Glutamine synthetase protects against neuronal degeneration in injured retinal tissue

Abstract

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 ...

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

“…This possibility is supported by the finding that an increase in GS expression in glial cells, caused by induction of its endogenous gene, has neuroprotective benefits (Gorovits et al 1997) and that GS expression and/or activity is down-regulated under certain neuropathological conditions (Lewis et al 1989;Oliver et al 1990;Smith et al 1991;Lewis et al 1994;Grosche et al 1995;Hartig et al 1995). We used the embryonic chicken retina for this study because the level of GS in this tissue can be closely regulated by glucocorticoids.…”

“…The observed role of GS expression in glutamate uptake suggests that a decline in GS expression at the site of injury may reduce the ability of glial cells to remove extracellular glutamate, thereby exacerbating the process of neuronal degeneration. Indeed, our previous studies demonstrated that GS is a potent neuroprotectant and that elevation of GS expression in glial cells protects against neuronal degeneration in injured retinal tissue (Gorovits et al 1997). The ability of GS to regulate the removal of extracellular glutamate is an important aspect of neuronal excitotoxicity, and research on ways to increase GS expression at the site of injury may offer novel strategies for neuroprotection.…”

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.…”

Manipulating Glia to Protect Retinal Ganglion Cells in Glaucoma

“…A similar upregulation of GS expression occurs in hepatic retinopathy where the GS detoxifies the retina of accumulated ammonia (Reichenbach et al, 1995). Upregulation of GS in Müller glia is neuroprotective against glutamate neurotoxicity (Gorovits et al, 1997), suggesting that the increased expression in DBA/2J retina may be a protective mechanism. As noted in the retinal astrocyte section, changes in GS have implications for managing oxidative stress and maintaining appropriate glutamate levels in retina.…”