Arraying the Pro-Life and Neurotoxic Responses to NMDA Receptor Signaling

Abstract

Glutamate receptors of the N -methyl-D-asparate (NMDA) type are implicated in physiological neuronal signaling (for example, neuroprotection and synaptic plasticity) as well as in pathological neuronal processes (for example, excitotoxicity and neurodegenerative disease). Zhang et al . performed microarray analysis to monitor gene expression in cultured hippocampal neurons in response to two different paradigms that stimulated NMDA receptors, which are calcium-conducting ion channels. In one, the NMDA receptors were activated by synaptic activity through the pharmacological blockade of inhibitory receptors, and in the other, both synaptic and extrasynaptic receptors were activated by glutamate added to the bath. The NMDA-specific transcriptional profile was assessed by comparing the genes stimulated or repressed in the presence or absence of NMDA-specific inhibitors. The activity-dependent profile resulted in a total of 478 induced genes, with 115 of those specific to NMDA receptor activation and 108 of those unique to the synaptic activation profile--that is, they were not also activated by bath-applied glutamate. Similar analysis showed that of 106 total genes activated by bath-applied glutamate, only 11 were specific to the extrasynaptic activation of the receptors. Only a single gene was specifically repressed by extrasynaptic NMDA receptors, whereas 34 genes were specifically repressed by synaptic activation of NMDA receptors. Using gene ontology to classify the regulated genes, the authors noted that synaptic receptor activation seemed to stimulate a prosurvival, antiapoptotic profile by enhancing genes associated with survival and repressing genes associated with cell death. The functions of two genes up-regulated by the synaptic activity, Bcl6 and Btg2 , were further assessed by overexpression studies and RNA inhibition studies. Bcl6 encodes a transcriptional repressor that has been implicated in B cell lymphomas, where it has been reported to repress p53 expression and prevent cell death due to DNA damage. The exact function of the protein encoded by Btg2 is unknown, but it has been implicated in protection of PC12 cells from cell death. Overexpression of either of these proteins in the hippocampal cultures protected the neurons from death induced by growth factor withdrawal or by staurosporine, and coexpression was most effective. Knockdown of Btg2 by RNA interference prevented activity-dependent protection from staurosporine, and knockdown of Bcl6 resulted in enhanced cell death in the absence of any stressful stimuli. Both of these genes required a nuclear calcium signal for transcriptional activation, and their activity-dependent transcription was blocked by overexpression of nuclear calcium-calmodulin binding protein. Analysis of the consequence of overexpression or knockdown of the Clca1 gene, which showed increased transcription in response to extrasynaptic glutamate and encodes a calcium-activated chloride channel, revealed that overexpression promoted cell death but knockdown did not promote survival. Altogether, these results help explain how a single receptor can promote opposing responses and emphasize the importance of spatially restricted signaling. S.-J. Zhang, M. N. Steijaert, D. Lau, G. Schütz, C. Delucinge-Vivier, P. Descombes, H. Bading, Decoding NMDA receptor signaling: Identification of genomic programs specifying neuronal survival and death. Neuron 53 , 549-562 (2007). [PubMed]