147 research outputs found
Atomic Force Microscopy of height fluctuations of fibroblast cells
We investigated the nanometer scale height fluctuations of 3T3 fibroblast
cells with the atomic force microscope (AFM) under physiological conditions.
Correlation between these fluctuations and lateral cellular motility can be
observed. Fluctuations measured on leading edges appear to be predominantly
related to actin polymerization-depolymerization processes. We found fast (5
Hz) pulsatory behavior with 1--2 nm amplitude on a cell with low motility
showing emphasized structure of stress fibres. Myosin driven contractions of
stress fibres are thought to induce this pulsation.Comment: 6 pages, 5 figures, 1 tabl
The Force-Velocity Relation for Growing Biopolymers
The process of force generation by the growth of biopolymers is simulated via
a Langevin-dynamics approach. The interaction forces are taken to have simple
forms that favor the growth of straight fibers from solution. The
force-velocity relation is obtained from the simulations for two versions of
the monomer-monomer force field. It is found that the growth rate drops off
more rapidly with applied force than expected from the simplest theories based
on thermal motion of the obstacle. The discrepancies amount to a factor of
three or more when the applied force exceeds 2.5kT/a, where a is the step size
for the polymer growth. These results are explained on the basis of restricted
diffusion of monomers near the fiber tip. It is also found that the mobility of
the obstacle has little effect on the growth rate, over a broad range.Comment: Latex source, 9 postscript figures, uses psfig.st
Photo-nuclear cross sections on Au
A method was developed for measuring photonuclear reactions concurrently at
several discrete photon beam energies on a stack of different target materials
via a single irradiation. Concentric ring targets of the materials (in order
from front to back targets: Au, TiO, Zn, Os, and Au) were irradiated at the
High Intensity Gamma-ray Source (HIS). As a proof of principle, we
report the result of the cross section measurements from the front Au target.
The excitation functions of the Au(,n)Au and
Au(,3n)Au reactions were determined in the incident
photon energy range of 13-31 MeV using quasi-monoenergetic photon beams
provided at HIS. The cross sections of the combined ground state
(2) and short-lived first isomeric state (m1, 5), and of the second
isomeric state (m2, 12) in the Au production are obtained
separately by subtracting the rays from the internal conversion of the
second isomeric state. The excitation function of the second isomeric state via
the photon-induced reaction Au(,n)Au was measured for
the first time. By using the activation method rather than direct neutron
counting, the exclusive cross sections for the (,n) and (,3n)
reactions were determined. Comparing the yields from the front and back gold
targets validates our ability to simulate the effect of photon scattering in
the target stack and provides a method for assessing the systematic uncertainty
of our technique
Contractility Dominates Adhesive Ligand Density in Regulating Cellular De-adhesion and Retraction Kinetics
Cells that are enzymatically detached from a solid substrate rapidly round up as the tensile prestress in the cytoskeleton is suddenly unopposed by cell–ECM adhesions. We recently showed that this retraction follows sigmoidal kinetics with time constants that correlate closely with cortical stiffness values. This raises the promising prospect that these de-adhesion measurements may be used for high-throughput screening of cell mechanical properties; however, an important limitation to doing so is the possibility that the retraction kinetics may also be influenced and potentially rate-limited by the time needed to sever matrix adhesions. In this study, we address this open question by separating contributions of contractility and adhesion to cellular de-adhesion and retraction kinetics. We first develop serum-free conditions under which U373 MG glioma cells can be cultured on substrates of fixed fibronectin density without direct matrix contributions from the medium. We show that while spreading area increases with ECM protein density, cortical stiffness and the time constants of retraction do not. Conversely, addition of lysophosphatidic acid (LPA) to stimulate cell contractility strongly speeds retraction, independent of the initial matrix protein density and LPA’s contributions to spreading area. All of these trends hold in serum-rich medium commonly used in tissue culture, with the time constants of retraction much more closely tracking cortical stiffness than adhesive ligand density or cell spreading. These results support the use of cellular de-adhesion measurements to track cellular mechanical properties
Bargaining over a common categorisation
Two agents endowed with different categorisations engage in bargaining to reach an understanding and agree on a common categorisation. We model the process as a simple non-cooperative game and demonstrate three results. When the initial disagreement is focused, the bargaining process has a zero-sum structure. When the disagreement is widespread, the zero-sum structure disappears and the unique equilibrium requires a retraction of consensus: two agents who individually associate a region with the same category end up rebranding it under a different category. Finally, we show that this last equilibrium outcome is Pareto dominated by a cooperative solution that avoids retraction; that is, the unique equilibrium agreement may be inefficient
Amyloid-b peptide on sialyl-LewisX-selectin-mediated membrane tether mechanics at the cerebral endothelial cell surface
Increased deposition of amyloid-b peptide (Ab) at the cerebral endothelial cell (CEC) surface has been implicated in enhancement of transmigration of monocytes across the brain blood barrier (BBB) in Alzheimer’s disease (AD). In this study, quantitative immunofluorescence microscopy (QIM) and atomic force microscopy (AFM) with cantilevers biofunctionalized
by sialyl-Lewisx (sLex) were employed to investigate Ab-altered mechanics of membrane tethers formed by bonding
between sLex and p-selectin at the CEC surface, the initial mechanical step governing the transmigration of monocytes. QIM results indicated the ability for Ab to increase p-selectin expression at the cell surface and promote actin polymerization in both bEND3 cells (immortalized mouse CECs) and human primary CECs. AFM data also showed the ability for Ab to increase cell stiffness and adhesion probability in bEND3 cells. On the contrary, Ab lowered the overall force of membrane tether formation (Fmtf), and produced a bimodal population of Fmtf, suggesting subcellular mechanical alterations in membrane tethering. The lower Fmtf population was similar to the results obtained from cells treated with an F-actin-disrupting drug, latrunculin A. Indeed, AFM results also showed that both Ab and latrunculin A decreased membrane stiffness, suggesting a
lower membrane-cytoskeleton adhesion, a factor resulting in lower Fmtf. In addition, these cerebral endothelial alterations induced by Ab were abrogated by lovastatin, consistent with its anti-inflammatory effects. In sum, these results demonstrated the ability for Ab to enhance p-selectin expression at the CEC surface and induce cytoskeleton
reorganization, which in turn, resulted in changes in membrane-cytoskeleton adhesion and membrane tethering, mechanical factors important in transmigration of monocytes through the BBB.This work was supported by Alzheimer Association Grant NIRG-06-24448; NIH Grant 1P01 AG18357, R21NS052385, 5R21AG032579 and in part by
1P01HL095486 and AHA 0835676N; ‘‘Bolashak’’ scholarship and Ministry of Education and Science of the Republic of Kazakhstan 1029/GF2. The funders had no
role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
Integrin-Specific Mechanoresponses to Compression and Extension Probed by Cylindrical Flat-Ended AFM Tips in Lung Cells
Cells from lung and other tissues are subjected to forces of opposing directions that are largely transmitted through integrin-mediated adhesions. How cells respond to force bidirectionality remains ill defined. To address this question, we nanofabricated flat-ended cylindrical Atomic Force Microscopy (AFM) tips with ∼1 µm2 cross-section area. Tips were uncoated or coated with either integrin-specific (RGD) or non-specific (RGE/BSA) molecules, brought into contact with lung epithelial cells or fibroblasts for 30 s to form focal adhesion precursors, and used to probe cell resistance to deformation in compression and extension. We found that cell resistance to compression was globally higher than to extension regardless of the tip coating. In contrast, both tip-cell adhesion strength and resistance to compression and extension were the highest when probed at integrin-specific adhesions. These integrin-specific mechanoresponses required an intact actin cytoskeleton, and were dependent on tyrosine phosphatases and Ca2+ signaling. Cell asymmetric mechanoresponse to compression and extension remained after 5 minutes of tip-cell adhesion, revealing that asymmetric resistance to force directionality is an intrinsic property of lung cells, as in most soft tissues. Our findings provide new insights on how lung cells probe the mechanochemical properties of the microenvironment, an important process for migration, repair and tissue homeostasis
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