"The motor symptoms of Parkinson disease are primarily caused by the progressive degeneration of nigrostriatal dopamine neurons of the substantia nigra pars compacta. While these neurons are susceptible, tuberoinfundibular DA neurons are not affected. Exposure to the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahyropyridine (MPTP) results in a similar pattern of DA neuronal susceptibility. The recovery of tuberoinfundibular DA neurons is dependent upon the increased expression of parkin following MPTP exposure. Parkin is an E3 ligase that contains a ubiquitin like domain that can bind to the 26S proteasome and directly enhance its activity. The purpose of this dissertation was to test the hypothesis that increased parkin expression is both necessary and sufficient to maintain proteasome activity following oxidative insult by MPTP. MPTP caused similar oxidative damage to proteins in regions containing axons terminals of nigrostiatal (striatum; ST) and tuberoinfundibular (median eminence; ME) DA neurons, however proteasome activity followed the differential pattern of parkin expression and was decreased in the ST and maintained in the ME. The ST of parkin knockout mice had decreased proteasome activity, accumulation of ubiquitinated TH, and increased oxidatively modified proteins, while these were not affected in the ME. Proteasome activity in the ME of parkin deficient mice was decreased following MPTP but oxidatively modified proteins were not changed in either the ME or ST. To determine if parkin up-regulation could rescue proteasome activity following exposure to MPTP, rAAV expressing Flag-tagged human parkin was injected into the substantia nigra containing cell bodies of nigrostriatal DA neurons and proteasome activity was measured 24 h post MPTP. Parkin overexpression in the substantia nigra increased basal proteasome activity and prevented MPTP-induced loss of proteasome activity in the ST. rAAV-mediated parkin expression resulted in activation of 26S proteasome activity that was due to an increase in the amount of assembled 26S proteasome. Furthermore, increased parkin expression resulted in the rescue of proteasome mediated turnover of ubiquitinated TH. Although parkin over-expression was not sufficient to rescue NSDA axon terminal DA stores following MPTP exposure, DA turnover was decreased. The data presented here reveal that parkin is necessary and sufficient for the maintenance of 26S proteasome activity in dopaminergic axon terminal regions following acute MPTP exposure. Over-expression of parkin may be beneficial in protecting nigrostriatal DA neurons from cytosolic DA related oxidative damage. In addition, the discovery of small molecules that positively modulate the 26S proteasome may have potential therapeutic applications in the treatment of Parkinson disease."--Pages ii-iii.
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