Actomyosin contractility rotates the cell nucleus (original) (raw)

1987

Cell locomotion begins with a protrusion from the leading periphery of the cell. What drives this extension? Here we present a model for the extension of cell protuberances that unifies certain aspects of this phenomenon, and is based on the hypothesis that osmotic pressure drives cell extensions. This pressure arises from membrane-associated reactions, which liberate osmoticallv active particles, and from the swelling of the actin network that underlies the membrane. in t r o d u c t io n : w hat are the forces driving cell motility? Cells possess several mechanisms for exerting forces on their surroundings. In particular, a number of mechanochemical enzymes have been identified, including myosin, dynein and kinesin; other molecules will probably be identified in the future. All of these molecules share a common characteristic: they enable the cell to exert only contractile forces. This is a puzzling situation, since in order to move about cells must also be capable of generating protrusive forces. Placing cells in hypertonic media seems to suppress all protrusive activity, suggesting that protrusive force generation may be produced by simple osmotic pressure (Harris, 1973; Trinkaus, 1984 Trinkaus, , 1985. But osmotic pressure is an isotropic force: it acts equally in all directions. Therefore, in order to use pressure for protrusion, the cell must devise means to focus the force in particular directions. In the next section we propose a mechanism by which osmotic forces drive cell protrusion, and which is coordinated with the polymerization of the actin network that fills such protrusions. This model is an extension of two previous models we have proposed for lamellipod and acrosomal extension.

The cytoskeleton and cell motility

International Journal of Biochemistry, 1991

A. Swimming B. Crawling C. Extensions of cell motility IV. The Cell Cytoskeleton A. Biopolymers B. Molecular motors C. Motor families D. Other cytoskeleton-associated proteins E. Cell anchoring and regulatory pathways F. The prokaryotic cytoskeleton V. Filament-Driven Motility A. Microtubule growth and catastrophes B. Actin gels C. Modeling polymerization forces D. A model system for studying actin-based motility: The bacterium Listeria monocytogenes E. Another example of filament-driven amoeboid motility: The nematode sperm cell VI. Motor-Driven Motility A. Generic considerations B. Phenomenological description close to thermodynamic equilibrium C. Hopping and transport models D. The two-state model E. Coupled motors and spontaneous oscillations F. Axonemal beating VII. Putting It Together: Active Polymer Solutions A. Mesoscopic approaches B. Microscopic approaches C. Macroscopic phenomenological approaches: The active gels D. Comparisons of the different approaches to describing active polymer solutions VIII. Extensions and Future Directions

Dynamics and Stability of the Contractile Actomyosin Ring in the Cell

Physical Review Letters

Contraction of the cytokinetic ring during cell division leads to physical partitioning of a eukaryotic cell into two daughter cells. This involves flows of actin filaments and myosin motors in the growing membrane interface at the mid-plane of the dividing cell. Assuming boundary driven alignment of the acto-myosin filaments at the inner edge of the iterface we explore how the resulting active stresses influence the flow. Using the continuum gel theory framework, we obtain exact axisymmetric solutions of the dynamical equations. These solutions are consistent with experimental observations on closure rate. Using these solutions we perform linear stability analysis for the contracting ring under non-axisymmetric deformations. Our analysis shows that few low wave number modes, which are unstable during onset of the constriction, later on become stable when the ring shrinks to smaller radii, which is a generic feature of actomyosin ring closure. Our theory also captures how the effective tension in the ring decreases with its radius causing significant slow down in the contraction process at later times.

Direct Observations of the Mechanical Behaviors of the Cytoskeleton in Living Fibroblasts

The Journal of Cell Biology, 1999

Cytoskeletal proteins tagged with green fluorescent protein were used to directly visualize the mechanical role of the cytoskeleton in determining cell shape. Rat embryo (REF 52) fibroblasts were deformed using glass needles either uncoated for purely physical manipulations, or coated with laminin to induce attachment to the cell surface. Cells responded to uncoated probes in accordance with a three-layer model in which a highly elastic nucleus is surrounded by cytoplasmic microtubules that behave as a jelly-like viscoelastic fluid. The third, outermost cortical layer is an elastic shell under sustained tension. Adhesive, laminin-coated needles caused focal recruitment of actin filaments to the contacted surface region and increased the cortical layer stiffness. This direct visualization of actin recruit-ment confirms a widely postulated model for mechanical connections between extracellular matrix proteins and the actin cytoskeleton. Cells tethered to laminintreated needles strongly resisted elongation by actively contracting. Whether using uncoated probes to apply simple deformations or laminin-coated probes to induce surface-to-cytoskeleton interaction we observed that experimentally applied forces produced exclusively local responses by both the actin and microtubule cytoskeleton. This local accomodation and dissipation of force is inconsistent with the proposal that cellular tensegrity determines cell shape.

Dynamics and stability of contractile actomyosin ring in the cell

arXiv (Cornell University), 2020

Eukaryotic CRFK cells motion characterized with atomic force microscopy

bioRxiv, 2021

We performed a time-lapse imaging with Atomic Force Microscopy (AFM) of the motion of eukaryotic CRFK (Crandell-Rees Feline Kidney) cells adhered onto a glass surface and anchored to other cells in culture medium at 37°C. The main finding is a gradient in the spring constant of the actomyosin cortex along the cells axis. The rigidity increases at the rear of the cells during motion. This observation as well as a dramatic decrease of the volume suggests that cells may organize a dissymmetry in the skeleton network to expulse water and drive actively the rear edge.

Size-and speed-dependent mechanical behavior in living mammalian cytoplasm

PNAS (Proceeding of the National Academy of Sciences of the United States of America), 2017

Active transport in the cytoplasm plays critical roles in living cell physiology. However, the mechanical resistance that intracellular compartments experience, which is governed by the cytoplasmic material property, remains elusive, especially its dependence on size and speed. Here we use optical tweezers to drag a bead in the cytoplasm and directly probe the mechanical resistance with varying size a and speed V. We introduce a method, combining the direct measurement and a simple scaling analysis, to reveal different origins of the size-and speed-dependent resistance in living mammalian cy-toplasm. We show that the cytoplasm exhibits size-independent vis-coelasticity as long as the effective strain rate V/a is maintained in a relatively low range (0.1 s −1 < V/a < 2 s −1) and exhibits size-dependent poroelasticity at a high effective strain rate regime (5 s −1 < V/a < 80 s −1). Moreover, the cytoplasmic modulus is found to be positively correlated with only V/a in the viscoelastic regime but also increases with the bead size at a constant V/a in the poroelastic regime. Based on our measurements, we obtain a full-scale state diagram of the living mammalian cytoplasm, which shows that the cytoplasm changes from a viscous fluid to an elastic solid, as well as from compressible material to incompressible material, with increases in the values of two dimensionless parameters, respectively. This state diagram is useful to understand the underlying mechanical nature of the cytoplasm in a variety of cellular processes over a broad range of speed and size scales. cell mechanics | poroelasticity | viscoelasticity | cytoplasmic state diagram T he cytoplasm of living mammalian cells is a crowded, yet dynamic , environment (1). There are continuous intracellular movements that are vital for cell physiology, such as transport of vesicles and other organelles. While biological motors and other enzymatic processes provide key driving forces for these activities, the mechanical properties of the cytoplasm are crucial for determining the mechanical resistance that cellular compartments experience. Indeed, both the active driving force and appropriate mechanical environment are critical for shaping the living cellular machinery. However, while the force that molecular motors generate both individually and collectively has been extensively studied (2, 3), the mechanical properties of the cytoplasmic environment remain elusive. In addition, the impact of object size and velocity on the mechanical resistance that active forces need to overcome to enable transport remains unclear. Such characterization is essential for understanding the physical environment and numerous key dynamic processes inside living cells. The cytoplasm is composed of cytoskeletal networks and many proteins, as well as organelles and vesicles. Materials with such complex microstructure are expected to display time-dependent or frequency-dependent properties (4). Indeed, it has been revealed by many experimental approaches that the mechanical properties of living cells exhibit clear frequency dependency (5, 6); the cell response follows a power-law rheology behavior within a broad frequency range (3, 7, 8). A common view is that cells are visco-elastic materials (9-13), and the observed mechanical properties depend on the timescale over which the deformation occurs during the measurement. One important feature of a viscoelastic material is that its mechanical property does not depend on any length scales of the observation (14). Interestingly, it has recently been demonstrated that living cells may also behave like a poroelastic gel at short timescales (15-17); the stress relaxation of cells can be entirely determined by migration of cytosol through cytoskeletal networks. In the framework of poroelasticity, the measured mechanical property strongly depends on the size of the probe, as it takes a longer time for cytosol to move over a longer distance, therefore the stress relaxes slower. The size-dependent poroelastic behavior is in direct contrast to viscoelasticity whose relaxation is a material property and is independent of probe size, set by the time-dependent response of the materials' macromolecular and supra-molecular constituents. More importantly, most of previous attempts to study cell mechanics probe cells from the exterior, such as by using atomic force microscopy or an optical stretcher, and thus the measurement depends more on the stiff actin-rich cell cortex rather than on the much softer cytoplasm (13, 15, 18). Therefore, it remains unclear if viscoelasticity or poroelasticity better describes the rate-dependent resistance of the cytoplasm of living mammalian cells, or if both are required. In this paper, we use optical tweezers to drag a plastic bead in the cytoplasm of a living mammalian cell and directly measure the force (denoted by F) and displacement (denoted by x) relationship, which reflects the mechanical behavior of the cytoplasm. Considering both viscoelasticity and poroelasticity, we identify two independent dimensional parameters in the experiments: V/a and Va, where V and a represent the speed and diameter of the probe bead, respectively. Using these two control parameters, and through a combination of experimental measurement and scaling analysis, we Significance Although the driving force generated by motor proteins to deliver intracellular cargos is widely studied, the mechanical nature of cytoplasm, which is also important for intracellular processes by providing mechanical resistance, remains unclear. We use optical tweezers to directly characterize the resistance to transport in living mammalian cytoplasm. Using scaling analysis, we successfully distinguish between the underlying mechanisms governing the resistance to mechanical deformation, that is, among viscosity, viscoelasticity, poroelasticity, or pure elasticity, depending on the speed and size of the probe. Moreover, a cytoplasmic state diagram is obtained to illustrate different mechanical behaviors as a function of two dimensionless parameters; with this, the underlying mechanics of various cellular processes over a broad range of speed and size scales is revealed.

Rheological properties of the Eukaryotic cell cytoskeleton

Physics Reports-review Section of Physics Letters, 2007

The mechanical properties of cells are dominated by the cytoskeleton, a crosslinked network of protein filaments. Unlike synthetic polymer networks, the cell cytoskeleton is a highly dynamical system due to the on-off kinetics of the crosslinking proteins and the polymerization-depolymerization cycles of the filaments themselves. More remarkably, some of the crosslinkers are motor proteins, which are capable of generating forces and directional motion between the filaments by consuming chemical energy. Thus, the cell cytoskeleton is a highly complex and active polymer gel, which is responsible for the unique cell mechanical properties, which make it crawl, divide and assemble to higher functional units. This complexity of the cytoskeleton demands the development of novel experimental techniques as well as theoretical ideas. This review will deal with the recent technological, experimental and theoretical developments in the field of cell mechanics, and current trends. It will be shown that despite the cytoskeletal complexity, cells show some very general rheological properties.

Actomyosin contractility rotates the cell nucleus (original) (raw)

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