Distribution of glutamate transporters in the human placenta

Glutamate metabolism is known to be important for growth and development of the human fetus. The glutamate transporters EAAT1, EAAT2 and EAAT3 are key components of the glutamate-glutamine cycle and responsible for active transport of glutamate over the cell membrane. The placenta is thought to regulate glutamate transport during fetal development. Glutamate transporters have been found in placentae of rats, but their distribution in the human placenta is unknown. Therefore, the distribution of glutamate transporters EAAT1, EAAT2 and EAAT3 were analysed in the human placenta during normal pregnancies ending between 8 and 40 weeks of gestation and in placentae of intrauterine growth restricted infants with gestational ages between 28 and 35 weeks of pregnancy. Using immunohistochemistry, EAAT1 expression was found in the syncytiotrophoblast layer, while EAAT2 was detected in the syncytiotrophoblast layer and in endothelial cells of about 5 per cent of all fetal blood vessels. EAAT3 was observed in the endothelium of the fetal blood vessels in all placentae examined. However, expression was also found in the syncytio- and the cytotrophoblast layer of the fetal villi at 8 weeks of gestational age. The expression patterns of EAAT1, EAAT2 and EAAT3 suggest involvement in active transport of glutamate between the fetal and maternal blood circulation. No differences were found in the distribution of the glutamate transporters between control and IUGR placentae. Our data show specific localization of EAAT1, EAAT2 and EAAT3 in the human placenta during development.