Direct evidence for Ca++-induced lateral phase separation in black membranes of lipid mixtures by the analysis of gramicidin A single-channels

Regulation of Gramicidin Channel Function Solely by Changes in Lipid Intrinsic Curvature

Membrane protein function is regulated by the lipid bilayer composition. In many cases the changes in function correlate with changes in the lipid intrinsic curvature (c 0), and c 0 is considered a determinant of protein function. Yet, water-soluble amphiphiles that cause either negative or positive changes in curvature have similar effects on membrane protein function, showing that changes in lipid bilayer properties other than c 0 are important-and may be dominant. To further investigate the mechanisms underlying the bilayer regulation of protein function, we examined how maneuvers that alter phospholipid head groups effective "size"-and thereby c 0-alter gramicidin (gA) channel function. Using dioleoylphospholipids and planar bilayers, we varied the head groups' physical volume and the electrostatic repulsion among head groups (and thus their effective size). When 1,2-dioleyol-sn-glycero-3-phosphocholine (DOPC), was replaced by 1,2-dioleyol-sn-glycero-3-phosphoethanolamine (DOPE) with a smaller head group (causing a more negative c 0), the channel lifetime (τ) is decreased. When the pH of the solution bathing a 1,2-dioleyol-sn-glycero-3-phosphoserine (DOPS) bilayer is decreased from 7 to 3 (causing decreased head group repulsion and a more negative c 0), τ is decreased. When some DOPS head groups are replaced by zwitterionic head groups, τ is similarly decreased. These effects do not depend on the sign of the change in surface charge. In DOPE:DOPC (3:1) bilayers, pH changes from 5→9 to 5→0 (both increasing head group electrostatic repulsion, thereby causing a less negative c 0) both increase τ. Nor do the effects depend on the use of planar, hydrocarbon-containing bilayers, as similar changes were observed in hydrocarbon-free lipid vesicles. Altering the interactions among phospholipid head groups may alter also other bilayer properties such as thickness or elastic moduli. Such changes could be excluded using capacitance measurements and single channel measurements on gA channels of different lengths. We conclude that changes gA channel function caused by changes in head group effective size can be predicted from the expected changes in c 0.

The ability of carbon nanoparticles to increase transmembrane current of cations coincides with impaired synaptic neurotransmission

Here, carbon nanodots synthesized from β-alanine (Ala-CDs) and detonation nanodiamonds (NDs) were assessed using (1) radiolabeled excitatory neurotransmitters L-[¹⁴C]glutamate, D-[2,3³H]aspartate, and inhibitory ones [³H]GABA, [³H]glycine for registration of their extracellular concentrations in rat cortex nerve terminals; (2) the fluorescent ratiometric probe NR12S and pH-sensitive probe acridine orange for registration of the membrane lipid order and synaptic vesicle acidification, respectively; (3) suspended bilayer lipid membrane (BLM) to monitor changes in transmembrane current. In nerve terminals, Ala-CDs and NDs increased the extracellular concentrations of neurotransmitters and decreased acidification of synaptic vesicles, whereas have not changed sufficiently the lipid order of membrane. Both nanoparticles, Ala-CDs and NDs, were capable of increasing the conductance of the BLM by inducing stable potential-dependent cation-selective pores. Introduction of divalent cations, Zn²⁺ or Cd²⁺ on the particles` application side (cis-side) increased the rate of Ala-CDs pore-formation in the BLM. The application of positive potential (+100 mV) to the cis-chamber with Ala-CDs or NDs also activated the insertion as compared with the negative potential (-100 mV). The Ala-CD pores exhibited a wide-range distribution of conductances between 10 and 60 pS and consecutive increase in conductance of each major peak by ~10 pS, which suggest the clustering of the same basic ion-conductive structure. NDs also formed ion-conductive pores ranging from 6 pS to 60 pS with the major peak of conductance at ~12 pS in cholesterol-containing membrane. Observed Ala-CDs and NDs-induced increase in transmembrane current coincides with disturbance of excitatory and inhibitory neurotransmitter transport in nerve terminals.

Microscopic Picture of Calcium-Assisted Lipid Demixing and Membrane Remodeling Using Multiscale Simulations

The specificity of anionic phospholipids-calcium ion interaction and lipid demixing has been established as a key regulatory mechanism in several cellular signaling processes. The mechanism and implications of this calcium-assisted demixing have not been elucidated from a microscopic point of view. Here, we present an overview of atomic interactions between calcium and phospholipids that can drive nonideal mixing of lipid molecules in a model lipid bilayer composed of zwitterionic (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)) and anionic (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS)) lipids with computer simulations at multiple resolutions. Lipid nanodomain formation and growth were driven by calcium-enabled lipid bridging of the charged phosphatidylserine (PS) headgroups, which were favored against inter-POPS dipole interactions. Consistent with several experimental studies of calcium-associated membrane sculpting, our analyses also suggest modifications in local membrane curvature and cross-leaflet couplings as a response to such induced lateral heterogeneity. In addition, reverse mapping to a complementary atomistic description revealed structural insights in the presence of anionic nanodomains, at timescales not accessed by previous computational studies. This work bridges information across multiple scales to reveal a mechanistic picture of calcium ion's impact on membrane biophysics.

Can adding oil control domain formation in binary amphiphile bilayers?

Bilayers formed of two species of amphiphile of different chain lengths may segregate into thinner and thicker domains composed predominantly of the respective species. Using a coarse-grained mean-field model, we investigate how mixing oil with the amphiphiles affects the structure and thickness of the bilayer at and on either side of the boundary between two neighbouring domains. In particular, we find that oil molecules whose chain length is close to that of the shorter amphiphiles segregate to the thicker domain. This smooths the surface of the hydrophobic bilayer core on this side of the boundary, reducing its area and curvature and their associated free-energy penalties. The smoothing effect is weaker for oil molecules that are shorter or longer than this optimum value: short molecules spread evenly through the bilayer, while long molecules swell the thicker domain, increasing the surface area and curvature of the bilayer core in the interfacial region. Our results show that adding an appropriate oil could make the formation of domain boundaries more or less favourable, raising the possibility of controlling the domain size distribution.

Proton and calcium-gated ionic mesochannels: phosphate-bearing polymer brushes hosted in mesoporous thin films as biomimetic interfacial architectures

Rational construction of interfaces based on multicomponent responsive systems in which molecular transport is mediated by structures of nanoscale dimensions has become a very fertile research area in biomimetic supramolecular chemistry. Herein, we describe the creation of hybrid mesostructured interfaces with reversible gate-like transport properties that can be controlled by chemical inputs, such as protons or calcium ions. This was accomplished by taking advantage of the surface-initiated polymerization of 2-(methacryloyloxy)ethyl phosphate (MEP) monomer units into and onto mesoporous silica thin films. In this way, phosphate-bearing polymer brushes were used as "gatekeepers" located not only on the outer surface of mesoporous thin films but also in the inner environment of the porous scaffold. Pore-confined PMEP brushes respond to the external triggering chemical signals not only by altering their physicochemical properties but also by switching the transport properties of the mesoporous film. The ion-gate response/operation was based on the protonation and/or chelation of phosphate monomer units in which the polymer brush works as an off-on switch in response to the presence of protons or Ca(2+) ions. The hybrid meso-architectured interface and their functional features were studied by a combination of experimental techniques including ellipso-porosimetry, cyclic voltammetry, X-ray reflectivity, grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, and in situ atomic force microscopy. In this context, we believe that the integration of stimuli-responsive polymer brushes into nanoscopic supramolecular architectures would provide new routes toward multifunctional biomimetic nanosystems displaying transport properties similar to those encountered in biological ligand-gated ion channels.

Lipid-mediated inactivation of colicin E1 channels by calcium ions

Based on the model of a toroidal protein-lipid pore, the effect of calcium ions on colicin E1 channel was predicted. In electrophysiological experiments Ca2+ suppressed the activity of colicin E1 channels in membranes formed of diphytanoylphosphatidylglycerol, whereas no desorption of the protein occurred from the membrane surface. The effect of Ca2+ was not observed on membranes formed of diphytanoylphosphatidylcholine. Single-channel measurements revealed that Ca2+-induced reduction of the colicin-induced current across the negatively charged membrane was due to a decrease in the number of open colicin channels and not changes in their properties. In line with the toroidal model, the effect of Ca2+ on the colicin E1 channel-forming activity is explained by alteration of the membrane lipid curvature caused by electrostatic interaction of Ca2+ with negatively charged lipid head groups.

Clostridium perfringens beta-toxin forms potential-dependent, cation-selective channels in lipid bilayers

Recombinant beta-toxin from Clostridium perfringens type C was found to increase the conductance of bilayer lipid membranes (BLMs) by inducing channel activity. The channels exhibited a distribution of conductances within the range of 10 to 380 pS, with the majority of the channels falling into two categories of conductance at 110 and 60 pS. The radii of beta-toxin pores found for the conductance states of 110 and 60 pS were 12.7 and 11.1 A, respectively. The single channels and the steady-state currents induced by beta-toxin across the BLMs exhibited ideal monovalent cation selectivity. Addition of divalent cations (Zn(2+), Cd(2+), or Mg(2+)) at a concentration of 2 mM increased the rate of beta-toxin insertion into BLMs and the single-channel conductance, while application of 5 mM Zn(2+) to a beta-toxin-induced steady-state current decreased the inward current by approximately 45%. The mutation of arginine 212 of beta-toxin to aspartate, previously shown to increase the 50% lethal dose of beta-toxin for mice nearly 13-fold, significantly reduced the ability of beta-toxin to form channels. These data support the hypothesis that the lethal action of beta-toxin is based on the formation of cation-selective pores in susceptible cells.

Lipid bilayer electrostatic energy, curvature stress, and assembly of gramicidin channels

Hydrophobic interactions between lipid bilayers and imbedded membrane proteins couple protein conformation to the mechanical properties of the bilayer. This coupling is widely assumed to account for the regulation of membrane protein function by the membrane lipids' propensity to form nonbilayer phases, which will produce a curvature stress in the bilayer. Nevertheless, there is only limited experimental evidence for an effect of bilayer curvature stress on membrane protein structure. We show that alterations in curvature stress, due to alterations in the electrostatic energy of dioleoylphosphatidylserine bilayers, modulate the structurally well-defined gramicidin A monomer <--> dimer reaction. Maneuvers that decrease the electrostatic energy of the unperturbed bilayer promote channel dissociation; we measure the change in interaction energy. The bilayer electrostatic energy thus can affect membrane protein structure by a mechanism that does not involve the electrostatic field across the bilayer, but rather electrostatic interactions among the phospholipid head groups in each monolayer which affect the bilayer curvature stress. These results provide further evidence for the importance of mechanical interactions between a bilayer and its imbedded proteins for protein structure and function.

Lateral inhomogeneous lipid membranes: theoretical aspects

The physical concepts underlying the lateral distribution of the components forming a lamellar assembly of amphiphiles are discussed in this review. The role of amphiphiles' molecular structure and/or aqueous environment (ionic strength, water soluble substances) on formation and stability of lateral patterns is investigated. A considerable effort is devoted to the analysis of the properties of patterned structure which can be different from those of randomly mixed multi-component lamellae. Examples include adhesion and fusion among laterally inhomogeneous bilayers, enhanced interfacial adsorption of ions and polymers, enhanced transport across the bilayer, modified mechanical properties, local stabilization of non-planar geometries (pores, edges) and related phenomena (electroporation, budding transition and so on). Furthermore, an analysis of chemical reactivity within or at the water interface of a laterally inhomogeneous bilayer is briefly discussed. A link between these concepts and experimental findings taken from the biological literature is attempted throughout the review.

The seventh Datta Lecture. Membrane bending energy concept of vesicle- and cell-shapes and shape-transitions

The main objective of this lecture is to discuss the role of lipid-bilayer elasticity (1) for the self-organization of lipid/protein-bilayers (2) for the stabilization of domain structures and shapes of cell membranes and (3) for the control of shape transitions (e.g. bud- and pit-formation) and shape instabilities (vesicle fission). It is demonstrated that many complex shape transitions of cell membranes can be mimicked by single lipid bilayer vesicles by simply varying the area-to-volume ratio or by chemically induced bending moments suggesting that these processes are governed by the universal minimum bending energy concept of closed shells composed of stratified membranes. The essential role of the coupling between curvature and phase separation in mixed membranes for the formation and stabilization of local pits and buds or the fission of budded vesicles is demonstrated. Finally, we discuss the consequences of the pronounced thermally excited bending undulations of the hyperelastic membranes for the membrane tension, the material exchange at membrane surfaces and the control of the adhesion of vesicles (or cells) on solid substrates.

Electron microscopic investigations on free-standing mixed lipid Langmuir-Blodgett-Kuhn monolayers: phase separation and aging process

Lipid monolayers were prepared by the Langmuir-Blodgett-Kuhn technique (LBK) as free-standing films spanning a diameter of up to 1 micron. These films were investigated by electron spectroscopic imaging (ESI) and electron energy loss spectroscopy (EELS). The free-standing monolayer is shown to be in a transient state in which an aging process is proceeding: after storage for two weeks in air at room temperature the films tear off the edges of the perforated supporting film. Ca2+ ions induced lateral phase separation in these films prepared from a 50:50 mixture of lecithin/glycerol could be visualized by means of ESI, i.e., by comparing micrographs below and above the Ca absorption edge in the EEL spectrum. The domain sizes of the demixed phases were determined to vary between 30 and 60 nm. In addition it was shown that the counter ion of the negatively charged glycerol in these films is Ca2+ and not Na+.

Correlations between changes in membrane capacitance induced by changes in ionic environment and the conductance of channels incorporated into bilayer lipid membranes

The action of metal polycations and pH on ionic channels produced in bilayer lipid membranes (BLM) by three different toxins was studied by measuring membrane capacitance and channel conductance. Here, we show that critical concentrations of Cd2+, La3+ or Tb3+ induce complex changes in membrane capacitance. The time course of capacitance changes is similar to the time course of channel blocking by these ions at low concentration. No changes in BLM capacitance or conductance were observed in the range of pH 5.8-9.0. A pH shift from 7.4 to 3-4 or 11-12 induced large changes in BLM capacitance and channel conductance. For all studied channel-forming proteins, the initial capacitance increase preceded the conductance decrease caused by addition of polycations or by a change in pH. A close relationship between membrane lipid packing and ion channel protein is suggested.

Ca(2+)-induced lateral phase separation in black lipid membranes and its coupling to the ion translocation by gramicidin

We analyze the single-channel current fluctuations of gramicidin incorporated into biomolecular lipid membranes (BLM) of binary mixtures of phosphatidylcholine (PC) and phosphatidylglycerol (PG) as a function of the Ca2+ concentration in the electrolyte (0.5 M CsCl, pH 6) solution. At low Ca2+ levels (cCa2+ < 10(-6) M) a monomodal conductance histogram and a single average lifetime suggests a homogeneous mixture over the full range of composition (PG(1-x)PCx, 0 < or = x < or = 1). At higher Ca2+ concentrations phase separation processes are inferred from the appearance of bimodal conductance histograms. The two channel populations (in the two coexisting phases) can also be distinguished through their different average lifetimes. By a systematic variation of the mole fractions of the two lipid components we derive the respective phase boundaries and thus the full Ca2+ concentration-composition phase diagram.

Calcium-ganglioside interactions and synaptic plasticity: effect of calcium on specific ganglioside/peptide (valinomycin, gramicidin A)-complexes in mixed mono- and bilayers

A controlled exchange of calcium between the extracellular space (mM Ca2+) and the neuroplasm (microM Ca2+) is considered to be an essential prerequisite for almost every stage of neuronal activity. Our research interest is focused on those compounds, which due to their physico-chemical properties and localization within the synaptic membrane might fulfill the task as neuromodulators for functional synaptic proteins. Because of this specific binding properties towards calcium and their peculiar interactions with calcium in model systems gangliosides (amphiphilic sialic acid containing glycosphingolipids) are favorite candidates for a functional involvement in synaptic transmission of information. In this study we used monolayers to investigate the molecular packing and surface potential at the air/water interface, the interaction of gangliosides with the depsipeptide valinomycin (= monovalent ion carrier), and its influenceability by calcium. Furthermore we looked at calcium effects on the single channel conductance and mean channel life-time of the monovalent ion channel gramicidin A in mixed PC/ganglioside bilayers. In pure ganglioside monolayers the addition of 0.01 mM Ca2+ induces monolayer condensation, a rise in collapse pressure (= higher film stability), a shift of phase transition (= change of conformation), and a more negative head group potential (change of electric properties). In mixed ganglioside-valinomycin monolayers the addition of Ca2+ causes phase separation and/or aggregate formation between the ganglioside and the peptide. Single channel conductance fluctuations as well as mean channel life-time were analyzed for gramicidin A incorporated into binary mixed black lipid membranes of negatively charged gangliosides (GM1, GD1a, GT1b, GMix) and neutral lecithin (DOPC) in different molar ratios. At monovalent electrolyte concentrations up to < 250 mM CsCl the single channel conductance was significantly larger in the negatively charged mixed DOPC/ganglioside membranes than in the neutral DOPC membrane. Additionally, in the presence of gangliosides the mean channel life-time is increased. The addition of calcium (0.05 mM) induced a reduction of single channel conductance of gramicidin A in DOPC- and mixed DOPC/ganglioside membranes. These physico-chemical data in connection with new electromicroscopical evidences for a precise localization of calcium, a calcium pump (Ca(2+)-ATPase), a clustered arrangement of gangliosides in synaptic terminals, and biochemical results with regard to activatory nature of exogenous gangliosides for neuronal protein phosphorylation and ATPases, support the hypothesis of a modulatory function of gangliosides in synaptic transmission.

Lateral order in binary lipid alloys and its coupling to membrane functions

Densitometry, Raman spectroscopy and small angle neutron scattering are employed to elucidate the miscibility behavior of lipid mixtures organized as liposomal dispersions. First, temperature-composition-phase diagrams for several binary alloys of dialkyl-lecithins differing in chain lengths by an increasing number of CH2-groups are derived. A mixture of dimyristoyllecithin and distearoyllecithin (delta CH2 = 4) shows a peritectic phase behavior with a miscibility gap in the gel state. In the fluid phase, at high enough temperatures, homogeneous mixtures of the two components are formed at all molar ratios. However, upon approaching a critical point by either lowering the temperature or increasing the hydrostatic pressure, critical concentration fluctuations are observed. If one component of a binary mixture is charged, electrostatic interactions can be used to induce phase separation at constant temperature. This is demonstrated for Ca2(+)-driven demixing in alloys of lecithin and negatively charged phosphatidylglycerol. The influence of the various concepts for the induction of lateral structure formation in lipid membranes on integral functional units like ionophores is demonstrated by analysing the single channel current fluctuations of gramicidin in bimolecular lipid membranes. Ca2+, as well as polyelectrolyte-induced phase separations are shown and discussed as examples for the important (lateral) order-function relationship in biomembranes.

pH-control of the miscibility properties of a binary lipid alloy and its influence on the ion transport by gramicidin

We studied the coupling of a membrane function (the transport of ions by the pore forming polypeptide gramicidin) to chemically driven phase changes in black membranes of binary lipid mixtures. In particular, we investigated the influence of the aqueous pH value on the fluid-fluid demixing effect of Ca2+ to phosphatidylcholine/phosphatidylglycerol bilayers. It is found that one can switch, under certain conditions, between a homogeneously mixed and a phase separated membrane by changing the pH. We interpret this as being caused by the change in the degree of dissociation of one of the lipid components.

Phase separation in bimolecular mixed lipid membranes induced by polylysine

We demonstrate, for the first time, polylysine-induced phase separation in a bimolecular lipid membrane of a lecithin/phosphatidylglycerol-mixture by analysing the single channel current fluctuations of gramicidin. The bimodal conductance histograms are direct evidence for the incorporation of the transport system into the two coexisting phases of different composition.

Lateral order in mixed lipid bilayers and its influence on ion translocation by gramicidin: a model for the structure-function relationship in membranes

The temperature-composition phase diagram of dimyristoylphosphatidylcholine and dipentadecylphosphatidylglycerol (DiC15PG) was determined by mass densitometry. For a mixture containing 30 mol% DiC15PG, the homogeneous distribution of the 2 components is demonstrated in the fluid state at T = 35 degrees C by small-angle neutron scattering in combination with the inverse contrast variation method. By the same technique, the coexistence of fluid and condensed phases at T = 23.3 degrees C could be shown in agreement with the densitometric data. Furthermore, it is demonstrated that Ca++ induces, even at T = 35 degrees C, separation into 2 fluid phases. A corresponding phase separation is found in bimolecular lipid membranes ("black films") by analysis of the single-channel conductance fluctuations of gramicidin A incorporated into an equimolarly mixed membrane of neutral lecithin and charged phosphatidic acid. The results are discussed as primary examples on the model-membrane level for the important structure-function relationship of biomembranes.