α-Synuclein in salivary extracellular vesicles as a potential biomarker of Parkinson’s disease

Background Detection of α-synuclein (α-syn) in biological fluids such as saliva may serve as potential biomarker of PD. α-syn pertaining to extracellular vesicles (EVs) has been recently studied in plasma, but not in other biological fluids such as saliva. Aim 1) To investigate the presence of exosomes in PD saliva; 2) to explore the value of α-syn in salivary EVs as potential biomarker in PD. Methods Saliva samples were obtained from 74 PD and 60 healthy controls (HCs). The EVs were extracted from saliva by XYCQ EV Enrichment KIT. Western blot and Nanosight 300 were used to validate the existence of exosomes in EVs and to analyze the size of salivary EVs. Salivary EVs α-syn levels, including total α-syn (α-synTotal), oligomeric α-syn (α-synOlig) and phosphorylated-ser129 α-syn (α-synPS129), were measured by Electrochemiluminescence (ECL) assays. Diagnostic value and clinical relevance of salivary EVs α-syn were assessed by Receiver Operator Characteristic (ROC) curve and Spearman correlation. Results Alix and CD9 positive EVs, representing the presence of exosomes, was detected in PD salivary samples. Size of salivary EVs was about 30–400 nm. The levels of α-synOlig and α-synOlig/α-synTotal in the salivary EVs were higher in PD than in HCs (10.39 ± 1.46 pg/ng vs.1.37 ± 0.24 pg/ng, p＜0.001;1.70 ± 0.52 pg/ng vs.0.67 ± 0.26 pg/ng, p＜0.001). There was no significant difference in α-synTotal、α-synPS129、 α-synPS129/α-synTotal ratio between PD and HCs (P = 0.723, 0.634, 0.734, respectively). α-synOlig 2.05 pg/ng distinguished PD from HCs with sensitivity 92% and specificity 86%; α-synOlig /α-synTotal 0.18 pg/ng differentiated PD from HCs with sensitivity 81% and specificity 71%. No significant correlation between salivary EVs α-synOlig, α-synOlig/α-synTotal and disease severity was found. Conclusions Exosomes are present in PD saliva. The α-synOlig and α-synOlig/α-synTotal ratio in salivary EVs may serve as potential diagnostic biomarkers for PD.