Exploring mechanochemical processes in the cell with optical tweezers

Force and torque, stress and strain or work are examples of mechanical and elastic actions which are intimately linked to chemical reactions in the cell. Optical tweezers are a light-based method which allows the real-time manipulation of single molecules and cells to measure their interactions. We describe the technique, briefly reviewing the operating principles and the potential capabilities to the study of biological processes. Additional emphasis is given to the importance of fluctuations in biology and how single-molecule techniques allow access to them. We illustrate the applications by addressing experimental configurations and recent progresses in molecular and cell biology.     

Detection and characterization of individual intermolecular bonds using optical tweezers

The development of scanning probe techniques has made it possible to examine protein-protein interactions at the level of individual molecular pairs. A calibrated optical tweezers, along with immunoglobulin G (IgG)-coated polystyrene microspheres, has been used to detect individual surface-linked Staphylococcus protein A (SpA) molecules and to characterize the strength of the noncovalent IgG-SpA bond. Microspheres containing, on average, less than one IgG per contact area were held in the optical trap while an SpA-coated substrate was scanned beneath them at a distance of approximately 50 nm. This geometry allows the trapped bead to make contact with the surface, from bond formation to rupture, and results in an enhancement of the force applied to a bond due to leverage supplied by the bead itself. Experiments yielded median single-bond rupture forces from 25 to 44 pN for IgG from four mammalian species, in general agreement with predictions based on free energies of association obtained from solution equilibrium constants.     

Controlled rotation of biological microscopic objects using optical line tweezers

Controlled, continuous rotation of cells or intracellular objects was achieved using optical tweezers with an elliptic beam profile (line tweezers), which was generated by placing a cylindrical lens in the path of the trapping beam. By rotating the cylindrical lens, rotation of the elliptic trapping beam and hence of the object trapped therein was achieved. Compared to previously reported techniques for rotation of microscopic objects, this approach is much simpler, gives better utilization of available laser power and also allows much easier control of the trap beam profile. We have used this approach for rotation of biological objects varying in size from 2 to 40 m. At 25 mW trapping beam power at the object plane E. coli bacteria could be rotated at speeds approaching 10 Hz and an intracellular object (presumably a calcium oxalate crystal) trapped inside Elodea densa plant cell could be rotated with speeds of up to 4 Hz. To our knowledge, this is the first report for rotation of an intracellular object.     

High-speed high-resolution imaging of intercellular immune synapses using optical tweezers

Imaging in any plane other than horizontal in a microscope typically requires a reconstruction from multiple optical slices that significantly decreases the spatial and temporal resolution that can be achieved. This can limit the precision with which molecular events can be detected, for example, at intercellular contacts. This has been a major issue for the imaging of immune synapses between live cells, which has generally required the reconstruction of en face intercellular synapses, yielding spatial resolution significantly above the diffraction limit and updating at only a few frames per minute. Strategies to address this issue have usually involved using artificial activating substrates such as antibody-coated slides or supported planar lipid bilayers, but synapses with these surrogate stimuli may not wholly resemble immune synapses between two cells. Here, we combine optical tweezers and confocal microscopy to realize generally applicable, high-speed, high-resolution imaging of almost any arbitrary plane of interest. Applied to imaging immune synapses in live-cell conjugates, this has enabled the characterization of complex behavior of highly dynamic clusters of T cell receptors at the T cell/antigen-presenting cell intercellular immune synapse and revealed the presence of numerous, highly dynamic long receptor-rich filopodial structures within inhibitory Natural Killer cell immune synapses.     

Naphthalimide intercalators with chiral amino side chains: effects of chirality on DNA binding, photodamage and antitumor cytotoxicity

Several novel heterocyclic-fused naphthalimides intercalators with chiral amino side chains were investigated. Their side chains' chiral configuration determines DNA binding activities in the order: S-enantiomers > R-enantiomers. And their DNA photodamaging activities were in good agreement with their DNA binding constants, the S-enantiomers could photocleave circular supercoiled pBR322 DNA more efficiently than their R-enantiomers. S-enantiomer B(3) could photodamage DNA at 0.2 microM and cleave supercoiled plasmid DNA from form I to form II completely at 50 microM. Almost all of these intercalators showed effective cytoxicities against human lung cancer cells and murine leukemia cells. S-enantiomers showed different antitumor cytotoxicity by comparison with R-enantiomers. This work may provide additional information for the role of amino side chains on intercalators as antitumor agents.     

Single-molecule manipulation of macromolecules on GUV or SUV membranes using optical tweezers

Despite their wide applications in soluble macromolecules, optical tweezers have rarely been used to characterize the dynamics of membrane proteins, mainly due to the lack of model membranes compatible with optical trapping. Here, we examined optical trapping and mechanical properties of two potential model membranes, giant and small unilamellar vesicles (GUVs and SUVs, respectively) for studies of membrane protein dynamics. We found that optical tweezers can stably trap GUVs containing iodixanol with controlled membrane tension. The trapped GUVs with high membrane tension can serve as a force sensor to accurately detect reversible folding of a DNA hairpin or membrane binding of synaptotagmin-1 C2AB domain attached to the GUV. We also observed that SUVs are rigid enough to resist large pulling forces and are suitable for detecting protein conformational changes induced by force. Our methodologies may facilitate single-molecule manipulation studies of membrane proteins using optical tweezers.     

The elasticity of single titin molecules using a two-bead optical tweezers assay

Titin is responsible for the passive elasticity of the muscle sarcomere. The mechanical properties of skeletal and cardiac muscle titin were characterized in single molecules using a novel dual optical tweezers assay. Antibody pairs were attached to beads and used to select the whole molecule, I-band, A-band, a tandem-immunoglobulin (Ig) segment, and the PEVK region. A construct from the PEVK region expressing >25% of the full-length skeletal muscle isoform was chemically conjugated to beads and similarly characterized. By elucidating the elasticity of the different regions, we showed directly for the first time, to our knowledge, that two entropic components act in series in the skeletal muscle titin I-band (confirming previous speculations), one associated with tandem-immunoglobulin domains and the other with the PEVK region, with persistence lengths of 2.9 nm and 0.76 nm, respectively (150 mM ionic strength, 22 degrees C). Novel findings were: the persistence length of the PEVK component rose (0.4-2.7 nm) with an increase in ionic strength (15-300 mM) and fell (3.0-0.3 nm) with a temperature increase (10-60 degrees C); stress-relaxation in 10-12-nm steps was observed in the PEVK construct and hysteresis in the native PEVK region. The region may not be a pure random coil, as previously thought, but contains structured elements, possibly with hydrophobic interactions.     

Adherence of Staphylococcus aureus fibronectin binding protein A mutants: an investigation using optical tweezers

Bacterial adhesion to extracellular matrix proteins plays a major role in infections of host tissue and medical devices. In some species of gram-positive cocci, this adhesion is mediated by specific molecules present on the bacterial cell surface. We have used optical tweezers to dynamically measure the adhesive force between an individual Staphylococcus aureus bacterium and a fibronectin-coated surface. A bacterium was optically trapped and brought in contact with a 10-microm diameter polystyrene microsphere coated with fibronectin. The force required to detach the cell from the microsphere was measured by tracking the displacement signals of the trapped cell on a quadrant photodiode throughout the detachment process for a series of S. aureus strains expressing fibronectin-binding proteins with various degrees of mutation. The single-bond rupture forces ranged between 15 and 26 pN depending on the extent of mutation. No binding was observed in the strain with the highest degree of mutation. These results confirm that multiple regions of the S. aureus fibronectin adhesin participate in the binding process and provide further insight into the role of these regions in the adhesive process.     

Micromanipulation of mitotic chromosomes in PTK2 cells using laser-induced optical forces ("optical tweezers")

To study the potential use of optical forces to manipulate chromosome movement, we have used a Nd:YAG laser at a wavelength of 1.06 microns focused into a phase contrast microscope. Metaphase and anaphase chromosomes were exposed while being monitored by video microscopy. The results indicated that when optical forces were applied to late-moving metaphase chromosomes on the side closest to the nearest spindle pole, the trapped chromosomes initiated movement to the metaphase plate. The chromosome velocities were two to eight times the normal rate depending on the chromosome size, geometry, and trapping site. At the initiation of anaphase, a pair of chromatids could be held by the optical trap and kept motionless throughout anaphase while the other pairs of chromatids separated and moved to opposite spindle poles. As a result, the trapped chromosome either was incorporated into one of the daughter cells or was lost in the cleavage furrow, or the two chromatids eventually separated and moved to their respective daughter cells. If the trap was removed at the beginning of anaphase B, the chromosome moved back to the poles. Our experiments demonstrate that the laser-induced optical force trap is a potential new technique to study noninvasively the mitotic spindle of living cells.     

Force unfolding kinetics of RNA using optical tweezers. II. Modeling experiments

By exerting mechanical force, it is possible to unfold/refold RNA molecules one at a time. In a small range of forces, an RNA molecule can hop between the folded and the unfolded state with force-dependent kinetic rates. Here, we introduce a mesoscopic model to analyze the hopping kinetics of RNA hairpins in an optical tweezers setup. The model includes different elements of the experimental setup (beads, handles, and RNA sequence) and limitations of the instrument (time lag of the force-feedback mechanism and finite bandwidth of data acquisition). We investigated the influence of the instrument on the measured hopping rates. Results from the model are in good agreement with the experiments reported in the companion article. The comparison between theory and experiments allowed us to infer the values of the intrinsic molecular rates of the RNA hairpin alone and to search for the optimal experimental conditions to do the measurements. We conclude that the longest handles and softest traps that allow detection of the folding/unfolding signal (handles approximately 5-10 Kbp and traps approximately 0.03 pN/nm) represent the best conditions to obtain the intrinsic molecular rates. The methodology and rationale presented here can be applied to other experimental setups and other molecules.     

A new determination of the shear modulus of the human erythrocyte membrane using optical tweezers.

Optical tweezers are used to apply calibrated forces to human erythrocytes, via small silica beads bound to their membrane. The shear modulus mu of the membrane is inferred from measurements of the cell deformation in the small strain linear regime. We find the same result mu = 2.5 +/- 0.4 microN/m for both discotic and nearly spherical swollen cells. This value is smaller than the one deduced from micropipettes experiments. However the two methods do not operate in the same deformation regime and are not expected to lead to the same result.     

Spatiotemporal analysis of cell response to a rigidity gradient: a quantitative study using multiple optical tweezers

We investigate the dynamic response of single cells to weak and local rigidities, applied at controlled adhesion sites. Using multiple latex beads functionalized with fibronectin, and each trapped in its own optical trap, we study the reaction in real time of single 3T3 fibroblast cells to asymmetrical tensions in the tens of pN · μm⁻¹ range. We show that the cell feels a rigidity gradient even at this low range of tension, and over time develops an adapted change in the force exerted on each adhesion site. The rate at which force increases is proportional to trap stiffness. Actomyosin recruitment is regulated in space and time along the rigidity gradient, resulting in a linear relationship between the amount of recruited actin and the force developed independently in trap stiffness. This time-regulated actomyosin behavior sustains a constant and rigidity-independent velocity of beads inside the traps. Our results show that the strengthening of extracellular matrix-cytoskeleton linkages along a rigidity gradient is regulated by controlling adhesion area and actomyosin recruitment, to maintain a constant deformation of the extracellular matrix.     

Distinct membrane mechanical properties of human mesenchymal stem cells determined using laser optical tweezers

The therapeutic efficacy of mesenchymal stem cells (MSCs) in tissue engineering and regenerative medicine is determined by their unique biological, mechanical, and physicochemical characteristics, which are yet to be fully explored. Cell membrane mechanics, for example, has been shown to critically influence MSC differentiation. In this study, we used laser optical tweezers to measure the membrane mechanics of human MSCs and terminally differentiated fibroblasts by extracting tethers from the outer cell membrane. The average tether lengths were 10.6+/-1.1 microm (hMSC) and 3.0+/-0.5 microm (fibroblasts). The tether extraction force did not increase during tether formation, which suggests existence of a membrane reservoir intended to buffer membrane tension fluctuations. Cytoskeleton disruption resulted in a fourfold tether length increase in fibroblasts but had no effect in hMSCs, indicating weak association between the cell membrane and hMSC actin cytoskeleton. Cholesterol depletion, known to decrease lipid bilayer stiffness, caused an increase in the tether length both in fibroblasts and hMSCs, as does the treatment of cells with DMSO. We postulate that whereas fibroblasts use both the membrane rigidity and membrane-cytoskeleton association to regulate their membrane reservoir, hMSC cytoskeleton has only a minor impact on stem cell membrane mechanics.     

POTATO: Automated pipeline for batch analysis of optical tweezers data

Optical tweezers are a single-molecule technique that allows probing of intra- and intermolecular interactions that govern complex biological processes involving molecular motors, protein-nucleic acid interactions, and protein/RNA folding. Recent developments in instrumentation eased and accelerated optical tweezers data acquisition, but analysis of the data remains challenging. Here, to enable high-throughput data analysis, we developed an automated python-based analysis pipeline called POTATO (practical optical tweezers analysis tool). POTATO automatically processes the high-frequency raw data generated by force-ramp experiments and identifies (un)folding events using predefined parameters. After segmentation of the force-distance trajectories at the identified (un)folding events, sections of the curve can be fitted independently to a worm-like chain and freely jointed chain models, and the work applied on the molecule can be calculated by numerical integration. Furthermore, the tool allows plotting of constant force data and fitting of the Gaussian distance distribution over time. All these features are wrapped in a user-friendly graphical interface, which allows researchers without programming knowledge to perform sophisticated data analysis.     

Raman study of mechanically induced oxygenation state transition of red blood cells using optical tweezers

Raman spectroscopy was used to monitor changes in the oxygenation state of human red blood cells while they were placed under mechanical stress with the use of optical tweezers. The applied force is intended to simulate the stretching and compression that cells experience as they pass through vessels and smaller capillaries. In this work, spectroscopic evidence of a transition between the oxygenation and deoxygenation states, which is induced by stretching the cell with optical tweezers, is presented. The transition is due to enhanced hemoglobin-membrane and hemoglobin neighbor-neighbor interactions, and the latter was further studied by modeling the electrostatic binding of two of the protein structures.     

Studies of plasma membrane mechanics and plasma membrane-cytoskeleton interactions using optical tweezers and fluorescence imaging

We use optical tweezers in conjunction with an optical position-sensing system, which spectrally filters signals generated by a trapped fluorescent microsphere to study plasma membrane (PM) mechanics and its interactions with cytoskeleton. We dynamically measure the PM tethering force on human embryonic kidney cells that are a standard cultured cell line. Recorded tethering force vs. PM displacement profiles, revealed the tether formation process, initiated with linear deformation of the PM, followed by a nonlinear regime and terminated with the local separation of PM. Tethering force vs. displacement profiles were used to estimate tether formation force and stiffness parameter of the PM. Integration of the force-displacement profiles yielded the work of tether formation, including linear and nonlinear components. Our results demonstrate that spectral filtering of the optically trapped fluorescent microsphere image formed on the position-sensing system overcomes the artifacts introduced by the transillumination imaging and allows accurate measures of PM mechanics before and during the initial stages of tether formation.     

Spectrin-level modeling of the cytoskeleton and optical tweezers stretching of the erythrocyte

We present a three-dimensional computational study of whole-cell equilibrium shape and deformation of human red blood cell (RBC) using spectrin-level energetics. Random network models consisting of degree-2, 3, ..., 9 junction complexes and spectrin links are used to populate spherical and biconcave surfaces and intermediate shapes, and coarse-grained molecular dynamics simulations are then performed with spectrin connectivities fixed. A sphere is first filled with cytosol and gradually deflated while preserving its total surface area, until cytosol volume consistent with the real RBC is reached. The equilibrium shape is determined through energy minimization by assuming that the spectrin tetramer links satisfy the worm-like chain free-energy model. Subsequently, direct stretching by optical tweezers of the initial equilibrium shape is simulated to extract the variation of axial and transverse diameters with the stretch force. At persistence length p = 7.5 nm for the spectrin tetramer molecule and corresponding in-plane shear modulus mu(0) approximately 8.3 microN/m, our models show reasonable agreement with recent experimental measurements on the large deformation of RBC with optical tweezers. We find that the choice of the reference state used for the in-plane elastic energy is critical for determining the equilibrium shape. If a position-independent material reference state such as a full sphere is used in defining the in-plane energy, then the bending modulus kappa needs to be at least a decade larger than the widely accepted value of 2 x 10(-19) J to stabilize the biconcave shape against the cup shape. We demonstrate through detailed computations that this paradox can be avoided by invoking the physical hypothesis that the spectrin network undergoes constant remodeling to always relax the in-plane shear elastic energy to zero at any macroscopic shape, at some slow characteristic timescale. We have devised and implemented a liquefied network structure evolution algorithm that relaxes shear stress everywhere in the network and generates cytoskeleton structures that mimic experimental observations.     

Single-molecule manipulation of double-stranded DNA using optical tweezers: interaction studies of DNA with RecA and YOYO-1.

By using optical tweezers and a specially designed flow cell with an integrated glass micropipette, we constructed a setup similar to that of Smith et al. (Science 271:795-799, 1996) in which an individual double-stranded DNA (dsDNA) molecule can be captured between two polystyrene beads. The first bead is immobilized by the optical tweezers and the second by the micropipette. Movement of the micropipette allows manipulation and stretching of the DNA molecule, and the force exerted on it can be monitored simultaneously with the optical tweezers. We used this setup to study elongation of dsDNA by RecA protein and YOYO-1 dye molecules. We found that the stability of the different DNA-ligand complexes and their binding kinetics were quite different. The length of the DNA molecule was extended by 45% when RecA protein was added. Interestingly, the speed of elongation was dependent on the external force applied to the DNA molecule. In experiments in which YOYO-1 was added, a 10-20% extension of the DNA molecule length was observed. Moreover, these experiments showed that a change in the applied external force results in a time-dependent structural change of the DNA-YOYO-1 complex, with a time constant of approximately 35 s (1/e2). Because the setup provides an oriented DNA molecule, we determined the orientation of the transition dipole moment of YOYO-1 within DNA by using fluorescence polarization. The angle of the transition dipole moment with respect to the helical axis of the DNA molecule was 69 degrees +/- 3.     

Characterization of bacterial spore germination using phase-contrast and fluorescence microscopy, Raman spectroscopy and optical tweezers

This protocol describes a method combining phase-contrast and fluorescence microscopy, Raman spectroscopy and optical tweezers to characterize the germination of single bacterial spores. The characterization consists of the following steps: (i) loading heat-activated dormant spores into a temperature-controlled microscope sample holder containing a germinant solution plus a nucleic acid stain; (ii) capturing a single spore with optical tweezers; (iii) simultaneously measuring phase-contrast images, Raman spectra and fluorescence images of the optically captured spore at 2- to 10-s intervals; and (iv) analyzing the acquired data for the loss of spore refractility, changes in spore-specific molecules (in particular, dipicolinic acid) and uptake of the nucleic acid stain. This information leads to precise correlations between various germination events, and takes 1-2 h to complete. The method can also be adapted to use multi-trap Raman spectroscopy or phase-contrast microscopy of spores adhered on a cover slip to simultaneously obtain germination parameters for multiple individual spores.