Involvement of Synapsin III in the onset of alpha-synuclein synaptic pathology in Parkinson’s disease

Parkinson’s Disease (PD) is one the most common neurodegenerative movement disorder. The brain of affected patients is characterized by progressive degeneration of the dopaminergic nigrostriatal system and intraneuronal inclusions mainly composed by fibrillary aggregates of alpha-synuclein, known as Lewy bodies. Recently, we demonstrated a reciprocal modulatory interaction between alpha-synuclein and synapsin III (Syn III), a synaptic vesicles-associated phosphoprotein that plays a crucial role in dopamine (DA) release and synaptic vesicle arrangement. Although Syn III alterations have been found to be related to the onset of neuropsychiatric disorders and multiple sclerosis, studies describing whether this protein is altered in the PD brain were missing. The aim of this study was to investigate whether Syn III may participate to alpha-synuclein pathology in experimental models of disease and in the brain of PD subjects. We used two-month-old wild type (wt) C57BL/6J or Syn III KO mice stereotaxically injected either with AAV-alpha-syn or AAV-GFP control vector in the right substantia nigra (SN). Mice were sacrificed at 8 weeks post-injection to be used for immunohistochemistry (IHC) or molecular biology studies in order to probe alpha-synuclein aggregation, nigrostriatal neuron degeneration and the levels and distribution of striatal synaptic proteins. Postmortem brain samples obtained by the Parkinson’s UK brain Bank and human cerebrospinal fluids (CSF) samples from the Neurology Unit of Spedali Civili, Brescia were used for western blotting (WB), co-immunoprecipitation and IHC. We found that in the AAV-alpha-synuclein-injected wt mice the overexpression of alpha-synuclein changed the distribution of Syn III and of several other synaptic-vesicle associated proteins such as Rabs and SNARES in the striatum. However, no changes were revealed in the expression and localization of the other synaptic proteins such as synapsin I and II. In parallel, these mice also showed a statistically significant decrease in the number of TH-positive neurons in the SN and striatal DAT-positive terminals. Interestingly, by analyzing these same parameters in the AAV-alpha-synuclein-injected Syn III KO mice we observed that these mice showed different accumulation of alpha-synuclein in the nigrostriatal system and nigrostriatal damage when compared to AAV-alpha-synuclein-injected wt mice. By IHC and WB, we observed a marked accumulation of Syn III in the caudate/putamen and SN of patients affected by PD when compared to age matched controls. Co-immunoprecipitation studies revealed the presence of Syn III/alpha-synuclein interaction in the cerebrospinal fluids of PD patients. Collectively, these evidences suggest that Syn III may be crucially involved in the onset of alpha-synuclein-related synaptic damage in PD