A role for the vegetally expressed Xenopus gene Mix.1 in endoderm formation and in the restriction of mesoderm to the marginal zone.
We have studied the role of the activin immediate-early response gene Mix.1 in mesoderm and endoderm formation. In early gastrulae, Mix.1 is expressed throughout the vegetal hemisphere, including marginal-zone cells expressing the trunk mesodermal marker Xbra. During gastrulation, the expression domains of Xbra and Mix.1 become progressively exclusive as a result of the establishment of a negative regulatory loop between these two genes. This mutual repression is important for the specification of the embryonic body plan as ectopic expression of Mix.1 in the Xbra domain suppresses mesoderm differentiation. The same effect was obtained by overexpressing VP16Mix.1, a fusion protein comprising the strong activator domain of viral VP16 and the homeodomain of Mix.1, suggesting that Mix.1 acts as a transcriptional activator. Mix.1 also has a role in endoderm formation. It cooperates with the dorsal vegetal homeobox gene Siamois to activate the endodermal markers edd, Xlhbox8 and cerberus in animal caps. Conversely, vegetal overexpression of enRMix.1, an antimorphic Mix.1 mutant, leads to a loss of endoderm differentiation. Finally, by targeting enRMix.1 expression to the anterior endoderm, we could test the role of this tissue during embryogenesis and show that it is required for head formation.
Lemaire
P
P
Laboratoire de Génétique et Physiologie du Développement, Institut de Biologie du Développement de Marseille, CNRS-INSERM-Université de la Méditérannée-AP de Marseille, Cedex 9, France. lemaire@lgpd.univ-mrs.fr
Darras
S
S
Caillol
D
D
Kodjabachian
L
L
D000818:Animals; D019521:Body Patterning; D002454:Cell Differentiation; D004268:DNA-Binding Proteins / Q000096:biosynthesis; D004625:Embryo, Nonmammalian / Q000502:physiology*; D004627:Embryonic Induction; D004707:Endoderm / Q000166:cytology / Q000502:physiology*; D005326:Fetal Proteins; D005775:Gastrula / Q000502:physiology*; D018507:Gene Expression Regulation, Developmental; D005801:Genes, Homeobox; D018398:Homeodomain Proteins / Q000096:biosynthesis / Q000235:genetics*; D008648:Mesoderm / Q000502:physiology; D016133:Polymerase Chain Reaction; D012315:RNA Caps / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D020825:T-Box Domain Proteins; D015534:Trans-Activators / Q000096:biosynthesis; D014157:Transcription Factors / Q000096:biosynthesis; D014981:Xenopus; D029867:Xenopus Proteins; D000098908:Brachyury Protein
D004268:DNA-Binding Proteins; D005326:Fetal Proteins; D018398:Homeodomain Proteins; D012315:RNA Caps; D011993:Recombinant Fusion Proteins; D020825:T-Box Domain Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D029867:Xenopus Proteins; D000098908:Brachyury Protein; C100368:Mix1 protein, Xenopus