a-synuclein (a-syn) mutations or gene amplification cause a small subset of Parkinson's disease (PD), with Lewy body (LB) formation, neurodegeneration and often an associated dementia. a-syn aggregation in LB is also widespread in sporadic PD and other LB diseases without apparent upregulation of transcription. a-syn overexpression leads to its aggregation and/or neurotoxicity in various animal models. Reduction of a-syn is neuroprotective in animal models. These observations suggest a therapeutic potential of reducing a-syn in treatment of PD and other LB diseases. Our long-term objective is to determine the mechanisms and regulation of a-synucleinopathy in neurodegenerative diseases, and to provide novel and effective treatment strategies. We focused on the lysosomal function in modulating neuronal a-syn aggregation and toxicity, because lysosomes are high capacity organelles responsible for clearance of damaged and aggregated proteins, and are implicated in aging and several neurodegenerative diseases. We found that mice with deficient lysosomal cathepsin D (CD) exhibited significant a-syn accumulation in the brains, indicating a critical role for CD in mediating a-syn metabolism. In vitro we have shown that overexpression of CD reduces a-syn aggregation and protects against a-syn-mediated toxicity. To further establish CD as a therapeutic target against a-synucleinopathy, we will examine its effects in vivo, and define the mechanisms of its action in vitro. We hypothesize that CD protects against a-syn neurotoxicity by increasing autophagic clearance of toxic species of a-syn. We will test this hypothesis by performing experiments with the following aims: 1. Test the hypothesis that CD haploinsufficiency increases sensitivity to a-syn-induced neurotoxicity in vivo. 2. Test the hypothesis that stereotaxic delivery of AAV-CD to the SN attenuates a-syn-mediated neurotoxicity in vivo. 3. Test the hypothesis that neuroprotection by CD is through clearance of a-syn by autophagy. Completion of these studies will determine the effect of partial loss-of-CD in a-syn-induced DA neuron death, and the effect of gain-of-function of CD in neuroprotection in vivo. Furthermore, insights into the potential molecular mechanisms of CD-mediated neuroprotection and the molecular mechanisms and regulation of clearance of aggregation-prone proteins, will be gained that will permit further refinement of CD therapeutic strategies.