Post-mortem analyses of brains from patients with Parkinson disease who received fetal mesencephalic transplants show that α-synuclein-containing (α-syn-containing) Lewy bodies gradually appear in grafted neurons. Here, we explored whether intercellular transfer of α-syn from host to graft, followed by seeding of α-syn aggregation in recipient neurons, can contribute to this phenomenon. We assessed α-syn cell-to-cell transfer using microscopy, flow cytometry, and high-content screening in several coculture model systems. Coculturing cells engineered to express either GFP-or DsRed-tagged α-syn resulted in a gradual increase in double-labeled cells. Importantly, α-syn-GFP derived from 1 neuroblastoma cell line localized to red fluorescent aggregates in other cells expressing DsRed-α-syn, suggesting a seeding effect of transmitted α-syn. Extracellular α-syn was taken up by cells through endocytosis and interacted with intracellular α-syn. Next, following intracortical injection of recombinant α-syn in rats, we found neuronal uptake was attenuated by coinjection of an endocytosis inhibitor. Finally, we demonstrated in vivo transfer of α-syn between host cells and grafted dopaminergic neurons in mice overexpressing human α-syn. In summary, intercellularly transferred α-syn interacts with cytoplasmic α-syn and can propagate α-syn pathology. These results suggest that α-syn propagation is a key element in the progression of Parkinson disease pathology.

Rey et al. generate a mouse model for the spatial propagation of α-synuclein pathology that mimics prodromal Parkinson's disease.

Parkinson’s disease is characterized by degeneration of substantia nigra dopamine neurons and by intraneuronal aggregates, primarily composed of misfolded α-synuclein. The α-synuclein aggregates in Parkinson’s patients are suggested to first appear in the olfactory bulb and enteric nerves and then propagate, following a stereotypic pattern, via neural pathways to numerous regions across the brain. We recently demonstrated that after injection of either mouse or human α-synuclein fibrils into the olfactory bulb of wild-type mice, α-synuclein fibrils recruited endogenous α-synuclein into pathological aggregates that spread transneuronally to over 40 other brain regions and subregions, over 12 months. We previously reported the progressive spreading of α-synuclein aggregates, between 1 and 12 months following α-synuclein fibril injections, and now report how far the pathology has spread 18- and 23-month post-injection in this model. Our data show that between 12 and 18 months, there is a further increase in the number of brain regions exhibiting pathology after human, and to a lesser extent mouse, α-synuclein fibril injections. At both 18 and 23 months after injection of mouse and human α-synuclein fibrils, we observed a reduction in the density of α-synuclein aggregates in some brain regions compared to others at 12 months. At 23 months, no additional brain regions exhibited α-synuclein aggregates compared to earlier time points. In addition, we also demonstrate that the induced α-synucleinopathy triggered a significant early neuron loss in the anterior olfactory nucleus. By contrast, there was no loss of mitral neurons in the olfactory bulb, even at 18 month post-injection. We speculate that the lack of continued progression of α-synuclein pathology is due to compromise of the neural circuitry, consequential to neuron loss and possibly to the activation of proteolytic mechanisms in resilient neurons of wild-type mice that counterbalances the spread and seeding by degrading pathogenic α-synuclein.Electronic supplementary materialThe online version of this article (10.1007/s00401-017-1792-9) contains supplementary material, which is available to authorized users.

Several people with Parkinson’s disease have been treated with intrastriatal grafts of fetal dopaminergic neurons. Following autopsy, 10–22 years after surgery, some of the grafted neurons contained Lewy bodies similar to those observed in the host brain. Numerous studies have attempted to explain these findings in cell and animal models. In cell culture, α-synuclein has been found to transfer from one cell to another, via mechanisms that include exosomal transport and endocytosis, and in certain cases seed aggregation in the recipient cell. In animal models, transfer of α-synuclein from host brain cells to grafted neurons has been shown, but the reported frequency of the event has been relatively low and little is known about the underlying mechanisms as well as the fate of the transferred α-synuclein. We now demonstrate frequent transfer of α-synuclein from a rat brain engineered to overexpress human α-synuclein to grafted dopaminergic neurons. Further, we show that this model can be used to explore mechanisms underlying cell-to-cell transfer of α-synuclein. Thus, we present evidence both for the involvement of endocytosis in α-synuclein uptake in viv o, and for seeding of aggregation of endogenous α-synuclein in the recipient neuron by the transferred α-synuclein. Finally, we show that, at least in a subset of the studied cells, the transmitted α-synuclein is sensitive to proteinase K. Our new model system could be used to test compounds that inhibit cell-to-cell transfer of α-synuclein and therefore might retard progression of Parkinson neuropathology.