Novel BMP Antagonists and Neural Induction in the Mouse Novel BMP Antagonists and Neural Induction in the Mouse

Abstract

At induction, the embryonic ectoderm is transformed via a series of morphogenetic changes and alterations in gene expression to form the neural ectoderm. A transient structure, the node, expresses factors including noggin, chordin, and follistatin that antagonize signaling by bone morphogenetic proteins (BMPs), causing the midline embryonic ectoderm to express neural genes. The cells at the border between the epidermal and neural ectoderm give rise to the future neural crest cells and are located at the site of neural tube closure; thus, tight regulation of signaling molecules in this area is crucial. What additional genes are involved in inducing and patterning the neural ectoderm remains to be determined. To test the hypothesis that antagonism of BMP signaling sets the boundary between the neural and epidermal ectoderm as well as to identify novel effectors of this process, a series of independent experiments was carried out. Misexpression of the BMP antagonist noggin or knock-down of BMP4 expression using shRNAs both expanded the neural ectoderm at the expense of the epidermal ectoderm. In a differential display-RT-PCR screen for genes downstream of noggin, we identified the bi-functional protein Geminin. In the early post-implantation embryo, Geminin is required for epithelial to mesenchymal transition at gastrulation as well as neural crest differentiation and migration. Geminin knock-down inhibited the migration of endoderm and mesoderm from the primitive streak and decreased the size of the neural crest cell population while overexpression expanded the neural ectoderm and neural crest. We also identified uncharacterized genes potentially involved in neural induction using laser capture microdissection of neural ectoderm, embryonic ectoderm, primitive streak, node, and anterior visceral endoderm, followed by RNA extraction and microarray analysis. Overall, this work indicates that inhibition of BMP signaling plays a critical role in setting the neural ectoderm/ epidermal ectoderm boundary in the mouse embryo and sets the stage to examine the role of additional genes in this process. These results have implications in understanding normal development, the genesis of birth defects, and may suggest novel strategies for the neural differentiation of stem cells.