title: A role for the extraembryonic yolk syncytial layer in patterning the zebrafish embryo suggested by properties of the hex gene.
abstract: Recent studies in mouse suggest that the extraembryonic endoderm has an important role in early embryonic patterning [1]. To analyze whether similar mechanisms operate in other vertebrates, we cloned the zebrafish homologue of Hex, a homeobox gene that is expressed asymmetrically in the mouse visceral endoderm [2]. Early expression of zebrafish hex is restricted to the dorsal portion of the yolk syncytial layer (YSL), an extraembryonic tissue. By the onset of gastrulation, hex is expressed in the entire dorsal half of the YSL, which directly underlies the cells fated to form the neural plate. We show that hex expression is initially regulated by the maternal Wnt pathway and later by a Bmp-mediated pathway. Overexpression experiments of wild-type and chimeric Hex constructs indicate that Hex functions as a transcriptional repressor and its overexpression led to the downregulation of bmp2b and wnt8 expression and the expansion of chordin expression. These findings provide further evidence that the zebrafish YSL is the functional equivalent of the mouse visceral endoderm and that extraembryonic structures may regulate early embryonic patterning in many vertebrates.
authors.affiliation: Department of Biochemistry and Biophysics University of California at San Francisco San Francisco, California, 94143-0448, USA.
authors.forename: C Y
authors.initials: CY
authors.lastname: Ho
authors.forename: C
authors.initials: C
authors.lastname: Houart
authors.forename: S W
authors.initials: SW
authors.lastname: Wilson
authors.forename: D Y
authors.initials: DY
authors.lastname: Stainier
mesh_terms: D000818:Animals; D019521:Body Patterning / Q000235:genetics; D019485:Bone Morphogenetic Proteins / Q000378:metabolism; D004625:Embryo, Nonmammalian / Q000196:embryology* / Q000502:physiology; D004627:Embryonic Induction; D004707:Endoderm / Q000502:physiology*; D018507:Gene Expression Regulation, Developmental; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D012097:Repressor Proteins; D013997:Time Factors; D014157:Transcription Factors; D051153:Wnt Proteins; D015017:Yolk Sac / Q000502:physiology*; D015027:Zebrafish / Q000196:embryology* / Q000235:genetics; D029961:Zebrafish Proteins
chemical_list: D019485:Bone Morphogenetic Proteins; C513080:Hhex protein, mouse; C513077:Hhex protein, zebrafish; D018398:Homeodomain Proteins; D011518:Proto-Oncogene Proteins; D012097:Repressor Proteins; D014157:Transcription Factors; D051153:Wnt Proteins; D029961:Zebrafish Proteins; C094522:wnt8b protein, zebrafish