A novel assay for detecting fusion pore formation: implications for the fusion mechanism

The explicit role of interfacial hydration during polyethylene glycol induced lipid fusion: a THz spectroscopic investigation

While the phenomenon of excipient mediated membrane fusion has been studied widely, the inherent role of interfacial hydration involved in the process has mostly remained unaddressed. Here we report the experimental validation of the fact that PEG-induced membrane fusion is associated with the dehydration of the membrane(s). We explore the explicit hydration behavior at three different lipids (DOPC, POPC and DPPC) membranes with different aliphatic tails as they undergo fusogenic transition in the presence of PEG of average molecular weight of 4000 using THz-FTIR spectroscopy in the frequency window of 1.5-13.5 THz. Dynamic light scattering and electron microscopic measurements confirm the formation of different intermediate steps of the liposomes during the fusion process: bilayer aggregation, destabilization and finally lipid fusion. We observe that membrane hydration follows a systematic trend with the lipid specificity as the fusion process sets in.

Leaflet by Leaflet Synergistic Effects of Antimicrobial Peptides on Bacterial and Mammalian Membrane Models

Antimicrobial peptides (AMPs) offer significant hope in the fight against antibiotic resistance. Operating via a mechanism different from that of antibiotics, they target the microbial membrane and ideally should damage it without impacting mammalian cells. Here, the interactions of two AMPs, magainin 2 and PGLa, and their synergistic effects on bacterial and mammalian membrane models were studied using electrochemical impedance spectroscopy, atomic force microscopy (AFM), and fluorescence correlation spectroscopy. Toroidal pore formation was observed by AFM when the two AMPs were combined, while individually AMP effects were confined to the exterior leaflet of the bacterial membrane analogue. Using microcavity-supported lipid bilayers, the diffusivity of each bilayer leaflet could be studied independently, and we observed that combined, the AMPs penetrate both leaflets of the bacterial model but individually each peptide had a limited impact on the proximal leaflet of the bacterial model. The impact of AMPs on a ternary, mammalian mimetic membrane was much weaker.

Combination of Oleic Acid and the gp41 Fusion Peptide Switches the Phosphatidylethanolamine-Induced Membrane Fusion Mechanism from a Nonclassical to a Classical Stalk Model

Membrane fusion is considered to be one of the crucial processes for the existence of eukaryotes and the entry of enveloped viruses into host cells. The fusion mechanism depends on the lipid composition of the membrane as well as the properties of fusion proteins or peptides. The gp41 fusion peptide from the human immunodeficiency virus (HIV) is known to catalyze membrane fusion by altering the physical properties of the membrane. Earlier, we demonstrated that a membrane containing 30 mol % phosphatidylethanolamine (PE) circumvents the classical stalk model because of its intrinsic negative curvature. In this work, we demonstrated how the gp41 fusion peptide influences the fusion mechanism of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dioleoyl-sn-glycero-3-phos-pho¬ethanolamine (DOPE) (70/30 mol %) membranes. We further evaluated the effect of the same peptide on the mechanism of fusion for membranes containing 30 mol % PE and a fatty acid with an intrinsic positive curvature (oleic acid (OA)). Our results show that gp41 switches the fusion mechanism from a nonclassical to a classical stalk model when membranes contain OA, but fails to do so for DOPC/DOPE membranes. This could be due to the extreme influence of the intrinsic negative curvature of PE, which is partially downregulated in the presence of OA.

Elucidation of the mechanism and energy barrier for anesthetic triggered membrane fusion in model membranes

Membrane fusion is vital for cellular function and is generally mediated via fusogenic proteins and peptides. The mechanistic details and subsequently the transition state dynamics of membrane fusion will be dependent on the type of the fusogenic agent. We have previously established the potential of general anesthetics as a new class of fusion triggering agents in model membranes. We employed two-photon excitation fluorescence cross-correlation spectroscopy (TPE-FCCS) to report on vesicle association kinetics and steady-state fluorescence dequenching assays to monitor lipid mixing kinetics. Using halothane to trigger fusion in 110 nm diameter dioleoylphosphatidylcholine (DOPC) liposomes, we found that lipid rearrangement towards the formation of the fusion stalk was rate limiting. The activation barrier for halothane induced membrane fusion in 110 nm vesicles was found to be ∼40 kJ mol−1. We calculated the enthalpy and entropy of the transition state to be ∼40 kJ mol−1and ∼180 J mol−1K−1, respectively. We have found that the addition of halothane effectively lowers the energy barrier for membrane fusion in less curved vesicles largely due to entropic advantages.