Aescin Incorporation and Nanodomain Formation in DMPC Model Membranes

Yeast lacking the sterol C-5 desaturase Erg3 are tolerant to the anti-inflammatory triterpenoid saponin escin

Escin is a mixture of over 30 glycosylated triterpenoid (saponin) structures, extracted from the dried fruit of horse chestnuts. Escin is currently used as an anti-inflammatory, and has potential applications in the treatment of arthritis and cancer. Engineered yeast would enable production of specific bioactive components of escin at industrial scale, however many saponins have been shown to be toxic to yeast. Here we report that a Saccharomyces cerevisiae strain specifically lacking the sterol C-5 desaturase gene ERG3, exhibits striking enhanced tolerance to escin treatment. Transcriptome analyses, as well as pre-mixing of escin with sterols, support the hypothesis that escin interacts directly with ergosterol, but not as strongly with the altered sterols present in erg3Δ. A diverse range of saponins are of commercial interest, and this research highlights the value of screening lipidome mutants to identify appropriate hosts for engineering the industrial production of saponins.

Interactions between DMPC Model Membranes, the Drug Naproxen, and the Saponin β-Aescin

In this study, the interplay among the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) as a model membrane, the nonsteroidal anti-inflammatory drug naproxen, and the saponin β-aescin are investigated. The naproxen amount was fixed to 10 mol%, and the saponin amount varies from 0.0 to 1.0 mol%. Both substances are common ingredients in pharmaceutics; therefore, it is important to obtain deeper knowledge of their impact on lipid membranes. The size and properties of the DMPC model membrane upon naproxen and aescin addition were characterized with differential scanning calorimetry (DSC), small- and wide-angle X-ray scattering (SAXS, WAXS), and photon correlation spectroscopy (PCS) in a temperature-dependent study. The interaction of all substances was dependent on the lipid phase state, which itself depends on the lipid's main phase transition temperature Tm. The incorporation of naproxen and aescin distorted the lipid membrane structure and lowers Tm. Below Tm, the DMPC-naproxen-aescin mixtures showed a vesicle structure, and the insertion of naproxen and aescin influenced neither the lipid chain-chain correlation distance nor the membrane thickness. Above Tm, the insertion of both molecules instead induced the formation of correlated bilayers and a decrease in the chain-chain correlation distance. The presented data clearly confirm the interaction of naproxen and aescin with DMPC model membranes. Moreover, the incorporation of both additives into the model membranes is evidenced.

A Thermodynamic Study on the Interaction between RH-23 Peptide and DMPC-Based Biomembrane Models

Investigation of the interaction between drugs and biomembrane models, as a strategy to study and eventually improve drug/substrate interactions, is a crucial factor in preliminary screening. Synthesized peptides represent a source of potential anticancer and theragnostic drugs. In this study, we investigated the interaction of a novel synthesized peptide, called RH-23, with a simplified dimyristoylphosphatidylcholine (DMPC) model of the cellular membrane. The interaction of RH-23 with DMPC, organized either in multilamellar vesicles (MLVs) and in Langmuir-Blodgett (LB) monolayers, was assessed using thermodynamic techniques, namely differential scanning calorimetry (DSC) and LB. The calorimetric evaluations showed that RH-23 inserted into MLVs, causing a stabilization of the phospholipid gel phase that increased with the molar fraction of RH-23. Interplay with LB monolayers revealed that RH-23 interacted with DMPC molecules. This work represents the first experimental thermodynamic study on the interaction between RH-23 and a simplified model of the lipid membrane, thus providing a basis for further evaluations of the effect of RH-23 on biological membranes and its therapeutic/diagnostic potential.

Small-Angle Neutron Scattering for Studying Lipid Bilayer Membranes

Small-angle neutron scattering (SANS) is a powerful tool for studying biological membranes and model lipid bilayer membranes. The length scales probed by SANS, being from 1 nm to over 100 nm, are well-matched to the relevant length scales of the bilayer, particularly when it is in the form of a vesicle. However, it is the ability of SANS to differentiate between isotopes of hydrogen as well as the availability of deuterium labeled lipids that truly enable SANS to reveal details of membranes that are not accessible with the use of other techniques, such as small-angle X-ray scattering. In this work, an overview of the use of SANS for studying unilamellar lipid bilayer vesicles is presented. The technique is briefly presented, and the power of selective deuteration and contrast variation methods is discussed. Approaches to modeling SANS data from unilamellar lipid bilayer vesicles are presented. Finally, recent examples are discussed. While the emphasis is on studies of unilamellar vesicles, examples of the use of SANS to study intact cells are also presented.

The Past and the Future of Langmuir and Langmuir-Blodgett Films

The Langmuir-Blodgett (LB) technique, through which monolayers are transferred from the air/water interface onto a solid substrate, was the first method to allow for the controlled assembly of organic molecules. With its almost 100 year history, it has been the inspiration for most methods to functionalize surfaces and produce nanocoatings, in addition to serving to explore concepts in molecular electronics and nanoarchitectonics. This paper provides an overview of the history of Langmuir monolayers and LB films, including the potential use in devices and a discussion on why LB films are seldom considered for practical applications today. Emphasis is then given to two areas where these films offer unique opportunities, namely, in mimicking cell membrane models and exploiting nanoarchitectonics concepts to produce sensors, investigate molecular recognitions, and assemble molecular machines. The most promising topics for the short- and long-term prospects of the LB technique are also highlighted.

Stable DOPG/Glycyrrhizin Vesicles with a Wide Range of Mixing Ratios: Structure and Stability as Seen by Scattering Experiments and Cryo-TEM

Phosphatidylglycerols represent a large share of the lipids in the plasmamembrane of procaryotes. Therefore, this study investigates the role of charged lipids in the plasma membrane with respect to the interaction of the antiviral saponin glycyrrhizin with such membranes. Glycyrrhizin is a natural triterpenic-based surfactant found in licorice. Vesicles made of 1,2-dioleoyl-sn-glycero-3-phospho-rac-(1’-glycerol) (DOPG)/glycyrrhizin are characterized by small-angle scattering with neutrons and X-rays (SANS and SAXS). Small-angle scattering data are first evaluated by the model-independent modified Kratky–Porod method and afterwards fitted by a model describing the shape of small unilamellar vesicles (SUV) with an internal head-tail contrast. Complete miscibility of DOPG and glycyrrhizin was revealed even at a ratio of lipid:saponin of 1:1. Additional information about the chain-chain correlation distance of the lipid/saponin mixtures in the SUV structures is obtained from wide-angle X-ray scattering (WAXS).

Is the Membrane Lipid Matrix a Key Target for Action of Pharmacologically Active Plant Saponins?

This study was focused on the molecular mechanisms of action of saponins and related compounds (sapogenins and alkaloids) on model lipid membranes. Steroids and triterpenes were tested. A systematic analysis of the effects of these chemicals on the physicochemical properties of the lipid bilayers and on the formation and functionality of the reconstituted ion channels induced by antimicrobial agents was performed. It was found that digitonin, tribulosin, and dioscin substantially reduced the boundary potential of the phosphatidylcholine membranes. We concluded that saponins might affect the membrane boundary potential by restructuring the membrane hydration layer. Moreover, an increase in the conductance and lifetime of gramicidin A channels in the presence of tribulosin was due to an alteration in the membrane dipole potential. Differential scanning microcalorimetry data indicated the key role of the sapogenin core structure (steroid or triterpenic) in affecting lipid melting and disordering. We showed that an alteration in pore forming activity of syringomycin E by dioscin might be due to amendments in the lipid packing. We also found that the ability of saponins to disengage the fluorescent marker calcein from lipid vesicles might be also determined by their ability to induce a positive curvature stress.

Aescin - a natural soap for the formation of lipid nanodiscs with tunable size

The saponin β-aescin from the seed extract of the horse chestnut tree Aesculus hippocastanum has demonstrated a beneficial role in clinical therapy which is in part related to its strong interaction with biological membranes. In this context the present work investigates the self-assembly of nm-sized discoidal lipid nanoparticles composed of β-aescin and the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The discoidal lipid nanoparticles reassemble from small discs into larger discs, ribbons and finally stacks of sheets upon heating from gel-phase to fluid phase DMPC. The morphological transition of the lipid nano-particles is mainly triggered by the phospholipid phase state change. The final morphology depends on the phospholipid-to-saponin ratio and the actual temperature. The study is conducted by small-angle X-ray scattering (SAXS) and transmission (TEM) and freeze fracture electron microscopy (FFEM) are used to cover larger length scales. Two different models, representing a disc and ribbon-like shape are applied to the SAXS data, evaluating possible geometries and molecular mixing of the nano-particles. The stacked sheets are analysed by the Caillé theory.

Moulding hydrodynamic 2D-crystals upon parametric Faraday waves in shear-functionalized water surfaces

Faraday waves, or surface waves oscillating at half of the natural frequency when a liquid is vertically vibrated, are archetypes of ordering transitions on liquid surfaces. Although unbounded Faraday waves patterns sustained upon bulk frictional stresses have been reported in highly viscous fluids, the role of surface rigidity has not been investigated so far. Here, we demonstrate that dynamically frozen Faraday waves-that we call 2D-hydrodynamic crystals-do appear as ordered patterns of nonlinear gravity-capillary modes in water surfaces functionalized with soluble (bio)surfactants endowing in-plane shear stiffness. The phase coherence in conjunction with the increased surface rigidity bears the Faraday waves ordering transition, upon which the hydrodynamic crystals were reversibly molded under parametric control of their degree of order, unit cell size and symmetry. The hydrodynamic crystals here discovered could be exploited in touchless strategies of soft matter and biological scaffolding ameliorated under external control of Faraday waves coherence.

DMPC Phospholipid Bilayer as a Potential Interface for Human Cystatin C Oligomerization: Analysis of Protein-Liposome Interactions Using NMR Spectroscopy

Studies revolving around mechanisms responsible for the development of amyloid-based diseases lay the foundations for the recognition of molecular targets of future to-be-developed treatments. However, the vast number of peptides and proteins known to be responsible for fibril formation, combined with their complexity and complexity of their interactions with various cellular components, renders this task extremely difficult and time-consuming. One of these proteins, human cystatin C (hCC), is a well-known and studied cysteine-protease inhibitor. While being a monomer in physiological conditions, under the necessary stimulus—usually a mutation, it tends to form fibrils, which later participate in the disease development. This process can potentially be regulated (in several ways) by many cellular components and it is being hypothesized that the cell membrane might play a key role in the oligomerization pathway. Studies involving cell membranes pose several difficulties; therefore, an alternative in the form of membrane mimetics is a very attractive solution. Here, we would like to present the first study on hCC oligomerization under the influence of phospholipid liposomes, acting as a membrane mimetic. The protein–mimetic interactions are studied utilizing circular dichroism, nuclear magnetic resonance, and size exclusion chromatography.

Heating-Induced DMPC/Glycyrrhizin Bicelle-to-Vesicle Transition: A X-Ray Contrast Variation Study

In this study, we investigated the conversion of lipid bicelles into vesicles in the case of a system composed of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and the saponin glycyrrhizin in the presence of sucrose. Glycyrrhizin is a biosurfactant present in the licorice root and possesses a triterpenic hydrophobic backbone and a hydrophilic headgroup built from two sugar molecules. The aim of this study is to determine the initial bicelle size at temperatures below the lipid's main phase transition temperature T_m and, based on these results, characteristics of the temperature-induced bicelle-to-vesicle transition. Moreover, the influence of the heating rate on this transition is followed. The general picture concluded from photon correlation spectroscopy and small angle X-ray scattering was confirmed by additional imaging with cryogenic transmission electron microscopy. Small angle X-ray scattering was especially used to determine size parameters of the existing structures. To enhance the contrast for X-rays, a buffer containing 25 wt% sucrose was used. It was found that larger vesicles were formed from smaller precursor particles and that monodisperse precursors are required for formation of very monodisperse vesicles upon temperature increase. At high glycyrrhizin contents and above a critical heating rate of ∼5°C min^-1, the polydispersity of these vesicles is decoupled from both parameters, glycyrrhizin content and heating rate. However, the vesicle size stays tunable by the glycyrrhizin content and increases upon increasing the glycyrrhizin concentration. Therefore, vesicles of defined size and with a rather low polydispersity of ∼12-14% can be formed.

Surface activity and foaming properties of saponin-rich plants extracts

Saponins are amphiphilic glycosidic secondary metabolites produced by numerous plants. So far only few of them have been thoroughly analyzed and even less have found industrial applications as biosurfactants. In this contribution we screen 45 plants from different families, reported to be rich in saponins, for their surface activity and foaming properties. For this purpose, the room-temperature aqueous extracts (macerates) from the alleged saponin-rich plant organs were prepared and spray-dried under the same conditions, in presence of sodium benzoate and potassium sorbate as preservatives and drying aids. For 15 selected plants, the extraction was also performed using hot water (decoction for 15 min) but high temperature in most cases deteriorated surface activity of the extracts. To our knowledge, for most of the extracts this is the first quantitative report on their surface activity. Among the tested plants, only 3 showed the ability to reduce surface tension of their solutions by more than 20 mN/m at 1% dry extract mass content. The adsorption layers forming spontaneously on the surface of these extracts showed a broad range of surface dilational rheology responses - from null to very high, with surface dilational elasticity modulus, E' in excess of 100 mN/m for 5 plants. In all cases the surface dilational response was dominated by the elastic contribution, typical for saponins and other biosurfactants. Almost all extracts showed the ability to froth, but only 32 could sustain the foam for more than 1 min (for 11 extracts the foams were stable during at least 10 min). In general, the ability to lower surface tension and to produce adsorbed layers with high surface elasticity did not correlate well with the ability to form and sustain the foam. Based on the overall characteristics, Saponaria officinalis L. (soapwort), Avena sativa L. (oat), Aesculus hippocastanum L. (horse chestnut), Chenopodium quinoa Willd. (quinoa), Vaccaria hispanica (Mill.) Rauschert (cowherb) and Glycine max (L.) Merr. (soybean) are proposed as the best potential sources of saponins for surfactant applications in natural cosmetic and household products.

Smart microgels as drug delivery vehicles for the natural drug aescin: uptake, release and interactions

In the present study, we show how acrylamide-based microgels can be employed for the uptake and release of the drug β-aescin, a widely used natural product with a variety of pharmacological effects. We show how aescin is incorporated into the microgel particles. It has an important influence on the structure of the microgels, by reducing their natural network-density gradient in the swollen state. Moreover, temperature-dependent measurements reveal how the incorporation of aescin stabilizes the microgel particles, while the volume phase transition temperature (VPTT) is almost constant, which is very important for the intended drug release. Finally, it is shown that upon increase of the temperature above the VPTT the particles are able to release aescin from their network, encouraging the use of this particular drug delivery system for hypothermia treatments.

Development and Percutaneous Permeation Study of Escinosomes, Escin-Based Nanovesicles Loaded with Berberine Chloride

Escin is a natural saponin, clinically used for the anti-edematous and anti-inflammatory effects. The aim of the study was to explore the possibility of converting escin into vesicle bilayer-forming component. The hyaluronidase inhibition activity of escin was evaluated after its formulation in escinosomes. Berberine chloride, a natural quaternary isoquinoline alkaloid isolated from several medicinal plants that is traditionally used for various skin conditions was loaded in the vesicles. The developed nanovesicles were characterized in terms of diameter, polydispersity, ζ-potential, deformability, recovery, encapsulation efficiency, stability, and release kinetics. Nanovesicle permeation properties through artificial membranes and rabbit ear skin were investigated using skin-PAMPA^TM and Franz cells were also evaluated. Escinosomes, made of phosphatidylcholine and escin, were loaded with berberine chloride. These nanovesicles displayed the best characteristics for skin application, particularly optimal polydispersity (0.17) and deformability, high negative ζ-potential value, great encapsulation efficiency (about 67%), high stability, and the best release properties of berberine chloride (about 75% after 24 h). In conclusion, escinosomes seem to be new vesicular carriers, capable to maintain escin properties such as hyaluronidase inhibition activity, and able to load other active molecules such as berberine chloride, in order to enhance or expand the activity of the loaded drug.

Aescin-Induced Conversion of Gel-Phase Lipid Membranes into Bicelle-like Lipid Nanoparticles

Mixtures of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and the saponin β-aescin spontaneously form monodisperse, bilayered discoidal micelles (also known as "bicelles" or "nanodisks") in aqueous solution. Such bicelles form below the melting temperature of DMPC when the phospholipids are in the rigid L_β' state and are precursors of spontaneously formed vesicles. The aescin concentration must be far above the cmc_aescin (≈0.3-0.4 mM). It was found that the shape and size of the bicelles are tunable by composition. High amounts of aescin decrease the size of the bicelles from diameters of ∼300 Å at 7 mol % to ∼120 Å at 30 mol % β-aescin. The structures are scrutinized by complementary small-angle X-ray and neutron scattering experiments. The scattering curves are subsequently analyzed by a model-independent (indirect Fourier transform analysis) and a model-based approach where bicelles are described as polydisperse bilayer disks encircled by a β-aescin rim. Moreover, the monomodal distribution and low polydispersity of the samples were confirmed by photon correlation spectroscopy. The discoidal structures were visualized by transmission electron microscopy.

Self-Assembly of the Bio-Surfactant Aescin in Solution: A Small-Angle X-ray Scattering and Fluorescence Study

This work investigates the temperature-dependent micelle formation as well as the micellar structure of the saponin aescin. The critical micelle concentration ( c m c ) of aescin is determined from the concentration-dependent autofluorescence (AF) of aescin. Values between c m c aescin , AF (10 ∘ C) = 0.38 ± 0.09 mM and c m c aescin , AF (50 ∘ C) = 0.32 ± 0.13 mM were obtained. The significance of this method is verified by tensiometry measurements. The value determined from this method is within the experimental error identical with values obtained from autofluorescence ( c m c aescin , T ( WP ) (23 ∘ C) = 0.33 ± 0.02 mM). The structure of the aescin micelles was investigated by small-angle X-ray scattering (SAXS) at 10 and 40 ∘ C. At low temperature, the aescin micelles are rod-like, whereas at high temperature the structure is ellipsoidal. The radii of gyration were determined to ≈31 Å (rods) and ≈21 Å (ellipsoid). The rod-like shape of the aescin micelles at low temperature was confirmed by transmission electron microscopy (TEM). All investigations were performed at a constant pH of 7.4, because the acidic aescin has the ability to lower the pH value in aqueous solution.

Aescin-Cholesterol Complexes in DMPC Model Membranes: A DSC and Temperature-Dependent Scattering Study

The saponin aescin, a mixture of triterpenoid saponins, is obtained from the seeds of the horse chestnut tree Aesculus hippocastanum. The β-form employed in this study is haemolytically active. The haemolytic activity results from the ability of aescin to form strong complexes with cholesterol in the red blood cell membrane. In this study, we provide a structural analysis on the complex formation of aescin and cholesterol when embedded in a phospholipid model membrane formed by 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). In this work, the temperatures investigated extend from DMPC’s L_β′ to its L_α phase in dependence of different amounts of the saponin (0–6 mol% for calorimetric and 0–1 mol% for structural analyses) and the steroid (1–10 mol%). At these aescin contents model membranes are conserved in the form of small unilamellar vesicles (SUVs) and major overall structural modifications are avoided. Additionally, interactions between aescin and cholesterol can be studied for both phase states of the lipid, the gel and the fluid state. From calorimetric experiments by differential scanning calorimetry (DSC), it could be shown that both, the steroid and the saponin content, have a significant impact on the cooperative phase transition behaviour of the DMPC molecules. In addition, it becomes clearly visible that the entire phase behaviour is dominated by phase separation which indeed also depends on the complexes formed between aescin and cholesterol. We show by various methods that the addition of cholesterol alters the impact of aescin on structural parameters ranging from the acyl chain correlation to vesicle-vesicle interactions. While the specific saponin-phospholipid interaction is reduced, addition of cholesterol leads to deformation of SUVs. The analyses of the structures formed were performed by wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS).

Temperature dependent self-organization of DMPC membranes promoted by intermediate amounts of the saponin aescin

The plant-derived biosurfactant aescin is naturally present in many plants and is used for treatment of disorders such as varicose veins and inflammation of veins. The hemolytic activity of this saponin is attributed to its interaction with cholesterol in the red blood cell membrane. This work investigates the phase and aggregation behavior of saponin-containing model membranes consisting of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The aescin concentrations studied range from 1 mol% to 7 mol% with respect to the total lipid content. The methods of choice to elucidate the structural picture are small-angle scattering of X-rays (SAXS) and neutrons (SANS) and cryogenic transmission electron microscopy (cryo-TEM). SANS and SAXS revealed that at lower aescin contents vesicular structures are conserved and vesicles tend to aggregate already at aescin contents of around 1 mol%. Aggregation and vesicle deformation effects are found to be stronger when the phospholipids are in the L [Formula: see text] phase. With increasing aescin content, mixed structures, i.e. aggregated and deformed vesicles and solubilized bilayer fragments, are present. This was proven for a sample with 4 mol% aescin by cryo-TEM. An increasing aescin amount leads to membrane decomposition and free standing bilayers which tend to build stacks at high temperature. These stacks are characterized by SAXS using the modified Caillé theory. Analyses and model dependent fitting reveal formation of well-defined structures beginning at 7 mol% aescin.

Interaction of the Saponin Aescin with Ibuprofen in DMPC Model Membranes

In the present work, we study the interaction of the saponin aescin with the nonsteroidal anti-inflammatory drug (NSAID) ibuprofen at concentrations of 1.2-2.5 mM. These amounts are higher than those usually used for medication (10-300 μM) to show possible structures and formulations for orally absorbed drug delivery systems. It is shown how the interaction of both substances, separately or together, alters the thermotropic phase behavior of the 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) bilayer in the presence of different amounts of aescin, ranging from 20 μM to 1 mM. The methods of choice are differential scanning calorimetry (DSC), and additionally wide-angle (WAXS) and small-angle X-ray scattering (SAXS). We found that these two additives, aescin and ibuprofen, alter the temperature-dependent structural appearance of the DMPC membrane depending on the aescin and drug content. The presence of the saponin and the drug become visible on different length scales, i.e., ranging from a global structural change to inner-membrane interactions. DSC reveals that the drug and saponin alter the cooperativity of the DMPC phase transition in a concentration-dependent manner. Furthermore, there is a significant difference between the drug-containing compared to the drug-free systems. By WAXS, we could resolve that aescin reverses the strong impact of ibuprofen on the diffraction peak of DMPC. Both molecules interact strongly with the phospholipid headgroups. This becomes visible in a changing area per lipid and shifting phase transition to higher temperatures. SAXS experiments reveal that the addition of ibuprofen leads to major morphological changes in the phospholipid bilayer. SAXS experiments performed on representative samples do not only cover the drug-saponin interaction within the bilayer from the structural perspective but also confirm the visually observed macroscopic concentration and temperature-dependent phase behavior. Vesicular shape of extruded samples is conserved at low aescin contents. At intermediate aescin content, aggregation between vesicles occurs, whereby the strength of aggregation is reduced by ibuprofen. At high aescin contents, DMPC bilayers are solubilized. The kind of formed structures depends on temperature and drug content. At low temperature, separated bilayer sheets are formed. Their size increases with ibuprofen in a concentration-dependent manner. At high temperature, the drug-free system reorganizes into stacked sheets. Whereas sheets at 5 mol % ibuprofen close to vesicles, the ones with 10 mol % of the drug increase massively in size. Altogether, ibuprofen was found to rather enhance than inhibit structural and thermotropic membrane modifications induced by the aescin on the DMPC model membrane.

DMPC vesicle structure and dynamics in the presence of low amounts of the saponin aescin

Vesicle shape and bilayer parameters are studied by small-angle X-ray (SAXS) and small-angle neutron (SANS) scattering in the presence of the saponin aescin. Bilayer dynamics is studied by neutron spin-echo (NSE) spectroscopy.