Reconstitution of non-ionic monoalkyl amphiphile-cholesterol vesicles by dilution of lipids-octylglucoside mixed micelles

Surfactant-Mediated Assembly of Precision-Size Liposomes

Liposomes can greatly improve the pharmacokinetics of therapeutic agents due to their ability to encapsulate drugs and accumulate in target tissues. Considerable effort has been focused on methods to synthesize these nanocarriers in the past decades. However, most methods fail to controllably generate lipid vesicles at specific sizes and with low polydispersity, especially via scalable approaches suitable for clinical product manufacturing. Here, we report a surfactant-assisted liposome assembly method enabling the precise production of monodisperse liposomes with diameters ranging from 50 nm to 1 μm. To overcome scalability limitations, we used tangential flow filtration, a scalable size-based separation technique, to readily concentrate and purify the liposomal samples from more than 99.9% of detergent. Further, we propose two modes of liposome self-assembly following detergent dilution to explain the wide range of liposome size control, one in which phase separation into lipid-rich and detergent-rich phases drives the formation of large bilayer liposomes and a second where the rate of detergent monomer partitioning into solution controls bilayer leaflet imbalances that promote fusion into larger vesicles. We demonstrate the utility of controlled size assembly of liposomes by evaluating nanoparticle uptake in macrophages, where we observe a clear linear relationship between vesicle size and total nanoparticle uptake.

[Mechanism of micelle-vesicle transformation and control of vesicular sizes and properties]

The mechanism of vesicle-to-micelle or micelle-to-vesicle transition was studied in order to control sizes and fluidities of vesicles during periods of preparation. Dependence of particle sizes measured by quasi-elastic light scattering, turbidities, fluidity parameters monitored by ESR spectroscopy, and morphological changes of mixed aggregates of egg yolk phosphatidylcholine (EPC) and a detergent (octylglucoside (OG) or sodium cholate (Na-chol)) on detergent concentration provided a model of vesicle destruction. It possessed three phase transition points, and proceeded in a stepwise fashion: vesicles, small particles containing large amounts of detergents (SUV(*)), intermediate structures, and mixed micelles. Vesicle formation on removal of detergents from micelles proceeded oppositely. Micelle-vesicle transition mechanism was common to all detergents examined. The feature of the mechanism was the presence of SUV(*). Next, SUV(*) was prepared by adding appropriate amount of a detergent to small unilamellar vesicles obtained by sonication. Time-dependent size growth of the SUV(*) was remarkable in the case of OG-containing SUV(*), but was insignificant in the case of Na-chol-containing SUV(*), suggesting the size determining step to be the stage of the SUV(*). The tendency to produce large or small vesicles from micelles was related to the absence or presence, respectively, of a net charge in the detergent molecule. The fluidities of EPC micelles containing small amounts of a detergent possessing a steroidal structure (e.g., Na-chol or CHAPS) were significantly smaller than the corresponding values of a detergent without a steroidal structure (e.g., OG), suggesting a method of control of orderliness of hydrocarbon chains in EPC vesicles.

Use of a dialyzable short-chain phospholipid for efficient solubilization and reconstitution of influenza virus envelopes

Virosomes are reconstituted viral envelopes that can serve as vaccines and as vehicles for cellular delivery of various macromolecules. To further advance the use of virosomes, we developed a novel dialysis procedure for the reconstitution of influenza virus membranes that is easily applicable to industrial production and compatible with encapsulation of a variety of compounds. This procedure relies on the use of 1,2-dicaproyl-sn-glycero-3-phosphocholine (DCPC) as a solubilizing agent. DCPC is a short-chain lecithin with detergent-like properties and with a critical micelle concentration of 14 mM. DCPC effectively dissolved the influenza virus membranes after which the nucleocapsids could be removed by ultracentrifugation. The solubilized membrane components were reconstituted either by removal of DCPC by dialysis or by a procedure involving initial dilution of the solubilized membrane components followed by dialysis. Both protocols resulted in removal of 99.9% of DCPC and simultaneous formation of virosomes. Analysis of the virosome preparations by equilibrium sucrose density gradient centrifugation revealed co-migration of phospholipid and protein for virosomes produced by either method. Moreover, both virosome preparations showed morphological and fusogenic characteristics similar to native influenza virus. Size, homogeneity and spike density of the virosomes varied with the two different reconstitution procedures employed. The recovery of viral membrane proteins and phospholipids in the virosomes was found to be higher for the dilution/dialysis procedure than for the simple dialysis protocol. This novel procedure for the production of virosomes is straightforward and robust and allows further exploitation of virosomes as vaccines or as drug delivery vehicles not only in academia, but also in industrial settings.

Characterization of loaded liposomes by size exclusion chromatography

This review focuses on the use of conventional (SEC) and high performance (HPSEC) size exclusion chromatography for the analysis of liposomes. The suitability of both techniques is examined regarding the field of liposome applications. The potentiality of conventional SEC is strongly improved by using a HPLC system associated to gel columns with a size selectivity range allowing liposome characterization in addition to particle fractionation. Practical aspects of size exclusion chromatography are described and a methodology based on HPSEC coupled to multidetection modes for on-line analysis of liposomes via label or substance encapsulation is presented. Examples of conventional SEC and HPSEC applications are described which concern polydispersity, size and encapsulation stability, bilayer permeabilization, liposome formation and reconstitution, incorporation of amphiphilic molecules. Size exclusion chromatography is a simple and powerful technique for investigation of encapsulation, insertion/interaction of substances from small solutes (ions, surfactants, drugs, etc.) up to large molecules (proteins, peptides and nucleic acids) in liposomes.

Vesicle reconstitution from lipid-detergent mixed micelles

The process of formation of lipid vesicles using the technique of detergent removal from mixed-micelles is examined. Recent studies on the solubilization and reconstitution of liposomes participated to our knowledge of the structure and properties of mixed lipid-detergent systems. The mechanisms involved in both the lipid self assembly and the micelle-vesicle transition are first reviewed. The simplistic three step minimum scheme is described and criticized in relation with isothermal as well as a function of the [det]/[lip] ratio, phase diagram explorations. The techniques of detergent elimination are reviewed and criticized for advantages and disadvantages. New methods inducing micelle-vesicle transition using enzymatic reaction and T-jump are also described and compared to more classical ones. Future developments of these techniques and improvements resulting of their combinations are also considered. Proper reconstitution of membrane constituents such as proteins and drugs into liposomes are examined in the light of our actual understanding of the micelle-vesicle transition.

New sterically stabilized vesicles based on nonionic surfactant, cholesterol, and poly(ethylene glycol)-cholesterol conjugates

Monomethoxypoly(ethylene glycol) cholesteryl carbonates (M-PEG-Chol) with polymer chain molecular weights of 1000 (M-PEG1000-Chol) and 2000 (M-PEG2000-Chol) have been newly synthesized and characterized. Their aggregation behavior in mixture with diglycerol hexadecyl ether (C16G2) and cholesterol has been examined by cryotransmission electron microscopy, high-performance gel exclusion chromatography, and quasielastic light scattering. Nonaggregated, stable, unilamellar vesicles were obtained at low polymer levels with optimal shape and size homogeneity at cholesteryl conjugate/ lipids ratios of 10 mol% M-PEG1000-Chol or 5 mol% M-PEG2000-Chol, corresponding to the theoretically predicted brush conformational state of the PEG chains. At 20 mol% M-PEG1000-Chol or 10 mol% M-PEG2000-Chol, the saturation threshold of the C16G2/cholesterol membrane in polymer is exceeded, and open disk-shaped aggregates are seen in coexistence with closed vesicles. Higher levels up to 30 mol% lead to the complete solubilization of the vesicles into disk-like structures of decreasing size with increasing PEG content. This study underlines the bivalent role of M-PEG-Chol derivatives: while behaving as solubilizing surfactants, they provide an efficient steric barrier, preventing the vesicles from aggregation and fusion over a period of at least 2 weeks.

Self-evolving microstructured systems upon enzymatic catalysis

The consequences of cell microstructuration on enzyme functions is discussed in the framework of self-evolving microstructured systems. Molecular assemblies of amphiphiles or lipids are spontaneously formed by self-organisation. Among these different structures, reversed micelles, liquid crystalline mesophases and vesicles are hosts for enzymatic reaction studies. Inside a living cell, phospholipid metabolism is responsible for membrane structural modifications; the catalytic behaviour of lipolytic enzymes, mainly phospholipase (PL) A2, is described in relation with structural aspects of biological membranes. The implication in cellular regulation events of PLC and PLD is discussed in relation with the role of their reaction products as second messengers in membrane fusion processes. The in vitro synthesis of dialkyl phosphatidylcholines, via the enzymatic 'salvage pathway' which leads to the formation of vesicles upon phospholipid formation, is considered in relation with autopoiesis. More recent studies on self-evolving systems based on enzyme-surfactants reactions are detailed. The interactions between amphiphilic aggregates and enzymes allow to explore the OG/octanol/water phase diagram. Enzymatic formation of dipalmitoylphosphatidylcholine (DPPC) liposomes and non-ionic surfactant vesicles (NSV), starting from mixed micelles or open structures, finally sets an example of a biomimetic self-evolving system.

Partition coefficient of a surfactant between aggregates and solution: application to the micelle-vesicle transition of egg phosphatidylcholine and octyl beta-D-glucopyranoside

The mechanism of the solubilization of egg phosphatidylcholine containing 10% (M/M) of egg phosphatidic acid unilamellar vesicles by the nonionic detergent, octyl beta-D-glucopyranoside, has been investigated at both molecular and supramolecular levels by using fluorescence and turbidity measurements. In the lamellar region of the transition, the solubilization process has been shown to be first a function of the initial size before reaching an equilibrium aggregation state at the end of this region (the onset of the micellization process). The analysis during the solubilization process of the evolution of both the fluorescence energy transfer between N-(7-nitro-2,1,3-benzoxadiazol-4-yl)-phosphatidylethanolamine (NBD-PE) and N-(lissamine rhodamine B sulfonyl)-phosphatidylethanolamine (Rho-PE) and the fluorescence of 6-dodecanoyl-2-dimethylaminoaphtalene (Laurdan) has allowed us to determine the evolution of the detergent partitioning between the aqueous and the lipidic phases, i.e., the evolution of the molar fraction of OG in the aggregates (XOG/Lip) with its monomeric detergent concentration in equilibrium ([OG]H2O), throughout the vesicle-to-micelle transition without isolating the aqueous medium from the aggregates. The curve described by XOG/Lip versus [OG]H2O shows that the partition coefficient of OG is changing throughout the solubilization process. From this curve, which tends to a value of 1/(critical micellar concentration), five different domains have been delimited: two in the lamellar part of the transition (for 0 < [OG]H2O < 15.6 mM), one in the micellization part, and finally two in the pure micellar region (for 16.5 < [OG]H2O < 21 mM). The first domain in the lamellar part of the transition is characterized by a continuous variation of the partition coefficient. In the second domain, a linear relation relates XOG/Lip and [OG]H2O, indicating the existence of a biphasic domain for which the detergent presents a constant partition coefficient of 18.2 M-1. From the onset to the end of the solubilization process (domain 3), the evolution of (XOG/Lip) with [OG]H2O can be fitted by a model corresponding to the coexistence of detergent-saturated lamellar phase with lipid-saturated mixed micelles, both in equilibrium with an aqueous phase, i.e., a three-phase domain. The micellar region is characterized first by a small two-phase domain (domain 4) with a constant partition coefficient of 21 M-1, followed by a one-phase mixed-micellar domain for which XOG/Lip no longer linearly depends on [OG]H2O. The results are discussed in terms of a phase diagram.

Ethanol-induced aggregation and fusion of small phosphatidylcholine liposome: participation of interdigitated membrane formation in their processes

The mechanism for the ethanol-induced aggregation/fusion of uniform-sized small liposomes comprised of dipalmitoyl (DPPC) or egg yolk (eggPC) phosphatidylcholines was studied by measuring the average size using a photon correlation spectroscopy, by observing directly the states in the liposomal solutions using freeze-fracture electron microscopy and by attempting resonance energy transfer using flurophore-labeled phospholipids. Abrupt increases in the apparent size of DPPC liposomes were observed in the presence of above 44 mg/ml ethanol, where microscopically plateau membranes form interdigitated structure, in which the acyl chains fully interpenetrate the hydrocarbon chains of the apposing monolayer. On the contrary, in the eggPC liposome, where the membranes cannot form interdigitated structures even in the presence of high concentration of ethanol, such intense aggregation and fusion were not observed, suggesting their intimate relation to the interdigitated structure formation. The formation of interdigitated structures in the adhering region leads to an increase in the interfacial area and an exposure of hydrophobic acyl chain terminal on the surface area, and enhances hydrophobic interactions between two interdigitated bilayers. Thus, the resultant interdigitated structure makes the aggregated state stable and partially initiates the bilayer mixing between the two apposed membranes, leading to fusion.