Inducible trophoblast-specific knockdown of mechanistic target of rapamycin impairs placental folate transport in mice.

Folate deficiency in pregnancy is strongly associated with fetal growth restriction (FGR). Fetal folate availability is determined by maternal folate intake and the capacity of the placenta to transport folate. However, the mechanisms regulating placental folate transport remain poorly understood. The mechanistic target of rapamycin (mTOR) regulates placental function and fetal growth, but it is unknown if mTOR regulates folate transport in vivo. We hypothesized that trophoblast-specific mTOR knockdown inhibits placental folate transport in mice. We generated transgenic mice with a doxycycline-inducible, trophoblast-specific Mtor knockdown using PiggyBac transposase-enhanced pronuclear injection. Doxycycline administration on embryonic day (E) 14.5 induced placental-specific Mtor knockdown, resulting in reduced fetal weight, placental weight, and fetal-to-placental weight ratio. Functionally, mTOR knockdown reduced folate uptake in isolated trophoblast plasma membranes (TPM), decreased the TPM protein expression of the three main placental folate transporters (FRα, RFC, and PCFT) without affecting protein expression of theses transporters in placental homogenates, and lowered fetal plasma folate concentration. In conclusion, mTOR signaling is a positive regulator of the three main placental folate transporters in vivo mediated by posttranslational mechanisms, likely involving effects of plasma membrane trafficking. Trophoblast mTOR signaling is essential for maintaining adequate fetal folate supply and we propose that the inhibition of placental mTOR signaling reported in FGR contributes to fetal folate deficiency and decreased fetal growth in this pregnancy complication. Our data supports the potential to target placental mTOR signaling as a novel intervention in pregnancies affected by abnormal fetal growth.