Mechanical properties of cytoskeletal polymers

Mechanosensitivity of phase separation in an elastic gel

Liquid-liquid phase separation (LLPS) in binary or multi-component solutions is a well-studied subject in soft matter with extensive applications in biological systems. In recent years, several experimental studies focused on LLPS of solutes in hydrated gels, where the formation of coexisting domains induces elastic deformations within the gel. While the experimental studies report unique physical characteristics of these systems, such as sensitivity to mechanical forces and stabilization of multiple, periodic phase-separated domains, the theoretical understanding of such systems and the role of long-range interactions have not emphasized the nonlinear nature of the equilibrium binodal for strong segregation of the solute. In this paper, we formulate a generic, mean-field theory of a hydrated gel in the presence of an additional solute which changes the elastic properties of the gel. We derive equations for the equilibrium binodal of the phase separation of the solvent and solute and show that the deformations induced by the solute can result in effective long-range interactions between phase-separating solutes that can either enhance or, in the case of externally applied pressure, suppress phase separation of the solute relative to the case where there is no gel. This causes the coexisting concentrations at the binodal to depend on the system-wide average concentration, in contrast to the situation for phase separation in the absence of the gel.

Structural Features and Nonlinear Rheology of Self-Assembled Networks of Cross-Linked Semiflexible Polymers

Disordered networks of semiflexible filaments are common support structures in biology. Familiar examples include fibrous matrices in blood clots, bacterial biofilms, and essential components of cells and tissues of plants, animals, and fungi. Despite the ubiquity of these networks in biomaterials, we have only a limited understanding of the relationship between their structural features and their highly strain-sensitive mechanical properties. In this work, we perform simulations of three-dimensional networks produced by the irreversible formation of cross-links between linker-decorated semiflexible filaments. We characterize the structure of networks formed by a simple diffusion-dependent assembly process and measure their associated steady-state rheological features at finite temperature over a range of applied prestrains that encompass the strain-stiffening transition. We quantify the dependence of network connectivity on cross-linker availability and detail the associated connectivity dependence of both linear elasticity and nonlinear strain-stiffening behavior, drawing comparisons with prior experimental measurements of the cross-linker concentration-dependent elasticity of actin gels.

The nonlinear motion of cells subject to external forces

To develop a minimal model for a cell moving in a crowded environment such as in tissue, we investigate the response of a liquid drop of active matter moving on a flat rigid substrate to forces applied at its boundaries. We consider two different self-propulsion mechanisms, active stresses and treadmilling polymerisation, and we investigate how the active drop motion is altered by these surface forces. We find a highly non-linear response to forces that we characterise using drop velocity, drop shape, and the traction between the drop and the substrate. Each self-propulsion mechanism gives rise to two main modes of motion: a long thin drop with zero traction in the bulk, mostly occurring under strong stretching forces, and a parabolic drop with finite traction in the bulk, mostly occurring under strong squeezing forces. In each case there is a sharp transition between parabolic, and long thin drops as a function of the applied forces and indications of drop break-up where large forces stretch the drop.

Cell wall and cytoskeletal contributions in single cell biomechanics of Nicotiana tabacum

Studies on the mechanics of plant cells usually focus on understanding the effects of turgor pressure and properties of the cell wall (CW). While the functional roles of the underlying cytoskeleton have been studied, the extent to which it contributes to the mechanical properties of cells is not elucidated. Here, we study the contributions of the CW, microtubules (MTs) and actin filaments (AFs), in the mechanical properties of Nicotiana tabacum cells. We use a multiscale biomechanical assay comprised of atomic force microscopy and micro-indentation in solutions that (i) remove MTs and AFs and (ii) alter osmotic pressures in the cells. To compare measurements obtained by the two mechanical tests, we develop two generative statistical models to describe the cell's behaviour using one or both datasets. Our results illustrate that MTs and AFs contribute significantly to cell stiffness and dissipated energy, while confirming the dominant role of turgor pressure.

Protein Assembly by Design

Proteins are nature's primary building blocks for the construction of sophisticated molecular machines and dynamic materials, ranging from protein complexes such as photosystem II and nitrogenase that drive biogeochemical cycles to cytoskeletal assemblies and muscle fibers for motion. Such natural systems have inspired extensive efforts in the rational design of artificial protein assemblies in the last two decades. As molecular building blocks, proteins are highly complex, in terms of both their three-dimensional structures and chemical compositions. To enable control over the self-assembly of such complex molecules, scientists have devised many creative strategies by combining tools and principles of experimental and computational biophysics, supramolecular chemistry, inorganic chemistry, materials science, and polymer chemistry, among others. Owing to these innovative strategies, what started as a purely structure-building exercise two decades ago has, in short order, led to artificial protein assemblies with unprecedented structures and functions and protein-based materials with unusual properties. Our goal in this review is to give an overview of this exciting and highly interdisciplinary area of research, first outlining the design strategies and tools that have been devised for controlling protein self-assembly, then describing the diverse structures of artificial protein assemblies, and finally highlighting the emergent properties and functions of these assemblies.

Opening doors with ultrasound and microbubbles: Beating biological barriers to promote drug delivery

Apart from its clinical use in imaging, ultrasound has been thoroughly investigated as a tool to enhance drug delivery in a wide variety of applications. Therapeutic ultrasound, as such or combined with cavitating nuclei or microbubbles, has been explored to cross or permeabilize different biological barriers. This ability to access otherwise impermeable tissues in the body makes the combination of ultrasound and therapeutics very appealing to enhance drug delivery in situ. This review gives an overview of the most important biological barriers that can be tackled using ultrasound and aims to provide insight on how ultrasound has shown to improve accessibility as well as the biggest hurdles. In addition, we discuss the clinical applicability of therapeutic ultrasound with respect to the main challenges that must be addressed to enable the further progression of therapeutic ultrasound towards an effective, safe and easy-to-use treatment tailored for drug delivery in patients.

Critical Frequency and Critical Stretching Rate for Reorientation of Cells on a Cyclically Stretched Polymer in a Microfluidic Chip

The ability of cells to sense and respond to mechanical signals from their surrounding microenvironments is one of the key issues in tissue engineering and regeneration, yet a fundamental study of cells with both cell observation and mechanical stimulus is challenging and should be based upon an appropriate microdevice. Herein we designed and fabricated a two-layer microfluidic chip to enable simultaneous observation of live cells and cyclic stretching of an elastic polymer, polydimethylsiloxane (PDMS), with a modified surface for enhanced cell adhesion. Human mesenchymal stem cells (hMSCs) were examined with a series of frequencies from 0.00003 to 2 Hz and varied amplitudes of 2%, 5%, or 10%. The cells with an initial random orientation were confirmed to be reoriented perpendicular to the stretching direction at frequencies greater than a threshold value, which we term critical frequency (f_c); additionally, the critical frequency f_c was amplitude-dependent. We further introduced the concept of critical stretching rate (R_c) and found that this quantity can unify both frequency and amplitude dependences. The reciprocal value of R_c in this study reads 8.3 min, which is consistent with the turnover time of actin filaments reported in the literature, suggesting that the supramolecular relaxation in the cytoskeleton within a cell might be responsible for the underlying cell mechanotransduction. The theoretical calculation of cell reorientation based on a two-dimensional tensegrity model under uniaxial cyclic stretching is well consistent with our experiments. The above findings provide new insight into the crucial role of critical frequency and critical stretching rate in regulating cells on biomaterials under biomechanical stimuli.

Mechanical Adaptability of Tumor Cells in Metastasis

The most dangerous aspect of cancer lies in metastatic progression. Tumor cells will successfully form life-threatening metastases when they undergo sequential steps along a journey from the primary tumor to distant organs. From a biomechanics standpoint, growth, invasion, intravasation, circulation, arrest/adhesion, and extravasation of tumor cells demand particular cell-mechanical properties in order to survive and complete the metastatic cascade. With metastatic cells usually being softer than their non-malignant counterparts, high deformability for both the cell and its nucleus is thought to offer a significant advantage for metastatic potential. However, it is still unclear whether there is a finely tuned but fixed mechanical state that accommodates all mechanical features required for survival throughout the cascade or whether tumor cells need to dynamically refine their properties and intracellular components at each new step encountered. Here, we review the various mechanical requirements successful cancer cells might need to fulfill along their journey and speculate on the possibility that they dynamically adapt their properties accordingly. The mechanical signature of a successful cancer cell might actually be its ability to adapt to the successive microenvironmental constraints along the different steps of the journey.

The Coxiella burnetii T4SS Effector AnkF Is Important for Intracellular Replication

Coxiella burnetii is an obligate intracellular pathogen and the causative agent of the zoonotic disease Q fever. Following uptake by alveolar macrophages, the pathogen replicates in an acidic phagolysosomal vacuole, the C. burnetii-containing vacuole (CCV). Effector proteins translocated into the host cell by the type IV secretion system (T4SS) are important for the establishment of the CCV. Here we focus on the effector protein AnkF and its role in establishing the CCV. The C. burnetii AnkF knock out mutant invades host cells as efficiently as wild-type C. burnetii, but this mutant is hampered in its ability to replicate intracellularly, indicating that AnkF might be involved in the development of a replicative CCV. To unravel the underlying reason(s), we searched for AnkF interactors in host cells and identified vimentin through a yeast two-hybrid approach. While AnkF does not alter vimentin expression at the mRNA or protein levels, the presence of AnkF results in structural reorganization and vesicular co-localization with recombinant vimentin. Ectopically expressed AnkF partially accumulates around the established CCV and endogenous vimentin is recruited to the CCV in a time-dependent manner, suggesting that AnkF might attract vimentin to the CCV. However, knocking-down endogenous vimentin does not affect intracellular replication of C. burnetii. Other cytoskeletal components are recruited to the CCV and might compensate for the lack of vimentin. Taken together, AnkF is essential for the establishment of the replicative CCV, however, its mode of action is still elusive.

Non-affinity in multi-material mechanical metamaterials

Non-affine deformations enable mechanical metamaterials to achieve their unusual properties while imposing implications for their structural integrity. The presence of multiple phases with different mechanical properties results in additional non-affinity of the deformations, a phenomenon that has never been studied before in the area of extremal mechanical metamaterials. Here, we studied the degree of non-affinity, [Formula: see text], resulting from the random substitution of a fraction of the struts,[Formula: see text], that make up a lattice structure and are printed using a soft material (elastic modulus = [Formula: see text]) by those printed using a hard material ([Formula: see text]). Depending on the unit cell angle (i.e., [Formula: see text] = 60°, 90°, or 120°), the lattice structures exhibited negative, near-zero, or positive values of the Poisson's ratio, respectively. We found that the auxetic structures exhibit the highest levels of non-affinity, followed by the structures with positive and near-zero values of the Poisson's ratio. We also observed an increase in [Formula: see text] with [Formula: see text] and [Formula: see text] until [Formula: see text] =10⁴ and [Formula: see text]= 75%-90% after which [Formula: see text] saturated. The dependency of [Formula: see text] upon [Formula: see text] was therefore found to be highly asymmetric. The positive and negative values of the Poisson's ratio were strongly correlated with [Formula: see text]. Interestingly, achieving extremely high or extremely low values of the Poisson's ratio required highly affine deformations. In conclusion, our results clearly show the importance of considering non-affinity when trying to achieve a specific set of mechanical properties and underscore the structural integrity implications in multi-material mechanical metamaterials.

Cryoelectron Tomography Reveals Nanoscale Organization of the Cytoskeleton and Its Relation to Microtubule Curvature Inside Cells

Microtubules (MTs) are the most rigid elements of the cytoskeleton with in vitro persistence lengths (L_p) in the range of 1-6 mm. In cellular environments, however, MTs often appear strongly curved. This has been attributed to the forces acting upon them in situ where they are embedded in composite networks of different cytoskeletal elements. Hitherto, the nanoscale organization of these networks has remained largely uncharacterized. Cryo-electron tomography (cryo-ET) allowed to visualize and analyze the in situ structure of cytoskeletal networks in pristinely preserved cellular environments and at high resolution. Here, we studied the molecular organization of MTs and their interactions with the composite cytoskeleton in frozen-hydrated HeLa and P19 cells at different cell-cycle stages. We describe modulation of MT curvature correlated with the surrounding molecular architecture, and show that nanoscale defects occur in curved MTs. The data presented here contribute to constructing realistic models of cytoskeletal biomechanics.

Effect of F-actin and Microtubules on Cellular Mechanical Behavior Studied Using Atomic Force Microscope and an Image Recognition-Based Cytoskeleton Quantification Approach

Cytoskeleton morphology plays a key role in regulating cell mechanics. Particularly, cellular mechanical properties are directly regulated by the highly cross-linked and dynamic cytoskeletal structure of F-actin and microtubules presented in the cytoplasm. Although great efforts have been devoted to investigating the qualitative relation between the cellular cytoskeleton state and cell mechanical properties, comprehensive quantification results of how the states of F-actin and microtubules affect mechanical behavior are still lacking. In this study, the effect of both F-actin and microtubules morphology on cellular mechanical properties was quantified using atomic force microscope indentation experiments together with the proposed image recognition-based cytoskeleton quantification approach. Young’s modulus and diffusion coefficient of NIH/3T3 cells with different cytoskeleton states were quantified at different length scales. It was found that the living NIH/3T3 cells sense and adapt to the F-actin and microtubules states: both the cellular elasticity and poroelasticity are closely correlated to the depolymerization degree of F-actin and microtubules at all measured indentation depths. Moreover, the significance of the quantitative effects of F-actin and microtubules in affecting cellular mechanical behavior is depth-dependent.

The Mechanical Properties of Bacteria and Why they Matter

I review recent techniques to measure the mechanical properties of bacterial cells and their subcellular components, and then discuss what these techniques have revealed about the constitutive mechanical properties of whole bacterial cells and subcellular material, as well as the molecular basis for these properties.

Random fiber networks with inclusions: The mechanism of reinforcement

The mechanical behavior of athermal random fiber networks embedding particulate inclusions is studied in this work. Composites in which the filler size is comparable with the mean segment length of the network are considered. Inclusions are randomly distributed in the network at various volume fractions, and cases in which fibers are rigidly bonded to fillers and in which no such bonding is imposed are studied separately. In the presence of inclusions, the small strain modulus increases, while the ability of the network to strain stiffen decreases relative to the unfilled network case. The reinforcement induced by fillers is most pronounced in sparse networks of floppier filaments that deform in the bending-dominated mode in the unfilled state. As the unfilled network density or the bending stiffness of fibers increases, the effect of filling diminishes rapidly. Fillers lead to a transition from the soft, bending-dominated, to the stiffer, stretching-dominated, deformation mode of the network, a transition which is primarily responsible for the observed overall reinforcement. The confinement, i.e., the restriction on network kinematics imposed by fillers, causes this transition. These results provide a justification for the observed difference in reinforcement obtained in sparsely versus densely cross-linked networks at a given filling fraction and provide guidance for the further development of network-based materials.

The long persistence length of model tubules

Young's elastic modulus and the persistence length are calculated for a coarse-grained model of tubule forming polymers. The model uses a wedge shaped composite of particles that previously has been shown to self-assemble into tubules. These calculations demonstrate that the model yields very large persistence lengths (corresponding to 78-126 μm) that are comparable to that observed in experiments for the microtubule lengths accessible to the calculations. The source for the stiffness is the restricted rotation of the monomer due to the excluded volume interactions between bonded macromolecular monomers as well as the binding between monomers. For this reason, large persistence lengths are common in tubule systems with a macromolecule as the monomer. The persistence length increases linearly with increased binding strength in the filament direction. No dependence in the persistence length is found for varying the tubule pitch for geometries with the protofilaments remaining straight.

Mitochondrial damage and cytoskeleton reorganization in human dermal fibroblasts exposed to artificial visible light similar to screen-emitted light

BACKGROUND

Artificial visible light is everywhere in modern life. Social communication confronts us with screens of all kinds, and their use is on the rise. We are therefore increasingly exposed to artificial visible light, the effects of which on skin are poorly known.

OBJECTIVE

The purpose of this study was to model the artificial visible light emitted by electronic devices and assess its effect on normal human fibroblasts.

METHODS

The spectral irradiance emitted by electronic devices was optically measured and equipment was developed to accurately reproduce such artificial visible light. Effects on normal human fibroblasts were analyzed on human genome microarray-based gene expression analysis. At cellular level, visualization and image analysis were performed on the mitochondrial network and F-actin cytoskeleton. Cell proliferation, ATP release and type I procollagen secretion were also measured.

RESULTS

We developed a device consisting of 36 LEDs simultaneously emitting blue, green and red light at distinct wavelengths (450 nm, 525 nm and 625 nm) with narrow spectra and equivalent radiant power for the three colors. A dose of 99 J/cm2 artificial visible light was selected so as not to induce cell mortality following exposure. Microarray analysis revealed 2984 light-modulated transcripts. Functional annotation of light-responsive genes revealed several enriched functions including, amongst others, the "mitochondria" and "integrin signaling" categories. Selected results were confirmed by real-time quantitative PCR, analyzing 24 genes representing these two categories. Analysis of micro-patterned culture plates showed marked fragmentation of the mitochondrial network and disorganization of the F-actin cytoskeleton following exposure. Functionally, there was considerable impairment of cell growth and spread, ATP release and type I procollagen secretion in exposed fibroblasts.

CONCLUSION

Artificial visible light induces drastic molecular and cellular changes in normal human fibroblasts. This may impede normal cellular functions and contribute to premature skin aging. The present results extend our knowledge of the effects of the low-energy wavelengths that are increasingly used to treat skin disorders.

The Physiological and Pathological Implications of the Formation of Hydrogels, with a Specific Focus on Amyloid Polypeptides

Hydrogels are water-swollen and viscoelastic three-dimensional cross-linked polymeric network originating from monomer polymerisation. Hydrogel-forming polypeptides are widely found in nature and, at a cellular and organismal level, they provide a wide range of functions for the organism making them. Amyloid structures, arising from polypeptide aggregation, can be damaging or beneficial to different types of organisms. Although the best-known amyloids are those associated with human pathologies, this underlying structure is commonly used by higher eukaryotes to maintain normal cellular activities, and also by microbial communities to promote their survival and growth. Amyloidogenesis occurs by nucleation-dependent polymerisation, which includes several species (monomers, nuclei, oligomers, and fibrils). Oligomers of pathological amyloids are considered the toxic species through cellular membrane perturbation, with the fibrils thought to represent a protective sink for toxic species. However, both functional and disease-associated amyloids use fibril cross-linking to form hydrogels. The properties of amyloid hydrogels can be exploited by organisms to fulfil specific physiological functions. Non-physiological hydrogelation by pathological amyloids may provide additional toxic mechanism(s), outside of membrane toxicity by oligomers, such as physical changes to the intracellular and extracellular environments, with wide-spread consequences for many structural and dynamic processes, and overall effects on cell survival.

Contribution of the cytoskeleton to mechanosensitivity reported by dinoflagellate bioluminescence

The cytoskeleton is crucial to cell mechanics and sensing the extracellular physical environment. The objective of this study was to examine the role of the cortical cytoskeleton in mechanosensitivity in a unicellular protist, the marine dinoflagellate Lingulodinium polyedra, using its intrinsic bioluminescence as a rapid reporter of mechanotransduction. Pharmacological treatments resolved effects due to immediate cytoskeleton disruption from those due to cytoskeletal remodeling during the light to dark phase transition. The cytoskeleton was visualized by confocal laser scanning microscopy of immunohistochemically labeled microtubules and phalloidin labeled F-actin, and mechanosensitivity assessed based on the bioluminescence response to mechanical stimulation measured during the dark phase. Latrunculin B treatment after the transition from the light to dark phase resulted in some disruption of cortical F-actin, no observed effect on the cortical microtubules, and partial inhibition of the bioluminescence response. Treatment with oryzalin, which depolarizes microtubules, completely disrupted the microtubule network and cortical F-actin, and partially inhibited bioluminescence. These results demonstrate that cells retain some mechanosensitivity despite a disrupted cytoskeleton; link mechanosensitivity to intact F-actin; show a close connection between F-actin and microtubules comprising the cortical cytoskeleton; confirm a strong contribution of the actin cytoskeleton to the translocation of scintillons, vesicles containing the luminescent chemistry; and support the role of the actin cytoskeleton in the association of scintillons with the vacuole membrane.

Push or Pull? The light-weight architecture of the Daphnia pulex carapace is adapted to withstand tension, not compression

Daphnia (Crustacea, Cladocera) are well known for their ability to form morphological adaptations to defend against predators. In addition to spines and helmets, the carapace itself is a protective structure encapsulating the main body, but not the head. It is formed by a double layer of the integument interconnected by small pillars and hemolymphatic space in between. A second function of the carapace is respiration, which is performed through its proximal integument. The interconnecting pillars were previously described as providing higher mechanical stability against compressive forces. Following this hypothesis, we analyzed the carapace structure of D. pulex using histochemistry in combination with light and electron microscopy. We found the distal integument of the carapace to be significantly thicker than the proximal. The pillars appear fibrous with slim waists and broad, sometimes branched bases where they meet the integument layers. The fibrous structure and the slim-waisted shape of the pillars indicate a high capacity for withstanding tensile rather than compressive forces. In conclusion they are more ligaments than pillars. Therefore, we measured the hemolymphatic gauge pressure in D. longicephala and indeed found the hemocoel to have a pressure above ambient. Our results offer a new mechanistic explanation of the high rigidity of the daphniid carapace, which is probably the result of a light-weight construction consisting of two integuments bound together by ligaments and inflated by a hydrostatic hyper-pressure in the hemocoel. J. Morphol. 277:1320-1328, 2016. © 2016 Wiley Periodicals, Inc.

Significance of 1B and 2B domains in modulating elastic properties of lamin A

Nuclear lamins are type V intermediate filament proteins which form an elastic meshwork underlying the inner nuclear membrane. Lamins directly contribute to maintain the nuclear shape and elasticity. More than 400 mutations have been reported in lamin A that are involved in diseases known as laminopathies. These mutations are scattered mainly in the lamin rod domain along with some in its C-terminal domain. The contribution of the rod domain towards the elasticity of lamin A molecule was hitherto unknown. Here, we have elucidated the significance of the 1B and 2B domains of the rod in modulating the elastic behavior of lamin A by single-molecule force spectroscopy. In addition, we have also studied the network forming capacity of these domains and their corresponding viscoelastic behavior. We have shown that the 1B domain has the ability to form a lamin-like network and resists larger deformation. However at the single-molecular level, both the domains have comparable mechanical properties. The self-assembly of the 1B domain contributes to the elasticity of the lamin A network.

Elastic regimes of subisostatic athermal fiber networks

Athermal models of disordered fibrous networks are highly useful for studying the mechanics of elastic networks composed of stiff biopolymers. The underlying network architecture is a key aspect that can affect the elastic properties of these systems, which include rich linear and nonlinear elasticity. Existing computational approaches have focused on both lattice-based and off-lattice networks obtained from the random placement of rods. It is not obvious, a priori, whether the two architectures have fundamentally similar or different mechanics. If they are different, it is not clear which of these represents a better model for biological networks. Here, we show that both approaches are essentially equivalent for the same network connectivity, provided the networks are subisostatic with respect to central force interactions. Moreover, for a given subisostatic connectivity, we even find that lattice-based networks in both two and three dimensions exhibit nearly identical nonlinear elastic response. We provide a description of the linear mechanics for both architectures in terms of a scaling function. We also show that the nonlinear regime is dominated by fiber bending and that stiffening originates from the stabilization of subisostatic networks by stress. We propose a generalized relation for this regime in terms of the self-generated normal stresses that develop under deformation. Different network architectures have different susceptibilities to the normal stress but essentially exhibit the same nonlinear mechanics. Such a stiffening mechanism has been shown to successfully capture the nonlinear mechanics of collagen networks.

Automated multidimensional image analysis reveals a role for Abl in embryonic wound repair

The embryonic epidermis displays a remarkable ability to repair wounds rapidly. Embryonic wound repair is driven by the evolutionary conserved redistribution of cytoskeletal and junctional proteins around the wound. Drosophila has emerged as a model to screen for factors implicated in wound closure. However, genetic screens have been limited by the use of manual analysis methods. We introduce MEDUSA, a novel image-analysis tool for the automated quantification of multicellular and molecular dynamics from time-lapse confocal microscopy data. We validate MEDUSA by quantifying wound closure in Drosophila embryos, and we show that the results of our automated analysis are comparable to analysis by manual delineation and tracking of the wounds, while significantly reducing the processing time. We demonstrate that MEDUSA can also be applied to the investigation of cellular behaviors in three and four dimensions. Using MEDUSA, we find that the conserved nonreceptor tyrosine kinase Abelson (Abl) contributes to rapid embryonic wound closure. We demonstrate that Abl plays a role in the organization of filamentous actin and the redistribution of the junctional protein β-catenin at the wound margin during embryonic wound repair. Finally, we discuss different models for the role of Abl in the regulation of actin architecture and adhesion dynamics at the wound margin.

Healing of subcutaneous tendons: Influence of the mechanical environment at the suture line on the healing process

Tendon ruptures remain a significant musculoskeletal injury. Despite advances in surgical techniques and procedures, traditional repair techniques maintain a high incidence of rerupture or tendon elongation. Mechanical loading and biochemical signaling both control tissue healing. This has led some researchers to consider using a technique based on tension regulation at the suture line for obtaining good healing. However, it is unknown how they interact and to what extent mechanics control biochemistry. This review will open the way for understanding the interplay between mechanical loading and the process of tendon healing.

Age-Related Changes in the Mechanical Properties of Human Fibroblasts and Its Prospective Reversal After Anti-Wrinkle Tripeptide Treatment

One of an essential characteristic of human skin are time dependent mechanical properties. Here, we demonstrate that stiffness of human dermal fibroblast correlates with age and it can be restored after anti-wrinkle tripeptide treatment. The stiffness of human fibroblasts isolated from donors of 30-, 40- and 60 years old were examined. Additionally the effect of anti- wrinkle tripeptide of latter cells was investigated. The atomic force microscopy measurements were performed on untreated fibroblast as well as on treated with the peptide. The Young's modulus for two indentation depths 200 and 600 nm of each cell type was determined. The Young's modulus increases with age of the cells. The highest values of Young's modulus were obtained for fibroblasts collected from 60 years old donors, for indentation depth of ~200 nm. For larger indentation depth of 600 nm there are no significant differences in stiffness between cells. Fibroblasts treated with the anti-wrinkle tripeptide exhibit lower Young's modulus. The cells derived from 40- and 60-years old donors restored stiffness characteristic to the level of 30 years old subjects. The results show correlation between stiffness and age of the human fibroblast as well as impact of anti-wrinkle tripeptide on the mechanical properties of skin cells.

Natural stiffening increases flaw tolerance of biological fibers

Many fibers in biomaterials such as tendon, elastin, or silk feature a nonlinear stiffening behavior of the stress-strain relationship, where the rigidity of the material increases severely as the material is being stretched. Here we show that such nonlinear stiffening is beneficial for a fiber's ability to withstand cracks, leading to a flaw tolerant state in which stress concentrations around cracks are diminished. Our findings, established by molecular mechanics and the derivation of a theoretical scaling law, explain experimentally observed fiber sizes in a range of biomaterials and point to the importance of nonlinear stiffening to enhance their fracture properties. Our study suggests that nonlinear stiffening provides a mechanism by which nanoscale mechanical properties can be scaled up, providing a means towards bioinspired fibrous material and structural design.

Altered cell mechanics from the inside: dispersed single wall carbon nanotubes integrate with and restructure actin

With a range of desirable mechanical and optical properties, single wall carbon nanotubes (SWCNTs) are a promising material for nanobiotechnologies. SWCNTs also have potential as biomaterials for modulation of cellular structures. Previously, we showed that highly purified, dispersed SWCNTs grossly alter F-actin inside cells. F-actin plays critical roles in the maintenance of cell structure, force transduction, transport and cytokinesis. Thus, quantification of SWCNT-actin interactions ranging from molecular, sub-cellular and cellular levels with both structure and function is critical for developing SWCNT-based biotechnologies. Further, this interaction can be exploited, using SWCNTs as a unique actin-altering material. Here, we utilized molecular dynamics simulations to explore the interactions of SWCNTs with actin filaments. Fluorescence lifetime imaging microscopy confirmed that SWCNTs were located within ~5 nm of F-actin in cells but did not interact with G-actin. SWCNTs did not alter myosin II sub-cellular localization, and SWCNT treatment in cells led to significantly shorter actin filaments. Functionally, cells with internalized SWCNTs had greatly reduced cell traction force. Combined, these results demonstrate direct, specific SWCNT alteration of F-actin structures which can be exploited for SWCNT-based biotechnologies and utilized as a new method to probe fundamental actin-related cellular processes and biophysics.

Probing the cell membrane by magnetic particle actuation and Euler angle tracking

The mechanical properties of the cell membrane and the subjacent actin cortex are determinants of a variety of processes in immunity and cell division. The lipid bilayer itself and its connection to the actin cortex are anisotropic. An accurate description of the mechanical structure of the cell membrane and the involved dynamics therefore necessitates a measurement technique that can capture the inherent anisotropy of the system. Here, we combine magnetic particle actuation with rotational and translational particle tracking to simultaneously measure the mechanical stiffness of monocytic cells in three rotational and two translational directions. When using particles that bind via integrins to the cell membrane and the subjacent cortex, we measured an isotropic stiffness and a characteristic power-law dependence of the shear modulus on the applied frequency. When using particles functionalized with immunoglobulin G, we measured an anisotropic stiffness with a 10-fold-reduced value in one dimension. We suggest that the observed reduced stiffness in the plane of the cell membrane is caused by a local detachment of the lipid bilayer from the subjacent cytoskeletal cortex. We expect that our technique will enable new insights into the mechanical properties of the cell membrane that will help us to better understand membrane processes such as phagocytosis and blebbing.

Mutations in the N-terminal actin-binding domain of filamin C cause a distal myopathy

Linkage analysis of the dominant distal myopathy we previously identified in a large Australian family demonstrated one significant linkage region located on chromosome 7 and encompassing 18.6 Mbp and 151 genes. The strongest candidate gene was FLNC because filamin C, the encoded protein, is muscle-specific and associated with myofibrillar myopathy. Sequencing of FLNC cDNA identified a c.752T>C (p.Met251Thr) mutation in the N-terminal actin-binding domain (ABD); this mutation segregated with the disease and was absent in 200 controls. We identified an Italian family with the same phenotype and found a c.577G>A (p.Ala193Thr) filamin C ABD mutation that segregated with the disease. Filamin C ABD mutations have not been described, although filamin A and filamin B ABD mutations cause multiple musculoskeletal disorders. The distal myopathy phenotype and muscle pathology in the two families differ from myofibrillar myopathies caused by filamin C rod and dimerization domain mutations because of the distinct involvement of hand muscles and lack of pathological protein aggregation. Thus, like the position of FLNA and B mutations, the position of the FLNC mutation determines disease phenotype. The two filamin C ABD mutations increase actin-binding affinity in a manner similar to filamin A and filamin B ABD mutations. Cell-culture expression of the c.752T>C (p.Met251)Thr mutant filamin C ABD demonstrated reduced nuclear localization as did mutant filamin A and filamin B ABDs. Expression of both filamin C ABD mutants as full-length proteins induced increased aggregation of filamin. We conclude filamin C ABD mutations cause a recognizable distal myopathy, most likely through increased actin affinity, similar to the pathological mechanism of filamin A and filamin B ABD mutations.

Stiffening of human skin fibroblasts with age

Changes in mechanical properties are an essential characteristic of the aging process of human skin. Previous studies attribute these changes predominantly to the altered collagen and elastin organization and density of the extracellular matrix. Here, we show that individual dermal fibroblasts also exhibit a significant increase in stiffness during aging in vivo. With the laser-based optical cell stretcher we examined the viscoelastic biomechanics of dermal fibroblasts isolated from 14 human donors aged 27 to 80. Increasing age was clearly accompanied by a stiffening of the investigated cells. We found that fibroblasts from old donors exhibited an increase in rigidity of ∼60% with respect to cells of the youngest donors. A FACS analysis of the content of the cytoskeletal polymers shows a shift from monomeric G-actin to polymerized, filamentous F-actin, but no significant changes in the vimentin and microtubule content. The rheological analysis of fibroblast-populated collagen gels demonstrates that cell stiffening directly results in altered viscoelastic properties of the collagen matrix. These results identify a new mechanism that may contribute to the age-related impairment of elastic properties in human skin. The altered mechanical behavior might influence cell functions involving the cytoskeleton, such as contractility, motility, and proliferation, which are essential for reorganization of the extracellular matrix.

Macroscopic stiffening of embryonic tissues via microtubules, RhoGEF and the assembly of contractile bundles of actomyosin

During morphogenesis, forces generated by cells are coordinated and channeled by the viscoelastic properties of the embryo. Microtubules and F-actin are considered to be two of the most important structural elements within living cells accounting for both force production and mechanical stiffness. In this paper, we investigate the contribution of microtubules to the stiffness of converging and extending dorsal tissues in Xenopus laevis embryos using cell biological, biophysical and embryological techniques. Surprisingly, we discovered that depolymerizing microtubules stiffens embryonic tissues by three- to fourfold. We attribute tissue stiffening to Xlfc, a previously identified RhoGEF, which binds microtubules and regulates the actomyosin cytoskeleton. Combining drug treatments and Xlfc activation and knockdown lead us to the conclusion that mechanical properties of tissues such as viscoelasticity can be regulated through RhoGTPase pathways and rule out a direct contribution of microtubules to tissue stiffness in the frog embryo. We can rescue nocodazole-induced stiffening with drugs that reduce actomyosin contractility and can partially rescue morphogenetic defects that affect stiffened embryos. We support these conclusions with a multi-scale analysis of cytoskeletal dynamics, tissue-scale traction and measurements of tissue stiffness to separate the role of microtubules from RhoGEF activation. These findings suggest a re-evaluation of the effects of nocodazole and increased focus on the role of Rho family GTPases as regulators of the mechanical properties of cells and their mechanical interactions with surrounding tissues.

Multi-scale models of cell and tissue dynamics

Cell and tissue movement are essential processes at various stages in the life cycle of most organisms. The early development of multi-cellular organisms involves individual and collective cell movement; leukocytes must migrate towards sites of infection as part of the immune response; and in cancer, directed movement is involved in invasion and metastasis. The forces needed to drive movement arise from actin polymerization, molecular motors and other processes, but understanding the cell- or tissue-level organization of these processes that is needed to produce the forces necessary for directed movement at the appropriate point in the cell or tissue is a major challenge. In this paper, we present three models that deal with the mechanics of cells and tissues: a model of an arbitrarily deformable single cell, a discrete model of the onset of tumour growth in which each cell is treated individually, and a hybrid continuum-discrete model of the later stages of tumour growth. While the models are different in scope, their underlying mechanical and mathematical principles are similar and can be applied to a variety of biological systems.

Self-organization of keratin intermediate filaments into cross-linked networks

Keratins, the largest subgroup of intermediate filament (IF) proteins, form a network of 10-nm filaments built from type I/II heterodimers in epithelial cells. A major function of keratin IFs is to protect epithelial cells from mechanical stress. Like filamentous actin, keratin IFs must be cross-linked in vitro to achieve the high level of mechanical resilience characteristic of live cells. Keratins 5 and 14 (K5 and K14), the main pairing occurring in the basal progenitor layer of epidermis and related epithelia, can readily self-organize into large filament bundles in vitro and in vivo. Here, we show that filament self-organization is mediated by multivalent interactions involving distinct regions in K5 and K14 proteins. Self-organization is determined independently of polymerization into 10-nm filaments, but involves specific type I–type II keratin complementarity. We propose that self-organization is a key determinant of the structural support function of keratin IFs in vivo.

Antimitotic chemotherapeutics promote adhesive responses in detached and circulating tumor cells

In the clinical treatment of breast cancer, antimitotic cytotoxic agents are one of the most commonly employed chemotherapies, owing largely to their antiproliferative effects on the growth and survival of adherent cells in studies that model primary tumor growth. Importantly, the manner in which these chemotherapeutics impact the metastatic process remains unclear. Furthermore, since dissemination of tumor cells through the systemic circulation and lymphatics necessitates periods of detached survival, it is equally important to consider how circulating tumor cells respond to such compounds. To address this question, we exposed both nontumorigenic and tumor-derived epithelial cell lines to two antitumor compounds, jasplakinolide and paclitaxel (Taxol), in a series of attached and detached states. We report here that jasplakinolide promoted the extension of microtubule-based projections and microtentacle protrusions in adherent and suspended cells, respectively. These protrusions were specifically enriched by upregulation of a stable post-translationally modified form of alpha-tubulin, and this occurred prior to, and independently of any reductions in cellular viability. Microtubule stabilization with Taxol significantly enhanced these effects. Additionally, Taxol promoted the attachment and spreading of suspended tumor cell populations on extracellular matrix. While the antiproliferative effects of these compounds are well recognized and clinically valuable, our findings that microfilament and microtubule binding chemotherapeutics rapidly increase the mechanisms that promote endothelial adhesion of circulating tumor cells warrant caution to avoid inadvertently enhancing metastatic potential, while targeting cell division.

A MILD ACUTE COMPRESSION INDUCES NEURAPRAXIA IN RAT SCIATIC NERVE

The pressure that induces neurapraxia in rat remains unrevealed. To determine the appropriate force to induce neurapraxia, two types of clips were applied to the sciatic nerve and were evaluated with functional, electrophysiological, and histological examinations. With a compression of 60 g/mm2, walking track analysis showed complete sciatic nerve paralysis one day postoperatively, but became normal in 14 days. Electrophysiologically, complete conduction block occurred one day post operatively, whereas the motor conduction velocity (MCV) below the compression site remained normal. Histologically, only limited signs of Wallerian degeneration were seen. The model in this study exhibited the features of neurapraxia.

Particle-tracking microrheology of living cells: principles and applications

A multitude of cellular and subcellular processes depend critically on the mechanical deformability of the cytoplasm. We have recently introduced the method of particle-tracking microrheology, which measures the viscoelastic properties of the cytoplasm locally and with high spatiotemporal resolution. Here we establish the basic principles of particle-tracking microrheology, describing the advantages of this approach over more conventional approaches to cell mechanics. We present basic concepts of molecular mechanics and polymer physics relevant to the microrheological response of cells. Particle-tracking microrheology can probe the mechanical properties of live cells in experimentally difficult, yet more physiological, environments, including cells embedded inside a 3D matrix, adherent cells subjected to shear flows, and cells inside a developing embryo. Particle-tracking microrheology can readily reveal the lost ability of diseased cells to resist shear forces.

Mechanical loading by fluid shear is sufficient to alter the cytoskeletal composition of osteoblastic cells

Many structural modifications have been observed as a part of the cellular response to mechanical loading in a variety of cell types. Although changes in morphology and cytoskeletal rearrangement have been widely reported, few studies have investigated the change in cytoskeletal composition. Measuring how the amounts of specific structural proteins in the cytoskeleton change in response to mechanical loading will help to elucidate cellular mechanisms of functional adaptation to the applied forces. Therefore, the overall hypothesis of this study was that osteoblasts would respond to fluid shear stress by altering the amount of specific cross-linking proteins in the composition of the cytoskeleton. Mouse osteoblats cell line MC3T3-E1 and human fetal osteoblasts (hFOB) were exposed to 2 Pa of steady fluid shear for 2 h in a parallel plate flow chamber, and then the amount of actin, vimentin, α-actinin, filamin, and talin in the cytoskeleton was measured using Western blot analyses. After mechanical loading, there was no change in the amount of actin monomers in the cytoskeleton, but the cross-linking proteins α-actinin and filamin that cofractionated with the cytoskeleton increased by 29% ( P < 0.01) and 18% ( P < 0.02), respectively. Localization of the cross-linking proteins by fluorescent microscopy revealed that they were more widely distributed throughout the cell after exposure to fluid shear. The amount of vimentin in the cytoskeleton also increased by 15% ( P < 0.01). These results indicate that osteoblasts responded to mechanical loading by altering the cytoskeletal composition, which included an increase in specific proteins that would likely enhance the mechanical resistance of the cytoskeleton.

Over-expression of alpha-actinin with a GFP fusion protein is sufficient to increase whole-cell stiffness in human osteoblasts

Osteoblasts respond to shear stress by simultaneously increasing their whole-cell stiffness and up-regulating the cytoskeletal crosslinking protein alpha-actinin. The stiffness of reconstituted cytoskeletal networks increases following the addition of alpha-actinin, but the effect of alpha-actinin on whole-cell mechanical behavior has not been investigated. The hypothesis of this study was that increasing alpha-actinin in the cytoskeleton would be sufficient to increase whole-cell stiffness. hFOB osteoblasts were transfected with a plasmid for GFP-tagged alpha-actinin, resulting in a 150% increase in the amount of alpha-actinin. The GFP-alpha-actinin fusion protein co-fractionated with the cytoskeleton and co-localized to the same regions of the cytoskeleton as endogenous alpha-actinin. Whole-cell mechanical behavior was measured by atomic force microscopy using a 25 mum diameter microsphere as an indenter. The whole-cell stiffness of cells over-expressing GFP-alpha-actinin was 60% higher than cells expressing only endogenous alpha-actinin (p < 0.002), which was within the range of mechanical behavior observed in osteoblastic cells exposed to 1 and 2 Pa of fluid shear. These results indicate that the up-regulation of alpha-actinin synthesis in osteoblasts is sufficient to alter the whole-cell mechanical behavior and highlights the potential role of alpha-actinin to reinforce cells against mechanical loads.

Scaling of nonaffine deformation in random semiflexible fiber networks

Random networks of semiflexible filaments play a crucial role in the mechanics of many systems, including the cell. To understand the response of semiflexible networks to stress, a strain-based measure of nonaffine deformation is introduced and used to probe the mechanics on various scales. This measure provides a unified description of nonaffinity in both strain and rotation and indicates that the level of nonaffinity in these quantities in a given system is comparable. The degree of nonaffinity decreases as the scale of observation increases. This scaling is a power law with different exponents for length scales smaller and larger than a characteristic length scale proportional to the fiber length. The fiber bending stiffness controls the scaling at small length scales, while the large length scale scaling exponent is independent of fiber density and stiffness.

High-throughput rheological measurements with an optical stretcher

The cytoskeleton is a major determinant of the mechanical strength and morphology of most cells. The composition and assembly state of this intracellular polymer network evolve during the differentiation of cells, and the structure is involved in many cellular functions and is characteristically altered in many diseases, including cancer. Here we exploit the deformability of the cytoskeleton as a link between molecular structure and biological function, to distinguish between cells in different states by using a laser-based optical stretcher (OS) coupled with microfluidic handling of cells. An OS is a cell-sized, dual-beam laser trap designed to nondestructively test the deformability of single suspended cells. Combined with microfluidic delivery, many cells can be measured serially in a short amount of time. With this tool it could be shown that optical deformability is sensitive enough to monitor subtle changes during the progression of cells from normal to cancerous and even a metastatic state. Stem cells can also be distinguished from more differentiated cells. The surprisingly low number of cells required for this assay reflects the tight regulation of the cytoskeleton by the cell. This suggests the possibility of using optical deformability as an inherent cell marker for basic cell biological investigation, diagnosis of disease, and sorting of stem cells from heterogeneous populations, obviating the need for external markers or special preparation. Many additional biological assays can be easily adapted to utilize this innovative physical method. This chapter details the setup and use of the microfluidic OS, the analysis and interpretation of data, and the results of a typical experiment.

Effective medium theory of semiflexible filamentous networks

We develop an effective medium approach to the mechanics of disordered, semiflexible polymer networks and study the response of such networks to uniform and nonuniform strain. We identify distinct elastic regimes in which the contributions of either filament bending or stretching to the macroscopic modulus vanish. We also show that our effective medium theory predicts a crossover between affine and nonaffine strain, consistent with both prior numerical studies and scaling theory.