Modulation of MCT3 expression during wound healing of the retinal pigment epithelium.

The retina is one of the most metabolically active tissues in the body and produces large quantities of lactate by the metabolism of glucose through aerobic glycolysis.1,2 Lactate produced by Muller cells is used by photoreceptors to fuel oxidative phosphorylation, whereas excess lactate is transported out of the retina to the choroidal blood supply by the retinal pigment epithelium (RPE).3 In this capacity, the RPE serves as the gatekeeper of the outer retina, regulating the flux of ions, metabolites, and fluid between the outer retina and the choroidal blood supply. Transepithelial transport of lactate is facilitated by monocarboxylate transporters (MCTs) in the apical and basolateral membranes of the RPE.4 The MCTs are members of the SLC16A gene family of transporters. Both the avian eye and the mammalian eye express a variety of these transporters, including MCT1 and MCT3 in the RPE and MCT4 in the retina.5 Although MCT3 is expressed preferentially in the basolateral membrane of the RPE, MCT1 is widely expressed in tissues throughout the body, and MCT4 is expressed in cells and tissues that are highly glycolytic.6 MCT1, MCT3, and MCT4 form a heterodimeric complex with CD147,6–9 a type I glycoprotein and member of the immunoglobulin superfamily.10 CD147 was first characterized as an extracellular matrix metalloproteinase inducer and a blood-brain barrier antigen.11–13 Assembly of the MCT/CD147 complex in the endoplasmic reticulum is required for the efficient trafficking of the mature transporter to the plasma membrane because in the absence of one subunit the other is targeted for degradation.8,9 The importance of MCTs in maintaining normal vision has been demonstrated in mice with targeted deletion of either CD147 (Bsg) or MCT3 (Slc16a8) genes. In Bsg−/− mice there was a decrease in the amplitudes of scotopic and photopic electroretinograms (ERGs) before degeneration of photoreceptor cells.14 Work from our laboratory suggested that the change in visual function observed in the Bsg−/− mice was caused by the loss of expression of MCT1, MCT3, and MCT4 in the RPE and retina and the disruption of metabolic coupling between Muller cells and photoreceptor cells.5 This hypothesis was supported by our recent study demonstrating that mice with a targeted deletion of the Slc16a8 (MCT3) gene exhibited decreased amplitudes of the light-stimulated ERG, whereas lactate levels in the retina were increased.15 The expression of RPE-specific genes can be altered in response to disease and trauma, thereby contributing to pathologic changes in this tissue.16 Removing RPE cells from eyes and placing them in culture also results in changes in gene expression that can vary with the time in culture and culture conditions.17–22 Our laboratory previously reported that MCT3 mRNA and protein were not detected in chick RPE cells when cultured on plastic or transwell inserts.23 These findings suggested that cell-cell, cell-substrate, or RPE-photoreceptor interactions are essential to regulate the expression of RPE specific genes. To obtain further insight into the regulation of MCT3 in RPE cells, we examined the expression of MCT3 in chick RPE/choroid explant cultures and hfRPE cells after the integrity of the epithelial monolayer was disrupted by mechanical wounding. Our findings show that disruption of cell-cell interactions leads to a downregulation of MCT3 and an up-regulation of MCT4. In addition, we reported that MCT3 was expressed after re-epithelialization but that the rapidity of MCT3 reexpression may be reliant on factors from the basal lamina. Taken together, these results indicate that the integrity of the RPE monolayer is essential for maintaining expression of MCT3 and that the basal lamina may contribute to the modulation of MCT expression in the RPE.