Toxicity of extracellular alpha-synuclein is independent of intracellular alpha-synuclein

Parkinson′s disease (PD) pathology progresses throughout the nervous system affecting numerous neuronal structures. It has been postulated that the progression of the pathology is based on a prion-like disease mechanism partly due to the seeding effect of endocytosed alpha-synuclein (ASYN) on the endogenous ASYN. The appearance of the pathology in dopaminergic neurons leads to neuronal cell death and motor symptoms. However, the effect on other neuronal structures is more inconsistent, leading to a higher variability in the prevalence of non-motor symptoms. Thus, the sensitivity to the pathology seems to vary among neuronal subtypes. Here, we analyzed the role of endogenous ASYN in the progression of PD-like pathology and the effect of monomeric and oligomeric ASYN as well as paraquat and rotenone on primary enteric, dopaminergic and cortical neurons from wild-type mice. Our results showed that pathology progression did not occur in the absence of endogenous ASYN and that dopaminergic neurons were more sensitive to ASYN and rotenone when compared to all other neuronal subtypes. Remarkably, the toxic effect of ASYN was independent of the presence of endogenous ASYN and directly related to the disturbance of the mitochondrial membrane potential. Thus, we suggest that the interaction between ASYN and mitochondria plays an important role in the toxicity of trans-synaptically transported ASYN and in the progression of PD pathology. These results question the prion-disease hypothesis and propose that endocytosed ASYN impairs the host′s mitochondrial function thereby also contributing to PD-pathology progression. ### Competing Interest Statement Johannes Levin reports speaker fees from Bayer Vital, Biogen and Roche, consulting fees from Axon Neuroscience and Biogen, author fees from Thieme medical publishers and W. Kohlhammer GmbH medical publishers, non-financial support from Abbvie and compensation for duty as part-time CMO from MODAG, outside the submitted work. Francisco Pan-Montojo reports consulting fees as external CSO from NEUREVO GmbH, also outside the submitted work. All other authors declare no conflict of interest. * ASYN : alpha-synuclein PD : Parkinson’s Disease BS : Blocking solution CNS : Central nervous system DIV : Day in vitro DMV : Motor nucleus of the vagus DS : Donkey serum ECAR : Extracellular acidification rate ENS : Enteric nervous system FACS : Fluorescence Activated Cell Sorting FCCP : Carbonyl cyanide-4 (trifluoromethoxy) phenylhydrazone GFP : Green-fluorescent protein IML : Intermediolateral nucleus of the spinal cord KO : Knock-out LB : Lewy bodies LN : Lewy neurites MFI : Mean fluorescence intensity NeuN : Neuronal nuclei NGF : Nerve growth factor OB : Olfactory bulb OCR : Mitochondrial Oxygen consumption rate PBS : Phosphate buffered saline PFA : Paraformaldehyde PGP9.5 : Protein gene product 9.5 RBD : REM sleep behaviour disorder REM : Rapid eye movement RO : Rotenone RT : Room temperature SNc : Substantia nigra pars compacta TH : Tyrosine hydroxylase TMRE : Tetramethylrhodamine, ethyl ester WT : Wild-type