Diffusing wave spectroscopy microrheology of actin filament networks.

Filamentous actin (F-actin), one of the constituents of the cytoskeleton, is believed to be the most important participant in the motion and mechanical integrity of eukaryotic cells. Traditionally, the viscoelastic moduli of F-actin networks have been measured by imposing a small mechanical strain and quantifying the resulting stress. The magnitude of the viscoelastic moduli, their concentration dependence and strain dependence, as well as the viscoelastic nature (solid-like or liquid-like) of networks of uncross-linked F-actin, have been the subjects of debate. Although this paper helps to resolve the debate and establishes the extent of the linear regime of F-actin networks' rheology, we report novel measurements of the high-frequency behavior of networks of F-actin, using a noninvasive light-scattering based technique, diffusing wave spectroscopy (DWS). Because no external strain is applied, our optical assay generates measurements of the mechanical properties of F-actin networks that avoid many ambiguities inherent in mechanical measurements. We observe that the elastic modulus has a small magnitude, no strain dependence, and a weak concentration dependence. Therefore, F-actin alone is not sufficient to generate the elastic modulus necessary to sustain the structural rigidity of most cells or support new cellular protrusions. Unlike previous studies, our measurements show that the mechanical properties of F-actin are highly dependent on the frequency content of the deformation. We show that the loss modulus unexpectedly dominates the elastic modulus at high frequencies, which are key for fast transitions. Finally, the measured mean square displacement of the optical probes, which is also generated by DWS measurements, offers new insight into the local bending fluctuations of the individual actin filaments and shows how they generate enhanced dissipation at short time scales.     

Characterization of cytoskeleton mechanical properties and 3D-actin structure in twisted adherent epithelial cells

Evaluation of the cytoskeleton mechanical properties requires specific micromanipulation techniques such as the magnetic twisting cytometry technique, in which microbeads are specifically linked to the cytoskeleton via transmembrane receptors. The aim of the study was to assess the structural relationship between the bead and the cytoskeleton structure. The spatial arrangement of the CSK network was therefore studied in fixed cells probed by beads and stained for F-actin by rhodamined phallo?dine. The spatial character of the actin CSK network, both in the bead neighborhood and at the cell scale, could then be studied for various degrees of fluorescent intensity from 3D-images of the actin structure, reconstructed from z-stack views obtained by confocal microscopy. Results show the feasibility of the staining/reconstruction technique which allows to reveal the three-dimensional organization of the cytoskeleton structure including an internal cytosolic structure with a high fluorescent F-actin intensity, and a sub-membranous cortical structure with a low fluorescent F-actin intensity.     

Mechanics and multiple-particle tracking microheterogeneity of alpha-actinin-cross-linked actin filament networks

Cell morphology is controlled by the actin cytoskeleton organization and mechanical properties, which are regulated by the available contents in actin and actin regulatory proteins. Using rheometry and the recently developed multiple-particle tracking method, we compare the mechanical properties and microheterogeneity of actin filament networks containing the F-actin cross-linking protein alpha-actinin. The elasticity of F-actin/alpha-actinin networks increases with actin concentration more rapidly for a fixed molar ratio of actin to alpha-actinin than in the absence of alpha-actinin, for networks of fixed alpha-actinin concentration and of fixed actin concentration, but more slowly than theoretically predicted for a homogeneous cross-linked semiflexible polymer network. These rheological measurements are complemented by multiple-particle tracking of fluorescent microspheres imbedded in the networks. The distribution of the mean squared displacements of these microspheres becomes progressively more asymmetric and wider for increasing concentration in alpha-actinin and, to a lesser extent, for increasing actin concentration, which suggests that F-actin networks become progressively heterogeneous for increasing protein content. This may explain the slower-than-predicted rise in elasticity of F-actin/alpha-actinin networks. Together these in vitro results suggest that actin and alpha-actinin provides the cell with an unsuspected range of regulatory pathways to modulate its cytoskeleton's micromechanics and local organization in vivo.     

Torsional motion of eosin-labeled F-actin as detected in the time-resolved anisotropy decay of the probe in the sub-millisecond time range

The internal motion of F-actin in the time range from 10(-6) to 10(-3) second has been explored by measuring the transient absorption anisotropy of eosin-labeled F-actin using laser flash photolysis. The transient absorption anisotropy of eosin-F-actin at 20 degrees C has a component that decays in the submicrosecond time scale to an anisotropy of about 0.3. This anisotropy then decays with a relaxation time of about 450 microseconds to a residual anisotropy of about 0.1 after 2 ms. When the concentration of eosin-F-actin was varied in the range from 7 to 28 microM, the transient absorption anisotropy curves obtained were almost indistinguishable from each other. These results show that the anisotropy decay arises from internal motion of eosin-F-actin. Analysis of the transient absorption anisotropy curves indicates that the internal motion detected by the decay in anisotropy is primarily a twisting of actin protomers in the F-actin helix; bending of the actin filament makes a minor contribution only to the measured decay. The torsional rigidity calculated from the transient absorption anisotropy is 0.2 X 10(-17) dyn cm2 at 20 degrees C, which is about an order of magnitude smaller than the flexural rigidity determined from previous studies. Thus, we conclude that F-actin is more flexible in twisting than in bending. The calculated root-mean-square fluctuation of the torsional angle between adjacent actin protomers in the actin helix is about 4 degrees at 20 degrees C. We also found that the torsional rigidity is approximately constant in the temperature range from 5 to approximately 35 degrees C, and that the binding of phalloidin does not appreciably affect the torsional motion of F-actin.     

Cell Elasticity Is Regulated by the Tropomyosin Isoform Composition of the Actin Cytoskeleton

The actin cytoskeleton is the primary polymer system within cells responsible for regulating cellular stiffness. While various actin binding proteins regulate the organization and dynamics of the actin cytoskeleton, the proteins responsible for regulating the mechanical properties of cells are still not fully understood. In the present study, we have addressed the significance of the actin associated protein, tropomyosin (Tpm), in influencing the mechanical properties of cells. Tpms belong to a multi-gene family that form a co-polymer with actin filaments and differentially regulate actin filament stability, function and organization. Tpm isoform expression is highly regulated and together with the ability to sort to specific intracellular sites, result in the generation of distinct Tpm isoform-containing actin filament populations. Nanomechanical measurements conducted with an Atomic Force Microscope using indentation in Peak Force Tapping in indentation/ramping mode, demonstrated that Tpm impacts on cell stiffness and the observed effect occurred in a Tpm isoform-specific manner. Quantitative analysis of the cellular filamentous actin (F-actin) pool conducted both biochemically and with the use of a linear detection algorithm to evaluate actin structures revealed that an altered F-actin pool does not absolutely predict changes in cell stiffness. Inhibition of non-muscle myosin II revealed that intracellular tension generated by myosin II is required for the observed increase in cell stiffness. Lastly, we show that the observed increase in cell stiffness is partially recapitulated in vivo as detected in epididymal fat pads isolated from a Tpm3.1 transgenic mouse line. Together these data are consistent with a role for Tpm in regulating cell stiffness via the generation of specific populations of Tpm isoform-containing actin filaments.     

MARCKS-related protein binds to actin without significantly affecting actin polymerization or network structure. Myristoylated alanine-rich C kinase substrate

Actinis a 42-kDa protein which, due to its ability to polymerize into filaments (F-actin), is one of the major constituents of the cytoskeleton. It has been proposed that MARCKS (an acronym for myristoylated alanine-rich C kinase substrate) proteins play an important role in regulating the structure and mechanical properties of the actin cytoskeleton by cross-linking actin filaments. We have recently reported that peptides corresponding to the effector domain of MARCKS proteins promote actin polymerization and cause massive bundling of actin filaments. We now investigate the effect of MARCKS-related protein, a 20-kDa member of the MARCKS family, on both filament structure and the kinetics of actin polymerization in vitro. Our experiments document that MRP binds to F-actin with micromolar affinity and that the myristoyl chain at the N-terminus of MRP is not required for this interaction. In marked contrast to the effector peptide, binding of MRP is not accompanied by an acceleration of actin polymerization kinetics, and we also could not reliably observe an actin cross-linking activity of MRP.     

Strain hardening of actin filament networks. Regulation by the dynamic cross-linking protein alpha-actinin

Mechanical stresses applied to the plasma membrane of an adherent cell induces strain hardening of the cytoskeleton, i.e. the elasticity of the cytoskeleton increases with its deformation. Strain hardening is thought to mediate the transduction of mechanical signals across the plasma membrane through the cytoskeleton. Here, we describe the strain dependence of a model system consisting of actin filaments (F-actin), a major component of the cytoskeleton, and the F-actin cross-linking protein alpha-actinin, which localizes along contractile stress fibers and at focal adhesions. We show that the amplitude and rate of shear deformations regulate the resilience of F-actin networks. At low temperatures, for which the lifetime of binding of alpha-actinin to F-actin is long, F-actin/alpha-actinin networks exhibit strong strain hardening at short time scales and soften at long time scales. For F-actin networks in the absence of alpha-actinin or for F-actin/alpha-actinin networks at high temperatures, strain hardening appears only at very short time scales. We propose a model of strain hardening for F-actin networks, based on both the intrinsic rigidity of F-actin and dynamic topological constraints formed by the cross-linkers located at filaments entanglements. This model offers an explanation for the origin of strain hardening observed when shear stresses are applied against the cellular membrane.     

Local call: from integrins to actin assembly

Integrins link the extracellular matrix to the actin cytoskeleton by triggering the assembly of different types of adhesion complex. One of their major components is filamentous actin (F-actin), and they are important signaling hubs for actin cytoskeleton reorganization in response to chemical and mechanical signals. In an exciting publication, Butler et al. have demonstrated for the first time that purified adhesion complexes possess the entire machinery necessary to actively assemble F-actin as a function of integrin activity and clustering.     

Biomechanics of actin filaments: a computational multi-level study

The actin microfilament (F-actin) is a structural and functional component of the cell cytoskeleton. Notwithstanding the primary role it plays for the mechanics of the cell, the mechanical behaviour of F-actin is still not totally explored. In particular, the relationship between the mechanics of F-actin and its molecular architecture is not completely understood. In this study, the mechanical properties of F-actin were related to the molecular topology of its building monomers (G-actin) by employing a computational multi-level approach. F-actins with lengths up to 500 nm were modelled and characterized, using a combination of equilibrium molecular dynamics (MD) simulations and normal mode analysis (NMA). MD simulations were performed to analyze the molecular rearrangements of G-actin in physiological conditions; NMA was applied to compute the macroscopic properties of F-actin from its vibrational modes of motion. Results from this multi-level approach showed that bending stiffness, bending modulus and persistence length are independent from the length of F-actin. On the contrary, the orientations and motions of selected groups of residues of G-actin play a primary role in determining the filament flexibility. In conclusion, this study (i) demonstrated that a combined computational approach of MD and NMA allows to investigate the biomechanics of F-actin taking into account the molecular topology of the filament (i.e., the molecular conformations of G-actin) and (ii) that this can be done using only crystallographic G-actin, without the need of introducing experimental parameters nor of reducing the number of residues.     

F-actin慢生长端的盖帽蛋白:红细胞原肌球调节蛋白

红细胞原肌球调节蛋白(erythrocyte tropomodulin,E-Tmod)是从红细胞膜中提取的原肌球蛋白(tropomyosin,TM)的结合蛋白.其N-端有两个TM结合位点和一个TM依赖的actin结合位点,C-端有5个富含亮氨酸的重复序列和一个TM非依赖的actin结合位点.作为F-actin慢生长端唯一的盖帽蛋白,E-Tmod与TM的N-端结合并同时与actin结合,减慢由TM包被的F-actin的解聚速度.E-Tmod编码基因高度保守,在红细胞、心肌细胞等细胞中广泛表达.E-Tmod对于F-actin和细胞骨架的组织以及对细胞力学特性的保持具有至关重要的作用.     

Latrunculin B has different effects on pollen germination and tube growth

The actin cytoskeleton is absolutely required for pollen germination and tube growth, but little is known about the regulation of actin polymer concentrations or dynamics in pollen. Here, we report that latrunculin B (LATB), a potent inhibitor of actin polymerization, had effects on pollen that were distinct from those of cytochalasin D. The equilibrium dissociation constant measured for LATB binding to maize pollen actin was determined to be 74 nM. This high affinity for pollen actin suggested that treatment of pollen with LATB would have marked effects on actin function. Indeed, LATB inhibited maize pollen germination half-maximally at 50 nM, yet it blocked pollen tube growth at one-tenth of that concentration. Low concentrations of LATB also caused partial disruption of the actin cytoskeleton in germinated maize pollen, as visualized by light microscopy and fluorescent-phalloidin staining. The amounts of filamentous actin (F-actin) in pollen were quantified by measuring phalloidin binding sites, a sensitive assay that had not been used previously for plant cells. The amount of F-actin in maize pollen increased slightly upon germination, whereas the total actin protein level did not change. LATB treatment caused a dose-dependent depolymerization of F-actin in populations of maize pollen grains and tubes. Moreover, the same concentrations of LATB caused similar depolymerization in pollen grains before germination and in pollen tubes. These data indicate that the increased sensitivity of pollen tube growth to LATB was not due to general destabilization of the actin cytoskeleton or to decreases in F-actin amounts after germination. We postulate that germination is less sensitive to LATB than tube extension because the presence of a small population of LATB-sensitive actin filaments is critical for maintenance of tip growth but not for germination of pollen, or because germination is less sensitive to partial depolymerization of the actin cytoskeleton.     

The effect of the 540-kilodalton actin cross-linking protein, actin-binding protein, on the mechanical properties of F-actin

This study describes the effect of actin-binding protein derived from rabbit lung macrophages on the mechanical properties of F-actin. The dynamic storage modulus, G'(omega), and loss modulus, G"(omega) of F-actin, at concentrations from 1 to 4 mg/ml, in the absence or presence of actin-binding protein at molar ratios to actin of 1:1000 to 1:125, were measured at frequencies ranging from 3 X 10(-3) to 0.5 Hz. Actin-binding protein increased the dynamic moduli of F-actin, but this increase was much greater as either the actin-binding protein/actin ratio or the total protein concentration increased. Moreover, there was a convergence of the values of G' and G" at high frequencies for F-actin which became more prominent upon the addition of actin-binding protein. The value of the modulus obtained by an extrapolation of these data to actin concentrations similar to that found in the cell cortex was close to values which have been obtained by direct measurements. The addition of actin-binding protein to an F-actin solution enabled it to reach an equilibrium strain following the application of a stress, in contrast to pure F-actin. These data allow a more rigorous definition of the "sol" to "gel" transition and suggest that the cross-linking of actin filaments by actin-binding protein leads to the formation of a network structure whose underlying mechanism of mechanical behavior is short range intrafilament bending in contrast to the classical rubber network.     

Mobility of Molecular Motors Regulates Contractile Behaviors of Actin Networks

Cells generate mechanical forces primarily from interactions between F-actin, cross-linking proteins, myosin motors, and other actin-binding proteins in the cytoskeleton. To understand how molecular interactions between the cytoskeletal elements generate forces, a number of in vitro experiments have been performed but are limited in their ability to accurately reproduce the diversity of motor mobility. In myosin motility assays, myosin heads are fixed on a surface and glide F-actin. By contrast, in reconstituted gels, the motion of both myosin and F-actin is unrestricted. Because only these two extreme conditions have been used, the importance of mobility of motors for network behaviors has remained unclear. In this study, to illuminate the impacts of motor mobility on the contractile behaviors of the actin cytoskeleton, we employed an agent-based computational model based on Brownian dynamics. We find that if motors can bind to only one F-actin like myosin I, networks are most contractile at intermediate mobility. In this case, less motor mobility helps motors stably pull F-actins to generate tensile forces, whereas higher motor mobility allows F-actins to aggregate into larger clustering structures. The optimal intermediate motor mobility depends on the stall force and affinity of motors that are regulated by mechanochemical rates. In addition, we find that the role of motor mobility can vary drastically if motors can bind to a pair of F-actins. A network can exhibit large contraction with high motor mobility because motors bound to antiparallel pairs of F-actins can exert similar forces regardless of their mobility. Results from this study imply that the mobility of molecular motors may critically regulate contractile behaviors of actin networks in cells.     

The cytoskeleton of the resting human blood platelet: structure of the membrane skeleton and its attachment to actin filaments

We used high-resolution EM and immunocytochemistry in combination with different specimen preparation techniques to resolve the ultrastructure of the resting platelet cytoskeleton. The periphery of the cytoskeleton, an electron-dense subplasmalemmal region in thin section electron micrographs, is a tightly woven planar sheet composed of a spectrin-rich network whose interstices contain GPIb/IX-actin-binding protein (ABP) complexes. This membrane skeleton connects to a system of curved actin filaments (F-actin) that emanate from a central oval core of F-actin cross-linked by ABP. The predominant interaction of the radial actin filaments with the membrane skeleton is along their sides, and the strongest connection between the membrane skeleton and F-actin is via ABP-GPIb ligands, although there is evidence for spectrin attaching to the ends of the radial actin filaments as well. Since a mechanical separation of the F-actin cores and radial F-actin-GPIb-ABP complexes from the underlying spectrin-rich skeleton leads to the latter's expansion, it follows that the spectrin-based skeleton of the resting cell may be held in a compressed form by interdigitating GPIb/IX complexes which are immobilized by radial F-actin-ABP anchors.     

Endocytosis and the development of cell polarity in yeast require a dynamic F-actin cytoskeleton

Studies using drugs that cause the disassembly of filamentous actin (F-actin) have demonstrated the importance of an intact actin cytoskeleton for polarised secretion by yeast cells [1,2]. To address the level of dynamic turnover needed for such processes, however, drugs or mutants that confer stabilising properties on F-actin are needed. Jasplakinolide is the only readily available drug that stabilises F-actin structures both in vivo and in vitro [3-6]. Yeast strains have been generated in which two of the ABC multidrug resistance transporter genes have been deleted, rendering normally jasplakinolide-resistant yeast cells sensitive to its effects. Treatment of these cells with jasplakinolide caused rapid and dramatic effects on the actin cytoskeleton, resulting in the accumulation of single large actin structures in cells. These structures, however, still contained components that are normally associated with cortical actin patches. A dynamic actin cytoskeleton was found to be critical for the generation of cell polarity and endocytosis.     

Micromechanics and ultrastructure of actin filament networks crosslinked by human fascin: a comparison with alpha-actinin.

Fascin is an actin crosslinking protein that organizes actin filaments into tightly packed bundles believed to mediate the formation of cellular protrusions and to provide mechanical support to stress fibers. Using quantitative rheological methods, we studied the evolution of the mechanical behavior of filamentous actin (F-actin) networks assembled in the presence of human fascin. The mechanical properties of F-actin/fascin networks were directly compared with those formed by alpha-actinin, a prototypical actin filament crosslinking/bundling protein. Gelation of F-actin networks in the presence of fascin (fascin to actin molar ratio >1:50) exhibits a non-monotonic behavior characterized by a burst of elasticity followed by a slow decline over time. Moreover, the rate of gelation shows a non-monotonic dependence on fascin concentration. In contrast, alpha-actinin increased the F-actin network elasticity and the rate of gelation monotonically. Time-resolved multiple-angle light scattering and confocal and electron microscopies suggest that this unique behavior is due to competition between fascin-mediated crosslinking and side-branching of actin filaments and bundles, on the one hand, and delayed actin assembly and enhanced network micro-heterogeneity, on the other hand. The behavior of F-actin/fascin solutions under oscillatory shear of different frequencies, which mimics the cell's response to forces applied at different rates, supports a key role for fascin-mediated F-actin side-branching. F-actin side-branching promotes the formation of interconnected networks, which completely inhibits the motion of actin filaments and bundles. Our results therefore show that despite sharing seemingly similar F-actin crosslinking/bundling activity, alpha-actinin and fascin display completely different mechanical behavior. When viewed in the context of recent microrheological measurements in living cells, these results provide the basis for understanding the synergy between multiple crosslinking proteins, and in particular the complementary mechanical roles of fascin and alpha-actinin in vivo.     

Multiple-particle tracking measurements of heterogeneities in solutions of actin filaments and actin bundles

One of the central functions of actin cytoskeleton is to provide the mechanical support required for the establishment and maintenance of cell morphology. The mechanical properties of actin filament assemblies are a consequence of both the available polymer concentration and the actin regulatory proteins that direct the formation of higher order structures. By monitoring the displacement of well-dispersed microspheres via fluorescence microscopy, we probe the degree of spatial heterogeneity of F-actin gels and networks in vitro. We compare the distribution of the time-dependent mean-square displacement (MSD) of polystyrene microspheres imbedded in low- and high-concentration F-actin solutions, in the presence and absence of the F-actin-bundling protein fascin. The MSD distribution of a 2. 6-microM F-actin solution is symmetric and its standard deviation is similar to that of a homogeneous solution of glycerol of similar zero-shear viscosity. However, increasing actin concentration renders the MSD distribution wide and asymmetric, an effect enhanced by fascin. Quantitative changes in the shape of the MSD distribution correlate qualitatively with the presence of large heterogeneities in F-actin solutions produced by increased filament concentration and the presence of actin bundles, as detected by confocal microscopy. Multiple-particle tracking offers a new, quantitative method to characterize the organization of biopolymers in solution.     

MARCKS-Related Protein Binds to Actin without Significantly Affecting Actin Polymerization or Network Structure

Actinis a 42-kDa protein which, due to its ability to polymerize into filaments (F-actin), is one of the major constituents of the cytoskeleton. It has been proposed that MARCKS (an acronym for myristoylated alanine-rich C kinase substrate) proteins play an important role in regulating the structure and mechanical properties of the actin cytoskeleton by cross-linking actin filaments. We have recently reported that peptides corresponding to the effector domain of MARCKS proteins promote actin polymerization and cause massive bundling of actin filaments. We now investigate the effect of MARCKS-related protein, a 20-kDa member of the MARCKS family, on both filament structure and the kinetics of actin polymerization in vitro. Our experiments document that MRP binds to F-actin with micromolar affinity and that the myristoyl chain at the N-terminus of MRP is not required for this interaction. In marked contrast to the effector peptide, binding of MRP is not accompanied by an acceleration of actin polymerization kinetics, and we also could not reliably observe an actin cross-linking activity of MRP.     

The mechanics of F-actin microenvironments depend on the chemistry of probing surfaces

To understand the microscopic mechanical properties of actin networks, we monitor the motion of embedded particles with controlled surface properties. The highly resolved Brownian motions of these particles reveal the viscoelastic character of the microenvironments around them. In both non-cross-linked and highly cross-linked actin networks, particles that bind F-actin report viscoelastic moduli comparable to those determined by macroscopic rheology experiments. By contrast, particles modified to prevent actin binding have weak microenvironments that are surprisingly insensitive to the introduction of filament cross-links. Even when adjacent in the same cross-linked gel, actin-binding and nonbinding particles report viscoelastic moduli that differ by two orders of magnitude at low frequencies (0.5-1.5 rad/s) but converge at high frequencies (> 10(4) rad/s). For all particle chemistries, electron and light microscopies show no F-actin recruitment or depletion, so F-actin microheterogeneities cannot explain the deep penetration (approximately 100 nm) of nonbinding particles. Instead, we hypothesize that a local depletion of cross-linking around nonbinding particles explains the phenomena. With implications for organelle mobility in cells, our results show that actin binding is required for microenvironments to reflect macroscopic properties, and conversely, releasing actin enhances particle mobility beyond the effects of mere biochemical untethering.     

The subspine organization of actin fibers regulates the structure and plasticity of dendritic spines

Synapse function and plasticity depend on the physical structure of dendritic spines as determined by the actin cytoskeleton. We have investigated the organization of filamentous (F-) actin within individual spines on CA1 pyramidal neurons in rat hippocampal slices. Using two-photon photoactivation of green fluorescent protein fused to beta-actin, we found that a dynamic pool of F-actin at the tip of the spine quickly treadmilled to generate an expansive force. The size of a stable F-actin pool at the base of the spine depended on spine volume. Repeated two-photon uncaging of glutamate formed a third pool of F-actin and enlarged the spine. The spine often released this "enlargement pool" into the dendritic shaft, but the pool had to be physically confined by a spine neck for the enlargement to be long-lasting. Ca2+/calmodulin-dependent protein kinase II regulated this confinement. Thus, spines have an elaborate mechanical nature that is regulated by actin fibers.