IDENTIFICATION OF MEMBRANE CA2+ CHANNELS ACTIVATED BY PROTEIN MISFOLDED OLIGOMERS AND THEIR ACTIVATION MECHANISM

Alzheimer’s disease is the most common form of dementia, characterized by the aggregation of the amyloid β peptide (Aβ) in the brain and by an impairment of calcium homeostasis caused by excessive activation of glutamatergic receptors, named excitotoxicity. Here, we studied the effects on calcium homeostasis caused by the formation of Aβ oligomeric assemblies, formed with Aβ40 and Aβ42 peptide, and by the model protein HypF-N. We found that these oligomers cause a rapid influx of calcium ions (Ca2+) across the cell membrane by rapidly activating extrasynaptic N-methyl-D-aspartate receptors (NMDAr) and, to a lower extent, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAr), and also by causing a perforation of lipid bilayers. Instead, none of the Ca2+ channels, including those found in previous interactome studies to physically interact with the oligomers, were found to participate to the observed Ca2+ influx. We also observed, however, that misfolded oligomers do not interact directly with NMDAr and AMPAr. Further experiments with lysophosphatidylcholine and arachidonic acid, which cause membrane compression and stretch, respectively, indicated that these receptors are activated through a change in membrane tension induced by the oligomers and transmitted mechanically to the receptors via the lipid bilayer. Indeed, lysophosphatidylcholine is able to neutralise the oligomer-induced activation of the NMDAr, whereas arachidonic acid activates the receptors similarly to the oligomers with no additive effects. An increased rotational freedom observed for a fluorescent probe embedded within the membrane in the presence of the oligomers also indicates a membrane stretch. These results reveal a further mechanism of toxicity of Aβ oligomers in Alzheimer’s disease through the perturbation of the mechanical properties of lipid membranes sensed by NMDAr and AMPAr, in addition to others based on the direct binding to membrane receptors and on permeabilization of lipid membranes.