Regulation of RALDH-1, RALDH-3 and CYP26A1 by transcription factors cVax/Vax2 and Tbx5 in the embryonic chick retina.

In vertebrate development, polarity of the dorsal-ventral eye axis is determined by the spatially restricted expression of transcription factors cVax/Vax2 in the ventral retina and Tbx5 in the dorsal retina. In addition, an early gradient of retinoic acid (RA) has been suggested to be involved in this process. Using retroviral transfection of the early eye anlage of chick embryos we investigated whether cVax/Vax2 and Tbx5 regulate the expression of enzymes that determine the retinal distribution of RA. Transfection of cVax/Vax2 completely abolished the spatial expression pattern of the RA synthesizing enzyme RALDH-1, of the RA degrading enzyme Cyp26A1 and caused an ectopic expression of RALDH-3 in the dorsal retina. Transfection of Tbx5 did not significantly alter the distribution of RALDH-1 and RALDH-3 but caused additional patches of Cyp26 outside of its normal domain of expression. The distribution of RA was experimentally changed either by ectopic expression of Cyp26A1, or by applications of RA. These manipulations did not affect the distribution of cVax/Vax2 or interfere with development of the retinotectal projection from dorsal retinal ganglion cells. The results show that in the chick retina the compartmentalized expression of RA metabolizing enzymes requires the spatially restricted expression of the transcription factors cVax/Vax2 and Tbx5. On the other hand, RA may not be decisive for the correct retinal projection toward the optic tectum.

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