Association of a fluorescent amphiphile with lipid bilayer vesicles in regions of solid-liquid-disordered phase coexistence

Microfluidic-assisted fabrication of phosphatidylcholine-based liposomes for controlled drug delivery of chemotherapeutics

Microfluidic enables precise control over the continuous mixing of fluid phases at the micrometre scale, aiming to optimize the processing parameters and to facilitate scale-up feasibility. The optimization of parameters to obtain monodispersed drug-loaded liposomes however is challenging. In this work, two phosphatidylcholines (PC) differing in acyl chain length were selected, and used to control the release of the chemotherapeutic agent doxorubicin hydrochloride, an effective drug used to treat breast cancer. Microfluidics was used to rapidly screen manufacturing parameters and PC formulations to obtain monodispersed unilamellar liposomal formulations with a reproducible size (i.e. < 200 nm). Cholesterol was included in all liposomal formulations; some formulations also contained DMPC(1,2-dimyristoyl-sn-glycero-3-phosphocholine) and/or DSPC(1,2-distearoyl-sn-glycero-3-phosphocholine). Systematic variations in microfluidics total flow rate (TFR) settings were performed, while keeping a constant flow rate ratio (FRR). A total of six PC-based liposomes were fabricated using the optimal manufacturing parameters (TFR 500 μL/min, FRR 0.1) for the production of reproducible, stable liposome formulations with a narrow size distribution. Liposomes actively encapsulating doxorubicin exhibited high encapsulation efficiencies (>80%) for most of the six formulations, and sustained drug release profiles in vitro over 48 h. Drug release profiles varied as a function of the DMPC/DSPC mol content in the lipid bilayer, with DMPC-based liposomes exhibiting a sustained release of doxorubicin when compared to DSPC liposomes. The PC-based liposomes, with a slower release of doxorubicin, were tested in vitro, as to investigate their cytotoxic activity against three human breast cancer cell lines: the non-metastatic ER+/PR + MCF7 cells, the triple-negative aggressive MDA-MB 231 cells, and the metastatic HER2-overexpressing/PR + BT474 cells. Similar cytotoxicity levels to that of free doxorubicin were reported for DMPC₅ and DMPC₃ binary liposomes (IC₅₀ ~ 1 μM), whereas liposomes composed of a single PC were less cytotoxic (IC₅₀ ~ 3-4 μM). These results highlight that microfluidics is suitable for the manufacture of monodispersed and size-specific PC-based liposomes in a controlled single-step; furthermore, selected PC-based liposome represent promising nanomedicines for the prolonged release of chemotherapeutics, with the aim of improving outcomes for patients.

Water Soluble Coumarin Quaternary Ammonium Chlorides: Synthesis and Biological Evaluation

In the present study, coumarin-bearing three pyridinium and three tetra-alkyl ammonium salts were synthesized. The compounds were fully characterized by ¹ H- and ¹³ C-NMR, LC/MS and IR spectroscopic methods and elemental analyses. The cytotoxic properties of all compounds were tested against human liver cancer (HepG2), human colorectal cancer (Caco-2) and non-cancer mouse fibroblast (L-929) cell lines. Some compounds performed comparable cytotoxicity with standard drug cisplatin. Antibacterial properties of the compounds were tested against Gram-negative Escherichia coli and Gram-positive Bacillus subtilis bacteria, but the compounds did not have any antibacterial effect against both bacteria. Enzyme inhibitory properties of all compounds were tested on the activities of human carbonic anhydrase I and II, and xanthine oxidase. All compounds inhibited both enzymes more effectively than standard drugs, acetazolamide and allopurinol, respectively. The biological evaluation results showed that ionic and water soluble coumarin derivatives are promising structures for further investigations especially on enzyme inhibition field.

Lipid vesicle composition influences the incorporation and fluorescence properties of the lipophilic sulphonated carbocyanine dye SP-DiO

Lipid membrane composition influences insertion efficiency and photophysical properties of lipophilic membrane-inserting dyes.

Effect of Acyl Chain Length on the Rate of Phospholipid Flip-Flop and Intermembrane Transfer

The rate at which phospholipids equilibrate between different membranes and between the non-polar environments in biological fluids is of high importance in the understanding of biomembrane diversity, as well as in the development of liposomes for drug delivery. In this work, we characterize the rate of insertion into and desorption from POPC bilayers for a homologous series of amphiphiles with the fluorescent NBD group attached to phosphoethanolamines of different acyl chain lengths, NBD-diC _n -PE with n = 6, 8, 10, and 12. The rate of translocation between bilayer leaflets was also characterized, providing all the relevant parameters for their interaction with lipid bilayers. The results are complemented with data for NBD-diC₁₄-PE obtained from literature (Abreu et al. Biophys J 87:353-365, 2004; Moreno et al. Biophys J 91:873-881, 2006). The rate of translocation between the POPC leaflets is not dependent on the length of the acyl chains, while this affects strongly the rate of desorption from the bilayer. Insertion in the POPC bilayer is not diffusion controlled showing a significant dependence on the acyl chain length and on temperature. The results obtained are compared with those previously reported for NBD-LysoC₁₄-PE (Sampaio et al. Biophys J 88:4064-4071, 2005), and with the homologous series of single chain amphiphiles NBD-C _n (Cardoso et al. J Phys Chem B 114:16337-16346, 2010; J Phys Chem B 115:10098-10108, 2011). This allows the establishment of important relations between the rate constants for interaction with the lipid bilayers and the structural properties of the amphiphiles, namely the total surface and the cross-section of their non-polar region.

Phase Behavior of Planar Supported Lipid Membranes Composed of Cholesterol and 1,2-Distearoyl-sn-Glycerol-3-Phosphocholine Examined by Sum-Frequency Vibrational Spectroscopy

The influence of cholesterol (CHO) on the phase behavior of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) planar supported lipid bilayers (PSLBs) was investigated by sum-frequency vibrational spectroscopy (SFVS). The intrinsic symmetry constraints of SFVS were exploited to measure the asymmetric distribution of phase segregated phospholipid domains in the proximal and distal layers of DSPC + CHO binary mixtures as a function of CHO content and temperature. The SFVS results suggest that cholesterol significantly affects the phase segregation and domain distribution in PSLBs of DSPC in a concentration dependent manner, similar to that found in bulk suspensions. The SFVS spectroscopic measurements of phase segregation and structure change in the binary mixture indicate that membrane asymmetry must be present in order for the changes in SFVS signal to be observed. These results therefore provide important evidence for the delocalization and segregation of different phase domain structures in PSLBs due to the interaction of cholesterol and phospholipids.

Remodeling of ordered membrane domains by GPI-anchored intestinal alkaline phosphatase

Glycosylphosphatidyl-inositol (GPI)-anchored proteins preferentially localize in the most ordered regions of the cell plasma membrane. Acyl and alkyl chain composition of GPI anchors influence the association with the ordered domains. This suggests that, conversely, changes in the fluid and in the ordered domains lipid composition affect the interaction of GPI-anchored proteins with membrane microdomains. Validity of this hypothesis was examined by investigating the spontaneous insertion of the GPI-anchored intestinal alkaline phophatase (BIAP) into the solid (gel) phase domains of preformed supported membranes made of dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC), DOPC/sphingomyelin (DOPC/SM), and palmitoyloleoylphosphatidylcholine/SM (POPC/SM). Atomic force microscopy (AFM) showed that BIAP inserted in the gel phases of the three mixtures. However, changes in the lipid composition of membranes had a marked effect on the protein containing bilayer topography. Moreover, BIAP insertion was associated with a net transfer of phospholipids from the fluid to the gel (DOPC/DPPC) or from the gel to the fluid (POPC/SM) phases. For DOPC/SM bilayers, transfer of lipids was dependent on the homogeneity of the gel SM phase. The data strongly suggest that BIAP interacts with the most ordered lipid species present in the gel phases of phase-separated membranes. They also suggest that GPI-anchored proteins might contribute to the selection of their own microdomain environment.

Fluorescence probe partitioning between Lo/Ld phases in lipid membranes

Fluorescence microscopy imaging is an important technique for studying lipid membranes and is increasingly being used for examining lipid bilayer membranes, especially those showing macroscopic coexisting domains. Lipid phase coexistence is a phenomenon of potential biological significance. The identification of lipid membrane heterogeneity by fluorescence microscopy relies on membrane markers with well-defined partitioning behavior. While the partitioning of fluorophores between gel and liquid-disordered phases has been extensively characterized, the same is not true for coexisting liquid phases. We have used fluorescence microscopy imaging to examine a large variety of lipid membrane markers for their liquid phase partitioning in membranes with various lipid compositions. Most fluorescent lipid analogs are found to partition strongly into the liquid-disordered (L(d)) phase. In contrast, some fluorescent polycyclic aromatic hydrocarbons with a flat ring system were found to partition equally, but others partition preferentially into liquid-ordered (L(o)) phases. We have found these fluorescent markers effective for identification of coexisting macroscopic membrane phases in ternary lipid systems composed of phospholipids and cholesterol.

Kinetics and thermodynamics of association of a fluorescent lysophospholipid derivative with lipid bilayers in liquid-ordered and liquid-disordered phases

We have measured the rates of insertion into, desorption from, and spontaneous interlayer translocation (flip-flop) of the fluorescent lysophospholipid derivative NBD-lyso-1-myristoylphosphatidylethanolamine in l(d) and l(o) phase lipid bilayer membranes. The lipid bilayers, studied as LUV, were prepared from pure 1-palmitoyl-2-oleoylphosphatidylcholine, in the l(d) phase; and from two Chol-containing binary lipid mixtures, 1-palmitoyl-2-oleoylphosphatidylcholine and Chol (molar ratio of 1:1) and SpM and Chol (molar ratio of 6:4), both in the l(o) phase. Insertion, desorption, and translocation rate constants and equilibrium constants for association of the amphiphile monomer with the lipid bilayers were measured between 15 degrees C and 35 degrees C, and the standard free energies, enthalpies, and entropies, as well as the activation energies for these processes were derived from these data. The equilibrium partition coefficients for partitioning of the amphiphile between the aqueous phase and the different membrane phases were also derived, and an estimation was made of hypothetical partition coefficients and the respective energetic parameters for partitioning between the different lipid phases if these were to coexist in the same membrane. We show that, contrary to general belief, the association of NBD-lysoMPE with lipid bilayers is not a diffusion-controlled process, the rate-limiting step in insertion being the formation of a free area in the membrane surface of an adequate size for insertion to occur.

"Uncorking" of liposomes by matrix metalloproteinase-9

A triggered release methodology of liposomal contents via the enzyme MMP-9 is described.

Kinetics and thermodynamics of association of a phospholipid derivative with lipid bilayers in liquid-disordered and liquid-ordered phases

We have measured the rates of insertion into, desorption from, and spontaneous interlayer translocation (flip-flop) in liquid-disordered and liquid-ordered phase lipid bilayer membranes, of the fluorescent phospholipid derivative NBD-dimyristoylphosphatidyl ethanolamine. This study made use of a recently described method that exploits a detailed knowledge of the binding kinetics of an amphiphile to bovine serum albumin, to recover the insertion and desorption rate constants when the albumin-bound amphiphile is transferred through the aqueous phase to the membrane and vice versa. The lipid bilayers, studied as large unilamellar vesicles, were prepared from pure 1-palmitoyl-2-oleoylphosphatidylcholine in the liquid-disordered phase; and from two cholesterol-containing binary lipid mixtures, 1-palmitoyl-2-oleoylphosphatidylcholine and cholesterol (molar ratio of 1:1), and egg sphingomyelin and cholesterol (molar ratio of 6:4), both in the liquid-ordered phase. Insertion, desorption, and translocation rate constants and equilibrium constants for association of the amphiphile monomer with the lipid bilayers were directly measured between 15 degrees and 35 degrees C, and the standard free energies, enthalpies, and entropies, as well as the activation energies for these processes, were derived from this data. The equilibrium partition coefficients for partitioning of the amphiphile between the aqueous phase and the different membrane phases were also derived, and permitted the estimation of hypothetical partition coefficients and the respective energetic parameters for partitioning between the different lipid phases if these were to coexist in the same membrane.

Binding of a fluorescent lipid amphiphile to albumin and its transfer to lipid bilayer membranes

Kinetics and thermodynamics of the binding of a fluorescent lipid amphiphile, Rhodamine Green(TM)-tetradecylamide (RG-C(14:0)), to bovine serum albumin were characterized in an equilibrium titration and by stopped-flow fluorimetry. The binding equilibrium of RG-C(14:0) to albumin was then used to reduce its concentration in the aqueous phase to a value below its critical micelle concentration. Under these conditions, the only two species of RG-C(14:0) in the system were the monomer in aqueous solution in equilibrium with the protein-bound species. After previous determination of the kinetic and thermodynamic parameters for association of RG-C(14:0) with albumin, the kinetics of insertion of the amphiphile into and desorption off lipid bilayer membranes in different phases (solid, liquid-ordered, and liquid-disordered phases, presented as large unilamellar vesicles) were studied by stopped-flow fluorimetry at 30 degrees C. Insertion and desorption rate constants for association of the RG-C(14:0) monomer with the lipid bilayers were used to obtain lipid/water equilibrium partition coefficients for this fluorescent amphiphile. The direct measurement of these partition coefficients is shown to provide a new method for the indirect determination of the equilibrium partition coefficient of similar molecules between two defined lipid phases if they coexist in the same membrane.

Substrate-induced deformation and adhesion of phospholipid vesicles at the main phase transition

The physiochemical properties of phospholipid vesicle, e.g. permeability, elasticity, etc., are directly modulated by the chain-melting transition of the lipid bilayer. Currently, there is a lack of understanding in the relationship between thermotropic transition, mechanical deformation and adhesion strength for an adherent vesicle at temperature close to main phase transition temperature T(m). In this study, the contact mechanics of dimyristoyl-phosphatidylcholine (DMPC) vesicle at the main phase transition are probed by confocal reflectance interference contrast microscopy in combination with phase contrast microscopy. It is shown that DMPC vesicles strongly adhere on pure fused silica substrate at T(m) and the degree of deformation as well as the adhesion energy is a decreasing function against the mid-plane diameter of the vesicles. Furthermore, an increase of osmotic pressure at the gel/liquid crystalline phase co-existence imposes insignificant changes in both the degree of deformation and adhesion energy of adherent vesicles when the lipid bilayer permeability is maximized. With the reverse of substrate charge, the mechanical deformation and adhesion strength for larger vesicles (mid-plane diameter >18 microm) are significantly reduced. By monitoring the parametric response of substrate-induced vesicle adhesion during main phase transition, it is shown that the degree of deformation and adhesion energy of adhering vesicle is increased and unchanged, respectively, against the increase of temperature.

Mechanism and kinetics of delta-lysin interaction with phospholipid vesicles

Delta-lysin is a 26 amino acid, hemolytic peptide toxin secreted by Staphylococcus aureus. It has been reported to form an amphipathic helix upon binding to lipid bilayers and is often cited as a typical example of the barrel-stave model for pore formation in lipid bilayer membranes. However, the exact mechanism by which it lyses cells and the physical basis of its target specificity are still unknown. Moreover, the evidence for delta-lysin insertion and pore formation in the membrane stems largely from theoretical modeling of the toxin and lacks experimental confirmation. We investigated binding and insertion of delta-lysin into phospholipid bilayer vesicles. The kinetics of these processes were studied by stopped-flow fluorescence with two types of experiments: (a) carboxyfluorescein release from the vesicles upon peptide-vesicle interaction, with concomitant relief of dye self-quenching; (b) fluorescence energy transfer from the intrinsic tryptophan of the peptide to a membrane-bound lipid probe. We formulated a detailed kinetic mechanism with explicit molecular rate constants for peptide binding, association, and insertion, obtaining a quantitative description of the experimental results. delta-Lysin insertion is strongly dependent on the peptide-to-lipid ratio, suggesting that association of a critical number of monomers on the membrane is required for activity. However, we found no evidence for a stable membrane-inserted pore. Rather, the peptide appears to cross the membrane rapidly and reversibly and cause release of the lipid vesicle contents in this process.

Solubility of amphiphiles in membranes: influence of phase properties and amphiphile head group

The solubilities of two fluorescent lipid amphiphiles with comparable apolar structures and different polar head groups, NBD-hexadecylamine and RG-tetradecylamine (or -octadecylamine), were compared in lipid bilayers at a molar ratio of 1/50 at 23 degrees C. Bilayers examined were in the solid, liquid-disordered, or liquid-ordered phases. While NBD-hexadecylamine was soluble in all the examined bilayer membrane phases, RG-tetradecylamine was stably soluble only in the liquid-disordered phase. RG-tetradecylamine insolubility in solid and liquid-ordered phases manifests itself as an aggregation of the amphiphile over a period of several days and the kinetics of aggregation were studied. Solubility of these amphiphiles in the different phases examined seems to be related to the dipole moment of the amphiphile (in particular, of the polar fluorophore) and its orientation relative to the dipolar potential of the membrane. We propose that amphiphilic molecules inserted into membranes (including lipid-attached proteins) partition into different coexisting membrane phases based upon: (1) nature of the apolar structure (chain length, degree of saturation, and chain branching as has been proposed in the literature); (2) magnitude and orientation of the dipole moment of the polar portion of the molecules relative to the membrane dipolar potential; and (3) hydration forces that are a consequence of ordering of water dipoles at the membrane surface.