Liposomes can mimic virus membranes

Amphiphilic Liquid Crystalline Polymer Micelles That Exhibit a Phase Transition at Body Temperature

Liquid crystalline polymers (LCPs), which exhibit unique structures and properties intermediate between those of liquids and solids, are widely utilized as functional and advanced materials for fabricating optical devices and high-performance fibers. This utility stems from their ability to abruptly change their organized structures and mobilities at their liquid crystalline-isotropic phase transition temperatures, similar to the properties of biological membranes. Despite these numerous potential applications of LCPs, no study on their use in medical applications such as drug delivery has been reported. In the present study, we synthesized amphiphilic side-chain LCPs (LCP-g-OEGs, where OEG is oligo(ethylene glycol)) for medical applications, where the LCP-g-OEGs undergo a nematic-isotropic phase transition at body temperature. The LCP-g-OEGs formed micelles with a diameter of approximately 130 nm in aqueous media. The micelles were stable and did not dissociate in aqueous media even when the temperature exceeded the nematic-isotropic phase transition temperature (T_NI). Although the release of a dye as a model drug from micelles was suppressed at temperatures lower than T_NI, their dye release was drastically enhanced at temperatures higher than T_NI. The LCP-g-OEG micelles regulated dye release reversibly in accordance with stepwise changes in temperature, without undergoing dissociation, differing from the behavior of standard temperature-responsive micelles. The temperature-responsive dye release behavior is induced by dramatic changes in their well-organized and dynamic structures as a result of the nematic-isotropic phase transition. These results demonstrate that the LCP-g-OEG micelles have a lot of medical applications as reversibly stimuli-responsive drug carriers.

DNA-Directed Patterning for Versatile Validation and Characterization of a Lipid-Based Nanoparticle Model of SARS-CoV-2

Lipid-based nanoparticles have been applied extensively in drug delivery and vaccine strategies and are finding diverse applications in the coronavirus disease 2019 (COVID-19) pandemic-from vaccine-component encapsulation to modeling the virus, itself. High-throughput, highly flexible methods for characterization are of great benefit to the development of liposomes featuring surface proteins. DNA-directed patterning is one such method that offers versatility in immobilizing and segregating lipid-based nanoparticles for subsequent analysis. Here, oligonucleotides are selectively conjugated onto a glass substrate and then hybridized to complementary oligonucleotides tagged to liposomes, patterning them with great control and precision. The power of DNA-directed patterning is demonstrated by characterizing a novel recapitulative lipid-based nanoparticle model of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-S-liposomes-that presents the SARS-CoV-2 spike (S) protein on its surface. Patterning a mixture of S-liposomes and liposomes that display the tetraspanin CD63 to discrete regions of a substrate shows that angiotensin-converting enzyme 2 (ACE2) specifically binds to S-liposomes. Subsequent introduction of S-liposomes to ACE2-expressing cells tests the biological function of S-liposomes and shows agreement with DNA-directed patterning-based assays. Finally, multiplexed patterning of S-liposomes verifies the performance of commercially available neutralizing antibodies against the two S variants. Overall, DNA-directed patterning enables a wide variety of custom assays for the characterization of any lipid-based nanoparticle.

Nanomedicine strategies to target coronavirus

With the severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002, the middle east respiratory syndrome CoV (MERS-CoV) in 2012 and the recently discovered SARS-CoV-2 in December 2019, the 21st first century has so far faced the outbreak of three major coronaviruses (CoVs). In particular, SARS-CoV-2 spread rapidly over the globe affecting nearly 25.000.000 people up to date. Recent evidences pointing towards mutations within the viral spike proteins of SARS-CoV-2 that are considered the cause for this rapid spread and currently around 300 clinical trials are running to find a treatment for SARS-CoV-2 infections. Nanomedicine, the application of nanocarriers to deliver drugs specifically to a target sites, has been applied for different diseases, such as cancer but also in viral infections. Nanocarriers can be designed to encapsulate vaccines and deliver them towards antigen presenting cells or function as antigen-presenting carriers themselves. Furthermore, drugs can be encapsulated into such carriers to directly target them to infected cells. In particular, virus-mimicking nanoparticles (NPs) such as self-assembled viral proteins, virus-like particles or liposomes, are able to replicate the infection mechanism and can not only be used as delivery system but also to study viral infections and related mechanisms. This review will provide a detailed description of the composition and replication strategy of CoVs, an overview of the therapeutics currently evaluated in clinical trials against SARS-CoV-2 and will discuss the potential of NP-based vaccines, targeted delivery of therapeutics using nanocarriers as well as using NPs to further investigate underlying biological processes in greater detail.

Soft matter science and the COVID-19 pandemic

Much of the science underpinning the global response to the COVID-19 pandemic lies in the soft matter domain. Coronaviruses are composite particles with a core of nucleic acids complexed to proteins surrounded by a protein-studded lipid bilayer shell. A dominant route for transmission is via air-borne aerosols and droplets. Viral interaction with polymeric body fluids, particularly mucus, and cell membranes controls their infectivity, while their interaction with skin and artificial surfaces underpins cleaning and disinfection and the efficacy of masks and other personal protective equipment. The global response to COVID-19 has highlighted gaps in the soft matter knowledge base. We survey these gaps, especially as pertaining to the transmission of the disease, and suggest questions that can (and need to) be tackled, both in response to COVID-19 and to better prepare for future viral pandemics.

A low voltage nanopipette dielectrophoretic device for rapid entrapment of nanoparticles and exosomes extracted from plasma of healthy donors

An insulator-based dielectrophoresis (iDEP) is a label-free method that has been extensively utilized for manipulation of nanoparticles, cells, and biomolecules. Here, we present a new iDEP approach that can rapidly trap nanoparticles at the close proximity of a glass nanopipette’s tip by applying 10 V/cm direct current (DC) across the pipette’s length. The trapping mechanism was systemically studied using both numerical modeling and experimental observations. The results showed that the particle trapping was determined to be controlled by three dominant electrokinetic forces including dielectrophoretic, electrophoretic and electroosmotic force. Furthermore, the effect of the ionic strength, the pipette’s geometry, and the applied electric field on the entrapment efficiency was investigated. To show the application of our device in biomedical sciences, we demonstrated the successful entrapment of fluorescently tagged liposomes and unlabeled plasma-driven exosomes from the PBS solution. Also, to illustrate the selective entrapment capability of our device, 100 nm liposomes were extracted from the PBS solution containing 500 nm polystyrene particles at the tip of the pipette as the voltage polarity was reversed.

Self-assembly and functionalization of alternating copolymer vesicles

This work reports novel alternating copolymer vesicles and their facile functionalization with carboxyl and amino groups through click copolymerization.

Use of solid-state nanopores for sensing co-translocational deformation of nano-liposomes

This works reports detection of electric field and hydrodynamic stress induced deformation of sub-100 nm liposomes during translocation through solid-state nanopore.

Organelle-mimicking liposome dissociates G-quadruplexes and facilitates transcription

Important biological reactions involving nucleic acids occur near the surface of membranes such as the nuclear membrane (NM) and rough endoplasmic reticulum (ER); however, the interactions between biomembranes and nucleic acids are poorly understood. We report here that transcription was facilitated in solution with liposomes, which mimic a biomembrane surface, relative to the reaction in a homogeneous aqueous solution when the template was able to form a G-quadruplex. The G-quadruplex is known to be an inhibitor of transcription, but the stability of the G-quadruplex was decreased at the liposome surface because of unfavourable enthalpy. The destabilization of the G-quadruplex was greater at the surface of NM- and ER-mimicking liposomes than at the surfaces of liposomes designed to mimic other organelles. Thermodynamic analyses revealed that the G-rich oligonucleotides adopted an extended structure at the liposome surface, whereas in solution the compact G-quadruplex was formed. Our data suggest that changes in structure and stability of nucleic acids regulate biological reactions at membrane surfaces.

Cholesterol, regulated exocytosis and the physiological fusion machine

Exocytosis is a highly conserved and essential process. Although numerous proteins are involved throughout the exocytotic process, the defining membrane fusion step appears to occur through a lipid-dominated mechanism. Here we review and integrate the current literature on protein and lipid roles in exocytosis, with emphasis on the multiple roles of cholesterol in exocytosis and membrane fusion, in an effort to promote a more molecular systems-level view of the as yet poorly understood process of Ca2+-triggered membrane mergers.

Physical and biological properties of cationic triesters of phosphatidylcholine

The properties of a new class of phospholipids, alkyl phosphocholine triesters, are described. These compounds were prepared from phosphatidylcholines through substitution of the phosphate oxygen by reaction with alkyl trifluoromethylsulfonates. Their unusual behavior is ascribed to their net positive charge and absence of intermolecular hydrogen bonding. The O-ethyl, unsaturated derivatives hydrated to generate large, unilamellar liposomes. The phase transition temperature of the saturated derivatives is very similar to that of the precursor phosphatidylcholine and quite insensitive to ionic strength. The dissociation of single molecules from bilayers is unusually facile, as revealed by the surface activity of aqueous liposome dispersions. Vesicles of cationic phospholipids fused with vesicles of anionic lipids. Liquid crystalline cationic phospholipids such as 1, 2-dioleoyl-sn-glycero-3-ethylphosphocholine triflate formed normal lipid bilayers in aqueous phases that interacted with short, linear DNA and supercoiled plasmid DNA to form a sandwich-structured complex in which bilayers were separated by strands of DNA. DNA in a 1:1 (mol) complex with cationic lipid was shielded from the aqueous phase, but was released by neutralizing the cationic charge with anionic lipid. DNA-lipid complexes transfected DNA into cells very effectively. Transfection efficiency depended upon the form of the lipid dispersion used to generate DNA-lipid complexes; in the case of the O-ethyl derivative described here, large vesicle preparations in the liquid crystalline phase were most effective.

O-ethylphosphatidylcholine: A metabolizable cationic phospholipid which is a serum-compatible DNA transfection agent

1,2-dioleoyl-sn-glycero-3-ethylphosphocholine was prepared in a one-step reaction from phosphatidylcholine by reaction with ethyl trifluoromethanesulfonate. This and related O-alkyl phosphatidylcholines constitute the first chemically stable triesters of biological lipid structures and the first cationic derivatives of phospholipids consisting entirely of biological metabolites linked with ester bonds. The complex of cationic phospholipid and plasmid DNA transfected cells with high efficiency. Maximum efficiency of transfection was obtained with complexes in which the positive charge was a few percent in excess over the negative charge. Modest stimulation of transfection of common cell lines was obtained by continuous culture in the presence of 10% serum. Incubation of the phospholipid complex for at least 2 h at 37 degrees C in nearly pure serum had no deleterious effects on transfection efficiency. The lipid has low toxicity; BHK cells tolerated amounts of 2 mg/2 x 10(6) cells at concentrations of 1 mg/mL. The lipid is biodegradable; it was hydrolyzed by phospholipase A(2) in vitro and was metabolized with a half-life of a few days in cells in culture. The synthetic route to cationic phospholipids is well suited to the preparation of derivatives that are tailor-made to have a wide variety of different properties.

To fuse or not to fuse: the effects of electrostatic interactions, hydrophobic forces, and structural amphiphilicity on protein-mediated membrane destabilization

The development of lipid-based delivery vehicles for therapeutic molecules has become a topic of intense research. Recently, much of this effort has been directed towards mimicking the characteristics of viruses that give them an advantage for the delivery of genetic medicines. One of the most desirable properties of viral-based vectors is the ability to promote the destabilization of the host cell membrane to allow the entry of the genetic medicine into the target cell. This has been found to be largely controlled by the coat proteins on the surface of enveloped viruses. Although the exact mechanism by which proteins involved in the fusion process are able to promote the destabilization of membranes has yet to be elucidated, much understanding based upon information gained from a wide variety of studies is advancing the state of knowledge in this area. Parameters such as hydrophobic and electrostatic interactions as well as structural amphiphilicity, control to a large extent, the nature of the interaction of proteins with membranes. Thus, membrane fusion is mediated primarily by these forces acting in concert with one another. Ultimately, the knowledge gained from these studies will help to develop the ideal delivery system for the next generation of therapeutics.

Cationic liposomes (Lipofectin) mediate retroviral infection in the absence of specific receptors

We have used cationic liposomes (Lipofectin) to facilitate retrovirus infection of cells lacking the homologous viral receptor. Ecotropic murine leukemia virus and packaged retroviral vectors were shown to infect mink cells, and amphotropic packaged retroviral vectors were shown to infect hamster cells in the presence of Lipofectin but not in the presence of Polybrene. Lipofectin-mediated infection of cells lacking the homologous receptor results in a titer approximately 0.1% of the titer in cells with the homologous receptor, using the standard Polybrene protocol. The use of Lipofectin may provide a simple means to experimentally infect a wide variety of cells with viruses not normally infectious for the species, tissue, or cell type of interest.

Delivery of nitroxide spin label to cultured cells by liposomes

The positively charged nitroxide spin label, 2,2,6,6-tetramethyl-piperidine-N-oxyl-4-trimethylammonium (Cat1), was encapsulated in two types of liposomes, phosphatidylserine/phosphatidylcholine (PS/PC) and phosphatidylserine/distearoylphosphatidylcholine/dipalmitoylphosphatidyl choline (PS/DSPC/DPPC). The liposomes were incubated with mouse thymus-bone marrow (TB) cells to study the uptake and metabolism of nitroxides entrapped in liposomes. The effects of temperature, metabolic inhibitors, and fixation of cells were investigated. The results indicate that different mechanisms are involved in the uptake of these two types of liposomes. PS/PC liposomes interact predominantely with the plasma membrane of TB cells and release Cat1 continuously, whereas the majority of PS/DSPC/DPPC liposomes are taken into the cells intact via endocytosis. These findings suggest that it may be possible to deliver nitroxides selectively, either to the membrane of cells or to their interior by manipulating the lipid composition of the liposomes. This study also found that the rate of reduction of Cat1 delivered using liposomes was increased under hypoxic conditions. Thus, the use of liposomes for in vivo delivery of nitroxides has the potential to provide NMR contrast that reflects different metabolic conditions.

Activation of purified simian virus 40 virions by free amino-group containing phospholipid liposomes

The infectivity of the simian virus 40 (SV40) virions purified after treatment with sodium deoxycholate is activated by mixing, prior to infection, the virions with the liposomes composed of phosphatidylserine or a mixture of phosphatidylethanolamine and phosphatidylcholine (H. Shimura, and G. Kimura (1985), Virology 144, 268-272). The sucrose-CsCl cushion sedimentation analysis of the virions mixed with the liposomes revealed that the density of the radiolabeled virions became lower and that of the radiolabeled liposomes became higher to give a similar range, suggesting the binding of virions with the liposomes. Electron microscopy revealed the side-to-side association of virions with liposomes. The efficiency of adsorption of the virions to monkey kidney BSC-1 cells varied depending on phospholipid types mixed with virions and did not always become high. In the case of phosphatidylethanolamine liposomes, the free amino group in the phospholipid molecule was essential for the activation of the virion infectivity, because mono- and di-methylated phosphatidylethanolamine failed to activate the infectivity. Fluid nature of phospholipids seemed to be necessary also for the infectivity activation, because dipalmitoyl and distearoyl phospholipids did not activate virion infectivity at 37 degrees, the temperature at which the liposomes of these phospholipids are supposed to be in a solid state. Presence of free amino groups and difference in acyl groups of the phospholipids did not influence the adsorption of the virions to cells. These results suggest that events which occur after adsorption of virions to cells are responsible for the activation of the SV40 virion infectivity by the liposomes composed of free amino-group containing phospholipids.

Dynamics of antibody- and lectin-mediated endocytosis of hapten-containing liposomes by murine macrophages

The uptake by murine macrophages of liposomes, exhibiting one of a variety of haptenic groups on their surfaces, was greatly enhanced by the addition of an intact antibody or a lectin specific for the incorporated hapten. The uptake of untreated liposomes was slow and linear over long periods, whereas upon addition of the antibody or lectin, over 30-fold increase in the maximal rate of uptake was observed. The process reached a plateau after 90-120 min. The interaction of the antibody- or lectin-treated liposome with the macrophages apparently resulted in an active endocytosis of soluble fluorescent, intraliposomal marker had a granular intracellular pattern in treated cells. The uptake was sensitive to azide and the liposome constituents could not be detected at the cell surface. The size of the liposomes as well as the state of stimulation of the macrophages (thioglycollate stimulated vs. normal) did not seem to have a major effect on the phagocytic process. The time required to reach the plateau in uptake was independent of liposome composition or antibody concentration and is, apparently, an intrinsic property of the cells. The implication of this phenomenon on the dynamics of the relevant macrophage receptors is discussed.

Immunospecific targeting of liposomes to cells: a novel and efficient method for covalent attachment of Fab' fragments via disulfide bonds

An efficient method for covalently cross-linking 50K Fab' antibody fragments to the surface of lipid vesicles is reported. Coupling up to 600 microgram of Fab'/mumol of phospholipid (about 6000 Fab' molecules per 0.2-micrometer vesicle) is achieved via a disulfide interchange reaction between the thiol group exposed on each Fab' fragment and a pyridyldithio-derivative of phosphatidylethanolamine present in low concentration in the membranes of preformed large unilamellar vesicles. The coupling reaction is efficient, proceeds rapidly under mild conditions, and yields well-defined products. Each vesicle-linked Fab' fragment retains its original antigenic specificity and full capacity to bind antigen. We have used Fab' fragments, coupled to vesicles by this method, to achieve immunospecific targeting of liposomes to cells in vitro. Vesicles bearing antihuman erythrocyte Fab' fragments bind quantitatively to human erythrocytes (at multiplicities up to 5000 0.2-micrometer vesicles per cell) while essentially no binding is observed to sheep or ox red blood cells. Vesicle-cell binding is stable over a pH range from 6 to 8 and is virtually unaffected by the presence of human serum (50%). Cell-bound vesicles retain their aqueous contents and can be eluted intact from cells by treatment with reducing agents (dithiothreitol or mercaptoethanol) at alkaline pH.

On the mode of liposome-cell interactions. Biotin-conjugated lipids as ultrastructural probes

An efficient method for labeling and visualizing phospholipids at the ultrastructural level is described. Biotin was coupled to the amines of appropriate phospholipids via the N-hydroxysuccinimide ester. The biotinylated lipid could be specifically labeled by ferritin-avidin conjugates and detected by transmission electron microscopy. The lipid derivatives were analyzed and evaluated in terms of their resemblance to the original lipid. Although differing in some aspects from the parent lipid molecules, the biotinyl derivatives still retain the basic characteristics of lipids vis-a-vis their orientation and position in the membrane bilayer. The latter property renders the biotinylated lipid qualitatively suitable for tracing the fate of the lipid component(s) of liposomes during their interaction with biological membranes of various cell types. Using this system, we propose that the extent and pattern of the liposome-cell interaction depends, at least in part, on the cell type employed. This observation may be due to intrinsic variations in cell surface structure and properties relative to those of the liposome.

Fusion of isolated myoblast plasma membranes. An approach to the mechanism

Fusion of plasma membranes isolated from myoblasts grown in culture has been investigated. 1. Membrane fusion was specifically dependent of Ca2+ at physiological concentrations. However, at higher concentrations of cations, fusion could be triggered not only by Ca2+, but by Mg2+ and Sr2+ as well. 2. The amount of fusion was directly proportional to temperature. 3. Fusion was found to depend on the state of maturation of the myoblast membranes. 4. Experiments with chemically and enzymatically modified membranes and with membranes derived from myoblasts grown in the presence of inhibitors of protein biosynthesis suggest the participation of proteinaceous membrane components in the fusion mechanism.

Direct modification of the glycocalyx of a cultured muscle cell line by incorporation of foreign gangliosides and an integral membrane glycoprotein

As part of a program to better understand the cause-or-effect nature of the relationship between cell surface carbohydrate and cell properties and behaviour, experiments have been carried out on direct modification of the glycocalyx of cultured cells. Modification was by incorporation of gangliosides and an integral membrane glycoprotein chosen to be dissimilar to species occurring naturally in the cell line. Two methods of incorporation were investigated: simple addition of the new components to the culture medium for various times, or assembly of the components into the walls of lipid vesicles which were subsequently fused with cells. Gangliosides from beef brain and glycophorin, the major human erythrocyte sialoglycoprotein, were successfully added to the surface of myoblasts in quantities sufficient to represent a significant perturbation. Changes in cell adhesion, morphology, and viability were observed which seem to be a direct result of glycocalyx modification.

Phospholipid exchange between bilayer membranes

The mode of interaction of aqueous dispersions of phospholipid vesicles is investigated. The vesicles (average diameter 950 A) are prepared from total lipid extracts of Escherichia coli composed of phosphatidylethanolamine, phosphatidylglycerol and cardiolipin. One type of vesicle contains trans-delta 9-octadecenoate, the other type trans-delta 9-hexadecenoate as predominant acyl chain component. The vesicles show order in equilibrium disorder transitions at transition temperatures, Tt = 42 degrees C and Tt = 29 degrees C, respectively. A mixture of these vesicles is incubated at 45 degrees C and lipid transfer is studied as a function of time using the phase transition as an indicator. The system reveals the following properties: Lipids are transferred between the two vesicle types giving rise to a vesicle population where both lipid components are homogeneously mixed. Lipid transfer is asymmetric, i.e. trans-delta 9-hexadecenoate-containing lipid molecules appear more rapidly in the trans-delta 9-octadecenoate-containing vesicles than vice versa. At a given molar ratio of the two types of vesicles the rate of lipid transfer is independent of the total vesicle concentration. It is concluded that lipid exchange through the water phase by way of single molecules or micelles is the mode of communication of these negatively charged lipid vesicles.

The plasma membrane consists of two polar-nonpolar-polar leaflets

The implications of a double polar-nonpolar-polar leaflet construction of the plasma membrane are investigated. Experimental data from transmission electron microscopic and enzymologic characterization of plasma membranes are advantageously interpreted in these terms compared to interpretation in terms of lipid bilayer. X-ray diffraction and electron spin resonance studies do not differentiate between the present and previous models for the structure of plasma membranes but electron spin resonance data that fail to indicate a statistical distribution of spin labels also fail to support the fluid mosaic model for cell membranes. Results from experiments involving vectorial digestion and labelling of plasma membranes as well as freeze fracture electron microscopic data are compatible with the present model. The molecular composition of the human erythrocyte membrane is investigated whereby the band III protein and glycophorin are suggested to be the structural proteins of the outer leaflet and the spectrins those of the inner leaflet.

Lipid vesicle-cell interactions. II. Induction of cell fusion

The ability of lipid vesicles of simple composition (lecithin, lysolecithin, and stearylamine) to induce cells of various types to fuse has been investigated. One in every three or four cells in monolayer cultures can be induced to fuse with a vesicle dose of about 100 per cell. At such dosages and for exposures of 15 min to 1 h, vesicles have essentially no effect on cell viability. Under anaerobic conditions, these cells lyse rather than fuse. Avian erythrocytes are readily fused with lipid vesicles in the presence of dextran. Fusion indices increase linearly with the zeta potential of the vesicles (increasing stearylamine content), indicating that contact between vesicle and cell membrane is required. Fusion indices increase sublinearly with increasing lysolecithin content. Divalent cations increase fusion indices at high vesicle doses. The data presented are consistent with the hypothesis that cell fusion occurs via simultaneous fusion of a vesicle with two adhering cell membranes.

Lipid vesicle-cell interactions. I. Hemagglutination and hemolysis

The interaction of lipid vesicles (liposomes) of several different compositions with erythrocytes has been investigated. Lecithin liposomes, rendered positively charged with stearylamine, exhibit potent hemagglutination activity in media containing low concentrations of electrolytes. The hemagglutination titer is found to be a linear function of the zeta potential of the lipid vesicles. Hemagglutination is reduced when the surface potential of the cells is made more positive by pH adjustment or enzyme treatment. Similarly, hemagglutination is reduced by increasing concentrations of electrolytes. Hemagglutination is examined theoretically and is shown to be consistent with vesicle-cell interactions that are due to only electrostatic forces. Vesicles containing lysolecithin in addition to lecithin and stearylamine cause lysis of erythrocytes, provided the lipids of the vesicles are above the crystal-liquid crystal phase transition temperature. In addition, hemolysis requires close juxtaposition of the vesicle to the cell membrane; vesicles precoated with antibodies exhibit severely diminished hemolytic activities, only a small fraction of which can be attributed to a reduction in hemagglutination titer. Evidence is presented indicating that a single vesicle is sufficient to lyse one cell. With regard to hemagglutination and hemolysis, lipid vesicles of simple composition mimic paramyxoviruses such as Sendai virus.

Lipid vesicles as carriers for introducing materials into cultured cells: influence of vesicle lipid composition on mechanism(s) of vesicle incorporation into cells

The mechanisms involved in the uptake of uni- and multi-lamellar lipid vesicles by BALB/c mouse 3T3 cells have been investigated. Vesicles are incorporated into cells both by endocytosis and by a nonendocytotic mechanism which we propose involves fusion of vesicles with the plasma membrane. The nonendocytotic pathway predominates in the uptake of negatively charged vesicles composed of phospholipids that are "fluid" (phosphatidylserine/phosphatidylcholine) at 37 degrees. Neutral fluid vesicles (phosphatidylcholine) and negatively charged vesicles prepared from "solid" phospholipids (phosphatidylserine/distearylphosphatidylcholine/dipalmitoylphosphatidylcholine) are instead incorporated largely by endocytosis. Uptake of the latter classes of vesicle is reduced (80-90% inhibition) by inhibitors of cellular energy metabolism and by cytochalasin B.

Fusion of dipalmitoylphosphatidylcholine vesicle membranes induced by concanavalin A

The temperature dependence of fatty acid spin label resonance spectra and freeze fracture micrographs of sonicated dipalmitoylphosphatidylcholine vesicles in the absence and presence of concanavalin A demonstrate a strong interaction of concanavalin A with these lipid membranes, which results in fusion of the vesicles. The rate of this reaction as followed with use of magnetic resonance exhibits a pronounced maximum at 36 degrees, the midpoint of the phase transition range of dipalmitoylphosphatidylcholine vesicles. This maximum is discussed in terms of structural fluctuations, which are maximal in the phase transition range of the membranes.

Interaction of phospholipid vesicles with cultured mammalian cells. II. Studies of mechanism

The mechanism of interaction of artificially generated lipid vesicles (approximately 500 A diameter) with Chinese hamster V79 cells bathed in a simple balanced salt solution was investigated. The major pathways of exogenous lipid incorporation in vesicle-treated cells are vesicle-cell fusion and vesicle-cell lipid exchange. At 37 degrees C, the fusion process is dominant, while at 2 degrees C or with energy depleted cells, exchange of lipids between vesicles and cells is important. The fusion mechanism was demonstrated using vesicles of [14C]lecithin containing trapped [13H]inulin. Consistent with a fusion hypothesis, both components became cell associated at 37 degrees C in nearly the same proportions as they were present in the applied vesicles. Additional arguments in favor of vesicle-cell fusion and against phagocytosis or adsorption of intact vesicles are presented. At 2 degrees C or with inhibitor-treated cells, the [3H]inulin uptake was largely suppressed, while the lipid uptake was reduced to a lesser extent. Evidence for vesicle-cell lipid exchange was obtained using V79 cells grown on 3H precursors for cellular lipids. [14C]lecithin vesicles, incubated with such cells, showed no change in their elution properties when subjected to molecular sieve chromatography on Sepharose 4B. However, radioactivity and thin-layer chromatographic analyses revealed that a variety of cell lipiids had been exchanged into the uniamellar vesicles. Further evidence for the fusion and exchange processes was obtained using vesicles prepared from mixtures of [3H]lecithin and [14C]cholesterol. A two-step fusion mechanism consistent with the present findings is proposed as a working model for other fusion studies.

Interaction of phospholipid vesicles with cells. Endocytosis and fusion as alternate mechanisms for the uptake of lipid-soluble and water-soluble molecules

Depending on their phospholipid composition, liposomes are endocytosed by, or fuse with, the plasma membrane, of Acanthamoeba castellanii. Unilamellar egg lecithin vesicles are endocytosed by amoeba at 28 degrees C with equal uptake of the phospholipid bilayer and the contents of the internal aqueous space of the vesicles. Uptake is inhibited almost completely by incubation at 4 degrees C or in the presence of dinitrophenol. After uptake at 28 degrees C, the vesicle phospholipid can be visualized by electron microscope autoradiography within cytoplasmic vacuoles. In contrast, uptake of unilamellar dipalmitoyl lecithin vesicles and multilamellar dipalmitoyl lecithin liposomes is only partially inhibited at 4 degrees C, by dinitrophenol and by prior fixation of the amoebae with glutaraldehyde, each of which inhibits pinocytosis. Vesicle contents are taken up only about 40% as well as the phospholipid bilayer. Electron micrographs are compatible with the interpretation that dipalmitoyl lecithin vesicles fuse with the amoeba plasma membrane, adding their phospholipid to the cell surface, while their contents enter the cell cytoplasm. Dimyristoyl lecithin vesicles behave like egg lecithin vesicles while distearoyl lecithin vesicles behave like dipalmitoyl lecithin vesicles.