Lipid peroxides promote large rafts: effects of excitation of probes in fluorescence microscopy and electrochemical reactions during vesicle formation

How Membrane Phospholipids Containing Long-Chain Polyunsaturated Fatty Acids and Their Oxidation Products Orchestrate Lipid Raft Dynamics to Control Inflammation

Long-chain PUFA (LC-PUFA) influence varying aspects of inflammation. One mechanism by which they regulate inflammation is by controlling the size and molecular composition of lipid rafts. Lipid rafts are sphingolipid/cholesterol-enriched plasma membrane microdomains that compartmentalize signaling proteins and thereby control downstream inflammatory gene expression and cytokine production. This review summarizes developments in our understanding of how LC-PUFA acyl chains of phospholipids, in addition to oxidized derivatives of LC-PUFAs such as oxidized 1-palmitoyl-2-arachidonyl-phosphatidylcholine (oxPAPC), manipulate formation of lipid rafts and thereby inflammation. We reviewed the literature, largely from the past 2 decades, on the impact of LC-PUFA acyl chains and oxidized products of LC-PUFAs on lipid raft biophysical organization of myeloid and lymphoid cells. The majority of the studies are based on rodent or cellular experiments with supporting mechanistic studies using biomimetic membranes and molecular dynamic simulations. These studies have focused largely on the LC-PUFA docosahexaenoic acid, with some studies addressing eicosapentaenoic acid. A few studies have investigated the role of oxidized phospholipids on rafts. The biophysical literature suggests a model in which n-3 LC-PUFAs, in addition to oxPAPC, localize predominately to nonraft regions and impart a disordering effect in this environment. Rafts become larger because of the ensuing increase in the difference in order between raft and nonrafts. Biochemical studies suggest that some n-3 LC-PUFAs can be found within rafts. This deviation from homeostasis is a potential trigger for controlling aspects of innate and adaptive immunity. Overall, select LC-PUFA acyl chains and oxidized acyl chains of phospholipids control lipid raft dynamics and downstream inflammation. Gaps in knowledge remain, particularly on underlying molecular mechanisms by which plasma membrane receptor organization is controlled in response to oxidized LC-PUFA acyl chains of membrane phospholipids. Validation in humans is also an area for future study.

Light-controllable cell-membrane disturbance for intracellular delivery

Highly polar and charged molecules, such as oligonucleotides, face significant barriers in crossing the cell membrane to access the cytoplasm. To address this problem, we developed a light-triggered twistable tetraphenylethene (TPE) derivative, TPE-C-N, to facilitate the intracellular delivery of charged molecules through an endocytosis-independent pathway. The central double bond of TPE in TPE-C-N is planar in the ground state but becomes twisted in the excited state. Under light irradiation, this planar-to-twisted structural change induces continuous cell membrane disturbances. Such disturbance does not lead to permanent damage to the cell membrane. TPE-C-N significantly enhanced the intracellular delivery of negatively charged molecules under light irradiation when endocytosis was inhibited through low-temperature treatment, confirming the endocytosis-independent nature of this delivery method. We have successfully demonstrated that the TPE-C-N-mediated light-controllable method can efficiently promote the intracellular delivery of charged molecules, such as peptides and oligonucleotides, with molecular weights ranging from 1000 to 5000 Da.

Lipid Peroxidation Drives Liquid-Liquid Phase Separation and Disrupts Raft Protein Partitioning in Biological Membranes

The peroxidation of membrane lipids by free radicals contributes to aging, numerous diseases, and ferroptosis, an iron-dependent form of cell death. Peroxidation changes the structure and physicochemical properties of lipids, leading to bilayer thinning, altered fluidity, and increased permeability of membranes in model systems. Whether and how lipid peroxidation impacts the lateral organization of proteins and lipids in biological membranes, however, remains poorly understood. Here, we employ cell-derived giant plasma membrane vesicles (GPMVs) as a model to investigate the impact of lipid peroxidation on ordered membrane domains, often termed membrane rafts. We show that lipid peroxidation induced by the Fenton reaction dramatically enhances the phase separation propensity of GPMVs into coexisting liquid-ordered (Lo) and liquid-disordered (Ld) domains and increases the relative abundance of the disordered phase. Peroxidation also leads to preferential accumulation of peroxidized lipids and 4-hydroxynonenal (4-HNE) adducts in the disordered phase, decreased lipid packing in both Lo and Ld domains, and translocation of multiple classes of raft proteins out of ordered domains. These findings indicate that the peroxidation of plasma membrane lipids disturbs many aspects of membrane rafts, including their stability, abundance, packing, and protein and lipid composition. We propose that these disruptions contribute to the pathological consequences of lipid peroxidation during aging and disease and thus serve as potential targets for therapeutic intervention.

Structure and function of the membrane microdomains in osteoclasts

The cell membrane structure is closely related to the occurrence and progression of many metabolic bone diseases observed in the clinic and is an important target to the development of therapeutic strategies for these diseases. Strong experimental evidence supports the existence of membrane microdomains in osteoclasts (OCs). However, the potential membrane microdomains and the crucial mechanisms underlying their roles in OCs have not been fully characterized. Membrane microdomain components, such as scaffolding proteins and the actin cytoskeleton, as well as the roles of individual membrane proteins, need to be elucidated. In this review, we discuss the compositions and critical functions of membrane microdomains that determine the biological behavior of OCs through the three main stages of the OC life cycle.

Toxic Effects of Penetrating Cations

As mitochondria are negatively charged organelles, penetrating cations are used as parts of chimeric molecules to deliver specific compounds into mitochondria. In other words, they are used as electrophilic carriers for such chemical moieties as antioxidants, dyes, etc., to transfer them inside mitochondria. However, unmodified penetrating cations affect different aspects of cellular physiology as well. In this review, we have attempted to summarise the data about the side effects of commonly used natural (e.g., berberine) and artificial (e.g., tetraphenylphosphonium, rhodamine, methylene blue) penetrating cations on cellular physiology. For instance, it was shown that such types of molecules can (1) facilitate proton transport across membranes; (2) react with redox groups of the respiratory chain; (3) induce DNA damage; (4) interfere with pleiotropic drug resistance; (5) disturb membrane integrity; and (6) inhibit enzymes. Also, the products of the biodegradation of penetrating cations can be toxic. As penetrating cations accumulate in mitochondria, their toxicity is mostly due to mitochondrial damage. Mitochondria from certain types of cancer cells appear to be especially sensitive to penetrating cations. Here, we discuss the molecular mechanisms of the toxic effects and the anti-cancer activity of penetrating cations.

Lipid peroxidation drives liquid-liquid phase separation and disrupts raft protein partitioning in biological membranes

The peroxidation of membrane lipids by free radicals contributes to aging, numerous diseases, and ferroptosis, an iron-dependent form of cell death. Peroxidation changes the structure, conformation and physicochemical properties of lipids, leading to major membrane alterations including bilayer thinning, altered fluidity, and increased permeability. Whether and how lipid peroxidation impacts the lateral organization of proteins and lipids in biological membranes, however, remains poorly understood. Here, we employ cell-derived giant plasma membrane vesicles (GPMVs) as a model to investigate the impact of lipid peroxidation on ordered membrane domains, often termed membrane rafts. We show that lipid peroxidation induced by the Fenton reaction dramatically enhances phase separation propensity of GPMVs into co-existing liquid ordered (raft) and liquid disordered (non-raft) domains and increases the relative abundance of the disordered, non-raft phase. Peroxidation also leads to preferential accumulation of peroxidized lipids and 4-hydroxynonenal (4-HNE) adducts in the disordered phase, decreased lipid packing in both raft and non-raft domains, and translocation of multiple classes of proteins out of rafts. These findings indicate that peroxidation of plasma membrane lipids disturbs many aspects of membrane rafts, including their stability, abundance, packing, and protein and lipid composition. We propose that these disruptions contribute to the pathological consequences of lipid peroxidation during aging and disease, and thus serve as potential targets for therapeutic intervention.

OxPAPC stabilizes liquid-ordered domains in biomimetic membranes

Long-chain polyunsaturated fatty acids (PUFAs) are prone to nonenzymatic oxidation in response to differing environmental stressors and endogenous cellular sources. There is increasing evidence that phospholipids containing oxidized PUFA acyl chains control the inflammatory response. However, the underlying mechanism(s) of action by which oxidized PUFAs exert their functional effects remain unclear. Herein, we tested the hypothesis that replacement of 1-palmitoyl-2-arachidonyl-phosphatidylcholine (PAPC) with oxidized 1-palmitoyl-2-arachidonyl-phosphatidylcholine (oxPAPC) regulates membrane architecture. Specifically, with solid-state 2H NMR of biomimetic membranes, we investigated how substituting oxPAPC for PAPC modulates the molecular organization of liquid-ordered (Lo) domains. 2H NMR spectra for bilayer mixtures of 1,2-dipalmitoylphosphatidylcholine-d62 (an analog of DPPC deuterated throughout sn-1 and -2 chains) and cholesterol to which PAPC or oxPAPC was added revealed that replacing PAPC with oxPAPC disrupted molecular organization, indicating that oxPAPC does not mix favorably in a tightly packed Lo phase. Furthermore, unlike PAPC, adding oxPAPC stabilized 1,2-dipalmitoylphosphatidylcholine-d6-rich/cholesterol-rich Lo domains formed in mixtures with 1,2-dioleoylphosphatidylcholine while decreasing the molecular order within 1,2-dioleoylphosphatidylcholine-rich liquid-disordered regions of the membrane. Collectively, these results suggest a mechanism in which oxPAPC stabilizes Lo domains-by disordering the surrounding liquid-disordered region. Changes in the structure, and thereby functionality, of Lo domains may underly regulation of plasma membrane-based inflammatory signaling by oxPAPC.

Singlet Oxygen Flux, Associated Lipid Photooxidation, and Membrane Expansion Dynamics Visualized on Giant Unilamellar Vesicles

The physical properties of lipid membranes depend on their lipid composition. Photosensitized singlet oxygen (¹O₂) provides a handle to spatiotemporally control the generation of lipid hydroperoxides via the ene reaction, enabling fundamental studies on membrane dynamics in response to chemical composition changes. Critical to relating the physical properties of the lipid membrane to hydroperoxide formation is the availability of a sensitive reporter to quantify the arrival of ¹O₂. Here, we show that a fluorogenic α-tocopherol analogue, H₄BPMHC, undergoes a >360-fold emission intensity enhancement in liposomes following a reaction with ¹O₂. Rapid quenching of ¹O₂ by the probe (k_q = 4.9 × 10⁸ M^-1 s^-1) ensures zero-order kinetics of probe consumption. The remarkable intensity enhancement of H₄BPMHC upon ¹O₂ trapping, its linear temporal behavior, and its protective role in outcompeting membrane damage provide a sensitive and reliable method to quantify the ¹O₂ flux on lipid membranes. Armed with this probe, fluorescence microscopy studies were devised to enable (i) monitoring the flux of photosensitized ¹O₂ into giant unilamellar vesicles (GUVs), (ii) establishing the onset of the ene reaction with the double bonds of monounsaturated lipids, and (iii) visualizing the ensuing collective membrane expansion dynamics associated with molecular changes in the lipid structure upon hydroperoxide formation. A correlation was observed between the time for antioxidant H₄BPMHC consumption by ¹O₂ and the onset of membrane fluctuations and surface expansion. Together, our imaging studies with H₄BPMHC in GUVs provide a methodology to explore the intimate relationship between photosensitizer activity, chemical insult, membrane morphology, and its collective dynamics.

Phase Separation in Liposomes Determined by Ergosterol and Classified Using Machine Learning

Recent studies indicated that ergosterol (Erg) helps form strongly ordered lipid domains in membranes that depend on their chemical characters. However, direct evidence of concentration-dependent interaction of Erg with lipid membranes has not been reported. We studied the Erg concentration-dependent changes in the phase behaviors of membranes using cell-sized liposomes containing 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). We observed the concentration range of phase separation in ternary membranes was significantly wider when Erg rather than cholesterol (Chol) was used as the sterol component. We used machine learning for the first time to analyze microscopic images of cell-sized liposomes and identify phase-separated structures. The automated method was successful in identifying homogeneous membranes but performance remained data-limited for the identification of phase separation domains characterized by more complex features.

Bending Rigidity, Capacitance, and Shear Viscosity of Giant Vesicle Membranes Prepared by Spontaneous Swelling, Electroformation, Gel-Assisted, and Phase Transfer Methods: A Comparative Study

Closed lipid bilayers in the form of giant unilamellar vesicles (GUVs) are commonly used membrane models. Various methods have been developed to prepare GUVs, however it is unknown if all approaches yield membranes with the same elastic, electric, and rheological properties. Here, we combine flickering spectroscopy and electrodefomation of GUVs to measure, at identical conditions, membrane capacitance, bending rigidity and shear surface viscosity of palmitoyloleoylphosphatidylcholine (POPC) membranes formed by several commonly used preparation methods: thin film hydration (spontaneous swelling), electroformation, gel-assisted swelling using poly(vinyl alcohol) (PVA) or agarose, and phase-transfer. We find relatively similar bending rigidity value across all the methods except for the agarose hydration method. In addition, the capacitance values are similar except for vesicles prepared via PVA gel hydration. Intriguingly, membranes prepared by the gel-assisted and phase-transfer methods exhibit much higher shear viscosity compared to electroformation and spontaneous swelling, likely due to remnants of polymers (PVA and agarose) and oils (hexadecane and mineral) in the lipid bilayer structure.

Oxidation of Membrane Lipids Alters the Activity of the Human Serotonin 1A Receptor

Lipid oxidation has significant effects on lipid bilayer properties; these effects can be expected to extend to interactions between the lipid bilayer and integral membrane proteins. Given that G protein-coupled receptor (GPCR) activity is known to depend on the properties of the surrounding lipid bilayer, these proteins represent an intriguing class of molecules in which the impact of lipid oxidation on protein behavior is studied. Here, we study the effects of lipid oxidation on the human serotonin 1A receptor (5-HT_1AR). Giant unilamellar vesicles (GUVs) containing integral 5-HT_1AR were fabricated by the hydrogel swelling method; these GUVs contained polyunsaturated 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (PLinPC) and its oxidation product 1-palmitoyl-2-(9'-oxo-nonanoyl)-sn-glycero-3-phosphocholine (PoxnoPC) at various ratios. 5-HT_1AR-integrated GUVs were also fabricated from lipid mixtures that had been oxidized by extended exposure to the atmosphere. Both types of vesicles were used to evaluate 5-HT_1AR activity using an assay to quantify GDP-GTP exchange by the coupled G protein α subunit. Results indicated that 5-HT_1AR activity increases significantly in bilayers containing oxidized lipids. This work is an important step in understanding how hyperbaric oxidation can change plasma membrane properties and lead to physiological dysfunction.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.

Hydrophobic Mismatch Controls the Mode of Membrane-Mediated Interactions of Transmembrane Peptides

Various cellular processes require the concerted cooperative action of proteins. The possibility for such synchronization implies the occurrence of specific long-range interactions between the involved protein participants. Bilayer lipid membranes can mediate protein-protein interactions via relatively long-range elastic deformations induced by the incorporated proteins. We considered the interactions between transmembrane peptides mediated by elastic deformations using the framework of the theory of elasticity of lipid membranes. An effective peptide shape was assumed to be cylindrical, hourglass-like, or barrel-like. The interaction potentials were obtained for membranes of different thicknesses and elastic rigidities. Cylindrically shaped peptides manifest almost neutral average interactions-they attract each other at short distances and repel at large ones, independently of membrane thickness or rigidity. The hourglass-like peptides repel each other in thin bilayers and strongly attract each other in thicker bilayers. On the contrary, the barrel-like peptides repel each other in thick bilayers and attract each other in thinner membranes. These results potentially provide possible mechanisms of control for the mode of protein-protein interactions in membrane domains with different bilayer thicknesses.

MEMBRANE PROTEIN STRUCTURES AND INTERACTIONS FROM COVALENT LABELING COUPLED WITH MASS SPECTROMETRY

Membrane proteins are incredibly important biomolecules because they mediate interactions between a cell's external and internal environment. Obtaining information about membrane protein structure and interactions is thus important for understanding these essential biomolecules. Compared with the analyses of water-soluble proteins, the structural analysis of membrane proteins is more challenging owing to their unique chemical properties and the presence of lipid components that are necessary to solubilize them. The combination of covalent labeling (CL) and mass spectrometry (MS) has recently been applied with great success to study membrane protein structure and interactions. These studies have demonstrated the many advantages that CL-MS methods have over other traditional biophysical techniques. In this review, we discuss both amino acid-specific and non-specific labeling approaches and the special considerations needed to address the unique challenges associated with interrogating membrane proteins. This review highlights the aspects of this approach that require special care to be applied correctly and provides a comprehensive review of the membrane protein systems that have been studied by CL-MS. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Assessing Photosensitized Membrane Damage: Available Tools and Comprehensive Mechanisms

Lipids are important targets of the photosensitized oxidation reactions, forming important signaling molecules, disorganizing and permeabilizing membranes, and consequently inducing a variety of biological responses. Although the initial steps of the photosensitized oxidative damage in lipids are known to occur by both Type I and Type II mechanisms, the progression of the peroxidation reaction, which leads to important end-point biological responses, is poorly known. There are many experimental tools used to study the products of lipid oxidation, but neither the methods nor their resulting observations were critically compared. In this article, we will review the tools most frequently used and the key concepts raised by them in order to rationalize a comprehensive model for the initiation and the progression steps of the photoinduced lipid oxidation.

Giant Unilamellar Vesicle Electroformation: What to Use, What to Avoid, and How to Quantify the Results

Since its inception more than thirty years ago, electroformation has become the most commonly used method for growing giant unilamellar vesicles (GUVs). Although the method seems quite straightforward at first, researchers must consider the interplay of a large number of parameters, different lipid compositions, and internal solutions in order to avoid artifactual results or reproducibility problems. These issues motivated us to write a short review of the most recent methodological developments and possible pitfalls. Additionally, since traditional manual analysis can lead to biased results, we have included a discussion on methods for automatic analysis of GUVs. Finally, we discuss possible improvements in the preparation of GUVs containing high cholesterol contents in order to avoid the formation of artifactual cholesterol crystals. We intend this review to be a reference for those trying to decide what parameters to use as well as an overview providing insight into problems not yet addressed or solved.

Broad lipid phase transitions in mammalian cell membranes measured by Laurdan fluorescence spectroscopy

Employing fluorescence spectroscopy and the membrane-embedded dye Laurdan we experimentally show that linear changes of cell membrane order in the physiological temperature regime are part of broad order-disorder-phase transitions which extend over a much broader temperature range. Even though these extreme temperatures are usually not object of live science research due to failure of cellular functions, our findings help to understand and predict cell membrane properties under physiological conditions as they explain the underlying physics of a broad order-disorder phase transition. Therefore, we analyzed the membranes of various cell lines, red blood cell ghosts and lipid vesicles by spectral decomposition in a custom-made setup in a temperature range from -40 °C to +90 °C. While the generalized polarization as a measure for membrane order of artificial lipid membranes like phosphatidylcholine show sharp transitions as known from calorimetry measurements, living cells in a physiological temperature range do only show linear changes. However, extending the temperature range shows the existence of broad transitions and their sensitivity to cholesterol content, pH and anaesthetic. Moreover, adaptation to culture conditions like decreased temperature and morphological changes like detachment of adherent cells or dendrite growth are accompanied by changes in membrane order as well. The observed changes of the generalized polarization are equivalent to temperature changes dT in the range of +12 K < dT < -6 K.

Melatonin: Regulation of Biomolecular Condensates in Neurodegenerative Disorders

Biomolecular condensates are membraneless organelles (MLOs) that form dynamic, chemically distinct subcellular compartments organizing macromolecules such as proteins, RNA, and DNA in unicellular prokaryotic bacteria and complex eukaryotic cells. Separated from surrounding environments, MLOs in the nucleoplasm, cytoplasm, and mitochondria assemble by liquid-liquid phase separation (LLPS) into transient, non-static, liquid-like droplets that regulate essential molecular functions. LLPS is primarily controlled by post-translational modifications (PTMs) that fine-tune the balance between attractive and repulsive charge states and/or binding motifs of proteins. Aberrant phase separation due to dysregulated membrane lipid rafts and/or PTMs, as well as the absence of adequate hydrotropic small molecules such as ATP, or the presence of specific RNA proteins can cause pathological protein aggregation in neurodegenerative disorders. Melatonin may exert a dominant influence over phase separation in biomolecular condensates by optimizing membrane and MLO interdependent reactions through stabilizing lipid raft domains, reducing line tension, and maintaining negative membrane curvature and fluidity. As a potent antioxidant, melatonin protects cardiolipin and other membrane lipids from peroxidation cascades, supporting protein trafficking, signaling, ion channel activities, and ATPase functionality during condensate coacervation or dissolution. Melatonin may even control condensate LLPS through PTM and balance mRNA- and RNA-binding protein composition by regulating N6-methyladenosine (m6A) modifications. There is currently a lack of pharmaceuticals targeting neurodegenerative disorders via the regulation of phase separation. The potential of melatonin in the modulation of biomolecular condensate in the attenuation of aberrant condensate aggregation in neurodegenerative disorders is discussed in this review.

ESCRT-III induces phase separation in model membranes prior to budding and causes invagination of the liquid-ordered phase

Membrane fission triggered by the endosomal sorting complex required for transport (ESCRT) is an important process observed in several pathogenic and non-pathogenic cellular events. From a synthetic-biology viewpoint, ESCRT proteins represent an interesting machinery for the construction of cell mimetic sub-compartments produced by fission. Since their discovery, the studies on ESCRT-III-mediated action, have mainly focused on protein dynamics, ignoring the role of lipid organization and membrane phase state. Recently, it has been suggested that membrane buds formed by the action of ESCRT-III are generated from transient microdomains in endosomal membranes. However, the interplay between membrane domain formation and ESCRT remodeling pathways has not been investigated. Here, giant unilamellar vesicles made of ternary lipid mixtures, either homogeneous in phase or exhibiting liquid-ordered/liquid-disordered phase coexistence, were employed as a model membrane system. These vesicles were incubated with purified recombinant ESCRT-III proteins from the parasite Entamoeba histolytica. In homogeneous membranes, we observe that EhVps32 can trigger domain formation while EhVps20 preferentially co-localizes in the liquid disordered phase. The addition of EhVps24 appears to induce the formation of intraluminal vesicles produced from the liquid-ordered phase. In phase separated membranes, the intraluminal vesicles are also generated from the liquid-ordered phase and presumably emerge from the phase boundary region. Our findings reinforce the hypothesis that ESCRT-mediated remodeling depends on the membrane phase state. Furthermore, the obtained results point to a potential synthetic biology approach for establishing eukaryotic mimics of artificial cells with microcompartments of specific membrane composition, which can also differ from that of the mother vesicle.

Molecular organization in hydroperoxidized POPC bilayers

Lipid hydroperoxides are the primary reaction products of lipid oxidation, a natural outcome of life under oxygen. While playing a major role in cell metabolism, the microscopic origins of the effects of lipid hydroperoxidation on biomembranes remain elusive. Here we probe the polar structure of partially to fully hydroperoxidized bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) by a combination of environment-sensitive fluorescent probes and coarse-grained Martini numerical simulations. We find that the inserted organic hydroperoxide group -OOH migrates preferentially to the surface for bilayers with small fractions of hydroperoxidized lipids, but populates also significantly the bilayer interior for larger fractions. Our findings suggest that by modifying the intimate polarity of biomembranes, lipid peroxidation will have a significant impact on the activity of transmembrane proteins and on the bio-medical efficiency of membrane active molecules such as cell-penetrating and antimicrobial peptides.

Tubulation of Supported Lipid Bilayer Membranes Induced by Photosensitized Lipid Oxidation

We show that photosensitized phospholipid oxidation, initiated by the lipid-conjugated fluorophore TopFluor-PC, causes defects, namely, membrane tubes and vesicle-like structures, in supported lipid bilayers (SLBs). Lipid oxidation is detrimental to the integrity of the lipid molecules; when oxidized, they undergo a conformational expansion, which causes membrane tubes to protrude from the SLB. Lipid oxidation is verified by FT-IR spectroscopy, and area expansion is observed in Langmuir trough experiments. Upon growing to a critical length, the membrane tubes arising from SLBs rapidly undergo transition to vesicle-like structures. We find a correlation between the maximum tube length and the diameter of the resulting vesicle, suggesting the conservation of the surface area between these features. We use geometric modeling and the measured tube length and vesicle radius to calculate the tube radius; our calculated mean tube diameter of 243 nm is comparable to other groups' experimental findings. In the presence of fluid flow, membrane tubes can be extended to tens to hundreds of microns in length. SLBs composed of saturated lipids resist light-induced tubulation, and the inclusion of the lipophilic antioxidant α-tocopherol attenuates the tubulation process and increases the light intensity threshold for tubulation.

Unveiling a Hidden Event in Fluorescence Correlative Microscopy by AFM Nanomechanical Analysis

Fluorescent imaging combined with atomic force microscopy (AFM), namely AFM-fluorescence correlative microscopy, is a popular technology in life science. However, the influence of involved fluorophores on obtained mechanical information is normally underestimated, and such subtle changes are still challenging to detect. Herein, we combined AFM with laser light excitation to perform a mechanical quantitative analysis of a model membrane system labeled with a commonly used fluorophore. Mechanical quantification was additionally validated by finite element simulations. Upon staining, we noticed fluorophores forming a diffuse weakly organized overlayer on phospholipid supported membrane, easily detected by AFM mechanics. The laser was found to cause a degradation of mechanical stability of the membrane synergically with presence of fluorophore. In particular, a 30 min laser irradiation, with intensity similar to that in typical confocal scanning microscopy experiment, was found to result in a ∼40% decrease in the breakthrough force of the stained phospholipid bilayer along with a ∼30% reduction in its apparent elastic modulus. The findings highlight the significance of analytical power provided by AFM, which will allow us to “see” the “unseen” in correlative microscopy, as well as the necessity to consider photothermal effects when using fluorescent dyes to investigate, for example, the deformability and permeability of phospholipid membranes.

Cholesterol-induced microdomain formation in lipid bilayer membranes consisting of completely miscible lipids

Recently, we reported that a ternary lipid bilayer comprising phosphatidylethanolamine (PE), phosphatidylcholine (PC), which were both derived from chicken egg, and cholesterol (Chol) generates microdomains that function as specific fusion sites for proteoliposomes. Chol-induced microdomain formation in a completely miscible lipid bilayer is an exceptional phenomenon. Numerous studies have elucidated the formation of domains in liquid ordered (L_o) and liquid disordered (L_d) phases of ternary bilayers, which comprise two partially miscible lipids and Chol. Herein, we investigated the composition and mechanism of formation of these unique microdomains in supported lipid bilayers (SLBs) using a fluorescence microscope and an atomic force microscope (AFM). We prepared ternary SLBs using egg-derived PC (eggPC), Chol and three different types of PE: egg-derived PE, 1-palmitoyl-2-oleoyl-PE, and 1,2-didocosahexaenoyl-PE (diDHPE). Fluorescence microscopy observations revealed that fluid and continuous SLBs were formed at PE concentrations (C_PE) of ≥6 mol%. Fluorescence recovery after photobleaching measurement revealed that the microdomain was more fluid than the surrounding region that showed typical diffusion coefficient of the L_o phase. The microdomains were observed as depressions in the AFM topographies. Their area fraction (θ) increased with C_PE, and diDHPE produced a significantly large θ among the three PEs. The microdomains in the PE+eggPC+Chol-SLBs were rich in polyunsaturated PE and were in the L_d-like phase. Associating eggPC and Chol caused polyunsaturated PE to segregate, resulting in a microdomain formation by conferring the umbrella effect on Chol, entropic effect of disordered acyl chains, and π-π interactions in the hydrophobic core.

Amyloid-β Interactions with Lipid Rafts in Biomimetic Systems: A Review of Laboratory Methods

Biomimetic lipid bilayer systems are a useful tool for modeling specific properties of cellular membranes in order to answer key questions about their structure and functions. This approach has prompted scientists from all over the world to create more and more sophisticated model systems in order to decipher the complex lateral and transverse organization of cellular plasma membranes. Among a variety of existing biomembrane domains, lipid rafts are defined as small, dynamic, and ordered assemblies of lipids and proteins, enriched in cholesterol and sphingolipids. Lipid rafts appear to be involved in the development of Alzheimer's disease (AD) by affecting the aggregation of the amyloid-β (Aβ) peptide at neuronal membranes thereby forming toxic oligomeric species. In this review, we summarize the laboratory methods which allow to study the interaction of Aβ with lipid rafts. We describe step by step protocols to form giant (GUVs) and large unilamellar vesicles (LUVs) containing raft-mimicking domains surrounded by membrane nonraft regions. Using fluorescence microscopy GUV imaging protocols, one can design experiments to visualize micron-scale raft-like domains, to determine the micron-scale demixing temperature of a given lipid mixture, construct phase diagram, and photogenerate domains in order to assess the dynamics of raft formation and raft size distribution. LUV fluorescence spectroscopy protocols with proper data analysis can be used to measure molecular packing of raft/nonraft regions of the membrane, to report on nanoscale raft formation and determine nanoscale demixing temperature. Because handling of the Aβ requires dedicated laboratory experience, we present illustrated protocols for Aβ-stock aliquoting, Aβ aqueous solubilization, oligomer preparation, determination of the Aβ concentration before and after filtration. Thioflavin binding, dynamic light scattering, and transmission electron microscopy protocols are described as complementary methods to detect Aβ aggregation kinetics, aggregate sizes, and morphologies of observed aggregates.

Quantitative Label-Free Imaging of Lipid Domains in Single Bilayers by Hyperspectral Coherent Raman Scattering

Lipid phase separation in cellular membranes is thought to play an important role in many biological functions. This has prompted the development of synthetic membranes to study lipid-lipid interactions in vitro, alongside optical microscopy techniques aimed at directly visualizing phase partitioning. In this context, there is a need to overcome the limitations of fluorescence microscopy, where added fluorophores can significantly perturb lipid packing. Raman-based optical imaging is a promising analytical tool for label-free chemically specific microscopy of lipid bilayers. In this work, we demonstrate the application of hyperspectral coherent Raman scattering microscopy combined with a quantitative unsupervised data analysis methodology developed in-house to visualize lipid partitioning in single planar membrane bilayers exhibiting liquid-ordered and liquid-disordered domains. Two home-built instruments were utilized, featuring coherent anti-Stokes Raman scattering and stimulated Raman scattering modalities. Ternary mixtures of dioleoylphosphatidylcholine, sphingomyelin, and cholesterol were used to form phase-separated domains. We show that domains are consistently resolved, both chemically and spatially, in a completely label-free manner. Quantitative Raman susceptibility spectra of the domains are provided alongside their spatially resolved concentration maps.

Single-lipid dynamics in phase-separated supported lipid bilayers

Phase separation is a fundamental organizing mechanism on cellular membranes. Lipid phases have complex dependencies on the membrane composition, curvature, tension, and temperature. Lipid diffusion rates vary by up to ten-fold between liquid-disordered (L_d) and liquid-ordered (L_o) phases depending on the membrane composition, measurement technique, and the surrounding environment. This manuscript reports the lipid diffusion on phase-separated supported lipid bilayers (SLBs) with varying temperature, composition, and lipid phase. Lipid diffusion is measured by single-particle tracking (SPT) and fluorescence correlation spectroscopy (FCS) via custom data acquisition and analysis protocols that apply to diverse membranes systems. Traditionally, SPT is sensitive to diffuser aggregation, whereas the diffusion rates reported by FCS are unaffected by the presence of immobile aggregates. Within this manuscript, we report (1) improved single-particle tracking analysis of lipid diffusion, (2) comparison and consistency between diffusion measurement methods for non-Brownian diffusers, and (3) the application of these methods to measure the phase, temperature, and composition dependencies in lipid diffusion. We demonstrate improved SPT analysis methods that yield consistent FCS and SPT diffusion results even when most fluorescent lipids are frequently confined within aggregates within the membrane. With varying membrane composition and temperature, we demonstrate differences in diffusion between the L_d and L_o phases of SLBs.

C24:0 and C24:1 sphingolipids in cholesterol-containing, five- and six-component lipid membranes

The biophysical properties of sphingolipids containing lignoceric (C24:0) or nervonic (C24:1) fatty acyl residues have been studied in multicomponent lipid bilayers containing cholesterol (Chol), by means of confocal microscopy, differential scanning calorimetry and atomic force microscopy. Lipid membranes composed of dioleoyl phosphatidylcholine and cholesterol were prepared, with the addition of different combinations of ceramides (C24:0 and/or C24:1) and sphingomyelins (C24:0 and/or C24:1). Results point to C24:0 sphingolipids, namely lignoceroyl sphingomyelin (lSM) and lignoceroyl ceramide (lCer), having higher membrane rigidifying properties than their C24:1 homologues (nervonoyl SM, nSM, or nervonoyl Cer, nCer), although with a similar strong capacity to induce segregated gel phases. In the case of the lSM-lCer multicomponent system, the segregated phases have a peculiar fibrillar or fern-like morphology. Moreover, the combination of C24:0 and C24:1 sphingolipids generates interesting events, such as a generalized bilayer dynamism/instability of supported planar bilayers. In some cases, these sphingolipids give rise to exothermic curves in thermograms. These peculiar features were not present in previous studies of C24:1 combined with C16:0 sphingolipids. Conclusions of our study point to nSM as a key factor governing the relative distribution of ceramides when both lCer and nCer are present. The data indicate that lCer could be easier to accommodate in multicomponent bilayers than its C16:0 counterpart. These results are relevant for events of membrane platform formation, in the context of sphingolipid-based signaling cascades.

Electroformation of liposomes and phytosomes using copper electrode

A novel method for electroformation of liposomes and phytosomes using copper electrode is described. Liposomes made at 2 V and 10 Hz AC field from L-α-egg-phosphatidylcholine (egg-PC), K. pneumoniae phosphatidylethanolamine, K. pneumoniae polar lipids and E. coli polar lipids on copper electrode were (777.9 ± 118.4), (370.2 ± 100.5), (825.3 ± 21.54), and (281.3 ± 42.3) nm in diameter, respectively. Giant vesicles were formed at 30 V and 10 Hz AC field from polar lipids of K. pneumoniae and E. coli were (106 ± 29.7) and (86 ± 24.3) µm in diameter, respectively. All liposomes were unilamellar as indicated by their unilamellar indices of 50 ± 2, had surface charge comparable to vesicles made from lipid(s) with similar composition and exhibited only 1-2 mol% of oxidized lipids. Cu concentration in the liposomal samples was <1.5 ppm for large unilamellar vesicles (LUVs) and ˂5 ppm for giant unilamellar vesicles (GUVs). The vesicles were stable for >15 d without loss of their size, charge, or unilamellarity. The method was successfully applied to prepare phytosomes from egg-PC and a phytochemical fraction of Dimorphocalyx glabellus, a medicinal plant with anti-diuretic properties. Phytosomes formed were 1000-1500 nm in diameter and exhibited altered fluorescence and absorbance properties compared to the unencapsulated phytochemical. Phytosomes with phytochemical: egg-PC ratio from 0.15 to 1.5 had encapsulation efficiency ranging 90-30%, respectively, and was stable for 1 month. Our method is easy, inexpensive and convenient that will prove to be useful for preparation of liposomes and phytosomes.

Direct imaging of liquid domains in membranes by cryo-electron tomography

Images of micrometer-scale domains in lipid bilayers have provided the gold standard of model-free evidence to understand the domains' shapes, sizes, and distributions. Corresponding techniques to directly and quantitatively assess smaller (nanoscale and submicron) liquid domains have been limited. Researchers commonly seek to correlate activities of membrane proteins with attributes of the domains in which they reside; doing so hinges on identification and characterization of membrane domains. Although some features of membrane domains can be probed by indirect methods, these methods are often constrained by the limitation that data must be analyzed in the context of models that require multiple assumptions or parameters. Here, we address this challenge by developing and testing two methods of identifying submicron domains in biomimetic membranes. Both methods leverage cryo-electron tomograms of ternary membranes under vitrified, hydrated conditions. The first method is optimized for probe-free applications: Domains are directly distinguished from the surrounding membrane by their thickness. This technique quantitatively and accurately measures area fractions of domains, in excellent agreement with known phase diagrams. The second method is optimized for applications in which a single label is deployed for imaging membranes by both high-resolution cryo-electron tomography and diffraction-limited optical microscopy. For this method, we test a panel of probes, find that a trimeric mCherry label performs best, and specify criteria for developing future high-performance, dual-use probes. These developments have led to direct and quantitative imaging of submicron membrane domains in vitrified, hydrated vesicles.

Lipid Hydroperoxidation Effect on the Dynamical Evolution of the Conductance Process in Bilayer Lipid Membranes: A Condition Toward Criticality

Cell membranes are one of the main targets of oxidative processes mediated by reactive oxygen species (ROS). These chemical species interact with unsaturated fatty acids of membrane lipids, triggering an autocatalytic chain reaction, producing lipid hydroperoxides (LOOHs) as the first relatively stable product of the ROS-mediated lipid peroxidation (LPO) process. Numerous biophysical and computational studies have been carried out to elucidate the LPO impact on the structure and organization of lipid membranes. However, although LOOHs are the major primary product of LPO of polyunsaturated fatty acids (PUFAs), to the best of our knowledge, there is no experimental evidence on the effects of the accumulation of these LPO byproducts on the electrical properties and the underlying dynamics of lipid membranes. In this work, bilayer lipid membranes (BLMs) containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocoline (POPC) with increasing hydroperoxidized POPC (POPC-OOH) molar proportions (BLM_PC/PC-OOH) are used as model membranes to investigate the effect of LOOH-mediated LPO propagation on the electrical behavior of lipid bilayers. Voltage-induced ion current signals are analyzed by applying the fractal method of power spectrum density (PSD) analysis. We experimentally prove that, when certain LOOH concentration and energy threshold are overcome, oxidized membranes evolve toward a critical state characterized by the emergence of non-linear electrical behavior dynamics and the pore-type metastable structures formation. PSD analysis shows that temporal dynamics exhibiting "white" noise (non-time correlations) reflects a linear relationship between the input and output signals, while long-term correlations (β > 0.5) begin to be observed closely to the transition (critical point) from linear (Ohmic) to nonlinear (non-Ohmic) behavior. The generation of lipid pores appears to arise as an optimized energy dissipation mechanism based on the system's ability to self-organize and generate ordered structures capable of dissipating energy gradients more efficiently under stressful oxidative conditions.

Mechanical characterization of vesicles and cells: A review

Vesicles perform many essential functions in all living organisms. They respond like a transducer to mechanical stress in converting the applied force into mechanical and biological responses. At the same time, both biochemical and biophysical signals influence the vesicular response in bearing mechanical loads. In recent years, liposomes, artificial lipid vesicles, have gained substantial attention from the pharmaceutical industry as a prospective drug carrier which can also serve as an artificial cell-mimetic system. The ability of these vesicles to enter through pores of even smaller size makes them ideal candidates for therapeutic agents to reach the infected sites effectively. Engineering of vesicles with desired mechanical properties that can encapsulate drugs and release as required is the prime challenge in this field. This requirement has led to the modifications of the composition of the bilayer membrane by adding cholesterol, sphingomyelin, etc. In this article, we review the manufacturing and characterization techniques of various artificial/synthetic vesicles. We particularly focus on the electric field-driven characterization techniques to determine different properties of vesicle and its membranes, such as bending rigidity, viscosity, capacitance, conductance, etc., which are indicators of their content and mobility. Similarities and differences between artificial vesicles, natural vesicles, and cells are highlighted throughout the manuscript since most of these artificial vesicles are intended for cell mimetic functions.

Elastic deformations mediate interaction of the raft boundary with membrane inclusions leading to their effective lateral sorting

Liquid-ordered lipid domains represent a lateral inhomogeneity in cellular membranes. These domains have elastic and physicochemical properties different from those of the surrounding membrane. In particular, their thickness exceeds that of the disordered membrane. Thus, elastic deformations arise at the domain boundary in order to compensate for the thickness mismatch. In equilibrium, the deformations lead to an incomplete register of monolayer ordered domains: the elastic energy is minimal if domains in opposing monolayers lie on the top of each other, and their boundaries are laterally shifted by about 3 nm. This configuration introduces a region, composed of one ordered and one disordered monolayers, with an intermediate bilayer thickness. Besides, a jump in a local monolayer curvature takes place in this intermediate region, concentrating here most of the elastic stress. This region can participate in a lateral sorting of membrane inclusions by offering them an optimal bilayer thickness and local curvature conditions. In the present study, we consider the sorting of deformable lipid inclusions, undeformable peripheral and deeply incorporated peptide inclusions, and undeformable transmembrane inclusions of different molecular geometry. With rare exceptions, all types of inclusions have an affinity to the ordered domain boundary as compared to the bulk phases. The optimal lateral distribution of inclusions allows relaxing the elastic stress at the boundary of domains.

The Effect of Transmembrane Protein Shape on Surrounding Lipid Domain Formation by Wetting

Signal transduction through cellular membranes requires the highly specific and coordinated work of specialized proteins. Proper functioning of these proteins is provided by an interplay between them and the lipid environment. Liquid-ordered lipid domains are believed to be important players here, however, it is still unclear whether conditions for a phase separation required for lipid domain formation exist in cellular membranes. Moreover, membrane leaflets are compositionally asymmetric, that could be an obstacle for the formation of symmetric domains spanning the lipid bilayer. We theoretically show that the presence of protein in the membrane leads to the formation of a stable liquid-ordered lipid phase around it by the mechanism of protein wetting by lipids, even in the absence of conditions necessary for the global phase separation in the membrane. Moreover, we show that protein shape plays a crucial role in this process, and protein conformational rearrangement can lead to changes in the size and characteristics of surrounding lipid domains.

Lipid Bilayer Thickness Measured by Quantitative DIC Reveals Phase Transitions and Effects of Substrate Hydrophilicity

Quantitative differential interference contrast microscopy is demonstrated here as a label-free method, which is able to image and measure the thickness of lipid bilayers with 0.1 nm precision. We investigate the influence of the substrate on the thickness of fluid-phase 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)-supported lipid bilayers and find a thinning of up to 10%, depending on substrate hydrophilicity, local bilayer coverage, and ionic strength of the medium. With fluorescently labeled lipid bilayers, we also observe changes in the bilayer thickness depending on the choice of fluorophore. Furthermore, liquid-ordered domains in bilayers, formed from DOPC, cholesterol, and sphingomyelin, are measured, and the corresponding thickness change between the liquid-ordered and liquid-disordered phases is accurately determined. Again, the thickness difference is found to be dependent on the presence of the fluorophore label, highlighting the need for quantitative label-free techniques.

Excitation of Fluorescent Lipid Probes Accelerates Supported Lipid Bilayer Formation via Photosensitized Lipid Oxidation

Fluorescent lipid probes are commonly used to label membranes of cells and model membranes like giant vesicles, liposomes, and supported lipid bilayers (SLB). Here, we show that excitation of fluorescent lipid probes with BODIPY-like conjugates results in a significant acceleration of the rupture and SLB formation process for unsaturated phospholipid vesicles on SiO₂ surfaces. The resulting SLBs also have smaller measured masses, which is indicative of a reduction in membrane thickness and/or membrane density. The excitation of fluorescent probes with NBD and Texas Red conjugates does not accelerate the SLB formation process. In the absence of fluorescent probes or light, the inclusion of oxidized phospholipids also accelerates SLB formation. The excitation-induced acceleration caused by BODIPY-like probes is eliminated when the probes are present with saturated phospholipids not susceptible to oxidation, and it is attenuated when a lipophilic antioxidant (α-tocopherol) is present. These results suggest that BODIPY-phospholipid conjugates are photosensitizers, and their excitation causes oxidation of lipid membranes, which significantly alters membrane properties.

PIP2 Reshapes Membranes through Asymmetric Desorption

Phosphatidylinositol-4,5-bisphosphate (PIP2) is an important signaling lipid in eukaryotic cell plasma membranes, playing an essential role in diverse cellular processes. The headgroup of PIP2 is highly negatively charged, and this lipid displays a high critical micellar concentration compared to housekeeping phospholipid analogs. Given the crucial role of PIP2, it is imperative to study its localization, interaction with proteins, and membrane-shaping properties. Biomimetic membranes have served extensively to elucidate structural and functional aspects of cell membranes including protein-lipid and lipid-lipid interactions, as well as membrane mechanics. Incorporation of PIP2 into biomimetic membranes, however, has at times resulted in discrepant findings described in the literature. With the goal to elucidate the mechanical consequences of PIP2 incorporation, we studied the desorption of PIP2 from biomimetic giant unilamellar vesicles by means of a fluorescent marker. A decrease in fluorescence intensity with the age of the vesicles suggested that PIP2 lipids were being desorbed from the outer leaflet of the membrane. To evaluate whether this desorption was asymmetric, the vesicles were systematically diluted. This resulted in an increase in the number of internally tubulated vesicles within minutes after dilution, suggesting that the desorption was asymmetric and also generated membrane curvature. By means of a saturated chain homolog of PIP2, we showed that the fast desorption of PIP2 is facilitated by presence of an arachidonic lipid tail and is possibly due to its oxidation. Through measurements of the pulling force of membrane tethers, we quantified the effect of this asymmetric desorption on the spontaneous membrane curvature. Furthermore, we found that the spontaneous curvature could be modulated by externally increasing the concentration of PIP2 micelles. Given that the local concentration of PIP2 in biological membranes is variable, spontaneous curvature generated by PIP2 may affect the formation of highly curved structures that can serve as initiators for signaling events.

Giant Vesicles and Their Use in Assays for Assessing Membrane Phase State, Curvature, Mechanics, and Electrical Properties

Giant unilamellar vesicles represent a promising and extremely useful model biomembrane system for systematic measurements of mechanical, thermodynamic, electrical, and rheological properties of lipid bilayers as a function of membrane composition, surrounding media, and temperature. The most important advantage of giant vesicles over other model membrane systems is that the membrane responses to external factors such as ions, (macro)molecules, hydrodynamic flows, or electromagnetic fields can be directly observed under the microscope. Here, we briefly review approaches for giant vesicle preparation and describe several assays used for deducing the membrane phase state and measuring a number of material properties, with further emphasis on membrane reshaping and curvature.

A "Clickable" Photoconvertible Small Fluorescent Molecule as a Minimalist Probe for Tracking Individual Biomolecule Complexes

Photoconvertible fluorophores can enable the visualization and tracking of a specific biomolecules, complexes, and cellular compartments with precise spatiotemporal control. The field of photoconvertible probes is dominated by fluorescent protein variants, which can introduce perturbations to the target biomolecules due to their large size. Here, we present a photoconvertible small molecule, termed CPX, that can be conjugated to any target through azide-alkyne cycloaddition ("click" reaction). To demonstrate its utility, we have applied CPX to study (1) trafficking of biologically relevant synthetic vesicles and (2) intracellular processes involved in transmission of α-synuclein (αS) pathology. Our results demonstrate that CPX can serve as a minimally perturbing probe for tracking the dynamics of biomolecules.

Contrasting roles of oxidized lipids in modulating membrane microdomains

Lipid rafts display a lateral heterogeneity forming membrane microdomains that hold a fundamental role on biological membranes and are indispensable to physiological functions of cells. Oxidative stress in cellular environments may cause lipid oxidation, changing membrane composition and organization, thus implying in effects in cell signaling and even loss of homeostasis. The individual contribution of oxidized lipid species to the formation or disruption of lipid rafts in membranes still remains unknown. Here, we investigate the role of different structures of oxidized phospholipids on rafts microdomains by carefully controlling the membrane composition. Our experimental approach based on fluorescence microscopy of giant unilamellar vesicles (GUV) enables the direct visualization of the impact of hydroperoxidized POPC lipid (referred to as POPCOOH) and shortened chain lipid PazePC (1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine) on phase separation. We found that the molecular structure of oxidized lipid is of paramount importance on lipid mixing and/or demixing. The hydrophobic mismatch promoted by POPCOOH coupled to its cylindrical molecular shape favor microdomains formation. In contrast, the conical shape of PazePC causes disarrangement of lipid 2D organized platforms. Our findings contribute to better unraveling how oxidized phospholipids can trigger formation or disruption of lipid rafts. As a consequence, phospholipid oxidation may indirectly affect association or dissociation of key biomolecules in the rafts thus altering cell signaling and homeostasis.

The Alzheimer's disease amyloid-β peptide affects the size-dynamics of raft-mimicking Lo domains in GM1-containing lipid bilayers

Alzheimer's disease (AD) is characterized by the overproduction of the amyloid-β peptide (Aβ) which forms fibrils under the influence of raft microdomains containing the ganglioside GM1. Raft-mimicking artificial liquid ordered (Lo) domains containing GM1 enhance amyloid-β polymerization. Other experiments suggest that Aβ binds preferably to the non-raft liquid disordered (Ld) phase rather than to the Lo phase in the presence of GM1. Here, the interaction of Aβ(1-42) with GM1-containing biphasic Lo-Ld giant vesicles was investigated. Fluorescence colocalisation experiments confirm that Aβ(1-42) binds preferentially to the Ld phase. The effect of Aβ(1-42) on the Lo-Ld size dynamics was studied using photoinduced spinodal decomposition which mimics the nanodomain-microdomain raft coalescence. Aβ affects the kinetics of the coarsening phase and the size of the resulting microdomains. The effect depends on which phase is in a majority: when the Lo microdomains are formed inside an Ld phase, their growth rate becomes slower and their final size smaller in the presence of Aβ(1-42), whereas when the Ld microdomains are formed inside an Lo phase, the growth rate becomes faster and the final size larger. Fluorimetric measurements on large vesicles using the probe Laurdan indicate that Aβ(1-42) binding respectively increases or decreases the packing of the Ld phase in the presence or absence of GM1. The differential effects of Aβ on spinodal decomposition are accordingly interpreted as resulting from distinct effects of the peptide on the Lo-Ld line tension modulated by GM1. Such modulating effect of Aβ on domain dynamics could be important for lipid rafts in signaling disorders in AD as well as in Aβ fibrillation.

Influence of the Membrane Dye R18 and of DMSO on Cell Penetration of Guanidinium-Rich Peptides

A quantitative analysis by confocal fluorescence microscopy of the entry into HEK293 and MCF-7 cells by fluorescein-labeled octaarginine (1) and by three octa-Adp derivatives (2 - 4, octamers of the β-Asp-Arg-dipeptide, derived from the biopolymer cyanophycin) is described, including the effects of the membrane dye R18 and of DMSO on cell penetration.