|M.Sc Student||Michaelov Ariel|
|Subject||Regulation of Hindbrain or Spinal Cord Cell-Fate Choices|
during Early Xenopus Laevis Development
|Department||Department of Medicine||Supervisor||Professor Dale Frank|
|Full Thesis text|
During embryonic development of the amphibian Xenopus laevis, the central nervous system (CNS) is induced by a two-step process. The first neural inducing step, "activation", initiates during gastrulation. During "activation", Bone Morphogenetic Protein (BMP) antagonists, such as noggin, chordin and follistatin secreted from the dorsal "Spemann organizer" region induce anterior neural structures such as forebrain in adjacent ectoderm. During the following "transformation" step, this anterior neural tissue in the posterior fated regions is respecified to hindbrain and spinal cord fates. Three signaling molecules, Wnt, Fibroblast Growth Factors (FGF) and Retinoic Acid (RA) are well established as "posteriorizers" that transform anterior neural tissue to these more posterior neural cell fates. Two additional posterior cell types, primary neurons and neural crest (NC) are also induced in neural plate/folds region by these same "posteriorizers".
The Wnt network activates a number of embryonic signaling pathways. The canonical Wnt/b-catenin pathway is required for the induction of posterior neural cell fates, such as hindbrain, primary neuron and neural crest. For over a decade, the accepted "classic" model for the neural caudalizing process suggested that canonical Wnt/β-catenin signaling forms a gradient along the antero-posterior (AP) axis that interacts with a constant level of BMP antagonism to pattern the CNS. The highest Wnt levels induce the spinal cord in the most posterior; intermediate levels induce the hindbrain and midbrain, while low/no activity induces the forebrain in the anterior. In addition, it was suggested that there is constant level of neural inducing BMP antagonists along the AP axis.
In this study, we demonstrate experimental proof that contradicts and challenges this "classic" Wnt/β catenin gradient model. Inhibition of embryonic canonical Wnt signaling did not strongly affect spinal cord or forebrain formation, yet hindbrain formation was seriously perturbed. In explants, ectopic BMP4 expression on the background of neural caudalizing factors stimulated spinal cord marker gene expression while simultaneously inhibiting hindbrain and forebrain markers. Expression of some spinal cord markers was also stimulated solely by BMP4, with no added caudalizer. Thus, this study shows that canonical Wnt signaling is not necessarily the key regulator of spinal cord induction and that BMP4 signaling may positively regulate spinal cord formation. Our results suggest that there needs to be a serious experimentally based re-evaluation of the models describing the induction of hindbrain versus spinal cord cell fates in the developing posterior CNS.