Internalized α-synuclein fibrils become truncated and resist degradation in neurons while glial cells rapidly degrade α-synuclein fibrils.

Parkinson's disease (PD) and other α-synucleinopathies are characterized by the intracellular aggregates of α-synuclein (αS) believed to spread via the cell-to-cell transmission. To understand the contributions of various brain cells to the spreading of αS pathology, we examined the metabolism of αS aggregates in neuronal and glial cells. In neurons, while the full-length αS rapidly disappeared following αS PFF uptake, truncated αS accumulated with a half-life of days rather than hours. Epitope mapping and fractionation studies indicate that αS fibrils internalized by neurons was truncated at the C-terminal region and remained insoluble. In contrast, microglia and astrocytes rapidly metabolized αS fibrils as the half-lives of αS fibrils in these glial cells were <6 hours. Differential uptake and processing of αS fibrils by neurons and glia was recapitulated in vivo where injection of fluorescently labeled αS fibrils initially accumulated in glial cells followed by rapid clearance while neurons stably accumulated αS fibrils at slower rate. Immunolocalization and subcellular fractionation studies show that internalized αS PFF is initially localized to endosomes followed by lysosomes. The lysosome is largely responsible for the degradation of internalized αS PFF as the inhibition of lysosomal function leads to the stabilization of αS in all cell types. Significantly, αS PFF causes lysosomal dysfunction in neurons. In summary, we show that neurons are inefficient in metabolizing internalized αS aggregates, partially because αS aggregates cause lysosomal dysfunction, potentially generating aggregation-prone truncated αS. In contrast, glial cells may protect neurons from αS aggregates by rapidly clearing αS aggregates.