FYCO1 mediates clearance of α-synuclein aggregates through a Rab7-dependent mechanism

Parkinson's disease can be caused by mutations in the α‐synuclein gene and is characterized by aggregates of α‐synuclein protein. We have previously shown that over‐expression of the small GTPase Rab7 can induce clearance of α‐synuclein aggregates. In this study, we investigate which Rab7 effectors mediate this effect. To model Parkinson's disease, we expressed the pathogenic A53T mutant of α‐synuclein in HEK293T cells and Drosophila melanogaster. We tested the Rab7 effectors FYVE and coiled‐coil domain‐containing protein 1 (FYCO1) and Rab‐interacting lysosomal protein (RILP). FYCO1‐EGFP‐decorated vesicles containing α‐synuclein. RILP‐EGFP also decorated vesicular structures, but they did not contain α‐synuclein. FYCO1 over‐expression reduced the number of cells with α‐synuclein aggregates, defined as visible particles of EGFP‐tagged α‐synuclein, whereas RILP did not. FYCO1 but not RILP reduced the amount of α‐synuclein protein as assayed by western blot, increased the disappearance of α‐synuclein aggregates in time‐lapse microscopy and decreased α‐synuclein‐induced toxicity assayed by the Trypan blue assay. siRNA‐mediated knockdown of FYCO1 but not RILP reduced Rab7‐induced aggregate clearance. Collectively, these findings indicate that FYCO1 and not RILP mediates Rab7‐induced aggregate clearance. The effect of FYCO1 on aggregate clearance was blocked by dominant negative Rab7 indicating that FYCO1 requires active Rab7 to function. Electron microscopic analysis and insertion of lysosomal membranes into the plasma membrane indicate that FYCO1 could lead to secretion of α‐synuclein aggregates. Extracellular α‐synuclein as assayed by ELISA was, however, not increased with FYCO1. Coexpression of FYCO1 in the fly model decreased α‐synuclein aggregates as shown by the filter trap assay and rescued the locomotor deficit resulting from neuronal A53T‐α‐synuclein expression. This latter finding confirms that a pathway involving Rab7 and FYCO1 stimulates degradation of α‐synuclein and could be beneficial in patients with Parkinson's disease