Degradation of estrogen receptor α in activated blastocysts is associated with implantation in the delayed implantation mouse model

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 receptora (ERa) is expressed in blastocysts, its targeted disruption leaves embryonic development and implantation unaffected. There- fore, the role of ERa in implanting blastocysts remains unclear. Using a delayed implantation model in mice, we showed increased expression of ERa in implantation-induced (activated) blastocysts; however, this ERa expression in activated blastocysts decreased within 6-h culture. In con- trast, 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 ERa expression in activated blastocysts. Embryo transfer of MG132-treated activated blastocysts into recipient mice on the morning of Day 4 of pseudopregnancy (Day 1 ¼ vaginal plug) 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 ERa in activated blastocyst is associated with completion of blastocyst implantation after embryo transfer on the morning of Day 4 of pseudopregnancy. Our results also suggest that selective protein turnover, such as that of ERa, occurs in activated blastocysts, while expression of other proteins, including Brca1, is maintained at the same stage.