Notch Signaling Pathway

Overview of Notch Signaling pathway

Notch-1, -2, -3, and -4 are highly conserved proteins that regulate a variety of physiological processes, including cell fate, proliferation, angiogenesis, cell survival, and the immune response. As is the case with a large number of other proteins involved in these processes, abnormal Notch activity has been shown to have complex and context-dependent effects on tumorigenesis. Notch undergoes several pre-processing steps during its transport to the membrane following translation. These include the addition of O-fucose in the endoplasmic reticulum by O-fucosyltransferase 1 (POFUT1) and the addition of N-acetylglucosamine in the Golgi by any of three N-acetylglucosaminetransferases found in vertebrates: Lunatic Fringe (LFNG), Manic Fringe (MFNG), and Radical Fringe (RFNG). Notch is also cleaved in the Golgi by Furin, resulting in the formation of a heterodimer composed of the Notch intracellular domain (NICD) and the Notch extracellular domain (NICD) (NECD). After transport to the membrane, the heterodimer exists as a single pass transmembrane protein. Notch is thought to cycle between endocytosis and reinsertion into the membrane, or it may be targeted for lysosomal degradation.

Notch Pathway Activation

Notch is activated through a novel process involving ligand binding and multistep proteolytic processing. Delta, Serrate, and Lag2 (DSL) are Notch ligands found in invertebrates, whereas Delta-like (DLL)-1, -3, -4, Jagged 1, and Jagged 2 are Notch ligands found in mammals. DSL ligands, like Notch, are single pass transmembrane receptors, and Notch activation typically involves direct cell-cell interaction (trans-activation). Following Notch binding, the Notch ligand's intracellular domain is ubiquitinated via the E3 ligase Mind Bomb-1. This initiates the Notch ligand/NECD complex's endocytosis into the ligand-expressing cell. Clathrin, Dynamin, Epsin, and Picalm are all endocytic factors that have been implicated in this process. The mechanical forces generated by these endocytosis-related events may be critical for the Notch pathway's subsequent steps, which include Notch's sequential proteolytic cleavage. TACE/ADAM17 cleaves Notch first, followed by the gamma-Secretase complex composed of Presenilin, PSENEN/PEN-2, APH1, and Nicastrin. It is unknown whether gamma-Secretase cleavage occurs at the membrane or in the endosomal compartment. The NICD is released into the cytosol and translocated to the nucleus following cleavage.

Composition of the Notch Signaling Pathway

Notch signaling is composed primarily of three components: the Notch receptor, the Notch ligand (DSL protein), and an intracellular effector molecule (CSL-DNA binding protein).

Mammalian notch receptors are classified as NOTCH1, NOTCH2, NOTCH3, and NOTCH4. Their primary function is to associate with the ligand and initiate the Notch signaling pathway. Notch proteins (Notch1–Notch4 in vertebrates) are single-pass receptors that are activated by membrane-bound ligands belonging to the Delta (or Delta-like) and Jagged/Serrate families.

The Notch ligand, alternatively referred to as the DSL protein, is a transmembrane protein with a highly conserved molecular structure. Currently, five Notch ligands are known: Delta-like (DLL1, DLL3, DLL4), Jagged1, and Jagged 2. CBF-1 (C-promoter binding factor-1), also known as RBP-J (recombination signal binding protein-j) in mammals. It is a transcription inhibitor. CBF-1 recognizes Notch signaling and binds to specific DNA sequences (GTGGGAA) on gene promoters.