Aβ42 Neurotoxicity Is Mediated by Ongoing Nucleated Polymerization Process Rather than by Discrete Aβ42 Species

The identification of toxic A beta species and/or the process of their formation is crucial for understanding the mechanism(s) of A beta neurotoxicity in Alzheimer disease and also for the development of effective diagnostic and therapeutic interventions. To elucidate the structural basis of A beta toxicity, we developed different procedures to isolate A beta species of defined size and morphology distribution, and we investigated their toxicity in different cell lines and primary neurons. We observed that crude A beta 42 preparations, containing a monomeric and heterogeneous mixture of A beta 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 beta 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 beta 42 into purified protofibril fractions restored amyloid formation and enhanced their toxicity. Selective removal of monomeric A beta 42 from these preparations, using insulin-degrading enzyme, reversed the toxicity of A beta 42 protofibrils. Together, our findings demonstrate that A beta 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 beta 42. These findings contribute significantly to the understanding of amyloid formation and toxicity in Alzheimer disease, provide novel insight into mechanisms of A beta protofibril toxicity, and important implications for designing anti-amyloid therapies.