Stability and liquid-liquid phase separation in mixed saturated lipid bilayers

Body centered tetragonal nanoparticle superlattices: why and when they form?

Body centered tetragonal (BCT) phases are structural intermediates between body centered cubic (BCC) and face centered cubic (FCC) structures. However, BCC ↔ FCC transitions may or may not involve a stable BCT intermediate. Interestingly, nanoparticle superlattices usually crystallize in BCT structures, but this phase is much less frequent for colloidal crystals of micrometer-sized particles. Two origins have been proposed for the formation of BCT NPSLs: (i) the influence of the substrate on which the nanoparticle superlattice is deposited, and (ii) non-spherical nanoparticle shapes, combined with the fact that different crystal facets have different ligand organizations. Notably, none of these two mechanisms alone is able to explain the set of available experimental observations. In this work, these two hypotheses were independently tested using a recently developed molecular theory for nanoparticle superlattices that explicitly captures the degrees of freedom associated with the ligands on the nanoparticle surface and the crystallization solvent. We show that the presence of a substrate can stabilize the BCT structure for spherical nanoparticles, but only for very specific combinations of parameters. On the other hand, a truncated-octahedron nanoparticle shape strongly stabilizes BCT structures in a wide region of the phase diagram. In the latter case, we show that the stabilization of BCT results from the geometry of the system and it does not require different crystal facets to have different ligand properties, as previously proposed. These results shed light on the mechanisms of BCT stabilization in nanoparticle superlattices and provide guidelines to control its formation.

Influence of drug/lipid interaction on the entrapment efficiency of isoniazid in liposomes for antitubercular therapy: a multi-faced investigation

Isoniazid (INH) is one of the primary drugs used in tuberculosis treatment and its encapsulation in liposomal vesicles can both improve its therapeutic index and minimize toxicity. Here we consider mixtures of hydrogenated soy phosphatidylcholine-phosphatidylglycerol (HSPC-DPPG) to get novel biocompatible liposomes for INH delivery. We determined INH encapsulation efficiency by coupling for the first time UV and Laser Transmission Spectroscopy and we showed that HSPC-DPPG liposomes can load more INH than expected from simple geometrical arguments, thus suggesting the presence of drug-lipid association. To focus on this aspect, which has never been explored in liposomal formulations, we employed several complementary techniques, such as dynamic and static light scattering, calorimetry and surface pressure measurements on lipid monolayers. We find that INH-lipid interaction increases the entrapment capability of liposomes due to INH adsorption. Moreover, the preferential INH-HSPC dipole-dipole interaction promotes the modification of lipid ordering, favoring the formation of HSPC-richer domains in excess of DPPG. Our findings highlight how investigating the fundamental aspects of drug-lipid interactions is of paramount importance for the optimal design of liposomal nanocarriers.

Development of Lipid Nanoparticles for the Delivery of Macromolecules Based on the Molecular Design of pH-Sensitive Cationic Lipids

Considerable efforts have been made on the development of lipid nanoparticles (LNPs) for delivering of nucleic acids in LNP-based medicines, including a first-ever short interfering RNA (siRNA) medicine, Onpattro, and the mRNA vaccines against the coronavirus disease 2019 (COVID-19), which have been approved and are currently in use worldwide. The successful rational design of ionizable cationic lipids was a major breakthrough that dramatically increased delivery efficiency in this field. The LNPs would be expected to be useful as a platform technology for the delivery of various therapeutic modalities for genome editing and even for undiscovered therapeutic mechanisms. In this review, the current progress of my research, including the molecular design of pH-sensitive cationic lipids, their applications for various tissues and cell types, and for delivering various macromolecules, including siRNA, antisense oligonucleotide, mRNA, and the clustered regularly interspaced short palindromic repeats (CRISPR)-associated (Cas) system will be described. Mechanistic studies regarding relationships between the physicochemical properties of LNPs, drug delivery, and biosafety are also summarized. Furthermore, current issues that need to be addressed for next generation drug delivery systems are discussed.

The Phase Behavior of Nanoparticle Superlattices in the Presence of a Solvent

Superlattices of nanoparticles coated by alkyl-chain ligands are usually prepared from a stable solution by evaporation, therefore the pathway of superlattice self-assembly critically depends on the amount of solvent present within it. This work addresses the role of the solvent on the structure and the relative stability of the different supercrystalline phases of single-component superlattices (simple cubic, body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed). The study is performed with a molecular theory for nanoparticle superlattices introduced in this work, which predicts the structure and thermodynamics of the supercrystals explicitly treating the presence and molecular details of the solvent and the ligands. The theory predicts a FCC-BCC transition with decreasing solvent content due to the competition between the translational entropy of the solvent and the entropy and internal energy of the ligands. This result provides an explanation for recent experimental observations by in situ X-ray scattering, which reported a FCC-BCC transition during solvent evaporation. The theory also predicts the effects of the length and surface coverage of the ligands and the radius of the core on the phase behavior in agreement with experimental evidence and previous molecular dynamics simulations. These results validate the use of the dimensionless softness parameter λ (ratio of ligand length to core radius) to predict the phase behavior of wet superlattices. Our results stress the importance of explicitly considering the presence of the solvent in order to reach a complete picture of the mechanisms that mediate the self-assembly of nanoparticle superlattices.

Hydrophobic scaffolds of pH-sensitive cationic lipids contribute to miscibility with phospholipids and improve the efficiency of delivering short interfering RNA by small-sized lipid nanoparticles

Despite the fact that small-sized lipid nanoparticles (LNPs) are important for improved tissue penetration and efficient drug delivery, their poor stability and intracellular trafficking significantly hinders their use as potent small-sized LNPs. It has been reported that both the diffusion of lipid components from LNPs and the adsorption of proteins on the surface of LNPs are responsible for their decreased potency. To overcome this issue, we focused on the chemical structure of hydrophobic scaffolds of pH-sensitive cationic lipids with various lengths and shapes. LNPs composed of a pH-sensitive cationic lipid with long, linear scaffolds induced gene silencing in a dose-dependent manner, while LNPs with a classical scaffold length (C18) failed. Replacing the helper lipid from cholesterol to egg sphingomyelin (ESM) resulted in the formation of smaller LNPs with a diameter of ~22 nm and enhanced gene silencing activity. Most of the ESMs were located in the outer layer and functioned to stabilize the LNPs. Long, linear scaffolds contributed to immiscibility with phosphocholine-containing lipids including ESM. This contribution was dependent on the scaffold length of pH-sensitive cationic lipids. Although phosphocholine-containing lipids usually inhibit membrane fusion-mediated endosomal escape, long, linear scaffolds contributed to avoiding the inhibitory effect and to enhance the potency of the LNPs. These findings provide useful information needed for the rational design of pH-sensitive cationic lipid structures and the selection of appropriate helper lipids and will facilitate the development of highly potent small-sized LNPs. STATEMENT OF SIGNIFICANCE: Despite the fact that small-sized lipid nanoparticles (LNPs) are important for improved tissue penetration and efficient drug delivery, the size reduction-associated decrease in the stability and intracellular trafficking significantly hinders the development of potent small-sized LNPs. Our limited understanding of the mechanism underlying the reduced potency has also hindered the development of more potent small-sized LNPs. The findings of the present study indicate that long and linear hydrophobic scaffolds of pH-sensitive cationic lipids could overcome the loss of efficiency for nucleic acid delivery. In addition, the long hydrophobic scaffolds led to immiscibility with neutral phospholipids, resulting in efficient endosomal escape. These findings provide useful information needed for the rational design of pH-sensitive cationic lipid structures and will facilitate the development of highly potent small-sized LNPs.

The Phase Behavior and Organization of Sphingomyelin/Cholesterol Membranes: A Deuterium NMR Study

We have studied the dependence of the phase and domain characteristics of sphingomyelin (SM)/cholesterol model membranes on sterol content and temperature using deuterium nuclear magnetic resonance. NMR spectra of N-palmitoyl(D31)-D-erythro-sphingosylphosphorylcholine (PSM-d31) were taken for temperatures from 25 to 70°C and cholesterol concentrations of 0-40%. Analogous experiments were performed using 1-palmitoyl,2-palmitoyl(D31)-sn-glycero-3-phosphocholine (DPPC-d31)/cholesterol membranes to carefully compare the data obtained using palmitoyl chains that have similar "kinked" conformations. The constructed phase diagrams exhibit both solid-ordered (so) + liquid-ordered (lo) and liquid-disordered (ld) + lo phase-coexistence regions with a clear three-phase line. Macroscopic (micron-sized) coexistence of ld and lo phases was not observed; instead, line-broadening in the ld+lo region was characterized by intermediate exchange of lipids between the two types of domains. The length scales associated with the domains were estimated to be 75-150 nm for PSM-d31/cholesterol and DPPC-d31/cholesterol model membranes.

Cholesterol Effects on the Physical Properties of Lipid Membranes Viewed by Solid-state NMR Spectroscopy

In this chapter, we review the physical properties of lipid/cholesterol mixtures involving studies of model membranes using solid-state NMR spectroscopy. The approach allows one to quantify the average membrane structure, fluctuations, and elastic deformation upon cholesterol interaction. Emphasis is placed on understanding the membrane structural deformation and emergent fluctuations at an atomistic level. Lineshape measurements using solid-state NMR spectroscopy give equilibrium structural properties, while relaxation time measurements study the molecular dynamics over a wide timescale range. The equilibrium properties of glycerophospholipids, sphingolipids, and their binary and tertiary mixtures with cholesterol are accessible. Nonideal mixing of cholesterol with other lipids explains the occurrence of liquid-ordered domains. The entropic loss upon addition of cholesterol to sphingolipids is less than for glycerophospholipids, and may drive formation of lipid rafts. The functional dependence of ²H NMR spin-lattice relaxation (R _1Z) rates on segmental order parameters (S _CD) for lipid membranes is indicative of emergent viscoelastic properties. Addition of cholesterol shows stiffening of the bilayer relative to the pure lipids and this effect is diminished for lanosterol. Opposite influences of cholesterol and detergents on collective dynamics and elasticity at an atomistic scale can potentially affect lipid raft formation in cellular membranes.

Scaffolds that mimic antigen-presenting cells enable ex vivo expansion of primary T cells

Therapeutic ex vivo T-cell expansion is limited by low rates and T-cell products of limited functionality. Here we describe a system that mimics natural antigen-presenting cells (APCs) and consists of a fluid lipid bilayer supported by mesoporous silica micro-rods. The lipid bilayer presents membrane-bound cues for T-cell receptor stimulation and costimulation, while the micro-rods enable sustained release of soluble paracrine cues. Using anti-CD3, anti-CD28, and interleukin-2, we show that the APC-mimetic scaffolds (APC-ms) promote two- to tenfold greater polyclonal expansion of primary mouse and human T cells compared with commercial expansion beads (Dynabeads). The efficiency of expansion depends on the density of stimulatory cues and the amount of material in the starting culture. Following a single stimulation, APC-ms enables antigen-specific expansion of rare cytotoxic T-cell subpopulations at a greater magnitude than autologous monocyte-derived dendritic cells after 2 weeks. APC-ms support over fivefold greater expansion of restimulated CD19 CAR-T cells than Dynabeads, with similar efficacy in a xenograft lymphoma model.

Super-swelled lyotropic single crystals

Lipids self-assemble into diverse supramolecular structures that exhibit thermotropic and/or lyotropic behavior. Lyotropic mesophases, where membranes conform to periodic minimal surfaces dividing two nonpenetrating aqueous subspaces, are arguably one of the most intriguing phases of lipid materials. Traditional 3D bicontinuous cubic lipid materials appear as a polycrystal of varying degrees of order. When exposed to water, the properties of the molecular building blocks of the membrane determine specific swelling limits setting the lattice dimensions at about 15 nm. This limited swelling severely impairs their application as delivery vehicles of large drugs or as matrices for guiding protein crystallization. We report the discovery of self-assembly strategies leading to the emergence of lipid bicontinuous single crystals with unprecedented swelling capacity. The conventional strategy to increase unit cell size is tweaking membrane composition to include charged building blocks, a process to achieve electrostatic-driven swelling. In this paper, we demonstrate that controlling self-assembly external conditions when coupled to membrane composition yields 3D bicontinuous cubic phases that swell up to lattice dimensions of 68 nm. Importantly, and contrary to what is perceived for soft lyotropic materials in general, the self-assembly methodology enables the development of large super-swelled monocrystals. Utilizing small-angle X-ray scattering and cryoelectron microscopy, we underpin three crucial factors dictating the stabilization of super-swelled lipid bicontinuous cubic single crystals: (i) organic solvent drying speed, (ii) membrane charge density, and (iii) polyethylene glycol-conjugated lipids amount.

Registered and antiregistered phase separation of mixed amphiphilic bilayers

We derive a mean-field free energy for the phase behavior of coupled bilayer leaflets, which is implicated in cellular processes and important to the design of artificial membranes. Our model accounts for amphiphile-level structural features, particularly hydrophobic mismatch, which promotes antiregistration, in competition with the direct transmidplane coupling usually studied, which promotes registration. We show that the phase diagram of coupled leaflets allows multiple metastable coexistences, and we illustrate the kinetic implications of this with a detailed study of a bilayer of equimolar overall composition. For approximate parameters estimated to apply to phospholipids, equilibrium coexistence is typically registered, but metastable antiregistered phases can be kinetically favored by hydrophobic mismatch. Thus, a bilayer in the spinodal region can require nucleation to equilibrate, in a novel manifestation of Ostwald's rule of stages. Our results provide a framework for understanding disparate existing observations in the literature, elucidating a subtle competition of couplings and a key role for phase-transition kinetics in bilayer phase behavior.

Spontaneous formation of heterogeneous patches on polymer-lipid core-shell particle surfaces during self-assembly

Spontaneous formation of heterogeneous patches on the surface of lipid-based nanoparticles (NPs) and microparticles (MPs) due to the segregation of two different functional groups. Patch formation is observed when tracing the functional groups with quantum dots, gold nanoparticles, and fluorescent dyes. This discovery could have important implications for the future design of self-assembled NPs and MPs for different biomedical applications.

Crystalline lipid domains: characterization by X-ray diffraction and their relation to biology

Biological membranes comprise thousands of different lipids, differing in their alkyl chains, headgroups, and degree of saturation. It is estimated that 5% of the genes in the human genome are responsible for regulating the lipid composition of cell membranes. Conceivably, the functional explanation for this diversity is found, at least in part, in the propensity of lipids to segregate into distinct domains, which are important for cell function. X-ray diffraction has been used increasingly to characterize the packing and phase behavior of lipids in membranes. Crystalline domains have been studied in synthetic membranes using wide- and small-angle X-ray scattering, and grazing incidence X-ray diffraction. Herein we summarize recent results obtained using the various X-ray methods, discuss the correlation between crystalline domains and liquid ordered domains studied with other techniques, and the relevance of crystalline domains to functional lipid domains in biological membranes.

The thermodynamics of simple biomembrane mimetic systems

Insight into the forces governing a system is essential for understanding its behavior and function. Thermodynamic investigations provide a wealth of information that is not, or is hardly, available from other methods. This article reviews thermodynamic approaches and assays to measure collective properties such as heat adsorption / emission and volume variations. These methods can be successfully applied to the study of lipid vesicles (liposomes) and biological membranes. With respect to instrumentation, differential scanning calorimetry, pressure perturbation calorimetry, isothermal titration calorimetry, dilatometry, and acoustic techniques aimed at measuring the isothermal and adiabatic processes, two- and three-dimensional compressibilities are considered. Applications of these techniques to lipid systems include the measurement of different thermodynamic parameters and a detailed characterization of thermotropic, barotropic, and lyotropic phase behavior. The membrane binding and / or partitioning of solutes (proteins, peptides, drugs, surfactants, ions, etc.) can also be quantified and modeled. Many thermodynamic assays are available for studying the effect of proteins and other additives on membranes, characterizing non-ideal mixing, domain formation, bilayer stability, curvature strain, permeability, solubilization, and fusion. Studies of membrane proteins in lipid environments elucidate lipid-protein interactions in membranes. Finally, a plethora of relaxation phenomena toward equilibrium thermodynamic structures can be also investigated. The systems are described in terms of enthalpic and entropic forces, equilibrium constants, heat capacities, partial volume changes, volume and area compressibility, and so on, also shedding light on the stability of the structures and the molecular origin and mechanism of the structural changes.

Phase separation in biological membranes: integration of theory and experiment

Lipid bilayer model membranes that contain a single lipid species can undergo transitions between ordered and disordered phases, and membranes that contain a mixture of lipid species can undergo phase separations. Studies of these transformations are of interest for what they can tell us about the interaction energies of lipid molecules of different species and conformations. Nanoscopic phases (<200 nm) can provide a model for membrane rafts, specialized membrane domains enriched in cholesterol and sphingomyelin, which are believed to have essential biological functions in cell membranes. Crucial questions are whether lipid nanodomains can exist in stable equilibrium in membranes and what is the distribution of their sizes and lifetimes in membranes of different composition. Theoretical methods have supplied much information on these questions, but better experimental methods are needed to detect and characterize nanodomains under normal membrane conditions. This review summarizes linkages between theoretical and experimental studies of phase separation in lipid bilayer model membranes.

AFM investigations of phase separation in supported membranes of binary mixtures of POPC and an eicosanyl-based bisphosphocholine bolalipid

Supported membranes prepared from binary mixtures of DOPC and the bolalipid C(20)BAS have been examined by atomic force microscopy (AFM). The supported membranes are phase separated to give a thicker DOPC-rich phase and a thinner bolalipid-rich phase for a range of lipid compositions. These results confirm an earlier prediction from mean field theory that phase separation is the thermodynamically stable state for membranes containing approximately equimolar C(20)BAS and double chain monopolar lipids with chain lengths exceeding 15 carbons. Hydrophobic mismatch between the monopolar lipid hydrocarbon chains and the membrane spanning bolalipid chains was suggested to provide the driving force for phase separation. The AFM results also show that the morphology of the mixed POPC:C(20)BAS supported membranes varies significantly with the conditions used to prepare the vesicles and supported membrane samples. The complex membrane morphologies observed are attributed to the interplay of several factors, including a compositionally heterogeneous vesicle population, exchange of lipid between the vesicle solution and solid substrate during formation of the supported membrane, and slow equilibration of domains due to pinning of the lipids to the solid support.