Soluble Amyloid β-Oligomers Affect Dielectric Membrane Properties by Bilayer Insertion and Domain Formation: Implications for Cell Toxicity

Abstract

It is well established that Alzheimer's amyloid beta-peptides reduce the membrane barrier to ion transport. The prevailing model ascribes the resulting interference with ion homeostasis to the formation of peptide pores across the bilayer. In this work, we examine the interaction of soluble prefibrillar amyloid beta (Abeta(1-42))-oligomers with bilayer models, observing also dramatic increases in ion current at micromolar peptide concentrations. We demonstrate that the Abeta-induced ion conductances across free-standing membranes and across substrate-supported "tethered" bilayers are quantitatively similar and depend on membrane composition. However, characteristic signatures of the molecular transport mechanism were distinctly different from ion transfer through water-filled pores, as shown by a quantitative comparison of the membrane response to Abeta-oligomers and to the bacterial toxin alpha-hemolysin. Neutron reflection from tethered membranes showed that Abeta-oligomers insert into the bilayer, affecting both membrane leaflets. By measuring the capacitance of peptide-free membranes, as well as their geometrical thicknesses, the dielectric constants in the aliphatic cores of 1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-diphytanoyl-sn-glycero-3-phosphocholine bilayers were determined to be epsilon = 2.8 and 2.2, respectively. The magnitude of the Abeta-induced increase in epsilon indicates that Abeta-oligomers affect membranes by inducing lateral heterogeneity in the bilayers, but an increase in the water content of the bilayers was not observed. The activation energy for Abeta-induced ion transport across the membrane is at least three times higher than that measured for membranes reconstituted with alpha-hemolysin pores, E(a) = 36.8 vs. 9.9 kJ/mol, indicating that the molecular mechanisms underlying both transport processes are fundamentally different. The Abeta-induced membrane conductance shows a nonlinear dependence on the peptide concentration in the membrane. Moreover, E(a) depends on peptide concentration. These observations suggest that cooperativity and/or conformational changes of the Abeta-oligomer particles upon transfer from the aqueous to the hydrocarbon environment play a prominent role in the interaction of the peptide with the membrane. A model in which Abeta-oligomers insert into the hydrophobic core of the membrane-where they lead to a local increase in epsilon and a concomitant reduction of the membrane barrier-describes the experimental data quantitatively.

Amyloid fibers provide structural integrity to Bacillus subtilis biofilms

“…Quite interestingly, the small aggregates that are precursors to amyloid fibers have been reported to be toxic (35,39). There is a generic structural similarity not only between amyloid fibrils formed from diverse proteins, but also their intermediate structures.…”

Mechanism of Membrane Interaction and Disruption by α-Synuclein

“…4) However, the membrane thinning, transient alterations of its structure and lateral diffusion properties alone may be sufficient to trigger a cascade of events leading to cell death. 22,23,27,28 In comparison to WT α-Syn, the concentration-dependent process of aggregation and membrane disruption is most pronounced for the highly toxic variant E57K followed by the familial variant A53T.…”

“…25 In addition, membrane thinning was proposed as a possible toxic mechanism for other amyloidogenic proteins including Aβ. 27 Conventionally, interactions between aggregates of α−Syn and lipids are studied by following the adsorption of protein to ULVs in solution and observing the effects by spatially averaging spectroscopic or fluorescent techniques. 22,28 Although this approach has provided valuable knowledge of the interactions of α−Syn with lipids, it does not allow the direct visualization of protein aggregates on the membrane.…”

Interfacial assembly of proteins and peptides: recent examples studied by neutron reflection

“…Neutron reflection has also been used to investigate the lipid-binding behaviour of antimicrobial peptides (Lad et al 2007) and amyloid-b peptides (Chi et al 2008;Valincius et al 2008). By combining the measurements of surface pressure, FTIR spectroscopy and neutron reflection, Lad et al showed that their cationic peptides (melittin, magainin II and cecropin P1) interacted more strongly with the anionic lipid 1,2-dihexadecanoyl-sn-glycerol-3-(phosphor-rac-(1-glycerol)) (DPPG) compared with the zwitterionic lipid 1,2-dihexadecanoyl-sn-glycerol-3-phosphocholine (DPPC), demonstrating the different effects of electrostatic interaction coupled with other effects implicated from molecular structures.…”

“…The biomolecular systems that have so far been studied include proteins (Lu et al ,d, 1999a(Lu et al ,b,c, 2003bSu et al 1998aSu et al ,b,c, 1999Su et al , 2000Fragneto et al 2000b;Holt et al 2000;Nylander et al 2001;Armstrong et al, 2004;Cooper et al 2005;Xu et al 2006bXu et al , 2007Xu et al , 2008Pan et al 2008), peptides (Holt et al 2000;Lu et al 2003aLu et al , 2004Lad et al 2007;Middelberg et al 2008;Zhao et al 2009), biosurfactants (Follows et al 2007) and lipid bilayers (Fragneto et al 2000a. A number of neutron reflection studies have also focused on interfacial interactions from binary mixtures of protein/surfactant (Lu et al 1998a,b;Green et al 2000aGreen et al ,b, 2001, protein or peptide/lipid (Schmidt et al 1992;Lu et al 2001;Miano et al 2007;Valincius et al 2008), protein/polymer Hollmann et al 2007Hollmann et al , 2008; Ganzevles et al 2008), DNA/polymer (Zhao et al 2008a), DNA/surfactant (Zhang et al 2008) and DNA/ lipid (Wu et al 2006). Several recent reviews have been dedicated to covering most of these systems (Lu 1999(Lu , 2002Fragneto-Cusani 2001;Krueger 2001;Thomas 2004;…”