Model Phospholipid Liposomes to Study the β-Amyloid-Peptide-Induced Membrane Disruption

Model phospholipid liposomes have been utilized widely to study the molecular interactions between peptides and membrane bilayers. In the mechanistic study of Alzheimer's disease (AD), disruption of neuronal cell membranes has been considered as a major contribution for the β-amyloid (Aβ) peptides' neurotoxicity. However, clear interpretation of the Aβ-induced cellular membrane at high-resolution level is challenging because of the co-existence of multiple pathways. Here we present the generation of simplified model liposome systems that will facilitate the in-depth mechanistic studies. Protocols for the preparation of model liposomes and the characterization of individual membrane disruption effects will be described.

The influenza fusion peptide promotes lipid polar head intrusion through hydrogen bonding with phosphates and N-terminal membrane insertion depth

Influenza infection requires fusion between the virus envelope and a host cell endosomal membrane. The influenza hemagglutinin fusion peptide (FP) is essential to viral membrane fusion. It was recently proposed that FPs would fuse membranes by increasing lipid tail protrusion, a membrane fusion transition state. The details of how FPs induce lipid tail protrusion, however, remain to be elucidated. To decipher the molecular mechanism by which FPs promote lipid tail protrusion, we performed molecular dynamics simulations of the wild-type (WT) FP, fusogenic mutant F9A, and nonfusogenic mutant W14A in model bilayers. This article presents the peptide-lipid interaction responsible for lipid tail protrusion and a related lipid perturbation, polar head intrusion, where polar heads are sunk under the membrane surface. The backbone amides from the four N-terminal peptide residues, deeply inserted in the membrane, promoted both perturbations through H bonding with lipid phosphates. Polar head intrusion correlated with peptides N-terminal insertion depth and activity: the N-termini of WT and F9A were inserted deeper into the membrane than nonfusogenic W14A. Based on these results, we propose that FP-induced polar head intrusion would complement lipid tail protrusion in catalyzing membrane fusion by reducing repulsions between juxtaposed membranes headgroups. The presented model provides a framework for further research on membrane fusion and influenza antivirals.

Peptides containing membrane-interacting motifs inhibit herpes simplex virus type 1 infectivity

Herpes simplex virus (HSV) membrane fusion represents an attractive target for anti-HSV therapy. To investigate the structural basis of HSV membrane fusion and identify new targets for inhibition, we have investigated the different membranotropic domains of HSV-1 gH envelope glycoprotein. We observed that fusion peptides when added exogenously are able to inhibit viral fusion likely by intercalating with viral fusion peptides upon adopting functional structure in membranes. Interestingly, peptides analogous to the predicted HSV-1 gH loop region inhibited viral plaque formation more significantly. Their inhibitory effect appears to be a consequence of their ability to partition into membranes and aggregate within them. Circular dichroism spectra showed that peptides self-associate in aqueous and lipidic solutions, therefore the inhibition of viral entry may occur via peptides association with their counterpart on wild-type gH. The antiviral activity of HSV-1 peptides tested provides an attractive basis for the development of new fusion peptide inhibitors corresponding to regions outside the fusion protein heptad repeat regions.

Close is not enough: SNARE-dependent membrane fusion requires an active mechanism that transduces force to membrane anchors

Is membrane fusion an essentially passive or an active process? It could be that fusion proteins simply need to pin two bilayers together long enough, and the bilayers could do the rest spontaneously. Or, it could be that the fusion proteins play an active role after pinning two bilayers, exerting force in the bilayer in one or another way to direct the fusion process. To distinguish these alternatives, we replaced one or both of the peptidic membrane anchors of exocytic vesicle (v)- and target membrane (t)-SNAREs (soluble N-ethylmaleimide-sensitive fusion protein [NSF] attachment protein [SNAP] receptor) with covalently attached lipids. Replacing either anchor with a phospholipid prevented fusion of liposomes by the isolated SNAREs, but still allowed assembly of trans-SNARE complexes docking vesicles. This result implies an active mechanism; if fusion occurred passively, simply holding the bilayers together long enough would have been sufficient. Studies using polyisoprenoid anchors ranging from 15-55 carbons and multiple phospholipid-containing anchors reveal distinct requirements for anchors of v- and t-SNAREs to function: v-SNAREs require anchors capable of spanning both leaflets, whereas t-SNAREs do not, so long as the anchor is sufficiently hydrophobic. These data, together with previous results showing fusion is inhibited as the length of the linker connecting the helical bundle-containing rod of the SNARE complex to the anchors is increased (McNew, J.A., T. Weber, D.M. Engelman, T.H. Sollner, and J.E. Rothman, 1999. Mol. Cell. 4:415-421), suggests a model in which one activity of the SNARE complex promoting fusion is to exert force on the anchors by pulling on the linkers. This motion would lead to the simultaneous inward movement of lipids from both bilayers, and in the case of the v-SNARE, from both leaflets.