Maternal VegT and ß-Catenin: Patterning the Xenopus Blastula

Abstract

Loss of the maternal T-box transcription factor VegT has a devastating effect on development. Embryos fail to gastrulate, lack the expression of all early zygotic genes characteristic of the endoderm germ layer and also fail to activate ventral, general and dorsal mesodermal gene expression (Zhang et al. 1998; Kofron et al. 1999; Xanthos et al. 2001, 2002). All activity in the activin receptor/Smad 2 signaling pathway is lost (Lee et al. 2001). Embryos depleted of maternal ß-catenin (and therefore deprived of maternal Wnt signaling) also have severe defects. Gastrulation is delayed, embryos develop without heads, dorsal axes and tails and lack neural, dorsal mesodermal and dorsal endodermal gene expression (Heasman et al. 1994; Wylie et al. 1996; Xanthos et al. 2002). Many early zygotic genes have been shown to be targets of these two signaling pathways (see Xanthos et al. 2002 for the expression profiles of zygotic genes in VegT^- and β-catenin^-embryos). The challenge now is to understand the networks downstream of VegT and ß-catenin that are responsible for embryonic patterning. In particular, two aspects of patterning will be considered here: cell fate specification in the animal-vegetal axis, and asymmetrical gene expression in the dorso-ventral axis of the embryo during the late blastula to early gastrula stages.