Distinct Annular Oligomers Captured along the Assembly and Disassembly Pathways of Transthyretin Amyloid Protofibrils

Background: Defects in protein folding may lead to severe degenerative diseases characterized by the appearance of amyloid fibril deposits. Cytotoxicity in amyloidoses has been linked to poration of the cell membrane that may involve interactions with amyloid intermediates of annular shape. Although annular oligomers have been detected in many amyloidogenic systems, their universality, function and molecular mechanisms of appearance are debated. Methodology/Principal Findings: We investigated with high-resolution in situ atomic force microscopy the assembly and disassembly of transthyretin (TTR) amyloid protofibrils formed of the native protein by pH shift. Annular oligomers were the first morphologically distinct intermediates observed in the TTR aggregation pathway. Morphological analysis suggests that they can assemble into a double-stack of octameric rings with a 16 +/- 2 nm diameter, and displaying the tendency to form linear structures. According to light scattering data coupled to AFM imaging, annular oligomers appeared to undergo a collapse type of structural transition into spheroid oligomers containing 8-16 monomers. Disassembly of TTR amyloid protofibrils also resulted in the rapid appearance of annular oligomers but with a morphology quite distinct from that observed in the assembly pathway. Conclusions/Significance: Our observations indicate that annular oligomers are key dynamic intermediates not only in the assembly but also in the disassembly of TTR protofibrils. The balance between annular and more compact forms of aggregation could be relevant for cytotoxicity in amyloidogenic disorders.