Detection of Bilayer Packing Stress and Its Release in Lamellar-Cubic Phase Transition by Time-Resolved Fluorescence Anisotropy

Exploring pH-Triggered Lamellar to Cubic Phase Transition in 2-Hydroxyoleic Acid/Monoolein Nanodispersions: Insights into Membrane Physicochemical Properties

Self-assembled lipid nanoparticles (LNPs) are essential nanocarriers for drug delivery. Functionalization of LNPs with ionizable lipids creates pH-responsive nanoparticles that change structures under varying pH conditions, enabling pH-triggered drug release. Typically, bicontinuous cubic phase nanoparticles (Cubosomes) and lamellar structured vesicles (Liposomes) differ in lipid packing statuses, affecting drug release and cellular uptake. However, most research predominantly focuses on elucidating lattice structure changes of these LNPs without a deep investigation of lipid-membrane properties. Addressing this gap, our study delves into the lipid-membrane physicochemical property variations during the lamellar-to-cubic phase transition. Here, we prepared pH-responsive LNPs using 2-hydroxyoleic acid/monoolein (2-OHOA/MO) binary components. Small-angle X-ray scattering (SAXS) revealed a phase transition from lamellar vesicles (Lα) to cubosomes (Im3m/Pn3m) with pH reduction. Laurdan and 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence probes tracked the lipid-water interfacial polarity and lipid-membrane fluidity variations during the phase transition. Raman spectroscopy provided further insights into lipid-membrane lipid chain packing and chain torsion. We observed that the changes in lipid-membrane properties coincided with the lamellar-to-cubic phase transition, emphasizing the interplay between the phase structure and lipid-membrane behaviors in the 2-OHOA/MO system. This study provides insights into the lipid-membrane properties variation during the pH-triggered phase transition in the 2-OHOA/MO system, guiding future research toward more effective and reliable pH-responsive drug delivery platforms.

The stabilization of primitive bicontinuous cubic phases with tunable swelling over a wide composition range

In this paper we investigate the pseudo-ternary phase diagram of glycerol monooleate (GMO), a cationic lipid (DOTAP - 1,2-dioleoyl-3-trimethylammonium propane), and a "PEGylated" lipid (DOPE-PEG - 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000 kDa]) in excess water. We use small angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (Cryo-EM) to map out a phase diagram in a regime of low DOPE-PEG content (1-5 mol%), which is pertinent for the application of lipid systems as carriers of biomolecular cargo to cells. Pure GMO is known to self-assemble into bicontinuous cubic phases of the gyroid type at low water content and of the diamond type in excess water. These complex structures have numerous advantages reaching beyond drug delivery, e.g. as protein crystallization matrices, but their formulation is challenging as very small contents of guest molecules can shift the phase behavior towards other geometries such as the lamellar phase. In this work, we show that the ternary GMO/DOTAP/DOPE-PEG system allows the stabilization of bicontinuous cubic phases in excess water over a wide composition range. The symmetry of the phase can be tuned by varying the amount of PEGylated lipid, with the primitive type dominating at low DOPE-PEG content (1-3 mol%) and the diamond phase arising at 5 mol% DOPE-PEG. In addition, we found that the diamond phase is virtually non-responsive to electrostatic swelling. In contrast, primitive bicontinuous cubic lattice dimensions swell up in equilibrium to 650 Å with increased cationic lipid content.

Cubosomes: remarkable drug delivery potential

Cubosomes are nanostructured liquid crystalline particles, made of certain amphiphilic lipids in definite proportions, known as biocompatible carriers in drug delivery. Cubosomes comprise curved bicontinuous lipid bilayers that are organized in three dimensions as honeycombed structures and divided into two internal aqueous channels that can be exploited by various bioactive ingredients, such as chemical drugs, peptides and proteins. Owing to unique properties such as thermodynamic stability, bioadhesion, the ability of encapsulating hydrophilic, hydrophobic and amphiphilic substances, and the potential for controlled release through functionalization, cubosomes are regarded as promising vehicles for different routes of administration. Based on the most recent reports, this review introduces cubosomes focusing on their structure, preparation methods, mechanism of release and potential routes of administration.

Radio Frequency-Activated Nanoliposomes for Controlled Combination Drug Delivery

This work was conducted in order to design, characterize, and evaluate stable liposomes containing the hydrophobic drug raloxifene HCl (RAL) and hydrophilic doxycycline HCl (DOX), two potentially synergistic agents for treating osteoporosis and other bone lesions, in conjunction with a radio frequency-induced, hydrophobic magnetic nanoparticle-dependent triggering mechanism for drug release. Both drugs were successfully incorporated into liposomes by lipid film hydration, although combination drug loading compromised liposome stability. Liposome stability was improved by reducing the drug load and by including Pluronics® (PL) in the formulations. DOX did not appear to interact with the phospholipid membranes comprising the liposomes, and its release was maximized in the presence of radio frequency (RF) heating. In contrast, differential scanning calorimetry (DSC) and phosphorus-31 nuclear magnetic resonance ((31)P-NMR) analysis revealed that RAL developed strong interactions with the phospholipid membranes, most notably with lipid phosphate head groups, resulting in significant changes in membrane thermodynamics. Likewise, RAL release from liposomes was minimal, even in the presence of RF heating. These studies may offer useful insights into the design and optimization of multidrug containing liposomes. The effects of RAL on liposome characteristics and drug release performance underscore the importance of appropriate physical-chemical analysis in order to identify and characterize drug-lipid interactions that may profoundly affect liposome properties and performance early in the formulation development process.

Engineering nanomedicines for improved melanoma therapy: progress and promises

Once metastatic, melanoma remains one of the most aggressive and morbid malignancies. Moreover, in past decades, the overall survival for advanced unresectable melanoma exhibited a constancy of poor prognosis. Low response rates and serious adverse effects have been characteristic of standard therapy based on a combination of chemotherapeutic agents or immunotherapy with IL-2. For example, the chemotherapy including dacarbazine, carmustin, cisplatin and tamoxifen is known as 'Dartmouth regimen' while the CVD regimen comprises carmustine, vinblastine and dacarbazine. Thus, there is an urgent and critical need to reformulate these bioactive agents using nanoscience and nanotechnology as alternative strategies. This article overviews current design and evaluation of nanomedicine undertaken to address this unmet medical need. The nanomedicines studied include polymeric nanoparticles, liposomes, polymersomes, dendrimers, cubosomes, niosomes and nanodiamonds. In this preclinical article, nanotechnology provides hope for effective treatment of this aggressive and largely treatment-resistant disease.

Ordered nanostructured amphiphile self-assembly materials from endogenous nonionic unsaturated monoethanolamide lipids in water

The self-assembly, solid state and lyotropic liquid crystalline phase behavior of a series of endogenous n-acylethanolamides (NAEs) with differing degrees of unsaturation, viz., oleoyl monoethanolamide, linoleoyl monoethanolamide, and linolenoyl monoethanolamide, have been examined. The studied molecules are known to possess inherent biological function. Both the monoethanolamide headgroup and the unsaturated hydrophobe are found to be important in dictating the self-assembly behavior of these molecules. In addition, all three molecules form lyotropic liquid crystalline phases in water, including the inverse bicontinuous cubic diamond (Q(II)(D)) and gyroid (Q(II)(G)) phases. The ability of the NAE's to form inverse cubic phases and to be dispersed into ordered nanostructured colloidal particles, cubosomes, in excess water, combined with their endogenous nature and natural medicinal properties, makes this new class of soft mesoporous amphiphile self-assembly materials suitable candidates for investigation in a variety of advanced multifunctional applications, including encapsulation and controlled release of therapeutic agents and incorporation of medical imaging agents.

[Elucidation of lipid complex formation mechanisms by static/dynamic structural evaluation]

Biomembrane is a place for signal transduction, where protein-membrane interactions are controlled by the membrane environment. This environment could be modified by lipid dynamics, such as interbilayer transport and transbilayer movement, which are governed by lipid transfer proteins and translocase enzymes, respectively. Thus, static and dynamic structural evaluations of the membranes are important to understand the links among function, structure, and dynamics of lipid membranes. This review describes our recent studies for 1) the production of lipid nanoparticles with nonlamellar liquid crystalline phases, 2) membrane-protein interaction that relates to the biogenesis of high-density lipoproteins, and 3) the characterization of lipid transfer dynamics by small-angle neutron scattering (SANS). It was demonstrated that different phospholipids have individual effect on membrane-apolipoprotein A-I (apoA-I) interactions that bring about a discoidal lipid-protein complex formation: Phosphatidylethanolamine, possessing the negative spontaneous curvature, increases both the degree of hydration at the membrane interface and the acyl chain order, and enhances the binding of amphipathic helices. A gel phase-forming lipid, sphingomyelin, forms heterogeneous interface of the mixed membranes and facilitates the discoidal particle formation with apoA-I. Phosphatidylserine partly contributes to a reduction in pH at membrane surface, which induces the conformational change of apoA-I and accelerates the discoidal complex formation. It was also demonstrated that SANS is available as a novel method to determine the dynamics of membrane lipids. This technique is perfectly suited to evaluate the activity of proteins relevant to lipid migrations.

Adsorption behaviors of DPPC/MO aggregates on SiO2 surfaces

The adsorption kinetics of extruded 1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine (DPPC)/1-(cis-9-octadecenoyl)- rac-glycerol (monoolein, MO) aggregates on SiO 2 surface at 25 degrees C is investigated in real time, using the dissipative quartz crystal microbalance (QCM) technique. Four adsorption pathways have been identified depending on the molar fraction of MO in the DPPC/MO system: (I) intact vesicle adsorption, (II) vesicle reorganization on a SiO 2 surface, (III) supported lipid bilayer (SLB) formation, and (IV) cubosome adsorption. The results can be understood by the fact that DPPC is a lamellar phase-forming lipid, whereas MO prefers the cubic phase. Therefore, the incorporation of MO in DPPC increases the packing parameter. Equally important, MO also increases the mobility of lipid molecules and lateral pressure in the bilayers as a result of the presence of a unique cis- double bond. Before extrusion, the vesicles size increases with the MO content when X MO <or= 0.7 and cubosomes are formed for X MO >or= 0.8. The extruded DPPC/MO suspensions consist of reformed vesicles for X MO <or= 0.7 and filtered cubosomes for X MO >or= 0.8, all with a uniform diameter of approximately 100 nm. Differential scanning calorimetry (DSC) further indicates that the addition of MO lowers the main phase transition temperature of DPPC and thus makes the hydrophobic interior more fluid.

Calcium triggered L alpha-H2 phase transition monitored by combined rapid mixing and time-resolved synchrotron SAXS

Background

Awad et al. [1] reported on the Ca²⁺-induced transitions of dioleoyl-phosphatidylglycerol (DOPG)/monoolein (MO) vesicles to bicontinuous cubic phases at equilibrium conditions. In the present study, the combination of rapid mixing and time-resolved synchrotron small-angle X-ray scattering (SAXS) was applied for the in-situ investigations of fast structural transitions of diluted DOPG/MO vesicles into well-ordered nanostructures by the addition of low concentrated Ca²⁺ solutions.

Methodology/Principal Findings

Under static conditions and the in absence of the divalent cations, the DOPG/MO system forms large vesicles composed of weakly correlated bilayers with a d-spacing of ∼140 Å (L_α-phase). The utilization of a stopped-flow apparatus allowed mixing these DOPG/MO vesicles with a solution of Ca²⁺ ions within 10 milliseconds (ms). In such a way the dynamics of negatively charged PG to divalent cation interactions, and the kinetics of the induced structural transitions were studied. Ca²⁺ ions have a very strong impact on the lipidic nanostructures. Intriguingly, already at low salt concentrations (DOPG/Ca²⁺>2), Ca²⁺ ions trigger the transformation from bilayers to monolayer nanotubes (inverted hexagonal phase, H₂). Our results reveal that a binding ratio of 1 Ca²⁺ per 8 DOPG is sufficient for the formation of the H₂ phase. At 50°C a direct transition from the vesicles to the H₂ phase was observed, whereas at ambient temperature (20°C) a short lived intermediate phase (possibly the cubic Pn3m phase) coexisting with the H₂ phase was detected.

Conclusions/Significance

The strong binding of the divalent cations to the negatively charged DOPG molecules enhances the negative spontaneous curvature of the monolayers and causes a rapid collapsing of the vesicles. The rapid loss of the bilayer stability and the reorganization of the lipid molecules within ms support the argument that the transition mechanism is based on a leaky fusion of the vesicles.

Interaction of an amphipathic peptide with phosphatidycholine/phosphatidylethanolamine mixed membranes

The effect of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) in mixed membranes with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) on interaction with a class A amphipathic peptide, Ac-DWLKAFYDKVAEKLKEAF-NH(2) (Ac-18A-NH(2)), was investigated. The fluorescence lifetime of 2-(9-anthroyloxy)stearic acid and (2)H NMR spectra were used to evaluate the penetration of water molecules into the membrane interface and the order of lipid acyl chains, respectively. The results demonstrated that DOPE in the mixed membranes decreased the fluorescence lifetime and increased the acyl-chain order, and that Ac-18A-NH(2) affected them more for membranes with higher DOPE fractions. The partition coefficient (K(p)) of the peptide to the mixed membranes was increased with the increase in the DOPE mole fractions. From the temperature dependence of the K(p) values, the binding of Ac-18A-NH(2) to POPC/DOPE mixed membranes was found to be entropy-driven. The formation of an alpha-helix at the membrane's surface is supposed to induce positive curvature strain, which decreases the headgroup hydration and acyl-chain order of lipids. Thus, the binding of Ac-18A-NH(2) to membranes is entropically more favorable at higher DOPE fractions since the peptide's insertion into the membrane can decrease the order parameter and unfavorable headgroup hydration, which explains the enhanced peptide binding.

Effects of an Amphipathic α-Helical Peptide on Lateral Pressure and Water Penetration in Phosphatidylcholine and Monoolein Mixed Membranes

The physicochemical properties of mixed membranes of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and a nonlamellar-forming lipid, 1-monoolein (MO), and the effects of an amphipathic alpha-helical peptide, 18A (DWLKAFYDKVAEKLKEAF), on the membranes were investigated by fluorescence measurements and 31P NMR. The intramolecular excimer formation of dipyrenylphosphatidylcholines showed that the increased lateral pressure near the bilayer center by MO is reduced by the lamellar-cubic phase transition at an MO mole fraction of 0.7, while the lateral pressure near the polar-apolar interface increases even through the phase transition. The fluorescence lifetime of 2-(9-anthroyloxy)stearic acid revealed that water penetration into the interface region increases with the MO fraction. The insertion of the 18A peptide into the membrane interface region decreased both the lateral pressure near the interface and water penetration, and shifted the lamellar-cubic phase transition to a higher MO fraction. This suggests that 18A induces a positive curvature strain and lowers the lateral pressure and water penetration. Furthermore, the increase in the MO fraction in POPC/MO LUV promoted partitioning of 18A to the membranes. This preferential binding to the MO-containing membranes is presumably ascribed to the propensity of 18A to reduce the membrane strain.