Two isoforms of Xenopus retinoic acid receptor γ2 (B) exhibit differential expression and sensitivity to retinoic acid during embryogenesis

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Developmental Genetics





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We report the isolation of two retinoic acid receptor isoforms (RARγ), which differ only in the 5′ untranslated and putative N-terminus A regions. The two isoforms appear to serve as early markers for the presumptive neural axis; however, their expression patterns differ. RARγ2, 1 is first expressed at gastrulation at the dorsal lip and subsequently along the presumptive neural axis. RARγ2.2 represents the full-length sequence of a receptor cDNA already partially characterized and present as a maternal transcript [Ellinger-Ziegelbauer and Dreyer (1991); Genes Dev 5:94-104, (1993): Mech Dev 41:31-46; Pfeffer and DeRobertis, (1994) Mech Dev: 45:147-153]. Unlike RARγ2.2, the 2.1 variant is not expressed either in pre-somitic mesoderm or notochord. RARγ2.1 is strongly expressed in branchial arches and to a lesser extent in the neural floor plate. The two isoforms also exhibit differential sensitivity to retinoic acid. Constitutive expression of RARγ2.2 following neurulation appears to be depressed by treatment with retinoic acid, but domains of highest expression, namely, the head and tail, remain relatively unaffected, as do patterns of expression prior to late neurulation. By contrast, RARγ2.1 is not transcribed in retinoid-inhibited structures. Using microinjection techniques, we show that changes of RARγ2.1 expression in presumptive head structures occur as an early and local consequence of retinoic acid administration. Since RARγ2.1 expression is inhibited by retinoic acid, we tested to see if other treatments that perturb axis formation had any effect. Surprisingly, UV irradiation did not suppress expression of the RARγ2.1 transcript, suggesting that its inhibition by retinoic acid is not due solely to inhibition of anterior neural development. These experiments demonstrate a new subdivision of isoforms that undergo differential expression during development and that exhibit differential sensitivity to retinoic acid and to UV. This sensitivity and the presence of this isoform variant in regions that are known to exhibit polarizing activity strengthen the hypothesis that these receptors play a primary role during morphogenesis.