ABSTRACT: alpha-synuclein (alpha-syn) is a main component of Lewy bodies (LB) that occur in many neurodegenerative diseases, including Parkinson's disease (PD), dementia with LB (DLB) and multi-system atrophy. alpha-syn mutations or amplifications are responsible for a subset of autosomal dominant familial PD cases, and overexpression causes neurodegeneration and motor disturbances in animals. To investigate mechanisms for alpha-syn accumulation and toxicity, we studied a mouse model of lysosomal enzyme cathepsin D (CD) deficiency, and found extensive accumulation of endogenous alpha-syn in neurons without overabundance of alpha-syn mRNA. In addition to impaired macroautophagy, CD deficiency reduced proteasome activity, suggesting an essential role for lysosomal CD function in regulating multiple proteolytic pathways that are important for alpha-syn metabolism. Conversely, CD overexpression reduces alpha-syn aggregation and is neuroprotective against alpha-syn overexpression-induced cell death in vitro. In a C. elegans model, CD deficiency exacerbates alpha-syn accumulation while its overexpression is protective against alpha-syn-induced dopaminergic neurodegeneration. Mutated CD with diminished enzymatic activity or overexpression of cathepsins B (CB) or L (CL) is not protective in the worm model, indicating a unique requirement for enzymatically active CD. Our data identify a conserved CD function in alpha-syn degradation and identify CD as a novel target for LB disease therapeutics.