Journal
GENES TO CELLS
Volume 20, Issue 10, Pages 771-788Publisher
WILEY
DOI: 10.1111/gtc.12278
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- Grants-in-Aid for Scientific Research [25660209] Funding Source: KAKEN
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It is well accepted that numerous RNAs derived from endogenous retroviruses (ERVs) are expressed in mammalian reproductive structures, particularly in the uterus, trophoblast, and placenta. Syncytin 1 and syncytin 2 in humans and syncytin A and syncytin B in mice are membrane proteins originating from Env genes of ERVs. These ERVs are involved in the fusion of trophoblast cells, resulting in multinucleated syncytiotrophoblast formation. Evidence accumulated indicates that syncytin-like fusogenic proteins are expressed in the placenta of rabbits, dogs/cats, ruminant ungulates, tenrecs, and opossums. The syncytin genes so far characterized are known to be endogenized to the host genome only within the past 12-80 million years, more recently than the appearance of mammalian placentas, estimated to be 160-180 million years ago. We speculate that ERVs including syncytin-like gene variants integrated into mammalian genomes in a locus-specific manner have replaced the genes previously responsible for cell fusion. We therefore propose the baton pass' hypothesis, in which multiple successive ERV variants take over' cell-fusion roles, resulting in increased trophoblast cell fusion, morphological variations in placental structures, and enhanced reproductive success in placental mammals.
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