Active repression of RAR signaling is required for head formation

The retinoic acid receptors (RARs) recruit coactivator and corepressor prot eins to activate or repress the transcription of target genes depending on the presence of retinoic acid (RA). Despite a detailed molecular understand ing of how corepressor complexes function, there is no in vivo evidence to support a necessary function for RAR-mediated repression. Signaling through RARs is required for patterning along the anteroposterior (A-P) axis, part icularly in the hindbrain and posterior, although the absence of RA is requ ired for correct anterior patterning. Because RARs and corepressors are pre sent in regions in which RA is absent, we hypothesized that repression medi ated through unliganded RARs might be important for anterior patterning. To test this hypothesis, specific reagents were used that either reduce or au gment RAR-mediated repression. Derepression of RAR signaling by expressing a dominant-negative corepressor resulted in embryos that exhibited phenotyp es similar to those treated by RA. Anterior structures such as forebrain an d cement gland were greatly reduced, as was the expression of molecular mar kers. Enhancement of target gene repression using an RAR inverse agonist re sulted in up-regulation of anterior neural markers and expansion of anterio r structures. Morpholino antisense oligonucleotide-mediated RAR alpha loss- of-function phenocopied the effects of RA treatment and dominant-negative c orepressor expression. Microinjection of wild-type or dominant-negative RAR alpha rescued the morpholino phenotype, confirming that RAR is functioning anteriorly as a transcriptional repressor. Lastly, increasing RAR-mediated repression potentiated head-inducing activity of the growth factor inhibit or cerberus, whereas releasing RAR-mediated repression blocked cerberus fro m inducing ectopic heads. We conclude that RAR-mediated repression of targe t genes is critical for head formation. This requirement establishes an imp ortant biological role for active repression of target genes by nuclear hor mone receptors and illustrates a novel function for RARs during vertebrate development.