2 ISOFORMS OF XENOPUS RETINOIC ACID RECEPTOR GAMMA-2(B) EXHIBIT DIFFERENTIAL EXPRESSION AND SENSITIVITY TO RETINOIC ACID DURING EMBRYOGENESIS

We report the isolation of two retinoic acid receptor isoforms (RAR ga mma), 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. RA R-gamma 2.1 is first expressed at gastrulation at the dorsal lip and s ubsequently along the presumptive neural axis. RAR-gamma 2.2 represent s the full-length sequence of a receptor cDNA already partially charac terized 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 RAR-gamma 2.2, t he 2.1 variant is not expressed either in pre-semitic mesoderm or noto chord. RAR-gamma 2.1 is strongly expressed in branchial arches and to a lesser extent in the neural floor plate. The two isoforms also exhib it differential sensitivity to retinoic acid. Constitutive expression of RAR-gamma 2.2 following neurulation appears to be depressed by trea tment with retinoic acid, but domains of highest expression, namely, t he head and tail, remain relatively unaffected, as do patterns of expr ession prior to late neurulation. By contrast, RAR-gamma 2.1 is not tr anscribed in retinoid-inhibited structures. Using microinjection techn iques, we show that changes of RAR-gamma 2.1 expression in presumptive head structures occur as an early and local consequence of retinoic a cid administration. Since RAR-gamma 2.1 expression is inhibited by ret inoic acid, we tested to see if other treatments that perturb axis for mation had any effect. Surprisingly, UV irradiation did not suppress e xpression of the RAR-gamma 2.1 transcript, suggesting that its inhibit ion by retinoic acid is not due solely to inhibition of anterior neura l development. These experiments demonstrate a new subdivision of isof orms that undergo differential expression during development and that exhibit differential sensitivity to retinoic acid and to UV. This sens itivity and the presence of this isoform variant in regions that are k nown to exhibit polarizing activity strengthen the hypothesis that the se receptors play a primary role during morphogenesis. (C) 1995 Wiley- Liss, Inc.