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P. M. Martin, B. Prasad, B. Mysona, Y. Dun, P. Roon, S. B. Smith, V. Ganapathy; Expression of SMCT2, a Low-Affinity Sodium-Coupled Monocarboxylate Transporter, in Retina. Invest. Ophthalmol. Vis. Sci. 2007;48(13):63.
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© ARVO (1962-2015); The Authors (2016-present)
Retinal neurons have high energy demands. In addition to glucose, these cells use lactate as an energy substrate. Müller glial cells produce lactate from glucose and release it into the extracellular space. Neuronal cells take up lactate and use it for energy production. Monocarboxylate transporters (MCTs) have been implicated both in the release and uptake of lactate in retinal cells. Recently, we identified two Sodium-coupled MonoCarboxylate Transporters (SMCT1 and SMCT2) that are capable of Na+-coupled electrogenic transport of lactate, pyruvate, and ß-hydroxybutyrate. SMCT1 is a high-affinity transporter whereas SMCT2 is a low-affinity transporter. Expression of SMCT1 and SMCT2 has been studied in kidney and intestine, and we previously demonstrated the neuron-specific expression of SMCT1 in brain and retina. SMCT2 expression in retina has not been studied. In the present study, we asked whether SMCT2 is expressed also in retina, and if so, in which particular retinal cell types.
Expression of SMCT2 mRNA in neural retina and RPE/eyecup was determined by RT-PCR. SMCT2 protein was localized in intact retina via immunofluorescence. Similar techniques were used to analyze expression of SMCT2 mRNA and protein in ARPE-19 (RPE), RGC-5 (ganglion) and rMC-1 (Müller) cells. Data obtained using transformed retinal cell lines were confirmed using primary cultures of ganglion and Müller cells isolated from mouse retina. Additionally, the glial-specific localization of SMCT2 was analyzed in mixed primary cultures of cortical neurons and astrocytes isolated from rat brain.
RT-PCR analysis established that SMCT2 mRNA is expressed in neural retina, but not in RPE/eyecup. Immunofluorescence analysis of SMCT2 protein displayed a pattern of localization consistent with labeling of Müller cells. Expression of SMCT2 mRNA and protein was readily evident in rMC-1, primary Müller cells, and primary astrocytes, but not in RGC-5, ARPE-19, primary ganglion cells, or primary cortical neurons.
In retina, SMCT2 is expressed exclusively in Müller cells. Since glial cells are thought to produce/release lactate rather than take it up, further studies are needed to determine the functional significance of the Müller cell-specific expression of SMCT2 in retina. These studies suggest that SMCT1 (neurons) and SMCT2 (Müller glia) play a differential role in monocarboxylate transport in retina in a cell type-specific manner.
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