Amyloid-β Ion Channels in Artificial Lipid Bilayers and Neuronal Cells. Resolving a Controversy

Abstract

One of the current hypotheses for the pathology of Alzheimer's disease (AD) proposes that amyloid-beta (Aβ) peptides induce uncontrolled, neurotoxic ion flux across cellular membranes. The resulting inability of neurons to regulate their intracellular concentration of ions, in particular calcium ions, has been associated with cell death and may thus contribute to cognitive impairment typical for AD. The exact biophysical mechanism of this ion flux is subject of an ongoing and unresolved controversy. Two mechanisms are currently debated. One proposed mechanism suggests that Aβ assembles into pore-like structures in lipid membranes, leading to stepwise fluctuations of transmembrane current that is typical for ion channels (ion channel hypothesis). The other proposed mechanism postulates a generalized and gradually increasing ion flux as a result of Aβ-induced thinning of membranes.Here, we resolve this controversy by examining, in detail, the two pivotal protocols for preparing and measuring Aβ induced conductance through planar lipid bilayers and cell membranes. The results clarify that Aβ induces stepwise ion flux across planar lipid bilayers as opposed to a gradual increase in transmembrane current; they show that the previously reported gradual increase in transmembrane current arises from residues of the solvent hexafluoroisopropanol, which is commonly used for the preparation of amyloid samples.We also examined the effect of Aβ samples on cell membranes. We exposed SH-SY5Y neuroblastoma cells and mouse cortical primary neurons to Aβ at resting potential in the presence and absence of typical ion channel blockers. The results provide additional evidence suggesting that Aβ peptides can form ion channels in cellular membranes that are independent from the postulated ability of Aβ to modulate intrinsic cellular ion channels or transporter proteins.