(770a) Human Embryonic Stem Cell Models of Early Human Placental Development

The placenta is a complex organ at the maternal-fetal interface, and is critical for fetal development during pregnancy. The trophectoderm layer of the blastocyst stage embryo is the precursor for all trophoblast (TB) cell types in the placenta. Abnormalities in TB development lead to placental pathology, and are associated with adverse outcomes during pregnancy such as preeclampsia and recurrent loss of pregnancy. Moreover, it is now generally accepted that effects on fetal development due to improper placentation may result in adverse health outcomes in later life. Yet, despite its importance to maternal and fetal health, early human TB development remains poorly understood. Mechanistic studies on TB development are difficult due to challenges in research with human embryos and limited availability of placental samples from early gestation. Furthermore, human placental samples collected during the first trimester â?? typically the earliest samples are ~ 6 weeks (~ 28 days post-fertilization) â?? are associated with heterogeneity, containing multiple TB and non-TB cell types, and have likely undergone adaptation to the maternal environment due to the influence of cytokines and chemokines present in the maternal decidua.

The use of human embryonic stem cells (hESCs) to develop TB models has gained significant interest since it was first reported that hESCs treated with Bone Morphogenetic Protein (BMP) differentiate into TB-like cells. The use of hESC-derived TB as a model system is attractive because it overcomes several challenges associated with studies on TB development discussed above. However, lack of understanding of the mechanistic basis of TB differentiation of hESCs is a major impediment for widespread acceptance of hESC-derived TB as a model system for early placental development. Why hESCs, believed to be epiblast-like, can access TB fate remains an enigma. In contrast to mouse embryonic stem cells (mESCs), hESCs have been reported to readily differentiate to TB-like cells upon treatment with Bone Morphogenetic Protein (BMP) and/or inhibitors of the Activin/Nodal. However, whether hESCs can give rise to bona fide TB has been a subject of intense debate. A role for BMP in trophectodermal differentiation has not been identified in the mouse. It has been postulated that mESCs are epigenetically restricted from TB differentiation through hypermethylation of the Elf5 promoter region. Analogously, hESCs are hypermethylated at the ELF5-2b promoter locus, whereas early placental TB is hypomethylated, suggesting a similar epigenetic restriction. Furthermore, unlike placental TB, hESC-derived TB has been reported to not downregulate HLA class I antigens.

We have shown that bona fide TB can indeed be obtained from hESCs, and they possess key markers of placental TB such as hypomethylation of the ELF5 promoter locus and downregulation of HLA class I antigens. Whether the hESC-derived TB can function as bona fide trophoblasts during placental development cannot be experimentally tested. Nevertheless, a surrogate criterion is whether the response of hESC-derived TB towards their signaling environment is similar to those of primary TB. Transforming Growth factor-beta (TGFβ) suppresses epithelial to mesenchymal transformation (EMT) and invasive TB formation from placental TBs and epidermal growth factor (EGF) triggers syncytialization of placental TBs. Our studies show that hESC-derived TB behave similarly, i.e. EGF treatment leads to formation of multinucleate syncytiotrophoblast (STB) and inhibition of Activin/Nodal signaling leads to EMT and formation of invasive TBs. Importantly, we have also identified conditions for selective differentiation of hESC-derived TB to invasive TB or STB lineages in monolayer cultures. Here we specifically discuss the role of BMP signaling in TB fate acquisition in hESCs, and subsequent differentiation to specialized TB subtypes.