The use of giant unilamellar vesicles to study functional properties of pore-forming toxins

Molecular Mechanisms of Action of Dendrimers with Antibacterial Activities on Model Lipid Membranes

In the last decades, numerous dendrimers with a variety of potential biomedical applications have been developed and investigated. The aim of the present study was to evaluate the molecular mechanisms of interaction between two dendrimers with proven antibacterial activity (4-N,N-dimethylamino-1,8-naphthalimide (Dab) and 3-bromo-Dab (Dab-Br)) and POPC (1-palmitoyl-2-oleoylphosphatidylcholine) model membranes (monolayers and liposomes). The pressure-area isotherms and the compressional modulus of the monolayers revealed that Dab is likely to penetrate the hydrophobic region of POPC, whereas Dab-Br inserts mainly into the lipid headgroup area. This assumption was confirmed by FTIR-ATR of POPC liposomes containing Dab and Dab-Br dendrimers. In addition, Dab induced a higher lipid order in POPC large unilamellar vesicles (LUVs) compared to Dab-Br. Moreover, both dendrimers changed the negative zeta potential of POPC vesicles to positive values, with slightly higher effect of Dab-Br, indicating electrostatic interactions between the lipid headgroups and dendrimers. Furthermore, Dab was able to reduce the average POPC LUVs' size, unlike Dab-Br. The visualization of giant unilamellar vesicles revealed that the increasing dendrimer concentration induced model membrane shrinking and complete disintegration, which was more prominent for Dab. Based on the experimental results, new fundamental knowledge about the destabilizing effect of dendrimers on model lipid membranes will be acquired with a focus on their application in pharmacology and clinical practice.

Fluorescent Rhein-Loaded Liposomes for In Vivo Biodistribution Study

Objectives: This work aimed to develop and investigate liposomes incorporating Rhein (Lip-Rh) into the liposomal membrane to enhance the compound's water solubility and oral bioavailability. Methods: Liposomes were produced by the thin lipid film technique, with a phosphatidylcholine-to-cholesterol molar ratio of 5:1, dissolved in chloroform and methanol, and thereafter hydrated with ultrapure water and subjected to sonication. The resultant liposomes were studied from a physicochemical perspective using DLS, zeta potential, STEM, UV-Vis, and fluorescence spectroscopies, while oral bioavailability was assessed by fluorescence imaging. Additionally, cell viability assays were performed on tumour cells (MCF-7) in comparison to normal cells (HGFs). Results: The resultant nanoparticles exhibited relatively uniform sizes and narrow size distribution. In vivo fluorescence imaging studies performed on Wistar rats demonstrated significantly enhanced oral bioavailability for Lip-Rh, with rapid absorption into the bloodstream observed one hour after administration, in contrast to the free compound dissolved in vegetable oil. Cell viability assays demonstrated higher cytotoxicity of Lip-Rh towards MCF-7 cells compared to HGF cells, highlighting the selective therapeutic potential of the product. Moreover, we determined that the optimal dose of Rhein per kilogram of body weight, when encapsulated in liposomes, is approximately 2.5 times less than when Rhein is delivered in its unencapsulated form. Conclusions: Lip-Rh is a promising candidate for oncological treatments, presenting three key advantages: increased cytotoxicity towards tumour cells, protection of normal tissues, and the practicality of oral delivery. Additional investigation is required to explore its application in anticancer therapy, whether as monotherapy or as a complementary treatment.

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.

Genetically programmed synthetic cells for thermo-responsive protein synthesis and cargo release

Synthetic cells containing genetic programs and protein expression machinery are increasingly recognized as powerful counterparts to engineered living cells in the context of biotechnology, therapeutics and cellular modelling. So far, genetic regulation of synthetic cell activity has been largely confined to chemical stimuli; to unlock their potential in applied settings, engineering stimuli-responsive synthetic cells under genetic regulation is imperative. Here we report the development of temperature-sensitive synthetic cells that control protein production by exploiting heat-responsive mRNA elements. This is achieved by combining RNA thermometer technology, cell-free protein expression and vesicle-based synthetic cell design to create cell-sized capsules able to initiate synthesis of both soluble proteins and membrane proteins at defined temperatures. We show that the latter allows for temperature-controlled cargo release phenomena with potential implications for biomedicine. Platforms like the one presented here can pave the way for customizable, genetically programmed synthetic cells under thermal control to be used in biotechnology.

Droplet Microfluidics for High-Throughput Screening and Directed Evolution of Biomolecules

Directed evolution is a powerful technique for creating biomolecules such as proteins and nucleic acids with tailor-made properties for therapeutic and industrial applications by mimicking the natural evolution processes in the laboratory. Droplet microfluidics improved classical directed evolution by enabling time-consuming and laborious steps in this iterative process to be performed within monodispersed droplets in a highly controlled and automated manner. Droplet microfluidic chips can generate, manipulate, and sort individual droplets at kilohertz rates in a user-defined microchannel geometry, allowing new strategies for high-throughput screening and evolution of biomolecules. In this review, we discuss directed evolution studies in which droplet-based microfluidic systems were used to screen and improve the functional properties of biomolecules. We provide a systematic overview of basic on-chip fluidic operations, including reagent mixing by merging continuous fluid streams and droplet pairs, reagent addition by picoinjection, droplet generation, droplet incubation in delay lines, chambers and hydrodynamic traps, and droplet sorting techniques. Various microfluidic strategies for directed evolution using single and multiple emulsions and biomimetic materials (giant lipid vesicles, microgels, and microcapsules) are highlighted. Completely cell-free microfluidic-assisted in vitro compartmentalization methods that eliminate the need to clone DNA into cells after each round of mutagenesis are also presented.

Binding of His-tagged fluorophores to lipid bilayers of giant vesicles

His-tagged molecules can be attached to lipid bilayers via certain anchor lipids, a method that has been widely used for the biofunctionalization of membranes and vesicles. To observe the membrane-bound molecules, it is useful to consider His-tagged molecules that are fluorescent as well. Here, we study two such molecules, green fluorescence protein (GFP) and green-fluorescent fluorescein isothiocyanate (FITC), both of which are tagged with a chain of six histidines (6H) that bind to the anchor lipids within the bilayers. The His-tag 6H is much smaller than the GFP molecule but somewhat larger than the FITC dye. The lipid bilayers form giant unilamellar vesicles (GUVs), the behavior of which can be directly observed in the optical microscope. We apply and compare three well-established preparation methods for GUVs: electroformation on platinum wire, polyvinyl alcohol (PVA) hydrogel swelling, and electroformation on indium tin oxide (ITO) glass. Microfluidics is used to expose the GUVs to a constant fluorophore concentration in the exterior solution. The brightness of membrane-bound 6H-GFP exceeds the brightness of membrane-bound 6H-FITC, in contrast to the quantum yields of the two fluorophores in solution. In fact, 6H-FITC is observed to be strongly quenched by the anchor lipids which bind the fluorophores via Ni²⁺ ions. For both 6H-GFP and 6H-FITC, the membrane fluorescence is measured as a function of the fluorophores' molar concentration. The theoretical analysis of these data leads to the equilibrium dissociation constants K_d = 37.5 nM for 6H-GFP and K_d = 18.5 nM for 6H-FITC. We also observe a strong pH-dependence of the membrane fluorescence.

A Fluorescence-based Assay for Measuring Phospholipid Scramblase Activity in Giant Unilamellar Vesicles

Transbilayer movement of phospholipids in biological membranes is mediated by a diverse set of lipid transporters. Among them are scramblases that facilitate rapid bi-directional movement of lipids without metabolic energy input. In this protocol, we describe the incorporation of phospholipid scramblases into giant unilamellar vesicles (GUVs) formed from scramblase-containing large unilamellar vesicles by electroformation. We also describe how to analyze their activity using membrane-impermeant sodium dithionite, to bleach symmetrically incorporated fluorescent ATTO488-conjugated phospholipids. The fluorescence-based readout allows single vesicle tracking for a large number of settled/immobilized GUVs, and provides a well-defined experimental setup to directly characterize these lipid transporters at the molecular level. Graphic abstract: Giant unilamellar vesicles (GUVs) are formed by electroformation from large unilamellar vesicles (LUVs) containing phospholipid scramblases (purple) and trace amounts of a fluorescent lipid reporter (green). The scramblase activity is analyzed by a fluorescence-based assay of single GUVs, using the membrane-impermeant quencher dithionite. Sizes not to scale. Modified from Mathiassen et al. (2021).

Endoplasmic reticulum phospholipid scramblase activity revealed after protein reconstitution into giant unilamellar vesicles containing a photostable lipid reporter

Transbilayer movement of phospholipids in biological membranes is mediated by a diverse set of lipid transporters. Among them are scramblases that facilitate a rapid bi-directional movement of lipids without metabolic energy input. Here, we established a new fluorescence microscopy-based assay for detecting phospholipid scramblase activity of membrane proteins upon their reconstitution into giant unilamellar vesicles formed from proteoliposomes by electroformation. The assay is based on chemical bleaching of fluorescence of a photostable ATTO-dye labeled phospholipid with the membrane-impermeant reductant sodium dithionite. We demonstrate that this new methodology is suitable for the study of the scramblase activity of the yeast endoplasmic reticulum at single vesicle level.