In both humans and mice (Fig. 2), 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 although both are able to induce cell–cell fusion.33 Syncytin-A plays an important biological role in syncytiotrophoblast
development, because syncytin-A null mice die in utero because of the failure of trophoblast cells to fuse and form one of GPCR & G Protein inhibitor the two syncytiotrophoblast layers present in the mouse placenta39 that play a key role in transport of nutrients for the developing conceptus.29 Given that two syncytins are immunosuppressive, they may play a role in maternofetal tolerance, although this concept has not been mechanistically tested in vivo.33 Recently, Heidmann et al.24 identified an env gene of retroviral origin in the rabbit Oryctolagus cuniculus, termed syncytin-Ory1, with the characteristic features of human syncytin (Fig. 2). An in silico search for full-length env genes with an uninterrupted open reading frame within the rabbit genome resulted in the identification of an env gene with placenta-specific expression and belonging to a family Kinase Inhibitor Library cell assay of endogenous retroelements present at a limited copy number in the rabbit genome. The placenta-expressed env gene demonstrated fusogenic activity
in an ex vivo cell–cell fusion assay. Interestingly, the receptor for the rabbit syncytin-Ory1 was found to be the same as that for human syncytin 1, i.e. the previously identified sodium-dependent neutral amino acid transporter type 2 (SLC1A5). Syncytin-Ory1 mRNA was specifically present at the level of the junctional zone of the placenta, 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 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 were positively selected for a convergent physiological role in development of the placenta.24 Domestic sheep have at least either 27 copies of ERVs in their genome, termed enJSRVs (Fig. 1), because they are highly related to the exogenous and pathogenic JSRV.6,40 JSRV is the causative agent of ovine pulmonary adenocarcinoma, a transmissible lung cancer of sheep.41 A unique feature of JSRV among oncogenic retroviruses is that its Env glycoprotein is the main determinant of cell transformation both in vitro and in vivo.42–48 Expression of the JSRV Env alone is able to transform a variety of cell lines in vitro, including mouse, rat, and chicken fibroblasts as well as human bronchial, canine, and rat epithelial cells.