In contrast, ‘ancient’ ERVs invaded the genomes before speciation and, consequently, are present in every individual at the same genomic location of phylogenetically related species.8 The biological significance of ERVs has been debated for several decades, and in the past they were generally thought to be ‘junk DNA’.9 However, recent studies suggest that ERVs have a variety of beneficial roles to their host.10–12 At the very least, the abundance of these elements in the host genome suggests that they contribute to genome plasticity. Moreover,

the presence of transcriptionally active ERVs with intact open reading frames conserved million of years after integration supports the idea that some ERVs were exapted by the host for specific biological roles. In this review, we will focus on the biological roles MAPK inhibitor of ERVs in development of the placenta and then highlight the biological role of sheep JSRV-related endogenous betaretroviruses (enJSRVs) in conceptus (embryo and PS-341 purchase associated extraembryonic membranes) development. ERVs have been speculated to play a physiological role in placenta morphogenesis for almost three decades, considering that retroviral particles have been frequently observed in the reproductive

tract.13–18 In fact, ERVs are abundant in the genital tract and placenta of various animal species.17,19 Baf-A1 The presence of intact env genes that are expressed in the multinucleated syncytiotrophoblasts of the placenta and preserved over thousands of years, together with the observation that they elicit fusion of cells in vitro, led to the speculation that ERVs play an essential role in placental development and were positively selected for a fundamental role in the evolution of placental

mammals and development of viviparity.20–24 HERV-W (ERVWE1), HERV-FRD, and ERV-3 are three human ERVs (HERV) whose intact env genes are expressed in the human placenta.25–27 HERV-W is not present in the human genome as a complete provirus; however, its env gene (ERVWE1), encoding a protein termed syncytin 1, is preferentially expressed in the syncytiotrophoblast. The syncytiotrophoblast is a multinucleated cell that lines the outer surface of the placenta, is derived by intercellular fusion of trophoblast cells, and is responsible for the transport of oxygen, nutrients, and waste products, production of hormones, and immune tolerance.28,29 Syncytin 1 is a glycosylated protein and possesses characteristic features of a retroviral Env protein, such as the presence of a leader peptide, a potential furin cleavage site, a fusion peptide-like sequence, and a putative immunosuppressive region (Fig. 2). It also contains a hydrophobic membrane-spanning domain, suggesting it could be inserted into the plasma membrane.