Charged Lipid Bilayers Research Articles

Interaction of Semiflexible Filamentous Virus Particles with Freestanding Lipid Membranes

Understanding of the mechanisms of interaction of macromolecules and colloidal particles with lipid membranes is far from complete, and the questions related to role of local perturbation of the membrane properties in these interactions are still largely unsolved. Previously, we have found [1] that interaction of DNA molecules with strongly charged freestanding cationic lipid bilayers [2] leads to an unexpected phenomenon of membrane-mediated coil-globule transition of membrane-absorbed DNA macromolecules. To elucidate the effect of the persistence length in these phenomena, we study the behavior of much stiffer semiflexible fd virus particles (persistence length ∼2.2 μm) electrostatically adsorbed on freestanding cationic lipid membranes. At low membrane charge densities, membrane-adsorbed fd virus particles behave as semiflexible filaments in 2D. On the other hand, we find that membrane-driven interactions at higher membrane charge densities are strong enough to induce the membrane-mediated coil-globule transition of the relatively stiff fd virus particles, which agrees with the recent theoretical predictions [3]. Further, for fd virus particles adsorbed at higher surface densities on weakly charged membranes we observe a new unexpected phenomenon of membrane-driven self-organization of the filamentous virus particles into long linear chain aggregates. [1] C. Herold, P. Schwille, and E. P. Petrov, Phys. Rev. Lett.104 (2010) 148102. [2] C. Herold, G. Chwastek, P. Schwille, and E. P. Petrov, Langmuir28 (2012) 5518. [3] A. G. Cherstvy and E. P. Petrov (2013) submitted.

Molecular Mechanism of Interfacial Adsorption of Disordered Cytoplasmic Tail of Immune Receptors to Membrane

Important immune responses are linked to Src-mediated phosphorylation of cytoplasmic tyrosine-based motifs (ITAMs). However, the mechanism of how receptor ligation translates into ITAM phosphorylation remains elusive. Here, we use molecular dynamic simulations to explore the potential regulatory involvement of lipid membranes and their influence on the structure and behavior of the cytoplasmic portion of the CD3ɛ chain of the T-cell receptor. It has been hypothesized that the accessibility of ITAM motifs, such as those in the CD3ɛ cytoplasmic tail, can be blocked by ionic interactions between positively charged amino acids in receptor tails and negatively charged lipid head groups. This interesting hypothesis represents a previously unrecognized mechanism for control of receptor activation. Our simulations support the notion that the net charge of the lipids present in the membrane can affect peptide-membrane interactions. Results are consistent with experimental findings that show increase interfacial absorption in negatively charged lipid bilayer. Our simulations revealed the conformational variability of the disordered tail, which led to an additional focus on quantifying the interaction by free energy calculations, combined with long time-scale simulations using coarse-grained (CG) approaches. These studies will be extended to address how changes in ionic conditions can modulate phosphorylation of ITAM motifs and lead to regulation of activation of the TCR and other ITAM-bearing immunoreceptors.

Simulations of Anionic Lipid Membranes: Development of Interaction-Specific Ion Parameters and Validation Using NMR Data

Overbinding of ions to lipid head groups is a potentially serious artifact in simulations of charged lipid bilayers. In this study, the Lennard-Jones radii in the CHARMM force field for interactions of Na(+) and lipid oxygen atoms of carboxyl, phosphate, and ester groups were revised to match osmotic pressure data on sodium acetate and electrophoresis data on palmitoyloleoyl phosphatidylcholine (POPC) vesicles. The new parameters were then validated by successfully reproducing previously published experimental NMR deuterium order parameters for dimyristoyl phosphatidylglycerol (DMPG) and newly obtained values for palmitoyloleoyl phosphatidylserine (POPS). Although the increases in Lennard-Jones diameters are only 0.02-0.12 Å, they are sufficient to reduce Na+ binding, and thereby increase surface areas per lipid by 5-10% compared with the unmodified parameters.

Interaction between Dipolar Lipid Headgroups and Charged Nanoparticles Mediated by Water Dipoles and Ions

In this work, a theoretical model describing the interaction between a positively or negatively charged nanoparticle and neutral zwitterionic lipid bilayers is presented. It is shown that in the close vicinity of the positively charged nanoparticle, the zwitterionic lipid headgroups are less extended in the direction perpendicular to the membrane surface, while in the vicinity of the negatively charged nanoparticle, the headgroups are more extended. This result coincides with the calculated increase in the osmotic pressure between the zwitterionic lipid surface and positively charged nanoparticle and the decrease of osmotic pressure between the zwitterionic lipid surface and the negatively charged nanoparticle. Our theoretical predictions agree well with the experimentally determined fluidity of a lipid bilayer membrane in contact with positively or negatively charged nanoparticles. The prospective significance of the present work is mainly to contribute to better understanding of the interactions of charged nanoparticles with a zwitterionic lipid bilayer, which may be important in the efficient design of the lipid/nanoparticle nanostructures (like liposomes with encapsulated nanoparticles), which have diverse biomedical applications, including targeted therapy (drug delivery) and imaging of cancer cells.

Phosphate Ions Promoting Association between Peptide and Modeling Cell Membrane Revealed by Sum Frequency Generation Vibrational Spectroscopy

Phosphate ion is one of the most important anions present in the intracellular and extracellular fluid. It can form strongly hydrogen-bonded and salt-bridged complexes with arginine and lysine to activate the voltage gated channel protein. A molecular-level insight into how the phosphate anions mediate the interaction between peptides and cell membrane is critical to understand membrane-bound peptide actions. In this study, sum frequency generation vibrational spectroscopy (SFG-VS) has been applied to characterize interactions between mastoparan (MP, a G-protein-activating peptide) and different charged lipid bilayers in situ. It is found that phosphate ions can greatly promote the association of MP with lipid bilayers and accelerate the conformation transition of membrane-bound MP from aggregation into α-helical structure. In phosphate buffer solution, MP can insert not only into negatively and neutrally charged lipid bilayers but also into positively charged lipid bilayers. In neutrally and negatively charged lipid bilayers, the tilt angle of α-helical structure becomes smaller with increasing buffer concentration, while MP adopts a multiple orientation distribution in the positively charged lipid bilayer. MP interacts with lipid bilayers in the salt solution environment most likely by formation of toroidal pores inside the bilayer matrix. Results from our studies will provide insight into the MP action mechanism and offer some ideas to deliver exogenous protein into the cytosol.

Domain organization of membrane-bound factor VIII.

Factor VIII (FVIII) is the blood coagulation protein which when defective or deficient causes for hemophilia A, a severe hereditary bleeding disorder. Activated FVIII (FVIIIa) is the cofactor to the serine protease factor IXa (FIXa) within the membrane-bound Tenase complex, responsible for amplifying its proteolytic activity more than 100,000 times, necessary for normal clot formation. FVIII is composed of two noncovalently linked peptide chains: a light chain (LC) holding the membrane interaction sites and a heavy chain (HC) holding the main FIXa interaction sites. The interplay between the light and heavy chains (HCs) in the membrane-bound state is critical for the biological efficiency of FVIII. Here, we present our cryo-electron microscopy (EM) and structure analysis studies of human FVIII-LC, when helically assembled onto negatively charged single lipid bilayer nanotubes. The resolved FVIII-LC membrane-bound structure supports aspects of our previously proposed FVIII structure from membrane-bound two-dimensional (2D) crystals, such as only the C2 domain interacts directly with the membrane. The LC is oriented differently in the FVIII membrane-bound helical and 2D crystal structures based on EM data, and the existing X-ray structures. This flexibility of the FVIII-LC domain organization in different states is discussed in the light of the FVIIIa-FIXa complex assembly and function.

Interactions of antileishmanial drugs with monolayers of lipids used in the development of amphotericin B–miltefosine-loaded nanocochleates

The emergence of strains of Leishmania resistant to existing drugs complicates the treatment of life-threatening visceral leishmaniasis. The development of new lipid formulation (nanocochleates), containing two active drugs: amphotericin B (AmB) and miltefosine (hexadecylphosphocholine, HePC), could increase effectiveness, decrease toxicity and reduce the risk of appearance of resistance. Nanocochleates are cigar-shaped structures of rolled negatively charged lipid bilayers bridged by calcium, prepared from dioleoylphosphatidylserine (DOPS) and cholesterol (Cho) and able to accommodate drugs. To determine the interaction, the orientation and the stability of the amphiphilic drugs in the lipid mixture and the optimal drugs/lipids ratio, the Langmuir film balance and BAM (Brewster angle microscopy) were used. The drugs were mixed with the lipids (DOPS or 9DOPS/1Cho) and spread at the air-water interface. A stability study showed that DOPS maintained HePC at the interface at low molar fraction of HePC; this effect became more marked in the presence of Cho. The fact that HePC can be stably associated with the monolayer at low molar fraction (below 10%) suggests that in the nanocochleates HePC is inserted between the lipid molecules rather than between the bilayers. Phase diagrams and BAM images showed that, even at low pressure, DOPS maintains AmB at low molar fraction (below 10%) in the “erect” rather than the horizontal form at the interface and that the presence of Cho reinforces this effect. These results allowed us to predict the organization and the orientation of these drugs in the nanocochleates and to determine the optimal drugs/lipids ratio.

Domain Organization of Membrane-Bound Factor VIII

Factor VIII (FVIII) is the blood coagulation protein which when defective or deficient causes for hemophilia A, a severe hereditary bleeding disorder. Activated FVIII (FVIIIa) is the co-factor to the serine protease Factor IXa (FIXa) within the membrane-bound Tenase complex, responsible for amplifying its proteolytic activity more than 100,000 times, necessary for normal blood clotting. FVIII is composed of two non-covalently linked peptide chains: a light chain holding the membrane interaction sites and a heavy chain holding the main FIXa interaction sites. The interplay between the light and heavy chains in the membrane-bound state is critical for FVIII biological efficiency. Here, we present our cryo-electron microscopy and structure analysis studies of human FVIII light chain (LC), as helically assembled onto negatively charged single lipid bilayer nanotubes (LNT). The resolved FVIII-LC membrane-bound structure at 20 Å, supports aspects of our previously proposed FVIII structure from membrane-bound two-dimensional (2D) crystals, such as only the C2 domain interacts directly with the membrane [Stoilova-McPhie 2002]. The light chain is oriented differently in the FVIII membrane-bound helical and 2D crystal structures based on electron microscopy data and the 3D structure solved by X-ray [Ngo 2008]. The flexibility of the FVIII-LC domain organization in different crystal packing (3D, 2D and helical) is essential to understand the FVIII membrane-bound organization and its significance for hemostasis. SSM acknowledge Professor Ed Egelman from the University of Virginia at Charlotseville for help and support with the IHRSR algorithm and helical analysis. This work is supported by a National Scientist Development grant from the American Heart Association: 10SDG3500034 and UTMB start up funds to SSM. The Cryo-EM and initial structure analysis was carried out at the NCMI facility, courtesy of Professor Wah Chiu, supported by a National Center for Research Resources grant P41RR02250.

Molecular interactions of Alzheimer amyloid-β oligomers with neutral and negatively charged lipid bilayers

Interaction of p3 (Aβ(17-42)) peptides with cell membranes is crucial for the understanding of amyloid toxicity associated with Alzheimer's disease (AD). Such p3-membrane interactions are considered to induce the disruption of membrane permeability and integrity, but the exact mechanisms of how p3 aggregates, particularly small p3 oligomers, induce receptor-independent membrane disruption are not yet completely understood. Here, we investigate the adsorption, orientation, and surface interaction of the p3 pentamer with lipid bilayers composed of both pure zwitterionic POPC (palmitoyl-oleoyl-phosphatidylcholine) and mixed anionic POPC-POPG (palmitoyl-oleoyl-phosphatidylglycerol) (3 : 1) lipids using explicit-solvent molecular dynamics (MD) simulations. MD simulation results show that the p3 pentamer has much stronger interactions with mixed POPC-POPG lipids than pure POPC lipids, consistent with experimental observation that Aβ adsorption and fibrillation are enhanced on anionic lipid bilayers. Although electrostatic interactions are main attractive forces to drive the p3 pentamer to adsorb on the bilayer surface, the adsorption of the p3 pentamer on the lipid bilayer with C-terminal β-strands facing toward the bilayer surface is a net outcome of different competitions between p3 peptides-lipid bilayer and ions-p3-bilayer interactions. More importantly, Ca(2+) ions are found to form ionic bridges to associate negatively charged residues of p3 with anionic headgroups of the lipid bilayer, resulting in Aβ-Ca(2+)-PO4(-) complexes. Intensive Ca(2+) bound to the lipid bilayer and Ca(2+) ionic bridges may lead to Ca(2+) hemostasis responsible for neuronal dysfunction and death. This work provides insights into the mutual structure, dynamics, and interactions of both Aβ peptides and lipid bilayers at the atomic level, which expand our understanding of the complex behavior of amyloid-induced membrane disruption.

Determining the Role of NS4B in Membrane Remodelling during Hcv Replication

Hepatitis C is an RNA virus that replicates in association with intracellular membranous structures call membranous webs (MWs). Viral protein NS4B is a key organizer of replication, one crucial function being the induction of MWs. The mechanisms of MW formation are unknown, but it clearly involves induction of membrane curvature, which may require NS4B oligomerisation and possibly hydrophobic wedging. NS4B is known to oligomerise, and the N-terminal amphipathic helix AH2 has been implied as a major determinant of self-association. In order to understand the process of MW induction, we aimed to determine AH2's capacity to remodel membranes by studying the interaction of AH2 with membranes mimicking those found within the cell using 2H and 31P solid-state NMR. Our results show changes in membrane morphology, induced by AH2 in negatively charged vesicles, an effect not observed in neutral bilayers indicating a requirement for negatively charged lipids. Chemical cross-linking studies of AH2 in lipids vesicles confirms AH2 homo-oligomerisation and suggests a charge dependency; with larger oligomers observed in neutral lipid bilayers compared to negatively charged lipid bilayers and lipid mixtures mimicking cellular membranes. These results suggest that AH2 plays a crucial role in NS4B's capacity to alter membrane morphology.

Experimental study of the bending elasticity of charged lipid bilayers in aqueous solutions with pH5

Exposure to high concentrations of contaminations due to air polluting gases, vapours and aerosols and possibly altering the normal pH in the body could lead to undesirable changes in the properties of biological cells. Here, we study experimentally the mechanical properties of synthetic phospholipid bilayers containing increasing molar fractions (up to 0.15) of charged lipid (synthetic phosphatidylserine) in aqueous solutions with controlled ionic strength and at pH 5, which is slightly lower than the physiological values of pH. Our observations in phase contrast and fluorescence testified to the coexistence of two phases in membranes for temperatures below 29°C. Micro-sized inhomogeneities in vesicle membranes were systematically observed at temperatures lower than 29°C and for molar fractions of phosphatidylserine in the bilayer higher than 0.1. For the quantitative determination of the membrane bending rigidity, we applied thermal fluctuation analysis of the shape of quasispherical lipid vesicles. As far as the liquid-crystalline state of the bilayer is a necessary condition for the application of the experimental method, only vesicles satisfying this requirement were processed for determination of their membrane bending rigidity. The value obtained for the bending modulus of bilayers with 0.15 molar content of charged lipid is about two times higher than the bending modulus of uncharged membranes in the same bathing solution. These findings are in qualitative agreement with our previous results for the bending rigidity of charged bilayers, measured by vesicle micromanipulation.

Molecular-dynamic simulation of phospholipid bilayers: Ion distribution at the surface of neutral and charged bilayer in the liquid crystalline state

The electric field and ion distribution at the surface of neutral and charged lipid bilayers (BeCl2 and dipalmitoyl phosphatidylcholine/dipalmitoyl phosphatidylserine (DPPC/DPPS) + KCl) were studied with molecular dynamic (MD) methods. It is shown that the contributions of lipid molecules, water and ions to the electric potential compensate each other in the region of the diffuse double layer and decrease the potential value close to zero. It is also demonstrated that the ion distribution at the charged surface is determined not only by the electrostatic ion-medium interaction. The total energy of this interaction was compared with the potential of mean ion force. It was shown that cations and anions have a different effect on the state of water molecules at the surface. The order parameter of water in the system DPPC + BeCl2 and the Clion distribution have the extremum at the distance of 10 α atoms of the phospholipid glycerol. This position was chosen as the “electrical” interface of the electrical double layer (EDL) for all lipid systems studied. The potential of mean force of counter ions in EDL allows us to obtain the value of potential at the lipid surface suitable for experimental test of the MD data. This surface potential and surface charge density was found from MD simulation different electrolyte concentrations and DPPS content of 20, 40 and 60% in the mixture with DPPC and was shown to be in a good agreement with the Gouy-Chapman-Stern model upon fitting parameters close to their experimental values.

Charge renormalization of bilayer elastic properties

By combining molecular dynamics simulations and analytical arguments, we investigate the elastic properties of charged lipid bilayers. We show that electrostatic interactions between the head groups can lead to solidification of the lipid bilayer that would otherwise be in a liquid state if the charges were absent. All elastic parameters of the bilayer such as the bending rigidity κ and the two-dimensional bulk modulus λ and Young's modulus Y are found to depend on the values of the charges assigned to the lipid head groups. To extract κ and λ, we fit the molecular dynamics data to a standard elastic model for lipid bilayers. Moreover, we analytically obtain the dependence of the Young modulus Y on the relative strengths of electrostatic and van der Waals interactions in the zero temperature limit.

Role of Nanoparticle Surface Functionality in the Disruption of Model Cell Membranes

Lipid bilayers are biomembranes common to cellular life and constitute a continuous barrier between cells and their environment. Understanding the interaction of engineered nanomaterials (ENMs) with lipid bilayers is an important step toward predicting subsequent biological effects. In this study, we assess the effect of varying the surface functionality and concentration of 10-nm-diameter gold (Au) and titanium dioxide (TiO(2)) ENMs on the disruption of negatively charged lipid bilayer vesicles (liposomes) using a dye-leakage assay. Our findings show that Au ENMs having both positive and negative surface charge induce leakage that reaches a steady state after several hours. Positively charged particles with identical surface functionality and different core compositions show similar leakage effects and result in faster and greater leakage than negatively charged particles, which suggests that surface functionality, not particle core composition, is a critical factor in determining the interaction between ENMs and lipid bilayers. The results suggest that particles permanently adsorb to bilayers and that only one positively charged particle is required to disrupt a liposome and trigger the leakage of its entire contents in contrast to mellitin molecules, the most widely studied membrane lytic peptide, which requires hundred of molecules to generate leakage.

Unraveling Dendrimer Translocation Across Cell Membrane Mimics

Poly(amidoamine) (PAMAM) dendrimers are promising candidates in several applications within the medical field. However, it is still to date not fully understood whether they are able to passively translocate across lipid bilayers. Recently, we used fluorescence microscopy to show that PAMAM dendrimers induced changes in the permeability of lipid membranes but the dendrimers themselves could not translocate to be released into the vesicle lumen. Because of the lack of resolution, these experiments could not assess whether the dendrimers were able to translocate but remained attached to the membrane. Using quartz crystal microbalance with dissipation monitoring and neutron reflectivity, a structural investigation was performed to determine how dendrimers interact with zwitterionic and negatively charged lipid bilayers. We hereby show that dendrimers adsorb on top of lipid bilayers without significant dendrimer translocation, regardless of the lipid membrane surface charge. Thus, most likely dendrimers are actively transported through cell membranes by protein-mediated endocytosis in agreement with previous cell studies. Finally, the higher activity of PAMAM dendrimers for phosphoglycerol-containing membranes is in line with their high antimicrobial activity against Gram-negative bacteria.

Supramolecular Structure of Membrane-Associated Polypeptides by Combining Solid-State NMR and Molecular Dynamics Simulations

Elemental biological functions such as molecular signal transduction are determined by the dynamic interplay between polypeptides and the membrane environment. Determining such supramolecular arrangements poses a significant challenge for classical structural biology methods. We introduce an iterative approach that combines magic-angle spinning solid-state NMR spectroscopy and atomistic molecular dynamics simulations for the determination of the structure and topology of membrane-bound systems with a resolution and level of accuracy difficult to obtain by either method alone. Our study focuses on the Shaker B ball peptide that is representative for rapid N-type inactivating domains of voltage-gated K+ channels, associated with negatively charged lipid bilayers.

Kinetics and Thermodynamics of Chlorpromazine Interaction with Lipid Bilayers: Effect of Charge and Cholesterol

Passive transport across cell membranes is the major route for the permeation of xenobiotics through tight endothelia such as the blood–brain barrier. The rate of passive permeation through lipid bilayers for a given drug is therefore a critical step in the prediction of its pharmacodynamics. We describe a detailed study on the kinetics and thermodynamics for the interaction of chlorpromazine (CPZ), an antipsychotic drug used in the treatment of schizophrenia, with neutral and negatively charged lipid bilayers. Isothermal titration calorimetry was used to study the partition and translocation of CPZ in lipid membranes composed of pure POPC, POPC:POPS (9:1), and POPC:Chol:POPS (6:3:1). The membrane charge due to the presence of POPS as well as the additional charge resulting from the introduction of CPZ in the membrane were taken into account, allowing the calculation of the intrinsic partition coefficients (K(P)) and the enthalpy change (ΔH) associated with the process. The enthalpy change upon partition to all lipid bilayers studied is negative, but a significant entropy contribution was also observed for partition to the neutral membrane. Because of the positive charge of CPZ, the presence of negatively charged lipids in the bilayer increases both the observed amount of CPZ that partitions to the membrane (KP(obs)) and the magnitude of ΔH. However, when the electrostatic effects are discounted, the intrinsic partition coefficient was smaller, indicating that the hydrophobic contribution was less significant for the negatively charged membrane. The presence of cholesterol strongly decreases the affinity of CPZ for the bilayer in terms of both the amount of CPZ that associates with the membrane and the interaction enthalpy. A quantitative characterization of the rate of CPZ translocation through membranes composed of pure POPC and POPC:POPS (9:1) was also performed using an innovative methodology developed in this work based on the kinetics of the heat evolved due to the interaction of CPZ with the membranes.

Automation Highlights from the Literature

Automation Highlights from the Literature

Lysozyme interaction with negatively charged lipid bilayers: protein aggregation and membrane fusion

We report on the interaction of hen egg-white lysozyme (HEWL) with lipid vesicles in terms of surface-induced protein conformational variation and subsequent aggregation. In particular, we investigated the variations of the secondary structure of native lysozyme in the presence of liposomes with different surface charge density, resulting from different molar ratios of the zwitterionic POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and the negatively charged POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1′-rac-glycerol)). It is well known that the main driving force involved in the interaction between globally anionic liposomes and lysozyme is electrostatic compensation, which, in some cases, produces extended aggregation. Moreover the presence of membranes can induce unfolding in the protein. In order to understand the main determinants of such phenomena, we probed simultaneously lysozyme-induced vesicle fusion events, variations in the secondary structure of the protein and its effect on liposomal membrane fluidity. We found that above a charge-density threshold, the association with vesicles results in modifications of the native structure associated with a decrease of liposomal membrane fluidity. Electron microscopy images revealed that the above described interactions result in mesoscopic structural changes, i.e. liposome clustering and fusion, together with the appearance of elongated structures, reminiscent of fibrillar aggregates. Additionally, a confocal microscopy analysis revealed that upon interaction with giant unilamellar vesicles (GUVs) of the same lipid composition where the above interactions were observed, a prompt insertion of lysozyme in the membrane occurs, leading to vesicle clustering, with the appearance of elongated structures where both the lipid and the protein are present.

Interactions of Lithium Ions with Lipid Membranes

Lithium is a naturally occurring alkali metal which is very easily ionized to Li+, especially in aqueous solutions. Lithium is found mainly in mineral and seawater and in trace amounts in the human body. Lithium ions are being used in various fields ranging from energy storage to the treatment of mental illnesses including bipolar disorder and schizophrenia. The reason why lithium is such a peculiar ion is not exactly known except that its mode of interactions with charged surfaces depends on its hydration properties (i.e. interaction with nearby water molecules). We report two kinds of experimental measurements of lithium interactions with charged lipid membranes. One set of experiments involves X-ray scattering of multilamellar lipid vesicles in solution for which we measure how lithium salts modify van der Waals and electrostatic forces between neighboring membranes. The other set of measurements compare the effect of added lithium on the activity of gramicidin channels in neutral and negatively charged lipid bilayers. We find that in both x-ray and channel measurements, lithium ions modify lipid interactions in a manner that differs from other monovalent posititive ions.