SummaryThe AP-2 family of transcription factors consists of five different proteins in humans and mice: AP-2␣, AP-2␤, AP-2␥, AP-2␦ and AP-2⑀. Frogs and fish have known orthologs of some but not all of these proteins, and homologs of the family are also found in protochordates, insects and nematodes. The proteins have a characteristic helix-span-helix motif at the carboxyl terminus, which, together with a central basic region, mediates dimerization and DNA binding. The amino terminus contains the transactivation domain. AP-2 proteins are first expressed in primitive ectoderm of invertebrates and vertebrates; in vertebrates, they are also expressed in the emerging neural-crest cells, and AP-2␣ -/-animals have impairments in neural-crest-derived facial structures. AP-2␤ is indispensable for kidney development and AP-2␥ is necessary for the formation of trophectoderm cells shortly after implantation; AP-2␣ and AP-2␥ levels are elevated in human mammary carcinoma and seminoma. The general functions of the family appear to be the cell-type-specific stimulation of proliferation and the suppression of terminal differentiation during embryonic development. Gene organization and evolutionary historyThe AP-2 family of transcription factors (Ensembl Family ENSF00000001105) consists in humans and mice of five members, AP-2␣, AP-2␤, AP-2␥, AP-2␦ and AP-2⑀; frogs and fish have some of these proteins, and homologs are also known in invertebrates. The chromosomal locations and accession numbers of the family are given in Tables 1 and 2, respectively. All mammalian AP-2 proteins except AP-2␦ are encoded by seven exons and share a characteristic domain structure (reviewed in [1]; for AP-2␦ see [2] and for AP-2⑀ see [3,4]). Orthologs show a similarity between 60 and 99% at the amino-acid level, whereas paralogs show a similarity between 56 and 78%.Analysis of the phylogenetic tree (Figure 1) reveals that the vertebrate AP-2 proteins are grouped together and are divided into five groups. The single Xenopus AP-2 is most closely related to mammalian AP-2␣ proteins. As the genes AP-2␤ and AP-2␦ are found on the same chromosome in chickens, rodents and humans (Table 1), it is likely that they are the result of an internal duplication. According to the phylogenetic tree, AP-2␦ genes appear to have separated from the rest of the family early in the vertebrate clade and to have evolved separately (Figure 1). A BLAST search of the puffer fish Fugu rubripes fourth genome assembly database [5] suggests that there are orthologs of AP-2␣, AP-2␤, AP-2␥ and AP-2⑀ but not AP-2␦ genes in bony fish, although only orthologs of AP-2␣ and AP-2␤ have been found in zebrafish.In the genome of the protochordate Ciona intestinalis a single AP-2 gene has been predicted; the phylogenetic tree shows that the protein evolved before the split of the AP-2␣, AP-2␤, AP-2␥ and AP-2⑀ proteins, with the highest sequence similarity with the AP-2␣ group, suggesting that AP-2␣ might be most similar to the ancestor of AP-2 proteins. This hypothesis is further supported b...

In mammals, cell lineage specification is established at the blastocyst stage. At this stage, transcription factor Cdx2 represses pluripotency genes, thus promoting extraembryonic trophoblast fate. Recently, transcription factor Gata3 was shown to act in a parallel pathway in promoting trophoblast cell fate, suggesting that there are more factors working in the trophoblast lineage. Here, we report that the transcription factor Tcfap2c is expressed at a high level in the trophectoderm and is able to induce trophoblast fate in embryonic stem cells. Trophoblast fate induced by Tcfap2c does not require Cdx2 and vice versa, suggesting that the molecules act in alternative pathways. However, both Tcfap2c and Cdx2 are required for the upregulation of Elf5, a marker of trophoblast stem cell maintenance, suggesting that both factors are required for stable trophoblast induction. Tcfap2c-induced trophoblast-like cells are stable in long-term culture, indicating that they are capable of self-renewal. Tcfap2c-controlled trophoblast maintenance involves the induction of Cdx2 and the repression of the pluripotency factor Nanog. Tcfap2c-induced trophoblast-like cells differentiate to trophoblast derivatives in vitro and contribute to the trophectoderm in blastocysts in vivo. Taken together, these observations suggest that Tcfap2c and Cdx2 cooperate to override the pluripotency program and establish the extraembryonic trophoblast maintenance program in murine embryos.The earliest cell fate decision during mammalian development is the establishment of the first two cell lineages of the blastocyst prior to implantation. The inner cell mass (ICM) forms the embryo proper as well as extraembryonic endodermal components of the placenta, whereas the trophectoderm (TE) gives rise to the fetal portion of the placenta, a structure unique to mammalian development (11). Self-renewing embryonic stem cell (ESC) and trophoblast stem cell (TSC) lines have been derived from each of these lineages in vitro (10,23,36). TSCs exhibit the potential to differentiate into multiple trophoblastic cell types in vitro, participate in the normal development of chimeras, and contribute exclusively to the trophoblastic component of the placenta in vivo (28,36).At the genetic level, key factors that establish and maintain the TE lineage in the early embryo have been identified. Based on so far unknown positional information, the Hippo signaling pathway component YAP (Yes kinase-associated protein 1, a coactivator of Tead4) is phosphorylated by LATS (large tumor suppressor, a Ser/Thr kinase that belongs to the Ndr/LATS subfamily of protein kinase A/PKG/PKC kinases) and becomes cytoplasmic in the inner cells of the morula. In outer blastomeres, YAP remains in the nucleus and associates with and activates TEAD4 (26), which in turn transactivates the expression of the transcription factor CDX2 (caudal-related homeobox 2) (27, 40). CDX2 represses pluripotency markers such as Oct3/4 (Pou5f1) and Nanog (and vice versa), which leads to the maintenance of the restrict...