Tamas Bartfai, Ph. D.
Scripps Research Institute

Chronic Neuroprotection during Aging by UCP Mediated Simultaneous Reduction of Free Radical Formation and Exocitotoxicity.

The present proposal is aimed at testing the hypothesis that activation of brain specific Uncoupling Proteins (UCPs) could prove chronically neuroprotective by providing a combination of two protective measures a) reducing free radical formation, b) reducing the vulnerability of mitochondria to excitotoxic glutamate mediated overloading with Ca2.

Aging increases the vulnerability of the nervous system to damage by free radicals and by glutamate-mediated excitotoxicity following ischemia and reperfusion. Ischemia-reperfusion injuries, transient ischemic attacks and ischemic stroke are more common above 60 than with younger men and women.

The increased vulnerability of the nervous system is expressed by an increased number of neuronal cells undergoing free radical and/or glutamate caused necrotic and apoptotic cell death. The loss of those central nervous system neurons is manifested by neurological motor and cognitive deficits; which increase with aging.

Vulnerability of mitochondria during ischemia and its role in cell death are now being appreciated. The mitochondrial Uncoupling Proteins UCPs are integral membrane proteins which can dissipate the proton gradient across the mitochondrial innermemebrane and act to coordinate respiration and ATP synthesis and free radical formation and thus balance risks of oxygen toxicity and energy (ATP) need of the cell. UCP activity may play a key role in the mitochondrial stress response and defense and hence our focus on UCP activation.

UCP is upregulated in several neuronal and cellular toxicity models, and we believe that its upregulation, at least initially, is part of the stress response aimed at protecting the mitochondria and thereby the whole neuron from excitotoxicity. The roles of UCP in reducing free radical formation have been known. In vitro data on liver, heart and brain mitochondria clearly shows that uncoupling respiration from ATP synthesis by pharmacological means (DNP) or by activation of UCP reduces free radical formation at Complex I and III of the respiratory chain. Uncoupling also partially depolarizes the mitochondria so it is NOT able to take up as much Ca2+ as it would if its membrane potential were intact. Partially uncoupled mitochondria were shown to survive excitotoxic glutamate mediated Ca2+ influx without opening the mitochondrial transition pore and without committing the cell towards an apoptotic program via cytochrome C release.

The proposal is centered on achieving a proof principle for UCP activation as a acute and chronic neuroprotective measure in aging by using 3 transgenic mouse strains that overexpress in brain mitochondria UCP2: 1.) in all forebrain neurons under the PDGFb promoter, 2.) specifically in catecholaminergic neurons under tyrosine hydroxylase promoter and 3.) only in the hypothalamus under hypocretin promoter. We propose this transgenic approach since we do not have any CNS-specific and CNS penetrant activator of UCP to test this hypothesis pharmacologically today. By studying the neuronal damage, in UCP overexpressing and wild type mice, caused by ischemia (focal and global) and by MPTP, that via free radical mechanisms kills dopaminergic neurons, we will be able to conclude if increased UCP activity is neuroprotective in vivo and if the combination of protection from both free radical and excitotoxic is indeed producing robust synergistic neuroprotection as expected theoretically.

Contact Dr. Bartfai.