Localization of organic anion transporting polypeptide (Oatp) 1a4 and Oatp1c1 at the rat blood-retinal barrier

Background: Organic anion transporting polypeptide (Oatp) transporters at the blood-brain barrier (BBB) and the blood-retinal barrier (BRB), which consists of retinal capillary endothelial cells and retinal pigment epithelial cells, are major determinants of the control of anionic drugs into the brain and retina. Although Oatp1a4 (Slco1a4) and Oatp1c1 (Slco1c1) are known to be expressed in the abluminal and luminal membrane of the rat BBB and Oatp1a4 is known to be expressed at the BRB, the expression and localization of Oatp1c1 at the BRB and subcellular localization of Oatp1a4 at the BRB have received little attention. Therefore, the purpose of present study was to determine the cellular and subcellular localization of Oatp1a4 and 1c1 at the BRB. Methods: We used guinea pig polyclonal antibodies to Oatp1a4 and 1c1 for immunoblot and immunohistochemical analysis to determine their cellular and subcellular distributions in the rat retina. We compared these distributions with those of the glucose transporter 1 (GLUT1/Slc2a1). Whole brain, brain capillary fractions and kidney were used as control. Results: Oatp1a4 and 1c1 immunoreactivities were detected in the rat retinal capillaries and co-localized with GLUT1, suggesting that both proteins are located on the abluminal and luminal membrane of the retinal capillary endothelial cells. Oatp1a4 and 1c1 immunoreactivities were preferentially detected on the apical and basolateral membrane of rat retinal pigment epithelial cells, respectively, suggesting that Oatp1a4 and 1c1 are localized on the apical membrane and the basolateral membrane of the retinal pigment epithelial cells, respectively. Conclusion: Oatp1a4 and 1c1 are present at the BRB and contribute to the transcellular transport of amphipathic organic anions across the BRB.