C-terminal -synuclein truncations are linked to cysteine cathepsin activity in Parkinson's disease

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 SNCA(A53T) 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 SNCA(A53T) 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.