In situ resonance energy transfer microscopy: monitoring membrane fusion in living cells

Fusion between fluid liposomes and intact bacteria: study of driving parameters and in vitro bactericidal efficacy

Background

Pseudomonas aeruginosa represents a good model of antibiotic resistance. These organisms have an outer membrane with a low level of permeability to drugs that is often combined with multidrug efflux pumps, enzymatic inactivation of the drug, or alteration of its molecular target. The acute and growing problem of antibiotic resistance of bacteria to conventional antibiotics made it imperative to develop new liposome formulations for antibiotics, and investigate the fusion between liposome and bacterium.

Methods

In this study, the factors involved in fluid liposome interaction with bacteria have been investigated. We also demonstrated a mechanism of fusion between liposomes (1,2-dipa lmitoyl-sn-glycero-3-phosphocholine [DPPC]/dimyristoylphosphatidylglycerol [DMPG] 9:1, mol/mol) in a fluid state, and intact bacterial cells, by lipid mixing assay.

Results

The observed fusion process is shown to be mainly dependent on several key factors. Perturbation of liposome fluidity by addition of cholesterol dramatically decreased the degree of fusion with P. aeruginosa from 44% to 5%. It was observed that fusion between fluid liposomes and bacteria and also the bactericidal activities were strongly dependent upon the properties of the bacteria themselves. The level of fusion detected when fluid liposomes were mixed with Escherichia coli (66%) or P. aeruginosa (44%) seems to be correlated to their outer membrane phosphatidylethanolamine (PE) phospholipids composition (91% and 71%, respectively). Divalent cations increased the degree of fusion in the sequence Fe²⁺ > Mg²⁺ > Ca²⁺ > Ba²⁺ whereas temperatures lower than the phase transition temperature of DPPC/DMPG (9:1) vesicles decreased their fusion capacity. Acidic as well as basic pHs conferred higher degrees of fusion (54% and 45%, respectively) when compared to neutral pH (35%).

Conclusion

Based on the results of this study, a possible mechanism involving cationic bridging between bacterial negatively charged lipopolysaccharide and fluid liposomes DMPG phospholipids was outlined. Furthermore, the fluid liposomal-encapsulated tobramycin was prepared, and the in vitro bactericidal effects were also investigated.

SNARE-mediated fusion of liposomes

Lipid-mixing assay is now commonly used to study protein, temperature and ion-dependent membrane fusion events. This assay has been crucial to demonstrate the ability of neuronal and non-neuronal soluble NSF attachment receptor (SNARE) to promote spontaneous fusion of liposomes. This lipid-mixing assay is based on the fluorescence resonance energy transfer (FRET) capability between a donor fluorescent lipid and a quenching lipid. When fusion between donor fluorescent liposomes and nonfluorescent acceptor liposome occurred, FRET decreases. This assay allows a real-time reading of SNARE-mediated liposome fusion.

Distribution of a glycosylphosphatidylinositol-anchored protein at the apical surface of MDCK cells examined at a resolution of <100 A using imaging fluorescence resonance energy transfer

Membrane microdomains ("lipid rafts") enriched in glycosylphosphatidylinositol (GPI)-anchored proteins, glycosphingolipids, and cholesterol have been implicated in events ranging from membrane trafficking to signal transduction. Although there is biochemical evidence for such membrane microdomains, they have not been visualized by light or electron microscopy. To probe for microdomains enriched in GPI- anchored proteins in intact cell membranes, we used a novel form of digital microscopy, imaging fluorescence resonance energy transfer (FRET), which extends the resolution of fluorescence microscopy to the molecular level (<100 A). We detected significant energy transfer between donor- and acceptor-labeled antibodies against the GPI-anchored protein 5' nucleotidase (5' NT) at the apical membrane of MDCK cells. The efficiency of energy transfer correlated strongly with the surface density of the acceptor-labeled antibody. The FRET data conformed to theoretical predictions for two-dimensional FRET between randomly distributed molecules and were inconsistent with a model in which 5' NT is constitutively clustered. Though we cannot completely exclude the possibility that some 5' NT is in clusters, the data imply that most 5' NT molecules are randomly distributed across the apical surface of MDCK cells. These findings constrain current models for lipid rafts and the membrane organization of GPI-anchored proteins.

Fluorescent probes for living cells

The functional characteristics of fluorescent probes used for imaging and measuring dynamic processes in living cells are reviewed. Initial consideration is given to general design requirements for delivery, targeting, detectability and fluorescence readout, and current technologies for attaining them. Discussion then proceeds to the more application-specific properties of intracellular ion indicators, membrane potential sensors, probes for proteins and lipids, and cell viability markers.