Colloidal probe microscopy of membrane–membrane interactions: From ligand–receptor recognition to fusion events

Membrane fusion studied by colloidal probes

Membrane-coated colloidal probes combine the benefits of solid-supported membranes with a more complex three-dimensional geometry. This combination makes them a powerful model system that enables the visualization of dynamic biological processes with high throughput and minimal reliance on fluorescent labels. Here, we want to review recent applications of colloidal probes for the study of membrane fusion. After discussing the advantages and disadvantages of some classical vesicle-based fusion assays, we introduce an assay using optical detection of fusion between membrane-coated glass microspheres in a quasi two-dimensional assembly. Then, we discuss free energy considerations of membrane fusion between supported bilayers, and show how colloidal probes can be combined with atomic force microscopy or optical tweezers to access the fusion process with even greater detail.

Precipitation of Calcium Carbonate Inside Giant Unilamellar Vesicles Composed of Fluid-Phase Lipids

Biomineralization of CaCO₃ commonly involves the formation of amorphous CaCO₃ precursor particles that are produced in a confined space surrounded by a lipid bilayer. While the influence of confinement itself has been investigated with different model systems, the impact of an enclosing continuous lipid bilayer on CaCO₃ formation in a confined space is still poorly understood as appropriate model systems are rare. Here, we present a new versatile method based on droplet-based microfluidics to produce fluid-phase giant unilamellar vesicles (GUVs) in the presence of high CaCl₂ concentrations. These GUVs can be readily investigated by means of confocal laser scanning microscopy in combination with bright-field microscopy, demonstrating that the formed CaCO₃ particles are in conformal contact with the fluid-phase lipid bilayer and thus suggesting a strong interaction between the particle and the membrane. Atomic force microscopy adhesion studies with membrane-coated spheres on different CaCO₃ crystals corroborated this notion of a strong interaction between the lipids and CaCO₃.

Using Force Spectroscopy to Probe Coiled-Coil Assembly and Membrane Fusion

Force spectroscopy allows the manipulation of single molecules and the characterization of their properties and interactions thereby rendering it a powerful tool for biological sciences. Force spectroscopy at the level of individual molecules requires force resolution in the piconewton regime as achieved by optical tweezers (OT), magnetic tweezers (MT), and atomic force microscopy (AFM) with AFM providing the largest force range from tenth of piconewton to several micronewton. In membrane probe spectroscopy the commonly used sharp cantilever tip is replaced by a lipid-coated glass sphere. This technique expands the scope of force spectroscopy to processes at and between lipid bilayers, like the formation of coiled coils between SNARE (soluble N-ethylmaleimide-sensitive factor attachment receptor) proteins as well as subsequent membrane fusion. To this end, two solid-supported membranes equipped with SNARE proteins or fusion peptides are separately deposited on a flat glassy surface and on a micrometer glass sphere attached to the end of a tipless AFM cantilever. These two membranes are rapidly brought into contact until a defined force is reached. The AFM deflection readout is used to monitor the distance between the two bilayers, which allows to observe and identify fusion processes of the two lipid membranes, while the forces needed to separate the two surfaces give insights into the formation of SNARE complexes. By changing the contact pressure one can access fusion kinetics and to some extent reconstruct the energy landscape of membrane fusion. In this chapter we describe the preparation of membrane-coated colloidal probes attached to AFM cantilevers, experimental procedures, and necessary data analysis to perform membrane probe spectroscopy in the presence of fusogenic peptides or proteins.

Measurement of Forces between Supported Cationic Bilayers by Colloid Probe Atomic Force Microscopy: Electrolyte Concentration and Composition

The interactions between supported cationic surfactant bilayers were measured by colloidal probe atomic force spectroscopy, and the effect of different halide salts was investigated. Di(alkylisopropylester)dimethylammonium methylsulfate (DIPEDMAMS) bilayers were fabricated by the vesicle fusion technique on muscovite mica. The interactions between the bilayers were measured in increasing concentrations of NaCl, NaBr, NaI, and CaCl₂. In NaCl, the bilayer interactions were repulsive at all concentrations investigated, and the Debye length and surface potential were observed to decrease with increasing concentration. The interactions were found to follow the electrical double layer (EDL) component of DLVO theory well. However, van der Waals forces were not detected; instead, a strong hydration repulsion was observed at short separations. CaCl₂ had a similar effect on the interactions as NaCl. NaBr and NaI were observed to be more efficient at decreasing surface potential than the chloride salts, with the efficacy increasing with the ionic radius.

Size, Kinetics, and Free Energy of Clusters Formed by Ultraweak Carbohydrate-Carbohydrate Bonds

Weak noncovalent intermolecular interactions play a pivotal role in many biological processes such as cell adhesion or immunology, where the overall binding strength is controlled through bond association and dissociation dynamics as well as the cooperative action of many parallel bonds. Among the various molecules participating in weak bonds, carbohydrate-carbohydrate interactions are probably the most ancient ones allowing individual cells to reversibly enter the multicellular state and to tell apart self and nonself cells. Here, we scrutinized the kinetics and thermodynamics of small homomeric Lewis X-Lewis X ensembles formed in the contact zone of a membrane-coated colloidal probe and a solid supported membrane ensuring minimal nonspecific background interactions. We used an atomic force microscope to measure force distance curves at Piconewton resolution, which allowed us to measure the force due to unbinding of the colloidal probe and the planar membrane as a function of contact time. Applying a contact model, we could estimate the free binding energy of the formed adhesion cluster as a function of dwell time and thereby determine the precise size of the contact zone, the number of participating bonds, and the intrinsic rates of association and dissociation in the presence of calcium ions. The unbinding energy per bond was found to be on the order of 1 kBT. Approximately 30 bonds were opened simultaneously at an off-rate of koff = 7 ± 0.2 s(-1).

SNARE-mediated membrane fusion trajectories derived from force-clamp experiments

Fusion of lipid bilayers is usually prevented by large energy barriers arising from removal of the hydration shell, formation of highly curved structures, and, eventually, fusion pore widening. Here, we measured the force-dependent lifetime of fusion intermediates using membrane-coated silica spheres attached to cantilevers of an atomic-force microscope. Analysis of time traces obtained from force-clamp experiments allowed us to unequivocally assign steps in deflection of the cantilever to membrane states during the SNARE-mediated fusion with solid-supported lipid bilayers. Force-dependent lifetime distributions of the various intermediate fusion states allowed us to propose the likelihood of different fusion pathways and to assess the main free energy barrier, which was found to be related to passing of the hydration barrier and splaying of lipids to eventually enter either the fully fused state or a long-lived hemifusion intermediate. The results were compared with SNARE mutants that arrest adjacent bilayers in the docked state and membranes in the absence of SNAREs but presence of PEG or calcium. Only with the WT SNARE construct was appreciable merging of both bilayers observed.

Direct observation of intermediate states in model membrane fusion

We introduce a novel assay for membrane fusion of solid supported membranes on silica beads and on coverslips. Fusion of the lipid bilayers is induced by bringing an optically trapped bead in contact with the coverslip surface while observing the bead's thermal motion with microsecond temporal and nanometer spatial resolution using a three-dimensional position detector. The probability of fusion is controlled by the membrane tension on the particle. We show that the progression of fusion can be monitored by changes in the three-dimensional position histograms of the bead and in its rate of diffusion. We were able to observe all fusion intermediates including transient fusion, formation of a stalk, hemifusion and the completion of a fusion pore. Fusion intermediates are characterized by axial but not lateral confinement of the motion of the bead and independently by the change of its rate of diffusion due to the additional drag from the stalk-like connection between the two membranes. The detailed information provided by this assay makes it ideally suited for studies of early events in pure lipid bilayer fusion or fusion assisted by fusogenic molecules.

Nanomechanical response of bacterial cells to cationic antimicrobial peptides

The effectiveness of antimicrobial compounds can be easily screened, however their mechanism of action is much more difficult to determine. Many compounds act by compromising the mechanical integrity of the bacterial cell envelope, and our study introduces an AFM-based creep deformation technique to evaluate changes in the time-dependent mechanical properties of Pseudomonas aeruginosa PAO1 bacterial cells upon exposure to two different but structurally related antimicrobial peptides. We observed a distinctive signature for the loss of integrity of the bacterial cell envelope following exposure to the peptides. Measurements performed before and after exposure, as well as time-resolved measurements and those performed at different concentrations, revealed large changes to the viscoelastic parameters that are consistent with differences in the membrane permeabilizing effects of the peptides. The AFM creep deformation measurement provides new, unique insight into the kinetics and mechanism of action of antimicrobial peptides on bacteria.

Phosphatidylinositol 4,5-bisphosphate alters the number of attachment sites between ezrin and actin filaments: a colloidal probe study

Direct linkage between the plasma membrane and the actin cytoskeleton is controlled by the protein ezrin, a member of the ezrin-radixin-moesin protein family. To function as a membrane-cytoskeleton linker, ezrin needs to be activated in a process that involves binding of ezrin to phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphorylation of a conserved threonine residue. Here, we used colloidal probe microscopy to quantitatively analyze the interaction between ezrin and F-actin as a function of these activating factors. We show that the measured individual unbinding forces between ezrin and F-actin are independent of the activating parameters, in the range of approximately 50 piconewtons. However, the cumulative adhesion energy greatly increases in the presence of PIP2 demonstrating that a larger number of bonds between ezrin and F-actin has formed. In contrast, the phosphorylation state, represented by phosphor-mimetic mutants of ezrin, only plays a minor role in the activation process. These results are in line with in vivo experiments demonstrating that an increase in PIP2 concentration recruits more ezrin to the apical plasma membrane of polarized cells and significantly increases the membrane tension serving as a measure of the adhesion sites between the plasma membrane and the F-actin network.

The nanomechanical properties of lipid membranes are significantly influenced by the presence of ethanol

Ethanol has a profound impact on biological systems and is moreover used in various medical and nonmedical applications. Its interaction with the lipid part of biological membranes has been the subject of intensive studies, but surprisingly, to our knowledge, no study has examined the influence of ethanol on lipid bilayer nanomechanics. We performed atomic force microscopy-based measurements to assess the influence of ethanol on the nanomechanical properties of fluid supported lipid bilayers. Ethanol significantly reduces membrane stability, bilayer thickness, Young's modulus, area stretch modulus, and bending stiffness. Altogether, our data suggest that ethanol addition to supported lipid bilayers supports both the hydrophobic and the hydrophilic permeation pathways by a decrease of bilayer thickness and reduced stability, respectively.

Coiled-coil formation on lipid bilayers--implications for docking and fusion efficiency

Coiled-coil formation of four different oligopeptides was characterized in solution, on hydrogels, and on membranes by employing circular dichroism spectroscopy, surface plasmon resonance spectroscopy, attenuated total reflection infrared spectroscopy, and ellipsometry. Peptide sequences rich in either glutamic acid (E: E3Cys, i-E3Cys) or lysine (K: K3Cys, i-K3Cys) were used to represent minimal mimics of eukaryotic SNARE motifs. Half of the peptides were synthesized in reverse sequence, so that parallel and antiparallel heptad coiled-coil structures were formed. Either E-peptides or K-peptides were attached covalently to phospholipid anchors via maleimide chemistry, and served as receptors for the recognition of the corresponding binding partners added to solution. Attenuated total reflection infrared spectroscopy of single bilayers confirmed the formation of coiled-coil complexes at the membrane interface. Coiled-coil formation in solution, as compared with association at the membrane surface, displays considerably larger binding constants that are largely attributed to loss of translational entropy at the interface. Finally, the fusogenicity of the various coiled-coil motifs was explored, and the results provide clear evidence that hemifusion followed by full fusion requires a parallel orientation of α-helices, whereas antiparallel oriented coiled-coil motifs display only docking.

Interaction forces between DPPC bilayers on glass

The surface force apparatus (SFA) was utilized to obtain force-distance profiles between silica-supported membranes formed by Langmuir-Blodgett deposition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). In the absence of a membrane, a long-range electrostatic repulsion and short-range steric repulsion are measured as a result of the deprotonation of silica in water and the roughness of the silica film. The electrostatic repulsion is partially screened by the lipid membrane, and a van der Waals adhesion comparable to that measured with well-packed DPPC membranes on mica is measured. This finding suggest that electrostatic interactions due to the underlying negatively charged silica are likely present in other systems of glass-supported membranes. In contrast, the charge of an underlying mica substrate is almost completely screened when a lipid membrane is deposited on the mica. The difference in the two systems is attributed to the stronger physisorption of zwitterionic lipids to molecularly smooth mica compared to physisorption to rougher silica.

Impact of peptide clustering on unbinding forces in the context of fusion mimetics

Coiled-coil zipping and unzipping is a pivotal process in SNARE-regulated membrane fusion. In this study we examine this process mediated by a minimal model for coiled-coil formation employing force spectroscopy in the context of membrane-coated surfaces and probes. The interaction forces of several hundred pN are surprisingly low considering the proposed amount of molecular bonds in the contact zone. However, by means of high-resolution imaging employing atomic force microscopy and studying the lateral mobility of lipids and peptides as a function of coiled-coil formation, we are able to supply a detailed view on processes occurring on the membrane surfaces during force measurements. The interaction forces determined here are not only dependent on the peptide concentration on the surface, but also on the regional organization of lateral peptide clusters found prior to coiled-coil formation.

Model system for cell adhesion mediated by weak carbohydrate-carbohydrate interactions

The multivalent carbohydrate-carbohydrate interaction between membrane-anchored epitopes derived from the marine sponge Microciona prolifera has been explored by colloidal probe microscopy. An in situ coupling of sulfated and non-sulfated disaccharides to membrane-coated surfaces was employed to mimic native cell-cell contacts.The dynamic strength of the homomeric self-association was measured as a function of calcium ions and loading rate. A deterministic model was used to estimate the basic energy landscape and number of participating bonds in the contact zone.

In situ hydrodynamic lateral force calibration of AFM colloidal probes

Lateral force microscopy (LFM) is an application of atomic force microscopy (AFM) to sense lateral forces applied to the AFM probe tip. Recent advances in tissue engineering and functional biomaterials have shown a need for the surface characterization of their material and biochemical properties under the application of lateral forces. LFM equipped with colloidal probes of well-defined tip geometries has been a natural fit to address these needs but has remained limited to provide primarily qualitative results. For quantitative measurements, LFM requires the successful determination of the lateral force or torque conversion factor of the probe. Usually, force calibration results obtained in air are used for force measurements in liquids, but refractive index differences between air and liquids induce changes in the conversion factor. Furthermore, in the case of biochemically functionalized tips, damage can occur during calibration because tip-surface contact is inevitable in most calibration methods. Therefore, a nondestructive in situ lateral force calibration is desirable for LFM applications in liquids. Here we present an in situ hydrodynamic lateral force calibration method for AFM colloidal probes. In this method, the laterally scanned substrate surface generated a creeping Couette flow, which deformed the probe under torsion. The spherical geometry of the tip enabled the calculation of tip drag forces, and the lateral torque conversion factor was calibrated from the lateral voltage change and estimated torque. Comparisons with lateral force calibrations performed in air show that the hydrodynamic lateral force calibration method enables quantitative lateral force measurements in liquid using colloidal probes.