Glutamate transport by retinal Müller 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 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. [³H]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.