Objective: Embryo implantation is a complex process requiring coordinated interactions between a well-developed embryo and a receptive endometrium. However, the fundamental mechanisms governing endometrial receptivity, particularly at the luminal surface where an embryo first interacts with, are not well understood. This study aimed to identify key factors that control human endometrial epithelial receptivity and to determine their functional importance.
Methods: Primary human endometrial epithelial cells were isolated from women and analyzed by proteomics for adhesion-related membrane proteins. Novel candidates of interest were then investigated for in vivo expression pattern and cellular localization in the human endometrium across the menstrual cycle using immunohistochemistry and for hormonal regulation using cell culture. Functional importance was next determined using in vitro models of human embryo attachment and invasion, and using endometrial tissues obtained from women undergoing IVF treatment.
Results: Podocalyxin (PODXL) was identified as a novel endometrial epithelial receptivity marker. PODXL was highly expressed on the apical surface of all epithelial and endothelial cells in the non-receptive endometrium, but selectively and specifically down-regulated in the luminal epithelium at receptivity; this down-regulation was confirmed to be mediated by progesterone. In in vitro implantation models, endometrial epithelial PODXL inhibited not only the attachment but also the invasion of both human embryo mimics (trophoblast spheroids) and actual human embryos, demonstrating that the above described down-regulation of PODXL is essential for endometrial surface receptivity. Clinically, inadequate luminal epithelial down-regulation of PODXL at the time of embryo transfer was associated with implantation failure in women undergoing IVF treatment.
Conclusions: PODXL is a key and previously unknown regulator of human endometrial epithelial receptivity. PODXL inhibits embryo implantation and its down-regulation in the luminal epithelium opens the window of implantation.