Toxic Protein Spread in Neurodegeneration: Reality versus Fantasy

Over the past decade, the importance of the propagation of amyloidogenic proteins such as α-synuclein and tau in the pathogenesis of neurodegenerative diseases has been supported by numerous neuropathological and experimental studies. While these proteins behave similarly to prions, recent evidence suggests the existence of fundamental differences, as they can propagate in the absence of endogenous template, they do not exhibit a strict ‘strain' behavior, and they may not be transmissible between individuals. We therefore propose to name these proteins ‘prionoids'. In this review we critically assess the extent of the overlap between these two entities and evaluate how the propagation of prionoids can fit into the wider system dysfunction seen in the brains of patients with Alzheimer's and Parkinson's diseases.