Expression and function of glutamine transporters SN1 (SNAT3) and SN2 (SNAT5) in retinal Müller cells

PURPOSE. The expression and function of the glutamine transporters ATA1 and ATA2 (isoforms of system A), SN1 and SN2 (isoforms of system N), and LAT1 and LAT2 (isoforms of system L) were investigated in Müller cells in a rat Müller cell line (rMC1) and primary cultures of mouse Müller cells. METHODS. Glutamine uptake in rMC1 cells and primary Müller cells was measured. The relative contributions of various transport systems to glutamine uptake were determined based on the differential substrate specificities and Na⁺ dependence of individual transport systems. RT-PCR was used to analyze the expression of transporter-specific mRNAs. RESULTS. Three different transport systems participated in glutamine uptake in rMC1 cells: system L (Na⁺-independent), system A (Na⁺-dependent and α-(methylamino)isobutyric acid [MeAIB]-sensitive), and system N (Na ⁺-dependent and MeAIB-insensitive). System N was the principal contributor (∼70%); the contributions of systems A and L were relatively lesser (20% and <10%, respectively). The functional features of Na ⁺-dependent and MeAIB-insensitive glutamine uptake were similar to the known characteristics of clones of SN1 and SN2. Glutamine uptake in primary Müller cells behaved in a manner similar to that in rMC1 cells. mRNA transcripts specific for ATA1, ATA2, SN1, SN2, LAT1, and LAT2 were expressed in Müller cells. CONCLUSIONS. System N (SN1 as well as SN2) is responsible for most of the glutamine uptake in Müller cells. Because system N is capable of mediating the release of glutamine from the cells, its abundant expression in Müller cells is of importance in the handling of glutamine in the retina.