Vol. 61, Issue 6, 1289-1296, June 2002

ATA2 Is Predominantly Expressed as System A at the Blood-Brain Barrier and Acts as Brain-to-Blood Efflux Transport for L-Proline

Hitomi Takanaga, Noriyo Tokuda, Sumio Ohtsuki, Ken-ichi Hosoya, and Tetsuya Terasaki

Department of Molecular Biopharmacy and Genetics, Graduate School of Pharmaceutical Sciences (H.T., N.T., S.O., T.T.), and New Industry Creation Hatchery Center (H.T., S.O., T.T.), Tohoku University, Sendai, Japan; Core Research for Evolutional Science and Technology of Japan Science and Technology Corporation, Kawaguchi City, Japan (H.T., S.O., K.H., T.T.); and Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University, Toyama, Japan (K.H.)

Although system A is present at the blood-brain barrier (BBB), the physiological roles of system A have not been clarified. The efflux transport of the substrates of system A, such as L-proline (L-Pro), glycine (Gly), and -methylaminoisobutyric acid (MeAIB), across the BBB was investigated using the in vivo Brain Efflux Index method. Over a period of 40 min, L-[³H]Pro and [³H]Gly underwent efflux from the brain, whereas [³H]MeAIB did not. The efflux of L-[³H]Pro was inhibited by the presence of unlabeled L-Pro and MeAIB, suggesting that carrier-mediated efflux transport of L-Pro across the BBB is involved in system A. L-[³H]Pro uptake by TR-BBB cells, used as an in vitro BBB model, was Na⁺-dependent with high-affinity (K_m1 = 425 µM) and low-affinity (K_m2 = 10.8 mM) saturable processes. The manner of inhibition of L-[³H]Pro uptake for amino acids was consistent with system A. Although GlnT, ATA2, and ATA3 mRNA were all expressed in TR-BBB cells, ATA2 mRNA was predominant. Under hypertonic conditions, ATA2 mRNA in TR-BBB cells was induced by up to 373%, and it activated [³H]MeAIB uptake. In light of these observations, our results indicate that L-Pro and Gly are transported from the brain across the BBB, whereas MeAIB is retained in the brain. System A is involved in efflux transport for L-Pro at the BBB. The predominantly expressed ATA2 mRNA at the BBB may play a role in maintaining the concentration of small neutral amino acids and cerebral osmotic pressure in the brain under pathological conditions.