Molecular mechanisms underlying specificity of excitotoxic signaling in neurons.

The central role of glutamate receptors in mediating excitotoxic neuronal death in stroke, epilepsy and trauma has been well established. Glutamate is the major excitatory amino acid transmitter within the CNS and it's signaling is mediated by a number of postsynaptic ionotropic and metabotropic receptors. Although calcium ions are considered key regulators of excitotoxicity, new evidence suggests that specific second messenger pathways rather than total Ca(2+) load, are responsible for mediating neuronal degeneration. Glutamate receptors are found localized at the synapse within electron dense structures known as the postsynaptic density (PSD). Localization at the PSD is mediated by binding of glutamate receptors to submembrane proteins such as actin and PDZ containing proteins. PDZ domains are conserved motifs that mediate protein-protein interactions and self-association. In addition to glutamate receptors PDZ-containing proteins bind a multitude of intracellular signal molecules including nitric oxide synthase. In this way PDZ proteins provide a mechanism for clustering glutamate receptors at the synapse together with their corresponding signal transduction proteins. PSD organization may thus facilitate the individual neurotoxic signal mechanisms downstream of receptors during glutamate overactivity. Evidence exists showing that inhibiting signals downstream of glutamate receptors, such as nitric oxide and PARP-1 can reduce excitotoxic insult. Furthermore we have shown that uncoupling the interaction between specific glutamate receptors from their PDZ proteins protects neurons against glutamate-mediated excitotoxicity. These findings have significant implications for the treatment of neurodegenerative diseases using therapeutics that specifically target intracellular protein-protein interactions.