Glutamate transport by retinal Muller cells in glutamate/aspartate transporter-knockout mice.
Glutamate transporters are involved in maintaining extracellular glutamate at a low level to ensure a high signal-to-noise ratio for glutamatergic neurotransmission and to protect neurons from excitotoxic damage. The mammalian retina is known to express the excitatory amino acid transporters, EAAT1-5; however, their specific role in glutamate homeostasis is poorly understood. To examine the role of the glial glutamate/aspartate transporter (GLAST) in the retina, we have studied glutamate transport by Muller cells in GLAST-/- mice, using biochemical, electrophysiological, and immunocytochemical techniques. Glutamate uptake assays indicated that the Km value for glutamate uptake was similar in wild-type and GLAST-/- mouse retinas, but the Vmax was approximately 50% lower in the mutant. In Na+-free medium, the Vmax was further reduced by 40%. In patch-clamp recordings of dissociated Muller cells from GLAST-/- mice, application of 0.1 mM glutamate evoked no current showing that the cells lacked functional electrogenic glutamate transporters. The result also indicated that there was no compensatory upregulation of EAATs in Muller cells. [3H]D-Aspartate uptake autoradiography, however, showed that Na+-dependent, high-affinity transporters account for most of the glutamate uptake by Muller cells, and that Na+-independent glutamate transport is negligible. Additional experiments showed that the residual glutamate uptake in Muller cells in the GLAST-/- mouse retina is not due to known glutamate transporters-cystine-glutamate exchanger, ASCT-1, AGT-1, or other heteroexchangers. The present study shows that while several known glutamate transporters are expressed by mammalian Muller cells, new Na+-dependent, high-affinity glutamate transporters remain to be identified.
Sarthy
Vijay P
VP
Department of Ophthalmology, Northwestern University Feinberg School of Medicine, Chicago, Illinois 60093, USA. vjsarthy@northwestern.edu
Pignataro
Leonardo
L
Pannicke
Thomas
T
Weick
Michael
M
Reichenbach
Andreas
A
Harada
Takayuki
T
Tanaka
Kohichi
K
Marc
Robert
R
D026962:Amino Acid Transport System ASC / Q000378:metabolism; D027322:Amino Acid Transport System X-AG / Q000235:genetics*; D026905:Amino Acid Transport Systems / Q000378:metabolism; D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism; D001692:Biological Transport / Q000235:genetics / Q000502:physiology; D002478:Cells, Cultured; D027341:Excitatory Amino Acid Transporter 1; D050491:Glutamate Plasma Membrane Transport Proteins; D018698:Glutamic Acid / Q000378:metabolism* / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000235:genetics; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009154:Mutation / Q000235:genetics; D009457:Neuroglia / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D012160:Retina / Q000166:cytology / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism; D027981:Symporters / Q000235:genetics*; D015854:Up-Regulation / Q000187:drug effects / Q000235:genetics
D026962:Amino Acid Transport System ASC; D027322:Amino Acid Transport System X-AG; D026905:Amino Acid Transport Systems; D027341:Excitatory Amino Acid Transporter 1; D050491:Glutamate Plasma Membrane Transport Proteins; C493393:Slc1a3 protein, mouse; C498847:Slc1a4 protein, mouse; C459184:Slc7a13 protein, mouse; D015222:Sodium Channels; D027981:Symporters; D001224:Aspartic Acid; D018698:Glutamic Acid; D012964:Sodium