Heterologous cell-cell junctions support transepithelial endometrial trophoblast invasion in human

The first step of embryonic trophoblast invasion involves penetration of the endometrial epithelium, which is a unique biological process involving the junction-free apical surfaces of two distinct epithelia. Although cell-cell junctions have been shown to play a role in the establishment of endometrial receptivity, data on their role in the invasion process are lacking. To examine whether human transepithelial trophoblast invasion is linked to heterologous junction formation between trophoblast and endometrial epithelial cells, we used a three-dimensional in vitro implantation model. Hybridoma-derived AC-1M88 extravillous trophoblast spheroids of defined size (300 µm) were placed either on endometrial adenocarcinoma-derived Ishikawa monolayers or on biopsy-derived primary endometrial epithelial cell monolayers. Labelling trophoblast and endometrial cells with different fluorescent dyes allowed differentiation of the two cell populations and monitoring of the invasion process for up to two days after attachment. Trophoblast invasion was tracked by fluorescence microscopy and quantified with the help of image analysis tools. We observed collective transepithelial invasion of the trophoblast into the endometrial monolayers. Junction formation was then assessed by transmission electron microscopy and immunofluorescence microscopy. We observed that transepithelial trophoblast invasion was accompanied by de novo formation of tight junctions, adherens junctions and desmosomes between trophoblast and endometrial cells. The relevance of desmosomal adhesion for invasion was furthermore examined by using blocking antibodies against the desmosomal cadherin desmoglein 2. We could demonstrate that the blocking antibodies stalled the invasion process. Together, we conclude that formation of heterologous cell-cell junctions between trophoblast and endometrial epithelial cells is a robust and important mechanism that facilitates implantation. These findings extend previous studies and offer completely new insights into the functional contribution of cell-cell junction rearrangement to embryo implantation.