Rapid preparation of giant unilamellar vesicles

Modular microfluidics for double emulsion formation

For many engineering applications such as manipulating two phase flows, generating single and double emulsions, and passively propelling liquids through channels, control over the surface energy of microfluidic channels is essential. In particular, double emulsion formation, which benefits from alternating hydrophobic and hydrophilic sections of channel, represents a challenge in fabricating controlled microfluidic channel surface properties. As double emulsions find further applications in single-cell handling and analysis, straightforward methods for generating them increase in value. Here, we present a method for generating double emulsions in microfluidic channels fabricated from modular fluidic blocks. By using a vapor-phase polymer coating technology-initiated chemical vapor deposition-we are able to fabricate blocks with varying surface properties. Assembling these blocks together then creates step-like changes in surface energy within a microchannel.

Kinetically Stable Triglyceride-Based Nanodroplets and Their Interactions with Lipid-Specific Proteins

Understanding of the interactions between proteins and natural and artificially prepared lipid membrane surfaces and embedded nonpolar cores is important in studies of physiological processes and their pathologies and is applicable to nanotechnologies. In particular, rapidly growing interest in cellular droplets defines the need for simplified biomimetic lipid model systems to overcome in vivo complexity and variability. We present a protocol for the preparation of kinetically stable nanoemulsions with nanodroplets composed of sphingomyelin (SM) and cholesterol (Chol), as amphiphilic surfactants, and trioleoylglycerol (TOG), at various molar ratios. To prepare stable SM/Chol-coated monodisperse lipid nanodroplets, we modified a reverse phase evaporation method and combined it with ultrasonication. Lipid composition, ζ-potential, gyration and hydrodynamic radius, shape, and temporal stability of the lipid nanodroplets were characterized and compared to extruded SM/Chol large unilamellar vesicles. Lipid nanodroplets and large unilamellar vesicles with theoretical SM/Chol/TOG molar ratios of 1/1/4.7 and 4/1/11.7 were further investigated for the orientational order of their interfacial water molecules using a second harmonic scattering technique, and for interactions with the SM-binding and Chol-binding pore-forming toxins equinatoxin II and perfringolysin O, respectively. The surface characteristics (ζ-potential, orientational order of interfacial water molecules) and binding of these proteins to the nanodroplet SM/Chol monolayers were similar to those for the SM/Chol bilayers of the large unilamellar vesicles and SM/Chol Langmuir monolayers, in terms of their surface structures. We propose that such SM/Chol/TOG nanoparticles with the required lipid compositions can serve as experimental models for monolayer membrane to provide a system that imitates the natural lipid droplets.

Membrane Remodeling of Giant Vesicles in Response to Localized Calcium Ion Gradients

In a wide variety of fundamental cell processes, such as membrane trafficking and apoptosis, cell membrane shape transitions occur concurrently with local variations in calcium ion concentration. The main molecular components involved in these processes have been identified; however, the specific interplay between calcium ion gradients and the lipids within the cell membrane is far less known, mainly due to the complex nature of biological cells and the difficultly of observation schemes. To bridge this gap, a synthetic approach is successfully implemented to reveal the localized effect of calcium ions on cell membrane mimics. Establishing a mimic to resemble the conditions within a cell is a severalfold problem. First, an adequate biomimetic model with appropriate dimensions and membrane composition is required to capture the physical properties of cells. Second, a micromanipulation setup is needed to deliver a small amount of calcium ions to a particular membrane location. Finally, an observation scheme is required to detect and record the response of the lipid membrane to the external stimulation. This article offers a detailed biomimetic approach for studying the calcium ion-membrane interaction, where a lipid vesicle system, consisting of a giant unilamellar vesicle (GUV) connected to a multilamellar vesicle (MLV), is exposed to a localized calcium gradient formed using a microinjection system. The dynamics of the ionic influence on the membrane were observed using fluorescence microscopy and recorded at video frame rates. As a result of the membrane stimulation, highly curved membrane tubular protrusions (MTPs) formed inside the GUV, oriented away from the membrane. The described approach induces the remodeling of the lipid membrane and MTP production in an entirely contactless and controlled manner. This approach introduces a means to address the details of calcium ion-membrane interactions, providing new avenues to study the mechanisms of cell membrane reshaping.

Receptor-Based Peptides for Inhibition of Leukotoxin Activity

The Gram-negative bacterium Aggregatibacter actinomycetemcomitans, commonly associated with localized aggressive periodontitis (LAP), secretes an RTX (repeats-in-toxin) protein leukotoxin (LtxA) that targets human white blood cells, an interaction that is driven by its recognition of the lymphocyte function-associated antigen-1 (LFA-1) integrin. In this study, we report on the inhibition of LtxA-LFA-1 binding as an antivirulence strategy to inhibit LtxA-mediated cytotoxicity. Specifically, we designed and synthesized peptides corresponding to the reported LtxA binding domain on LFA-1 and characterized their capability to inhibit LtxA binding to LFA-1 and subsequent cytotoxic activity in human immune cells. We found that several of these peptides, corresponding to sequential β-strands in the LtxA-binding domain of LFA-1, inhibit LtxA activity, demonstrating the effectiveness of this approach. Further investigations into the mechanism by which these peptides inhibit LtxA binding to LFA-1 reveal a correlation between toxin-peptide affinity and LtxA-mediated cytotoxicity, leading to a diminished association between LtxA and LFA-1 on the cell membrane. Our results demonstrate the possibility of using target-based peptides to inhibit LtxA activity, and we expect that a similar approach could be used to hinder the activity of other RTX toxins.

Mastering Complexity: Towards Bottom-up Construction of Multifunctional Eukaryotic Synthetic Cells

With the ultimate aim to construct a living cell, bottom-up synthetic biology strives to reconstitute cellular phenomena in vitro - disentangled from the complex environment of a cell. Recent work towards this ambitious goal has provided new insights into the mechanisms governing life. With the fast-growing library of functional modules for synthetic cells, their classification and integration become increasingly important. We discuss strategies to reverse-engineer and recombine functional parts for synthetic eukaryotes, mimicking the characteristics of nature's own prototype. Particularly, we focus on large outer compartments, complex endomembrane systems with organelles, and versatile cytoskeletons as hallmarks of eukaryotic life. Moreover, we identify microfluidics and DNA nanotechnology as two technologies that can integrate these functional modules into sophisticated multifunctional synthetic cells.

Synthesis of sericin-based conjugates by click chemistry: enhancement of sunitinib bioavailability and cell membrane permeation

Sericin is a natural protein that has been used in biomedical and pharmaceutical fields as raw material for polypeptide-based drug delivery systems (DDSs). In this paper, it has been employed as pharmaceutical biopolymer for the production of sunitinib-polypeptide conjugate. The synthesis has been carried out by simple click reaction in water, using the redox couple l-ascorbic acid/hydrogen peroxide as a free radical grafting initiator. The bioconjugate molecular weight (50 kDa < Mw < 75 kDa) was obtained by SDS-PAGE, while the spectroscopic characteristics have been studied in order to reveal the presence of grafted sunitinib. In both FT-IR and UV/Vis spectra, signals corresponding to sunitinib functional groups have been identified. Since sunitinib is an anticancer drug characterized by low bioavailability and low permeability, the bioconjugation aimed at their enhancement. In vitro studies demonstrated that bioavailability has been increased to almost 74%, compared with commercial formulation. Also cell membrane permeability has been augmented in in vitro tests, in which membrane models have been used to determine the lipid membrane/physiological fluid partition coefficient (Kp). The log(Kp) value of the bioconjugate was increased to over 4. This effect resulted in a three-fold decrease of IC50 value against MCF-7 cells.

Lipid-Bilayer Dynamics Probed by a Carbon Dot-Phospholipid Conjugate

Elucidating the dynamic properties of membranes is important for understanding fundamental cellular processes and for shedding light on the interactions of proteins, drugs, and viruses with the cell surface. Dynamic studies of lipid bilayers have been constrained, however, by the relatively small number of pertinent molecular probes and the limited physicochemical properties of the probes. We show that a lipid conjugate comprised of a fluorescent carbon dot (C-dot) covalently attached to a phospholipid constitutes a versatile and effective vehicle for studying bilayer dynamics. The C-dot-modified phospholipids readily incorporated within biomimetic membranes, including solid-supported bilayers and small and giant vesicles, and inserted into actual cellular membranes. We employed the C-dot-phospholipid probe to elucidate the effects of polymyxin-B (a cytolytic peptide), valproic acid (a lipophilic drug), and amyloid-β (a peptide associated with Alzheimer's disease) upon bilayer fluidity and lipid dynamics through the application of various biophysical techniques.

Influence of the environmental tonicity perturbations on the release of model compounds from large unilamellar vesicles (LUVs): A mechanistic investigation

In this work, the influence of environmental tonicity perturbations on the size and release kinetics of model markers from liposomes (calcein and rhodamine) was investigated. Large unilamellar vesicles (LUVs) were prepared from a mixture composed of organic solvents containing dissolved phosphatidylcholine and phosphate buffered saline (PBS, pH 7.4). Organic phase was removed by rotary evaporation and the obtained liposomal dispersions were extruded to reduce the liposomal sizes to approx. 400 nm. The LUVs were exposed to PBS of different tonicity to induce water migration, and consequently, generate an osmotic pressure on the vesicle membranes. The markers release kinetics were studied by the dialysis method employing Franz diffusion cells. LUVs appeared to be more susceptible to the osmotic swelling than the shrinking and the size changes were significantly more pronounced for calcein-loaded LUVs in comparison to rhodamine-loaded LUVs. The calcein release from LUVs was highly affected by the water influx/efflux, whereas rhodamine release was less affected by the tonicity perturbations. Mechanistically, it appeared that hydrophilic molecules (calcein) followed the water flux, whereas lipophilic molecules (rhodamine) seemed to be more affected by the changes in LUVs size and consequent alteration of the tightness of the phospholipid bilayer (where the lipophilic marker was imbedded in). These results demonstrate that the different tonicity (within the inner core and external environment of vesicles) can enhance/hamper the diffusion of a marker from LUVs and that osmotically active liposomes could be used as a novel controlled drug delivery system.

Polymalic Acid Tritryptophan Copolymer Interacts with Lipid Membrane Resulting in Membrane Solubilization

Anionic polymers with membrane permeation functionalities are highly desirable for secure cytoplasmic drug delivery. We have developed tritryptophan containing copolymer (P/WWW) of polymalic acid (PMLA) that permeates membranes by a mechanism different from previously described PMLA copolymers of trileucine (P/LLL) and leucine ethyl ester (P/LOEt) that use the "barrel stave" and "carpet" mechanism, respectively. The novel mechanism leads to solubilization of membranes by forming copolymer "belts" around planar membrane "packages." The formation of such packages is supported by results obtained from studies including size-exclusion chromatography, confocal microscopy, and fluorescence energy transfer. According to this "belt" mechanism, it is hypothesized that P/WWW first attaches to the membrane surface. Subsequently the hydrophobic tryptophan side chains translocate into the periphery and insert into the lipid bilayer thereby cutting the membrane into packages. The reaction is driven by the high affinity between the tryptophan residues and lipid side chains resulting in a stable configuration. The formation of the membrane packages requires physical agitation suggesting that the success of the translocation depends on the fluidity of the membrane. It is emphasized that the "belt" mechanism could specifically function in the recognition of abnormal cells with high membrane fluidity and in response to hyperthermia.

Giant vesicles (GV) in colloidal system under the optical polarization microscope (OPM)

This paper discusses the unprecedented microscopic findings of micellar growth in colloidal system (CS) of catalyzed piperidinolysis of ionized phenyl salicylate (PS^-). The giant vesicles (GV) was observed under the optical polarization microscope (OPM) at [NaX]=0.1M where X=3-isopropC₆H₄O^-. The conditions were rationalized from pseudo-first-order rate constant, k_obs of PS^- of micellar phase at 31.1×10^-3s^-1 reported in previous publication. The overall diameter of GV (57.6μm) in CS (CTABr/NaX/H₂O)-catalyzed piperidinolysis (where X=3-isopropC₆H₄O) of ionized phenyl salicylate were found as giant unilamellar vesicles (GUV) and giant multilamellar vesicles (GMV). The findings were also validated by means of rheological analysis.

Microscopic Evidence for the Correlation of Micellar Structures and Counterion Binding Constant for Flexible Nanoparticle Catalyzed Piperidinolysis of PS− in Colloidal System

The interest to determine the ionization degree of counterion initiates the need to study the relationship between counterion binding constant (RX Br) and the growth of self-assembly micellar structure. This paper discussed the microscopic evidence for the correlation of counterion binding constant values and micellar structure of cationic surfactants in the presence of phenolates and 2-ethyl phenolates ions in flexible nanoparticle (FN)-system of ionized phenyl salicylate-catalyzed piperidinolysis. The sizes of micelles were determined by particle size analysis. Transmission electron microscopy (TEM) results showed a spherical micellar structure for phenolates and 2-ethyl phenolates ions at counterion binding constant values 6.3 ± 1.0 and 24.0 ± 1.1 respectively. A study using a semi-emperical kinetic technique in published article proved that the suggested micellar structures at respective counterion binding constant values corresponded with the present microscopic evidence.

Production of Isolated Giant Unilamellar Vesicles under High Salt Concentrations

The cell membrane forms a dynamic and complex barrier between the living cell and its environment. However, its in vivo studies are difficult because it consists of a high variety of lipids and proteins and is continuously reorganized by the cell. Therefore, membrane model systems with precisely controlled composition are used to investigate fundamental interactions of membrane components under well-defined conditions. Giant unilamellar vesicles (GUVs) offer a powerful model system for the cell membrane, but many previous studies have been performed in unphysiologically low ionic strength solutions which might lead to altered membrane properties, protein stability and lipid-protein interaction. In the present work, we give an overview of the existing methods for GUV production and present our efforts on forming single, free floating vesicles up to several tens of μm in diameter and at high yield in various buffer solutions with physiological ionic strength and pH.

Production of Isolated Giant Unilamellar Vesicles under High Salt Concentrations

The cell membrane forms a dynamic and complex barrier between the living cell and its environment. However, its in vivo studies are difficult because it consists of a high variety of lipids and proteins and is continuously reorganized by the cell. Therefore, membrane model systems with precisely controlled composition are used to investigate fundamental interactions of membrane components under well-defined conditions. Giant unilamellar vesicles (GUVs) offer a powerful model system for the cell membrane, but many previous studies have been performed in unphysiologically low ionic strength solutions which might lead to altered membrane properties, protein stability and lipid-protein interaction. In the present work, we give an overview of the existing methods for GUV production and present our efforts on forming single, free floating vesicles up to several tens of μm in diameter and at high yield in various buffer solutions with physiological ionic strength and pH.

Using Scaffold Liposomes to Reconstitute Lipid-proximal Protein-protein Interactions In Vitro

Studies of integral membrane proteins in vitro are frequently complicated by the presence of a hydrophobic transmembrane domain. Further complicating these studies, reincorporation of detergent-solubilized membrane proteins into liposomes is a stochastic process where protein topology is impossible to enforce. This paper offers an alternative method to these challenging techniques that utilizes a liposome-based scaffold. Protein solubility is enhanced by deletion of the transmembrane domain, and these amino acids are replaced with a tethering moiety, such as a His-tag. This tether interacts with an anchoring group (Ni²⁺ coordinated by nitrilotriacetic acid (NTA(Ni²⁺)) for His-tagged proteins), which enforces a uniform protein topology at the surface of the liposome. An example is presented wherein the interaction between Dynamin-related protein 1 (Drp1) with an integral membrane protein, Mitochondrial Fission Factor (Mff), was investigated using this scaffold liposome method. In this work, we have demonstrated the ability of Mff to efficiently recruit soluble Drp1 to the surface of liposomes, which stimulated its GTPase activity. Moreover, Drp1 was able to tubulate the Mff-decorated lipid template in the presence of specific lipids. This example demonstrates the effectiveness of scaffold liposomes using structural and functional assays and highlights the role of Mff in regulating Drp1 activity.

Surface-enhanced Raman scattering measurement from a lipid bilayer encapsulating a single decahedral nanoparticle mediated by an optical trap

We present a new technique for the study of model membranes on the length-scale of a single nano-sized liposome. Silver decahedral nanoparticles have been encapsulated by a model unilamellar lipid bilayer creating nano-sized lipid vesicles. The metal core has two roles (i) increasing the polarizability of vesicles, enabling a single vesicle to be isolated and confined in an optical trap, and (ii) enhancing Raman scattering from the bilayer, via the high surface-plasmon field at the sharp vertices of the decahedral particles. Combined this has allowed us to measure a Raman fingerprint from a single vesicle of 50 nm-diameter, containing just ∼10⁴ lipid molecules in a bilayer membrane over a surface area of <0.01 μm², equivalent to a volume of approximately 1 zepto-litre. Raman scattering is a weak and inefficient process and previous studies have required either a substantially larger bilayer area in order to obtain a detectable signal, or the tagging of lipid molecules with a chromophore to provide an indirect probe of the bilayer. Our approach is fully label-free and bio-compatible and, in the future, it will enable much more localized studies of the heterogeneous structure of lipid bilayers and of membrane-bound components than is currently possible.

Micro-motors: A motile bacteria based system for liposome cargo transport

Biological micro-motors (microorganisms) have potential applications in energy utilization and nanotechnology. However, harnessing the power generated by such motors to execute desired work is extremely difficult. Here, we employ the power of motile bacteria to transport small, large, and giant unilamellar vesicles (SUVs, LUVs, and GUVs). Furthermore, we demonstrate bacteria–bilayer interactions by probing glycolipids inside the model membrane scaffold. Fluorescence Resonance Energy Transfer (FRET) spectroscopic and microscopic methods were utilized for understanding these interactions. We found that motile bacteria could successfully propel SUVs and LUVs with a velocity of 28 μm s⁻¹ and 13 μm s⁻¹, respectively. GUVs, however, displayed Brownian motion and could not be propelled by attached bacteria. Bacterial velocity decreased with the larger loaded cargo, which agrees with our calculations of loaded bacteria swimming at low Reynolds number.

Direct Measurement of Pore Dynamics and Leakage Induced by a Model Antimicrobial Peptide in Single Vesicles and Cells

Antimicrobial peptides are promising therapeutic alternatives to counter growing antimicrobial resistance. Their precise mechanism of action remains elusive, however, particularly with respect to live bacterial cells. We investigated the interaction of a fluorescent melittin analogue with single giant unilamellar vesicles, giant multilamellar vesicles, and bilamellar Gram-negative Escherichia coli (E. coli) bacteria. Time-lapse fluorescence lifetime imaging microscopy was employed to determine the population distribution of the fluorescent melittin analogue between pore state and membrane surface state, and simultaneously measure the leakage of entrapped fluorescent species from the vesicle (or bacterium) interior. In giant unilamellar vesicles, leakage from vesicle interior was correlated with an increase in level of pore states, consistent with a stable pore formation mechanism. In giant multilamellar vesicles, vesicle leakage occurred more gradually and did not appear to correlate with increased pore states. Instead pore levels remained at a low steady-state level, which is more in line with coupled equilibria. Finally, in single bacterial cells, significant increases in pore levels were observed over time, which were correlated with only partial loss of cytosolic contents. These observations suggested that pore formation, as opposed to complete dissolution of membrane, was responsible for the leakage of contents in these systems, and that the bacterial membrane has an adaptive capacity that resists peptide attack. We interpret the three distinct pore dynamics regimes in the context of the increasing physical and biological complexity of the membranes.

Formulation and stabilization of norfloxacin in liposomal preparations

A number of liposomal preparations of norfloxacin (NF) containing variable concentrations of phosphatidylcholine (PC) (10.8-16.2mM) have been formulated and an entrapment of NF to the extent of 41.7-56.2% was achieved. The values of apparent first-order rate constants (kobs) for the photodegradation of NF in liposomes (pH7.4) lie in the range of 1.05-2.40×10(-3)min(-1) compared to a value of 8.13×10(-3)min(-1) for the photodegradation of NF in aqueous solution (pH7.4). The values of kobs are a linear function of PC concentration indicating an interaction of PC and NF during the reaction. The second-order rate constant for the photochemical interaction of PC and NF has been determined as 8.92×10(-2)M(-1)min(-1). Fluorescence measurements on NF in liposomes indicate a decrease in fluorescence with an increase in PC concentration as a result of formation of NF(-) species which exhibits poor fluorescence. Dynamic light scattering has shown an increase in the size of NF encapsulated liposomes with an increase in PC concentration. The stabilization of NF in liposomes is achieved by the formation of a charge-transfer complex between NF and PC.

Distinct Membrane Disruption Pathways Are Induced by 40-Residue β-Amyloid Peptides

Cellular membrane disruption induced by β-amyloid (Aβ) peptides has been considered one of the major pathological mechanisms for Alzheimer disease. Mechanistic studies of the membrane disruption process at a high-resolution level, on the other hand, are hindered by the co-existence of multiple possible pathways, even in simplified model systems such as the phospholipid liposome. Therefore, separation of these pathways is crucial to achieve an in-depth understanding of the Aβ-induced membrane disruption process. This study, which utilized a combination of multiple biophysical techniques, shows that the peptide-to-lipid (P:L) molar ratio is an important factor that regulates the selection of dominant membrane disruption pathways in the presence of 40-residue Aβ peptides in liposomes. Three distinct pathways (fibrillation with membrane content leakage, vesicle fusion, and lipid uptake through a temporarily stable ionic channel) become dominant in model liposome systems under specific conditions. These individual systems are characterized by both the initial states of Aβ peptides and the P:L molar ratio. Our results demonstrated the possibility to generate simplified Aβ-membrane model systems with a homogeneous membrane disruption pathway, which will benefit high-resolution mechanistic studies in the future. Fundamentally, the possibility of pathway selection controlled by P:L suggests that the driving forces for Aβ aggregation and Aβ-membrane interactions may be similar at the molecular level.

Phospholipid and Hydrocarbon Interactions with a Charged Electrode Interface

Using a combination of molecular dynamics simulations and experiments we examined the interactions of alkanes and phospholipids at charged interfaces in order to understand how interfacial charge densities affect the association of these two representative molecules with electrodes. Consistent with theory and experiment, these model systems reveal interfacial associations mediated through a combination of Coulombic and van der Waals forces. van der Waals forces, in particular, mediate rapid binding of decane to neutral electrodes. No decane binding was observed at high surface charge densities because of interfacial water polarization, which screens hydrophobic attractions. The positively charged choline moiety of the phospholipid palmitoyloleoylphosphatidylcholine (POPC) is primarily responsible for POPC attraction by a moderately negatively charged electrode. The hydrocarbon tails of POPC interact with the hydrophobic electrode interface similarly to decane. Previously reported electrochemical results confirm these findings by demonstrating bipolar displacement currents from PC vesicles adhering to moderately negatively charged interfaces, originating from the choline interactions observed in simulations. At more negatively charged interfaces, choline-to-surface binding was stronger. In both simulations and experiments the maximal interaction of anionic PS occurs with a positively charged interface, provided that the electrostatic forces outweigh local Lennard-Jones interactions. Direct comparisons between the binding affinities measured in experiments and those obtained in simulations reveal previously unobserved atomic interactions that facilitate lipid vesicle adhesion to charged interfaces. Moreover, the implementation of a charged interface in molecular dynamics simulations provides an alternative method for the generation of large electric fields across phospholipid bilayers, especially for systems with periodic boundary conditions, and may be useful for simulations of membrane electropermeabilization.

Membrane interactions of proline-rich antimicrobial peptide, Chex1-Arg20, multimers

The increasing prevalence of antibiotic-resistant pathogens requires the development of new antibiotics. Proline-rich antimicrobial peptides (PrAMPs), including native apidaecins, Bac7, and oncocins or designed A3APO, show multi-modal actions against pathogens together with immunostimulatory activities. The interactions of the designed PrAMP, Chex1-Arg20, and its dimeric and tetrameric oligomers with different model membranes were investigated by circular dichroism spectroscopy, dynamic light scattering, zeta potential, differential scanning calorimetry, and dye leakage. Chex1-Arg20 oligomers showed stronger affinity and preferential binding to negatively charged phospholipid bilayers and led to lipid aggregation and neutralization. Fluorescence microscopy of negatively charged giant unilamellar vesicles with AlexFluor-647-labeled Chex1-Arg20 dimers and tetramers displayed aggregation at a peptide/lipid low ratio of 1:200 and at higher peptide concentrations (1:100/1:50) for Chex1-Arg20 monomer. Such interactions, aggregation, and neutralization of PrAMP oligomers additionally showed the importance of interactions of PrAMPs with negatively charged membranes.

A biomimetic DNA-based channel for the ligand-controlled transport of charged molecular cargo across a biological membrane

Biological ion channels are molecular gatekeepers that control transport across cell membranes. Recreating the functional principle of such systems and extending it beyond physiological ionic cargo is both scientifically exciting and technologically relevant to sensing or drug release. However, fabricating synthetic channels with a predictable structure remains a significant challenge. Here, we use DNA as a building material to create an atomistically determined molecular valve that can control when and which cargo is transported across a bilayer. The valve, which is made from seven concatenated DNA strands, can bind a specific ligand and, in response, undergo a nanomechanical change to open up the membrane-spanning channel. It is also able to distinguish with high selectivity the transport of small organic molecules that differ by the presence of a positively or negatively charged group. The DNA device could be used for controlled drug release and the building of synthetic cell-like or logic ionic networks.

Synthesis, characterization and thermotropic phase behavior of a homologous series of N-acyl-L-alaninols and interaction of N-myristoyl L-alaninol with dimyristoylphosphatidylcholine

N-acylethanolamines (NAEs) and their precursors, N-acylphosphatidylethanolamines are present in the cell membranes of a variety of species and exhibit interesting biological properties. N-acyl-L-alaninols (NAAOHs) are chiral homologues of NAEs and reduced forms of N-acyl-L-alanines (NAAs) and were reported to induce apoptosis in human lymphocytes. In the present study, we have synthesized and characterized a homologous series of N-acyl-L-alaninols (n=9-20). In DSC studies in the dry as well as hydrated states NAAOHs with different chain lengths showed single sharp transitions similar to N-acyl-L-alanines. Transition enthalpies (ΔHt) and entropies (ΔSt) of NAAOHs are linearly dependent on the acyl chain length in both dry and hydrated states. Powder X-ray diffraction studies showed that the d-spacings of NAAOHs exhibit linear dependence on the chain length and the incremental increase in the d values suggest that they may be packed in a tilted bilayer pattern. Studies on the interaction of N-myristoyl L-alaninol (NMAOH) with DMPC revealed that the two amphiphiles mix well up to 45 mol% of NMAOH, whereas phase separation is observed at higher contents of the alaninol. Transmission electron microscopic studies show that the NMAOH:DMPC (45:55, mol/mol) mixture forms unilamellar vesicles of about 120-150 nm in diameter.

Formation of Giant Unilamellar Proteo-Liposomes by Osmotic Shock

Giant unilamellar vesicles (GUVs), composed of a phospholipid bilayer, are often used as a model system for cell membranes. However, the study of proteo-membrane interactions in this system is limited as the incorporation of integral and lipid-anchored proteins into GUVs remains challenging. Here, we present a simple generic method to incorporate proteins into GUVs. The basic principle is to break proteo-liposomes with an osmotic shock. They subsequently reseal into larger vesicles which, if necessary, can endure the same to obtain even larger proteo-GUVs. This process does not require specific lipids or reagents, works under physiological conditions with high concentrations of protein, the proteins remains functional after incorporation. The resulting proteo-GUVs can be micromanipulated. Moreover, our protocol is valid for a wide range of protein substrates. We have successfully reconstituted three structurally different proteins, two trans-membrane proteins (TolC and the neuronal t-SNARE), and one lipid-anchored peripheral protein (GABARAP-Like 1 (GL1)). In each case, we verified that the protein remains active after incorporation and in its correctly folded state. We also measured their mobility by performing diffusion measurements via fluorescence recovery after photobleaching (FRAP) experiments on micromanipulated single GUVs. The diffusion coefficients are in agreement with previous data.

Membrane analysis with amphiphilic carbon dots

Newly-synthesized amphiphilic carbon dots were used for spectroscopic analysis and multicolour microscopic imaging of membranes and live cells. We show that Förster resonance energy transfer (FRET) occurred from the amphiphilic carbon dots to different membrane-associated fluorescence acceptors. The amphiphilic carbon dots enabled imaging of membrane disruption by the beta-amyloid peptide.

Site of fluorescent label modifies interaction of melittin with live cells and model membranes

The mechanism of membrane disruption by melittin (MLT) of giant unilamellar vesicles (GUVs) and live cells was studied using fluorescence microscopy and two fluorescent synthetic analogues of MLT. The N-terminus of one of these was acylated with thiopropionic acid to enable labeling with maleimido-AlexaFluor 430 to study the interaction of MLT with live cells. It was compared with a second analogue labeled at P14C. The results indicated that the fluorescent peptides adhered to the membrane bilayer of phosphatidylcholine GUVs and inserted into the plasma membrane of HeLa cells. Fluorescence and light microscopy revealed changes in cell morphology after exposure to MLT peptides and showed bleb formation in the plasma membrane of HeLa cells. However, the membrane disruptive effect was dependent upon the location of the fluorescent label on the peptide and was greater when MLT was labeled at the N-terminus. Proline at position 14 appeared to be important for antimicrobial activity, hemolysis and cytotoxicity, but not essential for cell membrane disruption.

Analysis of liposome model systems by time-of-flight secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is an important technique for studying chemical composition of micrometer scale objects due to its high spatial resolution imaging capabilities and chemical specificity. In this work we focus on the application of ToF-SIMS to gain insight into the chemistry of micrometer size liposomes as a potential model for neurotransmitter vesicles. Two models of giant liposomes were analyzed: histamine and aqueous two phase system (ATPS)-containing liposomes. Characterization of the internal structure of single fixed liposomes was done both with the Bi₃⁺ and C₆₀⁺ ion sources. The depth profiling capability of ToF-SIMS was used to investigate the liposome interior.

Resveratrol nanoformulations: challenges and opportunities

Resveratrol, a naturally occurring polyphenol and phytoalexin, has received significant attention in recent years due to its vast therapeutic effects including anticancer, antioxidant and anti-inflammatory effects. However, poor pharmacokinetic properties such as low aqueous solubility, low photostability and extensive first pass metabolism result in poor bioavailability, hindering its immense potential. Conventional dosage forms such as dry powder capsules and injections have met with limited success, demonstrating challenges faced in developing an effective formulation. Recently, nanotechnology-based formulations (nanoformulations) are being looked upon as a novel method for improving the pharmacokinetic properties, as well as enhancing targetability and bioavailability of resveratrol. This review outlines the therapeutic potential of resveratrol, explores its mechanisms of action and pharmacokinetic limitations, and discusses the success and challenges of resveratrol-encapsulated nanoparticles in the last decade. Potential techniques to improve encapsulation of the drug within nanoparticles, thereby enhancing its clinical potential are highlighted.

Evaluation of physical integrity of lipid bilayer under oxidative stress: application of fluorescence microscopy and digital image processing

Membrane damage as a result of oxidative stress is quantified using digital image heterogeneity analysis of single giant unilamellar vesicles (GUVs) composed of soy phosphatidylcholine (PC), which were found to undergo budding when containing chlorophyll a (Chla) as photosensitizer in the lipid bilayer. Based on digital image heterogeneity analysis, a dimensionless scalar parameter "entropy" for the budding process was found to change linearly during an initial budding stage. Photo-induced peroxidation of PC to form linoleoyl hydroperoxides, further leading to domains of higher polarities in GUVs, was suggested to initiate the budding process. The effect on budding process of GUVs was suggested for use in assays for evaluation of potential protectors of lipid bilayer integrity under oxidative stress, and "entropy" seemed to be a valid descriptor of such membranal integrity. The one-step procedure for quantification of prooxidative effects and antioxidative protection provided by drug candidates and potential food ingredients in membranes could be easily automated for direct measurement of oxidative and antioxidative effects on cellular integrity.

Double-edged sword in cells: chemical biology studies of the vital role of cytochrome c in the intrinsic pre-apoptotic mitochondria leakage pathway

Besides functioning as an electron transporter in the mitochondrial electron transport chain, cytochrome c (cyt c) is also one of the determinants in the execution of cell death.

In situ vesicle formation by native chemical ligation

Phospholipid vesicles are of intense fundamental and practical interest, yet methods for their de novo generation from reactive precursors are limited. A non-enzymatic and chemoselective method to spontaneously generate phospholipid membranes from water-soluble starting materials would be a powerful tool for generating vesicles and studying lipid membranes. Here we describe the use of native chemical ligation (NCL) to rapidly prepare phospholipids spontaneously from thioesters. While NCL is one of the most popular tools for synthesizing proteins and nucleic acids, to our knowledge this is the first example of using NCL to generate phospholipids de novo. The lipids are capable of in situ synthesis and self-assembly into vesicles that can grow to several microns in diameter. The selectivity of the NCL reaction makes in situ membrane formation compatible with biological materials such as proteins. This work expands the application of NCL to the formation of phospholipid membranes.

A new approach for the fabrication of microscale lipid bilayers at glass pipets: application to quantitative passive permeation visualization

A new method of planar bilayer lipid membrane (BLM) formation is presented that allows stable, solvent-free lipid bilayers exhibiting high seal resistances to be formed rapidly, easily and reproducibly. Using these bilayers the passive permeation of a series of carboxylic acids is investigated, to determine quantitatively the trend in permeability with lipophilicity of the acid. BLMs are formed at the tip openings of pulled theta pipets, and the rate of permeation of each carboxylic acid across the bilayer, from within the pipet into the bulk solution is determined. This is achieved through spatially-resolved measurements of the pH change that occurs upon the permeation of the weak acid, visualized using a pH-sensitive fluorophore with a confocal laser scanning microscope. The extracted fluorescence profiles are matched to finite element method (FEM) simulations, to allow the associated permeation coefficient for each weak acid to be determined with high accuracy, since this is the only adjustable parameter used to fit the experimental data. For bilayers formed in this way, the weak acids show increasing permeability with lipophilicity. Furthermore, the arrangement allows the effect of a trans-membrane electric field on permeation to be explored. For both propanoic and hexanoic acid it is found that an applied electric field enhances molecular transport, which is attributed to the formation of pores within the membrane.

Giant unilamellar vesicles formed by hybrid films of agarose and lipids display altered mechanical properties

Giant unilamellar vesicles (GUVs) are presumably the current most popular biomimetic membrane model. Preparation of GUVs in physiological conditions using the classical electroformation method is challenging. To circumvent these difficulties, a new method was recently reported, by which GUVs spontaneously swell from hybrid films of agarose and lipids. However, agarose is left encapsulated in the vesicles in different amounts. In this work, we thoroughly characterize the mechanical properties of these agarose-GUVs in response to electric pulses, which induce vesicle deformation and can lead to membrane poration. We show that the relaxation dynamics of deformed vesicles, both in the presence and absence of poration, is significantly slowed down for agarose-GUVs when compared to agarose-free GUVs. In the presence of poration, agarose polymers prevent complete pore closure and lead to high membrane permeability. A fraction of the vesicles were found to encapsulate agarose in the form of a gel-like meshwork. These vesicles rupture and open up after electroporation and the meshwork is expelled through a macropore. When the agarose-GUVs are heated above the melting temperature of agarose for 2 h before use, vesicle response is (partially) recovered due to substantial release of encapsulated agarose during temperature treatment. Our findings reveal potential artifactual behavior of agarose-GUVs in processes involving morphological changes in the membrane as well as poration.