August 9, 1996;
XTcf-3 transcription factor mediates beta-catenin-induced axis formation in Xenopus embryos.
XTcf-3 is a maternally expressed Xenopus homolog of the mammalian HMG box factors Tcf-1 and Lef-1. The N-terminus of XTcf-3 binds to beta-catenin. Microinjection of XTcf-3 mRNA in embryos results in nuclear translocation of beta-catenin. The beta-catenin-XTcf-3 complex activates transcription in a transient reporter gene assay, while XTcf-3 by itself is silent. N-terminal deletion of XTcf-3 (delta N) abrogates the interaction with beta-catenin, as well as the consequent transcription activation. This dominant-negative delta N mutant suppresses the induction of axis duplication by microinjected beta-catenin. It also suppresses endogenous axis specification upon injection into the dorsal blastomeres of a 4-cell-stage embryo
. We propose that signaling by beta-catenin involves complex formation with XTcf-3, followed by nuclear translocation and activation of specific XTcf-3 target genes.
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Expression Pattern of XTcf-3 in Early Embryonic Stages
(A) Northern blot analysis of RNA extracted from unfertilized eggs (lane 1) and embryos of the stages 6, 8, and 12. The blot was probed for the 3.5 kb XTcf-3 mRNA (top) and subsequently for actin (bottom).
(B and C) In situ hybridization for XTcf-3 expression in early embryonic stages. (B) shows a stage-7 cleared embryo, viewed laterally. (C) shows a section of a stage-7 embryo.
Physical Interaction of XTcf-3 with β-Catenin
In vitro translated XTcf-3 was subjected to gel retardation with an optimal Tcf/Lef probe (see Experimental Procedures). A nonspecific band (N.S.) resulted from an endogenous protein in the transcription/translation lysate (lane 1). F.P., free probe. Cotranslated full-length β-catenin and the deletion mutant ΔN/C β-catenin yielded supershifted bands (lanes 2–4). Deletion of the first 31 amino acids of XTcf-3 (ΔN) abrogated the interaction with β-catenin (lanes 5–7).
Nuclear Translocation of Injected β-Catenin by Coinjected XTcf-3
Stage-7 pre-MBT Xenopus embryos, analyzed by whole-mount immunohistochemistry. Three optical sections (Z-step, 5 μm) were projected onto each other for each picture. At left are the following: Myc-tagged β-catenin mRNA was injected alone (500 pg of RNA) (top) or in combination with 250 pg of RNA encoding wild-type XTcf-3 (middle) or ΔN (bottom). β-Catenin was visualized by staining for the Myc tag. Corresponding BO-PRO-3 nuclear staining is shown to the right.
Suppression of β-Catenin-Induced Axis Formation in Xenopus Embryos
Stage-35 embryos injected at the 4-cell stage in a single ventral blastomere with 250 pg of β-catenin RNA alone (A) or in combination with 250 pg of ΔN RNA (B).
Suppression of Endogenous Axis Formation
(A–D) Embryos allowed to develop until sibling stage 27. Anterior is to the left. (A) shows phenotypes of embryos injected with 250 pg of ΔN RNA in both dorsal blastomeres at the 4-cell stage. (B) shows a horizontal section of an embryo as in (A), with blastopore to the right.
(C and D) Embryos injected with a combination of 250 pg of ΔN RNA and 250 pg of β-gal RNA (C) or β-gal RNA alone (D).
(E and F) In situ hybridization with goosecoid on embryos injected with 250 pg of ΔN (E) or 250 pg of β-gal (F) analyzed at stage 11. Dorsal view of cleared embryos with the dorsal blastopore lip to the bottom. Some nonspecific staining occurs in the blastocoel cavity in (E).
Figure 1. Four Novel Tcf/Lef-like Genes Expressed in Xenopus Embryos
(A) Four highly homologous gene products (XTcf-3, XTcf-3b, XTcf-3c, and XTcf-3d) were identified in a stage-17 embryonic cDNA library. These are compared with human Tcf-1 and Lef-1. The HMG domains are boxed, as is the conserved N-terminal region. The sequences diverge in the region C-terminal to the HMG box. A putative nuclear localization signal (NLS) overlaps with the C-terminus of the HMG box (asterisks; Prieve et al. 1996).
(B) XTcf-3 and β-catenin constructs used in this study.