All D-Amino Acids Amyloid β Forms Ion Channels in Lipid Bilayers and is Toxic to Cells

Abstract

A current hypothesis for Alzheimer's disease (AD) proposes that increased levels of amyloid-beta (Aβ) peptides induce uncontrolled, neurotoxic ion flux across cellular membranes. The resulting dysregulation of ion homeostasis, in particular calcium ions, has been associated with neuronal degeneration and eventual death and may thus contribute to cognitive impairment typical for AD. Aβ-dependent ionic dysregulation may occur by direct membrane permeabilization; or via Aβ binding to cell membrane receptors with subsequent opening of existing ion channels or transporters; or via a combination of both mechanisms. Typically receptor binding interactions are stereo-specific. Here we have taken advantage of this feature and used it to interrogate the mechanism(s) of amyloid toxicity by comparing whether the all D-amino acids Aβ(1-42)behaves like the natural all L-Aβ(1-42). Using planar lipid bilayer (PLB) electrophysiological recordings, cell toxicity assays and molecular dynamics (MD) simulations, we present evidence showing that: the D-Aβ isomer exhibits a bilayer behavior indistinguishable from that described previously for L-Aβ isomer. Namely, both peptides form channel-like pores with heterogeneous conductances, similar cation selectivity and both are blocked by Zn2+. MD simulations show comparable β-barrel-like Aβ channels stability for D- and L-isomers, and both Aβ barrels isomers exhibit higher probability to cations in the solvated pore. Cell toxicity assays show similar toxicity levels for both Aβ isomers, consistent with earlier reports. The combined results suggest that Aβ cell toxicity is predominantly receptor-independent, non-stereo selective and imply a mechanism where increased levels of endogenous L-Aβ are toxic to cells mostly via direct pore formation. The findings presented lend support to the AD hypothesis suggesting cellular ion-dysregulation via Aβ channel formation.Funded by NCI Contract HHSN261200800001E (RN) and NIH (National Institute on Aging AG028709) extramural program (RL).