title: Syncytin-A and syncytin-B, two fusogenic placenta-specific murine envelope genes of retroviral origin conserved in Muridae.
abstract: 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 placenta-specific 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 approximately 20 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.
authors.affiliation: Unité des Rétrovirus Endogènes et Eléments Rétroïdes des Eucaryotes Supérieurs, Unité Mixte de Recherche, 8122 Centre National de la Recherche Scientifique, Institut Gustave Roussy, 94805 Villejuif, France.
authors.forename: Anne
authors.initials: A
authors.lastname: Dupressoir
authors.forename: Geoffroy
authors.initials: G
authors.lastname: Marceau
authors.forename: Cécile
authors.initials: C
authors.lastname: Vernochet
authors.forename: Laurence
authors.initials: L
authors.lastname: Bénit
authors.forename: Colette
authors.initials: C
authors.lastname: Kanellopoulos
authors.forename: Vincent
authors.initials: V
authors.lastname: Sapin
authors.forename: Thierry
authors.initials: T
authors.lastname: Heidmann
mesh_terms: D000818:Animals; D001483:Base Sequence; D002459:Cell Fusion; D020077:Endogenous Retroviruses / Q000235:genetics*; D019143:Evolution, Molecular; D015686:Gene Products, env / Q000235:genetics / Q000502:physiology*; D015752:Genes, env; D051379:Mice; D008969:Molecular Sequence Data; D009928:Organ Specificity; D010920:Placenta / Q000737:chemistry* / Q000166:cytology; D011257:Pregnancy Proteins / Q000235:genetics / Q000502:physiology*; D012333:RNA, Messenger / Q000032:analysis; D012641:Selection, Genetic; D014327:Trophoblasts / Q000166:cytology
chemical_list: D015686:Gene Products, env; D011257:Pregnancy Proteins; D012333:RNA, Messenger; C405335:syncytin; C497514:syncytin-A protein, mouse; C497515:syncytin-B protein, mouse