| Description |
For our eyes to see, they must establish topographic connections to visual processing centers in the brain, a process critically preceded by patterning of the retina. Gene expression gradients divide the retina along both the dorsal-ventral and the nasaltemporal axes, leading to a retina in which each retinal ganglion cell expresses a unique combination of these genes, depending on its location within the retina, which gives them positional identity. These gene expression domains lead to the downstream expression of axon guidance molecules on RGC axons, which are used to guide the axons to their correct termination points within the brain. Retinal patterning begins with the early expression of tbx5 in the dorsal retina at 12 hpf in zebrafish, likely triggered by extraocular Bmp ligands. Slightly later, multiple Bmp genes are expressed in the dorsal retina and guide expression of downstream dorsal genes. We show that following Wnt-independent initiation of tbx5, Bmp genes and downstream pathways members, including tbx5 and ephrinB2, are dependent on Wnt signaling in the dorsal RPE during a maintenance phase from approximately 15 hpf to 24 hpf. Finally, we demonstrate that activation of Bmp signaling rescues dorsal retinal markers lost from inhibition of Wnt signaling, but activation of Wnt signaling cannot rescue dorsal markers lost from inhibition of Bmp signaling, showing that Wnts signal through bmp expression to maintain dorsal identity. Tcf7 is one of five Tcfs present in the zebrafish genome, and we report its cloning and expression patterns. We show that tcf7 mRNA is present as at least three different C terminal splice forms in the zebrafish embryo, and report that another, putative repressor splice form, is present in whole-embryos, but not specifically in the eye, suggesting that Tcf7 likely functions as a transcriptional activator of Wnt target genes in the eye. Using morpholino loss-of-function approaches, we originally saw an expansion of dorsal retinal genes, suggesting that Tcf7 functions to restrict dorsal retinal gene expression, but analysis of a tcf7 mutant zebrafish line failed to reveal a retinal phenotype. Tcf7 does, however, play a role redundantly with Lefl during fin outgrowth. Collectively, this work provides significant insight into the role of Wnt signaling during dorsal-ventral retinal patterning. |
