Autophagy modulates SNCA/α-synuclein release, thereby generating a hostile microenvironment

Abstract

SNCA/α-synuclein aggregation plays a crucial role in synucleinopathies such as Parkinson disease and dementia with Lewy bodies. Aggregating and nonaggregating SNCA species are degraded by the autophagy-lysosomal pathway (ALP). Previously, we have shown that the ALP is not only responsible for SNCA degradation but is also involved in the intracellular aggregation process of SNCA. An additional role of extracellular SNCA in the pathology of synucleinopathies substantiating a prion-like propagation hypothesis has been suggested since released SNCA species and spreading of SNCA pathology throughout neural cells have been observed. However, the molecular interplay between intracellular pathways, SNCA aggregation, release, and response of the local microenvironment remains unknown. Here, we attributed SNCA-induced toxicity mainly to secreted species in a cell culture model of SNCA aggregation and in SNCA transgenic mice: We showed that ALP inhibition by bafilomycinA1 reduced intracellular SNCA aggregation but increased secretion of smaller oligomers that exacerbated microenvironmental response including uptake, inflammation, and cellular damage. Low-aggregated SNCA was predominantly released by exosomes and RAB11A-associated pathways whereas high-aggregated SNCA was secreted by membrane shedding. In summary, our study revealed a novel role of the ALP by linking protein degradation to nonclassical secretion for toxic SNCA species. Thus, impaired ALP in the diseased brain not only limits intracellular degradation of misfolded proteins, but also leads to a detrimental microenvironmental response due to enhanced SNCA secretion. These findings suggest that the major toxic role of SNCA is related to its extracellular species and further supports a protective role of intracellular SNCA aggregation.

Hallmarks of Brain Aging: Adaptive and Pathological Modification by Metabolic States

“…One potential mechanism involves extracellular vesicles (EVs). Lysosome dysfunction and oxidative stress can trigger the release of EVs containing aggregating α-synuclein, and those EVs can then be internalized and trigger the accumulation of toxic α-synuclein aggregates in previously healthy neurons (Emmanouilidou et al, 2010; Poehler et al, 2014; Zhang et al, 2018). Another potential mechanism for spreading of α-synuclein pathology is suggested by a study showing that misfolded preformed fibrils bind to LAG3 (lymphocyte-activation gene 3) and that LAG3 initiates α-synuclein preformed fibril endocytosis, transmission, and cytotoxicity (Mao et al, 2016).…”

Exosomes in the Pathology of Neurodegenerative Diseases

“…The GTPase Rab11, which is required for late endosomal vesicle formation, has been found to be important in this process. Overexpression of Rab11 has been shown to reduce aggregate formation in cells (48) while also increasing the release of exosomal ␣-synuclein (47).…”

Emerging nexus between RAB GTPases, autophagy and neurodegeneration

“…27 These examples do not come as a surprise, as overexpression of RAB proteins alleviates neurodegeneration in Table 1. List of neurodegenerative disorders with abnormal activities for RAB GTPase.…”