Cholesterol Dimers Research Articles

Dimerization of cholesterol permits its binding to lipid-free apoA-I

Background and Aims : To model the interaction of lipid-free apoA-I with cholesterol molecules that exist in various self-associated forms in extracellular space. Cholesterol dimerization is exploited to reconcile the existing experimental data on cholesterol binding to apoA-I with extremely low critical micelle concentration of cholesterol.Methods: The interaction of differently self-associated lipid-free apoA-I with cholesterol monomer and tail-to-tail (TT) or face-to-face (FF) cholesterol dimer was modelled with Schrödinger package. Two crystal structures of 1-43 N-truncated apolipoprotein Δ(1-43)A-I tetramer (PDB ID: 1AV1, structure B), 185-243 C-truncated apolipoprotein Δ(185-243)A-I dimer (PDB ID: 3R2P, structure M) were exploited.Results: Cholesterol monomers bind to multiple binding sites in apoA-I monomer, dimer and tetramer with low, moderate and high energy, still insufficient to overcome the thermodynamic restriction by cholesterol micellization (-52.8 kJ/mol). However, apoA-I monomer and dimer existing in structure B, that contain nonoverlapping and non-interacting pairs of binding sites with high affinity for TT and FF cholesterol dimers, can bind in common 14 cholesterol molecules that correspond to existing values. ApoA-I monomer and dimer in structure M can bind in common 6 cholesterol molecules. The values of respective total energy of cholesterol binding for both B and M structures exceed the free energy of cholesterol micellization.Conclusions: Cholesterol dimers may simultaneously interact with extracellular monomer and dimer of lipid-free apoA-I, that accumulate at acid pH in atheroma. The thermodynamically allowed apolipoprotein-cholesterol interaction outside the macrophage may represent a new mechanism of cholesterol transport by apoA-I from atheroma, in addition to ABCA1-mediated cholesterol efflux. Background and Aims : To model the interaction of lipid-free apoA-I with cholesterol molecules that exist in various self-associated forms in extracellular space. Cholesterol dimerization is exploited to reconcile the existing experimental data on cholesterol binding to apoA-I with extremely low critical micelle concentration of cholesterol. Methods: The interaction of differently self-associated lipid-free apoA-I with cholesterol monomer and tail-to-tail (TT) or face-to-face (FF) cholesterol dimer was modelled with Schrödinger package. Two crystal structures of 1-43 N-truncated apolipoprotein Δ(1-43)A-I tetramer (PDB ID: 1AV1, structure B), 185-243 C-truncated apolipoprotein Δ(185-243)A-I dimer (PDB ID: 3R2P, structure M) were exploited. Results: Cholesterol monomers bind to multiple binding sites in apoA-I monomer, dimer and tetramer with low, moderate and high energy, still insufficient to overcome the thermodynamic restriction by cholesterol micellization (-52.8 kJ/mol). However, apoA-I monomer and dimer existing in structure B, that contain nonoverlapping and non-interacting pairs of binding sites with high affinity for TT and FF cholesterol dimers, can bind in common 14 cholesterol molecules that correspond to existing values. ApoA-I monomer and dimer in structure M can bind in common 6 cholesterol molecules. The values of respective total energy of cholesterol binding for both B and M structures exceed the free energy of cholesterol micellization. Conclusions: Cholesterol dimers may simultaneously interact with extracellular monomer and dimer of lipid-free apoA-I, that accumulate at acid pH in atheroma. The thermodynamically allowed apolipoprotein-cholesterol interaction outside the macrophage may represent a new mechanism of cholesterol transport by apoA-I from atheroma, in addition to ABCA1-mediated cholesterol efflux.

A Cholesterol Dimer Stabilizes the Inactivated State of an Inward-Rectifier Potassium Channel.

Cholesterol oligomers reside in multiple membrane protein X-ray crystal structures. Yet, there is no direct link between these oligomers and a biological function. Here we present the structural and functional details of a cholesterol dimer that stabilizes the inactivated state of an inward-rectifier potassium channel KirBac1.1. K+ efflux assays confirm that high cholesterol concentration reduces K+ conductance. We then determine the structure of the cholesterol-KirBac1.1 complex using Xplor-NIH simulated annealing calculations driven by solid-state NMR distance measurements. These calculations identified an α-α cholesterol dimer docked to a cleft formed by adjacent subunits of the homotetrameric protein. We compare these results to coarse grain molecular dynamics simulations. This is one of the first examples of a cholesterol oligomer performing a distinct biological function and structural characterization of a conserved promiscuous lipid binding region.

A Cholesterol Dimer Stabilizes the Inactivated State of an Inward‐Rectifier Potassium Channel

AbstractCholesterol oligomers reside in multiple membrane protein X‐ray crystal structures. Yet, there is no direct link between these oligomers and a biological function. Here we present the structural and functional details of a cholesterol dimer that stabilizes the inactivated state of an inward‐rectifier potassium channel KirBac1.1. K+ efflux assays confirm that high cholesterol concentration reduces K+ conductance. We then determine the structure of the cholesterol‐KirBac1.1 complex using Xplor‐NIH simulated annealing calculations driven by solid‐state NMR distance measurements. These calculations identified an α–α cholesterol dimer docked to a cleft formed by adjacent subunits of the homotetrameric protein. We compare these results to coarse grain molecular dynamics simulations. This is one of the first examples of a cholesterol oligomer performing a distinct biological function and structural characterization of a conserved promiscuous lipid binding region.

Direct Measurement of Intermolecular Mechanical Force for Nonspecific Interactions between Small Molecules.

Mechanical force can evaluate intramolecular interactions in macromolecules. Because of the rapid motion of small molecules, it is extremely challenging to measure mechanical forces of nonspecific intermolecular interactions. Here, we used optical tweezers to directly examine the intermolecular mechanical force (IMMF) of nonspecific interactions between two cholesterols. We found that IMMFs of dimeric cholesterol complexes were dependent on the orientation of the interaction. The surprisingly high IMMF in cholesterol dimers (∼30 pN) is comparable to the mechanical stability of DNA secondary structures. Using Hess-like cycles, we quantified that changes in free energy of solubilizing cholesterol (ΔGsolubility) by β-cyclodextrin (βCD) and methylated βCD (Me-βCD) were as low as -16 and -27 kcal/mol, respectively. Compared to the ΔGsolubility of cholesterols in water (5.1 kcal/mol), these values indicated that cyclodextrins can easily solubilize cholesterols. Our results demonstrated that the IMMF can serve as a generic and multipurpose variable to dissect nonspecific intermolecular interactions among small molecules into orientational components.

Direct Observation of Cholesterol Dimers and Tetramers in Lipid Bilayers.

Cholesterol is a ubiquitous component of mammalian cell membranes and affects membrane protein function. Although cholesterol-mediated formation of ordered membrane domains has been extensively studied, molecular-level structural information about cholesterol self-association has been absent. Here, we combine solid-state nuclear magnetic resonance (NMR) spectroscopy with all-atom molecular dynamics simulations to determine the oligomeric structure of cholesterol in phospholipid bilayers. Two-dimensional 13C-13C correlation spectra of differentially labeled cholesterol indicate that cholesterol self-associates in a face-to-face fashion at membrane concentrations from 17 to 44 mol %. 2D 13C and 19F spin-counting experiments allowed us to measure the average oligomeric number of these cholesterol clusters. At low cholesterol concentrations of ∼20%, the average cluster size is centered on dimers. At a high cholesterol concentration of 44%, which is representative of virus lipid envelopes and liquid-ordered domains of cell membranes, both dimers and tetramers are observed. The cholesterol dimers are found in both phase-separated membranes that contain sphingomyelin and in disordered and miscible membranes that are free of sphingomyelin. Molecular dynamics simulations support these experimental observations and moreover provide the lifetimes, stabilities, distributions, and structures of these nanoscopic cholesterol clusters. Taken together, these NMR and MD data strongly suggest that dimers are the basic structural unit of cholesterol in phospholipid bilayers. The direct observation of cholesterol dimers and tetramers provides a revised framework for studying cholesterol interactions with membrane proteins to regulate protein functions and for understanding the pathogenic role of cholesterol in diseases.

Interaction of lipid-free apolipoprotein A-I with cholesterol revealed by molecular modeling

We report the modeling of the interaction of differently self-associated lipid-free apoA-I with cholesterol monomer and tail-to-tail (TT) or face-to-face (FF) cholesterol dimer. Cholesterol dimerization is exploited to reconcile the existing experimental data on cholesterol binding to apoA-I with extremely low critical micelle concentration of cholesterol. Two crystal structures of 1–43 N-truncated apolipoprotein Δ(1-43)A-I tetramer (PDB ID: 1AV1, structure B), 185–243 C-truncated apolipoprotein Δ(185-243)A-I dimer (PDB ID: 3R2P, structure M) were analyzed. Cholesterol monomers bind to multiple binding sites in apoA-I monomer, dimer and tetramer with low, moderate and high energy (−10 to −28 kJ/mol with Schrödinger package), still insufficient to overcome the thermodynamic restriction by cholesterol micellization (−52.8 kJ/mol). The binding sites partially coincide with the putative cholesterol-binding motifs. However, apoA-I monomer and dimer existing in structure B, that contain nonoverlapping and non-interacting pairs of binding sites with high affinity for TT and FF cholesterol dimers, can bind in common 14 cholesterol molecules that correspond to existing values. ApoA-I monomer and dimer in structure M can bind in common 6 cholesterol molecules. The values of respective total energy of cholesterol binding up to 64.5 and 67.0 kJ/mol for both B and M structures exceed the free energy of cholesterol micellization. We hypothesize that cholesterol dimers may simultaneously interact with extracellular monomer and dimer of lipid-free apoA-I, that accumulate at acid pH in atheroma. The thermodynamically allowed apolipoprotein-cholesterol interaction outside the macrophage may represent a new mechanism of cholesterol transport by apoA-I from atheroma, in addition to ABCA1-mediated cholesterol efflux.

Breaking the Barrier of Polynucleotide Size, Type, and Topology in Smad2 Antisense Therapy Using a Cationic Cholesterol Dimer with Flexible Spacer.

A liposomal formulation comprising a dicationic cholesterol based lipid, Chol+-(CH2)5-Chol+, and a helper zwitterionic lipid, DOPE (1:4), was prepared to deliver polynucleotides of different topologies, molecular weights, and backbones. This formulation was used to transfect HeLa cells with circular and linearized plasmid pEGFP-C3. The transfection efficiency of the dicationic cholesterol based coliposomal formulation Chol+-(CH2)5-Chol+/DOPE (1:4) was observed to be better when compared against different commercial delivery agents, Lipofectamine2000, Effectene, and a known oligonucleotide delivery agent, Oligofectamine. The efficacy was also compared with the respective monocationic cholesterol based liposomal formulations. Western blot analysis for Smad2 protein detection showed almost 100% downregulation of the Smad2 protein by polynucleotides delivered by Chol+-(CH2)5-Chol+/DOPE (1:4), which was better than that with Oligofectamine and Effectene. Similarly, semiquantitative RT-PCR showed the downregulation of Smad2 RNA along with that of a downstream target of Smad2, Id2. The higher efficiency of different types of nucleic acid delivery was also evident with Chol+-(CH2)5-Chol+/DOPE (1:4) in A549 cells. As an added benefit, the formulation Chol+-(CH2)5-Chol+/DOPE (1:4) was found to be highly biocompatible at all the compositions investigated herein.

Exploring the Structure and Stability of Cholesterol Dimer Formation in Multicomponent Lipid Bilayers

For 40 years, the existence and possible functional importance of cholesterol dimer formation has been discussed. Due to challenges associated with structural studies of membrane lipids, there has as yet been no direct experimental verification of the existence and relevance of the cholesterol dimer. Building on recent advances in lipid force fields for molecular simulation, in this work the structure and stability of the cholesterol dimer is characterized in POPC bilayers in absence and presence of sphingomyelin. The cholesterol dimer structural ensemble is found to consist of sub-states that reflect, but also differ from, previously proposed dimer structures. While face-to-face dimer structures predominate, no evidence is found for the existence of tail-to-tail dimers in POPC lipid bilayers. Near stoichiometric complex formation of cholesterol with sphingomyelin is found to effect cholesterol dimer structure without impacting population. Comparison with NMR-derived order parameters provide validation for the simulation model employed and conclusions drawn related to the structure and stability of cholesterol dimers in multicomponent lipid bilayers. © 2016 Wiley Periodicals, Inc.

Formation of Cholesterol Oxidation Products, Cholesterol Dimers and Cholestadienes After Thermal Processing of Cholesterol Standards and Butter

The aim of the study is to compare the transformation cholesterol standard and cholesterol present in a food matrix (butter) during thermal treatment at 150 and 180 °C for 1 h, which results in the formation of cholesterol oxidation products (0.5–189.3 mg g−1 of unheated standard and 0.044–0.063 mg g−1 of unheated butter) and cholesterol dimers (72.3–120.3 mg g−1 of unheated standard and 0.208 mg g−1 of unheated butter). The presence of cholesta‐3,5‐diene is investigated, especially in free cholesterol and cholesteryl palmitate unheated samples (105.3–116.4 mg g−1), as well as in thermally processed butter (0.009 mg g−1). Moreover, the processes of the extensive degradation of cholesterol are observed in standards samples (34.7–98.8% losses) compared to butter samples (25.5–73.5% losses). The qualitative and quantitative analyses of the above mentioned compounds are performed with the use of solid‐phase extraction, gas chromatography coupled with mass spectrometry and gel permeation chromatography. In conclusion, the thermal processing of butter and products containing cholesterol should be performed at mild temperatures, for example, 150 °C, because fewer negative components (e.g., oxysterols, cholesterol dimers) are formed in those conditions compared to heat processing at higher temperatures.Practical Application: The methods described in this study may be useful for the determination of sterols, oxysterols, cholesterol dimers, and cholesta‐3,5‐diene in standard cholesterol samples and cholesterol present in food products. The presented results data are crucial for the assessment of cholesterol changes during processing, especially the observation that some of the components have adverse effects on the human metabolism of oxysterols or cholesterol dimers. The investigations show that the thermal processing of products containing cholesterol should be performed at the lowest possible temperatures, for example, 150 °C, because this protects cholesterol from dimerization, oxidation, and degradation.Thermal processing of cholesterol can lead to the formation of cholesterol dimers, cholesterol oxidation products or cholestadienes, which can have adverse effect of the human health.

Liquid crystalline oligomers derived from cholesterol: synthesis and columnar mesomorphism

ABSTRACTThe novel columnar liquid crystals based on symmetric hairpin-shaped cholesterol tetramers bridged by rigid or hydrogen-bonding Schiff-base spacers were prepared by reacting cholesterol dimer with phenylenediamines or dihydrazides in yields of 70–85%. Structural characterisation of these new compounds had been achieved by infrared, nuclear magnetic resonance and mass spectrometry. Their mesomorphic properties were investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. They exhibited good hexagonal columnar mesophase with three molecules in a disc of the columnar hexagonal state. This study implied that the symmetric cholesterol tetramers with rigid cores or hydrogen-bonding cores prefer to columnar mesophase.

Nonsymmetrical cholesterol dimers constituting regioisomeric oxadiazole and thiadiazole cores: an investigation of the structure–property correlation

The effect of a bent angle on the stabilization of liquid crystalline and organogel self-assemblies.

Exploring the structure and stability of cholesterol dimer formation in multicomponent lipid bilayers.

For 40 years, the existence and possible functional importance of cholesterol dimer formation has been discussed. Due to challenges associated with structural studies of membrane lipids, there has as yet been no direct experimental verification of the existence and relevance of the cholesterol dimer. Building on recent advances in lipid force fields for molecular simulation, in this work the structure and stability of the cholesterol dimer is characterized in POPC bilayers in absence and presence of sphingomyelin. The cholesterol dimer structural ensemble is found to consist of sub-states that reflect, but also differ from, previously proposed dimer structures. While face-to-face dimer structures predominate, no evidence is found for the existence of tail-to-tail dimers in POPC lipid bilayers. Near stoichiometric complex formation of cholesterol with sphingomyelin is found to effect cholesterol dimer structure without impacting population. Comparison with NMR-derived order parameters provide validation for the simulation model employed and conclusions drawn related to the structure and stability of cholesterol dimers in multicomponent lipid bilayers. © 2016 Wiley Periodicals, Inc.

Unsymmetrical cholesterol and benzoxazole-based liquid crystalline dimers: synthesis and characterisation

The synthesis of a number of novel liquid crystalline materials has been achieved by connecting the 2-phenyl substituted benzoxazole unit to a cholesterol unit having an alkyl spacer through an imine linkage. All the synthesised compounds exhibited liquid crystalline property. Compounds 10b and 10d possess both the blue phase and the smectic C* phase.

The Liebermann–Burchard Reaction: Sulfonation, Desaturation, and Rearrangment of Cholesterol in Acid

In the Liebermann-Burchard (LB) colorimetric assay, treatment of cholesterol with sulfuric acid, acetic anhydride, and acetic acid elicits a blue color. We studied the reactivity of cholesterol under LB conditions and provide definitive NMR characterization for approximately 20 products, whose structure and distribution suggest the following mechanistic picture. The major reaction pathways do not involve cholestadienes, i-steroids, or cholesterol dimers, as proposed previously. Instead, cholesterol and its acetate and sulfate derivatives undergo sulfonation at a variety of positions, often with skeletal rearrangements. Elimination of an SO(3)H group as H(2)SO(3) generates a new double bond. Repetition of this desaturation process leads to polyenes and ultimately to aromatic steroids. Linearly conjugated polyene cations can appear blue but form too slowly to account for the LB color response, whose chemical origin remains unidentified. Nevertheless, the classical polyene cation model is not excluded for Salkowski conditions (sulfuric acid), which immediately generate considerable amounts of cholesta-3,5-diene. Some rearrangements of cholesterol in H(2)SO(4) resemble the diagenesis pathways of sterols and may furnish useful lipid biomarkers for characterizing geological systems.

An effect of antibiotic amphotericin B on ion transport across model lipid membranes and tonoplast membranes

A pH sensitive fluorescence probe piranine trisulfonate, entrapped inside small unilamellar liposomes formed with egg yolk phosphatidylcholine, was applied to investigate effect of polyene antibiotic amphotericin B (AmB) on proton transport across lipid membranes. Time dependencies of fluorescence-monitored pH changes inside lipid vesicles, upon sudden acidification of the liposome suspension, were analyzed in terms of two-exponential kinetics. It appears that addition of AmB at 3 mol%, with respect to lipid, considerably increases the rate constant of the fast component of proton transport (a change from (60 to 149) × 10 −3 s −1) and decreases the rate constant of the slow component (a change from (11 to 5) × 10 −3 s −1). Incorporation of 0.1 mol% AmB results in the decrease of both parameters (to (33 and 2) × 10 −3 s −1, respectively). The increase in the rate of proton transfer across the lipid membrane is interpreted as related to the formation of membrane channels by AmB, at higher concentration of the drug or nonspecific destabilization of the membrane structure. At low concentrations, at which formation of molecular structures of AmB is not possible, the antibiotic molecules are oriented horizontally with respect to the plane of the membrane and act in making the membrane more compact and less permeable to ions. The presence of sterols (cholesterol, ergosterol and cholesterol dimer) in the lipid phase, in the concentration 3 mol% and lower, decreased the rate constants of proton transfer across the membranes but did not influence significantly the effect of AmB on the ion transport. The presence of AmB in the bathing solutions of tonoplast membranes isolated from Conocephalum conicum at the concentrations range 1 × 10 −7 to 3.6 × 10 −5 does not influence considerably the ion current, as monitored by means of the patch-clamp technique.

Cholesterol Organization in Membranes at Low Concentrations: Effects of Curvature Stress and Membrane Thickness

Cholesterol is often found distributed nonrandomly in domains in biological and model membranes and has been reported to be distributed heterogeneously among various intracellular membranes. Although a large body of literature exists on the organization of cholesterol in plasma membranes or membranes with high cholesterol content, very little is known about organization of cholesterol in membranes containing low amounts of cholesterol. Using a fluorescent cholesterol analog (25-[ N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-methyl]amino]-27-norcholesterol, or NBD-cholesterol), we have previously shown that cholesterol may exhibit local organization even at very low concentrations in membranes, which could possibly be attributable to transbilayer tail-to-tail dimers. This is supported by similar observations reported by other groups using cholesterol or dehydroergosterol, a naturally occurring fluorescent cholesterol analog which closely mimics cholesterol. In this paper, we have tested the basic features of cholesterol organization in membranes at low concentrations using spectral features of dehydroergosterol. More importantly, we have investigated the role of membrane surface curvature and thickness on transbilayer dimer arrangement of cholesterol using NBD-cholesterol. We find that dimerization is not favored in membranes with high curvature. However, cholesterol dimers are observed again if the curvature stress is relieved. Further, we have monitored the effect of membrane thickness on the dimerization process. Our results show that the dimerization process is stringently controlled by a narrow window of membrane thickness. Interestingly, this type of local organization of NBD-cholesterol at low concentrations is also observed in sphingomyelin-containing membranes. These results could be significant in membranes that have very low cholesterol content, such as the endoplasmic reticulum and the inner mitochondrial membrane, and in trafficking and sorting of cellular cholesterol.

The promise of enzymes in therapy of hyperlipidemia

Treatment of hyperlipidemias must be commenced in that intestinal segment where alimentary lipoprotein aggregate generation is initiated postprandially and the aggregates are still in nascent form. This is to prevent the formation of potentially pathological units that do not equilibrate with the blood colloid. The colloid chemical state of the alimentary lipoprotein entities in the systemic circulation is decisive for maintenance of thair stability and for their disposal from the blood. The causes of impaired, “unripe,” lipoprotein aggregate formation include: (a) the action of human pancreatic lipase is even normally a restricted and vulnerable process, (b) inadequate ratio of protein to fat to stabilize the aggregates, (c) defective proteolysis in the small intestine, and (d) the dual behavior of the surface tension-lowering agent, the lecithin antagonist cholesterol. On the one hand, cholesterol represents physicochemically the weakest link in the lipoprotein interfaces. On the other hand, an excess of cholesterol in lipoprotein interfaces decreases the stability of the surface film. This leads to changes in the decay of the entities and the cell surface-active lipid constituents of the “unripe” plasma lipoprotein colloid complexes can easily be adsorbed to the endothelial cell surface plasma membrane: the excess aggregate-cholesterol molecules simply join, or dissolve in, the cell membrane-cholesterol which thus acts as its own receptor or solvent: cholesterol dimers occur. This causes rigidity of the endothelial cell surface membrane and leads to impaired cell metabolism. The intima recognizes the impairment as a foreign body and initiates a physiological defense reaction including phagocytosis. The event may be still more dangerous if the adrenergic receptors of the autonomous nervous system involved in lipolysis with subsequent plasma efflux are simultaneously stimulated. The therapeutic measure indicated is production of balanced alimentary lipoprotein aggregates. This can be achieved by oral administration of (a) protease, and (b) non-specific micellar lipase. The lipase functions without restriction and can perform all the activities that human pancreatic lipase cannot. The enzymes utilized are of mold origin and generally available.

Models for lipid organization in cholesterol-phospholipid bilayers including cholesterol dimer formation.

Three new structural models, which account for abrupt changes in physical properties observed at several molar concentrations of cholesterol in phospholipid bilayers, are described. Cholesterol monomers, each surrounded by its own envelope of unshared acyl hydrocarbon chains of the phospholipid, can accommodate 22% cholesterol. Cholesterol dimers, each surrounded by its envelope of unshared acyl hydrocarbon chains, can accommodate 31% cholesterol. When surrounded by shared acyl hydrocarbon chains, cholesterol dimmers can accommodate about 47% cholesterol. At greater concentrations, cholesterol aggregation occurs, the system is unstable, and cholesterol forms a separate phase.