LAUSR

21 S100P, a ubiquitously expressed protein involved in many tumors, has recently been 22 implicated in endometrial receptivity. However, the precise biological function of 23 S100P in the endometrium is still unclear. We aimed to study the dynamic expression 24 of S100P in the endometrium of mice during the peri-implantation period and its role 25 in implantation. Immunostaining revealed that S100P was remarkably up-regulated 26 after pregnancy and peaked at embryo implantation, whereas only low levels were 27 observed in non-pregnant uteri. S100P localized mainly in the cytoplasm and nuclei 28 of subepithelial stromal cells surrounding the embryo at the beginning of implantation 29 and subsequently to the nuclei of stromal cells in the decidua surrounding the embryo. 30 S100P expression also increased in the pseudopregnant uterus, but the 31 immunostaining was much weaker than that in the pregnant uterus, particularly in the 32 luminal and glandular epithelium. Confocal analysis demonstrated that S100P co33 localized with ezrin, with the highest overlap at the maternal-fetal interface. In vitro 34 studies revealed that S100P overexpression increased embryo adhesion and the 35 migration of endometrial cells, and ezrin knockdown reversed S100P-induced cell 36 migration. These data suggest that high levels of S100P are important for endometrial 37 receptivity and fetal-maternal interactions mediated by crosstalk with ezrin. 38