DOMAINS OF RETINOID SIGNALING AND NEURECTODERMAL EXPRESSION OF ZEBRAFISH OTX1 AND GOOSECOID ARE MUTUALLY EXCLUSIVE

Retinoid signalling plays an important role in embryonic pattern forma tion. Excess of retinoic acid during gastrulation results in axial def ects in vertebrate embryos, suggesting that retinoids are involved in early anteroposterior patterning. To study retinoid signalling in zebr afish embryos, we developed a novel method to detect endogenous retino ids in situ in embryos, using a fusion protein of the ligand inducible transactivation domain of a retinoic acid receptor and a heterologous DNA binding domain. Using this method, we show that retinoid signalli ng is localized in zebrafish embryos in the region of the embryonic sh ield, and towards the end of gastrulation in a posterior dorsal domain . To investigate the relationships between the spatial distribution of retinoid signalling and the regulation of retinoid target genes, we s tudied the downregulation by retinoic acid of two genes expressed in a nterior regions of the embryo, goosecoid and otxl. These experiments s how that expression of both genes is strongly downregulated in the ant erior neurectoderm of zebrafish embryos treated with retinoic acid, wh ereas mesendodermal expression is only mildly affected. Interestingly, a significant downregulation of goosecoid expression by retinoic acid was observed only during midgastrulation but not in earlier stages. I n agreement with these results, spatial expression of goosecoid and or x1 does not overlap with the region of retinoid signalling in the late gastrula. Our data support the hypothesis that a localized retinoid s ignal is involved in axial patterning during early development, at lea st in part through the repression of anterior genes in posterior regio ns of the embryo. Furthermore, our data suggest that the action of ret inoids is spatially as well as temporally regulated in the developing embryo.