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Simulating Actin Cytoskeleton Remodelling in the Shearing of Chondrocytes
Dowling, E.P. 1 , Ronan, W. 1 , Athanasiou, K.A. 2 , McGarry, J.P. 1
1 Department of Mechanical and Biomedical Engineering, NUI Galway
2 Biomedical Engineering Department, University of California, Davis, CA
e.dowling1@nuigalway.ie
Abstract
In this study, the role of actin fibres in the response of
chondrocytes to shear is experimentally determined. An
active model that describes the assembly of the actin
cytoskeleton in response to cell signalling, and the
dissociation of the actin cytoskeleton in response to a
reduction of intracellular tension is used to simulate the
experimental measurements.
1. Introduction
The actin cytoskeleton, formed via the polymerisation
of actin filaments and phosphorylation of myosin motors,
play a crucial role in the response of cells to mechanical
stimuli. In vitro investigations of the response of cells to
mechanical stimuli provide limited insight into these
mechanisms without the use of active computational
modelling.
2. Methods
Experimental: Chondrocytes were isolated from
bovine articular cartilage tissue, seeded onto glass slides
and positioned adjacent to a tungsten probe. A
piezoelectric motor then drove the probe laterally, leading
to deformation of the cell. This shear event was videorecorded
using a CCD camera. Beam theory was used to
determine the reaction force of the cell at various strain
levels. An additional series of shear experiments were
carried out on cells following the disruption of actin fibres
in the cells by the addition of cytochalasin D (2 µM) to the
media.
Computational: The contractile response of the actinmyosin
fibres is captured using a sliding filament model.
The formation of contractile actin-myosin fibres is
parameterised by the activation level η, which is governed
by a first order kinetic equation [1] :
Ck
f ⎡ σ ⎤ k b
η = [ 1−
η]
− ⎢1
− ⎥η
θ ⎣ σ 0 ⎦ θ
This equation describes the assembly of actin-myosin
units in response to a signal C and dissociation due to a
reduction in tension σ. 3D cell geometries are recreated
from in vitro images (Fig.1a).
3. Results
A plot of probe force versus probe indentation is shown
in Fig. 1b. Firstly, considering the experimental results: for
untreated cells in which actin fibres are intact, the reaction
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force increases dramatically upon initial probe indentation.
A yield point is reached, after which further indentation
leads to a lower increase in force levels. In contrast, cells
in which actin fibres have been disrupted, do not exhibit
such yield behaviour. Instead a linear force-indentation
relationship is observed, with forces being considerably
lower than untreated cells.
The active cell model captures very accurately the
characteristic yielding shape of untreated cells. In contrast,
a passive hyperelastic model accurately represents the
response of cells in which actin fibres have been disrupted.
Simulations also reveal that there are greater stresses in the
cell nucleus in the case of the active model (Fig. 1a).
Figure 1 (a) Contour plots of the stresses (kPa) experienced
by the nucleus after 7 µm of probe indentation. (b) Probe
force versus probe indentation curves for sheared cells.
4. Discussion
The yielding behaviour observed experimentally is
captured in the active computational model. This response
is due to the actin-myosin sliding filament behaviour and
the dissociation of actin fibres at the front of the cell due to
a localised reduction in tension. The present study shows
that active remodelling and contractility of the actin
cytoskeleton plays a critical role in the deformation of
chondrocytes. Furthermore, a passive computational model
of cell behaviour cannot be used to accurately compute
stresses in a cell or the evolution of the actin cytoskeleton.
5. References & Acknowledgements
[1] V.S. Deshpande, et al., "A bio-chemo-mechanical model for
cell contractility", PNAS, 2006, 103(38): pp. 14015-14020.
IRCSET; SFI-RFP (SFI-RFP/ENM1726); ICHEC.