Two isoforms of Xenopus retinoic acid receptor gamma 2 (B) exhibit differential
expression and sensitivity to retinoic acid during embryogenesis.

Crawford MJ, Liversage RA, Varmuza SL

Ramsay Wright Zoological Laboratories, University of Toronto, Ontario, Canada.

We report the isolation of two retinoic acid receptor isoforms (RAR gamma),
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. RAR-gamma 2.1 is first expressed at
gastrulation at the dorsal lip and subsequently along the presumptive neural
axis. RAR- gamma 2.2 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 RAR-gamma
2.2, the 2.1 variant is not expressed either in pre-somitic mesoderm or
notochord. RAR-gamma 2.1 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 RAR gamma
2.2 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, RAR-gamma 2.1 is not transcribed in retinoid-inhibited structures.
Using microinjection techniques, we show that changes of RAR-gamma 2.1
expression in presumptive head structures occur as an early and local
consequence of retinoic acid administration. Since RAR-gamma 2.1 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
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.

PMID: 8641047, UI: 96191149