Capturing the illusive ring-shaped intermediates in A42 amyloid formation.

Protein/peptide amyloid fibril formation is associated with various neurodegenerative diseases and, hence, has been the subject of extensive studies. From a structure-evolution point of view, we now know a great deal about the initial and final states of this process; however, we know very little about its intermediate states. Herein, we employ liquid-phase transmission electron microscopy to directly visualize the formation of one of the intermediates formed during the aggregation process of an amyloid-forming peptide. As shown in figure, we find that Aβ42, the amyloid formation of which has been linked to the development of Alzheimer's disease, can populate a ring-shaped intermediate structure with a diameter of tens of nanometers; additionally, the air-liquid interface can "catalyze" the formation of amyloid fibrils.