Notch Pathway

Canonical Notch Signaling | Non-Canonical Notch Signaling | Gene Data | Related Information 

The Notch signaling pathway is highly conserved and has a wide range of physiological roles including:

• Intercellular communication
• Regulation of cellular proliferation, differentiation, and apoptosis
• Complex and context-dependent effects on tumorigenesis and cancer

The main function of Notch signaling is regulation of cell fate specification, proliferation, and death for neuronal, cardiac, and endocrine systems, primarily in stem cells. This determines the biological orientation of cells throughout development, allocating different cell types within a tissue. Notch signaling is widely accepted as being involved in the development of most tissues in most species.

While the pathway of Notch appears straightforward, further research has found that Notch can signal via non-canonical mechanisms which may activate or suppress Notch signaling depending on the context.

Notch Receptors

Canonical Notch Signaling

Key steps in the canonical Notch signaling pathway:

1. Notch receptors are generated and glycosylated in the endoplasmic reticulum (ER) then transported to the Golgi apparatus.
2. In the Golgi apparatus, the Notch receptors are cleaved through S1 cleavage and then moved to the cell membrane.
3. The ligand from the sending cell binds to the Notch receptor of the receiving cell.
4. The Notch extracellular domain (NECD) is cleaved by the ADAM protease through S2 cleavage.
5. The Notch intracellular domain (NICD) is cleaved by gamma-Secretase through S3 cleavage.
6. The NICD translocates into the nucleus and forms a complex with CSL/RBPj, Mastermind-like (MAML), and a transcriptional co-activator which induces the transcription of Notch target genes.
7. The NICD activity is regulated by ubiquitination and phosphorylation.

Notch Receptor Pre-Processing

Post-translation, Notch receptors are pre-processed during transport to the membrane. Glycans are attached to the extracellular domain of Notch receptors at the EGF-like repeat domain including O-fucose in the endoplasmic reticulum and N-acetylglucosamine in the Golgi apparatus. Also within the Golgi, the Notch receptor is cleaved through S1 cleavage into a heterodimer of the intracellular domain (NICD) and extracellular domain (NECD) prior to its movement to the cellular membrane.

Notch Glycosylation Targets

• Lunatic Fringe/LFNG
• MFNG
• RFNG
• Protein O-Fucosyltransferase 1/POFUT1
• Protein O-Glucosyltransferase 1/POGLUT1

Notch Cycling and Endocytosis

Notch is thought to be in a cycling state that includes endocytosis and re-insertion into the membrane, NICD cleavage, or lysosomal degradation. Endocytosis mediates the activation of canonical Notch ligands, promotes the recycling or lysosomal degradation of inactive Notch, and has a direct influence on the activity and regulation of Notch.

Notch Endocytosis Targets

• Clathrin Heavy Chain 1/CHC17
• Clathrin Heavy Chain 2/CHC22
• Dynamin
• Mind Bomb 1/MIB1
• Numb

Notch Activation

The Notch signaling pathway is activated through binding of a canonical Notch ligand with the NECD. Mammalian Notch ligands include Delta-like (DLL) and Jagged family proteins, while invertebrate Notch ligands include Delta, Serrate, and Lag2 (DSL). These ligands bind directly from the sending cell to the NECD of the Notch receptor then subsequently undergo S2 cleavage by ADAM protease. After binding, the intracellular domain of the Notch ligand is ubiquitinated via the E3 ligase Mind Bomb-1. This initiates endocytosis of the Notch ligand/NECD complex into the ligand-expressing cell.

Notch Receptors and Canonical Notch Ligands

Notch Pathway Regulators

Notch-related Proteolytic Processing

There are 3 cleavage steps in the canonical Notch signaling pathway:

• S1 cleavage occurs within the Golgi apparatus during the pre-processing of the Notch receptor. This cleaves Notch into a heterodimer of the intracellular (NICD) and extracellular (NECD) domains of Notch.
• S2 cleavage excises the NECD via ADAM protease after the NECD binds with the canonical Notch ligand.
• S3 cleavage is executed by the enzyme gamma-Secretase, which cleaves the NICD into the cytosol before translocation to the nucleus.

Notch-related Proteolytic Processing Targets

Transcription of Notch Target Genes

After S3 cleavage via gamma-Secretase, the NICD is released into the cytosol and translocated to the nucleus. There, the NICD displaces transcriptional repressors and forms a Notch activator complex with CSL/RBPj, Mastermind-like (MAML) and a transcriptional co-activator. This activator complex enables the transcription of Notch target genes. A variety of transcription factors positively or negatively regulate NICD, maintaining a complex equilibrium of Notch activity.

Notch signaling is deactivated through the turnover of the NICD. The NICD is phosphorylated by CDK8 for recognition by the E3 ligase FBW7. This results in ubiquitination and subsequent proteasomal degradation of the NICD. 

Non-Canonical Notch Signaling

The non-canonical signaling pathway refers to a ligand-independent and transcription-independent function of Notch. Non-canonical Notch ligands can activate or inhibit Notch signaling, and have a role in antagonizing Wnt/beta-Catenin signaling.

Notch Gene Data

Related Information

• Interactive Pathway: Notch Signaling Pathways
• Morphogens, Embryonic Patterning and Axis Formation
• Taking Advantage of Notch in Stem Cell Expansion