Novel Increase of N‐ and O‐linked Sialylation in Alzheimer’s Disease

Abstract

AbstractBackgroundResearch shows evidence of increased sialylation in post‐mortem Alzheimer’s Disease (AD) cases. Sialylation is a form of glycosylation with the attachment of N‐acetyl‐neuraminic acid or sialic acid (SA) to glycoproteins and lipids. There are two major forms of sialylation, N‐ and O‐linked SA, modifications that regulate microglia cellular interactions and immune response. Yet it is unclear how these forms of sialylation contribute to AD pathology and modify disease progression. We hypothesize specific microglia sialylation patterns early in disease progression contribute to pathological burden and dampen immune response.MethodsWe utilized brain tissue from 10 post‐mortem human AD cases, focused on frontal, hippocampal, and cerebellar regions, and 5XFAD mouse model of amyloid pathology. To Identify N‐linked SA bonds, we focused on α‐2,6 SA which was recognized by a plant‐derived lectin. Aβ plaques, neurofibrillary tangles, microglia, and α‐2,6 SA were labelled with immunofluorescent markers in cortical brain tissue. Samples were imaged with confocal microscopy. To identify O‐linked SA bonds, Alcian Blue and Periodic Acid Schiff stains were used in novel ways to differentiate subsets of neutral and charged O‐linked SA bonds. Samples were imaged on a Aperio microscope slide scanner.Results12‐month‐old 5XFAD mice have significantly higher percent of α‐2,6 SA coverage in the plaque microenvironment compared to WT littermates (p<0.001). Pearson correlation analysis determined significantly greater α‐2,6 microglia sialylation (r = 0.522; p<0.0001) compared to plaque sialylation (r = 0.262) in the 5XFAD mouse model. Post‐mortem human AD cases displayed similar microglia sialylation patterns. Yet there was no significant correlation of between tau and α‐2,6 SA (r = ‐0.143; p<0.0001) in human AD compared to microglia sialylation (r = 0.279). Qualitatively, 5XFAD mice and human AD display similar recognition of Aβ plaque with O‐linked glycosylation patterns.ConclusionsWe observed significant α‐2,6 sialylation of microglia in 5XFAD mice and post‐mortem human AD which was independent of tau pathology. Additionally, intriguing patterns of O‐linked SA are present surrounding Aβ plaques using novel histological techniques. Next, we will closely investigate subsets of sialylated microglia to determine what forms of O‐linked SA may reduce clearance of debris by microglia while actively comparing Aβ plaque sialylation in human AD cases.