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

The differential polarized distribution of the reduced-folate transporter ( RFT-1) and folate receptor alpha (FR alpha), the two proteins involved in t he transport of folate, has been characterized in normal mouse retinal pigm ent epithelium (RPE) and in cultured human RPE cells. RPE cells mediate the vectorial transfer of nutrients from choroidal blood to neural retina. Whe reas FR alpha is known to be present in many cell types of the neural retin a, in situ hybridization analysis in the present study demonstrated that RF T-1 is present only in RPE, Laser-scanning confocal microscopy using antibo dies specific for RFT-1 demonstrated an apical distribution of this protein in cultured human and intact mouse RPE, which contrasts with the basolater al distribution of FR alpha 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 N-5-meth yltetrahydrofolate (the predominant folate derivative in blood) and folate as substrates, have shown that RFT-1 functions in a Na+- and Cl--independen t manner. The transporter is specific for folate and its analogs, A transme mbrane H+ gradient influences the transport function of this protein marked ly; the transport mechanism is Likely to be either folate/H+ co-transport o r folate/OH- exchange. Based on the differential polarization of FR alpha a nd RFT-1 in RPE, we suggest that these two proteins work in a concerted man ner to bring about the vectorial transfer of folate across the RPE cell lay er from the choroidal blood to the neural retina. This constitutes the firs t report of the differential polarization of the two folate transport prote ins in any polarized epithelium.