Structure and cohesive properties of sphingomyelin/cholesterol bilayers

Sterol-lipids enable large-scale, liquid-liquid phase separation in bilayer membranes of only 2 components

Despite longstanding excitement and progress toward understanding liquid-liquid phase separation in natural and artificial membranes, fundamental questions have persisted about which molecules are required for this phenomenon. Except in extraordinary circumstances, the smallest number of components that has produced large-scale, liquid-liquid phase separation in bilayers has stubbornly remained at three: a sterol, a phospholipid with ordered chains, and a phospholipid with disordered chains. This requirement of three components is puzzling because only two components are required for liquid-liquid phase separation in lipid monolayers, which resemble half of a bilayer. Inspired by reports that sterols interact closely with lipids with ordered chains, we tested whether phase separation would occur in bilayers in which a sterol and lipid were replaced by a single, joined sterol-lipid. By evaluating a panel of sterol-lipids, some of which are found in bacteria, we discovered a minimal bilayer of only two components (PChemsPC and diPhyPC) that robustly demixes into micron-scale, liquid phases. It suggests a new role for sterol-lipids in nature, and it reveals a membrane in which tie-lines (and, therefore, the lipid composition of each phase) are straightforward to determine and will be consistent across multiple laboratories.

Significance Statement

A wide diversity of bilayer membranes, from those with hundreds of lipids (e.g., vacuoles of living yeast cells) to those with very few (e.g., artificial vesicles) phase separate into micron-scale liquid domains. The number of components required for liquid-liquid phase separation has been perplexing: only two should be necessary, but more are required except in extraordinary circumstances. What minimal set of molecular characteristics leads to liquid-liquid phase separation in bilayer membranes? This question inspired us to search for single, joined "sterol-lipid" molecules to replace both a sterol and a phospholipid in membranes undergoing liquid-liquid phase separation. By producing phase-separating membranes with only two components, we mitigate experimental challenges in determining tie-lines and in maintaining constant chemical potentials of lipids.

Modulating a model membrane of sphingomyelin by a tricyclic antidepressant drug

Tricyclic medicine such as amitriptyline (AMT) hydrochloride, initially developed to treat depression, is also used to treat neuropathic pain, anxiety disorder, and migraines. The mechanism of functioning of this type of drugs is ambiguous. Understanding the mechanism is important for designing new drug molecules with higher pharmacological efficiency. Hence, in the present study, biophysical approaches have been taken to shed light on their interactions with a model cellular membrane of brain sphingomyelin in the form of monolayer and multi-lamellar vesicles. The surface pressure-area isotherm infers the partitioning of a drug molecule into the lipid monolayer at the air water interface, providing a higher surface area per molecule and reducing the in-plane elasticity. Further, the surface electrostatic potential of the lipid monolayer is found to increase due to the insertion of drug molecule. The interfacial rheology revealed a reduction of the in-plane viscoelasticity of the lipid film, which, depends on the adsorption of the drug molecule onto the film. Small-angle X-ray scattering (SAXS) measurements on multilamellar vesicles (MLVs) have revealed that the AMT molecules partition into the hydrophobic core of the lipid membrane, modifying the organization of lipids in the membrane. The modified physical state of less rigid membrane and the transformed electrostatics of the membrane could influence its interaction with synaptic vesicles and neurotransmitters making higher availability of the neurotransmitters in the synaptic cleft.

Research on the loading and release kinetics of the vincristine sulfate liposomes and its anti-breast cancer activity

Vincristine (VCR), as a cytotoxic drug, is used clinically to treat acute lymphatic leukemia and breast cancer, and commonly used clinically as vincristine sulfate (VCRS). However, its clinical use is limited by unpredictable pharmacologic characteristics, a narrow therapeutic index, and neurotoxicity. The pH gradient method was used for active drug loading of VCRS, and the process route mainly includes the preparation of blank liposomes and drug-loaded liposomes. VCRS liposomes had suitable particle size, high encapsulation efficiency and good stability. The loading and release kinetics of VCRS liposomes were explored. By calculating the changes of encapsulation efficiency with time at different temperatures, it was confirmed that the drug-loading process of liposomes exhibited a first-order kinetic feature, and the activation energy required for the reaction was determined as 20.6 kcal/mol. The release behavior at different pH was also investigated, and it was demonstrated that the release behavior conformed to the first-order model, suggesting that the release mechanism of VCRS was simple transmembrane diffusion. VCRS liposomes also enhanced in vitro and in vivo antitumor activity. Thus, VCRS liposomes showed great potential for VCRS delivery, and the loading and release kinetics were well researched to provide a reference for investigating active drug loading liposomes.

Targeting the sphingolipid rheostat in IDH1 mut glioma alters cholesterol homeostasis and triggers apoptosis via membrane degradation

The cross-regulation of metabolism and trafficking is not well understood for the vital sphingolipids and cholesterol constituents of cellular compartments. While reports are starting to surface on how sphingolipids like sphingomyelin (SM) dysregulate cholesterol levels in different cellular compartments (Jiang et al., 2022), limited research is available on the mechanisms driving the relationship between sphingolipids and cholesterol homeostasis, or its biological implications. Previously, we have identified sphingolipid metabolism as a unique vulnerability for IDH1 mut gliomas via a rational drug design. Herein, we show how modulating sphingolipid levels affects cholesterol homeostasis in brain tumors. However, we unexpectedly discovered for the first time that C17 sphingosine and NDMS addition to cancer cells alters cholesterol homeostasis by impacting its cellular synthesis, uptake, and efflux leading to a net decrease in cholesterol levels and inducing apoptosis. Our results reflect a reverse correlation between the levels of sphingosines, NDMS, and unesterified, free cholesterol in the cells. We show that increasing sphingosine and NDMS (a sphingosine analog) levels alter not only the trafficking of cholesterol between membranes but also the efflux and synthesis of cholesterol. We also demonstrate that despite the effort to remove free cholesterol by ABCA1-mediated efflux or by suppressing machinery for the influx (LDLR) and biosynthetic pathway (HMGCR), apoptosis is inevitable for IDH1 mut glioma cells. This is the first study that shows how altering sphingosine levels directly affects cholesterol homeostasis in cancer cells and can be used to manipulate this relationship to induce apoptosis in IDH1 mut gliomas.

MαCD-based plasma membrane outer leaflet lipid exchange in mammalian cells to study insulin receptor activity

Signaling receptors on the plasma membrane, such as insulin receptor, can have their activity modulated to some extent by their surrounding lipids. Studying the contribution of membrane lipid properties such as presence of ordered lipid domains or bilayer thickness on the activity of receptors has been a challenging objective in living cells. Using methyl-alpha cyclodextrin-mediated lipid exchange, we are able to alter the lipids of the outer leaflet plasma membrane of mammalian cells to investigate the effect of the properties of the exchanged lipid upon receptor function in live cells. In this article, we describe the technique of lipid exchange in detail and how it can be applied to better understand lipid-mediated regulation of insulin receptor activity in cells.

The effect of membrane composition on the interaction between human CYP51 and its flavonoid inhibitor - luteolin 7,3'-disulfate

Cytochromes P450 (CYP) are a family of membrane proteins involved in the production of endogenous molecules and the metabolism of xenobiotics. It is well-known that the composition of the membrane can influence the activity and orientation of CYP proteins. However, little is known about how membrane composition affects the ligand binding properties of CYP. In this study, we utilized surface plasmon resonance and fluorescence lifetime analysis to examine the impact of membrane micro-environment composition on the interaction between human microsomal CYP51 (CYP51A1) and its inhibitor, luteolin 7,3'-disulphate (LDS). We observed that membranes containing cholesterol or sphingomyelin exhibited the lowest apparent equilibrium dissociation constant for the CYP51A1-LDS complex. Additionally, the tendency for relation between kinetic parameters of the CYP51A1-LDS complex and membrane viscosity and overall charge was observed. These findings suggest that the specific composition of the membrane, particularly the presence of cholesterol and sphingomyelin, plays a vital role in regulating the interaction between CYP enzymes and their ligands.

Liposomal Doxorubicin: the Sphingomyelin/Cholesterol System Significantly Enhances the Antitumor Efficacy of Doxorubicin

Doxorubicin (DOX) has a cytotoxic effect on many tumor cells; however, its clinical application is limited owing to its strong side effects. Although Doxil® reduces the cardiotoxicity of free DOX, it has also introduced a new dose-limiting toxicity. In a previous study, a sialic acid-cholesterol conjugate (SA-CH) was synthesized and modified onto the surface of DOX-loaded liposomes to target tumor-associated macrophages (TAMs), further improving the efficacy of DOX-loaded liposomes over that of Doxil®. Meanwhile, the good retention characteristics and promising antitumor ability of sphingomyelin/cholesterol (SM/CH) system for water-soluble drugs have attracted wide attention. Therefore, we aimed to use SA-CH as the target and hydrogenated soybean phosphatidylcholine (HSPC) or egg sphingomyelin (ESM) as the membrane material to develop a more stable DOX-loaded liposome with stronger antitumor activity. The liposomes were evaluated for particle size, polydispersity index, zeta potential, entrapment efficiency, in vitro release, long-term storage, cytotoxicity, cellular uptake, pharmacokinetics, tumor targetability, and in vivo antitumor activity. In the liposomes prepared using HSPC/CH, sialic acid (SA) modification considerably increased the accumulation of DOX-loaded liposomes in the tumor, thus exerting a better antitumor effect. However, SA modification in DOX-ESL (SA-CH-modified DOX-loaded liposomes prepared by ESM/CH) destroyed the strong retention effect of the ESM/CH system on DOX, resulting in a reduced antitumor effect. Notably, DOX-ECL (DOX-loaded liposome prepared by ESM/CH) had the optimal storage stability, lowest toxicity, and optimal antitumor effect due to better drug retention properties. Thus, the ESM/CH liposome of DOX is a potential drug delivery system. Sketch of the effect of two DOX-loaded liposomes with hydrogenated soybean phospholipid (HSPC) and egg sphingomyelin (ESM) as lipid membrane material and surface-modified SA derivative on tumor growth inhibition.

Impact of sphingomyelin acyl chain heterogeneity upon properties of raft-like membranes

Sphingomyelin (SM) is a main component of lipid rafts and characteristic of abundance of long and saturated acyl chains. Recently, we reported that fluorescence-labeled lipids including C16:0 and C18:0SMs retained membrane behaviors of inherent lipids. Here, we newly prepared fluorescent SMs with longer acyl chains, C22:0 and C24:1, for observing their partition and diffusion in SM/cholesterol (chol)/dioleoylphosphatidylcholine (DOPC) bilayers. Although fluorescent C24:1SM underwent a uniform distribution between ordered (Lo) and disordered (Ld) phases, other fluorescent SMs with saturated acyl chains were preferentially distributed in the Lo phase. Interestingly, when the acyl chains of fluorescent and membrane SMs are different, distribution of fluorescent SM to the Lo phase was reduced compared to when the acyl chains are the same. This tendency was also observed for C16:0SM/C22:0SM/chol/DOPC quaternary bilayers, where the minor SM was more excluded out of the Lo phase than the major SM. We also found that the coexistence of SMs induces SM efflux out of the Lo phase and simultaneous DOPC influx to the Lo phase, consequently reducing the difference in fluidity between the two phases. These results suggest that physicochemical properties of lipid rafts are regulated by the acyl chain heterogeneity of SMs.

Phospholipid exchange shows insulin receptor activity is supported by both the propensity to form wide bilayers and ordered raft domains

Insulin receptor (IR) is a membrane tyrosine kinase that mediates the response of cells to insulin. IR activity has been shown to be modulated by changes in plasma membrane lipid composition, but the properties and structural determinants of lipids mediating IR activity are poorly understood. Here, using efficient methyl-alpha-cyclodextrin mediated lipid exchange, we studied the effect of altering plasma membrane outer leaflet phospholipid composition upon the activity of IR in mammalian cells. After substitution of endogenous lipids with lipids having an ability to form liquid ordered (Lo) domains (sphingomyelins) or liquid disordered (Ld) domains (unsaturated phosphatidylcholines (PCs)), we found that the propensity of lipids to form ordered domains is required for high IR activity. Additional substitution experiments using a series of saturated PCs showed that IR activity increased substantially with increasing acyl chain length, which increases both bilayer width and the propensity to form ordered domains. Incorporating purified IR into alkyl maltoside micelles with increasing hydrocarbon lengths also increased IR activity, but more modestly than by increasing lipid acyl chain length in cells. These results suggest that the ability to form Lo domains as well as wide bilayer width contributes to increased IR activity. Inhibition of phosphatases showed that some of the lipid dependence of IR activity upon lipid structure reflected protection from phosphatases by lipids that support Lo domain formation. These results are consistent with a model in which a combination of bilayer width and ordered domain formation modulates IR activity via IR conformation and accessibility to phosphatases.

Identification of a critical lipid ratio in raft-like phases exposed to nitric oxide: An AFM study

Lipid rafts are discrete, heterogeneous domains of phospholipids, sphingolipids, and sterols that are present in the cell membrane. They are responsible for conducting cell signaling and maintaining lipid-protein functionality. Redox-stress-induced modifications to any of their components can severely alter the mechanics and dynamics of the membrane causing impairment to the lipid-protein functionality. Here, we report on the effect of sphingomyelin (SM) in controlling membrane permeability and its role as a regulatory lipid in the presence of nitric oxide (NO). Force spectroscopy and atomic force microscopy imaging of raft-like phases (referring here to the coexistence of "liquid-ordered" and "liquid-disordered" phases in model bilayer membranes) prepared from lipids: 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC):SM:cholesterol (CH) (at three ratios) showed that the adhesion forces to pull the tip out of the membrane increased with increasing SM concentration, indicating decreased membrane permeability. However, in the presence of NO radical (1 and 5 μM), the adhesion forces decreased depending on SM concentration. The membrane was found to be stable at the ratio POPC:SM:CH (2:1:1) even when exposed to 1 μM NO. We believe that this is a critical ratio needed by the raft-like phases to maintain homeostasis under stress conditions. The stability could be due to an interplay existing between SM and CH. However, at 5 μM NO, membrane deteriorations were detected. For POPC:SM:CH (2:2:1) ratio, NO displayed a pro-oxidant behavior and damaged the membrane at both radical concentrations. These changes were reflected by the differences in the height profiles of the raft-like phases observed by atomic force microscopy imaging. Malondialdehyde (a peroxidation product) detection suggests that lipids may have undergone lipid nitroxidation. The changes were instantaneous and independent of radical concentration and incubation time. Our study underlines the need for identifying appropriate ratios in the lipid rafts of the cell membranes to withstand redox imbalances caused by radicals such as NO.

Morphologies and Structure of Brain Lipid Membrane Dispersions

This study aims to explore the variety of previously unknown morphologies that brain lipids form in aqueous solutions. We study how these structures are dependent on cholesterol content, salt solution composition, and temperature. For this purpose, dispersions of porcine sphingomyelin with varying amounts of cholesterol as well as dispersions of porcine brain lipid extracts were investigated. We used cryo-TEM to investigate the dispersions at high-salt solution content together with small-angle (SAXD) and wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) for dispersions in the corresponding salt solution at high lipid content. Sphingomyelin forms multilamellar vesicles in large excess of aqueous salt solution. These vesicles appear as double rippled bilayers in the images and as split Bragg peaks in SAXD together with a very distinct lamellar phase pattern. These features disappear with increasing temperature, and addition of cholesterol as the WAXD data shows that the peak corresponding to the chain crystallinity disappears. The dispersions of sphingomyelin at high cholesterol content form large vesicular type of structures with smooth bilayers. The repeat distance of the lamellar phase depends on temperature, salt solution composition, and slightly with cholesterol content. The brain lipid extracts form large multilamellar vesicles often attached to assemblies of higher electron density. We think that this is probably an example of supra self-assembly with a multiple-layered vesicle surrounding an interior cubic microphase. This is challenging to resolve. DSC shows the presence of different kinds of water bound to the lipid aggregates as a function of the lipid content. Comparison with the effect of lithium, sodium, and calcium salts on the structural parameters of the sphingomyelin and the morphologies of brain lipid extract morphologies demonstrate that lithium has remarkable effects also at low content.

Effect of dipole moment on amphiphile solubility and partition into liquid ordered and liquid disordered phases in lipid bilayers

Association of amphiphiles with biomembranes is important for their availability at specific locations in organisms and cells, being critical for their biological function. A prominent role is usually attributed to the hydrophobic effect, and to electrostatic interactions between charged amphiphiles and lipids. This work explores a closely related and complementary aspect, namely the contribution made by dipole moments to the strength of the interactions established. Two xanthene amphiphiles with opposite relative orientations of their dipole and amphiphilic moments have been selected (Rhodamine-C₁₄ and Carboxyfluorescein-C₁₄). The membranes studied have distinct lipid compositions, representing typical cell membrane pools, ranging from internal membranes to the outer and inner leaflet of the plasma membrane. A comprehensive study is reported, including the affinity of the amphiphiles for the different membranes, the stability of the amphiphiles as monomers and their tendency to form small clusters, as well as their transverse location in the membrane. The orientation of the amphiphile dipole moment, which determines whether its interaction with the membrane dipole potential is repulsive or attractive, is found to exert a large influence on the association of the amphiphile with ordered lipid membranes. These interactions are also responsible for the formation of small clusters or stabilization of amphiphile monomers in the membrane. The results obtained allow understanding the prevalence of protein lipidation at the N-terminal for efficient targeting to the plasma membrane, as well as the tendency of GPI-anchored proteins (usually lipidated at the C-terminal) to form small clusters in the membrane ordered domains.

Cell invasion by intracellular parasites - the many roads to infection

Intracellular parasites from the genera Toxoplasma, Plasmodium, Trypanosoma, Leishmania and from the phylum Microsporidia are, respectively, the causative agents of toxoplasmosis, malaria, Chagas disease, leishmaniasis and microsporidiosis, illnesses that kill millions of people around the globe. Crossing the host cell plasma membrane (PM) is an obstacle these parasites must overcome to establish themselves intracellularly and so cause diseases. The mechanisms of cell invasion are quite diverse and include (1) formation of moving junctions that drive parasites into host cells, as for the protozoans Toxoplasma gondii and Plasmodium spp., (2) subversion of endocytic pathways used by the host cell to repair PM, as for Trypanosoma cruzi and Leishmania, (3) induction of phagocytosis as for Leishmania or (4) endocytosis of parasites induced by specialized structures, such as the polar tubes present in microsporidian species. Understanding the early steps of cell entry is essential for the development of vaccines and drugs for the prevention or treatment of these diseases, and thus enormous research efforts have been made to unveil their underlying biological mechanisms. This Review will focus on these mechanisms and the factors involved, with an emphasis on the recent insights into the cell biology of invasion by these pathogens.

In Human and Mouse Spino-Cerebellar Tissue, Ataxin-2 Expansion Affects Ceramide-Sphingomyelin Metabolism

Ataxin-2 (human gene symbol ATXN2) acts during stress responses, modulating mRNA translation and nutrient metabolism. Ataxin-2 knockout mice exhibit progressive obesity, dyslipidemia, and insulin resistance. Conversely, the progressive ATXN2 gain of function due to the fact of polyglutamine (polyQ) expansions leads to a dominantly inherited neurodegenerative process named spinocerebellar ataxia type 2 (SCA2) with early adipose tissue loss and late muscle atrophy. We tried to understand lipid dysregulation in a SCA2 patient brain and in an authentic mouse model. Thin layer chromatography of a patient cerebellum was compared to the lipid metabolome of Atxn2-CAG100-Knockin (KIN) mouse spinocerebellar tissue. The human pathology caused deficits of sulfatide, galactosylceramide, cholesterol, C22/24-sphingomyelin, and gangliosides GM1a/GD1b despite quite normal levels of C18-sphingomyelin. Cerebellum and spinal cord from the KIN mouse showed a consistent decrease of various ceramides with a significant elevation of sphingosine in the more severely affected spinal cord. Deficiency of C24/26-sphingomyelins contrasted with excess C18/20-sphingomyelin. Spinocerebellar expression profiling revealed consistent reductions of CERS protein isoforms, Sptlc2 and Smpd3, but upregulation of Cers2 mRNA, as prominent anomalies in the ceramide-sphingosine metabolism. Reduction of Asah2 mRNA correlated to deficient S1P levels. In addition, downregulations for the elongase Elovl1, Elovl4, Elovl5 mRNAs and ELOVL4 protein explain the deficit of very long-chain sphingomyelin. Reduced ASMase protein levels correlated to the accumulation of long-chain sphingomyelin. Overall, a deficit of myelin lipids was prominent in SCA2 nervous tissue at prefinal stage and not compensated by transcriptional adaptation of several metabolic enzymes. Myelination is controlled by mTORC1 signals; thus, our human and murine observations are in agreement with the known role of ATXN2 yeast, nematode, and mouse orthologs as mTORC1 inhibitors and autophagy promoters.

Plasma membrane repair involvement in parasitic and other pathogen infections

Intracellular pathogens depend on specific mechanisms to be able to gain entry and survive into their host cells. For this, they subvert pathways involved in physiological cellular processes. Here we are going to focus on how two protozoan parasites, Trypanosoma cruzi and Leishmania sp, which may cause severe diseases in humans, use plasma membrane repair (PMR) mechanisms to gain entry in host intracellular environment. T. cruzi is the causative agent of Chagas disease, a disease originally endemic of central and South America, but that has become widespread around the globe. T. cruzi is able to invade any nucleated cell, but muscle cells are usually the main targets during chronic disease. During host cell contact, the parasite interacts with proteins at the host cell surface and may cause damage to their membrane, which has been shown to be responsible for inducing intracellular calcium increase and PMR-related events that culminate with parasite internalization. The same was recently observed for Leishmania sp, when infecting nonprofessional phagocytic cells, such as fibroblasts. Other pathogens, such as viruses or bacteria may also use PMR-related events for invasion and vacuole escape/maturation. In some cases, PMR may also be responsible to modulate pathogen intracellular development. These other PMR roles in pathogen infections will also be briefly discussed.

Mechanism of local anesthetic-induced disruption of raft-like ordered membrane domains

BACKGROUND

Because ordered membrane domains, called lipid rafts, regulate activation of ion channels related to the nerve pulse, lipids rafts are thought to be a possible target for anesthetic molecules. To understand the mechanism of anesthetic action, we examined influence of representative local anesthetics (LAs); dibucaine, tetracaine, and lidocaine, on raft-like liquid-ordered (L_o)/non-raft-like liquid-disordered (L_d) phase separation.

METHODS

Impact of LAs on the phase separation was observed by fluorescent microscopy. LA-induced perturbation of the L_o and L_d membranes was examined by DPH anisotropy measurements. Incorporation of LAs to the membranes was examined by fluorescent anisotropy of LAs. The biding location of the LAs was indicated by small angle x-ray diffraction (SAXD).

RESULTS

Fluorescent experiments showed that dibucaine eliminated the phase separation the most effectively, followed by tetracaine and lidocaine. The disruption of the phase separation can be explained by their disordering effects on the L_o membrane. SAXD and other experiments further suggested that dibucaine's most potent perturbation of the L_o membrane is attributable to its deeper immersion and bulky molecular structure. Tetracaine, albeit immersed in the L_o membrane as deeply as dibucaine, less perturbs the L_o membrane probably because of its smaller bulkiness. Lidocaine hardly reaches the hydrophobic region, resulting in the weakest L_o membrane perturbation.

CONCLUSION

Dibcaine perturbs the L_o membrane the most effectively, followed by tetracaine and lidocaine. This ranking correlates with their anesthetic potency.

GENERAL SIGNIFICANCE

This study suggests a possible mechanistic link between anesthetic action and perturbation of lipid rafts.

Molecular dissection of box jellyfish venom cytotoxicity highlights an effective venom antidote

The box jellyfish Chironex fleckeri is extremely venomous, and envenoming causes tissue necrosis, extreme pain and death within minutes after severe exposure. Despite rapid and potent venom action, basic mechanistic insight is lacking. Here we perform molecular dissection of a jellyfish venom-induced cell death pathway by screening for host components required for venom exposure-induced cell death using genome-scale lenti-CRISPR mutagenesis. We identify the peripheral membrane protein ATP2B1, a calcium transporting ATPase, as one host factor required for venom cytotoxicity. Targeting ATP2B1 prevents venom action and confers long lasting protection. Informatics analysis of host genes required for venom cytotoxicity reveal pathways not previously implicated in cell death. We also discover a venom antidote that functions up to 15 minutes after exposure and suppresses tissue necrosis and pain in mice. These results highlight the power of whole genome CRISPR screening to investigate venom mechanisms of action and to rapidly identify new medicines.

Box jellyfish venom causes tissue damage, pain, and death through unknown molecular mechanisms. Here, Lau et al. perform a CRISPR screen to identify genes required for venom action and use this information to develop an antidote that blocks venom-induced pain and tissue damage in vivo.

Membrane Composition Modulates Fusion by Altering Membrane Properties and Fusion Peptide Structure

Membrane fusion, one of the most essential processes in the life of eukaryotes, occurs when two separate lipid bilayers merge into a continuous bilayer and internal contents of two separated membranes mingle. There is a certain class of proteins that assist the binding of the viral envelope to the target host cell and catalyzing fusion. All class I viral fusion proteins contain a highly conserved 20–25 amino-acid amphipathic peptide at the N-terminus, which is essential for fusion activity and is termed as the ‘fusion peptide’. It has been shown that insertion of fusion peptides into the host membrane and the perturbation in the membrane generated thereby is crucial for membrane fusion. Significant efforts have been given in the last couple of decades to understand the lipid-dependence of structure and function of the fusion peptide in membranes to understand the role of lipid compositions in membrane fusion. In addition, the lipid compositions further change the membrane physical properties and alter the mechanism and extent of membrane fusion. Therefore, lipid compositions modulate membrane fusion by changing membrane physical properties and altering structure of the fusion peptide.

Role of Sphingomyelin in Alphaherpesvirus Entry

Bovine herpesvirus 1 (BoHV-1) is an alphaherpesvirus that causes disease in cattle populations worldwide. Sphingomyelin (SM) is the most abundant sphingolipid in the mammalian cell membrane, where it preferentially associates with cholesterol to form lipid raft domains. SM is a substrate for the lysosome-resident enzyme acid sphingomyelinase, which plays a role in cell membrane repair following injury. Treatment of cells with noncytotoxic concentrations of Staphylococcus aureus-derived sphingomyelinase successfully reduced cell surface-exposed sphingomyelin but did not significantly inhibit BoHV-1 entry and infection, as measured by the beta-galactosidase reporter assay. Interestingly, entry of the porcine alphaherpesvirus pseudorabies virus (PRV) was inhibited by sphingomyelin-depletion of cells. Treatment of BoHV-1 particles with sphingomyelinase inhibited viral entry activity, suggesting that viral SM plays a role in BoHV-1 entry, while cellular SM does not. Treatment of cells with noncytotoxic concentrations of the functional inhibitors of host acid sphingomyelinase, imipramine and amitriptyline, which induce degradation of the cellular enzyme, did not significantly inhibit BoHV-1 entry. In contrast, inhibition of cellular acid sphingomyelinase inhibited PRV entry. Entry of the human alphaherpesvirus herpes simplex virus 1 (HSV-1) was independent of both host SM and acid sphingomyelinase, in a manner similar to BoHV-1. Together, the results suggest that among the alphaherpesviruses, there is variability in entry requirements for cellular sphingomyelin and acid sphingomyelinase activity.IMPORTANCE Bovine herpesvirus 1 (BoHV-1) is an ubiquitous pathogen affecting cattle populations worldwide. Infection can result in complicated, polymicrobial infections due to the immunosuppressive properties of the virus. Available vaccines limit disease severity and spread but do not prevent infection. The financial and animal welfare ramifications of BoHV-1 are significant. In order to develop more effective prevention and treatment regimens, a more complete understanding of the initial steps in viral infection is necessary. We recently identified a low pH endocytosis pathway for BoHV-1. Here, we examine the role of cellular factors responsible for membrane integrity and repair in alphaherpesviral entry. This study allows comparisons of the BoHV-1 entry pathway with those of other alphaherpesviruses (pseudorabies virus [PRV] and herpes simplex virus 1 [HSV-1]). Lastly, this is the first report of sphingomyelin and lysosomal sphingomyelinase playing a role in the entry of a herpesvirus. The results may lead to the development of more effective prevention and treatment regimens.

Advances and challenges in liposome digestion: Surface interaction, biological fate, and GIT modeling

During the past 50 years, there has been increased interest in liposomes as carriers of pharmaceutical, cosmetic, and agricultural products. More recently, much progress has been made in the use of surface-modified formulas in experimental food matrices. However, before the viability and the applications of nutrients in liposomal form in the edible field can be determined, the digestion behavior along the human gastrointestinal tract (GIT) must be clarified. In vitro digestion models, from static models to dynamic mono-/bi-/multi-compartmental models, are increasingly being developed and applied as alternatives to in vivo assays. This review describes the surface interactions of liposomes with their encapsulated ingredients and with external food components and updates the biological fate of liposomes after ingestion. It summarizes current models for the human stomach and intestine that are available and their relevance in nutritional studies. It highlights limitations and challenges in the use of these models for liposomal colloid system digestion and discusses crucial factors, such as enzymes and bile salts, that affect liposomal bilayer degradation.

Molecular bionics - engineering biomaterials at the molecular level using biological principles

Life and biological units are the result of the supramolecular arrangement of many different types of molecules, all of them combined with exquisite precision to achieve specific functions. Taking inspiration from the design principles of nature allows engineering more efficient and compatible biomaterials. Indeed, bionic (from bion-, unit of life and -ic, like) materials have gained increasing attention in the last decades due to their ability to mimic some of the characteristics of nature systems, such as dynamism, selectivity, or signalling. However, there are still many challenges when it comes to their interaction with the human body, which hinder their further clinical development. Here we review some of the recent progress in the field of molecular bionics with the final aim of providing with design rules to ensure their stability in biological media as well as to engineer novel functionalities which enable navigating the human body.

Liposome: composition, characterisation, preparation, and recent innovation in clinical applications

In the last decades, pharmaceutical interested researches aimed to develop novel and innovative drug delivery techniques in the medical and pharmaceutical fields. Recently, phospholipid vesicles (Liposomes) are the most known versatile assemblies in the drug delivery systems. The discovery of liposomes arises from self-forming enclosed phospholipid bilayer upon coming in contact with the aqueous solution. Liposomes are uni or multilamellar vesicles consisting of phospholipids produced naturally or synthetically, which are readily non-toxic, biodegradable, and are readily produced on a large scale. Various phospholipids, for instance, soybean, egg yolk, synthetic, and hydrogenated phosphatidylcholine consider the most popular types used in different kinds of formulations. This review summarises liposomes composition, characterisation, methods of preparation, and their applications in different medical fields including cancer therapy, vaccine, ocular delivery, wound healing, and some dermatological applications.

Anticancer Agent Edelfosine Exhibits a High Affinity for Cholesterol and Disorganizes Liquid-Ordered Membrane Structures

Edelfosine is an anticancer drug with an asymmetric structure because, being a derivative of glycerol, it possesses two hydrophobic substituents of very different lengths. We showed that edelfosine destabilizes liquid-ordered membranes formed by either 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine, sphingomyelin (SM), and cholesterol (1:1:1 molar ratio) or SM and cholesterol (2:1 molar ratio). This was observed by differential scanning calorimetry in which phase transition arises from either of these membrane systems after the addition of edelfosine. The alteration in the liquid-ordered domains was characterized by using a small-angle X-ray diffraction that revealed the formation of gel phases as a consequence of the addition of edelfosine at low temperatures and by a wide-angle X-ray diffraction that confirmed changes in the membranes, indicating the formation of these gel phases. The increase in phase transition derived by the edelfosine addition was further confirmed by Fourier-transform infrared spectroscopy. The effect of edelfosine was compared with that of structurally analogue lipids: platelet-activating factor and 1-palmitoyl-2-acetyl- sn-glycero-3-phosphocholine, which also have the capacity of destabilizing liquid-ordered domains, although they are less potent than edelfosine for this activity, and lysophosphatidylcholine, which lacks this capacity. It was concluded that edelfosine may be associated with cholesterol favorably competing with sphingomyelin, and that this sets sphingomyelin free to undergo a phase transition. Finally, the experimental observations can be described by molecular dynamics calculations in terms of intermolecular interaction energies in phospholipid-cholesterol membranes. Higher interaction energies between asymmetric phospholipids and cholesterol than between sphingomyelin and cholesterol were obtained. These results are interesting because they biophysically characterize one of the main molecular mechanisms to trigger apoptosis of the cancer cells.

Multilamellar nanovesicles show distinct mechanical properties depending on their degree of lamellarity

Small multilamellar vesicles may have benefits over unilamellar vesicles for drug delivery, such as an increased volume for hydrophobic drugs. In addition, their altered mechanical properties might be beneficial for cellular uptake. Here, we show how atomic force microscopy (AFM) can be used to detect and characterize multilamellar vesicles. We quantify the size of each break event occurring during AFM nanoindentations, which shows good agreement with the thickness of supported lipid bilayers. Analyzing the size and number of these events for individual vesicles allows us to distinguish between vesicles consisting of 1 up to 5 bilayers. We validate these results by comparison with correlative cryo-electron microscopy (cryo-EM) data at the vesicle population level. Finally, we quantify the vesicle geometry and mechanical properties, and show that with additional bilayers adherent vesicles are more spherical and stiffer. Surprisingly, at ∼20% stiffening for each additional bilayer, the vesicle stiffness scales only weakly with lamellarity. Our results show the potential of AFM for studying liposomal nanoparticles and suggest that small multilamellar vesicles may have beneficial mechanical properties for cellular uptake.

Food Ingredients That Inhibit Cholesterol Absorption

Cholesterol is a vital component of the human body. It stabilizes cell membranes and is the precursor of bile acids, vitamin D and steroid hormones. However, cholesterol accumulation in the bloodstream (hypercholesterolemia) can cause atherosclerotic plaques within artery walls, leading to heart attacks and strokes. The efficiency of cholesterol absorption in the small intestine is of great interest because human and animal studies have linked cholesterol absorption with plasma concentration of total and low density lipoprotein cholesterol. Cholesterol absorption is highly regulated and influenced by particular compounds in the food supply. Therefore, it is desirable to learn more about natural food components that inhibit cholesterol absorption so that food ingredients and dietary supplements can be developed for consumers who wish to manage their plasma cholesterol levels by non-pharmacological means. Food components thus far identified as inhibitors of cholesterol absorption include phytosterols, soluble fibers, phospholipids, and stearic acid.

Lipid Interactions and Organization in Complex Bilayer Membranes

Bilayer lipids influence the lateral structure of the membranes, but the relationship between lipid properties and the lateral structure formed is not always understood. Model membrane studies on bilayers containing cholesterol and various phospholipids (PLs) suggest that high and low temperature melting PLs may segregate, especially in the presence of cholesterol. The effect of different PL headgroups on lateral structure of bilayers is also not clear. Here, we have examined the formation of lateral heterogeneity in increasingly complex (up to five-component) multilamellar bilayers. We have used time-resolved fluorescence spectroscopy with domain-selective fluorescent probes (PL-conjugated trans-parinaric acid), and (2)H NMR spectroscopy with site or perdeuterated PLs. We have measured changes in bilayer order using such domain-selective probes both as a function of temperature and composition. Our results from time-resolved fluorescence and (2)H NMR showed that in ternary bilayers, acyl chain order and thermostability in sphingomyelin-rich domains were not affected to any greater extent by the headgroup structure of the monounsaturated PLs (phosphatidylcholine, phosphatidylethanolamine, or phosphatidylserine) in the bilayer. In the complex five-component bilayers, we could not detect major differences between the different monounsaturated PLs regarding cholesterol-induced ordering. However, cholesterol clearly influenced deuterated N-palmitoyl sphingomyelin differently than the other deuterated PLs, suggesting that cholesterol favored N-palmitoyl sphingomyelin over the other PLs. Taken together, both the fluorescence spectroscopy and (2)H NMR data suggest that the complex five-component membranes displayed lateral heterogeneity, at least in the lower temperature regimen examined.

Resveratrol Interaction with Lipid Bilayers: A Synchrotron X-ray Scattering Study

Resveratrol belongs to the large group of biologically active polyphenol compounds, with several beneficial health effects including antioxidant activity, anti-inflammatory action, cardiovascular protection, neuroprotection, and cancer chemoprevention. In the present study, the possibility that the effects of resveratrol described above are caused by resveratrol membrane interactions and structural modifications of lipid bilayers is evaluated. In this context, it is possible that resveratrol interacts selectively with lipid domains present in biological membranes, thereby modulating the localization of the anchored proteins and controlling their intracellular cascades. This study was conducted in a synchrotron particle accelerator, where the influence of resveratrol in the structural organization of lipid domains in bilayers was investigated using small- and wide-angle X-ray scattering (SAXS and WAXS) techniques. Membrane mimetic systems composed of egg l-α-phosphatidylcholine (EPC), cholesterol (CHOL), and sphingomyelin (SM), with different molar ratios, were used to access the effects of resveratrol on the order and structure of the membrane. The results revealed that resveratrol induces phase separation, promoting the formation of lipid domains in EPC, EPC:CHOL [4:1], and EPC:CHOL:SM [1:1:1] bilayers, which brings some structural organization to membranes. Therefore, resveratrol controls lipid packing of bilayers by inducing the organization of lipid rafts. Moreover, the formation of lipid domains is important for modulating the activity of many receptors, transmembrane proteins, and enzymes whose activity depends on the structural organization of the membrane and on the presence or absence of these organized domains. This evidence can thereby explain the therapeutic effects of resveratrol.

Intestinal permeation enhancers for oral peptide delivery

Intestinal permeation enhancers (PEs) are one of the most widely tested strategies to improve oral delivery of therapeutic peptides. This article assesses the intestinal permeation enhancement action of over 250 PEs that have been tested in intestinal delivery models. In depth analysis of pre-clinical data is presented for PEs as components of proprietary delivery systems that have progressed to clinical trials. Given the importance of co-presentation of sufficiently high concentrations of PE and peptide at the small intestinal epithelium, there is an emphasis on studies where PEs have been formulated with poorly permeable molecules in solid dosage forms and lipoidal dispersions.

Cholesterol interactions with ceramide and sphingomyelin

Sphingolipids contain in their polar heads chemical groups allowing them to establish a complex network of H-bonds (through different OH and NHgroups) with other lipids in the bilayer. In the recent years the specific interaction of sphingomyelin (SM) with cholesterol (Chol) has been examined, largely in the context of the "lipid raft" hypothesis. Formation of SM-Ceramide (Cer) complexes, proposed to exist in cell membranes in response to stress, has also been described. More recently, a delicate balance of phase formation and transformation in ternary mixtures of SM, Chol and Cer, with mutual displacement of Chol and Cer from their interaction with SM is considered to exist. In addition, data demonstrating direct Chol-Cer interaction are becoming available.

Examination of the specificity of tumor cell derived exosomes with tumor cells in vitro

Small endogenous vesicles called exosomes are beginning to be explored as drug delivery vehicles. The in vivo targets of exosomes are poorly understood; however, they are believed to be important in cell-to-cell communication and may play a prominent role in cancer metastasis. We aimed to elucidate whether cancer derived exosomes can be used as drug delivery vehicles that innately target tumors over normal tissue. Our in vitro results suggest that while there is some specificity towards cancer cells over "immortalized" cells, it is unclear if the difference is sufficient to achieve precise in vivo targeting. Additionally, we found that exosomes associate with their cellular targets to a significantly greater extent (>10-fold) than liposomes of a similar size. Studies on the association of liposomes mimicking the unique lipid content of exosomes revealed that the lipid composition contributes significantly to cellular adherence/internalization. Cleavage of exosome surface proteins yielded exosomes exhibiting reduced association with their cellular targets, demonstrating the importance of proteins in binding/internalization. Furthermore, although acidic conditions are known to augment the metastatic potential of tumors, we found that cells cultured at low pH released exosomes with significantly less potential for cellular association than cells cultured at physiological pH.

Effects of sphingomyelin/ceramide ratio on the permeability and microstructure of model stratum corneum lipid membranes

The conversion of sphingomyelin (SM) to a ceramide (Cer) by acid sphingomyelinase (aSMase) is an important event in skin barrier development. A deficiency in aSMase in diseases such as Niemann-Pick disease and atopic dermatitis coincides with impaired skin barrier recovery after disruption. We studied how an increased SM/Cer ratio influences the barrier function and microstructure of model stratum corneum (SC) lipid membranes. In the membranes composed of isolated human SC Cer (hCer)/cholesterol/free fatty acids/cholesteryl sulfate, partial or full replacement of hCer by SM increased water loss. Partial replacement of 25% and 50% of hCer by SM also increased the membrane permeability to theophylline and alternating electric current, while a higher SM content either did not alter or even decreased the membrane permeability. In contrast, in a simple membrane model with only one type of Cer (nonhydroxyacyl sphingosine, CerNS), an increased SM/Cer ratio provided a similar or better barrier against the permeation of various markers. X-ray powder diffraction revealed that the replacement of hCer by SM interferes with the formation of the long periodicity lamellar phase with a repeat distance of d=12.7nm. Our results suggest that SM-to-Cer processing in the human epidermis is essential for preventing excessive water loss, while the permeability barrier to exogenous compounds is less sensitive to the presence of sphingomyelin.

Characterization of the water defect at the HIV-1 gp41 membrane spanning domain in bilayers with and without cholesterol using molecular simulations

The membrane spanning domain (MSD) of human immunodeficiency virus 1 (HIV-1) envelope glycoprotein gp41 is important for fusion and infection. We used molecular dynamics (MD) simulations (3.4 μs total) to relate membrane and peptide properties that lead to water solvation of the α-helical gp41 MSD's midspan arginine in pure dipalmitoylphosphatidylcholine (DPPC) and in 50/50 DPPC/cholesterol membranes. We find that the midspan arginine is solvated by water that penetrates the inner leaflet, leading to a so-called water defect. The water defect is surprisingly robust across initial conditions and membrane compositions, but the presence of cholesterol modulates its behavior in several key ways. In the cholesterol-containing membranes, fluctuations in membrane thickness and water penetration depth are localized near the midspan arginine, and the MSD helices display a tightly regulated tilt angle. In the cholesterol-free membranes, thickness fluctuations are not as strongly correlated to the peptide position and tilt angles vary significantly depending on protein position relative to boundaries between domains of differing thickness. Cholesterol in an HIV-1 viral membrane is required for infection. Therefore, this work suggests that the colocalized water defect and membrane thickness fluctuations in cholesterol-containing viral membranes play an important role in fusion by bringing the membrane closer to a stability limit that must be crossed for fusion to occur.

Long and short lipid molecules experience the same interleaflet drag in lipid bilayers

Membrane interleaflet viscosity ηe affects tether formation, phase separation into domains, cell shape changes, and budding. Contrary to the expected contribution to interleaflet coupling from interdigitation, the slide of lipid patches in opposing monolayers conferred the same value ηe≈3×10(9)  J s m-4 for the friction experienced by the ends of both short and long chain fluorescent lipid analogues. Consistent with the weak dependence of the translational diffusion coefficient on lipid length, the in-layer viscosity was, albeit length dependent, much smaller than ηe.

Biological functions of sphingomyelins

Sphingomyelin (SM) is a dominant sphingolipid in membranes of mammalian cells and this lipid class is specifically enriched in the plasma membrane, the endocytic recycling compartment, and the trans Golgi network. The distribution of SM and cholesterol among cellular compartments correlate. Sphingolipids have extensive hydrogen-bonding capabilities which together with their saturated nature facilitate the formation of sphingolipid and SM-enriched lateral domains in membranes. Cholesterol prefers to interact with SMs and this interaction has many important functional consequences. In this review, the synthesis, regulation, and intracellular distribution of SMs are discussed. The many direct roles played by membrane SM in various cellular functions and processes will also be discussed. These include involvement in the regulation of endocytosis and receptor-mediated ligand uptake, in ion channel and G-protein coupled receptor function, in protein sorting, and functioning as receptor molecules for various bacterial toxins, and for non-bacterial pore-forming toxins. SM is also an important constituent of the eye lens membrane, and is believed to participate in the regulation of various nuclear functions. SM is an independent risk factor in the development of cardiovascular disease, and new studies have shed light on possible mechanism behind its role in atherogenesis.

Molecular properties of various structurally defined sphingomyelins -- correlation of structure with function

Sphingomyelins are important phospholipids in plasma membranes of most cells. Because of their dominantly saturated nature, they affect the lateral structure of membranes, and contribute to the regulation of cholesterol distribution within membranes, and in cells. However, the abundance of molecular species present in cells also implies that sphingomyelins have other, more specific functions. Many of these functions are currently unknown, but are under extensive study. Mostly model membrane studies have shown that sphingomyelins (and other sphingolipids), in contrast to glycerophospholipids, have important hydrogen bonding properties which in several important ways confer specific functional properties to this abundant class of membrane phospholipids. The often very asymmetric nature of sphingomyelins, arising from mismatch in length between the long chain base and N-acyl chains, also impose specific properties (e.g., interdigitation) to sphingomyelins not seen with glycerophospholipids. In this review, the latest sphingomyelin literature will be scrutinized, and an effort will be made to correlate the molecular structure of sphingomyelin with functional properties. In particular, the effects of head group properties, interfacial hydrogen bonding, long chain base hydroxylation, N-acyl chain hydroxylation, and N-acyl chain methyl-branching will be discussed.

Ebolavirus requires acid sphingomyelinase activity and plasma membrane sphingomyelin for infection

Acid sphingomyelinase (ASMase) converts the lipid sphingomyelin (SM) to phosphocholine and ceramide and has optimum activity at acidic pH. Normally, ASMase is located in lysosomes and endosomes, but membrane damage or the interaction with some bacterial and viral pathogens can trigger its recruitment to the plasma membrane. Rhinovirus and measles viruses each require ASMase activity during early stages of infection. Both sphingomyelin and ceramide are important components of lipid rafts and are potent signaling molecules. Each plays roles in mediating macropinocytosis, which has been shown to be important for ebolavirus (EBOV) infection. Here, we investigated the role of ASMase and its substrate, SM, in EBOV infection. The work was performed at biosafety level 4 with wild-type virus with specificity and mechanistic analysis performed using virus pseudotypes and virus-like particles. We found that virus particles strongly associate with the SM-rich regions of the cell membrane and depletion of SM reduces EBOV infection. ASM-specific drugs and multiple small interfering RNAs strongly inhibit the infection by EBOV and EBOV glycoprotein pseudotyped viruses but not by the pseudotypes bearing the glycoprotein of vesicular stomatitis virus. Interestingly, the binding of virus-like particles to cells is strongly associated with surface-localized ASMase as well as SM-enriched sites. Our work suggests that ASMase activity and SM presence are necessary for efficient infection of cells by EBOV. The inhibition of this pathway may provide new avenues for drug treatment.

Sterols have higher affinity for sphingomyelin than for phosphatidylcholine bilayers even at equal acyl-chain order

The interaction between cholesterol and phospholipids in bilayer membranes is important for the formation and maintenance of membrane structure and function. However, cholesterol does not interact favorably with all types of phospholipids and, for example, prefers more ordered sphingomyelins (SMs) over phosphatidylcholines (PCs). The reason for this preference is not clear. Here we have studied whether acyl-chain order could be responsible for the preferred sterol interaction with SMs. Acyl-chain order was deduced from diphenylhexatriene anisotropy and from the deuterium order parameter obtained by (2)H-NMR on bilayers made from either 14:0/14:0((d27))-PC, or 14:0((d27))-SM. Sterol/phospholipid interaction was determined from sterol bilayer partitioning. Cholestatrienol (CTL) was used as a fluorescence probe for cholesterol, because its relative membrane partitioning is similar to cholesterol. When CTL was allowed to reach equilibrium partitioning between cyclodextrins and unilamellar vesicles made from either 14:0/14:0-PC or 14:0-SM, the molar-fraction partitioning coefficient (K(x)) was approximately twofold higher for SM bilayers than for PC bilayers. This was even the case when the temperature in the SM samples was raised to achieve equal acyl-chain order, as determined from 1,6-diphenyl-1,3,5-hexatriene (DPH) anisotropy and the deuterium order parameter. Although the K(x) did increase with acyl-chain order, the higher K(x) for SM bilayers was always evident. At equal acyl-chain order parameter (DPH anisotropy), the K(x) was also higher for 14:0-SM bilayers than for bilayers made from either 14:0/15:0-PC or 15:0-/14:0-PC, suggesting that minor differences in chain length or molecular asymmetry are not responsible for the difference in K(x). We conclude that acyl-chain order affects the bilayer affinity of CTL (and thus cholesterol), but that it is not the cause for the preferred affinity of sterols for SMs over matched PCs. Instead, it is likely that the interfacial properties of SMs influence and stabilize interactions with sterols in bilayer membranes.

Effect of hydrophobic mismatch and interdigitation on sterol/sphingomyelin interaction in ternary bilayer membranes

Sphingomyelin (SM) is a major phospholipid in most cell membranes. SMs are composed of a long-chain base (often sphingosine, 18:1(Δ4t)), and N-linked acyl chains (often 16:0, 18:0 or 24:1(Δ15c)). Cholesterol interacts with SM in cell membranes, but the acyl chain preference of this interaction is not fully elucidated. In this study we have examined the effects of hydrophobic mismatch and interdigitation on cholesterol/sphingomyelin interaction in complex bilayer membranes. We measured the capacity of cholestatrienol (CTL) and cholesterol to form sterol-enriched ordered domains with saturated SM species having different chain lengths (14 to 24 carbons) in ternary bilayer membranes. We also determined the equilibrium bilayer partitioning coefficient of CTL with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes containing 20mol% of saturated SM analogs. Ours results show that while CTL and cholesterol formed sterol-enriched domains with both short and long-chain SM species, the sterols preferred interaction with 16:0-SM over any other saturated chain length SM analog. When CTL membrane partitioning was determined with fluid POPC bilayers containing 20mol% of a saturated chain length SM analog, the highest affinity was seen with 16:0-SM (both at 23 and 37°C). These results indicate that hydrophobic mismatch and/or interdigitation attenuate sterol/SM association and thus affect lateral distribution of sterols in the bilayer membrane.

Hemagglutinin of influenza virus partitions into the nonraft domain of model membranes

The HA of influenza virus is a paradigm for a transmembrane protein thought to be associated with membrane-rafts, liquid-ordered like nanodomains of the plasma membrane enriched in cholesterol, glycosphingolipids, and saturated phospholipids. Due to their submicron size in cells, rafts can not be visualized directly and raft-association of HA was hitherto analyzed by indirect methods. In this study, we have used GUVs and GPMVs, showing liquid disordered and liquid ordered domains, to directly visualize partition of HA by fluorescence microscopy. We show that HA is exclusively (GUVs) or predominantly (GPMVs) present in the liquid disordered domain, regardless of whether authentic HA or domains containing its raft targeting signals were reconstituted into model membranes. The preferential partition of HA into ld domains and the difference between lo partition in GUV and GPMV are discussed with respect to differences in packaging of lipids in membranes of model systems and living cells suggesting that physical properties of lipid domains in biological membranes are tightly regulated by protein-lipid interactions.

Yeast lipids can phase-separate into micrometer-scale membrane domains

The lipid raft concept proposes that biological membranes have the potential to form functional domains based on a selective interaction between sphingolipids and sterols. These domains seem to be involved in signal transduction and vesicular sorting of proteins and lipids. Although there is biochemical evidence for lipid raft-dependent protein and lipid sorting in the yeast Saccharomyces cerevisiae, direct evidence for an interaction between yeast sphingolipids and the yeast sterol ergosterol, resulting in membrane domain formation, is lacking. Here we show that model membranes formed from yeast total lipid extracts possess an inherent self-organization potential resulting in liquid-disordered-liquid-ordered phase coexistence at physiologically relevant temperature. Analyses of lipid extracts from mutants defective in sphingolipid metabolism as well as reconstitution of purified yeast lipids in model membranes of defined composition suggest that membrane domain formation depends on specific interactions between yeast sphingolipids and ergosterol. Taken together, these results provide a mechanistic explanation for lipid raft-dependent lipid and protein sorting in yeast.

Sphingomyelin analogs with branched N-acyl chains: the position of branching dramatically affects acyl chain order and sterol interactions in bilayer membranes

Sphingolipids have been found to have single methyl branchings both in their long-chain base and in their N-linked acyl chains. In this study we determined how methyl-branching in the N-linked acyl chain of sphingomyelin (SM) affected their membrane properties. SM analogs with a single methyl-branching at carbon 15 (of a 17:0 acyl chain; anteiso) had a lower gel-liquid transition temperature as compared to an iso-branched SM analog. Phytanoyl SM (methyls at carbons 3, 7, 11 and 15) as well as a SM analog with a methyl on carbon 10 in a hexadecanoyl chain failed to show a gel-liquid transition above 10 degrees C. Only the two distally branched SM analogs (iso and anteiso) formed ordered domains with cholesterol in a 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer. However, domains formed by the branched SM analogs appeared to contain less sterol when compared to palmitoyl SM (PSM) as the saturated phospholipid. Sterol-enriched domains formed by the anteiso SM analog were also less stable against temperature than domains formed by PSM. Both the 10-methyl and phytanoyl SM analogs failed to form sterol-enriched domains in the POPC bilayer. Acyl chain branching weakened SM/sterol interactions markedly when compared to PSM, as also evidenced from the decreased affinity of cholestatrienol to bilayers containing branched SM analogs. Our results show that methyl-branching weakened intermolecular interactions in a position-dependent manner.