Demographic considerations predict an overwhelming burden on the U.S. health care system within the next few decades due to an influx of elderly patients with neurodegenerative disease. The most prevalent of these, Alzheimer's and Parkinson's diseases (AD and PD), are predicted to affect tens of millions of Americans by 2040. Both of these diseases involve progressively more aberrant brain proteostasis, associated with massive neuronal cell death. A variety of cytosolic and secreted brain chaperones contribute to maintenance of neuronal proteostasis in these and other neurodegenerative proteinopathies; of these, the secretory chaperone proSAAS has many compelling features. ProSAAS is expressed only in neurons and endocrine cells; because it traffics through the regulated secretory pathway, it becomes concentrated within dense core synaptic granules, and is released during neuronal activity. ProSAAS has been identified by five proteomics groups as a potential biomarker in neurodegenerative disease, and is found associated with aggregated proteins in the substantia nigra of PD patients as well as with amyloid plaques in AD‐affected individuals. ProSAAS blocks the aggregation of both Abeta and alpha synuclein at highly substoichiometric ratios, and both endogenous overexpression as well as exogenous application reduce Abeta‐ and alpha synuclein‐mediated neurotoxicity in primary neurons and cell lines. Most recently, we have shown that proSAAS overexpression is also functionally protective in vivo in a rat model of alpha‐synuclein overexpression. In the proposed work, we will investigate the likely common mechanisms by which proSAAS protects neurons from neurotoxic aggregating proteins and peptides such as alpha synuclein and Abeta 1‐42. We hypothesize that secreted proSAAS sequesters cytotoxic oligomers and fibrils extracellularly, reducing their concentrations at the synapse. Secondly, we hypothesize that endocytosed proSAAS acts intracellularly to similarly sequester cytotoxic proteins, speeding their degradation. Using cultured primary hippocampal and nigral neurons, we will determine whether proSAAS is involved in intracellular and extracellular Abeta and alpha synuclein sequestration. We will also determine whether intracellular expression of proSAAS confers a cytoprotective advantage compared to extracellular addition. Lastly we will assess whether endocytosed proSAAS accelerates the intracellular degradation of Abeta and alpha synuclein. In parallel, we will expand our exciting in vivo results to include the alpha‐synuclein preformed fibril model to tease apart the potential sites of action of proSAAS in substantia nigra and striatum. Pre‐degenerative changes in dopamine homeostasis, assessed using fast‐scan‐cyclic‐voltammetry, will be correlated with proSAAS‐mediated neuroprotection. Similarly, a mouse model of AD will be used to test the effects of proSAAS AAV‐mediated over‐ and underexpression on the development of amyloid pathology. Col...