Misfolding and aggregation of the proteins amyloid-β (Aβ), tau and alpha-synuclein (α-syn) is the predominant pathology underlying the neurodegenerative disorders, Alzheimer's and Parkinson's disease. Whilst end stage insoluble products of aggregation have been well characterised in human and animal models of disease, accumulating evidence from biophysical, cellular and in vivo studies has shown that soluble intermediates of aggregation, or oligomers, may be the key species that mediate toxicity and underlie seeding and spreading in disease. Here we review the process of protein misfolding, and the intrinsic and extrinsic processes that cause the native states of the key aggregating proteins to undergo conformational change to form oligomers and ultimately fibrils. We discuss the structural features of the key toxic intermediate, and describe the putative mechanisms by which oligomers may cause cell toxicity. Finally we explore the potential therapeutic approaches raised by the oligomer hypothesis in neurodegenerative disease. This article is protected by copyright. All rights reserved
