An envelope glycoprotein of the human endogenous retrovirus HERV-W is expressed in the human placenta and fuses cells expressing the type D mammalian retrovirus receptor

Bovine placental extracellular vesicles carry the fusogenic syncytin BERV-K1

Syncytins are endogenous retroviral envelope proteins which induce the fusion of membranes. A human representative of this group, endogenous retrovirus group W member 1 envelope (ERVW-1) or syncytin-1 is present in trophoblast-derived extracellular vesicles and supports the incorporation of these extracellular vesicles into recipient cells. During pregnancy, placenta-derived extracellular vesicles participate in feto-maternal communication. Bovine fetal binucleate trophoblast cells express the syncytin, bovine endogenous retroviral envelope protein K1 (BERV-K1). These cells release extracellular vesicles into the maternal stroma, but it is unclear whether BERV-K1 is included in these extracellular vesicles. Here, extracellular vesicles were isolated from bovine placental tissue using collagenase digestion, ultracentrifugation, and size exclusion chromatography. They were characterized with transmission electron microscopy, nanoparticle tracking analysis, immunoblotting and mass spectrometry. Immunohistochemistry and immunoelectron microscopy were used to localize BERV-K1 within the bovine placental tissue. The isolated extracellular vesicles range between 50 and 300 nm, carrying multiple extracellular vesicle biomarkers. Proteomic analysis and immunoelectron microscopy confirmed BERV-K1 presence on the isolated extracellular vesicles. Further, BERV-K1 was localized on intraluminal vesicles in secretory granules of binucleate trophoblast cells. The presence of BERV-K1 on bovine placental extracellular vesicles suggests their role in feto-maternal communication and potential involvement of BERV-K1 in uptake of extracellular vesicles by target cells.

TFEB controls syncytiotrophoblast formation and hormone production in placenta

TFEB, a bHLH-leucine zipper transcription factor belonging to the MiT/TFE family, globally modulates cell metabolism by regulating autophagy and lysosomal functions. Remarkably, loss of TFEB in mice causes embryonic lethality due to severe defects in placentation associated with aberrant vascularization and resulting hypoxia. However, the molecular mechanism underlying this phenotype has remained elusive. By integrating in vivo analyses with multi-omics approaches and functional assays, we have uncovered an unprecedented function for TFEB in promoting the formation of a functional syncytiotrophoblast in the placenta. Our findings demonstrate that constitutive loss of TFEB in knock-out mice is associated with defective formation of the syncytiotrophoblast layer. Indeed, using in vitro models of syncytialization, we demonstrated that TFEB translocates into the nucleus during syncytiotrophoblast formation and binds to the promoters of crucial placental genes, including genes encoding fusogenic proteins (Syncytin-1 and Syncytin-2) and enzymes involved in steroidogenic pathways, such as CYP19A1, the rate-limiting enzyme for the synthesis of 17β-Estradiol (E2). Conversely, TFEB depletion impairs both syncytial fusion and endocrine properties of syncytiotrophoblast, as demonstrated by a significant decrease in the secretion of placental hormones and E2 production. Notably, restoration of TFEB expression resets syncytiotrophoblast identity. Our findings identify that TFEB controls placental development and function by orchestrating both the transcriptional program underlying trophoblast fusion and the acquisition of endocrine function, which are crucial for the bioenergetic requirements of embryonic development.

Molecular dissection of HERV-W dependent microglial- and astroglial cell polarization

The endogenous retrovirus type W (HERV-W) is a human-specific entity, which was initially discovered in multiple sclerosis (MS) patient derived cells. We initially found that the HERV-W envelope (ENV) protein negatively affects oligodendrogenesis and controls microglial cell polarization towards a myelinated axon associated and damaging phenotype. Such first functional assessments were conducted ex vivo, given the human-specific origin of HERV-W. Recent experimental evidence gathered on a novel transgenic mouse model, mimicking activation and expression of the HERV-W ENV protein, revealed that all glial cell types are impacted and that cellular fates, differentiation, and functions were changed. In order to identify HERV-W-specific signatures in glial cells, the current study analyzed the transcriptome of ENV protein stimulated microglial- and astroglial cells and compared the transcriptomic signatures to lipopolysaccharide (LPS) stimulated cells, owing to the fact that both ligands can activate toll-like receptor-4 (TLR-4). Additionally, a comparison between published disease associated glial signatures and the transcriptome of HERV-W ENV stimulated glial cells was conducted. We, therefore, provide here for the first time a detailed molecular description of specific HERV-W ENV evoked effects on those glial cell populations that are involved in smoldering neuroinflammatory processes relevant for progression of neurodegenerative diseases.

Dynamic label-free analysis of SARS-CoV-2 infection reveals virus-induced subcellular remodeling

Assessing the impact of SARS-CoV-2 on organelle dynamics allows a better understanding of the mechanisms of viral replication. We combine label-free holotomographic microscopy with Artificial Intelligence to visualize and quantify the subcellular changes triggered by SARS-CoV-2 infection. We study the dynamics of shape, position and dry mass of nucleoli, nuclei, lipid droplets and mitochondria within hundreds of single cells from early infection to syncytia formation and death. SARS-CoV-2 infection enlarges nucleoli, perturbs lipid droplets, changes mitochondrial shape and dry mass, and separates lipid droplets from mitochondria. We then used Bayesian network modeling on organelle dry mass states to define organelle cross-regulation networks and report modifications of organelle cross-regulation that are triggered by infection and syncytia formation. Our work highlights the subcellular remodeling induced by SARS-CoV-2 infection and provides an Artificial Intelligence-enhanced, label-free methodology to study in real-time the dynamics of cell populations and their content.

Acute strength exercise training impacts differently the HERV-W expression and inflammatory biomarkers in resistance exercise training individuals

BACKGROUND

Human Endogenous Retroviruses (HERVs) are fossil viruses that composes 8% of the human genome and plays several important roles in human physiology, including muscle repair/myogenesis. It is believed that inflammation may also regulate HERV expression, and therefore may contribute in the muscle repair, especially after training exercise. Hence, this study aimed to assess the level of HERVs expression and inflammation profile in practitioners' resistance exercises after an acute strength training session.

METHODS

Healthy volunteers were separated in regular practitioners of resistance exercise training group (REG, n = 27) and non-trained individuals (Control Group, n = 20). All individuals performed a strength exercise section. Blood samples were collected before the exercise (T0) and 45 minutes after the training session (T1). HERV-K (HML1-10) and W were relatively quantified, cytokine concentration and circulating microparticles were assessed.

RESULTS

REG presented higher level of HERV-W expression (~2.5 fold change) than CG at T1 (p<0.01). No difference was observed in the levels of HERV-K expression between the groups as well as the time points. Higher serum TNF-α and IL-10 levels were verified post-training session in REG and CG (p<0.01), and in REG was found a positive correlation between the levels of TNF-α at T1 and IL-10 at T0 (p = 0.01). Finally, a lower endothelial microparticle percentage was observed in REG at T1 than in T0 (p = 0.04).

CONCLUSION

REG individuals exhibited a significant upregulation of HERV-W and modulation of inflammatory markers when compared to CG. This combined effect could potentially support the process of skeletal muscle repair in the exercised individuals.

Receptor-recognition and antiviral mechanisms of retrovirus-derived human proteins

Human syncytin-1 and suppressyn are cellular proteins of retroviral origin involved in cell-cell fusion events to establish the maternal-fetal interface in the placenta. In cell culture, they restrict infections from members of the largest interference group of vertebrate retroviruses, and are regarded as host immunity factors expressed during development. At the core of the syncytin-1 and suppressyn functions are poorly understood mechanisms to recognize a common cellular receptor, the membrane transporter ASCT2. Here, we present cryo-electron microscopy structures of human ASCT2 in complexes with the receptor-binding domains of syncytin-1 and suppressyn. Despite their evolutionary divergence, the two placental proteins occupy similar positions in ASCT2, and are stabilized by the formation of a hybrid β-sheet or 'clamp' with the receptor. Structural predictions of the receptor-binding domains of extant retroviruses indicate overlapping binding interfaces and clamping sites with ASCT2, revealing a competition mechanism between the placental proteins and the retroviruses. Our work uncovers a common ASCT2 recognition mechanism by a large group of endogenous and disease-causing retroviruses, and provides high-resolution views on how placental human proteins exert morphological and immunological functions.

Endogenous retrovirus HERVH-derived lncRNA UCA1 controls human trophoblast development

Endogenous retroviruses (ERVs) are frequently reactivated in mammalian placenta. It has been proposed that ERVs contribute to shaping the gene regulatory network of mammalian trophoblasts, dominantly acting as species- and placental-specific enhancers. However, whether and how ERVs control human trophoblast development through alternative pathways remains poorly understood. Besides the well-recognized function of human endogenous retrovirus-H (HERVH) in maintaining pluripotency of early human epiblast, here we present a unique role of HERVH on trophoblast lineage development. We found that the LTR7C/HERVH subfamily exhibits an accessible chromatin state in the human trophoblast lineage. Particularly, the LTR7C/HERVH-derived Urothelial Cancer Associated 1 (UCA1), a primate-specific long non-coding RNA (lncRNA), is transcribed in human trophoblasts and promotes the proliferation of human trophoblast stem cells (hTSCs), whereas its ectopic expression compromises human trophoblast syncytialization coinciding with increased interferon signaling pathway. Importantly, UCA1 upregulation is detectable in placental samples from early-onset preeclampsia (EO-PE) patients and the transcriptome of EO-PE placenta exhibits considerable similarities to that of the syncytiotrophoblasts differentiated from UCA1-overexpressing hTSCs, supporting up-regulated UCA1 as a potential biomarker of this disease. Altogether, our data shed light on the versatile regulatory role of HERVH in early human development and provide a unique mechanism whereby ERVs exert a function in human placentation and placental syndromes.

HtrA4 is required for human trophoblast stem cell differentiation into syncytiotrophoblast

INTRODUCTION

The syncytiotrophoblast (STB) of the human placenta facilitates vital maternal-fetal communication and is maintained by fusion (syncytialization) of cytotrophoblasts. Serine protease HtrA4 (high temperature requirement factor A4) is highly expressed only in the human placenta and was previously reported to be important for BeWo fusion. This study investigated whether HtrA4 is critical for differentiation of human trophoblast stem cells (TSCs) into STB.

METHODS

Primary TSCs were isolated from first trimester placentas (n = 5) and validated by immunofluorescence (IF) for CD49f, CK7 and vimentin. TSCs were then differentiated into STB and the success of syncytialization was confirmed by RT-PCR, IF and ELISA of known markers. TSCs were next stably transfected with a HtrA4-targetting CRISPR/Cas9 plasmid, and cells with severe HtrA4 knockdown (HtrA4-KD) were analyzed to investigate the impact on STB differentiation.

RESULTS

Primary TSCs were confirmed to be of high purity by staining positively for CD49f and CK7 but negatively for vimentin. These TSCs readily syncytialized when stimulated for STB differentiation, significantly increasing β-hCG and syncytin-1, substantially decreasing E-cadherin, and markedly losing cell borders. While TSCs produced very low levels of HtrA4, upon stimulation for STB differentiation the cells drastically upregulated HtrA4 expression; secretion of HtrA4 protein also increased sharply, correlating positively and significantly with that of β-hCG. The HtrA4-KD TSCs, however, failed to show this surge of HtrA4 production upon stimulation, and ultimately remained primarily mononucleated with no significant STB differentiation.

DISCUSSION

This study demonstrates that HtrA4 plays a critical role in TSC differentiation into syncytiotrophoblast.

A Replication-Competent Retroviral Vector Expressing the HERV-W Envelope Glycoprotein is a Potential Tool for Cancer Gene Therapy

The fusogenic membrane glycoprotein (FMG) derived from the human endogenous retrovirus-W (HERV-W) exhibits fusogenic properties, making it a promising candidate for cancer gene therapy. When cells are transfected with HERV-W FMG, they can fuse with neighboring cells expressing the receptor, resulting in the formation of syncytia. These syncytia eventually undergo cell death within a few days. In addition, it has been observed that an HERV-W env mutant, which is truncated after amino acid 483, displays increased fusogenicity compared to the wild-type HERV-W env. In this study, we observed syncytium formation upon transfection of HeLa and TE671 human cancer cells with plasmids containing the HERV-W 483 gene. To explore the potential of a semi-replication-competent retroviral (s-RCR) vector encoding HERV-W 483 for FMG-mediated cancer gene therapy, we developed two replication-defective retroviral vectors: a gag-pol vector encoding HERV-W 483 (MoMLV-HERV-W 483) and an env vector encoding VSV-G (pCLXSN-VSV-G-EGFP). When MoMLV-HERV-W 483 and pCLXSN-VSV-G-EGFP were co-transfected into HEK293T cells to produce the s-RCR vector, gradual syncytium formation was observed. However, the titers of the s-RCR virus remained consistently low. To enhance gene transfer efficiency, we constructed an RCR vector encoding HERV-W 483 (MoMLV-10A1-HERV-W 483), which demonstrated replication ability in HEK293T cells. Infection of A549 and HT1080 human cancer cell lines with this RCR vector induced syncytium formation and subsequent cell death. Consequently, both the s-RCR vector and RCR encoding HERV-W 483 hold promise as valuable tools for cancer gene therapy.

Retrotransposon-derived transcripts and their functions in immunity and disease

Retrotransposons, which account for approximately 42% of the human genome, have been increasingly recognized as "non-self" pathogen-associated molecular patterns (PAMPs) due to their virus-like sequences. In abnormal conditions such as cancer and viral infections, retrotransposons that are aberrantly expressed due to impaired epigenetic suppression display PAMPs, leading to their recognition by pattern recognition receptors (PRRs) of the innate immune system and triggering inflammation. This viral mimicry mechanism has been observed in various human diseases, including aging and autoimmune disorders. However, recent evidence suggests that retrotransposons possess highly regulated immune reactivity and play important roles in the development and function of the immune system. In this review, I discuss a wide range of retrotransposon-derived transcripts, their role as targets in immune recognition, and the diseases associated with retrotransposon activity. Furthermore, I explore the implications of chimeric transcripts formed between retrotransposons and known gene mRNAs, which have been previously underestimated, for the increase of immune-related gene isoforms and their influence on immune function. Retrotransposon-derived transcripts have profound and multifaceted effects on immune system function. The aim of this comprehensive review is to provide a better understanding of the complex relationship between retrotransposon transcripts and immune defense.

Correlation between antibodies against the pathogenic pHERV-W envelope protein and the inflammatory phase of multiple sclerosis

The role of retroviral envelope proteins belonging to the Human Endogenous Retroviral family 'W' (HERV-W), specifically syncytin-1 and pathogenic HERV-W (pHERV-W), as potential risk factors in multiple sclerosis (MS) has been established. This study aimed to investigate the humoral response to syncytin-1 and pHERV-W-derived peptides in a group of relapsing remitting MS patients categorized as having acute or stable disease. Furthermore, an inhibition assay was conducted to assess the extent of cross-reactivity between the two epitopes. The findings revealed that MS patients in the acute phase exhibited a higher specific antibody response to the pHERV-W env epitope compared to syncytin-1. This suggests a potential pathogenic role for pHERV-W env during the inflammatory stages of central nervous system involvement, and these antibody responses could serve as useful biomarkers for monitoring the progression of the disease.

Virus-Induced Cell Fusion and Syncytia Formation

Most enveloped viruses encode viral fusion proteins to penetrate host cell by membrane fusion. Interestingly, many enveloped viruses can also use viral fusion proteins to induce cell-cell fusion, both in vitro and in vivo, leading to the formation of syncytia or multinucleated giant cells (MGCs). In addition, some non-enveloped viruses encode specialized viral proteins that induce cell-cell fusion to facilitate viral spread. Overall, viruses that can induce cell-cell fusion are nearly ubiquitous in mammals. Virus cell-to-cell spread by inducing cell-cell fusion may overcome entry and post-entry blocks in target cells and allow evasion of neutralizing antibodies. However, molecular mechanisms of virus-induced cell-cell fusion remain largely unknown. Here, I summarize the current understanding of virus-induced cell fusion and syncytia formation.

Cell Fusion and Syncytia Formation in Cancer

The natural phenomenon of cell-cell fusion does not only take place in physiological processes, such as placentation, myogenesis, or osteoclastogenesis, but also in pathophysiological processes, such as cancer. More than a century ago postulated, today the hypothesis that the fusion of cancer cells with normal cells leads to the formation of cancer hybrid cells with altered properties is in scientific consensus. Some studies that have investigated the mechanisms and conditions for the fusion of cancer cells with other cells, as well as studies that have characterized the resulting cancer hybrid cells, are presented in this review. Hypoxia and the cytokine TNFα, for example, have been found to promote cell fusion. In addition, it has been found that both the protein Syncytin-1, which normally plays a role in placentation, and phosphatidylserine signaling on the cell membrane are involved in the fusion of cancer cells with other cells. In human cancer, cancer hybrid cells were detected not only in the primary tumor, but also in the circulation of patients as so-called circulating hybrid cells, where they often correlated with a worse outcome. Although some data are available, the questions of how and especially why cancer cells fuse with other cells are still not fully answered.

Early Syncytialization of the Ovine Placenta Revisited

Placentation is the development of a temporary arrangement between the maternal uterus and blastocyst-derived placental tissues designed to transport nutrients, gases, and other products from the mother to the embryo and fetus. Placentation differs histologically among species, but all types of placentation share the common trait of utilizing highly complex cell-to-cell and tissue-to-tissue morphological and biochemical interactions to remodel the uterine-placental interface. An elegant series of electron microscopy (EM) images supports the classification of ovine placentation as synepitheliochorial, because uterine luminal epithelial (LE) cells are maintained at the uterine-placental interface through incorporation into trophoblast syncytial plaques. In this review, we utilize immunofluorescence microscopy to provide further insights into early syncytialization of the ovine placenta. These observations, based on results using immunofluorescence microscopy, complement and expand, not replace, our understanding of syncytialization in sheep.

Galectin-1 Modulates the Fusogenic Activity of Placental Endogenous Retroviral Envelopes

Syncytin-1 and -2 are glycoproteins encoded by human endogenous retrovirus (hERV) that, through their fusogenic properties, are needed for the formation of the placental syncytiotrophoblast. Previous studies suggested that these proteins, in addition to the EnvP(b) envelope protein, are also involved in other cell fusion events. Since galectin-1 is a β-galactoside-binding protein associated with cytotrophoblast fusion during placental development, we previously tested its effect on Syncytin-mediated cell fusion and showed that this protein differently modulates the fusogenic potential of Syncytin-1 and -2. Herein, we were interested in comparing the impact of galectin-1 on hERV envelope proteins in different cellular contexts. Using a syncytium assay, we first demonstrated that galectin-1 increased the fusion of Syncytin-2- and EnvP(b)-expressing cells. We then tested the infectivity of Syncytin-1 and -2 vs. VSV-G-pseudotyped viruses toward Cos-7 and various human cell lines. In the presence of galectin-1, infection of Syncytin-2-pseudotyped viruses augmented for all cell lines. In contrast, the impact of galectin-1 on the infectivity of Syncytin-1-pseudotyped viruses varied, being cell- and dose-dependent. In this study, we report the functional associations between three hERV envelope proteins and galectin-1, which should provide information on the fusogenic activity of these proteins in the placenta and other biological and pathological processes.

The interaction of human immunodeficiency virus-1 and human endogenous retroviruses in patients (primary cell cultures) and cell line models

IMPORTANCE

In this work, we demonstrated that human immunodeficiency virus (HIV) infection leads to the modification of the human endogenous retrovirus (HERV) expression. Differential expression of multiple HERVs was found in peripheral blood mononuclear cells derived from HIV-infected patients compared to healthy donors and HIV-infected T cell cultures compared to non-infected. The effect of HIV presence on HERV expression appears to be more restricted in cells of monocytic origin, as only deregulation of HERV-W and HERV-K (HML-6) was found in these cell cultures after their infection with HIV. Multiple factors contribute to this aberrant HERV expression, and its levels appear to be modified in a time-dependent manner. Further studies and the development of optimized in vitro protocols are warranted to elucidate the interactions between HIV and HERVs in detail.

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

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

Progressive Exaptation of Endogenous Retroviruses in Placental Evolution in Cattle

Viviparity is made possible by the placenta, a structure acquired relatively recently in the evolutionary history of eutherian mammals. Compared to oviparity, it increases the survival rate of the fetus, owing to the eutherian placenta. Questions such as "How was the placenta acquired?" and "Why is there diversity in placental morphology among mammalian species?" remain largely unsolved. Our present understanding of the molecules regulating placental development remains unclear, owing in no small part to the persistent obscurity surrounding the molecular mechanisms underlying placental acquisition. Numerous genes associated with the development of eutherian placental morphology likely evolved to function at the fetal-maternal interface in conjunction with those participating in embryogenesis. Therefore, identifying these genes, how they were acquired, and how they came to be expressed specifically at the fetal-maternal interface will shed light on some crucial molecular mechanisms underlying placental evolution. Exhaustive studies support the hypothesis that endogenous retroviruses (ERVs) could be evolutional driving forces for trophoblast cell fusion and placental structure in mammalian placentas including those of the bovine species. This review focuses on bovine ERVs (BERVs) and their expression and function in the placenta.

Controlling Trophoblast Cell Fusion in the Human Placenta-Transcriptional Regulation of Suppressyn, an Endogenous Inhibitor of Syncytin-1

Cell fusion in the placenta is tightly regulated. Suppressyn is a human placental endogenous retroviral protein that inhibits the profusogenic activities of another well-described endogenous retroviral protein, syncytin-1. In this study, we aimed to elucidate the mechanisms underlying suppressyn's placenta-specific expression. We identified the promoter region and a novel enhancer region for the gene encoding suppressyn, ERVH48-1, and examined their regulation via DNA methylation and their responses to changes in the oxygen concentration. Like other endogenous retroviral genes, the ERVH48-1 promoter sequence is found within a characteristic retroviral 5' LTR sequence. The novel enhancer sequence we describe here is downstream of this LTR sequence (designated EIEs: ERV internal enhancer sequence) and governs placental expression. The placenta-specific expression of ERVH48-1 is tightly controlled by DNA methylation and further regulated by oxygen concentration-dependent, hypoxia-induced transcription factors (HIF1α and HIF2α). Our findings highlight the involvement of (1) tissue specificity through DNA methylation, (2) expression specificity through placenta-specific enhancer regions, and (3) the regulation of suppressyn expression in differing oxygen conditions by HIF1α and HIF2α. We suggest that these regulatory mechanisms are central to normal and abnormal placental development, including the development of disorders of pregnancy involving altered oxygenation, such as preeclampsia, pregnancy-induced hypertension, and fetal growth restriction.

Acquisition and Exaptation of Endogenous Retroviruses in Mammalian Placenta

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

Retrovirus-Derived RTL/SIRH: Their Diverse Roles in the Current Eutherian Developmental System and Contribution to Eutherian Evolution

Eutherians have 11 retrotransposon Gag-like (RTL)/sushi-ichi retrotransposon homolog (SIRH) genes presumably derived from a certain retrovirus. Accumulating evidence indicates that the RTL/SIRH genes play a variety of roles in the current mammalian developmental system, such as in the placenta, brain, and innate immune system, in a eutherian-specific manner. It has been shown that the functional role of Paternally Expressed 10 (PEG10) in placental formation is unique to the therian mammals, as are the eutherian-specific roles of PEG10 and PEG11/RTL1 in maintaining the fetal capillary network and the endocrine regulation of RTL7/SIRH7 (aka Leucine Zipper Down-Regulated in Cancer 1 (LDOCK1)) in the placenta. In the brain, PEG11/RTL1 is expressed in the corticospinal tract and hippocampal commissure, mammalian-specific structures, and in the corpus callosum, a eutherian-specific structure. Unexpectedly, at least three RTL/SIRH genes, RTL5/SIRH8, RTL6/SIRH3, and RTL9/SIRH10, play important roles in combating a variety of pathogens, namely viruses, bacteria, and fungi, respectively, suggesting that the innate immunity system of the brain in eutherians has been enhanced by the emergence of these new components. In this review, we will summarize the function of 10 out of the 11 RTL/SIRH genes and discuss their roles in eutherian development and evolution.

The important biological roles of Syncytin-1 of human endogenous retrovirus W (HERV-W) and Syncytin-2 of HERV-FRD in the human placenta development

BACKGROUND

 Human endogenous retroviruses (HERVs) entered the germ line by retroviral infection from a distant ancestor over 30 million years ago and constitute 8% of the human genome. The majorities of HERVs are non-protein coding and lack function because of the accumulation of mutations, insertions, deletions, and/or truncations. However, a small number of HERV genes carried ORFs with beneficial functions for the host.

METHODS & RESULTS

In this review, we summarize the structural and important biological roles of two HERV gene products termed Syncytin-1 and Syncytin-2 in human placenta development. Indeed, two retroviral gene products that have important roles in mammalian development, Syncytin-1 (HERV-W) and Syncytin-2 (HERV-FRD), are prime examples encoded by env genes and expressed in the placental trophoblasts. Several pivotal studies revealed that Syncytins are fundamental genes implicated in regulating trophoblast fusion and placenta morphogenesis.

CONCLUSION

 Interestingly, it has been suggested that syncytins may also be implicated in non-fusogenic activities leading to apoptosis, proliferation, and immunosuppressive activities.

Envelope Recombination: A Major Driver in Shaping Retroviral Diversification and Evolution within the Host Genome

Endogenous retroviruses (ERVs) are integrated into host DNA as the result of ancient germ line infections, primarily by extinct exogenous retroviruses. Thus, vertebrates' genomes contain thousands of ERV copies, providing a "fossil" record for ancestral retroviral diversity and its evolution within the host genome. Like other retroviruses, the ERV proviral sequence consists of gag, pro, pol, and env genes flanked by long terminal repeats (LTRs). Particularly, the env gene encodes for the envelope proteins that initiate the infection process by binding to the host cellular receptor(s), causing membrane fusion. For this reason, a major element in understanding ERVs' evolutionary trajectory is the characterization of env changes over time. Most of the studies dedicated to ERVs' env have been aimed at finding an "actual" physiological or pathological function, while few of them have focused on how these genes were once acquired and modified within the host. Once acquired into the organism, genome ERVs undergo common cellular events, including recombination. Indeed, genome recombination plays a role in ERV evolutionary dynamics. Retroviral recombination events that might have been involved in env divergence include the acquisition of env genes from distantly related retroviruses, env swapping facilitating multiple cross-species transmission over millions of years, ectopic recombination between the homologous sequences present in different positions in the chromosomes, and template switching during transcriptional events. The occurrence of these recombinational events might have aided in shaping retroviral diversification and evolution until the present day. Hence, this review describes and discusses in detail the reported recombination events involving ERV env to provide the basis for further studies in the field.

Reactivated endogenous retroviruses promote protein aggregate spreading

Prion-like spreading of protein misfolding is a characteristic of neurodegenerative diseases, but the exact mechanisms of intercellular protein aggregate dissemination remain unresolved. Evidence accumulates that endogenous retroviruses, remnants of viral germline infections that are normally epigenetically silenced, become upregulated in neurodegenerative diseases such as amyotrophic lateral sclerosis and tauopathies. Here we uncover that activation of endogenous retroviruses affects prion-like spreading of proteopathic seeds. We show that upregulation of endogenous retroviruses drastically increases the dissemination of protein aggregates between cells in culture, a process that can be inhibited by targeting the viral envelope protein or viral protein processing. Human endogenous retrovirus envelopes of four different clades also elevate intercellular spreading of proteopathic seeds, including pathological Tau. Our data support a role of endogenous retroviruses in protein misfolding diseases and suggest that antiviral drugs could represent promising candidates for inhibiting protein aggregate spreading.

Trophoblast Differentiation: Mechanisms and Implications for Pregnancy Complications

Placental development is a tightly controlled event, in which cell expansion from the trophectoderm occurs in a spatiotemporal manner. Proper trophoblast differentiation is crucial to the vitality of this gestational organ. Obstructions to its development can lead to pregnancy complications, such as preeclampsia, fetal growth restriction, and preterm birth, posing severe health risks to both the mother and offspring. Currently, the only known treatment strategy for these complications is delivery, making it an important area of research. The aim of this review was to summarize the known information on the development and mechanistic regulation of trophoblast differentiation and highlight the similarities in these processes between the human and mouse placenta. Additionally, the known biomarkers for each cell type were compiled to aid in the analysis of sequencing technologies.

The Insertion of an Evolutionary Lost Four-Amino-Acid Cytoplasmic Tail Peptide into a Syncytin-1 Vaccine Increases T- and B-Cell Responses in Mice

Human endogenous retrovirus type W (HERV-W) is expressed in various cancers. We previously developed an adenovirus-vectored cancer vaccine targeting HERV-W by encoding an assembled HERV-W group-specific antigen sequence and the HERV-W envelope sequence Syncytin-1. Syncytin-1 is constitutively fusogenic and forms large multinucleated cell fusions when overexpressed. Consequently, immunising humans with a vaccine encoding Syncytin-1 can lead to the formation of extensive syncytia, which is undesirable and poses a potential safety issue. Here, we show experiments in cell lines that restoring an evolutionary lost cleavage site of the fusion inhibitory R-peptide of Syncytin-1 inhibit cell fusion. Interestingly, this modification of the HERV-W vaccine's fusogenicity increased the expression of the vaccine antigens in vitro. It also enhanced Syncytin-1-specific antibody responses and CD8+-mediated T-cell responses compared to the wildtype vaccine in vaccinated mice, with a notable enhancement in responses to subdominant T-cell epitopes but equal responses to dominant epitopes and similar rates of survival following a tumour challenge. The impairment of cell-cell fusion and the enhanced immunogenicity profile of this HERV-W vaccine strengthens the prospects of obtaining a meaningful immune response against HERV-W in patients with HERV-W-overexpressing cancers.

Children with Chronic Immune Thrombocytopenia Exhibit High Expression of Human Endogenous Retroviruses TRIM28 and SETDB1

Chronic immune thrombocytopenia (CITP) is an autoimmune disease whose underlying biologic mechanisms remain elusive. Human endogenous retroviruses (HERVs) derive from ancestral infections and constitute about 8% of our genome. A wealth of clinical and experimental studies highlights their pivotal pathogenetic role in autoimmune diseases. Epigenetic mechanisms, such as those modulated by TRIM28 and SETDB1, are involved in HERV activation and regulation of immune response. We assessed, through a polymerase chain reaction real-time Taqman amplification assay, the transcription levels of pol genes of HERV-H, HERV-K, and HERV-W; env genes of Syncytin (SYN)1, SYN2, and HERV-W; as well as TRIM28 and SETDB1 in whole blood from 34 children with CITP and age-matched healthy controls (HC). The transcriptional levels of all HERV sequences, with the exception of HERV-W-env, were significantly enhanced in children with CITP as compared to HC. Patients on eltrombopag treatment exhibited lower expression of SYN1, SYN2, and HERV-W-env as compared to untreated patients. The mRNA concentrations of TRIM28 and SETDB1 were significantly higher and were positively correlated with those of HERVs in CITP patients. The over-expressions of HERVs and TRIM28/SETDB1 and their positive correlations in patients with CITP are suggestive clues of their contribution to the pathogenesis of the disease and support innovative interventions to inhibit HERV and TRIM28/SETDB1 expressions in patients unresponsive to standard therapies.

Single-cell analysis reveals transcriptomic and epigenomic impacts on the maternal-fetal interface following SARS-CoV-2 infection

During pregnancy the maternal-fetal interface plays vital roles in fetal development. Its disruption is frequently found in pregnancy complications. Recent studies show increased incidences of adverse pregnancy outcomes in patients with COVID-19; however, the mechanism remains unclear. Here we analysed the molecular impacts of SARS-CoV-2 infection on the maternal-fetal interface. Generating bulk and single-nucleus transcriptomic and epigenomic profiles from patients with COVID-19 and control samples, we discovered aberrant immune activation and angiogenesis patterns in distinct cells from patients. Surprisingly, retrotransposons were also dysregulated in specific cell types. Notably, reduced enhancer activities of LTR8B elements were functionally linked to the downregulation of pregnancy-specific glycoprotein genes in syncytiotrophoblasts. Our findings revealed that SARS-CoV-2 infection induced substantial changes to the epigenome and transcriptome at the maternal-fetal interface, which may be associated with pregnancy complications.

Molecular mechanisms of syncytin-1 in tumors and placental development related diseases

Human endogenous retroviruses (HERVs) have evolved from exogenous retroviruses and account for approximately 8% of the human genome. A growing number of findings suggest that the abnormal expression of HERV genes is associated with schizophrenia, multiple sclerosis, endometriosis, breast cancer, bladder cancer and other diseases. HERV-W env (syncytin-1) is a membrane glycoprotein which plays an important role in placental development. It includes embryo implantation, fusion of syncytiotrophoblasts and of fertilized eggs, and immune response. The abnormal expression of syncytin-1 is related to placental development-related diseases such as preeclampsia, infertility, and intrauterine growth restriction, as well as tumors such as neuroblastoma, endometrial cancer, and endometriosis. This review mainly focused on the molecular interactions of syncytin-1 in placental development-related diseases and tumors, to explore whether syncytin-1 can be an emerging biological marker and potential therapeutic target.

ERVW-1 Activates ATF6-Mediated Unfolded Protein Response by Decreasing GANAB in Recent-Onset Schizophrenia

Schizophrenia, a mental disorder, afflicts 1% of the worldwide population. The dysregulation of homeostasis in the endoplasmic reticulum (ER) has been implicated in schizophrenia. Moreover, recent studies indicate that ER stress and the unfolded protein response (UPR) are linked to this mental disorder. Our previous research has verified that endogenous retrovirus group W member 1 envelope (ERVW-1), a risk factor for schizophrenia, is elevated in individuals with schizophrenia. Nevertheless, no literature is available regarding the underlying relationship between ER stress and ERVW-1 in schizophrenia. The aim of our research was to investigate the molecular mechanism connecting ER stress and ERVW-1 in schizophrenia. Here, we employed Gene Differential Expression Analysis to predict differentially expressed genes (DEGs) in the human prefrontal cortex of schizophrenic patients and identified aberrant expression of UPR-related genes. Subsequent research indicated that the UPR gene called XBP1 had a positive correlation with ATF6, BCL-2, and ERVW-1 in individuals with schizophrenia using Spearman correlation analysis. Furthermore, results from the enzyme-linked immunosorbent assay (ELISA) suggested increased serum protein levels of ATF6 and XBP1 in schizophrenic patients compared with healthy controls, exhibiting a strong correlation with ERVW-1 using median analysis and Mann-Whitney U analysis. However, serum GANAB levels were decreased in schizophrenic patients compared with controls and showed a significant negative correlation with ERVW-1, ATF6, and XBP1 in schizophrenic patients. Interestingly, in vitro experiments verified that ERVW-1 indeed increased ATF6 and XBP1 expression while decreasing GANAB expression. Additionally, the confocal microscope experiment suggested that ERVW-1 could impact the shape of the ER, leading to ER stress. GANAB was found to participate in ER stress regulated by ERVW-1. In conclusion, ERVW-1 induced ER stress by suppressing GANAB expression, thereby upregulating the expression of ATF6 and XBP1 and ultimately contributing to the development of schizophrenia.

Exploring the Expression of the «Dark Matter» of the Genome in Mesothelioma for Potentially Predictive Biomarkers for Prognosis and Immunotherapy

Recent high-throughput RNA sequencing technologies have confirmed that a large part of the non-coding genome is transcribed. The priority for further investigations is nevertheless generally given in cancer to coding sequences, due to the obvious interest of finding therapeutic targets. In addition, several RNA-sequencing pipelines eliminate repetitive sequences, which are difficult to analyze. In this review, we shall focus on endogenous retroviruses. These sequences are remnants of ancestral germline infections by exogenous retroviruses. These sequences represent 8% of human genome, meaning four-fold the fraction of the genome encoding for proteins. These sequences are generally mostly repressed in normal adult tissues, but pathological conditions lead to their de-repression. Specific mesothelioma-associated endogenous retrovirus expression and their association to clinical outcome is discussed.

Antiviral activity of the human endogenous retrovirus‐R envelope protein against SARS‐CoV‐2

Coronavirus‐induced disease‐19 (COVID‐19), caused by SARS‐CoV‐2, is still a major global health challenge. Human endogenous retroviruses (HERVs) represent retroviral elements that were integrated into the ancestral human genome. HERVs are important in embryonic development as well as in the manifestation of diseases, including cancer, inflammation, and viral infections. Here, we analyze the expression of several HERVs in SARS‐CoV‐2‐infected cells and observe increased activity of HERV‐E, HERV‐V, HERV‐FRD, HERV‐MER34, HERV‐W, and HERV‐K‐HML2. In contrast, the HERV‐R envelope is downregulated in cell‐based models and PBMCs of COVID‐19 patients. Overexpression of HERV‐R inhibits SARS‐CoV‐2 replication, suggesting its antiviral activity. Further analyses demonstrate the role of the extracellular signal‐regulated kinase (ERK) in regulating HERV‐R antiviral activity. Lastly, our data indicate that the crosstalk between ERK and p38 MAPK controls the synthesis of the HERV‐R envelope protein, which in turn modulates SARS‐CoV‐2 replication. These findings suggest the role of the HERV‐R envelope as a prosurvival host factor against SARS‐CoV‐2 and illustrate a possible advantage of integration and evolutionary maintenance of retroviral elements in the human genome.

Intrinsic signalling factors associated with cancer cell-cell fusion

Cellular fusion e.g. between cancer cells and normal cells represents a stepwise process that is tightly regulated. During a pre-hybrid preparation program somatic cells and/or cancer cells are promoted to a pro-fusogenic state as a prerequisite to prepare a fusion process. A pro-fusogenic state requires significant changes including restructure of the cytoskeleton, e.g., by the formation of F-actin. Moreover, distinct plasma membrane lipids such as phosphatidylserine play an important role during cell fusion. In addition, the expression of distinct fusogenic factors such as syncytins and corresponding receptors are of fundamental importance to enable cellular mergers. Subsequent hybrid formation and fusion are followed by a post-hybrid selection process. Fusion among normal cells is important and often required during organismal development. Cancer cells fusion appears more rarely and is associated with the generation of new cancer hybrid cell populations. These cancer hybrid cells contribute to an elevated tumour plasticity by altered metastatic behaviour, changes in therapeutic and apoptotic responses, and even in the formation of cancer stem/ initiating cells. While many parts within this multi-step cascade are still poorly understood, this review article predominantly focusses on the intracellular necessities for fusion among cancer cells or with other cell populations of the tumour microenvironment. Video Abstract.

Anti-HERV-K Drugs and Vaccines, Possible Therapies against Tumors

The footprint of human endogenous retroviruses (HERV), specifically HERV-K, has been found in malignancies, such as melanoma, teratocarcinoma, osteosarcoma, breast cancer, lymphoma, and ovary and prostate cancers. HERV-K is characterized as the most biologically active HERV due to possession of open reading frames (ORF) for all Gag, Pol, and Env genes, which enables it to be more infective and obstructive towards specific cell lines and other exogenous viruses, respectively. Some factors might contribute to carcinogenicity and at least one of them has been recognized in various tumors, including overexpression/methylation of long interspersed nuclear element 1 (LINE-1), HERV-K Gag, and Env genes themselves plus their transcripts and protein products, and HERV-K reverse transcriptase (RT). Therapies effective for HERV-K-associated tumors mostly target invasive autoimmune responses or growth of tumors through suppression of HERV-K Gag or Env protein and RT. To design new therapeutic options, more studies are needed to better understand whether HERV-K and its products (Gag/Env transcripts and HERV-K proteins/RT) are the initiators of tumor formation or just the disorder’s developers. Accordingly, this review aims to present evidence that highlights the association between HERV-K and tumorigenicity and introduces some of the available or potential therapies against HERV-K-induced tumors.

Virus-Free Method to Control and Enhance Extracellular Vesicle Cargo Loading and Delivery

Extracellular vesicles (EVs)─including exosomes and microvesicles─are involved in cell-cell communication. EVs encapsulate different types of molecules such as proteins or nucleotides and are long-lasting contenders for the establishment of personalized drug delivery systems. Recent studies suggest that the intrinsic capacities for uptake and cargo delivery of basic EVs might be too limited to serve as a potent delivery system. Here, we develop two synergistic methods to, respectively, control EV cargo loading and enhance EV cargo delivery through fusion without requirement for any viral fusogenic protein. Briefly, cargo loading is enabled through a reversible drug-inducible system that triggers the interaction between a cargo of interest and CD63, a well-established transmembrane EV marker. Enhanced cargo delivery is promoted by overexpressing Syncytin-1, an endogenous retrovirus envelop protein with fusogenic properties encoded by the human genome. We validate our bioengineered EVs in a qualitative and quantitative manner. Finally, we utilize this method to develop highly potent killer EVs, which contain a lethal toxin responsible for protein translation arrest and acceptor cell death. These advanced methods and future downstream applications may open promising doors in the manufacture of virus-free and EV-based delivery systems.

Identification and Characterization of the HERV-K (HML-8) Group of Human Endogenous Retroviruses in the Genome

Human endogenous retroviruses (HERVs) can be vertically transmitted in a Mendelian fashion, are stably maintained in the human genome, and are estimated to constitute ∼8% of the genome. HERVs affect human physiology and pathology through their provirus-encoded protein or long terminal repeat (LTR) element effect. Characterization of the genomic distribution is an essential step to understanding the relationships between endogenous retrovirus expression and diseases. However, the poor characterization of human MMTV-like (HML)-8 prevents a detailed understanding of the regulation of the expression of this family in humans and its impact on the host genome. In light of this, the definition of an accurate and updated HERV-K HML-8 genomic map is urgently needed. In this study, we report the results of a comprehensive analysis of HERV-K HML-8 sequence presence and distribution within the human genome and hominoids, with a detailed description of the different structural and phylogenetic aspects characterizing the group. A total of 40 proviruses and 5 solo LTR elements for human were characterized, which included a detailed description of provirus structure, integration time, potentially regulated genes, transcription factor-binding sites, and primer-binding site features. Besides, 9 chimpanzee sequences, 8 gorilla sequences, and 10 orangutan sequences belonging to the HML-8 subgroup were identified. The integration time results showed that the HML-8 elements were integrated into the primate lineage around 35 and 42 million years ago (mya), during primates evolutionary speciation. Overall, the results clarified the composition of the HML-8 groups, providing an exhaustive background for subsequent functional studies.

Pregnancy Is Associated with Impaired Transcription of Human Endogenous Retroviruses and of TRIM28 and SETDB1, Particularly in Mothers Affected by Multiple Sclerosis

Accumulating evidence highlights the pathogenetic role of human endogenous retroviruses (HERVs) in eliciting and maintaining multiple sclerosis (MS). Epigenetic mechanisms, such as those regulated by TRIM 28 and SETDB1, are implicated in HERV activation and in neuroinflammatory disorders, including MS. Pregnancy markedly improves the course of MS, but no study explored the expressions of HERVs and of TRIM28 and SETDB1 during gestation. Using a polymerase chain reaction real-time Taqman amplification assay, we assessed and compared the transcriptional levels of pol genes of HERV-H, HERV-K, HERV-W; of env genes of Syncytin (SYN)1, SYN2, and multiple sclerosis associated retrovirus (MSRV); and of TRIM28 and SETDB1 in peripheral blood and placenta from 20 mothers affected by MS; from 27 healthy mothers, in cord blood from their neonates; and in blood from healthy women of child-bearing age. The HERV mRNA levels were significantly lower in pregnant than in nonpregnant women. Expressions of all HERVs were downregulated in the chorion and in the decidua basalis of MS mothers compared to healthy mothers. The former also showed lower mRNA levels of HERV-K-pol and of SYN1, SYN2, and MSRV in peripheral blood. Significantly lower expressions of TRIM28 and SETDB1 also emerged in pregnant vs. nonpregnant women and in blood, chorion, and decidua of mothers with MS vs. healthy mothers. In contrast, HERV and TRIM28/SETDB1 expressions were comparable between their neonates. These results show that gestation is characterized by impaired expressions of HERVs and TRIM28/SETDB1, particularly in mothers with MS. Given the beneficial effects of pregnancy on MS and the wealth of data suggesting the putative contribution of HERVs and epigenetic processes in the pathogenesis of the disease, our findings may further support innovative therapeutic interventions to block HERV activation and to control aberrant epigenetic pathways in MS-affected patients.

Expressions of Type I and III Interferons, Endogenous Retroviruses, TRIM28, and SETDB1 in Children with Respiratory Syncytial Virus Bronchiolitis

Interferons (IFNs) and IFN-stimulated genes (ISGs) play essential roles for the control of viral infections. Their expression in infants with respiratory syncytial virus (RSV) bronchiolitis is poorly defined. Human endogenous retroviruses (HERVs) represent 8% of our genome and modulate inflammatory and immune reactions. TRIM28 and SETDB1 participate in the epigenetic regulation of genes involved in the immune response, including IFNs and HERVs. No study has explored the expression of HERVs, TRIM28, and SETDB1 during RSV bronchiolitis. We assessed, through a PCR real-time Taqman amplification assay, the transcription levels of six IFN-I ISGs, four IFNλs, the pol genes of HERV-H, -K, and -W families, the env genes of Syncytin (SYN)1 and SYN2, and of TRIM28/SETDB1 in whole blood from 37 children hospitalized for severe RSV bronchiolitis and in healthy children (HC). The expression of most IFN-I ISGs was significantly higher in RSV+ patients than in age-matched HC, but it was inhibited by steroid therapy. The mRNA concentrations of IFN-λs were comparable between patients and age-matched HC. This lack of RSV-driven IFN-III activation may result in the defective protection of the airway mucosal surface leading to severe bronchiolitis. The expression of IFN-III showed a positive correlation with age in HC, that could account for the high susceptibility of young children to viral respiratory tract infections. The transcription levels of every HERV gene were significantly lower in RSV+ patients than in HC, while the expressions of TRIM28/SETDB1 were overlapping. Given the negative impact of HERVs and the positive effects of TRIM28/SETDB1 on innate and adaptive immune responses, the downregulation of the former and the normal expression of the latter may contribute to preserving immune functions against infection.

Unique Structure and Distinctive Properties of the Ancient and Ubiquitous Gamma-Type Envelope Glycoprotein

After the onset of the AIDS pandemic, HIV-1 (genus Lentivirus) became the predominant model for studying retrovirus Env glycoproteins and their role in entry. However, HIV Env is an inadequate model for understanding entry of viruses in the Alpharetrovirus, Gammaretrovirus and Deltaretrovirus genera. For example, oncogenic model system viruses such as Rous sarcoma virus (RSV, Alpharetrovirus), murine leukemia virus (MLV, Gammaretrovirus) and human T-cell leukemia viruses (HTLV-I and HTLV-II, Deltaretrovirus) encode Envs that are structurally and functionally distinct from HIV Env. We refer to these as Gamma-type Envs. Gamma-type Envs are probably the most widespread retroviral Envs in nature. They are found in exogenous and endogenous retroviruses representing a broad spectrum of vertebrate hosts including amphibians, birds, reptiles, mammals and fish. In endogenous form, gamma-type Envs have been evolutionarily coopted numerous times, most notably as placental syncytins (e.g., human SYNC1 and SYNC2). Remarkably, gamma-type Envs are also found outside of the Retroviridae. Gp2 proteins of filoviruses (e.g., Ebolavirus) and snake arenaviruses in the genus Reptarenavirus are gamma-type Env homologs, products of ancient recombination events involving viruses of different Baltimore classes. Distinctive hallmarks of gamma-type Envs include a labile disulfide bond linking the surface and transmembrane subunits, a multi-stage attachment and fusion mechanism, a highly conserved (but poorly understood) "immunosuppressive domain", and activation by the viral protease during virion maturation. Here, we synthesize work from diverse retrovirus model systems to illustrate these distinctive properties and to highlight avenues for further exploration of gamma-type Env structure and function.

Identification, comprehensive characterization, and comparative genomics of the HERV-K(HML8) integrations in the human genome

Around 8% of the human genome is composed by Human Endogenous Retroviruses (HERVs), ancient viral sequences inherited from the primate germ line after their infection by now extinct retroviruses. Given the still underexplored physiological and pathological roles of HERVs, it is fundamental to increase our information about the genomic composition of the different groups, to lay reliable foundation for functional studies. Among HERVs, the most characterized elements belong to the beta-like superfamily HERV-K, comprising 10 groups (HML1-10) with HML2 being the most recent and studied one. Among HMLs, the HML8 group is the only one still lacking a comprehensive genomic description. In the present work, we investigated HML8 sequences' distribution in the human genome (GRCh38/hg38), identifying 23 novel proviruses and characterizing the overall 78 HML8 proviruses in terms of genome structure, phylogeny, and integration pattern. HML8 elements were significantly enriched in human chromosomes 8 and X (p<0.005) while chromosomes 17 and 20 showed fewer integrations than expected (p<0.025 and p<0.005, respectively). Phylogenetic analyses classified HML8 members into 3 clusters, corresponding to the three LTR types MER11A, MER11B and MER11C. Besides different LTR types, common signatures in the internal structure suggested the potential existence of three different ancestral HML8 variants. Accordingly, time of integration estimation coupled with comparative genomics revealed that these three clusters have a different time of integration in the primates' genome, with MER11C elements being significantly younger than MER11A- and MER11B associated proviruses (p<0.005 and p<0.05, respectively). Approximately 30% of the HML8 elements were found co-localized within human genes, sometimes in exonic portions and with the same orientation, deserving further studies for their possible effects on gene expression. Overall, we provide the first detailed picture of the HML8 group distribution and variety among the genome, creating the backbone for the specific analysis of their transcriptional activity in healthy and diseased conditions.

Interplay between activation of endogenous retroviruses and inflammation as common pathogenic mechanism in neurological and psychiatric disorders

Human endogenous retroviruses (ERVs) are ancestorial retroviral elements that were integrated into our genome through germline infections and insertions during evolution. They have repeatedly been implicated in the aetiology and pathophysiology of numerous human disorders, particularly in those that affect the central nervous system. In addition to the known association of ERVs with multiple sclerosis and amyotrophic lateral sclerosis, a growing number of studies links the induction and expression of these retroviral elements with the onset and severity of neurodevelopmental and psychiatric disorders. Although these disorders differ in terms of overall disease pathology and causalities, a certain degree of (subclinical) chronic inflammation can be identified in all of them. Based on these commonalities, we discuss the bidirectional relationship between ERV expression and inflammation and highlight that numerous entry points to this reciprocal sequence of events exist, including initial infections with ERV-activating pathogens, exposure to non-infectious inflammatory stimuli, and conditions in which epigenetic silencing of ERV elements is disrupted.

A comprehensive investigation of human endogenous retroviral syncytin proteins and their receptors in men with normozoospermia and impaired semen quality

PURPOSE

The study aims to investigate first the presence of Syncytin 2 and its receptor, MFSD2, in human sperm, and second whether the expressions of Syncytin 1, Syncytin 2, and their receptors, SLC1A5 and MFSD2, differ between normozoospermic, asthenozoospermic, oligozoospermic, and oligoasthenozoospermic human sperm samples.

METHODS

The localization patterns and expression levels of syncytins and their receptors were evaluated in normozoospermic (concentration = 88.9 ± 5.5 × 10⁶, motility = 79.2 ± 3.15%, n = 30), asthenozoospermic (concentration = 51.7 ± 7.18 × 10⁶, motility = 24.0 ± 3.12%, n = 15), mild oligozoospermic (concentration = 13.5 ± 2.17 × 10⁶, motility = 72.1 ± 6.5%, n = 15), moderate oligozoospermic (concentration = 8.4 ± 3.21 × 10⁶, motility = 65.1 ± 8.9%, n = 15), severe oligozoospermic (concentration = 2.1 ± 1.01 × 10⁶, motility = 67.5 ± 3.2%, n = 15), and oligoasthenozoospermic (concentration = 5.5 ± 3.21 × 10⁶, motility = 18.5 ± 1.2%, n = 15) samples by immunofluorescence staining and western blot.

RESULTS

Syncytins and their receptors visualized by immunofluorescence showed similar staining patterns with slight staining of the tail in all spermatozoa regardless of normozoospermia, asthenozoospermia, oligozoospermia, or oligoasthenozoospermia. The localization patterns were categorized as equatorial segment, midpiece region, acrosome, and post-acrosomal areas. The combined staining patterns were also detected as acrosomal cap plus post acrosomal region, the midpiece plus equatorial segment, and midpiece plus acrosomal region. However, some sperm cells were categorized as non-stained. Both syncytin proteins were most intensely localized in the midpiece region, while their receptors were predominantly present in the midpiece plus acrosomal region. Conspicuously, syncytins and their receptors showed decreased expression in asthenozospermic, oligozoospermic, and oligoasthenozoospermic samples compared to normozoospermic samples.

CONCLUSION

The expression patterns of HERV-derived syncytins and their receptors were identical regardless of the spermatozoa in men with normozoospermia versus impaired semen quality. Further, asthenozoospermia, oligozoospermia, and oligoasthenozoospermia as male fertility issues are associated with decreased expression of both syncytins and their receptors.

[Plants and animals biological functions obtained from viruses]

Viruses can provide new biological functions to plants and animals. Some viruses persisting at low levels in plants might confer resistance to stress and parasites. In animals, more numerous examples of genes originating from viruses and used by different organisms have been described. For examples these genes might contribute to protect from new infections, or to ensure communication between neurons or to enable placenta development. In parasitic wasps, a complex viral machinery has been conserved as an endogenous virus dispersed in the wasp genome, which produces virions. These virions infect the parasitized host resulting in the production of virulence factors that inhibit defense mechanisms against the parasite. Different organisms have used the same viral functions repeatedly during animal evolution.

Human endogenous retroviruses and the inflammatory response: A vicious circle associated with health and illness

Human Endogenous Retroviruses (HERVs) are derived from ancient exogenous retroviral infections that have infected our ancestors' germline cells, underwent endogenization process, and were passed throughout the generations by retrotransposition and hereditary transmission. HERVs comprise 8% of the human genome and are critical for several physiological activities. Yet, HERVs reactivation is involved in pathological process as cancer and autoimmune diseases. In this review, we summarize the multiple aspects of HERVs' role within the human genome, as well as virological and molecular aspects, and their fusogenic property. We also discuss possibilities of how the HERVs are possibly transactivated and participate in modulating the inflammatory response in health conditions. An update on their role in several autoimmune, inflammatory, and aging-related diseases is also presented.

A novel function for the sperm adhesion protein IZUMO1 in cell-cell fusion

Mammalian sperm-egg adhesion depends on the trans-interaction between the sperm-specific type I glycoprotein IZUMO1 and its oocyte-specific GPI-anchored receptor JUNO. However, the mechanisms and proteins (fusogens) that mediate the following step of gamete fusion remain unknown. Using live imaging and content mixing assays in a heterologous system and structure-guided mutagenesis, we unveil an unexpected function for IZUMO1 in cell-to-cell fusion. We show that IZUMO1 alone is sufficient to induce fusion, and that this ability is retained in a mutant unable to bind JUNO. On the other hand, a triple mutation in exposed aromatic residues prevents this fusogenic activity without impairing JUNO interaction. Our findings suggest a second function for IZUMO1 as a unilateral mouse gamete fusogen.

The origins and formation of bone-resorbing osteoclasts

Osteoclasts (OCLs) are hematopoietic cells whose physiological function is to degrade bone. OCLs are key players in the processes that determine and maintain the mass, shape, and physical properties of bone. OCLs adhere to bone tightly and degrade its matrix by secreting protons and proteases onto the underlying surface. The combination of low pH and proteases degrades the mineral and protein components of the matrix and forms a resorption pit; the degraded material is internalized by the cell and then secreted into the circulation. Insufficient or excessive activity of OCLs can lead to significant changes in bone and either cause or exacerbate symptoms of diseases, as in osteoporosis, osteopetrosis, and cancer-induced bone lysis. OCLs are derived from monocyte-macrophage precursor cells whose origins are in two distinct embryonic cell lineages - erythromyeloid progenitor cells of the yolk sac, and hematopoietic stem cells. OCLs are formed in a multi-stage process that is induced by the cytokines M-CSF and RANKL, during which the cells differentiate, fuse to form multi-nucleated cells, and then differentiate further to become mature, bone-resorbing OCLs. Recent studies indicate that OCLs can undergo fission in vivo to generate smaller cells, called "osteomorphs", that can be "re-cycled" by fusing with other cells to form new OCLs. In this review we describe OCLs and discuss their cellular origins and the cellular and molecular events that drive osteoclastogenesis.

Stealing genes and facing consequences

The human genome contains a domesticated viral envelope gene with antiviral activity

Endogenous Jaagsiekte sheep retrovirus envelope protein promotes sheep trophoblast cell fusion by activating PKA/MEK/ERK1/2 signaling

BACKGROUND

Endogenous Jaagsiekte sheep retrovirus envelope protein (enJSRV-Env) plays an important role in trophoblast cell fusion in sheep. However, the underlying mechanism remains unclear.

METHODS

Primary endometrial luminal epithelial cells (LECs) were isolated from the sheep uterus and cocultured with sheep trophoblast cells (STCs). Giemsa staining was conducted to count multinucleated cells in the coculture system. Gain- and loss-of-function assays were performed to explore the role of enJSRV-Env in trophoblast cell fusion in the coculture system. Co-immunoprecipitation and mass spectrometry were carried out to identify the interacting partner of enJSRV-Env in the cocultures. Western blot analysis were conducted to determine the activation of protein kinase A (PKA)/mitogen-activated extracellular signal-regulated kinase (MEK)/extracellular signal-regulated kinase 1/2 (ERK1/2) signaling.

RESULTS

Primary LECs were identified by the expression of epithelial marker cytokeratin 18. Overexpression of enJSRV-Env promoted the formation of multinucleated cells in the coculture system. enJSRV-Env activated and physically interacted with PKA, along with the activation of MEK/ERK1/2 signaling. PKA inhibition completely reversed enJSRV-Env-induced MEK/ERK1/2 activation, and ERK1/2 inhibition abolished enJSRV-Env-induced formation of multinucleated cells in the coculture system.

CONCLUSION

enJSRV-Env promotes trophoblast cell fusion in the sheep placenta by activating PKA/MEK/ERK1/2 signaling. This finding reveals a novel mechanism underlying the contribution of enJSRV-Env to trophoblast cell fusion during placental morphogenesis.

Monotreme-Specific Conserved Putative Proteins Derived from Retroviral Reverse Transcriptase

Endogenous retroviruses (ERVs) have played an essential role in the evolution of mammals. ERV-derived genes are reported in the therians, many of which are involved in placental development; however, the contribution of the ERV-derived genes in monotremes, which are oviparous mammals, remains to be uncovered. Here, we conducted a comprehensive search for possible ERV-derived genes in platypus and echidna genomes and identified three reverse transcriptase-like genes named RTOM1, RTOM2, and RTOM3 clustered in the GRIP2 intron. Comparative genomic analyses revealed that RTOM1, RTOM2, and RTOM3 are strongly conserved and are under purifying selection between these species. These could be generated by tandem duplications before the divergence of platypus and echidna. All RTOM transcripts were specifically expressed in the testis, possibly suggesting their physiological importance. This is the first study reporting monotreme-specific de novo gene candidates derived from ERVs, which provides new insights into the unique evolution of monotremes.

How trophoblasts fuse: an in-depth look into placental syncytiotrophoblast formation

In humans, cell fusion is restricted to only a few cell types under normal conditions. In the placenta, cell fusion is a critical process for generating syncytiotrophoblast: the giant multinucleated trophoblast lineage containing billions of nuclei within an interconnected cytoplasm that forms the primary interface separating maternal blood from fetal tissue. The unique morphology of syncytiotrophoblast ensures that nutrients and gases can be efficiently transferred between maternal and fetal tissue while simultaneously restricting entry of potentially damaging substances and maternal immune cells through intercellular junctions. To maintain integrity of the syncytiotrophoblast layer, underlying cytotrophoblast progenitor cells terminate their capability for self-renewal, upregulate expression of genes needed for differentiation, and then fuse into the overlying syncytium. These processes are disrupted in a variety of obstetric complications, underscoring the importance of proper syncytiotrophoblast formation for pregnancy health. Herein, an overview of key mechanisms underlying human trophoblast fusion and syncytiotrophoblast development is discussed.