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Abstract

Aggregation and toxicity of the amyloid beta-peptide (A beta) are considered as critical events in the initiation and progression of Alzheimer's disease (AD). Recent evidence indicated that soluble oligomeric A beta assemblies exert pronounced toxicity, rather than larger fibrillar aggregates that deposit in the forms of extracellular plaques. While some rare mutations in the A beta sequence that cause early-onset AD promote the oligomerization, molecular mechanisms that induce the formation or stabilization of oligomers of the wild-type A beta remain unclear. Here, we identified an A beta variant phosphorylated at Ser26 residue (pSer26A beta) in transgenic mouse models of AD and in human brain that shows contrasting spatio-temporal distribution as compared to non-phosphorylated A beta (npA beta) or other modified A beta species. pSer26A beta is particularly abundant in intraneuronal deposits at very early stages of AD, but much less in extracellular plaques. pSer26A beta assembles into a specific oligomeric form that does not proceed further into larger fibrillar aggregates, and accumulates in characteristic intracellular compartments of granulovacuolar degeneration together with TDP-43 and phosphorylated tau. Importantly, pSer26A beta oligomers exert increased toxicity in human neurons as compared to other known A beta species. Thus, pSer26A beta could represent a critical species in the neurodegeneration during AD pathogenesis.