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Bistability 2
Published in James E. Ferrell, Systems Biology of Cell Signaling, 2021
The transition that takes place at kfeedback ~10 is called a pitchfork bifurcation, because the curves shown in Figure 9.9b sort of look like a pitchfork, and it is designated by the Greek letter Ψ, because Ψ resembles a pitchfork. (In cardiology, the splitting of a blood vessel into three is called a trifurcation, which makes etymological sense, but in nonlinear dynamics the term bifurcation is used). A familiar example of a pitchfork bifurcation from everyday life would be a vertical ruler being pushed on from above; above some critical strength of push, the straight ruler becomes unstable and two alternative bowed states become stable (Figure 9.10a). Related processes in biology include the buckling of epithelial sheets (Figure 9.10b) as well as certain signaling processes. For example, the Notch protein and its ligand Delta are cell surface proteins involved in a phenomenon termed lateral inhibition. Delta on one cell can repress, via its binding to Notch, the expression of Delta in its neighbors (Figure 9.10c). At low expression levels, the antagonistic Delta proteins on adjacent cells may be able to co-exist, but once the mutual inhibition becomes strong enough, in theory the system will traverse a pitchfork bifurcation and one or another of the two cells will win out.
Agrochemical-mediated cardiotoxicity in zebrafish embryos/larvae: What we do and where we go
Published in Critical Reviews in Environmental Science and Technology, 2023
Yang Yang, Yue Tao, Zixu Li, Yunhe Cui, Jinzhu Zhang, Ying Zhang
At the cardiac developmental level, endocardium-specific expression of Notch signaling regulates cardiac morphogenesis by interacting with multiple signals from the myocardium and epicardium. Disturbances in normal Notch signaling expression induce congenital heart disease and cardiomyopathy (Guillermo et al., 2016; Niessen & Karsan, 2008). In contrast to in humans, Notch receptors and ligands are more abundant in zebrafish but the core activation mechanisms and functions of the Notch signaling pathway are highly conserved across species (Masek & Andersson, 2017). Unlike in the general signaling pathway, both the receptor and ligand of the Notch signaling pathway are membrane proteins, which function as contact points between two cells. When the Notch signaling pathway is activated, the Notch receptor is cleaved three times (1: in the cytoplasm, furin protease in the Golgi apparatus converts Notch protein single-chain precursors into heterodimers via calcium-dependent non-covalent bonding by cleaving the s1 site at the extracellular end of the Notch transmembrane region; 2: when the Notch ligand binds to the extracellular domain of a heterodimer translocated to the cell membrane, ADAM metalloproteinase releases part of the extracellular fragment by cleaving the S2 site on the receptor; 3: γ-secretase cleaves the S3 site on the remaining heterodimer adhered to the cell membrane and forms the Notch intracellular domain) and eventually releases this domain, which forms a transcriptional complex with the CSL in the nucleus, thereby activating expression of the relevant genes (Nemir & Pedrazzini, 2008) (Figure 4).