The hamster as a model for embryo implantation: insights into a multifaceted process

Molecular regulators of human blastocyst development and hatching: Their significance in implantation and pregnancy outcome

In humans, blastocyst hatching and implantation events are two sequential, critically linked and rate-limiting events for a prospective pregnancy. These events are regulated by embryo-endometrium derived molecular factors which include hormones, growth factors, cytokines, immune-modulators, cell adhesion molecules and proteases. Due to poor viability of blastocysts, they fail to hatch and implant, leading to a low 'Live Birth Rates', majorly contributing to infertility. Here, embryo-derived biomarkers analysis plays a key role to assess potential biological viability of blastocysts which are capable of implantation and prospective pregnancy. Thus far, embryo-derived biomarkers examined are mostly immune-modulators which are thought to be associated with blastocyst development-implantation and progression of pregnancy, leading to live births. There is an urgent need to develop a quantitative and a reliable non-invasive approach aiding embryo selection for elective single embryo transfer and to minimize recurrent pregnancy loss and multiple pregnancies. In this article, we provide a comprehensive review on our current knowledge and understanding of potential embryo-derived molecular regulators, that is, biomarkers, of development of human blastocysts, their hatching and implantation. We discuss their potential implications in the assessment of blastocyst implantation potential and pregnancy outcome in terms of live births in humans.

Reliability of Rodent and Rabbit Models in Preeclampsia Research

In vivo studies on the pathology of gestation, including preeclampsia, often use small mammals such as rabbits or rodents, i.e., mice, rats, hamsters, and guinea pigs. The key advantage of these animals is their short reproductive cycle; in addition, similar to humans, they also develop a haemochorial placenta and present a similar transformation of maternal spiral arteries. Interestingly, pregnant dams also demonstrate a similar reaction to inflammatory factors and placentally derived antiangiogenic factors, i.e., soluble fms-like tyrosine kinase 1 (sFlt-1) or soluble endoglin-1 (sEng), as preeclamptic women: all animals present an increase in blood pressure and usually proteinuria. These constitute the classical duet that allows for the recognition of preeclampsia. However, the time of initiation of maternal vessel remodelling and the depth of trophoblast invasion differs between rabbits, rodents, and humans. Unfortunately, at present, no known animal replicates a human pregnancy exactly, and hence, the use of rabbit and rodent models is restricted to the investigation of individual aspects of human gestation only. This article compares the process of placentation in rodents, rabbits, and humans, which should be considered when planning experiments on preeclampsia; these aspects might determine the success, or failure, of the study. The report also reviews the rodent and rabbit models used to investigate certain aspects of the pathomechanism of human preeclampsia, especially those related to incorrect trophoblast invasion, placental hypoxia, inflammation, or maternal endothelial dysfunction.

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.

Insight on Polyunsaturated Fatty Acids in Endometrial Receptivity

Endometrial receptivity plays a crucial role in fertilization as well as pregnancy outcome in patients faced with fertility challenges. The optimization of endometrial receptivity may help with normal implantation of the embryo, and endometrial receptivity may be affected by numerous factors. Recently, the role of lipids in pregnancy has been increasingly recognized. Fatty acids and their metabolites may be involved in all stages of pregnancy and play a role in supporting cell proliferation and development, participating in cell signaling and regulating cell function. Polyunsaturated fatty acids, in particular, are essential fatty acids for the human body that can affect the receptivity of the endometrium through in a variety of methods, such as producing prostaglandins, estrogen and progesterone, among others. Additionally, polyunsaturated fatty acids are also involved in immunity and the regulation of endometrial decidualization. Fatty acids are essential for fetal placental growth and development. The interrelationship of polyunsaturated fatty acids with these substances and how they may affect endometrial receptivity will be reviewed in this article.

Enabling Hamster Embryo Culture System: Development of Preimplantation Embryos

Development of preimplantation embryos, from fertilization to hatched-blastocyst stage, has been a challenging task, regardless of the mammalian species being studied. While the mouse model has been versatile for studying in vitro development of early embryos, other rodent species are important to gain insights into comparative early embryogenesis. The golden hamster (Mesocricetus auratus) offers unique advantages to study cellular and molecular regulation of gamete maturation, fertilization and preimplantation development, including the phenomenon of blastocyst hatching. Achieving in vitro fertilization and first cleavage division is relatively easy; however, subsequent development past the two-/four-cell stage had been difficult in hamsters. Pioneering research, carried out over three decades has markedly enabled successful in vitro development of one-cell embryos to blastocysts. This article provides a comprehensive perspective (historical and current) on the embryo culture systems and details an optimized culture protocols to achieve normal and viable development of preimplantation embryos in the golden hamster.

Morphological, Ultrastructural, and Molecular Aspects of In Vitro Mouse Embryo Implantation on Human Endometrial Mesenchymal Stromal Cells in The Presence of Steroid Hormones as An Implantation Model

Objective

This experimental study aimed to evaluate the effects of 17β-estradiol (E2) and progesterone (P4) on the interaction between mouse embryo and human endometrial mesenchymal stromal cells, and gene expressions related to implantation [αV and β3 integrins, interleukin-1 receptor (IL-1R), and leukemia inhibitory factor receptor (LIFR)] using an in vitro two- dimensional model.

Materials and Methods

In this experimental study, the endometrial stromal cells were isolated enzymatically and mechanically, and cultured to the fourth passage. Next, their immunophenotype was confirmed by flow cytometric analysis as mesenchymal stromal cells. The cells were cultured as either the experimental group in the presence of E2 (0.3 nmol) and P4 (63.5 nmol) or control group without any hormone treatment. Mouse blastocysts were co-cultured with endometrial mesenchymal stromal cells in both groups for 48 hours. Their interaction was assessed under an inverted microscope and scanning electron microscopy (SEM). Expressions of αV and β3 integrins, LIFR, and IL-1R genes were analyzed by real-time reverse transcription-polymerase chain reaction (RT-PCR).

Results

Similar observations were seen in both groups by light microscopy and SEM. We observed the presence of pinopode-like structures and cell secretions on the apical surfaces of endometrial mesenchymal stromal cells in both groups. The trophoblastic cells expanded and interacted with the mesenchymal monolayer cells. At the molecular level, expression of IL-1R significantly increased in the hormonal treated group compared to the control (P≤0.05). Expressions of the other genes did not differ.

Conclusion

This study has shown that co-culture of endometrial mesenchymal stromal cells with mouse embryo in media that contained E2 (0.3 nmol) and P4 (63.5 nmol) could effectively increase the expression of IL-1R, which is involved in embryo implantation. However, there were no significant effects on expressions of αV and β3 integrins, LIFR, and on the morphology and ultrastructure of endometrial mesenchymal stromal cells.

Gene profiling the window of implantation: Microarray analyses from human and rodent models

Poor uterine receptivity leads to implantation defects or failure. Identification of uterine molecules crucial to uterine receptivity and/or embryo implantation provides the opportunity to design a diagnostic screening toolkit for uterine receptivity or targeted drug discovery for treating implantation-based infertility. In this regard, gene-profiling studies performed in humans and rodents have identified numerous genes involved in the transcriptional regulation of uterine receptivity and embryo implantation. In this article, we compared available uterine microarray datasets collected during the time of uterine receptivity and implantation in humans, mice and hamsters to uncover conserved gene sets. We also compared the transcriptome signature of women with unexplained infertility (UIF) and recurrent implantation failure (RIF) to gain insight into genes potentially dysregulated during endometrial receptivity or embryo implantation. Among numerous differentially expressed genes, few were revealed that might have molecular diagnostic screening potential for identifying the uterine receptive state during the time of implantation. Finally, functional annotation of gene sets uncovered altered uterine apoptosis or cell adhesion pathways in women with UIF and RIF, respectively. These conserved or divergent gene sets provide insights into the uterine receptive state for supporting blastocyst implantation.

Expression of ER-α and ER-β during peri-implantation period in uterus is essential for implantation and decidualization in golden hamster

AIMS

The role of estrogen in embryo implantation in golden hamster (Mesocricetus auratus) is still ambiguous. In order to clarify it, we investigated the spatial distribution and expression of estrogen receptors, ER-α and ER-β in the uterus of pregnant hamster during peri-implantation period and identified the effect of estrogen receptor antagonist ICI-182,780 on the embryo implantation.

MAIN METHODS

We performed in vivo experiments on early pregnant hamsters involving treatment with ICI-182,780, an estrogen receptor antagonist. Immunohistochemistry, western blot analysis and quantitative PCR were employed to evaluate the spatio-temporal distribution and expression of ER-α and ER-β in the uterus of normal early pregnant and treated hamsters.

KEY FINDINGS

Results showed that embryo implantation was completely absent in ICI-182,780 treated uterine horn while, normal implantation occurred in control and vehicle treated horns. Both the receptors were differentially expressed in the uterus of hamster from day 1 (D1) to day7 (D7). In contrast, treated horns without any implantation site showed no trace of any receptors. Protein and mRNA expression of both the receptors were high around the day of implantation while, ER-β expression was up-regulated on D7 of embryo implantation. P value˂0.05 is considered significant.

SIGNIFICANCE

Spatio-temporal expression of ERs in the uterus during peri-implantation period have crucial role for endometrium receptivity and implantation in hamster. Recurrent implantation failure is the devastating problem among the desirable couple and is mainly due to defect in endometrium receptivity. This study may provide a new insight to manage the problem of idiopathic infertility.

Cross-species transcriptomic approach reveals genes in hamster implantation sites

The mouse model has greatly contributed to understanding molecular mechanisms involved in the regulation of progesterone (P4) plus estrogen (E)-dependent blastocyst implantation process. However, little is known about contributory molecular mechanisms of the P4-only-dependent blastocyst implantation process that occurs in species such as hamsters, guineapigs, rabbits, pigs, rhesus monkeys, and perhaps humans. We used the hamster as a model of P4-only-dependent blastocyst implantation and carried out cross-species microarray (CSM) analyses to reveal differentially expressed genes at the blastocyst implantation site (BIS), in order to advance the understanding of molecular mechanisms of implantation. Upregulation of 112 genes and downregulation of 77 genes at the BIS were identified using a mouse microarray platform, while use of the human microarray revealed 62 up- and 38 down-regulated genes at the BIS. Excitingly, a sizable number of genes (30 up- and 11 down-regulated genes) were identified as a shared pool by both CSMs. Real-time RT-PCR and in situ hybridization validated the expression patterns of several up- and down-regulated genes identified by both CSMs at the hamster and mouse BIS to demonstrate the merit of CSM findings across species, in addition to revealing genes specific to hamsters. Functional annotation analysis found that genes involved in the spliceosome, proteasome, and ubiquination pathways are enriched at the hamster BIS, while genes associated with tight junction, SAPK/JNK signaling, and PPARα/RXRα signalings are repressed at the BIS. Overall, this study provides a pool of genes and evidence of their participation in up- and down-regulated cellular functions/pathways at the hamster BIS.

Alkaline phosphatases contribute to uterine receptivity, implantation, decidualization, and defense against bacterial endotoxin in hamsters

Alkaline phosphatase (AP) activity has been demonstrated in the uterus of several species, but its importance in the uterus, in general and during pregnancy, is yet to be revealed. In this study, we focused on identifying AP isozyme types and their hormonal regulation, cell type, and event-specific expression and possible functions in the hamster uterus during the cycle and early pregnancy. Our RT-PCR and in situ hybridization studies demonstrated that among the known Akp2, Akp3, Akp5, and Akp6 murine AP isozyme genes, hamster uteri express only Akp2 and Akp6; both genes are co-expressed in luminal epithelial cells. Studies in cyclic and ovariectomized hamsters established that while progesterone (P₄) is the major uterine Akp2 inducer, both P₄ and estrogen are strong Akp6 regulators. Studies in preimplantation uteri showed induction of both genes and the activity of their encoded isozymes in luminal epithelial cells during uterine receptivity. However, at the beginning of implantation, Akp2 showed reduced expression in luminal epithelial cells surrounding the implanted embryo. By contrast, expression of Akp6 and its isozyme was maintained in luminal epithelial cells adjacent to, but not away from, the implanted embryo. Following implantation, stromal transformation to decidua was associated with induced expressions of only Akp2 and its isozyme. We next demonstrated that uterine APs dephosphorylate and detoxify endotoxin lipopolysaccharide at their sites of production and activity. Taken together, our findings suggest that uterine APs contribute to uterine receptivity, implantation, and decidualization in addition to their role in protection of the uterus and pregnancy against bacterial infection.

Domestic animal models for biomedical research

Experimental animals in biomedical research provide insights into disease mechanisms and models for determining the efficacy and safety of new therapies and for discovery of corresponding biomarkers. Although mouse and rat models are most widely used, observations in these species cannot always be faithfully extrapolated to human patients. Thus, a number of domestic species are additionally used in specific disease areas. This review summarizes the most important applications of domestic animal models and emphasizes the new possibilities genetic tailoring of disease models, specifically in pigs, provides.

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.

Adherens junction proteins in the hamster uterus: their contributions to the success of implantation

The adherens junction (AJ) is important for maintaining uterine structural integrity, composition of the luminal environment, and initiation of implantation by virtue of its properties of cell-cell recognition, adhesion, and establishment of cell polarity and permeability barriers. In this study, we investigated the uterine changes of AJ components E-cadherin, beta-catenin, and alpha-catenin at their mRNA and protein levels, together with the cellular distribution of meprinbeta, phospho-beta-catenin, and active beta-catenin proteins, in hamsters that show only ovarian progesterone-dependent uterine receptivity and implantation. By in situ hybridization and immunofluorescence, we have demonstrated that uterine epithelial cells expressed three of these AJ proteins and their mRNAs prior to and during the initial phase of implantation. Immunofluorescence study showed no change in epithelial expression patterns of uterine AJ proteins from Days 1 to 5 of pregnancy. With advancement of the implantation process, AJ components were primarily expressed in cells of the secondary decidual zone (SDZ), but not in the primary decidual zone (PDZ). In contrast, we noted strong expression of beta-catenin and alpha-catenin proteins in the PDZ, but not in the SDZ, of mice. Taken together, these results suggest that AJ proteins contribute to uterine barrier functions by cell-cell adhesion to ensure protection of the embryo. In addition, cleavage of E-cadherin by meprinbeta might contribute to weakening uterine epithelial cell-cell contact for blastocyst implantation. We also report that the nuclear localization of active beta-catenin from Day 4 onward in hamsters implies that beta-catenin/Wnt-signal transduction is activated in the uterus during implantation and decidualization.

Dynamics of zonula occludens-2 expression during preimplantation embryonic development in the hamster

The objective was to study the expression of zonula occludens-2, a tight junction protein, during preimplantation hamster embryonic development, to predict its possible localization, source, and roles in trophectoderm differentiation and blastocyst formation in this species. Comparison of zonula occludens-2 expression pattern between the hamster and mouse preimplantation embryos from the zygote up to the blastocyst stage was also an objective of this study. Zonula occludens-2 localization was noted in nuclei of blastomeres in all stages of hamster and mouse embryonic development. Compared to mice, where zonula occludens-2 was first localized in the interblastomere membrane at the morula stage, hamster embryos had membranous zonula occludens-2 localization from the 2-cell stage onwards. Based on combined results of immunolocalization study in parthenogenic embryos and ovarian and epididymal sections, and quantitative PCR done in oocytes and all developmental stages of preimplantation embryos, perhaps there was a carry-over of zonula occludens-2 proteins or mRNA from the dam to the embryo. Based on these findings, we inferred that maternally derived zonula occludens-2 was involved in nuclear functions, as well as differentiation of blastomeres and blastocoel formation during preimplantation embryonic development in the hamster.

Temporal expression pattern of progesterone receptor in the uterine luminal epithelium suggests its requirement during early events of implantation

OBJECTIVE

To determine the precise timing of progesterone receptor (PR) disappearing from the uterine luminal epithelium (LE) to help understand the significance of the dynamic PR expression in the LE during embryo implantation.

DESIGN

Experimental rodent models.

SETTING

University research laboratories.

ANIMAL(S)

Mice and hamsters.

INTERVENTION(S)

Pseudopregnancy and artificial decidualization.

MAIN OUTCOME MEASURE(S)

Blue dye injection for detecting embryo attachment; immunohistochemistry, immunofluorescence, and in situ hybridization for detecting gene expression.

RESULT(S)

Progesterone receptor remained expressed in the LE up to 6 hours after the initial detection of blue dye reaction in mice (day 3, 22:00 hours), but disappeared first from LE cells at the implantation site and subsequently from the entire LE layer by day 4, 06:00 hours, when uterine stromal decidualization had become obvious. Progesterone receptor remained highly expressed in the LE of day 4 at 11:00 hours in pseudopregnant mice, but it disappeared from the entire LE layer by day 4 at 06:00 hours in artificially decidualized pseudopregnant mice.

CONCLUSION(S)

Progesterone receptor disappears from the LE after implantation has initiated and before the histologic decidualization manifests, suggesting an active role of continued PR expression in the LE for the initial implantation process. The disappearance of PR expression in the LE is regulated by uterine factor(s) produced upon embryo attachment.

Regional development of uterine decidualization: molecular signaling by Hoxa-10

Uterine decidualization, a key event in implantation, is critically controlled by stromal cell proliferation and differentiation. Although the molecular mechanism that controls this event is not well understood, the general consensus is that the factors derived locally at the site of implantation influence aspects of decidualization. Hoxa-10, a developmentally regulated homeobox transcription factor, is highly expressed in decidualizing stromal cells, and targeted deletion of Hoxa-10 in mice shows severe decidualization defects, primarily due to the reduced stromal cell responsiveness to progesterone (P(4)). While the increased stromal cell proliferation is considered to be an initiator of decidualization, the establishment of a full-grown functional decidua appears to depend on the aspects of regional proliferation and differentiation. In this regard, this article provides an overview of potential signaling mechanisms mediated by Hoxa-10 that can influence a host of genes and cell functions necessary for propagating regional decidual development.