Determination of lipid spontaneous curvature from X-ray examinations of inverted hexagonal phases

Bicontinuous inverted cubic phase stabilization as an index of antimicrobial and membrane fusion peptide activity

Some antimicrobial peptides (AMPs) and membrane fusion-catalyzing peptides (FPs) stabilize bicontinuous inverted cubic (QII) phases. Previous authors proposed a topological rationale: since AMP-induced pores, fusion intermediates, and QII phases all have negative Gaussian curvature (NGC), peptides which produce NGC in one structure also do it in another. This assumes that peptides change the curvature energy of the lipid membranes. Here I test this with a Helfrich curvature energy model. First, experimentally, I show that lipid systems often used to study peptide NGC have NGC without peptides at higher temperatures. To determine the net effect of an AMP on NGC, the equilibrium phase behavior of the host lipids must be determined. Second, the model shows that AMPs must make large changes in the curvature energy to stabilize AMP-induced pores. Peptide-induced changes in elastic constants affect pores and QII phase differently. Changes in spontaneous curvature affect them in opposite ways. The observed correlation between QII phase stabilization and AMP activity doesn't show that AMPs act by lowering pore curvature energy. A different rationale is proposed. In theory, AMPs could simultaneously stabilize QII phase and pores by drastically changing two particular elastic constants. This could be tested by measuring AMP effects on the individual constants. I propose experiments to do that. Unlike AMPs, FPs must make only small changes in the curvature energy to catalyze fusion. It they act in this way, their fusion activity should correlate with their ability to stabilize QII phases.

Does membrane curvature elastic energy play a role in mediating oxidative stress in lipid membranes?

The effects of oxidative stress on cells are associated with a wide range of pathologies. Oxidative stress is predominantly initiated by the action of reactive oxygen species and/or lipoxygenases on polyunsaturated fatty acid containing lipids. The downstream products are oxidised phospholipids, bioactive aldehydes and a range of Schiff base by-products between aldehydes and lipids, or other biomacromolecules. In this review we assess the impact of oxidative stress on lipid membranes, focusing on the changes that occur to the curvature preference (lipid spontaneous curvature) and elastic properties of membranes, since these biophysical properties modulate phospholipid homeostasis. Studies show that the lipid products of oxidative stress reduce stored curvature elastic energy in membranes. Based upon this observation, we hypothesize that the effects of oxidative stress on lipid membranes will be reduced by compounds that increase stored curvature elastic energy. We find a strong correlation appears across literature studies that we have reviewed, such that many compounds like vitamin E, Curcumin, Coenzyme Q10 and vitamin A show behaviour consistent with this hypothesis. Finally, we consider whether age-related changes in lipid composition represent the homeostatic response of cells to compensate for the accumulation of in vivo lipid oxidation products.

Lipid monolayer spontaneous curvatures: A collection of published values

Lipid monolayer spontaneous curvatures (or lipid intrinsic curvatures) are one of several material properties of lipids that enable the stored curvature elastic energy in a lipid aggregate to be determined. Stored curvature elastic energy is important since it can modulate the function of membrane proteins and plays a role in the regulatory pathways of phospholipid homeostasis. Due to the large number of different lipid molecules that might theoretically exist in nature, very few lipid spontaneous curvatures have been determined. Herein the values of lipid spontaneous curvatures that exist in the literature are collected, alongside key experimental details. Where possible, trends in the data are discussed and finally, obvious gaps in the knowledge are signposted.

Interferon-Induced Transmembrane Protein 3 Blocks Fusion of Diverse Enveloped Viruses by Altering Mechanical Properties of Cell Membranes

Interferon-induced transmembrane protein 3 (IFITM3) potently inhibits entry of diverse enveloped viruses by trapping the viral fusion at a hemifusion stage, but the underlying mechanism remains unclear. Here, we show that recombinant IFITM3 reconstituted into lipid vesicles induces negative membrane curvature and that this effect maps to its small amphipathic helix (AH). We demonstrate that AH (i) partitions into lipid-disordered domains where IAV fusion occurs, (ii) induces negative membrane curvature, and (iii) increases lipid order and membrane stiffness. These effects on membrane properties correlate with the fusion-inhibitory activity, as targeting the ectopically expressed AH peptide to the cytoplasmic leaflet of the cell plasma membrane diminishes IAV-cell surface fusion induced by exposure to acidic pH. Our results thus imply that IFITM3 inhibits the transition from hemifusion to full fusion by imposing an unfavorable membrane curvature and increasing the order and stiffness of the cytoplasmic leaflet of endosomal membranes. Our findings reveal a universal mechanism by which cells block entry of diverse enveloped viruses.

Lipid-protein interactions for ECA1 an N-ANTH domain protein involved in stress signaling in plants

Epsin-like Clathrin Adaptor 1 (ECA1/ PICALM1A) is an A/ENTH domain protein that acts as an adaptor protein in clathrin-mediated endocytosis. ECA1 is recruited to the membrane during salt stress signaling in plants in a phosphatidic acid (PA)-dependent manner. PA is a lipid second messenger that rapidly and transiently increases in concentration under stress stimuli. Upon an increase in PA concentration another lipid, diacylglycerol pyrophosphate (DGPP), starts to accumulate. The accumulation of DGPP is suggested to be a cue for attenuating PA signaling during stress in plants. We showed in vitro that ECA1-PA binding is modulated as a function of membrane curvature stress and charge. In this work, we investigate ECA1 binding to DGPP in comparison with PA. We show that ECA1 has more affinity for the less charged PA, and this binding is pH dependent. Additionally, plant PA binding proteins SnRK2.10, TGD2C, and PDK1-PH2 were investigated for their interaction with DGPP, since no known DGPP binding proteins are available in the literature to date. Our results shed further light on DGPP and its interactions with membrane proteins which brings us closer toward understanding the complexity of protein interactions with anionic lipids, especially the enigmatic anionic lipid DGPP.

Generalization of the swelling method to measure the intrinsic curvature of lipids

Via computer simulation of a coarse-grained model of two-component lipid bilayers, we compare two methods of measuring the intrinsic curvatures of the constituting monolayers. The first one is a generalization of the swelling method that, in addition to the assumption that the spontaneous curvature linearly depends on the composition of the lipid mixture, incorporates contributions from its elastic energy. The second method measures the effective curvature-composition coupling between the apposing leaflets of bilayer structures (planar bilayers or cylindrical tethers) to extract the spontaneous curvature. Our findings demonstrate that both methods yield consistent results. However, we highlight that the two-leaflet structure inherent to the latter method has the advantage of allowing measurements for mixed lipid systems up to their critical point of demixing as well as in the regime of high concentration (of either species).

Molecular Motor Dnm1 Synergistically Induces Membrane Curvature To Facilitate Mitochondrial Fission

Dnm1 and Fis1 are prototypical proteins that regulate yeast mitochondrial morphology by controlling fission, the dysregulation of which can result in developmental disorders and neurodegenerative diseases in humans. Loss of Dnm1 blocks the formation of fission complexes and leads to elongated mitochondria in the form of interconnected networks, while overproduction of Dnm1 results in excessive mitochondrial fragmentation. In the current model, Dnm1 is essentially a GTP hydrolysis-driven molecular motor that self-assembles into ring-like oligomeric structures that encircle and pinch the outer mitochondrial membrane at sites of fission. In this work, we use machine learning and synchrotron small-angle X-ray scattering (SAXS) to investigate whether the motor Dnm1 can synergistically facilitate mitochondrial fission by membrane remodeling. A support vector machine (SVM)-based classifier trained to detect sequences with membrane-restructuring activity identifies a helical Dnm1 domain capable of generating negative Gaussian curvature (NGC), the type of saddle-shaped local surface curvature found on scission necks during fission events. Furthermore, this domain is highly conserved in Dnm1 homologues with fission activity. Synchrotron SAXS measurements reveal that Dnm1 restructures membranes into phases rich in NGC, and is capable of inducing a fission neck with a diameter of 12.6 nm. Through in silico mutational analysis, we find that the helical Dnm1 domain is locally optimized for membrane curvature generation, and phylogenetic analysis suggests that dynamin superfamily proteins that are close relatives of human dynamin Dyn1 have evolved the capacity to restructure membranes via the induction of curvature mitochondrial fission. In addition, we observe that Fis1, an adaptor protein, is able to inhibit the pro-fission membrane activity of Dnm1, which points to the antagonistic roles of the two proteins in the regulation of mitochondrial fission.

Using Curvature Power To Map the Domain of Inverse Micellar Cubic Phases: The Case of Aliphatic Aldehydes in 1,2-Dioleoyl-sn-glycero-3-phosphoethanolamine

Oxylipins, or fatty aldehydes, are a class of molecules produced from membrane lipids as a result of oxidative stress or enzyme-mediated peroxidation. Here we report the effects of two biologically important fatty aldehydes, trans,trans-2,4-decanedienal (DD) and cis-11-hexadecenal (HD), on the phase behavior of the lipid 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) in water. We compare the phase behavior of DD/DOPE and HD/DOPE mixtures to the phase behavior of oleic acid/DOPE mixtures and show that DD, HD, and oleic acid have similar effects on the phase diagrams of DOPE. Notably, both DD and HD, like oleic acid, induce the formation of Fd3m inverse micellar cubic phases in DOPE/water mixtures. This is the first time that Fd3m phases in fatty aldehyde-containing mixtures have been reported. We assess the effects of DD, HD, and oleic acid on DOPE in terms of lipid spontaneous curvatures and propose a method to predict the formation of Fd3m phases from the curvature power of amphiphiles. This methodology predicts that Fd3m phases will become stable if the spontaneous curvature of a lipid mixture is -0.48 ± 0.05 nm^-1 or less.

Lipid Spontaneous Curvatures Estimated from Temperature-Dependent Changes in Inverse Hexagonal Phase Lattice Parameters: Effects of Metal Cations

Recently we reported a method for estimating the spontaneous curvatures of lipids from temperature-dependent changes in the lattice parameter of inverse hexagonal liquid crystal phases of binary lipid mixtures. This method makes use of 1,2-dioleoyl-sn-glycerol-3-phosphoethanolamine (DOPE) as a host lipid, which preferentially forms an inverse hexagonal phase to which a guest lipid of unknown spontaneous curvature is added. The lattice parameters of these binary lipid mixtures are determined by small-angle X-ray diffraction at a range of temperatures and the spontaneous curvature of the guest lipid is determined from these data. Here we report the use of this method on a wide range of lipids under different ionic conditions. We demonstrate that our method provides spontaneous curvature values for DOPE, cholesterol, and monoolein that are within the range of values reported in the literature. Anionic lipids 1,2-dioleoyl-sn-glycerol-3-phosphatidic acid (DOPA) and 1,2-dioleoyl-sn-glycerol-3-phosphoserine (DOPS) were found to exhibit spontaneous curvatures that depend on the concentration of divalent cations present in the mixtures. We show that the range of curvatures estimated experimentally for DOPA and DOPS can be explained by a series of equilibria arising from lipid-cation exchange reactions. Our data indicate a universal relationship between the spontaneous curvature of a lipid and the extent to which it affects the lattice parameter of the hexagonal phase of DOPE when it is part of a binary mixture. This universal relationship affords a rapid way of estimating the spontaneous curvatures of lipids that are expensive, only available in small amounts, or are of limited chemical stability.

Implementation of a methodology for determining elastic properties of lipid assemblies from molecular dynamics simulations

Background

The importance of the material properties of membranes for diverse cellular processes is well established. Notably, the elastic properties of the membrane, which depend on its composition, can directly influence membrane reshaping and fusion processes as well as the organisation and function of membrane proteins. Determining these properties is therefore key for a mechanistic understanding of how the cell functions.

Results

We have developed a method to determine the bending rigidity and tilt modulus, for lipidic assemblies of arbitrary lipid composition and shape, from molecular dynamics simulations. The method extracts the elastic moduli from the distributions of microscopic tilts and splays of the lipid components. We present here an open source implementation of the method as a set of Python modules using the computational framework OpenStructure. These modules offer diverse algorithms typically used in the calculatation the elastic moduli, including routines to align MD trajectories of complex lipidic systems, to determine the water/lipid interface, to calculate lipid tilts and splays, as well as to fit the corresponding distributions to extract the elastic properties. We detail the implementation of the method and give several examples of how to use the modules in specific cases.

Conclusions

The method presented here is, to our knowledge, the only available computational approach allowing to quantify the elastic properties of lipidic assemblies of arbitrary shape and composition (including lipid mixtures). The implementation as python modules offers flexibility, which has already allowed the method to be applied to diverse lipid assembly types, ranging from bilayers in the liquid ordered and disordered phases to a study of the inverted-hexagonal phase, and with different force-fields (both all-atom and coarse grained representations). The modules are freely available through GitHub at https://github.com/njohner/ost_pymodules/ while OpenStructure can be obtained at http://www.openstructure.org.

Hydrophobic surfactant proteins strongly induce negative curvature

The hydrophobic surfactant proteins SP-B and SP-C greatly accelerate the adsorption of vesicles containing the surfactant lipids to form a film that lowers the surface tension of the air/water interface in the lungs. Pulmonary surfactant enters the interface by a process analogous to the fusion of two vesicles. As with fusion, several factors affect adsorption according to how they alter the curvature of lipid leaflets, suggesting that adsorption proceeds via a rate-limiting structure with negative curvature, in which the hydrophilic face of the phospholipid leaflets is concave. In the studies reported here, we tested whether the surfactant proteins might promote adsorption by inducing lipids to adopt a more negative curvature, closer to the configuration of the hypothetical intermediate. Our experiments used x-ray diffraction to determine how the proteins in their physiological ratio affect the radius of cylindrical monolayers in the negatively curved, inverse hexagonal phase. With binary mixtures of dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC), the proteins produced a dose-related effect on curvature that depended on the phospholipid composition. With DOPE alone, the proteins produced no change. With an increasing mol fraction of DOPC, the response to the proteins increased, reaching a maximum 50% reduction in cylindrical radius at 5% (w/w) protein. This change represented a doubling of curvature at the outer cylindrical surface. The change in spontaneous curvature, defined at approximately the level of the glycerol group, would be greater. Analysis of the results in terms of a Langmuir model for binding to a surface suggests that the effect of the lipids is consistent with a change in the maximum binding capacity. Our findings show that surfactant proteins can promote negative curvature, and support the possibility that they facilitate adsorption by that mechanism.

Use of isotropically tumbling bicelles to measure curvature induced by membrane components

Isotropically tumbling discoidal bicelles are a useful biophysical tool for the study of lipids and proteins by NMR, dynamic light scattering, and small-angle X-ray scattering. Isotropically tumbling bicelles present a low-curvature central region, typically enriched with DMPC in the lamellar state, and a highly curved detergent rim, typically composed of DHPC. In this report, we study the impact of the partitioning and induced curvature of a few molecules of a foreign lipid on the bicelle size, structure, and curvature. Previous approaches for studying curvature have focused on macroscopic and bulk properties of membrane curvature. In the approach presented here, we show that the conical shape of the DOPE lipid and the inverted-conical shape of the DPC lipid induce measurable curvature changes in the bicelle size. Bicelles with an average of 1.8 molecules of DOPE have marked increases in the size of bicelles, consistent with negative membrane curvature in the central region of the bicelle. With bicelle curvature models, radii of curvature on the order of -100 Å and below are measured, with a greater degree of curvature observed in the more pliable Lα state above the phase-transition temperature of DMPC. Bicelles with an average of 1.8 molecules of DPC are reduced in size, consistent with positive membrane curvature in the rim, and at higher temperatures, DPC is distributed in the central region to form mixed-micelle structures. We use translational and rotational diffusion measurements by NMR, size-exclusion chromatography, and structural models to quantitate changes in bicelle size, curvature, and lipid dynamics.

An anionic phospholipid enables the hydrophobic surfactant proteins to alter spontaneous curvature

The hydrophobic surfactant proteins, SP-B and SP-C, greatly accelerate the adsorption of the surfactant lipids to an air/water interface. Previous studies of factors that affect curvature suggest that vesicles may adsorb via a rate-limiting structure with prominent negative curvature, in which the hydrophilic face of the lipid leaflets is concave. To determine if SP-B and SP-C might promote adsorption by inducing negative curvature, we used small-angle x-ray scattering to test whether the physiological mixture of the two proteins affects the radius of cylindrical monolayers in the inverse hexagonal phase. With dioleoyl phosphatidylethanolamine alone, the proteins had no effect on the hexagonal lattice constant, suggesting that the proteins fail to insert into the cylindrical monolayers. The surfactant lipids also contain ∼10% anionic phospholipids, which might allow incorporation of the cationic proteins. With 10% of the anionic dioleoyl phosphatidylglycerol added to dioleoyl phosphatidylethanolamine, the proteins induced a dose-related decrease in the hexagonal lattice constant. At 30°C, the reduction reached a maximum of 8% relative to the lipids alone at ∼1% (w/w) protein. Variation of NaCl concentration tested whether the effect of the protein represented a strictly electrostatic effect that screening by electrolyte would eliminate. With concentrations up to 3 M NaCl, the dose-related change in the hexagonal lattice constant decreased but persisted. Measurements at different hydrations determined the location of the pivotal plane and proved that the change in the lattice constant produced by the proteins resulted from a shift in spontaneous curvature. These results provide the most direct evidence yet that the surfactant proteins can induce negative curvature in lipid leaflets. This finding supports the model in which the proteins promote adsorption by facilitating the formation of a negatively curved, rate-limiting structure.

Membrane fusion induced by small molecules and ions

Membrane fusion is a key event in many biological processes. These processes are controlled by various fusogenic agents of which proteins and peptides from the principal group. The fusion process is characterized by three major steps, namely, inter membrane contact, lipid mixing forming the intermediate step, pore opening and finally mixing of inner contents of the cells/vesicles. These steps are governed by energy barriers, which need to be overcome to complete fusion. Structural reorganization of big molecules like proteins/peptides, supplies the required driving force to overcome the energy barrier of the different intermediate steps. Small molecules/ions do not share this advantage. Hence fusion induced by small molecules/ions is expected to be different from that induced by proteins/peptides. Although several reviews exist on membrane fusion, no recent review is devoted solely to small moleculs/ions induced membrane fusion. Here we intend to present, how a variety of small molecules/ions act as independent fusogens. The detailed mechanism of some are well understood but for many it is still an unanswered question. Clearer understanding of how a particular small molecule can control fusion will open up a vista to use these moleucles instead of proteins/peptides to induce fusion both in vivo and in vitro fusion processes.

Elastic deformations in hexagonal phases studied by small-angle X-ray diffraction and simulations

In this study we present experimental and theoretical results which concern the deviations from circularity of the pivotal plane in the inverse hexagonal phases (H(II)) of phospholipid self-assemblies. Due to packing constraints, the cross-section of the polar/apolar interface deviates from a circle, which we studied in minute detail by analysing small-angle X-ray diffraction data of dioleoyl-phosphatidylethanolamine (DOPE) and stearoyl-oleoyl-phosphatidylethanolamine (SOPE), respectively. On this structural basis, Monte Carlo (MC) simulated annealing variations of the free energy were carried out, both on the formation of the H(II)-phase and on the particular shape of the cross-section in the H(II)-phase. The equilibrium of the H(II)-phase pivotal plane contour and the corresponding values of the mean intrinsic curvature, H(m), and the hydrocarbon chain stiffness, τ, were determined from MC calculations. The results of these calculations were tested by solving the corresponding system of non-linear differential equations derived using variational calculus. Here our main aim is to predict the range of possible values of H(m) and τ. Comparing the measured structural data with predictions from MC calculations including lipid anisotropy, and accounting for the elastic deformations of the pivotal plane allowed us to determine a relationship between the bending deformation and stretching of hydrocarbon chains.