Abstract
<jats:title>Abstract</jats:title> <jats:sec> <jats:title>Importance</jats:title> <jats:p>Alzheimer disease (AD) is frequently accompanied by co-pathologies such as Lewy bodies, intracellular protein aggregates consisting of misfolded α-synuclein, ubiquitin and several other proteins. These aggregated proteins contribute to the pathological and clinical heterogeneity of AD and are associated with a more rapid progression. Cerebrospinal fluid (CSF) measurement of aggregated α-synuclein or skin biopsy histochemistry are the current biomarkers for Lewy body pathology in individuals with Parkinson’s disease and related synucleinopathies but are invasive, not easily scalable for large studies or practical in clinical practice. Neuron-derived extracellular vesicles (nEVs) provide a feasible and tolerable alternative to detect Lewy body pathology. nEVs exit across the blood-brain barrier and deliver neuron-derived proteins into the systemic circulation, offering direct access to brain-level protein concentrations from peripheral blood, not afforded by conventional plasma biomarkers.</jats:p> </jats:sec> <jats:sec> <jats:title>Objective</jats:title> <jats:p>To use nEVs extracted from plasma to measure brain-derived α-synuclein levels as a clinical biomarker of Lewy body pathology and synucleinopathy across clinically and pathologically characterized cohorts.</jats:p> </jats:sec> <jats:sec> <jats:title>Design, Setting, and Participants</jats:title> <jats:p>This multicohort observational study evaluated plasma nEV-derived α-synuclein levels in 1,304 individuals, including 794 (61%) individuals without dementia, 469 (36%) individuals with AD, and 41 (3%) individuals with PD across four cohorts. Postmortem validation was performed from autopsy data in 127 individuals, including 96 (76%) individuals without, and 31 (24%) with, Lewy body pathology. Cerebrospinal fluid (CSF) α-synuclein seed amplification assay data were available in a small subset of 54 (4.1%) of the group. Amyloid positron emission tomography (PET) data were available in 901(69.1%) of the total group.</jats:p> </jats:sec> <jats:sec> <jats:title>Exposure</jats:title> <jats:p>Clinical diagnosis, AD biomarker positivity, Lewy body pathology, CSF α-synuclein seed assay status, and amyloid PET positivity.</jats:p> </jats:sec> <jats:sec> <jats:title>Main Outcomes and Measures</jats:title> <jats:p>Plasma nEV-derived α-synuclein levels normalized to CD9 and assessed in relation to clinical diagnosis, AD biomarker status, neuropathologically confirmed Lewy body pathology, CSF seed assay results, and amyloid PET positivity.</jats:p> </jats:sec> <jats:sec> <jats:title>Results</jats:title> <jats:p> Compared with controls, plasma α-synuclein levels from nEVs were significantly elevated among individuals with PD (controls mean 0.51, SD=0.24 vs PD mean=0.69, SD=0.23, <jats:italic>P</jats:italic> =8.66×10 <jats:sup>-</jats:sup> □). nEV-derived α-synuclein levels were also significantly higher in P-tau181 and P- tau217 positive individuals with and without cognitive impairment compared to P-tau negative individuals. Elevated nEV-derived α-synuclein levels were subsequently validated in individuals with postmortem Lewy body pathology (28.6% higher mean levels, <jats:italic>P</jats:italic> =0.02). Among 54 individuals with CSF α-synuclein seed amplification assay, 8 (15%) were positive and showed a 18.2% increase in mean nEV α-synuclein levels compared to individuals with negative α- synuclein seed amplification. nEV-derived α-synuclein levels were also significantly elevated in amyloid PET positive individuals with and without dementia (5.8% higher mean levels, <jats:italic>P</jats:italic> =8.50E-03) and clinical AD (9.3% higher mean levels, <jats:italic>P</jats:italic> =6.38E-12). </jats:p> </jats:sec> <jats:sec> <jats:title>Conclusions and Relevance</jats:title> <jats:p>Plasma α-synuclein levels from nEVs likely reflect underlying synuclein pathology in the brain and have potential as a blood-based biomarker for detecting Lewy body pathology in PD and in AD and related dementias. nEVs cross the blood–brain barrier and carry neuron-derived cargo directly into the bloodstream, thus providing a unique window into brain α-synuclein levels that is not accessible through conventional plasma biomarkers.</jats:p> </jats:sec>