Approximately half of
Alzheimer's disease (AD) brains have concomitant Lewy pathology at autopsy, suggesting that α-
synuclein (α-SYN) aggregation is a regulated event in the pathogenesis of AD. Genome-wide association studies revealed that the ε4 allele of the
apolipoprotein E (
APOE4) gene, the strongest genetic risk factor for AD, is also the most replicated genetic risk factor for
Lewy body dementia (LBD), signifying an important role of
APOE4 in both
amyloid-β (Aβ) and α-SYN pathogenesis. How
APOE4 modulates α-SYN aggregation in AD is unclear. In this study, we aimed to determine how α-SYN is associated with AD-related pathology and how
APOE4 impacts α-SYN seeding and toxicity. We measured α-SYN levels and their association with other established AD-related markers in brain samples from autopsy-confirmed AD patients (N = 469), where 54% had concomitant LB pathology (AD + LB). We found significant correlations between the levels of α-SYN and those of Aβ40, Aβ42, tau and
APOE, particularly in insoluble fractions of AD + LB. Using a real-time quaking-induced conversion (RT-QuIC) assay, we measured the seeding activity of soluble α-SYN and found that α-SYN seeding was exacerbated by
APOE4 in the AD cohort, as well as a small cohort of autopsy-confirmed LBD brains with minimal Alzheimer type pathology. We further fractionated the soluble AD brain lysates by size exclusion chromatography (SEC) ran on fast
protein liquid chromatography (FPLC) and identified the α-SYN species (~ 96 kDa) that showed the strongest seeding activity. Finally, using human induced pluripotent stem cell (iPSC)-derived neurons, we showed that amplified α-SYN aggregates from AD + LB brain of patients with
APOE4 were highly toxic to neurons, whereas the same amount of α-SYN monomer was not toxic. Our findings suggest that the presence of LB pathology correlates with AD-related pathologies and that
APOE4 exacerbates α-SYN seeding activity and neurotoxicity, providing mechanistic insight into how
APOE4 affects α-SYN pathogenesis in AD.