Alterations in phospholipid polymorphism by polyethylene glycol

Technology used to build and transfer mammalian chromosomes

In the near twenty-year existence of the human and mammalian artificial chromosome field, the technologies for artificial chromosome construction and installation into desired cell types or organisms have evolved with the rest of modern molecular and synthetic biology. Medical, industrial, pharmaceutical, agricultural, and basic research scientists seek the as yet unrealized promise of human and mammalian artificial chromosomes. Existing technologies for both top-down and bottom-up approaches to construct these artificial chromosomes for use in higher eukaryotes are very different but aspire to achieve similar results. New capacity for production of chromosome sized synthetic DNA will likely shift the field towards more bottom-up approaches, but not completely. Similarly, new approaches to install human and mammalian artificial chromosomes in target cells will compete with the microcell mediated cell transfer methods that currently dominate the field.

Disordering of phospholipid headgroups induced by a small amount of polyethylene oxide

We present a (31)P NMR spectroscopy study of planar glass-plate-oriented multi-bilayers of dimyristoylphosphatidylcholine (DMPC) with addition of polyethylene oxide (PEO). This work revealed the presence of a new component in the spectra that appeared only with addition of a small fraction of PEO (up to one PEO segment per dimyristoylphosphatidylcholine molecule) and disappeared when larger amounts of PEO were added. We explained this phenomenon as an effect of an inhomogeneous force field induced by the PEO molecules located at a certain depth in the lipid membrane interface region.

State of the art in PEGylation: the great versatility achieved after forty years of research

In the recent years, protein PEGylation has become an established and highly refined technology by moving forward from initial simple random coupling approaches based on conjugation at the level of lysine ε-amino group. Nevertheless, amino PEGylation is still yielding important conjugates, currently in clinical practice, where the degree of homogeneity was improved by optimizing the reaction conditions and implementing the purification processes. However, the current research is mainly focused on methods of site-selective PEGylation that allow the obtainment of a single isomer, thus highly increasing the degree of homogeneity and the preservation of bioactivity. Protein N-terminus and free cysteines were the first sites exploited for selective PEGylation but currently further positions can be addressed thanks to approaches like bridging PEGylation (disulphide bridges), enzymatic PEGylation (glutamines and C-terminus) and glycoPEGylation (sites of O- and N-glycosylation or the glycans of a glycoprotein). Furthermore, by combining the tools of genetic engineering with specific PEGylation approaches, the polymer can be basically coupled at any position on the protein surface, owing to the substitution of a properly chosen amino acid in the sequence with a natural or unnatural amino acid bearing an orthogonal reactive group. On the other hand, PEGylation has not achieved the same success in the delivery of small drugs, despite the large interest and several studies in this field. Targeted conjugates and PEGs for combination therapy might represent the promising answers for the so far unmet needs of PEG as carrier of small drugs. This review presents a thorough panorama of recent advances in the field of PEGylation.

PEG as a tool to gain insight into membrane fusion

Thirty years ago, Klaus Arnold and others showed that the action of PEG in promoting cell-cell fusion was not due to such effects as surface absorption, cross-linking, solubilization, etc. Instead PEG acted simply by volume exclusion, resulting in an osmotic force driving membranes into close contact in a dehydrated region. This simple observation, based on a number of physical measurements and the use of PEG-based detergents that insert into membranes, spawned several important areas of research. One such area is the use of PEG to bring membranes into contact so that the role of different lipids and fusion proteins in membrane fusion can be examined in detail. We have summarized here insights into the fusion mechanism that have been obtained by this approach. This evidence indicates that fusion of model membranes (and probably cell membranes) occurs via severely bent lipidic structures formed at the point of sufficiently close contact between membranes of appropriate lipid composition. This line of research has also suggested that fusion proteins seem to catalyze fusion in part by reducing the free energy of hydrophobic interstices inherent to the lipidic fusion intermediate structures.

Solid-state NMR investigation of the depth of insertion of protegrin-1 in lipid bilayers using paramagnetic Mn2+

The depth of insertion of an antimicrobial peptide, protegrin-1 (PG-1), in lipid bilayers is investigated using solid-state NMR. Paramagnetic Mn(2+) ions bind to the surface of lipid bilayers and induce distance-dependent dipolar relaxation of nuclear spins. By comparing the signal dephasing of the peptide with that of the lipids, whose segmental depths of insertion are known, we determined the depths of several residues of PG-1 in 1,2 dilauryl-sn-glycero-3-phosphotidylcholine (DLPC) bilayers. We found that residues G2 at the N-terminus and F12 at the beta-turn of the peptide reside near the membrane surface, whereas L5 and V16 are embedded in the acyl chain region. The depths increase in the order of G2 < F12 < L5 < V16. These intensity-dephasing results are confirmed by direct measurement of the paramagnetically enhanced (13)C transverse relaxation rates. The relative depths indicate that PG-1 is tilted from the bilayer normal, which is consistent with independent solid-state NMR measurements of PG-1 orientation in the same lipids (Yamaguchi et al., 2001). They also indicate that PG-1 is fully immersed in the lipid bilayer. However, a quantitative mismatch between the bilayer thickness and PG-1 length suggests a local thinning of the DLPC bilayer by 8-10 A. The depth sensitivity of this Mn(2+) dephasing technique is tunable with the Mn(2+) concentration to focus on different regions of the lipid bilayer.

Probing Polyethylene Glycol-Phospholipid Membrane Interactions Using Enzymes

Interaction of polyethylene glycols (PEGs) with phospholipid membranes leads to aggregation and fusion of membranes. The structural basis of these events in membranes, especially in contact with low-molecular-weight PEGs, is uncertain. Using phsopholipases, a class of interfacially active enzymes, we demonstrate enhanced accessibility of lipid hydrophobic portions in the presence of PEGs. All three phospholipases, i.e., A(2), C, and D, show enhancement of activity in the presence of PEGs. Enhancement of activity does not depend on the size of the vesicle or the presence of proteins in the membrane. Fluorescence quenching of probes buried in the membrane supports the phospholipase data. The utility of phospholipases as probes to monitor local and fine structural changes in the membranes is discussed. Copyright 1999 Academic Press.

Merocyanine 540 as a fluorescence indicator for molecular packing stress at the onset of lamellar-hexagonal transition of phosphatidylethanolamine bilayers

The fluorescence of Merocyanine 540 (MC 540) is sensitive to the molecular packing of membrane lipids. Therefore, the fluorescence of MC 540 is expected to be sensitive to the curvature-related packing stress at the onset of the lamellar-hexagonal phase transition. We measured the fluorescence intensity of MC 540 when the temperatures of lipid bilayers approached their lamellar-hexagonal phase transitions. The fluorescence of MC 540 in the presence of egg and dioleoylphosphatidylethanolamine bilayers increased at the respective lamellar-hexagonal phase transitions of these lipids. Furthermore, increases in fluorescence intensity were also observed at temperatures just below their phase transitions. The enhanced fluorescence was not due to the specific interaction of the dye with the ethanolamine headgroup, because no such increase was observed when the probe was exposed to phosphatidylethanolamines which do not form hexagonal phase within the range of applied temperature. In addition, when the temperature of the lamellar-hexagonal phase transition was shifted, by the addition of a small amount of phosphatidylcholine, the dependence of the fluorescence intensity on temperature was modified accordingly. We postulate that the change of MC 540 fluorescence intensity at temperatures approaching the lamellar-hexagonal phase transition reflects changes in the partition of MC 540 into the fluid lipid phase. The change in partition is influenced by the curvature stress in bilayers at temperatures just below the lamellar-hexagonal phase transition.

Electron-spin resonance study of aggregation of gramicidin in dipalmitoylphosphatidylcholine bilayers and hydrophobic mismatch

The effect of aggregation of gramicidin A' (GA) on the phase structure of dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles was studied by cw-ESR using a chain-labeled lipid (16PC) at temperatures between 30 degrees and 45 degreesC that span the main phase transition of DPPC. Boundary lipids were observed only in dispersions with GA/DPPC molar ratios >1:15, where GA aggregates. Detailed fits by nonlinear least squares (NLLS) methods are consistent with the boundary lipid being characterized by a large negative order parameter ( approximately -0.4), indicative of a dynamic bending of the end of the acyl chain, and a substantially reduced motion, about an order of magnitude slower than that of the bulk lipid. The NLLS analysis compares favorably with a recent two-dimensional Fourier transform ESR study on DPPC/GA vesicles, which accurately discerned the bulk lipid. The detailed ESR observables are discussed in terms of the ordering effect of GA at low concentration of GA, the dissociation of the GA channel and the dynamic bending of the end chain segment of boundary lipid at high concentration of GA, and of HII phase formation induced by GA. It is suggested that these phenomena can be interpreted in terms of the combined effects of partial dehydration of the lipid headgroup by the GA and of the hydrophobic mismatch between GA and DPPC molecules. Substantial hysteresis is observed for heating versus cooling cycles, but only for a GA/DPPC molar ratio >1:15. This is consistent with the aggregation of GA molecules at high concentrations.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Effects of lipid headgroup and packing stress on poly(ethylene glycol)-induced phospholipid vesicle aggregation and fusion

The effect of lipid headgroup and curvature-related acyl packing stress on PEG-induced phospholipid vesicle aggregation and fusion were studied by measuring vesicle and aggregate sizes using the quasi-elastic light scattering and fluorescence energy transfer techniques. The effect of the lipid headgroup was monitored by varying the relative phosphatidylcholine (PC) and phosphatidylethanolamine (PE) contents in the vesicles, and the influence of hydrocarbon chain packing stress was controlled either by the relative amount of PE and PC content in the vesicles, or by the degree of unsaturation of the acyl chains of a series of PEs, e.g., dilinoleoylphosphatidylethanolamine (dilin-PE), lysophosphatidylethanolamine (lyso-PE), and transacylated egg phosphatidylethanolamine (TPE). The PEG threshold for aggregation depends only weakly on the headgroup composition of vesicles. However, in addition to the lipid headgroup, the curvature stress of the monolayer that forms the vesicle walls plays a very important role in fusion. Highly stressed vesicles, i.e., vesicles containing PE with highly unsaturated chains, need less PEG to induce fusion. This finding applies to the fusion of both small unilamellar vesicles and large unilamellar vesicles. The effect of electrostatic charge on vesicle aggregation and fusion were studied by changing the pH of the vesicle suspension media. At pH 9, when PE headgroups are weakly charged, increasing electrostatic repulsion between headgroups on the same bilayer surface reduces curvature stress, whereas increasing electrostatic repulsion between apposing bilayer headgroups hinders intervesicle approach, both of which inhibit aggregation and fusion, as expected.

Temperature-controlled release property of phospholipid vesicles bearing a thermo-sensitive polymer

As a novel temperature-sensitive liposome, dioleoylphosphatidylethanolamine vesicles bearing poly(N-isopropylacrylamide), which shows a lower critical solution temperature (LCST) near 32 degrees C, were designed. Poly(N-isopropylacrylamide) having long alkyl chains which are anchors to the lipid membranes was prepared by radical copolymerization of N-isopropylacrylamide and octadecyl acrylate using azobisisobutyronitrile as the initiator. The copolymer obtained revealed the LCST at about 30 degrees C in an aqueous solution. Dioleoylphosphatidylethanolamine vesicles coated with the copolymer was prepared and release property of the copolymer-coated vesicles was investigated. While release of calcein encapsulated in the copolymer-coated vesicles was limited below 30 degrees C, the release was drastically enhanced between 30 and 35 degrees C. Complete release from the vesicles was achieved within several seconds at 40 degrees C. This temperature-controlled release property of the vesicles can be attributable to stabilization and destabilization of the vesicle membranes induced by the copolymer fixed on the vesicles below and above the LCST, respectively. Moreover, the fluorometric measurement using dioleoyl-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phosphatidylethan ola mine suggested that the extensive release of calcein observed above the LCST is resulted from the bilayer to HII phase transition of the vesicle membranes. Since LCST of the copolymer is controllable, these vesicles might have potential usefulness as a drug delivery system with high temperature-sensitivity.

Bending, hydration and interstitial energies quantitatively account for the hexagonal-lamellar-hexagonal reentrant phase transition in dioleoylphosphatidylethanolamine

We have accounted for the unusual structural change wherein dioleoylphosphatidylethanolamine undergoes a hexagonal-lamellar-hexagonal transition sequence as the water content is reduced systematically. We describe the role played by the energies of bending, hydration, voids in hexagonal interstices, and van der Waals interaction in this transition sequence. We have used the X-ray diffraction and osmotic stress experiments on the two phases to derive the structural parameters and all of the force constants defining the energetics of the hexagonal and lamellar phases. We have calculated the chemical potentials of lipid and water in both phases and derived the phase diagram of the lipid with no free, adjustable parameters. The calculated temperature/osmotic stress and temperature/composition diagrams quantitatively agree with experiment. The reentrant transition appears to be driven by a delicate balance between the hydration energy in the lamellar phase and bending energy in the hexagonal phase, whereas the energy of voids in hexagonal interstices defines its energy scale and temperature range. Van der Waals attraction between the bilayers in the lamellar phase does not appear to be important in this transition.

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.

Structural dimensions and their changes in a reentrant hexagonal-lamellar transition of phospholipids

A hexagonal-lamellar-hexagonal (HII-L-HII) reentrant phase transition sequence on dehydration of dioleoylphosphatidylethanolamine occurs below 22 degrees C. This provides an unusual opportunity to measure how several structural dimensions change during this transition. Using x-ray diffraction, we have measured these dimensions with a hope of gaining some clue about the accompanying internal stresses. The principal dimensions described are molecular areas and molecular lengths projected onto the hexagonal lattice. In contrast with large changes in average area at the polar and hydrocarbon ends of the molecule, a position near the polar group/hydrocarbon interface is one of constant molecular area. It remains constant both as the monolayers curl from changing water content and in the transition from one structure to the other. In the L-to-HII transition, the most obvious change in molecular length is a 25% decrease in the distance between aqueous cylinders, the interaxial direction. There is little change in the interstitial direction, the direction toward the interstice equidistant from three aqueous cylinders. As the hexagonal phase is dehydrated, a number of internal changes in molecular lengths are described. Increases in the interaxial direction are much larger than in the interstitial. Simultaneously however, hydrocarbon chain lengths decrease, and polar group lengths increase. It is likely that molecules move axially and the cylinders become longer with dehydration. These dimensions and their changes might be used in the search for a better understanding of the energetics of molecular packing, of the interpretation of spectroscopic measurements of these phases, and of the mechanics of lipid layers.

Bilayer curvature and certain amphipaths promote poly(ethylene glycol)-induced fusion of dipalmitoylphosphatidylcholine unilamellar vesicles

Unilamellar vesicles of varying and reasonably uniform size were prepared from 1,2-dipalmitoyl-3-sn-phosphatidylcholine (DPPC) by the extrusion procedure and sonication. Quasi-elastic light scattering was used to show that different vesicle preparations had mean (Z-averaged) diameters of 1340, 900, 770, 630, and 358 A (sonicated). Bilayer-phase behavior as detected by differential scanning calorimetry was consistent with the existence of essentially uniform vesicle populations of different sizes. The response of these different vesicles to treatment with poly(ethylene glycol) (PEG) was monitored using fluorescence assays for lipid transfer, contents leakage, and contents mixing, as well as quasi-elastic light scattering. No fusion, as judged by vesicle contents mixing and change in vesicle size, was detected for vesicles of diameter greater than 770 A. The diameters of smaller vesicles increased dramatically when treated with high concentrations of PEG, although mixing of their contents could not be detected both because of their small trapped volumes and because of the extensive leakage induced in small vesicles by high concentrations of PEG. Lipid transfer was detected between vesicles of all sizes. We conclude the high bilayer curvature does encourage fusion of closely juxtaposed membrane bilayers but that highly curved vesicles appear also to rupture and form larger structures when diluted from high PEG concentration, a process that can be confused with fusion. Despite the failure of PEG to induce fusion of large, uncurved vesicles composed of a single phosphatidylcholine, these vesicles can be induced to fuse when they contain small amounts of certain amphiphathic compounds thought to play a role in cellular fusion processes. Thus, vesicles which contained 0.5 mol % L-alpha-lysopalmitoylphosphatidylcholine, 5 mol % platelet activating factor, or 0.5 mol % palmitic acid fused in the presence of 30%, 25%, and 20% (w/w) PEG, respectively. However, vesicles containing 1,2-dipalmitoyl-sn-glycerol, 1,2-dioleoyl-sn-glycerol, 1-oleoyl-2-acetyl-sn-glycerol, or monooleoyl-rac-glycerol at surface concentrations up to 5 mol % did not fuse in the presence or absence of PEG. There was no correlation between the abilities of these amphipaths to induce phase separation or nonlamellar phases and their abilities to support fusion of pure DPPC unilamellar vesicles in the presence of high concentrations of PEG. The results are discussed in terms of the type of disrupted lipid packing that could be expected to favor PEG-mediated fusion.

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.

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.

Extended hydrogen-bonded structures of phosphatidylethanolamine

The structure of phosphatidylethanolamine in pure dry hexane was studied. Viscosity measurements show that the hexane solution of PE has a very high viscosity, while freeze fracture electron microscopy revealed extensive fibre-like structures. These extended structures are disrupted by the addition of small amounts of water or organic solvents which are capable of hydrogen-bonding. The Fourier transform infrared spectra of the lipid solutions in dry and hydrated hexane show considerable differences in the phosphate and ethanolamine absorption bands, and demonstrate that the viscous fibre-like structures formed by phosphatidylethanolamine in dry hexane consist of extended intermolecular hydrogen-bonds, similar to those found in the solid lipid, with the ammonium group as the hydrogen-donor and the phosphate group as the hydrogen-acceptor. The high viscosity is not observed in hexane solution of phosphatidylcholine.

Ethylene glycol causes acyl chain disordering in liquid-crystalline, unsaturated phospholipid model membranes, as measured by 2H NMR

2H NMR has been used to probe the effects of ethylene glycol at the level of the acyl chains in liposomes prepared from dioleoylphosphatidic acid or dioleoylphosphatidylcholine, labeled with 2H at the 11-position of both oleic acid chains. Increasing concentrations of ethylene glycol lead to a proportional and substantial decrease in the quadrupolar splittings, measured from the 2H NMR spectra of both liposomal systems, indicative of acyl chain disordering.

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.