C-terminus-dependent detection of lysosomal alpha-synuclein in nigral Parkinson's disease human brain neurons

The abnormal accumulation of alpha-Synuclein (αSyn) within neurons is a hallmark of synucleinopathies, such as Parkinson's disease (PD), and could stem from impaired protein degradation. Genetic, in vitro, and post-mortem studies have suggested that lysosomal dysfunction and impaired proteolytic activity play important roles in the pathogenesis of PD. Lysosomes have been proposed as key sites for αSyn degradation, but direct evidence of the lysosomal localization of endogenous αSyn in the human brain is limited. This study aimed to investigate the localization of αSyn proteoforms, including different post-translational modifications (PTMs), within lysosomes of post-mortem human nigral neurons. We analyzed formalin-fixed, paraffin-embedded brain tissue from donors diagnosed with PD, PD with Dementia (PDD) or incidental Lewy body disease (iLBD). Substantia nigra sections were assessed using an extensive panel of αSyn-specific antibodies, including PTM-specific antibodies, and selected lysosomal markers via multiplex immunofluorescence, confocal and stimulated emission depletion (STED) microscopy. Here, we demonstrate widespread accumulation of αSyn within lysosomes in nigral dopaminergic neuron somas of donors with PD/PDD and iLBD. This lysosomal αSyn appeared morphologically distinct from cytosolic inclusions such as Lewy bodies (LBs) and related macro-aggregates, and was present both in cells with and without these larger αSyn deposits. When present, macro-aggregates were consistently accompanied by ring-shaped lysosomal structures. Compared to other neuronal morphologies, lysosomal αSyn was the most frequent morphology at early Braak stages (1-4), with a decline at later stages (5-6). Interestingly, lysosomal αSyn was detected solely by targeting the N-terminus or the NAC domain of αSyn, and not with antibodies targeting Serine 129-phosphorylated αSyn or other epitopes at the C-terminus (CT), suggesting that lysosome-associated αSyn lacks the CT. Our findings reveal two co-existing pools of neuronal somatic αSyn: a CT-negative lysosome-associated form, and a primarily non-lysosomal CT-positive form. Overall, we provide direct evidence of lysosomal involvement in cellular αSyn metabolism in post-mortem human PD brain.