Tcf3: a transcriptional regulator of axis induction in the early embryo

Abstract

The roles of Lef/Tcf proteins in determining cell fate characteristics have been described in many contexts during vertebrate embryogenesis, organ and tissue homeostasis, and cancer formation. Although much of the accumulated work on these proteins involves their ability to transactivate target genes when stimulated by β-catenin, Lef/Tcf proteins can repress target genes in the absence of stabilized β-catenin. By ablating Tcf3 function, we have uncovered an important requirement for a repressor function of Lef/Tcf proteins during early mouse development. Tcf3-/- embryos proceed through gastrulation to form mesoderm, but they develop expanded and often duplicated axial mesoderm structures, including nodes and notochords. These duplications are preceded by ectopic expression of Foxa2, an axial mesoderm gene involved in node specification, with a concomitant reduction in Lefty2, a marker for lateral mesoderm. By contrast,expression of a β-catenin-dependent, Lef/Tcf reporter (TOPGal), is not ectopically activated but is faithfully maintained in the primitive streak. Taken together, these data reveal a unique requirement for Tcf3 repressor function in restricting induction of the anterior-posterior axis.

Development of the upper lip: Morphogenetic and molecular mechanisms

“…3E). Furthermore, we found that canonical Wnt signaling is specifically activated in the prefusion epithelia and in the underlying mesenchyme in the medial nasal, lateral nasal, and maxillary processes, as demonstrated by expression of the specifically responsive TOPGAL transgene (DasGupta and Fuchs, 1999;Merrill et al, 2004;Ryan et al, manuscript submitted, Fig. 3F).…”

The β-Catenin/T-Cell Factor/Lymphocyte Enhancer Factor Signaling Pathway Is Required for Normal and Stress-Induced Cardiac Hypertrophy

“…Given the vast experience of numerous investigators employing inhibitory mutants of Tcf/Lef factors to modulate activity of the pathway in model systems from Drosophila melanogaster to mammalian cells (9,24,29,32,41,57,58,61), we chose cardiac-specific transgenic expression of a dominant inhibitory mutant of Lef-1. The alternative of employing Tcf3/ Tcf4 gene-targeted mice was limited by the early lethality of these models, with Tcf3 Ϫ/Ϫ embryos dying at approximately embryonic day 9.5 (expanded and duplicated axial mesoderm structures [39]) and Tcf4 Ϫ/Ϫ mice dying shortly after birth with a phenotype of complete absence of the stem cell compartment in the crypts of the small intestine (28). Other models were considered, including transgenic expression of inhibitors of Wnt signaling (e.g., secreted frizzled-related proteins or Dickkopf proteins), but these seemed illogical since, as noted, our data suggest that hypertrophic stress-induced stabilization of ␤-catenin is Wnt-independent (21).…”

Transgenic Wnt/TCF pathway reporters: all you need is Lef?

“…The activity of TOPGAL was initially described in the context of developing hair follicles, where it partially overlaps with TCF gene expression patterns, correlates with nuclear b-catenin and is inducible by a misexpressed stabilized form of b-catenin Fuchs, 1999, Merrill et al, 2001;Jamora et al, 2003). The reporter is also active in the primitive streak and node of gastrulating embryos (Merrill et al, 2004;Nakaya et al, 2005), in developing lung epithelia (De Langhe et al, 2005;Shu et al, 2005) and during wound healing in skin (Fathke et al, 2006); in all cases, there is evidence for Wnt signaling in these tissues, if not in all TOPGAL-positive cells.…”

“…In some cases, default repression by TCFs appears to be at least as important as Wnt/TCFmediated activation for proper target gene regulation (e.g., Brannon et al, 1997;Crawford et al, 1999;Yang et al, 2000;Knirr and Frasch, 2001;Merrill et al, 2001;Park et al, 2005). It is quite possible that Wnt signaling events that direct the de-repression of TCF target genes, but that do not trigger robust activation by TCFs, might not be detectable by multimerized TCF reporters, which have a baseline of zero and thus cannot register changes in repression (Merrill et al, 2004).…”

“…Although all three of these 'rules' have been broken repeatedly in the recent literature (no names given), many reports (for example, Dorsky et al, 2002;Maretto et al, 2003;Merrill et al, 2004;Dessimoz et al, 2005) have been careful to acknowledge the potential limitations of these useful genetic tools. Perhaps in the future, direct comparisons of different TCF reporters, which were originally designed to be simple indicators of signaling, will help to illuminate the molecular mechanisms of Wnt target gene regulation.…”