RAPID COMMUNICATION: Expression of an endogenous retroviral element, syncytin-Rum1, during early gestation in beef heifers1

Endogenous retroviral gene elements have been implicated in development and formation of the feto-maternal interface. A variant of the syncytin endogenous retroviral envelope gene family, syncytin-Rum1, was recently found in ruminants. We hypothesized that syncytin-Rum1 mRNA would be differentially expressed in utero-placental tissues and would fluctuate during key time points of early gestation in beef heifers. Commercial Angus crossbred heifers (n = 46; ~15 mo of age; BW = 362.3 ± 34.7 kg) housed in 6-animal pens were fed daily with native grass hay and supplemented with cracked corn to gain 0.3 kg/d. The heifers were estrus synchronized, artificially inseminated, (d of breeding = d 0) and ovariohysterectomized on d 16, 22, 28, 34, 40, and 50 (n = 9, 6, 6, 7, 6, and 5, respectively) of gestation and at d 16 of the estrous cycle for non-bred, non-pregnant controls (NP; n = 7). Harvested tissues were separated into maternal caruncle (CAR), intercarunclar endometrium (ICAR), and fetal membranes, (FM; chorioallantois, d 22 and later). All tissues were obtained from the ipsilateral uterine horn to the CL. Statistical analyses were conducted via the GLM procedure of SAS. Maternal CAR expression of syncytin-Rum1 was greater (P = 0.003) on d 50 by 81.5-fold compared to NP controls. At d 50 expression of syncytin-Rum1 in CAR was 190.3-fold greater than (P < 0.0001) ICAR. Fetal membranes had greater (P < 0.002) expression of syncytin-Rum1 from d 22 until d 50 of gestation compared to maternal ICAR (d 16 not analyzed). Expression of syncytin-Rum1 in FM was greater (P < 0.004) than in CAR until d 40 of gestation. Therefore, we conclude that syncytin-Rum1 is differentially expressed in utero-placental tissues and may be involved in the establishment of pregnancy. The expression of syncytin-Rum1 in maternal tissues is completely novel and indicates unique functions of syncytin in ruminant pregnancy.