Force measurements on myelin basic protein adsorbed to mica and lipid bilayer surfaces done with the atomic force microscope

Artificial Extracellular Vesicles Generated from T Cells Using Different Induction Techniques

Cell therapy is at the forefront of biomedicine in oncology and regenerative medicine. However, there are still significant challenges to their wider clinical application such as limited efficacy, side effects, and logistical difficulties. One of the potential approaches that could overcome these problems is based on extracellular vesicles (EVs) as a cell-free therapy modality. One of the major obstacles in the translation of EVs into practice is their low yield of production, which is insufficient to achieve therapeutic amounts. Here, we evaluated two primary approaches of artificial vesicle induction in primary T cells and the SupT1 cell line-cytochalasin B as a chemical inducer and ultrasonication as a physical inducer. We found that both methods are capable of producing artificial vesicles, but cytochalasin B induction leads to vesicle yield compared to natural secretion, while ultrasonication leads to a three-fold increase in particle yield. Cytochalasin B induces the formation of vesicles full of cytoplasmic compartments without nuclear fraction, while ultrasonication induces the formation of particles rich in membranes and membrane-related components such as CD3 or HLAII proteins. The most effective approach for T-cell induction in terms of the number of vesicles seems to be the combination of anti-CD3/CD28 antibody activation with ultrasonication, which leads to a seven-fold yield increase in particles with a high content of functionally important proteins (CD3, granzyme B, and HLA II).

Role of Cel5H protein surface amino acids in binding with clay minerals and measurements of its forces

Our previous study on the binding activity between Cel5H and clay minerals showed highest binding efficiency among other cellulase enzymes cloned. Here, based on previous studies, we hypothesized that the positive amino acids on the surface of Cel5H protein may play an important role in binding to clay surfaces. To examine this, protein sequences of Bacillus licheniformis Cel5H (BlCel5H) and Paenibacillus polymyxa Cel5A (PpCel5A) were analyzed and then selected amino acids were mutated. These mutated proteins were investigated for binding activity and force measurement via atomic force microscopy (AFM). A total of seven amino acids which are only present in BlCel5H but not in PpCel5A were selected for mutational studies and the positive residues which are present in both were omitted. Of the seven selected surface lysine residues, only three mutants K196A(M2), K54A(M3) and K157T(M4) showed 12%, 7% and 8% less clay mineral binding ability, respectively compared with wild-type. The probable reason why other mutants did not show altered binding efficiency might be due to relative location of amino acids on the protein surface. Meanwhile, measurement of adhesion forces on mica sheets showed a well-defined maximum at 69 ± 19 pN for wild-type, 58 ± 19 pN for M2, 53 ± 19 pN for M3, and 49 ± 19 pN for M4 proteins. Hence, our results demonstrated that relative location of surface amino acids of Cel5H protein especially positive charged amino acids are important in the process of clay mineral-protein binding interaction through electrostatic exchange of charges.

Mechanical matching between a ligand and receptor

Interactions between ligands and receptors and subsequent "locking" must involve some resistance to unbinding, manifesting itself as an interaction force. At body temperature, spontaneous unbinding will occur, however, external forces are required to accelerate this process. Bearing in mind the potential forces that the receptor-ligand complex is likely to be subjected to in a biological environment, it might be hypothesised that there is some mechanical matching between the receptor and ligand. To test this hypothesis, various receptor and ligand pairs were unfolded in their entirety in order to determine their total unfolding force. In this way, the total force to unfold the protein could be determined, allowing a comparison between ligand and receptor pairs. The interest of this work is to examine the interaction between five proteins and a mica surface by AFM without any modification to preserve the natural elastic properties of the protein molecules during the force measurements. The results showed a mechanical matching between GP120 (ligand) and CD4 (receptor) when analysing the total force required to unfold the same number of domains or events shown by the force distance curves of these proteins.

Adsorption mechanism of myelin basic protein on model substrates and its bridging interaction between the two surfaces

Myelin basic protein (MBP) is an intrinsically disordered (unstructured) protein known to play an important role in the stability of myelin's multilamellar membrane structure in the central nervous system. The adsorption of MBP and its capacity to interact with and bridge solid substrates has been studied using a surface forces apparatus (SFA) and a quartz crystal microbalance with dissipation (QCM-D). Adsorption experiments show that MBP molecules adsorb to the surfaces in a swollen state before undergoing a conformational change into a more compact structure with a thickness of ∼3 nm. Moreover, this compact structure is able to interact with nearby mica surfaces to form adhesive bridges. The measured adhesion force (energy) between two bridged surfaces is 1.0 ± 0.1 mN/m, (Ead = 0.21 ± 0.02 mJ/m(2)), which is slightly smaller than our previously reported adhesion force of 1.7 mN/m (Ead = 0.36 mJ/m(2)) for MBP adsorbed on two supported lipid bilayers (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775). The saturated surface concentration of compact MBP on a single SiO2 surface reaches a stable value of 310 ± 10 ng/cm(2) regardless of the bulk MBP concentration. A kinetic three-step adsorption model was developed that accurately fits the adsorption data. The developed model is a general model, not limited to intrinsically disordered proteins, that can be extended to the adsorption of various chemical compounds that undergo chemical reactions and/or conformational changes upon adsorbing to surfaces. Taken together with our previously published data (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775), the present results confirm that conformational changes of MBP upon adsorption are a key for strong adhesion, and that such conformational changes are strongly dependent on the nature of the surfaces.

Lipid domains control myelin basic protein adsorption and membrane interactions between model myelin lipid bilayers

The surface forces apparatus and atomic force microscope were used to study the effects of lipid composition and concentrations of myelin basic protein (MBP) on the structure of model lipid bilayers, as well as the interaction forces and adhesion between them. The lipid bilayers had a lipid composition characteristic of the cytoplasmic leaflets of myelin from "normal" (healthy) and "disease-like" [experimental allergic encephalomyelitis (EAE)] animals. They showed significant differences in the adsorption mechanism of MBP. MBP adsorbs on normal bilayers to form a compact film (3-4 nm) with strong intermembrane adhesion (∼0.36 mJ/m(2)), in contrast to its formation of thicker (7-8 nm) swelled films with weaker intermembrane adhesion (∼0.13 mJ/m(2)) on EAE bilayers. MBP preferentially adsorbs to liquid-disordered submicron domains within the lipid membranes, attributed to hydrophobic attractions. These results show a direct connection between the lipid composition of membranes and membrane-protein adsorption mechanisms that affects intermembrane spacing and adhesion and has direct implications for demyelinating diseases.

Myelin membrane assembly is driven by a phase transition of myelin basic proteins into a cohesive protein meshwork

Rapid conduction of nerve impulses requires coating of axons by myelin. To function as an electrical insulator, myelin is generated as a tightly packed, lipid-rich multilayered membrane sheath. Knowledge about the mechanisms that govern myelin membrane biogenesis is required to understand myelin disassembly as it occurs in diseases such as multiple sclerosis. Here, we show that myelin basic protein drives myelin biogenesis using weak forces arising from its inherent capacity to phase separate. The association of myelin basic protein molecules to the inner leaflet of the membrane bilayer induces a phase transition into a cohesive mesh-like protein network. The formation of this protein network shares features with amyloid fibril formation. The process is driven by phenylalanine-mediated hydrophobic and amyloid-like interactions that provide the molecular basis for protein extrusion and myelin membrane zippering. These findings uncover a physicochemical mechanism of how a cytosolic protein regulates the morphology of a complex membrane architecture. These results provide a key mechanism in myelin membrane biogenesis with implications for disabling demyelinating diseases of the central nervous system.

Preparation of DOPC and DPPC Supported Planar Lipid Bilayers for Atomic Force Microscopy and Atomic Force Spectroscopy

Cell membranes are typically very complex, consisting of a multitude of different lipids and proteins. Supported lipid bilayers are widely used as model systems to study biological membranes. Atomic force microscopy and force spectroscopy techniques are nanoscale methods that are successfully used to study supported lipid bilayers. These methods, especially force spectroscopy, require the reliable preparation of supported lipid bilayers with extended coverage. The unreliability and a lack of a complete understanding of the vesicle fusion process though have held back progress in this promising field. We document here robust protocols for the formation of fluid phase DOPC and gel phase DPPC bilayers on mica. Insights into the most crucial experimental parameters and a comparison between DOPC and DPPC preparation are presented. Finally, we demonstrate force spectroscopy measurements on DOPC surfaces and measure rupture forces and bilayer depths that agree well with X-ray diffraction data. We also believe our approach to decomposing the force-distance curves into depth sub-components provides a more reliable method for characterising the depth of fluid phase lipid bilayers, particularly in comparison with typical image analysis approaches.

Self-segregation of myelin membrane lipids in model membranes

Rapid conduction of nerve impulses requires coating of axons by myelin sheaths, which are multilamellar, lipid-rich membranes produced by oligodendrocytes in the central nervous system. To act as an insulator, myelin has to form a stable and firm membrane structure. In this study, we have analyzed the biophysical properties of myelin membranes prepared from wild-type mice and from mouse mutants that are unable to form stable myelin. Using C-Laurdan and fluorescence correlation spectroscopy, we find that lipids are tightly organized and highly ordered in myelin isolated from wild-type mice, but not from shiverer and ceramide synthase 2 null mice. Furthermore, only myelin lipids from wild-type mice laterally segregate into physically distinct lipid phases in giant unilamellar vesicles in a process that requires very long chain glycosphingolipids. Taken together, our findings suggest that oligodendrocytes exploit the potential of lipids to self-segregate to generate a highly ordered membrane for electrical insulation of axons.

Facilitating high-force single-polysaccharide stretching using covalent attachment of one end of the chain

Single polysaccharide force spectroscopy has yielded particularly interesting data, the interpretation of which requires the marriage of statistical-mechanical theories of polymer physics to the complexities afforded by possible force-induced conformational transitions of the constituent sugar rings. However, the difficulty of designing handles for the specific attachment of the different ends of polysaccharide chains to substrates, such as piezoelectric scanners, cantilevers or microbeads has meant that the majority of studies to date have been carried out with the polymer physisorbed to the substrates between which it is stretched, or at best chemically attached via bonds formed at uncontrolled locations along the length of the molecule. This means that the lengths of obtained polysaccharide stretches, as well as the forces that can be placed on the molecule without generating detachment, are generally smaller than those obtainable for polymers that offer the ability to be covalently attached to substrates specifically at their ends. As a consequence it is troublesome and tedious to record a statistically significant number of force curves that extend chains to high enough forces to investigate certain conformational transitions, such as the boat-to-inverted chair, exhibited by polysaccharides such as pectin. Herein, single molecule force-extension curves have been measured for the several pectin samples using AFM. The results are compared when either (1) the polymers have been physisorbed between the cantilever and the surface of the piezo-electric scanner, under several different solvent conditions of pH and ionic strength, or (2) the polymer molecule has been chemically attached at one end to the piezo surface using a recently reported coupling procedure. In fact, using such a chemical attachment to tether the end of the polysaccharide, reduced the frequency of successful stretching events obtained in a particular location, confirming the role of surface diffusion in the physisorbed experiments. Nevertheless, when polymer stretches were successfully recorded, the force that could be applied before detachment was significantly increased, indicating that this methodology has great potential for improving the acquisition of data reporting on force-induced conformational transitions of the sugar ring that require the application of significant stresses.

Dissociation rate constants of human fibronectin binding to fibronectin-binding proteins on living Staphylococcus aureus isolated from clinical patients

Staphylococcus aureus is part of the indigenous microbiota of humans. Sometimes, S. aureus bacteria enter the bloodstream, where they form infections on implanted cardiovascular devices. A critical, first step in such infections is a bond that forms between fibronectin-binding protein (FnBP) on S. aureus and host proteins, such as fibronectin (Fn), that coat the surface of implants in vivo. In this study, native FnBPs on living S. aureus were shown to form a mechanically strong conformational structure with Fn by atomic force microscopy. The tensile acuity of this bond was probed for 46 bloodstream isolates, each from a patient with a cardiovascular implant. By analyzing the force spectra with the worm-like chain model, we determined that the binding events were consistent with a multivalent, cluster bond consisting of ~10 or ~80 proteins in parallel. The dissociation rate constant (k(off), s(-1)) of each multibond complex was determined by measuring strength as a function of the loading rate, normalized by the number of bonds. The bond lifetime (1/k(off)) was two times longer for bloodstream isolates from patients with an infected device (1.79 or 69.47 s for the 10- or 80-bond clusters, respectively; n = 26 isolates) relative to those from patients with an uninfected device (0.96 or 34.02 s; n = 20 isolates). This distinction could not be explained by different amounts of FnBP, as confirmed by Western blots. Rather, amino acid polymorphisms within the Fn-binding repeats of FnBPA explain, at least partially, the statistically (p < 0.05) longer bond lifetime for isolates associated with an infected cardiovascular device.

ATOMIC FORCE MICROSCOPY MEASUREMENTS OF BOVINE SERUM ALBUMIN ADHESION FORCES ON SURFACES

A novel, direct technique has been developed to measure the interactions of bovine serum albumin (BSA) on surfaces by using atomic force microscopy (AFM) in a liquid environment. We have been able to measure adhesion forces between proteins and substrate surfaces in phosphate-buffered saline (PBS) solution directly, without any modification to the substrate and the AFM tip. Two different surfaces have been used in the measurements: mica (hydrophilic surface) and polystyrene (hydrophobic surface). The results show that a polystyrene surface has larger adhesion forces to BSA than a mica surface. This is consistent with previous research, which demonstrated that hydrophobic surfaces enhance protein adhesion but hydrophilic surfaces do not.

Enhanced molecular chaperone activity of the small heat-shock protein alphaB-cystallin following covalent immobilization onto a solid-phase support

The well-characterized small heat-shock protein, alphaB-crystallin, acts as a molecular chaperone by interacting with unfolding proteins to prevent their aggregation and precipitation. Structural perturbation (e.g., partial unfolding) enhances the in vitro chaperone activity of alphaB-crystallin. Proteins often undergo structural perturbations at the surface of a synthetic material, which may alter their biological activity. This study investigated the activity of alphaB-crystallin when covalently bound to a support surface; alphaB-crystallin was immobilized onto a range of solid material surfaces, and its characteristics and chaperone activity were assessed. Immobilization was achieved via a plasma-deposited thin polymeric interlayer containing aldehyde surface groups and reductive amination, leading to the covalent binding of alphaB-crystallin lysine residues to the surface aldehyde groups via Schiff-base linkages. Immobilized alphaB-crystallin was characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and quartz crystal microgravimetry, which showed that 300 ng cm(-2) (dry mass) of oligomeric alphaB-crystallin was bound to the surface. Immobilized alphaB-crystallin exhibited a significant enhancement (up to 5000-fold, when compared with the equivalent activity of alphaB-crystallin in solution) of its chaperone activity against various proteins undergoing both amorphous and amyloid fibril forms of aggregation. The enhanced molecular chaperone activity of immobilized alphaB-crystallin has potential applications in preventing protein misfolding, including against amyloid disease processes, such as dialysis-related amyloidosis, and for biodiagnostic detection of misfolded proteins.

Atomic force microscopy to study intermolecular forces and bonds associated with bacteria

Atomic force microscopy (AFM) operates on a very different principle than other forms of microscopy, such as optical microscopy or electron microscopy. The key component of an AFM is a cantilever that bends in response to forces that it experiences as it touches another surface. Forces as small as a few picoNewtons can be detected and probed with AFM. AFM has become very useful in biological sciences because it can be used on living cells that are immersed in water. AFM is particularly useful when the cantilever is modified with chemical groups (e.g. amine or carboxylic groups), small beads (e.g. glass or latex), or even a bacterium. This chapter describes how AFM can be used to measure forces and bonds between a bacterium and another surface. This paper also provides an example of the use of AFM on Staphylococcus aureus, a Gram-positive bacterium that is often associated with biofilms in humans.

A tactile response in Staphylococcus aureus

It is well established that bacteria are able to respond to temporal gradients (e.g., by chemotaxis). However, it is widely held that prokaryotes are too small to sense spatial gradients. This contradicts the common observation that the vast majority of bacteria live on the surface of a solid substrate (e.g., as a biofilm). Herein we report direct experimental evidence that the nonmotile bacterium Staphylococcus aureus possesses a tactile response, or primitive sense of touch, that allows it to respond to spatial gradients. Attached cells recognize their substrate interface and localize adhesins toward that region. Braille-like avidity maps reflect a cell's biochemical sensory response and reveal ultrastructural regions defined by the actual binding activity of specific proteins.

Bonds between fibronectin and fibronectin-binding proteins on Staphylococcus aureus and Lactococcus lactis

Bacterial cell-wall-associated fibronectin binding proteins A and B (FnBPA and FnBPB) form bonds with host fibronectin. This binding reaction is often the initial step in prosthetic device infections. Atomic force microscopy was used to evaluate binding interactions between a fibronectin-coated probe and laboratory-derived Staphylococcus aureus that are (i) defective in both FnBPA and FnBPB (fnbA fnbB double mutant, DU5883), (ii) capable of expressing only FnBPA (fnbA fnbB double mutant complemented with pFNBA4), or (iii) capable of expressing only FnBPB (fnbA fnbB double mutant complemented with pFNBB4). These experiments were repeated using Lactococcus lactis constructs expressing fnbA and fnbB genes from S. aureus. A distinct force signature was observed for those bacteria that expressed FnBPA or FnBPB. Analysis of this force signature with the biomechanical wormlike chain model suggests that parallel bonds form between fibronectin and FnBPs on a bacterium. The strength and covalence of bonds were evaluated via nonlinear regression of force profiles. Binding events were more frequent (p < 0.01) for S. aureus expressing FnBPA or FnBPB than for the S. aureus double mutant. The binding force, frequency, and profile were similar between the FnBPA and FnBPB expressing strains of S. aureus. The absence of both FnBPs from the surface of S. aureus removed its ability to form a detectable bond with fibronectin. By contrast, ectopic expression of FnBPA or FnBPB on the surface of L. lactis conferred fibronectin binding characteristics similar to those of S. aureus. These measurements demonstrate that fibronectin-binding adhesins FnBPA and FnBPB are necessary and sufficient for the binding of S. aureus to prosthetic devices that are coated with host fibronectin.

Myelin basic protein and myelin protein 2 act synergistically to cause stacking of lipid bilayers

Saltatory conduction of nerve impulses along axonal membranes depends on the presence of a multilayered membrane, myelin, that wraps around the axon. Myelin basic protein (MBP) and myelin protein 2 (P2) are intimately involved in the generation of the myelin sheath. They are also implicated in a number of neurological diseases, including autoimmune diseases of both the central and peripheral nervous systems. Here, we have used atomic force microsopy (AFM) to study the effects of MBP and P2 on lipid bilayers. MBP in association with a mica substrate appeared unstructured, and tended to coat the mica surface in the form of a monolayer. In contrast, P2 appeared as discrete particles, with molecular volumes consistent with the formation of both monomers and dimers. Either MBP or P2, at micromolar concentrations, caused stacking of brain lipid bilayers. This stacking effect was significantly potentiated when both proteins were added together. Bilayers composed of phosphatidylcholine (PC) and phosphatidylserine (PS) were stacked by MBP, provided that cholesterol was also present; in contrast, P2 did not stack PC/PS/cholesterol bilayers. Hence, the bilayer stacking effects of the two proteins have different lipid requirements.

Investigation of SNARE-Mediated Membrane Fusion Mechanism Using Atomic Force Microscopy

Membrane fusion is driven by specialized proteins that reduce the free energy penalty for the fusion process. In neurons and secretory cells, soluble N-ethylmaleimide-sensitive factor-attachment protein (SNAP) receptors (SNAREs) mediate vesicle fusion with the plasma membrane during vesicular content release. Although, SNAREs have been widely accepted as the minimal machinery for membrane fusion, the specific mechanism for SNARE-mediated membrane fusion remains an active area of research. Here, we summarize recent findings based on force measurements acquired in a novel experimental system that uses atomic force microscope (AFM) force spectroscopy to investigate the mechanism(s) of membrane fusion and the role of SNAREs in facilitating membrane hemifusion during SNARE-mediated fusion. In this system, protein-free and SNARE-reconstituted lipid bilayers are formed on opposite (trans) substrates and the forces required to induce membrane hemifusion and fusion or to unbind single v-/t-SNARE complexes are measured. The obtained results provide evidence for a mechanism by which the pulling force generated by interacting trans-SNAREs provides critical proximity between the membranes and destabilizes the bilayers at fusion sites by broadening the hemifusion energy barrier and consequently making the membranes more prone to fusion.

Protonated nanoparticle surface governing ligand tethering and cellular targeting

Nanoparticles have shown tremendous potential for effective drug delivery due to their tiny size and cell membrane penetration capabilities. Cellular targeting with nanoparticles is often achieved by surface modifications followed by ligand conjugation. However, the efficiency of the nanoparticles reaching the target cells and getting internalized depends on the stability of targeting ligands and the chemical nature of the ligand nanoparticle binding. Recent advancements in nanobiomaterials research have proven the superoxide dismutase (SOD) mimetic activity of cerium oxide nanoparticles (CNPs) in protecting cells against oxidative stress. Due to their excellent biocompatibility, CNPs can be used as a potential drug carrier that can transport and release drugs to the malignant sites. Here we combine single molecule force spectroscopy (SMFS) and density functional theory (DFT) simulations to understand the interaction between transferrin, a ligand protein overexpressed in cancer cells, and CNPs. SMFS studies demonstrate an increase in the transferrin adhesion to the nanoparticles' surface with an increase in positive zeta potential of CNPs. Binding energy values obtained from DFT calculations predict an increase in bond strength between the transferrin and CNPs upon surface protonation and charge modification. Transferrin-conjugated CNPs were tested for their binding stability and preferential cellular uptake efficiency by incubating them with human lung cancer cells (A549) and normal embryo lung cells (WI-38). The results demonstrate the importance of tuning the surface properties of nanoparticles for better ligand adsorption and cellular uptake.

Probing the influence of myelin and glia on the tensile properties of the spinal cord

Although glia have been historically classified as the structurally supporting cells of the central nervous system, their role in tissue mechanics is still largely unstudied. The influence of myelin and glia on the mechanical properties of spinal cord tissue was examined by testing embryonic day 18 chick embryo spinal cords in uniaxial tension following disruption of the glial matrix using either ethidium bromide (EB) or an antibody against galactocerebroside (alphaGalC) in the presence of complement. Demyelination was confirmed by myelin basic protein immunoreactivity and quantified using osmium tetroxide staining. A substantial loss of astrocytes and oligodendrocytes concurrent with demyelination was observed following EB injection but not alphaGalC injection. No morphological changes were observed following injection of saline or IgG with complement as controls for EB and alphaGalC. Demyelinated spinal cords demonstrated significantly lower stiffness and ultimate tensile stress than myelinated spinal cords. No significant differences were observed in the tensile response between the two demyelinating protocols. The results demonstrate that the glial matrix provides significant mechanical support to the spinal cord, and suggests that myelin and cellular coupling of axons via the glial matrix in large part dictates the tensile response of the tissue.

Molecular aspects of boundary lubrication by human lubricin: effect of disulfide bonds and enzymatic digestion

Lubricin (LUB) is a glycoprotein of the synovial cavity of human articular joints, where it serves as an antiadhesive, boundary lubricant, and regulating factor for the cartilage surface. It has been proposed that these properties are related to the presence of a long, extended, heavily glycosylated and highly hydrated mucinous domain in the central part of the LUB molecule. In this work, we show that LUB has a contour length of 220 +/- 30 nm and a persistence length of < or =10 nm. LUB molecules aggregate in oligomers where the protein extremities are linked by disulfide bonds. We have studied the effect of proteolytic digestion by chymotrypsin and removal of the disulfide bonds, both of which mainly affect the N- and C- terminals of the protein, on the adsorption, normal forces, friction (lubrication) forces, and wear of LUB layers adsorbed on smooth, negatively charged mica surfaces, where the protein naturally forms lubricating polymer brush-like layers. After in situ digestion, the surface coverage was drastically reduced, the normal forces were altered, and both the coefficient of friction and the wear were dramatically increased (the COF increased to mu = 1.1-1.9), indicating that the mucinous domain was removed from the surface. Removal of disulfide bonds did not change the surface coverage or the overall features of the normal forces; however, we find an increase in the friction coefficient from mu = 0.02-0.04 to mu = 0.13-1.17 in the pressure regime below 6 atm, which we attribute to a higher affinity of the protein terminals for the surface. The necessary condition for LUB to be a good lubricant is that the protein be adsorbed to the surface via its terminals, leaving the central mucin domain free to form a low-friction, surface-protecting layer. Our results suggest that this "end-anchoring" has to be strong enough to impart the layer a sufficient resistance to shear, but without excessively restricting the conformational freedom of the adsorbed proteins.

Thermal response of Langmuir-Blodgett films of dipalmitoylphosphatidylcholine studied by atomic force microscopy and force spectroscopy

The topographic evolution of supported dipalmitoylphosphatidylcholine (DPPC) monolayers with temperature has been followed by atomic force microscopy in liquid environment, revealing the presence of only one phase transition event at approximately 46 degrees C. This finding is a direct experimental proof that the two phase transitions observed in the corresponding bilayers correspond to the individual phase transition of the two leaflets composing the bilayer. The transition temperature and its dependency on the measuring medium (liquid saline solution or air) is discussed in terms of changes in van der Waals, hydration, and hydrophobic/hydrophilic interactions, and it is directly compared with the transition temperatures observed in the related bilayers under the same experimental conditions. Force spectroscopy allows us to probe the nanomechanical properties of such monolayers as a function of temperature. These measurements show that the force needed to puncture the monolayers is highly dependent on the temperature and on the phospholipid phase, ranging from 120+/-4 pN at room temperature (liquid condensed phase) to 49+/-2 pN at 65 degrees C (liquid expanded phase), which represents a two orders-of-magnitude decrease respective to the forces needed to puncture DPPC bilayers. The topographic study of the monolayers in air around the transition temperature revealed the presence of boundary domains in the monolayer surface forming 120 degrees angles between them, thus suggesting that the cooling process from the liquid-expanded to the liquid-condensed phase follows a nucleation and growth mechanism.

Axon kinematics change during growth and development

The microkinematic response of axons to mechanical stretch was examined in the developing chick embryo spinal cord during a period of rapid growth and myelination. Spinal cords were isolated at different days of embryonic (E) development post-fertilization (E12, E14, E16, and E18) and stretched 0%, 5%, 10%, 15%, and 20%, respectively. During this period, the spinal cord grew approximately 55% in length, and white matter tracts were myelinated significantly. The spinal cords were fixed with paraformaldehyde at the stretched length, sectioned, stained immunohistochemically for neurofilament proteins, and imaged with epifluorescence microscopy. Axons in unstretched spinal cords were undulated, or tortuous, to varying degrees, and appeared to straighten with stretch. The degree of tortuosity (ratio of the segment's pathlength to its end-to-end length) was quantified in each spinal cord by tracing several hundred randomly selected axons. The change in tortuosity distributions with stretch indicated that axons switched from non-affine, uncoupled behavior at low stretch levels to affine, coupled behavior at high stretch levels, which was consistent with previous reports of axon behavior in the adult guinea pig optic nerve (Bain, Shreiber, and Meaney, J. Biomech. Eng., 125(6), pp. 798-804). A mathematical model previously proposed by Bain et al. was applied to quantify the transition in kinematic behavior. The results indicated that significant percentages of axons demonstrated purely non-affine behavior at each stage, but that this percentage decreased from 64% at E12 to 30% at E18. The decrease correlated negatively to increases in both length and myelination with development, but the change in axon kinematics could not be explained by stretch applied during physical growth of the spinal cord. The relationship between tissue-level and axonal-level deformation changes with development, which can have important implications in the response to physiological forces experienced during growth and trauma.

Lateral reorganization of myelin lipid domains by myelin basic protein studied at the air-water interface

It has been speculated that adsorption of myelin basic protein (MBP) to the myelin lipid membrane leads to lateral reorganization of the lipid molecules within the myelin membrane. This hypothesis was tested in this study by surface pressure measurement and fluorescent imaging of a monolayer composed of a myelin lipid mixture. The properties of the lipid monolayer before and after addition of MBP into the subphase were monitored. Upon addition of MBP to the monolayer subphase, the surface pressure rose and significant rearrangement of the lipid domains was observed. These results suggest that binding and partial insertion of MBP into the lipid monolayer led to dramatic rearrangement and morphological changes of the lipid domains. A model of adsorption of MBP to the lipid domains and subsequent domain fusion promoted by minimization of electrostatic repulsion between the domains was proposed to account for the experimental observations. The significance of these results in light of the role of MBP in maintaining the myelin structural integrity is discussed.

Atomic force microscopy: Determination of unbinding force, off rate and energy barrier for protein–ligand interaction

Recently, atomic force microscopy (AFM) based force measurements have been applied biophysically and clinically to the field of molecular recognition as well as to the evaluation of dynamic parameters for various interactions between proteins and ligands in their native environment. The aim of this review is to describe the use of the AFM to measure the forces that control biological interaction, focusing especially on protein-ligand and protein-protein interaction modes. We first considered the measurements of specific and non-specific unbinding forces which together control protein-ligand interactions. As such, we will look at the theoretical background of AFM force measurement curves for evaluating the unbinding forces of protein-ligand complexes. Three AFM model dynamic parameters developed recently for use in protein-ligand interactions are reviewed: (i) unbinding forces, (ii) off rates, and (iii) binding energies. By reviewing the several techniques developed for measuring forces between biological structures and intermolecular forces in the literature, we show that use of an AFM for these applications provides an excellent tool in terms of spatial resolution and lateral resolution, especially for protein-protein and protein-ligand interactions.

Specific bonds between an iron oxide surface and outer membrane cytochromes MtrC and OmcA from Shewanella oneidensis MR-1

Shewanella oneidensis MR-1 is purported to express outer membrane cytochromes (e.g., MtrC and OmcA) that transfer electrons directly to Fe(III) in a mineral during anaerobic respiration. A prerequisite for this type of reaction would be the formation of a stable bond between a cytochrome and an iron oxide surface. Atomic force microscopy (AFM) was used to detect whether a specific bond forms between a hematite (Fe(2)O(3)) thin film, created with oxygen plasma-assisted molecular beam epitaxy, and recombinant MtrC or OmcA molecules coupled to gold substrates. Force spectra displayed a unique force signature indicative of a specific bond between each cytochrome and the hematite surface. The strength of the OmcA-hematite bond was approximately twice that of the MtrC-hematite bond, but direct binding to hematite was twice as favorable for MtrC. Reversible folding/unfolding reactions were observed for mechanically denatured MtrC molecules bound to hematite. The force measurements for the hematite-cytochrome pairs were compared to spectra collected for an iron oxide and S. oneidensis under anaerobic conditions. There is a strong correlation between the whole-cell and pure-protein force spectra, suggesting that the unique binding attributes of each cytochrome complement one another and allow both MtrC and OmcA to play a prominent role in the transfer of electrons to Fe(III) in minerals. Finally, by comparing the magnitudes of binding force for the whole-cell versus pure-protein data, we were able to estimate that a single bacterium of S. oneidensis (2 by 0.5 microm) expresses approximately 10(4) cytochromes on its outer surface.

Atomic force microscope studies of the fusion of floating lipid bilayers

This study investigated the fusion of apposing floating bilayers of egg L-alpha-phosphatidylcholine (egg PC) or 1,2-dimyristoyl-sn-glycero-3-phosphocholine. Atomic force microscope measurements of fusion forces under different compression rates were acquired to reveal the energy landscape of the fusion process under varied lipid composition and temperature. Between compression rates of approximately 1000 and approximately 100,000 pN/s, applied forces in the range from approximately 100 to approximately 500 pN resulted in fusion of floating bilayers. Our atomic force microscope measurements indicated that one main energy barrier dominated the fusion process. The acquired dynamic force spectra were fit with a simple model based on the transition state theory with the assumption that the fusion activation potential is linear. A significant shift in the energy landscape was observed when bilayer fluidity and composition were modified, respectively, by temperature and different cholesterol concentrations (15% < or = chol < or = 25%). Such modifications resulted in a more than twofold increase in the width of the fusion energy barrier for egg PC and 1,2-dimyristoyl-sn-glycero-3-phosphocholine floating bilayers. The addition of 25% cholesterol to egg PC bilayers increased the activation energy by approximately 1.0 k(B)T compared with that of bilayers with egg PC alone. These results reveal that widening of the energy barrier and consequently reduction in its slope facilitated membrane fusion.

Effect of cholesterol on the physical properties of pulmonary surfactant films: Atomic force measurements study

Atomic force measurements were performed on supported pulmonary surfactant (PS) films to address the effect of cholesterol on the physical properties of lung surfactant films. We recently found that cholesterol in excess of a physiological proportion abolishes surfactant function, and is the reason that surfactant fails to lower the surface tension upon compression. In this study, we investigated how the loss of mechanical stability observed earlier is related to the local mechanical properties of the film by local force measurements. The presence of 20% of cholesterol in bovine lipid extract surfactant (BLES) resulted in a decrease of the observed adhesive interaction, and an increase in rigidity of the film. We discuss the implication the increased rigidity might have on the functional failure of PS.

Nanomechanical force measurements of gliadin protein interactions

The strength and nature of interactions between monomeric gliadin proteins involving alpha-alpha, omega-omega, and alpha-omega interactions in 0.01M acetic acid, and the effect of urea has been investigated. It was shown by means of nanomechanical force measurements that the stretching events in the separation curve after adhesive phenomena originated from proteins. These stretching events displayed different responses of the alpha- and omega-gliadins to urea. While 2M urea caused the more globular alpha-gliadins to unfold, the beta-turn-rich omega-gliadins remained fairly stable even in 8M urea. This suggests different roles for gliadins in the formation of dough; while the omega-gliadins are still in a compact structure being responsible for the viscous flow, the alpha-gliadins have already started to participate in forming the network in dough.

Microstructural analysis of the effects of incorporation of myelin basic protein in phospholipid layers

We report an X-ray reflectivity study on the effects of adsorption of myelin basic protein (MBP) on Langmuir monolayers and on deposited Langmuir-Schaefer multilayers of the phospholipid dipalmitoyl phosphatidylglycerol (DPPG). We provide for the first time, direct microscopic evidence on the destructuring effects of MBP leading to plasticity of the DPPG layers supporting commonly accepted models of the stabilizing role of MBP in the myelin membrane. We also show how protein adsorption onto the layer is determined both by electrostatic and nonspecific hydrophobic interactions.

Sacrificial bonds in polymer brushes from rat tail tendon functioning as nanoscale velcro

Polymers play an important role in many biological systems, so a fundamental understanding of their cross-links is crucial not only for the development of medicines but also for the development of biomimetic materials. The biomechanical movements of all mammals are aided by tendon fibrils. The self-organization and biomechanical functions of tendon fibrils are determined by the properties of the cross-links between their individual molecules and the interactions among the cross-links. The cross-links of collagen and proteoglycan molecules are particularly important in tendons and, perhaps, bone. To probe cross-links between tendon molecules, we used the cantilever tip of an atomic force microscope in a pulling setup. Applying higher forces to rat tail tendon molecules with the tip led to a local disruption of the highly organized shell of tendon fibrils and to the formation or an increase of a polymer brush of molecules sticking out of the surface. The cross-linking between these molecules was influenced by divalent Ca2+ ions. Furthermore, the molecules of the polymer brush seemed to bind back to the fibrils in several minutes. We propose that sacrificial bonds significantly influence the tendon fibrils' self-organization and self-healing and therefore contribute to toughness and strength.

Simultaneous force and fluorescence measurements of a protein that forms a bond between a living bacterium and a solid surface

All microbial biofilms are initiated through direct physical contact between a bacterium and a solid surface, a step that is controlled by inter- and intramolecular forces. Atomic force microscopy and confocal laser scanning microscopy were used simultaneously to observe the formation of a bond between a fluorescent chimeric protein on the surface of a living Escherichia coli bacterium and a solid substrate in situ. The chimera was composed of a portion of outer membrane protein A (OmpA) fused to the cyan-fluorescent protein AmCyan. Sucrose gradient centrifugation and fluorescent confocal slices through bacteria demonstrated that the chimeric protein was targeted and anchored to the external cell surface. The wormlike chain theory predicted that this protein should exhibit a nonlinear force-extension "signature" consistent with the sequential unraveling of the AmCyan and OmpA domains. Experimentally measured force-extension curves revealed a unique pair of "sawtooth" features that were present when a bond formed between a silicon nitride surface (atomic force microscopy tip) and E. coli cells expressing the OmpA-AmCyan protein. The observed sawtooth pair closely matched the wormlike chain model prediction for the mechanical unfolding of the AmCyan and OmpA substructures in series. These sawteeth disappeared from the measured force-extension curves when cells were treated with proteinase K. Furthermore, these unique sawteeth were absent for a mutant stain of E. coli incapable of expressing the AmCyan protein on its outer surface. Together, these data show that specific proteins exhibit unique force signatures characteristic of the bond that is formed between a living bacterium and another surface.

Nanoscale properties of mixed fengycin/ceramide monolayers explored using atomic force microscopy

To gain insight into the interactions between fengycin and skin membrane lipids, mixed fengycin/ceramide monolayers were investigated using atomic force microscopy (AFM) (monolayers supported on mica) and surface pressure-area isotherms (monolayers at the air-water interface). AFM topographic images revealed phase separation in mixed monolayers prepared at 20 degrees C/pH 2 and composed of 0.25 and 0.5 fengycin molar ratios, in the form of two-dimensional (2-D) hexagonal crystalline domains of ceramide surrounded by a fengycin-enriched fluid phase. Surface pressure-area isotherms as well as friction and adhesion AFM images confirmed that the two phases had different molecular orientations: while ceramide formed a highly ordered phase with crystalline chain packing, fengycin exhibited a disordered fluid phase with the peptide ring lying horizontally on the substrate. Increasing the temperature and pH to values corresponding to the skin parameters, i.e., 37 degrees C/pH 5, was found to dramatically affect the film organization. At low fengycin molar ratio (0.25), the hexagonal ceramide domains transformed into round domains, while at higher ratio (0.5) these were shown to melt into a continuous fengycin/ceramide fluid phase. These observations were directly supported by the thermodynamic analysis (deviation from the additivity rule, excess of free energy) of the monolayer properties at the air-water interface. Accordingly, this study demonstrates that both the environmental conditions (temperature, pH) and fengycin concentration influence the molecular organization of mixed fengycin/ceramide monolayers. We believe that the ability to modulate the formation of 2-D domains in the skin membrane may be an important biological function of fengycin, which should be increasingly investigated in future pharmacological research.

Scrutiny of the failure of lipid membranes as a function of headgroups, chain length, and lamellarity measured by scanning force microscopy

A fast, quantitative, and unambiguous screening of material properties of biomembranes using scanning force microscopy in pulsed force mode on lipid membranes is presented. The spatially resolved study of breakthrough force, breakthrough distance, adhesion, stiffness, and topography of lipid membranes as determined simultaneously by digitalized pulsed force mode provides new insight into the structure-function relationship of model membranes, which are systematically analyzed by varying chain length, lipid headgroup, and lamellarity. For this purpose, a novel unbiased analysis method is presented. A strong correlation between adhesion and breakthrough events is found on lipid bilayers and multilayers and discussed in terms of structural stability and chemical and physical interactions. Our findings indicate that multilamellar 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine is mechanically strengthened with respect to material failure by calcium ions in solution.

Mechanism of glidants: Investigation of the effect of different colloidal silicon dioxide types on powder flow by atomic force and scanning electron microscopy

The effect of hydrophilic and hydrophobic colloidal silicon dioxide types (CSD) on the flow characteristics of microcrystalline cellulose (MCC) under different mixing conditions was macroscopically measured using the angle of repose method, the bulk and tapped densities. CSD ameliorated the flow characteristics in general, but hydrophobic CSD was more effective compared to the hydrophilic types under gentle mixing conditions. The macroscopic effect was explained on the particle level by scanning electron (SEM) and atomic force microscopy (AFM) studies. The CSD distribution on the MCC surface was more uniform for the hydrophobic type and was independent from the mixing conditions used in this study. From the cumulative adhesion force distributions of the mixtures, determined by AFM, the mean and the standard deviation of the adhesion force were calculated. The means were 44.8 nN for MCC alone, 25.2 and 28.3 nN for mixtures containing the two hydrophilic types, and 13.8 N for the hydrophobic CSD under gentle mixing conditions in a Turbula mixer. Stronger mixing in a plowshare mixer led to a further reduction to 17.5 and 17.4 nN for the two hydrophilic types, while the hydrophobic CSD showing a value of 13.9 nN was unchanged. A linear correlation between the angle of repose and the adhesion force could be established, indicating that for routine measurements of the efficiency of a glidant the simple angle of repose method is sufficient.

Synergistic interactions of lipids and myelin basic protein

This report describes force measurements and atomic force microscope imaging of lipid-protein interactions that determine the structure of a model membrane system that closely mimics the myelin sheath. Our results suggest that noncovalent, mainly electrostatic and hydrophobic, interactions are responsible for the multilamellar structure and stability of myelin. We find that myelin basic protein acts as a lipid coupler between two apposed bilayers and as a lipid "hole-filler," effectively preventing defect holes from developing. From our protein-mediated-adhesion and force-distance measurements, we develop a simple quantitative model that gives a reasonably accurate picture of the molecular mechanism and adhesion of bilayer-bridging proteins by means of noncovalent interactions. The results and model indicate that optimum myelin adhesion and stability depend on the difference between, rather than the product of, the opposite charges on the lipid bilayers and myelin basic protein, as well as on the repulsive forces associated with membrane fluidity, and that small changes in any of these parameters away from the synergistically optimum values can lead to large changes in the adhesion or even its total elimination. Our results also show that the often-asked question of which membrane species, the lipids or the proteins, are the "important ones" may be misplaced. Both components work synergistically to provide the adhesion and overall structure. A better appreciation of the mechanism of this synergy may allow for a better understanding of stacked and especially myelin membrane structures and may lead to better treatments for demyelinating diseases such as multiple sclerosis.

Force spectroscopy with a small dithering of AFM tip: a method of direct and continuous measurement of the spring constant of single molecules and molecular complexes

A new method of direct and continuous measurement of the spring constant of single molecule or molecular complex is elaborated. To that end the standard force spectroscopy technique with functionalized tips and samples is combined with a small dithering of the tip. The change of the dithering amplitude as a function of the pulling force is measured to extract the spring constant of the complex. The potentialities of this method are illustrated for the experiments with single bovine serum albumin-its polyclonal antibody (Ab-BSA) and fibrinogen-fibrinogen complexes.

Molecular dynamics exposes alpha-helices in myelin basic protein

Molecular dynamics simulations of models of unmodified and deiminated MBP (myelin basic protein) have been performed on solvated structures with added counterions, for 10 ns using AMBER (assisted model building with energy refinement). The protein structures became extended, and a considerable number of alpha-helical segments formed spontaneously. The degree of molecular extension was greater in the deiminated species, and the alpha-helices were more transient. These structural disruptions may be operative in vivo during multiple sclerosis.

Measuring forces with the AFM: polymeric surfaces in liquids

This review links together for the first time both the practicalities of force measurement and the work carried out to date on force detection between polymeric surfaces in liquids using the atomic force microscope (AFM). Also included is some of the recent work that has been carried out between surfactant surfaces and biologically coated surfaces with the AFM. The emphasis in this review is on the practical issues involved with force measurement between these types of surfaces, and the similarities and irregularities between the observed types of forces measured. Comparison is made between AFM and surface force apparatus (SFA) measurements, as there is a much longer history of work with the latter. Results indicate that forces between the surfaces reviewed here are a complicated mixture of steric-type repulsion, conformational behaviour on separation and long-range attraction, which is often ascribed to 'hydrophobic' forces. The origin of this latter force remains uncertain, despite its almost ubiquitous appearance in force measurements with these types of surfaces.

Intermolecular forces in bovine serum albumin solutions exhibiting solidlike mechanical behaviors

Mechanical properties of bovine serum albumin (BSA) solutions were analyzed to gain information on intermolecular forces that stabilize the system under normal physiological conditions. BSA solutions showed unexpectedly large zero shear viscosity values under steady shear flows but responded like solids to sinusoidal linear strains: the storage shear moduli were always larger than the loss shear moduli in the frequency range 1-100 rad/s. These results suggest that BSA solutions are so-called colloidal crystals in which colloidal particles are ordered in an array due to strong repulsive forces among particles. However, the pair potential between BSA molecules predicted based on the conventional Derjaguin-Landau-Verwey-Overbeek theory failed to explain these remarkable mechanical properties of BSA solutions. Additional repulsive forces other than electrostatic must be introduced to explain stability of BSA aqueous dispersions.

A physical origin for functional domain structure in nucleic acids as evidenced by cross-linking entropy: I

A global strategy for estimating the entropy of long sequences of RNA is proposed to help improve the predictive capacity of RNA secondary structure dynamic programming algorithm (DPA) free energy (FE) minimization methods. These DPA strategies only consider the effects that occur in the immediate (nearest neighbor) vicinity of a given base pair (bp) in a secondary structure plot. They are therefore defined as nearest-neighbor secondary structure (NNSS) strategies. The new approach utilizes the statistical properties of the Gaussian polymer chain model to introduce both local and global contributions to the entropy of a given secondary structure. These entropic contributions are primarily a function of the persistence length of the RNA. Limits on the domain size are strongly suggested by this model and these limits are a function of both the length and the percentage of bp enclosed within a given domain. The model generalizes the penalties found in the NNSS algorithms. The approach considers the importance of flexibility in the folding and stability of RNA by considering the role of the persistence length in a biopolymer structure. The theory also suggests that molecular machinery may also take advantage of this global entropic effect to bring about catalytic effects. The applications can also be extended to protein structure calculations with some additional considerations.

A physical origin for functional domain structure in nucleic acids as evidenced by cross-linking entropy: II

In Part I, cross-linking entropy (CLE) was proposed as a mechanism that limits the size of functional domains of RNA. To test this hypothesis, the theory is developed into an RNA secondary structure prediction filter which is applied to nearest-neighbor secondary structure (NNSS) algorithms that utilize a free energy (FE) minimization strategy. (The NNSS strategies are also referred to as the dynamic programming algorithm in the literature.) The cross-linking entropy for RNA is derived from a generalized Gaussian polymer chain model where the entropic contributions caused by the formation of base pairs (stacking) in RNA are analysed globally. Local entropic contributions are associated with the freezing out of degrees of freedom in the links. Both global and local entropic effects are strongly influenced by the persistence length. The cross-linking entropy provides a physical origin for the size of functional domains in long nucleic acid sequences and may go further to explain as to why the majority of the domain regions in typical sequences tend to be less than 600 nucleotides in length. In addition, improvements were observed in the "best guess" predictive capacity over NNSS prediction strategies. The thermodynamic distribution is more representative of the expected structures and is strongly governed by such physical parameters as the persistence length and the excluded volume. The CLE appears to generalize the tabulated penalties used in NNSS algorithms. The principal parameter influencing this entropy is the persistence length. The model is shown to accomodate a variable persistence length and is capable of describing the folding dynamics of RNA. A two-state kinetic model based on the CLE principle is used to help elucidate the folding kinetics of functional domains in the group I introns.

The effects of deimination of myelin basic protein on structures formed by its interaction with phosphoinositide-containing lipid monolayers

The recombinant 18.5-kDa charge isoform of murine myelin basic protein (rmMBP) is unmodified posttranslationally and was used to study the effects of deimination, i.e., the conversion of arginyl to citrullinyl residues, on the protein's interactions with itself and with lipids. The unmodified species rmMBP-Cit(0) (i.e., containing no citrullinyl residues) interacted with binary monolayers containing acidic (phosphatidylinositol) and nickel-chelating lipids to form paracrystalline arrays with 4.8-nm spacing. A sample of protein was deiminated to an average of 9 citrullinyl residues per molecule of protein, yielding rmMBP-Cit(9). Under both low- and high-salt conditions, this species formed better-ordered domains than rmMBP-Cit(0), viz., planar crystalline assemblies. Thus, deimination of MBP resulted in a significant alteration of its lipid-organizing and self-interaction properties that might be operative in myelin in vivo, especially in progression of the autoimmune disease multiple sclerosis. Comparisons of amino acid sequences indicated significant similarities of MBP with filaggrin, a protein that is deiminated in another autoimmune disease, rheumatoid arthritis, suggesting that comparable epitopes could be targeted in both pathologies. In contrast, binary lipid monolayers consisting of phosphatidylinositol-4-phosphate (or phosphatidylinositol-4,5-bisphosphate) and a nickel-chelating lipid formed helical tubular vesicular structures, which appeared to be induced and/or stabilized by rmMBP, especially in its deiminated form. Sequence comparisons with other actin- and phosphoinositide-binding proteins (vinculin, ActA, MARCKS) suggested that the carboxyl-terminal segment of MBP could form an amphipathic alpha helix and was the phosphoinositide binding site.