Selective Expression of the Large Neutral Amino Acid Transporter at the Blood-Brain Barrier

Amino acid supply in brain is regulated by the activity of the large neutral amino acid transporter (LAT) at the brain capillary endothelial cell, which forms the blood-brain barrier (BBB) in vivo. Bovine BBB poly(A)⁺ RNA was isolated from 2.0 kg of fresh bovine brain and size fractionated on a sucrose density gradient, and a size-fractionated bovine BBB cDNA library in the pSPORT vector was prepared. The full-length cDNA encoding the bovine BBB LAT was isolated from this library, and the predicted amino acid sequence was 89-92% identical to the LAT1 isoform. The bovine BBB LAT1 mRNA produced a 10-fold enhancement in tryptophan transport into frog oocytes coinjected with bovine BBB LAT1 mRNA and the mRNA for 4F2hc, which encodes the heavy chain of the heterodimer. Tryptophan transport into the mRNA-injected oocytes was sodium independent and was specifically inhibited by other large neutral amino acids, and the K_m of tryptophan transport was 31.5 ± 5.5 μM. Northern blotting with the bovine BBB LAT1 cDNA showed that the LAT1 mRNA is 100-fold higher in isolated bovine brain capillaries compared with C6 rat glioma cells or rat brain, and the LAT1 mRNA was not detected in rat liver, heart, lung, or kidney. These studies show that the LAT1 transcript is selectively expressed at the BBB compared with other tissues, and the abundance of the LAT1 mRNA at the BBB is manyfold higher than that of transcripts such as the 4F2hc antigen, actin, or the Glut1 glucose transporter.