1H-NMR study of the effect of synthetic polymers on the fluidity, transition temperature and fusion of dipalmitoyl phosphatidylcholine small vesicles

Task-based changes in proton MR spectroscopy signal during configural working memory in human medial temporal lobe

PURPOSE

To detect local cholinergic changes in human medial temporal lobe during configural working memory performance.

MATERIALS AND METHODS

Proton magnetic resonance spectroscopy (¹ H-MRS) measurements were acquired at 3T from a 2 × 2 × 3 cm voxel in right medial temporal lobe from 36 subjects during performance of a configural visual working memory task (cWMT). In order to compensate for expected task-based blood oxygenation level-dependent (BOLD) T₂ * effects, resonance signal changes of unbound choline-containing metabolites (Cho) were referenced to an internal standard of creatine + phosphocreatine metabolites (Cre) and compared between four task blocks: rest, memorization, active memory maintenance, and recognition. An unannounced memory retention test was conducted in 21 subjects. Quality assurance analyses examined task-based Cho and Cre individually as well as referenced to resonance signal from N-acetylaspartate (NAA).

RESULTS

Increases from a resting baseline in the Cho/Cre ratio were observed during 60-second blocks of active memory maintenance across the group (P = 0.0042). Behavioral accuracy during task performance correlated with memory retention (r = 0.48, P = 0.027). Quality assurance measures showed task-based changes in Cre resonance signal both individually (P = 0.00099) and when utilized as a noncholinergic internal reference (NAA/Cre, P = 0.00079).

CONCLUSION

Increases in human medial temporal lobe ¹ H-MRS Cho/Cre ratio occur during the maintenance of configural working memory information. However, interpretation of these results as driven by cholinergic activity cannot be assumed, as NAA, a noncholinergic metabolite, shows similar results when utilizing Cre as a reference. Caution is advised when considering Cre as an internal standard for task-based ¹ H-MRS measurements.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:682-691.

Polyelectrolyte-coated liposomes: stabilization of the interfacial complexes

Anionic liposomes, composed of egg lecithin (EL) or dipalmitoylphosphatidylcholine (DPPC) with 20 mol% of cardiolipin (CL(2-)), were mixed with cationic polymers, poly(4-vinylpyridine) fully quaternized with ethyl bromide (P2) or poly-L-lysine (PL). Polymer/liposome binding studies were carried out using electrophoretic mobility (EPM), fluorescence, and conductometry as the main analytical tools. Binding was also examined in the presence of added salt and polyacrylic acid (PAA). The following generalizations arose from the experiments: (a) Binding of P2 and PL to small EL/CL(2-) liposomes (60-80 nm in diameter) is electrostatic in nature and completely reversed by addition of salt or PAA. (b) Binding can be enhanced by hydrophobization of the polymer with cetyl groups. (c) Binding can also be enhanced by changing the phase state of the lipid bilayer from liquid to solid (i.e. going from EL to DPPC) or by increasing the size of the liposomes (i.e. going from 60-80 to 300 nm). By far the most promising systems, from the point of view of constructing polyelectrolyte multilayers on liposome cores without disruption of liposome integrity, involve small, liquid, anionic liposomes coated initially with polycations carrying pendant alkyl groups.

Biophysical studies on collagen-lipid interaction

The potential use of liposomes as a delivery system is still limited by the poor understanding of the interaction mechanisms of liposomes underlying with biological media. Interaction between liposomes and protein is important for the structure and function of cells. In the present work, the interaction between collagen and dipalmitoyl phosphatidylcholine (DPPC) liposomes was studied by solubilization using a nonionic detergent, octylglucoside (OG), as well as a monolayer technique. The solubilization of the liposomal membrane was found to proceed in three stages of transition from the vesicular form to the mixed micellar form. Moreover, the amount of detergent needed to completely solubilize the liposomal membrane was increased after the incubation of liposomes with collagen, indicating an increased membrane resistance to the detergent and hence, a change in the natural membrane permeation properties. The addition of collagen in the subphase of different monolayer films induced a considerable shift towards a larger area/molecule in the compression-isotherm curves. This is either due to the insertion of collagen into the monolayer via its hydrophobic residues or to adsorption causing a protein layer to be located parallel to the lipid monolayer. It was concluded that collagen significantly altered the physical state of the liposome membrane, which may be attributed to collagen interaction with the liposomal surface and/or to its incorporation within the bilayer membrane.

[Characterization and cytotoxicity of mixed PEG-DSG modified liposomes]

It is known that polyethyleneglycol (PEG) modification of the liposome surface leads to the formation of a fixed aqueous layer around the liposomes due to interaction between the PEG polymer and water molecules, which prevents the attraction of opsonins. When a combination of PEG-distearolyglycerol (PEG-DSG) whose characteristics are remarkably different is used, interaction between molecules occurs, leading to increased fixed aqueous layer thickness (FALT). From this speculation, we studied the effect of both modification of PEG900-DSG and PEG2000-DSG modified liposome on FALT, cell uptake and biodistribution. The FALT of mixed PEG modified liposome increased, compared to that of each single PEG modified liposome. In this mixed modification, maximum FALT was shown at liposome modified by added PEG-2000:PEG-900=2:1. This most suitable additional ratio was equal to actual incorporated ratio. On the other hand, cell uptake of mixed modified liposome containing doxorubicin (DOX) was similar with that of PEG2000 modified liposome. Furthermore, mixed PEG modification of liposome was tendency to increase cytotoxicity, compared to that of other modifications. After DOX contained liposome treatment, DOX distribution in the tumor and antitumor activity of DOX increase by mixed PEG modification. In conclusion, it was suggested that mixed PEG liposome (PEG-2000:PEG-900=2:1) was useful for cancer chemotherapy.

Study on the characterization of mixed polyethyleneglycol modified liposomes containing doxorubicin

Previously, we suggested that mixed polyethyleneglycol (PEG)-modification of liposomes with mixture of 1-monomethoxypolyethyleneglycol-2,3-distearoylglycerol (PEG-DSG) with short and long polyoxyethylene chains led to increasing fixed aqueous layer thickness (FALT) and was useful in vivo. FALT is expected to be one of the important factors that influence the pharmacokinetics of liposomes. In this study, we investigated the connection between FALT and some parameters in vitro. In both mixed and single PEG-modified liposomes, the incorporation ratios of PEG-DSGs were the same, the residual amount of PEG-DSGs with serum proteins was not affected by molecular weight. Consequently, the effective properties in vivo of mixed PEG-modification may be the increase in the absolute amount of PEG-DSGs on the liposome membranes, decrease of slipping out action taken PEG-DSGs with long polyoxyethylene chains by serum protein, and thus maintenance of FALT. While, it has been suggested that mixed PEG-modified liposomes are effective in vitro, too, because of the lower leakage and same level of doxorubicin (DOX) uptake as plain liposomal DOX.

Interactions between liposomes and hydroxypropylmethylcellulose

The characteristics of the adsorption process of hydroxypropylmethylcellulose (HPMC) of molecular weight 35400 Da and nominal viscosity 100 cps onto liposomes prepared with different egg lecithin-cholesterol molar ratios were examined. Adsorption isotherms were constructed and analysed to investigate the mechanisms implicated in the incorporation of the polymer to the interface. Only the isotherms obtained with cholesterol-free liposomes were fitted with Langmuir model. When cholesterol is present in the composition they present a sigmoidal slope. The mechanism of adsorption depends on liposome composition being the main force that drives polymer adsorption of hydrophobic nature. The apparent volumes of HPMC indicate that the conformation of the adsorbed macromolecules depends on liposome composition. Hydration enthalpy values show that adsorbed polymers do not give more hydrophilic systems after freeze-drying as expected with the hydrophilic characteristics of the HPMC.

Poly(ethylene glycol) (PEG)-mediated fusion between pure lipid bilayers: a mechanism in common with viral fusion and secretory vesicle release?

Membrane fusion is fundamental to the life of eukaryotic cells. Cellular trafficking and compartmentalization, import of food stuffs and export of waste, inter-cellular communication, sexual reproduction, and cell division are all dependent on this basic process. Yet, little is known about the molecular mechanism(s) by which fusion occurs. It is known that fusing membranes must somehow be docked and brought into close contact. Specific proteins, many of which have been identified within the past decade, accomplish this. An electrical connection or 'fusion pore' is established between compartments surrounded by the fusing membranes. Three primary views of the mechanism of pore formation during secretory and viral fusion have been proposed within the past decade. In one view, a protein ring forms an initial transient connection that expands slowly by recruiting lipid so as to form a lipidic junction. In another view, the initial fusion pore consists of a protein-lipid complex that transforms slowly until the fusion proteins dissociate from the complex to form an irreversible lipidic pore. In a third view, the initial pore is a transient lipid pore that fluctuates between open and closed states before either expanding irreversibly or closing. Recent work has helped define the mechanism by which poly(ethylene glycol) (PEG) mediates fusion of highly curved model membranes composed only of synthetic phospholipids. PEG is a highly hydrated polymer that can bring vesicle membranes to near molecular contact by making water between them thermodynamically unfavourable. Disrupted packing in the contacting monolayers of these vesicle membranes is necessary to induce fusion. The time course and sequence of molecular events of the ensuing fusion process have also been defined. This sequence of events involves the formation of an initial, transient intermediate in which outer leaflet lipids have mixed and small transient pores join fusing compartments ('stalk'). The transient intermediate transforms in 1-3 min to a fusion-committed, second intermediate ('septum') that then 'pops' to form the fusion pore. Inner leaflet mixing, which is shown to be distinct from outer leaflet mixing, accompanies contents mixing that marks formation of the fusion pore. Both the sequence of events and the activation energies of these events correspond well to those observed in viral membrane fusion and secretory granule fusion. These results strongly support the contention that both viral and secretory fusion events occur by lipid molecule rearrangements that can be studied and defined through the use of PEG-mediated vesicle fusion as a model system. A possible mechanism by which fusion proteins might mediate this lipidic process is described.

Lipid vesicles and membrane fusion

Membrane fusion is essential for cell survival and has attracted a great deal of both theoretical and experimental interest. Fluorescence (de)quenching measurements were designed to distinguish between bilayermerging and vesicle-mixing. Theoretical studies and various microscopic and diffraction methods have elucidated the mechanism of membrane fusion. These have revealed that membrane proximity and high defect density in the adjacent bilayers are the only prerequisites for fusion. Intermediates, such as stalk or inverse micellar structures can, but need not, be involved in vesicle fusion. Nonlamellar phase creation is accompanied by massive membrane fusion although it is not a requirement for bilayer merging. Propensity for membrane fusion is increased by increasing the local membrane disorder as well by performing manipulations that bring bilayers closer together. Membrane rigidification and enlarged bilayer separation opposes this trend. Membrane fusion is promoted by defects created in the bilayer due to the vicinity of lipid phase transition, lateral phase separation or domain generation, high local membrane curvature, osmotic or electric stress in or on the membrane; the addition of amphiphats or macromolecules which insert themselves into the membrane, freezing or other mechanical membrane perturbation have similar effects. Lowering the water activity by the addition of water soluble polymers or by partial system dehydration invokes membrane aggregation and hence facilitates fusion; as does the membrane charge neutralization after proton or other ion binding to the lipids and intermembrane scaffolding by proteins or other macromolecules. The alignment of defect rich domains and polypeptides or protein binding is pluripotent: not only does it increase the number of proximal defects in the bilayers, it triggers the vesicle aggregation and is fusogenic. Exceptions are the bound molecules that create steric or electrical barriers between the membranes which prevent fusion. Membrane fusion can be non-leaky but it is very common to lose material from the vesicle interior during the later stages of membrane unification, that is, after a few hundred microseconds following the induction of fusion.

Interaction of type-I collagen with phospholipid monolayer

The effects of type-I collagen on dipalmitoyl phosphatidylcholine (DPPC) and dimyristoyl phosphatidylcholine (DMPC) monolayer films with different compositions were studied using monolayer technique. The addition of collagen in the subphase of different monolayer films induced a considerable shift towards larger area/molecule in the compression-isotherm curves. This is either referred to the insertion of collagen into the monolayer by its hydrophobic residues or to an adsorption process causing a protein layer to be located parallel to the lipid monolayer [1]. The variation of collagen interaction with different lipid compositions was also verified through the penetration-kinetics experiment. Comparing our results to the results of Pajean et al. [2] and Pajean and Herbage [3] on the effect of collagen on the stability of lipid vesicles implies that the collagen induced stability could be explained on the basis of collagen-lipid monolayer interaction.

Interaction of Doxorubicin with phospholipid monolayer and liposomes

The effect of Doxorubicin which is (an anthracycline antibiotic with a broad spectrum of antitumor activity) on the monolayer and bilayer in the form of large Multilamellar Vesicles (MLV's) of Dipalmitoyl phosphatidylcholine (DPPC) were studied by means of monolayer techniques (surface pressure, penetration kinetics, and association constant) and light scattering technique. The monolayer technique showed that addition of DXR to a lipid film composed of (DPPC/CHOL/PEG-PE) at a molar ratio of (100:0:0) produced a less condensed Monolayer. In the (pie-A) curves, DXR induced shift towards larger area/molecule, where the area/molecule was shifted from 61 to 89 A2, and 116 A2 in the presence of 20 and 40 nM DXR, respectively. The three curves collapsed at a pressure pi = 45 mN/m. In penetration kinetics experiment (delta pi-t), the change in pressure with time was 8 and 14 mN/m for a DXR concentration of 20 and 40 nM, respectively, and the increase in surface pressure presented a plateau over a period of 30 min. The measured association constant (K) was found to be 5 x 10(5)/M. In the light scattering experiment, there was a shift of the transition temperature (Tm) of (MLV's) of the same composition of the monolayer towards a smaller value from 40.5 degrees to 34.5 degrees C. Incorporation of CHOL and PEG-PE as DPPC/CHOL/PEG-PE at a molar ratio of (100:20:0), (100:20:4) and (100:20:4) greatly counteracted the effect of DXR and made the lipid membrane more condense and rigid. Moreover, the penetration of DXR into the membrane was greatly reduced. There was a very small shift for the (pi-A) and (delta pi-t) curves, and the association constant of the drug for these different lipid compositions was greatly reduced down to 2.5 x 10(5)/M and the transition temperature (Tm) was increased up to (42.5 degrees C) in the presence of 40 nM DXR. Our results suggest that DXR has a great effect on the phospholipid membrane, and that addition of CHOL or PEG-PE to the phospholipid membrane causes stabilization for the membrane, and reduces the interaction with Doxorubicin.

Fusion of liposomes due to transient and lasting perturbation induced by synthetic amphiphilic peptides

The membrane-fusion activities of amphiphilic peptides of H-(Leu-Aib-Lys-Aib-Aib-Lys-Aib)n-Ala-N(C18H37)2 (n = 1, P7D and n = 3, P21D) immobilized on liposome were investigated. P7D, which takes a random conformation, induced fusion of DPPC SUV, but P7D immobilized on the DPPC SUV did not show the fusion activity. On the other hand, P21D showed a high activity of membrane fusion either in the free peptide or in the immobilized state. CF-Leakage experiments revealed that the peptides caused a transient perturbation of the membrane structure on binding to the membrane. A lasting and steady perturbation was also caused by P21D embedded in the membrane, which was indicated by EU3+ permeation through the membrane. This type of membrane perturbation was very slight in the case of P7D embedded in the membrane. A conclusion was reached that the different activities in the membrane fusion are based on the transient perturbation in the membrane at the peptide binding to the membrane surface as well as the steady perturbation caused by the peptide embedded in the membrane.

Cytoplasmic and peroxisomal catalases of the guinea pig liver: evidence for two distinct proteins

Catalase, a peroxisomal marker enzyme in the liver of most mammals, is found by immuno-electron microscopy in guinea pig (GP) hepatocytes not only in peroxisomes, but also in the cytoplasm (Beier et al. (1988) Eur. J. Cell Biol. 46, 129-135). We have been able to distinguish in GP liver homogenates between the cytosolic catalase and that part of the enzyme activity which is due to leakage of the enzyme from peroxisomes by adding 4% polyethylene glycol to the homogenization medium. This approach revealed that approximately 40% of the total catalase activity and almost all of alpha-hydroxy-acid oxidases are peroxisomal, while 60% of catalase is of genuine cytosolic origin. The cytosolic and peroxisomal catalases of guinea pig were purified to homogeneity and were analyzed by SDS-PAGE and isoelectric focussing. The cytosolic catalase exhibited a slightly higher Mr (approximately 1000) and a less acidic pI than the peroxisomal enzyme. Limited proteolysis and amino-acid analysis revealed also slight differences between the two molecular forms of catalase. Total RNA was isolated from guinea pig liver and translated in vitro by using a rabbit reticulocyte lysate system. Immunoprecipitation with an antibody against guinea pig catalase followed by high-resolution polyacrylamide gel electrophoresis revealed two polypeptide bands differing slightly in Mr. These observations suggest strongly, that cytoplasmic and peroxisomal catalases in guinea pig liver are two closely related but distinct proteins.

Interaction of poly(ethylene-glycols) with air-water interfaces and lipid monolayers: investigations on surface pressure and surface potential

We have characterized the surface activity of different-sized poly(ethylene-glycols) (PEG; M(r) 200-100,000 Da) in the presence or absence of lipid monolayers and over a wide range of bulk PEG concentrations (10(-8)-10% w/v). Measurements of the surface potential and surface pressure demonstrate that PEGs interact with the air-water and lipid-water interfaces. Without lipid, PEG added either to the subphase or to the air-water interface forms relatively stable monolayers. Except for very low molecular weight polymers (PEGs < 1000 Da), low concentrations of PEG in the subphase (between 10(-5) and 10(-4)% w/v) increase the surface potential from zero (with respect to the potential of a pure air-water interface) to a plateau value of approximately 440 mV. At much higher polymer concentrations, > 10(-1)% (w/v), depending on the molecular weight of the PEG and corresponding to the concentration at which the polymers in solution are likely to overlap, the surface potential decreases. High concentrations of PEG in the subphase cause a similar decrease in the surface potential of densely packed lipid monolayers spread from either diphytanoyl phosphatidylcholine (DPhPC), dipalmitoyl phosphatidylcholine (DPPC), or dioleoyl phosphatidylserine (DOPS). Adding PEG as a monolayer at the air-water interface also affects the surface activity of DPhPC or DPPC monolayers. At low lipid concentration, the surface pressure and potential are determined by the polymer. For intermediate lipid concentrations, the surface pressure-area and surface potential-area isotherms show that the effects due to lipid and PEG are not always additive and that the polymer's effect is distinct for the two lipids. When PEG-lipid-mixed monolayers are compressed to surface pressures greater than the collapse pressure for a PEG monolayer, the surface pressure-area and surface potential-area isotherms approach that of the lipid alone, suggesting that for this experimental condition PEG is expelled from the interface.

Polymer-induced membrane fusion: potential mechanism and relation to cell fusion events

Poly(ethylene glycol) (PEG) is used widely to mediate cell-cell fusion in the production of somatic cell hybrids and in the fusion injection of macromolecules into cultured cells from erythrocytes or liposomes. However, little is known about the mechanisms by which PEG induces fusion of cell membranes, making its use much more an art than a science. This article considers possible molecular events involved in biomembrane fusion and summarizes what we have learned about these in recent years from studies of fusion of well-defined model membranes. In addition, it recounts observations made over the past several years about the process of PEG-mediated fusion of model membranes. These observations have defined the process to an extent sufficient to allow us to propose a model for the molecular events involved in the process. It is suggested that dehydration leads to asymmetry in the lipid packing pressure in the two leaflets of the membrane bilayer leading to formation of a single bilayer septum at a point of close apposition of two membranes. The single bilayer septum then decays during formation of the initial fusion pore. Agents that enhance or alleviate the dehydration-induced asymmetric packing stress will favor or inhibit fusion. Although the proposed picture is consistent with much accumulated data, it is not yet proven; experiments must now be devised to test its details. Finally, the proposed model is discussed in terms of potential implications for the mechanisms available to a cell in controlling more complex in vivo cell fusion processes such as endocytosis, exocytosis, protein sorting/transport, and viral budding/infection.

Changes in the lipid dynamics of liposomal membranes induced by poly(ethylene glycol): free volume alterations revealed by inter- and intramolecular excimer-forming phospholipid analogs

Influence of osmotic shrinkage, swelling, and dehydration on large unilamellar liposomes (LUVs) of 1,2-dioleoylsn-glycero-3-phosphocholine (DOPC) was investigated using the fluorescent lipid probes 1-palmitoyl-2-[10-(pyren-1-yl)]-decanoyl-sn-glycero-3-phosphocholi ne (PPDPC) and 1,2-bis[10-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (bisPDPC). Increasing concentrations of poly(ethylene glycol) (PEG, average molecular weight of 6000) producing osmotic gradients delta omega up to 250 mOsm/kg were first added to the outside of LUV labeled with 0.1 mol% of either of the above fluorescent phospholipids. The resulting osmotic shrinkage was accompanied by a progressive reduction in the lateral diffusion of the membrane-incorporated PPDPC, evident as a decrease in the rate of its intermolecular excimer formation. In contrast, under the same conditions the rate of intramolecular excimer formation by bisPDPC increased. Notably, signals opposite to those described above were observed for both of the fluorescent probes upon osmotic swelling of DOPC liposomes with encapsulated PEG. The lateral diffusion of PPDPC became progressively reduced upon membrane dehydration due to increasing concentrations of symmetrically distributed PEG (with equal polymer concentrations inside and outside of the liposomes) when neither shrinkage nor swelling occurs while enhanced excimer formation by bisPDPC was evident. The later results were interpreted in terms of osmotically induced changes in the hydration of lipids. In brief, the removal of water from the phospholipid hydration shell diminishes the effective size of the polar headgroup, which subsequently allows for an enhanced lateral packing of the phospholipid acyl chains. Our findings are readily compatible with membrane free volume Vf changes due to osmotic forces under three different kinds of stress (shrinkage, swelling, and dehydration) applied on the lipid bilayers.

Lipid-alginate interactions render changes in phospholipid bilayer permeability

Lipid vesicles, e.g. liposomes, generally release their contents in a continuous manner. However, when these vesicles are entrapped in Ca-alginate and coated with poly(L-lysine), they release their contents in an unusual fashion, in 'bursts'. Molecular-level studies indicated that lipid-alginate interactions are responsible for changes in the barrier properties of lipid vesicles. Differential scanning calorimetry revealed that exposure of liposomes to alginate resulted in a 4-fold reduction in the phase transition enthalpy, with no change in the melting temperature. Size-exclusion chromatography of liposomes-in-alginate gave an additional liposomal peak with a smaller elution volume. These studies suggested that alginate is inserted into the lipid bilayer of vesicles. Lipid-alginate interactions were highly dependent on phospholipid head group charge and the phase transition temperature of the phospholipid. Based on these interactions, a mechanism to explain the 'burst' from these entrapped liposomes is suggested.

Polymer-mediated electrostatic interactions between charged lipid assemblies and electrolyte solutions: a tentative model of the polyethylene glycol-induced cell fusion

We developed a theoretical model to investigate the interaction between charged lipid aggregates and a water solution containing ions and uncharged polymers. The local concentration of ions and polymer chains around the lipid aggregate have been treated as variational parameters which can be found by minimizing the total energy of the system. We divided the energy into the following main contributions: (a) Solvation energy of the ions. This depends on the local polymer concentration through the variation of the solvent dielectric properties. (b) Ions-lipid aggregate interactions. These depend on the local concentrations both of the ion cloud and polymer chains. (c) Conformational energy of the polymer. This term is related to the inhomogeneous spatial density of the polymer segments. Any direct interaction between the charged lipid surface and the polymer coils has been intentionally neglected. The minimization procedure leads to a non-linear Poisson-Boltzmann equation coupled with a non-linear algebraic equation describing the polymer distribution. The solution of the above system allows one to calculate the ions and polymer spatial distribution around the lipid aggregate. The knowledge of such parameters is useful to predict the effect of non-ionic polymers on the structure and properties of lipid assemblies such as the mean area per lipid molecule, the aggregation number, the critical micellar concentration and the formation of immiscibility gaps in mixed lipid systems. A possible involvement of these parameters into the fusion process between lipid vesicles is discussed.

The influence of poly(ethylene glycol) on the molecular dynamics within the glycocalyx

Interaction of polymers with cell surfaces is a question of general interest for cell aggregation and fusion. The molecular dynamics within the surface coat of human erythrocytes as well as alterations of membrane protein arrangement (IMPs) in the presence of poly(ethylene glycol) (PEG) were investigated by EPR spin labeling techniques and freeze-fracture electron microscopy, respectively. AT PEG concentrations which induce aggregation of erythrocytes the surface coat and the protein arrangement is not disturbed by the polymer. This implicate an exclusion of the polymer from the cell surface.

Effect of poly(ethylene glycol) on the Ca2+-induced fusion of didodecyl phosphate vesicles

This paper reports a study of the effect of the dehydrating agent poly(ethylene glycol) (PEG) on didodecyl phosphate (DDP) bilayers and on the fusion activity of DDP vesicles as a function of the molecular weight of PEG. PEG 8K in a concentration of 10 wt % does not induce fusion. However, Ca2+-induced fusion is promoted as reflected by a lowering of the Ca2+ threshold concentration. This effect can most likely be attributed to the dehydrating capacity of the polymer. Interestingly, low concentrations (0.1 wt %) of PEG 20 K induce a moderate fusion capacity. At higher concentrations (0.5 wt %) fusion is inhibited, irrespective of the presence of Ca2+. These molecular weight dependent effects can be rationalized by taking into account that the clouding temperature differs for PEGs of different molecular weights. In the case of PEG 20K a microscopic phase separation will occur at the bilayer-water interface because PEG-PEG interactions and presumably PEG-DDP interactions are favored over PEG-water interactions. As a consequence, the DDP vesicle surface becomes covered with PEG 20K, resulting in a steric stabilization of the vesicles. This will impede or prevent, depending on the polymer concentration, the vesicles from approaching each other sufficiently close for fusion to occur.

A model for rouleaux pattern formation of red blood cells

Human red blood cells (RBCs) in a solution form rouleaux patterns under various conditions. The degree of rouleaux formation depends on, for example, the concentration and molecular weight of added large molecules. We present a two-dimensional discrete cellular space model in which an RBC is represented by a rectangle and differential adhesion is assumed among the longer (a-site), the shorter (b-site) sides of the rectangle and the solvent. The total sum of the adhesion energy is assumed to guide the step-by-step change of the model cell configuration and also define absolutely stable patterns. We compare the set of absolutely stable patterns and cell aggregate patterns for both actual and computer-simulated cases to obtain the basic validity of our framework. Then we proceed to assess the effects of added high polymers to the adhesion parameters. We first note that under suitable conditions, decrease in a-site-solvent affinity is necessary to have complex patterns rather than increase of a-a affinity. The hypothesis that addition of high polymers reduce the a-site-solvent affinity is concomitant with a newly proposed osmotic stress theory. The parameter fitting results for the experimental phase change curves can also be interpreted as supporting more the new theory than existing traditional explanations.

Ultrastructure of reconstituted membranes containing the muscarinic receptor

In this paper the demonstration is made that membrane vesicles (liposomes) containing the muscarinic receptor can be formed by polyethylene glycol (PEG) precipitation of detergent extracts of bovine atrial membranes. The incorporation of the muscarinic receptor in these vesicles may be related to the restoration of the heterogeneity and nucleotide modulation of muscarinic agonist binding by PEG precipitation of atrial detergent extracts, previously reported. Vesicles are also formed when detergent solubilized asolectin lipids, alone or in combination with membrane detergent extracts, are precipitated by PEG. The structure of the vesicles seems depend on their lipid and protein composition and the procedure employed for the removal of the dispersing medium. These results indicate that PEG precipitation could be used for the reconstitution of the muscarinic receptor into the liposomes of exogenous lipids.

Alterations in phospholipid polymorphism by polyethylene glycol

The fusogen polyethylene glycol is shown to alter the polymorphism of dimyristoyl phosphatidylcholine, soybean phosphatidylethanolamine, bovine phosphatidylserine, egg phosphatidylcholine/cholesterol mixture, dilinoleoylphosphatidylethanolamine/palmitoyl-oleoylphosphatidy lcholine mixture, and egg lysolecithin. Suspension of these lipids in 50% polyethylene glycol (mol wt = 6000) reduces both the lamellar and the hexagonal II repeat spacings as measured by X-ray diffraction. An increase in the gel to liquid crystalline and bilayer to hexagonal transition temperatures are observed by freeze-fracture, X-ray diffraction, differential scanning calorimetry and 31P NMR. Freeze-fracture electron micrographs revealed different bilayer defects depending on the physical states of the lipid. Lipidic particles in mixtures containing unsaturated phosphatidylethanolamine is eliminated. Some of the influences of polyethylene glycol on lipids may be explained by its dehydrating effect. However, other nonfusogenic dehydrating agents failed to produce similar results. These findings are consistent with the proposal that close bilayer contact and the formation of bilayer defects are associated with the fusogenic properties of polyethylene glycol.

Liposomal heme as oxygen carrier under semi-physiological conditions. Orientation study of heme embedded in a phospholipid bilayer by an electrooptical method

The meso-tetra(alpha,alpha,alpha,alpha(o-pivalamidophenyl]porphinato iron-mono(1-lauryl-2-methylimidazole) complex embedded in the bilayer of dimyristoylphosphatidylcholine (liposomal heme) binds molecular oxygen reversibly at pH 7 and 37 degrees C. Orientation of the iron porphyrin complex in the phospholipid bilayer was studied by electric birefringence and dichroism. It was observed that both the phospholipid bibilayer of liposome and the porphyrin plane are oriented nearly in parallel to the electric field. Therefore the angle between the porphyrin plane and the bilayer is considered to be practically small.

Effect of poly(ethylene glycol) on the polarity of aqueous solutions and on the structure of vesicle membranes

The partitioning of TEMPO into phosphatidylcholine vesicle membranes is reduced upon addition of poly(ethylene glycol). This is caused by reduced polarity of the aqueous phase as well as decreased membrane fluidity in the presence of poly(ethylene glycol). The isotropic hyperfine splitting of TEMPO in aqueous poly(ethylene glycol) solutions was used as a measure of solvent polarity. The alterations of the membrane fluidity were detected by means of two different fatty acid spin labels. The influences of physicochemical properties of an aqueous poly(ethylene glycol) phase on the membrane structure of cells and vesicles are discussed in the light of membrane fusion.

Evidence for an electrogenic adenosine-triphosphatase in Hevea tonoplast vesicles

The function of the Mg-dependent ATPase of Hevea tonoplast in active proton transport was investigated by using a purified tonoplast fraction containing tightly sealed vesicles. In the used experimental conditions, the uptake of [(14)C]triphenylmethyl-phosphonium ion ([(14)C]TPMP(+)) and [(3)H]tetraphenyl-phosphonium ion ([(3)H]TPP(+)) by the vesicles indicated a transmembrane potential difference, negative inside. In parallel, the uptake of [(14)C]methylamine into the vesicles monitored a transmembrane pH gradient, interior acid. The addition of 5 mM Mg-ATP markedly depolarized the membrane and increased the magnitude of trnasmembrane pH gradient. These ATP-driven events were substrate specific for Mg-ATP. They were strongly inhibited by ATPase inhibitors such as N, N'-dicyclohexylcar-bodiimide. They were completely eliminated by proton conductors such as carbonylcyanide-p-trifluoromethoxy-phenylhydrazone and 5-chloro-3-tert-butyl-2'-chloro-4-nitro-salicylanilide. They depended on the pH of the medium, the maximum being reached at about pH 7.0. These data provide in vitro evidence that the Mg-ATPase localized at tonoplast level is an electrogenic pump. They are consistent with the hypothesis that an electrogenic H(+) pump is catalyzed by the tonoplast ATPase of higher plants.

Effect of poly(ethylene glycol) on phospholipid hydration and polarity of the external phase

The hydration properties of phosphatidylcholine (PC)/water dispersions on the addition of poly(ethylene glycol) were studied by means of 2H-NMR. The quadrupole splittings and their temperature dependences correspond to measurements of PC/water dispersions at low water content. It is concluded that the bound water is partly extracted by poly(ethylene glycol) but the binding properties of the water in the inner hydration shell of about five water molecules are not changed. The ability of some phospholipid/water dispersions to undergo phase transitions to nonlamellar structures upon dehydration is discussed. Dipalmitoylphosphatidylcholine (DPPC) and egg phosphatidylcholine do not form nonlamellar structures on addition of purified poly(ethylene glycol), as was demonstrated by means of 31P-NMR. Poly(ethylene glycol) decreases the polarity of the aqueous phase and the partition of hydrophobic molecules between the membrane and the external phase is changed. This was demonstrated using the excimer fluorescence of pyrene in a ghost suspension. It is suggested that the changes in polarity and hydration on the addition of poly(ethylene glycol) can contribute to the alterations in the membrane surface observed under conditions of membrane contact and fusion.

Fluorescence polarization study on the increase of membrane fluidity of human erythrocyte ghosts induced by synthetic water-soluble polymers

The effect of water-soluble polymers on the membrane fluidity of human erythrocyte ghosts was investigated and was compared with that of concanavalin A by means of the fluorescence polarization technique. 8-Anilino-1-naphthalene sulfonic acid sodium salt and 1,6-diphenyl-1,3,5-hexatriene were used as probe molecules. The membrane fluidity was increased by the addition of polycations with concentrations of less than 2 x 10(-3) wt% 60 min after mixing. The fluidity changes were affected by the chemical structure (hydrophobicity, charge density, etc.) of polycations. Thus, the membrane fluidity increased markedly with increasing charge density on the chain backbone of polycations. On the other hand, nonionic polymers such as poly(ethylene glycol) and poly(N-vinyl-2-pyrrolidone) changed the membrane fluidity in a biphasic manner. That is, the fluidity of human erythrocyte ghost was temporarily increased and then decrease. For example, 20 wt.% of poly(ethylene glycol) gave a maximum fluidity 15 min after mixing with erythrocyte ghosts. A similar fluidity change was observed by adding concanavalin A. Such fluidity changes were not observed when lipid bilayer vesicles were used instead of cell membranes. These results suggested that the increase of membrane fluidity resulted from the intramembraneous aggregation of membrane-bound proteins which was induced by the added polymers. Cell agglutination was also induced by the addition of a large amount of polymers. This agglutination was considered to be due to the intermembraneous aggregation of membrane-bound proteins.