Design and use of model membranes to study biomolecular interactions using complementary surface-sensitive techniques

Illuminating the nanostructure of diffuse interfaces: Recent advances and future directions in reflectometry techniques

Diffuse soft matter interfaces take many forms, from end-tethered polymer brushes or adsorbed surfactants to self-assembled layers of lipids. These interfaces play crucial roles across a multitude of fields, including materials science, biophysics, and nanotechnology. Understanding the nanostructure and properties of these interfaces is fundamental for optimising their performance and designing novel functional materials. In recent years, reflectometry techniques, in particular neutron reflectometry, have emerged as powerful tools for elucidating the intricate nanostructure of soft matter interfaces with remarkable precision and depth. This review provides an overview of selected recent developments in reflectometry and their applications for illuminating the nanostructure of diffuse interfaces. We explore various principles and methods of neutron and X-ray reflectometry, as well as ellipsometry, and discuss advances in their experimental setups and data analysis approaches. Improvements to experimental neutron reflectometry methods have enabled greater time resolution in kinetic measurements and elucidation of diffuse structure under shear or confinement, while innovation in analysis protocols has significantly reduced data processing times, facilitated co-refinement of reflectometry data from multiple instruments and provided greater-than-ever confidence in proposed structural models. Furthermore, we highlight some significant research findings enabled by these techniques, revealing the organisation, dynamics, and interfacial phenomena at the nanoscale. We also discuss future directions and potential advancements in reflectometry techniques. By shedding light on the nanostructure of diffuse interfaces, reflectometry techniques enable the rational design and tailoring of interfaces with enhanced properties and functionalities.

Unravelling the interactions between small molecules and liposomal bilayers via molecular dynamics and thermodynamic modelling

Lipid-based drug delivery systems hold immense promise in addressing critical medical needs, from cancer and neurodegenerative diseases to infectious diseases. By encapsulating active pharmaceutical ingredients - ranging from small molecule drugs to proteins and nucleic acids - these nanocarriers enhance treatment efficacy and safety. However, their commercial success faces hurdles, such as the lack of a systematic design approach and the issues related to scalability and reproducibility. This work aims to provide insights into the drug-phospholipid interaction by combining molecular dynamic simulations and thermodynamic modelling techniques. In particular, we have made a connection between the structural properties of the drug-phospholipid system and the physicochemical performance of the drug-loaded liposomal nanoformulations. We have considered two prototypical drugs, felodipine (FEL) and naproxen (NPX), and one model hydrogenated soy phosphatidylcholine (HSPC) bilayer membrane. Molecular dynamic simulations revealed which regions within the phospholipid bilayers are most and least favoured by the drug molecules. NPX tends to reside at the water-phospholipid interface and is characterized by a lower free energy barrier for bilayer membrane permeation. Meanwhile, FEL prefers to sit within the hydrophobic tails of the phospholipids and is characterized by a higher free energy barrier for membrane permeation. Flory-Huggins thermodynamic modelling, small angle X-ray scattering, dynamic light scattering, TEM, and drug release studies of these liposomal nanoformulations confirmed this drug-phospholipid structural difference. The naproxen-phospholipid system has a lower free energy barrier for permeation, higher drug miscibility with the bilayer, larger liposomal nanoparticle size, and faster drug release in the aqueous medium than felodipine. We suggest that this combination of molecular dynamics and thermodynamics approach may offer a new tool for designing and developing lipid-based nanocarriers for unmet medical applications.

Distance tuneable integral membrane protein containing floating bilayers via in situ directed self-assembly

Model membranes allow for structural and biophysical studies on membrane biochemistry at the molecular level, albeit on systems of reduced complexity which can limit biological accuracy. Floating supported bilayers offer a means of producing planar lipid membrane models not adhered to a surface, which allows for improved accuracy compared to other model membranes. Here we communicate the incorporation of an integral membrane protein complex, the multidomain β-barrel assembly machinery (Bam), into our recently developed in situ self-assembled floating supported bilayers. Using neutron reflectometry and quartz crystal microbalance measurements we show this sample system can be fabricated using a two-step self-assembly process. We then demonstrate the complexity of the model membrane and tuneability of the membrane-to-surface distance using changes in the salt concentration of the bulk solution. Results demonstrate an easily fabricated, biologically accurate and tuneable membrane assay system which can be utilized for studies on integral membrane proteins within their native lipid matrix.

N-Formylation modifies membrane damage associated with PSMα3 interfacial fibrillation

The virulence of Staphylococcus aureus, a multi-drug resistant pathogen, notably depends on the expression of the phenol soluble modulins α3 (PSMα3) peptides, able to self-assemble into amyloid-like cross-α fibrils. Despite remarkable advances evidencing the crucial, yet insufficient, role of fibrils in PSMα3 cytotoxic activities towards host cells, the relationship between its molecular structures, assembly propensities, and modes of action remains an open intriguing problem. In this study, combining atomic force microscopy (AFM) imaging and infrared spectroscopy, we first demonstrated in vitro that the charge provided by the N-terminal capping of PSMα3 alters its interactions with model membranes of controlled lipid composition without compromising its fibrillation kinetics or morphology. N-formylation eventually dictates PSMα3-membrane binding via electrostatic interactions with the lipid head groups. Furthermore, PSMα3 insertion within the lipid bilayer is favoured by hydrophobic interactions with the lipid acyl chains only in the fluid phase of membranes and not in the gel-like ordered domains. Strikingly, our real-time AFM imaging emphasizes how intermediate protofibrillar entities, formed along PSMα3 self-assembly and promoted at the membrane interface, likely disrupt membrane integrity via peptide accumulation and subsequent membrane thinning in a peptide concentration and lipid-dependent manner. Overall, our multiscale and multimodal approach sheds new light on the key roles of N-formylation and intermediate self-assembling entities, rather than mature fibrils, in dictating deleterious interactions of PSMα3 with membrane lipids, likely underscoring its ultimate cellular toxicity in vivo, and in turn S. aureus pathogenesis.

Translational use of homing peptides: Tumor and placental targeting

HYPOTHESIS

Tissue-specific homing peptides have been shown to improve chemotherapeutic efficacy due to their trophism for tumor cells. Other sequences that selectively home to the placenta are providing new and safer therapeutics to treat complications in pregnancy. Our hypothesis is that the placental homing peptide RSGVAKS (RSG) may have binding affinity to cancer cells, and that insight can be gained into the binding mechanisms of RSG and the tumor homing peptide CGKRK to model membranes that mimic the primary lipid compositions of the respective cells.

EXPERIMENTS

Following cell culture studies on the binding efficacy of the peptides on a breast cancer cell line, a systematic translational characterization is delivered using ellipsometry, Brewster angle microscopy and neutron reflectometry of the extents, structures, and dynamics of the interactions of the peptides with the model membranes on a Langmuir trough.

FINDINGS

We start by revealing that RSG does indeed have binding affinity to breast cancer cells. The peptide is then shown to exhibit stronger interactions and greater penetration than CGKRK into both model membranes, combined with greater disruption to the lipid component. RSG also forms aggregates bound to the model membranes, yet both peptides bind to a greater extent to the placental than cancer model membranes. The results demonstrate the potential for varying local reservoirs of peptide within cell membranes that may influence receptor binding. The innovative nature of our findings motivates the urgent need for more studies involving multifaceted experimental platforms to explore the use of specific peptide sequences to home to different cellular targets.

Neutron reflectometry as a powerful tool to elucidate membrane interactions of drug delivery systems

The last couple of decades have seen an explosion of novel colloidal drug delivery systems, which have been demonstrated to increase drug efficacy, reduce side-effects, and provide various other advantages for both small-molecule and biomacromolecular drugs. The interactions of delivery systems with biomembranes are increasingly recognized to play a key role for efficient eradication of pathogens and cancer cells, as well as for intracellular delivery of protein and nucleic acid drugs. In parallel, there has been a broadening of methodologies for investigating such systems. For example, advanced microscopy, mass-spectroscopic "omic"-techniques, as well as small-angle X-ray and neutron scattering techniques, which only a few years ago were largely restricted to rather specialized areas within basic research, are currently seeing increased interest from researchers within wide application fields. In the present discussion, focus is placed on the use of neutron reflectometry to investigate membrane interactions of colloidal drug delivery systems. Although the technique is still less extensively employed for investigations of drug delivery systems than, e.g., X-ray scattering, such studies may provide key mechanistic information regarding membrane binding, re-modelling, translocation, and permeation, of key importance for efficacy and toxicity of antimicrobial, cancer, and other therapeutics. In the following, examples of this are discussed and gaps/opportunities in the research field identified.

Unraveling How Membrane Nanostructure Changes Impact the Eye Irritation of Nonionic Alkyl Ethoxylate Surfactants

Nonionic surfactants used in agri-spraying processes may cause varying degrees of corneal irritation when they come in direct contact with farmers' eyes, and the exact irritations are thought to be determined by how surfactants interact with corneal cell membranes. However, how nonionic surfactants interact with cell membranes at the molecular and nano levels remains largely unexplored. In this study, the interactions between nonionic surfactants (alkyl ethoxylate, C12Em) and lipid membranes were examined by membrane permeability measurement, quartz crystal microbalance with dissipation, dual polarization interferometry, confocal laser scanning microscopy, and neutron reflection, aiming to reveal complementary structural features at the molecular and nano levels. Apart from the extremely hydrophobic surfactant C12E2, all nonionic surfactants studied could penetrate the model cell membrane composed of a phosphocholine lipid bilayer. Nonionic surfactants with intermediate amphiphilicity (C12E6) rapidly fused into the lipid membrane and stimulated the formation of pores across the lipid bilayer, consistent with the cytoplasm leakage and fast cell necrosis observed from the cytotoxicity study of corneal cells. In comparison, while hydrophobic and hydrophilic surfactants [those with long and short ethoxylates (C12E4,12,23)] could cause mild structural alteration to the outer lipid layer of the membrane, these structural changes were insufficient to elicit large cytoplasmic leakage rapidly and instead cell death occurred over longer periods of time due to changes in the membrane permeability. These results reveal the strong link of surfactant-lipid membrane interactions to surfactant cytotoxicity and the association with amphiphilicity of nonionic surfactants.

Neutron reflection and scattering in characterising peptide assemblies

Self-assemblies of de novo designed short peptides at interface and in bulk solution provide potential platforms for developing applications in many medical and technological areas. However, characterising how bioinspired supramolecular nanostructures evolve with dynamic self-assembling processes and respond to different stimuli remains challenging. Neutron scattering technologies including small angle neutron scattering (SANS) and neutron reflection (NR) can be advantageous and complementary to other state-of-the-art techniques in tracing structural changes under different conditions. With more neutron sources now available, SANS and NR are becoming increasingly popular in studying self-assembling processes of diverse peptide and protein systems, but the difficulty in experimental manipulation and data analysis can deter beginners. This review will introduce the basic theory, general experimental setup and data analysis of SANS and NR, followed by provision of their applications in characterising interfacial and solution self-assemblies of representative peptides and proteins. SANS and NR are remarkably effective in determining the morphological features self-assembled short peptides, especially size and shape transitions as a result of either sequence changes or in response to environmental stimuli, demonstrating the unique capability of NR and SANS in unravelling the interactive processes. These examples highlight the potential of NR and SANS in supporting the development of novel short peptides and proteins as biopharmaceutical candidates in the fight against many diseases and infections that share common features of membrane interactive processes.

Recent advances in melittin-based nanoparticles for antitumor treatment: from mechanisms to targeted delivery strategies

As a naturally occurring cytolytic peptide, melittin (MLT) not only exhibits a potent direct tumor cell-killing effect but also possesses various immunomodulatory functions. MLT shows minimal chances for developing resistance and has been recognized as a promising broad-spectrum antitumor drug because of this unique dual mechanism of action. However, MLT still displays obvious toxic side effects during treatment, such as nonspecific cytolytic activity, hemolytic toxicity, coagulation disorders, and allergic reactions, seriously hampering its broad clinical applications. With thorough research on antitumor mechanisms and the rapid development of nanotechnology, significant effort has been devoted to shielding against toxicity and achieving tumor-directed drug delivery to improve the therapeutic efficacy of MLT. Herein, we mainly summarize the potential antitumor mechanisms of MLT and recent progress in the targeted delivery strategies for tumor therapy, such as passive targeting, active targeting and stimulus-responsive targeting. Additionally, we also highlight the prospects and challenges of realizing the full potential of MLT in the field of tumor therapy. By exploring the antitumor molecular mechanisms and delivery strategies of MLT, this comprehensive review may inspire new ideas for tumor multimechanism synergistic therapy.

Structural and functional insights into the delivery of a bacterial Rhs pore-forming toxin to the membrane

Bacterial competition is a significant driver of toxin polymorphism, which allows continual compensatory evolution between toxins and the resistance developed to overcome their activity. Bacterial Rearrangement hot spot (Rhs) proteins represent a widespread example of toxin polymorphism. Here, we present the 2.45 Å cryo-electron microscopy structure of Tse5, an Rhs protein central to Pseudomonas aeruginosa type VI secretion system-mediated bacterial competition. This structural insight, coupled with an extensive array of biophysical and genetic investigations, unravels the multifaceted functional mechanisms of Tse5. The data suggest that interfacial Tse5-membrane binding delivers its encapsulated pore-forming toxin fragment to the target bacterial membrane, where it assembles pores that cause cell depolarisation and, ultimately, bacterial death.

Electrostatic interactions control the adsorption of extracellular vesicles onto supported lipid bilayers

Communication between cells located in different parts of an organism is often mediated by membrane-enveloped nanoparticles, such as extracellular vesicles (EVs). EV binding and cell uptake mechanisms depend on the heterogeneous composition of the EV membrane. From a colloidal perspective, the EV membrane interacts with other biological interfaces via both specific and non-specific interactions, where the latter include long-ranged electrostatic and van der Waals forces, and short-ranged repulsive "steric-hydration" forces. While electrostatic forces are generally exploited in most EV immobilization protocols, the roles played by various colloidal forces in controlling EV adsorption on surfaces have not yet been thoroughly addressed. In the present work, we study the adsorption of EVs onto supported lipid bilayers (SLBs) carrying different surface charge densities using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and confocal laser scanning microscopy (CLSM). We demonstrate that EV adsorption onto lipid membranes can be controlled by varying the strength of electrostatic forces and we theoretically describe the observed phenomena within the framework of nonlinear Poisson-Boltzmann theory. Our modelling results confirm the experimental observations and highlight the crucial role played by attractive electrostatics in EV adsorption onto lipid membranes. They furthermore show that simplified theories developed for model lipid systems can be successfully applied to the study of their biological analogues and provide new fundamental insights into EV-membrane interactions with potential use in developing novel EV separation and immobilization strategies.

Biomacromolecular Characterizations Using State-of-the-Art Quartz Crystal Microbalance with Dissipation

Biomolecular characterization is essential in fields such as drug discovery, glycomics, and cell biology. This feature article focuses on the experimental use of quartz crystal microbalance with dissipation (QCM-D) as a powerful analytical technique to probe biological events ranging from biomacromolecular interactions and conformational changes of biomacromolecules to surface immobilization of biomacromolecules and cell morphological changes.

Engineering phosphatidylinositol-4,5-bisphosphate model membranes enriched in endocytic cargo: A neutron reflectometry, AFM and QCM-D structural study

The combination of in vitro models of biological membranes based on solid-supported lipid bilayers (SLBs) and of surface sensitive techniques, such as neutron reflectometry (NR), atomic force microscopy (AFM) and quartz crystal microbalance with dissipation monitoring (QCM-D), is well suited to provide quantitative information about molecular level interactions and lipid spatial distributions. In this work, cellular plasma membranes have been mimicked by designing complex SLB, containing phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2) lipids as well as incorporating synthetic lipo-peptides that simulate the cytoplasmic tails of transmembrane proteins. The QCM-D results revealed that the adsorption and fusion kinetics of PtdIns4,5P2 are highly dependent of Mg2+. Additionally, it was shown that increasing concentrations of PtdIns4,5P2 leads to the formation of SLBs with higher homogeneity. The presence of PtdIns4,5P2 clusters was visualized by AFM. NR provided important insights about the structural organization of the various components within the SLB, highlighting that the leaflet symmetry of these SLBs is broken by the presence of CD4-derived cargo peptides. Finally, we foresee our study to be a starting point for more sophisticated in vitro models of biological membranes with the incorporation of inositol phospholipids and synthetic endocytic motifs.

Creation of distinctive Bax-lipid complexes at mitochondrial membrane surfaces drives pore formation to initiate apoptosis

Apotosis is an essential process tightly regulated by the Bcl-2 protein family where proapoptotic Bax triggers cell death by perforating the mitochondrial outer membrane. Although intensively studied, the molecular mechanism by which these proteins create apoptotic pores remains elusive. Here, we show that Bax creates pores by extracting lipids from outer mitochondrial membrane mimics by formation of Bax/lipid clusters that are deposited on the membrane surface. Time-resolved neutron reflectometry and Fourier transform infrared spectroscopy revealed two kinetically distinct phases in the pore formation process, both of which were critically dependent on cardiolipin levels. The initially fast adsorption of Bax on the mitochondrial membrane surface is followed by a slower formation of pores and Bax-lipid clusters on the membrane surface. Our findings provide a robust molecular understanding of mitochondrial membrane perforation by cell-killing Bax protein and illuminate the initial phases of programmed cellular death.

Bilayer lipid membrane formation on surface assemblies with sparsely distributed tethers

A combined computational and experimental study of small unilamellar vesicle (SUV) fusion on mixed self-assembled monolayers (SAMs) terminated with different deuterated tether moieties (-(CD₂)₇CD₃ or -(CD₂)₁₅CD₃) is reported. Tethered bilayer lipid membrane (tBLM) formation of synthetic 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine was initially probed on SAMs with controlled tether (d-alkyl tail) surface densities and lateral molecular packing using quartz crystal microbalance with dissipation monitoring (QCM-D). Long time-scale coarse-grained molecular dynamics (MD) simulations were then employed to elucidate the mechanisms behind the interaction between the SUVs and the different phases formed by the -(CD₂)₇CD₃ and -(CD₂)₁₅CD₃ tethers. Furthermore, a series of real time kinetics was recorded under different osmotic conditions using QCM-D to determine the accumulated lipid mass and for probing the fusion process. It is shown that the key factors driving the SUV fusion and tBLM formation on this type of surfaces involve tether insertion into the SUVs along with vesicle deformation. It is also evident that surface densities of the tethers as small as a few mol% are sufficient to obtain stable tBLMs with a high reproducibility. The described "sparsely tethered" tBLM system can be advantageous in studying different biophysical phenomena, such as membrane protein insertion, effects of receptor clustering, and raft formation.

Molecular Insights for Alzheimer's Disease: An Unexplored Storyline on the Nanoscale Impact of Nascent Aβ1-42 toward the Lipid Membrane

Deciphering the mechanism of Alzheimer's disease is a key element for designing an efficient therapeutic strategy. Molecular dynamics (MD) calculations, atomic force microscopy, and infrared spectroscopy were combined to investigate β-amyloid (Aβ_1-42) peptide interactions with supported lipid bilayers (SLBs). The MD simulations showed that nascent Aβ_1-42 monomers remain anchored within a model phospholipid bilayer's hydrophobic core, which suggests their stability in their native environment. We tested this prediction experimentally by studying the behavior of Aβ_1-42 monomers and oligomers when interacting with SLBs. When Aβ_1-42 monomers and oligomers were self-assembled with a lipid bilayer and deposited as an SLB, they remain within the bilayers. Their presence in the bilayers induces destabilization of the model membranes. No specific interactions between Aβ_1-42 and the SLBs were detected when SLBs free of Aβ_1-42 were exposed to Aβ_1-42. This study suggests that Aβ can remain in the membrane after cleavage by γ-secretase and cause severe damage to the membrane.

Structural Characterization of Nanoparticle-Supported Lipid Bilayer Arrays by Grazing Incidence X-ray and Neutron Scattering

Arrays of nanoparticle-supported lipid bilayers (nanoSLB) are lipid-coated nanopatterned interfaces that provide a platform to study curved model biological membranes using surface-sensitive techniques. We combined scattering techniques with direct imaging, to gain access to sub-nanometer scale structural information on stable nanoparticle monolayers assembled on silicon crystals in a noncovalent manner using a Langmuir-Schaefer deposition. The structure of supported lipid bilayers formed on the nanoparticle arrays via vesicle fusion was investigated using a combination of grazing incidence X-ray and neutron scattering techniques complemented by fluorescence microscopy imaging. Ordered nanoparticle assemblies were shown to be suitable and stable substrates for the formation of curved and fluid lipid bilayers that retained lateral mobility, as shown by fluorescence recovery after photobleaching and quartz crystal microbalance measurements. Neutron reflectometry revealed the formation of high-coverage lipid bilayers around the spherical particles together with a flat lipid bilayer on the substrate below the nanoparticles. The presence of coexisting flat and curved supported lipid bilayers on the same substrate, combined with the sub-nanometer accuracy and isotopic sensitivity of grazing incidence neutron scattering, provides a promising novel approach to investigate curvature-dependent membrane phenomena on supported lipid bilayers.

Quartz crystal microbalance and atomic force microscopy to characterize mimetic systems based on supported lipids bilayer

Quartz Crystal Microbalance (QCM) with dissipation and Atomic Force Microscopy (AFM) are two characterization techniques that allow describing processes taking place at solid-liquid interfaces. Both are label-free and, when used in combination, provide kinetic, thermodynamic and structural information at the nanometer scale of events taking place at surfaces. Here we describe the basic operation principles of both techniques, addressing a non-specialized audience, and provide some examples of their use for describing biological events taking place at supported lipid bilayers (SLBs). The aim is to illustrate current strengths and limitations of the techniques and to show their potential as biophysical characterization techniques.

Nucleic Acid-Loaded Lipid Nanoparticle Interactions with Model Endosomal Membranes

Lipid nanoparticles (LNPs) are important delivery systems for RNA-based therapeutics, yet the mechanism of their interaction with endosomal membranes remains unclear. Here, the interactions of nucleic acid-loaded LNPs that contain an ionizable lipid with models of the early and late endosomal membranes are studied, for the first time, using different reflectometry techniques. Novel insight is provided with respect to the subphase pH, the stage of the endosome, and the nature of the nucleic acid cargo. It is found that the insertion of lipids from the LNPs into the model membrane is greatest at pH 6.5 and 5.5, whereas at higher pH, lipid insertion is suppressed with evidence instead for the binding of intact LNPs, demonstrating the importance of the pH in the fusion of LNPs undergoing the endosomal pathway. Furthermore, and independently of the pH, the effect of the early- versus late-stage endosomal models is minimal, suggesting that the increased fluidity and anionic nature of the late endosome has little effect on the extent of LNP interaction. Last, there is greater nucleic acid delivery from LNPs containing mRNA than Poly(A), indicating that the extent of interaction can be tuned according to the nature of the nucleic acid cargo. Such new information on the relative impact of factors influencing nucleic acid delivery by LNP interactions with endosomal membranes is important in the design and tuning of vehicles with improved nucleic acid delivery capacities.

Formation of a Fully Anionic Supported Lipid Bilayer to Model Bacterial Inner Membrane for QCM-D Studies

Supported lipid bilayers (SLBs) on quartz crystals are employed as versatile model systems for studying cell membrane behavior with the use of the highly sensitive technique of quartz crystal microbalance with dissipation monitoring (QCM-D). Since the lipids constituting cell membranes vary from predominantly zwitterionic lipids in mammalian cells to predominantly anionic lipids in the inner membrane of Gram-positive bacteria, the ability to create SLBs of different lipid compositions is essential for representing different cell membranes. While methods to generate stable zwitterionic SLBs and zwitterionic-dominant mixed zwitterionic–anionic SLBs on quartz crystals have been well established, there are no reports of being able to form predominantly or fully anionic SLBs. We describe here a method for forming entirely anionic SLBs by treating the quartz crystal with cationic (3-aminopropyl) trimethoxysilane (APTMS). The formation of the anionic SLB was tracked using QCM-D by monitoring the adsorption of anionic lipid vesicles to a quartz surface and subsequent bilayer formation. Anionic egg L-α-phosphatidylglycerol (PG) vesicles adsorbed on the surface-treated quartz crystal, but did not undergo the vesicle-to-bilayer transition to create an SLB. However, when PG was mixed with 10–40 mole% 1-palmitoyl-2-hydroxy-sn-glycero-3-phospho-(1′-rac-glycerol) (LPG), the mixed vesicles led to the formation of stable SLBs. The dynamics of SLB formation monitored by QCM-D showed that while SLB formation by zwitterionic lipids followed a two-step process of vesicle adsorption followed by the breakdown of the adsorbed vesicles (which in turn is a result of multiple events) to create the SLB, the PG/LPG mixed vesicles ruptured immediately on contacting the quartz surface resulting in a one-step process of SLB formation. The QCM-D data also enabled the quantitative characterization of the SLB by allowing estimation of the lipid surface density as well as the thickness of the hydrophobic region of the SLB. These fully anionic SLBs are valuable model systems to conduct QCM-D studies of the interactions of extraneous substances such as antimicrobial peptides and nanoparticles with Gram-positive bacterial membranes.

Deciphering the role of plant plasma membrane lipids in response to invasion patterns: how could biology and biophysics help?

Plants have to constantly face pathogen attacks. To cope with diseases, they have to detect the invading pathogen as early as possible via the sensing of conserved motifs called invasion patterns. The first step of perception occurs at the plasma membrane. While many invasion patterns are perceived by specific proteinaceous immune receptors, several studies have highlighted the influence of the lipid composition and dynamics of the plasma membrane in the sensing of invasion patterns. In this review, we summarize current knowledge on how some microbial invasion patterns could interact with the lipids of the plasma membrane, leading to a plant immune response. Depending on the invasion pattern, different mechanisms are involved. This review outlines the potential of combining biological with biophysical approaches to decipher how plasma membrane lipids are involved in the perception of microbial invasion patterns.

The interaction of styrene maleic acid copolymers with phospholipids in Langmuir monolayers, vesicles and nanodiscs; a structural study

HYPOTHESIS

Self-assembly of amphipathic styrene maleic acid copolymers with phospholipids in aqueous solution results in the formation of 'nanodiscs' containing a planar segment of phospholipid bilayer encapsulated by a polymer belt. Recently, studies have reported that lipids rapidly exchange between both nanodiscs in solution and external sources of lipids. Outstanding questions remain regarding details of polymer-lipid interactions, factors influencing lipid exchange and structural effects of such exchange processes. Here, the dynamic behaviour of nanodiscs is investigated, specifically the role of membrane charge and polymer chemistry.

EXPERIMENTS

Two model systems are investigated: fluorescently labelled phospholipid vesicles, and Langmuir monolayers of phospholipids. Using fluorescence spectroscopy and time-resolved neutron reflectometry, the membrane potential, monolayer structure and composition are monitored with respect to time upon polymer and nanodisc interactions.

FINDINGS

In the presence of external lipids, polymer chains embed throughout lipid membranes, the extent of which is governed by the net membrane charge. Nanodiscs stabilised by three different polymers will all exchange lipids and polymer with monolayers to differing extents, related to the properties of the stabilising polymer belt. These results demonstrate the dynamic nature of nanodiscs which interact with the local environment and are likely to deposit both lipids and polymer at all stages of use.

Lipopolysaccharides at Solid and Liquid Interfaces: Models for Biophysical Studies of the Gram-negative Bacterial Outer Membrane

Lipopolysaccharides (LPSs) are a constitutive element of the cell envelope of Gram-negative bacteria, representing the main lipid in the external leaflet of their outer membrane (OM) lipid bilayer. These unique surface-exposed glycolipids play a central role in the interactions of Gram-negative organisms with their surrounding environment and represent a key element for protection against antimicrobials and the development of antibiotic resistance. The biophysical investigation of a wide range of different types of in vitro model membranes containing reconstituted LPS has revealed functional and structural properties of these peculiar membrane lipids, providing molecular-level details of their interaction with antimicrobial compounds. LPS assemblies reconstituted at interfaces represent a versatile tool to study the properties of the Gram-negative OM by exploiting several surface-sensitive techniques, in particular X-ray and neutron scattering, which can probe the structure of thin films with sub-nanometer resolution. This review provides an overview of different approaches employed to investigate structural and biophysical properties of LPS, focusing on studies on Langmuir monolayers of LPS at the air/liquid interface and a range of supported LPS-containing model membranes reconstituted at solid/liquid interfaces.

Fluid Films as Models for Understanding the Impact of Inhaled Particles in Lung Surfactant Layers

Pollution is currently a public health problem associated with different cardiovascular and respiratory diseases. These are commonly originated as a result of the pollutant transport to the alveolar cavity after their inhalation. Once pollutants enter the alveolar cavity, they are deposited on the lung surfactant (LS) film, altering their mechanical performance which increases the respiratory work and can induce a premature alveolar collapse. Furthermore, the interactions of pollutants with LS can induce the formation of an LS corona decorating the pollutant surface, favoring their penetration into the bloodstream and distribution along different organs. Therefore, it is necessary to understand the most fundamental aspects of the interaction of particulate pollutants with LS to mitigate their effects, and design therapeutic strategies. However, the use of animal models is often invasive, and requires a careful examination of different bioethics aspects. This makes it necessary to design in vitro models mimicking some physico-chemical aspects with relevance for LS performance, which can be done by exploiting the tools provided by the science and technology of interfaces to shed light on the most fundamental physico-chemical bases governing the interaction between LS and particulate matter. This review provides an updated perspective of the use of fluid films of LS models for shedding light on the potential impact of particulate matter in the performance of LS film. It should be noted that even though the used model systems cannot account for some physiological aspects, it is expected that the information contained in this review can contribute on the understanding of the potential toxicological effects of air pollution.

Model architectures for bacterial membranes

The complex composition of bacterial membranes has a significant impact on the understanding of pathogen function and their development towards antibiotic resistance. In addition to the inherent complexity and biosafety risks of studying biological pathogen membranes, the continual rise of antibiotic resistance and its significant economical and clinical consequences has motivated the development of numerous in vitro model membrane systems with tuneable compositions, geometries, and sizes. Approaches discussed in this review include liposomes, solid-supported bilayers, and computational simulations which have been used to explore various processes including drug-membrane interactions, lipid-protein interactions, host-pathogen interactions, and structure-induced bacterial pathogenesis. The advantages, limitations, and applicable analytical tools of all architectures are summarised with a perspective for future research efforts in architectural improvement and elucidation of resistance development strategies and membrane-targeting antibiotic mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-021-00913-7.

Investigating Membrane‐Mediated Antimicrobial Peptide Interactions with Synchrotron Radiation Far‐Infrared Spectroscopy

Synchrotron radiation‐based Fourier transform infrared spectroscopy enables access to vibrational information from mid over far infrared to even terahertz domains. This information may prove critical for the elucidation of fundamental bio‐molecular phenomena including folding‐mediated innate host defence mechanisms. Antimicrobial peptides (AMPs) represent one of such phenomena. These are major effector molecules of the innate immune system, which favour attack on microbial membranes. AMPs recognise and bind to the membranes whereupon they assemble into pores or channels destabilising the membranes leading to cell death. However, specific molecular interactions responsible for antimicrobial activities have yet to be fully understood. Herein we probe such interactions by assessing molecular specific variations in the near‐THz 400–40 cm⁻¹ range for defined helical AMP templates in reconstituted phospholipid membranes. In particular, we show that a temperature‐dependent spectroscopic analysis, supported by 2D correlative tools, provides direct evidence for the membrane‐induced and folding‐mediated activity of AMPs. The far‐FTIR study offers a direct and information‐rich probe of membrane‐related antimicrobial interactions.

Screening of the binding affinity of serum proteins to lipid nanoparticles in a cell free environment

Lipid nanoparticles (LNPs) are promising drug and gene carriers. Upon intravenous administration, LNPs' experience different degree of cellular uptake depending on their formulation. Currently, in vitro and in vivo studies are the gold standard for assessing the fate of nano carriers once administered, but they are time consuming and expensive. In this work, we propose a time and cost-effective method to screen a wide range of LNP formulations and select the most promising candidates for in vitro and in vivo studies. Two different approaches were explored to investigate the binding affinity between LNPs and serum proteins using sensor functionalisation with either protein specific antibody or PEG specific antibody. The first approach allowed to identify the presence of a specific protein in the protein corona of lipid particles (reconstituted and native high-density lipoproteins (rHDL and HDL), and low-density lipoproteins LDL); while the second one provided a versatile platform for the immobilisation of pegylated-particles in order to follow the interaction with serum proteins and hence predict the composition of LNP protein corona. Sensing was done using Quartz Crystal Microbalance with Dissipation (QCM-D) but the approach is extendable to other surface sensing techniques such as Surface Plasmon Resonance (SPR) or ellipsometry.

Stiffness of Fluid and Gel Phase Lipid Nanovesicles: Weighting the Contributions of Membrane Bending Modulus and Luminal Pressurization

The mechanical properties of biogenic membranous compartments are thought to be relevant in numerous biological processes; however, their quantitative measurement remains challenging for most of the already available force spectroscopy (FS)-based techniques. In particular, the debate on the mechanics of lipid nanovesicles and on the interpretation of their mechanical response to an applied force is still open. This is mostly due to the current lack of a unified model being able to describe the mechanical response of both gel and fluid phase lipid vesicles and to disentangle the contributions of membrane rigidity and luminal pressure. In this framework, we herein propose a simple model in which the interplay of membrane rigidity and luminal pressure to the overall vesicle stiffness is described as a series of springs; this approach allows estimating these two contributions for both gel and fluid phase liposomes. Atomic force microscopy-based FS, performed on both vesicles and supported lipid bilayers, is exploited for obtaining all the parameters involved in the model. Moreover, the use of coarse-grained full-scale molecular dynamics simulations allowed for better understanding of the differences in the mechanical responses of gel and fluid phase bilayers and supported the experimental findings. The results suggest that the pressure contribution is similar among all the probed vesicle types; however, it plays a dominant role in the mechanical response of lipid nanovesicles presenting a fluid phase membrane, while its contribution becomes comparable to the one of membrane rigidity in nanovesicles with a gel phase lipid membrane. The results presented herein offer a simple way to quantify two of the most important parameters in vesicle nanomechanics (membrane rigidity and internal pressurization), and as such represent a first step toward a currently unavailable, unified model for the mechanical response of gel and fluid phase lipid nanovesicles.

A bactericide peptide changing the static and dilatational surface elasticity properties of zwitterionic lipids at the air-water interface: Relationship with the thermodynamic, structural and morphological properties

In this paper, we studied how different hydrophilicity degrees of the polar groups of the lipids dipalmitoylphosphatidylcholine (DPPC) and dipalmitoyl phosphatidylethanolamine (DPPE) influence the interaction of the antibiotic peptide vancomycin (VC), affecting the physicochemical properties of the monolayers, including thermodynamic, rheological, structural and morphological ones. Lipid Langmuir monolayers were prepared at air-water interfaces with VC aqueous solution as subphase and characterized with tensiometry, Brewster angle microscopy, infrared spectroscopy, dilatational, and interfacial shear rheology. The presence of PC or PE groups as polar head groups of the phospholipid monolayers modulated the interaction of VC adsorbing from the aqueous subphase since for DPPC, vancomycin condenses the monolayer, making it less stable, fluid, and more disordered. In contrast, for DPPE, vancomycin expands the monolayer, making it more stable, keeping the compressibility, and leading to the formation of interfacial aggregates, which are not observed for DPPC. We concluded thatelectrostatic interactions induced the insertion of the peptide into the polar heads of the monolayers (DPPE), while hydrophobic interactions, in addition to ion-dipole interactions, induced the adsorption of the peptide onto the polar head of the monolayers (DPPC).

Dissimilar action of tamoxifen and 4-hydroxytamoxifen on phosphatidylcholine model membranes

The anticancer drug tamoxifen and its primary metabolite 4-hydroxytamoxifen tend to accumulate in membranes due to its strong hydrophobic character. Thus, in this work we have carried out a systematic study to investigate their effects on model phosphatidylcholine membranes. Tamoxifen and 4-hydroxytamoxifen affect the phase behaviour of phosphatidylcholine model membranes, giving rise to formation of drug/dipalmitoylphosphatidylcholine domains, which is more evident in the case of 4-hydroxytamoxifen. These drugs have differential effects on the polar and apolar regions of the phospholipid supporting a different location of both compounds within the bilayer. Both compounds induce contents leakage in fluid phosphatidylcholine unilamellar liposomes, the effect of 4-hydroxytamoxifen being negligible as compared to that of tamoxifen. Molecular dynamics confirmed the tendency of both drugs to form clusters, tamoxifen locating all along the bilayer, whereas 4-hydroxytamoxifen mostly locates near the lipid/water interface, which can explain the different effects of both drugs in fluid phosphatidylcholine membranes.

Investigation of Self-Emulsifying Drug-Delivery System Interaction with a Biomimetic Membrane under Conditions Relevant to the Small Intestine

Self-emulsifying drug-delivery systems (SEDDS) have been extensively shown to increase oral absorption of solvation-limited compounds. However, there has been little clinical and commercial use of these formulations, in large part because the demonstrated advantages of SEDDS have been outweighed by our inability to precisely predict drug absorption from SEDDS using current in vitro assays. To overcome this limitation and increase the biological relevancy of in vitro assays, an absorption function can be incorporated using biomimetic membranes. However, the effects that SEDDS have on the integrity of a biomimetic membrane are not known. In this study, a quartz crystal microbalance with dissipation monitoring and total internal reflection fluorescence microscopy were employed as complementary methods to in vitro lipolysis-permeation assays to characterize the interaction of various actively digested SEDDS with a liquescent artificial membrane comprising lecithin in dodecane (LiDo). Observations from surface analysis showed that interactions between the digesting SEDDS and LiDo membrane coincided with inflection points in the digestion profiles. Importantly, no indications of membrane damage could be observed, which was supported by flux profiles of the lipophilic model drug felodipine (FEL) and impermeable marker Lucifer yellow on the basal side of the membrane. There was a correlation between the digestion kinetics of the SEDDS and the flux of FEL, but no clear correlation between solubilization and absorption profiles. Membrane interactions were dependent on the composition of lipids within each SEDDS, with the more digestible lipids leading to more pronounced interactions, but in all cases, the integrity of the membrane was maintained. These insights demonstrate that LiDo membranes are compatible with in vitro lipolysis assays for improving predictions of drug absorption from lipid-based formulations.

Large scale model lipid membrane movement induced by a cation switch

A biomembrane sample system where millimolar changes of cations induce reversible large scale (≥ 200 Å) changes in the membrane-to-surface distance is described. The system composes of a free-floating bilayer, formed adjacent to a self-assembled monolayer (SAM). To examine the membrane movements, differently charged floating bilayers in the presence and absence of Ca2+ and Na+, respectively, were examined using neutron reflectivity and quartz crystal microbalance measurements, alongside molecular dynamics simulations. In neutron reflectivity the variation of Ca2+ and Na+ concentration enabled precision manipulation of the membrane-to-surface distance. Simulations suggest that Ca2+ ions bridge between SAM and bilayer whereas the more diffuse binding of Na+, especially to bilayers, is unable to fully overcome the repulsion between anionic floating bilayer and anionic SAM. Reproduced neutron reflectivity results with quartz crystal microbalance demonstrate the potential of this easily producible sample system to become a standard analysis tool for e.g. investigating membrane binding effects, endocytosis and cell signaling.

Studying the surfaces of bacteria using neutron scattering: finding new openings for antibiotics

The use of neutrons as a scattering probe to investigate biological membranes has steadily grown in the past three decades, shedding light on the structure and behaviour of this ubiquitous and fundamental biological barrier. Meanwhile, the rise of antibiotic resistance has catalysed a renewed interest in understanding the mechanisms underlying the dynamics of antibiotics interaction with the bacterial cell envelope. It is widely recognised that the key reason behind the remarkable success of Gram-negative pathogens in developing antibiotic resistance lies in the effectiveness of their outer membrane (OM) in defending the cell from antibacterial compounds. Critical to its function, the highly asymmetric lipid distribution between the inner and outer bilayer leaflets of the OM, adds an extra level of complexity to the study of this crucial defence barrier. Here we review the opportunities offered by neutron scattering techniques, in particular reflectometry, to provide structural information on the interactions of antimicrobials with in vitro models of the OM. The differential sensitivity of neutrons towards hydrogen and deuterium makes them a unique probe to study the structure and behaviour of asymmetric membranes. Molecular-level understanding of the interactions between antimicrobials and the Gram-negative OM provides valuable insights that can aid drug development and broaden our knowledge of this critically important biological barrier.

Investigating lipid headgroup composition within epithelial membranes: a systematic review

Membrane lipid composition is often quoted within the literature, but with very little insight into how or why these compositions vary when compared to other biological membranes. One prominent area that lacks understanding in terms of rationale for lipid variability is the human gastro-intestinal tract (GIT). We have carried out a comprehensive systematic literature search to ascertain the key lipid components of epithelial membranes, with a particular focus on addressing the human GIT and to use compositional data to understand structural aspects of biological membranes. Both bacterial outer membranes and the human erythrocyte membrane were used as a comparison for the mammalian [epithelial] membranes and to understand variations in lipid presence. We show that phosphatidylcholine (PC) lipid types tend to dominate (33%) with phosphatidylethanolamines (PE) and cholesterol having very similar abundances (25 and 23% respectively). This systematic review presents a detailed insight into lipid headgroup composition and roles in various membrane types, with a summary of the distinction between the major lipid bilayer forming lipids and how peripheral lipids regulate charge and fluidity. The variety of lipids present in biological membranes is discussed and rationalised in terms function as well as cellular position.

Unravelling the structural complexity of protein-lipid interactions with neutron reflectometry

Neutron reflectometry (NR) is a large-facility technique used to examine structure at interfaces. In this brief review an introduction to the utilisation of NR in the study of protein-lipid interactions is given. Cold neutron beams penetrate matter deeply, have low energies, wavelengths in the Ångstrom regime and are sensitive to light elements. High differential hydrogen sensitivity (between protium and deuterium) enables solution and sample isotopic labelling to be utilised to enhance or diminish the scattering signal of individual components within complex biological structures. The combination of these effects means NR can probe buried structures such as those at the solid-liquid interface and encode molecular level structural information on interfacial protein-lipid complexes revealing the relative distribution of components as well as the overall structure. Model biological membrane sample systems can be structurally probed to examine phenomena such as antimicrobial mode of activity, as well as structural and mechanistic properties peripheral/integral proteins within membrane complexes. Here, the example of the antimicrobial protein α1-purothionin binding to a model Gram negative bacterial outer membrane is used to highlight the utilisation of this technique, detailing how changes in the protein/lipid distributions across the membrane before and after the protein interaction can be easily encoded using hydrogen isotope labelling.

In situ determination of the structure and composition of Langmuir monolayers at the air/water interface by neutron and X-ray reflectivity and ellipsometry

This review focuses on the description of the structure and composition of a variety of Langmuir monolayers (LMs) deposited at the air/water interface by using ellipsometry, Brewster Angle microscopy and scattering techniques, mainly neutron and X-ray reflectometry. Since the first experiment done by Angels Pockels with a homemade trough in her home kitchen until today, LMs of different materials have been extensively studied providing not only relevant model systems in biology, physics and chemistry but also precursors of novel materials via their deposition on solid substrates. There is a vast amount of surface-active materials that can form LMs and, therefore, far from a revision of the state-of-the-art, we will emphasize here: (i) some fundamental aspects to understand the physics behind the molecular deposition at the air/water interface; (ii) the advantages in using in situ techniques, such as reflectometry or ellipsometry, to resolve the interfacial architecture and conformation of molecular films; and, finally, (iii) a summary of several systems that have certain interest from the experimental or conceptual point of view. Concretely, we will report here advances in polymers confined to interfaces and surfactants, from fatty acids and phospholipids monolayers to more unconventional ones such as graphene oxide.

Pore-Spanning Plasma Membranes Derived from Giant Plasma Membrane Vesicles

Giant plasma membrane vesicles (GPMVs) are a highly promising model system for the eukaryotic plasma membrane. The unresolved challenge, however, is a path to surface-based structures that allows accessibility to both sides of the plasma membrane through high-resolution techniques. Such an approach would pave the way to advanced chip-based technologies for the analysis of complex cell surfaces to study the roles of membrane proteins, host-pathogen interactions, and many other bioanalytical and sensing applications. This study reports the generation of planar supported plasma membranes and for the first-time pore-spanning plasma membranes (PSPMs) derived from pure GPMVs that are spread on activated solid and highly ordered porous silicon substrates. GPMVs were produced by two different vesiculation agents and were first investigated with respect to their growth behavior and phase separation. Second, these GPMVs were spread onto silicon substrates to form planar supported plasma membrane patches. PSPMs were obtained by spreading of pure GPMVs on oxygen-plasma activated porous substrates with pore diameters of 3.5 μm. Fluorescence micrographs unambiguously showed that the PSPMs partially phase separate in a mobile ordered phase surrounded by a disordered phase, which was supported by cholesterol extraction using methyl-β-cyclodextrin.

Why Do Tethered-Bilayer Lipid Membranes Suit for Functional Membrane Protein Reincorporation?

Membrane proteins (MPs) are essential for cellular functions. Understanding the functions of MPs is crucial as they constitute an important class of drug targets. However, MPs are a challenging class of biomolecules to analyze because they cannot be studied outside their native environment. Their structure, function and activity are highly dependent on the local lipid environment, and these properties are compromised when the protein does not reside in the cell membrane. Mammalian cell membranes are complex and composed of different lipid species. Model membranes have been developed to provide an adequate environment to envisage MP reconstitution. Among them, tethered-Bilayer Lipid Membranes (tBLMs) appear as the best model because they allow the lipid bilayer to be decoupled from the support. Thus, they provide a sufficient aqueous space to envisage the proper accommodation of large extra-membranous domains of MPs, extending outside. Additionally, as the bilayer remains attached to tethers covalently fixed to the solid support, they can be investigated by a wide variety of surface-sensitive analytical techniques. This review provides an overview of the different approaches developed over the last two decades to achieve sophisticated tBLMs, with a more and more complex lipid composition and adapted for functional MP reconstitution.

ApoE and ApoE Nascent-Like HDL Particles at Model Cellular Membranes: Effect of Protein Isoform and Membrane Composition

Apolipoprotein E (ApoE), an important mediator of lipid transportation in plasma and the nervous system, plays a large role in diseases such as atherosclerosis and Alzheimer's. The major allele variants ApoE3 and ApoE4 differ only by one amino acid. However, this difference has major consequences for the physiological behaviour of each variant. In this paper, we follow (i) the initial interaction of lipid-free ApoE variants with model membranes as a function of lipid saturation, (ii) the formation of reconstituted High-Density Lipoprotein-like particles (rHDL) and their structural characterisation, and (iii) the rHDL ability to exchange lipids with model membranes made of saturated lipids in the presence and absence of cholesterol [1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) with and without 20 mol% cholesterol]. Our neutron reflection results demonstrate that the protein variants interact differently with the model membranes, adopting different protein conformations. Moreover, the ApoE3 structure at the model membrane is sensitive to the level of lipid unsaturation. Small-angle neutron scattering shows that the ApoE containing lipid particles form elliptical disc-like structures, similar in shape but larger than nascent or discoidal HDL based on Apolipoprotein A1 (ApoA1). Neutron reflection shows that ApoE-rHDL do not remove cholesterol but rather exchange saturated lipids, as occurs in the brain. In contrast, ApoA1-containing particles remove and exchange lipids to a greater extent as occurs elsewhere in the body.

Structural elucidation upon binding of antimicrobial peptides into binary mixed lipid monolayers mimicking bacterial membranes

HYPOTHESIS

Antimicrobial peptides (AMPs) kill microorganisms by causing structural damage to bacterial membranes. Different microorganisms often require a different type and concentration of an AMP to achieve full microbial killing. We hypothesise that the difference is caused by different membrane structure and composition.

EXPERIMENTS

Given the complexities of bacterial membranes, we have used monolayers of the binary DPPG/TMCL mixture to mimic the cytoplasmic membrane of Gram-positive bacteria and the binary DPPG/DPPE mixture to mimic the cytoplasmic membrane of Gram-negative bacteria, where DPPG, TMCL and DPPE stand for 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol), 1',3'-bis[1,2-dimyristoyl-sn-glycero-3-phospho]-sn-glycerol, and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, respectively. A Langmuir trough was specially designed to control the spread lipid monolayers and facilitate neutron reflectivity measurements.

FINDINGS

Surface pressure-area isotherm analysis revealed that all binary lipid systems mix non-ideally, but mixing is thermodynamically favoured. An increase in the surface pressure encourages demixing, resulting in phase separation and formation of clusters. Neutron reflectivity measurements were undertaken to study the binding of an antimicrobial peptide G(IIKK)₄-I-NH₂ (G₄) to the binary DPPG/TMCL and DPPG/DPPE monolayer mixtures at the molar ratios of 6/4 and 3/7, respectively. The results revealed stronger binding and penetration of G₄ to the DPPG/TMCL monolayer, indicating greater affinity of the antimicrobial peptide due to the electrostatic interaction and more extensive penetration into the more loosely packed lipid film. This work helps explain how AMPs attack different bacterial membranes, and the results are discussed in the context of other lipid models and antibacterial studies.

Impact of antimicrobial peptides on E. coli-mimicking lipid model membranes: correlating structural and dynamic effects using scattering methods

Using X-rays and neutrons we address the effect of AMPs on structure and dynamics of lipids in bacterial model membranes.

Mimicking the Mammalian Plasma Membrane: An Overview of Lipid Membrane Models for Biophysical Studies

Cell membranes are very complex biological systems including a large variety of lipids and proteins. Therefore, they are difficult to extract and directly investigate with biophysical methods. For many decades, the characterization of simpler biomimetic lipid membranes, which contain only a few lipid species, provided important physico-chemical information on the most abundant lipid species in cell membranes. These studies described physical and chemical properties that are most likely similar to those of real cell membranes. Indeed, biomimetic lipid membranes can be easily prepared in the lab and are compatible with multiple biophysical techniques. Lipid phase transitions, the bilayer structure, the impact of cholesterol on the structure and dynamics of lipid bilayers, and the selective recognition of target lipids by proteins, peptides, and drugs are all examples of the detailed information about cell membranes obtained by the investigation of biomimetic lipid membranes. This review focuses specifically on the advances that were achieved during the last decade in the field of biomimetic lipid membranes mimicking the mammalian plasma membrane. In particular, we provide a description of the most common types of lipid membrane models used for biophysical characterization, i.e., lipid membranes in solution and on surfaces, as well as recent examples of their applications for the investigation of protein-lipid and drug-lipid interactions. Altogether, promising directions for future developments of biomimetic lipid membranes are the further implementation of natural lipid mixtures for the development of more biologically relevant lipid membranes, as well as the development of sample preparation protocols that enable the incorporation of membrane proteins in the biomimetic lipid membranes.

Understanding the Structure and Dynamics of Complex Biomembrane Interactions by Neutron Scattering Techniques

The membrane is one of the key structural materials of biology at the cellular level. Composed predominantly of a bilayer of lipids with embedded and bound proteins, it defines the boundaries of the cell and many organelles essential to life and therefore is involved in almost all biological processes. Membrane-specific interactions, such as drug binding to a membrane receptor or the interactions of an antimicrobial compound with the lipid matrix of a pathogen membrane, are of interest across the scientific disciplines. Herein we present a review, aimed at nonexperts, of the major neutron scattering techniques used in membrane studies: small-angle neutron scattering, neutron membrane diffraction, neutron reflectometry, quasielastic neutron scattering, and neutron spin echo. Neutron scattering techniques are well suited to studying biological membranes. The nondestructive nature of cold neutrons means that samples can be measured for long periods without fear of beam damage from ultraviolet, electron, or X-ray radiation, and neutron beams are highly penetrating, thus offering flexibility in samples and sample environments. Most important is the strong difference in neutron scattering lengths between the two most abundant forms of hydrogen, protium and deuterium. Changing the relative amounts of protium/deuterium in a sample allows the production of a series of neutron scattering data sets, enabling the observation of differing components within complex membrane architectures. This approach can be as simple as using the naturally occurring neutron contrast between different biomolecules to study components in a complex by changing the solution H₂O/D₂O ratio or as complex as selectively labeling individual components with hydrogen isotopes. This review presents an overview of each experimental technique with the neutron instrument configuration, related sample preparation and sample environment, and data analysis, highlighted by a special emphasis on using prominent neutron contrast to understand structure and dynamics. This review gives researchers a practical introduction to the often enigmatic suite of neutron beamlines, thereby lowering the barrier to taking advantage of these large-facility techniques to achieve new understandings of membranes and their interactions with other molecules.

Manufacturing drug co-loaded liposomal formulations targeting breast cancer: Influence of preparative method on liposomes characteristics and in vitro toxicity

Developing more efficient manufacturing methods for nano therapeutic systems is becoming important, not only to better control their physico-chemical characteristics and therapeutic efficacy but also to ensure scale-up is cost-effective. The principle of cross-flow chemistry allows precise control over manufacturing parameters for the fabrication of uniform liposomal formulations, as well as providing reproducible manufacturing scale-up compared to conventional methods. We have herein investigated the use of microfluidics to produce PEGylated DSPC liposomes loaded with doxorubicin and compared their performance against identical formulations prepared by the thin-film method. The isoprenylated coumarin umbelliprenin was selected as a co-therapeutic. Umbelliprenin-loaded and doxorubicin:umbelliprenin co-loaded liposomes were fabricated using the optimised microfluidic set-up. The role of umbelliprenin as lipid bilayer fluidity modulation was characterized, and we investigated its role on liposomes size, size distribution, shape and stability compared to doxorubicin-loaded liposomes. Finally, the toxicity of all liposomal formulations was tested on a panel of human breast cancer cells (MCF-7, MDA-MB 231, BT-474) to identify the most potent formulation by liposomal fabrication method and loaded compound(s). We herein show that the microfluidic system is an alternative method to produce doxorubicin:umbelliprenin co-loaded liposomes, allowing fine control over liposome size (100-250 nm), shape, uniformity and doxorubicin drug loading (>80%). Umbelliprenin was shown to confer fluidity to model lipid biomembranes, which helps to explain the more homogeneous size and shape of co-loaded liposomes compared to liposomes without umbelliprenin. The toxicity of doxorubicin:umbelliprenin co-loaded liposomes was lower than that of free doxorubicin, due to the delayed release of doxorubicin from liposomes. An alternative, rapid and easy manufacturing method for the production of liposomes has been established using microfluidics to effectively produce uniform doxorubicin:umbelliprenin co-loaded liposomal formulations with proven cytotoxicity in human breast cancer cell lines in vitro.

Lipid membrane interactions of self-assembling antimicrobial nanofibers: effect of PEGylation

Supramolecular assembly and PEGylation (attachment of a polyethylene glycol polymer chain) of peptides can be an effective strategy to develop antimicrobial peptides with increased stability, antimicrobial efficacy and hemocompatibility. However, how the self-assembly properties and PEGylation affect their lipid membrane interaction is still an unanswered question. In this work, we use state-of-the-art small angle X-ray and neutron scattering (SAXS/SANS) together with neutron reflectometry (NR) to study the membrane interaction of a series of multidomain peptides, with and without PEGylation, known to self-assemble into nanofibers. Our approach allows us to study both how the structure of the peptide and the membrane are affected by the peptide-lipid interactions. When comparing self-assembled peptides with monomeric peptides that are not able to undergo assembly due to shorter chain length, we found that the nanofibers interact more strongly with the membrane. They were found to insert into the core of the membrane as well as to absorb as intact fibres on the surface. Based on the presented results, PEGylation of the multidomain peptides leads to a slight net decrease in the membrane interaction, while the distribution of the peptide at the interface is similar to the non-PEGylated peptides. Based on the structural information, we showed that nanofibers were partially disrupted upon interaction with phospholipid membranes. This is in contrast with the considerable physical stability of the peptide in solution, which is desirable for an extended in vivo circulation time.

Direct Observation of Sophorolipid Micelle Docking in Model Membranes and Cells by Single Particle Studies Reveals Optimal Fusion Conditions

Sophorolipids (SLs) are naturally produced glycolipids that acts as drug delivery for a spectrum of biomedical applications, including as an antibacterial antifungal and anticancer agent, where they induce apoptosis selectively in cancerous cells. Despite their utility, the mechanisms underlying their membrane interactions, and consequently cell entry, remains unknown. Here, we combined a single liposome assay to observe directly and quantify the kinetics of interaction of SL micelles with model membrane systems, and single particle studies on live cells to record their interaction with cell membranes and their cytotoxicity. Our single particle readouts revealed several repetitive docking events on individual liposomes and quantified how pH and membrane charges, which are known to vary in cancer cells, affect the docking of SL micelles on model membranes. Docking of sophorolipids micelles was found to be optimal at pH 6.5 and for membranes with -5% negatively charge lipids. Single particle studies on mammalian cells reveled a two-fold increased interaction on Hela cells as compared to HEK-293 cells. This is in line with our cell viability readouts recording an approximate two-fold increased cytotoxicity by SLs interactions for Hela cells as compared to HEK-293 cells. The combined in vitro and cell assays thus support the increased cytotoxicity of SLs on cancer cells to originate from optimal charge and pH interactions between membranes and SL assemblies. We anticipate studies combining quantitative single particle studies on model membranes and live cell may reveal hitherto unknown molecular insights on the interactions of sophorolipid and additional nanocarriers mechanism.

Interactions of anticancer drugs doxorubicin and idarubicin with lipid monolayers: New insight into the composition, structure and morphology

We quantify directly here for the first time the extents of interactions of two different anthracycline drugs with pure and mixed lipid monolayers with respect to the surface pressure and elucidate differences in the resulting interaction mechanisms. The work concerns interactions of doxorubicin (DOx) and idarubicin (IDA) with monolayers of the zwitterionic DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) and negatively charged DMPS (1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (sodium salt)) as well as a 7:3 mixture of the two lipids. These drugs are used in current cancer treatments, while the lipid systems were chosen as phosphocholines are the major lipid component of healthy cell membranes, and phosphoserines are the major lipid component that is externalized into the outer leaflet of cancerous cell membranes. It is shown that DOx interacts with DMPS monolayers to a greater extent than with DMPC monolayers by lower limits of a factor of 5 at a surface pressure of 10 mN/m and a factor of 12 at 30 mN/m. With increasing surface pressure, the small amount of drug (~0.3 µmol/m²) bound to DMPC monolayers is excluded from the interface, yet its interaction with DMPS monolayers is enhanced until there is even more drug (~3.2 µmol/m²) than lipid (~2.6 µmol/m²) at the interface. Direct evidence is presented for all systems studied that upon surface area compression lipid is reproducibly expelled from the monolayer, which we infer to be in the form of drug-lipid aggregates, yet the nature of adsorption of material back to the monolayer upon expansion is system-dependent. At 30 mN/m, most relevant to human physiology, the interactions of DOx and IDA are starkly different. For DOx, there is a conformational change in the interfacial layer driven by aggregation, resulting in the formation of lateral domains that have extended layers of drug. For the more lipophilic IDA, there is penetration of the drug into the hydrophobic acyl chain region of the monolayer and no indication of lateral segregation. In addition to the Langmuir technique, these advances were made as a result of direct measurements of the interfacial composition, structure and morphology using two different implementations of neutron reflectometry and Brewster angle microscopy. The results provide new insight into key processes that determine the uptake of drugs such as limited drug penetration through cell membranes by passive diffusion as well as activation of drug removal mechanisms related to multidrug resistance.