Abstract
The primitive endoderm (PrE) appears as an epithelium on the surface of inner cell mass (ICM) at the late blastocyst stage (E4.5). This is the second lineage segregation event in mouse development. The transcription factor gata6 and the Ras/MAPK signaling pathway are essential for the commitment of PrE. To further study the PrE lineage establishment, we generated a mutant mouse line by replacing gata6-coding exons with H2BGFP and monitored gata6 promoter activity in PrE development. We observed that the gata6 promoter driven GFP fluorescence signal was uniformly present initially in heterozygous blastocysts (gata6H2BGFP/+) at the 32-cell stage. However, the signal diverged into two populations by the 64-cell stage; the GFP intensity in cells of ICM either intensified or declined upon differentiation into PrE or epiblast (EPI), respectively. The PrE commitment was also accompanied by a reduced cell motility as the cells sorted to assemble into an epithelium. In homozygous mutant blastocysts (gata6H2BGFP/H2BGFP), initially strong GFP fluorescence throughout the ICM subsided and no PrE formed, suggesting the Gata6 protein is required to autoactivate its promoter activity. In embryonic stem cells in vitro, ectopically expressed Gata6 associated with its endogenous promoter and rescued the activity. Mutation of the motif phosphorylated by Erk1/2 at serine residue (S264) of Gata6 abolished its ability to transactivate promoter activity and PrE differentiation. The results indicate that MAPK phosphorylation of Gata6 protein enhances its association and activation of its own promoter, which then drives a positive feedback regulation and subsequent commitment to PrE lineage. As the blastocyst implants on the uterus, the pluripotent EPI cells will transform into a cup-shaped epithelium with proamniotic cavity formation in the center of the embryo. A recent study using a new technical approach to culture blastocysts in a matrix support in vitro suggests that the proamniotic lumen forms as a result of epithelial organization and rosette morphogenesis without cell death. Nevertheless, so far no gene mutation is known to impact directly the formation of proamniotic lumen as to provide mechanistic support to either model of embryonic cavitation. Phosphatase and tensin homolog (Pten), a lipid phosphatase originally identified as a tumor-suppressor gene, regulates the phosphoinositol 3 signaling pathway and impacts cell death and proliferation. We analyzed Pten mutant embryos in detail and discovered that the formation of the proamniotic cavity is impaired. Embryoid bodies derived from Pten-null embryonic stem (ES) cells failed to undergo cavitation, reproducing the embryonic phenotype in vitro. Analysis of embryoid bodies and embryos revealed a role of Pten in the initiation of the focal point of epithelial rosette that develops into the proamniotic lumen, and in establishing epithelial polarity to transform the amorphous epiblast cells into a polarized epithelium. We conclude that Pten is required for proamniotic cavity formation by establishing polarity for epiblast cells to form a rosette that expands into the proamnotic lumen, rather than facilitating apoptosis to create the cavity.