Mechanics of membrane fusion

Facile lipid nanoparticle size engineering approach via controllable fusion induced by depletion forces

Lipid nanoparticles (LNPs) are among the most promising drug delivery carriers in research and development, with one major clinical application being messenger RNA (mRNA) vaccine. Current LNP production methods have the limit of generating low polydispersity index (PDI; PDI < 0.1) only for relatively small particles (<100 nm). It is known that larger LNPs have desirable properties, for example, particles with diameters in the 100 to 200 nm range have good immunogenicity. Yet, these larger particles' large PDI limits their clinical translation because of concerns about manufacturing reproducibility and possible side effects. We report here a facile approach to produce large and monodisperse (100-200 nm, PDI < 0.1) LNPs. The approach is based on adding 10 kDa polyethylene glycol (PEG) to a solution containing smaller LNPs. We show that PEG-induced depletion forces lead to the fusion of LNPs to form particles of approximately double the original size while keeping the same starting PDI. We discuss the fusion mechanism and show the parameters it depends on. In particular, we show that the fusion leads to a decrease in the fraction of empty LNPs. We show that the purification for PEG after fusion is simple and complete, thus, we believe that this is a method for the production of large LNP with low PDI that has a lot of potential to find industrial use.

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.

Importance of the Number and Position of Tryptophan-Aspartate Repeats in Designing Peptide-Based Membrane Fusion Inhibitors

Enveloped viruses utilize membrane fusion to enter the host cell and cause viral diseases. Most conventional inhibitors are target-specific and ineffective against broad-spectrum viral infections. Therefore, the current scenario demands developing inhibitors to attenuate the membrane properties instead of targeting the viral proteins. In the present work, we have designed a new WD-containing peptide (dmTG-23) by introducing a WD repeat at the center of the peptide sequence using mTG-23 peptide as a template and a 14-amino acid peptide with seven WD repeats (WD-14), and tested their efficacies against the polyethylene glycol (PEG)-induced fusion assay. Our findings show that both dmTG-23 and WD-14 prevent PEG-induced fusion of small unilamellar vesicles (SUVs) with varying cholesterol concentrations, with a higher effect for dmTG-23. Moreover, we have compared our results with our previous observations on TG-23 and mTG-23 to evaluate the importance of the number and position of WD repeats in the peptide sequence to impart the highest inhibitory activity. Our results demonstrate that three WD repeat-containing dmTG-23 shows the highest efficacy against the PEG-induced fusion of SUVs by enhancing the membrane order at the acyl chain region. This result is significantly important in designing peptide-based broad-spectrum fusion inhibitors.

Size Dependent Liposomal Fusion and H-Bonded Sticky Aggregation Induced by Gold Nanoclusters

We present a novel approach to quantitatively control liposomal fusion and aggregation, depending upon the surface charge and functionalities, using fluorescent gold nanoclusters (AuNCs). We show that ligand-protected ultrasmall AuNCs are excellent fusogenic materials that follow renal clearance pathways after induction of liposomal fusion, especially with dipalmitoylphosphatidylcholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) mixed in a 3:1 molar ratio mimicking the brain cells. AuNCs show interesting photophysics and emit from visible to near-infrared (NIR) regions depending upon the quantum confinement effect that is directly related to the Fermi wavelength and size of the NCs. Herein, we have synthesized orange-emitting l-glutathione (GSH) coated Au25(GSH)18 NCs (GSH-AuNCs) to be applied on lipid membranes to observe the fusogenic property on small and giant unilamellar vesicles (SUVs and GUVs). We noticed that the GSH-AuNCs easily attach to the surface of the DPPC liposomes and facilitate complete fusion through favorable lipid mixing. The fusion rate is higher (80%) for the DPPC SUVs compared to that for the GUVs, which has been explained experimentally. On the contrary, the DMPG SUVs aggregate, with minimal lipid mixing, in the presence of the GSH-AuNCs. We noted a ∼70% fusion efficiency for SUVs with mixed DPPC:DMPG composition (3:1 molar ratio). This is the first report on fusion and aggregation of liposomes guided by ligand functionalities and surface charge of AuNCs.

Fusion of asymmetric membranes: the emergence of a preferred direction

The fusion of lipid membranes progresses through a series of intermediate steps with two significant energy barriers: hemifusion-stalk formation and fusion-pore expansion. The cell's ability to tune these energy barriers is crucial as they determine the rate of many biological processes involving membrane fusion. However, a mechanism that allows the cell to manipulate both barriers in the same direction remains elusive, since membrane properties that the cell could dynamically tune during its life cycle, such as the lipids' spontaneous curvatures and membrane tension, have an opposite effect on the two barriers: tension inhibits stalk formation while promoting fusion-pore expansion. In contrast, increasing the total membrane concentration of lipids with negative intrinsic curvatures, such as cholesterol, promotes hemifusion-stalk formation while inhibiting pore expansion, and vice versa for lipids with positive intrinsic curvatures. Therefore, changes in these membrane properties increase one energy barrier at the expense of the other, resulting in a mixed effect on the fusion reaction. A possible mechanism to change both barriers in the same direction is by inducing lipid composition asymmetry, which results in tension and spontaneous curvature differences between the monolayers. To test the feasibility of this mechanism, a continuum elastic model was used to simulate the fusion intermediates and calculate the changes in the energy barriers. The calculations showed that a reasonable lipid composition asymmetry could lead to a 10-20kBT difference in both energy barriers, depending on the direction from which fusion occurs. We further provide experimental support to the model predictions, demonstrating changes in the time to hemifusion upon asymmetry introduction. These results indicate that biological membranes, which are asymmetric, have a preferred direction for fusion.

Effect of membrane tension on pore formation induced by antimicrobial peptides and other membrane-active peptides

Membrane tension plays an important role in various aspects of the dynamics and functions of cells. Here, we review recent studies of the effect of membrane tension on pore formation in lipid bilayers and pore formation induced by membrane-active peptides (MAPs) including antimicrobial peptides (AMPs). For this purpose, the micropipette aspiration method using a patch of cell membrane/lipid bilayers and a giant unilamellar vesicle (GUV)/a total cell, and the application of osmotic pressure (Π) to suspensions of large unilamellar vesicles (LUVs) have been used. However, these conventional methods have some drawbacks for the investigation of the effect of membrane tension on the actions of MAPs such as AMPs. Recently, to overcome these drawbacks, a new Π method using GUVs has been developed. Here, we focus on this Π method as a new technique for revealing the effect of membrane tension on the MAPs-induced pore formation. Firstly, we review studies of the effect of membrane tension on pore formation in lipid bilayers as determined by conventional methods. Secondly, after a brief review of studies of the effect of Π on LUVs, we describe the estimation of membrane tension in GUVs induced by Π and the Π-induced pore formation. Thirdly, after a review of the effect of membrane tension on the MAPs-induced pore formation as obtained by the conventional methods, we describe an application of the Π method to studies of the effect of membrane tension on AMP-induced pore formation. Finally, we discuss the advantages of the Π method over conventional methods and consider future perspectives.

Fusion of Human Synovium-Derived Mesenchymal Stem/Stromal Cells with Primary Human Chondrocytes Using the Modified Adherence Method (MAM)

Cell fusion is a complex phenomenon that is key in maintaining tissue homeostasis, particularly in aiding tissue regeneration processes. Studies show that mesenchymal stem/stromal cells (MSCs) are capable of restoring damaged tissue by adopting the phenotype of various cell types via cell fusion. As cell fusion of MSCs with cells of different origin remains poorly researched, we have developed a protocol that allows successful electrofusion between human synovium-derived MSCs and human chondrocytes. Building on from our protocol can help researchers study the cell fusion processes in the in vitro environment and could set basis for development of fusion cell-based advanced therapy medicinal products (ATMPs). In our protocol, we provide a detailed description on how to culture both of the fusion partner cells, how to carry out the modified adherence method (MAM) to achieve a high yield of successfully fused cells, and how to determine the yield of cell fusion using either methyl violet or fluorescent cell trackers.

Anomalous membrane organization by omega-6 and omega-9 fatty acids

Omega fatty acids are currently being marketed as healthy food supplements as they have been implicated in multiple pathophysiological conditions, such as reducing plaque formation of Aβ peptide and inhibiting SARS-CoV-2 infection. Their mode of action has been hypothesized to be via membrane reorganization by the unsaturated acyl chains, leading to the modulation of lipid-protein cross-talk. However, the lack of molecular details led us to evaluate the molecular effect of omega-6 (linolenic acid) and omega-9 (oleic acid) fatty acids on membrane organization using a consolidated approach of fluorescence spectroscopy and all-atom molecular dynamics simulation. Our results show that the effect of these omega fatty acids is sensitive to their protonation states. Contrary to the accepted notion that chain unsaturation causes membrane disordering, both experimental and simulation results demonstrate that protonated linoleic acid promotes membrane ordering, despite having two unsaturations at the fatty acyl chain. However, protonated oleic fatty acid, with reduced unsaturation, disordered the acyl chain area of the lipid membranes. Equally surprisingly, deprotonated oleic acid orders, whereas deprotonated linoleic acid disorders, the membrane core region. Interestingly, while the lipid order parameter measurements from simulations did not capture these subtle differences, the calculated rotational autocorrelation function of a membrane dye was in line with experimentally measured apparent rotational correlation times. Our work provides a comprehensive revised molecular picture of the effect of omega fatty acids on membranes and highlights the importance of rigorous comparative approaches, as experimental and simulation studies in isolation can sometimes lead to inconsistent results.

Ceftriaxone-Loaded Liposomal Nanoparticles for Pulmonary Delivery Against Lower Respiratory Tract Infections: Development and Characterization

Background/Objectives: Herein, we demonstrate the development and characterization of ceftriaxone (CTX)-loaded liposomal nanoparticles (NPs) intended to be applicable to the management of lower respiratory tract infections (LRTIs) associated with resistant bacteria. Methods: The CTX-loaded liposomal NPs were fabricated by a thin film hydration approach. Results: The particle size of the NPs, determined by a Zetasizer, was within the range of 90-536 nm. Microscopic examination by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that particles are spherical in shape and have retained their original morphology even after freeze-drying. Attenuated total reflection-Fourier transform infrared (ATR-FTIR), differential scanning calorimetry (DSC), thermogravimetric (TG), and powder X-ray diffraction (PXRD) spectra exhibited that CTX is incorporated into the liposomes with no possible interaction between drug and excipients. The formation of the CTX-loaded liposomal NPs was dependent on the concentrations of phospholipids, cholesterol and mannitol; however, no considerable differences were observed in entrapment efficiency and loading capacity of CTX formulations (F6-F10). Using a twin-stage impinger (TSI), the in vitro aerosolization of the formulations were carried out at a flow rate of 60 ± 5 L/min and CTX was determined by a validated HPLC method and the prepared liposomal formulations produced promising fine particle fraction (FPF) between 47 and 62%. The prepared formulation (F6) showed prolonged CTX release of 94.0% ± 5.7 and 95.9% ± 3.9 at 24 h and 48 h, respectively. The drug release followed the Hixon-Crowell model, with CTX being transported through Fickian diffusion. Conclusions: These results highlight the prepared CTX-loaded inhaled liposomal formulation would be suitable for pulmonary delivery and extend the successful antibiotic delivery strategies for the effective management of LRTIs.

Unveiling the Complete Spectrum of SARS-CoV-2 Fusion Stages by In Situ Cryo-ET

SARS-CoV-2 entry into host cells is mediated by the spike protein, which drives membrane fusion. While cryo-EM has revealed stable prefusion and postfusion conformations of the spike, the transient intermediate states during the fusion process have remained poorly understood. Here, we designed a near-native viral fusion system that recapitulates SARS-CoV-2 entry and used cryo-electron tomography (cryo-ET) to capture fusion intermediates leading to complete fusion. The spike protein undergoes extensive structural rearrangements, progressing through extended, partially folded, and fully folded intermediates prior to fusion-pore formation, a process that is dependent on protease cleavage and inhibited by the WS6 S2 antibody. Upon interaction with ACE2 receptor dimer, spikes cluster at membrane interfaces and following S2' cleavage concurrently transition to postfusion conformations encircling the hemifusion and pre-fusion pores in a distinct conical arrangement. Subtomogram averaging revealed that the WS6 S2 antibody binds to the spike's stem-helix, crosslinks and clusters prefusion spikes and inhibits refolding of fusion intermediates. These findings elucidate the complete process of spike-mediated fusion and SARS-CoV-2 entry, highlighting the neutralizing mechanism of S2-targeting antibodies.

Interaction of Squalamine with Lipid Membranes

Squalamine is an aminosterol from dogfish shark which has drawn attention, besides its antimicrobial activity, as a drug candidate in the treatment of Parkinson's disease due to its ability to prevent binding of α-synuclein to lipid membranes. To get insight into the mode of action of this steroid, we studied the influence of squalamine on lipid bilayers and whether it could inhibit the binding of a model peptide. Solid-state 19F NMR of labeled [KIGAKI]3 indicated that, indeed, this peptide no longer binds as a flexible chain to the bilayer in the presence of squalamine. When the cationic squalamine was added to lipid vesicles containing phosphatidylglycerol lipids, the aminosterol was found in differential scanning calorimetry and solid-state 31P NMR experiments to lower the gel-to-fluid phase transition and cause the phase separation of domains enriched in anionic lipids. Squalamine had only a little influence on 2H NMR relaxation and on the order parameters of the chains. These findings indicate that the aminosterol does not affect the molecular mobility of the hydrophobic core of the bilayer; hence, it does not insert into the membrane, nor causes thinning as found for molecules inserting in the headgroup region. On the other hand, squalamine was found to interact with lipid headgroups through electrostatic interactions, as seen by solid-state 2H NMR on headgroup-labeled lipids. Furthermore, 31P NMR showed that squalamine shifted the lamellar-to-hexagonal phase transition of phosphatidylethanolamine lipids to higher temperatures, indicating a preference for positively curved membranes. Altogether, our experiments indicate a strong interaction of the cationic squalamine with lipid headgroups, in particular with anionic lipids. This affinity for membranes is strong enough to efficiently displace cationic polypeptides, confirming the proposed action mechanism in Parkinson treatment. Notably, supported by 1H-1H NOESY experiments, it was found that squalamine does not insert into the bilayer, but rather acts as facial amphiphile binding to the membrane surface. The binding to membranes may be envisaged in the form of oligomeric or micellar assemblies, which can disrupt the membrane at high concentrations, thereby explaining the antimicrobial and antifungal activities of squalamine.

SARS-CoV-2 FP1 Destabilizes Lipid Membranes and Facilitates Pore Formation

SARS-CoV-2 viral entry requires membrane fusion, which is facilitated by the fusion peptides within its spike protein. These predominantly hydrophobic peptides insert into target membranes; however, their precise mechanistic role in membrane fusion remains incompletely understood. Here, we investigate how FP1 (SFIEDLLFNKVTLADAGFIK), the N-terminal fusion peptide, modulates membrane stability and barrier function across various model membrane systems. Through a complementary suite of biophysical techniques-including electrophysiology, fluorescence spectroscopy, and atomic force microscopy-we demonstrate that FP1 significantly promotes pore formation and alters the membrane's mechanical properties. Our findings reveal that FP1 reduces the energy barrier for membrane defect formation and stimulates the appearance of stable conducting pores, with effects modulated by membrane composition and mechanical stress. The observed membrane-destabilizing activity suggests that, beyond its anchoring function, FP1 may facilitate viral fusion by locally disrupting membrane integrity. These results provide mechanistic insights into SARS-CoV-2 membrane fusion mechanisms and highlight the complex interplay between fusion peptides and target membranes during viral entry.

Mechanistic Insights into Synaptotagmin-1 Mediated Membrane Fusion and Interactions

We present two innovative approaches to investigate the dynamics of membrane fusion and the strength of protein-membrane interactions. The first approach employs pore-spanning membranes (PSMs), which allow for the observation of protein-assisted fusion processes. The second approach utilizes colloidal probe microscopy with membrane-coated probes with reconstituted proteins. PSMs enable one to obtain detailed information about the fusion process with particular emphasis on fusion intermediates and fusion pore formation. We demonstrate the potential of the PSM system using SNARE-mediated fusion. Accompanied by colloidal probe microscopy, molecular information can be gathered on how full-length synaptotagmin-1 (syt-1) contributes to the fusion process. We propose that syt-1 engages with anionic bilayers, significantly modifying the adhesion between membranes. The introduction of Ca2+ transforms these interactions, shifting from a state of minimal interaction force between bilayers to one of pronounced strength. This syt-1 interaction facilitates fusion in the presence of Ca2+ with a significant reduction in the occurrence of stalled intermediate fusion states. Moreover, the presence of Ca2+ significantly accelerates the fusion process, an effect that is further amplified by the addition of multivalent anions such as ATP.

FRAP Assay to Trace Lipid Mixing of the Inner and Outer Leaflet of Yeast Vacuoles: Assessing the Fusion State in Live Cells

Fluorescence recovery after photobleaching (FRAP) can be employed to investigate membrane lipid mixing of vacuoles in live budding yeast cells and distinguish the fused, hemi-fused or non-fused states of these organelles under physiological conditions. Here, we describe a protocol for labeling the outer and inner leaflets of vacuoles in live cells that allow to detect hemifusion intermediates and, thus, identify components necessary for fusion pore opening.

Insertion and Anchoring of the HIV-1 Fusion Peptide into a Complex Membrane Mimicking the Human T-Cell

A fundamental understanding of how the HIV-1 envelope (Env) protein facilitates fusion is still lacking. The HIV-1 fusion peptide, consisting of 15 to 22 residues, is the N-terminus of the gp41 subunit of the Env protein. Further, this peptide, a promising vaccine candidate, initiates viral entry into target cells by inserting and anchoring into human immune cells. The influence of membrane lipid reorganization and the conformational changes of the fusion peptide during the membrane insertion and anchoring processes, which can significantly affect HIV-1 cell entry, remains largely unexplored due to the limitations of experimental measurements. In this work, we investigate the insertion of the fusion peptide into an immune cell membrane mimic through multiscale molecular dynamics simulations. We mimic the native T-cell by constructing a nine-lipid asymmetric membrane, along with geometrical restraints accounting for insertion in the context of gp41. To account for the slow time scale of lipid mixing while enabling conformational changes, we implement a protocol to go back and forth between atomistic and coarse-grained simulations. Our study provides a molecular understanding of the interactions between the HIV-1 fusion peptide and the T-cell membrane, highlighting the importance of the conformational flexibility of fusion peptides and local lipid reorganization in stabilizing the anchoring of gp41 into the targeted host membrane during the early events of HIV-1 cell entry. Importantly, we identify a motif within the fusion peptide critical for fusion that can be further manipulated in future immunological studies.

Boosting Lipofection Efficiency Through Enhanced Membrane Fusion Mechanisms

Gene transfection is a fundamental technique in the fields of biological research and therapeutic innovation. Due to their biocompatibility and membrane-mimetic properties, lipid vectors serve as essential tools in transfection. The successful delivery of genetic material into the cytoplasm is contingent upon the fusion of the vector and cellular membranes, which enables hydrophilic polynucleic acids to traverse the hydrophobic barriers of two intervening membranes. This review examines the critical role of membrane fusion in lipofection efficiency, with a particular focus on the molecular mechanisms that govern lipoplex-membrane interactions. This analysis will examine the key challenges inherent to the fusion process, from achieving initial membrane proximity to facilitating final content release through membrane remodeling. In contrast to viral vectors, which utilize specialized fusion proteins, lipid vectors necessitate a strategic formulation and environmental optimization to enhance their fusogenicity. This review discusses recent advances in vector design and fusion-promoting strategies, emphasizing their potential to improve gene delivery yield. It highlights the importance of understanding lipoplex-membrane fusion mechanisms for developing next-generation delivery systems and emphasizes the need for continued fundamental research to advance lipid-mediated transfection technology.

Capturing intermediates and membrane remodeling in class III viral fusion

Enveloped viruses enter cells by fusing their envelopes to host cell membranes. Vesicular stomatitis virus (VSV) glycoprotein (G) is a prototype for class III fusion proteins. Although structures of the stable pre- and postfusion ectodomain of G are known, its fusogenic intermediates are insufficiently characterized. Here, we incubated VSV virions with late endosome-mimicking liposomes at pH 5.5 and used cryo-electron tomography (cryo-ET) to visualize stages of VSV's membrane fusion pathway, capture refolding intermediates of G, and reconstruct a sequence of G conformational changes. We observe that the G trimer disassembles into monomers and parallel dimers that explore a broad conformational space. Extended intermediates engage target membranes and mediate fusion, resulting in viral uncoating and linearization of the ribonucleoprotein genome. These viral fusion intermediates provide mechanistic insights into class III viral fusion processes, opening avenues for future research and structure-based design of fusion inhibition-based antiviral therapeutics.

Local Stress in Cylindrically Curved Lipid Membrane: Insights into Local Versus Global Lateral Fluidity Models

Lipid membranes, which are fundamental to cellular function, undergo various mechanical deformations. Accurate modeling of these processes necessitates a thorough understanding of membrane elasticity. The lateral shear modulus, a critical parameter describing membrane resistance to lateral stresses, remains elusive due to the membrane's fluid nature. Two contrasting hypotheses, local fluidity and global fluidity, have been proposed. While the former suggests a zero local lateral shear modulus anywhere within lipid monolayers, the latter posits that only the integral of this modulus over the monolayer thickness vanishes. These differing models lead to distinct estimations of other elastic moduli and affect the modeling of biological processes, such as membrane fusion/fission and membrane-mediated interactions. Notably, they predict distinct local stress distributions in cylindrically curved membranes. The local fluidity model proposes isotropic local lateral stress, whereas the global fluidity model predicts anisotropy due to anisotropic local lateral stretching of lipid monolayers. Using molecular dynamics simulations, this study directly investigates these models by analyzing local stress in a cylindrically curved membrane. The results conclusively demonstrate the existence of static local lateral shear stress and anisotropy in local lateral stress within the monolayers of the cylindrical membrane, strongly supporting the global fluidity model. These findings have significant implications for the calculation of surface elastic moduli and offer novel insights into the fundamental principles governing lipid membrane elasticity.

Recalibration of MARTINI-3 Parameters for Improved Interactions between Peripheral Proteins and Lipid Bilayers

The MARTINI force field is one of the most used coarse-grained models for biomolecular simulations. Many limitations of the model including the protein-protein overaggregation have been improved in its latest version, MARTINI-3. In this study, we investigate the efficacy of the MARTINI-3 parameters for capturing the interactions of peripheral proteins with model plasma membranes. Particularly, we consider two classes of proteins, namely, annexin and epsin, which are known to generate negative and positive membrane curvatures, respectively. We find that current MARTINI-3 parameters are not able to correctly describe the protein-membrane interface and the protein-induced membrane curvatures for any of these proteins. The problem arises due to the lack of proper hydrophobic interactions between the protein residues and lipid tails. Making systematic adjustments, we show that a combination of reduction in the protein-water interactions and enhancement of protein-lipid hydrophobic interactions is essential for accurate prediction of the interfacial structure including the protein-induced membrane curvature. Next, we apply our model to a couple of other peripheral proteins, namely, Snf7, a core component of the ESCRT-III complex, and the PH domain of evectin-2. We find that our model captures the protein-membrane interfacial structure much more accurately than the MARTINI-3 model for all of the peripheral proteins considered in this study. However, the strategy described in this study may not be suitable for oligomeric transmembrane proteins where protein-protein hydrophobic interactions should be increased instead of protein-lipid hydrophobic interactions.

Lys716 in the transmembrane domain of yeast mitofusin Fzo1 modulates anchoring and fusion

Outer mitochondrial membrane fusion, a vital cellular process, is mediated by mitofusins. However, the underlying molecular mechanism remains elusive. We have performed extensive multiscale molecular dynamics simulations to predict a model of the transmembrane (TM) domain of the yeast mitofusin Fzo1. Coarse-grained simulations of the two TM domain helices, TM1 and TM2, reveal a stable interface, which is controlled by the charge status of residue Lys716. Atomistic replica-exchange simulations further tune our model, which is confirmed by a remarkable agreement with an independent AlphaFold2 (AF2) prediction of Fzo1 in complex with its fusion partner Ugo1. Furthermore, the presence of the TM domain destabilizes the membrane, even more if Lys716 is charged, which can be an asset for initiating fusion. The functional role of Lys716 was confirmed with yeast experiments, which show that mutating Lys716 to a hydrophobic residue prevents mitochondrial fusion.

Mechanisms of extracellular vesicle uptake and implications for the design of cancer therapeutics

The translation of pre-clinical anti-cancer therapies to regulatory approval has been promising, but slower than hoped. While innovative and effective treatments continue to achieve or seek approval, setbacks are often attributed to a lack of efficacy, failure to achieve clinical endpoints, and dose-limiting toxicities. Successful efforts have been characterized by the development of therapeutics designed to specifically deliver optimal and effective dosing to tumour cells while minimizing off-target toxicity. Much effort has been devoted to the rational design and application of synthetic nanoparticles to serve as targeted therapeutic delivery vehicles. Several challenges to the successful application of this modality as delivery vehicles include the induction of a protracted immune response that results in their rapid systemic clearance, manufacturing cost, lack of stability, and their biocompatibility. Extracellular vesicles (EVs) are a heterogeneous class of endogenous biologically produced lipid bilayer nanoparticles that mediate intercellular communication by carrying bioactive macromolecules capable of modifying cellular phenotypes to local and distant cells. By genetic, chemical, or metabolic methods, extracellular vesicles (EVs) can be engineered to display targeting moieties on their surface while transporting specific cargo to modulate pathological processes following uptake by target cell populations. This review will survey the types of EVs, their composition and cargoes, strategies employed to increase their targeting, uptake, and cargo release, and their potential as targeted anti-cancer therapeutic delivery vehicles.

Curvature fluctuations of fluid vesicles reveal hydrodynamic dissipation within the bilayer

The biological function of membranes is closely related to their softness, which is often studied through the membranes' thermally driven fluctuations. Typically, the analysis assumes that the relaxation rate of a pure bending deformation is determined by the competition between membrane bending rigidity and viscous dissipation in the surrounding medium. Here, we reexamine this assumption and demonstrate that viscous flows within the membrane dominate the dynamics of bending fluctuations of nonplanar membranes with a radius of curvature smaller than the Saffman-Delbrück length. Using flickering spectroscopy of giant vesicles made of dipalmitoylphosphatidylcholine, DPPC:cholesterol mixtures and pure diblock-copolymer membranes, we experimentally detect the signature of membrane dissipation in curvature fluctuations. We show that membrane viscosity can be reliably obtained from the short time behavior of the shape time correlations. The results indicate that the DPPC:cholesterol membranes behave as a Newtonian fluid, while the polymer membranes exhibit more complex rheology. Our study provides physical insights into the time scales of curvature remodeling of biological and synthetic membranes.

Actin dynamics switches two distinct modes of endosomal fusion in yolk sac visceral endoderm cells

Membranes undergo various patterns of deformation during vesicle fusion, but how this membrane deformation is regulated and contributes to fusion remains unknown. In this study, we developed a new method of observing the fusion of individual late endosomes and lysosomes by using mouse yolk sac visceral endoderm cells that have huge endocytic vesicles. We found that there were two distinct fusion modes that were differently regulated. In homotypic fusion, two late endosomes fused quickly, whereas in heterotypic fusion they fused to lysosomes slowly. Mathematical modeling showed that vesicle size is a critical determinant of these fusion types and that membrane fluctuation forces can overcome the vesicle size effects. We found that actin filaments were bound to late endosomes and forces derived from dynamic actin remodeling were necessary for quick fusion during homotypic fusion. Furthermore, cofilin played a role in endocytic fusion by regulating actin turnover. These data suggest that actin promotes vesicle fusion for efficient membrane trafficking in visceral endoderm cells.

Lipid redistribution due to a cell-cell fusion pore

We consider the redistribution of lipids comprising the plasma membranes during cell-cell fusion, particularly due to the presence of a fusion pore. Assuming the membranes are of constant thickness, we find that the mole fraction of cholesterol increases in the directly apposed exoplasmic leaflets, and is decreased in the cytoplasmic leaflets. The redistribution of the phospholipids is obtained as well.

Dynamic formation of the protein-lipid prefusion complex

Synaptic vesicles (SVs) fuse with the presynaptic membrane (PM) to release neuronal transmitters. The SV protein synaptotagmin 1 (Syt1) serves as a Ca2+ sensor for evoked fusion. Syt1 is thought to trigger fusion by penetrating the PM upon Ca2+ binding; however, the mechanistic detail of this process is still debated. Syt1 interacts with the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) complex, a coiled-coil four-helical bundle that enables the SV-PM attachment. The SNARE-associated protein complexin (Cpx) promotes Ca2+-dependent fusion, possibly interacting with Syt1. We employed all-atom molecular dynamics to investigate the formation of the Syt1-SNARE-Cpx complex interacting with the lipid bilayers of the PM and SVs. Our simulations demonstrated that the PM-Syt1-SNARE-Cpx complex can transition to a "dead-end" state, wherein Syt1 attaches tightly to the PM but does not immerse into it, as opposed to a prefusion state, which has the tips of the Ca2+-bound C2 domains of Syt1 inserted into the PM. Our simulations unraveled the sequence of Syt1 conformational transitions, including the simultaneous docking of Syt1 to the SNARE-Cpx bundle and the PM, followed by Ca2+ chelation and the penetration of the tips of Syt1 domains into the PM, leading to the prefusion state of the protein-lipid complex. Importantly, we found that direct Syt1-Cpx interactions are required to promote these transitions. Thus, we developed the all-atom dynamic model of the conformational transitions that lead to the formation of the prefusion PM-Syt1-SNARE-Cpx complex. Our simulations also revealed an alternative dead-end state of the protein-lipid complex that can be formed if this pathway is disrupted.

Fusogenic Liposomes for the Intracellular Delivery of Phosphocreatine

Background/Objective: Maintaining intracellular adenosine triphosphate (ATP) levels is essential for numerous cellular functions, including energy metabolism, muscle contraction, and nerve impulse transmission. ATP is primarily synthesized in mitochondria through oxidative phosphorylation. It is also generated in the cytosol under anaerobic conditions using phosphocreatine (PCr) as a phosphate donor to adenosine diphosphate. However, the intracellular delivery of exogenous PCr is challenging as it does not readily cross the plasma membrane. This complicates the use of PCr as a therapeutic agent to maintain energy homeostasis or to treat conditions like cerebral creatine deficiency syndrome (CDS), which results from defective creatine transporters. Methods: This study employs the use of fusogenic liposomes to deliver PCr directly into the cytosol, bypassing membrane impermeability issues. We engineered various 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP)-based fusogenic liposomes, incorporating phospholipids such as 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) in combination with phospholipid-aromatic dye components to facilitate membrane fusion and to enhance the delivery of the PCr cargo. Liposomal formulations were co-loaded with membrane-impermeable chromophores and PCr and studied on live cells using confocal microscopy. Conclusions: We demonstrated the successful intracellular delivery of these agents and observed a 23% increase in intracellular ATP levels in cells treated with PCr-loaded liposomes. This increase was not observed with free PCr, confirming the effectiveness of the liposome-based delivery system. Additionally, cell viability assays showed minimal toxicity from the liposomes. Our results indicate that fusogenic liposomes are a promising method for the delivery of PCr (and potentially other cell-impermeable therapeutic agents) to the cellular cytosol. The approach demonstrated here could be advantageous for treating energy-related disorders and improving cellular energy homeostasis.

Cdc42 mobility and membrane flows regulate fission yeast cell shape and survival

Polarized exocytosis induced by local Cdc42 GTPase activity results in membrane flows that deplete low-mobility membrane-associated proteins. A reaction-diffusion particle model comprising Cdc42 positive feedback activation, hydrolysis by GTPase-activating proteins (GAPs), and flow-induced displacement by exo/endocytosis shows that flow-induced depletion of low mobility GAPs promotes polarization. We modified Cdc42 mobility in Schizosaccharomyces pombe by replacing its prenylation site with 1, 2 or 3 repeats of the Rit C-terminal membrane-binding domain (ritC), yielding alleles with progressively lower mobility and increased flow-coupling. While Cdc42-1ritC cells are viable and polarized, Cdc42-2ritC polarize poorly and Cdc42-3ritC are inviable, in agreement with model's predictions. Deletion of Cdc42 GAPs restores viability to Cdc42-3ritC cells, verifying the model's prediction that GAP deletion increases Cdc42 activity at the expense of polarization. Our work demonstrates how membrane flows are an integral part of Cdc42-driven pattern formation and require Cdc42-GTP to turn over faster than the surface on which it forms.

Strategic Design of Tryptophan-Aspartic Acid-Containing Peptide Inhibitors Using Coronin 1 as a Template: Inhibition of Fusion by Enhancing Acyl Chain Order

Enveloped viruses enter the host cell by fusing at the cell membrane or entering the cell via endocytosis and fusing at the endosome. Conventional inhibitors target the viral fusion protein to inactivate it for inducing fusion. These target-specific vis-à-vis virus-specific inhibitors fail to display their inhibitory efficacy against emerging and remerging viral infections. This necessitates the need to develop broad-spectrum entry inhibitors that are effective irrespective of the virus. Using a broad range of targeting techniques, the fusion inhibitors can modify the physical characteristics of the viral membrane, making it less prone to fusion. We have previously shown that two tryptophan-aspartic acid (WD)-containing hydrophobic peptides, TG-23 and GG-21, from coronin 1, a phagosomal protein, inhibit membrane fusion by modulating membrane organization and dynamics. In the present work, we designed two WD-containing hydrophilic peptides, QG-22 and AG-22, using coronin 1 as a template and evaluated their fusion inhibitory efficacies in the absence and presence of membrane cholesterol. Our results demonstrate that QG-22 and AG-22 inhibit membrane fusion irrespective of the concentration of membrane cholesterol. Our measurements of depth-dependent membrane organization and dynamics reveal that they impede fusion by enhancing the acyl chain order. Overall, our results validate the hypothesis of designing fusion inhibitors by modulating the membrane's physical properties. In addition, it demonstrates that chain hydrophobicity might not be a critical determinant for the development of peptide-based fusion inhibitors.

Lipid remodeling in acrosome exocytosis: unraveling key players in the human sperm

It has long been thought that exocytosis was driven exclusively by well-studied fusion proteins. Some decades ago, the role of lipids became evident and escalated interest in the field. Our laboratory chose a particular cell to face this issue: the human sperm. What makes this cell special? Sperm, as terminal cells, are characterized by their scarcity of organelles and the complete absence of transcriptional and translational activities. They are specialized for a singular membrane fusion occurrence: the exocytosis of the acrosome. This unique trait makes them invaluable for the study of exocytosis in isolation. We will discuss the lipids' role in human sperm acrosome exocytosis from various perspectives, with a primary emphasis on our contributions to the field. Sperm cells have a unique lipid composition, very rare and not observed in many cell types, comprising a high content of plasmalogens, long-chain, and very-long-chain polyunsaturated fatty acids that are particular constituents of some sphingolipids. This review endeavors to unravel the impact of membrane lipid composition on the proper functioning of the exocytic pathway in human sperm and how this lipid dynamic influences its fertilizing capability. Evidence from our and other laboratories allowed unveiling the role and importance of multiple lipids that drive exocytosis. This review highlights the role of cholesterol, diacylglycerol, and particular phospholipids like phosphatidic acid, phosphatidylinositol 4,5-bisphosphate, and sphingolipids in driving sperm acrosome exocytosis. Furthermore, we provide a comprehensive overview of the factors and enzymes that regulate lipid turnover during the exocytic course. A more thorough grasp of the role played by lipids transferred from sperm can provide insights into certain causes of male infertility. It may lead to enhancements in diagnosing infertility and techniques like assisted reproductive technology (ART).

Structure and inhibition of SARS-CoV-2 spike refolding in membranes

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein binds the receptor angiotensin converting enzyme 2 (ACE2) and drives virus-host membrane fusion through refolding of its S2 domain. Whereas the S1 domain contains high sequence variability, the S2 domain is conserved and is a promising pan-betacoronavirus vaccine target. We applied cryo-electron tomography to capture intermediates of S2 refolding and understand inhibition by antibodies to the S2 stem-helix. Subtomogram averaging revealed ACE2 dimers cross-linking spikes before transitioning into S2 intermediates, which were captured at various stages of refolding. Pan-betacoronavirus neutralizing antibodies targeting the S2 stem-helix bound to and inhibited refolding of spike prehairpin intermediates. Combined with molecular dynamics simulations, these structures elucidate the process of SARS-CoV-2 entry and reveal how pan-betacoronavirus S2-targeting antibodies neutralize infectivity by arresting prehairpin intermediates.

A Fluorogenic Pseudoinfection Assay to Probe Transfer and Distribution of Influenza Viral Contents to Target Vesicles

Fusion of enveloped viruses with endosomal membranes and subsequent release of the viral genome into the cytoplasm are crucial to the viral infection cycle. It is often modeled by performing fusion between virus particles and target lipid vesicles. We utilized fluorescence microscopy to characterize the kinetic aspects of the transfer of influenza viral ribonucleoprotein (vRNP) complexes to target vesicles and their spatial distribution within the fused volumes to gain deeper insight into the mechanistic aspects of endosomal escape. The fluorogenic RNA-binding dye QuantiFluor (Promega) was found to be well-suited for direct and sensitive microscopic observation of vRNPs which facilitated background-free detection and kinetic analysis of fusion events on a single particle level. To determine the extent to which the viral contents are transferred to the target vesicles through the fusion pore, we carried out virus-vesicle fusion in a side-by-side fashion. Measurement of the Euclidean distances between the centroids of superlocalized membrane and content dye signals within the fused volumes allowed determination of any symmetry (or the lack thereof) between them as expected in the event of transfer (or the lack thereof) of vRNPs, respectively. We found that, in the case of fusion between viruses and 100 nm target vesicles, ∼39% of the events led to transfer of viral contents to the target vesicles. This methodology provides a rapid, generic, and cell-free way to assess the inhibitory effects of antiviral drugs and therapeutics on the endosomal escape behavior of enveloped viruses.

Insertion and Anchoring of HIV-1 Fusion Peptide into Complex Membrane Mimicking Human T-cell

A fundamental understanding of how HIV-1 envelope (Env) protein facilitates fusion is still lacking. The HIV-1 fusion peptide, consisting of 15 to 22 residues, is the N-terminus of the gp41 subunit of the Env protein. Further, this peptide, a promising vaccine candidate, initiates viral entry into target cells by inserting and anchoring into human immune cells. The influence of membrane lipid reorganization and the conformational changes of the fusion peptide during the membrane insertion and anchoring processes, which can significantly affect HIV-1 cell entry, remains largely unexplored due to the limitations of experimental measurements. In this work, we investigate the insertion of the fusion peptide into an immune cell membrane mimic through multiscale molecular dynamics simulations. We mimic the native T-cell by constructing a 9-lipid asymmetric membrane, along with geometrical restraints accounting for insertion in the context of gp41. To account for the slow timescale of lipid mixing while enabling conformational changes, we implement a protocol to go back and forth between atomistic and coarse-grained simulations. Our study provides a molecular understanding of the interactions between the HIV-1 fusion peptide and the T-cell membrane, highlighting the importance of conformational flexibility of fusion peptides and local lipid reorganization in stabilizing the anchoring of gp41 into the targeted host membrane during the early events of HIV-1 cell entry. Importantly, we identify a motif within the fusion peptide critical for fusion that can be further manipulated in future immunological studies.

Role of isotropic lipid phase in the fusion of photosystem II membranes

It has been thoroughly documented, by using 31P-NMR spectroscopy, that plant thylakoid membranes (TMs), in addition to the bilayer (or lamellar, L) phase, contain at least two isotropic (I) lipid phases and an inverted hexagonal (HII) phase. However, our knowledge concerning the structural and functional roles of the non-bilayer phases is still rudimentary. The objective of the present study is to elucidate the origin of I phases which have been hypothesized to arise, in part, from the fusion of TMs (Garab et al. 2022 Progr Lipid Res 101,163). We take advantage of the selectivity of wheat germ lipase (WGL) in eliminating the I phases of TMs (Dlouhý et al. 2022 Cells 11: 2681), and the tendency of the so-called BBY particles, stacked photosystem II (PSII) enriched membrane pairs of 300-500 nm in diameter, to form large laterally fused sheets (Dunahay et al. 1984 BBA 764: 179). Our 31P-NMR spectroscopy data show that BBY membranes contain L and I phases. Similar to TMs, WGL selectively eliminated the I phases, which at the same time exerted no effect on the molecular organization and functional activity of PSII membranes. As revealed by sucrose-density centrifugation, magnetic linear dichroism spectroscopy and scanning electron microscopy, WGL disassembled the large laterally fused sheets. These data provide direct experimental evidence on the involvement of I phase(s) in the fusion of stacked PSII membrane pairs, and strongly suggest the role of non-bilayer lipids in the self-assembly of the TM system.

The diffuse interface description of fluid lipid membranes captures key features of the hemifusion pathway and lateral stress profile

Topological transitions of lipid membranes are ubiquitous in key biological processes for cell life, like neurotransmission, fertilization, morphogenesis, and viral infections. Despite this, they are not well understood due to their multiscale nature, which limits the use of molecular models and calls for a mesoscopic approach such as the celebrated Canham-Helfrich one. Unfortunately, such a model cannot handle topological transitions, hiding the crucial involved forces and the appearance of the experimentally observed hemifused intermediates. In this work, we describe the membrane as a diffuse interface preserving the Canham-Helfrich elasticity. We show that pivotal features of the hemifusion pathway are captured by this mesoscopic approach, e.g. a (meta)stable hemifusion state and the fusogenic behavior of negative monolayer spontaneous curvatures. The membrane lateral stress profile is calculated as a function of the elastic rigidities, yielding a coarse-grained version of molecular models findings. Insights into the fusogenic mechanism are reported and discussed.

Probing the structural evolution on the surface of cardiolipin vesicles with an amphiphilic second harmonic generation and fluorescence probe

Investigating the influence of the ambient chemical environment on molecular behaviors in liposomes is crucial for understanding and manipulating cellular vitality as well as the capabilities of lipid drug carriers in various environments. Here, we designed and synthesized a second harmonic generation (SHG) and fluorescence probe molecule called Pyr-Py+-N+ (PPN), which possesses membrane-targeting capability. We employed PPN to investigate the response of lipid vesicles composed of cardiolipin to the presence of exogenous salt. The kinetic behaviors, including the adsorption and embedding of PPN on the surface of small unilamellar vesicles (SUVs) composed of cardiolipin, were analyzed. The response of the SUVs to the addition of NaCl was also monitored. A rapid decrease in vesicle size can be evidenced through the rapid drop in SHG emission originating from PPN located on the vesicle surface.

A lever hypothesis for Synaptotagmin-1 action in neurotransmitter release

Neurotransmitter release is triggered in microseconds by Ca2+-binding to the Synaptotagmin-1 C2 domains and by SNARE complexes that form four-helix bundles between synaptic vesicles and plasma membranes, but the coupling mechanism between Ca2+-sensing and membrane fusion is unknown. Release requires extension of SNARE helices into juxtamembrane linkers that precede transmembrane regions (linker zippering) and binding of the Synaptotagmin-1 C2B domain to SNARE complexes through a 'primary interface' comprising two regions (I and II). The Synaptotagmin-1 Ca2+-binding loops were believed to accelerate membrane fusion by inducing membrane curvature, perturbing lipid bilayers or helping bridge the membranes, but SNARE complex binding orients the Ca2+-binding loops away from the fusion site, hindering these putative activities. Molecular dynamics simulations now suggest that Synaptotagmin-1 C2 domains near the site of fusion hinder SNARE action, providing an explanation for this paradox and arguing against previous models of Sytnaptotagmin-1 action. NMR experiments reveal that binding of C2B domain arginines to SNARE acidic residues at region II remains after disruption of region I. These results and fluorescence resonance energy transfer assays, together with previous data, suggest that Ca2+ causes reorientation of the C2B domain on the membrane and dissociation from the SNAREs at region I but not region II. Based on these results and molecular modeling, we propose that Synaptotagmin-1 acts as a lever that pulls the SNARE complex when Ca2+ causes reorientation of the C2B domain, facilitating linker zippering and fast membrane fusion. This hypothesis is supported by the electrophysiological data described in the accompanying paper.

Nanoparticle induced fusion of lipid membranes

Membrane fusion is crucial for infection of enveloped viruses, cellular transport, and drug delivery via liposomes. Nanoparticles can serve as fusogenic agents facilitating such membrane fusion for direct transmembrane transport. However, the underlying mechanisms of nanoparticle-induced fusion and the ideal properties of such nanoparticles remain largely unknown. Here, we used molecular dynamics simulations to investigate the efficacy of spheroidal nanoparticles with different size, prolateness, and ligand interaction strengths to enhance fusion between vesicles. By systematically varying nanoparticle properties, we identified how each parameter affects the fusion process and determined the optimal parameter range that promotes fusion. These findings provide valuable insights for the design and optimization of fusogenic nanoparticles with potential biotechnological and biomedical applications.

Control of artificial membrane fusion in physiological ionic solutions beyond the limits of electroformation

Membrane fusion, merging two lipid bilayers, is crucial for fabricating artificial membrane structures. Over the past 40 years, in contrast to precise and controllable membrane fusion in-vivo through specific molecules such as SNAREs, controlling the fusion in-vitro while fabricating artificial membrane structures in physiological ionic solutions without fusion proteins has been a challenge, becoming a significant obstacle to practical applications. We present an approach consisting of an electric field and a few kPa hydraulic pressure as an additional variable to physically control the fusion, enabling tuning of the shape and size of the 3D freestanding lipid bilayers in physiological ionic solutions. Mechanical model analysis reveals that pressure-induced parallel/normal tensions enhance fusion among membranes in the microwell. In-vitro peptide-membrane assay, mimicking vesicular transport via pressure-assisted fusion, and stability of 38 days with in-chip pressure control via pore size-regulated hydrogel highlight the potential for diverse biological applications.

A Coronin 1-Derived Peptide Inhibits Membrane Fusion by Modulating Membrane Organization and Dynamics

Membrane fusion is considered the first step in the entry of enveloped viruses into the host cell. Several targeted strategies have been implemented to block viral entry by limiting the fusion protein to form a six-helix bundle, which is a prerequisite for fusion. Nonetheless, the development of broad-spectrum fusion inhibitors is essential to combat emerging and re-emerging viral infections. TG-23, a coronin 1, a tryptophan-aspartate-rich phagosomal protein-derived peptide, demonstrated inhibition of fusion between small unilamellar vesicles (SUVs) by modulating the membrane's physical properties. However, its inhibitory efficacy reduces with an increasing concentration of membrane cholesterol. The present work aims to develop a fusion inhibitor whose efficacy would be unaltered in the presence of membrane cholesterol. A stretch of the tryptophan-aspartic acid-containing peptide with a similar secondary structure and hydrophobicity profile of TG-23 from coronin 1 was synthesized, and its ability to inhibit SUV-SUV fusion with varying concentrations of membrane cholesterol was evaluated. Our results demonstrate that the GG-21 peptide inhibits fusion irrespective of the cholesterol content of the membrane. We have further evaluated the peptide-induced change in the membrane organization and dynamics utilizing arrays of steady-state and time-resolved fluorescence measurements and correlated these results with their effect on fusion. Interestingly, GG-21 displays inhibitory efficacy in a wide variety of lipid compositions despite having a secondary structure and physical properties similar to those of TG-23. Overall, our results advocate that the secondary structure and physical properties of the peptide may not be sufficient to predict its inhibitory efficacy.

In Situ Detection of Nucleic Acids in Extracellular Vesicles via Membrane Fusion

Extracellular vesicles (EVs) carry diverse biomolecules (e. g., nucleic acids, proteins) for intercellular communication, serving as important markers for diseases. Analyzing nucleic acids derived from EVs enables non-invasive disease diagnosis and prognosis evaluation. Membrane fusion, a fundamental cellular process wherein two lipid membranes merge, facilitates cell communication and cargo transport. Building on this natural phenomenon, recent years have witnessed the emergence of membrane fusion-based strategies for the detection of nucleic acids within EVs. These strategies entail the encapsulation of detection probes within either artificial or natural vesicles, followed by the induction of membrane fusion with EVs to deliver probes. This innovative approach not only enables in situ detection of nucleic acids within EVs but also ensures the maintenance of structural integrity of EVs, thus preventing nucleic acid degradation and minimizing the interference from free nucleic acids. This concept categorizes approaches into universal and targeted membrane fusion strategies, and discusses their application potential, and challenges and future prospects.

Novel non-helical antimicrobial peptides insert into and fuse lipid model membranes

This research addresses the growing menace of antibiotic resistance by exploring antimicrobial peptides (AMPs) as alternatives to conventional antibiotics. Specifically, we investigate two linear amphipathic AMPs, LE-53 (12-mer) and LE-55 (16-mer), finding that the shorter LE-53 exhibits greater bactericidal activity against both Gram-negative (G(-)) and Gram-positive (G(+)) bacteria. Remarkably, both AMPs are non-toxic to eukaryotic cells. The heightened effectiveness of LE-53 is attributed to its increased hydrophobicity (H) compared to LE-55. Circular dichroism (CD) reveals that LE-53 and LE-55 both adopt β-sheet and random coil structures in lipid model membranes (LMMs) mimicking G(-) and G(+) bacteria, so secondary structure is not the cause of the potency difference. X-ray diffuse scattering (XDS) reveals increased lipid chain order in LE-53, a potential key distinction. Additionally, XDS study uncovers a significant link between LE-53's upper hydrocarbon location in G(-) and G(+) LMMs and its efficacy. Neutron reflectometry (NR) confirms the AMP locations determined using XDS. Solution small angle X-ray scattering (SAXS) demonstrates LE-53's ability to induce vesicle fusion in bacterial LMMs without affecting eukaryotic LMMs, offering a promising strategy to combat antibiotic-resistant strains while preserving human cell integrity, whereas LE-55 has a smaller ability to induce fusion.

Dynamic Formation of the Protein-Lipid Pre-fusion Complex

Synaptic vesicles (SVs) fuse with the presynaptic membrane (PM) to release neuronal transmitters. The SV protein Synaptotagmin 1 (Syt1) serves as a Ca2+ sensor for evoked fusion. Syt1 is thought to trigger fusion by penetrating into PM upon Ca2+ binding, however the mechanistic detail of this process is still debated. Syt1 interacts with the SNARE complex, a coiled-coil four-helical bundle that enables the SV-PM attachment. The SNARE-associated protein Complexin (Cpx) promotes the Ca2+-dependent fusion, possibly interacting with Syt1. We employed all-atom molecular dynamics (MD) to investigate the formation of the Syt1-SNARE-Cpx complex interacting with the lipid bilayers of PM and SV. Our simulations demonstrated that the PM-Syt1-SNARE-Cpx complex can transition to a "dead-end" state, wherein Syt1 attaches tightly to PM but does not immerse into it, as opposed to a pre-fusion state, which has the tips of the Ca2+-bound C2 domains of Syt1 inserted into PM. Our simulations unraveled the sequence of Syt1 conformational transitions, including the simultaneous Syt1 docking to the SNARE-Cpx bundle and PM, followed by the Ca2+ chelation and the penetration of the tips of Syt1 domains into PM, leading to the pre-fusion state of the protein-lipid complex. Importantly, we found that the direct Syt1-Cpx interactions are required to promote these transitions. Thus, we developed the all-atom dynamic model of the conformational transitions that lead to the formation of the pre-fusion PM-Syt1-SNARE-Cpx complex. Our simulations also revealed an alternative "dead-end" state of the protein-lipid complex that can be formed if this pathway is disrupted.

The Antiviral Activity of Interferon-Induced Transmembrane Proteins and Virus Evasion Strategies

Interferons (IFNs) are antiviral cytokines that defend against viral infections by inducing the expression of interferon-stimulated genes (ISGs). Interferon-inducible transmembrane proteins (IFITMs) 1, 2, and 3 are crucial ISG products and members of the CD225 protein family. Compelling evidence shows that IFITMs restrict the infection of many unrelated viruses by inhibiting the virus-cell membrane fusion at the virus entry step via the modulation of lipid composition and membrane properties. Meanwhile, viruses can evade IFITMs' restrictions by either directly interacting with IFITMs via viral glycoproteins or by altering the native entry pathway. At the same time, cumulative evidence suggests context-dependent and multifaceted roles of IFITMs in modulating virus infections and cell signaling. Here, we review the diverse antiviral mechanisms of IFITMs, the viral antagonizing strategies, and the regulation of IFITM activity in host cells. The mechanisms behind the antiviral activity of IFITMs could aid the development of broad-spectrum antivirals and enhance preparedness for future pandemics.

Overcoming Therapeutic Challenges of Antibiotic Delivery with Cubosome Lipid Nanocarriers

Low discovery rates for new antibiotics, commercial disincentives to invest, and inappropriate use of existing drugs have created a perfect storm of antimicrobial resistance (AMR). This "silent pandemic" of AMR looms as an immense, global threat to human health. In tandem, many potential novel drug candidates are not progressed due to elevated hydrophobicity, which may result in poor intracellular internalization and undesirable serum protein binding. With a reducing arsenal of effective antibiotics, enabling technology platforms that improve the outcome of treatments, such as repurposing existing bioactive agents, is a prospective option. Nanocarrier (NC) mediated drug delivery is one avenue for amplifying the therapeutic outcome. Here, the performance of several antibiotic classes encapsulated within the lipid-based cubosomes is examined. The findings demonstrate that encapsulation affords significant improvements in drug concentration:inhibition outcomes and assists in other therapeutic challenges associated with internalization, enzyme degradation, and protein binding. We emphasize that a currently sidelined compound, novobiocin, became active and revealed a significant increase in inhibition against the pathogenic Gram-negative strain, Pseudomonas aeruginosa. Encapsulation affords co-delivery of multiple bioactives as a strategy for mitigating failure of monotherapies and tackling resistance. The rationale in optimized drug selection and nanocarrier choice is examined by transport modeling which agrees with experimental inhibition results. The results demonstrate that lipid nanocarrier encapsulation may alleviate a range of challenges faced by antibiotic therapies and increase the range of antibiotics available to treat bacterial infections.

Molecular mechanism underlying SNARE-mediated membrane fusion enlightened by all-atom molecular dynamics simulations

The SNAP receptor (SNARE) proteins syntaxin-1, SNAP-25, and synaptobrevin mediate neurotransmitter release by forming tight SNARE complexes that fuse synaptic vesicles with the plasma membranes in microseconds. Membrane fusion is generally explained by the action of proteins on macroscopic membrane properties such as curvature, elastic modulus, and tension, and a widespread model envisions that the SNARE motifs, juxtamembrane linkers, and C-terminal transmembrane regions of synaptobrevin and syntaxin-1 form continuous helices that act mechanically as semirigid rods, squeezing the membranes together as they assemble ("zipper") from the N to the C termini. However, the mechanism underlying fast SNARE-induced membrane fusion remains unknown. We have used all-atom molecular dynamics simulations to investigate this mechanism. Our results need to be interpreted with caution because of the limited number and length of the simulations, but they suggest a model of membrane fusion that has a natural physicochemical basis, emphasizes local molecular events over general membrane properties, and explains extensive experimental data. In this model, the central event that initiates fast (microsecond scale) membrane fusion occurs when the SNARE helices zipper into the juxtamembrane linkers which, together with the adjacent transmembrane regions, promote encounters of acyl chains from both bilayers at the polar interface. The resulting hydrophobic nucleus rapidly expands into stalk-like structures that gradually progress to form a fusion pore, aided by the SNARE transmembrane regions and without clearly discernible intermediates. The propensity of polyunsaturated lipids to participate in encounters that initiate fusion suggests that these lipids may be important for the high speed of neurotransmitter release.

Structurally Dynamic Monocyte-Liposome Hybrid Vesicles as an Anticancer Drug Delivery Vehicle: A Crucial Correlation of Microscopic Elasticity and Ultrafast Dynamics

A biomimetic cell-based carrier system based on monocyte membranes and liposomes has been designed to create a hybrid "Monocyte-LP" which inherits the surface antigens of the monocytes along with the drug encapsulation property of the liposome. Förster resonance energy transfer (FRET) and polarization gated anisotropy measurements show the stiffness of the vesicles obtained from monocyte membranes (Mons), phosphatidylcholine membranes (LP), and Monocyte-LP to follow an increasing order of Mons > Monocyte-LP > LP. The dynamics of interface bound water molecules plays a key role in the elasticity of the vesicles, which in turn imparts higher delivery efficacy to the hybrid Monocyte-LP for a model anticancer drug doxorubicin than the other two vesicles, indicating a critical balance between flexibility and rigidity for an efficient cellular uptake. The present work provides insight on the influence of elasticity of delivery vehicles for enhanced drug delivery.

Myomerger Induces Membrane Hemifusion and Regulates Fusion Pore Expansion

Membrane fusion is a crucial mechanism in a wide variety of important events in cell biology from viral infection to exocytosis. However, despite many efforts and much progress, cell-cell fusion has remained elusive to our understanding. Along the life of the fusion pore, large conformational changes take place from the initial lipid bilayer bending, passing through the hemifusion intermediates, and ending with the formation of the first nascent fusion pore. In this sense, computer simulations are an ideal technique for describing such complex lipid remodeling at the molecular level. In this work, we studied the role played by the muscle-specific membrane protein Myomerger during the formation of the fusion pore. We have conducted μs length atomistic and coarse-grained molecular dynamics, together with free-energy calculations using ad hoc collective variables. Our results show that Myomerger favors the hemifusion diaphragm-stalk transition, reduces the nucleation-expansion energy difference, and promotes the formation of nonenlarging fusion pores.

Molecular regulation of myocyte fusion

Myocyte fusion is a pivotal process in the development and regeneration of skeletal muscle. Failure during fusion can lead to a range of developmental as well as pathological consequences. This review aims to comprehensively explore the intricate processes underlying myocyte fusion, from the molecular to tissue scale. We shed light on key players, such as the muscle-specific fusogens - Myomaker and Myomixer, in addition to some lesser studied molecules contributing to myocyte fusion. Conserved across vertebrates, Myomaker and Myomixer play a crucial role in driving the merger of plasma membranes of fusing myocytes, ensuring the formation of functional muscle syncytia. Our multiscale approach also delves into broader cell and tissue dynamics that orchestrate the timing and positioning of fusion events. In addition, we explore the relevance of muscle fusogens to human health and disease. Mutations in fusogen genes have been linked to congenital myopathies, providing unique insights into the molecular basis of muscle diseases. We conclude with a discussion on potential therapeutic avenues that may emerge from manipulating the myocyte fusion process to remediate skeletal muscle disorders.

Nanotherapeutics for prostate cancer treatment: A comprehensive review

Prostate cancer (PCa) is the most prevalent solid organ malignancy and seriously affects male health. The adverse effects of prostate cancer therapeutics can cause secondary damage to patients. Nanotherapeutics, which have special targeting abilities and controlled therapeutic release profiles, may serve as alternative agents for PCa treatment. At present, many nanotherapeutics have been developed to treat PCa and have shown better treatment effects in animals than traditional therapeutics. Although PCa nanotherapeutics are highly attractive, few successful cases have been reported in clinical practice. To help researchers design valuable nanotherapeutics for PCa treatment and avoid useless efforts, herein, we first reviewed the strategies and challenges involved in prostate cancer treatment. Subsequently, we presented a comprehensive review of nanotherapeutics for PCa treatment, including their targeting methods, controlled release strategies, therapeutic approaches and mechanisms. Finally, we proposed the future prospects of nanotherapeutics for PCa treatment.

Membrane lipids couple synaptotagmin to SNARE-mediated granule fusion in insulin-secreting cells

Insulin secretion depends on the Ca2+-regulated fusion of granules with the plasma membrane. A recent model of Ca2+-triggered exocytosis in secretory cells proposes that lipids in the plasma membrane couple the calcium sensor Syt1 to the membrane fusion machinery (Kiessling et al., 2018). Specifically, Ca2+-mediated binding of Syt1's C2 domains to the cell membrane shifts the membrane-anchored SNARE syntaxin-1a to a more fusogenic conformation, straightening its juxtamembrane linker. To test this model in live cells and extend it to insulin secretion, we enriched INS1 cells with a panel of lipids with different acyl chain compositions. Fluorescence lifetime measurements demonstrate that cells with more disordered membranes show an increase in fusion efficiency, and vice versa. Experiments with granules purified from INS1 cells and recombinant SNARE proteins reconstituted in supported membranes confirmed that lipid acyl chain composition determines SNARE conformation and that lipid disordering correlates with increased fusion. Addition of Syt1's C2AB domains significantly decreased lipid order in target membranes and increased SNARE-mediated fusion probability. Strikingly, Syt's action on both fusion and lipid order could be partially bypassed by artificially increasing unsaturated phosphatidylserines in the target membrane. Thus, plasma membrane lipids actively participate in coupling Ca2+/synaptotagmin-sensing to the SNARE fusion machinery in cells.