SEQUENTIAL ROLES FOR OTX2 IN VISCERAL ENDODERM AND NEUROECTODERM FOR FOREBRAIN AND MIDBRAIN INDUCTION AND SPECIFICATION

The homeobox gene Otx2 is a mouse cognate of the Drosophila orthodenti cle gene, which is required for development of the brain, rostral to r hombomere three. We have investigated the mechanisms involved in this neural function and specifically the requirement for Otx2 in the visce ral endoderm and the neuroectoderm using chimeric analysis in mice and explant recombination assay. Analyses of chimeric embryos composed of more than 90% of Otx2(-/-) ES cells identified an essential function for Otx2 in the visceral endoderm for induction of the forebrain and m idbrain. The chimeric studies also demonstrated that an anterior neura l plate can form without expressing Otx2. However, in the absence of O tx2, expression of important regulatory genes, such as Hesx1/Rpx, Six3 , Pax2, Wnt1 and En, fail to be initiated or maintained in the neural plate. Using explant-recombination assay, we could further demonstrate that Otx2 is required in the neuroectodem for expression of En. Altog ether, these results demonstrate that Otx2 is first required in the vi sceral endoderm for the induction, and subsequently in the neuroectode rm for the specification of forebrain and midbrain territories.