XOtx5b and XOtx2regulate photoreceptor and bipolar fates in theXenopusretina

Abstract

Photoreceptor and bipolar cells are molecularly related cell types in the vertebrate retina. XOtx5b is expressed in both photoreceptors and bipolars, while a closely related member of the same family of transcription factors, XOtx2, is expressed in bipolar cells only. Lipofection of retinal precursors with XOtx5b biases them toward photoreceptor fates whereas a similar experiment with XOtx2 promotes bipolar cell fates. Domain swap experiments show that the ability to specify different cell fates is largely contained in the divergent sequence C-terminal to the homeodomain, while the more homologous N-terminal and homeodomain regions of both genes, when fused to VP16 activators, promote only photoreceptor fates. XOtx5b is closely related to Crx and like Crx it drives expression from an opsin reporter in vivo. XOtx2 suppresses this XOtx5b-driven reporter activity providing a possible explanation for why bipolars do not express opsin. Similarly, co-lipofection of XOtx2 with XOtx5b overrides the latter's ability to promote photoreceptor fates and the combination drives bipolar fates. The results suggest that the shared and divergent parts of these homologous genes may be involved in specifying the shared and distinct characters of related cell types in the vertebrate retina.

Conserved Transcriptional Activators of the Xenopus Rhodopsin Gene

“…All of these experiments indicate that the regulation of retinaspecific genes share some conserved features between vertebrates. Taken together with phylogenetic studies and analysis of expression patterns, these experiments provide convincing evidence that Otx5 is involved in rhodopsin regulation in a manner similar to mammalian Crx (28,29,34,48,49). Experiments in this report show this conservation functionally for the first time using a non-mammalian promoter.…”

“…Several phylogenetic studies of the Otx gene families support this conclusion and another suggests that XOtx5 can bias neural retinal cells to a photoreceptor fate in Xenopus retinas (49). The importance of Crx during photoreceptor development is clear from knock-out experiments.…”

“…During neurulation the expression pattern is dynamic, first disappearing from the presumptive eye field at stage 13 and later reappearing in the developing eye and pineal gland at stage 24 (34). Expression in the eye is restricted to the photoreceptor layer, the bipolar cell layer, and the RPE by stage 41 (28,49). The spatial restriction and later onset of expression suggests that Crx has evolved a specialized role in retinal differentiation (29).…”

“…28, 29, and 48, and Supplementary Materials) and embryonic expression patterns (28,35,49). Xenopus laevis contains two paralogous Otx5 genes (29), which are expressed in photoreceptors and bipolar cells in embryonic retina (28,35,49).…”

The homeobox geneXbh1cooperates with proneural genes to specify ganglion cell fate within theXenopusneural retina

“…Both the results of combined Xath5 and Xbh1 lipofections, in which more than 60% of early RPCs are driven to a RGC fate, and of the late lipofections, are completely consistent with this. The later activities of Xath5, and of other bHLH factors involved in cell fate specification within the retina, may instead involve other cooperative factors, such as Chx10 or Xotx2, which were shown to favour bipolar cell fate (Hatakeyama et al, 2001;Viczian et al, 2003), or Crx and Xotx5b, which are involved in photoreceptor specification and differentiation (Furukawa et al, 1997;Furukawa et al, 1999;Viczian et al, 2003).…”