Separation of liquid phases in giant vesicles of ternary mixtures of phospholipids and cholesterol

Viscosity-Sensitive Membrane Dyes as Tools To Estimate the Crystalline Structure of Lipid Bilayers

Lipid membranes are crucial for cellular integrity and regulation, and tight control of their structural and mechanical properties is vital to ensure that they function properly. Fluorescent probes sensitive to the membrane's microenvironment are useful for investigating lipid membrane properties; however, there is currently a lack of quantitative correlation between the exact parameters of lipid organization and a readout from these dyes. Here, we investigate this relationship for "molecular rotors", or microviscosity sensors, by simultaneously measuring their fluorescence lifetime to determine the membrane viscosity, while using X-ray diffraction to determine the membrane's structural properties. Our results reveal a phase-dependent correlation between the membrane's structural parameters and mechanical properties measured by a BODIPY-based molecular rotor, giving excellent predictive power for the structural descriptors of the lipid bilayer. We also demonstrate that differences in membrane thickness between different lipid phases are not a prerequisite for the formation of lipid microdomains and that this requirement can be disrupted by the presence of line-active molecules. Our results underpin the use of membrane-sensitive dyes as reporters of the structure of lipid membranes.

Phase-Separated Giant Liposomes for Stable Elevation of α-Hemolysin Concentration in Lipid Membranes

Staphylococcus aureus α-hemolysin (αHL) is one of the most popular proteins in nanopore experiments within lipid membranes. Higher concentrations of αHL within the lipid membrane are desirable to enhance the mass transport capacity through nanopores. However, the reconstitution of αHL at high concentrations is associated with the problem of membrane lytic disruption. In this study, we present a method that effectively increases αHL concentration while maintaining membrane stability. This method is achieved by using phase-separated giant liposomes, where coexisting liquid-disordered (Ld) and liquid-ordered phases (Lo) are enriched in unsaturated lipids and saturated lipids with cholesterol (Chol), respectively. Fluorescence observation of αHL in liposomes revealed that the presence of Chol facilitates αHL insertion into the membrane. Despite the preferential localization of αHL in the Ld phase rather than the Lo phase, the coexistence of both Lo and Ld phases prevents membrane disruption in the presence of concentrated αHL. We have explained this stabilization mechanism considering the lower membrane tension exhibited by phase-separated liposomes compared to homogeneous liposomes. Under hypertonic conditions, we have successfully increased the local concentration of αHL by invagination of the lipid-only region in the Ld phase, leaving αHL behind. This method exhibits potential for the reconstitution of various nanochannels and membrane proteins that prefer the Ld phase over the Lo phase, thus enabling the production of giant liposomes at high concentrations and the replication of the membrane-crowding condition observed in cells.

Unsaturated Lipids Facilitate Partitioning of Transmembrane Peptides into the Liquid Ordered Phase

The affinity of single-pass transmembrane (TM) proteins for ordered membrane phases has been reported to depend on their lipidation, TM length, and lipid accessible surface area. In this work, the raft affinities of the TM domain of the linker for activation of T cells and its depalmitoylated variant are assessed using free energy simulations in a binary bilayer system composed of two laterally patched bilayers of ternary liquid ordered (Lo) and liquid disordered (Ld) phases. These phases are modeled by distinct compositions of distearoylphosphatidylcholine, palmitoyloleoylphosphatidylcholine (POPC), and cholesterol, and the simulations were carried out for 4.5 μs/window. Both peptides are shown to preferentially partition into the Ld phase in agreement with model membrane experiments and previous simulations on ternary lipid mixtures but not with measurements on giant plasma membrane vesicles where the Lo is slightly preferred. However, the 500 ns average relaxation time of lipid rearrangement around the peptide precluded a quantitative analysis of free energy differences arising from peptide palmitoylation and two different lipid compositions. When in the Lo phase, peptides reside in regions rich in POPC and interact preferentially with its unsaturated tail. Hence, the detailed substructure of the Lo phase is an important modulator of peptide partitioning, in addition to the inherent properties of the peptide.

Advances in giant unilamellar vesicle preparation techniques and applications

Giant unilamellar vesicles (GUVs) are versatile and promising cell-sized bio-membrane mimetic platforms. Their applications range from understanding and quantifying membrane biophysical processes to acting as elementary blocks in the bottom-up assembly of synthetic cells. Definite properties and requisite goals in GUVs are dictated by the preparation techniques critical to the success of their applications. Here, we review key advances in giant unilamellar vesicle preparation techniques and discuss their formation mechanisms. Developments in lipid hydration and emulsion techniques for GUV preparation are described. Novel microfluidic-based techniques involving lipid or surfactant-stabilized emulsions are outlined. GUV immobilization strategies are summarized, including gravity-based settling, covalent linking, and immobilization by microfluidic, electric, and magnetic barriers. Moreover, some of the key applications of GUVs as biomimetic and synthetic cell platforms during the last decade have been identified. Membrane interface processes like phase separation, membrane protein reconstitution, and membrane bending have been deciphered using GUVs. In addition, vesicles are also employed as building blocks to construct synthetic cells with defined cell-like functions comprising compartments, metabolic reactors, and abilities to grow and divide. We critically discuss the pros and cons of preparation technologies and the properties they confer to the GUVs and identify potential techniques for dedicated applications.

Raman Imaging Reveals Insights into Membrane Phase Biophysics in Cells

Cellular membranes are essential components of all living organisms. They are composed of a complex mixture of lipids with diverse chemical structures and crucial biological functions. The dynamic and heterogeneous nature of cellular membranes presents a challenge for studying their biophysical properties and organization in vivo. Raman imaging, particularly coherent Raman scattering techniques such as stimulated Raman scattering (SRS) microscopy, have emerged as powerful tools for studying cellular membranes with high spatial and temporal resolution and minimal perturbation. In this Review, we discuss the scientific importance and technical challenges of characterizing membrane composition in cellular contexts and how the advances of Raman imaging can provide unique insights into membrane phase behavior and organization. We also highlight recent applications of Raman imaging in studying cellular membranes and implications in diseases. In particular, the discovery of phase separation and a solid-phase intracellular membrane on endoplasmic reticulum is reviewed in detail, shedding light on the biology of lipotoxicity.

Lipid packing in biological membranes governs protein localization and membrane permeability

Plasma membrane (PM) heterogeneity has long been implicated in various cellular functions. However, mechanistic principles governing functional regulations of lipid environment are not well understood due to the inherent complexities associated with the relevant length and timescales that limit both direct experimental measurements and their interpretation. In this context, computer simulations hold immense potential to investigate molecular-level interactions and mechanisms that lead to PM heterogeneity and its functions. Herein, we investigate spatial and dynamic heterogeneity in model membranes with coexisting liquid ordered and liquid disordered phases and characterize the membrane order in terms of the local topological changes in lipid environment using the nonaffine deformation framework. Furthermore, we probe the packing defects in these membranes, which can be considered as the conjugate of membrane order assessed in terms of the nonaffine parameter. In doing so, we formalize the connection between membrane packing and local membrane order and use that to explore the mechanistic principles behind their functions. Our observations suggest that heterogeneity in mixed phase membranes is a consequence of local lipid topology and its temporal evolution, which give rise to disparate lipid packing in ordered and disordered domains. This in turn governs the distinct nature of packing defects in these domains that can play a crucial role in preferential localization of proteins in mixed phase membranes. Furthermore, we observe that lipid packing also leads to contrasting distribution of free volume in the membrane core region in ordered and disordered membranes, which can lead to distinctive membrane permeability of small molecules. Our results, thus, indicate that heterogeneity in mixed phase membranes closely governs the membrane functions that may emerge from packing-related basic design principles.

Impact of Truncated Oxidized Phosphatidylcholines on Phospholipase A2 Activity in Mono- and Polyunsaturated Biomimetic Vesicles

The interplay between inflammatory and redox processes is a ubiquitous and critical phenomenon in cell biology that involves numerous biological factors. Among them, secretory phospholipases A₂ (sPLA₂) that catalyze the hydrolysis of the sn-2 ester bond of phospholipids are key players. They can interact or be modulated by the presence of truncated oxidized phosphatidylcholines (OxPCs) produced under oxidative stress from phosphatidylcholine (PC) species. The present study examined this important, but rarely considered, sPLA₂ modulation induced by the changes in biophysical properties of PC vesicles comprising various OxPC ratios in mono- or poly-unsaturated PCs. Being the most physiologically active OxPCs, 1-palmitoyl-2-(5'-oxo-valeroyl)-sn-glycero-3-phosphocholine (POVPC) and 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (PGPC) have been selected for our study. Using fluorescence spectroscopy methods, we compared the effect of OxPCs on the lipid order as well as sPLA₂ activity in large unilamellar vesicles (LUVs) made of the heteroacid PC, either monounsaturated [1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)], or polyunsaturated [1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (PDPC)] at a physiological temperature. The effect of OxPCs on vesicle size was also assessed in both the mono- and polyunsaturated PC matrices. Results: OxPCs decrease the membrane lipid order of POPC and PDPC mixtures with PGPC inducing a much larger decrease in comparison with POVPC, indicative that the difference takes place at the glycerol level. Compared with POPC, PDPC was able to inhibit sPLA₂ activity showing a protective effect of PDPC against enzyme hydrolysis. Furthermore, sPLA₂ activity on its PC substrates was modulated by the OxPC membrane content. POVPC down-regulated sPLA₂ activity, suggesting anti-inflammatory properties of this truncated oxidized lipid. Interestingly, PGPC had a dual and opposite effect, either inhibitory or enhancing on sPLA₂ activity, depending on the protocol of lipid mixing. This difference may result from the chemical properties of the shortened sn-2-acyl chain residues (aldehyde group for POVPC, and carboxyl for PGPC), being, respectively, zwitterionic or anionic under hydration at physiological conditions.

Bio-enabled Engineering of Multifunctional "Living" Surfaces

Through the magic of "active matter"─matter that converts chemical energy into mechanical work to drive emergent properties─biology solves a myriad of seemingly enormous physical challenges. Using active matter surfaces, for example, our lungs clear an astronomically large number of particulate contaminants that accompany each of the 10,000 L of air we respire per day, thus ensuring that the lungs' gas exchange surfaces remain functional. In this Perspective, we describe our efforts to engineer artificial active surfaces that mimic active matter surfaces in biology. Specifically, we seek to assemble the basic active matter components─mechanical motor, driven constituent, and energy source─to design surfaces that support the continuous operation of molecular sensing, recognition, and exchange. The successful realization of this technology would generate multifunctional, "living" surfaces that combine the dynamic programmability of active matter and the molecular specificity of biological surfaces and apply them to applications in biosensors, chemical diagnostics, and other surface transport and catalytic processes. We describe our recent efforts in bio-enabled engineering of living surfaces through the design of molecular probes to understand and integrate native biological membranes into synthetic materials.

Investigating the cut-off effect of n-alcohols on lipid movement: a biophysical study

Cellular membranes are responsible for absorbing the effects of external perturbants for the cell's survival. Such perturbants include small ubiquitous molecules like n-alcohols which were observed to exhibit anesthetic capabilities, with this effect tapering off at a cut-off alcohol chain length. To explain this cut-off effect and complement prior biochemical studies, we investigated a series of n-alcohols (with carbon lengths 2-18) and their impact on several bilayer properties, including lipid flip-flop, intervesicular exchange, diffusion, membrane bending rigidity and more. To this end, we employed an array of biophysical techniques such as time-resolved small angle neutron scattering (TR-SANS), small angle X-ray scattering (SAXS), all atomistic and coarse-grained molecular dynamics (MD) simulations, and calcein leakage assays. At an alcohol concentration of 30 mol% of the overall lipid content, TR-SANS showed 1-hexanol (C6OH) increased transverse lipid diffusion, i.e. flip-flop. As alcohol chain length increased from C6 to C10 and longer, lipid flip-flop slowed by factors of 5.6 to 32.2. Intervesicular lipid exchange contrasted these results with only a slight cut-off at alcohol concentrations of 30 mol% but not 10 mol%. SAXS, MD simulations, and leakage assays revealed changes to key bilayer properties, such as bilayer thickness and fluidity, that correlate well with the effects on lipid flip-flop rates. Finally, we tie our results to a defect-mediated pathway for alcohol-induced lipid flip-flop.

Effect of the Lipid Landscape on the Efficacy of Cell-Penetrating Peptides

Every cell biological textbook teaches us that the main role of the plasma membrane is to separate cells from their neighborhood to allow for a controlled composition of the intracellular space. The mostly hydrophobic nature of the cell membrane presents an impenetrable barrier for most hydrophilic molecules larger than 1 kDa. On the other hand, cell-penetrating peptides (CPPs) are capable of traversing this barrier without compromising membrane integrity, and they can do so on their own or coupled to cargos. Coupling biologically and medically relevant cargos to CPPs holds great promise of delivering membrane-impermeable drugs into cells. If the cargo is able to interact with certain cell types, uptake of the CPP-drug complex can be tailored to be cell-type-specific. Besides outlining the major membrane penetration pathways of CPPs, this review is aimed at deciphering how properties of the membrane influence the uptake mechanisms of CPPs. By summarizing an extensive body of experimental evidence, we argue that a more ordered, less flexible membrane structure, often present in the very diseases planned to be treated with CPPs, decreases their cellular uptake. These correlations are not only relevant for understanding the cellular biology of CPPs, but also for rationally improving their value in translational or clinical applications.

Designing a green-emitting viscosity-sensitive 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) probe for plasma membrane viscosity imaging

Viscosity is a key characteristic of lipid membranes - it governs the passive diffusion of solutes and affects the lipid raft formation and membrane fluidity. Precise determination of viscosity values in biological systems is of great interest and viscosity-sensitive fluorescent probes offer a convenient solution for this task. In this work we present a novel membrane-targeting and water-soluble viscosity probe BODIPY-PM, which is based on one of the most frequently used probes BODIPY-C10. Despite its regular use, BODIPY-C10 suffers from poor integration into liquid-ordered lipid phases and lack of water solubility. Here, we investigate the photophysical characteristics of BODIPY-PM and demonstrate that solvent polarity only slightly affects the viscosity-sensing qualities of BODIPY-PM. In addition, with fluorescence lifetime imaging microscopy (FLIM), we imaged microviscosity in complex biological systems - large unilamellar vesicles (LUVs), tethered bilayer membranes (tBLMs) and live lung cancer cells. Our study showcases that BODIPY-PM preferentially stains the plasma membranes of live cells, equally well partitions into both liquid-ordered and liquid-disordered phases and reliably distinguishes lipid phase separation in tBLMs and LUVs.

Dynamics of asymmetric membranes and interleaflet coupling as intermediates in membrane fusion

Membrane fusion is a tool to increase the complexity of model membrane systems. Here, we use silica nanoparticles to fuse liquid-disordered DOPC giant unilamellar vesicles (GUVs) and liquid-ordered DPPC:cholesterol (7:3) GUVs. After fusion, GUVs display large membrane domains as confirmed by fluorescence confocal microscopy. Laurdan spectral imaging of the membrane phases in the fused GUVs shows differences compared with the initial vesicles indicating some lipid redistribution between phase domains as dictated by the tie lines of the phase diagram. Remarkably, using real-time confocal microscopy we were able to record the dynamics of formation of asymmetric membrane domains in hemifused GUVs and detected interleaflet coupling phenomena by which the DOPC-rich liquid-disordered domains in outer monolayer modulates the phase state of the DPPC:cholesterol inner membrane leaflet which transitions from liquid-ordered to liquid-disordered phase. We find that internal membrane stresses generated by membrane asymmetry enhance the efficiency of full fusion compared with our previous studies on symmetric vesicle fusion. Furthermore, under these conditions, the liquid-disordered monolayer dictates the bilayer phase state of asymmetric membrane domains in >90% of observed cases. By comparison to the findings of previous literature, we suggest that the monolayer phase that dominates the bilayer properties could be a mechanoresponsive signaling mechanism sensitive to the local membrane environment.

DNA-Origami Line-Actants Control Domain Organization and Fission in Synthetic Membranes

Cells can precisely program the shape and lateral organization of their membranes using protein machinery. Aiming to replicate a comparable degree of control, here we introduce DNA-origami line-actants (DOLAs) as synthetic analogues of membrane-sculpting proteins. DOLAs are designed to selectively accumulate at the line-interface between coexisting domains in phase-separated lipid membranes, modulating the tendency of the domains to coalesce. With experiments and coarse-grained simulations, we demonstrate that DOLAs can reversibly stabilize two-dimensional analogues of Pickering emulsions on synthetic giant liposomes, enabling dynamic programming of membrane lateral organization. The control afforded over membrane structure by DOLAs extends to three-dimensional morphology, as exemplified by a proof-of-concept synthetic pathway leading to vesicle fission. With DOLAs we lay the foundations for mimicking, in synthetic systems, some of the critical membrane-hosted functionalities of biological cells, including signaling, trafficking, sensing, and division.

Antibody binding reports spatial heterogeneities in cell membrane organization

The spatial organization of cell membrane glycoproteins and glycolipids is critical for mediating the binding of ligands, receptors, and macromolecules on the plasma membrane. However, we currently do not have the methods to quantify the spatial heterogeneities of macromolecular crowding on live cell surfaces. In this work, we combine experiment and simulation to report crowding heterogeneities on reconstituted membranes and live cell membranes with nanometer spatial resolution. By quantifying the effective binding affinity of IgG monoclonal antibodies to engineered antigen sensors, we discover sharp gradients in crowding within a few nanometers of the crowded membrane surface. Our measurements on human cancer cells support the hypothesis that raft-like membrane domains exclude bulky membrane proteins and glycoproteins. Our facile and high-throughput method to quantify spatial crowding heterogeneities on live cell membranes may facilitate monoclonal antibody design and provide a mechanistic understanding of plasma membrane biophysical organization.

Physical Concept to Explain the Regulation of Lipid Membrane Phase Separation under Isothermal Conditions

Lateral phase separation within lipid bilayer membranes has attracted considerable attention in the fields of biophysics and cell biology. Living cells organize laterally segregated compartments, such as raft domains in an ordered phase, and regulate their dynamic structures under isothermal conditions to promote cellular functions. Model membrane systems with minimum components are powerful tools for investigating the basic phenomena of membrane phase separation. With the use of such model systems, several physicochemical characteristics of phase separation have been revealed. This review focuses on the isothermal triggering of membrane phase separation from a physical point of view. We consider the free energy of the membrane that describes lateral phase separation and explain the experimental results of model membranes to regulate domain formation under isothermal conditions. Three possible regulation factors are discussed: electrostatic interactions, chemical reactions and membrane tension. These findings may contribute to a better understanding of membrane lateral organization within living cells that function under isothermal conditions and could be useful for the development of artificial cell engineering.

Effect of Cholesterol on Biomimetic Membrane Curvature and Coronavirus Fusion Peptide Encapsulation

Biomimetic cubic phases can be used for protein encapsulation in a variety of applications such as biosensors and drug delivery. Cubic phases with a high concentration of cholesterol and phospholipids were obtained herein. It is shown that the cubic phase structure can be maintained with a higher concentration of biomimetic membrane additives than has been reported previously. Opposing effects on the curvature of the membrane were observed upon the addition of phospholipids and cholesterol. Furthermore, the coronavirus fusion peptide significantly increased the negative curvature of the biomimetic membrane with cholesterol. We show that the viral fusion peptide can undergo structural changes leading to the formation of hydrophobic α-helices that insert into the lipid bilayer. This is of high importance, as a fusion peptide that induces increased negative curvature as shown by the formation of inverse hexagonal phases allows for greater contact area between two membranes, which is required for viral fusion to occur. The cytotoxicity assay showed that the toxicity toward HeLa cells was dramatically decreased when the cholesterol or peptide level in the nanoparticles increased. This suggests that the addition of cholesterol can improve the biocompatibility of the cubic phase nanoparticles, making them safer for use in biomedical applications. As the results, this work improves the potential for the biomedical end-use applications of the nonlamellar lipid nanoparticles and shows the need of systematic formulation studies due to the complex interplay of all components.

Tuning Targeted Liposome Avidity to Cells via Lipid Phase Separation

The addition of both cell-targeting moieties and polyethylene glycol (PEG) to nanoparticle (NP) drug delivery systems is a standard approach to improve the biodistribution, specificity, and uptake of therapeutic cargo. The spatial presentation of these molecules affects avidity of the NP to target cells in part through an interplay between the local ligand concentration and the steric hindrance imposed by PEG molecules. Here, we show that lipid phase separation in nanoparticles can modulate liposome avidity by changing the proximity of PEG and targeting protein molecules on a nanoparticle surface. Using lipid-anchored nickel-nitrilotriacetic acid (Ni-NTA) as a model ligand, we demonstrate that the attachment of lipid anchored Ni-NTA and PEG molecules to distinct lipid domains in nanoparticles can enhance liposome binding to cancer cells by increasing ligand clustering and reducing steric hindrance. We then use this technique to enhance the binding of RGD-modified liposomes, which can bind to integrins overexpressed on many cancer cells. These results demonstrate the potential of lipid phase separation to modulate the spatial presentation of targeting and shielding molecules on lipid nanocarriers, offering a powerful tool to enhance the efficacy of NP drug delivery systems.

A simple method to make, trap and deform a vesicle in a gel

We present a simple method to produce giant lipid pseudo-vesicles (vesicles with an oily cap on the top), trapped in an agarose gel. The method can be implemented using only a regular micropipette and relies on the formation of a water/oil/water double droplet in liquid agarose. We characterize the produced vesicle with fluorescence imaging and establish the presence and integrity of the lipid bilayer by the successful insertion of [Formula: see text]-Hemolysin transmembrane proteins. Finally, we show that the vesicle can be easily mechanically deformed, non-intrusively, by indenting the surface of the gel.

A lattice model of ternary mixtures of lipids and cholesterol with tunable domain sizes

Much of our understanding of the physical properties of raft domains in biological membranes, and some insight into the mechanisms underlying their formation stem from atomistic simulations of simple model systems, especially ternary mixtures consisting of saturated and unsaturated lipids, and cholesterol (Chol). To explore the properties of such systems at large spatial scales, we here present a simple ternary mixture lattice model, involving a small number of nearest neighbor interaction terms. Monte Carlo simulations of mixtures with different compositions show excellent agreement with experimental and atomistic simulation observations across multiple scales, ranging from the local distributions of lipids to the phase diagram of the system. The simplicity of the model allows us to identify the roles played by the different interactions between components, and the interplay between them. Importantly, by changing the value of one of the model parameters, we can tune the size of the liquid-ordered domains, thereby simulating both Type II mixtures exhibiting macroscopic phase separation and Type I mixtures with nanoscopic domains. The Type II mixture simulation results fit well to the experimentally determined phase diagram of mixtures containing saturated DPPC/unsaturated DOPC/Chol. When the tunable parameter is changed, we obtain the Type I version of DPPC/DOPC/Chol, i.e., a mixture not showing thermodynamic phase transitions but one that may be fitted to the same phase diagram if local measures are used to distinguish between the different states. Our model results suggest that short range packing is likely to be a key regulator of the stability and size distribution of biological rafts.

Remodeling of yeast vacuole membrane lipidomes from the log (one phase) to stationary stage (two phases)

Upon nutrient limitation, budding yeast of Saccharomyces cerevisiae shift from fast growth (the log stage) to quiescence (the stationary stage). This shift is accompanied by liquid-liquid phase separation in the membrane of the vacuole, an endosomal organelle. Recent work indicates that the resulting micrometer-scale domains in vacuole membranes enable yeast to survive periods of stress. An outstanding question is which molecular changes might cause this membrane phase separation. Here, we conduct lipidomics of vacuole membranes in both the log and stationary stages. Isolation of pure vacuole membranes is challenging in the stationary stage, when lipid droplets are in close contact with vacuoles. Immuno-isolation has previously been shown to successfully purify log-stage vacuole membranes with high organelle specificity, but it was not previously possible to immuno-isolate stationary-stage vacuole membranes. Here, we develop Mam3 as a bait protein for vacuole immuno-isolation, and demonstrate low contamination by non-vacuolar membranes. We find that stationary-stage vacuole membranes contain surprisingly high fractions of phosphatidylcholine lipids (∼40%), roughly twice as much as log-stage membranes. Moreover, in the stationary stage, these lipids have higher melting temperatures, due to longer and more saturated acyl chains. Another surprise is that no significant change in sterol content is observed. These lipidomic changes, which are largely reflected on the whole-cell level, fit within the predominant view that phase separation in membranes requires at least three types of molecules to be present: lipids with high melting temperatures, lipids with low melting temperatures, and sterols.

Chemical potential measurements constrain models of cholesterol-phosphatidylcholine interactions

Bilayer membranes composed of cholesterol and phospholipids exhibit diverse forms of nonideal mixing. In particular, many previous studies document macroscopic liquid-liquid phase separation as well as nanometer-scale heterogeneity in membranes of phosphatidylcholine (PC) lipids and cholesterol. Here, we present experimental measurements of cholesterol chemical potential (μc) in binary membranes containing dioleoyl PC (DOPC), 1-palmitoyl-2-oleoyl PC (POPC), or dipalmitoyl PC (DPPC), and in ternary membranes of DOPC and DPPC, referenced to crystalline cholesterol. μc is the thermodynamic quantity that dictates the availability of cholesterol to bind other factors, and notably must be equal between coexisting phases of a phase separated mixture. It is simply related to concentration under conditions of ideal mixing, but is far from ideal for the majority of lipid mixtures investigated here. Measurements of μc can vary with phospholipid composition by 1.5 kBT at constant cholesterol mole fraction implying a more than fivefold change in its availability for binding receptors and other reactions. Experimental measurements are fit to thermodynamic models including cholesterol-DPPC complexes or pairwise interactions between lipid species to provide intuition about the magnitude of interactions. These findings reinforce that μc depends on membrane composition overall, suggesting avenues for cells to alter the availability of cholesterol without varying cholesterol concentration.

Structural diversity of photoswitchable sphingolipids for optodynamic control of lipid microdomains

Sphingolipids are a structurally diverse class of lipids predominantly found in the plasma membrane of eukaryotic cells. These lipids can laterally segregate with other rigid lipids and cholesterol into liquid-ordered domains that act as organizing centers within biomembranes. Owing the vital role of sphingolipids for lipid segregation, controlling their lateral organization is of utmost significance. Hence, we made use of the light-induced trans-cis isomerization of azobenzene-modified acyl chains to develop a set of photoswitchable sphingolipids with different headgroups (hydroxyl, galactosyl, phosphocholine) and backbones (sphingosine, phytosphingosine, tetrahydropyran-blocked sphingosine) that are able to shuttle between liquid-ordered and liquid-disordered regions of model membranes upon irradiation with UV-A (λ = 365 nm) and blue (λ = 470 nm) light, respectively. Using combined high-speed atomic force microscopy, fluorescence microscopy, and force spectroscopy, we investigated how these active sphingolipids laterally remodel supported bilayers upon photoisomerization, notably in terms of domain area changes, height mismatch, line tension, and membrane piercing. Hereby, we show that the sphingosine-based (Azo-β-Gal-Cer, Azo-SM, Azo-Cer) and phytosphingosine-based (Azo-α-Gal-PhCer, Azo-PhCer) photoswitchable lipids promote a reduction in liquid-ordered microdomain area when in the UV-adapted cis-isoform. In contrast, azo-sphingolipids having tetrahydropyran groups that block H-bonding at the sphingosine backbone (lipids named Azo-THP-SM, Azo-THP-Cer) induce an increase in the liquid-ordered domain area when in cis, accompanied by a major rise in height mismatch and line tension. These changes were fully reversible upon blue light-triggered isomerization of the various lipids back to trans, pinpointing the role of interfacial interactions for the formation of stable liquid-ordered domains.

Phase Separation in Biology and Disease; Current Perspectives and Open Questions

In the past almost 15 years, we witnessed the birth of a new scientific field focused on the existence, formation, biological functions, and disease associations of membraneless bodies in cells, now referred to as biomolecular condensates. Pioneering studies from several laboratories [reviewed in1-3] supported a model wherein biomolecular condensates associated with diverse biological processes form through the process of phase separation. These and other findings that followed have revolutionized our understanding of how biomolecules are organized in space and time within cells to perform myriad biological functions, including cell fate determination, signal transduction, endocytosis, regulation of gene expression and protein translation, and regulation of RNA metabolism. Further, condensates formed through aberrant phase transitions have been associated with numerous human diseases, prominently including neurodegeneration and cancer. While in some cases, rigorous evidence supports links between formation of biomolecular condensates through phase separation and biological functions, in many others such links are less robustly supported, which has led to rightful scrutiny of the generality of the roles of phase separation in biology and disease.4-7 During a week-long workshop in March 2022 at the Telluride Science Research Center (TSRC) in Telluride, Colorado, ∼25 scientists addressed key questions surrounding the biomolecular condensates field. Herein, we present insights gained through these discussions, addressing topics including, roles of condensates in diverse biological processes and systems, and normal and disease cell states, their applications to synthetic biology, and the potential for therapeutically targeting biomolecular condensates.

Remodeling of Biomembranes and Vesicles by Adhesion of Condensate Droplets

Condensate droplets are formed in aqueous solutions of macromolecules that undergo phase separation into two liquid phases. A well-studied example are solutions of the two polymers PEG and dextran which have been used for a long time in biochemical analysis and biotechnology. More recently, phase separation has also been observed in living cells where it leads to membrane-less or droplet-like organelles. In the latter case, the condensate droplets are enriched in certain types of proteins. Generic features of condensate droplets can be studied in simple binary mixtures, using molecular dynamics simulations. In this review, I address the interactions of condensate droplets with biomimetic and biological membranes. When a condensate droplet adheres to such a membrane, the membrane forms a contact line with the droplet and acquires a very high curvature close to this line. The contact angles along the contact line can be observed via light microscopy, lead to a classification of the possible adhesion morphologies, and determine the affinity contrast between the two coexisting liquid phases and the membrane. The remodeling processes generated by condensate droplets include wetting transitions, formation of membrane nanotubes as well as complete engulfment and endocytosis of the droplets by the membranes.

Dynamic correlations in lipid bilayer membranes over finite time intervals

Recent single-molecule measurements [Schoch et al., Proc. Natl. Acad. Sci. U. S. A. 118, e2113202118 (2021)] have observed dynamic lipid-lipid correlations in membranes with submicrometer spatial resolution and submillisecond temporal resolution. While short from an instrumentation standpoint, these length and time scales remain long compared to microscopic molecular motions. Theoretical expressions are derived to infer experimentally measurable correlations from the two-body diffusion matrix appropriate for membrane-bound bodies coupled by hydrodynamic interactions. The temporal (and associated spatial) averaging resulting from finite acquisition times has the effect of washing out correlations as compared to naive predictions (i.e., the bare elements of the diffusion matrix), which would be expected to hold for instantaneous measurements. The theoretical predictions are shown to be in excellent agreement with Brownian dynamics simulations of experimental measurements. Numerical results suggest that the experimental measurement of membrane protein diffusion, in complement to lipid diffusion measurements, might help to resolve the experimental ambiguities encountered for certain black lipid membranes.

Membrane domain formation induced by binding/unbinding of curvature-inducing molecules on both membrane surfaces

The domain formation of curvature-inducing molecules, such as peripheral or transmembrane proteins and conical surfactants, is studied in thermal equilibrium and nonequilibrium steady states using meshless membrane simulations. These molecules can bind to both surfaces of a bilayer membrane and also move to the opposite leaflet by a flip-flop. Under symmetric conditions for the two leaflets, the membrane domains form checkerboard patterns in addition to striped and spot patterns. The unbound membrane stabilizes the vertices of the checkerboard. Under asymmetric conditions, the domains form kagome-lattice and thread-like patterns. In the nonequilibrium steady states, a flow of the binding molecules between the upper and lower solutions can occur via flip-flop.

DisGUVery: A Versatile Open-Source Software for High-Throughput Image Analysis of Giant Unilamellar Vesicles

Giant unilamellar vesicles (GUVs) are cell-sized aqueous compartments enclosed by a phospholipid bilayer. Due to their cell-mimicking properties, GUVs have become a widespread experimental tool in synthetic biology to study membrane properties and cellular processes. In stark contrast to the experimental progress, quantitative analysis of GUV microscopy images has received much less attention. Currently, most analysis is performed either manually or with custom-made scripts, which makes analysis time-consuming and results difficult to compare across studies. To make quantitative GUV analysis accessible and fast, we present DisGUVery, an open-source, versatile software that encapsulates multiple algorithms for automated detection and analysis of GUVs in microscopy images. With a performance analysis, we demonstrate that DisGUVery's three vesicle detection modules successfully identify GUVs in images obtained with a wide range of imaging sources, in various typical GUV experiments. Multiple predefined analysis modules allow the user to extract properties such as membrane fluorescence, vesicle shape, and internal fluorescence from large populations. A new membrane segmentation algorithm facilitates spatial fluorescence analysis of nonspherical vesicles. Altogether, DisGUVery provides an accessible tool to enable high-throughput automated analysis of GUVs, and thereby to promote quantitative data analysis in synthetic cell research.

Optical Manipulation of Gb3 Enriched Lipid Domains: Impact of Isomerization on Gb3 -Shiga Toxin B Interaction

The plasma membrane is a complex assembly of proteins and lipids that can self-assemble in submicroscopic domains commonly termed "lipid rafts", which are implicated in membrane signaling and trafficking. Recently, photo-sensitive lipids were introduced to study membrane domain organization, and photo-isomerization was shown to trigger the mixing and de-mixing of liquid-ordered (lo ) domains in artificial phase-separated membranes. Here, we synthesized globotriaosylceramide (Gb3 ) glycosphingolipids that harbor an azobenzene moiety at different positions of the fatty acid to investigate light-induced membrane domain reorganization, and that serve as specific receptors for the protein Shiga toxin (STx). Using phase-separated supported lipid bilayers on mica surfaces doped with four different photo-Gb3 molecules, we found by fluorescence microscopy and atomic force microscopy that liquid disordered (ld ) domains were formed within lo domains upon trans-cis photo-isomerization. The fraction and size of these ld domains were largest for Gb3 molecules with the azobenzene group at the end of the fatty acid. We further investigated the impact of domain reorganization on the interaction of the B-subunits of STx with the photo-Gb3 . Fluorescence and atomic force micrographs clearly demonstrated that STxB binds to the lo phase if Gb3 is in the trans-configuration, whereas two STxB populations are formed if the photo-Gb3 is switched to the cis-configuration highlighting the idea of manipulating lipid-protein interactions with a light stimulus.

Directly imaging emergence of phase separation in peroxidized lipid membranes

Lipid peroxidation is a process which is key in cell signaling and disease, it is exploited in cancer therapy in the form of photodynamic therapy. The appearance of hydrophilic moieties within the bilayer's hydrocarbon core will dramatically alter the structure and mechanical behavior of membranes. Here, we combine viscosity sensitive fluorophores, advanced microscopy, and X-ray diffraction and molecular simulations to directly and quantitatively measure the bilayer's structural and viscoelastic properties, and correlate these with atomistic molecular modelling. Our results indicate an increase in microviscosity and a decrease in the bending rigidity upon peroxidation of the membranes, contrary to the trend observed with non-oxidized lipids. Fluorescence lifetime imaging microscopy and MD simulations give evidence for the presence of membrane regions of different local order in the oxidized membranes. We hypothesize that oxidation promotes stronger lipid-lipid interactions, which lead to an increase in the lateral heterogeneity within the bilayer and the creation of lipid clusters of higher order.

Life in Multi-Extreme Environments: Brines, Osmotic and Hydrostatic Pressure─A Physicochemical View

Elucidating the details of the formation, stability, interactions, and reactivity of biomolecular systems under extreme environmental conditions, including high salt concentrations in brines and high osmotic and high hydrostatic pressures, is of fundamental biological, astrobiological, and biotechnological importance. Bacteria and archaea are able to survive in the deep ocean or subsurface of Earth, where pressures of up to 1 kbar are reached. The deep subsurface of Mars may host high concentrations of ions in brines, such as perchlorates, but we know little about how these conditions and the resulting osmotic stress conditions would affect the habitability of such environments for cellular life. We discuss the combined effects of osmotic (salts, organic cosolvents) and hydrostatic pressures on the structure, stability, and reactivity of biomolecular systems, including membranes, proteins, and nucleic acids. To this end, a variety of biophysical techniques have been applied, including calorimetry, UV/vis, FTIR and fluorescence spectroscopy, and neutron and X-ray scattering, in conjunction with high pressure techniques. Knowledge of these effects is essential to our understanding of life exposed to such harsh conditions, and of the physical limits of life in general. Finally, we discuss strategies that not only help us understand the adaptive mechanisms of organisms that thrive in such harsh geological settings but could also have important ramifications in biotechnological and pharmaceutical applications.

Exclusion of Anchor-Matched Peptide Nucleic Acid from Liquid-Ordered Domains by Hybridization with Complementary Flavin-Labeled DNA

Membrane-anchored proteins and their mimics, such as peptide nucleic acids (PNAs), are known to partition preferentially into either lipid raft/liquid-ordered (lo) domains or into non-raft/liquid-disordered (ld) domains, depending on their lipophilic anchors. Here, anchor-matched PNA was demonstrated to be excluded from the lo microdomains of giant unilamellar vesicles by hybridization with the complementary flavin-labeled DNA. As shown in control experiments using Alexa Fluor 488-labeled DNA, which showed that the preferential partitioning was the lo domain, the domain distribution of PNA was not only dependent on the lipophilic anchor but also on the structure of the hybridized DNA or PNA pair. In such systems, the main factors that influence changes in the domain selectivity of the probes are most likely to also be interactivity (i.e., steric bulkiness), hydrophilicity, and self-assembling ability. These findings may have the potential to contribute to the elucidation of membrane-active peptides, the method of their activation, and their applications in medicine such as antimicrobial use, especially with regard to their actions at the interface between the lo and ld domains in cells.

Rafting on the Plasma Membrane: Lipid Rafts in Signaling and Disease

The plasma membrane is not a uniform phospholipid bilayer; it has specialized membrane nano- or microdomains called lipid rafts. Lipid rafts are small cholesterol and sphingolipid-rich plasma membrane islands. Although their existence was long debated, their presence in the plasma membrane of living cells is now well accepted with the advent of super-resolution imaging techniques. It is interesting to note that lipid rafts function to compartmentalize receptors and their regulators and substantially modulate cellular signaling. In this review, we will examine the role of lipid rafts and caveolae-lipid raft-like microdomains with a distinct 3D morphology-in cellular signaling. Moreover, we will investigate how raft compartmentalized signaling regulates diverse physiological processes such as proliferation, apoptosis, immune signaling, and development. Also, the deregulation of lipid raft-mediated signaling during tumorigenesis and metastasis will be explored.

Suppression of Amyloid-β Adsorption on Endoplasmic Reticulum Stress-Mimicking Membranes by α-Tocopherol and α-Tocotrienol

Two forms of hydrophobic vitamin E (VE), α-tocopherol (Toc) and α-tocotrienol (Toc3), have been proposed to be effective against Alzheimer's disease (AD), the etiology of which is thought to involve endoplasmic reticulum (ER) stress. However, previous studies reported conflicting effects of Toc and Toc3 on the risk of AD. We prepared liposomes mimicking the phase separation of the ER membrane (solid-ordered/liquid-disordered phase separation) and studied how VE can influence the interaction between amyloid-β (Aβ) and the ER membrane. We found that Toc could inhibit the formation of the solid-ordered phase more significantly than Toc3. Furthermore, Aβ protofibril adsorption on ER stress-mimicking membranes was more strongly suppressed by Toc compared with Toc3. Therefore, we concluded that VE can relieve ER stress by destabilizing the solid-ordered phase of the ER membrane and subsequently reducing the amount of Aβ adsorbed on the membrane. Moreover, Toc exerted a stronger effect than Toc3.

Orientation of Cholesterol in Polyunsaturated Lipid Bilayers

The local normal to the fluid liquid crystalline phase of the lipid membrane is an axis of motional symmetry for the molecules that make up the bilayer. The presence of cholesterol in the membrane increases not only the lipid hydrocarbon chain order but also the strength of the membrane's orienting potential. Cholesterol undergoes rapid reorientation about a diffusion axis that is roughly aligned with the long molecular axis, but there is also a slower reorientation of the diffusion axis, or "wobble", relative to the local bilayer normal. The extent of this second, slower motion depends on the degree of order of the lipids that make up the bilayer. We use ²H nuclear magnetic resonance of deuterium-labeled cholesterol to investigate quantitatively the effect of lipid chain unsaturation on cholesterol orientation in a series of phospholipid bilayers. We find that the hydrocarbon chains in membranes composed of polyunsaturated lipids are much more highly disordered than those in membranes composed of saturated lipids but that cholesterol remains aligned roughly along the bilayer normal.

Domain dynamics of phase-separated lipid membranes under shear flow

The dynamical behaviour of lateral domains on phase-separated lipid vesicles under external flow is reported. A microfluidic chamber was used for the immobilization of vesicles and the application of shear. Microscopic observation revealed that domains tended to be localized at the vortex center and to exhibit a stripe morphology as the flow speed increased. We clarified the dependency of domain behaviors on the flow speed and lipid mixing fraction. The cholesterol ratio in the membrane affected these domain behaviors. Next, we investigated the growth of domains under flow. We discuss the mechanism of these trends by considering the free energy of phase separation, and reproduce the experimental results by numerical simulations. These findings may lead to a better understanding of the dynamical properties of the membrane under nonequilibrium situations and the biophysical mechanism of cellular mechanotransduction.

Impact of sphingomyelin acyl chain heterogeneity upon properties of raft-like membranes

Sphingomyelin (SM) is a main component of lipid rafts and characteristic of abundance of long and saturated acyl chains. Recently, we reported that fluorescence-labeled lipids including C16:0 and C18:0SMs retained membrane behaviors of inherent lipids. Here, we newly prepared fluorescent SMs with longer acyl chains, C22:0 and C24:1, for observing their partition and diffusion in SM/cholesterol (chol)/dioleoylphosphatidylcholine (DOPC) bilayers. Although fluorescent C24:1SM underwent a uniform distribution between ordered (Lo) and disordered (Ld) phases, other fluorescent SMs with saturated acyl chains were preferentially distributed in the Lo phase. Interestingly, when the acyl chains of fluorescent and membrane SMs are different, distribution of fluorescent SM to the Lo phase was reduced compared to when the acyl chains are the same. This tendency was also observed for C16:0SM/C22:0SM/chol/DOPC quaternary bilayers, where the minor SM was more excluded out of the Lo phase than the major SM. We also found that the coexistence of SMs induces SM efflux out of the Lo phase and simultaneous DOPC influx to the Lo phase, consequently reducing the difference in fluidity between the two phases. These results suggest that physicochemical properties of lipid rafts are regulated by the acyl chain heterogeneity of SMs.

Charged Lipids Influence Phase Separation in Cell-Sized Liposomes Containing Cholesterol or Ergosterol

Positively charged ion species and charged lipids play specific roles in biochemical processes, especially those involving cell membranes. The cell membrane and phase separation domains are attractive research targets to study signal transduction. The phase separation structure and functions of cell-sized liposomes containing charged lipids and cholesterol have been investigated earlier, and the domain structure has also been studied in a membrane model, containing the yeast sterol ergosterol. The present study investigates phase-separated domain structure alterations in membranes containing charged lipids when cholesterol is substituted with ergosterol. This study finds that ergosterol increases the homogeneity of membranes containing charged lipids. Cholesterol-containing membranes are more sensitive to a charged state, and ergosterol-containing liposomes show lower responses to charged lipids. These findings may improve our understanding of the differences in both yeast and mammalian cells, as well as the interactions of proteins with lipids during signal transduction.

Formation of lipid raft nanodomains in homogeneous ternary lipid mixture of POPC/DPSM/cholesterol: Theoretical insights

Lipid rafts, in biological membranes, are cholesterol-rich nanodomains that regulate many protein activities and cellular processes. Understanding the formation of the lipid-raft nanodomains helps us elucidate many complex interactions in the cell. In this study, the formation of lipid-raft nanodomains in a ternary palmitoyl-oleoyl-phosphatidylcholine/stearoyl-sphingomyelin/cholesterol (POPC/DPSM/Chol) lipid mixture, the most realistic surrogate model for biological membranes, has been successfully observed for the first time in-silico using microsecond timescale molecular dynamics simulations. The model reveals the formation of cholesterol-induced nanodomains with raft-like characteristics and their underlying mechanism: the cholesterol molecules segregate themselves into cholesterol nanodomains and then enrich the cholesterol-rich domain with sphingomyelin molecules to form a lipid raft thanks to the weak bonding of cholesterol with sphingomyelin. Besides, it is found that the increase in cholesterol concentration enhances the biophysical properties (e.g., bilayer thickness, area per lipid headgroup, and order parameter) of the lipid raft nanodomains. Such findings suggest that the POPC/DPSM/Chol bilayer is a suitable model to fundamentally extend the nanodomain evolution to investigate their lifetime and kinetics as well as to study protein-lipid interaction, protein-protein interaction, and selection of therapeutic molecules in the presence of lipid rafts.

Studying actin-induced cell shape changes using Giant Unilamellar Vesicles and reconstituted actin networks

Cell shape changes that are fuelled by the dynamics of the actomyosin cytoskeleton control cellular processes such as motility and division. However, the mechanisms of interplay between cell membranes and actomyosin are complicated to decipher in the complex environment of the cytoplasm. Using biomimetic systems offers an alternative approach to studying cell shape changes in assays with controlled biochemical composition. Biomimetic systems allow quantitative experiments that can help to build physical models describing the processes of cell shape changes. This article reviews works in which actin networks are reconstructed inside or outside cell-sized Giant Unilamellar Vesicles (GUVs), which are models of cell membranes. We show how various actin networks affect the shape and mechanics of GUVs and how some cell shape changes can be reproduced in vitro using these minimal systems.

Distribution of cholesterol in asymmetric membranes driven by composition and differential stress

Many lipid membranes of eukaryotic cells are asymmetric, which means the two leaflets differ in at least one physical property, such as lipid composition or lateral stress. Maintaining this asymmetry is helped by the fact that ordinary phospholipids rarely transition between leaflets, but cholesterol is an exception: its flip-flop times are in the microsecond range, so that its distribution between leaflets is determined by a chemical equilibrium. In particular, preferential partitioning can draw cholesterol into a more saturated leaflet, and phospholipid number asymmetry can force it out of a compressed leaflet. Combining highly coarse-grained membrane simulations with theoretical modeling, we investigate how these two driving forces play against each other until cholesterol's chemical potential is equilibrated. The theory includes two coupled elastic sheets and a Flory-Huggins mixing free energy with a χ parameter. We obtain a relationship between χ and the interaction strength between cholesterol and lipids in either of the two leaflets, and we find that it depends, albeit weakly, on lipid number asymmetry. The differential stress measurements under various asymmetry conditions agree with our theoretical predictions. Using the two kinds of asymmetries in combination, we find that it is possible to counteract the phospholipid number bias, and the resultant stress in the membrane, via the control of cholesterol mixing in the leaflets.

Possible Role of Bent Structure of Methylated Lithocholic Acid on Artificial and Plasma Membranes

Bile acids form micelles that are essential for the absorption of dietary lipids. However, excessive bile acid micelles can disrupt the plasma membrane by removing phospholipids, resulting in cell death. We hypothesized that the bent geometrical structure of the steroid scaffold of bile acids decreases the lipid order (similar to unsaturated phospholipids with cis double bonds), disrupting the plasma membrane. Here, lithocholic acid (LCA), a bile acid, was methylated to prevent micellization. Methylated lithocholic acid (Me-LCA) was mixed with a thin phase-separated lipid bilayer comprising 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol (Chol). Me-LCA was not localized in the DPPC-rich rigid phase but localized in the DOPC-rich fluid phase, and excess Me-LCA did not affect the phase separation. Me-LCA is distributed in the plasma and organelle membranes. However, Me-LCA with bent structure did not affect the membrane properties, membrane fluidity, and hydrophobicity of liposomes composed of DOPC, DPPC, and Chol and also did not affect the proliferation of cells.

Vitamin E Does Not Disturb Polyunsaturated Fatty Acid Lipid Domains

The function of vitamin E in biomembranes remains a prominent topic of discussion. As its limitations as an antioxidant persist and novel functions are discovered, our understanding of the role of vitamin E becomes increasingly enigmatic. As a group of lipophilic molecules (tocopherols and tocotrienols), vitamin E has been shown to influence the properties of its host membrane, and a wealth of research has connected vitamin E to polyunsaturated fatty acid (PUFA) lipids. Here, we use contrast-matched small-angle neutron scattering and differential scanning calorimetry to integrate these fields by examining the influence of vitamin E on lipid domain stability in PUFA-based lipid mixtures. The influence of α-tocopherol, γ-tocopherol, and α-tocopherylquinone on the lateral organization of a 1:1 lipid mixture of saturated distearoylphosphatidylcholine (DSPC) and polyunsaturated palmitoyl-linoleoylphosphatidylcholine (PLiPC) with cholesterol provides a complement to our growing understanding of the influence of tocopherol on lipid phases. Characterization of domain melting suggests a slight depression in the transition temperature and a decrease in transition cooperativity. Tocopherol concentrations that are an order of magnitude higher than anticipated physiological concentrations (2 mol percent) do not significantly perturb lipid domains; however, addition of 10 mol percent is able to destabilize domains and promote lipid mixing. In contrast to this behavior, increasing concentrations of the oxidized product of α-tocopherol (α-tocopherylquinone) induces a proportional increase in domain stabilization. We speculate how the contrasting effect of the oxidized product may supplement the antioxidant response of vitamin E.

Brillouin Spectroscopy of Binary Phospholipid-Cholesterol Bilayers

Multicomponent lipid bilayers are used as models for searching the origin of spatial heterogeneities in biomembranes called lipid rafts, implying the coexistence of domains of different phases and compositions within the lipid bilayer. The spatial organization of multicomponent lipid bilayers on a scale of a hundred nanometers remains unknown. Brillouin spectroscopy providing information about the acoustic phonons with the wavelength of several hundred nanometers has an unexplored potential for this problem. Here, we applied Brillouin spectroscopy for three binary bilayers composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol. The Brillouin experiment for the oriented planar multibilayers was realized for two scattering geometries involving phonons for the lateral and normal directions of the propagation. The DPPC-DOPC mixtures known for the coexistence of the solid-ordered and liquid-disordered phases had bimodal Brillouin peaks, revealing the phase domains with sizes more than a hundred nanometers. Analysis of the Brillouin data for the binary mixtures concluded that the lateral phonons are preferable for testing the lateral homogeneity of the bilayers, while the phonons spreading across the bilayers are sensitive to the layered packing at the mesoscopic scale.

Modulating membrane fusion through the design of fusogenic DNA circuits and bilayer composition

Membrane fusion is a ubiquitous phenomenon linked to many biological processes, and represents a crucial step in liposome-based drug delivery strategies. The ability to control, ever more precisely, membrane fusion pathways would thus be highly valuable for next generation nano-medical solutions and, more generally, the design of advanced biomimetic systems such as synthetic cells. In this article, we present fusogenic nanostructures constructed from synthetic DNA which, different from previous solutions, unlock routes for modulating the rate of fusion and making it conditional to the presence of soluble DNA molecules, thus demonstrating how membrane fusion can be controlled through simple DNA-based molecular circuits. We then systematically explore the relationship between lipid-membrane composition, its biophysical properties, and measured fusion efficiency, linking our observations to the stability of transition states in the fusion pathway. Finally, we observe that specific lipid compositions lead to the emergence of complex bilayer architectures in the fusion products, such as nested morphologies, which are accompanied by alterations in biophysical behaviour. Our findings provide multiple, orthogonal strategies to program lipid-membrane fusion, which leverage the design of either the fusogenic DNA constructs or the physico/chemical properties of the membranes, and could thus be valuable in applications where some design parameters are constrained by other factors such as material cost and biocompatibility, as it is often the case in biotechnological applications.

The Flavonoid Molecule Procyanidin Reduces Phase Separation in Model Membranes

Procyanidin extracted from fruits, such as apples, has been shown to improve lipid metabolization. Recently, studies have revealed that procyanidin interacts with lipid molecules in membranes to enhance lipid metabolism; however, direct evidence of the interaction between procyanidin and lipid membranes has not been demonstrated. In this study, the phase behaviors and changes in the membrane fluidity of cell-sized liposomes containing apple procyanidin, procyanidin B2 (PB2), were demonstrated for the first time. Phase separation in 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/cholesterol ternary membranes significantly decreased after the addition of PB2. The prospect of applying procyanidin content measurements, using the results of this study, to commercial apple juice was also assessed. Specifically, the PB2 concentrations were 50%, 33%, and 0% for pure apple juice, 2-fold diluted apple juice, and pure water, respectively. The results of the actual juice were correlated with PB2 concentrations and phase-separated liposomes ratios, as well as with the results of experiments involving pure chemicals. In conclusion, the mechanism through which procyanidin improves lipid metabolism through the regulation of membrane fluidity was established.

Mitochondrial membrane models built from native lipid extracts: Interfacial and transport properties

The mitochondrion is an essential organelle enclosed by two membranes whose functionalities depend on their very specific protein and lipid compositions. Proteins from the outer mitochondrial membrane (OMM) are specialized in mitochondrial dynamics and mitophagy, whereas proteins of the inner mitochondrial membrane (IMM) have dedicated functions in cellular respiration and apoptosis. As for lipids, the OMM is enriched in glycerophosphatidyl choline but cardiolipin is exclusively found within the IMM. Though the lipid topology and distribution of the OMM and IMM are known since more than four decades, little is known about the interfacial and dynamic properties of the IMM and OMM lipid extracts. Here we build monolayers, supported bilayers and giant unilamellar vesicles (GUVs) of native OMM and IMM lipids extracts from porcine heart. Additionally, we perform a comparative analysis on the interfacial, phase immiscibility and mechanical properties of both types of extract. Our results show that IMM lipids form more expanded and softer membranes than OMM lipids, allowing a better understanding of the physicochemical and biophysical properties of mitochondrial membranes.