Two-component Vesicles Research Articles

Active membrane recycling induced morphology changes in vesicles

Membranes of organelles in the intracellular trafficking pathway continuously undergo recycling through fission and fusion processes. The effect of these recycling processes on the large-scale morphology of organelles is not well understood. Using a dynamically triangulated surface model, we developed a membrane morphology simulator that allows for membrane trafficking, and analyzed the steady state shape of vesicles subjected to such active remodeling. We study a two-component vesicle composed of 1) active species which can have nonzero spontaneous curvature and participate in the recycling and 2) inactive species which do not participate in the recycling. We obtain a plethora of steady state morphologies as a function of the activity rate, spontaneous curvature, and the strength of interaction between species. We observe that morphology changes, as a function of rate of activity, are diametrically opposite for the two signs of the spontaneous curvature, but only have a weak effect on its magnitude. The interplay between the in-plane diffusion, the activity rate, and the spontaneous curvature are shown to determine the vesicle morphology at the steady state. It is shown that the spontaneous curvature and activity inhibits the formation of clusters of active species on the surface. We carry out linear stability analysis of a continuum model and show that the spherical shape of a vesicle is indeed unstable when subjected to active membrane recycling above a certain activity rate.

Dynamic response on a nanometer scale of binary phospholipid-cholesterol vesicles: Low-frequency Raman scattering insight.

Low-frequency Raman spectroscopy was used to study the dynamic response on a nanometer scale of aqueous suspensions of two-component lipid vesicles. Binary mixtures of saturated phospholipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) and cholesterol are interesting for possible coexistence of solidlike and liquid-ordered phases, while the phase coexistence was not reported for unsaturated phospholipid (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC) and cholesterol mixtures. The DOPC-DPPC mixtures represent the well-documented case of coexisting domains of solidlike and liquid-disordered phases. These three series of lipid mixtures are studied here. A broad peak with the maximum in the range of 30-50cm^{-1} and a narrow peak near 10cm^{-1} are observed in the Raman susceptibility of the binary mixtures and attributed to the acousticlike vibrational density of states and layer modes, respectively. Parameters of the broad and narrow peaks are sensitive to lateral and conformational hydrocarbon chain ordering. It was also demonstrated that the low-frequency Raman susceptibility of multicomponent lipid bilayers allows one to determine the phase state of lipid bilayers and distinguish the homogeneous distribution of molecular complexes from coexisting domains with sizes above several nanometers. Thus, the low-frequency Raman spectroscopy provides unique information in studying phase coexistence in lipid bilayers.

Theoretical simulations of deformation coupling with phase separation of two-component charged vesicles in a two-dimensional plane

The real bio-membranes are of multi-component, and they usually carry a certain quantity of charges. Therefore, it is of great biological significance to study charged multicomponent vesicles. However, the charged multi-component vesicles have been not yet systematically studied due mainly to the following two reasons: first, there are too many factors that will influence the behaviors of charged multi-component vesicles; second, theoretically it is difficult to deal with the phase separation of the multiple components from the Coulomb interaction of charged components at the same time. This work shows that the combination of the discrete-spatial variational method and dissipative dynamics can be used to address the above issues. For simplicity, we will consider only the deformation coupled with the phase separation of two-component charged vesicles in a two-dimensional plane rather than in three-dimensional space, which can present us more systematic research results. Besides, we have not considered the screening effects of counter ions or salts in this work, or equivalently we consider only the case where the screening length is relatively big. The charged vesicle is composed of two components A and B, where component A is negatively charged while component B is neutral. In particular, the charges on the vesicle can freely move in the membrane, which may be described by a time-dependent Ginzburg Landau equation. Initially, the two components are uniformly distributed on the vesicle.In this work, we specially focus on the influence of the electrostatic interaction on the compatibility of different components. It is found that introduction of charges will promote the apparent miscibility between different components. This could explain that the electrostatic interactions may contribute to the increase of the compatibility of different biomolecules in biological system. When temperature is relatively high, the electrostatic interaction will completely inhibit the phase separation which actually prevents the same component from being clustered. When temperature is relatively low, the electrostatic interaction will increase the number of phase domains, which actually turns the original macro phase separation into the micro one, thus reducing the cluster size. In this work, we also systematically study the influences of other factors, such as temperature, charge density of charged components, and the averaged concentration of charged component, on the final configuration of charged multicomponent vesicle. In particular, a phase diagram of the temperature and the averaged concentration of the charged component is obtained, and it is found that the number of phase domains will increase with the increase of charge density of component A. These conclusions are also qualitatively applicable to three-dimensional two-component charged vesicles.

Shape Morphologies of Icosahedral Two-Component Vesicles.

What are the features of partitioning of crystalline materials on the surface of a two-component icosahedral vesicle? We model the response of the rigid hardly stretchable crystalline icosahedra upon addition of a softer component on its surface. We demonstrate how the soft phase "invades" the shell regions with the highest elastic energy density around 12 5-fold topological defects. We explore the phase diagram of these inhomogeneous shells as a function of the soft material fraction, shell radius, and elastic moduli of the two phases. The findings are compared with the recent computer simulation findings, and their biological relevance, for example, for the structure of icosahedral viruses, is also discussed.

1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)-rich domain formation in binary phospholipid vesicle membranes: two-dimensional nucleation and growth.

Decades of study have probed phase transitions in model phospholipid bilayers and vesicles, especially in the context of the equilibrium phase diagram. Critical to the response of vesicles to environmental triggers, to the ultimate domain morphology, and to the approach to equilibrium (or not), we present here a study of domain formation in vesicles, focusing on a mechanism by which the cooling rate, tension, and composition affect the first appearance (nucleation) and subsequent growth of solid membrane domains. Employing a popular mixed membrane model based on DOPC and DPPC (1,2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, respectively), we examined phase separation in giant two-component vesicles that were cooled from the one-phase fluid (Lα) region of the phase diagram into a region of fluid (Lα)-solid coexistence. At moderate and low membrane tensions, cooling produced solid DPPC-rich domains appearing as compact patches or irregular hexagons and likely with a Pβ' (ripple) arrangement. (The compact solid domains in this study differed distinctly from striped domains in vesicles of the same composition, in terms of molecular organization and conditions of first appearance during cooling.) The amounts of these solid domains were shown to adhere to the lever arm rule for a tie line on the phase diagram, with a solid composition near 95 mol % DPPC. The nucleation of the compact solid domains occurred in a short period, followed by rapid addition of ordered molecules to the nucleated domains, excluding tracer dye. The two-dimensional nucleation density of these compact solid domains (in the range of 10(-2)-10(-1) μm(-2)) was found to increase with the cooling rate (equivalent to the quench depth) with a greater than linear dependence. The 2-D nucleation density was also seen to decrease with membrane tension, presumably because membrane tension increases the line tension around a domain that opposes nucleation. A sigmoidal dependence of the nucleation density on the DPPC concentration was also found. With cooling rates in excess of ∼1 °C/min, solid domains persisted down to room temperature, likely passing from a preferred equilibrium to a local equilibrium with continued cooling. As a result of the persistence of the originally nucleated domains and the conservation of DPPC in the membrane, we observed an increasingly greater number of smaller domains with increased cooling rates. The domains in these vesicles were compact or hexagonal-shaped in contrast to flower-shaped dendritic domains in the same membrane system in a supported membrane configuration.

Large effect of membrane tension on the fluid–solid phase transitions of two-component phosphatidylcholine vesicles

Model phospholipid membranes and vesicles have long provided insight into the nature of confined materials and membranes while also providing a platform for drug delivery. The rich thermodynamic behavior and interesting domain shapes in these membranes have previously been mapped in extensive studies that vary temperature and composition; however, the thermodynamic impact of tension on bilayers has been restricted to recent reports of subtly reduced fluid-fluid transition temperatures. In two-component phosphatidylcholine unilamellar vesicles [1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)], we report a dramatic influence of tension on the fluid-solid transition and resulting phases: At fixed composition, systematic variations in tension produce differently shaped solid domains (striped or irregular hexagons), shift fluid-solid transition temperatures, and produce a triple-point-like intersection of coexistence curves at elevated tensions, about 3 mN/m for 30% DOPC/70% DPPC. Tension therefore represents a potential switch of microstructure in responsive engineered materials; it is an important morphology-determining variable in confined systems, and, in biological membranes, it may provide a means to regulate dynamic structure.

On the Role of Anisotropy of Membrane Components in Formation and Stabilization of Tubular Structures in Multicomponent Membranes

Influence of isotropic and anisotropic properties of membrane constituents (nanodomains) on formation of tubular membrane structures in two-component vesicle is numerically investigated by minimization of the free energy functional based on the deviatoric-elasticity model of the membrane. It is shown that the lateral redistribution and segregation of membrane components may induce substantial change in membrane curvature resulting in the growth of highly curved tubular structures.

Adhesion-induced phase behavior of two-component membranes and vesicles.

The interplay of adhesion and phase separation is studied theoretically for two-component membranes that can phase separate into two fluid phases such as liquid-ordered and liquid-disordered phases. Many adhesion geometries provide two different environments for these membranes and then partition the membranes into two segments that differ in their composition. Examples are provided by adhering vesicles, by hole- or pore-spanning membranes, and by membranes supported by chemically patterned surfaces. Generalizing a lattice model for binary mixtures to these adhesion geometries, we show that the phase behavior of the adhering membranes depends, apart from composition and temperature, on two additional parameters, the area fraction of one membrane segment and the affinity contrast between the two segments. For the generic case of non-vanishing affinity contrast, the adhering membranes undergo two distinct phase transitions and the phase diagrams in the composition/temperature plane have a generic topology that consists of two two-phase coexistence regions separated by an intermediate one-phase region. As a consequence, phase separation and domain formation is predicted to occur separately in each of the two membrane segments but not in both segments simultaneously. Furthermore, adhesion is also predicted to suppress the phase separation process for certain regions of the phase diagrams. These generic features of the adhesion-induced phase behavior are accessible to experiment.

Budding Behavior of Multi-Component Vesicles

For multicomponent vesicles, the line tension of domain boundaries and the component-dependent elastic properties (e.g., spontaneous curvatures) are the two most important factors that mediate the budding behaviors of these vesicles. This paper especially focuses on their effects on the budding types and the budding number of a two-component vesicle. We found that the budding number is mainly determined by the component-dependent elastic properties while the budding types mediated by line tensions. A phase diagram is also obtained showing three different types of phase regions: (i) partially budding, (ii) partially bud-off, and (iii) totally bud-off regions. These inspiring results are, however, derived from a very simple spherical-cap model, and have been tested by computer simulations showing good agreement. We emphasize that, besides testing the spherical-cap model, the computer simulation techniques developed in the current work can be easily extended to other systems involving multicomponent vesicles.

The Role of “Raft-Like” Membranes on Antimicrobial Peptide-Lipid Bilayer Interactions

In vitro studies of antimicrobial peptides have largely focused on simple two-component vesicle systems consisting of two miscible lipids. However, real membranes are vastly more complex. In this study, we investigate the in vitro interaction between antimicrobial peptides and non-homogenous, cholesterol-containing model membranes with fluorescence-based and other biophysical assays. Phase mappings of ternary and binary mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyol-sn-glycero-3-phosphocholine (DPPC), and cholesterol have shown that separate phasal domains can exist in liposomes at a broad range of lipid compositions. The ability of these domains to form liquid-liquid coexisting phases, particularly liquid-disordered (ld)-liquid-ordered (lo) regions sometimes referred to as membrane “rafts”, raises the question of their stability or integrity in relation to similar cholesterol containing membranes. We therefore have studied the interactions of these membranes in response to antimicrobial peptide attack, using a series of antimicrobial peptides specifically chosen to disrupt membranes by a variety of mechanisms. We have found that liposomes containing these raft domains display far greater membrane disruption than similar homogenous model membranes containing equal mole percentages of cholesterol. Pyrene-PC translocation assays demonstrate an increased susceptibility of these raft-like membranes to form toroidal-pore-like structures in the bilayer. Binding parameters obtained from isothermal titration calorimetry measurements and circular dichroism spectroscopy indicate that preferential peptide-lipid binding interactions alone are not responsible for the differences in membrane disruption. These combined results indicate that an increase in local peptide concentration as a consequence of the peptide partitioning preferentially to ld domains allows for increased antimicrobial activity by pore formation or fragmentation of the membrane.

Numerical investigations of the dynamics of two-component vesicles

We examined the dynamics of the deformation and phase separation of two-componentvesicles. First, we numerically investigated the effects of (i) thermal noise, (ii)hydrodynamic flow induced by the line tension of the domain boundary and (iii)composition-dependent bending rigidity on the coarsening dynamics of a phase-separatedpattern on the surfaces of vesicles with fixed shapes. The dynamical exponentz (NDB ∼ t − z, the total length of the domain boundaries) of the coarseningof the phase-separated pattern was found to decrease fromz = 1/3 under nothermal noise to 1/5 < z < 1/4 when including the effects of thermal noise. We also found that the hydrodynamiceffect enhances the coarsening in a bicontinuous phase separation for a sphericalvesicle. In phase separations of a shape-fixed tubular vesicle, a band-like phaseseparation with periodicity along the longer axis of the tube occurs because of thecomposition-dependent bending rigidity and the higher curvatures at the tube end-caps.Second, we also explored the dynamics of shape deformation coupled with phaseseparation through the bending rigidity of the membrane which depends on thelocal composition in lipids and found that the composition-dependent bendingrigidity crucially influences the phase separation and deformation of the vesicle.The results of simulations are in good agreement with experimentally observedbehavior known as ‘shape convergence’ (Yanagisawa et al 2008 Phys. Rev. Lett. 100148102).

Effective line tension and contact angles between membrane domains in biphasic vesicles

Inhomogeneities in membranes give rise to localized interactions at the interface between domains in two-component vesicles. The corresponding energy is expressed as a line tension between the two phases. In this paper we study the implications of the thickness mismatch between domains which has been experimentally reported to be of order 20-30% and the conditions under which the induced line tension can destabilize the domains in inhomogeneous vesicles. For asymmetric lipidic membranes we prove an increase of the line tension and the existence of a contact angle. Adsorption of impurities is also examined, our scope being the extension of the Canham-Helfrich model to describe elastic deformations and chemical interactions arising at microscopic scales. This mismatch effect may have important consequences for the stability of very small domains.

Adhesion of multicomponent vesicle membranes

In this work, we study the adhesion of multicomponent vesicle membrane to both flat and curved substrates, based on the conventional Helfrich bending energy for multicomponent vesicles and adhesion potentials of different forms. A phase field formulation is used to describe the different components of the vesicle. For the axisymmetric case, a number of representative equilibrium vesicle shapes are computed and a few energy diagrams are presented which reveal the dependence of the calculated shapes and solution branches on various parameters including both bending moduli and spontaneous curvatures as well as the adhesion potential constants. Our computation also confirms a recent experimental observation that the adhesion may promote phase separation in two-component vesicle membranes.

Computer Simulations of Vesicle Fission Induced by External Amphipathic Inclusions

Fission is an important biological phenomenon to maintain normal functions of cells, but its mechanism is not quite clear. In this paper, the fission behaviors of a two-component lipid vesicle induced by external amphipathic inclusions are studied by the dissipative particle dynamics method. We find that the fission of the lipid domain the in a spherical vesicle will occur only when the concentration of amphipathic inclusions is beyond a threshold concentration. It reveals that this kind of fission is induced by the combination of the domain's line tension and the asymmetric distribution of the amphipathic inclusions in the membrane's inner and outer leaflets. Furthermore, we also find that the fission behaviors are influenced by the area-to-volume ratio of vesicles and the structure characters of amphipathic inclusions (e.g., chain length and rigidity). Especially, the chain length and rigidity of amphipathic inclusions have complicated effects on the vesicle fission. With the variation of these factors, the fission process will be accelerated or delayed. The present study is helpful to understand the possible fission mechanism of lipid bilayers with embedded proteins.

Fusion and fission pathways of vesicles from amphiphilic triblock copolymers: a dissipative particle dynamics simulation study

Dissipative particle dynamics (DPD) simulations are employed to study the fusion and fission dynamics of polymeric vesicles formed from amphiphilic triblock copolymers. The amphiphilic molecule is built from two hydrophilic blocks and a hydrophobic middle block. Two different pathways for both fusion and fission processes of two-component vesicles with polymer-based symmetric membranes have been found in the simulations. For each of the pathways, the conditions required to obtain complete fusion and fission have been investigated. Moreover, the fission process of single-component vesicles with polymer-based asymmetric membranes has also been studied in the simulations. Interestingly, the daughter vesicles have the same composition as the parent vesicle and only one fission pathway has been observed. Furthermore, the fusion and fission pathways have been compared and distinct vesicle recycling pathways have been suggested according to the results from the simulations. These findings may be helpful in explaining the fusion and fission dynamics of vesicles and understanding the general principle of membranes.

Equilibrium Theory and Geometrical Constraint Equation for Two-Component Lipid Bilayer Vesicles

This paper aims at the general mathematical framework for the equilibrium theory of two-component lipid bilayer vesicles. To take into account the influences of the local compositions together with the mean curvature and Gaussian curvature of the membrane surface, a general potential functional is constructed. We introduce two kinds of virtual displacement modes: the normal one and the tangential one. By minimizing the potential functional, the equilibrium differential equations and the boundary conditions of two-component lipid vesicles are derived. Additionally, the geometrical constraint equation and geometrically permissible condition for the two-component lipid vesicles are presented. The physical, mathematical, and biological meanings of the equilibrium differential equations and the geometrical constraint equations are discussed. The influences of physical parameters on the geometrically permissible phase diagrams are predicted. Numerical results can be used to explain recent experiments.

Lipid redistribution in phosphatidylserine-containing vesicles adsorbing on titania

Lipid vesicles (liposomes) exhibit a wide range of behavior at inorganic (oxide) surfaces. A complete understanding of the vesicle-surface interactions, and of the ensuing transformations surface adsorbed liposomes undergo, has proven elusive. This is at least in part due to the large number of degrees of freedom of the system comprising vesicles with their molecular constituents, substrate surface, and electrolyte solution. The least investigated among these degrees of freedom are those intrinsic to the vesicles themselves, involving rearrangements of lipid molecules. In this study, the adsorption of two-component vesicles (phosphatidylcholine:phosphatidylserine) on titanium dioxide was investigated by dual polarization interferometry. Mixtures of these two lipids containing more than 20% of phosphatidylserine form supported bilayers on titania, with phosphatidylserine predominantly facing the surface of the oxide. The purpose of this investigation is to ascertain whether redistribution of phosphatidylserine occurs already in the adsorbing vesicles. Indeed, this was found to be the case. A possible mechanism of this process is discussed.

The Interface Width of Separated Two-Component Lipid Membranes

We study two-component vesicles with coherent domains of the components, where the domains are separated by an interface. The components are characterized by different spontaneous curvatures. No line tension term or interactions between components are included in the model. The influence of the interface width and interface location on the bending energy and shape of the vesicles is studied. How the spontaneous curvature of one component influences the concentration profile is examined. The vesicles of oblate and prolate geometries are investigated.

Equilibrium Shape Equation and Geometrically Permissible Condition for Two-Component Lipid Bilayer Vesicles

Equilibrium shapes of vesicles composed of a mixture of partially miscible amphiphiles are investigated. To take into account the influences of the composition, a simple phenomenological coupling between the co mposition and the curvatures, including the mean curvature and the Gauss curvature of the membrane surface, is suggested. By minimizing the potential functional, the general shape equation is obtained and solved analytically for vesicles with simple shapes. Besides, the geometrical constraint equation and geometrically permissible condition for the two-component lipid vesicles are put forward. The influences of physical parameters on the geometrically permissible phase diagrams are predicted. The close relations between the predictions and existing experimental phenomena published recently are shown.

Effects of carbon chain difference and lipid composition on the contact mechanics of two-component vesicle

The composition of phospholipid mixture is known to influence the lateral organization of the resulting biomembrane. In this study, the substrate-induced adhesion of two component vesicles composed of dimyristoyl-phosphatidylcholine/distearoyl-phosphatidylcholine (DMPC/DSPC) is probed by confocal reflectance interference microscopy (C-RICM), cross-polarized light microscopy and contact mechanics modeling. First, our contact mechanics model of adherent vesicle is validated for the two-component DMPC/DSPC vesicles. The incorporation of 10 mol% of DSPC in DMPC bilayer enhances both the degree of vesicle deformation and adhesion energy of the resulting two-component vesicle in comparison with those of one-component DMPC or DSPC vesicles. It is further illustrated that the compositional variation of the binary DMPC/DSPC mixture does not affect the contact mechanics parameters of adherent vesicles when the mole fraction of DSPC increases from 0 to 70%. Interestingly, the difference in the acyl chain length between the two lipids within the phospholipid mixture is directly correlated with the degree of vesicle deformation and adhesion energy. Overall, the biophysical responses of adherent vesicles are shown to be tunable by the bio-membrane composition and the carbon chain difference.