I used zebrafish as a model organism to study the function of <italic>Rx</italic> in retinal development. First, I analyzed expression patterns of three zebrafish <italic> Rx</italic> genes (<italic>rx1, rx2, rx3</italic>) in embryos and adults. All three genes show dynamic spatiotemporal patterns of expression and initially their expression domains are coextensive with the region in the anterior neural plate identified as the retinal field by published fate mapping studies. As development proceeds, the expression of <italic>rx1</italic>/<italic>2</italic> is restricted to the neural retina while the expression of <italic>rx3</italic> can also be detected in the hypothalamus. In the differentiated neural retina, <italic> rx1</italic>/<italic>2</italic> are expressed only in the cone photoreceptors and marginal germinal zone whereas <italic>rx3</italic> is expressed in the bipolar cells. Using <italic>rx</italic> as a marker for retinal precursors and <italic>emx1</italic> or <italic>BF-1</italic> as a marker for telencephalic precursors, I demonstrated that the separation of the retinal field and/or the formation of bilateral optic primordia probably require relative cell movements between forebrain and retinal precursors. Overexpression of either <italic> rx</italic>1 or <italic>rx</italic>2 but not <italic>rx3</italic> results in the loss of forebrain tissue and the ectopic formation of retinal tissue. The mechanism involves changes of cell fates based on following observations. Increased cell death is not associated with the loss of forebrain and the ectopic expression of retinal marker is not associated with the increase of mitotic cells. In addition, changes of gene expression patterns are evident in embryos with ectopic <italic>rx2</italic> at neural keel stage. Results from deletion constructs experiments further suggest that <italic>rx2</italic> may require cofactors) to promote retinal development. These data not only reveal the mechanism that may lead to the formation of bilateral optic primordia but also suggest that regulated expression of zebrafish <italic>rx</italic>1 and <italic>rx</italic>2 helps to define the region of the forebrain fated to give rise to retinal tissue.