Alzheimer brains: increased soluble amyloid peptides, but decreased lipid rafts

Abstract

AbstractBackgroundPET imaging studies of AD patients show progressive increases of fibrillar Aβ‐ amyloid. Because current PET ligands underestimate non‐fibrillar forms, we assayed soluble Aβ in AD and controls. To identify mechanisms responsible for soluble Aβ in AD brains, we examined lipid rafts (LR), where APP is enzymatically processed. Frontal cortex was compared with cerebellum, which has minimal AD pathology.MethodPostmortem samples (N=48) were age‐and sex matched for Braak scores. Soluble Aβ was extracted from homogenates of full thickness frontal cortex by isotonic buffer and mild detergent (RIPA; 1% NP40) and centrifuged (10,000g/1h). Supernates were assayed by dot blots with antibodies for Aβ40 and Aβ42 (Biolegend, San Diego, CA). Aβ was assayed as relative fluorescent units per gm brain. LR fractions were isolated from these same brains by kit (Invent Biotechnologies; LR‐039), validated by ultracentrifugal fractionation.ResultCompared with cognitively normal controls (Braak 0‐1), elevations of soluble Aβ40 and Aβ42 were similar for intermediate and later stage AD (Braak 2‐3 and 4‐6). Clinical grade AD had greater increase of soluble Aβ40 (+400%) than Aβ42 (+50%) (p<0.05). LR raft yield per gram AD frontal cortex was 20% below controls (p<0.05), while cerebellar LR did not differ by Braak score. The composition of LR had a similar pattern, with 35% AD decreases of total protein and cholesterol (p<0.05); cerebellar LR again did not differ by Braak score. However, LR 4‐HNE, a marker of oxidative damage, was similarly increased in AD cortex and AD cerebellum.ConclusionThe extensive overlap of soluble Aβ levels in controls with AD contrasts with the PET findings on fibrillar Aβ. These findings further support fibrillar Aβ as a biomarker for AD treatments and show the need for more detailed postmortem analysis of diverse soluble and insoluble Aβ aggregates in relation to PET. The decreased LR in AD frontal cortex is consistent with its extensive neuronal loss, in contrast to the cerebellum. Further studies will examine LR enzymes of APP processing as a source of the increased soluble Aβ in AD, and subcellular clearance processes, which may be impaired in association with the oxidative damage indicated by increased 4‐HNE.