RECEPTOR-MEDIATED TRANSCYTOSIS OF TRANSFERRIN THROUGH BLOOD-BRAIN-BARRIER ENDOTHELIAL-CELLS

A cell culture model of the blood-brain barrier consisting of a cocult ure of bovine brain capillary endothelial cells (BBCECs) and astrocyte s has been used to examine the mechanism of iron transport to the brai n. Binding experiments showed that BBCECs express 35,000 high-affinity (concn at 50% receptor saturation = 11.3 +/- 2.1 nM) transferrin (Tf) receptors per cell. In contrast to apo-transferrin (apoTf), we observ ed a specific transport of holo-transferrin (holoTf) across BBCECs. Th is transport was inhibited completly at low temperature. Moreover, the anti-Tf receptor antibody (OX-26) competitively inhibited holoTf upta ke by BBCECs. Pulse-chase experiments demonstrated that only 10% of Tf was recycled to the luminal side of the cells, whereas the majority o f Tf was transcytosed to the abluminal side; double-labeling experimen ts clearly demonstrated that iron crosses BBCECs bound to Tf. No intra endothelial degradation of Tf was observed, suggesting that the intrae ndothelial pathway through BBCECs bypasses the lysosomal compartment. These results clearly show that the iron-Tf complex is transcytosed ac ross brain capillary endothelial cells by a receptor-mediated pathway without any degradation.