Advances in giant unilamellar vesicle preparation techniques and applications

On the coupling between membrane bending and stretching in lipid vesicles

The formation of a lipid vesicle from a lamellar phase involves a cost in bending energy of 100-1000 times the thermal energy for values of the membrane bending rigidity κ typical for phospholipid bilayers. The bending rigidity of a bilayer is however a strongly decreasing function of its thickness h, and the bilayer can thus reduce its bending energy by stretching (and thus thinning) the bilayer. In this paper, we construct a simple model to describe this mechanism for the coupling between bending and stretching and analyse its effect on the bending energy and thermal fluctuations of spherical lipid vesicles. We show that the bilayer thinning becomes significant for small vesicles, and for a vesicle with radius there is a sizeable thinning of the bilayer compared to the planar state. We furthermore demonstrate how this thinning is associated with a significant decrease in free energy due to the thermally excited bending modes. We argue that this previously unexplored effect can explain the experimentally observed lower limit of achievable vesicle sizes, which eventually become unstable due to the thinning of the bilayer. We also sketch how this effect provides a potential generic mechanism for the strong curvature dependence of protein adsorption to lipid membranes.

Microfluidics-based stable production of monodisperse giant unilamellar vesicles by oil-phase removal from double emulsion

Giant liposomes, or giant unilamellar vesicles (GUVs), have been utilized as cell-size bioreactors to replicate the physical and chemical properties of biological cells. However, conventional methods for preparing GUVs typically lack precise control over their size. Several research groups have recently developed microfluidic techniques to create monodisperse GUVs by generating water-in-oil-in-water (W/O/W) droplets with a thin oil layer that subsequently transform into GUVs. However, the formation of a thin oil shell requires the intricate control of the flow rate, which can be both challenging and unstable. In this study, we investigated the design of a two-step flow-focusing microfluidic channel to produce stable W/O/W droplets. These droplets underwent substantial oil layer reduction through spontaneous removal by fluidic shear forces. Consequently, the majority of the oil layer in the W/O/W droplets was reduced, improving uniformity of GUVs.

Extracellular Vesicle-Based Drug Delivery Systems in Cancer Therapy

Extracellular vesicles (EVs) are lipid bilayer-enclosed particles secreted by cells and ubiquitously present in various biofluids. They not only mediate intercellular communication but also serve as promising drug carriers that are capable of delivering therapeutic agents to target cells through their inherent physicochemical properties. In this review, we summarized the recent advances in EV isolation techniques and innovative drug-loading strategies. Furthermore, we emphasized the distinct advantages and therapeutic applications of EVs derived from different cellular sources in cancer treatment. Finally, we critically evaluated the ongoing clinical trials utilizing EVs for drug delivery and systematically assessed both the opportunities and challenges associated with implementing EV-based drug delivery systems in cancer therapy.

Fabrication of millimetre-sized amphoteric chitosan/alginate hollow vesicles for the adsorption of anionic and cationic dyes

In the face of complex dye wastewater, the development of amphoteric adsorbents for effective removal is imperative. Herein, millimetre-sized amphoteric chitosan/sodium alginate hollow vesicles were successfully prepared via a facile hydrothermal stirring method. The abundant functional groups, uniformly distributed micropores, and inner cavities contribute to enhance the adsorption capacity towards various types of charged dyes, including methylene blue (144.8 mg/g), acid blue-113 (654.9 mg/g), and methyl orange (400.3 mg/g). The vesicles worked well in the pH range of 3-11 and salt concentration up to 100 mmol/L, and remained >92 % of the original adsorption capacity after four cycles. The adsorption is a thermodynamic spontaneous process and the adsorption mechanism is monolayer chemisorption. Importantly, the vesicles possess semi-permeable membrane properties and excellent selectivity, making them favourable for complex wastewater treatment. The environment-friendly and high-performance amphoteric vesicles made from natural marine polysaccharides offer new possibilities for the development of effective absorbents.

µ-Raman Spectroscopic Temperature Dependence Study of Biomimetic Lipid 1,2-Diphytanoyl-sn-glycero-3-phosphocholine

Raman spectroscopy is one of the best techniques for obtaining information concerning the physical-chemical interactions between a lipid and a solvent. Phospholipids in water are the main elements of cell membranes and, by means of their chemical and physical structures, their cells can interact with other biological molecules (i.e., proteins and vitamins) and express their own biological functions. Phospholipids, due to their amphiphilic structure, form biomimetic membranes which are useful for studying cellular interactions and drug delivery. Synthetic systems such as DPhPC-based liposomes replicate the key properties of biological membranes. Among the different models, phospholipid mimetic membrane models of lamellar vesicles have been greatly supported. In this work, a biomimetic system, a deuterium solution (50 mM) of the synthetic phospholipid 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhDC), is studied using μ-Raman spectroscopy in a wide temperature range from -181.15 °C up to 22.15 °C, including the following temperatures: -181.15 °C, -146.15 °C, -111.15 °C, -76.15 °C, -61.15 °C, -46.15 °C, -31.15 °C, -16.15 °C, -1.15 °C, 14.15 °C, and 22.15 °C. Based on the Raman evidence, phase transitions as a function of temperature are shown and grouped into five classes, where the corresponding Raman modes describe the stretching of the (C-N) bond in the choline head group (gauche) and the asymmetric stretching of the (O-P-O) bond. The acquisition temperature of each Raman spectrum characterizes the rocking mode of the methylene of the acyl chain. These findings enhance our understanding of the role of artificial biomimetic lipids in complex phospholipid membranes and provide valuable insights for optimizing their use in biosensing applications. Although the phase stability of DPhPC is known, the collected Raman data suggest subtle molecular rearrangements, possibly due to hydration and second-order transitions, which are relevant for membrane modeling and biosensing applications.

Engineering of lipid membranes asymmetrically functionalized with chondroitin sulfate

One of the defining properties of the eukaryotic plasma membrane is the glycocalyx, which is formed by glycosylated lipids and proteins. The glycocalyx is arranged asymmetrically, as it is exclusive to the extracellular side of the membrane. Membrane asymmetry therefore includes both lipid and carbohydrate asymmetry, whereby the latter has the most skewed trans-leaflet imbalance. The glycocalyx plays a structural role that protects cell integrity and it also participates in mechanosensing and other cellular processes. However, our understanding of glycocalyx function is hampered by the lack of suitable model systems to perform biophysical investigation. Here, we describe the engineering of lipid bilayers that are chemically conjugated at the outer surface with one of the most abundant glycocalyx components, chondroitin sulfate (CS). Membranes were doped with a reactive phospholipid, which allowed thiol-maleimide conjugation of thiol-modified CS at the lipid headgroup. Our data show that we achieved CS conjugation of large unilamellar vesicles, supported lipid bilayers, and giant unilamellar vesicles. CS conjugation of vesicles allowed electrostatic recruitment of poly-L-lysine, which could recruit other CS-coated vesicles or CS in solution. Overall, we describe a simple and robust method for polysaccharide functionalization of vesicles which can be applied to gain new mechanistic understanding of the pathophysiological role of the glycocalyx.

Empowering bacteria with light: Optogenetically engineered bacteria for light-controlled disease theranostics and regulation

Bacterial therapy has emerged as a promising approach for disease treatment due to its environmental sensitivity, immunogenicity, and modifiability. However, the clinical application of engineered bacteria is limited by differences of expression levels in patients and possible off-targeting. Optogenetics, which combines optics and genetics, offers key advantages such as remote controllability, non-invasiveness, and precise spatiotemporal control. By utilizing optogenetic tools, the behavior of engineered bacteria can be finely regulated, enabling on-demand control of the dosage and location of their therapeutic products. In this review, we highlight the latest advancements in the optogenetic engineering of bacteria for light-controlled disease theranostics and therapeutic regulation. By constructing a three-dimensional analytical framework of "sense-produce-apply", we begin by discussing the key components of bacterial optogenetic systems, categorizing them based on their photosensitive protein response to blue, green, and red light. Next, we introduce innovative light-producing tools that extend beyond traditional light sources. Then, special emphasis is placed on the biomedical applications of optogenetically engineered bacteria in treating diseases such as cancer, intestinal inflammation and systemic disease regulation. Finally, we address the challenges and future prospects of bacterial optogenetics, outlining potential directions for enhancing the safety and efficacy of light-controlled bacterial therapies. This review aims to provide insights and strategies for researchers working to advance the application of optogenetically engineered bacteria in drug delivery, precision medicine and therapeutic regulation.

Interaction of an Oomycete Nep1-like Cytolysin with Natural and Plant Cell-Mimicking Membranes

Plants are attacked by various pathogens that secrete a variety of effectors to damage host cells and facilitate infection. One of the largest and so far understudied microbial protein families of effectors is necrosis- and ethylene-inducing peptide-1-like proteins (NLPs), which are involved in important plant diseases. Many NLPs act as cytolytic toxins that cause cell death and tissue necrosis by disrupting the plant's plasma membrane. Their mechanism of action is unique and leads to the formation of small, transient membrane ruptures. Here, we capture the interaction of the cytotoxic model NLP from the oomycete Pythium aphanidermatum, NLPPya, with plant cell-mimicking membranes of giant unilamellar vesicles (GUVs) and tobacco protoplasts using confocal fluorescence microscopy. We show that the permeabilization of GUVs by NLPPya is concentration- and time-dependent, confirm the small size of the pores by observing the inability of NLPPya monomers to pass through them, image the morphological changes of GUVs at higher concentrations of NLPPya and confirm its oligomerization on the membrane of GUVs. In addition, NLPPya bound to plasma membranes of protoplasts, which showed varying responses. Our results provide new insights into the interaction of NLPPya with model lipid membranes containing plant-derived sphingolipids.

Ultrahigh yields of giant vesicles obtained through mesophase evolution and breakup

Self-assembly of dry amphiphilic lipid films on surfaces upon hydration is a crucial step in the formation of cell-like giant unilamellar vesicles (GUVs). GUVs are useful as biophysical models, as soft materials, as chassis for bottom-up synthetic biology, and in biomedical applications. Here via combined quantitative measurements of the molar yield and distributions of sizes and high-resolution imaging of the evolution of thin lipid films on surfaces, we report the discovery of a previously unknown pathway of lipid self-assembly which can lead to ultrahigh yields of GUVs of >50%. This yield is about 60% higher than any GUV yield reported to date. The "shear-induced fragmentation" pathway occurs in membranes containing 3 mol% of the poly(ethylene glycol) modified lipid PEG2000-DSPE (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000]), when a lipid-dense foam-like mesophase forms upon hydration. The membranes in the mesophase fragment and close to form GUVs upon application of fluid shear. Experiments with varying mol% of PEG2000-DSPE and with lipids with partial molecular similarity to PEG2000-DSPE show that ultrahigh yields are only achievable under conditions where the lipid-dense mesophase forms. The increased yield of GUVs compared to mixtures without PEG2000-DSPE was general to flat supporting surfaces such as stainless steel sheets and to various lipid mixtures. In addition to increasing their accessibility as soft materials, these results demonstrate a route to obtaining ultrahigh yields of cell-sized liposomes using longstanding clinically-approved lipid formulations that could be useful for biomedical applications.

Effect of leaflet asymmetry on the stretching elasticity of lipid bilayers with phosphatidic acid

The asymmetry of membranes has a significant impact on their biophysical characteristics and behavior. This study investigates the composition and mechanical properties of symmetric and asymmetric membranes in giant unilamellar vesicles (GUVs) made of palmitoyloleoyl phosphatidylcholine (POPC) and palmitoyloleoyl phosphatidic acid (POPA). A combination of fluorescence quantification, zeta potential measurements, micropipette aspiration, and bilayer molecular dynamics simulations are used to characterize these membranes. The outer leaflet composition in vesicles is found consistent across the two preparation methods we employed, namely electroformation and inverted emulsion transfer. However, characterizing the inner leaflet poses challenges. Micropipette aspiration of GUVs show that oil residues do not substantially alter membrane elasticity, but simulations reveal increased membrane thickness and decreased interleaflet coupling in the presence of oil. Asymmetric membranes with a POPC:POPA mixture in the outer leaflet and POPC in the inner leaflet display similar stretching elasticity values to symmetric POPC:POPA membranes, suggesting potential POPA insertion into the inner leaflet during vesicle formation and suppressed asymmetry. The inverse compositional asymmetry, with POPC in the outer leaflet and POPC:POPA in the inner one yield less stretchable membranes with higher compressibility modulus compared with their symmetric counterparts. Challenges in achieving and predicting compositional correspondence highlight the limitations of phase-transfer-based methods. In addition, caution is advised when using fluorescently labeled lipids (even at low fractions of 0.5 mol %), as unexpected gel-like domains in symmetric POPC:POPA membranes were observed only with a specific type of labeled DOPE (dioleoylphosphatidylethanolamine) and the same fraction of unlabeled DOPE. The latter suggest that such domain formation may result from interactions between lipids and membrane fluorescent probes. Overall, this study underscores the complexity of factors influencing GUV membrane asymmetry, emphasizing the need for further research and improvement of characterization techniques.

Exogenous polyserine fibrils change membrane properties of phosphatidylcholine-liposome and red blood cells

The causative genes for neurodegenerative polyglutamine (polyQ) diseases produce homopolymeric polyglutamine (polyQ), polyserine (polyS), polyalanine (polyA), polycysteine (polyC), and polyleucine (polyL) sequences by repeat-associated non-AUG (RAN) translation. The cytotoxicity of the intracellular polyQ and RAN products has been extensively investigated. However, little is known about the toxicity of the extracellular polyQ and RAN products on the membranes of viable cells. Because polyQ aggregates induce a deflated morphology of a model membrane, we hypothesized that extracellular polyQ and RAN products might affect the membrane properties of viable cells. In this study, we demonstrated that exogenous polyS fibrils but not polyS or polyQ non-fibril aggregates altered the thermal phase transition behavior of a model membrane composed of a phosphatidylcholine bilayer using differential scanning calorimetry. PolyS fibrils induced morphological changes in viable red blood cells (RBCs). However, both polyS and polyQ non-fibril aggregates had no effects on RBCs. These results highlight the possibility that extracellular fibrils generated from RAN products may alter the properties of neuronal cell membranes, which may contribute to changes in the brain pathology.

Dynamics of giant vesicle assembly from thin lipid films

MOTIVATION

Giant unilamellar vesicles (GUVs), cell-like synthetic micrometer size structures, assemble when thin lipid films are hydrated in aqueous solutions. Quantitative measurements of static yields and distribution of sizes of GUVs obtained from thin film hydration methods were recently reported. Dynamic data such as the time evolution of yields and distribution of sizes, however, is not known. Dynamic data can provide insights into the assembly pathway of GUVs and guidelines for choosing conditions to obtain populations with desired size distributions.

APPROACH

We develop the 'stopped-time' technique to characterize the time evolution of the distribution of sizes and molar yields of populations of free-floating GUVs. We additionally capture high resolution time-lapse images of surface-attached GUV buds on the lipid films. We systematically study the dynamics of assembly of GUVs from three widely used thin film hydration methods, PAPYRUS (Paper-Abetted amPhiphile hYdRation in aqUeous Solutions), gentle hydration, and electroformation.

FINDINGS

We find that the molar yield versus time curves of GUVs demonstrate a characteristic sigmoidal shape, with an initial yield, a transient, and then a steady state plateau for all three methods. The population of GUVs showed a right-skewed distribution of diameters. The variance of the distributions increased with time. The systems reached steady state within 120 min. We rationalize the dynamics using the thermodynamically motivated budding and merging (BNM) model. These results further the understanding of lipid dynamics and provide for the first-time practical parameters to tailor the production of GUVs of specific sizes for applications.

Food liposomes: Structures, components, preparations, and applications

This review explores liposomes, focusing on their structure, components, the characteristics influencing their stability and applicability in foods, and preparation methods. The role of phospholipids and liposome modulators in preparing liposomes of desired structure and size is emphasized. The potential of liposomes to enhance food value through liposomal encapsulation and delivery of functional substances is reviewed. Conventional and advanced liposome preparation methods are reviewed, underscoring their impact on the marketability of liposomes. The review highlights the need for research into lecithin properties and modulators that enhance liposome stability. The need to develop cost-effective and rapid liposome preparation methods is identified as a key factor in improving the marketability of food liposomes and promoting their use in foods.

Engineering semi-permeable giant liposomes

Giant unilamellar vesicles (GUVs) with a semi-permeable nature are prerequisites for constructing synthetic cells. Here we engineer semi-permeable GUVs by the inclusion of DOTAP lipid in vesicles. Diffusion of molecules of different charge and size across GUVs are reported. Control over size-selective permeability is demonstrated by modulating the DOTAP lipid composition in different lipid systems without reconstituting membrane proteins. Such semi-permeable GUVs have immense applications for constructing synthetic cells.

QS21-Initiated Fusion of Liposomal Small Unilamellar Vesicles to Form ALFQ Results in Concentration of Most of the Monophosphoryl Lipid A, QS21, and Cholesterol in Giant Unilamellar Vesicles

Army Liposome Formulation with QS21 (ALFQ), a vaccine adjuvant preparation, comprises liposomes containing saturated phospholipids, with 55 mol% cholesterol relative to the phospholipids, and two adjuvants, monophosphoryl lipid A (MPLA) and QS21 saponin. A unique feature of ALFQ is the formation of giant unilamellar vesicles (GUVs) having diameters >1.0 µm, due to a remarkable fusion event initiated during the addition of QS21 to nanoliposomes containing MPLA and 55 mol% cholesterol relative to the total phospholipids. This results in a polydisperse size distribution of ALFQ particles, with diameters ranging from ~50 nm to ~30,000 nm. The purpose of this work was to gain insights into the unique fusion reaction of nanovesicles leading to GUVs induced by QS21. This fusion reaction was probed by comparing the lipid compositions and structures of vesicles purified from ALFQ, which were >1 µm (i.e., GUVs) and the smaller vesicles with diameter <1 µm. Here, we demonstrate that after differential centrifugation, cholesterol, MPLA, and QS21 in the liposomal phospholipid bilayers were present mainly in GUVs (in the pellet). Presumably, this occurred by rapid lateral diffusion during the transition from nanosize to microsize particles. While liposomal phospholipid recoveries by weight in the pellet and supernatant were 44% and 36%, respectively, higher percentages by weight of the cholesterol (~88%), MPLA (94%), and QS21 (96%) were recovered in the pellet containing GUVs, and ≤10% of these individual liposomal constituents were recovered in the supernatant. Despite the polydispersity of ALFQ, most of the cholesterol, and almost all of the adjuvant molecules, were present in the GUVs. We hypothesize that the binding of QS21 to cholesterol caused new structural nanodomains, and subsequent interleaflet coupling in the lipid bilayer might have initiated the fusion process, leading to creation of GUVs. However, the polar regions of MPLA and QS21 together have a "sugar lawn" of ten sugars, the hydrophilicity of which might have provided a driving force for rapid lateral diffusion and concentration of the MPLA and QS21 in the GUVs.

Imaging Giant Vesicle Membrane Domains with a Luminescent Europium Tetracycline Complex

Microdomains in lipid bilayer membranes are routinely imaged using organic fluorophores that preferentially partition into one of the lipid phases, resulting in fluorescence contrast. Here, we show that membrane microdomains in giant unilamellar vesicles (GUVs) can be visualized with europium luminescence using a complex of europium III (Eu3+) and tetracycline (EuTc). EuTc is unlike typical organic lipid probes in that it is a coordination complex with a unique excitation/emission wavelength combination (396/617 nm), a very large Stokes shift (221 nm), and a very narrow emission bandwidth (8 nm). The probe preferentially interacts with liquid disordered domains in GUVs, which results in intensity contrast across the surface of phase-separated GUVs. Interestingly, EuTc also alters GM1 ganglioside partitioning. GM1 typically partitions into liquid ordered domains, but after labeling phase-separated GUVs with EuTc, cholera toxin B-subunit (CTxB), which binds GM1, labels liquid disordered domains. We also demonstrate that EuTc, but not free Eu3+ or Tc, significantly reduces lipid diffusion coefficients. Finally, we show that EuTc can be used to label cellular membranes similar to a traditional membrane probe. EuTc may find utility as a membrane imaging probe where its large Stokes shift and sharp emission band would enable multicolor imaging.