The effect of sterol structure on the permeability of lipomes to glucose, glycerol and Rb +

PI(4,5)P2 and Cholesterol: Synthesis, Regulation, and Functions

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is the most abundant membrane phosphoinositide and cholesterol is an essential component of the plasma membrane (PM). Both lipids play key roles in a variety of cellular functions including as signaling molecules and major regulators of protein function. This chapter provides an overview of these two important lipids. Starting from a brief description of their structure, synthesis, and regulation, the chapter continues to describe the primary functions and signaling processes in which PI(4,5)P₂ and cholesterol are involved. While PI(4,5)P₂ and cholesterol can act independently, they often act in concert or affect each other's impact. The chapters in this volume on "Cholesterol and PI(4,5)P₂ in Vital Biological Functions: From Coexistence to Crosstalk" focus on the emerging relationship between cholesterol and PI(4,5)P₂ in a variety of biological systems and processes. In this chapter, the next section provides examples from the ion channel field demonstrating that PI(4,5)P₂ and cholesterol can act via common mechanisms. The chapter ends with a discussion of future directions.

Inward Rectifier Potassium Channels: Membrane Lipid-Dependent Mechanosensitive Gates in Brain Vascular Cells

Cerebral arteries contain two primary and interacting cell types, smooth muscle (SMCs) and endothelial cells (ECs), which are each capable of sensing particular hemodynamic forces to set basal tone and brain perfusion. These biomechanical stimuli help confer tone within arterial networks upon which local neurovascular stimuli function. Tone development is intimately tied to arterial membrane potential (VM) and changes in intracellular [Ca2+] driven by voltage-gated Ca2+ channels (VGCCs). Arterial VM is in turn set by the dynamic interplay among ion channel species, the strongly inward rectifying K+ (Kir) channel being of special interest. Kir2 channels possess a unique biophysical signature in that they strongly rectify, display negative slope conductance, respond to elevated extracellular K+ and are blocked by micromolar Ba2+. While functional Kir2 channels are expressed in both smooth muscle and endothelium, they lack classic regulatory control, thus are often viewed as a simple background conductance. Recent literature has provided new insight, with two membrane lipids, phosphatidylinositol 4,5-bisphosphate (PIP2) and cholesterol, noted to (1) stabilize Kir2 channels in a preferred open or closed state, respectively, and (2) confer, in association with the cytoskeleton, caveolin-1 (Cav1) and syntrophin, hemodynamic sensitivity. It is these aspects of vascular Kir2 channels that will be the primary focus of this review.

Development of a novel spatiotemporal depletion system for cellular cholesterol

Cholesterol is an essential component of mammalian cell membranes whose subcellular concentration and function are tightly regulated by de novo biosynthesis, transport, and storage. Although recent reports have suggested diverse functions of cellular cholesterol in different subcellular membranes, systematic investigation of its site-specific roles has been hampered by the lack of a methodology for spatiotemporal manipulation of cellular cholesterol levels. Here, we report the development of a new cholesterol depletion system that allows for spatiotemporal manipulation of intracellular cholesterol levels. This system utilizes a genetically encoded cholesterol oxidase whose intrinsic membrane binding activity is engineered in such a way that its membrane targeting can be controlled in a spatiotemporally specific manner via chemically induced dimerization. In combination with in situ quantitative imaging of cholesterol and signaling activity measurements, this system allows for unambiguous determination of site-specific functions of cholesterol in different membranes, including the plasma membrane and the lysosomal membrane.

An In Situ Fluorescence Assay for Cholesterol Transporter Activity of the Patched

We recently developed a simultaneous in situ quantitative imaging technique for cholesterol in both leaflets of the plasma membrane of mammalian cells. This ratiometric fluorescence technique allows real-time monitoring of the cholesterol transporter activity of plasma membrane-resident proteins in living cells. When applied to the hedgehog signaling system, it enables direct quantitative measurement of the cholesterol transporter activity of Patched1 and the effect of the hedgehog ligand on this activity.

Drug delivery, biodistribution and anti-EGFR activity: theragnostic nanoparticles for simultaneous in vivo delivery of tyrosine kinase inhibitors and kinase activity biosensors

In vivo delivery of small molecule therapeutics to cancer cells, assessment of the selectivity of administration, and measuring the efficacity of the drug in question at the molecule level, are important ongoing challenges in developing new classes of cancer chemotherapeutics. One approach that has the potential to provide targeted delivery, tracking of biodistribution and readout of efficacy, is to use multimodal theragnostic nanoparticles to deliver the small molecule therapeutic. In this paper, we report the development of targeted theragnostic lipid/peptide/DNA lipopolyplexes. These simultaneously deliver an inhibitor of the EGFR tyrosine kinase, and plasmid DNA coding for a Crk-based biosensor, Picchu-X, which when expressed in the target cells can be used to quantify the inhibition of EGFR in vivo in a mouse colorectal cancer xenograft model. Reversible bioconjugation of a known analogue of the tyrosine kinase inhibitor Mo-IPQA to a cationic peptide, and co-formulation with peptides containing both EGFR-binding and cationic sequences, allowed for good levels of inhibitor encapsulation with targeted delivery to LIM1215 colon cancer cells. Furthermore, high levels of expression of the Picchu-X biosensor in the LIM1215 cells in vivo allowed us to demonstrate, using fluorescence lifetime microscopy (FLIM)-based biosensing, that EGFR activity can be successfully suppressed by the tyrosine kinase inhibitor, released from the lipopolyplexes. Finally, we measured the biodistribution of lipopolyplexes containing 125I-labelled inhibitors and were able to demonstrate that the lipopolyplexes gave significantly higher drug delivery to the tumors compared with free drug.

Evaluation of the available cholesterol concentration in the inner leaflet of the plasma membrane of mammalian cells

Cholesterol is an essential component of the mammalian plasma membrane involved in diverse cellular processes. Our recent quantitative imaging analysis using ratiometric cholesterol sensors showed that the available cholesterol concentration in the inner leaflet of the plasma membrane (IPM) is low in unstimulated cells and increased in a stimulus-specific manner to trigger cell signaling events. However, the transbilayer distribution of cholesterol in the plasma membrane of mammalian cells remains controversial. Here we report a systematic and rigorous evaluation of basal IPM cholesterol levels in a wide range of mammalian cells with different properties employing cholesterol sensors derived from the D4 domain of the Perfringolysin O toxin and a sterol-transfer protein, Osh4. Results consistently showed that, although basal IPM cholesterol levels vary significantly among cells, they remain significantly lower than cholesterol levels in the outer leaflets. We found that IPM cholesterol levels were particularly low in all tested primary cells. These results support the universality of the low basal IPM cholesterol concentration under physiological conditions. We also report here the presence of sequestered IPM cholesterol pools, which may become available to cytosolic proteins under certain physiological conditions. We hypothesize that these pools may partly account for the low basal level of available IPM cholesterol. In conclusion, we provide new experimental data that confirm the asymmetric transbilayer distribution of the plasma membrane cholesterol, which may contribute to regulation of various cellular signaling processes at the plasma membrane.

Tail-Oxidized Cholesterol Enhances Membrane Permeability for Small Solutes

Cholesterol renders mammalian cell membranes more compact by reducing the amount of voids in the membrane structure. Because of this, cholesterol is known to regulate the ability of cell membranes to prevent the permeation of water and water-soluble molecules through the membranes. Meanwhile, it is also known that even seemingly tiny modifications in the chemical structure of cholesterol can lead to notable changes in membrane properties. The question is, how significantly do these small changes in cholesterol structure affect the permeability barrier function of cell membranes? In this work, we applied fluorescence methods as well as atomistic molecular dynamics simulations to characterize changes in lipid membrane permeability induced by cholesterol oxidation. The studied 7β-hydroxycholesterol (7β-OH-chol) and 27-hydroxycholesterol (27-OH-chol) represent two distinct groups of oxysterols, namely, ring- and tail-oxidized cholesterols, respectively. Our previous research showed that the oxidation of the cholesterol tail has only a marginal effect on the structure of a lipid bilayer; however, oxidation was found to disturb membrane dynamics by introducing a mechanism that allows sterol molecules to move rapidly back and forth across the membrane-bobbing. Herein, we show that bobbing of 27-OH-chol accelerates fluorescence quenching of NBD-lipid probes in the inner leaflet of liposomes by dithionite added to the liposomal suspension. Systematic experiments using fluorescence quenching spectroscopy and microscopy led to the conclusion that the presence of 27-OH-chol increases membrane permeability to the dithionite anion. Atomistic molecular dynamics simulations demonstrated that 27-OH-chol also facilitates water transport across the membrane. The results support the view that oxysterol bobbing gives rise to successive perturbations to the hydrophobic core of the membrane, and these perturbations promote the permeation of water and small water-soluble molecules through a lipid bilayer. The observed impairment of permeability can have important consequences for eukaryotic organisms. The effects described for 27-OH-chol were not observed for 7β-OH-chol which represents ring-oxidized sterols.

Controlling the size and shape of liposomal ciprofloxacin nanocrystals by varying the lipid bilayer composition and drug to lipid ratio

Drug nanocrystals precipitated inside liposomes are of increasing interest in liposomal drug delivery. For liposomal nanocrystal formulations, the size and shape of the drug nanocrystals can influence the apparent drug release properties, providing opportunities for developing tailored liposomal drug release systems. Small angle X-ray scattering (SAXS) and quantitative transmission electron microscopy (TEM) can be used to analyse the size distributions of the nanoparticles. In this study, by changing the fluidity of the membrane through the use of different membrane phospholipids with varying cholesterol content, the impact of lipid phase, fluidity and permeability on the size distribution of ciprofloxacin nanocrystals were investigated using standard TEM and SAXS as orthogonal techniques. The results show that the phospholipid phase behaviour has a direct effect on the nanocrystal size distribution, where shorter and thinner nanocrystals were formed in liposomes made from hydrogenated soy phosphatidylcholine (HSPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) phospholipids with higher phase transition temperatures than 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) with lower transition temperatures. This is mainly due to the phase behaviour of the liposome during nanocrystal formation. The addition of cholesterol that reduces fluidity and permeability of the DOPC liposomes was also shown to restrict the growth of the ciprofloxacin nanocrystals. Moreover, increasing the drug loading of the liposomes made from HSPC and DPPC produced longer and wider nanocrystals. The findings open new opportunities to tailor nanocrystal size distributions, as well as the aspect ratio of the enclosing liposomes with potential to alter drug release and in vivo behaviour.

Chirality Effect on Cholesterol Modulation of Protein Function

Cholesterol is a key steroidal, lipid biomolecule found abundantly in plasma membranes of eukaryotic cells. It is an important structural component of cellular membranes and regulates membrane fluidity and permeability. Cholesterol is also essential for normal functioning of key proteins including ion-channels, G protein-coupled receptors (GPCRs), membrane bound steroid receptors, and receptor kinases. It is thought that cholesterol exerts its actions via specific binding to chiral proteins and lipids as well as through non-specific physiochemical interactions. Distinguishing between the specific and the non-specific interactions can be difficult. Although much remains unclear, progress has been made in recent years by utilizing ent-cholesterol, the enantiomer of natural cholesterol (nat-cholesterol) as a probe. Ent-Cholesterol is the non-superimposable mirror image of nat-cholesterol and exhibits identical physiochemical properties as nat-cholesterol. Hence, if the biological effects of cholesterol result solely due to membrane effects, it is expected that there will be no difference between ent-cholesterol and nat-cholesterol. However, when direct binding with chiral proteins and lipids is involved, the enantiomer is expected to potentially elicit significantly different, measurable effects due to formation of diastereomeric complexes. In this chapter, we have reviewed the literature related to ent-cholesterol and its use as a probe for various biophysical and biological interactions of cholesterol.

Rational design of novel fluorescent analogues of cholesterol: a “step-by-step” computational study

State-of-the-art quantum chemical and molecular dynamics simulations are used as guidelines in design of novel fluorescent analogues of cholesterol.

Niosomes, an alternative for liposomal delivery

Niosomes are used in studies for drug delivery or gene transfer. However, their physical properties and features relative to liposomes are not well documented. To characterize and more rationally optimize niosome formulations, the properties of these vesicle systems are compared to those of liposomes composed of phosphatidylcholine and phosphatidylethanolamine lipids plus cholesterol. Niosomes are highly stable and only slightly more leaky than liposomes as assayed by calcein leakage; the permeability for ions (KCl) is higher than that of liposomes. Contrary to liposomes, the size of niosomes decreases substantially upon freezing in liquid nitrogen and subsequent thawing, as shown by cryo-EM and dynamic light scattering. The packing of niosomal membranes was determined by laurdan fluorescence and is slightly lower than that of liposomes. We did not succeed in the functional reconstitution of the L-arginine/L-ornithine antiporter ArcD2 in niosomes, which we attribute to the non-ionic nature of the surfactants. The antimicrobial peptides alamethicin and melittin act similarly on niosomes and liposomes composed of unsaturated components, whereas both niosomes and liposomes are unaffected when saturated amphiphiles are used. In conclusion, in terms of stability and permeability for drug-size molecules niosomes are comparable to liposomes and they may offer an excellent, inexpensive alternative for delivery purposes.

The Potential of α-Spinasterol to Mimic the Membrane Properties of Natural Cholesterol

Sterols play a unique role for the structural and dynamical organization of membranes. The current study reports data on the membrane properties of the phytosterol (3β,5α,22E)-stigmasta-7,22-dien-3-β-ol (α-spinasterol), which represents an important component of argan oil and have not been investigated so far in molecular detail. In particular, the impact of α-spinasterol on the structure and organization of lipid membranes was investigated and compared with those of cholesterol. Various membrane parameters such as the molecular packing of the phospholipid fatty acyl chains, the membrane permeability toward polar molecules, and the formation of lateral membrane domains were studied. The experiments were performed on lipid vesicles using methods of NMR spectroscopy and fluorescence spectroscopy and microscopy. The results show that α-spinasterol resembles the membrane behavior of cholesterol to some degree.

Differential Effects of Sterols on Ion Channels: Stereospecific Binding vs Stereospecific Response

Numerous ion channels have been shown to be regulated by the level of membrane cholesterol, but the mechanisms responsible for these effects are still not well understood. The key question in the field is how to discriminate between the contributions of the two central mechanisms that might be responsible for the sensitivity of ion channels to cholesterol: specific sterol-protein interactions or regulation of channels by the bilayer physical properties. Comparative analysis of cholesterol and its isomers on the function of an ion channel is a powerful tool to achieve this goal. An increasing number of studies show that cholesterol regulates several types of ion channels in a stereospecific manner, suggesting an involvement of specific sterol-protein interactions. However in this chapter, we present evidence that the stereospecificity of cholesterol-ion channel interactions may be mediated, not by a lack of binding, as has been generally assumed, but by the specificity of the interaction, which results in a functional effect, in the case of native cholesterol, and a lack of functional effect, in the case of a cholesterol isomer. In other words, accumulating evidence suggests that the structural requirements of ion channel cholesterol-binding sites are lax, allowing chiral isomers of cholesterol to bind to the same site in a nonstereospecific way, but the ability of a sterol to confer a functional effect on the channel activity can still be stereospecific. This is an important distinction both conceptually and methodologically. Indeed, our analysis shows that the orientations of cholesterol and its chiral isomer ent-cholesterol within a hydrophobic binding pocket of Kir2.2 are significantly different, and we propose that this difference may underlie distinct functional outcomes.

Molecular dynamics simulations of ternary lipid bilayers containing plant sterol and glucosylceramide

An atomic-level molecular dynamics simulation was carried out to study the effects of a plant sterol (sitosterol) and glucosylceramide (GlcCer) on a 1,2-dilinoleoylposphocholine (DLiPC) membrane. Initially, a membrane containing 50mol% sitosterol was compared with that containing the same ratio of cholesterol. These simulations showed differential condensing and ordering effects of sitosterol and cholesterol, with cholesterol being slightly more efficient than sitosterol in packing the membrane more tightly to a liquid ordered phase. By incorporation of 9.3% GlcCer on DLiPC/sterol membrane no notable change was observed in terms of area per lipid, bilayer thickness, order parameter and lateral diffusion. Some clusters of GlcCer/sterol were observed at higher ratio of GlcCer (15.5%), supporting the existence of GlcCer/sitosterol-enriched L_o-domains on the nanometer scale in the plant lipid mixture.

Thermodynamics of cell-penetrating HIV1 TAT peptide insertion into PC/PS/CHOL model bilayers through transmembrane pores: the roles of cholesterol and anionic lipids

Efficient delivery of pharmaceutically active molecules across cellular membranes using cell penetrating peptides (CPPs), such as the cationic human immunodeficiency virus-1 trans-acting activator of transcription peptide (HIV-1 TAT), continues to attract scientific attention in drug design and disease treatment. Experimental results show that the TAT peptide is not only capable of directly penetrating the biological membrane in a passive manner, but also forming physical, membrane-spanning pores that may facilitate transport. Experiments further show that anionic lipids accelerate peptide permeation within a range of mole percentage composition. In this work, we explored the structures and translocation thermodynamics of the cationic TAT peptide across a series of DPPC/DPPS model membranes with the presence of 0-30 mol% cholesterol. We computed the potentials of the mean force by using umbrella sampling molecular dynamics simulations coupled to the Martini coarse-grained force field. We systematically investigated the roles of cholesterol and anionic lipids (membrane surface charge) in TAT peptide translocation. In qualitative agreement with experimental findings, the barrier heights were significantly reduced in the presence of anionic lipids. A toroidal hydrophilic pore was strongly suggested by membrane structure analysis. Cholesterol stabilizes the liquid-ordered (Lo) phase of membranes and increases the elastic stiffness of bilayers. Consequently, it hinders transmembrane pore formation and thus modulates solute permeability, since the liquid-ordered phase suppresses reorientation of the lipid molecules on simulation time scales. Though cholesterol contributes marginally to the total free energy associated with peptide permeation, the coordination of cholesterol to the peptide weakens more favorable peptide-lipid interactions. The addition of the anionic lipid DPPS to the neutral DPPC bilayer leads to the emergence and further enhancement of an interfacially stable state of the peptide due to the favorable peptide-anionic lipid interactions. Translocation free energy barriers decrease in lockstep with increasing DPPS composition in the model bilayers simulated. Finally, we investigated the size of hydrophilic pores emerging in our simulations, as well as the qualitative mobility of the peptide on the membrane surface.

Heterogeneous Inhibition in Macroscopic Current Responses of Four Nicotinic Acetylcholine Receptor Subtypes by Cholesterol Enrichment

The nicotinic acetylcholine receptor (nAChR), located in the cell membranes of neurons and muscle cells, mediates the transmission of nerve impulses across cholinergic synapses. In addition, the nAChR is also found in the electric organs of electric rays (e.g., the genus Torpedo). Cholesterol, which is a key lipid for maintaining the correct functionality of membrane proteins, has been found to alter the nAChR function. We were thus interested to probe the changes in the functionality of different nAChRs expressed in a model membrane with modified cholesterol to phospholipid ratios (C/P). In this study, we examined the effect of increasing the C/P ratio in Xenopus laevis oocytes expressing the neuronal α7, α4β2, muscle-type, and Torpedo californica nAChRs in their macroscopic current responses. Using the two-electrode voltage clamp technique, it was found that the neuronal α7 and Torpedo nAChRs are significantly more sensitive to small increases in C/P than the muscle-type nAChR. The peak current versus C/P profiles during enrichment display different behaviors; α7 and Torpedo nAChRs display a hyperbolic decay with two clear components, whereas muscle-type and α4β2 nAChRs display simple monophasic decays with different slopes. This study clearly illustrates that a physiologically relevant increase in membrane cholesterol concentration produces a remarkable reduction in the macroscopic current responses of the neuronal α7 and Torpedo nAChRs functionality, whereas the muscle nAChR appears to be the most resistant to cholesterol inhibition among all four nAChR subtypes. Overall, the present study demonstrates differential profiles for cholesterol inhibition among the different types of nAChR to physiological cholesterol increments in the plasmatic membrane. This is the first study to report a cross-correlation analysis of cholesterol sensitivity among different nAChR subtypes in a model membrane.

Functionalized lipids and surfactants for specific applications

Synthetic lipids and surfactants that do not exist in biological systems have been used for the last few decades in both basic and applied science. The most notable applications for synthetic lipids and surfactants are drug delivery, gene transfection, as reporting molecules, and as support for structural lipid biology. In this review, we describe the potential of the synergistic combination of computational and experimental methodologies to study the behavior of synthetic lipids and surfactants embedded in lipid membranes and liposomes. We focused on select cases in which molecular dynamics simulations were used to complement experimental studies aiming to understand the structure and properties of new compounds at the atomistic level. We also describe cases in which molecular dynamics simulations were used to design new synthetic lipids and surfactants, as well as emerging fields for the application of these compounds. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Effects of Stretching Speed on Mechanical Rupture of Phospholipid/Cholesterol Bilayers: Molecular Dynamics Simulation

Rupture of biological cell membrane under mechanical stresses is critical for cell viability. It is triggered by local rearrangements of membrane molecules. We investigated the effects of stretching speed on mechanical rupture of phospholipid/cholesterol bilayers using unsteady molecular dynamics simulations. We focused on pore formation, the trigger of rupture, in a 40 mol% cholesterol-including bilayer. The unsteady stretching was modeled by proportional and temporal scaling of atom positions at stretching speeds from 0.025 to 30 m/s. The effects of the stretching speed on the critical areal strain, where the pore forms, is composed of two regimes. At low speeds (<1.0 m/s), the critical areal strain is insensitive to speed, whereas it significantly increases at higher speeds. Also, the strain is larger than that of a pure bilayer, regardless of the stretching speeds, which qualitatively agrees with available experimental data. Transient recovery of the cholesterol and phospholipid molecular orientations was evident at lower speeds, suggesting the formation of a stretch-induced interdigitated gel-like phase. However, this recovery was not confirmed at higher speeds or for the pure bilayer. The different responses of the molecular orientations may help explain the two regimes for the effect of stretching speed on pore formation.

Building Synthetic Sterols Computationally - Unlocking the Secrets of Evolution?

Cholesterol is vital in regulating the physical properties of animal cell membranes. While it remains unclear what renders cholesterol so unique, it is known that other sterols are less capable in modulating membrane properties, and there are membrane proteins whose function is dependent on cholesterol. Practical applications of cholesterol include its use in liposomes in drug delivery and cosmetics, cholesterol-based detergents in membrane protein crystallography, its fluorescent analogs in studies of cholesterol transport in cells and tissues, etc. Clearly, in spite of their difficult synthesis, producing the synthetic analogs of cholesterol is of great commercial and scientific interest. In this article, we discuss how synthetic sterols non-existent in nature can be used to elucidate the roles of cholesterol’s structural elements. To this end, we discuss recent atomistic molecular dynamics simulation studies that have predicted new synthetic sterols with properties comparable to those of cholesterol. We also discuss more recent experimental studies that have vindicated these predictions. The paper highlights the strength of computational simulations in making predictions for synthetic biology, thereby guiding experiments.

Differential effect of plant lipids on membrane organization: specificities of phytosphingolipids and phytosterols

The high diversity of the plant lipid mixture raises the question of their respective involvement in the definition of membrane organization. This is particularly the case for plant plasma membrane, which is enriched in specific lipids, such as free and conjugated forms of phytosterols and typical phytosphingolipids, such as glycosylinositolphosphoceramides. This question was here addressed extensively by characterizing the order level of membrane from vesicles prepared using various plant lipid mixtures and labeled with an environment-sensitive probe. Fluorescence spectroscopy experiments showed that among major phytosterols, campesterol exhibits a stronger ability than β-sitosterol and stigmasterol to order model membranes. Multispectral confocal microscopy, allowing spatial analysis of membrane organization, demonstrated accordingly the strong ability of campesterol to promote ordered domain formation and to organize their spatial distribution at the membrane surface. Conjugated sterol forms, alone and in synergy with free sterols, exhibit a striking ability to order membrane. Plant sphingolipids, particularly glycosylinositolphosphoceramides, enhanced the sterol-induced ordering effect, emphasizing the formation and increasing the size of sterol-dependent ordered domains. Altogether, our results support a differential involvement of free and conjugated phytosterols in the formation of ordered domains and suggest that the diversity of plant lipids, allowing various local combinations of lipid species, could be a major contributor to membrane organization in particular through the formation of sphingolipid-sterol interacting domains.

Membrane properties of cholesterol analogs with an unbranched aliphatic side chain

The interactions between cholesterol and other membrane molecules determine important membrane properties. It was shown that even small changes in the molecular structure of cholesterol have a crucial influence on these interactions. We recently reported that in addition to alterations in the tetracyclic ring structure, the iso-branched side chain of cholesterol also has a significant impact on membrane properties (Scheidt et al., 2013). Here we used synthetic cholesterol analogs to investigate the influence of an unbranched aliphatic side chain of different length. The (2)H NMR order parameter of the phospholipid chains and therefore the molecular packing of the phospholipid molecules shows a significant dependence on the sterol's alkyl side chain length, while, membrane permeation studied by a dithionite ion permeation assay and lateral diffusion measured by (1)H MAS pulsed field gradient NMR are less influenced. To achieve the same molecular packing effect similar to that of an iso-branched aliphatic side chain, a longer unbranched side chain (n-dodecyl instead of n-octyl) at C17 of cholesterol is required. Obviously, sterols having a branched iso-alkyl chain with two terminal methyl groups exhibit altered cholesterol-phospholipid interactions compared to analogous molecules with a straight unbranched chain.

Local partition coefficients govern solute permeability of cholesterol-containing membranes

The permeability of lipid membranes for metabolic molecules or drugs is routinely estimated from the solute's oil/water partition coefficient. However, the molecular determinants that modulate the permeability in different lipid compositions have remained unclear. Here, we combine scanning electrochemical microscopy and molecular-dynamics simulations to study the effect of cholesterol on membrane permeability, because cholesterol is abundant in all animal membranes. The permeability of membranes from natural lipid mixtures to both hydrophilic and hydrophobic solutes monotonously decreases with cholesterol concentration [Chol]. The same is true for hydrophilic solutes and planar bilayers composed of dioleoyl-phosphatidylcholine or dioleoyl-phosphatidyl-ethanolamine. However, these synthetic lipids give rise to a bell-shaped dependence of membrane permeability on [Chol] for very hydrophobic solutes. The simulations indicate that cholesterol does not affect the diffusion constant inside the membrane. Instead, local partition coefficients at the lipid headgroups and at the lipid tails are modulated oppositely by cholesterol, explaining the experimental findings. Structurally, these modulations are induced by looser packing at the lipid headgroups and tighter packing at the tails upon the addition of cholesterol.

Designer lipids for drug delivery: from heads to tails

For four decades, liposomes composed of both naturally occurring and synthetic lipids have been investigated as delivery vehicles for low molecular weight and macromolecular drugs. These studies paved the way for the clinical and commercial success of a number of liposomal drugs, each of which required a tailored formulation; one liposome size does not fit all drugs! Instead, the physicochemical properties of the liposome must be matched to the pharmacology of the drug. An extensive biophysical literature demonstrates that varying lipid composition can influence the size, membrane stability, in vivo interactions, and drug release properties of a liposome. In this review we focus on recently described synthetic lipid headgroups, linkers and hydrophobic domains that can provide control over the intermolecular forces, phase preference, and macroscopic behavior of liposomes. These synthetic lipids further our understanding of lipid biophysics, promote targeted drug delivery and improve liposome stability. We further highlight the immune reactivity of novel synthetic headgroups as a key design consideration. For instance it was originally thought that synthetic PEGylated lipids were immunologically inert; however, it's been observed that under certain conditions PEGylated lipids induce humoral immunity. Such immune activation may be a limitation to the use of other engineered lipid headgroups for drug delivery. In addition to the potential immunogenicity of engineered lipids, future investigations on liposome drugs in vivo should pay particular attention to the location and dynamics of payload release.

tert-Butyl hydroperoxide-induced differing plasma membrane and oxidative stress processes in yeast strains BY4741 and erg5Δ

The molecular mechanism of tert-butyl hydroperoxide (t-BuOOH) elicited cytotoxicity and the background of t-BuOOH sensitivity were studied in the Saccharomyces cerevisiae ergosterol-less gene deletion mutant erg5Δ and its parental strain BY4741. In comparison to BY4741, untreated erg5Δ cells exhibited alterations in sterol and fatty acid compositions of the plasma membrane, as reflected by the inherent amphotericin B resistance, an elevated level (31%) of plasma membrane rigidity and a decreased uptake of glycerol. Surprisingly, the untreated erg5Δ cells exhibited an unbalanced intracellular redox state, accompanied by the continuous upregulation of the antioxidant enzymes Mn superoxide dismutase, catalase, and glutathione S-transferase, which resulted in decreased specific concentrations of superoxide and peroxides and elevated levels of the hydroxyl radical and thiols. The 2.5-fold sensitivity of erg5Δ to t-BuOOH suggested that the oxidative stress adaptation processes of the mutant could not restore the redox homeostasis of the cells and there is an overlap between sterol and redox homeostases. t-BuOOH treatment of both strains induced adaptive modification of the sterol and fatty acid compositions, increased the plasma membrane fluidity and elevated the specific activities of most antioxidant enzymes through specific regulation processes in a strain-dependent manner.

Cholesterol binding to ion channels

Numerous studies demonstrated that membrane cholesterol is a major regulator of ion channel function. The goal of this review is to discuss significant advances that have been recently achieved in elucidating the mechanisms responsible for cholesterol regulation of ion channels. The first major insight that comes from growing number of studies that based on the sterol specificity of cholesterol effects, show that several types of ion channels (nAChR, Kir, BK, TRPV) are regulated by specific sterol-protein interactions. This conclusion is supported by demonstrating direct saturable binding of cholesterol to a bacterial Kir channel. The second major advance in the field is the identification of putative cholesterol binding sites in several types of ion channels. These include sites at locations associated with the well-known cholesterol binding motif CRAC and its reversed form CARC in nAChR, BK, and TRPV, as well as novel cholesterol binding regions in Kir channels. Notably, in the majority of these channels, cholesterol is suggested to interact mainly with hydrophobic residues in non-annular regions of the channels being embedded in between transmembrane protein helices. We also discuss how identification of putative cholesterol binding sites is an essential step to understand the mechanistic basis of cholesterol-induced channel regulation. Clearly, however, these are only the first few steps in obtaining a general understanding of cholesterol-ion channels interactions and their roles in cellular and organ functions.

Effects of amphiphile topology on the aggregation of oligocholates in lipid membranes: macrocyclic versus linear amphiphiles

A macrocyclic and a linear trimer of a facially amphiphilic cholate building block were labeled with a fluorescent dansyl group. The environmentally sensitive fluorophore enabled the aggregation of the two oligocholates in lipid membranes to be studied by fluorescence spectroscopy. Concentration-dependent emission wavelength and intensity revealed a higher concentration of water for the cyclic compound. Both compounds were shown by the red-edge excitation shift (REES) to be located near the membrane/water interface at low concentrations, but the cyclic trimer was better able to migrate into the hydrophobic core of the membrane than the linear trimer. Fluorescent quenching by a water-soluble (NaI) and a lipid-soluble (TEMPO) quencher indicated that the cyclic trimer penetrated into the hydrophobic region of the membrane more readily than the linear trimer, which preferred to stay close to the membrane surface. The fluorescent data corroborated with the previous leakage assays that suggested the stacking of the macrocyclic cholate trimer into transmembrane nanopores, driven by the strong associative interactions of water molecules inside the macrocycles in a nonpolar environment.

Protection/Deprotection of surface activity and its applications in the controlled release of liposomal contents

The micelles of two tripropargylammonium-functionalized cationic surfactants were cross-linked by a disulfide-containing diazido cross-linker in the presence of Cu(I) catalysts. With multiple residual alkyne groups on the surface, the resulting surface cross-linked micelles (SCMs) were postfunctionalized by reaction with 2-azidoethanol and an azido-terminated poly(ethylene glycol), respectively, via the alkyne-azide click reaction. The water-soluble nanoparticles obtained had low surface activity due to the buried hydrophobic tails. Cleavage of the disulfide cross-links by dithiothreitol (DTT) exposed the hydrophobic tails and resumed surface activity of the "caged" surfactants within 2 min after DTT addition. The controlled breakage of the SCMs was used to lower the surface tension of aqueous solutions and trigger the release of liposomal contents on demand.

Interaction of 3β-amino-5-cholestene with phospholipids in binary and ternary bilayer membranes

3β-Amino-5-cholestene (aminocholesterol) is a synthetic sterol whose properties in bilayer membranes have been examined. In fluid palmitoyl sphingomyelin (PSM) bilayers, aminocholesterol and cholesterol were equally effective in increasing acyl chain order, based on changes in diphenylhexatriene (DPH) anisotropy. In fluid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, aminocholesterol ordered acyl chains, but slightly less efficiently than cholesterol. Aminocholesterol eliminated the PSM and DPPC gel-to-liquid crystalline phase transition enthalpy linearly with concentration, and the enthalpy approached zero at 30 mol % sterol. Whereas cholesterol was able to increase the thermostability of ordered PSM domains in a fluid bilayer, aminocholesterol under equal conditions failed to do this, suggesting that its interaction with PSM was not as favorable as cholesterols. In ternary mixed bilayers, containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), PSM or DPPC, and cholesterol at proportions to contain a liquid-ordered phase (60:40 by mol of POPC and PSM or DPPC, and 30 mol % cholesterol), the average lifetime of trans-parinaric acid (tPA) was close to 20 ns. When cholesterol was replaced with aminocholesterol in such mixed bilayers, the average lifetime of tPA was only marginally shorter (about 18 ns). This observation, together with acyl chain ordering data, clearly shows that aminocholesterol was able to form a liquid-ordered phase with saturated PSM or DPPC. We conclude that aminocholesterol should be a good sterol replacement in model membrane systems for which a partial positive charge is deemed beneficial.

Amphotericin B/sterol co-loaded PEG-phospholipid micelles: effects of sterols on aggregation state and hemolytic activity of amphotericin B

PURPOSE

To elucidate the effect of sterols on the aggregation of amphotericin B (AmB) in PEG-phospholipid micelles and its consequences on the hemolytic activity of AmB.

METHODS

AmB-incorporated PEG-phospholipid micelles co-loaded with ergosterol, cholesterol, or 7-dehydrocholesterol were prepared at 4:1:1 and 20:5:1 ratios of polymer-to-sterol-to-AmB. The aggregation state of AmB was elucidated by UV-vis spectroscopy. AmB/sterol co-loaded PEG-phospholipid micelles were incubated with red blood cells and the hemolytic activity of AmB assessed by measurement of free hemoglobin.

RESULTS

AmB in PEG-phospholipid micelles stayed mostly in a deaggregated state in the absence of sterol or with cholesterol, but aggregated in the presence of ergosterol or 7-dehydrocholesterol. The fraction of aggregated AmB in PEG-phospholipid micelles was lower at the 20:5:1 ratio. In an aggregated state or in the absence of sterol, AmB caused rapid and complete hemolysis. In contrast, deaggregated AmB co-loaded with cholesterol caused slower and incomplete hemolysis, especially at a 20:5:1 ratio.

CONCLUSIONS

The aggregation state of AmB in PEG-phospholipid micelles was sterol dependant. AmB/cholesterol co-loaded PEG-phospholipid micelles caused low in vitro hemolysis due to deaggregation of AmB and micellar stability, presumably owing to cholesterol/phospholipid versus cholesterol/AmB interactions in the interior core region.

Chronopotentiometric studies of phosphatidylcholine bilayers modified by ergosterol

We have monitored the effect of ergosterol on electrical capacitance and electrical resistance of the phosphatidylcholine bilayer membranes using chronopotentiometry method. The chronopotentiometric characteristic of the bilayers depends on constant-current flow through the membranes. For low current values, no electroporation takes place and the membrane voltage rises exponentially to a constant value described by the Ohm's law. Based on these kinds of chronopotentiometric curves, a method of the membrane capacitance and the membrane resistance calculations is presented.

Translocation of hydrophilic molecules across lipid bilayers by salt-bridged oligocholates

Macrocyclic oligocholates were found in a previous work (Cho, H.; Widanapathirana, L.; Zhao, Y. J. Am. Chem. Soc.2011, 133, 141-147) to stack on top of one another in lipid membranes to form nanopores. Pore formation was driven by a strong tendency of the water molecules in the interior of the amphiphilic macrocycles to aggregate in a nonpolar environment. In this work, cholate oligomers terminated with guanidinium and carboxylate groups were found to cause efflux of hydrophilic molecules such as glucose, maltotriose, and carboxyfluorescein (CF) from POPC/POPG liposomes. The cholate trimer outperformed other oligomers in the transport. Lipid-mixing assays and dynamic light scattering ruled out fusion as the cause of leakage. The strong dependence on chain length argues against random intermolecular aggregates as the active transporters. The efflux of glucose triggered by these compounds increased significantly when the bilayers contained 30 mol% cholesterol. Hill analysis suggested that the active transporter consisted of four molecules. The oligocholates were proposed to fold into "noncovalent macrocycles" by the guanidinium-carboxylate salt bridge and stack on top of one another to form similar transmembrane pores as their covalent counterparts.

Amphotericin B membrane action: role for two types of ion channels in eliciting cell survival and lethal effects

The formation of aqueous pores by the polyene antibiotic amphotericin B (AmB) is at the basis of its fungicidal and leishmanicidal action. However, other types of nonlethal and dose-dependent biphasic effects that have been associated with the AmB action in different cells, including a variety of survival responses, are difficult to reconcile with the formation of a unique type of ion channel by the antibiotic. In this respect, there is increasing evidence indicating that AmB forms nonaqueous (cation-selective) channels at concentrations below the threshold at which aqueous pores are formed. The main foci of this review will be (1) to provide a summary of the evidence supporting the formation of cation-selective ion channels and aqueous pores by AmB in lipid membrane models and in the membranes of eukaryotic cells; (2) to discuss the influence of membrane parameters such as thickness fluctuations, the type of sterol present and the existence of sterol-rich specialized lipid raft microdomains in the formation process of such channels; and (3) to develop a cell model that serves as a framework for understanding how the intracellular K(+) and Na(+) concentration changes induced by the cation-selective AmB channels enhance multiple survival response pathways before they are overcome by the more sustained ion fluxes, Ca(2+)-dependent apoptotic events and cell lysis effects that are associated with the formation of AmB aqueous pores.