Expression and differential polarization of the reduced-folate transporter-1 and the folate receptor α in mammalian retinal pigment epithelium

The differential polarized distribution of the reduced-folate transporter (RFT-1) and folate receptor α (FRα), the two proteins involved in the transport of folate, has been characterized in normal mouse retinal pigment epithelium (RPE) and in cultured human RPE cells. RPE cells mediate the vectorial transfer of nutrients from choroidal blood to neural retina. Whereas FRα is known to be present in many cell types of the neural retina, in situ hybridization analysis in the present study demonstrated that RFT-1 is present only in RPE. Laser-scanning confocal microscopy using antibodies specific for RFT-1 demonstrated an apical distribution of this protein in cultured human and intact mouse RPE, which contrasts with the basolateral distribution of FRα in these cells. The expression of RFT-1 in the RPE cell apical membrane was confirmed by functional studies with purified apical membrane vesicles from bovine RPE. These studies, done with N⁵- methyltetrahydrofolate (the predominant folate derivative in blood) and folate as substrates, have shown that RFT-1 functions in a Na^+- and C1^- independent manner. The transporter is specific for folate and its analogs. A transmembrane H⁺ gradient influences the transport function of this protein markedly; the transport mechanism is likely to be either folate/H⁺ co- transport or folate/OH^- exchange. Based on the differential polarization of FRα and RFT-1 in RPE, we suggest that these two proteins work in a concerted manner to bring about the vectorial transfer of folate across the RPE cell layer from the choroidal blood to the neural retina. This constitutes the first report of the differential polarization of the two folate transport proteins in any polarized epithelium.