POU transcription factors have been shown to be important in the regulation of cell fate decisions during development. We have focused on understanding the function of one POU domain gene, XlPOU 2 , in early neural development. XlPOU 2 is immediately downstream in the neural inductive signaling pathway for noggin. Our current goal is to identify noggin inducible elements in the XlPOU 2 promoter. We have examined the role of XlPOU 2 in Spemann's organizer by overexpressing the XlPOU 2 gene product in embryos. The overexpression of XlPOU 2 leads to a suppression of dorsal gene function in gastrulating Xenopus embryos, which results in a """"""""headless"""""""" phenotype. Thus, XlPOU2's action appears to be restricted to early gastrulation. XlPOU2 interferes with the formation of a 2 degrees axis induced within the Wnt signaling cascade. The phenotype XlPOU2 can be rescued by chordin, another signaling molecule present in the organzier. XlPOU2 blocks the activin-induced expression of mesoderm markers, such as the immediate response gene, gsc. XlPOU2 may suppress gsc directly, since it binds to two elements in the gsc promoter, both of which are specific for class III POU genes. We are testing whether XLPOU2 can suppress the activation of the gsc promoter. We have also characterized a novel POU gene in zebrafish, taichi. The expression pattern of taichi during development and in the adult brain is consistent with its potential role in regulating the establishment and/or maintenance of neural stem cells. Taichi expression is associated with proliferating, non-differentiated cells of the brain. This cell population can be labeled by BrdU, and does not express two markers of neural differentiation, acetylated tubulin and neurofilament protein. We are characterizing these cells further using another marker that is co-expressed with taichi, NRP-1 (an RNA binding protein). To further understand the role of stem cells in mammalian development, we are attempting to clone a mouse homolog of taichi.
