Aquaporins and placenta.

Water is the major component of cells and tissues. The fetal body consists of about 70-90% water and its fluid balance is dependent on the mother. In fact, abortion, premature birth, amniotic fluid volume abnormality, malformation and fetal growth restrictions might result when the homeostasis of the maternal-fetal fluid exchange is disrupted. Thus, maternal-fetal fluid balance is critical during pregnancy. In this sense, several mechanisms, including aquaporins (AQPs) have been reported to play important roles in maternal-fetal fluid balance. AQPs are small membrane proteins (about 30kDa), present in different organs, that increase the permeability of water, as well as other small uncharged molecules to be transported across the bilayer cell membranes. Several aquaporins are expressed in placenta, and aquaporins play key roles in the placental function. Even though aquaporins have a proven crucial role in water homeostasis, the physiological and pathological importance of aquaporins as glycerol channels is not fully understood. This review focuses on advances in our knowledge of the roles of aquaporins in placental cells, particularly the roles of AQP3 and AQP9 in placental metabolism and points to the pathophysiological importance of glycerol channels in placenta, as well as the signal transduction pathways activated by them. Moreover, the regulation of aquaporins expression by different placental hormones, such as leptin and the mechanisms involved will be discussed.

Leptin upregulates aquaporin 9 expression in human placenta in vitro.

Aquaporins are integral membrane proteins that have permeability functions in many tissues. Aquaporin 9 may transport not only water but also small molecules, such as glycerol, monocarboxylates, purines and pyrimidines. Aquaporin 9 is expressed in syncytiotrophoblast of human term placenta, and it may contribute to the embryonic/fetal growth and survival. We have previously found that Aquaporin 9 expression levels seem to be increased in placenta from gestational diabetes. Since leptin plasma levels and leptin expression are increased in placenta from gestational diabetes, we aimed to study the possible role of leptin on Aquaporin 9 expression in human placenta in vitro. The present work shows that leptin produces a dose-dependent increase of Aquaporin 9 expression, resulting in an increase in Aquaporin-9 protein in human trophoblast explants.

Activity of NA(+)/H(+) exchangers alters aquaporin-mediated water transport in human placenta.

The intracellular pH (pHi) of syncytiotrophoblasts is regulated, in part, by Na(+)/H(+) exchanger (NHE)-1, NHE-2, and NHE-3. Failures in pHi homeostasis could alter critical cellular functions such as water transport and cell volume. Here, we evaluated whether alterations in syncytiotrophoblast pHi could modify water uptake mediated by aquaporins (AQPs) and the contribution of NHEs to this mechanism. We showed that changes in syncytiotrophoblast pHi did not affect water uptake in the presence of functional NHEs. However, inhibition of NHEs alters transcellular water transport mediated by AQPs in acidosis. These results suggest an interaction between placental AQPs and NHEs in the regulation of water uptake during acidotic states.

High levels of human chorionic gonadotropin (hCG) correlate with increased aquaporin-9 (AQP9) expression in explants from human preeclamptic placenta.

Trophoblastic abnormalities have a central role in the pathophysiology of preeclampsia, and some placental hormones, such as human chorionic gonadotropin (hCG), could affect the placental function. Here, we hypothesized that the elevated serum levels of hCG may be involved in the increased aquaporin-9 (AQP9) protein expression in preeclamptic placentas via adenosine 3('),5(')-cyclic phosphate (cAMP) pathways. Normal placental explants were cultured with different concentrations of recombinant hCG or 8-Br-cAMP, a potent analogue of cAMP. We evaluated AQP9 protein expression and localization. After both treatments, we localized AQP9 in the apical membrane of syncytiotrophoblast and in the cytoplasm. We also observed a concentration-dependent effect on AQP9 protein expression. In addition, water uptake increased 1.6-fold in explants treated with hCG. Our results suggest that hCG may increase AQP9 protein expression and functionality via cAMP pathways. Although, in preeclamptic placentas high levels of hCG may upregulate AQP9 protein expression, AQP9 functionality was reduced possibly by other factors.