Controls Neuroectoderm Specification via Dual Circuits of Direct Repression and Indirect Stimulation in Embryos.

Spemann organizer is a center of dorsal mesoderm and itself retains the mesoderm character, but it has a stimulatory role for neighboring ectoderm cells in becoming neuroectoderm in gastrula embryos. () overexpression in ventral region promotes secondary axis formation including neural tissues, but the role of in neural specification could be indirect. We examined the neural inhibitory and stimulatory roles of in the same cell and neighboring cells contexts. In the animal cap explant system, overexpression inhibited expression of neural specific genes including , , , and . Genome-wide chromatin immunoprecipitation sequencing (ChIP-seq) and promoter analysis of early neural genes of and were performed to show that the neural inhibitory mode of was direct. Site-directed mutagenesis and serially deleted construct studies of promoter revealed that Gsc directly binds within the promoter to repress its expression. Conjugation assay of animal cap explants was also performed to demonstrate an indirect neural stimulatory role for The genes for secretory molecules, Chordin and Noggin were up-regulated in injected cells with the neural fate only achieved in uninjected neighboring cells. These experiments suggested that regulates neuroectoderm formation negatively when expressed in the same cell and positively in neighboring cells via soluble factors. One is a direct suppressive circuit of neural genes in expressing mesoderm cells and the other is an indirect stimulatory circuit for neurogenesis in neighboring ectoderm cells via secreted BMP antagonizers.