Osmoregulation of Taurine Transporter Function and Expression in Retinal Pigment Epithelial, Ganglion, and Müller Cells

PURPOSE. To determine whether taurine transporter (TauT) activity and expression are regulated by hyperosmolarity in RPE, ganglion, and Müller cells. METHODS. Uptake of taurine was measured in ARPE-19 cells cultured in DMEM-F12 medium without or with the addition of 50 mM NaCl or 100 mM mannitol. The kinetics of the transport were analyzed. RT-PCR and Northern and Western blot analyses were used to assess TauT mRNA and protein levels. The influence of hyperosmolarity on the uptake of taurine, myo-inositol, and γ-aminobutyric acid GABA was studied in RPE, RGC-5, and rMC1 cells. RESULTS. TauT activity was abundant in RPE and was stimulated (3.5-fold) when the cells were exposed to hyperosmolar conditions (DMEM-F12 culture medium plus 50 mM NaCl or 100 mM mannitol). Peak stimulation of taurine uptake occurred after 17 hours of exposure to hyperosmolar medium. Kinetic analysis revealed that the hyperosmolarity-induced stimulation was associated with an increase in V_max of TauT with no change in K_m. TauT mRNA and protein levels increased in RPE cells exposed to hyperosmolar conditions. Hyperosmolarity also stimulated the uptake of myo-inositol (∼15-fold); GABA uptake was influenced less markedly. Immunofluorescence and functional studies showed that TauT is present in cultured RGC-5 and rMC1 cells. TauT activity was robust in these cells in normal osmolar conditions and increased by approximately twofold in hyperosmolar conditions. CONCLUSIONS. These studies provide the first evidence that hyperosmolarity regulates TauT activity and expression in RPE and that TauT is present in ganglion and Müller cells and is regulated by hypertonicity. The data are relevant to diseases such as diabetes, macular degeneration, and neurodegeneration, in which retinal cell volumes may fluctuate dramatically.