AD synapses contain abundant Aß monomer and multiple soluble oligomers, including a 56-kDa assembly.

ABSTRACT

Much evidence indicates that soluble amyloid beta (Aß) oligomers are key mediators of early cognitive loss, but the localization and key peptide species remain unclear. We have used flow cytometry analysis to demonstrate that surviving Alzheimer's disease (AD) synapses accumulate both Aß and phosphorylated tau (p-tau). The present experiments use peptide-specific X-map assays and Western blot analyses to identify the Aß peptide species in synaptosome-enriched samples from normal human subjects, neurologic controls, and AD cases. Aß40 peptide levels did not vary, but both Aß42 and Aß oligomers were increased in soluble AD extracts, with oligomer levels 20-fold higher in aqueous compared with detergent extracts. In Western blot analysis, a ladder of sodium dodecyl sulfate (SDS)-stable oligomers was observed in AD cases, varying in size from monomer, the major peptide observed, to larger assemblies up to about 200 kDa and larger. Multiple oligomers, including monomer, small oligomers, a 56-kDa assembly, and amyloid precursor protein (APP) were correlated with the Aß level measured in flow cytometry-purified synaptosomes. These results suggest that multiple amyloid precursor protein processing pathways are active in AD synapses and multiple soluble oligomeric assemblies may contribute to synaptic dysfunction.