Molecular Mechanisms of OATP/ SLCO -mediated Organic Anion Clearance at the Blood-Cerebrospinal Fluid Barrier.

The blood-cerebrospinal fluid barrier (BCSFB), formed by the choroid plexus epithelial (CPE) cells, plays an active role in removing drugs and metabolic wastes from the brain. Recent functional studies in isolated mouse choroid plexus (CP) tissues suggested the existence of organic anion transporting polypeptides (OATPs, encoded by SLCOs) at the apical membrane of BCSFB, which may clear large organic anions from the cerebrospinal fluid (CSF). However, the specific OATP isoform involved is unclear. Using quantitative fluorescence imaging, we showed that the fluorescent anions, sulforhodamine101 (SR101), fluorescein methotrexate (FL-MTX), and 8-fluorescein-cAMP (Fluo-cAMP), are actively transported from the CSF to the subepithelial space in CP tissues isolated from wild-type mice. In contrast, transepithelial transport of these compounds across the CPE cells was abolished in Oatp1a/1b -/- mice due to impaired apical uptake. Using transporter-expressing cell lines, SR101, FL-MTX and Fluo-cAMP were additionally shown to be transported by mouse OATP1A5 and its human counterpart OATP1A2. Kinetic analysis showed that estrone-3-sulfate and SR101 are transported by OATP1A2 and OATP1A5 with similar K m values. Immunofluorescence staining further revealed the presence of OATP1A2 protein in human CP tissues. Together, our results suggest that large organic anions in the CSF are actively transported into CPE cells by apical OATP1A2 (OATP1A5 in mice), then subsequently effluxed into the blood by basolateral multidrug resistance associated proteins (MRPs). As OATP1A2 transports a wide array of endogenous compounds and xenobiotics, the presence of this transporter at the BCSFB may imply a novel route for removing neurohormones, drugs and toxins from the CSF. Significance Statement Transporters at the blood-cerebrospinal fluid (CSF) barrier play an important role in brain drug disposition. Quantitative fluorescence imaging and molecular analyses revealed a functional role of rodent OATP1A5 in clearing large organic anions from the CSF and suggested a similar role for OATP1A2 at the human blood-CSF barrier. Delineating the mechanisms governing organic anion clearance from the CSF may help improve the prediction of central nervous system (CNS) pharmacokinetics and inform the identification of drug candidates with favorable CNS pharmacokinetics.