The interaction of calcium with gangliosides in bilayer membranes

Buffer mobility and the regulation of neuronal calcium domains

The diffusion of calcium inside neurons is determined in part by the intracellular calcium binding species that rapidly bind to free calcium ions upon entry. It has long been known that some portion of a neuron's intracellular calcium binding capacity must be fixed or poorly mobile, as calcium diffusion is strongly slowed in the intracellular environment relative to diffusion in cytosolic extract. The working assumption was that these immobile calcium binding sites are provided by structural proteins bound to the cytoskeleton or intracellular membranes and may thereby be relatively similar in composition and capacity across different cell types. However, recent evidence suggests that the immobile buffering capacity can vary greatly between cell types and that some mobile calcium binding proteins may alter their mobility upon binding calcium, thus blurring the line between mobile and immobile. The ways in which immobile buffering capacity might be relevant to different calcium domains within neurons has been explored primarily through modeling. In certain regimes, the presence of immobile buffers and the interaction between mobile and immobile buffers have been shown to result in complex spatiotemporal patterns of free calcium. In total, these experimental and modeling findings call for a more nuanced consideration of the local intracellular calcium microenvironment. In this review we focus on the different amounts, affinities, and mobilities of immobile calcium binding species; propose a new conceptual category of physically diffusible but functionally immobile buffers; and discuss how these buffers might interact with mobile calcium binding partners to generate characteristic calcium domains.

Cholesterol-dependent formation of GM1 ganglioside-bound amyloid beta-protein, an endogenous seed for Alzheimer amyloid

GM1 ganglioside-bound amyloid beta-protein (GM1/Abeta), found in brains exhibiting early pathological changes of Alzheimer's disease (AD) including diffuse plaques, has been suggested to be involved in the initiation of amyloid fibril formation in vivo by acting as a seed. To elucidate the molecular mechanism underlying GM1/Abeta formation, the effects of lipid composition on the binding of Abeta to GM1-containing lipid bilayers were examined in detail using fluorescent dye-labeled human Abeta-(1-40). Increases in not only GM1 but also cholesterol contents in the lipid bilayers facilitated the binding of Abeta to the membranes by altering the binding capacity but not the binding affinity. An increase in membrane-bound Abeta concentration triggered its conformational transition from helix-rich to beta-sheet-rich structures. Excimer formation of fluorescent dye-labeled GM1 suggested that Abeta recognizes a GM1 "cluster" in membranes, the formation of which is facilitated by cholesterol. The results of the present study strongly suggested that increases in intramembrane cholesterol content, which are likely to occur during aging, appear to be a risk factor for amyloid fibril formation.

Neuropeptides interact with glycolipid receptors: a surface plasmon resonance study

Using Surface Plasmon Resonance (SPR) we investigated the interaction of seven neuropeptides with different characteristics and beta-amyloid (Abeta42) peptide, with membranes containing gangliosides. A wide range of affinities characterized the bindings (K(D) = 10(-3)- 10(-7) M), following the scheme: for GM1, Abeta42 > DYN > SP = GAL = SOM = BRD > OXY = ENK; for GD1a, Abeta42 = DYN = GAL > SP = SOM = BRD = OXY > ENK and for GT1b, Abeta42 > DYN > SP = GAL > SOM = BRD = OXY > ENK. The ganglioside sugar moiety, specifically the sialic acid, had an important role in the interactions. In general the affinities were higher with polysialo, than with monosialo gangliosides. The sensorgrams describing the interactions of Abeta42 and SP with gangliosides differed from the interactions of the other studied peptides. Ca(2+) promoted changes in peptide-glycolipid interactions.

Gangliosides affect membrane-channel activities dependent on ambient temperature

1. The functional properties of biological membranes depend on their molecular composition. In regard to this, charged glycosphingolipids play an outstanding role in the functional adaptation of membranes to different temperatures. 2. In order to shed some light on the respective functional properties of complex membraneous glycosphingolipids, the effects of altered temperatures (5-40 degrees C) on planar lipid bilayers made from diphytanoylphosphatidylcholine (DPPC) and alamethicin as an ion channel was analyzed in the presence of either a sialoglycosphingolipid (less polar disialoganglioside GD1a or highly polar tetrasialoganglioside GQ1b) or phosphatidylserine (PS; as control). 3. Different to the control bilayers made from DPPC or DPPC + PS, the bilayers containing gangliosides had specific maxima in alamethicin conductance and stabile life times. Changes in pore-state conductances indicate structural effects based on an interaction of the large (negatively charged) ganglioside headgroups with the alamethicin pores. 4. The results concerning open time and closed time of channels seem to be based on the gangliosides changing the viscosity of the bilayer and possibly introducing phase transitions. 5. Thus, the findings suggest that gangliosides (1) directly affect channel molecules via their headgroups and (2) may additionally affect the fluidity of membranes in order to maintain membrane homeoviscosity in areas surrounding ion channels independent from the environmental temperature. 6. The effects of gangliosides may be of special interest in describing the ability of neuronal adaptation of vertebrates to temperature and more general regarding the functional adaptation of neurons.

Differential involvement of gangliosides versus phospholipids in the process of temperature adaptation in vertebrates. A comparative phenomenological and physicochemical study

The data presented support the idea that gangliosides are involved in thermal adaptation of neuronal membranes. Brain ganglioside patterns from cold-blooded vertebrate species living in different climates and from mammals during ontogenetical or seasonal changes in their body temperature were compared. The general rule "the lower the environmental temperature the more polar is the composition of brain gangliosides" as derived from these data was confirmed by the changes in ganglioside patterns evoked by experimentally induced cold acclimation of fish. To assess whether gangliosides are able to modulate the temperature-dependent properties of membranes, artificial mono- and bilayer membrane model systems were used. Incorporation of gangliosides in the model bilayer membranes evoked drastic changes in the dynamics of a peptide channel, suggesting that gangliosides are able to modulate basic membrane properties. In addition, data on thermosensitivity of ganglioside-calcium interactions and on surface behavior of gangliosides in monolayers are reviewed.

Oriented 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine/ganglioside membranes: a Fourier transform infrared attenuated total reflection spectroscopic study. Band assignments; orientational, hydrational, and phase behavior; and effects of Ca2+ binding

Fourier transform infrared (FTIR) attenuated total reflection (ATR) spectroscopy was used to elucidate the hydration behavior and molecular order of phospholipid/ganglioside bilayers. We examined dry and hydrated films of the gangliosides GM1, deacetyl-GM1, lyso-GM1, deacetyllyso-GM1, and GM3 and oriented mixed films of these gangliosides with 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) using polarized light. Analysis of the amide I frequencies reveals that the amide groups are involved in intermolecular interactions via hydrogen bonds of varying strengths. The tilt angle of the acyl chains of the lipids in mixed films was determined as a function of ganglioside structure. Deacetylation of the sialic acid in the headgroup has a stronger influence on the tilt angle than the removal of the ganglioside fatty acid. The phase behavior was examined by FTIR ATR spectroscopy and by differential scanning calorimetry (DSC) measurements on lipid suspensions. At the same molar concentration, lyso-gangliosides have less effect on changes of transition temperature compared to the double-chain analogs. Distinct differences in the amide band shapes were observed between mixtures with lyso-gangliosides and normal double-chain gangliosides. Determined from the dicroic ratio RATR, the orientation of the COO- group in all DMPC/ganglioside mixtures was found to be relatively fixed with respect to the membrane normal. In 4:1 mixtures of DMPC with GM1 and deacetyl-GM1, the binding of Ca2+ leads to a slight decrease in chain tilt in the gel phase, probably caused by a dehydration of the membrane-water interface. In mixtures of DMPC with GM3 and deacetyl-lyso-GM1, a slight increase in chain tilt is observed. The chain tilt in DMPC/lyso-GM1 mixtures is unchanged. Analysis of the COO- band reveals that Ca2+ does not bind to the carboxylate group of the sialic acid of GM1 and deacetyl-GM1, the mixtures in which a decrease in chain tilt was observed. Binding to the sialic acid was only observed for mixtures of DMPC with GM3, lyso-GM1, and deacetyl-lyso-GM1. Ca2+ obviously accumulates at the bilayer-water interface and leads to partial dehydration of the headgroup region in the gel as well as in the liquid-crystalline phase. This can be concluded from the changes in the amide I band shapes. With the exception of DMPC/deacetyl-GM1, the effects on the ester C==O bands are small. The addition of Ca2+ has minor effects on the phase behavior, with the exception of the DMPC/GM1 mixture.

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.

Oligosaccharide behavior of complex natural glycosphingolipids in multicomponent model membranes

Wideline 2H NMR of model membranes was used to consider the molecular consequences of factors often suggested as modulators of complex glycosphingolipid oligosaccharide arrangement and motional characteristics at cell surfaces. GM1, asialo-GM1, and globoside were studied as examples of plasma membrane recognition sites. The experimental approach involved substitution of deuterons (D) for protons at specific locations within the carbohydrate chains. Deuterated glycolipids were then dispersed at 7-10 mol% in unsonicated bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine. Factors tested for their significance to carbohydrate chain conformation and dynamics included glycolipid natural alkyl and acyl chain variability, membrane fluidity, and the presence of cholesterol and a charged sugar residue (neuraminic acid). Effects of Ca2+ and membrane-associated protein were briefly considered. Two distinct strategies were employed in substituting deuterons for selected protons of carbohydrate residues. Neither approach necessitated alteration of the glycolipid natural fatty acid composition. (i) Protons of the exocyclic hydroxymethyl group on the terminal Gal residue of GM1 and asialo-GM1, and on the terminal N-acetylgalactosamine (GalNAc) residue of globoside, were replaced with deuterium (producing -CDHOH) by an enzymatic oxidation/reduction cycle. This represents the first application of such an approach to deuteration of complex neutral glycolipids. Spectral results were compared to those obtained for the similarly-deuterated monoglycosyl lipid, galactosylceramide (GalCer), with natural fatty acid composition. Efficacy of this labeling method may in principle be influenced by structural variations within a given glycolipid family. Also, asymmetric rotation of the deuterated group made it less attractive than the second method for relating spectral features to receptor geometry. (ii) A general synthetic, nonenzymatic method was investigated for replacing amino sugar N-acetyl groups with deuterated acetate (-COCD3). The acetate group of the GalNAc residue of globoside, GM1, and asialo-GM1, as well as that on neuraminic acid in GM1, was replaced with -COCD3. This second method afforded better signal-to-noise--an important consideration for 2H NMR. The NMR technique employed had the potential for detecting changes of as little as 10% in oligosaccharide orientation or motional order. Each glycolipid demonstrated clear evidence of preferred average oligosaccharide conformations in all (fluid) membrane environments examined. The most striking observation was that, in fluid matrices, conformation and motional order of the complex oligosaccharide chains were only modestly influenced by factors tested, including natural variation in the glycolipid hydrocarbon chains, membrane fluidity, temperature, and the presence of cholesterol or the N-acetylneuraminic acid (NeuAc) residue on GM1.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular modeling of the conformational and sodium ion binding properties of the oligosaccharide component of ganglioside GM1

The receptor-like recognition behavior of the GM1 ganglioside has been examined theoretically in terms of conformational and binding properties. Modeling was conducted at two limiting conditions of dielectric constant in order to determine sensitivity to scaling of coulombic interactions. A systematic conformational search of the GM1 oligosaccharide in the absence of explicit solvent molecules indicates that there are many inherently low energy conformational states. Up to 39 conformers were found with energies within 5 kcal/mole of the observed lowest energy conformer. Using a dielectric constant of 80, a systematic search of sodium binding sites on GM1 identified 37 sites where a positively charged group might bind, while at least 12 sites were identified using a dielectric constant of 1. Notably important binding sites include pockets formed by the proximity of glycosidic (O1), sugar ring (O5), and exocyclic methylene hydroxyl (OH6) oxygens on the sugars. The oxygens of acetyl groups attached to sugars also contribute to the binding. Direct coordination with the carboxylate of sialic acid is not a prerequisite for cationic binding. The large number of conformational states and binding sites for the GM1 oligosaccharide are paradoxical to the specific recognition behavior of the molecule. This paradox can be explained in terms of bridging ligands, which are found from molecular dynamics to be capable of stabilizing molecular conformation.

Modulation of the activity of Clostridium perfringens neuraminidase by the molecular organization of gangliosides in monolayers

The activity of Clostridium perfringens neuraminidase against gangliosides GM3, GD1a and GM1 was studied in lipid monolayers at the air-buffer solution interface. The enzyme activity assay against pure ganglioside monolayers is based on the markedly different molecular packing areas of the substrate gangliosides and the resulting product glycosphingolipids. This allows to control and monitor the surface pressure and the ganglioside intermolecular organization (cross-sectional packing areas and dipole potentials) in a continuous manner during the catalytic process. It was found that the rate and the extent of the enzymatic reaction depended markedly on the lateral surface pressure. In general, the activity of neuraminidase against GM3 and GD1a was higher at lower surface pressure. This corresponded to larger intermolecular spacings among the ganglioside molecules. Both the activity and the extent of the reaction against GM3 were higher than toward GD1a. GM1 could not be degraded by the enzyme, irrespective of the surface pressure but the enzyme could interact with this ganglioside. A latency period, longer for GM3 than for GD1a, was observed prior to the onset of rapid degradation; this indicates that pre-catalytic steps are occurring at the interface before effective ganglioside degradation takes place. The latency period, the total amount of ganglioside degraded, and the velocity of the reaction varied with the surface pressure in different manners. Our data indicate that the different steps of the catalytic reaction occurring at the surface (i.e., substrate recognition and interfacial adsorption, catalysis, maximum extent of substrate conversion) are independently regulated by the molecular organization of the substrate gangliosides.

Multiphasic modelling of ligand/acceptor interactions. The hydrophobicity-dependent binding of relatively small amphiphilic substances to acceptor proteins and the nature and facedness of acceptor sites

The modelling of multiphasic ligand/acceptor equilibrium binding systems proceeds at three logically distinct levels: (1) A suitable response quantity, e.g. the amount of acceptor-bound ligand nEL, is expressed as a function of the ligand concentrations [Li] (L = A,B,...) in the compartment i that contains the acceptor sites. One thus obtains a response function nEL = f1([Li]). In general, the equilibrium constants KL contained in such mathematical models are physically ill-defined. (2) Each local concentration [Li] is further expressed as a function of [Laq], the corresponding concentration in the aqueous phase, leading to nEL = f2([Laq]). In this way, the constants KL are transformed into effective constants K'L which (i) can be assessed experimentally and (ii) depend on ligand hydrophobicity in a way that is characteristic of the binding site. Formulation of the functions f1 and F2 only requires knowledge of the reactions in which the acceptor sites participate directly. (3) For each ligand, the experimentally accessible total ligand concentration Lt is expressed as a function of [Laq], leading to concentration balance equations Lt = Lt([Laq]). The latter transformation takes account of any reactions, distinct from ligand/acceptor interaction, in which the ligands are involved, e.g. binding to additional protein sites. As a result of steps 2 and 3, each binding system is described by a set of simultaneous equations dependent on the auxiliary variable [Laq]: (i) the response function f2([Laq]) and (ii) a concentration balance for each ligand Lt = Lt([Laq]). The formulae are rendered more conscise and their discussion and application to data fitting are simplified by introducing, for each ligand L, a function FL characterising the distribution of unbound monomeric ligand over the various partition compartments. When the acceptor acts on unbound ligand, the formulae are further expressed in terms of a new auxiliary variable i.e. the total concentration of unbound monomeric ligand microL. In contrast to data analysis as a function of local concentrations, analysis in terms of total ligand concentrations avoids losing sight of alternate hypotheses about the nature of the binding sites. The present formulation has also permitted clarification of several consequences of the multiphasic nature of the binding systems that, as yet, have been poorly recognised.(ABSTRACT TRUNCATED AT 400 WORDS)

[Brain gangliosides and memory formation]

It is generally accepted that the process of molecular facilitation (= Bahnung) of neuronal circuits by means of stabilization of synaptic contacts represents the structural basis for memory formation. On the basis of physiological, electron microscopic, biochemical, and physicochemical data, a concept is presented speaking in favor of the idea that interactions between brain gangliosides (= amphiphilic sialic acid-containing glycosphingolipids with peculiar physicochemical properties), calcium, and functional membraneous proteins (ion channels, ion pumps, receptors, kinases) play a crucial modulatory role in the transmission and storage of information.

Effects of exogenous GM1 ganglioside on LTP in rat hippocampal slices perfused with different concentrations of calcium

Combined actions of GM1 ganglioside molecules and Ca2+ ions on long-term potentiation (LTP) of the hippocampal CA1 synapses were examined in slice preparations. Application of exogenous GM1 ganglioside significantly enhanced LTP. Antigenic blocking of endogenous GM1 ganglioside by applying anti-GM1 antiserum appeared to attenuate LTP. Exogenous GM1 treatment not only antagonized LTP suppression caused by low Ca2+ (1.0 microM) in the perfusion medium, but also amplified LTP at high Ca2+ (5.0 microM) condition. Thus, GM1 may stabilize intracellular calcium levels to enhance synaptic potentiation.

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.

Analysis of membrane-bound acceptors. A correction function for non-specific accumulation of poorly water-soluble hydrophobic or amphipathic ligands based on the ligand partition concept

Non-specific ligand accumulation into membrane material, which may contribute considerably to the experimental signal obtained in binding studies with labelled amphipathic and hydrophobic ligands, may be accounted for by linear partition of the ligands into the membrane phase. For application to binding data obtained at a single membrane-lipid concentration, a fitting procedure is proposed which allows one to correct for non-specific ligand partition. If the assumption is met that the amount of acceptor-bound ligand is small compared to the total amount present in the system, one can validly interpret the data in terms of total ligand concentrations. The apparent dissociation constants Kd(app) thus obtained should be corrected for the often large effects of the size of the partition compartment(s), by performing assays at several membrane-lipid concentrations. The importance of the latter correction is stressed and an approach for obtaining the characteristic effective dissociation constants Kd' is indicated. The procedure also yields estimates of the ligand/membrane partition coefficients.

Calcium regulation of neuronal differentiation: the role of calcium in GM1-mediated neuritogenesis

Cultures of mouse Neuro-2a neuroblastoma cells treated with 3-6 mM extracellular Ca2+ exhibited enhanced neurite extension characterized by increased neurite numbers and lengths. The ganglioside GM1 potentiated the effect of extracellular Ca2+ by increasing further the number and length of the neurites formed in response to exogenous Ca2+. Maximal neuritic numbers were achieved with 4 mM Ca2+ while the longest neurites were observed in medium containing 4-6 mM Ca2+. Stimulation of the Ca2+ influx with the ionophore A23187 or the amino acid taurine also enhanced neurite formation and GM1 potentiated these actions. Transmission electron microscopy revealed numerous microtubules and neurofilaments in neurites and microfilaments with the spine-like processes along fine neuritic branches and in the filopodia of growth cones. Neuritic varicosities and growth cones contained a variety of vesicles. All of these structures were increased in the presence of GM1 and were increased further by extracellular Ca2+ or A23187. The ability of GM1 to enhance neuritogenesis was diminished by EGTA or Ruthenium red. Similarly, the effect of GM1 was diminished or abolished by Ca2+ channel blockers such as CdCl2 or LaCl3. X-ray microprobe analysis revealed that GM1 alone enhanced intracellular levels of total ionic and membrane bound Ca2+, perhaps accounting for the increased neuritogenesis observed under conditions in which Ca2+ was manipulated. The present study suggest that the neuritogenic action of GM1 is Ca2+ dependent.

Membrane electrostatics

In conclusion, charged membrane together with their adjacent electrolyte solution form a thermodynamic and physico-chemical entity. Their surfaces represent an exceptionally complicated interfacial system owing to intrinsic membrane complexity, as well as to the polarity and often large thickness of the interfacial region. Despite this, charged membranes can be described reasonably accurately within the framework of available theoretical models, provided that the latter are chosen on the basis of suitable criteria, which are briefly discussed in Section A. Interion correlations are likely to be important for the regular and/or rigid, thin membrane-solution interfaces. Lateral distribution of the structural membrane charge is seldom and charge distribution perpendicular to the membranes is nearly always electrostatically important. So is the interfacial hydration, which to a large extent determines the properties of the innermost part of the interfacial region, with a thickness of 2-3 nm. Fine structure of the ion double-layer and the interfacial smearing of the structural membrane charge decrease whilst the surface hydration increases the calculated value of the electrostatic membrane potential relative to the result of common Gouy-Chapman approximation. In some cases these effects partly cancel-out; simple electrostatic models are then fairly accurate. Notwithstanding this, it is at present difficult to draw detailed molecular conclusions from a large part of the published data, mainly owing to the lack of really stringent controls or calibrations. Ion binding to the membrane surface is a complicated process which involves charge-charge as well as charge-solvent interactions. Its efficiency normally increases with the ion valency and with the membrane charge density, but it is also strongly dependent on the physico-chemical and thermodynamic state of the membrane. Except in the case of the stereospecific ion binding to a membrane, the relatively easily accessible phosphate and carboxylic groups on lipids and integral membrane proteins are the main cation binding sites. Anions bind preferentially to the amine groups, even on zwitterionic molecules. Membrane structure is apt to change upon ion binding but not always in the same direction: membranes with bound ions can either expand or become more condensed, depending on the final hydrophilicity (polarity) of the membrane surface. The more polar membranes, as a rule, are less tightly packed and more fluid. Diffusive ion flow across a membrane depends on the transmembrane potential and concentration gradients, but also on the coulombic and hydration potentials at the membrane surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of gangliosides on membrane permeability studied by enzymic and fluorescence-spectroscopy techniques

The effect of gangliosides on membrane permeability was investigated by studying the kinetic properties of cytochrome c oxidase, the activity of which, when the enzyme is reconstituted in phospholipid vesicles, is dependent on membrane permeability to H+ and K+. The experiments indicate that three different gangliosides (GM1, DD1a, GT1b) incorporated into cytochrome c oxidase-containing phospholipid vesicles stimulate enzymic activity, in the absence of ionophores, most probably by disorganizing the bilayer lipid assembly and increasing its permeability to ions. This interpretation was confirmed by fluorescence-spectroscopy experiments in which the rate of passive leakage of carboxyfluorescein entrapped in the vesicles was measured. Cholera toxin, or its isolated B-subunit, added to GM1-containing proteoliposomes inhibited cytochrome c oxidase activity, indicating the lack of formation, under these experimental conditions, of channels freely permeable to H+ or K+.

Interaction of ganglioside-containing planar bilayers with serotonin and inorganic cations

The binding of serotonin and inorganic cations K+, Na+, Ca2+, Mg2+ to planar bilayers formed from mixtures of phosphatidylcholine and mono-, di- and trisialogangliosides was studied by the potentiodynamic and nonactin-induced potassium conductivity method. The theoretical analysis of the results obtained was made taking into account (1) protrusion of the ganglioside charges from the membrane surface and (2) simultaneous adsorption of ions on the bilayer surface and on the ganglioside charges protruding into the solution. It was shown that there was no specific binding of K+ and Na+. The binding constants for Ca2+, Mg2+ were determined. These constants for all the gangliosides studied were equal to 500 M-1. The determined binding constants of serotonin to various gangliosides diminish in the following order: GD3 greater than GT1b greater than GD1a greater than GM1.

Modelling of chemical reactions catalysed by membrane-bound enzymes. Determination and significance of the kinetic constants

A model of multiphasic systems, based on the assumption of zero-order partition of substrates and products into the membranes, is applied to reversible mono-substrate and bi-substrate reactions catalysed by membrane-bound enzymes. Apart from replacement of single-phase kinetic constants by apparent kinetic constants, the derived kinetic expressions are formally identical with those for corresponding single-phase systems. The model confers to the apparent kinetic constants an experimentally verifiable meaning. For full characterization of membrane-kinetic systems, experiments at various concentrations of enzyme-embedding phospholipid are required. Extrapolation to zero phospholipid concentration of each Km app then yields the corresponding true kinetic constant characteristic of the membrane-bound enzyme and also provides a technique for determination of the membrane-partition constants. The procedure implies that the phospholipid content should be assayed for full characterization of membrane-bound enzymes. If, for practical reasons, the assays have to be limited to a single enzyme concentration, correction of the apparent kinetic constants is still possible provided the phospholipid concentration and the partition constants of the reactants are known. The model has permitted prediction of a number of previous observations reflecting the multiphasic nature of the systems. The assumptions, underlying the model, and their implications are examined as well as some commonly used experimental designs for determination of the type of enzymic site.

Electro-insertion of xeno-glycophorin into the red blood cell membrane

The electroporation technique, with field strengths slightly below the critical value Ec for electroporation of red blood cells (RBC), enables the insertion of xeno-proteins into the RBC membrane without damaging the cells. The electro-insertion has been used to insert biotinylated human glycophorin into human RBC membrane and human glycophorin into murine RBC membrane. Binding anti-human glycophorin antibody (10F7) to the murine RBC bearing human glycophorin indicates extracellular orientation of inserted glycophorin. Insertion of about 10(5) glycophorin molecule per cell has been estimated by whole cell ELISA.

Acetylcholine receptors and concanavalin A-binding sites on cultured Xenopus muscle cells: electrophoresis, diffusion, and aggregation

Using digitally analyzed fluorescence videomicroscopy, we have examined the behavior of acetylcholine receptors and concanavalin A binding sites in response to externally applied electric fields. The distributions of these molecules on cultured Xenopus myoballs were used to test a simple model which assumes that electrophoresis and diffusion are the only important processes involved. The model describes the distribution of concanavalin A sites quite well over a fourfold range of electric field strengths; the results suggest an average diffusion constant of approximately 2.3 X 10(-9) cm2/s. At higher electric field strengths, the asymmetry seen is substantially less than that predicted by the model. Acetylcholine receptors subjected to electric fields show distributions substantially different from those predicted on the basis of simple electrophoresis and diffusion, and evidence a marked tendency to aggregate. Our results suggest that this aggregation is due to lateral migration of surface acetylcholine receptors, and is dependent on surface interactions, rather than the rearrangement of microfilaments or microtubules. The data are consistent with a diffusion-trap mechanism of receptor aggregation, and suggest that the event triggering receptor localization is a local increase in the concentration of acetylcholine receptors, or the electrophoretic concentration of some other molecular species. These observations suggest that, whatever mechanism(s) trigger initial clustering events in vivo, the accumulation of acetylcholine receptors can be substantially enhanced by passive, diffusion-mediated aggregation.

Liposomes as carriers of poorly water-soluble substrates: linear modelling of membrane systems with catalytic or binding sites of different facedness. Significance of experimental membrane partition coefficients and of kinetic and equilibrium parameters

1. A multiphasic modelling approach to systems containing membrane-bound receptors or catalytic sites and a liposomal preparation as a substrate carrier is described. Kinetic expressions are derived for a single-substrate enzymic reaction operating at constant liposome concentration or at a fixed substrate/liposome concentration ratio. 2. The assumption that accumulation of exchangeable components into the phospholipid bilayers can be described by linear bulk-phase partition leads to simple relationships between the initial reaction rate and (a) two kinetic coefficients (V and K'm), (b) the partition coefficients of the solutes for the lipid compartments of the membrane (Pms) and liposomal preparations (P1s) and (c) the total concentrations of substrate, membrane lipid and liposomal lipid. K'm is called the effective Michaelis constant. 3. For correct estimation of the coefficients V, K', Pms and P1s extrapolation to zero lipid concentration is required. 4. The distinction is introduced between hydrophilic and hydrophobic aqueous-faced sites, lipid-faced sites and mixed sites, i.e. sites overlapping an aqueous and a lipid region. For hydrophilic aqueous-faced sites K'm is equal to the true Km and for the other types of site to Km/Ps. For lipid-faced and for mixed sites Ps corresponds to the membrane partition coefficient Pms. For binding of homologous compounds to a hydrophobic aqueous-faced binding pocket Ps is the incremental site partition coefficient Pbss, which takes into account the energetic contribution to the binding process due to the hydrophobic tail of the ligands. 5. K'm accounts for any effects due to the facedness and nature of the enzymic sites. The dependence of the systems on the size of the lipidic partition compartment(s) is expressed exclusively by a distribution function F.6. When enzyme assays are performed with a series of chemically different substrates containing the same catalytically sensitive group, independence of K'm from partition indicates a hydrophilic aqueous-faced binding site. For the low-molecular-mass members of the homologous series a linear increase in -log (K'm) with the logarithm of the partition coefficient will be observed with any of the other site types considered 7. Equilibrium relationships for binding of a ligand to a membrane-bound receptor are also derived. 8. The significance of experimental membrane partition coefficients is discussed.

Neutron diffraction studies of digalactosyldiacylglycerol

The structure of the digalactosyldiacylglycerol bilayer is calculated using neutron diffraction data. The polar head group of this lipid is oriented parallel to the plane of the bilayer such that the galactose moieties are tightly packed at the bilayer surface into a 0.8 nm thick polar layer. The thickness of this layer is independent of water activity over a wide range (15-100% relative humidity). The constant thickness of both the galactose layer and the hydrocarbon layer constrain the structure factor amplitudes to lie on a single continuous transform for repeat periods between 4 and 5 nm.

Characterization of the channel properties of tetanus toxin in planar lipid bilayers

A detailed characterization of the properties of the channel formed by tetanus toxin in planar lipid bilayers is presented. Channel formation proceeds at neutral pH. However, an acidic pH is required to detect the presence of channels in the membrane rapidly and effectively. Acid pH markedly lowers the single-channel conductance, for phosphatidylserine at 0.5 M KCl gamma = 89 pS at pH 7.0 while at pH 4.8, gamma = 30 pS. The toxin channel is cation selective without significant selectivity between potassium and sodium (gamma [K+]/gamma [Na+] greater than or equal to 1.35). In all the lipids studied gamma is larger at positive than at negative voltages. The toxin channel is voltage dependent both at neutral and acidic pH: for phosphatidylserine membranes, the probability of the channel being open is much greater at positive than at negative voltage. In different phospholipids the channel exhibits different voltage dependence. In phosphatidylserine membranes the channel is inactivated at negative voltages, whereas in diphytanoylphosphatidylcholine membranes channels are more active at negative voltages than at positive. The presence of acidic phospholipids in the bilayers increases both the single-channel conductance as well as the probability of the channel being open at positive voltage. A subconductance state is readily identifiable in the single-channel recordings. Accordingly, single-channel conductance histograms are best fitted with a sum of 3 Gaussian distributions corresponding to the closed state, the open subconductance state and the full open state. Channel activity occurs in bursts of openings separated by long closings. Probability density analysis of the open dwell times of the toxin channel indicate the existence of a single open state with a lifetime greater than or equal to 1 ms in all lipids studied. Analysis of intra-bursts closing lifetimes reveals the existence of two components; the slow component is of the order of 1 ms, the fast one is less than or equal to 0.5 ms. The channel activity induced by tetanus toxin in lipid bilayers suggests a mechanism for its neurotoxicity: a voltage dependent, cation selective channel inserted in the postsynaptic membrane would lead to continuous depolarization and, therefore, persistent activation of the postsynaptic cell.

Semiquantitative evaluation of histochemically detectable calcium binding in mammalian brain slices

A method is described for the electron histochemical demonstration of Ca in the central nervous system, based on fixation on Ca-containing paraformaldehyde solution and the subsequent complexing of Ca by ammonium oxalate. The method resulted in highly electron-dense deposits, with good ultrastructural preservation. The Ca content of the deposits was proved by physico-chemical analysis. The high electron density permitted the counting of deposits and thereby an estimation of their numerical density, via planimetry of electron micrographs. Since pre- and postsynaptic localizations could be distinguished on the basis of ultrastructure, this procedure is regarded as a unique semiquantitative method for estimation of the tissue Ca binding of mammalian brain slices.

Effect of calcium ions on the thermotropic behaviour of neutral and anionic glycosphingolipids

In the concentration range of 10(-5) to 10(-1) M Ca2+ modulates the thermotropic properties of several neutral and anionic glycosphingolipids (galactosylceramide, asialo-GM1, sulfatide, GM1, GD1a, GT1b) and of their mixtures with dipalmitoylphosphatidylcholine. The transition temperature of gangliosides is not appreciably changed while the transition enthalpy increases by 20% in the presence of Ca2+. The more marked effect of Ca2+ is on the thermotropic behavior of systems containing sulfatide. Increasing concentrations of Ca2+ between 10(-5) and 10(-3) M (up to a molar ratio of Ca2+/sulfatide 1:2) induce a progressive increase of both the transition temperature and enthalpy. Further increases up to 10(-1) M Ca2+ induce a new phase transition at a lower temperature. No evidence is found for induction of phase separation of pure glycosphingolipid-Ca2+ domains in mixtures of any of the glycosphingolipids with dipalmitoylphosphatidylcholine. The modification of the phase behavior of anionic glycosphingolipids by Ca2+ does not involve detectable variations of the intermolecular packing but is accompanied by marked modifications of the dipolar properties of the polar head group region.

The effect of antibodies to gangliosides on Ca2+ channel-linked release of gamma-aminobutyric acid in rat brain slices

Antibodies to GM1 ganglioside enhance the release of gamma-aminobutyric acid (GABA) from rat brain slices induced by depolarization with either 40 mM K+ or 200 microM veratrine. Three new observations are now reported. (a) GABA release induced by the Ca2+ ionophore A23187 was not affected by these antibodies. Because this Ca2+ ionophore causes transmitter release by bypassing depolarization-induced opening of Ca2+ channels, this result suggests that gangliosides participate either in the functioning of such Ca2+ channels or in the Na+ channels involved in depolarization. (b) The enhancement (by antibodies to GM1 ganglioside) of GABA release induced by high K+ levels occurred in the presence of tetrodotoxin (0.01 microM). (c) GABA release induced by veratrine in the absence of Ca2+ was not affected by the antibodies. These latter two observations indicate that Na+ channels are not involved in the action of the antibodies. We conclude that this evidence points to the participation of gangliosides in Ca2+ channel functions involved in GABA release in rat brain slices.