Mechanisms of Alpha-Synuclein-Seeded Aggregation in Neurons Revealed by Fluorescence Lifetime Imaging.

The brains of Parkinson's disease (PD) patients are characterized by the presence of Lewy body inclusions enriched with fibrillar forms of the presynaptic protein alpha-synuclein (aSyn). Despite related evidence that Lewy pathology spreads across different brain regions as the disease progresses, the underlying mechanism, and hence the fundamental cause of PD progression, is unknown. The propagation of aSyn pathology is thought to potentially occur through the release of aSyn aggregates from diseased neurons, their uptake by neighboring healthy neurons via endocytosis, and subsequent seeding of native aSyn aggregation in the cytosol. A critical aspect of this process is believed to involve the escape of internalized aggregates from the endolysosomal compartment, though direct evidence of this mechanism in cultured neuron models remains lacking. In this study, we utilize a custom-built, time-gated fluorescence lifetime imaging microscopy (FLIM) system to investigate the progression of seeded aggregation over time in live cortical neurons. By establishing fluorescence lifetime sensitivity to aSyn aggregation levels, we are able to monitor the protein's aggregation state. Through a FLIM analysis of neurons expressing aSyn-mVenus and exposed to aSyn preformed fibrils labeled with the acid-responsive dye pHrodo, we reveal the protein's aggregation state in both the cytosol and the endolysosomal compartment. The results indicate that aSyn seeds undergo partial disassembly prior to escaping the endocytic pathway and that this escape is closely linked to the aggregation of cytosolic aSyn. In certain neurons, monomeric aSyn is found to translocate from the cytosol into the endolysosomal compartment, where it apparently forms aggregates in proximity to retained seeds. Additional analyses reveal zones of neuritic aSyn aggregates that overlap with regions of microtubule disruption. Collectively, these findings enhance our understanding of aSyn pathology propagation in PD and other synucleinopathies, motivate additional experiments along these lines, and offer a path to guide the development of disease-modifying therapies.