OVEREXPRESSION OF A CELLULAR RETINOIC ACID-BINDING PROTEIN (XCRABP) CAUSES ANTEROPOSTERIOR DEFECTS IN DEVELOPING XENOPUS-EMBRYOS

We have isolated the first Xenopus laevis cDNA coding for a cellular r etinoicacid binding protein (xCRABP). xCRABP contains a single open re ading frame, coding for an approximately 15x10(3) Mr protein. Northern blot analysis shows that this cDNA hybridizes to a mRNA that is expre ssed both maternally and zygotically and which already reaches maximal expression during gastrulation (much earlier than previously describe d CRABP genes from other species). Insitu hybridisation showed that at the onset of gastrulation, xCRABP mRNA is localised at the dorsal sid e of the embryo, in the ectoderm and in invaginatingmesoderm. xCRABP e xpression then rapidly resolves into two domains; a neural domain, whi ch becomes localised in the anterior hindbrain, and a posterior domain in neuroectoderm and mesoderm. These two domains were already evident by the mid-gastrula stage. We investigated the function of xCRABP by injecting fertilized eggs with an excess of sense xCRABP mRNA and exam ined the effects on development. We observed embryos with clear antero posterior defects, many of which resembled the effects of treating Xen opus gastrulae with all-trans retinoic acid. Notably, the heart was de leted, anterior brain structures and the tailwere reduced, and segment ation of the hindbrain was inhibited. The effects ofinjecting xCRABP t ranscripts are compatible with the idea that xCRABP overexpression mod ulates the action of an endogenous retinoid, thereby regulating theexp ression of retinoid target genes, such as Hox genes. In support of thi s, we showed that the expression of two Xenopus Herb genes, Hoxb-9 and Hoxb-4, is strongly enhanced by xCRABP overexpression. These results suggest that xCRABP expression may help to specify the anteroposterior axis during the early development of Xenopus laevis.