Acceleration of trophoblast differentiation by heparin-binding EGF-like growth factor is dependent on the stage-specific activation of calcium influx by ErbB receptors in developing mouse blastocysts

Endometrial extracellular vesicles regulate processes related to embryo development and implantation in human blastocysts

STUDY QUESTION

What is the transcriptomic response of human blastocysts following internalization of extracellular vesicles (EVs) secreted by the human endometrium?

SUMMARY ANSWER

EVs secreted by the maternal endometrium induce a transcriptomic response in human embryos that modulates molecular mechanisms related to embryo development and implantation.

WHAT IS KNOWN ALREADY

EVs mediate intercellular communication by transporting various molecules, and endometrial EVs have been postulated to be involved in the molecular regulation of embryo implantation. Our previous studies showed that endometrial EVs carry miRNAs and proteins associated with implantation events that can be taken up by human blastocysts; however, no studies have yet investigated the transcriptomic response of human embryos to this EV uptake, which is crucial to demonstrate the functional significance of this communication system.

STUDY DESIGN, SIZE, DURATION

A prospective descriptive study was performed. Primary human endometrial epithelial cells (pHEECs), derived from endometrial biopsies collected from fertile oocyte donors (n = 20), were cultured in vitro to isolate secreted EVs. Following EV characterization, Day 5 human blastocysts (n = 24) were cultured in the presence or absence of the EVs for 24 h and evaluated by RNA-sequencing.

PARTICIPANTS/MATERIALS, SETTING, METHODS

EVs were isolated from the conditioned culture media using ultracentrifugation, and characterization was performed using western blot, nanoparticle tracking analysis, and transmission electron microscopy. Human blastocysts were devitrified, divided into two groups (n = 12/group), and cultured in vitro for 24 h with or without previously isolated EVs. RNA-sequencing analysis was performed, and DESeq2 was used to identify differentially expressed genes (DEGs) (FDR < 0.05). QIAGEN Ingenuity Pathway Analysis was used to perform the functional enrichment analysis and integration with our recently published data from the pHEECs' EV-miRNA cargo.

MAIN RESULTS AND THE ROLE OF CHANCE

Characterization confirmed the isolation of EVs from pHEECs' conditioned culture media. Among the DEGs in blastocysts co-cultured with EVs, we found 519 were significantly upregulated and 395 were significantly downregulated. These DEGs were significantly enriched in upregulated functions related to embryonic development, cellular invasion and migration, cell cycle, cellular organization and assembly, gene expression, and cell viability; and downregulated functions related to cell death and DNA fragmentation. Further, the intracellular signaling pathways regulated by the internalization of endometrial EVs were previously related to early embryo development and implantation potential, for their role in pluripotency, cellular homeostasis, early embryogenesis, and implantation-related processes. Finally, integrating data from miRNA cargo of EVs, we found that the miRNAs carried by endometrial EVs targeted nearly 80% of the DEGs in human blastocysts.

LIMITATIONS, REASONS FOR CAUTION

This is an in vitro study in which conditions of endometrial cell culture could not mimic the intrauterine environment.

WIDER IMPLICATIONS OF THE FINDINGS

This study provides novel insights into the functional relevance of EVs secreted by the human endometrium, and particularly the role of EV-miRNA regulation on global transcriptome behavior of human blastocysts during early embryogenesis and embryo implantation. It provides potential biomarkers that could become useful diagnostic targets for predicting implantation success.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the Spanish Ministry of Education through FPU awarded to M.S.-B. (FPU18/03735), Generalitat Valenciana through VALi+d Programme awarded to M.C.C.-G. (ACIF/2019/139), and Instituto de Salud Carlos III and cofounded by the European Social Fund (ESF) "Investing in your future" through the Miguel Servet Program (CP20/00120 [H.F.]; CP19/00149 [I.C.]). The authors have no conflicts of interest to disclose.

TRIAL REGISTRATION NUMBER

N/A.

Molecular and cellular regulators of embryo implantation and their application in improving the implantation potential of IVF-derived blastocysts

Background

In vitro fertilization (IVF) and embryo transfer (ET) are widely used in reproductive biology. Despite the transfer of high-quality blastocysts, the implantation rate of IVF-derived blastocysts remains low after ET.

Methods

This article provides a comprehensive review of current research on embryo implantation regulators and their application to improve the implantation potential of IVF-derived blastocysts.

Main Findings

The in vivo mouse model revealed selective proteolysis immediately after expression in activated blastocysts, that is, degradation of ERα expression in activated blastocysts regulated by the ubiquitin-proteasome pathway, followed by completion of blastocyst implantation. Treatment of blastocysts to induce appropriate protein expression during in vitro culture prior to ET is a useful approach for improving implantation rates. This approach showed that combined treatment with PRL, EGF, and 4-OH-E2 (PEC) improved the blastocyst implantation rates. Furthermore, arginine and leucine drive reactive oxygen species (ROS)-mediated integrin α5β1 expression and promote blastocyst implantation.

Conclusion

Findings based on analysis of molecular and cellular regulators are useful for improving the implantation potential of IVF-derived blastocysts. These approaches may help to elucidate the mechanisms underlying the completion of the blastocyst implantation, although further investigation is required to improve the success of implantation and pregnancy.

Uterine WNTS modulates fibronectin binding activity required for blastocyst attachment through the WNT/CA2+ signaling pathway in mice

Adhesion of the implanting blastocyst involves the interaction between integrin proteins expressed by trophoblast cells and components present in the basement membrane of the endometrial luminal epithelium. Although several factors regulating integrins and their adhesion to fibronectin are already known, we showed that Wnt signaling is involved in the regulation of blastocyst adhesion through the trafficking of integrins expressed by trophoblast cells. Localization of Itgα5β1 by immunofluorescence and FN-binding assays were conducted on peri-implantation blastocysts treated with either Wnt5a or Wnt7a proteins. Both Wnt5a and Wnt7a induced a translocation of Itgα5β1 at the surface of the blastocyst and an increase in FN-binding activity. We further demonstrated that uterine fluid is capable of inducing integrin translocation and this activity can be specifically inhibited by the Wnt inhibitor sFRP2. To identify the Wnt signaling pathway involved in this activity, blastocysts were incubated with inhibitors of either p38MAPK, PI3K pathway or CamKII prior to the addition of Wnts. Whereas inhibition of p38MAPK and PI3K had not effect, inhibition of CamKII reduced FN-binding activity induced by Wnts. Finally, we demonstrated that inhibition of Wnts by sFRP2 reduced the binding efficiency of the blastocyst to uterine epithelial cells. Our findings provide new insight into the mechanism that regulates integrin trafficking and FN-binding activity and identifies Wnts as a key player in blastocyst attachment to the uterine epithelium.

Integrins and their potential roles in mammalian pregnancy

Integrins are a highly complex family of receptors that, when expressed on the surface of cells, can mediate reciprocal cell-to-cell and cell-to-extracellular matrix (ECM) interactions leading to assembly of integrin adhesion complexes (IACs) that initiate many signaling functions both at the membrane and deeper within the cytoplasm to coordinate processes including cell adhesion, migration, proliferation, survival, differentiation, and metabolism. All metazoan organisms possess integrins, and it is generally agreed that integrins were associated with the evolution of multicellularity, being essential for the association of cells with their neighbors and surroundings, during embryonic development and many aspects of cellular and molecular biology. Integrins have important roles in many aspects of embryonic development, normal physiology, and disease processes with a multitude of functions discovered and elucidated for integrins that directly influence many areas of biology and medicine, including mammalian pregnancy, in particular implantation of the blastocyst to the uterine wall, subsequent placentation and conceptus (embryo/fetus and associated placental membranes) development. This review provides a succinct overview of integrin structure, ligand binding, and signaling followed with a concise overview of embryonic development, implantation, and early placentation in pigs, sheep, humans, and mice as an example for rodents. A brief timeline of the initial localization of integrin subunits to the uterine luminal epithelium (LE) and conceptus trophoblast is then presented, followed by sequential summaries of integrin expression and function during gestation in pigs, sheep, humans, and rodents. As appropriate for this journal, summaries of integrin expression and function during gestation in pigs and sheep are in depth, whereas summaries for humans and rodents are brief. Because similar models to those illustrated in Fig. 1, 2, 3, 4, 5 and 6 are present throughout the scientific literature, the illustrations in this manuscript are drafted as Viking imagery for entertainment purposes.

Elevated homocysteine activates unfolded protein responses and causes aberrant trophoblast differentiation and mouse blastocyst development

Hyperhomocysteinemia may arise from folate/vitamin B12 deficiency, genetic polymorphisms, kidney disease, or hypothyroidism. It is associated with an increased risk of early pregnancy loss and placenta-related complications of pregnancy, including pre-eclampsia and fetal growth restriction. While the majority of studies of hyperhomocysteinemia focus on epigenetic changes secondary to metabolic disruption, the effects of homocysteine toxicity on placental development remain unexplored. Here, we investigated the influence of hyperhomocysteinemia on early blastocyst development and trophoblast differentiation. Exposure of cultured blastocysts to high homocysteine levels reduces cell number in the trophectoderm layer, most likely through increased apoptosis. Homocysteine also promotes differentiation of a trophoblast stem cell line. Both effects diminish the stem cell pool, and are mediated in an endoplasmic reticulum (ER) unfolded protein response (UPRER )-dependent manner. Targeted alleviation of UPRER may therefore provide a new therapeutic intervention to improve pregnancy outcome in women with hyperhomocysteinemia.

The Ontogeny and Function of Placental Macrophages

The placenta is a fetal-derived organ whose function is crucial for both maternal and fetal health. The human placenta contains a population of fetal macrophages termed Hofbauer cells. These macrophages play diverse roles, aiding in placental development, function and defence. The outer layer of the human placenta is formed by syncytiotrophoblast cells, that fuse to form the syncytium. Adhered to the syncytium at sites of damage, on the maternal side of the placenta, is a population of macrophages termed placenta associated maternal macrophages (PAMM1a). Here we discuss recent developments that have led to renewed insight into our understanding of the ontogeny, phenotype and function of placental macrophages. Finally, we discuss how the application of new technologies within placental research are helping us to further understand these cells.

Trophectoderm cell failure leads to peri-implantation lethality in Trpm7-deficient mouse embryos

Early embryogenesis depends on proper control of intracellular homeostasis of ions including Ca²⁺ and Mg²⁺. Deletion of the Ca²⁺ and Mg²⁺ conducting the TRPM7 channel is embryonically lethal in mice but leaves compaction, blastomere polarization, blastocoel formation, and correct specification of the lineages of the trophectoderm and inner cell mass unaltered despite that free cytoplasmic Ca²⁺ and Mg²⁺ is reduced at the two-cell stage. Although Trpm7^-/- embryos are able to hatch from the zona pellucida, no expansion of Trpm7^-/- trophoblast cells can be observed, and Trpm7^-/- embryos are not identifiable in utero at E6.5 or later. Given the proliferation and adhesion defect of Trpm7^-/- trophoblast stem cells and the ability of Trpm7^-/- ESCs to develop to embryos in tetraploid embryo complementation assays, we postulate a critical role of TRPM7 in trophectoderm cells and their failure during implantation as the most likely explanation of the developmental arrest of Trpm7-deficient mouse embryos.

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.

Perturbed differentiation of murine embryonic stem cells upon Pelota deletion due to dysregulated FOXO1/β-catenin signaling

Differentiation of the embryonic stem cells (ESCs) is regulated by a variety of different signaling pathways. Genetic depletion of murine Pelota gene (Pelo) leads to early embryonic lethality. Here, we aimed at determining the embryonic stage and deciphering the dysregulated signaling pathways affected upon Pelo deletion. We found that development of PELO-null embryos is perturbed between the embryonic days E4.5 and E5.5, at which first differentiation process of ESCs takes place. Molecular analysis revealed enhanced activity of phosphoinositide 3-kinase-protein kinase B/ AKT (PI3K-PKB/AKT) signaling, but nuclear accumulation of forkhead box O1 (FOXO1), and upregulation of the pluripotency-related gene, Oct4, in mutant ESCs cultured under differentiation condition. Despite increased levels of nuclear β-catenin in PELO-null ESCs as a result of decreased activity of glycogen synthase kinase-3β, the activity of the canonical wingless (Wnt)/β-catenin/T-cell factor (TCF) was significantly attenuated as judged by the promoter reporter assay, downregulated Wnt/β-catenin target genes, and impaired cell proliferation. Interestingly, we demonstrated an increased binding of β-catenin to FOXO1 in PELO-mutant ESCs cultured under differentiation condition that could explain, on one side, the nuclear accumulation of FOXO1 protein and hence persistent pluripotency of PELO-mutant ESCs, and on the other side, the dysregulated transcriptional activity of β-catenin/TCF and therefore attenuated PELO-null ESC self-renewal. Taken together, our results strongly suggest that PELO deletion averts ESC differentiation through promoting FOXO1/β-catenin binding with subsequent dysregulation of FOXO1 and canonical β-catenin/TCF signaling pathways.

Endothelins and their receptors in embryo implantation

As a critical stage of pregnancy, the implantation of blastocysts into the endometrium is a progressive, excessively regulated local tissue remodeling step involving a complex sequence of genetic and cellular interplay executed within an optimal time frame. For better understanding the causes of infertility and, more importantly, for developing powerful strategies for successful implantations and combating infertility, an increasing number of recent studies have been focused on the identification and study of newly described substances in the reproductive tree. The endothelins (ET), a 21-aminoacidic family of genes, have been reported to be responsible for the contraction of vascular and nonvascular smooth muscles, including the smooth muscles of the uterus. Therefore, this review aims to comprehensively discuss the physiological role of endothelins and signaling through their receptors, as well as their probable involvement in the implantation process.

Improvement of implantation potential in mouse blastocysts derived from IVF by combined treatment with prolactin, epidermal growth factor and 4-hydroxyestradiol

STUDY QUESTION

Can supplementation of medium with prolactin (PRL), epidermal growth factor (EGF) and 4-hydroxyestradiol (4-OH-E2) prior to embryo transfer improve implantation potential in mouse blastocysts derived from IVF?

SUMMARY ANSWER

Combined treatment with PRL, EGF and 4-OH-E2 improves mouse blastocyst implantation rates, while alone, each factor is ineffective.

WHAT IS KNOWN ALREADY

Blastocyst dormancy during delayed implantation caused by ovariectomy is maintained by continued progesterone treatment in mice, and estrogen injection rapidly activates blastocysts to implantation-induced status in vivo. While the expression of many proteins is upregulated in implantation-induced blastocysts, selective proteolysis by proteasomes, such as estrogen receptor α (ESR1), occurs in implantation-induced blastocysts to achieve implantation-competent status. It is worth evaluating the proteins expressed during these periods to identify humoral factors that might improve the implantation potential of IVF-derived blastocysts because the poor quality of embryos obtained by IVF is one of the major causes of implantation failure.

STUDY DESIGN, SIZE, DURATION

Superovulated oocytes from ICR mice were fertilized with spermatozoa and then cultured in vitro in potassium simplex optimized medium (KSOM) without phenol red (KSOM-P) for 90-96 h. Blastocysts were treated with PRL (10 or 20 mIU/mL), EGF (5 or 10 ng/mL) or 4-OH-E2 (1 or 10 nM) in KSOM-P for 24 h.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Levels of breast cancer 1 (BRCA1), EGF receptor (EGFR, also known as ERBB1), ERBB4, tubulointerstitial nephritis antigen-like 1 (TINAGL1) and ESR1 protein were examined with immunohistochemical analysis using immunofluorescence methods and confocal laser scanning microscopy. For embryo transfer, six blastocysts were suspended in HEPES-buffered KSOM-P medium and transferred into the uteri of recipient mice on the morning of Day 4 (0900-1000 h) of pseudopregnancy (Day 1 = vaginal plug). The number of implantation sites was then recorded on Day 6 using the blue dye method.

MAIN RESULTS AND THE ROLE OF CHANCE

PRL, EGF and 4-OH-E2 each promoted BRCA1 protein level in the trophectoderm (TE). While PRL treatment resulted in an increase in EGFR, EGF increased both EGFR and ERBB4 in the blastocyst TE. TINAGL1 in the TE was enhanced by 4-OH-E2, which also increased localization of this protein to the basement membrane. Treatment with PRL, EGF or 4-OH-E2 alone did not improve blastocyst implantation rates. Combined treatment with PRL, EGF and 4-OH-E2 resulted in increased levels of EGFR, ERBB4, TINAGL1 and BRCA1 in the TE, whereas ESR1 was not upregulated in the treated blastocysts. Furthermore, combined treatment with PRL, EGF and 4-OH-E2 improved blastocyst implantation rates versus control (P = 0.009).

LARGE SCALE DATA

Not applicable.

LIMITATIONS, REASONS FOR CAUTION

Our studies were carried out in a mouse model, and the conclusions were drawn from limited results obtained from one species. Whether the increase in EGFR, ERBB4 and TINAGL1 protein in the TE improves implantation potential of blastocysts needs to be further studied experimentally by assessing other expressed proteins. The influence of combined supplementation in vitro of PRL, EGF and 4-OH-E2 on implantation also requires further examination and optimization in human blastocysts before it can be considered for clinical use in ART.

WIDER IMPLICATIONS OF THE FINDINGS

Enhanced implantation potential by combined treatment with PRL, EGF and 4-OH-E2 appears to result in the upregulation of at least two distinct mechanisms, namely signaling via EGF receptors and basement membrane formation during the peri-implantation period in mice. While PRL, EGF and 4-OH-E2 each promoted BRCA1 protein level in the TE, treatment with each alone did not improve blastocyst implantation. Therefore, BRCA1 protein appears to be unnecessary for the attachment reaction in blastocysts in mice Combined supplementation of PRL, EGF and 4-OH-E2 might also be of relevance for embryo transfer of human IVF-derived blastocysts for ART.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported in part by the JSPS KAKENHI [Grant numbers 22580316 and 25450390 (to H.M.)] and the Joint Research Project of Japan-U.S. Cooperative Science Program (to H.M.). The authors have no conflict of interest to declare.

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.

Molecular and cellular events during blastocyst implantation in the receptive uterus: clues from mouse models

The success of implantation is an interactive process between the blastocyst and the uterus. Synchronized development of embryos with uterine differentiation to a receptive state is necessary to complete pregnancy. The period of uterine receptivity for implantation is limited and referred to as the “implantation window”, which is regulated by ovarian steroid hormones. Implantation process is complicated due to the many signaling molecules in the hierarchical mechanisms with the embryo-uterine dialogue. The mouse is widely used in animal research, and is uniquely suited for reproductive studies, i.e., having a large litter size and brief estrous cycles. This review first describes why the mouse is the preferred model for implantation studies, focusing on uterine morphology and physiological traits, and then highlights the knowledge on uterine receptivity and the hormonal regulation of blastocyst implantation in mice. Our recent study revealed that selective proteolysis in the activated blastocyst is associated with the completion of blastocyst implantation after embryo transfer. Furthermore, in the context of blastocyst implantation in the mouse, this review discusses the window of uterine receptivity, hormonal regulation, uterine vascular permeability and angiogenesis, the delayed-implantation mouse model, morphogens, adhesion molecules, crosslinker proteins, extracellular matrix, and matricellular proteins. A better understanding of uterine and blastocyst biology during the peri-implantation period should facilitate further development of reproductive technology.

Studies on lysophosphatidic acid action during in vitro preimplantation embryo development

Assisted reproductive technologies, including in vitro embryo production (IVP), have been successfully used in animal reproduction to optimize breeding strategies for improved production and health in animal husbandry. Despite the progress in IVP techniques over the years, further improvements in in vitro embryo culture systems are required for the enhancement of oocyte and embryo developmental competence. One of the most important issues associated with IVP procedures is the optimization of the in vitro culture of oocytes and embryos. Studies in different species of animals and in humans have identified important roles for receptor-mediated lysophosphatidic acid (LPA) signaling in multiple aspects of human and animal reproductive tract function. The data on LPA signaling in the ovary and uterus suggest that LPA can directly contribute to embryo-maternal interactions via its influence on early embryo development beginning from the influence of the ovarian environment on the oocyte to the influence of the uterine environment on the preimplantation embryo. This review discusses the current status of LPA as a potential supplement in oocyte maturation, fertilization, and embryo culture media and current views on the potential involvement of the LPA signaling pathway in early embryo development.

Annexin A2 Acts as an Adhesion Molecule on the Endometrial Epithelium during Implantation in Mice

To determine the function of Annexin A2 (Axna2) in mouse embryo implantation in vivo, experimental manipulation of Axna2 activities was performed in mouse endometrial tissue in vivo and in vitro. Histological examination of endometrial tissues was performed throughout the reproduction cycle and after steroid treatment. Embryo implantation was determined after blockage of the Axna2 activities by siRNA or anti-Axna2 antibody. The expression of Axna2 immunoreactivies in the endometrial luminal epithelium changed cyclically in the estrus cycle and was upregulated by estrogen. After nidatory estrogen surge, there was a concentration of Axna2 immunoreactivities at the interface between the implanting embryo and the luminal epithelium. The phenomenon was likely to be induced by the implanting embryos as no such concentration of signal was observed in the inter-implantation sites and in pseudopregnancy. Knockdown of Axna2 by siRNA reduced attachment of mouse blastocysts onto endometrial tissues in vitro. Consistently, the number of implantation sites was significantly reduced after infusion of anti-Axna2 antibody into the uterine cavity. Steroids and embryos modulate the expression of Axna2 in the endometrial epithelium. Axna2 may function as an adhesion molecule during embryo implantation in mice.

Molecular and cellular events involved in the completion of blastocyst implantation

Blastocyst implantation is an interactive process between the embryo and the uterus. The synchronization of embryonic development with uterine differentiation to a receptive state is essential for a successful pregnancy. The period of uterine receptivity for implantation is limited. Although implantation involves the interaction of numerous signaling molecules, our understanding of the hierarchical mechanisms that coordinate with the embryo-uterine dialogue is not yet sufficient to prevent infertility caused by implantation failure. This review highlights our knowledge on uterine receptivity and hormonal regulation of blastocyst implantation in mice. We also discuss the adhesion molecules, cross-linker proteins, extracellular proteins, and matricellular proteins involved in blastocyst implantation. Furthermore, our recent study reveals that selective proteolysis in an activated blastocyst is associated with the completion of blastocyst implantation after embryo transfer. A better understanding of uterine and blastocyst biology during the peri-implantation period would facilitate further development of reproductive technology.

Female tract cytokines and developmental programming in embryos

In the physiological situation, cytokines are pivotal mediators of communication between the maternal tract and the embryo. Compelling evidence shows that cytokines emanating from the oviduct and uterus confer a sophisticated mechanism for 'fine-tuning' of embryo development, influencing a range of cellular events from cell survival and metabolism, through division and differentiation, and potentially exerting long-term impact through epigenetic remodelling. The balance between survival agents, including GM-CSF, CSF1, LIF, HB-EGF and IGFII, against apoptosis-inducing factors such as TNFα, TRAIL and IFNg, influence the course of preimplantation development, causing embryos to develop normally, adapt to varying maternal environments, or in some cases to arrest and undergo demise. Maternal cytokine-mediated pathways help mediate the biological effects of embryo programming, embryo plasticity and adaptation, and maternal tract quality control. Thus maternal cytokines exert influence not only on fertility and pregnancy progression but on the developmental trajectory and health of offspring. Defining a clear understanding of the biology of cytokine networks influencing the embryo is essential to support optimal outcomes in natural and assisted conception.

Intracellular Ca2+ signaling and preimplantation development

The key, versatile role of intracellular Ca2+ signaling during egg activation after fertilization has been appreciated for several decades. More recently, evidence has accumulated supporting the concept that cytoplasmic Ca2+ is also a major signaling nexus during subsequent development of the fertilized ovum. This chapter will review the molecular reactions that regulate intracellular Ca2+ levels and cell function, the role of Ca2+ signaling during egg activation and specific examples of repetitive Ca2+ signaling found throughout pre- and peri-implantation development. Many of the upstream and downstream pathways utilized during egg activation are also critical for specific processes that take place during embryonic development. Much remains to be done to elucidate the full complexity of Ca2+ signaling mechanisms in preimplantation embryos to the level of detail accomplished for egg activation. However, an emerging concept is that because this second messenger can be modulated downstream of numerous receptors and is able to bind and activate multiple cytoplasmic signaling proteins, it can help the coordination of development through up- and downstream pathways that change with each embryonic stage.

Insulin-like growth factor 1 increases apical fibronectin in blastocysts to increase blastocyst attachment to endometrial epithelial cells in vitro

STUDY QUESTION

Does insulin-like growth factor 1 (IGF1) increase adhesion competency of blastocysts to increase attachment to uterine epithelial cells in vitro?

SUMMARY ANSWER

IGF1 increases apical fibronectin on blastocysts to increase attachment and invasion in an in vitro model of implantation.

WHAT IS KNOWN ALREADY

Fibronectin integrin interactions are important in attachment of blastocysts to uterine epithelial cells at implantation.

STUDY DESIGN, SIZE, DURATION

Mouse blastocysts (hatched or near completion of hatching) were cultured in serum starved (SS) medium with varying treatments for 24, 48 or 72 h. Treatments included 10 ng/ml IGF1 in the presence or absence of the PI3 kinase inhibitor LY294002, an IGF1 receptor (IGF1R) neutralizing antibody or fibronectin. Effects of treatments on blastocysts were measured by attachment of blastocysts to Ishikawa cells, blastocyst outgrowth and fibronectin and focal adhesion kinase (FAK) localization and expression. Blastocysts were randomly allocated into control and treatment groups and experiments were repeated a minimum of three times with varying numbers of blastocysts used in each experiment. FAK and integrin protein expression on Ishikawa cells was quantified in the presence or absence of IGF1.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Fibronectin expression and localization in blastocysts was studied using immunofluorescence and confocal microscopy. Global surface expression of integrin αvβ3, β3 and β1 was measured in Ishikawa cells using flow cytometry. Expression levels of phosphorylated FAK and total FAK were measured in Ishikawa cells and blastocysts by western blot and image J analysis. Blastocyst outgrowth was quantified using image J analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

The presence of IGF1 significantly increased mouse blastocyst attachment to Ishikawa cells compared with SS conditions (P < 0.01). IGF1 treatment resulted in distinct apical fibronectin staining on blastocysts, which was reduced by the PI3 kinase inhibitor LY294002. This coincided with a significant increase in blastocyst outgrowth in the presence of IGF1 (P < 0.01) or fibronectin (P < 0.001), which was abolished by LY294002 (P < 0.001). Apical expression of integrin αvβ3, β3 and β1 in Ishikawa cells was unaltered by IGF1. However, IGF1 increased phosphorylated FAK (P < 0.05) and total FAK expression in Ishikawa cells. FAK signalling is linked to integrin activation and can affect the integrins' ability to bind and recognize extracellular matrix proteins such as fibronectin. Treatment of blastocysts with IGF1 before co-culture with Ishikawa cells increased their attachment (P < 0.05). This effect was abolished in the presence of LY294002 (P < 0.001) or an IGF1R neutralizing antibody (P < 0.05).

LIMITATIONS, REASONS FOR CAUTION

This study uses an in vitro model of attachment that uses mouse blastocysts and human endometrial cells. This involves a species crossover and although this use has been well documented as a model for attachment (as human embryo numbers are limited) the results should be interpreted carefully.

WIDER IMPLICATIONS OF THE FINDINGS

This study presents mechanisms by which IGF1 improves attachment of blastocysts to Ishikawa cells and documents for the first time how IGF1 can increase adhesion competency in blastocysts. Failure of the blastocyst to implant is the major cause of human assisted reproductive technology (ART) failure. As growth factors are absent during embryo culture, their addition to embryo culture medium is a potential avenue to improve IVF success. In particular, IGF1 could prove to be a potential treatment for blastocysts before transfer to the uterus in an ART setting.

Ion channels in the endometrium: regulation of endometrial receptivity and embryo implantation

BACKGROUND

Although embryo implantation is a prerequisite for human reproduction, it remains a poorly understood process. The molecular mechanisms regulating endometrial receptivity and/or embryo implantation are still largely unclear.

METHODS

Pubmed and Medline literature databases were searched for articles in English published up to December 2013 with relevant keywords including 'endometrium', 'Na(+), Cl(-), K(+), or Ca(2+) channels', 'ion channels', 'endometrial receptivity', 'blastocyst implantation' and 'embryo implantation'.

RESULTS

At the time of writing, more than 14 types of ion channels, including the cystic fibrosis transmembrane conductance regulator, epithelial sodium channel and various Ca(2+) and K(+) channels, had been reported to be expressed in the endometrium or cells of endometrial origin. In vitro and/or in vivo studies conducted on different species, including rodents, pigs and humans, demonstrated the involvement of various ion channels in the process of embryo implantation by regulating: (i) uterine luminal fluid volume; (ii) decidualization; and (iii) the expression of the genes associated with implantation. Importantly, abnormal ion channel expression was found to be associated with implantation failure in IVF patients.

CONCLUSIONS

Ion channels in the endometrium are emerging as important players in regulating endometrial receptivity and embryo implantation. Abnormal expression or function of ion channels in the endometrium may lead to impaired endometrial receptivity and/or implantation failure. Further investigation into the roles of endometrial ion channels may provide a better understanding of the complex process of embryo implantation and thus reveal novel targets for diagnosis and treatment of implantation failure.

LIF-STAT signaling and trophoblast biology

Leukemia inhibitory factor (LIF) is a pleiotropic growth factor that regulates several biological functions. This review focuses on the LIF-dependent STAT activation and its impact on modulation of trophoblast functions during embryo implantation. LIF is mainly produced by the maternal endometrium at the time of implantation while its receptors are present both on the endometrium and trophoblasts. It might influence blastocyst attachment through STAT3 activation and expression of integrins. After attachment of the blastocyst, trophoblasts undergo proliferation and differentiation into invasive EVTs and non-invasive STBs. Under in vitro conditions, LIF regulates all these processes through activation of STAT- and MAPK-dependent signaling pathways. The observations that LIF and STAT3 knockout mice are infertile further strengthen the notion about the critical involvement of LIF-mediated signaling during embryo implantation. Hence, a better understanding of LIF-STAT signaling would help in improving fertility as use of LIF in in vitro blastocyst culture improves the implanting ability of blastocyst after IVF.

Physiological and molecular determinants of embryo implantation

Embryo implantation involves the intimate interaction between an implantation-competent blastocyst and a receptive uterus, which occurs in a limited time period known as the window of implantation. Emerging evidence shows that defects originating during embryo implantation induce ripple effects with adverse consequences on later gestation events, highlighting the significance of this event for pregnancy success. Although a multitude of cellular events and molecular pathways involved in embryo-uterine crosstalk during implantation have been identified through gene expression studies and genetically engineered mouse models, a comprehensive understanding of the nature of embryo implantation is still missing. This review focuses on recent progress with particular attention to physiological and molecular determinants of blastocyst activation, uterine receptivity, blastocyst attachment and uterine decidualization. A better understanding of underlying mechanisms governing embryo implantation should generate new strategies to rectify implantation failure and improve pregnancy rates in women.

Desmosomal adhesiveness is developmentally regulated in the mouse embryo and modulated during trophectoderm migration

During embryonic development tissues remain malleable to participate in morphogenetic movements but on completion of morphogenesis they must acquire the toughness essential for independent adult life. Desmosomes are cell-cell junctions that maintain tissue integrity especially where resistance to mechanical stress is required. Desmosomes in adult tissues are termed hyper-adhesive because they adhere strongly and are experimentally resistant to extracellular calcium chelation. Wounding results in weakening of desmosomal adhesion to a calcium-dependent state, presumably to facilitate cell migration and wound closure. Since desmosomes appear early in mouse tissue development we hypothesised that initial weak adhesion would be followed by acquisition of hyper-adhesion, the opposite of what happens on wounding. We show that epidermal desmosomes are calcium-dependent until embryonic day 12 (E12) and become hyper-adhesive by E14. Similarly, trophectodermal desmosomes change from calcium-dependence to hyper-adhesiveness as blastocyst development proceeds from E3 to E4.5. In both, development of hyper-adhesion is accompanied by the appearance of a midline between the plasma membranes supporting previous evidence that hyper-adhesiveness depends on the organised arrangement of desmosomal cadherins. By contrast, adherens junctions remain calcium-dependent throughout but tight junctions become calcium-independent as desmosomes mature. Using protein kinase C (PKC) activation and PKCα-/- mice, we provide evidence suggesting that conventional PKC isoforms are involved in developmental progression to hyper-adhesiveness. We demonstrate that modulation of desmosomal adhesion by PKC can regulate migration of trophectoderm. It appears that tissue stabilisation is one of several roles played by desmosomes in animal development.

High throughput, cell type-specific analysis of key proteins in human endometrial biopsies of women from fertile and infertile couples

BACKGROUND

Although histological dating of endometrial biopsies provides little help for prediction or diagnosis of infertility, analysis of individual endometrial proteins, proteomic profiling and transcriptome analysis have suggested several biomarkers with altered expression arising from intrinsic abnormalities, inadequate stimulation by or in response to gonadal steroids or altered function due to systemic disorders. The objective of this study was to delineate the developmental dynamics of potentially important proteins in the secretory phase of the menstrual cycle, utilizing a collection of endometrial biopsies from women of fertile (n = 89) and infertile (n = 89) couples.

METHODS AND RESULTS

Progesterone receptor-B (PGR-B), leukemia inhibitory factor, glycodelin/progestagen-associated endometrial protein (PAEP), homeobox A10, heparin-binding EGF-like growth factor, calcitonin and chemokine ligand 14 (CXCL14) were measured using a high-throughput, quantitative immunohistochemical method. Significant cyclic and tissue-specific regulation was documented for each protein, as well as their dysregulation in women of infertile couples. Infertile patients demonstrated a delay early in the secretory phase in the decline of PGR-B (P < 0.05) and premature mid-secretory increases in PAEP (P < 0.05) and CXCL14 (P < 0.05), suggesting that the implantation interval could be closing early. Correlation analysis identified potential interactions among certain proteins that were disrupted by infertility.

CONCLUSIONS

This approach overcomes the limitations of a small sample number. Protein expression and localization provided important insights into the potential roles of these proteins in normal and pathological development of the endometrium that is not attainable from transcriptome analysis, establishing a basis for biomarker, diagnostic and targeted drug development for women with infertility.

Leucine and arginine regulate trophoblast motility through mTOR-dependent and independent pathways in the preimplantation mouse embryo

Uterine implantation is a critical element of mammalian reproduction and is a tightly and highly coordinated event. An intricate and reciprocal uterine-embryo dialog exists to synchronize uterine receptivity with the concomitant activation of the blastocyst, maximizing implantation success. While a number of pathways involved in regulating uterine receptivity have been identified in the mouse, less is understood about blastocyst activation, the process by which the trophectoderm (TE) receives extrinsic cues that initiate new characteristics essential for implantation. Amino acids (AA) have been found to regulate blastocyst activation and TE motility in vitro. In particular, we find that arginine and leucine alone are necessary and sufficient to induce TE motility. Both arginine and leucine act individually and additively to propagate signals that are dependent on the activity of the mammalian target of rapamycin complex 1 (mTORC1). The activities of the well-established downstream targets of mTORC1, p70S6K and 4EBP, do not correlate with trophoblast motility, suggesting that an independent-rapamycin-sensitive pathway operates to induce trophoblast motility, or that other, parallel amino acid-dependent pathways are also involved. We find that endogenous uterine factors act to induce mTORC1 activation and trophoblast motility at a specific time during pregnancy, and that this uterine signal is later than the previously defined signal that induces the attachment reaction. In vivo matured blastocysts exhibit competence to respond to an 8-hour AA stimulus by activating mTOR and subsequently undergoing trophoblast outgrowth by the morning of day 4.5 of pregnancy, but not on day 3.5. By the late afternoon of day 4.5, the embryos no longer require any exposure to AA to undergo trophoblast outgrowth in vitro, demonstrating the existence and timing of an equivalent in vivo signal. These results suggest that there are two separate uterine signals regulating implantation, one that primes the embryo for the attachment reaction and another that activates mTOR and initiates invasive behavior.

Integrin β3 in rat blastocysts and epithelial cells is essential for implantation in vitro: studies with Ishikawa cells and small interfering RNA transfection

BACKGROUND

Integrins are involved in the process of embryo-endometrium interaction during implantation. We investigated the localization of integrin β3 in the rat blastocyst and Ishikawa cells using an in vitro co-culture model of implantation.

METHODS

Zona pellucida-free rat blastocysts were co-cultured with the Ishikawa cells (endometrial adenocarcinoma cell line) to observe the attachment between the embryo and endometrium. Immunofluorescence staining was used to investigate the localization of integrin β3 in rat embryos at different stages of development (each n= 3 embryos) and at the embryo/endometrium interface, observed by confocal microscopy. The Ishikawa cells were transfected with integrin β3 small interfering RNA (siRNA) for 48 h and then co-cultured with Day 5 rat blastocysts to observe the effect on attachment.

RESULTS

Integrin β3 staining in the rat embryos increased at the blastocyst stage being highly concentrated in the cytoplasm of trophoblast cells (n= 9 embryos). Integrin β3 was localized on the apical surface of the Ishikawa cells (n= 3 experiments). However, integrin β3 relocated to the apical membrane of trophoblast cells in response to attachment to Ishikawa cells (n= 6 embryos). Moreover, when Ishikawa cells were transfected with integrin β3 siRNA, blastocyst attachment was significantly reduced compared with those transfected with control siRNA (16.7 versus 92.3%, respectively, P< 0.05).

CONCLUSIONS

Integrin β3, localized apically in the blastocyst and the Ishikawa cells, is important during initial attachment of the blastocyst to endometrial cells. This study provides further evidence of the importance of integrins during implantation and may aid in elucidating the molecular mechanism of implantation failure and infertility in women.

Expression of calbindin-D28k and its regulation by estrogen in the human endometrium during the menstrual cycle

Human endometrium resists embryo implantation except during the 'window of receptivity'. A change in endometrial gene expression is required for the development of receptivity. Uterine calbindin-D28k (CaBP-28k) is involved in the regulation of endometrial receptivity by intracellular Ca2+. Currently, this protein is known to be mainly expressed in brain, kidneys, and pancreas, but potential role(s) of CaBP-28k in the human uterus during the menstrual cycle remain to be clarified. Thus, in this study we demonstrated the expression of CaBP-28k in the human endometrium in distinct menstrual phases. During the human menstrual cycle, uterine expression levels of CaBP-28k mRNA and protein increased in the proliferative phase and fluctuated in these tissues, compared with that observed in other phases. We assessed the effects of two sex-steroid hormones, 17beta-estradiol (E2) and progesterone (P4), on the expression of CaBP-28k in Ishikawa cells. A significant increase in the expression of CaBP-28k mRNA was observed at the concentrations of E2 (10(-9 to -7) M). In addition, spatial expression of CaBP-28k protein was detected by immunohistochemistry. CaBP-28k was abundantly localized in the cytoplasm of the luminal and glandular epithelial cells during the proliferative phases (early-, mid-, late-) and early-secretory phase of menstrual cycle. Taken together, these results indicate that CaBP-28k, a uterine calcium binding protein, is abundantly expressed in the human endometrium, suggesting that uterine expression of CaBP-28k may be involved in reproductive function during the human menstrual cycle.

Calcium signaling in placenta

The placenta sustains the developing fetus throughout gestation and its major functions include nutrition, gas and waste exchange via a variety of passive or active mechanisms. Up to 30 g of calcium (Ca(2+)) actively crosses the trophoblast layer during human pregnancy. The Ca(2+) ion not only plays an important role for skeletal development but is also an essential second messenger. This review is intended to highlight the implications of Ca(2+) signaling during reproduction and specifically placentation. Initially, a Ca(2+) wave induces fertilization of the oocyte. The intracellular Ca(2+) concentration is key for the blastocyst implantation, proper placental development and function. Current knowledge of many proteins involved in placental Ca(2+) regulation and their function in pathologic conditions is largely limited. Recent studies, however, point to alterations in Ca(2+) homeostasis in placental pathologies such as pre-eclampsia (PE) and intrauterine growth restriction (IUGR). A broader understanding of the role of Ca(2+) signaling during human reproduction may offer insight into impaired pregnancy outcomes.

Transient {beta}2-adrenoceptor activation confers pregnancy loss by disrupting embryo spacing at implantation

Pregnancy loss is a serious social and medical issue, with one important cause associated with aberrant embryo implantation during early pregnancy. However, whether and how the process of embryo implantation is affected by environmental factors such as stress-induced sympathetic activation remained elusive. Here we report an unexpected, transient effect of β₂-adrenoreceptor (β₂-AR) activation (day 4 postcoitus) in disrupting embryo spacing at implantation, leading to substantially increased midterm pregnancy loss. The abnormal embryo spacing could be prevented by pretreatment of β₂-AR antagonist or genetic ablation of β-AR. Similar β₂-AR activation at day 5 postcoitus, when implantation sites have been established, did not affect embryo spacing or pregnancy outcome, indicating that the adverse effect of β₂-AR activation is limited to the preimplantation period before embryo attachment. In vitro and in vivo studies demonstrated that the transient β₂-AR activation abolished normal preimplantation uterine contractility without adversely affecting blastocyst quality. The contractility inhibition is mediated by activation of the cAMP-PKA pathway and accompanied by specific down-regulation of lpa3, a gene previously found to be critical for uterine contraction and embryo spacing. These results indicated that normal uterine contraction-mediated correct intrauterine embryo distribution is crucial for successful ongoing pregnancy. Abnormal β₂-AR activation at early pregnancy provided a molecular clue in explaining how maternal stress at early stages could adversely affect the pregnancy outcome.

Function-specific intracellular signaling pathways downstream of heparin-binding EGF-like growth factor utilized by human trophoblasts

Heparin-binding EGF-like growth factor (HBEGF) is expressed by trophoblast cells throughout gestation. First-trimester cytotrophoblast cells are protected from hypoxia-induced apoptosis because of the accumulation of HBEGF through a posttranscriptional autocrine mechanism. Exogenous application of HBEGF is cytoprotective in a hypoxia/reoxygenation (H/R) injury model and initiates trophoblast extravillous differentiation to an invasive phenotype. The downstream signaling pathways induced by HBEGF that mediate these various cellular activities were identified using two human first-trimester cytotrophoblast cell lines, HTR-8/SVneo and SW.71, with similar results. Recombinant HBEGF (1 nM) induced transient phosphorylation of MAPK3/1 (ERK), MAPK14 (p38), and AKT within 15 min and JNK after 1-2 h. To determine which downstream pathways regulate the various functions of HBEGF, cells were treated with specific inhibitors of the ERK upstream regulator MEK (U0126), the AKT upstream regulator phosphoinositide-3 (PI3)-kinase (LY294002), MAPK14 (SB203580), and JNK (SP600125), as well as with inactive structural analogues. Only SB203580 specifically prevented HBEGF-mediated rescue during H/R, while each inhibitor attenuated HBEGF-stimulated cell migration. Accumulation of HBEGF at reduced oxygen was blocked only by a combination of U0126, SB203580, and SP600125. We conclude that HBEGF advances trophoblast extravillous differentiation through coordinate activation of PI3 kinase, ERK, MAPK14, and JNK, while only MAPK14 is required for its antiapoptotic activity. Additionally, hypoxia induces an autocrine increase in HBEGF protein levels through MAPK14, JNK or ERK. These experiments reveal a complexity of the intracellular signaling circuitry that regulates trophoblast functions critical for implantation and placentation.

Diverse functions of HBEGF during pregnancy

The establishment of pregnancy requires an intimate physical interaction and a molecular dialogue between the conceptus and the maternal reproductive tract that commences at implantation and continues until the placenta is formed and fully functional. Failure of the regulatory processes that ensure the fidelity of this relationship can precipitate a catastrophic pregnancy loss. One of the earliest identified molecular mediators of blastocyst implantation is heparin-binding epidermal growth factor (EGF)-like growth factor (HBEGF), which signals between the endometrium and implanting trophoblast cells to synchronize their corresponding developmental programs. HBEGF expression by trophoblast cells of the developing placenta appears to regulate extravillous differentiation and provide cytoprotection in a sometimes-hostile environment. This versatile member of the EGF signaling system will be examined in light of its associations with key events during early pregnancy.

Calmodulin-mediated regulation of the epidermal growth factor receptor

In this review, we first describe the mechanisms by which the epidermal growth factor receptor generates a Ca(2+) signal and, subsequently, we compile the available experimental evidence regarding the role that the Ca(2+)/calmodulin complex, formed after the rise in cytosolic free Ca(2+) concentration, exerts on the receptor. We focus not only on the indirect action that Ca(2+)/calmodulin exerts on the epidermal growth factor receptor, as a result of the activation of distinct calmodulin-dependent kinases, but also, and more extensively, on the direct interaction of Ca(2+)/calmodulin with the receptor. We also describe several mechanistic models that could account for the Ca(2+)/calmodulin-mediated regulation of epidermal growth factor receptor activity. The control exerted by calmodulin on distinct epidermal growth factor receptor-mediated cellular functions is also discussed. Finally, the phosphorylation of this Ca(2+) sensor by the epidermal growth factor receptor is highlighted.

Expression of heparin-binding EGF-like growth factor in term chorionic villous explants and its role in trophoblast survival

Heparin-binding EGF-like growth factor (HBEGF) induces trophoblast extravillous differentiation and prevents apoptosis. These functions are compromised in preeclampsia. Because HBEGF is downregulated in placentas delivered by women with preeclampsia, we have examined its expression and cytoprotective activity in term villous explants. Chorionic villous explants prepared from non-pathological placentas collected by cesarean section at term were cultured at either 20% or 2% O2 and treated with the HBEGF antagonist CRM197 or recombinant HBEGF. Paraffin sections were assayed for trophoblast death, proliferation and HBEGF expression using the TUNEL method, immunohistochemistry for nuclear Ki67 expression and semi-quantitative immunohistochemistry with image analysis, respectively. Trophoblast cell death was increased significantly after 8h of culture with CRM197 or by culture for 2h at 2% O2. Exogenous HBEGF prevented cell death due to hypoxia. Proliferative capacity was not affected by culture at either 20% or 2% O2. Contrary to first trimester placenta, term trophoblasts do not elevate HBEGF expression in response to hypoxia. However, low endogenous levels of HBEGF are required to maintain survival. Therefore, HBEGF-mediated signaling significantly reduces trophoblast cell death at term and its deficiency in preeclampsia could negatively impact trophoblast survival.

HB-EGF: a unique mediator of embryo-uterine interactions during implantation

An implantation-competent blastocyst, several hours prior to its attachment on the uterine wall, transmits signals to surrounding uterine cells and vice-versa to initiate a two-way interaction. The language of this precocious dialogue is versatile, taking advantage of secreted molecules for long-range interactions and membrane-bound molecules for more immediate interactions. Heparin-binding epidermal growth factor-like growth factor (HB-EGF) was identified as an early messenger of implantation which uses both modes of communication. In this review, we discuss the footprint of HB-EGF as to how it was initially identified as a mediator of implantation and how it initiates embryo-uterine interactions during this process.

Expression of genes involved in early cell fate decisions in human embryos and their regulation by growth factors

Little is understood about the regulation of gene expression in human preimplantation embryos. We set out to examine the expression in human preimplantation embryos of a number of genes known to be critical for early development of the murine embryo. The expression profile of these genes was analysed throughout preimplantation development and in response to growth factor (GF) stimulation. Developmental expression of a number of genes was similar to that seen in murine embryos (OCT3B/4,CDX2,NANOG). However,GATA6is expressed throughout preimplantation development in the human. Embryos were cultured in IGF-I, leukaemia inhibitory factor (LIF) or heparin-binding EGF-like growth factor (HBEGF), all of which are known to stimulate the development of human embryos. Our data show that culture in HBEGF and LIF appears to facilitate human embryo expression of a number of genes:ERBB4(LIF) andLIFRandDSC2(HBEGF) while in the presence of HBEGF no blastocysts expressedEOMESand when cultured with LIF only two out of nine blastocysts expressedTBN. These data improve our knowledge of the similarities between human and murine embryos and the influence of GFs on human embryo gene expression. Results from this study will improve the understanding of cell fate decisions in early human embryos, which has important implications for both IVF treatment and the derivation of human embryonic stem cells.

Basic aspects of implantation

Implantation, a critical step for establishing pregnancy, requires molecular and cellular events resulting in healthy uterine growth and differentiation, blastocyst adhesion, invasion and placental formation. Successful implantation requires a receptive endometrium, a normal and functional embryo at the blastocyst stage and a synchronized dialogue between maternal and embryonic tissues. In addition to the main role of sex steroids, the complexity of embryo implantation and placentation is exemplified by the number of cytokines and growth factors with demonstrated roles in these processes. Disturbances of the normal expression and action of these cytokines result in absolute or partial failure of implantation and abnormal placental formation in mice and humans. Members of the gp130 cytokine family, interleukin (IL)-11 and leukaemia inhibitory factor, the transforming growth factor-beta superfamily, colony-stimulating factors, and the IL-1 and IL-15 systems are all crucial for successful implantation. In addition, chemokines are important both in recruiting specific cohorts of leukocytes to the implantation site, and in trophoblast trafficking and differentiation. This review provides discussion on embryonic and uterine factors that are involved in the process of implantation in autocrine, paracrine and/or juxtacrine manners at hormonal, cellular, and molecular levels.

Maternal heparin-binding-EGF deficiency limits pregnancy success in mice

An intimate discourse between the blastocyst and uterus is essential for successful implantation. However, the molecular basis of this interaction is not clearly understood. Exploiting genomic Hbegf mutant mice, we show here that maternal deficiency of heparin-binding EGF-like growth factor (HB-EGF) defers on-time implantation, leading to compromised pregnancy outcome. We also demonstrate that amphiregulin, but not epiregulin, partially compensates for the loss of HB-EGF during implantation. In search of the mechanism of this compensation, we found that reduced preimplantation estrogen secretion from ovarian HB-EGF deficiency is a cause of sustained expression of uterine amphiregulin before the initiation of implantation. To explore the significance specifically of uterine HB-EGF in implantation, we examined this event in mice with conditional deletion of uterine HB-EGF and found that this specific loss of HB-EGF in the uterus still defers on-time implantation without altering preimplantation ovarian estrogen secretion. The observation of normal induction of uterine amphiregulin surrounding the blastocyst at the time of attachment in these conditional mutant mice suggests a compensatory role of amphiregulin for uterine loss of HB-EGF, preventing complete failure of pregnancy. Our study provides genetic evidence that HB-EGF is critical for normal implantation. This finding has high clinical relevance, because HB-EGF signaling is known to be important for human implantation.

Reaching out for signals: filopodia sense EGF and respond by directed retrograde transport of activated receptors

ErbB1 receptors situated on cellular filopodia undergo systematic retrograde transport after binding of the epidermal growth factor (EGF) and activation of the receptor tyrosine kinase. Specific inhibitors of the erbB1 receptor tyrosine kinase as well as cytochalasin D, a disruptor of the actin cytoskeleton, abolish transport but not free diffusion of the receptor–ligand complex. Diffusion constants and transport rates were determined with single molecule sensitivity by tracking receptors labeled with EGF conjugated to fluorescent quantum dots. Retrograde transport precedes receptor endocytosis, which occurs at the base of the filopodia. Initiation of transport requires the interaction and concerted activation of at least two liganded receptors and proceeds at a constant rate mediated by association with actin. These findings suggest a mechanism by which filopodia detect the presence and concentration of effector molecules far from the cell body and mediate cellular responses via directed transport of activated receptors.

Review of factors essential for blastocyst implantation for their modulating effects on the maternal immune system

Pituitary and ovarian hormones prepare the endometrium for successful blastocyst implantation and support its process directly or indirectly through the action of growth factors, cytokines and other molecules. Many of the blastocyst implantation essential factors (BIEFs) are modulators of the maternal immune system. Since little is known as to the action of these molecules on the uterine lymphocytes, its clarification is imperative to the understanding of the process of blastocyst implantation.

Secreted phosphoprotein 1 (osteopontin) is expressed by stromal macrophages in cyclic and pregnant endometrium of mice, but is induced by estrogen in luminal epithelium during conceptus attachment for implantation

Secreted phosphoprotein 1 (SPP1, osteopontin) is the most highly upregulated extracellular matrix/adhesion molecule/cytokine in the receptive phase human uterus, and Spp1 null mice manifest decreased pregnancy rates during mid-gestation as compared with wild-type counterparts. We hypothesize that Spp1 is required for proliferation, migration, survival, adhesion, and remodeling of cells at the conceptus-maternal interface. Our objective was to define the temporal/spatial distribution and steroid regulation of Spp1 in mouse uterus during estrous cycle and early gestation. In situ hybridization localized Spp1 to luminal epithelium (LE) and immune cells. LE expression was prominent at proestrus, decreased by estrus, and was nearly undetectable at diestrus. During pregnancy, Spp1 mRNA was not detected in LE until day 4.5 (day 1 = vaginal plug). Spp1-expressing immune cells were scattered within the endometrial stroma throughout the estrous cycle and early pregnancy. Immunoreactive Spp1 was prominent at the apical LE surface by day 4.5 of pregnancy and Spp1 protein was also co-localized with subsets of CD45-positive (leukocytes) and F4/80-positive (macrophages) cells. In ovariectomized mice, estrogen, but not progesterone, induced Spp1 mRNA, whereas estrogen plus progesterone did not induce Spp1 in LE. These results establish that estrogen regulates Spp1 in mouse LE and are the first to identify macrophages that produce Spp1 within the peri-implantation endometrium of any species. We suggest that Spp1 at the apical surface of LE provides a mechanism to bridge conceptus to LE during implantation, and that Spp1-positive macrophages within the stroma may be involved in uterine remodeling for conceptus invasion.

Trophoblast adhesion of the peri-implantation mouse blastocyst is regulated by integrin signaling that targets phospholipase C

Integrin signaling modulates trophoblast adhesion to extracellular matrices during blastocyst implantation. Fibronectin (FN)-binding activity on the apical surface of trophoblast cells is strengthened after elevation of intracellular Ca(2+) downstream of integrin ligation by FN. We report here that phosphoinositide-specific phospholipase C (PLC) mediates Ca(2+) signaling in response to FN. Pharmacological agents used to antagonize PLC (U73122) or the inositol phosphate receptor (Xestospongin C) inhibited FN-induced elevation of intracellular Ca(2+) and prevented the upregulation of FN-binding activity. In contrast, inhibitors of Ca(2+) influx through either voltage-gated or non-voltage-gated Ca(2+) channels were without effect. Inhibition of protein tyrosine kinase activity by genistein, but not G-protein inhibition by suramin, blocked FN-induced intracellular Ca(2+) signaling and upregulation of adhesion, consistent with involvement of PLC-gamma. Confocal immunofluorescence imaging of peri-implantation blastocysts demonstrated that PLC-gamma2, but not PLC-gamma1 nor PLC-beta1, accumulated near the outer surface of the embryo. Phosphotyrosine site-directed antibodies revealed phosphorylation of PLC-gamma2, but not PLC-gamma1, upon integrin ligation by FN. These data suggest that integrin-mediated activation of PLC-gamma to initiate phosphoinositide signaling and intracellular Ca(2+) mobilization is required for blastocyst adhesion to FN. Signaling cascades regulating PLC-gamma could, therefore, control a critical feature of trophoblast differentiation during peri-implantation development.

Kinetics of bromhexine-mediated down-regulation of focal adhesive molecules of uterus and trophectoderm affecting conception in the rat

PURPOSE

Quantitative evaluation of properties of bromhexine (B) for expression of uterine proteins in ovariectomized (OVX) and pregnant rats.

MATERIALS AND METHODS

Expression of proteins through SDS-PAGE, along with incorporation of glycosidic moieties, was conducted in pregnant and OVX rats under B influence. These findings were corroborated with other tests such as implantation sites, fetal and litter sizes in pregnant rats.

RESULTS

In OVX animals, even under the influence of estradiol dipropionate and progesterone, the B recreated a condition akin to OVX animals. It also induced 50-80% inhibition in the incorporation of glycosidic moieties to polypeptide chain. Distinct reduction in implantation sites, fetal sizes and interference in the conception (16/46) in pregnant rats substantiated the results of the action of B as an antiimplantation agent.

CONCLUSION

Bromhexine has shown interference in blastocyst attachment, conception, reduction in number of implantation sites and dwarfing of fetuses; hence, it is a potential candidate for antiimplantation.

Roadmap to embryo implantation: clues from mouse models

Implantation involves an intricate discourse between the embryo and uterus and is a gateway to further embryonic development. Synchronizing embryonic development until the blastocyst stage with the uterine differentiation that takes place to produce the receptive state is crucial to successful implantation, and therefore to pregnancy outcome. Although implantation involves the interplay of numerous signalling molecules, the hierarchical instructions that coordinate the embryo-uterine dialogue are not well understood. This review highlights our knowledge about the molecular development of preimplantation and implantation and the future challenges of the field. A better understanding of periimplantation biology could alleviate female infertility and help to develop novel contraceptives.