Influence of transbilayer area asymmetry on the morphology of large unilamellar vesicles

Effects of Normal and Lateral Electric Fields on Membrane Mechanical Properties

As a core component of biological and synthetic membranes, lipid bilayers are key to compartmentalizing chemical processes. Bilayer morphology and mechanical properties are heavily influenced by electric fields, such as those caused by biological ion concentration gradients. We present atomistic simulations exploring the effects of electric fields applied normally and laterally to lipid bilayers. We find that normal fields decrease membrane tension, while lateral fields increase it. Free energy perturbation calculations indicate the importance of dipole-dipole interactions to these tension changes, especially for lateral fields. We additionally show that membrane area compressibilities can be related to their cohesive energies, allowing us to estimate changes in membrane bending rigidity under applied fields. We find that normal and lateral fields decrease and increase bending rigidity, respectively. These results point to the use of directed electric fields to locally control membrane stiffness, thereby modulating associated cellular processes.

Methyl-β-cyclodextrin asymmetrically extracts phospholipid from bilayers, granting tunable control over differential stress in lipid vesicles

Lipid asymmetry - that is, a nonuniform lipid distribution between the leaflets of a bilayer - is a ubiquitous feature of biomembranes and is implicated in several cellular phenomena. Differential tension - that is, unequal lateral monolayer tensions comparing the leaflets of a bilayer- is closely associated with lipid asymmetry underlying these varied roles. Because differential tension is not directly measurable in combination with the fact that common methods to adjust this quantity grant only semi-quantitative control over it, a detailed understanding of lipid asymmetry and differential tension are impeded. To overcome these challenges, we leveraged reversible complexation of phospholipid by methyl-β-cyclodextrin (mbCD) to tune the direction and magnitude of lipid asymmetry in synthetic vesicles. Lipid asymmetry generated in our study induced (i) vesicle shape changes and (ii) gel-liquid phase coexistence in 1-component vesicles. By applying mass-action considerations to interpret our findings, we discuss how this approach provides access to phospholipid thermodynamic potentials in bilayers containing lipid asymmetry (which are coupled to the differential tension of a bilayer). Because lipid asymmetry yielded by our approach is (i) tunable and (ii) maintained over minute to hour timescales, we anticipate that this approach will be a valuable addition to the experimental toolbox for systematic investigation into the biophysical role(s) of lipid asymmetry (and differential tension).

Multiscale remodeling of biomembranes and vesicles

Biomembranes and vesicles cover a wide range of length scales. Indeed, small nanovesicles have a diameter of a few tens of nanometers whereas giant vesicles can have diameters up to hundreds of micrometers. The remodeling of giant vesicles on the micron scale can be observed by light microscopy and understood by the theory of curvature elasticity, which represents a top-down approach. The theory predicts the formation of multispherical shapes as recently observed experimentally. On the nanometer scale, much insight has been obtained via coarse-grained molecular dynamics simulations of nanovesicles, which provides a bottom-up approach based on the lipid numbers assembled in the two bilayer leaflets and the resulting leaflet tensions. The remodeling processes discussed here include the shape transformations of vesicles, their morphological responses to the adhesion of condensate droplets, the instabilities of lipid bilayers and nanovesicles, as well as the topological transformations of vesicles by membrane fission and fusion. The latter processes determine the complex topology of the endoplasmic reticulum.

Leaflet Tensions Control the Spatio-Temporal Remodeling of Lipid Bilayers and Nanovesicles

Biological and biomimetic membranes are based on lipid bilayers, which consist of two monolayers or leaflets. To avoid bilayer edges, which form when the hydrophobic core of such a bilayer is exposed to the surrounding aqueous solution, a single bilayer closes up into a unilamellar vesicle, thereby separating an interior from an exterior aqueous compartment. Synthetic nanovesicles with a size below 100 nanometers, traditionally called small unilamellar vesicles, have emerged as potent platforms for the delivery of drugs and vaccines. Cellular nanovesicles of a similar size are released from almost every type of living cell. The nanovesicle morphology has been studied by electron microscopy methods but these methods are limited to a single snapshot of each vesicle. Here, we review recent results of molecular dynamics simulations, by which one can monitor and elucidate the spatio-temporal remodeling of individual bilayers and nanovesicles. We emphasize the new concept of leaflet tensions, which control the bilayers' stability and instability, the transition rates of lipid flip-flops between the two leaflets, the shape transformations of nanovesicles, the engulfment and endocytosis of condensate droplets and rigid nanoparticles, as well as nanovesicle adhesion and fusion. To actually compute the leaflet tensions, one has to determine the bilayer's midsurface, which represents the average position of the interface between the two leaflets. Two particularly useful methods to determine this midsurface are based on the density profile of the hydrophobic lipid chains and on the molecular volumes.

Enhanced permeation by amphiphilic surfactant is spatially heterogenous at membrane and cell level

In the context of increased interest in permeability enhancement technologies to achieve mucosal delivery of drugs and biologics, we report our study on effects of the amphiphilic surfactant at cell membrane and cell population levels. Our results show that modulation in membrane order and fluidity initially occurs on insertion of individual surfactant molecules into the outer leaflet of membrane lipid bilayer; a process occurring at concentrations below surfactant's critical micellar concentration. The surfactant insertion, and consequent increase in membrane fluidity, are observed to be spatially heterogenous, i.e. manifested as 'patches' of increased membrane fluidity. At the cell population level, spatially heterogeneous activity of surfactant is also manifested, with certain cells displaying high permeability amongst a 'background' population. We propose that this heterogeneity is further manifested in a broad profile of intracellular and nuclear exposure levels to a model drug (doxorubicin) observed in cell population. The study points to heterogeneous nature of surfactant effects at cell membrane and cells in population levels.

Remodeling of Membrane Shape and Topology by Curvature Elasticity and Membrane Tension

Cellular membranes exhibit a fascinating variety of different morphologies, which are continuously remodeled by transformations of membrane shape and topology. This remodeling is essential for important biological processes (cell division, intracellular vesicle trafficking, endocytosis) and can be elucidated in a systematic and quantitative manner using synthetic membrane systems. Here, recent insights obtained from such synthetic systems are reviewed, integrating experimental observations and molecular dynamics simulations with the theory of membrane elasticity. The study starts from the polymorphism of biomembranes as observed for giant vesicles by optical microscopy and small nanovesicles in simulations. This polymorphism reflects the unusual elasticity of fluid membranes and includes the formation of membrane necks or fluid 'worm holes'. The proliferation of membrane necks generates stable multi-spherical shapes, which can form tubules and tubular junctions. Membrane necks are also essential for the remodeling of membrane topology via membrane fission and fusion. Neck fission can be induced by fine-tuning of membrane curvature, which leads to the controlled division of giant vesicles, and by adhesion-induced membrane tension as observed for small nanovesicles. Challenges for future research include the interplay of curvature elasticity and membrane tension during membrane fusion and the localization of fission and fusion processes within intramembrane domains.

Influence of NaCl on shape deformation of polymersomes

Polymersomes frequently appear in the literature as promising candidates for a wide range of applications from targeted drug delivery to nanoreactors. From a cell mimetic point of view, it is important to understand the size and shape changes of the vesicles in the physiological environment since that can influence the drug delivery mechanism. In this work we studied the structural features of polymersomes consisting of poly(ethylene glycol)-poly(dimethylsiloxane)-poly(ethylene glycol) at the nanoscopic length scale in the presence of NaCl, which is a very common molecule in the biotic aqueous environment. We used dynamic light scattering (DLS), cryo-TEM, small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS). We observed transformation of polymersomes from spherical to elongated vesicles at low salt concentration and into multivesicular structures at high salt concentration. Model fitting analysis of SANS data indicated a reduction of vesicle radius up to 47% and from the SAXS data we observed an increase in membrane thickness up to 8% and an increase of the PDMS hydrophobic segment up to 11% indicating stretching of the membrane due to osmotic imbalance. Also, from the increase in the interlamellar repeat distance up to 98% under high salt concentrations, we concluded that the shape and structural changes observed in the polymersomes are a combined result of osmotic pressure change and ion-membrane interactions.

Generating biomembrane-like local curvature in polymersomes via dynamic polymer insertion

Biomembrane curvature formation has long been observed to be essential in the change of membrane morphology and intracellular processes. The significant importance of curvature formation has attracted scientists from different backgrounds to study it. Although magnificent progress has been achieved using liposome models, the instability of these models restrict further exploration. Here, we report a new approach to mimic biomembrane curvature formation using polymersomes as a model, and poly(N-isopropylacrylamide) to induce the local curvature based on its co-nonsolvency phenomenon. Curvatures form when poly(N-isopropylacrylamide) becomes hydrophobic and inserts into the membrane through solvent addition. The insertion area can be fine-tuned by adjusting the poly(N-isopropylacrylamide) concentration, accompanied by the formation of new polymersome-based non-axisymmetric shapes. Moreover, a systematic view of curvature formation is provided through investigation of the segregation, local distribution and dissociation of inserted poly(N-isopropylacrylamide). This strategy successfully mimicks biomembrane curvature formation in polymersomes and a detailed observation of the insertion can be beneficial for a further understanding of the curvature formation process. Furthermore, polymer insertion induced shape changing could open up new routes for the design of non-axisymmetric nanocarriers and nanomachines to enrich the boundless possibilities of nanotechnology.

Investigating biomembrane curvature formation is important for studying intracellular processes, but the instability of liposome models mimicking these membranes restricts exploration of membrane processes. Here, the authors demonstrate control over the curvature formation in polymersome membranes by insertion of PNIPAm as stimuli responsive polymer.

Liposomes as vehicles for topical ophthalmic drug delivery and ocular surface protection

Introduction: The development of ophthalmic formulations able to deliver hydrophilic and hydrophobic drugs to the inner structures of the eye and restore the preocular tear film has been a leading topic of discussion over the last few years. In this sense, liposomes represent a suitable strategy to achieve these objectives in ocular drug delivery.Areas covered: Knowledge of the different physiological and anatomical eye structures, and specially the ocular surface are critical to better understanding and comprehending the characteristics required for the development of topical ophthalmic liposomal formulations. In this review, several features of liposomes are discussed such as the main materials used for their fabrication, basic structure and preparation methods, from already established to novel techniques, allowing the control and design of special characteristics. Besides, physicochemical properties, purification processes and strategies to overcome delivery or encapsulation challenges are also presented. Expert opinion: Regarding ocular drug delivery of liposomes, there are some features that can be redesigned. Specific biocompatible and biodegradable materials presenting therapeutic properties, such as lipidic compounds or polymers significantly change the way of tackling ophthalmic diseases. Besides, liposomes entail an effective, safe and versatile strategy for the treatment of diseases in the clinical practice.

Membrane Curvature, Trans-Membrane Area Asymmetry, Budding, Fission and Organelle Geometry

In biology, the modern scientific fashion is to mostly study proteins. Much less attention is paid to lipids. However, lipids themselves are extremely important for the formation and functioning of cellular membrane organelles. Here, the role of the geometry of the lipid bilayer in regulation of organelle shape is analyzed. It is proposed that during rapid shape transition, the number of lipid heads and their size (i.e., due to the change in lipid head charge) inside lipid leaflets modulates the geometrical properties of organelles, in particular their membrane curvature. Insertion of proteins into a lipid bilayer and the shape of protein trans-membrane domains also affect the trans-membrane asymmetry between surface areas of luminal and cytosol leaflets of the membrane. In the cases where lipid molecules with a specific shape are not predominant, the shape of lipids (cylindrical, conical, or wedge-like) is less important for the regulation of membrane curvature, due to the flexibility of their acyl chains and their high ability to diffuse.

Spherical Nanovesicles Transform into a Multitude of Nonspherical Shapes

Nanovesicles are closed, bubblelike surfaces with a diameter between 20 and 200 nm, formed by lipid bilayers and biomembranes. Electron microscopy (EM) studies have shown that these vesicles can attain both spherical and nonspherical shapes. One disadvantage of EM methods is that they provide only a single snapshot of each vesicle. Here, we use molecular dynamics simulations to monitor the morphological transformations of individual nanovesicles. We start with the assembly of spherical vesicles that enclose a certain volume of water and contain a certain total number of lipids. When we reduce their volume, the spherical vesicles are observed to transform into a multitude of nonspherical shapes such as oblates and stomatocytes as well as prolates and dumbbells. This surprising polymorphism can be controlled by redistributing a small fraction of lipids between the inner and outer leaflets of the bilayer membranes. As a consequence, the inner and the outer leaflets experience different mechanical tensions. Small changes in the vesicle volume reduce the overall bilayer tension by 2 orders of magnitude, thereby producing tensionless bilayers. In addition, we show how to determine, for a certain total number of lipids, the unique spherical vesicle for which both leaflet tensions vanish individually. We also compute the local spontaneous curvature of the spherical membranes by identifying the first moment of the spherically symmetric stress profiles across the lipid bilayers with the nanoscopic torque as derived from curvature elasticity. Our study can be extended to other types of lipid membranes and sheds new light on cellular nanovesicles such as exosomes, which are increasingly used as biomarkers and drug delivery systems.

Morphology Transformation of Supramolecular Structures in Aqueous Mixtures of Two Oppositely Charged Amphiphiles

The co-assembly of oppositely charged amphiphiles provides a fascinating approach for forming complex supramolecular structures, which are interesting from both fundamental and technological viewpoints. Here, we report a stepwise morphology transformation of co-assembled supramolecular structures in the aqueous mixture of lithocholic acid (LCA) and cetyltrimethylammonium bromide (CTAB) at mixed molar ratios of 1:1 and 2:1. The co-assembly of LCA and CTAB initially forms multilamellar vesicles followed by the spontaneous growth of membrane tubes from the vesicles. The vesicle-to-tube transition is accompanied by a fluidic-to-crystalline phase transition. After being aged, the membrane tubes twist into left-handed helices, which then intertwine into left-handed double helices and multihelix bundles. The single handedness of these supramolecular structures is a reflection of the amplification of the chirality of LCA. An understanding of the co-assembly mechanism and pathway is a key step toward producing supramolecular structures with distinguished morphologies.

Preparation of asymmetric phospholipid vesicles for use as cell membrane models

Freely suspended liposomes are widely used as model membranes for studying lipid-lipid and protein-lipid interactions. Liposomes prepared by conventional methods have chemically identical bilayer leaflets. By contrast, living cells actively maintain different lipid compositions in the two leaflets of the plasma membrane, resulting in asymmetric membrane properties that are critical for normal cell function. Here, we present a protocol for the preparation of unilamellar asymmetric phospholipid vesicles that better mimic biological membranes. Asymmetry is generated by methyl-β-cyclodextrin-catalyzed exchange of the outer leaflet lipids between vesicle pools of differing lipid composition. Lipid destined for the outer leaflet of the asymmetric vesicles is provided by heavy-donor multilamellar vesicles containing a dense sucrose core. Donor lipid is exchanged into extruded unilamellar acceptor vesicles that lack the sucrose core, facilitating the post-exchange separation of the donor and acceptor pools by centrifugation because of differences in vesicle size and density. We present two complementary assays allowing quantification of each leaflet's lipid composition: the overall lipid composition is determined by gas chromatography-mass spectrometry, whereas the lipid distribution between the two leaflets is determined by NMR, using the lanthanide shift reagent Pr³⁺. The preparation protocol and the chromatographic assay can be applied to any type of phospholipid bilayer, whereas the NMR assay is specific to lipids with choline-containing headgroups, such as phosphatidylcholine and sphingomyelin. In ~12 h, the protocol can produce a large yield of asymmetric vesicles (up to 20 mg) suitable for a wide range of biophysical studies.

Carbon Nanotube Porins in Amphiphilic Block Copolymers as Fully Synthetic Mimics of Biological Membranes

Biological membranes provide a fascinating example of a separation system that is multifunctional, tunable, precise, and efficient. Biomimetic membranes, which mimic the architecture of cellular membranes, have the potential to deliver significant improvements in specificity and permeability. Here, a fully synthetic biomimetic membrane is reported that incorporates ultra-efficient 1.5 nm diameter carbon nanotube porin (CNTPs) channels in a block-copolymer matrix. It is demonstrated that CNTPs maintain high proton and water permeability in these membranes. CNTPs can also mimic the behavior of biological gap junctions by forming bridges between vesicular compartments that allow transport of small molecules.

Migration of Deformable Vesicles Induced by Ionic Stimuli

We have investigated the dynamics of phospholipid vesicles composed of 1,2-dioleoyl- sn-glycero-3-phosphocholine triggered by ionic stimuli using electrolytes such as CaCl₂, NaCl, and NaOH. The ionic stimuli induce two characteristic vesicle dynamics, deformation due to the ion binding to the lipids in the outer leaflet of the vesicle and migration due to the concentration gradient of ions, that is, diffusiophoresis or the interfacial energy gradient mechanism. We examined the deformation pathway for each electrolyte as a function of time and analyzed it based on the surface dissociation model and the area difference elasticity model, which reveals the change of the cross-sectional area of the phospholipid by the ion binding. The metal ions such as Ca²⁺ and Na⁺ encourage inward budding deformation by decreasing the cross-sectional area of a lipid, whereas the hydroxide ion (OH^-) encourages outward budding deformation by increasing the cross-sectional area of a lipid. When we microinjected these electrolytes toward the vesicles, a strong coupling between the deformation and the migration of the vesicle was observed for CaCl₂ and NaOH, whereas for NaCl, the coupling was very weak. This difference probably originates from the binding constants of the ions.

Entropic elasticity based coarse-grained model of lipid membranes

Various models for lipid bilayer membranes have been presented to investigate their morphologies. Among them, the aggressive coarse-grained models, where the membrane is represented by a single layer of particles, are computationally efficient and of practical importance for simulating membrane dynamics at the microscopic scale. In these models, soft potentials between particle pairs are used to maintain the fluidity of membranes, but the underlying mechanism of the softening requires further clarification. We have analyzed the membrane area decrease due to thermal fluctuations, and the results demonstrate that the intraparticle part of entropic elasticity is responsible for the softening of the potential. Based on the stretching response of the membrane, a bottom-up model is developed with an entropic effect explicitly involved. The model reproduces several essential properties of the lipid membrane, including the fluid state and a plateau in the stretching curve. In addition, the area compressibility modulus, bending rigidity, and spontaneous curvature display linear dependence on model parameters. As a demonstration, we have investigated the closure and morphology evolution of membrane systems driven by spontaneous curvature, and vesicle shapes observed experimentally are faithfully reproduced.

pH sensing by lipids in membranes: The fundamentals of pH-driven migration, polarization and deformations of lipid bilayer assemblies

Most biological molecules contain acido-basic groups that modulate their structure and interactions. A consequence is that pH gradients, local heterogeneities and dynamic variations are used by cells and organisms to drive or regulate specific biological functions including energetic metabolism, vesicular traffic, migration and spatial patterning of tissues in development. While the direct or regulatory role of pH in protein function is well documented, the role of hydrogen and hydroxyl ions in modulating the properties of lipid assemblies such as bilayer membranes is only beginning to be understood. Here, we review approaches using artificial lipid vesicles that have been instrumental in providing an understanding of the influence of pH gradients and local variations on membrane vectorial motional processes: migration, membrane curvature effects promoting global or local deformations, crowding generation by segregative polarization processes. In the case of pH induced local deformations, an extensive theoretical framework is given and an application to a specific biological issue, namely the structure and stability of mitochondrial cristae, is described. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

The lipolytic degradation of highly structured cubic micellar nanoparticles of soy phosphatidylcholine and glycerol dioleate by phospholipase A2 and triacylglycerol lipase

The effects of different lipolytic enzymes on the structure of lipid liquid crystalline nano-particles (LCNP) have been investigated by cryogenic transmission electron microscopy (cryo-TEM) and synchrotron small angle X-ray diffraction (SAXD). Here we used highly structured cubic micellar (Fd3m) nanoparticles of 50/50 (wt%/wt%) soy phosphatidyl choline (SPC)/glycerol dioleate (GDO) as substrate. Two types of lipolytic enzymes were used, phospholipase A2 (PLA2) that catalyses degradation of the phospholipid component, SPC, and porcine pancreatic triacylglycerol lipase (TGL) that facilitate the hydrolysis of the diglyceride, GDO. Evolution of the structure was found to be very different and linked to specificity of the two types of enzymes. PLA2, which hydrolyses the lamellar forming component, SPC, induces a reversed micellar lipid phase, while TGL which hydrolysis the reverse phase forming compound, GDO, induces a lamellar phase.

Determination of Nonspherical Morphology of Doxorubicin-Loaded Liposomes by Atomic Force Microscopy

The 3-D morphology of doxorubicin (DOX)-loaded liposomes with a size of circa 100 nm was characterized by atomic force microscopy in an aqueous environment. Prolate liposomes appear in accordance with linear expansion of DOX fiber bundles precipitated inside liposomes. Oblate and concave liposomes were simultaneously observed with increased DOX concentrations; however, their morphologies were not readily determined by 2-D cryo-TEM imaging. Precise data analysis of the 3-D parameters of each liposome allowed semiquantitative evaluation of the transformation of spherical liposomes into nonspherical-prolate, oblate, and concave liposomes. In addition, nonspherical liposomes became spherical on the replacement of the liposomal outer phase consisting of a sucrose solution, with water and subsequent water influx. All spherical liposomes transformed into oblate and concave liposomes with a return to hyperosmotic conditions, when transferred from water to sucrose solution. Furthermore, the concave liposomes did not appear under DOX incubation conditions (65°C), which could be due to the amorphous and supersaturated DOX inside the liposomes that restrained liposomal shrinkage. As atomic force microscopy has improved our ability to image 3-D morphologies of liposomes in various conditions, it is an alternative analytical tool to cryo-TEM and may have future applications in regulatory tests for quality control and assurance.

Systems biology of cellular membranes: a convergence with biophysics

Systems biology and systems medicine have played an important role in the last two decades in shaping our understanding of biological processes. While systems biology is synonymous with network maps and '-omics' approaches, it is not often associated with mechanical processes. Here, we make the case for considering the mechanical and geometrical aspects of biological membranes as a key step in pushing the frontiers of systems biology of cellular membranes forward. We begin by introducing the basic components of cellular membranes, and highlight their dynamical aspects. We then survey the functions of the plasma membrane and the endomembrane system in signaling, and discuss the role and origin of membrane curvature in these diverse cellular processes. We further give an overview of the experimental and modeling approaches to study membrane phenomena. We close with a perspective on the converging futures of systems biology and membrane biophysics, invoking the need to include physical variables such as location and geometry in the study of cellular membranes. WIREs Syst Biol Med 2017, 9:e1386. doi: 10.1002/wsbm.1386 For further resources related to this article, please visit the WIREs website.

Spontaneous formation of nanometer scale tubular vesicles in aqueous mixtures of lipid and block copolymer amphiphiles

Many common amphiphiles self-assemble in water to produce heterogeneous populations of discrete and symmetric but polydisperse and multilamellar vesicles isolating the encapsulated aqueous core from the surrounding bulk. But when mixtures of amphiphiles of vastly different elastic properties co-assemble, their non-uniform molecular organization can stabilize lower symmetries and produce novel shapes. Here, using high resolution electron cryomicroscopy and tomography, we identify the spontaneous formation of a membrane morphology consisting of unilamellar tubular vesicles in dilute aqueous solutions of binary mixtures of two different amphiphiles of vastly different origins. Our results show that aqueous phase mixtures of a fluid-phase phospholipid and an amphiphilic block copolymer spontaneously assume a bimodal polymorphic character in a composition dependent manner: over a broad range of compositions (15-85 mol% polymer component), a tubular morphology co-exists with spherical vesicles. Strikingly, in the vicinity of equimolar compositions, an exclusively tubular morphology (L_t; diameter, ∼15 nm; length, >1 μm; core, ∼2.0 nm; wall, ∼5-6 nm) emerges in an apparent steady state. Theory suggests that the spontaneous stabilization of cylindrical vesicles, unaided by extraneous forces, requires a significant spontaneous bilayer curvature, which in turn necessitates a strongly asymmetric membrane composition. We confirm that such dramatic compositional asymmetry is indeed produced spontaneously in aqueous mixtures of a lipid and polymer through two independent biochemical assays - (1) reduction in the quenching of fluorophore-labeled lipids and (2) inhibition in the activity of externally added lipid-hydrolyzing phospholipase A2, resulting in a significant enrichment of the polymer component in the outer leaflet. Taken together, these results illustrate the coupling of the membrane shape with local composition through spontaneous curvature generation under conditions of asymmetric distribution of mixtures of disparate amphiphiles.

Exploring the shape deformation of biomembrane tubes with theoretical analysis and computer simulation

The shape deformation of membrane nanotubes is studied by a combination of theoretical analysis and molecular simulation. First we perform free energy analysis to demonstrate the effects of various factors on two ideal states for the pearling transition, and then we carry out dissipative particle dynamics simulations, through which various types of membrane tube deformation are found, including membrane pearling, buckling, and bulging. Different models for inducing tube deformation, including the osmotic pressure, area difference and spontaneous curvature models, are considered to investigate tubular instabilities. Combined with free energy analysis, our simulations show that the origin of the deformation of membrane tubes in different models can be classified into two categories: effective spontaneous curvature and membrane tension. We further demonstrate that for different models, a positive membrane tension is required for the pearling transition. Finally we show that different models can be coupled to effectively deform the membrane tube.

Membrane mechanical properties of synthetic asymmetric phospholipid vesicles

Synthetic lipid vesicles have served as important model systems to study cellular membrane biology. Research has shown that the mechanical properties of bilayer membranes significantly affects their biological behavior. The properties of a lipid bilayer are governed by lipid acyl chain length, headgroup type, and the presence of membrane proteins. However, few studies have explored how membrane architecture, in particular trans-bilayer lipid asymmetry, influences membrane mechanical properties. In this study, we investigated the effects of lipid bilayer architecture (i.e. asymmetry) on the mechanical properties of biological membranes. This was achieved using a customized micropipette aspiration system and a novel microfluidic technique previously developed by our team for building asymmetric phospholipid vesicles with tailored bilayer architecture. We found that the bending modulus and area expansion modulus of the synthetic asymmetric bilayers were up to 50% larger than the values acquired for symmetric bilayers. This was caused by the dissimilar lipid distribution in each leaflet of the bilayer for the asymmetric membrane. To the best of our knowledge, this is the first report on the impact of trans-bilayer asymmetry on the area expansion modulus of synthetic bilayer membranes. Since the mechanical properties of bilayer membranes play an important role in numerous cellular processes, these results have significant implications for membrane biology studies.

Straining soft colloids in aqueous nematic liquid crystals

Liquid crystals (LCs), because of their long-range molecular ordering, are anisotropic, elastic fluids. Herein, we report that elastic stresses imparted by nematic LCs can dynamically shape soft colloids and tune their physical properties. Specifically, we use giant unilamellar vesicles (GUVs) as soft colloids and explore the interplay of mechanical strain when the GUVs are confined within aqueous chromonic LC phases. Accompanying thermal quenching from isotropic to LC phases, we observe the elasticity of the LC phases to transform initially spherical GUVs (diameters of 2-50 µm) into two distinct populations of GUVs with spindle-like shapes and aspect ratios as large as 10. Large GUVs are strained to a small extent (R/r < 1.54, where R and r are the major and minor radii, respectively), consistent with an LC elasticity-induced expansion of lipid membrane surface area of up to 3% and conservation of the internal GUV volume. Small GUVs, in contrast, form highly elongated spindles (1.54 < R/r < 10) that arise from an efflux of LCs from the GUVs during the shape transformation, consistent with LC-induced straining of the membrane leading to transient membrane pore formation. A thermodynamic analysis of both populations of GUVs reveals that the final shapes adopted by these soft colloids are dominated by a competition between the LC elasticity and an energy (∼0.01 mN/m) associated with the GUV-LC interface. Overall, these results provide insight into the coupling of strain in soft materials and suggest previously unidentified designs of LC-based responsive and reconfigurable materials.

Interplay of packing and flip-flop in local bilayer deformation. How phosphatidylglycerol could rescue mitochondrial function in a cardiolipin-deficient yeast mutant

In a previous work, we have shown that a spatially localized transmembrane pH gradient, produced by acid micro-injection near the external side of cardiolipin-containing giant unilamellar vesicles, leads to the formation of tubules that retract after the dissipation of this gradient. These tubules have morphologies similar to mitochondrial cristae. The tubulation effect is attributable to direct phospholipid packing modification in the outer leaflet, that is promoted by protonation of cardiolipin headgroups. In this study, we compare the case of cardiolipin-containing giant unilamellar vesicles with that of giant unilamellar vesicles that contain phosphatidylglycerol (PG). Local acidification also promotes formation of tubules in the latter. However, compared with cardiolipin-containing giant unilamellar vesicles the tubules are longer, exhibit a visible pearling, and have a much longer lifetime after acid micro-injection is stopped. We attribute these differences to an additional mechanism that increases monolayer surface imbalance, namely inward PG flip-flop promoted by the local transmembrane pH gradient. Simulations using a fully nonlinear membrane model as well as geometrical calculations are in agreement with this hypothesis. Interestingly, among yeast mutants deficient in cardiolipin biosynthesis, only the crd1-null mutant, which accumulates phosphatidylglycerol, displays significant mitochondrial activity. Our work provides a possible explanation of such a property and further emphasizes the salient role of specific lipids in mitochondrial function.

Divided we stand: splitting synthetic cells for their proliferation

With the recent dawn of synthetic biology, the old idea of man-made artificial life has gained renewed interest. In the context of a bottom-up approach, this entails the de novo construction of synthetic cells that can autonomously sustain themselves and proliferate. Reproduction of a synthetic cell involves the synthesis of its inner content, replication of its information module, and growth and division of its shell. Theoretical and experimental analysis of natural cells shows that, whereas the core synthesis machinery of the information module is highly conserved, a wide range of solutions have been realized in order to accomplish division. It is therefore to be expected that there are multiple ways to engineer division of synthetic cells. Here we survey the field and review potential routes that can be explored to accomplish the division of bottom-up designed synthetic cells. We cover a range of complexities from simple abiotic mechanisms involving splitting of lipid-membrane-encapsulated vesicles due to physical or chemical principles, to potential division mechanisms of synthetic cells that are based on prokaryotic division machineries.

Recognition of membrane-bound fusion-peptide/MPER complexes by the HIV-1 neutralizing 2F5 antibody: implications for anti-2F5 immunogenicity

The membrane proximal external region (MPER) of the fusogenic HIV-1 glycoprotein-41 harbors the epitope sequence recognized by 2F5, a broadly neutralizing antibody isolated from an infected individual. Structural mimicry of the conserved MPER 2F5 epitope constitutes a pursued goal in the field of anti-HIV vaccine development. It has been proposed that 2F5 epitope folding into its native state is attained in the vicinity of the membrane interface and might involve interactions with other viral structures. Here we present results indicating that oligomeric complexes established between MPER and the conserved amino-terminal fusion peptide (FP) can partition into lipid vesicles and be specifically bound by the 2F5 antibody at their surfaces. Cryo-transmission electron microscopy of liposomes doped with MPER:FP peptide mixtures provided the structural grounds for complex recognition by antibody at lipid bilayer surfaces. Supporting the immunogenicity of the membrane-bound complex, these MPER:FP peptide-vesicle formulations could trigger cross-reactive anti-MPER antibodies in rabbits. Thus, our observations suggest that contacts with N-terminal regions of gp41 may stabilize the 2F5 epitope as a membrane-surface antigen.

Yeast vacuoles fragment in an asymmetrical two-phase process with distinct protein requirements

Yeast vacuoles fragment and fuse in response to environmental conditions, such as changes in osmotic conditions or nutrient availability. Here we analyze osmotically induced vacuole fragmentation by time-lapse microscopy. Small fragmentation products originate directly from the large central vacuole. This happens by asymmetrical scission rather than by consecutive equal divisions. Fragmentation occurs in two distinct phases. Initially, vacuoles shrink and generate deep invaginations that leave behind tubular structures in their vicinity. Already this invagination requires the dynamin-like GTPase Vps1p and the vacuolar proton gradient. Invaginations are stabilized by phosphatidylinositol 3-phosphate (PI(3)P) produced by the phosphoinositide 3-kinase complex II. Subsequently, vesicles pinch off from the tips of the tubular structures in a polarized manner, directly generating fragmentation products of the final size. This phase depends on the production of phosphatidylinositol-3,5-bisphosphate and the Fab1 complex. It is accelerated by the PI(3)P- and phosphatidylinositol 3,5-bisphosphate-binding protein Atg18p. Thus vacuoles fragment in two steps with distinct protein and lipid requirements.

Lipid polymorphisms and membrane shape

Morphological plasticity of biological membrane is critical for cellular life, as cells need to quickly rearrange their membranes. Yet, these rearrangements are constrained in two ways. First, membrane transformations may not lead to undesirable mixing of, or leakage from, the participating cellular compartments. Second, membrane systems should be metastable at large length scales, ensuring the correct function of the particular organelle and its turnover during cellular division. Lipids, through their ability to exist with many shapes (polymorphism), provide an adequate construction material for cellular membranes. They can self-assemble into shells that are very flexible, albeit hardly stretchable, which allows for their far-reaching morphological and topological behaviors. In this article, we will discuss the importance of lipid polymorphisms in the shaping of membranes and its role in controlling cellular membrane morphology.

Toward understanding protocell mechanosensation

Mechanosensitive (MS) channels can prevent bacterial bursting during hypo-osmotic shocks by responding to increases in lateral tension at the membrane level through an integrated and coordinated opening mechanism. Mechanical regulation in protocells could have been one of the first mechanisms to evolve in order to preserve their integrity against changing environmental conditions. How has the rich functional diversity found in present cells been created throughout evolution, and what did the primordial MS channels look like? This review has been written with the aim of identifying which factors may have been important for the appearance of the first osmotic valve in a prebiotic context, and what this valve may have been like. It highlights the mechanical properties of lipid bilayers, the association of peptides as aggregates in membranes, and the conservation of sequence motifs as central aspects to understand the evolution of proteins that gate below the tension required for spontaneous pore formation and membrane rupture. The arguments developed here apply to both MscL and MscS homologs, but could be valid to mechano-susceptible proteins in general.

Role of iPLA(2) in the regulation of Src trafficking and microglia chemotaxis

Microglia are immune effector cells in the central nervous system (CNS) and their activation, migration and proliferation play crucial roles in brain injuries and diseases. We examined the role of intracellular Ca(2+) -independent phospholipase A(2) (iPLA(2)) in the regulation of microglia chemotaxis toward ADP. Inhibition of iPLA(2) by 4-bromoenol lactone (BEL) or iPLA(2) knockdown exerted a significant inhibition on phosphatidylinositol-3-kinase (PI3K) activation and chemotaxis. Further examination revealed that iPLA(2) knockdown abrogated Src activation, which is required for PI3K activation and chemotaxis. Colocalization studies showed that cSrc-GFP was retained in the endosomal recycling compartment (ERC) in iPLA(2) knockdown cells, but the addition of arachidonic acid (AA) could restore cSrc trafficking to the plasma membrane by allowing the formation/release of recycling endosomes associated with cSrc-GFP. Using BODIPY-AA, we showed that AA is selectively enriched in recycling endosomes. These results suggest that AA is required for the cSrc trafficking to the plasma membrane by controlling the formation/release of recycling endosomes from the ERC.

Membrane Curvature Induced by Sugar and Polymer Solutions

We monitor the effect of transversal membrane asymmetry on the morphology of giant uni-lamellar vesicles in sugar and polymer solutions. The shapes of fluid lipid vesicles are governed by the bending elasticity of their membrane which is characterized by the bending modulus and the spontaneous curvature of the bilayer. We present a recently developed technique for the measurement of the spontaneous curvature using quantitative phase contrast microscopy. Different mechanisms for elastic membrane asymmetry and the role of the bending energy concept for the morphology of cellular organelles are discussed.

Periodic modulation of tubular vesicles induced by phase separation

We investigated changes in the shape of tubular ternary vesicles induced by phase separation into liquid-ordered and liquid-disordered phases. Tubular vesicles transform into two types of periodically modulated vesicles depending on the area fraction of the liquid-ordered phase. One type is a necklace of oblate subunits with two circular domains of the liquid-order phase, and the other is a periodically modulated tube with stripes of the liquid-order phase. The transition between the circular and striped domains is governed by the domain boundary energy, whereas the periodicity of modulated vesicles is determined geometrically based on the fixed volume and area constraints. The observed multidomain vesicles are kinetically trapped in metastable states, and all domains show budding to reduce the boundary energy.

Multiscale simulation of protein mediated membrane remodeling

Proteins interacting with membranes can result in substantial membrane deformations and curvatures. This effect is known in its broadest terms as membrane remodeling. This review article will survey current multiscale simulation methodologies that have been employed to examine protein mediated membrane remodeling.

Interactions of surfactants with lipid membranes

Surfactants are surface-active, amphiphilic compounds that are water-soluble in the micro- to millimolar range, and self-assemble to form micelles or other aggregates above a critical concentration. This definition comprises synthetic detergents as well as amphiphilic peptides and lipopeptides, bile salts and many other compounds. This paper reviews the biophysics of the interactions of surfactants with membranes of insoluble, naturally occurring lipids. It discusses structural, thermodynamic and kinetic aspects of membrane-water partitioning, changes in membrane properties induced by surfactants, membrane solubilisation to micelles and other phases formed by lipid-surfactant systems. Each section defines and derives key parameters, mentions experimental methods for their measurement and compiles and discusses published data. Additionally, a brief overview is given of surfactant-like effects in biological systems, technical applications of surfactants that involve membrane interactions, and surfactant-based protocols to study biological membranes.