Spontaneous Motions Research Articles

Cytoskeleton dynamics: Fluctuations within the network

Out-of-equilibrium systems, such as the dynamics of a living cytoskeleton (CSK), are inherently noisy with fluctuations arising from the stochastic nature of the underlying biochemical and molecular events. Recently, such fluctuations within the cell were characterized by observing spontaneous nano-scale motions of an RGD-coated microbead bound to the cell surface [Bursac et al., Nat. Mater. 4 (2005) 557–561]. While these reported anomalous bead motions represent a molecular level reorganization (remodeling) of microstructures in contact with the bead, a precise nature of these cytoskeletal constituents and forces that drive their remodeling dynamics are largely unclear. Here, we focused upon spontaneous motions of an RGD-coated bead and, in particular, assessed to what extent these motions are attributable to (i) bulk cell movement (cell crawling), (ii) dynamics of focal adhesions, (iii) dynamics of lipid membrane, and/or (iv) dynamics of the underlying actin CSK driven by myosin motors.

Structure, dynamics, and elasticity of free 16s rRNA helix 44 studied by molecular dynamics simulations

Molecular dynamics (MD) simulations were employed to investigate the structure, dynamics, and local base-pair step deformability of the free 16S ribosomal helix 44 from Thermus thermophilus and of a canonical A-RNA double helix. While helix 44 is bent in the crystal structure of the small ribosomal subunit, the simulated helix 44 is intrinsically straight. It shows, however, substantial instantaneous bends that are isotropic. The spontaneous motions seen in simulations achieve large degrees of bending seen in the X-ray structure and would be entirely sufficient to allow the dynamics of the upper part of helix 44 evidenced by cryo-electron microscopic studies. Analysis of local base-pair step deformability reveals a patch of flexible steps in the upper part of helix 44 and in the area proximal to the bulge bases, suggesting that the upper part of helix 44 has enhanced flexibility. The simulations identify two conformational substates of the second bulge area (bottom part of the helix) with distinct base pairing. In agreement with nuclear magnetic resonance (NMR) and X-ray studies, a flipped out conformational substate of conserved 1492A is seen in the first bulge area. Molecular dynamics (MD) simulations reveal a number of reversible alpha-gamma backbone flips that correspond to transitions between two known A-RNA backbone families. The flipped substates do not cumulate along the trajectory and lead to a modest transient reduction of helical twist with no significant influence on the overall geometry of the duplexes. Despite their considerable flexibility, the simulated structures are very stable with no indication of substantial force field inaccuracies.

Filamin-A and Rheological Properties of Cultured Melanoma Cells

Here we report the rheological properties of cultured hsFLNa (filamin-A)-expressing (FIL+) and hsFLNa-deficient (FIL−) melanoma cells. Using magnetic twisting cytometry over a wide range of probing frequencies, and targeting either cortical or deeper cytoskeletal structures, we found that differences in stiffness of FIL+ versus FIL− cells were remarkably small. When probed through deep cytoskeletal structures, FIL+ cells were, at most, 30% stiffer than FIL− cells, whereas when probed through more peripheral cytoskeletal structures FIL− cells were not different except at very high frequencies. The loss tangent, expressed as an effective cytoskeletal temperature, was systematically greater in FIL− than FIL+ cells, but these differences were small and showed that the FIL+ cells were only slightly closer to a solidlike state. To quantify cytoskeletal remodeling, we measured spontaneous motions of beads bound to cortical cytoskeletal structures and found no difference in FIL+ versus FIL− cells. Although mechanical differences between FIL+ and FIL− cells were evident both in cortical and deeper structures, these differences were far smaller than expected based on measurements of the rheology of purified actin-filamin solutions. These findings do not rule out an important contribution of filamin to the mechanical properties of the cortical cytoskeleton, but suggest that effects of filamin in the cortex are not exerted on the length scale of the probe used here. These findings would appear to rule out any important contribution of filamin to the bulk mechanical properties of the cytoplasm, however. Although filamin is present in the cytoplasm, it may be inactive, its mechanical effects may be small compared with other crosslinkers, or mechanical properties of the matrix may be dominated by an overriding role of cytoskeletal prestress.

Do Biophysical Properties of the Airway Smooth Muscle in Culture Predict Airway Hyperresponsiveness?

Airway hyperresponsiveness is a cardinal feature of asthma but remains largely unexplained. In asthma, the key end-effector of acute airway narrowing is the airway smooth muscle (ASM) cell. Here we report novel biophysical properties of the ASM cell isolated from the relatively hyporesponsive Lewis rat versus the relatively hyperresponsive Fisher rat. We focused upon the ability of the cytoskeleton (CSK) of the ASM cell to stiffen, to generate contractile forces, and to remodel. We used optical magnetic twisting cytometry to measure cell stiffness and traction microscopy to measure contractile forces. To measure remodeling dynamics, we quantified spontaneous nanoscale motions of a microbead tightly anchored to the CSK. In response to a panel of contractile and relaxing agonists, Fisher ASM cells showed greater stiffening, bigger contractile forces, and faster CSK remodeling; they also exhibited higher effective temperature of the CSK matrix. These physical differences measured at the level of the single cell in vitro were consistent with strain-related differences in airway responsiveness in vivo. As such, comprehensive biophysical characterizations of CSK dynamics at the level of the cell in culture may provide novel perspectives on the ASM and its contributions to the excessive airway narrowing in asthma.

Force fluctuation and microrehology in endothelial cells

Important processes of living cells, including intracellular transport, cell crawling, contraction, division and mechanochemical signal transduction, are controlled by cytoskeletal (CSK) dynamics. Some aspects of CSK dynamics have been studied by following the spontaneous motion of CSK-bound particles. Such particle exhibit a superdiffusive behavior that is believed to arise from random local ATP-driven intracellular force fluctuations generated by polymerization processes and motor proteins. (Lau et al, Phys Rev Lett 91:198101). Here we report simultaneous measurements of spontaneous particle motions and cellular force fluctuations. Human vascular endothelial cells were plated onto collagen coated elastic polyacrylamide hydrogels. Force fluctuations at the basal cell membrane(cell tractions) were computed from the displacements of gel-embedded fluorescent beads. Spontaneous particle motion was measured using fibronectin coated fluorescent beads that were bound to the apicell cell membrane via integrin receptors. Bead motion of both CSK-bound and ECM-bound beads were measured with nanometer-resolution and expressed as mean square displacement (MSD). The MSD of both CSK-bound and ECM-bound beads displayed a superdiffusive behavior that was well described by a power law: MSD = a*t^b. In contradiction to existing theories of stress dissipation within the CSK, we found an identical power law exponent for both CSK-bound and ECM-bound beads of b = 1.6. This finding suggests that the spontaneous motion of CSK-bound beads is driven not by random, local stress fluctuations within a viscoelastic continuum, but rather by large scale stress fluctuations within a CSK network that transmits these stresses with little or no dissipation to the ECM.

Cytoskeletal remodelling and slow dynamics in the living cell

The cytoskeleton (CSK) is a crowded network of structural proteins that stabilizes cell shape and drives cell motions. Recent studies on the dynamics of the CSK have established that a wide variety of cell types exhibit rheology in which responses are not tied to any particular relaxation times and are thus scale-free. Scale-free rheology is often found in a class of materials called soft glasses, but not all materials expressing scale-free rheology are glassy (see plastics, wood, concrete or some metals for example). As such, the extent to which dynamics of the CSK might be regarded as glassy remained an open question. Here we report both forced and spontaneous motions of microbeads tightly bound to the CSK of human muscle cells. Large oscillatory shear fluidized the CSK matrix, which was followed by slow scale-free recovery of rheological properties (aging). Spontaneous bead motions were subdiffusive at short times but superdiffusive at longer times; intermittent motions reflecting nanoscale CSK rearrangements depended on both the approach to kinetic arrest and energy release due to ATP hydrolysis. Aging, intermittency, and approach to kinetic arrest establish a striking analogy between the behaviour of the living CSK and that of inert non-equilibrium systems, including soft glasses, but with important differences that are highly ATP-dependent. These mesoscale dynamics link integrative CSK functions to underlying molecular events, and represent an important intersection of topical issues in condensed matter physics and systems biology.

Hypoxia alters biophysical properties of endothelial cells via p38 MAPK- and Rho kinase-dependent pathways

Hypoxia alters the barrier function of the endothelial cells that line the pulmonary vasculature, but underlying biophysical mechanisms remain unclear. Using rat pulmonary microvascular endothelial cells (RPMEC) in culture, we report herein changes in biophysical properties, both in space and in time, that occur in response to hypoxia. We address also the molecular basis of these changes. At the level of the single cell, we measured cell stiffness, the distribution of traction forces exerted by the cell on its substrate, and spontaneous nanoscale motions of microbeads tightly bound to the cytoskeleton (CSK). Hypoxia increased cell stiffness and traction forces by a mechanism that was dependent on the activation of Rho kinase. These changes were followed by p38-mediated decreases in spontaneous bead motions, indicating stabilization of local cellular-extracellular matrix (ECM) tethering interactions. Cells overexpressing phospho-mimicking small heat shock protein (HSP27-PM), a downstream effector of p38, exhibited decreases in spontaneous bead motions that correlated with increases in actin polymerization in these cells. Together, these findings suggest that hypoxia differentially regulates endothelial cell contraction and cellular-ECM adhesion.

Tai Chi synthesizer: a motion synthesis framework based on key‐postures and motion instructions

AbstractHuman spontaneous motions such as walking and runing are seldom described in detail. However, some well designed motions such as conditioning and rehabilitation exercises are usually described in detail and edited into exercise manuals for training purpose. This paper presents a novel framework for synthesizing new motions based on given motion manuals and corresponding motion capture examples. First, using text analysis method, a set of basic motion texts is extracted from exercise manuals and converted into text normal form. Then we introduce an annotation method to build the mapping table between basic motion texts and their corresponding motion clips in the given examples. To facilitate the search for proper motion clips, a multi‐dimensional index structure based on posture parameters is proposed. Then a new motion with given textual description can be synthesized by converting the description sentences into a sequence of text normal forms and then concatenating the corresponding motion clips to form the desired motion. Moreover, to realize the smooth concatenation, we show that it can be achieved by finding an appropriate path in the proposed multi‐dimensional index space. Several experimental examples are given to demonstrate the proposed method is effective in synthesizing desired motions according to given descriptions. Copyright © 2004 John Wiley & Sons, Ltd.

Role of heat shock protein 27 in cytoskeletal remodeling of the airway smooth muscle cell.

Remodeling of the airway smooth muscle (ASM) cell has been proposed to play an important role in airway hyperresponsiveness. Using a functional assay, we have assessed remodeling of the cultured rat ASM cell and the role of heat shock protein (HSP) 27 in that process. To probe remodeling dynamics, we measured spontaneous motions of an individual Arg-Gly-Asp-coated microbead that was anchored to the cytoskeleton. We reasoned that the bead could not move unless the microstructure to which it is attached rearranged; if so, then its mean square displacement (MSD) would report ongoing internal reorganizations over time. Each bead displayed a random, superdiffusive motion; MSD increased with time as approximately t(1.7), whereas an exponent of unity would be expected for a simple passive diffusion. Increasing concentrations of cytochalasin-D or latrunculin-A caused marked increases in the MSD, whereas colchicine did not. Treatments with PDGF or IL-1beta, but not transforming growth factor-beta, caused decreases in the MSD, the extent of which rank-ordered with the relative potency of these agents in eliciting the phosphorylation of HSP27. The chemical stressors anisomycin and arsenite each increased the levels of HSP27 phosphorylation and, at the same time, decreased bead motions. In particular, arsenite prevented and even reversed the effects of cytochalasin-D on bead motions. Finally, ASM cells overexpressing phospho-mimicking human HSP27, but not wild-type or phosphorylation-deficient HSP27, exhibited decreases in bead motions that were comparable to the arsenite response. Taken together, these results show that phosphorylated HSP27 favors reduced bead motions that are probably due to stabilization of the actin cytoskeleton.

Motions of Si(100) Steps at Low Temperature. A Study by Molecular Dynamics

ABSTRACTAn isothermal Molecular Dynamics simulation method has been applied to the study of motions of steps on Si(100). The steps have a monolayer height and their temperature is below the roughening transition (i.e. T≤ 1000K). The calculations predict spontaneous motions even in this low temperature range and describe the functional dependence of the displacements of step-edge atoms on the step geometry, size and temperature and on the nature of the interatomic forces. Owing to the use of the Fourier analysis, these results can be applied to structures of realistic size by a proper scaling of the wavevectors.

Acoustic energy dissipation during phase separation in oversaturated solid solutions

The phase separation in oversaturated solid solutions Al-4.0 at.% Cu and Al-6.5 at.% Zn is accompanied by an acoustic emission (AE). The AE signals were detected in the stage of formation of the concentration inhomogeneities (Guinier-Preston zones) in the former alloy and in the early phase separation stage in the latter. There are two possible mechanisms of the AE in the course of the phase separation: (i) dynamic energy relaxation related to spontaneous motions of the interphase boundary or (ii) plastic relaxation of the elastic energy related to the nucleation of misfit dislocations at the sites of coherency breakage.

Acoustic emission during heating of strained aluminum

The phenomenon of acoustic emission during annealing of preliminarily strained polycrystalline aluminum was observed. Although processes accompanying the metal annealing belong to thermoactivated (microscopic motion, diffusion) phenomena, the acoustic oscillations are generated in the events of spontaneous macroscopic motions of grain boundaries. This is confirmed by the results of analyzing the effective activation energy and the investigation of the grain boundary structure.

Toddlers' Postural Adaptations to Different Support Surfaces

This study was undertaken to determine whether young children, after only a few weeks standing experience, could respond adaptively to the dynamical constraints imposed by different support surfaces. The spontaneous postural motions of young children (13-14 months old) were observed as they stood on surfaces that differed in length, friction, and rigidity. There were no externally imposed perturbations to stance. Children's postural control was remarkably adaptive: There were few falls on any of the surfaces. Moreover, the children showed surface-specific utilization of manual postural control (holding onto wooden poles), suggesting that manual control is an adaptive strategy for postural control. Finally, kinematic analysis suggested that, in some instances, children were able to employ independent control of the hips, contrary to previous models which had suggested that hip motions could not be controlled before the age of 3 years. Small, slow hip movements useful in controlling spontaneous sway (unperturbed stance) may serve as a basis for the development of larger, faster hip movements that are associated with imposed perturbations.

Passive Deformations and Active Motions of Leukocytes

The purpose of this paper is to review the development of continuum mechanics models of single leukocytes in both passive deformations and active motions and to indicate some future directions. Models of passive deformations describe the overall rheological behavior of single leukocytes under externally applied forces and predict the average mechanical properties from experimental data. Various "apparent" viscoelastic coefficients are obtained depending on the models assumed and the types of test used. Models of spontaneous motions postulate active driving mechanisms which must be derived internally from the cell itself and probably have different bases for different kind of motions. For pseudopod protrusion on leukocytes, energy transduction from chemical potential to mechanical work associated with actin polymerization at the tip of the projection is assumed to supply the motive power. For pseudopod retraction, active contraction due to actin-myosin interaction is assumed to be the driving force. The feasibility of the hypotheses are tested via numerical examples and comparison of the theoretical results with experimental measurements.

Kinetics and instabilities in the extraction of Co(II) by sodium di-(2-ethylhexyl) phosphate at the water/toluene interface: I. Kinetics and mechanism

A toluenic solution of sodium di-(2-ethylhexyl) phosphate (NaD2EHP) was allowed to react with an aqueous solution of Co(II) in a nonagitated system. Two layers of NaD2EHP emulsions in water appeared: a dilute emulsion A and an opaque emulsion B located below the toluenic phase. Furthermore, spontaneous convective motions were observed. The kinetics of complexation was investigated by spectrophotometry. The influence of four factors was studied: the Co 2+ concentration C m, the NaD2EHP concentration C d, the inert anti-emulsifier (KBr) concentration C e, and the hydration degree I h of the surfactant. When C m was increased, the reaction rate w exhibited a maximum at C m = 15 moles m −3 and then a minimum at C m = 30 moles m −3. The whole dependence w( C m, C d, C e, I h) is explained by a double location of a superficial reaction with proper reaction orders of 1 relative to Co 2+ and 0 relative to NaD2EHP. The results show that, in the absence of forced convection, the rate w of the Co(II) extraction is the sum w 1 + w 2 where w 1 is the reaction rate at the surface of the droplets in layer B and w 2 represents the reaction rate at the macroscopic W/O interface. An increase in C m resulted in a strong reduction of the intensity E of the emulsification. Conversely, an increase in E led to an increase in the droplet area and therefore an increase in w 1, while w 2 was reduced via an increase in the resistance to the diffusion of Co 2+ through the aqueous interstices of emulsion B.

Effect of calcium paradox on spontaneous sarcomere motions in isolated rat cardiac muscle

Spontaneous sarcomere motions in isolated rat cardiac muscle were measured by recording the light intensity at a point of magnified muscle image. The spontaneous sarcomere motions were located near the cut end, and not evenly distributed along the muscle. Calcium depletion markedly decreased the amplitude of the light intensity fluctuations. In all cases, calcium readmission after 5 min calcium depletion (calcium paradox), which has been known to injure myocardium, increased the amplitude of the light intensity fluctuation than that under control conditions. Thus, cell injury enhances spontaneous sarcomere motions. While it remains unclear whether or not cell injury is a prerequisite for spontaneous sarcomere motions, under certain conditions the area of myocardial damage can be detected by the existence of spontaneous sarcomere motions.

The contribution of chemistry to new Marangoni mass-transfer instabilities at the oil/water interface

Spontaneous interfacial motions appear at an interface between two immiscible phases in a state far from equilibrium, e.g. an aqueous phase of an organic acid or a complex ion and an organic phase of a long-chain surfactant molecule. The instabilities observed are related to variations in the interfacial tension. It is shown that they resemble the well known Marangoni effect by considering interfacial convection and the coupling between diffusion and convection fluxes, but they differ by the presence of chemical reactions. The notion of assisted desorption is defined in order to interpret the experiments.The analysis of this phenomenon could be useful in determining the optimal conditions for obtaining convective interfacial transfer to enhance liquid–liquid extraction processes.

Scattered-light intensity fluctuations in diastolic rat cardiac muscle caused by spontaneous Ca++-dependent cellular mechanical oscillations.

Laser light scattered by nonstimulated rat cardiac muscle bathed in physiological saline containing a [Ca++] of 0.4-2.5 mM displays scattered-light intensity fluctuations (SLIF); the frequencies of both SLIF and resting force are Ca++ dependent. Direct inspection of these muscles by phase-contrast microscopy under incoherent illumination revealed the presence of spontaneous asynchronous cellular motions that are also Ca++ dependent. The physical properties of the scattered light are compatible with the hypothesis that SLIF are due to the diastolic motion, except for the dependence on scattering angle, which may be perturbed because the muscles are optically thick. To determine whether diastolic SLIF and motion are an intrinsic property of activated myofilaments, photon-counting auto-correlation of the scattered light was performed both in rat right-ventricular papillary muscles skinned with the detergent Triton X-100 (1%) and in muscles with intact membranes under conditions that alter cellular Ca++ fluxes. In skinned muscles activated over a range of Ca++ from threshold to maximum force production, neither SLIF nor asynchronous motion was observed when Ca++ was buffered to constant values. In intact muscles the frequency of SLIF and the amplitude of diastolic motion were (a) markedly increased by substituting K+ or Li+ for Na+ in the bath; (b) not altered by verapamil (1 microM); and (c) reversibly abolished by caffeine (greater than or equal to 10 mM). These properties are exactly those of mechanical oscillations that have been observed in isolated cardiac cell fragments, which are the result Ca++ oscillations caused by Ca++ release from the sarcoplasmic reticulum (SR). We infer that mechanical oscillations caused by spontaneous Ca++-induced Ca++ release from the SR occur in intact nonstimulated cardiac muscle even in the absence of Ca++ overload and are the principle cause of SLIF, and that myoplasmic [Ca++] in "resting" muscle is not in a microscopic steady state.

Ultrasonic absorption evidence of structural fluctuations in viral capsids.

When the coat protein of the small icosahedral virus, brome mosaic virus, reassembles into capsids, the ultrasonic absorption of the solution greatly increases. Submitting the solution to an ultrasonic field thus appears to reveal spontaneous molecular motions within a protein assembly. Confirmatory evidence of a dynamics of a protein shell comes from measurements on brome mosaic virus at various degrees of swelling and on tomato bushy stunt virus treated with the crosslinking agent glutaraldehyde. The detected fluctuations may be related either with cooperative deformational motion in the capsid or with more localized structural changes. Such structural changes may help liberate the RNA at an early stage of viral infection.

Posture Changes in Continuously Working Conditions

In enduring a posture in actual living conditions, spontaneously appearing posture changes are considered to play an important role, In this paper, patterns of subsidiary postural changes while working sustaining a certain posture for hours were discussed by analyzing the factors leading to increase of those changes. During a prolonged experimental work period of numeral inspection that demanded short-term memories, variations in frequency of posture changes were apparently dependent on the day's condition and changes in alertness. The rate of those postural changes which recurred at variable intervals of one to several minutes was revealed to be greatly influenced by when and how long the state of sustained attention was affected. In the case of printing workers working while continuously standing, the patterns of posture changes differed between different types of standing conditions. Postural changes enabling resting of the lower extremities or active muscle groups increased during later hours in the afternoon as a result of compensating fatigue, the leg motion rate being significantly related with the rate of subsidiary upper limb activities, whereas the spontaneous leg motions of workeres inserting work-oriented steppings were less frequent and had no correlation with upper limb motions. Postural changes of sitting key-hoard operators were much less and related with other types of subsidiary behavior. These results indicate that the patterns of postural changes are a function of complex factors of the task performing situation that requires a surtained position, and that changes in the central nervous system and in orienting behavior being relevant to motor control functions should be taken into account. Further studies are needed to elucidate conditions of provoking spontaneous changes in human posture in order to discuss their relevance to the mechanism of sustaining an erect standing or a sitting posture for a long period.