We have previously shown that the inhibition of fibroblast growth factor (FGF) signaling induced endodermal gene expression in the animal cap and caused the expansion of the endodermal mass in Xenopus embryos. However, we still do not know whether or not the alteration of FGF signaling controls embryonic cell fate, or when FGF signal blocking is required for endoderm formation in Xenopus. Here, we show that FGF signal blocking in embryonic cells causes their descendants to move into the endodermal region and to express endodermal genes. It is also interesting that blocking FGF signaling between fertilization and embryonic stage 10.5 promotes endoderm formation, but persistent FGF signaling blocking after stage 10.5 restricts endoderm formation and differentiation.
Animals ; Endoderm/drug effects/embryology/*metabolism ; Fibroblast Growth Factors/antagonists & inhibitors/genetics/*physiology ; Gene Expression Regulation, Developmental/drug effects ; In Situ Hybridization ; Pyrroles/administration & dosage/pharmacology ; Receptors, Fibroblast Growth Factor/genetics/physiology ; Reverse Transcriptase Polymerase Chain Reaction ; Signal Transduction/drug effects ; Xenopus Proteins/antagonists & inhibitors/genetics/*physiology ; Xenopus laevis/embryology/genetics/*physiology

The heterodimeric c-Jun/c-Fos, an activator protein-1 (AP-1) has been implicated in mesoderm induction (Dong et al., 1996; Kim et al., 1998) whereas the homodimer of c-Jun was reported to be involved in neural inhibition during the early development of Xenopus embryos. During the early vertebrate development AP-1 involvement in the neural induction is still not clearly understood. We report here that AP-1 has a role in Zic3 expression, a critical proneural gene and a primary regulator of neural and neural crest development (Nakata et al., 1997; Nakata et al., 1998). AP-1 was able to induce the Zic3 gene in a dose dependent manner but other homo- or hetero-dimeric proteins, such as c-Jun/c-Jun, JunD/FosB or JunD/Fra-1 were not. The inhibition of AP-1 activity using morpholino antisenses of c-jun mRNAs blocked the Zic3 expression induced by activin. In addition, co-injection of c-jun mRNA rescued the down-regulated Zic3 expression. The promoter region of isolated Zic3 genomic DNA was found to possess several consensus-binding site of AP-1. Thus, in the functional assays, AP-1 could increase promoter activity of Zic3 gene. These findings suggest that proneural gene, Zic3 may be regulated by heterodimeric AP-1(c-Jun/c-Fos) and it may have a role in activin signaling for the regulation of neural specific gene, Zic3.
Activins/pharmacology/physiology ; Animals ; Base Sequence ; Binding Sites/genetics ; Consensus Sequence/genetics ; Dimerization ; Embryo, Nonmammalian/metabolism ; *Gene Expression Regulation, Developmental ; Homeodomain Proteins/*genetics ; Molecular Sequence Data ; Promoter Regions (Genetics)/genetics ; Proto-Oncogene Proteins c-fos/genetics/physiology ; Proto-Oncogene Proteins c-jun/genetics/physiology ; RNA, Antisense/genetics ; Research Support, Non-U.S. Gov't ; Transcription Factor AP-1/genetics/*physiology ; Transcription Factors/*genetics ; *Transcription, Genetic ; Up-Regulation ; Xenopus Proteins/*genetics ; Xenopus laevis/*embryology/*genetics