In amphibia the egg is radially symmetric, but fertilization triggers a rotation of the egg cortex that determines the aids of bilateral symmetry. On the dorsal side the large yolk platelets move upwards, coming in intimate contact with animal pole cytoplasm. Our working hypothesis is that the dorsal yolk could release small hydrophobic molecules that activate transcription factors of the nuclear receptor family. Eight nuclear receptors of the retinoic acid (RA)/thyroid hormone family have been isolated from a cDNA library prepared from unfertilized Xenopus eggs. Using functional assays we have shown that two of them are retinoic acid receptors (RARs) of the classical type and two others are members of the novel retinoid """"""""X""""""""receptor family (RXRs). An antibody against one of the latter proteins shows that it becomes localized to a small subset of blastula nuclei that gives rise to mesoderm, particularly on the dorsal side. The discovery of maternal retinoid receptors, which become asymmetrically distributed during development, may lead to a better understanding of the potent teratogenic effects of RA in early vertebrate development. The other receptors correspond to so-called orphan receptors, for which the ligand is not yet known. The sequence of their DNA binding regions strongly suggests that they may bind to an overlapping subset of the gene network activated by retinoid receptors. The identification of ligands for the maternal orphan receptors could result in the identification of new morphogens and teratogens that cause congenital malformation in vertebrates. The objective of this proposal is to ascertain the developmental function of maternal retinoid and orphan receptors, including the identification of the endogenous ligand molecules involved, the study of their teratogenic effects, their mode of action, the localization of receptors and their ligands in the developing egg, and the molecular mechanisms of their segregation. A number of molecular techniques will be used in this analysis, exploiting the accessibility of amphibian embryos to experimental manipulation at the early stages of development, and of transgenic mice at later stages.
