Epithelial-mesenchymal plasticity in trophoblast differentiation

The differentiation of cytotrophoblast (CTB) into extravillous trophoblast (EVT) is central to development of the human placenta. This review explores the process by which CTB differentiate into invasive EVT. At its core is a mechanism known as the epithelial-mesenchymal transition (EMT), whereby immobile epithelial cells are transformed into an invasive, mesenchymal phenotype. Analysis of EMT-associated genes in first trimester CTB and EVT shows clear evidence of an EMT, promoting transition from CTB to EVT. The same analysis of third trimester cells shows evidence of an MET (mesenchymal-epithelial transition); EVT regress from the invasive first trimester mesenchymal phenotype to one characterized as mesenchymal but non-motile and non-proliferative. Of the EMT-master regulators, only the expression of the ZEB2 transcription factor showed a major increase in expression in first trimester EVT, which then reversed in third trimester cells. Overexpression of ZEB2 in trophoblast cell lines led to a pro-EMT shift in molecular markers, a more mesenchymal morphology and increased invasiveness, confirming its probable role in stimulating first trimester EMT. To confirm and compare the EMT observed in EVT with the several known types of EMT, we conducted a deeper gene expression analysis, enabling development of a trophoblast-specific EMT signature. Analysis of DNA methylation revealed genome-wide hypomethylation of third trimester EVT but also a small geneset with gains of methylation. EMT-associated genes showing a gain of methylation and differential expression comprised both pro-EMT and pro-MET genes, suggesting a regulated balance, maintaining the third trimester phenotype. The existence of multiple phenotypes on the EMT spectrum supports the concept of trophoblast epithelial-mesenchymal plasticity (EMP), with EMT-mediated generation of invasive first trimester cells followed by MET-assisted regression to the third trimester phenotype.