A ZEBRAFISH RETINOIC ACID RECEPTOR EXPRESSED IN THE REGENERATING CAUDAL FIN

Retinoic acid (RA) is an important signalling molecule in vertebrate p attern formation both in developing and regenerating tissues. The effe cts of RA are due largely to regulation of gene transcription, mediate d by retinoic acid receptors (RAR-alpha, RAR-beta, RAR-gamma) and reti noid X receptors (RXR-alpha, RXR-beta, RXR-gamma). We have been using zebrafish as a model of regeneration to study the role of retinoic aci d and its receptors in vertebrate pattern formation. In this report, w e describe the molecular cloning and characterization of one of the ze brafish RARs that is the predominant receptor in the regenerating caud al fin and corresponds most closely to the RAR-gamma subtype isolated from mouse and human and to RAR-delta from newt. Zebrafish RAR-gamma ( zfRAR-gamma) exhibits both structural and functional conservation with its mammalian counterparts. Studies utilizing both normal and regener ating caudal fins of the zebrafish have indicated that it is the RAR-g amma subtype, compared to RAR-alpha or RAR-beta, which is expressed at the highest levels in the tail fin. To localize the expression patter n of RAR-gamma during fin regeneration, we have carried out whole-moun t in situ hybridization. ZfRAR-gamma transcripts, during fin regenerat ion, are localized in the blastemal tissue formed at the distal ends o f the bony rays following amputation. Treatment of fish with RA during fin regeneration induces a number of striking morphological effects o n the regenerate. When amputations are performed distal to the branch points or dichotomies, where a single ray bifurcates to extend two ind ividual 'daughter' rays, RA treatment causes a dichotomy reduction whe re the two 'daughter' rays fuse to once again form a single ray. The s ingle ray subsequently bifurcates in a comparatively normal manner. Ou r data suggest that exogenous RA can respecify pattern in the regenera ting caudal fin and identifies the blastemae as possible RA target tis sues.