A two-step model for the fate determination of presumptive endodermal blastomeres in Xenopus embryos.
BACKGROUND
In Xenopus, the endoderm germ layer is derived from the vegetal blastomeres of cleavage-stage embryos. Cell transplantation experiments have revealed that the endodermal fate becomes gradually fixed during the late blastula stages. Sox17alpha, Mix.1, Mixer and GATA-4 encode vegetal zygotic transcription factors with endoderm-inducing activity. The accumulation of their transcripts during the late blastula stages may cause determination of the endodermal fate. VegT, a T-box transcription factor, the maternal transcripts of which are vegetally localised, is also required for endoderm formation.


RESULTS
We analysed the events leading to the progressive accumulation of the transcripts for Sox17alpha, Mix.1, Mixer and GATA-4. Two phases could be distinguished in the endodermal programme. In phase 1, Sox17alpha, Mix.1, and the genes encoding transforming growth factor beta-related signalling molecules Xnr1, Xnr2 and Derrière were activated cell-autonomously at around the mid-blastula transition (MBT) by maternal determinants. In phase 2, TGFbeta signalling, possibly involving Xnr1, Xnr2 and Derrière, led to the activation of Mixer and GATA-4 in late blastula stages and to the reinforcement of the expression of Sox17alpha and Mix.1. Overexpression of VegT in animal caps triggered a developmental programme qualitatively similar to that observed in vegetal blastomeres, except that Xnr1 and GATA-4 were not activated by the early gastrula stage.


CONCLUSIONS
Our results support a two-step model for endoderm determination between fertilisation and the onset of gastrulation. The initial cell-autonomous activation of early endodermal genes by maternal determinants including, but not limited to, VegT is relayed by the action of zygotic TGFbetas such as Xnr1, Xnr2 and Derrière.
Yasuo
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Laboratoire de Génétique et Physiologie du Développement, Institut de Biologie du Développement de Marseille, CNRS-INSERM-Université de la Méditerannée-AP de Marseille, Campus de Luminy, Case 907, F-13288, Marseille, Cedex 9, France.
Lemaire
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D000818:Animals; D015415:Biomarkers; D001757:Blastomeres / Q000378:metabolism*; D002450:Cell Communication; D002874:Chromosome Mapping; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D047108:Embryonic Development; D004707:Endoderm / Q000378:metabolism*; D050991:GATA4 Transcription Factor; D018507:Gene Expression Regulation, Developmental; D006133:Growth Substances / Q000235:genetics / Q000378:metabolism; D006609:High Mobility Group Proteins; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism; D036341:Intercellular Signaling Peptides and Proteins; D055456:Nodal Signaling Ligands; D016133:Polymerase Chain Reaction; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D055760:SOXF Transcription Factors; D020825:T-Box Domain Proteins / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000235:genetics / Q000378:metabolism; D014981:Xenopus / Q000196:embryology*; D029867:Xenopus Proteins
D015415:Biomarkers; D004268:DNA-Binding Proteins; D050991:GATA4 Transcription Factor; C494751:GATA4 protein, Xenopus; C118849:GDF3 protein, Xenopus; D006133:Growth Substances; D006609:High Mobility Group Proteins; D018398:Homeodomain Proteins; D036341:Intercellular Signaling Peptides and Proteins; C100368:Mix1 protein, Xenopus; C113775:Ndr2 protein, vertebrate; D055456:Nodal Signaling Ligands; D011506:Proteins; D055760:SOXF Transcription Factors; C485173:STK38L protein, Xenopus; D020825:T-Box Domain Proteins; D014157:Transcription Factors; D016212:Transforming Growth Factor beta; C103966:VegT protein, Xenopus; D029867:Xenopus Proteins; C512833:nodal1 protein, Xenopus; C491602:sox17a protein, Xenopus; C491601:sox17b.1 protein, Xenopus