(442k) Microscopic and Coarse Grained Stochastic Simulation of Epidermal Growth Factor Receptor Diffusion on Corralled Membrane Surfaces

The epidermal growth factor receptor (EGFR) is a transmembrane receptor which, when bound to its ligand (EGF), dimerizes, and a trans-autophosphorylation occurs, leading to the activation of the MAPK cascade [1]. When activated, the last kinase in this cascade, ERK, enters the nucleus and activates transcription factors, controlling genes involved in regulating cell growth and other cell processes [2]. Tumors containing mutated EGFRs lead to more aggressive cancers than those with normal EGFR. Studies of ovarian, cervical, bladder, and esophageal cancers show that patients with increased expression of EGFR have lower survival rates than patients with normal EGFR expression levels [3].

Because dimerization is required for phosphorylation of the intracellular domains of EGF receptors, monomers are usually incapable of signaling. In order to efficiently signal, EGFR must dimerize. The cell membrane contains most of its EGFR population in several small regions of high receptor concentration [4]. This clustering leads to amplification of the signal because the receptors are close enough to dimerize and share ligands [5]. Data from single particle tracking [6, 7] and laser tweezer experiments [8, 9] have suggested that the plasma membrane is consists of corrals separated by barriers associated with the actin cytoskeleton within the cell. These fences and corrals are possibly the mechanism by which receptors are localized to specific areas of the plasma membrane, and an understanding of how these fences work will aid in finding ways to disrupt over-amplified signals, which lead to cancer.

A stochastic, spatial model has been developed to simulate the effect of corrals on the diffusion of EGFR on the plasma membrane. These barriers are modeled as lines with a low transition probability of crossing. The fence barrier in this model is defined as the ratio of the transition probability per unit time for diffusion across a boundary to that within a corral. The results of this simulation indicate that a fence barrier on the order of 10-3 recreates the experimentally determined factor of 5-50 difference between diffusion on artificial and natural plasma membranes [6].

Because the time scale for diffusion across barriers is three orders of magnitude greater than that of diffusion within a corral, most of the computational time is spent simulating diffusion within a corral. In order to decrease the time required for simulations, an expression for the macroscopic diffusivity of receptors affected by barriers was derived and applied to a coarse-grained simulation of the model. Values of diffusivity calculated from this theoretical model closely match those obtained from simulation results and capture the functionality of each parameter on the macroscopic diffusivity. This expression was applied to experimental data for corral size and residency times for various cell types [7], and, under the assumption that the microscopic diffusion is the same in all plasma membranes, it was found that all cell types tested exhibit fence barriers of the same order of magnitude. From this result, it is proposed that barriers to plasma membrane diffusion have the same or similar structure in all cells, but that the corral size varies.

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