RPE Lactate Transporters: A Role in Retinal Survival
The retinal pigment epithelium (RPE) transports ions, metabolites, and fluid between the outer retina and the choroidal blood supply. By so doing, the RPE controls the microenvironment of the photoreceptor cells and thus participates in their functional activity. The long-term goals of this proposal are to study the regulated expression, cellular distribution, and functional activity of RPE transport proteins and how these proteins may modulate metabolic and pH homeostasis in the outer retina. The neural retina is one of the most metabolically active tissues in the body and produces large quantities of lactate via aerobic glycolysis. Because photoreceptor cell function is dependent on tight pH regulation, it is essential that lactate be removed from the subretinal space (SRS). Studies from our lab have shown that the RPE expresses two proton-coupled monocarboxylate transporters that facilitate the transepithelial movement of lactate from the subretinal space to the choroid;MCT1 in the apical membrane and MCT3 in the basolateral membrane. Both transporters are linked to CD147, an accessory protein required for proper trafficking to the plasma membrane. Mice with targeted deletion of genes encoding either MCT3 (Slc16A8) or CD147 (Bsg) have reduced visual function. In the Mct3-/- mouse, lactate levels in the retina are four-fold higher than in retinas from wt mice. This finding supports the hypothesis that MCT3 is the gatekeeper that regulates lactate transport out of the retina thereby regulating the pH and the osmolarity of the SRS. Our recent studies of these mice show they have swelling of the RPE with altered organization of the actin cytoskeleton and membrane proteins in the basolateral membrane. These findings underscore the importance of further characterizing how the open and closed state of these transporters is regulated by protein-protein interactions and how autocrine/paracrine signals and hormones modulate the co-transport of lactate, H+, and H2O from the SRS to the choroid. Therefore, we propose an integrated series of experiments to investigate: 1) how the functional activity of MCTs contributes to the pH homeostasis of the outer retina and the structural integrity of the outer blood retinal barrier, 2) how autocrine/paracrine signals and hormones regulate the kinetic activity of MCTs thereby modulating the transport of lactate from the subretinal space to the choroid, 3) how specific motifs in the C-terminal domains of MCTs orchestrate their assembly into macromolecular complexes which stabilize their polarized distribution in the plasma membrane and through signaling pathways regulate their open and closed state. These studies will enhance our understanding of how these heteromeric transporters (MCT/CD147) contribute to maintaining normal visual functions. Additionally, these studies will further our understanding of how changes in expression and activity of MCTs contribute to pathophysiological changes found in aging and diseased eyes.