The team blocked expression of the envelope of the enJSRVs using morpholino antisense oligonucleotides, which inhibit translation of specific messenger RNA. When production of the envelope protein was blocked in the early placenta, the growth of the placenta was reduced and a certain cell type, termed giant binucleate cells, did not develop.
The result was that embryos could not implant and the sheep miscarried, Spencer said.
Miscarriage is a serious medical problem for all mammals, including humans.
"Our research supports the idea that endogenous retroviruses shaped the evolution of the placenta in mammals and then became indispensable for pregnancy, and thus may be why they are expressed in the placenta of many mammals," he said.
"The enJSRVs arose from ancient infections of small ruminants during their evolution," said. Dr. Massimo Palmarini, a virologist at The University of Glasgow Veterinary School. "This infection was beneficial to the host and was then positively selected for during evolution. In other words, animals with enJSRVs were better equipped than those without. Therefore, enJSRVs became a permanent part of the sheep genome and, in these days, sheep can't do without them."
The research team is trying to determine exactly how enJSRVs function in development of the sheep placenta. Their results should have implications for both human health and animal production.