Nonequilibrium mechanics of active cytoskeletal networks

Cells both actively generate and sensitively react to forces through their mechanical framework, the cytoskeleton, which is a nonequilibrium composite material including polymers and motor proteins. We measured the dynamics and mechanical properties of a simple three-component model system consisting of myosin II, actin filaments, and cross-linkers. In this system, stresses arising from motor activity controlled the cytoskeletal network mechanics, increasing stiffness by a factor of nearly 100 and qualitatively changing the viscoelastic response of the network in an adenosine triphosphate-dependent manner. We present a quantitative theoretical model connecting the large-scale properties of this active gel to molecular force generation.

Tau protein binding forms a 1 nm thick layer along protofilaments without affecting the radial elasticity of microtubules

Tau is one of the most abundant microtubule-associated proteins involved in kinetic stabilization and bundling of axonal microtubules. Although intense research has revealed much about tau function and its involvement in Alzheimer's disease during the past years, it still remains unclear how exactly tau binds on microtubules and if the kinetic stabilization of microtubules by tau is accompanied, at least in part, by a mechanical reinforcement of microtubules. In this paper, we have used atomic force microscopy to address both aspects by visualizing and mechanically analyzing microtubules in the presence of native tau isoforms. We could show that tau at saturating concentrations forms a 1 nm thick layer around the microtubule, but leaves the protofilament structure well visible. The latter observation argues for tau binding mainly along and not across the protofilaments. The radial elasticity of microtubules was almost unaffected by tau, consistent with tau binding along the tops of the protofilaments. Tau did increase the resistance of microtubules against rupture. Finite-element calculations confirmed our findings.

Failure of viral shells

We report a combined theoretical and experimental study of the structural failure of viral shells under mechanical stress. We find that discontinuities in the force-indentation curve associated with failure should appear when the so-called Föppl-von Kármán (FvK) number exceeds a critical value. A nanoindentation study of a viral shell subject to a soft-mode instability, where the stiffness of the shell decreases with increasing pH, confirms the predicted onset of failure as a function of the FvK number.

Bio imaging of intracellular NO production in single bone cells after mechanical stimulation

We show the intracellular upregulation of NO production after mechanical stimulation, an essential chemical signal in bone remodeling. This is done in real time using the fluorescent chromophore DAR-4M AM. Differences in cellular response to mechanical stimulation of different regions of a single cell were observed.

INTRODUCTION

Osteocytes are the most abundant bone cells that are believed to be the mechanosensors of bone, responding to mechanical stresses in interstitial fluid flow through the canaliculi. Under mechanical load, chemical signals such as NO play a key role in the activity of osteoblasts/osteoclasts that regulate bone remodeling. Despite the importance of NO in signaling, its real-time detection has proved challenging. This is largely because of the short NO half-life (typically approximately 0.1-5 s). Here, we show the upregulation of intracellular NO production in single osteocytes under localized mechanical stimulation.

MATERIALS AND METHODS

We used the chromophore DAR-4M AM for NO detection. This is loaded into surface-attached MLO-Y4 osteocyte-like and MC3T3-E1 osteoblast-like cells that are subjected to a localized mechanical stimulation using optical tweezers or a microneedle tip. DAR-4M AM is membrane-permeable and chelates NO, forming a stable, fluorescent compound, which is visible with a rhodamine filter.

RESULTS

Nonstimulated MLO-Y4 and MC3T3-E1 cells showed basal NO production levels, as indicated by a gradual increase in their fluorescence intensity. Localized mechanical stimulation of single MC3T3-E1 cells and MLO-Y4 cells by optical tweezers (150-550 pN, 0.5-3 Hz, 1 minute) showed a nearly 15-30% increase, whereas MLO-Y4 cells stimulated by a microneedle (10-20 nN, 1 minute) showed nearly 15-16% increase relative to their nonstimulated state. Furthermore, stimulation of a single cell process by a microneedle resulted in a 2-10% increase in the fluorescence intensity.

CONCLUSIONS

NO is essential for mechanically induced bone remodeling and is a meaningful parameter for measuring bone cell activation after mechanical loading. Here we show NO upregulation in individual bone cells after a localized mechanical stimulation. We also show that both the cell body and the cell processes might be involved in mechanosensing. This technique allows characterization of the mechanosensitivity of different parts of a single osteocyte. This opens up the possibility to uncover the complexities and function of single osteocytes in the dynamic process of bone remodeling.

Allosteric inhibition of kinesin-5 modulates its processive directional motility

Small-molecule inhibitors of kinesin-5 (refs. 1-3), a protein essential for eukaryotic cell division, represent alternatives to antimitotic agents that target tubulin. While tubulin is needed for multiple intracellular processes, the known functions of kinesin-5 are limited to dividing cells, making it likely that kinesin-5 inhibitors would have fewer side effects than do tubulin-targeting drugs. Kinesin-5 inhibitors, such as monastrol, act through poorly understood allosteric mechanisms, not competing with ATP binding. Moreover, the microscopic mechanism of full-length kinesin-5 motility is not known. Here we characterize the motile properties and allosteric inhibition of Eg5, a vertebrate kinesin-5, using a GFP fusion protein in single-molecule fluorescence assays. We find that Eg5 is a processive kinesin whose motility includes, in addition to ATP-dependent directional motion, a diffusive component not requiring ATP hydrolysis. Monastrol suppresses the directional processive motility of microtubule-bound Eg5. These data on Eg5's allosteric inhibition will impact these inhibitors' use as probes and development as chemotherapeutic agents.

Correlated fluctuations of microparticles in viscoelastic solutions: quantitative measurement of material properties by microrheology in the presence of optical traps

The Brownian motions of microscopic particles in viscous or viscoelastic fluids can be used to measure rheological properties. This is the basis of recently developed one- and two-particle microrheology techniques. For increased temporal and spatial resolution, some microrheology techniques employ optical traps, which introduce additional forces on the particles. We have systematically studied the effect that confinement of particles by optical traps has on their auto- and cross-correlated fluctuations. We show that trapping causes anticorrelations in the motion of two particles at low frequencies. We demonstrate how these anticorrelations depend on trap strength and the shear modulus of viscoelastic media. We present a method to account for the effects of optical traps, which permits the quantitative measurement of viscoelastic properties in one- and two-particle microrheology over an extended frequency range in a variety of viscous and viscoelastic media.

Elastic response, buckling, and instability of microtubules under radial indentation

We tested the mechanical properties of single microtubules by lateral indentation with the tip of an atomic force microscope. Indentations up to approximately 3.6 nm, i.e., 15% of the microtubule diameter, resulted in an approximately linear elastic response, and indentations were reversible without hysteresis. At an indentation force of around 0.3 nN we observed an instability corresponding to an approximately 1-nm indentation step in the taxol-stabilized microtubules, which could be due to partial or complete rupture of a relatively small number of lateral or axial tubulin-tubulin bonds. These indentations were reversible with hysteresis when the tip was retracted and no trace of damage was observed in subsequent high-resolution images. Higher forces caused substantial damage to the microtubules, which either led to depolymerization or, occasionally, to slowly reannealing holes in the microtubule wall. We modeled the experimental results using finite-element methods and find that the simple assumption of a homogeneous isotropic material, albeit structured with the characteristic protofilament corrugations, is sufficient to explain the linear elastic response of microtubules.

High-frequency stress relaxation in semiflexible polymer solutions and networks

We measure the linear viscoelasticity of sterically entangled and chemically cross-linked networks of actin filaments over more than five decades of frequency. The high-frequency response reveals rich dynamics unique to semiflexible polymers, including a previously unobserved relaxation due to rapid axial tension propagation. For high molecular weight, and for cross-linked gels, we obtain quantitative agreement with predicted shear moduli in both amplitude and frequency dependence.

Structural and mechanical study of a self-assembling protein nanotube

We report a structural characterization of self-assembling nanostructures. Using atomic force microscopy (AFM), we discovered that partially hydrolyzed alpha-lactalbumin organizes in a 10-start helix forming tubes with diameters of only 21 nm. We probed the mechanical strength of these nanotubes by locally indenting them with an AFM tip. To extract the material properties of the nanotubes, we modeled the experiment using finite element methods. Our study shows that artificial helical protein self-assembly can yield very stable, strong structures that can function either as a model system for artificial self-assembly or as a nanostructure with potential for practical applications.

Optical trap stiffness in the presence and absence of spherical aberrations

Optical traps are commonly constructed with high-numerical-aperture objectives. Oil-immersion objectives suffer from spherical aberrations when used for imaging in aqueous solutions. The effect of spherical aberrations on trapping strength has been modeled by approximation, and only a few experimental results are available in the case of micrometer-sized particles. We present an experimental study of the dependence of lateral and axial optical-trap stiffness on focusing depth for polystyrene and silica beads of 2 microm diameter by using oil- and water-immersion objectives. We demonstrate a strong depth dependence of trap stiffness with the oil-immersion objective, whereas no depth dependence was observed with the water-immersion objective.

Rapid chiral assembly of rigid DNA building blocks for molecular nanofabrication

Practical components for three-dimensional molecular nanofabrication must be simple to produce, stereopure, rigid, and adaptable. We report a family of DNA tetrahedra, less than 10 nanometers on a side, that can self-assemble in seconds with near-quantitative yield of one diastereomer. They can be connected by programmable DNA linkers. Their triangulated architecture confers structural stability; by compressing a DNA tetrahedron with an atomic force microscope, we have measured the axial compressibility of DNA and observed the buckling of the double helix under high loads.

Short-time inertial response of viscoelastic fluids: observation of vortex propagation

We probe the response of viscous and viscoelastic fluids on micrometer and microsecond length and time scales using two optically trapped beads. In this way we resolve the flow field, which exhibits clear effects of fluid inertia. Specifically, we resolve the short-time vortex flow and the corresponding evolution of this vortex, which propagates diffusively for simple liquids. For viscoelastic fluids, this propagation is shown to be faster than diffusive and the displacement correlations reflect the frequency-dependent shear modulus of the medium.

High-frequency microrheology of wormlike micelles

We have measured the frequency-dependent shear modulus of entangled solutions of wormlike micelles by high-frequency microrheology and have compared the results with those from macrorheology experiments done on the same samples. Using optical microrheology based on laser interferometry we have measured loss and storage moduli over six decades in frequency up to about 100 kHz. We present data over a decade in concentration in the entangled regime and find good agreement between micro- and macrorheology, thus validating recently developed microrheology techniques. By collapsing data for different concentrations, we furthermore determine both the concentration scaling of the plateau modulus and a power-law exponent of the complex shear modulus at high frequencies.

The bipolar mitotic kinesin Eg5 moves on both microtubules that it crosslinks

During cell division, mitotic spindles are assembled by microtubule-based motor proteins. The bipolar organization of spindles is essential for proper segregation of chromosomes, and requires plus-end-directed homotetrameric motor proteins of the widely conserved kinesin-5 (BimC) family. Hypotheses for bipolar spindle formation include the 'push-pull mitotic muscle' model, in which kinesin-5 and opposing motor proteins act between overlapping microtubules. However, the precise roles of kinesin-5 during this process are unknown. Here we show that the vertebrate kinesin-5 Eg5 drives the sliding of microtubules depending on their relative orientation. We found in controlled in vitro assays that Eg5 has the remarkable capability of simultaneously moving at approximately 20 nm s(-1) towards the plus-ends of each of the two microtubules it crosslinks. For anti-parallel microtubules, this results in relative sliding at approximately 40 nm s(-1), comparable to spindle pole separation rates in vivo. Furthermore, we found that Eg5 can tether microtubule plus-ends, suggesting an additional microtubule-binding mode for Eg5. Our results demonstrate how members of the kinesin-5 family are likely to function in mitosis, pushing apart interpolar microtubules as well as recruiting microtubules into bundles that are subsequently polarized by relative sliding.

Microrheology of solutions of semiflexible biopolymer filaments using laser tweezers interferometry

Semiflexible polymers are of great biological importance in determining the mechanical properties of cells. Techniques collectively known as microrheology have recently been developed to measure the viscoelastic properties of solutions of submicroliter volumes. We employ one such technique, which uses a focused laser beam to trap a micron-sized silica bead and interferometric photodiode detection to measure passively the position fluctuations of the trapped bead with nanometer resolution and high bandwidth. The frequency-dependent complex shear modulus G*(f) can be extracted from the position fluctuations via the fluctuation-dissipation theorem and the generalized Stokes-Einstein relation. Using particle tracking microrheology, we report measurements of shear moduli of solutions of fd viruses, which are filamentous, semiflexible, and monodisperse bacteriophages, each 0.9 microm long, 7 nm in diameter, and having a persistence length of 2.2 microm. Recent theoretical treatments of semiflexible polymer dynamics provide quantitative predictions of the rheological properties of such a model system. The fd samples measured span the dilute, semidilute, and concentrated regimes. In the dilute regime G*(f) is dominated by (rigid rod) rotational relaxation, whereas the high-frequency regime reflects single-semiflexible filament dynamics consistent with the theoretical prediction. Due to the short length of fd viruses used in this study, the intermediate regime does not exhibit a well-developed plateau. A dynamic scaling analysis gives rise to a concentration scaling of c(1.36) (r=0.99) in the transition regime and a frequency scaling of f(0.63) (r=0.98) at high frequencies.

Bacteriophage capsids: tough nanoshells with complex elastic properties

The shell of bacteriophages protects the viral DNA during host-to-host transfer and serves as a high-pressure container storing energy for DNA injection into a host bacterium. Here, we probe the mechanical properties of nanometer-sized bacteriophage phi 29 shells by applying point forces. We show that empty shells withstand nanonewton forces while being indented up to 30% of their height. The elastic response varies across the surface, reflecting the arrangement of shell proteins. The measured Young's modulus (approximately 1.8 GPa) is comparable with that of hard plastic. We also observe fatigue and breakage of capsids after probing them repetitively. These results illustrate the mechanoprotection that viral shells provide and also suggest design principles for nanotechnology.