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1 Division of Developmental Biology, Children’s Hospital Medical Center, 3333 Burnet Avenue, Cincinnati, OH 45229-3039, USA
2 Department of Pediatrics, University of Minnesota Medical School, MMC 742, Minneapolis MN 55455, USA
3 NIOB/Hubrecht Laboratory, Uppsalalaan 8, 3584 CT, Utrecht, The Netherlands
* These authors contributed equally to this work
Author for correspondence (e-mail: heabq9{at}chmcc.org)
Accepted 29 May 2002
In the early Xenopus embryo, the dorsal axis is specified by a Wnt signal transduction pathway, involving the movement of ß-catenin into dorsal cell nuclei and its functional association with the LEF-type transcription factor XTcf3. The subsequent function of XTcf3 is uncertain. Overexpression data has suggested that it can be both an activator and repressor of downstream genes. XTcf3 mRNA is synthesized during oogenesis in Xenopus and is stored in the egg. To identify its role in dorsal axis specification, we depleted this maternal store in full-grown oocytes using antisense deoxyoligonucleotides, and fertilized them. The developmental effects of XTcf3 depletion, both on morphogenesis and the expression of marker genes, show that primarily, XTcf3 is an inhibitor, preventing both dorsal and ventral cells of the late blastula from expressing dorsal genes. We also show that simple relief from the repression is not the only factor required for dorsal gene expression. To demonstrate this, we fertilized eggs that had been depleted of both XTcf3 and the maternal transcription factor VegT. Dorsal genes normally repressed by XTcf3 are not activated in these embryos. These data show that normal dorsal gene expression in the embryo requires the transcriptional activator VegT, whilst XTcf3 prevents their inappropriate expression on the ventral side of the embryo.
Key words: XTcf3, Antisense oligo, Xenopus, Organizer, VegT
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