Phospholipid/cholesterol model membranes: formation of cholesterol crystallites

Graphene-based hybrid composites for cancer diagnostic and therapy

The application of graphene-based nanocomposites for therapeutic and diagnostic reasons has advanced considerably in recent years due to advancements in the synthesis and design of graphene-based nanocomposites, giving rise to a new field of nano-cancer diagnosis and treatment. Nano-graphene is being utilized more often in the field of cancer therapy, where it is employed in conjunction with diagnostics and treatment to address the complex clinical obstacles and problems associated with this life-threatening illness. When compared to other nanomaterials, graphene derivatives stand out due to their remarkable structural, mechanical, electrical, optical, and thermal capabilities. The high specific surface area of these materials makes them useful as carriers in controlled release systems that respond to external stimuli; these compounds include drugs and biomolecules like nucleic acid sequences (DNA and RNA). Furthermore, the presence of distinctive sheet-like nanostructures and the capacity for photothermal conversion have rendered graphene-based nanocomposites highly favorable for optical therapeutic applications, including photothermal treatment (PTT), photodynamic therapy (PDT), and theranostics. This review highlights the current state and benefits of using graphene-based nanocomposites in cancer diagnosis and therapy and discusses the obstacles and prospects of their future development. Then we focus on graphene-based nanocomposites applications in cancer treatment, including smart drug delivery systems, PTT, and PDT. Lastly, the biocompatibility of graphene-based nanocomposites is also discussed to provide a unique overview of the topic.

Is Lipid Metabolism of Value in Cancer Research and Treatment? Part I- Lipid Metabolism in Cancer

For either healthy or diseased organisms, lipids are key components for cellular membranes; they play important roles in numerous cellular processes including cell growth, proliferation, differentiation, energy storage and signaling. Exercise and disease development are examples of cellular environment alterations which produce changes in these networks. There are indications that alterations in lipid metabolism contribute to the development and progression of a variety of cancers. Measuring such alterations and understanding the pathways involved is critical to fully understand cellular metabolism. The demands for this information have led to the emergence of lipidomics, which enables the large-scale study of lipids using mass spectrometry (MS) techniques. Mass spectrometry has been widely used in lipidomics and allows us to analyze detailed lipid profiles of cancers. In this article, we discuss emerging strategies for lipidomics by mass spectrometry; targeted, as opposed to global, lipid analysis provides an exciting new alternative method. Additionally, we provide an introduction to lipidomics, lipid categories and their major biological functions, along with lipidomics studies by mass spectrometry in cancer samples. Further, we summarize the importance of lipid metabolism in oncology and tumor microenvironment, some of the challenges for lipodomics, and the potential for targeted approaches for screening pharmaceutical candidates to improve the therapeutic efficacy of treatment in cancer patients.

Lipid metabolism in idiopathic pulmonary fibrosis: From pathogenesis to therapy

Idiopathic pulmonary fibrosis (IPF) is a chronic irreversible interstitial lung disease characterized by a progressive decline in lung function. The etiology of IPF is unknown, which poses a significant challenge to the treatment of IPF. Recent studies have identified a strong association between lipid metabolism and the development of IPF. Qualitative and quantitative analysis of small molecule metabolites using lipidomics reveals that lipid metabolic reprogramming plays a role in the pathogenesis of IPF. Lipids such as fatty acids, cholesterol, arachidonic acid metabolites, and phospholipids are involved in the onset and progression of IPF by inducing endoplasmic reticulum stress, promoting cell apoptosis, and enhancing the expression of pro-fibrotic biomarkers. Therefore, targeting lipid metabolism can provide a promising therapeutic strategy for pulmonary fibrosis. This review focuses on lipid metabolism in the pathogenesis of pulmonary fibrosis.

Simulation study of domain formation in a model bacterial membrane

Recent experimental studies revealed that functional membrane microdomains (FMMs) are formed in prokaryotic cells which are structurally and functionally similar to the lipid rafts formed in eukaryotic cells. In this study, we employ coarse-grained molecular dynamics simulations to investigate the mechanism of domain formation and its physiochemical properties in a model methicillin-resistant staphylococcus aureus (MRSA) cell membrane. We find that domains are formed through lateral segregation of staphyloxanthin (STX), a carotenoid which shields the bacteria from the host's immune because of its antioxidant nature. Simulation results suggest that membrane integrity increases with the size of the domain, which is assessed by computing bond order parameter of the lipid tails, membrane expansion modulus and water permeability across the membrane. Various membrane domain proteins such as flotillin-like protein floA and penicillin binding protein (PBP2a) preferentially bind with the STX and accumulate in the membrane domain which is consistent with the recent experimental results.

In-Membrane Nanostructuring of Cationic Amphiphiles Affects Their Antimicrobial Efficacy and Cytotoxicity: A Comparison Study between a De Novo Antimicrobial Lipopeptide and Traditional Biocides

Cationic biocides have been widely used as active ingredients in personal care and healthcare products for infection control and wound treatment for a long time, but there are concerns over their cytotoxicity and antimicrobial resistance. Designed lipopeptides are potential candidates for alleviating these issues because of their mildness to mammalian host cells and their high efficacy against pathogenic microbial membranes. In this study, antimicrobial and cytotoxic properties of a de novo designed lipopeptide, CH₃(CH₂)₁₂CO-Lys-Lys-Gly-Gly-Ile-Ile-NH₂ (C₁₄KKGGII), were assessed against that of two traditional cationic biocides C_nTAB (n = 12 and 14), with different critical aggregation concentrations (CACs). C₁₄KKGGII was shown to be more potent against both bacteria and fungi but milder to fibroblast host cells than the two biocides. Biophysical measurements mimicking the main features of microbial and host cell membranes were obtained for both lipid monolayer models using neutron reflection and small unilamellar vesicles (SUVs) using fluorescein leakage and zeta potential changes. The results revealed selective binding to anionic lipid membranes from the lipopeptide and in-membrane nanostructuring that is distinctly different from the co-assembly of the conventional C_nTAB. Furthermore, C_nTAB binding to the model membranes showed low selectivity, and its high cytotoxicity could be attributed to both membrane lysis and chemical toxicity. This work demonstrates the advantages of the lipopeptides and their potential for further development toward clinical application.

Pathways and Mechanisms of Cellular Cholesterol Efflux-Insight From Imaging

Cholesterol is an essential molecule in cellular membranes, but too much cholesterol can be toxic. Therefore, mammalian cells have developed complex mechanisms to remove excess cholesterol. In this review article, we discuss what is known about such efflux pathways including a discussion of reverse cholesterol transport and formation of high-density lipoprotein, the function of ABC transporters and other sterol efflux proteins, and we highlight their role in human diseases. Attention is paid to the biophysical principles governing efflux of sterols from cells. We also discuss recent evidence for cholesterol efflux by the release of exosomes, microvesicles, and migrasomes. The role of the endo-lysosomal network, lipophagy, and selected lysosomal transporters, such as Niemann Pick type C proteins in cholesterol export from cells is elucidated. Since oxysterols are important regulators of cellular cholesterol efflux, their formation, trafficking, and secretion are described briefly. In addition to discussing results obtained with traditional biochemical methods, focus is on studies that use established and novel bioimaging approaches to obtain insight into cholesterol efflux pathways, including fluorescence and electron microscopy, atomic force microscopy, X-ray tomography as well as mass spectrometry imaging.

Designing a Useful Lipid Raft Model Membrane for Electrochemical and Surface Analytical Studies

A model biomimetic system for the study of protein reconstitution or drug interactions should include lipid rafts in the mixed lipid monolayer, since they are usually the domains embedding membrane proteins and peptides. Four model lipid films composed of three components: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), cholesterol (Chol) and sphingomyelin (SM) mixed in different molar ratios were proposed and investigated using surface pressure measurements and thermodynamic analysis of the monolayers at the air-water interface and imaged by Brewster angle microscopy. The ternary monolayers were transferred from the air-water onto the gold electrodes to form bilayer films and were studied for the first time by electrochemical methods: alternative current voltammetry and electrochemical impedance spectroscopy and imaged by atomic force microscopy. In excess of DOPC, the ternary systems remained too liquid for the raft region to be stable, while in the excess of cholesterol the layers were too solid. The layers with SM in excess lead to the formation of Chol:SM complexes but the amount of the fluid matrix was very low. The equimolar content of the three components lead to the formation of a stable and well-organized assembly with well-developed raft microdomains of larger thickness, surrounded by the more fluid part of the bilayer. The latter is proposed as a convenient raft model membrane for further physicochemical studies of interactions with drugs or pollutants or incorporation of membrane proteins.

Non-viral nanoparticles for RNA interference: Principles of design and practical guidelines

Ribonucleic acid interference (RNAi) is an innovative treatment strategy for a myriad of indications. Non-viral synthetic nanoparticles (NPs) have drawn extensive attention as vectors for RNAi due to their potential advantages, including improved safety, high delivery efficiency and economic feasibility. However, the complex natural process of RNAi and the susceptible nature of oligonucleotides render the NPs subject to particular design principles and requirements for practical fabrication. Here, we summarize the requirements and obstacles for fabricating non-viral nano-vectors for efficient RNAi. To address the delivery challenges, we discuss practical guidelines for materials selection and NP synthesis in order to maximize RNA encapsulation efficiency and protection against degradation, and to facilitate the cytosolic release of oligonucleotides. The current status of clinical translation of RNAi-based therapies and further perspectives for reducing the potential side effects are also reviewed.

Lipid Metabolism and Lipidomics Applications in Cancer Research

Lipids are the critical components of cellular and plasma membrane, which constitute an impermeable barrier of cellular compartments, and play important roles on numerous cellular processes including cell growth, proliferation, differentiation, and signaling. Alterations in lipid metabolism have been implicated in the development and progression of cancers. However, unlike other biomolecules, the diversity in the structures and characteristics of lipid species results in the limited understanding of their metabolic alterations in cancers. Lipidomics is an emerging discipline that studies lipids in a large scale based on analytical chemistry principles and technological tools. Multidimensional mass spectrometry-based shotgun lipidomics (MDMS-SL) uses direct infusion to avoid difficulties from alterations in concentration, chromatographic anomalies, and ion-pairing alterations to improve resolution and achieve rapid and accurate qualitative and quantitative analysis. In this chapter, lipids and lipid metabolism relevant to cancer research are introduced, followed by a brief description of MDMS-SL and other shotgun lipidomics techniques and some applications for cancer research.

Metabolic Reprogramming-A New Era How to Prevent and Treat Graft Versus Host Disease After Allogeneic Hematopoietic Stem Cell Transplantation Has Begun

Allogeneic hematopoietic stem cell transplantation (HSCT) is the solitary therapeutic therapy for many types of hematological cancers. The benefits of this procedure are challenged by graft vs. host disease (GVHD), causing significant morbidity and mortality. Recent advances in the metabolomics field have revolutionized our understanding of complex human diseases, clinical diagnostics and allow to trace the de novo biosynthesis of metabolites. There is growing evidence for metabolomics playing a role in different aspects of GVHD, and therefore metabolomic reprogramming presents a novel tool for this disease. Pre-transplant cytokine profiles and metabolic status of allogeneic transplant recipients is shown to be linked with a threat of acute GVHD. Immune reactions underlying the pathophysiology of GVHD involve higher proliferation and migration of immune cells to the target site, requiring shifts in energy supply and demand. Metabolic changes and reduced availability of oxygen result in tissue and cellular hypoxia which is extensive enough to trigger transcriptional and translational changes. T cells, major players in acute GVHD pathophysiology, show increased glucose uptake and glycolytic activity. Effector T (Teff) cells activated during nutrient limiting conditions in vitro or multiplying during GVHD in vivo, depend more on oxidative phosphorylation (OXPHOS) and fatty acid oxidation (FAO). Dyslipidemia, such as the increase of medium and long chain fatty and polyunsaturated acids in plasma of GVHD patients, has been observed. Sphingolipids associate with inflammatory conditions and cancer. Chronic GVHD (cGVHD) patients show reduced branched-chain amino acids (BCAAs) and increased sulfur-containing metabolites post HSCT. Microbiota-derived metabolites such as aryl hydrocarbon receptor (AhR) ligands, bile acids, plasmalogens and short chain fatty acids vary significantly and affect allogeneic immune responses during acute GVHD. Considering the multitude of possibilities, how altered metabolomics are involved in GVHD biology, multi-timepoints related and multivariable biomarker panels for prognosticating and understanding GVHD are needed. In this review, we will discuss the recent work addressing metabolomics reprogramming to control GVHD in detail.

Role of pH level on the morphology and growth rate of myelin figures

The myelin figure (MF) is one of the basic structures of lipids, and the study of their formation and the effect of various parameters on their growth is useful in understanding several biological processes. In this paper, we address the influence of the pH degree of the surrounding medium on MF dynamics. We introduce a tunable shearing digital holographic microscopy arrangement to obtain quantitative and volumetric information about the complex growth of MFs. Our results show that (1) the time evolution of relative length and volume changes of MFs follows a power-law, (2) the acidity facilitates the growth rate, and (3) the acidic environment causes the formation of thicker MFs.

The role of lipids in the central nervous system and their pathological implications in amyotrophic lateral sclerosis

Lipids play an important role in the central nervous system (CNS). They contribute to the structural integrity and physical characteristics of cell and organelle membranes, act as bioactive signalling molecules, and are utilised as fuel sources for mitochondrial metabolism. The intricate homeostatic mechanisms underpinning lipid handling and metabolism across two major CNS cell types; neurons and astrocytes, are integral for cellular health and maintenance. Here, we explore the various roles of lipids in these two cell types. Given that changes in lipid metabolism have been identified in a number of neurodegenerative diseases, we also discuss changes in lipid handling and utilisation in the context of amyotrophic lateral sclerosis (ALS), in order to identify key cellular processes affected by the disease, and inform future areas of research.

Effect of ergosterol on the interlamellar spacing of deuterated yeast phospholipid multilayers

Sterols regulate several physico-chemical properties of biological membranes that are considered to be linked to function. Ergosterol is the main sterol molecule found in the cell membranes of yeasts and other fungi. Like the cholesterol found in mammalian cells, ergosterol has been proposed to have an ordering and condensing effect on saturated phospholipid membranes. The effects of cholesterol have been investigated extensively and result in an increase in the membrane thickness and the lipid acyl chain order. Less information is available on the effects of ergosterol on phospholipid membranes. Neutron Diffraction (ND) was used to characterize the effect of ergosterol on lipid multilayers prepared with deuterated natural phospholipids extracted from the yeast Pichia pastoris. The data show that the effect of ergosterol on membranes prepared from the natural phospholipid extract rich in unsaturated acyl chains, differs from what has been observed previously in membranes rich in saturated phospholipids. In contrast to cholesterol in synthetic phospholipid membranes, the presence of ergosterol up to 30 mol % in yeast phospholipid membranes only slightly altered the multilayer structure. In particular, only a small decrease in the multilayer d-spacing was observed as function of increasing ergosterol concentrations. This result highlights the need for further investigation to elucidate the effects of ergosterol in biological lipid mixtures.

The host-defense peptide piscidin P1 reorganizes lipid domains in membranes and decreases activation energies in mechanosensitive ion channels

The host-defense peptide (HDP) piscidin 1 (P1), isolated from the mast cells of striped bass, has potent activities against bacteria, viruses, fungi, and cancer cells and can also modulate the activity of membrane receptors. Given its broad pharmacological potential, here we used several approaches to better understand its interactions with multicomponent bilayers representing models of bacterial (phosphatidylethanolamine (PE)/phosphatidylglycerol) and mammalian (phosphatidylcholine/cholesterol (PC/Chol)) membranes. Using solid-state NMR, we solved the structure of P1 bound to PC/Chol and compared it with that of P3, a less potent homolog. The comparison disclosed that although both peptides are interfacially bound and α-helical, they differ in bilayer orientations and depths of insertion, and these differences depend on bilayer composition. Although Chol is thought to make mammalian membranes less susceptible to HDP-mediated destabilization, we found that Chol does not affect the permeabilization effects of P1. X-ray diffraction experiments revealed that both piscidins produce a demixing effect in PC/Chol membranes by increasing the fraction of the Chol-depleted phase. Furthermore, P1 increased the temperature required for the lamellar-to-hexagonal phase transition in PE bilayers, suggesting that it imposes positive membrane curvature. Patch-clamp measurements on the inner Escherichia coli membrane showed that P1 and P3, at concentrations sufficient for antimicrobial activity, substantially decrease the activating tension for bacterial mechanosensitive channels. This indicated that piscidins can cause lipid redistribution and restructuring in the microenvironment near proteins. We conclude that the mechanism of piscidin's antimicrobial activity extends beyond simple membrane destabilization, helping to rationalize its broader spectrum of pharmacological effects.

On the role of helper lipids in lipid nanoparticle formulations of siRNA

Onpattro, the first RNAi-based therapeutic to receive FDA approval, is enabled by a lipid nanoparticle (LNP) system that facilitates siRNA delivery into the cytoplasm of target cells (hepatocytes) following intravenous (i.v.) administration. These LNP-siRNA systems consist of four lipid components (ionizable cationic lipid, distearolyphosphatidycholine or DSPC, cholesterol, and PEG-lipid) and siRNA. The ionizable cationic lipid has been optimised for RNA encapsulation and intracellular delivery, and the PEG-lipids have been engineered to regulate LNP size and transfection potency. The roles of the other "helper" lipids, DSPC and cholesterol, remain less clear. Here we show that in empty LNP systems that do not contain siRNA, DSPC-cholesterol resides in outer layers, whereas in loaded systems a portion of the DSPC-cholesterol is internalised together with siRNA. It is concluded that the presence of internalised helper lipid is vital to the stable encapsulation of siRNA in the LNP and thus to LNP-siRNA function.

A molecular dynamics study proposing the existence of statistical structural heterogeneity due to chain orientation in the POPC-cholesterol bilayer

We performed molecular dynamics simulations of a lipid bilayer consisting of POPC and cholesterol at temperatures from 283 to 308K and cholesterol concentrations from 0 to 50% mol/mol. The purpose of this study was to look for the existence of structural differences in the region delimited by these parameters and, in particular, in a region where coexistence of liquid disordered and liquid ordered phases has been proposed. Our interest in this range of concentration and temperature responds to the fact that polyene ionophore activity varies considerably along it. Two force fields, CHARMM36 and Slipids, were compared in order to determine the most suitable. Both force fields predict non-monotonic behaviors consistent with the existence of phase transitions. We found the presence of lateral structural heterogeneity, statistical in nature, in some of the bilayers occurring in this range of temperatures and sterol concentrations. This heterogeneity was produced by correlated ordering of the POPC tails and not due to cholesterol enrichment, and lasts for tens of nanoseconds. We relate these observations to the action of polyenes in these membranes.

Graphene Oxide increases mammalian spermatozoa fertilizing ability by extracting cholesterol from their membranes and promoting capacitation

Graphene Oxide (GO) is a widely used biomaterial with an amazing variety of applications in biology and medicine. Recently, we reported the ability of GO to improve the in vitro fertilization (IVF) outcomes in swine, a validated animal model with a high predictive value for human fertility. For that reason, here we characterized the mechanisms involved in this positive interaction by adopting an experimental approach combining biological methods (confocal microscopy analysis on single cell, flow cytometry on cell populations and co-incubation with epithelial oviductal cells), physical-chemical techniques (Differential Scanning Calorimetry and Thermogravimetric Analysis), and chemical methods (mass spectrometry and lipid measurement). As a result, we propose a model in which GO is able to extract cholesterol from the spermatozoa membrane without causing any detrimental effect. In this way, the cholesterol extraction promotes a change in membrane chemical-physical properties that could positively affect male gamete function, modulating sperm signalling function and increasing in this way the fertilizing potential, without losing the ability to physiologically interact with the female environment. In conclusion, these data seem to suggest new intriguing possibilities in engineering sperm membrane for improving assisted reproduction technologies outcomes, even in human medicine.

Lipid metabolism in inflammation-related diseases

There are thousands of lipid species existing in cells, which belong to eight different categories. Lipids are the essential building blocks of cells. Recent studies have started to unveil the important functions of lipids in regulating cell metabolism. However, we are still at a very early stage in fully understanding the physiological and pathological functions of lipids. The application of lipidomics for studying lipid metabolism can provide a direct readout of the cellular status and broadens our understanding of the mechanisms that underpin metabolic disease states. This review provides an introduction to lipid metabolism and its role in modulating homeostasis and immunity. We also describe representative applications of lipidomics for studying lipid metabolism in inflammation-related diseases.

Anomalous interaction of tri-acyl ester derivatives of uridine nucleoside with a l-α-dimyristoylphosphatidylcholine biomembrane model: a differential scanning calorimetry study

OBJECTIVES

Uridine was conjugated with fatty acids to improve the drug lipophilicity and the interaction with phospholipid bilayers.

METHODS

The esterification reaction using carbodiimides compounds as coupling agents and a nucleophilic catalyst allowed us to synthesize tri-acyl ester derivatives of uridine with fatty acids. Analysis of molecular interactions between these tri-acyl ester derivatives and l-α-dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles (MLV) - as a mammalian cell membrane model - have been performed by differential scanning calorimetry (DSC).

KEY FINDINGS

The DSC thermograms suggest that nucleoside and uridine triacetate softly interact with phospholipidic multilamellar vesicles which are predominantly located between the polar phase, whereas the tri-acyl ester derivatives with fatty acids (myristic and stearic acids) present a strongly interaction with the DMPC bilayer due to the nucleoside and aliphatic chains parts which are oriented towards the polar and lipophilic phases of the phospholipidic bilayer, respectively. However, the effects caused by the tri-myristoyl uridine and tri-stearoyl uridine are different.

CONCLUSIONS

We show how the structural changes of uridine modulate the calorimetric behaviour of DMPC shedding light on their affinity with the phospholipidic biomembrane model.

Molecular AFM imaging of Hsp70-1A association with dipalmitoyl phosphatidylserine reveals membrane blebbing in the presence of cholesterol

Hsp70-1A-the major stress-inducible member of the HSP70 chaperone family-is being implicated in cancer diseases with the development of resistances to standard therapies. In normal cells, the protein is purely cytosolic, but in a growing number of tumor cells, a significant fraction can be identified on to the cell surface. The anchoring mechanism is still under debate, as Hsp70-1A lacks conventional signaling sequences for translocation from the cytosol to exoplasmic leaflet of the plasma membrane and common membrane binding domains. Recent reports propose a lipid-mediated anchoring mechanism based on a specific interaction with charged, saturated lipids such as dipalmitoyl phosphatidylserine (DPPS). Here, we prepared planar supported lipid bilayers (SLBs) to visualize the association of Hsp70-1A directly and on the single molecule level by atomic force microscopy (AFM). The single molecule sensitivity of our approach allowed us to explore the low concentration range of 0.05 to 1.0 μg/ml of Hsp70-1A which was not studied before. We compared the binding of the protein to bilayers with 20% DPPS lipid content both in the absence and presence of cholesterol. Hsp70-1A inserted exclusively into DPPS domains and assembled in clusters with increasing protein density. A critical density was reached for incubation with 0.5 μg/ml (7 nM); at higher concentrations, membrane defects were observed that originated from cluster centers. In the presence of cholesterol, this critical concentration leads to the formation of membrane blebs, which burst at higher concentrations supporting a previously proposed non-classical pathway for the export of Hsp70-1A by tumor cells. In the discussion of our data, we attempt to link the lipid-mediated plasma membrane localization of Hsp70-1A to its potential involvement in the development of resistances to radiation and chemotherapy based on our own findings and the current literature.

Hybrid nanoparticles as a new technological approach to enhance the delivery of cholesterol into the brain

Restoration of the Chol homeostasis in the Central Nervous System (CNS) could be beneficial for the treatment of Huntington's Disease (HD), a progressive, fatal, adult-onset, neurodegenerative disorder. Unfortunately, Chol is unable to cross the blood-brain barrier (BBB), thus a novel strategy for a targeted delivery of Chol into the brain is highly desired. This article aims to investigate the production of hybrid nanoparticles composed by Chol and PLGA (MIX-NPs) modified with g7 ligand for BBB crossing. We described the impact of ratio between components (Chol and PLGA) and formulation process (nanoprecipitation or single emulsion process) on physico-chemical and structural characteristics, we tested MIX-NPs in vitro using primary hippocampal cell cultures evaluating possible toxicity, uptake, and the ability to influence excitatory synaptic receptors. Our results elucidated that both formulation processes produce MIX-NPs with a Chol content higher that 40%, meaning that Chol is a structural particle component and active compound at the same time. The formulation strategy impacted the architecture and reorganization of components leading to some differences in Chol availability between the two types of g7 MIX-NPs. Our results identified that both kinds of MIX-NPs are efficiently taken up by neurons, able to escape lysosomes and release Chol into the cells resulting in an efficient modification in expression of synaptic receptors that could be beneficial in HD.

Neuronal Lipid Metabolism: Multiple Pathways Driving Functional Outcomes in Health and Disease

Lipids are a fundamental class of organic molecules implicated in a wide range of biological processes related to their structural diversity, and based on this can be broadly classified into five categories; fatty acids, triacylglycerols (TAGs), phospholipids, sterol lipids and sphingolipids. Different lipid classes play major roles in neuronal cell populations; they can be used as energy substrates, act as building blocks for cellular structural machinery, serve as bioactive molecules, or a combination of each. In amyotrophic lateral sclerosis (ALS), dysfunctions in lipid metabolism and function have been identified as potential drivers of pathogenesis. In particular, aberrant lipid metabolism is proposed to underlie denervation of neuromuscular junctions, mitochondrial dysfunction, excitotoxicity, impaired neuronal transport, cytoskeletal defects, inflammation and reduced neurotransmitter release. Here we review current knowledge of the roles of lipid metabolism and function in the CNS and discuss how modulating these pathways may offer novel therapeutic options for treating ALS.

Structural insights on biologically relevant cationic membranes by ESR spectroscopy

Cationic bilayers have been used as models to study membrane fusion, templates for polymerization and deposition of materials, carriers of nucleic acids and hydrophobic drugs, microbicidal agents and vaccine adjuvants. The versatility of these membranes depends on their structure. Electron spin resonance (ESR) spectroscopy is a powerful technique that employs hydrophobic spin labels to probe membrane structure and packing. The focus of this review is the extensive structural characterization of cationic membranes prepared with dioctadecyldimethylammonium bromide or diC14-amidine to illustrate how ESR spectroscopy can provide important structural information on bilayer thermotropic behavior, gel and fluid phases, phase coexistence, presence of bilayer interdigitation, membrane fusion and interactions with other biologically relevant molecules.

Stereochemistry- and concentration-dependent effects of phosphatidylserine enrichment on platelet function

Platelets are small (1-2μm in diameter), circulating anuclear cell fragments with important roles in hemostasis and thrombosis that provide an excellent platform for studying the role of membrane components in cellular communication. Platelets use several forms of communication including exocytosis of three distinct granule populations, formation of bioactive lipid mediators, and shape change (allowing for adhesion). This work explores the role of stereochemistry and concentration of exogenous phosphatidylserine (PS) on platelet exocytosis and adhesion. PS, most commonly found in the phosphatidyl-l-serine (l-PS) form, is exposed on the outer leaflet of the cell membrane after the platelet is activated. Knowledge about the impact of exogenous phosphatidylserine on cell-to-cell communication is limited (particularly concentration and stereochemistry effects). This study found that platelets incubated in l-PS or phosphatidyl-d-serine (d-PS) are enriched to the same extent with their respective incubated PS. All levels of l-PS enrichment also showed an increase in platelet cholesterol, but only the 50μM d-PS incubation showed an increase in cholesterol. The uptake of d-PS induced the secretion of granules and manufactured platelet activating factor (PAF) in otherwise unstimulated platelets. The uptake of l-PS had a greater impact on platelet stimulation by decreasing both the amount of δ-granule secretion and the amount of PAF that was manufactured.

Physical Chemistry of Bile: Detailed Pathogenesis of Cholelithiasis

Despite the overwhelming prevalence of cholelithiasis, many health care professionals are not familiar with the basic pathophysiology of gallstone formation. This article provides an overview of the biochemical pathways related to bile, with a focus on the physical chemistry of bile. We describe the important factors in bile synthesis and secretion that affect the composition of bile and consequently its liquid state. Within this biochemical background lies the foundation for understanding the clinical and sonographic manifestation of cholelithiasis, including the pathophysiology of cholesterol crystallization, gallbladder sludge, and gallstones. There is a brief discussion of the clinical manifestations of inflammatory and obstructive cholestasis and the impact on bile metabolism and subsequently on liver function tests. Despite being the key modality in diagnosing cholelithiasis, ultrasound has a limited role in the characterization of stone composition.

Phase Behavior of Mixed Lipid Monolayers on Perfluorocarbon Nanoemulsions and its Effect on Acoustic Contrast

Lipid-stabilized nanoemulsions containing a volatile liquid perfluorocarbon core have been studied as ultrasound contrast agents owing to their ability to transform into high-contrast microbubbles when subjected to high intensity focused ultrasound (HIFU). However, while there have been several studies on the effect of acoustic parameters on contrast, the effect of the droplet's stabilizing shell has not been studied as extensively. Inspired by previous studies showing lateral phase separation in microbubbles and vesicles, nanodroplets were formulated with a perfluorohexane core and a shell composed of varying amounts of saturated (DPPC) phospholipids, unsaturated (DOPC) phospholipids, and cholesterol, which were fractionated to obtain nanodroplets of mean diameter 300-400 nm and were stable over one week. When the DOPC content was increased to 40 mol%, ultrasound contrast increased by about one order of magnitude over DPPC-only droplets. Based on fluorescence microscopy results of lateral lipid phase separation on the droplet surface, the various combinations of DPPC, DOPC, and cholesterol were assigned to three regimes on the ternary phase diagram: solid-liquid ordered (low contrast), liquid ordered-liquid disordered (medium contrast), and solid-liquid disordered (high contrast). These regimes were confirmed by TEM analysis of nanoscale droplets. Droplets containing mixed lipid monolayers were also found to produce a significantly greater yield than single-component droplets. The discovery of the dependence of acoustic response on lipid phase separation will help to understand the formulation, behavior, and vaporization mechanism of acoustically-responsive nanoemulsions.

Determination of the sputtering yield of cholesterol using Arn(+) and C60(+(+)) cluster ions

The sputtering yield of cholesterol films on silicon wafers is measured using Arn(+) and C60(+(+)) ions in popular energy (E) and cluster size (n) ranges. It is shown that the C60(+(+)) ions form a surface layer that stabilizes the film so that a well-behaved profile is obtained. On the other hand, the Arn(+) gas clusters leave the material very clean but, at room temperature, the layer readily restructures into molecular bilayers, so that, although a useful measure may be made of the sputtering yield, the profiles become much more complex. This restructuring does not occur at room temperature normally but results from the actions of the beams in the sputtering process for profiling in secondary ion mass spectrometry. Better profiles may be made by reducing the sample temperature to -100 °C. This is likely to be necessary for many lower molecular weight materials (below 1000 Da) to avoid the movement of molecules. Measurements for cholesterol films on 37 nm of amiodarone on silicon are even better behaved and show the same sputtering yields at room temperature as those films directly on silicon at -100 °C. The yields for both C60(+(+)) and Arn(+) fit the Universal Equation to a standard deviation of 11%.

Effect of Sterol Structure on the Physical Properties of 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine Membranes Determined Using (2)H Nuclear Magnetic Resonance

The effect of a series of phytosterols on lipid chain ordering in 1-palmitoyl((2)H31)-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d31) multibilayer vesicles was examined by (2)H NMR spectroscopy at 25 °C. These results, along with existing data for other sterols, indicate that the ordering power of sterols in POPC-d31 depends on subtle aspects of sterol structure. Cholesterol, 7-dehydrocholesterol (7-DHC), campesterol, β-sitosterol, ergosterol, brassicasterol, and stigmasterol all increase the lipid chain order as sterol concentration is increased. However, saturation of the ordering occurs at different sterol concentrations for ergosterol (as previously reported), brassicasterol, β-sitosterol, and stigmasterol. Here our interest lies in finding which part of the sterol structure is responsible for the observed saturation of the palmitoyl chain order as a function of sterol concentration. In particular, we propose that the saturation of the ordering of POPC-d31/brassicasterol and POPC-d31/stigmasterol membranes at quite low sterol concentrations is due to the presence of a double bond at C22. We also discuss how the structural differences between the sterols affect their ability to intercalate between the POPC acyl chains. Furthermore, the effective solubility of sterols in POPC is discussed in relation to the dependence of maximum POPC-d31 chain order vs sterol concentration.

EXPLOITING THE VERSATILITY OF CHOLESTEROL IN NANOPARTICLES FORMULATION

The biocompatibility of polymers, lipids and surfactants used to formulate is crucial for the safe and sustainable development of nanocarriers (nanoparticles, liposomes, micelles, and other nanocarriers). In this study, Cholesterol (Chol), a typical biocompatible component of liposomal systems, was formulated in Chol-based solid nanoparticles (NPs) stabilized by the action of surfactant and without the help of any other formulative component. Parameters as type (Solutol HS 15, cholic acid sodium salt, poly vinyl alcohol and Pluronic-F68), concentration (0.2; 0.5 and 1% w/v) of surfactant and working temperature (r.t. and 45°C) were optimized and all samples characterized in terms of size, zeta potential, composition, thermal behavior and structure. Results demonstrated that only Pluronic-F68 (0.5% w/v) favors the organization of Chol chains in structured NPs with mean diameter less than 400nm. Moreover, we demonstrated the pivotal role of working temperature on surfactant aggregation state/architecture/stability of Chol-based nanoparticles. At room temperature, Pluronic-F68 exists in solution as individual coils. In this condition, nanoprecipitation of Chol formed the less stable NPs with a 14±3% (w/w) of Pluronic-F68 prevalently on surface (NP-Chol/0.5). On the contrary, working near the critical micelle temperature (CMT) of surfactant (45°C), Chol precipitates with Pluronic-F68 (9±5% w/w) in a compact stable matricial structure (NP-Chol/0.5-45). In vitro studies highlight the low toxicity and the affinity of NP-Chol/0.5-45 for neuronal cells suggesting their potential applicability in pathologies with a demonstrated alteration of neuronal plasticity and synaptic communication (i.e. Huntington's disease).

Sterol-Phospholipid Hybrids at the Air/Water Interface: Studies on Properties and Interactions with Parent Lipid Molecules

The two synthetic sterol-phospholipid hybrids DCholPC and PCholPC were investigated in monolayers at the air/water interface. Study was based on π-A isotherm analysis complemented with application of grazing incidence X-ray diffraction. It was found that both compounds are capable of forming stable, highly condensed monolayers of a surface characteristics typical for sterols. GIXD studies show that the crystallographic area for DCholPC monolayer is very similar to that found for cholesterol (37.1 vs 38.0 Å(2)), while for PCholPC (28.8 Å(2)) it is significantly smaller as compared to area for the mixed Chol/DPPC 2/1 monolayer (33.4 Å(2)). In our study the problem of interactions between investigated sterol-phospholipid hybrids and natural membrane lipid components was for the first time analyzed in planar lipid systems. Studies on mixed monolayers showed that both hybrids, similarly to cholesterol, reveal a condensing effect toward DPPC acyl chains; however, DCholPC having two steroid moieties in the molecule was found to be more efficient. On the other hand, the sterol moiety and the hydrocarbon chain of PCholPC molecule are packed in the 2D crystalline phase extremely tight. Our studies showed that the investigated compounds can be applied as biocompatible components of stable liposomes.

Niosomes as Drug Nanovectors: Multiscale pH-Dependent Structural Response

The use of nanocarriers, which respond to different stimuli controlling their physicochemical properties and biological responsivness, shows a growing interest in pharmaceutical science. The stimuli are activated by targeting tissues and biological compartments, e.g., pH modification, temperature, redox condition, enzymatic activity, or can be physically applied, e.g., a magnetic field and ultrasound. pH modification represents the easiest method of passive targeting, which is actually used to accumulate nanocarriers in cells and tissues. The aim of this paper was to physicochemically characterize pH-sensitive niosomes using different experimental conditions and demonstrate the effect of surfactant composition on the supramolecular structure of niosomes. In this attempt, niosomes, made from commercial (Tween21) and synthetic surfactants (Tween20 derivatives), were physicochemically characterized by using different techniques, e.g., transmission electron microscopy, Raman spectroscopy, and small-angle X-ray scattering. The changes of niosome structure at different pHs depend on surfactants, which can affect the supramolecular structure of colloidal nanocarriers and their potential use both in vitro and in vivo. At pH 7.4, the shape and structure of niosomes have been maintained; however, niosomes show some differences in terms of bilayer thicknesses, water penetration, membrane coupling, and cholesterol dispersion. The acid pH (5.5) can increase the bilayer fluidity, and affect the cholesterol depletion. In fact, Tween21 niosomes form large vesicles with lower curvature radius at acid pH; while Tween20-derivative niosomes increase the intrachain mobility within a more interchain correlated membrane. These results demonstrate that the use of multiple physicochemical procedures provides more information about supramolecular structures of niosomes and improves the opportunity to deeply investigate the effect of stimuli responsiveness on the niosome structure.

Filter-extruded liposomes revisited: a study into size distributions and morphologies in relation to lipid-composition and process parameters

Filter-extrusion is a widely used technique for down-sizing of phospholipid vesicles. In order to gain a detailed insight into size and size distributions of filter-extruded vesicles composed of egg phosphatidyl-choline (with varying fractions of cholesterol)--in relation to extrusion-parameters (pore-size, number of filter passages, and flow-rate), flow field-flow fractionation in conjunction with multi-angle laser light scattering (AF4-MALLS, Wyatt Technology Corp., Santa Barbara, CA) was employed. Liposome size-distributions determined by AF4-MALLS were compared with those of dynamic light scattering and correlated with cryo-transmission electron microscopy and (31)P-NMR-analysis of lamellarity. Both the mean size of liposome and the width of size distribution were found to decrease with sequential extrusion through smaller pore size filters, starting at a size range of ≈70-415 nm upon repeated extrusion through 400 nm pore-filters, eventually ending with a size range from ≈30 to 85 nm upon extrusion through 30 nm pore size filters. While for small pores sizes (50 nm), increased flow rates resulted in smaller vesicles, no significant influence of flow rate on mean vesicle size was seen with larger pores. Cholesterol at increasing mol fractions up to 0.45 yielded bigger vesicles (at identical process conditions). For a cholesterol mol fraction of 0.5 in combination with small filter pore size, a bimodal size distribution was seen indicating cholesterol micro-crystallites. Finally, a protocol is suggested to prepare large (∼ 300 nm) liposomes with rather narrow size distribution, based on the filter extrusion at defined flow-rates in combination with freeze-/thaw-cycling and bench-top centrifugation.

Comment on "Cholesterol solubility limit in lipid membranes probed by small angle neutron scattering and MD simulations" by S. Garg et al., Soft Matter, 2014, 10, 9313

In a recent article, Garg et al. used neutron scattering techniques to determine the limiting amount of cholesterol which vesicles of either POPS or POPC can accommodate. This amount was called "the cholesterol solubility limit". In light of extensive literature on cholesterol phase separation in phospholipid bilayers, the way in which "solubility limit" is defined in this article and the conclusions derived are misleading and require some clarification.

WITHDRAWN: Modulation of the kinetics of 3β-hydroxy-5-oxo-5,6-secocholestan-6-al/phosphatidylethanolamine Schiff base formation by cholesterol and cholesterol crystallization

The Publisher regrets that this article is an accidental duplication of an article that has already been published, http://dx.doi.org/10.1016/j.chemphyslip.2015.01.001. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.