title: Xwnt-8 and lithium can act upon either dorsal mesodermal or neurectodermal cells to cause a loss of forebrain in Xenopus embryos.
abstract: When Xenopus gastrulae are made to misexpress Xwnt-8, or are exposed to lithium ions, they develop with a loss of anterior structures. In the current study, we have characterized the neural defects produced by either Xwnt-8 or lithium and have examined potential cellular mechanisms underlying this anterior truncation. We find that the primary defect in embryos exposed to lithium at successively earlier stages during gastrulation is a progressive rostral to caudal deletion of the forebrain, while hindbrain and spinal regions of the CNS remain intact. Misexpression of Xwnt-8 during gastrulation produces an identical loss of forebrain. Our results demonstrate that lithium and Wnts can act upon either prospective neural ectodermal cells, or upon dorsal mesodermal cells, to cause a loss of anterior pattern. Specifically, ectodermal cells isolated from lithium- or Wnt-exposed embryos are unable to form anterior neural tissue in response to inductive signals from normal dorsal mesoderm. In addition, although dorsal mesodermal cells from lithium- or Wnt-exposed embryos are specified properly, and produce normal levels of the anterior neural inducing molecules noggin and chordin, they show a greatly reduced capacity to induce anterior neural tissue in conjugated ectoderm. Taken together, our results are consistent with a model in which Wnt- or lithium-mediated signals can induce either mesodermal or ectodermal cells to produce a dominant posteriorizing morphogen which respecifies anterior neural tissue as posterior.
authors.affiliation: Department of Cell and Developmental Biology, Oregon Health Sciences University, Portland 97201, USA.
authors.forename: J R
authors.initials: JR
authors.lastname: Fredieu
authors.forename: Y
authors.initials: Y
authors.lastname: Cui
authors.forename: D
authors.initials: D
authors.lastname: Maier
authors.forename: M V
authors.initials: MV
authors.lastname: Danilchik
authors.forename: J L
authors.initials: JL
authors.lastname: Christian
mesh_terms: D000818:Animals; D004475:Ectoderm / Q000187:drug effects*; D005775:Gastrula / Q000187:drug effects; D018507:Gene Expression Regulation, Developmental / Q000187:drug effects; D008094:Lithium / Q000633:toxicity*; D008636:Mesencephalon / Q000187:drug effects; D008648:Mesoderm / Q000187:drug effects*; D008934:Mitogens / Q000633:toxicity*; D009420:Nervous System / Q000187:drug effects; D016548:Prosencephalon / Q000002:abnormalities* / Q000196:embryology; D011505:Protein-Tyrosine Kinases / Q000633:toxicity*; D011518:Proto-Oncogene Proteins / Q000633:toxicity*; D051153:Wnt Proteins; D029867:Xenopus Proteins; D014982:Xenopus laevis / Q000196:embryology*; D029961:Zebrafish Proteins
chemical_list: D008934:Mitogens; D011518:Proto-Oncogene Proteins; D051153:Wnt Proteins; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; C495248:wnt8a protein, Xenopus; D008094:Lithium; D011505:Protein-Tyrosine Kinases