Lipid Bilayers and Membranes

TPP-based mitocans: a potent strategy for anticancer drug design

Cancer is one of the most important problems that endanger human health. The number of cancer patients is increasing rapidly worldwide. Compared with normal cells, cancer cells exhibit abnormal metabolism (abnormal glycolysis and oxidative phosphorylation, high levels of reactive oxygen species, anti-apoptosis, high mitochondrial membrane potential, and so on), and specific targeting of these metabolic abnormalities would be a promising drug design direction. These physiological characteristics are closely related to tumorigenesis and development, which are mainly regulated by mitochondria. Therefore, mitochondria have become important anticancer drug targets, attracting much attention in recent years. In this review, we systematically summarize various mitochondrial anticancer drugs developed, especially mitocans based on triphenylphosphonium (TPP), and discuss the advantages of TPP in endowing mitochondrial targeting function.

Effect of Transmembrane Electric Field on GM1 Containing DMPC-Cholesterol Monolayer: A Computational Study

Transmembrane electric potentials and membrane curvature have always provided pathways to mediate different cellular processes. We present results of molecular dynamics (MD) simulations of lipid monolayer composed of 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol (CHOL) under a transverse electric field to monitor the effect of electric field on membrane containing ganglioside monosialo 1 (GM1). Four systems were studied with membrane monolayer in the presence and absence of GM1 with and without applying electric field along the normal of the monolayer. The applied transmembrane electric field was 0.4 mV/Å which corresponds to the action potential of animal cell. Our results indicate that the electric field induces a considerable lateral stress on the monolayer in the presence of GM1, which is evident from the lateral pressure profiles. It was found that due to the application of electric field major perturbation was caused to the system containing GM1, manifested by the bending of the monolayer. We believe this study provides correlation between electric field and spontaneous membrane bending, specially based on the membrane composition. The consequences of these MD simulations provide considerable insights to different biological phenomenon and lipid membrane models.

Evaluation of bending modulus of lipid bilayers using undulation and orientation analysis

In the current paper, phospholipid bilayers are modeled using coarse-grained molecular dynamics simulations with the MARTINI force field. The extracted molecular trajectories are analyzed using Fourier analysis of the undulations and orientation vectors to establish the differences between the two approaches for evaluating the bending modulus. The current work evaluates and extends the implementation of the Fourier analysis for molecular trajectories using a weighted horizon-based averaging approach. The effect of numerical parameters in the analysis of these trajectories is explored by conducting parametric studies. Computational modeling results are validated against experimentally characterized bending modulus of lipid membranes using a shape fluctuation analysis. The computational framework is then used to estimate the bending moduli for different types of lipids (phosphocholine, phosphoethanolamine, and phosphoglycerol). This work provides greater insight into the numerical aspects of evaluating the bilayer bending modulus, provides validation for the orientation analysis technique, and explores differences in bending moduli based on differences in the lipid nanostructures.

GM1 Softens POPC Membranes and Induces the Formation of Micron-Sized Domains

The influence of the glycolipid GM1 on the physical properties of POPC membranes was studied systematically by using different methods applied to giant and large unilamellar vesicles. The charge per GM1 molecule in the membrane was estimated from electrophoretic mobility measurements. Optical microscopy and differential scanning calorimetry were employed to construct a partial phase diagram of the GM1/POPC system. At room temperature, phase separation in the membrane was detected for GM1 fractions at and above ∼5 mol %, whereby GM1-rich gel-like domains were observed by fluorescent microscopy. Fluctuation analysis, vesicle electrodeformation, and micropipette aspiration were used to assess the bending rigidity of the membrane as a function of GM1 content. In the fluid phase, GM1 was shown to strongly soften the bilayer. In the region of coexistence of fluid and gel-like domains, the micropipette aspiration technique allowed measurements of the bending rigidity of the fluid phase only, whereas electrodeformation and fluctuation analysis were affected by the presence of the gel-phase domains. The observation that GM1 decreased the bilayer bending rigidity is important for understanding the role of this ganglioside in the flexibility of neuronal membranes.

Sugar does not affect the bending and tilt moduli of simple lipid bilayers

The diffuse X-ray scattering method has been applied to samples composed of SOPC, DOPC, DMPC, and POPC with added sugar, either sucrose, glucose, fructose, maltose, or trehalose. Several sugar concentrations in the range 200-500 mM were investigated for each of the lipid/sugar samples. We observed no systematic change in the bending modulus KC or in the tilt modulus Kθ with increasing sugar concentration. The average values of both these moduli were the same as those of the respective pure lipid controls within statistical uncertainty of 2%. These results are inconsistent with previous reports of sugar concentration dependent values of KC.

Solvent-assisted lipid self-assembly at hydrophilic surfaces: factors influencing the formation of supported membranes

As a simple and efficient technique, the solvent-assisted lipid bilayer (SALB) formation method offers a versatile approach to fabricating a planar lipid bilayer on solid supports. Corresponding mechanistic aspects and the role of various governing parameters remain, however, to be better understood. Herein, we first scrutinized the effect of lipid concentration (0.01 to 5 mg/mL) and solvent type (isopropanol, n-propanol, or ethanol) on SALB formation on silicon oxide in order to identify optimal conditions for this process. The obtained fluid-phase lipid layers on silicon oxide were investigated by using the quartz crystal microbalance with dissipation monitoring, epifluorescence microscopy, and atomic force microscopy. The experimental results indicate that, in alcohol, lipid attachment to the substrate is reversible and reaches equilibrium in accordance with the bulk lipid concentration. During the solvent-exchange step, the water fraction increases and the deposited lipids are converted into planar bilayer fragments, along with the concurrent adsorption and rupture of micelles within an optimal lipid concentration range. In addition, fluid-phase lipid bilayers were successfully formed on other substrates (e.g., chrome, indium tin oxide, and titanium oxide) that are largely intractable to conventional methods (e.g., vesicle fusion). Moreover, gel-phase lipid bilayers were fabricated as well. Depending on the phase state of the lipid bilayer during fabrication, the corresponding adlayer mass varied by approximately 20% between the fluid- and gel-phase states in a manner which is consistent with the molecular packing of lipids in the two arrangements. Taken together, our findings help to explain the mechanistic details of SALB formation, optimize the corresponding procedure, and demonstrate the general utility for fabricating gel- and fluid-phase planar lipid bilayers.

Contribution of temperature to deformation of adsorbed vesicles studied by nanoplasmonic biosensing

With increasing temperature, biological macromolecules and nanometer-sized aggregates typically undergo complex and poorly understood reconfigurations, especially in the adsorbed state. Herein, we demonstrate the strong potential of using localized surface plasmon resonance (LSPR) sensors to address challenging questions related to this topic. By employing an LSPR-based gold nanodisk array platform, we have studied the adsorption of sub-100-nm diameter 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid vesicles on titanium oxide at two temperatures, 23 and 50 °C. Inside this temperature range, DPPC lipid vesicles undergo the gel-to-fluid phase transition accompanied by membrane area expansion, while DOPC lipid vesicles remain in the fluid-phase state. To interpret the corresponding measurement results, we have derived general equations describing the effect of deformation of adsorbed vesicles on the LSPR signal. At the two temperatures, the shape of adsorbed DPPC lipid vesicles on titanium oxide remains nearly equivalent, while DOPC lipid vesicles become less deformed at higher temperature. Adsorption and rupture of DPPC lipid vesicles on silicon oxide were also studied for comparison. In contrast to the results obtained on titanium oxide, adsorbed vesicles on silicon oxide become more deformed at higher temperature. Collectively, the findings demonstrate that increasing temperature may ultimately promote, hinder, or have negligible effect on the deformation of adsorbed vesicles. The physics behind these observations is discussed, and helps to clarify the interplay of various, often hidden, factors involved in adsorption of biological macromolecules at interfaces.

Bending lipid membranes: experiments after W. Helfrich's model

Current description of biomembrane mechanics originates for a large part from W. Helfrich's model. Based on his continuum theory, many experiments have been performed in the past four decades on simplified membranes in order to characterize the mechanical properties of lipid membranes and the contribution of polymers or proteins. The long-term goal was to develop a better understanding of the mechanical properties of cell membranes. In this paper, we will review representative experimental approaches that were developed during this period and the main results that were obtained.

What are the true values of the bending modulus of simple lipid bilayers?

Values of the bending modulus KC are reviewed, and possible causes for the considerable differences are discussed. One possible cause is the use of glucose and sucrose in the classical micromechanical manipulation and shape analysis methods. New data, using the more recent low angle X-ray method, are presented that do not support an effect of glucose or sucrose on KC. Another possible cause is using an incomplete theory to interpret the data. Adding a tilt term to the theory clearly does not affect the value obtained from the shape analysis method. It is shown that a tilt term, using a value of the modulus Kθ indicated by simulations, theory, and estimated from order parameters obtained from NMR and from the wide angle X-ray method, should also not affect the value obtained using the micromechanical manipulation method, although it does require a small correction when determining the value of the area compressibility modulus KA. It is still being studied whether including a tilt term will significantly affect the values of KC obtained using low angle X-ray data. It remains unclear what causes the differences in the experimental values of KC for simple lipid bilayers.