The dynamics of giant unilamellar vesicle oxidation probed by morphological transitions

Membrane nanotubes transform into double-membrane sheets at condensate droplets

Cellular membranes exhibit a multitude of highly curved morphologies such as buds, nanotubes, cisterna-like sheets defining the outlines of organelles. Here, we mimic cell compartmentation using an aqueous two-phase system of dextran and poly(ethylene glycol) encapsulated in giant vesicles. Upon osmotic deflation, the vesicle membrane forms nanotubes, which undergo surprising morphological transformations at the liquid-liquid interfaces inside the vesicles. At these interfaces, the nanotubes transform into cisterna-like double-membrane sheets (DMS) connected to the mother vesicle via short membrane necks. Using super-resolution (stimulated emission depletion) microscopy and theoretical considerations, we construct a morphology diagram predicting the tube-to-sheet transformation, which is driven by a decrease in the free energy. Nanotube knots can prohibit the tube-to-sheet transformation by blocking water influx into the tubes. Because both nanotubes and DMSs are frequently formed by cellular membranes, understanding the formation and transformation between these membrane morphologies provides insight into the origin and evolution of cellular organelles.

Giant vesicles form in physiological saline and encapsulate pDNA by the modified electroformation method

Giant vesicles (GVs) are used to study the structures and functions of cells and cell membranes. Electroformation is the most commonly used method for GV preparation. However, the electroformation of GVs is hindered in highly concentrated ionic solutions, limiting their application as cell models for research under physiological conditions. In this study, giant multilayer vesicles were successfully generated in physiological saline using a modified electroformation device by adding an insulating layer between the two electrode plates. The influence of the electric frequency and strength on the electroformation of GVs in physiological saline was explored, and a possible mechanism for this improvement was assessed. It has been shown that an insulating layer between the two electrodes can improve the electroformation of GVs in physiological saline by increasing the electrical impedance, which is weakened by the saline solution, thereby restoring the reduced effective electric field strength. Furthermore, macromolecular plasmid DNA (pDNA) was successfully encapsulated in the electroformed GVs of the modified device. This modified electroformation method may be useful for generating eukaryotic cell models under physiological conditions.

Lysosomal damage sensing and lysophagy initiation by SPG20-ITCH

Cells respond to lysosomal membrane permeabilization by membrane repair or selective macroautophagy of damaged lysosomes, termed lysophagy, but it is not fully understood how this decision is made. Here, we uncover a pathway in human cells that detects lipid bilayer perturbations in the limiting membrane of compromised lysosomes, which fail to be repaired, and then initiates ubiquitin-triggered lysophagy. We find that SPG20 binds the repair factor IST1 on damaged lysosomes and, importantly, integrates that with the detection of damage-associated lipid-packing defects of the lysosomal membrane. Detection occurs via sensory amphipathic helices in SPG20 before rupture of the membrane. If lipid-packing defects are extensive, such as during lipid peroxidation, SPG20 recruits and activates ITCH, which marks the damaged lysosome with lysine-63-linked ubiquitin chains to initiate lysophagy and thus triages the lysosome for destruction. With SPG20 being linked to neurodegeneration, these findings highlight the relevance of a coordinated lysosomal damage response for cellular homeostasis.

The Chemical Reactivity of Membrane Lipids

It is well-known that aqueous dispersions of phospholipids spontaneously assemble into bilayer structures. These structures have numerous applications across chemistry and materials science and form the fundamental structural unit of the biological membrane. The particular environment of the lipid bilayer, with a water-poor low dielectric core surrounded by a more polar and better hydrated interfacial region, gives the membrane particular biophysical and physicochemical properties and presents a unique environment for chemical reactions to occur. Many different types of molecule spanning a range of sizes, from dissolved gases through small organics to proteins, are able to interact with membranes and promote chemical changes to lipids that subsequently affect the physicochemical properties of the bilayer. This Review describes the chemical reactivity exhibited by lipids in their membrane form, with an emphasis on conditions where the lipids are well hydrated in the form of bilayers. Key topics include the following: lytic reactions of glyceryl esters, including hydrolysis, aminolysis, and transesterification; oxidation reactions of alkenes in unsaturated fatty acids and sterols, including autoxidation and oxidation by singlet oxygen; reactivity of headgroups, particularly with reactive carbonyl species; and E/Z isomerization of alkenes. The consequences of reactivity for biological activity and biophysical properties are also discussed.

Nanoparticles of Push-Pull Triphenylamine-Based Molecules for Light-Controlled Stimulation of Neuronal Activity

Organic semiconductor materials with a unique set of properties are very attractive for interfacing biological objects and can be used for noninvasive therapy or detection of biological signals. Here, we describe the synthesis and investigation of a novel series of organic push-pull conjugated molecules with the star-shaped architecture, consisting of triphenylamine as a branching electron donor core linked through the thiophene π-spacer to electron-withdrawing alkyl-dicyanovinyl groups. The molecules could form stable aqueous dispersions of nanoparticles (NPs) without the addition of any surfactants or amphiphilic polymer matrixes with the average size distribution varying from 40 to 120 nm and absorption spectra very similar to those of human eye retina pigments such as rods and green cones. Variation of the terminal alkyl chain length of the molecules forming NPs from 1 to 12 carbon atoms was found to be an efficient tool to modulate their lipophilic and biological properties. Possibilities of using the NPs as light nanoactuators in biological systems or as artificial pigments for therapy of degenerative retinal diseases were studied both on the model planar bilayer lipid membranes and on the rat cortical neurons. In the planar bilayer system, the photodynamic activity of these NPs led to photoinactivation of ion channels formed by pentadecapeptide gramicidin A. Treatment of rat cortical neurons with the NPs caused depolarization of cell membranes upon light irradiation, which could also be due to the photodynamic activity of the NPs. The results of the work gave more insight into the mechanisms of light-controlled stimulation of neuronal activity and for the first time showed that fine-tuning of the lipophilic affinity of NPs based on organic conjugated molecules is of high importance for creating a bioelectronic interface for biomedical applications.

The role of lipid oxidation on electrical properties of planar lipid bilayers and its importance for understanding electroporation

Electroporation is a useful tool for the manipulation with the cell membrane permeability. Underlying physicochemical processes taking place at the molecular level during electroporation are relatively well studied. However, various processes remain unknown, one of them is lipid oxidation, a chain reaction that causes degradation of lipids, and might explain the long-lasting membrane permeability after the electric field has ceased. The aim of our study was to observe the differences in the electrical properties of planar lipid bilayers, as in vitro cell membrane models, due to lipid oxidation. Phospholipids were chemically oxidized and oxidation products were analysed using mass spectrometry. Electrical properties, resistance R (Ω) and capacitance C (F) were measured using an LCR meter. Using a previously developed measuring device, a linear increasing signal was applied to a stable bilayer in order to measure its breakdown voltage U_br (V) and lifetime t_br (µs). We observed an increase in conductance and capacitance of the oxidized planar lipid bilayers when compared to their non-oxidized counterparts. With increasing lipid oxidation, the core of the bilayer becomes more polar, and consequently more permeable. Our findings can explain the long-lasting permeability of the cell membrane after electroporation.

Directly imaging emergence of phase separation in peroxidized lipid membranes

Lipid peroxidation is a process which is key in cell signaling and disease, it is exploited in cancer therapy in the form of photodynamic therapy. The appearance of hydrophilic moieties within the bilayer's hydrocarbon core will dramatically alter the structure and mechanical behavior of membranes. Here, we combine viscosity sensitive fluorophores, advanced microscopy, and X-ray diffraction and molecular simulations to directly and quantitatively measure the bilayer's structural and viscoelastic properties, and correlate these with atomistic molecular modelling. Our results indicate an increase in microviscosity and a decrease in the bending rigidity upon peroxidation of the membranes, contrary to the trend observed with non-oxidized lipids. Fluorescence lifetime imaging microscopy and MD simulations give evidence for the presence of membrane regions of different local order in the oxidized membranes. We hypothesize that oxidation promotes stronger lipid-lipid interactions, which lead to an increase in the lateral heterogeneity within the bilayer and the creation of lipid clusters of higher order.

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.

Methods to mechanically perturb and characterize GUV-based minimal cell models

Cells shield organelles and the cytosol via an active boundary predominantly made of phospholipids and membrane proteins, yet allowing communication between the intracellular and extracellular environment. Micron-sized liposome compartments commonly known as giant unilamellar vesicles (GUVs) are used to model the cell membrane and encapsulate biological materials and processes in a cell-like confinement. In the field of bottom-up synthetic biology, many have utilized GUVs as substrates to study various biological processes such as protein-lipid interactions, cytoskeletal assembly, and dynamics of protein synthesis. Like cells, it is ideal that GUVs are also mechanically durable and able to stay intact when the inner and outer environment changes. As a result, studies have demonstrated approaches to tune the mechanical properties of GUVs by modulating membrane composition and lumenal material property. In this context, there have been many different methods developed to test the mechanical properties of GUVs. In this review, we will survey various perturbation techniques employed to mechanically characterize GUVs.

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.

Deuterated polyunsaturated fatty acids inhibit photoirradiation-induced lipid peroxidation in lipid bilayers

Lipid peroxidation (LPO) plays a key role in many age-related neurodegenerative conditions and other disorders. Light irradiation can initiate LPO through various mechanisms and is of importance in retinal and dermatological pathologies. The introduction of deuterated polyunsaturated fatty acids (D-PUFA) into membrane lipids is a promising approach for protection against LPO. Here, we report the protective effects of D-PUFA against the photodynamically induced LPO, using illumination in the presence of the photosensitizer trisulfonated aluminum phthalocyanine (AlPcS₃) in liposomes and giant unilamellar vesicles (GUV), as assessed in four experimental models: 1) sulforhodamine B leakage from liposomes, detected with fluorescence correlation spectroscopy (FCS); 2) formation of diene conjugates in liposomal membranes, measured by absorbance at 234 nm; 3) membrane leakage in GUV assessed by optical phase-contrast intensity observations; 4) UPLC-MS/MS method to detect oxidized linoleic acid (Lin)-derived metabolites. Specifically, in liposomes or GUV containing H-PUFA (dilinoleyl-sn-glycero-3-phosphatidylcholine), light irradiation led to an extensive oxidative damage to bilayers. By contrast, no damage was observed in lipid bilayers containing 20% or more D-PUFA (D2-Lin or D10-docosahexanenoic acid). Remarkably, addition of tocopherol increased the dye leakage from liposomes in H-PUFA bilayers compared to photoirradiation alone, signifying tocopherol's pro-oxidant properties. However, in the presence of D-PUFA the opposite effect was observed, whereby adding tocopherol increased the resistance to LPO. These findings suggest a method to augment the protective effects of D-PUFA, which are currently undergoing clinical trials in several neurological and retinal diseases that involve LPO.

The pore-forming activity of sticholysin I is enhanced by the presence of a phospholipid hydroperoxide in membrane

Sticholysin I (StI) is a pore-forming toxin (PFT) belonging to the actinoporin protein family characterized by high permeabilizing activity in membranes. StI readily associates with sphingomyelin (SM)-containing membranes originating pores that can lead to cell death. Binding and pore-formation are critically dependent on the physicochemical properties of membrane. 1-palmitoyl-2-oleoylphosphatidylcholine hydroperoxide (POPC-OOH) is an oxidized phospholipid (OxPL) containing an -OOH moiety in the unsaturated hydrocarbon chain which orientates towards the bilayer interface. This orientation causes an increase in the lipid molecular area, lateral expansion and decrease in bilayer thickness, elastic and bending modulus, as well as modification of lipid packing. Taking advantage of membrane structural changes promoted by POPC-OOH, we investigated its influence on the permeabilizing ability of StI. Here we report the action of StI on Giant Unilamellar Vesicles (GUVs) made of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and SM containing increasing amount of POPC-OOH to assess vesicle permeability changes when compared to OxPL-lacking membranes. Inclusion of POPC-OOH in membranes did not promote spontaneous vesicle leaking but resulted in increased membrane permeability due to StI action. StI activity did not modify the fluid-gel phase coexistence boundaries neither in POPC:SM or POPC-OOH:SM membranes. However, the StI insertion mechanism in membrane seems to differ between POPC:SM and POPC-OOH:SM mixtures as suggested by changes in the time course of monolayer surface tension measurements, even though a preferable binding of the toxin to OxPL-containing systems could not be here demonstrated. In summary, modifications in the membrane imposed by lipid hydroperoxidation favor StI permeabilizing activity.

Visualising UV-A light-induced damage to plasma membranes of eye lens

An eye lens is constantly exposed to the solar UV radiation, which is considered the most important external source of age-related changes to eye lens constituents. The accumulation of modifications of proteins and lipids with age can eventually lead to the development of progressive lens opacifications, such as cataracts. Though the impact of solar UV radiation on the structure and function of proteins is actively studied, little is known about the effect of photodamage on plasma membranes of lens cells. In this work we exploit Fluorescence Lifetime Imaging Microscopy (FLIM), together with viscosity-sensitive fluorophores termed molecular rotors, to study the changes in viscosity of plasma membranes of porcine eye lens resulting from two different types of photodamage: Type I (electron transfer) and Type II (singlet oxygen) reactions. We demonstrate that these two types of photodamage result in clearly distinct changes in viscosity - a decrease in the case of Type I damage and an increase in the case of Type II processes. Finally, to simulate age-related changes that occur in vivo, we expose an intact eye lens to UV-A light under anaerobic conditions. The observed decrease in viscosity within plasma membranes is consistent with the ability of eye lens constituents to sensitize Type I photodamage under natural irradiation conditions. These changes are likely to alter the transport of metabolites and predispose the whole tissue to the development of pathological processes such as cataracts.

Light-Triggered Cargo Loading and Division of DNA-Containing Giant Unilamellar Lipid Vesicles

A minimal synthetic cell should contain a substrate for information storage and have the capability to divide. Notable efforts were made to assemble functional synthetic cells from the bottom up, however often lacking the capability to reproduce. Here, we develop a mechanism to fully control reversible cargo loading and division of DNA-containing giant unilamellar vesicles (GUVs) with light. We make use of the photosensitizer Chlorin e6 (Ce6) which self-assembles into lipid bilayers and leads to local lipid peroxidation upon illumination. On the time scale of minutes, illumination induces the formation of transient pores, which we exploit for cargo encapsulation or controlled release. In combination with osmosis, complete division of two daughter GUVs can be triggered within seconds of illumination due to a spontaneous curvature increase. We ultimately demonstrate the division of a selected DNA-containing GUV with full spatiotemporal control-proving the relevance of the division mechanism for bottom-up synthetic biology.

Molecular dynamics simulations of the effects of lipid oxidation on the permeability of cell membranes

The formation of transient pores in their membranes is a well-known mechanism of permeabilization of cells exposed to high-intensity electric pulses. However, the formation of such pores is not able to explain all aspects of the so-called electroporation phenomenon. In particular, the reasons for sustained permeability of cell membranes, persisting long after the pulses' application, remain elusive. The complete resealing of cell membranes takes indeed orders of magnitude longer than the time for electropore closure as reported from molecular dynamics (MD) investigations. Lipid peroxidation has been suggested as a possible mechanism to explain the sustainable permeability of cell membranes. However, theoretical investigations of membrane lesions containing excess amounts of hydroperoxides have shown that the conductivities of such lesions were not high enough to account for the experimental measurements. Here, expanding on these studies, we investigate quantitatively the permeability of cell membrane lesions that underwent secondary oxidation. MD simulations and free energy calculations of lipid bilayers show that such lesions provide a better model of post-pulse permeable and conductive electropermeabilized cells. These results are further discussed in the context of sonoporation and ferroptosis, respectively a procedure and a phenomenon, among others, in which, alike electroporation, substantial lipid oxidation might be triggered.

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.

Composition effects on photooxidative membrane destabilization by TiO2 nanoparticles

Membrane interactions and photooxidative membrane destabilization of titanium dioxide (TiO₂) nanoparticles were investigated, focusing on the effects of membrane composition, notably phospholipid headgroup charge and presence of cholesterol. For this, we employed a battery of state-of-the-art methods for studies of bilayers formed by zwitterionic palmitoyloleoylphosphatidylcholine (POPC) containing also polyunsaturated palmitoylarachidonoylphosphocholine (PAPC), as well as its mixtures with anionic palmitoyloleoylphosphatidylglycerol (POPG) and cholesterol. It was found that the TiO₂ nanoparticles display close to zero charge at pH 7.4, resulting in aggregation. At pH 3.4, in contrast, the 6 nm TiO₂ nanoparticles are well dispersed due to a strongly positive ζ-potential. Mirroring this pH dependence, TiO₂ nanoparticles were observed to bind to negatively charged lipid bilayers at pH 3.4, but much less so at pH 7.4. While nanoparticle binding has some destabilizing effect alone, illumination with ultraviolet (UV) light accentuates membrane destabilization, a result of oxidative stress caused by generated reactive oxygen species (ROS). Neutron reflectivity (NR), quartz crystal microbalance (QCM), and small-angle X-ray scattering (SAXS) results all demonstrate that membrane composition strongly influences membrane interactions and photooxidative destabilization of lipid bilayers. In particular, the presence of anionic POPG makes the bilayers more sensitive to oxidative destabilization, whereas a stabilizing effect was observed in the presence of cholesterol. Also, structural aspects of peroxidation were found to depend strongly on membrane composition, notably the presence of anionic phospholipids. The results show that membrane interactions and UV-induced ROS generation act in concert and need to be considered together to understand effects of lipid membrane composition on UV-triggered oxidative destabilization by TiO₂ nanoparticles, e.g., in the context of oxidative damage of bacteria and cells.

Photoinduced Endosomal Escape Mechanism: A View from Photochemical Internalization Mediated by CPP-Photosensitizer Conjugates

Endosomal escape in cell-penetrating peptide (CPP)-based drug/macromolecule delivery systems is frequently insufficient. The CPP-fused molecules tend to remain trapped inside endosomes and end up being degraded rather than delivered into the cytosol. One of the methods for endosomal escape of CPP-fused molecules is photochemical internalization (PCI), which is based on the use of light and a photosensitizer and relies on photoinduced endosomal membrane destabilization to release the cargo molecule. Currently, it remains unclear how this delivery strategy behaves after photostimulation. Recent findings, including our studies using CPP-cargo-photosensitizer conjugates, have shed light on the photoinduced endosomal escape mechanism. In this review, we discuss the structural design of CPP-photosensitizer and CPP-cargo-photosensitizer conjugates, and the PCI mechanism underlying their application.

A combination of electrochemistry and mass spectrometry to monitor the interaction of reactive species with supported lipid bilayers

Reactive oxygen and nitrogen species (RONS), e.g. generated by cold physical plasma (CPP) or photodynamic therapy, interfere with redox signaling pathways of mammalian cells, inducing downstream consequences spanning from migratory impairment to apoptotic cell death. However, the more austere impact of RONS on cancer cells remains yet to be clarified. In the present study, a combination of electrochemistry and high-resolution mass spectrometry was developed to investigate the resilience of solid-supported lipid bilayers towards plasma-derived reactive species in dependence of their composition. A 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayer was undisturbed by 200 µM H2O2 (control) but showed full permeability after CPP treatment and space-occupying oxidation products such as PoxnoPC, PAzePC, and POPC hydroperoxide were found. Electron paramagnetic resonance spectroscopy demonstrated the presence of hydroxyl radicals and superoxide anion/hydroperoxyl radicals during the treatment. In contrast, small amounts of the intramembrane antioxidant coenzyme Q10 protected the bilayer to 50% and LysoPC was the only POPC derivative found, confirming the membrane protective effect of Q10. Such, the lipid membrane composition including the presence of antioxidants determines the impact of pro-oxidant signals. Given the differences in membrane composition of cancer and healthy cells, this supports the application of cold physical plasma for cancer treatment. In addition, the developed model using the combination of electrochemistry and mass spectrometry could be a promising method to study the effect of reactive species or mixes thereof generated by chemical or physical sources.

Mechanisms of Photosensitized Lipid Oxidation and Membrane Permeabilization

Lipid oxidation encompasses chemical transformations affecting animals and plants in many ways, and light is one of the most common triggers of lipid oxidation in our habitat. Still, the molecular mechanisms and biological consequences of photoinduced lipid oxidation were only recently understood at the molecular level. In this review, we focus on the main mechanisms of photosensitized lipid oxidation and membrane permeabilization, dissecting the consequences of both singlet oxygen and contact-dependent pathways and discussing how these reactions contribute to chemical and biophysical changes in lipid membranes. We aim to enable scientists to develop novel and more efficient photosensitizers in photomedicine, as well as better strategies for sun protection.

Role of Oxidized Lipids in Permeation of H2O2 Through a Lipid Membrane: Molecular Mechanism of an Inhibitor to Promoter Switch

H₂O₂ permeation through a cell membrane significantly affects living organisms, and permeation is controlled by the physico-chemical nature of lipids and other membrane components. We investigated the molecular relationship between H₂O₂ permeation and lipid membrane structure using three oxidized lipids. POVPC and PazePC act as intra- and inter-molecular permeation promoters, respectively; however, their underlying mechanisms were different. The former changed the partition equilibrium, while the latter changed the permeation pathway. PoxnoPC inhibited permeation under our experimental conditions via an intra-molecular configuration change. Thus, both intra- and inter-molecular processes were found to control the role of oxidized lipids as inhibitors and promoters towards H₂O₂ permeation with different mechanisms depending on structure and composition. Here, we identified two independent H₂O₂ permeation routes: (i) permeation through lipid membrane with increased partition coefficient by intra-molecular configurational change and (ii) diffusion through pores (water channels) formed by inter-molecular configurational change of oxidized lipids. We provide new insight into how biological cells control permeation of molecules through intra- and inter-molecular configurational changes in the lipid membrane. Thus, by employing a rational design for both oxidized lipids and other components, the permeation behaviour of H₂O₂ and other ions and molecules through a lipid membrane could be controlled.

Oxidative modification of skin lipids by cold atmospheric plasma (CAP): A standardizable approach using RP-LC/MS2 and DI-ESI/MS2

Cold atmospheric plasma (CAP) is an emerging source for the locally defined delivery of reactive species, and its clinical potential has been identified in the control of inflammatory processes, such as acute and chronic wounds, or cancerous lesions. Lipids, due to their localization and chemical structure as ideal targets for oxidative species, are relevant modifiers of physiological processes. Human forehead lipids collected on a target were treated by an argon plasma jet and immediately analyzed by direct-infusion high-resolution tandem mass spectrometry (DI-MS²) or liquid chromatography-tandem MS (RP-LC/MS²). Subsequent data analysis was performed by LipidHunter (University of Leipzig), LipidXplorer (Max Planck Institute of Molecular Cell Biology and Genetics, Dresden), and LipidSearch (Thermo Scientific). With either MS method, all major lipid classes of sebum lipids were detected. Significant differences regarding triacylglycerols (predominantly identified in RP-LC/MS²) and ceramides (predominantly identified in DI-MS²) indicate experimental- or approach-inherent distinctions. A CAP-driven oxidation of triacyclglycerols, ceramides, and cholesteryl esters was detected such as truncations and hydroperoxylations, but at a significantly lower extent than expected. Scavenging of reactive species due to naturally present antioxidants in the samples and the absence of a liquid interphase to allow reactive species deposition by the CAP will have contributed to the limited amount of oxidation products observed. In addition, limitations of the software's capability of identifying unexpected oxidized lipids potentially led to an underestimation of the CAP impact on skin lipids, indicating a need for further software development. With respect to the clinical application of CAP, the result indicates that intact skin with its sebum/epidermal lipid overlay is well protected and that moderate treatment will yield limited (if any) functional consequences in the dermal tissue.

Lipid Unsaturation Properties Govern the Sensitivity of Membranes to Photoinduced Oxidative Stress

Unsaturated lipid oxidation is a fundamental process involved in different aspects of cellular bioenergetics; dysregulation of lipid oxidation is often associated with cell aging and death. To study how lipid oxidation affects membrane biophysics, we used a chlorin photosensitizer to oxidize vesicles of various lipid compositions and degrees of unsaturation in a controlled manner. We observed different shape transitions that can be interpreted as an increase in the area of the targeted membrane followed by a decrease. These area modifications induced by the chemical modification of the membrane upon oxidation were followed in situ by Raman tweezers microspectroscopy. We found that the membrane area increase corresponds to the lipids' peroxidation and is initiated by the delocalization of the targeted double bonds in the tails of the lipids. The subsequent decrease of membrane area can be explained by the formation of cleaved secondary products. As a result of these area changes, we observe vesicle permeabilization after a time lag that is characterized in relation with the level of unsaturation. The evolution of photosensitized vesicle radius was measured and yields an estimation of the mechanical changes of the membrane over oxidation time. The membrane is both weakened and permeabilized by the oxidation. Interestingly, the effect of unsaturation level on the dynamics of vesicles undergoing photooxidation is not trivial and thus carefully discussed. Our findings shed light on the fundamental dynamic mechanisms underlying the oxidation of lipid membranes and highlight the role of unsaturations on their physical and chemical properties.

Singlet oxygen partition between the outer-, inner- and membrane-phases of photo/chemotherapeutic liposomes

Herein, we developed a strategy to quantify the fraction of singlet oxygen lifetime spent in the three distinct local liposomal environments through the combination of direct and indirect singlet oxygen detection approaches.

Giant Lipid Vesicle Formation Using Vapor-Deposited Charged Porous Polymers

In this study, we prepare giant lipid vesicles using vapor-deposited charged microporous poly(methacrylic acid- co-ethylene glycol diacrylate) polymer membranes with different morphologies and thicknesses. Our results suggest that vesicle formation is favored by thinner, more structured porous hydrogel substrates. Electrostatic interactions between the polymer and the lipid head groups affect vesicle yield and size distribution. Repulsive electrostatic interactions between the hydrogel and the lipid head groups promote vesicle formation; attractive electrostatic interactions suppress vesicle formation. Ionic strength and sugar concentration are also major parameters affecting the yield and size of giant vesicles. The presence of both ions and sugars in the hydration buffer results in increased vesicle yields. These results indicate that lipid-polymer interactions and osmotic effects in addition to the substrate morphology and surface charge are key factors affecting vesicle formation. Our data suggest that surface chemistry should be designed to tune electrostatic interactions with the lipid mixture of interest to promote vesicle formation. This vapor-deposited hydrogel fabrication technique offers tunability over the physicochemical properties of the hydrogel substrate for the production of giant vesicles with different sizes and compositions.

Solubilization kinetics determines the pulsatory dynamics of lipid vesicles exposed to surfactant

We establish a biophysical model for the dynamics of lipid vesicles exposed to surfactants. The solubilization of the lipid membrane due to the insertion of surfactant molecules induces a reduction of membrane surface area at almost constant vesicle volume. This results in a rate-dependent increase of membrane tension and leads to the opening of a micron-sized pore. We show that solubilization kinetics due to surfactants can determine the regime of pore dynamics: either the pores open and reseal within a second (short-lived pore), or the pore stays open up to a few minutes (long-lived pore). First, we validate our model with previously published experimental measurements of pore dynamics. Then, we investigate how the solubilization kinetics and membrane properties affect the dynamics of the pore and construct a phase diagram for short and long-lived pores. Finally, we examine the dynamics of sequential pore openings and show that cyclic short-lived pores occur with a period inversely proportional to the solubilization rate. By deriving a theoretical expression for the cycle period, we provide an analytical tool to estimate the solubilization rate of lipid vesicles by surfactants. Our findings shed light on some fundamental biophysical mechanisms that allow simple cell-like structures to sustain their integrity against environmental stresses, and have the potential to aid the design of vesicle-based drug delivery systems. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Photolithographic patterned surface forms size-controlled lipid vesicles

Using traditional 2-D photolithographic methods, surface patterns are made on agarose and used to form lipid vesicles with controlled size and layout. Depending on the size and layout of the patterned structures, the lipid bilayer vesicle size can be tuned and placement can be predetermined. Vesicles formed on 2-D patterned surfaces can be harvested for further investigations or can be assayed directly on the patterned surface. Lipid vesicles on the patterned surface are assayed for unilamellarity and protein incorporation, and vesicles are indeed unilamellar as observed from outer leaflet fluorescence quenching. Vesicles successfully incorporate the integral membrane protein α-hemolysin and maintain its membrane transport function.

Light-induced evolution of microaggregates: transformation to vesicles, cyclic growth and collapse and vesicle fusion

The self-assembled dynamic microaggregates were obtained in one pot via PISA and underwent visible light-induced evolutionary behaviors in the presence of nile red or rhodamine.

Extracellular Vesicles Released by Cardiomyocytes in a Doxorubicin-Induced Cardiac Injury Mouse Model Contain Protein Biomarkers of Early Cardiac Injury

Purpose: Cardiac injury is a major cause of death in cancer survivors, and biomarkers for it are detectable only after tissue injury has occurred. Extracellular vesicles (EV) remove toxic biomolecules from tissues and can be detected in the blood. Here, we evaluate the potential of using circulating EVs as early diagnostic markers for long-term cardiac injury.Experimental Design: Using a mouse model of doxorubicin (DOX)-induced cardiac injury, we quantified serum EVs, analyzed proteomes, measured oxidized protein levels in serum EVs released after DOX treatment, and investigated the alteration of EV content.Results: Treatment with DOX caused a significant increase in circulating EVs (DOX_EV) compared with saline-treated controls. DOX_EVs exhibited a higher level of 4-hydroxynonenal adducted proteins, a lipid peroxidation product linked to DOX-induced cardiotoxicity. Proteomic profiling of DOX_EVs revealed the distinctive presence of brain/heart, muscle, and liver isoforms of glycogen phosphorylase (GP), and their origins were verified to be heart, skeletal muscle, and liver, respectively. The presence of brain/heart GP (PYGB) in DOX_EVs correlated with a reduction of PYGB in heart, but not brain tissues. Manganese superoxide dismutase (MnSOD) overexpression, as well as pretreatment with cardioprotective agents and MnSOD mimetics, resulted in a reduction of EV-associated PYGB in mice treated with DOX. Kinetic studies indicated that EVs containing PYGB were released prior to the rise of cardiac troponin in the blood after DOX treatment, suggesting that PYGB is an early indicator of cardiac injury.Conclusions: EVs containing PYGB are an early and sensitive biomarker of cardiac injury. Clin Cancer Res; 24(7); 1644-53. ©2017 AACRSee related commentary by Zhu and Gius, p. 1516.

Systems biology of cellular membranes: a convergence with biophysics

Systems biology and systems medicine have played an important role in the last two decades in shaping our understanding of biological processes. While systems biology is synonymous with network maps and '-omics' approaches, it is not often associated with mechanical processes. Here, we make the case for considering the mechanical and geometrical aspects of biological membranes as a key step in pushing the frontiers of systems biology of cellular membranes forward. We begin by introducing the basic components of cellular membranes, and highlight their dynamical aspects. We then survey the functions of the plasma membrane and the endomembrane system in signaling, and discuss the role and origin of membrane curvature in these diverse cellular processes. We further give an overview of the experimental and modeling approaches to study membrane phenomena. We close with a perspective on the converging futures of systems biology and membrane biophysics, invoking the need to include physical variables such as location and geometry in the study of cellular membranes. WIREs Syst Biol Med 2017, 9:e1386. doi: 10.1002/wsbm.1386 For further resources related to this article, please visit the WIREs website.

Membrane Oxidation in Cell Delivery and Cell Killing Applications

Cell delivery or cell killing processes often involve the crossing or disruption of cellular membranes. We review how, by modifying the composition and properties of membranes, membrane oxidation can be exploited to enhance the delivery of macromolecular cargoes into live human cells. We also describe how membrane oxidation can be utilized to achieve efficient killing of bacteria by antimicrobial peptides. Finally, we present recent evidence highlighting how membrane oxidation is intimately engaged in natural biological processes such as antigen delivery in dendritic cells and in the killing of bacteria by antimicrobial peptides. Overall, the insights that have been recently gained in this area should facilitate the development of more effective delivery technologies and antimicrobial therapeutic approaches.

Production of Isolated Giant Unilamellar Vesicles under High Salt Concentrations

The cell membrane forms a dynamic and complex barrier between the living cell and its environment. However, its in vivo studies are difficult because it consists of a high variety of lipids and proteins and is continuously reorganized by the cell. Therefore, membrane model systems with precisely controlled composition are used to investigate fundamental interactions of membrane components under well-defined conditions. Giant unilamellar vesicles (GUVs) offer a powerful model system for the cell membrane, but many previous studies have been performed in unphysiologically low ionic strength solutions which might lead to altered membrane properties, protein stability and lipid-protein interaction. In the present work, we give an overview of the existing methods for GUV production and present our efforts on forming single, free floating vesicles up to several tens of μm in diameter and at high yield in various buffer solutions with physiological ionic strength and pH.

Production of Isolated Giant Unilamellar Vesicles under High Salt Concentrations

The cell membrane forms a dynamic and complex barrier between the living cell and its environment. However, its in vivo studies are difficult because it consists of a high variety of lipids and proteins and is continuously reorganized by the cell. Therefore, membrane model systems with precisely controlled composition are used to investigate fundamental interactions of membrane components under well-defined conditions. Giant unilamellar vesicles (GUVs) offer a powerful model system for the cell membrane, but many previous studies have been performed in unphysiologically low ionic strength solutions which might lead to altered membrane properties, protein stability and lipid-protein interaction. In the present work, we give an overview of the existing methods for GUV production and present our efforts on forming single, free floating vesicles up to several tens of μm in diameter and at high yield in various buffer solutions with physiological ionic strength and pH.

A Molecular Rotor that Measures Dynamic Changes of Lipid Bilayer Viscosity Caused by Oxidative Stress

Oxidation of cellular structures is typically an undesirable process that can be a hallmark of certain diseases. On the other hand, photooxidation is a necessary step of photodynamic therapy (PDT), a cancer treatment causing cell death upon light irradiation. Here, the effect of photooxidation on the microscopic viscosity of model lipid bilayers constructed of 1,2-dioleoyl-sn-glycero-3-phosphocholine has been studied. A molecular rotor has been employed that displays a viscosity-dependent fluorescence lifetime as a quantitative probe of the bilayer's viscosity. Thus, spatially-resolved viscosity maps of lipid photooxidation in giant unilamellar vesicles (GUVs) were obtained, testing the effect of the positioning of the oxidant relative to the rotor in the bilayer. It was found that PDT has a strong impact on viscoelastic properties of lipid bilayers, which 'travels' through the bilayer to areas that have not been irradiated directly. A dramatic difference in viscoelastic properties of oxidized GUVs by Type I (electron transfer) and Type II (singlet oxygen-based) photosensitisers was also detected.

An overview of molecular dynamics simulations of oxidized lipid systems, with a comparison of ELBA and MARTINI force fields for coarse grained lipid simulations

Biological membranes and model lipid systems containing high amounts of unsaturated lipids and sterols are subject to chemical and/or photo-induced lipid oxidation, which leads to the creation of exotic oxidized lipid products (OxPLs). OxPLs are known to have significant physiological impact in cellular systems and also affect physical properties of both biological and model lipid bilayers. In this paper we (i) provide a perspective on the existing literature on simulations of lipid bilayer systems containing oxidized lipid species as well as the main related experimental results, (ii) describe our new data of all-atom and coarse-grained simulations of hydroperoxidized lipid monolayer and bilayer systems and (iii) provide a comparison of the MARTINI and ELBA coarse grained force fields for lipid bilayer systems. We show that the better electrostatic treatment of interactions in ELBA is able to resolve previous conflicts between experiments and simulations. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Oxidation of Membrane Curvature-Regulating Phosphatidylethanolamine Lipid Results in Formation of Bilayer and Cubic Structures

Oxidation is associated with conditions related to chronic inflammations and aging. Cubic structures have been observed in the smooth endoplasmic reticulum and mitochondrial membranes of cells under oxidative stress (e.g., tumor cells and virus-infected cells). It has been previously suspected that oxidation can result in the rearrangement of lipids from a fluid lamellar phase to a cubic structure in organelles containing membranes enriched with amphiphiles that have nonzero intrinsic curvature, such as phosphatidylethanolamine (PE) and cardiolipin. This study focuses on the oxidation of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), a lipid that natively forms an inverted hexagonal phase at physiological conditions. The oxidized samples contain an approximately 3:2 molar ratio of nonoxidized to oxidized DOPE. Optical microscopy images collected during the hydration of this mixture from a dried film suggest that the system evolves into a coexistence of a stable fluid lamellar phase and transient square lattice structures with unit cell sizes of 500-600 nm. Small-angle X-ray scattering of the same lipid mixture yielded a body-centered Im3m cubic phase with the lattice parameter of 14.04 nm. On average, the effective packing parameter of the oxidized DOPE species was estimated to be 0.657 ± 0.069 (standard deviation). This suggests that the oxidation of PE leads to a group of species with inverted molecular intrinsic curvature. Oxidation can create amphiphilic subpopulations that potently impact the integrity of the membrane, since negative Gaussian curvature intrinsic to cubic phases can enable membrane destabilization processes.

Addition of Cleaved Tail Fragments during Lipid Oxidation Stabilizes Membrane Permeability Behavior

Lipid oxidation has been linked to plasma membrane damage leading to cell death. In previous work, we examined the effect of oxidation on bilayer permeability by replacing defined amounts of an unsaturated lipid species with the corresponding phospholipid product that would result from oxidative tail scission of that species. This study adds the cleaved tail fragment, better mimicking the chemical results of oxidation. Permeability of PEG12-NBD, a small, uncharged molecule, was measured for vesicles with oxidation concentration corresponding to between 0 and 18 mol % of total lipid content. Permeability was measured using a microfluidic trap to capture the vesicles and spinning disk confocal microscopy (SDCM) to measure the transport of fluorescent PEG12-NBD at the equatorial plane. The thicknesses of lipid bilayers containing oxidized species were estimated by measuring capacitance of a black lipid membrane while simultaneously measuring bilayer area. We found that relative to chemically modeled oxidized bilayers without tail fragments, bilayers containing cleaved tail groups were less permeable for the same degree of oxidation. Curiously, membrane capacitance measurements indicated that the addition of tail fragments to chemically modeled oxidized bilayers also thinned these bilayers relative to samples with no tail fragments; in other words, the more permeable membranes were thicker. Above 12.5% chemically modeled oxidation, compositions both with and without the cleaved tail groups showed pore formation. This work highlights the complexity of the relationship between chemically modeled lipid bilayer oxidation and cell membrane properties.

Peroxidised phospholipid bilayers: insight from coarse-grained molecular dynamics simulations

An original coarse-grained model for peroxidised phospholipids is presented, based on the MARTINI lipid force field. This model results from a combination of thermodynamic modelling and structural information on the area per lipid, which have been made available recently. The resulting coarse-grained lipid molecules form stable bilayers, and a set of elastic coefficients (compressibility and bending moduli) is obtained. We compare the compressibility coefficient to the experimental values [Weber et al., Soft Matter, 2014, 10, 4241]. Predictions for the mechanical properties, membrane thickness and lateral distribution of hydroperoxide groups in the phospholipid bilayer are presented.

Bilayer Deformation, Pores, and Micellation Induced by Oxidized Lipids

The influence of different oxidized lipids on lipid bilayers was investigated with 16 individual 1 μs atomistic molecular dynamics (MD) simulations. Binary mixtures of lipid bilayers of 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine (PLPC) and its peroxide and aldehyde products were performed at different concentrations. In addition, an asymmetrical short chain lipid, 1-palmitoyl-2-decanoyl-sn-glycero-3-phosphatidylcholine (PDPC), was used to compare the effects of polar/apolar groups in the lipid tail on lipid bilayer. Although water defects occurred with both aldehyde and peroxide lipids, full pore formation was observed only for aldehyde lipids. At medium concentrations the pores were stable. At higher concentrations, however, the pores became unstable and micellation occurred. Data analysis shows that aldehyde lipids' propensity for pore formation is due to their shorter and highly mobile tail. The highly polar peroxide lipids are stabilized by strong hydrogen bonds with interfacial water.

Stability and instability for low refractive-index-contrast particle trapping in a dual-beam optical trap

A dual-beam optical trap is used to trap and manipulate dielectric particles. When the refractive index of these particles is comparable to that of the surrounding medium, equilibrium trapping locations within the system shift from stable to unstable depending on fiber separation and particle size. This is due to to the relationship between gradient and scattering forces. We experimentally and computationally study the transitions between stable and unstable trapping of poly(methyl methacrylate) beads for a range of parameters relevant to experimental setups involving giant unilamellar vesicles. We present stability maps for various fiber separations and particle sizes, and find that careful attention to particle size and configuration is necessary to obtain reproducible quantitative results for soft matter stretching experiments.