C-terminal {alpha}-synuclein truncations are linked to cysteine cathepsin activity in Parkinson`s disease [Molecular Bases of Disease] (Journal of Biological Chemistry)

A pathological feature of Parkinson`s disease (PD) is Lewy bodies (LBs) composed of ?-synuclein (?-syn) amyloid fibrils. ?-Syn is a 140 amino acids-long protein, but truncated ?-syn is enriched in LBs. The proteolytic processes that generate these truncations are not well-understood. On the basis of our previous work, we propose that these truncations could originate from lysosomal activity attributable to cysteine cathepsins (Cts). Here, using a transgenic SNCAA53T mouse model, overexpressing the PD-associated ?-syn variant A53T, we compared levels of ?-syn species in purified brain lysosomes from nonsymptomatic mice with those in age-matched symptomatic mice. In the symptomatic mice, antibody epitope mapping revealed enrichment of C-terminal truncations, resulting from CtsB, CtsL, and asparagine endopeptidase. We did not observe changes in individual cathepsin activities, suggesting that the increased levels of C-terminal ?-syn truncations are because of the burden of aggregated ?-syn. Using LC-MS and purified ?-syn, we identified C-terminal truncations corresponding to amino acids 1-122 and 1-90 from the SNCAA53T lysosomes. Feeding rat dopaminergic N27 cells with exogenous ?-syn fibrils confirmed that these fragments originate from incomplete fibril degradation in lysosomes. We mimicked these events in situ by asparagine endopeptidase degradation of ?-syn fibrils. Importantly, the resulting C-terminally truncated fibrils acted as superior seeds in stimulating ?-syn aggregation compared with that of the full-length fibrils. These results unequivocally show that C-terminal ?-syn truncations in LBs are linked to Cts activities, promote amyloid formation, and contribute to PD pathogenesis.