Intrastriatal injection of α-synuclein can lead to widespread synucleinopathy independent of neuroanatomic connectivity

Background: Prionoid transmission of alpha-synuclein (alpha Syn) aggregates along neuroanatomically connected projections is posited to underlie disease progression in alpha-synucleinopathies. Here, we specifically wanted to study whether this prionoid progression occurs via direct inter-neuronal transfer and, if so, would intrastriatal injection of aSyn aggregates lead to nigral degeneration. Methods: To test prionoid transmission of alpha Syn aggregates along the nigro-striatal pathway, we injected amyloidogenic alpha Syn aggregates into two different regions of the striatum of adult human wild type alpha Syn transgenic mice (Line M20) or non-transgenic (NTG) mice and aged for 4 months. Results: M20 mice injected in internal capsule (IC) or caudate putamen (CPu) regions of the striatum showed florid alpha Syn inclusion pathology distributed throughout the neuraxis, irrespective of anatomic connectivity. These alpha Syn inclusions were found in different cell types including neurons, astrocytes and even ependymal cells. On the other hand, intrastriatal injection of alpha Syn fibrils into NTG mice resulted in sparse alpha Syn pathology, mostly localized in the striatum and entorhinal cortex. Interestingly, NTG mice injected with preformed human alpha Syn fibrils showed no induction of alpha Syn inclusion pathology, suggesting the presence of a species barrier for alpha Syn fibrillar seeds. Modest levels of nigral dopaminergic (DA) neuronal loss was observed exclusively in substantia nigra (SN) of M20 cohorts injected in the IC, even in the absence of frank alpha Syn inclusions in DA neurons. None of the NTG mice or CPu-injected M20 mice showed DA neurodegeneration. Interestingly, the pattern and distribution of induced alpha Syn pathology corresponded with neuroinflammation especially in the SN of M20 cohorts. Hypermorphic reactive astrocytes laden with alpha Syn inclusions were abundantly present in the brains of M20 mice. Conclusions: Overall, our findings show that the pattern and extent of dissemination of alpha Syn pathology does not necessarily follow expected neuroanatomic connectivity. Further, the presence of intra-astrocytic alpha Syn pathology implies that glial cells participate in alpha Syn transmission and possibly have a role in non-cell autonomous disease modification.