Glutamine synthetase enhances the clearance of extracellular glutamate by the neural retina

Abstract

Clearance of synaptic glutamate by glial cells is required for the normal function of excitatory synapses and for prevention of neurotoxicity. Although the regulatory role of glial glutamate transporters in glutamate clearance is well established, little is known about the influence of glial glutamate metabolism on this process. This study examines whether glutamine synthetase (GS), a glial-specific enzyme that amidates glutamate to glutamine, affects the uptake of glutamate. Retinal explants were incubated in the presence of induction of the endogenous gene, did not affect the amount of glutamate accumulated within the cells, but led to a dramatic increase in the amount of glutamine released. This increase, which was directly correlated with the level of GS expression, was dependent on the presence of external sodium ions, and could be completely abolished by methionine sulfoximine, a specific inhibitor of GS activity. Our results demonstrate that GS activity significantly influences the uptake of glutamate by the neural retina and suggest that this enzyme may represent an important target for neuroprotective strategies.

Amino-acid levels in subretinal and vitreous fluid of patients with retinal detachment

“…GS has also been shown to influence glutamate clearance from the synaptic cleft and amplified expression of GS has provided protection from excess glutamate. 45,46 Retinal ischaemia secondary to RD may play a role in disrupting the mechanisms of reuptake and degradation responsible for maintaining glutamate homeostasis.…”

Ischemic tolerance in chemical preconditioning: Possible role of astrocytic glutamine synthetase buffering glutamate‐mediated neurotoxicity

“…In earlier study, Eddelstone and Mucke (1993) reported that reactive astrocytes upregulated their GS levels and enhanced a capacity to metabolize glutamate to glutamine. In the retinal Mü ller cells (a counterpart of astrocytes that express a high level of GS), the metabolic conversion of glutamate to glutamine might reduce the intracellular concentration of glutamate, and lead to a dramatic increase in the amount of glutamine released (Shaked et al, 2002). In addition, an increase of glutamine could be completely abolished by methionine sulfoximine, a specific inhibitor of GS activity (Shaked et al, 2002).…”

“…In the retinal Mü ller cells (a counterpart of astrocytes that express a high level of GS), the metabolic conversion of glutamate to glutamine might reduce the intracellular concentration of glutamate, and lead to a dramatic increase in the amount of glutamine released (Shaked et al, 2002). In addition, an increase of glutamine could be completely abolished by methionine sulfoximine, a specific inhibitor of GS activity (Shaked et al, 2002). It was reported recently that loss of astrocytic GS in the human epileptogenic hippocampus is a possible molecular basis for extracellular glutamate accumulation and seizure generation (Eid et al, 2004).…”

“…Glutamine synthetase is also considered as a critical enzyme for the regulation of the concentration of glutamate in the brain (D'Amelio et al, 1990;Sonnewald et al, 1997;Hertz et al, 1999), and as an enhancer of extracellular glutamate scavenge system (Eddelstone and Mucke, 1993;Toki et al, 1998;Shaked et al, 2002). The post-ischemic increases in astrocytic GS suggest that the capacity to uptake glutamate and to convert glutamate to glutamine is enhanced after ischemia (Petito et al, 1992;Louw et al, 1998).…”

Excitotoxicity and Glaucoma

“…The activity of glutamine synthetase is stimulated by glucocorticoids [76,77]. In chick embryo retinas the stimulated glutamine synthetase by glucocorticoids is correlated with amounts of released glutamine from Müller glia, suggesting that the activity of glutamine synthetase influences glutamate uptake [78].…”