Molecular alterations associated with the NMDA preconditioning-induced neuroprotective mechanism against glutamate cytotoxicity

Exposure of the brain to sub-lethal concentrations of glutamate activates, through stimulation of the glutamate N-methyl-D-aspartate (NMDA) receptors, an endogenous brain protective mechanism (NMDA preconditioning) against glutamate cytotoxicity and various other injurious stimuli. Selective drug activation of this mechanism is considered to be a promising neuroprotective treatment against the devastating consequences of stroke and other traumatic brain insults. Although some properties of this mechanism have been characterized, many aspects concerning it are yet to be elucidated. In order to improve our understanding of the NMDA preconditioning mechanism, we have established an experimental in vitro model of primary rat neuronal cultures, in which NMDA preconditioning completely abolishes the glutamic acid insultinduced neuronal damage. Employing this model, we have monitored in the present study the level of activation or expression of several signal transducing proteins, assumed to be involved in the NMDA-activated protective mechanism, at various time points during the three successive periods of the model, preconditioning, insult, and reperfusion. The results demonstrated that the NMDA preconditioning-induced neuroprotective mechanism is associated with inactivation of p66ShcA, prevention of the insult-induced inactivation of Src, activation of AKT, inactivation followed by reactivation of FKHR-L1, and with increased expression of p52ShcA, EGFR, and MnSOD. The essential role of Src activity in the protective mechanism was further indicated by the demonstration that decreasing Src activation level by the Src inhibitor PP2 attenuated the NMDA preconditioning-induced protection. The alterations detailed above in the activation status or level of expression of the studied proteins are suggested to be part of the NMDA preconditioning-induced neuroprotective mechanism.