176 research outputs found
A migrating epithelial monolayer flows like a Maxwell viscoelastic liquid
We perform a bidimensional Stokes experiment in an active cellular material:
an autonomously migrating monolayer of Madin-Darby Canine Kidney (MDCK)
epithelial cells flows around a circular obstacle within a long and narrow
channel, involving an interplay between cell shape changes and neighbour
rearrangements. Based on image analysis of tissue flow and coarse-grained cell
anisotropy, we determine the tissue strain rate, cell deformation and
rearrangement rate fields, which are spatially heterogeneous. We find that the
cell deformation and rearrangement rate fields correlate strongly, which is
compatible with a Maxwell viscoelastic liquid behaviour (and not with a
Kelvin-Voigt viscoelastic solid behaviour). The value of the associated
relaxation time is measured as ~min, is observed to be
independent of obstacle size and division rate, and is increased by inhibiting
myosin activity. In this experiment, the monolayer behaves as a flowing
material with a Weissenberg number close to one which shows that both elastic
and viscous effects can have comparable contributions in the process of
collective cell migration.Comment: 17 pages, 15 figure
Cell division: a source of active stress in cellular monolayers
We introduce the notion of cell division-induced activity and show that the
cell division generates extensile forces and drives dynamical patterns in cell
assemblies. Extending the hydrodynamic models of lyotropic active nematics we
describe turbulent-like velocity fields that are generated by the cell division
in a confluent monolayer of cells. We show that the experimentally measured
flow field of dividing Madin-Darby Canine Kidney (MDCK) cells is reproduced by
our modeling approach. Division-induced activity acts together with intrinsic
activity of the cells in extensile and contractile cell assemblies to change
the flow and director patterns and the density of topological defects. Finally
we model the evolution of the boundary of a cellular colony and compare the
fingering instabilities induced by cell division to experimental observations
on the expansion of MDCK cell cultures.Comment: Accepted Manuscript for Celebrating Soft Matter's 10th Anniversar
Polymers in linear shear flow: a numerical study
We study the dynamics of a single polymer subject to thermal fluctuations in
a linear shear flow. The polymer is modeled as a finitely extendable nonlinear
elastic FENE dumbbell. Both orientation and elongation dynamics are
investigated numerically as a function of the shear strength, by means of a new
efficient integration algorithm. The results are in agreement with recent
experiments.Comment: 7 pages, see also the preceding paper
(http://arxiv.org/nlin.CD/0503028
Tension Dynamics and Linear Viscoelastic Behavior of a Single Semiflexible Polymer Chain
We study the dynamical response of a single semiflexible polymer chain based
on the theory developed by Hallatschek et al. for the wormlike-chain model. The
linear viscoelastic response under oscillatory forces acting at the two chain
ends is derived analytically as a function of the oscillation frequency . We
shall show that the real part of the complex compliance in the low frequency
limit is consistent with the static result of Marko and Siggia whereas the
imaginary part exhibits the power-law dependence +1/2. On the other hand, these
compliances decrease as the power law -7/8 for the high frequency limit. These
are different from those of the Rouse dynamics. A scaling argument is developed
to understand these novel results.Comment: 23 pages, 6 figure
Single-molecule experiments in biological physics: methods and applications
I review single-molecule experiments (SME) in biological physics. Recent
technological developments have provided the tools to design and build
scientific instruments of high enough sensitivity and precision to manipulate
and visualize individual molecules and measure microscopic forces. Using SME it
is possible to: manipulate molecules one at a time and measure distributions
describing molecular properties; characterize the kinetics of biomolecular
reactions and; detect molecular intermediates. SME provide the additional
information about thermodynamics and kinetics of biomolecular processes. This
complements information obtained in traditional bulk assays. In SME it is also
possible to measure small energies and detect large Brownian deviations in
biomolecular reactions, thereby offering new methods and systems to scrutinize
the basic foundations of statistical mechanics. This review is written at a
very introductory level emphasizing the importance of SME to scientists
interested in knowing the common playground of ideas and the interdisciplinary
topics accessible by these techniques. The review discusses SME from an
experimental perspective, first exposing the most common experimental
methodologies and later presenting various molecular systems where such
techniques have been applied. I briefly discuss experimental techniques such as
atomic-force microscopy (AFM), laser optical tweezers (LOT), magnetic tweezers
(MT), biomembrane force probe (BFP) and single-molecule fluorescence (SMF). I
then present several applications of SME to the study of nucleic acids (DNA,
RNA and DNA condensation), proteins (protein-protein interactions, protein
folding and molecular motors). Finally, I discuss applications of SME to the
study of the nonequilibrium thermodynamics of small systems and the
experimental verification of fluctuation theorems. I conclude with a discussion
of open questions and future perspectives.Comment: Latex, 60 pages, 12 figures, Topical Review for J. Phys. C (Cond.
Matt
Cadherin exits the junction by switching its adhesive bond
Intercellular traction forces or lateral alignment of cadherin molecules can influence adherens junction dynamics by altering the cadherin dimerization interface
Dynamic Mechanisms of Cell Rigidity Sensing: Insights from a Computational Model of Actomyosin Networks
Cells modulate themselves in response to the surrounding environment like substrate elasticity, exhibiting structural reorganization driven by the contractility of cytoskeleton. The cytoskeleton is the scaffolding structure of eukaryotic cells, playing a central role in many mechanical and biological functions. It is composed of a network of actins, actin cross-linking proteins (ACPs), and molecular motors. The motors generate contractile forces by sliding couples of actin filaments in a polar fashion, and the contractile response of the cytoskeleton network is known to be modulated also by external stimuli, such as substrate stiffness. This implies an important role of actomyosin contractility in the cell mechano-sensing. However, how cells sense matrix stiffness via the contractility remains an open question. Here, we present a 3-D Brownian dynamics computational model of a cross-linked actin network including the dynamics of molecular motors and ACPs. The mechano-sensing properties of this active network are investigated by evaluating contraction and stress in response to different substrate stiffness. Results demonstrate two mechanisms that act to limit internal stress: (i) In stiff substrates, motors walk until they exert their maximum force, leading to a plateau stress that is independent of substrate stiffness, whereas (ii) in soft substrates, motors walk until they become blocked by other motors or ACPs, leading to submaximal stress levels. Therefore, this study provides new insights into the role of molecular motors in the contraction and rigidity sensing of cells
Particular distribution and expression pattern of endoglin (CD105) in the liver of patients with hepatocellular carcinoma
<p>Abstract</p> <p>Background</p> <p>Endoglin (CD105) has been considered a prognostic marker for hepatocellular carcinoma (HCC), and widely used as an appropriate targeting for antiangenesis therapy in some cancers. Our aim was to evaluate the distribution and expression of CD105 in the liver of patients with HCC, and to discuss whether CD105 may be used as an appropriate targeting for antiangenesis therapy in HCC.</p> <p>Methods</p> <p>Three parts of liver tissues from each of 64 patients with HCC were collected: tumor tissues (TT), adjacent non-tumor (AT) liver tissues within 2 cm, and tumor free tissues (TF) 5 cm far from the tumor edge. Liver samples from 8 patients without liver diseases served as healthy controls (HC). The distribution and expression of CD105 in tissues were evaluated by immunohistochemistry, Western blotting analysis, and real-time PCR. HIF-1alpha and VEGF<sub>165 </sub>protein levels in tissues were analyzed by Immunohistochemistry and Western blotting analysis or ELISA.</p> <p>Results</p> <p>CD105 was positively stained mostly in a subset of microvessels 'endothelial sprouts' in TT of all patients while CD105 showed diffuse positive staining, predominantly on hepatic sinus endothelial cells in the surrounding of draining veins in TF and AT. The mean score of MVD-CD105 (mean ± SD/0.74 mm<sup>2</sup>) was 19.00 ± 9.08 in HC, 153.12 ± 53.26 in TF, 191.12 ± 59.17 in AT, and 85.43 ± 44.71 in TT, respectively. Using a paired <it>t </it>test, the expression of CD105 in AT and TF was higher than in TT at protein (MVD, <it>p </it>= 0.012 and <it>p </it>= 0.007, respectively) and mRNA levels (<it>p </it>< 0.001 and <it>p </it>= 0.009, respectively). Moreover, distribution and expression of CD105 protein were consistent with those of HIF-1alpha and VEGF<sub>165 </sub>protein in liver of patients with HCC. The level of <it>CD105 </it>mRNA correlated with VEGF<sub>165 </sub>level in TF (r = 0.790, <it>p </it>= 0.002), AT (r = 0.723, <it>p </it>< 0.001), and TT (r = 0.473, <it>p </it>= 0.048), respectively.</p> <p>Conclusion</p> <p>It is demonstrated that CD105 was not only present in neovessels in tumor tissues, but also more abundant in hepatic sinus endothelium in non-tumor tissues with cirrhosis. Therefore, CD105 may not be an appropriate targeting for antiangenesis therapy in HCC, especially with cirrhosis.</p
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