BackgroundHuman infertility has become a serious and social issue all over the world, especially in developed countries. Numerous types of assisted reproductive technology have been developed and are widely used to treat infertility. However, pregnancy outcomes require further improvement. It is essential to understand the cross‐talk between the uterus (mother) and the embryo (fetus) in pregnancy, which is a very complicated event.MethodsThe mammalian uterus requires many physiological and morphological changes for pregnancy‐associated events, including implantation, decidualization, placentation, and parturition, to occur. Here is discussed recent advances in the knowledge of the molecular mechanisms underlying these reproductive events — in particular, embryonic implantation and decidualization — based on original and review articles.Main findings (Results)In mice, embryonic implantation and decidualization are regulated by two steroid hormones: estrogen and progesterone. Along with these hormones, cytokines, cell‐cycle regulators, growth factors, and transcription factors have essential roles in implantation and decidualization in mice.ConclusionRecent studies using the gene manipulation of mice have given considerable insight into the molecular mechanisms underlying embryonic implantation and decidualization. However, as most of the findings are based on mice, comparative research using different mammalian species will be useful for a better understanding of the species‐dependent differences that are associated with reproductive events, including embryonic implantation.

Vitrification is a powerful tool for the efficient production of offspring derived from cryopreserved oocytes or embryos in mammalian species including domestic animals. Genome editing technologies such as transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeats (CRISPR)/ CRISPR-associated (Cas)9 are now available even for domestic species, suggesting that the vitrification of embryos at the pronuclear stage (PN) will be more important because they could provide genomic host cells to be targeted by TALENs or CRISPR/Cas9. Although we reported the successful production of piglets derived from vitrified PN embryos by a solid-surface vitrification method with glutathione supplementation, further improvements are required. The cryoprotective agent (CPA) carboxylated ε-poly-L-lysine (COOH-PLL) was introduced in 2009. COOH-PLL reduces the physical and physiological damage caused by cryopreservation in mammalian stem cells and the vitrification of mouse oocytes and embryos. Those results suggested that vitrification of COOH-PLL may help improve the developmental ability of pig embryos vitrified at the PN stage. However, it remains unclear whether COOH-PLL is available as a CPA for the vitrification of embryos in domestic species. In this study, we evaluated COOH-PLL as a CPA with ethylene glycol (EG) and Cryotop as a device for the vitrification of PN pig embryos. Exposure to vitrification solution supplemented with COOH-PLL up to 30% did not decrease developmental ability to the 2-cell stage and the blastocyst stage. After warming, most of the vitrified embryos survived regardless of the concentration of COOH-PLL (76.0 ± 11.8% to 91.8 ± 4.6%). However, the vitrified embryos without COOH-PLL showed a lower development rate up to the blastocyst stage (1.3 ± 1.0%) compared to the fresh embryos (28.4 ± 5.0%) (p<0.05). In contrast, supplementation of 20% (w/v) COOH-PLL in the vitrification solution dramatically improved the developmental ability to blastocysts of the vitrified embryos (19.4 ± 4.6%) compared to those without COOH-PLL (p<0.05). After the transfer of embryos vitrified with 30% (v/v) EG and 20% (w/v) COOH-PLL, we successfully obtained 15 piglets from 8 recipients. Taken together, our present findings demonstrate for the first time that COOH-PLL is an effective CPA for embryo vitrification in the pig. COOH-PLL is a promising CPA for further improvements in the vitrification of oocytes and embryos in mammalian species.

In mammals, spermatozoa activate oocytes by triggering a series of intracellular Ca2+ oscillations with phospholipase C zeta (PLCζ), a sperm-borne oocyte-activating factor. Because the introduction of PLCζ alone can induce oocyte activation, it might be a promising reagent for assisted reproductive technologies. To test this possibility, we injected human PLCζ (hPLCζ) mRNA into mouse oocytes at different concentrations. We observed the oocyte activation and subsequent embryonic development. Efficient oocyte activation and embryonic development to the blastocyst stage was achieved only with a limited range of mRNA concentrations (0.1 ng/μl). Higher concentrations of mRNA caused developmental arrest of most embryos, suggesting that excessive PLCζ protein might be harmful at this stage. In a second series of experiments, we aimed to regulate the PLCζ protein concentration in oocytes by applying auxin-inducible degron (AID) technology that allows rapid degradation of the target protein tagged with AID induced by auxin. Injection of the hPLCζ protein tagged with AID and enhanced green fluorescent protein (hPLCζ-AID-EGFP) demonstrated that high EGFP expression levels at the late 1-cell stage were efficiently reduced by auxin treatment, suggesting efficient hPLCζ degradation by this system. Furthermore, the defective development observed with higher concentrations of hPLCζ-AID-EGFP mRNA was rescued following auxin treatment. Full-term offspring were obtained by round spermatid injection with optimized hPLCζ-AID activation. Our results indicate that this AID technology can be applied to regulate the protein levels in mouse oocytes and that our optimized PLCζ system could be used for assisted fertilization in mammals.

In mice, the conditional knockout strategy using the Cre-loxP system is useful for various types of research. The Cre mouse line with progesterone receptor promoter (Pgr Cre ) has been widely used to produce specific uterine gene-deficient mice, but in the Cre line, endogenous Pgr gene is replaced by Cre recombinase gene, which makes the breeding of homozygous mice (Pgr Cre/Cre ) difficult because they are infertile. Yang et al. (2013,

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Leukemia inhibitory factor (LIF) receptor, an interleukin 6 cytokine family signal transducer (Il6st, also known as Gp130) that is expressed in the uterine epithelium and stroma, has been recognized to play an essential role in embryo implantation. However, the molecular mechanism underlying Gp130-mediated LIF signaling in the uterine epithelium during embryo implantation has not been elucidated. In this study, we generated mice with uterine epithelium specific deletion of Gp130 (Gp130 ecKO). Gp130 ecKO females were infertile due to the failure of embryo attachment and decidualization. Histomorphological observation revealed that the endometrial shape and embryo position from Gp130 ecKO were comparable to those of the control, and uterine epithelial cell proliferation, whose attenuation is essential for embryo implantation, was controlled in Gp130 ecKO. Comprehensive gene expression analysis using RNA-seq indicates that epithelial Gp130 regulates the expression of estrogen- and progesterone-responsive genes in conjunction with immune response during embryo implantation. We also found that an epithelial remodeling factor, snail family transcriptional repressor 1 (Snai1), was markedly reduced in the pre-implantation uterus from Gp130 ecKO. These results suggest that not only the suppression of uterine epithelial cell proliferation, but also Gp130-mediated epithelial remodeling is required for successful implantation in mice.

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