Monitoring α-synuclein ubiquitination dynamics reveals key endosomal effectors mediating its trafficking and degradation

While defective α-synuclein homeostasis is central to the pathogenesis of Parkinson’s, fundamental questions about its intracellular trafficking remain unresolved. We have developed a bimolecular fluorescence complementation assay in living cells to monitor de novo ubiquitination of α-synuclein. We identified lysine residues 45, 58 and 60 as critical ubiquitination sites for the degradation of α-synuclein. This is mediated by NBR1 binding to ubiquitinated α-synuclein and entry into endosomes in a process that involves ESCRT I-III complexes for subsequent lysosomal degradation. Autophagy or the autophagic chaperone Hsc70 are dispensable for this pathway. Antibodies against diglycine-modified α-synuclein peptides confirmed that endogenous α-synuclein is similarly ubiquitinated in brain and targeted to the lysosome in rat primary and human iPSC-derived neurons. Ubiquitinated α-synuclein was detected in Lewy bodies and cellular models of aggregation suggesting that it may be entrapped with endo/lysosomes in inclusion bodies. Collectively, our data elucidate the intracellular trafficking of de novo ubiquitinated α-synuclein and provide the tools for investigating the rapidly turned-over fraction of this disease-causing protein.