Cadence of procreation: orchestrating embryo-uterine interactions

Effects of blastocyst elongation and implantation chamber formation on the alignment of the embryonic axis and uterine axis in mice

Embryo implantation involves a series of events that bring the embryo and maternal tissues into contact to support post-implantation development in mammals. During implantation, alignment of the embryonic-abembryonic (E-Ab) axis of the blastocyst with the mesometrial-antimesometrial (M-AM) axis of the uterus precedes post-implantation embryonic development and placentation. In the present study, we observed the morphological changes in blastocysts and the endometrial luminal epithelium (LE) that occur during the alignment of the embryonic and the uterine axes. We found that at the time that the blastocysts attached to the LE at the mural trophectoderm, the embryonic axis was not aligned with the uterine axis. Alignment of the embryonic E-Ab axis with the uterine M-AM axis occurred after E4.0, and the embryo was significantly elongated during the process. The depth of the implantation chamber (IC) correlated with the degree of alignment, suggesting that elongated embryos are oriented along the M-AM axis during IC formation. Transplantation of the Concanavalin A (Con A)-coated beads induced IC formation, and the alignment of two Con A-coated beads present in the same IC in the M-AM direction suggested that elongated materials can align along the M-AM axis. These data suggest that an elongated shape of the embryo and IC formation coordinate the alignment of the embryonic and uterine axes.

Transcriptome profiling reveals superovulation with the gonadotropin-releasing hormone agonist trigger impaired embryo implantation in mice

Introduction

Superovulation is a critical step in assisted reproductive technology, but the use of human chorionic gonadotropin (hCG) as a trigger for superovulation can result in ovarian hyperstimulation. Thus, the use of Gonadotropin-releasing hormone agonist (GnRHa) trigger has been increasingly adopted, although it has been associated with a higher rate of pregnancy failure compared to natural cycles. This study aimed to investigate the effect of GnRHa trigger on embryo implantation in a mouse model.

Methods

Mice in the superovulation (PG) group were administered 7.5 IU of PMSG, followed by the injection of 3.5 μg of GnRHa (Leuprorelin) 48 h later, while mice in the control group (CTR) mated naturally. We compared the number of oocytes, blastocysts, and corpus luteum between the two groups and the implantation sites after the transfer of natural blastocysts. Ovaries, uterus, and serum 2 and 4 days after mating were collected for qRT-PCR, transcriptome sequencing, and hormone assays.

Results

The PG group had more oocytes, blastocysts, and corpus luteum after superovulation than the CTR group. However, the mRNA expression of leukemia inhibitory factor (Lif) and the number of implantation sites were reduced in the PG group. The ELISA assay revealed that superovulation increased ovarian estrogen secretion. The transcriptome analysis showed that superphysiological estrogen led to a response of the uterus to a high estrogen signal, resulting in abnormal endometrium and extracellular matrix remodeling and up-regulation of ion transport and inflammation-related genes.

Conclusion

Our findings suggest that a combination of PMSG and GnRHa trigger impaired embryo implantation in mice, as the excessive uterine response to superphysiological estrogen levels can lead to the change of gene expression related to endometrial remodeling, abnormal expression of uterine ion transport genes and excessive immune-related genes.

Uterus proliferative period ceRNA network of Yunshang black goat reveals candidate genes on different kidding number trait

Pregnancy loss that occurs in the uterus is an important and widespread problem in humans and farm animals and is also a key factor affecting the fecundity of livestock. Understanding the differences in the fecundity of goats may be helpful in guiding the breeding of goats with high fecundity. In this study, we performed RNA sequencing and bioinformatics analysis to study the uterus of Yunshang black goats with high and low fecundity in the proliferative period. We identified mRNAs, long non-coding RNAs (lncRNAs), and microRNAs (miRNAs) by analyzing the uterine transcriptomes. The target genes of the identified miRNAs and lncRNAs were predicted, and miRNA-mRNA interaction and competitive endogenous RNA (ceRNA) networks were constructed. By comparisons between low- and high-fecundity groups, we identified 1,674 differentially expressed mRNAs (914 were upregulated, and 760 were downregulated), 288 differentially expressed lncRNAs (149 were upregulated, and 139 were downregulated), and 17 differentially expressed miRNAs (4 were upregulated, and 13 were downregulated). In addition, 49 miRNA-mRNA pairs and 45 miRNA-lncRNA pairs were predicted in the interaction networks. We successfully constructed a ceRNA interaction network with 108 edges that contained 19 miRNAs, 11 mRNAs, and 73 lncRNAs. Five candidate genes (PLEKHA7, FAT2, FN1, SYK, and ITPR2) that were annotated as cell adhesion or calcium membrane channel protein were identified. Our results provide the overall expression profiles of mRNAs, lncRNAs, and miRNAs in the goat uterus during the proliferative period and are a valuable reference for studies into the mechanisms associated with the high fecundity, which may be helpful to guide goat to reduce pregnancy loss.

Pre-implantation mouse embryo movement under hormonally altered conditions

Pre-implantation embryo movement is crucial to pregnancy success, but the role of ovarian hormones in modulating embryo movement is not understood. We ascertain the effects of altered hormonal environment on embryo location using two delayed implantation mouse models: natural lactational diapause (ND); and artificially induced diapause (AD), a laboratory version of ND generated by ovary removal and provision of supplemental progesterone (P4). Previously, we showed that embryos in a natural pregnancy (NP) first display unidirectional clustered movement, followed by bidirectional scattering and spacing movement. In the ND model, we discovered that embryos are present as clusters near the oviductal-uterine junction for ∼24 h longer than NP, followed by locations consistent with a unidirectional scattering and spacing movement. Intriguingly, the AD model resembles embryo location in NP and not ND. When measuring serum hormone levels, unlike the popular paradigm of reduced estrogen (E2) levels in diapause, we observed that E2 levels are comparable across NP, ND and AD. P4 levels are reduced in ND and highly increased in AD when compared to NP. Further, exogenous administration of E2 or P4 modifies embryo location during the unidirectional phase, while E2 treatment also affects embryo location in the bidirectional phase. Taken together, our data suggest that embryo movement can be modulated by both P4 and E2. Understanding natural hormonal adaptation in diapause provides an opportunity to determine key players that regulate embryo location, thus impacting implantation success. This knowledge can be leveraged to understand pregnancy survival and implantation success in hormonally altered conditions in the clinic.

Upregulation of iNOS and phosphorylated eNOS in the implantation-induced blastocysts of mice

Purpose

This study aimed to examine expressions of iNOS and phosphorylated eNOS (p-eNOS) in implantation-induced blastocysts. We also examined the upstream of p-eNOS.

Methods

To address the protein expressions in implantation-induced blastocysts, we performed immunohistochemical analysis using a delayed implantation mouse model. Immunostaining for iNOS, p-eNOS, and p-Akt was done. To address the relationship between p-eNOS and p-Akt, activated blastocysts were treated with an Akt inhibitor, MK-2206.

Results

iNOS expression was at low levels in dormant blastocysts, whereas the expression was significantly increased in the activated blastocysts. Double staining of p-eNOS and p-Akt in individual blastocysts showed colocalization of p-eNOS and p-Akt of the trophectoderm. p-eNOS and p-Akt expressions were at low levels in dormant blastocysts, whereas both of them were significantly increased in the activated blastocysts. Both dormant and activated blastocysts showed significant positive correlations between p-eNOS and p-Akt. MK-2206 treatment for activated blastocysts showed that blastocysts with lower p-Akt had significantly lower p-eNOS levels.

Conclusions

iNOS and p-eNOS, Ca2+ independent NOS, are upregulated by E2 in the blastocysts during implantation activation. Furthermore, p-eNOS is upregulated in implantation-induced blastocysts downstream of p-Akt.

Gastruloids: A Novel System for Disease Modelling and Drug Testing

By virtue of its inaccessible nature, mammalian implantation stage development has remained one of the most enigmatic and hard to investigate periods of embryogenesis. Derived from pluripotent stem cells, gastruloids recapitulate key aspects of gastrula-stage embryos and have emerged as a powerful in vitro tool to study the architectural features of early post-implantation embryos. While the majority of the work in this emerging field has focused on the use of gastruloids to model embryogenesis, their tractable nature and suitability for high-throughput scaling, has presented an unprecedented opportunity to investigate both developmental and environmental aberrations to the embryo as they occur in vitro. This review summarises the recent developments in the use of gastruloids to model congenital anomalies, their usage in teratogenicity testing, and the current limitations of this emerging field.

Targeted depletion of uterine glandular Foxa2 induces embryonic diapause in mice

Embryonic diapause is a reproductive strategy in which embryo development and growth is temporarily arrested within the uterus to ensure the survival of neonates and mothers during unfavorable conditions. Pregnancy is reinitiated when conditions become favorable for neonatal survival. The mechanism of how the uterus enters diapause in various species remains unclear. Mice with uterine depletion of Foxa2, a transcription factor, are infertile. In this study, we show that dormant blastocysts are recovered from these mice on day 8 of pregnancy with persistent expression of uterine Msx1, a gene critical to maintaining the uterine quiescent state, suggesting that these mice enter embryonic diapause. Leukemia inhibitory factor (LIF) can resume implantation in these mice. Although estrogen is critical for implantation in progesterone-primed uterus, our current model reveals that FOXA2-independent estrogenic effects are detrimental to sustaining uterine quiescence. Interestingly, progesterone and anti-estrogen can prolong uterine quiescence in the absence of FOXA2. Although we find that Msx1 expression persists in the uterus deficient in Foxa2, the complex relationship of FOXA2 with Msx genes and estrogen receptors remains to be explored.

Delayed Implantation Induced by Letrozole in Mice

Implantation timing is critical for a successful pregnancy. A short delay in embryo implantation caused by targeted gene ablation produced a cascading problem in the later stages of the pregnancy. Although several delayed implantation models have been established in wild mice, almost none of them is suitable for investigating the early delay's effects on the late events of pregnancy. Here, we report a new delayed implantation model established by the intraperitoneal administration of letrozole at 5 mg/kg body weight on day 3 of pregnancy. In these mice, initiation of implantation was induced at will by the injection of estradiol (E2). When the estradiol (3 ng) was injected on day 4 of pregnancy (i.e., without delay), the embryo implantation restarted, and the pregnancy continued normally. However, 25 ng estrogen caused compromised implantation. We also found that 67% of the female mice could be pregnant normally and finally gave birth when the estradiol injection (3 ng) was on day 5 of pregnancy (i.e., 1-day delay). Most failed pregnancies had impaired decidualization, decreased serum progesterone levels, and compromised angiogenesis. Progesterone supplementation could rescue decidualization failure in the mice. Collectively, we established a new model of delayed implantation by letrozole, which can be easily applied to study the effect and mechanisms of delay of embryo implantation on the progression of late pregnancy events.

Specification and role of extraembryonic endoderm lineages in the periimplantation mouse embryo

During mammalian embryo development, the correct formation of the first extraembryonic endoderm lineages is fundamental for successful development. In the periimplantation blastocyst, the primitive endoderm (PrE) is formed, which gives rise to the parietal endoderm (PE) and visceral endoderm (VE) during further developmental stages. These PrE-derived lineages show significant differences in both their formation and roles. Whereas differentiation of the PE as a migratory lineage has been suggested to represent the first epithelial-to-mesenchymal transition (EMT) in development, organisation of the epithelial VE is of utmost importance for the correct axis definition and patterning of the embryo. Despite sharing a common origin, the striking differences between the VE and PE are indicative of their distinct roles in early development. However, there is a significant disparity in the current knowledge of each lineage, which reflects the need for a deeper understanding of their respective specification processes. In this review, we will discuss the origin and maturation of the PrE, PE, and VE during the periimplantation period using the mouse model as an example. Additionally, we consider the latest findings regarding the role of the PrE-derived lineages and early embryo morphogenesis, as obtained from the most recent in vitro models.

Analysis of heart and neural crest derivatives-expressed protein 2 (HAND2)-progesterone interactions in peri-implantation endometrium†

Implantation is restricted to a narrow window when the local endometrial microenvironment is supportive of the invading embryo. The ovarian steroid hormones estrogen (E) and progesterone (P) are principal regulators of uterine receptivity. Suppression of E-dependent proliferation of luminal epithelium (LE) by P is mandatory for embryo implantation. Here, we report that the balance of E receptor α (ERα) and P receptors (PR) activity controls HAND2 expression, a key transcription factor that determines the fate of the implanting embryo and thereby pregnancy outcome. As a model, we used wild-type mice as well as mice in which either both PR isoforms or the A-isoform was genetically ablated (PRKO and PRAKO, respectively). Detailed spatiotemporal analyses of PR, HAND2, and ERα expression at implantation site demonstrated co-expression of HAND2 and PR but not ERα. Furthermore, in hormonally treated ovariectomized WT, PRAKO and PRKO mice, E suppresses endometrial HAND2 expression. Adding P together with E partially rescues HAND2 expression in WT, but not PRAKO and PRKO animals. Therefore, infertility in PRAKO mice is at least in part associated with the loss of PR-A-regulated HAND2 expression.

Lipid droplets in mammalian eggs are utilized during embryonic diapause

Lipid droplets (LDs) are stored in the embryo throughout the preimplantation development. Yet, the role of LDs in the embryo remains unknown. Embryonic diapause (ED) is a temporary arrest of an embryo when it waits for the uterine receptivity signal to implant. We provide evidence that LDs play a crucial role in maintaining ED. Diapausing embryos exhibit increased release of exosomes reflecting elevated embryonic signaling to the mother. During ED, the decrease in lipid is caused by a switch from carbohydrate metabolism to lipid catabolism. We have also shown that presence of LDs in the oocytes of various mammals positively corelates with their species-specific length of diapause. Our results reveal the functional role of LDs in embryonic development.

Embryonic diapause (ED) is a temporary arrest of an embryo at the blastocyst stage when it waits for the uterine receptivity signal to implant. ED used by over 100 species may also occur in normally “nondiapausing” mammals when the uterine receptivity signal is blocked or delayed. A large number of lipid droplets (LDs) are stored throughout the preimplantation embryo development, but the amount of lipids varies greatly across different mammalian species. Yet, the role of LDs in the mammalian egg and embryo remains unknown. Here, using a mouse model, we provide evidence that LDs play a crucial role in maintaining ED. By mechanical removal of LDs from zygotes, we demonstrated that delipidated embryos are unable to survive during ED. LDs are not essential for normal prompt implantation, without ED. We further demonstrated that with the progression of ED, the amount of intracellular lipid reduces, and composition changes. This decrease in lipid is caused by a switch from carbohydrate metabolism to lipid catabolism in diapausing blastocysts, which also exhibit increased release of exosomes reflecting elevated embryonic signaling to the mother. We have also shown that presence of LDs in the oocytes of various mammals positively corelates with their species-specific length of diapause. Our results reveal the functional role of LDs in embryonic development. These results can help to develop diagnostic techniques and treatment of recurrent implantation failure and will likely ignite further studies in developmental biology and reproductive medicine fields.

Signaling through retinoic acid receptors is essential for mammalian uterine receptivity and decidualization

Retinoic acid (RA) signaling has long been speculated to regulate embryo implantation, because many enzymes and proteins responsible for maintaining RA homeostasis and transducing RA signals are tightly regulated in the endometrium during this critical period. However, due to a lack of genetic data, it was unclear whether RA signaling is truly required for implantation and which specific RA signaling cascades are at play. Herein we utilize a genetic murine model that expresses a dominant-negative form of RA receptor (RAR) specifically in female reproductive organs to show that functional RA signaling is fundamental to female fertility, particularly implantation and decidualization. Reduction in RA signaling activity severely affects the ability of the uterus to achieve receptive status and decidualize, partially through dampening follistatin expression and downstream activin B/bone morphogenetic protein 2 signaling. To confirm translational relevance of these findings to humans, human endometrial stromal cells (hESCs) were treated with a pan-RAR antagonist to show that in vitro decidualization is impaired. RNA interference perturbation of individual RAR transcripts in hESCs revealed that RARα in particular was essential for proper decidualization. These data provide direct functional evidence that uterine RAR-mediated RA signaling was crucial for mammalian embryo implantation, and its disruption led to failure of uterine receptivity and decidualization, resulting in severely compromised fertility.

Progesterone promotes embryo adhesion by upregulating c-Fos/c-Jun transcription factor-mediated poFUT1 expression†

The proliferation and adhesion abilities of placental trophoblasts are critical for embryo implantation and successful pregnancy. Protein O-fucosyltransferase 1 (poFUT1) and the transcription factor c-Fos/c-Jun have been found to promote trophoblastic cell invade into the endometrium. Progesterone is critical to the regulation of embryonic implantation. However, the exact role of poFUT1 in embryo proliferation and adhesion to the endometrium, and the relationship between progesterone, c-Fos/c-Jun, and poFUT1 has not been studied in detail. In the current study, we found that the serum levels of poFUT1 and progesterone significantly was decreased in miscarriage patients compared with those in normal pregnancy women, and there is a positive correlation between the changes in progesterone and poFUT1. Employing a human embryo trophoblastic cell line (JAR), we showed that progesterone facilitated the activation of c-Fos/c-Jun. Using an electrophoretic mobility shift assay and chromatin immunoprecipitation, we confirmed that the specific transcription factor c-Fos/c-Jun regulated the poFUT1 promoter, which could enhance poFUT1 transcriptional activity, thus further increasing trophoblast cell proliferation and adhesion potential. Taking these findings together, progesterone upregulates poFUT1 expression via the specific transcription factor c-Fos/c-Jun, and then increase trophoblast cell proliferation and adhesion potential. poFUT1 and progesterone may be used together as potential markers of miscarriage, and they may be diagnostic and therapeutic targets for miscarriage.

Melatonin improves uterine-conceptus interaction via regulation of SIRT1 during early pregnancy

Melatonin has been shown to improve in vitro fertilization and offspring survival after bacterial infection, but its role in regulating maternal-fetal communication during early pregnancy has not been investigated. Results of this study demonstrated expression of abundant melatonin receptors in conceptus and endometrium during early pregnancy. In gilts, expression of melatonin receptor 1A (MTNR1A or MT1) and melatonin receptor 1B (MTNR1B or MT2) increased in trophectoderm (Tr) and uterine luminal epithelium (LE) with advancing days during early pregnancy in a different manner. Melatonin increased proliferation and migration of porcine trophectoderm (pTr) cell, the percent pTr cells in the G2 phase of the cell cycle, and the expression of implantation-related genes by pTr cells and endometrial luminal epithelium (pLE). Melatonin also attenuated the production of LPS-induced pro-inflammatory cytokines and tunicamycin-induced endoplasmic reticulum (ER) stress-sensing proteins. The expression of sirtuin 1 (SIRT1) as a potential target of melatonin increased between Days 9 and 14 of gestation. Co-treatment with SIRT1 inhibitor EX527 and melatonin restored cell-cell interactions through PI3K and MAPK signaling. Knockdown of SIRT1 decreased the expression of implantation-related genes, as well as migration of pTr and pLE cells. The expression of microRNAs regulated by SIRT1 was suppressed in response to melatonin. Furthermore, melatonin significantly increased lipopolysaccharide (LPS)-reduced fertilization and embryogenesis in zebrafish model. These results suggest that melatonin may improve the uterine-conceptus interactions via the regulation of SIRT1 during early pregnancy.

A role for Msx genes in mammalian embryonic diapause

Mammalian embryonic diapause is a reproductive phenomenon defined by the reversible arrest in blastocyst development and metabolic activity within the uterus which synchronously becomes quiescent to implantation. This natural strategy, evident in over 130 species across eight orders, can temporally uncouple conception from delivery until conditions are favorable for the survival of the mother and newborn. While the maternal endocrine milieu has been shown to be important for this process, the local molecular mechanisms by which the uterus and embryo achieve quiescence, maintain blastocyst survival and then resumes blastocyst activation with subsequent implantation in response to endocrine cues remains unclear. Here we review the first evidence that the proximal molecular control of embryonic diapause is conserved in three unrelated mammalian species which employ different endocrine programs to initiate diapause. In particular, uterine expression of muscle segment homeobox (Msx) genes Msx1 or Msx2 persists during diapause, followed by downregulation with blastocyst reactivation and implantation. Mice (Mus musculus) with conditional inactivation of Msx1 and Msx2 in the uterus fail to achieve diapause and reactivation. Remarkably, the mink (Neovison vison) and tammar wallaby (Macropus eugenii) share this pattern of MSX1 or MSX2 expression as in mice during delay - it persists during diapause and is rapidly downregulated upon implantation. Therefore, these findings were the first to provide evidence that there are common conserved molecular regulators in the uterus of unrelated mammals during embryonic diapause.

Distinct dormancy progression depending on embryonic regions during mouse embryonic diapause†

Many mammalian species undergo embryonic diapause and suspend development at the blastocyst stage before implantation, which is also known as delayed implantation. We studied the process of how mouse embryos enter a dormancy status at a cellular level. Immunofluorescent analysis of differentiation markers for epiblast, primitive endoderm, and trophectoderm suggested that cell differentiation status was maintained during 7 days in diapause. To understand the progression of cellular dormancy during diapause, we examined the expression of a transgenic cell cycle marker Fucci2 and Ki67 by antibody staining, in addition to direct counting of nuclei in embryos. From these analyses, embryos during diapause were categorized into four stages by cell number and cell cycle. Cell cycle arrest occurred from the ab-embryonic region and from the trophectoderm to the ICM in the embryonic side. We also observed cell cycle transition by live imaging of Fucci2 embryos during the reactivation in culture from dormant status. Cell cycle was initially recovered from the embryonic side of embryos and eventually spread throughout the whole embryo. We also found that embryos in later stages of diapause required a longer period of time for reactivation. From these observations, it was shown that entrance into and exit from dormant status varied depending on cell types and location of cells in an embryo. These results suggest that embryonic diapause includes multiple steps and the mechanisms involved in cellular dormancy may be distinct between embryonic regions.

Characterization of C-C motif chemokine ligand 4 in the porcine endometrium during the presence of the maternal-fetal interface

Chemokines and their receptors play a crucial role in embryo implantation at the maternal-fetal interface during pregnancy. In this study, we investigated the role of CCL4 in development of the porcine endometrium in the early gestational period. Porcine CCL4 showed high similarity with the human counterpart, and mRNA expression of CCL4 and its receptor (CCR5) was predominantly present in the endometrium during early pregnancy. Treatment with CCL4 increased proliferation of porcine uterine luminal epithelial (pLE) cells by activation of PI3K and MAPK signal transduction. In addition, CCL4 recovered the endoplasmic-reticulum stress-reduced proliferation and decreased the unfolded protein response in pLE cells. Besides, the lipopolysaccharide-activated NF-κB pathway was suppressed in response to CCL4 in pLE cells. Inhibition of CCR5 decreased the proliferation of pLE cells and activation of the PI3K and MAPK pathways by CCL4. Furthermore, CCL4 enhanced conceptus-maternal interactions between porcine trophectoderm (pTr) cells and pLE cells during early pregnancy by activating expression of migration and implantation-related genes. Collectively, the results suggest that CCL4 may improve successful implantation in early pregnancy in pigs.

Adrenomedullin improves fertility and promotes pinopodes and cell junctions in the peri-implantation endometrium

Implantation is a complex event demanding contributions from both embryo and endometrium. Despite advances in assisted reproduction, endometrial receptivity defects persist as a barrier to successful implantation in women with infertility. We previously demonstrated that maternal haploinsufficiency for the endocrine peptide adrenomedullin (AM) in mice confers a subfertility phenotype characterized by defective uterine receptivity and sparse epithelial pinopode coverage. The strong link between AM and implantation suggested the compelling hypothesis that administration of AM prior to implantation may improve fertility, protect against pregnancy complications, and ultimately lead to better maternal and fetal outcomes. Here, we demonstrate that intrauterine delivery of AM prior to blastocyst transfer improves the embryo implantation rate and spacing within the uterus. We then use genetic decrease-of-function and pharmacologic gain-of-function mouse models to identify potential mechanisms by which AM confers enhanced implantation success. In epithelium, we find that AM accelerates the kinetics of pinopode formation and water transport and that, in stroma, AM promotes connexin 43 expression, gap junction communication, and barrier integrity of the primary decidual zone. Ultimately, our findings advance our understanding of the contributions of AM to uterine receptivity and suggest potential broad use for AM as therapy to encourage healthy embryo implantation, for example, in combination with in vitro fertilization.

Homeobox genes for embryo implantation: From mouse to human

The proper development of uterus to a state of receptivity and the attainment of implantation competency for blastocyst are 2 indispensable aspects for implantation, which is considered to be a critical event for successful pregnancy. Like many developmental processes, a large number of transcription factors, such as homeobox genes, have been shown to orchestrate this complicated but highly organized physiological process during implantation. In this review, we focus on progress in studies of the role of homeobox genes, especially the Hox and Msx gene families, during implantation, together with subsequent development of post-implantation uterus and related reproductive defects in both mouse models and humans, that have led to better understanding of how implantation is precisely regulated and provide new insights into infertility.

Regulation of Placental Development and Its Impact on Fetal Growth-New Insights From Mouse Models

The placenta is the chief regulator of nutrient supply to the growing embryo during gestation. As such, adequate placental function is instrumental for developmental progression throughout intrauterine development. One of the most common complications during pregnancy is insufficient growth of the fetus, a problem termed intrauterine growth restriction (IUGR) that is most frequently rooted in a malfunctional placenta. Together with conventional gene targeting approaches, recent advances in screening mouse mutants for placental defects, combined with the ability to rapidly induce mutations in vitro and in vivo by CRISPR-Cas9 technology, has provided new insights into the contribution of the genome to normal placental development. Most importantly, these data have demonstrated that far more genes are required for normal placentation than previously appreciated. Here, we provide a summary of common types of placental defects in established mouse mutants, which will help us gain a better understanding of the genes impacting on human placentation. Based on a recent mouse mutant screen, we then provide examples on how these data can be mined to identify novel molecular hubs that may be critical for placental development. Given the close association between placental defects and abnormal cardiovascular and brain development, these functional nodes may also shed light onto the etiology of birth defects that co-occur with placental malformations. Taken together, recent insights into the regulation of mouse placental development have opened up new avenues for research that will promote the study of human pregnancy conditions, notably those based on defects in placentation that underlie the most common pregnancy pathologies such as IUGR and pre-eclampsia.

Regulation of porcupine-dependent Wnt signaling is essential for uterine development and function

Six members of the Wnt family are expressed in the female reproductive tract. Their collective function ensures proper development of the uterus, preparing it for pregnancy during adulthood. Here, we take advantage of the fact that Porcn, a prerequisite for all Wnt secretion, is located on the X chromosome, to generate females that were mosaic for Porcn throughout the reproductive tract. Porcn^flox/+ females were mated with progesterone receptor (Pgr)-Cre males (Pgr^Cre/+ ) to generate females that were heterozygous for Porcupine in all tissues of the female reproductive tract, resulting in mosaicism due to random X-inactivation. We demonstrated that Porcn mosaic females are extremely subfertile and exhibit a large spectrum of phenotypes ranging from morphologically normal uteri to uteri with extremely enlarged cystic glands. Decreased fertility in Porcupine mosaic females was not associated with phenotype severity and was observed regardless of whether or not cystic glands were enlarged. By crossing-in a GFP reporter on the wild-type X chromosome, we were able to correlate endometrial gland hyperplasia with a mostly Porcupine mutant stroma, demonstrating the role of stromal Wnts in the regulation of endometrial gland proliferation. Finally, we demonstrated that fertility issues within mosaic females were due to a reduced response to estrogen and to abnormal Tcf/Lef signaling across the mesometrial-anti-mesometrial axis during the window of implantation.

The enigma of embryonic diapause

Embryonic diapause – a period of embryonic suspension at the blastocyst stage – is a fascinating phenomenon that occurs in over 130 species of mammals, ranging from bears and badgers to mice and marsupials. It might even occur in humans. During diapause, there is minimal cell division and greatly reduced metabolism, and development is put on hold. Yet there are no ill effects for the pregnancy when it eventually continues. Multiple factors can induce diapause, including seasonal supplies of food, temperature, photoperiod and lactation. The successful reactivation and continuation of pregnancy then requires a viable embryo, a receptive uterus and effective molecular communication between the two. But how do the blastocysts survive and remain viable during this period of time, which can be up to a year in some cases? And what are the signals that bring it out of suspended animation? Here, we provide an overview of the process of diapause and address these questions, focussing on recent molecular data.

Apposition to endometrial epithelial cells activates mouse blastocysts for implantation

STUDY QUESTION

How do interactions between blastocyst-stage embryos and endometrial epithelial cells regulate the early stages of implantation in an in vitro model?

SUMMARY ANSWER

Mouse blastocyst apposition with human endometrial epithelial cells initiates trophectoderm differentiation to trophoblast, which goes on to breach the endometrial epithelium.

WHAT IS KNOWN ALREADY

In vitro models using mouse blastocysts and human endometrial cell lines have proven invaluable in the molecular characterisation of embryo attachment to endometrial epithelium at the onset of implantation. Genes involved in embryonic breaching of the endometrial epithelium have not been investigated in such in vitro models.

STUDY DESIGN, SIZE, DURATION

This study used an established in vitro model of implantation to examine cellular and molecular interactions during blastocyst attachment to endometrial epithelial cells.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Mouse blastocysts developed from embryonic day (E) 1.5 in vitro were hatched and co-cultured with confluent human endometrial adenocarcinoma-derived Ishikawa cells in serum-free medium. A scale of attachment stability based on blastocyst oscillation upon agitation was devised. Blastocysts were monitored for 48 h to establish the kinetics of implantation, and optical sectioning using fluorescence microscopy revealed attachment and invasion interfaces. Quantitative PCR was used to determine blastocyst gene expression. Data from a total of 680 mouse blastocysts are reported, with 3-6 experimental replicates. T-test and ANOVA analyses established statistical significance at P < 0.05, P < 0.01 and P < 0.001.

MAIN RESULTS AND THE ROLE OF CHANCE

Hatched E4.5 mouse blastocysts exhibited weak attachment to confluent Ishikawa cells over the first 24 h of co-culture, with intermediate and stable attachment occurring from 28 h (E5.5 + 4 h) in a hormone-independent manner. Attached embryos fixed after 48 h (E6.5) frequently exhibited outgrowths, characterised morphologically and with antibody markers as trophoblast giant cells (TGCs), which had breached the Ishikawa cell layer. Beginning co-culture at E5.5 also resulted in intermediate and stable attachment from E5.5 + 4 h; however, these embryos did not go on to breach the Ishikawa cell layer, even when co-culture was extended to E7.5 (P < 0.01). Blastocysts cultured from E4.5 in permeable transwell inserts above Ishikawa cells before transfer to direct co-culture at E5.5 went on to attach but failed to breach the Ishikawa cell layer by E6.5 (P < 0.01). Gene expression analysis at E5.5 demonstrated that direct co-culture with Ishikawa cells from E4.5 resulted in downregulation of trophectoderm transcription factors Cdx2 (P < 0.05) and Gata3 (P < 0.05) and upregulation of the TGC transcription factor Hand1 (P < 0.05). Co-culture with non-endometrial human fibroblasts did not alter the expression of these genes.

LARGE SCALE DATA

None.

LIMITATIONS, REASONS FOR CAUTION

The in vitro model used here combines human carcinoma-derived endometrial cells with mouse embryos, in which the cellular interactions observed may not fully recapitulate those in vivo. The data gleaned from such models can be regarded as hypothesis-generating, and research is now needed to develop more sophisticated models of human implantation combining multiple primary endometrial cell types with surrogate and real human embryos.

WIDER IMPLICATIONS OF THE FINDINGS

This study implicates blastocyst apposition to endometrial epithelial cells as a critical step in trophoblast differentiation required for implantation. Understanding this maternal regulation of the embryonic developmental programme may lead to novel treatments for infertility.

STUDY FUNDING AND COMPETING INTEREST(S)

This work was supported by funds from the charities Wellbeing of Women (RG1442) and Diabetes UK (15/0005207), and studentship support for SCB from the Anatomical Society. No conflict of interest is declared.

Embryo-epithelium interactions during implantation at a glance

At implantation, with the acquisition of a receptive phenotype in the uterine epithelium, an initial tenuous attachment of embryonic trophectoderm initiates reorganisation of epithelial polarity to enable stable embryo attachment and the differentiation of invasive trophoblasts. In this Cell Science at a Glance article, we describe cellular and molecular events during the epithelial phase of implantation in rodent, drawing on morphological studies both in vivo and in vitro, and genetic models. Evidence is emerging for a repertoire of transcription factors downstream of the master steroidal regulators estrogen and progesterone that coordinate alterations in epithelial polarity, delivery of signals to the stroma and epithelial cell death or displacement. We discuss what is known of the cell interactions that occur during implantation, before considering specific adhesion molecules. We compare the rodent data with our much more limited knowledge of the human system, where direct mechanistic evidence is hard to obtain. In the accompanying poster, we represent the embryo-epithelium interactions in humans and laboratory rodents, highlighting similarities and differences, as well as depict some of the key cell biological events that enable interstitial implantation to occur.

Hormone dependent uterine epithelial-stromal communication for pregnancy support

Human fertility is a relatively inefficient process. Despite the presence of visibly healthy embryos, 30% of pregnancies generated by assisted reproductive technology (ART) fail before the second trimester. The uterine microenvironment plays a critical role in establishing and maintaining a successful pregnancy that requires coordinated communication between the epithelial and stromal cells of the endometrium. The epithelial cells must cease proliferation and become permissive for the conceptus (embryo and associated extraembryonic membranes), while the stromal cells undergoes mesenchymal-to-epithelioid transformation to form the decidua in support of subsequent embryo development. The ovarian steroids Estrogen (E2) and Progesterone (P4) are the major hormones governing these processes. These hormones act via their nuclear receptors, the estrogen receptor, ESR1, and progesterone receptor, PGR, to direct the transcription of genes that orchestrate epithelial and stromal cell communication. This review will discuss the molecular mechanisms utilized by steroid hormones that regulate uterine receptivity, as well, establish and maintain pregnancy.

Introduction: Endometrial function: facts, urban legends, and an eye to the future

The embryo alone, though very important, is not sufficient to explain successful or failed implantation. Human embryonic implantation is less efficient than in nonmenstruating species. The main difference lies in the decidual control of early implantation events and the subsequent course of pregnancy versus embryo control in nonmenstruating species. In this article, we introduce the facts behind the low efficiency of this crucial process, address urban legends routinely considered without high clinical quality evidence, and provide a vision of how the endometrial field will develop in the near future.

Circadian clock-related genetic risk scores and risk of placental abruption

INTRODUCTION

The circadian clock plays an important role in several aspects of female reproductive biology. Evidence linking circadian clock-related genes to pregnancy outcomes has been inconsistent. We sought to examine whether variations in single nucleotide polymorphisms (SNPs) of circadian clock genes are associated with PA risk.

METHODS

Maternal blood samples were collected from 470 PA case and 473 controls. Genotyping was performed using the Illumina Cardio-MetaboChip platform. We examined 119 SNPs in 13 candidate genes known to control circadian rhythms (e.g., CRY2, ARNTL, and RORA). Univariate and penalized logistic regression models were fit to estimate odds ratios (ORs); and the combined effect of multiple SNPs on PA risk was estimated using a weighted genetic risk score (wGRS).

RESULTS

A common SNP in the RORA gene (rs2899663) was associated with a 21% reduced odds of PA (P < 0.05). The odds of PA increased with increasing wGRS (Ptrend < 0.001). The corresponding ORs were 1.00, 1.83, 2.81 and 5.13 across wGRS quartiles. Participants in the highest wGRS quartile had a 5.13-fold (95% confidence interval: 3.21-8.21) higher odds of PA compared to those in the lowest quartile. Although the test for interaction was not significant, the odds of PA was substantially elevated for preeclamptics with the highest wGRS quartile (OR = 14.44, 95%CI: 6.62-31.53) compared to normotensive women in the lowest wGRS quartile.

DISCUSSION

Genetic variants in circadian rhythm genes may be associated with PA risk. Larger studies are needed to corroborate these findings and to further elucidate the pathogenesis of this important obstetrical complication.

Baicalin promotes embryo adhesion and implantation by upregulating fucosyltransferase IV (FUT4) via Wnt/beta-catenin signaling pathway

Glycosylation plays a significant role in determining the receptivity of the uterine endometrium to embryo. Fucosyltransferase IV (FUT4) is expressed stage-specifically in the uterine endometrium of mammalians, and considered as a marker of the endometrial receptivity. Baicalin, a monomer of flavonoids, is known to have functions in improving reproduction. However, the mechanism by which baicalin regulates the expression of FUT4 in embryo-endometrium adhesion remains unclear. Our results showed that baicalin significantly increased FUT4 mRNA and protein expression levels both in human endometrial cells and mouse endometrial tissue, and consistently elevated embryo adhesion rate during implantation in vitro and embryonic implantation competence in pregnant mouse. This study suggests that baicalin facilitates endometrial reproduction via elevating FUT4 expression through Wnt/β-catenin signaling pathway.

Transmembrane Mucin Expression and Function in Embryo Implantation and Placentation

Transmembrane mucins (TMs) are extremely large, complex glycoproteins that line the apical surfaces of simple epithelia including those of the female reproductive tract. TMs provide a physical barrier consistent with their role as part of the innate immune system. This barrier function must be overcome in the context of embryo implantation to permit blastocyst attachment. Three major TMs have been identified in uterine epithelia of multiple species: MUC1, MUC4, and MUC16. MUC1 has been found in all species studied to date, whereas expression of MUC4 and MUC16 have been less well studied and may be species specific. The strategies for removing mucins to permit embryo attachment also vary in a species-specific way and include both hormonal suppression of TM gene expression and membrane clearance via cell surface proteases. Studies emerging from the cancer literature indicate that TMs can modulate a surprisingly wide variety of signal transduction processes. Furthermore, various cell surface proteins have been identified that bind either the oligosaccharide or protein motifs of TMs suggesting that these molecules may support cell attachment in some contexts, including trophoblast interactions with cells of the immune system. The intimate association of TMs at sites of embryo-maternal interaction and the varied functions these complex molecules can play make them key players in embryo implantation and placentation processes.