Pharmacological inactivation of the endothelin type A receptor in the early chick embryo: a model of mispatterning of the branchial arch derivatives.

In the present study, we have applied an antagonist treatment to the chick embryo in ovo in order to demonstrate and dissect the essential roles of the endothelin type A (ETA) receptor in the embryonic development. We have cloned, sequenced and expressed the cDNA of the chick ETA receptor and shown that its affinity for endothelin antagonists is very similar to that shown by its mammalian counterparts. We have studied the spatio-temporal expression pattern of this receptor by in situ hybridization and shown that there is a high level of its mRNA within the mesenchyme of the branchial arches at E3-E5, in keeping with the direct effect of endothelin-1 (ET-1) on the fate of this region of the embryo. Unlike the endothelin type B (ETB) receptor mRNA, ETA mRNA is not expressed in neural crest cells during emigration from the neural tube, but is detected in neural crest-derived ectomesenchyme of the branchial arches. Finally, the functional involvement of this receptor in craniofacial and cardiovascular organogenesis was assessed by selectively inactivating the ETA receptor with specific antagonists applied during the time period corresponding to the expression of the ETA receptor and colonisation of the branchial arches. Embryos treated by these antagonists show a severe reduction and dysmorphogenesis of the hypobranchial skeleton, as well as heart and aortic arch derivative defects. This phenotype is very similar to that obtained in mice by gene inactivations of ET-1 and ETA. These results are observed with ETA antagonists but not with an ETB antagonist, and are dependent on the dose of the antagonists used and on the time of application to the embryo. Altogether, these data strongly show that the ET-1/ETA pathway, in chicken as in mammals, is a major factor involved directly and functionally in morphogenesis of the face and heart. This experimental model of pharmacological inactivation of a gene product described in this study offers a simple and rapid alternative to gene inactivation in mouse. This strategy can be applied to other ligand-receptor systems and extended to compounds of various chemical and functional natures.