Non‐classical exocytosis of α‐synuclein is sensitive to folding states and promoted under stress conditions

Abstract

J. Neurochem. (2010) 113, 1263–1274. Abstract Parkinson’s disease is characterized by deposition of misfolded/aggregated α‐synuclein proteins in multiple regions of the brain. Neurons can release α‐synuclein; through this release, pathological forms of α‐synuclein are propagated between neurons, and also cause neuroinflammation. In this study, we demonstrate that release of α‐synuclein is consistently increased under various protein misfolding stress conditions in both neuroblastoma and primary neuron models. This release is mediated by a non‐classical, endoplasmic reticulum (ER)/Golgi‐independent exocytosis, and stress‐induced release coincides with increased translocation of α‐synuclein into vesicles. Both vesicle translocation and secretion were blocked by attachment of a highly stable, globular protein to α‐synuclein, whereas forced protein misfolding resulted in an increase in both of these activities. Mass spectrometry analysis showed a higher degree of oxidative modification in secreted α‐synuclein than in the cellular protein. Together, these results suggest that structurally abnormal, damaged α‐synuclein proteins translocate preferentially into vesicles and are released from neuronal cells via exocytosis.

A deadly spread: cellular mechanisms of α-synuclein transfer

“…10,15 The intraluminal vesicular localization of a-syn further supports this role for exocytosis, although the classical endoplasmic reticulum/Golgi-dependent pathway is likely not involved. 10,15 Cellular stressors, such as serum deprivation, proteosomal or lysosomal inhibition, and hydrogen peroxide promote Figure 2a). Finally, induction of misfolding also increases association of a-syn with both vesicles and secretion, whereas medium-derived a-syn shows more oxidative modifications than the cellular forms, suggesting that neurons preferentially release damaged and aggregationprone a-syn proteins.…”

“…Monomers and aggregated forms of a-syn are secreted from neuroblastoma cells overexpressing a-syn and from rat primary cortical neurons. 10,15 This secretion is inhibited by low temperature, suggesting an exocytotic process. 10,15 The intraluminal vesicular localization of a-syn further supports this role for exocytosis, although the classical endoplasmic reticulum/Golgi-dependent pathway is likely not involved.…”

“…10,15 This secretion is inhibited by low temperature, suggesting an exocytotic process. 10,15 The intraluminal vesicular localization of a-syn further supports this role for exocytosis, although the classical endoplasmic reticulum/Golgi-dependent pathway is likely not involved. 10,15 Cellular stressors, such as serum deprivation, proteosomal or lysosomal inhibition, and hydrogen peroxide promote Figure 2a).…”

“…[6][7][8] Furthermore, overexpressed and/or misfolded a-syn is pathogenic to cells while a-syn can be secreted from cells, enter other cells, and seed small intracellular aggregates, demonstrating a connection between a-syn and pathogenic mechanisms of PD. [9][10][11][12][13][14][15][16] In parallel, a much-discussed hypothesis by Braak and colleagues 17 states that a pathogenic agent, introduced via ingestion and/or inhalation, may transfer from the entry site along known long, unmyelinated axons to basal brain areas and eventually to brain stem and cortical regions. Although a-syn is unlikely to be this initial pathogen, it might be the initial target of the unknown agent.…”

Glucocerebrosidase, a new player changing the old rules in Lewy body diseases

“…Exocytosis of these vesicles would generate vesicle-free extracellular α -synuclein. Previous studies have shown that α -synuclein is secreted via unconventional exocytosis (Lee et al , 2005 ;Jang et al , 2010 ), and exosome-associated release has also been suggested (Emmanouilidou et al , 2010 ;Alvarez -Erviti et al, 2011 ;Danzer et al , 2012 ). In recipient cells, internalization of extracellular α -synuclein aggregates may occur through endocytosis of the free protein or through direct fusion of exosomes to the plasma membrane.…”