The Notch signaling pathway is a highly conserved cell signaling system present in most animals. Mammals possess four different notch receptors, referred to as NOTCH1, NOTCH2, NOTCH3, and NOTCH4. The notch receptor is a single-pass transmembrane receptor protein. It is a hetero-oligomer composed of a large extracellular portion, which associates in a calcium-dependent, non-covalent interaction with a smaller piece of the notch protein composed of a short extracellular region, a single transmembrane-pass, and a small intracellular region. Notch signaling promotes proliferative signaling during neurogenesis, and its activity is inhibited by Numb to promote neural differentiation. It plays a major role in the regulation of embryonic development. Notch signaling is dysregulated in many cancers, and faulty notch signaling is implicated in many diseases, including T-cell acute lymphoblastic leukemia (T-ALL), cerebral autosomal-dominant arteriopathy with sub-cortical infarcts and leukoencephalopathy (CADASIL), multiple sclerosis, Tetralogy of Fallot, and Alagille syndrome. Inhibition of notch signaling inhibits the proliferation of T-cell acute lymphoblastic leukemia in both cultured cells and a mouse model.
Discovery In 1914, John S. Dexter noticed the appearance of a notch in the wings of the fruit fly Drosophila ampelophila. The alleles of the gene were identified in 1917 by American evolutionary biologist Thomas Hunt Morgan. Its molecular analysis and sequencing was independently undertaken in the 1980s by Spyros Artavanis-Tsakonas and Michael W. Young. Alleles of the two C. elegans Notch genes were identified based on developmental phenotypes: lin-12 and glp-1. The cloning and partial sequence of lin-12 was reported at the same time as Drosophila Notch by Iva Greenwald.
Mechanism
The Notch protein spans the cell membrane, with part of it inside and part outside. Ligand proteins binding to the extracellular domain induce proteolytic cleavage and release of the intracellular domain, which enters the cell nucleus to modify gene expression. The cleavage model was first proposed in 1993 based on work done with Drosophila Notch and C. elegans lin-12, informed by the first oncogenic mutation affecting a human Notch gene. Compelling evidence for this model was provided in 1998 by in vivo analysis in Drosophila by Gary Struhl and in cell culture by Raphael Kopan. Although this model was initially disputed, the evidence in favor of the model was irrefutable by 2001. The receptor is normally triggered via direct cell-to-cell contact, in which the transmembrane proteins of the cells in direct contact form the ligands that bind the notch receptor. The Notch binding allows groups of cells to organize themselves such that, if one cell expresses a given trait, this may be switched off in neighbouring cells by the intercellular notch signal. In this way, groups of cells influence one another to make large structures. Thus, lateral inhibition mechanisms are key to Notch signaling. lin-12 and Notch mediate binary cell fate decisions, and lateral inhibition involves feedback mechanisms to amplify initial differences. The Notch cascade consists of Notch and Notch ligands, as well as intracellular proteins transmitting the notch signal to the cell's nucleus. The Notch/Lin-12/Glp-1 receptor family was found to be involved in the specification of cell fates during development in Drosophila and C. elegans. The intracellular domain of Notch forms a complex with CBF1 and Mastermind to activate transcription of target genes. The structure of the complex has been determined.
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