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

Abstract

Syncytins are envelope genes of retroviral origin that have been coopted 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.synepitheliochorial | placental evolution | phylogeny | placentome | ERV

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

“…In situ hybridization experiments using syncytin-Ten1 sequences as a probe clearly show that specific expression takes place at the level of the multinucleate syncytial structures generated at the feto-maternal interface of the placenta, consistent with this fusogenic syncytin gene having a direct role in syncytium formation. Syncytin-Ten1 adds to the syncytin genes previously identified in the Euarchontoglires and Laurasiatheria clades, with the two primate syncytin-1 and -2 genes (3-5), the two Muroidea syncytin-A and -B genes (7), the Leporidae syncytin-Ory1 gene (8), and the syncytin-Car1 and syncytin-Rum1 genes recently identified in the Carnivora and Ruminantia, respectively (9,10). Identification of syncytinTen1 clearly establishes that syncytin capture extends beyond the Boreoeutheria superorder within eutherian mammals, with syncytin-Ten1 being, to our knowledge, the first syncytin gene identified to date in the ancestrally diverged Atlantogenata superorder.…”

“…To identify putative env-derived syncytin genes in Afrotheria, we made use of the available lesser hedgehog tenrec genome sequence [2× coverage assembly of the E. telfairi genome, National Center for Biotechnology Information (NCBI) EchTel2.0, November 2012] and of the method that we previously devised to screen the cow genome for such genes (10). Basically, a BLAST search for ORFs (from the Met start codon to the stop codon) longer than 400 aa was performed using a selected series of env sequences, including all presently identified syncytins (Materials and Methods).…”

“…Recently, the direct involvement of syncytins in placentation has been demonstrated unambiguously through the generation of knockout mice for syncytin-A and -B (12,13), whose embryonic placenta displayed defects in cell-cell fusion, resulting in decreased maternal-fetal exchanges and impaired embryo survival. Interestingly, other captured env genes have been reported to be expressed specifically in the placenta [e.g., syncytin-like env-Cav1 (14), bosenv4/fematrin-1 (10,15), and enJSRV env (16) genes], with the last one reported to be involved in peri-implantation placental morphogenesis (17).…”

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

“…Along this line, it seems relevant to ask in what measure, depending on expression pattern, intrinsic fusogenic activity or receptor identity and localization, the syncytin genes present in each species may account for the observed variability in the morphology and organization of the utero-placental interface. In this context, it is of interest to note that a seventh syncytin gene was recently discovered in the suborder Ruminantia [64], in whose species the placenta lacks an extended syncytium layer, but displays synepitheliochorial placentation, which involves a very limited cell fusion process, with only trinucleate cells (or small multinucleated syncytial plaques in the case of sheep) being formed and with evidence for heterologous fusion between cells of fetal and maternal origin, a feature not found among the other eutherian mammals. This gene, named syncytin-Rum1, was first identified in the cow, where it was shown to be specifically expressed in the placenta.…”

Involvement of TLR7 and TLR8 in conceptus development and establishment of pregnancy in sheep

“…During pregnancy, viral particles shed into the uterine lumen transfect conceptus trophectoderm (Black et al 2010) as early as Day 12 (Dunlap et al 2005) and influence development, IFNT production, and BNC formation (Dunlap et al 2006a). Ovine enJSRV Envelope protein is a member of the syncytin family of retroviral proteins with high fusogenic activity responsible for inducing formation of the syncytiotrophoblasts in placenta of various species (Mi et al 2000, Cornelis et al 2013. In the ovine conceptus, mononuclear trophectoderm cells express abundant enJSRV-Env protein by Day 16 of pregnancy (Dunlap et al 2006a).…”