title: Regulation of hhex expression in the yolk syncytial layer, the potential Nieuwkoop center homolog in zebrafish.
abstract: The Nieuwkoop center is the earliest signaling center during dorsal-ventral pattern formation in amphibian embryos and has been implied to function in induction of the Spemann-Mangold organizer. In zebrafish, Nieuwkoop-center-like activity resides in the dorsal yolk syncytial layer (YSL) at the interface of the vegetal yolk cell and the blastoderm. hex homologs are expressed in the anterior endomesoderm in frogs (Xhex), the anterior visceral endoderm in mice, and the dorsal YSL in zebrafish (hhex). Here, we investigate the control of hhex expression in the YSL. We demonstrate that bozozok (boz) is absolutely required for early hhex expression, while overexpression of boz causes ectopic hhex expression. Activation of Wnt/beta-catenin signaling by LiCl induces hhex expression in wild-type YSL but not in boz mutant embryos, revealing that boz activity is required downstream of Wnt/beta-catenin signaling for hhex expression. Further, we show that the boz-mediated induction of hhex is independent of the Boz-mediated repression of bmp2b. Our data reveal that repressive effects of both Vega1 and Vega2 may be responsible for the exclusion of hhex expression from the ventral and lateral parts of the YSL. In summary, zebrafish hhex appears to be activated by Wnt/beta-catenin in the dorsal YSL, where Boz acts in a permissive way to limit repression of hhex by Vega1 and Vega2.
authors.affiliation: Developmental Biology, Institute of Biology 1, University of Freiburg, Haupstrasse 1, D-79104 Freiburg, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Bischof
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Driever
mesh_terms: D000818:Animals; D019521:Body Patterning; D055396:Bone Morphogenetic Protein 2; D019485:Bone Morphogenetic Proteins / Q000235:genetics / Q000378:metabolism; D003598:Cytoskeletal Proteins / Q000378:metabolism; D004530:Egg Yolk / Q000166:cytology / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism*; D017403:In Situ Hybridization; D055457:Nodal Protein; D020897:Organizers, Embryonic; D011506:Proteins / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D015534:Trans-Activators / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000235:genetics / Q000378:metabolism; D051153:Wnt Proteins; D015027:Zebrafish / Q000196:embryology* / Q000235:genetics; D029961:Zebrafish Proteins / Q000235:genetics / Q000378:metabolism*; D051176:beta Catenin
chemical_list: D055396:Bone Morphogenetic Protein 2; D019485:Bone Morphogenetic Proteins; D003598:Cytoskeletal Proteins; C513077:Hhex protein, zebrafish; D018398:Homeodomain Proteins; D055457:Nodal Protein; C526008:Nodal protein, mouse; D011506:Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; D016212:Transforming Growth Factor beta; C434249:Vox protein, zebrafish; D051153:Wnt Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin; C405936:bmp2b protein, zebrafish; C495269:ctnnb1 protein, zebrafish; C113874:dharma protein, zebrafish; C434250:vent protein, zebrafish; C094521:wnt8a protein, zebrafish