Glucocerebrosidase reduces the spread of protein aggregation in aDrosophila melanogastermodel of neurodegeneration by regulating proteins trafficked by extracellular vesicles
Authors: Jewett Kathryn A. , Thomas Ruth E. , Phan Chi Q. , Milstein Gillian , Yu Selina , Pallanck Leo J. , Davis Marie Y. 
Published: May 2020

Abnormal protein aggregation within neurons is a key pathologic feature of Parkinson’s disease (PD). The spread of protein aggregates in the brain is associated with clinical disease progression, but how this occurs remains unclear. Mutations in the geneglucosidase, beta acid 1(GBA), which encodes the lysosomal enzyme glucocerebrosidase (GCase), are the most penetrant common genetic risk factor for PD and dementia with Lewy bodies, and also associate with faster disease progression. To explore the mechanism by which mutations inGBAinfluence pathogenesis of these diseases, we previously created aDrosophilamodel ofGBAdeficiency (Gba1b) that manifests neurodegeneration, motor and cognitive deficits, and accelerated protein aggregation. Proteomic analysis ofGba1bmutants revealed dysregulation of proteins involved in extracellular vesicle (EV) biology, and we found altered protein composition of EVs fromGba1bmutants. To further investigate this novel mechanism, we hypothesized thatGBAmay influence the spread of pathogenic protein aggregates throughout the brain via EVs. We found that protein aggregation is reduced cell-autonomously and non-cell-autonomously by expressing wildtype GCase in specific tissues. In particular, accumulation of insoluble ubiquitinated proteins and Ref(2)P in the brains ofGba1bflies are reduced by ectopic expression of GCase in muscle tissue. Neuronal expression of GCase also cell-autonomously rescued protein aggregation in brain as well as non-cell-autonomously rescued protein aggregation in muscle. Muscle-specificGBAexpression rescued the elevated levels of EV-intrinsic proteins and Ref(2)P found in EVs fromGba1bflies. Genetically perturbing EV biogenesis in specific tissues in the absence of GCase revealed differential cell-autonomous effects on protein aggregation but could not replicate the non-cell-autonomous rescue observed with tissue-specificGBAexpression. Additionally, we identified ectopically expressed GCase within isolated EVs. Together, our findings suggest that GCase deficiency mediates accelerated spread of protein aggregates between cells and tissues via dysregulated EVs, and EV-mediated trafficking of GCase may partially account for the reduction in aggregate spread.

Author’s Summary

Parkinson’s disease (PD) is a common neurodegenerative disease characterized by abnormal clumps of proteins (aggregates) within the brain and other tissues which can lead to cellular dysfunction and death. Mutations in the geneGBA, which encodes glucocerebrosidase (GCase), are the strongest genetic risk factor for PD, and are associated with faster disease progression. GCase-deficient mutant flies display features suggestive of PD including increased protein aggregation in brain and muscle. We found that restoring GCase protein in the muscle of mutant flies reduced protein aggregation in muscle and the brain, suggesting a mechanism involving interaction between tissues. Previous work indicated thatGBAinfluences extracellular vesicles (EVs) – small membrane-bound structures released by cells to communicate and/or transport cargo from cell to cell. Here, we found increased aggregated proteins within EVs of mutant flies, which was reduced by restoring GCase in muscle. In addition, we found GCase within the EVs, possibly explaining how GCase in one tissue such as muscle could reduce protein aggregation in a distant tissue like the brain. Our findings suggest that GCase influences proteins within EVs, affecting the spread of protein aggregation. This may be important to understanding PD progression and could uncover new targets to slow neurodegeneration.