Are exosomes the vehicle for protein aggregate propagation in neurodegenerative diseases?

Abnormal protein aggregation has been implicated in neurodegenerative processes in human neurological disorders, such as Alzheimer’s disease and Parkinson’s disease. Recently, studies have established a novel concept that protein aggregates are transmitted among neuronal cells. By extension, such interneuronal aggregate transmission has been hypothesized to be the underlying mechanism for the pathological and clinical disease progression. However, the precise mechanism of the interneuronal aggregate transmission remains ill-defined. Recent reports have suggested that exosomes, a specific group of extracellular vesicles that are involved in intercellular transfer of cellular macromolecules such as proteins and RNAs, could play an important role in the aggregate transmission among neurons. Here, we review various types of extracellular vesicles and critically evaluate the evidence supporting the role of exosomes in interneuronal aggregate transmission and neurodegeneration. We also discuss the competing mechanisms other than the exosome-mediated transmission. By doing so, we aim to assess the current state of knowledge on the mechanism of interneuronal aggregate transmission and suggest the future directions of research towards understanding the mechanism.