A 42 Neurotoxicity Is Mediated by Ongoing Nucleated Polymerization Process Rather than by Discrete

The identification of toxic A species and/or the process of their formation is crucial for understanding the mechanism(s) of A neurotoxicity in Alzheimer disease and also for the development of effective diagnostic and therapeutic interventions. To elucidate the structural basis of A toxicity, we developed different procedures to isolate A species of defined size and morphology distribution, and we investigated their toxicity in different cell lines and primary neurons. We observed that crude A 42 preparations, containing a monomeric and heterogeneous mixture of A 42 oligomers, were more toxic than purified monomeric, protofibrillar fractions, or fibrils. The toxicity of protofibrils was directly linked to their interactions with monomeric A 42 and strongly dependent on their ability to convert into amyloid fibrils. Subfractionation of protofibrils diminished their fibrillization and toxicity, whereas reintroduction of monomeric A 42 into purified protofibril fractions restored amyloid formation and enhanced their toxicity. Selective removal of monomeric A 42 from these preparations, using insulin-degrading enzyme, reversed the toxicity of A 42 protofibrils. Together, our findings demonstrate that A 42 toxicity is not linked to specific prefibrillar aggregate(s) but rather to the ability of these species to grow and undergo fibril formation, which depends on the presence of monomeric A 42. These findings contribute significantly to the understanding of amyloid formation and toxicity in Alzheimer disease, provide novel insight into mechanisms of A protofibril toxicity, and important implications for designing anti-amyloid therapies.