Genes and Pathways Regulating Implantation: Comparative Insights from the Sheep.

Abstract

Implantation in all mammals involves shedding of the zona pellucida, followed by orientation, apposition, attachment, adhesion and invasion of the blastocyst to the endometrium. Endometrial invasion does not occur in domestic ruminants; thus, definitive implantation is achieved by adhesion of the mononuclear trophoblast cells to the endometrial lumenal epithelium (LE) and formation of syncytia by the fusion of trophoblast binucleate cells with the LE. The noninvasive and protracted nature of implantation in domestic animals provides valuable opportunities to investigate fundamental processes of implantation that are shared among all mammals. After entry into the uterus on Day 4 and formation of a blastocyst by Day 6, the blastocyst sheds the zona pellucida (Day 8), elongates to a filamentous form (Days 11-16), and adheres to the endometrial LE (Day 16). Recurrent early pregnancy loss in the uterine gland knockout ewe model indicates that secretions of the endometrial epithelia have physiologic roles and are required for blastocyst elongation and implantation. Continuous exposure of the endometrium to progesterone for 8 to 10 days in early pregnancy downregulates progesterone receptors (PGR) in endometrial epithelia. The loss of epithelial PGR is associated with reductions in cell-surface mucin MUC1 and induction of genes in LE and superficial glandular epithelium (sGE) (CAT2, CST3, CTSL, HIF2A, IGFBP1, LGALS15, SLC2A1, SLC5A1, SLC5A11) and then GE (CST3, CTSL, GRP, SPP1, STC1). These genes encode proteases, protease inhibitors, secreted adhesion molecules, and transporters of amino acids and glucose that modify the uterine milieu to support survival and growth of the conceptus. Interferon tau (IFNT), the pregnancy recognition signal produced by the mononuclear trophectoderm cells of the conceptus, acts on the endometrium to induce genes in LE (WNT7A) or GE (GRP, PRLR), stimulate many of the progesterone-induced genes in LE/sGE, and induce many classical IFN-stimulated genes in the glands, stroma and resident immune cells. Interestingly, IFNT regulation of most epithelial genes requires progesterone, which is likely mediated by progestamedins (FGF7, FGF10 and/or HGF) from the PGR-positive stroma. Between Days 14 and 16 of pregnancy, the binucleate cells begin to differentiate in the trophoblast and subsequently migrate and fuse with the endometrial LE to form syncytia. In the endometrial glands, SPP1 and STC1, and likely other genes, are further stimulated by CSH1 (placental lactogen) from BNC. Thus, the sequential and combinatorial effects of ovarian progesterone and conceptus hormones (IFNT and CSH1) orchestrate changes in the uterine microenvironment that regulate conceptus development. Emerging evidence indicates that loss of the epithelial PGR and the actions of placental hormones in sheep and humans modify the local uterine microenvironment to establish uterine receptivity to support implantation, as well as ensure secretion of histotroph required for conceptus survival and growth. Further, common gene networks and pathways are being discovered across species during the peri-implantation period. A greater understanding of the cellular and molecular signals that regulate uterine receptivity and implantation can be used to identify causes of recurrent pregnancy loss and to improve pregnancy outcome in domestic animals and humans. Supported by grants from the NIH and USDA CSREES.