PBT2 inhibits glutamate-induced excitotoxicity in neurons through metal-mediated preconditioning

Excitotoxicity is the pathological process by which neuronal death occurs as a result of excessive stimulation of receptors at the excitatory synapse such as the NMDA receptor (NMDAR). Excitotoxicity has been implicated in the acute neurological damage from ischemia and traumatic brain injury and in the chronic neurodegeneration in Alzheimer's disease (AD) and Huntington's disease (HD). As a result NMDAR antagonists have become an attractive therapeutic strategy for the potential treatment of multiple neurodegenerative diseases. However NMDAR signaling is dichotomous in nature, with excessive increases in neuronal intracellular calcium through excessive NMDAR activity being lethal but moderate increases to intracellular calcium levels during normal synaptic function providing neuroprotection. Subsequently indiscriminant inhibition of this receptor is best avoided as was concluded from previous clinical trials of NMDAR antagonists. We show that the metal chaperone, PBT2, currently in clinical trials for HD, is able to protect against glutamate-induced excitotoxicity mediated through NMDARs. This was achieved by PBT2 inducing Zn(2+)-dependent increases in intracellular Ca(2+) levels resulting in preconditioning of neurons and inhibition of Ca(2+)-induced neurotoxic signaling cascade involving calpain-activated cleavage of calcineurin. Our study demonstrates that modulating intracellular Ca(2+) levels by a zinc ionophore is a valid therapeutic strategy to protect against the effects of excitotoxicity thought to underlie both acute and chronic neurodegenerative diseases.