Expression of the sodium-coupled monocarboxylate transporters SMCT1 (SLC5A8) and SMCT2 (SLC5A12) in retina

Purpose. Monocarboxylates are primary energy substrates in the retina. Recently, the authors identified two sodium-coupled monocarboxylate transporters (SMCTs), SMCT1 (a high- affinity transporter) and SMCT2 (a low-affinity transporter). Expression of SMCT1 and SMCT2 has been studied in several tissues; however, little is known about their expression in retina. In the present study, the authors asked whether SMCT1 and SMCT2 are also expressed in retina and, if so, in which particular retinal cell types. Methods. SMCT1 and SMCT2 expression was analyzed in intact mouse retina and cultured retinal cells (ganglion, Muller, RPE) by RT-PCR, in situ hybridization, and immunofluorescence. Uptake assays were performed to demonstrate SMCT1 (RGC-5 and ARPE-19 cells) and SMCT2 (rMC-1 cells) expression at the functional level. Results. SMCT1 mRNA and protein were detected in the ganglion cell layer, inner nuclear layer, inner/ outer plexiform layers, photoreceptor inner segments, and RPE. In RPE, the expression of SMCT1 was restricted to the basolateral membrane. SMCT2 mRNA and protein were detected only in neural retina, with a pattern of protein localization consistent with labeling of Muller cells. In vitro studies confirmed the cell type-specific expression of SMCT1 and SMCT2. Uptake assays demonstrated Na+-coupled monocarboxylate transport in RGC-5, ARPE-19, and rMC-1 cells. Conclusions. These data provide the first evidence for the expression of SMCT1 and SMCT2 in the retina and for the cell-type specific distribution of these transporters within the retina. These studies suggest that SMCT1 and SMCT2 play a differential role in monocarboxylate transport in the retina in a cell type-specific manner.