Osmoregulation of taurine transporter function and expression in retinal pigment epithelial, ganglion, and müller cells.
OBJECTIVE
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 gamma-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 ( approximately 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.
El-Sherbeny
Amira
A
Department of Cellular Biology and Anatomy, Medical College of Georgia, Augusta, Georgia 30912-2000, USA.
Naggar
Hany
H
Miyauchi
Seiji
S
Ola
M Shamsul
MS
Maddox
Dennis M
DM
Martin
Pamela Moore
PM
Ganapathy
Vadivel
V
Smith
Sylvia B
SB
D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002478:Cells, Cultured; D006982:Hypertonic Solutions / Q000494:pharmacology; D007294:Inositol / Q000378:metabolism; D008353:Mannitol / Q000494:pharmacology; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D026901:Membrane Transport Proteins; D009457:Neuroglia / Q000187:drug effects / Q000378:metabolism*; D009994:Osmolar Concentration; D010857:Pigment Epithelium of Eye / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D012165:Retinal Ganglion Cells / Q000187:drug effects / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012964:Sodium / Q000494:pharmacology; D012965:Sodium Chloride / Q000494:pharmacology; D013654:Taurine / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism; D000099247:Taurine Transporters
D002352:Carrier Proteins; D006982:Hypertonic Solutions; D007294:Inositol; D008353:Mannitol; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D012964:Sodium; D012965:Sodium Chloride; D013654:Taurine; D005680:gamma-Aminobutyric Acid; D000099247:Taurine Transporters