Defects of the induction and very early patterning of the nervous system are among the most severe and yet poorly understood conditions of early development. Despite this, little is currently known about the molecular histogenesis of the mammalian CNS, despite the identification of many genes involved in the induction and patterning of the nervous system in C. elegans, Drosophila and Xenopus embryos. This is likely due to both the inaccessibility of the mammalian embryo for manipulation and the evolutionary duplication of genes which subsume critical functions. In the current investigation, we will examine the role of the secreted molecules noggin, chordin, and cerberus in neural induction using mouse embryonic stem (ES) cells as an analogue of the amphibian animal cap assay, followed by studies of ectopic expression of these molecules in early primitive streak staged mouse embryos grown in whole embryo culture. The role of these secreted inhibitors will also be examined by interfering with the BMP signal transduction pathway using dominant/negative receptor constructs, and by inhibiting transduction of signal from receptor to nucleus by expressing an anti-Smad in ES cells. Because early wnt signaling contributes to the extinction of BMP expression in the ectoderm, axis formation and neural induction, the effects of both activating and inhibiting the wnt signaling pathway on neural differentiation will be determined. This in vitro followed by whole embryo approach should conclusively determine the roles of these secreted signaling molecules in the early induction of the mouse embryo CNS. In the longer term it should also allow us to identify their downstream targets and the role in neural induction and patterning of additional (yet unidentified) molecules expressed by the unique signaling centers of the early mouse embryo.
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