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PurposeThe 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.MethodsThe 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.ResultsSyncytins 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.ConclusionThe 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.

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.

Elevated levels of type I interferon (IFN) during pregnancy are associated with intrauterine growth retardation, preterm birth, and fetal demise through mechanisms that are not well understood. A critical step of placental development is the fusion of trophoblast cells into a multinucleated syncytiotrophoblast (ST) layer. Fusion is mediated by syncytins, proteins deriving from ancestral endogenous retroviral envelopes. Using cultures of human trophoblasts or mouse cells, we show that IFN-induced transmembrane proteins (IFITMs), a family of restriction factors blocking the entry step of many viruses, impair ST formation and inhibit syncytin-mediated fusion. Moreover, the IFN inducer polyinosinic:polycytidylic acid promotes fetal resorption and placental abnormalities in wild-type but not in Ifitm-deleted mice. Thus, excessive levels of IFITMs may mediate the pregnancy complications observed during congenital infections and other IFN-induced pathologies.

Syncytins are envelope genes from endogenous retroviruses that have been captured during evolution for a function in placentation. They have been found in all placental mammals in which they have been searched, including marsupials. Placental structures are not restricted to mammals but also emerged in some other vertebrates, most frequently in lizards, such as the viviparous Mabuya Scincidae. Here, we performed high-throughput RNA sequencing of a Mabuya placenta transcriptome and screened for the presence of retroviral env genes with a full-length ORF. We identified one such gene, which we named “syncytin-Mab1,” that has all the characteristics expected for a syncytin gene. It encodes a membrane-bound envelope protein with fusogenic activity ex vivo, is expressed at the placental level as revealed by in situ hybridization and immunohistochemistry, and is conserved in all Mabuya species tested, spanning over 25 My of evolution. Its cognate receptor, required for its fusogenic activity, was searched for by a screening assay using the GeneBridge4 human/Chinese hamster radiation hybrid panel and found to be the MPZL1 gene, previously identified in mammals as a signal-transducing transmembrane protein involved in cell migration. Together, these results show that syncytin capture is not restricted to placental mammals, but can also take place in the rare nonmammalian vertebrates in which a viviparous placentotrophic mode of reproduction emerged. It suggests that similar molecular tools have been used for the convergent evolution of placentation in independently evolved and highly distant vertebrates.

Pre-eclampsia (PE) is a pregnancy-specific disorder, characterized by hypertension and proteinuria. In PE, trophoblasts mediated inadequate remodeling of uterine spiral arteries seem to interrupt uteroplacental blood flow, one of the hallmarks in the early onset of PE (EO-PE). This, in turn, results in placental ischemia–reperfusion injury during hypoxia and reoxygenation episodes, leading to the generation of reactive oxygen species (ROS) and oxidative stress (OS). But still it is debatable if OS is a cause or consequence of PE. In this present study, we have investigated the effects of OS on PE placentae and trophoblast cell functions using BeWo and HTR8/SVneo cell lines. PE placental tissues showed abnormal ultrastructure, high level of reactive oxygen species (ROS) with altered unfolded protein responses (UPR) in compare with term placental tissues. Similar to PE placentae, during OS induction, the trophoblast cells showed altered invasion and migration properties with significantly variable expression of differentiation and invasion markers, e.g., syncytin and MMPs. The effect was rescued by antioxidant, N -acetyl cysteine, thereby implying a ROS-specific effect and in the trophoblast cells, OS triggers UPR pathway through IRE1α-XBP1 axis. Taken together, these findings highlight the harmful effect of unfolded protein response, which was induced due to OS on trophoblast cells and deformed invasion and differentiation programme and can be extended further to clinical settings to identify clinically approved antioxidants during pregnancy as a therapeutic measure to reduce the onset of PE.

Please see the correct Table 1 here. thumbnail Download: * PPT PowerPoint slide * PNG larger image * TIFF original image Table 1. (2013) Reduced Syncytin-1 Expression Levels in Placental Syndromes Correlates with Epigenetic Hypermethylation of the ERVW-1 Promoter Region.

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.

Abstract Endogenous retroviral envelope (ERV env) genes, notably syncytins, are known for driving placental development in mammals and lizards. However, their broader contributions to non-mammalian vertebrates, particularly viviparous fish, remain largely unexplored. Here, we present the discovery and functional characterization of three co-opted/captured ERV env gene clades, including a Percom-env clade comprising previously reported percomORF in the viviparous teleost Sebastes schlegelii. Our findings reveal that each gene clade plays a distinct and critical role in neural regulation, reproductive maturation, and viviparity. Notably, Percom-env percomORF retains fusogenic activity and its brain-specific expression is driven by conserved regulatory elements across Percomorpha. Meanwhile, Seb-env4 env genes, expressed uniquely in the testis, support seasonal gonadal maturation and Sertoli cell maintenance. Lastly, Seb-env3 env genes, localized at the maternofetal interface, retain a robust fusion capacity essential for follicular placentation, and have persisted for approximately 15 million years in the Sebastes genus, thus identified as candidate syncytin-Seb, likely underpinning the emergence of viviparity. These findings demonstrate env co-option/capture drives teleost adaptations, extending retroviral env-mediated placentation beyond mammals and lizards, and highlight conserved mechanisms in vertebrate reproductive evolution.

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.

Brain development and function require uptake of essential omega-3 fatty acids in the form of lysophosphatidylcholine via major-facilitator superfamily transporter MFSD2A, a potential pharmaceutical target to modulate blood–brain barrier (BBB) permeability. MFSD2A is also the receptor of endogenous retroviral envelope syncytin-2 (SYNC2) in human placenta, where it mediates cell–cell fusion and formation of the maternal–fetal interface. Here, we report a cryo-electron microscopy structure of the human MFSD2A–SYNC2 complex that reveals a large hydrophobic cavity in the transporter C-terminal domain to occlude long aliphatic chains. The transporter architecture suggests an alternating-access transport mechanism for lipid substrates in mammalian MFS transporters. SYNC2 establishes an extensive binding interface with MFSD2A, and a SYNC2-soluble fragment acts as a long-sought-after inhibitor of MFSD2A transport. Our work uncovers molecular mechanisms important to brain and placenta development and function, and SYNC2-mediated inhibition of MFSD2A transport suggests strategies to aid delivery of therapeutic macromolecules across the BBB. Major facilitator superfamily protein MFSD2A takes essential omega-3 fatty acids into the brain, and is the receptor of SYNC2 membrane fusogen in placenta. A cryo-EM structure of the human MFSD2A–SYNC2 complex reveals key aspects of MFSD2A transport and receptor mechanisms.