Abstract
The amyloid β (Aβ) peptide and its shorter variants, including a highly cytotoxic Aβ25–35 peptide, exert their neurotoxic effect during Alzheimer’s disease by various mechanisms, including cellular membrane permeabilization. The intrinsic polymorphism of Aβ has prevented the identification of the molecular basis of Aβ pore formation by direct structural methods, and computational studies have led to highly divergent pore models. Here, we have employed a set of biophysical techniques to directly monitor Ca2+-transporting Aβ25–35 pores in lipid membranes, to quantitatively characterize pore formation, and to identify the key structural features of the pore. Moreover, the effect of membrane cholesterol on pore formation and the structure of Aβ25–35 has been elucidated. The data suggest that the membrane-embedded peptide forms 6- or 8-stranded β-barrel like structures. The 8-stranded barrels may conduct Ca2+ ions through an inner cavity, whereas the tightly packed 6-stranded barrels need to assemble into supramolecular structures to form a central pore. Cholesterol affects Aβ25–35 pore formation by a dual mechanism, i.e., by direct interaction with the peptide and by affecting membrane structure. Collectively, our data illuminate the molecular basis of Aβ membrane pore formation, which should advance both basic and clinical research on Alzheimer’s disease and membrane-associated pathologies in general.
Highlights
Proteolytic cleavage of the amyloid precursor protein (APP) produces the amyloid β (Aβ) peptide, which forms extracellular fibrillar deposits in cross β-sheet conformation[1,2,3]
Aβ peptide occurs in brain tissue in various forms
Aβ25–35 binds to anionic membranes, promoted by its excess positive charge due to Lys[28], as well as to zwitterionic phosphatidylcholine (PC) membranes, less efficiently[28,29,30]
Summary
Under conditions that can be compared more directly with the vesicle experiments, i.e. in the presence of bulk aqueous buffer, the β-sheet structure reaches the maximum fraction of 40% at xchol = 0.05 and 0.40, while β-turns decrease to around 20% at these cholesterol contents (Fig. 5c). For membranes under aqueous buffer at xchol = 0.4, values of γ greater that 10° cannot be combined with the measured β-sheet dichroic ratio Rβ = 0.89 to solve Eq S6 for angle β This means that data analysis using γ = 0° is more reliable, or the β-barrel-like oligomeric structures are tilted from the membrane normal by less than 10°. Our data shed light on the elusive membrane pores formed by the Aβ25–35 peptide, which were modeled earlier by computational methods either as β-barrel-like structures or an α-helical bundle[26,27,36]. Transmembrane Ca2+ transport through Aβ25–35 pores is documented, and the kinetic parameters and the stoichiometry of pore formation and the key structural features of the pore are identified, which is a significant step forward in understanding the molecular basis of Aβ cytotoxicity through a membrane damaging mechanism
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