Notch is a juxtacrine cell-cell signaling system — short-range, contact-dependent — that controls binary cell-fate decisions and lateral inhibition. Four mammalian receptors (Notch1–4) + five DSL ligands (Delta-like 1/3/4, Jagged1/2). Mechanism: receptor and ligand are both transmembrane on adjacent cells. Ligand-receptor binding pulls the Notch extracellular domain off → exposes S2 cleavage site for ADAM10/17 (TACE) → produces a substrate for γ-secretase (the same complex that cleaves APP — relevant to Alzheimer pharmacology). γ-secretase performs S3 cleavage in the transmembrane domain → releases the Notch intracellular domain (NICD) → NICD translocates to nucleus → binds CSL (CBF-1/RBP-Jκ in mammals) → recruits MAML coactivator + p300 → activates Notch target genes (HES, HEY families — basic-helix-loop-helix repressors of differentiation programs). Output: maintains stem/progenitor states; drives lineage choice (e.g., T-cell vs B-cell at thymic entry; absorptive vs secretory enterocyte; arterial vs venous endothelial). Cancer relevance: NOTCH1 gain-of-function in ~50% of T-ALL (γ-secretase inhibitors trialed); NOTCH3 in vascular CADASIL; oncogene in some solid tumors but tumor suppressor in others (skin SCC). Therapeutics: γ-secretase inhibitors (GSIs — once trialed in Alzheimer for APP-cleavage; failed due to Notch-mediated GI toxicity — goblet-cell metaplasia from disrupted intestinal differentiation); anti-DLL3 (rovalpituzumab) for SCLC; anti-Notch1 antibody Brontictuzumab. Cross-links: cell cycle cdk (proliferation control), wnt beta catenin (parallel/contrasting morphogen), hedgehog smoothened (parallel).
Organ Systems
immune-hematologic
nervous
digestive
cardiovascular
Pathway Steps
DSL ligand (Delta/Jagged) on signal-sending cell → Notch receptor extracellular domain on adjacent cell — via mechanical pulling force; cis-inhibition by ligand on same cell. Notch is a juxtacrine system: a DSL ligand (Delta or Jagged) on one cell engages the Notch receptor on the touching neighbor — signaling needs direct cell-cell contact. Ligand endocytosis generates a mechanical pulling force that exposes the receptor’s cleavage site, while ligand on the same cell instead causes cis-inhibition.
Notch-ligand engagement → S2 cleavage by ADAM10/17 (TACE) — via extracellular domain released; substrate primed for γ-secretase. Ligand pulling exposes Notch’s negative regulatory region to the ADAM10/17 (TACE) metalloproteases, which make the S2 cut that sheds the extracellular domain. This primes the remaining membrane-bound fragment as a substrate for the next, intramembrane cleavage.
S2-cleaved Notch → S3 cleavage by γ-secretase → NICD release — via γ-secretase = presenilin-1/2 + nicastrin + APH-1 + PEN-2 (same as APP). γ-secretase (presenilin-1/2 + nicastrin + APH-1 + PEN-2) makes the S3 cut within the membrane, releasing the Notch intracellular domain (NICD). This is the same protease that processes APP — so γ-secretase inhibitors developed for Alzheimer’s also block Notch, causing characteristic GI/skin toxicity.
NICD (cytoplasmic) → nuclear translocation → CSL binding — via binds CSL → displaces CoR (CIR/SMRT) → recruits MAML + p300 coactivators. Free NICD translocates to the nucleus and binds the DNA-binding factor CSL (RBP-Jκ), converting it from a repressor to an activator by displacing corepressors (CIR/SMRT) and recruiting the coactivator MAML and p300. Notch is thus a membrane receptor that doubles as a direct transcriptional regulator.
NICD-CSL-MAML on enhancer → HES / HEY transcription — via bHLH repressors of differentiation; maintains progenitor state. The NICD-CSL-MAML complex drives the bHLH repressors of the HES/HEY families, which block differentiation genes and maintain progenitor/stem states. This output underlies lateral inhibition — neighboring cells adopting distinct fates — and binary cell-fate decisions throughout development.
NICD ubiquitination (FBXW7) → proteasomal degradation — via NICD half-life minutes; sustained signaling requires continuous receptor activation. Notch signaling is non-catalytic and self-limiting: each receptor is cleaved once and consumed, and nuclear NICD is rapidly phosphorylated and targeted by the FBXW7 ubiquitin ligase for degradation (half-life minutes). Sustained signaling therefore needs continuous ligand engagement — and FBXW7 loss is oncogenic.