Podocalyxin is a key negative regulator of human endometrial epithelial receptivity for embryo implantation.

STUDY QUESTION: How is endometrial epithelial receptivity, particularly adhesiveness, regulated at the luminal epithelial surface for embryo implantation in the human? SUMMARY ANSWER: Podocalyxin (PCX), a transmembrane protein, was identified as a key negative regulator of endometrial epithelial receptivity; specific downregulation of PCX in the luminal epithelium in the mid-secretory phase, likely mediated by progesterone, may act as a critical step in converting endometrial surface from a non-receptive to an implantation-permitting state. WHAT IS KNOWN ALREADY: The human endometrium must undergo major molecular and cellular changes to transform from a non-receptive to a receptive state to accommodate embryo implantation. However, the fundamental mechanisms governing receptivity, particularly at the luminal surface where the embryo first interacts with, are not well understood. A widely held view is that upregulation of adhesion-promoting molecules is important, but the details are not well characterized. STUDY DESIGN, SIZE, DURATION: This study first aimed to identify novel adhesion-related membrane proteins with potential roles in receptivity in primary human endometrial epithelial cells (HEECs). Further experiments were then conducted to determine candidates' in vivo expression pattern in the human endometrium across the menstrual cycle, regulation by progesterone using cell culture, and functional importance in receptivity using in vitro human embryo attachment and invasion models. PARTICIPANTS/MATERIALS, SETTING,
METHODS: Primary HEECs (n = 9) were isolated from the proliferative phase endometrial tissue, combined into three pools, subjected to plasma membrane protein enrichment by ultracentrifugation followed by proteomics analysis, which led to the discovery of PCX as a novel candidate of interest. Immunohistochemical analysis determined the in vivo expression pattern and cellular localization of PCX in the human endometrium across the menstrual cycle (n = 23). To investigate whether PCX is regulated by progesterone, the master driver of endometrial differentiation, primary HEECs were treated in culture with estradiol and progesterone and analyzed by RT-PCR (n = 5) and western blot (n = 4). To demonstrate that PCX acts as a negative regulator of receptivity, PCX was overexpressed in Ishikawa cells (a receptive line) and the impact on receptivity was determined using in vitro attachment (n = 3-5) and invasion models (n = 4-6), in which an Ishikawa monolayer mimicked the endometrial surface and primary human trophoblast spheroids mimicked embryos. Mann-Whitney U-test and ANOVA analyses established statistical significance at *P ≤ 0.05 and **P ≤ 0.01. MAIN RESULTS AND THE ROLE OF CHANCE: PCX was expressed on the apical surface of all epithelial and endothelial cells in the non-receptive endometrium, but selectively downregulated in the luminal epithelium from the mid-secretory phase coinciding with the establishment of receptivity. Progesterone was confirmed to be able to suppress PCX in primary HEECs, suggesting this hormone likely mediates the downregulation of luminal PCX in vivo for receptivity. Overexpression of PCX in Ishikawa monolayer inhibited not only the attachment but also the penetration of human embryo surrogates, demonstrating that PCX acts as an important negative regulator of epithelial receptivity for implantation. LIMITATIONS, REASONS FOR CAUTION: Primary HEECs isolated from the human endometrial tissue contained a mixture of luminal and glandular epithelial cells, as further purification into subtypes was not possible due to the lack of specific markers. Future study would need to investigate how progesterone differentially regulates PCX in endometrial epithelial subtypes. In addition, this study used primary human trophoblast spheroids as human embryo mimics and Ishikawa as endometrial epithelial cells in functional models, future studies with human blastocysts and primary epithelial cells would further validate the findings. WIDER IMPLICATIONS OF THE
FINDINGS: The findings of this study add important new knowledge to the understanding of human endometrial remodeling for receptivity. The identification of PCX as a negative regulator of epithelial receptivity and the knowledge that its specific downregulation in the luminal epithelium coincides with receptivity development may provide new avenues to assess endometrial receptivity and individualize endometrial preparation protocols in assisted reproductive technology (ART). The study also discovered PCX as progesterone target in HEECs, identifying a potentially useful functional biomarker to monitor progesterone action, such as in the optimization of progesterone type/dose/route of administration for luteal support. STUDY FUNDING/COMPETING INTEREST(S): Study funding was obtained from ESHRE, Monash IVF and NHMRC. LR reports potential conflict of interests (received grants from Ferring Australia; personal fees from Monash IVF Group and Ferring Australia; and non-financial support from Merck Serono, MSD, and Guerbet outside the submitted work. LR is also a minority shareholder and the Group Medical Director for Monash IVF Group, a provider of fertility preservation services). The remaining authors have no potential conflict of interest to declare. TRIAL REGISTRATION NUMBER: NA.