Almost all cells in the human body are constantly degrading old, often damaged proteins, a process that prevents the formation of protein aggregates and is vital to keep the cell alive and functioning. In healthy cells, proteins are degraded by the so-called ubiquitin/proteasome pathway (UPP). This is also true for dopamine neurons, the cells that die in Parkinson's disease. However, a hallmark of Parkinson's Disease is protein inclusion bodies in surviving dopaminergic neurons. These inclusion bodies, which consist of various proteins, are called Lewy-bodies, after the neuropathologist, Friedreich Lewy, who discovered them almost 100 years ago. The formation of these inclusion bodies is attributed to disturbed protein degradation by the neurons. Recent studies found that the activity of the ubiquitin/proteasome pathway is decreased in the aging Central Nervous System (CNS), and possibly even more so in Parkinson's disease. Similarly, the discovery of mutations in the Parkin gene, which appears to be related to the ubiquitin/proteasome pathway, in some forms of inherited (familial) Parkinson's disease further supports a central role of the ubiquitin-proteasome system in Parkinson's disease. To model the effect of reduced proteasome activity on neuronal protein degradation and inclusion body formation, we created a transgenic mouse in which a key subunit of the enzyme complex that actually digests the proteins, the 20S proteasome, has been modified. This modification leads to altered (reduced) proteasome activity. Prior to the creation of the transgenic mouse, we verified that the inactive mutant subunit is incorporated into the 20S core of the 26S proteasome and reduced its activity by expressing it in a cell line in culture. We will use this novel in vivo model of partial proteasome dysfunction to ask how impaired proteasome function affects the survival of DA neurons. Our hypothesis is that the changes in the proteasome activity will lead to altered protein removal in the cells of these mice, making them more susceptible to formation of protein aggregates, especially during aging or after stressors such as the Parkinson's disease-inducing neurotoxin MPTP are applied. The planned experiments will use our transgenic mice to better understand the role of reduced proteasome activity in the neurodegenerative process underlying Parkinson's disease. These studies may also lay the groundwork for future experiments such as cross-breeding of other transgenic models of Parkinson's, and provide a model in which neuroprotective drugs that address protein aggregation can be tested.