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Abstract TCF1 belongs to the family of LEF1/TCF transcription factors that regulate gene expression downstream of Wnt/β-catenin signaling, which is crucial for embryonic development and is involved in adult stem cell regulation and tumor growth. In early Xenopus embryos, tcf1 plays an important role in mesoderm induction and patterning. Foxd3 emerged as a potential tcf1 target gene in a microarray analysis of gastrula stage embryos. Because foxd3 and tcf1 are coexpressed during gastrulation, we investigated whether foxd3 is regulated by tcf1. By using morpholino-mediated knockdown, we show that during gastrulation foxd3 expression is dependent on tcf1. By chromatin immunoprecipitation, we also demonstrate direct interaction of β-catenin/tcf complexes with the foxd3 gene locus. Hence, our results indicate that tcf1 acts as an essential activator of foxd3, which is critical for dorsal mesoderm formation in early embryos.
Fig. 1 (left). Overexpression of tcf1 induces a dorsalized phe- notype in Xenopus embryos. (A) Xenopus tropicalis embryos overexpressing tcf1 display premature mesodermal involution. Stage 10.5 embryos, marginally injected in both blastomeres at the two-cell stage with 180 pg of tcf1 RNA, show premature mesodermal involution around the entire marginal zone (81%, n=62; red arrowhead, ventral). (B) Overexpression of tcf1 induces dorsalization. Expression of chordin (83%, n=58) and goosecoid (83%, n=59) has expanded all around the blastophore in Xenopus laevis embryos injected with tcf1 RNA, while expression of wnt8 (73%, n=64) and cdx1 (89%, n=55) is lost.
Fig. 2 (right).Tcf1 regulates the expression of foxd3 in the early mesoderm. (A) Endogenous expression of foxd3 at the dorsal blastoporus lip. (B) Injection of tcf1 RNA in each blastomere (total of 400 pg) in two-cell stage Xenopus laevis embryos leads to the expression of foxd3 all around the blastopore (75%, n=61). (C) Injection of 10 ng of tcf1 morpholino (MO) in both blastomeres at the two-cell stage abrogates foxd3 expression in Xenopus laevis (50%, n=86). (D) Unilateral injection of tcf1 MO (rightblastomere) at the two-cell stage inhibits foxd3 expression only on the injected side (51%, n=84). (E) Coinjection of Xtltcf1 RNA (rightblastomere) with tcf1 MO (both blastomeres) rescues foxd3 expression (rightblastomere) in Xenopus laevis; left side is indicated by a red arrow- head, right side is indicated by a black arrowhead. (NIC, non-injected stage 10.5 control embryo).
