Glutamine synthetase: Role in neuroprotection, mechanism for cell-type specific expression and regulation by cell-to-cell contact interaction
Glutamine synthetase (GS) is a key enzyme in the ‘small glutamate compartment’, which constitutes in neural tissues an endogenous neuroprotective mechanism that prevents the accumulation of neurotoxic amounts of glutamate in postsynaptic regions. Under normal conditions, glutamate is taken up by glial cells and converted by the GS enzyme into the non-toxic amino acid glutamine. Glutamine is then released by the glial cells and returns into neurons, where it is hydrolized by glutaminase to form glutamate again. In this way, glutamate neurotoxicity is prevented and the neurotransmitter pool is replenished. This biochemical pathway fails, however, to prevent glutamate neurotoxicity under pathological conditions. Glutamate neurotoxicity has been implicated in the process of neuronal degeneration following injury or focal ischemia and in the pathophysiology of a wide variety of neurological disorders. Considering the role of GS in glutamate detoxification, it has been postulated that glial cells cannot cope with glutamate neurotoxicity, because the level of GS is not high enough to catalyze the excessive amount of glutamate released by damaged neurons. Recent studies by us and others have indeed demonstrated that the removal of glutamate from the extracellular space is ultimately dependent on the level of GS expression in glial cells, and that neuronal injury is often accompanied by a decline in GS expression and/or activity. Most importantly, we have shown that an increase in the amount of GS in glial cells, via induction of the endogenous gene or exogenous supply of purified GS, protects against neuronal degeneration in injured retinal tissue. Based on these findings we postulate that elevation of GS expression in ovo, at the site of neural injury, will protect against neuronal degeneration. The objective of our current research effort is to test this hypothesis.
The significance of GS in neuroprotection led us to investigate the molecular mechanism that underlies the induction of high GS levels in glial cells. Cloning, sequencing and functional analysis of the upstream region of the GS gene identified two major regulatory elements: a glucocorticoid response element (GRE) and a neural restrictive silencer element (NRSE). Studies in the chicken retina revealed that glial specificity of GS expression is achieved by utilizing these positive (GRE) and negative (NRSE) regulatory elements, which are not glial-specific by themselves, but might establish a glial-specific pattern of expression through their mutual activity. We are currently investigating the interplay between the two transcription factors that mediates these activities: the glucocorticoid receptor protein, which is responsible for hormonal induction of GS expression in glial cells, and NRSF/REST, which represses the hormonal response in non-neural cells.
One of the most interesting aspects in the control of GS expression is the fact that hormonal induction of GS expression is ultimately dependent on glia-neuron cell contact. GS expression cannot be induced in separated glial cells. We demonstrated that in the absence of cell-to-cell contact, glial cells express a high level of the c-Jun, a transcription factor that controls a set of genes that regulate cell proliferation. The c-Jun protein can also render the glucocorticoid receptor transcriptionally inactive and thus inhibit GS induction. This inhibitory mechanism functions in proliferating retinal cells of early embryonic ages and is activated in the mature tissue by growth factors or oncogenes, and upon exposure of the tissue to trauma or ischemia. Under each of these conditions the c-Jun signaling pathway is activated, glial cells proliferate, the glucocorticoid receptor becomes transcriptionally inactive and GS expression cannot be induced. We are currently exploring the molecular mechanism that underlies the cell-to-cell contact control of c-Jun expression