Endogenous retroviruses expressed by ovine trophoblast

Urial Sheep (Ovis vignei)
Wikimedia Commons

Jaagsiekte sheep retrovirus (JSRV) is a nasty pathogen causing lung tumours in sheep. No fewer than 27 JSRV-related endogenous retroviruses (enJSRVs) occur in domestic breeds. Gene capture is an ongoing process and the distribution of enJSRVs has been exploited to test hypotheses about the domestication of sheep (here). For example, the most common variant (enJSRV-18) is absent in ancestral breeds such as the Urial (above) and Mouflon. Although found in goats, enJSRVs are not present in cattle so are much younger than a syncytin recently described by Cornelis et al. for ruminants (here).

Five enJSRVs are capable of transcription and like syncytins (previous post) they have an env gene. Are they then "nascent syncytins" as suggested by Cornelis et al., or do they already subserve a function in the ovine placenta?

A period of blastocyst elongation precedes implantation in sheep and binucleated trophoblast cells first appear then. enJSRV mRNAs are detectable at this stage and later are expressed in binucleate trophoblast cells as well as in the syncytial plaques formed by fusion of those cells with uterine epithelial cells (reviewed here).

Moreover, when antisense oligonucleotides were used to block enJSRV protein production, differentiation of binucleate cells was inhibited (here). The more i read about endogenous retroviruses the more intrigued I become.