Biology subjects

Stephan, M.

Publications and source records attributed to Stephan, M..

3 recordsLinked to original sources

bioRxiv2023

Tunable biomimetic bacterial membranes from binary and ternary lipid mixtures and their application in antimicrobial testing

Reconstruction of accurate yet simplified mimetic models of cell membranes is a very challenging goal of synthetic biology. To date, most of the research focuses on the development of eukaryotic cell membranes, while reconstitution of their prokaryotic counterparts has not been fully addressed, and the proposed models do not reflect well the complexity of bacterial cell envelopes. Here, we describe the reconstitution of biomimetic bacterial membranes with an increasing level of complexity, developed from binary and ternary lipid mixtures. Giant unilamellar vesicles composed of phosphatidylcholine (PC) and phosphatidylethanolamine (PE); PC and phosphatidylglycerol (PG); PE and PG; PE, PG and cardiolipin (CA) at varying molar ratios were successfully prepared by the electroformation method. Each of the proposed mimetic models focuses on reproducing specific membrane features such as membrane charge, curvature, leaflets asymmetry, or the presence of phase separation. GUVs were characterized in terms of size distribution, surface charge, and lateral organization. Finally, the developed models were tested against the lipopeptide antibiotic daptomycin. The obtained results showed a clear dependency of daptomycin binding efficiency on the amount of negatively charged lipid species present in the membrane. We anticipate that the models proposed here can be applied not only in antimicrobial testing but also serve as platforms for studying fundamental biological processes in bacteria as well as their interaction with physiologically relevant biomolecules.

biophysics↗

bioRxiv2023

Bilayer charge asymmetry destabilizes membranes upon poration

Transmembrane asymmetry is ubiquitous in cells, particularly with respect to lipids, where charged lipids are mainly restricted to one monolayer. We investigate the influence of anionic lipid asymmetry on the stability of giant unilamellar vesicles (GUVs), minimal plasma membrane models. To quantify asymmetry, we apply the fluorescence quenching assay, which is often difficult to reproduce and caution in handling the quencher is generally underestimated. Thus, we first optimize this assay and then apply it to GUVs prepared with the inverted emulsion transfer protocol using increasing fractions of anionic lipids restricted to one leaflet. This protocol is found to produce highly asymmetric bilayers, but with [~]20% interleaflet mixing. To probe the stability of asymmetric vs symmetric membranes, we expose the GUVs to porating electric pulses and monitor the fraction of destabilized vesicles. The pulses open macropores, and the GUVs either completely recover or exhibit leakage or bursting/collapse. Residual oil destabilizes porated membranes and destabilization is even more pronounced in asymmetrically charged membranes. This is corroborated by the measured pore edge tension decrease with increasing charge asymmetry. Using GUVs with membranes with imposed transmembrane pH asymmetry, we confirm that poration-triggered destabilization does not depend on the approach used to generate membrane asymmetry.

biophysics↗

bioRxiv2022

Effect of leaflet asymmetry on mechanical properties of lipid bilayers with phosphatidic acid

The asymmetry of membranes has a significant impact on their biophysical characteristics and behavior. This study investigates the composition and mechanical properties of symmetric and asymmetric membranes in giant unilamellar vesicles (GUVs) made of phosphatidylcholine (POPC) and phosphatidic acid (POPA). A combination of fluorescence quantification, zeta potential measurements, micropipette aspiration and bilayer molecular dynamics simulations are used to characterize these membranes. The outer leaflet composition in vesicles is found consistent across the two preparation methods we employed, namely electroformation and inverted emulsion transfer. However, characterizing the inner leaflet poses challenges. Micropipette aspiration of GUVs show that oil residues do not substantially alter membrane elasticity, but simulations reveal increased membrane thickness and decreased interleaflet coupling in the presence of oil. Asymmetric membranes with a POPC:POPA mixture in the outer leaflet and POPC in the inner leaflet display similar stretching elasticity values to symmetric POPC:POPA membranes, suggesting potential POPA insertion into the inner leaflet during vesicle formation and suppressed asymmetry. The inverse compositional asymmetry, with POPC in the outer leaflet and POPC:POPA in the inner yield less stretchable membranes with higher compressibility modulus compared to their symmetric counterparts. Challenges in achieving and predicting compositional correspondence highlight the limitations of phase-transfer-based methods. Additionally, caution is advised when using fluorescently labeled lipids (even at low fractions of 0.5 mol%), as unexpected gel-like domains in symmetric POPC:POPA membranes were observed only with a specific type of labeled DOPE (dioleoylphosphatidylethanolamine) and the same fraction of unlabeled DOPE. The latter suggest that such phase separation may result from interactions between lipids and membrane fluorescent probes. Overall, this study underscores the complexity of factors influencing GUV membrane asymmetry, emphasizing the need for further research and improvement of characterization techniques. SIGNIFICANCEAsymmetrically charged lipid bilayer models are superior to commonly used symmetrical ones, exhibiting naturally present asymmetry, thereby exhibiting a more adequate range of biophysical membrane characteristics better reflecting biological membranes. This study focuses on the mechanical properties of phosphatidic acid (PA)-enriched membranes, a crucial lipid for cellular lipid metabolism, e.g. glycerophospholipid synthesis, and for signal transduction. Micropipette aspiration, fluorescent PA-sensor, and zeta potential studies demonstrate that asymmetric membranes are less stretchable than symmetric ones. Accompanying in silico studies on the symmetric membranes confirm that oil impurities do not influence the membrane stretching elasticity but increase its thickness and decrease the coupling of the two leaflets, which sheds light on the elastic behavior of experimental models of asymmetric lipid bilayers.

biophysics↗