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P. M. Martin, J. Gnana Prakasam, S. Ananth, B. Mysona, Y. Dun, P. Roon, S. B. Smith, V. Ganapathy; Expression of OCTN2, a Sodium-Dependent Carnitine Transporter, in Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5193.
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Carnitine, synthesized in the liver and also obtained from the diet, is essential for metabolism of long-chain fatty acids. Carnitine is known to provide protection against oxidative damage in a number of ocular diseases including glaucoma, cataract, macular degeneration, and diabetic retinopathy. SLC22A5, also known as OCTN2 (novel organic cationic transporter 2), is a Na+-coupled transporter for the uptake of carnitine in many tissues. Mutations in OCTN2 are the major cause of primary carnitine deficiency in humans. Expression of OCTN2 has been described in a number of tissues including skeletal muscle, heart, brain, and liver, but not in retina. In the present study, we asked whether OCTN2 is expressed also in retina, and if so, in which particular retinal cell types.
Expression of OCTN2 mRNA in neural retina and RPE/eyecup was determined by RT-PCR and in situ hybridization. Immunohistochemistry was performed to localize OCTN2 protein in intact retina. Similar techniques were used to analyze the expression of OCTN2 mRNA and protein in transformed retinal cell lines: HRPE, ARPE-19 (RPE), RGC-5 (ganglion), and rMC-1 (Müller). These data were confirmed using primary cultures of ganglion, Müller and RPE cells isolated from mouse retina. Uptake of [3H]carnitine was monitored to analyze the function of OCTN2 in the transformed retinal cell lines.
RT-PCR analysis established that OCTN2 mRNA is expressed in both neural retina and RPE/eyecup. Immunofluorescence analysis of OCTN2 protein in intact retina detected an expression pattern consistent with labeling of Müller cells. OCTN2 protein was detected also in the basolateral membrane of the RPE, and in the ganglion cell layer. Expression of OCTN2 mRNA and protein was readily evident in all cells tested: RGC-5, rMC-1, ARPE-19, HRPE, primary Müller cells, primary ganglion cells, and primary RPE cells. Functional assays with Na-coupled uptake of carnitine, confirmed the expression of OCTN2 in transformed retinal cell lines.
OCTN2 is expressed in Müller, ganglion, and RPE cells. Expression of OCTN2 in Müller cells supports the known ability of this cell type to metabolize long-chain fatty acids. OCTN2 in RPE may be involved in the transfer of carnitine across the outer blood-retinal barrier. Neurons are thought to have little ability to metabolize long-chain fatty acids; therefore, the functional significance of OCTN2 in ganglion cells needs further investigation.
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