Correlation between the ripple phase and stripe domains in membranes

Incorporation of Triacylglycerol and Cholesteryl Ester Droplets in Phase-Separated Giant Unilamellar Vesicles

Cytoplasmic lipid droplets form from the endoplasmic reticulum (ER). Because the ER membrane can undergo phase separation, the interaction of lipid droplets with phase-separated bilayers is of significant interest. In this study, we used fluorescence microscopy to investigate the incorporation of droplets composed of triolein, trilinolein, trimyristolein, trieicosenoin, and cholesteryl arachidonate in the bilayers of giant unilamellar vesicles (GUVs) consisting of a mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol. After the triacylglycerol droplets were incorporated, the DOPC/DPPC/cholesterol (3:3:2) GUVs, which exhibited liquid-disordered (Ld) and liquid-ordered (Lo) phase separation, retained their phase-separated state. The triacylglycerol droplets were predominantly partitioned in the Ld domains. To elucidate the basis of this preferential partitioning, we investigated the surface pressures of DOPC, DPPC, and cholesterol monolayers containing triolein at the air/water interface using a Langmuir trough. From these measurements, we determined the interfacial tension at the monolayer-covered triolein/water interface. The results showed that DOPC most effectively reduced the interfacial tension. Thus, the droplet sorting into the DOPC-enriched Ld domains likely arose from the difference in the abilities of the two phases to stabilize the droplet interface. In contrast, cholesteryl arachidonate had a profound effect on bilayer phase behavior. Fluorescence images of the DOPC/DPPC/cholesterol (3:3:2) GUVs showed that the domain structures disappeared after droplet incorporation. Additionally, surface pressure measurements of DOPC/DPPC/cholesterol (3:3:2) monolayers containing cholesteryl arachidonate at the air/water interface suggested that cholesteryl arachidonate weakened the lipid-lipid interaction. The results indicate that the cholesteryl arachidonate molecules diffused across the bilayer to hinder the bilayer phase separation.

Melittin-Induced Structural Transformations in DMPG and DMPS Lipid Membranes: A Langmuir Monolayer and AFM Study

In this study, we explore the interactions between melittin, a cationic antimicrobial peptide, and model lipid membranes composed of the negatively charged phospholipids 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) and 1,2-dimyristoyl-sn-glycero-3-phosphoserine (DMPS). Using the Langmuir monolayer technique and atomic force microscopy (AFM), we reveal novel insights into these interactions. Our key finding is the observation of the ripple phase in the DMPS bilayer on mica, a phenomenon not previously reported for negatively charged single bilayers. This discovery is significant given the critical role of phosphatidylserine (PS) in cancer biology and the potential of melittin as an anticancer agent. We also highlight the importance of subphase composition, as melittin interacts preferentially with lipids in the liquid-condensed phase; thus, selecting the appropriate subphase composition is crucial because it affects lipid behavior and consequently melittin interactions. Our results show that melittin incorporates into lipid monolayers in both liquid-expanded and liquid-condensed phases, enhancing membrane fluidity and disorder, but is expelled from DMPS in the solid phase. AFM imaging further reveals that melittin induces substantial structural changes in the DMPG membrane and forms the ripple phase in the DMPS bilayers. Despite these alterations, melittin does not cause pore formation or membrane rupture, suggesting strong electrostatic adsorption on the membrane surface that prevents penetration. These findings highlight the differential impacts of melittin on lipid monolayers and bilayers and underscore its potential for interacting with membranes without causing disruption.

Mechanical characterization of freestanding lipid bilayers with temperature-controlled phase

Coexistence of lipid domains in cell membranes is associated with vital biological processes. Here, we investigate two such membranes: a multi-component membrane composed of DOPC and DPPC lipids with gel and fluid separated domains, and a single component membrane composed of PMPC lipids forming ripples. We characterize their mechanical properties below their melting point, where ordered and disordered regions coexist, and above their melting point, where they are in fluid phase. To conduct these inquiries, we create lipid bilayers in a microfluidic chip interfaced with a heating system and optical tweezers. The chip features a bubble trap and enables high-throughput formation of planar bilayers. Optical tweezers experiments reveal interfacial hydrodynamics (fluid-slip) and elastic properties (membrane tension and bending rigidity) at various temperatures. For PMPC bilayers, we demonstrate a higher fluid slip at the interface in the fluid-phase compared to the ripple phase, while for the DOPC:DPPC mixture, similar fluid slip is measured below and above the transition point. Membrane tension for both compositions increases after thermal fluidization. Bending rigidity is also measured using the forces required to extend a lipid nanotube pushed out of the freestanding membranes. This novel temperature-controlled microfluidic platform opens numerous possibilities for thermomechanical studies on freestanding planar membranes.

Deciphering the origin of the melting profile of unilamellar phosphatidylcholine liposomes by measuring the turbidity of its suspensions

The elucidation of the thermal properties of phosphatidylcholine liposomes is often based on the analysis of the thermal capacity profiles of multilamellar liposomes (MLV), which may qualitatively disagree with those of unilamellar liposomes (LUV). Experiments and interpretation of LUV liposomes is further complicated by aggregation and lamellarization of lipid bilayers in a short time period, which makes it almost impossible to distinguish the signatures of the two types of bilayers. To characterize independently MLV and LUV of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), the latter were prepared with the addition of small amounts of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylglycerol (DPPG) which, due to the sterical hindrance and negative charge at a given pH value, cause LUV repellence and contribute to their stability. Differential scanning calorimetry curves and temperature-dependent UV/Vis spectra of the prepared MLV and LUV were measured. Multivariate analysis of spectrophotometric data determined the phase transition temperatures (pretransition at T_p and the main phase transition at T_m), and based on the changes in turbidities, the thickness of the lipid bilayer in LUV was determined. The obtained data suggested that the curvature change is a key distinguishing factor in MLV and LUV heat capacity profiles. By combining the experimental results and those obtained by MD simulations, the interfacial water layer was characterized and its contribution to the thermal properties of LUV was discussed.

Atomic force microscopy of phase separation on ruptured, giant unilamellar vesicles, and a mechanical pathway for the co-existence of lipid gel phases

Giant unilamellar vesicles (GUVs) and supported lipid bilayers (SLBs) are synthetic model systems widely used in biophysical studies of lipid membranes. Although SLBs are advantageous for biophysical analysis, phase separation behaviors of lipid species in these two model systems can differ due to the lipid-substrate interactions that are present only for SLBs. In the present study, we report that in binary systems, certain phase domains on GUVs retain their original shapes and patterns after the GUVs rupture on glass surfaces. This enabled atomic force microscopy (AFM) experiments on phase domains, a procedure difficult to perform and interpret when applied to GUVs. Unusual phase behavior was evident in binary GUVs containing DLPC and either DPPC or DSPC. These DLPC/DSPC and DLPC/DPPC GUVs both presented the thermodynamic anomaly of having two co-existing gel phases. One phase (a bright phase) included a relatively high concentration of DiI-C20 but excluded Bodipy-HPC, and the other (dark phase) excluded both probes. The bright phases are of interest because they seem to stabilize dark phases against coalescence. Results suggested that the gel phases labeled by DiIC20 in the DLPC/DSPC membrane, which surround the dark gel phase, is an extra layer of membrane, indicating a highly curved structure that might stabilize the interior dark domains, thereby enabling the co-existence of two different gel phases. Results show the utility of AFM on collapsed GUVs, and suggest a possible mechanism for stabilization of lipid domains.

Molecular engineering of lanthanide ion chelating phospholipids generating assemblies with a switched magnetic susceptibility

Lanthanide ion (Ln³⁺) chelating amphiphiles are powerful molecules for tailoring the magnetic response of polymolecular assemblies. Mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine-diethylene triaminepentaacetate (DMPE-DTPA) complexed to Ln³⁺ deliver highly magnetically responsive bicelles. Their magnetic properties are readily tuned by changing the bicellar size or the magnetic susceptibility Δχ of the bilayer lipids. The former technique is intrinsically bound to the region of the phase diagram guarantying the formation of bicelles. Methods aiming towards manipulating the Δχ of the bilayer are comparatively more robust, flexible and lacking. Herein, we synthesized a new Ln³⁺ chelating phospholipid using glutamic acid as a backbone: DMPE-Glu-DTPA. The chelate polyhedron was specifically engineered to alter the Δχ, whilst remaining geometrically similar to DMPE-DTPA. Planar asymmetric assemblies hundreds of nanometers in size were achieved presenting unprecedented magnetic alignments. The DMPE-Glu-DTPA/Ln³⁺ complex switched the Δχ, achieving perpendicular alignment of assemblies containing Dy³⁺ and parallel alignment of those containing Tm³⁺. Moreover, samples with chelated Yb³⁺ were more alignable than the Tm³⁺ chelating counterparts. Such a possibility has never been demonstrated for planar Ln³⁺ chelating polymolecular assemblies. The physico-chemical properties of these novel assemblies were further studied by monitoring the alignment behavior at different temperatures and by including 16 mol% of cholesterol (Chol-OH) in the phospholipid bilayer. The DMPE-Glu-DTPA/Ln³⁺ complex and the resulting assemblies are promising candidates for applications in numerous fields including pharmaceutical technologies, structural characterization of membrane biomolecules by NMR spectroscopy, as contrasting agents for magnetic resonance imaging, and for the development of smart optical gels.

Pathological levels of glucosylceramide change the biophysical properties of artificial and cell membranes

Accumulation of glucosylceramide decreases membrane fluidity in artificial membranes and in cell models of Gaucher disease.

Polymorphic transformation towards formation of nanotubes by self-assembly of an achiral molecule

In this paper, nanotubes with a uniform diameter were prepared by self-assembly of an achiral azobenzene-containing fatty acid. The polymorphic transformation of the assemblies during the cooling process was systematically studied. By controlling the incubation temperature, different morphologies, such as membranes, stripes, helical ribbons and tubes, were all obtained in our experiment. These elements were all predicted by Selinger et al. in the theoretical model of the formation of nanotubes. To the best of our knowledge, this is the first experimental example to fully support their theory.

Hybrid copolymer-phospholipid vesicles: phase separation resembling mixed phospholipid lamellae, but with mechanical stability and control

Vesicles whose bilayer membranes contain phospholipids mixed with co-polymers or surfactants comprise new hybrid materials having potential applications in drug delivery, sensors, and biomaterials. Here we describe a model polymer-phospholipid hybrid membrane system exhibiting strong similarities to binary phospholipid mixtures, but with more robust membrane mechanics. A lamella-forming graft copolymer, PDMS-co-PEO (polydimethylsiloxane-co-polyethylene oxide) was blended with a high melting temperature phospholipid, DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), over a broad compositional range. The resulting giant hybrid unilamellar vesicles were compared qualitatively and quantitatively to analogous mixed phospholipid membranes in which a low melting temperature phospholipid, DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine), was blended with DPPC. The mechanical properties of the hybrid vesicles, even when phase separated, were robust with high lysis stresses and strains approaching those of the pure copolymer vesicles. The temperature-composition phase diagram of the hybrid vesicles closely resembled that of the mixed phospholipids; with only slightly greater nonidealities in the hybrid compared with DOPC/DPPC mixed membranes. In both systems, it was demonstrated that tension could be used to manipulate DPPC solidification into domains of patchy or striped morphologies that exhibited different tracer incorporation. The patch and stripe-shaped domains are thought to be different solid DPPC polymorphys: ripple and tilt (or gel). This work demonstrates that in mixed-phospholipid bilayers where a high-melting phospholipid solidifies on cooling, the lower-melting phospholipid may be substituted by an appropriate copolymer to improve mechanical properties while retaining the underlying membrane physics.

Nature's lessons in design: nanomachines to scaffold, remodel and shape membrane compartments

Our understanding of the membrane sculpting capabilities of proteins from experimental model systems could be used to construct functional compartmentalised architectures for the engineering of synthetic cells.

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.

Influence of nanoparticle-membrane electrostatic interactions on membrane fluidity and bending elasticity

The aim of this work is to investigate the effect of electrostatic interactions between the nanoparticles and the membrane lipids on altering the physical properties of the liposomal membrane such as fluidity and bending elasticity. For this purpose, we have used nanoparticles and lipids with different surface charges. Positively charged iron oxide (γ-Fe2O3) nanoparticles, neutral and negatively charged cobalt ferrite (CoFe2O4) nanoparticles were encapsulated in neutral lipid 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine and negatively charged 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine lipid mixture. Membrane fluidity was assessed through the anisotropy measurements using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene. Though the interaction of both the types of nanoparticles reduced the membrane fluidity, the results were more pronounced in the negatively charged liposomes encapsulated with positively charged iron oxide nanoparticles due to strong electrostatic attractions. X-ray photoelectron spectroscopy results also confirmed the presence of significant quantity of positively charged iron oxide nanoparticles in negatively charged liposomes. Through thermally induced shape fluctuation measurements of the giant liposomes, a considerable reduction in the bending elasticity modulus was observed for cobalt ferrite nanoparticles. The experimental results were supported by the simulation studies using modified Langevin-Poisson-Boltzmann model.