The molecular mechanisms governing invasive differentiation of human trophoblasts remain largely elusive. Here, we investigated the role of Wnt-␤-catenin-T-cell factor (TCF) signaling in this process. Reverse transcriptase-polymerase chain reaction and Western blot analyses demonstrated expression of Wnt ligands, frizzled receptors, LRP-6, and TCF-3/4 transcription factors in total placenta and different trophoblast cell models. Immunohistochemistry of placental tissues and differentiating villous explant cultures showed that expression of TCF-3/4 strongly increased in invading trophoblasts. Some of these cells also accumulated dephosphorylated ␤-catenin in the nucleus. Wnt3A treatment of primary cytotrophoblasts and SGHPL-5 cells induced activity of TCF-luciferase reporters. Accordingly, the ligand provoked interaction of TCF-3/4 with ␤-catenin as assessed in electrophoretic mobility shift assays (EMSAs) and upregulation of Wnt/TCF target genes as observed by Western blot analyses. Wnt3A stimulated trophoblast migration and invasion through Matrigel, which could be blocked by addition of Dickkopf-1, mediating inhibition of canonical Wnt signaling. Dickkopf-1 also reduced basal migration, invasion, and proliferation of cytotrophoblasts, suggesting expression of endogenous Wnt ligand(s). Immunohistochemistry revealed that the percentage of extravillous trophoblasts containing nuclear ␤-catenin was significantly higher in placentas of complete hydatidiform mole pregnancies as compared to normal placentas. Thus, canonical Wnt signaling may promote invasive trophoblast differentiation, and exaggerated activation of the pathway could contribute to trophoblastic hyperplasia and local invasion.

Protein phosphatase 2A (PP2A) holoenzymes consist of a catalytic C subunit, a scaffolding A subunit, and one of several regulatory B subunits that recruit the AC dimer to substrates. PP2A is required for chromosome segregation, but PP2A's substrates in this process remain unknown. To identify PP2A substrates, we carried out a two-hybrid screen with the regulatory B/PR55 subunit. We isolated a human homolog of C. elegans HCP6, a protein distantly related to the condensin subunit hCAP-D2, and we named this homolog hHCP-6. Both C. elegans HCP-6 and condensin are required for chromosome organization and segregation. HCP-6 binding partners are unknown, whereas condensin is composed of the structural maintenance of chromosomes proteins SMC2 and SMC4 and of three non-SMC subunits. Here we show that hHCP-6 becomes phosphorylated during mitosis and that its dephosphorylation by PP2A in vitro depends on B/PR55, suggesting that hHCP-6 is a B/PR55-specific substrate of PP2A. Unlike condensin, hHCP-6 is localized in the nucleus in interphase, but similar to condensin, hHCP-6 associates with chromosomes during mitosis. hHCP-6 is part of a complex that contains SMC2, SMC4, kleisin-beta, and the previously uncharacterized HEAT repeat protein FLJ20311. hHCP-6 is therefore part of a condensin-related complex that associates with chromosomes in mitosis and may be regulated by PP2A.

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The basic helix-loop-helix (bHLH) transcription factor, Hand1, plays an important role in the development of the murine extra-embryonic trophoblast cell lineage. In the present study, we have analysed the expression of Hand1 in human extra-embryonic cell types and determined its binding specificity and transcriptional activity upon interaction with different class A bHLH factors. Northern blotting and in situ hybridization showed that Hand1 mRNA is specifically expressed in amnion cells at different stages of gestation. Accordingly, we demonstrate that the protein is exclusively produced in the amniotic epithelium in vivo and in purified amnion cells in vitro using a novel polyclonal Hand1 antiserum. Reverse transcriptase-PCR and immunohistochemical staining of blastocysts revealed the production of Hand1 mRNA and polypeptide in the trophectodermal cell layer. In the presence of E12/E47, Hand1 stimulated the transcription of luciferase reporters harbouring degenerate E-boxes, suggesting that E-proteins are potential dimerization partners in trophoblastic tumour and amnion cells. In contrast, Hand1 diminished E12/E47-dependent transcription of reporters containing perfect E-boxes by inhibiting the interaction of Hand1/E-protein heterodimers with the palindromic cognate sequence. Furthermore, we show that Hand1 down-regulated GAL—E12-dependent reporter expression, indicating that the protein can also act directly as a transcriptional repressor. Mutational analyses of GAL-Hand1 suggested that two protein regions located within its N-terminal portion mainly confer the repressing activity. In conclusion, human Hand1 may play an important role in the differentiation of the amniotic membrane and the pre-implanting trophoblast. Furthermore, the data suggest that Hand1 can act as a repressor by two independent mechanisms; sequestration of class A bHLH factors from E-boxes and inhibition of their transcriptional activity.