Fig. 2.Transformants releasing E␤C suffered less damage than control lines when EPNs were present. (A) Root damage measured on plants that had received neither WCR eggs nor nematodes was minimal, and there was no difference between transformed and nontransformed plants (n ϭ 5, P ϭ 0.87). (B) Root damage on plants that received only WCR eggs, but no nematodes, was substantial. Again, no significant difference was found between the transformed and nontransformed plants (n ϭ 5, P ϭ 0.18). (C) In plots that received WCR eggs and H. megidis, roots from transformed plants (pooled) had significantly less damage than roots from control lines (n ϭ 30, P ϭ 0.007). Approximately one-quarter of the transformed plants were found not to emit E␤C. Removing these plants from the statistical analysis did not significantly affect the results. The letters above the bars indicate significant differences within a graph. Error bars indicate standard errors.

Abnormal neuronal aggregation of ␣-synuclein is implicated in the development of many neurological disorders, including Parkinson disease and dementia with Lewy bodies. Glial cells also show extensive ␣-synuclein pathology and may contribute to disease progression. However, the mechanism that produces the glial ␣-synuclein pathology and the interaction between neurons and glia in the disease-inflicted microenvironment remain unknown. Here, we show that ␣-synuclein proteins released from neuronal cells are taken up by astrocytes through endocytosis and form inclusion bodies. The glial accumulation of ␣-synuclein through the transmission of the neuronal protein was also demonstrated in a transgenic mouse model expressing human ␣-synuclein. Furthermore, astrocytes that were exposed to neuronal ␣-synuclein underwent changes in the gene expression profile reflecting an inflammatory response. Induction of pro-inflammatory cytokines and chemokines correlated with the extent of glial accumulation of ␣-synuclein. Together, these results suggest that astroglial ␣-synuclein pathology is produced by direct transmission of neuronal ␣-synuclein aggregates, causing inflammatory responses. This transmission step is thus an important mediator of pathogenic glial responses and could qualify as a new therapeutic target.

␣-Synuclein (␣-syn), particularly in its aggregated forms, is implicated in the pathogenesis of Parkinson's disease and other related neurological disorders. However, the normal biology of ␣-syn and how it relates to the aggregation of the protein are not clearly understood. Because of the lack of the signal sequence and its predominant localization in the cytosol, ␣-syn is generally considered exclusively an intracellular protein. Contrary to this assumption, here, we show that a small percentage of newly synthesized ␣-syn is rapidly secreted from cells via unconventional, endoplasmic reticulum/Golgi-independent exocytosis. Consistent with this finding, we also demonstrate that a portion of cellular ␣-syn is present in the lumen of vesicles. Importantly, the intravesicular ␣-syn is more prone to aggregation than the cytosolic protein, and aggregated forms of ␣-syn are also secreted from cells. Furthermore, secretion of both monomeric and aggregated ␣-syn is elevated in response to proteasomal and mitochondrial dysfunction, cellular defects that are associated with Parkinson's pathogenesis. Thus, intravesicular localization and secretion are part of normal life cycle of ␣-syn and might also contribute to pathological function of this protein.

Abnormal aggregation of α-synuclein and sustained microglial activation are important contributors to the pathogenic processes in Parkinson's disease. However, the relationship between disease-associated protein aggregation and microglia-mediated neuroinflammation remains unknown. Here, using a combination of in silico, in vitro, and in vivo approaches, we show that extracellular α-synuclein released from neuronal cells is an endogenous agonist for toll-like receptor 2 (TLR2), which activates inflammatory responses in microglia. TLR2 ligand activity of α-synuclein is conformation-sensitive; only specific types of oligomer can interact with and activate TLR2. This paracrine interaction between neuron-released oligomeric α-synuclein and TLR2 in microglia suggests that both of these proteins are novel therapeutic targets for modification of neuroinflammation in Parkinson's disease and related neurological diseases.