Larsson LI. (eds) Cell Fusions. Springer, Dordrecht
The strongest candidates for developmentally regulated cellular fusogens in mammals are Syncytins which contribute to cell–cell fusion leading to placental syncytiotrophoblast in higher primates, rodents, lagomorphs and sheeps. They consist of domesticated endogenous retroviral envelope glycoproteins (Env) whose fusion properties depend on the initial recognition of a specific receptor. In order to clearly understand Syncytins characteristics, we will first illustrate molecular details characterizing the maturation of class I fusion proteins by introducing envelope-driven fusion in an infectious context, i.e. virus cell fusion, exemplifying each step that lead to functional virions with the most relevant model such as HIV-1 lentivirus or MLV and type D interference group retroviruses. In a second part, we will comparatively present the current knowledge concerning Syncytins and the associated three levels of complexity. First, the placenta is probably more variable in structure than any of the mammalian organs. Second, Syncytins recognize specific and highly function-divergent/unrelated receptors. Third, some Syncytins were shown to exhibit other functions than fusion, such as proliferation, immunomodulation, receptor interference and anti-apoptotic properties. We will conclude by a brief overview of the consequences of Syncytin expression outside of its privileged tissue.