Editors' ChoiceNeurodegeneration

Multiple paths spread toxic α-synuclein aggregates

Sci. Signal.  11 Oct 2016:
Vol. 9, Issue 449, pp. ec234
DOI: 10.1126/scisignal.aal1604

Monomeric, soluble α-synuclein is normally present in neurons but misfolding can cause α-synuclein to oligomerize and form fibrils. These α-synuclein fibrils are a major component of Lewy bodies, which are intracellular protein aggregates associated with neurodegenerative conditions, such as Parkinson’s disease and Lewy body dementia. Pathogenic, oligomeric α-synuclein spreads from neuron to neuron and can seed the formation of protein aggregates in naïve cells. Mao et al. report that the neuronal transmembrane protein LAG3 (lymphocyte-activation gene 3) bound to in vitro-synthesized α-synuclein fibrils, but not monomeric α-synuclein, and was required for endocytosis of fibrillar α-synuclein into mouse cortical neurons and cultured human neuroblastoma cells. Endocytosis of fibrillar α-synuclein, cell-to-cell transmission of fibrillar α-synuclein, and cell death were all reduced in cortical neurons isolated from LAG3–/– mice compared with these processes in neurons isolated from wild-type mice. Experiments in which fibrillar α-synuclein was injected into the brains of LAG3–/– and wild-type mice demonstrated that loss of LAG3 inhibited the spread of fibrillar α-synuclein to neurons distant from the injection site and reduced both neuronal death and motor defects. Antibodies that prevented α-synuclein binding to LAG3 reduced the toxicity and cell-to-cell transmission of fibrillar α-synuclein between cultured human neuroblastoma cells, suggesting LAG3 as a potential therapeutic target for slowing the progression of synucleinopathies in human patients (see Jucker and Heikenwalder).

Abounit et al. found that, in addition to entering cells through endocytosis, fibrillar α-synuclein can be transferred between cells through nanotubes. Cultured mouse catecholaminergic tumor cells took up in vitro-synthesized fibrillar α-synuclein that was labelled with the fluorescent dye Alexa 488. When cells loaded with fluorescent fibrillar α-synuclein (donor cells) were cocultured with cells that had not been exposed to the dye-labeled protein (acceptor cells), fibrillar α-synuclein accumulated in the acceptor cells. Furthermore, when the acceptor cells expressed a form of soluble α-synuclein fused to the fluorescent protein Cherry (Ch), the acceptor cells accumulated aggregates that contained both the Alexa 488 and Ch fluorophores, indicating that fibrillar collagen transferred from one cell could recruit monomeric α-synuclein into aggregates inside the acceptor cells. Uptake of extracellular fibrillar α-synuclein by the donor cells was blocked by expression of a dominant-negative form of dynamin-1. In contrast, the transfer of fibrillar α-synuclein from one cell to another was unaffected, suggesting that donor uptake required endocytosis but transfer between the donor and acceptor involved a different process. In cells that had both taken up fibrillar α-synuclein by endocytosis and received fibrillar α-synuclein by cell transfer, the two populations of α-synuclein fibrils did not colocalize, implying differential intracellular trafficking of these two populations of fibrillar α-synuclein. Indeed, fibrillar α-synuclein was transferred from donor to acceptor cells through nanotubes. Fluorescent fibrillar α-synuclein was present in nanotubes emanating from donor cells; culturing cells at very low density inhibited the transfer of fibrillar α-synuclein from cell to cell; and culturing cells in conditions that favored or inhibited the formation of nanotubes increased or decreased, respectively, the transfer of α-synuclein fibrils from donor cells to acceptor cells. The α-synuclein fibrils that were transferred between cells through nanotubes colocalized with lysosomal markers. This nanotube-mediated transfer appeared likely to occur with primary mouse cortical neurons, which exhibited localization of transferred fibrillar α-synuclein in lysosomes and a dependence on cell proximity. Nanotubes between primary neurons were not detectable, possibly for technical reasons, but cultured neuronal tumor cells transferred fibrillar α-synuclein to cortical neurons through nanotubes. These two studies demonstrate that there may be multiple modes of cell-to-cell transmission of pathogenic α-synuclein between neurons. Thus, halting the progression of synucleinopathies may require combinatorial approaches.

X. Mao, M. T. Ou, S. S. Karuppagounder, T.-I. Kam, X. Yin, Y. Xiong, P. Ge, G. E. Umanah, S. Brahmachari, J.-H. Shin, H. C. Kang, J. Zhang, J. Xu, R. Chen, H. Park, S. A. Andrabi, S. U. Kang, R. A. Gonçalves, Y. Liang, S. Zhang, C. Qi, S. Lam, J. A. Keiler, J. Tyson, D. Kim, N. Panicker, S. P. Yun, C. J. Workman, D. A. A. Vignali, V. L. Dawson, H. S. Ko, T. M. Dawson, Pathological α-synuclein transmission initiated by binding lymphocyte-activation gene 3. Science 353, aah3374 (2016). [Abstract]

M. Jucker, M. Heikenwalder, Immune receptor for pathogenic α-synuclein. Science 353, 1498–1499 (2016). [Summary]

S. Abounit, L. Bousset, F. Loria, S. Zhu, F. de Chaumont, L. Pieri, J.-C. Olivo-Marin, R. Melki, C. Zurzolo, Tunneling nanotubes spread fibrillar α-synuclein by intercellular trafficking of lysosomes. EMBO J. 35, 2120–2138 (2016). [PubMed]