Capture of syncytin-Mar1, a Fusogenic Endogenous Retroviral Envelope Gene Involved in Placentation in the Rodentia Squirrel-Related Clade

Abstract

Syncytin genes are fusogenic envelope protein (env) genes of retroviral origin that have been captured for a function in placentation. Within rodents, two such genes have previously been identified in the mouse-related clade, allowing a demonstration of their essential role via knockout mice. Here, we searched for similar genes in a second major clade of the Rodentia order, the squirrel-related clade, taking advantage of the complete sequencing of the ground squirrel Ictidomys tridecemlineatus genome. In silico search for env genes with full coding capacity identified several candidate genes with one displaying placenta-specific expression, as revealed by quantitative reverse transcription-PCR analysis of a large panel of tissues. This gene belongs to a degenerate endogenous retroviral element, with recognizable hallmarks of an integrated provirus. Cloning of the gene in an expression vector for ex vivo cell-cell fusion and pseudotype assays demonstrated fusogenicity on a large panel of mammalian cells. In situ hybridization on placenta sections showed specific expression in domains where trophoblast cells fuse into a syncytiotrophoblast at the fetomaternal interface, consistent with a role in syncytium formation. Finally, we show that the gene is conserved among the tribe Marmotini, thus dating its capture back to about at least 25 million years ago, with evidence for purifying selection and conservation of fusogenic activity. This gene that we named syncytin-Mar1 is distinct from all seven syncytin genes identified to date in eutherian mammals and is likely to be a major effector of placentation in its related clade. IMPORTANCESyncytin genes are fusogenic envelope genes of retroviral origin, ancestrally captured for a function in placentation. Within rodents, two such genes had been previously identified in the mouse-related clade. Here, in the squirrel-related rodent clade, we identified the envelope gene of an endogenous retrovirus with all the features of a syncytin: it is specifically expressed in the placenta of the woodchuck Marmota monax, at the level of cells fusing into a syncytium; it can trigger cell-cell and virus-cell fusion ex vivo; and it has been conserved for >25 million years of evolution, suggesting an essential role in its host physiology. Remarkably, syncytin-Mar1 is unrelated to all other syncytin genes identified thus far in mammals (primates, muroids, carnivores, and ruminants). These results extend the range of retroviral envelope gene "domestication" in mammals and show that these events occurred independently, on multiple occasions during evolution to improve placental development in a process of convergent evolution.

Endogenous Retroviruses: With Us and against Us

“…Syncytin-A and Syncytin-B appear to function like human Syncytins within the mouse placenta and are known to have entered the murine (Muridae) lineages approximately 20 million years ago (Dupressoir et al, 2005). Similarly, Syncytin-Ory1 has been discovered in rabbits and hares (Leporidae; Heidmann et al, 2009), Syncytin-Car1 within 26 different species of carnivorans (Carnivora; Cornelis et al, 2012), Syncytin-Mar1 within the squirrel-related clade (either Scuridae or Marmotini; Redelsperger et al, 2014), Syncytin-Ten1 within tenrec (Tenrecidae; Cornelis et al, 2014), Syncytin-Rum1 in ruminants (Ruminantia; Cornelis et al, 2013), and Syncytin-Opo1 within the short-lived placenta of opossum and kangaroo marsupials (Marsupialia; Cornelis et al, 2015). …”

An endogenous retroviral envelope syncytin and its cognate receptor identified in the viviparous placental Mabuya lizard

“…Syncytins have been found in all placental mammals in which they have been searched, with independently captured syncytins occurring across all major clades of placental mammals, including Euarchontoglires (primates, rodents, and lagomorphs), Laurasiatherians (ruminants and carnivores), Afrotherians (tenrec), and even Marsupials (opossum) ( Fig. 1) (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). This has led to the proposal that these genes, which are absolutely required for placentation as shown by the knockout mice experiments, are most likely involved in the emergence and evolution of placental mammals from egg-laying animals (1).…”

Expression and function of endogenous retroviruses in the placenta

“…It fulfilled all criteria of a syncytin (Table ), its capture was dated back 25 Mya and evidence for purifiying selection and conservation of the fusiogenic activity was obtained. It has been found conserved in the tribe Marmotini, explaining the name and has been found expressed in the woodchuck Marmota monax . In the chorioallantoic, discoidal placenta expression of syncytin Mar‐1 was observed only in regions where cellular fusion processes take place.…”

“…Syncytin‐A and syncytin‐B entered the mouse‐related clade 20 Mya (see above), env‐Cav1 entered guinea pigs and all Caviomorpha 30 Mya. Mar‐1 was found in the squirrel‐related clade of Rodentia taking advantage of the complete sequencing of the ground squirrel genome. It fulfilled all criteria of a syncytin (Table ), its capture was dated back 25 Mya and evidence for purifiying selection and conservation of the fusiogenic activity was obtained.…”