Surface behavior and lipid interaction of Alzheimer beta-amyloid peptide 1-42: a membrane-disrupting peptide

Abstract

Amyloid aggregates, found in patients that suffer from Alzheimer's disease, are composed of fibril-forming peptides in a beta-sheet conformation. One of the most abundant components in amyloid aggregates is the beta-amyloid peptide 1-42 (Abeta 1-42). Membrane alterations may proceed to cell death by either an oxidative stress mechanism, caused by the peptide and synergized by transition metal ions, or through formation of ion channels by peptide interfacial self-aggregation. Here we demonstrate that Langmuir films of Abeta 1-42, either in pure form or mixed with lipids, develop stable monomolecular arrays with a high surface stability. By using micropipette aspiration technique and confocal microscopy we show that Abeta 1-42 induces a strong membrane destabilization in giant unilamellar vesicles composed of palmitoyloleoyl-phosphatidylcholine, sphingomyelin, and cholesterol, lowering the critical tension of vesicle rupture. Additionally, Abeta 1-42 triggers the induction of a sequential leakage of low- and high-molecular-weight markers trapped inside the giant unilamellar vesicles, but preserving the vesicle shape. Consequently, the Abeta 1-42 sequence confers particular molecular properties to the peptide that, in turn, influence supramolecular properties associated to membranes that may result in toxicity, including: 1), an ability of the peptide to strongly associate with the membrane; 2), a reduction of lateral membrane cohesive forces; and 3), a capacity to break the transbilayer gradient and puncture sealed vesicles.

FIGURE 1

Surface behavior of pure Aβ 1–42 peptide monolayers. Lateral pressure area (solid line) and surface potential area (dashed line) isotherms of Aβ 1–42. The subphase was 145 mM NaCl, pH 6.

FIGURE 2

Surface behavior in mixed DPPC-Aβ 1–42 peptide interfaces. Lateral pressure area (solid line) and surface potential area (dashed line) isotherms. The corresponding peptide area as a proportion of the mixed monolayer was 25% (1), 50% (2), and 75% (3). Subphase 145 mM NaCl, pH 6. Arrows indicate the collapse pressure of the peptide-enriched phase. Note the effect of the peptide on the liquid expanded-liquid condensed phase transition of the DPPC-enriched phase (∼10 mN/m).

FIGURE 3

Videomicrographs of GUVs tested by micromanipulation after exposure to Aβ 1–42 peptide. Giant unilamellar vesicle composed of POPC (top) and composed of POPC/SM/Chol (1:1:1 molar ratio, bottom) held under constant, low membrane tension exposed to 5 μM Aβ 1–42 peptide. (A) Before and (B) after peptide exposure. τ_lyse is the critical tension to lyse the vesicle (mN/m) and is shown in the figure to denote the change in the membrane physical properties under the different conditions. It is the average tension when the vesicle breaks as the suction pressure was increased. Videomicrographs were taken optimizing the brightness and contrast setup to set the best condition for vesicle vision. The size bar at bottom-right of each panel indicates the scale of the pictures (5 μm). Inserted bars indicate the diameter of GUVs and the projection toward the inner of the suction pipette after peptide interaction.

FIGURE 4

Direct visualization of lytic action of Aβ 1–42 peptide. Confocal microscopy of POPC (top) and POPC/SM/Chol (1:1:1 molar ratio, bottom) GUVs, filled with Alexa⁴⁸⁸-Dextran (green) and Alexa⁵⁴⁶-Maleimide (red), exposed to Aβ 1–42 7.5 μM. Effect of lipid-peptide interaction from the time, 0 s: injection of the peptide. Arrows indicate the differential leakage of the fluorophores.

FIGURE 5

Dye release kinetics from inside GUVs after Aβ 1–42 peptide exposure. Alexa⁴⁸⁸-Dextran (squares) and Alexa⁵⁴⁶-Maleimide (circles) leakage from inside POPC (A) and POPC/SM/Chol (1:1:1 molar ratio) (B) GUVs after exposure to Aβ 1–42 7.5 μM. The fluorescence intensity was measured with MetaMorph software by quantifying the average gray value of Alexa⁴⁸⁸-Dextran and Alexa⁵⁴⁶-Maleimide of each picture inside the GUVs after exposition to Aβ 1–42 peptide. (A, inset) Alexa⁴⁸⁸-Dextran (squares) and Alexa⁵⁴⁶-Maleimide (circles) fluorescence intensity in absence of peptide (see text). Initial rates of leakage (slope of the initial linear behavior of dye leakage after peptide exposure): (A) Alexa⁴⁸⁸-Dextran: 0.72 AU/s, Alexa⁵⁴⁶-Maleimide: 1.45 AU/s; and (B) Alexa⁴⁸⁸-Dextran: 0.52 AU/s, Alexa⁵⁴⁶-Maleimide: 0.96 AU/s.