Detection of surface charge-related properties in model membrane systems by aqueous two-phase partition

Interactions between Phase-Separated Liquids and Membrane Surfaces

Liquid-liquid phase separation has recently emerged as an important fundamental organizational phenomenon in biological settings. Most studies of biological phase separation have focused on droplets that "condense" from solution above a critical concentration, forming so-called "membraneless organelles" suspended in solution. However, membranes are ubiquitous throughout cells, and many biomolecular condensates interact with membrane surfaces. Such membrane-associated phase-separated systems range from clusters of integral or peripheral membrane proteins in the plane of the membrane to free, spherical droplets wetting membrane surfaces to droplets containing small lipid vesicles. In this review, we consider phase-separated liquids that interact with membrane surfaces and we discuss the consequences of those interactions. The physical properties of distinct liquid phases in contact with bilayers can reshape the membrane, and liquid-liquid phase separation can construct membrane-associated protein structures, modulate their function, and organize collections of lipid vesicles dynamically. We summarize the common phenomena that arise in these systems of liquid phases and membranes.

Coenzyme Q10 in vesicles composed of archaeal ether lipids or conventional lipids enhances the immuno-adjuvanticity to encapsulated protein

Cellular accumulation, tissue distribution, and immuno-adjuvanticity were evaluated for liposomal CoQ10 prepared from either distearoylphosphatidylcholine:dicetylphosphate:cholesterol (4:1:5, mol. ratio) (conventional liposomes) or from the total polar lipids of the archaeon Methanosarcina mazei (archaeosomes). Liposomal CoQ10 vesicles of approximately 100 nm diameter, containing up to 179 mumol of CoQ10 per mg of lipid have been evaluated using J774A.1 macrophages and Balb/c mice. Archaeosomes uptake by J774A.1 macrophages was better than with the conventional liposome, and the incorporation of CoQ10 enhanced the uptake of both lipid vesicle types. All vesicle types were detected in the liver and spleen of mice (4-27% of injected dose) within 3 h of intraperitoneal injection. Moreover, incorporation of CoQ10 into lipid vesicles enhanced the immuno-adjuvanticity of both conventional liposomes and archaeosomes, to achieve approximately a doubling in the titres of BSA-specific antibody in sera to 169 and 430 micrograms ml-1, respectively. Increases in IgG1 and IgG2a/2b accounted for most of the CoQ10-induced increases in anti-BSA titres. These results are rationalized on the basis of surface hydrophobicity and opsonization changes induced by the presence of CoQ10 in vesicles. We suggest that liposomal CoQ10 has potential as a new generation of vaccine delivery system to enhance the immune response. Its use as a novel delivery system may be particularly effective under pathological conditions where the occurrence of an oxidative stress condition significantly impairs the immune system functions.

Poly(ethylene glycol) amphiphile adsorption and liposome partition

Surface localized poly(ethylene glycol) (PEG) amphiphiles of type C16:0-EO151 and C18:2-EO151 were studied via ellipsometry at macroscopic, flat methylated silica (MeSi), phosphatidic acid (PA), and phosphatidylcholine (PC) surfaces. At these surfaces the amphiphiles adsorb similarly, in a non-cooperative manner, achieving a plateau (approximately 0.1 PEG chains/nm2) well below amphiphile critical micelle concentration (CMC). The resultant PEG-enriched layers were 10-15 nm thick, with a polymer concentration (approximately 0.07 g/cm3) greater than the PEG-enriched phase of many dextran, PEG aqueous two-phase systems. PEG-amphiphile adsorption (mg/m2) at hydrophobic and phospholipid flat surfaces correlated with changes in the partition (log K) of PC liposomes in such two-phase systems. PEG-amphiphile adsorption at macroscopic surfaces appears to represent a balance between hydrophobic attraction and repulsive intra-chain interactions which promote chain elongation normal to the surface.

Partitioning of high-density lipoproteins in charge-sensitive two-phase systems

Aqueous polymer two-phase systems characterized by a difference in the electrical potential between the upper and the lower phase (charged systems) are useful tools for the detection of changes in the surface charge and hydrophobicity of high-density lipoproteins (HDL). While the large particle size of low-density lipoproteins (LDL) leads to accumulation at the interface, the smaller diameter and the higher surface charge density of the native HDL particles allows partitioning without aggregation at the interface. Charged two-phase systems can be used to check the native state of HDL samples. Moreover, these systems would be suitable for investigating the hydrophobicity and surface charge of modified HDL.

Influence of surface hydrophilicity of liposomes on their interaction with plasma protein and clearance from the circulation: studies with poly(ethylene glycol)-coated vesicles

Well-defined liposome systems have previously established the influence of size, surface charge lipid composition and surface ligands, on in vivo fate and behaviour of model compounds entrapped in liposomes. In the present study, preformed liposomes which quantitatively retain aqueous markers were covalenty coupled via dipalmitoylphosphatidyl-ethanolamine, to the hydrophilic polymer, monomethoxypoly(ethylene glycol) (MPEG 5000). Such liposomes retain the coating in the presence of plasma, and appear to adsorb plasma components more slowly than liposomes without the polymer, shown using an aqueous two-phase partitioning technique. MPEG-coupled liposomes were cleared from the blood circulation up to 30% more slowly than liposomes without MPEG after intravenous administration to mice, despite the unmodified liposomes being of a composition and size shown previously to favour achievement of maximum half-life. It is suggested that the polymer acts as a surface barrier to plasma factors which otherwise bind to liposomes in the blood and accelerate vesicle removal.