title: A gene regulatory network controlling hhex transcription in the anterior endoderm of the organizer.
abstract: The homeobox gene hhex is one of the earliest markers of the anterior endoderm, which gives rise to foregut organs such as the liver, ventral pancreas, thyroid, and lungs. The regulatory networks controlling hhex transcription are poorly understood. In an extensive cis-regulatory analysis of the Xenopus hhex promoter, we determined how the Nodal, Wnt, and BMP pathways and their downstream transcription factors regulate hhex expression in the gastrula organizer. We show that Nodal signaling, present throughout the endoderm, directly activates hhex transcription via FoxH1/Smad2 binding sites in the proximal -0.44 Kb promoter. This positive action of Nodal is suppressed in the ventral-posterior endoderm by Vent 1 and Vent2, homeodomain repressors that are induced by BMP signaling. Maternal Wnt/β-catenin on the dorsal side of the embryo cooperates with Nodal and indirectly activates hhex expression via the homeodomain activators Siamois and Twin. Siamois/Twin stimulate hhex transcription through two mechanisms: (1) they induce the expression of Otx2 and Lim1 and together Siamois, Twin, Otx2, and Lim1 appear to promote hhex transcription through homeobox sites in a Wnt-responsive element located between -0.65 to -0.55 Kb of the hhex promoter. (2) Siamois/Twin also induce the expression of the BMP-antagonists Chordin and Noggin, which are required to exclude Vents from the organizer allowing hhex transcription. This study reveals a complex network regulating anterior endoderm transcription in the early embryo.
authors.affiliation: Division of Developmental Biology, Cincinnati Children's Research Foundation and Department of Pediatrics, College of Medicine, University of Cincinnati, 3333 Burnet Avenue, Cincinnati, OH 45229, USA.
authors.forename: Scott A
authors.initials: SA
authors.lastname: Rankin
authors.forename: Jay
authors.initials: J
authors.lastname: Kormish
authors.forename: Matt
authors.initials: M
authors.lastname: Kofron
authors.forename: Anil
authors.initials: A
authors.lastname: Jegga
authors.forename: Aaron M
authors.initials: AM
authors.lastname: Zorn
mesh_terms: D000818:Animals; D004707:Endoderm / Q000378:metabolism*; D051858:Forkhead Transcription Factors / Q000502:physiology; D018507:Gene Expression Regulation, Developmental; D053263:Gene Regulatory Networks / Q000502:physiology*; D018398:Homeodomain Proteins / Q000235:genetics* / Q000502:physiology; D055457:Nodal Protein / Q000502:physiology; D051857:Otx Transcription Factors / Q000502:physiology; D020218:Response Elements; D015398:Signal Transduction; D051899:Smad2 Protein / Q000502:physiology; D014158:Transcription, Genetic; D051153:Wnt Proteins / Q000502:physiology; D029867:Xenopus Proteins / Q000235:genetics* / Q000502:physiology; D014982:Xenopus laevis / Q000196:embryology*; D051176:beta Catenin / Q000502:physiology
chemical_list: C102067:FOXH1 protein, Xenopus; D051858:Forkhead Transcription Factors; D018398:Homeodomain Proteins; D055457:Nodal Protein; D051857:Otx Transcription Factors; C497661:Otx2 protein, Xenopus; C093133:SIA1 protein, Xenopus; D051899:Smad2 Protein; C497785:Smad2 protein, Xenopus; D051153:Wnt Proteins; D029867:Xenopus Proteins; D051176:beta Catenin; C513078:hhex protein, Xenopus; C109979:sia2 protein, Xenopus