Active transport of organic anion (OA) and cations (OC) from CSF to blood is a critical determinant of the concentration of potentially toxic neurotransmitter metabolites, drugs, and xenobiotics within the brain. We have used a combination of isolated membrane vesicles, tissue fragments in vitro, and primary culture to study xenobiotic transport across the blood-CSF barrier (choroid plexus). Our working hypothesis was that the transport mechanisms utilized by the plexus would parallel those of the kidney, but with the polarity reversed since the direction of transport is reversed (i.e., from CSF to blood rather than from blood to urine). Initial focus was on the mechanism and subcellular location of OA transport by the plexus. We were able to demonstrate that apical (CSF-side) transport of OA is mediated by OA/a-ketoglutarate (aKG) exchange -- the same mechanism used at the basolateral face of the renal epithelium. However, the situation for OC is much more complicated. Initial studies of TEA transport in both isolated apical membrane vesicles from the bovine plexus or in primary cultures of choroid plexus cells from neonatal rats indicate the presence of proton/OC exchange at the apical membrane, i.e., the same transport activity seen in the apical membrane of the proximal tubule. Since the direction of transport is reversed (CSF to blood), TEA transport must take place by a mechanism distinct from that used in the kidney. In contrast, choline -- an important neurotransmitter precursor that is regulated by choroid plexus transport -- is handled in a very differently. It is taken up across the apical membrane by a potential driven mechanism, which we demonstrated to be mediated by OCT2. OCT1 is also capable of transporting choline, but, as shown by PCR, is not expressed in choroid plexus. OCT3 was present, but unlike OCT2, does not transport choline. Our OCT2-GFP construct was expressed exclusively in the apical membrane. Thus, for choline, like OAs, the polarity of transport in the plexus is reversed relative to kidney. We have also assessed OC transport in cultured retinal pigmented epithelium (RPE) from the human eye using verapamil as substrate. As judged by its unique substrate specificity, RPE verapamil transport appears to be a novel system. Its driving force was shown to be proton/verapamil exchange. Interestingly, this system was shown to be inducable by substrate in culture, a process blocked by inhibitors of both protein and RNA synthesis. Other transporters were not induced and their activity was unchanged by the protein and RNA synthesis inhibitors. This is the first demonstration of induction of one of the OA or OC drug transporters.
| Sweet, Douglas H; Miller, David S; Pritchard, John B et al. (2002) Impaired organic anion transport in kidney and choroid plexus of organic anion transporter 3 (Oat3 (Slc22a8)) knockout mice. J Biol Chem 277:26934-43 |
| Han, Y H; Sweet, D H; Hu, D N et al. (2001) Characterization of a novel cationic drug transporter in human retinal pigment epithelial cells. J Pharmacol Exp Ther 296:450-7 |
| Sweet, D H; Miller, D S; Pritchard, J B (2001) Ventricular choline transport: a role for organic cation transporter 2 expressed in choroid plexus. J Biol Chem 276:41611-9 |
| Miller, D S; Villalobos, A R; Pritchard, J B (1999) Organic cation transport in rat choroid plexus cells studied by fluorescence microscopy. Am J Physiol 276:C955-68 |
| Pritchard, J B; Sweet, D H; Miller, D S et al. (1999) Mechanism of organic anion transport across the apical membrane of choroid plexus. J Biol Chem 274:33382-7 |
