PEGylated lipids impede the lateral diffusion of adsorbed proteins at the surface of (magneto)liposomes

Protein binding to nanoparticles is a crucial issue in biomedicine, as it triggers their clearance from the bloodstream after intravenous injection. Many techniques are available for measuring strong protein binding interactions, but weak dynamic interactions are more difficult to assess. To tackle the latter problem, in the present work, cytochrome c was chosen as a representative model of a water-soluble protein and the adsorbing particulates were either small unilamellar phospholipid vesicles or 14 nm diameter solid superparamagnetic iron oxide cores onto which a phospholipid bilayer was strongly chemisorbed (so-called magnetoliposomes). Incorporation of cytochrome c oxidase into the phospholipid bilayer allowed the association of cytochrome c with the surface of the particles to be measured with high sensitivity by VIS-spectrophotometry. The impact of enzyme density as well as some of the physical features of the PEG corona (degree of PEGylation and PEG chain length) adjacent to the surface of the lipid structures on the overall kinetics was also investigated.

Cationic magnetoliposomes

Magnetoliposomes (MLs) consist of nanosized, magnetisable iron oxide cores (magnetite, Fe(3)O(4)) which are individually enveloped by a bilayer of phospholipid molecules. To generate these structures, the so-called water-compatible magnetic fluid is first synthesized by co-precipitation of Fe(2+) and Fe(3+) salts with ammonia and the resulting cores are subsequently stabilized with lauric acid molecules. Incubation and dialysis of this suspension with an excess of sonicated, small unilamellar vesicles, ultimately, results in phospholipid-Fe(3)O(4) complexes which can be readily captured from the solution by high-gradient magnetophoresis (HGM), reaching very high yields. Examination of the architecture of the phospholipid coat reveals the presence of a typical bilayered phospholipid arrangement. Cationic MLs are then produced by confronting MLs built up of zwitterionic phospholipids with vesicles containing the relevant cationic lipid, followed by fractionation of the mixture in a second HGM separation cycle. Data, published earlier by our group (Soenen et al., ChemBioChem 8:2067-2077, 2007) prove that these constructs are unequivocal biocompatible imaging agents resulting in a highly efficient labeling of biological cells.

A Successful Strategy for the Production of Cationic Magnetoliposomes

The present work describes a strategy and the mechanistic background for synthesizing magnetoliposomes (MLs) in which the bi-layered coating is partly composed of positively charged lipids. In a first step, neutral MLs are prepared from zwitterionic phosphatidylcholine vesicles and magnetite nanocores stabilized with an anionic surfactant, and then incubated with cationic vesicles formed from phosphatidylcholine and dioleoyltrimethylammoniumpropane. During this latter step, spontaneous intermembrane lipid transfer occurs. After reaching an equilibrium state, the desired cationic MLs are captured from the mixture in a high-gradient magnetophoresis setup.

Stable long-term intracellular labelling with fluorescently tagged cationic magnetoliposomes

Iron oxide nanocrystals that are dextran coated are widely exploited biomedically for magnetic resonance imaging (MRI), hyperthermia cancer treatment and drug or gene delivery. In this study, the use of an alternative coating consisting of a phospholipid bilayer directly attached to the magnetite core is described. The flexible nature of the magnetoliposome (ML) coat, together with the simple production procedure, allows rapid and easy modification of the coating, offering many exciting possibilities for the use of these particles in biomedical applications. Upon incubation of neutral MLs with an equimolar amount of cationic 1,2-distearoyl-3-trimethylammoniumpropane (DSTAP)-bearing vesicles, approximately one third of the cationic lipids are incorporated into the ML coat. This is in line with a theoretical model predicting transferability of only the outer leaflet phospholipids of bilayer structures. Most interestingly, the use of MLs containing 3.33 % DSTAP with a positive zeta-potential of (31.3+/-7.3) mV (mean +/-SD) at neutral pH, results in very heavy labelling of a variety of biological cells (up to (70.39+/-4.52) pg of Fe per cell, depending on the cell type) without cytotoxic effects. The results suggest the general applicability of these bionanocolloids for cell labelling. Mechanistically, the nanoparticles are primarily taken up by clathrin-mediated endocytosis and follow the endosomal pathway. The fate of the ML coat after internalisation has been studied with different fluorescent lipid conjugates, which because of the unique features of the ML coat can be differentially incorporated in either the inner or the outer layer of the ML bilayer. It is shown that, ultimately, iron oxide cores surrounded by an intact lipid bilayer appear in endosomal structures. Once internalised, MLs are not actively exocytosed and remain within the cell. The lack of exocytosis and the very high initial loading of the cells by MLs result in a highly persistent label, which can be detected, even in highly proliferative 3T3 fibroblasts, for up to at least one month (equivalent to approximately 30 cell doublings), which by far exceeds any values reported in the literature.

Steric stabilization of lipid/polymer particle assemblies by poly(ethylene glycol)-lipids

Biocompatible and biodegradable assemblies consisting of spherical particles coated with lipid layers were prepared from sub-micrometer poly(lactic acid) particles and lipid mixtures composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-3-trimethylammonium-propane. These original colloidal assemblies, named LipoParticles, are of a great interest in biotechnology and biomedicine. Nevertheless, a major limitation of their use is their poor colloidal stability toward ionic strength. Indeed, electrostatic repulsions failed to stabilize LipoParticles in aqueous solutions containing more than 10 mM NaCl. By analogy with the extensive use of poly(ethylene glycol) (PEG)-lipid conjugates to improve the circulation lifetime of liposomes in vivo, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)] with various PEG chain lengths was added to the lipid formulation. Here, we show that LipoParticle stabilization was enhanced at least up to 150 mM NaCl (for more than 1 year at 4 degrees C). To determine the structure of PEG-modified LipoParticles as a function of the PEG chain length and the PEG-lipid fraction in the lipid formulation, a thorough physicochemical characterization was carried out by means of many techniques including quasi-elastic light scattering, zeta potential measurements, transmission electron microscopy, 1H NMR spectroscopy, and small-angle X-ray scattering. Finally, an attempt was made to link the resulting structural data to the colloidal behavior of PEG-modified LipoParticles.

Surface functionalization of magnetoliposomes in view of improving iron oxide-based magnetic resonance imaging contrast agents: Anchoring of gadolinium ions to a lipophilic chelate

Small unilamellar phospholipid vesicles containing the phosphatidylethanolamine-diethylene triamine pentaacetic acid (PE-DTPA) conjugate as one of the building stones were constructed. The ability of these colloids to complex gadolinium(III) ions at the surface of both the inner and outer bilayer shells was verified using a colorimetric method with Arsenazo III as a dye indicator. On incubation of these functionalized vesicles with magnetoliposomes (MLs, nanometer-sized magnetite cores encapsulated in a phospholipid bilayer), PE-DTPA percolates into the ML coat. The PE-DTPA content could be fine-tuned by varying the conjugate concentration in the donor vesicles. In the experimental conditions applied, up to 500 Gd(3+) ions were immobilized per ML colloid. The resulting ML-Gd(3+) complexes might have great potential, for example, as a novel magnetic resonance imaging contrast agent.

An overview of lipid membrane supported by colloidal particles

In recent years, original hybrid assemblies composed of a particle core surrounded by a lipid shell emerged as promising entities for various biotechnological applications. Their broadened bio-potentialities, ranging from model membrane systems or biomolecule screening supports, to substance delivery reservoirs or therapeutic vectors, are furthered by their versatility of composition due to the possible wide variation in the particle nature and size, as well as in the lipid formulation. The synthesis, the characteristics, and the uses of these Lipid/Particle assemblies encountered in the literature so far are reviewed, and classified according to the spherical core size in order to highlight general trends. Moreover, several criteria are particularly discussed: i) the interactions involved between the particles and the lipids, and implicitly the assembly elaboration mechanism, ii) the most suited techniques for an accurate characterization of the entities from structural and physicochemical points of view, and iii) the remarkable properties of the solid-supported lipid membrane obtained.

Receptor-mediated biological responses are prolonged using hydrophobized ligands

Hormone-receptor interactions occur following three-dimensional diffusion of the ligand to the membrane-embedded receptor. However, prior hydrophobization of the ligand might restrict its movement to two dimensions along the membrane surface, and the biological response might therefore be modulated. This idea was tested using the C-terminal nonapeptide, CCK9, of the satiating hormone, cholecystokinin (CCK). The hormone was lipidated by linking it covalently to distearoylphosphatidylethanolamine via a poly(ethylene glycol) (PEG) spacer. The desired conjugate was isolated by thin-layer chromatography and incorporated into preformed small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles. The hormone-bearing vesicles were injected intraperitoneally into Wistar rats and food intake monitored. Compared to the biological effect elicited by the same amount of soluble non-derivatized CCK9, food intake reduction showed a delayed onset, but lasted for a significantly longer time. We believe this prolonged effect was due to the transfer of the derivatized CCK9 from the vesicles to the natural membrane containing the hormone receptor. Ultimately, this event may result in sustained receptor occupation and, thus, food intake reduction. The underlying mechanism for the physiological effects observed may be of relevance in interpreting results obtained using artificial measuring devices; for example, the signal produced by biosensors may be drastically affected by the hydrophobicity of the ligand.