Truncated β-amyloid peptide channels provide an alternative mechanism for Alzheimer's Disease and Down syndrome

Full-length amyloid beta peptides (Aβ_1-40/42) form neuritic amyloid plaques in Alzheimer's disease (AD) patients and are implicated in ADpathology.However, recent transgenic animalmodels castdoubt on their direct role in AD pathology. Nonamyloidogenic truncated amyloid-beta fragments (Aβ_11-42 and Aβ_17-42) are also found in amyloid plaques of AD and in the preamyloid lesions of Down syndrome, a model system for early-onset AD study. Very little is known about the structure and activity of these smaller peptides, although they could be the primary AD and Down syndrome pathological agents. Using complementary techniques of molecular dynamics simulations, atomic force microscopy, channel conductancemeasurements, calcium imaging, neuritic degeneration, and cell death assays, we show that nonamyloidogenic Aβ_9-42 and Aβ_17-42 peptides form ion channels with loosely attached subunits and elicit single-channel conductances. The subunits appear mobile, suggesting insertion of small oligomers, followed by dynamic channel assembly and dissociation. These channels allow calcium uptake in amyloid precursor protein-deficient cells. The channel mediated calcium uptake induces neurite degeneration in human cortical neurons. Channel conductance, calcium uptake, and neurite degeneration are selectively inhibited by zinc, a blocker of amyloid ion channel activity. Thus, truncated Aβ fragments could account for undefined roles played by full length Aβs and provide a unique mechanism of AD and Down syndrome pathologies. The toxicity of nonamyloidogenic peptides via an ion channelmechanism necessitates a reevaluation of the current therapeutic approaches targeting the nonamyloidogenic pathway as avenue for AD treatment.