Neuronal activity-dependent protection against apoptotic and oxidative insults
Baxter, Paul Stuart
Patterns of physiological electrical activity in the central nervous system (CNS) cause longlasting changes in gene expression that promote neuronal survival. These changes can be mediated by signalling pathways activated by Ca2+ influx through synaptic N-methyl DAspartate receptors (NMDARs). Identification and study of these, and other neuroprotective signalling pathways of the CNS, is invaluable; as this may one day lead to therapeutic strategies against the deleterious effects of CNS injury or degeneration. The data presented in this thesis focuses on activity-dependent neuroprotection and how it interacts with other signalling pathways to protect against apoptotic and oxidative insults. A previously unobserved role of activity-dependent neuroprotection in mediating the effects of the neuropeptide PACAP is demonstrated. By promoting cAMP/PKA signalling PACAP triggers neuronal firing activity, which is essential for the neuroprotective effects mediated by PACAP. This firing activity cooperates with direct signalling by PKA in promoting longlasting CREB-mediated gene expression. The molecular events associated with PACAP mediated stimulation of CRE-dependent gene expression are presented. Investigation of the control of neuronal antioxidant defences by neuronal activity, both on its own and in cooperation with astrocyte-derived support, was also investigated. Neuronal activity is demonstrated to strongly increase the capacity of the antioxidant glutathione (GSH) system, through a program of coordinated transcriptional events. The utilisation, biosynthesis and recycling of GSH is enhanced in neurons, leading to increased resistance against oxidative insults. Since several GSH pathway enzyme genes are regulated by the transcription factor Nrf2, the ability of CDDO-F3, a small molecule activator of Nrf2, to mimic the effect of firing activity on neuronal GSH levels was examined. CDDO-F3 sustains neuronal GSH levels and confers neuroprotection against oxidative insult. These actions are dependent on the presence of astrocytes; whereas Nrf2 mediated regulation of GSH pathway genes is essentially inactive in neurons. Neuronal activity and activation of the astrocytic Nrf2 pathway can cooperate, maintaining neuronal GSH levels and protecting neurons against strong oxidative insults. Collectively this work expands our knowledge as to the molecular mechanisms of activity-dependent neuroprotection, and how such signals may synergise with other protective pathways to promote neuronal health.