Identification of an endogenous retroviral envelope gene with fusogenic activity and placenta-specific expression in the rabbit: a new "syncytin" in a third order of mammals

Abstract

BackgroundSyncytins are envelope genes of retroviral origin that have been co-opted by the host to mediate a specialized function in placentation. Two of these genes have already been identified in primates, as well as two distinct, non orthologous genes in rodents.ResultsHere we identified within the rabbit Oryctolagus cuniculus-which belongs to the lagomorpha order- an envelope (env) gene of retroviral origin with the characteristic features of a bona fide syncytin, that we named syncytin-Ory1. An in silico search for full-length env genes with an uninterrupted open reading frame within the rabbit genome first identified two candidate genes that were tested for their specific expression in the placenta by quantitative RT-PCR of RNA isolated from a large set of tissues. This resulted in the identification of an env gene with placenta-specific expression and belonging to a family of endogenous retroelements present at a limited copy number in the rabbit genome. Functional characterization of the identified placenta-expressed env gene after cloning in a CMV-driven expression vector and transient transfection experiments, demonstrated both fusogenic activity in an ex vivo cell-cell fusion assay and infectivity of pseudotypes. The receptor for the rabbit syncytin-Ory1 was found to be the same as that for human syncytin-1, i.e. the previously identified ASCT2 transporter. This was demonstrated by a co-culture fusion assay between hamster A23 cells transduced with an expression vector for ASCT2 and A23 cells transduced with syncytin-Ory1. Finally, in situ hybridization of rabbit placenta sections with a syncytin-Ory1 probe revealed specific expression at the level of the junctional zone between the placental lobe and the maternal decidua, where the invading syncytial fetal tissue contacts the maternal decidua to form the labyrinth, consistent with a role in the formation of the syncytiotrophoblast. The syncytin-Ory1 gene is found in Leporidae but not in Ochotonidae, and should therefore have entered the lagomorpha order 12-30 million years ago.ConclusionThe identification of a novel syncytin gene within a third order of mammals displaying syncytiotrophoblast formation during placentation strongly supports the notion that on several occasions retroviral infections have resulted in the independent capture of genes that have been positively selected for a convergent physiological role.

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.…”

“…(7) Higher magnification of the views in 5 demonstrating the absence of a cellular membrane between nuclei (nu). (8 and 9) Higher magnifications of the cells in 6 centered on the interface between the syncytium and the uterine epithelium (8) or the maternal endothelial cell (9). Note the space between the syncytium and the epithelial or the endothelial cellular membranes (indicated by arrowheads), suggesting that the basal membrane of these cells is not degraded by the syncytium at this stage.…”

Human endogenous retrovirus K (HERV-K) is expressed in villous and extravillous cytotrophoblast cells of the human placenta

“…Knockout mice for the murine syncytin-A died in utero, indicating that this gene is essential for mouse embryonic development . Endogenous retroviral env genes were also "enslaved" for placental functions in sheep (Black et al, 2010) and rabbits (Heidmann et al, 2009). In the sheep, the envelope gene of endogenous retroviruses related to the Jaagsiekte sheep retrovirus was shown to be involved in placenta evolution and inhibition of their expression by RNA interference compromised conceptus elongation.…”

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

“…Furthermore, a fifth syncytin gene, syncytin-Ory1, distinct from each of those just listed, has been identified in yet another mammalian lineage, the Leporidae family (rabbit and hare) [59]. The syncytin-Ory1 gene codes for a placenta-specific envelope protein endowed with fusogenic activity and has been conserved for over 12 Myr.…”