Genetic Evidence for Adenylyl Cyclase 1 as a Target for Preventing Neuronal Excitotoxicity Mediated by N-Methyl-D-aspartate Receptors

Abstract

The excessive activation of N-methyl-D-aspartate (NMDA) receptors by glutamate results in neuronal excitotoxicity. cAMP is a key second messenger and contributes to NMDA receptordependent synaptic plasticity. Adenylyl cyclases 1 (AC1) and 8 (AC8) are the two major calcium-stimulated ACs in the central nervous system. Previous studies demonstrate AC1 and AC8 play important roles in synaptic plasticity, memory, and persistent pain. However, little is known about the possible roles of these two ACs in glutamate-induced neuronal excitotoxicity. Here, we report that genetic deletion of AC1 significantly attenuated neuronal death induced by glutamate in primary cultures of cortical neurons, whereas AC8 deletion did not produce a significant effect. AC1, but not AC8, contributes to intracellular cAMP production following NMDA receptor activation by glutamate in cultured cortical neurons. AC1 is involved in the dynamic modulation of cAMP-response element-binding protein activity in neuronal excitotoxicity. To explore the possible roles of AC1 in cell death in vivo, we studied neuronal excitotoxicity induced by an intracortical injection of NMDA. Cortical lesions induced by NMDA were significantly reduced in AC1 but not in AC8 knock-out mice. Our findings provide direct evidence that AC1 plays an important role in neuronal excitotoxicity and may serve as a therapeutic target for preventing excitotoxicity in stroke and neurodegenerative diseases.Neuronal death in stroke and other neurodegenerative diseases is mainly mediated by excitotoxicity. The central role of glutamate receptors in mediating excitotoxic neuronal death has been well established (1-5). Injury to the central nervous system triggers an abnormal release of glutamate and other excitatory amino acids that contributes significantly to the neurological outcome (5-8). The released glutamate causes an excessive activation of the NMDA 2 receptors, leading to calcium overload in neurons and subsequent neuronal death (1,3,6,9). The type of cell death in neuronal excitotoxicity seems to depend on the nature of the injury. Necrosis occurs after an acute insult, whereas apoptotic cell death is involved in propagation of the secondary injury (10 -12). Neuronal apoptosis induced by activation of NMDA receptors has been postulated to underlie the loss of neurons in many central nervous system disorders (13,14). Despite the initial promise of NMDA receptor antagonists and calcium channel blockers for the reduction of excitotoxic damage, clinical trials with these agents had to be abandoned because of toxic side-effects (15-17). The intervention of signaling events, which are downstream in excitotoxic cascade, might provide new strategies for the treatment of neuronal excitotoxicity. The cAMP signaling pathway has been shown to act downstream of NMDA receptor activation in synaptic plasticity (18 -22). The effect of the cAMP signaling pathway on cell survival depends on cell type and the specific type of cellular stress and may be influenced by cross-talk with other signal...

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