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Abstract

The molecular architecture of alpha -Synuclein (alpha -Syn) inclusions, pathognomonic of various neurodegenerative disorders, remains unclear. alpha -Syn inclusions were long thought to consist mainly of alpha -Syn fibrils, but recent reports pointed to intracellular membranes as the major inclusion component. Here, we use cryo-electron tomography (cryo-ET) to image neuronal alpha -Syn inclusions in situ at molecular resolution. We show that inclusions seeded by alpha -Syn aggregates produced recombinantly or purified from patient brain consist of alpha -Syn fibrils crisscrossing a variety of cellular organelles. Using gold-labeled seeds, we find that aggregate seeding is predominantly mediated by small alpha -Syn fibrils, from which cytoplasmic fibrils grow unidirectionally. Detailed analysis of membrane interactions revealed that alpha -Syn fibrils do not contact membranes directly, and that alpha -Syn does not drive membrane clustering. Altogether, we conclusively demonstrate that neuronal alpha -Syn inclusions consist of alpha -Syn fibrils intermixed with membranous organelles, and illuminate the mechanism of aggregate seeding and cellular interaction. The molecular architecture of alpha -Synuclein (alpha -Syn) inclusions, pathognomonic of various neurodegenerative disorders, remains unclear. Here, authors use cryo-electron tomography to image neuronal alpha -Syn inclusions in situ and find that inclusions consist of alpha -Syn fibrils intermixed with cellular organelles without interacting directly.