Wnt/ß-Catenin signalling regulates key cellular decisions, namely cell survival, proliferation and differentiation during development. It is also implicated in various cancers, with truncation of the tumour suppressor gene APC (a member of the wnt/ß-catenin pathway) strongly linked with colorectal cancer. The main function of the wnt/ß-Catenin pathway is usually described as being the regulation of ß-Catenin concentration. Although many of the proteins and protein interactions involved in the wnt/ß-catenin pathway are known, quantitative knowledge of protein concentrations in mammalian cells is limited. This lack of quantitative data limits a system-level understanding offered by a computational systems biology approach. Therefore we have measured the concentrations, intracellular spatial location and transient behaviour of key proteins of the Wnt pathway: APC, axin, GSK3ß, ß-Catenin and E-cadherin in mammalian kidney epithelial HEK293T cells. The measured concentrations showed substantial differences to that used in the Lee et al Xenopus computational Model (Lee et al, PLoS Biology, 1, 2003).

We have used our new data to inform the development of a new computational model of the Wnt/ß-Catenin signalling pathway in mammalian cells. The model is used to investigate the system behaviour of the Wnt/ß-Catenin signalling pathway under steady state and transient conditions. Computational strategies employed to interrogate the system include: Sensitivity analyses to identify parameters for optimisation; Parametric optimisations to correlate predicted ß-catenin distributions with measured steady state concentrations and to fit transient changes of predicted ß-catenin levels with measured transient data; Structural adjustments to pathway topology to include additional proteins complexes and revised protein binding kinetics, APC-independent ß-catenin degradation and the partitioning of ß-catenin to sequestering protein, the key cell-cell adhesion molecule E-cadherin. Our simulations and experimental suggest an alternate role for the Wnt/ß-Catenin signalling pathway in mammalian cells in contrast to that suggested in the Xenopus oocytes.