Metabolism of One-Carbon Units and Creatine in Uterine-Placental Tissues During Pregnancy in Sheep

Abstract

Early embryonic deaths and intrauterine growth restriction of fetuses are two major causes that limit the reproductive success of livestock. Both issues tie to the failure of proper transport of nutrients, gases, and signaling molecules at the maternal-placental interface. An efficient transplacental transport of these molecules in ruminants is ensured by optimal growth and development of the placenta, the foundation for which is laid in early gestation. Therefore, to better understand the physiology of developing conceptuses, studies were conducted using sheep as an animal model to determine the metabolism of one-carbon units and creatine at the uterine-placental interface that support mechanisms associated with conceptus elongation such as proliferation, migration, and cytoskeletal remodeling of trophectoderm cells.

The first study determined if serine hydroxymethyltransferase 2 (SHMT2), a key enzyme for one-carbon metabolism (OCM), is critical for ovine conceptus development by inhibiting translation of SHMT2 mRNA in vitro and in vivo. Results of this study revealed that conceptuses express enzymes of the OCM pathway and that knockdown of SHMT2 mRNA translation reduced proliferation and migration of ovine trophectoderm cells in vitro and tended to decrease pregnancy rates in ewes during the peri-implantation period of pregnancy in vivo. Therefore, SHMT2-mediated OCM likely supports survival and development of ovine conceptuses. In the second and third studies, the cell-, tissue-, and time-specific expression of enzymes and transporters for creatine metabolism were determined throughout gestation. Abundances of creatine and its precursor, guanidinoacetate, were greater in uterine flushings during the peri- than during peri implantation period of pregnancy, as they were in fetal fluids in late- than in early-gestation. The enzyme arginine glycine amidinotransferase (AGAT) for synthesis of guanidinoacetate was not detected in conceptuses or placentomes during gestation, however, AGAT was expressed in uterine luminal epithelia from Day 30 of pregnancy to the end of gestation. Interestingly, enzymes for the synthesis of Cr and the transporter of creatine were expressed in conceptus trophectoderm, syncytial plaques, and endometrial luminal epithelia cells throughout pregnancy. Transporters of guanidinoacetate and creatine were expressed in trophectoderm cells of conceptus, syncytial plaques or caruncular epithelial cells of placentomes and luminal epithelia of uterus throughout gestation.

Collectively, results from these studies indicate a critical role for SHMT2-mediated one carbon metabolism for conceptus development. Identification of the entire components of the creatine metabolism pathway demonstrates creatine to be a novel nutrient in uterine histotroph and suggests a role for this pathway in maintaining cellular bioenergetics for conceptus development.