Reproduction Abstracts

Introduction: Implantation of a blastocyst into a receptive uterus involves a series of highly coordinated cellular and molecular events directed by ovarian estrogen and progesterone. In particular, estrogen is essential for on-time uterine receptivity and blastocyst activation in mice. Although estrogen receptor α (ERα) is expressed in blastocysts, its targeted disruption leaves embryonic development and implantation unaffected. Therefore, the role of ERα in implanting blastocysts remains unclear.

Materials and methods: Delayed implantation was induced and maintained on daily injections of P₄. Implantation-induced (activated) blastocysts were collected 23 h after the E₂ injections. Immunohistochemical analysis was performed to examine the protein expressions. Blastocysts were transferred into recipient mice on the morning of Day 4 of pseudopregnancy (Day 1= vaginal plug).

Results and discussion: Expression of ERα was increased in activated blastocysts; however, this ERα expression in activated blastocysts decreased within 6-h culture. In contrast, breast cancer 1 (Brca1) was maintained in the blastocysts during the culture. The treatment of activated blastocysts with the proteasome inhibitor MG132 demonstrated that proteolysis is associated with down-regulation of ERα expression in activated blastocysts. Embryo transfer of MG132-treated activated blastocysts showed a decreased implantation rate, whereas combined treatment with MG132 and the ER antagonist, ICI 182 780, resulted in recovery of the rate of implantation. This study has revealed that down-regulation of ERα in activated blastocyst is associated with completion of blastocyst implantation. Our results also suggest that selective protein turnover, such as that of ERα, occurs in activated blastocysts, while expression of other proteins, including Brca1, is maintained at the same stage.