Xenopus Frizzled 5 Regulates the Transition of Retinal Progenitors to the Neural Precursor State

Purpose: A key step in retinal development is the transition of progenitors to neural precursors, defined by an increase in proliferation and acquisition of neural potential. The signals that regulate the formation of retinal neural precursors remain to be defined. Xenopus Frizzled 5 (Xfz5) transmembrane receptor is expressed in the presumptive neural retina at optic vesicle stages, and is thus poised to influence retinal progenitor development in the early neural retina. In this study we examine the function of Xfz5 in retinal progenitors. Methods: We used injected antisense morpholino oligonucleotides into early cleavage stage embryos to block Xfz5 protein translation, then assayed for changes in gene expression by in situ hybridization. We also examined effects on cell proliferation by BrDU incorporation and by staining for phosphorylated histone H3, a mitotic marker. To determine whether activation of the Wnt/beta-catenin signaling pathway is required during eye development, we selectively blocked this pathway in retinal progenitors by generating transgenic embryos expressing dnTCF3 under the control of the Rx promoter. We also monitored activity of the Wnt/beta-catenin signaling pathway during eye development by generating transgenic embryos using a reporter transgene (TOP:GFP) that is activated by Wnt/beta-catenin signaling in vivo. Results: We found that blocking either Xfz5 or canonical wnt signaling within the developing retina inhibited expression of Xsox2, a key regulator of neural competence. We also observed reduced cell proliferation and inhibition of proneural and neurogenic gene expression. Blocking Xsox2 function mimicked these effects. RNA rescue experiments established that Xsox2 is downstream of Xfz5. Inhibition of either Xfz5 or Xsox2 also affected the fate of individual retinal progenitors biasing them to adopt the non-neural Mueller glial fate. Conclusions: We conclude that Xfz5, acting through TCF-dependent Wnt/Fz signaling and regulation of Xsox2 expression, controls the transition of retinal progenitors to neural precursors in the developing Xenopus retina.