Syncytin-A and syncytin-B, two fusogenic placenta-specific murine envelope genes of retroviral origin conserved in Muridae

Effects of individually silenced N-glycosylation sites and non-synonymous single-nucleotide polymorphisms on the fusogenic function of human syncytin-2

The placental syncytiotrophoblast, which is formed by the fusion of cytotrophoblast cells, is indispensable for the establishment and maintenance of normal pregnancy. The human endogenous retrovirus envelope glycoprotein syncytin-2 is the most important player in mediating trophoblast cell-cell fusion as a fusogen. We constructed expression plasmids of wild-type and 21 single-amino-acid substitution mutants of syncytin-2, including 10 N-glycosylation sites individually silenced by mutagenizing N to Q, 1 naturally occurring single-nucleotide polymorphism (SNP) N118S that introduced an N-glycosylation site, and another 10 non-synonymous SNPs located within important functional domains. We observed that syncytin-2 was highly fusogenic and that the mutants had different capacities in merging 293T cells. Of the 21 mutants, N133Q, N312Q, N443Q, C46R (in the CXXC motif) and R417H (in the heptad repeat region and immunosuppressive domain) lost their fusogenicity, whereas N332Q, N118S, T367M (in the fusion peptide), V483I (in the transmembrane domain) and T522M (in the cytoplasmic domain) enhanced the fusogenic activity. We also proved that N133, N146, N177, N220, N241, N247, N312, N332 and N443 were all glycosylated in 293T cells. A co-immunoprecipitation assay showed compromised interaction between mutants N443Q, C46R, T367M, R417H and the receptor MFSD2A, whereas N118S was associated with more receptors. We also sequenced the coding sequence of syncytin-2 in 125 severe pre-eclamptic patients and 272 normal pregnant Chinese women. Surprisingly, only 1 non-synonymous SNP T522M was found and the frequencies of heterozygous carriers were not significantly different. Taken together, our results suggest that N-glycans at residues 133, 312, 332 and 443 of syncytin-2 are required for optimal fusion induction, and that SNPs C46R, N118S, T367M, R417H, V483I and T522M can alter the fusogenic function of syncytin-2.

State of the art in cell-cell fusion

Mammalian life begins with a cell-cell fusion event, i.e. the fusion of the spermatozoid with the oocyte and needs further cell-cell fusion processes for the development, growth, and maintenance of tissues and organs over the whole life span. Furthermore, cellular fusion plays a role in infection, cancer, and stem cell-dependent regeneration as well as including an expanded meaning of partial cellular fusion, nanotube formation, and microparticle-cell fusion. The cellular fusion process is highly regulated by proteins which carry the information to organize and regulate membranes allowing the merge of two separate lipid bilayers into one. The regulation of this genetically and epigenetically controlled process is achieved by different kinds of signals leading to communication of fusing cells. The local cellular and extracellular environment additionally initiates specific cell signaling necessary for the induction of the cell-cell fusion process. Common motifs exist in distinct cell-cell fusion processes and their regulation. However, there is specific regulation of different cell-cell fusion processes, e.g. myoblast, placental, osteoclast, and stem cell fusion. Hence, specialized fusion events vary between cell types and species. Molecular mechanisms remain largely unknown, especially limited knowledge is present for cancer and stem cell fusion mechanisms and regulation. More research is necessary for the understanding of cellular fusion processes which can lead to development of new therapeutic strategies grounding on cellular fusion regulation.

The basal chorionic trophoblast cell layer: An emerging coordinator of placenta development

During gestation, fetomaternal exchange occurs in the villous tree (labyrinth) of the placenta. Development of this structure depends on tightly coordinated cellular processes of branching morphogenesis and differentiation of specialized trophoblast cells. The basal chorionic trophoblast (BCT) cell layer that localizes next to the chorioallantoic interface is of critical importance for labyrinth morphogenesis in rodents. Gcm1-positive cell clusters within this layer initiate branching morphogenesis thereby guiding allantoic fetal blood vessels towards maternal blood sinuses. Later these cells differentiate and contribute to the syncytiotrophoblast of the fetomaternal barrier. Additional cells within the BCT layer sustain continued morphogenesis, possibly through a repopulating progenitor population. Several mouse mutants highlight the importance of a structurally intact BCT epithelium, and a growing number of studies addresses its patterning and epithelial architecture. Here, we review and discuss emerging concepts in labyrinth development focussing on the biology of the BCT cell layer.

[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.

Activation and regulation of endogenous retroviral genes in the human pituitary gland and related endocrine tumours

AIMS

Adenohypophysis (AH) hormone-producing cells represent the origin of diverse groups of pituitary adenomas (PA). Deregulation of hypothalamic hormone receptors, growth factors and cAMP signalling have been implicated in the aetiology of PA. Endogenous retroviruses (ERVs) are derived from past exogenous retroviral infections and represent more than 8% of the human genome. Some ERV genes encode open reading frames and produce functional proteins, for example, the ERVW-1 envelope gene Syncytin-1, essential for placentogenesis, but also deregulated in human tumours. Data concerning ERV expression in the AH and related endocrine tumours are missing.

METHODS

Syncytin-1 protein was analysed in normal AH (n = 15) and compared with five PA subtypes (n = 117) by immunohistochemistry. Absolute gene expression of 20 ERV functional envelope genes and ERVW-5 gag was measured. PA tissues were examined for Syncytin-1 and the cAMP signalling marker phospho-CREB-Ser133 using immunohistochemistry. Isolated primary human PA cells were treated with different hormones. Murine embryonic and adult pituitary gland ERV expressions were compared with human AH.

RESULTS

Syncytin-1 protein colocalized with corticotropic cells of AH. In contrast, all PA demonstrated significant Syncytin-1 protein overexpression, supporting deregulation. All other ERV genes showed significant up-regulations in different PA subtypes. Phospho-CREB-Ser133 and Syncytin-1 colocalized in PA cells. Cultivated primary PA cells with ACTH or CRH induced their respective receptors and ERV genes. Syncytin-A/-B, murine orthologues to human Syncytin-1/-2, localized to embryonic and adult pituitary glands demonstrating functional mammalian conservation.

CONCLUSIONS

Deregulated ERV genes may contribute to PA development via cAMP signalling.

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.

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.

Induction of ovine trophoblast cell fusion by fematrin-1 in vitro

Endogenous retroviruses present in the genomes take a specific role in placental formation in various vertebrates, including bovine and sheep. Fematrin-1, which is the envelope (Env) protein of bovine endogenous retrovirus found in bovine placenta, is involved in the formation of fetomaternal hybrid cells in cattle placenta. This study was conducted to clarify whether fematrin-1 possesses fusogenic activity in trophoblast cells. Another question is whether Env proteins only have species-specific activity or not. For this, fematrin-1 gene was transfected in ovine trophoblast cells, and we examined fusogenic activity with Cos-7 cells. Although fematrin-1 fusogenic activity was detected in both neutral and acidic pH conditions, acidic condition significantly enhanced it. These activities were rather weaker than those of vesicular stomatitis virus G protein as a positive control. However, the ratio of fematrin-1 and vesicular stomatitis virus G protein fusion index was confirmed similar to those in the previous reports. Some fusion cells showed multinucleate cells. These results imply that fematrin-1 is involved in the formation of trophoblast hybrid cells even in different species trophoblastic cells.

Conceptus implantation and placentation: molecules related to epithelial-mesenchymal transition, lymphocyte homing, endogenous retroviruses, and exosomes

Processes of conceptus implantation and placentation, unique to mammalian reproduction, have been extensively studied. It was once thought that processes of these events varied greatly, notably between invasive and noninvasive modes of implantation and/or placentation. Regardless of the mode of implantation, however, physiological and biochemical processes in conceptus implantation to the maternal endometrium including the kinds of gene expression and their products are now considered not to differ so much. Recent progress has identified that in addition to the hormones, cytokines, proteases and cell adhesion molecules classically characterized, epithelial-mesenchymal transition, molecules related to lymphocyte homing, the expression of endogenous retroviruses and possibly exosomes are all required for the progression of conceptus implantation to placentation. In this review, therefore, new findings related to these events are integrated into the context of conceptus implantation to the maternal endometrium.

The Roles of Syncytin-Like Proteins in Ruminant Placentation

Recent developments in genome sequencing techniques have led to the identification of huge numbers of endogenous retroviruses (ERV) in various mammals. ERVs, which occupy 8%–13% of mammalian genomes, are believed to affect mammalian evolution and biological diversity. Although the functional significance of most ERVs remains to be elucidated, several ERVs are thought to have pivotal roles in host physiology. We and other groups recently identified ERV envelope proteins (e.g., Fematrin-1, Syncytin-Rum1, endogenous Jaagsiekte sheep retrovirus Env) that may determine the morphogenesis of the unique fused trophoblast cells, termed trinucleate cells and syncytial plaques, found in ruminant placentas; however, there are still a number of outstanding issues with regard to the role of ERVs that remain to be resolved. Here, we review what is known about how these ERVs have contributed to the development of ruminant-specific trophoblast cells.

Involvement of nephrin in human placental trophoblast syncytialization

The placenta has numerous functions, such as transporting oxygen and nutrients and building the immune tolerance of the fetus. Cell fusion is an essential process for placental development and maturation. In human placental development, mononucleated cytotrophoblast (CTB) cells can fuse to form a multinucleated syncytiotrophoblast (STB), which is the outermost layer of the placenta. Nephrin is a transmembrane protein that belongs to the Ig superfamily. Previous studies have shown that nephrin contributes to the fusion of myoblasts into myotubes in zebrafish and mice, presenting a functional conservation with its Drosophila ortholog sticks and stones. However, whether nephrin is involved in trophoblast syncytialization remains unclear. In this study, we report that nephrin was localized predominantly in the CTB cells and STB of human placenta villi from first trimester to term pregnancy. Using a spontaneous fusion model of primary CTB cells, the expression of nephrin was found to be increased during trophoblast cell fusion. Moreover, the spontaneous syncytialization and the expression of syncytin 2, connexin 43, and human chorionic gonadotropin beta were significantly inhibited by nephrin-specific siRNAs. The above results demonstrate that nephrin plays an important role in trophoblast syncytialization.

Characterization of the fusion core in zebrafish endogenous retroviral envelope protein

Zebrafish endogenous retrovirus (ZFERV) is the unique endogenous retrovirus in zebrafish, as yet, containing intact open reading frames of its envelope protein gene in zebrafish genome. Similarly, several envelope proteins of endogenous retroviruses in human and other mammalian animal genomes (such as syncytin-1 and 2 in human, syncytin-A and B in mouse) were identified and shown to be functional in induction of cell-cell fusion involved in placental development. ZFERV envelope protein (Env) gene appears to be also functional in vivo because it is expressible. After sequence alignment, we found ZFERV Env shares similar structural profiles with syncytin and other type I viral envelopes, especially in the regions of N- and C-terminal heptad repeats (NHR and CHR) which were crucial for membrane fusion. We expressed the regions of N + C protein in the ZFERV Env (residues 459-567, including predicted NHR and CHR) to characterize the fusion core structure. We found N + C protein could form a stable coiled-coil trimer that consists of three helical NHR regions forming a central trimeric core, and three helical CHR regions packing into the grooves on the surface of the central core. The structural characterization of the fusion core revealed the possible mechanism of fusion mediated by ZFERV Env. These results gave comprehensive explanation of how the ancient virus infects the zebrafish and integrates into the genome million years ago, and showed a rational clue for discovery of physiological significance (e.g., medicate cell-cell fusion).

Detecting endogenous retrovirus-driven tissue-specific gene transcription

Transposable elements (TEs) comprise approximately half of the human genome, and several independent lines of investigation have demonstrated their role in rewiring gene expression during development, evolution, and oncogenesis. The identification of their regulatory effects has largely been idiosyncratic, by linking activity with isolated genes. Their distribution throughout the genome raises critical questions—do these elements contribute to broad tissue- and lineage-specific regulation? If so, in what manner, as enhancers, promoters, RNAs? Here, we devise a novel approach to systematically dissect the genome-wide consequences of TE insertion on gene expression, and test the hypothesis that classes of endogenous retrovirus long terminal repeats (LTRs) exert tissue-specific regulation of adjacent genes. Using correlation of expression patterns across 18 tissue types, we reveal the tissue-specific uncoupling of gene expression due to 62 different LTR classes. These patterns are specific to the retroviral insertion, as the same genes in species without the LTRs do not exhibit the same effect. Although the LTRs can be transcribed themselves, the most highly transcribed TEs do not have the largest effects on adjacent regulation of coding genes, suggesting they function predominantly as enhancers. Moreover, the tissue-specific patterns of gene expression that are detected by our method arise from a limited number of genes, rather than as a general consequence of LTR integration. These findings identify basic principles of co-opting LTRs for genome evolution, and support the utility of our method for the analysis of TE, or other specific gene sets, in relation to the rest of the genome.

A review of inter- and intraspecific variation in the eutherian placenta

The placenta is one of the most morphologically variable mammalian organs. Four major characteristics are typically discussed when comparing the placentas of different eutherian species: placental shape, maternal-fetal interdigitation, intimacy of the maternal-fetal interface and the pattern of maternal-fetal blood flow. Here, we describe the evolution of three of these features as well as other key aspects of eutherian placentation. In addition to interspecific anatomical variation, there is also variation in placental anatomy and function within a single species. Much of this intraspecific variation occurs in response to different environmental conditions such as altitude and poor maternal nutrition. Examinations of variation in the placenta from both intra- and interspecies perspectives elucidate different aspects of placental function and dysfunction at the maternal-fetal interface. Comparisons within species identify candidate mechanisms that are activated in response to environmental stressors ultimately contributing to the aetiology of obstetric syndromes such as pre-eclampsia. Comparisons above the species level identify the evolutionary lineages on which the potential for the development of obstetric syndromes emerged.

Retroviral envelope gene captures and syncytin exaptation for placentation in marsupials

Syncytins are genes of retroviral origin captured by eutherian mammals, with a role in placentation. Here we show that some marsupials-which are the closest living relatives to eutherian mammals, although they diverged from the latter ∼190 Mya-also possess a syncytin gene. The gene identified in the South American marsupial opossum and dubbed syncytin-Opo1 has all of the characteristic features of a bona fide syncytin gene: It is fusogenic in an ex vivo cell-cell fusion assay; it is specifically expressed in the short-lived placenta at the level of the syncytial feto-maternal interface; and it is conserved in a functional state in a series of Monodelphis species. We further identify a nonfusogenic retroviral envelope gene that has been conserved for >80 My of evolution among all marsupials (including the opossum and the Australian tammar wallaby), with evidence for purifying selection and conservation of a canonical immunosuppressive domain, but with only limited expression in the placenta. This unusual captured gene, together with a third class of envelope genes from recently endogenized retroviruses-displaying strong expression in the uterine glands where retroviral particles can be detected-plausibly correspond to the different evolutionary statuses of a captured retroviral envelope gene, with only syncytin-Opo1 being the present-day bona fide syncytin active in the opossum and related species. This study would accordingly recapitulate the natural history of syncytin exaptation and evolution in a single species, and definitely extends the presence of such genes to all major placental mammalian clades.

"Ménage à Trois": the evolutionary interplay between JSRV, enJSRVs and domestic sheep

Sheep betaretroviruses represent a fascinating model to study the complex evolutionary interplay between host and pathogen in natural settings. In infected sheep, the exogenous and pathogenic Jaagsiekte sheep retrovirus (JSRV) coexists with a variety of highly related endogenous JSRVs, referred to as enJSRVs. During evolution, some of them were co-opted by the host as they fulfilled important biological functions, including placental development and protection against related exogenous retroviruses. In particular, two enJSRV loci, enJS56A1 and enJSRV-20, were positively selected during sheep domestication due to their ability to interfere with the replication of related competent retroviruses. Interestingly, viruses escaping these transdominant enJSRVs have recently emerged, probably less than 200 years ago. Overall, these findings suggest that in sheep the process of endogenization is still ongoing and, therefore, the evolutionary interplay between endogenous and exogenous sheep betaretroviruses and their host has not yet reached an equilibrium.

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.

The captured retroviral envelope syncytin-A and syncytin-B genes are conserved in the Spalacidae together with hemotrichorial placentation

Syncytins are fusogenic envelope (env) genes of retroviral origin that have been captured for a function in placentation. Multiple independent events of syncytin gene capture were found to have occurred in primates, rodents, lagomorphs, carnivores, and ruminants. In the mouse, two syncytin-A and -B genes are present, which trigger the formation of the two-layered placental syncytiotrophoblast at the maternal-fetal interface, a structure classified as hemotrichorial. Here, we identified syncytin-A and -B orthologous genes in the genome of all Muroidea species analyzed, thus dating their capture back to about at least 40 million years ago, with evidence that they evolved under strong purifying selection. We further show, in the divergent Spalacidae lineage (blind mole rats [Spalax]), that both syncytins have conserved placenta-specific expression, as revealed by RT-PCR analysis of a panel of Spalax galili tissues, and display fusogenic activity, using ex vivo cell-cell fusion assays. Refined analysis of the placental architecture and ultrastructure revealed that the Spalax placenta displays a hemotrichorial organization of the interhemal membranes, as similarly observed for other Muroidea species, yet with only one trophoblastic cell layer being clearly syncytialized. In situ hybridization experiments further localized syncytin transcripts at the level of these differentiated interhemal membranes. These findings argue for a role of syncytin gene capture in the establishment of the original hemotrichorial placenta of Muroidea, and more generally in the diversity of placental structures among mammals.

Retroviral envelope syncytin capture in an ancestrally diverged mammalian clade for placentation in the primitive Afrotherian tenrecs

Syncytins are fusogenic envelope (env) genes of retroviral origin that have been captured for a function in placentation. Syncytins have been identified in Euarchontoglires (primates, rodents, Leporidae) and Laurasiatheria (Carnivora, ruminants) placental mammals. Here, we searched for similar genes in species that retained characteristic features of primitive mammals, namely the Malagasy and mainland African Tenrecidae. They belong to the superorder Afrotheria, an early lineage that diverged from Euarchotonglires and Laurasiatheria 100 Mya, during the Cretaceous terrestrial revolution. An in silico search for env genes with full coding capacity within a Tenrecidae genome identified several candidates, with one displaying placenta-specific expression as revealed by RT-PCR analysis of a large panel of Setifer setosus tissues. Cloning of this endogenous retroviral env gene demonstrated fusogenicity in an ex vivo cell-cell fusion assay on a panel of mammalian cells. Refined analysis of placental architecture and ultrastructure combined with in situ hybridization demonstrated specific expression of the gene in multinucleate cellular masses and layers at the materno-fetal interface, consistent with a role in syncytium formation. This gene, which we named "syncytin-Ten1," is conserved among Tenrecidae, with evidence of purifying selection and conservation of fusogenic activity. To our knowledge, it is the first syncytin identified to date within the ancestrally diverged Afrotheria superorder.

Role of the vasohibin family in the regulation of fetoplacental vascularization and syncytiotrophoblast formation

Vasohibin-1 (VASH1) and vasohibin-2 (VASH2), the 2 members of the vasohibin family, have been identified as novel regulators of angiogenesis. VASH1 ceases angiogenesis, whereas VASH2 stimulates sprouting. Here we characterized their functional role in the placenta. Immunohistochemical analysis of human placental tissue clarified their distinctive localization; VASH1 in endothelial cells and VASH2 in trophoblasts. We then used a mouse model to explore their function. Wild-type, Vash1^(−/−), and Vash2^(−/−) mice on a C57BL6 background were used in their first pregnancy. As expected, the fetal vascular area was increased in the Vash1^(−/−) mice, whereas it was decreased in the Vash2^(−/−) mice relative to wild-type. In addition, we noticed that the Vash2^(−/−) mice at 18.5dpc displayed thinner villi of the labyrinth and larger maternal lacunae. Careful observation by an electron microscopy revealed that the syncytiotrophoblast formation was defective in the Vash2^(−/−) mice. To test the possible involvement of VASH2 in the syncytiotrophoblast formation, we examined the fusion of BeWo cells, a human trophoblastoid choriocarcinoma cell line. The forskolin treatment induced the fusion of BeWo cells, and the knockdown of VASH2 expression significantly inhibited this cell fusion. Conversely, the overexpression of VASH2 by the infection with adenovirus vector encoding human VASH2 gene significantly increased the fusion of BeWo cells. Glial cell missing-1 and endogenous retrovirus envelope glycoprotein Syncytin 1 and Syncytin 2 are known to be involved in the fusion of trophoblasts. However, VASH2 did not alter their expression in BeWo cells. These results indicate that VASH1 and VASH2 showed distinctive localization and opposing function on the fetoplacental vascularization. Moreover, our study shows for the first time that VASH2 expressed in trophoblasts is involved in the regulation of cell fusion for syncytiotrophoblast formation.

Inhibition of Borna disease virus replication by an endogenous bornavirus-like element in the ground squirrel genome

Animal genomes contain endogenous viral sequences, such as endogenous retroviruses and retrotransposons. Recently, we and others discovered that nonretroviral viruses also have been endogenized in many vertebrate genomes. Bornaviruses belong to the Mononegavirales and have left endogenous fragments, called "endogenous bornavirus-like elements" (EBLs), in the genomes of many mammals. The striking features of EBLs are that they contain relatively long ORFs which have high sequence homology to the extant bornavirus proteins. Furthermore, some EBLs derived from bornavirus nucleoprotein (EBLNs) have been shown to be transcribed as mRNA and probably are translated into proteins. These features lead us to speculate that EBLs may function as cellular coopted genes. An EBLN element in the genome of the thirteen-lined ground squirrel (Ictidomys tridecemlineatus), itEBLN, encodes an ORF with 77% amino acid sequence identity to the current bornavirus nucleoprotein. In this study, we cloned itEBLN from the ground squirrel genome and investigated its involvement in Borna disease virus (BDV) replication. Interestingly, itEBLN, but not a human EBLN, colocalized with the viral factory in the nucleus and appeared to affect BDV polymerase activity by being incorporated into the viral ribonucleoprotein. Our data show that, as do certain endogenous retroviruses, itEBLN potentially may inhibit infection by related exogenous viruses in vivo.

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.

Essential role of the ERK/MAPK pathway in blood-placental barrier formation

The mammalian genome contains two ERK/MAP kinase kinase genes, Map2k1 and Map2k2, which encode dual-specificity kinases responsible for ERK activation. Loss of Map2k1 function in mouse causes embryonic lethality due to placental defects, whereas Map2k2 mutants have a normal lifespan. The majority of Map2k1(+/-) Map2k2(+/-) embryos die during gestation from the underdevelopment of the placenta labyrinth, demonstrating that both kinases are involved in placenta formation. Map2k1(+/-) Map2k2(+/-) mutants show reduced vascularization of the labyrinth and defective formation of syncytiotrophoblast layer II (SynT-II) leading to the accumulation of multinucleated trophoblast giant cells (MTGs). To define the cell type-specific contribution of the ERK/MAPK pathway to placenta development, we performed deletions of Map2k1 function in different Map2k1 Map2k2 allelic backgrounds. Loss of MAP kinase kinase activity in pericytes or in allantois-derived tissues worsens the MTG phenotype. These results define the contribution of the ERK/MAPK pathway in specific embryonic and extraembryonic cell populations for normal placentation. Our data also indicate that MTGs could result from the aberrant fusion of SynT-I and -II. Using mouse genetics, we demonstrate that the normal development of SynT-I into a thin layer of multinucleated cells depends on the presence of SynT-II. Lastly, the combined mutations of Map2k1 and Map2k2 alter the expression of several genes involved in cell fate specification, cell fusion and cell polarity. Thus, appropriate ERK/MAPK signaling in defined cell types is required for the proper growth, differentiation and morphogenesis of the placenta.

Sperm-egg fusion: a molecular enigma of mammalian reproduction

The mechanism of gamete fusion remains largely unknown on a molecular level despite its indisputable significance. Only a few of the molecules required for membrane interaction are known, among them IZUMO1, which is present on sperm, tetraspanin CD9, which is present on the egg, and the newly found oolema protein named Juno. A concept of a large multiprotein complex on both membranes forming fusion machinery has recently emerged. The Juno and IZUMO1, up to present, is the only known extracellular receptor pair in the process of fertilization, thus, facilitating the essential binding of gametes. However, neither IZUMO1 nor Juno appears to be the fusogenic protein. At the same time, the tetraspanin is expected to play a role in organizing the egg membrane order and to interact laterally with other factors. This review summarizes, to present, the known molecules involved in the process of sperm-egg fusion. The complexity and expected redundancy of the involved factors makes the process an intricate and still poorly understood mechanism, which is difficult to comprehend in its full distinction.

Capture of syncytin-Mar1, a fusogenic endogenous retroviral envelope gene involved in placentation in the Rodentia squirrel-related clade

Syncytin genes are fusogenic envelope protein (env) genes of retroviral origin that have been captured for a function in placentation. Within rodents, two such genes have previously been identified in the mouse-related clade, allowing a demonstration of their essential role via knockout mice. Here, we searched for similar genes in a second major clade of the Rodentia order, the squirrel-related clade, taking advantage of the complete sequencing of the ground squirrel Ictidomys tridecemlineatus genome. In silico search for env genes with full coding capacity identified several candidate genes with one displaying placenta-specific expression, as revealed by quantitative reverse transcription-PCR analysis of a large panel of tissues. This gene belongs to a degenerate endogenous retroviral element, with recognizable hallmarks of an integrated provirus. Cloning of the gene in an expression vector for ex vivo cell-cell fusion and pseudotype assays demonstrated fusogenicity on a large panel of mammalian cells. In situ hybridization on placenta sections showed specific expression in domains where trophoblast cells fuse into a syncytiotrophoblast at the fetomaternal interface, consistent with a role in syncytium formation. Finally, we show that the gene is conserved among the tribe Marmotini, thus dating its capture back to about at least 25 million years ago, with evidence for purifying selection and conservation of fusogenic activity. This gene that we named syncytin-Mar1 is distinct from all seven Syncytin genes identified to date in eutherian mammals and is likely to be a major effector of placentation in its related clade. Importance: Syncytin genes are fusogenic envelope genes of retroviral origin, ancestrally captured for a function in placentation. Within rodents, two such genes had been previously identified in the mouse-related clade. Here, in the squirrel-related rodent clade, we identified the envelope gene of an endogenous retrovirus with all the features of a Syncytin: it is specifically expressed in the placenta of the woodchuck Marmota monax, at the level of cells fusing into a syncytium; it can trigger cell-cell and virus-cell fusion ex vivo; and it has been conserved for >25 million years of evolution, suggesting an essential role in its host physiology. Remarkably, syncytin-Mar1 is unrelated to all other Syncytin genes identified thus far in mammals (primates, muroids, carnivores, and ruminants). These results extend the range of retroviral envelope gene "domestication" in mammals and show that these events occurred independently, on multiple occasions during evolution to improve placental development in a process of convergent evolution.

Preconception zinc deficiency disrupts postimplantation fetal and placental development in mice

Zinc is an essential nutrient for optimal fertility, but the effects of preconception zinc deficiency on postimplantation development are not known. Female mice were fed a control or a zinc-deficient diet (ZDD) for 4-5 days before ovulation (preconception). Embryonic and/or placental development were evaluated on Days 3.5, 6.5, 10.5, 12.5, and 16.5 of pregnancy. The findings show a decrease in embryo length (31%, Day 10.5; 13%, Day 12.5; 10%, Day 16.5) and weight (23%, Day 16.5) in embryos from mothers fed a ZDD preconception. Zinc deficiency also caused a high incidence of pregnancy loss (46%, Day 10.5; 34%, Day 12.5; 51%, Day 16.5) compared to control (2%, Day 10.5; 7%, Day 12.5; 9%, Day 16.5). ZDD embryos transferred to normal recipients were 38% smaller and implantation rate was only 10% compared to 40% for controls. Trophoblast cell differentiation and implantation on Day 6.5 of pregnancy were compromised by preconception zinc deficiency. On Day 12.5 of pregnancy, placenta weight and area of fetal placenta were decreased 37% and 31%, respectively, by preconception zinc deficiency. Consistent with a smaller fetal placenta, expression of key placental transcripts, including Ar, Esx1, Syna, Tfeb, Dlx3, and Gcm1 mRNA, but not Ctsq mRNA, were decreased 30%-70% in the ZDD group. Preconception zinc deficiency caused 41%-57% of embryos to exhibit delayed or aberrant neural tube development, as examined by light microscopy and magnetic resonance imaging. Collectively, the findings provide evidence for the importance of preconception zinc in promoting optimal fertility and oocyte developmental potential.

Dysfunction of bovine endogenous retrovirus K2 envelope glycoprotein is related to unsuccessful intracellular trafficking

Endogenous retroviruses (ERVs) are the remnants of retroviral infection of ancestral germ cells. Mutations introduced into ERVs halt the production of infectious agents, but their effects on the function of retroviral proteins are not fully understood. Retroviral envelope glycoproteins (Envs) are utilized in membrane fusion during viral entry, and we recently identified intact coding sequences for bovine endogenous retrovirus K1 (BERV-K1) and BERV-K2 Envs. Amino acid sequences of BERV-K1 Env (also called Fematrin-1) and BERV-K2 Env are similar, and both viruses are classified in the genus Betaretrovirus. While Fematrin-1 plays an important role in cell-to-cell fusion in bovine placenta, the BERV-K2 envelope gene is marginally expressed in vivo, and its recombinant Env protein is defective in membrane fusion due to inefficient cleavage of surface (SU) and transmembrane subunits. Here, we conducted chimeric analyses of Fematrin-1 and BERV-K2 Envs and revealed that defective maturation of BERV-K2 Env contributed to failed intracellular trafficking. Fluorescence microscopy and flow cytometric analysis suggested that in contrast to Fematrin-1 Env, BERV-K2 Env could not be transported from the endoplasmic reticulum to the trans-Golgi network, where cellular proteases required for processing retroviral Envs are localized. We also identified that one of the responsive regions of this phenomenon resided within a 65-amino-acid region of BERV-K2 SU. This is the first report to identify that retroviral Env SU is involved in the regulation of intracellular trafficking, and it may help to elucidate the maturation process of Fematrin-1 and other related Envs.

IMPORTANCE

Retroviruses utilize envelope glycoproteins (Envs) to enter host target cells. Mature retroviral Env is a heterodimer, which consists of surface (SU) and transmembrane (TM) subunits that are generated by the cleavage of an Env precursor protein in the trans-Golgi network. SU and TM mediate the recognition of the entry receptor and virus-host membrane fusion, respectively. However, unexplained issues remain for the maturation process of retroviral Env. We previously reported that bovine endogenous retrovirus K2 (BERV-K2) Env lost fusogenicity due to a defect in the cleavage of SU and TM. In this study, we identified that mutations residing in BERV-K2 SU disturbed intracellular trafficking of BERV-K2 Env and resulted its inefficient cleavage. Because SU is not known to play an important role in this process, our study may provide novel insights into the maturation mechanism of retroviral Envs.

Retroviruses facilitate the rapid evolution of the mammalian placenta

The mammalian placenta exhibits elevated expression of endogenous retroviruses (ERVs), but the evolutionary significance of this feature remains unclear. I propose that ERV-mediated regulatory evolution was, and continues to be, an important mechanism underlying the evolution of placental development. Many recent studies have focused on the co-option of ERV-derived genes for specific functional adaptations in the placenta. However, the co-option of ERV-derived regulatory elements could potentially lead to the incorporation of entire gene regulatory networks, which, I argue, would facilitate relatively rapid developmental evolution of the placenta. I suggest a model in which an ancient retroviral infection led to the establishment of the ancestral placental developmental gene network through the co-option of ERV-derived regulatory elements. Consequently, placental development would require elevated tolerance to ERV activity. This in turn would expose a continuous stream of novel ERV mutations that may have catalyzed the developmental diversification of the mammalian placenta.

HERV-K(HML-2), the Best Preserved Family of HERVs: Endogenization, Expression, and Implications in Health and Disease

Retroviruses that have the ability to infect germ line cells can become an integral and inherited part of the host genome. About 8% of the human chromosomal DNA consists of sequences derived from infections by retroviruses that presumably circulated 2-40 millions of years ago, and some elements are actually much older. Post-insertional recombinations, deletions, and mutations have rendered all known human endogenous retroviruses (HERVs) non-infectious. However some, particularly the most recently acquired proviruses of the HERV-K(HML-2) family, can expresses viral proteins and produce viral particles. In this review we will first discuss the major aspects of the endogenization process and peculiarities of the different HERV-K families. We will then focus on the genes and proteins encoded by HERV-K(HML-2) as well as inactivation of these proviruses by postinsertional mutations and their inhibition by antiretroviral factors. After describing the evolutionary interplay between host and endogenous retrovirus we will delve deeper into the currently limited understanding of HERV-K and its possible association with disease, particularly tumorigenesis.

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.

Comprehensive analysis of endogenous bornavirus-like elements in eukaryote genomes

Bornaviruses are the only animal RNA viruses that establish a persistent infection in their host cell nucleus. Studies of bornaviruses have provided unique information about viral replication strategies and virus-host interactions. Although bornaviruses do not integrate into the host genome during their replication cycle, we and others have recently reported that there are DNA sequences derived from the mRNAs of ancient bornaviruses in the genomes of vertebrates, including humans, and these have been designated endogenous borna-like (EBL) elements. Therefore, bornaviruses have been interacting with their hosts as driving forces in the evolution of host genomes in a previously unexpected way. Studies of EBL elements have provided new models for virology, evolutionary biology and general cell biology. In this review, we summarize the data on EBL elements including what we have newly identified in eukaryotes genomes, and discuss the biological significance of EBL elements, with a focus on EBL nucleoprotein elements in mammalian genomes. Surprisingly, EBL elements were detected in the genomes of invertebrates, suggesting that the host range of bornaviruses may be much wider than previously thought. We also review our new data on non-retroviral integration of Borna disease virus.

Exploring the effects of immunity and life history on the dynamics of an endogenous retrovirus

Mammalian DNA is littered with the signatures of past retroviral infections. For example, at least 8% of the human genome can be attributed to endogenous retroviruses (ERVs). We take a single-locus approach to develop a simple susceptible-infected-recovered model to investigate the circumstances under which a disease-causing retrovirus can become incorporated into the host genome and spread through the host population if it were to confer an immunological advantage. In the absence of any fitness benefit provided by the long terminal repeat (LTR), we conclude that signatures of ERVs are likely to go to fixation within a population when the probability of evolving cellular/humoral immunity to a related exogenous version of the virus is extremely small. We extend this model to examine whether changing the speed of the host life history influences the likelihood that an exogenous retrovirus will incorporate and spread to fixation. Our results reveal the parameter space under which incorporation of exogenous retroviruses into a host genome may be beneficial to the host. In our final model, we find that the likelihood of an LTR reaching fixation in a host population is not strongly affected by host life history.

'There and back again': revisiting the pathophysiological roles of human endogenous retroviruses in the post-genomic era

Almost 8% of the human genome comprises endogenous retroviruses (ERVs). While they have been shown to cause specific pathologies in animals, such as cancer, their association with disease in humans remains controversial. The limited evidence is partly due to the physical and bioethical restrictions surrounding the study of transposons in humans, coupled with the major experimental and bioinformatics challenges surrounding the association of ERVs with disease in general. Two biotechnological landmarks of the past decade provide us with unprecedented research artillery: (i) the ultra-fine sequencing of the human genome and (ii) the emergence of high-throughput sequencing technologies. Here, we critically assemble research about potential pathologies of ERVs in humans. We argue that the time is right to revisit the long-standing questions of human ERV pathogenesis within a robust and carefully structured framework that makes full use of genomic sequence data. We also pose two thought-provoking research questions on potential pathophysiological roles of ERVs with respect to immune escape and regulation.

Paleovirology of 'syncytins', retroviral env genes exapted for a role in placentation

The development of the emerging field of 'paleovirology' allows biologists to reconstruct the evolutionary history of fossil endogenous retroviral sequences integrated within the genome of living organisms and has led to the retrieval of conserved, ancient retroviral genes 'exapted' by ancestral hosts to fulfil essential physiological roles, syncytin genes being undoubtedly among the most remarkable examples of such a phenomenon. Indeed, syncytins are 'new' genes encoding proteins derived from the envelope protein of endogenous retroviral elements that have been captured and domesticated on multiple occasions and independently in diverse mammalian species, through a process of convergent evolution. Knockout of syncytin genes in mice provided evidence for their absolute requirement for placenta development and embryo survival, via formation by cell-cell fusion of syncytial cell layers at the fetal-maternal interface. These genes of exogenous origin, acquired 'by chance' and yet still 'necessary' to carry out a basic function in placental mammals, may have been pivotal in the emergence of mammalian ancestors with a placenta from egg-laying animals via the capture of a founding retroviral env gene, subsequently replaced in the diverse mammalian lineages by new env-derived syncytin genes, each providing its host with a positive selective advantage.

The transmembrane protein of the human endogenous retrovirus--K (HERV-K) modulates cytokine release and gene expression

Numerous copies of endogenous retroviruses are present in the genome of mammals including man. Although most of them are defective, some, e.g., the human endogenous retroviruses HERV-K, were found to be expressed under certain physiological conditions. For instance, HERV-K is expressed in germ cell tumours and melanomas as well as in the placenta. Most exogenous retroviruses including the human immunodeficiency virus HIV-1 induce severe immunodeficiencies and there is increasing evidence that the transmembrane envelope (TM) proteins of these retroviruses may be involved. We show here that HERV-K particles released from a human teratocarcinoma cell line, a recombinant TM protein and a peptide corresponding to a highly conserved so-called immunosuppressive domain in the TM protein of HERV-K inhibit the proliferation of human immune cells, induce modulation of the expression of numerous cytokines, and modulate the expression of cellular genes as detected by a microarray analysis. The changes in cytokine release and gene expression induced by the TM protein of HERV-K are similar to those found previously induced by the TM protein of HIV-1. These data suggest that the mechanism of immunosuppression may be similar for different retroviruses and that the expression of the TM protein in tumours and in the placenta may suppress immune responses and thus prevent rejection of the tumour and the embryo.

Fematrin-1 is involved in fetomaternal cell-to-cell fusion in Bovinae placenta and has contributed to diversity of ruminant placentation

During placentation, mammals employ different strategies for nourishing and supporting fetuses. Members of the Bovidae family, consisting of cloven-hoofed ruminants, utilize multiple maternal attachment points on the placenta, known as cotyledons, and hybrid cells, named trinucleate cells or syncytial plaques, made up of a fusion of fetal trophoblasts and maternal endometrial cells to provide essential hormones and maintain long gestation periods. These hybrid cells are unique to the Bovidae, as fetomaternal borders are clearly separated by syncytiotrophoblasts or epithelial cells in the placenta of other mammals. Recently, it was reported that Syncytin-Rum1 was inserted into ruminant genomes, including cattle and sheep, and was possibly involved in fetomaternal cell-to-cell fusion in both species. However, Syncytin-Rum1 alone is insufficient to explain the morphological diversity of the fetomaternal hybrids between Bovinae and Caprinae (i.e., trinucleate cells in Bovinae and syncytial plaques in Caprinae). Here we report that the bovine endogenous retrovirus K1 (BERV-K1) envelope, which we term Fematrin-1, was specifically expressed in binucleated trophoblasts throughout gestation in cattle and induced fusion with bovine endometrial cells in vitro at a significantly higher level than Syncytin-Rum1 under physiological conditions. Fematrin-1 was found to be integrated into intron 18 of FAT tumor suppressor homolog 2 (FAT2) about 18.3 to 25.4 million years ago and has been subject to purifying selection through the evolution of Bovinae. Phylogenetically, Fematrin-1 is distinct from Syncytin genes found in other mammalian species that form syncytiotrophoblasts. Our results suggest that the newly acquired endogenous retroelement has contributed to generating placentation diversity through ruminant evolution.

Regulation of murine placentogenesis by the retroviral genes Syncytin-A, Syncytin-B and Peg10

The murine placenta has a trichorial structure with two multinucleated syncytiotrophoblast (SCT) layers representing a barrier between the maternal and fetal blood system. Genes of endogenous retroviruses and retrotransposon-derived paternally expressed genes (Peg), remnants of past infections and integrations in the genome, have essential functions in placentogenesis. Previous studies showed that the envelope genes Syncytin-A and Syncytin-B were essential for cell-cell fusion of the SCT. The goal of this study was to analyze the temporal localization and expression of nine genes throughout placental development from embryonic day (E)8.5 to E18.5 using in situ-hybridization and absolute RNA-quantification. These included a comparison of previously characterized genes from the labyrinth Syncytin-A, Syncytin-B, Gcm1, the junctional zone PL-1, PL-2, Plf, Tpbpa with two further characterized genes Peg10 and Tpbpb. Syncytin-A and Syncytin-B RNA localized to SCT-I and SCT-II, respectively. Peg10 RNA localized to all extraembryonic tissues, specifically to the parietal and sinusoidal TGC of the labyrinth layer, which is in contact with SCT-I and the maternal blood. All three retroviral/retrotransposon-derived genes showed the highest expression at E16.5, but Peg10 with 188,917.1 molecules/ng cDNA was 208-fold and 106.8-fold higher expressed than Syncytin-A and Syncytin-B, respectively. Tpbpb localized to the junctional zone and showed the highest expression at E16.5 along with PL-2, Plf, Tpbpa, but not PL-1, which decreased in expression at E10.5. To investigate a role of Syncytin-A, Syncytin-B and Peg10 in cell-cell fusion, we established a cell culture system with fractionated primary trophoblasts from murine placentae. Culturing trophoblasts for up to 72h partly resembled trophoblast development in vivo according to the nine marker genes. Knockdown of Syncytin-A demonstrated a functional regulation of cell-cell fusion, where knockdown of Peg10 showed no involvement in cell fusion. Due to the expression of Peg10 in TGCs, we propose an essential functional role in the fetal-maternal blood system.

Sputnik, a virophage infecting the viral domain of life

This chapter discusses the astonishing discovery of the Sputnik virophage, a new virus infecting giant viruses of the genera Mimivirus and Mamavirus. While other virophages have also since been described, this chapter focuses mainly on Sputnik, which is the best described. We detail the general properties of the virophage life cycle, as well as its hosts, genomic characteristics, ecology, and origin. In addition to genetic, phylogenetic, and structural evidence, the existence of virophages has deeply altered our view of the tripartite division of life to include the addition of a fourth domain constituted of the nucleocytoplasmic large DNA viruses, an important point that is discussed.

Differential evolutionary fate of an ancestral primate endogenous retrovirus envelope gene, the EnvV syncytin, captured for a function in placentation

Syncytins are envelope genes of retroviral origin that have been co-opted for a role in placentation. They promote cell–cell fusion and are involved in the formation of a syncytium layer—the syncytiotrophoblast—at the materno-fetal interface. They were captured independently in eutherian mammals, and knockout mice demonstrated that they are absolutely required for placenta formation and embryo survival. Here we provide evidence that these “necessary” genes acquired “by chance” have a definite lifetime with diverse fates depending on the animal lineage, being both gained and lost in the course of evolution. Analysis of a retroviral envelope gene, the envV gene, present in primate genomes and belonging to the endogenous retrovirus type V (ERV-V) provirus, shows that this captured gene, which entered the primate lineage >45 million years ago, behaves as a syncytin in Old World monkeys, but lost its canonical fusogenic activity in other primate lineages, including humans. In the Old World monkeys, we show—by in situ analyses and ex vivo assays—that envV is both specifically expressed at the level of the placental syncytiotrophoblast and fusogenic, and that it further displays signs of purifying selection based on analysis of non-synonymous to synonymous substitution rates. We further show that purifying selection still operates in the primate lineages where the gene is no longer fusogenic, indicating that degeneracy of this ancestral syncytin is a slow, lineage-dependent, and multi-step process, in which the fusogenic activity would be the first canonical property of this retroviral envelope gene to be lost.

Syncytins are “new” genes encoding the envelope protein of captured endogenous retroviral elements. Their unambiguous status of “cellular gene” was recently demonstrated by knocking them out in genetically modified mice, showing their absolute requirement for placenta formation and embryo survival, via formation by cell–cell fusion of the feto-maternal syncytium interface. These genes are remarkable, as they are “necessary” for a basic function in placental mammals and yet they were acquired “by chance” on multiple occasions and independently in diverse mammalian species. We proposed that syncytins have been pivotal for the emergence of animals with a placenta from those laying eggs via the capture of a founding retroviral env gene, then subsequently replaced in the diverse mammalian lineages upon successive and independent germline infections by new retroviruses and co-optation of their env gene, each new gene providing its host with a positive selective advantage. This hypothesis would account for the diversity of the captured syncytins that can be currently found, concomitant with the diversity of placental architectures. A consequence of this paradigm is that evidence for “decaying syncytins” in eutherian mammals should exist, and this is precisely what we sought—and found—in this study.

Genetic basis of cell-cell fusion mechanisms

Cell-cell fusion in sexually reproducing organisms is a mechanism to merge gamete genomes and, in multicellular organisms, it is a strategy to sculpt organs, such as muscle, bone, and placenta. Moreover, this mechanism has been implicated in pathological conditions, such as infection and cancer. Studies of genetic model organisms have uncovered a unifying principle: cell fusion is a genetically programmed process. This process can be divided in three stages: competence (cell induction and differentiation); commitment (cell determination, migration, and adhesion); and cell fusion (membrane merging and cytoplasmic mixing). Recent work has led to the discovery of fusogens, which are cell fusion proteins that are necessary and sufficient to fuse cell membranes. Two unrelated families of fusogens have been discovered, one in mouse placenta and one in Caenorhabditis elegans (syncytins and F proteins, respectively). Current research aims to identify new fusogens and determine the mechanisms by which they merge membranes.

Captured retroviral envelope syncytin gene associated with the unique placental structure of higher ruminants

Syncytins are envelope genes of retroviral origin that have been co-opted for a role in placentation and likely contribute to the remarkable diversity of placental structures. Independent capture events have been identified in primates, rodents, lagomorphs, and carnivores, where they are involved in the formation of a syncytium layer at the fetomaternal interface via trophoblast cell-cell fusion. We searched for similar genes within the suborder Ruminantia where the placenta lacks an extended syncytium layer but displays a heterologous cell-fusion process unique among eutherian mammals. An in silico search for intact envelope genes within the Bos taurus genome identified 18 candidates belonging to five endogenous retrovirus families, with one gene displaying both placenta-specific expression, as assessed by quantitative RT-PCR analyses of a large panel of tissues, and conservation in the Ovis aries genome. Both the bovine and ovine orthologs displayed fusogenic activity by conferring infectivity on retroviral pseudotypes and triggering cell-cell fusion. In situ hybridization of placenta sections revealed specific expression in the trophoblast binucleate cells, consistent with a role in the formation--by heterologous cell fusion with uterine cells--of the trinucleate cells of the cow and the syncytial plaques of the ewe. Finally, we show that this gene, which we named "Syncytin-Rum1," is conserved among 16 representatives of higher ruminants, with evidence for purifying selection and conservation of its fusogenic properties, over 30 millions years of evolution. These data argue for syncytins being a major driving force in the emergence and diversity of the placenta.

During embryogenesis, esrp1 expression is restricted to a subset of epithelial cells and is associated with splicing of a number of developmentally important genes

BACKGROUND

Development of a mature organism from a single cell requires a series of important morphological changes, which is in part regulated by alternative splicing. In this article, we report the expression of Esrp1 during early mouse embryogenesis, a splicing factor implicated in epithelial to mesenchymal transitions.

RESULTS

By qRT-PCR, we find higher expression of Esrp1 and Esrp2 in placenta compared to the embryos. We also find a correlation between the expression of Esrp1 and alternative splicing of several known target exons. Using in situ RNA hybridization we show that while Esrp1 expression is ubiquitous in embryonic day (E)6.5 mouse embryos, expression becomes restricted to the chorion and definitive endoderm starting at E7.5. Esrp1 expression was consistently restricted to a subset of epithelial cell types in developing embryos from E9.5 to E13.5.

CONCLUSIONS

Our results suggest that Esrp1 could play an important role in the morphological changes underlying embryogenesis of the placenta and embryo.

Gestational protein restriction affects trophoblast differentiation

Whether and how gestational protein restriction (PR) affects placental development and function remain unknown. To test the hypothesis that PR can affect trophoblast differentiation in mid-and late pregnancy, rats were fed a 20% or an isocaloric 6% protein diet from Day 1 to 14 or 18 of pregnancy and effects of PR on trophoblast differentiation were determined by changes in expressions of marker gene(s) for trophoblast lineages. At Day 18 of pregnancy, PR increased expressions of Esrrb, Id1 andId2 (trophoblast stem cell markers), decreased expressions of Ascl2 (spongiotrophblast cell marker) and Prl2c1 (trophoblast giant cell marker), but did not alter expressions of Gjb3 and Pcdh12(glycogen cell markers) in the junctional zone (JZ). In the labyrinth zone (LZ), PR did not change expressions of Prl2b1 (trophoblast giant cell marker), Gcm1 and Syna (syncytiotrophoblast cell markers), but decrease expression of Ctsq (sinusoidal trophoblast giant cell marker). These results indicate that PR impairs the differentiation of trophoblast stem cell into spongiotrophoblast and trophoblast giant cells in JZ, and formation of sinusoidal trophoblast giant cells in LZ.

Free blastocyst and implantation stages in the European brown hare: correlation between ultrasound and histological data

The European brown hare (Lepus europaeus) is the only species with superconception, whereby the maternal reproductive tract hosts two sets of conceptuses at different developmental stages. The embryonic development of the hare has not yet been described. To understand the mechanism of superconception, we studied oviduct transport and implantation stages by embryo flushing and live high-resolution ultrasound. Ultrasound data of implantation stages is correlated with histology. In the oviduct, a mucin coat is deposited on the zona pellucida. The blastocysts enter the uterine horns on Day 5, 1 day later than in the rabbit, and directly expand approximately threefold. Spacing is accompanied by peristaltic movement of the endometrium. The mucin coat disappears and the conceptuses attach. The yolk-sac expands in the blastocoel and syncytial knobs invade the antimesometrial endometrium. Maternal blood lacunae appear in the mesometrial endometrial folds, which are subsequently invaded by the syncytiotrophoblast. The haemochorial chorioallantoic placenta forms. The yolk-sac cavity is gradually replaced by the allantois and finally by the exocoel. The different reproductive strategies of the precocial hare and the altricial rabbit are discussed. We assume that the lagomorph-specific mucin coat and the hare-specific delay of the oviduct–uterine transition are prerequisites for superconception.

[Genome virology: the novel interaction of RNA viruses and host genomes]

The origin of virus-like organisms probably dates back to the earliest forms of cellular life. Such a long coexistence between viruses and ourselves suggests that viruses may have crucially influenced the evolution of our species and vice versa. Sequences derived from retroviruses and retrotransposons have been shown to make up a substantial part of the human genome, suggesting a direct role of virus infection as a source of new genetic information and genomic innovation of the host species. Until very recently, retroviruses were the only viruses known to generate such endogenous copies in vertebrate genomes. However, we and others have reported recently that non-retroviral RNA viruses, including bornaviruses and filoviruses, have been endogenized repeatedly during mammalian evolution. These endogenous elements of RNA viruses not only provide evidence of ancient viral infections in each animal species but also offer novel paradigms for the interaction between RNA viruses and their hosts. Based on the presentation of the plenary lecture at the XV International Congress of Virology 2011, I will review here our recent findings regarding the generation and functions of endogenous bornavirus-like N elements in mammalian genomes, in order to reveal the unknown dynamics of RNA viruses in eukaryotic cells, and also discuss the evolutionary interaction between RNA viruses and hosts.