Visualization of a single cell. The components of the Wnt pathway are represented in red and the components of the Notch pathway in blue. The canonical Wnt pathway starts from the extracellular level of the short range signaling Wnt protein, which we represent by the Wntext variable. This short range molecule binds to the Frizzled receptor, which leads to an increase in the intracellular level of DSH. We therefore represent the interaction between these molecules by an activation from Wntext to Frizzled and then to DSH. The level of the scaffolding protein Axin is dependent on the level of DSH which acts as an inhibitor and Casein Kinase 1 α (represented by the variable Cask1α), which is assumed to be an activator. β-Catenin is phosphorylated by a complex composed, amongst others, by Axin. We separately represent the expression level of β-Catenin, β-Cateninexp. β-Catenin is activated by β-Cateninexp and inhibited by Axin. The family of downstream target genes of the canonical Wnt signaling pathway, (GT1), are transcriptionally activated by β-Catenin. The Notch pathway is based on the cell-cell interaction between the Notch receptor (whose level is represented by the variable Notch) and the ligands expressed on the membrane of the immediate neighboring cells (Ligandin). Since a successful binding between the receptor and the ligand leads to the cleavage of the intracellular part of Notch (Notch-IC) we use the minimum target function to model this interaction. We represent the level of the downstream target genes of Notch signaling as a single variable (GT2), which is activated by Notch-IC. Amongst these targets is the protein p21, which is activated by Notch-IC and inhibits Wnt. Wnt is an external variable of the cell. The downstream targets of Notch signaling activate the Jagged ligand.
Schaub et al. BMC Systems Biology 2007 1:4 doi:10.1186/1752-0509-1-4