A Mechanism for the Inhibition of Tau Neurotoxicity: Studies with Artificial Membranes, Isolated Mitochondria, and Intact Cells.

It is currently believed that molecular agents that specifically bind to and neutralize the toxic proteins/peptides, amyloid β (Aβ42), tau, and the tau-derived peptide PHF6, hold the key to attenuating the progression of Alzheimer's disease (AD). We thus tested our previously developed nonaggregating Aβ42 double mutant (Aβ42) as a multispecific binder for three AD-associated molecules, wild-type Aβ42, the tau mutant, and a synthetic PHF6 peptide. Aβ42 acted as a functional inhibitor of these molecules in assays and in neuronal cell-based models of AD. The double mutant bound both cytotoxic tau and synthetic PHF6 and protected neuronal cells from the accumulation of tau in cell lysates and mitochondria. Aβ42 also reduced toxic intracellular levels of calcium and the overall cell toxicity induced by overexpressed tau, synthetic PHF6, Aβ42, or a combination of PHF6and Aβ42. Aβ42 inhibited PHF6-induced overall mitochondrial dysfunction: In particular, Aβ42 inhibited PHF6-induced damage to submitochondrial particles (SMPs) and suppressed PHF6-induced elevation of the ζ-potential of inverted SMPs (proxy for the inner mitochondrial membrane, IMM). PHF6 reduced the lipid fluidity of cardiolipin/DOPC vesicles (that mimic the IMM) but not DOPC (which mimics the outer mitochondrial membrane), and this effect was inhibited by Aβ42. This inhibition may be explained by the conformational changes in PHF6 induced by Aβ42 in solution and in membrane mimetics. On this basis, the paper presents a mechanistic explanation for the inhibitory activity of Aβ42 against Aβ42- and tau-induced membrane permeability and cell toxicity and provides confirmatory evidence for its protective function in neuronal cells.