T (Brachyury) is a direct target of Wnt3a during paraxial mesoderm specification

Abstract

Wnt3a encodes a signal that is expressed in the primitive streak of the gastrulating mouse embryo and is required for paraxial mesoderm development. In its absence cells adopt ectopic neural fates. Embryos lacking the T-boxcontaining transcription factors, Brachyury or Tbx6, also lack paraxial mesoderm. Here we show that Brachyury is specifically down-regulated in Wnt3a mutants in cells fated to form paraxial mesoderm. Transgenic analysis of the T promoter identifies T (Brachyury) as a direct transcriptional target of the Wnt signaling pathway. Our results suggest that Wnt3a, signaling via Brachyury, modulates a balance between mesodermal and neural cell fates during gastrulation. Received September 13, 1999; revised version accepted October 13, 1999. The embryonic mesoderm of the mammalian embryo is formed by a series of inductive interactions first in the primitive streak, which gives rise to head and trunk mesoderm, and later in the tailbud, which generates the most posterior mesoderm of the tail. As development progresses, successively posterior structures are generated, leading to a posterior extension of the body axis. Previous studies have established that Wnt3a, which encodes a member of the Wnt family of secreted signaling molecules (for review, see Cadigan and Nusse 1997;Moon et al. 1997), is expressed in pluripotent ectoderm cells of the primitive streak during gastrulation (Takada et al. 1994). At early somite stages [8.0-8.5 days postcoitum (dpc)], the Wnt3a expression domain correlates with a domain of cells in the anterior primitive streak fated to give rise to paraxial mesoderm (for review, see Tam and Trainor 1994;Wilson and Beddington 1996). Moreover, the anterior and lateral limits of the Wnt3a expression domain lie between cells fated to give rise to paraxial mesoderm and cells that will give rise to neural ectoderm.A requirement for Wnt3a in the specification of trunk and tail paraxial somitic mesoderm fates has been demonstrated by mutant analyses. Wnt3a homozygous null mutant embryos lack all but the anterior-most seven to nine somites (Takada et al. 1994;Greco et al. 1996;Yoshikawa et al. 1997). As a consequence, only the most rostral cervical vertebrae are formed. Histological and molecular analyses demonstrate that ectopic neural structures form in place of posterior paraxial mesoderm (Yoshikawa et al. 1997 Mesoderm specification is thought to be regulated, at least in part, by members of the T-box gene family of DNA-binding transcription factors (Smith 1999). Two of these, T and Tbx6, are coexpressed with Wnt3a in the primitive streak during gastrulation (Takada et al. 1994;Chapman et al. 1996). Mutations in either gene lead to a loss of trunk and tail mesoderm (Chesley 1935;Chapman and Papaioannou 1998). Ectopic neural tubes form in place of paraxial mesoderm in the Tbx6 mutants, but it is not clear how similar the neural tube abnormalities noted in the T homozygotes are to the Wnt3a phenotype. Given the similarities between the Wnt3a and T-box mutant phenotypes, we have investiga...

Expression of all Wnt genes and their secreted antagonists during mouse blastocyst and postimplantation development

“…ϩ and Ϫ indicate whether the cited references (shown in parentheses) reported expression by ISH, although, published results coming from Northern blots, RT-PCR or microarray studies are shown in brackets. Due to lack of space, not all references could be mentioned, however, the appropriate references as well as expression profiles can be found at the Gene Expression Database web page (2) Bouillet et al (1996); 10, Lloyd et al (2003);11, Zakin et al (2000); 12, Yamaguchi et al (1999);13, Takada et al (1994);14, Hoang et al (1998);15, Lescher et al (1998);16, Leyns et al (1997);17, Finley et al (2003); 18, Glinka et al (1998);19, Monaghan et al (1999).…”

The DVE changes distal epiblast fate from definitive endoderm to neurectoderm by antagonizing nodal signaling

“…However, in DVE-ablated embryos, Wnt3 was expressed in the proximal epiblast after culture (7/7). Thus, the DVE/AVE is required to maintain the posterior expression of Wnt3 and its downstream target T (Yamaguchi et al, 1999) (Fig. 2C',D').…”

A zinc finger transcription factor, ZicL, is a direct activator ofBrachyuryin the notochord specification ofCiona intestinalis

“…A part of the Brachyury expression domain in the primitive streak depends on Wnt3a (Yamaguchi et al, 1999b) and the downstream effectors Lef1 and Tcf1 (Galceran et al, 2001). TCF-binding sites have been found in the mouse Brachyury promoter, and their mutation led to loss of Brachyury expression and Wnt responsiveness (Arnold et al, 2000;Yamaguchi et al, 1999b). However, cis-regulatory elements responsible for the notochord-specific expression of the mouse Brachyury have not been well characterized.…”

“…The regulatory sequences required for the expression of Brachyury in the primitive streak are present within 500 bp upstream of the transcription start site of this gene (Clements et al, 1996;Yamaguchi et al, 1999a). A part of the Brachyury expression domain in the primitive streak depends on Wnt3a (Yamaguchi et al, 1999b) and the downstream effectors Lef1 and Tcf1 (Galceran et al, 2001). TCF-binding sites have been found in the mouse Brachyury promoter, and their mutation led to loss of Brachyury expression and Wnt responsiveness (Arnold et al, 2000;Yamaguchi et al, 1999b).…”