Role of PEPT2 in the Choroid Plexus Uptake of Glycylsarcosine and 5-Aminolevulinic Acid: Studies in Wild-Type and Null Mice

Purpose. To determine the importance of PEPT2 in the uptake of glycylsarcosine (GlySar) and 5-aminolevulinic acid (5-ALA) in mouse choroid plexus whole tissue. Methods. Uptake studies were performed in bicarbonate artificial cerebrospinal fluid buffer using choroid plexuses isolated from PEPT2+/+ and PEPT2−/ − mice. [14C]GlySar and [14C]5-ALA were studied as a function of temperature, concentration, potential inhibi-tors, and low sodium conditions. Results. PEPT2−/ − mice exhibited a 90 % reduction in GlySar uptake (p < 0.001) and a 92 % reduction in 5-ALA uptake (p < 0.001) as compared to wild type animals. At 4°C (vs. 37°C), GlySar uptake was reduced by 95 % in PEPT2+/+ mice; no difference was observed in null animals. Unlabeled GlySar inhibited the uptake of [14C]GlySar in PEPT2+/+ mice (p < 0.01); self-inhibition did not occur in PEPT2−/− mice. GlySar demonstrated saturable uptake in PEPT2+/+ mice (Vmax 16.4 pmol mg−1 min−1, Km 70 M, Kd 0.014 l mg −1 min−1), however, uptake was linear in PEPT2−/ − mice (Kd 0.023 l mg −1 min−1). Low sodium buffer (1 mM) resulted in 75 % and 59 % reduc-tions, respectively, in GlySar (p < 0.001) and 5-ALA (p < 0.01) uptake in PEPT2+/+ mice; no differences were observed in PEPT2−/ − mice. Overall, about 90–95 % of the choroid plexus uptake of GlySar and 5-ALA was mediated by PEPT2, with about 5–10 % of the residual uptake occurring by nonspecific mechanisms. Conclusions. The results demonstrate that PEPT2 is the only trans-porter responsible for the choroid plexus uptake of GlySar and 5-ALA. They also suggest a role for PEPT2 in the clearance of di-peptides and endogenous peptidomimetics from cerebrospinal fluid. KEY WORDS: 5-ALA; choroid plexus; GlySar; knockout mouse; PEPT2