The primordial germ cells (PGCs) of the mouse are derived from proximal epiblast cells that are adjacent to the extraembryonic ectoderm during gastrulation. Previous studies have demonstrated that extraembryonic ectoderm-derived BMP4 and BMP8B are both required for PGC generation. Here we show that Bmp2, a member of the Dpp class of the Bmp superfamily, also plays a role in PGC generation. PGC number is significantly reduced in Bmp2 heterozygous and homozygous embryos at the N2 generation onto C57BL/6 background. Bmp2 homozygous embryos also have a short allantois and about 50% of them do not undergo normal chorioallantoic fusion. Using whole-mount in situ hybridization, we show that Bmp2 is primarily expressed in the endoderm of mouse pregastrula and gastrula embryos. Using a genetic approach, we further show that Bmp2 and Bmp4, but not Bmp2 and Bmp8b, have an additive effect on PGC generation. These results suggest that PGC generation in the mouse embryo is regulated not only by extraembryonic ectoderm-derived BMP4 and BMP8B, but also by endoderm-derived BMP2.

Extraembryonic ectoderm-derived factors instruct the pluripotent epiblast cells to develop toward a restricted primordial germ cell (PGC) fate during murine gastrulation. Genes encoding Bmp4 of the Dpp class and Bmp8b of the 60A class are expressed in the extraembryonic ectoderm and targeted mutation of either results in severe defects in PGC formation. It has been shown that heterodimers of DPP and 60A classes of bone morphogenetic proteins (BMPs) are more potent than each homodimers in bone and mesoderm induction in vitro, suggesting that BMP4 and BMP8B may form heterodimers to induce PGCs. To investigate how BMP4 and BMP8B interact and signal for PGC induction, we cocultured epiblasts of embryonic day 6.0 -6.25 embryos with BMP4 and BMP8B proteins produced by COS cells. Our data show that BMP4 or BMP8B homodimers alone cannot induce PGCs whereas they can in combination, providing evidence that two BMP pathways are simultaneously required for the generation of a given cell type in mammals and also providing a prototype method for PGC induction in vitro. Furthermore, the PGC defects of Bmp8b mutants can be rescued by BMP8B homodimers whereas BMP4 homodimers cannot mitigate the PGC defects of Bmp4 null mutants, suggesting that BMP4 proteins are also required for epiblast cells to gain germ-line competency before the synergistic action of BMP4 and BMP8B.embryogenesis ͉ mouse ͉ stem cells ͉ cell fate determination P rimordial germ cells (PGCs) are progenitors of all gametes.Maternal factors play critical roles in germ cell specification in Drosophila, Caenorhabditis elegans, zebrafish, and frogs (1-5). By contrast, mice appear to use rather different developmental mechanisms. Embryological studies using lineage tracing by Lawson and Hage (6) revealed that proximal epiblast within one or two cell diameters of the extraembryonic ectoderm at embryonic day 6.0-6.5 (E6.0-6.5) eventually generated PGCs. Furthermore, descendents of a labeled cell in the proximal epiblast were found among PGCs and extraembryonic mesoderm cells (in particular the allantois), suggesting that the PGC fate is not fixed before E6.5 and PGCs and allantois share common precursors. Epiblast transplantation experiments showed that epiblast cells at different topological sites before E6.5 gave rise to PGCs only if they were positioned in close proximity to the extraembryonic ectoderm (7). Therefore, it was hypothesized that factors produced by the extraembryonic ectoderm are required for the generation of PGC precursors that eventually migrate toward the primitive streak and then segregate into PGC and allantois lineages. This hypothesis was further supported by epiblast cocultures with extraembryonic ectoderm (8).Bone morphogenetic proteins (BMPs) are members of the transforming growth factor type ␤ superfamily of growth factors that function as homodimers or heterodimers to signal through heteromeric receptor complexes and downstream SMAD proteins (9 -11). Although significant progress has been made in delineating their signal pathways in general...

Aberrant activation of the WNT/beta-catenin signaling pathway is frequently involved in a broad spectrum of human malignancies. Alternative to genetic deletions and point mutations, epigenetic inactivation of negative WNT regulators, through DNA methylation of promoter CpG islands and/or histone modification, leads to the activation or amplification of aberrant WNT/beta-catenin signaling. In this review, we summarized the contribution of epigenetic dysregulation of WNT/beta-catenin signaling to tumorigenesis and highlighted the importance of epigenetic identification of negative regulators of this pathway as putative tumor suppressors. The reversal of these silenced regulators may be developed as potential cancer therapeutics.