Sodium gradient-dependent phosphate transport in placental brush border membrane vesicles

Slc20a2 deficiency results in fetal growth restriction and placental calcification associated with thickened basement membranes and novel CD13 and lamininα1 expressing cells

The essential nutrient phosphorus must be taken up by the mammalian embryo during gestation. The mechanism(s) and key proteins responsible for maternal to fetal phosphate transport have not been identified. Established parameters for placental phosphate transport match those of the type III phosphate transporters, Slc20a1 and Slc20a2. Both members are expressed in human placenta, and their altered expression is linked to preeclampsia. In this study, we tested the hypothesis that Slc20a2 is required for placental function. Indeed, complete deficiency of Slc20a2 in either the maternal or embryonic placental compartment results in fetal growth restriction. We found that Slc20a2 null mice can reproduce, but are subviable; ∼50% are lost prior to weaning age. We also observed that 23% of Slc20a2 deficient females develop pregnancy complications at full term, with tremors and placental abnormalities including abnormal vascular structure, increased basement membrane deposition, abundant calcification, and accumulation of novel CD13 and lamininα1 positive cells. Together these data support that Slc20a2 deficiency impacts both maternal and neonatal health, and Slc20a2 is required for normal placental function. In humans, decreased levels of placental Slc20a1 and Slc20a2 have been correlated with early onset preeclampsia, a disorder that can manifest from placental dysfunction. In addition, preterm placental calcification has been associated with poor pregnancy outcomes. We surveyed placental calcification in human preeclamptic placenta samples, and detected basement membrane-associated placental calcification as well as a comparable lamininα1 positive cell type, indicating that similar mechanisms may underlie both human and mouse placental calcification.

Regulation of nutrient transport across the placenta

Abnormal fetal growth, both growth restriction and overgrowth, is associated with perinatal complications and an increased risk of metabolic and cardiovascular disease later in life. Fetal growth is dependent on nutrient availability, which in turn is related to the capacity of the placenta to transport these nutrients. The activity of a range of nutrient transporters has been reported to be decreased in placentas of growth restricted fetuses, whereas at least some studies indicate that placental nutrient transport is upregulated in fetal overgrowth. These findings suggest that changes in placental nutrient transport may directly contribute to the development of abnormal fetal growth. Detailed information on the mechanisms by which placental nutrient transporters are regulated will therefore help us to better understand how important pregnancy complications develop and may provide a foundation for designing novel intervention strategies. In this paper we will focus on recent studies of regulatory mechanisms that modulate placental transport of amino acids, fatty acids, and glucose.

Placental transfer of phosphate in anaesthetized rats

Mechanisms of transfer of inorganic phosphate, Pi, across the placenta of rats at 21 days of gestation were studied using 32Pi. In one group of experiments the unidirectional transfer of Pi from mother to fetus was estimated from radioactivity in the fetus at various intervals after the tracer injection into the mother. At 15 min after tracer injection, the transfer rate was only slightly higher than the estimated rate of fetal accretion of Pi, and it decreased rapidly with the length of the experiment suggesting deterioration of the transfer mechanism under conditions of an acute experiment. In other experiments, transfer of 32Pi and 51Cr-EDTA (a marker of paracellular transfer) were measured across the dually-perfused placenta in the maternal-fetal direction. The transfer rate of 32Pi was an order of magnitude higher than the transfer of 51Cr-EDTA indicating that most of the Pi transfer is transcellular. The transfer of 32P decreased when the concentration of Na+ in the maternal perfusate was reduced, it was related inversely to the concentration of Pi on the fetal side of the placenta, and it was related directly to the concentration of Ca2+ on the fetal side. The maternal-fetal transfer of Pi exhibited saturation kinetics with a K(m) of about 0.4 mM suggesting that at a physiological concentration of Pi in maternal plasma the transfer mechanism is nearly saturated. The present observations are consistent with Pi being transferred in contransport with Na+. The maternal-fetal transport of Pi appears to be stabilized by the high affinity of the transport system to Pi, and controlled by a negative feedback between fetal concentration of Pi and the Pi transfer rate. It may also be controlled, to some degree, by the fetal plasma Ca2+.

Voltage-dependent phosphate transport in osteoblast-like cells

Phosphate ion (Pi) in sufficient concentrations is crucial for bone mineralization. The osteoblast (OB) may be responsible for the transport of Pi into the bone interstitium, where mineralization occurs. We previously characterized a Na(+)-dependent Pi transporter (NaPi) in the osteoblastic UMR-106-01 cell line. In the present study, the alteration of Na(+)-dependent Pi transport by changes in membrane potential was investigated. Depolarizing the cells with increasing concentrations of ambient K+ and valinomycin resulted in a progressive decline in Na(+)-dependent Pi uptake to a maximum of 28% at a membrane potential of -18 mV compared to control Na(+)-dependent Pi uptake at a membrane potential of approximately -60 mV. Hyperpolarizing the cells with SCN- increased Na(+)-dependent Pi uptake over control by 50% at an SCN- concentration of 70 mM. Determination of membrane potential by using the fluorescent probe, DiSC3(5), showed that the addition of Pi to cells in Na(+)-containing medium resulted in a small depolarization. These data show that NaPi activity can be altered by membrane potential changes and that the initiation of Na(+)-dependent Pi uptake is associated with depolarization of the plasma membrane of UMR-106-01 cells. Taken together, the cotransport of Na+ and Pi results in the movement of a net positive charge into the cell.

Influence of insulin on phosphate uptake by brush border membranes from human placenta

Regulation of phosphate transport by insulin was investigated in brush border membranes from human placenta at term. At 22 degrees C, a 45 min incubation of the total tissue with 10(-6) M insulin significantly decreased both the initial rate and the peak of sodium-dependent phosphate uptake by the corresponding brush border membranes. In contrast, Na+ transport was not influenced by the hormone. Increasing the insulin concentration from 0 to 10(-5) M resulted in a dose-dependent inhibition of phosphate uptake with half-maximal effect at 1.1 x 10(-9) M. The hormone decreased PO4 transport by decreasing the affinity of the carrier for the substrate (Km = 0.180 +/- 0.010 mM and 0.215 +/- 0.015 mM in absence and presence of 10(-6) M insulin respectively, P less than 0.05). The inhibitory effect of insulin required the presence of Mn2+ whereas neither Mn2+ nor insulin alone had any influence on PO4 uptake. It is therefore assumed that receptor phosphorylation, which needs the presence of Mn2+, is an intermediate step of insulin action on PO4 uptake by the subsequently isolated brush border membranes. In contrast, insulin had no effect on PO4 uptake when the membranes were directly incubated with the hormone prior to the transport measurement, suggesting that an intracellular messenger is needed for the inhibitory effect. This messenger is not cAMP since insulin at 10(-6) M concentration has no effect on cAMP content of the total placental tissue.(ABSTRACT TRUNCATED AT 250 WORDS)