THE CHOROID-PLEXUS EPITHELIUM IS THE SITE OF THE ORGANIC ANION TRANSPORT PROTEIN IN THE BRAIN

The mRNA for organic anion transport protein (oatp) was previously sho wn to be present in abundance in liver and kidney, and in small amount s in brain. Data obtained from experiments with reverse transcriptase- PCR techniques and in situ hybridization analysis showed that the oatp mRNA is present within the brain, localized to the choroid plexus. A sequence-specific antibody to the oatp polypeptide demonstrated the pr esence of the expected polypeptide with a molecular weight of 80,000 p lus an immunoreactive species with a higher molecular weight in prepar ations of choroid plexus membranes. Examination of the choroid plexus by fluorescence confocal microscopy revealed that immunoreactive oatp polypeptide is localized to the apical surface of the choroid plexus e pithelial cells, which contacts the cerebrospinal fluid. This localiza tion of oatp is consistent with previous experiments showing vectorial transport of organic anions between the choroid plexus and the cerebr ospinal fluid.