Quantitative targeted absolute proteomics of rat blood–cerebrospinal fluid barrier transporters: comparison with a human specimen

The purpose of this study was to determine absolute protein expression levels of transporters in rat choroid plexus, that is, the blood-cerebrospinal fluid barrier, and to compare them with the levels in the human choroid plexus. Plasma membrane fractions were prepared from pooled, freshly isolated choroid plexuses of 30 male Wistar rats and from frozen choroid plexus of one male human donor. Protein expression levels of 54 rat and 121 human molecules were measured, using a quantitative targeted absolute proteomics technique. In rat, oatp1a5 showed the most abundant protein expression (30.3 fmol/μg protein), and its expression level was 3.1-, 4.5-, 5.5-, 8.4-, 9.0-, 9.9-, 22-, 91-, and 95-fold greater than those of glut1, oatp1c1, mrp1, mct1, oat3, pept2, mrp4, bcrp, and mdr1a, respectively. OATP1A2 (a possible homolog of rat oatp1a5), OATP1C1 and PEPT2 were not detected in human choroid plexus. MRP1, OAT3, and MRP4 showed 4.0-, 1.8-, and 1.7-fold smaller expression levels in human than rat, respectively. MATE1 was detected in human, but not rat, and its expression level (8.61 fmol/μg protein) was the highest among the xenobiotic transporters examined in human choroid plexus. These findings should be useful for understanding rat blood-cerebrospinal fluid barrier function and its differences from that in human. This is the first study clarifying the absolute protein expression levels of many transporters in the plasma membrane fractions of rat and human choroid plexuses, that is, blood cerebrospinal fluid barrier, by means of quantitative targeted absolute proteomics (QTAP) technique. This study also identified the protein expressions of some transporters including MATE1 and ABCA8 in the choroid plexus for the first time.