ATA2 is predominantly expressed as system A at the blood-brain barrier and acts as brain-to-blood efflux transport for L-proline.
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 alpha-methylaminoisobutyric acid (MeAIB), across the BBB was investigated using the in vivo Brain Efflux Index method. Over a period of 40 min, L-[(3)H]Pro and [(3)H]Gly underwent efflux from the brain, whereas [(3)H]MeAIB did not. The efflux of L-[(3)H]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-[(3)H]Pro uptake by TR-BBB cells, used as an in vitro BBB model, was Na(+)-dependent with high-affinity (K(m1) = 425 microM) and low-affinity (K(m2) = 10.8 mM) saturable processes. The manner of inhibition of L-[(3)H]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 [(3)H]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.
Takanaga
Hitomi
H
Department of Molecular Biopharmacy and Genetics, Graduate School of Pharmaceutical Sciences, Tohoku University, Sendai, Japan.
Tokuda
Noriyo
N
Ohtsuki
Sumio
S
Hosoya
Ken-Ichi
K
Terasaki
Tetsuya
T
D026924:Amino Acid Transport System A / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D000596:Amino Acids / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport; D001812:Blood-Brain Barrier / Q000502:physiology*; D001921:Brain / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004730:Endothelium, Vascular / Q000187:drug effects / Q000378:metabolism; D005998:Glycine / Q000378:metabolism; D006982:Hypertonic Solutions / Q000494:pharmacology; D008297:Male; D011392:Proline / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism; D013997:Time Factors; D014316:Tritium; D015091:beta-Alanine / Q000031:analogs & derivatives* / Q000378:metabolism
D026924:Amino Acid Transport System A; D000596:Amino Acids; D006982:Hypertonic Solutions; D012333:RNA, Messenger; C486010:Slc38a4 protein, rat; D014316:Tritium; D015091:beta-Alanine; C012924:2,2-dimethyl-beta-alanine; D011392:Proline; D012964:Sodium; D005998:Glycine