Cryo-EM visualization of partial β-sheet formation in curved transthyretin protofibrils resistant to amyloid fibril formation.

The formation of amyloid fibrils by transthyretin (TTR) is linked to various fatal systemic amyloidoses (ATTR). Pre-fibrillar species such as protofibrils are promising diagnostic and therapeutic targets for preventing the onset of ATTR in the preclinical stage. However, their structural properties remain poorly understood. Here, using cryo-electron microscopy, we characterized the structural features of TTR protofibrils that emerge via a unique in vitro pathway under acidic conditions. Two-dimensional class averages directly visualized two stripes of continuous density lines spaced 4.8 Å apart, along the protofibril axis, surrounded by fuzzy outer regions. This finding is suggestive of central β-sheet cores stabilized by intermolecular hydrogen bonds. The partial β-sheet formation may help resist conversion into amyloid fibrils under seeding and agitation, and may also account for the curved, flexible morphology of the protofibrils, which are distinct from the straight amyloid fibrils in which β-sheets span the entire structure. Our findings suggest structural features of protofibrils that are amyloid-incompetent, and may help conceptualize how to target their β-sheet features in diagnostic and therapeutic strategies during the early stages of aggregation.