Placental syncytins: Genetic disjunction between the fusogenic and immunosuppressive activity of retroviral envelope proteins

Abstract

We have previously demonstrated that the envelope proteins of a murine and primate retrovirus are immunosuppressive in vivo. This property was manifested by the ability of the proteins, when expressed by allogeneic tumor cells normally rejected by engrafted mice, to have the env-expressing cells escape (at least transiently) immune rejection. Here, we analyzed the immunosuppressive activity of the human and murine syncytins. These are envelope genes from endogenous retroviruses independently coopted by ancestral hosts, conserved in evolution, specifically expressed in the placenta, and with a cell-cell fusogenic activity likely contributing to placenta morphogenesis. We show that in both humans and mice, one of the two syncytins (human syncytin-2 and mouse syncytin-B) is immunosuppressive and, rather unexpectedly, the other (human syncytin-1 and mouse syncytin-A) is not (albeit able to induce cell-cell fusion). Delineation of the immunosuppressive domain by deletion analysis, combined with a comparison between immunosuppressive and nonimmunosuppressive sequences, allowed us to derive a mutation rule targeted to specific amino acids, resulting in selective switch from immunosuppressive to nonimmunosuppressive envelope proteins and vice versa. These results unravel a critical function of retroviral envelopes, not necessarily ''individually'' selected for in the retrovirus endogenization process, albeit ''tandemly'' conserved in evolution for the syncytin pairs in primates and Muridae. Selective inactivation of immunosuppression, under conditions not affecting fusogenicity, should be important for understanding the role of this function in placental physiology and maternofetal tolerance.endogenous retrovirus ͉ fusogenicity ͉ immunosuppression ͉ HERV T he placenta is an autonomous and transient organ essentially intended for feeding and oxygenating the embryo and the fetus during intrauterine life. In several mammalian species, including Homo sapiens, the fusion of trophoblastic cells into a multinucleated layer called syncytiotrophoblast constitutes a key process of placental morphogenesis. The syncytiotrophoblast, being the main maternofetal barrier in direct contact with maternal blood, performs the essential trophic exchange functions between mother and fetus, along with the secretion of hormones and growth factors, maintenance of homeostasis, and the necessary inhibition of the mother's immune response against the allogeneic determinants of the fetus (1-3).Little is known about the molecular mechanisms involved in trophoblastic differentiation. However, a major advance has been made by the identification of envelope (Env) proteins encoded by endogenous retroviruses (ERVs) and likely involved in the formation of the syncytiotrophoblast (4-7). The human and murine genomes indeed harbor thousands of ERV elements that display a structure close to that of the integrated proviral form of exogenous retroviruses and that most probably are the remnants of past infections of the germ line by ancestral retroviruses (8-11)...

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

“…rstb.royalsocietypublishing.org Phil Trans R Soc B 368: 20120507 further substantiated in our laboratory using in vivo tumour rejection assays that measured the ability of the proteins, when expressed in allogeneic tumour cells normally rejected by engrafted mice, to allow these cells to escape, at least transiently, immune rejection [27]. This approach led to the delineation of the ISD within a 20 amino acid-long peptide and to the identification of two amino acid positions where introduction of specific mutations could abolish the immunosuppressive activity without affecting the fusogenicity and infectivity of the protein [28]. The ISD was next demonstrated to be critical for the propagation of the retrovirus itself [29]: the presence of the ISD-inactivating mutations within the env gene fully impaired the replication of Friend MLV in normal immunocompetent mice, whereas the mutant virus replicated with the same efficiency as the wild-type, both ex vivo in cultured cells and in vivo in irradiated immunocompromized mice.…”

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

“…Syncytin proteins are expected to participate in the formation of the placental syncytiotrophoblast (ST) at the maternal-fetal interface via fusion of the mononucleate cytotrophoblasts (CTs). Some of them also possess an immunosuppressive activity, as classically observed for infectious retroviral envelope glycoproteins, which may be involved in maternal-fetal tolerance (11). 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.…”

Les syncytines

“…Or, les syncytines auraient conservé cette propriété de leur ancêtre infectieux, puisqu'au moins une d'entre elles, à la fois chez les primates (syncytine-2) et chez les rongeurs (syncytine-B), s'est avérée être immunosuppressive, dans des tests in vivo basés sur l'inhibition du rejet de tumeurs par le système immunitaire de la souris [36]. Cette activité est portée par un domaine spécifique (ISU pour immunosuppressive domain) de la sous-unité transmembranaire de l'enveloppe ( Figure 1A), et un acide aminé critique pour cette fonction immunosuppressive a été identifié.…”

“…Elle pourrait même constituer la fonction première des syncytines, avant leur fonction de fusion. La possibilité de générer des souris exprimant une syncytine-B mutée qui a perdu sélectivement ses propriétés immunosuppressives mais garde intactes ses propriétés de fusion [36] …”