The role of Wnt-lin28/let7 signaling pathway in embryo implantation competency and trophoblast migration during early pregnancy

Abstract

Embryo implantation consists of a series of complicated and coordinated events, including attachment of the embryo at the blastocyst stage to the endometrial epithelium, epithelial-mesenchymal transition (EMT) and penetration of the blastocyst through the surface of the endometrium. The outer layer of the penetrated blastocyst will then differentiate into the trophoblast cells leading eventually to the formation of the placenta. Implantation failure is a major cause of infertility, while uncontrolled implantation leads to ectopic pregnancy. Despite the importance of these events, our knowledge on the regulation of implantation and placentation differentiation is seriously limited.

Lethal-7 (let-7) is a family of miRNAs highly conserved among animal species. Aberrant expression of miRNA including mir-let-7 has been reported in blastocysts of infertile couples. Lin28 proteins is a natural repressor of mir-let-7 family. Our previous micro-array data has demonstrated that several members of the let-7 family were up-regulated in the dormant blastocysts when compared with the activated blastocysts. Furthermore, by performing signaling pathway analysis, Wnt signaling was shown to be one of the major regulators of mir-let-7 expression. Therefore, the hypothesis of this project was that Wnt/β-catenin signaling regulates embryo implantation competency, implantation accompanying embryonic trophectoderm EMT and uterine receptivity through lin28/let7 pathway.

The first objective examined the roles of Wnt/β-catenin-lin28a/let-7 pathway in regulating the implantation competency of mouse embryo/human embryo surrogate by using both in vivo and in vitro implantation model. It was demonstrated that Wnt/β-catenin signaling pathway stimulates lin28a expression, and thereby facilitates embryo implantation competency through down-regulating mir-let-7 expression.

The second objective investigated the roles of Wnt/β-catenin-lin28a/let7 pathway in regulating the EMT process of mouse embryo/human trophoblast during implantation. EMT was demonstrated to be occurred during mouse embryo outgrowth which is accompanied with Wnt-signaling activation and lin28a up-regulation. On the other hand, mir-let-7 overexpression hinders EMT and thus the implantation rate. Similar observation can be obtained in human trophoblast.

The third objective investigated the roles of mir-let-7 in regulating uterine receptivity. My results demonstrated that mir-let-7 expression is increased in peri-implantation mouse uterine epithelial cells when compared with the pre-implantation period. Maximal mir-let-7 expression can be detected at the implantation site. Exogenous mir-let-7 up-regulation induces both human and mouse uterine epithelial cell receptivity. However, in contrast to embryo/trophoblast, no lin28 expression can be detected in uterus, indicating that mir-let-7 in uterus is not regulated by lin28. Estrogen (E2) and progesterone (P4) was subsequently demonstrated to be the regulators of mir-let-7 expression during peri-implantation uterine epithelium.

In conclusion, this thesis demonstrated that Wnt/β-catenin suppresses mir-let-7 activity through lin28 stimulation to augment embryo implantation competency and induces EMT in embryo. In uterus, on the other hand, high level of mir-let-7 which is regulated by E2 and P4, is essential for establishing uterine receptivity. Results from this study provide the first demonstration on the biological role of Wnt/β-catenin-lin28a/let-7 pathway in implantation. Aberrant expression of let-7 in human blastocysts of infertile couples has been reported. In the long term, the result would provide the scientific basis for possible manipulation of let-7 on enhancing the implantation process.