Microscopic examination of porcine conceptus-maternal interface between days 10 and 19 of pregnancy

Embryonic signals mediate extracellular vesicle biogenesis and trafficking at the embryo-maternal interface

BACKGROUND

Extracellular vesicles (EVs) are membrane-coated nanoparticles secreted by almost all cell types in living organisms. EVs, as paracrine mediators, are involved in intercellular communication, immune response, and several reproductive events, including the maintenance of pregnancy. Using a domestic animal model (Sus scrofa) with an epitheliochorial, superficial type of placentation, we focused on EV biogenesis pathway at the embryo-maternal interface, when the embryonic signaling occurs for maternal recognition and the maintenance of pregnancy.

RESULTS

Transmission electron microscopy was used during early pregnancy to visualize EVs and apocrine and/or merocrine pathways of secretion. Immunofluorescent staining localized proteins responsible for EV biogenesis and cell polarization at the embryo-maternal interface. The expression profiles of genes involved in biogenesis and the secretion of EVs pointed to the possible modulation of endometrial expression by embryonic signals. Further in vitro studies showed that factors of embryonic origin can regulate the expression of the ESCRT-II complex and EV trafficking within endometrial luminal epithelial cells. Moreover, miRNA-mediated rapid negative regulation of gene expression was abolished by delivered embryonic signals.

CONCLUSIONS

Our findings demonstrated that embryonic signals are potent modulators of ESCRT-dependent EV-mediated secretory activity of the endometrium during the critical stages of early pregnancy. Video Abstract.

Extracellular vesicles: Focus on peri-implantation period of pregnancy in pigs

The establishment of cell-to-cell communication between the endometrium and the developing embryo is the most important step in successful mammalian pregnancy. Close interaction between the uterine luminal epithelium and trophoblast cells requires triggering timely molecular dialog for successful maternal recognition of pregnancy, embryo implantation, and placenta development. Quite recently, extracellular vesicles (EVs) carrying unique molecular cargo emerged as evolutionarily conserved mediators of cell-to-cell communication during early pregnancy. To date, the presence of EVs at the embryo-maternal interface has been demonstrated in numerous mammals, including domestic livestock, such as pigs. However, few studies have focused on revealing the mechanism of EV-mediated crosstalk between developing early embryos and receptive endometrium. Over the past years, it has appeared that understanding the role of EVs in mammalian reproduction can substantially improve our understanding of the biological challenges of successful reproductive performance. This review describes current knowledge of EVs, specifically in relation to the peri-implantation period in pigs, characterized by common features of embryo implantation and high embryonic mortality in mammals.

Proteomic profiles and the function of RBP4 in endometrium during embryo implantation phases in pigs

BACKGROUND

Endometrial receptivity plays a vital role in the success of embryo implantation. However, the temporal proteomic profile of porcine endometrium during embryo implantation is still unclear.

RESULTS

In this study, the abundance of proteins in endometrium on days 9, 10, 11, 12, 13, 14, 15 and 18 of pregnancy (D9, 10, 11, 12, 13, 14, 15 and 18) was profiled via iTRAQ technology. The results showed that 25, 55, 103, 91, 100, 120, 149 proteins were up-regulated, and 24, 70, 169, 159, 164, 161, 198 proteins were down-regulated in porcine endometrium on D10, 11, 12, 13, 14, 15 and 18 compared with that on D9, respectively. Among these differentially abundance proteins (DAPs), Multiple Reaction Monitoring (MRM) results indicated that S100A9, S100A12, HRG and IFI6 were differentially abundance in endometrial during embryo implantation period. Bioinformatics analysis showed that the proteins differentially expressed in the 7 comparisons were involved in important processes and pathways related to immunization, endometrial remodeling, which have a vital effect on embryonic implantation.

CONCLUSION

Our results reveal that retinol binding protein 4 (RBP4) could regulate the cell proliferation, migration and apoptosis of endometrial epithelial cells and endometrial stromal cells to affect embryo implantation. This research also provides resources for studies of proteins in endometrium during early pregnancy.

Urinary metabolomics reveals the biological characteristics of early pregnancy in pigs

Background

Embryo implantation in sows is an important event during pregnancy. During this process, blastocysts undergo dramatic morphologic changes, and the endometrium becomes receptive. Studies have shown that developmental changes associated with the crosstalk between peri-implantation embryos and embryo-uterine are driven by various biomolecules secreted by the endometrium and embryos. In sows, changes in the uterus are also reflected in circulating body fluids and urine. Metabolomics reveals the metabolic state of cells, tissues, and organisms. In this study, we collected urine samples from large white sows during the peri-implantation period. The levels of urinary metabolites at different periods were analyzed using ultra-performance liquid chromatography/tandem mass spectrometry (UPLC–MS/MS) analysis techniques.

Results

A total of 32 samples were collected from 8 sows during the estrus period and at each phase of early pregnancy (9, 12, and 15 days of gestation). A total of 530 metabolites were identified with high confidence in all samples. Compared with samples collected during the estrus phase, 269 differential metabolites were found in samples obtained during early pregnancy.

Conclusions

The identified metabolites included lipids and lipid-like molecules, organic acids and their derivatives, organic oxygen compounds, organoheterocyclic compounds, benzenoids, among others. Metabolites, such as choline and pregnanediol-3-glucuronide, play important roles in pregnancy in sows and other animals. These results reveal the metabolic changes in urine of sows during early pregnancy phase. The differential urinary metabolites can be used for assessing peri-implantation status in sows. Understanding these metabolic changes may promote the management of pregnant sows through various interventions such as provision of proper nutrition.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40813-022-00256-z.

A review of the amino acid metabolism in placental function response to fetal loss and low birth weight in pigs

The fertility of sows mainly depends on the embryo losses during gestation and the survival rate of the post-farrowing piglets. The selection of highly-prolific sows has been mainly focused on the selection of genotypes with high ovulatory quota. However, in the early- and post-implantation stages, the rate of embryo losses was increased with the increase of zygotes. Among the various factors, placental growth and development is the vital determinant for fetal survival, growth, and development. Despite the potential survival of fetuses with deficient placental development, their life-conditions and growth can be damaged by a process termed intrauterine growth retardation (IUGR). The newborn piglets affected by IUGR are prone to increased morbidity and mortality rates; meanwhile, the growth, health and welfare of the surviving piglets will remain hampered by these conditions, with a tendency to exacerbate with age. Functional amino acids such as glycine, proline, and arginine continue to increase with the development of placenta, which are not only essential to placental growth (including vascular growth) and development, but can also be used as substrates for the production of glutathione, polyamines and nitric oxide to benefit placental function in many ways. However, the exact regulation mechanism of these amino acids in placental function has not yet been clarified. In this review, we provide evidence from literature and our own work for the role and mechanism of dietary functional amino acids during pregnancy in regulating the placental functional response to fetal loss and birth weight of piglets. This review will provide novel insights into the response of nutritionally nonessential amino acids (glycine and proline) to placental development as well as feasible strategies to enhance the fertility of sows.

Nutritional restriction during the peri-conceptional period alters the myometrial transcriptome during the peri-implantation period

This study hypothesized that female peri-conceptional undernutrition evokes transcriptomic alterations in the pig myometrium during the peri-implantation period. Myometrium was collected on days 15-16 of pregnancy from pigs fed a normal- (n = 4) or restricted-diet (n = 4) from conception until day 9th of pregnancy, and the transcriptomic profiles of the tissue were compared using Porcine (V2) Expression Microarrays 4 × 44 K. In restricted diet-fed pigs, 1021 differentially expressed genes (DEGs) with fold change ≥ 1.5, P ≤ 0.05 were revealed, and 708 of them were up-regulated. Based on the count score, the top within GOs was GO cellular components "extracellular exosome", and the top KEGG pathway was the metabolic pathway. Ten selected DEGs, i.e. hydroxysteroid (17β) dehydrogenase 8, cyclooxygenase 2, prostaglandin F receptor, progesterone receptor membrane component 1, progesterone receptor membrane component 2, annexin A2, homeobox A10, S-phase cyclin A-associated protein in the ER, SRC proto-oncogene, non-receptor tyrosine kinase, and proliferating cell nuclear antigen were conducted through qPCR to validate microarray data. In conclusion, dietary restriction during the peri-conceptional period causes alterations in the expression of genes encoding proteins involved i.a. in the endocrine activity of the myometrium, embryo-maternal interactions, and mechanisms regulating cell cycle and proliferation.

Insights Into Extracellular Vesicle/Exosome and miRNA Mediated Bi-Directional Communication During Porcine Pregnancy

Spontaneous fetal loss is one of the most important challenges that commercial pig industry is still facing in North America. Research over the decade provided significant insights into some of the associated mechanisms including uterine capacity, placental efficiency, deficits in vasculature, and immune-inflammatory alterations at the maternal-fetal interface. Pigs have unique epitheliochorial placentation where maternal and fetal layers lay in opposition without any invasion. This has provided researchers opportunities to accurately tease out some of the mechanisms associated with maternal-fetal interface adaptations to the constantly evolving needs of a developing conceptus. Another unique feature of porcine pregnancy is the conceptus derived recruitment of immune cells during the window of conceptus attachment. These immune cells in turn participate in pregnancy associated vascular changes and contribute toward tolerance to the semi-allogeneic fetus. However, the precise mechanism of how maternal-fetal cells communicate during the critical times in gestation is not fully understood. Recently, it has been established that bi-directional communication between fetal trophoblasts and maternal cells/tissues is mediated by extracellular vesicles (EVs) including exosomes. These EVs are detected in a variety of tissues and body fluids and their role has been described in modulating several physiological and pathological processes including vascularization, immune-modulation, and homeostasis. Recent literature also suggests that these EVs (exosomes) carry cargo (nucleic acids, protein, and lipids) as unique signatures associated with some of the pregnancy associated pathologies. In this review, we provide overview of important mechanisms in porcine pregnancy success and failure and summarize current knowledge about the unique cargo containing biomolecules in EVs. We also discuss how EVs (including exosomes) transfer their contents into other cells and regulate important biological pathways critical for pregnancy success.

The Early Stages of Implantation and Placentation in the Pig

Pregnancy in pigs includes the events of conceptus (embryo/fetus and placental membranes) elongation, implantation, and placentation. Placentation in pigs is defined microscopically as epitheliochorial and macroscopically as diffuse. In general, placentation can be defined as the juxtapositioning of the endometrial/uterine microvasculature to the chorioallantoic/placental microvasculature to facilitate the transport of nutrients from the mother to the fetus to support fetal development and growth. Establishment of epitheliochorial placentation in the pig is achieved by: (1) the secretions of uterine glands prior to conceptus attachment to the uterus; (2) the development of extensive folding of the uterine-placental interface to maximize the surface area for movement of nutrients across this surface; (3) increased angiogenesis of the vasculature that delivers both uterine and placental blood and, with it, nutrients to this interface; (4) the minimization of connective tissue that lies between these blood vessels and the uterine and placental epithelia; (5) interdigitation of microvilli between the uterine and placental epithelia; and (6) the secretions of the uterine glands, called histotroph, that accumulate in areolae for transport though the placenta to the fetus. Placentation in pigs is not achieved by invasive growth of the placenta into the uterus. In this chapter, we summarize current knowledge about the major events that occur during the early stages of implantation and placentation in the pig. We will focus on the microanatomy of porcine placentation that builds off the excellent histological work of Amoroso and others and provide a brief review of some of the key physiological, cellular, and molecular events that accompany the development of "implantation" in pigs.

Synergistic action of estradiol and PGE2 on endometrial transcriptome in vivo resembles pregnancy effects better than estradiol alone†

Successful pregnancy establishment in mammals depends on numerous interactions between embryos and the maternal organism. Estradiol-17β (E2) is the primary embryonic signal in the pig, and its importance has been questioned recently. However, E2 is not the only molecule of embryonic origin. In pigs, prostaglandin E2 (PGE2) is abundantly synthesized and secreted by conceptuses and endometrium. The present study aimed to determine the role of PGE2 and its simultaneous action with E2 in changes in porcine endometrial transcriptome during pregnancy establishment. The effects of PGE2 and PGE2 acting with E2 were studied using an in vivo model of intrauterine hormone infusions, and were compared to the effects of E2 alone and conceptuses' presence on day 12 of pregnancy. The endometrial transcriptome was profiled using gene expression microarrays followed by statistical analyses. Downstream analyses were performed using bioinformatics tools. Differential expression of selected genes was verified by quantitative polymerase chain reaction. Microarray analysis revealed 2413 differentially expressed genes (DEGs) in the endometrium treated simultaneously with PGE2 and E2 (P < 0.01). No significant effect of PGE2 administered alone on endometrial transcriptome was detected. Gene ontology annotations enriched for DEGs were related to multiple processes such as: focal adhesion, vascularization, cell migration and proliferation, glucose metabolism, tissue remodeling, and activation of immune response. Simultaneous administration of E2 and PGE2 induced more changes within endometrial transcriptome characteristic to pregnancy than infusion of E2 alone. The present findings suggest that synergistic action of estradiol-17β and PGE2 resembles the effects of pregnancy on endometrial transcriptome better than E2 alone.

Small RNA-seq analysis of extracellular vesicles from porcine uterine flushing fluids during peri-implantation

The extracellular vesicles (EVs) of uterine flushing fluids (UFs) mediate intrauterine communication between conceptus and uterus in pigs. The small RNAs of UFs-EVs are widely recognized as important factors that influence embryonic implantation. However, small RNAs expression profiles of porcine UFs-EVs during peri-implantation are still unknown. In this study, cup-shaped EVs of porcine UFs on days 10 (D10), 13 (D13) and 18 (D18) of pregnancy were isolated and characterized. The expression of small RNAs in these EVs was comprehensively profiled through sequencing. A total of 152 known microRNAs (miRNAs), 43 novel miRNAs, 6248 known Piwi-interacting RNAs (piRNAs) and 110 novel piRNAs were identified. Among these small RNAs, RT-qRCR results indicated that ssc-let-7f-5p, ssc-let-7i-5p and ssc-let-7g were differentially expressed during the three stages. Bioinformatics analysis showed that the miRNAs differentially expressed in the three comparisons (D10 vs D13, D13 vs D18 and D10 vs D18) were involved in important processes and pathways related to immunization, endometrial receptivity and embryo development, which play important roles in embryonic implantation. Our results reveal that EVs from porcine UFs contain various small RNAs with potentially vital effects on implantation. This research also provides resources for studies of miRNAs and piRNAs in the cross-talk between embryo and endometrium.

Expression of Insulin-Like Growth Factor 1 (IGF-1) and Epidermal Growth Factor (EGF) Receptors and the Effect of IGF-1 and EGF on Androgen and Estrogen Release in the Myometrium of Pigs-In Vitro Study

Porcine myometrium possesses steroidogenic activity but its regulation is not well understood. It was hypothesized that the regulators of myometrial steroidogenesis are insulin-like growth factor 1 (IGF-1) and epidermal growth factor (EGF), which were found to modulate the steroidogenic activity of the endometrium and embryos. Myometrial slices were collected from gravid and nongravid pigs on days 10 or 11, 12 or 13 and 15 or 16 and studied for: (1) the relative abundance of IGF-1R and EGFR mRNA transcripts and proteins, to determine myometrial readiness to response growth factors treatment and (2) the effect of IGF-1 or EGF on the myometrial release of androstenedione (A4), testosterone (T), estrone (E1) and estradiol-17β (E2). The results showed that the relative expression and abundance of IGF-1R and EGFR in the myometrium were altered regarding the female reproductive status. During the estrous cycle, EGF increased myometrial release of A4 on days 12-13 and E2 on days 15-16. In gravid pigs (days 15-16), IGF-1 and EGF increased the E1 release. In conclusion: (1) porcine myometrium possesses the potential to respond to IGF-1 and EGF treatment, (2) EGF significantly increases myometrial A4 and E2 release in cyclic pigs, while IGF-1 and EGF increase the E1 release in gravid pigs.

MiRNAs in the Peri-Implantation Period: Contribution to Embryo-Maternal Communication in Pigs

MicroRNAs (miRNAs) constitute a large family of noncoding RNAs, approximately 22 nucleotides long, which function as guide molecules in RNA silencing. Targeting most protein-coding transcripts, miRNAs are involved in nearly all developmental and pathophysiological processes in animals. To date, the regulatory roles of miRNAs in reproduction, such as fertilization, embryo development, implantation, and placenta formation, among others, have been demonstrated in numerous mammalian species, including domestic livestock such as pigs. Over the past years, it appeared that understanding the functions of miRNAs in mammalian reproduction can substantially improve our understanding of the biological challenges of successful reproductive performance. This review describes the current knowledge on miRNAs, specifically in relation to the peri-implantation period when the majority of embryonic mortality occurs in pigs. To present a broader picture of crucial peri-implantation events, we focus on the role of miRNA-processing machinery and miRNA–mRNA infarctions during the maternal recognition of pregnancy, leading to maintenance of the corpus luteum function and further embryo implantation. Furthermore, we summarize the current knowledge on cell-to-cell communication involving extracellular vesicles at the embryo–maternal interface in pigs. Finally, we discuss the potential of circulating miRNAs to serve as indicators of ongoing embryo–maternal crosstalk.

Spatial organization of endometrial gene expression at the onset of embryo attachment in pigs

BACKGROUND

During the preimplantation phase in the pig, the conceptus trophoblast elongates into a filamentous form and secretes estrogens, interleukin 1 beta 2, interferons, and other signaling molecules before attaching to the uterine epithelium. The processes in the uterine endometrium in response to conceptus signaling are complex. Thus, the objective of this study was to characterize transcriptome changes in porcine endometrium during the time of conceptus attachment considering the specific localization in different endometrial cell types.

RESULTS

Low-input RNA-sequencing was conducted for the main endometrial compartments, luminal epithelium (LE), glandular epithelium (GE), blood vessels (BV), and stroma. Samples were isolated from endometria collected on Day 14 of pregnancy and the estrous cycle (each group n = 4) by laser capture microdissection. The expression of 12,000, 11,903, 11,094, and 11,933 genes was detectable in LE, GE, BV, and stroma, respectively. Differential expression analysis was performed between the pregnant and cyclic group for each cell type as well as for a corresponding dataset for complete endometrium tissue samples. The highest number of differentially expressed genes (DEGs) was found for LE (1410) compared to GE, BV, and stroma (800, 1216, and 384). For the complete tissue, 3262 DEGs were obtained. The DEGs were assigned to Gene Ontology (GO) terms to find overrepresented functional categories and pathways specific for the individual endometrial compartments. GO classification revealed that DEGs in LE were involved in 'biosynthetic processes', 'related to ion transport', and 'apoptotic processes', whereas 'cell migration', 'cell growth', 'signaling', and 'metabolic/biosynthetic processes' categories were enriched for GE. For blood vessels, categories such as 'focal adhesion', 'actin cytoskeleton', 'cell junction', 'cell differentiation and development' were found as overrepresented, while for stromal samples, most DEGs were assigned to 'extracellular matrix', 'gap junction', and 'ER to Golgi vesicles'.

CONCLUSIONS

The localization of differential gene expression to different endometrial cell types provided a significantly improved view on the regulation of biological processes involved in conceptus implantation, such as the control of uterine fluid secretion, trophoblast attachment, growth regulation by Wnt signaling and other signaling pathways, as well as the modulation of the maternal immune system.

Expression and role of peroxisome proliferator-activated receptors in the porcine early placenta trophoblast

Peroxisome proliferator-activated receptors (PPARs) are members of a nuclear receptor family of ligand-dependent transcription factors. Three isoforms of PPAR named PPARα, PPARβ/δ, and PPARγ have been described, each encoded by a separate gene: PPARA, PPARD, and PPARG, respectively. In the present study, we examined the profiles of PPAR and retinoid X receptor (RXR; PPAR heterodimer partner) mRNA expression and PPAR DNA binding activity in porcine trophoblast tissue collected on days 15, 20, 25, and 30 of pregnancy and in day-20 embryos. Placenta trophoblast cells isolated on day 25 of pregnancy were used to determine effects of (1) cytokines on PPAR and RXR mRNA expression and (2) PPAR agonists on prostaglandin (PG) E2 synthesis and the expression of genes involved in steroidogenesis, fatty acid binding, and PG transport, as well as on cell proliferation. The mRNA expression of PPARA and RXRB was greater in trophoblast tissue collected on days 25 and 30 of pregnancy compared with day 15 (P < 0.05), while DNA binding activity of PPARα decreased between day 15 and 25 (P < 0.05). Increased concentrations of PPARD and RXRA transcripts were observed in trophoblasts collected on day 20 compared to trophoblasts from days 15 and 30 (P < 0.05). Moreover, concentrations of DNA-bound PPARβ/δ and PPARγ proteins increased in day-30 trophoblasts compared to day 15 (P < 0.01) and day 20 (P < 0.05), respectively. On day 20 of gestation, the mRNA expression of PPARD, PPARG, and RXRA and protein levels of PPARα and PPARγ isoforms were greater in trophoblast than embryonic tissue (P < 0.01). Interleukin 1β and/or interferon γ, but not IL6 and leukemia inhibitory factor, upregulated PPAR and RXR mRNA expression in placenta trophoblast cells in vitro (P < 0.05). Rosiglitazone (a PPARγ agonist) stimulated prostaglandin E synthase mRNA expression in trophoblast cells and PGE2 accumulation in incubation medium (P < 0.05). Moreover, activation of PPAR isoforms differentially affected the expression of genes involved in steroidogenesis, fatty acid binding, and PG transport in studied cells. Finally, PPARα and PPARγ agonists stimulated trophoblast cell proliferation (P < 0.05), and this effect was abolished by the addition of a respective PPAR antagonist (P < 0.05). Overall, these results point to a role of PPAR isoforms in porcine placenta development and function.

Cell type-specific analysis of transcriptome changes in the porcine endometrium on Day 12 of pregnancy

BACKGROUND

Along with trophoblast elongation (Days 10 to 12), estradiol is secreted in increasing amounts for recognition of pregnancy. Endometrial secretions driven by ovarian progesterone and conceptus signals are essential for conceptus growth and development. Results of transcriptome analyses of whole endometrial tissue samples in the pig indicated the need for cell type-specific endometrial gene expression analysis for a better understanding of transcriptome changes associated with establishment of pregnancy.

RESULTS

The most distinct transcriptome profile and the majority of differentially expressed genes (DEGs) were identified in luminal epithelium (LE). Many DEGs were found only in the cell type-specific analysis. The functional classification of DEGs identified in specific endometrial cell types revealed various distinct functions and pathways. Genes related to immune activation, estrogen signaling pathway, embryo development, and cell proliferation were upregulated in LE of pregnant gilts. Genes involved in sterol biosynthetic and metabolic processes and extracellular matrix were upregulated in stroma. Genes associated with cell communication such as via exosomes and vesicles were found as differential in LE, glandular epithelium (GE), and stroma (S).

CONCLUSIONS

This study revealed that conceptus signals induce different transcriptomic regulations in the endometrial compartments/cell types related to their specific function during recognition and establishment of pregnancy.

Endometrial response to conceptus-derived estrogen and interleukin-1β at the time of implantation in pigs

The establishment of pregnancy is a complex process that requires a well-coordinated interaction between the implanting conceptus and the maternal uterus. In pigs, the conceptus undergoes dramatic morphological and functional changes at the time of implantation and introduces various factors, including estrogens and cytokines, interleukin-1β2 (IL1B2), interferon-γ (IFNG), and IFN-δ (IFND), into the uterine lumen. In response to ovarian steroid hormones and conceptus-derived factors, the uterine endometrium becomes receptive to the implanting conceptus by changing its expression of cell adhesion molecules, secretory activity, and immune response. Conceptus-derived estrogens act as a signal for maternal recognition of pregnancy by changing the direction of prostaglandin (PG) F_2α from the uterine vasculature to the uterine lumen. Estrogens also induce the expression of many endometrial genes, including genes related to growth factors, the synthesis and transport of PGs, and immunity. IL1B2, a pro-inflammatory cytokine, is produced by the elongating conceptus. The direct effect of IL1B2 on endometrial function is not fully understood. IL1B activates the expression of endometrial genes, including the genes involved in IL1B signaling and PG synthesis and transport. In addition, estrogen or IL1B stimulates endometrial expression of IFN signaling molecules, suggesting that estrogen and IL1B act cooperatively in priming the endometrial function of conceptus-produced IFNG and IFND that, in turn, modulate endometrial immune response during early pregnancy. This review addresses information about maternal-conceptus interactions with respect to endometrial gene expression in response to conceptus-derived factors, focusing on the roles of estrogen and IL1B during early pregnancy in pigs.

Pregnancy and spontaneous fetal loss: A pig perspective

Pigs have a unique, non-invasive epitheliochorial placenta where maternal and fetal layers lay in apposition. Indentation of fetal capillaries into the trophoblasts and maternal capillaries into the uterine epithelium reduce the distance between the fetal and maternal blood, ensuring nutrient transfer for proper conceptus development. Another unique feature of pig pregnancy is conceptus-mediated immune cell enrichment during the early stages of conceptus attachment (around gestation Day 15). This period coincides with the development of vasculature networks at the maternal-fetal interface, which is critical for successful conceptus growth. Specific chemokines, their receptors, and chemokine decoy receptor networks coordinate this immune cell enrichment and the positioning at the maternal-fetal interface. The recruited immune cells, in turn, adopt a specialized phenotype to support key processes of maternal-fetal adaptations, including tolerance to the semi-allogeneic fetus and supporting vascularization. Disturbance in coordinated cross talk between the conceptus and maternal endometrium is an important mechanism associated with spontaneous fetal loss. The exact mechanism of fetal loss is still not yet identified, although research in the last two decades point to various factors including genetics, nutrition, uterine capacity, placental efficiency, and imbalanced immune factors at the maternal-fetal interface. In this review, we summarize some of the recent advances in endometrial immune cell functions and their regulation. We also provide insights into endometrial/placental transcriptome, microRNA biology, and extravesicular transport across the maternal-fetal interface, as well as their potential implications in porcine pregnancy success or failure.

In vitro porcine blastocyst development in three-dimensional alginate hydrogels

Appropriate embryonic and fetal development significantly impact pregnancy success and, therefore, the efficiency of swine production. The pre-implantation period of porcine pregnancy is characterized by several developmental hallmarks, which are initiated by the dramatic morphological change that occurs as pig blastocysts elongate from spherical to filamentous blastocysts. Deficiencies in blastocyst elongation contribute to approximately 20% of embryonic loss, and have a direct influence on within-litter birth weight variation. Although factors identified within the uterine environment may play a role in blastocyst elongation, little is known about the exact mechanisms by which porcine (or other species') blastocysts initiate and progress through the elongation process. This is partly due to the difficulty of replicating elongation in vitro, which would allow for its study in a controlled environment and in real-time. We developed a three dimensional (3-D) culture system using alginate hydrogel matrices that can encapsulate pig blastocysts, maintain viability and blastocyst architecture, and facilitate reproducible morphological changes with corresponding expression of steroidogenic enzyme transcripts and estrogen production, consistent with the initiation of elongation in vivo. This review highlights key aspects of the pre-implantation period of porcine pregnancy and the difficulty of studying blastocyst elongation in vivo or by using in vitro systems. This review also provides insights on the utility of 3-D hydrogels to study blastocyst elongation continuously and in real-time as a complementary and confirmatory approach to in vivo analysis.

Signs of embryo-maternal communication: miRNAs in the maternal serum of pregnant pigs

Circulating miRNAs were proposed to be indicators of normal or complicated pregnancies. Based on this knowledge and our recent transcriptomic approach showing expression of miRNAs in the porcine endometrium, conceptuses and uterine extracellular vesicles during pregnancy, we have hypothesized that signs of ongoing local embryo-maternal crosstalk involving miRNAs can be detected in the circulation of pregnant gilts as early as a few days after maternal recognition of pregnancy. By applying several molecular biology techniques that differ in dynamic range and precision in maternal serum of Day 16 pregnant pigs, we were able to show for the first time increased levels of several miRNAs, previously reported to be expressed in either conceptuses and extracellular vesicles (miR-26a and miR-125b) or pregnant endometrium (miR-23b). Our results clearly showed that real-time RT-PCR and digital PCR are the most reliable methods, being able to detect small-fold changes of low-abundant circulating miRNAs. Further validation in a separate group of gilts confirmed an increase in miR-23b and miR-125b levels. In silico analyses identified pregnancy-related biological processes and pathways affected by these miRNAs. Target prediction analysis revealed hundreds of porcine transcripts with conserved sites for these miRNAs, which were classified into signaling pathways relevant to pregnancy. We conclude that a unique set of miRNAs can already be observed in the circulation of pigs during the first weeks of pregnancy, as a result of the initiation of embryo-maternal communication.

Extracellular vesicle mediated intercellular communication at the porcine maternal-fetal interface: A new paradigm for conceptus-endometrial cross-talk

Exosomes and microvesicles are extracellular vesicles released from cells and can contain lipids, miRNAs and proteins that affect cells at distant sites. Recently, microvesicles containing miRNA have been implicated in uterine microenvironment of pigs, a species with unique epitheliochorial (non-invasive) placentation. Here we report a novel role of conceptus-derived exosomes/microvesicles (hereafter referred to as extracellular vesicles; EVs) in embryo-endometrial cross-talk. We also demonstrate the stimulatory effects of EVs (PTr2-Exo) derived from porcine trophectoderm-cells on various biological processes including the proliferation of maternal endothelial cells (PAOEC), potentially promoting angiogenesis. Transmission immuno-electron microscopy confirmed the presence of EVs in tissue biopsies, PTr2-Exo and PAOEC-derived EVs (PAOEC-Exo). RT-PCR detected 14 select miRNAs in CD63 positive EVs in which miR-126-5P, miR-296-5P, miR-16, and miR-17-5P were the most abundant angiogenic miRNAs. Proteomic analysis revealed EV proteins that play a role in angiogenesis. In-vitro experiments, using two representative cell lines of maternal-fetal interface, demonstrated bidirectional EVs shuttling between PTr2 and PAOEC cells. Importantly, these studies support the idea that PTr2-Exo and PAOEC-Exo containing select miRNAs and proteins can be successfully delivered to recipient cells and that they may have a biological role in conceptus-endometrial cross-talk crucial for the pregnancy success.

Interleukins, interferons, and establishment of pregnancy in pigs

Early pregnancy in mammals requires complex and highly orchestrated cellular and molecular interactions between specialized cells within the endometrium and the conceptus. Proinflammatory cytokines are small signaling proteins released by leukocytes that augment innate and adaptive immune responses. They are also released by the mammalian trophectoderm as the conceptus apposes the uterine surface for implantation. On approximately day 12 of development in pigs, the conceptus undergoes a rapid morphological transformation referred to as elongation while simultaneously releasing estrogens and a novel conceptus form of interleukin-1 beta (IL1β). Following elongation, pig conceptuses express interferon gamma (IFNγ) and, in lesser amounts, interferon delta (IFNδ). Significant IFN signaling takes place within the endometrium between day 14 and 18 of pregnancy as the conceptus intimately associates with the uterine epithelium. Based on studies carried out in pigs and other mammals, the combined spacio-temporal activities of conceptus estrogens, IL1β, and IFN set in motion a series of coordinated events that promote establishment of pregnancy. This is achieved through enhancement of conceptus development, uterine receptivity, maternal-fetal hemotropic exchange, and endometrial leukocyte function. These events require activation of specific signaling pathways within the uterine luminal epithelium, glandular epithelium, and stroma. Here, we review proinflammatory cytokine expression by pig conceptuses and the hypothesized actions of these molecules during establishment of pregnancy.

Expression of aquaporin 1 and 5 and their regulation by ovarian hormones, arachidonic acid, forskolin and cAMP during implantation in pigs

Aquaporin proteins (AQPs) are a family of channels expressed in numerous mammalian tissues, where they play a fundamental role in regulating water transport across cell membranes. Based on reports that AQPs are present in the reproductive system and participate in reproductive processes, our aim was to investigate the effect of progesterone (P(4)), estradiol (E(2)), oxytocin (OT), arachidonic acid (AA), forskolin (FSK) and cyclic adenosine monophosphate (cAMP) on AQP1 and AQP5 expression at mRNA and protein levels in porcine uterine explants from Days 14-16 of gestation in order to determine if they play a role in implantation period in pigs. Quantitative real time PCR and Western-blot analysis revealed that the uterine explants treated with FSK and cAMP produce delayed, but long-term effects on AQP1 abundance (24 h) while AQP5 had a rapid and sustained response to FSK and cAMP in protein content (3 and 24 h). AA increases gene and protein content of AQP1 after longer exposition whereas AQP5 increases after 3 h only at the protein level. Both AQPs potentially remains under control of steroid hormones. OT has been shown to increase AQP1, and decrease AQP5 mRNA, without visible changes in protein content. P(4), E(2), AA, FSK and cAMP caused the appearance of AQP5 expression in the basolateral plasma membrane of the epithelial cells. The staining represents most likely AQP5 functioning mechanism for both absorption and reabsorption across the glandular epithelium.

Difference in expression patterns of placental cholesterol transporters, ABCA1 and SR-BI, in Meishan and Yorkshire pigs with different placental efficiency

Cholesterol is a key cell membrane component and precursor of steroid hormones. The maternal cholesterol is an important exogenous cholesterol source for the developing embryos and its transportation is mediated by ABCA1 and SR-BI. Here we reported that during the peri-implantation period in pigs, ABCA1 was expressed by uterine luminal epithelium (LE) and interestingly, its expression was more abundantly in LE on mesometrial side of uterus. However, SR-BI was expressed primarily by LE, glandular epithelial cells (GE) and trophoblast cells (Tr). During the placentation period, the expression levels of ABCA1 and SR-BI proteins at epithelial bilayer and placental areolae were significantly higher in Chinese Meishan pigs compared to Yorkshire pigs. Consisitently, mRNA levels of HMGCR, the rate-limiting enzyme for cholesterol synthesis, were significantly higher in Meishan placentas than in Yorkshire placentas. Our findings revealed the routes of transplacental cholesterol transport mediated by ABCA1 and SR-BI in pigs and indicated that ABCA1 related pathway may participate in anchoring the conceptus to the mesometrial side of uterus. Additionally, an ABCA1 dependent compensatory mechanism related to the placental efficiency in response to the smaller placenta size in Meishan pigs was suggested.

Placentation, maternal-fetal interface, and conceptus loss in swine

Pregnancy is a delicate yet complex physiological process that requires fine-tuning of many factors (hormones, growth factors, cytokines, and receptors) between the mother and the conceptus to ensure the survival of the conceptus(es) to term. Any disturbance in the maternal-conceptus dialog can have detrimental effects on the affected conceptus or even the outcome of pregnancy as a whole. Being a litter-bearing species, such disruptions can lead to a loss of up to 45% of the totally healthy offspring during early (periattachment) and midgestation to late gestation in pigs. Although the exact mechanism is not entirely understood, several factors have been associated with the fetal loss including but not limited to uterine capacity, placental efficiency, genetics, nutrition, and deficits in vascularization at the maternal-fetal interface. Over the years, we investigated how immune cells are recruited to the porcine maternal-fetal interface and whether they contribute to vascularization. We also delineated how cytokines, chemokines, and cytokine destabilizing factors fine-tune inflammation and whether the cytokine shift from early to midpregnancy exists at the porcine maternal-fetal interface. Finally, we evaluated the role of microRNAs in regulating immune cell recruitment and their angiogenic functions during pregnancy. Collectively our research points out that the immune-angiogenesis axis at the porcine maternal interface is significantly involved in promoting new blood vessel development, regulating inflammatory responses and ultimately contributing to pregnancy success. In this review, we summarized current knowledge on spontaneous fetal loss in swine, with special attention to the mechanisms in immune reactivity and interplay at the maternal-fetal interface.

MicroRNAome of porcine conceptuses and trophoblasts: expression profile of micrornas and their potential to regulate genes crucial for establishment of pregnancy

Tightly coordinated, reciprocal embryo-maternal interactions affect gene expression during early pregnancy. Recently, microRNAs (miRNAs) have emerged as new players in the fine tuning of embryo development and implantation in mammals via posttranscriptional gene regulation mechanisms. Here, we integrated transcriptomic and computational approaches to profile miRNAs and miRNA synthesis and transport-related genes at different developmental stages of porcine conceptuses and trophoblast during early pregnancy in the pig. Using semiquantitative RT-PCR, we examined mRNA levels of 10 genes encoding proteins involved in miRNA synthesis and transport: DROSHA, DGCR8, XPO5, DICER1, TARBP2, TNRC6A, AGO1, AGO2, AGO3, and AGO4. Custom, multispecies microarrays were used to profile miRNAs. Prediction algorithms of miRNA-mRNA interactions allowed identification of target transcripts for the analyzed miRNAs. These included VEGF, LIF, PTGS2, and IL-6R, known to be crucial components of embryo-maternal interactions in the pig. Two selected miRNAs, miR-26a and miR-125b, were tested for the presence in the extracellular vesicles isolated from uterine luminal flushings during pregnancy. Results of in vitro study demonstrated that miRNAs, such as miR-125b, can regulate expression of genes crucial for embryo development and implantation in porcine endometrial luminal epithelial cells. For the first time, expression profiles of miRNAs and related genes in porcine conceptuses and trophoblast during maternal recognition of pregnancy and embryo implantation in the pig were described. Altogether, our results indicate potential roles of these small, noncoding RNAs in the early development of embryos and embryo-maternal cross-talk during early pregnancy in the pig.

Insulin-like growth factor I induces proliferation and migration of porcine trophectoderm cells through multiple cell signaling pathways, including protooncogenic protein kinase 1 and mitogen-activated protein kinase

During early pregnancy, the developing conceptus is dependent upon a wide range of growth factors and nutrients that are secreted by or transported by uterine epithelia into the uterus at the maternal-conceptus interface for successful implantation and placentation. Among these factors, insulin-like growth factor-I (IGF-I) is known to play an important role in development of the early embryo and uterine endometrium. However, few studies have been conducted with pigs to determine IGF-I-induced functional effects on peri-implantation embryos such as activation of cell signaling cascades responsible for growth, proliferation and differentiation of cells of the conceptus. Therefore, the aim of this study was to analyze mRNA expression of endometrial IGF-I and its receptor, to examine the functional role of IGF-I on primary porcine trophectoderm (pTr) cells and to assess potential signaling pathways responsible for biological activities of IGF-1. In the present study, expression of endometrial type I IGF receptor (IGF-IR) mRNA increased significantly from Day 10 to Day 12 of pregnancy and the increase was greater for pregnant than cyclic gilts. Both IGF-I and IGF-IR mRNAs were abundant in endometrial luminal-, glandular epithelia, and stratum compactum stroma on Day 12 of pregnancy. In addition, IGF-I significantly induced phosphorylation of AKT1, ERK1/2 and RPS6 in a time- and concentration-dependent manner in pTr cells. Immunofluorescence microscopy revealed that IGF-I treated pTr cells exhibited increased abundance of phosphorylated (p)-AKT1 and p-ERK1/2 MAPK proteins in the nucleus and cytoplasm, and p-RPS6 proteins in the cytosol as compared to non-treated pTr cells. In the presence of the ERK1/2 MAPK inhibitor (U0126), IGF-I-induced AKT1 phosphorylation was not affected, whereas the PI3K inhibitor (LY294002) decreased IGF-I-induced phosphorylation of ERK1/2 and AKT1 proteins, and both the PI3K-AKT1 and ERK1/2 MAPK pathways were blocked by LY294002. Furthermore, IGF-I significantly stimulated both proliferation and migration of pTr cells, but these effects were blocked by P38 inhibitor (SB203580), U0126, MTOR inhibitor (rapamycin) and LY294002. Taken together, these results indicate that IGF-I coordinately regulates multiple cell signaling pathways including PI3K-AKT1-RPS6 and ERK1/2 MAPK signaling pathways that are critical to proliferation, migration and survival of trophectoderm cells during early pregnancy in pigs.

Proliferation-stimulating effect of colony stimulating factor 2 on porcine trophectoderm cells is mediated by activation of phosphatidylinositol 3-kinase and extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase

Colony-stimulating factor 2 (CSF2), also known as granulocyte macrophage colony-stimulating factor, facilitates mammalian embryonic development and implantation. However, biological functions and regulatory mechanisms of action of porcine endometrial CSF2 in peri-implantation events have not been elucidated. The aim of present study was to determine changes in cellular activities induced by CSFs and to access CSF2-induced intracellular signaling in porcine primary trophectoderm (pTr) cells. Differences in expression of CSF2 mRNA in endometrium from cyclic and pregnant gilts were evaluated. Endometrial CSF2 mRNA expression increases during the peri-implantation period, Days 10 to 14 of pregnancy, as compared to the estrous cycle. pTr cells obtained in Day 12 of pregnancy were cultured in the presence or absence of CSF2 (20 ng/ml) and LY294002 (20 µM), U0126 (20 µM), rapamycin (20 nM), and SB203580 (20 µM). CSF2 in pTr cell culture medium at 20 ng/ml significantly induced phosphorylation of AKT1, ERK1/2, MTOR, p70RSK and RPS6 protein, but not STAT3 protein. Also, the PI3K specific inhibitor (LY294002) abolished CSF2-induced increases in p-ERK1/2 and p-MTOR proteins, as well as CSF2-induced phosphorylation of AKT1. Changes in proliferation and migration of pTr cells in response to CSF2 were examined in dose- and time-response experiments. CSF2 significantly stimulated pTr cell proliferation and, U0126, rapamycin and LY294002 blocked this CSF2-induced proliferation of pTr cells. Collectively, during the peri-implantation phase of pregnancy in pigs, endometrial CSF2 stimulates proliferation of trophectoderm cells by activation of the PI3K-and ERK1/2 MAPK-dependent MTOR signal transduction cascades.

Epidermal growth factor stimulates proliferation and migration of porcine trophectoderm cells through protooncogenic protein kinase 1 and extracellular-signal-regulated kinases 1/2 mitogen-activated protein kinase signal transduction cascades during early pregnancy

For successful implantation and establishment of early epitheliochorial placentation, porcine conceptuses require histotroph, including nutrients and growth factors, secreted by or transported into the lumen of the uterus. Epidermal growth factor (EGF), an essential component of histotroph, is known to have potential growth-promoting activities on the conceptus and uterine endometrium. However, little is known about its effects to transactivate cell signaling cascades responsible for proliferation, growth and differentiation of conceptus trophectoderm. In the present study, therefore, we determined that EGFR mRNA and protein were abundant in endometrial luminal and glandular epithelia, stratum compactum stroma and conceptus trophectoderm on days 13-14 of pregnancy, but not in any other cells of the uterus or conceptus. In addition, primary porcine trophectoderm (pTr) cells treated with EGF exhibited increased abundance of phosphorylated (p)-AKT1, p-ERK1/2 MAPK and p-P90RSK over basal levels within 5min, and effect that was maintained to between 30 and 120min. Immunofluorescence microscopy revealed abundant amounts of p-ERK1/2 MAPK and p-AKT1 proteins in the nucleus and, to a lesser extent, in the cytoplasm of pTr cells treated with EGF as compared to control cells. Furthermore, the abundance of p-AKT1 and p-ERK1/2 MAPK proteins was inhibited in control and EGF-treated pTr cells transfected with EGFR siRNA. Compared to the control siRNA transfected pTr cells, pTr cells transfected with EGFR siRNA exhibited an increase in expression of IFND and TGFB1, but there was no effect of expression of IFNG. Further, EGF stimulated proliferation and migration of pTr cells through activation of the PI3K-AKT1 and ERK1/2 MAPK-P90RSK cell signaling pathways. Collectively, these results support the hypothesis that EGF coordinately activates multiple cell signaling pathways critical to proliferation, migration and survival of trophectoderm cells that are critical to development of porcine conceptuses during implantation and placentation.

Effect of conceptus on transforming growth factor (TGF) β1 mRNA expression and protein concentration in the porcine endometrium--in vivo and in vitro studies

Transforming growth factor (TGF) β and its receptors are expressed at the conceptus-maternal interface during early pregnancy in the pig. The present studies were conducted to examine: (1) the effect of conceptus products on TGFβ1 mRNA expression and protein concentration in the porcine endometrium using in vivo and in vitro models, and (2) the effect of TGFβ1 on proliferation of porcine trophoblast cells in vitro. During in vivo experiments, gilts with one surgically detached uterine horn were slaughtered on days 11 or 14 of the estrous cycle and pregnancy. For in vitro studies, endometrial explants and luminal epithelial (LE) cells co-cultured with stromal (ST) cells were treated with conceptus-exposed medium (CEM). Moreover, porcine trophoblast cells were treated with TGFβ1, and the number of viable cells was measured. On day 11, the presence of conceptuses had no effect on TGFβ1 mRNA expression, but decreased the TGFβ1 protein concentration in the connected uterine horn compared with the detached uterine horn. In contrast to day 11, on day 14 after estrus, TGFβ1 mRNA expression and protein content in the endometrium collected from the gravid uterine horn were greater when compared with the contralateral uterine horn. The treatment of endometrial slices with CEM resulted in greater TGFβ1 mRNA expression and protein secretion. LE cells responded to CEM with an increased TGFβ1 mRNA level. Moreover, TGFβ1 stimulated the proliferation of day 14 trophoblast cells. In summary, porcine conceptuses may regulate TGFβ1 synthesis in the endometrium at the time of implantation. TGFβ1, in turn, may promote conceptus development by increasing the proliferation of trophoblast cells.

Deep sequencing of the porcine endometrial transcriptome on day 14 of pregnancy

In pigs, conceptus attachment to the uterine surface epithelium starts around Day 14 of pregnancy preceded by a pronounced vascularization at the implantation zones, initiating the epitheliochorial placentation. To characterize the complex transcriptome changes in the endometrium in the course of initial conceptus attachment, deep sequencing of endometrial RNA samples of pregnant animals (n = 4) and corresponding cyclic controls (n = 4) was performed using Illumina RNA-Seq. The obtained sequence reads were mapped to the porcine genome, and relative expression values were calculated for the analysis of differential gene expression. Statistical analysis revealed 1933 differentially expressed genes (false discovery rate 1%), 1229 with higher and 704 with lower mRNA concentration, in the samples from pregnant animals. Expression of selected genes was validated by the use of quantitative real-time RT-PCR. The RNA-Seq data were compared to results of a microarray study of bovine endometrium on Day 18 of pregnancy and additional related data sets. Bioinformatics analysis revealed for the genes with higher mRNA concentration in pregnant samples strong overrepresentation, particularly for immune-related functional terms but also for apoptosis and cell adhesion. Overrepresented terms for the genes with lower mRNA concentration in pregnant samples were related to extracellular region, ion transport, cell adhesion, and lipid and steroid metabolic process. In conclusion, RNA-Seq analysis revealed comprehensive transcriptome differences in porcine endometrium between Day 14 of pregnancy and corresponding cyclic endometrium and highlighted new processes and pathways probably involved in regulation of noninvasive implantation in the pig.

Regulation of expression and role of leukemia inhibitory factor and interleukin-6 in the uterus of early pregnant pigs

Cytokines, which are generally involved in the process of inflammation, may also play a critical role in conceptus implantation. We examined: (1) the expression profiles of leukemia inhibitory factor (LIF) and interleukin (IL)-6 mRNA and their protein content in the endometrium of cyclic and pregnant gilts on Days 10 to 18 after estrus; (2) the effect of conceptus-exposed medium on LIF and IL-6 synthesis in the endometrium; (3) the profiles of IL6R and LIFR mRNA expression in pig conceptuses collected on Days 10 to 18 of pregnancy; and (4) the effect of LIF and IL-6 on the attachment and proliferation of porcine trophoblast cells. The expression of LIF mRNA in the endometrium increased between Days 10 and 12 in both cyclic and pregnant gilts, and tended to be higher in Day 12 pregnant animals compared with nonpregnant ones. The LIF protein content in the uterine lumen peaked on Day 12 of pregnancy, and was higher than on Day 12 of the estrous cycle. Endometrial IL-6 mRNA expression was upregulated on Day 12 in pregnant gilts compared with nonpregnant animals. Moreover, a higher content of IL-6 protein was observed in pregnant than in cyclic gilts. The addition of conceptus-exposed medium resulted in up-regulation of LIF and IL6 mRNA expression, and increased IL-6 content in endometrial slices. In conceptuses, increased mRNA expression was detected on Days 10 to 14 for IL6R and on Day 14 for LIFR, when compared with other days studied. LIF and IL-6 stimulated the attachment and proliferation of trophoblast cells in vitro. In summary, LIF and IL-6 are important components of embryo-uterine interactions during early pregnancy in the pig, and may contribute to successful conceptus implantation.

Pregnancy-associated glycoprotein (PAG) family localized in chorionic cells within the epitheliochorial/diffuse placenta of the alpaca (Lama pacos)

Pregnancy-associated glycoproteins (PAGs) are abundant embryo-originated products expressed in the pre-placental trophoblast and later in the post-implantational chorionic epithelium of some ungulate species. This paper describes the cellular immunolocalization of the chorionic PAG family in the epitheliochorial placenta type of the alpaca (Lama pacos-Lp), in which the PAGs were named 'LpPAGs'. Placental Lp sections (5 μm) of different females near mid-pregnancy (150 days post coitum; dpc), advanced pregnancy (244-263 dpc) and late pregnancy (347 dpc) were used for cross-species (heterologous-ht) double fluorescent immunohistochemistry (htdF-IHC). The htdF-IHC was performed with primary rabbit polyvalent anti-porcine PAG polyclonals. The LpPAG immuno-complexes were visualized with secondary goat anti-rabbit immunoglobulins-conjugated with Alexa 488 fluorophore (green), among all nuclei of placental cells stained with propidium iodide (red). This is the first study reporting the immunolocalization of the LpPAG family identified by htdF-IHC at the feto/maternal interface during different pregnancy stages of the alpaca. The most dominant and strongest immune-positive LpPAG signals were found in the well-developed chorionic cell layer. Our htdF-IHC indicated relatively high epitope resemblance to that of the PAGs in camelids and pigs. These data increase our general knowledge of chorionic PAG localization during pregnancy-stage dependent development of the epitheliochorial diffuse placenta type in the alpaca.

Effect of conceptus secretions on HOXA10 and PTGS2 gene expression, and PGE2 release in co-cultured luminal epithelial and stromal cells of the porcine endometrium at the time of early implantation

Homeobox A10 (HOXA10) gene expression was demonstrated in the endometrium of adult porcine uteri, however there is little information concerning the role of this gene in the pig. Objectives of the present study were to examine: 1) the expression of HOXA10 in the endometrium of cyclic and early pregnant gilts; 2) the effect of estradiol (E(2)) and progesterone (P(4)) on HOXA10 expression in porcine luminal epithelial (LE) and stromal (ST) cells in vitro; 3) the effect of E(2) and conceptus-exposed medium (CEM) on HOXA10 and prostaglandin endoperoxide synthase (PTGS2) gene expression and prostaglandin (PG) E(2) secretion from LE and ST cells in a co-culture model. The abundance of HOXA10 mRNA was increased on day 15 of pregnancy in comparison to day 15 of the estrous cycle. Moreover, increased HOXA10 mRNA level was detected in ST cells after E(2) and P(4) treatment. E(2) stimulated the expression of HOXA10 in LE cells cultured on collagen and pre-treated with steroids, but not in LE on plastic surfaces. Addition of CEM to LE cells cultured in collagen-coated inserts of the co-culture system resulted in elevated HOXA10 and PTGS2 gene expression and PGE(2) secretion in these cells, but not in ST cells cultured in basal compartments. ST cells directly treated with E(2) or CEM showed higher levels of HOXA10 and PTGS2 expression. Blocking of estrogen receptors with ICI-182,780 did not influence the stimulatory effect of CEM. We conclude that HOXA10 expression in the porcine endometrium is closely related to the implantation process and stimulated by conceptus products. Moreover, the co-culture system of LE and ST cells is a promising model for the study of endometrial response to conceptus-derived factors.

Endometrial caspase 1 and interleukin-18 expression during the estrous cycle and peri-implantation period of porcine pregnancy and response to early exogenous estrogen administration

BACKGROUND

The role for endometrial secretion of cytokines during the establishment of pregnancy in a number of mammals is well established. The current study determined endometrial expression of caspase 1 (CASP1) and interleukin-18 (IL18) during the estrous cycle and early pregnancy, and following early estrogen administration, which induces conceptus loss during early development in pigs.

METHODS

Gilts were hysterectomized on either D 0, 5, 10, 12, 15 and 18 of the estrous cycle, or D 10, 12, 15 or 18 of pregnancy. The abundance of endometrial CASP1 mRNA was unaffected by day of the estrous cycle, however there was a 6 and 10-fold increase in expression on D 15 and 18 of pregnancy. Endometrial expression of IL18 mRNA increased 5-fold between D 10 to 18 in cyclic and pregnant gilts. Total recoverable IL18 in uterine flushings was greater in pregnant compared to cyclic gilts on D 15 and 18.In the second experiment, mated gilts were treated with either corn oil (CO) or estrogen (E) on D 9 and 10 and hysterectomized on either D 10, 12, 13, 15 or 17 of pregnancy. The current study localizes the presence of CASP1 to the epithelial layer of the endometrium for the first time. Further, a day x treatment interaction was detected for endometrial CASP1 mRNA and protein abundance as E stimulated an earlier increase on D 13 compared to CO gilts. Although IL18 mRNA expression remained unaltered from the E treatment, protein abundance was significantly attenuated on D 15 and 18 in response to E treatment.

CONCLUSIONS

Endometrial expression of CASP1 and IL18 is associated with establishment of pregnancy in pigs. Alteration of CASP1 and IL18 following premature exposure of the uterus to estrogen during early pregnancy may contribute to conceptus loss between Days 15 to 18 of pregnancy.

Transforming growth factor beta (TGFB) signaling is activated during porcine implantation: proposed role for latency-associated peptide interactions with integrins at the conceptus-maternal interface

The process of implantation is mediated by a complex network of signaling and adhesive factors. In the pig, latent and active transforming growth factor beta (TGFB), TGFB receptors (TGFBR), and integrins (ITGs) are present during the peri-implantation period. TGFB signals via TGFBR and activates downstream effector SMAD proteins 2 and 3 (p-SMAD2/3). Latency-associated peptide (LAP), part of the latent TGFB complex, is known to bind to ITG heterodimers and activate TGFB. We hypothesize that active TGFBs and TGFBRs along with LAP and ITGs functionally interact at the conceptus-maternal interface to mediate events essential for conceptus development and attachment in pigs. Uteri and conceptuses from days 10, 12, 16, 20, and 24 pregnant gilts were immunostained for TGFB, LAP, and ITG subunits (ITGAV, ITGB1, ITGB3, ITGB5, ITGB6, and ITGB8). Activation of TGFBRs was evaluated by the presence of phosphorylated downstream effector SMAD2/3. Binding of LAP to ITGs was also evaluated using porcine trophectoderm cells. Abundant active TGFB was detected at the apical surfaces of epithelia at the conceptus-maternal interface, and p-SMAD2/3 was detected at both conceptus attachment and nonattachment sites during implantation. Separate aggregates of LAP, ITGB1, ITGB5, and later ITGB3 were detected at the porcine conceptus-maternal interface, and binding of LAP to ITGs on apical surfaces was demonstrated. Results suggest that functional LAP-ITG adhesion complexes support conceptus attachment and promote TGFB activation leading to TGFB interaction with TGFBR supporting events of porcine implantation.

Pregnancy-associated glycoprotein (PAG) family: transcripts and gene amplicons in camelids

In this study, the placental localization of PAG-like transcripts and genomic existence of PAG-like amplicons in new-world (Lp, Lama pacos, alpaca) and old-world camelids (Cb, Camelus bactrianus, bactrian; Cd, Camelus dromedarius; dromedary) are reported for the first time. Sections of Lp (150-347 days post coitum), Cd (43-90 cm crown-rump length) and Cb (term) placentas were used for heterologous (ht; cross-species) autoradiographic in situ hybridization (aISH) with single-stranded diagnostic (antisense) or control (sense) [alpha-(35)S]dATP-labeled 323 nt porcine PAG8 (pPAG8) cDNA probes produced by asymmetric PCRs. The aISH with antisense (35)S-pPAG8 probe identified camelid PAG-like (LpPAG, CbPAG and CdPAG) mRNA expression restricted to chorionic epithelium cells within placentas of camelids. In addition, genomic DNA (gDNA), isolated from placental sections were used as templates for camelid PAG-like gene amplicon production by PCR. Specificity of the obtained multiple camelid gDNA PAG-like amplicons was confirmed by double ht-Southern hybridizations with [alpha-(32)P]dATP-labeled 611 bp pPAG5 and pPAG10 double-stranded cDNA probes. The double ht-Southern hybridizations of camelid gDNA amplicons (with pPAG5 and -10 probes) allowed the identification of length-polymorphism of LpPAG, CbPAG and CdPAG genes, coding catalytically active and potentially inactive forms. Such an application of porcine PAG probes may be advantageous for future identification of still undiscovered PAG-like families in other eutherian species.

Immunohistochemical Localization of Vascular Endothelial Growth Factor (VEGF) and its Two Receptors (Flt-I and KDR) in the Endometrium and Placenta of the Mare During the Oestrous Cycle and Pregnancy

Polyclonal antisera for vascular endothelial growth factor (VEGF) and its two main receptor molecules, VEGF-I (Flt) and VEGF-II (KDR), were used in a conventional immunocytochemical staining method to localize these angiogenic ligand molecules in the endometrium and placenta of the mare during the oestrous cycle and pregnancy. The anti-VEGF and anti-Flt sera both labelled the lumenal and glandular epithelia of the endometrium throughout the oestrous cycle and both the invasive trophoblast cells of the endometrial cups and the non-invasive trophoblast of the allantochorion in pregnancy. The anti-KDR serum likewise stained the maternal and foetal epithelial layers during the oestrous cycle and pregnancy and it also labelled fibroblast-like cells in the endometrial and allantoic stromas and the endothelium of foetal and maternal capillaries. The results demonstrated that constant supplies of the principal vasculogenic and angiogenic factor, VEGF, and its two major receptors, Flt and KDR, are available on both the maternal and foetal sides of the placental barrier throughout gestation in the mare. They are presumed to facilitate the continuing development of the extensive foetal and maternal capillary networks that are such prominent features within the microplacentomes of the diffuse, epitheliochorial equine placenta.

Porcine endometrial and conceptus tissue kallikrein 1, 4, 11, and 14 gene expression

Previous studies have suggested that the porcine endometrium may express several tissue kallikreins during the estrous cycle and early pregnancy. The present study investigated porcine endometrial and conceptus tissue kallikrein 1, 4, 11, and 14 mRNA expression during the estrous cycle and early pregnancy. Tissue kallikrein (KLK) gene expression was evaluated using quantitative RT-PCR and in situ hybridization. KLK1 expression was similar across the estrous cycle and early pregnancy, and localized to the endometrial luminal (L) and glandular (G) epithelium. KLK4 endometrial mRNA expression was greatest on days 0, 5, and 10 when compared with days 12, 15, and 17 of the estrous cycle and greater in cyclic compared with pregnant gilts. Expression of KLK4 was more intense in the stroma and uterine epithelium from days 0 to 10 of the estrous cycle. Endometrial KLK11 mRNA was not different between cyclic and pregnant gilts but the expression was greatest on days 10 and 12 compared with all other days evaluated. There was an increased intensity of KLK11 gene expression in the stratum compactum on day 10 of the estrous cycle and early pregnancy. Endometrial KLK14 mRNA expression was not detectable on days 5 and 10 but was expressed on days 0, 12, 15, and 17 of the estrous cycle and pregnancy. KLK14 expression was localized in the uterine L and G epithelium, and stroma throughout the endometrium after day 10. Conceptus KLK1 mRNA did not change from days 10 to 17 of gestation. However, conceptus KLK4, and 14 mRNA expression was greatest on day 10 with expression declining after day 14 of gestation. Expression of the various tissue kallikreins in the endometrium and conceptus during the estrous cycle and early pregnancy in the pig can serve in the activation of growth factors and tissue remodeling during the establishment of pregnancy.

Placental expression of the membrane form of folate binding protein during pregnancy in swine

Previous experiments indicated that secreted (s) and membrane (m) forms of folate binding protein (FBP) are present in the intrauterine environment of the pig. Our previous studies indicated that the two forms were produced sequentially; the secreted form was present in the intrauterine glands until Day 20 of gestation, whereas binding analysis indicated that folate binding increased dramatically in placental membranes until Day 50 of gestation. However, the cell types expressing mFBP have not been investigated. In this experiment, uterine wall sections from Day 20, 35, 50, 70, 90, and 105 of gestation were collected at slaughter and fixed, and subjected to in situ hybridization analysis for mFBP expression. The mFBP mRNA was expressed by both columnar and cuboidal epithelia of the placental folds and expression appeared to be similar throughout gestation. Therefore, the placenta expressed mFBP from Day 35 of gestation onward, consistent with the concept that sFBP and mFBP occur sequentially during gestation in swine, and that placental mFBP expression plays a role in folate transport after a functional chorioallantoic placenta is established (between Day 20 and 35).

Global characterization of porcine intrauterine proteins during early pregnancy

Total protein secreted in the intrauterine lumen increases between day 10 and 13 post-estrus in both cyclic and pregnant gilts. The objective of this experiment was to identify those intrauterine proteins whose secretion changes during this time period. Sixteen mature gilts were either mated (day 0) or remained cyclic and were slaughtered at either day 10 or day 13 (n = 4 per status by day). At slaughter, each uterine horn was flushed with 20 ml Minimal Essential Medium. Flushings were dialyzed extensively against distilled water. A 0.5 ml aliquot of each was lyophilized, subjected to two-dimensional PAGE, and protein spots were identified following Coomassie staining of each gel. Densitometry was used to compare relative amounts of each spot. After statistical analysis, spots that differed due to either day, status, or day by status interaction were excised and digested in-gel with trypsin. The resulting peptides were analyzed by tandem mass spectrometry (MS/MS). Using MS/MS data, protein identification for each spot was attempted. There were 280 matching spots, of which 132 were significantly (P < 0.05 or 0.01) affected by pregnancy status, day, or the day by status interaction. Most (73%) spots increased from day 10 to day 13 with no effect of pregnancy. Several spots were identified as proteases or their inhibitors. Others potentially modify glycolipids and/or glycoproteins. These results indicate that the concentrations of many proteins within the intrauterine environment during early pregnancy are independent of the conceptus and could play roles in regulating the endometrial or conceptus glycocalyx.

Expression of porcine endometrial prostaglandin synthase during the estrous cycle and early pregnancy, and following endocrine disruption of pregnancy

Porcine trophoblast attachment to the uterine surface is associated with increased conceptus and endometrial production of prostaglandins. Conceptus secretion of estrogen on Day 12 of gestation is important for establishment of pregnancy; however, early (Days 9 and 10) exposure to exogenous estrogens results in embryonic mortality. Present studies established the temporal and spatial pattern of endometrial PTGS1 (prostaglandin-endoperoxide synthase 1) and PTGS2 expression during the estrous cycle and early pregnancy and determined the effect of early estrogen treatment on endometrial PTGS expression in pregnant gilts. Endometrial PTGS1 mRNA expression increased 2- to 3-fold after Day 10 of the estrous cycle and pregnancy, whereas PTGS2 mRNA expression increased 76-fold between Days 5 and 15 of the estrous cycle and pregnancy. Increased expression of the PTGS2 transcript was detected in the lumenal epithelium after Day 10 in both cyclic and pregnant gilts. There was a 10- and 20-fold increase in endometrial PTGS2 protein expression between Days 5 and 18 of the estrous cycle and pregnancy respectively. Administration of estrogen on Days 9 and 10 of gestation increased endometrial PTGS2 mRNA and protein on Day 10, but decreased PTGS2 mRNA and protein in lumenal epithelium (LE) on Day 12 of gestation compared to vehicle-treated gilts. The present study demonstrates that an increase in uterine epithelial PTGS2 expression occurs after Day 10 of the estrous cycle and early pregnancy in the pig. The conceptus-independent increase in the uterine LE indicates that a novel pathway exists for endometrial induction PTGS2 expression before conceptus elongation and attachment to the uterine surface. Epithelial expression of PTGS2 may serve as one of the signals for placental attachment and embryo survival in the pig. Early administration of estrogen on Days 9 and 10 of pregnancy alters endometrial PTGS2 mRNA and protein expression, which may, at least in part, represent a mechanism by which endocrine disruption of pregnancy causes total embryonic loss during implantation in the pig.