Thermodynamic and Fluorescence Studies on the Interaction of Cholesterol with Palmitoyl-Oleoyl Phosphatidylcholine and Sphingomyelin

Effect of high sodium ion level on the interaction of AmB with a cholesterol-rich phospholipid monolayer

Invasive fungal infections are a primary reason for high mortality in immunocompromised people, especially in critically ill patients, such as intensive care unit (ICU) patients, advanced cancer patients, or severe burn patients. Hypernatremia also can increase mortality in severely ill patients. Amphotericin B (AmB) is the gold standard for treating infections, but in severely ill patients, AmB can cause hematotoxicity when administered intravenously due to its interaction with cholesterol on red blood cell membranes. This results in limited doses of AmB and affects the treatment of infections. The proportion of cholesterol molecules in membrane lipids in red blood cells is as high as 50 mol%, and the sodium ions can influence the interaction between AmB and lipids on the membrane. Therefore, in the complex clinical situation of a severely ill patient with a fungal infection and hypernatremia, the interaction between amphotericin B and the red blood cell membranes is worth studying in depth. In this work, the interaction between AmB and the dipalmitoyl phosphatidylcholine (DPPC)/cholesterol mixed monolayer in the presence of high sodium ion levels was studied when the proportion of cholesterol was 50%. The results show that the effect of AmB on reducing the monolayer's area at a high level of sodium ions is slightly stronger at 30 mN/m. The effect of AmB on reducing the elastic modulus of the DPPC/Chol monolayer is significantly weakened by a high sodium ion level, compared with the level of sodium ions at normal physiological concentration. The higher the sodium ion concentration, the weaker the intermolecular force of the DPPC/Chol/AmB mixed monolayers. The scanning electron microscope (SEM) and atomic force microscopy (AFM) observations suggest that at a high sodium ion level, the presence of AmB significantly reduces the surface roughness of the DPPC/Chol monolayer. AmB may bind to cholesterol molecules, and it isolates cholesterol from the monolayer, resulting in a reduced height of the cholesterol-rich monolayer and an increasingly dispersed monolayer region. The results are beneficial to understanding the mechanism of impact of a high sodium ion level on the relationship between AmB and red blood cell membranes rich in cholesterol and are valuable for understanding the hemolytic toxicity of AmB to red blood cells at a high sodium ion level.

Effect of Trastuzumab on the thermodynamic behavior and roughness of fluid membrane using unsaturated phospholipid/cholesterol mixed monolayer model

The microenvironment near the receptor on biological membrane plays an important role in regulating drug-receptor interaction, and the interaction between drugs and lipids on membrane can also affect the microenvironment of membrane, which may affect drugs' efficacy or cause the drug resistance. Trastuzumab (Tmab) is a monoclonal antibody, used to treat early breast cancer associated with the overexpression of Human Epidermal growth factor Receptor 2 (HER2). But its effectiveness is limited due to its tendency to make tumor cells resistant to the drug. In this work, the monolayer mixed by unsaturated phospholipids (DOPC, DOPE and DOPS) and cholesterol were used as a model to simulate the fluid membrane region on biological membrane. The phospholipid/cholesterol mixed monolayers in molar ratio 7:3 and 1:1, were respectively used to simulate the one layer of simplified normal cell membrane and tumor cell membrane. The influence of this drug on the phase behavior, elastic modulus, intermolecular force, relaxation and the surface roughness of the unsaturated phospholipid/cholesterol monolayer was investigated. The results show that at 30 mN/m the increase or decrease of the elastic modulus and surface roughness of the mixed monolayer caused by Tamb depends on the type of phospholipid, but the intensity of the effect depends on the content of cholesterol, and the intensity of influence is more significant at the presence of 50% cholesterol. However, the effect of Tmab on the ordering of the DOPC/cholesterol or DOPS/cholesterol mixed monolayer is stronger when the content of cholesterol is 30%, but it was stronger for the DOPE/cholesterol mixed monolayer when the content of cholesterol is 50%. This study is helpful to understand the effects of anticancer drugs on microenvironment of cell membrane, and it has a certain reference value for the design of drug delivery system and drug target identification.

Interaction between polyene antifungal drug and saturated phospholipid monolayer regulated by calcium ions at the air-water interface

Amphotericin B (AmB) is a polyene antifungal drug, which could directly form pores on the sterol-free phospholipid monolayers. The interaction between AmB and phospholipid can be affected by calcium ions, but the mechanism is still unclear. DPPC is a saturated phospholipid with -PC group, which is often used to simulate the outer cell membrane leaflet. And DPPC is also the main constituent of pulmonary surfactants. In this work, the DPPC monolayer was used as a model membrane to study the effect of calcium ions on the interaction between AmB and phospholipid. The influence of different concentration of calcium ions on the elastic modulus, mean molecular area increment, excess Gibbs free energy and stability of the AmB/DPPC mixed monolayer has been researched at the surface pressure of 7.5 mN/m, 12.5 mN/m and 22.5 mN/m. The AmB/DPPC monolayers at the air-water interface have been observed in real-time by Brewster angle microscope and the microstructure of the Langmuir-Blodgett monolayer films transferred onto the mica have been researched by scanning electron microscope and atomic force microscope. The results showed that calcium ions had a significant influence on the elastic modulus, mean molecular area increment, excess Gibbs free energy, stability and microstructure of the AmB/DPPC monolayer. It has been indicated that the influence of calcium ions on the interaction between AmB and DPPC molecules mainly depended on the effect of calcium ions on the orientation of AmB molecules. The calcium ions could regulate the effect of AmB to the stability of the DPPC monolayer. This regulatory role changed with the different concentrations of calcium ions and the different phase states of the monolayer. This work provides useful information to further understand the influence mechanism of calcium ions on the interaction between AmB and saturated phospholipid with -PC group, which is helpful to find out the effect mechanism of calcium ion on the interaction between AmB and the outer layer of cell membrane or pulmonary surfactants in different phase states and to understand the toxicity mechanism of AmB on the cell membrane or lungs.

Interaction of amphotericin B and saturated or unsaturated phospholipid monolayers containing cholesterol or ergosterol at the air-water interface

The antimicrobial activity of amphotericin B (AmB) depends on its interaction with ergosterol-containing cell membranes of fungus. Cholesterol is a sterol in mammalian cell membrane, and its structure is very similar to ergosterol, which caused to the toxic of amphotericin B to mammalian or human cell membranes. Even so, it is still the gold standard for the treatment of fungal infections. The mechanism of its toxicity to mammalian cell membrane has become a hot topic. The toxicity mechanism of amphotericin B on the cell membrane is also related to the phospholipids on the membrane. The effects of saturated and unsaturated fat chains on the interaction of amphotericin B with phospholipid monolayers containing cholesterol or ergosterol were studied at the molecular level using an air-water interface monolayer model. Both atomic force microscope and Brewster angle microscope were used to observe the surface morphology of the monolayer. The analysis of limiting molecular area suggested that the interaction between AmB and the two kinds of sterol is significantly different on the unsaturated lipid monolayer. According to the elastic modulus, the AmB molecules can increase the compressibility or viscoelasticity of the phospholipid/sterol monolayer. However, this impact of AmB on the DOPC/sterol monolayer containing ergosterol was stronger than that containing cholesterol at 25 ~ 50 mN/m. While this impact of AmB on the DPPC/sterol monolayer containing cholesterol was stronger than that containing ergosterol at 32 ~ 56 mN/m. The excess Gibbs free energy of the monolayer showed that, in the presence of saturated fat chain, amphotericin B could make the molecules of the DPPC/cholesterol monolayer and the DPPC/ergosterol monolayer arrange more closely and make intermolecular interaction stronger. There was no significant difference between DPPC/cholesterol monolayer and DPPC/ergosterol monolayer. However, in the presence of unsaturated chain, the effects of amphotericin B on the DOPC/cholesterol monolayer and the DOPC/ergosterol monolayer were significantly different. Amphotericin B made the molecular arrangement of DOPC/ergosterol monolayer more loosed, and the intermolecular force weakened at 5-35 mN/m. AFM images reflect that AmB can perforate the phospholipid-ergosterol monolayer, which was no significant correlation with saturation of the lipid monolayer. But the areas of dark areas shaped holes on the DPPC/ergosterol monolayer were larger than that on the DOPC/ergosterol monolayer. The adsorption of amphotericin B on lipid/sterol monolayer suggests that the orientation of amphotericin B may be different when it is inserted into the monolayer of phospholipid-sterol in the presence of saturated or unsaturated chains. The results are helpful to understand the complex mechanism of toxicity of amphotericin B to cell membrane.

Interaction of natural compounds with biomembrane models: A biophysical approach for the Alzheimer's disease therapy

Natural compounds such as caffeine (CA), gallic acid (GA) and tannic acid (TA) have been reported to be useful for Alzheimer's disease (AD) therapy. It was proved that some natural compounds inhibit the formation of senil plaques composed by beta-amyloid peptide (Aβ), a hallmark of AD. Evidences suggest that the therapeutic activity of compounds depends of their interaction with biological membranes. To understand why these compounds fail in vivo and in clinical trials, it is important to evaluate their pharmacokinetics properties. Thus, a biophysical approach to study drug-membrane interactions is essential to understand the mechanisms by which the drugs interact with the cellular membranes and affect the Aβ production, aggregation and clearance pathways. 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol (chol) were used to mimic the biophysical properties of cell membranes and study their interactions with these compounds. The partition coefficient, influence on membrane fluidity and location within the bilayer of the drugs were studied by derivative spectrophotometry, dynamic light scattering and fluorescence quenching, respectively. The results suggest that TA exhibited a significant higher partition than CA and GA and a preferential location near to the polar head of bilayer. The obtained results may explain the therapeutic mechanisms reported for these natural compounds.

New Insights on the Biophysical Interaction of Resveratrol with Biomembrane Models: Relevance for Its Biological Effects

Resveratrol has been widely studied because of its pleiotropic effects in cancer therapy, neuroprotection, and cardioprotection. It is believed that the interaction of resveratrol with biological membranes may play a key role in its therapeutic activity. The capacity of resveratrol to partition into lipid bilayers, its possible location within the membrane, and the influence of this compound on the membrane fluidity were investigated using membrane mimetic systems composed of egg l-α-phosphatidylcholine (EPC), cholesterol (CHOL), and sphingomyelin (SM). The results showed that resveratrol has greater affinity for the EPC bilayers than for EPC:CHOL [4:1] and EPC:CHOL:SM [1:1:1] membrane models. The increased difficulty in penetrating tight packed membranes is also demonstrated by fluorescence quenching of probes and by fluorescence anisotropy measurements. Resveratrol may be involved in the regulation of cell membrane fluidity, thereby contributing for cell homeostasis.

Properties of Langmuir and solid supported lipid films with sphingomyelin

Biological cell membranes play a crucial role in various biological processes and their functionality to some extent is determined by the hydrophilic/hydrophobic balance. A significant progress in understanding the membrane structure was the discovery of laterally segregated lipid domains, called the lipid rafts. These raft domains are of ordered lamellar liquid-crystalline phase, while rest of the membrane exists in a relatively disordered lamellar liquid-crystalline phase. Moreover, the chemical constitution of the lipid rafts consists of a higher content (up to 50%) of cholesterol (Chol) and sphingomyelin (SM). Sphingomyelin also plays a significant role in the red cells of blood and nerves, in some diseases, as a precursor to ceramides, and other sphingolipid metabolites. In this paper properties of Langmuir and solid supported mixed lipid films of DPPC/SM, DOPC/SM, and Chol/SM are described. Special attention has been paid to wetting properties (hydrophobic/hydrophilic balance) of these films transferred onto a hydrophilic glass surface. To our knowledge such results have not yet been published in the literature. The properties were determined via contact angle measurements and then calculation of the films' apparent surface free energy. The films' wettability and their apparent surface free energy strongly depend on their composition. The energy is affected by both the structure of hydrocarbon chains of glycerophospholipids (DPPC and DOPC) and their interactions with SM. Properties of mixed Chol/SM monolayer depend also on the film stoichiometry. At a low Chol content (XChol=0.25) the interactions between SM and Chol are strong and hence the formation of binary complex is possible. This is accompanied by a decrease in the film surface free energy in comparison to that of pure SM monolayer, contrary to a higher Chol content where the monolayer energy increases. This suggests that cholesterol is excluded from the membrane thus increasing the film hydrophilicity. These results are consistent with the literature data and somehow confirm the hypothesis of lipid raft formation. The roughness of the investigated monolayer surfaces was also determined using optical profilometry. The roughness parameters of the DPPC, SM, and mixed DPPC/SM generally correlate with the changes of their apparent surface free energy, i.e. with the decreasing roughness the apparent surface free energy also decreases. However, this is not the case for mixed DOPC/SM monolayers. Although the roughness increases with SM content the apparent surface free energy decreases. Therefore some other factors, like the presence of unsaturated bonds in the DOPC molecule, influence the film phase state and the energy too. More experiments are needed to explain this hypothesis.

Solid-state ¹³C NMR reveals annealing of raft-like membranes containing cholesterol by the intrinsically disordered protein α-Synuclein

Misfolding and aggregation of the intrinsically disordered protein α-Synuclein (αS) in Lewy body plaques are characteristic markers of late-stage Parkinson's disease. It is well established that membrane binding is initiated at the N-terminus of the protein and affects biasing of conformational ensembles of αS. However, little is understood about the effect of αS on the membrane lipid bilayer. One hypothesis is that intrinsically disordered αS alters the structural properties of the membrane, thereby stabilizing the bilayer against fusion. Here, we used two-dimensional (13)C separated local-field NMR to study interaction of the wild-type α-Synuclein (wt-αS) or its N-terminal (1-25) amino acid sequence (N-αS) with a cholesterol-enriched ternary membrane system. This lipid bilayer mimics cellular raft-like domains in the brain that are proposed to be involved in neuronal membrane fusion. The two-dimensional dipolar-recoupling pulse sequence DROSS (dipolar recoupling on-axis with scaling and shape preservation) was implemented to measure isotropic (13)C chemical shifts and (13)C-(1)H residual dipolar couplings under magic-angle spinning. Site-specific changes in NMR chemical shifts and segmental order parameters indicate that both wt-αS and N-αS bind to the membrane interface and change lipid packing within raft-like membranes. Mean-torque modeling of (13)C-(1)H NMR order parameters shows that αS induces a remarkable thinning of the bilayer (≈6Å), accompanied by an increase in phospholipid cross-sectional area (≈10Å(2)). This perturbation is characterized as membrane annealing and entails structural remodeling of the raft-like liquid-ordered phase. We propose this process is implicated in regulation of synaptic membrane fusion that may be altered by aggregation of αS in Parkinson's disease.