Screening human sequence databases for endogenous retroviral elements with coding envelope genes has revealed 16 candidate genes that we assayed for their fusogenic properties. All 16 genes were cloned in a eukaryotic expression vector and assayed for cell-cell fusion by using a large panel of mammalian cells in transient transfection assays. Fusion was observed for two human endogenous retrovirus (HERV) envelopes, the previously characterized HERV-W envelope, also called syncytin, and a previously uncharacterized gene from the HERV-FRD family. Cells prone to env-mediated fusion were different for the two envelopes, indicating different receptor usage. A search for the FRDenv gene in primates indicated that the corresponding proviral element is present in all simians, from New World monkeys to humans, being absent only in prosimians. Cloning of the corresponding env genes in simians disclosed conservation of the fully coding status of the gene, and most remarkably, conservation of its fusogenic property. Finally, a Northern blot analysis for the expression of the FRD family among a series of human tissues demonstrated specific expression in the placenta, as previously demonstrated for the other fusogenic human envelope of the HERV-W family. Altogether, the present data have identified a previously uncharacterized envelope (that we propose to name syncytin 2 after renaming syncytin as syncytin 1) with a potential role in placenta formation, and the identification of the complete set of retroviral envelopes with fusogenic properties now allows a definite analysis of the possible role of HERV in this physiological process, via classical genetic approaches.

Recently, we and others have identified two human endogenous retroviruses that entered the primate lineage 25-40 million years ago and that encode highly fusogenic retroviral envelope proteins (syncytin-1 and -2), possibly involved in the formation of the placenta syncytiotrophoblast layer generated by trophoblast cell fusion at the materno-fetal interface. A systematic in silico search throughout mouse genome databases presently identifies two fully coding envelope genes, present as unique copies and unrelated to any known murine endogenous retrovirus, that we named syncytin-A and -B. Quantitative RT-PCR demonstrates placentaspecific expression for both genes, with increasing transcript levels in this organ from 9.5 to 14.5 days postcoitum. In situ hybridization of placenta cryosections further localizes these transcripts in the syncytiotrophoblast-containing labyrinthine zona. Consistently, we show that both genes can trigger cell-cell fusion in ex vivo transfection assays, with distinct cell type specificities suggesting different receptor usage. Genes orthologous to syncytin-A and -B and disclosing a striking conservation of their coding status are found in all Muridae tested (mouse, rat, gerbil, vole, and hamster), dating their entry into the rodent lineage Ϸ20 million years ago. Together, these data strongly argue for a critical role of syncytin-A and -B in murine syncytiotrophoblast formation, thus unraveling a rather unique situation where two pairs of endogenous retroviruses, independently acquired by the primate and rodent lineages, would have been positively selected for a convergent physiological role.endogenous retrovirus ͉ placenta ͉ syncytiotrophoblast ͉ rodent E ndogenous retroviruses (ERVs) are present in the genome of all vertebrates (1-3). They are most probably the genomic traces of germ-line infections by exogenous retroviruses having taken place during evolution. Over time, increase in their copy number has occurred, at least for some of them, via retrotransposition or germ-cell reinfection. This generated multigenic families containing a few to several hundred elements and now accounting for a substantial fraction of the genome [8% for the human (4) and 10% for the murine (5) genomes]. In mice, eight families of ERVs, with a structure close to the integrated proviral form of exogenous retroviruses (gag-, pol-, and env-related regions bordered by two LTRs), have been described. Those with close exogenous relatives include type B proviruses related to the murine mammary tumor virus (6) and type C proviruses related to the Moloney murine leukemia virus (MLV), with the endogenous MLV, MuRRS, MuRVY, GLN, MuERVC, and MmERV family members (1,7,8). A family of sequences with no exogenous relatives, called IAPE (intracisternal A-particle sequences with an envelope gene), was also described (9). Although some of these ERVs have accumulated mutations, deletions, and͞or truncations and would therefore require recombination with exogenous counterparts to generate replication-competent retroviruses, oth...

Sequences of retroviral origin occupy approximately 8% of the human genome. Most of these "retroviral" genes have lost their coding capacities since their entry into our ancestral genome millions of years ago, but some reading frames have remained open, suggesting positive selection. The complete sequencing of the human genome allowed a systematic search for retroviral envelope genes containing an open reading frame and resulted in the identification of 16 genes that we have characterized. We further showed, by quantitative reverse transcriptase PCR using specifically devised primers which discriminate between coding and noncoding elements, that all 16 genes are expressed in at least some healthy human tissues, albeit at highly different levels. All envelope genes disclose significant expression in the testis, three of them have a very high level of expression in the placenta, and a fourth is expressed in the thyroid. Besides their primary role as key molecules for viral entry, the envelope genes of retroviruses can induce cell-cell fusion, elicit immunosuppressive effects, and even protect against infection, and as such, endogenous retroviral envelope proteins have been tentatively identified in several reports as being involved in both normal and pathological processes. The present study provides a comprehensive survey of candidate genes and tools for a precise evaluation of their involvement in these processes.