Abstract: In Alzheimer’s disease, microglial phagocytosis is engaged in the pathogenesis as it clears abnormal protein accumulations, debris, and apoptotic cells in the early stages of Alzheimer’s disease, but fuels neuroinflammation and accelerates disease progression in later stages. In vivo parabiosis experiments in aged animals have demonstrated that blood-born factors modulate synaptic plasticity, neurogenesis, and microglial responses. We hypothesize that peripheral factors can modulate microglial function and thereby possibly influence Alzheimer’s disease pathology. The objective of this study is to investigate the effects of Alzheimer’s disease serum on microglial phagocytosis. Here, we use an immortalized human microglial cell line in an in vitro parabiosis assay to investigate the impact of the serum from individuals diagnosed with Alzheimer’s disease (n = 30) and age-matched controls (n = 30) (PRODEM study) on microglial phagocytosis. Exposure to Alzheimer’s disease serum increased microglial phagocytic uptake of pH-sensitive fluorescent particles and downregulated expression of the lysosomal master regulator transcription factor EB (TFEB) and of ATPase H+ transporting lysosomal V1 subunit B2 (ATP6V1B2), a component of the vacuolar ATPase. To identify serum components that may relate to changes in phagocytosis, serum samples of the Three-City Study (3C Study) were used. In the 3C Study, blood samples were collected up to 12 years before the onset of cognitive decline or dementia and their serum metabolome is well-defined. Microglia exposed to the serum of future Alzheimer’s disease patients from the 3C Study displayed an increased phagocytic uptake compared with the serum of matched controls, depending on the presence of the apolipoprotein E ε4 allele in the Alzheimer’s disease patients. Furthermore, microglial phagocytosis correlated inversely with serum levels of the omega-3 fatty acid eicosapentaenoic acid. We confirmed this inverse correlation between eicosapentaenoic acid and phagocytosis in the serum samples of the PRODEM cohort. In addition, in vitro testing of eicosapentaenoic acid on microglial phagocytosis showed a concentration-dependent decrease in phagocytic uptake. In conclusion, following incubation with Alzheimer’s disease blood serum, we observed increased microglial phagocytic uptake and the downregulation of TFEB and ATP6V1B2, possibly indicating lysosomal dysfunction. Furthermore, microglial phagocytosis was inversely correlated with serum eicosapentaenoic acid levels, suggesting an important role for dietary eicosapentaenoic acid in microglial function.

Key words: Alzheimer’s disease, blood serum, eicosapentaenoic acid, in vitro parabiosis, metabolome, microglia, omega-3 fatty acids, phagocytosis