A placenta-specific receptor for the fusogenic, endogenous retrovirus-derived, human syncytin-2

Abstract

Syncytin-2 is an envelope gene from the human endogenous retrovirus FRD (HERV-FRD) co-opted by an ancestral primate host, conserved in evolution over >40 Myr, specifically expressed in the placenta, and with a cell-cell fusogenic activity likely contributing to placenta morphogenesis. Here, using the GeneBridge4 human/Chinese hamster radiation hybrid panel, we mapped and identified the human receptor for syncytin-2. This receptor-namely Major Facilitator Superfamily Domain Containing 2 (MFSD2)-belongs to a large family of presumptive carbohydrate transporters with 10 -12 membrane-spanning domains, is located at chromosomal position 1p34.2, and is conserved in evolution. An expression vector for MFSD2 confers fusogenicity to otherwise insusceptible cells upon transfection of syncytin-2. It also confers infectivity to syncytin-2 pseudotypes, consistent with this protein being the receptor for the ancestrally acquired HERV-FRD family of endogenous retroviruses. At variance with the human gene, neither mouse nor rat MFSD2 can mediate membrane fusion, which is consistent with the fact that the envelope-derived syncytin genes co-opted by rodents during evolution are not orthologous to the human syncytin genes. Remarkably, a real-time quantitative RT-PCR analysis of MFSD2 in various human tissues demonstrates specific expression in the placenta, as well as in the human BeWo choriocarcinoma cell line, which discloses enhancement of receptor expression upon induction by forskolin of cell-cell fusion and syncytium formation. In situ hybridization of human placental tissue using an MFSD2-specific probe further unambiguously demonstrates receptor expression at the level of the syncytiotrophoblast, again consistent with a role in placenta morphogenesis.envelope protein ͉ human endogenous retrovirus (HERV) ͉ major facilitator superfamily domain containing 2 (MFSD2) ͉ syncytiotrophoblast T he placenta is an autonomous and transient organ essentially aimed at mediating nutrient and gas exchange between mother and fetus during intrauterine life. In several mammalian species with a hemochorial placenta-including human-a key process of placental morphogenesis is the fusion of trophoblastic cells into a multinucleated layer called syncytiotrophoblast, which constitutes the main materno-fetal barrier in direct contact with maternal blood and fulfils essential trophic exchange functions (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 (4-8). The mammalian genomes indeed harbor thousands of ERV elements that display a structure close to that of the integrated proviral form of exogenous retroviruses (gag, pol, and env-related regions flanked by 2 long terminal repeats) and most probably are the remnants of past infections of the germ line by ancestral retroviruses (9, 10). Although the vast majority...

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

“…Both env genes display placenta-specific expression and induce syncytium formation when introduced into cultured cells (figure 2c), which led to their designation as syncytin-1 and syncytin-2, respectively. Each protein binds a distinct receptor: syncytin-1 interacts with the neutral amino acid transporter ASCT2 [36] and syncytin-2 with the multipass transmembrane protein MFSD2 [39]. Comparative analysis of the syntenic regions within the genomes from various species revealed that the syncytin-1 gene entered the genome of primates before the separation of Hominoids from Old World monkeys, being conserved only in Hominoids [40], whereas syncytin-2 was found to be conserved in the genome of all primates, except prosimians [38].…”

Les syncytines

“…Puis la sous-unité d'ancrage transmembranaire (TM) induit la fusion entre la membrane du virus et la membrane plasmique de la cellule cible grâce à un peptide de fusion ( Figure 1A et 1B Figure 1D), d'où leur appellation de syncytine-1 et syncytine-2. Les deux protéines interagissent avec des récepteurs distincts : le récepteur des rétrovirus de type D (RDR/ASCT-2, pour sodium-dependent neutral amino acid transporter type 2) pour la syncytine-1 [10] et une protéine transmembranaire MFSD2 (major facilitator superfamily domain containing 2A) pour la syncytine-2 [13]. Des analyses génomiques comparatives ont révélé que la syncytine-1 est apparue dans le génome primate après la spéciation Hominoïdae/singes de l'Ancien Monde, il y a environ 25 millions d'années (Ma) [14].…”

“…La syncytine-1, exprimée de manière peu spécifique dans le placenta, est détectée dans tous les trophoblastes des lignages villeux (cytotrophoblastes et syncytiotrophoblaste) et extravilleux [19,20] ( Figure 2B). En revanche, la syncytine-2 est détectée dans une fraction des cytotrophoblastes des villosités [21] et son récepteur, MFSD2, est exprimé dans le syncytiotrophoblaste [13] ( Figure 2B). L'expression très régulée des deux partenaires syncytine-2/MFSD2 dans des types cellulaires [21,37]) et du récepteur de la syncytine-2, MFSD2 (expériences d'hybridation in situ sur coupes de placenta du 3 e trimestre [13]), dans le placenta humain.…”

“…En revanche, la syncytine-2 est détectée dans une fraction des cytotrophoblastes des villosités [21] et son récepteur, MFSD2, est exprimé dans le syncytiotrophoblaste [13] ( Figure 2B). L'expression très régulée des deux partenaires syncytine-2/MFSD2 dans des types cellulaires [21,37]) et du récepteur de la syncytine-2, MFSD2 (expériences d'hybridation in situ sur coupes de placenta du 3 e trimestre [13]), dans le placenta humain. C. Fusion de cellules trophoblastiques humaines en culture primaire après 72 heures de culture, suite à l'extinction de l'expression de la syncytine-2 par siARN [23].…”

“…L'architecture placentaire de ces embryons est perturbée, avec une accumulation de trophoblastes non fusionnés, et un défaut de fusion d'une des deux couches distincts, permettrait l'incorporation, par fusion, des cytotrophoblastes mononucléés dans le syncytiotrophoblaste adjacent, tout en évitant que les cytotrophoblastes ne fusionnent entre eux. Ce processus permettrait la croissance et le renouvellement du tissu syncytial tout au long de la grossesse [13]. Dans des modèles de différenciation spontanée de cytotrophoblastes établis en culture primaire, l'extinction de la syncytine-2 et, dans une moindre mesure, celle de la syncytine-1, par ajout d'oligonucléotides antisens ou par ARN interférence (siARN), diminuent fortement l'efficacité de fusion et la production d'hCG associée [22,23] (Figure 2C).…”