AXIAL (HNF3-BETA) AND RETINOIC ACID RECEPTORS ARE REGULATORS OF THE ZEBRAFISH SONIC HEDGEHOG PROMOTER

The signalling molecule Sonic hedgehog is involved in a multitude of d istinct patterning processes during vertebrate embryogenesis. In the n ascent body axis of the zebrafish embryo, sonic hedgehog is co-express ed with axial (HNF3 beta in mammals), a transcription regulator of the winged helix family. We show here that misexpression of axial leads t o ectopic activation of sonic hedgehog expression in the zebrafish, su ggesting that axial is a regulator of sonic hedgehog transcription. Th e sonic hedgehog gene was cloned from zebrafish and its promoter was c haracterized with respect to activation by axial. Expression of axial or rat HNF3 beta. in HeLa cells results in activation of co-transfecte d sonic hedgehog promoter-CAT fusion genes. This effect is mediated by two Axial (HNF3 beta) recognition sequences. We furthermore identifie d a retinoic acid response element (RARE) in the sonic hedgehog upstre am region which can be bound by retinoic acid receptor (RAR) and retin oid X receptor (RXR) heterodimers in vitro and confers retinoic acid i nducibility to the sonic hedgehog promoter in the HeLa cell system. Ou r results suggest that both Axial (HNF3 beta) and retinoic acid recept ors are direct regulators of the sonic hedgehog gene.