# BioSysBio:abstracts/2007/Vilda Purutcuoglu

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# Bayesian Inference of a Realistic MAPK/ERK Pathway

**Author(s): **Vilda Purutcuoglu and Prof. Ernst Wit

**Affiliations:** Department of Mathematics and Statistics, Lancaster University

**Contact:**v.purutcuoglu@lancaster.ac.uk, e.wit@lancaster.ac.uk

**Keywords:** Bayesian inference, diffusion approximation, MAPK/ERK pathway, stochastic modelling

## Features of the MAPK pathway

All cellular activations are regulated by various signal transduction pathways. The cellular signal transduction is the process of carrying over a signal in the cell's environment for producing an associate response [10]. This transfer is initiated by an external stimulus of the pathway by binding of an external protein to a receptor, and ends with the production of the target protein. The MAPK (mitogen-activated protein kinase) or its synonymous ERK (extracellular signal regulated kinase) pathway is one of the major signal transduction systems which regulates the cellular growth control of all eukaryotes including cell proliferation, differentiation, and apoptosis [6]. The activation of this pathway is triggered by an external stimulus of the growth factor EGF which binds to activated tyrosine kinase receptors and ends with the transcription and translation of the c-Fos gene [9].

(Figure 1:Simple representation of the structure of MAPK/ERK pathway)

The MAPK/ERK pathway has several organizational properties, which explain its complexity. The ultrasensitivity, the bistability, the periodic behaviour, and the existence of location-depended proteins are some of these characteristics [9]. On the other hand the structure of this mechanism whose main components are Ras, Raf, and MEK proteins (Figure 1), includes a number of phosphorylations on the protein level. The functionality of these proteins is stochastic in nature and directed by positive and negative feedback loops that cause either activation or inhibition of other proteins [9,6]. Due to the large number of interactions at the protein level, the outcome of the signaling is not a simple linear function of the signal. Interaction maps of the proteins or simple representation of the pathway like given in Figure 1 are not sufficient to exhaustively describe this complex system.

## Representation of the pathway by quasi-reactions

Since the MAPK/ERK pathway is important in the cellular lifecycle, it has been intensively studied [8,5,6,1]. Some give a qualitative description of this regulatory mechanism [9,11]. However, none of the sources describe the system by an explicit set of reactions. We combine these qualitative sources for a representation of the pathway as a list of (quasi) reactions. We use multiple parametrizations to indicate different localization of the molecules in the cell and to describe the protein using different binding sites and various phosphorylations. The following set of equations, which describes the activation of the MAPK pathway by the EGF receptor, is part of the reaction list with 94 reactions and 51 substrates, representing 33 proteins and genes.

- EGF + Shc
_{m}(translocation)
EGF + Shc - Grb2 + SOS Grb2-SOS
- EGF + Grb2-SOS
_{m}(translocation)
EGF + Grb2-SOS - Shc
_{m}+ Grb2-SOS_{m}Shc-Grb2-SOS_{m} - Shc-Grb2-SOS
_{m}+ Ras.GDP Shc-Grb2-SOS_{m}+ Ras.GTP

Here, Grb2, SOS, Shc, EGF, Ras.GTP, and Ras.GDP are single
proteins, Grb2-SOS and Shc-Grb2-SOS are protein complexes in the
cytosol, and Shc-Grb2-SOS_{m} is a protein complex near the
membrane. As can be seen from the reactions, the translocation of
substrates to the membrane is expressed by the notation *m*. For
instance the protein Shc_{m} denotes the Shc protein translocated
from the cytosol towards the membrane. The different levels of the
phosphorylation, on the other hand, are denoted by the index p or p1
and p2 where the first two abbreviations show the
mono-phosphorylation and the latter implies the
double-phosphorylation of a protein. For example, MEK.p2 represents
the double-phosphorylated MEK protein on the S218 and S222 binding
sites.

## Modelling by diffusion approximation

Gene regulation is commonly modelled via ordinary differential equations (ODEs), which are based on the law of mass action and continuous concentrations of each chemical substrate. Although ODEs are successful to represent some reactions like linear production and degradation, they cannot describe the small system variability of the actual reactions. For biochemical systems, stochastic processes are a natural choice [2,12] as these kinds of dynamic formalization take into account the probabilistic manner of the different biological activations, such as the transcription of certain proteins, which occurs with low frequency in biological time [7].

In this study under the assumption that the probability distribution
of the number of the molecules of each species at *t* depends on the
continuous *t* and continuous number of molecules, we use the
diffusion approximation to explain the change of state of each
substrate at *t*. In this modelling the current state is found by a
Langevin approach, where a correlated noise term describes the
stochastic behaviour of the model over and above the drift term

in which *s* - dimensional vector representing
the change of a Brownian motion over time and *s* is the total
number of substrates in the system. and
are mean, or drift,
and variance, or diffusion, matrices, respectively, both depending
on the state of the system *Y* at time *t*, and the parameter vector
explicitly.
represents the stochastic rate
constant of the *j*th reaction and *r* denotes the total number of
reaction. Accordingly is the net effect matrix and
*r*-dimensional vector describes the hazard of
each reaction at time *t*. The algorithm computes the next state at
by replacing by [3,4].

## Diffusion approximation for inference

For estimating the model parameters, i.e. the stochastic rate constants, we apply the discretized version of diffusion approximation, which is known as Euler approximation,

where *s*-dimensional iid
random vector.

We define our data vector as a *(n+1)*x *s* matrix in which each
column indicates a vector of
and *n* stands for the
total number of observed time step. Finally *i* is the indicator of
the *i*th substrate.

Since the change in state for a given

has a multivariate normal distribution, the probability density function associated with this time increment is given by the following equation:

where . Then from
equation above, we derive the likelihood as

which is proportional to

where *i*th substrate at
time *t*.

As can be seen from the last expression of

, the conditional posterior densities of reaction rates does not have a known distribution. We compute the posterior distribution of using the MCMC method. Under a symmetric candidate generator for each , we apply a Metropolis-Hasting algorithm where the acceptance probability is found as follows

in which

is the candidate value.On simulated data the sampler converges well and is able to identify the dynamics of the MAPK/ERK pathway.

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