Syncytin expression in uterine endometrium and fetal membranes during early pregnancy in sheep

Abstract Text: Endogenous retroviruses may be involved in formation of the placental interface between the endometrium and fetal membranes (FM). When this interface is not adequately formed, fetal loss or growth retardation may occur. Syncytin is an integrated retroviral envelope gene thought to be involved in cell-cell fusion and immunosuppression within the mammalian placenta; however, in sheep the exact function of syncytin is unknown.  Ewes also possess ~20 copies of another endogenous retrovirus (enJSRV) which is closely related to the exogenous Jaagsiekte sheep retrovirus. Integration and expression of enJSRV strains, in the placenta, is breed specific. To examine syncytin and enJSRV expression at the maternal-fetal interface throughout early gestation, crossbred western whiteface (primarily Rambouillet, Targhee, and Columbia) ewes were naturally mated and gravid uteri were obtained on days 14, 16, 18, 20, 22, 24, 26, 28, and 30 (n = 6 to 8/d) after mating (day of mating = d 0). Nonpregnant, mid-luteal ewes (d 10; n = 8) were used as controls. Expression of syncytin and enJSRV-18 was determined with snap-frozen caruncular (maternal placental) tissue and FM (chorion on d16 and chorioallantois thereafter) using quantitative real-time RT-PCR. Statistical analyses used PROC GLM of SAS with orthogonal contrasts.  Fetal length increased three-fold (P < 0.001) from d 20 (5.50 ± 0.97 mm) to d 30 (19.29 ± 0.52 mm) of gestation.  Fetal membrane expression of syncytin decreased from d 16 thru d 20, was increased from d 22 until d 26, and decreased, again, to d 30 (P = 0.002). Both fetal growth patterns and FM expression of syncytin had significant linear (P < 0.005) and cubic (P < 0.001) orthogonal contrasts. Pregnant ewes had greater syncytin expression (P = 0.002) compared with non-pregnant ewes. Syncytin expression in caruncular tissues decreased from d 14 until d 20, then increased to d 24 and remained steady to d 30 of gestation (P = 0.009) resulting in quadratic (P = 0.01) and cubic (P = 0.001) orthogonal contrasts. Interestingly, neither FM nor caruncular tissues of western whiteface ewes expressed detectable levels of enJSRV-18. Therefore, syncytin but not enJSRV-18 is likely involved in the regulation of placental function and growth during pregnancy in western whiteface sheep.

Supported by USDA-NRI grant 2007-01215 to LPR and ATGB.

Keywords: early gestation, placenta, syncytin