XLPOU-2 IS A POTENTIAL REGULATOR OF SPEMANN ORGANIZER

XlPOU 2 a member of the class III POU-domain family, is expressed init ially at mid-blastula transition (MET) and during gastrulation in the entire marginal zone mesoderm, including Spemann's Organizer (the Orga nizer). To identify potential targets of XlPOU 2, the interaction of X lPOU 2 with other genes co-expressed in the Organizer was examined by microinjecting XlPOU 2's mRNA into the lineage of cells that contribut es to the Organizer, head mesenchyme and prechordal plate. XlPOU 2 sup presses the expression of a number of dorsal mesoderm-specific genes, including gsc, Xlim-1, Xotx2, noggin and chordin, but not Xnot. As a c onsequence of the suppression of dorsal mesoderm gene expression, bone morphogenetic factor-4 (Bmp-4), a potent inducer of ventral mesoderm, is activated in the Organizer. Gsc is a potential target of XlPOU 2. XlPOU 2 is capable of binding a class III POU protein binding site (CA TTAAT) that is located within the gsc promoter, in the activin-inducib le (distal) element. Furthermore, XlPOU 2 suppresses the activation of the gsc promoter by activin signaling. At the neurula and tailbud sta ges, dorsoanterior structures are affected: embryos displayed micropth almia and the loss of the first branchial arch, as detected by the exp ression of pax-6, Xotx2 and en-2. By examining events downstream from the Wnt and chordin pathways, we determined that XlPOU 2, when overexp ressed, acts specifically in the Organizer, downstream from GSK-3 beta of the Wnt pathway and upstream from chordin. The interference in dor salizing events caused by XlPOU 2 was rescued by chordin. Thus, in add ition to its direct neuralizing ability, in a different context, XlPOU 2 has the potential to antagonize dorsalizing events in the Organizer .