Formation of α-synuclein Lewy neurite-like aggregates in axons impedes the transport of distinct endosomes

Aggregates of alpha-synuclein (alpha-syn) accumulate in neurons in Parkinson's disease and other synucleinopathies. These inclusions predominantly localize to axons even in the early stages of the disease, but their affect on axon function has remained unknown. Previously we established a model in which the addition of preformed alpha-syn fibrils to primary neurons seeds formation of insoluble alpha-syn inclusions built from endogenously expressed alpha-syn that closely recapitulate the neuropathological phenotypes of Lewy neurites found in human diseased brains. Here we show, using live-cell imaging, that immobile alpha-syn inclusions accumulate in axons from the recruitment of alpha-syn located on mobile alpha-syn-positive vesicles. Ultrastructural analyses and live imaging demonstrate that alpha-syn accumulations do not cause a generalized defect in axonal transport; the inclusions do not fill the axonal cytoplasm, disrupt the microtubule cytoskeleton, or affect the transport of synaptophysin or mitochondria. However, the alpha-syn aggregates impair the transport of Rab7 and TrkB receptor-containing endosomes, as well as autophagosomes. In addition, the TrkB receptor-associated signaling molecule pERK5 accumulates in alpha-syn aggregate-bearing neurons. Thus alpha-syn pathology impairs axonal transport of signaling and degradative organelles. These early effects of alpha-syn accumulations may predict points of intervention in the neurodegenerative process.