Fig. 4

Schematic representation of CP epithelial cell showing amino acid transporters localized in this study. The amino acid—Na⁺ symporter SNAT1 (Slc38a1) is shown together with the marker protein anion exchanger 2 (AE2) at the vascular basolateral side of the CP epithelium. The amino acid antiporter LAT2 (Slc7a8) and the neutral amino acid—Na⁺ symporter/H⁺ antiporter SNAT3 (Slc38a3) are shown together with the marker protein Na⁺, K⁺-ATPase and a K⁺ channel at the CSF-facing luminal side of the CP epithelium. The sodium/proton exchanger NHE1 which might cooperate with SNAT3 to allow the efflux of Gln into CSF is also represented. Additionally, shown in dotted lines are amino acid transporters of which in this study only the mRNA has been detected in CP [moderately expressed antiporter y⁺LAT2 (Slc7a6) and low level expressed antiporter y⁺LAT1 (Slc7a7) and uniporters LAT4 (SLC43a2) and TAT1 (Slc16a10)] which may participate to the passage of neutral essential amino acids into CSF. Other amino acid transporters clearly detected at the mRNA level in the present study but not yet localized to a specific choroid plexus epithelium membrane [SIT1 (Slc6a20b) and ASC1 (Slc7a10) (high mRNA levels) as well as SNAT2 (Slc38a2), SNAT6 (Slc38a6) and EAAT1 (Slc1a3) (moderate mRNA levels)] and other potentially important amino acid transporters not tested in the present study are not included in the schema. CSF amino acids the concentration ratios of which (CSF/plasma, see Fig. 3) are changed ≥ 2-fold in LAT2 (Slc7a8) knockout mice are indicated with Students t-test p-values; (non-)ess. NAAs (non-)essential neutral amino acids