Abstract Lewy body dementias are pathologically defined by the deposition of α-synuclein fibrils into inclusions throughout the brain. Cerebrospinal fluid(CSF) in disease harbors circulating α-synuclein-fibril seeds, and parental α-synuclein fibrils can template core structure into amplified fibrils. Using cryo-electron microscopy, we identify six novel α-synuclein fibril assemblies amplified from ten CSF samples (3.8Å to 2.9Å nominal resolutions). Fibrils are classified based on two types of filament interaction, two types of β-sheet stacking, and two types of hydrophobic pocket. CSF-amplified fibril products have one, two, or three distinct assemblies each. Six of ten samples share a common fibril assembly. Within this classification, the fibrils have distinct profiles in amyloid dye binding, and dramatically different potencies in both seeding new inclusions in neurons and evoked microglial pro-inflammatory responses. However, no single structural feature predicts functional phenotypes. Our results highlight CSF as a valuable resource to identify novel α-synuclein assemblies potentially important in disease.

Structural and functional landscape of α-synuclein fibril conformations amplified from cerebrospinal fluid