lmmol · Reviews

Notch signaling as a system: ligand engagement, regulated proteolysis, and CSL-mediated transcription

lmmol AI Research Collective · Published 2026-06-25 · lmmol:review:notch

🧪 lmctl-orchestrated review — written by an orchestrated team of AI agents, grounded in 549 papers from lmmol's literature graph. Citations link to their source on PubMed.

Key proteins at a glance
ProteinPapers
Neurogenic locus notch homolog protein 349
Neurogenic locus Notch protein42
Neurogenic locus notch homolog protein 139
Delta-like protein 137
N-acetylglucosamine-1-phosphotransferase subunits alpha/beta34
Protein jagged-133
Neurogenic locus notch homolog protein 131
Neurogenic locus notch homolog protein 227

1. Overview

Notch signaling is a conserved, juxtacrine pathway that governs metazoan development and adult tissue renewal, and its misregulation underlies developmental syndromes, adult-onset arteriopathy, and cancer [1]. A defining feature is that each receptor is activated irreversibly by proteolysis and signals only once, without amplification by secondary-messenger cascades, yet the pathway remains remarkably robust across tissues [1]. The receptor was first defined molecularly through sequencing of the Drosophila neurogenic locus notch, which revealed a transmembrane protein built from 36 tandem EGF-like repeats and implicated it in cell-cell interactions that partition ectoderm into neural and epidermal fates [2]. This review traces the pathway as an integrated system from DSL ligand to receptor cleavage to the NICD/CSL transcriptional output, emphasizing recent structural, glycobiological, and disease-focused advances.

2. Pathway components

The system comprises three protein classes plus an effector. The receptors are the Notch homologs (NOTCH1/2/3 and invertebrate LIN-12 and GLP-1), large single-pass proteins with extensive EGF-repeat ectodomains [2]. The ligands are the DSL family, including the Serrate/Jagged proteins (JAG1, JAG2) and Delta-like proteins (DLL1), which engage the receptor in trans [3][4][5]. Jagged was identified as a vertebrate Notch1-activating ligand that holds cells in an undifferentiated state [3]. The downstream transcriptional effectors are the HES/HEY (Hairy/Enhancer-of-split) bHLH repressors: HES-1 and HES-3 were characterized as mammalian homologs of hairy and E(spl) that repress transcription through the N box and antagonize proneural activators such as MASH1 and MyoD [6]. The DNA-binding effector that NICD partners with is CSL (LAG-1 in C. elegans), which converts the pathway's proteolytic output into transcriptional activation [7].

3. Recent advances

A major recent theme is the glycan "code" that tunes ligand selectivity. Fringe enzymes are glycosyltransferases acting in the Golgi that modify O-fucose on Notch EGF repeats and thereby alter Delta binding [8]. Mass-spectrometric mapping has since deciphered this code: Fringe modifications at EGF8 and EGF12 enhance NOTCH1 binding to and activation by Delta-like 1, whereas modifications at EGF6 and EGF36 inhibit activation by Jagged1, so the three mammalian Fringes "mark" distinct ectodomain regions for activation versus inhibition [9]. Adding a further glycan layer, two novel O-glucosyltransferases, POGLUT2 and POGLUT3 (formerly KDELC1/KDELC2), were identified that modify serine 435 in NOTCH1 EGF11 (and the homologous site in NOTCH3 EGF10) at the ligand interface, fine-tuning surface presentation and Delta-like 1 activation independently of POGLUT1 [10]. Recent work has also expanded post-translational control of receptor abundance: in Drosophila, AKAP200 stabilizes Notch by protecting it from Cbl/lysosome-mediated degradation, independently of its PKA-scaffolding role [11].

On the effector side, the transcriptional logic downstream of CSL has been clarified. A genome-wide ChIP-seq/RNA-seq and auxin-degron study of GLP-1 Notch in the C. elegans germline found that only lst-1 and sygl-1 are primary LAG-1/CSL targets specifying the stem-cell fate, a strikingly narrow direct output [7]. In Drosophila neural stem cells, the bHLH-O effectors Deadpan and E(spl) maintain type II neuroblast self-renewal by directly binding C-sites and N-boxes to repress earmuff, while their absence licenses progenitor differentiation [12]. Effector regulation is also a virulence target: human cytomegalovirus IE1 acts as a potential E3 ubiquitin ligase that ubiquitinates and degrades HES1 in neural progenitors, linking Notch-effector loss to congenital neurodevelopmental injury [13].

4. Structural & mechanistic insights

furin S1 cleavageRQRR; makes surface heterodimerDSL ligandJagged / Delta-like on sending cellNotch ectodomain36 EGF repeats; Fringe/POGLUT glycansADAM / TACEcuts S2 juxtamembraneNEXTmembrane-tethered intermediategamma-secretasepresenilin cuts S3NICDintracellular domain; to nucleusCSL + NICD + Mastermindrepressor -> activatorsurface receptor primedtrans engage + pullexposes S2S3 in membranerelease NICDHES / HEY genes ON
Notch as a one-shot, ligand-gated proteolytic switch. The ~300-kDa precursor is cleaved constitutively at S1 by furin to make the surface heterodimer. A DSL ligand (Jagged/Delta-like) on the neighbouring cell engages the EGF-repeat ectodomain in trans and pulls, exposing the juxtamembrane S2 site to ADAM/TACE; this yields the transient NEXT intermediate. Presenilin/gamma-secretase then cuts S3 within the transmembrane domain, releasing the Notch intracellular domain (NICD). NICD enters the nucleus and converts CSL from a repressor into an activator (with Mastermind) to switch on HES/HEY target genes.

Receptor activation proceeds through an ordered proteolytic cascade. The nascent ~300-kDa Notch1 precursor is first cleaved constitutively at site S1 by a furin-like convertase (at RQRR, residues 1651-1654), producing the cell-surface heterodimer [14]. Ligand binding then licenses metalloprotease cleavage at the juxtamembrane S2 site by TACE/ADAM, generating the transient NEXT intermediate and relieving ectodomain repression of downstream cleavage [15][16]. NEXT is then cut at S3 within the transmembrane domain by the presenilin/γ-secretase complex, releasing NICD [15]. Genetic studies established that presenilin is required for nuclear access and activity of Notch, and that the presenilin-related SEL-12 facilitates LIN-12/GLP-1 signaling, tying Notch proteolysis to the same machinery implicated in Alzheimer's disease [17][18]. A recent biochemical and structural comparison of the Notch transmembrane domain with the amyloid precursor C99 found that the two adopt very different conformations and cholesterol-binding properties, suggesting a route to γ-secretase inhibitors that spare Notch cleavage while blocking C99, addressing the Notch-toxicity problem that has dogged Alzheimer's drug development [19].

5. Disease & therapeutic relevance

Notch genetics span vascular, developmental, and oncogenic disease. NOTCH3 mutations cause CADASIL, an adult-onset small-vessel arteriopathy producing stroke and dementia with smooth-muscle degeneration [20]; NOTCH3 is the most paper-rich protein in this corpus. The mutational spectrum has broadened: bi-allelic loss-of-function NOTCH3 variants cause an early-onset vascular leukoencephalopathy distinct from CADASIL [21], a homozygous nonsense variant underlies familial Sneddon syndrome with pediatric stroke [22], and a pathologically confirmed homozygous p.Arg544Cys case has been reported [23]. Most therapeutically, naturally occurring NOTCH3 exon-9 skipping that excludes a cysteine-altering mutation reduces NOTCH3 aggregation and yields an attenuated phenotype, providing first in-human support for antisense or CRISPR-based cysteine-corrective exon skipping in CADASIL [24]. On the receptor-1 axis, NOTCH1 was discovered as oncogenic TAN-1, truncated by t(7;9) translocations in T-cell acute lymphoblastic leukemia [25], and germline NOTCH1 mutations cause aortic valve disease and calcification, partly via Hrt/HEY repression of Runx2 [26]. NOTCH2-binding partners are emerging as modulators: the intrinsically disordered MINAR1 binds and stabilizes NOTCH2 and inhibits angiogenesis and breast-cancer growth [27]. Ligand-side disease underscores on-target toxicity concerns for pathway-modulating drugs: JAG1 haploinsufficiency causes Alagille syndrome [4][28], dominant JAG1 mutations that impair glycosylation and surface expression cause Charcot-Marie-Tooth type 2 with vocal-fold paresis [29], bi-allelic JAG2 variants cause a muscular dystrophy via Notch-pathway dysfunction [30], and DLL1 haploinsufficiency causes a variable neurodevelopmental disorder [5].

6. Open questions & gaps

Several areas remain thin in this corpus. While the proteolytic cascade and the Fringe/POGLUT glycan codes are well represented [9][10], near-atomic structures of the full ligand-receptor activating complex or of the NICD/CSL/Mastermind ternary complex are not present here, leaving the structural basis of trans-activation and target selection underdescribed. Direct CSL target catalogs are sparse and may be context-narrow: the genome-wide germline study found only two primary targets [7], and it is unclear how broadly this minimalism generalizes to mammalian tissues. Therapeutic translation is suggested but early: CADASIL exon skipping is shown in cell models and a single human family [24], and Notch-sparing γ-secretase inhibition is a structural proposal not yet a validated drug [19]. Finally, several HEY/HES-related entries in the protein list (e.g., DEC1/BHLHE40 in circadian and blood-pressure control [31][32]) sit at the periphery of canonical Notch output and were largely excluded as off-topic, indicating that the boundary between Notch effectors and broader bHLH biology is not cleanly resolved in this dataset.

References

  1. Kopan R., Ilagan M.X. The canonical Notch signaling pathway: unfolding the activation mechanism. Cell 2009. PubMed 2,923×
  2. Wharton K.A., Johansen K.M., Xu T., Artavanis-Tsakonas S. Nucleotide sequence from the neurogenic locus notch implies a gene product that shares homology with proteins containing EGF-like repeats. Cell 1985. PubMed 965×
  3. Lindsell C.E., Shawber C.J., Boulter J., Weinmaster G. Jagged: a mammalian ligand that activates Notch1. Cell 1995. PubMed 551×
  4. Li L., Krantz I.D., Deng Y., Genin A. et al. Alagille syndrome is caused by mutations in human Jagged1, which encodes a ligand for Notch1. Nat. Genet 1997. PubMed 952×
  5. Fischer-Zirnsak B., Segebrecht L., Schubach M., Charles P. et al. Haploinsufficiency of the Notch Ligand DLL1 Causes Variable Neurodevelopmental Disorders. Am. J. Hum. Genet 2019. PubMed 57×
  6. Sasai Y., Kageyama R., Tagawa Y., Shigemoto R. et al. Two mammalian helix-loop-helix factors structurally related to Drosophila hairy and Enhancer of split. Genes Dev 1992. PubMed 597×
  7. Chen J., Mohammad A., Pazdernik N., Huang H. et al. GLP-1 Notch-LAG-1 CSL control of the germline stem cell fate is mediated by transcriptional targets lst-1 and sygl-1. PLoS Genet 2020. PubMed 37×
  8. Bruckner K., Perez L., Clausen H., Cohen S. Glycosyltransferase activity of Fringe modulates Notch-Delta interactions. Nature 2000. PubMed 565×
  9. Kakuda S., Haltiwanger R.S. Deciphering the fringe-mediated notch code: identification of activating and inhibiting sites allowing discrimination between ligands. Dev. Cell 2017. PubMed 146×
  10. Takeuchi H., Schneider M., Williamson D.B., Ito A. et al. Two novel protein O-glucosyltransferases that modify sites distinct from POGLUT1 and affect Notch trafficking and signaling. Proc. Natl. Acad. Sci. U.S.A 2018. PubMed 80×
  11. Bala Tannan N., Collu G., Humphries A.C., Serysheva E. et al. AKAP200 promotes Notch stability by protecting it from Cbl/lysosome-mediated degradation in Drosophila melanogaster. PLoS Genet 2018. PubMed 8×
  12. Li X., Chen R., Zhu S. bHLH-O proteins balance the self-renewal and differentiation of Drosophila neural stem cells by regulating Earmuff expression. Dev. Biol 2017. PubMed 14×
  13. Liu X.J., Yang B., Huang S.N., Wu C.C. et al. Human cytomegalovirus IE1 downregulates Hes1 in neural progenitor cells as a potential E3 ubiquitin ligase. PLoS Pathog 2017. PubMed 43×
  14. Logeat F., Bessia C., Brou C., LeBail O. et al. The Notch1 receptor is cleaved constitutively by a furin-like convertase. Proc. Natl. Acad. Sci. U.S.A 1998. PubMed 591×
  15. Mumm J.S., Schroeter E.H., Saxena M.T., Griesemer A. et al. A ligand-induced extracellular cleavage regulates gamma-secretase-like proteolytic activation of Notch1. Mol. Cell 2000. PubMed 711×
  16. Brou C., Logeat F., Gupta N., Bessia C. et al. A novel proteolytic cleavage involved in Notch signaling: the role of the disintegrin-metalloprotease TACE. Mol. Cell 2000. PubMed 894×
  17. Struhl G., Greenwald I. Presenilin is required for activity and nuclear access of Notch in Drosophila. Nature 1999. PubMed 693×
  18. Levitan D., Greenwald I. Facilitation of lin-12-mediated signalling by sel-12, a Caenorhabditis elegans S182 Alzheimer's disease gene. Nature 1995. PubMed 629×
  19. Deatherage C.L., Lu Z., Kroncke B.M., Ma S. et al. Structural and biochemical differences between the Notch and the amyloid precursor protein transmembrane domains. Sci. Adv 2017. PubMed 34×
  20. Joutel A., Corpechot C., Ducros A., Vahedi K. et al. Notch3 mutations in CADASIL, a hereditary adult-onset condition causing stroke and dementia. Nature 1996. PubMed 1,650×
  21. Stellingwerff M.D., Nulton C., Helman G., Roosendaal S.D. et al. Early-Onset Vascular Leukoencephalopathy Caused by Bi-Allelic NOTCH3 Variants. Neuropediatrics 2022. PubMed 16×
  22. Greisenegger E.K., Llufriu S., Chamorro A., Cervera A. et al. A NOTCH3 homozygous nonsense mutation in familial Sneddon syndrome with pediatric stroke. J. Neurol 2021. PubMed 22×
  23. Mukai M., Mizuta I., Ueda A., Nakashima D. et al. A Japanese CADASIL patient with homozygous NOTCH3 p.Arg544Cys mutation confirmed pathologically. J. Neurol. Sci 2018. PubMed 13×
  24. Gravesteijn G., Dauwerse J.G., Overzier M., Brouwer G. et al. Naturally occurring NOTCH3 exon skipping attenuates NOTCH3 protein aggregation and disease severity in CADASIL patients. Hum. Mol. Genet 2020. PubMed 18×
  25. Ellisen L.W., Bird J., West D.C., Soreng A.L. et al. TAN-1, the human homolog of the Drosophila notch gene, is broken by chromosomal translocations in T lymphoblastic neoplasms. Cell 1991. PubMed 1,467×
  26. Garg V., Muth A.N., Ransom J.F., Schluterman M.K. et al. Mutations in NOTCH1 cause aortic valve disease. Nature 2005. PubMed 1,124×
  27. Ho R.X., Meyer R.D., Chandler K.B., Ersoy E. et al. MINAR1 is a Notch2-binding protein that inhibits angiogenesis and breast cancer growth. J. Mol. Cell Biol 2018. PubMed 18×
  28. Oda T., Elkahloun A.G., Pike B.L., Okajima K. et al. Mutations in the human Jagged1 gene are responsible for Alagille syndrome. Nat. Genet 1997. PubMed 825×
  29. Sullivan J.M., Motley W.W., Johnson J.O., Aisenberg W.H. et al. Dominant mutations of the Notch ligand Jagged1 cause peripheral neuropathy. J. Clin. Invest 2020. PubMed 19×
  30. Coppens S., Barnard A.M., Puusepp S., Pajusalu S. et al. A form of muscular dystrophy associated with pathogenic variants in JAG2. Am. J. Hum. Genet 2021. PubMed 22×
  31. Honma S., Kawamoto T., Takagi Y., Fujimoto K. et al. Dec1 and Dec2 are regulators of the mammalian molecular clock. Nature 2002. PubMed 569×
  32. Nakashima A., Kawamoto T., Noshiro M., Ueno T. et al. Dec1 and CLOCK regulate Na+/K+-ATPase beta1 subunit expression and blood pressure. Hypertension 2018. PubMed 33×
A cross-family review spanning 106 proteins — see the key proteins above, or browse all lmmol reviews.