The role of human endogenous retroviruses in trophoblast differentiation and placental development

Cadmium exposure causes trophoblast abnormal syncytization and endocrine dysfunction in preeclampsia

INTRODUCTION

Cadmium exposure increases the risk of preeclampsia. Present study aimed to explore the mechanism of abnormal syncytization of trophoblasts caused by cadmium exposure and its relationship with preeclampsia.

METHODS

RT-PCR, Western blot and Hematoxylin-Eosin were used to detect syncytization in placenta. BeWo cells were treated with forskolin (50 μM) and CdCl2 (20 μM) to verify the mechanism through which cadmium exposure led to abnormal syncytization and cadmium exposure subsequently caused endocrine dysfunction of syncytiotrophoblast.

RESULTS

Compared with normotension control, abnormal accumulation of syncytial knots was found in placenta of preeclampsia. Cell fusion rate was increased and gene expressions of GCM1, SYN-1, SYN-2, PLGF and β-hCG were elevated in FSK-treated cells. In BeWo cells co-treated with FSK and CdCl2 and in BeWo cells co-treated with FSK and GCM1 siRNA, cell fusion rate was decreased and gene expression of SYN-2 was reduced as compared with FSK-treated cells. Nuclear translocation level of GCM1 was lower in placenta of preeclampsia and BeWo cells co-treated with FSK and CdCl2. The mRNA level of GCM1 and β-hCG were decreased in CdCl2 treated JEG3 cells. The expression of sFlt1 was increased and the expression of PLGF was decreased in placenta of preeclampsia, CdCl2 treated and GCM1 siRNA transfected JEG3 cells. Compared with cells treated with CdCl2 and GCM1 siRNA, co-treatment with CdCl2 and GCM1 siRNA resulted in decreased expressions of β-hCG, PLGF and GCM1 and increased expression of sFlt1 in JEG3 cells.

CONCLUSION

The results indicated that cadmium exposure during pregnancy led to abnormal syncytization and endocrine dysfunction of trophoblasts and were related to the occurrence of preeclampsia.

Disruption of maternal vascular remodeling by a fetal endoretrovirus-derived gene in preeclampsia

BACKGROUND

Preeclampsia, one of the most lethal pregnancy-related diseases, is associated with the disruption of uterine spiral artery remodeling during placentation. However, the early molecular events leading to preeclampsia remain unknown.

RESULTS

By analyzing placentas from preeclampsia, non-preeclampsia, and twin pregnancies with selective intrauterine growth restriction, we show that the pathogenesis of preeclampsia is attributed to immature trophoblast and maldeveloped endothelial cells. Delayed epigenetic reprogramming during early extraembryonic tissue development leads to generation of excessive immature trophoblast cells. We find reduction of de novo DNA methylation in these trophoblast cells results in selective overexpression of maternally imprinted genes, including the endoretrovirus-derived gene PEG10 (paternally expressed gene 10). PEG10 forms virus-like particles, which are transferred from the trophoblast to the closely proximate endothelial cells. In normal pregnancy, only a low amount of PEG10 is transferred to maternal cells; however, in preeclampsia, excessive PEG10 disrupts maternal vascular development by inhibiting TGF-beta signaling.

CONCLUSIONS

Our study reveals the intricate epigenetic mechanisms that regulate trans-generational genetic conflict and ultimately ensure proper maternal-fetal interface formation.

Critically short telomeres derepress retrotransposons to promote genome instability in embryonic stem cells

Telomeres, at the ends of chromosomes, protect chromosomes from fusion and preserve genomic stability. However, the molecular mechanisms underlying telomere attrition-induced genome instability remain to be understood. We systematically analyzed the expression of retrotransposons and performed genomic sequencing of different cell and tissue types with telomeres of varying lengths due to telomerase deficiency. We found that critically short telomeres altered retrotransposon activity to promote genomic instability in mouse embryonic stem cells, as evidenced by elevated numbers of single nucleotide variants, indels and copy number variations (CNVs). Transpositions of retrotransposons such as LINE1 resulting from the short telomeres can also be found in these genomes with elevated number of mutations and CNVs. Retrotransposon activation is linked to increased chromatin accessibility, and reduced heterochromatin abundance correlates with short telomeres. Re-elongation of telomeres upon recovery of telomerase partly represses retrotransposons and heterochromatin accumulation. Together, our findings suggest a potential mechanism by which telomeres maintain genomic stability by suppressing chromatin accessibility and retrotransposon activity.

The Regulation and Functions of Endogenous Retrovirus in Embryo Development and Stem Cell Differentiation

Endogenous retroviruses (ERVs) are repetitive sequences in the genome, belonging to the retrotransposon family. During the course of life, ERVs are associated with multiple aspects of chromatin and transcriptional regulation in development and pathological conditions. In mammalian embryos, ERVs are extensively activated in early embryo development, but with a highly restricted spatial-temporal pattern; and they are drastically silenced during differentiation with exceptions in extraembryonic tissue and germlines. The dynamic activation pattern of ERVs raises questions about how ERVs are regulated in the life cycle and whether they are functionally important to cell fate decision during early embryo and somatic cell development. Therefore, in this review, we focus on the pieces of evidence demonstrating regulations and functions of ERVs during stem cell differentiation, which suggests that ERV activation is not a passive result of cell fate transition but the active epigenetic and transcriptional regulation during mammalian development and stem cell differentiation.

Silencing and Transcriptional Regulation of Endogenous Retroviruses: An Overview

Almost half of the human genome is made up of transposable elements (TEs), and about 8% consists of endogenous retroviruses (ERVs). ERVs are remnants of ancient exogenous retrovirus infections of the germ line. Most TEs are inactive and not detrimental to the host. They are tightly regulated to ensure genomic stability of the host and avoid deregulation of nearby gene loci. Histone-based posttranslational modifications such as H3K9 trimethylation are one of the main silencing mechanisms. Trim28 is one of the identified master regulators of silencing, which recruits most prominently the H3K9 methyltransferase Setdb1, among other factors. Sumoylation and ATP-dependent chromatin remodeling factors seem to contribute to proper localization of Trim28 to ERV sequences and promote Trim28 interaction with Setdb1. Additionally, DNA methylation as well as RNA-mediated targeting of TEs such as piRNA-based silencing play important roles in ERV regulation. Despite the involvement of ERV overexpression in several cancer types, autoimmune diseases, and viral pathologies, ERVs are now also appreciated for their potential positive role in evolution. ERVs can provide new regulatory gene elements or novel binding sites for transcription factors, and ERV gene products can even be repurposed for the benefit of the host.

Dynamics of trophoblast differentiation in peri-implantation-stage human embryos

Single-cell RNA sequencing of cells from cultured human blastocysts has enabled us to define the transcriptomic landscape of placental trophoblast (TB) that surrounds the epiblast and associated embryonic tissues during the enigmatic day 8 (D8) to D12 peri-implantation period before the villous placenta forms. We analyzed the transcriptomes of 3 early placental cell types, cytoTB (CTB), syncytioTB (STB), and migratoryTB (MTB), picked manually from cultured embryos dissociated with trypsin and were able to follow sublineages that emerged from proliferating CTB at the periphery of the conceptus. A unique form of CTB with some features of STB was detectable at D8, while mature STB was at its zenith at D10. A form of MTB with a mixed MTB/CTB phenotype arose around D10. By D12, STB generation was in decline, CTB had entered a new phase of proliferation, and mature MTB cells had begun to move from the main body of the conceptus. Notably, the MTB transcriptome at D12 indicated enrichment of transcripts associated with IFN signaling, migration, and invasion and up-regulation of HLA-C, HLA-E, and HLA-G. The STB, which is distinct from the STB of later villous STB, had a phenotype consistent with intense protein export and placental hormone production, as well as migration and invasion. The studies show that TB associated with human embryos is in rapid developmental flux during peri-implantation period when it must invade, signal robustly to the mother to ensure that the pregnancy continues, and make first contact with the maternal immune system.

Close to the Bedside: A Systematic Review of Endogenous Retroviruses and Their Impact in Oncology

BACKGROUND

Human endogenous retroviruses (HERVs) are genetic elements in the human genome, which resulted from ancient retroviral germline infections. HERVs have strong transcriptional promoters and enhancers that affect a cell's transcriptome. They also encode proteins that can exert effects in human cells. This review examines how our increased understanding of HERVs have led to their potential use as biomarkers and immunologic targets.

MATERIAL AND METHODS

PubMed/Medline, Embase, Web of Science, and Cochrane databases were used in a systematic search to identify all articles studying the potential impact of HERVs on surgical diseases. The search included studies that involved clinical patient samples in diseases including cancer, inflammatory conditions, and autoimmune disease. Articles focused on conditions not routinely managed by surgeons were excluded.

RESULTS

Eighty six articles met inclusion and quality criteria for this review and were included. Breast cancer and melanoma have robust evidence regarding the use of HERVs as potential tumor markers and immunologic targets. Reported evidence of the activity of HERVs in colorectal cancer, pancreatic cancer, hepatocellular cancer, prostate and ovarian cancer, germ cell tumors as well as idiopathic pulmonary hypertension, and the inflammatory response in burns was also reviewed.

CONCLUSIONS

Increasingly convincing evidence indicates that HERVs may play a role in solid organ malignancy and present important biomarkers or immunologic targets in multiple cancers. Innovative investigation of HERVs is a valuable focus of translational research and can deepen our understanding of cellular physiology and the effects of endogenous retroviruses on human biology. As strategies for treatment continue to focus on genome-based interventions, understanding the impact of endogenous retroviruses on human disease will be critical.

Exaptation at the molecular genetic level

The realization that body parts of animals and plants can be recruited or coopted for novel functions dates back to, or even predates the observations of Darwin. S.J. Gould and E.S. Vrba recognized a mode of evolution of characters that differs from adaptation. The umbrella term aptation was supplemented with the concept of exaptation. Unlike adaptations, which are restricted to features built by selection for their current role, exaptations are features that currently enhance fitness, even though their present role was not a result of natural selection. Exaptations can also arise from nonaptations; these are characters which had previously been evolving neutrally. All nonaptations are potential exaptations. The concept of exaptation was expanded to the molecular genetic level which aided greatly in understanding the enormous potential of neutrally evolving repetitive DNA-including transposed elements, formerly considered junk DNA-for the evolution of genes and genomes. The distinction between adaptations and exaptations is outlined in this review and examples are given. Also elaborated on is the fact that such distinctions are sometimes more difficult to determine; this is a widespread phenomenon in biology, where continua abound and clear borders between states and definitions are rare.

An endogenous adeno-associated virus element in elephants

An endogenous viral element derived from adeno-associated virus containing a nearly intact open reading frame (ORF) of the rep gene (enAAV-rep) has been identified in the genomes of various mammals including degu and African elephant. Particularly, in degu, mRNA expression of enAAV-rep has been observed specifically in the liver. Here we newly identified enAAV-rep in Asian elephant and rock hyrax, both of which are afrotherians. The enAAV-rep of African and Asian elephants appeared to be orthologous and originated from an integration event of the entire genome of AAV into the ancestral genome of elephants more than 6 million years ago, whereas that of rock hyrax appeared to have originated independently. Negative selection operating at the amino acid sequence level was detected for the ORF of enAAV-rep in elephants. As in degu, mRNA expression of enAAV-rep was specifically observed in the liver in Asian elephant. Integrations of enAAV-rep appeared to have occurred independently on the evolutionary lineages of elephants and degu, suggesting that the AAV Rep protein has been co-opted repeatedly in the mammalian liver.

Endogenous Retrovirus 3 - History, Physiology, and Pathology

Endogenous viral elements (EVE) seem to be present in all eukaryotic genomes. The composition of EVE varies between different species. The endogenous retrovirus 3 (ERV3) is one of these elements that is present only in humans and other Catarrhini. Conservation of ERV3 in most of the investigated Catarrhini and the expression pattern in normal tissues suggest a putative physiological role of ERV3. On the other hand, ERV3 has been implicated in the pathogenesis of auto-immunity and cancer. In the present review we summarize knowledge about this interesting EVE. We propose the model that expression of ERV3 (and probably other EVE loci) under pathological conditions might be part of a metazoan SOS response.

Endogenous retroviruses mobilized during friend murine leukemia virus infection

We have demonstrated in a mouse model that infection with a retrovirus can lead not only to the generation of recombinants between exogenous and endogenous gammaretrovirus, but also to the mobilization of endogenous proviruses by pseudotyping entire polytropic proviral transcripts and facilitating their infectious spread to new cells. However, the frequency of this occurrence, the kinetics, and the identity of mobilized endogenous proviruses was unclear. Here we find that these mobilized transcripts are detected after only one day of infection. They predominate over recombinant polytropic viruses early in infection, persist throughout the course of disease and are comprised of multiple different polytropic proviruses. Other endogenous retroviral elements such as intracisternal A particles (IAPs) were not detected. The integration of the endogenous transcripts into new cells could result in loss of transcriptional control and elevated expression which may facilitate pathogenesis, perhaps by contributing to the generation of polytropic recombinant viruses.

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.

Expression of Syncytin 1 (HERV-W), in the preimplantation human blastocyst, embryonic stem cells and trophoblast cells derived in vitro

STUDY QUESTION

As Syncytin 1 (human endogenous retrovirus (HERV-W)) is crucial for human embryo placentation is it expressed during preimplantation embryo development?

SUMMARY ANSWER

Syncytin 1 was expressed mainly in trophoblast cells of the blastocyst particularly in cells underlying the inner cell mass (ICM).

WHAT IS KNOWN ALREADY

Syncytin 1 (along with HERV-FRD or Syncytin 2) is expressed in first-trimester placenta and required for cell-cell fusion to enable formation of syncytiotrophoblast and effective placentation.

STUDY DESIGN, SIZE AND DURATION

Preimplantation human embryos donated for research were cultured in vitro and protein expression of Syncytin 1 at the blastocyst stage of development investigated. Comparisons were made with protein (Syncytin 1) and mRNA (Syncytin 1 and 2) expression in human embryonic stem cells (hESCs) undergoing differentiation to trophoblast-like cells in vitro. In total, 10 blastocysts (×3 or 4 replicates) were analysed and 4 hESC lines. The study was terminated after consistent observations of embryos were made.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Donated embryos were thawed and cultured to blastocyst, fixed with 4% w/v paraformaldehyde. Syncytin 1 protein expression was determined by immunofluorescent localisation and confocal microscopy. Additionally, hESCs were differentiated to trophoblast-like cells in standard and conditioned culture medium with growth factors (bone morphogenetic protein 4 (BMP4) or fibroblast growth factor 4 (FGF4) and assessed for mRNA (Syncytin 1 and 2) by quantitative polymerase chain reaction (qPCR) and protein expression by immunolocalization and western blot.

MAIN RESULTS AND ROLE OF CHANCE

Syncytin 1 was expressed in cytoplasm and on the cell surface of some trophoblast cells, and consistently the trophectoderm underlying the ICM of the blastocyst. There was weak but consistent expression of Syncytin 1 in cells on the periphery of the ICM also displaying pluripotency antibody marker (Tra-1-60). Three-dimensional reconstruction of confocal slice data provided good visualization of expression. The time course of expression of Syncytin 1 was replicated in hESCs differentiated in vitro confirming the embryo observations and providing statistically significant differences in protein and mRNA level (P= 0.002) and (P< 0.05), respectively.

LIMITATION, REASONS FOR CAUTION

Culture of a limited number of embryos to blastocyst in vitro may not replicate the range and quality of development in situ. Probes (antibodies, PCR) were tested for specificity, but might have non-specific reactions.

WIDER IMPLICATIONS OF FINDINGS

Syncytin expression is a prerequisite for embryo implantation and placentation. Understanding when expression first occurs during embryo development may be informative for understanding conditions of abnormal gestations such as pre-clampsia.

STUDY FUNDING/COMPETING INTERESTS

The study was supported partly by an ERASMUS training grant and grant G0801059 from the Medical Research Council, U.K. There were no competing interests.

Molecular functions of human endogenous retroviruses in health and disease

Human endogenous retroviruses (HERVs) and related genetic elements form 504 distinct families and occupy ~8% of human genome. Recent success of high-throughput experimental technologies facilitated understanding functional impact of HERVs for molecular machinery of human cells. HERVs encode active retroviral proteins, which may exert important physiological functions in the body, but also may be involved in the progression of cancer and numerous human autoimmune, neurological and infectious diseases. The spectrum of related malignancies includes, but not limits to, multiple sclerosis, psoriasis, lupus, schizophrenia, multiple cancer types and HIV. In addition, HERVs regulate expression of the neighboring host genes and modify genomic regulatory landscape, e.g., by providing regulatory modules like transcription factor binding sites (TFBS). Indeed, recent bioinformatic profiling identified ~110,000 regulatory active HERV elements, which formed at least ~320,000 human TFBS. These and other peculiarities of HERVs might have played an important role in human evolution and speciation. In this paper, we focus on the current progress in understanding of normal and pathological molecular niches of HERVs, on their implications in human evolution, normal physiology and disease. We also review the available databases dealing with various aspects of HERV genetics.

Inhibiting DNA Methylation Causes an Interferon Response in Cancer via dsRNA Including Endogenous Retroviruses

We show that DNA methyltransferase inhibitors (DNMTis) upregulate immune signaling in cancer through the viral defense pathway. In ovarian cancer (OC), DNMTis trigger cytosolic sensing of double-stranded RNA (dsRNA) causing a type I interferon response and apoptosis. Knocking down dsRNA sensors TLR3 and MAVS reduces this response 2-fold and blocking interferon beta or its receptor abrogates it. Upregulation of hypermethylated endogenous retrovirus (ERV) genes accompanies the response and ERV overexpression activates the response. Basal levels of ERV and viral defense gene expression significantly correlate in primary OC and the latter signature separates primary samples for multiple tumor types from The Cancer Genome Atlas into low versus high expression groups. In melanoma patients treated with an immune checkpoint therapy, high viral defense signature expression in tumors significantly associates with durable clinical response and DNMTi treatment sensitizes to anti-CTLA4 therapy in a pre-clinical melanoma model.

Recent advances in the study of active endogenous retrovirus envelope glycoproteins in the mammalian placenta

Endogenous retroviruses (ERVs) are a component of the vertebrate genome and originate from exogenous infections of retroviruses in the germline of the host. ERVs have coevolved with their hosts over millions of years. Envelope glycoproteins of endogenous retroviruses are often expressed in the mammalian placenta, and their potential function has aroused considerable research interest, including the manipulation of maternal physiology to benefit the fetus. In most mammalian species, trophoblast fusion in the placenta is an important event, involving the formation of a multinucleated syncytiotrophoblast layer to fulfill essential fetomaternal exchange functions. The key function in this process derives from the envelope genes of endogenous retroviruses, namely syncytins, which show fusogenic properties and placenta-specific expression. This review discusses the important role of the recognized endogenous retrovirus envelope glycoproteins in the mammalian placenta.

Therapeutic potential of antiviral drugs targeting chemorefractory colorectal adenocarcinoma cells overexpressing endogenous retroviral elements

Background

Endoretroviruses account for circa 8 % of all transposable elements found in the genome of humans and other animals. They represent a genetic footprint of ancestral germ-cell infections of exoviruses that is transmittable to the progeny by Mendelian segregation. Traces of human endogenous retroviruses are physiologically expressed in ovarial, testicular and placental tissues as well as in stem cells. In addition, a number of these fossil viral elements have also been related to carcinogenesis. However, a relation between endoretroviruses expression and chemoresistance has not been reported yet.

Methods

Twenty colorectal carcinoma patient samples were scrutinized for HERV-W_E1 and HERV-FRD₁ endoretroviruses using immunohistochemical approaches. In order to search for differential expression of these elements in chemotherapy refractory cells, a resistant HCT8 colon carcinoma subline was developed by serial etoposide exposure. Endoretroviral elements were detected by immunocytochemical staining, qPCR and ELISA. IC₅₀-values of antiviral and cytostatic drugs in HCT8 cells were determined by MTT proliferation assay. The antivirals-cytostatics interaction was evaluated by the isobologram method.

Results

In this work, we show for the first time that HERV-W_E1, HERV-FRD₁, HERV-3₁, and HERV-V₁ are a) simultaneously expressed in treatment-naïve colon carcinoma cells and b) upregulated after cytostatic exposure, suggesting that these retroviral elements are intimately related to chemotherapy resistance. We found a number of antiviral drugs to have cytotoxic activity and the ability to force the downregulation of HERV proteins in vitro. We also demonstrate that the use of different antiviral compounds alone or in combination with anticancer agents results in a synergistic antiproliferative effect and downregulation of different endoretroviral elements in highly chemotherapy-resistant colorectal tumor cells.

Conclusions

Enhanced HERV-expression is associated with chemoresistance in colon carcinomas which can be overcome by antiviral drugs alone or in combination with anticancer drugs. Therefore, the introduction of antiviral compounds to the current chemotherapy regimens potentially improves patient outcomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-015-0199-5) contains supplementary material, which is available to authorized users.

Human endogenous retrovirus envelope proteins target dendritic cells to suppress T-cell activation

Though mostly defective, human endogenous retroviruses (HERV) can retain open reading frames, which are especially expressed in the placenta. There, the envelope (env) proteins of HERV-W (Syncytin-1), HERV-FRD (Syncytin-2), and HERV-K (HML-2) were implicated in tolerance against the semi-allogenic fetus. Here, we show that the known HERV env-binding receptors ASCT-1 and -2 and MFSD2 are expressed by DCs and T-cells. When used as effectors in coculture systems, CHO cells transfected to express Syncytin-1, -2, or HML-2 did not affect T-cell expansion or overall LPS-driven phenotypic DC maturation, however, promoted release of IL-12 and TNF-α rather than IL-10. In contrast, HERV env expressing choriocarcinoma cell lines suppressed T-cell proliferation and LPS-induced TNF-α and IL-12 release, however, promoted IL-10 accumulation, indicating that these effects might not rely on HERV env interactions. However, DCs conditioned by choriocarcinoma, but also transgenic CHO cells failed to promote allogenic T-cell expansion. This was associated with a loss of DC/T-cell conjugate frequencies, impaired Ca(2+) mobilization, and aberrant patterning of f-actin and tyrosine phosphorylated proteins in T-cells. Altogether, these findings suggest that HERV env proteins target T-cell activation indirectly by modulating the stimulatory activity of DCs.

Role of placenta-specific protein 1 in trophoblast invasion and migration

Placenta-specific protein 1 (PLAC1), a placenta-specific gene, is known to be involved in the development of placenta in both humans and mice. However, the precise role of PLAC1 in placental trophoblast function remains unclear. In this study, the localization of PLAC1 in human placental tissues and its physiological significance in trophoblast invasion and migration are investigated by technical studies including real-time RT-PCR, in situ hybridization, immunohistochemistry, and functional studies by utilizing cell invasion and migration assays in the trophoblast cell line HTR8/SVneo as well as the primary inducing extravillous trophoblasts (EVTs). The results show that PLAC1 is mainly detected in the trophoblast columns and syncytiotrophoblast of the first-trimester human placental villi, as well as in the EVTs that invade into the maternal decidua. Knockdown of PLAC1 by RNA interference significantly suppresses the invasion and migration of HTR8/SVneo cells and shortens the distance of the outgrowth of the induced EVTs from the cytotrophoblast column of the explants. All the above data suggests that PLAC1 plays an important role in human placental trophoblast invasion and migration.

Footprint of APOBEC3 on the genome of human retroelements

Almost half of the human genome is composed of transposable elements. The genomic structures and life cycles of some of these elements suggest they are a result of waves of retroviral infection and transposition over millions of years. The reduction of retrotransposition activity in primates compared to that in nonprimates, such as mice, has been attributed to the positive selection of several antiretroviral factors, such as apolipoprotein B mRNA editing enzymes. Among these, APOBEC3G is known to mutate G to A within the context of GG in the genome of endogenous as well as several exogenous retroelements (the underlining marks the G that is mutated). On the other hand, APOBEC3F and to a lesser extent other APOBEC3 members induce G-to-A changes within the nucleotide GA. It is known that these enzymes can induce deleterious mutations in the genome of retroviral sequences, but the evolution and/or inactivation of retroelements as a result of mutation by these proteins is not clear. Here, we analyze the mutation signatures of these proteins on large populations of long interspersed nuclear element (LINE), short interspersed nuclear element (SINE), and endogenous retrovirus (ERV) families in the human genome to infer possible evolutionary pressure and/or hypermutation events. Sequence context dependency of mutation by APOBEC3 allows investigation of the changes in the genome of retroelements by inspecting the depletion of G and enrichment of A within the APOBEC3 target and product motifs, respectively. Analysis of approximately 22,000 LINE-1 (L1), 24,000 SINE Alu, and 3,000 ERV sequences showed a footprint of GG→AG mutation by APOBEC3G and GA→AA mutation by other members of the APOBEC3 family (e.g., APOBEC3F) on the genome of ERV-K and ERV-1 elements but not on those of ERV-L, LINE, or SINE.

Reactivation of codogenic endogenous retroviral (ERV) envelope genes in human endometrial carcinoma and prestages: Emergence of new molecular targets

Endometrial carcinoma (EnCa) is the most common invasive gynaecologic carcinoma. Over 85% of EnCa are classified as endometrioid, expressing steroid hormone receptors and mostly involving pathological prestages. Human endogenous retroviruses (ERV) are chromosomally integrated genes, account for about 8% of the human genome and are implicated in the etiology of carcinomas. The majority of ERV envelope (env) coding genes are either not present or not consistently represented between common gene expression microarrays. The aim of this study was to analyse the absolute gene expression of all known 21 ERV env genes including 19 codogenic and two env genes with premature stop codons in EnCa, endometrium as well as in hyperplasia and polyps. For EnCa seven env genes had high expression with >200 mol/ng cDNA (e.g. envH1-3, Syncytin-1, envT), two middle >50 mol/ng cDNA (envFc2, erv-3) and 12 low <50 mol/ng cDNA (e.g. Syncytin-2, envV2). Regarding tumor parameters, Syncytin-1 and Syncytin-2 were significantly over-expressed in advanced stage pT2 compared to pT1b. In less differentiated EnCa Syncytin-1, erv-3, envT and envFc2 were significantly over-expressed. Syncytin-1, Syncytin-2 and erv-3 were specific to glandular epithelial cells of polyps, hyperplasia and EnCa using immunohistochemistry. An analysis of 10 patient-matched EnCa with endometrium revealed that the ERV-W 5' long terminal repeat regulating Syncytin-1 was hypomethylated, including the ERE and CRE overlapping MeCP2 sites. Functional analyses showed that 10 env genes were regulated by methylation in EnCa using the RL95-2 cell line. In conclusion, over-expressed env genes could serve as indicators for pathological pre-stages and EnCa.

Human endogenous retroviruses and cancer prevention: evidence and prospects

Background

Cancer is a significant and growing problem worldwide. While this increase may, in part, be attributed to increasing longevity, improved case notifications and risk-enhancing lifestyle (such as smoking, diet and obesity), hygiene-related factors resulting in immuno-regulatory failure may also play a major role and call for a revision of vaccination strategies to protect against a range of cancers in addition to infections.

Discussion

Human endogenous retroviruses (HERVs) are a significant component of a wider family of retroelements that constitutes part of the human genome. They were originated by the integration of exogenous retroviruses into the human genome millions of years ago. HERVs are estimated to comprise about 8% of human DNA and are ubiquitous in somatic and germinal tissues.

Physiologic and pathologic processes are influenced by some biologically active HERV families. HERV antigens are only expressed at low levels by the host, but in circumstances of inappropriate control their genes may initiate or maintain pathological processes. Although the precise mechanism leading to abnormal HERVs gene expression has yet to be clearly elucidated, environmental factors seem to be involved by influencing the human immune system.

HERV-K expression has been detected in different types of tumors.

Among the various human endogenous retroviral families, the K series was the latest acquired by the human species. Probably because of its relatively recent origin, the HERV-K is the most complete and biologically active family.

The abnormal expression of HERV-K seemingly triggers pathological processes leading to melanoma onset, but also contributes to the morphological and functional cellular modifications implicated in melanoma maintenance and progression.

The HERV-K-MEL antigen is encoded by a pseudo-gene incorporated in the HERV-K env-gene. HERV-K-MEL is significantly expressed in the majority of dysplastic and normal naevi, as well as other tumors like sarcoma, lymphoma, bladder and breast cancer. An amino acid sequence similar to HERV-K-MEL, recognized to cause a significant protective effect against melanoma, is shared by the antigenic determinants expressed by some vaccines such as BCG, vaccinia virus and the yellow fever virus.

HERV-K are also reactivated in the majority of human breast cancers. Monoclonal and single-chain antibodies against the HERV-K Env protein recently proved capable of blocking the proliferation of human breast cancer cells in vitro, inhibiting tumor growth in mice bearing xenograft tumors.

Summary

A recent epidemiological study provided provisional evidence of how melanoma risk could possibly be reduced if the yellow fever virus vaccine (YFV) were received at least 10 years before, possibly preventing tumor initiation rather than culling melanoma cells already compromised. Further research is recommended to confirm the temporal pattern of this protection and eliminate/attenuate the potential role of relevant confounders as socio-economic status and other vaccinations.

It appears also appropriate to examine the potential protective effect of YFV against other malignancies expressing high levels of HERV-K antigens, namely breast cancer, sarcoma, lymphoma and bladder cancer.

Tumor immune-therapy, as described for the monoclonal antibodies against breast cancer, is indeed considered more complex and less advantageous than immune-prevention. Cellular immunity possibly triggered by vaccines as for YFV might also be involved in anti-cancer response, in addition to humoral immunity.

Retroelement demethylation associated with abnormal placentation in Mus musculus x Mus caroli hybrids

The proper functioning of the placenta requires specific patterns of methylation and the appropriate regulation of retroelements, some of which have been co-opted by the genome for placental-specific gene expression. Our inquiry was initiated to determine the causes of the placental defects observed in crosses between two species of mouse, Mus musculus and Mus caroli. M. musculus × M. caroli fetuses are rarely carried to term, possibly as a result of genomic incompatibility in the placenta. Taking into account that placental dysplasia is observed in Peromyscus and other Mus hybrids, and that endogenous retroviruses are expressed in the placental transcriptome, we hypothesized that these placental defects could result, in part, from failure of the genome defense mechanism, DNA methylation, to regulate the expression of retroelements. Hybrid M. musculus × M. caroli embryos were produced by artificial insemination, and dysplastic placentas were subjected to microarray and methylation screens. Aberrant overexpression of an X-linked Mus retroelement in these hybrid placentas is consistent with local demethylation of this retroelement, concomitant with genome instability, disruption of gene regulatory pathways, and dysgenesis. We propose that the placenta is a specific site of control that is disrupted by demethylation and retroelement activation in interspecific hybridization that occur as a result of species incompatibility of methylation machinery. To our knowledge, the present data provide the first report of retroelement activation linked to decreased methylation in a eutherian hybrid system.

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.

RhoE is regulated by cyclic AMP and promotes fusion of human BeWo choriocarcinoma cells

Fusion of placental villous cytotrophoblasts with the overlying syncytiotrophoblast is essential for the maintenance of successful pregnancy, and disturbances in this process have been implicated in pathological conditions such as pre-eclampsia and intra-uterine growth retardation. In this study we examined the role of the Rho GTPase family member RhoE in trophoblast differentiation and fusion using the BeWo choriocarcinoma cell line, a model of villous cytotrophoblast fusion. Treatment of BeWo cells with the cell permeable cyclic AMP analogue dibutyryl cyclic AMP (dbcAMP) resulted in a strong upregulation of RhoE at 24h, coinciding with the onset of fusion. Using the protein kinase A (PKA)-specific cAMP analogue N⁶-phenyl-cAMP, and a specific inhibitor of PKA (14–22 amide, PKI), we found that upregulation of RhoE by cAMP was mediated through activation of PKA signalling. Silencing of RhoE expression by RNA interference resulted in a significant decrease in dbcAMP-induced fusion. However, expression of differentiation markers human chorionic gonadotrophin and placental alkaline phosphatase was unaffected by RhoE silencing. Finally, we found that RhoE upregulation by dbcAMP was significantly reduced under hypoxic conditions in which cell fusion is impaired. These results show that induction of RhoE by cAMP is mediated through PKA and promotes BeWo cell fusion but has no effect on functional differentiation, supporting evidence that these two processes may be controlled by separate or diverging pathways.

Genipa americana (Rubiaceae) fruit extract affects mitogen-activated protein kinase cell pathways in human trophoblast-derived BeWo cells: implications for placental development

Genipa americana L. (Rubiaceae) is a fruit tree and a traditional medicine used to treat anemia, icterus, asthma, and liver and spleen problems. The aim of the present study was to verify the effect of G. americana fruit ethanolic extract on the mechanism for proliferation and differentiation of trophoblast-like cells. Qualitative analysis of G. americana fruit extract was performed, and BeWo cells, a well-established placental choriocarcinoma cell line that can undergo differentiation, were used to analyze cell viability and proliferation. Methods consisted of cytotoxic and proliferation measurement, detection of release of human chorionic gonadotrophins, cell fusion observation, and evaluation of cell-signaling pathways (production of cyclic adenosine monophosphate and phosphorylation of mitogen-activated protein kinases [MAPKs]). A stock solution of the extract was diluted in Ham's F-12 medium with 10% fetal bovine serum at concentrations ranging from 50 to 1000 μg/mL. Cells treated with dimethylsulfoxide, forskoline, and MAPK inhibitors (PD98059 or SB203580) were used as a control. Forskoline was used to induce the differentiation state in BeWo cells. Phytoanalysis indicated the presence of steroids only. Results showed that the G. americana fruit extract did not cause any cytotoxicity or interference in cell differentiation. However, a significant antiproliferative state related to inhibition and reactivation of ERK1/2 and p38 MAPK in BeWo cells was seen. These results suggest that steroids from G. americana may affect placental cell regulation.

Trophoblast fusion

The villous trophoblast of the human placenta is the epithelial cover of the fetal chorionic villi floating in maternal blood. This epithelial cover is organized in two distinct layers, the multinucleated syncytiotrophoblast directly facing maternal blood and a second layer of mononucleated cytotrophoblasts. During pregnancy single cytotrophoblasts continuously fuse with the overlying syncytiotrophoblast to preserve this end-differentiated layer until delivery. Syncytial fusion continuously supplies the syncytiotrophoblast with compounds of fusing cytotrophoblasts such as proteins, nucleic acids and lipids as well as organelles. At the same time the input of cytotrophoblastic components is counterbalanced by a continuous release of apoptotic material from the syncytiotrophoblast into maternal blood. Fusion is an essential step in maintaining the syncytiotrophoblast. Trophoblast fusion was shown to be dependant on and regulated by multiple factors such as fusion proteins, proteases and cytoskeletal proteins as well as cytokines, hormones and transcription factors. In this chapter we focus on factors that may be involved in the fusion process of trophoblast directly or that may prepare the cytotrophoblast to fuse.

Melanoma, Darwinian medicine and the inner world

INTRODUCTION

A diverse range of human diseases, including allergy, asthma, autoimmune disease, cancer and chronic neurologic diseases, notably multiple sclerosis and endogenous depression, is becoming more prevalent in industrialized countries. It has been postulated that environmental factors associated with improved standards of hygiene play a leading role in this process since the immune system seems to need extrinsic challenges for its proper maturation.

THE INNER WORLD

An added dimension has now emerged--the impact on disease of the inner world, principally the numerous endogenous retroviruses (HERVs) within the human genome. Taking melanoma as an example, we propose a framework for understanding how a complex infectious and immunological background can induce or inhibit expression of a HERV-related disease process. The central role of a failure to induce or to maintain certain populations of self-specific CD8(+) T-cells mediating immune surveillance, the expression of HERV-encoded peptides on affected cells and pathological mechanisms directly attributable to HERV proteins are discussed.

CONCLUSIONS

The presented concepts explain events preceding the clinical manifestation of diseases by several years and provide a rationale for the use of currently available vaccines to protect against certain HERV-induced diseases, especially melanoma. Criteria for establishing the causal role of HERVs in a given disease are proposed.

Retrotransposon silencing by DNA methylation contributed to the evolution of placentation and genomic imprinting in mammals

The evolution of two mammalian-specific traits, viviparous reproduction with a placenta and genomic imprinting, have been addressed by multiple studies of two retrotransposon derived, mammalian-specific genes. These gene targeting experiments in mice, together with recent comparative genomic analyses among three mammalian groups, suggest that extremely rare events; namely exaptations from retrotransposons, made crucial contributions to the establishment and diversification of mammals via placental formation. We propose that nearly neutral evolution, as well as Darwinian evolution (natural selection), plays an important role in the exaptation process. Comparative genomic analysis of various imprinted regions has also revealed that an imprinting control element essential for parent-of-origin specific monoallelic expression of imprinted genes emerged in each of the imprinted regions, possibly by the insertion of exogenous DNAs, such as retrotransposons. In both cases, DNA methylation in germ cells must have been of critical importance to repress the exogenous DNAs inserted into the genomes of mammalian ancestors. We propose that the ability of germ line DNA methylation enabled the emergence of certain mammalian-specific features during the course of evolution.

Endogenous retroviruses in trophoblast differentiation and placental development

Endogenous retroviruses (ERVs) are present in the genome of all vertebrates and originated from infections of the germline of the host by exogenous retroviruses. ERVs have coevolved with their hosts for millions of years and are recognized to contribute to genome plasticity, protect the host against infection of related pathogenic and exogenous retroviruses, and play a vital role in development of the placenta. Consequently, some ERVs have been positively selected and maintained in the host genome throughout evolution. This review will focus on the critical role of ERVs in development of the mammalian placenta and specifically highlight the biological role of sheep JSRV-related endogenous betaretroviruses in conceptus (embryo and associated extraembryonic membranes) development.

Downregulation of caveolin-1 enhances fusion of human BeWo choriocarcinoma cells

Background

Fusion of placental villous cytotrophoblasts with the overlying syncytiotrophoblast is essential for the maintenance of successful pregnancy, and disturbances in this process have been implicated in pathological conditions such as pre-eclampsia and intra-uterine growth retardation. Caveolin-1 has been shown to be expressed in human villous cytotrophoblast and to be downregulated during fusion into syncytiotrophoblast but it is unclear whether it plays a role in this process.

Methodology/Principal Findings

We used RNA interference to determine whether caveolin-1 plays a role in differentiation and fusion in the BeWo choriocarcinoma cell line, a model of villous cytotrophoblast fusion. Assessment of cell fusion by desmosomal protein immunostaining revealed that cells transfected with caveolin-1 siRNA showed significantly enhanced fusion in response to treatment with dibutyryl cyclic AMP compared with cells transfected with a non-silencing control. Furthermore, caveolin-1 knockdown alone was sufficient to promote spontaneous fusion. In addition, biochemical differentiation, assessed by expression of placental alkaline phosphatase, was upregulated in caveolin-1 siRNA-transfected cells, with or without dbcAMP treatment. Assessment of Akt phosphorylation showed that caveolin-1 knockdown resulted in a significant reduction in phosphorylation at Thr³⁰⁸.

Conclusions/Significance

Taken together, these results suggest that caveolin-1 regulates BeWo cell differentiation and fusion, possibly through a mechanism involving modulation of Akt activity.

The proprotein convertase furin in human trophoblast: Possible role in promoting trophoblast cell migration and invasion

Furin, a proprotein convertase (PC), is ubiquitously expressed and implicated in many physiological and pathological processes. This study is aimed to identify the role of furin in human trophoblast invasion and migration. Furin was found to be highly expressed in placental villi of both rhesus monkeys and human beings during early pregnancy. Specifically, furin was found in trophoblast column and trophoblast shell, regions where highly invasive cytotrophoblast cells invade the maternal decidua during human placentation. To determine whether furin plays any role in trophoblast invasion and migration, we employed human extravillous HTR8/SVneo cells in Matrigel invasion and transwell migration assays. Knocking-down furin expression by siRNA significantly inhibited invasion and migration of HTR8/SVneo cells (P<0.01), with corresponding decrease of matrix metalloproteinase-9 (MMP-9) activities. In contrast, over-expression of furin markedly increased cell invasion and migration (P<0.01), accompanied by significant increase of MMP-9 activities. Furthermore, furin siRNA significantly increased the levels of both tissue inhibitors of MMPs (TIMP)-1 and -2. Our results suggest that furin may play an important role in the invasion and migration of human trophoblast cells during early pregnancy.

Friendly viruses: the special relationship between endogenous retroviruses and their host

Endogenous retroviruses (ERVs) are present in the genome of all vertebrates and have coevolved with their hosts for millions of years. Some ERVs play a critical role in placental development, contribute to genome plasticity, and protect the host against infection of related pathogenic and exogenous retroviruses, thus some ERVs have been positively selected and maintained in the host genome. The sheep genome contains 27 endogenous retroviruses (enJSRVs) related to the pathogenic Jaagsiekte sheep retrovirus (JSRV), the causative agent of a transmissible lung cancer in sheep. enJSRVs are able to protect their host against JSRV infection by blocking different steps of the viral replication cycle. In addition, enJSRVs are absolutely required for sheep placental development. Thus, enJSRVs-JSRV provides a unique and interesting model to study the symbiotic relationship and interplay between host ERVs and evolution. This review will provide some examples of the biological functions of ERVs. In particular, the role of ERVs in reproductive biology and in protecting the host against pathogenic retrovirus infections will be emphasized using enJSRVs/JSRV and the sheep as a model.

Comparative methylation of ERVWE1/syncytin-1 and other human endogenous retrovirus LTRs in placenta tissues

Human endogenous retroviruses (HERVs) are globally silent in somatic cells. However, some HERVs display high transcription in physiological conditions. In particular, ERVWE1, ERVFRDE1 and ERV3, three proviruses of distinct families, are highly transcribed in placenta and produce envelope proteins associated with placenta development. As silencing of repeated elements is thought to occur mainly by DNA methylation, we compared the methylation of ERVWE1 and related HERVs to appreciate whether HERV methylation relies upon the family, the integration site, the tissue, the long terminal repeat (LTR) function or the associated gene function. CpG methylation of HERV-W LTRs in placenta-associated tissues was heterogeneous but a joint epigenetic control was found for ERVWE1 5'LTR and its juxtaposed enhancer, a mammalian apparent LTR retrotransposon. Additionally, ERVWE1, ERVFRDE1 and ERV3 5'LTRs were all essentially hypomethylated in cytotrophoblasts during pregnancy, but showed distinct and stage-dependent methylation profiles. In non-cytotrophoblastic cells, they also exhibited different methylation profiles, compatible with their respective transcriptional activities. Comparative analyses of transcriptional activity and LTR methylation in cell lines further sustained a role for methylation in the control of functional LTRs. These results suggest that HERV methylation might not be family related but copy-specific, and related to the LTR function and the tissue. In particular, ERVWE1 and ERV3 could be developmentally epigenetically regulated HERVs.

Mobilization of endogenous retroviruses in mice after infection with an exogenous retrovirus

Mammalian genomes harbor a large number of retroviral elements acquired as germ line insertions during evolution. Although many of the endogenous retroviruses are defective, several contain one or more intact viral genes that are expressed under certain physiological or pathological conditions. This is true of the endogenous polytropic retroviruses that generate recombinant polytropic murine leukemia viruses (MuLVs). In these recombinants the env gene sequences of exogenous ecotropic MuLVs are replaced with env gene sequences from an endogenous polytropic retrovirus. Although replication-competent endogenous polytropic retroviruses have not been observed, the recombinant polytropic viruses are capable of replicating in numerous species. Recombination occurs during reverse transcription of a virion RNA heterodimer comprised of an RNA transcript from an endogenous polytropic virus and an RNA transcript from an exogenous ecotropic MuLV RNA. It is possible that homodimers corresponding to two full-length endogenous RNA genomes are also packaged. Thus, infection by an exogenous virus may result not only in recombination with endogenous sequences, but also in the mobilization of complete endogenous retrovirus genomes via pseudotyping within exogenous retroviral virions. We report that the infection of mice with an ecotropic virus results in pseudotyping of intact endogenous viruses that have not undergone recombination. The endogenous retroviruses infect and are integrated into target cell genomes and subsequently replicate and spread as pseudotyped viruses. The mobilization of endogenous retroviruses upon infection with an exogenous retrovirus may represent a major interaction of exogenous retroviruses with endogenous retroviruses and may have profound effects on the pathogenicity of retroviral infections.

The evolutionary dynamics of human endogenous retroviral families

The capacity to integrate into the chromosomal DNA of germ-line cells has endowed retroviruses with the potential to be vertically transmitted from generation to generation and eventually become fixed in the genomes of the entire population. This has been independently accomplished by several ancient retroviruses that invaded the genomes of our early and more recent primate and hominoid ancestors. Some of the inherited elements then proliferated in the genome, resulting in a number of lineages with complex phylogenetic patterns. Although the vast majority of chromosomally integrated retroelements have suffered inactivating mutations and deletions, a significant impact on various aspects of human biology has been recently revealed and evidence for the present activity of at least one human endogenous retrovirus family continues to accumulate.

A large number of the human microRNAs target lentiviruses, retroviruses, and endogenous retroviruses

Retroelements (including transposons, retrotransposons, retroviruses, and lentiviruses) make up a significant portion of eukaryotic genomes. Given their ability to mutate genes these mobile elements always present a threat to the integrity of the host genomes. Recent studies have revealed complex molecular mechanisms that silence the mutagenic ability of these RE as well strategically express the pieces of the incorporated RE that are utilized to silence human endogenous retroviruses (HERVs) or invading exogenous retroviruses (IERV). We have hypothesized that small endogenous RNA originally evolved to quell "foreign" IERV-genes and subsequently emerged into elaborate silencing systems that include RNA interference, miRNA-based gene regulation and other gene silencing mechanisms. Here, we present evidence that the replication of complex RE are most likely silenced or regulated by homologous miRNA that are found as a part of the cellular repertoire. We analyzed Homo sapiens miRNAs for possible target genetic sequences in selected HERVs and IERV found in humans and other large primates. We identified several miRNAs that have >80% sequence homology with human HERVs; -L, -W, and -K, and IERV like SIVcpz, HTLV-1, and HTLV-2. We found an inverse correlation between the numbers and relative degree of homology of miRNAs to the relative replication capacity of a specific RE. Therefore, larger numbers of miRNAs with greater degree of homology are found against the least active RE and the least numbers of miRNAs with smaller degree of homology are found against the most active RE (i.e. HERV-K). Implications of these observations in RE disease and therapy are discussed.

Placenta trophoblast fusion

It has been known for more than 150 years that syncytial fusion is a normal feature in biological systems. In humans there are two larger syncytial tissues: skeletal muscles fibers and placental syncytiotrophoblast. Other fusion events take place as well from fertilization of the oocyte to infection of human cells by enveloped viruses (however, the latter does not necessarily lead to syncytium formation).Although knowledge of the fusion process is incomplete, it is clear that membranes do not fuse easily; specific proteins and other factors are required and are selectively activated. In this chapter, we describe the classic proteins, such as the syncytins, assumed to be involved in the fusion process. We also describe other factors that may play roles in the fusion process or in the preparation of the cells to fuse, such as charged phospholipids, divalent cations, and intracellular proteases. Finally, we speculate on why trophoblast cells fuse in vitro and deal with in vitro models of trophoblast fusion and how their fusion rates can be quantified.

Epigenetic activation of the human growth hormone gene cluster during placental cytotrophoblast differentiation

The hGH cluster contains a single human pituitary growth hormone gene (hGH-N) and four placenta-specific paralogs. Activation of the cluster in both tissues depends on 5' remote regulatory elements. The pituitary-specific locus control elements DNase I-hypersensitive site I (HSI) and HSII, located 14.5 kb 5' of the cluster (position -14.5), establish a continuous domain of histone acetylation that extends to and activates hGH-N in the pituitary gland. In contrast, histone modifications in placental chromatin are restricted to the more 5'-remote HSV-HSIII region (kb -28 to -32) and to the placentally expressed genes in the cluster, with minimal modification between these two regions. These data predict distinct modes of hGH cluster gene activation in the pituitary and placenta. Here we used cell culture models to track structural changes at the hGH locus through placental-gene activation. The data revealed that this process was initiated in primary cytotrophoblasts by histone H3K4 di- and trimethylation and H4 acetylation restricted to HSV and to the individual placental-gene repeat (PGR) units within the cluster. Later stages of transcriptional induction were accompanied by enhancement and extension of these modifications and by robust H3 acetylation at HSV, at HSIII, and throughout the placental-gene regions. These data suggested that elements restricted to HSIII-HSV regions and each individual PGR might be sufficient for activation of the hCS genes. This model was tested by comparing hCS transgene expression in the placentas of mouse embryos carrying a full hGH cluster to that in placentas in which the HSIII-HSV region was directly linked to the individual hCS-A PGR unit. The findings indicate that the HSIII-HSV region and the PGR units, although targeted for initial chromatin structural modifications, are insufficient to activate gene expression and that this process is dependent on additional, as-yet-unidentified chromatin determinants.

Pregnancy recognition and conceptus implantation in domestic ruminants: roles of progesterone, interferons and endogenous retroviruses

The present review highlights new information on pregnancy recognition and conceptus development and implantation in sheep with respect to regulation by progesterone, interferons and endogenous retroviruses. After formation of the corpus luteum, progesterone acts on the endometrium and stimulates blastocyst growth and elongation to a filamentous conceptus (embryo/fetus and associated extra-embryonic membranes). The envelope of endogenous retroviruses related to Jaagsiekte sheep retroviruses appears to intrinsically regulate mononuclear trophectoderm cell proliferation and differentiation into trophoblast giant binucleate cells. The mononuclear trophectoderm cells of elongating sheep conceptuses secrete interferon-tau, which acts on the endometrium to prevent development of the luteolytic mechanism by inhibiting transcription of the gene for the oestrogen receptor alpha in the luminal and superficial ductal glandular epithelia. These actions prevent oestrogen-induced transcription of the oxytocin receptor gene and, therefore, oxytocin-induced luteolytic pulses of prostaglandin F2alpha. Progesterone down regulation of its receptors in luminal and glandular epithelia correlates temporally with a reduction in anti-adhesive mucin land induction of secreted galectin 15 (LGALSI5) and secreted phosphoprotein 1, which are proposed to regulate trophectoderm proliferation and adhesion. Interferon-c acts on the endometrial lumenal epithelium to induce WNT7A and to stimulate LGALS 15, cathepsin L and cystatin C, which are candidate regulators of conceptus development and implantation. The number of potential contributors to maternal recognition and establishment of pregnancy continues to grow and this highlights our limited appreciation of the complexity of the key molecules and signal transduction pathways that intersect during these key developmental processes. The goal of improving reproductive efficiency by preventing embryonic losses that occur during the peri-implantation period of pregnancy in domestic ruminants provides the challenge to increase our knowledge of endometrial function and conceptus development.

C-Terminal truncations of syncytin-1 (ERVWE1 envelope) that increase its fusogenicity

Syncytin-1, the envelope protein of ERVWE1, an endogenous retrovirus of the HERV-W family, plays an important role in regulating fusion of the placental trophoblast. At least one of its receptors is expressed on a variety of human cell types. Its ability to fuse cells makes it an attractive candidate molecule in gene therapy against cancer. We studied the relevance of sequences in the cytoplasmic tail of syncytin-1 for inducing cell-cell fusion. We generated a series of C-terminally truncated syncytin-1 variants. Sequences immediately adjacent to the transmembrane region of syncytin-1 were necessary for inducing optimal fusion, whereas the extreme C-terminus of syncytin-1 partially inhibited its fusogenicity. Two variants of syncytin-1, truncated after residues 483 and 515, were significantly hyperfusogenic compared to wild-type syncytin-1. Cellular and cell-surface expression levels of these two variant proteins were similar to those of wild-type syncytin-1. In testing the latter we found that only a very minor portion of recombinantly expressed cellular syncytin-1 was fully mature and expressed on the cell surface. Our results contribute to the understanding of the structure-function relationship of syncytin-1, and might have implications for the use of this molecule in gene therapy.