Non-bilayer lipids and biological fusion intermediates

Membrane contacts with the endoplasmic reticulum modulate plastid morphology and behaviour

Plastid behaviour often occurs in tandem with endoplasmic reticulum (ER) dynamics. In order to understand the underlying basis for such linked behaviour we have used time-lapse imaging-based analysis of plastid movement and pleomorphy, including the extension and retraction of stromules. Stable transgenic plants that simultaneously express fluorescent fusion proteins targeted to the plastid stroma, and the ER along with BnCLIP1-eGFP, an independent plastid envelope localized membrane contact site (MCS) marker were utilized. Our experiments strongly suggest that transient MCS formed between the plastid envelope and the ER are responsible for their concomitant behaviour.

Membrane cholesterol regulates the oligomerization and fusogenicity of SARS-CoV fusion peptide: implications in viral entry

N-terminal residues (770-788) of the S2 glycoprotein of severe acute respiratory syndrome coronavirus (SARS-CoV) have been recognized as a potential fusion peptide that can be involved in the entry of the virus into the host cell. Membrane composition plays an important role in lipid-peptide interaction and the oligomeric status of the peptide. SARS-CoV fusion peptide (S2 fusion peptide) is known to undergo cholesterol-dependent oligomerization in the membrane; however, its significance in membrane fusion is still speculative. This study aimed to investigate the oligomerization of SARS-CoV fusion peptide in a membrane containing phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol, with varying concentrations of cholesterol, and to evaluate peptide-induced membrane fusion to correlate the importance of peptide oligomerization with membrane fusion. Peptide-induced modulation of membrane organization and dynamics was explored by steady-state and time-resolved fluorescence spectroscopic measurements using depth-dependent probes. The results clearly demonstrated the induction of S2 fusion peptide oligomerization by membrane cholesterol and the higher efficiency of the oligomer in promoting membrane fusion compared to its monomeric counterpart. Cholesterol-dependent peptide oligomerization and membrane fusion are important aspects of viral infection since the cholesterol level can change with age as well as with the onset of various pathophysiological conditions.

Cyanobacterial membrane dynamics in the light of eukaryotic principles

Intracellular compartmentalization is a hallmark of eukaryotic cells. Dynamic membrane remodeling, involving membrane fission/fusion events, clearly is crucial for cell viability and function, as well as membrane stabilization and/or repair, e.g., during or after injury. In recent decades, several proteins involved in membrane stabilization and/or dynamic membrane remodeling have been identified and described in eukaryotes. Yet, while typically not having a cellular organization as complex as eukaryotes, also bacteria can contain extra internal membrane systems besides the cytoplasmic membranes (CMs). Thus, also in bacteria mechanisms must have evolved to stabilize membranes and/or trigger dynamic membrane remodeling processes. In fact, in recent years proteins, which were initially defined being eukaryotic inventions, have been recognized also in bacteria, and likely these proteins shape membranes also in these organisms. One example of a complex prokaryotic inner membrane system is the thylakoid membrane (TM) of cyanobacteria, which contains the complexes of the photosynthesis light reaction. Cyanobacteria are evolutionary closely related to chloroplasts, and extensive remodeling of the internal membrane systems has been observed in chloroplasts and cyanobacteria during membrane biogenesis and/or at changing light conditions. We here discuss common principles guiding eukaryotic and prokaryotic membrane dynamics and the proteins involved, with a special focus on the dynamics of the cyanobacterial TMs and CMs.

Lipid network and moiety analysis for revealing enzymatic dysregulation and mechanistic alterations from lipidomics data

Lipidomics is of growing importance for clinical and biomedical research due to many associations between lipid metabolism and diseases. The discovery of these associations is facilitated by improved lipid identification and quantification. Sophisticated computational methods are advantageous for interpreting such large-scale data for understanding metabolic processes and their underlying (patho)mechanisms. To generate hypothesis about these mechanisms, the combination of metabolic networks and graph algorithms is a powerful option to pinpoint molecular disease drivers and their interactions. Here we present lipid network explorer (LINEX$^2$), a lipid network analysis framework that fuels biological interpretation of alterations in lipid compositions. By integrating lipid-metabolic reactions from public databases, we generate dataset-specific lipid interaction networks. To aid interpretation of these networks, we present an enrichment graph algorithm that infers changes in enzymatic activity in the context of their multispecificity from lipidomics data. Our inference method successfully recovered the MBOAT7 enzyme from knock-out data. Furthermore, we mechanistically interpret lipidomic alterations of adipocytes in obesity by leveraging network enrichment and lipid moieties. We address the general lack of lipidomics data mining options to elucidate potential disease mechanisms and make lipidomics more clinically relevant.

Generation of nanoscopic membrane curvature for membrane trafficking

Curved membranes are key features of intracellular organelles, and their generation involves dynamic protein complexes. Here we describe the fundamental mechanisms such as the hydrophobic insertion, scaffolding and crowding mechanisms these proteins use to produce membrane curvatures and complex shapes required to form intracellular organelles and vesicular structures involved in endocytosis and secretion. For each mechanism, we discuss its cellular functions as well as the underlying physical principles and the specific membrane properties required for the mechanism to be feasible. We propose that the integration of individual mechanisms into a highly controlled, robust process of curvature generation often relies on the assembly of proteins into coats. How cells unify and organize the curvature-generating factors at the nanoscale is presented for three ubiquitous coats central for membrane trafficking in eukaryotes: clathrin-coated pits, caveolae, and COPI and COPII coats. The emerging theme is that these coats arrange and coordinate curvature-generating factors in time and space to dynamically shape membranes to accomplish membrane trafficking within cells.

Recent Developments in Single-Virus Fusion Assay

Viral infection is a global health hazard. A crucial step in the infection cycle of enveloped viruses is the fusion of viral and host cellular membranes, which permits the transfer of the viral genome to the host cells. Membrane fusion is a ubiquitous process involved in sperm-egg fusion, exocytosis, vesicular trafficking, and viral entry to host cells. While different protein machineries catalyze the diverse fusion processes, the essential step, i.e., merging of two lipid bilayers against a kinetic energy barrier, is the same. Therefore, viral fusion machineries/pathways are not only the sites for antiviral drug development but also serve as model fusogens. Ensemble-based spectroscopic approaches or bulk fusion assays have yielded valuable insights regarding the fusion processes. However, due to the stochastic nature of the fusion events, ensemble-based assays do not permit synchronization of all the fusion events, and the molecular steps leading to fusion pore opening cannot be resolved entirely and correlated with the structural changes in viral fusion proteins. Several single-virus fusion assays have been developed to circumvent these issues. The review describes the recent advancements in single-virus/particle fusion assays using the Influenza virus as a paradigm.

Structural Entities Associated with Different Lipid Phases of Plant Thylakoid Membranes—Selective Susceptibilities to Different Lipases and Proteases

It is well established that plant thylakoid membranes (TMs), in addition to a bilayer, contain two isotropic lipid phases and an inverted hexagonal (H_II) phase. To elucidate the origin of non-bilayer lipid phases, we recorded the ³¹P-NMR spectra of isolated spinach plastoglobuli and TMs and tested their susceptibilities to lipases and proteases; the structural and functional characteristics of TMs were monitored using biophysical techniques and CN-PAGE. Phospholipase-A1 gradually destroyed all ³¹P-NMR-detectable lipid phases of isolated TMs, but the weak signal of isolated plastoglobuli was not affected. Parallel with the destabilization of their lamellar phase, TMs lost their impermeability; other effects, mainly on Photosystem-II, lagged behind the destruction of the original phases. Wheat-germ lipase selectively eliminated the isotropic phases but exerted little or no effect on the structural and functional parameters of TMs—indicating that the isotropic phases are located outside the protein-rich regions and might be involved in membrane fusion. Trypsin and Proteinase K selectively suppressed the H_II phase—suggesting that a large fraction of TM lipids encapsulate stroma-side proteins or polypeptides. We conclude that—in line with the Dynamic Exchange Model—the non-bilayer lipid phases of TMs are found in subdomains separated from but interconnected with the bilayer accommodating the main components of the photosynthetic machinery.

The beginning and the end of SNARE-induced membrane fusion

Membrane fusion is not a spontaneous process. Physiologically, the formation of coiled-coil protein complexes, the SNAREpins, bridges the membrane of a vesicle and a target membrane, brings them in close contact, and provides the energy necessary for their fusion. In this review, we utilize results from in vitro experiments and simple physics and chemistry models to dissect the kinetics and energetics of the fusion process from the encounter of the two membranes to the full expansion of a fusion pore. We find three main energy barriers that oppose the fusion process: SNAREpin initiation, fusion pore opening, and expansion. SNAREpin initiation is inherent to the proteins and makes in vitro fusion kinetic experiments rather slow. The kinetics are physiologically accelerated by effectors. The energy barriers that precede pore opening and pore expansion can be overcome by several SNAREpins acting in concert.

Physico-Chemical Mechanisms of the Functioning of Membrane-Active Proteins of Enveloped Viruses

Over the past few years, the attention of the whole world has been riveted to the emergence of new dangerous strains of viruses, among which a special place is occupied by coronaviruses that have overcome the interspecies barrier in the past 20 years: SARS viruses (SARS), Middle East respiratory syndrome (MERS), as well as a new coronavirus infection (SARS-CoV-2), which caused the largest pandemic since the Spanish flu in 1918. Coronaviruses are members of a class of enveloped viruses that have a lipoprotein envelope. This class also includes such serious pathogens as human immunodeficiency virus (HIV), hepatitis, Ebola virus, influenza, etc. Despite significant differences in the clinical picture of the course of disease caused by enveloped viruses, they themselves have a number of characteristic features, which determine their commonality. Regardless of the way of penetration into the cell—by endocytosis or direct fusion with the cell membrane—enveloped viruses are characterized by the following stages of interaction with the target cell: binding to receptors on the cell surface, interaction of the surface glycoproteins of the virus with the membrane structures of the infected cell, fusion of the lipid envelope of the virion with plasma or endosomal membrane, destruction of the protein capsid and its dissociation from the viral nucleoprotein. Subsequently, within the infected cell, the newly synthesized viral proteins must self-assemble on various membrane structures to form a progeny virion. Thus, both the initial stages of viral infection and the assembly and release of new viral particles are associated with the activity of viral proteins in relation to the cell membrane and its organelles. This review is devoted to the analysis of physicochemical mechanisms of functioning of the main structural proteins of a number of enveloped viruses in order to identify possible strategies for the membrane activity of such proteins at various stages of viral infection of the cell.

Cooperation of Conical and Polyunsaturated Lipids to Regulate Initiation and Processing of Membrane Fusion

The shape of lipids has long been suspected to be a critical determinant for the control of membrane fusion. To experimentally test this assertion, we used conical and malleable lipids and measured their influence on the fusion kinetics. We found that, as previously suspected, both types of lipids accelerate fusion. However, the implicated molecular mechanisms are strikingly different. Malleable lipids, with their ability to change shape with low energy cost, favor fusion by decreasing the overall activation energy. On the other hand, conical lipids, with their small polar head relative to the area occupied by the hydrophobic chains, tend to make fusion less energetically advantageous because they tend to migrate towards the most favorable lipid leaflet, hindering fusion pore opening. They could however facilitate fusion by generating hydrophobic defects on the membranes; this is suggested by the similar trend observed between the experimental rate of fusion nucleation and the surface occupied by hydrophobic defects obtained by molecular simulations. The synergy of dual-process, activation energy and nucleation kinetics, could facilitate membrane fusion regulation in vivo.

Lipid Polymorphism of the Subchloroplast-Granum and Stroma Thylakoid Membrane-Particles. II. Structure and Functions

In Part I, by using 31P-NMR spectroscopy, we have shown that isolated granum and stroma thylakoid membranes (TMs), in addition to the bilayer, display two isotropic phases and an inverted hexagonal (HII) phase; saturation transfer experiments and selective effects of lipase and thermal treatments have shown that these phases arise from distinct, yet interconnectable structural entities. To obtain information on the functional roles and origin of the different lipid phases, here we performed spectroscopic measurements and inspected the ultrastructure of these TM fragments. Circular dichroism, 77 K fluorescence emission spectroscopy, and variable chlorophyll-a fluorescence measurements revealed only minor lipase- or thermally induced changes in the photosynthetic machinery. Electrochromic absorbance transients showed that the TM fragments were re-sealed, and the vesicles largely retained their impermeabilities after lipase treatments-in line with the low susceptibility of the bilayer against the same treatment, as reflected by our 31P-NMR spectroscopy. Signatures of HII-phase could not be discerned with small-angle X-ray scattering-but traces of HII structures, without long-range order, were found by freeze-fracture electron microscopy (FF-EM) and cryo-electron tomography (CET). EM and CET images also revealed the presence of small vesicles and fusion of membrane particles, which might account for one of the isotropic phases. Interaction of VDE (violaxanthin de-epoxidase, detected by Western blot technique in both membrane fragments) with TM lipids might account for the other isotropic phase. In general, non-bilayer lipids are proposed to play role in the self-assembly of the highly organized yet dynamic TM network in chloroplasts.

The role of fusion peptides in depth-dependent membrane organization and dynamics in promoting membrane fusion

Membrane fusion is an important event in the life of eukaryotes; occurs in several processes such as endocytosis, exocytosis, cellular trafficking, compartmentalization, import of nutrients and export of waste, vesiculation, inter cellular communication, and fertilization. The enveloped viruses as well utilize fusion between the viral envelope and host cell membrane for infection. The stretch of 20-25 amino acids located at the N-terminus of the fusion protein, known as fusion peptide, plays a decisive role in the fusion process. The stalk model of membrane fusion postulated a common route of bilayer transformation for stalk, transmembrane contact, and pore formation; and fusion peptide is believed to facilitate bilayer transformation to promote membrane fusion. The peptide-induced change in depth-dependent organization and dynamics could provide important information in understanding the role of fusion peptide in membrane fusion. In this review, we have discussed about three depth-dependent properties of the membrane such as rigidity, polarity and heterogeneity, and the impact of fusion peptide on these three membrane properties.

Continuum Models of Membrane Fusion: Evolution of the Theory

Starting from fertilization, through tissue growth, hormone secretion, synaptic transmission, and sometimes morbid events of carcinogenesis and viral infections, membrane fusion regulates the whole life of high organisms. Despite that, a lot of fusion processes still lack well-established models and even a list of main actors. A merger of membranes requires their topological rearrangements controlled by elastic properties of a lipid bilayer. That is why continuum models based on theories of membrane elasticity are actively applied for the construction of physical models of membrane fusion. Started from the view on the membrane as a structureless film with postulated geometry of fusion intermediates, they developed along with experimental and computational techniques to a powerful tool for prediction of the whole process with molecular accuracy. In the present review, focusing on fusion processes occurring in eukaryotic cells, we scrutinize the history of these models, their evolution and complication, as well as open questions and remaining theoretical problems. We show that modern approaches in this field allow continuum models of membrane fusion to stand shoulder to shoulder with molecular dynamics simulations, and provide the deepest understanding of this process in multiple biological systems.

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.

Arachidonic acid and lysophosphatidylcholine inhibit multiple late steps of regulated exocytosis

The canonical Phospholipase A₂ (PLA₂) metabolites lysophosphatidylcholine (LPC) and arachidonic acid (ARA) affect regulated exocytosis in a wide variety of cells and are proposed to directly influence membrane merger owing to their respective spontaneous curvatures. According to the Stalk-pore hypothesis, negative curvature ARA inhibits and promotes bilayer merger upon introduction into the distal or proximal monolayers, respectively; in contrast, with positive curvature, LPC has the opposite effects. Using fully primed, release-ready native cortical secretory vesicles (CV), well-established fusion assays and standardized lipid analyses, we show that exogenous ARA and LPC, as well as their non-metabolizable analogous, ETYA and ET-18-OCH3, inhibit the docking/priming and membrane merger steps, respectively, of regulated exocytosis.

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.

Combined targeted Omic and Functional Assays Identify Phospholipases A₂ that Regulate Docking/Priming in Calcium-Triggered Exocytosis

The fundamental molecular mechanism underlying the membrane merger steps of regulated exocytosis is highly conserved across cell types. Although involvement of Phospholipase A₂ (PLA₂) in regulated exocytosis has long been suggested, its function or that of its metabolites-a lyso-phospholipid and a free fatty acid-remain somewhat speculative. Here, using a combined bioinformatics and top-down discovery proteomics approach, coupled with lipidomic analyses, PLA₂ were found to be associated with release-ready cortical secretory vesicles (CV) that possess the minimal molecular machinery for docking, Ca²⁺ sensing and membrane fusion. Tightly coupling the molecular analyses with well-established quantitative fusion assays, we show for the first time that inhibition of a CV surface calcium independent intracellular PLA₂ and a luminal secretory PLA₂ significantly reduce docking/priming in the late steps of regulated exocytosis, indicating key regulatory roles in the critical step(s) preceding membrane merger.

Synthetic lipids as a biocide candidate for disinfection of ballast water

The objective of this study is to propose the use of specific synthetic lipid as an active substance (biocide) in the control of harmful aquatic microorganisms, such as pathogens and non-indigenous species, transported in ships' ballast water. The biocide candidate, without metal or halogen components, was produced from a sub-product of the edible oil industry, the lecithin. Laboratory assays were conducted with phytoplankton, zooplankton, and marine bacteria to evaluate the efficiency of the biocide. The study also considers specific biocide's characteristics related to environmental risks, such as chemical composition, persistence, bioaccumulation, and toxicity. Results showed that, in the first 24 h of treatment, the biocide effectively reduced the concentration of the planktonic micro-organisms to very low levels. Additionally, a preliminary risk evaluation pointed that biocide candidate has a low residual toxicity, also a low potential for persistence and bioaccumulation in the environment.

Lipid-dependence of target membrane stability during influenza viral fusion

Although influenza kills about a half million people each year, even after excluding pandemics, there is only one set of antiviral drugs: neuraminidase inhibitors. By using a new approach utilizing giant unilamellar vesicles and infectious X-31 influenza virus, and testing for the newly identified pore intermediate of membrane fusion, we observed ∼30-87% poration, depending upon lipid composition. Testing the hypothesis that spontaneous curvature (SC) of the lipid monolayer controls membrane poration, our Poisson model and Boltzmann energetic considerations suggest a transition from a leaky to a non-leaky fusion pathway depending on the SC of the target membrane. When the target membrane SC is below approximately -0.20 nm^-1 fusion between influenza virus and target membrane is predominantly non-leaky while above that fusion is predominantly leaky, suggesting that influenza hemagglutinin (HA)-catalyzed topological conversion of target membranes during fusion is associated with a loss of membrane integrity.

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.

Exploring the Mechanism of Viral Peptide-Induced Membrane Fusion

Membrane fusion is essential in several cellular processes in the existence of eukaryotic cells such as cellular trafficking, compartmentalization, intercellular communication, sexual reproduction, cell division, and endo- and exocytosis. Membrane fusion proceeds in model membranes as well as biological membranes through the rearrangement of lipids. The stalk hypothesis provides a picture of the general nature of lipid rearrangement based on mechanical properties and phase behavior of water-lipid mesomorphic systems. In spite of extensive research on exploring the mechanism of membrane fusion, a clear molecular understanding of intermediate and pore formation is lacking. In addition, the mechanism by which proteins and peptides reduce the activation energy for stalk and pore formation is not yet clear though there are several propositions on how they catalyze membrane fusion. In this review, we have discussed about various putative functions of fusion peptides by which they reduce activation barrier and thus promote membrane fusion. A careful analysis of the discussed effects of fusion peptides on membranes might open up new possibilities for better understanding of the membrane fusion mechanism.

Lipid polymorphism in chloroplast thylakoid membranes - as revealed by 31P-NMR and time-resolved merocyanine fluorescence spectroscopy

Chloroplast thylakoid membranes contain virtually all components of the energy-converting photosynthetic machinery. Their energized state, driving ATP synthesis, is enabled by the bilayer organization of the membrane. However, their most abundant lipid species is a non-bilayer-forming lipid, monogalactosyl-diacylglycerol; the role of lipid polymorphism in these membranes is poorly understood. Earlier ³¹P-NMR experiments revealed the coexistence of a bilayer and a non-bilayer, isotropic lipid phase in spinach thylakoids. Packing of lipid molecules, tested by fluorescence spectroscopy of the lipophilic dye, merocyanine-540 (MC540), also displayed heterogeneity. Now, our ³¹P-NMR experiments on spinach thylakoids uncover the presence of a bilayer and three non-bilayer lipid phases; time-resolved fluorescence spectroscopy of MC540 also reveals the presence of multiple lipidic environments. It is also shown by ³¹P-NMR that: (i) some lipid phases are sensitive to the osmolarity and ionic strength of the medium, (ii) a lipid phase can be modulated by catalytic hydrogenation of fatty acids and (iii) a marked increase of one of the non-bilayer phases upon lowering the pH of the medium is observed. These data provide additional experimental evidence for the polymorphism of lipid phases in thylakoids and suggest that non-bilayer phases play an active role in the structural dynamics of thylakoid membranes.

Non-lamellar lipid assembly at interfaces: controlling layer structure by responsive nanogel particles

Biological membranes do not only occur as planar bilayer structures, but depending on the lipid composition, can also curve into intriguing three-dimensional structures. In order to fully understand the biological implications as well as to reveal the full potential for applications, e.g. for drug delivery and other biomedical devices, of such structures, well-defined model systems are required. Here, we discuss the formation of lipid non-lamellar liquid crystalline (LC) surface layers spin-coated from the constituting lipids followed by hydration of the lipid layer. We demonstrate that hybrid lipid polymer films can be formed with different properties compared with the neat lipid LC layers. The nanostructure and morphologies of the lipid films formed reflect those in the bulk. Most notably, mixed lipid layers, which are composed of glycerol monooleate and diglycerol monooleate with poly(N-isopropylacrylamide) nanogels, can form films of reverse cubic phases that are capable of responding to temperature stimulus. Owing to the presence of the nanogel particles, changing the temperature not only regulates the hydration of the cubic phase lipid films, but also the lateral organization of the lipid domains within the lipid self-assembled film. This opens up the possibility for new nanostructured materials based on lipid-polymer responsive layers.

Relationship between Structure and Fluctuations of Lipid Nonlamellar Phases Deposited at the Solid-Liquid Interface

The structure and dynamics of nanostructure films formed by mixtures of soy phosphatidylcholine and glycerol dioleate at the silicon-aqueous interface were studied by grazing incidence neutron spin echo spectroscopy (GINSES), specular and off-specular neutron reflectometry, and small-angle X-ray diffraction. Reverse hexagonal (H_II) and micellar cubic phase (Fd3m) layers at the solid-liquid interface have been identified with neutron reflectometry measurements. A preferred orientation of the liquid crystalline (LC) domains was observed only for the anisotropic H_II phase. The size of the LC domains was found to be about 1 micrometer as estimated from the width of the diffraction peaks. GINSES revealed that the cubic phase forms rather rigid films. In comparison, the H_II film was more flexible, appearing as a modified undulation spectrum of the cylinders due to the interaction with the substrate.

Interaction of phospholipase C with liposome: A conformation transition of the enzyme is critical and specific to liposome composition for burst hydrolysis and fusion in concert

Phospholipase C (PLC)¹ is known to help the pathogen B. cereus entry to the host cell and human PLC is over expressed in multiple cancers. Knowledge of dynamic activity of the enzyme PLC while in action on membrane lipids is essential and helpful to drug design and delivery. In view of this, interactions of PLC with liposome of various lipid compositions have been visualized by testing enzyme activity and microenvironments around the intrinsic fluorophores of the enzyme. Overall change of the protein's conformation has been monitored by fluorescence spectroscopy and circular dichroism (CD). Liposome aggregation and fusion were predicted by increase in turbidity and vesicle size. PLC in solution has high fluorescence and exhibit appreciable shift in its emission maxima, upon gradual change in excitation wavelength towards the red edge of the absorption band. REES fluorescence studies indicated that certain Trp fluorophores of inactive PLC are in motionally restricted compact/rigid environments in solution conformation. PLC fluorescence decreased in association with liposome and Trps loosed rigidity where liposome aggregation and fusion occurred. We argue that the structural flexibility is the cause of decrease of fluorescence, mostly to gain optimum conformation for maximum activity of the enzyme PLC. Further studies deciphered that the enzyme PLC undergoes change of conformation when mixed to LUVs prepared with specific lipids. CD data at the far-UV and near-UV regions of PLC in solution are in excellent agreement with the previous reports. CD analyses of PLC with LUVs, showed significant reduction of α-helices, increase of β-sheets; and confirmed dramatic change of orientations of Trps. In case of liposome composed of lipid raft like composition, the enzyme binds very fast, hydrolyze PC with higher rate, exhibit highest structural flexibility and promote vesicle fusion. These data strongly suggest marked differences in conformation transition induced PLC activation and liposome fusion on the lipid composition.

Coarse-grained molecular dynamics study of membrane fusion: Curvature effects on free energy barriers along the stalk mechanism

The effects of membrane curvature on the free energy barrier for membrane fusion have been investigated using coarse-grained molecular dynamics (CG-MD) simulations, assuming that fusion takes place through a stalk intermediate. Free energy barriers were estimated for stalk formation as well as for fusion pore formation using the guiding potential method. Specifically, the three different geometries of two apposed membranes were considered: vesicle-vesicle, vesicle-planar, and planar-planar membranes. The free energy barriers for the resulting fusion were found to depend importantly on the fusing membrane geometries; the lowest barrier was obtained for vesicular membranes. Further, lipid sorting was observed in fusion of the mixed membranes of dimyristoyl phosphatidylcholine and dioleoyl phosphatidylethanolamine (DOPE). Specifically, DOPE molecules were found to assemble around the stalk to support the highly negative curved membrane surface. A consistent result for lipid sorting was observed when a simple continuum model (CM) was used, where the Helfrich energy and mixing entropy of the lipids were taken into account. However, the CM predicts a much higher free energy barrier than found using CG-MD. This discrepancy originates from the conformational changes of lipids, which were not considered in the CM. The results of the CG-MD simulations reveal that a large conformational change in the lipid takes place around the stalk region, which results in a reduction of free energy barriers along the stalk mechanism of membrane fusion.

Addition of lysophospholipids with large head groups to cells inhibits Shiga toxin binding

Shiga toxin (Stx), an AB₅ toxin, binds specifically to the neutral glycosphingolipid Gb3 at the cell surface before being transported into cells. We here demonstrate that addition of conical lysophospholipids (LPLs) with large head groups inhibit Stx binding to cells whereas LPLs with small head groups do not. Lysophosphatidylinositol (LPI 18:0), the most efficient LPL with the largest head group, was selected for in-depth investigations to study how the binding of Stx is regulated. We show that the inhibition of Stx binding by LPI is reversible and possibly regulated by cholesterol since addition of methyl-β-cyclodextrin (mβCD) reversed the ability of LPI to inhibit binding. LPI-induced inhibition of Stx binding is independent of signalling and membrane turnover as it occurs in fixed cells as well as after depletion of cellular ATP. Furthermore, data obtained with fluorescent membrane dyes suggest that LPI treatment has a direct effect on plasma membrane lipid packing with shift towards a liquid disordered phase in the outer leaflet, while lysophosphoethanolamine (LPE), which has a small head group, does not. In conclusion, our data show that cellular treatment with conical LPLs with large head groups changes intrinsic properties of the plasma membrane and modulates Stx binding to Gb3.

Aggregation and fusion of low-density lipoproteins in vivo and in vitro

Low-density lipoproteins (LDLs, also known as 'bad cholesterol') are the major carriers of circulating cholesterol and the main causative risk factor of atherosclerosis. Plasma LDLs are 20- to 25-nm nanoparticles containing a core of cholesterol esters surrounded by a phospholipid monolayer and a single copy of apolipoprotein B (550 kDa). An early sign of atherosclerosis is the accumulation of LDL-derived lipid droplets in the arterial wall. According to the widely accepted 'response-to-retention hypothesis', LDL binding to the extracellular matrix proteoglycans in the arterial intima induces hydrolytic and oxidative modifications that promote LDL aggregation and fusion. This enhances LDL uptake by the arterial macrophages and triggers a cascade of pathogenic responses that culminate in the development of atherosclerotic lesions. Hence, LDL aggregation, fusion, and lipid droplet formation are important early steps in atherogenesis. In vitro, a variety of enzymatic and nonenzymatic modifications of LDL can induce these reactions and thereby provide useful models for their detailed analysis. Here, we summarize current knowledge of the in vivo and in vitro modifications of LDLs leading to their aggregation, fusion, and lipid droplet formation; outline the techniques used to study these reactions; and propose a molecular mechanism that underlies these pro-atherogenic processes. Such knowledge is essential in identifying endogenous and exogenous factors that can promote or prevent LDL aggregation and fusion in vivo and to help establish new potential therapeutic targets to decelerate or even block these pathogenic reactions.

The influenza hemagglutinin fusion domain is an amphipathic helical hairpin that functions by inducing membrane curvature

The highly conserved N-terminal 23 residues of the hemagglutinin glycoprotein, known as the fusion peptide domain (HAfp23), is vital to the membrane fusion and infection mechanism of the influenza virus. HAfp23 has a helical hairpin structure consisting of two tightly packed amphiphilic helices that rest on the membrane surface. We demonstrate that HAfp23 is a new class of amphipathic helix that functions by leveraging the negative curvature induced by two tightly packed helices on membranes. The helical hairpin structure has an inverted wedge shape characteristic of negative curvature lipids, with a bulky hydrophobic region and a relatively small hydrophilic head region. The F3G mutation reduces this inverted wedge shape by reducing the volume of its hydrophobic base. We show that despite maintaining identical backbone structures and dynamics as the wild type HAfp23, the F3G mutant has an attenuated fusion activity that is correlated to its reduced ability to induce negative membrane curvature. The inverted wedge shape of HAfp23 is likely to play a crucial role in the initial stages of membrane fusion by stabilizing negative curvature in the fusion stalk.

Model cell membranes: discerning lipid and protein contributions in shaping the cell

The high complexity of biological membranes has motivated the development and application of a wide range of model membrane systems to study biochemical and biophysical aspects of membranes in situ under well defined conditions. The aim is to provide fundamental understanding of processes controlled by membrane structure, permeability and curvature as well as membrane proteins by using a wide range of biochemical, biophysical and microscopic techniques. This review gives an overview of some currently used model biomembrane systems. We will also discuss some key membrane protein properties that are relevant for protein-membrane interactions in terms of protein structure and how it is affected by membrane composition, phase behavior and curvature.

pH-sensitive liposomes for drug delivery in cancer treatment

In recent years, liposomes have been employed with growing success as pharmaceutical carriers for antineoplastic drugs. One specific strategy used to enhance in vivo liposome-mediated drug delivery is the improvement of intracytoplasmic delivery. In this context, pH-sensitive liposomes (pHSLip) have been designed to explore the endosomal acidification process, which may lead to a destabilization of the liposomes, followed by a release of their contents into the cell cytoplasm. This review considers the current status of pHSLip development and its applicability in cancer treatment, focusing on the mechanisms of pH sensitivity and liposomal composition of pHSLip. The final section will discuss the application of these formulations in both in vitro and in vivo studies of antitumor efficacy.

GTP-dependent membrane fusion

Shape changes and topological remodeling of membranes are essential for the identity of organelles and membrane trafficking. Although all cellular membranes have common features, membranes of different organelles create unique environments that support specialized biological functions. The endoplasmic reticulum (ER) is a prime example of this specialization, as its lipid bilayer forms an interconnected system of cisternae, vesicles, and tubules, providing a highly compartmentalized structure for a multitude of biochemical processes. A variety of peripheral and integral membrane proteins that facilitate membrane curvature generation, fission, and/or fusion have been identified over the past two decades. Among these, the dynamin-related proteins (DRPs) have emerged as key players. Here, we review recent advances in our functional and molecular understanding of fusion DRPs, exemplified by atlastin, an ER-resident DRP that controls ER structure, function, and signaling.

Imaging mass spectrometry in neuroscience

Imaging mass spectrometry is an emerging technique of great potential for investigating the chemical architecture in biological matrices. Although the potential for studying neurobiological systems is evident, the relevance of the technique for application in neuroscience is still in its infancy. In the present Review, a principal overview of the different approaches, including matrix assisted laser desorption ionization and secondary ion mass spectrometry, is provided with particular focus on their strengths and limitations for studying different neurochemical species in situ and in vitro. The potential of the various approaches is discussed based on both fundamental and biomedical neuroscience research. This Review aims to serve as a general guide to familiarize the neuroscience community and other biomedical researchers with the technique, highlighting its great potential and suitability for comprehensive and specific chemical imaging.

Neutron diffraction with an excess-water cell

As part of a study of the molecular basis of membrane fusion by enveloped viruses, we have used neutron diffraction to study the lamellar (L(α)) to inverse hexagonal (H(II)) phase transition in the phospholipid N-methylated dioleoylphosphatidylethanolamine. This lipid was chosen because its phase transitions are particularly sensitive to the presence of agents that have been demonstrated to promote or inhibit membrane fusion. Two different geometries of neutron diffraction were used: small angle scattering (SANS) and a membrane diffractometer. The SANS measurements were carried out on the SWAN instrument at KEK, Japan, using dispersions of multilamellar vesicles (MLVs). The diffractometer measurements used the V1 instrument at BeNSC-HMI, Germany, with a specially-constructed cell that holds a stack of lipid bilayers in an excess-water state. The two approaches are compared and discussed. Although the diffractometer takes considerably longer to collect the data, it records much higher resolution than the SANS instrument. The samples recorded in the excess-water cell were shown to be well aligned, despite the lipids being fully hydrated, allowing for the production of high-resolution data. Trial measurements performed have demonstrated that sample alignment is preserved throughout the L(α) to H(II) phase transition, thereby opening up possibilities for obtaining high-resolution data from non-lamellar phases.

5-(Perylen-3-yl)ethynyl-arabino-uridine (aUY11), an arabino-based rigid amphipathic fusion inhibitor, targets virion envelope lipids to inhibit fusion of influenza virus, hepatitis C virus, and other enveloped viruses

Entry of enveloped viruses requires fusion of viral and cellular membranes. Fusion requires the formation of an intermediate stalk structure, in which only the outer leaflets are fused. The stalk structure, in turn, requires the lipid bilayer of the envelope to bend into negative curvature. This process is inhibited by enrichment in the outer leaflet of lipids with larger polar headgroups, which favor positive curvature. Accordingly, phospholipids with such shape inhibit viral fusion. We previously identified a compound, 5-(perylen-3-yl)ethynyl-2'-deoxy-uridine (dUY11), with overall shape and amphipathicity similar to those of these phospholipids. dUY11 inhibited the formation of the negative curvature necessary for stalk formation and the fusion of a model enveloped virus, vesicular stomatitis virus (VSV). We proposed that dUY11 acted by biophysical mechanisms as a result of its shape and amphipathicity. To test this model, we have now characterized the mechanisms against influenza virus and HCV of 5-(perylen-3-yl)ethynyl-arabino-uridine (aUY11), which has shape and amphipathicity similar to those of dUY11 but contains an arabino-nucleoside. aUY11 interacted with envelope lipids to inhibit the infectivity of influenza virus, hepatitis C virus (HCV), herpes simplex virus 1 and 2 (HSV-1/2), and other enveloped viruses. It specifically inhibited the fusion of influenza virus, HCV, VSV, and even protein-free liposomes to cells. Furthermore, aUY11 inhibited the formation of negative curvature in model lipid bilayers. In summary, the arabino-derived aUY11 and the deoxy-derived dUY11 act by the same antiviral mechanisms against several enveloped but otherwise unrelated viruses. Therefore, chemically unrelated compounds of appropriate shape and amphipathicity target virion envelope lipids to inhibit formation of the negative curvature required for fusion, inhibiting infectivity by biophysical, not biochemical, mechanisms.

Acute manipulation of diacylglycerol reveals roles in nuclear envelope assembly & endoplasmic reticulum morphology

The functions and morphology of cellular membranes are intimately related and depend not only on their protein content but also on the repertoire of lipids that comprise them. In the absence of in vivo data on lipid asymmetry in endomembranes, it has been argued that motors, scaffolding proteins or integral membrane proteins rather than non-lamellar bilayer lipids such as diacylglycerol (DAG), are responsible for shaping of organelles, local membrane curvature and fusion. The effects of direct alteration of levels of such lipids remain predominantly uninvestigated. Diacylglycerol (DAG) is a well documented second messenger. Here we demonstrate two additional conserved functions of DAG: a structural role in organelle morphology, and a role in localised extreme membrane curvature required for fusion for which proteins alone are insufficient. Acute and inducible DAG depletion results in failure of the nuclear envelope (NE) to reform at mitosis and reorganisation of the ER into multi-lamellar sheets as revealed by correlative light and electron microscopy and 3D reconstructions. Remarkably, depleted cells divide without a complete NE, and unless rescued by 1,2 or 1,3 DAG soon die. Attenuation of DAG levels by enzyme microinjection into echinoderm eggs and embryos also results in alterations of ER morphology and nuclear membrane fusion. Our findings demonstrate that DAG is an in vivo modulator of organelle morphology in mammalian and echinoderm cells, indicating a fundamental role conserved across the deuterostome superphylum.

Lysophosphatidylcholine inhibits membrane-associated SNARE complex disassembly

In cells, N-ethylmaleimide-sensitive factor (NSF) attachment protein receptors called SNAREs are involved in membrane fusion. In neurons, for example, target membrane proteins SNAP-25 and syntaxin called t-SNAREs present at the pre-synaptic membrane, and a synaptic vesicle-associated membrane protein (VAMP) or v-SNARE, is part of the conserved protein complex involved in neurotransmission. Cholesterol and LPC (L-α-lysophosphatidylcholine) are known to contribute to the negative and positive curvature respectively of membranes. In this study, using purified recombinant neuronal membrane-associated SNAREs, we demonstrate for the first time that membrane-curvature-influencing lipids profoundly influence SNARE complex disassembly. Exposure of cholesterol-associated t-SNARE and v-SNARE liposome mixtures to NSF-ATP results in dissociated vesicles. In contrast, exposure of LPC-associated t-SNARE and v-SNARE liposome mixtures to NSF-ATP, results in inhibition of t-/v-SNARE disassembly and the consequent accumulation of clustered vesicles. Similarly, exposure of isolated rat brain slices and pancreas to cholesterol or LPC, also demonstrates LPC-induced inhibition of SNARE complex disassembly. Earlier studies demonstrate a strong correlation between altered plasma LPC levels and cancer. The altered plasma LPC levels observed in various cancers may in part contribute to defects in SNARE assembly-disassembly and membrane fusion, consequently affecting protein maturation and secretion in cancer cells.