Embryo-epithelium interactions during implantation at a glance

Organoids to model the endometrium: implantation and beyond

Despite advances in assisted reproductive techniques in the 4 decades since the first human birth after in vitro fertilisation, 1–2% of couples experience recurrent implantation failure, and some will never achieve a successful pregnancy even in the absence of a confirmed dysfunction. Furthermore, 1–2% of couples who do conceive, either naturally or with assistance, will experience recurrent early loss of karyotypically normal pregnancies. In both cases, embryo-endometrial interaction is a clear candidate for exploration. The impossibility of studying implantation processes within the human body has necessitated the use of animal models and cell culture approaches. Recent advances in 3-dimensional modelling techniques, namely the advent of organoids, present an exciting opportunity to elucidate the unanswerable within human reproduction. In this review, we will explore the ontogeny of implantation modelling and propose a roadmap to application and discovery.

Preimplantation Endometrial Transcriptomics in Natural Conception Cycle of the Rhesus Monkey

There is no report on preimplantation phase endometrial transcriptomics in natural conception cycles of primates. In the present study, the whole-genome expression array of endometrium on Days 2, 4, and 6 post-ovulation (pov) in proven natural conception (Group 1; n = 12) and non-mated, ovulatory (Group 2; n = 12) cycles of rhesus monkeys was examined, compared, and validated. Of fifteen (15) genes showing differential expression (>2-fold; pFDR < 0.05), six genes (CHRND, FOXD3, GJD4, MAPK8IP3, MKS1, and NUP50) were upregulated, while eight genes (ADCY5, ADIPOR1, NNMT, PATL1, PIGV, TGFBR2, TOX2, and VWA5B1) were down regulated on Day 6 pov as compared to Day 2 pov in conception cycles. On Day 6 pov, four genes (ADCY5, NNMT, TOX2, and VWA5B1) were down regulated, and AVEN was upregulated in conception cycles compared with the non-conception cycle. These observations were orthogonally validated at protein expression level. Group-specifically expressed unique genes in conception cycles influence the process of induction of immune-tolerance, while the genes expressed in both groups influence processes of protein targeting and metabolism. A triad of timed-actions of progesterone, seminal plasma, and preimplantation embryo putatively regulate several input molecules to CREB, NF-kB, and STAT regulatory networks during secretory phase towards evolution of endometrial receptivity in the rhesus monkey.

Trophectoderm differentiation to invasive syncytiotrophoblast is promoted by endometrial epithelial cells during human embryo implantation

STUDY QUESTION

How does the human embryo breach the endometrial epithelium at implantation?

SUMMARY ANSWER

Embryo attachment to the endometrial epithelium promotes the formation of multinuclear syncytiotrophoblast from trophectoderm, which goes on to breach the epithelial layer.

WHAT IS KNOWN ALREADY

A significant proportion of natural conceptions and assisted reproduction treatments fail due to unsuccessful implantation. The trophectoderm lineage of the embryo attaches to the endometrial epithelium before breaching this barrier to implant into the endometrium. Trophectoderm-derived syncytiotrophoblast has been observed in recent in vitro cultures of peri-implantation embryos, and historical histology has shown invasive syncytiotrophoblast in embryos that have invaded beyond the epithelium, but the cell type mediating invasion of the epithelial layer at implantation is unknown.

STUDY DESIGN, SIZE, DURATION

Fresh and frozen human blastocyst-stage embryos (n = 46) or human trophoblast stem cell (TSC) spheroids were co-cultured with confluent monolayers of the Ishikawa endometrial epithelial cell line to model the epithelial phase of implantation in vitro. Systems biology approaches with published transcriptomic datasets were used to model the epithelial phase of implantation in silico.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Human embryos surplus to treatment requirements were consented for research. Day 6 blastocysts were co-cultured with Ishikawa cell layers until Day 8, and human TSC spheroids modelling blastocyst trophectoderm were co-cultured with Ishikawa cell layers for 48 h. Embryo and TSC morphology was assessed by immunofluorescence microscopy, and TSC differentiation by real-time quantitative PCR (RT-qPCR) and ELISA. Single-cell human blastocyst transcriptomes, and bulk transcriptomes of TSC and primary human endometrial epithelium were used to model the trophectoderm-epithelium interaction in silico. Hypernetworks, pathway analysis, random forest machine learning and RNA velocity were employed to identify gene networks associated with implantation.

MAIN RESULTS AND THE ROLE OF CHANCE

The majority of embryos co-cultured with Ishikawa cell layers from Day 6 to 8 breached the epithelial layer (37/46), and syncytiotrophoblast was seen in all of these. Syncytiotrophoblast was observed at the embryo-epithelium interface before breaching, and syncytiotrophoblast mediated all pioneering breaching events observed (7/7 events). Multiple independent syncytiotrophoblast regions were seen in 26/46 embryos, suggesting derivation from different regions of trophectoderm. Human TSC spheroids co-cultured with Ishikawa layers also exhibited syncytiotrophoblast formation upon invasion into the epithelium. RT-qPCR comparison of TSC spheroids in isolated culture and co-culture demonstrated epithelium-induced upregulation of syncytiotrophoblast genes CGB (P = 0.03) and SDC1 (P = 0.008), and ELISA revealed the induction of hCGβ secretion (P = 0.03). Secretory-phase primary endometrial epithelium surface transcriptomes were used to identify trophectoderm surface binding partners to model the embryo-epithelium interface. Hypernetwork analysis established a group of 25 epithelium-interacting trophectoderm genes that were highly connected to the rest of the trophectoderm transcriptome, and epithelium-coupled gene networks in cells of the polar region of the trophectoderm exhibited greater connectivity (P < 0.001) and more organized connections (P < 0.0001) than those in the mural region. Pathway analysis revealed a striking similarity with syncytiotrophoblast differentiation, as 4/6 most highly activated pathways upon TSC-syncytiotrophoblast differentiation (false discovery rate (FDR < 0.026)) were represented in the most enriched pathways of epithelium-coupled gene networks in both polar and mural trophectoderm (FDR < 0.001). Random forest machine learning also showed that 80% of the endometrial epithelium-interacting trophectoderm genes identified in the hypernetwork could be quantified as classifiers of TSC-syncytiotrophoblast differentiation. This multi-model approach suggests that invasive syncytiotrophoblast formation from both polar and mural trophectoderm is promoted by attachment to the endometrial epithelium to enable embryonic invasion.

LARGE SCALE DATA

No omics datasets were generated in this study, and those used from previously published studies are cited.

LIMITATIONS, REASONS FOR CAUTION

In vitro and in silico models may not recapitulate the dynamic embryo-endometrial interactions that occur in vivo. The influence of other cellular compartments in the endometrium, including decidual stromal cells and leukocytes, was not represented in these models.

WIDER IMPLICATIONS OF THE FINDINGS

Understanding the mechanism of human embryo breaching of the epithelium and the gene networks involved is crucial to improve implantation success rates after assisted reproduction. Moreover, early trophoblast lineages arising at the epithelial phase of implantation form the blueprint for the placenta and thus underpin foetal growth trajectories, pregnancy health and offspring health.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by grants from Wellbeing of Women, Diabetes UK, the NIHR Local Comprehensive Research Network and Manchester Clinical Research Facility, and the Department of Health Scientist Practitioner Training Scheme. None of the authors has any conflict of interest to declare.

Increased expression of HMGB1 in the implantation phase endometrium is related to recurrent implantation failure

BACKGROUND

Impaired endometrial receptivity was the main cause of recurrent implantation failure (RIF); however, its underlying mechanisms had not been elucidated. This study aimed to determine the expression level of high-mobility group box protein 1 (HMGB1) in the endometrium with RIF and its effect on endometrial receptivity.

METHODS AND RESULTS

Genome-wide expression profiling, real-time reverse transcription PCR, immunohistochemical staining, western blot, and in vitro assays were performed in this study. We found that HMGB1 expression was significantly decreased in the implantation phase endometrium in the control group (patients with tubal infertility and successfully achieve conception after the first embryo transfer) (P = 0.006). However, the expression levels of HMGB1 mRNA and protein were significantly upregulated during the implantation phase in endometrial tissues obtained from patients with RIF compared to that in the control group (P = 0.001), consistent with the results of the genome-wide expression profiling. Moreover, in vitro assays showed that increased expression of HMGB1 in human endometrial epithelial cells dramatically displayed a marked deficiency in supporting blastocysts and human embryonic JAR cells adhesion, which mimic the process of embryo adhesion.

CONCLUSION

These findings strongly indicated that increased HMGB1 levels suppressed the epithelial cell adhesion capability, therefore contributing to impaired endometrial receptivity in patients with recurrent implantation failure, which can be used as a target for the recognition and treatment of recurrent implantation failure in clinical practice.

Recent Insights into Human Endometrial Peptidases in Blastocyst Implantation via Shedding of Microvesicles

Blastocyst implantation involves multiple interactions with numerous molecules expressed in endometrial epithelial cells (EECs) during the implantation window; however, there is limited information regarding the molecular mechanism underlying the crosstalk. In blastocysts, fibronectin plays a major role in the adhesion of various types of cells by binding to extracellular matrix proteins via the Arg-Gly-Asp (RGD) motif. In EECs, RGD-recognizing integrins are important bridging receptors for fibronectin, whereas the non-RGD binding of fibronectin includes interactions with dipeptidyl peptidase IV (DPPIV)/cluster of differentiation (CD) 26. Fibronectin may also bind to aminopeptidase N (APN)/CD13, and in the endometrium, these peptidases are present in plasma membranes and lysosomal membranes. Blastocyst implantation is accompanied by lysosome exocytosis, which transports various peptidases and nutrients into the endometrial cavity to facilitate blastocyst implantation. Both DPPIV and APN are released into the uterine cavity via shedding of microvesicles (MVs) from EECs. Recently, extracellular vesicles derived from endometrial cells have been proposed to act on trophectoderm cells to promote implantation. MVs are also secreted from embryonal stem cells and may play an active role in implantation. Thus, crosstalk between the blastocyst and endometrium via extracellular vesicles is a new insight into the fundamental molecular basis of blastocyst implantation.

Podocalyxin promotes an impermeable epithelium and inhibits pro-implantation factors to negatively regulate endometrial receptivity

Embryo implantation is a key step in establishing pregnancy and a major limiting factor in IVF. Implantation requires a receptive endometrium but the mechanisms governing receptivity are not well understood. We have recently discovered that podocalyxin (PCX or PODXL) is a key negative regulator of human endometrial receptivity. PCX is expressed in all endometrial epithelial cells in the non-receptive endometrium but selectively down-regulated in the luminal epithelium at receptivity. We have further demonstrated that this down-regulation is essential for implantation because PCX inhibits embryo attachment and penetration. However, how PCX confers this role is unknown. In this study, through RNAseq analysis of Ishikawa cell line stably overexpressing PCX, we discovered that PCX suppresses expression of genes controlling cell adhesion and communication, but increases those governing epithelial barrier functions, especially the adherens and tight junctions. Moreover, PCX suppresses multiple factors such as LIF and signaling pathways including Wnt and calcium signaling that support receptivity but stimulates anti-implantation genes such as LEFTY2. Functional studies confirmed that PCX promotes epithelial barrier functions by increasing key epithelial junction proteins such as E-cadherin and claudin 4. PCX thus promotes an anti-adhesive and impermeable epithelium while impedes pro-implantation factors to negatively control endometrial receptivity for implantation.

Modulation of E-Cadherin and N-Cadherin by ovarian steroids and embryonic stimuli

Endometrium is a dynamic tissue that undergoes extensive remodelling to attain a receptive state which is further modulated in presence of an embryo for successful initiation of pregnancy. Cadherins are the proteins of the junctional complex of which E-cadherin (E-Cad) is crucial for maintaining epithelial cell state and integrity of the epithelial barrier; gain of N-cadherin (N-Cad) in epithelial cells leads to epithelial to mesenchymal transition (EMT). In the present study, we investigated the expression of E-Cad and N-Cad in the mouse endometrial luminal epithelium and its modulation by estrogen, progesterone, and embryonic stimuli. We observed that E-Cad is diffusely expressed in the luminal epithelium of mouse endometrium during the estrus stage and upon estrogen treatment. It is apico-laterally and basolaterally sorted at the diestrus stage and in response to the combined treatment of estrogen and progesterone. In 3D spheroids of human endometrial epithelial cells, combined treatment with estrogen and progesterone led to lateral sorting of E-Cad without any effects on its mRNA levels. at the time of embryo implantation, there is loss of E-Cad along with the gain of N-Cad and SNAIL expression suggestive of EMT in the luminal epithelium. This EMT is possibly driven by embryonic stimuli as treatment with estrogen and progesterone did not lead to the gain of N-Cad expression in the mouse endometrium in vivo or in human endometrial epithelial cells in vitro. In conclusion, the present study demonstrates that steroid hormones directly affect E-Cad sorting in the endometrial epithelium which undergo EMT in response to embryonic stimuli.

The role of fascin in carcinogenesis and embryo implantation

The cytoskeleton, with its actin bundling proteins, plays crucial roles in a host of cellular function, such as cancer metastasis, antigen presentation and trophoblast migration and invasion, as a result of cytoskeletal remodeling. A key player in cytoskeletal remodeling is fascin. Upregulation of fascin induces the transition of epithelial phenotypes to mesenchymal phenotypes through complex interaction with transcription factors. Fascin expression also regulates mitochondrial F-actin to promote oxidative phosphorylation (OXPHOS) in some cancer cells. Trophoblast cells, on the other hand, exhibit similar physiological functions, involving the upregulation of genes crucial for its migration and invasion. Owing to the similar tumor-like characteristics among cancer and trophoblats, we review recent studies on fascin in relation to cancer and trophoblast cell biology; and based on existing evidence, link fascin to the establishment of the maternal-fetal interface.

Insights into the Regulation of Implantation and Placentation in Humans, Rodents, Sheep, and Pigs

Precise cell-specific spatio-temporal molecular signaling cascades regulate the establishment and maintenance of pregnancy. Importantly, the mechanisms regulating uterine receptivity, conceptus apposition and adhesion to the uterine luminal epithelia/superficial glandular epithelia and, in some species, invasion into the endometrial stroma and decidualization of stromal cells, are critical prerequisite events for placentation which is essential for the appropriate regulation of feto-placental growth for the remainder of pregnancy. Dysregulation of these signaling cascades during this critical stage of pregnancy can lead to pregnancy loss, impaired growth and development of the conceptus, and alterations in the transplacental exchange of gasses and nutrients. While many of these processes are conserved across species, significant variations in the molecular mechanisms governing maternal recognition of pregnancy, conceptus implantation, and placentation exist. This review addresses the complexity of key mechanisms that are critical for the establishment and maintenance of a successful pregnancy in humans, rodents, sheep, and pigs. Improving understanding of the molecular mechanisms governing these processes is critical to enhancing the fertility and reproductive health of humans and livestock species.

Activation of SGLT3a in endometrial epithelial cells induces paracrine stromal cell decidualization

Endometrial epithelial cells (EECs) and stromal cells (ESCs) have a close functional association. During the peri-implantation period, EECs with enhanced functional activities secrete a variety of paracrine factors to promote the decidualization of ESCs. However, little is known about the specific process by which EECs secrete paracrine factors to induce the decidualization of ESCs. Some evidence suggests that the activation of sodium-glucose cotransporter 3a (SGLT3a) induces the depolarization of ESCs to affect their function. Therefore, SGLT3a acts as a sensor molecule in certain cell types. In this study, the expression of SGLT3a was investigated in EECs to determine whether its levels increased during the peri-implantation period in female mice. The activation of SGLT3a in mouse EECs induced Na⁺ -dependent depolarization of the cell membrane and an influx of extracellular Ca²⁺ , which further promoted the expression and release of the paracrine factors prostaglandin E2 (PGE2) and F2-alpha (PGF2α) by upregulating the expression of cyclooxygenase-2. In turn, PGE2 and PGF2α induced the decidualization of ESCs. Importantly, we identified SGLT3a as a key molecule involved in the cross-talk between EECs and ESCs during the process of uterine decidualization.

Main actors behind the endometrial receptivity and successful implantation

Embryo implantation occurs during a short period of time, the implantation window, in the mid-secretory phase of the menstrual cycle. The cross-talk between the endometrium and the embryo, at the stage of blastocyst, is a necessary condition for successful implantation. Till now, no single molecule or receptor has been identified to play an essential role on embryo implantation but a huge number of mediators, including cytokines, lipids, adhesion molecules, growth factors, and others, are reported to support the establishment of pregnancy. Therefore, the aim of this review is not only to describe the different actors involved in the implantation process, but also to try to characterize the relationships between these factors as well as their time-regulated activation. Moreover, the availability of in vitro culture systems to study the interactions between embryo and endometrium as well as the paracrine communication regulated by exosomal vesicles will be investigated, as an innovative approach for a more precise characterization of the interactions between the different molecules involved in this process. The in-depth knowledge of all these complex mechanisms will allow to address the reasons of implantation failure and infertility, thus providing new avenues for promoting the successful establishment of a pregnancy.

Loss of basigin expression in uterine cells leads to subfertility in female mice†

Basigin (BSG) is a transmembrane glycoprotein involved in cell proliferation, angiogenesis, and tissue remodeling. BSG has been shown to be essential for male and female reproduction although little is known about its role in normal uterine function. To study the potential function of BSG in the female reproductive tract, we generated mice with conditional knockout of Bsg in uterine cells using progesterone receptor-Cre and hypothesized that BSG is required for normal pregnancy in mice. Fertility study data showed that the conditional knockout mice had significantly reduced fertility compared to controls. Ovarian function of the conditional knockout mice appeared normal with no difference in the number of superovulated oocytes collected or in serum progesterone levels between the conditional knockout and the control mice. Uterine tissues collected at various times of gestation showed increased abnormalities in implantation, decidualization, placentation, and parturition in the conditional knockout mice. Uterine cross sections on Day 5 of pregnancy showed implantation failure and abnormal uterine epithelial differentiation in a large proportion of the conditional knockout mice. There was a compromised decidual response to artificial decidualization stimuli and decreased mRNA and protein levels for decidualization genes in the uteri of the conditional knockout mice. We also observed altered protein expression of monocarboxylate transporter 1 (MCT1), as well as impaired angiogenesis in the conditional knockout uteri compared to the controls. These results support that BSG is required for successful pregnancy through its functions in implantation and decidualization.

Alterations of miR-16, miR-let-7a and their target genes expression in human blastocysts following vitrification and re-vitrification

Embryo cryopreservation is a widely used technique in infertility management and today is an essential part of assisted reproductive technology (ART). In some cases, re-vitrification can be applied to good quality supernumerary warmed embryos that have not been transferred in the present cycle. However, there is no study about re-vitrification impact on microRNA and gene expression in human embryos. The purpose of this study is to evaluate miR-16, miR-let7a and target genes expression in in vitro produced human blastocysts following re-vitrification.Day3 embryos obtained from ICSI cycles of fertile couples referring for family balancing program were biopsied and cultured individually. On the fourth day (post-ICSI) male ones (choices of their parents) were transferred and the females (good quality embryos) were donated for research. Donated embryos were cultured to blastocyst stage and assigned to three groups: fresh, vitrified and re-vitrification. Embryos were vitrified on Cryotech carriers. Then blastocysts of three groups were individually assessed for expression of miR-16, miR-let7a and target genes.The results showed that re-vitrification of human blastocysts did not affect the ability to re-expand in culture. In addition, significant decrease was observed in miR-16 and miR-let7a expression in re-vitrified group compared to fresh (p < 0.05). A significant upregulation of the target genes ITGβ3 and BCL-2 in re-vitrified and vitrified embryos was observed compared to the fresh group (p < 0.05). The expression of BAX as a pro-apoptotic gene showed a significant decrease in re-vitrification group comparing with the fresh one (P < 0.05).The results of this research indicated that re-vitrification of embryos changes the expression of miR-16, miR-let-7a and their target genes. These alterations include increased expression of BCl-2 and ITGβ3 genes which play important roles in embryo survival and implantation, respectively. Clinical proof of these effects requires further research.

Interaction networks between the Fallopian tubes and the embryo in human tubal pregnancy: Current knowledge and perspectives

AIM

More than 90% of ectopic pregnancies occur in the Fallopian tubes. As the pathogenesis of tubal pregnancy remains largely unclear, the development of strategies to prevent and treat tubal pregnancy still represents a major clinical challenge. This review thoroughly summarizes the current data, aiming to determine the genes and signaling pathways that are involved in the pathophysiology of human tubal pregnancy.

METHODS

An electronic search from databases of PubMed, Google Scholar, and Chinese databases was carried out using key words pertaining to the pathogenesis of tubal pregnancy from the perspectives of both the Fallopian tubes and the embryo. A review of the literatures including review articles, experimental, and observational studies and case reports published between 1999 and 2021 was conducted.

RESULTS

Tubal pregnancy results from the interaction networks between the Fallopian tube and the embryo rather than from simple tubal abnormality. Furthermore, the embryo-maternal communication is supposed to start from the preimplantation period to the implantation period.

CONCLUSION

A greater understanding of the interaction networks between the Fallopian tubes and the embryo is of great significance for the prevention and medical treatment of tubal pregnancy.

Role of Decidual Natural Killer Cells in Human Pregnancy and Related Pregnancy Complications

Pregnancy is a unique type of immunological process. Healthy pregnancy is associated with a series of inflammatory events: implantation (inflammation), gestation (anti-inflammation), and parturition (inflammation). As the most abundant leukocytes during pregnancy, natural killer (NK) cells are recruited and activated by ovarian hormones and have pivotal roles throughout pregnancy. During the first trimester, NK cells represent up to 50–70% of decidua lymphocytes. Differently from peripheral-blood NK cells, decidual natural killer (dNK) cells are poorly cytolytic, and they release cytokines/chemokines that induce trophoblast invasion, tissue remodeling, embryonic development, and placentation. NK cells can also shift to a cytotoxic identity and carry out immune defense if infected in utero by pathogens. At late gestation, premature activation of NK cells can lead to a breakdown of tolerance of the maternal–fetal interface and, subsequently, can result in preterm birth. This review is focused on the role of dNK cells in normal pregnancy and pathological pregnancy, including preeclampsia, recurrent spontaneous abortion, endometriosis, and recurrent implantation failure. dNK cells could be targets for the treatment of pregnancy complications.

Expression of SGLT1 in the Mouse Endometrial Epithelium and its Role in Early Embryonic Development and Implantation

Many functional activities of endometrium epithelium are energy consuming which are very important for maintaining intrauterine environment needed by early embryonic development and establishment of implantation window. Glucose is a main energy supplier and one of the main components of intrauterine fluid. Obviously, glucose transports in endometrium epithelium involve in for these activities but their functions have not been elucidated. In this research, we observed a spatiotemporal pattern of sodium glucose transporter 1 (SGLT1) expression in the mouse endometrium. We also determined that progesterone can promote the expression of SGLT1 in the mouse endometrial epithelium in response to the action of oestrogen. Treatment with the SGLT1 inhibitor phlorizin or small interfering RNA specific for SGLT1 (SGLT1-siRNA) altered glucose uptake in primary cultured endometrial epithelial cells, which exhibited reduced ATP levels and AMPK activation. The injection of phlorizin or SGLT1-siRNA into one uterine horn of each mouse on day 2 of pregnancy led to an increased glucose concentration in the uterine fluid and decreased number of harvested normal blastocysts and decreased expression of integrin αVβ3 in endometrial epithelium and increased expression of mucin 1 and lactoferrin in endometrial epithelium and the uterine homogenates exhibited activated AMPK, a decreased ATP level on day 4, and a decreased number of implantation sites on day 5. In embryo transfer experiments, pre-treatment of the uterine horn with phlorizin or SGLT1-siRNA during the implantation window led to a decreased embryo implantation rate on day 5 of pregnancy, even when embryos from normal donor mice were used. In conclusion, SGLT1, which participates in glucose transport in the mouse endometrial epithelium, inhibition and/or reduced expression of SGLT1 affects early embryo development by altering the glucose concentration in the uterine fluid. Inhibition and/or reduced expression of SGLT1 also affects embryo implantation by influencing energy metabolism in epithelial cells, which consequently influences implantation-related functional activities.

GLUT4 in Mouse Endometrial Epithelium: Roles in Embryonic Development and Implantation

GLUT4 is involved in rapid glucose uptake among various kinds of cells to contribute to glucose homeostasis. Prior data have reported that aberrant glucose metabolism by GLUT4 dysfunction in the uterus could be responsible for infertility and increased miscarriage. However, the expression and precise functions of GLUT4 in the endometrium under physiological conditions remain unknown or controversial. In this study, we observed that GLUT4 exhibits a spatiotemporal expression in mouse uterus on pregnant days 1–4; its expression especially increased on pregnant day 4 during the window of implantation. We also determined that estrogen, in conjunction with progesterone, promotes the expression of GLUT4 in the endometrial epithelium in vivo or in vitro. GLUT4 is an important transporter that mediates glucose transport in endometrial epithelial cells (EECs) in vitro or in vivo. In vitro, glucose uptake decreased in mouse EECs when the cells were treated with GLUT4 small interfering RNA (siRNA). In vivo, the injection of GLUT4-siRNA into one side of the mouse uterine horns resulted in an increased glucose concentration in the uterine fluid on pregnant day 4, although it was still lower than in blood, and impaired endometrial receptivity by inhibiting pinopode formation and the expressions of leukemia inhibitory factor (LIF) and integrin ανβ3, finally affecting embryonic development and implantation. Overall, the obtained results indicate that GLUT4 in the endometrial epithelium affects embryo development by altering glucose concentration in the uterine fluid. It can also affect implantation by impairing endometrial receptivity due to dysfunction of GLUT4.

Cell dynamics in human villous trophoblast

BACKGROUND

Villous cytotrophoblast (vCTB) is a precursor cell population that supports the development of syncytiotrophoblast (vSTB), the high surface area barrier epithelium of the placental villus, and the primary interface between maternal and fetal tissue. In light of increasing evidence that the placenta can adapt to changing maternal environments or, under stress, can trigger maternal disease, we consider what properties of these cells empower them to exert a controlling influence on pregnancy progression and outcome.

OBJECTIVE AND RATIONALE

How are cytotrophoblast proliferation and differentiation regulated in the human placental villus to allow for the increasing demands of the fetal and environmental challenges and stresses that may arise during pregnancy?

SEARCH METHODS

PubMed was interrogated using relevant keywords and word roots combining trophoblast, villus/villous, syncytio/syncytium, placenta, stem, transcription factor (and the individual genes), signalling, apoptosis, autophagy (and the respective genes) from 1960 to the present. Since removal of trophoblast from its tissue environment is known to fundamentally change cell growth and differentiation kinetics, research that relied exclusively on cell culture has not been the main focus of this review, though it is mentioned where appropriate. Work on non-human placenta is not systematically covered, though mention is made where relevant hypotheses have emerged.

OUTCOMES

The synthesis of data from the literature has led to a new hypothesis for vCTB dynamics. We propose that a reversible transition can occur from a reserve population in G0 to a mitotically active state. Cells from the in-cycle population can then differentiate irreversibly to intermediate cells that leave the cycle and turn on genes that confer the capacity to fuse with the overlying vSTB as well as other functions associated with syncytial barrier and transport function. We speculate that alterations in the rate of entry to the cell cycle, or return of cells in the mitotic fraction to G0, can occur in response to environmental challenge. We also review evidence on the life cycle of trophoblast from the time that fusion occurs, and point to gaps in knowledge of how large quantities of fetal DNA arrive in maternal circulation. We critique historical methodology and make a case for research to re-address questions about trophoblast lifecycle and dynamics in normal pregnancy and the common diseases of pre-eclampsia and fetal growth restriction, where altered trophoblast kinetics have long been postulated.

WIDER IMPLICATIONS

The hypothesis requires experimental testing, moving research away from currently accepted methodology towards a new standard that includes representative cell and tissue sampling, assessment of cell cycle and differentiation parameters, and robust classification of cell subpopulations in villous trophoblast, with due attention to gestational age, maternal and fetal phenotype, disease and outcome.

Podocalyxin is a key negative regulator of human endometrial epithelial receptivity for embryo implantation

STUDY QUESTION

How is endometrial epithelial receptivity, particularly adhesiveness, regulated at the luminal epithelial surface for embryo implantation in the human?

SUMMARY ANSWER

Podocalyxin (PCX), a transmembrane protein, was identified as a key negative regulator of endometrial epithelial receptivity; specific downregulation of PCX in the luminal epithelium in the mid-secretory phase, likely mediated by progesterone, may act as a critical step in converting endometrial surface from a non-receptive to an implantation-permitting state.

WHAT IS KNOWN ALREADY

The human endometrium must undergo major molecular and cellular changes to transform from a non-receptive to a receptive state to accommodate embryo implantation. However, the fundamental mechanisms governing receptivity, particularly at the luminal surface where the embryo first interacts with, are not well understood. A widely held view is that upregulation of adhesion-promoting molecules is important, but the details are not well characterized.

STUDY DESIGN, SIZE, DURATION

This study first aimed to identify novel adhesion-related membrane proteins with potential roles in receptivity in primary human endometrial epithelial cells (HEECs). Further experiments were then conducted to determine candidates' in vivo expression pattern in the human endometrium across the menstrual cycle, regulation by progesterone using cell culture, and functional importance in receptivity using in vitro human embryo attachment and invasion models.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Primary HEECs (n = 9) were isolated from the proliferative phase endometrial tissue, combined into three pools, subjected to plasma membrane protein enrichment by ultracentrifugation followed by proteomics analysis, which led to the discovery of PCX as a novel candidate of interest. Immunohistochemical analysis determined the in vivo expression pattern and cellular localization of PCX in the human endometrium across the menstrual cycle (n = 23). To investigate whether PCX is regulated by progesterone, the master driver of endometrial differentiation, primary HEECs were treated in culture with estradiol and progesterone and analyzed by RT-PCR (n = 5) and western blot (n = 4). To demonstrate that PCX acts as a negative regulator of receptivity, PCX was overexpressed in Ishikawa cells (a receptive line) and the impact on receptivity was determined using in vitro attachment (n = 3-5) and invasion models (n = 4-6), in which an Ishikawa monolayer mimicked the endometrial surface and primary human trophoblast spheroids mimicked embryos. Mann-Whitney U-test and ANOVA analyses established statistical significance at *P ≤ 0.05 and **P ≤ 0.01.

MAIN RESULTS AND THE ROLE OF CHANCE

PCX was expressed on the apical surface of all epithelial and endothelial cells in the non-receptive endometrium, but selectively downregulated in the luminal epithelium from the mid-secretory phase coinciding with the establishment of receptivity. Progesterone was confirmed to be able to suppress PCX in primary HEECs, suggesting this hormone likely mediates the downregulation of luminal PCX in vivo for receptivity. Overexpression of PCX in Ishikawa monolayer inhibited not only the attachment but also the penetration of human embryo surrogates, demonstrating that PCX acts as an important negative regulator of epithelial receptivity for implantation.

LIMITATIONS, REASONS FOR CAUTION

Primary HEECs isolated from the human endometrial tissue contained a mixture of luminal and glandular epithelial cells, as further purification into subtypes was not possible due to the lack of specific markers. Future study would need to investigate how progesterone differentially regulates PCX in endometrial epithelial subtypes. In addition, this study used primary human trophoblast spheroids as human embryo mimics and Ishikawa as endometrial epithelial cells in functional models, future studies with human blastocysts and primary epithelial cells would further validate the findings.

WIDER IMPLICATIONS OF THE FINDINGS

The findings of this study add important new knowledge to the understanding of human endometrial remodeling for receptivity. The identification of PCX as a negative regulator of epithelial receptivity and the knowledge that its specific downregulation in the luminal epithelium coincides with receptivity development may provide new avenues to assess endometrial receptivity and individualize endometrial preparation protocols in assisted reproductive technology (ART). The study also discovered PCX as progesterone target in HEECs, identifying a potentially useful functional biomarker to monitor progesterone action, such as in the optimization of progesterone type/dose/route of administration for luteal support.

STUDY FUNDING/COMPETING INTEREST(S)

Study funding was obtained from ESHRE, Monash IVF and NHMRC. LR reports potential conflict of interests (received grants from Ferring Australia; personal fees from Monash IVF Group and Ferring Australia; and non-financial support from Merck Serono, MSD, and Guerbet outside the submitted work. LR is also a minority shareholder and the Group Medical Director for Monash IVF Group, a provider of fertility preservation services). The remaining authors have no potential conflict of interest to declare.

TRIAL REGISTRATION NUMBER

NA.

In Vitro Implantation Model Using Human Endometrial SUSD2+ Mesenchymal Stem Cells and Myometrial Smooth Muscle Cells

Objective

This study evaluated a novel in vitro implantation model using human endometrial mesenchymal stem cells (EMSCs), SUSD2⁺, and myometrial smooth muscle cells (SMCs) that were co-cultured with mouse blastocysts as the surrogate embryo.

Materials and Methods

In this experimental study, SUSD2⁺ MSCs were isolated from human endometrial cell suspensions (ECS) at the fourth passage by magnetic-activated cell sorting. The ECS and SUSD2⁺ cells were separately co-cultured with human myometrial muscle cells for five days. After collection of mouse blastocysts, the embryos were placed on top of the co-cultured cells for 48 hours. The interaction between the embryo and the cultured cells was assessed morphologically at the histological and ultrastructural levels, and by expression profiles of genes related to implantation.

Results

Photomicrographs showed that trophoblastic cells grew around the embryonic cells and attached to theECS and SUSD2⁺ cells. Ultrastructural observations revealed pinopode and microvilli-like structures on the surfaces of both the ECS and SUSD2⁺ cells. Morphologically, the embryos developed to the egg-cylinder stage in both groups. Gene expression analysis showed no significant differences between the two groups in the presence of an embryo, but an increased expression of αV was detected in SUSD2⁺ cells compared to ECS cells in the absence of an embryo.

Conclusion

This study showed that SUSD2⁺ cells co-cultured with SMCs could interact with mouse embryos. The co-cultured cells could potentially be used as an implantation model.

Integrated pseudotime analysis of human pre-implantation embryo single-cell transcriptomes reveals the dynamics of lineage specification

Understanding lineage specification during human pre-implantation development is a gateway to improving assisted reproductive technologies and stem cell research. Here we employ pseudotime analysis of single-cell RNA sequencing (scRNA-seq) data to reconstruct early mouse and human embryo development. Using time-lapse imaging of annotated embryos, we provide an integrated, ordered, and continuous analysis of transcriptomics changes throughout human development. We reveal that human trophectoderm/inner cell mass transcriptomes diverge at the transition from the B2 to the B3 blastocyst stage, just before blastocyst expansion. We explore the dynamics of the fate markers IFI16 and GATA4 and show that they gradually become mutually exclusive upon establishment of epiblast and primitive endoderm fates, respectively. We also provide evidence that NR2F2 marks trophectoderm maturation, initiating from the polar side, and subsequently spreads to all cells after implantation. Our study pinpoints the precise timing of lineage specification events in the human embryo and identifies transcriptomics hallmarks and cell fate markers.

Epithelial apical glycosylation changes associated with thin endometrium in women with infertility - a pilot observational study

BACKGROUND

Low endometrial receptivity is one of the major factors affecting successful implantation in assisted reproductive technologies (ART). Infertile patients with thin endometrium have a significantly lower cumulative clinical pregnancy rate than patients with normal endometrium. Molecular pathophysiology of low receptivity of thin endometrium remains understudied. We have investigated composition of glycocalyx of the apical surface of luminal and glandular epithelial cells in thin endometrium of infertile women.

METHODS

Thirty-two patients with tubal-peritoneal infertility undergoing in vitro fertilization (IVF) were included in the study. Endometrial samples were obtained in a natural menstrual cycle. Patients were divided into two groups: patients with normal endometrium (≥8 mm) and with thin endometrium (< 8 mm). Histochemical and immunohistochemical analysis of paraffin-embedded endometrial samples was performed using six biotinylated lectins (UEA-I, MAL-II, SNA, VVL, ECL, Con A) and anti-LeY and MECA-79 monoclonal antibodies (MAbs).

RESULTS

Complex glycans analysis taking into account the adjusted specificity of glycan-binding MAbs revealed 1.3 times less expression of MECA-79 glycans on the apical surface of the luminal epithelial cells of thin endometrium compared to normal endometrium; this deficiency may adversely affect implantation, since MECA-79 glycans are a ligand of L-selectin and mediate intercellular interactions. The glycans containing a type-2 unit Galβ1-4GlcNAcβ (LacNAc) but lacking sulfo-residues at 6-OH of GlcNAcβ, and binding to MECA-79 MAbs were found; they can be considered as potential markers of endometrium receptivity. Expression of the lectins-stained glycans on the apical surfaces of the luminal and glandular epithelial cells did not differ significantly. Correlation between the expression of difucosylated oligosaccharide LeY on the apical surfaces of the luminal and glandular epithelial cells was found in patients with thin endometrium and recurrent implantation failure. A similar relationship was shown for mannose-rich glycans.

CONCLUSIONS

Specific features of key glycans expression in epithelial compartments of thin endometrium may be essential for morphogenesis of the endometrial functional layer and explain its low receptivity.

Proteome reprogramming of endometrial epithelial cells by human trophectodermal small extracellular vesicles reveals key insights into embryo implantation

Embryo implantation into the receptive endometrium is critical in pregnancy establishment, initially requiring reciprocal signalling between outer layer of the blastocyst (trophectoderm cells) and endometrial epithelium; however, factors regulating this crosstalk remain poorly understood. Although endometrial extracellular vesicles (EVs) are known to signal to the embryo during implantation, the role of embryo-derived EVs remains largely unknown. Here, we provide a comprehensive proteomic characterisation of a major class of EVs, termed small EVs (sEVs), released by human trophectoderm cells (Tsc-sEVs) and their capacity to reprogram protein landscape of endometrial epithelium in vitro. Highly purified Tsc-sEVs (30-200 nm, ALIX⁺ , TSG101⁺ , CD9/63/81⁺ ) were enriched in known players of implantation (LIFR, ICAM1, TAGLN2, WNT5A, FZD7, ROR2, PRICKLE2), antioxidant activity (SOD1, PRDX1/4/6), tissue integrity (EZR, RAC1, RHOA, TNC), and focal adhesions (FAK, ITGA2/V, ITGB1/3). Functionally, Tsc-sEVs were taken up by endometrial cells, altered transepithelial electrical resistance, and upregulated proteins implicated in embryo attachment (ITGA2/V, ITGB1/3), immune regulation (CD59, CD276, LGALS3), and antioxidant activity (GPX1/3/4, PRDX1/2/4/5/6): processes that are critical for successful implantation. Collectively, we provide critical insights into Tsc-sEV-mediated regulation of endometrial function that contributes to our understanding of the molecular basis of implantation.

Culture strategy as a modulator of target assessments: Functionality of suspension versus hanging drop-derived choriocarcinoma spheroids as in vitro model of embryo implantation

The choriocarcinoma spheroid model has been amply applied to study the underlying molecular mechanism of implantation. Reproducibility and functionality of spheroid tumor models were addressed precisely. To mimic embryo-endometrium crosstalk, no functional characteristics of spheroids have been provided based on culture strategies. In this study, choriocarcinoma spheroids were provided as suspension culture (SC) or hanging drop culture (HDC). Primary assessments were performed based on morphology, cellular density, and hormonal secretion. Spheroid-endometrial cross talk was assessed as coculture procedures. Further, alkaline phosphatase (ALP) activity and expression of genes involved in attachment, invasion, and inducing migration were quantified. We found HDC spheroids provided a homogenous-shaped aggregate with a high grade of viability, cellular integration, hormonal secretion, and the dominant role of WNTs expression in their microarchitecture. SC spheroids showed a higher level of ALP activity and the expression of integrated genes in modulating attachment, invasion, and migration abilities. Spheroid confrontation assays clearly clarified the superiority of SC spheroids to crosstalk with epithelial and stromal cells of endometrium in addition to motivating an ideal endometrial response. Conclusively, culture strategies by affecting various molecular signaling pathways should be chosen precisely according to specific target assessments. Specifically, SC assumed as an ideal model in spheroid-endometrial cross talk.

Intermembrane distances at the feto-maternal interface in epitheliochorial placentation

INTRODUCTION

In an epitheliochorial placenta, the apical membranes of trophoblast cells and of uterine epithelial cells are in contact to each other (feto-maternal contact). In addition, there are also folds in which the trophoblast membrane is in contact with itself (feto-fetal contact) and areas where apical uterine epithelial membrane is in contact with itself (materno-maternal contact).

METHODS

We use transmission electron microscopy of placental samples from pigs. (n = 3), cows (n = 2), sheep (n = 2), goat (n = 2) and roe deer (n = 1) to study the intermembrane distance in these three contact types.

RESULTS

The measured intermembrane distances vary between 8 and 25 nm. One common feature is that the distance at feto-fetal contact sites is about 6-10 nm wider than at materno-maternal sites and feto-maternal sites show intermediate values.

DISCUSSION

This finding suggests that the membrane distance at feto-maternal contact sites is determined by heterophilic binding of larger fetal to smaller maternal binding molecules. Homophilic binding of smaller maternal or larger fetal molecules lead to the smaller or wider intermembrane distances at materno-maternal or feto-fetal contact sites respectively. The observation that this similar pattern of membrane distances is present in pigs and in ruminants suggest that an evolutionary mechanism is involved in determining the intermembrane distance in epitheliochorial placentas.

Long non-coding RNAs in endometrial physiology and pathophysiology

The endometrium is an essential component of the female uterus which provides the environment for pregnancy establishment and maintenance. Abnormalities of the endometrium not only lead to difficulties in establishing and maintaining pregnancy but also play a causative role in diseases of endometrial origin including endometriosis and endometrial cancer. Non-coding RNAs are proposed to play a role in regulating the genome in both normal endometrial physiology and pathophysiology. In this review, we first provide a general overview of non-coding RNAs and reproductive physiology of the endometrium. We then discuss the role on non-coding RNAs in normal endometrial physiology and pathophysiology of endometrial infertility. We then conclude with non-coding RNAs in the pathophysiology of endometriosis and endometrial cancer.

CXCL12 enhances pregnancy outcome via improvement of endometrial receptivity in mice

Successful pregnancy inevitably depends on the implantation of a competent embryo into a receptive endometrium. Although many substances have been suggested to improve the rate of embryo implantation targeting enhancement of endometrial receptivity, currently there rarely are effective evidence-based treatments to prevent or cure this condition. Here we strongly suggest minimally-invasive intra-uterine administration of embryo-secreted chemokine CXCL12 as an effective therapeutic intervention. Chemokine CXCL12 derived from pre- and peri-implanting embryos significantly enhances the rates of embryo attachment and promoted endothelial vessel formation and sprouting in vitro. Consistently, intra-uterine CXCL12 administration in C57BL/6 mice improved endometrial receptivity showing increased integrin β3 and its ligand osteopontin, and induced endometrial angiogenesis displaying increased numbers of vessel formation near the lining of endometrial epithelial layer with higher CD31 and CD34 expression. Furthermore, intra-uterine CXCL12 application dramatically promoted the rates of embryo implantation with no morphologically retarded embryos. Thus, our present study provides a novel evidence that improved uterine endometrial receptivity and enhanced angiogenesis induced by embryo-derived chemokine CXCL12 may aid to develop a minimally-invasive therapeutic strategy for clinical treatment or supplement for the patients with repeated implantation failure with less risk.

Organoids of the female reproductive tract

Healthy functioning of the female reproductive tract (FRT) depends on balanced and dynamic regulation by hormones during the menstrual cycle, pregnancy and childbirth. The mucosal epithelial lining of different regions of the FRT-ovaries, fallopian tubes, uterus, cervix and vagina-facilitates the selective transport of gametes and successful transfer of the zygote to the uterus where it implants and pregnancy takes place. It also prevents pathogen entry. Recent developments in three-dimensional (3D) organoid systems from the FRT now provide crucial experimental models that recapitulate the cellular heterogeneity and physiological, anatomical and functional properties of the organ in vitro. In this review, we summarise the state of the art on organoids generated from different regions of the FRT. We discuss the potential applications of these powerful in vitro models to study normal physiology, fertility, infections, diseases, drug discovery and personalised medicine.

Integrin β1 coordinates survival and morphogenesis of the embryonic lineage upon implantation and pluripotency transition

At implantation, the embryo establishes contacts with the maternal endometrium. This stage is associated with a high incidence of preclinical pregnancy losses. While the maternal factors underlying uterine receptivity have been investigated, the signals required by the embryo for successful peri-implantation development remain elusive. To explore these, we studied integrin β1 signaling, as embryos deficient for this receptor degenerate at implantation. We demonstrate that the coordinated action of pro-survival signals and localized actomyosin suppression via integrin β1 permits the development of the embryo beyond implantation. Failure of either process leads to developmental arrest and apoptosis. Pharmacological stimulation through fibroblast growth factor 2 (FGF2) and insulin-like growth factor 1 (IGF1), coupled with ROCK-mediated actomyosin inhibition, rescues the deficiency of integrin β1, promoting progression to post-implantation stages. Mutual exclusion between integrin β1 and actomyosin seems to be conserved in the human embryo, suggesting the possibility that these mechanisms could also underlie the transition of the human epiblast from pre- to post-implantation.

Non-invasive Intrauterine Administration of Botulinum Toxin A Enhances Endometrial Angiogenesis and Improves the Rates of Embryo Implantation

Endometrial angiogenesis plays crucial roles in determining the endometrial receptivity. Defects in endometrial receptivity often cause repeated implantation failure, which is one of the major unmet needs for infertility and contributes a major barrier to the assisted reproductive technology. Despite the numerous extensive research work, there are currently no effective evidence-based treatments to prevent or cure this condition. As a non-invasive treatment strategy, botulinum toxin A (BoTA) was administered into one side of mouse uterine horns, and saline was infused into the other side of horns for the control. Impact of BoTA was assessed in the endometrium at 3 or 8 days after infusion. We demonstrated that BoTA administration enhances the capacity of endothelial cell tube formation and sprouting. The intrauterine BoTA administration significantly induced endometrial angiogenesis displaying increased numbers of vessel formation and expression levels of related marker genes. Moreover, BoTA intrauterine application promoted the endometrial receptivity, and the rates of embryo implantation were improved with BoTA treatment with no morphologically retarded embryos. Intrauterine BoTA treatment has a beneficial effect on vascular reconstruction of functional endometrium prior to embryo implantation by increasing endometrial blood flow near the uterine cavity suggesting BoTA treatment as a potential therapeutic strategy for patients who are suffering from repeated implantation failure with the problems with endometrial receptivity.

Organoids as Novel Models for Embryo Implantation Study

In the last decade, organoids have become emerging novel models for biomedical research. Organoids are small, self-organized three-dimensional (3D) tissue cultures derived from stem cells that mimic certain tissues or organs. In reproductive medicine, researchers have generated numerous organoids including blastoid (blastocyst organoid), endometrial organoid, and trophoblast organoid. These organdies provide useful models for studying the embryo implantation mechanism through observation of cell differentiation, gene expression, and epigenetic profiles at the implantation stage. As in vitro tissue models, organoids could be coupled with many other frontier technologies such as gene editing and genomic sequencing. However, the main drawback of organoids is that they do not fully mimic their counterparts in vivo tissues. Furthermore, there is a consensus of research ethics on organoids that may limit the types of studies that scientists perform with. Nevertheless, all discoveries and efforts surrounding organoids still greatly benefit therapy development for reproductive clinics.

Biomechanics of Early Life in the Female Reproductive Tract

Early human life that starts at the onset of fertilization and ends with implantation of the embryo in the uterine wall is the foundation for a successful pregnancy. The different stages during this period require biomechanical mechanisms, which are mostly unknown due to difficulties to conduct in vivo studies in humans.

Highly glycosylated MUC1 mediates high affinity L-selectin binding at the human endometrial surface

Background

Sialyl-Lewis X/L-selectin high affinity binding interactions between transmembrane O-glycosylated mucins proteins and the embryo have been implicated in implantation processes within the human reproductive system. However, the adhesive properties of these mucins at the endometrial cell surface are difficult to resolve due to known discrepancies between in vivo models and the human reproductive system and a lack of sensitivity in current in vitro models. To overcome these limitations, an in vitro model of the human endometrial epithelial was interrogated with single molecule force spectroscopy (SMFS) to delineate the molecular configurations of mucin proteins that mediate the high affinity L-selectin binding required for human embryo implantation.

Results

This study reveals that MUC1 contributes to both the intrinsic and extrinsic adhesive properties of the HEC-1 cellular surface. High expression of MUC1 on the cell surface led to a significantly increased intrinsic adhesion force (148 pN vs. 271 pN, p < 0.001), whereas this adhesion force was significantly reduced (271 pN vs. 118 pN, p < 0.001) following siRNA mediated MUC1 ablation. Whilst high expression of MUC1 displaying elevated glycosylation led to strong extrinsic (> 400 pN) L-selectin binding at the cell surface, low expression of MUC1 with reduced glycosylation resulted in significantly less (≤200 pN) binding events.

Conclusions

An optimal level of MUC1 together with highly glycosylated decoration of the protein is critical for high affinity L-selectin binding. This study demonstrates that MUC1 contributes to cellular adhesive properties which may function to facilitate trophoblast binding to the endometrial cell surface through the L-selectin/sialyl-Lewis x adhesion system subsequent to implantation.

Extracellular vesicles in embryo implantation and disorders of the endometrium

Implantation of the embryo is a rate-limiting step for a successful pregnancy, and it requires an intricate crosstalk between the embryo and the endometrium. Extracellular vesicles (EVs) are membrane-enclosed, nano-sized structures produced by cells to mediate cell to cell communication and modulate a diverse set of biological processes. Herein, we review the involvement of EVs in the process of embryo implantation and endometrial diseases. EVs have been isolated from uterine fluid, cultured endometrial epithelial/stromal cells and trophectodermal cells. The endometrial epithelial and stromal/decidual cell-derived EVs and its cargo are internalized bythe trophoblast cells, and they regulate a diverse set of genes involved in adhesion, invasion and migration. Conversely, the embryo-derived EVs and its cargo are internalized by epithelial and immune cells of the endometrium for biosensing and immunomodulation required for successful implantation. EVs have also been shown to play a role in infertility, recurrent implantation failure, endometriosis, endometritis and endometrial cancer. Further research should set a stage for EVs as non-invasive "liquid biopsy" tools for assessment of endometrial health.

Integrating LCM-Based Spatio-Temporal Transcriptomics Uncovers Conceptus and Endometrial Luminal Epithelium Communication that Coordinates the Conceptus Attachment in Pigs

Attachment of conceptus to the endometrial luminal epithelium (LE) is a critical event for early placentation in Eutheria. Since the attachment occurs at a particular site within the uterus, a coordinated communication between three spatially distinct compartments (conceptus and endometrial LE from two anatomical regions of the uterus to which conceptus attaches and does not attach) is essential but remains to be fully characterized. Using the laser capture microdissection (LCM) technique, we firstly developed an approach that can allow us to pair the pig conceptus sample with its nearby endometrial epithelium sample without losing the native spatial information. Then, a comprehensive spatio-temporal transcriptomic profile without losing the original conceptus-endometrium coordinates was constructed. The analysis shows that an apparent difference in transcriptional responses to the conceptus exists between the endometrial LE from the two anatomically distinct regions in the uterus. In addition, we identified the communication pathways that link the conceptus and endometrial LE and found that these pathways have important roles in conceptus attachment. Furthermore, a number of genes whose expression is spatially restricted in the two different anatomical regions within the uterus were characterized for the first time and two of them (SULT2A1 and MEP1B) may cooperatively contribute to establish conceptus attachment in pigs. The results from our study have implications in understanding of conceptus/embryo attachment in pigs and other large polytocous species.

Embryo implantation in the laboratory: an update on current techniques

BACKGROUND

The embryo implantation process is crucial for the correct establishment and progress of pregnancy. During implantation, the blastocyst trophectoderm cells attach to the epithelium of the endometrium, triggering intense cell-to-cell crosstalk that leads to trophoblast outgrowth, invasion of the endometrial tissue, and formation of the placenta. However, this process, which is vital for embryo and foetal development in utero, is still elusive to experimentation because of its inaccessibility. Experimental implantation is cumbersome and impractical in adult animal models and is inconceivable in humans.

OBJECTIVE AND RATIONALE

A number of custom experimental solutions have been proposed to recreate different stages of the implantation process in vitro, by combining a human embryo (or a human embryo surrogate) and endometrial cells (or a surrogate for the endometrial tissue). In vitro models allow rapid high-throughput interrogation of embryos and cells, and efficient screening of molecules, such as cytokines, drugs, or transcription factors, that control embryo implantation and the receptivity of the endometrium. However, the broad selection of available in vitro systems makes it complicated to decide which system best fits the needs of a specific experiment or scientific question. To orient the reader, this review will explore the experimental options proposed in the literature, and classify them into amenable categories based on the embryo/cell pairs employed.The goal is to give an overview of the tools available to study the complex process of human embryo implantation, and explain the differences between them, including the advantages and disadvantages of each system.

SEARCH METHODS

We performed a comprehensive review of the literature to come up with different categories that mimic the different stages of embryo implantation in vitro, ranging from initial blastocyst apposition to later stages of trophoblast invasion or gastrulation. We will also review recent breakthrough advances on stem cells and organoids, assembling embryo-like structures and endometrial tissues.

OUTCOMES

We highlight the most relevant systems and describe the most significant experiments. We focus on in vitro systems that have contributed to the study of human reproduction by discovering molecules that control implantation, including hormones, signalling molecules, transcription factors and cytokines.

WIDER IMPLICATIONS

The momentum of this field is growing thanks to the use of stem cells to build embryo-like structures and endometrial tissues, and the use of bioengineering to extend the life of embryos in culture. We propose to merge bioengineering methods derived from the fields of stem cells and reproduction to develop new systems covering a wider window of the implantation process.

The Complicated Effects of Extracellular Vesicles and Their Cargos on Embryo Implantation

As a rate-limiting step in pregnancy, embryo implantation is highly dependent on intercellular communication. Extracellular vesicles (EVs) are newly identified to be important in the course of intercellular communication. EVs have been isolated from a wide variety of biofluids and tissues, including plasma, liver, uterine, semen, embryo, etc. The present and future use of EVs not only as biomarkers, but also as targeting drug delivery system, is promisingly pave the way for advanced comprehension of implantation failure in reproductive diseases. However, as the precise mechanisms of EVs in embryo implantation has not been elucidated yet. Herein, we summarize the current knowledge on the diverse effects of EVs from various sources and their cargos such as microRNA, long non-coding RNA, protein, etc. on embryo implantation, and the potential mechanisms of EVs in reproductive diseases such as recurrent implantation failure, polycystic ovary syndrome and endometriosis. It is essential to note that many of the biologically plausible functions of EVs in embryo implantation discussed in present literatures still need further research in vivo.

Proteome analysis of endometrial tissue from patients with PCOS reveals proteins predicted to impact the disease

Polycystic ovary syndrome (PCOS) is a complex disease that causes an ovulatory infertility in approximately 10% of reproductive-age women. We searched for candidate proteins that might contribute to endometrial receptivity defects in PCOS patients, and result in adverse reproductive outcomes. Shotgun proteomics approach was used to investigate the proteome profile of the endometrium at the luteal phase in PCOS patients compared to healthy fertile individuals. Biological process and pathway analyses were conducted to categorize the proteins with differential expressions. Confirmation was performed for a number of proteins via immunoblotting in new samples. 150 proteins with higher abundance, and 46 proteins with lower abundance were identified in the endometrial tissue from PCOS patients compared to healthy fertile individuals. The proteins with higher abundance were enriched in protein degradation, cell cycle, and signaling cascades. Proteins with lower abundance in PCOS patients were enriched in extracellular matrix (ECM) composition and function, as well as the salvage pathway of purine biosynthesis. Metabolism was the most affected biological process with over 100 up-regulated, and approximately 30 down-regulated proteins. Our results indicate significant imbalances in metabolism, proteasome, cell cycle, ECM related proteins, and signaling cascades in endometrial tissue of PCOS, which may contribute to poor reproductive outcomes in these patients. We postulate that the endometria in PCOS patients may not be well-differentiated and synchronized for implantation. Possible roles of the above-mentioned pathways that underlie implantation failure in PCOS will be discussed. Our findings need to be confirmed in larger populations.

Protein O-GlcNAcylation Promotes Trophoblast Differentiation at Implantation

Embryo implantation begins with blastocyst trophectoderm (TE) attachment to the endometrial epithelium, followed by the breaching of this barrier by TE-derived trophoblast. Dynamic protein modification with O-linked β-N-acetylglucosamine (O-GlcNAcylation) is mediated by O-GlcNAc transferase and O-GlcNAcase (OGA), and couples cellular metabolism to stress adaptation. O-GlcNAcylation is essential for blastocyst formation, but whether there is a role for this system at implantation remains unexplored. Here, we used OGA inhibitor thiamet g (TMG) to induce raised levels of O-GlcNAcylation in mouse blastocysts and human trophoblast cells. In an in vitro embryo implantation model, TMG promoted mouse blastocyst breaching of the endometrial epithelium. TMG reduced expression of TE transcription factors Cdx2, Gata2 and Gata3, suggesting that O-GlcNAcylation stimulated TE differentiation to invasive trophoblast. TMG upregulated transcription factors OVOL1 and GCM1, and cell fusion gene ERVFRD1, in a cell line model of syncytiotrophoblast differentiation from human TE at implantation. Therefore O-GlcNAcylation is a conserved pathway capable of driving trophoblast differentiation. TE and trophoblast are sensitive to physical, chemical and nutritive stress, which can occur as a consequence of maternal pathophysiology or during assisted reproduction, and may lead to adverse neonatal outcomes and associated adult health risks. Further investigation of how O-GlcNAcylation regulates trophoblast populations arising at implantation is required to understand how peri-implantation stress affects reproductive outcomes.

Sex steroids influence organizational but not functional decidualization of feline endometrial cells in a 3D culture system†

Successful implantation requires complex signaling between the uterine endometrium and the blastocyst. Prior to the blastocyst reaching the uterus, the endometrium is remodeled by sex steroids and other signals to render the endometrium receptive. In vitro models have facilitated major advances in our understanding of endometrium preparation and endometrial-blastocyst communication in mice and humans, but these systems have not been widely adapted for use in other models which might generate a deeper understanding of these processes. The objective of our study was to use a recently developed, three-dimensional culture system to identify specific roles of female sex steroids in remodeling the organization and function of feline endometrial cells. We treated endometrial cells with physiologically relevant concentrations of estradiol and progesterone, either in isolation or in combination, for 1 week. We then examined size and density of three-dimensional structures, and quantified expression of candidate genes known to vary in response to sex steroid treatments and that have functional relevance to the decidualization process. Combined sex steroid treatments recapitulated organizational patterns seen in vivo; however, sex steroid manipulations did not induce expected changes to expression of decidualization-related genes. Our results demonstrate that sex steroids may not be sufficient for complete decidualization and preparation of the feline endometrium, thereby highlighting key areas of opportunity for further study and suggesting some unique functions of felid uterine tissues.

Extracellular vesicles: Mediators of embryo-maternal crosstalk during pregnancy and a new weapon to fight against infertility

In modern-day life, infertility is one of the major issues that can affect an individual, both physically and psychologically. Several anatomical, physiological, and genetic factors might contribute to the infertility of an individual. Intercellular communication between trophectoderm and endometrial epithelium triggers successful embryo implantation and thereby establishes pregnancy. Recent studies demonstrate that Extracellular vesicles (EVs) are emerging as one of the crucial components that are involved in embryo-maternal communication and promote pregnancy. Membrane-bound EVs release several secreted factors within the uterine fluid, which mediates an intermolecular transfer of EVs' cargos between blastocysts and endometrium. Emerging evidences indicate that several events like imbalance in the release of endometrial or placenta-derived EVs (exosomes/MVs), uptake of their content, failure of embryo selection might lead to implantation failure. Here in this review, we have discussed the current knowledge of the involvement of EVs in maternal-fetal communications during implantation and also highlighted the EVs' rejuvenating ability to overcome infertility-related issues. We also discussed the alteration of the EVs' cargo in different pathological conditions that lead to infertility. Therefore, this review would give a better understanding of EVs' contribution in successful embryo implantation, which could help in the development of new diagnostic tools and cell-free biologics to improve the in vivo reproductive process and to treat infertility by restoring normal reproductive functions.

Tracking placental development in health and disease

Pre-eclampsia and fetal growth restriction arise from disorders of placental development and have some shared mechanistic features. Initiation is often rooted in the maldevelopment of a maternal-placental blood supply capable of providing for the growth requirements of the fetus in later pregnancy, without exerting undue stress on maternal body systems. Here, we review normal development of a placental bed with a safe and adequate blood supply and a villous placenta-blood interface from which nutrients and oxygen can be extracted for the growing fetus. We consider disease mechanisms that are intrinsic to the maternal environment, the placenta or the interaction between the two. Systemic signalling from the endocrine placenta targets the maternal endothelium and multiple organs to adjust metabolism for an optimal pregnancy and later lactation. This signalling capacity is skewed when placental damage occurs and can deliver a dangerous pathogenic stimulus. We discuss the placental secretome including glycoproteins, microRNAs and extracellular vesicles as potential biomarkers of disease. Angiomodulatory mediators, currently the only effective biomarkers, are discussed alongside non-invasive imaging approaches to the prediction of disease risk. Identifying the signs of impending pathology early enough to intervene and ameliorate disease in later pregnancy remains a complex and challenging objective.

Common principles of early mammalian embryo self-organisation

Pre-implantation mammalian development unites extreme plasticity with a robust outcome: the formation of a blastocyst, an organised multi-layered structure ready for implantation. The process of blastocyst formation is one of the best-known examples of self-organisation. The first three cell lineages in mammalian development specify and arrange themselves during the morphogenic process based on cell-cell interactions. Despite decades of research, the unifying principles driving early mammalian development are still not fully defined. Here, we discuss the role of physical forces, and molecular and cellular mechanisms, in driving self-organisation and lineage formation that are shared between eutherian mammals.

Rab11a drives adhesion molecules to the surface of endometrial epithelial cells

STUDY QUESTION

Is Rab11a GTPase, a regulator of intracellular trafficking, of significance in endometrial functions?

SUMMARY ANSWER

Rab11a is an important component of the cascades involved in equipping the endometrial epithelium (EE) with 'adhesiveness' and 'cohesiveness'.

WHAT IS KNOWN ALREADY

Cell adhesion molecules (CAMs) have been investigated extensively for modulation in their endometrial expression during the peri-implantation phase. However, the mechanisms by which CAMs are transported to the EE surface have not received the same attention. Rab11a facilitates transport of specific proteins to the plasma membrane in endothelial cells, fibroblasts, embryonic ectodermal cells, etc. However, its role in the transport of CAMs in EE remains unexplored.

STUDY DESIGN, SIZE, DURATION

In-vitro investigations were directed towards deciphering the role of Rab11a in trafficking of CAMs (integrins and E-cadherin) to the cell surface of Ishikawa, an EE cell line. Towards this, Rab11a stable knockdown (Rab-kd) and control clones of Ishikawa were generated. JAr (human trophoblastic cell line) cells were used to form multicellular spheroids. Pre-receptive (n = 6) and receptive (n = 6) phase endometrial tissues from women with proven fertility and receptive phase (n = 6) endometrial tissues from women with unexplained infertility were used.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Rab-kd and control clones were used for in-vitro assays. Live cells were used for biotinylation, JAr spheroid assays, flow cytometry, trans-epithelial electrical resistance assays and wound-healing assays. Lysosome and Golgi membranes were isolated by ultracentrifugation. Confocal microscopy, immunoblotting, qRT-PCR and immunohistochemistry were employed for assessing the expression of Rab11a, integrins and E-cadherin.

MAIN RESULTS AND THE ROLE OF CHANCE

shRNA-mediated attenuation of Rab11a expression led to a significant (P < 0.01) decline in the surface localization of αVβ3 integrin. Cell surface protein extracts of Rab-kd clones showed a significant (P < 0.05) reduction in the levels of αV integrin. Further, a significant (P < 0.01) decrease was observed in the percent JAr spheroids attached to Rab-kd clones, compared to control clones. Rab-kd clones also showed a significant (P < 0.001) decline in the total levels of E-cadherin. This was caused neither by reduced transcription nor by increased lysosomal degradation. The role of Rab11a in maintaining the epithelial nature of the cells was evident by a significant increase in the migratory potential, presence of stress-fibres and a decrease in the trans-epithelial resistance in Rab-kd monolayers. Further, the levels of endometrial Rab11a and E-cadherin in the receptive phase were found to be significantly (P < 0.05) lower in women with unexplained infertility compared to that in fertile women. Taken together, these observations hint at a key role of Rab11a in the trafficking of αVβ3 integrin and maintenance of E-cadherin levels at the surface of EE cells.

LARGE-SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

The in-vitro setting of the study is a limitation. Further immunohistochemical localizations of Rab11a and CAMs were conducted on a limited number of human endometrial samples.

WIDER IMPLICATIONS OF THE FINDINGS

Rab11a-mediated trafficking of endometrial CAMs in EE cells can be explored further for its potential as a target for fertility regulation or infertility management.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by the Indian Council of Medical Research (ICMR), the Department of Science and Technology (DST), the Council of Scientific and Industrial Research (CSIR), Government of India. No competing interests are declared.

Comparative secretome profile analysis of cultured immortalized human endometrial stromal cells supplemented with implanted versus nonimplanted blastocyst-conditioned medium: A preliminary analysis

AIM

Human endometrial stromal cells (HESCs) were previously shown to be capable of discriminating embryos with different qualities. Here we aimed to compare the specific response of the HESC secretome to implanted blastocyst-conditioned medium (BCM) versus nonimplanted medium and identify cytokine candidates useful for the assessment of blastocyst implantation.

METHODS

Cleavage embryos were individually cultured in one microdrop of medium for blastocyst formation. The BCM was collected after fresh blastocyst transfer on day 5 and used to supplement HESC culture medium. A high-throughput antibody array covering 440 cytokines was used to detect the secretory proteins of HESCs supplemented with implanted or nonimplanted BCM.

RESULTS

A total of 22 differentially expressed proteins were found out of 440 cytokines in the supernatant of HESCs supplemented with BCM from the implanted group compared to the nonimplanted group, including seven upregulated and 15 downregulated proteins. Gene Ontology enrichment analysis showed that the differentially expressed proteins were mainly involved in cell chemotaxis and motility, and ERK1/2 cascade regulation. Kyoto Encyclopedia of Genes and Genomes analysis suggested that the mitogen-activated protein kinase and phosphatidylinositol 3 kinase/Akt pathways were mainly involved.

CONCLUSION

HESCs specifically responded to BCM from different quality blastocysts, a finding that can be used to develop a novel approach for blastocyst quality assessment.

The role of extracellular matrix in normal and pathological pregnancy: Future applications of microphysiological systems in reproductive medicine

IMPACT STATEMENT

Extracellular matrix in the womb regulates the initiation, progression, and completion of a healthy pregnancy. The composition and physical properties of extracellular matrix in the uterus and at the maternal-fetal interface are remodeled at each gestational stage, while maladaptive matrix remodeling results in obstetric disease. As in vitro models of uterine and placental tissues, including micro-and milli-scale versions of these organs on chips, are developed to overcome the inherent limitations of studying human development in vivo, we can isolate the influence of cellular and extracellular components in healthy and pathological pregnancies. By understanding and recreating key aspects of the extracellular microenvironment at the maternal-fetal interface, we can engineer microphysiological systems to improve assisted reproduction, obstetric disease treatment, and prenatal drug safety.

Self-Organization of Mouse Stem Cells into an Extended Potential Blastoid

Mammalian blastocysts comprise three distinct cell lineages essential for development beyond implantation: the pluripotent epiblast, which generates the future embryo, and surrounding it the extra-embryonic primitive endoderm and the trophectoderm tissues. Embryonic stem cells can reintegrate into embryogenesis but contribute primarily to epiblast lineages. Here, we show that mouse embryonic stem cells cultured under extended pluripotent conditions (EPSCs) can be partnered with trophoblast stem cells to self-organize into blastocyst-like structures with all three embryonic and extra-embryonic lineages. Morphogenetic and transcriptome profiling analyses reveal that these blastocyst-like structures show distinct embryonic-abembryonic axes and primitive endoderm differentiation and can initiate the transition from the pre- to post-implantation egg cylinder morphology in vitro.

Endometrial profilin 1: a key player in embryo-endometrial crosstalk

Objective

Despite extensive research on implantation failure, little is known about the molecular mechanisms underlying the crosstalk between the embryo and the maternal endometrium, which is critical for successful pregnancy. Profilin 1 (PFN1), which is expressed both in the embryo and in the endometrial epithelium, acts as a potent regulator of actin polymerization and the cytoskeletal network. In this study, we identified the specific role of endometrial PFN1 during embryo implantation.

Methods

Morphological alterations depending on the status of PFN1 expression were assessed in PFN1-depleted or control cells grown on Matrigel-coated cover glass. Day-5 mouse embryos were cocultured with Ishikawa cells. Comparisons of the rates of F-actin formation and embryo attachment were performed by measuring the stability of the attached embryo onto PFN1-depleted or control cells.

Results

Depletion of PFN1 in endometrial epithelial cells induced a significant reduction in cell-cell adhesion displaying less formation of colonies and a more circular cell shape. Mouse embryos co-cultured with PFN1-depleted cells failed to form actin cytoskeletal networks, whereas more F-actin formation in the direction of surrounding PFN1-intact endometrial epithelial cells was detected. Furthermore, significantly lower embryo attachment stability was observed in PFN1-depleted cells than in control cells. This may have been due to reduced endometrial receptivity caused by impaired actin cytoskeletal networks associated with PFN1 deficiency.

Conclusion

These observations definitively demonstrate an important role of PFN1 in mediating cell-cell adhesion during the initial stage of embryo implantation and suggest a potential therapeutic target or novel biomarker for patients suffering from implantation failure.

Investigating the role of CD44 and hyaluronate in embryo-epithelial interaction using an in vitro model

Implantation failure is an important impediment to increasing success rates in assisted reproductive technologies. Knowledge of the cascade of morphological and molecular events at implantation remains limited. Cell surface CD44 and hyaluronate (HA) have been reported in the uterus, but a role in intercellular interaction at implantation remains to be evaluated. Mouse embryos were co-cultured with human Ishikawa endometrial epithelial monolayers over 2 days. Attachment was tenuous during the first 24 h, after which it became stable, leading to breaching of the monolayer. The effects of enzymatically reducing the density of HA, or introducing a function-blocking antibody to CD44, were monitored during progression from weak to stable embryonic attachment. Hyaluronidase-mediated removal of surface HA from the epithelial cells enhanced the speed of attachment, while a similar treatment of embryos had no effect. The antibody to CD44 caused retardation of initial attachment. These results suggest that CD44-HA binding could be employed by embryos during initial docking, but the persistence of HA in epithelial cells might be detrimental to later stages of implantation by retarding attainment of stable attachment.