Cellular Localization of Placenta-Specific Human Endogenous Retrovirus (HERV) Transcripts and their Possible Implication in Pregnancy-Induced Hypertension

Syncytin-1, syncytin-2 and suppressyn in human health and disease

In this review, we summarized the results of experimental and clinical studies about three human endogenous retroviruses and their products-syncytin-1, syncytin-2, and suppressyn in human physiology and pathophysiology. We summed up the described connection with various pathological processes and diseases, mainly with pregnancy-induced hypertensive diseases such as preeclampsia, oncogenesis, gestational trophoblastic disease, and multiple sclerosis. Supposed mechanisms of action and the potential of clinical applications are also described.

Acquisition and Exaptation of Endogenous Retroviruses in Mammalian Placenta

Endogenous retroviruses (ERVs) are retrovirus-like sequences that were previously integrated into the host genome. Although most ERVs are inactivated by mutations, deletions, or epigenetic regulation, some remain transcriptionally active and impact host physiology. Several ERV-encoded proteins, such as Syncytins and Suppressyn, contribute to placenta acquisition, a crucial adaptation in mammals that protects the fetus from external threats and other risks while enabling the maternal supply of oxygen, nutrients, and antibodies. In primates, Syncytin-1 and Syncytin-2 facilitate cell-cell fusion for placental formation. Suppressyn is the first ERV-derived protein that inhibits cell fusion by binding to ASCT2, the receptor for Syncytin-1. Furthermore, Syncytin-2 likely inserted into the genome of the common ancestor of Anthropoidea, whereas Syncytin-1 and Suppressyn likely inserted into the ancestor of catarrhines; however, they were inactivated in some lineages, suggesting that multiple exaptation events had occurred. This review discusses the role of ERV-encoded proteins, particularly Syncytins and Suppressyn, in placental development and function, focusing on the integration of ERVs into the host genome and their contribution to the genetic mechanisms underlying placentogenesis. This review provides valuable insights into the molecular and genetic aspects of placentation, potentially shedding light on broader evolutionary and physiological processes in mammals.

In Vitro Expression Analysis Reveals HML6-c14 to Be an Attractive Research Target

HML6-c14, a long terminal repeat (LTR)-type retrotransposon identified by expressed sequence tag (EST) database screening, was found to undergo RNA processing resembling that of placental tissue by in vitro expression analysis. Previous in situ hybridization studies using normal placental tissue showed that the transcript remained in the nucleus. However, among the transcripts forcedly expressed in cultured cells, the transcript that retained the 3.3 kb intron was observed in the nucleus, and a part of the spliced transcript was observed outside the nucleus. To verify whether this cytoplasmic transcript could be translated, we examined the coding potential of the open reading frame (ORF), consisting of 109 codons on the spliced transcript, along with two other putative ORFs detected in the intronic region. As a result, none of the ORF-derived products could be detected by Western blotting as fusion proteins tagged with the FLAG epitope, suggesting that HML6-c14 belongs to a group of long non-coding RNA (lncRNA) genes. Promoter analysis of the upstream 6.4 kb genomic region also suggested that the 5'-LTR itself potentially retains high promoter activity. Despite losing the ability to produce functional proteins, HML6-c14 continues to retain its transcriptional ability while converting to an lncRNA gene, which is an interesting subject for future research.

Dynamics of the Equine Placental DNA Methylome and Transcriptome from Mid- to Late Gestation

The placenta is a temporary organ that is essential for the survival of the fetus, with a lifelong effect on the health of both the offspring and the dam. The functions of the placenta are controlled by its dynamic gene expression during gestation. In this study, we aimed to investigate the equine placental DNA methylome as one of the fundamental mechanisms that controls the gene expression dynamic. Chorioallantois samples from four (4M), six (6M), and ten (10M) months of gestation were used to map the methylation pattern of the placenta. Globally, methylation levels increased toward the end of gestation. We identified 921 differentially methylated regions (DMRs) between 4M and 6M, 1225 DMRs between 4M and 10M, and 1026 DMRs between 6M and 10M. A total of 817 genes carried DMRs comparing 4M and 6M, 978 comparing 4M and 10M, and 804 comparing 6M and 10M. We compared the transcriptomes between the samples and found 1381 differentially expressed genes (DEGs) when comparing 4M and 6M, 1428 DEGs between 4M and 10M, and 741 DEGs between 6M and 10M. Finally, we overlapped the DEGs and genes carrying DMRs (DMRs-DEGs). Genes exhibiting (a) higher expression, low methylation and (b) low expression, high methylation at different time points were identified. The majority of these DMRs-DEGs were located in introns (48.4%), promoters (25.8%), and exons (17.7%) and were involved in changes in the extracellular matrix; regulation of epithelial cell migration; vascularization; and regulation of minerals, glucose, and metabolites, among other factors. Overall, this is the first report highlighting the dynamics in the equine placenta methylome during normal pregnancy. The findings presented serve as a foundation for future studies on the impact of abnormal methylation on the outcomes of equine pregnancies.

Fluidic Flow Enhances the Differentiation of Placental Trophoblast-Like 3D Tissue from hiPSCs in a Perfused Macrofluidic Device

The human placenta serves as a multifunctional organ to maintain the proper development of a fetus. However, our knowledge of the human placenta is limited due to the lack of appropriate experimental models. In this work, we created an in vitro placental trophoblast-like model via self-organization of human induced pluripotent stem cells (hiPSCs) in a perfused 3D culture macrofluidic device. This device allowed cell seeding, in situ trophoblast lineage differentiation, and formation of trophoblast-like tissues from hiPSCs in a biomimetic microenvironment. It incorporated extracellular matrix (ECM) and fluid flow in a single device. After trophoblast lineage differentiation, we were able to generate the 3D clusters with major cell types of the human placenta, including trophoblast progenitor cytotrophoblasts (CTBs), differentiated subtypes, syncytiotrophoblasts (STBs), and extravillous trophoblasts (EVTs) under long-term 3D culture (∼23 days). Moreover, the formed tissues exhibited enhanced expressions of CTB-, STB-, and EVT-related markers at the level of genes and proteins under a dynamic culture compared with static conditions. RNA-seq analysis revealed the higher expression of trophoblast-specific genes in 3D tissues, indicating the essential role of fluid flow to promote the trophoblast differentiation of hiPSCs. The established placental 3D model combined a bioengineering strategy with developmental principles, providing a promising platform for the study of placental biology in a biomimetic microenvironment in health and disease.

The Oldest Co-opted gag Gene of a Human Endogenous Retrovirus Shows Placenta-Specific Expression and Is Upregulated in Diffuse Large B-Cell Lymphomas

Vertebrate genomes contain endogenous retroviruses (ERVs) that represent remnants of past germline infections by ancient retroviruses. Despite comprising 8% of the human genome, the human ERVs (HERVs) do not encode a replication competent retrovirus. However, some HERV genes have been co-opted to serve host functions, most notably the viral envelope-derived syncytins involved in placentation. Here, we identify the oldest HERV intact gag gene with an open reading frame, gagV1. Its provirus contains an intact env, envV1, and the first open reading frame found in an HERV gag leader, pre-gagV1, which encodes a novel protein. This HERV is linked to a related gag gene, gagV3, and these three genes all show patterns of evolutionary conservation in primates. gagV1 and pre-gagV1 orthologs are present in all simian primate lineages indicating that this HERV entered the germline of the common simian primate ancestor at least 43 Ma, whereas gagV3 is found in Old and New World monkeys. gagV1 and gagV3 have undergone recurrent gene conversion events and positive selection. Expression of gagV1, gagV3, and pre-gagV1 is restricted to the placenta in humans and macaques suggesting co-option for placenta-specific host functions. Transcriptomic analysis of human tumors also found upregulated levels of gagV1 transcripts in diffuse large B-cell lymphomas. These findings suggest that these HERV-V genes may be useful markers for the most common type of non-Hodgkin's lymphoma and that they may have contributed to the successive domestications of env and gag genes in eutherians involved in the ongoing ERV-driven evolution of the placenta.

MicroRNA retrocopies generated via L1-mediated retrotransposition in placental mammals help to reveal how their parental genes were transcribed

In mammalian genomes, most retrocopies emerged via the L1 retrotransposition machinery. The hallmarks of an L1-mediated retrocopy, i.e., the intronlessness, the presence of a 3′ poly-A tail, and the TSDs at both ends, were frequently used to identify retrotransposition events. However, most previous studies only focused on protein-coding genes as their possible parental sources and thus only a few retrocopies derived from non-coding genes were reported. Remarkably, none of them was from microRNAs. Here in this study, we found several retrocopies generated from the mir-302–367 cluster gene (MIR302CHG), and identified a novel alternatively spliced exon encoding mir-302a. The other recognized microRNA retrotransposition events are primate-specific with mir-373 and mir-498 as their parental genes. The 3′ poly-A tracts of these two retrocopy groups were directly attached to the end of the microRNA precursor homologous regions, which suggests that their parental transcripts might alternatively terminate at the end of mir-373 and mir-498. All the three parental microRNAs are highly expressed in specific tissues with elevated retrotransposon activity, such as the embryonic stem cells and the placenta. This might be the reason that our first microRNA retrocopy findings were derived from these three microRNA genes.

Increased copy number of syncytin-1 in the trophectoderm is associated with implantation of the blastocyst

Background

A key step in embryo implantation is the adhesion to and invasion of the endometrium by the blastocyst trophectoderm. The envelope proteins of HERV-W and -FRD (human endogenous retrovirus-W and -FRD), syncytin-1 and syncytin-2, are mainly distributed in the placenta, and play important roles in the development of the placenta. The placenta originates from the trophectoderm of the blastocyst. It is unclear whether the envelope proteins of HERV-W and -FRD have an effect on the development of the trophectoderm and whether they have any association with the implantation of the blastocyst.

Methods

The whole-genome amplification products of the human blastocyst trophectoderm were used to measure the copy number of syncytin-1 and syncytin-2 using real time qPCR. In addition, clinical data associated with the outcome of pregnancies was collected, and included age, body mass index (BMI), basic follicle stimulating hormone(bFSH), rate of primary infertility and oligo-astheno-teratospermia, the thickness of the endometrium on the day of endometrial transformation, the levels of estrogen and progestin on the transfer day, the days and the morphological scores of the blastocysts. The expression of mRNA and the copy numbers of syncytin-1 and syncytin-2 in H1 stem cells, and in differentiated H1 cells, induced by BMP4, were measured using real time qPCR.

Results

The relative copy number of syncytin-1 in the pregnant group (median: 424%, quartile: 232%-463%, p < 0.05) was significantly higher than in the non-pregnant group (median: 100%, quartile: 81%-163%). There was a correlation (r _s  = 0.681, p < 0.001) between the copy number of syncytin-1 and blastocyst implantation after embryo transfer. As the stem cells differentiated, the expression of NANOG mRNA decreased, and the expression of caudal type homeobox 2(CDX2) and β-human chorionic gonadotropin (β-hCG) mRNAs increased. Compared to the undifferentiated cells, the relative expression of the syncytin-1 mRNA was 1.63 (quartile: 0.59-6.37, p > 0.05), 3.36 (quartile: 0.85-14.80, p > 0.05), 10.85 (quartile: 3.39-24.46, p < 0.05) and 67.81 (quartile: 54.07-85.48, p < 0.05) on day 1, 3, 5 and 7, respectively, after the differentiation. The relative expression of syncytin-2 was 5.34 (quartile: 4.50-10.30), 7.90 (quartile: 2.46-14.01), 57.44 (quartile: 38.35-103.87) and 344.76 (quartile: 267.72-440.10) on day 1, 3, 5 and 7, respectively, after the differentiation (p < 0.05). The copy number of syncytin-1 increased significantly during differentiation.

Conclusion

Preceding the transfer of frozen embryos, the increased copy number of syncytin-1 in the blastocyst trophectoderm was associated with good outcomes of pregnancies.

Sharpey-Schafer Lecture 2019: From retroviruses to human birth

NEW FINDINGS

What is the topic of this review? The timing of birth is an important determinant of future health and well-being. This review examines the role of endogenous retroviruses as upstream regulators of key biological functions of the placenta, including cell-cell fusion, modulation of the maternal immune system, and the production of key pregnancy hormones. What advances does it highlight? Endogenous retroviruses are an obligate requirement for successful human reproduction. The products of retroviral elements, incorporated into the germline millions of years ago, have been co-opted to serve vital biological roles within the placenta that ultimately dictate the length of human pregnancy and therefore well-being trajectories.

ABSTRACT

Gestational length at the time of birth is an important determinant of future health and well-being, yet the physiological regulation of the onset of labour in humans remains obscure. The evolution of egg formation and internal fertilisation in amniotes required a mechanism to suppress the contractile activity of the oviduct that is provided by progesterone. Delivery of the egg is then associated with the withdrawal of progesterone and a return of contractile activity to the reproductive tract. In mammals, the process of pregnancy is complicated further by the need to protect the fetus from potential attack by the maternal immune system. There is increasing evidence that retroviruses incorporated into the mammalian germline in the evolutionary past play a key role in suppressing the maternal immune reaction to the developing conceptus, organising the development of the placenta and perhaps, in humans, modulating the action of progesterone, determining gestational length and the onset of labour. It seems that the presence of an endogenous retrovirus is an obligate requirement for human reproduction.

Placental lncRNA expression associated with placental cadmium concentrations and birth weight

Heavy metal exposures, such as cadmium, can have negative effects on infant birth weight (BW)-among other developmental outcomes-with placental dysfunction potentially playing a role in these effects. In this study, we examined how differential placental expression of long non-coding RNAs (lncRNAs) may be associated with cadmium levels in placenta and whether differences in the expression of those lncRNAs were associated with fetal growth. In the Rhode Island Child Health Study, we used data from Illumina HiSeq whole transcriptome RNA sequencing (n = 199) to examine association between lncRNA expression and measures of infant BW as well as placental cadmium concentrations controlled for appropriate covariates. Of the 1191 lncRNAs sequenced, 46 demonstrated associations (q < 0.05) with BW in models controlling for infant sex, maternal age, BMI, maternal education, and smoking during pregnancy. Furthermore, four of these transcripts were associated with placental cadmium concentrations, with MIR22HG and ERVH48-1 demonstrating increases in expression associated with increasing cadmium exposure and elevated odds of small for gestational age birth, while AC114763.2 and LINC02595 demonstrated reduced expression associated with cadmium, but elevated odds of large for gestational age birth with increasing expression. We identified relationships between lncRNA expression with both placental cadmium concentrations and BW. This study provides evidence that disrupted placental expression of lncRNAs may be a part of cadmium's mechanisms of reproductive toxicity.

Human Endogenous Retroviruses Are Preferentially Expressed in Mononuclear Cells From Cord Blood Than From Maternal Blood and in the Fetal Part of Placenta

Background: Placenta shows high transcription levels of human endogenous retroviruses (HERVs) that are overexpressed during embryonic and fetal development. Methods: In order to gather further information on the degree of HERV activation in maternal and fetal tissues we assessed the transcription levels of pol genes of HERV-H, -K, and -W in PBMCs of newborns and their mothers as well as in chorion (fetal part) and decidua basalis (maternal part) of the placenta using a real time PCR assay. Results: Transcripts of pol genes of the three HERV families were significantly higher in mononuclear cells from cord blood than from maternal blood and in the fetal part than in the maternal part of the placenta. Conclusions: The HERV over-expressions in cells and tissues of the offspring are further clues that they play pivotal physiologic roles during early life events and suggest that HERV-driven abnormalities of pregnancy and fetal development may derive mostly from the conceptus, not from the mother.

Suppressyn localization and dynamic expression patterns in primary human tissues support a physiologic role in human placentation

We previously identified suppressyn (SUPYN), a placental protein that negatively regulates the cell fusion essential for trophoblast syncytialization via binding to the trophoblast receptor for syncytin-1, ASCT2, and hypothesized that SUPYN may thereby regulate cell-cell fusion in the placenta. Here, we redefine in vivo SUPYN localization using specific monoclonal antibodies in a rare early placental sample, showing SUPYN localization in villous and extravillous trophoblast subtypes, the decidua and even in placental debris in the maternal vasculature. In human trophoblast cell lines, we show SUPYN alters ASCT2 glycosylation within the secretory pathway and that this binding is associated with inhibition of cell fusion. Using newly-optimized trophoblast isolation protocols that allow tracking of ex vivo cell fusion, we present transcription and translation dynamics of fusion-related proteins over 96 hours in culture and the effects of changes in ambient oxygen levels on these processes. We report converse syncytin-1 and SUPYN transcriptional and translational responses to surrounding oxygen concentrations that suggest both are important in the effects of hypoxia and hyperoxia on placental syncytialization. Our results suggest that SUPYN’s anti-fusogenic properties may be exerted at several sites in the maternal body and its dysregulation may be associated with diseases of abnormal placentation.

Transcriptomic features of trophoblast lineage cells derived from human induced pluripotent stem cells treated with BMP 4

INTRODUCTION

Early development of the human placenta remains poorly understood due to the lack of proper model systems. Previous reports have demonstrated that human induced pluripotent stem cells (hiPSCs) treated with bone morphogenetic protein 4 (BMP4) can differentiate into extraembryonic tissues as useful models of the early stage of trophoblast (TB) differentiation. In our previous study, we optimized the culture conditions of hiPSC-derived TB lineages, but the differentiated cells were heterogeneous.

METHODS

In order to characterize the hiPSC-derived TB lineage cells, four types of hiPSCs were treated with 50 ng/mL of BMP4 for 10 days. Subsequently, cells that were positive for the pan-TB marker keratin 7(KRT7) were purified from the differentiated cells using flow cytometry and identified with a DNA microarray.

RESULTS

Comparisons of our microarray data with the human transcriptome in a previous large-scale analysis showed that the gene expression patterns of KRT7+ cells were similar to the placenta. In total, 259 upregulated genes were commonly expressed in all four KRT7+ groups, including well-known TB markers. Among these upregulated genes, several with poorly investigated expression patterns and functions were confirmed as expressed in the primary placenta. While only XAGE2 and KCNQ2 were expressed in TB layers, XAGE2 was expressed throughout pregnancy and KCNQ2 was expressed only in cytotrophoblasts of the first trimester placenta.

CONCLUSION

BMP4-treated KRT7+ cells were in the course of the human placental development. In addition, this approach allowed the identification of new genes that might be involved in placentation. However, further studies are needed to confirm their functions.

Regulation of human trophoblast cell syncytialization by transcription factors STAT5B and NR4A3

In human trophoblast cells, cyclic AMP or its inducer forskolin (FSK) activates two downstream signaling molecules, protein kinase A (PKA) and exchange protein directly activated by cAMP (EPAC), both of which induce syncytialization, cell fusion, and the production of human chorionic gonadotropin (hCG) and progesterone. However, a transcription factor other than GCM1 and molecular mechanisms associated with these events have not been well characterized. To identify novel transcription factors involved in syncytialization of cAMP-stimulated human choriocarcinoma BeWo cells, the microarray analysis was performed with RNAs extracted from PKA- or EPAC-selective cAMP analog-stimulated BeWo cells, from which two up-regulated transcription factors, STAT5 and NR4A3, were found. The knockdown of STAT5B decreased FSK-induced cell fusion and the expression of syncytialization markers, CGB, syncytin1, syncytin2, GCM1, and OVOL1, but NR4A3 knockdown increased FSK-induced cell fusion and the expression of CGB and syncytin2. These findings indicated that cAMP-PKA up-regulated STAT5B, followed by increase in syncytin2 expression through GCM1 and OVOL1, resulting in cell fusion and hCG production, while cAMP-PKA-up-regulated NR4A3 could decrease syncytin2 expression, and suggested that both positive and negative effects of STAT5B and NR4A3, respectively, are required to control the degree of syncytialization in human trophoblast cells.

Differential Methylation of Syncytin-1 and 2 Distinguishes Fetal Growth Restriction from Physiologic Small for Gestational Age

Objective  The retroviral genes encoding Syncytin-1 ( SYN1 ) and Syncytin-2 ( SYN2 ) are epigenetically regulated, uniquely expressed in the placenta and critical to placental function. We sought to determine if placental expression and methylation patterns of SYN1 and SYN2 from pregnancies complicated by fetal growth restriction (FGR) differed from physiologic small for gestational age (SGA) and appropriate for gestational age (AGA) controls. Study Design  Placental biopsies were obtained from AGA, SGA and FGR neonates delivered at >36 weeks gestation. SGA and FGR were defined as birth weight <10% with FGR additionally requiring abnormal fetal testing. We quantified DNA methylation of SYN1 and SYN2 by EpiTyper and gene expression by RT-qPCR. Results  We identified 10 AGA, 9 SGA and 7 FGR placentas. There was decreased methylation in SYN1 and SYN2 in FGR relative to AGA and SGA. When the sum of SYN1 and SYN2 methylation was used for prediction of FGR from SGA, the area under the receiver operator characteristic curve was 0.9048 (0.7602, 1). Conclusion   SYN1 and SYN2 methylation marks differ in FGR and SGA. We plan future studies to examine these markers in cell free DNA to determine if these methylation changes could be used as a biomarker for FGR.

Novel endogenous retrovirus-derived transcript expressed in the bovine placenta is regulated by WNT signaling

Endogenous retroviruses (ERVs) are involved in placentation; perhaps, the most well-known ERVs are the syncytins, actively transcribed env genes involved in cell–cell fusion and possible morphological variations. However, ERVs other than syncytins that play an important role in placental development have not been well characterized. To identify ERV genes expressed during the onset of placentation in the bovine species, we characterized the expression profiles of bovine conceptus transcripts during the peri-attachment period using RNA-seq analysis, and confirming some candidates through real-time PCR. Using in silico and PCR analyses, we identified a novel ERV proviral sequence derived from a gag region, designated bovine endogenous retroviruses (BERV)-K3, containing Gag_p10 and Gag_p24, zinc finger domain. Initial expression of this ERV in bovine conceptuses was on day 20 (day 0 = day of estrus), soon after conceptus attachment to the endometrial epithelium, and its high placental expression was maintained up to the middle of pregnancy. The BERV-K3 transcript was also found in the uterine luminal and glandular epithelia, liver, kidney, intestine, and skin. BERV-K3 is located on chromosome 7 and integrated within LOC100848658, from which noncoding RNA could be transcribed. Furthermore, the expression of endogenous BERV-K3 in bovine trophoblast cell lines was induced by a WNT agonist, a signaling system common to genes expressed in placentas. These data support the argument that during the evolutionary process, mammals incorporated not only similar ERV sequences, but also ERVs unique to individual species. BERV-K3 is in the latter case, likely providing functions unique to ruminant gestation.

The Phylogeny of Placental Evolution Through Dynamic Integrations of Retrotransposons

Trophoblasts, a major constituent of the placenta, are known to express genes derived from various endogenous retroviruses (ERVs) as well as LTR retrotransposons. However, the evolutionary significance of ERV-derived genes involved in placental development has not been well characterized. In this review, we catalog the diverse morphology of placental structure among mammalian species with note of counterintuitive developments. We then detail the history of ancient placenta development with paternally expressed gene 10 (Peg10/Sirh1), Peg11/Sirh2, and Sirh7/Ldoc1 as LTR retrotransposons, followed by independent captures of ERV-env-related genes such as Syncytin-1, -2, -A, -B, -Rum1, and Fematrin-1 responsible for trophoblast cell fusion, resulting in multinucleate syncytiotrophoblast formation, and possibly morphological diversification of placentas. Because the endogenization of retroviral infections has occurred multiple times independently in different mammalian lineages, and some use the same molecules in their transcriptional activation, we speculate that ERV gene variants integrated into mammalian genomes in a locus-specific manner have replaced the genes previously responsible for cell fusion. Moreover, ERVs also work as transcriptional regulators of various genes such as interferon (IFN)-stimulated genes. The "baton pass" hypothesis suggests that evolutionary events caused by multiple successive retrotransposon integrations, possibly resulting in effective fusogenic activity, downstream gene transcription in a temporal and spatial manner, and/or increased diversity of placental structures.

The role of syncytins in human reproduction and reproductive organ cancers

Human life begins with sperm and oocyte fusion. After fertilization, various fusion events occur during human embryogenesis and morphogenesis. For example, the fusion of trophoblastic cells constitutes a key process for normal placental development. Fusion in the placenta is facilitated by syncytin 1 and syncytin 2. These syncytins arose from retroviral sequences that entered the primate genome 25 million and more than 40 million years ago respectively. About 8% of the human genome consists of similar human endogenous retroviral (HERVs) sequences. Many are inactive because of mutations or deletions. However, the role of the few that remain transcriptionally active has not been fully elucidated. Syncytin proteins maintain cell-cell fusogenic activity based on ENV: gene-mediated viral cell entry. In this review, we summarize how syncytins and their receptors are involved in fusion events during human reproduction. The significance of syncytins in tumorigenesis is also discussed.

Viral symbiosis and the holobiontic nature of the human genome

The human genome is a holobiontic union of the mammalian nuclear genome, the mitochondrial genome and large numbers of endogenized retroviral genomes. This article defines and explores this symbiogenetic pattern of evolution, looking at the implications for human genetics, epigenetics, embryogenesis, physiology and the pathogenesis of inborn errors of metabolism and many other diseases.

High Expression of Endogenous Retroviral Envelope Gene in the Equine Fetal Part of the Placenta

Endogenous retroviruses (ERVs) are proviral phases of exogenous retroviruses that have co-evolved with vertebrate genomes for millions of years. Previous studies have identified the envelope (env) protein genes of retroviral origin preferentially expressed in the placenta which suggests a role in placentation based on their membrane fusogenic capacity and therefore they have been named syncytins. Until now, all the characterized syncytins have been associated with three invasive placentation types: the endotheliochorial (Carnivora), the synepitheliochorial (Ruminantia), and the hemochorial placentation (human, mouse) where they play a role in the syncytiotrophoblast formation. The purpose of the present study was to evaluate whether EqERV env RNA is expressed in horse tissues as well and investigate if the horse, possessing an epitheliochorial placenta, has "captured" a common retroviral env gene with syncytin-like properties in placental tissues. Interestingly, although in the equine placenta there is no syncytiotrophoblast layer at the maternal-fetal interface, our results showed that EqERV env RNA is highly expressed at that level, as expected for a candidate syncytin-like gene but with reduced abundance in the other somatic tissues (nearly 30-fold lower) thus suggesting a possible role in the placental tissue. Although the horse is one of the few domestic animals with a sequenced genome, few studies have been conducted about the EqERV and their expression in placental tissue has never been investigated.

[Placental Development and Endogenous Retroviruses]

A placenta as we know now is a relatively new invention in mammals. Data accumulated indicates that a major cell type of the placenta is trophoblast, in which elevated expression of genes derived from various endogenous retroviruses (ERVs) as well as LTR retrotransposons is seen. However, evolutionally significance of ERV expression in placental development has not been well characterized or sorted out. In this review, we describe diversity of placental structures among mammalian species, of which morphological and cells types are far more diverse than those expected from the lines of mammalian orders. We then describe paternally expressed gene 10 (Peg10/Sirh1) and Peg11/Sirh2 as ERVs associated with ancient placenta development, followed by env-related genes such as Syncytin-1, -2, -A, -B, -Rum1, and Fematrin-1 responsible for trophoblast cells fusion, resulting in multinucleate syncytiotrophoblast formation. Because the endogenization of retroviral infections has occurred multiple times in different mammalian lineages, and some of them use similar molecules in their transcriptional activation, we speculate that ERV gene variants integrated into mammalian genomes in a locus specific manner have replaced the genes previously responsible for cell fusion. The role of cell fusion achieved by multiple successive ERV integrations is now called ''baton pass'' hypothesis, possibly resulting in increased trophoblast cell fusion, morphological diversity in placental structures, and survivability of fetuses and/or reproductive advantage in placental mammals.

Baton pass hypothesis: successive incorporation of unconserved endogenous retroviral genes for placentation during mammalian evolution

It is well accepted that numerous RNAs derived from endogenous retroviruses (ERVs) are expressed in mammalian reproductive structures, particularly in the uterus, trophoblast, and placenta. Syncytin 1 and syncytin 2 in humans and syncytin A and syncytin B in mice are membrane proteins originating from Env genes of ERVs. These ERVs are involved in the fusion of trophoblast cells, resulting in multinucleated syncytiotrophoblast formation. Evidence accumulated indicates that syncytin-like fusogenic proteins are expressed in the placenta of rabbits, dogs/cats, ruminant ungulates, tenrecs, and opossums. The syncytin genes so far characterized are known to be endogenized to the host genome only within the past 12-80 million years, more recently than the appearance of mammalian placentas, estimated to be 160-180 million years ago. We speculate that ERVs including syncytin-like gene variants integrated into mammalian genomes in a locus-specific manner have replaced the genes previously responsible for cell fusion. We therefore propose the 'baton pass' hypothesis, in which multiple successive ERV variants 'take over' cell-fusion roles, resulting in increased trophoblast cell fusion, morphological variations in placental structures, and enhanced reproductive success in placental mammals.

A CRE/AP-1-like motif is essential for induced syncytin-2 expression and fusion in human trophoblast-like model

Syncytin-2 is encoded by the envelope gene of Endogenous Retrovirus-FRD (ERVFRD-1) and plays a critical role in fusion of placental trophoblasts leading to the formation of the multinucleated syncytiotrophoblast. Its expression is consequently regulated in a strict manner. In the present study, we have identified a forskolin-responsive region located between positions -300 to -150 in the Syncytin-2 promoter region. This 150 bp region in the context of a minimal promoter mediated an 80-fold induction of promoter activity following forskolin stimulation. EMSA analyses with competition experiments with nuclear extracts from forskolin-stimulated BeWo cells demonstrated that the -211 to -177 region specifically bound two forskolin-induced complexes, one of them containing a CRE/AP-1-like motif. Site-directed mutagenesis of the CRE/AP-1 binding site in the context of the Syncytin-2 promoter or a heterologous promoter showed that this motif was mostly essential for forskolin-induced promoter activity. Transfection experiments with dominant negative mutants and constitutively activated CREB expression vectors in addition to Chromatin Immunoprecipitation suggested that a CREB family member, CREB2 was binding and acting through the CRE/AP-1 motif. We further demonstrated the binding of JunD to this same motif. Similar to forskolin and soluble cAMP, CREB2 and JunD overexpression induced Syncytin-2 promoter activity in a CRE/AP-1-dependent manner and Syncytin-2 expression. In addition, BeWo cell fusion was induced by both CREB2 and JunD overexpression, while being repressed following silencing of either gene. These results thereby demonstrate that induced expression of Syncytin-2 is highly dependent on the interaction of bZIP-containing transcription factors to a CRE/AP-1 motif and that this element is important for the regulation of Syncytin-2 expression, which results in the formation of the peripheral syncytiotrophoblast layer.

ADAM "sequence" part II: hypothesis and speculation

Noted for centuries in humans, a relatively hairless mammal [e.g., Hallero, 1766; Hohl, 1828 in Klunker, 2003], the so-called amniotic deformities, adhesions, mutilations (ADAM) sequence remains causally and pathogenetically incognito. In 1930 Streeter stated " apodictically" that no evidence has been found that intra-uterine amputation is due to amniotic bands or adhesions …" and that his 16 cases provided (histological) evidence for a "germinal origin." He concluded that an amniotic cord was "not an adhesion or inflammatory product but … an anomalous developmental structure and present from the outset." In survivors the "traces" of damaged limb-buds "reveal the scars of poor germ-plasm." In 1958, Willis, in dismissing the amniotic origin of the ADAM defects (or "Streeter" or "Simonart" bands) quoted Keith [1940] to the effect that "(a)mniotic adhesions … are always produced by … the fetus – as a result of dysplasia in foetal tissues. They are the result, not the cause, of foetal malformations." Streeter [1930] mentions a potential familial case (56-year-old man and his mother), not controlled by photographs or other records and concluded "that the (ADAM) deformity is not easily transmissible," but "due to the constitution of the germ-plasm." Torpin [1968] concluded, as apodictically as Streeter and Willis, that "… proof of amnion rupture without damage to the chorionic sac is no longer "in question." Considering Torpin's decades-long study of the ADAM phenomenon and review of 494 references (missing many) it is surprising that he does not discuss the relationship between the apparent ADAM defects and other, internal anomalies that maybe present in an affected fetus or infant not evidently caused by the amniotic disruptions, adhesions or mutilations, unless his mind was made up. Our review of these internal and other presumed primary malformations in ADAM is ongoing. However, on a preliminary basis, it seems likely to us that: (1) there is an increased prevalence of such primary anomalies in the ADAM condition confirming the view and experience of others, for example Czeizel et al. [1993]; (2) these malformations (e.g., heterotaxy) may arise as early as gastrulation; (3) that, given the ADAM phenomenon is exclusively ascertained as the ADAM phenotype in fetuses and infants, that is, that its cause and ascertainment are completely congruent, then the apparent amniotic defect must also be regarded as a malformation; (4) that in such a case the ADAM phenomenon with associated primary malformation(s) is a form of syndromal pleiotropy due to one cause yet to be elucidated. To that end we recommend archiving DNA from all affected fetuses coming to autopsy and their parents and placentas and surgical tissues of all viable affected infants for ultimate exome or genome sequencing perhaps with special attention to the syncytin genes.

Implication of human endogenous retrovirus envelope proteins in placental functions

Human endogenous retroviruses (ERVs) represent 8% of the total human genome. Although the majority of these ancient proviral sequences have only retained non-coding long terminal repeats (LTRs), a number of “endogenized” retroviral genes encode functional proteins. Previous studies have underlined the implication of these ERV-derived proteins in the development and the function of the placenta. In this review, we summarize recent findings showing that two ERV genes, termed Syncytin-1 and Syncytin-2, which encode former envelope (Env) proteins, trigger fusion events between villous cytotrophoblasts and the peripheral multinucleated syncytiotrophoblast layer. Such fusion events maintain the stability of this latter cell structure, which plays an important role in fetal development by the active secretion of various soluble factors, gas exchange and regulation of fetomaternal immunotolerance. We also highlight new studies showing that these ERV proteins, in addition to their localization at the cell surface of cytotrophoblasts, are also incorporated on the surface of various extracellular microvesicles, including exosomes. Such exosome-associated proteins could be involved in the various functions attributed to these vesicles and could provide a form of tropism. Additionally, through their immunosuppressive domains, these ERV proteins could also contribute to fetomaternal immunotolerance in a local and more distal manner. These various aspects of the implication of Syncytin-1 and -2 in placental function are also addressed in the context of the placenta-related disorder, preeclampsia.

An integrative view on the physiology of human early placental villi

The placenta is an indispensable organ for intrauterine protection, development and growth of the embryo and fetus. It provides tight contact between mother and conceptus, enabling the exchange of gas, nutrients and waste products. The human placenta is discoidal in shape, and bears a hemo-monochorial interface as well as villous materno-fetal interdigitations. Since Peter Medawar's astonishment to the paradoxical nature of the mother-fetus relationship in 1953, substantial knowledge in the domain of placental physiology has been gathered. In the present essay, an attempt has been made to build an integrated understanding of morphological dynamics, cell biology, and functional aspects of genomic and proteomic expression of human early placental villous trophoblast cells followed by a commentary on the future directions of research in this field.

Detection of the human endogenous retrovirus ERV3-encoded Env-protein in human tissues using antibody-based proteomics

OBJECTIVES

There is growing evidence to suggest that human endogenous retroviruses (HERVs) have contributed to human evolution, being expressed in development, normal physiology and disease. A key difficulty in the scientific evaluation of this potential viral contribution is the accurate demonstration of virally expressed protein in specific human cells and tissues. In this study, we have adopted the endogenous retrovirus, ERV3, as our test model in developing a reliable high-capacity methodology for the expression of such endogenous retrovirus-coded protein.

DESIGN

Two affinity-purified polyclonal antibodies to ERV3 Env-encoded protein were generated to detect the corresponding protein expression pattern in specific human cells, tissues and organs.

PARTICIPANTS

Sampling included normal tissues from 144 individuals ranging from childhood to old age. This included more than forty different tissues and organs and some 216 different cancer tissues representing the twenty commonest forms of human cancer.

SETTING

The Rudbeck Laboratory, Uppsala University and Uppsala University Hospital, Uppsala, Sweden.

MAIN OUTCOME MEASURES

The potential expression at likely physiological level of the ERV3Env encoded protein in a wide range of human cells, tissues and organs.

RESULTS

We found that ERV3 encoded Env protein is expressed at substantive levels in placenta, testis, adrenal gland, corpus luteum, Fallopian tubes, sebaceous glands, astrocytes, bronchial epithelium and the ducts of the salivary glands. Substantive expression was also seen in a variety of epithelial cells as well as cells known to undergo fusion in inflammation and in normal physiology, including fused macrophages, myocardium and striated muscle. This contrasted strongly with the low levels expressed in other tissues types. These findings suggest that this virus plays a significant role in human physiology and may also play a possible role in disease.

CONCLUSION

This technique can now be extended to the study of other HERV genomes within the human chromosomes that may have contributed to human evolution, physiology and disease.

A novel human endogenous retroviral protein inhibits cell-cell fusion

While common in viral infections and neoplasia, spontaneous cell-cell fusion, or syncytialization, is quite restricted in healthy tissues. Such fusion is essential to human placental development, where interactions between trophoblast-specific human endogenous retroviral (HERV) envelope proteins, called syncytins, and their widely-distributed cell surface receptors are centrally involved. We have identified the first host cell-encoded protein that inhibits cell fusion in mammals. Like the syncytins, this protein, called suppressyn, is HERV-derived, placenta-specific and well-conserved over simian evolution. In vitro, suppressyn binds to the syn1 receptor and inhibits syn1-, but not syn2-mediated trophoblast syncytialization. Suppressyn knock-down promotes cell-cell fusion in trophoblast cells and cell-associated and secreted suppressyn binds to the syn1 receptor, ASCT2. Identification of the first host cell-encoded inhibitor of mammalian cell fusion may encourage improved understanding of cell fusion mechanisms, of placental morphogenesis and of diseases resulting from abnormal cell fusion.

Dynamic evolution of endogenous retrovirus-derived genes expressed in bovine conceptuses during the period of placentation

In evolution of mammals, some of essential genes for placental development are known to be of retroviral origin, as syncytin-1 derived from an envelope (env) gene of an endogenous retrovirus (ERV) aids in the cell fusion of placenta in humans. Although the placenta serves the same function in all placental mammals, env-derived genes responsible for trophoblast cell fusion and maternal immune tolerance differ among species and remain largely unidentified in the bovine species. To examine env-derived genes playing a role in the bovine placental development comprehensively, we determined the transcriptomic profiles of bovine conceptuses during three crucial windows of implantation periods using a high-throughput sequencer. The sequence reads were mapped into the bovine genome, in which ERV candidates were annotated using RetroTector^© (7,624 and 1,542 for ERV-derived and env-derived genes, respectively). The mapped reads showed that approximately 18% (284 genes) of env-derived genes in the genome were expressed during placenta formation, and approximately 4% (63 genes) were detected for all days examined. We verified three env-derived genes that are expressed in trophoblast cells by polymerase chain reaction. Out of these three, the sequence of env-derived gene with the longest open reading frame (named BERV-P env) was found to show high expression levels in trophoblast cell lines and to be similar to those of syncytin-Car1 genes found in dogs and cats, despite their disparate origins. These results suggest that placentation depends on various retrovirus-derived genes that could have replaced endogenous predecessors during evolution.

Role of the human endogenous retrovirus HERV-K18 in autoimmune disease susceptibility: study in the Spanish population and meta-analysis

Background

Human endogenous retroviruses (HERVs) are genomic sequences that resulted from ancestral germ-line infections by exogenous retroviruses and therefore are transmitted in a Mendelian fashion. Increased HERV expression and antibodies to HERV antigens have been found in various autoimmune diseases. HERV-K18 in chromosome 1 was previously associated with type one diabetes and multiple sclerosis (MS). The etiology of these complex conditions has not been completely elucidated even after the powerful genome wide association studies (GWAS) performed. Nonetheless, this approach does not scrutinize the repetitive sequences within the genome, and part of the missing heritability could lie behind these sequences. We aimed at evaluating the role of HERV-K18 in chromosome 1 on autoimmune disease susceptibility.

Methods

Two HERV-K18 SNPs (97Y/C and 154W/Stop substitutions) conforming three haplotypes were genotyped in Spanish cohorts of multiple sclerosis (n = 942), rheumatoid arthritis (n = 462) and ethnically matched controls (n = 601). Our findings were pooled in a meta-analysis including 5312 autoimmune patients and 4032 controls.

Results

Significant associations of both HERV-K18 polymorphisms in chromosome 1 with MS patients stratified by HLA-DRB1*15∶01 were observed [97Y/C p = 0.02; OR (95% CI) = 1.5 (1.04–2.17) and 154W/Stop: p = 0.001; OR (95% CI) = 1.6 (1.19–2.16)]. Combined meta-analysis of the previously published association studies of HERV-K18 with different autoimmune diseases, together with data derived from Spanish cohorts, yielded a significant association of the HERV-K18.3 haplotype [97Y–154W: p_M-H = 0.0008; OR_M-H (95% CI) = 1.22 (1.09–1.38)].

Conclusion

Association of the HERV-K18.3 haplotype in chromosome 1 with autoimmune-disease susceptibility was confirmed through meta-analysis.

From ancestral infectious retroviruses to bona fide cellular genes: role of the captured syncytins in placentation

During their replication, infectious retroviruses insert a reverse-transcribed cDNA copy of their genome, a "provirus", into the genome of their host. If the infected cell belongs to the germline, the integrated provirus can become "fixed" within the host genome as an endogenous retrovirus and be transmitted vertically to the progeny in a Mendelian fashion. Based on the numerous proviral sequences that are recovered within the genomic DNA of vertebrates--up to ten percent in the case of mammals--such events must have occurred repeatedly during the course of millions of years of evolution. Although most of the ancient proviral sequences have been disrupted, a few "endogenized" retroviral genes are conserved and still encode functional proteins. In this review, we focus on the recent discovery of genes derived from the envelope glycoprotein-encoding (env) genes of endogenous retroviruses that have been domesticated by mammals to carry out an essential function in placental development. They were called syncytins based on the membrane fusogenic capacity that they have kept from their parental env gene and which contributes to the formation of the placental fused cell layer called the syncytiotrophoblast, at the materno-fetal interface. Remarkably, the capture of syncytin or syncytin-like genes, sometimes as pairs, was found to have occurred independently from different endogenous retroviruses in diverse mammalian lineages such as primates--including humans--, muroids, leporids, carnivores, caviids, and ovis, between around 10 and 85 million years ago. Knocking out one or both mouse syncytin-A and -B genes provided evidence that they indeed play a critical role in placentation. We discuss the possibility that the immunosuppressive domain embedded within retroviral envelope glycoproteins and conserved in syncytin proteins, may be involved in the tolerance of the fetus by the maternal immune system. Finally, we speculate that the capture of a founding syncytin-like gene could have been instrumental in the dramatic transition from egg-laying to placental mammals.

Transcriptional and functional studies of Human Endogenous Retrovirus envelope EnvP(b) and EnvV genes in human trophoblasts

HERV (Human Endogenous Retrovirus)-encoded envelope proteins are implicated in the development of the placenta. Indeed, Syncytin-1 and -2 play a crucial role in the fusion of human trophoblasts, a key step in placentation. Other studies have identified two other HERV env proteins, namely EnvP(b) and EnvV, both expressed in the placenta. In this study, we have fully characterized both env transcripts and their expression pattern and have assessed their implication in trophoblast fusion. Through RACE analyses, standard spliced transcripts were detected, while EnvV transcripts demonstrated alternative splicing at its 3' end. Promoter activity and expression of both genes were induced in forskolin-stimulated BeWo cells and in primary trophoblasts. Although we have confirmed the fusogenic activity of EnvP(b), overexpression or silencing experiments revealed no impact of this protein on trophoblast fusion. Our results demonstrate that both env genes are expressed in human trophoblasts but are not required for syncytialization.

Reduced expression of both syncytin 1 and syncytin 2 correlates with severity of preeclampsia

Human endogenous retroviruses (HERVs) represent up to 8% of the human genome and express several of its genes in the placenta. Studies have demonstrated that HERV envelope proteins syncytins 1 and 2 play a crucial role in trophoblast fusion and placenta development. Here, we compared the levels of placental expression of syncytins with the severity of preeclampsia (PE) symptoms. Confocal microscopy experiments indicated a pronounced deficiency in cellular fusion in trophoblast cells from patients with PE when compared to controls. As determined by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and Western blot analyses, syncytin mRNA and protein levels were decreased in PE placentas versus controls. Interestingly, syncytin 2 levels were more importantly impaired than syncytin 1. Our results further highlighted the existence of a correlation between the extent of the decrease in the expression levels of both fusogenic proteins and the degree of severity of PE symptoms. These HERV proteins could thereby be used as potential markers for the early diagnosis of PE.

[Syncytins - retroviral envelope genes captured for the benefit of placental development]

In the past, germline infections by retroviruses have led to vertical transmission of "endogenized" retroviruses. Escaping genetic drift, some of the viral genes have been conserved until now because of beneficial effects on their host. Here we present the syncytin genes that encode envelope proteins from endogenous retroviruses. Syncytins have inherited fusogenic properties from their infectious ancestor and are specifically expressed in the placenta. Both properties have suggested their involvement in the formation of the syncytiotrophoblast, a multinucleated layer that mediates feto-maternal exchanges in the placenta. The capture of syncytin genes occurred on several independent occasions during evolution of mammals. Knock-out experiments of syncytins in the mouse definitively confirmed the role of these genes in placentation. Finally, a second function for syncytins, i.e. an immunosuppressive activity, could contribute to materno-fetal immune tolerance. This constitutes a remarkable example of convergent evolution where the properties of retroviral envelope genes are subverted to play a major physiological role.

Human trophoblast in trisomy 21: a model for cell-cell fusion dynamic investigation

Trophoblastic cell fusion is one essential step of the human trophoblast differentiation leading to formation of the syncytiotrophoblast, site of the numerous placental functions. This process is multifactorial and finely regulated. Using the physiological model of primary culture of trophoblastic cells isolated from human placenta, we have identified different membrane proteins directly involved in trophoblastic cell fusion: connexin 43, ZO-1 and recently syncytins. These fusogenic membrane retroviral envelop glycoproteins: syncytin-1 (encoded by the HERV-W gene) and syncytin-2 (encoded by the FRD gene) and their receptors are major factors involved in human placental development. Disturbances of syncytiotrophoblast formation are observed in trisomy 21-affected placentas. Overexpression of the copper/zinc superoxide dismutase (SOD-1), encoded by chromosome 21 as well as an abnormal hCG signaling are implicated in the defect of syncytiotrophoblast formation. This abnormal trophoblast fusion and differentiation in trisomy 21-affected placenta is reversible in vitro by different ways.

Functional characterization of the human placental fusogenic membrane protein syncytin 2

Fusion of cytotrophoblasts into the multinucleated syncytiotrophoblast layer is essential for the development of a functional placenta. The envelope protein of a human endogenous retrovirus W (HERV-W) family member, syncytin 1, has been shown to mediate placental cell fusion. Recently, the envelope protein of another HERV family member (HERV-FRD), syncytin 2, has been identified and shown to be highly expressed in the placenta. To better understand the biology of syncytin 2, in this study we first investigated syncytin 2 gene expression in normal and preeclamptic placentas and then characterized the functions of syncytin 2. The expression of syncytin 2 gene was decreased in preeclamptic placentas and could be stimulated by the cAMP stimulant forskolin. The endoprotease furin was found to be involved in the posttranslational cleavage of syncytin 1 and 2 polypeptides into surface and transmembrane subunits. In addition, proper association of the subunits of syncytins 1 and 2 is probably required for the functional integrity of each protein, because subunit swapping of syncytins 1 and 2 failed to generate fusogenic chimeras. Finally, we demonstrated that the disulfide bridge-forming CX(2)C and CX(7)C motifs found in syncytins 1 and 2 are essential for their fusogenic activities, because mutations in the CX(2)C motif not only abolished fusogenesis but also functioned as dominant-negative mutants. Our results suggest that syncytin 2 may function as a second fusogenic protein for placental cell fusion.