Retinoic acid receptors and nuclear orphan receptors in the development of Xenopus laevis.

Nuclear hormone receptors are ligand-activated transcription factors that regulate the expression of target genes by binding to hormone responsive elements (HRE) in their 5' upstream region. Retinoids which are known for their teratogenicity and which have a potential role in the specification of the anteroposterior axis of vertebrate embryos regulate transcription via a hormone-like mechanism by activating nuclear retinoic acid receptors, designated RAR and RXR. Of the several isoforms of RAR found in embryos of Xenopus laevis, xRAR gamma 2 appears to be the most abundant. During the early retinoic acid-sensitive period of development, the total amount of xRAR gamma 2 transcript and protein is increased and a highly specific pattern of expression emerges. During neurulation, the receptor is predominantly found in the dorsal posterior region, in the head endomesoderm, and in the rostral hindbrain. The dependence of this pattern on mesoderm induction and on neural induction is discussed. Contrasting with the elaborate pattern of xRAR gamma 2, the FTZ-F1-related nuclear orphan receptors (xFF1rA/B) are ubiquitous nuclear proteins in Xenopus embryos, as are the peroxisome proliferator-activated receptors xPPAR alpha and beta. PPARs are activated by polyunsaturated fatty acids and regulate the synthesis of enzymes involved in lipid metabolism. Later in development, the isoforms xPPAR alpha, beta, and gamma attain different tissue specificities.