Functional differences between Tcf1 isoforms in early Xenopus development

Abstract

In Xenopus gastrula stage embryos, four isoforms of Tcf1 (B, C, D and E) are present with high amino acid sequence conservation compared to fish, mice and human. We studied possible functional differences between these Tcf1 isoforms during early Xenopus development. After overexpression of single Tcf1 isoforms, two distinct phenotypes were observed. Overexpression of the B or D isoforms of Tcf1, which both lack a C-clamp, enhances early canonical Wnt signaling and induces ectopic dorsal mesoderm at the expense of ventrolateral mesoderm prior to gastrulation, causing severe antero-dorzalization of embryos. Overexpression of the E-isoform, which contains a complete C-clamp, does not induce ectopic dorsal mesoderm, but rather leads to severe caudal truncation. Overexpression of the C-isoform, which contains a partial C-clamp, induces a similar phenotype. Mutation of a single amino acid in the C-clamp, known to produce a hypomorphic mutant in D. melanogaster, led to a gain of function in inducing ectopic organizer tissue, as observed after overexpression of the B or D isoforms of Tcf1. Depletion of the C-clamp exon from the zygotic mRNA pool, by injection of a morpholino oligo that targets the splice acceptor site of the exon containing the C-clamp, caused a severe shortening of the AP-axis. Furthermore, embryos showed poor development of the CNS, paraxial mesoderm and primary blood vessels. In situ hybridization analysis showed that Lef1 expression was downregulated at the mid-hindbrain boundary, in the otic vesicles and the branchial arches. The results indicate that in post-gastrula stage Xenopus embryos, the E-tail of Tcf1 is required for expression of Lef1 and for blood vessel formation.

Two Homeobox Transcription Factors, Goosecoid and Ventx1.1, Oppositely Regulate Chordin Transcription in Xenopus Gastrula Embryos

“…We found that the WRE located in the proximal region of GRE1 within the chrd(-2250) promoter was critical for the Gsc-mediated activation of chrd transcription. Previous studies have reported that Tcf7 induces chrd expression in Xenopus embryos [ 27 , 28 ]. Additionally, a recent study demonstrated that the active Wnt/β-catenin signaling-mediated induction of organizer genes requires two steps [ 26 ].…”

“…These results support those of previous reports, wherein ectopic Tcf7 robustly induces chrd expression in Xenopus [ 28 ]. However, different Tcf7 isoforms show functional differences in activating organizer-specific genes and producing distinct Xenopus phenotypes [ 27 ]. We observed that Tcf7 recognizes only the WRE site, suggesting that Wnt/Tcf signaling activates organizer-specific genes, such as chrd, by binding to conversed cis-acting elements [ 25 , 26 , 27 ].…”

“…However, different Tcf7 isoforms show functional differences in activating organizer-specific genes and producing distinct Xenopus phenotypes [ 27 ]. We observed that Tcf7 recognizes only the WRE site, suggesting that Wnt/Tcf signaling activates organizer-specific genes, such as chrd, by binding to conversed cis-acting elements [ 25 , 26 , 27 ]. N-terminal truncated Tcf3 (ΔN-Tcf3) completely abolished chrd expression [ 47 ].…”