There is substantial evidence for the vectorial transport of solutes across the BRB. Various solute transport processes present in RPE are shown in Figure 10. Unlike other epithelial tissues and similar to the choroid plexus, RPE expresses Na + /K + -ATPase primarily in the apical membrane (Quinn and Miller, 1992). RPE cells secrete Na+ actively. Indeed, the active 22Na secretion was inhibited by apical ouabain (Miller and Edelman, 1990). RPE cells of human as well as rat origins express tetrodotoxin-sensitive Na + channels (Wen et al., 1994; Botchkin and Matthews, 1994). Besides Na + secretion, CP absorption is the principal contributor to the active ion transport across the RPE. CP absorption is primarily determined by furosemide-and bumetanide-sensitive Na + /K + /CP cotransporter, which allows apical CP entry (La Cour, 1992). Ca2+- cAMP-activated CP channels are present on the basolateral side to allow CP exit (Ueda and Steinberg, 1994; Strauss et al., 1996). Also, patch-clamp studies on single cells demonstrated a swelling-activated CP channel in RPE. Bovine and human RPE express CFTR, a cAMP-regulated CP channel (Miller et al., 1992). Na + /Ca2+ exchanger is also present in the apical membrane of bovine and dogfish RPE cells (Fijisawa et al., 1993).
RPE has a reverse polarization of Na + /K + -ATPase, because this pump is localized on the apical membrane as opposed to the basolateral membrane. To determine whether such reverse polarization is also the case with protein trafficking and sorting in RPE cells, Bok et al. (1992) determined the budding of viruses whose progeny bud from specific membrane domains in epithelial cells as directed by the sorting of their envelope gly-coprotein. Upon infection of human and bovine RPE with these envoloped viruses, cultured human and bovine RPE exhibited the same pattern of viral budding as has been observed in other polarized epithelia, with the influenza hemagglutinin sorted to the apical membrane and the vesicular stomatitis glycoprotein sorted to the basolateral membrane.
In addition to the above-mentioned ionic transport processes, there are specific transporters that regulate the transport of nutrients and metabolites such as glucose, amino acids, nucleosides, folic acid, lactic acid, ascorbic acid, and retinoids in the retina. A facilitated glucose transporter, GLUT1, a 50 kDa protein, is expressed in various cells, including retinal pigmented epithelial cells, choroidal cells, retinal Muller cells, and the outer segments of the photoreceptor cells in the adult eye. Immunofluorescence and immunogold studies revealed that GLUT1 is present on both apical and basolateral sites of RPE cells (Mantych et al., 1993). Immunoreactivity for GLUT3, a 50-55 kDa protein, was observed in the adult inner synaptic layer of the retina (Mantych et al., 1993).
In the blood-retinal barrier of rats, carrier systems exist for the transport of neutral and basic amino acids (Tornquist and Alm, 1986; Tornquist et al., 1990). Also, taurine and myo- inositol enter RPE cells via carrier-mediated transport mechanisms (Miyamoto et al., 1991). A purine nucleo-side transporter is present in RPE cells and retinal neurons, indicated by an increase in the accumulation of [3H]phenylisopropyl adenosine and [3H]adenosine in the presence of nitrobenzylthioinosine, an inhibitor of purine nucleoside transporter (Blazynski, 1991). The localization of these transporters is still not clear. The apical reduced-folate transporter (RFT- 1) and the basolateral folate receptor alpha (FRa) mediate vectorial transfer of reduced folate from choroidal blood to the neural retina in mouse and human RPE cells (Chancy et al., 2000).
To regulate lactate levels in the neural retina, RPE transports lactate between two tissue compartments, the interphotoreceptor matrix and the choriocapillaries. In isolated bovine RPE, Na + -lactate cotransporter located on the basolateral side moves lactate out of cells and the apically localized H + -lactate cotransporter moves lactate into the cells (Kenyon et al., 1994). The transport of lactate and other monocarboxylates in mammalian cells is mediated by a family of monocarboxylate transporters (MCTs), a group of highly homologous proteins that reside in the plasma membrane of almost all cells and mediate the 1:1 electroneutral transport of a proton and a lactate ion. MCT3 has been identified in RPE cells with basolateral distribution (Yoon et al., 1997). Unlike GLUTI, MCT1 is highly expressed in the apical processes of RPE and absent on the basal membrane of pigment epithelium (Philip et al., 1998; Gerhart et al., 1999; Bergersen et al., 1999). MCT1 is also associated with Müller cell microvilli, the plasma membranes of the rod inner segments, and all retinal layers between the inner and external limiting membranes. MCT1 functions to transport lactate between the retina and the blood at the level of retinal endothelium as well as the pigment epithelium. MCT2, on the other hand, is abundantly expressed on the inner (basal) plasma membrane of Müller cells and glial cells surrounding retinal micro-vessels. MCT4 is weekly expressed in RPE cells (Philip et al., 1998).
In primary or subcultured bovine and cat retinal pigment epithelium, ascorbate transport was observed to be coupled to the movement of sodium down its electrochemical gradient (Khatami et al., 1986). In cultured bovine capillary pericytes, ascorbate was transported via facilitated diffusion (Khatami, 1987). The uptake was specific for ascorbate, and this process was not sensitive to metabolic inhibition, the presence of ouabain, or the removal of Na+ from the bathing medium, consistent with ascorbate entry into the cells by facilitated diffusion.
RPE cells are critical in the maintenance of the visual or retinoid cycle, which involves the back-and-forth movement of vitamin A (retinol) and some of its derivatives (retinoids) between the rods and cones (photorecep-tors) and the RPE (Bok et al., 1984). Binding of retinol to retinol-binding proteins such as cellular retinol-binding protein and an interphotoreceptor retinoid-binding protein increases its solubility and delivers it to the RPE by a receptor-mediated process. The presence of cellular retinol binding protein and an interphotoreceptor retinoid-binding protein was shown in human, monkey, bovine, rat, and mouse retinas (Bunt-Milam and Saari, 1983; Bok et al., 1984). Cellular retinol-binding protein is predominantly localized in the space that surrounds the photoreceptor outer segments and the apical surface of RPE cells. It is also present in Müller glial cells. Interphotoreceptor retinoid-binding protein is localized in the space bordered by three cell types—RPE, photoreceptor, and Müller—which is consistent with its proposed role in the transport of retinoids among cells.
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