The neurotransmitter glutamate is neurotoxic when it is accumulated in a massive amount in the extracellular f luid. Excessive release of glutamate has been shown to be a major cause of neuronal degeneration after central nervous system injury. Under normal conditions, accumulation of synaptically released glutamate is prevented, at least in part, by a glial uptake system in which the glia-specific enzyme glutamine synthetase (GS) plays a key role. We postulated that glial cells cannot cope with glutamate neurotoxicity because the level of GS is not high enough to catalyze the excessive amounts of glutamate released by damaged neurons. We examined whether elevation of GS expression in glial cells protects against neuronal degeneration in injured retinal tissue. Analysis of lactate dehydrogenase eff lux, DNA fragmentation, and histological sections revealed that hormonal induction of the endogenous GS gene in retinal glial cells correlates with a decline in neuronal degeneration, whereas inhibition of GS activity by methionine sulfoximine leads to increased cell death. A supply of purified GS enzyme to the culture medium of retinal explants or directly to the embryo in ovo causes a dose-dependent decline in the extent of cell death. These results show that GS is a potent neuroprotectant and that elevation of GS expression in glial cells activates an endogenous mechanism whereby neurons are protected from the deleterious effects of excess glutamate in extracellular f luid after trauma or ischemia. Our results suggest new approaches to the clinical handling of neuronal degeneration.Glutamate neurotoxicity plays an important role in the process of neuronal degeneration after trauma or focal ischemia (for reviews, see refs. 1 and 2). Glutamate, a neurotransmitter that mediates normal excitatory synaptic transmission by interaction with postsynaptic receptors, is neurotoxic when present in excessive amounts. Injured neurons release massive amounts of glutamate, which induce neuronal cell death by continuous overexcitation of postsynaptic receptors. In this way, the initial trauma is amplified and causes the damage to spread to neighboring cells. Under normal conditions, the synaptically released glutamate is taken up into glial cells, where it is converted into glutamine by the glia-specific enzyme glutamine synthetase [GS; L-glutamate:ammonia ligase (ADPforming); EC 6.3.1.2]; glutamine reenters the neurons and is hydrolyzed by glutaminase to form glutamate, thus replenishing the neurotransmitter pool (3, 4). This biochemical pathway fails, however, to prevent glutamate neurotoxicity after insult. We hypothesized that GS is a limiting factor in this process and that its level in glial cells is not high enough to catalyze the excessive amounts of glutamate released by damaged cells. If this is the case, an increase in GS expression should have neuroprotective benefits.To examine the neuroprotective potential of GS, it is necessary to employ an experimental system in which expression of GS can be modulated. The ...

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Inducibility by glucocorticoids of the glutamine synthetase gene in chicken embryo retina and the transcriptional activity of the glucocorticoid receptor (GR) greatly increase between embryonic days 6 and 10 (E6, E10), although the level of GR does not markedly change during that time. This apparent discrepancy was investigated by exming the pattern of GR expression in undifferentiated E6 retina and in E10 retina, which consists mostly of differentiated cells. Two GR Isoforms, 90 and 95 kDa, were found to be expressed at both of these ages at a similar total level but in different proportions: in E6 retina the level of the 90-kDa isoform was higher, whereas in E10 retina the 95-kDa receptor was higher.However, following treatment of the retinas with cortisol, the 95-kDa isoform became the predominant receptor at both ages.Immunohistochemical analysis revealed that the cellular localization of GR markedly changed in the course of development: in the undifferentiated E6 retina GR was expressed in virtually all cells, whereas in the more differentiated E10 and E12 retina, GR was detected only in Mfiller glia cells. The latfer represent 20% of the cells in this tissue and are the only cells in which glucocorticoid hormone induces the glutamine synthetase gene.We suggest that the compatmentalization of GR in Muller glia is a majior aspect of the mechanism that modulates receptor activity during retina development and results in the temporal increase in the inducibility of glutamine synthetase and its specific localization in Mfuller glia cells.

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