Phospholipid vesicle aggregation: effect of monovalent and divalent ions

Lipid Exchange Promotes Fusion of Model Protocells

Vesicle fusion is an important process underlying cell division, transport, and membrane trafficking. In phospholipid systems, a range of fusogens including divalent cations and depletants have been shown to induce adhesion, hemifusion, and then full content fusion between vesicles. This work shows that these fusogens do not perform the same function for fatty acid vesicles, which are used as model protocells (primitive cells). Even when fatty acid vesicles appear adhered or hemifused to each other, the intervening barriers between vesicles do not rupture. This difference is likely because fatty acids have a single aliphatic tail, and are more dynamic than their phospholipid counterparts. To address this, it is postulated that fusion could instead occur under conditions, such as lipid exchange, that disrupt lipid packing. Using both experiments and molecular dynamics simulations, it is verified that fusion in fatty acid systems can indeed be induced by lipid exchange. These results begin to probe how membrane biophysics could constrain the evolutionary dynamics of protocells.

Monovalent ion-mediated charge-charge interactions drive aggregation of surface-functionalized gold nanoparticles

Monolayer-protected metal nanoparticles (NPs) are not only promising materials with a wide range of potential industrial and biological applications, but they are also a powerful tool to investigate the behaviour of matter at nanoscopic scales, including the stability of dispersions and colloidal systems. This stability is dependent on a delicate balance between attractive and repulsive interactions that occur in the solution, and it is described in quantitative terms by the classic Derjaguin-Landau-Vewey-Overbeek (DLVO) theory, that posits that aggregation between NPs is driven by van der Waals interactions and opposed by electrostatic interactions. To investigate the limits of this theory at the nanoscale, where the continuum assumptions required by the DLVO theory break down, here we investigate NP dimerization by computing the Potential of Mean Force (PMF) of this process using fully atomistic MD simulations. Serendipitously, we find that electrostatic interactions can lead to the formation of metastable NP dimers at physiological ion concentrations. These dimers are stabilized by complexes formed by negatively charged ligands belonging to distinct NPs that are bridged by positively charged monovalent ions present in solution. We validate our findings by collecting tomographic EM images of NPs in solution and by quantifying their radial distribution function, that shows a marked peak at interparticle distance comparable with that of MD simulations. Taken together, our results suggest that not only van der Waals interactions, but also electrostatic interactions mediated by monovalent ions at physiological concentrations, contribute to attraction between nano-sized charged objects at very short length scales.

New understanding of electrical activity brought by surface potential of cardiomyocytes

Aiming at the problem encountered in the previous research: during the electrical activity of cardiomyocytes, the influent ions do not seem to be directly derived from the extracellular fluid. We chose to cut in from the colloidal properties of the cells, follow the basic principles of physical chemistry, and establish hypotheses along the derivation of the structural characteristics of cardiomyocytes. Through the surface ion adsorption experiment and patch clamp experiment of living cells, under the condition of sequentially reducing the concentration of Na⁺ in the extracellular fluid, we observed the exchange and diffusion of adsorbed ions on the cell surface; the changes of inflow I_Na, I_Ca-L and action potential; and correlation between results. The results showed that the hypothesis is true. The observed parameter changes were consistent with the fact that during depolarization of cardiomyocytes, the ions of influx were derived from the inference of adsorbed ions on the cell surface; at the same time, it also provided an objective and realistic explanation for the generation of electrocardiogram.

Multivalent ions and biomolecules: Attempting a comprehensive perspective

Ions are ubiquitous in nature. They play a key role for many biological processes on the molecular scale, from molecular interactions, to mechanical properties, to folding, to self-organisation and assembly, to reaction equilibria, to signalling, to energy and material transport, to recognition etc. Going beyond monovalent ions to multivalent ions, the effects of the ions are frequently not only stronger (due to the obviously higher charge), but qualitatively different. A typical example is the process of binding of multivalent ions, such as Ca2+ , to a macromolecule and the consequences of this ion binding such as compaction, collapse, potential charge inversion and precipitation of the macromolecule. Here we review these effects and phenomena induced by multivalent ions for biological (macro)molecules, from the "atomistic/molecular" local picture of (potentially specific) interactions to the more global picture of phase behaviour including, e. g., crystallisation, phase separation, oligomerisation etc. Rather than attempting an encyclopedic list of systems, we rather aim for an embracing discussion using typical case studies. We try to cover predominantly three main classes: proteins, nucleic acids, and amphiphilic molecules including interface effects. We do not cover in detail, but make some comparisons to, ion channels, colloidal systems, and synthetic polymers. While there are obvious differences in the behaviour of, and the relevance of multivalent ions for, the three main classes of systems, we also point out analogies. Our attempt of a comprehensive discussion is guided by the idea that there are not only important differences and specific phenomena with regard to the effects of multivalent ions on the main systems, but also important similarities. We hope to bridge physico-chemical mechanisms, concepts of soft matter, and biological observations and connect the different communities further.

The crystal structure of Arabidopsis BON1 provides insights into the copine protein family

The Arabidopsis thaliana BON1 gene product is a member of the evolutionary conserved eukaryotic calcium-dependent membrane-binding protein family. The copine protein is composed of two C2 domains (C2A and C2B) followed by a vWA domain. The BON1 protein is localized on the plasma membrane, and is known to suppress the expression of immune receptor genes and to positively regulate stomatal closure. The first structure of this protein family has been determined to 2.5-Å resolution and shows the structural features of the three conserved domains C2A, C2B and vWA. The structure reveals the third Ca²⁺ -binding region in C2A domain is longer than classical C2 domains and a novel Ca²⁺ binding site in the vWA domain. The structure of BON1 bound to Mn²⁺ is also presented. The binding of the C2 domains to phospholipid (PSF) has been modeled and provides an insight into the lipid-binding mechanism of the copine proteins. Furthermore, the selectivity of the separate C2A and C2B domains and intact BON1 to bind to different phospholipids has been investigated, and we demonstrated that BON1 could mediate aggregation of liposomes in response to Ca²⁺ . These studies have formed the basis of further investigations into the important role that the copine proteins play in vivo.

Aggregation of Lipid A Variants: A Hybrid Particle-Field Model

Lipid A is one of the three components of bacterial lipopolysaccharides constituting the outer membrane of Gram-negative bacteria, and is recognized to have an important biological role in the inflammatory response of mammalians. Its biological activity is modulated by the number of acyl-chains that are present in the lipid and by the dielectric medium, i.e., the type of counter-ions, through electrostatic interactions. In this paper, we report on a coarse-grained model of chemical variants of Lipid A based on the hybrid particle-field/molecular dynamics approach (hPF-MD). In particular, we investigate the stability of Lipid A bilayers for two different hexa- and tetra-acylated structures. Comparing particle density profiles along bilayer cross-sections, we find good agreement between the hPF-MD model and reference all-atom simulation for both chemical variants of Lipid A. hPF-MD models of constituted bilayers composed by hexa-acylated Lipid A in water are stable within the simulation time. We further validate our model by verifying that the phase behavior of Lipid A/counterion/water mixtures is correctly reproduced. In particular, hPF-MD simulations predict the correct self-assembly of different lamellar and micellar phases from an initially random distribution of Lipid A molecules with counterions in water. Finally, it is possible to observe the spontaneous formation and stability of Lipid A vesicles by fusion of micellar aggregates.

Targeted delivery using membrane vesicles in prokaryotes

Membrane vesicles (MVs) are lumen-containing spheres of lipid bilayers secreted by all prokaryotes into the extracellular milieu. They have multifunctional roles in stress response, virulence transfer, biofilm formation, and microbial interactions. Remarkably, MVs contain various components, including lytic enzymes, genetic materials, and hydrophobic signals, at high concentrations and transfer them effectively to the target microbial cells. Therefore, MVs act as carriers for bactericidal effects, horizontal gene transfer, and quorum sensing. Although the purpose of secreted MVs remains unclear, recent reports have provided evidence that MVs selectively interact with microbial cells in order to transfer their content to the target species. Herein, we review microbial interactions using MVs and discuss MV-mediated selective delivery of their content to target microbial cells.

Reconstitution of membrane tethering mediated by Rab-family small GTPases

Membrane tethering is one of the most critical steps to determine the spatiotemporal specificity of membrane trafficking, which is the process to selectively transport proteins, lipids, and other biological molecules to the appropriate locations in eukaryotic cells, such as subcellular organelles, the plasma membrane, and the extracellular space. Based on genetic, cell biological, biochemical, and structural studies, Rab-family small GTPases and a number of Rab-interacting proteins (termed Rab effectors), including coiled-coil tethering proteins and multisubunit tethering complexes, have been proposed to be key protein components for membrane tethering. Nevertheless, indeed whether and how Rab GTPases and their specific Rab effectors directly act upon and catalyze membrane tethering still remains enigmatic. By chemically defined reconstitution of membrane tethering from purified Rab-family GTPase proteins and synthetic liposomal membranes, recent studies have revealed the intrinsic potency of Rab-family GTPases to physically and specifically tether two distinct lipid bilayers of liposomal membranes. Experimental evidence from these reconstitution studies support the novel working model in which Rab-family small GTPases act as a bona fide membrane tether for mediating membrane tethering events in eukaryotic membrane trafficking.

Human Rab small GTPase- and class V myosin-mediated membrane tethering in a chemically defined reconstitution system

Membrane tethering is a fundamental process essential for the compartmental specificity of intracellular membrane trafficking in eukaryotic cells. Rab-family small GTPases and specific sets of Rab-interacting effector proteins, including coiled-coil tethering proteins and multisubunit tethering complexes, are reported to be responsible for membrane tethering. However, whether and how these key components directly and specifically tether subcellular membranes remains enigmatic. Using chemically defined proteoliposomal systems reconstituted with purified human Rab proteins and synthetic liposomal membranes to study the molecular basis of membrane tethering, we established here that Rab-family GTPases have a highly conserved function to directly mediate membrane tethering, even in the absence of any types of Rab effectors such as the so-called tethering proteins. Moreover, we demonstrate that membrane tethering mediated by endosomal Rab11a is drastically and selectively stimulated by its cognate Rab effectors, class V myosins (Myo5A and Myo5B), in a GTP-dependent manner. Of note, Myo5A and Myo5B exclusively recognized and cooperated with the membrane-anchored form of their cognate Rab11a to support membrane tethering mediated by trans-Rab assemblies on opposing membranes. Our findings support the novel concept that Rab-family proteins provide a bona fide membrane tether to physically and specifically link two distinct lipid bilayers of subcellular membranes. They further indicate that Rab-interacting effector proteins, including class V myosins, can regulate these Rab-mediated membrane-tethering reactions.

Interaction of Bacterial Membrane Vesicles with Specific Species and Their Potential for Delivery to Target Cells

Membrane vesicles (MVs) are secreted from a wide range of microbial species and transfer their content to other cells. Although MVs play critical roles in bacterial communication, whether MVs selectively interact with bacterial cells in microbial communities is unclear. In this study, we investigated the specificity of the MV-cell interactions and evaluated the potential of MVs to target bacterial cells for delivery. MV association with bacterial cells was examined using a fluorescent membrane dye to label MVs. MVs derived from the enterobacterium Buttiauxella agrestis specifically interacted with cells of the parent strain but interacted less specifically with those of other genera tested in this study. Electron microscopic analyses showed that MVs were not only attached on B. agrestis cells but also fused to them. The interaction energy, which was characterized by hydrodynamic diameter and zeta potential based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, was significant low between MVs and cells in B. agrestis, compared to those between B. agrestis MVs and cells of other genera. Similar specific interaction was also occurred between B. agrestis MVs and cells of six other species belonging to Buttiauxella spp. B. agrestis harboring plasmid pBBR1MCS-1 secreted plasmid-containing MVs (p-MVs), and plasmid DNA in p-MVs was transferred to the same species. Moreover, antibiotic-associated MVs enabled effective killing of target species; the survival rate of B. agrestis was lower than those of Escherichia coli and Pseudomonas aeruginosa in the presence of gentamicin-associated MVs derived from B. agrestis. Altogether, we provide the evidence that MVs selectively interact with target bacterial cells and offer a new avenue for controlling specific bacterial species using bacterial MVs in microbial communities.

The dispersion and aggregation of graphene oxide in aqueous media

Graphene oxide (GO), as a typical two-dimensional material, possesses a range of oxygen-containing groups and shows surfactant and/or polyelectrolyte-like characteristics. Herein, GO sheets with narrow size distribution were prepared by an ultracentrifugation-based process and the aggregation behaviour of GO in pure water and an electrolyte aqueous solution were studied using laser light scattering (LLS). When adding common electrolytes, such as NaCl and MgCl2, into the GO dispersions, aggregation occurs and irreversible coagulation eventually occurs too. However, the GO dispersion can still remain stable when adding excess AlCl3. The zeta potential of the GO dispersion changes from negative to positive after the addition of access AlCl3, indicating that electrostatic repulsion is still responsible for the dispersion of GO, which is in good agreement with the LLS results. This finding on the dispersion of GO may be applied in the solution processing of GO. It also expands the scope of the design and preparation of new GO-based hybrid materials with different functions.

Membrane-anchored human Rab GTPases directly mediate membrane tethering in vitro

Rab GTPases are master regulators of eukaryotic endomembrane systems, particularly functioning in membrane tethering to confer the directionality of intracellular membrane trafficking. However, how exactly Rab GTPases themselves act upon membrane tethering processes has remained enigmatic. Here, we thoroughly tested seven purified Rab GTPases in human, which localize at the various representative organelles, for their capacity to support membrane tethering in vitro. Strikingly, we found that three specific human Rabs (endoplasmic reticulum/Golgi Rab2a, early endosomal Rab5a, and late endosomal/lysosomal Rab7a) strongly accelerated membrane aggregation of synthetic liposomes even in the absence of any additional components, such as classical tethers, tethering factors, and Rab effectors. This Rab-induced membrane aggregation was a reversible membrane tethering reaction that can be strictly controlled by the membrane recruitment of Rab proteins on both apposing membranes. Thus, our current reconstitution studies establish that membrane-anchored human Rab GTPases are an essential tethering factor to directly mediate membrane tethering events.

Open questions on the origin of life at anoxic geothermal fields

We have recently reconstructed the 'hatcheries' of the first cells by combining geochemical analysis with phylogenomic scrutiny of the inorganic ion requirements of universal components of modern cells (Mulkidjanian et al. Proc Natl Acad Sci U S A 109:E821-830, 2012). These ubiquitous, and by inference primordial, proteins and functional systems show affinity to and functional requirement for K⁺, Zn²⁺, Mn²⁺, and phosphate. Thus, protocells must have evolved in habitats with a high K⁺/Na⁺ ratio and relatively high concentrations of Zn, Mn and phosphorous compounds. Geochemical reconstruction shows that the ionic composition conducive to the origin of cells could not have existed in marine settings but is compatible with emissions of vapor-dominated zones of inland geothermal systems. Under an anoxic, CO₂-dominated atmosphere, the ionic composition of pools of cool, condensed vapor at anoxic geothermal fields would resemble the internal milieu of modern cells. Such pools would be lined with porous silicate minerals mixed with metal sulfides and enriched in K⁺ ions and phosphorous compounds. Here we address some questions that have appeared in print after the publication of our anoxic geothermal field scenario. We argue that anoxic geothermal fields, which were identified as likely cradles of life by using a top-down approach and phylogenomics analysis, could provide geochemical conditions similar to those which were suggested as most conducive for the emergence of life by the chemists who pursuit the complementary bottom-up strategy.

Mechanism and function of synaptotagmin-mediated membrane apposition

Synaptotagmin-I (syt) is a Ca²⁺ sensor that triggers synchronous neurotransmitter release. The first documented biochemical property of syt was its ability to aggregate membranes in response to Ca²⁺. However, the mechanism and function of syt-mediated membrane aggregation are poorly understood. Here, we demonstrate that syt-mediated vesicle aggregation is driven by trans interactions between syt molecules bound to different membranes. We observed a strong correlation between the ability of Ca²⁺-syt to aggregate vesicles and to stimulate SNARE-mediated membrane fusion. Moreover, artificial aggregation of membranes - using non-syt proteins - also efficiently promoted fusion of SNARE-bearing liposomes. Finally, using a modified fusion assay, we observed that syt drives the assembly of otherwise non-fusogenic individual t-SNARE proteins into fusion competent heterodimers, in an aggregation-independent manner. Thus, membrane aggregation and t-SNARE assembly appear to be two key aspects of Ca²⁺-syt-regulated, SNARE-catalyzed fusion reactions.

Stimulation of human red blood cells leads to Ca2+-mediated intercellular adhesion

Red blood cells (RBCs) are a major component of blood clots, which form physiologically as a response to injury or pathologically in thrombosis. The active participation of RBCs in thrombus solidification has been previously proposed but not yet experimentally proven. Holographic optical tweezers and single-cell force spectroscopy were used to study potential cell-cell adhesion between RBCs. Irreversible intercellular adhesion of RBCs could be induced by stimulation with lysophosphatidic acid (LPA), a compound known to be released by activated platelets. We identified Ca(2+) as an essential player in the signaling cascade by directly inducing Ca(2+) influx using A23187. Elevation of the internal Ca(2+) concentration leads to an intercellular adhesion of RBCs similar to that induced by LPA stimulation. Using single-cell force spectroscopy, the adhesion of the RBCs was identified to be approximately 100 pN, a value large enough to be of significance inside a blood clot or in pathological situations like the vasco-occlusive crisis in sickle cell disease patients.

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.

Two conserved residues are important for inducing highly ordered membrane domains by the transmembrane domain of influenza hemagglutinin

The interaction with lipids of a synthetic peptide corresponding to the transmembrane domain of influenza hemagglutinin was investigated by means of electron spin resonance. A detailed analysis of the electron spin resonance spectra from spin-labeled phospholipids revealed that the major effect of the peptide on the dynamic membrane structure is to induce highly ordered membrane domains that are associated with electrostatic interactions between the peptide and negatively charged lipids. Two highly conserved residues in the peptide were identified as being important for the membrane ordering effect. Aggregation of large unilamellar vesicles induced by the peptide was also found to be correlated with the membrane ordering effect of the peptide, indicating that an increase in membrane ordering, i.e., membrane dehydration, is important for vesicle aggregation. The possibility that hydrophobic interaction between the highly ordered membrane domains plays a role in vesicle aggregation and viral fusion is discussed.

Statistical determinants of selective ionic complexation: ions in solvent, transport proteins, and other "hosts"

To provide utility in understanding the molecular evolution of ion-selective biomembrane channels/transporters, globular proteins, and ionophoric compounds, as well as in guiding their modification and design, we present a statistical mechanical basis for deconstructing the impact of the coordination structure and chemistry of selective multidentate ionic complexes. The deconstruction augments familiar ideas in liquid structure theory to realize the ionic complex as an open ion-ligated system acting under the influence of an "external field" provided by the host (or surrounding medium). Using considerations derived from this basis, we show that selective complexation arises from exploitation of a particular ion's coordination preferences. These preferences derive from a balance of interactions much like that which dictates the Hofmeister effect. By analyzing the coordination-state space of small family IA and VIIA ions in simulated fluid media, we derive domains of coordinated states that confer selectivity for a given ion upon isolating and constraining particular attributes (order parameters) of a complex comprised of a given type of ligand. We demonstrate that such domains may be used to rationalize the ion-coordinated environments provided by selective ionophores and biological ion channels/transporters of known structure, and that they can serve as a means toward deriving rational design principles for ion-selective hosts.

A new class of inhibitors of peptide sorption and acylation in PLGA

Acylation of peptides occurring within controlled-release depots prepared from copolymers of lactic and glycolic acid (PLGA) is a degradation reaction that may compromise product safety and efficacy. As peptide sorption to PLGA is believed to be a common precursor to peptide acylation, a new method to inhibit acylation is presented involving disruptors of peptide sorption, namely, inorganic divalent cations. Kinetics of sorption of a model peptide, octreotide acetate, to free-acid end-group PLGA was monitored in the presence and absence of water-soluble inorganic divalent cationic salts in HEPES buffer solution (pH 7.4, 37 degrees C). Sorption of cations and octreotide attained pseudo-equilibrium by 24 h. From 24-h sorption isotherms, all cations studied inhibited octreotide sorption to PLGA-the inhibiting effect of the cations increased in the order: Na(+)<Mg(2+)<Ca(2+), Sr(2+)<Ni(2+)<Mn(2+). Long-term inhibition of octreotide sorption in the presence of 15 mM CaCl(2) and MnCl(2) translated to decreased acylated octreotide present in solution by greater than 50% at 21 days incubation, i.e., from 32% in the cation-free control to 14 and 13% for CaCl(2) and MnCl(2), respectively. Over one month in vitro release, PLGA implants encapsulating octreotide acetate and CaCl(2) or MnCl(2) also showed substantial inhibition of acylation relative to no-salt or NaCl controls, and similarly, strong inhibition of acylation upon divalent salt incorporation was observed during solvent extrusion of suspended peptide with polar organic carrier solvents. Hence, disrupting peptide sorption to PLGA with addition of inorganic divalent cations is a simple and viable strategy to inhibit acylation of peptides in PLGA delivery systems.

Influence of La3+ ions on the egg-yolk phosphatidylcholine and sodium taurocholate self-assemblies in aqueous suspension

In this study, the influence of La3+ on the mixed egg-yolk phosphatidylcholine (EYPC)-sodium taurocholate (TC) aggregates was investigated by using turbidity, quasi-elastic light-scattering (QELS) technique, Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM). La3+ ions exert different effects on different EYPC-TC aggregates. The EYPC-TC mixed spheroidal micelles are stable in the presence of La3+ ions. The EYPC-TC vesicles are aggregated by the interaction of La3+ with the EYPC phosphate group. Particularly, La3+ causes structural transition of the EYPC-TC mixed disk micelles by changing the colloidal properties of TC. This process includes two steps, destruction of mixed disk micelles and formation of mixed spheroidal micelle. Several intermediate structures such as multi-lamellar vesicles and cylindrical micelles were observed during the transition. The experimental data and analysis in the current study suggest that metal ion is one of the important factors to control the structure of the EYPC-TC self-assemblies. And the possible influence of metal ions on the properties of bile biocolloid has been discussed.

Diazeniumdiolate reactivity in model membrane systems

The effect of small unilamellar phospholipid vesicles on the acid-catalyzed dissociation of nitric oxide from diazeniumdiolate ions, R(1)R(2)N[N(O)NO](-), [1: R(1)=H(2)N(CH(2))(3)-, R(2)=H(2)N(CH(2))(3)NH(CH(2))(4)-; 2: R(1)=R(2)=H(2)N(CH(2))(3)-; 3: R(1)=n-butyl-, R(2)=n-butyl-NH2+(CH(2))(6)-; 4: R(1)=R(2)=nPr-] has been examined at pH 7.4 and 37 degrees C. NO release was catalyzed by anionic liposomes (DPPG, DOPG, DMPS, POPS and DOPA) and by mixed phosphatidylglycerol/phosphatidylcholine (DPPG/DPPC and DOPG/DPPC) covesicles, while cationic liposomes derived from 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and the zwitterionic liposome DMPC did not significantly affect the dissociation rates of the substrates examined. Enhancement of the dissociation rate constant in DPPG liposome media (0.010M phosphate buffer, pH 7.4, 37 degrees C) at 10mM phosphoglycerol levels, ranged from 37 for 1 to 1.2 for the anionic diazeniumdiolate 4, while DOPA effected the greatest rate enhancement, achieving 49-fold rate increases with 1 under similar conditions. The observed catalysis decreases with increase in the bulk concentration of electrolytes in the reaction media. Quantitative analysis of catalytic effects has been obtained through the application of pseudo-phase kinetic models and equilibrium binding constants at different liposome interfaces are compared. The stoichiometry of nitric oxide release from 1 and 2 in DPPG/DPPC liposome media has been obtained through oxyhemoglobin assay. DPPG=1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], DOPG=1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)], DMPS=1,2-dimyristoyl-sn-glycero-3-[phospho-l-serine], POPS=1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine], DOPA=1,2-dioleoyl-sn-glycero-3-phosphate; DPPC=1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DMPC=1,2-dimyristoyl-sn-glycero-3-phosphocholine, DOTAP=1,2-dioleoyl-3-trimethylammonium-propane.

Direct imaging of lipid-ion network formation under physiological conditions by frequency modulation atomic force microscopy

Various metal cations in physiological solutions interact with lipid headgroups in biological membranes, having an impact on their structure and stability, yet little is known about the molecular-scale dynamics of the lipid-ion interactions. Here we directly investigate the extensive lipid-ion interaction networks and their transient formation between headgroups in a dipalmitoylphosphatidylcholine bilayer under physiological conditions. The spatial distribution of ion occupancy is imaged in real space by frequency modulation atomic force microscopy with sub-Angstrom resolution.

Combined effect of surface electrostatic charge and poly(ethyl glycol) on the association of liposomes with colon carcinoma cells

Particulate drug formulations are considered to be a means that may improve the pharmacokinetics and biodistribution of active compounds. By using them, drug distribution is determined solely by the properties of the carrier. The surface properties of such supramolecular aggregates determine how they will interact with various biological structures. Among others, surface electrostatic charge and surface grafted polymers are considered to be among the major factors affecting its interaction with proteins and cells. In this article, we present experimental evidence that properly selected surface electrostatic charge and grafted polymers can alter the association of liposomes with colon cancer cells. The dependence of the adsorption of liposomes onto the cell surface on the quantity and length of surface grafted polymers for a certain surface charge density exhibits a distinct maximum. For example, when liposomes were formed with 20 mol% of DOTAP, PE-PEG350 increased liposome adsorption by up to 6 mol%. This adsorption maximum depends on both polymer length and charge type. Results presented in this article show that the interaction of liposomes with colon cancer cells can be tuned by a proper combination of liposome surface electrostatics and surface grafted polymers.

Partitioning of polychlorinated biphenyls in octanol/water, triolein/water, and membrane/water systems

The use of the octanol/water partition constant (Kow) as a surrogate parameter for lipid/water partitioning of persistent organic pollutants (POPs) was reassessed by comparing the measured Kow of 12 selected polychlorinated biphenyl congeners (PCBs) with partition constants in triolein/water (Ktw) and membrane/water (Kmw) systems. Kow and Ktw were measured by the slow-stirring method. Kmw was measured by an adaptation of the slow-stirring method using suspensions of phosphatidylcholine and phosphatidylserine liposomes. Partitioning of POPs to octanol, triolein, and liposomes is similar but not equal. The log-log correlation for Kow and Ktw is excellent (r2 = 0.982) and that for Kow and Kmw is somewhat weaker (r2 = 0.856). Ktw values are greater than Kow by a factor of 1.6. Kmw of some PCB congeners exceed both Kow and Ktw by an order of magnitude. The differences are attributed to different PCB activity coefficients in the different lipid phases. The results imply that Kow can be used as a reasonable conservative estimate of lipid/water partitioning. But the observed differences between Kow and Kmw also indicate that using Kow to predict accumulation of POPs, particularly highly hydrophobic ones, in the polar lipids of organisms will underestimate their concentrations at equilibrium.

Modulation of lysozyme charge influences interaction with phospholipid vesicles

Lysozyme is a globular protein which is known to bind to negatively charged phospholipid vesicles. In order to study the relationship between charge state of the protein and its interaction with negatively charged phospholipid membranes chemical modifications of the proteins were carried out. Succinylation and carbodiimide modification was used to shift the isoelectric point of lysozyme to lower and higher pH values, respectively. The binding of the modified lysozyme to phospholipid vesicles prepared from phosphatidic acid (PA) was determined using microelectrophoresis and ultracentrifugation. At acidic pH of the solution all lysozyme species reduced the surface charges of PA vesicles. Succinylated lysozyme (succ lysozyme) reduced the electrophoretic mobility (EPM) to nearly zero, whereas native lysozyme and carboxylated lysozyme (carbo lysozyme) changed the surface charge to positive values. At neutral pH, the reduction of surface charges was less for carbo lysozyme and unmodified lysozyme. Succ lysozyme did not change the EPM. Unmodified and carbo lysozyme decreased the magnitude of EPM, but the whole complex was still negatively charged. The bound fraction of all modified lysozyme to PA vesicles at high lysozyme/PA ratios was nearly constant at acidic pH. At low lysozyme/PA ratios the extent of bound lysozyme is changed in the order carbo>unmodified>succ lysozyme. Increasing the pH, the extent of bound lysozyme to PA large unilamellar vesicles (LUV) is reduced, at pH 9.0 only 35% of carbo lysozyme, 23% of unmodified lysozyme is bound, whereas succ lysozyme does not bind at pH 7.4 and 9.0. At low pH, addition of all lysozyme species resulted in a massive aggregation of PA liposomes, at neutral pH aggregation occurs at much higher lysozyme/PA ratios. Lysozyme binding to PA vesicles is accompanied by the penetration of lysozyme into the phospholipid membrane as measured by monolayer techniques. The penetration of lysozyme into the monolayer was modulated by pH and ionic strengths. The interaction of lysozyme with negatively charged vesicles leads to a decrease of the phospholipid vesicle surface hydration as measured by the shift of the maximum of the fluorescence signal of a headgroup labeled phospholipid. The binding of bis-ANS as an additional indicator for the change of surface hydrophobicity is increased at low pH after addition of lysozyme to the vesicles. More hydrophobic patches of the lysozyme-PA complex are exposed at low pH. At low pH the binding process of lysozyme to PA vesicles is followed by an extensive intermixing of phospholipids between the aggregated vesicles, accompanied by a massive leakage of the vesicle aqueous content. The extent of lysozyme interaction with PA LUV at neutral and acidic pH is in the order carbo lysozyme>lysozyme>succ lysozyme.

Interactions between titanium dioxide and phosphatidyl serine-containing liposomes: formation and patterning of supported phospholipid bilayers on the surface of a medically relevant material

Titanium is widely used in biomedical applications. Its mechanical properties and biocompatibility, conferred by a layer of oxide present on its surface, make titanium the material of choice for various implants (artificial hip and knee joints, dental prosthetics, vascular stents, heart valves). Furthermore, the high refractive index of titanium oxide is advantageous in biosensor applications based on optical detection methods. In both of the above fields of application, novel surface modification strategies leading to biointeractive interfaces (that trigger specific responses in biological systems) are continuously sought. In this report, we investigate the interactions between TiO2 and phosphatidyl serine-containing liposomes, present a novel approach for preparing supported phospholipid bilayers (SPBs) of various compositions on TiO2, and use the unique ability of liposomes to distinguish between different surfaces to create SPB corrals on SiO2/TiO2 structured substrates. These results represent an important first step toward the design of biointeractive interfaces on titanium oxide surfaces that are based on a cell membrane-like environment.

Effect of ion-binding and chemical phospholipid structure on the nanomechanics of lipid bilayers studied by force spectroscopy

The nanomechanical response of supported lipid bilayers has been studied by force spectroscopy with atomic force microscopy. We have experimentally proved that the amount of ions present in the measuring system has a strong effect on the force needed to puncture a 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer with an atomic force microscope tip, thus highlighting the role that monovalent cations (so far underestimated, e.g., Na(+)) play upon membrane stability. The increase in the yield threshold force has been related to the increase in lateral interactions (higher phospholipid-phospholipid interaction, decrease in area per lipid) promoted by ions bound into the membrane. The same tendency has also been observed for other phosphatidylcholine bilayers, namely, 2-dilauroyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, and 1,2-dioleoyl-sn-3-phosphocholine, and also for phosphatidylethanolamine bilayers such as 1-palmitoyl-2-oleoyl-sn-3-phosphoethanolamine. Finally, this effect has been also tested on a natural lipid bilayer (Escherichia coli lipid extract), showing the same overall tendency. The kinetics of the process has also been studied, together with the role of water upon membrane stability and its effect on membrane nanomechanics. Finally, the effect of the chemical structure of the phospholipid molecule on the nanomechanical response of the membrane has also been discussed.

Separation of membranes from acid-solubilized fish muscle proteins with the aid of calcium ions and organic acids

Calcium chloride, and to a lesser extent MgCl2, aided in the separation of membranes by centrifugation from cod (Gadus morhua) muscle homogenates solubilized at pH 3 in the presence of citric acid or malic acid but not lactic acid. Adding citric acid and Ca2+ before solubilizing the cod muscle homogenates was needed for the effect. At 1 mM citric acid, 70-80% of the phospholipid and 25-30% of the protein were removed at 10 mM Ca2+. At 8 mM Ca2+, citric acid showed an optimal effect on phospholipid removal at 5 mM with 90% of the phospholipid and 35% of the protein removed. The treatment with citric acid and Ca2+ was also effective in separating the membrane from solubilized herring (Clupea harengus) muscle homogenate. Ca2+ and citric acid might exert their influence by disconnecting linkages between membranes and cytoskeletal proteins.

Growth of giant membrane lobes mechanically driven by wetting fronts of phospholipid membranes at water-solid interfaces

We report on the growth of giant membrane lobes that is mechanically driven by wetting fronts of phospholipid membranes at water-solid interfaces and a strategy to control the two-dimensional structure of the membrane lobes on a solid surface. The growth of giant membrane lobes was observed on a single-lipid bilayer which spread from a lump of phospholipid deposited on a silica-glass substrate or an oxidized silicon wafer in aqueous solutions of NaCl, KCl, MgCl2, or CaCl2 at relatively high salt concentrations. Most of the membrane lobes were very flat unilamellar tubes elongating from the lump of phospholipid, and their length reached 1 mm in 5 h. Experimental findings clearly indicate that the membrane lobes are adherent to the surface of the single-lipid bilayer and are mechanically elongated from the lump of phospholipid by the sliding motion of the single-lipid bilayer. We could control the two-dimensional structure of the membrane lobes on the substrate by controlling the spreading direction of the single-lipid bilayer using Pt micropatterns that were deposited on the smooth surface of the oxidized silicon wafer.

Molecular dynamics simulation of a dipalmitoylphosphatidylcholine bilayer with NaCl

Molecular dynamics simulations are performed on two hydrated dipalmitoylphosphatidylcholine bilayer systems: one with pure water and one with added NaCl. Due to the rugged nature of the membrane/electrolyte interface, ion binding to the membrane surface is characterized by the loss of ion hydration. Using this structural characterization, binding of Na(+) and Cl(-) ions to the membrane is observed, although the binding of Cl(-) is seen to be slightly weaker than that of Na(+). Dehydration is seen to occur to a different extent for each type of ion. In addition, the excess binding of Na(+) gives rise to a net positive surface charge density just outside the bilayer. The positive density produces a positive electrostatic potential in this region, whereas the system without salt shows an electrostatic potential of zero.

Strength of Ca(2+) binding to retinal lipid membranes: consequences for lipid organization

There is evidence that membranes of rod outer segment (ROS) disks are a high-affinity Ca(2+) binding site. We were interested to see if the high occurrence of sixfold unsaturated docosahexaenoic acid in ROS lipids influences Ca(2+)-membrane interaction. Ca(2+) binding to polyunsaturated model membranes that mimic the lipid composition of ROS was studied by microelectrophoresis and (2)H NMR. Ca(2+) association constants of polyunsaturated membranes were found to be a factor of approximately 2 smaller than constants of monounsaturated membranes. Furthermore, strength of Ca(2+) binding to monounsaturated membranes increased with the addition of cholesterol, while binding to polyunsaturated lipids was unaffected. The data suggest that the lipid phosphate groups of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) in PC/PE/PS (4:4:1, mol/mol) are primary targets for Ca(2+). Negatively charged serine in PS controls Ca (2+) binding by lowering the electric surface potential and elevating cation concentration at the membrane/water interface. The influence of hydrocarbon chain unsaturation on Ca(2+) binding is secondary compared to membrane PS content. Order parameter analysis of individual lipids in the mixture revealed that Ca(2+) ions did not trigger lateral phase separation of lipid species as long as all lipids remained liquid-crystalline. However, depending on temperature and hydrocarbon chain unsaturation, the lipid with the highest chain melting temperature converted to the gel state, as observed for the monounsaturated phosphatidylethanolamine (PE) in PC/PE/PS (4:4:1, mol/mol) at 25 degrees C.

Aggregation and fusion of vesicles composed of N-palmitoyl derivatives of membrane phospholipids

N-Acylphosphatidylethanolamines and N-acylphosphatidylserines have been isolated from mammalian cells and have been associated with some tissue degenerative changes, although the relationship between their synthesis and the uncontrolled sequence of events that ends in irreversible tissue damage is not completely established. Our results show that monovalent and divalent cations induce aggregation and fusion of liposomes constituted by N-palmitoylphosphatidylethanolamine (NPPE) and N-palmitoylphosphatidylserine (NPPS). The effectiveness of cations to induce the aggregation of NPPE and NPPS liposomes is Ca2+ > Mg2+ >> Na+. NPPS liposomes aggregate at lower concentrations of divalent cations than NPPE liposomes, but with sodium NPPE liposomes aggregate to a higher extent than NPPS liposomes. The reaction order for the aggregation processes depends on the lipid and the cation nature and range from 1.04 to 1.64. Dynamic light scattering shows an irreversible increase of the size of the aggregates in the presence of all cations tested. The irreversibility of the aggregation process and the intermixing of bilayer lipids, as studied by resonance energy transfer assay, suggest that fusion, rather than aggregation, occurs. The existence of a real fusion was demonstrated by the coalescence of the aqueous contents of both NPPS and NPPE liposomes in the presence of either monovalent or divalent cations. The different binding sensitivity of Ca2+ to NPPS and NPPE liposomes, determined by zeta potential measurements, agrees with the results obtained in the aggregation and fusion assays. Our results suggest that the synthesis in vivo of N-acylated phospholipids can introduce important changes in membrane-mediated processes.

Dehydration of Hydrated Bilayer of Dipalmitoylphosphatidylcholine Caused by Beryllium Ion: Evidence from a Differential Scanning Calorimetry of Bilayer Phase Transition

The effect of polyvalent metal ions Be2+, Mg2+, Ca2+, Sr2+, Ba2+, and La3+ on the phase transition behavior of hydrated bilayer of dipalmitoylphosphatidylcholine (DPPC) was investigated by differential scanning calorimetry (DSC) in relation to their ability to induce the aggregation of DPPC vesicles. The addition of the metal ions other than Be2+ provided DSC thermograms characteristic to a fully hydrated DPPC bilayer. By the addition of Be2+, the endothermic peak associated with the bilayer phase transition was shifted to that corresponding to partially dehydrated DPPC bilayer, which was reported by Kodama et al. (Biochem. Biophys. Acta 689, 567, 1982). This demonstrates that Be2+ causes dehydration of DPPC head group in hydrated bilayer and supports the speculation that the unusual property of Be2+ to induce the aggregation of PC vesicles is attributed to the destruction of repulsive hydration force due to the partial dehydration of vesicular surface. Copyright 1998 Academic Press.

Ca2+-mediated interaction between dextran sulfate and dimyristoyl-sn-glycero-3-phosphocholine surfaces studied by 2H nuclear magnetic resonance

The binding of dextran sulfates (DSs) with varying chain lengths to phosphatidylcholine multilamellar vesicles was investigated as a function of polyelectrolyte, NaCl, and Ca2+ concentration. Attractive forces between negatively charged polyelectrolytes and zwitterionic phospholipids arise from the assembly of calcium bridges. The formation of calcium bridges between the sulfate groups on the dextran sulfate and the phosphate group of the lipid results in increased calcium binding in mixtures of DS and 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). At high NaCl concentration, the plateau adsorption of DS 500 is increased. The strength of dextran sulfate binding to DMPC is reflected in the changes of the 2H NMR quadrupolar splittings of the headgroup methylenes. Association forces increase with the number of calcium bridges formed. Low-molecular-weight DS does not bind to DMPC surfaces whereas longer-chain DSs strongly influence headgroup structure as a result of strong association. DS binding increases with increasing concentration; however, further association of the polyelectrolyte can be promoted only if negative charges are sufficiently screened. DS binding to lipid bilayers is a complicated balance of calcium bridging and charge screening. From our data we postulate that the structure of the adsorbed layer resembles a lattice of DS strands sandwiched between the bilayer lamellae.

N-stearoyl-phosphatidylserine: synthesis and role in divalent-cation-induced aggregation and fusion

N-Acylphosphatidylserines have been isolated from intact and injured tissues, but the participation of such acidic phospholipids in membrane aggregation and fusion has not been demonstrated. We have synthesized N-stearoylphosphatidylserine (NSPS) and examined divalent-cation-induced aggregation of NSPS-liposomes, which leads to membrane destabilization and fusion. The purified lipid was characterized by its chromatographic and spectroscopic (infrared and 1H nuclear magnetic resonance) properties and by its chemical degradation pattern. Aggregation of unilamellar NSPS-liposomes was studied as a function of calcium and magnesium concentration. The ability of calcium and magnesium to induce vesicle aggregation is higher for phosphatidylserine (PS)-liposomes (threshold concentration 1.5 mM for calcium and 4.6 mM for magnesium) than for NSPS-liposomes (threshold concentration 2.8 mM for calcium and 6.6 mM for magnesium). The irreversibility of the aggregation reactions after adding EDTA suggests that vesicle fusion might occur in the presence of calcium and magnesium. Preliminary studies, based on mixing of both lipid and internal aqueous contents, show that fusion rather than aggregation of NSPS-liposomes occurs in the presence of calcium ions. The tendency of NSPS-liposomes to aggregate at higher cation concentrations than PS-liposomes suggests that N-acylation of phosphatidylserine protects the membrane against degenerative damage caused by aggregation and fusion.