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W. Li, U. Siddaramappa, B. Mysona, T.K. Van Ells, V. Ganapathy, S.B. Smith; Expression and Function of the Glutamine Transporter, System N, in Retinal Müller Cells . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2977.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In the retina, Müller cells play a crucial role in the "glutamate–glutamine cycle" in which neurons release glutamate into the extracellular space, Müller cells take it up and convert it to glutamine via glutamine synthetase, and then release glutamine to the extracellular milieu for use by neurons. We were interested in studying the mechanism whereby Müller cells release glutamine and hypothesized that the glutamine transporter, System N, would play a prominent role in this process. Methods: A rat Müller cell line (rMC–1) and mouse primary Müller cells were used in molecular and functional studies of the known glutamine transporters: System N, System A and System L. RT–PCR was used to study the expression of these transporters. Functional studies used [3H]–glutamine to determine the Na+ and Cl––dependence, pH dependence, substrate specificity and kinetics of the transport system. Results: RT–PCR analysis of RNA isolated from rMC–1 and primary Müller cells provided evidence for the expression of System N (SN1 and SN2), System A (SNAT1 and SNAT2), and System L (LAT1/4F2hc and LAT2/4F2hc). Functional studies revealed that System N is the predominant contributor to glutamine uptake in these cells. Under conditions which allowed primarily the measurement of System N activity, glutamine uptake was Na+–dependent and Cl––independent, was greatest at pH 8.0 and decreased markedly at lower pH levels, and was inhibited most by glutamine compared with other amino acids. Kinetic analysis suggested that 2 Na+ molecules are transported with each glutamine consistent with transport by System N. Conclusions: Müller cells express three glutamine transport systems, System N, System L and System A. Of these, the predominant mechanism used by Müller cells for transport of glutamine is System N. Future studies will determine factors that regulate the expression and function of this transporter in Müller cells.
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