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Glutamate transport by retinal Müller cells in glutamate/aspartate transporter‐knockout mice
Author(s) -
Sarthy Vijay P.,
Pignataro Leonardo,
Pannicke Thomas,
Weick Michael,
Reichenbach Andreas,
Harada Takayuki,
Tanaka Kohichi,
Marc Robert
Publication year - 2004
Publication title -
glia
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.954
H-Index - 164
eISSN - 1098-1136
pISSN - 0894-1491
DOI - 10.1002/glia.20097
Subject(s) - glutamate receptor , biology , glutamate aspartate transporter , glutamatergic , transporter , metabotropic glutamate receptor 6 , microbiology and biotechnology , biochemistry , metabotropic glutamate receptor , biophysics , receptor , gene
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 Müller cells in GLAST −/− mice, using biochemical, electrophysiological, and immunocytochemical techniques. Glutamate uptake assays indicated that the K m value for glutamate uptake was similar in wild‐type and GLAST −/− mouse retinas, but the V max was ∼50% lower in the mutant. In Na + ‐free medium, the V max was further reduced by 40%. In patch‐clamp recordings of dissociated Müller 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 Müller cells. [ 3 H] D ‐Aspartate uptake autoradiography, however, showed that Na + ‐dependent, high‐affinity transporters account for most of the glutamate uptake by Müller cells, and that Na + ‐independent glutamate transport is negligible. Additional experiments showed that the residual glutamate uptake in Müller 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 Müller cells, new Na + ‐dependent, high‐affinity glutamate transporters remain to be identified. © 2004 Wiley‐Liss, Inc.