Molecular mechanisms of organic cation and anion transport at the blood‐CSF barrier

The blood‐cerebrospinal fluid (CSF) barrier, formed by epithelial cells of the choroid plexus (CP), plays an active role in removing neurotoxins, drugs, and metabolites from the brain. Organic cations and organic anions are known to be transported at the blood‐CSF barrier, but the underlying molecular mechanisms have not been fully understood. The goal of this study is to identify major organic cation and anion transporters expressed in human and mouse CP and to elucidate the functions of these transporters by live tissue imaging using fluorescent probe substrates. For organic cations, real‐time PCR analysis showed a predominant expression of plasma membrane monoamine transporter (PMAT) in both human and mouse CPs. In contrast, organic cation transporters 1–3 (OCT1–3) and the multidrug and toxin extrusion protein 1 and 2 (MATE1/2) are not or minimally expressed. In freshly isolated CP from wildtype mouse, live tissue imaging showed that IDT307, an analog of the organic cation MPP+, was transported into CP epithelial cells at the apical (CSF‐facing) membrane and sensitive to inhibition by the PMAT inhibitor quinine. Once inside CP epithelial cells, IDT307 was highly accumulated in the mitochondria with little efflux at the basolateral (blood‐facing) membrane. IDT307 uptake and intracellular accumulation was greatly attenuated by ~70% in CP tissue from the Pmat knockout mouse. For organic anions, real‐time PCR analysis showed the highest expression of the organic anion transporting polypeptides 1C1 (OATP1C1) and multidrug resistance‐associated protein 1 (MRP1). Organic anion transporter 3 (OAT3/Oat3), MRP4/Mrp4, Oatp1a4/5, OATP1A2 and OATP3A1 also showed significant expression in human or mouse CP. Unlike IDT307, the organic anion fluorescein‐methotrexate (FL‐MTX), which is a reported substrate of OATPs and MRPs, was rapidly taken up at the apical membrane and efficiently excreted into the capillary areas with little intracellular accumulation. Rifampicin, a pan‐inhibitor of OATPs, completely blocked FL‐MTX uptake into the CP tissue whereas MK571, a potent inhibitor of MRP1/4, abolished the basolateral efflux of FL‐MTX, resulting in its high accumulation within the CP epithelial cells. Taken together, our results established that organic cations are transported by PMAT from the CSF and highly accumulated in CP. Larger organic anions are removed from the CSF by OATP (e.g. OATP1C1)‐mediated apical uptake followed by MRP1/4‐mediated basolateral efflux into the blood. Our findings revealed the molecular mechanisms of organic cation and anion transport at the blood‐CSF barrier and suggested important roles of CP transporters in maintaining the homeostasis and normal function of the brain. Support or Funding Information This study is supported by National Institutes of Health Grant GM066233. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .