Interaction forces between ternary lipid bilayers containing cholesterol

Protective effect of trehalose sugar on amyloid-membrane interactions using BLM electrophysiology

Alzheimer's disease (AD) is a neurodegenerative disease characterized by dementia and memory loss in the elderly population. The amyloid-β peptide (Aβ) is one of the main pathogenic factors in AD and is known to cause damage to neuronal cellular membranes. There is no cure currently available for AD, and new approaches, including preventive strategies, are highly desirable. In this work, we explore the possibility of protecting neuronal membranes from amyloid-induced damage with naturally existing sugar trehalose. Trehalose has been shown to protect plant cellular membranes in extreme conditions and modify Aβ misfolding. We hypothesize that trehalose can protect the neuronal membrane from amyloid toxicity. In this work, we studied the protective effect of trehalose against Aβ1-42-induced damage in model lipid membranes (DPPC/POPC/cholesterol) using atomic force microscopy and black lipid membrane electrophysiology. Our results demonstrate that Aβ1-42 damaged membranes and led to ionic current leakage across these membranes due to the formation of various defects and pores. The presence of trehalose reduced the ion current across membranes caused by Aβ1-42 peptide damage, thus efficiently protecting the membranes. These findings suggest that the trehalose sugar can potentially be useful in protecting neuronal membranes against amyloid toxicity in AD.

Nanoscale visualization of phase separation in binary supported lipid monolayer using tip-enhanced Raman spectroscopy

Supported lipid membranes are an important model system to study the phase separation behavior at the nanoscale. However, the conventional nanoanalytical tools often fail to provide reliable chemical characterization of the phase separated domains in a non-destructive and label-free manner. This study demonstrates the application of scanning tunneling microscopy-based tip-enhanced Raman spectroscopy (TERS) to study the nanoscale phase separation in supported d62-DPPC : DOPC lipid monolayers. Hyperspectral TERS imaging successfully revealed a clear segregation of the d62-DPPC-rich and DOPC-rich domains. Interestingly, nanoscale deposits of d62-DPPC were observed inside the DOPC-rich domains and vice versa. High-resolution TERS imaging also revealed the presence of a 40-120 nm wide interfacial region between the d62-DPPC-rich and DOPC-rich domains signifying a smooth transition rather than a sharp boundary between them. The novel insights obtained in this study demonstrate the effectiveness of TERS in studying binary lipid monolayers at the nanoscale.

Dexamethasone and Dexamethasone Phosphate: Effect on DMPC Membrane Models

Dexamethasone (Dex) and Dexamethasone phosphate (Dex-P) are synthetic glucocorticoids with high anti-inflammatory and immunosuppressive actions that gained visibility because they reduce the mortality in critical patients with COVID-19 connected to assisted breathing. They have been widely used for the treatment of several diseases and in patients under chronic treatments, thus, it is important to understand their interaction with membranes, the first barrier when these drugs get into the body. Here, the effect of Dex and Dex-P on dimyiristoylphophatidylcholine (DMPC) membranes were studied using Langmuir films and vesicles. Our results indicate that the presence of Dex in DMPC monolayers makes them more compressible and less reflective, induces the appearance of aggregates, and suppresses the Liquid Expanded/Liquid Condensed (LE/LC) phase transition. The phosphorylated drug, Dex-P, also induces the formation of aggregates in DMPC/Dex-P films, but without disturbing the LE/LC phase transition and reflectivity. Insertion experiments demonstrate that Dex induces larger changes in surface pressure than Dex-P, due to its higher hydrophobic character. Both drugs can penetrate membranes at high lipid packings. Vesicle shape fluctuation analysis shows that Dex-P adsorption on GUVs of DMPC decreases membrane deformability. In conclusion, both drugs can penetrate and alter the mechanical properties of DMPC membranes.

Cholesterol's Condensing Effect: Unpacking a Century-Old Mystery

The ability of cholesterol to uncoil (i.e., condense) the acyl chains of phospholipids has been known for a century. Despite extensive studies of the interactions between cholesterol and phospholipids, a molecular-level understanding of this uncoiling phenomenon has remained elusive. Equally unclear has been whether cholesterol's two different faces (i.e., its relatively smooth α face and its relatively rough β face) contribute to its condensing power. Because cholesterol's condensing effect is believed to play a major role in controlling the fluidity, structure, and functioning of all animal cell membranes, its biological importance cannot be overstated. This Perspective focuses on experimental evidence that addresses (i) the credibility of a popular "umbrella" mechanism that has been used to account for cholesterol's condensing effect, (ii) the credibility of an alternate "template" mechanism, (iii) the importance of cholesterol two-faced character with respect to its condensing power, and (iv) the viability of a surface occupancy model.

Interaction of graphene oxide with artificial cell membranes: Role of anionic phospholipid and cholesterol in nanoparticle attachment and membrane disruption

A mechanistic understanding of the interaction of graphene oxide (GO) with cell membranes is critical for predicting the biological effects of GO following accidental exposure and biomedical applications. We herein used a quartz crystal microbalance with dissipation monitoring (QCM-D) to probe the interaction of GO with model cell membranes modified with anionic lipids or cholesterol under biologically relevant conditions. The attachment efficiency of GO on supported lipid bilayers (SLBs) decreased with increasing anionic lipid content and was unchanged with varying cholesterol content. In addition, the incorporation of anionic lipids to the SLBs rendered the attachment of GO partially reversible upon a decrease in solution ionic strength. These results demonstrate the critical role of lipid bilayer surface charge in controlling GO attachment and release. We also employed the fluorescent dye leakage technique to quantify the role of anionic lipids and cholesterol in vesicle disruption caused by GO. Notably, we observed a linear correlation between the amount of dye leakage from the vesicles and the attachment efficiencies of GO on the SLBs, confirming that membrane disruption is preceded by GO attachment. This study highlights the non-negligible role of lipid bilayer composition in controlling the physicochemical interactions between cell membranes and GO.

Direct Measurements of the Forces between Silver and Mica in Humic Substance-Rich Solutions

Deposition of engineered nanoparticles onto porous media from flowing suspensions is important for soil and groundwater quality. The deposition mechanism is controlled by interaction forces between particles and collectors. We investigated the origin and magnitude of opposing forces between silver and mica surfaces (representing nanosilver and sand grains) in solutions relevant to agricultural soils with direct measurements using a surface force apparatus. Solutions of variable NaNO₃, Ca(NO₃)₂, and humic acid (HA) concentrations were used to differentiate individual contributing forces and quantify surface properties. The measured Hamaker constant for silver-water-mica was consistent with Lifshitz theory. Our results indicate that HA forms an adsorbed surface layer, but its charge, thicknesses, compressibility, and mass are significantly larger on mica than silver. Ca²⁺ primarily reduced the differences between the initially adsorbed HA layer properties on each surface, making them more similar. Force-distance profiles indicate that, when silver-mica systems were exposed to HA, osmotic-steric, electrostatic, and van der Waals forces dominate. Soft particle theory was deemed inappropriate for this system. Derjaguin's approximation was utilized to translate force measurements into interaction energy between nanosilver particles and mica collectors. We propose attachment efficiency estimates from measured surface properties, which suggest high particle mobility when nanosilver is applied to HA-rich agricultural soils with modest ionic strength.

How Well Can You Tailor the Charge of Lipid Vesicles?

Knowledge and control of surface charge or potential is important for tailoring colloidal interactions. In this work, we compare widely used zeta potential (ζ) measurements of charged lipid vesicle surface potential to direct measurements using the surface force apparatus (SFA). Our measurements show good agreement between the two techniques. On varying the fraction of anionic lipids dimyristoylphosphatidylserine (DMPS) or dimyristoylphosphatidylglycerol (DMPG) mixed with zwitterionic dimyristoylphosphatidylcholine (DMPC) from 0 to 100 mol % we observed a near-linear increase in membrane surface charge or potential up to 20-30 mol % charged lipids beyond which charge saturation occurred in physiological (high) salt conditions. Similarly, in low salt concentrations, a linear increase in charge/potential was found but only up to ∼5-10 mol % charged lipids beyond which the surface charge or potential leveled off. While a lower degree of ionization is expected due to the lower dielectric constant (ε ∼ 4) of the lipid acyl chain environment, increasing intramembrane electrostatic repulsion between neighboring charged lipid head groups at higher charge loading contributes to charge suppression. Measured potentials in physiological salt solutions were consistent with predictions using the Gouy-Chapman-Stern-Grahame (GCSG) model of the electrical double layer with Langmuir binding of counterions, but in low salt conditions, the model significantly overestimated the surface charge/potential. The much lower ionization in low salt (maximum ∼1-2% of total lipids ionized) instead was consistent with counterion condensation at the bilayer surface which limited the charge that could be obtained. The strong interplay between membrane composition, lipid headgroup ionization, electrolyte concentration, and solution pH complicates exact prediction and tuning of membrane surface charge for applications. However, the theoretical frameworks used here can provide guidelines to understand this interplay and establish a range of achievable potentials for a system and predict the response to triggers like pH and salt concentration changes.

Functional Hydration Behavior: Interrelation between Hydration and Molecular Properties at Lipid Membrane Interfaces

Water is an abundant commodity and has various important functions. It stabilizes the structure of biological macromolecules, controls biochemical activities, and regulates interfacial/intermolecular interactions. Common aspects of interfacial water can be obtained by overviewing fundamental functions and properties at different temporal and spatial scales. It is important to understand the hydrogen bonding and structural properties of water and to evaluate the individual molecular species having different hydration properties. Water molecules form hydrogen bonds with biomolecules and contribute to the adjustment of their properties, such as surface charge, hydrophilicity, and structural flexibility. In this review, the fundamental properties of water molecules and the methods used for the analyses of water dynamics are summarized. In particular, the interrelation between the hydration properties, determined by molecules, and the properties of molecules, determined by their hydration properties, are discussed using the lipid membrane as an example. Accordingly, interesting water functions are introduced that provide beneficial information in the fields of biochemistry, medicine, and food chemistry.

Preparation and Characterization of Solid-Supported Lipid Bilayers Formed by Langmuir-Blodgett Deposition: A Tutorial

The structure, phase behavior, and properties of cellular membranes are derived from their composition, which includes phospholipids, sphingolipids, sterols, and proteins with various levels of glycosylation. Because of the intricate nature of cellular membranes, a plethora of in vitro studies have been carried out with model membrane systems that capture particular properties such as fluidity, permeability, and protein binding but vastly simplify the membrane composition in order to focus in detail on a specialized property or function. Supported lipid bilayers (SLB) are widely used as archetypes for cellular membranes, and this instructional review primarily focuses on the preparation and characterization of SLB systems formed by Langmuir deposition methods. Typical characterization methods, which take advantage of the planar orientation of SLBs, are illustrated, and references that go into more depth are included. This invited instructional review is written so that nonexperts can quickly gain in-depth knowledge regarding the preparation and characterization of SLBs. In addition, this work goes beyond traditional instructional reviews to provide expert readers with new results that cover a wider range of SLB systems than those previously reported in the literature. The quality of an SLB is frequently not well described, and details such as topological defects can influence the results and conclusions of an individual study. This article quantifies and compares the quality of SLBs fabricated from a variety of gel and fluid compositions, in correlation with preparation techniques and parameters, to generate general rules of thumb to guide the construction of designed SLB systems.

Drastic Swelling of Lipid Oligobilayers by Polyelectrolytes: A Potential Molecular Model for the Internal Structure of Lubricating Films in Mammalian Joints

Osteoarthritis is the most common arthropathy in western civilization. It is primarily caused by the degeneration of lipid-coated cartilage, leading to increased friction in joints. Hyaluronic acid (HA), a negatively charged polysaccharide and the main component of the synovial fluid, is held responsible for joint lubrication. It is believed that HA, adsorbed to the lipid-coated cartilage, forms a protective layer against wear. Studies have shown that the concentration and molecular weight (MW) of HA are reduced in joints suffering from osteoarthritis. On the basis of these observations, local joint injections of HA or mixtures of HA and surface-active phospholipids (SAPLs) have been applied as medical cures to restore the functionality of the joints in a procedure called viscosupplementation. However, this cure is still disputed, and no consensus has been reached with respect to optimum HA concentration and MW. To provide detailed insight in the structural rearrangement of lipid films upon contact with HA or polymeric analogues, we studied the interaction of the polyelectrolyte poly(allylamine hydrochloride) (PAH) with surface-bound oligobilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by neutron reflectivity (NR) and ellipsometry. Using this model system, we found a drastic swelling of the lipid films as a function of PAH concentration, whose strength compares to that in previous studies on HA incubation. In contrast, no significant dependence of film thickness on PAH MW was observed. A detailed picture of the film architecture was developed which inter alia shows that charged PAH is adsorbed to the lipid headgroups, leading to electrostatic repulsion. The swelling behavior is well explained by the equilibrium of Coulomb and van der Waals interactions in a DLVO-based model. Our detailed structural analysis of the PAH/lipid interfacial layer may help to elucidate the mechanisms of viscosupplementation and derive a structure-function relationship for the lubricating interface in mammalian joints.

Microinjection for the ex Vivo Modification of Cells with Artificial Organelles

Microinjection is extensively used across fields to deliver material intracellularly. Here we address the fundamental aspects of introducing exogenous organelles into cells to endow them with artificial functions. Nanocarriers encapsulating biologically active cargo or extreme intraluminal pH were injected directly into the cytosol of cells, where they bypassed subcellular processing pathways and remained intact for several days. Nanocarriers' size was found to dictate their intracellular distribution pattern upon injection, with larger vesicles adopting polarized agglomerated distributions and smaller colloids spreading evenly in the cytosol. This in turn determined the symmetry or asymmetry of their dilution following cell division, ultimately affecting the intracellular dose at a cell population level. As an example of microinjection's applicability, a cell type relevant for cell-based therapies (dendritic cells) was injected with vesicles, and its migratory properties were studied in a co-culture system mimicking lymphatic capillaries.

Preferential Adsorption of l-Histidine onto DOPC/Sphingomyelin/3β-[N-(N',N'-dimethylaminoethane)carbamoyl]cholesterol Liposomes in the Presence of Chiral Organic Acids

We investigated the effect of organic acids such as mandelic acid (MA) and tartaric acid (TA) on the adsorption behavior of both histidine (His) and propranolol (PPL) onto liposomes. A cationic and heterogeneous liposome prepared using 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/sphingomyelin (SM)/3β-[N-(N',N'-dimethylaminoethane)carbamoyl]cholesterol (DC-Ch) in a ratio of (4/3/3) showed the highest adsorption efficiency of MA and TA independent of chirality, while neutral liposome DOPC/SM/cholesterol = (4/3/3) showed low efficiency. As expected, electrostatic interactions were dominant in MA or TA adsorption onto DOPC/SM/DC-Ch = (4/3/3) liposomes, suggesting that organic acids had adsorbed onto SM/DC-Ch-enriched domains. The adsorption behaviors of organic acids onto DOPC/SM/DC-Ch = (4/3/3) were governed by Langmuir adsorption isotherms. For adsorption, the membrane polarities slightly decreased (i.e., membrane surface was hydrophilic), but no alterations in membrane fluidity were observed. In the presence of organic acids that had been preincubated with DOPC/SM/DC-Ch = (4/3/3), the adsorption of l- and d-His onto those liposomes was examined. Preferential l-His adsorption was dramatically prevented only in the presence of l-MA, suggesting that the adsorption sites for l-His and l-MA on DOPC/SM/DC-Ch = (4/3/3) liposomes are competitive, while those for l-His and d-MA, l-TA, and d-TA are isolated.

Interaction Forces between Lipid Rafts

Cellular membranes containing sphingolipids and cholesterol have been shown to self-organize into lipid rafts-specialized domains that host integral membrane proteins and modulate the bioactivity of cells. In this work, force-distance profiles between raft membranes in the liquid-ordered phase consisting of singly unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a complex mixture of brain sphingomyelin (BSM), and cholesterol were measured using the surface force apparatus (SFA). Two distinct force profiles were detected corresponding to uniform raft membranes and raft membranes with a higher level of topological membrane defects (heterogeneous) as corroborated by atomic force microscopy (AFM) scans. In all cases a weak, long-range electrostatic repulsion was observed with some variation in the surface charge density. The variation in electrostatic repulsion was attributed to charged lipid species primarily from the constituent lipids in the BSM mixture. The adhesion between the uniform raft membranes was comparable to our previous work with pure component, liquid-ordered POPC-DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine)-cholesterol membranes. Raft membranes with more topological defects adhered more strongly owing to hydrophobic attraction between exposed acyl chains. Even though the rafts were in the liquid-ordered phase and membrane defects were present in the contact region, the raft membranes were stable, and no structural rearrangement was observed throughout the measurements. Our findings demonstrate that liquid-ordered membranes are stable to mechanical loading and not particularly sensitive to compositional variation.

Quantitative Monitoring of Microphase Separation Behaviors in Cationic Liposomes Using HHC, DPH, and Laurdan: Estimation of the Local Electrostatic Potentials in Microdomains

Microphase separation behaviors of cationic liposomes have been investigated using a pH-sensitive fluorescent probe with 4-heptadecyl-7-hydroxycoumarin (HHC), 1,6-diphenyl-1,3,5-hexatriene, and 6-lauroyl-2-dimethylaminonaphthalene, and to estimate localized electrostatic potentials. Shifts of the apparent pKa values of HHC were observed in cationic liposomes in proportion to the amount of cationic lipids. Two pKa values were obtained with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/3β-[N(N',N'-dimethylaminoethane)-carbamoyl] cholesterol hydrochloride (DC-Ch) liposomes, while only one pKa value was generated with either DOPC/1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or DOPC/dimethyldioctadecylammonium-bromide (DODAB) liposomes. The physicochemical membrane property analyses, focusing on membrane fluidity and membrane polarity, revealed heterogeneity among DOPC/DC-Ch liposomes. By analyzing the pH titration curves using sigmoidal fitting, the localized electrostatic potentials were estimated. For DOPC/DOTAP = (7/3), the membrane was in the liquid-disordered phase and the density of cationic molecules was 0.41 cation/nm(2). For DOPC/DC-Ch = (7/3), the membrane was heterogeneous and the densities of cationic molecules in liquid-disordered and liquid-ordered phases were 0.25 and 1.24 cation/nm(2), respectively. We thereby conclude that the DC-Ch molecules can form nanodomains when these molecules are concentrated to 59%.

Engineered Nanostructures of Haptens Lead to Unexpected Formation of Membrane Nanotubes Connecting Rat Basophilic Leukemia Cells

A recent finding reports that co-stimulation of the high-affinity immunoglobulin E (IgE) receptor (FcεRI) and the chemokine receptor 1 (CCR1) triggered formation of membrane nanotubes among bone-marrow-derived mast cells. The co-stimulation was attained using corresponding ligands: IgE binding antigen and macrophage inflammatory protein 1α (MIP1 α), respectively. However, this approach failed to trigger formation of nanotubes among rat basophilic leukemia (RBL) cells due to the lack of CCR1 on the cell surface (Int. Immunol. 2010, 22 (2), 113-128). RBL cells are frequently used as a model for mast cells and are best known for antibody-mediated activation via FcεRI. This work reports the successful formation of membrane nanotubes among RBLs using only one stimulus, a hapten of 2,4-dinitrophenyl (DNP) molecules, which are presented as nanostructures with our designed spatial arrangements. This observation underlines the significance of the local presentation of ligands in the context of impacting the cellular signaling cascades. In the case of RBL, certain DNP nanostructures suppress antigen-induced degranulation and facilitate the rearrangement of the cytoskeleton to form nanotubes. These results demonstrate an important scientific concept; engineered nanostructures enable cellular signaling cascades, where current technologies encounter great difficulties. More importantly, nanotechnology offers a new platform to selectively activate and/or inhibit desired cellular signaling cascades.

Alendronate-coated long-circulating liposomes containing 99mtechnetium-ceftizoxime used to identify osteomyelitis

Osteomyelitis is a progressive destruction of bones caused by microorganisms. Inadequate or absent treatment increases the risk of bone growth inhibition, fractures, and sepsis. Among the diagnostic techniques, functional images are the most sensitive in detecting osteomyelitis in its early stages. However, these techniques do not have adequate specificity. By contrast, radiolabeled antibiotics could improve selectivity, since they are specifically recognized by the bacteria. The incorporation of these radiopharmaceuticals in drug-delivery systems with high affinity for bones could improve the overall uptake. In this work, long-circulating and alendronate-coated liposomes containing ^99mtechnetium-radiolabeled ceftizoxime were prepared and their ability to identify infectious foci (osteomyelitis) in animal models was evaluated. The effect of the presence of PEGylated lipids and surface-attached alendronate was evaluated. The bone-targeted long-circulating liposomal ^99mtechnetium–ceftizoxime showed higher uptake in regions of septic inflammation than did the non-long-circulating and/or alendronate-non-coated liposomes, showing that both the presence of PEGylated lipids and alendronate coating are important to optimize the bone targeting. Scintigraphic images of septic or aseptic inflammation-bearing Wistar rats, as well as healthy rats, were acquired at different time intervals after the intravenous administration of these liposomes. The target-to-non-target ratio proved to be significantly higher in the osteomyelitis-bearing animals for all investigated time intervals. Biodistribution studies were also performed after the intravenous administration of the formulation in osteomyelitis-bearing animals. A significant amount of liposomes were taken up by the organs of the mononuclear phagocyte system (liver and spleen). Intense renal excretion was also observed during the entire experiment period. Moreover, the liposome uptake by the infectious focus was significantly high. These results show that long-circulating and alendronate-coated liposomes containing ^99mtechnetium-radiolabeled ceftizoxime have a tropism for infectious foci.

Characterization of solid-supported dipalmitoylphosphatidylcholine membranes containing cholesterol

The incorporation of cholesterol into dipalmitoylphosphatidylcholine (DPPC) membranes, even in small amounts, has been shown to significantly alter the properties of the membrane. In this work, force-distance interaction profiles of DPPC membranes containing 8 mol % cholesterol obtained using the surface force apparatus are analyzed in the context of high-resolution structural characterization by atomic force microscopy and neutron reflectometry. The adhesion between the mixed membranes was greater than that for pure DPPC and was variable-depending on the number of defects in the outer membrane leaflets. These defects were only detectable by atomic force microscopy and had an average size of 230 ± 30 nm and 1-5% surface density in the outer leaflet. The adhesion between the membranes monotonically increased as the thickness of the membrane decreased-in direct correlation with the number of defects present (exposed hydrophobic groups) in the membrane contact region. Because of the low diffusion rate of gel-phase membranes, the interaction force profiles were stable and no membrane restructuring was observed.