Signal Transduction Pathways of Neuronal Injury

It is now clear that modulation of particular signal transduction pathways in the CNS can profoundly influence the extent of neuronal degeneration following injury. For example, the therapeutic window for the treatment of cerebral ischemia is narrow, less than 10 hours, and requires rapid reversal of the toxic cellular events. Although the core of an ischemic insult suffers from loss of cerebral blood flow and metabolism, the penumbral zone, i.e., the region surrounding the ischemic core, is characterized by decreased blood flow and patchy areas of hypermetabolism. Pharmacological manipulation with agents such as glutamate receptor antagonists and imidazole receptor binding agents have been shown to reduce the extent of ischemia within the penumbral zone. In addition, muscarinic agonists have been demonstrated to possibly influence neuronal plasticity.

Protection during neuronal injury also is thought to be a result of preserving calcium homeostasis. Several studies implicate the importance of glutamate toxicity and preventing elevations in intracellular calcium. We have demonstrated that during anoxia, calcium, either from external or internal calcium stores, appears to be one of the initial mediators of neurotoxicity in the ischemic cascade. Yet, subsequent neuronal degeneration appears to be ultimately dependent on the nitric oxide (NO) pathway. Generation of NO, by a mechanism that may require intracellular cellular calcium release, can lead to the death of neurons.

Other work has focused on the modulation of several signal transduction pathways which include protein kinase C (PKC) and protein kinase A (PKA). During neurodegeneration, agents that decrease PKC activity can prevent neuronal damage. Inhibition of PKC activity also has been shown to protect neurons during periods of glutamate and kainate toxicity. In addition, we have demonstrated that modulation of PKC activity and PKA activity is protective against both anoxia and NO exposure in primary hippocampal neuronal cultures and that these signal transduction pathways mediate, at least in part, the protective effects of bFGF and EGF.

NO production also has been linked to the neuronal death that occurs with cerebral injury. NO, a free radical, has recently been shown to decrease neuronal survival. NO is generated from the enzyme nitric oxide synthase (NOS) and has a half-life of approximately 30 seconds. NOS oxidizes the substrate L-arginine to yield NO and citrulline. NO also modulates the short-term effects of excitatory amino acids on brain development, learning, and memory. During periods of ischemia, NMDA receptor activation leads to both calcium influx into cells and the production of NO and cGMP. NO is toxic to neurons under these conditions. Studies have documented reduction in glutamate neurotoxicity in cortical and hippocampal neurons by the addition of inhibitors of NO production. NO also has been implicated as a mediator of neurodegeneration during in vivo models of cerebral ischemia. In some experimental animal models, infarction following middle cerebral artery occlusion can be reduced by 72% following intraperitoneal administration of an inhibitor of NO synthesis and mutant mice deficient in neuronal NOS activity have been shown to experience reduced infarct volumes. In addition, we have shown that NO contributes to anoxic neuronal death and that peptide growth factors are protective against NO toxicity. NO toxicity is at least partially mediated through the modulation of PKC and PKA.