Presence of CLA-1 and HDL Binding Sites on Syncytiotrophoblast Brush Border and Basal Plasma Membranes of Human Placenta

It is now known that rapid placental and fetal development is associated with elevated levels of circulating high density lipoprotein (HDL) in pregnant women. The main structure implicated in the maternal–fetal exchange is the syncytiotrophoblast, composed of a brush border membrane (BBM), facing the mother, and a basal plasma membrane (BPM), facing the fetus. In order to understand the mechanisms controlling the placental and fetal supplies of cholesterol, we purified both BBM and BPM and verified the presence of HDL binding sites in these membranes. Binding studies using125I-HDL3show a single affinity binding site on BPM which has a dissociation constant (Kd) of 3.45±0.43μg protein/ml and a maximal binding capacity (Bmax) of 5.46±1.69μg protein/mg membrane proteins. In BBM, we observed two affinity binding sites, one with a Kdof 0.62±0.03μg protein/ml and another one with a Kdof 6.57±0.87μg protein/ml. TheirBmax values were 0.54±0.11 and 2.34±0.39μg of HDL3/mg membrane proteins, respectively. CLA-1, a putative HDL-receptor of 85kDa, was detected on both BPM and BBM, together with two apo A-l binding sites of 110 and 96kDa on BPM and BBM, respectively. These results provide further evidence that human placenta possesses specific sites for HDL binding, underlining the important role of maternal HDL in the transfer of cholesterol from the maternal circulation to the placenta and the fetus.