The Notch pathway is a cell-to-cell signaling mechanism conserved in metazoans that regulates the ex
Regulation of Notch Signaling
Similar to other signaling pathways, the Notch pathway is highly regulated at various points in the cell to maintain Notch signaling at levels and intensities (i.e. signal duration) appropriate to the cell type and developmental event. The primary ways in which such regulation is accomplished is by controlling the amount of protein components available for signaling. This is done in a variety of ways, such as controlling the ex
The Notch pathway involves cis and trans interactions among ligands and receptors. In brief, cis interactions occur between ligands and receptors on the same cell, preventing either protein from participating in Notch signal transduction. Trans interactions occur between ligands and receptors on adjacent cells, resulting in activation of the pathway in the cell expressing the participating receptor. Both interactions will be discussed in greater detail later in this chapter. While the precise details of these cis-trans relationships are not fully understood, it is apparent the ratio of them is an essential regulatory mechanism in cells within an equivalence group, which are a group of undifferentiated cells with the potential to adopt various fates. A recent study within a mammalian cell culture system demonstrated it is the ratio of cis and trans interactions between neighboring cells that dictates which cell becomes the signal sending cell (a high Delta/Notch ratio) and which cell becomes the signal-receiving cell (a low Delta/Notch ratio) with activation of the Notch pathway.
Another significant regulatory mechanism of the Notch pathway is gene dosage sensitivity, in which an organism “counts” Notch gene dosage, such that either too much or too little Notch will result in altered function of that cell type. Sensitivity of the pathway to gene dosage was originally identified in flies, but additional studies have shown mammals also exhibit abnormal effects when gene dosage of pathway components is altered. Both the Notch receptor and the ligand Delta are haploinsufficient, meaning a single functional copy of the gene does not produce enough gene product in a diploid organism to function normally, resulting in an abnormal or disease state. Haploinsufficiency is uncommon in diploid organisms, demonstrating the Notch pathway is extremely sensitive to gene dosage for proper function. Similarly, Notch is one of only two genes in flies that are triplomutant, meaning three copies of the Notch gene, rather than the standard two copies, displays the characteristic mutant phenotype of notched wings. A possible reason the Notch pathway is so sensitive to gene dosage may be due to the stoichiometric interactions between pathway components, which suggest even a small stoichiometric difference in receptor and ligand levels may inappropriately restrict signaling in a cell population. Other signaling pathways lack this stoichiometric interaction, instead relying on enzymatic amplification step(s) to increase the amount of signal.
Ligands and Receptors in Notch pathway
Notch signaling is present in most multicellular organisms and has a fundamental role in metazoan development where cell-cell interaction through the Notch receptor can control gene action which dictates cell fate by managing intrinsic and extrinsic developmental cues. Mammals specifically have four types of Notch receptors (Notch1 through Notch 4) and five types of Notch ligands (Jagged1, Jagged 2, Dll1, Dll2, and Dll4) which are based on homology to their Drosophila models Delta and Serrate. The Notch receptor is expressed on the cell surface as a heterodimer where the extracellular domain is made up of epidermal growth factor (EGF)-like repeats engaged in ligand binding, while Lin-12/Notch repeats near the transmembrane region aid in Notch heterodimerization. The intracellular region of Notch contains a RAM 23 domain and cdc10/ankyrin repeats both involved in downstream signaling, as well as a PEST region involved in the degradation of Notch.
Notch ligand molecules Jagged and Dlls both contain Delta/Serrate/Lag-2 (DSL) domains and EGF-like repeats within their extracellular domain that are involved in receptor binding. However, Jagged family members also contain a cysteine-rich domain possibly required to control specificity of Notch receptor binding. A conserved motif called Delta and OSM-11-like proteins (DOS) has been identified within the first two EGF-like repeats, and may contribute in Notch binding.
A von Willebrand factor type C domain is also present on Jagged family members possibly involved in ligand dimerization. Although similarities in the overall organization of the extracellular domains of DSL ligands do exist, there are some structural differences such as in the number of EGF-like repeats and the spacing between these repeats. The intracellular domains of DSL ligands, with the exception of Dll3, contain multiple lysine residues that may be modified by distinct E3 ubiquitin ligases. This modification by ubiquitin is crucial for ligands to activation Notch ligands, however, the specific role for this modification is under developed. Furthermore, most of the DSL ligands contain a C-terminal PDZ (PSD-95/Dlg/ZO-1)-ligand region that promote interaction with the actin cytoskeleton. Dll3 has a collapsed DSL domain, lacking a DOS motif and intracellular domain lysine residues. The majority of Dll3 is found in the Golgi with low cell surface ex