The epidermal growth factor (EGF) receptor (EGFR) belongs to the family of receptor tyrosine kinases, also known as ErbB receptors. These receptors trigger a rich network of signaling pathways and regulate cell functions, such as proliferation, differentiation and migration, and play a key role in the genesis of several tumors (1). Therefore, a detailed understanding of the mechanisms of receptor activation is critical. The objective of this work is to analyze the influence of the spatial heterogeneities of the membrane EGFRs on the response of signaling events by modulating EGF binding and EGFR dimerization. The work has been motivated by recent advances in our understanding of the distribution of proteins in the plasma membrane (2, 3).
The equilibrium binding of EGF on EGFR, resulting in concave up Scatchard plot, has been an issue of debate for over a decade. We have developed a compartmental equilibrium model to represent heterogeneity in the local EGFR density (4). The model can explain the shape of the Scatchard plot, and fitting of experimental data suggests EGFR localization to extents in agreement with observed via microscopy studies.
In order to understand the dynamics of these processes, kinetic modeling of receptor diffusion, dimerization and ligand binding has also been carried out. An outstanding issue is the selection of a suitable model to study the kinetics of receptor dimerization. To meet this goal, a criterion for choosing a suitable model (spatially distributed vs. well-mixed and deterministic vs. stochastic) was developed. The study shows that dimerization reactions in the plasma membrane with Damköhler (Da) number > 0.1 (tested for typical receptor densities) require spatial modeling (5). Comparison with partial differential equations (PDEs) indicates that spatial Monte Carlo (MC) is needed to capture the effect of non-random receptor distribution created by receptor dimerization. We have developed a spatially distributed, multiscale MC based simulation framework to enable the simulation of receptor dynamics, and to bridge the spatial and temporal resolution of various microscopy techniques. The results from MC simulations are in excellent agreement with single particle tracking microscopy and biochemical data (6). The simulations reveal the dependence of sequence of EGF-EGFR reaction events on ligand concentration, receptor density and receptor mobility. Substantial differences in signaling between normal and cancer cells are observed due to localization. Overall, this work suggests the existence of a layer of control at the cell surface by altering the signal sensing mechanism.
References
1. Y. Yarden, M. X. Sliwkowski, Nature Reviews Molecular Cell Biology 2, 127-137 (2001).
2. G. Vereb et al., PNAS 100, 8053-8058 (2003).
3. A. Kusumi et al., Annual Review of Biophysics and Biomolecular Structure 34, 351-378 (2005).
4. K. Mayawala, D. G. Vlachos, J. S. Edwards, FEBS Letters 579, 3043-3047 (2005).
5. K. Mayawala, D. G. Vlachos, J. S. Edwards, Biophysical Chemistry, Accepted (2006).
6. K. Mayawala, D. G. Vlachos, J. S. Edwards, BMC Cell Biology 6, 41 (2005).