Cholesterol supports headgroup superlattice domain formation in fluid phospholipid/cholesterol bilayers

U18666A inhibits classical swine fever virus replication through interference with intracellular cholesterol trafficking

The level of cholesterol in host cells has been demonstrated to affect viral infection. Our previous studies showed that cholesterol-rich membrane rafts mediated the entry of classical swine fever virus (CSFV) into PK-15 or 3D4/21 cells, but the role of cholesterol post entry was still not clear. In this study, we found that CSFV replication before fusion was affected when the cholesterol trafficking in infected cells was disrupted using a cholesterol transport inhibitor, U18666A. Our data showed that U18666A affected both the fusion and replication steps in the life cycle of the virus, but not its binding and entry steps. The subsequent experiments confirmed that niemann-pick C1 (NPC1), a lysosomal membrane protein that helps cholesterol to leave the lysosome, was affected by U18666A, which led to the accumulation of cholesterol in lysosomes and inhibition of CSFV replication. Imipramine, a cationic hydrophobic amine similar to U18666A, also inhibited CSFV replication via similar mechanism. Surprisingly, the antiviral effect of U18666A was restored by the histone deacetylase inhibitor (HDACi), Vorinostat, which suggested that HDACi reverted the dysfunction of NPC1, and intra-cellular cholesterol accumulation disappeared and CSFV replicability resumed. Together, these data indicated that CSFV transformed from early endosome and late endosome into lysosome after endocytosis for further replication and that U18666A was a potential drug candidate for anti-pestivirus treatment.

Dansyl-labeled anionic amphiphile with a hexadecanoic carbon chain: synthesis and detection for shape transitions in organized molecular assemblies

The probing properties of a new fluorophore-labeled anionic surfactant, sodium 16-(N-dansyl)aminocetylate (16-DAN-ACA) were investigated systematically in molecular assemblies, especially in the transitions between micelles and vesicles. 16-DAN-ACA can efficiently differentiate the two different aggregate types in mixed cationic and anionic surfactant systems. The fluorescence anisotropy of 16-DAN-ACA was found to be sensitive for directly detecting the micellar growth in micelles containing oppositely charged surfactants; both cationic cetyltrimethylammonium bromide (CTAB) systems and anionic sodium dodecyl sulfate (SDS) systems were studied. The results indicated that the 16-DAN-ACA is a good fluorescent probe for differentiating the different aggregates, and even more can be used to detect the micellar growth.

Sphingolipids as modulators of membrane proteins

The diversity of the transmembranome of higher eukaryotes is matched by an enormous diversity of sphingolipid classes and molecular species. The intrinsic properties of sphingolipids are not only suited for orchestrating lateral architectures of biological membranes, but their molecular distinctions also allow for the evolution of protein motifs specifically recognising and interacting with individual lipids. Although various reports suggest a role of sphingolipids in membrane protein function, only a few cases have determined the specificity of these interactions. In this review we discuss examples of specific protein-sphingolipid interactions for which a modulator-like dependency on sphingolipids was assigned to specific proteins. These novel functions of sphingolipids in specific protein-lipid assemblies contribute to the complexity of the sphingolipid classes and other molecular species observed in animal cells. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Calorimetric behavior of phosphatidylcholine/phosphatidylethanolamine bilayers is compatible with the superlattice model

Differential scanning calorimetry was used to study the phase behavior of binary lipid bilayers consisting of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) of varying acyl chain length. A two-state transition model was used to resolve the individual transition components, and the two-state transition enthalpy, the relative enthalpy, and the transition temperature of each component were plotted as a function of composition. Intriguingly, abrupt changes in these thermodynamic parameters were observed at or close to many "critical" X(PE) values predicted by the superlattice model proposing that phospholipids with different headgroups tend to adopt regular rather than random lateral distributions. Statistical analysis indicated that the agreement between the observed and predicted "critical" compositions is highly significant. Accordingly, these data provide strong evidence that the molecules in PC/PE bilayers tend to adopt regular, superlattice-like lateral arrangements, which could be involved in the regulation of the lipid compositions of biological membranes.

The relationship among brain natriuretic peptide (BNP), cholesterol and lipoprotein

OBJECTIVE

To study the relationship among brain natriuretic peptide (BNP), cholesterol and lipoprotein.

DESIGN

A retrospective, cross-sectional study.

SETTING

Tokushima University Hospital area.

PATIENTS

A retrospective study of 46 patients (nine inpatients and 37 outpatients) with angina pectoris or arrhythmias who were seen at Tokushima University Hospital Cardiovascular Division and had measurements of their BNP, fatty acid and lipid profile. The average age of patients was 57±17 years, and 39% were male subjects.

MAIN OUTCOME MEASURES

BNP, dihomo-γ-linolenic acid, arachidonic acid, eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), apolipoproteinA1, apolipoprotein A2 (ApoA2), apolipoprotein B (ApoB), apolipoprotein C2, apolipoprotein C3, apolipoprotein E, total cholesterol (TC), triglyceride, high density lipoprotein cholesterol and low density lipoprotein cholesterol.

RESULTS

The baseline characteristics of the patients were shown in table 1 and the data of lipoprotein were shown in table 2. Table 3 shows the relationship among BNP, cholesterol and lipoprotein. The authors found significant negative correlation between serum levels of BNP and ApoA2 (figure 1; r=-0.458, p=0.001), serum levels of BNP and ApoB (figure 2; r=-0.328, p=0.026) and serum levels of BNP and TC (figure 3; r=-0.383, p=0.010). There is a possibility that dietary EPA and DHA may modulate cardiac mitochondrial and autonomic nervous system dysfunction via fatty-acids-PPARs-PTEN-PI3K/Akt-SREBPs system and affect serum BNP levels indirectly.

CONCLUSION

BNP had significant negative correlation with ApoA2, ApoB and TC. The findings suggest that increasing serum levels of ApoA2, ApoB and TC may have an effect on improving heart function. But the mechanism is presently unclear.

U18666A, an intra-cellular cholesterol transport inhibitor, inhibits dengue virus entry and replication

The level of cholesterol in host cells has been shown to affect viral infection. However, it is still not understood why this level of regulation is important for successful infection. We have shown in this study that dengue virus infection was affected when the cholesterol intake in infected cells was disrupted using a cholesterol transport inhibitor, U18666A. The antiviral effect was found to result from two events: retarded viral trafficking in the cholesterol-loaded late endosomes/lysosomes and suppressed de novo sterol biosynthesis in treated infected cells. We also observed an additive antiviral effect of U18666A with C75, a fatty acid synthase inhibitor, suggesting dengue virus relies on both the host cholesterol and fatty acid biosynthesis for successful replication.

A Synthetic mirror image of kalata B1 reveals that cyclotide activity is independent of a protein receptor

Featuring a circular, knotted structure and diverse bioactivities, cyclotides are a fascinating family of peptides that have inspired applications in drug design. Most likely evolved to protect plants against pests and herbivores, cyclotides also exhibit anti-cancer, anti-HIV, and hemolytic activities. In all of these activities, cell membranes appear to play an important role. However, the question of whether the activity of cyclotides depends on the recognition of chiral receptors or is primarily modulated by the lipid-bilayer environment has remained unknown. To determine the importance of lipid membranes on the activity of the prototypic cyclotide, kalata B1, we synthesized its all-D enantiomer and assessed its bioactivities. After the all-D enantiomer had been confirmed by (1)H NMR to be the structural mirror image of the native kalata B1, it was tested for anti-HIV activity, cytotoxicity, and hemolytic properties. The all-D peptide is active in these assays, albeit with less efficiency; this reveals that kalata B1 does not require chiral recognition to be active. The lower activity than the native peptide correlates with a lower affinity for phospholipid bilayers in model membranes. These results exclude a chiral receptor mechanism and support the idea that interaction with phospholipid membranes plays a role in the activity of kalata B1. In addition, studies with mixtures of L and D enantiomers of kalata B1 suggested that biological activity depends on peptide oligomerization at the membrane surface, which determines affinity for membranes by modulating the association-dissociation equilibrium.

Acyl-chain mismatch driven superlattice arrangements in DPPC/DLPC/cholesterol bilayers

Fluorescence and infrared spectroscopy and cholesterol oxidase activity were employed to investigate the effect of phosphatidylcholine (PC) acyl chain length mismatch on the lateral organizations of lipids in liquid-ordered dipalmitoyl-PC/dilauroyl-PC/cholesterol (DPPC/DLPC/CHOL) bilayers. Plots of steady-state fluorescence emission anisotropy of diphenylhexatriene (DPH) labeled PC (DPH-PC) embedded in the DPPC/DLPC/CHOL bilayers revealed significant peaks at several DPPC mole fractions (Y(DPPC)) when the cholesterol mole fraction (X(CHOL)) was fixed to particular values. Analogously, the DPH-PC anisotropy peaked at several critical X(CHOL)'s when Y(DPPC) was fixed. Acyl chain C-H and C horizontal lineO vibrational peak frequencies of native PC as well as the activity of cholesterol oxidase also revealed dips and peaks at similar Y(DPPC)'s. Importantly, most of the observed peaks/dips coincide with the critical mole fractions predicted by the Superlattice (SL) model. A three-dimensional map of DPH-PC anisotropy versus composition in the range 0.32 <or= X(CHOL) <or= 0.50; 0.54 <or= Y(DPPC) <or= 0.72 revealed a prominent peak at (X(CHOL), Y(DPPC)) approximately (0.42, 0.64). This suggests a simultaneous presence of two different types of superlattices, one where cholesterol is the quest molecule in a PC host lattice and another where DPPC is the guest in the DLPC host lattice. Time-resolved measurements of DPH-PC fluorescence indicated the existence of an ordered, rotationally hindered environment of acyl chains at that "critical" composition consistent with the existence of SL arrangements. We propose that beside CHOL/PC superlattices, DPPC, and DLPC as well tend to adopt regular SL-like lateral distributions relative to each other, presumably because the less hydrophobic DLPC molecule is slightly displaced toward the aqueous phase, thus allowing more room and mobility for the head groups of both DPPC and DLPC as well as for the acyl chain tails of DPPC. The parallel presence of two kinds of superlattices, that is, CHOL/PC-SL and DPPC/DLPC-SL as demonstrated here, has intriguing implications regarding lipid homeostasis of eukaryote membranes.

Cholesterol modulates the interaction of beta-amyloid peptide with lipid bilayers

The interaction of an amphiphilic, 40-amino acid beta-amyloid (Abeta) peptide with liposomal membranes as a function of sterol mole fraction (X(sterol)) was studied based on the fluorescence anisotropy of a site-specific membrane sterol probe, dehydroergosterol (DHE), and fluorescence resonance energy transfer (FRET) from the native Tyr-10 residue of Abeta to DHE. Without Abeta, peaks or kinks in the DHE anisotropy versus X(sterol) plot were detected at X(sterol) approximately 0.25, 0.33, and 0.53. Monomeric Abeta preserved these peaks/kinks, but oligomeric Abeta suppressed them and created a new DHE anisotropy peak at X(sterol) approximately 0.38. The above critical X(sterol) values coincide favorably with the superlattice compositions predicted by the cholesterol superlattice model, suggesting that membrane cholesterol tends to adopt a regular lateral arrangement, or domain formation, in the lipid bilayers. For FRET, a peak was also detected at X(sterol) approximately 0.38 for both monomeric and oligomeric Abeta, implying increased penetration of Abeta into the lipid bilayer at this sterol mole fraction. We conclude that the interaction of Abeta with membranes is affected by the lateral organization of cholesterol, and hypothesize that the formation of an oligomeric Abeta/cholesterol domain complex may be linked to the toxicity of Abeta in neuronal membranes.

AFM studies of the effect of temperature and electric field on the structure of a DMPC-cholesterol bilayer supported on a Au(111) electrode surface

Atomic force microscopy (AFM) was used to characterize a phospholipid bilayer composed of 70 mol % 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 30 mol % cholesterol, at a Au(111) electrode surface. Results indicate that addition of cholesterol relaxes membrane elastic stress, increases membrane thickness, and reduces defect density. The thickness and thermotropic properties of the mixed DMPC-cholesterol bilayer supported at the gold electrode surface are quite similar to the properties of the mixed membrane in unilamellar vesicles. The stability of the supported membrane at potentials negative to the potential of zero charge E(pzc) was investigated. This study demonstrates that the bilayer supported at the gold electrode surface is stable provided the applied potential (E - E(pzc)) is less than -0.3 V. At larger polarizations, swelling of the membrane is observed. Polarizations larger than -1 V cause electrodewetting of the bilayer from the gold surface. At these negative potentials, the bilayer remains in close proximity to the metal surface, separated from it by a approximately 2 nm thick layer of electrolyte.

Effect of quinone on the fluorescence decay dynamics of endogenous flavin bound to bacterial luciferase

The interaction of quinone with luciferase from Photobacterium leiognathi was studied based on the fluorescence decay measurements of the endogenous flavin bound to the enzyme. Homologous 1,4-quinones, 1,4-benzoquinone, methyl-1,4-benzoquinone, 2-methyl-5-isopropyl-1,4-benzoquine and 1,4-naphthoquinone, were investigated. In the absence of quinone, the fluorescence intensity and anisotropy decays of the endogenous flavin exhibited two intensity decay lifetimes (~1 and 5 ns) and two anisotropy decay lifetimes (~0.2 and 20 ns), suggesting a heterogeneous quenching and a rotational mobility microenvironment of the active site of the luciferase, respectively. In the presence of quinone, the intensity decay heterogeneity was largely maintained, whereas the fraction of the short anisotropy decay component and the averaged rotational rate of FMN increased with the increasing hydrophobicity of the quinone. We hypothesize that the hydrophobicity of the quinone plays a role in the non-specific inhibition mechanism of xenobiotic molecules in the bacterial bioluminescence system via altering the rotational mobility of the endogenous flavin in the luciferase.

The superlattice model of lateral organization of membranes and its implications on membrane lipid homeostasis

Most biological membranes are extremely complex structures consisting of hundreds of different lipid and protein molecules. According to the famous fluid-mosaic model lipids and many proteins are free to diffuse very rapidly in the plane of the membrane. While such fast diffusion implies that different membrane lipids would be laterally randomly distributed, accumulating evidence indicates that in model and natural membranes the lipid components tend to adopt regular (superlattice-like) distributions. The superlattice model, put forward based on such evidence, is intriguing because it predicts that 1) there is a limited number of allowed compositions representing local minima in membrane free energy and 2) those energy minima could provide set-points for enzymes regulating membrane lipid compositions. Furthermore, the existence of a discrete number of allowed compositions could help to maintain organelle identity in the face of rapid inter-organelle membrane traffic.

Use of fluorescence to determine the effects of cholesterol on lipid behavior in sphingomyelin liposomes and erythrocyte membranes

The purpose of this study was to generate the equivalent of a cholesterol/temperature phase map for a biological membrane using fluorescence spectroscopy. The pseudo-phase map was created using human erythrocytes treated with various concentrations of methyl-beta-cyclodextrin to remove defined amounts of cholesterol and a trio of fluorescent probes that assess different membrane properties (laurdan, diphenylhexatriene, and merocyanine 540). Parallel experiments with two-photon microscopy suggested that changes in cellular cholesterol content affected the entire membrane rather than being localized to specific macroscopic domains. The various regions of the composite erythrocyte pseudo-phase map were interpreted using analogous data acquired from multilamellar vesicles that served as simplified models of cholesterol-dependent phases. The vesicles consisted of various concentrations of cholesterol (0 to 50 mol%) with either palmitoyl sphingomyelin, 1:1 dipalmitoylphosphatidylcholine and dioleoylphosphatidylcholine, or phospholipid mixtures intended to simulate either the inner or outer leaflet of erythrocyte membranes. Four distinguishable regions were observed in sphingomyelin phase maps corresponding to the traditional solid-ordered and liquid-disordered phases and two types of liquid-ordered behavior. Physical properties were less diverse in the mixed phospholipid vesicles, as expected, based on previous studies. Erythrocytes displayed five regions of different combinations of membrane properties along the phase map. Some of the observations identified similarities between the cells and liquid-ordered behavior observed in the various types of liposomes as well as some interesting differences.

Carbohydrate restriction and dietary cholesterol modulate the expression of HMG-CoA reductase and the LDL receptor in mononuclear cells from adult men

The liver is responsible for controlling cholesterol homeostasis in the body. HMG-CoA reductase and the LDL receptor (LDL-r) are involved in this regulation and are also ubiquitously expressed in all major tissues. We have previously shown in guinea pigs that there is a correlation in gene expression of HMG-CoA reductase and the LDL-r between liver and mononuclear cells. The present study evaluated human mononuclear cells as a surrogate for hepatic expression of these genes. The purpose was to evaluate the effect of dietary carbohydrate restriction with low and high cholesterol content on HMG-CoA reductase and LDL-r mRNA expression in mononuclear cells. All subjects were counseled to consume a carbohydrate restricted diet with 10–15% energy from carbohydrate, 30–35% energy from protein and 55–60% energy from fat. Subjects were randomly assigned to either EGG (640 mg/d additional dietary cholesterol) or SUB groups [equivalent amount of egg substitute (0 dietary cholesterol contributions) per day] for 12 weeks. At the end of the intervention, there were no changes in plasma total or LDL cholesterol (LDL-C) compared to baseline (P > 0.10) or differences in plasma total or LDL-C between groups. The mRNA abundance for HMG-CoA reductase and LDL-r were measured in mononuclear cells using real time PCR. The EGG group showed a significant decrease in HMG-CoA reductase mRNA (1.98 ± 1.26 to 1.32 ± 0.92 arbitrary units P < 0.05) while an increase was observed for the SUB group (1.13 ± 0.52 to 1.69 ± 1.61 arbitrary units P < 0.05). Additionally, the LDL-r mRNA abundance was decreased in the EGG group (1.72 ± 0.69 to 1.24 ± 0.55 arbitrary units P < 0.05) and significantly increased in the SUB group (1.00 ± 0.60 to 1.67 ± 1.94 arbitrary units P < 0.05). The findings indicate that dietary cholesterol during a weight loss intervention alters the expression of genes regulating cholesterol homeostasis.

Molecular dynamics studies of the molecular structure and interactions of cholesterol superlattices and random domains in an unsaturated phosphatidylcholine bilayer membrane

The effect of the molecular organization of lipid components on the properties of the bilayer membrane has been a topic of increasing interest. Several experimental and theoretical studies have suggested that cholesterol is not randomly distributed in the fluid-state lipid bilayer but forms nanoscale domains. Several cholesterol-enriched nanodomain structures have been proposed, including rafts, regular or maze arrays, complexes, and superlattices. At present, the molecular mechanisms by which lipid composition influences the formation and stability of lipid nanodomains remain unclear. In this study, we have used molecular dynamics (MD) simulations to investigate the effects of the molecular organization of cholesterol--superlattice versus random--on the structure of and interactions between lipids and water in lipid bilayers of cholesterol and 1-palmitoyl-2-oleoylphosphatidylcholine (cholesterol/POPC) at a fixed cholesterol mole fraction of 0.40. On the basis of four independent replicates of 200-ns MD simulations for a superlattice or random bilayer, statistically significant differences were observed in the lipid structural parameters, area per lipid, density profile, and acyl chain order profile, as well as the hydrogen bonding between various pairs (POPC and water, cholesterol and water, and POPC and cholesterol). The time evolution of the radial distribution of the cholesterol hydroxy oxygen suggests that the lateral distribution of cholesterol in the superlattice bilayer is more stable than that in the random bilayer. Furthermore, the results indicate that a relatively long simulation time, more than 100 ns, is required for these two-component bilayers to reach equilibrium and that this time is influenced by the initial lateral distribution of lipid components.

Lipid headgroup superlattice modulates the activity of surface-acting cholesterol oxidase in ternary phospholipid/cholesterol bilayers

The relationship between the molecular organization of lipid headgroups and the activity of surface-acting enzyme was examined using a bacterial cholesterol oxidase (COD) as a model. The initial rate of cholesterol oxidation by COD in fluid state 1-palmitoyl-2-oleoyl-phosphatidylethanolamine/1-palmitoyl-2-oleoyl-phosphatidylcholine/cholesterol (POPE/POPC/CHOL) bilayers was measured as a function of POPE-to-phospholipid mole ratio (X(PE)) and cholesterol-to-lipid mole ratio (X(CHOL)) at 37 degrees C. At X(PE) = 0, the COD activity changed abruptly at X(CHOL) approximately 0.40, whereas major activity peaks were detected at X(PE) approximately 0.18, 0.32, 0.50, 0.64, and 0.73 when X(CHOL) was fixed to 0.33 or 0.40. At a fixed X(CHOL) of 0.50, the COD activity increased progressively with PE content and exhibited small peaks or kinks at X(PE) approximately 0.40, 0.50, 0.58, 0.69, and 0.81. When X(PE) and X(CHOL) were systematically varied within a narrow 2-D lipid composition window, an onset of COD activity at X(CHOL) approximately 0.40 and the elimination of the activity peak at X(PE) approximately 0.64 for X(CHOL) >0.40 were clearly observed. Except for X(PE) approximately 0.40 and 0.58, the observed critical PE mole ratios agree closely (+/-0.03) with those predicted by a headgroup superlattice model (Virtanen, J.A., et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 4964-4969; Cannon, B., et al. (2006) J. Phys. Chem. B 110, 6339-6350), which proposes that lipids with headgroups of different sizes tend to adopt regular, superlattice-like distributions at discrete and predictable compositions in fluid lipid bilayers. Our results indicate that headgroup superlattice domains exist in lipid bilayers and that they may play a crucial role in modulating the activity of enzymes acting on the cell membrane surface.