Control of membrane fusion mechanism by lipid composition: predictions from ensemble molecular dynamics

Visualizing intermediate stages of viral membrane fusion by cryo-electron tomography

Protein-mediated membrane fusion is the dynamic process where specialized protein machinery undergoes dramatic conformational changes that drive two membrane bilayers together, leading to lipid mixing and opening of a fusion pore between previously separate membrane-bound compartments. Membrane fusion is an essential stage of enveloped virus entry that results in viral genome delivery into host cells. Recent studies applying cryo-electron microscopy techniques in a time-resolved fashion provide unprecedented glimpses into the interaction of viral fusion proteins and membranes, revealing fusion intermediate states from the initiation of fusion to release of the viral genome. In combination with complementary structural, biophysical, and computation modeling approaches, these advances are shedding new light on the mechanics and dynamics of protein-mediated membrane fusion.

Direct and cell-mediated EV-ECM interplay

Extracellular vesicles (EV) are a heterogeneous group of lipid particles excreted by cells. They play an important role in regeneration, development, inflammation, and cancer progression, together with the extracellular matrix (ECM), which they constantly interact with. In this review, we discuss direct and indirect interactions of EVs and the ECM and their impact on different physiological processes. The ECM affects the secretion of EVs, and the properties of the ECM and EVs modulate EVs' diffusion and adhesion. On the other hand, EVs can affect the ECM both directly through enzymes and indirectly through the modulation of the ECM synthesis and remodeling by cells. This review emphasizes recently discovered types of EVs bound to the ECM and isolated by enzymatic digestion, including matrix-bound nanovesicles (MBV) and tissue-derived EV (TiEV). In addition to the experimental studies, computer models of the EV-ECM-cell interactions, from all-atom models to quantitative pharmacology models aiming to improve our understanding of the interaction mechanisms, are also considered. STATEMENT OF SIGNIFICANCE: Application of extracellular vesicles in tissue engineering is an actively developing area. Vesicles not only affect cells themselves but also interact with the matrix and change it. The matrix also influences both cells and vesicles. In this review, different possible types of interactions between vesicles, matrix, and cells are discussed. Furthermore, the united EV-ECM system and its regulation through the cellular activity are presented.

Impaired cathepsin D in retinal pigment epithelium cells mediates Stargardt disease pathogenesis

Recessive Stargardt disease (STGD1) is an inherited juvenile maculopathy caused by mutations in the ABCA4 gene, for which there is no suitable treatment. Loss of functional ABCA4 in the retinal pigment epithelium (RPE) alone, without contribution from photoreceptor cells, was shown to induce STGD1 pathology. Here, we identified cathepsin D (CatD), the primary RPE lysosomal protease, as a key molecular player contributing to endo-lysosomal dysfunction in STGD1 using a newly developed "disease-in-a-dish" RPE model from confirmed STGD1 patients. Induced pluripotent stem cell (iPSC)-derived RPE originating from three STGD1 patients exhibited elevated lysosomal pH, as previously reported in Abca4-/- mice. CatD protein maturation and activity were impaired in RPE from STGD1 patients and Abca4-/- mice. Consequently, STGD1 RPE cells have reduced photoreceptor outer segment degradation and abnormal accumulation of α-synuclein, the natural substrate of CatD. Furthermore, dysfunctional ABCA4 in STGD1 RPE cells results in intracellular accumulation of autofluorescent material and phosphatidylethanolamine (PE). The altered distribution of PE associated with the internal membranes of STGD1 RPE cells presumably compromises LC3-associated phagocytosis, contributing to delayed endo-lysosomal degradation activity. Drug-mediated re-acidification of lysosomes in the RPE of STGD1 restores CatD functional activity and reduces the accumulation of immature CatD protein loads. This preclinical study validates the contribution of CatD deficiencies to STGD1 pathology and provides evidence for an efficacious therapeutic approach targeting RPE cells. Our findings support a cell-autonomous RPE-driven pathology, informing future research aimed at targeting RPE cells to treat ABCA4-mediated retinopathies.

Insights into membrane interactions and their therapeutic potential

Recent research into membrane interactions has uncovered a diverse range of therapeutic opportunities through the bioengineering of human and non-human macromolecules. Although the majority of this research is focussed on fundamental developments, emerging studies are showcasing promising new technologies to combat conditions such as cancer, Alzheimer's and inflammatory and immune-based disease, utilising the alteration of bacteriophage, adenovirus, bacterial toxins, type 6 secretion systems, annexins, mitochondrial antiviral signalling proteins and bacterial nano-syringes. To advance the field further, each of these opportunities need to be better understood, and the therapeutic models need to be further optimised. Here, we summarise the knowledge and insights into several membrane interactions and detail their current and potential uses therapeutically.

Carrier-Tumor Cell Membrane Interactions for Optimized Delivery of a Promising Drug, 4(RS)-4-F4t-Neuroprostane

Nanomedicines engineered to deliver molecules with therapeutic potentials, overcoming drawbacks such as poor solubility, toxicity or a short half-life, are targeted towards their cellular destination either passively or through various elements of cell membranes. The differences in the physicochemical properties of the cell membrane between tumor and nontumor cells have been reported, but they are not systematically used for drug delivery purposes. Thus, in this study, a new approach based on a match between the liposome compositions, i.e., membrane fluidity, to selectively interact with the targeted cell membrane was used. Lipid-based carriers of two different fluidities were designed and used to deliver 4(RS)-4-F4t-Neuroprostane (F4t-NeuroP), a potential antitumor molecule derived from docosahexaenoic acid (DHA). Based on its hydrophobic character, F4t-NeuroP was added to the lipid mixture prior to liposome formation, a protocol that yielded over 80% encapsulation efficiency in both rigid and fluid liposomes. The presence of the active molecule did not modify the liposome size but increased the liposome negative charge and the liposome membrane fluidity, which suggested that the active molecule was accommodated in the lipid membrane. F4t-NeuroP integration in liposomes with a fluid character allowed for the selective targeting of the metastatic prostate cell line PC-3 vs. fibroblast controls. A significant decrease in viability (40%) was observed for the PC-3 cancer line in the presence of F4t-NeuroP fluid liposomes, whereas rigid F4t-NeuroP liposomes did not alter the PC-3 cell viability. These findings demonstrate that liposomes encapsulating F4t-NeuroP or other related molecules may be an interesting model of drug carriers based on membrane fluidity.

Lipid Exchange Promotes Fusion of Model Protocells

Vesicle fusion is an important process underlying cell division, transport, and membrane trafficking. In phospholipid systems, a range of fusogens including divalent cations and depletants have been shown to induce adhesion, hemifusion, and then full content fusion between vesicles. This work shows that these fusogens do not perform the same function for fatty acid vesicles, which are used as model protocells (primitive cells). Even when fatty acid vesicles appear adhered or hemifused to each other, the intervening barriers between vesicles do not rupture. This difference is likely because fatty acids have a single aliphatic tail, and are more dynamic than their phospholipid counterparts. To address this, it is postulated that fusion could instead occur under conditions, such as lipid exchange, that disrupt lipid packing. Using both experiments and molecular dynamics simulations, it is verified that fusion in fatty acid systems can indeed be induced by lipid exchange. These results begin to probe how membrane biophysics could constrain the evolutionary dynamics of protocells.

Membrane Tension Inhibits Lipid Mixing by Increasing the Hemifusion Stalk Energy

Fusion of biological membranes is fundamental in various physiological events. The fusion process involves several intermediate stages with energy barriers that are tightly dependent on the mechanical and physical properties of the system, one of which is membrane tension. As previously established, the late stages of fusion, including hemifusion diaphragm and pore expansions, are favored by membrane tension. However, a current understanding of how the energy barrier of earlier fusion stages is affected by membrane tension is lacking. Here, we apply a newly developed experimental approach combining micropipette-aspirated giant unilamellar vesicles and optically trapped membrane-coated beads, revealing that membrane tension inhibits lipid mixing. We show that lipid mixing is 6 times slower under a tension of 0.12 mN/m compared with tension-free membranes. Furthermore, using continuum elastic theory, we calculate the dependence of the hemifusion stalk formation energy on membrane tension and intermembrane distance and find the increase in the corresponding energy barrier to be 1.6 kBT in our setting, which can explain the increase in lipid mixing time delay. Finally, we show that tension can be a significant factor in the stalk energy if the pre-fusion intermembrane distance is on the order of several nanometers, while for membranes that are tightly docked, tension has a negligible effect.

Tunable biomimetic bacterial membranes from binary and ternary lipid mixtures and their application in antimicrobial testing

The reconstruction of accurate yet simplified mimetic models of cell membranes is a very challenging goal of synthetic biology. To date, most of the research focuses on the development of eukaryotic cell membranes, while reconstitution of their prokaryotic counterparts has not been fully addressed, and the proposed models do not reflect well the complexity of bacterial cell envelopes. Here, we describe the reconstitution of biomimetic bacterial membranes with an increasing level of complexity, developed from binary and ternary lipid mixtures. Giant unilamellar vesicles composed of phosphatidylcholine (PC) and phosphatidylethanolamine (PE); PC and phosphatidylglycerol (PG); PE and PG; PE, PG and cardiolipin (CA) at varying molar ratios were successfully prepared by the electroformation method. Each of the proposed mimetic models focuses on reproducing specific membrane features such as membrane charge, curvature, leaflets asymmetry, or the presence of phase separation. GUVs were characterized in terms of size distribution, surface charge, and lateral organization. Finally, the developed models were tested against the lipopeptide antibiotic daptomycin. The obtained results showed a clear dependency of daptomycin binding efficiency on the amount of negatively charged lipid species present in the membrane. We anticipate that the models proposed here can be applied not only in antimicrobial testing but also serve as platforms for studying fundamental biological processes in bacteria as well as their interaction with physiologically relevant biomolecules.

Folding@home: Achievements from over 20 years of citizen science herald the exascale era

Simulations of biomolecules have enormous potential to inform our understanding of biology but require extremely demanding calculations. For over 20 years, the Folding@home distributed computing project has pioneered a massively parallel approach to biomolecular simulation, harnessing the resources of citizen scientists across the globe. Here, we summarize the scientific and technical advances this perspective has enabled. As the project's name implies, the early years of Folding@home focused on driving advances in our understanding of protein folding by developing statistical methods for capturing long-timescale processes and facilitating insight into complex dynamical processes. Success laid a foundation for broadening the scope of Folding@home to address other functionally relevant conformational changes, such as receptor signaling, enzyme dynamics, and ligand binding. Continued algorithmic advances, hardware developments such as graphics processing unit (GPU)-based computing, and the growing scale of Folding@home have enabled the project to focus on new areas where massively parallel sampling can be impactful. While previous work sought to expand toward larger proteins with slower conformational changes, new work focuses on large-scale comparative studies of different protein sequences and chemical compounds to better understand biology and inform the development of small-molecule drugs. Progress on these fronts enabled the community to pivot quickly in response to the COVID-19 pandemic, expanding to become the world's first exascale computer and deploying this massive resource to provide insight into the inner workings of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus and aid the development of new antivirals. This success provides a glimpse of what is to come as exascale supercomputers come online and as Folding@home continues its work.

High curvature promotes fusion of lipid membranes: Predictions from continuum elastic theory

The fusion of lipid membranes progresses through a series of hemifusion intermediates with two significant energy barriers related to the formation of stalk and fusion pore, respectively. These energy barriers determine the speed and success rate of many critical biological processes, including the fusion of highly curved membranes, for example synaptic vesicles and enveloped viruses. Here we use continuum elastic theory of lipid monolayers to determine the relationship between membrane shape and energy barriers to fusion. We find that the stalk formation energy decreases with curvature by up to 31 kBT in a 20-nm-radius vesicle compared with planar membranes and by up to 8 kBT in the fusion of highly curved, long, tubular membranes. In contrast, the fusion pore formation energy barrier shows a more complicated behavior. Immediately after stalk expansion to the hemifusion diaphragm, the fusion pore formation energy barrier is low (15-25 kBT) due to lipid stretching in the distal monolayers and increased tension in highly curved vesicles. Therefore, the opening of the fusion pore is faster. However, these stresses relax over time due to lipid flip-flop from the proximal monolayer, resulting in a larger hemifusion diaphragm and a higher fusion pore formation energy barrier, up to 35 kBT. Therefore, if the fusion pore fails to open before significant lipid flip-flop takes place, the reaction proceeds to an extended hemifusion diaphragm state, which is a dead-end configuration in the fusion process and can be used to prevent viral infections. In contrast, in the fusion of long tubular compartments, the surface tension does not accumulate due to the formation of the diaphragm, and the energy barrier for pore expansion increases with curvature by up to 11 kBT. This suggests that inhibition of polymorphic virus infection could particularly target this feature of the second barrier.

Structural basis for catalysis of human choline/ethanolamine phosphotransferase 1

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are two primary components of the eukaryotic membrane and play essential roles in the maintenance of membrane integrity, lipid droplet biogenesis, autophagosome formation, and lipoprotein formation and secretion. Choline/ethanolamine phosphotransferase 1 (CEPT1) catalyzes the last step of the biosynthesis of PC and PE in the Kennedy pathway by transferring the substituted phosphate group from CDP-choline/ethanolamine to diacylglycerol. Here, we present the cryo-EM structures of human CEPT1 and its complex with CDP-choline at resolutions of 3.7 Å and 3.8 Å, respectively. CEPT1 is a dimer with 10 transmembrane segments (TMs) in each protomer. TMs 1-6 constitute a conserved catalytic domain with an interior hydrophobic chamber accommodating a PC-like density. Structural observations and biochemical characterizations suggest that the hydrophobic chamber coordinates the acyl tails during the catalytic process. The PC-like density disappears in the structure of the complex with CDP-choline, suggesting a potential substrate-triggered product release mechanism.

Serinc5 Restricts HIV Membrane Fusion by Altering Lipid Order and Heterogeneity in the Viral Membrane

The host restriction factor, Serinc5, incorporates into budding HIV particles and inhibits their infection by an incompletely understood mechanism. We have previously reported that Serinc5 but not its paralogue, Serinc2, blocks HIV cell entry by membrane fusion, specifically by inhibiting fusion pore formation and dilation. A body of work suggests that Serinc5 may alter the conformation and clustering of the HIV fusion protein, Env. To contribute an additional perspective to the developing model of Serinc5 restriction, we assessed Serinc2 and Serinc5's effects on HIV pseudoviral membranes. By measuring pseudoviral membrane thickness via cryo-electron microscopy and order via the fluorescent dye, FLIPPER-TR, Serinc5 was found to increase membrane heterogeneity, skewing the distribution toward a larger fraction of the viral membrane in an ordered phase. We also directly observed for the first time the coexistence of membrane domains within individual viral membrane envelopes. Using a total internal reflection fluorescence-based single particle fusion assay, we found that treatment of HIV pseudoviral particles with phosphatidylethanolamine (PE) rescued HIV pseudovirus fusion from restriction by Serinc5, which was accompanied by decreased membrane heterogeneity and order. This effect was specific for PE and did not depend on acyl chain length or saturation. Together, these data suggest that Serinc5 alters multiple interrelated properties of the viral membrane─lipid chain order, rigidity, line tension, and lateral pressure─which decrease the accessibility of fusion intermediates and disfavor completion of fusion. These biophysical insights into Serinc5 restriction of HIV infectivity could contribute to the development of novel antivirals that exploit the same weaknesses.

IFITM3 blocks influenza virus entry by sorting lipids and stabilizing hemifusion

Interferon-induced transmembrane protein 3 (IFITM3) inhibits the entry of numerous viruses through undefined molecular mechanisms. IFITM3 localizes in the endosomal-lysosomal system and specifically affects virus fusion with target cell membranes. We found that IFITM3 induces local lipid sorting, resulting in an increased concentration of lipids disfavoring viral fusion at the hemifusion site. This increases the energy barrier for fusion pore formation and the hemifusion dwell time, promoting viral degradation in lysosomes. In situ cryo-electron tomography captured IFITM3-mediated arrest of influenza A virus membrane fusion. Observation of hemifusion diaphragms between viral particles and late endosomal membranes confirmed hemifusion stabilization as a molecular mechanism of IFITM3. The presence of the influenza fusion protein hemagglutinin in post-fusion conformation close to hemifusion sites further indicated that IFITM3 does not interfere with the viral fusion machinery. Collectively, these findings show that IFITM3 induces lipid sorting to stabilize hemifusion and prevent virus entry into target cells.

Effect of polyunsaturated free fatty acids on the membrane fusion mechanism

Membrane fusion is one of the important processes for the survival of eukaryotic cells and the entry of enveloped viruses into the host cells. Lipid composition plays a crucial role by modulating the organization and dynamics of the membrane, as well as the structure and conformation of membrane proteins. The diversity of the lipid acyl chain in its length and degree of unsaturation originates from the variation in free fatty acids (FFAs). We have studied the effect of linoleic (LA) and alpha-linolenic (ALA) acids on the depth-dependent organization, dynamics, and fusion of DOPC/DOPE (70/30 mol%) membranes utilizing steady-state and time-resolved fluorescence spectroscopic methods. Our results suggest that membranes with 5 mol% LA stabilize the stalk-intermediate and promote lipid mixing at the early stage of the process, i.e., the fusion follows the classical stalk model. Conversely, the extents of lipid and content mixing at the stalk intermediate are similar in the presence of 5 mol% of ALA, indicating the fusion mechanism as a nonclassical one like in the DOPC/DOPE (70/30 mol%) membranes. Our results provide an in-depth insight into the effect of the increasing degree of fatty acid tail unsaturation on membrane organization and dynamics and their impact on the membrane fusion mechanism.

Influenza Virus Membrane Fusion Is Promoted by the Endosome-Resident Phospholipid Bis(monoacylglycero)phosphate

The phospholipid bis(monoacylglycero)phosphate (BMP) is enriched in late endosomal and endolysosomal membranes and is believed to be involved in membrane deformation and generation of intralumenal vesicles within late endosomes. Previous studies have demonstrated that BMP promotes membrane fusion of several enveloped viruses, but a limited effect has been found on influenza virus. Here, we report the use of single-virus fusion assays to dissect BMP's effect on influenza virus fusion in greater depth. In agreement with prior reports, we found that hemifusion kinetics and efficiency were unaffected by the addition of 10-20 mol % BMP to the target membrane. However, using an assay for fusion pore formation and genome exposure, we found full fusion efficiency to be substantially enhanced by the addition of 10-20 mol % BMP to the target membrane, while the kinetics remained unaffected. By comparing BMP to other negatively charged phospholipids, we found the effect on fusion efficiency mainly attributable to headgroup charge, although we also hypothesize a role for BMP's unusual chemical structure. Our results suggest that BMP function as a permissive factor for a wider range of viruses than previously reported. We hypothesize that BMP may be a general cofactor for endosomal entry of enveloped viruses.

Fluorescence Spectroscopic Analysis of Lateral and Transbilayer Fluidity of Exosome Membranes

Exosomes are small extracellular vesicles (sEVs) involved in distal cell-cell communication and cancer migration by transferring functional cargo molecules. Membrane domains similar to lipid rafts are assumed to occur in exosome membranes and are involved in interactions with target cells. However, the bilayer membrane properties of these small vesicles have not been fully investigated. Therefore, we examined the fluidity, lateral domain separation, and transbilayer asymmetry of exosome membranes using fluorescence spectroscopy. Although there were some differences between the exosomes, TMA-DPH anisotropy showing moderate lipid chain order indicated that ordered phases comprised a significant proportion of exosome membranes. Selective TEMPO quenching of the TMA-DPH fluorescence in the liquid-disordered phase indicated that 40-50% of the exosome membrane area belonged to the ordered phase based on a phase-separated model. Furthermore, NBD-PC in the outer leaflet showed longer fluorescence lifetimes than those in the inner leaflets. Therefore, the exosome membranes maintained transbilayer asymmetry with a topology similar to that of the plasma membranes. In addition, the lateral and transbilayer orders of exosome membranes obtained from different cell lines varied, probably depending on the different membrane lipid components and compositions partially derived from donor cells. As these higher membrane orders and asymmetric topologies are similar to those of cell membranes with lipid rafts, raft-like functional domains are possibly enriched on exosome membranes. These domains likely play key roles in the biological functions and cellular uptake of exosomes by facilitating selective membrane interactions with target organs.

Membrane Fusion Biophysical Analysis of Fusogenic Liposomes

Liposomes represent important drug carrier vehicles in biological systems. A fusogenic liposomal system composed of equimolar mixtures of the cationic lipid DOTAP and the phospholipid DOPE showed high fusion and delivery efficiencies with cells and lipid vesicles. However, aspects of the thermodynamics involving the interaction of these fusogenic liposomes and biomimetic systems remain unclear. Here, we investigate the fusion of this system with large unilamellar vesicles (LUVs) composed of the zwitterionic lipid POPC and increasing fractions of the anionic lipid POPG and up to 30 mol % cholesterol. The focus here is to concomitantly follow changes in size, zeta-potential, and enthalpy binding upon membrane interaction and fusion. Isothermal titration calorimetry (ITC) data showed that membrane fusion in our system is an exothermic process in the absence of cholesterol, suggesting that electrostatic attraction is the driving force for fusion. An endothermic component appeared and eventually dominated the titration at 30 mol % cholesterol, which we propose is caused by membrane fluidification when cholesterol is diluted upon fusion. The inflection points of the ITC data occurred around 0.5-0.7 POPG/DOTAP for all systems, the same stoichiometry for which zeta-potential and dynamic light scattering measurements showed an increase in size coupled with charge neutralization of the system, which is consistent with the fact that fusion in our system is charge-mediated. Microscopy observations of the final mixtures revealed the presence of giant vesicles, which is a clear indication of fusion, coexisting with intermediate-sized objects that could be the result of both fusion and/or aggregation. The results show that the fusion efficiency of the DOTAP:DOPE fusogenic system is modulated by the charge and membrane packing of the acceptor membrane and explain why the system fuses very efficiently with cells.

Lipid/water interface of galactolipid bilayers in different lyotropic liquid-crystalline phases

In this study, carried out using computational methods, the organisation of the lipid/water interface of bilayers composed of galactolipids with both α-linolenoyl acyl chains is analysed and compared in three different lyotropic liquid-crystalline phases. These systems include the monogalactosyldiglyceride (MGDG) and digalactosyldiglyceride (DGDG) bilayers in the lamellar phase, the MGDG double bilayer during stalk phase formation and the inverse hexagonal MGDG phase. For each system, lipid-water and direct and water-mediated lipid-lipid interactions between the lipids of one bilayer leaflet and those of two apposing leaflets at the onset of new phase (stalk) formation, are identified. A network of interactions between DGDG molecules and its topological properties are derived and compared to those for the MGDG bilayer.

Molecular Dynamics Simulations of Curved Lipid Membranes

Eukaryotic cells contain membranes with various curvatures, from the near-plane plasma membrane to the highly curved membranes of organelles, vesicles, and membrane protrusions. These curvatures are generated and sustained by curvature-inducing proteins, peptides, and lipids, and describing these mechanisms is an important scientific challenge. In addition to that, some molecules can sense membrane curvature and thereby be trafficked to specific locations. The description of curvature sensing is another fundamental challenge. Curved lipid membranes and their interplay with membrane-associated proteins can be investigated with molecular dynamics (MD) simulations. Various methods for simulating curved membranes with MD are discussed here, including tools for setting up simulation of vesicles and methods for sustaining membrane curvature. The latter are divided into methods that exploit scaffolding virtual beads, methods that use curvature-inducing molecules, and methods applying virtual forces. The variety of simulation tools allow researcher to closely match the conditions of experimental studies of membrane curvatures.

Combination of Oleic Acid and the gp41 Fusion Peptide Switches the Phosphatidylethanolamine-Induced Membrane Fusion Mechanism from a Nonclassical to a Classical Stalk Model

Membrane fusion is considered to be one of the crucial processes for the existence of eukaryotes and the entry of enveloped viruses into host cells. The fusion mechanism depends on the lipid composition of the membrane as well as the properties of fusion proteins or peptides. The gp41 fusion peptide from the human immunodeficiency virus (HIV) is known to catalyze membrane fusion by altering the physical properties of the membrane. Earlier, we demonstrated that a membrane containing 30 mol % phosphatidylethanolamine (PE) circumvents the classical stalk model because of its intrinsic negative curvature. In this work, we demonstrated how the gp41 fusion peptide influences the fusion mechanism of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1,2-dioleoyl-sn-glycero-3-phos-pho¬ethanolamine (DOPE) (70/30 mol %) membranes. We further evaluated the effect of the same peptide on the mechanism of fusion for membranes containing 30 mol % PE and a fatty acid with an intrinsic positive curvature (oleic acid (OA)). Our results show that gp41 switches the fusion mechanism from a nonclassical to a classical stalk model when membranes contain OA, but fails to do so for DOPC/DOPE membranes. This could be due to the extreme influence of the intrinsic negative curvature of PE, which is partially downregulated in the presence of OA.

Mechanism of Membrane Fusion: Interplay of Lipid and Peptide

Membrane fusion is an essential process for the survival of eukaryotes and the entry of enveloped viruses into host cells. A proper understanding of the mechanism of membrane fusion would provide us a handle to manipulate several biological pathways, and design efficient vaccines against emerging and re-emerging viral infections. Although fusion proteins take the central stage in catalyzing the process, role of lipid composition is also of paramount importance. Lipid composition modulates membrane organization and dynamics and impacts the lipid–protein (peptide) interaction. Moreover, the intrinsic curvature of lipids has strong impact on the formation of stalk and hemifusion diaphragm. Detection of transiently stable intermediates remains the bottleneck in the understanding of fusion mechanism. In order to circumvent this challenge, analytical methods can be employed to determine the kinetic parameters from ensemble average measurements of observables, such as lipid mixing, content mixing, and content leakage. The current review aims to present an analytical method that would aid our understanding of the fusion mechanism, provides a better insight into the role of lipid shape, and discusses the interplay of lipid and peptide in membrane fusion.

Computational methods to study enveloped viral entry

The protein-membrane interactions that mediate viral infection occur via loosely ordered, transient assemblies, creating challenges for high-resolution structure determination. Computational methods and in particular molecular dynamics simulation have thus become important adjuncts for integrating experimental data, developing mechanistic models, and suggesting testable hypotheses regarding viral function. However, the large molecular scales of virus-host interaction also create challenges for detailed molecular simulation. For this reason, continuum membrane models have played a large historical role, although they have become less favored for high-resolution models of protein assemblies and lipid organization. Here, we review recent progress in the field, with an emphasis on the insight that has been gained using a mixture of coarse-grained and atomic-resolution molecular dynamics simulations. Based on successes and challenges to date, we suggest a multiresolution strategy that should yield the best mixture of computational efficiency and physical fidelity. This strategy may facilitate further simulations of viral entry by a broader range of viruses, helping illuminate the diversity of viral entry strategies and the essential common elements that can be targeted for antiviral therapies.

Effect of Phosphatidylethanolamine and Oleic Acid on Membrane Fusion: Phosphatidylethanolamine Circumvents the Classical Stalk Model

Membrane fusion is one of the most important processes for the survival of eukaryotic cells and entry of enveloped viruses to the host cells. Lipid composition plays a crucial role in the process by modulating the organization and dynamics of the membrane, as well as the structure and conformation of membrane proteins. Phosphatidylethanolamine (PE), a lipid molecule with intrinsic negative curvature, promotes membrane fusion by stabilizing the non-lamellar intermediate structures in the fusion process. Conversely, oleic acid (OA), with intrinsic positive curvature, inhibits membrane fusion. The current study aimed to investigate polyethylene glycol-mediated lipid mixing, content mixing, content leakage, and depth-dependent membrane organization and dynamics, using arrays of steady-state and time-resolved fluorescence techniques, to determine the causative role of PE and OA in membrane fusion. The results demonstrated that the presence of 30 mol % PE in the membrane promotes membrane fusion through a mechanism that circumvents the classical stalk model. On the contrary, membranes containing OA showed reduced rate and extent of fusion, despite following the same mechanism. Collectively, our findings in terms of membrane organization and dynamics indicated a plausible role of PE and OA in membrane fusion.

Myelinosome Organelles in the Retina of R6/1 Huntington Disease (HD) Mice: Ubiquitous Distribution and Possible Role in Disease Spreading

Visual deficit is one of the complications of Huntington disease (HD), a fatal neurological disorder caused by CAG trinucleotide expansions in the Huntingtin gene, leading to the production of mutant Huntingtin (mHTT) protein. Transgenic HD R6/1 mice expressing human HTT exon1 with 115 CAG repeats recapitulate major features of the human pathology and exhibit a degeneration of the retina. Our aim was to gain insight into the ultrastructure of the pathological HD R6/1 retina by electron microscopy (EM). We show that the HD R6/1 retina is enriched with unusual organelles myelinosomes, produced by retinal neurons and glia. Myelinosomes are present in all nuclear and plexiform layers, in the synaptic terminals of photoreceptors, in the processes of retinal neurons and glial cells, and in the subretinal space. In vitro study shows that myelinosomes secreted by human retinal glial Müller MIO-M1 cells transfected with EGFP-mHTT-exon1 carry EGFP-mHTT-exon1 protein, as revealed by immuno-EM and Western-blotting. Myelinosomes loaded with mHTT-exon1 are incorporated by naive neuronal/neuroblastoma SH-SY5Y cells. This results in the emergence of mHTT-exon1 in recipient cells. This process is blocked by membrane fusion inhibitor MDL 28170. Conclusion: Incorporation of myelinosomes carrying mHTT-exon1 in recipient cells may contribute to HD spreading in the retina. Exploring ocular fluids for myelinosome presence could bring an additional biomarker for HD diagnostics.

Non-raft submicron domain formation in cholesterol-containing lipid bilayers induced by polyunsaturated phosphatidylethanolamine

Domain formation in "HLC" ternary lipid bilayers, comprising a high transition temperature (High-T_m) lipid, a Low-T_m lipid, and cholesterol (Chol), has been extensively studied as raft-resembling systems. Recently, we reported the formation of submicron domains in an "LLC" lipid bilayer, encompassing Low-T_m phosphatidylethanolamine (PE), Low-T_m phosphatidylcholine (PC), and Chol. We hypothesized that the formation of this unique domain is driven by polyunsaturated PE. In this study, we explored the effects of the degree of PE unsaturation and the double bond distribution at the sn-position on the mechanism of formation and the composition of submicron domains. Supported lipid bilayers (SLBs), comprising PE with various degrees of unsaturation, monounsaturated PC (POPC), and Chol, were investigated using fluorescence microscopy, atomic force microscopy, and the force-distance curve measurement. The area fraction of submicron domains in PE+POPC+Chol-SLB increased with the PE concentration and degree of unsaturation of the PE acyl chain. The results indicated that the submicron domains were enriched with polyunsaturated PE and were in the liquid-disordered-like state, whereas their surrounding regions were in the liquid-ordered-like state. Segregation of polyunsaturated PE from the Chol-containing region generated submicron domains in the LLC lipid bilayer. We propose a mechanism for the formation of these submicron domains based on molecular interactions involving the hydrophobic and hydrophilic parts of the bilayer membrane.

Free energies of membrane stalk formation from a lipidomics perspective

Many biological membranes are asymmetric and exhibit complex lipid composition, comprising hundreds of distinct chemical species. Identifying the biological function and advantage of this complexity is a central goal of membrane biology. Here, we study how membrane complexity controls the energetics of the first steps of membrane fusions, that is, the formation of a stalk. We first present a computationally efficient method for simulating thermodynamically reversible pathways of stalk formation at coarse-grained resolution. The method reveals that the inner leaflet of a typical plasma membrane is far more fusogenic than the outer leaflet, which is likely an adaptation to evolutionary pressure. To rationalize these findings by the distinct lipid compositions, we computed ~200 free energies of stalk formation in membranes with different lipid head groups, tail lengths, tail unsaturations, and sterol content. In summary, the simulations reveal a drastic influence of the lipid composition on stalk formation and a comprehensive fusogenicity map of many biologically relevant lipid classes.

Fusion of cellular membranes begins with the formation of a stalk. Here, the authors develop a computationally efficient method for coarse-grained simulations of stalk formation and apply this approach to comprehensively analyse how stalk formation is influenced by the membrane lipid composition.

Monomethyl Auristatin E Grafted-Liposomes to Target Prostate Tumor Cell Lines

Novel nanomedicines have been engineered to deliver molecules with therapeutic potentials, overcoming drawbacks such as poor solubility, toxicity or short half-life. Lipid-based carriers such as liposomes represent one of the most advanced classes of drug delivery systems. A Monomethyl Auristatin E (MMAE) warhead was grafted on a lipid derivative and integrated in fusogenic liposomes, following the model of antibody drug conjugates. By modulating the liposome composition, we designed a set of particles characterized by different membrane fluidities as a key parameter to obtain selective uptake from fibroblast or prostate tumor cells. Only the fluid liposomes made of palmitoyl-oleoyl-phosphatidylcholine and dioleoyl-phosphatidylethanolamine, integrating the MMAE-lipid derivative, showed an effect on prostate tumor PC-3 and LNCaP cell viability. On the other hand, they exhibited negligible effects on the fibroblast NIH-3T3 cells, which only interacted with rigid liposomes. Therefore, fluid liposomes grafted with MMAE represent an interesting example of drug carriers, as they can be easily engineered to promote liposome fusion with the target membrane and ensure drug selectivity.

Fusogenic Effect of Cholesterol Prevails over the Inhibitory Effect of a Peptide-Based Membrane Fusion Inhibitor

Membrane fusion is the primary step in the entry of enveloped viruses into the host cell. Membrane composition modulates the membrane fusion by changing the organization dynamics of the fusion proteins, peptides, and membranes. The asymmetric lipid compositions of the viral envelope and the host cell influence the membrane fusion. Cholesterol is an important constituent of mammalian cells and plays a vital role in the entry of several viruses. In our pursuit of developing peptide-based general fusion inhibitors, we have previously shown that a coronin 1-derived peptide, TG-23, inhibited polyethylene glycol-induced fusion between symmetric membranes without cholesterol. In this work, we have studied the effect of TG-23 on the polyethylene glycol-mediated fusion between 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) (60/30/10 mol %) and DOPC/DOPE/DOPG/CH (50/30/10/10 mol %) membranes and between DOPC/DOPE/DOPG (60/30/10 mol %) and DOPC/DOPE/DOPG/CH (40/30/10/20 mol %) membranes. Our results demonstrate that the TG-23 peptide inhibited the fusion between membranes containing 0 and 10 mol % cholesterol though the efficacy is less than that of symmetric fusion between membranes devoid of cholesterol, and the inhibitory efficacy becomes negligible in the fusion between membranes containing 0 and 20 mol % cholesterol. Several steady-state and time-resolved fluorescence spectroscopic techniques have been successfully utilized to evaluate the organization, dynamics, and membrane penetration of the TG-23 peptide. Taken together, our results demonstrate that the reduction of the inhibitory effect of TG-23 in asymmetric membrane fusion containing cholesterol of varying concentrations is not due to the altered peptide structure, organization, and dynamics, rather owing to the intrinsic negative curvature-inducing property of cholesterol. Therefore, the membrane composition is an added complexity in the journey of developing peptide-based membrane fusion inhibitors.

Light-induced lipid mixing implies a causal role of lipid splay in membrane fusion

The fusion of lipid membranes is central to many biological processes and requires substantial structural reorganization of lipids brought about by the action of fusogenic proteins. Previous molecular dynamics simulations have suggested that splayed lipids, whose tails transiently contact the headgroup region of the bilayer, initiate lipid mixing. Here, we explore the lipid splay hypothesis experimentally. We show that the light-induced trans/cis conversion of the azobenzene-based tail of a model lipid molecule enhances the probability by which its own acyl chains, or the acyl chains of the host lipid, transiently contact the lipid headgroup in a liposomal bilayer. At the same time, the trans/cis conversion triggers lipid mixing of sonicated or extruded liposomes, without requiring fusogenic proteins. This establishes a causal relationship between lipid splay and membrane fusion.

The Application of Coarse-Grained Molecular Dynamics to the Evaluation of Liposome Physical Stability

Physical stability is one of critical characteristics of liposome, especially to its clinical application. Vesicle fusion was one of the common physical stability phenomena that occurred during the long storage period. Because vesicle fusion could be easily checked by the change of vesicle size, it was widely applied in the evaluation of liposome physical stability. However, since the method requires the liposome to be placed under certain conditions for long-term observation, a liposome physical stability test usually takes several weeks, which greatly hinders the research efficiency. In this study, to speed up the research efficiency, coarse-grained molecular dynamics was first applied in the study of liposome physical stability. By analyzing the microprocess of vesicle fusion, two parameters including diffusion constant and the total time of the vesicle morphology transition process were employed to study the liposome physical stability. Then, in order to verify the applicability of two parameters, the physical stability of elastic liposomes and conventional liposomes was compared at 3 different temperatures. It was found that the fusion probability and speed of elastic liposomes were higher than those of conventional liposomes. Thus, elastic liposomes showed a worse physical stability compared with that of conventional liposomes, which was consistent with former research. Through this research, a new efficient method based on coarse-grained molecular dynamics was proposed for the study of liposome physical stability.

Calcium-Lipid Interactions Observed with Isotope-Edited Infrared Spectroscopy

Calcium ions bind to lipid membranes containing anionic lipids; however, characterizing the specific ion-lipid interactions in multicomponent membranes has remained challenging because it requires nonperturbative lipid-specific probes. Here, using a combination of isotope-edited infrared spectroscopy and molecular dynamics simulations, we characterize the effects of a physiologically relevant (2 mM) Ca²⁺ concentration on zwitterionic phosphatidylcholine and anionic phosphatidylserine lipids in mixed lipid membranes. We show that Ca²⁺ alters hydrogen bonding between water and lipid headgroups by forming a coordination complex involving the lipid headgroups and water. These interactions distort interfacial water orientations and prevent hydrogen bonding with lipid ester carbonyls. We demonstrate, experimentally, that these effects are more pronounced for the anionic phosphatidylserine lipids than for zwitterionic phosphatidylcholine lipids in the same membrane.

Phospholipases in neuronal function: A role in learning and memory?

Despite the human brain being made of nearly 60% fat, the vast majority of studies on the mechanisms of neuronal communication which underpin cognition, memory and learning, primarily focus on proteins and/or (epi)genetic mechanisms. Phospholipids are the main component of all cellular membranes and function as substrates for numerous phospholipid-modifying enzymes, including phospholipases, which release free fatty acids (FFAs) and other lipid metabolites that can alter the intrinsic properties of the membranes, recruit and activate critical proteins, and act as lipid signalling molecules. Here, we will review brain specific phospholipases, their roles in membrane remodelling, neuronal function, learning and memory, as well as their disease implications. In particular, we will highlight key roles of unsaturated FFAs, particularly arachidonic acid, in neurotransmitter release, neuroinflammation and memory. In light of recent findings, we will also discuss the emerging role of phospholipase A₁ and the creation of saturated FFAs in the brain.

Plasma membrane asymmetry of lipid organization: fluorescence lifetime microscopy and correlation spectroscopy analysis

A fundamental feature of the eukaryotic cell membrane is the asymmetric arrangement of lipids in its two leaflets. A cell invests significant energy to maintain this asymmetry and uses it to regulate important biological processes, such as apoptosis and vesiculation. The dynamic coupling of the inner or cytoplasmic and outer or exofacial leaflets is a challenging open question in membrane biology. Here, we combined fluorescence lifetime imaging microscopy (FLIM) with imaging total internal reflection fluorescence correlation spectroscopy (ITIR-FCS) to differentiate the dynamics and organization of the two leaflets of live mammalian cells. We characterized the biophysical properties of fluorescent analogs of phosphatidylcholine, sphingomyelin, and phosphatidylserine in the plasma membrane of two mammalian cell lines (CHO-K1 and RBL-2H3). Because of their specific transverse membrane distribution, these probes allowed leaflet-specific investigation of the plasma membrane. We compared the results of the two methods having different temporal and spatial resolution. Fluorescence lifetimes of fluorescent lipid analogs were in ranges characteristic for the liquid ordered phase in the outer leaflet and for the liquid disordered phase in the inner leaflet. The observation of a more fluid inner leaflet was supported by free diffusion in the inner leaflet, with high average diffusion coefficients. The liquid ordered phase in the outer leaflet was accompanied by slower diffusion and diffusion with intermittent transient trapping. Our results show that the combination of FLIM and ITIR-FCS with specific fluorescent lipid analogs is a powerful tool for investigating lateral and transbilayer characteristics of plasma membrane in live cell lines.

The Role of a Crystallographically Unresolved Cytoplasmic Loop in Stabilizing the Bacterial Membrane Insertase YidC2

YidC, a bacterial member of the YidC/Alb3/Oxa1 insertase family, mediates membrane protein assembly and insertion. Cytoplasmic loops are known to have functional significance in membrane proteins such as YidC. Employing microsecond-level molecular dynamics (MD) simulations, we show that the crystallographically unresolved C2 loop plays a crucial role in the structural dynamics of Bacillus halodurans YidC2. We have modeled the C2 loop and used all- atom MD simulations to investigate the structural dynamics of YidC2 in its apo form, both with and without the C2 loop. The C2 loop was found to stabilize the entire protein and particularly the C1 region. C2 was also found to stabilize the alpha-helical character of the C-terminal region. Interestingly, the highly polar or charged lipid head groups of the simulated membranes were found to interact with and stabilize the C2 loop. These findings demonstrate that the crystallographically unresolved loops of membrane proteins could be important for the stabilization of the protein despite the apparent lack of structure, which could be due to the absence of the relevant lipids to stabilize them in crystallographic conditions.

Membrane Composition Modulates Fusion by Altering Membrane Properties and Fusion Peptide Structure

Membrane fusion, one of the most essential processes in the life of eukaryotes, occurs when two separate lipid bilayers merge into a continuous bilayer and internal contents of two separated membranes mingle. There is a certain class of proteins that assist the binding of the viral envelope to the target host cell and catalyzing fusion. All class I viral fusion proteins contain a highly conserved 20–25 amino-acid amphipathic peptide at the N-terminus, which is essential for fusion activity and is termed as the ‘fusion peptide’. It has been shown that insertion of fusion peptides into the host membrane and the perturbation in the membrane generated thereby is crucial for membrane fusion. Significant efforts have been given in the last couple of decades to understand the lipid-dependence of structure and function of the fusion peptide in membranes to understand the role of lipid compositions in membrane fusion. In addition, the lipid compositions further change the membrane physical properties and alter the mechanism and extent of membrane fusion. Therefore, lipid compositions modulate membrane fusion by changing membrane physical properties and altering structure of the fusion peptide.

Self-Assembled Nanostructured Lipid Systems: Is There a Link between Structure and Cytotoxicity?

Self-assembly of lipid-based liquid crystalline (LLC) nanoparticles is a formulation art arising from the hydrophilic-lipophilic qualities and the geometric packing of amphiphilic lipid molecules in an aqueous environment. The diversity of commercialized amphiphilic lipids and an increased understanding of the physicochemical factors dictating their membrane curvature has enabled versatile architectural design and engineering of LLC nanoparticles. While these exotic nanostructured materials are hypothesized to form the next generation of smart therapeutics for a broad field of biomedical applications, biological knowledge particularly on the systemic biocompatibility or cytotoxicity of LLC materials remains unclear. Here, an overview on the interactions between LLCs of different internal nanostructures and biological components (including soluble plasma constituents, blood cells, and isolated tissue cell lines) is provided. Factors affecting cell-nanoparticle tolerability such as the type of lipids, type of steric stabilizers, nanoparticle surface charges, and internal nanostructures (or lipid phase behaviors) are elucidated. The mechanisms of cellular uptake and lipid transfer between neighboring membrane domains are also reviewed. A critical analysis of these studies sheds light on future strategies to transform LLC materials into a viable therapeutic entity ideal for internal applications.

A potential mechanism for first-person internal sensation of memory provides evidence for the relationship between learning and LTP induction

Studies conducted to verify learning-induced changes anticipated from Hebb's postulate led to the finding of long-term potentiation (LTP). Even though several correlations have been found between behavioural markers of memory retrieval and LTP, it is not known how memories are retrieved using learning-induced changes. In this context, the following non-correlated findings between learning and LTP induction provide constraints for discovering the mechanism: 1) Requirement of high stimulus intensity for LTP induction in contrast to what is expected for a learning mechanism, 2) Delay of at least 20 to 30 s from stimulation to LTP induction, in contrast to mere milliseconds for associative learning, and 3) A sudden drop in peak-potentiated effect (short-term potentiation) that matches with short-lasting changes expected during working memory and occurs only at the time of delayed LTP induction. When memories are viewed as first-person internal sensations, a newly uncovered mechanism provides explanation for the relationship between memory and LTP. This work interconnects large number of findings from the fields of neuroscience and psychology and provides a further verifiable mechanism of learning.

Cholesterol suppresses membrane leakage by decreasing water penetrability

Membrane fusion is a fundamental biological process that lies at the heart of enveloped virus infection, synaptic signaling, intracellular vesicle trafficking, gamete fertilization, and cell-cell fusion. Membrane fusion is initiated as two apposed membranes merge to a single bilayer called a hemifusion diaphragm. It is believed that the contents of the two fusing membranes are released through a fusion pore formed at the hemifusion diaphragm, and yet another possible pathway has been proposed in which an undefined pore may form outside the hemifusion diaphragm at the apposed membranes, leading to the so-called leaky fusion. Here, we performed all-atom molecular dynamics simulations to study the evolution of the hemifusion diaphragm structure with various lipid compositions. We found that the lipid cholesterol decreased water penetrability to inhibit leakage pore formation. Biochemical leakage experiments support these simulation results. This study may shed light on the underlying mechanism of the evolution pathways of the hemifusion structure, especially the understanding of content leakage during membrane fusion.

Rate of hemifusion diaphragm dissipation and ability to form three-junction bound HD determined by lipid composition

Though the hemifusion diaphragm (HD) is widely accepted as an intermediate in bilayer membrane fusion, lipid contributions toward HD stability and dynamics are still not fully understood. In this paper, we study large, binary, protein-free HD systems at varying compositions of negative intrinsic curvature (NIC) lipids using molecular dynamics (MD) simulations of a solvent-free coarse-grained lipid model. Under MD, initially created HDs are found to relax to three major end states depending on the composition and lipid intrinsic curvature. Low compositions of NIC lipids or weak intrinsic curvature result in double-bilayer end states, and moderate compositions of moderate to strong NIC lipids result in metastable fusion pores. Importantly, high compositions of moderate NIC lipids result in a metastable HD that persists beyond μs time scales. NIC lipids stabilize the HD by filling the junction core around the HD. Sorting of NIC lipids toward the three-junction region occurs in fused-endpoint systems, but no significant sorting was seen in systems that end in a double bilayer indicating that high line tension at the triple junction drives HD dissipation faster than sorting can enrich that junction enough to lower that line tension. The appearance of three end states dependent on the NIC lipid composition highlights the necessity of NIC lipids for non-leaky fusion.

Detection of Curvature-Radius-Dependent Interfacial pH/Polarity for Amphiphilic Self-Assemblies: Positive versus Negative Curvature

It is possible that a defined curvature at the membrane interface controls its pH/polarity to exhibit specific bioactivity. By utilizing an interface-interacting spiro-rhodamine pH probe and the Schiff base polarity probe, we have shown that the pH deviation from the bulk phase to the interface (ΔpH)/interfacial dielectric constant (κ(i)) for amphiphilic self-assemblies can be regulated by the curvature geometry (positive/negative) and its radius. According to ¹H NMR and fluorescence anisotropy investigations, the probes selectively interact with an anionic interfacial Stern layer. The ΔpH/κ(i) values for the Stern layer are estimated by UV-vis absorption and fluorescence studies. For the anionic sodium bis-2-ethylhexyl-sulfosuccinate (AOT) inverted micellar (IM) negative interface, the highly restricted water and proton penetration into the Stern layer owing to tight surfactant packing or a reduced water-exposed headgroup area may be responsible for the much lower ΔpH ≈ -0.45 and κ(i) ≈ 28 in comparison to ∼-2.35 and ∼44, respectively, for the anionic sodium dodecyl sulfate (SDS) micellar positive interface with a close similar Stern layer. With increasing AOT IM water-pool radius (1.7-9.5 nm) or [water]/[AOT] ratio ( w₀) (8.0-43.0), the ΔpH and κ(i) increase maximally up to ∼-1.22 and ∼45, respectively, due to a greater water-exposed headgroup area. However, the unchanged ΔpH ≈ -0.65 and κ(i) ≈ 53.0 within radii ∼3.5-8.0 nm for the positive interface of a mixed Triton X-100 (TX-100)/SDS (4:1) micelle justify its packing flexibility. Interestingly, the continuously increasing ΔpH trend for IM up to its largest possible water-pool radius of ∼9.5 nm may rationalize the increase in ΔpH (∼-1.4 to -1.6) with the change in the curvature radii (∼15 to 50 nm) for sodium 1,2-dimyristoyl- sn-glycero-3-phosphorylglycerol (DMPG)/1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) (2:1) large unilamellar vesicles (LUV) owing to its negative interface. Whereas, similar to the micellar positive interface, the unchanged ΔpH at the positive LUV interface was confirmed by fluorescence microscopic studies with giant unilamellar vesicles of identical lipids composition. The present study offers a unique and simple method of monitoring the curvature-radius-dependent interfacial pH/polarity for biologically related membranes.

Camptothecin-loaded fusogenic nanodroplets as ultrasound theranostic agent in stem cell-mediated drug-delivery system

Adipose-derived stem cells (ADSCs) have been utilized in cellular delivery systems to carry therapeutic agents into tumors by migration. Drug-loaded nanodroplets release drugs and form bubbles after acoustic droplet vaporization (ADV) triggered by ultrasound stimulation, providing a system for ultrasound-induced cellular delivery of theranostic agents. In order to improve the efficiency of drug release, fusogenic nanodroplets were designed to go from nano to micron size upon uptake by ADSCs for reducing ADV threshold. The purpose of our study was to demonstrate the utility of camptothecin-loaded fusogenic nanodroplets (CPT-FNDs) as ultrasound theranostic agents in an ADSCs delivery system. CPT-FNDs showed an increase in size from 81.6 ± 3.5 to 1043.5 ± 28.3 nm and improved CPT release from 22.0 ± 1.8% to 37.6 ± 2.1%, demonstrating the fusion ability of CPT-FNDs. CPT-FNDs-loaded ADSCs demonstrated a cell viability of 77 ± 4%, and the in vitro migration ability was 3.2 ± 1.2-fold for the tumor condition compared to the cell growth condition. Ultrasound enhancement imaging showed intratumoral ADV-generated bubble formation (increasing 3.24 ± 0.47 dB) triggered by ultrasound after CPT-FNDs-loaded ADSCs migration into B16F0 tumors. Histological images revealed intratumoral distribution of CPT-FNDs-loaded ADSCs and tissue damage due to the ADV. The CPT-FNDs can be used as theranostic agents in an ADSCs delivery system to provide the ultrasound contrast imaging and deliver combination therapy of drug release and physical damage after ADV.

Evaluation of bending modulus of lipid bilayers using undulation and orientation analysis

In the current paper, phospholipid bilayers are modeled using coarse-grained molecular dynamics simulations with the MARTINI force field. The extracted molecular trajectories are analyzed using Fourier analysis of the undulations and orientation vectors to establish the differences between the two approaches for evaluating the bending modulus. The current work evaluates and extends the implementation of the Fourier analysis for molecular trajectories using a weighted horizon-based averaging approach. The effect of numerical parameters in the analysis of these trajectories is explored by conducting parametric studies. Computational modeling results are validated against experimentally characterized bending modulus of lipid membranes using a shape fluctuation analysis. The computational framework is then used to estimate the bending moduli for different types of lipids (phosphocholine, phosphoethanolamine, and phosphoglycerol). This work provides greater insight into the numerical aspects of evaluating the bilayer bending modulus, provides validation for the orientation analysis technique, and explores differences in bending moduli based on differences in the lipid nanostructures.

Hemagglutinin-Mediated Membrane Fusion: A Biophysical Perspective

Influenza hemagglutinin (HA) is a viral membrane protein responsible for the initial steps of the entry of influenza virus into the host cell. It mediates binding of the virus particle to the host-cell membrane and catalyzes fusion of the viral membrane with that of the host. HA is therefore a major target in the development of antiviral strategies. The fusion of two membranes involves high activation barriers and proceeds through several intermediate states. Here, we provide a biophysical description of the membrane fusion process, relating its kinetic and thermodynamic properties to the large conformational changes taking place in HA and placing these in the context of multiple HA proteins working together to mediate fusion. Furthermore, we highlight the role of novel single-particle experiments and computational approaches in understanding the fusion process and their complementarity with other biophysical approaches.

Asymmetric Phosphatidylethanolamine Distribution Controls Fusion Pore Lifetime and Probability

Little attention has been given to how the asymmetric lipid distribution of the plasma membrane might facilitate fusion pore formation during exocytosis. Phosphatidylethanolamine (PE), a cone-shaped phospholipid, is predominantly located in the inner leaflet of the plasma membrane and has been proposed to promote membrane deformation and stabilize fusion pores during exocytotic events. To explore this possibility, we modeled exocytosis using plasma membrane SNARE-containing planar-supported bilayers and purified neuroendocrine dense core vesicles (DCVs) as fusion partners, and we examined how different PE distributions between the two leaflets of the supported bilayers affected SNARE-mediated fusion. Using total internal reflection fluorescence microscopy, the fusion of single DCVs with the planar-supported bilayer was monitored by observing DCV-associated neuropeptide Y tagged with a fluorescent protein. The time-dependent line shape of the fluorescent signal enables detection of DCV docking, fusion-pore opening, and vesicle collapse into the planar membrane. Four different distributions of PE in the planar bilayer mimicking the plasma membrane were examined: exclusively in the leaflet facing the DCVs; exclusively in the opposite leaflet; equally distributed in both leaflets; and absent from both leaflets. With PE in the leaflet facing the DCVs, overall fusion was most efficient and the extended fusion pore lifetime (0.7 s) enabled notable detection of content release preceding vesicle collapse. All other PE distributions decreased fusion efficiency, altered pore lifetime, and reduced content release. With PE exclusively in the opposite leaflet, resolution of pore opening and content release was lost.

Toward Chemically Resolved Computer Simulations of Dynamics and Remodeling of Biological Membranes

Cellular membranes are fundamental constituents of living organisms. Apart from defining the boundaries of the cells, they are involved in a wide range of biological functions, associated with both their structural and the dynamical properties. Biomembranes can undergo large-scale transformations when subject to specific environmental changes, including gel-liquid phase transitions, change of aggregation structure, formation of microtubules, or rupture into vesicles. All of these processes are dependent on a delicate interplay between intermolecular forces, molecular crowding, and entropy, and their understanding requires approaches that are able to capture and rationalize the details of all of the involved interactions. Molecular dynamics-based computational models at atom-level resolution are, in principle, the best way to perform such investigations. Unfortunately, the relevant spatial and time dimensionalities involved in membrane remodeling phenomena would require computational costs that are today unaffordable on a routinely basis. Such hurdles can be removed by coarse-graining the representations of the individual molecular components of the systems. This procedure anyway reduces the possibility of describing the chemical variations in the lipid mixtures composing biological membranes. New hybrid particle field multiscale approaches offer today a promising alternative to the more traditional particle-based simulations methods. By combining chemically distinguishable molecular representations with mesoscale-based computationally affordable potentials, they appear as one of the most promising ways to keep an accurate description of the chemical complexity of biological membranes and, at the same time, cover the required scales to describe remodeling events.