ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

static load and a dynamic load. A static load generates a transient response where the
peak is reached after approximately 10 minutes. A dynamic load gives rise to a
sustained upregulation of the concentration of phosphorylated ERK [2]. We attempt to
capture these effects by means of a model integrating the external stimuli and its effect
on intracellular signaling in order to gain a more quantitative understanding of
underlying fundamental processes.
3. METHODS
The model comprises an Ordinary Differential Equations (ODE) model of the ERK
signalling pathway (adapted from [3]), coupled to a 1D rheological model of cell-matrix
mechanics (adapted from [4]) that considers extracellular matrix (ECM), integrin,
cytoskeletal and nuclear (visco)elasticity and actomyosin contractility. The ERK
pathway is activated by stretching of integrin molecules. Integrin activation was made
proportional to its deformation. Several feedback mechanisms between the intracellular
and the mechanical model were implemented.
3.1 Model of the ERK pathway
Faratian et al. [3] presented a model of the ERK pathway to investigate the resistance to
receptor tyrosine kinase (RTK) inhibitor therapies. The model contains the ERK
pathway as part of a larger MAPK and Phosphatidylinositol 3-kinase (PI3K) signaling
network. Despite the fact that the cell type and goal of the model are different, the ERK
pathway in this work can be used as an estimation of the ERK pathway employed in
mechanotransduction in bone cells. The ERK pathway in the Faratian model starts at
Ras-GDP and ends with phosphorylated
ERK. The mechanical stimulus will promote
the phosphorylation of Ras-GDP, leading to
the posphorylation of ERK. Faratian et al.
[3] use protein phosphatase 2 (PP2A) as
their dephosphorylation agent. However
they consider the concentration of PP2A to
be a constant, meaning that
dephosphorylation is not influenced from
the outside.
In the model presented here, PP2A activity
is affected by the activation of the serine
protein kinase Akt. Akt suppresses PP2A
through the activation of mammalian target
of rapamycin (mTOR) [5]. The
phosphorylation of Akt by PI3K is
stimulated by mechanical load through
integrin activation in marrow stromal cells (MSCs) [6]. Hence the model is expanded
with the PI3K/Akt/mTOR pathway. Mechanical load will activate integrin, this
activated integrin will stimulate Ras-GDP and Akt phosphorylation thereby
simultaneously stimulating ERK phosphorylation and inhibiting ERK
dephosphorylation. Figure 1 gives an overview of the modelled pathway.
3.2 Mechanical model of the cell
Fig. 1 Overview of the modelled pathways: the
activated integrin initiates the ERK pathway and
suppresses the dephosphorylation of ERK and the
MAPKs.