Quantification of Multivalent Interactions by Tracking Single Biological Nanoparticle Mobility on a Lipid Membrane

Ligand Separation on Nanoconstructs Affects Targeting Selectivity to Protein Dimers on Cell Membranes

This work demonstrates that targeting ligand density on nanoparticles can affect interactions between the nanoconstructs and cell membrane receptors. We discovered that when the separation between covalently grafted DNA aptamers on gold nanostars was comparable to the distance between binding sites on a receptor dimer (matched density; MD), nanoconstructs exhibited a higher selectivity for binding to the dimeric form of the protein. Single-particle dynamics of MD nanoconstructs showed slower rotational rates and larger translational footprints on cancer cells expressing more dimeric forms of receptors (dimer+) compared with cells having more monomeric forms (dimer-). In contrast, nanoconstructs with either increased (nonmatched density; NDlow) or decreased ligand spacing (NDhigh) had minimal changes in dynamics on either dimer+ or dimer- cells. Real-time, single-particle analyses can reveal the importance of nanoconstruct ligand density for the selective targeting of membrane receptors in live cells.

Single Nanovesicles Tracking Reveals Their Heterogeneous Extracellular Adsorptions

The vigorous nanomedicine offers significant possibilities for effective therapeutics of various diseases, and nanovesicles (NVs) represented by artificial liposomes and natural exosomes and cytomembranes especially show great potential. However, their complex interactions with cells, particularly the heterogeneous extracellular adsorptions, are difficult to analyze spatiotemporally due to the transient dynamics. In this study, by single NVs tracking, the extracellular NVs adsorptions are directly observed and their heterogeneous characteristics are revealed. Briefly, plenty of NVs adsorbed on HCT116 cells are tracked and classified, and it is discovered that they exhibit various diffusion properties from different extracellular regions: stable adsorptions on the rear surface and restricted adsorptions on the front protrusion. After the hydrolysis of hyaluronic acid in the extracellular matrix by hyaluronidase, the restricted adsorptions are further weakened and manifested as dissociative adsorptions, which demonstrated reduced total NVs adsorptions from a single-cell and single-particle perspective. Compared with traditional static analysis, the spatiotemporal tracking and heterogeneous results not only reveal the extracellular NVs-cell interactions but also inspire a wide variety of nanomedicine and their nano-investigations.

PDK1:PKCα heterodimer association-dissociation dynamics in single-molecule diffusion tracks on a target membrane

Previous studies have documented the formation of a heterodimer between the two protein kinases PDK1 and PKCα on a lipid bilayer containing their target lipids. This work investigates the association-dissociation kinetics of this PDK1:PKCα heterodimer. The approach monitors the two-dimensional diffusion of single, membrane-associated PDK1 molecules for diffusivity changes as PKCα molecules bind and unbind. In the absence of PKCα, a membrane-associated PDK1 molecule exhibits high diffusivity (or large diffusion constant, D) because its membrane-contacting PH domain binds the target PIP3 lipid headgroup with little bilayer penetration, yielding minimal frictional drag against the bilayer. In contrast, membrane-associated PKCα contacts the bilayer via its C1A, C1B, and C2 domains, which each bind at least one target lipid with significant bilayer insertion, yielding a large frictional drag and low diffusivity. The present findings reveal that individual fluor-PDK1 molecules freely diffusing on the membrane surface undergo reversible switching between distinct high and low diffusivity states, corresponding to the PDK1 monomer and the PDK1:PKCα heterodimer, respectively. The observed single-molecule diffusion trajectories are converted to step length time courses, then subjected to two-state, hidden Markov modeling and dwell time analysis. The findings reveal that both the PDK1 monomer state and the PDK1:PKCα heterodimer state decay via simple exponential kinetics, yielding estimates of rate constants for state switching in both directions. Notably, the PDK1:PKCα heterodimer has been shown to competitively inhibit PDK1 phosphoactivation of AKT1, and is believed to play a tumor suppressor role by limiting excess activation of the highly oncogenic PDK1/AKT1/mTOR pathway. Thus, the present elucidation of the PDK1:PKCα association-dissociation kinetics has important biological and medical implications. More broadly, the findings illustrate the power of single-molecule diffusion measurements to reveal the kinetics of association-dissociation events in membrane signaling reactions that yield a large change in diffusive mobility.

pH-Modulated Nanoarchitectonics for Enhancement of Multivalency-Induced Vesicle Shape Deformation at Receptor-Presenting Lipid Membrane Interfaces

Multivalent ligand-receptor interactions between receptor-presenting lipid membranes and ligand-modified biological and biomimetic nanoparticles influence cellular entry and fusion processes. Environmental pH changes can drive these membrane-related interactions by affecting membrane nanomechanical properties. Quantitatively, however, the corresponding effects on high-curvature, sub-100 nm lipid vesicles are scarcely understood, especially in the multivalent binding context. Herein, we employed the label-free localized surface plasmon resonance (LSPR) sensing technique to track the multivalent attachment kinetics, shape deformation, and surface coverage of biotin ligand-functionalized, zwitterionic lipid vesicles with different ligand densities on a streptavidin receptor-coated supported lipid bilayer under varying pH conditions (4.5, 6, 7.5). Our results demonstrate that more extensive multivalent interactions caused greater vesicle shape deformation across the tested pH conditions, which affected vesicle surface packing as well. Notably, there were also pH-specific differences, i.e., a higher degree of vesicle shape deformation was triggered at a lower multivalent binding energy in pH 4.5 than in pH 6 and 7.5 conditions. These findings support that the nanomechanical properties of high-curvature lipid membranes, especially the membrane bending energy and the corresponding responsiveness to multivalent binding interactions, are sensitive to solution pH, and indicate that multivalency-induced vesicle shape deformation occurs slightly more readily in acidic pH conditions relevant to biological environments.

How clathrin-coated pits control nanoparticle avidity for cells

The paramount relevance of clathrin-coated pits (CCPs) to receptor-mediated endocytosis of nanoparticles, extracellular vesicles, and viruses has made them the focus of many studies; however, the role of CCP geometry in the ligand-receptor interactions between multivalent nanoparticles and cells has not been investigated. We hypothesized the general dependence of nanoparticle binding energy on local membrane curvature to be expandable to the specific case of ligand-functionalized nanoparticles binding cell membranes, in the sense that membrane structures whose curvature matches that of the particle (e.g., CCPs) signficantly contribute to binding avidity. We investigated this hypothesis with nanoparticles that bind multivalently to angiotensin II receptor type 1, which is subject to clathrin-mediated endocytosis. When we used cholesterol extraction to prevent the action of CCPs, we found a 67 to 100-fold loss in avidity. We created a theoretical model that predicts this decrease based on the loss of ligand-receptor interactions when CCPs, which perfectly match nanoparticle geometry, are absent. Our findings shed new light on how cells "see" nanoparticles. The presence or absence of CPPs is so influential on how cells interact with nanoparticles that the number of particles required to be visible to cells changes by two orders of magnitude depending on CCP presence.

Protein-coated nanoparticles exhibit Lévy flights on a suspended lipid bilayer

Lateral diffusion of nano-objects on lipid membranes is a crucial process in cell biology. Recent studies indicate that nanoparticle lateral diffusion is affected by the presence of membrane proteins and deviates from Brownian motion. Gold nanoparticles (Au NPs) stabilized by short thiol ligands were dispersed near a free-standing bilayer formed in a 3D microfluidic chip. Using dark-field microscopy, the position of single NPs at the bilayer surface was tracked over time. Numerical analysis of the NP trajectories shows that NP diffusion on the bilayer surface corresponds to Brownian motion. The addition of bovine serum albumin (BSA) protein to the solution led to the formation of a protein corona on the NP surface. We found that protein-coated NPs show anomalous superdiffusion and that the distribution of their relative displacement obeys Lévy flight statistics. This superdiffusive motion is attributed to a drastic reduction in adhesive energies between the NPs and the bilayer in the presence of the protein corona. This hypothesis was confirmed by numerical simulations mimicking the random walk of a single particle near a weakly adhesive surface. These results may be generalized to other classes of nano-objects that experience adsorption-desorption behaviour with a weakly adhesive surface.

Significance of Receptor Mobility in Multivalent Binding on Lipid Membranes

Numerous key biological processes rely on the concept of multivalency, where ligands achieve stable binding only upon engaging multiple receptors. These processes, like viral entry or immune synapse formation, occur on the diffusive cellular membrane. One crucial, yet underexplored aspect of multivalent binding is the mobility of coupled receptors. Here, we discuss the consequences of mobility in multivalent processes from four perspectives: (I) The facilitation of receptor recruitment by the multivalent ligand due to their diffusivity prior to binding. (II) The effects of receptor preassembly, which allows their local accumulation. (III) The consequences of changes in mobility upon the formation of receptor/ligand complex. (IV) The changes in the diffusivity of lipid environment surrounding engaged receptors. We demonstrate how understanding mobility is essential for fully unravelling the principles of multivalent membrane processes, leading to further development in studies on receptor interactions, and guide the design of new generations of multivalent ligands.

Modulating the Influenza A Virus-Target Membrane Fusion Interface With Synthetic DNA-Lipid Receptors

Influenza A virus (IAV) binds to sialylated glycans on the cell membrane before endocytosis and fusion. Cell-surface glycans are highly heterogeneous in length and glycosylation density, which leads to variations in the distance and rigidity with which IAV is held away from the cell membrane. To gain mechanistic insight into how receptor length and rigidity impact the mechanism of IAV entry, we employed synthetic DNA-lipids as highly tunable surrogate receptors. We tethered IAV to target membranes with a panel of DNA-lipids to investigate the effects of the distance and tether flexibility between virions and target membranes on the kinetics of IAV binding and fusion. Tether length and the presence of a flexible linker led to higher rates of IAV binding, while the efficiencies of lipid and content mixing were typically lower for longer and more rigid DNA tethers. For all DNA tether modifications, we found that the rates of IAV lipid and content mixing were unchanged. These results suggest that variations in the interface between IAV and a target membrane do not significantly impact the rate-limiting step of fusion or the low-pH-triggered engagement of viral fusion peptides with the target membrane. However, our results imply that the flexibility of the viral receptor is important for ensuring that hemifusion events are able to successfully proceed to pore formation.

Multivalency-Induced Shape Deformation of Nanoscale Lipid Vesicles: Size-Dependent Membrane Bending Effects

The size of membrane-enveloped virus particles, exosomes, and lipid vesicles strongly impacts functional properties in biological and applied contexts. Multivalent ligand-receptor interactions involving nanoparticle shape deformation are critical to such functions, yet the corresponding effect of nanoparticle size remains largely elusive. Herein, using an indirect nanoplasmonic sensing approach, we investigated how the nanoscale size properties of ligand-modified lipid vesicles affect real-time binding interactions, especially vesicle deformation processes, with a receptor-modified, cell membrane-mimicking platform. Together with theoretical analyses, our findings reveal a pronounced, size-dependent transition in the membrane bending properties of nanoscale lipid vesicles between 60 and 180 nm in diameter. For smaller vesicles, a large membrane bending energy enhanced vesicle stiffness while the osmotic pressure energy was the dominant modulating factor for larger, less stiff vesicles. These findings advance our fundamental understanding of how nanoparticle size affects multivalency-induced nanoparticle shape deformation and can provide guidance for the design of biomimetic nanoparticles with tailored nanomechanical properties.

Diffusion of Lipid Nanovesicles Bound to a Lipid Membrane Is Associated with the Partial-Slip Boundary Condition

During diffusion of nanoparticles bound to a cellular membrane by ligand-receptor pairs, the distance to the laterally mobile interface is sufficiently short for their motion to depend not only on the membrane-mediated diffusivity of the tethers but also in a not yet fully understood manner on nanoparticle size and interfacial hydrodynamics. By quantifying diffusivity, velocity, and size of individual membrane-bound liposomes subjected to a hydrodynamic shear flow, we have successfully separated the diffusivity contributions from particle size and number of tethers. The obtained diffusion-size relations for synthetic and extracellular lipid vesicles are not well-described by the conventional no-slip boundary condition, suggesting partial slip as well as a significant diffusivity dependence on the distance to the lipid bilayer. These insights, extending the understanding of diffusion of biological nanoparticles at lipid bilayers, are of relevance for processes such as cellular uptake of viruses and lipid nanoparticles or labeling of cell-membrane-residing molecules.

Physicochemical tools for studying virus interactions with targeted cell membranes in a molecular and spatiotemporally resolved context

The objective of this critical review is to provide an overview of how emerging bioanalytical techniques are expanding our understanding of the complex physicochemical nature of virus interactions with host cell surfaces. Herein, selected model viruses representing both non-enveloped (simian virus 40 and human norovirus) and enveloped (influenza A virus, human herpes simplex virus, and human immunodeficiency virus type 1) viruses are highlighted. The technologies covered utilize a wide range of cell membrane mimics, from supported lipid bilayers (SLBs) containing a single purified host membrane component to SLBs derived from the plasma membrane of a target cell, which can be compared with live-cell experiments to better understand the role of individual interaction pairs in virus attachment and entry. These platforms are used to quantify binding strengths, residence times, diffusion characteristics, and binding kinetics down to the single virus particle and single receptor, and even to provide assessments of multivalent interactions. The technologies covered herein are surface plasmon resonance (SPR), quartz crystal microbalance with dissipation (QCM-D), dynamic force spectroscopy (DFS), total internal reflection fluorescence (TIRF) microscopy combined with equilibrium fluctuation analysis (EFA) and single particle tracking (SPT), and finally confocal microscopy using multi-labeling techniques to visualize entry of individual virus particles in live cells. Considering the growing scientific and societal needs for untangling, and interfering with, the complex mechanisms of virus binding and entry, we hope that this review will stimulate the community to implement these emerging tools and strategies in conjunction with more traditional methods. The gained knowledge will not only contribute to a better understanding of the virus biology, but may also facilitate the design of effective inhibitors to block virus entry.

Molecular Communication Theoretical Modeling and Analysis of SARS-CoV2 Transmission in Human Respiratory System

Severe Acute Respiratory Syndrome-CoronaVirus 2 (SARS-CoV2) caused the ongoing pandemic. This pandemic devastated the world by killing more than a million people, as of October 2020. It is imperative to understand the transmission dynamics of SARS-CoV2 so that novel and interdisciplinary prevention, diagnostic, and therapeutic techniques could be developed. In this work, we model and analyze the transmission of SARS-CoV2 through the human respiratory tract from a molecular communication perspective. We consider that virus diffusion occurs in the mucus layer so that the shape of the tract does not have a significant effect on the transmission. Hence, this model reduces the inherent complexity of the human respiratory system. We further provide the impulse response of SARS-CoV2-ACE2 receptor binding event to determine the proportion of the virus population reaching different regions of the respiratory tract. Our findings confirm the results in the experimental literature on higher mucus flow rate causing virus migration to the lower respiratory tract. These results are especially important to understand the effect of SARS-CoV2 on the different human populations at different ages who have different mucus flow rates and ACE2 receptor concentrations in the different regions of the respiratory tract.

Avidity and surface mobility in multivalent ligand-receptor binding

Targeted drug delivery relies on two physical processes: the selective binding of a therapeutic particle to receptors on a specific cell membrane, followed by transport of the particle across the membrane. In this article, we address some of the challenges in controlling the thermodynamics and dynamics of these two processes by combining a simple experimental system with a statistical mechanical model. Specifically, we characterize and model multivalent ligand-receptor binding between colloidal particles and fluid lipid bilayers, as well as the surface mobility of membrane-bound particles. We show that the mobility of the receptors within the fluid membrane is key to both the thermodynamics and dynamics of binding. First, we find that the particle-membrane binding free energy-or avidity-is a strongly nonlinear function of the ligand-receptor affinity. We attribute the nonlinearity to a combination of multivalency and recruitment of fluid receptors to the binding site. Our results also suggest that partial wrapping of the bound particles by the membrane enhances avidity further. Second, we demonstrate that the lateral mobility of membrane-bound particles is also strongly influenced by the recruitment of receptors. Specifically, we find that the lateral diffusion coefficient of a membrane-bound particle is dominated by the hydrodynamic drag against the aggregate of receptors within the membrane. These results provide one of the first direct validations of the working theoretical framework for multivalent interactions. They also highlight that the fluidity and elasticity of the membrane are as important as the ligand-receptor affinity in determining the binding and transport of small particles attached to membranes.

Tracking single particles for hours via continuous DNA-mediated fluorophore exchange

Monitoring biomolecules in single-particle tracking experiments is typically achieved by employing fixed organic dyes or fluorescent fusion proteins linked to a target of interest. However, photobleaching typically limits observation times to merely a few seconds, restricting downstream statistical analysis and observation of rare biological events. Here, we overcome this inherent limitation via continuous fluorophore exchange using DNA-PAINT, where fluorescently-labeled oligonucleotides reversibly bind to a single-stranded DNA handle attached to the target molecule. Such versatile and facile labeling allows uninterrupted monitoring of single molecules for extended durations. We demonstrate the power of our approach by observing DNA origami on membranes for tens of minutes, providing perspectives for investigating cellular processes on physiologically relevant timescales.

The length of single-particle tracking experiments are limited due to photobleaching. Here the authors achieve long-term single-particle tracking with continuous fluorophore exchange in DNA-PAINT and use this to observe DNA origami on lipid bilayers for tens of minutes.

Rational Fabrication of a Smart Electrochemiluminescent Sensor: Synergistic Effect of a Self-Luminous Faraday Cage and Biomimetic Magnetic Vesicles

A smart electrochemiluminescent (ECL) sensor has been designed in this work. The sensing system consisted of Ag NPs-Ti₃AlC₂ nanosheets (Ag-TACS) as the self-luminous Faraday cage and biomimetic magnetic vesicles as the functional substrate. By engineering the structure and properties of Ti₃AlC₂ nanosheets to induce the Faraday-cage effect, the outer Helmholtz plane (OHP) was extended to contribute to ECL enhancement. Compared with the Faraday cage that further incorporated luminous materials, the self-luminous Faraday cage in the "direct label" model kept all the luminous materials on the OHP. Meanwhile, biomimetic magneticvesicles with highly efficient fluidity were used to improve the sensing efficiency and obtain a perfect Faraday-cage structure to enhance the ECL signals. The highest ECL enhancement (ca. 25 times) has been achieved by the synergistic effect of the Faraday cage and biomimetic magnetic vesicles. This sensing system was used to detect the wild-type K-ras gene in the colorectal tumor tissue. It provides not only an important guide for the novel ECL sensing concept but also a smart modulation system of the electromagnetic field.

A Dynamic, Supramolecular View on the Multivalent Interaction between Influenza Virus and Host Cell

Understanding how influenza viruses traverse the mucus and recognize host cells is critical for evaluating their zoonotic potential, and for prevention and treatment of the disease. The surface of the influenza A virus is covered with the receptor-binding protein hemagglutinin and the receptor-cleaving enzyme neuraminidase, which jointly control the interactions between the virus and the host cell. These proteins are organized in closely spaced trimers and tetramers to facilitate multivalent interactions with sialic acid-terminated glycans. This review shows that the individually weak multivalent interactions of influenza viruses allow superselective binding, virus-induced recruitment of receptors, and the formation of dynamic complexes that facilitate molecular walking. Techniques to measure the avidity and receptor specificity of influenza viruses are reviewed, and the pivotal role of multivalent interactions with their emergent properties in crossing the mucus and entering host cells is discussed. A model is proposed for the initiation of cell entry through virus-induced receptor clustering. The multivalent interactions of influenza viruses are maintained in a dynamic regime by a functional balance between binding and cleaving.

A quantitative view on multivalent nanomedicine targeting

Although the concept of selective delivery has been postulated over 100 years ago, no targeted nanomedicine has been clinically approved so far. Nanoparticles modified with targeting ligands to promote the selective delivery of therapeutics towards a specific cell population have been extensively reported. However, the rational design of selective particles is still challenging. One of the main reasons for this is the lack of quantitative theoretical and experimental understanding of the interactions involved in cell targeting. In this review, we discuss new theoretical models and experimental methods that provide a quantitative view of targeting. We show the new advancements in multivalency theory enabling the rational design of super-selective nanoparticles. Furthermore, we present the innovative approaches to obtain key targeting parameters at the single-cell and single molecule level and their role in the design of targeting nanoparticles. We believe that the combination of new theoretical multivalent design and experimental methods to quantify receptors and ligands aids in the rational design and clinical translation of targeted nanomedicines.

Characterising the diffusion of biological nanoparticles on fluid and cross-linked membranes

Tracing the motion of macromolecules, viruses, and nanoparticles adsorbed onto cell membranes is currently the most direct way of probing the complex dynamic interactions behind vital biological processes, including cell signalling, trafficking, and viral infection. The resulting trajectories are usually consistent with some type of anomalous diffusion, but the molecular origins behind the observed anomalous behaviour are usually not obvious. Here we use coarse-grained molecular dynamics simulations to help identify the physical mechanisms that can give rise to experimentally observed trajectories of nanoscopic objects moving on biological membranes. We find that diffusion on membranes of high fluidities typically results in normal diffusion of the adsorbed nanoparticle, irrespective of the concentration of receptors, receptor clustering, or multivalent interactions between the particle and membrane receptors. Gel-like membranes on the other hand result in anomalous diffusion of the particle, which becomes more pronounced at higher receptor concentrations. This anomalous diffusion is characterised by local particle trapping in the regions of high receptor concentrations and fast hopping between such regions. The normal diffusion is recovered in the limit where the gel membrane is saturated with receptors. We conclude that hindered receptor diffusivity can be a common reason behind the observed anomalous diffusion of viruses, vesicles, and nanoparticles adsorbed on cell and model membranes. Our results enable direct comparison with experiments and offer a new route for interpreting motility experiments on cell membranes.

Virions and respiratory droplets in air: Diffusion, drift, and contact with the epithelium

Some infections, including e.g. influenza and currently active COVID 19, may be transmitted via air during sneezing, coughing, and talking. This pathway occurs via diffusion and gravity-induced drift of single virions and respiratory droplets consisting primarily of water, including small fraction of nonvolatile matter, and containing virions. These processes are accompanied by water evaporation resulting in reduction of the droplet size. The manifold of information concerning these steps is presented in textbooks and articles not related to virology and the focus is there frequently on biologically irrelevant conditions and/or droplet sizes. In this brief review, we systematically describe the behavior of virions and virion-carrying droplets in air with emphasis on various regimes of diffusion, drift, and evaporation, and estimate the rates of all these steps under virologically relevant conditions. In addition, we discuss the kinetic aspects of the first steps of infection after attachment of virions or virion-carrying droplets to the epithelium, i.e., virion diffusion in the mucus and periciliary layers, penetration into the cells, and the early stage of replication. The presentation is oriented to virologists who are interested in the corresponding physics and to physicists who are interested in application of the physics to virology.

Differential Diffusional Properties in Loose and Tight Docking Prior to Membrane Fusion

Fusion of biological membranes, although mediated by divergent proteins, is believed to follow a common pathway. It proceeds through distinct steps, including docking, merger of proximal leaflets (stalk formation), and formation of a fusion pore. However, the structure of these intermediates is difficult to study because of their short lifetime. Previously, we observed a loosely and tightly docked state preceding leaflet merger using arresting point mutations in SNARE proteins, but the nature of these states remained elusive. Here, we used interferometric scattering (iSCAT) microscopy to monitor diffusion of single vesicles across the surface of giant unilamellar vesicles (GUVs). We observed that the diffusion coefficients of arrested vesicles decreased during progression through the intermediate states. Modeling allowed for predicting the number of tethering SNARE complexes upon loose docking and the size of the interacting membrane patches upon tight docking. These results shed new light on the nature of membrane-membrane interactions immediately before fusion.

Independent Size and Fluorescence Emission Determination of Individual Biological Nanoparticles Reveals that Lipophilic Dye Incorporation Does Not Scale with Particle Size

Advancements in nanoparticle characterization techniques are critical for improving the understanding of how biological nanoparticles (BNPs) contribute to different cellular processes, such as cellular communication, viral infection, as well as various drug-delivery applications. Since BNPs are intrinsically heterogeneous, there is a need for characterization methods that are capable of providing information about multiple parameters simultaneously, preferably at the single-nanoparticle level. In this work, fluorescence microscopy was combined with surface-based two-dimensional flow nanometry, allowing for simultaneous and independent determination of size and fluorescence emission of individual BNPs. In this way, the dependence of the fluorescence emission of the commonly used self-inserting lipophilic dye 3,3'-dioctadecyl-5,5'-di(4-sulfophenyl)oxacarbocyanine (SP-DiO) could successfully be correlated with nanoparticle size for different types of BNPs, including synthetic lipid vesicles, lipid vesicles derived from cellular membrane extracts, and extracellular vesicles derived from human SH-SY5Y cell cultures; all vesicles had a radius, r, of ∼50 nm and similar size distributions. The results demonstrate that the dependence of fluorescence emission of SP-DiO on nanoparticle size varies significantly between the different types of BNPs, with the expected dependence on membrane area, r², being observed for synthetic lipid vesicles, while a significant weaker dependence on size was observed for BNPs with more complex composition. The latter observation is attributed to a size-dependent difference in membrane composition, which may influence either the optical properties of the dye and/or the insertion efficiency, indicating that the fluorescence emission of this type of self-inserting dye may not be reliable for determining size or size distribution of BNPs with complex lipid compositions.

Quantification of Multivalent Interactions between Sialic Acid and Influenza A Virus Spike Proteins by Single-Molecule Force Spectroscopy

Multivalency is a key principle in reinforcing reversible molecular interactions through the formation of multiple bonds. The influenza A virus deploys this strategy to bind strongly to cell surface receptors. We performed single-molecule force spectroscopy (SMFS) to investigate the rupture force required to break individual and multiple bonds formed between synthetic sialic acid (SA) receptors and the two principal spike proteins of the influenza A virus (H3N2): hemagglutinin (H3) and neuraminidase (N2). Kinetic parameters such as the rupture length (χ_β) and dissociation rate (k_off) are extracted using the model by Friddle, De Yoreo, and Noy. We found that a monovalent SA receptor binds to N2 with a significantly higher bond lifetime (270 ms) compared to that for H3 (36 ms). By extending the single-bond rupture analysis to a multibond system of n protein-receptor pairs, we provide an unprecedented quantification of the mechanistic features of multivalency between H3 and N2 with SA receptors and show that the stability of the multivalent connection increases with the number of bonds from tens to hundreds of milliseconds. Association rates (k_on) are also provided, and an estimation of the dissociation constants (K_D) between the SA receptors to both proteins indicate a 17-fold higher binding affinity for the SA-N2 bond with respect to that of SA-H3. An optimal designed multivalent SA receptor showed a higher binding stability to the H3 protein of the influenza A virus than to the monovalent SA receptor. Our study emphasizes the influence of the scaffold on the presentation of receptors during multivalent binding.

Membrane-tethered mucin-like polypeptides sterically inhibit binding and slow fusion kinetics of influenza A virus

The mechanism(s) by which cell-tethered mucins modulate infection by influenza A viruses (IAVs) remain an open question. Mucins form both a protective barrier that can block virus binding and recruit IAVs to bind cells via the sialic acids of cell-tethered mucins. To elucidate the molecular role of mucins in flu pathogenesis, we constructed a synthetic glycocalyx to investigate membrane-tethered mucins in the context of IAV binding and fusion. We designed and synthesized lipid-tethered glycopolypeptide mimics of mucins and added them to lipid bilayers, allowing chemical control of length, glycosylation, and surface density of a model glycocalyx. We observed that the mucin mimics undergo a conformational change at high surface densities from a compact to an extended architecture. At high surface densities, asialo mucin mimics inhibited IAV binding to underlying glycolipid receptors, and this density correlated to the mucin mimic's conformational transition. Using a single virus fusion assay, we observed that while fusion of virions bound to vesicles coated with sialylated mucin mimics was possible, the kinetics of fusion was slowed in a mucin density-dependent manner. These data provide a molecular model for a protective mechanism by mucins in IAV infection, and therefore this synthetic glycocalyx provides a useful reductionist model for studying the complex interface of host-pathogen interactions.

First-order 'hyper-selective' binding transition of multivalent particles under force

Multivalent particles bind to targets via many independent ligand-receptor bonding interactions. This microscopic design spans length scales in both synthetic and biological systems. Classic examples include interactions between cells, virus binding, synthetic ligand-coated micrometer-scale vesicles or smaller nano-particles, functionalised polymers, and toxins. Equilibrium multivalent binding is a continuous yet super-selective transition with respect to the number of ligands and receptors involved in the interaction. Increasing the ligand or receptor density on the two particles leads to sharp growth in the number of bound particles at equilibrium. Here we present a theory and Monte Carlo simulations to show that applying mechanical force to multivalent particles causes their adsorption/desorption isotherm on a surface to become sharper and more selective, with respect to variation in the number of ligands and receptors on the two objects. When the force is only applied to particles bound to the surface by one or more ligands, then the transition can become infinitely sharp and first-order-a new binding regime which we term 'hyper-selective'. Force may be imposed by, e.g. flow of solvent around the particles, a magnetic field, chemical gradients, or triggered uncoiling of inert oligomers/polymers tethered to the particles to provide a steric repulsion to the surface. This physical principle is a step towards 'all or nothing' binding selectivity in the design of multivalent constructs.

Modeling the dynamics of phospholipids in the fluid phase of liposomes

We present the derivation of a new model to describe neutron spin echo spectroscopy and quasi-elastic neutron scattering data on liposomes. We compare the new model with existing approaches and benchmark it with experimental data. The analysis indicates the importance of including all major contributions in the modeling of the intermediate scattering function. Simultaneous analysis of the experimental data on lipids with full contrast and tail contrast matched samples reveals highly confined lipid tail motion. A comparison of their dynamics demonstrates the statistical independence of tail-motion and height-height correlation of the membrane. A more detailed analysis indicates that the lipid tail relaxation is confined to a potential with cylindrical symmetry, in addition to the undulation and diffusive motion of the liposome. Despite substantial differences in the chemistry of the fatty acid tails, the observation indicates a universal behavior. The analysis of partially deuterated systems confirms the strong contribution of the lipid tail to the intermediate scattering function. Within the time range from 5 to 100 ns, the intermediate scattering function can be described by the height-height correlation function. The existence of the fast-localized tail motion and the contribution of slow translational diffusion of liposomes determine the intermediate scattering function for t < 5 ns and t > 100 ns, respectively. Taking into account the limited time window lowers the bending moduli by a factor of 1.3 (DOPC) to 2 (DMPC) compared to the full range.

Recruitment of receptors at supported lipid bilayers promoted by the multivalent binding of ligand-modified unilamellar vesicles

The development of model systems that mimic biological interactions and allow the control of both receptor and ligand densities, is essential for a better understanding of biomolecular processes, such as the recruitment of receptors at interfaces, at the molecular level. Here we report a model system based on supported lipid bilayers (SLBs) for the investigation of the clustering of receptors at their interface. Biotinylated SLBs, used as cell membrane mimics, were functionalized with streptavidin (SAv), used here as receptor. Subsequently, biotinylated small (SUVs) and giant (GUVs) unilamellar vesicles were bound to the SAv-functionalized SLBs by multivalent interactions and found to induce the recruitment of both SAv on the SLB surface and the biotin moieties in the vesicles. The recruitment of receptors was investigated with quartz crystal microbalance with dissipation monitoring (QCM-D), which allowed the identification of the biotin and SAv densities necessary to obtain receptor recruitment. At approx. 0.6% of biotin in the vesicles, a transition between dense and low vesicle packing was observed, which coincided with the transitions between recruitment in the vesicles vs. recruitment in the SLB and between full and partial use of the biotin moieties in the vesicle. Direct optical visualization of the clustering at the interface of individual GUVs with the SLB platform was achieved with fluorescence microscopy, showing recruitment of SAv at the contact area as well as the deformation of the vesicles upon binding. Different vesicle binding regimes were observed for lower and higher biotin densities in the vesicles and at the SLBs. A more quantitative analysis of the molecular parameters implied in the interaction, indicated that approx. 10% of the vesicle area constitutes the contact area. Moreover, the SUV binding and recruitment appeared to be fast on the analysis time scale, whereas the binding of GUVs is slower due to the larger SLB area over which SAv recruitment needs to occur. The mechanisms revealed in this study may provide insight in biological processes in which recruitment occurs.

The development of model systems that mimic biological interactions and allow the control of both receptor and ligand densities, is essential for a molecular understanding of biomolecular processes, such as the recruitment of receptors at interfaces.

Analysis and refinement of 2D single-particle tracking experiments

In recent decades, single particle tracking (SPT) has been developed into a sophisticated analytical approach involving complex instruments and data analysis schemes to extract information from time-resolved particle trajectories. Very often, mobility-related properties are extracted from these particle trajectories, as they often contain information about local interactions experienced by the particles while moving through the sample. This tutorial aims to provide a comprehensive overview about the accuracies that can be achieved when extracting mobility-related properties from 2D particle trajectories and how these accuracies depend on experimental parameters. Proper interpretation of SPT data requires an assessment of whether the obtained accuracies are sufficient to resolve the effect under investigation. This is demonstrated by calculating mean square displacement curves that show an apparent super- or subdiffusive behavior due to poor measurement statistics instead of the presence of true anomalous diffusion. Furthermore, the refinement of parameters involved in the design or analysis of SPT experiments is discussed and an approach is proposed in which square displacement distributions are inspected to evaluate the quality of SPT data and to extract information about the maximum distance over which particles should be tracked during the linking process.

Surface Modification with Control over Ligand Density for the Study of Multivalent Biological Systems

In the study of multivalent interactions at interfaces, as occur for example at cell membranes, the density of the ligands or receptors displayed at the interface plays a pivotal role, affecting both the overall binding affinities and the valencies involved in the interactions. In order to control the ligand density at the interface, several approaches have been developed, and they concern the functionalization of a wide range of materials. Here, different methods employed in the modification of surfaces with controlled densities of ligands are being reviewed. Examples of such methods encompass the formation of self-assembled monolayers (SAMs), supported lipid bilayers (SLBs) and polymeric layers on surfaces. Particular emphasis is given to the methods employed in the study of different types of multivalent biological interactions occurring at the functionalized surfaces and their working principles.

Monovalent and Oriented Labeling of Gold Nanoprobes for the High-Resolution Tracking of a Single-Membrane Molecule

Single-molecule tracking is a powerful method to study molecular dynamics in living systems including biological membranes. High-resolution single-molecule tracking requires a bright and stable signal, which has typically been facilitated by nanoparticles due to their superb optical properties. However, there are concerns about using a nanoparticle to label a single molecule because of its relatively large size and the possibility of cross-linking multiple target molecules, both of which could affect the original molecular dynamics. In this work, using various labeling schemes, we investigate the effects using nanoparticles to measure the diffusion of single-membrane molecules. By conjugating a low density of streptavidin (sAv) to gold nanoparticles (AuNPs) of different sizes (10, 15, 20, 30, and 40 nm), we isolate and quantify the effect of the particle size on the diffusion of biotinylated lipids in supported lipid bilayers (SLBs). We find that single sAv tends to cross-link two biotinylated lipids, leading to a much slower diffusion in SLBs. We further demonstrate a simple and robust strategy for the monovalent and oriented labeling of a single lipid molecule with a AuNP by using naturally dimeric rhizavidin (rAv) as a bridge, thus connecting the biotinylated nanoparticle surface and biotinylated target molecule. The rAv-AuNP conjugate demonstrates fast and free diffusion in SLBs (2-3 μm²/s for rAv-AuNP sizes of 10-40 nm), which is comparable to the diffusion of dye-labeled lipids, indicating that the adverse size and cross-linking effects are successfully avoided. We also note that the diffusion of dye-labeled lipids critically depends on the choice of dye, which could report different diffusion coefficients by about 20% (2.2 μm²/s of ATTO647N and 2.6 μm²/s of ATTO532). By comparing the diffusion of the uniformly and randomly oriented labeling of a single lipid molecule with a AuNP, we conclude that oriented labeling is favorable for measuring the diffusion of single-membrane molecules. Our work shows that the measured diffusion of the membrane molecule is highly sensitive to the molecular design of the cross-linker for labeling. The demonstrated approach of monovalent and oriented AuNP labeling provides the opportunity to study single-molecule membrane dynamics at much higher spatiotemporal resolutions and, most importantly, without labeling artifacts.

A Dynamic DNA Machine via Free Walker Movement on Lipid Bilayer for Ultrasensitive Electrochemiluminescent Bioassay

Herein, an ultrasensitive electrochemiluminescent (ECL) strategy was proposed based on a highly efficient dynamic DNA machine based on microRNA triggered free movement on the lipid bilayer interface. Typically, the lipid bilayer is constructed on the electrode surface modified with nafion@ECL luminophore and gold nanoparticles to immobilize the DNA walker labeled with cholesterol and hairpin nucleotides labeled with cholesterol and ferrocene (Fc), based on the cholesterol-lipid interaction. On this state, Fc was close to the ECL luminophore, performing a quenched ECL emission. In the presence of target microRNA 21, it could trigger the entropy beacon-based DNA amplification to convert microRNA to massive special DNA sequences, which could further hybridize with the blocking DNA on DNA walker to reactivate the DNA walker and thus trigger the DNA walker-based amplification to make Fc to be far from the ECL luminophore, performing a recovered ECL emission related with the concentration of microRNA 21. Compared with the conventional DNA walker immobilized on the interface via chemical bonds or physical adsorption, a higher reaction efficiency could be achieved due to the free movements of DNA walker and its substrates on the interface. As expected, satisfactory performances for the detection of microRNA 21 were achieved with a detection limit of 0.4 fM and quantitative estimation in cells. Furthermore, this dynamic DNA machine-based ECL strategy could be readily expanded for the detection of other biomarkers for clinical diagnosis.

Intracellular RNA delivery by lipid nanoparticles: Diffusion, degradation, and release

Various RNAs (siRNAs, miRNAs, or mRNAs) can be delivered into cells by lipid nanoparticles (LNPs) of 50-150 nm in diameter. The subsequent RNA release from LNPs may occur via various scenarios. Herein, two related kinetic models are proposed. The first model takes into account that LNPs are often porous so that RNA molecules diffuse in and detach from nanopores. The analysis is focused on RNA diffusion from a pore. The analytical expression obtained for the RNA escape rate constant is used to identify the difference in the release of siRNAs, miRNAs, and mRNAs. The key message here is that the mRNA diffusion from pores appears to be too slow, and accordingly the mRNA release seems to occur primarily via degradation of LNPs. The second coarse-grained model describes the diffusion-mediated release of RNA from a LNP in the situation when this process is accompanied by the LNP degradation at the lipid-solution interface. The corresponding kinetics are shown in detail at different relative rates of the RNA diffusion and LNP degradation. Potentially, this can help to interpret drug plasma levels after various dosing regimens.

Single-Molecule Detection with Lightguiding Nanowires: Determination of Protein Concentration and Diffusivity in Supported Lipid Bilayers

Determining the surface concentration and diffusivity of cell-membrane-bound molecules is central to the understanding of numerous important biochemical processes taking place at cell membranes. Here we use the high aspect ratio and lightguiding properties of semiconductor nanowires (NWs) to detect the presence of single freely diffusing proteins bound to a lipid bilayer covering the NW surface. Simultaneous observation of light-emission dynamics of hundreds of individual NWs occurring on the time scale of only a few seconds is interpreted using analytical models and employed to determine both surface concentration and diffusivity of cholera toxin subunit B (CTxB) bound to GM1 gangliosides in supported lipid bilayer (SLB) at surface concentrations down to below one CTxB per μm². In particular, a decrease in diffusivity was observed with increasing GM1 content in the SLB, suggesting increasing multivalent binding of CTxB to GM1. The lightguiding capability of the NWs makes the method compatible with conventional epifluorescence microscopy, and it is shown to work well for both photostable and photosensitive dyes. These features make the concept an interesting complement to existing techniques for studying the diffusivity of low-abundance cell-membrane-bound molecules, expanding the rapidly growing use of semiconductor NWs in various bioanalytical sensor applications and live cell studies.

Mobility-Based Quantification of Multivalent Virus-Receptor Interactions: New Insights Into Influenza A Virus Binding Mode

Viruses, such as influenza A, typically bind to the plasma membrane of their host by engaging multiple membrane receptors in parallel, thereby forming so-called multivalent interactions that are created by the collective action of multiple weak ligand-receptor bonds. The overall interaction strength can be modulated by changing the number of engaged receptors. This feature is used by viruses to achieve a sufficiently firm attachment to the host's plasma membrane but also allows progeny viruses to leave the plasma membrane after completing the virus replication cycle. Design of strategies to prevent infection, for example, by disturbing these attachment and detachment processes upon application of antivirals, requires quantification of the underlying multivalent interaction in absence and presence of antivirals. This is still an unresolved problem, as there is currently no approach available that allows for determining the valency (i.e., of the number of receptors bound to a particular virus) on the level of single viruses under equilibrium conditions. Herein, we track the motion of single influenza A/X31 viruses (IAVs; interacting with the ganglioside GD1a incorporated in a supported lipid bilayer) using total internal reflection fluorescence microscopy and show that IAV residence time distributions can be deconvoluted from valency effects by taking the IAV mobility into account. The so-derived off-rate distributions, expressed in dependence of an average, apparent valency, show the expected decrease in off-rate with increasing valency but also show an unexpected peak structure, which can be linked to a competition in the opposing functionalities of the two influenza A virus spike proteins, hemagglutinin (HA), and neuraminidase (NA). By application of the antiviral zanamivir that inhibits the activity of NA, we provide direct evidence, how the HA/NA balance modulates this virus-receptor interaction, allowing us to assess the inhibition concentration of zanamivir based on its effect on the multivalent interaction.

Single Virion Tracking Microscopy for the Study of Virus Entry Processes in Live Cells and Biomimetic Platforms

The most widely-used assays for studying viral entry, including infectivity, cofloatation, and cell-cell fusion assays, yield functional information but provide low resolution of individual entry steps. Structural characterization provides high-resolution conformational information, but on its own is unable to address the functional significance of these conformations. Single virion tracking microscopy techniques provide more detail on the intermediate entry steps than infection assays and more functional information than structural methods, bridging the gap between these methods. In addition, single virion approaches also provide dynamic information about the kinetics of entry processes. This chapter reviews single virion tracking techniques and describes how they can be applied to study specific virus entry steps. These techniques provide information complementary to traditional ensemble approaches. Single virion techniques may either probe virion behavior in live cells or in biomimetic platforms. Synthesizing information from ensemble, structural, and single virion techniques ultimately yields a more complete understanding of the viral entry process than can be achieved by any single method alone.

Single-Vesicle Assays Using Liposomes and Cell-Derived Vesicles: From Modeling Complex Membrane Processes to Synthetic Biology and Biomedical Applications

The plasma membrane is of central importance for defining the closed volume of cells in contradistinction to the extracellular environment. The plasma membrane not only serves as a boundary, but it also mediates the exchange of physical and chemical information between the cell and its environment in order to maintain intra- and intercellular functions. Artificial lipid- and cell-derived membrane vesicles have been used as closed-volume containers, representing the simplest cell model systems to study transmembrane processes and intracellular biochemistry. Classical examples are studies of membrane translocation processes in plasma membrane vesicles and proteoliposomes mediated by transport proteins and ion channels. Liposomes and native membrane vesicles are widely used as model membranes for investigating the binding and bilayer insertion of proteins, the structure and function of membrane proteins, the intramembrane composition and distribution of lipids and proteins, and the intermembrane interactions during exo- and endocytosis. In addition, natural cell-released microvesicles have gained importance for early detection of diseases and for their use as nanoreactors and minimal protocells. Yet, in most studies, ensembles of vesicles have been employed. More recently, new micro- and nanotechnological tools as well as novel developments in both optical and electron microscopy have allowed the isolation and investigation of individual (sub)micrometer-sized vesicles. Such single-vesicle experiments have revealed large heterogeneities in the structure and function of membrane components of single vesicles, which were hidden in ensemble studies. These results have opened enormous possibilities for bioanalysis and biotechnological applications involving unprecedented miniaturization at the nanometer and attoliter range. This review will cover important developments toward single-vesicle analysis and the central discoveries made in this exciting field of research.

Brownian Motion at Lipid Membranes: A Comparison of Hydrodynamic Models Describing and Experiments Quantifying Diffusion within Lipid Bilayers

The capability of lipid bilayers to exhibit fluid-phase behavior is a fascinating property, which enables, for example, membrane-associated components, such as lipids (domains) and transmembrane proteins, to diffuse within the membrane. These diffusion processes are of paramount importance for cells, as they are for example involved in cell signaling processes or the recycling of membrane components, but also for recently developed analytical approaches, which use differences in the mobility for certain analytical purposes, such as in-membrane purification of membrane proteins or the analysis of multivalent interactions. Here, models describing the Brownian motion of membrane inclusions (lipids, peptides, proteins, and complexes thereof) in model bilayers (giant unilamellar vesicles, black lipid membranes, supported lipid bilayers) are summarized and model predictions are compared with the available experimental data, thereby allowing for evaluating the validity of the introduced models. It will be shown that models describing the diffusion in freestanding (Saffman-Delbrück and Hughes-Pailthorpe-White model) and supported bilayers (the Evans-Sackmann model) are well supported by experiments, though only few experimental studies have been published so far for the latter case, calling for additional tests to reach the same level of experimental confirmation that is currently available for the case of freestanding bilayers.

Single Viruses on the Fluorescence Microscope: Imaging Molecular Mobility, Interactions and Structure Sheds New Light on Viral Replication

Viruses are simple agents exhibiting complex reproductive mechanisms. Decades of research have provided crucial basic insights, antiviral medication and moderately successful gene therapy trials. The most infectious viral particle is, however, not always the most abundant one in a population, questioning the utility of classic ensemble-averaging virology. Indeed, viral replication is often not particularly efficient, prone to errors or containing parallel routes. Here, we review different single-molecule sensitive fluorescence methods that we employ routinely to investigate viruses. We provide a brief overview of the microscopy hardware needed and discuss the different methods and their application. In particular, we review how we applied (i) single-molecule Förster resonance energy transfer (smFRET) to probe the subviral human immunodeficiency virus (HIV-1) integrase (IN) quaternary structure; (ii) single particle tracking to study interactions of the simian virus 40 with membranes; (iii) 3D confocal microscopy and smFRET to quantify the HIV-1 pre-integration complex content and quaternary structure; (iv) image correlation spectroscopy to quantify the cytosolic HIV-1 Gag assembly, and finally; (v) super-resolution microscopy to characterize the interaction of HIV-1 with tetherin during assembly. We hope this review is an incentive for setting up and applying similar single-virus imaging studies in daily virology practice.

Affinity Purification and Single-Molecule Analysis of Integral Membrane Proteins from Crude Cell-Membrane Preparations

The function of integral membrane proteins is critically dependent on their naturally surrounding lipid membrane. Detergent-solubilized and purified membrane proteins are therefore often reconstituted into cell-membrane mimics and analyzed for their function with single-molecule microscopy. Expansion of this approach toward a broad range of pharmaceutically interesting drug targets and biomarkers however remains hampered by the fact that these proteins have low expression levels, and that detergent solubilization and reconstitution often cause protein conformational changes and loss of membrane-specific cofactors, which may impair protein function. To overcome this limitation, we here demonstrate how antibody-modified nanoparticles can be used to achieve affinity purification and enrichment of selected integral membrane proteins directly from cell membrane preparations. Nanoparticles were first bound to the ectodomain of β-secretase 1 (BACE1) contained in cell-derived membrane vesicles. In a subsequent step, these were merged into a continuous supported membrane in a microfluidic channel. Through the extended nanoparticle tag, a weak (∼fN) hydrodynamic force could be applied, inducing directed in-membrane movement of targeted BACE1 exclusively. This enabled selective thousand-fold enrichment of the targeted membrane protein while preserving a natural lipid environment. In addition, nanoparticle-targeting also enabled simultaneous tracking analysis of each individual manipulated protein, revealing how their mobility changed when moved from one lipid environment to another. We therefore believe this approach will be particularly useful for separation in-line with single-molecule analysis, eventually opening up for membrane-protein sorting devices analogous to fluorescence-activated cell sorting.

Multivalent ligand-receptor-mediated interaction of small filled vesicles with a cellular membrane

The ligand-receptor-mediated contacts of small sub-100-nm-sized lipid vesicles (or nanoparticles) with the cellular membrane are of interest in the contexts of cell-to-cell communication, endocytosis of membrane-coated virions, and drug (RNA) delivery. In all these cases, the interior of vesicles is filled by biologically relevant content. Despite the diversity of such systems, the corresponding ligand-receptor interaction possesses universal features. One of them is that the vesicle-membrane contacts can be accompanied by the redistribution of ligands and receptors between the contact and contact-free regions. In particular, the concentrations of ligands and receptors may become appreciably higher in the contact regions and their composition may there be different compared to that in the suspended state in the solution. A statistical model presented herein describes the corresponding distribution of various ligands and receptors and allows one to calculate the related change of the free energy with variation of the vesicle-engulfment extent. The results obtained are used to clarify the necessary conditions for the vesicle-assisted pathway of drug delivery.

Binding Kinetics and Lateral Mobility of HSV-1 on End-Grafted Sulfated Glycosaminoglycans

Many viruses, including herpes simplex (HSV), are recruited to their host cells via interaction between their envelope glycoproteins and cell-surface glycosaminoglycans (GAGs). This initial attachment is of a multivalent nature, i.e., it requires the establishment of multiple bonds between amino acids of viral glycoproteins and sulfated saccharides on the GAG chain. To gain understanding of how this binding process is modulated, we performed binding kinetics and mobility studies using end-grafted GAG chains that mimic the end attachment of these chains to proteoglycans. Total internal reflection fluorescence microscopy was used to probe binding and release, as well as the diffusion of single HSV-1 particles. To verify the hypothesis that the degree of sulfation, but also the arrangement of sulfate groups along the GAG chain, plays a key role in HSV binding, we tested two native GAGs (chondroitin sulfate and heparan sulfate) and compared our results to chemically sulfated hyaluronan. HSV-1 recognized all sulfated GAGs, but not the nonsulfated hyaluronan, indicating that binding is specific to the presence of sulfate groups. Furthermore we observed that a notable fraction of GAG-bound virions exhibit lateral mobility, although the multivalent binding to the immobilized GAG brushes ensures firm virus attachment to the interface. Diffusion was faster on the two native GAGs, one of which, chondroitin sulfate, was also characterized by the highest association rate per GAG chain. This highlights the complexity of multivalent virus-GAG interactions and suggests that the spatial arrangement of sulfates along native GAG chains may play a role in modulating the characteristics of the HSV-GAG interaction. Altogether, these results, obtained with a minimal and well-controlled model of the cell membrane, provide, to our knowledge, new insights into the dynamics of the HSV-GAG interaction.

Detachment of Membrane Bound Virions by Competitive Ligand Binding Induced Receptor Depletion

Multivalent receptor-mediated interactions between virions and a lipid membrane can be weakened using competitive nonpathogenic ligand binding. In particular, the subsequent binding of such ligands can induce detachment of bound virions, a phenomenon of crucial relevance for the development of new antiviral drugs. Focusing on the simian virus 40 (SV40) and recombinant cholera toxin B subunit (rCTB), and using (monosialotetrahexosyl)ganglioside (GM1) as their common receptor in a supported lipid bilayer (SLB), we present the first detailed investigation of this phenomenon by employing the quartz crystal microbalance with dissipation (QCM-D) and total internal reflection fluorescence (TIRF) microscopy assisted 2D single particle tracking (SPT) techniques. Analysis of the QCM-D-measured release kinetics made it possible to determine the binding strength of a single SV40-GM1 pair. The release dynamics of SV40, monitored by SPT, revealed that a notable fraction of SV40 becomes mobile just before the release, allowing to estimate the distribution of SV40-bound GM1 receptors just prior to release.

Recent advances in hybrid measurement methods based on atomic force microscopy and surface sensitive measurement techniques

This review summaries the recent progress of the combination of optical and non-optical surface sensitive techniques with the atomic force microscopy.

Hydrodynamic Propulsion of Liposomes Electrostatically Attracted to a Lipid Membrane Reveals Size-Dependent Conformational Changes

The efficiency of lipid nanoparticle uptake across cellular membranes is strongly dependent on the very first interaction step. Detailed understanding of this step is in part hampered by the large heterogeneity in the physicochemical properties of lipid nanoparticles, such as liposomes, making conventional ensemble-averaging methods too blunt to address details of this complex process. Here, we contribute a means to explore whether individual liposomes become deformed upon binding to fluid cell-membrane mimics. This was accomplished by using hydrodynamic forces to control the propulsion of nanoscale liposomes electrostatically attracted to a supported lipid bilayer. In this way, the size of individual liposomes could be determined by simultaneously measuring both their individual drift velocity and diffusivity, revealing that for a radius of ∼45 nm, a close agreement with dynamic light scattering data was observed, while larger liposomes (radius ∼75 nm) displayed a significant deformation unless composed of a gel-phase lipid. The relevance of being able to extract this type of information is discussed in the context of membrane fusion and cellular uptake.