[Can prion-like propagation occur in neurodegenerative diseases?: in view of transmissible systemic amyloidosis].

Common neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD), are now considered as "protein misfolding diseases," because the misfolding of a small number of proteins is a key event in the pathogenesis and progression of these diseases. Proteins that are prone to misfolding and thereby associated with neurodegenerative diseases include amyloid β (AD), tau (AD and tauopathy), α-synuclein (PD, dementia with Lewy bodies, etc.), polyglutamine proteins (Huntington's disease, spinocerebellar ataxia, etc.), and superoxide dismutase 1 (amyotrophic lateral sclerosis). These proteins share certain essential properties with prions. Similar to abnormal prions, misfolded proteins function as a template to catalyze the misfolding of the native proteins and assemble into insoluble, β-sheet-rich, fibrillar aggregates termed as "amyloids." Furthermore, there is enough evidence supporting the intercellular transfer of misfolded protein aggregates. The transmission of these aggregates from one cell to another may be in accordance with the concept that neuropathological changes propagate along neuronal circuits in neurodegenerative diseases. Prion-like propagation mechanisms have been extensively analyzed in connection with systemic amyloidoses such as amyloid A (AA) amyloidosis and amyloid apolipoprotein AII (AApoAII) amyloidosis. Studies have shown that AA and AApoAII amyloidoses are transmitted from one organism to another through amyloid fibrils. However, studies have not yet proved that protein misfolding diseases, except for prion diseases, are infectious. Given the intercellular transfer of misfolded protein aggregates, we cannot ignore the possibility that disease-specific, misfolded proteins can be transmitted between individuals through surgical procedures or tissue transplantation. Importantly, cell non-autonomous mechanisms underlying the pathogenesis of neurodegenerative diseases may represent a more readily accessible target for novel disease-modifying therapies. In the present review, we discuss some aspects of the prion-like propagation of neurodegenerative diseases, taking into consideration the accumulated evidence supporting the transmissibility of systemic amyloidoses.