Rate constant of tension-induced pore formation in lipid membranes

Free Energy Analysis of Peptide-Induced Pore Formation in Lipid Membranes by Bridging Atomistic and Coarse-Grained Simulations

Antimicrobial peptides (AMPs) are attractive materials for combating the antimicrobial resistance crisis because they can kill target microbes by directly disrupting cell membranes. Although thousands of AMPs have been discovered, their molecular mechanisms of action are still poorly understood. One broad mechanism for membrane disruption is the formation of membrane-spanning hydrophilic pores which can be stabilized by AMPs. In this study, we use molecular dynamics simulations to investigate the thermodynamics of pore formation in model single-component lipid membranes in the presence of one of three AMPs: aurein 1.2, melittin and magainin 2. To overcome the general challenge of modeling long time scale membrane-related behaviors, including AMP binding, clustering, and pore formation, we develop a generalizable methodology for sampling AMP-induced pore formation. This approach involves the long equilibration of peptides around a pore created with a nucleation collective variable by performing coarse-grained simulations, then backmapping equilibrated AMP-membrane configurations to all-atom resolution. We then perform all-atom simulations to resolve free energy profiles for pore formation while accurately modeling the interplay of lipid-peptide-solvent interactions that dictate pore formation free energies. Using this approach, we quantify free energy barriers for pore formation without direct biases on peptides or whole lipids, allowing us to investigate mechanisms of pore formation for these 3 AMPs that are a consequence of unbiased peptide diffusion and clustering. Further analysis of simulation trajectories then relates variations in pore lining by AMPs, AMP-induced lipid disruptions, and salt bridges between AMPs to the observed pore formation free energies and corresponding mechanisms. This methodology and mechanistic analysis have the potential to generalize beyond the AMPs in this study to improve our understanding of pore formation by AMPs and related antimicrobial materials.

Interaction of anionic Fe3O4 nanoparticles with lipid vesicles: a review on deformation and poration under various conditions

This review focuses on the deformation and poration of lipid vesicles caused by the interaction of anionic magnetite nanoparticles (MNPs). Effects of various factors, such as surface charge density, salt and sugar concentrations in buffer, membrane cholesterol content, polymer-grafted phospholipid, and membrane potential have been discussed for the interaction of MNPs with lipid vesicles. To quantify these effects on the vesicles, compactness, fraction of deformation and poration, dynamics of membrane permeation, and kinetics of membrane permeation have been critically evaluated. The review explores the potential advancements as well as future directions of the research field in the biomedical application of MNPs.

Probability and kinetics of rupture and electrofusion in giant unilamellar vesicles under various frequencies of direct current pulses

Irreversible electroporation induces permanent permeabilization of lipid membranes of vesicles, resulting in vesicle rupture upon the application of a pulsed electric field. Electrofusion is a phenomenon wherein neighboring vesicles can be induced to fuse by exposing them to a pulsed electric field. We focus how the frequency of direct current (DC) pulses of electric field impacts rupture and electrofusion in cell-sized giant unilamellar vesicles (GUVs) prepared in a physiological buffer. The average time, probability, and kinetics of rupture and electrofusion in GUVs have been explored at frequency 500, 800, 1050, and 1250 Hz. The average time of rupture of many 'single GUVs' decreases with the increase in frequency, whereas electrofusion shows the opposite trend. At 500 Hz, the rupture probability stands at 0.45 ± 0.02, while the electrofusion probability is 0.71 ± 0.01. However, at 1250 Hz, the rupture probability increases to 0.69 ± 0.03, whereas the electrofusion probability decreases to 0.46 ± 0.03. Furthermore, when considering kinetics, at 500 Hz, the rate constant of rupture is (0.8 ± 0.1)×10-2 s-1, and the rate constant of fusion is (2.4 ± 0.1)×10-2 s-1. In contrast, at 1250 Hz, the rate constant of rupture is (2.3 ± 0.8)×10-2 s-1, and the rate constant of electrofusion is (1.0 ± 0.1)×10-2 s-1. These results are discussed by considering the electrical model of the lipid bilayer and the energy barrier of a prepore.

Processes and mechanisms underlying burst of giant unilamellar vesicles induced by antimicrobial peptides and compounds

To clarify the damage of lipid bilayer region in bacterial cell membrane caused by antimicrobial peptides (AMPs) and antimicrobial compounds (AMCs), their interactions with giant unilamellar vesicles (GUVs) of various lipid compositions have been examined. The findings revealed two main causes for the leakage: nanopore formation in the membrane and burst of GUVs. Although GUV burst has been explained previously based on the carpet model, the supporting evidence is limited. In this review, to better clarify the mechanism of GUV burst by AMPs, AMCs, and other membrane-active peptides, we described current knowledge of the conditions, characteristics, and detailed processes of GUV burst and the changes in the shape of the GUVs during burst. We identified several physical factors that affect GUV burst, such as membrane tension, electrostatic interaction, structural changes of GUV membrane such as membrane folding, and oil in the membrane. We also clarified one of the physical mechanisms underlying the instability of lipid bilayers that are associated with leakage in the carpet model. Based on these results, we propose a mechanism underlying some types of GUV burst induced by these substances: the growth of a nanopore to a micropore, resulting in GUV burst.

Effect of Phosphatidylethanolamine on Pore Formation Induced by the Antimicrobial Peptide PGLa

Most antimicrobial peptides (AMPs) induce pore formation and a burst of lipid bilayers and plasma membranes. This causes severe leakage of the internal contents and cell death. The AMP PGLa forms nanopores in giant unilamellar vesicles (GUVs) comprising dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylglycerol (DOPG). We here elucidated the effect of the line tension of a prepore rim on PGLa-induced nanopore formation by investigating the interaction of PGLa with single GUVs comprising dioleoylphosphatidylethanolamine (DOPE)/DOPG (6:4) in buffer using the single GUV method. We found that PGLa forms nanopores in the GUV membrane, which evolved into a local burst and burst of GUVs. The rate of pore formation in DOPE/DOPG-GUVs was smaller than that in DOPC/DOPG-GUVs. PGLa is located only in the outer leaflet of a GUV bilayer just before a fluorescent probe AF647 leakage from the inside, indicating that this asymmetric distribution induces nanopore formation. PGLa-induced local burst and burst of GUVs were observed at 10 ms-time resolution. After nanopore formation started, dense particles and small vesicles appeared in the GUVs, followed by a decrease in the GUV diameter. The GUV was finally converted into smaller GUV or lipid membrane aggregates. We discuss the mechanisms of PGLa-induced nanopore formation and its direct evolution to a GUV burst.

Determination of pore edge tension from the kinetics of rupture of giant unilamellar vesicles using the Arrhenius equation: effects of sugar concentration, surface charge and cholesterol

The pore edge tension (Γ) of a membrane closely intertwines with membrane stability and plays a vital role in the mechanisms that facilitate membrane resealing following pore formation caused by electrical and mechanical tensions. We have explored a straightforward procedure to determine Γ by fitting the inverse of the tension-dependent logarithm of the rate constant of rupture of giant unilamellar vesicles (GUVs) using the Arrhenius equation. The GUVs were prepared using a combination of 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) and 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) in a physiological environment. The effects of sugar concentration, membrane surface charge density, and membrane cholesterol concentration on Γ have been investigated. The values of Γ increase with sugar concentration in the physiological buffer, measuring 9.6 ± 0.3, 10.4 ± 0.1, and 16.2 ± 0.1 pN for 40, 100, and 300 mM, respectively. A higher concentration of anionic lipids (70 mol% of DOPG) significantly reduces Γ. An increasing trend of Γ with cholesterol content was observed; specifically, the values of Γ were 11.9 ± 0.9, 13.9 ± 0.7, and 16.2 ± 0.4 pN for 15, 29, and 40 mol% cholesterol, respectively. Thus, the presence of higher anionic lipids in the bilayer led to a decrease in membrane stability. In contrast, the presence of higher sugar concentrations in the buffer and increased cholesterol concentration in the membranes enhanced membrane stability.

Estimation of negative membrane tension in lipid bilayers and its effect on antimicrobial peptide magainin 2-induced pore formation

Positive membrane tension in the stretched plasma membrane of cells and in the stretched lipid bilayer of vesicles has been well analyzed quantitatively, whereas there is limited quantitative information on negative membrane tension in compressed plasma membranes and lipid bilayers. Here, we examined negative membrane tension quantitatively. First, we developed a theory to describe negative membrane tension by analyzing the free energy of lipid bilayers to obtain a theoretical equation for negative membrane tension. This allowed us to obtain an equation describing the negative membrane tension (σosm) for giant unilamellar vesicles (GUVs) in hypertonic solutions due to negative osmotic pressure (Π). Then, we experimentally estimated the negative membrane tension for GUVs in hypertonic solutions by measuring the rate constant (kr) of rupture of the GUVs induced by the constant tension (σex) due to an external force as a function of σex. We found that larger σex values were required to induce the rupture of GUVs under negative Π compared with GUVs in isotonic solution and quantitatively determined the negative membrane tension induced by Π (σosm) by the difference between these σex values. At small negative Π, the experimental values of negative σosm agree with their theoretical values within experimental error, but as negative Π increases, the deviation increases. Negative tension increased the stability of GUVs because higher tensions were required for GUV rupture, and the rate constant of antimicrobial peptide magainin 2-induced pore formation decreased.

Effects of membrane potentials on the electroporation of giant unilamellar vesicles

Living organisms maintain a resting membrane potential, which plays an important role in various biophysical and biological processes. In the context of medical applications, irreversible electroporation (IRE) is a non-thermal and minimally invasive technique that utilizes precisely controlled electric field pulses of micro- to millisecond durations to effectively ablate cancer and tumor cells. Previous studies on IRE-induced rupture of cell-mimetic giant unilamellar vesicles (GUVs) have primarily been conducted in the absence of membrane potentials. In this study, we investigated the electroporation of GUVs, including parameters such as the rate constant of rupture and the probability of rupture, in the presence of various negative membrane potentials. The membranes of GUVs were prepared using lipids and channel forming proteins. As the membrane potential increased from 0 to -90 mV, the rate constant of rupture showed a significant increase from (7.5 ± 1.6)×10-3 to (35.6 ± 5.5)×10-3 s-1. The corresponding probability of rupture also exhibited a notable increase from 0.40 ± 0.05 to 0.68 ± 0.05. To estimate the pore edge tension, the electric tension-dependent logarithm of the rate constant was fitted with the Arrhenius equation for different membrane potentials. The presence of membrane potential did not lead to any significant changes in the pore edge tension. The increase in electroporation is reasonably explained by the decrease in the prepore free energy barrier. The choice of buffer used in GUVs can significantly influence the kinetics of electroporation. This study provides valuable insights that can contribute to the application of electroporation techniques in the biomedical field.

Self-Assembly of Lipid Molecules under Shear Flows: A Dissipative Particle Dynamics Simulation Study

The self-assembly of lipid molecules in aqueous solution under shear flows was investigated using the dissipative particle dynamics simulation method. Three cases were considered: zero shear flow, weak shear flow and strong shear flow. Various self-assembled structures, such as double layers, perforated double layers, hierarchical discs, micelles, and vesicles, were observed. The self-assembly behavior was investigated in equilibrium by constructing phase diagrams based on chain lengths. Results showed the remarkable influence of chain length, shear flow and solution concentration on the self-assembly process. Furthermore, the self-assembly behavior of lipid molecules was analyzed using the system energy, particle number and shape factor during the dynamic processes, where the self-assembly pathways were observed and analyzed for the typical structures. The results enhance our understanding of biomacromolecule self-assembly in a solution and hold the potential for applications in biomedicine.

Kinetics of Nisin-Induced Pore Formation in Giant Unilamellar Vesicles

We have studied the kinetics of pore formation in giant unilamellar vesicles (GUV) with the antimicrobial peptide nisin. The role of charged lipid composition in the rate of pore formation by nisin in the vesicle membrane is investigated using fluorescence microscopy. We propose a model and obtain an analytical expression for the variation in the fluorescence intensity of a GUV as a function of time. We find that the analytical equation fits well to the experimental data, and the membrane surface potential can be estimated from the fit parameters. We further show that the formation of multiple pores on the vesicle membrane is affected by the charged lipid composition of the membrane.

Interaction of Macromolecular Chain with Phospholipid Membranes in Solutions: A Dissipative Particle Dynamics Simulation Study

The interaction between macromolecular chains and phospholipid membranes in aqueous solution was investigated using dissipative particle dynamics simulations. Two cases were considered, one in which the macromolecular chains were pulled along parallel to the membrane surfaces and another in which they were pulled vertical to the membrane surfaces. Several parameters, including the radius of gyration, shape factor, particle number, and order parameter, were used to investigate the interaction mechanisms during the dynamics processes by adjusting the pulling force strength of the chains. In both cases, the results showed that the macromolecular chains undergo conformational transitions from a coiled to a rod-like structure. Furthermore, the simulations revealed that the membranes can be damaged and repaired during the dynamic processes. The role of the pulling forces and the adsorption interactions between the chains and membranes differed in the parallel and perpendicular pulling cases. These findings contribute to our understanding of the interaction mechanisms between macromolecules and membranes, and they may have potential applications in biology and medicine.

Effect of Poloxamer Binding on the Elasticity and Toughness of Model Lipid Bilayers

Poloxamers, also known by their trade name, Pluronics, are known to mitigate damage to cellular membranes. However, the mechanism underlying this protection is still unclear. We investigated the effect of poloxamer molar mass, hydrophobicity, and concentration on the mechanical properties of giant unilamellar vesicles, composed of 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine, using micropipette aspiration (MPA). Properties including the membrane bending modulus (κ), stretching modulus (K), and toughness are reported. We found that poloxamers tend to decrease K, with an impact largely dictated by their membrane affinity, i.e., both a high molar mass and less hydrophilic poloxamers depress K at lower concentrations. However, a statistically significant effect on κ was not observed. Several poloxamers studied here showed evidence of membrane toughening. Additional pulsed-field gradient NMR measurements provided insight into how polymer binding affinity connects to the trends observed by MPA. This model study provides important insights into how poloxamers interact with lipid membranes to further understanding of how they protect cells from various types of stress. Furthermore, this information may prove useful for the modification of lipid vesicles for other applications, including use in drug delivery or as nanoreactors.

Antimicrobial peptide magainin 2-induced rupture of single giant unilamellar vesicles comprising E. coli polar lipids

Most antimicrobial peptides (AMPs) damage the cell membrane of bacterial cells and induce rapid leakage of the internal cell contents, which is a main cause of their bactericidal activity. One of the AMPs, magainin 2 (Mag), forms nanopores in giant unilamellar vesicles (GUVs) comprising phosphatidylcholine (PC) and phosphatidylglycerol (PG), inducing leakage of fluorescent probes. In this study, to elucidate the Mag-induced pore formation in lipid bilayer region in E. coli cell membrane, we examined the interaction of Mag with single GUVs comprising E. coli polar lipids (E. coli-lipid-GUVs). First, we investigated the Mag-induced leakage of a fluorescent probe AF488 from single E. coli-lipid-GUVs, and found that Mag caused rupture of GUVs, inducing rapid AF488 leakage. The rate constant of Mag-induced GUV rupture increased with the Mag concentration. Using fluorescence microscopy with a time resolution of 5 ms, we revealed the GUV rupture process: first, a small micropore was observed in the GUV membrane, then the pore radius increased within 50 ms without changing the GUV diameter, the thickness of the membrane at the pore rim concomitantly increased, and eventually membrane aggregates were formed. Mag bound to only the outer monolayer of the GUV before GUV rupture, which increased the area of the GUV bilayer. We also examined the physical properties of E. coli-lipid-GUVs themselves. We found that the rate constant of the constant tension-induced rupture of E. coli-lipid-GUVs was higher than that of PG/PC-GUVs. Based on these results, we discussed the Mag-induced rupture of E. coli-lipid-GUVs and its mechanism.

Effect of monolayer spontaneous curvature on constant tension-induced pore formation in lipid bilayers

The methodology of constant tension-induced rupture of giant unilamellar vesicles (GUVs) has provided information on tension-induced pore formation. This method was used to investigate the effect of spontaneous curvature (H₀) for a lipid monolayer on the rate constant (k_r) for constant tension (σ)-induced rupture, which originates from pore formation in lipid bilayers. Lipids were incorporated with different H₀ values into GUV membranes to change the overall H₀ value for the GUV monolayer. The dioleoylphosphatidylglycerol (DOPG)/dioleoylphosphatidylethanolamine (DOPE) (4/6, molar ratio, here and elsewhere) monolayer has a negative H₀, whereas the DOPG/dioleoylphosphatidylcholine (DOPC) (4/6) monolayer has an essentially zero H₀. A higher tension was required to induce the rupture of DOPG/DOPE (4/6)-GUVs compared with DOPG/DOPC (4/6)-GUVs. The line tension (Γ) for a pre-pore in DOPG/DOPE (4/6)-GUVs, determined by the analysis of the tension dependence of k_r, was 1.5 times larger than that in DOPG/DOPC (4/6)-GUVs. The k_r values for GUVs comprising DOPG/DOPC/18:1 lysophosphatidylcholine (LPC) (40/55/10), which has a positive H₀, were larger than those for DOPG/DOPC (4/6)-GUVs under the same tension. The Γ value for DOPG/DOPC/LPC (40/55/10)-GUVs was almost half that for DOPG/DOPC (4/6)-GUVs. These results indicate that Γ decreases with increasing H₀, which results in an increase in k_r. Based on these results, the effect of H₀ on k_r and Γ is discussed.

Self-Assembly of Lipid Mixtures in Solutions: Structures, Dynamics Processes and Mechanical Properties

The self-assembly of lipid mixtures in aqueous solution was investigated by dissipative particle dynamics simulation. Two types of lipid molecules were modelled, where three mixed structures, i.e., the membrane, perforated membrane and vesicle, were determined in the self-assembly processes. Phase behaviour was investigated by using the phase diagrams based on the tail chain lengths for the two types of lipids. Several parameters, such as chain number and average radius of gyration, were employed to explore the structural formations of the membrane and perforated membrane in the dynamic processes. Interface tension was used to demonstrate the mechanical properties of the membrane and perforated membrane in the equilibrium state and dynamics processes. Results help us to understand the self-assembly mechanism of the biomolecule mixtures, which has a potential application for designing the lipid molecule-based bio-membranes in solutions.

Effects of sugar concentration on the electroporation, size distribution and average size of charged giant unilamellar vesicles

We investigated the effects of sugar concentration on the electroporation, size distribution and average size of giant unilamellar vesicles (GUVs). GUVs were prepared from 40 mol% of 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) and 60 mol% of 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipids. Pulsed electric field was applied to the 40%DOPG/60%DOPC-GUVs and it induced lateral electric tension (σ_c) in the membranes of vesicles. The σ_c-induced probability of rupture (P_pore) and the rate constant of rupture (k_p) of GUVs under the sugar concentration, c = 40, 100 and 300 mM, were determined. Both the P_pore and k_p increased with the increase of σ_c, but higher tension was required to generate the same values of P_pore and k_p with increasing c. We also investigated average sizes of GUVs from the size distribution of vesicles under various sugar concentrations. With the increase of c, the peak of the size distribution histograms shifted to the region of smaller vesicles. The average size decreased 1.6-fold when c increased from 10 to 300 mM. These investigations help to understand various biomedical, biophysical, and biochemical processes in vesicles and cells. Electroporation, size distribution and average size of charged GUVs were investigated under various sugar concentrations. The sugar concentration influences the electroporation of vesicles and the average size of GUVs.

Quantification of pulsed electric field for the rupture of giant vesicles with various surface charges, cholesterols and osmotic pressures

Electropermeabilization is a promising phenomenon that occurs when pulsed electric field with high frequency is applied to cells/vesicles. We quantify the required values of pulsed electric fields for the rupture of cell-sized giant unilamellar vesicles (GUVs) which are prepared under various surface charges, cholesterol contents and osmotic pressures. The probability of rupture and the average time of rupture are evaluated under these conditions. The electric field changes from 500 to 410 Vcm^-1 by varying the anionic lipid mole fraction from 0 to 0.60 for getting the maximum probability of rupture (i.e., 1.0). In contrast, the same probability of rupture is obtained for changing the electric field from 410 to 630 Vcm^-1 by varying the cholesterol mole fraction in the membranes from 0 to 0.40. These results suggest that the required electric field for the rupture decreases with the increase of surface charge density but increases with the increase of cholesterol. We also quantify the electric field for the rupture of GUVs containing anionic mole fraction of 0.40 under various osmotic pressures. In the absence of osmotic pressure, the electric field for the rupture is obtained 430 Vcm^-1, whereas the field is 300 Vcm^-1 in the presence of 17 mOsmL^-1, indicating the instability of GUVs at higher osmotic pressures. These investigations open an avenue of possibilities for finding the electric field dependent rupture of cell-like vesicles along with the insight of biophysical and biochemical processes.

Lipid Membrane Binding and Cell Protection Efficacy of Poly(1,2-butylene oxide)-b-poly(ethylene oxide) Copolymers

Poloxamers consisting of poly(ethylene oxide) (PEO) and poly(propylene oxide) segments can protect cell membranes against various forms of stress. We investigated the role of the hydrophobic block chemistry on polymer/membrane binding and cell membrane protection by comparing a series of poly(butylene oxide)-b-PEO (PBO-b-PEO) copolymers to poloxamer analogues, using a combination of pulsed-field-gradient (PFG) NMR experiments and a lactate dehydrogenase (LDH) cell assay. We found that the more hydrophobic PBO-b-PEO copolymers bound more significantly to model liposomes composed of 1-palmitol-2-oleoyl-glycero-3-phosphocholine (POPC) compared to poly(propylene oxide) (PPO)/PEO copolymers. However, both classes of polymers performed similarly when compared by an LDH assay. These results present an important comparison between polymers with similar structures but with different binding affinities. They also provide mechanistic insight as enhanced polymer/lipid membrane binding did not directly translate to increased cell protection in the LDH assay, and therefore, additional factors need to be considered when trying to achieve greater membrane protection efficacy.

Effect of Osmotic Pressure on Pore Formation in Lipid bilayers by the Antimicrobial Peptide Magainin 2

Osmotic pressure (Π) induces membrane tension in cells and lipid vesicles, which may affect the activity of antimicrobial peptides (AMPs) by an unknown mechanism. We recently quantitated the membrane tension...

PoET: automated approach for measuring pore edge tension in giant unilamellar vesicles

Motivation

A reliable characterization of the membrane pore edge tension of single giant unilamellar vesicles (GUVs) requires the measurement of micrometer sized pores in hundreds to thousands of images. When manually performed, this procedure has shown to be extremely time-consuming and to generate inconsistent results among different users and imaging systems. A user-friendly software for such analysis allowing quick processing and generation of reproducible data had not yet been reported.

Results

We have developed a software (PoET) for automatic pore edge tension measurements on GUVs. The required image processing steps and the characterization of the pore dynamics are performed automatically within the software and its use allowed for a 30-fold reduction in the analysis time. We demonstrate the applicability of the software by comparing the pore edge tension of GUVs of different membrane compositions and surface charges. The approach was applied to electroporated GUVs but is applicable to other means of pore formation.

Availability and implementation

The complete software is implemented in Python and available for Windows at https://dx.doi.org/10.17617/3.7h.

Supplementary information

Supplementary data are available at Bioinformatics Advances online.

Recent developments in the kinetics of ruptures of giant vesicles under constant tension

External tension in membranes plays a vital role in numerous physiological and physicochemical phenomena. In this review, recent developments in the constant electric- and mechanical-tension-induced rupture of giant unilamellar vesicles (GUVs) are considered. We summarize the results relating to the kinetics of GUV rupture as a function of membrane surface charge, ions in the bathing solution, lipid composition, cholesterol content in the membrane, and osmotic pressure. The mechanical stability and line tension of the membrane under these conditions are discussed. The membrane tension due to osmotic pressure and the critical tension of rupture for various membrane compositions are also discussed. The results and their analysis provide a biophysical description of the kinetics of rupture, along with insight into biological processes. Future directions and possible developments in this research area are included.

Translocation of the nonlabeled antimicrobial peptide PGLa across lipid bilayers and its entry into vesicle lumens without pore formation

Fluorescent-probe-labeled peptides are used to study the interactions of peptides with cells and lipid vesicles but labeling peptides with fluorescent probes can significantly change these interactions. We recently developed a new method to detect the entry of nonlabeled peptides into the lumen of single giant unilamellar vesicles (GUVs). Here we applied this method to examine the interaction of the antimicrobial peptide PGLa with single GUVs to elucidate whether PGLa can enter the GUV lumen without pore formation. First, we examined the interaction of nonlabeled PGLa with single GUVs comprising dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC) (4/6) whose lumens contain the fluorescent probe AF647 and DOPG/DOPC (8/2)-large unilamellar vesicles encapsulating a high concentration of calcein. After a large lag period from starting the interaction with PGLa, the fluorescence intensity of the GUV lumen due to calcein (I_calcein) increased gradually without leakage of AF647, indicating that PGLa enters the GUV lumen without pore formation in the GUV membrane. The fraction of entry of PGLa increased with increasing PGLa concentration. Simultaneous measurement of the fractional area change of the GUV membrane (δ) and PGLa-induced increase in I_calcein showed that the entry of PGLa occurs only during the second increase in δ, indicating that PGLa enters the lumen during its translocation from the outer leaflet to the inner leaflet. The fraction of entry of PGLa without pore formation increased with increasing membrane tension. Based on these results, we discuss the elementary processes and the mechanism of the entry of PGLa into the GUV lumen.

To Close or to Collapse: The Role of Charges on Membrane Stability upon Pore Formation

Resealing of membrane pores is crucial for cell survival. Membrane surface charge and medium composition are studied as defining regulators of membrane stability. Pores are generated by electric field or detergents. Giant vesicles composed of zwitterionic and negatively charged lipids mixed at varying ratios are subjected to a strong electric pulse. Interestingly, charged vesicles appear prone to catastrophic collapse transforming them into tubular structures. The spectrum of destabilization responses includes the generation of long-living submicroscopic pores and partial vesicle bursting. The origin of these phenomena is related to the membrane edge tension, which governs pore closure. This edge tension significantly decreases as a function of the fraction of charged lipids. Destabilization of charged vesicles upon pore formation is universal-it is also observed with other poration stimuli. Disruption propensity is enhanced for membranes made of lipids with higher degree of unsaturation. It can be reversed by screening membrane charge in the presence of calcium ions. The observed findings in light of theories of stability and curvature generation are interpreted and mechanisms acting in cells to prevent total membrane collapse upon poration are discussed. Enhanced membrane stability is crucial for the success of electroporation-based technologies for cancer treatment and gene transfer.

Effects of osmotic pressure on the irreversible electroporation in giant lipid vesicles

Irreversible electroporation (IRE) is a nonthermal tumor/cell ablation technique in which a series of high-voltage short pulses are used. As a new approach, we aimed to investigate the rupture of giant unilamellar vesicles (GUVs) using the IRE technique under different osmotic pressures (Π), and estimated the membrane tension due to Π. Two categories of GUVs were used in this study. One was prepared with a mixture of dioleoylphosphatidylglycerol (DOPG), dioleoylphosphatidylcholine (DOPC) and cholesterol (chol) for obtaining more biological relevance while other with a mixture of DOPG and DOPC, with specific molar ratios. We determined the rate constant (kp) of rupture of DOPG/DOPC/chol (46/39/15)-GUVs and DOPG/DOPC (40/60)-GUVs induced by constant electric tension (σc) under different Π. The σc dependent kp values were fitted with a theoretical equation, and the corresponding membrane tension (σoseq) at swelling equilibrium under Π was estimated. The estimated membrane tension agreed well with the theoretical calculation within the experimental error. Interestingly, the values of σoseq were almost same for both types of synthesized GUVs under same osmotic pressure. We also examined the sucrose leakage, due to large osmotic pressure-induced pore formation, from the inside of DOPG/DOPC/chol(46/39/15)-GUVs. The estimated membrane tension due to large Π at which sucrose leaked out was very similar to the electric tension at which GUVs were ruptured without Π. We explained the σc and Π induced pore formation in the lipid membranes of GUVs.

Effect of membrane potential on entry of lactoferricin B-derived 6-residue antimicrobial peptide into single Escherichia coli cells and lipid vesicles

The antimicrobial peptide (AMP) derived from lactoferricin B, LfcinB (4-9) (RRWQWR), and lissamine rhodamine B red-labeled peptide (Rh-LfcinB (4-9)) exhibit strong antimicrobial activities, and they can enter Escherichia coli cells without damaging the cell membranes. Thus, these peptides are cell-penetrating peptide (CPP) -type AMPs. In this study, to elucidate the effect of the membrane potential (Δφ) on the action of the CPP-type AMP, Rh-LfcinB (4-9), we investigated the interactions of Rh-LfcinB (4-9) with single E. coli cells and spheroplasts containing calcein in the cytosol using confocal laser scanning microscopy. At low peptide concentrations, Rh-LfcinB (4-9) entered the cytosol of single E. coli cells and spheroplasts without damaging the cell membranes, and the H⁺-ionophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) suppressed its entry. The studies using the time-kill method indicate that these low concentrations of peptide exhibit antimicrobial activity but CCCP inhibits this activity. Next, we investigated the effect of Δφ on the interaction of Rh-LfcinB (4-9) with single giant unilamellar vesicles (GUVs) comprising E. coli polar lipid extracts and containing a fluorescent probe, Alexa Fluor 647 hydrazide. At low concentrations (0.2-0.5 μM), Rh-LfcinB (4-9) showed significant entry to the single GUV lumen without pore formation in the presence of Δφ. The fraction of entry of peptide increased with increasing negative membrane potential, indicating that the rate of peptide entry into the GUV lumen increased with increasing negative membrane potential. These results indicate that Δφ enhances the entry of Rh-LfcinB (4-9) into single E. coli cells, spheroplasts, and GUVs and its antimicrobial activity.IMPORTANCE: Bacterial cells have a membrane potential (Δφ), but the effect of Δφ on action of cell-penetrating peptide-type antimicrobial peptides (AMPs) is not clear. Here, we investigated the effect of Δφ on the action of fluorescent probe-labeled AMP derived from lactoferricin B, Rh-LfcinB (4-9). At low peptide concentrations, Rh-LfcinB (4-9) enters the cytosol of Escherichia coli cells and spheroplasts without damaging their cell membrane, but a protonophore suppresses this entry and its antimicrobial activity. The rate of entry of Rh-LfcinB (4-9) into the giant unilamellar vesicles (GUVs) comprising E. coli lipids without pore formation increases with increasing Δφ. These results indicate that Δφ enhances the antimicrobial activity of Rh-LfcinB (4-9) and hence LfcinB (4-9) by increasing the rate of their entry into the cytosol.

An investigation into the critical tension of electroporation in anionic lipid vesicles

Irreversible electroporation (IRE) is a technique for the disruption of localized cells or vesicles by a series of short and high-frequency electric pulses which has been used for tissue ablation and treatment in certain diseases. It is well reported that IRE induces lateral tension in the membranes of giant unilamellar vesicles (GUVs). The GUVs are prepared by a mixture of anionic lipid dioleoylphosphatidylglycerol (DOPG) and neutral lipid dioleoylphosphatidylcholine (DOPC) using the natural swelling method. Here the influence of DOPG mole fraction, XDOPG, on the critical tension of electroporation in GUVs has been investigated in sodium chloride-containing PIPES buffer. The critical tension decreases from 9.0 ± 0.3 to 6.0 ± 0.2 mN/m with the increase of XDOPG from 0.0 to 0.60 in the membranes of GUVs. Hence an increase in XDOPG greatly decreases the mechanical stability of membranes. We develop a theoretical equation that fits the XDOPG dependent normalized critical tension, and obtain a binding constant for the lipid-ion interaction of 0.75 M-1. The decrease in the energy barrier for formation of the nano-size nascent or prepore state, due to the increase in XDOPG, is the main factor explaining the decrease in critical tension of electroporation in vesicles.

Interaction between Bottlebrush Polymers and Phospholipid Membranes in Solutions

In this work, the interactions between bottlebrush polymers and phospholipid membranes were investigated using dissipative particle dynamics simulations. The weak and strong adsorption phenomena between the polymers and membranes were examined by calculating the system parameters. A spring model was introduced to explain the variances in the shape factors and the radius of gyration of the bottlebrush polymers, as well as the order parameters of the phospholipid membrane in the pulling processes. This work provides further understanding for the application of bottlebrush polymers in biological processes.

Electrostatic effects on the electrical tension-induced irreversible pore formation in giant unilamellar vesicles

Irreversible electroporation (IRE) is a new technique in which a series of short pulses with high frequency electrical energy is applied on the targeted regions of cells or vesicles for their destruction or rupture formation. IRE induces lateral tension in the membranes of vesicles. We have investigated the electrostatic interaction effects on the constant electrical tension-induced rate constant of irreversible pore formation in the membranes of giant unilamellar vesicles (GUVs). The electrostatic interaction has been varied by changing the salt concentration in buffer and the surface charge density of membranes. The membranes of GUVs are synthesized by a mixture of negatively charged lipid dioleoylphosphatidylglycerol (DOPG) and neutral lipid dioleoylphosphatidylcholine (DOPC) using the natural swelling method. The rate constant of pore formation increases with the decrease of salt concentration in buffer along with the increase of surface charge density of membranes. The tension dependent probability of pore formation and the rate constant of pore formation are fitted to the theoretical equation, and obtained the line tension of membranes. The decrease in energy barrier of a prepore due to electrostatic interaction is the key factor causing an increase of rate constant of pore formation.

Influence of cholesterol on electroporation in lipid membranes of giant vesicles

Irreversible electroporation (IRE) is primarily a nonthermal ablative technology that uses a series of high-voltage and ultra-short pulses with high-frequency electrical energy to induce cell death. This paper presents the influence of cholesterol on the IRE-induced probability of pore formation and the rate constant of pore formation in giant unilamellar vesicles (GUVs). The GUVs are prepared by a mixture of dioleoylphosphatidylglycerol (DOPG), dioleoylphosphatidylcholine (DOPC) and cholesterol using the natural swelling method. An IRE signal of frequency 1.1 kHz is applied to the membranes of GUVs. The probability of pore formation and the rate constant of pore formation events are obtained using statistical analysis from several single GUVs. The time-dependent fraction of intact GUVs among all those examined is fitted to a single exponential decay function from where the rate constant of pore formation is calculated. The probability of pore formation and the rate constant of pore formation decreases with an increase in cholesterol content in the membranes of GUVs. Theoretical equations are fitted to the tension-dependent rate constant of pore formation and to the probability of pore formation, which allows us to obtain the line tension of membranes. The obtained line tension increases with an increase in cholesterol in the membranes. The increase in the energy barrier of the prepore state, due to the increase of cholesterol in membranes, is the main factor explaining the decrease in the rate constant of pore formation.

Kinetics of irreversible pore formation under constant electrical tension in giant unilamellar vesicles

Stretching in the plasma membranes of cells and lipid membranes of vesicles plays important roles in various physiological and physicochemical phenomena. Irreversible electroporation (IRE) is a minimally invasive non-thermal tumor ablation technique where a series of short electrical energy pulses with high frequency is applied to destabilize the cell membranes. IRE also induces lateral tension due to stretching in the membranes of giant unilamellar vesicles (GUVs). Here, the kinetics of irreversible pore formation under constant electrical tension in GUVs has been investigated. The GUVs are prepared by a mixture of dioleoylphosphatidylglycerol and dioleoylphosphatidylcholine using the natural swelling method. An IRE signal of frequency 1.1 kHz is applied to the GUVs through a gold-coated electrode system. Stochastic pore formation is observed for several 'single GUVs' at a particular constant tension. The time course of the fraction of intact GUVs among all the examined GUVs is fitted with a single-exponential decay function from which the rate constant of pore formation in the vesicle, k_p, is calculated. The value of k_p increases with an increase of membrane tension. An increase in the proportion of negatively charged lipids in a membrane gives a higher k_p. Theoretical equations are fitted to the tension-dependent k_p and to the probability of pore formation, which allows us to obtain the line tension of the membranes. The decrease in the energy barrier for formation of the nano-size nascent or prepore state, due to the increase in electrical tension, is the main factor explaining the increase of k_p.

Deformation and poration of giant unilamellar vesicles induced by anionic nanoparticles

The interaction of anionic magnetite nanoparticles (MNPs) of size 18 nm with negatively charged giant unilamellar vesicles (GUVs) formed from a mixture of neutral dioleoylphosphatidylcholine (DOPC) and negatively charged dioleoylphosphatidylglycerol (DOPG) lipids has been investigated. It has been obtained that NPs induces the deformation of spherical GUVs. The reaction of other GUVs on NPs consists in the appearance of pores in their membranes. We focused the effect of electrostatics on the interaction of charged membranes with MNPs. To study the influence of the surface charge of GUVs on the processes under consideration, we varied the fraction of DOPG in the vesicles from 0 to 100%. We examined the influence of salt concentration in the range of 50-300 mM NaCl concentration. To describe the degree of deformation, a special parameter compactness was introduced. The pore formation in the membranes of GUVs was investigated by the leakage of sucrose. The compactness increases with time and also NPs concentration. The fraction of deformed GUVs increases with the increase of surface charge density of membranes as well as the decrease of salt concentration in buffer. The value of compactness for neutral membrane is 1.25 times higher than that of charged ones. The fraction of deformed GUVs become constant after 20 min, however it increases with NPs concentration. The time taken for stochastic pore formation is less for charged membrane than neutral one. The physical mechanism explaining the experimental results obtained in these investigations.

Role of Membrane Potential on Entry of Cell-Penetrating Peptide Transportan 10 into Single Vesicles

Cell-penetrating peptides (CPPs) can translocate across plasma membranes to enter the cytosol of eukaryotic cells without decreasing cell viability. We revealed the mechanism underlying this translocation by examining the effect of membrane potential, φ_m, on the entry of a CPP, transportan 10 (TP10), into the lumen of single giant unilamellar vesicles (GUVs). For this purpose, we used the single GUV method to detect the entry of carboxyfluorescein (CF)-labeled TP10 (CF-TP10) into the lumen of single GUVs. First, we used various K⁺ concentration differences to apply different negative membrane potentials on single GUVs containing gramicidin A in their membrane and confirmed these potentials using the φ_m-sensitive fluorescent probe 3,3'-dihexyloxacarbocyanine iodine. The fluorescence intensity of the GUV membranes (i.e., the rim intensity) due to 3,3'-dihexyloxacarbocyanine iodine increased with |φ_m| up to 118 mV, and its dependence on |φ_m| less than 28 mV agreed with a theoretical estimation (i.e., the dye concentration in the inner leaflet of a GUV is larger than that in the outer leaflet according to the Boltzmann distribution). We then examined the effect of φ_m on the entry of CF-TP10 into GUVs using single GUVs containing small GUVs or large unilamellar vesicles inside the mother GUV lumen. We found that CF-TP10 entered the GUV lumen without pore formation and the rate of entry of CF-TP10 into the GUV lumen, V_entry, increased with an increase in |φ_m|. The rim intensity due to CF-TP10 increased with an increase in |φ_m|, indicating that the CF-TP10 concentration in the inner leaflet of the GUV increased with |φ_m|. These results indicate that the φ_m-induced elevation in V_entry can be explained by the increase in CF-TP10 concentration in the inner leaflet with |φ_m|. We discuss the mechanism underlying this effect of membrane potential based on the pre-pore model of the translocation of CF-TP10 across a GUV membrane.

Effects of electrically-induced constant tension on giant unilamellar vesicles using irreversible electroporation

Stretching in membranes of cells and vesicles plays important roles in various physiological and physicochemical phenomena. Irreversible electroporation (IRE) is the irreversible permeabilization of the membrane through the application of a series of electrical field pulses of micro- to millisecond duration. IRE induces lateral tension due to stretching in the membranes of giant unilamellar vesicles (GUVs). However, the effects of electrically induced (i.e., IRE) constant tension in the membranes of GUVs have not been investigated yet in detail. To explore the effects of electrically induced tension on GUVs, firstly a microcontroller-based IRE technique is developed which produces electric field pulses (332 V/cm) with pulse width 200 µs. Then the electrodeformation, electrofusion and membrane rupture of GUVs are investigated at various constant tensions in which the membranes of GUVs are composed of dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC). Stochastic electropore formation is observed in the membranes at an electrically induced constant tension in which the probability of pore formation is increased with the increase of tension from 2.5 to 7.0 mN/m. The results of pore formation at different electrically-induced constant tensions are in agreement with those reported for mechanically-induced constant tension. The decrease in the energy barrier of the pre-pore state due to the increase of electrically-induced tension is the main factor increasing the probability of electropore formation. These investigations help to provide an understanding of the complex behavior of cells/vesicles in electric field pulses and can form the basis for practical applications in biomedical technology.

Understanding Cell Penetration of Cyclic Peptides

Approximately 75% of all disease-relevant human proteins, including those involved in intracellular protein-protein interactions (PPIs), are undruggable with the current drug modalities (i.e., small molecules and biologics). Macrocyclic peptides provide a potential solution to these undruggable targets because their larger sizes (relative to conventional small molecules) endow them the capability of binding to flat PPI interfaces with antibody-like affinity and specificity. Powerful combinatorial library technologies have been developed to routinely identify cyclic peptides as potent, specific inhibitors against proteins including PPI targets. However, with the exception of a very small set of sequences, the vast majority of cyclic peptides are impermeable to the cell membrane, preventing their application against intracellular targets. This Review examines common structural features that render most cyclic peptides membrane impermeable, as well as the unique features that allow the minority of sequences to enter the cell interior by passive diffusion, endocytosis/endosomal escape, or other mechanisms. We also present the current state of knowledge about the molecular mechanisms of cell penetration, the various strategies for designing cell-permeable, biologically active cyclic peptides against intracellular targets, and the assay methods available to quantify their cell-permeability.

Spontaneous and Stress-Induced Pore Formation in Membranes: Theory, Experiments and Simulations

The large plasticity, dynamics and adaptability of biological membranes allow different modes of intrinsic and inducible permeability. These phenomena are of physiological importance for a number of natural functions related to cell death and can also be manipulated artificially for practical purposes like gene transfer, drug delivery, prevention of infections or anticancer therapy. For these advances to develop in a controllable and specific way, we need a sufficient understanding of the membrane permeability phenomena. Since the formulation of early concepts of pore formation, there has been an enormous effort to describe membrane permeability by using theory, simulations and experiments. A major breakthrough has come recently through theoretical developments that allow building continuous trajectories of pore formation both in the absence and presence of stress conditions. The new model provides a coherent quantitative view of membrane permeabilization, useful to test the impact of known lipid properties, make predictions and postulate specific pore intermediates that can be studied by simulations. For example, this theory predicts unprecedented dependencies of the line tension on the pore radius and on applied lateral tension which explain previous puzzling results. In parallel, important concepts have also come from molecular dynamics simulations, of which the role of water for membrane permeabilization is of special interest. These advances open new challenges and perspectives for future progress in the study of membrane permeability, as experiments and simulations will need to test the theoretical predictions, while theory achieves new refinements that provide a physical ground for observations.

Nanotube-Mediated Path to Protocell Formation

Cellular compartments are membrane-enclosed, spatially distinct microenvironments that confine and protect biochemical reactions in the biological cell. On the early Earth, the autonomous formation of compartments is thought to have led to the encapsulation of nucleotides, thereby satisfying a starting condition for the emergence of life. Recently, surfaces have come into focus as potential platforms for the self-assembly of prebiotic compartments, as significantly enhanced vesicle formation was reported in the presence of solid interfaces. The detailed mechanism of such formation at the mesoscale is still under discussion. We report here on the spontaneous transformation of solid-surface-adhered lipid deposits to unilamellar membrane compartments through a straightforward sequence of topological changes, proceeding via a network of interconnected lipid nanotubes. We show that this transformation is entirely driven by surface-free energy minimization and does not require hydrolysis of organic molecules or external stimuli such as electrical currents or mechanical agitation. The vesicular structures take up and encapsulate their external environment during formation and can subsequently separate and migrate upon exposure to hydrodynamic flow. This may link the self-directed transition from weakly organized bioamphiphile assemblies on solid surfaces to protocells with secluded internal contents.

The role of membrane tension in the action of antimicrobial peptides and cell-penetrating peptides in biomembranes

For antimicrobial peptides (AMPs) with antimicrobial and bactericidal activities and cell-penetrating peptides (CPPs) with activity to permeate through plasma membrane, their interactions with lipid bilayer region in plasma membrane play important roles in these functions. However, the elementary processes and mechanisms of their functions have not been clear. The single giant unilamellar vesicle (GUV) method has revealed the details of elementary processes of interaction of some AMPs and CPPs with lipid vesicles. In this review, we summarize the mode of action of AMPs such as magainin 2 (Mag) and CPPs such as transportan 10 (TP10), revealed by the single GUV methods, and especially we focus on the role of membrane tension in actions of Mag and TP10 and the mechanisms of their actions. First, we explain the characteristics of the single GUV method briefly. Next, we summarize the recent view on the effect of tension on physical properties of lipid bilayers and describe the role of tension in actions of Mag and TP10. Some experimental results indicate that Mag-induced pore is a stretch-activated pore. The effect of packing of transbilayer asymmetric lipid on Mag-induced pore formation is described. On the other hand, entry of fluorescent dye, carboxyfluorescein (CF)-labeled TP10 (i.e., CF-TP10), into single GUVs without pore formation is affected by tension and high concentration of cholesterol. Pre-pore model for translocation of CF-TP10 across lipid bilayer is described. The experimental methods and their analysis described here are useful for investigation of functions of the other types of AMPs, CPPs, and proteins.

Free energy analysis of membrane pore formation process in the presence of multiple melittin peptides

Understanding the molecular mechanism underlying pore formation in lipid membranes by antimicrobial peptides is of great importance in biological sciences as well as in drug design applications. Melittin has been widely studied as a pore forming peptide, though the molecular mechanism for pore formation is still illusive. We examined the free energy barrier for the creation of a pore in lipid membranes with and without multiple melittin peptides. It was found that six melittin peptides significantly stabilized a pore, though a small barrier (a few k_BT) for the formation still existed. With five melittin peptides or fewer, the pore formation barrier was much higher, though the established pore was in a local energy minimum. Although seven melittins effectively reduced the free energy barrier, a single melittin peptide left the pore after a long time MD simulation probably because of the overcrowded environment around the bilayer pore. Thus, it is highly selective for the number of melittin peptides to stabilize the membrane pore, as was also suggested by the line tension evaluations. The free energy cost required to insert a single melittin into the membrane is too high to explain the one-by-one insertion mechanism for pore formation, which also supports the collective melittin mechanism for pore formation.

Interaction of Giant Unilamellar Vesicles with the Surface Nanostructures on Dragonfly Wings

The waxy epicuticle of dragonfly wings contains a unique nanostructured pattern that exhibits bactericidal properties. In light of emerging concerns of antibiotic resistance, these mechano-bactericidal surfaces represent a particularly novel solution by which bacterial colonization and the formation of biofilms on biomedical devices can be prevented. Pathogenic bacterial biofilms on medical implant surfaces cause a significant number of human deaths every year. The proposed mechanism of bactericidal activity is through mechanical cell rupture; however, this is not yet well understood and has not been well characterized. In this study, we used giant unilamellar vesicles (GUVs) as a simplified cell membrane model to investigate the nature of their interaction with the surface of the wings of two dragonfly species, Austrothemis nigrescens and Trithemis annulata, sourced from Victoria, Australia, and the Baix Ebre and Terra Alta regions of Catalonia, Spain. Confocal laser scanning microscopy and cryo-scanning electron microscopy techniques were used to visualize the interactions between the GUVs and the wing surfaces. When exposed to both natural and gold-coated wing surfaces, the GUVs were adsorbed on the surface, exhibiting significant deformation, in the process of membrane rupture. Differences between the tensile rupture limit of GUVs composed of 1,2-dioleoyl- sn-glycero-3-phosphocholine and the isotropic tension generated from the internal osmotic pressure were used to indirectly determine the membrane tensions, generated by the nanostructures present on the wing surfaces. These were estimated as being in excess of 6.8 mN m^-1, the first experimental estimate of such mechano-bactericidal surfaces. This simple model provides a convenient bottom-up approach toward understanding and characterizing the bactericidal properties of nanostructured surfaces.

Elementary Processes and Mechanisms of Interactions of Antimicrobial Peptides with Membranes-Single Giant Unilamellar Vesicle Studies

To elucidate the mechanisms of action of antimicrobial peptides (AMPs) and to develop de novo designed peptides with activities similar to those of AMPs, it is essential to elucidate the detailed processes of AMP interactions with plasma membranes of bacterial and fungal cells and model membranes (lipid bilayers). In this mini-review, we summarize the present state of knowledge of the interactions of AMPs with lipid vesicles obtained using the single giant unilamellar vesicle (GUV) method. Currently, three modes of action of AMPs on GUVs have been defined. The elementary processes of interactions of AMPs with lipid vesicles revealed by the single GUV method, and the advantages of this technique, are described and discussed. For example, the single GUV method can be used to determine rate constants of AMP-induced pore formation or local rupture and membrane permeation of internal contents through the pore or the local rupture, the transbilayer movement of lipids, and the relationship between the location of AMPs and pore formation. Effects of membrane tension and of asymmetric lipid packing in the bilayer on AMP-induced pore formation also are described. On the basis of these data, we discuss the present state of understanding of the interaction of AMPs with lipid bilayers and future prospects for AMP studies.

Nanoparticle-Engendered Rupture of Lipid Membranes

Tension-induced rupture of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) lipid membranes with encapsulated hydrophobic nanoparticles is elucidated using dissipative particle dynamics simulations. The dynamics of hole formation is studied, and a nanoparticle size-dependent relationship is established for the probability of membrane rupture within a given time as a function of the membrane tension. Two mechanisms of hole formation are explored: homogeneous nucleation and heterogeneous nucleation at the nanoparticle surface. While the kinetics of homogeneous nucleation in unloaded membranes complies with the predictions of the classical Deryagin-Gutop (DG) theory, the heterogeneous nucleation causes progressively lower lysis tensions as the particle size increases. The thermodynamics of heterogeneous nucleation is treated by introducing an effective contact angle at the three-phase, solvent-membrane-solid boundary into the DG theory. The proposed approach helps quantitatively interpret the simulation results and predict the membrane stability in real experiments with significantly larger (by many orders of magnitude) observation times and spatial scales.

Dynamic Fingering in Adhered Lipid Membranes

Artificial lipid membranes incorporating proteins have frequently been used as models for the dynamic organization of biological structures in living cells as well as in the development of biology-inspired technologies. We report here on the experimental demonstration and characterization of a pattern-forming process that occurs in a lipid bilayer membrane adhered via biotin-avidin binding to a second lipid membrane that is supported by a solid substrate. Adhesion regions are roughly circular with a diameter of about 25 μm. Using confocal fluorescence microscopy, we record time series of dynamic fingering patterns that grow in the upper lipid membrane and intermembrane biotin-avidin bonds. The fingers are micrometer-scale elongated pores that grow from the edge of an already-stabilized hole. Finger growth is saltatory on the scale of tens of seconds. We find that as the fingers grow and the density of adhesion proteins increases, the rate of finger growth decreases exponentially and the width of newly formed fingers decreases linearly. We show that these findings are consistent with a thermodynamic description of dynamic pore formation and stabilization.

Mechanism of Initial Stage of Pore Formation Induced by Antimicrobial Peptide Magainin 2

Antimicrobial peptide magainin 2 forms pores in lipid bilayers, a property that is considered the main cause of its bactericidal activity. Recent data suggest that tension or stretching of the inner monolayer plays an important role in magainin 2-induced pore formation in lipid bilayers. Here, to elucidate the mechanism of magainin 2-induced pore formation, we investigated the effect on pore formation of asymmetric lipid distribution in two monolayers. First, we developed a method to prepare giant unilamellar vesicles (GUVs) composed of dioleoylphosphatidylglycerol (DOPG), dioleoylphosphatidylcholine (DOPC), and lyso-PC (LPC) in the inner monolayer and of DOPG/DOPC in the outer monolayer. We consider that in these GUVs, the lipid packing in the inner monolayer was larger than that in the outer monolayer. Next, we investigated the interaction of magainin 2 with these GUVs with an asymmetric distribution of LPC using the single GUV method, and found that the rate constant of magainin 2-induced pore formation, k_p, decreased with increasing LPC concentration in the inner monolayer. We constructed a quantitative model of magainin 2-induced pore formation, whereby the binding of magainin 2 to the outer monolayer of a GUV induces stretching of the inner monolayer, causing pore formation. A theoretical equation defining k_p as a function of magainin 2 surface concentration, X, reasonably explains the experimental relationship between k_p and X. This model quantitatively explains the effect on k_p of the LPC concentration in the inner monolayer. On the basis of these results, we discuss the mechanism of the initial stage of magainin 2-induced pore formation.

Pore formation in lipid membrane II: Energy landscape under external stress

Lipid membranes are extremely stable envelopes allowing cells to survive in various environments and to maintain desired internal composition. Membrane permeation through formation of transversal pores requires substantial external stress. Practically, pores are usually formed by application of lateral tension or transmembrane voltage. Using the same approach as was used for obtaining continuous trajectory of pore formation in the stress-less membrane in the previous article, we now consider the process of pore formation under the external stress. The waiting time to pore formation proved a non-monotonous function of the lateral tension, dropping from infinity at zero tension to a minimum at the tension of several millinewtons per meter. Transmembrane voltage, on the contrary, caused the waiting time to decrease monotonously. Analysis of pore formation trajectories for several lipid species with different spontaneous curvatures and elastic moduli under various external conditions provided instrumental insights into the mechanisms underlying some experimentally observed phenomena.

Pore formation in lipid membrane I: Continuous reversible trajectory from intact bilayer through hydrophobic defect to transversal pore

Lipid membranes serve as effective barriers allowing cells to maintain internal composition differing from that of extracellular medium. Membrane permeation, both natural and artificial, can take place via appearance of transversal pores. The rearrangements of lipids leading to pore formation in the intact membrane are not yet understood in details. We applied continuum elasticity theory to obtain continuous trajectory of pore formation and closure, and analyzed molecular dynamics trajectories of pre-formed pore reseal. We hypothesized that a transversal pore is preceded by a hydrophobic defect: intermediate structure spanning through the membrane, the side walls of which are partially aligned by lipid tails. This prediction was confirmed by our molecular dynamics simulations. Conversion of the hydrophobic defect into the hydrophilic pore required surmounting some energy barrier. A metastable state was found for the hydrophilic pore at the radius of a few nanometers. The dependence of the energy on radius was approximately quadratic for hydrophobic defect and small hydrophilic pore, while for large radii it depended on the radius linearly. The pore energy related to its perimeter, line tension, thus depends of the pore radius. Calculated values of the line tension for large pores were in quantitative agreement with available experimental data.