Synchronous human embryo development and endometrial preparation for implantation (receptivity) are essential for successful pregnancy. However, understanding of the critical factors that regulate blastocyst-endometrial interactions is very limited. We recently discovered that human blastocysts that fail to implant following IVF, secrete elevated levels of microRNA (miR)-661, which is taken up by primary human endometrial epithelial cells (HEEC) to alter gene expression and adhesive capability. The overall aim was to investigate the mechanisms by which miR-661 regulates endometrial receptivity in humans. MiR-661 down-regulates mouse double minute homolog 2 (MDM2) in breast cancer cells, however MDM2 localization and function in the endometrium have not been studied.
We immunolocalized MDM2 in fertile and infertile (primary/unexplained infertility) endometrial tissue during the receptive phase of the menstrual cycle (n=8/group). Primary HEECs and Ishikawa (endometrial epithelial cell line) cells were transfected with miR-661 mimic (synthetic miR) or control. The effect on MDM2 gene expression was determined (n=4/group). To model endometrial-blastocyst adhesion, we investigated the effect of HTR8/SVneo (trophoblast cell line) spheroid adhesion to Ishikawa cells, or HEECs following MDM2 knockdown by small interfering (siRNA) (n=3/group).
MDM2 localized to the endometrial glandular and luminal epithelium (site of blastocyst attachment) during the receptive phase. MDM2 immunostaining was significantly decreased in endometrial epithelium from infertile versus fertile women (p<0.05). MiR-661 down-regulated MDM2 mRNA in Ishikawa and HEECs. MDM2 knockdown in Ishikawa and primary HEECs reduced HTR8/SVneo spheroid adhesion to both (p<0.05).
This demonstrated that human blastocyst-secreted miR-661 reduces endometrial adhesion via down-regulation of MDM2, suggesting that MDM2 contributes to endometrial-blastocyst adhesion, implantation, and infertility in women. This study highlights a potential new mechanism by which human blastocysts destined to not implant, impair endometrial receptivity and contributes to implantation failure and infertility. This has important implications in developing biomarkers for embryo implantation potential and novel treatments of implantation failure.