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.