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. [C-14] GlySar and [C-14] 5-ALA were studied as a function of temperature, concentration, potential inhibitors, 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 4degreesC (vs. 37degreesC), GlySar uptake was reduced by 95% in PEPT2(+/+) mice; no difference was observed in null animals. Unlabeled GlySar inhibited the uptake of [C-14] GlySar in PEPT2(+/+) mice (p<0.01); self-inhibition did not occur in PEPT2(-/-) mice. GlySar demonstrated saturable uptake in PEPT2(+/+) mice (V-max=16.4 pmol mg(-1) min(-1), K-m=70 mu M, K-d=0.014 mu l mg(-1) min(-1)), however, uptake was linear in PEPT2(-/-) mice (K-d=0.023 mu l mg(-1) min(-1)). Low sodium buffer (1 mM) resulted in 75% and 59% reductions, 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 transporter responsible for the choroid plexus uptake of GlySar and 5-ALA. They also suggest a role for PEPT2 in the clearance of dipeptides and endogenous peptidomimetics from cerebrospinal fluid.