13 research outputs found
Hysteresis in the cell response to time-dependent substrate stiffness
Mechanical cues like the rigidity of the substrate are main determinants for
the decision making of adherent cells. Here we use a mechano-chemical model to
predict the cellular response to varying substrate stiffness. The model
equations combine the mechanics of contractile actin filament bundles with a
model for the Rho-signaling pathway triggered by forces at cell-matrix
contacts. A bifurcation analysis of cellular contractility as a function of
substrate stiffness reveals a bistable response, thus defining a lower
threshold of stiffness, below which cells are not able to build up contractile
forces, and an upper threshold of stiffness, above which cells are always in a
strongly contracted state. Using the full dynamical model, we predict that
rate-dependent hysteresis will occur in the cellular traction forces when cells
are exposed to substrates of time-dependent stiffness.Comment: Revtex, 4 PDF figure
Modeling the Coupling of Mechanics and Biochemistry in Cell Adhesion
The actin cytoskeleton, which is a filament system made of actin biopolymers, mainly determines the mechanical properties of biological cells. In turn, the actin cytoskeleton is itself regulated by various biochemical signaling pathways. To advance the theoretical understanding of this coupling between mechanics and biochemistry, we developed a model for stress fibers which constitute the typical morphology of the actin cytoskeleton in mature cell adhesion. The mechanics of a stress fiber is described by a chain of viscoelastic elements that in addition may locally contract. The initial discrete model is transformed to a partial differential equation by performing a continuum limit. The biochemical regulation is modeled by a system of reaction diffusion equations that couple to the mechanics via the contractile activity along the fiber. In the first part of this thesis, the mechanical stress fiber equation is solved analytically and in particular the complex modulus is exactly calculated. The model is then used for the analysis of experimental data, measured by collaborators in experiments on stress fiber laser nanosurgery. It turns out that stress fibers are considerably crosslinked to their environment and that the localization of certain mechanosensitive proteins correlates with the theoretically predicted stress distribution within the actin cytoskeleton. Finally, the complete model is used to describe cellular behavior on elastic substrates. By performing a bifurcation analysis theoretical predictions are derived that can be tested in future experiments, in particular, the model predicts bistability and hysteresis in cell adhesion
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Substrate stiffness regulates cadherin-dependent collective migration through myosin-II contractility
The mechanical microenvironment is known to influence single-cell migration; however, the extent to which mechanical cues affect collective migration of adherent cells is not well understood. We measured the effects of varying substrate compliance on individual cell migratory properties in an epithelial wound-healing assay. Increasing substrate stiffness increased collective cell migration speed, persistence, and directionality as well as the coordination of cell movements. Dynamic analysis revealed that wounding initiated a wave of motion coordination from the wound edge into the sheet. This was accompanied by a front-to-back gradient of myosin-II activation and establishment of cell polarity. The propagation was faster and farther reaching on stiff substrates, indicating that substrate stiffness affects the transmission of directional cues. Manipulation of myosin-II activity and cadherin–catenin complexes revealed that this transmission is mediated by coupling of contractile forces between neighboring cells. Thus, our findings suggest that the mechanical environment integrates in a feedback with cell contractility and cell–cell adhesion to regulate collective migration
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Mapping the dynamics of force transduction at cell–cell junctions of epithelial clusters
Force transduction at cell-cell adhesions regulates tissue development, maintenance and adaptation. We developed computational and experimental approaches to quantify, with both sub-cellular and multi-cellular resolution, the dynamics of force transmission in cell clusters. Applying this technology to spontaneously-forming adherent epithelial cell clusters, we found that basal force fluctuations were coupled to E-cadherin localization at the level of individual cell-cell junctions. At the multi-cellular scale, cell-cell force exchange depended on the cell position within a cluster, and was adaptive to reconfigurations due to cell divisions or positional rearrangements. Importantly, force transmission through a cell required coordinated modulation of cell-matrix adhesion and actomyosin contractility in the cell and its neighbors. These data provide insights into mechanisms that could control mechanical stress homeostasis in dynamic epithelial tissues, and highlight our methods as a resource for the study of mechanotransduction in cell-cell adhesions. DOI: http://dx.doi.org/10.7554/eLife.03282.00
Spatial activity profiling along a fixed bed of powder catalyst during selective oxidation of propylene to acrolein
Spatial profiling of the reactant and product concentration including the gas phase temperature during the selective oxidation of propylene to acrolein along a catalyst bed allowed locating and distinguishing between specific processes occurring in each individual point of a chemical reactor. For this purpose, a lab-scale testing setup capable of resolving concentration and temperature gradients in a fixed-bed reactor was developed. The local gas phase composition and temperature were determined using a sampling capillary and mass spectrometry along a multicomponent Bi–Mo–Co–Fe oxide catalyst bed during selective oxidation of propylene to acrolein under high conversion conditions. In this way, the reaction progress in terms of conversion, selectivities and yields along the reactor was revealed. While ca. 66% of the integral propylene conversion occurred in the first third of the catalyst bed with high selectivity towards acrolein, the latter third of the bed was dominated by the formation of acrylic acid and CO as further and total oxidation products, respectively. Acrylic acid, which originates from the sequential oxidation of propylene to acrolein, was the by-product with the highest yield and especially formed above 440 °C. CO and CO were observed directly from propylene, along with consecutive pathways of propylene oxidation, which favor CO formation. The numerous insights obtained by even a single profile highlight the strong capabilities of spatially resolved activity and temperature measurements for diagnostics of packed-bed reactors and identifying the reaction pathways occurring within
Viscoelastic response of contractile filament bundles
The actin cytoskeleton of adherent tissue cells often condenses into filament
bundles contracted by myosin motors, so-called stress fibers, which play a
crucial role in the mechanical interaction of cells with their environment.
Stress fibers are usually attached to their environment at the endpoints, but
possibly also along their whole length. We introduce a theoretical model for
such contractile filament bundles which combines passive viscoelasticity with
active contractility. The model equations are solved analytically for two
different types of boundary conditions. A free boundary corresponds to stress
fiber contraction dynamics after laser surgery and results in good agreement
with experimental data. Imposing cyclic varying boundary forces allows us to
calculate the complex modulus of a single stress fiber.Comment: Revtex with 24 pages, 7 Postscript figures included, accepted for
publication in Phys. Rev.
Coupling biochemistry and mechanics in cell adhesion: a model for inhomogeneous stress fiber contraction
Biochemistry and mechanics are closely coupled in cell adhesion. At sites of
cell-matrix adhesion, mechanical force triggers signaling through the
Rho-pathway, which leads to structural reinforcement and increased
contractility in the actin cytoskeleton. The resulting force acts back to the
sites of adhesion, resulting in a positive feedback loop for mature adhesion.
Here we model this biochemical-mechanical feedback loop for the special case
when the actin cytoskeleton is organized in stress fibers, which are
contractile bundles of actin filaments. Activation of myosin II molecular
motors through the Rho-pathway is described by a system of reaction-diffusion
equations, which are coupled into a viscoelastic model for a contractile actin
bundle. We find strong spatial gradients in the activation of contractility and
in the corresponding deformation pattern of the stress fiber, in good agreement
with experimental findings.Comment: Revtex, 35 pages, 13 Postscript figures included, in press with New
Journal of Physics, Special Issue on The Physics of the Cytoskeleto
High Refractive Index Silicone Gels for Simultaneous Total Internal Reflection Fluorescence and Traction Force Microscopy of Adherent Cells
Substrate rigidity profoundly impacts cellular behaviors such as migration, gene expression, and cell fate. Total Internal Reflection Fluorescence (TIRF) microscopy enables selective visualization of the dynamics of substrate adhesions, vesicle trafficking, and biochemical signaling at the cell-substrate interface. Here we apply high-refractive-index silicone gels to perform TIRF microscopy on substrates with a wide range of physiological elastic moduli and simultaneously measure traction forces exerted by cells on the substrate
Ovarian cancer spheroids use myosin-generated force to clear the mesothelium
Dissemination of ovarian tumors involves the implantation of cancer spheroids into the mesothelial monolayer on the walls of peritoneal and pleural cavity organs. Biopsies of tumors attached to peritoneal organs show that mesothelial cells are not present under tumor masses. We have developed a live, image-based
in vitro
model in which interactions between tumor spheroids and mesothelial cells can be monitored in real time to provide spatial and temporal understanding of mesothelial clearance. Here we provide evidence that ovarian cancer spheroids utilize integrin – and talin - dependent activation of myosin and traction force to promote mesothelial cells displacement from underneath a tumor cell spheroid. These results suggest that ovarian tumor cell clusters gain access to the sub-mesothelial environment by exerting force on the mesothelial cells lining target organs, driving migration and clearance of the mesothelial cells