Several eye-field transcription factors (EFTFs) are expressed in the anterior region of the vertebrate neural plate and are essential for eye formation. The Xenopus EFTFs ET, Rx1, Pax6, Six3, Lhx2, tll and Optx2 are expressed in a dynamic, overlapping pattern in the presumptive eye field. Expression of an EFTF cocktail with Otx2 is sufficient to induce ectopic eyes outside the nervous system at high frequency. Using both cocktail subsets and functional (inductive) analysis of individual EFTFs, we have revealed a genetic network regulating vertebrate eye field specification. Our results support a model of progressive tissue specification in which neural induction then Otx2-driven neural patterning primes the anterior neural plate for eye field formation. Next, the EFTFs form a self-regulating feedback network that specifies the vertebrate eye field. We find striking similarities and differences to the network of homologous Drosophila genes that specify the eye imaginal disc, a finding that is consistent with the idea of a partial evolutionary conservation of eye formation.
"Noggin has been shown to induce the EFTFs and anterior neural marker, otx2 (Zuber et al., 2003). If SB43+DM treatment is sufficient to mimic Noggin's ability to generate retina, then we would expect that SB43+DM treatment is sufficient to increase EFTF expression in animal caps. "
"The topographic location of the eye field corresponds to an anterior region where BMP, Wnt, and Nodal signals are repressed and IGF and FGF signaling is active (reviewed in Andreazzoli, 2009). The specific combination of these signals allows the concomitant expression of the Eye Field Transcription Factors (EFTFs), which are responsible for determining the features of early retinal progenitors and to trigger the following phases of eye development (Zuber et al., 2003). In fact, co-injection of EFTFs, or their activator Noggin, is sufficient to induce a retinal fate in pluripotent ectodermal cells, which, following transplantation, are able to generate a complete retina (Viczian et al., 2009; Lan et al., 2009). "
[Show abstract][Hide abstract] ABSTRACT: Background:
The transcription factor Rx1, also known as Rax, controls key properties of retinal precursors including migration behavior, proliferation, and maintenance of multipotency. However, Rx1 effector genes are largely unknown.
To identify genes controlled by Rx1 in early retinal precursors, we compared the transcriptome of Xenopus embryos overexpressing Rx1 to that of embryos in which Rx1 was knocked-down. In particular, we selected 52 genes coherently regulated, i.e., actived in Rx1 gain of function and repressed in Rx1 loss of function experiments, or vice versa. RT-qPCR and in situ hybridization confirmed the trend of regulation predicted by microarray data for the selected genes. Most of the genes upregulated by Rx1 are coexpressed with this transcription factor, while downregulated genes are either not expressed or expressed at very low levels in the early developing retina. Putative direct Rx1 target genes, activated by GR-Rx1 in the absence of protein synthesis, include Ephrin B1 and Sh2d3c, an interactor of ephrinB1 receptor, which represent candidate novel effectors for the migration promoting activity of Rx1.
This study identifies previously undescribed Rx1 regulated genes mainly involved in transcription regulation, cell migration/adhesion, and cell proliferation that contribute to delineate the molecular mechanisms underlying Rx1 activities.
"Rx functions as part of a highly conserved network of transcription factors, collectively referred to as the eye field transcription factors, and includes Pax6, Six3, Optx2, Tlx, Lhx2, and ET
. A small number of zebrafish Rx3 targets have previously been identified by comparing candidate gene expression in rx3-/- mutants and wild-type phenotype siblings around somitogenesis. "
[Show abstract][Hide abstract] ABSTRACT: Background
The genetic cascades underpinning vertebrate early eye morphogenesis are poorly understood. One gene family essential for eye morphogenesis encodes the retinal homeobox (Rx) transcription factors. Mutations in the human retinal homeobox gene (RAX) can lead to gross morphological phenotypes ranging from microphthalmia to anophthalmia. Zebrafish rx3 null mutants produce a similar striking eyeless phenotype with an associated expanded forebrain. Thus, we used zebrafish rx3-/- mutants as a model to uncover an Rx3-regulated gene network during early eye morphogenesis.
Rx3-regulated genes were identified using whole transcriptomic sequencing (RNA-seq) of rx3-/- mutants and morphologically wild-type siblings during optic vesicle morphogenesis. A gene co-expression network was then constructed for the Rx3-regulated genes, identifying gene cross-talk during early eye development. Genes highly connected in the network are hub genes, which tend to exhibit higher expression changes between rx3-/- mutants and normal phenotype siblings. Hub genes down-regulated in rx3-/- mutants encompass homeodomain transcription factors and mediators of retinoid-signaling, both associated with eye development and known human eye disorders. In contrast, genes up-regulated in rx3-/- mutants are centered on Wnt signaling pathways, associated with brain development and disorders. The temporal expression pattern of Rx3-regulated genes was further profiled during early development from maternal stage until visual function is fully mature. Rx3-regulated genes exhibited synchronized expression patterns, and a transition of gene expression during the early segmentation stage when Rx3 was highly expressed. Furthermore, most of these deregulated genes are enriched with multiple RAX-binding motif sequences on the gene promoter.
Here, we assembled a comprehensive model of Rx3-regulated genes during early eye morphogenesis. Rx3 promotes optic vesicle morphogenesis and represses brain development through a highly correlated and modulated network, exhibiting repression of genes mediating Wnt signaling and concomitant enhanced expression of homeodomain transcription factors and retinoid-signaling genes.
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